Uptake, processing and release of retinoids by cultured human retinal pigment epithelium

Single particle cryo-EM of the complex between interphotoreceptor retinoid-binding protein and a monoclonal antibody

Interphotoreceptor retinoid‐binding protein (IRBP) is a highly expressed protein secreted by rod and cone photoreceptors that has major roles in photoreceptor homeostasis as well as retinoid and polyunsaturated fatty acid transport between the neural retina and retinal pigment epithelium. Despite two crystal structures reported on fragments of IRBP and decades of research, the overall structure of IRBP and function within the visual cycle remain unsolved. Here, we studied the structure of native bovine IRBP in complex with a monoclonal antibody (mAb5) by cryo‐electron microscopy, revealing the tertiary and quaternary structure at sufficient resolution to clearly identify the complex components. Complementary mass spectrometry experiments revealed the structure and locations of N‐linked carbohydrate post‐translational modifications. This work provides insight into the structure of IRBP, displaying an elongated, flexible three‐dimensional architecture not seen among other retinoid‐binding proteins. This work is the first step in elucidation of the function of this enigmatic protein.

Aberrant RNA splicing is the major pathogenic effect in a knock-in mouse model of the dominantly inherited c.1430A>G human RPE65 mutation

Human RPE65 mutations cause a spectrum of retinal dystrophies that result in blindness. While RPE65 mutations have been almost invariably recessively inherited, a c.1430A>G (p.(D477G)) mutation has been reported to cause autosomal dominant retinitis pigmentosa (adRP). To study the pathogenesis of this human mutation, we have replicated the mutation in a knock-in (KI) mouse model using CRISPR/Cas9-mediated genome editing. Significantly, in contrast to human patients, heterozygous KI mice do not exhibit any phenotypes in visual function tests. When raised in regular vivarium conditions, homozygous KI mice display relatively undisturbed visual functions with minimal retinal structural changes. However, KI/KI mouse retinae are more sensitive to light exposure and exhibit signs of degenerative features when subjected to light stress. We find that instead of merely producing a missense mutant protein, the A>G nucleotide substitution greatly affects appropriate splicing of Rpe65 mRNA by generating an ectopic splice site in comparable context to the canonical one, thereby disrupting RPE65 protein expression. Similar splicing defects were also confirmed for the human RPE65 c.1430G mutant in an in vitro Exontrap assay. Our data demonstrate that a splicing defect is associated with c.1430G pathogenesis, and therefore provide insights in the therapeutic strategy for human patients.

Complement modulation in the retinal pigment epithelium rescues photoreceptor degeneration in a mouse model of Stargardt disease

Recessive Stargardt macular degeneration (STGD1) is caused by mutations in the gene for the ABCA4 transporter in photoreceptor outer segments. STGD1 patients and Abca4^-/- (STGD1) mice exhibit buildup of bisretinoid-containing lipofuscin pigments in the retinal pigment epithelium (RPE), increased oxidative stress, augmented complement activation and slow degeneration of photoreceptors. A reduction in complement negative regulatory proteins (CRPs), possibly owing to bisretinoid accumulation, may be responsible for the increased complement activation seen on the RPE of STGD1 mice. CRPs prevent attack on host cells by the complement system, and complement receptor 1-like protein y (CRRY) is an important CRP in mice. Here we attempted to rescue the phenotype in STGD1 mice by increasing expression of CRRY in the RPE using a gene therapy approach. We injected recombinant adeno-associated virus containing the CRRY coding sequence (AAV-CRRY) into the subretinal space of 4-wk-old Abca4^-/- mice. This resulted in sustained, several-fold increased expression of CRRY in the RPE, which significantly reduced the complement factors C3/C3b in the RPE. Unexpectedly, AAV-CRRY-treated STGD1 mice also showed reduced accumulation of bisretinoids compared with sham-injected STGD1 control mice. Furthermore, we observed slower photoreceptor degeneration and increased visual chromophore in 1-y-old AAV-CRRY-treated STGD1 mice. Rescue of the STGD1 phenotype by AAV-CRRY gene therapy suggests that complement attack on the RPE is an important etiologic factor in STGD1. Modulation of the complement system by locally increasing CRP expression using targeted gene therapy represents a potential treatment strategy for STGD1 and other retinopathies associated with complement dysregulation.

Signalling beyond photon absorption: extracellular retinoids and growth factors modulate rod photoreceptor sensitivity

KEY POINTS

We propose that the end product of chromophore bleaching in rod photoreceptors, all-trans retinol, is part of a feedback loop that increases the sensitivity of the phototransduction cascade in rods. A previously described light-induced hypersensitivity in rods, termed adaptive potentiation, is reduced by exogenously applied all-trans retinol but not all-trans retinal. This potentiation is produced by insulin-like growth factor-1, whose binding proteins are located in the extracellular matrix, even in our isolated retina preparation after removal of the retinal pigmented epithelium. Simple modelling suggests that the light stimuli used in the present study will produce sufficient all-trans retinol within the interphotoreceptor matrix to explain the potentiation effect.

ABSTRACT

Photoreceptors translate the absorption of photons into electrical signals for propagation through the visual system. Mammalian photoreceptor signalling has largely been studied in isolated cells, and such studies have necessarily avoided the complex environment of supportive proteins that surround the photoreceptors. The interphotoreceptor matrix (IPM) contains an array of proteins that aid in both structural maintenance and cellular homeostasis, including chromophore turnover. In signalling photon absorption, the chromophore 11-cis retinal is first isomerized to all-trans retinal, followed by conversion to all-trans retinol (ROL) for removal from the photoreceptor. Interphotoreceptor retinoid-binding protein (IRBP) is the most abundant protein in the IPM, and it promotes the removal of bleached chromophores and recycling in the nearby retinal pigment epithelium. By studying the light responses of isolated mouse retinas, we demonstrate that ROL can act as a feedback signal onto photoreceptors that influences the sensitivity of phototransduction. In addition to IRBP, the IPM also contains insulin-like growth factor-1 (IGF-1) and its associated binding proteins, although their functions have not yet been described. We demonstrate that extracellular application of physiological concentrations of IGF-1 can increase rod photoreceptor sensitivity in mammalian retinas. We also determine that chromophores and growth factors can limit the range of a newly described form of photoreceptor light adaptation. Finally, fluorescent antibodies demonstrate the presence of IRBP and IGFBP-3 in isolated retinas. A simple model of the formation and release of ROL into the extracellular space quantitatively describes this novel feedback loop.

