Vitamin A and the role of the pigment epithelium during bleaching and regeneration of rhodopsin in the frog eye

Non-viral therapeutic approaches to ocular diseases: An overview and future directions

Currently there are no viable treatment options for patients with debilitating inherited retinal degeneration. The vast variability in disease-inducing mutations and resulting phenotypes has hampered the development of therapeutic interventions. Gene therapy is a logical approach, and recent work has focused on ways to optimize vector design and packaging to promote optimized expression and phenotypic rescue after intraocular delivery. In this review, we discuss ongoing ocular clinical trials, which currently use viral gene delivery, but focus primarily on new advancements in optimizing the efficacy of non-viral gene delivery for ocular diseases. Non-viral delivery systems are highly customizable, allowing functionalization to improve cellular and nuclear uptake, bypassing cellular degradative machinery, and improving gene expression in the nucleus. Non-viral vectors often yield transgene expression levels lower than viral counterparts, however their favorable safety/immune profiles and large DNA capacity (critical for the delivery of large ocular disease genes) make their further development a research priority. Recent work on particle coating and vector engineering presents exciting ways to overcome limitations of transient/low gene expression levels, but also highlights the fact that further refinements are needed before use in the clinic.

Ciliary ectosomes: transmissions from the cell's antenna

The cilium is the site of function for a variety of membrane receptors, enzymes and signal transduction modules crucial for a spectrum of cellular processes. Through targeted transport and selective gating mechanisms, the cell localizes specific proteins to the cilium that equip it for the role of sensory antenna. This capacity of the cilium to serve as a specialized compartment where specific proteins can be readily concentrated for sensory reception also makes it an ideal organelle to employ for the regulated emission of specific biological material and information. In this review we present and discuss an emerging body of evidence centered on ciliary ectosomes - bioactive vesicles released from the surface of the cilium.

The culture and maintenance of functional retinal pigment epithelial monolayers from adult human eye

The retinal pigment epithelium (RPE) is implicated in many eye diseases, including age-related macular degeneration, and therefore isolating and culturing these cells from recently deceased adult human donors is the ideal source for disease studies. Adult RPE could also be used as a cell source for transplantation therapy for RPE degenerative disease, likely requiring first in vitro expansion of the cells obtained from a patient. Previous protocols have successfully extracted RPE from adult donors; however improvements in yield, cell survival, and functionality are needed. We describe here a protocol optimized for adult human tissue that yields expanded cultures of RPE with morphological, phenotypic, and functional characteristics similar to freshly isolated RPE. These cells can be expanded and cultured for several months without senescence, gross cell death, or undergoing morphological changes. The protocol takes around a month to obtain functional RPE monolayers with accurate morphological characteristics and normal protein expression, as shown through immunohistochemistry analysis, RNA expression profiles via quantitative PCR (qPCR), and transepithelial resistance (TER) measurements. Included in this chapter are steps used to extract RPE from human adult globes, cell culture, cell splitting, cell bleaching, immunohistochemistry, and qPCR for RPE markers, and TER measurements as functional test.

The cone-specific visual cycle

Cone photoreceptors mediate our daytime vision and function under bright and rapidly-changing light conditions. As their visual pigment is destroyed in the process of photoactivation, the continuous function of cones imposes the need for rapid recycling of their chromophore and regeneration of their pigment. The canonical retinoid visual cycle through the retinal pigment epithelium cells recycles chromophore and supplies it to both rods and cones. However, shortcomings of this pathway, including its slow rate and competition with rods for chromophore, have led to the suggestion that cones might use a separate mechanism for recycling of chromophore. In the past four decades biochemical studies have identified enzymatic activities consistent with recycling chromophore in the retinas of cone-dominant animals, such as chicken and ground squirrel. These studies have led to the hypothesis of a cone-specific retina visual cycle. The physiological relevance of these studies was controversial for a long time and evidence for the function of this visual cycle emerged only in very recent studies and will be the focus of this review. The retina visual cycle supplies chromophore and promotes pigment regeneration only in cones but not in rods. This pathway is independent of the pigment epithelium and instead involves the Müller cells in the retina, where chromophore is recycled and supplied selectively to cones. The rapid supply of chromophore through the retina visual cycle is critical for extending the dynamic range of cones to bright light and for their rapid dark adaptation following exposure to light. The importance of the retina visual cycle is emphasized also by its preservation through evolution as its function has now been demonstrated in species ranging from salamander to zebrafish, mouse, primate, and human.

Exon-skipping variant of RGR opsin in human retina and pigment epithelium

An extraneous exon-skipping mRNA encodes an altered form of a light-absorbing opsin in human retina and pigment epithelium (RPE). The predicted protein variant differs from full-length RPE-retinal G protein-coupled receptor (RGR) by having an in-frame deletion of exon 6, which contains the entire sixth transmembrane domain. To verify that the exon 6-deleted RGR protein (RGR-d) exists in human retinas, we have produced RGR-d antibody probes. In Western blot assays, the RGR-d protein was detected in retinas of a large proportion ( approximately 53%) of individual donors, including patients with age-related macular degeneration (AMD). The relative abundance of RGR-d varied significantly between individuals. The altered protein is expressed in RPE cells and has a more basal subcellular localization that is remarkably different from that of normal RGR opsin. The presence of this exon-skipping variant of RGR in humans may contribute to the progressive derangement of the RPE.

