Mullerian glia in dystrophic rodent retinas: an immunocytochemical analysis

Intravitreal and subretinal syngeneic bone marrow mononuclear stem cell transplantation improves photoreceptor survival but does not ameliorate retinal function in two rat models of retinal degeneration

PURPOSE

To study and compare effects of syngeneic bone marrow mononuclear stem cells (BM-MNCs) transplants on inherited retinal degeneration in two animal models with different etiologies: the RCS and the P23H-1 rats. To compare the safety and efficacy of two methods of intraocular delivery: subretinal and/or intravitreal.

METHODS

A suspension of BM-MNCs was injected subretinally or intravitreally in the left eyes of P23H-1 and RCS rats at post-natal day (P) 21. At different survival intervals after the injection: 7, 15, 30 or 60 days, the retinas were cross-sectioned, and photoreceptor survival and glial cell responses were investigated using immunodetection of cones (anti-cone arrestin), synaptic connections (anti-bassoon), microglia (anti-Iba-1), astrocytes and Müller cells (anti-GFAP). Electroretinographic function was also assessed longitudinally.

RESULTS

Intravitreal injections (IVIs) or subretinal injections (SRIs) of BM-MNCs did not produce adverse effects. The transplanted cells survived for up to 15 days but did not penetrate the retina. Both IVIs and SRIs increased photoreceptor survival, decreased synaptic degeneration and glial fibrillary acidic protein (GFAP) expression in Müller cells but did not modify microglial cell activation and migration or the electroretinographic responses.

CONCLUSIONS

Intravitreal and subretinal syngeneic BM-MNCs transplantation decreases photoreceptor degeneration and shows anti-gliotic effects on Müller cells but does not ameliorate retinal function. Moreover, syngeneic BM-MNCs transplants are more effective than the xenotransplants of these cells. BM-MNC transplantation has potential therapeutic effects that merit further investigation.

Multiple intrinsic factors act in concert with Lhx2 to direct retinal gliogenesis

Müller glia (MG) are the principal glial cell type in the vertebrate retina. Recent work has identified the LIM homeodomain factor encoding gene Lhx2 as necessary for both Notch signaling and MG differentiation in late-stage retinal progenitor cells (RPCs). However, the extent to which Lhx2 interacts with other intrinsic regulators of MG differentiation is unclear. We investigated this question by investigating the effects of overexpression of multiple transcriptional regulators that are either known or hypothesized to control MG formation, in both wildtype and Lhx2-deficient RPCs. We observe that constitutively elevated Notch signaling, induced by N1ICD electroporation, inhibited gliogenesis in wildtype animals, but rescued MG development in Lhx2-deficient retinas. Electroporation of Nfia promoted the formation of cells with MG-like radial morphology, but did not drive expression of MG molecular markers. Plagl1 and Sox9 did not induce gliogenesis in wildtype animals, but nonetheless activated expression of the Müller marker P27^Kip1 in Lhx2-deficient cells. Finally, Sox2, Sox8, and Sox9 promoted amacrine cell formation in Lhx2-deficient cells, but not in wildtype retinas. These findings demonstrate that overexpression of individual gliogenic factors typically regulates only a subset of characteristic MG markers, and that these effects are differentially modulated by Lhx2.

Müller glia activation in response to inherited retinal degeneration is highly varied and disease-specific

