Received 7 September 2010 | Accepted 16 December 2010 | Published 31 December 2010
1Gavin Herbert Eye Institute and Stem Cell Research Center, University of California, Irvine, CA; 2Department of Ophthalmology, Shanghai Ninth People's Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai, China
Correspondence to: Henry Klassen, Sue & Bill Gross Hall - a CIRM Institute, 845 Health Sciences Road, Room 2035, University of California-Irvine, Irvine, CA 92697-1705; Phone: +1 (949) 824-7750; FAX: +1 714-456-5073; email: email@example.com
Purpose: Glial cell line-derived neurotrophic factor (GDNF) is neuroprotective of retinal neurons, and transduced retinal progenitor cells (RPCs) can deliver this cytokine for the treatment of retinal diseases, yet the potential effects of GDNF on RPCs have received little attention.
Methods: Murine RPCs were assessed under multiple conditions in the presence or absence of epidermal growth factor (EGF, 20 ng/ml) and/or GDNF (10 ng/ml) using a variety of techniques, including live-cell imaging, caspase-3 activity assay, whole genome microarray, quantitative polymerase chain reaction (qPCR), and western blotting.
Results: Live monitoring revealed that formation of initial aggregates resulted largely from the collision and adherence of dissociated RPCs, as opposed to clonal proliferation. Spheres enlarged in size and number, with more reaching the threshold criteria for cross-sectional areas in the EGF+GDNF condition. Proliferation was measurably augmented in association with EGF+GDNF, and Ki-67 expression was modestly increased (1.07 fold), as were hairy and enhancer of split 5 (Hes5), mammalian achaete-scute homolog 1 (Mash1), and Vimentin. However, global gene expression did not reveal a notable treatment-related response, and the expression of the majority of progenitor and lineage markers examined remained stable. GDNF reduced RPC apoptosis, compared to complete growth-factor withdrawal, although it could not by itself sustain mitotic activity.
Conclusions: These data support the feasibility of developing GDNF-transduced RPCs as potential therapeutic agents for use in retinal diseases.
The neural retina is subject to a range of degenerative conditions, including those like retinitis pigmentosa and age-related macular degeneration that involve photoreceptor loss. Similar to other compartments of the central nervous system, the mammalian retina lacks the capacity to regenerate following injury, and there are at present no restorative therapies available for diseases involving photoreceptor loss. Retinal progenitor cells (RPCs) actively generate the mature neurons and Müller glia of the neural retina during eye development. A large body of evidence has now shown that RPCs can be isolated, expanded [1-4], and transplanted in various animal models [1,5-9]. The transplantation of RPCs to the abnormal retina has become an important strategy in retinal regeneration research.
Studies on avian and mammalian embryos have identified factors that influence the behavior of RPCs during normal retinal development and in response to injury. Particular interest has been directed toward the subset of factors with demonstrated neuroprotective efficacy because of their potential clinical relevance. One important effective molecule known to rescue retinal neurons, including photoreceptors, in multiple animal models is the glial cell line-derived neurotrophic factor (GDNF) [10-12]. GDNF is a distant member of the transforming growth factor-β family of growth factors and a member of the GDNF family, which also includes neurturin, persephin, and artemin . GDNF was originally purified and characterized in 1993 and found to be a survival factor for embryonic dopaminergic midbrain neurons in culture . It later became clear that GDNF also acts as a potent neurotrophic factor in a variety of other contexts. GDNF is widely distributed in the developing central nervous system and has been confirmed to subserve pivotal roles in many additional tissues, including the peripheral nervous system, inner ear, embryonic kidney, gastrointestinal tract, skeletal muscle [15-17] and spermatogonial stem cells .
In the past few years, the potential of GDNF as a therapeutic agent for the treatment of neurodegenerative conditions such as Parkinson disease, which is characterized by dopaminergic cell loss, has been actively explored [19,20]. In the eye, it has been shown that both GDNF and its receptors are synthesized in the retina [15,21], thereby suggesting that this factor has an innate neurotrophic role in this tissue. Several lines of work, using recombinant protein, knockout, or overexpression methods, have implicated GDNF in the regulation of cell-cycle progression, specifically in terms of promoting neuroblast proliferation at early stages of development, as well as promoting the survival and differentiation of retinal photoreceptors [22,23]. In addition to these roles, GDNF may also delay the onset of apoptosis and participate in the regulation of cellular migration, although in this regard the literature is more variable. For instance, Clarkson and colleagues  showed that GDNF had no discernable effect on apoptosis in astrocytes derived from the embryonic mesencephalon or from the neonatal cortex. Iwashita and colleagues  also showed that GDNF did not affect the survival or proliferation of neural crest stem cells. Together, these results indicate that although the neurotrophic role is well established, the effects of GDNF do not entirely generalize across cell types or developmental stages.