Reduction of all-trans retinal to all-trans retinol in the outer segments of frog and mouse rod photoreceptors

The first step in the Visual Cycle, the series of reactions that regenerate the vertebrate visual pigment rhodopsin, is the reduction of all-trans retinal to all-trans retinol, a reaction that requires NADPH. We have used the fluorescence of all-trans retinol to study this reduction in living rod photoreceptors. After the bleaching of rhodopsin, fluorescence (excitation, 360 nm; emission, 457 or 540 nm) appears in frog and wild-type mouse rod outer segments reaching a maximum in 30-60 min at room temperature. With this excitation and emission, the mitochondrial-rich ellipsoid region of the cells shows strong fluorescence as well. Fluorescence measurements at different emission wavelengths establish that the outer segment and ellipsoid signals originate from all-trans retinol and reduced pyridine nucleotides, respectively. Using outer segment fluorescence as a measure of all-trans retinol formation, we find that in frog rod photoreceptors the NADPH necessary for the reduction of all-trans retinal can be supplied by both cytoplasmic and mitochondrial metabolic pathways. Inhibition of the reduction reaction, either by retinoic acid or through suppression of metabolic activity, reduced the formation of retinol. Finally, there are no significant fluorescence changes after bleaching in the rod outer segments of Rpe65(-/-) mice, which lack 11-cis retinal.

A Palmitoylation Switch Mechanism in the Regulation of the Visual Cycle

RPE65 is essential for the biosynthesis of 11-cis-retinal, the chromophore of rhodopsin. Here, we show that the membrane-associated form (mRPE65) is triply palmitoylated and is a chaperone for all-trans-retinyl esters, allowing their entry into the visual cycle for processing into 11-cis-retinal. The soluble form of RPE65 (sRPE65) is not palmitoylated and is a chaperone for vitamin A, rather than all-trans-retinyl esters. Thus, the palmitoylation of RPE65 controls its ligand binding selectivity. The two chaperones are interconverted by lecithin retinol acyl transferase (LRAT) acting as a molecular switch. Here mRPE65 is a palmitoyl donor, revealing a new acyl carrier protein role for palmitoylated proteins. When chromophore synthesis is not required, mRPE65 is converted into sRPE65 by LRAT, and further chromophore synthesis is blocked. The studies reveal new roles for palmitoylated proteins as molecular switches and LRAT as a palmitoyl transferase whose role is to catalyze the mRPE65 to sRPE65 conversion.

Rpe65 Is a Retinyl Ester Binding Protein That Presents Insoluble Substrate to the Isomerase in Retinal Pigment Epithelial Cells

Photon capture by a rhodopsin pigment molecule induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. To restore light sensitivity, the all-trans-retinaldehyde must be chemically re-isomerized by an enzyme pathway called the visual cycle. Rpe65, an abundant protein in retinal pigment epithelial (RPE) cells and a homolog of beta-carotene dioxygenase, appears to play a role in this pathway. Rpe65-/- knockout mice massively accumulate all-trans-retinyl esters but lack 11-cis-retinoids and rhodopsin visual pigment in their retinas. Mutations in the human RPE65 gene cause a severe recessive blinding disease called Leber's congenital amaurosis. The function of Rpe65, however, is unknown. Here we show that Rpe65 specifically binds all-trans-retinyl palmitate but not 11-cis-retinyl palmitate by a spectral-shift assay, by co-elution during gel filtration, and by co-immunoprecipitation. Using a novel fluorescent resonance energy transfer (FRET) binding assay in liposomes, we demonstrate that Rpe65 extracts all-trans-retinyl esters from phospholipid membranes. Assays of isomerase activity reveal that Rpe65 strongly stimulates the enzymatic conversion of all-trans-retinyl palmitate to 11-cis-retinol in microsomes from bovine RPE cells. Moreover, we show that addition of Rpe65 to membranes from rpe65-/- mice, which possess no detectable isomerase activity, restores isomerase activity to wild-type levels. Rpe65 by itself, however, has no intrinsic isomerase activity. These observations suggest that Rpe65 presents retinyl esters as substrate to the isomerase for synthesis of visual chromophore. This proposed function explains the phenotype in mice and humans lacking Rpe65.

P2Y(2) receptor agonist INS37217 enhances functional recovery after detachment caused by subretinal injection in normal and rds mice

PURPOSE

To evaluate the effects of INS37217 on the recovery of retinal function after experimental retinal detachment induced by subretinal injection.

