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Cell Imaging Techniques (Methods in Molecular Biology)
Douglas J. Taatjes
Brooke T. Mossman
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METHODS IN MOLECULAR BIOLOGY ™
Cell Imaging Techniques Methods and Protocols Edited by
Douglas J. Taatjes Brooke T. Mossman
Cell Imaging Techniques
M E T H O D S I N M O L E C U L A R B I O L O G Y™
John M. Walker, SERIES EDITOR 337 337. Ion Channels: Methods and Protocols, edited by J. D. Stockand and Mark S. Shapiro, 2006 336 336. Clinical Applications of PCR: Second Edition, edited by Y. M. Dennis Lo, Rossa W. K. Chiu, and K. C. Allen Chan, 2006 335. Fluorescent Energy Transfer Nucleic Acid 335 Probes: Designs and Protocols, edited by Vladimir V. Didenko, 2006 334. PRINS and In Situ PCR Protocols: Second 334 Edition, edited by Franck Pellestor, 2006 333. Transplantation Immunology: Methods and 333 Protocols, edited by Philip Hornick and Marlene Rose, 2006 332. Transmembrane Signaling Protocols: Second 332 Edition, edited by Hydar Ali and Haribabu Bodduluri, 2006 331 331. Human Embryonic Stem Cell Protocols, edited by Kursad Turksen, 2006 330. Nonhuman Embryonic Stem Cell Protocols, Vol. 330 II: Differentiation Models, edited by Kursad Turksen, 2006 329. 329 Nonhuman Embryonic Stem Cell Protocols, Vol. I: Isolation and Characterization, edited by Kursad Turksen, 2006 328 328. New and Emerging Proteomic Techniques, edited by Dobrin Nedelkov and Randall W. Nelson, 2006 327 327. Epidermal Growth Factor: Methods and Protocols, edited by Tarun B. Patel and Paul J. Bertics, 2006 326. In Situ Hybridization Protocols, Third Edition, 326 edited by Ian A. Darby and Tim D. Hewitson, 2006 325 325. Nuclear Reprogramming: Methods and Protocols, edited by Steve Pells, 2006 324. Hormone Assays in Biological Fluids, edited by 324 Michael J. Wheeler and J. S. Morley Hutchinson, 2006 323. Arabidopsis Protocols, Second Edition, edited by 323 Julio Salinas and Jose J. Sanchez-Serrano, 2006 322. Xenopus Protocols: Cell Biology and Signal 322 Transduction, edited by X. Johné Liu, 2006 321. Microfluidic Techniques: Reviews and Protocols, 321 edited by Shelley D. Minteer, 2006 320 320. Cytochrome P450 Protocols, Second Edition, edited by Ian R. Phillips and Elizabeth A. Shephard, 2006 319 319. Cell Imaging Techniques, Methods and Protocols, edited by Douglas J. Taatjes and Brooke T. Mossman, 2006 318. Plant Cell Culture Protocols, Second Edition, edited 318 by Victor M. Loyola-Vargas and Felipe Vázquez-Flota, 2005 317. Differential Display Methods and Protocols, Second 317 Edition, edited by Peng Liang, Jonathan Meade, and Arthur B. Pardee, 2005
316. 316 Bioinformatics and Drug Discovery, edited by Richard S. Larson, 2005 315. 315 Mast Cells: Methods and Protocols, edited by Guha Krishnaswamy and David S. Chi, 2005 314. 314 DNA Repair Protocols: Mammalian Systems, Second Edition, edited by Daryl S. Henderson, 2006 313. 313 Yeast Protocols: Second Edition, edited by Wei Xiao, 2005 312. 312 Calcium Signaling Protocols: Second Edition, edited by David G. Lambert, 2005 311. 311 Pharmacogenomics: Methods and Protocols, edited by Federico Innocenti, 2005 310. 310 Chemical Genomics: Reviews and Protocols, edited by Edward D. Zanders, 2005 309. 309 RNA Silencing: Methods and Protocols, edited by Gordon Carmichael, 2005 308 Therapeutic Proteins: Methods and Protocols, 308. edited by C. Mark Smales and David C. James, 2005 307 Phosphodiesterase Methods and Protocols, 307. edited by Claire Lugnier, 2005 306 Receptor Binding Techniques: Second Edition, 306. edited by Anthony P. Davenport, 2005 305. 305 Protein–Ligand Interactions: Methods and Applications, edited by G. Ulrich Nienhaus, 2005 304. 304 Human Retrovirus Protocols: Virology and Molecular Biology, edited by Tuofu Zhu, 2005 303. 303 NanoBiotechnology Protocols, edited by Sandra J. Rosenthal and David W. Wright, 2005 302. 302 Handbook of ELISPOT: Methods and Protocols, edited by Alexander E. Kalyuzhny, 2005 301. 301 Ubiquitin–Proteasome Protocols, edited by Cam Patterson and Douglas M. Cyr, 2005 300. 300 Protein Nanotechnology: Protocols, Instrumentation, and Applications, edited by Tuan Vo-Dinh, 2005 299 Amyloid Proteins: Methods and Protocols, 299. edited by Einar M. Sigurdsson, 2005 298. 298 Peptide Synthesis and Application, edited by John Howl, 2005 297. 297 Forensic DNA Typing Protocols, edited by Angel Carracedo, 2005 296 Cell Cycle Control: Mechanisms and Protocols, 296. edited by Tim Humphrey and Gavin Brooks, 2005 295. 295 Immunochemical Protocols, Third Edition, edited by Robert Burns, 2005 294. 294 Cell Migration: Developmental Methods and Protocols, edited by Jun-Lin Guan, 2005 293 Laser Capture Microdissection: Methods and 293. Protocols, edited by Graeme I. Murray and Stephanie Curran, 2005 292 DNA Viruses: Methods and Protocols, edited by 292. Paul M. Lieberman, 2005 291 Molecular Toxicology Protocols, edited by 291. Phouthone Keohavong and Stephen G. Grant, 2005
M ET H O D S I N M O L E C U L A R B I O L O GY™
Cell Imaging Techniques Methods and Protocols
Douglas J. Taatjes Brooke T. Mossman Department of Pathology University of Vermont Burlington, VT
© 2006 Humana Press Inc. 999 Riverview Drive, Suite 208 Totowa, New Jersey 07512 www.humanapress.com All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise without written permission from the Publisher. Methods in Molecular BiologyTM is a trademark of The Humana Press Inc. All papers, comments, opinions, conclusions, or recommendations are those of the author(s), and do not necessarily reflect the views of the publisher. This publication is printed on acid-free paper. ∞ ANSI Z39.48-1984 (American Standards Institute)Permanence of Paper for Printed Library Materials. Cover design by Patricia F. Cleary Cover illustration: Background: whole-cell experiment combined with a pre-embedding method: CLS-RCM image of a 90-nm Epon section (Chap. 18, Fig. 15D, p. 397). Inset, upper left: electron micrograph depicting a porosome close to a microvillus at the apical plasma membrane of a pancreatic acinar cell (Chap. 15, Fig. 2B, p. 303). Upper right: NSOM image of an antibody-labeled mouse macrophage (Chap. 14, Fig. 3C, p. 282). Lower left: metaphase analysis with multiple single-gene probes (Chap. 12, Fig. 1, p. 238). Lower right: adeno-associated virus serotype 5 (Chap. 7, Fig. 9, p. 161). For additional copies, pricing for bulk purchases, and/or information about other Humana titles, contact Humana at the above address or at any of the following numbers: Tel.: 973-256-1699; Fax: 973-256-8341; E-mail: [email protected]; or visit our Website: www.humanapress.com Photocopy Authorization Policy: Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by Humana Press Inc., provided that the base fee of US $30.00 per copy is paid directly to the Copyright Clearance Center at 222 Rosewood Drive, Danvers, MA 01923. For those organizations that have been granted a photocopy license from the CCC, a separate system of payment has been arranged and is acceptable to Humana Press Inc. The fee code for users of the Transactional Reporting Service is: [1-58829-157-X/06 $30.00]. Printed in the United States of America. 10 9 8 7 6 5 4 3 2 1 eISBN 1-59259-993-1 ISSN 1064-3745 Library of Congress Cataloging-in-Publication Data Cell imaging techniques: methods and protocols / edited by Douglas J. Taatjes, Brooke T. Mossman. p. cm. -- (Methods in molecular biology ; 319) Includes bibliographical references and index. ISBN 1-58829-157-X (alk. paper) 1. Cytology. 2. Microscopy. 3. Molecular biology. 4. Imaging systems in biology. I. Taatjes, Douglas J. II. Mossman, Brooke T., 1947- III. Methods in molecular biology (Clifton, N.J.) ; 319 QH585.C463 2005 611'.0181--dc22 2005046145
Preface In 1665, a book was published that inaugurated the use of the microscope to investigate the natural world. The author was Robert Hooke, a talented artist, architect, and amateur scientist. Hooke wrote Micrographia: Or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries Thereupon, at the behest of the newly chartered Royal Society in London, for whom he was working as curator of scientific experiments. In Micrographia, he presented the first detailed observations of everyday objects made with his self-constructed light microscope. Although this book contains a treasure-trove of drawings (in Hooke's own hand) of the appearance of various animate and inanimate specimens as viewed in magnified form, one of the drawings and its associated description stands out as particularly germane to our present topic of cell imaging techniques. In his description of a thin piece of clear cork cut with a penknife and observed with his microscope, Hooke described the honey-comb-like appearance of the cork with "pores" or "cells" representing the basic structural unit. This represents the first printed reference of the term "cell" to describe a unit structure of an organism. In the 340 years since the publication of Micrographia, a multitude of new microscopy-based systems have evolved for the observation of cells. Indeed, many of these techniques have been developed in the past few decades. Recent books have sought to present single volumes detailing methods for specific types of microscopy, such as confocal scanning laser microscopy, atomic force microscopy, and electron microscopy. In the present book, we have sought to present an eclectic collection of what we consider some of the essential stateof-the-art methods for imaging cells and molecules. Cell Imaging Techniques: Methods and Protocols has been organized to begin with light microscopic methods to observe molecules such as mRNA, calcium, and collagen. Chapters covering confocal scanning laser microscopy, quantitative computer-assisted image analysis, laser scanning cytometry, laser capture microdissection, microarray image scanning, near-field scanning optical microscopy, atomic force microscopy, and reflection contrast microscopy follow. The book then finishes with chapters on preparative methods for transmission electron microscopy of particles and cells. We have tried to arrange the chapters in a logical format, beginning with light microscopy techniques, proceeding through scanning probe-type v
techniques, and ending with electron microscopy. The chapter on reflection contrast microscopy serves as a link between light and electron microscopy. Although Cell Imaging Techniques: Methods and Protocols is primarily intended to convey detailed methods and protocols for cell imaging, we have also included some review-type chapters to set the stage for the protocol-driven chapters. Moreover, given the incredible breadth of microscopy-based imaging techniques available today, we tried to include many that might not have been covered in detail in previous books. By necessity, we have had to exclude many valuable and marvelous techniques (multiphoton confocal microscopy, for one), but are secure in the knowledge that they have been comprehensively treated in other volumes of the Methods in Molecular Biology series. We believe that Cell Imaging Techniques: Methods and Protocols will be useful for those involved in seeking a variety of microscopy-based techniques for imaging cells and molecules. With the proliferation of core "cell imaging facilities" at universities, hospitals, and pharmaceutical and biotechnology companies throughout the world, this volume should provide a handy reference or starting point for researchers seeking the latest information and protocols for a wide variety of cell imaging techniques. We hope that readers will find value in the techniques presented herein and might even be tempted to try some techniques they had not considered previously. Finally, we would like to thank those associated with the production of this book. First, the authors themselves for agreeing to take the time to prepare their chapters in a timely manner and in a form filled with technical details not usually present in a research publication. It was not an easy task, and we thank them for their efforts. Second, we would like to thank Professor John Walker, the series editor, for his helpful insights, interest in the book, and his timely response to our queries. Third, we would like to express our appreciation to Craig Adams and the staff at Humana Press for their patience and editorial efforts in the production of the book, and to Marilyn Wadsworth at the University of Vermont for her invaluable assistance in our editorial tasks. Finally, the color reproduction of images, so important in a volume like this, would not have been possible without the generous financial support of the Optical Analysis Corporation (Nashua, NH), JMAR Technologies, Inc. (South Burlington, VT), and the Department of Pathology, University of Vermont (Burlington, VT). Douglas J. Taatjes Brooke T. Mossman
Contents Preface ........................................................................................................ v Contributors ............................................................................................... ix Color Plates ............................................................................................ xiii 1 Molecular Beacons: Fluorescent Probes for Detection of Endogenous mRNAs in Living Cells Diana P. Bratu ................................................................................ 1 2 Second-Harmonic Imaging of Collagen Guy Cox and Eleanor Kable .......................................................... 15 3 Visualizing Calcium Signaling in Cells by Digitized Wide-Field and Confocal Fluorescent Microscopy Michael Wm. Roe, Jerome F. Fiekers, Louis H. Philipson, and Vytautas P. Bindokas ......................................................... 37 4 Multifluorescence Labeling Techniques and Confocal Laser Scanning Microscopy on Lung Tissue Maria Stern, Douglas J. Taatjes, and Brooke T. Mossman ........... 67 5 Evaluation of Confocal Microscopy System Performance Robert M. Zucker ......................................................................... 77 6 Quantitative Analysis of Atherosclerotic Lesion Composition in Mice Marilyn P. Wadsworth, Burton E. Sobel, David J. Schneider, Wendy Tra, Hans van Hirtum, and Douglas J. Taatjes ............. 137 7 Applications of Microscopy to Genetic Therapy of Cystic Fibrosis and Other Human Diseases Thomas O. Moninger, Randy A. Nessler, and Kenneth C. Moore ........................................................... 153 8 Laser Scanning Cytometry: Principles and Applications Piotr Pozarowski, Elena Holden, and Zbigniew Darzynkiewicz .............................................. 165 9 Near-Clinical Applications of Laser Scanning Cytometry David A. Rew, Gerrit Woltmann, and Davinder Kaur ............... 193 10 Laser Capture Microdissection Virginia Espina, John Milia, Glendon Wu, Stacy Cowherd, and Lance A. Liotta ................................................................. 213 vii
11 Analysis of Asbestos-Induced Gene Expression Changes in Bronchiolar Epithelial Cells Using Laser Capture Microdissection and Quantitative Reverse Transcriptase–Polymerase Chain Reaction Christopher B. Manning, Brooke T. Mossman, and Douglas J. Taatjes ............................................................ 231 12 New Approaches to Fluorescence In Situ Hybridization Sabita K. Murthy and Douglas J. Demetrick .............................. 237 13 Microarray Image Scanning Latha Ramdas and Wei Zhang .................................................... 261 14 Near-Field Scanning Optical Microscopy in Cell Biology and Cytogenetics Michael Hausmann, Birgit Perner, Alexander Rapp, Leo Wollweber, Harry Scherthan, and Karl-Otto Greulich ...... 275 15 Porosome: The Fusion Pore Revealed by Multiple Imaging Modalities Bhanu P. Jena .............................................................................. 295 16 Secretory Vesicle Swelling by Atomic Force Microscopy Sang-Joon Cho and Bhanu P. Jena .............................................. 317 17 Imaging and Probing Cell Mechanical Properties With the Atomic Force Microscope Kevin D. Costa ............................................................................ 331 18 Reflection Contrast Microscopy: The Bridge Between Light and Electron Microscopy F. A. Prins, I. Cornelese-ten Velde, and E. de Heer ................... 363 19 Three-Dimensional Analysis of Single Particles by Electron Microscopy: Sample Preparation and Data Acquisition Teresa Ruiz and Michael Radermacher ...................................... 403 20 Three-Dimensional Reconstruction of Single Particles in Electron Microscopy: Image Processing Michael Radermacher and Teresa Ruiz ...................................... 427 21 A New Microbiopsy System Enables Rapid Preparation of Tissue for High-Pressure Freezing Dimitri Vanhecke, Peter Eggli, Werner Graber, and Daniel Studer ................................................................... 463 Index ...................................................................................................... 479
Contributors VYTAUTAS P. BINDOKAS • Department of Neurobiology, Pharmacology, and Physiology, University of Chicago, Chicago, IL DIANA P. BRATU • Department of Molecular Genetics, Public Health Research Institute, Newark, NJ; Program of Molecular Medicine, University of Massachusetts Medical School, Worcester, MA SANG-JOON CHO • Department of Physiology, Wayne State University School of Medicine, Detroit, MI I. CORNELESE-TEN VELDE • Department of Pathology, Leiden University Hospital, Leiden, The Netherlands KEVIN D. COSTA • Department of Biomedical Engineering, Columbia University, New York, NY STACY COWHERD • Laboratory of Pathology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD GUY COX • Electron Microscope Unit, University of Sydney, Sydney, Australia ZBIGNIEW DARZYNKIEWICZ • The Brander Cancer Research Institute, New York Medical College, Valhalla, NY E. DE HEER • Department of Pathology, Leiden University Hospital, Leiden, The Netherlands DOUGLAS J. DEMETRICK • Departments of Pathology and Laboratory Medicine, Oncology and Biochemistry and Molecular Biology, University of Calgary, Alberta, Canada PETER EGGLI • Anatomical Institute, University of Bern, Bern, Switzerland VIRGINIA ESPINA • Center for Applied Proteomics and Molecular Medicine, George Mason University, Manassas, VA JEROME F. FIEKERS • Department of Anatomy and Neurobiology, University of Vermont, Burlington, VT WERNER GRABER • Anatomical Institute, University of Bern, Bern, Switzerland KARL-OTTO GREULICH • Department of Single Cell and Single Molecule Techniques, Institute of Molecular Biotechnology, Jena, Germany MICHAEL HAUSMANN • Kirchoff Institute of Physics, University of Heidelberg, Heidelberg, Germany ELENA HOLDEN • CompuCyte Corporation, Cambridge, MA BHANU P. JENA • Department of Physiology, Wayne State University School of Medicine, Detroit, MI ix
ELEANOR KABLE • Electron Microscope Unit, University of Sydney, Sydney, Australia DAVINDER KAUR • Laser Cytometry Facility, Glenfield Hospital, University of Leicester, UK LANCE A. LIOTTA • Center for Applied Proteomics and Molecular Medicine, George Mason University, Manassas, VA CHRISTOPHER B. MANNING • Department of Pathology, University of Vermont, Burlington, VT JOHN MILIA • Arcturus Bioscience Inc., Mountain View, CA THOMAS O. MONINGER • Central Microscopy Research Facility and Department of Internal Medicine, University of Iowa, Iowa City, IA KENNETH C. MOORE • Central Microscopy Research Facility, University of Iowa, Iowa City, IA BROOKE T. MOSSMAN • Department of Pathology, University of Vermont, Burlington, VT SABITA K. MURTHY • Head of Division, Medical Genetics, Al Wasl Hospital, Dubai, United Arab Emirates RANDY A. NESSLER • Central Microscopy Research Facility and Department of Pediatrics, University of Iowa, Iowa City, IA BIRGIT PERNER • Department of Single Cell and Single Molecule Techniques, Institute of Molecular Biotechnology, Jena, Germany LOUIS H. PHILIPSON • Department of Medicine, University of Chicago, Chicago, IL PIOTR POZAROWSKI • The Brander Cancer Research Institute, New York Medical College, Valhalla, NY, and Department of Clinical Immunology, School of Medicine, Lublin, Poland F. A. PRINS • Department of Pathology, Leiden University Hospital, Leiden, The Netherlands MICHAEL RADERMACHER • Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT LATHA RAMDAS • Cancer Genomics Core Laboratory, University of Texas M.D. Anderson Cancer Center, Houston, TX ALEXANDER RAPP • Department of Single Cell and Single Molecule Techniques, Institute of Molecular Biotechnology, Jena, Germany DAVID A. REW • Southampton University Hospitals, Southampton, UK MICHAEL WM. ROE • Department of Medicine, University of Chicago, Chicago, IL TERESA RUIZ • Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT HARRY SCHERTHAN • Max-Planck-Institute for Molecular Genetics, Berlin, Germany
DAVID J. SCHNEIDER • Department of Medicine, University of Vermont, Burlington, VT BURTON E. SOBEL • Department of Medicine, University of Vermont, Burlington, VT MARIA STERN • Department of Pathology, University of Vermont, Burlington, VT DANIEL STUDER • Anatomical Institute, University of Bern, Bern, Switzerland DOUGLAS J. TAATJES • Department of Pathology, and Microscopy Imaging Center, University of Vermont, Burlington, VT WENDY TRA • Department of Pathology, and Microscopy Imaging Center, University of Vermont, Burlington, VT DIMITRI VANHECKE • Anatomical Institute, University of Bern, Bern, Switzerland HANS VAN HIRTUM • Department of Pathology, and Microscopy Imaging Center, University of Vermont, Burlington, VT MARILYN P. WADSWORTH • Department of Pathology, and Microscopy Imaging Center, University of Vermont, Burlington, VT LEO WOLLWEBER • Department of Single Cell and Single Molecule Techniques, Institute of Molecular Biotechnology, Jena, Germany GERRIT WOLTMANN • Glenfield Hospital, University of Leicester, UK GLENDON WU • Laboratory of Pathology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD WEI ZHANG • Cancer Genomics Core Laboratory, University of Texas M.D. Anderson Cancer Center, Houston, TX ROBERT M. ZUCKER • Reproductive Toxicology Division, National Health and Environmental Effects Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC
Color Plates Color Plates follow p. 274. COLOR PLATE 1 COLOR PLATE 2
COLOR PLATE 3
COLOR PLATE 10
Pseudocolored image of mouse islets loaded with Fura-2. (Chapter 3, Fig. 1; see full caption on p. 52 and discussion on p. 51.) Expression of mitochondrially-targeted ratiometric pericam (RPC-mt) in neuroendocrine cells: laser scanning confocal images of AtT20 cells. (Chapter 3, Fig. 3; see full caption and discussion on p. 58.) Triple-labeling using mouse monoclonal PKC (blue), rabbit polyclonal p-ERK (red), and rat monoclonal Ki-67 (green) antibodies on sham animals (A) and animals exposed to crocidolite asbestos for 4 days (B,C). (Chapter 4, Fig. 1; see full caption on p. 74 and discussion on p. 73.) Methods used to check spectral registration of different laser lines. (Chapter 5, Fig. 6; see full caption on p. 93 and discussion on p. 92.) Gray-scale fluorescent capture of DAPI-stained nuclei in mouse atherosclerotic lesion, and with red pseudocolor overlay. (Chapter 6, Fig. 2; see full caption on p. 144 and discussion on p. 142.) Polarized light microscopy assists in more accurately discriminating atherosclerotic lesion borders. (Chapter 6, Fig. 3; see full caption on p. 149 and discussion on p. 146.) Assay of cells incubated with either free drug or drug-encapsulated microspheres by laser scanning cytometry. (Chapter 9, Fig. 3; see full caption on p. 201 and discussion on p. 200.) Time-course experiments of doxorubicin encapsulated microspheres. (Chapter 9, Fig. 4; see full caption on p. 203 and discussion on p. 202.) Gate settings for the analysis of sputum bronchial epithelial cells. (Chapter 9, Fig. 9; see full caption on p. 210 and discussion on p. 207.) Annotation of stitched images on AutoPix system. (Chapter 10, Fig. 6; see full caption on p. 224 and discussion on p. 223.)
COLOR PLATE 11 Metaphase analysis with multiple single-gene probes. Multicolor FISH showing simultaneous localization of three human genomic BAC probes. (Chapter 12, Fig. 1; see full caption on p. 238 and discussion on p. 237.) COLOR PLATE 12 FISH of tissue microarray breast carcinoma specimens. (Chapter 12, Fig. 2; see full caption on p. 243 and discussion on p. 242.) COLOR PLATE 13 FISH of LCM-prepared nuclei. (Chapter 12, Fig. 3; see full caption on p. 244 and discussion on p. 242.) COLOR PLATE 14 Setting parameters for microarray image scanning: images for different gains and corresponding scatterplots. (Chapter 13, Fig. 3; see full caption on p. 272 and discussion on p. 271.) COLOR PLATE 15 NSOM images of the telomeric region of a human meiotic chromosome core after immunostaining of TRF2 by Cy3-labeled antibodies. (Chapter 14, Fig. 6; see full caption on p. 288 and discussion on p. 287.) COLOR PLATE 16 Porosomes: dynamics of depressions following stimulation of secretion. (Chapter 15, Fig. 3; see full caption on p. 304 and discussion on p. 302.) COLOR PLATE 17 Electron micrograph of porosomes in neurons. (Chapter 15, Fig. 5; see full caption on p. 306 and discussion on p. 304.) COLOR PLATE 18 Monitoring height and width of zymogen granule (arrow) after exposure to GTP. (Chapter 16, Fig. 6; see full caption and discussion on p. 325.) COLOR PLATE 19 Immunoblot assay demonstrating the presence of AQP1 antibody in SLO permeabilized zymogen granule (Chapter 16, Fig. 7; see full caption on p. 327 and discussion on p. 326.)
1 Molecular Beacons Fluorescent Probes for Detection of Endogenous mRNAs in Living Cells Diana P. Bratu Summary A novel approach for detecting nucleic acid in solution has been adopted for real-time imaging of native mRNAs in living cells. This method utilizes hybridization probes, called "molecular beacons," that generate fluorescent signals only when they are hybridized to a complementary target sequence. Nuclease-resistant molecular beacons are designed to efficiently hybridize to accessible regions within RNAs and then be detected via fluorescence microscopy. The target regions chosen for probe binding are selected using two computer algorithms, mfold and OligoWalk, that predict the secondary structure of RNAs and help narrow down sequence stretches to which the probes should bind with high affinity in vivo. As an example, molecular beacons were designed against regions of oskar mRNA, microinjected into living Drosophila melanogaster oocytes and imaged via confocal microscopy. Key Words: Molecular beacons; fluorescent probes; hybridization; secondary structure prediction; RNA localization; live-cell imaging.
1. Introduction The direct visualization of specific mRNAs in living cells has been desirable for some time for accelerating the studies of intracellular RNA trafficking and localization, just as green fluorescent protein has stimulated the study of specific proteins in vivo. Tyagi and Kramer have developed a general method using hybridization probes called "molecular beacons," which generate fluorescence signals as they hybridize to complementary nucleic acid target sequences (see Fig. 1) (1). Unbound molecular beacons are nonfluorescent and it is not necessary to remove excess probes to detect the hybrids. These probes bind to their targets spontaneously at physiological temperatures; thus, their introduction into cells is sufficient to illuminate target mRNAs (2–4). From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Bratu
Fig. 1. Principle of operation. Molecular beacons are oligonucleotides that possess complementary sequences on either end of a probe sequence, enabling the molecule to assume a hairpin configuration, in which a fluorophore and a quencher are held in close proximity. When these probes hybridize to a complemetary target, the formation of a probe–target hybrid disrupts the hairpin stem, removing the fluorophore from the vicinity of the quencher, and restoring the probe's fluorescence.
Theoretically, any sequence within a target RNA can be chosen as a site for molecular beacon binding. The endless possibilities give one the confidence that such regions are easily identified. However, the extent of target accessibility is primarily a consequence of complex secondary and tertiary intramolecular structures, which are difficult to predict and can mask many of these regions. Furthermore, inside the cell, mRNAs exist in association with proteins that further occlude parts of the mRNA. Although regions involved in protein binding can only be identified by experimental analysis, reasonable attempts can be made to predict the regions that are not involved in tight secondary structures. So far, several in vitro assays and theoretical algorithms are available to help identify putative target sites within mRNA sequences, as well as probes with high affinity for binding (5–8). The mfold RNA-folding algorithm is used to predict the most thermodynamically stable secondary structure along with an ensemble of suboptimal structures (9). Because none of these structures can be considered to represent the naturally occurring conformation, the parameters that describe the entire ensemble are analyzed. The number of candidate sites is winnowed down by employing a second algorithm. OligoWalk scans the folded RNA sequence for regions to which various-length oligonucleotides are capable of binding (10). With consideration of the base composition of each oligonucleotide and of the predicted
Molecular Beacons
secondary structure of the RNA, the output provides information about the stability of the expected hybrid and thus identifies potential target regions. Once identified, molecular beacons specific for those regions are designed and synthesized. To detect an intracellular target, these probes need to be stable inside the cell and not perturb or destroy its target. Conventional molecular beacons synthesized from deoxyribonucleotides are not suitable for use inside living cells because the single-stranded probe sequence of conventional molecular beacons are subject to digestion by single-strand-specific cellular deoxyribonucleases. Therefore, for in vivo studies, molecular beacons are synthesized from unnatural nucleotides possessing an oxymethyl group in place of a hydrogen atom at the 2′-position of their pentose moiety. These modified molecular beacons are resistant to cellular nucleases, and upon binding to RNA, they do not form a substrate for ribonuclease H, thus maintaining the integrity of the target molecule (2). This chapter describes the steps necessary to find accessible probe-binding target sites within an RNA sequence and to design efficient molecular beacons for RNA detection in vivo. The last section provides an example of RNA imaging in living Drosophila oocytes. 2. Materials 1. 2. 3. 4. 5. 6.
8. 9. 10.
RNAstructure (free download on http://rna.chem.rochester.edu/index.html for PC only). Spectrofluorometer (Photon Technology International, South Brunswick, NJ). Spetrofluorometric thermal cycler (Applied Biosystems Prism 7700). Nuclease-free water (Ambion Inc., Austin, TX). TE buffer: 1 mM EDTA, 10 mM Tris-HCl, pH 8.0. 10X Phosphate-buffered saline (PBS) with CaCl2 and MgCl2 (Sigma, St. Louis, MO). Dilute to 1X with nuclease-free water or diethyl pyrocarbonate (DEPC)treated water. Store at room temperature. Molecular beacon. Dissolve stock solutions in nuclease-free water and store at –20°C. Dilute working solutions in 1X PBS, keep protected from light, and store at –20°C for 1 mo. Oligonucleotide target complimentary to the probe sequence of the molecular beacon. Dissolve in TE buffer and store at –20°C. In vitro-transcribed RNA. Hybridization buffer: 1 mM MgCl2, 20 mM Tris-HCl, pH 8.0.
3. Methods 3.1. Selection of RNA Target Regions 3.1.1. Secondary Structure Prediction Using mfold Input the mRNA sequence on the mfold RNA server (http://www.bioinfo. rpi.edu/~zukerm/) and fold it using the default settings (9). An immediate output is obtained unless the RNA is longer than 800 bp, in which case it is
folded as a "batch." The completed job is posted within a couple of hours, depending on the server's availability. The algorithm predicts the most thermodynamically stable secondary structure along with an ensemble of suboptimal structures (the maximum limit can be chosen prior to folding the RNA; a default value of 50 is sufficient for this analysis). Because none of these structures can be considered to represent a naturally occurring conformation, the parameters describing the entire ensemble must be analyzed. The first parameter is ss-count, which defines the probability of a nucleotide to be singlestranded. The second parameter is P-num, a value that denotes the total number of different basepairs that can be formed by a particular base within the set of foldings. These values are assigned to each nucleotide in the mRNA sequence (see Note 1). Once the set of foldings is generated, obtain the following files to be analyzed. • To view the predicted secondary structure of the RNA, select the jpg option of the first listed structure (the most stable structure in the set). • To view this folded structure representing the ss-count values, choose the "ss-count" annotation, where each nucleotide (annotated as a dot or character) will be indicated by a color representing the value found in the "ss-count table." Clicking on the structure image converts this to a color-coded fold (see Note 2). • This window also offers access to the "ss-count table" file. To obtain the actual values, open the file and save it. Open this file in Microsoft Excel; this will provide the opportunity to graph the ss-count value for each nucleotide. • The P-num values are found in a file accessible from the first output screen. Follow the same steps as for the "ss-count table" to view the secondary structure, save the file, and plot the graph of P-num values for the entire sequence.
3.1.1.1. CRITERIA
SELECTING ACCESSIBLE TARGET REGIONS
The ss-count/P-num graph indicates regions of structural plasticity. • Plot both ss-count and P-mm for each number nucleotide. Adjust ss-count values by x before plotting. The ss-count values are adjusted in order to make them comparable with the range of values for P-num. x = P-mmmax / (ss-countmax + 1) ss-countadj = (ss-count + 1) × x • Evaluate the graph and choose regions with the following criteria: ss-count = high (single-stranded) P-mm = low (well determined) OR high (poorly determined)
Values for ss-count of more than 50% (probability of being singlestranded) are considered high and are represented by warm colors in the color-annotated secondary structure. Nucleotides involved in double-stranded regions are represented by cool colors. If these regions are well determined,
the P-num values for the nucleotides involved are very low. If a region is flexible, meaning the nucleotides are not found in the same basepair but they form several kinds of basepairs within the predicted structures, then the P-mm is high, representing a poorly determined structure. Such regions are easily determined when evaluating the P-num color-annotated secondary structure (see Note 3). 3.1.2. RNAstructure: OligoWalk The RNAstructure program is a faithful recreation of mfold for Windows (10). The OligoWalk program overcomes the need to synthesize a large number of oligonucleotides by using thermodynamic data to find oligonucleotides that bind strongly to a target RNA while offering a fast search interface. Utilizing the most current set of thermodynamic parameters for nucleic acid secondary structure, this algorithm calculates the equilibrium affinity of each set-length complementary oligonucleotide and predicts its overall free energy of binding while taking into account duplex stability, local secondary structure within the target RNA, as well as intermolecular and intramolecular secondary structures formed by the oligonucleotide (10–15). For each oligonucleotide chosen, the following thermodynamic parameters (∆G) are calculated in kcal/mol: ∆Goverall: net ∆G of oligonucleotide–target binding (with consideration of all contributions, including breaking target structure and oligonucleotide-self structure) ∆Gduplex (∆Gbinding): ∆G of the oligonucleotide–target hybrid from unstructured states ∆Gbreak target: energy penalty resulting from the breaking of intramolecular target basepairs when the oligonucleotide is bound ∆Goligo-self: ∆G of the intramolecular oligonucleotide structure ∆Goligo-oligo: ∆G of the intermolecular oligonucleotide structure
Before running the OligoWalk module, several selections must be made. First, a set of optimal and suboptimal structures of the RNA is predicted, using the folding feature of RNAstructure, creating a '.ct' file (connect table) that contains the sequence and basepair information (the sequence must be in caps). Then, the "mode" that determines the types of ∆G calculation for a given sequence is chosen. Use the default mode, "break local structure." In this mode, the target sequence breaks wherever the oligonucleotide binds (∆Gbreak target), oligonucleotides lose pairs in self-structures (∆Goligo-oligo and ∆Goligo-self) and gain pairs in oligonucleotide–target binding (∆Gduplex). The option to include all suboptimal structures of the target is chosen to determine the total free-energy loss of the target. Each structure's free-energy loss is weighed according to the free energy of the structure.
Once these options are set, OligoWalk calculations are run for an oligonucleotide concentration of 100 ng/µL, ranging in length from 18 to 25 nucleotides. These oligonucleotides are "walked" along the RNA sequence, one nucleotide at a time, until the criteria below are met (see Note 4). It is possible for a longer region of the target to be chosen as favorable, not just the region comprising the length of the oligonucleotide. ∆Goligo-self and ∆Goligo-oligo are disregarded, as the molecular beacon structure is analyzed separately using mfold (see subheading 3.2.1.). 3.1.2.1. CRITERIA
SELECTING TARGET REGIONS
*∆Goverall and ∆Gduplex = most negative (–) kcal/mol values; *∆Gduplex – ∆Goverall ≤ –10 kcal/mol; *∆Gbreak = lowest positive (+) kcal/mol values.
3.2. Molecular Beacons 3.2.1. Design The probe length comprising the hairpin loop can range between 15 and 25 nucleotides. This is dependent on the target region length chosen to be accessible for probe binding. Longer sequence stretches permit probe design flexibility. The %GC composition of the probe sequence should range between 40% and 55%. Two arm sequences designed to be complementary to each other are then added at the respective ends of the probe sequence. Because the rate of hybridization of a molecular beacon is influenced by the stability of its stem, the composition of these sequences is also very important. In practice, the length of the arm sequences is four or five nucleotides and are composed mostly of G/C's (see Note 5). Using Zucker's mfold folding program, predict the structure of the molecular beacon. The sequence should not form unwanted secondary structures (see Note 6). 3.2.2. Signal-to-Background Ratios Signal-to-background ratio measurements indicate the purity of a molecular beacon preparation. Because of the presence of oligonucleotides that possess only a fluorophore and not a quencher, or of inefficient quenching between the two moieties, the fluorescence signal resulting from the presence of the target is obscured by background fluorescence, leading to inaccurate hybridization measurements (16). The fluorescence of the molecular beacon in a 150-µL solution of 1 mM MgCl2, 20 mM Tris-HCl, pH 8.0 is determined (Fbuffer) using the optimal excitation and emission wavelength of the fluorophore. After 10 µL of a 0.1 µM molecular beacon solution is added, a new fluorescence level is recorded (Fclosed). The increase in fluorescence is monitored
after a fivefold molar excess of a complementary oligonucleotide is added to the solution. Fopen is the maximum level of fluorescence reached after the reaction is complete. The signal-to-background ratio equals (Fopen – Fbuffer)/(Fclosed – Fbuffer). Good molecular beacons generate a fluorescent signal at least 30 times more intense than in the absence of target. 3.2.3. Determination of Thermal Denaturation Profiles The design of molecular beacons is highly permissive. They can be designed so that they are stable and function under a broad set of conditions. Combinations of loop and stem lengths can be chosen to detect various nucleic acid targets (see Fig. 2). The fluorescence of molecular beacon solutions should be measured over a wide range of temperatures. Equimolar molecular beacon solutions are prepared in the hybridization buffer. Add a fivefold molar excess of an oligonucleotide that is perfectly complementary to the molecular beacon's probe sequence. In a spectrofluorometric thermal cycler, decrease the temperature from 95°C to 25°C in increments of 1°C. To ensure that equilibrium is reached at each temperature step, the steps should last at least 30 s. The melting profiles of the probes alone and of their hybrids should indicate correct molecular beacon characteristics (see Fig. 2). 3.2.4. Testing Putative Probes: In Vitro Hybridization to RNA Prior to using molecular beacons in living cells, in vitro-transcribed fulllength RNA is used as a target molecule for in vitro hybridization reactions to determine whether the molecular beacons that were predicted to work well by the computer programs were in fact able to bind to their intended target sequences. The intensity of the fluorescence signal and the rate of hybridization are measured in a spectrofluorometer. These reactions are performed with "naked" RNA under plausible physiological conditions and are not carried out in the presence of a cellular extract that contains the various factors that might have an affinity for the mRNA. Each molecular beacon should be tested for its ability to hybridize to the mRNA in vitro. Even though each molecular beacon could bind spontaneously to an oligonucleotide that is complementary to its probe sequence, only a few might bind efficiently to full-length transcripts. 1. Prepare full-length RNA by in vitro transcription. 2. Measure the fluorescence intensity of a 125-µL solution containing 1 mM MgCl2, 20 mM Tris-HCl, pH 8.0, and 80 nM of molecular beacon at 25°C until no change in fluorescence occurs.
Fig. 2. Thermal denaturation profiles of DNA molecular beacons, alone (dotted lines) or bound to target (continuous lines), where fluorescence intensity was measured as a function of temperature. (A) The loop length of a molecular beacon, comprising the probe sequence, was increased in increments of three nucleotides at a time while maintaining a constant stem length of six nucleotides. The melting transition of the probe–target hybrids shifted toward higher temperatures, indicating an increase in the stability of the hybrids. (B) The stem length was increased 1 nucleotide at a time while maintaining a constant loop length of 15 nucleotides. The melting transition of the probe–target hybrids shifted toward lower temperatures, indicating a destabilization of the probe–target hybrids, while the melting transition of the molecular beacon stem shifted toward higher temperatures.
3. Add a 5-µL aliquot of 100 nM transcribed mRNA to this solution and measure the subsequent change in fluorescence as a function of time at 25°C (see Note 7). The time-course of hybridization is recorded at the optimal excitation and emission wavelengths of the fluorophore coupled to the molecular beacon (see Note 8).
3.3. In Vivo Detection of Oskar mRNA The mRNA sequence encoded by the Drosophila melanogaster maternal gene oskar is shown as a model target for in vivo detection with molecular beacons. Oskar mRNA localizes during mid-oogenesis in a tight sliver at the posterior pole of the oocyte. Utilizing the RNA secondary structure prediction program mfold and applying the aforementioned criteria for selection of accessible target regions, sequences within the mRNA were identified. These regions are either single-stranded or paired with distant sequences in most of the thermodynamically favorable foldings and contained both flexible and stable structures. Next, using OligoWalk's thermodynamic outputs, which reflected the degree of stability of all RNA–probe hybrids formed from scanning oligonucleotides, the choice of target sequences was narrowed down to 11 regions (see Fig. 3). Among the 11 regions selected for oskar mRNA, 4 regions (osk 62–87, osk 964, osk 2209, and osk 2579) had high values for both ss-count and P-num. The threshold value for the selection of these regions was 220, which represents a 50% probability that the respective region is single-stranded and is a measure of how "well determined" the structure formed by the sequence is. The remaining seven regions had low P-num values, indicating better determined and more stable regions. This theoretical analysis helped reduce the number of potentially accessible target regions and, with good confidence, provided sequences for oskar mRNA-specific molecular beacons. A 100 ng/µL-solution of the oskar mRNA-specific molecular beacon dissolved in nuclease free water was microinjected into a stage 9 Drosophila melanogaster oocyte (see Fig. 4) (17). The images acquired at various time intervals revealed a homogenous distribution of the molecular beacons with an increased fluorescent signal at the posterior end of the oocyte (see Note 9) (2). 4. Notes 1. mfold values:
• ss-count is the number of times that the ith base is single-stranded in n predicted foldings. • P-num (i) = ∑k < i φ (∆G(k,i) ≤ ∆G + ∆∆G) + ∑i < j φ (∆G(i,j) ≤ ∆G + ∆∆G), where i, j, and k are bases and φ is defined as 1 when expression is true and 0 otherwise.
10 Fig. 3. Most stable secondary structure of oskar mRNA showing the location of molecular beacon target sequences. The computerfolded structure is depicted in three segments, with the connecting nucleotides indicated at the break points. The bold nomenclatures represent regions that were most accessible by the molecular beacons.
Fig. 4. Stage 9 Drosophila oocyte injected with a molecular beacon solution specific for oskar mRNA. After 10 min, an image was acquired using a confocal microscope. Anterior is to the left and posterior is to the right of the image.
• ∆G is the overall minimum free-energy of any folding that contains the i,j basepair: ∆G = min1≤i < j ≤ n ∆G(i,j).
• ∆∆G is a user-selected free-energy increment set at 1 ≤ ∆∆G ≤ 12 (kcal/mol) ∆∆G = ∆G × P/100.
• P is the parameter that controls the value of the free-energy increment ∆∆G. The default value is 5. 2. To save this structure as a ".pdf" file, choose the output option as a "postscript" file. Click on the structure and the option to "save as" is available in a pop-up window. Save this as a ".cgi" file, which can then be converted to a ".pdf" file with Acrobat Distiller. View and manipulate in Adobe Illustrator. 3. Unlike predictions of local hairpins, long-range interactions or multibranch junctions remain poorly determined. Such predicted structures could provide insights into regions of potential structural plasticity within an RNA molecule and thus reveal potential accessible sites for antisense probes. Therefore, when considering probes for targeting an RNA sequence, it is very helpful to pay close attention to the information offered by a secondary structure fold. 4. In vivo, the mRNA structure is dependent on the intracellular environment. For example, mRNA accessibility becomes less predictable, as the structures of actively translated mRNAs are transiently altered by passing ribosomes. Also, the stability of the duplexes formed by probes and complementary nucleic acids is influenced by the presence of RNA-binding proteins. These might mask target
Bratu sites, preventing hybridization from occurring. Therefore, a number of different regions distributed throughout the mRNA sequences should be considered as target sites for molecular beacons. Because the 2´-O-methyl backbone renders molecular beacons capable of forming stronger stems, the sequence composition of the complementary arms are either four nucleotides for shorter-length probe sequences or five nucleotides for longer hairpin loops. For example, molecular beacons with 15–18 nucleotides composing the hairpin loops should have stems such as CGGC (or a variation of this order). For longer loops, design stems such as GATCC or GGAGC. A 100% GC five- nucleotide stem is too strong and the molecular beacon would not open at room temperature under cellular conditions. The structures generated by the folding program should have the intended hairpin structure. Any other structure that does not form the set-length stem hybrid will generate inefficient binding probes (i.e., the stem is longer, or the nucleotides within the hairpin loop form basepairs) or high-background signals (when the nucleotides at the 5´ and 3´ ends of the sequence do not form a pair, the fluorophore is removed from the vicinity of the quencher). In either instance, both the probe and stem sequences can be manipulated. If the chosen region on the RNA permits, shift the frame of the probe along the target sequence until a probe structure with minimal self-complementarity is obtained. For a longer hairpin loop, a small interior stem of two to three nucleotides long does not significantly affect the performance of a molecular beacon. If the alternative structures arise from the choice of the stem sequence, change its composition to ensure a hairpin conformation. The stem can also be formed by terminal nucleosides of the probe sequence. For each molecular beacon tested, the kinetic characteristics of hybridization as well as the level of fluorescence generated might be different. The most accessible target sequences induce the fastest increase in fluorescence. However, the efficiency of hybridization and stability of binding can depend on more than the secondary structure of the RNA. The probe's size and sequence composition are important determinants of the stability of the resulting hybrid. For oskar mRNA, each probe has approx 40% GC composition and is 18 to 25 nucleotides in length. The rate of binding might also be influenced by the tertiary structure of the RNA. The sites accessible for targeting could be masked by the three-dimensional folding of the molecule or by other tertiary interactions, such as prestacking of the single-stranded regions. For these reactions, the molecular beacon molar concentration exceeds that of the target RNA to ensure that all RNA molecules are bound by a probe. Molecular beacons can be labeled with a wide range of fluorophores, which can be efficiently quenched by the same quencher, such as dabcyl, BHQ1, or BHQ2 (16,18). A list of companies licensed to synthesize molecular beacons as well as a list of the available fluorescent labels can be found at http://www.molecularbeacons.org/. When images are acquired by a confocal fluorescence microscope, the background signal from other focal planes generated by the nonspecific binding of cellular components to probes that changes their conformation and causes them to fluoresce is
eliminated. When conventional fluorescence microscopy is implemented, the images can be enhanced via deconvolution analysis, where the fluorescence contribution from other focal planes is removed, or by ratio imaging, where the intensity of the fluorescence generated by the specific molecular beacon is divided by the intensity of the spectrally distinguishable fluorescence of a second, nonspecific molecular beacon that is coinjected in an equimolar cocktail with the specific molecular beacon.
Acknowledgments The author thanks Sanjay Tyagi and Fred Russell Kramer for their assistance and advice and Salvatore A.E. Marras for help with molecular beacon synthesis. This work was supported by National Institutes of Health grants ES-10536 and EB-00277. References 1. Tyagi, S. and Kramer, F. R. (1996) Molecular beacons: probes that fluoresce upon hybridization. Nature Biotechnol. 14, 303–308. 2. Bratu, D. P., Cha, B. J., Mhlanga, M. M., Kramer, F. R., and Tyagi, S. (2003) Visualizing the distribution and transport of mRNAs in living cells. Proc. Natl. Acad. Sci. USA 100, 13,308–13,313. 3. Matsuo, T. (1998) In situ visualization of messenger RNA for basic fibroblast growth factor in living cells. Biochem. Biophys. Acta 1379, 178–184. 4. Sokol, D. L., Zhang, X., Lu, P., and Gewirtz, A. M. (1998) Real time detection of DNA.RNA hybridization in living cells. Proc. Natl. Acad. Sci. USA 95, 11,538–11,543. 5. Southern, E. M., Milner, N., and Mir, K. U. (1997) Discovering antisense reagents by hybridization of RNA to oligonucleotide arrays. Ciba Found. Symp. 209, 38–44; discussion 44–46. 6. Ho, S. P., Bao, Y., Lesher, T., et al. (1998) Mapping of RNA accessible sites for antisense experiments with oligonucleotide libraries. Nature Biotechnol. 16, 59–63. 7. Lima, W. F., Mohan, V., and Crooke, S. T. (1997) The influence of antisense oligonucleotide-induced RNA structure on Escherichia coli RNase H1 activity. J. Biol. Chem. 272, 18,191–18,199. 8. Milner, N., Mir, K. U., and Southern, E. M. (1997) Selecting effective antisense reagents on combinatorial oligonucleotide arrays. Nature Biotechnol. 15, 537–541. 9. Zuker, M. (2003) Mfold web server for nucleic acid folding and hybridization prediction. Nucleic Acids Res. 31, 1–10. 10. Mathews, D. H., Burkard, M. E., Freier, S. M., Wyatt, J. R., and Turner, D. H. (1999) Predicting oligonucleotide affinity to nucleic acid targets. RNA 5, 1458–1469. 11. Peyret, N., Seneviratne, P. A., Allawi, H. T., and SantaLucia, J., Jr. (1999) Nearestneighbor thermodynamics and NMR of DNA sequences with internal A.A, C.C, G.G, and T.T mismatches. Biochemistry 38, 3468–3477.
12. SantaLucia, J., Jr. (1998) A unified view of polymer, dumbbell, and oligonucleotide DNA nearest-neighbor thermodynamics. Proc. Natl. Acad. Sci. USA 95, 1460–1465. 13. SantaLucia, J., Jr., Allawi, H. T., and Seneviratne, P. A. (1996) Improved nearestneighbor parameters for predicting DNA duplex stability. Biochemistry 35, 3555–3562. 14. Sugimoto, N., Nakano, S., Katoh, M., et al. (1995) Thermodynamic parameters to predict stability of RNA/DNA hybrid duplexes. Biochemistry 34, 11,211–11,216. 15. Xia, T., SantaLucia, J., Jr., Burkard, M. E., et al. (1998) Thermodynamic parameters for an expanded nearest-neighbor model for formation of RNA duplexes with Watson-Crick base pairs. Biochemistry 37, 14,719–14,735. 16. Marras, S. A., Kramer, F. R., and Tyagi, S. (2002) Efficiencies of fluorescence resonance energy transfer and contact-mediated quenching in oligonucleotide probes. Nucleic Acids Res. 30, e122. 17. Bratu, D. P. (2003) Imaging Native mRNAs in Living Drosophila Oocytes Using Molecular Beacons. New York University, UMI Dissertation Service, New York, New York. 18. Tyagi, S., Bratu, D. P., and Kramer, F. R. (1998) Multicolor molecular beacons for allele discrimination. Nature Biotechnol. 16, 49–53.
2 Second-Harmonic Imaging of Collagen Guy Cox and Eleanor Kable
Summary Molecules that have no center of symmetry are able to convert light to its second harmonic, at twice the frequency and half the wavelength. This only happens with any efficiency at very high light intensities such as are given by a pulsed laser, and because the efficiency of the process depends on the square of the intensity, it will be focal plane selective in exactly the same way as two-photon excitation of fluorescence. Because of its unusual molecular structure and its high degree of crystallinity, collagen is, by far, the strongest source of second harmonics in animal tissue. Because collagen is also the most important structural protein in the mammalian body, this provides a very useful imaging tool for studying its distribution. No energy is lost in second-harmonic imaging, so the image will not fade, and because it is at a shorter wavelength than can be excited by two-photon fluorescence, it can be separated easily from multiple fluorescent probes. It is already proving useful in imaging collagen with high sensitivity in various tissues, including cirrhotic liver, normal and carious teeth, and surgical repair of tendons. Key Words: Collagen; second harmonic; structural proteins; biological imaging; 3D imaging; non-linear microscopy; matrix.
1. Introduction 1.1. Second-Harmonic Generation Theodore Maiman won a much publicized race to make a working laser when he built a pulsed ruby laser in 1960. This was the world's first laser and it produced pulses of deep red (697 nm) light, just within the visible range. Almost immediately, it was found that shining pulses of ruby laser light through a quartz crystal produced near-ultraviolet light at 348 nm, the second harmonic of the original light (1). Second-harmonic generation (SHG) takes place when the electric field of the exciting light is sufficiently strong to deform a molecule. If the molecule is not symmetrical, the resulting anisotropy creates an oscillating field at twice the From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Cox and Kable
frequency, the second harmonic (2). This means that the ability to generate second harmonics is peculiar to molecules that are not centrosymmetric. SHG will also take place at interfaces where there is a huge difference in refractive index, such as metal surfaces. For a more technical description, see Note 1. The resulting beam of light at twice the frequency/half the wavelength usually travels in the same direction as the incident beam and in phase with it, although it commonly has a different plane of polarization. Different samples and illumination conditions will modify this behavior; the whole question of beam propagation is discussed in more detail in Subheadings 2.3. and 3.1. Crystals of nonsymmetric molecules such as potassium deuterium hydrogen phosphate are very effective generators of second harmonics, and particularly when the angle of the crystal lattice is carefully matched to the incoming beam, they can give extremely high second-harmonic yields. The production of suitable crystals (potassium, cesium, or rubidium titanyl phosphate or arsenate are other common examples) is an important industry; and the crystals are in everyday use as frequency doublers in the laser industry—even humble items such as green laser pointers contain a frequency-doubling crystal. Second-harmonic generation was first used in microscopy as long ago as 1974 (3) (see Note 2), but the more practically useful technique of scanned second-harmonic microscopy was first achieved 4 yr later, by Gannaway and Sheppard (4), using a continuous-wave laser. All modern applications stem from this work. As they pointed out, in the scanning mode the image is focal plane selective because the signal depends on the square of the incident beam power. It is, thus, effectively equivalent in imaging properties to confocal microscopy and two-photon fluorescence; the fact that it is coherent rather than incoherent does cause small differences, but they are only of theoretical interest so far as biological applications are concerned. Scanning the beam across the sample means that high intensities only have to be present in a very small region at any one time, but, even so, the continuous-wave laser used by Gannaway and Sheppard necessitated power densities at the sample that could not be tolerated by biological specimens. It was clear even then that pulsed lasers would make the technique more practicable, and the introduction of second-harmonic microscopy as a practical technique had to await the availability of pulsed lasers with very short pulse lengths. These now offer very high instantaneous powers in conjunction with low total power averaged over time, at a repetition rate fast enough not to restrict scan speed. Second-harmonic-generation microscopy has many features in common with two-photon fluorescence (TPF) microscopy, and the hardware required is very similar, but there are some key differences (summarized in Table 1). Fluorescence always involves some loss of energy in the sample, and the fact that electrons are raised to excited states means that bleaching is inevitable.