Despite different aetiologies, most inherited retinal disorders culminate in photoreceptor loss, which induces concomitant changes in the neural retina, one of the most striking being reactive gliosis by Müller cells. It is typically assumed that photoreceptor loss leads to an upregulation of glial fibrilliary acidic protein (Gfap) and other intermediate filament proteins, together with other gliosis-related changes, including loss of integrity of the outer limiting membrane (OLM) and deposition of proteoglycans. However, this is based on a mix of both injury-induced and genetic causes of photoreceptor loss. There are very few longitudinal studies of gliosis in the retina and none comparing these changes across models over time. Here, we present a comprehensive spatiotemporal assessment of features of gliosis in the degenerating murine retina that involves Müller glia. Specifically, we assessed Gfap, vimentin and chondroitin sulphate proteoglycan (CSPG) levels and outer limiting membrane (OLM) integrity over time in four murine models of inherited photoreceptor degeneration that encompass a range of disease severities (Crb1rd8/rd8, Prph2+/Δ307, Rho-/-, Pde6brd1/rd1). These features underwent very different changes, depending upon the disease-causing mutation, and that these changes are not correlated with disease severity. Intermediate filament expression did indeed increase with disease progression in Crb1rd8/rd8 and Prph2+/Δ307, but decreased in the Prph2+/Δ307 and Pde6brd1/rd1 models. CSPG deposition usually, but not always, followed the trends in intermediate filament expression. The OLM adherens junctions underwent significant remodelling in all models, but with differences in the composition of the resulting junctions; in Rho-/- mice, the adherens junctions maintained the typical rod-Müller glia interactions, while in the Pde6brd1/rd1 model they formed predominantly between Müller cells in late stage of degeneration. Together, these results show that gliosis and its associated processes are variable and disease-dependent.

Retinal morphological and functional changes in an animal model of retinitis pigmentosa

The P23H-1 transgenic rat carries a mutated mouse opsin gene, in addition to endogenous opsin genes, and undergoes progressive photoreceptor loss that is generally characteristic of human autosomal dominant retinitis pigmentosa (RP). Here, we examined morphological changes correlated with visual function that is comparable to clinical application in the pigmented P23H-1 rat retina as photoreceptor degeneration progressed. We found that rod function was compromised as early as postnatal day 28 and was a good indicator for tracking retinal degeneration. Cone function was normal and did not change until the thickness of the photoreceptor layer was reduced by 75%. Similar to the threshold versus intensity curves used to evaluate vision of RP patients, light-adaptation curves showed that cone thresholds depended on the number of remaining functioning cones, but not on its length of outer segments (OS). By 1 year of age, both rod and cone functions were significantly compromised. Correlating with early abnormal rod function, rods and related secondary neurons also underwent progressive degeneration, including shortening of inner and OS of photoreceptors, loss of rod bipolar and horizontal cell dendrites, thickening of the outer Müller cell processes, and reduced density of pre- and postsynaptic markers. Similar early morphological modifications were also observed in cones and their related secondary neurons. However, cone function was maintained at nearly normal level for a long period. The dramatic loss of rods at late stage of degeneration may contribute to the dysfunction of cones. Attention has to be focused on preserving cone function and identifying factors that damage cones when therapeutic regimes are applied to treat retinal degeneration. As such, these findings provide a foundation for future studies involving treatments to counter photoreceptor loss.

Transplanted olfactory ensheathing cells reduce the gliotic injury response of Müller cells in a rat model of retinitis pigmentosa

Retinitis pigmentosa leads to progressive visual loss and blindness. Until now there has been no effective therapy. Experimental studies have shown that transplantation of olfactory ensheathing cells into CNS lesions reorganizes the glial scar and stimulates axon regeneration. We now report that cultured syngeneic adult rat olfactory ensheathing cells transplanted into the subretinal space of pigmented Royal College of Surgeon rats survived for a minimum of 8weeks, migrated into surrounding space, and suppressed the gliotic injury response of the Müller cells. Further investigation of intraretinal transplantation of olfactory ensheathing cells may suggest a possible future route for protection of the retina in retinitis pigmentosa.

Cell transplantation strategies for retinal repair

Cell transplantation is a novel therapeutic strategy to restore visual responses to the degenerate adult neural retina and represents an exciting area of regenerative neurotherapy. So far, it has been shown that transplanted postmitotic photoreceptor precursors are able to functionally integrate into the adult mouse neural retina. In this review, we discuss the differentiation of photoreceptor cells from both adult and embryonic-derived stem cells and their potential for retinal cell transplantation. We also discuss the strategies used to overcome barriers present in the degenerate neural retina and improve retinal cell integration. Finally, we consider the future translation of retinal cell therapy as a therapeutic strategy to treat retinal degeneration.