METHODS

Subretinal injections of 1 micro L of fluorescent microbeads, saline, or INS37217 (1-200 micro M) were made by the transvitreal method in normal (C57BL/6) mice and in mice heterozygous for the retinal degeneration slow (rds) gene. Control, mock-injected animals underwent corneal puncture without injection. Histologic and ERG evaluations were made at 0 to 1 and 8 hours, and 1, 3, 7, 10, 14, and 60 days post injection (PI). DNA fragmentation was evaluated by terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling (TUNEL).

RESULTS

A single subretinal injection of saline solution containing fluorescent beads caused a histologically evident retinal detachment and distributed the microbeads to almost all the subretinal space. Spontaneous reattachment occurred within 24 hours after injection and was accompanied by evident retinal folding that appeared largely resolved by 6 days later. Relative to controls, injection of saline resulted in approximately 40% recovery of dark-adapted a-wave amplitude at 24 hours PI and gradually improved to approximately 90% of controls at 2 months PI. Subretinal injection of saline containing INS37217 (10 micro M) significantly increased rod and cone ERG of normal and rds(+/-) mice at 1 and 10 days PI, when compared with injection of saline alone. Additionally, INS37217 reduced the number of TUNEL-positive photoreceptors and the enhanced rate of reattachment.

CONCLUSIONS

Enhancement of ERG recovery by INS37217 is likely due to reduced retinal folding and cell death associated with detachment. These results support the use of INS37217 to help restore function after therapies that involve subretinal administration of drugs in animal models of retinal diseases.

Synthesis of the all-trans-retinal chromophore of retinal G protein-coupled receptor opsin in cultured pigment epithelial cells

Light-dependent production of 11-cis-retinal by the retinal pigment epithelium (RPE) and normal regeneration of rhodopsin under photic conditions involve the RPE retinal G protein-coupled receptor (RGR) opsin. This microsomal opsin is bound to all-trans-retinal which, upon illumination, isomerizes stereospecifically to the 11-cis isomer. In this paper, we investigate the synthesis of the all-trans-retinal chromophore of RGR in cultured ARPE-hRGR and freshly isolated bovine RPE cells. Exogenous all-trans-[(3)H]retinol is incorporated into intact RPE cells and converted mainly into retinyl esters and all-trans-retinal. The intracellular processing of all-trans-[(3)H]retinol results in physiological binding to RGR of a radiolabeled retinoid, identified as all-trans-[(3)H]retinal. The ARPE-hRGR cells contain a membrane-bound NADPH-dependent retinol dehydrogenase that reacts efficiently with all-trans-retinol but not the 11-cis isomer. The NADPH-dependent all-trans-retinol dehydrogenase activity in isolated RPE microsomal membranes can be linked in vitro to specific binding of the chromophore to RGR. These findings provide confirmation that RGR opsin binds the chromophore, all-trans-retinal, in the dark. A novel all-trans-retinol dehydrogenase exists in the RPE and performs a critical function in chromophore biosynthesis.

A photic visual cycle of rhodopsin regeneration is dependent on Rgr

During visual excitation, rhodopsin undergoes photoactivation and bleaches to opsin and all-trans-retinal. To regenerate rhodopsin and maintain normal visual sensitivity, the all-trans isomer must be metabolized and reisomerized to produce the chromophore 11-cis-retinal in biochemical steps that constitute the visual cycle and involve the retinal pigment epithelium (RPE; refs. 3-8). A key step in the visual cycle is isomerization of an all-trans retinoid to 11-cis-retinol in the RPE (refs. 9-11). It could be that the retinochrome-like opsins, peropsin, or the retinal G protein-coupled receptor (RGR) opsin12-16 are isomerases in the RPE. In contrast to visual pigments, RGR is bound predominantly to endogenous all-trans-retinal, and irradiation of RGR in vitro results in stereospecific conversion of the bound all-trans isomer to 11-cis-retinal. Here we show that RGR is involved in the formation of 11-cis-retinal in mice and functions in a light-dependent pathway of the rod visual cycle. Mutations in the human gene encoding RGR are associated with retinitis pigmentosa.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

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.

The endogenous chromophore of retinal G protein-coupled receptor opsin from the pigment epithelium

The recent identification of nonvisual opsins has revealed an expanding family of vertebrate opsin genes. The retinal pigment epithelium (RPE) and Müller cells contain a blue and UV light-absorbing opsin, the RPE retinal G protein-coupled receptor (RGR, or RGR opsin). The spectral properties of RGR purified from bovine RPE suggest that RGR is conjugated in vivo to a retinal chromophore through a covalent Schiff base bond. In this study, the isomeric structure of the endogenous chromophore of RGR was identified by the hydroxylamine derivatization method. The retinaloximes derived from RGR in the dark consisted predominantly of the all-trans isomer. Irradiation of RGR with 470-nm monochromatic or near-UV light resulted in stereospecific isomerization of the bound all-trans-retinal to an 11-cis configuration. The stereospecificity of photoisomerization of the all-trans-retinal chromophore of RGR was lost by denaturation of the protein in SDS. Under the in vitro conditions, the photosensitivity of RGR is at least 34% that of bovine rhodopsin. These results provide evidence that RGR is bound in vivo primarily to all-trans-retinal and is capable of operating as a stereospecific photoisomerase that generates 11-cis-retinal in the pigment epithelium.

