Interphotoreceptor retinoid-binding protein: role in delivery of retinol to the pigment epithelium

CryoEM structure and small-angle X-ray scattering analyses of porcine retinol-binding protein 3

The vertebrate visual cycle hinges on enzymatically converting all-trans-retinol (at-ROL) into 11-cis-retinal (11c-RAL), the chromophore that binds to opsins in photoreceptors, forming light-responsive pigments. When struck by a photon, these pigments activate the phototransduction pathway and initiate the process of vision. The enzymatic isomerization of at-ROL, crucial for restoring the visual pigments and preparing them to receive new light stimuli, relies on various enzymes found in both the photoreceptors and retinal pigment epithelium cells. To function effectively, retinoids must shuttle between these two cell types. Retinol-binding protein 3 (RBP3), located in the interphotoreceptor matrix, probably plays a pivotal role in this transport mechanism. Comprised of four retinoid-binding modules, RBP3 also binds fatty acids, potentially aiding retinal function by facilitating the loading and unloading of different retinoids at specific cell types thereby directing the cycle. In this study, we present a 3.67 Å cryoEM structure of porcine RBP3, along with molecular docking analysis and corroborative in-solution small-angle X-ray scattering data for titration of RBP3 with relevant ligands, that also give insights on RBP3 conformational adaptability.

Retinal Responses to Visual Stimuli in Interphotoreceptor Retinoid Binding-Protein Knock-Out Mice

Interphotoreceptor retinoid-binding protein (IRBP) is an abundant glycoprotein in the subretinal space bound by the photoreceptor (PR) outer segments and the processes of the retinal pigmented epithelium (RPE). IRBP binds retinoids, including 11-cis-retinal and all-trans-retinol. In this study, visual function for demanding visual tasks was assessed in IRBP knock-out (KO) mice. Surprisingly, IRBP KO mice showed no differences in scotopic critical flicker frequency (CFF) compared to wildtype (WT). However, they did have lower photopic CFF than WT. IRBP KO mice had reduced scotopic and photopic acuity and contrast sensitivity compared to WT. IRBP KO mice had a significant reduction in outer nuclear layer (ONL) thickness, PR outer and inner segment, and full retinal thickness (FRT) compared to WT. There were fewer cones in IRBP KO mice. Overall, these results confirm substantial loss of rods and significant loss of cones within 30 days. Absence of IRBP resulted in cone circuit damage, reducing photopic flicker, contrast sensitivity, and spatial frequency sensitivity. The c-wave was reduced and accelerated in response to bright steps of light. This result also suggests altered retinal pigment epithelium activity. There appears to be a compensatory mechanism such as higher synaptic gain between PRs and bipolar cells since the loss of the b-wave did not linearly follow the loss of rods, or the a-wave. Scotopic CFF is normal despite thinning of ONL and reduced scotopic electroretinogram (ERG) in IRBP KO mice, suggesting either a redundancy or plasticity in circuits detecting (encoding) scotopic flicker at threshold even with substantial rod loss.

Interphotoreceptor Retinol-Binding Protein Ameliorates Diabetes-Induced Retinal Dysfunction and Neurodegeneration Through Rhodopsin

Patients with diabetes often experience visual defects before any retinal pathologies are detected. The molecular mechanism for the visual defects in early diabetes has not been elucidated. Our previous study reported that in early diabetic retinopathy (DR), rhodopsin levels were reduced due to impaired 11-cis-retinal regeneration. Interphotoreceptor retinol-binding protein (IRBP) is a visual cycle protein and important for 11-cis-retinal generation. IRBP levels are decreased in the vitreous and retina of DR patients and animal models. To determine the role of IRBP downregulation in the visual defects in early DR, we induced diabetes in transgenic mice overexpressing IRBP in the retina. IRBP overexpression prevented diabetes-induced decline of retinal function. Furthermore, IRBP overexpression also prevented decreases of rhodopsin levels and 11-cis-retinal generation in diabetic mice. Diabetic IRBP transgenic mice also showed ameliorated retinal oxidative stress, inflammation, apoptosis, and retinal degeneration compared with diabetic wild-type mice. These findings suggest that diabetes-induced IRBP downregulation impairs the regeneration of 11-cis-retinal and rhodopsin, leading to retinal dysfunction in early DR. Furthermore, increased 11-cis-retinal-free opsin constitutively activates the phototransduction pathway, leading to increased oxidative stress and retinal neurodegeneration. Therefore, restored IRBP expression in the diabetic retina may confer a protective effect against retinal degeneration in DR.

