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From the Department of Ophthalmology, Yamanashi Medical University, Tamaho Yamanashi, Japan.
| Abstract |
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METHODS. RGCs from the retinas of 2-day-old rats were isolated in a two-step panning and cultured in a serum-free medium. BDNF (1, 10, and 100 pg/ml or 1, 10, and 100 ng/ml), NT-4 (0.1, 1, 10, and 100 ng/ml) or their vehicle, phosphate-buffered saline, were individually added to aliquots of the medium to be cultured for 48 hours. Then, after adding 5-chloromethylfluorescein diacetate, the survival of RGCs was evaluated using flow cytometry.
RESULTS. The method used allowed the authors to analyze 10,000 RGCs per sample in approximately 2 minutes, so that a much larger number of cells was evaluated in a shorter period than with previously reported methods. RGCs were classified into either large or small RGCs, and the survival of each of these groups was determined objectively by the amount of fluorescent emission. BDNF improved the survival rate of RGCs concentration-dependently. In particular, the survival rate of small RGCs was greatly improved. BDNF at 100 ng/ml increased the survival rate of small RGCs by 17.4% and that of large RGCs by 7.8% in comparison to the controls. NT-4 did not significantly improve the survival rates of either large or small RGCs.
CONCLUSIONS. BDNF improved the survival rate of RGCs, particularly of small RGCs, concentration-dependently, but NT-4 had little influence on the survival rate. The current method was useful in evaluating the effects of neuroprotective factors or neurotoxic factors on cultured RGCs.
| Introduction |
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Brain-derived neurotrophic factor (BDNF) discovered in 1975 is one of the most important neurotrophic factors and is involved in the classification and survival of neurons.4 5 6 7 8 9 It has been reported that BDNF improved RGC survival in vitro and that BDNF suppressed RGC loss in axotomized animals and in the ocular hypertensive model.6 7 10 Neurotrophin (NT)-4 is another major neurotrophic factor that has been reported to increase the RGC survival rate and is involved in retinal development, but these results are not necessarily consistent.5 8 11 12 13 14
Rat RGCs have been classified into subgroups by their cell size,2 7 15 and RGCs of different sizes had different susceptibilities to the loading conditions of these factors.7 However, it has been difficult to evaluate precisely the effects of loading conditions on RGCs separated by their cell size.
Flow cytometry has been recently developed to analyze cells very quickly and objectively. Cells can be rapidly separated according to their size, cell cycle, and other parameters with this method.
In the present study, isolated RGCs cultured in serum-free medium were subject to flow cytometric analysis to investigate quantitatively and objectively the neurotrophic effects of BDNF and NT-4 on RGCs.
| Methods |
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Isolation of Retinal Ganglion Cells
RGCs were isolated according to the two-step panning method
reported.1
Briefly, 2-day-old rats (SpragueDawley) were
euthanatized to obtain approximately 60 eyes for each experiment.
Retinas were separated from enucleated eyeballs, incubated for 20
minutes in a solution containing 5 mg/ml papain to dissociate the
cells, and incubated for 5 minutes with rabbit antimacrophage antibody
(Inter-Cell Technologies, Hopewell, NJ). Cell suspensions were
treated for 45 minutes in 100-mm Petri dishes coated with goat
anti-rabbit IgG (L + H chain) antibody (Southern Biotechnology
Associates, Birmingham, AL). Suspensions containing cells that did not
adhere to the Petri dish were treated for 1 hour in 100-mm Petri dishes
coated with anti-Thy 1.1 antibody (from hybridoma T11D7e2; American
Type Culture Collection, Rockville, MD).
Cells that adhered to the second Petri dish were collected after treatment with 0.125% trypsin for 10 minutes for avulsion and incubated in 24-well plates.
