Endocytosis and degradation of interstitial retinol-binding protein: differential capabilities of cells that border the interphotoreceptor matrix

Seeing the Future: A Review of Ocular Therapy

Ocular diseases present a unique challenge and opportunity for therapeutic development. The eye has distinct advantages as a therapy target given its accessibility, compartmentalization, immune privilege, and size. Various methodologies for therapeutic delivery in ocular diseases are under investigation that impact long-term efficacy, toxicity, invasiveness, and delivery range. While gene, cell, and antibody therapy and nanoparticle delivery directly treat regions that have been damaged by disease, they can be limited in the duration of the therapeutic delivery and have a focal effect. In contrast, contact lenses and ocular implants can more effectively achieve sustained and widespread delivery of therapies; however, they can increase dilution of therapeutics, which may result in reduced effectiveness. Current therapies either offer a sustained release or a broad therapeutic effect, and future directions should aim toward achieving both. This review discusses current ocular therapy delivery systems and their applications, mechanisms for delivering therapeutic products to ocular tissues, advantages and challenges associated with each delivery system, current approved therapies, and clinical trials. Future directions for the improvement in existing ocular therapies include combination therapies, such as combined cell and gene therapies, as well as AI-driven devices, such as cortical implants that directly transmit visual information to the cortex.

Extracellular vesicles highlight many cases of photoreceptor degeneration

The release of extracellular vesicles is observed across numerous cell types and serves a range of biological functions including intercellular communication and waste disposal. One cell type which stands out for its robust capacity to release extracellular vesicles is the vertebrate photoreceptor cell. For decades, the release of extracellular vesicles by photoreceptors has been documented in many different animal models of photoreceptor degeneration and, more recently, in wild type photoreceptors. Here, I review all studies describing extracellular vesicle release by photoreceptors and discuss the most unifying theme among them-a photoreceptor cell fully, or partially, diverts its light sensitive membrane material to extracellular vesicles when it has defects in the delivery or morphing of this material into the photoreceptor's highly organized light sensing organelle. Because photoreceptors generate an enormous amount of light sensitive membrane every day, the diversion of this material to extracellular vesicles can cause a massive accumulation of these membranes within the retina. Little is known about the uptake of photoreceptor derived extracellular vesicles, although in some cases the retinal pigment epithelial cells, microglia, Müller glia, and/or photoreceptor cells themselves have been shown to phagocytize them.

Extracellular vesicles in the retina - putative roles in physiology and disease

The retina encompasses a network of neurons, glia and epithelial and vascular endothelia cells, all coordinating visual function. Traditionally, molecular information exchange in this tissue was thought to be orchestrated by synapses and gap junctions. Recent findings have revealed that many cell types are able to package and share molecular information via extracellular vesicles (EVs) and the technological advancements in visualisation and tracking of these delicate nanostructures has shown that the role of EVs in cell communication is pleiotropic. EVs are released under physiological conditions by many cells but they are also released during various disease stages, potentially reflecting the health status of the cells in their cargo. Little is known about the physiological role of EV release in the retina. However, administration of exogenous EVs in vivo after injury suggest a neurotrophic role, whilst photoreceptor transplantation in early stages of retina degeneration, EVs may facilitate interactions between photoreceptors and Müller glia cells. In this review, we consider some of the proposed roles for EVs in retinal physiology and discuss current evidence regarding their potential impact on ocular therapies via gene or cell replacement strategies and direct intraocular administration in the diseased eye.

The Ocular Gene Delivery Landscape

The eye is at the forefront of developing therapies for genetic diseases. With the FDA approval of the first gene-therapy drug for a form of congenital blindness, numerous studies have been initiated to develop gene therapies for other forms of eye diseases. These examinations have revealed new information about the benefits as well as restrictions to using drug-delivery routes to the different parts of the eye. In this article, we will discuss a brief history of gene therapy and its importance to the eye and ocular delivery landscape that is currently being investigated, and provide insights into their advantages and disadvantages. Efficient delivery routes and vehicle are crucial for an effective, safe, and longer-lasting therapy.

