<title>1. INTRODUCTION</title> <p>Studies on primary beta cells
are required in almost all fields of islet research. Unfortunately, it has been
far from simple to obtain high numbers of pure and viable beta-cells.
Previously published methods for purification of beta cells from other
endocrine islet cells are based on the sedimentation velocity [##REF##6114890##1##], labelling of beta-cell
surface antigens [##REF##11043864##2##], or the endogenous
(auto) fluorescence of beta cells [##REF##6130019##3##–##REF##6129116##6##]. Most of these methods
have a limited applicability, as they are associated with low yields of beta-cells,
or permanent deterioration of cellular function caused by mandatory labelling
of the cellular membrane.</p> <p>A technology that we felt might be developed into an efficacious beta-cell
purification method, with high yield, and without significant deterioration of
function, is the sorting of beta cells based on their autofluorescence. Beta cells
show higher autofluorescence than other islet cells. This autofluorescence is
caused by the high cellular content of flavin adenine dinucleotide (FAD) [##REF##17392##7##–##UREF##0##9##], an electron acceptor
in the metabolic oxidative phosphorylation. The principle applicability of
isolation of beta cells by autofluorescence was shown by Rabinovitch et al. [##REF##6129168##4##] and Van
de Winkel et al. [##REF##6123526##10##], but
resulted in a preparation with considerable amounts of alpha-cells.</p> <p>In this study, we investigated whether beta-cell yield and purity can be improved by
manipulating islet cell metabolism and FAD autofluorescence. In addition, we
investigated whether selection based on nicotinamide-adenine dinucleotide
(phosphate) (NAD(P)H) fluorescence can be a valuable secondary isolation
parameter. We show that preincubation of islet cells in culture media
containing graded loads of D-glucose and amino acids alters cellular
autofluorescence, beta-cell purity and yield after isolation.</p>
<title>2. MATERIALS AND METHODS</title> <title>2.1. Experimental animals</title> <p>Pathogen-free inbred male Lewis rats weighing 300–320 g were used
as islet donors. All experimental animals were obtained from Harlan (Horst, The
Netherlands). The animals were fed standard rat chow and acidified water ad
libitum. All animal experiments were performed after receiving approval of the
institutional Animal Care Committee of the Groningen University
and all animals received human care in compliance with the Dutch Law on
Experimental Animal Care.</p> <title>2.2. Preparation of islet cells</title> <p>Rat islets were isolated as previously described [##REF##15211119##11##]. Islet isolation was performed in Krebs Ringer HEPES
(KRH) buffered with 25 mmol/L HEPES containing 10% (wt/vol.) Bovine serum albumin
(BSA). After ductal distention of the pancreas with KRH 10% BSA, the organ was
chopped into pieces of 1-mm<sup>2</sup>. Next, the tissue fragments were
subjected to 3 successive washing steps with KRH 10% BSA to remove proteases
that might have leaked out of the exocrine part of the pancreas. Subsequently,
the chopped pancreas was brought into a 25 mL Erlenmeyer flask and incubated at
37°C with 1.0 mg/mL of collagenase with KRH 10% BSA. The total volume of tissue
and buffer was always 13 mL. After 10 minutes, the tissue fragments were placed
in a 50 mL tube with 30 mL KRH 10% BSA at 4°C. The digested tissue was washed
and sedimented 2 times to remove collagenase and exocrine-derived proteases.
Next, the tissue fragments were placed again in a 25 mL Erlenmeyer flask and
incubated at 37°C with 0.7 mg/mL of collagenase with KRH 10% BSA in a total
volume of 13 mL. After 8 minutes, the tissue fragments were placed in a 50 mL
tube with 30 mL KRH 10% BSA at 4°C. Finally, the digest was allowed to sediment
and was washed twice with RPMI 1640 containing 1% (wt/vol.) BSA. Islets
were separated from exocrine tissue by centrifugation over a discontinuous
dextran gradient [##REF##1702970##12##] and further purified by
handpicking. Purified islets were
washed in RPMI 1640 (Gibco) containing 1% BSA at least 8 times and cultured
overnight in CMRL (Gibco) supplemented with 1375 mM D-glucose, penicillin,
streptomycin, and 10% fetal calf serum (FCS) (Gibco, Calif, USA) at 37°C, 5% CO<sub>2</sub>.</p> <p>The next morning, islets were collected and
washed in phosphate-buffered saline twice before being dispersed into single
cells by mechanical shaking at 37°C for 3 minutes in 0.05% trypsin, 0.5 mM
EDTA (Gibco, Calif, USA). The enzymatic reaction was stopped by adding KRH 10% BSA. Cells were washed,
and remaining cell clumps were removed by filtering the cell mixture over a 35 <italic>μ</italic>m pore size filter (BD Falcon).</p> <p>On average, 550 islets per pancreas, and 1500 to 2000 cells per islet were
obtained. Exclusion of trypane blue positive cells indicated that more than 95%