Rhodopsin and phototransduction

Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.

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

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

Retinol kinetics in the isolated retina determined by retinoid extraction and HPLC

Suzuki et al. [Vis. Res. 26, 425-9 (1986); Vis. Res. 28, 1061-70 (1988)] have described a formaldehyde-based (HCHO-based) extraction procedure that efficiently recovers 11-cis retinal initially present as rhodopsin chromophore in photoreceptor membranes. Using the isolated retina of the toad (Bufo marinus), we tested whether this procedure ('HCHO' method), in combination with a formaldehyde-free extraction procedure ('i/h' method) and the analysis of extracted retinoids by high performance liquid chromatography (HPLC), can account quantitatively for light-induced changes in retinoid levels and thus serve as an alternative to spectrophotometry for tracking the formation of all-trans retinol in this intact rod preparation. Initially dark-adapted retinas were incubated in bright light or in darkness and then analysed by homogenization and extraction using the HCHO and i/h methods. Combined data obtained using the two extraction procedures indicated a near-conservation of total retinoid recovered from dark-incubated and illuminated retinas, and thus accounted for light-induced changes in retinoid levels. The HCHO procedure, employing formaldehyde, isopropanol and hexane, was similar to that described by Suzuki et al. and recovered retinaldehydes including chromophoric 11-cis retinal. The i/h procedure utilized isopropanol and hexane and, unlike the HCHO method, efficiently recovered all-trans retinol. Illumination (onset at time zero) that produced an approximately exponential decline of 11-cis retinal (time constant of 24 s) led to an increase and then a gradual decline in all-trans retinal. The normalized peak level of all-trans retinal, representing about 0.54 of the total molar quantity of recovered retinoid, developed with illumination periods of 10-80 s. The normalized level of all-trans retinol reached approximately 0.3 in retinas illuminated for 1 min and, with longer illuminations (up to 30 min), exhibited an approximately exponential further growth to approximately 0.9 with a time constant of 9.2 min. The results indicate the workability of the HCHO and i/h extraction procedures for tracking the in situ conversion of all-trans retinal to all-trans retinol, a reaction thought to be important for both operation of the retinoid visual cycle and shut-off of the phototransduction cascade.

Identification and characterization of P2Y2 nucleotide receptors in human retinal pigment epithelial cells

P2 nucleotide receptor expression in cultured human retinal pigment epithelial (RPE) cells was investigated using the photoaffinity ATP analog BzATP, polymerase chain reaction of reverse-transcribed RNA (RT-PCR) and fura-2 fluorescence measurement of changes in intracellular free calcium concentration ([Ca2+]i). In experiments carried out in RPE cells at passage 10-15, addition of micromolar concentrations of ATP, UTP, and ATPgammaS to RPE cells resulted in a rapid, transient 3.5-fold increase in [Ca2+]i followed by a prolonged elevation that was twofold above the original baseline. Similar results were obtained from cells at passage 2. Characteristics of nucleotide-stimulated calcium mobilization in RPE cells, including partial inhibition by pertussis toxin, suggest that a G protein-coupled receptor mediates this response. Consistent with the expression of a P2Y2 nucleotide receptor subtype in RPE cells, [alpha-32P]BzATP labeled a 53-kDa protein in plasma membranes, and RT-PCR revealed the presence of P2Y2 receptor RNA. Adenosine had no effect on [Ca2+]i in RPE cells, indicating that the A2 subtype of P1 receptor described previously in human RPE is not involved in the response to nucleotides. Together the results indicate that human RPE cells express functional P2Y2 nucleotide receptors.

Rod-opsin immunoreaction in the pineal organ of the pigmented mouse does not indicate the presence of a functional photopigment

The aim of the present study was to characterize the rod-opsin immunoreaction in the mammalian pineal organ. Pigmented mice (strain C57BL) were selected as the animal model. Immunocytochemical investigations involving the use of highly specific polyclonal and monoclonal antibodies against bovine rod-opsin (the apoprotein of the photopigment rhodopsin) showed that approximately 25% of all pinealocytes were rod-opsin immunoreactive. Immunoblotting techniques revealed three protein bands of approximately 40, 75, and 110 kDa; these were detected by the monoclonal antibody and the polyclonal antiserum in retinal and pineal extracts. These protein bands presumably represented the monomeric, dimeric and trimeric forms of rod-opsin. The amount of rod-opsin in retina and pineal organ was quantified by means of an enzyme-linked immunosorbent assay. This yielded 570 +/- 30 pmoles rod-opsin per eye and 0.3 +/- 0.05 pmoles rod-opsin per pineal organ. High pressure liquid chromatography analysis of whole eye extracts demonstrated the chromophoric group of the photopigment rhodopsin, 11-cis retinal, and its isomer, all-trans-retinal. A shift from 11-cis retinal to all-trans-retinal was found upon light adaptation. No retinals were detected in the pineal organ. Autoradiographic investigations showed that 3H-retinol, intraperitoneally injected into the animals, was incorporated into the outer and inner segments of retinal photoreceptors, but not into the pineal organ. It is concluded that the mouse pineal organ contains the authentic apoprotein of rhodopsin but that it lacks retinal derivatives as essential components of all known vertebrate photopigments.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinol esterification in bovine retinal pigment epithelium: reversibility of lecithin:retinol acyltransferase