Daily mitochondrial dynamics in cone photoreceptors

Cone photoreceptors in the retina are exposed to intense daylight and have higher energy demands in darkness. Cones produce energy using a large cluster of mitochondria. Mitochondria are susceptible to oxidative damage, and healthy mitochondrial populations are maintained by regular turnover. Daily cycles of light exposure and energy consumption suggest that mitochondrial turnover is important for cone health. We investigated the three-dimensional (3D) ultrastructure and metabolic function of zebrafish cone mitochondria throughout the day. At night retinas undergo a mitochondrial biogenesis event, corresponding to an increase in the number of smaller, simpler mitochondria and increased metabolic activity in cones. In the daytime, endoplasmic reticula (ER) and autophagosomes associate more with mitochondria, and mitochondrial size distribution across the cluster changes. We also report dense material shared between cone mitochondria that is extruded from the cell at night, sometimes forming extracellular structures. Our findings reveal an elaborate set of daily changes to cone mitochondrial structure and function.

Membrane trafficking in the retinal pigment epithelium at a glance

The retinal pigment epithelium (RPE) is a highly specialised pigmented monolayer sandwiched between the choroid and the photoreceptors in the retina. Key functions of the RPE include transport of nutrients to the neural retina, removal of waste products and water from the retina to the blood, recycling of retinal chromophores, absorption of scattered light and phagocytosis of the tips of the photoreceptor outer segments. These functions place a considerable membrane trafficking burden on the RPE. In this Cell Science at a Glance article and the accompanying poster, we focus on RPE-specific adaptations of trafficking pathways. We outline mechanisms underlying the polarised expression of membrane proteins, melanosome biogenesis and movement, and endocytic trafficking, as well as photoreceptor outer segment phagocytosis and degradation. We also briefly discuss theories of how dysfunction in trafficking pathways contributes to retinal disease.

Gene Therapy for Inherited Retinal Degeneration

Inherited retinal degeneration (IRD), a group of rare retinal diseases that primarily lead to the progressive loss of retinal photoreceptor cells, can be inherited in all modes of inheritance: autosomal dominant (AD), autosomal recessive (AR), X-linked (XL), and mitochondrial. Based on the pattern of inheritance of the dystrophy, retinal gene therapy has 2 main strategies. AR, XL, and AD IRDs with haploinsufficiency can be treated by inserting a functional copy of the gene using either viral or nonviral vectors (gene augmentation). Different types of viral vectors and nonviral vectors are used to transfer plasmid DNA both in vitro and in vivo. AD IRDs with gain-of-function mutations or dominant-negative mutations can be treated by disrupting the mutant allele with (and occasionally without) gene augmentation. This review article aims to provide an overview of ocular gene therapy for treating IRDs using gene augmentation with viral or nonviral vectors or gene disruption through different gene-editing tools, especially with the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system.

Nanoparticle-based technologies for retinal gene therapy

For patients with hereditary retinal diseases, retinal gene therapy offers significant promise for the prevention of retinal degeneration. While adeno-associated virus (AAV)-based systems remain the most popular gene delivery method due to their high efficiency and successful clinical results, other delivery systems, such as non-viral nanoparticles (NPs) are being developed as additional therapeutic options. NP technologies come in several categories (e.g., polymer, liposomes, peptide compacted DNA), several of which have been tested in mouse models of retinal disease. Here, we discuss the key biochemical features of the different NPs that influence how they are internalized into cells, escape from endosomes, and are delivered into the nucleus. We review the primary mechanism of NP uptake by retinal cells and highlight various NPs that have been successfully used for in vivo gene delivery to the retina and RPE. Finally, we consider the various strategies that can be implemented in the plasmid DNA to generate persistent, high levels of gene expression.

Cone outer segment extracellular matrix as binding domain for interphotoreceptor retinoid-binding protein

Cones are critically dependent on interphotoreceptor retinoid-binding protein (IRBP) for retinoid delivery in the visual cycle. Cone-dominant vertebrates offer an opportunity to uncover the molecular basis of IRBP's role in this process. Here, we explore the association of IRBP with the interphotoreceptor matrix (IPM) of cones vs. rods in cone dominant retinas from chicken (Gallus domesticus), turkey (Meleagris gallopavo), and pig (Sus scrofa). Retinas were detached and fixed directly or washed in saline prior to fixation. Disassociated photoreceptors with adherent matrix were also prepared. Under 2 mM CaCl(2) , insoluble matrix was delaminated from saline washed retinas. The distribution of IRBP, as well as glycans binding peanut agglutinin (cone matrix) and wheat germ agglutinin (rod/cone matrix), was defined by confocal microscopy. Retina flat mounts showed IRBP diffusely distributed in an interconnecting, lattice-like pattern throughout the entire matrix. Saline wash replaced this pattern with fluorescent annuli surrounding individual cone outer segments. In isolated cones and matrix sheets, IRBP colocalized with the peanut agglutinin binding matrix glycans. Our results reveal a wash-resistant association of IRBP with a matrix domain immediately surrounding cone outer segments. The cone matrix sheath may be responsible for IRBP-mediated cone targeting of 11-cis retinoids.

