Pigmented retinal epithelium involvement in photoreceptor development and function

A treatment within sight: challenges in the development of stem cell-derived photoreceptor therapies for retinal degenerative diseases

Stem cell therapies can potentially treat various retinal degenerative diseases, including age-related macular degeneration (AMD) and inherited retinal diseases like retinitis pigmentosa. For these diseases, transplanted cells may include stem cell-derived retinal pigmented epithelial (RPE) cells, photoreceptors, or a combination of both. Although stem cell-derived RPE cells have progressed to human clinical trials, therapies using photoreceptors and other retinal cell types are lagging. In this review, we discuss the potential use of human pluripotent stem cell (hPSC)-derived photoreceptors for the treatment of retinal degeneration and highlight the progress and challenges for their efficient production and clinical application in regenerative medicine.

New Prospects for Retinal Pigment Epithelium Transplantation

ABSTRACT

Retinal pigment epithelium (RPE) transplants rescue photoreceptors in selected animal models of retinal degenerative disease. Early clinical studies of RPE transplants as treatment for age-related macular degeneration (AMD) included autologous and allogeneic transplants of RPE suspensions and RPE sheets for atrophic and neovascular complications of AMD. Subsequent studies explored autologous RPE-Bruch membrane-choroid transplants in patients with neovascular AMD with occasional marked visual benefit, which establishes a rationale for RPE transplants in late-stage AMD. More recent work has involved transplantation of autologous and allogeneic stem cell-derived RPE for patients with AMD and those with Stargardt disease. These early-stage clinical trials have employed RPE suspensions and RPE monolayers on biocompatible scaffolds. Safety has been well documented, but evidence of efficacy is variable. Current research involves development of better scaffolds, improved modulation of immune surveillance, and modification of the extracellular milieu to improve RPE survival and integration with host retina.

Retinal Cell Transplantation, Biomaterials, and In Vitro Models for Developing Next-generation Therapies of Age-related Macular Degeneration

Retinal pigment epithelium (RPE) cells grown on a scaffold, an RPE patch, have potential to ameliorate visual impairment in a limited number of retinal degenerative conditions. This tissue-replacement therapy is suited for age-related macular degeneration (AMD), and related diseases. RPE cells must be transplanted before the disease reaches a point of no return, represented by the loss of photoreceptors. Photoreceptors are specialized, terminally differentiated neurosensory cells that must interact with RPE's apical processes to be functional. Human photoreceptors are not known to regenerate. On the RPE's basal side, the RPE transplant must induce the reformation of the choriocapillaris, thereby re-establishing the outer blood-retinal barrier. Because the scaffold is positioned between the RPE and choriocapillaris, it should ideally degrade and be replaced by the natural extracellular matrix that separates these tissues. Besides biodegradable, the scaffolds need to be nontoxic, thin enough to not affect the focal length of the eye, strong enough to survive the transplant procedure, yet flexible enough to conform to the curvature of the retina. The challenge is patients with progressing AMD treasure their remaining vision and fear that a risky surgical procedure will further degrade their vision. Accordingly, clinical trials only treat eyes with severe impairment that have few photoreceptors to interact with the transplanted patch. Although safety has been demonstrated, the cell-replacement mechanism and efficacy remain difficult to validate. This review covers the structure of the retina, the pathology of AMD, the limitations of cell therapy approaches, and the recent progress in developing retinal therapies using biomaterials.

A defined subset of clonal retinal stem cell spheres is biased to RPE differentiation

Retinal stem cells (RSCs) are rare pigmented cells found in the pigmented ciliary layer of the mammalian retina. Studies show that RSCs can replicate to maintain the stem cell pool and produce retinal progenitors that differentiate into all retinal cell types. We classified RSCs based on their level and distribution of pigment into heavily pigmented (HP), lightly pigmented (LP), and centrally pigmented (CP) spheres. We report that CP spheres are capable of generating large cobblestone lawns of retinal pigment epithelial (RPE) cells. The other clonal sphere types (HP and LP) primarily produce cells with neural morphology and fewer RPE cells. The RSCs are homogeneous, but their downstream progenitors are different. We found that CP spheres contain highly proliferative populations of early RPE progenitors that respond to proliferative signals from the surrounding non-pigmented cells. HP and LP spheres contain late RPE progenitors which are not affected by proliferative signals.

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Three types of clonal retinal stem cell spheres form from the same single stem cell

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Centrally pigmented spheres contain populations of early RPE progenitors

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Heavily and lightly pigmented spheres contain populations of late RPE progenitors

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Downstream RPE progenitors are different due to extrinsic and intrinsic factors

Interactions of the choroid, Bruch's membrane, retinal pigment epithelium, and neurosensory retina collaborate to form the outer blood-retinal-barrier

The three interacting components of the outer blood-retinal barrier are the retinal pigment epithelium (RPE), choriocapillaris, and Bruch's membrane, the extracellular matrix that lies between them. Although previously reviewed independently, this review integrates these components into a more wholistic view of the barrier and discusses reconstitution models to explore the interactions among them. After updating our understanding of each component's contribution to barrier function, we discuss recent efforts to examine how the components interact. Recent studies demonstrate that claudin-19 regulates multiple aspects of RPE's barrier function and identifies a barrier function whereby mutations of claudin-19 affect retinal development. Co-culture approaches to reconstitute components of the outer blood-retinal barrier are beginning to reveal two-way interactions between the RPE and choriocapillaris. These interactions affect barrier function and the composition of the intervening Bruch's membrane. Normal or disease models of Bruch's membrane, reconstituted with healthy or diseased RPE, demonstrate adverse effects of diseased matrix on RPE metabolism. A stumbling block for reconstitution studies is the substrates typically used to culture cells are inadequate substitutes for Bruch's membrane. Together with human stem cells, the alternative substrates that have been designed offer an opportunity to engineer second-generation culture models of the outer blood-retinal barrier.

Usher syndrome type 1-associated cadherins shape the photoreceptor outer segment

Usher syndrome type 1 (USH1) causes combined hearing and sight defects, but USH1 protein function in the retina is unclear. Schietroma et al. use Xenopus to model the deficiency in two USH1 proteins—protocadherin-15 and cadherin-23—and identify crucial roles for these molecules in shaping the photoreceptor outer segment.

Usher syndrome type 1 (USH1) causes combined hearing and sight defects, but how mutations in USH1 genes lead to retinal dystrophy in patients remains elusive. The USH1 protein complex is associated with calyceal processes, which are microvilli of unknown function surrounding the base of the photoreceptor outer segment. We show that in Xenopus tropicalis, these processes are connected to the outer-segment membrane by links composed of protocadherin-15 (USH1F protein). Protocadherin-15 deficiency, obtained by a knockdown approach, leads to impaired photoreceptor function and abnormally shaped photoreceptor outer segments. Rod basal outer disks displayed excessive outgrowth, and cone outer segments were curved, with lamellae of heterogeneous sizes, defects also observed upon knockdown of Cdh23, encoding cadherin-23 (USH1D protein). The calyceal processes were virtually absent in cones and displayed markedly reduced F-actin content in rods, suggesting that protocadherin-15–containing links are essential for their development and/or maintenance. We propose that calyceal processes, together with their associated links, control the sizing of rod disks and cone lamellae throughout their daily renewal.