Esterification of all-trans-retinol is a key reaction of the vertebrate visual cycle, since it produces an insoluble, relatively non-toxic, form of the vitamin for storage and supplies substrate for the isomerization reaction. CoA-dependent and -independent pathways have been described for retinol esterification in retinal pigment epithelium (RPE). The CoA-independent reaction, catalysed by lecithin:retinol acyltransferase (LRAT) was examined in more detail in this study. Addition of retinol to RPE microsomes results in a burst of retinyl ester synthesis, followed by a rapid apparent cessation of the reaction. However, [3H]retinol, added when retinyl ester synthesis has apparently ceased, is rapidly incorporated into retinyl ester without a net increase in the amount of ester. The specific radioactivities of [3H]retinol and [3H]retinyl ester reach the same value. [14C]Palmitate from palmitoyl-CoA is incorporated into preexisting retinyl ester in the absence of net ester synthesis, too. These exchange reactions suggest that the reaction has reached equilibrium at the plateau of the progress curve and that only the accumulation of retinyl ester, and not its synthesis, has stopped during this phase of the reaction. Studies with geometrical isomers of retinol revealed that the rate of exchange of all-trans-retinol with all-trans-retinyl esters was about 6 times more rapid than exchange of 11-cis-retinol with 11-cis-retinyl ester. This is the first demonstration of the reversibility of LRAT and the first example of stereospecificity of retinyl ester synthesis in the visual system. Reversal of the LRAT reaction could contribute to the mobilization of 11-cis-retinol from 11-cis-retinyl ester pools.

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.

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.

Structural alterations in retinal tissues from rats deficient in vitamin E and selenium and treated with hyperbaric oxygen

Vitamin E and selenium play key roles in preventing in vitro lipid peroxidation and free radical damage to retinal tissues. In this research, we studied the effects of hyperbaric oxygen on retinal structure in rats fed diets deficient in vitamin E and/or selenium. We also correlated any alterations in retinal structure with previously measured alterations in electroretinograms (ERGs). Age-matched rats were fed a basal diet deficient in both vitamin E and selenium (B diet), a basal diet supplemented with vitamin E alone (B+E diet), or selenium alone (B+Se diet), or with both micronutrients (B+E+Se). Half the rats in each group were treated (+HBO) with hyperbaric oxygen (100% O2 at 3 ATA for 1.5 per hr day, 5 days per week) and half were not (-HBO). We previously found that the rats fed the B diet for 6 weeks and treated with HBO for 4 weeks (B+HBO group) had diminished a-wave ERG amplitudes. At this time point all rats in the B group and half of the rats in the B+E+Se group were killed for the structural studies reported here. Surprisingly, we found no evidence of photoreceptor cell necrosis [i.e. a decreased thickness of the outer nuclear layer (ONL)] in retinas from rats in the B+HBO group despite the diminished amplitude of the a-wave which arises from this retinal layer. Quantitative structural analyses of retinas from rats in the B+HBO, B-HBO, B+E+Se-HBO and B+E+Se+HBO groups also failed to reveal any significant differences in the cell height of the retinal pigmented epithelium (nasal, central or temporal regions) or the number of mitochondria, phagosomas or inclusion bodies in the central retinal pigment epithelium (RPE). The inner nuclear layer (INL) thickness was, however, consistently decreased in all retinal regions for the rats in the B+HBO group. Our previous work also showed that only rats fed the B+Se diet for 17 weeks and treated with HBO for 15 weeks (B+Se+HBO group) showed diminished a-wave and b-wave ERG amplitudes. At this time point rats in the B+E+Se, B+E, and B+Se groups were killed for structural studies reported here. Only rats in the B+Se+HBO group showed a significantly decreased (about 20%) thickness of the central ONL. This evidence of photoreceptor cell necrosis correlated very well with our previous observation of diminished a- and b-wave amplitudes only in the B+Se+HBO group (at week 17).(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal photoisomerase: role in invertebrate visual cells

In invertebrate visual cells, the rhodopsin content is maintained at a high level by the fast process of photoregeneration during daylight. Rhodopsin is converted by photoabsorption to metarhodopsin, which is reconverted to rhodopsin by light. In addition, rhodopsin is regenerated by a slow process of renewal which takes days to complete and involves the biosynthesis of opsin. It is well known that rhodopsin can be formed from opsin only when 11-cis-retinal is present; this requires the existence of an isomerizing enzyme which is capable of transforming all-trans-retinal, released from the degradation of metarhodopsin, into the 11-cis-retinal isomer. In some invertebrate visual systems, experiments on rhodopsin regeneration have been interpreted by assuming that the isomerization reaction is a light-dependent process involving a retinal-protein complex. Two retinal photoisomerases which have been well characterized, i.e. bee photoisomerase and cephalopod retinochrome, are reviewed here. Their properties are compared in order to determine their physiological role, which is likely to be in the renewal of visual pigment rhodopsin. To conclude, a visual pigment cycle is proposed in which rhodopsin regeneration follows two light-dependent pathways. This greatly simplifies the rhodopsin regeneration scheme for invertebrate visual systems.