Structure of zebrafish IRBP reveals fatty acid binding

Interphotoreceptor retinoid-binding protein (IRBP) has a remarkable role in targeting and protecting all-trans and 11-cis retinol, and 11-cis retinal during the rod and cone visual cycles. Little is known about how the correct retinoid is efficiently delivered and removed from the correct cell at the required time. It has been proposed that different fatty composition at that the outer-segments and retinal-pigmented epithelium have an important role is regulating the delivery and uptake of the visual cycle retinoids at the cell-interphotoreceptor-matrix interface. Although this suggests intriguing mechanisms for the role of local fatty acids in visual-cycle retinoid trafficking, nothing is known about the structural basis of IRBP-fatty acid interactions. Such regulation may be mediated through IRBP's unusual repeating homologous modules, each containing about 300 amino acids. We have been investigating structure-function relationships of Zebrafish IRBP (zIRBP), which has only two tandem modules (z1 and z2), as a model for the more complex four-module mammalian IRBP's. Here we report the first X-ray crystal structure of a teleost IRBP, and the only structure with a bound ligand. The X-ray structure of z1, determined at 1.90 Å resolution, reveals a two-domain organization of the module (domains A and B). A deep hydrophobic pocket with a single bound molecule of oleic acid was identified within the N-terminal domain A. In fluorescence titrations assays, oleic acid displaced all-trans retinol from zIRBP. Our study, which provides the first structure of an IRBP with bound ligand, supports a potential role for fatty acids in regulating retinoid binding.

Plasma membrane protein polarity and trafficking in RPE cells: past, present and future

The retinal pigment epithelium (RPE) comprises a monolayer of polarized pigmented epithelial cells that is strategically interposed between the neural retina and the fenestrated choroid capillaries. The RPE performs a variety of vectorial transport functions (water, ions, metabolites, nutrients and waste products) that regulate the composition of the subretinal space and support the functions of photoreceptors (PRs) and other cells in the neural retina. To this end, RPE cells display a polarized distribution of channels, transporters and receptors in their plasma membrane (PM) that is remarkably different from that found in conventional extra-ocular epithelia, e.g. intestine, kidney, and gall bladder. This characteristic PM protein polarity of RPE cells depends on the interplay of sorting signals in the RPE PM proteins and sorting mechanisms and biosynthetic/recycling trafficking routes in the RPE cell. Although considerable progress has been made in our understanding of the RPE trafficking machinery, most available data have been obtained from immortalized RPE cell lines that only partially maintain the RPE phenotype and by extrapolation of data obtained in the prototype Madin-Darby Canine Kidney (MDCK) cell line. The increasing availability of RPE cell cultures that more closely resemble the RPE in vivo together with the advent of advanced live imaging microscopy techniques provides a platform and an opportunity to rapidly expand our understanding of how polarized protein trafficking contributes to RPE PM polarity.

Cultured primary human fetal retinal pigment epithelium (hfRPE) as a model for evaluating RPE metabolism

Mitochondrial dysfunction has been shown to contribute to age-related and proliferative retinal diseases. Over the past decade, the primary human fetal RPE (hfRPE) culture model has emerged as an effective tool for studying RPE function and mechanisms of retinal diseases. This model system has been rigorously characterized and shown to closely resemble native RPE cells at the genomic and protein level, and that they are capable of accomplishing the characteristic functions of a healthy native RPE (e.g., rod phagocytosis, ion and fluid transport, and retinoid cycle). In this review, we demonstrated that the metabolic activity of the RPE is an indicator of its health and state of differentiation, and present the hfRPE culture model as a valuable in vitro system for evaluating RPE metabolism in the context of RPE differentiation and retinal disease.

Retinoid uptake, processing, and secretion in human iPS-RPE support the visual cycle

PURPOSE

Retinal pigmented epithelium derived from human induced pluripotent stem (iPS) cells (iPS-RPE) may be a source of cells for transplantation. For this reason, it is essential to determine the functional competence of iPS-RPE. One key role of the RPE is uptake and processing of retinoids via the visual cycle. The purpose of this study is to investigate the expression of visual cycle proteins and the functional ability of the visual cycle in iPS-RPE.

METHODS

iPS-RPE was derived from human iPS cells. Immunocytochemistry, RT-PCR, and Western blot analysis were used to detect expression of RPE genes lecithin-retinol acyl transferase (LRAT), RPE65, cellular retinaldehyde-binding protein (CRALBP), and pigment epithelium-derived factor (PEDF). All-trans retinol was delivered to cultured cells or whole cell homogenate to assess the ability of the iPS-RPE to process retinoids.

RESULTS

Cultured iPS-RPE expresses visual cycle genes LRAT, CRALBP, and RPE65. After incubation with all-trans retinol, iPS-RPE synthesized up to 2942 ± 551 pmol/mg protein all-trans retinyl esters. Inhibition of LRAT with N-ethylmaleimide (NEM) prevented retinyl ester synthesis. Significantly, after incubation with all-trans retinol, iPS-RPE released 188 ± 88 pmol/mg protein 11-cis retinaldehyde into the culture media.

CONCLUSIONS

iPS-RPE develops classic RPE characteristics and maintains expression of visual cycle proteins. The results of this study confirm that iPS-RPE possesses the machinery to process retinoids for support of visual pigment regeneration. Inhibition of all-trans retinyl ester accumulation by NEM confirms LRAT is active in iPS-RPE. Finally, the detection of 11-cis retinaldehyde in the culture medium demonstrates the cells' ability to process retinoids through the visual cycle. This study demonstrates expression of key visual cycle machinery and complete visual cycle activity in iPS-RPE.