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

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

Interphotoreceptor retinoid-binding protein removes all-trans-retinol and retinal from rod outer segments, preventing lipofuscin precursor formation

Interphotoreceptor retinoid-binding protein (IRBP) is a specialized lipophilic carrier that binds the all-trans and 11-cis isomers of retinal and retinol, and this facilitates their transport between photoreceptors and cells in the retina. One of these retinoids, all-trans-retinal, is released in the rod outer segment by photoactivated rhodopsin after light excitation. Following its release, all-trans-retinal is reduced by the retinol dehydrogenase RDH8 to all-trans-retinol in an NADPH-dependent reaction. However, all-trans-retinal can also react with outer segment components, sometimes forming lipofuscin precursors, which after conversion to lipofuscin accumulate in the lysosomes of the retinal pigment epithelium and display cytotoxic effects. Here, we have imaged the fluorescence of all-trans-retinol, all-trans-retinal, and lipofuscin precursors in real time in single isolated mouse rod photoreceptors. We found that IRBP removes all-trans-retinol from individual rod photoreceptors in a concentration-dependent manner. The rate constant for retinol removal increased linearly with IRBP concentration with a slope of 0.012 min^-1 μm^-1 IRBP also removed all-trans-retinal, but with much less efficacy, indicating that the reduction of retinal to retinol promotes faster clearance of the photoisomerized rhodopsin chromophore. The presence of physiological IRBP concentrations in the extracellular medium resulted in lower levels of all-trans-retinal and retinol in rod outer segments following light exposure. It also prevented light-induced lipofuscin precursor formation, but it did not remove precursors that were already present. These findings reveal an important and previously unappreciated role of IRBP in protecting the photoreceptor cells against the cytotoxic effects of accumulated all-trans-retinal.

Formation and Clearance of All-Trans-Retinol in Rods Investigated in the Living Primate Eye With Two-Photon Ophthalmoscopy

Purpose

Two-photon excited fluorescence (TPEF) imaging has potential as a functional tool for tracking visual pigment regeneration in the living eye. Previous studies have shown that all-trans-retinol is likely the chief source of time-varying TPEF from photoreceptors. Endogenous TPEF from retinol could provide the specificity desired for tracking the visual cycle. However, in vivo characterization of native retinol kinetics is complicated by visual stimulation from the imaging beam. We have developed an imaging scheme for overcoming these challenges and monitored the formation and clearance of retinol.

Methods

Three macaques were imaged by using an in vivo two-photon ophthalmoscope. Endogenous TPEF was excited at 730 nm and recorded through the eye's pupil for more than 90 seconds. Two-photon excited fluorescence increased with onset of light and plateaued within 40 seconds, at which point, brief incremental stimuli were delivered at 561 nm. The responses of rods to stimulation were analyzed by using first-order kinetics.

Results

Two-photon excited fluorescence resulting from retinol production corresponded to the fraction of rhodopsin bleached. The photosensitivity of rhodopsin was estimated to be 6.88 ± 5.50 log scotopic troland. The rate of retinol clearance depended on intensity of incremental stimulation. Clearance was faster for stronger stimuli and time constants ranged from 50 to 300 seconds.