GDNF has shown considerable potential as a novel therapeutic agent for the treatment of neurodegenerative diseases, including those of the retina; however, a significant challenge to implementation has been the need for sustained local delivery over lengthy time periods. An attractive strategy for neutrophic factor delivery that has been considered is the transplantation of stem or progenitor cells that have been genetically modified to overexpress the particular gene of interest [26,27]. When contemplating such a strategy, it is important to consider the impact of the delivered factor on the behavior of the cell used for delivery. In addition, it is of interest to know how the factor might influence tissue-specific stem cells residing within the recipient host tissue. At present, little is known regarding the influence of GDNF on cultured RPCs. Here we investigate the influence of GDNF on a previously characterized and transplantable population of mitotically active progenitor cells derived from the immature murine retina.
RPCs were previously isolated from the neural retina of postnatal day 1GFP transgenic mice . Briefly, retinas were harvested from newborn GFP transgenic C57BL/6 mice (gift from Masaru Okabe, University of Osaka, Osaka, Japan)  and subjected to several cycles of collagenase digestion to dissociate the tissue. Cells were then forced through a nylon mesh of 100 µm pore size, centrifuged, and resuspended in standard culture medium, containing Dulbecco’s modified Eagle’s medium/Ham’s F12 1:1 (Omega Scientific, Tarzana, CA) supplemented containing 1% (by vol.) N2 neural supplement (Gibco, Invitrogen, Carlsbad, CA), 2 mM L-glutamine (Sigma Aldrich, St. Louis, MO), 2,000 U nystatin (Gibco), 50 mg/ml penicillin-streptomycin (Sigma), and 20 ng/ml recombinant human epidermal growth factor (EGF; Invitrogen, Carlsbad, CA). GFP+ neurospheres appeared within the first 2 to 3 days. Culture media were changed every 2 days, and proliferating cells were passaged at regular intervals of 4–5 days. These cells were immunoreactive for nestin (a marker for neural progenitor cells) and Ki-67 (a marker for cell proliferation).
To first determine the optimal concentration of GDNF for use in the present study, we tested the effects of four different concentrations of GDNF (2.5, 5, 10, and 20 ng/ml) on the gene expression profile of genetically modified RPCs. Cells were treated with GDNF for 5 days, RNA was extracted, and a quantitative PCR (qPCR) assay was performed. The progenitor markers remained at similar levels to those of untreated controls under the different concentrations of GDNF examined (Figure 1). Given this general equivalence, combined with our interest in looking for a possible physiologic (as opposed to toxic) influence, we chose 10 ng/ml for use in our system.
RPCs were cultured in standard base media with the only difference being the presence or absence of EGF (20 ng/ml) and/or 10 ng/ml recombinant human GDNF (PHC7045; Biosource, Invitrogen) as follows:
1) No recombinant factors (nonmitogenic conditions, negative control),
2) EGF alone (proliferation conditions, positive control, i.e., identical to “standard medium”),
3) GDNF alone, or
4) EGF+GDNF together.
Cellular morphology was recorded on treatment day 1, 3, and 5 using a microscope-mounted camera and imaging system. In addition, cellular proliferation and sphere formation were also recorded and quantified every 45 min using an IncuCyte live-cell imaging system (Essen Instruments, Ann Arbor, MI) located within the incubator.
RPCs were cultured under four different conditions consisting of the base medium and either no added growth factors, EGF alone, GDNF alone, or EGF+GDNF, for 5 days. Cell viability was measured using a Cell Counting Kit-8 assay (Dojindo, Kumamoto, Japan). In brief, cells were suspended at a final concentration of 1×104 cells/well and cultured in 96-well, flat-bottomed microplates. At the end of each treatment day, 10 μl WST-8 [2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt] was added to each well. The plates were incubated for an additional 4 h at 37 °C to convert WST-8 into formazan. The absorbance was then measured at 450 nm with a spectrophotometer. Cell viability is in direct proportion to the absorbance at 450 nm; therefore, viability was expressed as the A450 value. All experiments were performed in 96-well plates (ten wells/condition).