Signaling by vitamin A and retinol-binding protein in regulation of insulin responses and lipid homeostasis

Vitamin A, retinol, circulates in blood bound to serum retinol binding protein (RBP) and is transported into cells by a membrane protein termed stimulated by retinoic acid 6 (STRA6). It was reported that serum levels of RBP are elevated in obese rodents and humans, and that increased level of RBP in blood causes insulin resistance. A molecular mechanism by which RBP can exert such an effect is suggested by the recent discovery that STRA6 is not only a vitamin A transporter but also functions as a surface signaling receptor. Binding of RBP-ROH to STRA6 induces the phosphorylation of a tyrosine residue in the receptor C-terminus, thereby activating a JAK/STAT signaling cascade. Consequently, in STRA6-expressing cells such as adipocytes, RBP-ROH induces the expression of STAT target genes, including SOCS3, which suppresses insulin signaling, and PPARγ, which enhances lipid accumulation. RBP-retinol thus joins the myriad of cytokines, growth factors and hormones which regulate gene transcription by activating cell surface receptors that signal through activation of Janus kinases and their associated transcription factors STATs. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.

Ca++-switch induction of RPE differentiation

Cultured retinal pigment epithelial (RPE) cells are commonly used as a model of the tissue to study their involvement in visual diseases. Unfortunately, cultured RPE often lose their differentiated phenotype reducing their usefulness as a model of the RPE in vivo. In this study, we used a Ca++-switch protocol to initiate the patterned expression of several phenotypic and functional markers of RPE differentiation. Cultured RPE cells from adult donors were maintained through at least six serial passages prior to assay to minimize their differentiated properties. The cells were then subjected to the Ca++-switch protocol and maintained at confluence for up to 4 months. Paired control and Ca++-switch cells were examined for phenotype, pigmentation, and the expression of tyrosinase, CRABP, myocilin, and bestrophin by western blot analysis. The Ca++-switch protocol led to a rapid restriction of N-cadherin to lateral cell borders, and to expression of tyrosinase by day 4. After 8 weeks, the experimental RPE monolayers began to accumulate visible pigment, and after 12 weeks CRABP expression was observed. Myocilin was observed at 4 months after the Ca++-switch but bestrophin was not detected at any time point. Our results suggest this protocol may drive epithelial morphogenesis in RPE cells. We note two specific differences in cells plated in low Ca++, reduced spreading on the substrate and coordinated development of cadherin adhesion when the Ca++-concentration is returned to normal. Thus, we suggest that this method produces phenotypic changes through multiple cell signalling pathways.

Mutation of key residues of RPE65 abolishes its enzymatic role as isomerohydrolase in the visual cycle

RPE65 is essential for isomerization of vitamin A to the visual chromophore. Mutations in RPE65 cause early-onset blindness, and Rpe65-deficient mice lack 11-cis-retinal but overaccumulate alltrans-retinyl esters in the retinal pigment epithelium (RPE). RPE65 is proposed to be a substrate chaperone but may have an enzymatic role because it is closely related to carotenoid oxygenases. We hypothesize that, by analogy with other carotenoid oxygenases, the predicted iron-coordinating residues of RPE65 are essential for retinoid isomerization. To clarify RPE65's role in isomerization, we reconstituted a robust minimal visual cycle in 293-F cells. Only cells transfected with RPE65 constructs produced 11-cis-retinoids, but coexpression with lecithin:retinol acyltransferase was needed for high-level production. Accumulation was significant, amounting to >2 nmol of 11-cis-retinol per culture. Transfection with constructs harboring mutations in residues of RPE65 homologous to those required for interlinked enzymatic activity and iron coordination in related enzymes abolish this isomerization. Iron chelation also abolished isomerization activity. Mutating cysteines implicated in palmitoylation of RPE65 had generally little effect on isomerization activity. Mutations associated with Leber congenital amaurosis/early-onset blindness cause partial to total loss of isomerization activity in direct relation to their clinical effects. These findings establish a catalytic role, in conjunction with lecithin:retinol acyltransferase, for RPE65 in synthesis of 11-cis-retinol, and its identity as the isomerohydrolase.

Uptake and esterification of vitamin A by RCS rat retinal pigment epithelial cells in primary culture

We investigated the capacity of Royal College of Surgeons (RCS) rat retinal pigment epithelial (RPE) cells to take up all-trans-retinol (ROL) (vitamin A) and to metabolize it into retinyl esters (RE). Cultures of RPE cells were established from RCS and control newborn rats. All-trans-ROL was delivered to the apical surface of the RPE monolayer. Retinoids were analyzed by high-performance liquid chromatography. The cellular retinol-binding protein type I (CRBP-I) was assessed by Western blotting. Before supplementation with ROL, RE were lower in RCS rats. After ROL supplementation, esters increased and reached values that were similar in the two strains, but the increase, expressed relative to the initial value, was higher in RCS rats. The uptake of ROL and the level of CRBP-I were greater in RCS rats. Our results provide evidence of a functional retinol esterifying enzyme in cultured RCS RPE cells and suggest that CRBP-I could play a role in the uptake and esterification of ROL in the RPE cells.

Inherited multifocal RPE-diseases: mechanisms for local dysfunction in global retinoid cycle gene defects

Alterations of retinoid cycle genes are known to cause retinal diseases characterized by focal white dot fundus lesions. Fundus appearances reveal circumscribed RPE-changes, although generalized metabolic defects and global functional abnormalities are present. As a possible explanation, topographic inhomogeneities of the human photoreceptor mosaic and the role of a cone specific visual cycle will be discussed. Due to particular characteristics of photoreceptor subtypes as well as different pathways for photopigment regeneration the metabolic demand of individual RPE cells might differ. In "flecked retina diseases" heterogeneity of metabolic demand in individual RPE cells could therefore be responsible for their multifocal appearance.