Classical complement activation and acquired immune response pathways are not essential for retinal degeneration in the rd1 mouse

Misregulation of the innate immune response and other immune-related processes have been suggested to play a critical role in the pathogenesis of a number of different neurodegenerative diseases, including age related macular degeneration. In an animal model for photoreceptor degeneration, several genes of the innate and acquired immune system were found to be differentially regulated in the retina during the degenerative process. In addition to this differential regulation of individual genes, we found that in the rd1 retina a significantly higher number of genes involved in immune-related responses were expressed at any given time during the degenerative period. The peak of immune-related gene expression was at postnatal day 14, coinciding with the peak of photoreceptor apoptosis in the rd1 mouse. We directly tested the potential involvement of acquired and innate immune responses in initiation and progression of photoreceptor degeneration by analyzing double mutant animals. Retinal morphology and photoreceptor apoptosis of rd1 mice on a SCID genetic background (no mature T- and B-cells) or in combination with a RAG1 (no functional B- and T-cells) or a C1qalpha (no functional classical complement activation pathway) knockout was followed during the degenerative process using light microscopy or TUNEL staining, respectively. Although complement factor C1qalpha was highly up-regulated in the rd1 retina concomitantly with the degenerative process, lack of this protein did not protect the rd1 retina. Similarly, retinal degeneration and photoreceptor apoptosis appeared to proceed normally in the rd1 mouse lacking functional B- and T-cells. Our results suggest that both, the classical complement system of innate immunity and a functional acquired immune response are not essential for the degenerative process in the rd1 mouse retina.

Developmental abnormalities in the Nuc1 rat retina: a spontaneous mutation that affects neuronal and vascular remodeling and retinal function

The retina serves as an excellent model in which to study vertebrate CNS development. We have discovered a spontaneous mutation in the Sprague-Dawley rat that results in a novel and unusual ocular phenotype, including retinal abnormalities, that we have named Nuc1. We have previously shown that the Nuc1 mutation appears to suppress programmed cell death in the developing retina. Here we report that maturation of both the retinal neurons and the retinal vessels is abnormal in Nuc1 homozygous rats. The developmental changes in the retinal neurons and vasculature are correlated with regard to degree of abnormality. As Nuc1 homozygotes mature, focal retinal detachment begins at approximately 3 months after birth, and near total traction retinal detachment, associated with pre-retinal fibrosis and neovascularization, is evident by 18 months. Electroretinographic studies at 2.5 months of age indicate that functional retinal degeneration precedes retinal detachment. The functional abnormality is most evident in rods and the inner retina, and is present in homozygous but not heterozygous mutants. Immunocytochemical studies of rod and cone photoreceptors indicate abnormalities in rod, but not cone, photoreceptors in Nuc1 homozygotes, consistent with the electroretinographic findings. In Nuc1 animals, the Muller cells are activated. Although such activation may result from inflammation, Muller cells in Nuc1 may be reacting to a neuronal influence. It appears that the Nuc1 mutation plays a regulatory role in both developing and maturing ocular tissues. The Nuc1 mutation may also serve as an important genetic tool to explore the relationships that may exist among gliosis, normal neuronal development, and normal vascular development and how abnormalities in these associations lead to common retinal diseases.