of the cells were viable after the procedure.</p> <title>2.3. Islet cell incubation</title> <p>In the first experiment,
dispersed islet cells were incubated in our standard medium, that is, RPMI 1640
(Gibco, Calif, USA) containing 1% BSA for one hour at 37°C before beta-cell sorting. Subsequently,
in the second experiment, to study the influence of various concentrations of
glucose or amino acids on beta-cell purity, dispersed islets were incubated in
the following media containing different glucose and amino acids concentrations
(see Tables <xref ref-type="table" rid="tab1">1</xref>(a) and <xref ref-type="table" rid="tab1">1</xref>(b)) MEM (Gibco, Calif, USA), RPMI 1640 (Gibco, Calif,
USA), CMRL (Gibco, Calif, USA), or DMEM-F12 (Gibco, Calif, USA) all
supplemented with 1% BSA for one hour at 37°C before cell sorting. In the third
experiment, we evaluated the effects of glucose and amino acids on beta-cell
purity separately; therefore, dispersed islets were incubated in unsupplemented
MEM medium (5.5 mmol/L D-glucose and 553 mg/L amino acids), or MEM medium
supplemented to either 10, 15, or 20 mmol/L D-Glucose (pH 7.4), or 1000, 1500,
or 2000 mg/L amino acids (pH 7,4) for one hour at 37°C prior to flow cytometry.</p> <p>In all experiments, twenty minutes prior to analysis 2 <italic>μ</italic>g/mL propidium iodide (PI) (Sigma-Aldrich, Mo, USA) was added to the cell mixtures to exclude
apoptotic cells.</p> <title>2.4. Flow cytometry and cell sorting</title> <p>In experiments 1 and 2, high and low fluorescent cells were sorted on a fluorescence activated 
MoFlo high speed cell sorter (Dakocytomation BV, Heverlee, Belgium). In experiment 3, flow
Cytometric analysis was performed on the LSR II system (BD Biosciences, Calif, USA). In all experiments,
side-scattered light was collected at an angle of 90°, and signals were
linearly amplified. The forward and side scatter patterns of the cells were analyzed
at a wave length of 488 nm. The flavin adenine dinucleotide (FAD) content of
the cells was analyzed at an excitation wave length of 488 nm, and collected at
525 nm. The PI staining was analyzed at an excitation wavelength of 488 nm, and
collected at 675 nm. The nicotinamide-adenine dinucleotide (phosphate)
(NAD(P)H) content was analyzed at an excitation wave length of 350 nm (UV)
using an Argon laser, and collected at 450 nm. The 488 nm laser was operated at
80 mW and the Argon laser at 25 mW. The flow rate was typically a few hundred
cells per second.</p> <p>Flow cytometry results were analyzed using standardized
gating on living cells using Winlist 5.0 software (Verity Software House, Topsham, Me, USA).
After gating on the cell in the forward and side scatter plot, this gate was
copied to a second plot in which we selected the living cells (i.e., propidium
iodide negative cells) and both gates are then copied to a histogram showing
FAD fluorescence (see ##FIG##0##Figure 1(a)##). Analysis of the single cells by flow
cytometry at an excitation wavelength of 488 nm resulted in two distinct
autofluorescent cell populations. The first cell population showed low
autofluorescence (7.24 ± 0.93 fluorescence intensity units (FIUs)). The second
cell population showed high autofluorescence (29.94 ± 4.79 FIU) (see 
##FIG##0##Figure 1(a)##).
In experiment 3, we measured the mean fluorescent intensity of the high and low
fluorescent cells and evaluated the increase or decrease of fluorescent
intensity (i.e., fold change) induced by glucose or amino acids versus unsupplemented
medium.</p> <title>2.5. Immunostaining</title> <p>Both the high and low fluorescent isolated cell fractions were spotted on glass slides. Per slide, 50 000 cells were spotted, 
using a cytospin centrifuge. Cells were spun down at
500 rpm for 5 minutes at room temperature. Spots were dried, and the cells were
fixed in Bouin fixative for 3 minutes at room temperature followed by
immunostaining for the following pancreatic hormones or a combination of these
hormones: insulin, glucagon, and somatostatin-pancreatic polypeptide (PP/somatostatin).</p> <p>Briefly, slides stained for PP/somatostatin were blocked for endogenous peroxidase for
20 minutes in methanol containing 0.9% (vol./vol.) H<sub>2</sub>O<sub>2</sub>.
Slides were washed and incubated with the primary antibody (polyclonal rabbit antirat
PP (Abcam Inc., Mass, USA)
1:600, and monoclonal rabbit antirat somatostatin (Sigma-Aldrich, Mo, USA)
1:800) for 1 hour at room temperature. After washing in phosphate buffered
saline, secondary (goat antirabbit IgG horse radish peroxidase (HRP) conjugated
(Dako, Glostrup, Denmark) (1:50) in the presence of 1% goat serum) and tertiary antibodies (rabbit anti-goat
IgG conjugated to HRP (Dako, Glostrup, Denmark) (1:50) in the presence of 1% Rabbit Serum) were incubated for 30 minutes at
room temperature. The presence of an antibody-PP/somatostatin interaction was
visualized after an enzymatic reaction using 3-amino-9-ethyl-carbazole (Sigma-Aldrich, Mo, USA).