MicroRNAs are essential for differentiation of the retinal pigmented epithelium and maturation of adjacent photoreceptors

Dysfunction of the retinal pigmented epithelium (RPE) results in degeneration of photoreceptors and vision loss and is correlated with common blinding disorders in humans. Although many protein-coding genes are known to be expressed in RPE and are important for its development and maintenance, virtually nothing is known about the in vivo roles of non-coding transcripts. The expression patterns of microRNAs (miRNAs) have been analyzed in a variety of ocular tissues, and a few were implicated to play role in RPE based on studies in cell lines. Here, through RPE-specific conditional mutagenesis of Dicer1 or Dgcr8 in mice, the importance of miRNAs for RPE differentiation was uncovered. miRNAs were found to be dispensable for maintaining RPE fate and survival, and yet they are essential for the acquisition of important RPE properties such as the expression of genes involved in the visual cycle pathway, pigmentation and cell adhesion. Importantly, miRNAs of the RPE are required for maturation of adjacent photoreceptors, specifically for the morphogenesis of the outer segments. The alterations in the miRNA and mRNA profiles in the Dicer1-deficient RPE point to a key role of miR-204 in regulation of the RPE differentiation program in vivo and uncover the importance of additional novel RPE miRNAs. This study reveals the combined regulatory activity of miRNAs that is required for RPE differentiation and for the development of the adjacent neuroretina.

Early stages of retinal development depend on Sec13 function

ER-to-Golgi transport of proteins destined for the extracellular space or intracellular compartments depends on the COPII vesicle coat and is constitutive in all translationally active cells. Nevertheless, there is emerging evidence that this process is regulated on a cell- and tissue-specific basis, which means that components of the COPII coat will be of differential importance to certain cell types. The COPII coat consists of an inner layer, Sec23/24 and an outer shell, Sec13/31. We have shown previously that knock-down of Sec13 results in concomitant loss of Sec31. In zebrafish and cultured human cells this leads to impaired trafficking of large cargo, namely procollagens, and is causative for defects in craniofacial and gut development. It is now widely accepted that the outer COPII coat is key to the architecture and stability of ER export vesicles containing large, unusual cargo proteins. Here, we investigate zebrafish eye development following Sec13 depletion. We find that photoreceptors degenerate or fail to develop from the onset. Impaired collagen trafficking from the retinal pigment epithelium and defects in overall retinal lamination also seen in Sec13-depleted zebrafish might have been caused by increased apoptosis and reduced topical proliferation in the retina. Our data show that the outer layer of the COPII coat is also necessary for the transport of large amounts of cargo proteins, in this case rhodopsin, rather than just large cargo as previously thought.

RPE-secreted factors: influence differentiation in human retinal cell line in dose- and density-dependent manner

Retinal pigment epithelial (RPE) cells play an important role in normal functioning of retina and photoreceptors, and some retinal degenerations arise due to malfunctioning RPE. Retinal pigment epithelium transplantation is being explored as a strategy to rescue degenerating photoreceptors in diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Additionally, RPE-secreted factors could rescue degenerating photoreceptors by prolonging survival or by their ability to differentiate and give rise to photoreceptors by transdifferentiation. In this study, we have explored what role cell density could play in differentiation induced in a human retinal progenitor cell line, in response to RPE-secreted growth factors. Retinal progenitors plated at low (1 × 10(4) cells/cm(2)), medium (2-4 × 10(4) cells/cm(2)), and high (1 × 10(5) cells/cm(2)) cell density were exposed to various dilutions of RPE-conditioned medium (secreted factors) under conditions of defined medium culture. Progenitor cell differentiation was monitored phenotypically (morphological, biochemical analysis, and immunophenotyping, and western blot analysis were performed). Our data show that differentiation in response to RPE-secreted factors is modulated by cell density and dilutions of conditioned medium. We conclude that before embarking on RPE transplantation as a modality for treatment of RP and AMD, one will have to determine the role that cell density and inhibitory and stimulatory neurotrophins secreted by RPE could play in the efficacy of survival of transplants. We report that RPE-conditioned medium enhances neuronal phenotype (photoreceptors, bipolars) at the lowest cell density in the absence of cell-cell contact. Eighty percent to 90% of progenitor cells differentiate into photoreceptors and bipolars at 50% concentration of conditioned medium, while exposure to 100% conditioned medium might increase multipolar neurons (ganglionic and amacrine phenotypes) to a small degree. However, no clear-cut pattern of differentiation in response to RPE-secreted factors is noted at higher cell densities.

Cellular retinol binding protein 1 modulates photoreceptor outer segment folding in the isolated eye

In a previous study, we used differential proteomics to identify retinal proteins whose steady-state levels were altered in an experimental system in which photoreceptor outer segments were improperly folded. We determined that the steady-state level of cellular retinol binding protein 1 (CRBP1) was downregulated in eyes lacking organized outer segments. The purpose of this study was to determine if CRBP1 is a plausible candidate for regulating outer segment assembly. We used Morpholinos to directly test the hypothesis that a decreased level of CRBP1 protein was associated with the misfolding of outer segments. Results from these studies indicate that downregulation of CRBP1 protein resulted in aberrant assembly of outer segments. Because CRBP1 plays a dual role in the retina-retinal recycling and generation of retinoic acid-we evaluated both possibilities. Our data demonstrate that outer segment folding was not modified by 11-cis retinal supplementation, suggesting that CRBP1 influences outer segment assembly through a mechanism unrelated to rhodopsin regeneration. In contrast, retinoic acid is required for the proper organization of nascent outer segment membranes. The localization of CRBP1 within Muller cells and the RPE and its demonstrated role in modulating the proper folding of nascent outer segment membranes through retinoic acid further elucidates the role of these cells in directly influencing photoreceptor physiology.

Embryonic retinal cells and support to mature retinal neurons

Purpose. There is a paucity of neuron replacement studies for retinal ganglion cells. Given the complex phenotype of these neurons, replacement of ganglion cells may be impossible. However, transplanted embryonic cells could provide factors that promote the survival of these neurons. The authors sought to determine whether transplanted embryonic retinal cells from various stages of development influence the survival of mature ganglion cells Methods. Acutely dissociated retinal cells, obtained from chick embryos, were transplanted into the vitreous chamber of posthatch chicken eyes after the ganglion cells were selectively damaged. Eight days after transplantation, numbers of ganglion cells were determined Results. Embryonic retinal cells from embryonic day (E)7, E10, and E11 promoted the survival of ganglion cells, whereas cells from earlier or later stages of development or from other tissue sources did not. The environment provided by the posthatch eye did not support the proliferation of the embryo-derived cells, unlike the environment provided by culture conditions. Furthermore, cells that migrated into the retina failed to express neuronal or glial markers; those that remained in the vitreous formed aggregates of neuronal and glial cells Conclusions. The environment provided within the mature retina does not support the differentiation and proliferation of retinal progenitors. Furthermore, embryo-derived cells likely produce secreted factors that promote the survival of damaged ganglion cells. Therefore, embryonic retinal cells could be applied as a cell-based survival therapy to treat neurodegenerative diseases of the retina.

Proteomic analysis of the retina: removal of RPE alters outer segment assembly and retinal protein expression

The mechanisms that regulate the complex physiological task of photoreceptor outer segment assembly remain an enigma. One limiting factor in revealing the mechanism(s) by which this process is modulated is that not all of the role players who participate in this process are known. The purpose of this study was to determine some of the retinal proteins that likely play a critical role in regulating photoreceptor outer segment assembly. To do so, we analyzed and compared the proteome map of tadpole Xenopus laevis retinal pigment epithelium (RPE)-supported retinas containing organized outer segments with that of RPE-deprived retinas containing disorganized outer segments. Solubilized proteins were labeled with CyDye fluors followed by multiplexed two-dimensional separation. The intensity of protein spots and comparison of proteome maps was performed using DeCyder software. Identification of differentially regulated proteins was determined using nanoLC-ESI-MS/MS analysis. We found a total of 27 protein spots, 21 of which were unique proteins, which were differentially expressed in retinas with disorganized outer segments. We predict that in the absence of the RPE, oxidative stress initiates an unfolded protein response. Subsequently, downregulation of several candidate Müller glial cell proteins may explain the inability of photoreceptors to properly fold their outer segment membranes. In this study, we have used identification and bioinformatics assessment of proteins that are differentially expressed in retinas with disorganized outer segments as a first step in determining probable key molecules involved in regulating photoreceptor outer segment assembly.