Morphological changes in the Royal College of Surgeons rat retina during photoreceptor degeneration and after cell-based therapy

There are concomitant morphological and functional changes in the inner retina during the course of photoreceptor degeneration in a range of animal models of retina degeneration and in humans with eye disease. One concern that has been raised is that the changes occurring in the inner retina might compromise attempts to rescue or restore visual input by various interventional approaches. It is known that cell-based therapy can preserve significant visual capability for many months. In this study, we examine the overall changes in the Royal College of Surgeons (RCS) rat during degeneration and the effects of cell transplantation by means of immunohistochemistry and confocal microscopy. The degenerative changes are complex, and they progress with age. They involve the neurons with which both rods and cones interconnect--retinal second- and third-order neurons underwent dramatic modification, including sprouting, retraction as photoreceptor loss progressed--as well as Müller glia and secondary vascular changes, which were associated at later times with neuronal migration. The pathological vascular changes led to major disruption of inner retina. After introducing a retinal pigment epithelial cell line to the subretinal space early in the progress of photoreceptor degeneration, most inner retinal changes were held in abeyance for up to at least 10 months of age. Given the concern that has been raised regarding whether inner retinal changes might compromise any graft-related benefit, this is an encouraging finding.

Leber congenital amaurosis linked to AIPL1: a mouse model reveals destabilization of cGMP phosphodiesterase

Leber congenital amaurosis (LCA4) has been linked to mutations in the photoreceptor-specific gene Aryl hydrocarbon interacting protein like 1 (Aipl1). To investigate the essential role of AIPL1 in retina, we generated a mouse model of LCA by inactivating the Aipl1 gene. In Aipl1(-/-) retinas, the outer nuclear layer develops normally, but rods and cones then quickly degenerate. Aipl1(-/-) mice have highly disorganized, short, fragmented photoreceptor outer segments and lack both rod and cone electroretinogram responses. Recent biochemical evidence indicates that AIPL1 can enhance protein farnesylation. Our study reveals that rod cGMP phosphodiesterase, a farnesylated protein, is absent and cGMP levels are elevated in AIPL1(-/-) retinas before the onset of degeneration. Our findings demonstrate that AIPL1 enhances the stability of phosphodiesterase and is essential for photoreceptor viability.

Correlation between retinal ganglion cell death and chronically developing inherited glaucoma in a new rat mutant

Glaucoma is a progressive optic neuropathy with characteristic optic disc changes, retinal ganglion cell loss and progressive visual field defects. Elevated intraocular pressure is considered to be a major risk factor in glaucomatous neuropathy. This study aimed to characterize and document a new chronic glaucoma model in the rat with respect to the effect of elevated intraocular pressure on overall retinal dysfunction and retinal ganglion cell loss, and to elucidate the possible mechanisms underlying this cell loss. Intraocular pressure (IOP) was measured in rats using a Tonopen. RGCs were retrogradely labeled with the fluorescent dye, 4-[didecylaminostyryl]-N-methyl-pyridinium-iodide (4-Di-10 ASP) and quantified on retinal flat mounts using fluorescence microscopy. The optic nerve head was examined fundoscopically. Changes in the histological appearance of the whole eyes was studied in paraffin sections, and immunohistochemistry was carried out on cryostat sections. The levels of mRNA for several genes were compared between control and glaucomatous retinae using semi-quantitative RT-PCR. Mutant animals are affected with either a unilateral or bilateral enlargement of the globes having an IOP that ranged from 25 to 45 mmHg, as compared to control values of 12-16 mmHg. The IOP of glaucomatous eyes increased significantly with age to attain a value of 35+/-7.3 at 1.5 years. Concomitant with the rise in IOP, the number of labeled RGCs continued to decrease in number with age. A total of 1887+/-117RGC mm(-2) could be labeled in wild-type control and juvenile mutant pre-glaucomatous retinas, whereas this number dropped to 92+/-26RGC mm(-2) at 1.5 years. Ophthalmoscopy revealed atrophied optic nerve heads in the affected eyes. The pars plicata and the pars plana of the ciliary body of glaucomatous eyes were hypertrophied and elongated, respectively. The anterior chamber was narrow and the irido-corneal angle open in glaucoma eyes. The mRNA of glial-fibrillary-acidic protein, endothelin-1, STAT-3 and STAT-6 increased in the retinas correlating with the severity and duration of the disease. Changes in the expression of GFAP and endothelin-1 could be confirmed using immunohistochemistry. This model may help to address several fundamental issues in the pathogenesis of glaucoma and aid in the development of neuroprotective strategies.