Conclusions

This study demonstrates a method for rapidly measuring the rate of clearance of retinol in vivo. Moreover, TPEF generated due to retinol can be used as a measure of rhodopsin depletion, similar to densitometry. This enhances the utility of two-photon ophthalmoscopy as a technique for evaluating the visual cycle in the living eye.

RPE65 and the Accumulation of Retinyl Esters in Mouse Retinal Pigment Epithelium

The RPE65 protein of the retinal pigment epithelium (RPE) enables the conversion of retinyl esters to the visual pigment chromophore 11-cis retinal. Fresh 11-cis retinal is generated from retinyl esters following photoisomerization of the visual pigment chromophore to all-trans during light detection. Large amounts of esters accumulate in Rpe65-/- mice, indicating their continuous formation when 11-cis retinal generation is blocked. We hypothesized that absence of light, by limiting the conversion of esters to 11-cis retinal, would also result in the build-up of retinyl esters in the RPE of wild-type mice. We used HPLC to quantify ester levels in organic extracts of the RPE from wild-type and Rpe65-/- mice. Retinyl ester levels in Sv/129 wild-type mice that were dark adapted for various intervals over a 4-week period were similar to those in mice raised in cyclic light. In C57BL/6 mice however, which contain less Rpe65 protein, dark adaptation was accompanied by an increase in ester levels compared to cyclic light controls. Retinyl ester levels were much higher in Rpe65-/- mice compared to wild type and kept increasing with age. The results suggest that the RPE65 role in retinyl ester homeostasis extends beyond enabling the formation of 11-cis retinal.

Vitamin A and Vision

Visual systems detect light by monitoring the effect of photoisomerization of a chromophore on the release of a neurotransmitter from sensory neurons, known as rod and cone photoreceptor cells in vertebrate retina. In all known visual systems, the chromophore is 11-cis-retinal complexed with a protein, called opsin, and photoisomerization produces all-trans-retinal. In mammals, regeneration of 11-cis-retinal following photoisomerization occurs by a thermally driven isomerization reaction. Additional reactions are required during regeneration to protect cells from the toxicity of aldehyde forms of vitamin A that are essential to the visual process. Photochemical and phototransduction reactions in rods and cones are identical; however, reactions of the rod and cone visual pigment regeneration cycles differ, and perplexingly, rod and cone regeneration cycles appear to use different mechanisms to overcome the energy barrier involved in converting all-trans- to 11-cis-retinoid. Abnormal processing of all-trans-retinal in the rod regeneration cycle leads to retinal degeneration, suggesting that excessive amounts of the retinoid itself or its derivatives are toxic. This line of reasoning led to the development of various approaches to modifying the activity of the rod visual cycle as a possible therapeutic approach to delay or prevent retinal degeneration in inherited retinal diseases and perhaps in the dry form of macular degeneration (geographic atrophy). In spite of great progress in understanding the functioning of rod and cone regeneration cycles at a molecular level, resolution of a number of remaining puzzling issues will offer insight into the amelioration of several blinding retinal diseases.

Kinetics of rhodopsin's chromophore monitored in a single photoreceptor

Absorption of light isomerizes the retinyl chromophore of the photoreceptor pigment rhodopsin from 11-cis to all-trans, generating the photoactivated rhodopsin form. The photoisomerization of the chromophore however destroys rhodopsin, and its regeneration requires the removal of the all-trans and the supply of fresh 11-cis chromophore. The all-trans chromophore is removed through a series of steps beginning with its release from photoactivated rhodopsin in the form of all-trans-retinal, leaving behind the apoprotein opsin. All-trans-retinal is then reduced to all-trans-retinol, which is transported out of the photoreceptor. Rhodopsin is regenerated from opsin and fresh 11-cis-retinal arriving to the photoreceptor from the retinal pigment epithelium. Both all-trans and 11-cis-retinal can form precursors of lipofuscin, a pigment that accumulates with age in the lysosomal compartment of the retinal pigment epithelium. All-trans-retinal, all-trans-retinol, and lipofuscin precursors all emit significant and distinct fluorescence signals, allowing their monitoring in single photoreceptor cells with fluorescence imaging. Here we describe the procedures for measuring these fluorophores in single mouse rod photoreceptors.