Autologous transplantation of genetically modified iris pigment epithelial cells: a promising concept for the treatment of age-related macular degeneration and other disorders of the eye

Age-related macular degeneration (ARMD) is the leading cause for visual impairment and blindness in the elder population. Laser photocoagulation, photodynamic therapy and excision of neovascular membranes have met with limited success. Submacular transplantation of autologous iris pigment epithelial (IPE) cells has been proposed to replace the damaged retinal pigment epithelium following surgical removal of the membranes. We tested our hypothesis that the subretinal transplantation of genetically modified autologous IPE cells expressing biological therapeutics might be a promising strategy for the treatment of ARMD and other retinal disorders. Pigment epithelium-derived factor (PEDF) has strong antiangiogenic and neuroprotective activities in the eye. Subretinal transplantation of PEDF expressing IPE cells inhibited pathological choroidal neovascularization in rat models of laser-induced rupture of Bruch's membrane and of oxygen induced ischemic retinopathy. PEDF expressing IPE transplants also increased the survival and preserved rhodopsin expression of photoreceptor cells in the RCS rat, a model of retinal degeneration. These findings suggest a promising concept for the treatment of ARMD and other retinal disorders.

Interphotoreceptor retinoid-binding protein contains three retinoid binding sites

Interphotoreceptor retinoid binding protein (IRBP), the major soluble protein component of the interphotoreceptor matrix, is believed to participate in the visual cycle by transporting retinoids between retinal pigment epithelium and photoreceptor cells in the eye. IRBP can associate with several chemical and isomeric forms of retinoids but displays the highest affinity towards the retinoids that are important in the visual cycle, 11-cis-retinal and all-trans-retinol. It was previously reported that IRBP can associate with 2 mol of all-trans-retinol or 2 mol of 11-cis-retinal per mol of protein. One of the retinoid binding sites, termed 'site 1', was found to display a broad ligand selectivity and to bind either all-trans-retinol or 11-cis-retinal with similar affinities. Here, the retinoid-binding properties of IRBP were further examined. The data demonstrate that IRBP contains three distinct retinoid binding sites. The promiscuous 'site 1', and two additional sites with a stricter selectivity. One of the latter sites appears to be selective towards all-trans-retinol, while the other is specific for 11-cis-retinal.

Development and cellular functions of the iris pigment epithelium

A number of studies have shown that transplantation of retinal pigment epithelial (RPE) cells to the subretinal space offers a promising treatment modality for retinal degenerative diseases. However, it is necessary to transplant autologous cells to avoid rejection; unfortunately, obtaining autologous RPE cells necessitates such traumatic surgical intervention as to make this approach irrelevant. It has been hypothesized that iris pigment epithelial (IPE) cells may be a possible substitute for RPE cells for transplantation into the subretinal space. The iris pigment epithelium, which has the same embryonic origin as retinal pigment epithelium, has not received much attention from visual scientists. Even though it forms a highly specialized tissue, it is not clear whether the iris pigment epithelium contributes critical functions to the health of the visual system. In vivo the IPE does not appear to have any of the functions characteristic of RPE; however, in vitro cultured IPE cells do acquire functions, such as specific phagocytosis of rod outer segments, that are characteristic of RPE cells, and have been shown to have the potential to carry out many functions characteristic of RPE cells, e.g., retinol metabolism. This review outlines the development and cellular functions of the IPE with special emphasis on the modulation of those functions that can allow the IPE cells to be transplanted to the subretinal space where they appear to acquire differentiated properties of retinal pigment epithelium (RPE).

Maintenance of retinoid metabolism in human retinal pigment epithelium cell culture

If transplantation of cultured retinal pigment epithelium (RPE) or iris pigment epithelium (IPE) is to be successful in the treatment of ocular disease, it is imperative to demonstrate that these cells can perform all of their necessary metabolic functions. Unfortunately, a critical function of the RPE, retinoid metabolism, is often lost rapidly in culture. We have examined whether or not nonspecific proteolytic enzymes commonly used in cell isolation and serial passaging may be responsible for this loss of function, and we have investigated novel isolation and passaging techniques which can alleviate this loss of retinoid metabolism.RPE cells were obtained from human donor eyes by enzymatic and nonenzymatic methods. Cells were cultured either on control tissue culture inserts or on inserts coated with a layer of thermally responsive poly(N -isopropylacrylamide-co-cinnamoylcarbamidemethylstyrene). Upon confluence, cells were detached either by trypsinization or by lowering dish temperature. Retinoid metabolism of cells was assessed after isolation and culture by incubating membrane fractions with3H-all- trans -retinol. Retinoid metabolism was also measured in freshly isolated IPE, corneal endothelium (CE), an RPE cell line (D407), and two hepatocyte cell lines (Hepa 6 and HepG2). Membrane fractions from cells isolated nonenzymatically or using collagenase/hyaluronidase formed 11- cis -retinol, retinal isomers and retinyl esters. Retinoid metabolism of RPE cells freshly isolated by trypsinization showed no 11- cis -retinal and little 11- cis -retinol formation. Nondamaged cells cultured on thermally responsive surfaces detached in sheets upon temperature change. They showed metabolism similar to that of cells freshly isolated by nonenzymatic means. After trypsinization, confluent cultures dissociated into individual cells, but these cells showed poor retinoid metabolism, including no detectable retinyl esters or 11- cis -retinoid isomers. IPE, CE and Hepa 6 did not show any retinoid metabolism. D407 and HepG2 produced retinals, but not the 11- cis isomer.RPE cells isolated using trypsin lose the ability to form critical intermediates in the visual cycle. Collagenase/hyaluronidase or nonenzymatic cell isolation techniques enable these functions to be maintained. After cell culture, thermally responsive surfaces allow nonenzymatic cell detachment and excellent maintenance of retinoid metabolism.