Remodeling of second-order neurons in the retina of rd/rd mutant mice

This is a brief review of data obtained by analyzing the morphology and the physiology of the retinas in rd/rd and normal, wt mice, aged 10-90 days. Second-order neurons of the rd/rd show abnormalities that start with the anomalous development of rod bipolar cells around P10 and culminate with the atrophy of dendrites in cone bipolar cells, mostly evident at P90. Horizontal cells remodel considerably. Cone-mediated ERGs, (recorded between 13 and 16 days of age) have reduced a-wave and b-wave amplitudes and longer b-wave latency and duration. B-wave abnormalities indicate specific postreceptoral dysfunction. Morphological and ERG changes in rd/rd retinas are consistent with substantial inner retinal remodeling associated to photoreceptor degeneration.

Morphological and functional abnormalities in the inner retina of the rd/rd mouse

We investigated the effects of photoreceptor degeneration on the anatomy and physiology of inner retinal neurons in a mouse model of retinitis pigmentosa, the retinal degeneration (rd) mutant mouse. Although there is a general assumption that the inner retinal cells do not suffer from photoreceptor death, we confirmed major changes both accompanying and after this process. Changes include sprouting of horizontal cells, lack of development of dendrites of rod bipolar cells, and progressive atrophy of dendrites in cone bipolar cells. Electrophysiological recordings demonstrate a selective impairment of second-order neurons that is not predictable on the basis of a pure photoreceptor dysfunction. Our data point out the necessity to prove integrity of the inner retina before attempting restoring visual function through photoreceptor intervention. This is even more important when considering that although intervention can be performed before the onset of any symptoms in animals carrying inherited retinopathies, this is obviously not true for human subjects.

Expression of p75(NTR) in photoreceptor cells of dystrophic rat retinas

Although a gene mutation in the Royal College of Surgeons (RCS) dystrophic rat results in defective phagocytosis and in accumulation of debris in the subretinal space, the molecular mechanisms leading to photoreceptor cell death remain unclear. In this study, the expression of p75(NTR), the low-affinity neurotrophin receptor incriminated in the apoptosis of developing neurons, was investigated at various stages of retinal degeneration in dystrophic rats using immunohistochemistry, in situ reverse transcription polymerase chain reaction (RT-PCR), Western blot, and relative RT-PCR. In normal adult retinas, p75(NTR) immunolabeling was observed mainly in the outer limiting membrane, with punctate labeling in the inner nuclear and ganglion cell layers. In 18- to 30-day-old dystrophic retinas, the immunostaining appeared to increase especially in the photoreceptor outer and inner segments. Dense staining was also observed in the retinal pigment epithelium (RPE) and choroid. In 60-day-old dystrophic rat retinas, the density of immunolabeling for p75(NTR) increased dramatically in the remaining inner retina, especially in the inner nuclear, inner plexiform, and ganglion cell layers. Post-embedding immunogold labeling of normal retinas verified the distribution of p75(NTR) in photoreceptor cells within the inner segments, cell bodies, and outer segments. The apparent increased intensity in p75(NTR) immunostaining in dystrophic retinas was verified by Western blots and densitometric analyses. In situ RT-PCR and relative RT-PCR further established increased synthesis of p75(NTR) in dystrophic retinas. The increased levels of p75(NTR) in the RPE and photoreceptor cells, the initial sites of injury, during retinal degeneration in dystrophic rats strongly suggest that altered expression of p75(NTR) may be directly involved in photoreceptor death.