Retinal pigment epithelial cells promote spatial reorganization and differentiation of retina photoreceptors

Retina differentiation involves the acquisition of a precise layered arrangement, with RPE cells in the first layer in intimate contact with photoreceptors in the second layer. Here, we developed an in vitro coculture model, to test the hypothesis that RPE cells play a pivotal role in organizing the spatial structure of the retina. We cocultured rat retinal neurons with ARPE-19 epithelial cells under various experimental conditions. Strikingly, when seeded over RPE cells, photoreceptors attached to their apical surfaces and proceeded with their development, including the increased synthesis of rhodopsin. Conversely, when we seeded RPE cells over neurons, the RPE cells rapidly detached photoreceptors from their substrata and positioned themselves underneath, thus restoring the normal in vivo arrangement. Treatment with the metalloproteinase inhibitor TIMP-1 blocked this reorganization, suggesting the involvement of metalloproteinases in this process. Reorganization was highly selective for photoreceptors because 98% of photoreceptors but very few amacrine neurons were found to redistribute on top of RPE cells. Interestingly, RPE cells were much more efficient than other epithelial or nonepithelial cells in promoting this reorganization. RPE cells also promoted the growth of photoreceptor axons away from them. An additional factor that contributed to the distal arrangement of photoreceptor axons was the migration of photoreceptor cell bodies along their own neurites toward the RPE cells. Our results demonstrate that RPE and photoreceptor cells interact in vitro in very specific ways. They also show that in vitro studies may provide important insights into the process of pattern formation in the retina.

PEDF and GDNF are key regulators of photoreceptor development and retinal neurogenesis in reaggregates from chick embryonic retina

Here, role(s) of pigment epithelial-derived factor (PEDF) and glial-derived neurotrophic factor (GDNF) on photoreceptor development in three-dimensional reaggregates from the retinae of the E6 chick embryo (rosetted spheroids) was investigated. Fully dispersed cells were reaggregated under serum-reduced conditions and supplemented with 50 ng/ml PEDF alone or in combination with 50 ng/ml GDNF. The spheroids were analyzed for cell growth, differentiation, and death using proliferating cell nuclear antigen, terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling, and other immunocytochemical stainings and semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) methods. PEDF strongly promoted synthesis of the messenger RNAs for blue and violet cone opsins and to a lesser extent on the red and green cone opsins. This correlated with an increase in the number of cone photoreceptors, as determined by the cone cell marker CERN906. Likewise, PEDF nearly completely inhibited rod differentiation, as detected by immunostaining with anti-rho4D2 and RT-PCR. Furthermore, PEDF accelerated proliferation of cells in the spheroids and inhibited apoptosis. As negative effects, PEDF inhibited the normal histotypic tissue formation of retinal aggregates and reduced the frequency of photoreceptor rosettes and IPL-like areas. Noticeably, supplementation of PEDF-treated cultures with GDNF reversed the effects of PEDF on spheroid morphology and on rod differentiation. This study establishes that PEDF strongly affects three-dimensional retinogenesis in vitro, most notably by inhibiting rod development and supporting proliferation and differentiation of cones, effects which are partially counteracted by GDNF.

Module structure of interphotoreceptor retinoid-binding protein (IRBP) may provide bases for its complex role in the visual cycle - structure/function study of Xenopus IRBP

BACKGROUND

Interphotoreceptor retinoid-binding protein's (IRBP) remarkable module structure may be critical to its role in mediating the transport of all-trans and 11-cis retinol, and 11-cis retinal between rods, cones, RPE and Müller cells during the visual cycle. We isolated cDNAs for Xenopus IRBP, and expressed and purified its individual modules, module combinations, and the full-length polypeptide. Binding of all-trans retinol, 11-cis retinal and 9-(9-anthroyloxy) stearic acid were characterized by fluorescence spectroscopy monitoring ligand-fluorescence enhancement, quenching of endogenous protein fluorescence, and energy transfer. Finally, the X-ray crystal structure of module-2 was used to predict the location of the ligand-binding sites, and compare their structures among modules using homology modeling.

RESULTS

The full-length Xenopus IRBP cDNA codes for a polypeptide of 1,197 amino acid residues beginning with a signal peptide followed by four homologous modules each approximately 300 amino acid residues in length. Modules 1 and 3 are more closely related to each other than either is to modules 2 and 4. Modules 1 and 4 are most similar to the N- and C-terminal modules of the two module IRBP of teleosts. Our data are consistent with the model that vertebrate IRBPs arose through two genetic duplication events, but that the middle two modules were lost during the evolution of the ray finned fish. The sequence of the expressed full-length IRBP was confirmed by liquid chromatography-tandem mass spectrometry. The recombinant full-length Xenopus IRBP bound all-trans retinol and 11-cis retinaldehyde at 3 to 4 sites with Kd's of 0.2 to 0.3 microM, and was active in protecting all-trans retinol from degradation. Module 2 showed selectivity for all-trans retinol over 11-cis retinaldehyde. The binding data are correlated to the results of docking of all-trans-retinol to the crystal structure of Xenopus module 2 suggesting two ligand-binding sites. However, homology modeling of modules 1, 3 and 4 indicate that both sites may not be available for binding of ligands in all four modules.

CONCLUSION

Although its four modules are homologous and each capable of supporting ligand-binding activity, structural differences between their ligand-binding domains, and interactions between the modules themselves will be critical to understanding IRBP's complex role in the visual cycle.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Expression of tissue plasminogen activator during eye development

Tissue plasminogen activator (tPA) is a serine protease responsible for the activation of plasminogen to plasmin as well as extracellular matrix remodeling. While tPA is used clinically to treat some retinal disorders and it is expressed at low levels in the adult eye, its expression pattern during eye development had never been determined. tPA protein is broadly dispersed in the lens placode and optic vesicle of the mouse eye and it becomes highly localized to the apical surfaces of both the lens pit and the optic cup as they invaginate. In the lens, tPA remains at the apical tips of both lens epithelial and fiber cells from the lens vesicle stage until birth in the mouse, when it begins to downregulate to barely detectable levels in adults. In humans, tPA is found in a similar pattern in the lens vesicle and early lens, however, appreciable protein is also detected in the cytoplasm of lens epithelial cells until adulthood. In the retina, tPA is found at the apical interface between the developing retinal pigmented epithelium and neural retina, then begins to downregulate once the photoreceptors have differentiated. In conclusion, tPA protein is found in a different pattern in embryonic versus adult eyes and may be involved in remodeling of the extracellular environment during eye development.