Harmonic Imaging of Collagen
Table 1 Comparison of SHG and TPF SHG Exactly double the original frequency Largely frequency independent Virtually instantaneous, approx 1 fs Propagated forward Coherent (in phase with exciting light) No energy loss or damage Requires short laser pulses
TPF Spectrum of frequencies less than two times the original Strongly frequency dependent Lifetime in nanoseconds Propagated in all directions Incoherent Always energy loss and associated damage Requires short laser pulses
SHG dissipates no energy in the sample and there is no excitation or bleaching. Of course, that does not imply that no damage can occur; there are several mechanisms through which intense laser pulses can damage the sample, but these are totally independent of the imaging process. Serious biological use of SHG microscopy has been a recent phenomenon, with few papers dated prior to 2001. Gauderon et al. (2) were able to image the DNA of polytene chromosomes of Drosophila. Campagnola et al. (5) applied the known SHG properties of styryl potentiometric dyes to the microscope, continuing from earlier work in which these properties had been investigated in the cuvet. They also imaged collagen and other structural proteins in a variety of tissues at resolutions of up to 1 µm (5,6). Membranes have been labeled with second-harmonic generating dyes, providing a sensitive probe of membrane separation (7,8). 1.2. Collagen and SHG Individual noncentrosymmetric molecules will generate a second-harmonic signal, but molecules arranged in a crystalline array will give a very much stronger response. Hence, any biological material that is both crystalline and noncentrosymmetric is likely to be suitable for SHG microscopy. Furthermore, SHG microscopy of such materials should be capable of providing information about orientation and crystallinity, as well as morphology. Collagen is the most important extracellular structural protein of the vertebrate body, making up around 6% of the body mass, mainly in bone, cartilage, skin, interstitial tissues, and basal laminae. The collagen molecule is nonsymmetric and is arranged in a triple helix (9). There are many different forms of collagen, coded separately in the genome. Minor amino acid changes affect the final conformation and, therefore, give the different collagen types different functions in the living organism. Four types (I, II, III, and V) form fibrils, type
IV forms sheets in basal laminae, and types VI and IX act as links, binding fibrillar collagen to other cell components. Type I, in particular, is highly crystalline and is very important as a structural component of the skeleton, cartilage, and soft tissue. This unusual structure makes type I collagen an effective generator of second harmonics, a fact that has been appreciated for over 20 yr (10). The signal is, to a large extent, wavelength independent over a wide range of wavelengths in the infrared region (11,12). Furthermore, it is possible from this signal to determine the polarity of the collagen helix (13). Differences in signal between normal and experimentally damaged collagen have also been reported (14), raising the hope that pathological changes might be detected. Very recently, spectral changes in the second-harmonic signal from normal and cancerous tissue have been described, opening up a possibility that it might become a useful diagnostic tool in oncology (15). We have shown that SHG can be an exquisitely sensitive tool for detecting and imaging collagen at high resolution, with diffraction-limited resolution (sub-300 nm) easily achievable (12,16). There seems to be the potential to distinguish between different collagen types, with highly crystalline type I collagen giving a much stronger second-harmonic signal than type III, even though both stain identically with collagen-specific dyes (12). 2. A Microscope for SHG Imaging The basic requirements for SHG microscopy are those for TPF microscopy: a scanning microscope (usually a confocal microscope, although confocal optics are irrelevant) coupled to a pulsed infrared laser. However, the different nature of signal generation in SHG microscopy means that some simple and straightforward modifications to a normal multiphoton microscope will be needed. 2.1. The Laser A pulsed titanium–sapphire (Ti-S) laser is the normal choice, although neodymium yttrium aluminium garnet (Nd YAG) lasers have been used (11), and, in principle, other lasers such as chromium forsterite are possible and might come into wider use in the future. Because SHG depends on the square of the flux density, shorter pulses will mean a substantial reduction in the average power required. Therefore, in principle at least, a "femtosecond" Ti-S laser (typically delivering 100- to 200-fs pulses) will usually be preferred over a "picosecond" version, which delivers 1- to 2-ps pulses. However, to gain the full benefit of short pulses, it is essential that they remain short. Light travels approx 30 µm during a 100-fs laser pulse, so the pulse will contain fewer than 40 waves at 800 nm. With such a small number
of waves in a pulse, the wavelength cannot be specified with great precision and, consequently, there will be a range of wavelengths present in a Gaussian distribution with a full width at half-maximum (FWHM) range of about 2 nm. This makes no difference in terms of biological imaging, as the spread is narrower than the band passed by current detection systems, whether filter based or spectrometric. It is also trivial in terms of TPF excitation. However, it is very much wider than the completely monochromatic light produced by a continuous-wave laser. There is an important practical consequence of this wavelength spread: It will broaden the pulse if it travels through a dense medium such as glass. All glass has dispersion; that is, its refractive index is wavelength dependent, so that longer wavelengths travel more slowly than shorter ones. In a very short laser pulse, therefore, the different wavelengths will travel at different speeds and the pulse will broaden. Longer pulses have a much smaller wavelength spread, so they will not be affected to the same extent; in practical terms, pulse broadening is only a problem in the femtosecond region. A consequence of this is that the laser needs to be directly coupled to the microscope; transmitting the light through an optical fibre will broaden the pulse to an unacceptable extent (see Note 3). Any additional optical elements in the beam path—even such useful items as beam expanders—will lengthen the pulse, as will objective lenses; so the less glass in the objective, the better. This implies that there is little point in using complex lenses such as apochromats, especially because color correction is irrelevant (only one wavelength has to be brought to a focus). Fluorite lenses typically offer comparable numerical apertures with many fewer elements, and because their basis is the low-dispersion mineral fluorite (calcium fluoride), pulse broadening is minimal. Femtosecond Ti-S lasers fall into three categories: 1. Fully tuneable between 700 and 1000 nm. These lasers typically have a range of manual adjustments that need attention in normal operation, and more than one set of optical components might be needed to obtain the widest tuning range, although 700 to 950 nm or 750 to 1000 nm are typically now available with a single optics set. 2. Tunable over a limited range, but fully automated so that no external adjustments need to be made. The tunable range available with these is increasing rapidly, and although fully tuneable lasers still offer a wider range at the time of writing (June 2003), the automated models are now very close to them in performance and might well match them before long. 3. Fixed wavelength, commonly 800 nm.
Fixed-wavelength lasers are not very suitable—unless set to a rather longer wavelength than 800 nm—because detection of the second harmonic at 400 nm is often blocked or attenuated by other optical elements in the microscope. Automated lasers are probably now the most effective solutions, unless other
usage requirements dictate a wider tuning range. Because the SHG signal from collagen shows little wavelength dependence within the tuning range of a Ti-S laser, wider range lasers do offer the facility of optimizing laser wavelength for multiphoton fluorescence excitation without compromising SHG detection. In particular, effective two-photon excitation of many members of the GFP (green fluorescent protein) family requires long wavelengths that lie beyond the range of automated lasers (see Note 4). Tunable lasers are tuned by moving a birefringent element, but this, in turn, will require adjustments to a prism to compensate group velocity dispersion— ensuring that all wavelengths remain together within the laser cavity—and there will also be alignments to mirrors and the slit that acts to impose mode locking on the pulses. Hence, a fully tunable laser requires not only an investment in technology but also an investment in expertise to keep it correctly adjusted. 2.2. Ancillaries The coupling system to the microscope, whether lab built or supplied by the microscope maker, will need to divert part of the beam to diagnostic equipment. A spectrum analyzer is the most useful ancillary, showing not only the actual wavelength tuned but also the spread of wavelengths present. This is a very effective visual indication of the pulse length, and software is available that will compute an estimated pulse length from the spread, although this does depend on some assumptions about the pulse shape. An autocorrelator will give a more precise measure of the pulse length, but is a fiddly and complex instrument to use, although, again, rapid strides are being made in improving usability for the biologist. Recently, models have been produced that provide a remote head to measure the actual pulse width emerging from the objective. A power meter is also valuable, even if the diagnostics built into the laser itself gives a power reading, because an independent power meter can pick up losses further along the beam path. Often, a decline in power will be a useful warning that either the laser is not perfectly aligned or that the internal optics are in need of cleaning. As discussed in Subheading 2.1., the beam delivery to the microscope must be direct, not through an optical fiber. Normally, therefore, the microscope and laser will both be rigidly mounted to an optical table. Microscope manufacturers might have some degree of standardization in their hardware, but these systems are still largely custom installations, so the user will have a fair degree of choice. Points to consider include the following: 1. The range and convenience of alignment of the beam, because the pointing stability of Ti-S lasers is often rather worse than is normal for continuous-wave lasers. 2. The control over beam power. Although neutral density filters will provide attenuation, an electronic control such as a Pockel Cell or electro-optic modulator (EOM) will allow beam blanking on flyback (thus reducing the total beam exposure of the sample) as well as patterned irradiation.
3. The efficiency of the total throughput. Often much light is lost before it even reaches the microscope. Many partial beam splitters (used to divert part of the beam to diagnostic equipment) have a surprisingly high absorbance in the 700- to 900-nm range. A piece of plain, uncoated glass is often at least as effective as a custom beam splitter. If much power is disappearing, it is worth carrying out an audit with a power meter to see where the losses are.
2.3. The Microscope Whereas second harmonic light is generally propagated forward, geometric considerations (the Gouy phase shift in a focused spot) might mean that much of the flux emanating from the scanning spot is in the off-axis directions (7,8). Therefore, for effective collection, the condenser lens must have a numerical aperture (NA) equal to or larger than that of the objective. An oil-immersion condenser is a necessity. Equally important is that Köhler illumination be set up with scrupulous precision. Without this, a large proportion of the second-harmonic light will never reach the detector. Having a totally immersed system also reduces the amount of stray room light that can enter, an important consideration with any form of nondescanned detector. Although they provide geometrically perfect focusing, galvanometer stage movements do not function well in this environment. The combination of short working distances and viscous drag from the immersion media impede the free motion of the stage, so that it often fails to follow the instructions given by the controlling computer. Even though, in theory, moving the sample between a stationary objective and condenser achieves optical perfection, it is preferable in practice to focusing with the nosepiece; a high precision is achieved with electronic or piezo-driven nosepieces. Generally, the precision with which a series can be collected will be very high, but the reproducibility will be less so; thus, there might be visible errors in returning to a given focal plane. Unless one wishes to work in total darkness, every attempt should be made to prevent room light from reaching any of the nondescanned detectors. Figure 1 shows the Leica screening arrangements as fitted to our microscope. Around the condenser, we have a large (10 cm) matt-black disk, which serves a dual function in excluding room light as well as safeguarding against accidental laser exposure. From the condenser to the lamp housing is a close-fitting matt black telescopic tube, which keeps out all room light above the condenser. The slider that blocks off the wide-field epifluorescence illuminator (highpressure mercury or argon arc lamp) often passes a surprisingly large amount of light when closed. Although this is not noticeable in conventional epifluorescence microscopy, it can interfere substantially with two-photon or second-harmonic detection. Because it is often not convenient to turn off the mercury lamp, a blanking plate should be made up to fit in the lamp housing. We use a simple
Fig. 1. Schematic arrangement of lens and detector arrangements on our Leica TCS2 MP/DMIRB inverted-microscope system. The two dual-channel detectors are identical and have removable filter cubes, so that different combinations are easy to assemble. The 650DCLP dichroic is mounted in a cube of the wide-field epifluorescence filter changer and directs all fluorescence returning through the objective to the detector. When this is rotated out of position, the normal confocal detection system can be used.
Fig. 2. (A) Curves of several infrared-blocking filters. The interference filters provide a sharp cutoff and good transmission. However, the e650 SP not only provides total blocking of the Ti-S laser light but also cuts off even the violet part of the visible spectrum. The e700 SP provides all of the visible spectrum but still cuts off at 400 nm and will also allow some laser light through at the short end of the tuning range. The colored glass filter BG38 is obviously very imperfect, allowing some leakage from 750 nm on and severely attenuating the red part of the visible spectrum; nevertheless, it transmits 80% at 400 nm and a usable fraction a little further into the ultraviolet. (B) Curves of two narrow-band filters suitable for detecting SHG signals. Because of the absorbance of the coating materials, only approx 35% transmission is attainable at 405 nm, so it will be preferable to tune the laser to 830 nm and use the 415-nm filter, which passes more than 60%. All spectra are reproduced by kind permission of Chroma Technology Corp.
aluminum disk, painted matt black, mounted in a filter holder, which will slip into one of the ventilation slots in front of the housing. Filters for SHG detection need some careful consideration. As Fig. 2A shows, most infrared-blocking filters (essential to block transmitted laser light)
Fig. 3. Patterns of radiation from (A) single dipole, (B) array normal to the incident beam, (C) array in line with the incident beam, and (D) a bulk array. (D, from ref. 18 by permission of the authors and The Biophysical Society). (E) Shape of the excitation point-spread function (psf) superimposed on typical collagen spacing in connective tissue. (A–D, from ref. 12 with permission from Elsevier). (F) Randomly spaced fibers will propagate the second harmonic in a forward direction. Lower (red) wave is the excitation beam at 800 nm, propagating in the direction of the red arrow. The upper row shows second-harmonic radiation generated by the randomly spaced dipoles indicated by vertical arrows. Light propagated in the forward direction will be in step both with
also block light shorter than 400 nm. Therefore, even though most lasers can be tuned to wavelengths shorter than 800 nm, it will not be easy to detect the SHG signals in this part of the spectrum. Furthermore, the coatings used in most interference filters start to have strong absorption at wavelengths shorter than about 420 nm. At 415 nm, a 10-nm bandpass filter will transmit about 60% at its peak; at 405 nm, the equivalent filter will only transmit 30% (see Fig. 2B). Therefore, other things being equal, it is best to use an excitation wavelength of 830 nm or longer. We normally use 830-nm excitation and a 415-nm barrier filter. 3. Methods 3.1. Signal Propagation and Properties Typically, depending on the extent to which the sample scatters light, between 80% and 90% of the SHG signal from collagen in a tissue sample will be propagated forward (12). Figure 3 shows the direction of propagation of the signal from dipoles that are small compared to the wavelength propagated. Parts A–D were drawn to represent coherent anti-Stokes Raman scattering (CARS) microscopy (18) but are equally applicable to SHG. A single dipole (Fig. 3A) radiates in all directions except normal to the incident beam. A single, 40- to 50-nm collagen fiber would be expected to behave in this way. A planar array normal to the incident beam will radiate strongly forward and backward, because waves in both of these directions will be in phase (Fig. 3B). There will be little lateral propagation because in this direction, the wave from one dipole will not be in phase with the wave from its neighbor. An array orientated along the direction of the beam (Fig. 3C) will propagate strongly forward, because, regardless of the actual spacing (and even if it is completely irregular), all dipoles will have the same phase relationship to the exciting beam in the forward direction but will be randomly out of phase in the reverse direction (Fig. 3F). This same effect will be even more marked in a bulk array of dipoles (Fig. 3D), which will propagate the harmonic virtually exclusively forward. In SHG microscopy, the excited volume is elliptical, with the long axis along the direction of the beam, so that in any grouping of collagen fibers, there will be more excited dipoles in line with the beam than across it (Fig. 3E). Thus, whereas an isolated individual fiber may radiate its signal both forward and backward, overall the signal is predominantly propagated forward. In experiments with cryo-sections of human endometrium, we have found that only
Fig. 3. (Continued) the exciting radiation and light from other dipoles, whereas in the reverse direction, radiation from each dipole will have a random phase relation with any other, so little propagation will take place.
10–12% of the detectable SHG signal is propagated back through the objective lens (12), the remainder passing through the condenser to the transmitted light detector. Although SHG uses long wavelengths, the resolution achievable is quite respectable. Calculated resolution values at 830 nm are around 250 nm (19), and measurements close to that have been reported in practice (12,16,19). Figure 5C shows resolution of this order in a histological section of skin. The depth resolution will be identical to that of TPF, and under best conditions, around 800 nm should be achieved. This is more than adequate for effective three-dimensional reconstructions (see Fig. 6). 3.2. Signal Detection and Imaging 3.2.1. Suitable Samples The SHG signal from type I collagen is very strong—typically stronger than most two-photon excited fluorescence in biological samples—so that low excitation levels can be used. When fluorescence is being detected at the same time, the laser intensity needed will generally be determined by the fluorescent signal. SHG from collagen seems to be unaffected by most preparation techniques. Good images can be obtained from histological paraffin sections, whether unstained (see Fig. 4) or stained with routine histological stains such as hematoxylin and eosin or Masson's trichrome (Figs. 5,6; refs. 12,16). Sirius Red, which stains collagen and also enhances its birefringence (20,21), also has no effect on the SHG, although it is noticeable that the second-harmonic signal does not exactly colocalize with two-photon excited fluorescence from Sirius Red (12) or with the autofluorescence from aldehyde-fixed collagen (see Fig. 4). We have suggested that only the highly crystalline collagen gives the SHG signal and stains (and aldehydes) have more effect on surrounding less crystalline material; colocalization analyses support this (22). One common preparation technique does seem to compromise the SHG signal from collagen. It is severely depleted in epoxy resin-embedded blocks of tissue fixed in glutaraldehyde and osmium tetroxide for electron microscopy, even when sectioned at 500 nm thickness for the optical microscope. Even though some signal is still present, on the basis of our preliminary trials we cannot recommend this material for SHG imaging; a disappointing conclusion for those needing to combine optical and electron microscope imaging of collagen. The image formation in three dimensions is effectively equivalent to that in multiphoton fluorescence; therefore depth penetration in turbid and scattering samples will be substantially better than in single-photon confocal microscopy, the predominantly forward propagation of the signal does impose some restrictions on sample thickness for optimal imaging. Whereas in multiphoton mode one can simply probe down into a thick specimen until the point of no signal is
Fig. 4. Micrographs of kangaroo-tail tendon transplanted into rat muscle. Projection from 24 optical sections. Aldehyde-fixed, unstained paraffin section cleared in xylene and permanently mounted. For more details of the sample, see refs. 12 and 18. The implanted tendon is seen in the lower left half of the picture, and the host tissue (including some type III collagen from the host response) is seen in the upper right. (A) Forward-propagated signal, almost exclusively SHG at 410 nm, although detected over a wide spectral range (400–550 nm). (B) Back-propagated signal in the spectral range 500- to 550-nm showing aldehyde-induced autofluorescence. The photomultiplier tube voltage was set 100 V higher than in (A) in order to pick up the much weaker autofluorescence. Previous collection of an image at a higher zoom setting has caused slight bleaching in the fluorescent image (arrowed), but the second-harmonic image is unaffected. Note that no SHG signal is visible in the type III collagen (upper right), implying that if it either produces no, SHG signal or a much weaker, SHG signal.
reached, for optimal imaging in SHG microscopy the signal has to be able to pass through the entire sample. Thus, in either case, the penetration depth might be 200 µm; however, in SHG, the sample itself cannot be too much thicker than this. With thicker samples, one often can image down to a considerable depth at the edge of the specimen when no signal can be detected in the center—a consequence of the large angle through which the signal is generated and propagated. Alternatively, one can simply use the backscattered image; even though up to 90% of the signal might be lost, in a suitable sample it might still be possible to collect a good image (23). 3.2.2. Detection Strategies Because the SH signal is by definition shorter than any two-photon excited fluorescence, a 10-nm FWHM filter will completely exclude any fluorescent signal. Figure 6 shows SH images collected in this way. Although this does
Fig. 5. Histological paraffin section of skin, stained with Masson's trichrome. (A) and (B) taken with 20× NA 0.5 lens, using 800-nm excitation and a broad-band collection
Fig. 6. Rendered image (simulated fluorescence projection at an oblique angle) from 20 optical sections through a histological section of the periodontal membrane of a developing tooth. SHG image, showing collagen only, captured with BP 415/10 filter, 25× NA 0.75 oil-immersion objective, 830-nm excitation.
Fig. 5. (Continued) filter. (A) Transmitted image; (B) back-propagated image of collagen around hair follicles in mouse skin. (A) shows both fluorescence and SHG, but the SHG is substantially brighter and is easily recognisable without any formal separation. (C) High-power image (×100 NA 1.4 objective) taken using 830-nm excitation and a BP 415/10 filter (Fig. 2) to exclude all fluorescence. Only collagen is seen and the resolution is excellent (arrowheads are spaced 330 nm apart).
30 Fig. 7. Separation of signals by image subtraction, human cirrhotic liver biopsy. (A) Signal collected between 400 and 560 nm in the transmission detector, containing SHG and fluorescence. (B) Signal collected between 500 and 560 nm in the back-propagated nondescanned detector, showing only fluorescence. (C) Image B scaled to match intensities on the hepatocytes (h) and subtracted from image A, giving an image that shows only the SHG signal.
make detection absolutely unambiguous, it restricts excitation to wavelengths for which the appropriate filter is available. Sometimes excitation of other fluorochromes makes it desirable to tune the laser to wavelengths for which the user does not have a suitable filter or for which a good filter is unavailable (such as close to 800 nm to excite DAPI or Hoechst). Also, a researcher wishing to find out whether SHG imaging can be useful in a particular project will probably not want to purchase special filters initially. Fortunately, the fact that the wavelength must always be shorter than that of any two-photon excited fluorescence (as fluorescence always involves a Stokes shift) means that it is often quite easy to distinguish an SHG signal without custom filters. This is particularly handy for those cases in which one is not looking at the SHG signal in isolation. Often, an experiment will require one or more fluorochromes to be imaged in the same sample. Because the SHG signal from collagen can be excited anywhere within the tunable range of a Ti-S laser, it is often desirable to optimize the excitation for the fluorochrome rather than having the collagen signal at the wavelength of a particular bandpass filter. Particularly if there is not much autofluorescence (which tends to have a broad spectrum) present, one might be able to use the shorter wavelength of the collagen signal to distinguish it with nothing more than a conventional short-pass/dichroic/long-pass combination. For example, in a specimen stained with routine fluorochromes (FITC, TRITC, etc., without DAPI), there will be very little fluorescence (even autofluorescence) shorter than 500 nm, so a 500 DCLP dichroic in the detector will give a signal that is predominantly SHG in the short-wavelength channel. Figure 4 shows an example of this; even with a 560 nm dichroic, the short part of the spectrum is totally dominated by the SHG signal while a strong fluorescent signal is detected in the range 560–650 nm. (In this sample, which is unstained, this is partly a function of the strength of the SHG signal from the highly crystalline kangaroo-tail collagen and partly a reflection if the aldehyde-induced autofluorescence tends to peak around 580 nm). Where fluorescence is stronger in the short wavelengths, so that this strategy will not work, it is simple to use the directionality of the SHG signal to detect it, because fluorescence will propagate equally in all directions. If matching filters are used, the transmitted detector will show a combination of fluorescence and SHG signals (Fig. 5A), whereas back-propagated detectors (see Fig. 1), whether confocal or nondescanned, will show mostly or entirely fluorescence (Fig. 5B). Therefore, subtracting the "reflected" signal from the "transmitted" signal will leave only the SHG signal. Figure 7 shows an example of this. The sample is a cirrhotic liver, which has a very high level of autofluorescence. Cirrhosis (fibrosis) of the liver is a proliferation of collagen fibers through the liver, and SHG microscopy is proving to be a powerful tool
to study it (12,23). The transmitted detector collects both SHG and fluorescent signals (see Fig. 7A), whereas the back-propagated signal has only the fluorescence (see Fig. 7B). The simple procedure to isolate the SH signal can be done in any image analysis or image processing program. The steps are as follows: 1. Measure intensity in both images of an area known to show only fluorescence (in this example, the hepatocytes labeled h). 2. Scale the intensity of the fluorescence-only image (Fig. 7B) so that it matches the corresponding area in the combined image (Fig. 7A). In this case, the image in Fig. 7B needs to be scaled up by approx 10%. 3. Subtract the image, in Fig.7B from the image in Fig. 7A. The resulting image (Fig.7C) shows only the SHG signal.
The two-color combination of this SHG-only image with the fluorescenceonly image can be seen as Fig. 7B of ref. 12. In principle, one could also use fluorescent lifetime to separate the SH signal from fluorescence, although this does not seem to have been done in practice. Generation of the SH signal is virtually instantaneous, whereas, typically, fluorescence does not appear for several hundred picoseconds and then decays over 3–12 ns. With a suitably high time resolution, the signals should therefore be quite distinct. 3.3. Conclusion: Applications for SHG Imaging of Collagen Although the ability of collagen to excite second harmonics has been known for 20 yr (10), and putative uses for the technique have been proposed as long as 16 yr ago (13), it has only been in the past 2 yr that advances in microscope and laser technology have brought SHG microscopy at high resolution into the hands of histologists and cell biologists. At this stage, it might be premature to predict what its final use will be. Two broad areas seem likely: the use of SHG imaging as a tool to study the three-dimensional architecture of collagen in fine detail at the microscopic level in both healthy and diseased tissue, and its use to investigate changes in collagen in disease. On the former front, SHG has already been used to study the arrangement of collagen in human endometrium as part of a long-term study of the glandular and vascular structure in three dimensions (12,16,24). It is also being developed as a method for detecting and quantifying collagen invasion in cirrhotic liver (12,25). We have shown (12) that SHG imaging appears to be able to distinguish between at least some collagen types (see Fig. 4). Current research in our laboratory aims to quantify and characterize this by using pure and well-defined collagen samples (22). SHG has also been used as a tool to detect the polarity of collagen in connective tissue (13) and this might also become applicable at higher resolution with current developments in instrumentation. Experimentally
induced damage has been shown to affect the SH signal from collagen (14) and, very recently, pathologic changes have also been shown to be detectable by the SHG properties (15). Teeth have a well-characterized collagen structure, and the SH signal is detectable in sectioned tooth material with and without decalcification (see Fig. 6). A study is currently in progress using SHG to study changes in collagen associated with dental caries. Hereditary collagen diseases are another area, as yet unexplored, in which SHG imaging could have a significant impact. Now that multiphoton microscopes are in wide use, it seems inescapable that the associated technique of SHG microscopy will grow steadily more important in future years. 4. Notes 1. The process of second harmonic generation (SHG) has been summarized in a recent paper by Gauderon et al. (2). Briefly, as electromagnetic radiation propagates through matter, the electric field (E) exerts forces on the sample's internal charge distribution. The consequent redistribution of charge generates an additional field component. The resultant dipole moment per unit volume is referred to as the electric polarization (P) and can be expressed as a sum of linear and nonlinear terms. The nonlinear components only become significant at very high light intensities. The primary nonlinear effect is a polarization of second order in the electric field and is given by (27) 2ω Eω Eω Pi2ω = χijk j k
where subscripts denote Cartesian components and superscripts denote the relevant frequencies. Is a (3×3×3) third-rank tensor, termed the second-order nonlinear optical susceptibility, whose elements sum to zero for material with inversion symmetry. 2. In 1974, Hellwarth and Christiansen (3) looked at crystals illuminated by focused laser light in a conventional wide-field microscope. This is not an effective use of the nonlinear imaging properties of SHG; it is not depth-selective and the photon flux required to excite the entire visible field at the same time is extreme. Further, the difficulty of excluding exciting light from the image is considerable. It is, therefore, not a practically useful technique, although it does have the advantage, compared to scanning techniques, that the wavelength of the emitted light (the second harmonic) determines the resolution. 3. "Prechirping" (i.e., retarding the shorter wavelengths before the pulse enters the fiber) has been used to compensate for this but has the practical inconvenience that the "prechirp" must be adjusted for different wavelengths and pulse lengths. This, in turn, is likely to mean realigning the beam entering the microscope. Changing wavelengths—one of the key merits if the Ti-S laser—is thus made difficult and tedious; therefore, this solution has not gained general acceptance. 4. Two-photon excitation maxima for common members of the Clontech GFP family are as follows: eCFP = 860 nm, eGFP = 920 nm, eYFP = 960 nm, Ds-Red = approx 975 nm (17).
Acknowledgments We are very grateful to Allan Jones, Frank Manconi, Anne Swan, and Mark Gorrell for samples, discussion, and collaboration. We also owe a deep debt to Colin Sheppard for introducing us to second-harmonic microscopy, and to Régis Gauderon and Paul Xu for working to develop and evaluate the technique. The microscope was purchased through a Research Infrastructure (Equipment and Facilities) grant from the Australian Research Council. We thank Sunney Xie and colleagues, the Biophysical Journal, and the Biophysical Society for allowing us to reproduce Fig. 3 A–D, and the Journal of Structural Biology for allowing us to reproduce Fig. 3E. We likewise thank Chroma Technology Corp. for permission to reproduce the filter curves shown in Fig. 2. We are very grateful to José Feijó for critically reading and substantially improving the final manuscript. References 1. Franken, P. A., Hill, A. E., Peters, C. W., and Weinreich, G. (1961) Generation of optical harmonics. Phys. Rev. Lett. 7, 118–119. 2. Gauderon, R., Lukins, P. B., and Sheppard, C. J. R. (2001) Simultaneous multichannel nonlinear imaging: combined two-photon excited fluorescence and second harmonic generation microscopy. Micron 32, 685–689. 3. Hellwarth, R. and Christensen, P. (1974) Nonlinear microscopic examination of structure in polycrystalline ZnSe. Opt. Commun. 12, 318–322. 4. Gannaway, J. N. and Sheppard, C. J. R. (1978) Second harmonic imaging in the scanning optical microscope. Opt. Quant. Electron. 10, 435. 5. Campagnola, P., Clark, H. A., Mohler, W. A., Lewis, A., and Loew, L. M. (2001) Second-harmonic imaging of living cells. J. Biomed. Opt. 6, 277–286. 6. Campagnola, P. J., Millard, A. C., Terasaki, M., Hoppe, P. E., Malone, C. J., and Mohler, W. A. (2002) Three-dimensional high-resolution second-harmonic generation imaging of endogenous structural proteins in biological tissues. Biophys. J. 81, 493–508. 7. Mertz, J. and Moreaux, L. (2001) Multi-harmonic light microscopy: theory and applications to membrane imaging, in Multiphoton Microscopy in the Biomedical Sciences (A. Periasamy and P.T.C. So, eds.), Proceedings of SPIE 4262, 9–17. 8. Moreaux, L., Sandre, O., Charpak, S., Blanchard-Desce, M., and Mertz, J. (2001) Coherent scattering in multi-harmonic microscopy. Biophys. J. 80, 1568–1574. 9. Lodish, H., Berk, A., Lipursky, S. L., Matsudaira, P., Baltimore, D., and Darrell, J. (2000) Molecular Cell Biology, 4th ed., W. H. Freeman, New York. 10. Roth, S. and Freund, I. (1981) Optical second-harmonic scattering in rat-tail tendon. Biopolymers 20, 1271–1290. 11. Georgiou, E., Theodossiou, T., Hovhannisya, V., Politopoulos, K., Rapti, G. S., and Yova, D. (2000) Second and third optical harmonic generation in type I collagen, by nanosecond laser irradiation, over a broad spectral region. Opt. Commun. 176, 253–260.
12. Cox, G., Kable, E., Jones, A., Fraser, I., Manconi, F., and Gorrell, M. (2002) 3-dimensional imaging of collagen using second harmonic generation. J. Struct. Biol. 141, 53–62. 13. Freund, I., Deutsch, M., and Sprecher, A. (1986) Connective tissue polarity. Optical second-harmonic microscopy, crossed-beam summation, and small-angle scattering in rat-tail tendon. Biophys. J. 50, 693–712. 14. Kim, B. M., Eichler, J., Reiser, K. M., Rubenchik, A. M., and Da Silva, L. B. (2000) Collagen structure and nonlinear susceptibility: effects of heat, glycation, and enzymatic cleavage on second harmonic signal intensity. Lasers Surg. Med. 27, 329–335. 15. Deng, X., Williams, E. D., Thompson, E. W., Gan, X., and Gu, M. (2002) Second harmonic generation from biological tissues: effect of excitation wavelength. Scanning 24, 175–178. 16. Cox, G. C., Manconi, F., and Kable, E. (2002) Second harmonic imaging of collagen in mammalian tissue. Proc. SPIE 4620, 148–156. 17. Blab, G. A., Lommerse, P. H. M., Cognet, L., Harms, G. S., and Schmidt, T. (2001) Two-photon excitation action cross-sections of the autofluorescent proteins. Chem. Phys. Lett. 350, 71–77. 18. Ji-Xin Cheng, Kevin Jia, Y., Gengfeng Zheng, and Sunney Xie, X. (2002) Laserscanning coherent anti-Stokes Raman scattering microscopy and applications to cell biology. Biophys. J. 83, 502–509. 19. Cox, G., Kable, E., Sheppard, C. J. R., and Xu, P. (2002) Resolution of second harmonic generation microscopy. Durban, South Africa. Proc. 15th Int. Cong. Electron Microscopy 2, 331–332. 20. Schräpler, V. R., Schmidt, T., Schultka, R., and Hepp, W-D. (1991) Farbstoffanalytische Untersuchungen zum polarisationsmikropischen Nachweis von Kollagen mit Solaminrot 4B (Teil II). Acta Histochem. 90, 75–85. 21. Milthorpe, B. K. (1994) Xenografts for tendon and ligament repair. Biomaterials 15, 745–752. 22. Cox, G. C., Xu, P., Sheppard, C. J. R., and Ramshaw, J. (2003) Characterization of the second harmonic signal from collagen. Proceedings of SPIE 4963, 32–40. 23. Zipfel, W. R., Williams, R. M., Christie, R., Nitikin, A. Y., Hyman, B. T., and Webb, W. W. (2003) Live tissue intrinsic emission microscopy using multiphoton excited native fluorescence and second harmonic generation. Proc. Natl. Acad. Sci. USA 100, 7075–7080. 24. Manconi, F., Cox, G., Kable, E., Markham R., and Fraser, I. S. (2001) Computergenerated three-dimensional reconstruction of uterine histological parallel serial sections displaying microvascular and glandular structures in human endometrium. Micron 32, 449–453. 25. Gorrell, M. D., Wang, X. M., Levy, M. T., et al. (2003) Intrahepatic expression of collagen and fibroblast activation protein (FAP) in hepatitis c virus infection. Adv. Exp. Med. Biol. 524, 235–243. 26. Yariv, A. (1967) Quantum Electronics, Wiley, New York.
3 Visualizing Calcium Signaling in Cells by Digitized Wide-Field and Confocal Fluorescent Microscopy Michael Wm. Roe, Jerome F. Fiekers, Louis H. Philipson, and Vytautas P. Bindokas Summary Calcium (Ca2+) is a fundamentally important component of cellular signal transduction. Dynamic changes in the concentration of Ca2+ ([Ca2+]) in the cytoplasm and within organelles are tightly controlled and regulate a diverse array of biological activities, including fertilization, cell division, gene expression, cellular metabolism, protein biosynthesis, secretion, muscle contraction, intercellular communication, and cell death. Measurement of intracellular [Ca2+] is essential to understanding the role of Ca2+ and for defining the underlying regulatory mechanisms in any cellular process. A broad range of synthetic and biosynthetic fluorescent Ca2+ sensors are available that enable the visualization and quantification of subcellular spatio-temporal [Ca2+] gradients. This chapter describes the application of wide-field digitized video fluorescence microfluorometry and confocal microscopy to quantitatively image Ca2+ in cells with high temporal and spatial resolution. Key Words: Intracellular calcium; signal transduction; cellular imaging; fluorescence; confocal microscopy; Fura-2; Fluo-3; cameleon; pericam; biosensor; transfection.
1. Introduction Calcium is an essential second-messenger signal in all cell types. Many cellular processes, including mitosis, gene transcription, protein biosynthesis and processing, energy metabolism, membrane electrical activity, exocytosis, motility, receptor-mediated signal transduction, and intercellular communication, are regulated by highly coordinated, spatio-temporal gradients of the concentration of intracellular Ca2+ ([Ca2+]i), which can be transient, sustained, and oscillatory. Ca2+ homeostasis and signal transduction are precisely controlled by transport mechanisms and Ca2+-binding proteins. Crucial to the regulation of Ca2+ signaling are mechanisms that control Ca2+ fluxes in the plasma membrane, endoplasmic reticulum, mitochondria, and Golgi complex. From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Roe et al.
Understanding the role of Ca2+ in cell physiology and elucidation of the underlying regulatory mechanisms requires measurements of [Ca2+]i. In recent years, new technical developments in microscopy, digital cameras, and Ca2+ indicators have made the study of high-resolution, rapid changes in subcellular [Ca2+] possible. In this chapter, we describe the methods we employ to visualize Ca2+ signals in the cytoplasm and within organelles of electrically excitable endocrine cells in vitro. Since the early 1980s, fluorescent synthetic Ca2+ dyes have been the most frequently used indicators to detect and visualize spatio-temporal [Ca2+] gradients in the cytoplasm and organelles of mammalian cells. Commercially available synthetic indicators such as Fura-2, Fluo-3, MagFura-2, and Rhod-2 can be easily loaded into cell suspensions and attached cells. The biophysical properties of synthetic fluorescent Ca2+ indicators are summarized in Table 1. The general procedure for loading cells with a synthetic Ca2+ indicator involves incubation of cells in culture medium or physiological buffer that contains a membrane-permeable, acetoxymethyl ester (AM) precursor of the dye. Cellular nonspecific esterase activity facilitates removal of the ester and unmasks the negatively charged Ca2+-binding site of the indicator. Consequently, the Ca2+sensitive anionic dye is trapped within the cytoplasm. This procedure, originally described by Tsien and his colleagues (1–5), allows labeling of large populations of cells with fluorescent Ca2+ indicators. However, because the lipophilic derivative of the indicator can partition across any membrane-surrounded organelle, it is extremely difficult to control the compartmentalization of the dye. This problem has long been recognized and methods for optimizing and evaluating dye loading conditions involving the evaluation of the effects of incubation time, temperature, and dye precursor concentration are described in detail elsewhere (6,7). Notwithstanding these refinements, employing lipophilic precursors to load Ca2+ dyes into specific subcellular compartments remains problematic. Specific labeling of the cytoplasm with a fluorescent Ca2+ dye can be achieved by microinjection of individual cells with a membrane-impermeable form of the dye, reversibly permeabilizing the plasma membrane of a population of cells in the presence of the potassium salt form of the dye using scrape loading or osmotic techniques (see ref. 7 for a review). Microinjection is labor intensive, technically demanding, and does not allow the study of a large number of cells simultaneously. The latter approaches risk damaging cells and do not enable loading of subcellular compartments such as organelles with Ca2+ indicators. Beginning in the 1990s, genetically targeted recombinant chemiluminescent and fluorescent biosynthetic Ca2+ sensors have been used with great success to study Ca2+ signaling (8–10). By utilizing specific targeting sequences that direct expression of the sensors to various locations within cells, Ca2+ biosensors have provided measurements of [Ca2+] in the cytoplasm, mitochondrial
Table 1 Properties of Synthetic Fluorescent Calcium Indicators
Fura-2 Indo-1 Fluo-3 Fluo-4 Fura Red Calcium Green Calcium Orange Calcium Crimson
DWX DWM SW SW DWX; DWM SW SW SW
MagFura-2 (Furaptra) MagFluo-4
DWX DWX
Rhod-2
Plasma membrane Fura-2FF SW Calcium Green C18 SW
K′d (µM) 0.15–0.22 0.23 0.4 0.34 0.14 0.19 0.185 0.185
25 22 1 38 0.28
X1 (nm)
M1 (nm)
340 340 488; 505 488; 495 420 488; 505 550 590
365; 380
510 400 530 > 510 > 640 530 575 615
340 488; 490
550 340 488; 510
> 505 > 510 575
M2 (nm) 475
Dynamic range 13–25 20–80 40–100 > 100 5–12 ~ 14 ~3 ~ 2.5
Abbreviations: DWX, dual-wavelength excitation ratiometric dye; DWM, dual-wavelength emission ratiometric dye; SW, single-wavelength dye. Note: The dynamic range is the fold change in fluorescence intensity or the ratio of fluorescence values measured under conditions of saturating and low calcium concentrations. Note that Fura red is normally used as DWX (ratio 420/480) but can be DWM when coloaded into cells with Fluo-3 or Fluo-4 (ratio 530/640). Source: Adapted from refs. 7, 25, and 26.
matrix, endoplasmic reticulum lumen, Golgi complex, nucleus, secretory granule surface, caveolae, and cytoplasmic surface of the plasma membrane (9,11–23). Genes encoding biosensors based on recombinant aequorin, a Ca2+-sensitive bioluminescent protein, as well as mutants of green fluorescent protein can be readily transfected into cells. Table 2 summarizes the biophysical properties of genetically targeted fluorescent Ca2+ biosensors. Constructs encoding recombinant aequorin chimeras are available commercially and can be targeted to specific locations within cells. Aequorin measures [Ca2+] across a wide range (0.1–100 µM). Unfortunately, the brightness of aequorin emission is very low compared with fluorescent indicators. Experiments are generally performed using cell suspensions or by integrating light signals from many cells grown as a monolayer. Imaging aequorin signals in single cells with current technology provides poor spatial and limited temporal resolution. Detection of aequorin luminescence requires photomultiplier tubes housed within light-tight containers or expensive high-sensitivity photon counting arrays. The light-generating reaction of Ca2+ with aequorin depends on coelenterazine, a cofactor that must be present during the experiments. In addition, aequorin molecules are irreversibly consumed by the reaction with Ca2+. This raises two problems. First, prior to the Ca2+ measurements, cells must be incubated for prolonged periods of time in solutions that do not contain Ca2+; this prevents consumption of aequorin by the high [Ca2+] in extracellular solutions. Second, irreversible consumption of the indicator induces a measurement artifact that mimics lowering of [Ca2+], thus limiting the amount of time for an experiment, especially when measuring Ca2+ signals from subcellular regions, which have high [Ca2+] such as the lumen of the endoplasmic reticulum. Genetically targeted biosynthetic fluorescent Ca2+ sensors overcome many of the limitations encountered with synthetic and chemiluminescent Ca2+ indicators. First described in 1997 by Tsien and his colleagues, cameleons provide investigators the tools necessary to record Ca2+ signals from specific subcellular compartments (9). Cameleons are fluorescent biosynthetic Ca2+ indicators constructed by inserting a Ca2+ sensor (Xenopus laevis calmodulin and M13, a calmodulin-binding protein) between two mutated forms of green fluorescent protein (GFP). Cameleon fluorescence is affected by differences in the concentration of Ca2+ that alter the amount of fluorescence resonance energy transfer (FRET) between mutant forms of GFP. This process is influenced by a Ca2+-induced change in the conformation of calmodulin–M13, which, consequently, alters the relative angular displacement between the two mutant GFPs, bringing them closer together, for example, following an increase in [Ca2+]. The increase in FRET is a direct function of [Ca2+]. In contrast to cameleons, camgaroos and pericams do not utilize FRET; Ca2+ binding results in a direct electrostatic change in the environment of these circularly permuted fluorescent proteins, causing a Ca2+-dependent shift in
Table 2 Biosynthetic Fluorescent Calcium Sensors Target Cytoplasm
Cameleon-1 Cameleon-1/E104Q Cameleon-1/E31Q Cameleon-2 Yellow Cameleon-2 (YC2) Yellow Cameleon-2 (YC2) Yellow Cameleon-2.1 (YC2.1) Yellow Cameleon-2.1 (YC2.1) Yellow Cameleon-2.3 (YC2.3) Yellow Cameleon-2.12 (YC2.12) Yellow Cameleon-3.1 (YC3.1) Yellow Cameleon-3.2 (YC3.2) Yellow Cameleon-3.3 (YC3.3) Yellow Cameleon-6.1 (YC6.1) Yellow Red Cameleon-2 (YRC2) Cyan Red Cameleon-2 (CRC2) Sapphire Red Cameleon-2 (SapRC2) Camgaroo-1 Camgaroo-2 Ratiometric-Pericam (RPC) Flash-Pericam Inverse-Pericam
FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET CPSW CPSW CPDWX CPSW CPSW
K′d (µM)
11 4.4–5.4 700 11 11 1.24 4.3
1 0.76 0.87 1 1 0.79 0.6
Dynamic range 1.7–1.8 1.7–1.8 1.7–1.8 1.7–1.8 1.5 2 2
0.11 0.2–0.4 0.2–0.4 0.2–0.4
2 1.85 2 2 0.1–0.4 0.1–0.4 0.1–0.4 8 7 10 8 7
Ref. 9 9 9 9 9 34 15 16 13 19 15 24 13 22 17 17 17 24 13 18 18 18
Table 2 (Continued) Target
Golgi Nucleus
G-CaMP pGA Yellow Cameleon-3er (YC3er)
CPSW CRET FRET
Yellow Cameleon-4er (YC4er) Yellow Cameleon-4er (YC4er) Yellow Cameleon-4er (YC4er) Yellow Cameleon-6.2er (YC6.2er) Yellow Cameleon-2mt (YC2mt) Yellow Cameleon-2.1mt (YC2.1mt) Yellow Cameleon-3mt (YC3mt) Yellow Cameleon-3.1mt (YC3.1mt) Yellow Cameleon-4mt (YC4mt) Yellow Cameleon-4.1mt (YC4.1mt) Camgaroo-2mt Ratiometric-Pericam-mt (RPC-mt) Galactosyltransferase-Yellow Cameleon-3.3 (GT-YC3.3) Cameleon-2nu Yellow Cameleon-3.1nu (YC3.1nu) Yellow Cameleon-6.1nu (YC6.1nu)
FRET FRET FRET FRET FRET FRET FRET FRET FRET FRET CPSW CPDWX FRET
FRET FRET FRET
10 4.4–5.4
104 1.7 1.5
10 2 1.7–1.8 2
Ref. 20 8 9 9 34 12 22 34 34 34 34 34 34 13 18 13 9 16 22
Secretory Granules Caveolae Plasma Membrane
Ratiometric-Pericam-nu (RPC-nu) Phogrin-Yellow Cameleon-2 (phogrin-YC2) Caveolin-1-Yellow Cameleon-2.1 (CYC2.1) Neuromodulin-Yellow Cameleon-2.1 (NYC2.1) Ratiometric-Pericam-synaptosome associated protein of 25 kDa (RPC-SNAP25)
CPDWX FRET
CPDWX
Abbreviations: CPSW, circularly permuted, single-wavelength excitation; CPDWX, circularly permuted, dual-wavelength excitation; CRET, chemilumenescence resonance energy transfer. Note: Subcellular location, apparent dissociation constant (K′d) for Ca2+ and Hill coefficient (n) is listed for the sensors. Cameleon-1, Cameleon1/E31Q, Cameleon-2, Cameleon-2nu, YC2.1, YC4er, and YC4.1mt display biphasic Ca2+-binding curves and high- and low-affinity Ca2+ binding. The reader should note that in some cases, investigators have determined K'd and n values in vitro and in vivo and should consult the references for additional details. An estimation of the dynamic range of the sensors is provided. The dynamic range in this case is defined as the fold change in fluorescent ratio or intensity of the sensor exposed to nominally 0 M [Ca2+] and saturating [Ca2+] (10–20 mM). Biosensors constructed from circularly permuted mutations of EYFP, Camgaroo-1, Camgaroo-2, Pericams, and G-CaMP have a much higher dynamic range than the fluorescence resonance energy transfer (FRET) sensors.
fluorescence emission (13,18,20,22–24). Ca2+ biosensors exhibit different biophysical properties, such as pH sensitivity and Ca2+ affinities, that enable measurement of Ca2+ signals in different cellular compartments using single- and dual-wavelength emission and excitation microfluorometry. The choice of the most appropriate indicator or indicators and measurement systems for the experiments is dictated by several considerations, including the source or location of the Ca2+ signal to be studied and whether direct visualization of spatial changes in subcellular [Ca2+] is necessary (7,25–27). In addition, certain constraints specific to the cell type or tissue under study might be important. For example, relatively homogeneous loading of populations of dispersed adherent cells with synthetic Ca2+ indicators is readily accomplished using membrane-permeable fluorescent derivatives, but loading thick biological tissues such as explants of islets of Langerhans, which consist of clusters of 1000–3000 cells, is problematic. Dye compartmentalization can be a significant problem (ability to cross one membrane implies multiple membranes might be crossed if esterase activity is low; targeted probes could offer an advantage here). Moreover, in some cases, loading primary cultures of dispersed cells has proven difficult. In this chapter, we describe protocols we employ to measure [Ca2+] spatio-temporal gradients in endocrine cell lines and islets of Langerhans. 2. Materials What follows is a list of the basic items needed to perform quantitative imaging of Ca2+ signals in cells. We find that two laboratory areas are required for these experiments: an area to perform cell culture and preparation of solutions used in the experiments, and a separate area in the laboratory or small room devoted to housing the microscope and imaging system. For our experiments, the imaging studies are performed in a darkened room (~10 m2) adjacent to the main laboratory. If space is limited, a small region of the laboratory can be darkened using black curtains suspended from the ceiling or a light-tight box can be constructed or purchased to fit over the microscope. 1. Laminar flow, tissue culture hood with ultraviolet (UV) light. 2. Temperature-regulated, humidified incubator. 3. 95% O2: 5% CO2 gas tanks (for bubbling bicarbonate-buffered perifusion solutions). 4. Complete cell culture medium. 5. Trypsin-EDTA cell dissociation solution. 6. Phosphate-buffered saline (PBS) without Mg2+ and Ca2+. 7. Hemocytometer. 8. Tissue culture flasks or circular cell culture plates. 9. Six-well tissue culture plates. 10. Glass cover slips and coating reagent if required.
Imaging Ca2+ in Cells 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Pipet tips and micropipets. Siliconized, 1.5-mL Eppendorf microcentrifuge tubes. Serological pipets and handheld dispenser (1, 5, 10, 25 mL). Fine-point jeweler's forceps. Microperifusion system and chamber. Temperature-regulated water bath and thermometer. Peristaltic pump and tubing for perifusion system (various sizes; chemically resistant). Vibration-free isolation table for microscope. Pipette Aid motor or aquarium bubbler. Two 500-mL vacuum Ehrlenmeyer flasks and rubber stoppers and desiccant. Inverted or upright microscope equipped for epifluorescence (for standard widefield fluorescence imaging), confocal laser scanning microscope, or spinning disk optical microscope. Fluorescence objectives (×10, ×20, ×40, ×60, ×100). Fluorescence light source (xenon or mercury) or laser. Optical filters (neutral density, excitation, dichroic, and emission). High-speed excitation and emission filter changing devices with computer-interfaced controller. High-sensitivity, high-resolution cooled digital camera (charge-coupled device [CCD]). Computer workstation with a 17- to 20-in. monitor, local area network access, high-capacity data storage peripheral devices (0.06–1 Tbyte capacity) for image and data archiving, and color printer. Image acquisition and analysis software with appropriate video card and driver for CCD. Chemical salts and buffers to prepare perifusion solutions, glass bottles, and conical centrifuge tubes. Fluorescent Ca2+-sensitive dyes (membrane permeable as well as potassium salt forms for performing in vitro calibration). Mammalian expression plasmids encoding biosynthetic Ca2+ sensors. Competent Escherichia coli strains for plasmid transformation, growth medium, sterile flasks, Luria–Bertani (LB) agar plates, transfer loop, bunsen burner, rotating temperature-regulated culture chamber. Plasmid cDNA preparation kits. Mammalian cell transfection reagents. Ca2+ calibration reagents and buffers.
3. Methods 3.1. Tissue Culture Our experiments are performed on endocrine cell lines and primary cells. For imaging studies using fluorescent synthetic or biosynthetic Ca2+ indicators, we seed cells onto uncoated glass cover slips in six-well tissue culture plates
24–72 h before the experiments; cells are maintained under normal culture conditions (see Note 1). Typically, we seed at a cell density to achieve 60–80% confluence by the day of the experiment in order to image [Ca2+] transients in single cells and also to have sufficient cell-free regions to obtain background fluorescence measurements. 3.2. Synthetic Ca2+ Indicators Lipophilic (membrane permeable) and hydrophilic (membrane impermeable) derivatives of synthetic fluorescent Ca2+ dyes can be obtained from many commercial sources. Membrane-impermeable Ca2+ indicators can be loaded into cells by microinjection, osmotic methods, or scrape-loading and remain in compartments into which they are introduced. They can also be used for in vitro calibrations. For our cell imaging experiments, we load cells with AM derivatives of the indicators (see Note 2). 3.3. Loading Cells With Synthetic Ca2+ Dyes The basic protocol to load neuroendocrine cells with Fura-2, Fluo-3, Fluo-4, Calcium Crimson, Fura Red, Calcium Orange, Calcium Green, MagFura-2, and Rhod-2 is outlined here. 1. Using fine-point jeweler's forceps, we gently remove a cover slip from the sixwell tissue culture plate and transfer it into a microperifusion chamber (Harvard Apparatus). 2. Cells are incubated in the chamber with Krebs-Ringer bicarbonate (KRB) or Krebs-Ringer HEPES (KRH) buffer solution containing 5 µM AM form of the dye and 0.0125–0.025% Pluronic F127 (see Note 3). Cell loading is conducted for 15–20 min in a humidified CO2/O2 incubator at 37oC. Following this period of time, the microperifusion chamber is mounted onto the temperature-controlled specimen stage of an inverted fluorescence microscope equipped for either standard wide-field epifluorescence imaging or optical spinning disk laser confocal microscopy. The cells are rinsed for 15–30 min with KRB or KRH to remove all extracellular AM form of the dye and to provide time for complete de-esterification.
Evaluation of the quality of dye loading is essential. Whereas the use of AM membrane-permeable forms of fluorescent Ca2+ indicators enables large numbers of cells to be loaded simultaneously, assessment of the compartmentalization of the dye is essential for achieving reproducible results and for reliable interpretation of the imaging data. Dye loading is affected by AM dye concentration, loading temperature and time, and cell type. Several reviews are available that discuss in detail the evaluation of dye loading and optimization of loading conditions (6,7). Optimal conditions for dye loading vary depending on cell type and must be determined empirically.