Retina-derived fetuin (RDF): analysis by immunocytochemistry, reverse transcriptase-polymerase chain reaction and Southern blot

PURPOSE

This study was undertaken to determine the presence of retina-derived fetuin (RDF) protein and its message in retinal tissues and retinal pigment epithelial (RPE) cells. The techniques utilized in this study included light micros-copy, immunochemistry, Western blot, reverse transcriptase-polymerase chain reaction (RT-PCR) and Southern blot.

METHODS

Retinal tissues and sections from embryonic, early postnatal and adult normal rats and retinal pigment epithe-lial (RPE) cells from postnatal rats were immunostained for fetuin with a polyclonal fetuin antibody and a peroxidase conjugated-secondary antibody using immunocytochemistry and Western blot analyses. The cDNA generated from RNA isolated from early postnatal rat retinas and RPE was probed with primers for rat fetuin, amplified by PCR and the PCR products were analyzed by Southern blot.

RESULTS

Fetuin (RDF) was immunolocalized to cells of the neuroepithelium in retinas of early postnatal rats and most prominently in the nuclei and perinuclear region of cultured neonatal rat RPE cells. In adult retinas, ganglion cells, inner segments of photoreceptor cells, some components of the outer plexiform layer, ganglion cells and optic nerve processes were immunoreactive for the fetuin protein. As shown by Western blot, fetuin (RDF) was higher in embryonic and early postnatal retinas than in late postnatal retinas, indicating that this protein may be developmentally regulated. Using RT-PCR, the message for rat fetuin was demonstrated in the retina and RPE of normal postnatal rats. Southern blot confirmed that the PCR product from the retina and RPE was generated from rat fetuin mRNA as well as from rat liver, the primary source of fetuin.

CONCLUSIONS

Fetuin, termed retina-derived fetuin (RDF), is reported for the first time in retinal tissues. Fetuin is a cysteine protease inhibitor that may play a role in support of neuronal cell survival during early retinal development and the maintenance of neuronal activity. RDF may interact with other growth factors and cytokines in providing trophic support for neurons and possibly other cells of the developing retina.

Immunolocalization of CD44 in the dystrophic rat retina

The distribution of the cell surface adhesion/receptor molecule CD44 was studied in retinas of the Royal College of Surgeons (RCS) rat which exhibits an inherited retinal dystrophy. In this animal model, the retinal pigment epithelium fails to phagocytize shed photoreceptor outer segment material, a membranous debris layer accumulates in the subretinal space and the photoreceptor cells degenerate. Using immunoperoxidase and immunogold labeling, CD44 was localized to Müller cell apical microvilli in normal rat retinas, as noted in other species. For the RCS rat, immunoperoxidase labeling of 18 day and 1 month retinas showed the typical microvillar labeling pattern. At 2 months postnatal, following degeneration of most of the photoreceptors, a more condensed band of microvillar label was observed. At 3 months, when photoreceptor degeneration was virtually complete, only distinct regions of dense label remained between the neural retina and debris zone. Upon ultrastructural and immunogold analysis, these regions were found to contain closely packed Müller cell microvilli. At all ages studied, labeling for CD44 in the inner retina did not increase, as it does in other forms of retinal degeneration which lack a debris zone. However, by 3 months the debris zone was labeled for CD44 indicating that CD44 molecules remain on Müller cell microvilli and processes which have extended into and become part of the debris zone. This may be caused by an altered distribution of still undetermined ligands for CD44 which are present within the interphotoreceptor matrix of the RCS rat retina.