Culture Conditions
Culture plates were coated with 0.1 mg/ml of polyornithine (Sigma
Chemical Co., St. Louis, MO) for 5 hours or longer and then coated
overnight with 5 µg/ml of EHS-laminin (Upstate Biotechnology, Lake
Placid, NY). The medium developed by Politi et
al.16 for monolayer culture of mixed mouse retinal
neurons was modified for use in this experiment. The medium used was
Dulbeccos modified Eagles medium to which the following were added:
insulin (1.6 x 10-6 M), progesterone
(4.0 x 10-8 M), selenite (6.0 x
10-8 M), transferrin (12.5 x
10-8 M), putrescine (2 x
10-4 M), hydrocortisone (1.0 x
10-7 M), cytidine-5'-diphosphocholine (5.2 x 10-6 M) and cytidine-5'-diphosphoethanolamine
(2.9 x 10-6 M). The seeding density was
1200 cells/mm2. The cells were incubated at
37°C in humidified 10% CO2 and 90% air.
BDNF or NT-4, 10 µl, was added to each well containing RGCs, after preparation with phosphate-buffered saline (PBS) to make final concentrations of 1, 10, and 100 pg/ml and 1, 10, and 100 ng/ml for BDNF or 0.1, 1, 10, and 100 ng/ml for NT-4, respectively. As the control, 10 µl PBS without neurotrophic factors was added to each well.
Treatment of Cells for Evaluation
After incubation for 48 hours, cells were treated for 10 minutes,
keeping the same culture condition with 5-chloromethylfluorescein
diacetate (Molecular Probes, Inc., Eugene, OR). After the reaction, the
cells were detached from the culture dish by gently pipetting 20 to 30
times. To avoid cell damage as much as possible, pipetting was
performed gently without making bubbles in the incubator. Then, RGCs in
the supernatant were subjected quickly to flow cytometry. It took
approximately 3 to 4 minutes from cell avulsion to complete the
analysis for each culture well. The survival rate of RGCs was measured
quantitatively by the amount of fluorescence. The condition of the RGCs
was observed microscopically before and immediately after avulsion, and
no attached cells were confirmed on the culture dish.
Conditions of Flow Cytometry
Cells were evaluated using a flow cytometer (Facscaliber; Becton
Co., San Jose, CA). The measurement conditions were determined
from the preliminary experimental data as follows. Forward scatter
(FSC) value reflecting cell size: voltage E00, amplifier gain 2.3; side
scatter (SSC) value: reflecting cytoplasmic structure: voltage 400,
amplifier gain 2.0; and fluorescent intensity: voltage 450, amplifier
gain 1.5; flow rate: high. Ten thousand cells were evaluated in every
sample automatically. Two sizes of standard particles, 6 and 10 µm,
were measured to determine cell size before cell evaluation.
Statistical Tests
The survival rates for the various conditions were tested with
ANOVA and the post hoc method. A significant difference was defined as
P < 0.05.
| Results |
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Effects of NT-4
NT-4 slightly improved the survival rate of small RGCs in a
somewhat dose-dependent manner. The survival rate of small RGCs with
100 ng/ml of NT-4 increased by 4.1% from that of the vehicle control.
However, this improvement was not significant. The survival rate of
large RGCs did not change with any of the NT-4 concentrations used.
| Discussion |
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The evaluation of RGC survival using fluorescence intensity or morphologic observation in the previous reports had a limit of evaluated number of cells and took some time, during which the cell conditions may have changed. Some reports evaluated RGC survival according to the degree of process elongation.3 However, many cells with short processes were seen emitting strong fluorescence of 5-chloromethylfluorescein diacetate, indicating that RGC survival could not necessarily be evaluated accurately by measuring the length of the process. It is not easy to draw the line accurately between survived cell and nonsurvived cells by previous methods. The present method can classify a total of 10,000 RGCs in approximately 2 minutes per sample automatically by their cell size and fluorescence intensity. Therefore, we believe that the present method evaluates cell conditions much more objectively and accurately than previous methods. However, the present method requires approximately twice the cell density needed in previous methods.1 2
In observations of RGCs under a fluorescent microscope before and after avulsion, cells immediately after avulsion were found to be round with a short process. However, the intensity of fluorescence did not change, suggesting that there should be no problem in evaluating of cell survival. In preliminary studies, we tried to use some enzymes to dissociate and recover cells from the culture dish. In these cases, however, cell viability was decreased and not suitable for evaluation.