Imaging Ca2+ in Cells
3.4. Expressing Biosynthetic Ca2+ Biosensors in Cells Fluorescent Ca2+ biosensors are not yet available commercially. It has been our experience that, without exception, investigators who initially designed, constructed, and characterized a biosensor will provide samples of the plasmid constructs. In some cases, completing an interinstitutional materials transfer agreement will be required before obtaining the biosensors. The reader is urged to directly contact the investigator for procedures to obtain the constructs. Following receipt of samples (usually 1–5 µL of a 1- to 2-µg/µL stock cDNA solution, we use commercial kits to produce larger volumes of the plasmid cDNA (Qiagen Maxi Prep) and to transiently transfect cells using liposomal methods. Cells are seeded onto 15-mm or 25-mm glass cover slips 1 d prior to transfection and incubated in complete growth medium. On d 2, cells are transfected with 1–2 µg of cDNA for 4 h (see Note 4). Imaging studies are preformed 2–4 d after transfection. 3.5. Microperifusion Precise control of the contents, temperature, and volume of the external bathing solutions is absolutely necessary to ensure reproducible experiments. Most investigators employ temperature-regulated, microperifusion chambers mounted on the specimen stage of inverted and upright fluorescent microscopes. Our experience indicates that continuous superfusion of cells with different solutions, rather than by direct, manual application of chemical reagents into the chamber and onto cells using micropipets, enables more precise control and reproducibility in altering experimental conditions. In our studies, cells are constantly superfused by defined buffer solutions administered 2–5 mL/min; in a 1-mL microperifusion chamber, higher flow rates (>5 mL/min) could dislodge cells, whereas slower fluid flow (100 µL) use 25-mm circular cover slips. Superfusion of cells is initiated immediately after placing them into the imaging chamber and maintained throughout the experiment. The cells are visualized with a ×40 or ×60 oil-immersion fluorescence objective. Larger tissues such as single intact islets of Langerhans are studied using a ×10 or ×20 fluorescence objective. Objectives with a high numerical aperture and long working distance are preferred with inverted microscopes. If you use an upright microscope, a ×60 water-immersion objective is highly recommended for experiments with living cells. A 75-W xenon or mercury bulb is used for conventional wide-field fluorescence imaging. Excitation light is attenuated with neutral-density filters and the excitation wavelength is controlled by optical filters. Currently, we employ a Sutter or Prior filter wheel to set excitation wavelengths. The filter wheel is attached to a filter/shutter control device connected to a computer. Image acquisition and analytical software controls filter wheel settings and shutter activity. A separate emission filter wheel is utilized to change emission light wavelengths and to perform multiparameter emission microphotometry. Fluorescent signals from cells are captured with an intensified video camera or a digital CCD. There are many cameras available for fluorescence microscopy. Because of space limitations, we will not review the wide range of digital cameras here. The choice of camera depends on resolution and acquisition speed requirements as well as spectral properties. The reader is encouraged to investigate individual specifications of the digital cameras currently
available from manufacturers' websites for additional information. A video camera is suitable for Fura and some single-wavelength dyes, but the limited 8-bit digitization range makes it ill-suited for FRET probes as well as dyes with high dynamic response, such as Fluo-3 and Fluo-4. 3.6.1. Measuring [Ca2+]i Using Fura-2 1. Make all experimental solutions fresh daily for imaging studies. Pre-equilibrate solutions to required temperature, and if bicarbonate-buffered perfusates are used, then bubble solutions with O2 : CO2 gas 15–30 min before experiments. 2. Remove cover slips with monolayers of cells from multiwell tissue culture plates and place them into the microperifusion chamber. 3. Load cells with Fura-2AM. We limit exposure of Fura-2 to bright light in the laboratory, and we load cells with Fura-2 either in a cell incubator with bicarbonate buffered solutions or, if loading at room temperature with HEPES-buffered solutions, use a light-tight box. See above comments about loading conditions. Do not use bicarbonate buffers in room air. 4. During Fura-2 loading, set the controller for the desired chamber temperature. Make certain that all fluids flow freely in all tubing. Degassing of solutions in tubing can form bubbles that restrict/occlude flow. Adjust and set fluid inflow rate. Set acquisition parameters using image acquisition and analysis software. Set excitation filter wavelengths (using the image acquisition software). For Fura-2, use 340-nm and 380-nm excitation filters, 400DCLP dichroic mirror, and 510/40-nm emission filter. For typical experiments, we record image pairs (exposure time of 50–250 ms for each excitation wavelength) at 0.20- to 10-s intervals. Set the shutter to automatically close between image pair exposures to reduce the amount of time cells are exposed to UV light. The duration of our experiments varies from 5 min to 2 h. It is important to completely plan the acquisition profile prior to running an experiment, and some software packages allow users to program and store protocols for experiments. This can greatly simplify the procedures during an experiment and aid in repeating experimental protocols with precision. 5. Mount chamber onto the microscope stage and immediately begin rinsing cells for 10–15 min with standard external solution to remove excess Fura-2AM. Adjust and set fluid efflux rate to match influx rate. This prevents large fluctuations in perifusate volume in the chamber. Make certain to maintain a constant vigil on the status of the fluid efflux from the chamber in order to prevent overflow accidents, which can severely damage an inverted microscope and the objectives. 6. Prior to exposing cells to excitation light, preliminary focus can be accomplished using bright-field light and the binocular eyepieces. Be certain to focus the objective with great caution. Approach the cover slip with the objective slowly; once the oil has contacted the surface of the cover glass, make adjustments only with the fine-focus controller. Do not extend the objective too far. Impacting the cover glass with the metal surface of the objective will break the cover glass; the cover slips are extremely fragile and remarkably little force is required to shatter them with the objective. Not only will this cause loss of a potentially important biological
Roe et al. sample, but it also risks severe (and quite costly!) damage to the internal components of the microscope because of contamination with saline solutions. For most cells, we set the focal plane about halfway through the cells. Insert a neutral-density filter (Chroma UV filters, 0.3–2.0) into the excitation light path before opening the excitation light shutter (see Note 6). Open the shutter and visualize cells using fluorescent excitation light. Choose an appropriate field-of-view. Use the acquisition software to refocus using either the 340-nm or 380-nm excitation light. In general, we use the 380-nm illuminated cells for focus because this generally is a brighter image than the 340 nm. Next, set the camera gain. This can be accomplished using either a peripheral camera illumination controller or the camera gain controls in the image acquisition software (see Note 7). Once set, the camera gain is not readjusted during the experiment. It is important to be consistent here because arbitrary gain changes can affect conversion of ratio values to ion concentration unless in vivo calibrations are used. Finally, use the acquisition software to delineate regions of interest (ROIs) in which imaging data are collected and displayed during the experiment. These can consist of individual cells, groups of cells, and/or subcellular regions in one or multiple cells. Recheck the experimental protocol, acquisition settings, perifusion solutions, fluid flow, and image focus. Begin the experiment, changing experimental conditions as required. During each experiment, we simultaneously display the 340-nm, 380-nm, and ratio images and graphical depictions of the time-dependent changes in ratio values. A cell-free or in vitro calibration method can be performed to convert the F340/F380 ratio into molar [Ca2+]. A calibration curve plotting the F340/F380 ratio as a function of [Ca2+] can be used to provide an estimate of [Ca2+]i. To construct the curve, the F340/F380 ratio is measured in a series of buffered solutions with defined [Ca2+] and the potassium salt of Fura-2 (usually 1–5 µM). This calibration method, however, does not take into account possible effects of intracellular environment on Fura-2 (6). At the end of each experiment, we perform an in vivo (or in situ) calibration. To determine the fluorescence intensity values at saturating [Ca2+], the cells are exposed to 10–20 µM ionomycin in the presence of extracellular Ca2+. Under these conditions, the fluorescence maximum value for the intensity ratio is reached in about 30–60 s. After this, ionomycin, in solutions containing 10–20 mM EGTA and no added Ca2+, is applied. The amount of time required to reach the minimum fluorescence intensity for the ratios varies and can take 5–60 min before reaching a steadystate value. Some labs prefer to use 4-Br-A23187 as the ionophore because it aids in setting Rmin. Do not use the nonbromonated form of this ionophore because its intrinsic fluorescence will interfere with the calibration of Fura-2. To convert the fluorescence intensity ratio into molar [Ca2+], we use the following equation (1): [Ca2+] = K′d β [(R – Rmin)/(Rmax – R)] where K′d is the relative Ca2+ dissociation constant of Fura-2 (150–220 nM), β is the ratio of the 380-nm fluorescence intensity recorded at 0 and saturating [Ca2+], R is the ratio of Fura-2 fluorescence intensities at 340 nm and 380 nm, Rmax is the fluorescence ratio at saturating [Ca2+], and Rmin is the fluorescence ratio at 0 [Ca2+].
11. We save all images to the computer hard drive and program the software to automatically export and save the intensity data to spread sheets. After each experiment, the data and images are archived onto CD-R or DVD-R storage media for offline analysis at computer workstations in the main laboratory. Also, the computers in the imaging suite are networked to the workstations to facilitate downloading experimental data to offline sites. At the end of each session, we suggest deleting imaging files or transferring them to CD-R or DVD-R storage media because they can rapidly fill most computer hard drives with 60–80 gigabytes storage capacity.
3.7. Measuring [Ca2+]i Using Laser Scanning Confocal Microscopy Confocal laser scanning microscopy (CLSM) allows visualization of subcellular [Ca2+] gradients with high spatial and temporal resolution. The primary advantage of CLSM over standard, wide-field epifluorescence imaging is the reduction of fluorescence emission that originates from outside the plane of focus. The contribution of out-of-focus fluorescence reduces image contrast and resolution of fine microanatomical structures, confounds interpretation of quantitative imaging experiments, and hinders visualization of signaling events occurring within specific subcellular locations. This can be especially problematic in thick biological samples, such as intact single pancreatic islets of Langerhans (which are spherical clusters of cells 100–300 µM in diameter consisting of 1000–3000 cells) or in tissue slices. An example of this artifact is illustrated in Fig. 1 (see Color Plate 1, following p. 274), which shows the distribution of Fura-2 fluorescence in mouse islets of Langerhans in vitro; Fura-2 fluorescence appears to be present throughout the islet cells. CLSM images from a consecutive series of optical z-axis sections through a Fura-2-loaded mouse islet however clearly reveal that Fura-2 fluorescence is confined to the outer cell layers of the islet and not present within the core of the islet (see Fig. 2). Single-wavelength excitation and ratiometric dual-wavelength emission imaging can be conducted with CLSM. Although a wide range of fluorescent Ca2+ indicators and genetically targeted Ca2+ biosensors are available for CLSM, most studies have utilized single-wavelength excitation Ca2+-sensitive dyes such as Fluo-3 or Fluo-4. The choice of a specific Ca2+ indicator depends on the laser excitation wavelengths available. Many CLSM systems utilize an argon–krypton laser light source, which produces three discrete excitation lines: 488 nm, 568 nm, and 647 nm. Alternatively, many confocal systems use argon and He–Ne lasers (488- and 543- or 633-nm lines). Until recently, UV laser sources were uncommon because the lasers required high power and suffered from short life. Solid-state diode lasers are now available that could soon expand the wavelengths available in laser-based confocal systems. It is reasonable to question the need for ratiometric imaging for quantitative measurements of [Ca2+] in confocal systems because the in-focus volume, which can be many voxels (three-dimensional pixels), is a constant and much
Fig. 1. Pseudocolored image of mouse islets loaded with Fura-2. The islets were imaged by conventional wide-field epifluorescence using a ×10 objective. The apparent distribution of the Fura-2 fluorescence throughout the islets is an artifact resulting from the contribution of fluorescence from above and below the focal plane. (See Color Plate 1, following p . 274.)
less than the volume of a cell. The main rationale is to account for dye dilution caused by changes in cell volume during measurements (25,28–30). Most scanning confocal systems contain at least two emission detector channels that permit emission ratio-based measurements. Emission ratio dyes like Indo-1 can be used in confocal imaging. Also, one can perform ratiometric imaging of cells coloaded with dyes that increase and decrease fluorescence intensity with [Ca2+], such as Fluo-3 and Fura red, respectively. With appropriate sets of dichroic and emission filters, simultaneous recordings of fluorescence emission intensities (stimulated by a single wavelength of excitation light) from cells loaded with multiple Ca2+-sensitive dyes (e.g., Fluo-3 and Fura red) have been used to perform ratiometric dual-wavelength emission measurements of [Ca2+] (28–30). Excitation-ratio dyes like Fura-2 are not used because appropriate UV lasers are not commonplace and fast excitation line selection and switching requires expensive hardware.
Fig. 2. Serial optical z-sections of a mouse islet loaded with Fura-2. The islet was imaged using a UV confocal scanning laser microscope. Note the annular fluorescence and the absence of fluorescence within the central regions of the islet. Spacing between each section was 1 µm.
Ca2+ signals can be studied using the confocal microscope operating in one of three possible imaging modes. The single line-scanning mode (xt-scanning, where x represents a line one pixel wide and t is time in seconds) provides the highest temporal resolution. Depending on scanner design, line sample rates range from about 667 to 4000 lines/s. In this mode, the laser beam rapidly and repeatedly scans along the line across the same region of an optical field. This approach has greatly facilitated the study of elementary Ca2+ signals and the initiation and propagation of intracellular Ca2+ waves (27). More recently, new CLSM systems allow for multiple lines (and different shapes) to be employed in xt-scanning. It is important to minimize laser intensity because bleaching and photodamage are especially high as a result of the energy being confined to tiny cell volumes. A common solution might be to decrease confocality by opening the system pinhole(s). Analogous to the wide-field epifluorescence method described earlier, spatiotemporal [Ca2+] gradients in a thin optical section of cells can be imaged using the xyt-scanning mode. Although the temporal resolution using xyt-scanning is
slower than line-scanning, more spatial information can be collected. Confocal imaging systems allow the operator to adjust the scanning rate, sampling density, and size of the optical area for xyt imaging of biological samples. Temporal and spatial resolution is dependent on the laser scanning speed: the slower the scan rate, the lower the temporal resolution, the brighter the image (up to the point where all dye molecules have been excited), and the higher the spatial resolution (increased pixel dwell time decreases the impact of detector noise). Excessively slow scans will rapidly bleach the probes and will cause photodamage. Because the images in the xyt-scanning mode are confocal, contribution of fluorescence from outside the focal plane is minimized; this important optical property greatly facilitates the identification of Ca2+ signals originating from different subcellular compartments with precision. Requirements for spatial and temporal resolution will dictate the type of confocal to be used for xyt-scanning. If high temporal resolution is required (such as that necessary to visualize initialization and propagation of Ca2+ waves through the entire cell or a collection of cells), then high frame or image acquisition rates should be employed. Standard galvanometer mirror-scanning confocal microscopy systems can scan a full frame (512 × 512 pixels) in approx 1 s and are capable of 10- to 30-Hz scanning small xy regions (e.g., 64 × 64 and 128 × 128 pixels). However, the image intensity and contrast are low and spatial resolution is limited. A better solution for high-speed, high-resolution imaging is to use a spinning disk confocal microscope and image capture using high-sensitivity, high-speed CCD digital cameras. These microscopes are capable of data acquisition rates between 30 and120 Hz with excellent full-frame spatial resolution. One disadvantage is that these systems typically are designed to acquire a single emission wavelength. Acquisition of multiple emission images either requires multiple CCD detectors or a device to record all wavelengths on different portions of the same camera chip. The third mode, xyzt-scanning, allows resolution of spatio-temporal [Ca2+] gradients in the volume of the cell. Dynamic three-dimensional cell imaging involves repeated and rapid acquisition of xy images in multiple z-axis optical sections over time. The changes in z-axis steps can be accomplished by (a) a stage-stepping motor attached to the focus knob that precisely and reproducibly alters the position of the microscope stage, (b) an intrinsic/internal motorized focus (on some automated microscopes), (c) a high-speed piezoelectric device that controls the position of the viewing objective without moving the microscope stage, or (d) a galvanometer stage plate that can accurately displace the stage up to approximately 100 µM. 3.7.1. Measuring [Ca2+]i by CLSM in the Line-Scanning Mode 1. Grow cells on cover slips and load with single-wavelength excitation Ca2+-sensitive fluorescent dyes (e.g., Fluo-3 or Fluo-4). Place the coverslip into a microperifusion chamber mounted on the specimen stage of the confocal microscope
system and superfuse cells with buffers for 15–30 min to remove excess indicator. Initial focus of the cells should be accomplished using condenser light. 2. Use xy scans to identify the cell or cells to be studied. Refocus the image under fluorescent laser excitation light (for Fluo-3, excitation line 488 nm from the Ar/Kr laser and emission wavelength detected at 510–530 nm). Set the gain (30–60% pixel saturation), black levels, and pinhole/slit width. Use minimal laser power settings (1–10%) to avoid photobleaching and phototoxicity. Configure the line-scanning acquisition in accordance with the CLSM system you are using; adjust position of the line (or lines) as required. 3. Begin the experiment. For rapid solution exchanges, we suggest using a gravityfed perifusion system or microspritzer devices. 4. Archive the data for offline analysis. A wide choice of imaging software is available for offline analysis of line scan data. Data can be expressed as a ratio of the pixel fluorescence intensity (F) along the line scan measured at each time-point relative to baseline (F0). The pseudoratio value (F/F0) represents the foldchange in F relative to F0 and can be converted to [Ca2+] by the equation (1,31): [Ca2+] = K′d [(βF/F0) – (1/α)]/(1 – βF/F0)] where K′d is the apparent Ca2+ dissociation constant of the dye, α is the ratio of Fmax/Fmin, and β is the ratio of F0/Fmax. Fmax is the maximum fluorescence intensity value (following application of 10 µM ionomycin) and Fmin is the minimum fluorescence intensity in the absence of Ca2+ (10 µM ionomycin, 10–20 mM EGTA, and no added Ca2+). The data can be displayed as an image consisting of a combined collection of the consecutive line scans showing the changes in F, [Ca2+], or R (depicted in gray scales or pseudocolored, respectively) or graphically, expressing the pseudoratio, R, or [Ca2+] as a function of time.
3.7.2. Single-Wavelength Excitation Measurements of [Ca2+]i Using CLSM 1. Grow cells on glass cover slips and load with single-wavelength Ca2+ indicator. Like the line-scanning experiments, the fluorophore of choice for xyt-scanning is Fluo-3 or Fluo-4, although many others are commercially available. Fluo-3 fluorescence intensity is very low at resting baseline [Ca2+]i (50–100 nM) and increases 4-fold to 10-fold at saturating [Ca2+]. We typically load cells in KRB or KRBH containing 1–5 µM Fluo-3/AM for 15–20 min at room temperature or at 37°C. 2. After the dye loading, wash cells with extracellular solutions for 10–20 min by continuous superfusion. 3. Set up the confocal excitation and emission wavelengths, and adjust photomultiplier gain, slit width, and black levels in accordance with the confocal system operator's guide. Identify a field-of-view with sufficient number of cells and a cell-free area for background ROI. Use acquisition software to create ROIs that can consists of single cells, cell clusters, or single or multiple subcellular regions. Set up the acquisition program (duration of experiment, number of images to be acquired, and online graphical plotting formats, if available).
4. Perform the experiment and, afterward, conduct an in vivo calibration. Be sure to carefully monitor fluid exchange in the microperifusion chamber to avoid accidental flooding and exposure of microscope to saline solutions. If an accident does occur, immediately cease the experiment. Dry the microscope with absorbent paper, lightly cleaning all affected surfaces with a pad of absorbent paper moistened with distilled water. If saline solution has entered the body of the microscope or the interior of an objective, do not hesitate to contact the microscope technician for immediate cleaning and repair. Objectives used on confocal microscopes can easily cost over $10,000 each and can be seriously damaged if saline solution is wicked inside them. Let the system administrator determine whether fluid has entered any part of the system. 5. Imaging data are stored and analyzed as earlier.
3.7.3. Dual-Wavelength Ratiometric Emission Measurements of [Ca2+]i Using CLSM 1. Culture cells on cover slips. Load cells with Fluo-3/AM and Fura red/AM for 15–20 min at room temperature or at 37 oC. Fluo-3 and Fluo-4 produces a much brighter signal than Fura red. For loading rat hepatoma cells, we used a 1:3 ratio of Fluo-3/AM and Fura red/AM For other cell types (mouse insulinoma cells), we used a 1:5 ratio. 2. Rinse cells 15–30 min with perifusate by continuous superfusion. During this time, perform the initial focus, identify a field-of-view, and set up the confocal system and data acquisition; identify ROIs and remember that the ROI used for background subtraction should be located in a cell-free area. Both dyes are excited by the 488-nm line of an Ar or Ar/Kr laser, and emission intensities are monitored simultaneously at 535 nm (Fluo-3) and 622 nm (Fura Red) (see Note 8). Recheck focus before performing the experiment. 3. Conduct the experiment, monitoring the status of the perifusion system throughout. 4. Save image and data files to CD-R or DVD-R storage media. Backgroundsubtracted data can be expressed as the time-dependent change in the ratio of Fluo-3 (FIFL3) and Fura Red (FIFR) fluorescence intensity: As [Ca2+] increases, FIFL3 and FIFR increase and decrease, respectively. Converting FIFL3/FIFR into [Ca2+] can be accomplished using in vivo calibration (28,29).
3.8. Measuring [Ca2+] in Organelles Ca2+ signaling is regulated by localized control of Ca2+ fluxes. To fully define and understand subcellular Ca2+ signaling, the contribution of the [Ca2+] within organelles must be directly studied. [Ca2+] gradients in mitochondria and endoplasmic reticulum (ER) have been imaged using synthetic Ca2+ indicators Rhod-2 and MagFura-2, respectively, and are loaded into cells using AM derivatives of the dyes. Cationic Rhod-2 partitions into mitochondria because of inner mitochondrial membrane hyperpolarization. Mag-Fura-2 (or Furaptra) has a low affinity for Ca2+, thus making it a suitable indicator of Ca2+ fluxes in
regions of high [Ca2+]. This indicator, however, distributes throughout the cytosol and organelles, depending on loading conditions (low temperature favors dye compartmentalization because esterase activity is lowered while diffusion rates are not appreciable affected). Measurements of ER Ca2+ fluxes with Mag-Fura-2 are generally conducted in permeabilized cells in order to eliminate the contribution of fluorescence originating from the dye distributed in the cytosol. Alternatively, cells are incubated at 37oC for as long as overnight to allow transporters to clear the cytosol of indicator. Mag-Fura-2 has been used to detect ER Ca2+ signaling in intact cells. This involves loading cells with the AM form of the indicator, followed by an overnight incubation in growth medium (without Mag-Fura-2/AM), during which time cytosolic but not organelle-sequestered Mag-Fura-2 is removed by plasma membrane anion transporters. Genetically targeted Ca2+ biosensors represent the most recent approach to directly image intraorganelle Ca2+ signals. Recombinant forms of aequorin and fluorescent fusion protein chimeras have been expressed in a wide range of eukaryotic cells. By including a targeting sequence in the expression vector, the Ca2+ biosensors can be expressed selectively in specific subcellular compartments such as the ER, mitochondria, Golgi, and nucleus. Single-wavelength excitation, dual-wavelength excitation, and dual-wavelength emission microspectrofluorometry can be used to quantitatively image subcellular Ca2+ signaling with genetically targeted fluorescent biosynthetic Ca2+ sensors. Although wide-field and confocal imaging can be used, we recommend CLSM or spinning disk optical confocal imaging because these approaches allow for the more reliable identification of the location of the fluorescent signal than standard wide-field imaging. This is especially important if the goal of a study is to record mitochondrial flux vs cytosolic (or ER) fluxes (see Note 9). 3.8.1. Quantitative Dual-Wavelength Ratiometric Imaging of [Ca2+]organelle Using Cameleon and Ratiometric Pericam Ca2+ Biosensors 1. Culture cells onto glass coverslips the day before transfecting with the biosensors. In 6-well culture plates, we seed each well (containing a 25-mm circular glass coverslip) with 0.5-1 × 106 cells. 2. Day 2, transfect cells with 1–2 µg DNA per well. There are several choices for gene transfer and the most optimal reagent or method will depend on cell type. We have used liposomal methods for transiently transfecting insulin-secreting cell lines, hepatoma cells, neuroendocrine cell lines, and primary vascular smooth muscle cells; the transfection efficiency ranged between 5 and 30%. 3. We perform imaging experiments with the transfected cells 48–96 h after transfection using conventional wide-field or single-spinning disk confocal microfluorometry to measure spatio-temporal [Ca2+] gradients in the cytoplasm, ER,
Fig. 3. Expression of mitochondrially targeted ratiometric pericam (RPC-mt) in neuroendocrine cells. Laser scanning confocal images of AtT20 cells colabeled with (A) RPC-mt (green) and (B) MitoTracker Red. (C) An overlay image constructed using Adobe Photoshop. The yellow color indicates colocalization of the two dyes. Note that some cells in the field-of-view do not express RPC-mt and remain red in the overlay image. (D) Map of colocalization using Bio-Rad confocal image processing software. Blue indicates regions of overlap between green and red channel fluorescence. (see Color Plate 2, following p. 274.) mitochondria, or nucleus. The subcellular distribution of biosensors targeted to the mitochondria (see Fig. 3; Color Plate 2, following p. 274) and ER (see Fig. 4) can be easily visualized with confocal imaging. An example of real-time measurements of [Ca2+] oscillations in the cytoplasm of an insulin-secreting βTC3 cell expressing YC2.1 is shown in Fig. 5. Note the inverse relationship between the intensities of the FRET donor (see Fig. 5, top panel) and FRET acceptor (see Fig. 5, middle panel). For imaging cameleons, we recommend 436–440 nm excitation, 455DCLP dichroic, and 485 nm and 535 nm emission. Preferred filters for the ratiometric pericams are 410–415 nm and 480–485 nm excitation, 505DRLP-XR dichroic, and 535 nm emission. See Table 3 for filter sets. To minimize photobleaching (and phototoxicty), we insert neutral-density filters into the excitation light path; attenuation of light between 50 and 99% seems to work best for the biosensors. We choose cells with intermediate brightness to study; in our experience, brightly fluorescent cells tend to be unresponsive, possibly reflecting an adverse effect of overexpression of the calmodulin-containing biosensor and buffering of Ca2+ transients (16). A
Fig. 4. Confocal micrograph of AtT20 cells expressing YC4er. Cells were transfected with YC4er using lipofectamine and imaged 2 d after transfection. Note the reticular pattern of distribution of the biosensor throughout the cells and absence of expression in the nuclei. Image obtained with a Bio-Rad 1024 MRC confocal laser scanning system mounted on an upright fluorescence microscope and a 60× water immersion objective using 488-nm excitation and 520-nm emission. protocol to estimate the intracellular concentration of the biosensor has been described by Miyawaki and his colleagues (16). 4. We convert the ratio data into a molar value of [Ca2+] by in vivo calibration using 10 µM ionomycin in the presence and absence of Ca2+ (plus EGTA). For the ER cameleon calibration, we add ionomycin in the presence of 10–20 mM [Ca2+] in the extracellular solution to evoke maximum ratio, then for minimum ratio, we administer an extracellular solution containing 10–20 mM EGTA and no added Ca2+. The data are converted to molar [Ca2+] using the following equation (16):
Fig. 5. [Ca2+] oscillations in the cytoplasm of a βTC3 cell expressing YC2.1. The oscillatory changes in fluorescence intensity of the FRET acceptor (upper panel; FI535), FRET donor (middle panel; FI480), and ratio (lower panel; Ratio FI 535/480) induced by glucose (KR2; 2 mM) and tetraethylammonium (TEA) (open bar) were measured using conventional wide-field fluorescence microscopy. YC2.1 excitation was 440 nm; FRET donor and acceptor emissions were recorded at 480 nm and 535 nm, respectively. Time (s) is indicated on the x-axis.
Table 3 Filters for Imaging Fluorescence Calcium Biosensors Biosensor Yellow Cameleon Red Cameleon YRC2 CRC2 SapRC2 Camgaroo Pericam
440DF10
455DRLP
480DF10 440DF20 400DF15 480DF30 480DF10
505DRLP 455DRLP 455DRLP 505DCLP 505DRLP-XR
535DF25 480DF30 510WB40 535DF25 535DF25
565EFLP 565EFLP 565EFLP
Note: Our suggestions for excitation (X1 and X2), dichroic (DC), and emission (M1 and M2) filters for biosynthetic Ca2+ biosensors. Consult filter manufacturer for product details and recommended alternatives.
[Ca2+] = K′d [(R – Rmin)/(Rmax – R)](1/n) where K′d is the apparent dissociation constant and n is the Hill coefficient. Published values for biosensor K′d and n are summarized in Table 2.
4. Notes 1. Prior to seeding cells, cover slips are cleaned and sterilized in absolute ethanol, air-dried while exposed to UV light in a tissue culture hood for 1 h. Coating cover glasses with substrates, such as poly-L-lysine or collagen, is not required for attachment and growth of the cells we study. However, this is not the case for all cell types and use of coated glass cover slips might be necessary. The size and shape of the cover slips depends on the specimen chamber that will house the samples on the microscope stage of the imaging system. We use Harvard Apparatus and Warner Instruments microperifusion chambers. For the Harvard MP-4 system, the microperifusion chamber employs 25-mm circular glass cover slips, whereas 15-mm circular cover glass is used in the Warner PC-21 microperifusion system. 2. Manufacturers provide detailed instructions regarding storage, stability, reconstitution conditions, and handling precautions for the indicators. Unless instructed otherwise, we recommend that immediately upon receipt, the AM form of the dye be frozen at –20oC in a desiccator until ready for resuspension. Acetoxymethyl ester dye derivatives are shipped in multiple vials or a single vial containing 50 µg or 1 mg, respectively, and are soluble in dimethylsulfoxide (DMSO), ethanol, or methanol. Our common practice is to obtain multiple vials of the dye (50 µg per vial) and reconstitute the dye on the day of the experiments. We use water-free DMSO (stored at room temperature in a desiccator) to dissolve the membrane-permeable form of the dye to yield a final concentration of 1 mM (e.g., 50 µl DMSO/50 µg Fura-2AM; Mr = 1002). This volume and concentration provides enough indicator to load cells on 10–50 cover slips. The reconstituted AM form of the dye can be frozen and reused. Although repeated
freeze–thawing cycles do not seem to affect Fura-2, Fluo-3, and Fura red loading or fluorescent properties, we advise using newly reconstituted AM batches of the indicator at least once per week. 3. Pluronic is a nonionic detergent that helps to evenly disperse the AM form of the dye in an aqueous solution. Alternatively, bovine serum albumin can be used to keep the AM esters in solution; AM has limited aqueous solubility and precipitated dye will not enter cells, causing poor loading. Thus, increasing the dye concentration thus can decrease loading if the dye begins to precipitate. Precipitated dye is typically seen as bright debris over the surface of the cells and cover slip that rapidly bleaches from view. Metabolism of the cleaved AM moiety can produce toxicity in some cell types. 4. We have found that time in culture is a critical factor that determines transfection efficiency when using a lipofectamine method: a 12- to18-h culture period prior to transfection appeared optimal for cell types we have studied, including insulinoma, pituitary, and hepatoma cell lines. Even so, the maximum transfection efficiency we have achieved by the liposomal approach was approx 30% and ranged from 5 to 30%. Transfection of primary cells like pancreatic β-cells and rodent vascular smooth muscle cells with a liposome-based method was problematic, usually yielding an extremely low transfection efficiency. An alternative approach is to transfect cells with the biosensors using a viral-mediated gene shuttle vector. This requires construction of a viral vector because most of the biosensors are currently available as a plasmid cDNA. Notwithstanding this limitation, transfection efficiency is greatly improved (80–90%) and allows expression of the sensors in cell lines and primary cells as well as more complex biological tissues such as brain slices, pancreatic islets of Langerhans, and vascular preparations. Another key advantage of viral transfection is the potential for selective expression of a biosensor in a specific cell type within a multicellular tissue. This is accomplished by infecting tissues with viral vectors whose expression is under the control of a cell-specific promoter. For example, by driving adenovirus- and baculovirusmediated expression with the rat insulin promoter sequence (RIP1), we have selectively and specifically labeled pancreatic β-cells in intact mouse islets of Langerhans with GFP-labeled fusion proteins. Finally, although it is conceivable that a transgenic approach might be used to express biosensors in cells in vivo, there is little evidence in the literature that this method will be useful in mammalian models. Transgenic expression of YC2 and camgaroo-2 in Caenorhabditis elegans and Drosophila neurons, respectively, has been reported (32,33). 5. All solutions, including KRB and KRH buffers, should be freshly prepared on the same day as the imaging experiments. We strongly recommend avoiding direct application of drugs or other reagents into the specimen chamber by micropipetting; this method does not allow precise control of drug concentration and risks mechanical disruption of cells, which can affect Ca2+ signaling. Other methods are available for drug or reagent delivery to cells, including microelectrodes attached to a pressurized microspritzer device or a gravity-fed U-tube connected to a computercontrolled manifold, which receives fluid input from one or more reservoirs. These
methods have the advantage of allowing the administration of reagents to highly localized regions or to individual cells and are especially useful when administering drugs that are available in limited amounts. Under no circumstances should you expose the cells (or your eyes) to unattenuated arc lamp light. Doing so risks photobleaching of the indicator and cell photodamage. Make certain that the digital camera gain is not activated before diverting the emission light path from the binocular eyepieces to the camera. Also, turn off room lights and the condenser lamp before activating the camera. Unfortunately, some intensified digital cameras have limited protection circuitry to prevent damage to the intensifier caused by a sudden exposure to bright light. Apart from outright failure, strong light also greatly decreases the life of Gen-3 intensifiers or can "burn-in" patterns. For Fura-2 imaging using the intensified video camera, we adjust the camera gain using the 380-nm image by initially increasing the camera gain to near saturation, then setting the final adjustment 20–30% below saturation. This is important because as [Ca2+] increases and the 380-nm signal decreases, but as the [Ca2+] returns to baseline or overshoots baseline (which can occur in some Ca2+ signaling responses), the 380-nm intensity increases. Setting the camera gain to saturation reduces image quality and impairs reliable quantification of the imaging intensity data. Using the CCD cameras, the 340-nm and 380-nm image gains can be set independently. Use laser power setting of 3–10%, although up to 30% can be required depending on the quality of dye loading. Because emission intensity of Fluo-3 is higher than Fura red, gain settings for the red channel (Fura red) will be higher than the green channel (Fluo-3). As a general rule, we set the gain so that cell fluorescence intensity was at least fivefold above background. The optimal pinhole size, determined by objective numerical aperture (NA) and wavelength, should ideally have different diameters, but many systems have only one pinhole. For live cell work, one must typically open the pinhole wide enough to get adequate signal intensity while keeping the laser intensity low to prevent photobleaching and phototoxicity. Targeted Ca2+ biosensors seem to offer an advantage in compartment selectivity; however, compartment physical characteristics, low expression levels, general phototoxicity, and perhaps chaperone content might still seriously hamper measurements. Cameleon probes are not well suited for CLSM using the 488-nm laser because CFP is poorly excited and YFP is directly excited. Some Ar lasers provide a 457-nm line that has been used to excite cameleon probes; however, this line is typically very weak and directly excited YFP. Recent utilization of 405-nm diode lasers in some confocal systems permits strong excitation of CFP with minimal YFP excitation and offers promise that targeted cameleon probes will become more widely used for confocal measurements of compartmental Ca2+ fluxes in single cells, tissue slices, or multicellular preparations.
Acknowledgments This work was supported by an American Diabetes Association Research Award (MWR), NIH DK64162 (MWR), and NIH DK63493 (LHP and MWR).
References 1. Grynkiewicz, G., Poenie, M., and Tsien, R. Y. (1985) A new generation of Ca2+ indicators with greatly improved fluorescence properties. J. Biol. Chem. 260, 3440–3450. 2. Kao, J. P. Y., Harootunian, A. T., and Tsien, R. Y. (1989) Photochemically generated cytosolic calcium pulses and their detection by fluo-3. J. Biol. Chem. 264, 8179–8184. 3. Minta, A., Kao, J. P. Y., and Tsien, R. Y. (1989) Fluorescent indicators for cytosolic calcium based on rhodamine and fluorescein chromophores. J. Biol. Chem. 264, 8171–8178. 4. Tsien, R. Y. (1981) A non-disruptive technique for loading calcium buffers and indicators into cells. Nature 290, 527–528. 5. Tsien, R. Y., Pozzan, T., and Rink, T. G. (1982) Calcium homeostasis in intact lymphocytes: cytoplasmic free calcium monitored with a new, intracellularly trapped fluorescent indicator. J. Cell Biol. 94, 325–334. 6. Roe, M. W., Lemasters, J. J., and Herman, B. (1990) Assessment of Fura-2 for measurements of cytosolic free calcium. Cell Calcium 11, 63–73. 7. Takahashi, A., Camacho, P., Lechleiter, J. D., and Herman, B. (1999) Measurement of intracellular calcium. Physiol. Rev. 79, 1089–1125. 8. Baubet, V., Le Mouellic, H., Campbell, A. K., Lucas-Meunier, E., Fossier, P., and Brulet, P. (2000) Chimeric green fluorescent protein-aequorin as bioluminescent Ca2+ reporters at the single-cell level. Proc. Natl. Acad. Sci. USA 97, 7260–7265. 9. Miyawaki, A., Llopis, J., Heim, R., et al. (1997) Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin. Nature 388, 882–887. 10. Rizzuto, R., Simpson, A. W., Brini, M., and Pozzan, T. (1992) Rapid changes of mitochondrial Ca2+ revealed by specifically targeted recombinant aequorin. Nature 358, 325–327. 11. Emmanouilidou, E., Teschemacher, A. G., Pouli, A. E., Nicholls, L. I., Seward, E. P., and Rutter, G. A. (1999) Imaging Ca2+ concentration changes at the secretory vesicle surface with a recombinant targeted cameleon. Curr. Biol. 9, 915–918. 12. Foyouzi-Youssefi, R., Arnaudeau, S., Borner, C., et al. (2000) Bcl-2 decreases the free Ca2+ concentration within the endoplasmic reticulum. Proc. Natl. Acad. Sci. USA 97, 5723–5728. 13. Griesbeck, O., Baird, G. S., Campbell, R. E., Zacharias, D. A., and Tsien, R. Y. (2001) Reducing the environmental sensitivity of yellow fluorescent protein: mechanisms and applications. Proc. Natl. Acad. Sci. USA 276, 29,188–29,194. 14. Isshiki, M., Ying, Y. -S., Fujita, T., and Anderson, R. G. W. (2002) A molecular sensor detects signal transduction from caveolae in living cells. J. Biol. Chem. 277, 43,389–43,398. 15. Miyawaki, A., Griesbeck, O., Heim, R., and Tsien, R. Y. (1999) Dynamic and quantitative Ca2+ measurements using improved cameleons. Proc. Natl. Acad. Sci. USA 96, 2135–2140.
16. Miyawaki, A., Mizuno, H., Llopis, J., Tsien, R. Y., and Jalink, K. (2001) Cameleons as cytosolic and intra-organellar calcium probes, in Calcium Signalling, 2nd ed. (Tepikin, A.V., ed.), Oxford University Press, Oxford, pp. 3–16. 17. Mizuno, H., Sawano, A., Eli, P., Hama, H., and Miyawaki, A. (2001) Red fluorescent protein from Discosoma as a fusion tag and pardner for fluorescence energy transfer. Biochemistry 40, 2502–2510. 18. Nagai, T., Sawano, A., Park, E. S., and Miyawaki, A. (2001) Circularly permuted green fluorescent proteins engineered to sense Ca2+. Proc. Natl. Acad. Sci. USA 98, 3197–3202. 19. Nagai, T., Ibata, K., Park, E. S., Kubota, M., Mikoshiba, K., and Miyawaki, A. (2002) A variant of yellow fluorescent protein with fast and efficient maturation for cell-biological applications. Nature Biotechnol. 20, 87–90 20. Nakai, J., Ohkura, M., and Imoto, K. (2001) A high signal-to-noise Ca2+ probe composed of a single green fluorescent protein. Nat. Biotechnol. 19, 137–141. 21. Pinton, P., Tsuboi, T., Ainscow, E. K., Pozzan, T., Rizzuto, R., and Rutter, G. A. (2002) Dynamics of glucose-induced membrane recruitment of protein kinase C βII in living pancreatic islet β-cells. J. Biol. Chem. 277, 37,702–37,710. 22. Truong, K., Sawano, A., Mizuno, H., et al. (2001) FRET-based in vivo Ca2+ imaging by a new calmodulin-GFP fusion molecule. Nature Struct. Biol. 8, 1069–1073. 23: Zhang, J., Campbell, R. E., Ting, A. Y., and Tsien, R. Y. (2002) Creating new fluorescent probes for cell biology. Nature Rev. Cell Biol. 3, 906–918. 24. Baird, G. S., Zacharias, D. A., and Tsien, R. Y. (1999) Circular permutations and receptor insertion within green fluorescent proteins. Proc. Natl. Acad. Sci. USA 96, 11,241–11,246. 25. Dustin, L. B. (2000) Ratiometric analysis of calcium mobilization. Clin. Appl. Immunol. Rev. 1, 5–15. 26. Hallett, M. B., Hodges, R., Cadman, M., et al. (1999) Techniques for measuring and manipulating free Ca2+ in the cytosol and organelles of neutrophils. J. Immunol. Methods 232, 77–88. 27. Thomas, D., Tovey, S. C., Collins, T. J., Bootman, M. D., Berridge, M. J., and Lipp, P. (2000) A comparison of fluorescent Ca2+ indicator properties and their use in measuring elementary and global Ca2+ signals. Cell Calcium 28, 213–223. 28. Lipp, P. and Niggli, E. (1993) Ratiometric confocal Ca2+ measurements with visible wavelength indicators in isolated cardiac myocytes. Cell Calcium 14, 359–372. 29. Lipp, P., Luscher, C., and Niggli, E. (1996) Photolysis of caged compounds characterized by ratiometric confocal microscopy: a new approach to homogeneously control and measure the calcium concentration in cardiac myocytes. Cell Calcium 19, 255–266. 30. Roe, M. W., Moore, A. L., and Lidofsky, S. D. (2001) Purinergic-independent calcium signaling mediates recovery from hepatocellular swelling: implications for volume regulation. J. Biol. Chem. 276, 30,871–30,877. 31. David, G., Talbot, J., and Barrett, E. F. (2003) Quantitative estimate of mitochondrial [Ca2+] in stimulated motor neurons. Cell Calcium 33, 197–206.
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4 Multifluorescence Labeling Techniques and Confocal Laser Scanning Microscopy on Lung Tissue Maria Stern, Douglas J. Taatjes, and Brooke T. Mossman Summary Lung tissue consists of more than 40 individual cell types that might interact to produce adverse pathologies. After injury, a number of signaling proteins expressed in various epithelial and other cell types have been linked to the advent of apoptosis, compensatory proliferation, and adaptation to stress. We describe here the use of immunochemistry and multifluorescence approaches using confocal laser scanning microscopy to define the signaling pathways (protein kinases C and mitogen-activated protein kinases) activated by asbestos fibers after inhalation. Using these approaches, we are able to localize signaling events in distinct cell types of the lung and determine their status in the cell cycle (resting or nonresting). Moreover, we are able to determine whether various signaling proteins colocalize in cells and the sites affected by asbestos fibers. Key Words: Immunofluorescence; confocal laser scanning microscopy; asbestos; extracellular signal-regulated kinases (ERKs); PKCδ; proliferation marker; Ki-67.
1. Introduction Confocal laser scanning microscopy (CLSM) is a laser-based imaging technology widely used in pathology and cell biology research. This approach can be used to identify lesions and affected cell types. CLSM provides increased resolution over conventional wide-field microscopy and has the ability to reject out-of-focus fluorescence. It also allows a decrease in autofluorescence signal caused by collagen deposits in some tissues, such as lung, by the use of fluorescent probes that excite at higher wavelengths (1). It is a powerful technique for studying cell signaling by environmental agents in the initiation and pathology of lung disease and/or repair and adaptation. Inhaled environmental agents such as asbestos might elicit cell signaling pathways at the cell membrane. Using antibodies specific to phosphorylated (i.e., activated) signaling proteins,
From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Stern, Taatjes, and Mossman
related signaling events and gene transactivation by signaling proteins can be documented in vivo. The mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinases (ERKs) as well as some isotypes of protein kinase C (i.e., PKCδ) have been linked to both cell proliferation and the development of apoptosis in response to toxic agents such as oxidants and asbestos fibers. In this chapter, we describe multifluorescence techniques to document nuclear and membrane translocation and increased activation of PKCδ and ERK 1/2 in lung tissue after inhalation of asbestos (2,3). Use of a Ki-67 antibody specific for nonresting epithelial cells indicates that these protein kinases are often colocalized in proliferating cells (4). 2. Materials 2.1. Processing and Sectioning Mice are administered a lethal dose of sodium pentobarbital (Abbot Laboratories, Chicago, IL) before the chest cavity is opened, a polyurethane catheter is inserted into the trachea, and the lungs are instilled with 1X phosphatebuffered saline (PBS) at a pressure of 25 cm water. The unfixed lungs are separated by suturing, removed from the chest, snap-frozen by plunging into liquid-nitrogen-cooled isopentane, placed in OCT embedding compound (Tissue Tek, Torrance, CA), and frozen at –80°C. Sections are cut at –23°C in a cryostat using disposable knives and then retrieved onto Superfrost +/+ slides. Sections are quickly examined under the light microscope to assure that the tissue structure has not been damaged during the sectioning process. Slides are stored in a slide box at –80°C until use. 2.2. Reagents 1. 10X PBS (for 1 L): Add 2.76 g of sodium phosphate monobasic and 14.1 g of sodium phosphate dibasic (anhydrous) to 500 mL of deionized water (dH2O). Add 90 g of sodium chloride and bring volume to 1L, followed by adjustment of the pH to 7.4. For 1X PBS, dilute 1 : 10 with distilled water (dH2O) and adjust pH to 7.4 if necessary. Both solutions are stable at room temperature (RT). 2. PBS-PFA fixative: 3.7% paraformaldehyde (PFA), 1X PBS. PFA should be handled under a fume hood and glove protection is required. Weigh 3.7 g of PFA and add to 100 mL of 1X PBS. Heat stirring solution at 60°C until PFA dissolves. When solution becomes clear, remove flask from the heating plate, cool to RT, and filter through Whatman filter paper. This solution is made fresh each time and chilled prior to use. 3. Sodium dodecyl sulfate (SDS): 0.5% SDS, 1X PBS. Procedure should be handled with respiratory mask and gloves. Weigh 0.05 g of SDS and add to 10 mL of 1X PBS. Shake the tube until mix goes into solution. This solution is stable at RT. 4. 1% Bovine serum albumin (BSA)/PBS: 1 mg/mL BSA, 1X PBS, 0.02% sodium azide. Store solution at 4°C. This solution is stable until evidence of bacterial growth.
Multifluorescence Labeling Techniques and CLSM
5. 10% Normal goat serum (from secondary antibody host) in 1X PBS. Solution is made fresh prior to each use. 6. 5% Blocking reagent from mouse-on-mouse kit (M.O.M. kit; Vector Laboratories, Burlingame, CA) in 1X PBS. Solution is made fresh prior to each use. 7. Mounting medium: AquaPolyMount (Polysciences Inc., Warrington, PA) should be stored at 4°C and warmed to RT for a few minutes before each use.
2.3. Equipment A humid environment is necessary to prevent evaporation of reagents during prolonged incubations. Inexpensive humidity chambers can be created in the lab. For instance, a large (6-in. diameter) glass Petri dish with a moistened filter paper on the bottom can be used as a humid chamber. Because slides should not touch wet paper directly, a piece of parafilm elevated 2–4 mm above the filter paper on which place the slides can be used (5). 2.4. Controls Staining controls omitting primary antibodies are required for each of the secondary antibodies for every staining. Negative controls using isotype control antibodies or nonimmune serum from the same species as the primary antibody are also helpful for validating the specificity of staining results. 2.5. Antibody Sources 1. PKCδ: rabbit polyclonal nPKC δ (C-20) antibody (Santa Cruz, cat. no. sc-937). 2. p-ERK: rabbit polyclonal Phospho-p44/42 MAP Kinase (Thr202/Tyr204) (Antibody, Cell Signaling Inc., cat. no. 9101). 3. p-JNK: rabbit polyclonal JNK (SAPK) [pTpY183/185] (Biosource, cat. no. 44682). 4. Cytokeratin: pan antibody produced in mouse (Sigma, cat. no. C2562). 5. MAC3: rat anti-mouse antibody (BD Biosciences Pharmingen, cat. no. 553322). 6. proSP-C: rabbit anti-human prosurfactant protein C polyclonal antibody (Chemicon, cat. no. AB3786). 7. Ki-67: monoclonal rat anti-mouse, clone TEC3 antibody (DAKO, cat. no. M7249).
3. Methods 3.1. Single Immunofluorescence Labeling 3.1.1. MAPK and PKC Because anti-p-ERK and anti-p-JNK antibodies used in our laboratory are all derived from rabbits, they can be detected using the same secondary antibody. Therefore, an identical procedure can be used to successfully obtain each labeling. The anti-PKCδ antibody is a mouse monoclonal; thus, it should be used in conjunction with the mouse-on-mouse kit.
3.1.1.1. P-ERK, P-JNK ANTIBODIES, RABBIT POLYCLONAL 1. For fixation, slides are placed into a Coplin jar filled with fresh 3.7% PFA for 10-min at RT followed by two 5-min washes in 1X PBS (see Note 1). 2. Wash slides in 1X PBS twice for 5 min. 3. Place slides into –20°C methanol for 10 min for permeabilization. Permeabilization is advised to ensure free access of the antibody to its antigen. Storage of methanol as well as incubation of the tissue should take place at –20°C. 4. Wash slides in 1X PBS twice for 5 min. 5. Circumscribe the tissue section with a Pap-pen. Using a Q-tip, dry around the section and circumscribe the tissue section with a Pap-pen to create a hydrophobic border. This hydrophobic border allows the use of small volumes of solutions to cover the entire section without wasting unreasonably large amounts of expensive reagents. 6. Place the slides in a humid chamber. 7. For antigen retrieval, sections can be treated with 1% SDS in 1X PBS for 5 min at RT to enhance antibody staining (6,7). Apply 1% SDS for 5 min on each section. 8. Wash sections twice for 5 min in 1X PBS in a Coplin jar. 9. Block in 10% normal goat serum in 1X PBS for 1 h at RT. Serum used for the blocking step has to be from the animal species in which the secondary antibody was raised in to prevent nonspecific binding. Therefore, for primary antibodies raised in other animal species than mouse, 10% normal serum diluted in 1X PBS should be used for 1-h incubation. 10. Wash twice in 1X PBS for 5 min. 11. Apply rabbit polyclonal antibody diluted in 1% BSA/1X PBS (PKCδ, 3 µg/mL; p-ERK, 1 : 250; p-JNK, 2 µg/mL). In most instances, we recommend applying primary antibodies overnight at 4°C to enhance staining intensities while lowering nonspecific background staining. 12. Wash slides twice in 1X PBS for 5 min. 13. Cover sections with biotinylated goat anti-rabbit IgG (Vector Laboratories) for 1 h at RT (see Notes 2–4). 14. Wash slides twice in 1X PBS for 5 min. 15. Incubate sections in strepavidin Alexa 568 conjugate at 1 : 400 dilution in 1X PBS for 1 h at RT in the dark. During the incubation with fluorophore-conjugated secondary antibodies, the humid chamber should be covered with aluminum foil to avoid exposure to room light and potential fluorophore photobleaching. Secondary antibodies are chosen with the desired conjugated fluorophore and to bind to the primary antibody. 16. Rinse slides in 1X PBS for 5 min at RT. 17. Rinse twice with dH2O for 1 min. 18. Mount section with cover slip using AquaPolyMount. Use no. 1-1/2 cover slips. Apply a drop of AquaPolyMount to cover the section and attach a cover slip. Using forceps, apply minimal pressure to the top of the cover slip to remove air bubbles from underneath and to create a tight seal (see Notes 6,7). 19. Store slides in a lightproof slide box at 4°C until analysis.
3.1.1.2. PKCδ ANTIBODY, MOUSE MONOCLONAL 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17.
Take slides out of –80°C and immediately place them in 4% PFA for 10 min at RT. Wash slides in 1X PBS twice for 5 min. Place slides into –20°C methanol for 10 min. Wash slides in 1X PBS twice for 5 min. Circumscribe the tissue section with a Pap-pen. Place the slides in a humid chamber. For antigen retrieval, apply 1% SDS for 5 min on each section. Wash sections twice for 5 min in 1X PBS in a Coplin jar. To minimize nonspecific antibody binding of mouse monoclonal antibodies on murine tissue, slides are incubated in 5% blocking reagent from a mouse-onmouse kit (Vector Laboratories, Burlingame, CA) in 1X PBS for 1 h. Wash twice in 1X PBS for 5 min. Pre-incubate sections in 10% concentrated protein from a mouse-on-mouse kit in 1X PBS for 5 min. Apply 1 : 250 dilution of PKCδ antibody in 10% concentrated protein from a mouse-on-mouse kit in 1X PBS overnight at 4°C. Wash twice in 1X PBS for 5 min. Apply 1 : 400 dilution of goat anti-mouse IgG conjugated to Alexa 647 in 1% BSA/1X PBS for 1 h at RT in the dark. Rinse slides in 1X PBS for 5 min at RT. Rinse with dH2O for twice for 1 min. Mount section with cover slip using AquaPolyMount.
3.1.2. Cell Type Markers To establish whether the protein of interest is specific to bronchiolar and alveolar epithelial cells vs macrophages that infiltrate lung after an injury, lung sections can be colabeled with cell-type-specific markers. 3.1.2.1. ANTI-CYTOKERATIN; EPITHELIAL CELL MARKER; MOUSE MONOCLONAL PAN (MIXTURE) ANTIBODY 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Take slides out of –80°C and immediately place them in 4% PFA for 10 min at RT. Wash slides in 1X PBS twice for 5 min. Place slides into –20°C methanol for 10 min. Wash slides in 1X PBS twice for 5 min. Circumscribe the tissue section with a Pap-pen. Place the slides in a humid chamber. For antigen retrieval, apply 1% SDS for 5 min on each section. Wash sections twice for 5 min in 1X PBS in a Coplin jar. Block in 5% blocking reagent from a mouse-on-mouse kit in 1X PBS for 1 h at RT. Wash twice in 1X PBS for 5 min. Pre-incubate sections in 10% concentrated protein from a mouse-on-mouse kit in 1X PBS for 5 min.
12. Apply 1:100 dilution of anti-cytokeratin in 10% concentrated protein from a mouse-on-mouse kit in 1X PBS overnight at 4°C. 13. Wash twice in 1X PBS for 5 min. 14. Apply 1:400 dilution of goat anti-mouse IgG conjugated to Alexa 647 in 1% BSA/1X PBS for 1 h at RT in the dark. 15. Rinse slides in 1X PBS for 5 min at RT. 16. Rinse twice with dH2O for 1 min. 17. Mount section with cover slip using AquaPolyMount.
3.1.2.2. MAC-3; RAT ANTI-MOUSE M3/84 ANTIBODY THAT REACTS WITH MAC-3 ANTIGEN EXPRESSED ON MOUSE MONONUCLEAR PHAGOCYTES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Take slides out of –80°C and immediately place them in 4% PFA for 10 min at RT. Wash slides in 1X PBS twice for 5 min. Place slides into –20°C methanol for 10 min. Wash slides in 1X PBS twice for 5 min. Circumscribe the tissue section with a Pap-pen. Place the slides in a humid chamber. For antigen retrieval, apply 1% SDS for 5 min on each section. Wash sections twice for 5 min in 1X PBS in a Coplin jar. Incubate sections in 10% normal goat serum in a humid chamber on orbital shaker for 1 h at RT for blocking. Wash twice in 1X PBS for 5 min. Apply 20 µg/mL solution of MAC-3 antibody in 1% BSA/PBS overnight at 4°C. Wash twice in 1X PBS for 5 min. Apply 1:400 dilution of goat anti-rat IgG conjugated to Alexa 647 in 1% BSA/PBS for 30 min at RT in the dark. Rinse slides in 1X PBS for 5 min at RT. Rinse twice with dH2O for 1 min. Mount section with cover slip using AquaPolyMount.