In addition, growth of RPCs in medium with EGF alone, or with EGF and GDNF, was assessed over a one-month time course. Cells were passaged before reaching confluence. At each passage, the cell number was counted using a hemocytometer, and a population doubling level (PDL) was determined as current PDL=Log(final harvest cell number/initial seeded cell number)×3.33.
After 24 h culture, RPCs exposed to the four different conditions mentioned above were collected and washed in cold phosphate buffered saline (PBS; 2.68 mM KCI, 1.47 mM KH2PO4, 135.60 mM NaCl, 8.10 mM Na2HPO4). Cell pellets were resuspended in 100 μl lysis buffer. After 20 min incubation on ice, each cell lysate was spun at 18,000× g at 4 °C for 15 min. The supernatant was collected and 5 μl of cell lysate from each condition was diluted to 100 μl in the assay buffer consisting of 20 mM Hepes at pH 7.4, 0.1% CHAPS, 5 mM dithiothreitol, and 2 mM EDTA, supplemented with 2 mM of the caspase-3 substrate Ac-DEVD-pNA. The enzymatic activity was determined spectrophotometrically at 405 nm (molar absorptivity of p-nitroaniline, εmM=10.5 at 405 nm). Caspase-3 activity was determined as µmol pNA released per minute per milligram of protein.
RPCs were grown in culture media that contained EGF or EGF+GDNF for 5 days. Total RNA was extracted from each sample using the RNeasy Mini Kit (Cat. No. 74104; Qiagen, Valencia, CA). After RNA isolation, samples were treated by DNaseI (Macherey-Nagel, Düren, Germany) to digest and eliminate any contaminating genomic DNA. RNA concentration was measured for each sample at a wavelength of 260 nm (A260), and the purity of extracted total RNA was determined by the A260/A280 ratio. Quantitative reverse-transcriptase PCR analyses and microarray analysis were only performed on samples with A260/A280 ratios between 1.9 and 2.1.
After RNA isolation (two treatments, with three samples per treatment), 1 μg total RNA from each sample was used to prepare material for hybridization with each of the six Affymetrix mouse gene 1.0 ST arrays at the UCI Genomics High-Throughput Facility at the University of California, Irvine, as recommended by the manufacturer (Affymetrix Genechip Whole Transcript Sense Target Labeling Assay Manual; Affymetrix, Inc., Santa Clara, CA). The integrity and concentration of total RNA were measured, followed by hybridization, scanning, and generation of raw expression data, which were subsequently normalized using a standard technique by the same facility, as follows.
All starting total RNA samples were quality assessed before beginning the target preparation/processing steps by running out a small amount of each sample (typically 25–250 ng/well) onto a RNA 6000 Nano LabChip that was evaluated on an Agilent Bioanalyzer 2100 (Agilent Technologies, Palo Alto, CA). Single-stranded, then double-stranded, cDNA was synthesized from the poly(A)+mRNA present in the isolated total RNA (typically 100 ng total RNA starting material for each sample reaction) using the GeneChip WT cDNA Synthesis Kit (Affymetrix, Inc., Santa Clara, CA) and random hexamers tagged with a T7 promoter sequence. The double-stranded cDNA was then used as a template to generate many copies of antisense cRNA from an in vitro transcription reaction for 16 h in the presence of T7 RNA polymerase using the Affymetrix Genechip WT cDNA Amplification Kit. Ten micrograms of cRNA were input into the second-cycle cDNA reaction with random hexamers that were used to reverse-transcribe the cRNA from the first cycle to produce single-stranded DNA in the sense orientation.
The single-stranded DNA sample was fragmented (WT Terminal Labeling Kit, Affymetrix) to an average strand length of 60 bases (range 40–70 bp) following prescribed protocols (Affymetrix GeneChip WT Sense Target Labeling Assay Manual). The fragmented single-stranded DNA was subsequently labeled with recombinant terminal deoxynucleotidyl transferase and the Affymetrix proprietary DNA Labeling Reagent, which is covalently linked to biotin. Following the recommended procedure, 0.54 μg of this fragmented single-stranded target cDNA was hybridized at 45 °C with rotation for 17 h (Affymetrix GeneChip Hybridization Oven 640) to probe sets present on an Affymetrix mouse-gene 1.0 ST array. The GeneChip arrays were washed and then stained (streptavidin-phycoerythrin) on an Affymetrix Fluidics Station 450 (Fluidics protocol FS450_007). Arrays were scanned using the GeneChip Scanner 3000 7G and GeneChip Operating Software v1.4 to produce CEL intensity files.