Contribution of Müller cells toward the regulation of photoreceptor outer segment assembly

The assembly of photoreceptor outer segments into stacked discs is a complicated process, the precise regulation of which remains a mystery. It is known that the integrity of the outer segment is heavily dependent upon surrounding cell types including the retinal pigment epithelium and Müller cells; however the role played by Müller cells within this photoreceptor-specific process has not been fully explored. Using an RPE-deprived but otherwise intact Xenopus laevis eye rudiment preparation, we reveal that Müller cell involvement in outer segment assembly is dependent upon the stimulus provided to the retina. Pigment epithelium-derived factor is able to support proper membrane folding after inhibition of Müller cell metabolism by alpha-aminoadipic acid, while isopropyl beta-D-thiogalactoside, a permissive glycan, requires intact Müller cell function. These results demonstrate that both intrinsic and extrinsic redundant mechanisms exist to support the ability of photoreceptors to properly assemble their outer segments. Our study further suggests that the receptor for pigment epithelium-derived factor resides in photoreceptors themselves while that for permissive glycans is likely localized to Müller cells, which in turn communicate with photoreceptors to promote proper membrane assembly.

Adenoviral-mediated gene transfer to retinal explants during development and degeneration

Naturally occurring mutations of the beta subunit of the cyclic guanosine monophosphate (cGMP) phosphodiesterase (beta-PDE) gene in rod photoreceptors of mice and dogs are similar to one of the inherited retinal degenerations termed retinitis pigmentosa in humans. Defects in the rod beta-PDE gene leading to photoreceptor cell degeneration in retinal degenerative (rd) mice can be corrected by transfer of a wild type beta-PDE gene. However, the rapid photoreceptor degeneration in this mutant makes the study of gene therapy difficult. Since the retinal degeneration is slowed in vitro, we have employed retinal explants from rd mice to study factors influencing viral transduction. Retinal explants provide a rapid, efficient method to compare the transduction efficiency of adenoviral vector-mediated reporter gene delivery at different ages in normal and rd mice. Retinal explants from postnatal day (P)2 to P28 control (C57BL/6J) and P2-P42 rd mice were exposed for 20 hr to 2.5 x 10(8) plaque forming units (pfu) ml(-1) of adenoviral vector with a beta-galactosidase (Lac Z) reporter gene (Ad-CMV-Lac Z). After incubation in vector-free media for an additional 3 days, the explants were fixed and histochemically stained for beta-galactosidase to reveal Lac Z gene expression. The explants were also embedded and sectioned for light microscopic observation. Transduction efficiency was higher in rd mice than in controls on all postnatal days examined. In normal retinal explants, expression of the Lac Z gene increased from P2 to a peak around P7-P8, then decreased at subsequent ages; little transduction could be found after P17. In rd mice transduction efficiency of Ad-CMV-Lac Z increased from P2 to P7, decreased by P10 and increased again after P10. The most dramatic increase in the transduction efficiency occurred in the rd retina between P10 and P15 when Lac Z was intensely expressed throughout the retina. Microscopic examination of retinal sections revealed the types and distribution of Lac Z-positive cells responsible for the deep blue staining in the retinal whole mount. In normal and rd mice, Lac Z-positive cells were located throughout the retina. However, larger numbers of Lac Z-positive cells were present at all ages examined in retinal explants from rd mice compared to normal mice. These data indicate a difference in transduction efficiency between normal and rd mice, especially after P12, and suggest efficient adenovirus-mediated gene transfer is more attainable in developing or degenerating retina. Thus, transduction efficiency in rd mice depends on the relationship between development, maturation and the degenerative state of the photoreceptor cells.

Interphotoreceptor retinoid-binding protein--an old gene for new eyes

Evolving 40 times independently, eyes are striking examples of convergent evolution in that 11-cis retinaldehyde is always used for photon capture, yet the mechanism for its regeneration may be dramatically different in between systems. In particular, insects, cephalopods and vertebrates show varying physical separation of the cis-->trans photoisomerization and chromphore reisomerization. In the vertebrate retina, these two processes are actually distributed between different cells. This compartmentalization is made possible by the phylogenetic innovation of the two-layered optic cup of the vertebrate retina. This unprecedented design created the subretinal space as a novel anatomical compartment allowing photoreceptors access to the retinal pigment epithelium (RPE) and Müller cells, the two cell types which share the burden of 11-cis retinoid regeneration. To take advantage of this arrangement, early vertebrates appear to have recruited for retinoid binding, the betabetaalpha-spiral fold proven useful in enoyl-CoA isomerase/hydratases, and the carboxy-terminal proteases for stabilizing hydrophobic ligands. Quadruplication of this functional domain within a single polypeptide lead to the emergence of interphotoreceptor retinoid-binding protein (IRBP). IRBP is the main soluble component of the IPM, and is prevented from diffusing out of the subretinal space because its large size excludes it from the photoreceptor/Müller cell zonulae adheretes. Despite this physical entrapment, IRBP is rapidly turned over within the IPM through a process that coordinates secretion of the protein by the photoreceptors, and its removal from the matrix by RPE and photoreceptor endocytosis. The present review will summarize what is known about the structure and function of IRBP to anticipate future avenues of research.

Developmental plasticity of photoreceptors

During development, retinal ganglion cells undergo conspicuous structural remodeling as they gradually attain their mature morphology and connectivity. Alterations in their dendritic organization and in their axonal projections can also be achieved following early insult to their targets or their afferents. Other retinal cell types are thought not to display this same degree of developmental plasticity. The present review will consider the evidence, drawn largely from recent experimental studies in the carnivore retina, that photoreceptors also undergo structural remodeling, extending their terminals transiently into inner plexiform layer before retracting to the outer plexiform layer. The determinants of this transient targeting to the inner plexiform layer are considered, and the role of cholinergic amacrine cells is discussed. The factors triggering this retraction are also considered, including the concurrent maturational changes in outer segment formation and in the differentiation of the outer plexiform layer. These results provide new insight into the life history of the photoreceptor cell and its connectivity, and suggest a transient role for the photoreceptors in the circuitry of the inner retina during early development, prior to the onset of phototransduction.

Lazy eyes zebrafish mutation affects Müller glial cells, compromising photoreceptor function and causing partial blindness

A behavioral assay based on the optokinetic reflex was used to screen chemically mutagenized zebrafish larvae for deficits in visual function. A homozygous recessive mutation, lazy eyes (lze), was isolated based on the observation that 5-day postfertilization (dpf) mutants displayed weaker and less frequent eye movements than wild-type fish in response to moving stripes. Electroretinographic (ERG) recordings revealed that mutants had severely reduced a- and b-wave amplitudes relative to wild-type fish, indicating outer retinal dysfunction. Retinal lamination and cellular differentiation were normal in the lze retina; however, mutant photoreceptor cells had small outer segments and pyknotic nuclei were occasionally observed in the outer retina and the marginal zone of lze. Cone, rod, amacrine, bipolar, and Müller cell marker analyses indicated that the typical lze retina contained fewer rod photoreceptors and fewer Müller cells than wild-type fish at 5 dpf. At 3 dpf, however, mutant retinas had normal numbers of rod photoreceptors and Müller cells, suggesting that the initial differentiation of these cell types occurred normally. Rod photoreceptor histology was normal at this early stage, but Müller cells were often hypertrophied, suggesting that they were unhealthy. Constant light rearing of mutant animals accelerated the Müller cell degeneration, severely worsened the visual deficit, but had no obvious affect on the photoreceptors. When ERG responses and Müller cell degeneration from the same mutant animals were analyzed, the extent of the Müller cell loss matched closely the degree to which ERG responses were reduced. In summary, the lze gene appears to be required for Müller cell viability and normal visual function. The lze mutant may be a model for the study of the involvement of Müller cells in photoreceptor development and function.