3.1.2.3. ANTI-SP-C; RABBIT PROSURFACTANT PROTEIN C POLYCLONAL ANTIBODY, DETECTS ALVEOLAR TYPE II EPITHELIAL CELLS IN MOUSE LUNG TISSUE 1. 2. 3. 4. 5. 6. 7. 8. 9.
Take slides out of –80°C and immediately place them in 4% PFA for 10 min at RT. Wash slides in 1X PBS twice for 5 min. Place slides into –20°C methanol for 10 min. Wash slides in 1X PBS twice for 5 min. Circumscribe the tissue section with a Pap-pen. Place the slides in a humid chamber. For antigen retrieval, apply 1% SDS for 5 min on each section. Wash sections twice for 5 min in 1X PBS in a Coplin jar. Incubate sections in 10% normal goat serum in a humid chamber on orbital shaker for 1 h at RT for blocking. 10. Wash twice in 1X PBS for 5 min. 11. Apply 3 µg/mL Solution of anti-SP-C antibody in 1% BSA/PBS overnight at 4°C. 12. Wash twice in 1X PBS for 5 min.
13. Apply 1:400 dilution of goat anti-rabbit IgG conjugated to Alexa 647 in 1% BSA/1X PBS for 30 min at RT in the dark. 14. Rinse slides in 1X PBS for 5 min at RT. 15. Rinse twice with dH2O for 1 min. 16. Mount section with cover slip using AquaPolyMount.
3.1.3. Proliferation Marker: Ki-67; Clone TEC-3, Cell Proliferation Marker, Rat Anti-Mouse Monoclonal 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Take slides out of –80°C and immediately place them in 4% PFA for 10 min at RT. Wash slides in 1X PBS twice for 5 min. Place slides into –20°C methanol for 10 min. Wash slides in 1X PBS twice for 5 min. Circumscribe the tissue section with a Pap-pen. Place the slides in a humid chamber. For antigen retrieval, apply 1% SDS for 5 min on each section. Wash sections twice for 5 min in 1X PBS in a Coplin jar. Incubate sections in 10% normal goat serum in a humid chamber on orbital shaker for 1 h at RT for blocking. Wash twice in 1X PBS for 5 min. Apply 1:25 dilution of Ki-67 antibody in 1X PBS overnight at 4°C. Wash twice in 1X PBS for 5 min. Apply 1:300 dilution of goat anti-rat IgG conjugated to Alexa 647 in 1% BSA/PBS for 30 min at RT in the dark. Rinse slides in 1X PBS for 5 min at RT. Rinse twice with dH2O for 1 min. Mount section with cover slip using AquaPolyMount.
3.2. Double and/or Triple Labeling To simultaneously detect two or three primary antibodies raised in different host animal species (e.g., rabbit polyclonal p-ERK, mouse monoclonal PKCδ, and rat monoclonal Ki-67 (see Fig. 1; see Color Plate 3, following p. 274), antibodies are combined together and applied on preblocked slides (see Notes 1,5, and 8) overnight at 4°C in a humid chamber. For detection of primary antibodies, a cocktail of two or three fluorophore-conjugated secondary antibodies can be applied to the sections or, alternatively, a sequential incubation with each of the secondary antibodies separately (see Notes 3,9). For examples of labeling techniques described above, see refs. 2 and 3. 3.3. Fluorescent Detection of Cell Nuclei There are a number of fluorescent dyes for cell nuclei, including SYTOX Green and propidium iodide (PI). SYTOX Green, a green nucleic acid-binding fluorescent dye, is also useful to demonstrate the general cellularity of a tissue. Diluted in 1X PBS to 5 µM, it is applied directly on a tissue section for 15 min
Fig. 1. Triple-labeling using mouse monoclonal PKCδ (blue), rabbit polyclonal p-ERK (red), and rat monoclonal Ki-67 (green) antibodies on sham animals (A) and animals exposed to crocidolite asbestos for 4 d (B,C). White pixels appear at the areas of colocalization of all three antibodies. Colocalization of blue and red colors demonstrate colocalization of p-ERK and PKCδ. A negative control omitting primary antibody is also provided and shows no fluorescence signal (C). (See Color Plate 3, following p. 274.)
after the excess secondary antibody has been washed off. The sections should then be washed at least two times for 10–15 min in 1X PBS. SYTOX Green is excited by 488-nm wavelength light (see Note 10), leading to a potential overlap with tissue autofluorescence. However, because the signal emission of SYTOX Green is quite intense, autofluorescence is not a major problem. In addition, this leaves two wavelengths free for staining with antibodies against proteins of interest. (See Note 11.) Propidium iodide (PI), a nuclear stain for cells in the G1 phase of the cell cycle, can also be used as a nuclear marker of cellularity. The working solution consists of 10 µg/mL of PI and 10 µg/mL of RNase A diluted in 1X PBS. Tissue sections should be incubated for 30 min at RT in PI, followed by two 10-min washes in 1X PBS. PI is excited by yellow light, resulting in red fluorescence emission. 4. Notes 1. Depending on what staining procedures the sample is going to be used for, the tissue might receive PFA fixation prior to OCT. If this is the case, the PFA fixation step can be withdrawn from the staining procedure. No other changes are needed. 2. Always optimize antibody dilutions using recommended concentrations as a guideline. Every system is slightly different, and trial-and-error experiments might be required to optimize the antibody or a technique for your specific system. Try doing a dilution series for all primary and secondary antibodies. 3. There are a few approaches to overnight incubation with a primary antibody. Incubations can be done at 4°C in a refrigerator or in a cold room on an orbital shaker to assure even application of the antibody. If an orbital shaker is used, cover the section with a piece of parafilm to ensure even distribution and minimal
evaporation of the antibody solution. As a rule, most primary antibody incubations can also be performed for 1 h at RT on an orbital shaker. However, it is not generally recommended because of the possible loss of quality in labeling and resulting unevenness of binding. The background signal can also be decreased by adding a second blocking step using the same blocking reagent after washing away the primary antibody and prior to incubation with the secondary antibody. To amplify the signal, a biotin/strepavidin system can be employed. Biotin has a very specific affinity for strepavidin/avidin that allows a decrease in background fluorescence as a result of nonspecific binding of a secondary antibody. This system is especially beneficial when doing multiple labeling to match the signal intensities of different antibodies. Remember to prewarm the AquaPolyMount to RT prior to use to avoid bubbles when applying to a tissue section. Other mounting media containing antifade reagents are widely available from commercial sources. After the cover slip is mounted, remove excess medium around the cover slip on the slide and/or on the cover slip itself by gently blotting with a bibulous paper. In the case of double or triple labeling, the possibility exists that different fixation conditions are required for preservation of the various antigens. Try to perform the entire staining procedure using a fixative that is specifically required for one of the antibodies. If a required technique is much harsher, attempt to do a sequence labeling by adding a second blocking step to decrease the fluorescent background and to even out the balance between signaling intensities. When blocking sections for multilabeling using an antibody that requires specific reagents (e.g., colabeling of a rabbit polyclonal antibody with a mouse monoclonal antibody), label both probes using only reagents from a mouse-on-mouse kit needed for mouse antibody to simplify the procedure. If binding of a nonmurineraised antibody fails, perform a sequence labeling using different blocking reagents and antibody diluents for each probe. SYTOX Green can emit at the higher wavelength and be detected by the red channel. This leads to detection of nuclear labeling using a wide-band green filter. However, it does not cause any problems while imaging with the confocal microscope using the sequential scanning mode. To minimize or avoid autofluorescence, use those probes that fluoresce at higher wavelengths (i.e., 568 nm and 647 nm rather than 488 nm).
References 1. Taatjes, D. J., Palmer, C. J., Pantano, C., Hoffmann, S. B., Cummins, A., and Mossman, B. T. (2001) Laser-based microscopic approaches: application to cell signaling in environmental lung disease. Biotechniques 31(4), 880–894. 2. Cummins, A. B., Palmer, C., Mossman, B. T., and Taatjes, D. J. (2003) Persistent localization of activated extracellular signal-regulated kinases (ERK1/2) is epithelial cell-specific in an inhalation model of asbestosis. Am. J. Pathol. 162(3), 713–720.
3. Lounsbury, K. M., Stern, M., Taatjes, D., Jaken, S., and Mossman, B. T. (2002) Increased localization and substrate activation of protein kinase C delta in lung epithelial cells following exposure to asbestos. Am. J. Pathol. 160(6), 1991–2000. 4. Schmidt, M. H., Broll, R., Bruch, H. P., Bogler, O., and Duchrow, M. (2003) The proliferation marker pKi-67 organizes the nucleolus during the cell cycle depending on Ran and cyclin B. J. Pathol. 199(1), 18–27. 5. Carson, F. L. (1996) Histotechnology: A Self-Instructional Text, ASCP Chicago, IL. 6. Brown, D., Lydon, J., McLaughlin, M., Stuart-Tilley, A., Tyszkowski, R., and Alper, S. (1996) Antigen retrieval in cryostat tissue sections and cultured cells by treatment with sodium dodecyl sulfate (SDS). Histochem. Cell. Biol. 105(4), 261–267. 7. Wilson, D. M., 3rd and Bianchi, C. (1999) Improved immunodetection of nuclear antigens after sodium dodecyl sulfate treatment of formaldehyde-fixed cells. J. Histochem. Cytochem. 47(8), 1095–1100.
5 Evaluation of Confocal Microscopy System Performance Robert M. Zucker Summary The confocal laser scanning microscope (CLSM) has enormous potential in many biological fields. When tests are made to evaluate the performance of a CLSM, the usual subjective assessment is accomplished by using a histological test slide to create a "pretty picture." Without the use of functional tests, many of the machines could be working at suboptimal performance levels, delivering suboptimum performance and possibly misleading data. To replace the subjectivity in evaluating a confocal microscope, tests were derived or perfected that measure field illumination, lens clarity, laser power, laser stability, dichroic functionality, spectral registration, axial resolution, scanning stability, photomultiplier tube quality, overall machine stability, and system noise. These tests will help serve as a guide for other investigators to ensure that their machines are working correctly to provide data that are accurate with the necessary resolution, sensitivity, and precision. Utilization of this proposed testing approach will help eliminate the subjective nature of assessing the CLSM and allow different machines to be compared. These tests are essential if one is to make intensity measurements. Key Words: Confocal microscope; lasers; coefficient of variation; photomultiplier tubes; field illumination; axial resolution; spectral registration; laser stability; beads; microscope lenses; quality assurance; quantification; spectroscopy
1. Introduction The confocal laser scanning microscope (CLSM) has been evaluated by using the following biological test samples: beads, spores, pollens, diatoms, fluorescent plastic slides, fluorescence dye slides, silicone chips, and histological slides from plants or animals (1–10). In most cases, the test sample is of biological origin and is sometimes used in the course of research in the individual's research laboratory. This is a testing procedure recommended by the manufacturers of most CLSM equipment. In our opinion, this is too arbitrary a test when applications (i.e., intensity measurements or colocalization studies) other than "pretty pictures" are needed. Unfortunately, this technology differs from flow From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
cytometry and does not have a universal standard with which to evaluate the CLSM or the image quality derived from a CLSM. It would be advantageous to have better methods to measure system performance and image quality. The CLSM consists of a standard high-end microscope with very good objectives, different lasers to excite the sample, fiber optics to deliver the laser light to the stage, acoustical transmission optical filters (AOTFs) to regulate the laser light onto the stage, barrier filters, dichroics, and pinholes to control the light, electronic scanning devices (galvanometers), detection devices to measure photons (i.e., photomultiplier tubes [PMTs]) and various other electronic components. For this system to operate correctly, it is important for it to be properly aligned and to have all of the components function correctly. Instrument performance tests that have been devised include the following: laser power, laser stability, field illumination, spectral registration, lateral resolution, axial Z resolution, lens cleanliness, lens functionality, and Z-drive reproducibility (1–10). This list is not inclusive and additional parameters might be needed to assess if the CLSM is working properly. Because a confocal microscope can provide spectacular three-dimensional (3D) data of biological structures, one can have a tendency to overlook many of the quality assurance (QA) parameters that might be necessary as controls. The CLSM could function at suboptimum conditions for long periods of time, delivering inferior data, with the problems being resolved only when the investigator cannot achieve the desired images or there is a hard failure of the system necessitating a service personnel visit. Inferior performance of a CLSM could be attributed to sample preparation. However, the image data might also be incorrectly interpreted if the CLSM is not working correctly. Because all CLSM images are digital and made with sophisticated optical equipment, it is now possible to derive tests that can evaluate some of the components installed in the machine. The CLSMs should not be evaluated by using only a subjective "pretty" image on a histological slide. QA on the CLSM is essential to ensure that it is performing properly and delivering accurate and reproducible data. This review attempts to incorporate QA procedures into the operation/maintenance of confocal microscopes. This review also emphasizes that scientists need to evaluate their CLSM system performance to ensure that it is working properly. 2. Materials 2.1. Field Illumination: Fluorescent Slides The field illumination test slides consisted of three fluorescent plastic slides (Delta; Applied Precision Inc, Issaquah, Washington) that had excitation peak wavelengths of 408 nm (blue), 488 nm (orange), and 590 nm (red) and emission peak wavelengths of 440 nm, 519 nm, and 650 nm, respectively. The orange slides (488 nm) were used to test for visible field illumination and alignment. The blue
Confocal QA
slides (408 nm) were used for ultraviolet (UV) field illumination and alignment. Field illumination can also be measured using four Fluor-ref slides (Microscopy Education, Springfield, MA) or four Chroma slides (Chroma, Brattleboro, VT). Although extensive tests were not made with these slides, in preliminary studies they appear to work well if the surface is clean and free of debris. 2.2. Power Meter The power meter used to measure light on the microscope stage was either a Fieldmaster or a Lasermate Q (Coherent, Auburn, CA) with visible (LN36) and UV detectors (L818). A power meter (1830C) from Newport Corporation with an SL 818 visible wand detector can also be used for power measurements. A remote control box for the Coherent UV Enterprise laser was used to regulate UV laser power (model 0163-662-00; Coherent, Santa Clara, CA). On most confocal systems, there is a 10× lens: Zeiss uses a 10× Plan Neofluar (numerical aperture [NA] of 0.3) and a Leica has a 10× Plan Fluorotar (NA of 0.3) or 10× Plan Apo (NA = 0.4). The dry 10× lens was used to take power measurements. The machinist's plans for building the power detector holder are available by e-mailing the author. 2.3. Beads Various bead types are useful as test particles to access machine functionality. The beads were obtained from Spherotech (Libertyville, IL), Molecular Probes (Eugene, OR) or Coulter electronics (Hialeah, FL). Spherotech beads that were used included the 10-µm Rainbow (EX 365, 488, 568) fluorescent particles (FPS-10057). The following beads were used for preliminary field illumination tests: yellow beads (5.5 µm, FPS-5052) EX 488 for visible field illumination; UV beads (5.5 µm, FPS-5040) EX 365 for UV field illumination; blue beads (5.5 µm FPS-5070, EX 647) for 647 nm field illumination. The 6.2-µm Rainbow beads with three different intensities (FPS-6057-3) were used for early statistical PMT tests. The following PSF Rainbow beads were used: 0.16 µm, FP-02557-2s; 0.5 µm, FP 0857-2; and 1.0 µm, FP-0557-2. The polystyrene 10-µm beads (refractive index [RI] = 1.59) were mounted with optical cement (RI=1.56) on a slide using a no. 1.5 size cover glass. The Leica immersion oil has a refractive index of 1.518. The following Molecular Probes beads were used: TetraSpeck™ beads (T7282 0.5 or 1 µm, EX 365, 488, 568, 647) were used for spectral registration tests and point spread functions (PSF); PS-Speck Microscope Point Source Kit, consisting of beads (175 nm, P-7220) of different wavelengths, were used for acute deconvolution PSF measurements; 15-µm FocalCheck microspheres (F24634 kit) consisting of an orange ring and blue throughout (F7236) for UV and visible colocalization or green, orange, and dark red ring stains (F7235) for visible colocalization of the 488, 543, and 633 laser lines. Fluorospheres (10 µm, Fullbright
Green II; Coulter, Hialeah, FL; EX 488) were used for preliminary statistical PMT tests. The 10-µm Spherotech Rainbow beads were substituted for these beads in later experiments. Bead slides were made by dropping 3–5 µL of diluted beads onto a slide, allowing the liquid to dry and then covering the spot with Permount, glycerol, water, or oil and sealing it with a 1.5 cover glass. Antifade from Vector (Vectoshield H-1000) or from Molecular Probes (Slowfade light S-7461) is useful to decrease bleaching. 2.4. Biological Test Slides FluoCells 1 (F-14780; Molecular Probes, Eugene, OR) were stained with three fluorochomes (Mitotracker Red CMXRos, BODIPY FL phalloidin, DAPI) and used as biological test slides. Additional slides were made in our laboratory with cells grown on cover slips, fixed with paraformaldehyde, and stained with DAPI for UV excitation or other suitable fluorochromes for visible excitation. 2.5. Axial (Z) Resolution Test The axial resolution of the CLSM is tested using a single reflecting mirror obtained from Leica or Edmonds Scientific. A 21-mm square (#31008; Edmonds Scientific, Philadelphia, PA) was glued onto a microscope slide and a cover glass (#1.5; Fischer, Pittsburgh, PA) was placed on top of the slide with a drop of immersion oil (Leica Immersion oil, n = 1.518) The cover slip is placed firmly onto the mirror to remove all excessive oil. This type of standard test slide can also be obtained from a confocal manufacturer or Spherotech (Libertyville, IL). 2.6. Square Sampling Galvanometer Check It is important to ascertain whether there was square sampling or rectangular sampling in an image. A computer chip was glued onto a glass slide and used as a test substrate. A commercial grid product can also be obtained from MicroBrightField (Williston, VT) or Geller MicroScientific (Topsfield, MA). A digital tagged image file format (TIFF) image was obtained using a dry 20× objective and the amount of small boxes observed was counted by eye in the vertical and horizontal directions. If there is the same number of boxes per inch in the vertical and horizontal directions, then it can be assumed that the sampling of pixels is square. If they are not equivalent, then the sampling of pixels is rectangular, which is not desired. Problems with the galvanometer can also be detected with this grid. 2.7. Laser Beam Shape The UV laser beam can be checked using an inexpensive lens (12 mm outer diameter [od]; B1099; Melles Griot) held in a lens holder (13 mm inner diameter [id]; H1089,) and focusing the beam onto a white piece of paper to show its configuration mode.
2.8. Confocal Microscope The majority of data presented in this chapter was derived on either a Leica TCS-SP1 or a Leica TCS4D (Heidelberg, Germany) confocal microscope system. These systems contained an argon–krypton laser (Melles Griot; Omnichrome) emitting 488-, 568-, and 647-nm lines and a Coherent Enterprise UV laser emitting 351- and 365-nm lines. The system contains an AOTF and the following three dichroics for visible light applications: single dichroic (RSP500); double dichroic (DD); and triple dichroic (TD). The Leica-derived tests were shown to be applicable to other point scanning systems that contain different types of laser, objective, or other hardware configuration. For comparison purposes, similar tests were made on two different Zeiss 510 units containing three lasers (argon 488 [25 mW], HENE 543 [1 mW], and HENE 633 [5 mW]) with a merge module and an AOTF. In addition, for comparison purposes, similar tests were made on a Leica SP2 unit that contained three lasers (argon 488 [50 mW], HENE 543 [1.2 mW], and HENE 633 [10 mW]) with a merge module and an AOTF. 2.9. Software Analysis The analysis of the images was made on workstations that contained Leica, Zeiss, and Bitplane (Zurich Switzerland) software packages. If necessary, the TIFF images were imported into Image Pro Plus (Media Cybernetics, Silver Springs, MD) or Image J (NIH) for more intensive measurements and analysis. 2.10. PMT Spectral Check The PMT spectral response was measured over a large spectral region using an inexpensive Pariss fluorescence calibration lamp consisting of a defined mixed-ion gas (model 816025; LightForm Inc, Hillsborough, NJ). 3. Methods 3.1. Field Illumination The fluorescent slide was placed on the stage and the maximum intensity was found on the surface of the slide. It is important to measure the field illumination at a specific depth in the plastic slide, as the intensity distribution might change from the surface to the interior of the slide. The depth of focus was adjusted between 30 and 100 µm, depending on the objective that was used: (5× [100 µm], 10× [75 µm], 20× [50 µm], 40× [40 µm], 63× [30 µm], 100× [30 µm]). Investigators should also be careful not to observe an illumination field deep within the plastic slide samples, as it will usually yield a better field illumination than regions closer to the surface because of various optical distortion factors.
Fig. 1. Field illumination. Field illumination pattern of visible (A) and UV (B) excitation using a 20× (PlanApo; NA = 0.7) lens. The visible field illumination shows uniform illumination, with the brightest intensity being in the center of the objective. The line running diagonally in panels A and B measures the histogram intensity of the field illumination graphically represented in panels C and D. The variation in intensity from the left to right side of the field is less than 10% for visible excitation and more than 150% for UV excitation. Acceptable field illumination has the brightest intensity in the center of the objective, decreasing less than 25% across the field. The intensity regions were prepared by using Image Pro Plus to divide the grey scale value into 10 equal regions and a median filter was used for additional processing.
Data derived from a 20× PlanApo lens (NA = 0.7) zoomed to a factor of 1.2 is used to illustrate good visible field illumination (488 nm), and a misaligned UV (365 nm) system is used to yield a poor field illumination (see Fig. 1). The images were obtained with either a UV plastic slide (excitation [ex] 365 nm; emission [em] 440–480 nm) or a visible plastic slide (ex 488 nm; em 505–550 nm) located securely on the stage. The original images were contoured into 10 intensity ranges using Image Pro Plus software. The line running diagonally in Fig. 1A,B measures the histogram intensity of the field that is represented in the graphs in Fig. 1C,D. The maximum intensity should be in the center of the objective and decreasing less than 25% across the field in all directions, as shown with visible 488-nm excitation in Fig. 1A. It should not be in the bottom corner, as illustrated with
UV illumination in Fig. 1B. As shown in Fig. 1, the visible light (Fig. 1C) had less than a 10% decease in intensity across the field, whereas the UV light (Fig. 1D) had a 150% decrease across the field. If the maximum light intensity is not located in the center of the field, there is an alignment problem that needs to be addressed. The nonuniform pattern shown in Fig. 1 with UV illumination clearly illustrates a field illumination problem, which will affect intensity measurements in an image. Although Fig. 1 was obtained with UV optics, it represents the type of field illumination that can also occur with visible excitation. This pattern is unacceptable with any CLSM optical system, as the maximum intensity should be in the center of the objective and not in a corner. Each laser line must be checked to ensure that they are aligned properly, as they use different dichroics to ensure that the beams are colocalized. In addition, the field illumination of one lens is not necessary identical to the field illumination of the other lenses, necessitating that each lens be checked with the suitable dichroic that will be used in the experiment. In our Leica system, the three visible wavelengths of light are derived from one Omnichrome argon–krypton laser. This enables the field illumination to be tested at one wavelength (488 nm) and allows one to assume that it is equivalent to testing field illumination with the other wavelengths. Because the UV line is derived from a different laser (Enterprise, Coherent), it is essential to check all objectives for proper field illumination (Fig. 1) at the 365-nm excitation in addition to the 488-nm excitation. Newer designed confocal systems (Leica SP2, Leica AOBS, Zeiss 510, Zeiss 510 Meta, Nikon C1, Olympus FluoView FV1000, and Bio-Rad Radiance 2100) use three individual lasers with a merge module, which requires that all laser wavelengths have correctly aligned beams emitted from the merge modules. In these systems, all three lines have to be individually tested for field illumination. One laser line might be perfectly aligned, yielding acceptable field illumination, whereas the other laser lines might be misaligned, yielding intensity values for which the brightest region is not in the center of the field, as illustrated in Fig. 1. 3.2. Power Meter The equipment used to acquire power readings include a detector, a machineshop-built detector holder, and a portable power meter, which are illustrated in Fig. 2. To measure the power output of the different wavelengths, either a UV or visible probe (Coherent probe detectors [L818, LN36]) or Newport corporation wand visible probe detector (SL 818) is secured in a special holder that fits onto the microscope stage during the measurement of either UV or visible laser light (see Fig. 2). The test should be done with a dry objective (2.5× to 20×; preferably 10×) at a fixed position, usually at the top of its movable tract.
Fig. 2. Power meter and detector holders. A holder to secure detectors on the microscope stage was made in the machine shop in a size similar to a 3×1 microscope slide. The detector holders for the Coherent (LN 36) or Newport wand (SL818) are represented in the photographs. With small objective circumferences (i.e., Leica 10× PlanApo (NA = 0.4), the Coherent detectors can be placed on top of the objective. A Coherent power meter and detector are used to measure laser power on the microscope stage.
Currently, most systems will have a dry 10× lens (NA = 0.3 or 0.4) that can be used to access power using a power meter detector. The use of a lens that has a different lens design, magnification, or NA will affect the laser power transmission and measurement.
The test was made using a 10× (NA = 0.3) or a 10× (NA = 0.4) objective in the following manner. The lens is raised to its maximum specified height. The detector is secured on the stage and grossly centered using either laser light or mercury fluorescent light. The detector position is then more accurately adjusted to achieve maximum signal intensity by using the microscope's x/y joystick. The power meter is adjusted to the specific wavelength (365, 488, 568, or 647 nm) and the maximum power of laser light is read on the digital scale. The CLSM zoom factor is set from 8 to 32 to reduce the beam scan and to focus it into the "sweet spot" of the detector. The scanner is set at bidirectional slow speed to reduce the time period that the power meter is reading 0. The maximum digital reading from the power meter was recorded. The power derived from this measurement is dependent on the magnification and NA of the lens used. Each lens will have a unique set of values, which is dependent on the objective's NA and other transmission factors. The power meter diode was not reliable and could only be used as a crude estimate of the functioning of the laser. This might change in the future with better designed systems. The Coherent probe detectors LN818 and LN36 can also be placed directly on top of a 10× objective without a holder. A comparison of the maximum power output derived from different lasers and different optical systems was made on a Zeiss 510, a Leica TCS-SP1, and a Leica TCS-SP2. The maximum power was measured with a Coherent power meter using two different 10× (NA = 0.3, SP1 and Z510) lenses and one 10× (NA = 0.4, SP2) lens on three different types of CLSM system: Zeiss 510, Leica TCS-SP2, and Leica TCS-SP1. The Leica TCS-SP1 system has one 75-mW argon–krypton laser (model 643) emitting three laser wavelength lines. The newer CLSM systems from all manufacturers are designed with three lasers that use different dichroic components to merge the different laser wavelengths. The Zeiss 510 contained three different lasers (30-mW argon, 1 mW HeNe [543 nm], and 5-mW HeNe [633 nm]) with the multiple wavelengths aligned with a merge module. The Leica SP2 contained three different lasers (50-mW argon, 1.2-mW HeNe [543 nm], and 10-mW HeNe [633 nm]) with the multiple wavelengths aligned with a merge module. Table 1 shows a comparison of power with three different systems from Leica and Zeiss using a triple dichroic to reflect different wavelengths of laser light to the stage. The power meter values shown in Table 1 can be used to determine both the maximum power output and dichroic reflectance and, thus, functionality in the system. The values reported in Table 1 were acquired from functioning systems that were subjectively assessed to be problem-free, aligned correctly, and functioning properly. There are no assurances that these machines were producing ideal power readings or were perfectly aligned when these tests were made. However, we believe that the values demonstrate the type of data that can be achieved with this test. The acquired data can be used to compare
Table 1 Comparison of Relative Power on Microscope Stage
Laser 488 nm 543–568 nm 633–647 nm
Leica SP1
Zeiss 510
Ar–krypton 1.1 1.45 1.6
(3-Laser) 4.5 0.22 2.8
(3-Laser) 3.13 0.26 1.21
Note: The maximum power was measured in milliwatts with a LN 36 detector and a Coherent Lasermate power meter adjusted to the specific excitation wavelengths. The power was measured on the stage of a Zeiss 510, a Leica SP2, and a Leica SP1 system. This test can be used to determine if the system has acceptable laser power by measuring the power on the microscope stage and not opening up the scan head.
similar machines and internally control an individual machine for proper performance over a period of time. 3.3. UV Power Test The test was carried out in a manner similar to that described for the visible power measurement. A power meter (Lasermate/Q with UV detector [L818]; Coherent, Santa Clara, CA) was used to measure the light emitted from a UV Enterprise laser. A Coherent UV, 60-mW, 3-yr-old Enterprise laser delivered normal power output at the laser head (over 40 mW of laser power), but only about 500 µW of maximum power through a Plan Fluor 10× (NA = 0.3). However, it should be noted that when our system had insufficient output under these conditions (approx 500 µW through a 10× Leica [NA = 0.3] lens), we also had insufficient light for many UV experiments using higher magnification objectives (40×, 63×, and 100×). 3.4. UV Beam Shape In addition to power requirements of a UV system, it is important that the UV beam have the correct mode. The beam should be radially symmetric with a Gaussian intensity distribution. The beam should have a TEM00 configuration (transverse excitation mode or Gaussian mode). The UV laser beam can be checked using an inexpensive lens (12 mm od; B1099; Melles Griot) held in a lens holder (13 mm id; H1089) and focusing the beam onto a white piece of paper to show its configuration mode. 3.5. Bead or Histological Power Meter The crude power of the system can also be assessed by recording the PMT voltage necessary to acquire an image at almost saturation values by using standard histological samples like the FluoCell 1 slide (F-14780; Molecular Probes,
Eugene, OR), beads like the 10-µm Spherotech beads (FPS-10057-100), or Applied Precision fluorescent plastic slides. If conditions are identical between machines, this PMT value can be used as a reference value to compare CLSM units and to establish their acceptable performance levels. Leica technicians routinely use a 40× lens to measure the fluorescence saturation of a histological plant sample. If it saturates in the PMT range between 600 and 700 units in PMT 1, the system is passed as having adequate power. Because of the optical limitations of the stage, the power meter detector cannot be used with higher power optics (40×, 63×, 100×). Thus, it will be useful to use a test sample (fluorescence histological slide, colored plastic, or large beads) to assess UV and visible power with these higher-magnification lenses. Using maximum UV power, it was found that the 10-µm Spherotech bead saturates PMT 1 (low-noise PMT, R6857) at a voltage setting of 650 using a 100× PlanApo lens (NA = 1.4). When saturation occurred at the higher values, it indicated less power throughput in the CLSM system, and when saturation occurred at relatively lower PMT values, it indicated greater power throughput. The saturation values on other substrates or slides (biological or histological specimens) can also be used and have to be defined by the user's laboratory, as the degree of staining can be different. 3.6. Dichroic Functionality The dichroic comparison test was accomplished in the following manner. An API fluorescent plastic slide (orange) was placed on the stage using either 488-nm or 568-nm excitation light. After the dichroic was switched into position, the PMT was kept constant and the mean gray scale value (GSV) of a region of interest (ROI) in the image was determined for each acquisition condition. The values for the three dichroics can be compared to determine which one has the highest reflectivity (see Table 2). The maximum values are normalized to 1 and the other values are reported as a percentage of the maximum GSV. It is important to use a bandpass or barrier filter to collect the desired light. Light at different regions of the spectra may have unique information and it is sometimes advisable to use a narrower bandpass with higher reflection to get the desired fluorescent information. 3.7. Axial (Z) Resolution (Mirror) The axial resolution test is considered the "gold standard" of resolution in confocal microscopy (1,3,7,8). Although it is not the only criterion for a good image, the axial resolution of the system should be maximized to yield a minimal axial Z-resolution value. The reflected surface of the mirror can be found in either xy or xz scanning by observing the brighter spot with an open aperture. Initially, axial resolution is tested in the reflection mode with the 100× objective
Table 2 Comparison of Relative Dichroic Reflectance Wavelength 488
10× (0.3/NA)
SD DD TD SD DD TD
Note: The relative laser power was measured with the 488-nm and 568-nm wavelengths using six different magnification objectives and three dichroics. The 10× and 63× data are shown for clarity. The table demonstrates the relative reflectivity of the dichroics in the system. The test was accomplished by measuring the intensity in an ROI of an image using either 488-nm or 568-nm excitation light, a fluorescent plastic slide, and one of three specific dichroic and maintaining the PMT at a constant voltage. An ROI of the image yielded the mean value for each acquisition condition (wavelength, objective, and dichroic). The GSV of the two images are divided to yield a ratio that is expressed in the table as a fraction. The value of 1.00 is the maximum dichroic refection and is expressed as a bold number. The dichroic with the maximum reflection should be used when only one fluorochrome is required. Unexpectedly, the double dichroic (DD) yielded the best reflectivity with 488-nm wavelength light with most lenses and the triple dichroic (TD) yielded the best reflectively with the 568 nm wavelength light (30% more light reflected than the DD) with most objectives. This table can be used to choose the dichroic that should be used with each excitation wavelength for optimized reflection.
(NA = 1.4; PlanApo lens), zoom of 16× to 24×, a large pinhole diameter opening, and minimum laser power. After the reflected surface is found by scanning in the xz mode, the pinhole aperture is reduced to a minimum size. The reflected image is then obtained with a frame averaging of 2–4, and the intensity profile across the reflected surface is determined as shown in Fig. 3. The maximum of the peak is determined and then the half-maximum intensity value of the profile is obtained to determine the full width at half-maximum (FWHM) distance to determine the axial resolution. The data can be observed graphically or it can be transferred into Excel to measure the peak and the half-maximum values. The specification for axial registration in a Leica TCS-SP system is below 350 nm. The pattern of the axial resolution curve is also important. One looks for a symmetrical large peak and smaller peaks and valleys to the left of it, indicating diffraction patterns of an acceptable lens (Fig. 3B). The axial Z-resolution of three different lenses on an aligned Leica TCS-SP1 system was the following: a 40× (Fluor, NA = 1.0) was 610 nm; a 63× waterimmersion lens (PlanApo, NA = 1.2) was 390 nm; and a 63× oil-immersion lens (PlanApo, NA = 1.32) was 315 nm. These are good values for high-resolution work on any confocal microscope. These axial registration values will change depending on lens quality and system alignment.
Fig. 3. (A) Axial resolution. The axial resolution was made with two 100× lenses (NA = 1.4) on the same Leica TCS-SP1 confocal system. The peak intensity of the histogram is 245 and the half-maximum intensity is measured at 122.5. One lens gave an excellent full width at half-maximum (FWHM) of 190 nm, whereas the other lens yielded a poor value of 410 nm. The system was aligned properly in both cases. (B) The axial resolution of a PlanApo 63× (NA = 1.32, 330 nm) showing a symmetrical major peak and a diffraction pattern consisting of smaller peaks and valleys. This pattern is suggestive of an excellent-quality lens.
Fig. 4. Field alignment. A computer chip that is mounted on a slide shows 25 boxes per inch. The x and y difference should be equivalent, yielding square boxes and not rectangles. To align the system for bidirectional scanning, the scanning lines should show a smooth transition (A) and not the individual scan lines in the inset (B).
3.8. Axial (Z) Registration (Beads) One-micrometer beads from Molecular Probes (Tetraspec, T7284) or Spherotech (Rainbow, FP–0857-2) are first located in the xy direction and then scanned in the xz direction. The power is adjusted for saturation and then they are zoomed approx 8× and averaged: 4×. The size of the bead in the horizontal is compared to the vertical size. The difference between the two numbers will yield the axial resolution of the lens. This method is slightly more subjective than the gold standard axial mirror test, but it does yield similar values. Sometimes the values for unknown reasons are better and sometimes they are worse. 3.9. Square Pixels, Phase Alignment, and Galvanometer Evaluation The pixel size and symmetry in XY directional field scanning can be checked by using a computer chip attached to a glass slide or a slide obtained from Microbright Field or Geller MicroScientific (see Fig. 4). The confocal should be set up in the reflective mode using a 10× or 20× dry lens. The small boxes in the
vertical and horizontal direction should be compared by counting or by a measured standard line. If there are the same number of boxes per inch in the vertical and horizontal directions, then it can be assumed that the sampling of pixels is a square. If they are not equivalent, then the sampling of pixels will be rectangular, which is not desired. This test ensures that the scanning in the X and Y directions yields a perfect square and the information will be registered correctly. This test can be used to assure that alignment exists in bidirectional scanning. This test can also be used to evaluate galvanometer function. The vertical lines should be straight without wiggles. One should be equivalent to multiple averaged scans. 3.10. Spectral Registration With Beads (1 µm) (UV and Visible) The 1 µm multiple wavelength fluorescent beads (Tetraspec, T7284 Molecular Probes, or Rainbow beads, FP–0857-2 Spherotech) were used to monitor the visible spectral registration of lenses (100× PlanApo, NA = 1.4; 63× PlanApo, NA = 1.2; PlanFluor 40×, NA = 1.0) or the registration between multiple beams (UV and 568 nm in our case). The bead was located at low zoom in xy and the gain and offset were adjusted to their respective optimum image quality levels. Next, an xz scan was obtained at the proper zoom magnification. Care was made to make adjustments at the lower power levels to reduce possible bleaching effects. By balancing laser light intensity with the AOTF, the laser crossover between the detection channels was minimized. To check for laser crosstalk, one laser light line is closed with the AOTF and the signal is observed in the other channels with the PMT setting that is necessary to acquire the proper signal with the respective excitation lines. Subjectively, it can be ascertained how much crossover occurs. If too much crossover exists, the test will be invalidated. The bead was imaged (xy and xz scans) with an 8× to 24× zoom, a slow to medium scanning rate, and frame averaged four to eight times. The registration of bead fluorescence images between the 365-nm UV wavelengths and the 568-nm visible wavelengths in an aligned system was almost superimposable (see Fig. 5A). Figure 5B shows the xz registration between the 365-nm UV line and the 568-nm visible line in a misaligned system. The data for Fig. 5B were acquired at a different time using the same 100× lens, suggesting that the system is misaligned, as the difference between the peaks was 650 nm (acceptable difference is 210 nm). The 568-nm line was chosen instead of the 488-nm line to minimize the crossover fluorescence between the visible and UV wavelengths. 3.11. Focal Check Beads (UV and Visible Lines) Molecular Probes produces a series of beads (Focal Check) that can be used to assess the different visible wavelengths from multiple lasers in a confocal system. These beads have different fluorescent excitation rings and core colors that can be used to assess colocalization of laser light from multiple lasers. We used these beads
Fig. 5. Spectral registration (UV and visible). The xz spectral colocalization of this UV (365 nm) and visible wavelength (568nm) was evaluated with a 100× PlanApo NA = 1.4 lens using a 1-µm multiple wavelength fluorescent bead (TetraSpec T7284; Molecular Probes). An aligned system has a FWHM of less than 210 nm (A), whereas a misaligned system has a FWHM difference of 650 nm (B). The bead was imaged using xz scans with a 24× zoom, a slow scanning rate, and averaged eight times. The 568-nm line was chosen instead of the 488-nm line to minimize the crossover between the visible and UV wavelengths.
to examine the UV (365 nm) and visible lines (568 nm) in our TCS-SP1 confocal system that had an argon–krypton laser emitting three lines and a UV Enterprise laser. A 15-µm bead with UV interior and orange fluorescence ring exterior (F7236; Molecular Probes) was used to show that the UV and 568-nm lines were aligned (see Fig. 6; see Color Plate 4, following p. 274). In a similar manner, the F7237 bead consists of a UV blue core and a green ring and this can be used to show
Fig. 6. Spectral registration methods. Three methods are used to check spectral registration of different laser lines. A mirror can be used for visible wavelengths on a Leica TCS-SP system (left). A bead scanned in the xz directions can be used on all confocal systems (middle) to show alignment of laser beams. Focal check beads (right) can be used to show alignment of multiple laser beams. In all cases, it is useful to make measurements on the peak height position to determine the spectral registration. (See Color Plate 4, following p. 274.)
colocalization of the 488-nm and 365-nm laser lines. This bead might have slightly more crosstalk than the blue core with a red ring (F7236). Normally, the bead should reveal concentric fluorescent rings that have maximum values in the same focal plane with either an xy or xz scan. The laser power and AOTF should be adjusted to reduce crosstalk between the emitted fluorescence. Any deviation between the concentric localization of the rings or the maximum diameter of the rings and core bead size suggests misalignment. In the newer confocal systems that have three lasers and a merge module, it is recommended to test for colocalization with a focal check bead (F7235) that has three rings representing green, orange, and red or a focal check bead (F7239) that has a red ring and a green core. Individual pinholes in a Zeiss system might have to be realigned monthly and this test can indicate if the pinhole needs adjustments. Smaller beads should yield more accuracy. 3.12. Lens or System Spectral Registration (Beads or Reflective Mirror) This spectral registration test demonstrates the ability of the CLSM to colocalize different wavelengths of fluorescence in the same plane. To evaluate the spectral registration of the 365-, 488-, 568-, and 647-nm lines, either a 1-µm
Fig. 7. Spectral registration (visible). The visible spectral registration of a 100× Plan Apo NA = 1.4 objective was evaluated using a front-surface, single-reflection mirror with the same lens at different times. A 10-nm slit was put over each wavelength and the reflection of each line was measured sequentially. The AOTF and PMT intensity was adjusted so that the maximum intensity of each line was 250 GSV. Lens B was sent back to the factory, as it did not meet the following: (1) spectral registration for UV (365 nm) and visible (568 nm); (2) spectral registration for the three visible lines; and (3) axial resolution specifications. The refurbished Lens A showed excellent registration among the three visible lines with the difference being less than 220 µm. Refurbished Lens A also had an axial registration below 350 nm. This single-reflection mirror test will yield slightly better spectral registration than 1-µm bead data for the 647-nm excitation line, as the fluorescence emission occurs in the far-red range (>660 nm) and many lenses have difficulty colocalizing this far-red emitted light with the fluorescence emitted from the 488-nm and 568-nm wavelength excitation.
multicolored bead (Tetraspec or Rainbow) or a front-surface, single-reflective mirror (see Fig. 7) was used. The front-surface, single-reflective mirror can be used to check visible spectral registration in a Leica TCS-SP system, in a similar manner to what was described for Fig. 3 for axial Z registration. In the Leica SP system, a 10-nm refection bandwidth is put over each excitation wavelength and the reflection is measured sequentially. By tweaking the AOTF and PMT voltage adjustments, the intensity of each reflected line was adjusted so that the maximum intensity of the image was approx 250 GSV. This mirror test is more accurate than the bead tests, but the data obtained skew the results slightly toward better values in normal operating conditions. The emission from either specimens or beads are normally recorded at least 10–40 nm above the excitation wavelengths and not at the excitation wavelength. Many lenses have difficulty in colocalizing far-red fluorescence with the blue and green fluorescence, so measuring the emission at 647 ± 10 nm will yield better resolution than measuring the emission at 660–700 nm. Figure 7 represents a 100× lens measured over a time period of approx 6 mo on the same CLSM system. On testing Lens B, problems in axial resolution and
spectral registration existed. The separation between the 488-nm and 647-nm line was 305 nm in lens B and the axial registration was 410 nm. This lens (B) was returned to the factory to correct this spectral registration problem in visible light and a spectral registration problem between UV and visible light. Upon repair, lens A showed perfect colocalization between the 488-nm and 647-nm lines and acceptable registration among the 488-, 568-, and 647-nm lines. The UV and 568 nm (see Fig. 5) also showed acceptable registration after repair. Depending on the laser configuration, this test can reveal characteristics of the lens' spectral registration or system alignment. In an argon–krypton laser system, because all three lines are derived simultaneously, the test will reveal the lens characteristics. If a three-laser system with a merge module is used, it will reveal the combination of both lens characteristics and laser alignment. It is useful to make this test with more than one objective, as it will be rare that a system will contain multiple lenses with axial and spectral registration problems. A 1-µm bead can be used to test visible wavelengths for colocalizing in a similar manner to that described for colocalizing UV and visible light. Using the bead test, it is necessary to reduce the laser light to reduce bead bleaching and then adjust the laser light with the AOTF and PMT voltage so fluorescence crosstalk between the emitted wavelengths from different laser lines is minimized. It is also useful to suspend the beads in an antifade medium to reduce bleaching for this test, as they will be zoomed at high magnification, which increases photobleaching. 3.13. Laser Stability (Visible, Long Term [Hours]) Laser stability measurements were made over hours to evaluate the possible fluctuations in power (1,11). The laser power fluctuations were initially measured both in PMT 1 (blue light sensitive, low noise, R6857) and in the transmission detector, using a fluorescent plastic slide with very low laser power that was reduced with either neutral-density filters or with the AOTF adjustments. The transmission optical system without a slide showed results similar to PMT 1 with fluorescent plastic slides, and this was the desired optical system to perform this test if it exists on the system, as it eliminated any possibility of bleaching or laser interaction with the substrate. To measure laser stability using the transmission optical system, the microscope is first aligned for Kohler illumination with a histological slide, which is then removed from the optical path. The image intensity is measured using the transmission detector for the three wavelengths of the argon–krypton laser by sequentially measuring the laser light with the 488-, 568-, and 647-nm wavelengths, with the power being adjusted by the AOTF transmission control so that the transmission detector voltage remains constant for all three wavelengths. The test usually takes a few hundred scans separated by 10–30 s over a period of 2 h. The intensity of a large ROI of the field is averaged and plotted
Fig. 8. Laser noise. The periodic change in laser power was measured using transmitted interference optics without a slide in the optical path. The two panels show a visible system delivering low-power fluctuations and one delivering high-laser-power fluctuations over time. The 488-nm and 568-nm lines are cycling periodically directly opposite the 647-nm line. Variations in this power intensity occur over hours and never seem to stabilize. To measure laser stability over time, the PMT was kept constant and the laser power of the three lines was adjusted with the AOTF. Next, over 200 samples were sequentially measured every 15 s. After the test was complete, the intensity of a large ROI was evaluated and plotted over time. The three lines are designated the following R (red, 647 nm), G (green, 568 nm), and B (blue, 488 nm). The laser power instability might be the result of either laser light entering a fiber with an incorrect laser polarization, thermal instability in the AOTF, or a poorly aligned system. The reason for the source of power instability is not known.
over time for the three wavelengths. The goal of this test is to have a straight line with no variations in power to ensure accuracy in the intensity measurements. It is not necessary to save the scan, but it is useful to save the data measurements in an Excel file, text file, or equivalent. The type of laser power stability data that can be achieved with this test is represented in Fig. 8. It shows a relatively stable visible system yielding lowpower fluctuations (Fig. 8A; 15%), yielding high-laser-power fluctuations (source of noise is not identified). The goal of this test is to achieve a flat stable line. There is periodic noise in the laser system that exceeds the manufacturer's (Ominichrome) laser stability fluctuations specifications of less that 0.5% over a 2-h time period. The 488-nm and 568-nm lines have a periodic cycle that is directly opposite the 647-nm lines, and stability is never achieved (Fig. 8). The AOTF may be reponsible for these power fluctuations. The fiberoptics might influence artifacts resulting from deterioration with time or improper polarization alignment. Fiberoptical problems will attenuate the laser power and necessitate using higher laser power or higher PMT settings in the operation of the CLSM. One test procedure recommended by the manufacturer to
Fig. 9. Laser Stability of a Coherent Enterprise UV laser. The Coherent Enterprise UV laser delivers less than 1% peak-to-peak noise. The laser was connected to an LP 20 water–water cooler, which should be set at least 10°C above the cooling water of the building. It should also be set above the ambient temperature of the room. Improper set points for laser cooling resulted in poor thermal regulation of the laser (trace B). Improper fiber alignment resulted in additional laser intensity variations (trace C). The elimination of the temperature and polarization issues resulted in proper laser stability (trace A, 3% power variation over time). The test was conducted by measuring the laser power stability in PMT 1 using a fluorescent plastic slide. Neutral-density filters were used to reduce the power and, thus, minimize slide bleaching. The transmission detector optics gave similar results to the UV fluorescent plastic slide and was also used to measure UV laser stability.
ensure that polarization is correct after the alignment procedures is to wiggle the fiberoptic and see if the image returns to the same intensity, suggesting that the polarization is correct. This is a fairly crude test, but it will demonstrate whether the system fiberoptic needs further polarization alignment. Better procedures are being tested by the manufacturers to eliminate this polarization alignment problem. 3.14. Laser Power Stability (UV Long Term) The Coherent Enterprise laser delivers less than 1% peak-to-peak noise and is considered a very quiet and stable laser. The Coherent laser was tested in a manner similar to that described for visible lasers using the transmitted optical system or the PMT system with blue-colored fluorescent plastic slides and very low laser power. A relatively stable line showing minimal fluctuations should be obtained (see Fig. 9, trace A). The temperature of the cooling system must be regulated properly or power fluctuations will occur (Fig. 9, traces B and C). One source of power instability appears to be way the laser is cooled and how the laser heat is dissipated. This was illustrated with our Coherent Enterprise
UV laser that was connected to a Coherent LP 20 water–water exchange cooler. This cooler should be set at least 10°C above the circulating cooling water of the building and it should be set above the ambient temperature of the room. Improper set points for the LP 20 cooler resulted in temperature regulation problems of the circulating cooling water in the laser, which, in turn, resulted in the improper regulation of the laser power (Fig. 9, trace B). 3.15. Laser Power Stability (Short Term [Seconds]) To measure laser stability using the transmission optical system, the microscope is first aligned for Kohler illumination with a histological slide, which is then removed from the optical path. The image intensity is measured using the transmission detector for the three wavelengths of the argon–krypton laser by sequentially measuring the laser light with the 488-, 568-, and 647-nm wavelengths, with the power being adjusted by the AOTF transmission control so that the transmission detector voltage remains constant for all three wavelengths. In contrast to the longterm stability test, the short-term test uses only one scan for each wavelength. The intensity of a single-line scan is averaged and the mean, standard deviation, and coefficient of variation (CV) are determined (see Fig. 10). The stability of the laser and the system noise while scanning is determined by this test. This test will measure the noise in the laser and the system and how much averaging should be necessary to improve image quality at the PMT setting that is used. The goal of this test is to have a straight line with no variations in power. If power fluctuations exist, they can be averaged to increase image quality. Averaging will reduce the image CV. If transmission optics are unavailable on the system, this test can also be done with fluorescent plastic using very low laser power to reduce bleaching. 3.16. CV Principle One of the major elements of a poor image in a confocal system is related to using the PMT at high voltage values. This can be the result of insufficient sample staining, a misaligned system, or a failing hardware component or attenuation of light through a fiberoptic. If the unit could be operated at lower PMT values, then the image quality would be improved and the PMT noise decreased. The noise present in the system can be evaluated using a large (10-µm) bead (Spherotech) or an Applied Precision fluorescent substrate slide. The 10-µm bead consisting of nearly uniform size and intensity (CV = 5% by flow cytometry) is zoomed 4× to increase the number of pixels contained in the ROI with either a 100× objective (PlanApo, NA = 1.4) or a 63× (water [NA = 1.2] or oil [NA = 1.32]) lens. The bead is located in the center of the field and the image of the bead is obtained at the center cross-section of the bead, which relates to its maximum diameter. This large bead suspended in an antifade solution permitted repeated measurements with minimal bleaching of the bead sample. A fixed ROI was
Fig. 10. Laser noise (short time). The transmission PMT was set to a constant value and the intensity of the laser lines was adjusted with the AOTF to bring the intensity to approximately the same point for graphic representation. The intensities of the three lines were made from one individual scan at each wavelength and it demonstrated that 488 nm (CV = 2.70), 568 nm (CV = 1.45), and 647 nm (CV = 1.37) had variations in intensity at very low PMT settings. The blue line (488 nm) was twice as noisy as the green (568 nm) or red (647 nm) lines. The more intensity variations, the more averaging that will be necessary to create a good image.
determined in the bead image that consisted of approximately half of the bead's area. The mean and standard deviation of the pixel intensities in the ROI of the bead were determined to yield the CV. This can also be done with a clean fluorescent substrate slide measured at a specific depth, so the intensity is reproducible. The CV is defined as the standard deviation (σ)/mean (µ). It is important to maintain the machine variables (pinhole = size Airy disk, PMT voltage, averaging, etc.) at reproducible values for these studies. The laser power was set at a constant value that allowed the mean intensity level of the bead to be approx 150 (out of 255) for each PMT setting. The noise associated with the various settings can be evaluated by varying PMT settings, frame averaging, scan speed, image size, and laser power (1–3). The CV that we are measuring is actually the variation of pixel intensity within the bead, as opposed to the variation of intensity among a population of beads, as measured on a flow cytometer. An increase in CV might imply that there is either a decrease in laser power or a system alignment problem that results in higher PMT values. A noisy laser can also be detected by this test.
Fig. 11. Bead pixel variations. TIFF images of a 10-µm Spherotech bead were obtained with two different PMT settings (PMT = 400, PMT = 600) with a zoom of 4 and no frame averaging using a 100× PlanApo lens (NA = 1.4). A ROI was drawn in the interior of the bead and the histogram of the population of pixel intensities is displayed in the bottom panels. The mean pixel intensity in both images was approx 150 intensity levels and was obtained by keeping the PMT at 400 or 600 and adjusting the laser power with the AOTF.