Normalization was performed using the probe logarithmic intensity error (PLIER) estimation method, which includes a quantile normalization protocol within the associated software algorithm. Briefly, the probe cell intensity files (*.CEL) generated above were analyzed using Affymetrix Expression Console software v1.1 using the PLIER algorithm to generate probe-level summarization files (*.CHP). The algorithm used was from PLIER v2.0 (quantification scale: linear; quantification type: signal and detection p value; background: PM-GCBG; normalization method: sketch-quantile).
Statistical analysis was subsequently conducted using JMP Genomic 3 software (SAS Institute Inc.) The raw data were log-2 transformed and imported into the software for analysis. The strategy used was to compare the gene expression profile of the control group (EGF alone) with the experimental group (EGF+GDNF). Data from the arrays were analyzed by the clustering of differences between treatments and identification of significant changes. Changes were considered statistically significant if the difference in expression between the groups had a p value less than 0.05.
To further assess expression changes, transcription levels of selected genes were examined by performing semiquantitative real-time PCR using an Applied Biosystems 7500 Fast Real-Time PCR Detection System (Applied Biosystems, Foster, CA). Two micrograms of total RNA in a 20 μl reaction volume were reverse-transcribed using an Omniscript cDNA Synthesis Kit (Qiagen). Oligonucleotide primer sequences (Table 1) were designed using Primer 3 software. The primers were synthesized commercially (Invitrogen) and qPCR was performed in 20 μl total volume containing 10 μl of 2× Power SYBR Green PCR Master Mix (Applied Biosystems), 10 μl of cDNA, and 300 nM of gene-specific primers. Cycling parameters for qPCR were as follows: initial denaturation at 95 °C for 10 min, followed by 40 cycles of 15 s at 95 °C and of 1 min at 60 °C. To normalize template input, the β-actin transcript level was measured for each sample (endogenous control). The efficiency of the PCR reaction was measured with primers using serial dilution of cDNA (1:1, 1:5, 1:25, 1:125, 1:625, and 1:3,125). The relative expression of the gene of interest, (Etarget) ΔCt target (control-treated)/(Eref)ΔCt ref (control-treated) , was then evaluated by the Pfaffl method . The value obtained for crossing threshold (Ct) represents the number of PCR cycles at which an increase in fluorescence signal (and therefore cDNA) can be detected above the background. The increase is exponential for the particular gene. Data are expressed as fold-change relative to untreated controls, after normalizing to β-actin (our choice from among a set of “house-keeping” genes).
Samples for protein analyses were homogenized in CelLytic M (C2978; Sigma), and proteins were measured using the Bio-Rad detergent-compatible protein assay (500–0006; Bio-Rad). Proteins (50 µg) were electrophorised using a 3%–8% Tris-acetate gel (EA0375BOX; Invitrogen) for 70 min at 150 V and then transferred from the gel to a polyvinyldifluoride membrane using an iBot Gel Transfer Device (IB1001; Invitrogen). Western blotting was then performed using the WesternBreeze Chromogenic Western Blot Immunodetection Kit (WB7103, Invitrogen) according to the manufacturer’s protocol. After blockade of nonspecific binding sites using Blocker/Diluent solutions provided, the polyvinylfluoride membranes were incubated for 2 h at room temperature with mouse primary antibodies directed against either Ki-67 (556003, 1:200; BD), nestin (611658, 1:200; BD), or β-actin (A5441, 1:200; Sigma). This was followed by washing and 1 h incubation with an alkaline phosphatase-conjugated antimouse IgG secondary antibody provided in the kit. Immunoblots were developed using chromogenic substrate for 10 min. The reactive bands were scanned and quantified by densitometry using UN-SCAN-IT Gel 6.1 (Silk Scientific, Inc. Orem, UT) The density of product from the EGF treatment group was defined as 1, and the data from the EGF+GDNF condition was expressed as the fold-change of this value.