Structure of zebrafish IRBP reveals fatty acid binding

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

Dysregulation of inter-photoreceptor retinoid-binding protein (IRBP) after induced Müller cell disruption

Reduced expression of a ~150 kDa protein was unexpectedly observed while investigating Norrin protein in a transgenic murine model in which Müller cells can be selectively and inducibly disrupted. Isolation of this unknown protein via ion exchange and hydrophobic interaction chromatography followed by Tandem mass spectrometry identified it as Inter-photoreceptor retinoid-binding protein (IRBP). Significantly reduced IRBP mRNA expression was observed at the early and late stages after Müller cell disruption. IRBP protein expression was also consistently reduced to 5.7% of the control level as early as 1 week after Müller cell disruption. This down-regulation of IRBP was accompanied by focal hyperfluorescent dots and cytotoxic N-retinylidene-N-retinylethanolamine (A2E) accumulation. In vitro treatment of cone photoreceptor cell lines with conditioned medium collected from stressed Müller cells suggested that Müller cells regulated photoreceptors expression of IRBP via secreted factor(s). In vivo studies suggested that one of these secreted factors was tumour necrosis factor alpha (TNFα). These findings suggest that dysregulation of IRBP expression caused by Müller cell dysfunction may be an important early event in photoreceptor degeneration in some retinal diseases. This study reports down-regulation of inter-photoreceptor retinoid-binding protein (IRBP) in photoreceptors and retinoid cycle derangement after Müller cell disruption in a transgenic mouse model. The findings indicate that Müller cells communicate with photoreceptors in response to stress by secreting soluble protein factor(s). We propose that down-regulation of IRBP may represent an early and novel pathogenic mechanism in degenerative retinal diseases.

Stem cells as source for retinal pigment epithelium transplantation

Inherited maculopathies, age related macular degeneration and some forms of retinitis pigmentosa are associated with impaired function or loss of the retinal pigment epithelium (RPE). Among potential treatments, transplantation approaches are particularly promising. The arrangement of RPE cells in a well-defined tissue layer makes the RPE amenable to cell or tissue sheet transplantation. Different cell sources have been suggested for RPE transplantation but the development of a clinical protocol faces several obstacles. The source should provide a sufficient number of cells to at least recover the macula area. Secondly, cells should be plastic enough to be able to integrate in the host tissue. Tissue sheets should be considered as well, but the substrate on which RPE cells are cultured needs to be carefully evaluated. Immunogenicity can also be an obstacle for effective transplantation as well as tumorigenicity of not fully differentiated cells. Finally, ethical concerns may represent drawbacks when embryo-derived cells are proposed for RPE transplantation. Here we discuss different cell sources that became available in recent years and their different properties. We also present data on a new source of human RPE. We provide a protocol for RPE differentiation of retinal stem cells derived from adult ciliary bodies of post-mortem donors. We show molecular characterization of the in vitro differentiated RPE tissue and demonstrate its functionality based on a phagocytosis assay. This new source may provide tissue for allogenic transplantation based on best matches through histocompatibility testing.

Reduction of all-trans-retinal in vertebrate rod photoreceptors requires the combined action of RDH8 and RDH12