Figure 11 illustrates the pixel distribution of a 10-µm bead that was measured with a PMT voltage setting of 400 and 600 and a zoom of 4× and an Airy disk of 1. These two bead images were obtained in the following manner. The mean intensity value in the ROI within the bead was set at channel no. 150 by adjusting the AOTF manipulation, instead of actually lowering/raising the laser power. The higher PMT voltage yielded a broader histogram, which translated into more pixel intensity variations. Because the CV (σ/µ) is defined as the standard deviation (σ) divided by the mean (µ), one can compare the quality of images using this technique. As the quality (less bead noise) of the images increases, the CV of the population of pixel intensities within the bead decreases. To compare image quality between different confocal microscopes, it is critical that as many variables as possible be kept constant (1,2). The relationship between PMT voltages and frame averaging influenced the CV value and image quality. If the laser power was kept constant and the system power was adjusted with the AOTF, it was found that either increased frame averaging or lower PMT voltages could decrease the bead CV value and, thus, improve image quality. The relationship shown in Fig. 12 demonstrates that at higher PMT set-
Fig. 12. Relationship among averaging, PMT, and CV. The noise present in the system was evaluated using a 10-µm bead with a 100× PlanApo objective (NA = 1.4). If the mean is assumed to be constant (channel no. 150), the histogram distributions generated by averaging can be produced and the CV and standard deviation (SD) calculated. This test was made by decreasing the PMT value and adjusting the laser power with the AOTF to ensure that the mean pixel intensity was at a value of 150. The higher PMT values were taken first to minimize possible bleaching. Images corresponding to 1, 2, 4, 8, 16, and 32 were obtained at each PMT setting. The CV is defined as the standard deviation (σ) of pixels in a bead divided by the mean (µ) intensity (CV = σ/µ). The noise at a specific setting can be reduced if frame averaging is increased. The averaging decreases the pixel variation, which lowers the SD and decreases the CV. Theoretically, by averaging the image n times, the CV and SD are decreased by the square root of n.
tings, it is necessary to frame average more to reduce the CV and increase image quality. The same relationship exists in biological samples (see Fig. 14). 3.17. PMT Function The PMT is the detecting system and there exists different quality PMTs of the same type in a confocal system. This CV bead test method can be used to access the operation and quality of the PMTs in the confocal system. The use of the Leica SP system easily allowed for switching of PMTs and pairing them with different excitation wavelengths. In effect, any PMT could be used in conjunction with any of the four excitation wavelengths. Although the PMT position will affect the CV, it is not considered to be a major contributor, and in this assessment, all of the PMTs were considered equivalent. PMT 1 and PMT 3 have an extra mirror present to reflect the light, whereas, PMT 2 collects the light directly after the prism. There are two types of PMT used in the Leica system: PMT 1 is considered low
Table 3 PMT Comparison and Noise Excitation 488 nm
PMT#
PMT Voltage
CV(%)
Relative CV
505–555 nm
6.06 6.58 6.23 6.02 7.00 6.46 4.00 4.88 4.49 20.30 22.70 20.30
100 108.65 102.86 100 116.25 107.17 100 122 112.11 100 111.68 100.12
Note: The noise of the system was evaluated using a 10-µm bead (Spherotech) and a 100× PlanApo (NA = 1.4) objective. The intensity of a 10-µm bead was determined at a constant laser power, a zoom of 4, and no averaging using various PMT settings. The emitted light was measured in each of three PMTs. The pixels in each ROI were set to a mean of approx 150 and the standard deviation of pixel distribution was measured to determine the CV. The CV of the pixel intensity within the bead was measured at each PMT setting. PMT 1 is low noise and blue sensitive, whereas PMT 2 and PMT 3 are far-red sensitive. The quality and part of the performance of each PMT can be measured with this test.
noise (R6357) and PMT 2 and PMT 3 (R6358) have high efficiency and sensitivity in the far-red wavelength regions. Zeiss also has different types of PMT in their system. The system was set up with a triple dichroic (TD) using 488-nm, 568-nm, and 647-nm wavelength excitation. The three PMTs were adjusted to allow the mean pixel intensities to be at channel 150. The relative intensities were measured with the three PMTs for all conditions (see Table 3). Lower CVs will translate into better image quality and will require less frame averaging to produce the equivalent image quality. This is a test that is useful to determine system quality and identify a possible problem in PMT performance prior to a hard failure. The test can be done with dichroics and barrier filters to assess the efficiency of each filter in the light path. It is important to define the bandpass region that is being evaluated, as dichroics will eliminate specific regions in the transmission. A similar type of test can be made on filter systems to evaluate the PMTs. 3.18. Spectral Scanning: PMT Comparison The PMT performance was measured using the Leica spectral scan feature. This feature allows for a sequential scan across the whole range of 400–800 nm in units as small as 5 nm. The test used an inexpensive, defined mixed-ion light source, (LightForm Inc., Hillsborough, NJ) to measure the spectral response of
Fig. 13. Spectral scanning. The PMT performance was measured across the 400- to 650-nm spectral range using the Leica spectral scan feature. The slide holder was removed and the light source (LightForm Inc., Hillsborough, NJ) was positioned on the stage. A spectral scan consisting of 50 increments of 5 nm each was made between 400 and 650 nm using a 10× (NA = 0.4) PlanApo lens without averaging and an Airy disk of 1. A large ROI was used to analyze the intensity of each 5-nm scan and the data are displayed as an intensity curve between 400 and 650 nm. Note the broadness in the peaks acquired from PMT 1 relative to the tighter and smaller CVs in PMT 2. PMT 3 is similar to PMT 1 and is not shown for figure clarity. This scan shows a suppression of PMT 2 wavelengths below 450 nm. It also demonstrates a lack of resolution of the PMT 1 peaks relative to PMT 2 peaks (24).
a PMT across a large spectral region. The slide holder was removed and the light source positioned on the stage. A spectral scan consisting of 50 increments of 5 nm each was made between 400 and 650 nm using a 10× (NA = 0.4) PlanApo lens without averaging and an Airy disk of 1. The efficiency of the light-collection system was low, necessitating that a large PMT voltage setting be used. The intensity of each 5-nm scan was calculated as the mean from a large ROI and the data were displayed as an intensity graph between 400 and 650 nm (see Fig. 13). A similar test can be made on a Zeiss 510 Meta using a 10.7 nm scan size. If necessary, the pinhole can be opened to let in more light.
3.19. Biological Samples The CV technique developed on beads was applied to biological specimens (MP FluoCells; chicken cells stained with Acridine Orange) to observe if the same relationship between PMT voltage and averaging described on beads are maintained on biological cells. FluoCells® (F-14780, Molecular Probes) were excited with a 568-nm laser line and detected with a 580 to 630-nm bandpass filter in PMT 2. The difference in image quality illustrated by averaging 1, 4, or 32 times at 2 different PMT settings (552 or 799) is shown in Fig. 14. Figure 13B–F shows images zoomed 4× using Image Pro Plus to clearly illustrate the individual pixels. The CVs of a selected ROI in the nucleus varied with the number of frames averaged and the PMT voltage. The best image quality (low CV) consisted of either low PMT voltages (Fig. 14B,C) with minimal amounts of frame averaging, or high PMT voltages with 32 frames averaged (Fig. 14F). High PMT settings (Fig. 14D,E) with minimal amounts of frame averaging (one or four) demonstrated high CVs and poor image quality. In all cases, the increase in averaging resulted in a decrease in the CV and a corresponding increase in image quality. In contrast, raising the PMT voltages increased the CV and decreased image quality. The higher PMT settings necessitated the use of greater frame averaging to increase image quality. Figure 14 shows the relationship among PMT voltage, frame averaging, and CV on image quality on cells, which was similar to that described with beads in Fig. 11. The noise in this figure is also reduced as the square root of the frames averaged (12,13). The CVs of a ROI in the nucleus of the various panels of Fig. 14 was the following: B, 49%; C, 40%; D, 212%; E, 109%; F, 49%. 3.20. Sensitivity The sensitivity of a confocal microscope is an important parameter to determine, as its value influences the acquisition settings of the PMT voltage, laser power, and frame that are used to acquire images. The values derived will relate Fig. 14. PMT and averaging of cells. FluoCells (F-14780; Molecular Probes) were excited with a 568-nm laser line and detected with a 580- to 630-nm bandpass filter in PMT 2. Averaging 1 measured the resolution, 4 or 32 times at two PMT settings (552 or 799). (A) shows the distribution of three cells at normal magnification, whereas (B)–(F) show one cell located in the box in (A) that was zoomed 4× with Image Pro Plus. The settings in the different panels were the following: (A) control (PMT = 552, AV = 1); (B) (PMT = 552, AV = 1), (C) (PMT = 552, AV = 4), (D) (PMT = 799 AV = 1), (E) (PMT = 799, AV = 4), and (F) (PMT = 799, AV = 32). Note the difference in pixel variations in the six panels of the same cell acquired at different PMT/averaging settings. The CVs of a ROI in the nucleus of the various panels were the following: (B) 49%; (C) 40%; (D) 212%; (E) 109%; (F) 49%.
Table 4 Confocal Laser scanning Microscope Sensitivity: Relationship Between Laser Power and CV Fixed Power Comparison Laser type Argon Krypton (75mW, Leica) Argon 25mW HeNe 1mW (Zeiss)
Wavelength mW
Power mW
CV-Bead% SD/Mean
1 0.2 1 0.2
Maximum Power Comparison Argon Krypton (75mW, Leica) Argon 25mW HeNe 1mW
1.1 1.45 3.2 0.23
3.8 2.6 1 1.9
Note: The sensitivity from a Leica TCS-SP1 containing an argon–krypton laser emitting three laser lines and a Zeiss 510 containing three individual lasers and a merge module are represented. The CVs were obtained from a 10-µm bead using a 100× PlanApo objective (NA = 1.4) The laser power was derived by using a 10× (NA = 0.3) objective and a power meter situated on the stage. By setting the power to a fixed value of either 1 mW of 488-nm laser light or 0.2 mW laser of 543-nm laser light on the stage, the sensitivity of two machines was measured. The CV of the bead was observed to be almost three times lower with the 488-nm laser lines using the Zeiss 510 system compared to the Leica TCS-SP1 system. By increasing the lasers to their maximum power, the CV values were decreased. Sometime maximum power measurements are useful to indicate alignment of the system and functionality of different components. There are many explanations for this difference.
to alignment, equipment functionality, and general performance of the CLSM. Table 4 represents two confocal microscopes that have different configurations: a Leica TCS-SP1 containing one argon–krypton laser emitting three laser lines, and a Zeiss 510 system that contains three individual lasers with a merge module. The test particle was a 10-µm Spherotech bead and measurements were made using a 100× PlanApo objective (NA = 1.4) with a zoom factor of 4 with 488-nm excitation in both systems. The laser power in both systems was measured on the stage using a 10× (NA = 0.3) objective and a power meter detector secured on the stage. It is essential that the acquisition parameters be as equivalent as possible when trying to compare machines from the same or different manufacturer. Because there are considerable differences between the designs of different machines, extreme care must be made when interpreting the results of data obtained from different manufacturers' machines. The variables that
must be considered included specific wavelength, laser type, objective lens, optical components, pinhole size, scan speed, zoom, and pixel size. Other variables not mentioned could also affect the measurement. It should be emphasized again that it is extremely difficult to compare different manufacturers' machines because of the number of variables that must be kept constant, and conclusions drawn from this head-to-head test can only be made if all the variables are nearly equivalent. To illustrate the test, we used a Leica machine that has an older Omnichrome argon–krypton laser emitting three laser lines and a new Zeiss 510 CLSM containing three relatively stable lasers. It would be expected that by using different lasers, the tests would reveal different sensitivity values (see Table 4). The sensitivity of two machines was made by maintaining the laser power at a constant value of 1 mW for 488-nm wavelengths and 0.2 mW for 568/543-nm wavelengths. The data from this test revealed that 1 mW of 488-nm power measured on the scan head yielded a CV value of 4% with a Leica TCS-SP1 and a CV value of 1.3% with a Zeiss 510. Comparable power readings on both systems showed the CV to be almost three times lower with the 488-nm and 568-nm lines with the Zeiss 510 system as with the Leica TCS-SP system containing an Omnichrome argon–krypton laser. The significance of a higher CV value means that the samples will have to be frame averaged to increase the image quality. These CV values will change if the laser power and PMT values are changed. The quality of lasers will also affect the CV values, as shown by the data in this test. Finally, increasing the laser power to a maximum value resulted in the CV being lowered with both the Zeiss 510 and Leica TCS-SP1 systems. However, it is not recommended to operate lasers at maximum power because of sample bleaching considerations and laser lifetime considerations. This CV sensitivity data could be considered an initial reference point that can be used by other investigators to compare their CLSM performance with similar machines. It seems to be possible with this approach that the sensitivity of systems in different laboratories can be compared. Because of the number of variables that must be kept constant, extreme care must be made when interpreting this test in the comparison of different manufacturers' machines. However, there is no reason why similar machines from the same manufacturer cannot be compared if they are run under nearly identical conditions. 4. Notes 4.1. System Performance Tests Image quality is an important parameter in the evaluation of a confocal microscope performance. Unfortunately, image quality is too often used as the "gold standard" to evaluate confocal microscope functionality and performance. Variables that effect image quality should be assessed to ensure that the
system is delivering its optimum performance. In the cases in which intensity measurements are required, it is essential that the machine be stable to deliver reproducible data. A series of tests were either adapted from the literature or devised in our laboratory to measure the system performance of the confocal microscope (1–8). These tests include the following: laser power measured at the stage, field illumination, laser stability, dichroic performance, PMT performance, system linearity, axial resolution, spectral registration, sensitivity, and lens quality. This list is not inclusive but represents what can be tested and interpreted to ensure that the machine is operating properly. 4.1.1. Field Illumination Field illumination is one of the easiest and most important tests to make on a confocal microscope. Many CLSM units in laboratories that have been checked for field illumination using a plastic fluorescent slide (Applied Precision) have demonstrated unacceptable field illumination patterns. This test should be made with all objectives and all wavelengths of visible and UV light to ensure that the machine is delivering proper field illumination. Field illumination should be relatively uniform, with the maximum intensity being in the center of the objective and decreasing less than 25% across the field according to one manufacturer. The light should come in the center of the objective and it should decrease in all directions in a similar manner. The decrease is dependent on the characteristics of the objective and its magnification. Most alignment procedures are made using high-magnification objectives (40×). However, this does not always translate into good performance with different magnification objectives. The illumination intensity across the observation field can be measured with different types of test specimen in order to ensure that a homogeneous field illumination exists. The following test substrates have been used: concentrated fluorescent dye suspended in a hanging-drop well slide, small concentrated fluorescent beads (1–3 µm) or large concentrated fluorescent beads (10 µm) (Spherotech), fluorescent specimens, uranyl glass slides, Altuglas, or plastic fluorescent slides (Applied Precision), a piece of tissue paper stained with fluorescent dye or fluorescent dye solution (Fluorescein [F-7505] or Rhodamine B [R-6626]; Sigma, St Louis, MO) and mixed with immersion oil (Leica Immersion oil, n = 1.518) (1). A histological sample derived from plant or animal, which is usually the choice of service field engineers (1,3,8). The plastic slides (Applied Precision) were found to be the most consistent sample to test field illumination. We use the blue slide for UV excitation and the orange slide for 488-nm and 568-nm excitation. The red slide was found to bleach rapidly with 568-nm excitation; therefore, it is preferable to use the orange slide for this wavelength. The Chroma red slide was chosen for the 365-nm, 488-nm, 543-nm, and 568-nm excitations. The green slide can also be used for 488-nm excitation.
Initially, the surface of the slide is determined as it is the region that emits the most intense fluorescence in the z axial direction. It is important to measure the field illumination at a specific depth in the plastic slide, as the intensity distribution might change from the surface to the interior of the slide. Investigators should also be careful not to observe illumination fields deep within the plastic slide samples, as it will usually yield a better field illumination than regions closer to the surface because of various optical distortion factors (14). It is also important that the plastic slide be placed on a firm surface to eliminate any possibility of substrate flex. Although it has been proposed to use this fluorescent slide to take daily measurements for system stability, the factor of reproducible depth in addition to instability of laser power makes this application questionable. There are many test substrates that have been used to measure field illumination. Some criticisms of these tests are as follows. Uranyl glass has previously been used to check field illumination, but it is currently difficult to obtain and we have observed that plastic slides have higher fluorescent efficiency than the uranyl glass at all wavelengths. A field of small or large beads on a slide (Spherotech) can be used, but it is essential that all of these beads be located at the same plane or the image will be inaccurate. To eliminate this potential error, a stack of images can be obtained from the beads, followed by a maximum projection of the stack to obtain an image of the beads that represents field intensity. However, the downside of this method is that it is very time-consuming. Histological samples derived from plant or animal can also be used to measure field illumination, and these are usually the choice of service field engineers. Histological samples will show the illumination pattern indicating proper or uneven illumination. In our experience, histological samples are not sensitive enough to properly measure field illumination. It usually will yield a sense of false security for the investigator. Solutions of fluoresce fluid are unstable and may shift with time. If there is a discrepancy between the plastic substrate test slide and other test slides in measuring field illumination, it might be the result of a greater sensitivity on the plastic substrate. In our system, the three visible wavelengths of light are derived from one Omnichrome argon–krypton laser, which allows us to test the field illumination at one wavelength (488 nm) and assume that it is equivalent to testing field illumination with the other wavelengths. Because the UV line is derived from a different laser (Enterprise, Coherent) it is essential to check all objectives for proper field illumination at the 365-nm excitation in addition to the visible 488-nm excitation. Newer designed confocal systems use three individual lasers with merge modules, requiring that all laser wavelengths have correctly aligned beams emitted from the merge modules. In these systems, each of the three lines has to be individually tested. One laser line might be perfectly aligned,
yielding acceptable field illumination, whereas the other laser lines might be misaligned, yielding intensity values in which the brightest region is not in the center of the field, as illustrated in Fig. 1 with UV illumination. 4.1.1.1. OBJECTIVES
The lenses are the engines that drive this technology and their selection is very critical for optimum performance. Some of the factors to consider are the following: high numerical aperture (NA) relative to the specific magnification, flat field objectives, long working distance (WD) relative to NA, good fluorescence transmission, and good achromatic correction at desired wavelengths (12). Currently, microscope manufacturers make lenses that go though extratight QA procedures and have been classified as confocal objectives. These should be bought, as they are believed to be of a higher quality than ordinary fine microscope objectives. As a general rule, one should use the smallest magnification and the largest NA lens to acquire images (13). These lenses offer a larger field-of-view and better light transmission. Although it is critical to reject out-of-focus light for confocality, it is also necessary to have sufficient laser light entering the system, and lenses should be chosen that have good fluorescent transmission, characteristics. Opening the pinhole can increase transmission, but this should be done only if insufficient light is present, as this parameter decreases the confocality of the system. The choice of a good fluorescent transmission lens is sometimes chosen over PlanApo and spectral precision. It is suggested that a confocal core lab should have a full range of high-quality microscope objectives consisting of air, water, multi-immersion, and oil lenses (typically planapochromat of the highest numerical apertures available) to satisfy the multiple applications in a core laboratory. It is necessary to match lowmagnification oil and water objectives with higher-power water and oil objectives. Water-immersion objectives increase the working distance of the higherpower objectives. A complete set might be the following: 5× dry, 10× dry, 20× dry— used for low magnification imaging of macroscopic specimens (e.g., wholemount mouse brains and fetuses), a 10× oil, 20×, 40× oil, 63× oil––used for conventional microscopy and 10× and 20× multi-immersion lenses combined with a 63× water lens and a 63× oil lens. A 100× lens has problems with alignment and low light yield and UV incompatibility. It is suggested that a 63× lens be used and zoomed slightly to address the 100× magnification range. Objectives have unique characteristics and should be chosen for the specific applications accordingly. For instance, the Leica 100× (Fig. 2; NA = 1.4) is not recommended for UV applications, as it has a bull's-eye pattern with UV excitation. Because of the design of the 100× objective, it is recommended to use a zoom of 2× when using UV light in order to achieve a drop off of less than 25% across the field. Leica recommends that the PlanApo 63× (NA = 1.32) be used
for UV work, as it has more uniform UV field illumination and better UV light transmission. Other objectives that are useful in conventional microscopy might have a fluorescence bull's-eye pattern, making these lenses unsuitable of confocal applications (1,3). It is important to acquire lenses that are compatible for confocal microscopy applications and test the lenses for field illumination accuracy using UV and visible excitation wavelengths. This field illumination test allows for a system evaluation consisting of both the objective properties and the confocal microscope laser alignment. This bull's-eye intensity profile has been obtained with different magnification objectives using all manufacturers' systems (Biorad, Nikon, Leica, Zeiss). The incompatibility of different lenses with confocal microscope systems can increase this bull's-eye effect and this parameter should be considered in choosing lenses. The problem might be the result of the lasers under filling the objective, which results in operating a lens under suboptimum conditions, resulting in the field illumination problems. One recommended solution to poor field illumination or bull's-eye illumination is to increase the zoom factor. However, this enlarges the illumination center and pushes the lower intensities off the field-of-view. Increasing zoom also increases the magnification and bleaching rate of the sample and this might defeat the purpose of using a low-magnification objective to observe a large field-of-view or might rapidly bleach the sample. This field illumination effect has to be monitored with each laser wavelength and each objective, as the alignment, wavelength, and lens design can influence the field illumination pattern. In summary, to eliminate field illumination problems, the system should be aligned correctly, with the brightest light being focused into the center of the field and decreasing less that 25% in all directions equally from the center with most lenses. Not all problems with the field illumination test are the result of poor alignment, lens design/quality, or incompatibility of a lens with specific wavelengths of light. A dirty lens will yield both poor field illumination and poor resolution. If a lens is dirty or covered with dried oil, then it would yield a nonuniform pattern (3). In one example, the intensity of the field from a 20× (NA = 0.6) dirty lens varied by as much as 70%, with the maximum intensity being off center on the right side of the image. After cleaning the lens to remove oil and other particles, an acceptable illumination pattern was obtained, with the maximum intensity being located in the center of the image and decreasing in intensity by less than 10% from the center maximum (3). 4.2. Power Meter Readings This power test appears to be one of the most useful tests because it quickly evaluates both the system alignment and performance. For the adequate operation of a CLSM, a sufficient laser power must excite the specimen. If a system
is misaligned or functioning suboptimally, it can be assessed by a test that measures laser power. The power test can indicate if the system is aligned properly up to the plane of excitation on the stage or if the machine has a defective component (i.e., a dying laser or a defective fiber). It should be emphasized that this test is performed at the microscope stage prior to the light reflecting the dichroics for a second time and penetrating through the emission pinhole and the emission barrier filters or prisms (if they exists in the system) and into the PMT. In our experience, without sufficient power throughput in the system, the voltages will have to be increased to high values to visualize fluorescence derived from specimens, which will introduce PMT noise. In addition, the cause of the decreased laser power might result in other problems (i.e., laser instability, loss of axial resolution, increased laser noise, increased PMT noise, fiber polarization, broken fibers, AOTF malfunction). The laser power measurements are listed in Table 1 and are useful to illustrate the maximum power that can be obtained from a CLSM having these different laser configurations. These measurements serve as a valuable reference for an individual lab to QA their system over time and as a comparison with other similar CLSM for adequate performance. In a three-laser system, it appears that there is sufficient power with the 633-nm and 488-nm lasers, but because the maximum power of the 543-nm laser is so low and the attenuation of the 543-nm laser is so great, at least 0.2 mW of power are necessary from the 1.2-mW HeNe 543-nm laser to provide sufficient light to excite the samples. If the system is not aligned properly, the laser output will be decreased. With insufficient power, the PMTs will need to be operated at high-voltage settings, which increases the system noise and produces unacceptable images. Less power throughput in the confocal system suggests a problematic laser, a fiber polarization problem, defective AOTF, or just a poorly aligned system. The maximum laser power is dependent on the laser, optical configuration, and the specific objective used. Using a 10× objective (NA = 0.3), it is desirable to have at least 1 mW of power on the microscope stage for each laser line derived from a 75-mW Omnichrome 643 argon–krypton laser. Other confocal systems with different laser configurations will naturally have different power values, as indicated in Table 1. It is important to measure the power output to evaluate system performance for all three lines after installation to make sure that the system is aligned properly and the laser is functioning correctly. These power values will not only serve as a reference to ensure the system is performing properly but can be useful to notify the confocal manufacturer of deviations from acceptable values, which will mean either laser failure or misalignment. A new Omnichrome 75-mW argon–krypton mixed-gas laser delivered the following power outputs: 488 nm, 1.10 mW; 568 nm, 2.68 mW; 647 nm, 1.60 mW. After time and proper laser
alignment, almost 3 mW for each line have been achieved in this system. However, on another day the maximum power with a Leica TCS-SP1 system using a Plan 10× Fluor (NA = 0.3) was the following: 488 nm (1.1 mW), 568 nm (1.45 mW), and 647 nm (1.65 mW). Similarly, the maximum power measured on installation with a Leica SP2 system using a Plan 10× Fluor (NA = 0.3) was the following: 4.6 mW (TD, 488 nm), 6.5 mW (DD, 514 nm), 0.22 mW (TD, 543 nm), and 2.8 mW (TD, 633 nm). The values on the same machine taken a few months later were the following: 2.87 mW (TD, 488 nm), 3.9 mW (DD, 514 nm), 0.093 mW (TD, 543 nm), and 1.45 mW (TD, 633 nm). The reason for the fluctuations is unknown, but it appears that they might be attributed to fiber polarization problems, and AOTF instability problems. 4.3. Bead/Histological Power Meter Sample If a power meter is not available, the crude power of the system can be assessed using standard histological samples like the FluoCells slide (F-14780; Molecular Probes) or beads like the 10-µm Spherotech beads (FPS-10057-100) and recording the PMT value that is necessary to acquire an image at almost saturation values If conditions are identical between machines. This PMT value can be used as a crude reference value to compare CLSM units and to establish their acceptable performance levels. Scientists desiring a more accurate method to test performance will find major problems with this type of testing because of the large range of acceptable PMT voltage values. Another reason to doubt the data from using a histological sample is that individual PMTs and samples can vary greatly in quality. Two similar PMTs on our machine had almost 100 PMT unit differences in the 700 PMT range. What is even more troubling with this test is that the PMT voltage is expressed as a logarithmic relationship relative to an intensity increase, which means that the difference of only a few PMT units can be translated into a huge difference in intensity. In our opinion, these tests are very crude and subjective because of the acceptability of such a large range of PMT amplification values, the variations in staining between different plant samples, variations in PMT characteristics, and the logarithmic relationship between PMT and intensity (1). It does, however, yield a rough estimate to determine if there is sufficient power in the system. It is useful for each laboratory to have a reference slide to determine PMT saturation values with different laser wavelengths to determine if there is adequate power in the system and if the CLSM is adequately performing. The comparison of two lenses to transmit light at an Airy disk of 1 can be relatively compared using a saturation test with almost any uniformly stained sample. On a Leica system, the 100× (NA = 1.4) does not transmit as much light as a 63× (NA = 1.32) lens; thus, for low-light applications, this 100× lens should not be used when there is insufficient staining.
4.4. Laser Adjustments Argon, UV, and argon–krypton lasers need to be aligned and adjusted on a regular basis, as they occasionally go out of alignment. The investigator can easily measure laser power over time using a power meter positioned on the stage (see Table 1). Either the loss of laser power or poor optical alignment will reduce the laser power in the system, necessitating an increase in PMT to compensate for lack of laser light intensity. If minor adjustments are made to the mirrors with the horizontal and vertical knobs located on the back of these lasers, the laser power sometimes can be increased. However, these visible lasers are usually enclosed in a box, with the rear knobs being inaccessible for adjustment by the investigator. In fact, most confocal manufactures do not allow the user to adjust these controls, as it is the responsibility of the manufacturer on a service contract to ensure that the lasers and system are functioning properly. For example, in our system it is possible to tweak the Coherent UV Enterprise laser, but it is not possible to adjust the argon–krypton laser, as it is enclosed in a box that the manufacturer required not be open or the service contract would be invalidated. The investigator usually will not notice a problem with laser power or alignment, but will continually have to increase the laser power to compensate for the reduced system laser power. This use of increased laser power will not only shorten the life of the laser, but will not correct the CLSM system problems that are indicated by reduced power. The reduced laser power might result in a system yielding poor resolution and system noise. If there is insufficient light entering the system, a careful realignment of the laser beam might be required (a separate procedure done by qualified personnel) to increase the laser output. If this alignment does not solve the insufficient system power values (similar to data shown in Table 1), then the laser might need to be replaced. Knowing specifications of laser power output on a stage is a critical parameter to assess system performance. In the future, it is suggested that the different manufacturers should specify these power values obtained on the microscope stage for different laser configurations, thus allowing investigators to determine their CLSM performance in their laboratories. 4.5. UV Power Test One of the major problems that occur with UV confocal systems is that there is insufficient UV power output. The UV power transmission can be decreased from a number of factors, which include misalignment, aging fiberoptic, polarization mismatch between fiber and laser, unfocused collimator lens, and dying laser. This measurement of UV power helps assess the system performance and determines if adequate UV power is being transmitted through the system and if the fiber is in good condition. Although it is recommended to take the measurements
at the back objective aperture region to eliminate the characteristics of the lens from influencing the test, we were not able to mount our detector probes in a sufficiently stable manner to allow for repeatable measurements. An objective designed with good UV transmission characteristics should be used to increase the power throughput. Attenuation of the laser light through the low-power optics of the system will still occur and the power values obtained are relative values that are highly dependent on the specific type of objective used. Because our power detector does not work with higher-power optics (40×, 63×, 100×) because of optical limitations of the stage, it will be useful to use a histological test slide sample, fluorescence slides, or bead sample to assess UV power with these higher-power lenses. Experiments can also be done with histological test slides or fluorescent colored glass to approximate the laser output with higherpower objectives. For instance, by using maximum UV laser power, it was found that the 10-µm Spherotech bead saturates PMT 1 (low-noise PMT) at a setting of 650 using a 100× PlanApo lens (NA = 1.4). Leica technicians routinely use a 40× lens to measure the fluorescence saturation of a histological plant sample. If it saturates in the PMT range between 600 and 700 units in PMT 1, the system is passed as having adequate power. In our opinion, this test is very crude and subjective because of the acceptability of such a large range of PMT amplification values, the variations in staining between different plant samples, variations in PMT characteristics and the logarithmic relationship between PMT and intensity (1,3). It does, however, yield a rough estimate to determine if there is sufficient laser power in the CLSM. It would be useful to measure these values on a user histological test slide or bead when the machine is first installed and deemed working acceptably by the manufacturer's service and sales representative. 4.6. Dichroic Reflectance (Single-Wavelength Excitation) Dichroic filters are made to reflect or reject specific wavelengths of light and pass the desired excitation/emission wavelengths of light. Dichroics do not always perform the way they were designed to perform in a CLSM (see Table 2). Dichroic tests should be made to determine the optimum system efficiency and ascertain the performance of individual dichroics with a variety of objectives and wavelengths. In certain images, it is important to have narrow bandpasses and not broad long passes in acquiring the fluorescent emission. It is important to use the dichroics that reflect the maximum amount of light at the desired wavelengths to increase the efficiency of the CLSM (1,3,6,7). By placing a fluorescent slide on the stage and measuring the relative intensity of an image, the efficiency to reflect light in a confocal system of the specific dichroic filter can be evaluated. The double dichroics (DD) and triple dichroics (TD) are more complicated dichroics than the single dichroics (SD) and, in theory, should reflect less light, as they were made to reject more light and pass fewer specific wavelengths of light. The placing of either
a single, double, or triple dichroics in the light path should reflect successively less light using the 488-nm excitation line. Using a Leica confocal filter system, normally the 488-nm line should use the SD (RSP500), the 568-nm line should use the DD (488/568), and the 647-nm line should use the TD (488/568/647). However, the DD reflected the 488-nm light best and the TD reflected the 568-nm light best with all objectives. In principle, the better the reflection, the more efficient the dichroic. Using 568-nm excitation, the TD reflected 30% more light than the recommended DD dichroic. Comparing the RSP500 and DD dichroics with 488-nm excitation shows the efficiency difference between the low-power and high-power objectives, necessitating the need to test all of the objectives. From these data, it can be surmised that when using single-wavelength excitation, the DD should be used in preference to the RSP for 488-nm excitation for all lenses, and the TD should be used instead of the DD for 568-nm excitation for all lenses. This is a QA test to determine the efficiency of the dichroics in CLSM and help in determining which dichroic should be used in a single-wavelength excitation experiment. This dichroic test is used with single-wavelength excitation and application of the data allows the system to be run at lower PMT values, which translates into less noise and better performance. With multiple excitation wavelengths, the dichroics have to be chosen to balance the power of the emitted fluorescence from each fluorochrome, but this test is helpful in making the decision. A lambda scan of the dichroics will indicate their actual spectra in the system and whether there are alignment problems. 4.7. Axial (Z) Resolution The axial resolution test is considered the "gold standard" of resolution in confocal microscopy (1,3,7,8). The axial resolution test is made using a 100× PlanApo (NA = 1.4) objective and has yielded below 350 nm. It should be emphasized that this is the only performance specification in 2006 that a company has said it will guarantee on a confocal microscope. Normally in a functioning system, values between 280 nm and 350 nm with a 100× (NA = 1.4) lens were obtained. A 63× PlanApo (NA = 1.32) lens should meet the specification of 400 nm, although Leica does not currently guarantee this value on a TCS-SP system. If the laboratory does not have a 100× PlanApo (NA = 1.4) objective and it is not possible to borrow one for comparison purposes from another confocal facility, it is useful to have other lenses as a reference point. The axial Z resolution of three different lenses was the following: a 40× (Fluor, NA = 1.0) was 610 nm; a 63× water-immersion lens (PlanApo, NA = 1.2) was 390 nm; and a 63× oil-immersion (PlanApo, NA = 1.32) was 315 nm. The excellent resolution that was obtained with the 40× and 63× lenses on our aligned system can serve as a system standard for axial resolution in a correctly aligned machine for other investigators using Leica TCS-SP equipment. It is important that the lenses achieve good values or the resolution in the system will
be inadequate. It is also important that the pattern of the axial resolution be symmetrical with suitable diffraction regions (peaks and valleys) to the left of the major peak (see Fig. 3B). Normally, the axial registration does not change over time assuming the laser lines are stable. However if alterations are made in the scan head (i.e., galvanometer replaced) or when the lasers in the system are replaced, it will be necessary to realign the system and measure the axial resolution again. The quality of the lens by this test will relate to the quality of the biological image and that is why it is called the "gold standard." It is important to compare the user-determined test slide with that of the service technician's slide to ensure that both specimens are yielding the same value. It should be emphasized that not all lenses are created equal and some will yield better resolution than others, as clearly illustrated in Fig. 3. If possible, lenses should be chosen from the manufacturer for excellent quality. Currently, there is a grade of lenses defined as confocal grade by one manufacturer. These lenses should be acquired, as these lenses undergo higher QA procedures in the factory and they are guaranteed to show excellent axial resolution, spectral registration, and other excellent lens characteristics. Other manufacturers should let you evaluate the lenses that are purchased prior to acceptance of the CLSM system. It is very important to have the best quality objectives on a CLSM. 4.8. Axial Registration (Beads) This method is slightly more subjective than the axial z mirror test, but it does yield similar values most of the time. For unknown reasons, the values might be better or worse than the mirror-derived values. The mirror test is more accurate and should be used if available. 4.9. Square Pixels The changing of a galvanometer stage will require that the symmetry in XY directional field scanning be measured. Measurements of objects will be inaccurate if the xy scanning does not yield a perfect square. This value should stay constant, but it must be checked and adjusted periodically. The phase adjustment in bidirectional scanning can also be checked and adjusted by this test. A lack of adequate phase alignment will result in a decrease in resolution. In cases that require the highest-resolution image, unidirectional scanning should be done, knowing that the scanning time will double. The galvanometer stability can also be checked using the square pixel test 4.10. Spectral Registration (UV and Visible) The 1-µm multiple wavelength fluorescent beads (Tetraspec, T7284, Molecular Probes; or Rainbow beads, Spherotech) were used to monitor the UV and visible colocalization. The registration of bead fluorescence images between
the 365-nm UV wavelengths and the 568-nm visible wavelengths in an aligned system was almost superimposable (see Fig. 5A), whereas in a misaligned system (see Fig. 5B), the difference between the peaks was 650 nm (acceptable difference is only 210 nm). The 568-nm line was chosen instead of the 488-nm line to minimize the crossover fluorescence between the visible and UV wavelengths. If the wavelengths are not aligned, then colocalization and fluorescence resonance energy transfer (FRET) studies cannot be effectively made on the machine. The image data must also be expressed as a maximum projection to eliminate the spectral mismatch. It appears for an unknown reason that if there is proper spectral registration between UV and visible wavelengths, then the UV field illumination might not be uniform, and vice versa. Both field illumination and spectral registration parameters must be checked. In addition, the confocal machines have separate collimator lenses that are used to align the UV light for different magnification lenses. It is very difficult to get all of the objectives to show proper spectral registration between UV and visible wavelengths. The spectral registration of the 365-, 488-, 568-, and 647-nm lines can be made with either a small (0.5- or 1.0-µm multicolored bead) or a front-surface, single-reflective mirror (see Fig. 7). The spectral registration with the mirror on a Leica system is a superior test to the bead, as the laser light can be measured sequentially or simultaneously to eliminate any crosstalk between adjacent emission wavelengths. In addition, no bleaching occurs at high zoom magnifications with the mirror. With new confocal systems that contain three visible lasers, the spectral registration test measures both the lens spectral registration and the laser spectral registration. It would be useful to measure a few different objectives to determine if the spectral registration of the lasers is correct or if a pattern of misalignment occurs. It is highly unlikely that different lenses will show the same spectral mismatch and, thus, the pattern observed should indicate if there are potential problems with either the lasers or with the objectives. Molecular Probes produces a series of different-sized beads (Focal Check 1 µm, 6 µm, 15 µm) with different fluorescent rings and core bodies to assess colocalization from multiple lasers. These data should reveal if the laser lines are aligned correctly. The smaller bead should be more accurate but slightly harder to use. 4.11. Laser Power Stability Power stability in a CLSM can be influenced by many factors, which include the lasers, PMTs, electronics, electronic component failure, fiberoptics transmission, fiberoptical polarizations incompatibility, AOTF thermal regulation, thermal heat dissipation, optical components, and galvanometers. The data obtained from this power stability test alert the investigator to possible errors that might exist in the acquisition of intensity measurements in biological and physiological
experiments (1,11). The reason for such high variations in CLSM systems is unknown. For an investigator, it is not initially important to know where the source of instability is being generated, but only that it exists. Once the problem is identified, trained microscope service personnel will be able to troubleshoot the system and hopefully remove the source of the power instability. Laser stability measurements should be made on the CLSM for the investigator to have confidence that the CLSM is not introducing artifacts in experiments requiring intensity measurements or time-dependent physiological experiments. In our system, there was periodic noise in the laser power tested by the transmitted light detectors that exceeds the manufacturer's (Ominichrome) laser stability fluctuations specifications of less that 0.5% over a 2-h time period. The 488-nm and 568-nm lines have a periodic cycle that is directly opposite the 647-nm line (see Fig. 9). Argon lasers and helium–neon lasers (543 nm or 633 nm) are considerably quieter than the argon–krypton laser and are definitely preferable. In a confocal microscope, there are different ways to measure power stability over time (hours). These include the following: (1) manufacturer-installed pin diodes; (2) laser meters on the microscope stage connected to a readout device; (3) fluorescent emission intensity from a plastic slide detected by a PMT; (4) transmission optical system detection. The pin diode test was not stable and should only be used as a subjective assessment of power. A laser power meter can be connected to a UV or visible (VIS) wavelength detector situated on the microscope stage and then be continuously used to monitor the power output with either a chart recorder or equivalent computer software (Coherent). Simultaneous comparison of the measurements using a pin diode in the Leica SP CLSM and either a power meter on the stage or the transmission average intensity (not shown) demonstrated that the pin diode has unstable power readings, whereas the other two measurements (transmission optics detection and power meter detection) were relatively stable over time. The pin diode should not be used as an absolute indicator for power or stability, as the power derived from it can vary in intensity over time. It can, however, be used as a subjective assessment of the laser performance and system alignment. A UV or VIS detector situated on the microscope stage and connected to a suitable power meter can monitor the CLSM laser power. The power output intensity is continuously monitored with either a chart recorder or equivalent computer software. Manual measurements are deemed not accurate enough and are very time-consuming. If transmission optics is not available on the system, a power test can be made that uses a fluorescent slide sample placed in the light path. However, the investigator must be aware that repeated samplings of a fluorescence slide could bleach the sample, which will decrease the fluorescence intensity and increase the transmission intensity. Therefore, the laser power should be decreased with the AOTF to minimum values to help reduce slide bleaching, as
decreasing the laser power with the power supply might result in laser instability. In addition, one must be aware of possible energy excitation of the fluorochrome in the slide. In our experience, the most reliable method to measure the laser power stability consisted of using the transmission optics of the CLSM without a fluorescence slide in the optical path. If transmission optics is not present, a fluorescent-colored slide can be used, but extreme care must be made to reduce the laser power with an AOTF to reduce possible interaction of the laser beam on the sample. Sometimes the output fluorescence intensity might increase as a result of repetitive additive excitation or decrease resulting from bleaching. Lasers used in flow cytometry or confocal microscopy equipment should be stable with low peak-to-peak noise and minimal power fluctuations over hours. Laser noise can originate from different sources, which include the AOTF, laser polarization mismatch, heat dissipation, and power supplies. One of the most likely causes is fiber polarization, which might be mismatched with the polarization of the laser. One of the most likely sources is a poor power-supply regulation that results in light output fluctuations at the frequency of line current used to run the power-supply (15). Noise in a helium–neon laser might be found at frequencies of a few hundred kilohertz as a result of either radio-frequency energies used to pump the laser medium or of fluctuations in the medium itself (15). The DC power supply used should be the correct type (Omnichrome power supply 171B or 176B with Omnichrome argon–krypton laser) to produce low noise and should be operated at "Light mode" (constant power) and not constant current mode (15). The 171B power supply had transformers and heating problems and has been replaced by the 176B model with rectifiers that regulate heat better. Typically, a Coherent Enterprise laser, Coherent 90-5 or Coherent 70-4, will have less than 1% peak-to-peak noise (15) and the power will not fluctuate over time. The air-cooled argon laser from Uniphase or Spectra Physics used in benchtop flow cytometers or confocal microscopes will also have less than 1% peak-to-peak noise according to the manufacturer's specifications. The argon–krypton (Melles Girot, Omnichrome 643) laser contains three simultaneous laser lines that yield power intensity fluctuations of less than 0.5% for 2 h (personal communication and website). If this is true, where do the fluctuations in power intensity in excess of 10% as shown in Figs. 8 and 9 come from if the laser is not generating it? Is it the AOTF or fiber polarization? Lasers can be checked with power meters in front of the beam or by special electronic boards that connect to the power supply to determine laser stability. However, this testing might not be possible by the investigator, as the laser is in a sealed compartment and the investigator is not allowed into this compartment or the service contract will be canceled.
4.12. Laser Power Stability (UV) The Coherent Enterprise laser delivers less than 1% peak-to-peak noise and is considered a very stable laser. The argon air-cooled lasers, HeNe lasers, and Spectrophysics argon–krypton laser are all rated at less than 1% peak-to-peak noise. However, even with the Coherent UV Enterprise laser or a HeNe laser (543 nm), periodic noise and large power fluctuations were observed. One source of power stability appears connected to the way the laser is cooled and how the laser heat that is generated is dissipated. This was illustrated with our Coherent Enterprise UV laser that was connected to a Coherent LP 20 water–water exchange cooler. This cooler should be set at least 10°C above the circulating cooling water of the building and it should be set above the ambient temperature of the room. Improper set points for the LP 20 cooler resulted in temperature-regulation problems of the circulating cooling water in the laser, which, in turn, resulted in the improper regulation of the laser power (see Fig. 9, trace B). In addition, problems with proper fiber alignment also appeared to occur with the UV system, resulting in power fluctuations (see Fig. 9, trace C). The elimination of these temperature and polarization issues resulted in proper laser cooling and laser stability (Fig. 9, trace A; 650 nm (far red). The position of the cells can be recorded and the cells of interest can successfully be relocated for reanalysis by storing the information on disk (6–9). A schematic representation of the LSC is illustrated in Fig. 1. The X-Y coordinates of the cell position on the slide allow recall, visual, or camera inspection of specific cells and the reevaluation and repositioning of cells after further staining. Rare events can be validated within complex cell populations. Cells can be restained and remeasured in studies with additional fluorochromes in multiparameter studies. The sample can be remeasured to generate time-sequenced data using the integral clock and time stamp on each measurement. More advanced instruments and software now offer up to three coaxial violet, blue and red/infrared lasers, more detectors, and more versatile image analysis software, including the capability for tissue section and FISH spot and multiparameter, high-content analysis. The methodologies that we describe were developed on a first-generation instrument from Compucyte (Cambridge, MA; www.compucyte.com). This was acquired in 1996 for a small, specialized cytometry research facility within a district hospital, for near-clinical and cytopathological research. The instrument was equipped with a single argon laser (488 nm) and a mercury lamp for excitation, an Olympus microscope, light collection optics, three color PMTs, and forward-scatter detector, and an early version of the Wincyte proprietary analysis software on a Windows-based Pentium computer. Its image analysis capabilities were enhanced through a charge-coupled device (CCD) camera linked to a Kontron image analysis system (8). 1.1. Specific Applications Our studies have addressed a range of these capabilities, including single- and dual-parameter DNA content analysis, the measurement of cell proliferation in
Rew, Woltmann, and Kaur
human tumors following in vivo bromodeoxyuridine labeling (9,10), viability studies of human tumor cell lines and of cells extracted from human tumors, chamber slide studies, micronucleus genotoxicity assays (3), rare-event analysis on asthmatic sputum samples, and studies on the subcellular distribution, metabolism, and drug-resistance mechanisms in breast cancer cells (11,12). In this chapter, we will describe our studies on intracellular cytotoxic drug distribution in encapsulated microspheres and on the detection of dispersed eosinophils in asthmatic sputum. 1.2. Laser Scanning Cytometry Studies of Fluorochromatic Drug Uptake in Human Tumor Cells Treatment strategies for cancers are often inadequate. We hypothesized that quantitative cytometry and, in particular, LSC can provide useful information about tumor biology and treatment sensitivity not otherwise available to the clinician. Specifically, the decision to use adjuvant chemotherapy for primary, metastatic, or recurrent tumors and the selection of particular cytotoxic agents thereafter is much influenced by empiricism in clinical oncology. This might be depriving some patients of benefits from individually targetted adjuvant chemotherapy. Multidrug-resistance (MDR) mechanisms are a potent cellular source of treatment failure, and other patients might be being treated with ineffectual agents. Tumor cells in suspension in blood such as leukemias and in malignant ascites are easy to study using FC (13). White blood cells in particular have specific size, scatter and phenotypic characteristics. Fresh and archival samples of solid tumors pose problems to FC by virtue of their heterogeneity of size, scatter, viability, and biomarker preservation (11). In contrast, visualization of complex populations and of individual cells by LSC allows verification of features and rare events in complex populations during quantitative cytometry. The anthracycline and anthraquinone families of cytotoxic agents include adriamycin (doxorubicin), daunorubicin, mitozantrone, epirubicin, and idarubicin. They have a number of potent cytotoxic actions, including intercalation of DNA and inhibition of DNA repair, and are widely used in oncology practice. They possess intrinsic fluorescence and absorb light around 470 nm and emit around 560 nm. They are expelled from the cell by the p170 glycoprotein MDR pump. Laser cytometry thus offers opportunities to assay the MDR capacity of cancer cells and its modification by MDR blocking agents (14), to identify those tumors that fail to concentrate the agents and spare their owners from ineffective therapy, to identify those drugs most reliably concentrated in cells from tumor biopsies from a panel of agents, and to study new drug delivery vehicles such as drug-loaded albumin microspheres. The assay of drug uptake alone is
Near-Clinical Applications of LSC
insufficient to indicate cell killing. For example, a fluorescent drug might be metabolised, sequestered, or otherwise rendered inert within target cells. Additional assays of drug efficacy are needed to show evidence of induced cell death or irreversible disruption of other vital functions, such as evidenced by mitotic disruption or apoptosis (15). We first describe methods to evaluate the binding and internalization of doxorubicin-loaded albumin microspheres to breast cancer cells in vitro using LSC. Doxorubicin microcapsules are a drug delivery system comprising 3-µm human serum albumin (HSA) microcapsules with doxorubicin covalently linked to their outer surface. They are available commercially or they can be produced in the laboratory. In this experiment, we investigate the binding of doxorubicin microcapsules and uptake of doxorubicin in both the wild-type (WT) and doxorubicin-resistant (R) MCF-7 breast cancer cell lines using an in vitro model and LSC. 2. Materials 2.1. Cytometric Reagents 2.1.1. Doxorubicin We use doxorubicin (Sigma-Aldrich Ltd, Nottingham, UK) as the anthracycline reporter molecule. The drug is used in lyophilized form, obtained in vials containing 10 mg doxorubicin hydrochloride. Each vial is reconstituted under sterile conditions in double-distilled sterile water (ddH2O) to create stock solutions of 1 mg/mL, which can be stored in polypropylene screw-top vials and frozen at –70°C. 2.1.2. Reagents Used to Bind Doxorubicin to HSA Microcapsules 1. 1-Ethyl-3 (3-dimethyl aminopropyl) carboiimide (EDC) linker, mannitol, pepsin, and Tween 80 (Sigma-Aldrich Ltd). 2. Hydrochloride acid (32%) (Fisher Scientific Ltd, Loughbrough, UK). 3. HSA microspheres.
2.1.3. Selection of Breast Cancer Cell Lines and Culture Conditions The reagents needed as follows (all purchased from Life Technologies Ltd [Paisley, Scotland]): 1. L-Glutamine. 2. Fetal calf serum (FCS), heat-activated, mycoplasm virus-free. 3. HEPES buffer: 1 M L-glutamine (2 mM). 4. Phosphate-buffered saline (PBS) without calcium magnesium or sodium biocarbonate. 5. RPMI 1640 medium without L-glutamine or phenol red.
6. Trypsin solution containing 0.25% trypsin. 7. 1 mM Ethylenediaminetetraacetic acid (EDTA).
3. Methods 3.1. Source and Production of HSA Microcapsules The HSA microcapsules have a mean diameter of 3.3 µm. Ours were a gift from Andaris Healthcare Ltd, as a 1 : 2 (w/w) suspension with mannitol (7.5×109/mL: number of microcapsules). The microcapsules are washed free of mannitol prior to crosslinking the doxorubicin to the microcapsules as follows. Microcapsules are suspended at 100 mg HSA/mL in a 1% (v/v) solution of Tween 80 in ddH2O, vortexed once for 30 s and then left to stand at room temperature for 30 min. The suspension is then centrifuged at 875g for 2 min; the supernatant is removed and discarded. The microcapsules are resuspended in 1 mL of ddH2O (50 mg/mL), vortexed for another 30 s, and centrifuged at 875g for 2 min. The supernatant is removed and discarded. This washing step is repeated twice. 3.2. Preparation of Doxorubicin Microcapsules Using 1-Ethyl-3 Carboiimide Linker All procedures are carried out under sterile conditions. A solution of doxorubicin (3 mg/mL ddH2O) is added to 100 mg of washed microcapsules; then the mixture is vortexed for 30 s. A solution of EDC (6 mg in 500 µL of ddH2O) is then added and the mixture is revortexed for 30 s. A magnetic stirrer bar is placed in the reaction vessel and left stirring at 37°C overnight, in the dark. The doxorubicin microcapsules are then collected by centrifugation at 875g for 5 min and the supernatant is removed and discarded. Unreacted doxorubicin is removed by resuspending the doxorubicin microcapsules in 5 mL ddH2O, vortexing, and then centrifuging the suspension at 875g for 5 min, with the supernatant then being discarded. This step is repeated until the supernatant appeared colorless to the eye. The drug-loaded albumin microcapsules are then resuspended in 1 mL of ddH2O and stored at 20°C in the dark. 3.3. Selection and Subculture of MCF-7/WT Cells and MCF-7/R Cells The MCF-7/WT human breast adenocarcinoma cell line no. 86012803 (16) can be obtained from the European Collection of Animal Cell Culture (Porton Down, UK). Cells are maintained in culture medium (RPMI 1640 without phenol red, supplemented with 10% [v/v] FCS, 2 mM L-glutamine, and 0.04 M HEPES buffer) and cultured at 37°C in a humidified incubator containing 5% CO2. Confluent cultures of MCF-7/WT cells are subcultured as follows. Cells are washed with PBS once, then trypsin/EDTA 0.1% is added (0.5 mL for a 25-well
plate, 1 mL for a 6-well plate, 3 mL for a 25-cm2 flask [T25], and 5 ml for a 75-cm2 flask (T75). The culture vessels are incubated in trypsin/EDTA for 3–5 min at 37°C until the cells become detached. The action of trypsin/EDTA is denatured by the addition of an equal volume of culture medium containing 10% FCS. The cells are then aspirated and placed into sterile 15-mL tubes, centrifuged at 450g for 8 min, resuspended in 1 mL of culture medium, and then counted using a hemocytometer. Our MCF-7/R cells (17) were kindly provided by Dr. Tim Gant (MRC, Toxicology Unit, University of Leicester). These cells can be cultured in the same way as the MCF-7/WT cells. Additionally, they are maintained throughout in the presence of 0.5 µM doxorubicin. This should be added to the culture medium and filter-sterilized at the time of subculturing. Both the MCF-7/WT and MCF-7/R cells are normally seeded at 4 × 104 cells/cm2, equivalent to 1 × 106 cells per T25 flask. 3.4. Drug Uptake Studies The cell lines can then be used to investigate the rate of uptake of free doxorubicin and the binding of doxorubicin microcapsules to the target cells, the time-course of uptake, the patterns of intracellular distribution, the dynamics of drug exclusion and microcapsule metabolism, and the effects on cell viability, cytostasis, and apoptosis over various time periods and using various incubation doses. One can further quantitate the handling of different fluorochromatic drugs and reagents by different cell types. The following subsections illustrate our experiments in this context. 3.5. Measurement the Uptake of Free Doxorubicin and Doxorubicin Microcapsules Cells are trypsinized (MCF-7/WT, MCF-7/R) and incubated at a cell density of either 2 × 105 or 5 × 105 cell/mL in serum-free culture medium in 5-mL polypropylene tubes at 37°C in a CO2 incubator for 1 h with 0.5 µM of either free doxorubicin or doxorubicin microcapsules (15). At 10-min intervals during the incubation, the cell suspensions should be mixed manually. After incubation, they should be washed twice in PBS, resuspended in 500 µL PBS, and then visualized using the LSC. Time-course experiments can be conducted to establish the rate of uptake of either free doxorubicin or doxorubicin microcapsules after the cells are seeded at 1×106 into the T25 flask incubated in 3 mL of culture medium at 37°C in CO2, and incubated for a further 2–96 h. Following incubation, adherent cells are washed with PBS and detached from the flasks with trypsin. The cell pellet is then resuspended in 1 mL culture medium, cell viability counts are performed, and the cell density are adjusted to
Fig. 2. Construction of a chamber slide.
