Kidney-failure (End stage renal disease, ESRD) is a debilitating disease with no treatments or therapeutics. Patients suffering from ESRD go on dialysis and require a kidney transplant in order to regain kidney function. A majority of kidney diseases and ESRD originate within the glomerulus and are associated with proteinuria. ESRD imposes a significant burden on the patients and the health care system worldwide in is in urgent need for effective therapies and better treatment options.
The study of kidney disease is complex because the onset is often undetected, diseases may be acute or chronic in nature, the genetic makeup of the host leads to variable clinical syndromes, and multiple organs are often involved simultaneously. Cell cultures and animal models are necessary to study disease susceptibility, mechanisms, prognosis, and potential therapies. In renal research, the use of experimental animal models has proved invaluable. Nevertheless, animal models are often limited because they do not always fully replicate the human diseases. For example, the current mouse models of diabetic nephropathy do not typically demonstrate the features of human diseases, such as Kimmelstiel-Wilson nodules.
One of the fastest moving areas of research progress in nephrology has been the appreciation of the importance of the visceral glomerular epithelial cell, hereinafter referred to as the podocyte, in health and disease. Podocytes form the final filtration barrier for blood in the glomerulus and play a central role in glomerular diseases that ultimately result in ESRD (Mundel P. and Reiser J. Kidney Int 2010, 77: 571-80). Podocytes are terminally differentiated pericyte-like cells that reside on the outer surface of the glomerular basement membrane and give rise to long major processes that branch into structures known as foot processes (FPs). FPs of adjacent podocyte cells interdigitate and form narrow filtration slits, a structure known as the slit diaphragm (SD), which forms a molecular sieve that the body uses to retain proteins in the blood, while filtering small molecules and other agents in to the urinary space. Thus, podocyte injury is a common theme in many proteinuric kidney diseases (Mundel P. and Reiser J. Kidney Int 2010, 77: 571-80). Additionally, because of this central role of podocytes in maintaining healthy kidney function, podocytes represent a key target for the development of novel therapeutics to treat a variety of kidney diseases.
Because podocytes play a key role in the prevention of proteinuria in the healthy situation, they are important targets of injury in a variety of renal diseases and are important determinants of outcome. Improved understanding of podocyte biology has come from two main arenas: first, molecular genetics of single gene disorders which lead to rare forms of congenital nephrotic syndrome; and second, focused study of this specialized cell type in vivo and in vitro. Research has also shed light on specific proteins, RNA and cell signaling mechanisms in the podocytes that represent good targets for drug discovery efforts, diagnostics and therapeutics. For example, recent studies have shown that stabilization of the podocyte actin cyctoskeleton, both in vitro and in vivo, can significantly protect the podocytes and ameliorate proteinuria in vivo, suggesting that such agents may have a therapeutic potential for treating proteinuria in humans.
Although podocyte cultures do not fully replicate the in vivo environment, they have several major advantages. These include the ability to directly study mechanistic events, to control the environment such that specific hypotheses can be tested, and that multiple experiments can be performed to validate the initial observations.
Currently, a significant limitation in using podocyte cultures is the difficulty in handling podocytes and using them in a medium- or high-throughput assay environment (Reiser, Gupta et al, Kidney Int. (2010) 77, 662). Assays for systematic, high-throughput screening of libraries of potential agents are not available. Podocytes are a challenging cell type for use in such assays.
A need exists for developing podocyte cultures that are suitable for drug screening (such as high throughput drug screening), and for studying molecular pathways involved in glomerular diseases.