In vertebrate rod cells, retinoid dehydrogenases/reductases (RDHs) are critical for reducing the reactive aldehyde all-trans-retinal that is released by photoactivated rhodopsin, to all-trans-retinol (vitamin A). Previous studies have shown that RDH8 localizes to photoreceptor outer segments and is a strong candidate for performing this role. However, RDH12 function in the photoreceptor inner segments is also key, because loss of function mutations cause retinal degeneration in some forms of Leber congenital amaurosis. To investigate the in vivo roles of RDH8 and RDH12, we used fluorescence imaging to examine all-trans-retinol production in single isolated rod cells from wild-type mice and knock-out mice lacking either one or both RDHs. Outer segments of rods deficient in Rdh8 failed to reduce all-trans-retinal, but those deficient in Rdh12 were unaffected. Following exposure to light, a leak of retinoids from outer to inner segments was detected in rods from both wild-type and knock-out mice. In cells lacking Rdh8 or Rdh12, this leak was mainly all-trans-retinal. Wild-type rods incubated with all-trans-retinal reduced moderate loads of retinal within the cell interior, but this ability was lost by cells deficient in Rdh8 or Rdh12. Our findings are consistent with localization of RDH8 to the outer segment where it provides most of the activity needed to reduce all-trans-retinal generated by the light response. In contrast, RDH12 in inner segments can protect vital cell organelles against aldehyde toxicity caused by an intracellular leak of all-trans-retinal, as well as other aldehydes originating both inside and outside the cell.

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.

All-trans retinal mediates light-induced oxidation in single living rod photoreceptors

All-trans retinal is a potent photosensitizer that is released in photoreceptor outer segments by the photoactivated visual pigment following the detection of light. Photoreceptor outer segments also contain high concentrations of polyunsaturated fatty acids, and are thus particularly susceptible to oxidative damage such as that initiated by light via a photosensitizer. Upon its release, all-trans retinal is reduced within the outer segment to all-trans retinol, through a reaction requiring metabolic input in the form of NADPH. The phototoxic potential of physiologically generated all-trans retinal was examined in single living rod photoreceptors obtained from frog (Rana pipiens) retinas. Light-induced oxidation was measured with fluorescence imaging using an oxidation-sensitive indicator dye from the shift in fluorescence between the intact and oxidized forms. Light-induced oxidation was highest in metabolically compromised rod outer segments following photoactivation of the visual pigment rhodopsin, and after a time interval, sufficiently long to ensure the release of all-trans retinal. Furthermore, light-induced oxidation increased with the concentration of exogenously added all-trans retinal. The results show that the all-trans retinal generated during the detection of light can mediate light-induced oxidation. Its removal through reduction to all-trans retinol protects photoreceptor outer segments against light-induced oxidative damage.

Exaggerated eye growth in IRBP-deficient mice in early development

PURPOSE

Because interphotoreceptor retinoid-binding protein (IRBP) is expressed before being needed in its presumptive role in the visual cycle, we tested whether it controls eye growth during development.

METHODS

The eyes of congenic IRBP knockout (KO) and C57BL/6J wild-type (WT) mice ranging in age from postnatal day (P)2 to P440 were compared by histology, laser micrometry, cycloplegic photorefractions, and partial coherence interferometry.

RESULTS

The size and weight of IRBP KO mouse eyes were greater than those of the WT mouse, even before eye-opening. Excessive ocular enlargement started between P7 and P10, with KO retinal arc lengths becoming greater compared with WT from P10 through P30 (18%; P < 0.01). The outer nuclear layer (ONL) of KO retinas became 20% thinner between P12 to P25, and progressed to 38% thinner at P30. At P30, there were 30% fewer cones per vertical section in KO than in WT retinas. Bromodeoxyuridine (BrdU) labeling indicated the same number of retinal cells were born in KO and WT mice. A spike in apoptosis was observed in KO outer nuclear layer at P25. These changes in size were accompanied by a large decrease in hyperopic refractive error, which reached -4.56 ± 0.70 diopters (D) versus +9.98 ± 0.993 D (mean ± SD) in WT, by postnatal day 60 (P60). CONCLUSIONS; In addition to its role in the visual cycle, IRBP is needed for normal eye development. How IRBP mediates ocular development is unknown.

Rod outer segment retinol formation is independent of Abca4, arrestin, rhodopsin kinase, and rhodopsin palmitylation

PURPOSE

The reactive aldehyde all-trans retinal is released in rod photoreceptor outer segments by photoactivated rhodopsin and is eliminated through reduction to all-trans retinol. This study was undertaken to determine whether all-trans retinol formation depends on Abca4, arrestin, rhodopsin kinase, and the palmitylation of rhodopsin, all of which are factors that affect the release and sequestration of all-trans retinal.