5 × 105 cells/mL in all cells. Samples are then analyzed and visualized by the LSC to capture the uptake and internalization of the doxorubicin microcapsules by the target cells. 3.6. Slide Preparation for LSC and Fluorescent Microscopy of Labeled Cells Chamber slides (see Fig. 2) (see Note 1) allow the study of spatially fixed but viable cells in fluid media under the microscope of the LSC. This allows a wide range of experimental options in the study of the response of cells to stimulants, toxins, or the binding of additional antibodies and fluorochromes. 3.7. Examination of Samples Using the LSC The 488-nm argon laser provides excitation. Proprietary WincyteTM software (Compucyte) is used to control the instrument. The target cells are identified and selected by contouring on laser light scatter and red fluorescence (as determined by the presence of doxorubicin and doxorubicin microcapsules). The integrated laser light scatter and red fluorescence of each cell are displayed in a dot plot of area vs red maximal pixel. The dot plot is very similar in concept and presentation to those familiar to flow cytometrists (see Fig. 3; Color Plate 7, following p. 274). The signal from each cell comprises some 200 pixels in 0.5-µm increments. The area measures the number of pixels in the field of interest, usually a single cell; thus, area represents the size of the cells or microcapsules. The maximum pixel value is represented the highest fluorescence signal from within the data contour of the individual cell. Each cell can be rescanned
Fig. 3. Assay of cells incubated with either free drug or drug-encapsulated microspheres by LSC. The size of the cells or particles is denoted on the Y-axis (Area). The intensity of red, doxorubicin-associated fluorescence, is denoted on the X-axis. (See Color Plate 7, following p. 274.)
numerous times, because the LSC records the X-Y position of each cell on the slide. Rescanning each cell from any selected dot plot region creates laser lightscatter images and red fluorescent images, which can be stored separately and later merged. This allows us to produce galleries of cells images (see Note 2). The specific channel-splitting facility of this program allows each image to be split into three 8-bit images comprising red, green, and blue color components. The coassembly of the laser scan and red fluorescent images of the cells allows false color to be produced. 3.8. Technical Observations These studies demonstrate the utility of LSC in correlating quantitative data on complex cell populations equivalent to that generated by FC, with highly informative qualitative imagery of the cells of interest.
Our work has been conducted on a first-generation commercial instrument (circa 1996) and early versions of the proprietary Wincyte software. Our system has been adapted for advanced image analysis using a simple link to the Kontron KS100 image analysis system. Our system has been robust, reliable, and straightforward to use. We understand that more recent versions of the instrument offer considerable advances in capability. We consider that LSC offers a considerable advance in many respects over conventional FC: • It allows for the visualization of cells of interest and the quantitative fluorometric analysis of cell, subcellular, and particulate fluorescence to the limits of resolution of the instrument. This allows validation of rare events and the study of spatially related and temporally discrete events. • It allows the study of viable and whole-cell preparations. • It allows the retention, reexamination, reinterrogation, and reexperimentation of samples and cells of interest. • It allows for novel forms of presentation of samples to the instrument, as, for example, in fluidic chamber slides. • It can be made interoperable with other compatible instruments, such as confocal microscopes made by Olympus Optical, whereby the coordinates of cells of interest can be transferred to the stage of the confocal microscope. • It makes possible broader forms of experimentation that are not possible on flow cytometers.
Specifically, in these studies, we have demonstrated, among others, the following: • The normal operational mode of the LSC, using single-laser excitation, forward scatter, and two-color analyses • Techniques of sample presentation on the microscope slide • The use of the instrument to create galleries of images
3.9. Specific Biological Observations Our ongoing series of studies have revealed the following: • The kinetics of uptake, distribution, and elimination of free doxorubicin and encapsulated doxorubicin in wild-type (drug-sensitive) and resistant breast cancer cell lines. Such studies can clearly be extended to any experimental cell type or any fluorochromatic drug or reagent handled by cells (see Fig. 4; Color Plate 8, following p. 274). • The facility to discriminate between single cells and aggregates of cells, and subcellular particles—in this instance, drug-loaded albumin microcapsules. • The use of fluorochromatic cytotoxic drugs as their own biomarkers of binding, uptake, distribution, exclusion, and cell damage in cancer cell lines in vitro. • The workings of drug-resistance mechanisms in cancer cells.
Fig. 4. Time-course experiments of doxorubicin-encapsulated microspheres. Microspheres were incubated with the target cells for 1 h under standard conditions. Cells were analyzed at various time intervals over 96 h. The images were captured on the LSC and then manipulated to produce the image galleries. The illustrations show the binding, uptake, and intracellular distribution of the bright red cytocaps and the gradual release of free doxorubicin into the cells. (See Color Plate 8, following p. 274.) • The induction of cytostasis and apoptosis in target cells. • The biological complexity of homogenous cell lines with respect to drug uptake and metabolism.
3.10. Quantitative Analysis of Bronchial and Peripheral Blood Eosinophils and Respiratory Epithelial Cells Using LSC The objective, quantitative measurement of specific cell types in complex samples of clinical material has important diagnostic and therapeutic applications. Microscope-based studies with trained technical observers, such as used in the cervical smear cytology program or in genotoxicity testing, are laborious, inefficient, subjective, and expensive. Automation would offer considerable advantages. The quantitative measurement of eosinophils in the sputum of asthmatic patients has valuable research and diagnostic functions. Cellular and fluid
markers of inflammation in sputum have been assessed by a number of investigators, and their validity, repeatability, and responsiveness have been demonstrated in small-scale studies. Sputum eosinophil counts as a marker of asthmatic airway inflammation have the potential for aiding diagnosis and monitoring treatment response to steroid medication in clinical asthma. Sputum cytology also aids the diagnosis of other diseases, including lung cancer. Sputum is a viscid and heterogeneous substance that is unsuitable for FC analysis. Cell preservation is poor and relatively few cells are found in a unit volume of sputum. Manual analysis is very tedious. We thus devised a procedure for objective measurement of sputum eosinophils and bronchial epithelial cells using the planar analysis system of the LSC. 3.10.1. Human Sputum Samples Samples of human sputum suitable for analysis are obtained in specialist respiratory practice by sequential saline aerosol inhalation at concentrations of 3%, 4%, and 5% for 5 min each. Subjects expectorated for approx 2 min and sputum samples were collected in a sterile 30-mL universal container on ice (18). Sputum was processed within 2 h of expectoration. Macroscopically visible sputum plugs were manually selected from saliva using blunt forceps and a Petri dish to minimize oral squamous cell contamination. The selected sample was weighed and processed on ice in 4 vol of 0.1% dithiothreitol (DTT) using gentle aspiration through a Pasteur pipet and subsequent vortexing for 15 s. Following this, the sample was rocked on a bench rocker on ice for 5 min and mixed thoroughly with equal volumes (to DTT) of Dulbecco's phosphate-buffered saline (D-PBS). The cell suspension was filtered through 45-µm nylon mesh and centrifuged for 10 min at 800g at 4°C. The cell pellet was resuspended in D-PBS to determine cell viability, oral squamous cell contamination, and absolute cell numbers. The volume of the cell suspension was adjusted for optimal cytospin dispersion with D-PBS to 0.25 × 106/mL. Cell samples can be extracted from sputum by processing on ice in 4 vol of 0.1% DTT, followed by 4 vol of D-PBS, filtering through a 45-µm nylon gauze, and centrifuging for 10 min at 800g at 4oC. Suspensions should be adjusted for optimal cytospin dispersion with D-PBS to 0.25 × 106/mL. 3.10.2. Use of the Octospot Cytospin System To accomplish time-efficient analysis by LSC without the need for frequent slide replacement, we used the novel Octospot® cytospin system (see Fig. 5). This system is compatible with the Shandon cytocentrifuge and allows transfer of cell suspensions (40–80 µL per well) from eight-well microtiter plate strips onto microscope slides, thus generating eight separate cytospins on a single slide. Octospot slides are covered with a solvent-resistant hydrophobic coating surrounding each well, allowing differential immunostaining of adjacent
Fig. 5. The octospot® slide system allows the generation of eight individual cytospins on a single microscope slide using the Shandon microcentrifuge and standard eight-well microtiter plate strips.
cytospins. A 40-µL sputum cell suspension (0.25 × 106/mL) was added to each well and spun onto slides at 450 rpm for 6 min in a Shandon cytocentrifuge. Cytospins were air-dried and frozen at –20° until further analysis. After thawing, slides were fixed in high-grade acetone/methanol (50/50). Individual cytospins on nonproprietary standard microscope slides were circled with a hydrophobic wax marker to allow immunostaining of adjoining cytospins with 50 mL of antibody solutions (see Note 3). Slides were incubated at room temperature for 1 h with mouse a human-MBP monoclonal antibody (Ab), mouse and human-cytokeratin Ab, or isotypematched control Ab in PBS + 0.1% BSA. Each cytospin was washed with 20 vol (1 mL) of PBS using simultaneous suction and pipetting to avoid overflow and cross-contamination between adjacent cytospins. To reduce nonspecific binding of fluorochrome-conjugated second Ab to highly charged eosinophil proteins, cytospins were blocked with Chromotrope2R for 15 min, followed by a further washing step and incubation with goat and mouse–Oregon Green®-conjugated second Ab (20 mg/mL) and 0.2 mg/mL propidium iodide for 1 h at room temperature (see Fig. 6) (see Notes 4–6). 3.10.3. Analysis Using the LSC Slides are scanned on the LSC stage using the 20× objective and argon laser light stimulation at 5 mW. Wincyte software is used to draw contours around areas of specified fluorescence intensity, relating to specific staining protocols. For all of the experiments, the red fluorescence channel was used for segmentation around propidium iodide (PI)-stained cell nuclei. Only the red and green sensors were used in channel 1 and 4, respectively. The scan data display in conjunction with scattergram windows for peak fluorescence (red and green max pixel) plotted against area was generally used for optimizing detector voltage and gain as well as background values (all set in Setup → LSC settings). Offset
Fig. 6. Overview of indirect immunostaining protocol of sputum cytospins using the Octospot® slide system. Immunostaining with primary antibody was followed by a block step with Chromotrope 2R to reduce unspecific binding of conjugated secondary antibody to highly charged eosinophil-derived proteins. Finally, secondary antibody solution (Oregon Green® conjugated) is applied. Note that negative controls and differential antibody staining is performed on the same slide, thereby reducing between-slide variability.
values of 2000 for red and 2035 for green fluorescence were used. The PMT power setting ("gain") was varied depending on fluorescent-staining intensity between 15 and 60 for red and green fluorescence aiming to utilize the dynamic range of the instrument optimally for individual experiments. For sputum cytospin analysis, red gain was adjusted to achieve mean area values for single cells of 100–200 (25–50 mm2 for scanning at ×20 magnification). To allow accurate immunophenotyping of sputum cytospins, detection of a maximal number of single-cell events within the initial cell suspension was critically important. To minimize software contouring around cell clumps and overlapping cells, great care was taken during cytospin preparation to achieve adequate cell dispersion on the glass slide. Preliminary experiments established that cell dispersion for sputum cytospins was optimal at initial cell concentrations of 0.5 × 106/mL. Acytospin index (CI),
CI =
SCC TCC
where SCC is the total number of single-cell events contoured and TCC is the total number of events contoured, was determined for all cytospins to monitor cell dispersion. Single-cell events were determined by combining the software algorithm for single-cell detection with a gating step based on nuclear size, as PI stained nuclei were used as contouring parameter. Cell dispersion within singlecytospin areas was variable but could be optimized by adding a second cardboard filter to the clip mechanism during cytocentrifugation. In the case of the Octospot slide, eight regions can be set around the eight octospots, thus allowing for automated analysis and comparison of the entire content of each cytospin on the slide. Thus, for example, controls can be analyzed alongside the test samples to minimize experimental variation and error (see Note 7). 3.10.4. Results In order to calculate the proportion of a particular cell subset as a percentage of the total blood and sputum samples were labeled with PI to utilize its bright red nuclear staining pattern for software contouring and, hence, capture of all nucleated cells or nucleated cell remnants. The WinCyte software that controls the LSC draws contours around areas of specified fluorescence intensity, if sufficiently contrasted from the background, and registers these contours as objects. Within the area of an object and within a customizable distance surrounding it, other cell parameters can be sampled. The software program distinguishes and optionally excludes cell aggregates based on bit-pattern data. By using this exclusion algorithm and, in addition, setting a nuclear size gate based on the measured area or total integrated red fluorescence of captured events, it was possible to focus on a relatively pure single-cell population within each cytospin (see Figs. 7B, 8A, and 9A). Green fluorescence was sampled within this single cell gate. For sputum eosinophils, the peak fluorescence signal within each data contour was plotted against contour (nuclear) size (see Figs. 7 and 8), whereas the more homogenous and generally brighter staining of sputum epithelial cells was plotted as integrated green fluorescence within each data contour against peak green fluorescence signal (see Fig. 9B; Color Plate 9 following p. 274). To exclude oral squamous epithelial cells, events within the high green peak/high green integrated fluorescence, gate were plotted in addition for their red peak and integrated fluorescence, as bronchial epithelial cells feature more uniform and generally brighter nuclear staining than contaminating oral squamous epithelial cells (see Fig. 9B). For all sputum samples, at least three cytospins per slide and antibody were analyzed and specific mean percentages were calculated after subtraction of negative controls. More detailed results and a method comparison using this method and traditional manual differential cell counting are presented elsewhere (18).
Fig. 7. Determination of blood eosinophil count by LSC. (A) x/y-position of propidium iodide labelled cells detected in a single cytospin. (B) Multiple cell objects are excluded on the basis of their higher integrated propidium iodide fluorescence. (C) Single cells are scanned for specific green fluorescence after labelling with α-MBP antibody and FITC-conjugated secondary antibody. (D) Histogram of plot in C with isotype matched control antibody overlaid.
3.10.5. Technical Observations: LSC in Sputum Analysis • Octospot cytospin slides are a valuable tool for the presentation and analysis of samples to the LSC. • Selective gating on peak and integral fluorescence and nuclear and whole-cell fluorescence allows discrimination of cell populations of interest, for both green and red fluorochromes. • Gating strategies can be validated by direct visualization of the selected cells. • Perseverance and attention to detail can allow quite challenging sample presentation problems to be overcome by LSC. • Sample dispersion and cytospin density are critical to the optimal analysis. • Propidium iodide staining for DNA is a valuable technique for gating on dispersed cells using their nuclear fluorescence. • Repeated scans are simple to perform and overcome the problem of interobserver error, which complicates manual counts.
Fig. 8. Gate settings for the analysis of sputum eosinophil cells by LSC. (A) Single cells gated by detection of niuclear staining with propidium iodide. (B) Eosinophil gate (major basic protein positive cells in gate 1). (C) Histogram of plot B with isotype matched control antibody overlaid. (D) Brightfield microscopy video captures of relocated cells within eosinophil gate restained with Romanowski stain. More than 90% of cells within gate displayed eosinophil morphology. Outside this gate cells of this morphology were rare.
3.10.6. Biological Observations: LSC in Sputum Analysis • Eosinophils and bronchial epithelial cells can be successfully identified and quantified on the LSC. • Compared to manual counting—the current gold standard—estimation of the sputum eosinophil count by LSC was equivalent. • The assay has demonstrated higher counts of eosinophils in the sputum of asthmatic than in normal subjects. • Immunophenotyping using techniques demonstrated in this study could be extended to other cell types in complex clinical samples. • The addition of a third photomultiplier tube to the instrument would allow double staining of other intracellular and extracellular antigens, such as is now possible.
Fig. 9. Gate settings for the analysis of sputum bronchial epithelial cells. (A) Single cells gated by detection of nuclear staining with PI. (B) Peak green fluorescence plotted against integrated green fluorescence for single cells stained with α-cytokeratin antibody (labeled with Oregon Green-conjugated secondary antibody. (C) Histogram of B with isotype-matched control antibody overlaid. (D) Cells within quadrant 2 of plot B are plotted for red fluorescence to exclude oral sqamous epithelial cells; (E) Epifluorescent video captures of relocated cells within quadrant 2 of plot D (typical of bronchial epithelial cells). In excess of 95% of cells within this gate had this morphology. Outside of this gate, cells with this morphology were very rare (See Color Plate 9, following p. 274.)
4. Notes 1. Chamber slides can be constructed from normal glass microscope slides. Multilayered adhesive tape is aligned across the two faces of the slide with a shorter piece of tape joining the two at the end. A glass cover slip is positioned carefully on the tape and the edges are sealed. A 20-µL aliquot of sample is pipetted onto the chamber slide at the end of the cover slip that is not sealed. The cells are allowed to settle for 5 min. The slide should be then sealed with transparent nail varnish before analysis. 2. To improve graphical presentation on older LSC instruments, digital color images of the cells can be captured in WinCyte and copied to a graphic package such as Paint Shop Pro 5TM for Windows as bitmap (bmp) images for display. There, the images of red fluorescence can be superimposed onto the laser light-scatter
images. False colors can be used to identify the cells (green or blue), doxorubicin (red), and doxorubicin microcapsules (red). Samples can be stored for later analysis by freezing at –20oC. Cytospins can be circled with a hydrophobic wax marker to allow immunostaining with 50 µL of antibody solutions. Oregon Green gives brighter staining and less photobleaching than FITC conjugates. Each cytospin can be washed with 1.0 mL PBS with simultaneous microsuction to avoid overflow and cross-contamination between adjacent cytospins. Slides should be cover-slipped with glycerol 25% in PBS. To reduce nonspecific binding of fluorochrome-conjugated secondary antibody to highly charged eosinophilic proteins, cytospins can be blocked with Chromotrope R (Sigma) for 15 min. There must be sufficient contrast between the cells of interest and the background to achieve satisfactory contouring. By focusing on the cell nucleus rather than the entire cell, better contrasts can be achieved.
Acknowledgments We thank Dr. Alison Goodall for her help with the doxorubicin microcapsule studies. Elements of our work have been supported in turn by the UK's Cancer Research Campaign, Wessex Cancer Trust, and the NHS Research and Development Executive. References 1. Kamentsky, L. A. and Kamentsky, L. D. (1991) Microscope based multiparameter laser scanning cytometer which yields data comparable to flow cytometry data. Cytometry 12, 381–387. 2. Kamentsky, L. A., Kamentsky, L. D., Fletcher, J. A., Kurose, A., and Sasaki, K. (1997) Methods for automatic multiparameter analysis of fluorescence in situ hybridised specimens with a laser scanning cytometer. Cytometry 27, 117–125. 3. Styles, J. A. and Rew, D. A. (2001) Automation of mouse micronucleus genotoxicity assays by laser scanning cytometry. Cytometry 44, 153–155. 4. Clatch, R. J., Foreman, J. R., and Walloch, J. L. (1998a) Simplified immunophenotypic analysis by laser scanning cytometry. Cytometry 34, 1, 3–16. 5. Clatch, R. J. and Foreman, J. R. (1998b) Five color immunophenotyping plus DNA content by laser scanning cytometry. Cytometry 34, 1, 36–38. 6. Sasaki, K., Kurose, A., Miura, Y., Sato, T., and Ikeda, E. (1996) DNA ploidy analysis by laser scanning cytometry in colorectal cancers, and comparison with flow cytometry. Cytometry 23, 106–109. 7. Reeve, L. and Rew, D.A. (1997) New technology in the analytical cell sciences: the laser scanning cytometer. Eur. J. Surg. Oncol. 23, 445–450. 8. Woltmann, G., Wardlaw, A. J., and Rew, D. A. (1998) Image analysis enhancement of the laser scanning cytometer. Cytometry 33, 362–365.
9. Rew, D. A., Reeve, L., and Wilson, G. D. (1998) A comparison of flow and laser scanning cytometry for the measurement of cell proliferation in human solid tumors. Cytometry 33, 355–361. 10. Rew, D. A. (2000) In (Darzynkiewicz. Z., Robinson, P. J., and Crissman, H. A., eds.), Cytometry; 3rd ed. Wiley, New York, Chap. 68. 11. Reeve, L. (2000) The development of tumor specific assays for cellular response to anthracycline drugs using laser cytometry. PhD thesis, University of Leicester, UK. 12. Kaur, D. (2002) Investigation of cellular and molecular mechanisms involved in targeted drug delivery systems for human cancers. Doctoral thesis, University of Leicester, UK. 13. Krishan, A. and Sauerteig, A. (1992) Flow cytometric monitoring of cellular resistance to cancer chemotherapy, in Flow Cytometry: Principles and Clinical Application (Bauer, K.D., Duque, R.E., and Shankey, T.V., eds.), Williams and Wilkins, New York, pp. 459–467. 14. Landon, T. M. (1997) Multidrug resistance assays, in Bioprobes, (Landon, T. M., ed.), Molecular Probes, Eugene, OR, pp. 21, 22, 25. 15. Muller, I., Jenner, A., Bruchelt, G., Niethammer, D., and Halliwell, B. (1997) Effect of concentration on the cytotoxic mechanism of doxorubicin—apoptosis and oxidative DNA damage. Biochem. Biophys. Res. Commun. 230, 254–257. 16. Soule, H. D., Vazguez, J., Long, A., Albert, S., and Brennan, M. (1973) A human cell line from a pleural effusion derived from a breast carcinoma. J. Natl. Cancer Inst. 51, 1409–1416. 17. Batist, G., Tulpule, A., Sinha, B. K., Katki, A. G., Myers, C. E., and Cowan, K. H. (1986) Overexpression of a novel anionic glutathione transferase in multidrugresistant human breast cancer cells. J. Biol. Chem. 261, 15544–15549. 18. Woltmann, G., Ward, R. J., Symon, F. A., Rew, D. A., Pavord, I., and Wardlaw, A. J. (1999) Objective quantitative analysis of eosinophils and bronchial epithelial cells in induced sputum by laser scanning cytometry. Thorax 54, 124–130.
10 Laser Capture Microdissection Virginia Espina, John Milia, Glendon Wu, Stacy Cowherd, and Lance A. Liotta Summary Laser capture microdissection (LCM) is a technique for isolating pure cell populations from a heterogeneous tissue section or cytological preparation via direct visualization of the cells. This technique is applicable to molecular profiling of diseased and disease-free tissue, permitting correlation of cellular molecular signatures with specific cell populations. DNA, RNA, or protein analysis can be performed with the microdissected tissue by any method with adequate sensitivity. The principle components of LCM technology are (1) visualization of the cells of interest via microscopy, (2) transfer of laser energy to a thermolabile polymer with formation of a polymer–cell composite, and (3) removal of the cells of interest from the heterogeneous tissue section. LCM is compatible with a variety of tissue types, cellular staining methods, and tissuepreservation protocols that allow microdissection of fresh or archival specimens. LCM platforms are available as a manual system (PixCell; Arcturus Bioscience) or as an automated system (AutoPix™). Key Words: Cancer; DNA; laser capture microdissection; molecular profiling; proteomics; protein; RNA; tissue heterogeneity.
1. Introduction Laser capture microdissection (LCM) is a technique for isolating pure cell populations from a heterogeneous tissue section or cytological preparation via direct visualization of the cells. A common problem encountered by genomic and proteomic researchers in the analysis of tissue arises from the heterogeneous nature of the tissue. Molecular profiling of a pure cell population, which is reflective of the cell population's in vivo genomic and proteomic state, is essential for correlating molecular signatures in diseased and disease-free cells (1–4). Direct microscopic visualization of the cells permits the selection of normal, premalignant, and malignant cells or disease and disease-free cells as
Espina et al.
Fig. 1. Principles of LCM. A thermolabile polymer supported on an optical-quality plastic cap is positioned directly above the surface of a tissue section that is mounted on a glass microscope slide. A stationary, near-infrared laser is pulsed in the vicinity of the cells of interest. The polymer melts, or wets, in the vicinity of the laser pulse, forming a polymer–cell composite. Removal of the cap, away from the tissue surface, results in microdissection of the desired cells.
distinct cell populations from the heterogeneous tissue. Heterogeneous tissue might confuse molecular analysis because it is currently impossible to discern which cells contribute which cellular constituents to a given tissue lysate. LCM enables researchers to isolate specific cells of interest, without contamination from surrounding cells (5–8). Laser capture microdissection instruments as developed at the National Institutes of Health exist in manual and automated (robotic) platforms (Arcturus Bioscience, Inc., Mountain View, CA) (9,10). The manual system, PixCell, and the automated system, AutoPix™ utilize identical principles for microdissection (see Fig. 1). The primary components of LCM technology are (1) visualization of the cells of interest via microscopy, (2) transfer of laser energy to a thermolabile polymer with formation of a polymer–cell composite, and (3) removal of the cells of interest from the heterogeneous tissue section. A stationary near-infrared laser mounted in the optical axis of the microscope stage is used for melting, or wetting, a thermolabile polymer film (see Note 1). The polymer film is manufactured on the bottom surface of an optical-quality plastic support or cap. The cap acts as an optic for focusing the laser in the same plane as the tissue section. The polymer melts only in the vicinity
of the laser pulse, forming a polymer–cell composite. A dye incorporated into the polymer serves two purposes: (1) It absorbs laser energy, preventing damage to the cellular constituents, and (2) It aids in visualizing areas of melted polymer. Removal of the polymer from the tissue surface shears the embedded cells of interest away from the heterogeneous tissue section. Extraction buffers applied to the polymer film solubilize the cells, liberating the molecules of interest. The microdissected cells can be analyzed for DNA, RNA, or protein by any method with appropriate sensitivity (6–9,11–15). Protein analysis of microdissected frozen-tissue sections will be used to illustrate LCM protocols. 2. Materials 2.1. Preparation of Tissue Sections or Cytospin Preps 1. Uncoated, precleaned glass microscope slides, 25 × 75 mm (A. Daigger & Co., Wheeling, IL). 2. Specimen for protein analysis of microdissected tissue: cytospin preps or frozen–tissue sections cut at 2–15 mm (see Note 2). 3. Specimen for DNA or RNA analysis of microdissected tissue: cytospin preps, frozen-tissue sections, ethanol- or formalin-fixed paraffin-embedded tissue sections cut at 2–15 mm (5–8 mm is optimal) (see Note 2). 4. Cryopreservation solution (OCT) (Sakura Finetek Corp., Torrance, CA).
2.2. H&E Staining of Tissue Sections 1. Mayer's Hematoxylin Solution (Sigma Diagnostics, St. Louis, MO). Hematoxylin is an inhalation and contact hazard. Wear gloves when handling. 2. Eosin Y Solution, alcoholic (Sigma Diagnostics). Eosin Y is flammable. Store away from heat, sparks, and open flames. Contact hazard; wear gloves when handling. 3. Scott's Tap Water Substitute Blueing Solution (Fisher Scientific, Pittsburgh, PA). 4. Ethanol gradient: 70% (v/v in purified H2O), 95% and 100% ethanol. Prepare fresh ethanol solutions weekly, or sooner if staining more than 20 slides/wk or if the ambient humidity is greater than 40%. 5. Ethyl alcohol, absolute, 200 proof for molecular biology (Sigma-Aldrich, Milwaukee, WI). Ethanol is flammable. Store away from heat, sparks, and open flames. Do not ingest. Contact hazard; wear gloves when handling. 6. Purified water (Type I reagent-grade water). 7. Xylene (Mallinckrodt Baker, Inc., Phillipsburg, NJ). Xylene vapor is harmful or fatal; use with appropriate ventilation and discard in appropriate hazardous waste container. Xylene is flammable; store and use away from heat, sparks, and open flame. Contact hazard; wear gloves when handling. 8. Protease inhibitors (Complete Protease Inhibitor Cocktail Tablets; Roche, Mannheim, Germany).
2.3. Laser Capture Microdissection 1. PixCell II, PixCell IIe, or AutoPix laser capture microdissection system (Arcturus Engineering, Mountain View, CA). 2. CapSure™ Macro LCM Caps (Arcturus Bioscience) (see Note 3). 3. PrepStrip™ Tissue Preparation Strip (Arcturus Bioscience). 4. CapSure™ Cleanup pad (Arcturus Bioscience). 5. 500-µL Microcentrifuge tubes: Safe-Lock Eppendorf tubes (cat. no. 22 36 361-1; Brinkmann Instruments Inc., Westbury, NY) or MicroAmp™ 500 µL Thinwalled PCR Reaction Tubes (cat. no. 9N801-0611; Applied Biosystems, Foster City, CA). 6. Extraction buffer for cellular constituent of interest.
3. Methods The following protocols illustrate (1) frozen-section sample preparation, (2) hematoxylin and eosin (H&E) tissue staining, (3) manual LCM for protein analysis, and (4) automated LCM. Alternative tissue preparation methods, such as ethanol or formalin fixation with paraffin embedding, are acceptable for DNA analysis of microdissected tissue (16). 3.1. Frozen-Tissue Sectioning Frozen surgical biopsy material should be embedded directly in a cryopreservative solution or liquid nitrogen as soon as the specimen is procured. Prompt preservation of the sample limits protein degradation as a result of protease activity. The samples should be cut to 2–15 mm thickness (5–8 mm is optimal) on plain, uncharged, precleaned glass microscope slides. Position the tissue section near the center of the slide, avoiding the top and bottom third of the slide (see Notes 2, 4, and 5). Do not allow the tissue section to dry on the slide. Place the slide directly on dry ice or keep the slide in the cryostat at –20ºC or colder until the slides can be stored at –80ºC (see Note 6). 3.2. H&E Staining Classic tissue-staining protocols allow visualization of the tissue or cells of interest with a standard inverted light microscope. Fixation of the tissue in 70% ethanol is followed by staining of the cellular constituents, with final dehydration in an ethanol gradient. Incorporation of protease inhibitors in the staining reagents, along with a microdissection session limited to 1 h, minimizes protein degradation during the staining process (17) (see Note 7). Most cellular staining protocols are compatible with LCM (see Note 8). Complete dehydration of the tissue is necessary for minimizing the upward adhesive forces between the tissue section and the slide (see Note 9). Fluorescent stains
are compatible with fluorescence-equipped PixCell systems and all AutoPix systems (18) (see Note 10). 3.2.1. H&E Staining for Frozen Tissue Sections 1. Remove slide from freezer and place on dry ice or directly into the 70% ethanol fixative bath. 2. Dip, or gently shake, the slide in each of the following solutions, for the time indicated. Blot the slide on absorbent paper in between each solution, preventing carryover from the previous solution. 3. 70% Ethanol fixative: 3–10 s. 4. Distilled (d)H2O: 10 s. 5. Mayer's hematoxylin: 15 s. 6. dH2O: 10 s. 7. Scott's tap water substitute: 10 s. 8. 70% Ethanol: 10 s. 9. Eosin Y (optional): 3–10 s. 10. 95% Ethanol: 10 s. 11. 95% Ethanol: 10 s. 12. 100% Ethanol: 30 s to 1 min. 13. 100% Ethanol: 30 s to 1 min. 14. Xylene: 30 s to 1 min. 15. Xylene: 30 s to 1 min.
3.2.2. H&E Staining for Formalin-Fixed Paraffin-Embedded Tissue Sections Paraffin-embedded tissue sections must be deparaffinized prior to staining. Xylene acts as a solvent, removing the paraffin. Rehydration of the slide allows staining of the tissue elements. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Xylene: 5 min. Xylene: 5 min. 100% Ethanol: 30 s. 95% Ethanol: 30 s. 70% Ethanol: 30 s. dH2O: 10 s. Mayer's hematoxylin: 15 s. dH2O: 10 s. Scott's tap water substitute: 10 s. 70% Ethanol: 10 s. Eosin Y (optional): 3–10 s. 95% Ethanol: 10 s. 95% Ethanol: 10 s. 100% Ethanol: 30 s to 1 min. 100% Ethanol: 30 s to 1 min.
Fig. 2. Comparison of index-matched and non-index-matched tissue images. (A) Fluid, such as xylene, on the surface of the tissue acts as a refraction medium, providing an index-matched image of the tissue. As the slide dries (B), the refractive index of the fluid is no longer present. The dry-slide image appears as shades of gray. This issue is overcome on the PixCell system by digitally saving the index-matched images of the tissue, or cells of interest, and using these images as a guide or map for microdissection. 16. Xylene: 30 s to 1 min. 17. Xylene: 30 s to 1 min.
3.3. Manual Laser Capture Microdissection (PixCell System) There is no warm-up period required for the PixCell II/IIe. The instrument is ready for operation after turning on the power. Visualization of cellular morphological features and the tissue, in general, are achievable with image magnifications up to ×40 via optical and color digital imaging. Cover slips and mounting media are not compatible with microdissection. In addition, a lack of immersion fluids on any of the optics prevents refraction of light from the tissue image. Thus, the color and detail of a given tissue stain is lost as the stained slide dries (see Fig. 2). Manual LCM methods capitalize on the index refraction of a wet
tissue slide for visualizing and reviewing an index-matched image of the tissue (19,20). An index-matched image or images can be digitally saved and used as a guide, or scout, to locate the cells of interest after the stained slide dries. 3.3.1. Slide Preparation Allow the stained slide to air-dry. Dust or debris on the slide can be removed by blotting the dried slide with a PrepStrip sample preparation strip. 3.3.2. PixCell Instrument Procedure for Microdissection 1. Load the CapSure cassette module with a CapSure cartridge. a. Remove the CapSure cassette module from the platform. b. Press in the locking pins on each end to hold the cassette in the load position. c. Slide a CapSure cartridge into the cassette until it stops. Two cartridges can be loaded onto the cassette module. d. After the cartridges are loaded, pull the locking pins out to lock the cartridges in place. Load the cassette module onto the PixCell II/IIe. 2. Access the LCM software program by double-clicking on the Arcturus software icon. 3. Enter your user name or select a name from the list. Click on "Acquire data." 4. Enter a study name or select a study name from the list. Click on "Select." 5. Enter the Slide # and Cap Lot #. If desired, notes concerning the slide or study can be entered as "Notes." 6. Click the checkbox for "Stamp images with name, date & time" if this information is to be imprinted on the images created during LCM. Click "Continue." 7. Move the joystick into the vertical position to ensure proper positioning of the cap in relation to the capture zone. 8. Place the microscope slide containing the prepared and stained specimen for microdissection on the stage. 9. Locate the cells of interest using either the oculars or the monitor. After the target area for dissection is in the viewing area, with the joystick still in the vertical position, press the "Vacuum" switch on the front of the Controller to activate the vacuum and hold the slide in place during microdissection. 10. The Live Video and PC screen displays the current image on the microscope. The images can be saved as you work by selecting the appropriate icon from the toolbar (see Note 11). Map image: low/high-power objective image of the general area or specific cells to be microdissected (see Fig. 2); Before image: intact tissue prior to microdissection; After image: tissue after microdissection; Cap image: microdissected tissue only. 11. Slide the CapSure cassette backward or forward so that a cap is sitting at the "Load" position. Swing the placement arm over the cap. While placing one hand over the counterbalance to prevent jarring of the cap and improper seating, lift the placement arm and place the cap onto the slide. This is the transfer position.
Fig. 3. Laser spot focus. A focused laser spot is essential for efficient microdissection. A focused laser appears as a bright, well-defined circle without a halo or corona. Unfocused laser spots appear as blurry spots with halos or coronas around a center bright spot. AutoPix instruments exhibit a satellite spot near the principle laser spot. The satellite spot is an optical phenomenon because of second-surface reflection of the laser beam as it propagates through the optical system. The satellite spot is a byproduct of the optical system but is helpful for determining the proper focus of the primary laser spot. 12. Enable the laser by turning the key switch located on the front of the controller and then press the "LASER ENABLE" button. Pressing this button will activate the target beam when the placement arm is in the transfer position. 13. Verify that the laser is in focus. a. Select the small spot size (7.5 µm), using the "Spot Size Adjust" lever found on the left side of the microscope. b. Rotate the objectives of the microscope until the 10x objective is in use. c. Using the joystick, move the slide under the laser so that target beam is located in an area without tissue. d. Reduce the intensity of the light through the optics until the field viewed on the monitor is almost dark and the target beam is easily viewed. e. Using the "Laser Focus Adjust" located just below the size adjustment lever, adjust the target beam until the beam reaches the point of sharpest intensity and most concentrated light with little or no "haloing" or coronas (see Fig. 3). The laser should now be focused for any of the three laser sizes and objectives. Select the laser spot size suitable for the microdissection and cell size. (See Notes 12 and 13.) 14. Press the red pendant button to fire a test laser pulse. Observe the wetted polymer after the laser is fired. Firing the laser pulse causes the polymer to melt in the vicinity of the laser pulse. There should be a distinct clear circle surrounded by a dark ring (see Fig. 4) (see Note 14).
Fig. 4. Wetted polymer for effective microdissection. Adequate power, duration, proper tissue thickness, and cap placement on the tissue are parameters affecting the melting of the polymer. An ideal wetted polymer appears as a distinct dark ring with a clear center (arrows). The presence of a dark area in the center of the spot indicates that the power and/or duration settings are too high (leftmost spot). Inadequate power and duration result in failure of the laser to melt the polymer, creating a spot that appears as a gray, fuzzy circle (rightmost spot). Adequate power with inadequate duration might lead to spots with minute diameters, as shown by the spot second from the right. 15. Adjust the "Power" and "Duration" of the laser pulse with the up and down arrows on the front of the controller to obtain a melted polymer spot with a diameter similar in size to the selected laser spot size. Use the suggested ranges as a reference point. These settings can be adjusted to customize the melted polymer spot to the type and thickness of the tissue to be dissected. The suggested settings for the PixCell II system with a Macro cap are as follows: Spot size
Small, 7.5 µm Medium, 15 µm Large, 30 µm
45 mW 35 mW 25 mW
750 µs 1.5 ms 5.0 ms
16. Single-cell microdissection is possible by adjusting the power and duration settings such that a very narrow area of the polymer is melted with each laser pulse. Suggested settings for single-cell microdissection are approx 45 mW power and 650 µsec duration for PixCell II and Macro caps.
Fig. 5. Microdissected tissue embedded in the polymer cap. After microdissection, the polymer–cell composite can be visualized by placing the cap on the glass microscope slide in an area lacking tissue. The presence of stained cellular material inside the wetted margins of the polymer indicates effective microdissection. 17. To perform the microdissection, visually locate the cells of interest. Align the target beam directly over the cells of interest. Using the target beam to guide the dissection, press the pendant switch for single shots. For a rapid fire of pulses, press and hold the pendent switch. The laser pulse frequency interval can be adjusted on the controller by selecting "REPEAT" and then selecting the desired time between laser pulses. 18. After the desired number of cells has been collected on a cap, remove the cap by lifting it off the slide using the placement arm (see Note 15). Swing the placement arm away from the slide. 19. If desired, the microdissected material can be viewed by placing the cap on an area of the slide without tissue. Turn the vacuum off by depressing the "vacuum" button on the controller. Position the slide so the tissue is not in view on the monitor or through the oculars. Swing the placement arm, containing the cap, back onto the slide. Use the joystick to manipulate the cap above the objective. Observe the cap for microdissection of the desired cells and for debris and/or adhesion of nonspecific tissue to the polymer surface (see Fig. 5). 20. If viewing the cap reveals debris or nonspecific tissue adhesion, the said material can be removed by blotting the polymer surface with the CapSure Cleanup pad or an adhesive note. 21. Lift and rotate the cap arm until the cap is over the "cap removal site." Lower the arm and then rotate the arm back toward the slide. The cap will remain at the cap removal site. Blot the cap as described in step 20 if necessary.
22. Insert the polymer end of the cap into the top of a 500-µL microcentrifuge tube. The sample is now ready for extraction of the desired components or the cap–tube assembly can be stored for extraction at a later date. 23. After all dissections are completed, the PixCell II/IIe should be put in shutdown by first pressing the "Laser Enable" button to disable the laser. Turn off the power to the PixCell II/IIe, the controller, and the video monitor.
3.3.3. Saving Images Images saved during the microdissection can be saved on the computer hard drive. 1. Click the "Done" button on the image toolbar. Another slide can be microdissected, another study initiated, or the program terminated. 2. Click "Save Images" to save the images on the C drive of the PC. The images can be copied to a PC-formatted, 100-Mb zip disk as a .JPEG or .TIFF format after saving the image on the C drive. The AutoPix system is equipped with a CD-RW drive for file transfer and sharing. 3. Click on "Save Data." 4. Archived LCM images on the PixCell system are stored under the following file directory: C:\\LCMdata\user name\study name\date.
3.3.4. Storing Samples for Downstream Analysis Microdissected cells for protein analysis can be stored at –80ºC prior to extraction. Microdissected cells for DNA analysis can be stored desiccated at room temperature up to 1 wk prior to extraction. Samples for RNA analysis should not be stored prior to extraction. Condensation in the microcentrifuge tube during storage can be a potential source of RNAse contamination. 3.4. Automated Laser Capture Microdissection (AutoPix System) The AutoPix combines robotics and optical scanning software for automated microdissection of selected cells (see Note 16). The AutoPix incorporates imaging software for creating index-matched, stitched images of the tissue, permitting more accurate identification of cellular morphology during cell selection. The resolution of the stitched images is constant, but the area of the images changes with magnification, permitting precise areas of tissue to be annotated for microdissection. Annotation software coupled with the index-matched image permits singlepoint dissection, line dissection or, polygon dissection (see Fig. 6; Color Plate 10, following p. 274). Algorithm-based, cell image recognition software on the AutoPix platform enhances the LCM technology. The algorithm is based on texture, morphology, size, color, and contrast of the tissue, permitting automated cell selection in addition to automated microdissection (21).
Fig. 6. Annotation of stitched images on AutoPix system. Annotation software permits single-point, line, or polygon area selection for microdissection on the indexmatched stitched image. The ability to definitively select the cells of interest enhances the accuracy of microdissection. (See Color Plate 10, following p. 274.)
The AutoPix visualization system does not include oculars because of the enclosed system configuration. Instead, a PAL-format color camera permits visualization of the slide as a "roadmap image" for determining the target area of microdissection (21). The enhancements of the automated system include multiple-slide capacity (three slides), area quantitation of microdissected tissue, wetted polymer spot measurement, and cell recognition software. 4. Notes 1. The short laser pulse widths utilized, the low laser power levels required, the absorption of the laser pulse by the polymer and dye, and the long elapsed time between laser pulses combine to prevent the experimenter from depositing any
significant amount of heat at the tissue surface that might affect later laboratory analysis. The near-infrared laser diode has a maximum laser output of 100 mW. Optimal tissue thickness for microdissection is 5–8 µm. Tissue sections less than 5 µm might not provide a full cell thickness, necessitating microdissection of more cells for a given assay. Tissue sections thicker than 8 µm might not microdissect completely, leaving integral cellular components adhering to the slide. CapSure HS caps (Arcturus Bioscience) are often utilized in microdissection of tissue for RNA analysis. A 12-µm rail on the surface of the polymer prevents the polymer from touching the tissue except in the vicinity of the laser pulse. The HS caps are designed with an extraction device, allowing extraction buffer to contact the polymer within a centrally designated area. These features limit any potential RNA contamination from surrounding cells. CapSure HS caps can be used successfully for DNA or protein extraction. In contrast, CapSure Macro caps are placed in direct contact with the tissue and are not equipped with an extraction device. Any cellular material on the surface of the polymer of a Macro cap will be available for extraction. The inverted light microscope in the PixCell platform utilizes a vacuum to immobilize the slide on the microscope stage. The size of the objective opening limits the microdissectable area on the microscope slide to the middle third of the slide. In contrast, the AutoPix stage has a capacity for one to three slides and the microdissectable area of the slide is approximately 19 × 45 mm. Lung tissue or other tissue with a thin, open architecture can be cut on charged or silanized slides to prevent the tissue from nonspecifically adhering to the polymer during microdissection. In general, coated slides are not used for microdissection because of the increased adhesive forces between the tissue and the slide. Effective microdissection is a balance between three adhesive forces: (1) maximizing downward adhesive forces between the polymer and the tissue, (2) minimizing lateral adhesive forces between the cells, and (3) minimizing upward adhesive forces between the slide and the tissue. Avoid repeated temperature fluctuations of the cut frozen sections. Store the frozen-section slides at –80ºC until the time of microdissection. Repeated fluctuations in temperature might cause the tissue to adhere more tightly to the slide, limiting the effectiveness of microdissection. Protease and or phosphatase inhibitors can be added to compatible staining solutions (17). Complete Protease Inhibitor Cocktail tablets are water soluble. For protease inhibitor addition to the 70% ethanol solution, dissolve the tablet in 15 mL of dH2O, and then add 35 mL ethanol. Limiting the time from staining to completion of microdissection also ensures preservation of cellular constituents. Frozensection samples for RNA and protein analysis should be stained and microdissected within 1 hr. Examples of LCM compatible stains are H&E, methylene blue, Wright-Giemsa or toluidine blue. Eosin staining of the cytoplasm is not necessary for visualization of cells during microdissection. Minimal staining times, in whichever staining protocol is utilized, limit potential protein alterations as result of contact with
Espina et al. the staining reagents. Selection of tissue-staining protocols should be based on compatibility with the downstream analysis to be performed with the microdissected tissue. Prolonging the 100% ethanol dehydration steps of the staining protocol to a maximum of 5 min could enhance tissue dehydration, maximizing microdissection efficiency. Skin tissue, cartilage, and samples prepared on charged slides might be difficult to microdissect. Additional slide or tissue treatments such as incorporation of glycerol slide coatings or modified staining protocols with glycerol might be required (22). The following is one such approach for frozen-tissue sections as adapted from ref. 22. Stain the slide in Mayer's hematoxylin for 30 s. Rinse in dH2O for 15 s. Fix the slide in 70% and 95% ethanol solutions for 10 s each. Rinse in dH2O for 10 s. Place the slide in Scott's Tap Water (Blueing) solution for 15 s. Dehydrate the slide in 70% ethanol for 2 min. Soak the slide in 3% glycerol in phosphate-buffered saline for 5–10 min. Dehydrate in two solutions of 100% ethanol: first solution for 10 s and second solution for 1 min. Clear the slide in two changes of xylene or xylene substitute for 1 min each. Allow the slide to air-dry before proceeding to microdissection. PixCell instruments equipped with fluorescent modules incorporate mercury vapor lamps with blue, green, and red filter cubes. Additional filter cube positions are available for end-user modifications. Blue filter cubes use 455- to 495-nm excitation wavelengths, with emission greater than 510 nm. Green filter cubes use 503- to 547-nm excitation wavelengths, with emission greater than 565 nm. The red filter cube excitation wavelengths are 590–650 nm, with emission greater than 667 nm. These filter cubes can be used with immuno-LCM protocols (18). A drawback of the PixCell system is the inability to microdissect directly from an index-matched image of the tissue (see Fig. 2). Map images can be saved while the image is wet, providing a guide for microdissection (19,20). This is not an issue with the AutoPix platform. Cells for microdissection are selected via annotation software directly from an index-matched, stitched image (see Fig. 6). Index-matched images can be obtained with either system by rewetting the tissue with a drop of xylene prior to microdissection. It is imperative that the slide be completely dry prior to cap placement for microdissection because xylene dissolves the polymer. The laser should not be refocused when changing objectives or spot sizes. It is only necessary to focus the laser, with the small 7.5-µm spot size setting and the 10× objective, for each initial cap placement and any time the cap is repositioned on the tissue. Microdissection can be performed with any suitable laser spot size and a 4×, 10× or 20× objective. Satellite laser spots are a phenomenon noted on the AutoPix platform (see Fig. 3). The satellite laser spot is a second-surface reflection of the laser as the laser beam propagates through the optics. The laser is reflected from the back (second) surface of a coated optic. The coating is required to allow the laser to change direction in the optical path. Imperfections in the coating allow a portion of the laser beam to pass through the coating and reflect back to the viewer from the second
optical surface. The power of the satellite spot is too low to melt the polymer and does not interfere with efficient microdissection. The satellite spot is a byproduct of the light amplification system and was not designed as a system component, but the satellite spot is helpful for accurately focusing the primary laser spot. If the satellite spot is in focus, the primary laser spot will be in focus. The dark ring produced by pulsing the laser is a combination of migration of the dye and changes in the thickness of the polymer wall at the site of the laser pulse (see Fig. 4). These changes in the polymer permit visualization of the melted polymer. The black ring should be sharp in appearance with a clear center. This pattern indicates: (1) proper laser focusing, (2) adequate laser operation, and (3) acceptable performance of the CapSure polymer. A "fuzzy" ring could indicate improper focusing of the laser, uneven placement of the CapSure cap on the tissue, or inadequate power and/or duration of the laser pulse. The first step in troubleshooting a poorly wetted polymer spot is repositioning the cap on the tissue. Often, the cap is crooked or uneven in relation to the tissue. The second step in resolving poorly wetted polymer spots is refocusing the laser. The third action to correct poor polymer wetting is adjustment of the power and duration. Increase the laser power by approx 10 mW and the duration by 2.0 ms and fire another laser test pulse. Observe the wetted polymer for the appropriate appearance. If the above steps fail to resolve the problem, discard the cap and repeat the process with a fresh cap. Assuming an average epithelial cell diameter of 7 µm and a 30-µm laser spot size, the operator can expect to collect, on average, five to six cells per laser pulse. Using this information, it is possible to estimate the number of cells captured based on the number of laser pulses counted during microdissection. The number of laser pulses is automatically counted on the toolbar on both the PixCell and AutoPix systems. a. 30-µm laser spot size: Number of pulses × 5 = total cells captured. b. 15-µm laser spot size: Number of pulses × 3 = total cells captured. c. 7.5-µm laser spot size: Number of pulses × 1 = total cells captured. The AutoPix requires a warm-up period of 1 h prior to use if the instrument has been turned off. The instrument power can remain on for daily operation eliminating the need for a warm-up period. The AutoPix is outfitted with a xenon lamp for fluorescence visualization, with similar filter cube configurations as the PixCell LCM system. The AutoPix is equipped with a high-sensitivity black-and-white camera in addition to the color camera.
Acknowledgments The authors thank Don Armstrong, Ph.D., Arcturus Bioscience Inc., for critical review of the manuscript. They also thank Geoff Goodrich, Ashi Malekafzali, and Theresa Taylor for consistent, helpful advice.
References 1. Liotta, L. A. and Kohn, E. C. (2001) The microenvironment of the tumour–host interface. Nature 411, 375–379. 2. Liotta, L. A. and Kohn, E. C. (2002) Stromal therapy: the next step in ovarian cancer treatment. J. Natl. Cancer Inst. 94, 1113–1114. 3. Michener, C. M., Ardekani, A. M., Petricoin, E. F., 3rd, Liotta, L. A., and Kohn, E. C. (2002) Genomics and proteomics: application of novel technology to early detection and prevention of cancer. Cancer Detect. Prev. 26, 249–255. 4. Wulfkuhle, J. D., Sgroi, D. C., Krutzsch, H., et al. (2002) Proteomics of human breast ductal carcinoma in situ. Cancer Res. 62, 6740–6749. 5. Emmert-Buck, M. R., Strausberg, R. L., Krizman, D. B., et al. (2000) Molecular profiling of clinical tissue specimens: feasibility and applications. Am. J. Pathol. 156, 1109–1115. 6. Gillespie, J. W., Ahram, M., Best, C. J., et al. (2001) The role of tissue microdissection in cancer research. Cancer J. 7, 32–39. 7. Paweletz, C. P., Liotta, L. A., and Petricoin, E. F., 3rd. (2001) New technologies for biomarker analysis of prostate cancer progression: laser capture microdissection and tissue proteomics. Urology 57, 160–163. 8. Simone, N. L., Paweletz, C. P., Charboneau, L., Petricoin, E. F., 3rd, and Liotta, L. A. (2000) Laser capture microdissection: beyond functional genomics to proteomics. Mol. Diagn. 5, 301–307. 9. Bonner, R. F., Emmert-Buck, M., Cole, K., et al. (1997) Laser capture microdissection: molecular analysis of tissue. Science 278, 1481–1483. 10. Emmert-Buck, M. R., Bonner, R. F., Smith, P. D., et al. (1996) Laser capture microdissection. Science 274, 998–1001. 11. Chuaqui, R., Vargas, M. P., Castiglioni, T., et al. (1996) Detection of heterozygosity loss in microdissected fine needle aspiration specimens of breast carcinoma. Acta. Cytol. 40, 642–648. 12. Bichsel, V. E., Petricion, E. F., 3rd, and Liotta, L. A. (2000) The state of the art microdissection and its downstream applications. J. Mol. Med. 78, B20. 13. Miura, K., Bowman, E. D., Simon, R. (2002) Laser capture microdissection and microarray expression analysis of lung adenocarcinoma reveals tobacco smokingand prognosis-related molecular profiles. Cancer Res. 62, 3244–3250. 14. Ornstein, D. K., Englert, C., Gillespie, J. W., et al. (2000) Characterization of intracellular prostate-specific antigen from laser capture microdissected benign and malignant prostatic epithelium. Clin. Cancer Res. 6, 353–356. 15. DiFrancesco, L. M., Murthy, S. K., Luider, J., and Demetrick, D. J. (2000) Laser capture microdissection-guided fluorescence in situ hybridization and flow cytometric cell cycle analysis of purified nuclei from paraffin sections. Mod. Pathol. 13, 705–711. 16. Gillespie, J. W., Best, C. J., Bichsel, V. E., et al. (2002) Evaluation of non-formalin tissue fixation for molecular profiling studies. Am. J. Pathol. 160, 449–457. 17. Simone, N. L., Remaley, A. T., Charboneau, L., et al. (2000) Sensitive immunoassay of tissue cell proteins procured by laser capture microdissection. Am. J. Pathol. 156, 445–452.
18. Fend, F., Emmert-Buck, M. R., Chuaqui, R., et al. (1999) Immuno-LCM: laser capture microdissection of immunostained frozen sections for mRNA analysis. Am. J. Pathol. 154, 61–66. 19. Wong, M. H., Saam, J. R., Stappenbeck, T. S., Rexer, C. H., and Gordon, J. I. (2000) Genetic mosaic analysis based on Cre recombinase and navigated laser capture microdissection. Proc. Natl. Acad. Sci. USA 97, 12,601–12,606. 20. Mouledous, L., Hunt, S., Harcourt, R., Harry, J., Williams, K. L., and Gutstein, H. B. (2003) Navigated laser capture microdissection as an alternative to direct histological staining for proteomic analysis of brain samples. Proteomics 3, 610–615. 21. Arcturus Engineering (2002) User Guide AutoPix Automated Laser Capture Microdissection System, Arcturus Engineering, Mountain View, CA, p. 5. 22. Agar, N. S., Halliday, G. M., Barnetson, R. S., and Jones, A. M. (2003) A novel technique for the examination of skin biopsies by laser capture microdissection. J. Cutan. Pathol. 30, 265–270.
11 Analysis of Asbestos-Induced Gene Expression Changes in Bronchiolar Epithelial Cells Using Laser Capture Microdissection and Quantitative Reverse Transcriptase–Polymerase Chain Reaction Christopher B. Manning, Brooke T. Mossman, and Douglas J. Taatjes Summary Laser capture microdissection (LCM) enables the removal of discrete microstructures or cell types from properly prepared histological sections. Extraction of RNA from microdissected tissue followed by quantitative reverse transcriptase–polymerase chain (QRT-PCR) reaction permits the analysis of cell-type or microstructure-specific gene expression changes that occur in response to various stimuli in the environment. In our lab, the combination of LCM and QRTPCR has proven very useful in the determination of the in vivo gene expression changes that occur in bronchiolar epithelium in response to inhalation of crocidolite asbestos. A detailed description of the preparation of cDNA from bronchiolar epithelial cells obtained by LCM is described in this work. Key Words: Asbestos; bronchiolar epithelium; environmental pathology; laser capture microdissection; lung disease; microgenomics; quantitative reverse transcriptase–polymerase chain reaction.