Role of the 3'-untranslated region of RPE65 mRNA in the translational regulation of the RPE65 gene: identification of a specific translation inhibitory element

Previously, we demonstrated that explanted bovine retinal pigment epithelium (RPE) cells lose RPE65 protein, a major microsomal protein specific to RPE, while the RPE65 mRNA remains, suggesting posttranscriptional regulation of RPE65 expression in vitro. Accordingly, we analyze here the effect of the 5'- and 3'-untranslated regions (UTRs) of RPE65 mRNA on translational efficiency using in vitro translation systems. We compared the levels of translation products and mRNA stability among RPE65 transcripts containing deletions of the 5'- and 3'-UTRs. First, the 5'-UTR does not affect translational efficiency. However, the 3'-UTR does influence translation efficiency. A putative translation inhibitory element (TIE) is contained within the 170-nucleotide (nt) sequence downstream of the stop codon. There is also a weak destabilizing effect that is associated with the region 3' to the putative TIE. But the effect of this is much less than that of the TIE. This TIE, however, does not inhibit translation of the heterologous chloramphenicol acetyltransferase gene, suggesting that a specific interaction with the upstream RPE65 coding sequence, or its product, may be required. Thus, the posttranscriptional regulation of RPE65 mRNA expression observed in cultured RPE may be via a mechanism of translational inhibition.

Early onset photoreceptor abnormalities induced by targeted disruption of the interphotoreceptor retinoid-binding protein gene

Vision in all vertebrates is dependent on an exchange of retinoids between the retinal pigment epithelium and the visual photoreceptors. It has been proposed that the interphotoreceptor retinoid-binding protein (IRBP) is essential for this intercellular exchange, and that it serves to prevent the potentially cytotoxic effects of retinoids. Although its precise function in vivo has yet to be defined, the early expression of IRBP suggests that it may also be required for normal photoreceptor development. To further assess the biological role of IRBP, we generated transgenic mice with targeted disruption of the IRBP gene (IRBP-/- mice). Specifically, homologous recombination was used to replace the first exon and promoter region of the IRBP gene with a phosphoglycerate kinase-promoted neomycin-resistant gene. Immunocytochemical and Western blot analyses demonstrated the absence of IRBP expression in the IRBP-/- mice. As early as postnatal day 11, histological examination of the retinas of IRBP-/- mice revealed a loss of photoreceptor nuclei and changes in the structural integrity of the receptor outer segments. At 30 d of age, the photoreceptor abnormalities in IRBP-/- mice were more severe, and electroretinographic recordings revealed a marked loss in photic sensitivity. In contrast, no morphological or electrophysiological changes were detected in age-matched heterozygotes. These observations indicate that normal photoreceptor development and function are highly dependent on the early expression of IRBP, and that in the absence of IRBP there is a slowly progressive degeneration of retinal photoreceptors.

Binding of retinol in both retinoid-binding sites of interphotoreceptor retinoid-binding protein (IRBP) is stabilized mainly by hydrophobic interactions

Interphotoreceptor retinoid-binding protein (IRBP) is an ocular protein which is believed to participate in the visual cycle by mediating transport of retinoids between pigment epithelium and photoreceptor cells. The molecular mechanism underlying the ability of IRBP to target particular retinoids to the specific cells that are their sites of action and metabolism is not completely clear, and little information is available regarding the structure of the protein's multiple ligand-binding sites. IRBP possesses two retinoid-binding sites, and it was reported that binding of the visual chromophore, 11-cis-retinal, in one of these sites, but not in the other, is tightly regulated by another IRBP ligand, docosahexaenoic acid (Chen, Y., Houghton, L. A., Brenna, J. T., and Noy, N. (1996) J. Biol. Chem. 271, 20507). The two sites are thus functionally distinct. Here, the thermodynamic parameters governing the interactions of retinol with the IRBP retinoid-binding sites were measured. The data demonstrate that the interactions of retinol with both sites are stabilized mainly by hydrophobic interactions, and that the hydroxyl head group of retinol is not involved in formation of protein-ligand complexes. Nevertheless, the data indicate that the two sites are structurally distinct, and that binding of retinol in them occurs by remarkably different modes of interactions.

Novel characteristics of a myosin isolated from mammalian retinal pigment epithelial and endothelial cells

We have isolated a novel, high Mr protein from human retinal pigment epithelial cells and endothelial cells by affinity chromatography on Sepharose 4B. Two polypeptides are present on SDS-gels of the 8 M urea eluent with apparent molecular mass of approximately 210 and 47 kDa. In the absence of dithiothreitol, the two polypeptides migrate as one protein band with an apparent molecular mass of approximately 550 kDa. "Piglet," as this molecule is tentatively named, is present in retinal pigment epithelial and endothelial cells of several species, but could not be detected in the nonepithelial cells we examined. Immunofluorescent localization using an antibody to the 210-kDa polypeptide revealed a filamentous network in the cytoplasm of cultured cells. This antibody was used to identify a cDNA for piglet in a bovine aortic endothelial cell expression library. Sequence data indicate a high degree of identity with non-muscle myosin II heavy chain. We subsequently found that piglet had an actin-activated ATPase activity, colocalized with actin in cells, and reacted on Western blots with a pan-non-muscle myosin II heavy chain antiserum. The protein was also recognized by antibodies specific for myosin heavy chain isoform A, but did not react with anti-isoform B antibodies. Although piglet has several features in common with known forms of non-muscle myosin II, the distinctly unconventional features it displays suggest that it is a novel myosin.

Docosahexaenoic acid modulates the interactions of the interphotoreceptor retinoid-binding protein with 11-cis-retinal

Rapid transport of retinoids across the interphotoreceptor matrix is a critical part of the visual cycle, since it serves to replenish bleached rhodopsin with its chromophore 11-cis-retinal. The transport of retinoids in the interphotoreceptor matrix is believed to be mediated by the interphotoreceptor retinoid-binding protein (IRBP), a protein that, in addition to possessing two retinoid-binding sites, associates in vivo with long chain fatty acids. Here, the interrelationships between binding of the two types of ligands to IRBP were studied. The composition of fatty acids associated with IRBP in bovine retina was determined, and it was found that polyunsaturated fatty acids constitute a significant fraction of those. It was further found that docosahexaenoic acid, but not palmitic acid, induced a rapid and specific release of 11-cis-retinal from one of the protein's retinoid-binding sites. Based on these results and on the additional observation that a steep concentration gradient of docosahexaenoic acid exists between photoreceptor and pigment epithelium cells, a model for the mechanism by which IRBP may target 11-cis-retinal to photoreceptor cells is proposed.