METHODS

Experiments were performed in isolated retinas and single living rods derived from 129/sv wild-type mice and Abca4-, arrestin-, and rhodopsin kinase-deficient mice and in genetically modified mice containing unpalmitylated rhodopsin. Formation of all-trans retinol was measured by imaging its fluorescence and by HPLC of retina extracts. The release of all-trans retinal from photoactivated rhodopsin was measured in purified rod outer segment membranes according to the increase in tryptophan fluorescence. All experiments were performed at 37°C.

RESULTS

The kinetics of all-trans retinol formation in the different types of genetically modified mice are in reasonable agreement with those in wild-type animals. The kinetics of all-trans retinol formation in 129/sv mice are similar to those in C57BL/6, although the latter are known to regenerate rhodopsin much more slowly. The release of all-trans retinal from rhodopsin in purified membranes is significantly faster than the formation of all-trans retinol in intact cells and is independent of the presence of the palmitate groups.

CONCLUSIONS

The regeneration of rhodopsin and the recycling of its chromophore are not strongly coupled. Neither the activities of Abca4, rhodopsin kinase, and arrestin, nor the palmitylation of rhodopsin affects the formation of all-trans retinol.

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.

Rapid formation of all-trans retinol after bleaching in frog and mouse rod photoreceptor outer segments

All-trans retinol is formed in the outer segments of vertebrate rod photoreceptors from the reduction of the all-trans retinal released by photoactivated rhodopsin. The reduction requires NADPH and is therefore dependent on metabolic input. In metabolically intact photoreceptors, a large increase in rod outer segment fluorescence, attributed to the fluorescence of all-trans retinol, follows rhodopsin photoactivation. The fluorescence increase is biphasic, including a rapid and a slow component. In metabolically compromised cells, there is a much smaller fluorescence increase following rhodopsin photoactivation, but it too contains a rapid component. We have measured the fluorescence signal in single living frog and mouse rod photoreceptors, and have characterized its dependence on the wavelengths of light selected for excitation and for collecting emission. We find that in metabolically intact cells, the excitation and emission properties of both the rapid and slow components of the fluorescence signal are in close agreement with those of all-trans retinol fluorescence. In metabolically compromised cells, however, the signal can only partially be due to all-trans retinol, and most of it is consistent with all-trans retinal. The results suggest that in the outer segments of living rod photoreceptors there is rapid release of all-trans retinal, which in metabolically intact cells is accompanied by rapid conversion to all-trans retinol.

2-Hydroxypropyl-beta-cyclodextrin removes all-trans retinol from frog rod photoreceptors in a concentration-dependent manner

PURPOSE

To determine whether a nonprotein lipophilic carrier, 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD), can remove all-trans retinol from rod photoreceptor outer segments. All-trans retinol is generated in rod outer segments after light exposure. It is highly insoluble, and its efficient transport across extra- and intracellular aqueous space requires specialized carriers.

METHODS

Experiments were carried out with isolated frog rod photoreceptor cells. The removal of all-trans retinol by different concentrations of this carrier was measured by imaging its fluorescence in single-rod photoreceptors.

RESULTS

HP-beta-CD concentrations >0.3 mM significantly increased the rate of all-trans retinol removal. The rate of removal increased linearly with carrier concentration, with a slope of 0.0058 min(-1)/mM.

CONCLUSIONS

The effectiveness of HP-beta-CD shows that a specialized interaction with the cell membrane is not necessary for the efficient transfer of all-trans retinol between the cell membrane and the carrier. The transfer occurs through a collision-based mechanism, as indicated by the linear increase of the rate of removal with the carrier concentration.