Vitamin neurotoxicity

Vitamins contain reactive functional groups necessary to their established roles as coenzymes and reducing agents. Their reactive potential may produce injury if vitamin concentration, distribution, or metabolism is altered. However, identification of vitamin toxicity has been difficult. The only well-established human vitamin neurotoxic effects are those due to hypervitaminosis A (pseudotumor cerebri) and pyridoxine (sensory neuropathy). In each case, the neurological effects of vitamin deficiency and vitamin excess are similar. Closely related to the neurological symptoms of hypervitaminosis A are symptoms including headache, pseudotumor cerebri, and embryotoxic effects reported in patients given vitamin A analogs or retinoids. Most tissues contain retinoic acid (RA) and vitamin D receptors, members of a steroid receptor superfamily known to regulate development and gene expression. Vitamin D3 effects on central nervous system (CNS) gene expression are predictable, in addition to the indirect effects owing to its influence on calcium and phosphorus homeostasis. Folates and thiamine cause seizures and excitation when administered in high dosage directly into the brain or cerebrospinal fluid (CSF) of experimental animals but have rarely been reported to cause human neurotoxicity, although fatal reactions to i.v. thiamine are well known. Ascorbic acid influences CNS function after peripheral administration and influences brain cell differentiation and 2-deoxyglucose accumulation by cultured glial cells. Biotin influences gene expression in animals that are not vitamin-deficient and alters astrocyte glucose utilization. The multiple enzymes and binding proteins involved in regeneration of retinal vitamin A illustrate the complexity of vitamin processing in the body. Vitamin A toxicity is also a good general model of vitamin neurotoxicity, because it shows the importance of the ratio of vitamin and vitamin-binding proteins in producing vitamin toxicity and of CNS permeability barriers. Because vitamin A and analogs enter the CNS better than most vitamins, and because retinoids have many effects on enzyme activity and gene expression, Vitamin A neurotoxicity is more likely than that of most, perhaps all other vitamins. Megadose vitamin therapy may cause injury that is confused with disease symptoms. High vitamin intake is more hazardous to peripheral organs than to the nervous system, because CNS vitamin entry is restricted. Vitamin administration into the brain or CSF, recommended in certain disease states, is hazardous and best avoided. The lack of controlled trials prevents us from defining the lowest human neurotoxic dose of any vitamin. Large differences in individual susceptibility to vitamin neurotoxicity probably exist, and ordinary vitamin doses may harm occasional patients with genetic disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

Guanine nucleotide-binding regulatory proteins in retinal pigment epithelial cells

The expression of GTP-binding regulatory proteins (G proteins) in retinal pigment epithelial (RPE) cells was analyzed by RNA blot hybridization and cDNA amplification. Both adult and fetal human RPE cells contain mRNA for multiple G protein alpha subunits (G alpha) including Gs alpha, Gi-1 alpha, Gi-2 alpha, Gi-3 alpha, and Gz alpha (or Gx alpha), where Gs and Gi are proteins that stimulate or inhibit adenylyl cyclase, respectively, and Gz is a protein that may mediate pertussis toxin-insensitive events. Other G alpha-related mRNA transcripts were detected in fetal RPE cells by low-stringency hybridization to Gi-2 alpha and Gs alpha protein-coding cDNA probes. The diversity of G proteins in RPE cells was further studied by cDNA amplification with reverse transcriptase and the polymerase chain reaction. This approach revealed that, besides the above mentioned members of the G alpha gene family, at least two other G alpha subunits are expressed in RPE cells. Human retinal cDNA clones that encode one of the additional G alpha subunits were isolated and characterized. The results indicate that this G alpha subunit belongs to a separate subfamily of G proteins that may be insensitive to inhibition by pertussis toxin.