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.

Chromatographic and electrophoretic analysis of biomedically important retinoids

The determination of retinol (vitamin A) and its metabolites, as well as synthetic retinoids, in biological samples is a challenging task due to the sensitivity of these compounds to light, heat and oxygen, high protein binding, separation of geometric isomers and determination of low endogenous levels. Numerous procedures for sample preparation have been published for biological fluids and tissues, consisting of solvent extraction, solid-phase extraction (off-line) and HPLC with column switching (on-line solid-phase extraction). The last-mentioned technique has several advantages, including a high degree of automation, no evaporation of extraction solvents, protection from light and higher sensitivity. Due to the favourable UV characteristics of most retinoids, HPLC with UV detection is most often employed, and photodiode array detection is becoming more and more popular. Fluorescence and electrochemical detection have found only a limited field of application, but the use of LC-MS resulted in a few highly sensitive methods. Reconsideration of GC through the use of better deactivated columns and cold on-column injection and evaluation of new promising separation methods, such as supercritical fluid chromatography and capillary electrophoresis, have shown preliminary encouraging results, but appear to reach the required sensitivity only by coupling to MS. Therefore, HPLC with UV detection is still the method of choice for highly sensitive and selective retinoid determination, as well as for high sample throughput and robustness.

Pineal opsin: a nonvisual opsin expressed in chick pineal

Pineal opsin (P-opsin), an opsin from chick that is highly expressed in pineal but is not detectable in retina, was cloned by the polymerase chain reaction. It is likely that the P-opsin lineage diverged from the retinal opsins early in opsin evolution. The amino acid sequence of P-opsin is 42 to 46 percent identical to that of the retinal opsins. P-opsin is a seven-membrane spanning, G protein-linked receptor with a Schiff-base lysine in the seventh membrane span and a Schiff-base counterion in the third membrane span. The primary sequence of P-opsin suggests that it will be maximally sensitive to approximately 500-nanometer light and produce a slow and prolonged phototransduction response consistent with the nonvisual function of pineal photoreception.

Platelet-derived growth factor is an autocrine growth stimulator in retinal pigmented epithelial cells

The retinal pigmented epithelium (RPE) plays a major role in normal and exaggerated retinal wound repair; the latter can result in epiretinal membrane formation and loss of vision. The RPE forms a stable monolayer of highly differentiated cells that proliferates only during wound repair. The mechanism underlying the change to the proliferating phenotype is unknown. When grown on a plastic substratum, cultured RPE cells mimic the proliferating phenotype in situ; they escape density arrest and proliferate in serum-free medium. In this study, we have demonstrated that a platelet-derived growth factor (PDGF) autocrine loop is involved in RPE growth in serum-free medium, because: (1) RPE cells secrete PDGF into their media and express PDGF receptors; (2) the PDGF receptors on RPE cells are autophosphorylated in serum-free medium and suramin, an agent that displaces PDGF and other growth factors from their receptors, blocks the autophosphorylation; and (3) a neutralizing antibody to PDGF significantly decreases RPE growth in serum-free medium. When a linear scrape is made in an RPE monolayer, the cells migrate and proliferate to fill in the gap mimicking wound repair in situ. Cells along the edge of the scrape show increased expression of PDGF and PDGF-beta receptors, and increased staining for proliferating cell nuclear antigen. Immunohistochemistry and in situ hybridization demonstrate expression of PDGF in ganglion cells and cells of retinal blood vessels. PDGF is not detected in the outer retina or RPE in untreated eyes, but is detected in RPE participating in wound repair, either adjacent to laser burns or underlying retinal detachment. PDGF and PDGF receptors are also expressed in RPE in epiretinal membranes removed during vitreous surgery. These data suggest that PDGF is an autocrine stimulator of growth in RPE that plays a role in retinal wound repair and epiretinal membrane formation.

Expression and characterization of the fourth repeat of Xenopus interphotoreceptor retinoid-binding protein in E. coli

Interphotoreceptor retinoid-binding protein (IRBP) is an extracellular glycolipoprotein which in higher vertebrates has a 4-repeat structure and carries endogenous vitamin A and fatty acids. The location of IRBP's 1-2 binding sites for retinol is unknown. To begin to understand which repeat(s) are responsible for ligand-binding, we expressed the fourth repeat of Xenopus IRBP in E. coli to determine if it could by itself bind all-trans retinol. Our expression studies used a polyhistidine fusion domain to purify the recombinant protein directly from inclusion bodies. The fusion protein could be renatured without aggregation if refolded at a sufficiently dilute concentration (< 3 microM). The recombinant fourth repeat of Xenopus IRBP binds [3H]all-trans retinol and the fluorescence of this ligand increases 8-fold upon binding. The binding is saturable with a Kd = 0.4 microM. The expression of recombinant IRBP fragments as fusion proteins in prokaryotes will be useful for defining the structural requirements for ligand binding by this interesting protein.

Synthesis and secretion of retinol-binding protein and transthyretin by cultured retinal pigment epithelium

Recent studies indicate that the retinal pigment epithelium (RPE) may serve as an extrahepatic source of retinol-binding protein (RBP) and transthyretin (TTR) for the retina by virtue of the fact that this cell layer is the exclusive retinal location for mRNA coding for these proteins [Herbert, J., et al. (1991) Invest. Ophthalmol. Vis. Sci. 32, 302-309; Cavallaro, T., et al. (1990) Invest. Ophthalmol. Vis. Sci. 31, 497-501], although the proteins themselves are present in a variety of retinal neurons. It is therefore necessary to determine whether these mRNAs are translated and whether their translated products are secreted like hepatic RBP and TTR. Metabolic labeling of cultured bovine RPE with [35S]cysteine and [35S]methionine and subsequent analysis of newly synthesized proteins in the conditioned medium by affinity chromatography, gel filtration, partial amino acid sequence analysis, and autoradiography of electrophoretograms indicate that both RBP and TTR are synthesized and secreted by the RPE. Moreover, for cells grown in chambers with permeable supports, the predominant direction for secretion was into the apical medium. The mean apical:basal ratio after 72 h of incubation was 9.2 for TTR and 4.5 for RBP. A function for these proteins in the neurosensory retina remains speculative. They could be involved in the delivery of all-trans-retinol to amacrine and Müller cells as a precursor for retinoic acid, since these cells are known to contain cellular retinoic acid binding protein [Gaur, V.P., et al. (1990) Exp. Eye Res. 50, 505-511; Milam et al. (1990) J. Comp. Neurol. 296, 123-129].(ABSTRACT TRUNCATED AT 250 WORDS)