Cell nuclei were stained using haematoxylin for 1 to 3 minutes. Slides were
embedded in Kaiser's glycerol gelatin (Merck &amp; Co., Inc., NJ, USA). At random, at least
500 cells per spot were analyzed under light microscopy.</p> <p>Slides stained for insulin or glucagon were incubated with the primary antibody
(monoclonal mouse anti-rat insulin (IgG<sub>1</sub>) (Sigma-Aldrich,
Mo, USA) 1:750; or monoclonal mouse anti-rat glucagon (IgG<sub>1</sub>) (Sigma-Aldrich, Mo,
USA) 1:2000 for 1 hour at room temperature. After washing, the secondary antibody (goat anti-mouse
IgG<sub>1</sub> FITC conjugated (Dako, Glostrup, Denmark) (1:50) in the presence of 1% goat serum) 
was incubated for 30 minutes at room
temperature in the dark. Cell nuclei were stained using 5 <italic>μ</italic>g/mL DAPI (Roche Inc., NJ, USA)
for 20 minutes at room temperature in the dark. Spots were embedded in cytifluor (Agar Scientifics, Essex, England). The
presence of insulin or glucagon staining was analyzed using a Leica DMRXA
fluorescent microscope and Leica Qwin Pro Software. At random, at least 500
cells were analyzed per spot.</p> <title>2.6. Statistics</title> <p>Results are displayed as the
mean ± standard error of the mean (SEM) of at least 4 experiments. The
Mann-Whitney <italic>U</italic> test was used to determine statistically significant
differences. <italic>P</italic>-values &lt;.05 were considered to be statistically
significant.</p>
<title>3. RESULTS</title> <title>3.1. Experiment 1: fluorescent patterns in islet cells cultured in standard medium (RPMI 1640)</title> <p>Isolated rat islets were
dispersed into a single cell suspension by enzymatic digestion and incubated in
RPMI 1640. On average, we obtained 1872 ± 507 viable cells (&gt;95% of the
total cell population) per islet (see ##TAB##2##Table 2##). Analysis of the single cells by
flow cytometry at an excitation wavelength of 488 nm [##REF##6129168##4##, ##REF##6123526##10##] resulted in two
distinct autofluorescent cell populations. The first cell population showed low
autofluorescence (7.24 ± 0.93 fluorescence intensity units (FIUs)), and
comprised 46.25 ± 14.37% of the total viable endocrine cell population. The second
cell population showed high autofluorescence (29.94 ± 4.79 FIU) and comprised
48.36 ± 6.96% of the total viable 
endocrine cell population (see ##FIG##0##Figure 1(a)##).</p> <p>In order to investigate the phenotype of these two distinct cell populations, the
high and low fluorescent cell populations were sorted after which
immunohistochemistry was performed. The low fluorescent endocrine cell
population was composed of 13.52 ± 2.08% insulin 
positive beta cells (see ##FIG##0##Figure 1(b)##) and 38.60 ± 12.95% glucagon 
positive alpha cells (see ##FIG##0##Figure 1(b)##). The
high autofluorescent cell population proved to be mainly beta cells (89.10 ± 5.45%)
(see ##FIG##0##Figure 1(b)##), but also contained a large portion of alpha-cells (18.65 ± 2.81%)
(see ##FIG##0##Figure 1(b)##).</p> <p>These results show that beta-cell sorting based on cellular autofluorescence is possible; 
however, the purity needs to be improved.</p> <title>3.2. Experiment 2: effect of preincubation with different media on beta-cell purity</title> <p>As shown in the previous section, the vast majority of the beta cells were found in the high
autofluorescent endocrine cell population. This population was, however,
contaminated with a considerable number of glucagon-positive alpha-cells (18.65 ± 2.81%). We questioned whether we could change the fluorescent pattern of the
high fluorescent beta-cell population and the low fluorescent nonbeta-cell
population by altering the metabolic state of the endocrine cells.</p> <p>The cellular metabolic state was altered in this second
experiment by short-term incubation in other culture media, containing
different amounts of D-glucose and amino acids. Both D-glucose and amino acids
are well-known stimuli of metabolic activity and therefore suitable candidates
to alter cellular flavin adenine dinucleotide (FAD) content and autofluorescence.
To this end, endocrine single cells were incubated in RPMI 1640, MEM, DMEM-F12,
and CMRL medium 1 hour prior to cell sorting. <xref ref-type="table" rid="tab1">Table 1</xref> shows the composition of
the applied culture media. Preincubation of endocrine single cells in culture
medium containing varying amounts of D-glucose and amino acids had a
considerable effect on the FAD fluorescence of both the high and low
autofluorescent cell population (see ##FIG##1##Figure 2##(a)). When incubated in RPMI 1640,
two partially overlapping fluorescent cell populations were observed. Preincubation
of the cells in either DMEM-F12 or CMRL medium resulted in a more scattered
fluorescence pattern in the low fluorescent cell population and a decreased
difference in fluorescence intensity between the high fluorescent beta cells
and the low fluorescent nonbeta-cell population (see ##FIG##1##Figure 2##(a)). Despite the
altered fluorescent pattern, sorting of the high FAD fluorescent cells still
resulted in a considerable alpha-cell contamination. When dispersed islets were
incubated in CMRL, alpha-cell contamination was not significantly different
from preincubation in RPMI 1640. When cells were incubated in DMEM-F12 medium, alpha-cell
contamination was reduced to 8.54 ± 0.36%, which was significantly lower
compared to preincubation in RPMI 1640 (<italic>P</italic> = .03 Mann-Whitney <italic>U</italic> 
test) (see ##FIG##1##Figure 2##(b)). However, preincubation of the endocrine single cells in MEM medium,
containing only 5.5 mM D-glucose and 553 mg/L amino acids, showed two very
distinct fluorescent cell populations with an increased difference in fluorescence
intensity between the 2 populations compared to RPMI 1640 preincubation (see 
##FIG##1##Figure 2##(a)). Moreover, sorting of the high fluorescent beta-cell fraction resulted in
only 3.18 ± 0.99% nonbeta-cell 
contamination (see Figures ##FIG##1##2##(b), ##FIG##1##2##(c), 
and ##FIG##1##2##(d)),
which was significantly less than the nonbeta-cell contamination observed when
the endocrine cells were preincubated in RPMI 1640 (<italic>P</italic> = .03 Mann-Whitney <italic>U</italic>
test), and also DMEM-F12 (<italic>P</italic> = .03, Mann-Whitney <italic>U</italic> test) (see 
##FIG##1##Figure 2##(b)).