Activated MAPK/ERK kinase (MEK-1) induces transdifferentiation of pigmented epithelium into neural retina

During vertebrate eye development, the optic vesicle originating from the neuroectoderm is partitioned into a domain that will give rise to the neural retina (NR) and another that will give rise to the retinal pigmented epithelium (RPE). Previous studies have shown that ectopic expression of FGFs in the RPE induces RPE-to-NR transdifferentiation. Similarly, a naturally occurring mutation of the transcription factor Mitf in mouse resulted in the formation of a second neural retina in place of the dorsal RPE, but the putative signaling pathway linking FGF to Mitf regulation is presently unknown. In cultures of neural crest-derived melanocytes, the MAPK pathway was recently shown to target the Mitf transcription factor for ubiquitin-dependent proteolysis, resulting in a rapid degradation and downregulation. In the present study, we show that ectopic expression of a constitutively activated allele of MEK-1, the immediate upstream activator of the MAPK ERK, in chicken embryonic retina in ovo, induces transdifferentiation of the RPE into a neural-like epithelium that is correlated with a downregulation of Mitf expression in the presumptive RPE.

Structure and function of photoreceptor and second-order cell mosaics in the retina of Xenopus

The structure, physiology, synaptology, and neurochemistry of photoreceptors and second-order (horizontal and bipolar) cells of Xenopus laevis retina is reviewed. Rods represent 53% of the photoreceptors; the majority (97%) are green light-sensitive. Cones belong to large long-wavelength-sensitive (86%), large short-wavelength-sensitive (10%), and miniature ultraviolet wavelength-sensitive (4%) groups. Photoreceptors release glutamate tonically in darkness, hyperpolarize upon light stimulation and their transmitter release decreases. Photoreceptors form ribbon synapses with second-order cells where postsynaptic elements are organized into triads. Their overall adaptational status is regulated by ambient light conditions and set by the extracellular dopamine concentration. The activity of photoreceptors is under circadian control and is independent of the central body clock. Bipolar cell density is about 6000 cells/mm2 They receive mixed inputs from rods and cones. Some bipolar cell types violate the rule of ON-OFF segregation, giving off terminal branches in both sublayers of the inner plexiform layer. The majority of them contain glutamate, a small fraction is GABA-positive and accumulates serotonin. Luminosity-type horizontal cells are more frequent (approximately 1,000 cells/mm2) than chromaticity cells (approximately 450 cells/mm2). The dendritic field size of the latter type was threefold bigger than that of the former. Luminosity cells contact all photoreceptor types, whereas chromatic cells receive their inputs from the short-wavelength-sensitive cones and rods. Luminosity cells are involved in generating depolarizing responses in chromatic horizontal cells by red light stimulation which form multiple synapses with blue-light-sensitive cones. Calculations indicate that convergence ratios in Xenopus are similar to those in central retinal regions of mammals, predicting comparable spatial resolution.

Downregulation of a unique photoreceptor protein correlates with improper outer segment assembly

A unique photoreceptor protein has been characterized. This protein, termed XAP-1 antigen, is expressed by photoreceptors exclusively under conditions in which the outer segment membranes are properly assembled. When the retinal pigment epithelium is adherent to the underlying neural retina, the XAP-1 antigen is localized to the plasma membrane that surrounds the inner and outer segments in the areas juxtaposed to the subretinal space. A similar labeling pattern is detected in retinal pigment epithelium-deprived retinas in which assembly of nascent outer segments is supported by lactose. In retinas that undergo degeneration subsequent to the removal of the retinal pigment epithelium, the expression of this protein is completely downregulated. Immunohistochemical analyses and subcellular fractionation along with Western blot analysis, indicate that the XAP-1 antigen is a membrane-associated soluble protein. Mass spectrometric analysis indicates that the XAP-1 antigen shares homology via 12 tryptic peptide masses with the gamma-crystallin (lens structural protein) subclasses, although it does not immunolocalize to the same ocular structures as reported for the gamma-crystallins. We propose that XAP-1 antigen is a unique protein that is expressed extensively by healthy photoreceptor cells; the expression of the XAP-1 antigen exclusively by photoreceptors with organized outer segments suggests that this protein may play a critical role in outer segment assembly.

A mutation of early photoreceptor development, mikre oko, reveals cell-cell interactions involved in the survival and differentiation of zebrafish photoreceptors

To gain insight into mechanisms involved in photoreceptor development, we characterized a zebrafish mutation in the mikre oko locus that produces early loss of photoreceptor cells. mikre oko photoreceptors lose their elongated morphology at the time of wild-type outer segment formation and undergo cell death within a few days. To investigate whether this phenotype involves cell-cell interaction defects, we performed analysis of genetically mosaic animals. Interactions of mikre oko photoreceptors with wild-type cells rescue several aspects of the mutant phenotype. When placed in a wild-type environment, mikre oko photoreceptor cells retain elongated morphology and survive longer. Moreover, although mutant mikre oko photoreceptor outer segments develop only infrequently and are usually disorganized, mikre oko cone and rod cells in mosaic retinas develop robust outer segments that closely resemble the wild type. In contrast to the outer segments, the proximal regions of mikre oko photoreceptor cells, including their inner segments, the nuclear regions, and the synaptic termini, retain the mutant appearance. mikre oko outer segment rescue is not mediated by interactions with the retinal pigment epithelium. These studies demonstrate that the differentiation of outer segments is surprisingly independent from the more proximal photoreceptor cell features and that outer segment development includes retinal pigment epithelium-independent cell-cell interactions.

Defects in the MITF(mi/mi) apical surface are associated with a failure of outer segment elongation

The loss of MITF function in the MITF(mi/mi)mouse affects not only RPE differentiation, but also the development of rod photoreceptor outer segments. Our data indicate that opsin immunoreactivity is detected in the cell membrane and along the ONL/RPE border of developing MITF(mi/mi)rod photoreceptors and that rod outer segment morphogenesis is initiated. Although molecules associated with the outer segment continued to be expressed, outer segments did not elongate and develop stacked organized discs perpendicular to the RPE. The MITF(mi/mi)RPE also failed to form apical microvilli and lacked the apical network of the phosphoprotein ezrin seen in wild type tissue. The MITF(mi/mi)RPE basal surface was loosely organized and retained ezrin labelling which indicated some degree of differentiation. The correlation seen in our data suggest that there may be a link between the failure of the RPE apical domain to form and lack of rod outer segment elongation.

Microenvironmental regulation of visual pigment expression in the chick retina

Visual pigment (VP) expression in the chick embryo retina was investigated in ovo, in dissociated and explant cultures, and in cDNAs from individual cells. While VP mRNA is not detectable by in situ hybridization until embryonic day (ED) 14-16 in ovo, analysis of VP expression by RT-PCR showed that VP messages are present in the retina as many as 7-10 days before they become detectable by in situ hybridization, and are also detected in other regions of the embryonic CNS. On the other hand, red opsin expression is markedly accelerated when cells are isolated from their intraocular microenvironment at ED 6, and placed in pigment epithelium-free dissociated or explant cultures. This acceleration occurs regardless of cell density, birth date, or serum presence in the medium, suggesting that many photoreceptors are already programmed to express red opsin on or before ED 6, and that microenvironmental inhibitory factors prevent implementation of this program until ED 14 in ovo. The selectivity of this phenomenon is suggested by the finding that other VPs are not observed by in situ hybridization in ED 6 cultures, although they are detectable in cultures of older retinas. Taken together, these findings suggest that red opsin expression may be constitutive for many developing photoreceptor cells in the chick.