Immunocytochemical characterisation of proteins secreted by retinal pigment epithelium in retinas of normal and Royal College of Surgeons dystrophic rats

In a previous study, an antigen consisting of proteins secreted by retinal pigment epithelial (RPE) cells was injected into a sheep and the specificity of the resulting antiserum was shown by Western blotting and its effects on retinal development were determined in vitro and in vivo. In the present study, the distribution of these secreted proteins was determined by light microscopy immunocytochemistry in cultured neonatal rat RPE cells and retinas of normal and Royal College of Surgeons (RCS) dystrophic rats and cerebrum of normal adult rats. Immunolabelling for these RPE-secreted proteins was detected in cytoplasmic vesicles surrounding nuclei and within processes of cultured normal and transformed rat RPE. In retinas of late postnatal and adult rats, dense immunostaining was found in the cytoplasm of RPE cells and ganglion cell bodies. In addition to RPE and ganglion cells, scattered photoreceptors within the thin outer nuclear layer and small structures within the debris zone were also densely immunoreactive in retinas of 2-mo-old RCS dystrophic rats. The numbers of immunostained ganglion cells appeared to decrease in retinas of older RCS rats, although the immunoreactivity within the RPE appeared to increase in density. No other neuron within the retina, i.e. bipolar, amacrine or horizontal, was immunoreactive for RPE-secreted proteins. In the cerebral cortex of adult rats, immunoreactivity for RPE-secreted proteins was primarily detected within large perikarya of pyramidal neurons and smaller granule neurons. In conclusion, we report an immunocytochemical analysis of an antiserum raised against secreted proteins of rat RPE. This antiserum recognised proteins within secretory-like vesicles of cultured neonatal normal and transformed rat RPE and showed a specificity for RPE and ganglion cells in normal rat retinas, that appeared to be developmentally regulated, and neuron perikarya in adult rat cerebrum.

A transformed neonatal rat retinal pigment epithelial cell line: secreted protein analysis and fibroblast growth factor and receptor expression

PURPOSE

A newly-derived transformed neonatal rat retinal pigment epithelial (tnrRPE) cell line was investigated: for secreted proteins by electrophoresis, and for basic and acidic fibroblast growth factor (FGF) by immunocytochemistry, Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR). The FGFR-1 (flg) receptor, which is recognized by aFGF and bFGF, was studied by RT-PCR.

METHODS

Retinal pigment epithelial (RPE) cells were isolated from 6-day-old pigmented normal Long Evans rats, and became spontaneously transformed after the second passage.

RESULTS

RPE cells at the 5th through 28th passages expressed the epithelial cell marker cytokeratin and cellular retinaldehyde binding protein (CRALBP), an RPE cell marker, but were negative for glial fibrillary acidic protein (GFAP), as shown by immunofluorescence. Secreted proteins of late passage tnrRPE cells were in a narrow molecular weight range of 60-80kDa, while early passage cells exhibited multiple proteins from 20-200kDa. These tnrRPE cells increased by 17-30 fold over a 4-day culture period. At 5th and 28th passage, immunostaining for bFGF and aFGF was dense within nuclei, but light and diffuse within the cytoplasm of transformed RPE cells. As shown by Northern blot, similar levels of message for bFGF were detected in 5th and 30th passage RPE cells. As shown by Northern blot, similar levels of message for bFGF were detected in 5th and 30th passage RPE cells. Furthermore, as shown by RT-PCR, bFGF mRNA was found in freshly isolated and transformed neonatal rat RPE cells. However, the message for FGFR-1(flg) receptor was detected only in the transformed RPE cells.

CONCLUSIONS

This study demonstrated a neonatal rat RPE cell line that proliferated rapidly in vitro, expressed high levels of message for hFGF and FGFR-1(flg) receptor, and continued to express RPE-cell characteristics. Importantly, mRNA levels of confluent cultures of these cells were sufficient for bFGF mRNA blot analysis, which eliminates the necessity for PCR and for using excessive numbers of animals for such studies.