The percentage of insulin positive cells
in the high FAD fluorescent cell fraction slightly increased when incubated in
either DMEM-F12 or MEM medium, but this increase was not statistically
significant (see ##FIG##1##Figure 2##(b)). Analysis of the presence of PP/Somatostatin
positive cells in the high and low fluorescent cell fractions after
preincubation in MEM medium and subsequent sorting showed the presence of these
cells in the low FAD fluorescent cell fraction, but not the high fluorescent beta-cell
fraction (see ##FIG##1##Figure 2##(d)).</p> <p>These data demonstrated that preincubation of endocrine single cells in culture
medium composed of high or low concentrations of amino acids or D-glucose
alters cellular FAD fluorescence. This altered FAD metabolism can be applied to
increase the beta-cell purity by flow
cytometry.</p> <p>Both D-glucose levels and amino acids appear to influence beta and nonbeta-cell
autofluorescence. Preincubation of single cell endocrine cells in MEM medium,
containing both low concentrations of amino acids and D-glucose, resulted in
higher beta-cell purity when compared to preincubation in standard RPMI 1640.
However, preincubation in MEM still results in an alpha-cell contamination in
the isolated beta-cell fraction.</p> <title>3.3. Experiment 3: effects of glucose and amino acids on beta-cell autofluorescence</title> <title>3.3.1. Effects of glucose and amino acids on FAD fluorescence</title> <p>As discussed in the previous section, preincubation of
endocrine cells in MEM medium resulted in a beta-cell population of relatively
high purity. However, the population still contained 3.18% alpha-cell
contamination.</p> <p>Both the D-glucose and amino acids differed
in the varying media, and it is well known, and shown by the previous
experiment, that both components influence the metabolic state, and thus the
FAD content of the endocrine cells. Beta-cell purity and yield might be further
optimized by preincubation in MEM medium supplemented with D-glucose or amino
acids. Dispersed islets were incubated in unsupplemented MEM medium, or MEM
medium supplemented with D-glucose to a concentration of 10, 15, and 20 mM, or
amino acids to a concentration of 1000, 1500, or 2000 mg/L.</p> <p>Incubation of dispersed islets with
increasing amounts of D-glucose showed an increased FAD fluorescence in the beta-cell
population (high FAD fluorescent cell population), which was significantly
increased at 20 mM D-glucose (FAD fluorescence fold induction of 1.26 ± 0.12 times when compared to unsupplemented MEM medium, <italic>P</italic> &lt; .05,
Mann-Whitney <italic>U</italic> test) (see 
##FIG##2##Figure 3(a)##). The FAD fluorescence of the nonbeta-cell
population was not significantly influenced by incubation with increasing
D-glucose concentrations (see ##FIG##2##Figure 3(a)##). Incubation of single cell endocrine
cells with increasing amounts of amino acids showed an increased FAD
fluorescence in both the beta-cell and the nonbeta-cell population, where only
the increased fluorescence of the beta-cell population was statistically
significant (1500 mg/L: FAD fluorescence fold induction of 1.44 ± 0.15 times
when compared to unsupplemented MEM medium, <italic>P</italic> &lt; .05, Mann-Whitney <italic>U</italic>
test) (see ##FIG##2##Figure 3(b)##).</p> <title>3.3.2. Effects of glucose and amino acids on NADPH fluorescence</title> <p>In addition to measuring FAD fluorescence, nicotinamide-adenine
dinucleotide (phosphate) (NAD(P)H) fluorescence was also assessed as a
secondary parameter to measure beta-cell metabolism by flow cytometry.
Measuring autofluorescence at 350 nm represents both the cellular NADH and
NADPH content.</p> <p>Flow cytometric analysis of the endocrine
single cell mixture incubated in unsupplemented MEM medium showed a single
NAD(P)H fluorescent population when excited at 350 nm (see ##FIG##3##Figure 4(a)##). When
incubated in MEM with increasing amounts of amino acids, a more fluorescent
cell population appeared as a minor shoulder (results not shown), whereas after
incubation in MEM containing increasing amounts of D-glucose a highly
fluorescent peak appeared (see ##FIG##3##Figure 4(b)##). Thus, both glucose and amino acids
appear to influence NAD(P)H fluorescence.</p> <title>3.3.3. The role of NADPH fluorescence in increasing the purity of the beta cells
population using FAD fluorescence</title> <p>Since both glucose and amino acids appeared to affect NAD(P)H fluorescence, the next
step was to analyze whether it would be possible to use NAD(P)H fluorescence
together with FAD fluorescence in order to get an even more purified beta-cell
population compared with the use of only FAD fluorescence. ##FIG##4##Figure 5(a)## shows
that if cells are incubated in unsupplemented MEM medium, the dot plot of FAD
fluorescence versus NAD(P)H fluorescence showed three populations: cells low in
both FAD and low in NAD(P)H fluorescence, cells high in FAD fluorescence and
low in NAD(P)H fluorescence, and a small population of cells high in both FAD
and NAD(P)H fluorescence. When dispersed islets were incubated in MEM
supplemented with D-glucose to 20 mmol/L D-glucose, the cell populations differ
(see ##FIG##4##Figure 5(b)##): the population of cells with high FAD, high NAD(P)H becomes
much larger. Since in our first experiment we have shown that cells with high
FAD fluorescent cells are beta cells and cells with low FAD fluorescent cells
are nonbeta-cells, this may suggest that after stimulation with glucose, beta cells
(cells high in FAD fluorescence) also become high in NAD(P)H fluorescence.</p> <p>In ##FIG##5##Figure 6## the effect of glucose and amino acids on NAD(P)H fluorescence of beta and nonbeta
cells was analyzed. According the to results of experiment 1, beta cells were
defined as cells high in FAD fluorescence, while nonbeta cells were defined as
low in FAD fluorescence. Analysis of the NAD(P)H fluorescence level of these 2