Investigating the mechanisms of retinal degenerations with antisense oligonucleotides

Utilizing antisense oligonucleotides coupled with an intact Xenopus eye rudiment model, we have effectively demonstrated that we are able to downregulate the expression of a photoreceptor-specific protein, rds/peripherin, and generate a loss-of-function model upon which to further study the function of the rds/peripherin gene. The ultrastructure and protein expression patterns very closely resemble those previously documented in both the rds mouse and in human autosomal dominant retinitis pigmentosa due to peripherin/RDS mutations. An identical strategy can be applied to any gene correlated with a degenerative retinal phenotype. As the entire array of genes is revealed through the various genome projects, including human and mouse, it is becoming increasingly critical to evaluate and determine the function of the corresponding gene products. Discovering which gene is responsible for a particular clinical phenotype is only the first of many steps in the development of a treatment or cure for that particular disease. Using our in vitro model, in which the retina is readily accessible to the antisense oligonucleotide yet the normal three-dimensional ultrastructure of the retina is maintained, we can evaluate the function of virtually any gene as the sequence becomes available. A thorough understanding of the function of individual genes will provide insights on the role of gene product in retinal health and pathophysiology. This experimental approach will also allow for specific therapeutic interventions to be evaluated so that targeted treatments can be designed for individuals with specific genetic mutations.

Cell differentiation, synaptogenesis, and influence of the retinal pigment epithelium in a rat neonatal organotypic retina culture

This study was focused on the analysis of cell type differentiation and synaptogenesis as well as outer segment formation in an organotypic culture of the neonatal rat retina during a 6-14 day period of in vitro development. Moreover, the effects of the retinal pigment epithelium (RPE) on these processes were investigated. The in vitro development resulted in a retinal architecture and lamination comparable to that of in vivo retinas. The RPE influences the proper alignment of photoreceptors as well as the formation of the outer limiting membrane (OLM), but not processes of cell differentiation, synaptogenesis and inner retinal lamination.

Pigment epithelium-derived factor supports normal development of photoreceptor neurons and opsin expression after retinal pigment epithelium removal

Dysfunction of the retinal pigment epithelium (RPE), its loss, or separation from the underlying neural retina results in severe photoreceptor degeneration. Pigment epithelium-derived factor (PEDF) is a glycoprotein with reported neuroprotective and differentiation properties that is secreted in abundance by RPE cells. The "pooling" of PEDF within the interphotoreceptor matrix places this molecule in a prime physical location to affect the underlying neural retina. The purpose of this study was to analyze the morphogenetic activity of PEDF in a model of photoreceptor dysmorphogenesis induced by removal of the RPE. Eyes were dissected from embryonic Xenopus laevis, and the RPE was removed before culturing in medium containing PEDF, PEDF plus anti-PEDF antibodies, or medium alone. Control retinas were maintained with an adherent RPE. Light and electron microscopic analysis was used to examine retinal ultrastructure. Opsin was localized immunocytochemically and quantified as an index of outer segment membranous material and photoreceptor protein expression. Removal of the RPE resulted in an aberrant assembly of photoreceptor outer segments, loss of fine subcellular ultrastructure in photoreceptors, and a reduction in opsin protein levels when compared with control retinas. The addition of PEDF prevented the dysmorphic photoreceptor changes induced by RPE removal. In particular, photoreceptor ultrastructure, outer segment membrane assembly, and steady-state levels of opsin were equivalent to control conditions. Anti-PEDF antibodies completely blocked the morphogenetic activity of PEDF. These results indicate that PEDF is able to mimic the supportive role of the RPE on photoreceptors during the final stages of retinal morphogenesis.

Targeted disruption of Müller cell metabolism induces photoreceptor dysmorphogenesis

Within the retina, the Müller cells and photoreceptors are in close physical proximity and are metabolically coupled. It is unknown, however, whether Müller cells affect photoreceptor differentiation and outer segment membrane assembly. The objective of this study was to determine whether targeted disruption of Müller cell metabolism would induce photoreceptor dysmorphogenesis. Intact isolated Xenopus laevis embryonic eyes were cultured in medium with or without Müller cell-specific inhibitors (i.e., alpha-aminoadipic acid and fluorocitrate). To assess Müller cell injury, the gross retinal morphology was examined along with immunocytochemical assessment of Müller cell-specific protein expression patterns. The steady-state levels of opsin were quantified to determine whether the Müller cell inhibitors negatively affected photoreceptor protein synthesis. Müller and photoreceptor cell ultrastructure was scrutinized and the organization of the outer segment membranes was graded. In control retinas, there was no swelling of Müller cell cytoplasm. Glial fibrillary acidic protein (GFAP) was undetectable, whereas glutamine synthetase was abundant. The steady-state level of opsin was high and photoreceptors elaborated properly folded outer segments. Exposure to both Müller cell-specific inhibitors induced swelling of Müller cell endfeet, cytoplasmic paling and alterations of Müller cell-specific protein expression patterns. The steady-state level of opsin in retinas exposed to alpha-aminoadipic acid was unchanged compared with control eyes, whereas, in eyes exposed to fluorocitrate, opsin levels were slightly reduced. The most significant finding was that targeted disruption of Müller cell metabolism adversely affected photoreceptor outer segment membrane assembly, causing dysmorphogenesis of nascent outer segments. These results suggest that the termination signal(s) necessary for proper outer segment folding were disrupted by targeted inhibition of Müller cells and support the hypothesis that Müller cells interact with photoreceptors through mechanisms that may regulate, at least in part, the assembly of photoreceptor outer segment membranes.

Photoreceptor classes and transmission at the photoreceptor synapse in the retina of the clawed frog, Xenopus laevis

The photoreceptor population in Xenopus consists of a green-sensitive rod (lambda(max) = 523 nm), a blue-sensitive rod (lambda(max) = 445 nm) and three classes of cone. The largest cone is red-sensitive (lambda(max) = 611 nm). The intermediate cone is presumed to be blue-sensitive based on physiological criteria, whereas the miniature cone may be UV-sensitive. Horizontal cells (HC) are of two sorts: axon-bearing and axonless. The axon-bearing HC is of the luminosity type and probably contacts all types of photoreceptor. The axonless HC is of the chromaticity type and contacts only intermediate (blue) cones and at least one type of rod. During development dendrites of HCs and bipolar neurons penetrate photoreceptor bases. A progressive maturation of HC and bipolar synapses with rods and cones occurs between tadpoles stages 37/8 and 46. Neighboring rods and cones are joined by gap junctions. During this same period, the outer segments are laid down and photopigments synthesized. A linear relation was found between the quantum capturing ability of the rod and its absolute threshold. Mature rods of the Xenopus retina release glutamate in a calcium-dependent manner. Glutamate release was found to be a linear function of calcium influx through L-type calcium channels. Both types of HC possess ionotropic glutamate receptors of the AMPA subtype.