Pathogenesis and reversibility of retinopathy induced by 1,4-bis (4-aminophenoxy)-2-phenylbenzene (2-phenyl-APB-144) in pigmented rats

Pigmented male rats were administered a suspension of 0, 25, or 100 mg/kg 2-phenyl-APB-144 in corn oil by gavage. The rats were killed at 4 and 12 h, and at 1, 2, 4, 7, 14, 28, 57, or 84 days after a single oral administration. The primary site of retinopathy appeared to be the retinal pigment epithelial (RPE) cells. The RPE cells showed necrosis within 12 h post-exposure (PE) at 25 mg/kg and within 4 h PE at 100 mg/kg. Subsequently, photoreceptor outer segments (POS) were disrupted with a hyperplastic RPE cell response within 2 days PE. Intact photoreceptor inner segments (PIS) and RPE cells apposing closely with distal POS were important determinants for reversibility of the damaged POS. The damaged RPE cells were regenerated prior to restoration of normal POS. At 25 mg/kg, both the RPE cells and POS were damaged, but PIS were intact. The damaged POS were regenerated from intact PIS with closely apposing RPE cells. By 14 days PE, the damaged POS had partially regenerated and attained approximately one third to one half of their normal length. By 57 and 84 days PE, the damaged retina had regained an essentially normal structure. In contrast, at 100 mg/kg, the POS and PIS were extensively disrupted, with marked RPE cell hyperplasia after 7 days PE resulting in the formation of multiple retinal arcades (foldings), and rosettes by 14 days PE. Subsequently, retinal arcades and rosettes gradually disappeared as the result of extensive loss of PIS and POS with progressive migration of photoreceptor nuclei toward the Bruch's membrane after 28 days PE. Focal regeneration of POS was observed by 57 days PE where intact PIS and a single layer of regenerated RPE cells were apposed closely with distal POS. The POS regeneration did not occur where the RPE cells were denuded or hyperplastic RPE cells were present. The hyperplastic RPE cells were devoid of slender apical processes, closely enclosing the distal POS. Approximately 20-30% of the retina had partially regained a normal structure by 84 days PE.

Photoreceptor differentiation in cerebellar medulloblastoma: evidence for a functional photopigment and authentic S-antigen (arrestin)

The aim of the present study was to evaluate the putative photoreceptor differentiation found in certain cerebellar medulloblastomas. The analyses were focussed on S-antigen, rod-opsin (the apoprotein of the visual pigment rhodopsin) and 11-cis retinal (the prosthetic group of rhodopsin). Fresh frozen and paraffin-embedded biopsy specimens of three medulloblastomas were investigated by means of immunocytochemistry, enzyme-linked immunosorbent assay (ELISA), high-pressure liquid chromatography (HPLC), and immunoblotting. As shown in paraffin sections, one out of the three tumors (tumor A) contained S-antigen- and rod-opsin-immunoreactive tumor cells. The immunoblotting technique revealed in this tumor a single protein band of approximately 48-50 kDa that reacted with the S-antigen antibody and three protein bands of approximately 40, 75 and 110 kDa recognized by the rod-opsin antibody. These bands could not be detected in the two remaining tumors (tumor B and C). The rod-opsin content of tumor A was quantified by the ELISA; 11.7 pmol rod-opsin were calculated for the biopsy. The HPLC demonstrated the presence of 11-cis- and all-trans-retinal in tumor A, but not in tumors B and C. Furthermore, it was shown that 11-cis-retinal was converted to all-trans-retinal upon illumination of the tumor extract. The ratio between 11-cis- and all-trans-retinal was approximately 1:1 before illumination and 3:5 after illumination. A total of 2-3 pmol of retinal was found in the biopsy of tumor A. In addition all-trans-retinol was present in this tumor. The results indicate that certain medulloblastomas express a functional photopigment and S-antigen, another protein of the phototransduction cascade. They strongly support the concept that medulloblastoma cells may differentiate along the photoreceptor cell lineage.

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.

Distribution of vitamin A compounds in bovine eyes after bleaching adaptation

A seasonal increase in the amount of bleached rhodopsin caused, in living animals, by the seasonal increase of the intensity of sunlight in the early morning before the calves are killed, was verified in the bovine eyes subjected to the present study. This was used as a means of assaying distribution and isomer composition of esterified and unesterified retinol in eyes from animals light-adapted to a different extent under environmental conditions. The progressive increase of bleached rhodopsin results in a parallel increase of all-trans-retinol in retina and of both all-trans- and 11-cis-retinyl esters in pigment epithelium. Analytical subcellular fractionation of RPE homogenate reveals that retinyl esters accumulate without an exclusive subcellular localization in nuclear, mitochondrial/lysosomal and microsomal fractions. Whatever the amount of bleached rhodopsin, only small and constant amounts of retinyl esters are found in the soluble fraction of RPE, entirely under the all-trans configuration. When a considerable portion of rhodopsin is bleached (about 70%), substantial amounts of all-trans-retinol, along with minor amounts of 11-cis-retinol, accumulate in RPE subcellular organelles. The in vitro bleaching of bovine eyes results in a distribution of retinoids between retina and RPE which appears different from that detected in eyes naturally bleached to the same extent.

Requirement of insulin or IGF-1 for the maintenance of retinyl ester synthetase activity by cultured retinal pigment epithelial cells

Previous work from these laboratories showed that the retention of retinyl ester synthetase activity by cultured human retinal pigment epithelium is up to tenfold greater with PM medium (Medium 199 plus insulin, other added defined components, 1% serum and 1% retina extract) than with conventional culture media. The present work shows that insulin is the component of PM medium required for maintenance of ester synthetase activity and that insulin-like growth factor type 1 (IGF-1) also is effective at maintaining ester synthesis. In addition, insulin can maintain ester synthetase activity in cultured rat RPE. Preliminary dose-response measurements provide additional support for these findings and strongly suggest that both insulin and IGF-1 are maximally effective at physiological concentrations (1-10 ng ml-1).