Effects of retinal pigment epithelial cell-secreted factors on neonatal rat retinal explant progenitor cells

This study demonstrates the effects of conditioned media from transformed neonatal rat retinal pigment epithelial cells (tnrRPE-CM) in a culture system consisting of neonatal rat retinal explants. For this study, retinal explants from postnatal day 2 (PN2) normal rats were cultured for over 3 weeks on a poly-D-L-ornithine-coated surface in RPE-CM only, 10% serum, or a serum-free defined media, and then examined by phase-contrast and scanning electron microscopy and immunocytochemistry. After 2 days in vitro, long ganglion cell-like neurites projected from retinal explants grown in tnrRPE-CM. These neurites increased in number and length with prolonged time in culture. In addition, by 5 days, round cells were observed adjacent to neonatal explants grown in tnrRPE-CM. By day 10, these round cells had increased in number and were seen along the neurites, in massive clusters immediately adjacent to these explants and dispersed throughout the culture-plate surface. Media conditioned by primary cultures of normal neonatal rat RPE cells caused a similar, but less robust, cellular response in retinal explants when compared to tnrRPE-CM. At 10 days, retinal explants grown in 10% serum showed only a few short processes, but no round cells, while those explants grown in defined media appeared to be degenerating. The round migrating cells are classified as retinal progenitor cells since they immunostained for opsin and interphotoreceptor retinoid-binding protein (IRBP), two photoreceptor cell markers, and a few for cellular retinaldehyde binding protein (CRALBP), a Muller cell marker. Neurite outgrowth and retinal progenitor cell production from explants were eliminated when the tnrRPE-CM was subjected to trypsin or heat treatment, indicating that the factor(s) responsible for promoting these cellular events was most likely proteinaceous. Growth factors, including basic fibroblast growth factor, were unable to generate long neurite outgrowth or progenitor cell production as observed in RPE-CM-supplemented explant cultures. We report that CM from cultures of primary and transformed neonatal rat RPE cells promoted ganglion cell-like neurites and the production of migrating retinal progenitor cells that primarily expressed photoreceptor-specific markers, from neonatal rat retinal explants. This evidence further confirms the important role of RPE in retinal development. The production of large numbers of progenitor cells by an RPE-secreted factor(s) may have important implications for possible therapeutic approaches to help correct retinal disease states by replacing lost cells through transplantation technology.

Influence of a retinal pigment epithelial cell factor(s) on rat retinal progenitor cells

Retinal development was studied by explant culture of retinas from embryonic and neonatal rats in response to medium conditioned (CM) by a transformed neonatal rat retinal pigment epithelial (tnrRPE) cell line. Retinal explants from embryonic days 16 and 18 and postnatal day 2 Long-Evans rats were cultured for over 3 weeks on a poly-D,L-ornithine-coated surface in RPE-CM only, 10% serum or a serum-free defined medium. By 2 days in vitro, round cells were seen emerging from both embryonic and neonatal retinal explants grown in tnrRPE-CM. With extended time in culture, these round cells had increased in number and were seen in large confluent clusters adjacent to the explants. After 2 weeks in culture, some of these cells had undergone a morphological differentiation as shown by process formation. Insignificant numbers of these same cells were seen in explant cultures grown in 10% serum or serum-free defined medium. When isolated and subcultured, approx. 80% of the round cells from embryonic and neonatal rat retinal explants were densely immunolabeled for opsin and arrestin, both photoreceptor cell markers and neuron-specific enolase, a marker for mature neurons. Cellular retinaldehyde-binding protein, a Müller cell marker, immunolabeled approx. 30% of the cells from embryonic and neonatal rat retinal explants. In addition, nestin, an intermediate filament protein found only in neuroepithelial cells, was present in approx. 70% of the embryonic cells, but in only less than 1% of the neonatal cells. Based on this immunocytochemical characterization, these round cells are termed retinal progenitor cells and because of their mitogenic capacity under these in vitro conditions, these cells appear to possess stem cell characteristics. Moreover, in a 3-day bioassay, tnrRPE-CM caused a twofold and greater increase in harvested progenitor cells from both neonatal and embryonic explants, while cell numbers in control and growth factor-supplemented cultures showed no increase above the initial plating density. In these studies, CM from cultures of transformed neonatal rat RPE cells promoted the production, survival, proliferation and maturation of retinal progenitor cells from neonatal and embryonic rat retinal explants.