Closer look at lactose-mediated support of retinal morphogenesis

We have previously shown in intact isolated eye rudiments from Xenopus laevis that lactose, but not mannose, permits the formation of organized photoreceptor outer segments in the absence of the retinal pigment epithelium (RPE). The purpose of this study was to determine, using electron microscopic analysis, the key ultrastructural differences between healthy retinas, lactose-protected retinas, and retinas that developed aberrantly to reveal which subcellular structures were exclusively present in healthy retinas. Filamentous actin was also localized in retinas to determine its distribution under the various conditions. In healthy retinas, calycal processes extending approximately three-fourths of the length of the outer segment surrounded highly organized photoreceptor outer segments. Adherens junctions were localized between adjacent photoreceptors and Müller cells at the outer limiting membrane. In addition, Müller cells possessed apical processes that extended for a short distance beyond the adherens junctions. These fine cytoarchitectural details were missing in retinas that completed differentiation in the absence of the RPE; both calycal and apical processes were no longer present and adherens junctions were sparsely intermittent. Müller cells appeared atrophic. Similarly, mannose promoted none of the fine cytoarchitectural details of the retina. Lactose, however, supported the formation of the proper subcellular cytoarchitecture of both photoreceptor and Müller cells. These results suggest that these subcellular structures may be fundamental for the proper assembly and stability of organized outer segments and are necessary to allow for normal cytogenesis of the outer retina.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

9-cis-retinoic acid in combination with retinal pigment epithelium induces apoptosis in cultured retinal explants only during early postnatal development

Retinoic acid is one of the active metabolites of vitamin A and has profound effects on the development of the CNS including retina. Previously, we have shown that rod-specific apoptosis is induced in retinal explants from neonatal mice by exposure to 9-cis-retinoic acid (9CRA) when the retinal pigment epithelium (RPE) is present. In explants lacking RPE, it instead has a differentiation-promoting effect seen as an accelerated opsin expression on postnatal day 3. To investigate the long-term effect of 9CRA exposure, we have explanted retinas from neonatal C3H mice with or without RPE attached and placed in organ culture. After 19 or 48 h in culture or 7, 8 or 13 days in culture, the explants were either fixed for histochemical examination or frozen for assay of DEVDase activity. We found that long-term exposure to 9CRA caused a decrease in the number of cell layers in the outer nuclear layer (ONL) only in explants with the RPE attached. When explants with RPE attached were exposed to 9CRA only during the second postnatal week, neither an increase in DEVDase activity, TUNEL-positive cells, nor a decrease in cell layers of the ONL could be demonstrated, indicating that the retina was insensitive to the apoptosis-inducing effect of 9CRA after the first postnatal week. The absence of RPE in control explants resulted in a higher number of rosettes and the extrusion of cells into the subretinal space.

A reliable method for organ culture of neonatal mouse retina with long-term survival

Organ culture systems of the central nervous system have proven to be useful tools for the study of development, differentiation, and degeneration. Some studies have been limited by the inability to maintain the cultures over an extended period. Here we describe an organ culture technique for the mouse retina. This method uses commercially available supplies and reproducible procedures to maintain healthy retinas with normal architecture for 4 weeks in vitro. The system is amenable to quantitative analysis. It can be used with both normal and retinal degeneration (rd) retinas to study of the role of various factors in photoreceptor degeneration in retinal cell fate determination and development.

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.

RPE secreted proteins and antibody influence photoreceptor cell survival and maturation

Proteins in media conditioned by retinal pigment epithelial cells (RPE-CM) and an antibody against these proteins (RPE-SP) were tested for their respective effects on rat retinal development in vitro and in vivo. Proteins of RPE-CM were separated in denaturing gels and evaluated by Western blot analysis. Retinal explants from postnatal day 2 (P2) rats were cultured in RPE-CM only or CM diluted with the RPE-SP antibody and, after 7 days, the explants were dissociated into single cells that were immunostained for opsin. RPE-CM or antibody was also injected into the vitreous of postnatal day 7 (P7) Long-Evans rats and analyzed 7 and 21 days later. Electrophoretic analysis of RPE-CM predominantly showed 60-70 kDa proteins; when these proteins were probed with RPE-SP antibody by Western blot, immunoreactive proteins were restricted to this narrow molecular weight range. In P2 retinal explant cultures supplemented with RPE-CM, long ganglion cell-like neurites were detected in 3 days. This activity was nullified in explant cultures grown in RPE-CM titrated with antibody, and these explants appeared to degenerate within 5 days. Over 80% of dissociated retinal cells from explants 7 days after treatment with RPE-CM expressed opsin, compared to only 20% of cells from explants grown in defined medium or serum. Retinas of P14 rats injected intravitreally with RPE-CM at P7 had increased numbers of ectopic photoreceptor cells within the inner nuclear layer when compared to retinas of sham-injected eyes. In contrast, retinas of eyes injected intravitreally with RPE-SP antibody exhibited shorter outer (OS) and inner (IS) segments and thinner outer nuclear (ONL) and outer plexiform (OPL) layers than retinas of sham-injected eyes. In conclusion, proteins in RPE-CM appeared to accelerate and maximize the development of rat photoreceptor cells in vitro, while intravitreal injections of its antibody caused an apparent retardation of outer segment maturation. These results suggest that a protein(s) secreted by RPE plays a key role in normal retinal development, particularly in photoreceptor cell survival and outer segment maturation.

A splice variant of trkB and brain-derived neurotrophic factor are co-expressed in retinal pigmented epithelial cells and promote differentiated characteristics

There is evidence suggesting reciprocal trophic interactions between photoreceptors and the retinal pigmented epithelium (RPE), but the factors involved have not been identified. In this study, we investigated the hypothesis that one or more known neurotrophic factors act upon the RPE. Cultured human and freshly isolated bovine RPE cells demonstrated saturable specific binding for [125I]labeled BDNF, NT-4/5 and NT-3 with little specific binding for CNTF and none for NGF. Cross-competition experiments showed that BDNF is the preferred ligand and cross-linking of [125I]BDNF resulted in a doublet at 160 kd that was increased in RPE cells incubated in all-trans retinoic acid. There was basal phosphorylation of a 145 kd protein recognized by an anti-trk antibody that was increased in RPE cells pulsed with BDNF. RT-PCR with primers spanning the transmembrane domain demonstrated that RPE cells express trkB mRNA lacking a region homologous to exon 9 of chicken trkB, a splice variant that has been demonstrated to preferentially interact with BDNF. Northern blots demonstrated that cultured RPE cells also express mRNA for BDNF. BDNF did not stimulate proliferation or increase survival of RPE cells in serum-free medium, but promoted a differentiated morphology and increased the expression of cellular retinaldehyde binding protein, a marker of the differentiated state in RPE cells. An RPE cell line that spontaneously shows differentiated features showed a high level of BDNF mRNA. These data demonstrate that RPE cells express a short splice variant of trkB whose activation correlates with expression of differentiated characteristics and the cells themselves are capable of producing a ligand for the receptors. Signaling through trkB could play a role in differentiation of RPE cells during development and maintenance of the differentiated state in adult RPE.

Modification of glutamine synthetase expression by mammalian Müller (glial) cells in retinal organ cultures

One of the key enzymes in glial-neuronal transmitter recycling is glutamine synthetase (GS). In the retina, GS is exclusively expressed by glial (Müller) cells where it serves to convert neuron-released active transmitter substances (glutamate and GABA) into glutamine. Experiments on avian retinae have shown that GS expression is developmentally regulated by glucocorticoid hormones and, to a lesser extent, by a non-hormonal control mechanism(s). Much less is known about GS regulation in mammalian retinae, although either increases or decreases of GS immunoreactivity have been observed in Müller cells in different forms of retinal pathologies. We studied GS expression in postnatal rabbit retinae both in vivo and explanted as wholemounts in vitro, using immunocytochemistry and Western immunoblotting. GS expression was detectable in vivo from the fourth postnatal day, and increased rapidly within the first weeks of life. Levels were lower in vitro than in vivo by an order of magnitude, and could be significantly stimulated (> 60-110%) in vitro by application of hydrocortisone, conditioned medium from cultured retinal pigment epithelium and glutamate or ammonia, but not GABA. It is concluded that GS expression in mammalian Müller cells is dependent on systemic control by glucocorticoid hormones, as observed in birds, but environmental (activity-dependent) factors may play a more important role in mammals.