Multivesicular bodies in neurons: distribution, protein content, and trafficking functions

Multivesicular bodies (MVBs) are intracellular endosomal organelles characterized by multiple internal vesicles that are enclosed within a single outer membrane. MVBs were initially regarded as purely prelysosomal structures along the degradative endosomal pathway of internalized proteins. MVBs are now known to be involved in numerous endocytic and trafficking functions, including protein sorting, recycling, transport, storage, and release. This review of neuronal MVBs summarizes their research history, morphology, distribution, accumulation of cargo and constitutive proteins, transport, and theories of functions of MVBs in neurons and glia. Due to their complex morphologies, neurons have expanded trafficking and signaling needs, beyond those of "geometrically simpler" cells, but it is not known whether neuronal MVBs perform additional transport and signaling functions. This review examines the concept of compartment-specific MVB functions in endosomal protein trafficking and signaling within synapses, axons, dendrites and cell bodies. We critically evaluate reports of the accumulation of neuronal MVBs based on evidence of stress-induced MVB formation. Furthermore, we discuss potential functions of neuronal and glial MVBs in development, in dystrophic neuritic syndromes, injury, disease, and aging. MVBs may play a role in Alzheimer's, Huntington's, and Niemann-Pick diseases, some types of frontotemporal dementia, prion and virus trafficking, as well as in adaptive responses of neurons to trauma and toxin or drug exposure. Functions of MVBs in neurons have been much neglected, and major gaps in knowledge currently exist. Developing truly MVB-specific markers would help to elucidate the roles of neuronal MVBs in intra- and intercellular signaling of normal and diseased neurons.

Purification of the full-length Xenopus interphotoreceptor retinoid binding protein and growth of diffraction-quality crystals

PURPOSE

Interphotoreceptor retinol-binding protein (IRBP), composed of two or four homologous modules in tandem, plays an important role in retinoid trafficking between the retinal pigmented epithelium, photoreceptors, and Müller cells. The exact nature of this role is not yet clear. Attempts to purify the full-length retinal IRBP to homogeneity for crystallization purposes have largely been unsuccessful, owing primarily to instability and denaturation of the protein at high concentrations in aqueous media.

METHODS

A bacterial expression system was used for the production of the recombinant full-length four modules-containing Xenopus IRBP (xIRBP; 1197 amino acids; 131 kDa). An optimized purification strategy and the presence of molar excesses of a thiol-based reducing agent yielded highly pure xIRBP in a soluble, stable and active form, free of its fusion partner. Binding of all-trans retinol was characterized by fluorescence spectroscopy monitoring ligand-fluorescence enhancement, quenching of endogenous protein fluorescence, and energy transfer.

RESULTS

We grew the first diffraction-quality crystal of xIRBP. We have gathered diffraction data from these crystals to 2.46 A resolution, sufficient to yield an atomic model of the tertiary structure of IRBP. Retinol-binding results determined by fluorescence spectroscopy show roughly one retinol-binding site per polypeptide chain.

CONCLUSIONS

The binding stoichiometry taken together with modeling data suggest that not all modules are functionally equivalent. Determination of the full-length IRBP structure will be a significant breakthrough in understanding the functional roles of IRBP in the visual cycle. The advances presented here will not only lead to the structure of the full-length IRBP, but will allow us to understand how the modules interact in the function of IRBP. Furthermore, these studies will allow characterization of the ligand-binding site(s) with bound ligand(s).