Interphotoreceptor retinoid-binding protein (IRBP), a major 124 kDa glycoprotein in the interphotoreceptor matrix of Xenopus laevis. Characterization, molecular cloning and biosynthesis

We have demonstrated that the neural retina of Xenopus laevis secretes into the extracellular matrix surrounding the inner and outer segments of its photoreceptors a glycoprotein containing hydrophobic domains conserved in mammalian interphotoreceptor retinoid-binding proteins (IRBPs). The soluble extract of the interphotoreceptor matrix contains a 124 kDa protein that cross-reacts with anti-bovine IRBP immunoglobulins. In vitro [3H]fucose incorporation studies combined with in vivo light and electron microscopic autoradiographic analysis, showed that the IRBP-like glycoprotein is synthesized by the neural retina and secreted into the interphotoreceptor matrix. A 1.2 kb Xenopus IRBP cDNA was isolated by screening a stage 42 (swimming tadpole) lambda Zap II library with a human IRBP cDNA under low-stringency conditions. The cDNA hybridizes with a 4.2 kb mRNA in adult Xenopus neural retina, tadpole heads as well as a less-abundant mRNA of the same size in brain. During development, IRBP and opsin mRNA expression correlates with photoreceptor differentiation. The translated amino acid sequence of the Xenopus IRBP clone has an overall 70% identity with the fourth repeat of the human protein. Sequence alignment with the four repeats of human IRBP showed three highly conserved regions, rich in hydrophobic residues. This focal conservation predicts domains important to the protein's function, which presumably is to facilitate the exchange of 11-cis retinal and all-trans retinol between the pigment epithelium and photoreceptors, and to the transport of fatty acids through the hydrophilic interphotoreceptor matrix.

Interphotoreceptor retinoid-binding protein (IRBP). Molecular biology and physiological role in the visual cycle of rhodopsin

The regeneration of visual pigment in rod photoreceptors of the vertebrate retina requires an exchange of retinoids between the neural retina and the retina pigment epithelium (RPE). It has been hypothesized that interphotoreceptor retinoid-binding protein (IRBP) functions as a two-way carrier of retinoid through the aqueous compartment (interphotoreceptor matrix) that separates the RPE and the photoreceptors. The first part of this review summarizes the cellular and molecular biology of IRBP. Work on the IRBP gene indicates that the protein contains a four-fold repeat structure that may be involved in binding multiple retinoid and fatty acid ligands. These repeats and other aspects of the gene structure indicate that the gene has had an active and complex evolutionary history. IRBP mRNA is detected only in retinal photoreceptors and in the pineal gland; expression is thus restricted to the two photosensitive tissues of vertebrate organisms. In the second part of this review, we consider the results obtained in experiments that have examined the activity of IRBP in the process of visual pigment regeneration. We also consider the results obtained on the bleaching and regeneration of rhodopsin in the acutely detached retina, as well as in experiments testing the ability of IRBP to protect its retinoid ligand from isomerization and oxidation. Taken together, the findings provide evidence that, in vivo, IRBP facilitates both the delivery of all-trans retinol to the RPE and the transfer of 11-cis retinal from the RPE to bleached rod photoreceptors, and thereby directly supports the regeneration of rhodopsin in the visual cycle.

A developmentally regulated microsomal protein specific for the pigment epithelium of the vertebrate retina

In the vertebrate retina, the retinal pigment epithelium (RPE) performs specific functions critical to the normal process of vision. Although some of these functions are well documented, molecular data are still scarce. Using the RPE-specific monoclonal antibody RPE9, raised against human RPE cells, we have identified a novel 65 kD protein, conserved in mammals, birds, and frogs. This RPE-specific protein was found to be nonglycosylated. It was most effectively solubilized in the presence of detergent suggesting that it is associated with the RPE cell membranes. Its partitioning in the detergent phase of Triton X-114 and its solubilization in 0.75 M and 1.0 M KCl suggest that it interacts with the membrane either through a polypeptide anchor or charged amino acids. Cell fractionation by differential solubilization and differential centrifugation demonstrated that the protein was preferentially associated with the microsomal membrane fraction, where it is the major protein. Developmental expression of this 65 kD protein was examined in neonatal rats. Morphologically well-differentiated RPE cells did not express the 65 kD protein at birth. However, expression was detectable at postnatal day 4, that is, one to two days before the photoreceptors develop their outer segments, suggesting that the expression of the 65 kD protein may be coordinated with other developmental events in the intact retina. This is further supported by the fact that RPE cells in confluent culture lose the expression of this protein within two weeks, while they maintain their characteristic epithelial morphology. Because of its specificity, its evolutionary conservation, and its timing of expression, it is possible that this protein may be involved in one of the key roles of RPE and as such is an important molecular marker for RPE differentiation.