The cell size of surviving cells was always slightly larger than nonsurviving cells, either in the small-cell group or large-cell group in the present study. Because it is well known that cells shrink their cell body by apoptosis, nonsurviving cells in the present study were thought to expire by apoptosis. Indeed, electron-microscopic observations of currently isolated RGCs showed the condensation or defragmentation of nuclei, which represent typical apoptotic changes (data not shown).
In the present study, BDNF improved the survival rate concentration-dependently, having nearly the same range as that reported in previous studies. The maximum improvements of the survival rates of small and large cells were 17.4% and 7.8%, respectively, which seem to be lower than the values of previously reported in in vivo studies.17 Several explanations of this difference are possible. Because previous studies were conducted by injection into the hyaloid body or by coculture, the effects of unknown factors were undeniable, and accurate quantitative analysis was difficult. Several factors have neurotrophic effects on RGCs, and multiple factors should be involved in neurotrophic action.7 8 10 18 19 20 21 22 Indeed, the number of RGCs in the BDNF knock-out mouse was the same as that of the wild type.19 23 In the present study, isolated RGCs were cultured in a serum-free medium, eliminating the participation of unknown factors. Second, the degree of improvement should differ depending on cell type. Moreover, the maturity of RGCs may greatly affect the results. Johnson et al.6 reported that BDNF rescued retinal neurons by 7% in E17 rat retina. However, this improvement decreased in retinas of rats older than E17.24 The number of trk-B receptors, a major BDNF receptor, was variable, depending on retinal maturity.25 26 27 28
It has been reported that rat RGCs can be classified into subgroups by cell size, and those reactions against neurotoxic events or neurotrophic factors were different depending on the cell size.7 15 Large RGCs were very resistant to axotomy in adult rats.7 The existence of trk-B receptors in retina is well known.25 26 28 29 Although many trk-B receptors exist as a homodimers in the cell membrane, heterodimers composed of truncated trk-B and full-length trk-B receptors have inhibitory effects on BDNF signaling,30 31 and those truncated trk-B receptors were detected to a much greater extent in large RGCs than in small RGCs in the adult rat.31 The trk-B receptor is also the major receptor for NT-4. This evidence may explain why the improvements in the survival of large RGCs with BDNF or NT-4 was lower than that for small RGCs in the present study.
The neuroprotective effects of NT-4 on RGCs are not necessarily consistent.5 8 11 12 The expression of NT-4 in the retina is weaker than that of BDNF. This may be the reason why significant neuroprotective effects of NT-4 on RGCs were not observed in the present study. Because it was expected that the neurotrophic effects should be clear when neurotoxic factors were challenged to the RGCs, 100 µM glutamate was administrated to RGCs with or without 100 ng/ml of NT-4. Glutamate decreased the survival rate in both small and large RGCs. However, the improvement of survival rate by NT-4 was not significant (data not shown). Overall, it is implied that the neuroprotective effect of NT-4 on RGCs is weaker than that of BDNF, at least in the postnatal rat in vitro model.
In addition to BDNF and NT-4, several other neuroprotective factors have been reported such as nerve growth factor (NGF), NT-3, and NT-6, ciliary neurotrophic factor (CNTF), basic-fibroblast growth factor (b-FGF). On the other hand, many neurotoxic factors like glutamate have been discovered. Using the current experimental system, the effects of various neuroprotective and neurotoxic factors on RGCs can be evaluated objectively and accurately.
| Footnotes |
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Submitted for publication November 3, 1999; revised February 9, 2000; accepted March 8, 2000.
Commercial relationships policy: N.
Corresponding author: Kenji Kashiwagi, Department of Ophthalmology, Yamanashi Medical University, 1110 Shimokato, Tamaho, Yamanashi 409-3898, Japan. kenjik{at}res.yamanashi-med.ac.jp
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