Glial transcytosis of a photoreceptor-secreted signaling protein, retinoschisin

In vitro studies have clearly shown that signaling/guidance proteins can diffuse to their targets. However, it is unclear whether they can travel by diffusion in vivo, or if they are distributed in the tissue by an active mechanism. Retinoschisin, a signaling molecule related to neuropilins, is synthesized and secreted by photoreceptor cells in the outer retina; then it interacts with inner retinal cells contributing to synaptic organization and optic nerve fiber integrity. We developed an assay to examine how retinoschisin, which is secreted a distance away, reaches its inner retinal targets. We found that retinoschisin is preferentially taken up and carried into the inner retina from the retinal outer border (the photoreceptor side) by Müller cells (the main glial cells of the vertebrate retina). This transcytosis is disrupted by DL-alpha-aminoadipic acid, a Müller cell/glia-specific toxin. Our results suggest that glial uptake/transcytosis can provide an effective and precise alternative for distributing signaling molecules in the nervous system.

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.

Internalization of interphotoreceptor retinoid-binding protein by theXenopus retinal pigment epithelium

Xenopus rods and cones secrete into the interphotoreceptor matrix (IPM) a 124-kDa glycoprotein termed interphotoreceptor retinoid-binding protein (IRBP; Hessler et al. [1996] J. Comp. Neurol. 367:329-341). IRBP is confined to the IPM, being too large to diffuse through the zonulae adherentes between adjacent photoreceptor and Müller cells. Despite this physical entrapment within the subretinal space, IRBP is rapidly cleared from the IPM by an unknown mechanism. Immunohistochemistry and immunoelectron microscopy were used to localize IRBP in intact and detached retina-retinal pigment epithelium (RPE) eyecups. The effects of light, dark, and time of day on the compartmentalization of IRBP were characterized by quantitative Western blot analysis and by immunoprecipitation of IRBP labeled in vivo by intraocular injection of [(35)S]methionine. Immunohistochemistry showed that the apparent intercellular IRBP in both the RPE and the photoreceptors is resistant to saline extraction, in contrast to that in the IPM. In the RPE, IRBP was associated with matrix material within phagosomes and endosomes. The IPM, RPE, and retina contained 75%, 18%, and 7% of the total IRBP in the eye, respectively. The IPM and RPE contain 130 +/- 14 pmoles and 34 +/- 4 pmoles of IRBP, respectively. The amounts of IRBP in the RPE at middark and midlight were the same. Furthermore, the in vivo uptake of [(35)S]methionine-labeled IRBP was light independent. Our studies suggest that IRBP is not strictly confined to the subretinal space but rather that significant amounts are present intracellularly, particularly within the RPE, which does not synthesize IRBP. Furthermore, IRBP secreted by the photoreceptors is taken up from the IPM mainly through a light-independent endocytic pathway separate from outer segment phagocytosis. The role of RPE endocytosis should be explored in relation to the function of IRBP.

VIP enhances the differentiation of retinal pigment epithelium in culture: from cAMP and pp60(c-src) to melanogenesis and development of fluid transport capacity

The retinal pigment epithelium (RPE) is a single cell layer juxtaposed between the neural retina and the choroid and functions as a blood-retina barrier. The RPE performs functions essential for photoreceptor (PR) survival. Although the regulation of these functions has remained unknown, it is a distinct possibility that the RPE is under constant regulation by signaling molecules coming from the choroid and the retina. Vasoactive intestinal peptide (VIP), a 28-amino acid neuropeptide present in the retina and in the choroid, has been shown to promote the growth and differentiation of a variety of cells in tissue and organ cultures. In cultured RPE cells, VIP is the one most effective stimulator of the cAMP signaling pathway among a long list of neurotransmitters and modulators tested. For example, VIP, at 1 microM, stimulates the intracellular cAMP to 80-100- and 20-fold in 3 min in RPE cells cultured from chick embryos and adult human donor eyes, respectively. In cultured chick embryonic RPE, VIP is also shown to be a potent and effective modulator of pp60(c-src), the non-receptor tyrosine kinase present in differentiating and terminally differentiated cells. VIP stimulates both overall phosphorylation at unknown sites and phosphotyrosine dephosphorylation in pp60(c-src). A 190-kDa microtubule-associated protein is known to be one of the downstream targets in VIP-modulated signaling pathways. At the cellular level, VIP stimulates cell proliferation modestly and melanogenesis pronouncedly in growing chick embryonic RPE cultures. Ultimately, the differentiation goal of RPE cells in vivo is to perform functions that are essential for photoreceptor survival. On bare permeable supports (that is, without biological material coating), the chick embryonic RPE cells grow to become RPE sheets with a cytoarchitecture that allows the display of two of the RPE functions. These cultures demonstrate structural polarity and are functionally polarized, allowing for proper macromolecule secretion and fluid transport. VIP is shown to stimulate macromolecule secretion at the apical surface (retina facing) and the development of the capacity for fluid transport from the apical to the basal surface of the RPE sheet. In conclusion, studies in our laboratory indicate that VIP is a differentiation promotor during the development of a functional RPE. Recent advances in the molecular biology of melanogenesis and the fluid transport-linked Na-K-2Cl cotransporter in other cells will allow future studies of VIP modulated events in the RPE at the molecular level. Finally, identification of RPE differentiation factors may prove essential for the ultimate success of RPE transplantation, thus promoting the rescue of photoreceptor cells in retinal degeneration.