Effect of phytanic acid on cultured retinal pigment epithelium: an in vitro model for Refsum's disease

Refsum's disease (heredopathia atactica polyneuritiformis) is an autosomal recessive retinitis pigmentosa syndrome caused by the excessive deposition of phytanic acid in ocular tissues. It is thought that phytanic acid causes retinal degeneration either by interfering with vitamin A metabolism in the retinal pigment epithelium or by altering photoreceptor cell membrane structure. Efforts to elucidate the molecular mechanism of phytanic acid's retinal toxicity have been hampered by the rarity of human pathological specimens and by the inability to reproduce the disease in living animal models. In this study, an in vitro model for Refsum's disease was established by exposing cultured human and bovine retinal pigment epithelial cells to phytanic acid bound to bovine serum albumin at concentrations comparable to levels found in affected humans. Ultrastructural studies show that these cells exhibit morphological changes consistent with those observed in pathological specimens from patients with Refsum's disease. Biochemical assays of retinoid metabolism by cell membranes from control cells and from cells exposed to 200 microM phytanic acid demonstrate that the ability to esterify retinol and to isomerize all-trans retinoids to 11-cis retinoids remains intact despite the deposition of large amounts of phytanic acid. The work described here is strong evidence against the hypothesis that phytanic acid inhibits vitamin A metabolism in the retinal pigment epithelium, and it demonstrates the potential use of cultured retinal pigment epithelial cells in modeling this and other degenerative diseases of the retina.

Rhodopsin regeneration in the normal and in the detached/replaced retina of the skate

The bleaching and regeneration of rhodopsin in the skate retina was studied by means of fundus reflectometry, both in the normal eyecup preparation and after the retina had been detached and then replaced on the surface of the pigment epithelium (RPE). After bleaching virtually all the rhodopsin in the retinal test area of the normal eyecup, more than 90% of the photopigment was reformed after about 2 hr in darkness; over most of this time course, rhodopsin density rose linearly at a rate of 0.875% min-1 with a half-time of 55 min. Detaching the retina from its pigment epithelium resulted in a number of abnormalities, both structural and functional. Histological examination of the detached/replaced (D/R) retina showed striking alterations in the structural integrity of the RPE cells at their interface with the neural retina. The cells appeared vacuolated and misshapen, and the apical processes of the RPE, which normally ensheath the receptor outer segments, were shredded and free of their association with the visual cells. These morphological changes, as well as dilution of the IRBP content of the subretinal space caused by separation of the tissues, appear to be the main factors contributing to the functional abnormalities in rhodopsin kinetics. But despite these abnormalities and the persistent detachment, the rate of regeneration and the amount of rhodopsin reformed after bleaching were reduced by less than 50% of their normal values. The fact that a significant fraction of the bleached rhodopsin was regenerated under these conditions indicates that 11-cis retinal formed in the RPE was able to traverse a much greater than normal subretinal space to reach the opsin-bearing photoreceptor membranes.

Muller cells of chicken retina synthesize 11-cis-retinol

The amounts of endogenous retinyl palmitate, retinol and retinaldehyde were measured in the neural retina and retinal pigment epithelium (RPE) of predominantly cone (chicken), rod (rat) and more mixed (cat, human) retinae. The ratio of 11-cis to all-trans isomers of retinyl palmitate and retinol in the neural retina and the RPE increases progressively with the increase in diurnality of the species from rat to chicken. The membrane fractions of both chicken and bovine RPE enzymically isomerize all-trans retinol to 11-cis-retinol. Chicken neural retina membranes enzymically form 11-cis-retinol and all-trans-retinyl palmitate from all-trans-retinol. Light and electron microscopy revealed no contamination of chicken neural retina by RPE. Muller cells from chicken retina were isolated, cultured and characterized by immunocytochemical localization of cellular retinaldehyde-binding protein. Cultured chicken Muller cells form all-trans-retinyl palmitate, 11-cis-retinol and 11-cis-retinyl palmitate from all-trans-retinol and release most of the 11-cis-retinol into the medium. The results indicate that chicken neural retina and Muller cells in particular synthesize 11-cis-retinoids from all-trans-retinol.

Functional properties of interphotoreceptor retinoid-binding protein

It has been hypothesized that interphotoreceptor retinoid-binding protein (IRBP) functions as a two-way carrier of retinoid between the retinal pigment epithelium (RPE) and rod photoreceptors in the vertebrate eye. This hypothesis has been tested in recent studies that have employed purified, initially ligand-free, bovine IRBP and the "RPE-eyecup" obtained from the toad (Bufo marinus) eye. The present experiments further characterize the IRBP/RPE-eyecup system with respect to (i) the solubilization and protection of retinol by IRBP, and (ii) the time course of IRBP-mediated release of 11-cis retinal by the RPE. The data, together with previous findings in the IRBP/RPE-eyecup preparation, support the view that 11-cis retinal is the principal retinoid released by the RPE into IRBP-supplemented aqueous medium, and that IRBP in vivo promotes the regeneration of rhodopsin by facilitating the exchange of retinoid between bleached rods and the RPE.

Effect of light on endogenous ligands carried by interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP) is a vitamin A carrier present only in the extracellular material lying between the neural retina and the retinal pigment epithelium of vertebrate eyes. The amount of retinol bound endogenously by IRBP in this interphotoreceptor space is known to increase upon illumination. This finding led to the hypothesis that IRBP may act as a shuttle for vitamin A during the visual cycle that regenerates rhodopsin. In the present work, we separated IRBP from other retinoid-binding proteins in bovine interphotoreceptor matrix preparations by means of size-exclusion chromatography. IRBP's endogenous ligands were retained during this procedure and were then extracted into hexane and analysed by normal-phase HPCL. We found that IRBP carries, in a light-dependent manner, all the retinoid isomers involved in the visual cycle. For dark-adapted eyes the amounts of bound ligands are (in nmol per eye) 0.09 all-trans retinol, 0.11 11-cis retinol, 0.04 all-trans retinal, 0.16 11-cis retinal, and 0.07 retinyl esters. For light-adapted eyes the amount of all-trans retinol was found to increase by a factor of five, and that of 11-cis retinal to decrease by a factor of four. (These eyes contain 3.1 nmol of IRBP, which does not change in amount with lighting conditions). Thus, the major endogenous ligand of IRBP is 11-cis retinaldehyde in the dark and all-trans retinol in the light. The data are consistent with a role for IRBP as a non-selective scavenger and stabilizer of retinoids released from photoreceptors and pigment epithelial cells. However, it cannot be concluded from these data that IRBP is involved in directed transport of retinoids across the interphotoreceptor matrix, since there is no evidence for the appropriate spatial gradients in the ligands bound to the protein.