1. Introduction Laser capture microdissection (LCM) is a relatively new and unique technique of increasing popularity that facilitates the identification of cell-type or microstructure-specific changes in gene expression by providing a means to selectively remove individual cells or small groups of cells from properly prepared tissue sections (1,2). The microdissection instrument that has been most widely used, the Arcturus PixCell II, relies on a polymer film that is placed over the tissue of interest. A short-duration pulse from an infrared laser is used to select the desired cells by melting the area of the film with which they are in
Manning, Mossman, and Taatjes
Fig. 1. Laser capture microdissection and RT–PCR. (A) Dehydrated tissue section and cap with polymer film on surface facing slide. (B) The cap is placed over the tissue to be microdissected. (C) An infrared laser pulse is directed at the film in contact with the cells of interest. (D) Removal of the cap after the film is melted also removes the microdissected cells. (E) The microdissected cells can be used for analysis of gene expression using RT-PCR.
contact. RNA can then be extracted from these cells for quantitation via quantitative reverse transcriptase–polymerase chain reaction (QRT-PCR) or qualitative assessment for the presence or absence of expression of a gene by gel-based RT-PCR. One of the challenges facing researchers interested in understanding the in vivo signaling mechanisms and changes in gene expression involved in the responses of the lung to environmental insults, such as asbestos, is its complex architecture. The lung is a highly heterogeneous organ with over 40 cell types. An important consequence of this heterogeneity is that the responses of individual cell types to pollutants are diluted in measurements of gene expression in relatively crude preparations such as whole-lung extracts. The combination of LCM and QRT-PCR (see Fig. 1) is particularly useful for measuring changes in expression of transcription factors and markers of proliferation and injury in lung epithelial cells following exposure to asbestos (3). In this chapter, the methodology for the isolation of DNA-free RNA from bronchiolar epithelial cells (see Fig. 2) and subsequent preparation of cDNA are described in detail (4–6). 2. Materials 2.1. Kits and Reagents 1. Arcturus PicoPure RNA isolation kit: Components include conditioning buffer (CB), extraction buffer (XB), 70% ethanol (EtOH), wash buffer 1 (W1), wash buffer 2 (W2), elution buffer (EB), and RNA purification columns. 2. Ethanol (ETOH) series dilutions (70% and 95% ETOH): Add 30 or 5 mL of autoclaved distilled-deionized water (ddH20) to prepare. 70 or 95 mL of 100% ETOH to 100 mL. 3. Arcturus CapSure LCM caps, CapSure pads, and prep strips (included with caps).
LCM of Lung Tissue
Fig. 2. Removal of a bronchiole from a lung section using laser capture microdissection: before microdissection (A), after microdissection (B), and the isolated bronchiole on the surface of the cap (C). 4. Promega RQ1 RNase-free DNase. 5. Promega Reverse Transcription system.
2.2. Equipment 1. A cryostat suitable for cutting 10-µm thick sections from tissue embedded in optical cutting temperature (OCT) at –20°C is required for generating sections that can be properly microdissected. 2. An Arcturus PixCell II laser capture microdissector is used to remove bronchiolar epithelium from prepared sections. 3. A dessicator, preferably with a vacuum pump, is needed to keep the prepared tissue sections dehydrated until use.
3. Methods 3.1. Specimen Preparation Following euthanasia by a combination of 5 mg sodium pentobarbital and pneumothorax, the lungs are inflated with calcium/magnesium-free phosphatebuffered saline (PBS) to a pressure of 30 cm H2O. After occluding the primary bronchi by tying off with surgical suture, one lung is immersed in a cryomold containing OCT embedding compound. The cryomold is then dipped into liquid-nitrogen-cooled isopentane until the appearance of the OCT changes from clear to opaque. Immediately remove the new specimen block and transfer to a –80°C freezer. 3.2. Slide Preparation 3.2.1. Sectioning Specimen OCT blocks should be equilibrated at –20°C for 20 min prior to sectioning. Sections should be cut at 10 µm and transferred within min to a –80°C freezer for storage or processed immediately for microdissection. The sections must be kept cold to minimize RNA degradation in the tissue of interest (see Notes 1 and 2).
3.2.2. Dehydration It is essential that tissue be completely dehydrated for efficient transfer of cells from the tissue section to the polymer film on the CapSure caps. The following procedure should result in tissue ready for microdissection (typically done in groups of 5–10 slides with Coplin jars): 1. 2. 3. 4. 5. 6. 7. 8.
Immerse in cold 70% ETOH (chilled to –20°C) for 60 s. Immerse in ddH20 for 30 s. Immerse in 70% ETOH for 60 s. Immerse in 95% ETOH for 60 s. Immerse in 100% ETOH for 2 min. Immerse in xylene twice for 10 min each. After removing sections from xylene, allow them to dry for 5 min in a fume hood. Keep slides in a vacuum dessicator until use (for no more than 12 h).
3.3. Microdissection 1. Prior to microdissection, weakly adhering tissue should be removed from the slide using the prep strips included with the CapSure caps. This is accomplished by peeling off the backing of the prep strip and pressing it onto the section. Next, apply pressure on the strip with two fingers and move along the strip in one direction, taking care not to apply so much force as to damage the tissue on the slide. 2. For bronchiolar epithelium, a 7.5-µm laser spot size is best to achieve capture. The power and duration of laser pulses should be adjusted as necessary to get good capture of the desired cells. To test if the settings are correct, position the laser over the lumen of a bronchiole and pulse the laser. If the settings are correct, the laser will leave a dark ring with a clear center in the film. 3. After collecting the cells of interest onto a cap, gently roll the film surface of the cap over the exposed surface of a CapSure pad. This will remove tissue that was not selected but adhered to the cap nonspecifically. 4. Immediately snap the cap into the opening of a 0.5-mL RNase-free Eppendorf tube that contains 50 µL of extraction buffer. Invert the tube and, in that position, incubate at 42°C for 30 min. 5. Centrifuge at 800g for 2 min to pull the extraction buffer to the bottom of the tube. Pull off the cap and close the lid to the tube. Immediately snap-freeze the extract using dry ice and place in a freezer at –80°C for storage. If desired, the rest of the isolation can be performed immediately. 6. Apply 250 µL of conditioning buffer to the surface of the filter in the purification columns. Allow the column to equilibrate for 5 min at room temperature. After the column is equilibrated, centrifuge at 16,000g for 1 min. 7. Thaw the samples and pipet 50 µL of 70% ethanol (from the kit) to each sample. Mix well using the pipet. 8. Add the mixture of ethanol and extract to the purification column and centrifuge for 2 min at 100g, followed by 30 s at 16,000g.
Add 100 µL of W1 to each column and centrifuge at 8000g for 1 min. Add 100 µL of W2 to each column and centrifuge at 8000g for 1 min. Add another 100 µL of W2 to each column and centrifuge at 16,000g for 1 min. Remove all filtrate from the tube and replace the purification filter. Centrifuge at 16,000g for 1 min to ensure that the filter is dry. 13. Transfer the purification filter to a new tube (from the kit) and elute by applying 24 µL of elution buffer to the filter, allowing equilibration for 1 min, and centrifuging for 1 min at 16,000g. 9. 10. 11. 12.
3.4. DNase Treatment 1. 2. 3. 4. 5.
Add 3 µL of Promega RQ1 DNase 10X reaction buffer to each sample. Add 3 µL of RQ1 RNase-Free DNase to each sample. Incubate at 37°C for 30 min. Add 3 µL of RQ1 DNase stop solution to each sample. Incubate at 65°C for 10 min.
3.5. Reverse Transcription To each sample add the following reagents from the Promega Reverse Transcription kit and follow steps 6–13: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
12 µL of 25 mM MgCl2. 6 µL of 10X reverse transcription buffer. 6 µL of 10 mM dNTP mixture. 1.5 µL of Rnasin. 3 µL of random hexamers. Mix by pipetting. Remove 10 µL from each sample to serve as that sample's "no reverse transcription" (NRT) control. Add 45 U AMV reverse transcriptase to each sample and 1 µL of nuclease-free water to each NRT control. Incubate all tubes at room temperature for 10 min. Incubate all tubes at 42°C for 1 h. Incubate all tubes at 95°C for 5 min. Cool tubes on ice for 5 min. Freeze tubes at –20°C. The cDNA is now ready for analysis using either Q-PCR or gel-based PCR.
4. Notes 1. Sectioning should be performed as quickly as possible to preserve the integrity of the RNA within specimen blocks. 2. As with any technique involving RNA, special care must be employed to prevent RNase contamination. Gloves should be changed regularly. Benchtops should be cleaned with an RNase-destroyed agent such as RnaseZap from Ambion. The surfaces of the cryostat should also be cleaned prior to sectioning. RNase-free filter tips for pipets and RNase-free tubes are also essential.
References 1. Emmert-Buck, M. R., Bonner, R. F., Smith, P. D., et al. (1996) Laser capture microdissection. Science 274, 998–1001. 2. Taatjes, D. J., Palmer, C. J., Pantano, C., Hoffmann, S.B., Cummins, A., and Mossman, B.T. (2001) Laser-based microscopic approaches: application to cell signaling in environmental lung disease. Biotechniques 31, 880–894. 3. Manning, C. B., Cummins, A. B., Jung, M. W., et al. (2002) A mutant epidermal growth factor receptor targeted to lung epithelium inhibits asbestos-induced proliferation and proto-oncogene expression. Cancer Res. 62, 4169–4175. 4. PicoPure RNA Isolation Kit User Guide Version B, Arcturus Engineering, Mountain View, CA. 5. RQ1 RNase-Free DNase Technical Bulletin TB518, Promega Corporation. 6. Reverse Transcription System Technical Bulletin TB099, Promega Corporation.
12 New Approaches to Fluorescence In Situ Hybridization Sabita K. Murthy and Douglas J. Demetrick Summary Fluorescence in situ hybridization (FISH) is a nonisotopic labeling and detection method that provides a direct way to determine the relative location or copy number of specific DNA sequences in nuclei or chromosomes. With recent advancements, this technique has found increased application in a number of research areas, including cytogenetics, prenatal diagnosis, cancer research and diagnosis, nuclear organization, gene loss and/or amplification, and gene mapping. The availability of different types of probe and the increasing number of FISH techniques has made it a widespread and diversely applied technology. Multicolor karyotyping by multicolor FISH and spectral karyotyping interphase FISH and comparative genomic hybridization allow genetic analysis of previously intractable targets. We present a brief overview of FISH technology and describe in detail methods of probe labeling and detection for different types of tissue sample, including microdissected nuclei from formalin-fixed paraffin-embedded tissue sections. Key Words: Fluorescence in situ hybridization (FISH); interphase FISH; laser capture microdissection (LCM); LCM-FISH.
1. Introduction Fluorescence in situ hybridization (FISH) is a powerful molecular–cytogenetic detection technique that utilizes a fluorescent-labeled DNA probe, which is hybridized to a genomic target in the nuclei of fixed cells, to ascertain the presence or absence of a particular DNA sequence. Hybridization to the target loci is visualized by the detection of fluorescent signals on metaphase chromosomes or interphase nuclei (see Fig. 1; Color Plate 11, following p. 274). Using appropriate probes, aberrations such as chromosomal aneuploidy, deletions, duplications, and translocations can be detected on a cell-to-cell basis in metaphase chromosome preparations or in nondividing interphase nuclei. Since the first demonstration of this fluorescence labeling technique (1–3), FISH has developed very rapidly to be a valuable tool in basic cytogenetic research and diagnosis. Advances in probe generation, hybridization technology, fluorescence From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Murthy and Demetrick
Fig. 1. Metaphase analysis with multiple single-gene probes. Multicolor FISH showing simultaneous localization of three human genomic BAC probes, CDKN2c (green), RB1 (red), and CCND1 (yellow), on a normal human metaphase spread. (See Color Plate 11, following p. 274.)
microscopy, and digital and spectral imaging have led to the rapid and improved development of molecular cytogenetic approaches that allow identification of chromosomal aberrations with unprecedented accuracy. Several reviews are available that emphasize the role of FISH in biological research as well as in genetic diagnostic applications (4–9). 1.1. FISH Probes 1.1.1. Centromeric Probes Specific or restricted specificity alpha satellite probes are used to identify repeat sequence targets in the centromeric region of each chromosome. Chromosome-specific centromeric probes are now readily available commercially, enabling us to identify individual chromosomes and to analyze numerical chromosomal abnormalities in interphase and metaphase preparations.
FISH Technology
1.1.2. Gene-Specific Probes Locus-specific or unique probes are cosmid (with insert of about 40 kb) and BAC, PAC, or P1 (with inserts of about 125 kb) genomic clones and are used for gene mapping, deletion, and amplification analysis, as well as analysis of chromosome rearrangements such as duplication and translocation. These probes have wide application in genetic diagnosis such as identification of microdeletions, deletion of tumor suppressor genes (10), amplification of oncogenes (11,12), and the demonstration of fusion genes involved in the various translocations in cancer (13–15). 1.1.3. Whole-Chromosome Paints Whole-chromosome painting probes are polymerase chain reaction (PCR)generated from flow-sorted genomic DNA libraries that are used to identify individual chromosomes or chromosome segments (16,17). Chromosome painting probes can also be generated from PCR-amplified DNA of microdissected chromosome segments or markers of unknown origin (18,19). These chromosome-specific probes are either labeled with a single fluorochrome or with specific combinations of two or more fluorochromes to generate increasing numbers of discernable targets beyond the number of fluorochromes that are available. Using combinatorial labeling with 5 different fluorochromes, up to 31 different targets can be distinguished (20,21). 1.1.4. Telomere Probes Cytogenetic analysis of chromosome ends is often challenging, as most of the chromosome ends are G-band negative and, thus, lightly stained. Cryptic rearrangements involving these regions are often difficult to detect by routine cytogenetic banding methods. Submicroscopic deletions or rearrangements of the telomeres have deleterious consequences leading to genomic instability and cancer. FISH probes specific to unique telomere sequences are now available commercially, which enable the identification of individual telomeres and their rearrangements. Also, peptide nucleic acid probes can be artificially generated and used instead of natural nucleic acid probes (22). 1.2. Applications of FISH 1.2.1. Multicolor FISH of Metaphase Preparations (M-FISH, SKY, COBRA-FISH, Rx-FISH, mBAND-FISH) Conventional cytogenetic analysis provides essential information of diagnostic and prognostic importance in patients with genetic abnormalities. However, it has major limitations with respect to the detection of subtle or cryptic chromosomal aberrations and the analysis of highly rearranged chromosomes,
particularly in poorly spread and/or contracted metaphase preparations. Complex chromosomal rearrangements (CCRs) involving whole chromosomes or parts of chromosomes are common in cancer cells, where it is often impossible to identify the chromosomes or chromosome bands that are involved in the rearrangement. New molecular cytogenetic approaches such as multicolor FISH (M-FISH), spectral karyotyping (SKY), combined binary ratio labeling FISH (COBRA-FISH), and multicolor banding (m-BAND FISH) (21,23–27) enable improved characterization of these CCRs. 1.2.1.1. SPECTRAL KARYOTYPING
Spectral karyotyping is a multicolor FISH technique that allows one to simultaneously visualize all human chromosomes represented in different colors by the computer. This facilitates karyotype analysis, particularly for the identification of ambiguous and hidden chromosomal abnormalities. Applications of SKY to screening genomes for chromosomal aberrations in human disease and cancer are numerous. It is useful in mapping chromosomal break points, detecting subtle translocations, identifying marker chromosomes, homogeneously stained regions, and double minutes, and characterizing CCRs (21,24). Although SKY enables the identification of individual chromosomes and small chromosome segments, it is usually limited to the analysis of chromosome translocations without further differentiation of chromosomal subregion. Subregional human genomic probes "painting" several colored chromosome bands have been established by microdissection (19) and "reverse painting" (18). However, these probes cannot identify intrachromosomal rearrangements such as inversions. Subchromosomal paints for human chromosomes derived by reverse painting of DNA from primates such as gibbons, which have very close homology, are found to be useful in delineating intrachromosomal rearrangements (28). 1.2.1.2. COMPARATIVE GENOMIC HYBRIDIZATION
Comparative genomic hybridization (CGH), yet another specialized method of FISH application, is widely used in cancer research and diagnosis (12,29–31). CGH utilizes the hybridization of differentially labeled tumor and reference DNA to normal metaphase chromosomes to generate a map of DNA copy number changes in tumor genomes in a single hybridization experiment. It has been used to complement immunohistochemical, DNA content measurement, and histomorphology to establish a phenotype/genotype correlation in solid tumor progression (32–37). CGH has now become a routine approach and is widely accepted in cancer research. It is the method of choice for obtaining an overview of genetic alterations in cancer cells, especially in solid tumors, where highly abnormal chromosomes and complex karyotypes are common,
but good metaphase preparations are difficult to obtain (37,38). CGH can also be applied to archival paraffin material for retrospective studies (39–41), and because of the ability to amplify and subsequently label small amounts of DNA by degenerate oligonucleotide primer PCR (DOP-PCR) (42), microdissected or laser capture microdissected material can also be used (43–46). 1.2.1.3. TELOMERE AND TELOMERE LENGTH MEASUREMENT (TEL-FISH, Q-FISH, FLOW-FISH)
Cryptic chromosome rearrangements that are otherwise sometimes missed by routine cytogenetic analysis can be easily identified by chromosome-specific telomeric probes (tel-FISH), which are available commercially. Telomeres containing noncoding DNA repeats at the end of the chromosomes are essential for chromosomal stability and are implicated in regulating the replication and senescence of cells. The gradual loss of telomere repeats in cells has been linked to aging and tumor development. The most important indicator of correct telomere function is telomere length maintenance within the range typical for each species. Quantitative fluorescence in situ hybridization (Q-FISH) provides an estimate of telomere length in each individual chromosome with a resolution of 200 basepairs (47). A variation of Q-FISH is the flow-FISH technique, in which a fluorescein isothiocyanate (FITC)-labeled telomere-specific peptide nucleic acid probe is hybridized in a quantitative way to telomere repeats, followed by telomere fluorescence measurements on individual cells by flow cytometry (48,49). It offers the advantage of looking at telomere fluorescence in different subpopulations of cells. 1.2.2. Interphase FISH One of the theoretical advantages of FISH over conventional cytogenetics is the ability to identify small genetic alterations in both dividing and nondividing cells. Interphase FISH has opened up a wide range of applications for clinical diagnosis. It can be used to define chromosomal numerical abnormalities, chromosome rearrangements involving amplification, deletion or translocation of specific chromosome regions, identification of marker chromosomes (50), and copy number of individual genes. Interphase FISH is of special interest in cancer diagnosis. Cancer tissues, particularly solid tumors, frequently fail to grow and divide in tissue culture and often have a very poor mitotic index and poor-quality metaphase chromosomes for cytogenetic analysis. Interphase FISH allows cytogenetic diagnosis from these specimens that cannot be used in conventional dye-banding analyses. Furthermore, translocation break-point analysis (51) and gene copy number analysis of cancer specimens (52), using single-gene probes, have proven to be useful predictors of treatment response and this area of molecular diagnosis is rapidly growing.
Unlike conventional blood or other specimens submitted for conventional genetic analysis, cancer specimens are usually contaminated with large numbers of normal cells which could lead to false-negative results in a test evaluating the presence of a particular cancer abnormality. 1.2.2.1. FISH
PARAFFIN-EMBEDDED TISSUES
Fluorescence in situ hybridization of paraffin sections has been described (53), and commercial kits are currently available to perform such analyses. If the FISH target is highly amplified, one may be able to directly identify amplification within the cancer cells of paraffin sections. Background noise from autofluorescence in the tissue section, difficulty assessing an appropriate tissue plane in a high-power view of a thick section, or poor probe penetration, however, could seriously compromise interpretation of low-level amplification, translocation, or deletion (54,55). However, improved techniques for analysis of such fixed tissues by FISH have been reported (55). High-throughput genetic profiling of tumor tissues by FISH on tissue microarray is a very promising improvement to conventional paraffin section FISH (56–58), where a large number of tissue sections can be labeled and analyzed from one slide (see Fig. 2; Color Plate 12 following p. 274). This is very economical, time saving and very useful for retrospective studies of prognostic or predictive markers. Wholesale purification of nuclei from paraffin sections could decrease the effects of tissue autofluorescence, probe penetration, or specimen thickness on FISH signal visualization (59); however, the in situ verification of the source of the cells is lost when the tissue is destroyed during nuclear isolation procedures. Laser capture microdissection (LCM), a new technique to purify individual or groups of cells from paraffin-embedded tissue sections (60), was used to prepare nuclei from breast carcinoma cells for both FISH and flow cytometric analysis (61). This technique, LCM-FISH, allows identification of normal interphase copy numbers, permitting detection of low-level amplification or potentially single-copy deletions, which would be very difficult in paraffin section FISH (see Fig. 3; Color Plate 13, following p. 274). LCM-FISH was also very useful in evaluating deletion as a mechanism for loss of heterozygosity (LOH) by allowing comparison between microsatellite analysis and interphase FISH in microdissected breast cancer cells (62). In our experience, purification of nuclei from either whole or microdissected tissue results in much less background staining, likely because most of the cellular RNA is removed from the specimen. 1.2.2.2. INTERPHASE FISH AND PRENATAL DIAGNOSIS
Interphase FISH also has considerable application to prenatal diagnosis. Prenatal assessments of common aneuploidy can be determined rapidly by interphase FISH, using uncultured amniocyte cells. Noninvasive prenatal diagnosis is also becoming possible with the analysis of fetal cells in maternal circulation.
Fig. 2. FISH of tissue microarray breast carcinoma specimens. A tissue microarray block was prepared from 1.5-mm plugs of paraffin-embedded, formalin-fixed human breast cancer specimens. A 6-µm section was cut, deparaffinized, and subjected to FISH with a human erbB-2 genomic BAC probe (red). A hematoxylin/eosin figure (A) shows the location of the higher-magnification FISH image (B), which illustrates amplification of the erbB-2 gene. (See Color Plate 12, following p. 274.)
FISH allows rapid detection of specific chromosome abnormalities in uncultured amniotic fluid cells within 2 d of amniocentesis. The technique is typically used in pregnancies at high genetic risk for which a quick result is important in future
Fig. 3. FISH of LCM-prepared nuclei. Cells were harvested from 30-µm sections of hematoxylin-stained human breast carcinoma by LCM. The nuclei were purified using organic solvents, rehydration, and proteinase K digestion, as described previously (61), dropped onto clean glass slides, and fixed. The specimens were photographed under phase contrast with the "naked nuclei" indicated by arrows (A). The scale bar indicates 10 µm. The cells were subjected to hybridization with a genomic BAC probe for human erbB-2 (red) and a human centromeric probe for chromosome 17 (green) and show amplification of erbB-2, as in Fig. 2. (See Color Plate 13, following p. 274.)
decision making (63–66). Interphase FISH is also a very useful follow-up tool on investigations of uncultured amniotic fluid (AF) cells after finding an uncertain chromosome aberration in a first chorionvilli (CV sample) or AF sample (67–69). 1.2.3. Fiber FISH Direct visualization of fluorescent probes on extended DNA has been used in high-resolution gene mapping and identification of chromosome break points (70). Recent progress in enhancing the resolution and sensitivity of the FISH technique has further facilitated the physical map construction (71–73) of the human genome. For example, fiber-FISH has been used to prepare contigs of the human Y chromosome covering areas that were resistant to easy physical mapping (74). This technique is now only rarely used. 1.3. Microarray-Based CGH Utilizing the recent advancement of expression microarray technology, microarray-based CGH provides a means to quantitatively measure DNA copy number aberrations and to map them directly onto genomic sequences where genomic DNA is hybridized onto arrayed genomic clones (75,76). Ligationmediated PCR is an improved method of generating homogenous and highly reproducible amplified DNA from a single cell, with fragment size of 0.2–2 kb (77,78). It has been demonstrated by these authors that arrays generated from ligation-mediated PCR of BAC genomic DNAs provided precise measurements in cell lines and clinical material so that high-level amplification and singlecopy alterations could be reliably detected in diploid, polyploid, and heterogeneous backgrounds. An alternate technique, namely hybridization onto cDNA microarrays, offers significant advantages, most importantly the ability to identify amplified genes rather than amplified genomic regions and to perform expression studies on the same arrays using standard expression microarray approaches (79–81). Thousands of mapped cDNAs are readily available, which facilitate amplicon mapping and identification of new cancer genes (http://www.bcgsc.bc.ca/; http://www.nhgri.nih.gov/; http://www.ncbi.nlm.nih.gov/; http://www.cephb. fr/bio/ ceph-genethon-map.html). A powerful approach to defining putative amplification target genes might combine CGH with results from cDNA microarray analysis, followed by a quick survey of large numbers of uncultured tumors with tissue microarray technology to study the clinical significance of such newly discovered gene amplifications. Array-based CGH, in which fluorescence ratios at arrayed DNA elements provide a locus-by-locus measure of DNA copy number variation, represents another means of achieving increased mapping resolution. Whole genomes of organisms can be prepared using bacterial artificial chromosomes and arrayed onto glass for CGH with 1 Mb resolution, including human (82) and mouse (83). It is hypothesized that matrix-based
DNA or RNA hybridization techniques could potentially replace the need for chromosome preparation (58,75,79–81). Although CGH has the advantage of requiring only genomic DNA and has proven to be an important initial screening test for chromosomal gains and losses in tumor progression, array CGH cannot at present detect structural alterations such as isochromosome formation, double minutes, homogenously stained regions, clonal heterogeneity, and so forth. 2. Materials A number of labeled FISH probes are available commercially (e.g., Vysis, Cytocell, etc.) that are more often used for clinical diagnostic purposes. They are used directly for hybridization as per the manufacturer's instructions. Here, detail labeling and detection protocols for FISH, much of which was previously described by Trask (84) and have been subsequently modified (85). 2.1. Reagents 1. Nick translation labeling kit (Gibco, cat. no.18160-010). 2. For indirect labeling: Digoxygenin-11-dUTP, Fluorescein-12-dUTP or Biotin-16dUTP (Roche, cat. nos. 1-093-088, 1-427-857, and 1-093-070, respectively). 3. For direct labeling: Cy3-dCTP, Cy5-dCTP, or FluorX-dCTP (Amersham, cat. nos. PA53021, PA55021, and PA58021, respectively). 4. DNAse I (Gibco, cat. nos.18047-019). 5. Microspin G50 column (Pharmacia, cat. no. 27-5330). 6. Human DNA: human Cot-1, 250 µL (1 mg/mL; Gibco, cat. no. 15279-011) + 50 µL. 7. Human placental DNA (10 mg/mL; Sigma, cat. no. D4642); Mix and ethanol precipitate with 30 µL 3M sodium acetate, pH 5.2. Wash with 70% ethanol and resuspend the pellet in 50 µL TE (see Subheading 2.2., item 9). 8. Herring sperm DNA (10 mg/mL) (Gibco, cat. no. 15634-017). 9. Sheep anti-digoxygenin Fab fragments: 1/100 dilution (Roche, cat. no. 1214 667). 10. Cy3-conjugated donkey anti-sheep IgG (Jackson ImmunoResearch, cat. no. 713165-147). 11. Antifade (Molecular Probes, cat. no. S7461). 12. Proteinase K buffer: 50 mM Tris-HCl, 10 mM NaCl, 10 mM EDTA, pH 8.0. 13. Proteinase K (Invitrogen, cat. no. 25530-031). 14. 4′-6-Diamidino-2-phenyindole dilactate (DAPI) stain 1 mg/mL (Sigma, cat. no. D9564). 15. Tissue culture media: modified Eagle's medium (MEM) and RPMI-1640 (Gibco, cat. nos. 11090-081 and 18875-093, respectively). 16. Fetal bovine serum (FBS) (Gibco, cat. no.10437-028). 17. Antibiotic/antimycotic 100X: 10,000 U penicillin/10,000 µg streptomycin/25 µg/mL fungizone (Gibco, cat. no. 15240-062). 18. HEPES buffer, 1 M (Gibco, cat. no. 15630-080). 19. L-Glutamine: 200 mM (100X) (Gibco, cat. no. 25030-081).
20. Phytohemagglutinin (PHA-M form): Reconstituted in 10 mL sterile water (Gibco, cat. no. 10576-015). 21. KaryoMaxTM Colcemid solution: 10 µg/mL (Gibco, cat. no. 15210-016). 22. Actinomycin D: 250 µg/mL in PBS (Sigma, cat. no. A1410). 23. RNAse A (Sigma, cat. no. P8038): Prepare 20 mg/mL in TE and heat-inactivate in a boiling water bath for 20 min to remove DNAse activity. 24. Drierite (BDH, cat. no. B26998).
2.2. Solutions 1. Hybridization mix: 2 mL of 50% dextran sulfate, 1 mL of 20X SSC (pH 7.0), and 5 mL good quality formamide; pH with 1 M HCl to 7.00. 2. Denaturing solution: 70 mL formamide, 10 mL 20X SSC (see item 8), and 20 mL high-quality distilled water (pH to 7.0). 3. Formamide wash solution: 50 mL formamide, 10 mL 20X SSC, and 40 mL highquality distilled water (pH solution to 7.0). 4. PBS (phosphate-buffered saline) 10X: 80 g NaCl, 2 g KCl, 11.5 g Na2HPO4 • 7H2O, 2 g KH2PO4. Make up to 1 L with distilled water and autoclave. 5. Stop buffer: 0.5 M EDTA (pH 8.0). 6. McIlvaine's buffer pH 7.0: 0.63 g citric acid (anhydrous) and 6.19 g sodium dibasic phosphate (anhydrous) in 500 mL water. 7. Counterstain: 2 µL DAPI stain (1 mg/mL stock) (Sigma, cat. no. D9564) in 50 mL McIlvaine's buffer. 8. 20X SSC: 175 g NaCl and 88 g Na-citrate (anhydrous) in 1 L water. 9. Tris-EDTA (TE): 10 mM Tris-HCl, 1 mM EDTA. 10. Antibody blocking solution: 0.1 M sodium phosphate buffer, pH 8.0, 0.1% NP-40 detergent, 0.02% sodium azide, 5% nonfat dried milk. Mix and let it stand several days at 4°C. Centrifuge for 15 min at 200g to remove insoluble milk proteins. Store at 4°C. 11. Antibody wash solution: 2X SSC, pH 7.00, and 0.005% 3[(cholamidipropyl) dimethyl ammonio]-propane sulfonate (CHAPS) detergent (Calbiochem, cat. no. 220 201). 12. Trypsin-EDTA: 0.25% trypsin and 1 mM EDTA (Gibco, cat. no. 25200-056). 13. Cytogenetics fixative: 1 : 3 glacial acetic acid and anhydrous methanol. 14. Hypotonic solution: 0.075 M KCl solution, pH 7.0. 15. Carlsberg solution: 0.1% Sigma protease XXIV, 0.1 M Tris-HCl, 0.07 M NaCl, pH 7.2.
3. Methods In this section, we discuss the actual methodology for specimen preparation and the performance of FISH for the more common applications that can be performed in the average laboratory or institution. We will discuss metaphase preparations from synchronized fibroblasts and lymphocytes and interphase preparations from tissue specimens. We will then discuss basic probe labeling, hybridization of the labeled probe to denatured genomic DNA in the specimen, detection of the labeled probe, and visualization of the specimen showing in situ
hybridization. Finally, there will be a discussion of specific applications for performing FISH on nuclei isolated from formalin-fixed, paraffin-embedded whole-tissue sections, performing FISH on formalin-fixed, paraffin-embedded tissue sections or tissue microarrays, and, finally, FISH on nuclei from LCM specimens. 3.1 Specimen Preparation 3.1.1. Metaphase Preparation From Fibroblast Culture 1. Two to three days prior to performing the chromosome spreads, split the fibroblast cells 1 : 3 into new flasks with 10 mL MEM culture media and HEPES buffer, 20% FBS, and 1% penicillin/streptomycin and allow the cells to grow to 50–60% confluency. Feed the cells with fresh media 12–14 h prior to harvesting. 2. Add Colcemid to a final concentration of 0.02 µg/mL for 3–4 h. 3. Transfer the media and add 0.5 mL of trypsin-EDTA to detach the cells (1–2 min). Collect the cells in a tube and centrifuge at 400g for 10 min. To the gently vortexresuspended pellet, add 10 mL of warm (37°C) freshly made KCl solution (0.075 M) and incubate at 37°C for 30 min. 4. Add 4–5 drops of cold fresh Cytogenetics fixative to the cells in KCl (see Note 1) and centrifuge at 400g for 10 min. To the gently vortex-resuspended pellet, add 10 mL of cold fixative dropwise and with gentle vortexing (4°C) and centrifuge again for 10 min. Repeat this twice. The pellet must be fully resuspended prior to adding the fixative in order to minimize clumps and maximize yield. 5. Resuspend the final pellet in a smaller volume of fixative (0.5–1 mL depending on the size of the pellet). Drop 1–2 drops of this final suspension onto a clean slide and immediately place it on a warm plate (at 37°C) to dry. Warm the slides for 4 h and let them age at room temperature for 3–4 d (see Note 2). Better spreading might require dipping the slides into cold water and dropping the suspension onto the wet slides and/or from greater height (0.3–1.0 m). Where available, a thermotron (Cytogenetic drying chamber) can be used, which provides an optimum and controlled temperature and humidity environment for achieving ideal spreading results. 6. Seal the slide box in a Ziploc™ plastic bag with silica gel or Drierite and store them at –20°C until further use (see Note 3).
3.1.2. Metaphase Preparation From Lymphocyte Culture 1. For metaphase preparations from lymphocytes, add 0.5 mL of whole blood in 10 mL of RPMI 1640 tissue culture media with 20% fetal calf serum, 1% penicillin/streptomycin, 1% L-glutamine, 20 U of heparin, 20 U of HEPES buffer, and 4% PHA. Incubate at 37°C for 72 h. 2. Add Colcemid to 1 µg/mL final concentration at the 68th h of culture. Incubate at 37°C for 4–5 h. 3. Centrifuge at 400g for 10 min. To the gently vortex-resuspended pellet, add 10 mL of prewarmed 0.075 M KCl and incubate at 37°C for 30 min. 4. Continue as in step 4 of Subheading 3.1.1.
3.1.3. Interphase Nuclei Preparation From Fresh or Frozen Tissue Samples 1. With a sterile scalpel, cut a small piece (0.1–0.2 cm) of fresh or frozen tissue and gently touch it onto four to five spots onto a clean slide (one might want to score a circle on the underside of the slide with a diamond pencil to mark the spot for hybridization and visualization purposes). 2. Semidry (see Note 4) and fix the cells in fresh Cytogenetics fixative for 10 min. Air-dry and then heat on a warm plate at 37°C for 4 h. Age the slides for 3 to 4 d at room temperature and store them until used further for FISH.
3.2. Probe Labeling, Hybridization, and Visualization Probes can be labeled either by direct or indirect labeling methods. In direct labeling, the detectable molecule (Cy3-dCTP or Cy5-dCTP) is bound directly to the nucleic acid probe so that after hybridization and posthybridization washing, it can be visualized immediately. This type of labeling gives less background, but the signal is often weak. In indirect labeling, the probe is first labeled enzymatically with a hapten (digoxygenin or biotin, Subheading 3.2.1., step 1), which in the second step is accessible to the respective antibody (antidigoxygenin or avidin, Subheading 3.2.2.3., step 3) to which the fluorescent molecule is tagged. This type of labeling gives a brighter signal because of the attachment of a large number of antibody molecules to the hapten, which leads to an amplification of the fluorescent signal. 3.2.1. Probe Labeling Perform the nick translation labeling method using a Gibco Nick Translation Kit (see Note 5). The following protocol is as previously published with some modifications (85): 1. In a microfuge tube, add the following: 500 ng probe DNA and dH2O to a total volume of 18.5 µL; 2.5 µL of dTTP minus dNTP mix for indirect labeling or dCTP minus dNTP mix for direct labeling; 1 µL Digoxygenin-11-dUTP for indirect labeling or 2.5 µL Cy3-dCTP for direct labeling. 2. In a separate tube, add 1 µL DNAse I to 20 mL of dH2O (see Note 6). Add 1 µL of this to the above mixture. Mix well and add 2.5 µL Pol I/DNAse I mix (0.5 U/µL DNA Polymerase I, 0.4 mU/µL DNAse I). Incubate at 16°C for 90 min. 3. Add 7 µL of FISH stop buffer (0.5 M EDTA, pH 8.0) and 25 µL TE. Desalt through a Microspin™ G50 column as per the manufacturer's instructions. 4. Dry the sample on a Speedvac™ and resuspend the sample in 20 µL TE. Labeled probes can be stored at –20°C in the dark for up to 1 yr.
3.2.2. Hybridization 3.2.2.1. HYBRIDIZATION MIX AND PROBE DENATURATION 3 µL of labeled probe (from the 20-µL labeled probe of Subheading 3.2.1.); 1 µL of human DNA (Human Cot + Placental DNA, final concentration per 20-µL reaction: 5 µg and 10 µg, respectively); 1 µL Herring sperm DNA (final concentration per 20-µL reaction: 10 µg); 15 µL Hybridization mix.
Mix well and denature at 80°C for 8 min (see Note 7). Incubate at 37°C for 15–30 min prior to application to the slide, to suppress repetitive DNA-associated background. 3.2.2.2. DENATURATION
TARGET DNA
Incubate aged slides (at least 3–4 d old) at 80°C for 4 min in 70% formamide solution (pH 7.0). Immediately quench in ice-cold 70% ethanol and dehydrate through 80%, 90%, and 100% ethanol and dry the slides on a warm plate (37°C) for 5 min. Add the hybridization mix with the probe from Subheading 3.2.2.1. onto the prewarmed, denatured slide and cover-slip it. Seal the cover slip with rubber cement. Incubate the slide in a humidified chamber at 37°C overnight (chamber can be made by wetting paper towels with water and putting the slides on a pipet-tip rack onto the towels within a sealed Tupperware™ or like container). 3.2.2.3. VISUALIZATION 3.2.2.3.1. Primary Antibody Reaction for Indirectly Labeled Slides 1. After overnight hybridization at 37°C, rinse in PBS to remove excess probe. Wash the slides with 50% formamide wash solution, three times for 5 min each, at 42°C and three times for 5 min each with 2X SSC at 42°C. 2. Add 100 µL antibody blocking solution, cover slip the slide, and incubate at 37°C for 30 min in a humidified chamber. 3. Make primary antibody solution by adding 1 µL sheep antidigoxygenin (1:100 dilution) to 99 µL antibody blocking solution. Centrifuge at 10,000g for 5 min. Add the supernatant to the slide after blocking for 30 min, cover slip, and incubate at room temperature for 1 h. 4. Rinse slide with PBS and wash three times with antibody wash solution for 5 min each. Again, block with blocking solution for 5 min. In a separate tube, add 1 µL Cy3-conjugated anti-sheep IgG (1:50 dilution) to 99 µL blocking solution and add this to the slide after centrifugation, as earlier, cover slip, and incubate for 45 min at room temperature. 5. Rinse in PBS and wash in antibody wash solution three times for 5 min each at room temperature. 6. Rinse in PBS and counterstain.
3.2.2.3.2. Posthybridization Washing for Direct Labeling
For directly labeled slides, after overnight hybridization, proceed as follows: 1. Rinse the slide in PBS and wash in 50% formamide solution at 42°C, three washes of 5 min each. 2. Rinse three times for 5 min each in 2X SSC. 3. Rinse in PBS and counterstain. 3.2.2.3.3. Counterstaining
After the washing steps for both of the labeling procedures, stain the metaphase chromosomes and/or interphase nuclei as follows. In 50 mL McIlvaine buffer, add 2 µL of DAPI stain (1 mg/mL stock). Stain the slides for 4 min for metaphase spreads and 2 min for interphase nuclei. The DAPI might need to be titrated to a much lower concentration (0.1–2 µL DAPI stock per 50 mL of buffer) to decrease very bright interphase nuclei staining. After staining with DAPI, treat the metaphase preparations with Actinomycin D for 5 min for DA-DAPI banding (not necessary for interphase specimens). Rinse in PBS, air-dry, and mount in glycerol-based antifade. Slides can be stored at 4°C in the dark until visualization, if less than 12 h (see Note 8), or at –20°C for longer periods (up to several months). Examples of multicolor FISH on metaphase chromosomes are shown in Fig. 1. 3.2.3. FISH Analysis of Formalin-Fixed, Paraffin-Embedded Tumor Samples Fluorescence in situ hybridization on a formalin-fixed paraffin-embedded tissue sample is difficult because of poor probe penetration as well as autofluorescence from the fixed tissue. The following protocol (55), which results in the isolation of whole nuclei, has demonstrated good FISH results in such tissues. For treatment of paraffin sections in which the morphology is to be retained, see Note 9. 1. Deparaffinize the tissue in xylene for 30 min (at least two changes of fresh xylene). 2. Rehydrate through a 100%, 95%, 70%, and 50% ethanol series and finally wash in dH2O. 3. Digest the tissue in Carlsberg solution for 1 h at room temperature. Rinse in water. 4. Treat with 100 µg/mL heat inactivated Rnase A (Sigma) in 10 mM Tris-HCl, pH 7.5, 1 mM EDTA, 0.3% NP-40 for 15 min at room temperature. 5. Remove nuclei (carefully draw supernatant through wide-bore pipet or filter through nylon mesh to remove tissue fragments). 6. Drop one to two drops of the above suspension onto a clean glass slide, air-dry, and fix in Cytogenetics fixative (can be stored at this time). Bake 4 h at 37°C and store as described previously. 7. Allow slides to come to room temperature and remove from storage box. 8. Incubate in 50% glycerol/0.1X SSC at 90°C for 3 min. 9. Cool to room temperature in 2X SSC.
10. Denature for 5 min at 80°C in 70% formamide in 2X SSC and quench in ice-cold 70% ethanol. 11. Dehydrate in ethanol series. 12. Incubate in Proteinase K (8 µg/mL) solution for 7.5 min at 37°C. 13. Dehydrate in ethanol series and air-dry. 14. Hybridize with probe and proceed as in routine FISH protocol for washing and visualizing.
3.2.4. FISH of LCM-Nuclei For FISH of nuclei for which the cell of origin must be known, isolation of nuclei from whole tissue (55) is not acceptable. Cells can be microdissected using an Arcturus PixCell instrument; then the capture polymer is dissolved to release the tissue fragment, allowing enzymatic release of nuclei and subsequent FISH. The procedure is basically as described previously (86). 1. Cut 20- to 30-µm-thick paraffin sections from the blocks of interest. It is best to start with thicker sections to minimize sectioning of the large nuclei common to neoplasms (see Note 9). 2. Extensively deparaffinize (three changes of fresh xylene over 10–30 min) and stain with hematoxylin (not eosin, which leads to a broad-band fluorescent background). 3. Laser capture microdissect the dehydrated slide using the PixCell microdissection instrument (Arcturus Engineering Inc., Mountainview, CA) according to the manufacturer's protocols (www.arctur.com). You will need to increase the pulse energy and pulse time over the usual to allow capture of cells from the thick section (see Note 10). 4. Extract nuclei from the LCM caps as follows (86), using care to avoid flames or sparks and an appropriate fume safety cabinet (see Note 11). Add 100 µL of fresh chloroform into a 0.5-mL microfuge tube and cap with a CapShur LCM cap containing the microdissected specimen. Invert the tube for 10 s and centrifuge at 3000g for 30 s to release the tissue from the "capture" polymer of the LCM cap (see Note 12). Remove the LCM cap and add 200 µL of anhydrous ethyl ether and mix by inversion (necessary to lower density of the solvent, allowing tissue to pellet). 5. Centrifuge and remove the supernatant. Wash the pellet three times with 400 µL of fresh xylene to remove dissolved LCM cap polymers. Wash the pellet with 400 µL of 100%, 95%, and 70% ethanol, and two washes with TE, pH 8.0. Finally, wash the pellet with Proteinase K buffer (50 mM Tris-HCl, 10 mM NaCl, 10 mM EDTA, pH 8.0) and resuspend in the same solution to a final volume of 100 µL. Add Proteinase K (50 µL at 0.015% to a final concentration of 0.005%). Digest for 60–120 min at 37°C with gentle finger-vortexing every 10–20 min. 6. For FISH analysis, dilute the cells with 350 µL of TE and gently remove the liquid into another 0.5-mL microfuge tube with a large-bore pipet tip (0.5–1 mm) to leave the undigested tissue fragments in the tube. Centrifuge for 5 min at 10,000g, followed by careful removal of the supernatant. Resuspend the pellet in 50 µL of TE and drop 2–3 drops of this suspension onto a clean slide. Air-dry the slides and
then fix them in fresh Cytogenetics fix for 5 min and air-dry again. Warm at 37°C for 4 h and either store as usual or proceed to hybridization. (See Notes 13 and 14.)
4. Notes 1. Slow fixation of cells helps to retain a better morphology of the metaphase chromosomes. 2. Aging of slides is important, as it leads to slow and complete fixation of the cells. This helps in retaining good chromosome/nuclei morphology during the succeeding denaturation and staining procedures. 3. Slides stored at –20°C under desiccation can be used for more than 1 yr without any distortion of the metaphases plates or interphase nuclei, or loss of signal. Remember to allow the slides to come to room temperature before unsealing the storage bag, to minimize condensation. 4. Do not dry the tissue completely after making the imprints. 5. Although dogma suggests that nick translation labeling must be used to ensure optimal penetration of the specimen by the labeled probe, our personal experience has not identified significant differences in FISH signal intensity between random primer or nick translation hapten-labeled genomic probes. 6. Dilution of DNAse I should be optimized by digesting and aliquot of probe with various dilutions of DNAse I (1/1000 to 1/20,000) and electrophoresing a small aliquot of the sample in a 1% agarose gel. Optimal DNA digestion will yield a smear between 200 and 600 bp. 7. If aminoallyl-labeled nucleotides are used for labeling, it is important to denature at 65°C instead of 80°C. A higher denaturation temperature might cause loss of fluorescence. 8. Photobleaching seems to be lessened and the fluor signal seems better if the slides are left overnight prior to visualization. 9. If purification of nuclei is not desirable, bypassing steps 3–7 will result in treatment of the tissue to improve probe penetration and decrease autofluorescence, without removing nuclei (see Fig. 2). 10. Some experimentation is required to optimize specimen thickness (for better visualization) vs nuclear yield. Do not use adhesive or other coated slides to pick up the paraffin sections from the microtome, or you will not be able to remove the microdissected cells from the glass slide. 11. Some experimentation is required to maximize capture, which will be dependent on section thickness and tissue composition. For the PixCell II, laser energy might require 70–80 mW or even higher for 30-µm-thick sections and pulse duration of 7–8 ms. 12. Be very careful using these solvents! Diethyl ether is highly volatile, flammable, and potentially explosive if not fresh or stored properly. 13. It is not uncommon for bits of the capture polymer to remain undissolved. Leave them in the tube for the following steps, but when resuspending the cells for harvest, add more TE up to 700 µL, or perform two pipet extractions (e.g., 350 µL each) to maximize the yield of nuclei, and then centrifuge the nuclei slightly longer (6 min instead of 5 min) to pellet them.
14. For LCM, familiarity with the specimen morphology is essential. For the inexperienced user, it might be helpful to have a picture of the tissue section marked up by a pathologist to guide the microdissection. The morphology of dried, 30-µm-thick tissue sections is often terrible. As well, the tissue thickness precludes dissections of single cells or tiny (four to eight) cell groups unless they are well separated from other cells, as the laser energy required to melt the polymer to penetrate a thick section usually expands the beam width beyond 30–40 µm. Make sure that collagen fibrils are not inadvertently captured, as they will cause lifting of adjacent tissue and contaminate the specimen. For poor yields, check that the section thickness is thick enough to avoid nuclear slicing, that the capture polymer is dissolving, that the released tissue sections are not floating and thus being removed with solvent supernatants, and that the centrifugation step is fast enough and long enough to ensure pelleting of the nuclei from your specimens. One can drop a sample of the nuclei on a slide and examine it after air-drying under phase-contrast optics (86) or stain it with methylene blue or hematoxylin to evaluate the quality of the preparation prior to using it for FISH.
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13 Microarray Image Scanning Latha Ramdas and Wei Zhang
Summary Of the technologies available for measuring gene expression, microarrays using cDNA targets is one of the most common and well-developed high-throughput techniques. With this technique, the expression levels of thousands of genes are measured simultaneously. DNA probes are immobilized on solid surfaces, either membrane-based or chemically coated glass surfaces. On glass arrays, the probes are hybridized with fluorescent-labeled target samples. Fluorescence intensities, which reflect gene expression levels, are detected by imaging the array using a laser or white-light source and capturing the image using photomultiplier tube detection or a charge-coupled device camera. Different laser-based scanners are used in laboratories to scan microarray images. This chapter discusses the imaging process and the protocols being developed. Key Words: Genomics; microarray; laser scanners; imaging; gene expression; fluorescence.
1. Introduction From data derived from completion of the Human Genome Project, we know that there are approx 40,000 genes in human cells and that hundreds and thousands of these genes interact to perform diverse cellular activities. Microarray or DNAchip technology is a recently developed technique that allows biologists to obtain a bird's-eye view of the expression of thousands of genes in a single experiment. This technique has recently been used to address many biological issues, such as discovery of genes involved in certain diseases and molecular classification of cancers (1–5). The two platforms used with this technique are the membrane-based (porous surfaces like nylon) and chemically coated glass arrays. In both cases, thousands of DNA probes are immobilized robotically and hybridized with either 32P- or 33P-labeled targets in the case of membrane arrays or fluorescent-labeled cDNA targets in the case of glass arrays. With the glass microarrays, where fluorescent-labeled samples are hybridized, the signal From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Ramdas and Zhang
intensities are measured with fluorescence laser scanners. Detection of signals in glass microarray images is accomplished by measuring fluorescence intensities with scanners that allow simultaneous determination of the relative expression levels of all the genes represented on the array. This technique is the focus in this chapter. The imaging technique requires light of a specific wavelength to excite the fluorophores in the sample, and scanners can accomplish this by using one of two techniques. Most scanners use laser sources to excite each fluorescent probe, and emitted light is detected by a photomultiplier tube (PMT). This method uses a laser-based scanning system that involves the rapid movement of small points of laser light across the sample, from which the image is then reconstructed. The second imaging technique is based on a charge-coupled device (CCD). Scanners that use this method have a white-light arc lamp as their illumination source. Because white light encompasses all wavelengths in the visible spectrum, it is possible to select the wavelength of choice. Fluorescent light emitted from the sample is collected and imaged using CCD cameras. In this chapter, the protocol for scanning will be elaborated for the laser scanner using PMT detection. 2. Materials A total of 2304 known sequence-verified human cDNAs were prepared by polymerase chain reaction (PCR) from the cDNA clone library (Research Genetics, Huntsville, AL) using two primers on the vector. The DNA clones, in 384-well plates, were spotted onto poly-L-lysine-coated microscope slides using an arrayer (Genomic Solutions, Ann Arbor, MI). All clones except those of the controls were duplicated on the array. After printing, the slides were dried and crosslinked by ultraviolet radiation at 650 J/cm2. The slides were then washed with water, dried, and stored for later use. Cyanine-dye-labeled reverse-transcribed target cDNA sample in 130 µL of total volume of Express hyb solution (Clontech, Palo Alto, CA) with a mixture of blocking reagents was hybridized to the glass microarray slides. After hybridization, the slides were preprocessed for scanning. Laser scanners with appropriate laser sources for scanning (532-nm excitation for Cy3 and 695-nm excitation for Cy5) were used for scanning the microarray image. Quantification of the spots was carried out using an image analysis software product, ArrayVision (Imaging Research, Inc. Ontario, Canada). 3. Methods The most frequently used method of labeling gene products or mRNA is to incorporate cyanine (Cy)-labeled nucleotides into cDNA during reverse
Scanners and Scanning
transcription of the mRNA. The most common fluorophores are Cy3 (greenexcitation light at 532 or 545 nm) and Cy5 (red-excitation light at 635 nm). Microarray scanners capture the Cy3 and Cy5 signals on each spot of the microarray, using either the laser light of the specific wavelength to excite the fluorophores or white light, for which the choice of wavelength is flexible. Superimposing the two images from Cy3 and Cy5 channels provides a composite image that allows straightforward visual estimation. If we color the Cy5 image red and the Cy3 image green, an intense green or red spot will signify a higher level of that particular gene in one of the two samples. A yellow spot will indicate similar expression of the gene in the two samples. Several microarray scanners are available and appropriate for use in this technique, including a DNA microarray scanner (Agilent Technologies, Palo Alto, CA), ArrayWorx (Applied Precision, Issaquah, WA), ChipReader (Virtek Vision Corporation, Ontario, Canada), GenePix 4000B (Axon Instruments Inc., Foster City, CA), GeneTac LSIV and UC-4 (Genomic Solutions Inc., Ann Arbor, MI), and ScanArray (Packard Bioscience, Billerica, MA). The specifications for these scanners are listed in Table 1. Each scanner has unique features, and several factors should be considered when choosing one (6). 3.1. Signal Detection 3.1.1. Light Source Laser light is a common light source in which a small point of light moves through the entire microarray slide and reconstructs the image from this point measurement. The laser source is preselected on the basis of excitation and the emission wavelength of the fluorescent probes. Another type of light source is broad-spectrum white light, which is used in scanners such as the ArrayWorx scanner. The advantage of this type of scanner is its flexibility: it offers selection from 330 to 700 nm for excitation and from 380 to 800 nm emission wavelengths via filter selection. 3.1.2. Photon Detection Two main optical modes are used to detect signals from the microarray slides. Most laser-based scanners use a PMT detector and a scanning process that involves several steps: Dye
Laser → Excitation
PMT Converter → Electrons → Signal Amplification Filtration
The classic photomultiplier contains a light-sensitive photocathode that generates electrons when exposed to light. These electrons strike a charged electrode, called a dynode, and produce additional electrons. PMTs contain several
Table 1 Specifications for Six Microarray Scanners Agilent scanner Light source
ArrayWorx
GenePix 4000B
Laser (532 and 635 nm)
Laser (for Cy3 and Cy5) PMT
Stitch-byposition
PMT and voltage adjustment Simultaneous
Field focus Dynamic (µM) autofocus Pixel 5, 10 µM resolution Scan speed ~8 min/slide Dimensions 24 × 36 × 24 l × d × h (in.) Type of 48-Slide autoloader carousell
60 (thick specimen) 5–100 µM
>10 min/slide 31 × 23 × 23 40-Slide carrier
Gene TAC LSIV
GeneTac UC-4
ScanArray Express
Laser (488, Laser (532 Laser 494, 532, and 635 nm) and 635 nm) PMT PMT PMT and laser power adjustment Dark-field, Dark-field, Confocal, sequential sequential sequential 100 100 Autofocus 1–100 µM
1–100 µM
~5 min/slide 13.5 × 8 × 17.5
~5 min/slide 36 × 24 × 22
24-Slide carrier
4-Slide carrier
ChipReader Laser
10 (small depth) 5, 10, 20, 30 5, 10, 20, and 50 µM 30 µM ~5 min/slide ~5 min/slide 30 × 16.5 × 14 11 × 9 × 12 Single slide
dynodes, so the electrons emitted are amplified at each dynode, thus amplifying the signal. The electrons are collected at the anode and output as a current that is proportional to the intensity of light that strikes the photocathode. Thus, the PMT amplifies and measures low levels of light. Some scanners, such as the GeneTac LSIV and UC-4 scanners, increase the efficiency of the PMT detector by using a side-window PMT detector to minimize photobleaching and darkcurrent noise. Some laser/PMT-based systems, such as the ScanArray scanners, use confocal optics to sharpen the focus of the laser beam. This type of scanner focuses light at both the excitation end on the substrate and at the emission end on the PMT detector. The laser light, focused through a pinhole to restrict the focal length and reduce imaging artifacts, induces fluorescence. The emitted light is then collected by an objective lens and converged through another pinhole to the detector. However, if the substrate surface has large variations in thickness, the intensity measurements can differ depending on the focal plane. The second method for collecting fluorescent light from a sample is by using a high-performance CCD camera. This is the technology used in the ArrayWorx scanner. A white-light beam is passed through an excitation filter, yielding monochromatic light of the appropriate wavelength to excite the molecules on the array slide. The emitted light is focused onto a CCD camera via emission filters. In the ArrayWorx scanner, the excitation beam is passed through a specifically configured fiberoptic bundle delivering highly stable light onto the sample. The CCD camera collects light from small panels on the slide, and these panels are tiled to reconstruct the image. This method is often referred to as "stitch-by-position." The disadvantage of this system is that fewer emission photons are generated than with the PMT, in which the signal is amplified. To overcome this weakness, CCD systems integrate the signals over a period of time, thus prolonging the scan time. This long exposure time increases the dark current, which is the generation of electrons from heat at a constant rate. Because dark-current electrons are indistinguishable from electrons generated by photons, more noise is introduced. To minimize this noise, the CCD system must be maintained at a low temperature. The above-discussed optics are shown in Fig. 1. 3.2. Scanning 3.2.1. Simultaneous and Sequential Scanning Among the laser-based scanning systems available, some scan two dyes simultaneously, whereas others (sequential scanners) scan one dye at a time. Simultaneous scanning is fast and requires no image alignment after the scanning is completed, whereas sequential scanning acquires one image at a time and builds a composite image. Crosstalk between the two optical channels is
Fig.1. (A) The PMT amplifies electrons through dynodes, using the side window to minimize photobleaching, as in the Genomic Solutions GeneTAC UC-4 system. (B) The optical path in a confocal imaging technique showing the objective lens and the detector lens, and the convergence of the incident and the emission light through pinholes. (C) In the optical path of a CCD system, a white-light beam is passed through an excitation filter, and the chosen wavelength of light illuminates the sample on a glass slide. The light emitted through the emission filter is then focused onto a CCD camera.
not a concern for sequential scanning but could cause problems with simultaneous scanning. To overcome this problem, simultaneous-scanning devices use a laser wavelength of 532 nm instead of 545 nm to excite Cy3 and, thus, allow better spectral separation between Cy3 and Cy5, whose excitation wavelength is 635 nm. 3.2.2. Features of Scanning Devices Two other scanning features are the stitch-by-position technology in CCD camera detection and the confocal scanning in PMT detection. Dark-field imaging and laser-beam scanning with PMT detection are combined in the GeneTAC LSIV and UC-4 scanners. In the dark-field approach, the laser beam is angled so that the backscattered light cannot reach the PMT, which improves the contrast and the signal sensitivity. As a drawback, these technologies yield images that are dimmer than those projected from other scanners, so the slides require longer exposure to the laser. However, the improved side-window PMT system compensates by preventing photobleaching. In addition, the system has an increased focal depth of approx 100 µm. Thus, there is no variability resulting from uneven slide surfaces. 3.3. Signal-to-Noise Ratio The goal of scanning is to produce a high-quality image from which gene expression information can be acquired. The quality of an image is judged by its signal-to-noise ratio. Common sources of noise are thermal emissions or dark current, both of which occur in the PMT and CCD camera. The GeneTAC LSIV and UC-4 scanners use side-window PMT detection to reduce this noise, the GenePix 4000B scanner reduces the time the laser spends on each pixel, the ScanArray scanner uses a small depth of focus, and the ArrayWorx scanner uses a thermoelectrically cooled camera to reduce the dark current. Another kind of noise, called shot noise, is caused by the particle nature of light. This is the most dominant type of noise affecting the signal-to-noise ratio. The absolute magnitude of this noise increases with increasing signal intensity; however, the noise only increases by the square root of the signal intensity; thus, the signal-tonoise ratio decreases as the signal intensity increases. 3.4. Comparison of Scanners The whole process of microarray technology involves many different steps, which is why many of the current protocols are standardized to minimize experimental variations. Standardized procedures are essential for compatible data production, quality control, and analysis. However, reliability of the data from experiment to experiment and comparison of data from laboratory to laboratory or instrument to instrument has not been extensively evaluated. A commonly accepted standard with which to compare the performances of various scanners is presently lacking.