Soluble expression in E. coli of a functional interphotoreceptor retinoid-binding protein module fused to thioredoxin: correlation of vitamin A binding regions with conserved domains of C-terminal processing proteases

The exchange of all-trans retinol and 11-cis retinal between the photoreceptors and retinal pigmented epithelium is mediated by interphotoreceptor retinoid-binding protein (IRBP). IRBP contains binding sites for retinoids, docosahexaenoic acid and probably cell surface and matrix receptors. IRBP arose through the quadruplication of an ancient protein, represented by its carboxy-terminal module (module 4 in amphibians and mammals). Module 4 has retinol binding activity and is composed of regions coded for by each of IRBP's four exons. Determining the function of the exons has been hampered by insoluble expression of module 4 in Escherichia coli. Here, we found that module 4 of Xenopus IRBP (X4IRBP), as well as its exon segments, can be expressed in a soluble form as thioredoxin fusion proteins. The recombinant proteins were purified by ion exchange and arsenical-based affinity chromatography. Liquid chromatography/mass spectrometry confirmed that the sequence of X4IRBP is correct. All-trans retinol binding was characterized by monitoring enhancement of retinol fluorescence, quenching of intrinsic protein fluorescence, and transfer of energy to the bound retinol. Retinol bound to X4IRBP at 2.20+/-0.29 sites with a KD=1.25+/-0.39. One of the two sites was localized to Exons(2+3) and had a KD=0.26+/-0.13 micron. This site, which supported protein quenching and energy transfer, probably contains at least one of the two conserved tryptophans present in this segment. The second site was localized to Exon 4. This site supported the enhancement of retinol fluorescence but not protein quenching or energy transfer and had a KD=1.94+/-0.20 micron. Exon 1 had no retinol binding activity. The location of the retinol binding regions correlated with the distribution of domains conserved between IRBPs and the newly recognized family of C-terminal processing proteases (CtpAs), proteins which bind and cleave non-polar carboxy termini.

Goldfish cones secrete a two-repeat interphotoreceptor retinoid-binding protein

Vitamin A and fatty acids are critical to photoreceptor structure, function, and development. The transport of these nutrients between the pigment epithelium and neural retina is mediated by interphotoreceptor retinoid-binding protein (IRBP). IRBP, a 133-kDa (human) glycolipoprotein, is the major protein component of the extracellular matrix separating these two cell layers. In amphibians and mammals, IRBP consists of four homologous repeats of about 300 amino acids which form two retinol and four fatty acid-binding sites. Here we show that IRBP in teleosts is a simpler protein composed of only two repeats. Western blot analysis shows that goldfish IRBP is half the size (70 kDa) of IRBP in higher vertebrates. Metabolic labeling studies employing Brefeldin A taken together with in situ hybridization studies and the presence of a signal peptide show that goldfish IRBP is secreted by the cone photoreceptors. The translated amino acid sequence has a calculated molecular weight of 66.7 kDa. The primary structure consists of only two homologous repeats with a similarity score of 52.5%. The last repeats of human and goldfish IRBPs are 69.1% similar with hydrophobic regions being the most similar. These data suggest that two repeats were lost during the evolution of the ray-finned fish (Actinopterygii), or that the IRBP gene duplicated between the emergence of bony fish (Osteichthyes) and amphibians. Acquisition of a multirepeat structure may reflect evolutionary pressure to efficiently transport higher levels of hydrophobic molecules within a finite space. Quadruplication of an ancestral IRBP gene may have been an important event in the evolution of photo-receptors in higher vertebrates.