cell population in dispersed islets showed that incubation in medium containing
increasing amounts of D-glucose increased the NAD(P)H fluorescence in the high
FAD fluorescent beta-cell population (1.21 ± 0.08, 1.31 ± 0.11, 1.31 ± 0.09 fold increase versus unsupplemented
medium, respectively, for 10, 15, and 20 mmol/L D-glucose, <italic>P</italic> &lt; .05,
Mann-Whitney <italic>U</italic> test, see 
##FIG##5##Figure 6(a)##), whereas the low FAD fluorescent nonbeta-cell
population was not influenced (see ##FIG##5##Figure 6(a)##).</p> <p>Incubation of dispersed islets in MEM supplemented with increasing amounts of amino acids
showed a minor increase in NAD(P)H fluorescence in the high FAD fluorescent beta-cell
population (1500 mg/L: 1.09 ± 0.02, <italic>P</italic> &lt; .05,
Mann-Whitney <italic>U</italic> test) (see 
##FIG##5##Figure 6(b)##). The low FAD fluorescent nonbeta-cell
population showed a decrease in NAD(P)H fluorescence in response to increasing
amounts of amino acids (see ##FIG##5##Figure 6(b)##). This decrease was significant at an
amino acid concentration of 2000 mg/L (0.88 ± 0.06, <italic>P</italic> &lt; .05, Mann-Whitney
<italic>U</italic> test).</p> <p>Next, the percentage of NAD(P)H high fluorescent cells within the beta-cell (high FAD
fluorescence) population was measured (see Figures ##FIG##5##6(c)##, ##FIG##5##6(d)##). These 
figures show that approximately 71.2 ± 7.4% of beta cells were low
NAD(P)H fluorescent cells, whereas 28.8 ± 7.4% of the beta cells were high NAD(P)H
fluorescent when cells were incubated in unsupplemented MEM medium. When cells
were incubated with MEM with increasing D-glucose concentrations (see ##FIG##5##Figure 6(c)##),
the percentage of high NAD(P)H fluorescent cells within the beta-cell
population increased, while the number of low fluorescent NAD(P)H cells within
the beta-cell population decreased (see ##FIG##5##Figure 6(c)##). When supplemented to 20 mM D-glucose, the
percentage of low NAD(P)H fluorescent beta cells decreased to 28.82 ± 7.41% (<italic>P</italic> = .06,
Mann-Whitney <italic>U</italic> test), whereas the percentage of high NAD(P)H fluorescent beta cells
increased to 61.12 ± 7.19% (<italic>P</italic> = .06, Mann-Whitney <italic>U</italic> test). After
incubation with increasing amounts of amino acids, the percentage of high
NAD(P)H fluorescent cells with the beta-cell population was unchanged (see ##FIG##5##Figure
6(d)##).</p> <p>Altogether, the results from experiment 3 show that D-glucose increased both beta-cell FAD
fluorescence and beta-cell NAD(P)H fluorescence. This resulted in an increased
percentage of highly NAD(P)H fluorescent cells within the beta-cell population
after incubation with increasing D-glucose concentrations. Nonbeta-cell FAD
fluorescence is only slightly increased after incubations with increasing
D-glucose concentrations, while NAD(P)H fluorescence of the nonbeta cells is unaffected by glucose.</p> <p>Amino acids slightly affect beta cells FAD and NAD(P)H fluorescence, but the
percentage high/low NAD(P)H fluorescent cells within the beta-cell population
remained unchanged. Nonbeta cells responded to incubations with increasing
amino acid concentration by increasing FAD fluorescence and decreasing NAD(P)H
fluorescence.</p>
<title>4. DISCUSSION</title> <p>In this study, we show that
both beta-cell and nonbeta-cell flavin adenine dinucleotide (FAD) and nicotinamide-adenine
dinucleotide (phosphate) (NAD(P)H) fluorescence can be manipulated by
preincubation of dispersed islet cells in culture medium containing graded
loads of D-glucose and amino acids. The purity and yield of the sorted beta-cell
fraction were optimized
by incubation of dispersed islets in culture medium containing 5.5 mM of
D-glucose and 553 mg/L amino acids. Under these conditions, the fluorescent
pattern of the beta-cell and the nonbeta-cell fraction showed minimal overlap,
resulting in a highly pure beta-cell fraction after sorting. Both beta-cell FAD
and NAD(P)H fluorescence could be significantly increased by incubating
dispersed islets in culture medium supplemented with 20 mM D-glucose. Nonbeta-cell
FAD and NAD(P)H fluorescence were not affected by these conditions. Sorting of beta
cells based on both the FAD and NAD(P)H fluorescence, after preincubation in
culture medium supplemented with D-glucose, could provide a method to isolate a
pure beta-cell fraction from rat islets.</p> <p>The
isolation method presented in this study clearly shows an improvement on the beta-cell
isolation procedure described by Rabinovitch et al. [##REF##6129168##4##] and Pipeleers and Pipeleers-Marichal [##REF##6114890##1##]. When only FAD
fluorescent sorting was applied, we could yield a beta-cell fraction with not
more than approximately 3% alpha-cell contamination which is fivefold lower
than the 16% contamination measured using the method described by Rabinovitch
et al. [##REF##6129168##4##] and Pipeleers and Pipeleers-Marichal [##REF##6114890##1##]. The purity degree may
be brought to 100% by applying an additional selection for NADP(H)
fluorescence.</p> <p>Our
study shows a clear effect of the D-glucose content of the applied media on the
FAD fluorescence and the ability to separate beta cells from nonbeta cells by flow
cytometry. When the FAD fluorescence of beta cells cultured in MEM containing
5.5 mM is compared to MEM supplemented with 15 or 20 mM of D-glucose (with
equal amino acid concentrations) (see ##FIG##2##Figure 3##), we see a clear cut difference
in the FAD fluorescence of beta and nonbeta cells. Although to a lesser extend,
also amino acids influence FAD fluorescence of beta and nonbeta cells. However,
the D-glucose and amino acid content of the culture medium are not the only
factor influencing the cellular FAD fluorescence. This was shown in our second
experiment in which we compared MEM medium to DMEM-F12 (with equal amino acid
concentrations) and CMRL (with equal D-glucose concentrations) (see ##FIG##1##Figure 2##).