The results represent the average of three experiments (±SE). Except where specifically indicated, each experiment was performed in triplicate. For cell viability and caspase-3 activity studies, ten fields per sample were analyzed for each condition. Statistical significance was determined by Student’s two-tailed t-test.
RPCs were subjected to different treatment conditions for up to 5 days. As a result, the morphology of RPCs clearly differed in response to the presence or absence of EGF (Figure 2). There was increased extension of processes in response to the absence of EGF (Figure 2A-F), whereas the appearance of undifferentiated RPCs cultured in either EGF alone or EGF+GDNF remained stable throughout the treatment period. Undifferentiated RPCs either adhered to the surface of the uncoated flasks or were observed floating in the culture medium, either as single cells or spheres of different sizes. From day 3, short processes with few, if any, branches extended from some of the adherent cells (Figure 2H,K). No obvious morphological differences were observed in response to the presence of 10 ng/ml GDNF (GDNF alone) when compared to culture conditions without EGF (no recombinant factors; Figure 2D-F). However, in the presence of GDNF (EGF+GDNF), more and larger spheres were observed, as compared to EGF alone (Figure 2I,L).
The proliferation of RPCs and formation of spherical cellular aggregates (spheres) were also monitored via the IncuCyte system over the five-day time course, allowing comparison of EGF and EGF+GDNF conditions. By way of the acquired video, dissociated RPCs were seen to exhibit a strong propensity to aggregate in culture, with the formation of spheres being evident as early as 5.5 h after seeding. Aggregates started with the movement of suspended single RPCs within the culture medium and adherence to other cells, as opposed to cell division and clonal expansion (see Appendix 1, Figure 3). Over time, the spheres enlarged and their movement lessened, consistent with an increased contribution of cell proliferation to sphere formation. The number of spheres with a cross-sectional area of >870 µm2 increased in both treatment conditions, beginning on day 0 and continuing to day 5, at which time greater numbers of spheres had reached threshold criteria in the GDNF-supplemented medium (p<0.05). In addition, more spheres had attained higher categories of cross-sectional area in this treatment condition as well, consistent with increased cellular proliferation (Figure 4).
Murine RPCs exhibited exponential growth in the presence of EGF alone, as well as with EGF+GDNF, but diminished asymptotically in the absence of both factors, or with GDNF alone. Cell viability was compared for the EGF alone versus the EGF+GDNF conditions using the cell-counting kit. By this method, the number of RPCs increased from day 1 to 4, although there was no difference between conditions. By day 5, however, there were significantly more viable cells in the EGF+GDNF compared to those in EGF alone (Figure 5A). Moreover, the growth over a one-month period confirmed that the addition of GDNF to a standard EGF-containing proliferation medium provides sustained augmentation of RPC numbers (Figure 5B).
We determined the level of caspase-3 activity under conditions with or without EGF or GDNF after 24 h treatment. Caspase-3 activity levels with no growth factors, as well as with GDNF alone, were substantially elevated compared to conditions that included EGF (p<0.05), and activity was significantly higher in the no-growth-factor condition than in GDNF alone (p<0.05). Between EGF and EGF+GDNF, no difference was observed (Figure 6).
The signal levels of individual cDNA from the experimental (EGF+GDNF) and control (EGF) groups were compared after hybridization on a mouse whole-genome, gene-level microarray that consisted of over 28,853 genes. The fold change and p value of each of the genes were calculated, and log2 (fold change) and −log10 (p value) were transformed. Genes with a p value of <0.05 and an absolute fold change value of >1 were arbitrarily defined as upregulated in GDNF-treated cells, whereas those with a p value <0.05 and an absolute fold change value of <1 were defined as downregulated. The distribution analysis was done as a quality control step. The overlayed kernel density estimates (Figure 7A) derived from the distribution analysis showed the raw univariate distributions for all arrays, and allowed the visualization of sources of variation attributed to technical procedures. The distributions for these arrays were very similar, indicating that this represented a high-quality data set requiring little if any normalization for further analysis. Principle component analysis revealed a broad degree of overlap in expression patterns between the two treatment conditions, with little suggestion of a treatment effect (Figure 7C). However, ANOVA analysis showed that of the approximately 25,189 genes that were expressed in the cultures treated with EGF+GDNF, 5,250 of these were associated with the presence of GDNF (Figure 7B). A total of 3,865 genes were upregulated and 1,385 genes down regulated following the addition of GDNF to EGF-containing standard medium. In this case, changes in 24 genes were detected in the EGF+GDNF condition (Table 2). Among those were 16 known and eight predicted genes, the latter not yet associated with any biologic functions (data not shown). Of the 16 known genes, six were olfactory receptors with three being upregulated and three down-regulated. There were two genes with possible associations to photoreceptor cilia, namely myosin IXa and a dynein (Dnahc7a); however, changes were slight and in opposite directions.