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.

Uptake and isomerization of all-trans retinol by isolated bovine retinal pigment epithelial cells: further clues to the visual cycle

The site and substrate for all-trans to 11-cis isomerization in the visual cycle have remained obscure for several decades. Only recently studies on a subcellular level have begun to shed some light on these phenomena. We have addressed this system on a cellular level by utilizing intact isolated bovine retinal pigment epithelial cells, maintained during short-term incubation in vitro. Supplementation with labeled all-trans retinol incorporated in a lipid vesicle carrier, in a range of 1-6 nmol per 10(6) cells, resulted in a rapid uptake of retinol. The majority of the internalized retinol was processed prior to mixing with endogenous retinoid pools and most of it was converted into all-trans retinylester. Up to 10% of the incorporated label was isomerized yielding 11-cis retinol, 11-cis retinaldehyde and 11-cis retinylester. The kinetics of the 11-cis retinoid formation indicated that 11-cis retinol is the first isomerization product. The level of 11-cis retinol apparently 'triggered' further processing into other 11-cis retinoids. An updated model with discussion topics is presented for the retinoid pathway relevant to the visual cycle.

Mannose-sensitive HRP endocytosis by the retinal pigment epithelium

Mannose-sensitive endocytosis by rat retinal pigment epithelium (RPE) explants was characterized using the mannose-rich glycoprotein horseradish peroxidase (HRP). The number of HRP-containing endosomes in the RPE was morphologically quantitated by light microscopy while the amount of HRP ingested was biochemically quantitated by enzyme assay. HRP internalized via a mannose-sensitive receptor was differentiated from fluid-phase uptake in competitive inhibition studies using D-mannose. Morphological results showed that most HRP-containing endosomes formed within the first 15 min of incubation and showed little increase in number during 4 hr of continued incubation with HRP. In contrast, the biochemical assay showed a steady increase in the amount of HRP in RPE endosomes measured over time. The addition of 10 mM D-mannose to the incubation medium was associated with a significant decrease in both the number of HRP-containing endosomes and the amount of HRP ingested by RPE explants. Values indicate that half of the total uptake of HRP is mediated by a mannose-sensitive receptor while the balance is ingested via non-specific fluid-phase endocytosis.

Early expression of the gene for interphotoreceptor retinol-binding protein during photoreceptor differentiation suggests a critical role for the interphotoreceptor matrix in retinal development