In this experiment, we found an effect of the medium on the FAD fluorescence of
both cell populations, but it never reached the strong and distinguishable FAD
fluorescence of beta and nonbeta cells as observed with supplemented MEM
medium. These results may be explained by the differences in amino acid
composition between the culture media. Different concentrations of amino acids
(see <xref ref-type="table" rid="tab1">Table 1</xref>(b)) that are known to stimulate beta cell insulin secretion,
requiring oxidative metabolism, that is, L-Alanine, L-Glutamine, L-Arginine
together with L-Leucine [##REF##14656214##13##–##REF##17956307##18##], may explain
differences in FAD fluorescence as compared to supplemented MEM medium. But, as
separate amino acids, concentrations are relatively low compared to earlier
described stimulatory concentrations, the amino acids may work in synergy or
DMEM-F12, and CMRL may contain other factors than D-glucose and amino acids
that work in concert on the FAD fluorescence and cellular metabolism.</p> <p>Our
method of isolating beta cells on the basis of FAD and NADPH metabolism is not
only applicable as a technique to purify beta cells but also as a fast and
direct measure for the beta-cell functional viability. Cellular
autofluorescence, measured at an excitation wave length of 488 nm, is dependent
on the cellular FAD content. FAD is produced from FADH<sub>2</sub> during ATP synthesis
in the mitochondrial oxidative phosphorylation. Autofluorescence measured at an
excitation wave length of 350 nm represents both the cellular NADH and NADPH
content. NADH is the major electron acceptor during ATP synthesis. During
active metabolism, FAD and NAD(P)H fluorescence are considered to be in redox
equilibrium which implies that increases in FAD fluorescence and decreases in
NADH fluorescence represent increases in cellular oxidative phosphorylation
rates [##REF##4306507##19##, ##REF##4333612##20##]. Analysis of the beta-cell
fraction showed no response in NAD(P)H fluorescence to increasing
concentrations of amino acids, but a marked increase in NAD(P)H fluorescence
when incubated with graded loads of D-glucose. These results suggest that
besides a stimulation of ATP production and consequently the reduction of FADH<sub>2</sub> to FAD and NADH to NAD<sup>+</sup>, other metabolic pathways are active leading
to an increased cellular NAD(P)H content and fluorescence. It is well known
that beta cells use alternative metabolic pathways, such as the pyruvate malate
shuttle [##REF##7821735##21##–##REF##11716357##27##], to increase cellular
NADPH levels in addition to the production of ATP. These pathways are
considered to contribute to both the synthesis and exocytosis of insulin. When beta
cells are stimulated with high concentrations of glucose, the pyruvate
carboxylase activity is favored over pyruvate dehydrogenase, favoring both
NADPH synthesis and ATP production. Therefore, measuring beta-cell FAD and NADPH responses to D-glucose can
specifically be applied as a fast measure for the functional viability of the beta-cell
preparation.</p> <p>The NAD(P)H fluorescent patterns in the nonbeta-cell fraction suggest that NAD(P)H
and FAD levels might not be fully in redox equilibrium in these cells. It might
also represent a contamination of the nonbeta-cell fraction with beta-cells.
Fluorescent staining of the nonbeta-cell fraction showed the presence of
approximately 10% insulin positive beta-cells. The presence of these beta cells
might skew the FAD/NAD(P)H equilibrium towards a more beta-cell like FAD/NAD(P)H
balance. Measuring decreasing NAD(P)H levels in response to increasing
concentrations of amino acids in the nonbeta-cell fraction confirmed the
increase in FAD fluorescence, that is, the FAD/NAD(P)H equilibrium. The
incubation of endocrine cells with increasing amounts of amino acids or
D-glucose confirmed the finding that nonbeta-cell autofluorescence decreased in
MEM which contains low concentrations of essential ingredients. The nonbeta cells
appeared to have a stronger FAD response to increasing amounts of amino acids
compared to the response to D-glucose, with an optimum at 1500 mg/L. In
contrast to the FAD fluorescence, which showed an optimum at 1500 mg/L amino
acids, the NAD(P)H decreased further when incubated with 2000 mg/L amino acids.</p> <p>The described technology also allows for a clear identification and isolation of beta
cells with different metabolic activity. Van Schravendijk et al. [##REF##1400446##28##] have shown that
different beta-cell subpopulations respond to either high or intermediate
glucose concentrations, and these cells are well known as “high-” and
“low”-responding beta-cells, respectively. With the combined application of FAD
autofluorescence and NAD(P)H fluorescence, we show two distinct beta-cell
populations. These two separate beta-cell populations might represent these “low”-responding
beta cells and “high”-responding beta-cells.</p> <p>The isolation approach described in this paper has advantages over earlier