In addition to checking for global changes, the potential influence of GDNF on a specific subset of genes with known associations to RPCs was also evaluated using quantitative PCR. Following treatment of RPCs with GDNF for up to 5 days, expression of most progenitor-associated markers remained effectively stable (Figure 8). At baseline, in the presence of EGF, cultured RPCs expressed the progenitor markers Nestin, Ki-67, ceh-10 homeo domain containing homolog (C. elegans) [Chx10], sex determining region Y-box 2 (Sox2), v-myc myelocytomatosis viral oncogene homolog (avian) [C-myc], and Vimentin. This pattern was sustained in the EGF+GDNF condition. Similarly, the expression levels of precursor and lineage-related markers including βIII-tubulin, microtubule-associated protein 2 (MAP2), doublecortin (DCX), cellular retinaldehyde binding protein (CRALBP), protein kinase C, alpha (PKC-α), and Recoverin. These were relatively unaffected by the addition of GDNF to the EGF-based proliferation medium (Figure 1). Of the above markers, a subset exhibited marginally increased expression, namely, Hes5 (1.03-fold), Ki-67 (1.07-fold), Mash 1 (1.12-fold) and Vimentin (1.20-fold; Figure 8). Of these, the microarrays showed no significant difference for Hes5, Ki-67, or Vimentin (Mash 1 was not included; Table 3). Taking the comparison further, the qPCR data did not confirm the marginal changes seen with the microarrays for Nanog, Notch1, or GFAP.
PCR detects changes in mRNA expression; however, the extent to which these changes are reflected at the level of proteins is also of interest. Western blotting was employed for this purpose on samples from the EGF alone and the EGF+GDNF treatment groups. Nestin was used as a marker of neural progenitor cells and Ki-67 as a marker of cell proliferation.
Anti-nestin identified two bands with the anticipated molecular weight, approximately 220 kDa and 200 kDa, with β-actin labeling used as the loading control. The upper band of nestin (approximately 220 kDa) was strongly labeled in both the EGF and EGF+GDNF conditions and the lower band (approximately 200 kDa) was little changed in the EGF+GDNF condition, compared with EGF alone. Overall, no significant difference between treatment conditions was detected by quantification of the entire blot-lane intensity. Two bands were detected by Ki-67 antibody, approximately 395 kDa and 345 kDa. Quantification analysis showed that the intensity of the entire blot lane was increased in the EGF+GDNF condition by 1.189±0.0095-fold (Figure 9). These results at the protein level were consistent with the mRNA results for Nestin and Ki-67, as determined by qPCR (Figure 8), namely, that the total Nestin mRNA did not change in response to EGF+GDNF treatment over 5 days, whereas Ki-67 was slightly upregulated (Figure 9).
Cytokines and stem cells are both powerful biologic agents and feature in many regenerative strategies currently being investigated. While these two categories of agents have evident therapeutic potential in their own right, the possibility that complementary or synergistic effects could be achieved using a combinatorial approach is also worthy of attention. Of particular interest in the setting of retinal degeneration is the use of tissue-specific progenitors, namely RPCs, to deliver neurotrophic factors with known neuroprotective activity, a notable example being GDNF. It has been established that cultured central nervous system progenitor cells can be transduced to express a variety of transgenes [31-33], including GDNF [27,34]. Because the role of GDNF can differ between cell types and developmental stages, it is important to understand the effect of this factor not only on the retina, but also on potential donor cells such as RPCs. Here, we have shown that exogenous GDNF has several subtle yet discernable influences on cultured murine RPCs, including augmentation of EGF-induced proliferation as well as a modest reduction in apoptosis when EGF is withdrawn.