Interphotoreceptor retinol-binding protein (IRBP), the major protein component of the subretinal space, is in a strategic position to mediate cellular interactions between the retinal pigmented epithelium (RPE) and the neural retina. While IRBP appears to be involved in vitamin A transport during the visual cycle in the adult, the role of this protein during eye development has not been determined. As a first step to understanding the role of IRBP during retinal development, we have studied the expression of the mRNA for this glycolipoprotein during photoreceptor differentiation in the rat. A rat neural retina cDNA library was prepared from which an IRBP clone was isolated. The clone contains an open reading frame followed by a 3' noncoding sequence ending in 10 adenosine residues. The coding region has an identity of 83.9 and 82.5% with the nucleotide sequence of human and bovine IRBP, respectively. Rats (Sprague-Dawley, Wistar, and Royal College of Surgeon pink-eyed controls) have a 6.4 and a 5.2-kb mRNA for IRBP which are present in a 1:4 ratio and thus are the only vertebrate known to definitely have more than one major form of the IRBP message. Genomic Southern blots are consistent with the hypothesis that there is only one allele of the IRBP gene, suggesting that the two forms are produced by alternative processing of the mRNA. To generate an antisense RNA probe for use in molecular titration assays and Northern blots, an Eco RI-Bam HI fragment from the coding region was subcloned in between flanking Sp6 and T7 promoters. Total RNA was prepared from undissected rat globes from postnatal days p0-p22. The expression of the mRNA for IRBP was studied by Northern blots and the level of the transcripts determined by solution hybridization assays. Approximately 10(5) IRBP mRNA transcripts/micrograms total eye RNA are present at birth. This increases to a final level of 3.1 X 10(6) transcripts/micrograms total RNA by p9. The one-half maximal level of the mRNA occurs at p4.2 which is 2 wk before the one-half maximal level of IRBP is reached in the subretinal space (Gonzalez-Fernandez, F., R. A. Landers, P. A. Glazebrook, S.-L. Fong, G. I. Liou, D. M. K. Lam, and C. D. B. Bridges. 1984. J. Cell Biol. 99:2092-2098). The expression of the mRNA for IRBP reflects the developmental emergence of the interphotoreceptor matrix as an important structure within the retina.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Upon absorption of a photon, the 11-cis retinaldehyde chromophore of rhodopsin is isomerized and reduced to all-trans retinol (vitamin A) in the photoreceptor outer segments, whereupon it leaves the photoreceptors, and moves to the retinal pigment epithelium (RPE). To clarify the function of the RPE in the regeneration of 11-cis retinaldehyde, we delivered all-trans retinol to monolayer cultures of human RPE. During delivery the retinol was associated with its putative natural carrier, interphotoreceptor retinoid binding protein (IRBP). IRBP has been proposed as a carrier protein involved in the exchange of retinoids between the photoreceptors and the retinal pigment epithelium. The retinoid composition of RPE cells and culture medium was analyzed by HPLC following several incubation periods. The RPE monolayer was found to process all-trans retinol into two distinct end-products: all-trans retinyl palmitate, which remained within the RPE monolayer: and 11-cis retinaldehyde which was released into the culture medium. These results demonstrate retinoid isomerase, retinol oxidoreductase and retinyl ester synthetase activity in human RPE cells cultured under the appropriate conditions. They show that IRBP can serve as a carrier of retinol through an aqueous medium to the RPE, and they illustrate that the visual cycle can be studied in vitro.

Involvement of the Golgi apparatus in sorting of materials to opposite ends of frog rod retinal photoreceptors

We have studied the rod cells of retinas of Rana pipiens by phosphatase cytochemistry and immunocytochemistry. We find that the Golgi apparatus of these cells, although different in its intracellular distribution from that of other neurons, has a cis-trans organization like that of other neurons as regards morphological features and the distribution of phosphatase activities. Antibodies against opsin bind to several sacs of the rod Golgi apparatus, especially those at the trans side of the Golgi stack. This suggests that Golgi involvement in the packaging of opsin for eventual delivery to the photoreceptive outer segments of the cell involves passage through trans Golgi systems. Proteins destined for the opposite end of the cell--the presynaptic terminal--also seem to pass through trans Golgi systems, as is indicated both by immunocytochemical localization of the synaptic vesicle protein p38 (synaptophysin) and by the presence of thiamine pyrophosphatase activity in some of the synaptic vesicles. Our findings suggest that sorting of membrane proteins destined for opposite ends of the photoreceptor takes place in systems at or near the trans Golgi face.

Endocytosis in the inner segment of rod photoreceptors: analysis of Xenopus laevis retinas using horseradish peroxidase

To evaluate endocytosis in rod photoreceptor inner segments, isolated retinas from Xenopus laevis were maintained in vitro in the presence of the tracer, horseradish peroxidase (HRP). HRP-positive vesicles, 110 +/- 82 nm in diam. (mean +/- S.D.), appeared in the inner segment cytoplasm within 2 min of incubation. Within 8-15 min, 20-30% of the vesicles in the ellipsoid were HRP-positive, reaching approx. 50% by 30-60 min and maintaining this level through 4 hr of culture. In retinas incubated 30 min with HRP and then cultured for an additional period without tracer, the HRP-labeled vesicles were reduced in number by 50- and 75% in the 1- and 2-hr chase incubations, respectively. In retinas cultured for 8 min or longer, multivesicular bodies in the inner segments also contained HRP reaction product, suggesting that the HRP-labeled endocytotic vesicles enter multivesicular bodies. Retinas cultured with HRP for greater than or equal to 1 hr contained significantly more vesicles (per unit area of ellipsoid cytoplasm) as compared with unincubated retinas, retinas incubated from 1 hr in normal media or retinas incubated 30 min with HRP followed by normal chase medium for greater than or equal to 30 min. Taken together, these data indicate that the rod photoreceptor inner segment is capable of extensive endocytotic activity, a process which may function to recover components of the inner segment plasma membrane and the interphotoreceptor matrix.