described beta-cell isolation methods. Isolation of beta cells based on the
detection of the high beta-cell FAD and NAD(P)H fluorescence provides a tool to
separate beta cells with minimal interference in cellular physiology. We showed
that the yield and purity of the isolated beta-cell fraction could be optimized
by preincubation of the dispersed islets in MEM medium supplemented with
D-glucose. Under these preincubation conditions, both beta-cell FAD and NAD(P)H
fluorescence were increased, whereas the nonbeta-cell fluorescence was
virtually the same.</p>
<p>Recommended by Bernard Portha</p> <p>Isolation of primary beta cells from other cells within in the pancreatic islets is of importance for many fields of islet research. However, up to now, no satisfactory method has been developed that gained high numbers of viable beta cells, without considerable alpha-cell contamination. In this study, we investigated whether rat beta cells can be isolated from nonbeta endocrine cells by manipulating the flavin adenine dinucleotide (FAD) and nicotinamide-adenine dinucleotide phosphate (NAD(P)H) autofluorescence. Beta cells were isolated from dispersed islets by flow cytometry, based on their high FAD and NAD(P)H fluorescence. To improve beta cell yield and purity, the cellular FAD and NAD(P)H contents were altered by preincubation in culture media containing varying amounts of D-glucose and amino acids. Manipulation of the cellular FAD and NAD(P)H fluorescence improves beta cell yield and purity after sorting. This method is also a fast and reliable method to measure beta cell functional viability. A conceivable application is assessing beta cell viability before transplantation.</p>
<title>ACKNOWLEDGMENT</title> <p>This research was sponsored by the Dutch Diabetes Foundation.</p>
<fig id="fig1" position="float"><label>Figure 1</label><caption><p>
<italic>Analysis of low and high fluorescent endocrine cells</italic>. Forward and side scatter analysis of endocrine cells showed a very diverse cell
population. Gating of propidium iodide negative cells showed two distinct, but overlapping cell
populations when excited at 488 nm (a). Insulin and glucagon staining of sorted
high and low fluorescent cell populations (b).</p></caption></fig> <fig id="fig2" position="float"><label>Figure 2</label><caption><p>
<italic>FAD fluorescence is altered by culture medium composition</italic>. Fluorescent patterns of dispersed islets
preincubated in MEM, RPMI, CMRL, or DMEM-F12 medium (a).
Analysis of glucagon-positive alpha cells present in the high fluorescent cell
fraction following preincubation in MEM, RPMI, CMRL, or DMEM-F12 and subsequent
sorting (b). Sorting of beta cells from the MEM incubated cell mixture resulted
in a more than 95% pure beta-cell population, whereas in the nonbeta-cell
fraction glucagon positive cells and a small amount of insulin positive cells
were observed (c). Analysis of PP/somatostatin-positive cells present in the
high and low fluorescent cell fractions following preincubation in MEM medium and subsequent
sorting (d). Results are presented as the mean ± SEM, and statistical
significance was determined using the Mann-Whitney <italic>U</italic> test. <italic>P</italic>-values &lt;.05
were considered to be statistically significant and are represented by (∗).</p></caption></fig> <fig id="fig3" position="float"><label>Figure 3</label><caption><p>
<italic>Changes in cellular FAD fluorescence</italic>. Beta-cell (black bars) and nonbeta-cell (dashed bars) FAD
fluorescence were measured in response to (a) increasing concentrations of
D-glucose and (b) amino acids. Fluorescence is represented as relative FAD
fluorescence, compared to medium containing 553 mg/L amino acids and 5.5 mM
D-Glucose. The graph represents four separate experiments, bars represent the
mean ± SEM, and statistical significance was calculated using the Mann-Whitney <italic>U</italic>
test. <italic>P</italic>-values &lt;.05 were considered to be statistically significant and are
represented by (∗).</p></caption></fig> <fig id="fig4" position="float"><label>Figure 4</label><caption><p>
<italic>Cellular NAD(P)H
fluorescence</italic>.
NAD(P)H fluorescent pattern of dispersed islets
following preincubation in (a) unsupplemented MEM medium and (b) MEM medium
supplemented to 20 mM of D-Glucose.</p></caption></fig> <fig id="fig5" position="float"><label>Figure 5</label><caption><p>
<italic>Cellular FAD
fluorescence versus NAD(P)H fluorescence</italic>.
FAD
and NAD(P)H fluorescent patterns of dispersed islets following preincubation in
(a) unsupplemented MEM medium and (b) MEM medium supplemented to 20 mM of
D-Glucose.</p></caption></fig> <fig id="fig6" position="float"><label>Figure 6</label><caption><p>
<italic>Changes in cellular NAD(P)H fluorescence</italic>.
Beta-cell
(black bars) and nonbeta-cell (dashed bars) NAD(P)H fluorescence were measured
in response to (a) increasing concentrations of D-glucose and (b) amino acids.