A growing body of work has shown that growth factors, alone or in combination, exert important effects on cellular behavior, particularly during development. For example, Zheng and colleagues  have shown that EGF, fibroblast growth factor, and hepatocyte growth factor all induce changes in fetal liver hepatocytes in vitro, and that different combinations of these growth factors show various effects on the proliferation and differentiation of the cells. Salient to the current report, Ahmad and colleagues have shown that retinal progenitors proliferate and remain undifferentiated in vitro in the presence of epidermal growth factor (EGF), and display properties similar to stem cells , as have we, having tested the same murine RPCs here . Ahmad and colleagues  have also shown that EGF can mediate suppression of retinal rod photoreceptor differentiation. Although the influence of EGF and ciliary neurotrophic factor (CNTF) on RPCs has been reported, the influence of GDNF has not, to our knowledge, been previously investigated. The relative lack of response of murine RPCs to GDNF found in the present study is in contrast to what we have previously shown for other treatment conditions, namely serum as well as a different cytokine, CNTF . Both serum and CNTF had notable differentiating influences on murine RPCs, whereas the present study indicates that GDNF does not, under comparable conditions. In the presence of EGF, exposure to GDNF appears to be compatible with maintaining the undifferentiated state of these cells.
GDNF has emerged as a particularly important cytokine by virtue of its powerful neuroprotective influence, as demonstrated in multiple models of neurodegeneration [39,40]. However, despite significant progress in the understanding of the GDNF signaling pathways and receptor interactions, the exact cellular mechanisms responsible for these neuroprotective effects are not yet fully understood. Furthermore, despite therapeutic promise in the clinical treatment of retinal and other diseases, this growth factor has yet to be validated in the treatment of patients. A major challenge to the use of cytokines is their rapid breakdown in vivo by endogenous proteases. Cell-based delivery of these molecules is attractive for this reason, and the present findings have implications for the development of GDNF-overexpressing RPCs.
In terms of the effects reported in the present study, a proliferative influence has been described previously, and it has been reported that GDNF is able to improve the survival and differentiation, both in vivo and in vitro, of a variety of brain neurons [41,42], as well as being essential for the proliferation of enteric precursor cells . Similarly, GDNF has been found to stimulate proliferation during renal development . Beyond this, a regulatory mitogenic effect has been demonstrated for rat glioma cells by adding exogenous GDNF or using antisense oligonucleotides for the suppression of endogenous GDNF. Moreover, a similar proliferative effect of GDNF has been established in photoreceptor-enriched rat monolayer cultures . Our current data extend these findings to the in vitro behavior of RPCs, a cell type of great developmental significance in the retina. It remains possible that different, and potentially deleterious, effects could be produced by exposure of RPCs to GDNF at substantially higher concentrations, as might occur with very high overexpression of an otherwise therapeutic transgene.
Of importance to the development of GDNF-overexpressing RPCs, neither augmentation of proliferation nor reduction in apoptosis pose an impediment to the expansion of these cells in culture. This allays one potential concern that arises when contemplating the genetic modification of a proliferative cell type for cytokine delivery. In other words, it is important that a constitutively expressed therapeutic transgene not impede the proliferative ability of the carrier cell, thereby necessitating repeated re-derivation and genetic modification. For instance, CNTF has demonstrated potential as a neurotrophic agent, however, the known differentiating influence of this factor on RPCs could pose a hindrance to the use of this candidate transgene. It is also important that the potential therapeutic molecule not induce undesirable changes in the phenotypic status of the cell used for delivery. In this context, it is reassuring that the global gene expression of murine RPCs was not substantially altered by exposure to GDNF.
This study shows that GDNF is not a potent inducer of RPC differentiation. Its results support the use of these cells for GDNF delivery to the diseased retina. Because the expression profile of cultured RPCs is not markedly altered by exposure to GDNF, it may be the case that transduced RPCs will retain their ability to repopulate local retinal populations following transplantation.
Appendix 1: Time-lapse video record of cultured transduced retinal progenitor cells.
The authors are grateful to Dr. Michael J. Young and his laboratory for providing the murine retinal progenitor cells, as well as to J. Denis Heck, Ph.D. and Seung-Ah Chung, SRA of the UCI Genomics High-Throughput Facility for sharing their analytical expertise. The authors thank the Lincy Foundation, the Andrei Olenicoff Memorial Foundation, the Polly and Michael Smith Foundation, and the Discovery Eye Foundation for financial support. This work was also supported in part by a Challenge Grant from RPB.