A phosphatase activity and a synaptic vesicle antigen in multivesicular bodies of frog retinal photoreceptor terminals

Previous work has suggested that multivesicular bodies participate in endocytosis and membrane cycling at nerve terminals, including the presynaptic terminals of retinal photoreceptors. We now have found that multivesicular bodies located in the presynaptic terminals of photoreceptors in retinae of Rana pipiens show reaction product in preparations incubated to demonstrate phosphatase activity at pH 5, using cytidine monophosphate as the substrate. Evidently, multivesicular bodies in photoreceptors can possess at least some hydrolytic enzymes during their sojourn in the terminals. We have also found that the multivesicular bodies in frog retinal photoreceptor terminal stain, immunocytochemically, for the presence of SV2, an antigen of synaptic vesicles. This observation supports the suggestion that, along with the extensive, repeated reuse of membrane components for synaptic vesicle recycling, there is some incorporation of the components into structures that are potentially degradative.

Chondroitin 6-sulfate glycosaminoglycan is a major constituent of primate cone photoreceptor matrix sheaths

Recent work suggests that chemically and structurally distinct domains of the interphotoreceptor matrix ("cone matrix sheaths") surround cone photoreceptor outer segments and ellipsoids. This specific regionalization of at least some molecular constituents of the interphotoreceptor matrix may establish a unique cone-associated microenvironment. Previous histochemical and biochemical investigations have shown that a variety of glycosaminoglycans are components of the interphotoreceptor matrix and that the structural integrity of cone sheaths is slightly disrupted by glycosaminoglycan-degrading enzymes. In order to pursue the possibility that specific glycosaminoglycan species establish cone matrix sheath domains, monoclonal antibodies directed against various unsaturated glycosaminoglycans have been screened on sections of primate retina. The results of these studies identify chondroitin 6-sulfate glycosaminoglycan as a specific component of primate cone matrix sheaths.

Identification of frog photoreceptor plasma and disk membrane proteins by radioiodination

Several functions have been identified for the plasma membrane of the rod outer segment, including control of light-dependent changes in sodium conductance and a sodium-calcium exchange mechanism. However, little is known about its constituent proteins. Intact rod outer segments substantially free of contaminants were prepared in the dark and purified on a density gradient of Percoll. Surface proteins were then labeled by lactoperoxidase-catalyzed radioiodination, and intact rod outer segments were reisolated. Membrane proteins were identified by polyacrylamide gel electrophoresis and autoradiography. The surface proteins labeled included rhodopsin, the major membrane protein, and 12 other proteins. Several control experiments indicated that the labeled proteins are integral membrane proteins and that label is limited to the plasma membrane. To compare the protein composition of plasma membrane with that of the internal disk membrane, purified rod outer segments were lysed by hypotonic disruption or freeze-thawing, and plasma plus disk membranes were radioiodinated. In these membrane preparations, rhodopsin was the major iodinated constituent, with 12 other proteins also labeled. Autoradiographic evidence indicated some differences in protein composition between disk and plasma membranes. A quantitative comparison of the two samples showed that labeling of two proteins, 24 kilodaltons (kDa) and 13 kDa, was enriched in the plasma membrane, while labeling of a 220-kDa protein was enriched in the disk membrane. These plasma membrane proteins may be associated with important functions such as the light-sensitive conductance and the sodium-calcium exchanger.

Disc shedding and autophagy in the cone-dominant ground squirrel retina

The temporal pattern of cone outer-segment disc shedding was examined in the retina of the California ground squirrel, Spermophilus beecheyi, under two different lighting conditions. Squirrels were entrained to either 10-180 lx (room lighting) or 1,400 lx. Cone shedding during the dark period was biphasic in both conditions, with increases occurring at 2-3 hr and 5-6 hr after light offset. Entrainment to 1400 lx resulted in an increase in shedding at 2 hr after light offset and a slight advance in the timing of both peaks. Dense granules were often found in photoreceptors, retinal neurons, Müller cells, microglia and vascular cells. These granules, which were found primarily during the dark period and early light period, were lipofuscin-like in their lipophilia, size and autofluorescence. Many of the granules were probably autophagic in origin, but some within Müller cells may have originated via endocytosis of extracellular material which was exocytosed by photoreceptors.