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.

Interphotoreceptor retinoid-binding protein promotes rhodopsin regeneration in toad photoreceptors

Interphotoreceptor retinoid-binding protein (IRBP) has been hypothesized to function as an intercellular shuttle in the vertebrate eye, serving to transport retinoids between the retinal pigment epithelium (RPE) and photoreceptors in the process by which visual pigment is regenerated after photolysis. This hypothesis was tested in preparations utilizing the toad (Bufo marinus) eye and purified, initially ligand-free IRBP obtained from the bovine eye. Rod outer segments (ROS) or neural retinas were isolated and bleached, then incubated with native RPE (RPE-eyecup) in the presence or absence of IRBP. The amount of rhodopsin present after incubation was determined by spectrophotometric analysis and compared with that in control preparations receiving bovine serum albumin or Ringer's solution only. Supplementation with IRBP enhanced the formation of rhodopsin in both the ROS/RPE-eyecup and retina/RPE-eyecup preparations. Regeneration in ROS/RPE-eyecups receiving IRBP (1.8 nmol) increased in a roughly linear manner with the period of incubation (0-4 hr), at a rate of 0.44 nmol/hr. The extent of regeneration was graded with the quantities of IRBP and opsin introduced into the RPE-eyecup. With increasing amounts of IRBP (up to 5.2 nmol) or of initially available opsin (up to 15.6 nmol), the amount of rhodopsin formed (3-hr incubation) approached the same plateau value, about 2.5 nmol. Analysis of IRBP-supplemented Ringer's solution incubated in the RPE-eyecup showed 11-cis-retinal to be virtually the only retinoid withdrawn from the RPE. With large quantities of IRBP (3.2-9.2 nmol), the amount of 11-cis-retinal (2.7 +/- 0.5 nmol) withdrawn from the RPE during a 3-hr incubation was similar to the plateau value of rhodopsin formed in the ROS/RPE-eyecup. No 11-cis-retinal was observed in albumin-supplemented Ringer's solution (0.4-11.2 nmol of bovine serum albumin) or in Ringer's alone after similar incubation in the RPE-eyecup. The results suggest that an IRBP-mediated transfer of 11-cis-retinal from the RPE to the rods supports rhodopsin regeneration in vivo.

Light-evoked changes in the interphotoreceptor matrix

The normal function of vertebrate photoreceptor cells depends on multiple interactions and transfer of substances between the photoreceptors and the retinal pigment epithelium (RPE), but the mechanisms of these interactions are poorly understood. Many are thought to be mediated by the interphotoreceptor matrix (IPM), a complex extracellular matrix that surrounds the photoreceptors and lies between them and the RPE. Histochemical, immunocytochemical, and lectin probes for several IPM constituents revealed that components of the IPM in the rat undergo a major shift in distribution or molecular conformation after the transition between light and dark. In the light, various IPM constituents concentrated in bands at the apical and basal regions of the outer segment zone; in the dark, they distributed much more uniformly throughout the zone. The change in IPM distribution was triggered by the light-dark transition; it was not a circadian event, and it was not driven by a systemic factor. The light-evoked change in IPM distribution may facilitate the transfer of substances between the photoreceptors and the RPE.

Beta-glucuronidase mediated pathway essential for retinal pigment epithelial degradation of glycosaminoglycans. Disease expression and in vitro disease correction using retroviral mediated cDNA transfer

A beta-glucuronidase mediated pathway for the degradation of glycosaminoglycans is present in the retinal pigment epithelium. The pathway has been defined using ocular tissues and cultured cells from mutant animals having a recessively inherited deficiency of the lysosomal enzyme. In situ, storage products accumulate in secondary lysosomes of the retinal pigment epithelium, the cytoplasm fills with inclusions and the cells hypertrophy; severity of the disease increases with aging. Deficient activity of beta-glucuronidase is present in primary and second passage cultures. Radiolabel studies with 35SO4 show a significant retention of cell layer label by mutant retinal pigment epithelial cells during a 72-hr pulse or 24-hr chase period. The labels is in newly synthesized chondroitin sulfate and heparan sulfate, which are natural substrates for the deficient enzyme. There is no difference from normal in the total radioactivity and electrophoretic profile of the glycosaminoglycans that are synthesized and released into the media. A retroviral vector was used to transfer normal rat beta-glucuronidase cDNA into the mutant cells. The vector treatment results in restoration of enzyme activity and correction of the degradative defect; 35SO4 labeling shows that chondroitin sulfate and heparan sulfate levels return to normal. The vector treatment studies indicate that a single gene defect determines the abnormal beta-glucuronidase mediated pathway in the mutant retinal pigment epithelium.