Pigmented epithelium sustains cell proliferation and decreases expression of opsins and acetylcholinesterase in reaggregated chicken retinospheroids

We investigated the effect of the retinal pigmented epithelium on cell proliferation and differentiation in rosetted retinospheroids, which are retina-like spheres reaggregated in the complete absence of retinal pigmented epithelium from dissociated retinal cells of 6-day-old chick embryos in a rotation culture system. In spheroids raised in the absence of retinal pigmented epithelium (controls), acetylcholinesterase was expressed in cells of an inner nuclear-like layer and their neuropil matrices. Moreover, the ratio between rods and cones was found to be approximately normal throughout the spheroid. When spheroids were cultured in the presence of retinal pigmented epithelium monolayers, cell proliferation in spheroids as determined by BrdU labelling was significantly increased and extended for 1 week, while acetylcholinesterase protein levels and specific activities in homogenates were decreased to approximately 30%. At the same time, opsin immunoreactivity was completely suppressed within the spheroid and appeared slowly in cells around its periphery; i.e. the proportion of rhodopsin-positive cells decreased from 14 to 3%. This study reveals that the retinal pigmented epithelium in vitro sustains cell proliferation but inhibits the differentiation of acetylcholinesterase-positive cells and of photoreceptors.

Photoreceptor repair in response to RPE transplants in RCS rats: outer segment regeneration

PURPOSE

We have previously shown that transplants of normal rat neonatal RPE cells rescued photoreceptor cells in retinas of Royal College of Surgeons (RCS) dystrophic rats for up to one year. In this study, we investigated the photoreceptor rescue effects in RCS rats within the first three weeks following transplantation in an attempt to determine if RPE transplants initiate repair mechanisms, specifically, outer segment (OS) regeneration.

METHODS

Freshly isolated RPE cells from neonatal pigmented Long Evans rats were transplanted into the subretinal space of 22-23 day-old RCS rats using a transscleral approach. For controls, vehicle was similarly injected.

RESULTS

When analyzed at 10 days post-transplantation, long inner segments were observed with short buds of outer segment growth in the area of the RPE-cell transplants. The outer segments were of insufficient length to be measured at 10 days, but by 14 and 21 days, OS were 2.02 +/- 0.32 microns and 18.80 +/- 2.78 microns, respectively. In vehicle-injected retinas from 10 to 21 days postsurgery, outer segments were not observed and the inner segments were three-fold shorter than in RPE-transplanted retinas. At 10 days post-transplantation, most RPE cells were seen in the subretinal space, but a few had attached to Bruch's membrane; however, by 21 days, many of the transplanted RPE cells had attached to Bruch's membrane, although a few were found free in the subretinal space.

CONCLUSIONS

This study has shown that transplants of normal rat neonatal RPE cells have the capacity to support not only photoreceptor cell survival but also initiate early repair mechanisms as exhibited by outer segment regeneration in RCS retinas. These results also conclusively show the important role that the RPE plays in outer segment growth and maturation.

Retinal dysplasia and degeneration in RARbeta2/RARgamma2 compound mutant mice

The eye is the organ whose development is the most frequently altered in response to maternal vitamin A deficiency [VAD; Warkany, J. and Schraffenberger, S. (1946). Archs Ophthalmol. 35, 150–169]. With the exception of prenatal retinal dysplasia, all the ocular abnormalities of the fetal VAD syndrome are recapitulated in mouse mutants lacking either RARalpha and RARbeta2, RARalpha and RARgamma, RARgamma and RARbeta2, or RXRalpha [Lohnes, D., Mark, M., Mendelsohn, C., Dolle, P., Dierich, A., Gorry, P., Gansmuller, A. and Chambon, P. (1994) Development 120, 2723–2748; Mendelsohn, C., Lohnes, D., Decimo, D., Lufkin, T., LeMeur, M., Chambon, P. and Mark, M. (1994) Development 120, 2749–2771; Kastner, P., Grondona, J. Mark, M., Gansmuller, A., LeMeur, M., Decimo, D., Vonesch, J.L., Dolle, P. and Chambon, P. (1994) Cell 78, 987–1003], thus demonstrating that retinoic acid (RA) is the active vitamin A metabolite during prenatal eye morphogenesis. Whether retinoids are also involved in postnatal eye development could not be investigated, as VAD newborns are not viable and the above RAR double null mutants and RXRalpha null mutants died in utero or at birth. We report here the generation of viable RARbeta2/RARgamma2 double null mutant mice, which exhibit several eye defects. The neural retina of newborn RARbeta2gamma2 mutants is thinner than normal due to a reduced rate of cell proliferation, and from day 4 shows multiple foci of disorganization of its layers. These RARbeta2gamma2 mutants represent the first genetically characterized model of retinal dysplasia and their phenotype demonstrates that RARs, and therefore RA, are required for retinal histogenesis. The RARbeta2gamma2 retinal pigment epithelium (RPE) cells display histological and/or ultrastructural alterations and/or fail to express cellular retinol binding protein I (CRBPI). Taken altogether, the early onset of the RPE histological defects and their striking colocalisation with areas of the neural retina displaying a faulty laminar organization, a reduced neuroblastic proliferation, and a lack of photoreceptor differentiation and/or increased apoptosis, make the RPE a likely target tissue of the RARbeta2gamma2 double null mutation. A degeneration of the adult neural retina, which may similarly be secondary to a defective RPE, is also observed in these mutants, thus demonstrating an essential role of RA in the survival of retinal cells. Moreover, all RARbeta2gamma2 mutants display defects in structures derived from the periocular mesenchyme including local agenesis of the choroid and of the sclera, small eyelids, and a persistence of the primary mesenchymal vitreous body. A majority of the RARbeta2 single null mutants also exhibit this latter defect, thus demonstrating that the RARbeta2 isoform plays a unique role in the formation of the definitive vitreous body.

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.

Interphotoreceptor retinoid-binding protein (IRBP): expression in the adult and developing Xenopus retina

Apposition of the neural retina and pigment epithelium is critical to photoreceptor development and function. Interphotoreceptor retinoid-binding protein (IRBP) is a major component of the extracellular matrix separating these epithelia in the African clawed frog Xenopus laevis (Gonzalez-Fernandez et al., [1993], J. Cell Sci. 105:7-21). In the adult retina, IRBP appears to mediate the transport of hydrophobic molecules, particularly retinoids and fatty acids, within the hydrophilic extracellular domain. In this paper, we compare the distribution of IRBP and its mRNA in adult and embryonic Xenopus retina. Xenopus IRBP antisense RNA, labeled with tritium or digoxigenin, was used for in situ hybridizaton studies. For immunohistochemistry, we used an antiserum against Xenopus IRBP expressed in Escherichia coli. In the adult, we found that IRBP is synthesized at similar levels by both rods and cones. The protein is restricted to the interphotoreceptor matrix, with lesser amounts in the pigment epithelial cytoplasm. In the embryo, expression of the mRNA for IRBP is restricted to the central retina, where photoreceptor differentiation has taken place. By contrast, the protein is distributed throughout the embryonic subretinal space. Therefore, the presence of IRBP precedes photoreceptor differentiation. In summary, IRBP is synthesized by both rods and cones and may be internalized by the pigment epithelium. In the embryo, IRBP is synthesized by the central retina and diffuses through the matrix, reaching the undifferentiated peripheral retina. In view of its ligand-binding properties, diffusion of IRBP may provide the peripheral neural retina with a vehicle to transport retinoids and docosahexaenoic acid (molecules critical to normal retinal development) from the pigment epithelium.