Fig. 2. Correlation analysis among three scanners at (A) 20 µm and (B) 10 µm resolution. The correlation coefficients (r) are labeled in each of the comparisons.
Fig. 2. (Continued) RA, RB, and RC represent data obtained from scanners A, B, and C, respectively. Log10(R) is the logarithm-transformed intensity ratio of the two channels. A linear regression line is shown in each plot.
To evaluate the performance of different scanners and compare the results, we scanned a single microarray slide on three different scanners. Our analysis involved the GeneTAC LSIV scanner (A), which combines a laser-beam source, dark-field imaging, sequential scanning, and PMT detection, the GenePix 4000B scanner (B), which uses a laser source, simultaneous scanning, and PMT detection, and the ScanArray scanner (C), a laser scanner with confocal-imaging technology and PMT detection (7). We scanned the slide on the three devices using comparable resolution, retrieved the tagged image file format (TIFF) images, and quantitated the thousands of spots on the array using the spot-finding software, ArrayVision (Imaging Research Inc., Ontario, Canada). The fluorescence intensities from each spot on the array were measured as sVol, which was the background-corrected volume, where volume is the density of each spot multiplied by its area and density is the average of all pixel intensities in the spot. The ratios of sVols from the two channels (Cy3 and Cy5) were used as measures of the relative expression levels, and the data were transformed into logarithmic values. The log-transformed intensity ratios from the three different instruments were then compared. When all three scanners were used, the correlation coefficient was between 0.90 and 0.96 (see Fig. 2); when images and data were obtained using the same scanner at different times, the correlation coefficient was approx 0.93. Thus, the instruments used did not cause any variability in results. We also compared the genes that were differentially expressed between the two cohybridized samples acquired from the three scanners. Among the 160 most differentially expressed genes, 95% of the genes were identified from all three analyses. Thus, all three instruments identified nearly all of the same differentially expressed genes. 3.5. Setting Scanning Parameters This subsection tests the steps that need to be considered when using a sequential scanner, which combines dark-field imaging and laser-beam scanning with PMT detection, to obtain raw signal intensity data. 1. The computer and the scanner are turned on in that order to prewarm for scanning. 2. The scanning program controls the action on the laser scanner. Thus, using the program, the machine is set ready to scan. The microarray slides are loaded on the slide holder and, when ready, the shutter is closed to latch (see Note 1). 3. Scan dimensions: Scan dimensions, which define the area on the slide to be scanned, are set and the image captured. For a standard slide (25 mm × 76 mm), the dimensions depend on the area on the slide that the actual array occupies (approx 20 mm × 70 mm) and the values 1248 × 2348 are set on the program for 20 µm resolution.
4. Scan resolution: The resolution of the image is set from 5 to 50 µm, which also determines the scan dimension. For a preview of the image, a low resolution is set, but for actual measurement, the spot size and the number of pixels in the spot determine the scan resolution. To obtain valid data, the pixel resolutions should be such that the spot size is greater than 5 times the pixel size and preferably 10 times. Thus, for a spot size of 100 µm, the pixel size should be 10 µm. 5. Pan and zoom: Pan and zoom is another option that is available to increase the resolution of a small area on the array. 6. Gains: With the sequential scanner discussed here, there are no adjustments to the power of the laser. However, an adjustment to the gains can be made, which dictate the amplification of the signal as it passes through the dynodes and thus influence the signal (see Note 2). 7. Palettes: Another option on this arrayer is the viewing palette, which can assist in determining the optimum gain to be set for scanning. Generally, setting a red or green palette for Cy5 and Cy3, respectively, will produce individual images with red and green spots and a composite image that is mostly yellow if most of the expression is the same in both the samples. However, detecting the saturation of the spots is not possible with this color setup because the human eye cannot differentiate shades of green and red very well. For this reason, there is a feature that allows one to set a color to indicate saturation as white (see Note 3). 8. Offset: This is another option to modify the background to better visualize the spots. By changing the zero value to a finite offset value, the contrast between the spots and the background can be improved. However, this causes a filtering effect, and in order to obtain all the raw data from the array, it is best that this number remain at zero during scanning.
Several other scanners provide the option of varying the PMT voltage and the laser power to improve the signal-to-noise ratio of the spots. For example, in the GenePix 4000 scanner, the lasers operate at full power; hence, the voltage of the PMT controls the gains of the PMT. 4. Notes 1. The glass arrays need to be handled very carefully. While scanning, the slides are always handled with gloves. Care has to be taken not to touch the inner surface of the slide, as this can destroy the poly-L-lysine coat on the surface. 2. Caution must to be taken in increasing the gain, because this affects both signal and noise without discrimination. Also there is serious loss of information from the spots on the microarray as the spots become saturated in intensity. For example, we scanned a slide at a particular gain and several five-increment intervals of gain and analyzed the data. As the gain increases, more and more spots fall in the saturated range and expression variation for all the spots goes to a minimum (see Fig. 3; Color Plate 14, following p. 274). For best results, the signal intensities from the two channels should fall in the linear range of detection (8).
Fig. 3. The images for different gains used for scanning and the corresponding scatterplots are shown. The saturated spots in the images are shown as white spots. The scatterplot is the comparison of the signal intensities between two gains, and as the gain increases, the number of spots that are saturated in intensity increases. (See Color Plate 14, following p. 274.)
3. The palette option of red or green to white, where white indicates saturated spots, is a good choice while scanning. Reasonably high gains need to be used while scanning in order to see the spots of low fluorescence intensities. The best choice is the spectral palette with spectral colors that cover the whole range in the 16-bit image.
References 1. Bittner, M., Meltzer, P., Chen, Y., et al. (2000) Molecular classification of cutaneous malignant melanoma by gene expression profiling. Nature 406, 536–540. 2. Eisen, M. B. and Brown, P. O. (1999) DNA arrays for analysis of gene expression. Methods Enzymol. 303, 179–205. 3. Fuller, G. N., Rhee, C. H., Hess, K. R., et al. (1999) Reactivation of insulin-like growth factor binding protein 2 expression in glioblastoma multiforme: a revelation by parallel gene expression profiling. Cancer Res. 59, 4228–4332. 4. Hegde, P., Qi, R., Abernathy, K., et al. (2000) A concise guide to cDNA micorarray analysis. Biotechniques 29, 548–562. 5. Golub, T. R., Slonim, D. K., Tamayo, P., et al. (1999) Molecular classification of cancer: Class discovery and class prediction by gene expression monitoring. Science 286, 531–537. 6. Ramdas, L. and Zhang W. (2002) What's happening inside your microarray scanner? Biophotonics Int. 9, 42–47. 7. Ramdas, L., Wang, J., Hu, L., Cogdell, D., Taylor, E., and Zhang, W. (2001) Comparative evaluation of laser-based microarray scanners. Biotechniques 31, 546–552. 8. Ramdas, L., Coombes, K. R., Baggerly, K., et al. (2001) Sources of nonlinearity in cDNA microarray expression measurements. Genome Biol. 2, 47.
14 Near-Field Scanning Optical Microscopy in Cell Biology and Cytogenetics Michael Hausmann, Birgit Perner, Alexander Rapp, Leo Wollweber, Harry Scherthan, and Karl-Otto Greulich Summary Light microscopy has proven to be one of the most versatile analytical tools in cell biology and cytogenetics. The growing spectrum of scientific knowledge demands a continuous improvement of the optical resolution of the instruments. In far-field light microscopy, the attainable resolution is dictated by the limit of diffraction, which, in practice, is about 250 nm for high-numericalaperture objective lenses. Near-field scanning optical microscopy (NSOM) was the first technique that has overcome this limit up to about one order of magnitude. Typically, the resolution range below 100 nm is accessed for biological applications. Using appropriately designed scanning probes allows for obtaining an extremely small near-field light excitation volume (some tens of nanometers in diameter). Because of the reduction of background illumination, high contrast imaging becomes feasible for light transmission and fluorescence microscopy. The height of the scanning probe is controlled by atomic force interactions between the specimen surface and the probe tip. The control signal can be used for the production of a topographic (nonoptical) image that can be acquired simultaneously. In this chapter, the principle of NSOM is described with respect to biological applications. A brief overview of some requirements in biology and applications described in the literature are given. Practical advice is focused on instruments with aperture-type illumination probes. Preparation protocols focussing on NSOM of cell surfaces and chromosomes are presented. Key Words: Near-field scanning optical microscopy; NSOM; applications in biology; cell surfaces; metaphase chromosomes; meiotic chromosomes
1. Introduction 1.1. Diffraction Limit in Far-Field Light Microscopy In biological settings, the usually applied microscope techniques (bright field, phase contrast, epifluorescence, confocal laser scanning, etc.) make use of the so-called far-field light microscopy. This means that the imaging process is From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Hausmann et al.
determined by the physical behavior of propagating electromagnetic waves for specimen illumination and detection. Compared to the wavelength of light (typically in the range 400–800 nm), the distances between illumination source or detector system, respectively, and the specimen are hundreds of micrometers up to millimeters, that is they are large ("far field"). Because of the finite aperture of the microscope optics, light waves are diffracted, resulting in the well-known effect that an infinitesimal small pointlike object appears as a spread image of minimum diameter. The image intensity distribution of a pointlike object is given by the point spread function (PSF), from which a measure of spatial resolution can be defined for far-field optical microscopes (1). The classical diffraction limited optical resolution is given by  λ  D = 0.61   NA 
(2,3) where λ is the wavelength of the light, NA is the numerical aperture of the objective lens, and D is the smallest distance between two pointlike objects that are imaged separately. A closely corresponding value also used as a measure of resolution is the full width at half-maximum of the PSF (4). In addition to the physical constraints of the microscope optics, the real optical conditions of the specimen have also considerable influence on the attainable resolution in practice. Thus, in routine biological applications, the spatial resolution of far-field light microscopy is limited at about 250 nm (5,6). During the last two decades, several approaches have been described to overcome these imaging restrictions of far-field light microscopy (7). The first technique that successfully surpassed the diffraction limit has been near-field scanning optical microscopy (NSOM) (for a review, see refs. 8 and 9). Meanwhile, NSOM techniques have become a special discipline in optics, in which many laboratories develop highly sophisticated instruments and investigate their physical behavior (see, e.g., the 71 articles dicussed in Journal of Microscopy vol. 202, 10). 1.2. Principle of Near-Field Scanning Optical Microscopy The principle of NSOM has been discovered and rediscovered several times. More than 70 yr ago, the theory of an instrument very similar in construction to modern NSOM systems was discussed (11) as a possibility for surpassing the diffraction limit of resolution (12). After the first experimental achievement of microscopic imaging with nearly atomic resolution by nonoptical scanning techniques (scanning tunneling microscopy, atomic force microscopy) (13,14) NSOM was realized (15,16). Similar to atomic force microscopy, a sharp probe physically scans the sample surface in NSOM. The height movement of the scanning probe is controlled by the atomic force interaction between sample and probe tip, which generates
NSOM in Biology
Fig. 1. Schematic representation of a NSOM probe.
a nonoptical image of the specimen topography. For biological applications, the most frequently applied NSOM probe consists of a metal (aluminum)-coated tapered optical fiber with a small aperture typically 20–130 nm in diameter at the end (see Note 1). This probe is used as a light source in optics, similar to the way the headset of a Walkman is used as a sound source in acoustics. In the latter case, the sound source has dimensions in centimeters, which is much smaller than the wavelengths of sound. Thus, the real acoustic figures and timber can only be heard if the ear is very close to the sound source (i.e., in a range much smaller than the wavelength). In a NSOM probe tip, the incident light wave coming from a laser light source via a glass fiber is propagating into a funnel of dimensions below the diffraction limit (see Fig. 1). The light emitted by the aperture is, therefore, predominantly composed of evanescent waves rather than propagating waves. The intensity of evanescent waves decays exponentially with the distance from the NSOM probe tip. The region around the tip in which a significant intensity level of the evanescent wave can be detected is called the near field (see Fig. 1). The near-field region typically has dimensions less than 100 nm. This limits the tipto-sample distance to considerably less than 100 nm in order to obtain highintensity evanescent waves. In principle, the NSOM probe tip illuminating the sample can also be used to detect the light emitted from the sample. This means that the probe tip works in the same way a doctor's stethoscope is used to detect sound generated by a lung or a heart. In this case, sound waves with a wavelength of meters are localized with an
accuracy of centimeters. Therefore, the NSOM has occasionally been termed an "optical stethoscope" (15). Nevertheless, in biological applications, near-field illumination systems are more frequent than near-field detection systems. In the case of near-field illumination systems, the light emitted by the NSOM tip is absorbed by the sample or by fluorochromes of the sample. The light subsequently transmitted through the sample (transmitted light) or emitted by the fluorochromes (fluorescent light) is predominantly composed of propagating waves and can be collected by conventional far-field optics and sensitive detectors. However, the NSOM probe is scanning the object point by point, so that the acquired image is the result of a sequence of intensity signals obtained from each precisely localized point. Therefore, the optical resolution is determined by the size of the illumination point. Because the near-field intensity decays rapidly with the probe distance, the aperture size is an appropriate parameter for estimating the optical resolution (17) (see Note 2). According to our experience, in practice, sub-hundred-nanometer resolution is feasible for many biological applications if the aperture has a diameter of 80 nm or less. 1.3. Requirements and Applications in Biology A NSOM image usually records a small section of a sample (typically less than 10 µm × 10 µm) with high topographic and high optical resolution (better than 100 nm). To obtain an overview of the sample and preparation conditions, it is often helpful to image the same sample with lower magnification and resolution. This requires the addition of a far-field microscope. Because image detection of NSOM is based on far-field optics, the scanning unit can be adapted to a far-field microscope, so that both imaging modes can be run with the same instrument under identical specimen conditions (see Note 3). Near-field scanning optical microscopic imaging is a combination of topographic (atomic force) imaging and high-resolution optical (light transmission, fluorescence) imaging. Both nonoptical and optical images are acquired simultaneously. Similar to atomic force microscopy, it is essential that the probe-tosample distance (typically in the range of 10 nm) is accurately controlled by using a force feedback loop. The optical image, however, is very sensitive to small axial movements of the tip, because the near-field illumination intensity decays exponentially with the tip-to-sample distance. Therefore, the most commonly applied control mechanism in NSOM is, in contrast to atomic force microscopy, the socalled shear-force feedback control. The NSOM probe is mounted into a piezoelectric tube so that it oscillates at its resonance frequency in a lateral vibrational mode with an amplitude typically less than 1 nm. In close proximity to the sample, the shear forces dampen this motion and induce a measurable change in the oscillation amplitude and phase, which can be used as a control signal in an electronic feedback system and for the generation of the topographic image.
The physical constraints of the shear-force feedback control require dry specimens, which seems to preclude biological applications in which a fluid environment is of central importance for structure conservation. However, structure-conserving drying procedures are known from electron microscopy (18) and can be modified in an appropriate way (see also Subheading 3.2.) (see Note 4). The requirement for specimen drying in NSOM fluorescence imaging imposes restrictions on the choice of fluorochromes that can be used for labeling. Furthermore, in dry samples, photobleaching might increase as a result of the direct contact with oxygen from air. Despite these so far existing limitations, NSOM has been utilized in many applications in different fields of biology and has proven a powerful technique when surface structures or structures very near to the surface need to be visualized by means of light microscopy at high spatial resolution and contrast. So far, NSOM has, for instance, been applied for the analysis of the following: 1. 2. 3. 4. 5. 6. 7.
Cytoskeletal actin (19). Green fluorescent proteins in bacteria (20). Cortical neurons (21). Membranes of erythrocytes (22) and skin fibroblasts (23). Cell surfaces of mouse fibroblasts (24) and lymphocytes (25). Cell surfaces of human breast tumor cells (26,27) and cardiomyocytes (28). Metaphase chromosomes after G-banding (29,30) and fluorescence in situ hybridization (FISH) labeling (31–34). 8. Meiotic chromosomes (33). 9. DNA conformation (35,36).
Especially in single-molecule fluorescence studies on cells and subcellular components (e.g., single green fluorescent protein photophysics) (19,32, 36–39), NSOM has a superior advantage over far-field light microscopy because the topographic (nonoptical) image simultaneously acquired with the fluorescence image allows a precise localization of the fluorescence signal on the specimen. 2. Materials 2.1. Instrumentation 1. Inverted far-field microscope with Hg-illumination system and appropriate filters for fluorescence microscopy (see Notes 5 and 6). 2. Charge-coupled device (CCD) camera and a frame grabber board (if necessary) for far-field image detection. 3. NSOM unit (see Fig. 2) implemented in the far-field microscope. 4. Motor-driven stage and sensitive detectors for light detection in near-filed imaging (photomultiplier, avalanche diode).
Fig. 2. Schematic representation of a NSOM unit implemented in the microscope condenser as used in a ß-type SNOM 210 from Carl Zeiss Jena (see also ref. 33). 5. Laser module (He–Ne laser, Ar laser, etc.) with an acousto-optical tunable filter (AOTF) to select the appropriate laser wavelength and to tune the intensity. 6. Laser fiber unit to couple the light into the NSOM illumination fiber (see Fig. 1). 7. NSOM control unit. 8. Computer with software to drive the NSOM unit. 9. Image detection and evaluation software for far-field images obtained by the CCD camera. 10. Image detection and evaluation software for near-field images obtained by point detectors (optical near-field signal) and the shear-force feedback control unit (atomic force signal).
Because the aforementioned components can be obtained from various companies, we do not indicate a special brand in this section. 2.2. Solutions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
PBS (phosphate-buffered saline). 3.7% and 4% Formaldehyde prepared from fresh paraformaldehyde. Ethanol (70%, 80%, 85%, 90%, 95%, 100%). HMDS (hexamethyldisilazane). Formamide (70%). 2X SSC, 0.5X SSC (standard sodium citrate). 4X SSC/0.2% Tween-20. 1X PBD (phosphate-buffered detergent). 10 mM HCl. MEM medium (Life Technologies) / 0.5% mammalian protease inhibitor (Sigma). 1% Lipsol. Fixative I: 3.7% acid-free formaldehyde, 0.1 M sucrose, pH 7.4.
13. PBS/0.5% Triton X-100. 14. PBTG: PBS, 0.1% Tween-20, 0.2% bovine serum albumin (BSA), 0.1% fish gelatin.
3. Methods 3.1. Image Acquisition The operations of the different instruments differ in many details and have to be adapted in detail to a particular setting. Therefore, the following protocol only describes essential steps. Further details have to be taken from the manufacturer's operating instructions for the given instrument. 1. Mount the NSOM probe into the NSOM unit of the microscope (see Note 7). 2. Connect the glass fiber to the laser unit and adjust the fiber so that enough light intensity is coupled into the fiber. 3. Measure the resonance frequency of the NSOM probe (see Note 8). 4. Adjust the shear-force feedback control to this frequency. 5. Move the specimen into the center of the far-field image. 6. Approach the NSOM probe to the specimen surface (see Note 9). 7. Select the field of detection. 8. Make a fast scan in order to adjust the detector gain. 9. Scan the specimen (see Note 10).
The following typical examples for the application of NSOM were performed on a SNOM 210 in the β-type version (Carl Zeiss Jena GmbH, Digital Instruments Veeco GmbH). The piezo scanning unit was integrated into the microscope condenser of an Axiovert 135 microscope. Microfabricated probes (Institut für Mikrotechnik Mainz) with silicon nitride tips coated with aluminum were mounted in a shear-force sensor support. The probes typically had an aperture of 80–100 nm. The instrument was equipped with an argon-ion laser (λ = 458 nm, 488 nm) and two He–Ne lasers (λ = 543 nm, 633 nm) for nearfield illumination. The illumination intensity was tuned by an AOTF for each laser wavelength independently. Fluorescence or transmission light was detected in air by an Achroplan long-distance objective 40×/NA 0.6 and transferred to a photomultiplier or an avalanche photodiode, respectively, using appropriate filter settings. The instrument was controlled by the NanoScope IIIa controller. Further details of the instrument are described in ref. 33. 3.2. NSOM of Cell Surfaces (25,27) For the examples shown here (see Figs. 3,4), two types of cell systems were used: 1. Peritoneal cells (mouse lymphocytes and macrophages) dropped on glass slides. 2. Breast cancer cells of the cell line T-47D (ATCC HTB 133) (see Note 11) grown on chambered glass slides.
Fig. 3. NSOM images of an antibody-labeled mouse B-lymphocyte (A,B) and a mouse macrophage (C) of the same preparation. In the topographic images (A,C), the different structures of the cell surfaces are visible; in the optical image (B), regions of high Cy3 fluorescence intensity are shown as dark gray regions. In (D), a far-field image of the same specimen is shown. The macrophages are larger than the B-lymphocytes. (Part of this figure was taken from ref. 25 with permission of SPIE.)
Prior to NSOM, the following preparation steps have to be done: 1. 2. 3. 4. 5. 6. 7. 8.
Seed and grow cells on ethanol-cleaned standard slides or cover glasses. Remove cell culture medium. Wash twice in PBS. Fix cells with 4% formaldehyde in PBS for 15 min at 4°C. Wash in PBS for 15 min at room temperature. Optional: Label cell membrane with fluorescent antibodies (see Note 12). Wash 15 min in PBS. Dehydrate by an ethanol series (70%, 80%, 90%, 100%) at room temperature for 5 min each (see Note 13).
Fig. 4. Surface sections (1.5 µm x 1.5 µm) of a breast cancer cell of the line T-47D are shown. In the topographic image (A), typical rosettelike cell surface structures are visible; in the optical image (B), the light transmission image obtained for the He–Ne laser at 543 nm is shown (dark = high absorbance, light = high transmission). (From ref. 25 with permission of SPIE.)
9. Expose to HMDS (see Note 13). HMDS is known to reduce surface tension and to induce crosslinking in proteins. 10. Dry specimen at room temperature.
As an example, surfaces of peritoneal cells (mouse) (see Fig. 3) and breast tumor cells (human) (see Fig. 4) were visualized by NSOM 210. The scans were performed with a velocity of the NSOM probe of less than 1 µm/s. The images were processed (flattened, low-pass filtered) and visualized in threedimensional topographic false color plots using the NanoScope IIIa software (version 4.42r1) running under Windows on a PC (Figs. 3A–C and 4). Because the instrument is based on a standard Zeiss Axiovert microscope, far-field imaging (see Fig. 3D) was also possible using a CCD camera and a frame grabber board (Scion-Imaging). Image recording was controlled by the Scion Imaging software running under Windows on a PC. The images obtained (see Figs. 3 and 4) show typical rosettelike and cylinderlike structures on the cell surface in the sub-hundred-nanometer range. Such studies will allow one to investigate the variations in surface morphology on a single-cell level, (e.g., after chemical or pharmacological treatment) (e.g., 27). 3.3. NSOM of Metaphase Chromosomes After FISH (29,33,34) Fluorescence in situ hybridization (40,41) has become a routine technique in biomedical research and clinical diagnostics. For NSOM, standard FISH techniques can be applied. However, after specific DNA labeling, the specimen has to be dried carefully, which precludes the use of certain fluorochromes in NSOM applications.
Fig. 5. NSOM images of human metaphase chromosomes after "standard" FISH (A–C) and low-temperature FISH (D–F) (see ref. 34) with a Cy3-labeled centromeric DNA probe. (A) The topographic near-field image reveals that chromosome arms display a collapsed structure, particularly of the central region of the chromatids, leaving higher rims at the chromosomal border. (B) NSOM transmitted light image at 543 nm showing a high intensity at the chromosomal border only, which verifies the topographic impression. (C) NSOM fluorescence image of Cy3 fluorescence in the centromeric region detected via a 590-nm long-pass filter. The high fluorescence intensity corresponds to the absorption at
The following protocol is based on a commercially available protocol: 1. Obtain chromosome spreads by standard fixation techniques (e.g., methanol/acetic acid fixation, formaldehyde fixation, etc.) (see Note 14). 2. Prepare and label respective DNA probe and dissolve in hybridization solution according to the protocol of the manufacturer of a labelling kit of your choice (see Note 15). 3. Denature preparation in 70% formamide/2X SSC at 70°C for 2 min, and DNA probe for 3 min at 95°C. 4. Dehydrate preparations through an ethanol series (70%, 80%, 95%) at 4°C for 2 min each. 5. Add DNA probe mixture to the slide. 6. Cover the hybridization mixture and preparations with a coverglass, seal with rubber cement, and hybridize in a humidified chamber at 37°C overnight. 7. Remove the seal and coverglass. 8. Wash in 0.5X SSC (pH 7.0) at 72°C for 5 min. 9. Transfer preparations to 1X PBD at room temperature. 10. Detect the hybrid molecules by incubation with fluorochrome-labeled secondary antibodies (see Note 16). 11. Counterstaining of the chromosomes is not necessary. 12. Rinse slides three times 2 min each in 1X PBD. 13. Air-dry preparations at room temperature.
This standard FISH technique involves denaturation of the probe and target DNA at temperatures ≥70°C, thereby creating single-stranded DNA molecules that form hybrids upon reassociation of the complementary DNA sequences. A typical NSOM image series of thermally denatured metaphase chromosome preparations and specific fluorescent labeling of all centromeres by FISH is shown in Fig. 5. The granularity of the chromosome morphology (see Fig. 5A–C) indicates that thermal denaturation processes of standard FISH techniques might have detrimental effects on chromatin organization. Therefore, an alternative to standard FISH protocols was introduced that omits denaturing chemical agents and thermal treatment ("low-temperature FISH") (42). The following protocol is particularly suitable for detecting repetitive centromere probes: 1. Obtain chromosome spreads and DNA probe as described above (see Note 14). 2. Treat specimen with RNase in 2X SSC at 37°C for 1 h.
Fig. 5. (Continued) the centromere in (b). (D–F) Human metaphase chromosome 1 after visualization of the 1q12 pericentromeric region by "low-temperature" FISH using the DNA probe pUC1.77. (D) Topographic near-field image indicates that the FISH method applied maintains a filled appearance of the chromatids. (E) NSOM transmitted light image at 543 nm. High transmission is apparent along the chromatids. (F) NSOM fluorescence image (detected via a 590-nm long-pass filter) reveals strong signal at the hybridisation site.
3. Wash 3 min in 2X SSC at 37°C. 4. Incubate in 10 mM HCl for 2 min. 5. Add freshly prepared pepsin (final conc. 0.2 mg/mL) to prewarmed 0.01 N HCl in a Coplin jar at 37°C. Submerse preparations and incubate for 10 min. 6. Rinse the slides with distilled H2O and wash twice in 2X SSC for 5 min. 7. Subject to a short fixation with freshly prepared 4% formaldehyde in PBS for 10 min at room temperature. 8. Wash twice again in 2X SSC. 9. Dehydrate through an ethanol series (70%, 85%, 95%) for 5 min each. 10. Air-dry preparations at room temperature. 11. Add 5 µL of the previously denatured probe mixture to the preparation and seal it with rubber cement under a plastic cover glass. 12. Hybridize for 15 h at 37°C. 13. Peel off rubber cement, float off cover glasses in 4X SSC/0.2% Tween-20 and wash preparations again for 5 min in the same solution at 37°C. 14. Incubate in 10% blocking solution in PBS or in 1.5% dry skim milk in PBS for 5 min. 15. Detect the hybrid molecules by an appropriate antibody–fluorochrome system (e.g., antidigoxigenin–Cy3 Fab fragments, or avidin–Cy3 in the case of biotin as the label of the DNA probe). 16. Wash in PBS for 10 min at room temperature. 17. Subject to another ethanol series (70%, 85%, 95%) for 5 min each. 18. Counterstaining of the chromosomes is not necessary for NSOM. 19. Air-dry the sample at room temperature.
A typical example of a metaphase chromosome after low-temperature FISH (42) is shown in Fig. 5D–F. In this case, chromosome 1 was labeled with a subcentromeric DNA probe (pUC1.77; see Note 17). The chromosome appears more voluminous with a "smooth" (i.e., less granular) surface as compared to thermal denaturation (compare Fig. 5A,D). After standard FISH, the chromosomes are generally very flat; signal and background intensities are at about the same levels. In contrast, an intensive transmitted light signal was restricted to the chromatids of the low-temperature FISH chromosomes, indicating an intensive near-field object interaction (see Fig. 5E). The fluorescence image shows a clearly visible labelling site at the centromeric region (see Fig. 5F). 3.4. NSOM of Meiotic Chromosomes (33) Meiosis is the cell division type that contributes to sexual reproduction by reducing the diploid chromosome number to the haploid. Meiotic chromosomes display a unique organization in that they exhibit a proteinaceuos chromosome core along replicated sister chromatids, and the DNA emanates as loops from these cores. Cores of homologous chromosomes are connected during pachytene by a protein zipper called the synaptonemal complex (SC). The ends of the SCs
are capped by the telomeres, which (in addition to other aspects) play an important role in the chromosome-pairing process at first meiotic prophase. So far, little is known about the relative distribution of telomere proteins within ends of the meiotic chromosome cores. NSOM might be a means to gain a better insight into the spatial organization of the meiotic telomere. The following (for details, see ref. 43) describes the preparation of meiotic chromosome spreads for NSOM: 1. Mince fresh or frozen testicular tissue in MEM medium/0.5% mammalian protease inhibitor (Sigma) at 4°C. 2. Remove tissue pieces and place a drop of the suspension on clean, aminosilanecoated glass slides (Super plus; Menzel Gläser). 3. Mix 50 µL of cell suspension and 150 µL of 1% Lipsol on a slide. 4. Briefly tilt the slide after each step to mix the solutions evenly within the resulting drop. 5. After 5 min, add fixative I and distribute the suspension evenly by streaking with the side of a pipet tip over the slide without touching the surface. 6. Fix specimen by adding 200 µL fixative I. Allow to air-dry in a fume hood. 7. Wash for 30 min with 0.5% Triton X-100/PBS. 8. Wash with PBS. 9. Incubate for 10 min in PBTG. 10. Incubate at 4°C overnight with 100 µL PBTG containing the primary antibody under a cover slip in a humid chamber (see Note 18). 11. Rinse three times 3 min with PBTG at 37°C. 12. Incubate for 30 min at 37°C with secondary, fluorochrome-conjugated antibody of choice (see Note 19). 13. Wash three times 3 min in PBS. 14. Dehydrate through an ethanol series (70%, 85%, 95% for 5 min each). 15. Air-dry the sample at room temperature.
For an example of successful labeling at a meiotic telomere, see Fig. 6 and Color Plate 15, following p. 274. The meandering parts of the SC are visible in the topographic and light transmission image. From the fluorescence image, TRF2 signals can be localized in the telomere knob. 3.5. Discussion and Perspective Many applications have shown that the intrinsic advantages of NSOM make this technique extremely useful for high-resolution imaging in life sciences. Nevertheless, it seems that NSOM is rarely applied in biology. Some features might be responsible for this situation: First, compared to far-field light microscopy, NSOM is a complex technique that requires detailed training of the operator before images of good quality can be obtained. Second, most nearfield microscopes are devices in experimental physics laboratories and not widely accessible to the biologist. The commercially available instruments are primarily designed for measurements of solid-state physics and are not always
Fig. 6. NSOM images of the telomeric region of a human meiotic chromosome core after immunostaining of TRF2 by Cy3-labeled antibodies. (A) The topographic image shows a thickened telomere at the end of the core (called the attachment plate); (B) a telomeric knob as regions of higher intensity of transmitted light at 543 nm; (C) Cy3 fluorescence of the TRF2 antibody labeling site, which was recorded via a 590-nm lowpass filter; (D) far-field fluorescence image of the synaptonemal complex and the TRF2 label at the telomere (see Color Plate 15, following p. 274).
adapted to the requirements of biological experiments—for instance, the use of standard glass slides as specimen carrier. Third, the resolution and sensitivity of NSOM depends not only on the quality of the probe but also on the quality of the specimen. Especially, the drying process required because of the shearforce feedback control has to be done with great care in order to conserve the native supra molecular structures of the specimen. Despite great efforts of several groups (44–46), it remains a technical challenge to develop an instrument that works in a liquid (i.e., more physiological environment) and allows for nearfield analysis of "soft objects." 4. Notes 1. The scanning probe is the most critical element of the complete technique. It is difficult to produce such probes in such a way that NSOM imaging can reproducibly be performed for the same specimen. Therefore, a reliable procedure to produce large quantities of high-quality probes is microfabrication of metalcoated silicon oxide or silicon nitride tips that are glued on glass fibers. However, examples of biological applications have been described using non-metal-coated aperture-type probes. Other types of probe referring to the so-called "apertureless" types are described in the literature, but, to our knowledge, they have not been applied for any routine application in biology. 2. As in far-field light microscopy, in practice the optical resolution of NSOM strongly depends on the experimental conditions applied (NSOM probe, light
intensity, optical conditions of the sample, sample topography, etc.). Structurally conserved surfaces of cells or organelles often show gross topological changes (typically several micrometers). In this case, the topography might modulate the optical signal because the axial tip-to-sample distance is not always the shortest tip-to-sample distance that controls the height movement of the NSOM probe. This effect can modulate the optical resolution within the image and could also cause artifacts in the image (47,48). However, theoretical approaches exist that allow for an approximate determination of the resolution of NSOM also from images of biological samples (49). The combination of far-field and near-field microscopy in one instrument is an advantage for the user, especially for biological applications, if switching between both instrumentation modes can be done without changing the sample or sample carrier. Therefore, it is of practical importance in biological applications that the standard glass slides or cover glasses, which are routinely applied in far-field light microscopy, can also be used for specimen preparation under NSOM conditions. The shear-force feedback control reacts also on transparent surface material. Hence, care has to be taken not to cover the specimen by some material that should not be imaged. The microscope has to be protected against low-frequency vibrations of the surroundings by means of an active optical table. If standard glass slides are used for far-field and near-field microscopy, longdistance objectives are useful to focus through the slide. Several manufacturers offer NSOM probes. They are usually delivered ready to use for a given instrument. In all cases, special care has to be taken in handling the NSOM probe in order not to destroy the probe tip. Each NSOM probe has its own resonance frequency that has to be determined after changing the NSOM probe. Sometimes more than one resonance can be detected. In this case, the frequency that displays the highest resonance amplitude should be utilized for the NSOM control. By visual far-field microscopy, the NSOM probe can be manually adjusted into a focal plane above the specimen surface. The final approach of the NSOM tip should be done by the instrument under shear-force control. Most biological samples are so-called soft samples (compared to solid-state surfaces) with large height modulations (typically in the micrometer range). Therefore, a very slow scan velocity is recommended (less than 1 µm/s). Breast cancer cells of the cell line T-47D (ATCC HTB 133) were grown on chambered glass slides (Nalge Nunc International, Naperville, IL USA) and cultivated in RPMI 1640 cell culture medium with 10% fetal calf serum at 37°C with 5% CO2 in a humidified atmosphere. Three to four days before reaching a confluent cell monolayer, the medium was changed to phenol red-free RPMI 1640, including 5% charcoal-stripped fetal calf serum, 1% antibiotic/antimycotic solution (GIbco BRL/Life Technologies), 2 mM glutamine, and 1% nonessential amino acids solution. After 6 d of further cultivation, aliquots of the cells were treated for
48 h with 17ß-estradiol (Sigma-Aldrich Chemie GmbH, Deisenhofen, Germany) at concentrations of 5 × 10–9 M, 5 × 10–7 M, or 5 × 10–5 M. The 17ß-estradiol stock solution was prepared in ethanol (27). 12. Mouse peritoneal cells: Immunoglobulin G on the mouse lymphocytes was visualized by biotinylated goat–anti-mouse IgG antibodies and Cy3-conjugated streptavidin. The incubation time was 15 min for each step. Cy3 is well suited for NSOM because of its high photostability (see Subheading 1.3.). 13. The duration of the ethanol and HMDS exposure is very critical on the conservation of cell surface structures. The appropriate exposure times have to be tested experimentally. For the examples described here, the optimum was as follows:
Fresh peritoneal cells Cancer cells of T-47D
Duration of each ethanol step
Duration of HMDS exposure
2.5 min 5.0 min
14. For the experiments presented here, lymphocytes were prepared from fresh peripheral blood and stimulated with phytohemagglutinin M to grow for 72 h. The cells were synchronized and arrested in mitosis by a colcemid block for the last 2 h of cultivation. After hypotonic treatment with prewarmed KCl (75 mM), the cells were fixed with cold methanol/acetic acid (3:1, v/v). Metaphase chromosomes and interphase cell nuclei were spread on precleaned slides. After evaporation of the fixative, the slides were stored in 100% ethanol at 4°C. Prior to FISH, they were rinsed with 100% ethanol and air-dried. 15. A commercially available DNA probe (Appligene Oncor) specific for all centromeres was used. This probe was labeled with digoxigenin. According to the manufacturer´s instruction, 1.5 µL of probe DNA was denatured in 30 µL Oncor Hybrisol VI (containing 50% formamide) at 72°C for 5 min. 16. For detection of the hybrid molecules, 60 µL of Cy3-labeled antidigoxigenin was applied and incubated under a plastic cover slip at 37°C for 45 min. 17. A DNA probe (pUC 1.77) specific for the region q12 on chromosome 1 was used for the experiments shown in Fig. 5D–F. The pUC 1.77 probe was labeled with biotin-11-dUTP. Five microliters of DNA probe (about 100 ng) were diluted with 3 µL 20X SSC, 3 µL of 10X HCl-Tris, and 19 µL of H2O and denatured at 95°C for 4 min. The hybridization solution was cooled down to approx 60°C and kept at this temperature until use. 18. To our current knowledge, the NSOM data described here and in ref. 33 are the very first experiments in which NSOM imaging was performed to visualize telomere proteins of meiotic chromosomes. The labeling pattern shown was obtained with different primary antibodies, such as rabbit anti-human TRF1 or rabbit antihuman TRF2 (43). 19. For the detection of the primary rabbit antibodies consecutive incubations with a secondary biotinylated anti-rabbit antibody and avidin–Cy3 were performed.
The Cy3 fluorochrome proved effective in all NSOM experiments and was superior to FITC or Cy5.
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15 Porosome The Fusion Pore Revealed by Multiple Imaging Modalities Bhanu P. Jena Summary Secretion occurs in all cells of multicellular organisms and involves the delivery of secretory products packaged in membrane-bound vesicles to the cell exterior. Specialized cells for neurotransmission, enzyme secretion, or hormone release utilize a highly regulated secretory process. Secretory vesicles are transported to specific sites at the plasma membrane, where they dock and fuse to release their contents. Similar to other cellular processes, cell secretion is found to be highly regulated and a precisely orchestrated event. It has been demonstrated that membranebound secretory vesicles dock and fuse at porosomes, which are specialized supramolecular structures at the cell plasma membrane. Swelling of secretory vesicles results in a buildup of pressure, allowing expulsion of intravesicular contents. The extent of secretory vesicle swelling dictates the amount of intravesicular contents expelled during secretion. The discovery of the porosome, its isolation, its structure and dynamics at nanometer resolution and in real time, and its biochemical composition and functional reconstitution into artificial lipid membrane have been determined. The molecular mechanism of secretory vesicle fusion at the base of porosomes and vesicle swelling have also been resolved. These findings reveal the molecular machinery and mechanism of cell secretion. In this chapter, the discovery of the porosome, its isolation, its structure and dynamics at nanometer resolution and in real time, and its biochemical composition and functional reconstitution into artificial lipid membrane are discussed. Key Words: Fusion pore; atomic force microscopy; electron microscopy.
1. Introduction Secretion and membrane fusion are fundamental cellular processes regulating endoplasmic reticulum-Golgi transport, plasma membrane recycling, cell division, sexual reproduction, acid secretion, and the release of enzymes, hormones, and neurotransmitters, to name just a few. It is, therefore, no surprise that defects in secretion and membrane fusion give rise to diseases like diabetes, From: Methods in Molecular Biology, vol. 319: Cell Imaging Techniques: Methods and Protocols Edited by: D. J. Taatjes and B. T. Mossman © Humana Press Inc., Totowa, NJ
Alzheimer's, Parkinson's, acute gastroduodenal diseases, gastroesophageal reflux disease, intestinal infections due to inhibition of gastric acid secretion, biliary diseases resulting from malfunction of secretion from hepatocytes, polycystic ovarian disease as a result of altered gonadotropin secretion, and Gitelman disease associated with growth hormone deficiency and disturbances in vasopressin secretion are only a few examples. Understanding cellular secretion and membrane fusion helps not only to advance our understanding of these vital cellular and physiological processes, but in the development of drugs also to help ameliorate secretory defects, provide insight into our understanding of cellular entry and exit of viruses and other pathogens, and in the development of smart drug delivery systems. Therefore, secretion and membrane fusion play an important role in health and disease. Studies (1–21), in the last decade demonstrate that membrane-bound secretory vesicles dock and transiently fuse at the base of specialized plasma membrane structures called porosomes or fusion pores, to expel vesicular contents. These studies further demonstrate that during secretion, secretory vesicles swell, enabling the expulsion of intravesicular contents through porosomes (16,19–21). With these findings (1–21), a new understanding of cell secretion has emerged and confirmed by a number of laboratories (22–27). Throughout history, the development of new imaging tools has provided new insights into our perceptions of the living world and profoundly impacted human health. The invention of the light microscope almost 300 yr ago was the first catalyst, propelling us into the era of modern biology and medicine. Using the light microscope, a giant step into the gates of modern medicine was made by the discovery of the unit of life—the cell. The structure and morphology of normal and diseased cells and of disease-causing microorganisms were revealed for the first time using the light microscope. Then, in 1938, with the birth of the electron microscope (EM), dawned a new era in biology and medicine. Through the mid-1940s and 1950s, a number of subcellular organelles were discovered and their functions determined using the EM. Viruses, the new life-forms were discovered and observed for the first time and implicated in diseases ranging from the common cold to acquired immune disease (aquired immune deficiency syndrome [AIDS]). Despite the capability of the EM to image biological samples at near-nanometer resolution, sample processing (fixation, dehydration, staining) results in morphological alterations and was a major concern. Then, in the mid-1980s, scanning probe microscopy evolved (1,28), further extending our perception of the living world to the near-atomic realm. One such scanning probe microscope, the atomic force microscope (AFM), has helped overcome both limitations of light and electron microscopy, enabling determination of the structure and dynamics of single biomolecules and live cells in three dimensions, at
Porosome
Fig. 1. Schematic diagram depicting key components of an atomic force microscope. (From ref. 12.)
near-angstrom resolution. This unique capability of the AFM has given rise to a new discipline of "nanobioscience," heralding a new era in biology and medicine. Using AFM in combination with conventional tools and techniques, this past decade has witnessed advances in our understanding of cell secretion (1–21) and membrane fusion (9,17,18,29), as noted earlier in the chapter. The resolving power of the light microscope is dependent on the wavelength of the light used and, therefore, 250–300 nm in lateral and much less in depth resolution can be achieved at best. The porosome or fusion pore in live secretory cells are cup-shaped structures, measuring 100–150 nm at its opening and 15–30 nm in relative depth in the exocrime pancreas, and just 10 nm at the presynaptic membrane of the nerve terminal. As a result, it had evaded visual detection until its discovery using the AFM (3–8,15). The development of the AFM (28) has enabled the imaging of live cells in physiological buffer at nanometer to subnanometer resolution. In AFM, a probe tip microfabricated from silicon or silicon nitride and mounted on a cantilever spring is used to scan the surface of the sample at a constant force. Either the probe or the sample can be precisely moved in a raster pattern using a xyz piezotube to scan the surface of the sample (see Fig. 1). The deflection of the cantilever, measured optically, is used to generate an isoforce relief of the sample (30). Thus, force is used to image surface profiles of objects by the AFM, allowing imaging of live cells and subcellular structures submerged in physiological buffer solutions. To image live cells, the scanning probe of the AFM operates in physiological
buffers and can do so under two modes: contact or tapping. In the contact mode, the probe is in direct contact with the sample surface as it scans at a constant vertical force. Although high-resolution AFM images can be obtained in this mode of AFM operation, sample height information generated might not be accurate because the vertical scanning force could depress the soft cell. However, information on the viscoelastic properties of the cell and the spring constant of the cantilever enables measurement of the cell height. In the tapping mode, on the other hand, the cantilever resonates and the tip makes brief contacts with the sample. In the tapping mode in fluid, lateral forces are virtually negligible. It is therefore important that the topology of living cells be obtained using both contact and tapping modes of AFM operation in fluid. The scanning rate of the tip over the sample also plays an important role on the quality of the image. Because cells are soft samples, a high scanning rate would influence its shape. Hence, a slow tip movement over the cell would be ideal and results in minimal distortion and better image resolution. Rapid cellular events might be further monitored by using section analysis. To examine isolated cells by the AFM, freshly cleaved mica coated with Cel-Tak has also been used with great success (3–8). Also, to obtain optimal resolution, the contents of the bathing medium as well as the cell surface to be scanned should be devoid of any debris. 2. Methods 2.1. Isolation of Pancreatic Acinar Cells Acinar cells for secretion experiments, light microscopy, AFM, and EM were isolated using a minor modification of a published procedure. For each experiment, a male Sprague–Dawley rat weighing 80–100 g was euthanized by CO2 inhalation. The pancreas was dissected and diced into 0.5-mm3 sections with a razor blade, mildly agitated for 10 min at 37°C in a siliconized glass tube with 5 mL of oxygenated buffer A (98 mM NaCl, 4.8 mM KCl, 2 mM CaCl2, 1.2 mM MgCl2, 0.1% bovine serum albumin, 0.01% soybean trypsin inhibitor, 25 mM HEPES, pH 7.4) containing 1000 units of collagenase. The suspension of acini was filtered through a 224-µm Spectra-Mesh (Spectrum Laboratory Products, Rancho Dominguez, CA) polyethylene filter to remove large clumps of acini and undissociated tissue. The acini were washed six times, 50 mL per wash, with ice-cold buffer A. Isolated rat pancreatic acini and acinar cells were plated on Cell-Tak-coated (Collaborative Biomedical Products, Bedford, MA) glass cover slips. Two to three hours after plating, cells were imaged with the AFM before and during stimulation of secretion. Isolated acinar cells and hemiacinar preparations were used in the study because fusion of secretory vesicles at the protein membrane (PM) in these cells occurs at the apical region facing the acinar lumen.
2.2. Pancreatic Plasma Membrane Preparation Rat pancreatic PM fractions were isolated using a modification of a published method. Male Sprague–Dawley rats weighing 70–100 g were euthanized by CO2 inhalation. The pancreases were removed and placed in ice-cold phosphate-buffered saline (PBS), pH 7.5. Adipose tissue was removed and the pancreases were diced into 0.5-mm3 pieces using a razor blade in a few drops of homogenization buffer A (1.25 M sucrose, 0.01% trypsin inhibitor, and 25 mM HEPES, pH 6.5). The diced tissue was homogenized in 15% (w/v) ice-cold homogenization buffer A using four strokes at maximum speed of a motordriven pestle (Wheaton overhead stirrer). One and a half milliliters of the homogenate was layered over a 125-µL cushion of 2 M sucrose and 500 µL of 0.3 M sucrose was layered onto the homogenate in Beckman centrifuge tubes. After centrifugation at 145,000g for 90 min in a Sorvall AH-650 rotor, the material banding between the 1.2 M and 0.3 M sucrose interface was collected and the protein concentration was determined. For each experiment, fresh PM was prepared and used the same day in all AFM experiments. 2.2.1. Isolation of Synaptosomes, Synaptosomal Membrane, and Synaptic Vesicles Synaptosomes, synaptosomal membrane, and synaptic vesicles were prepared from rat brains (31,32). Whole rat brains from Sprague–Dawley rats (100–150 g) were isolated and placed in ice-cold buffered sucrose solution (5 mM HEPES, pH 7.4, 0.32 M sucrose) supplemented with protease inhibitor cocktail (Sigma, St. Louis, MO) and homogenized using a Teflon–glass homogenizer (8–10 strokes). The total homogenate was centrifuged for 3 min at 2500g. The supernatant fraction was further centrifuged for 15 min at 14,500g, and the resultant pellet was resuspended in buffered sucrose solution, which was loaded onto 3-10-23% Percoll gradients. After centrifugation at 28,000g for 6 min, the enriched synaptosomal fraction was collected at the 10–23% Percoll gradient interface. To isolate synaptic vesicles and synaptosomal membrane (32), isolated synaptosomes were diluted with 9 vol of ice-cold H2O (hypotonic lysis of synaptosomes to release synaptic vesicles) and immediately homogenized with three strokes in Dounce homogenizer, followed by a 30-min incubation on ice. The homogenate was centrifuged for 20 min at 25,500g, and the resultant pellet (enriched synaptosomal membrane preparation) and supernatant (enriched synaptic vesicles preparation) were used in our studies. 2.2.2. Preparation of Lipid Membrane on Mica and Porosome Reconstitution To prepare lipid membrane on mica for AFM studies, freshly cleaved mica disks were placed in a fluid chamber. Two hundred microliters of the bilayer
bath solution, containing 140 mM NaCl, 10 mM HEPES, and 1 mM CaCl2, were placed at the center of the cleaved mica disk. Ten microliters of the brain lipid vesicles were added to the above bath solution. The mixture was then allowed to incubate for 60 min at room temperature, before washing (10X), using 100 µL bath solution/wash. The lipid membrane on mica was imaged by the AFM before and after the addition of immunoisolated porosomes. 2.3. Atomic Force Microscopy "Pits" and fusion pores at the PM in live pancreatic acinar secreting cells in PBS, pH 7.5, were imaged with the AFM (Bioscope III, Digital Instruments) using both contact and tapping modes. All images presented in this chapter were obtained in the "tapping" mode in fluid, using silicon nitride tips with a spring constant of 0.06 Nm, and an imaging force of
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