Fluorescence is represented as relative NAD(P)H fluorescence, compared to
medium containing 553 mg/L amino acids and 5.5 mM D-Glucose. The percentage of high
NAD(P)H fluorescent cells in response to (c) increasing concentrations of
D-glucose and (d) amino acids was measured within the FAD fluorescent beta-cell
population. Black bars represent the percentage of high NAD(P)H fluorescent
cells within the FAD fluorescent beta-cell population. The dotted bars
represent the low NAD(P)H fluorescent cells within the FAD fluorescent beta-cell
population. The graphs represent four separate experiments, bars represent the
mean ± SEM, and statistical significance was calculated using the Mann-Whitney <italic>U</italic>
test. <italic>P</italic>-values &lt;.05 were considered to be statistically significant and are
represented by (∗).</p></caption></fig>
<table-wrap id="tab1a" position="anchor"><label>(a)</label><table frame="hsides" rules="groups"><thead><tr><th align="left" rowspan="1" colspan="1"/><th align="center" rowspan="1" colspan="1">D-Glucose (mM)</th><th align="center" rowspan="1" colspan="1">Amino acid content (mg/L)</th><th align="center" rowspan="1" colspan="1">FAD (mg/L)</th><th align="center" rowspan="1" colspan="1">Riboflavin (mg/L)</th></tr></thead><tbody><tr><td align="left" rowspan="1" colspan="1">MEM</td><td align="center" rowspan="1" colspan="1">5.5</td><td align="center" rowspan="1" colspan="1">553</td><td align="center" rowspan="1" colspan="1">0</td><td align="center" rowspan="1" colspan="1">0.1</td></tr><tr><td align="left" rowspan="1" colspan="1">RPMI</td><td align="center" rowspan="1" colspan="1">11</td><td align="center" rowspan="1" colspan="1">1010</td><td align="center" rowspan="1" colspan="1">0</td><td align="center" rowspan="1" colspan="1">0.2</td></tr><tr><td align="left" rowspan="1" colspan="1">CMRL</td><td align="center" rowspan="1" colspan="1">5.5</td><td align="center" rowspan="1" colspan="1">860</td><td align="center" rowspan="1" colspan="1">1</td><td align="center" rowspan="1" colspan="1">0.01</td></tr><tr><td align="left" rowspan="1" colspan="1">DMEM-F12</td><td align="center" rowspan="1" colspan="1">17</td><td align="center" rowspan="1" colspan="1">553</td><td align="center" rowspan="1" colspan="1">0</td><td align="center" rowspan="1" colspan="1">0.219</td></tr></tbody></table></table-wrap> <table-wrap id="tab1b" position="anchor"><label>(b)</label><table frame="hsides" rules="groups"><thead><tr><th align="left" rowspan="2" colspan="1"/><th align="center" rowspan="1" colspan="1">L-Glutamine </th><th align="center" rowspan="1" colspan="1">L-Alanine </th><th align="center" rowspan="1" colspan="1">L-Leucine </th><th align="center" rowspan="1" colspan="1">L-Arginine-HCL </th></tr><tr><th align="center" rowspan="1" colspan="1">mM (mg/L)</th><th align="center" rowspan="1" colspan="1">mM (mg/L)</th><th align="center" rowspan="1" colspan="1">mM (mg/L)</th><th align="center" rowspan="1" colspan="1">mM (mg/L)</th></tr></thead><tbody><tr><td align="left" rowspan="1" colspan="1">MEM</td><td align="center" rowspan="1" colspan="1">0 (0)</td><td align="center" rowspan="1" colspan="1">0 (0)</td><td align="center" rowspan="1" colspan="1">0.4 (52.0)</td><td align="center" rowspan="1" colspan="1">0.6 (126)</td></tr><tr><td align="left" rowspan="1" colspan="1">RPMI</td><td align="center" rowspan="1" colspan="1">2.0 (300)</td><td align="center" rowspan="1" colspan="1">0 (0)</td><td align="center" rowspan="1" colspan="1">0.4 (50)</td><td align="center" rowspan="1" colspan="1">1.1 (240)</td></tr><tr><td align="left" rowspan="1" colspan="1">CMRL</td><td align="center" rowspan="1" colspan="1">0 (0)</td><td align="center" rowspan="1" colspan="1">0.3 (25)</td><td align="center" rowspan="1" colspan="1">0.5 (60)</td><td align="center" rowspan="1" colspan="1">0.3 (70)</td></tr><tr><td align="left" rowspan="1" colspan="1">DMEM-F12</td><td align="center" rowspan="1" colspan="1">2.5 (365)</td><td align="center" rowspan="1" colspan="1">0.05 (4.45)</td><td align="center" rowspan="1" colspan="1">0.5 (59.05)</td><td align="center" rowspan="1" colspan="1">0.7 (147.50)</td></tr></tbody></table></table-wrap> <table-wrap id="tab2" position="float"><label>Table 2</label><caption><p>Isolation characteristics.</p></caption><table frame="hsides" rules="groups"><thead><tr><th align="left" rowspan="1" colspan="1"/><th align="center" rowspan="1" colspan="1">Absolute numbers</th><th align="left" rowspan="1" colspan="1">Percentage of total cell number</th></tr></thead><tbody><tr><td align="left" rowspan="1" colspan="1">Islets per animal</td><td align="center" rowspan="1" colspan="1">550</td><td align="left" rowspan="1" colspan="1"/></tr><tr><td align="left" rowspan="1" colspan="1">Cells per islet</td><td align="center" rowspan="1" colspan="1">1872 ± 507</td><td align="left" rowspan="1" colspan="1">100%</td></tr><tr><td align="left" rowspan="1" colspan="1">Vitality (trypane blue)</td><td align="center" rowspan="1" colspan="1"/><td align="left" rowspan="1" colspan="1">&gt;95%</td></tr><tr><td align="left" rowspan="1" colspan="1">Low fluorescent cells</td><td align="center" rowspan="1" colspan="1"/><td align="left" rowspan="1" colspan="1">20.03 ± 1.20%</td></tr><tr><td align="left" rowspan="1" colspan="1">High fluorescent cells</td><td align="center" rowspan="1" colspan="1"/><td align="left" rowspan="1" colspan="1">21.40 ± 10.8%</td></tr></tbody></table></table-wrap>
<graphic xlink:href="EDR2008-165360.001"/> <graphic xlink:href="EDR2008-165360.002"/> <graphic xlink:href="EDR2008-165360.003"/> <graphic xlink:href="EDR2008-165360.004"/> <graphic xlink:href="EDR2008-165360.005"/> <graphic xlink:href="EDR2008-165360.006"/>
[{"label": ["9"], "surname": ["Burch", "Bessey", "Lowry"], "given-names": ["HB", "OA", "OH"], "article-title": ["Fluorometric measurements of riboflavin and its natural derivatives in small quantities of blood serum and cells"], "italic": ["The Journal of Biological Chemistry"], "year": ["1948"], "volume": ["179"], "fpage": ["457"], "lpage": ["470"]}]
Exp Diabetes Res. 2008 Jul 28; 2008:165360