The retinal pigment epithelium: something more than a constituent of the blood-retinal barrier--implications for the pathogenesis of diabetic retinopathy

The retinal pigment epithelium (RPE) is an specialized epithelium lying in the interface between the neural retina and the choriocapillaris where it forms the outer blood-retinal barrier (BRB). The main functions of the RPE are the following: (1) transport of nutrients, ions, and water, (2) absorption of light and protection against photooxidation, (3) reisomerization of all-trans-retinal into 11-cis-retinal, which is crucial for the visual cycle, (4) phagocytosis of shed photoreceptor membranes, and (5) secretion of essential factors for the structural integrity of the retina. An overview of these functions will be given. Most of the research on the physiopathology of diabetic retinopathy has been focused on the impairment of the neuroretina and the breakdown of the inner BRB. By contrast, the effects of diabetes on the RPE and in particular on its secretory activity have received less attention. In this regard, new therapeutic strategies addressed to modulating RPE impairment are warranted.

Retinol-binding site in interphotoreceptor retinoid-binding protein (IRBP): a novel hydrophobic cavity

PURPOSE

Interphotoreceptor retinoid-binding protein (IRBP) appears to target and protect retinoids during the visual cycle. X-ray crystallographic studies had noted a betabetaalpha-spiral fold shared with crotonases and C-terminal protein transferases. The shallow cleft formed by the fold was assumed to represent the retinol-binding site. However, a second hydrophobic site consisting of a highly restricted cavity was more recently appreciated during in silico ligand-docking studies. In this study, the ligand-binding environment within the second module of Xenopus IRBP (X2IRBP) is defined.

METHODS

Pristine recombinant polypeptide corresponding to X2IRBP was expressed in a soluble form and purified to homogeneity without its fusion tag. Phenylalanine was substituted for tryptophan at each of the putative retinol-binding domains (W450F, hydrophobic cavity; W587F, shallow cleft). Binding of 11-cis and all-trans retinol were observed in titrations monitoring retinol fluorescence enhancement, quenching of tryptophan fluorescence, and energy transfer. The effect of oleic acid on retinol binding was also examined.

RESULTS

A ligand-binding stoichiometry of approximately 1:1 was observed for 11-cis and all-trans with K(d) in the tens of nanomolar range. The substitution mutants showed little effect on retinol binding in titrations after fluorescence enhancement. However, the W450F and not the W587F mutant showed a markedly reduced capacity for fluorescence quenching for both 11-cis and all-trans retinol. Oleic acid inhibited the binding of 11-cis and all-trans retinol in an apparent noncompetitive manner.

CONCLUSIONS

The binding site for 11-cis and all-trans retinol is a novel hydrophobic cavity that is highly restrictive and probably distinct from the long chain fatty acid-binding site.

Formation of all-trans retinol after visual pigment bleaching in mouse photoreceptors

PURPOSE

To test whether the formation of all-trans retinol limits the regeneration of the visual pigment. all-trans retinol is formed after visual pigment bleaching through the reduction of all-trans retinal in a reaction involving NADPH. This reduction begins the recycling of the chromophore for the regeneration of the visual pigment.

METHODS

Experiments were performed with dark-adapted, isolated retinas and isolated photoreceptor cells from wild-type and Nrl(-/-) mice. The photoreceptors of Nrl(-/-) mice are conelike and contain only cone pigments. The formation of all-trans retinol after pigment bleaching was measured by quantitative HPLC of retinoids extracted from isolated retinas and by imaging the fluorescence of retinol in photoreceptor outer segments. Experiments were performed at 37 degrees C.

RESULTS

In rods, the formation of all-trans retinol proceeded with first-order kinetics, with a rate constant of 0.06 +/- 0.02 minute(-1), significantly faster than the reported rate constant for rhodopsin regeneration. In Nrl(-/-) photoreceptors, the formation of all-trans retinol occurred at least 100 times faster than in rods. For both cell types, the fraction of all-trans retinal converted to all-trans retinol at equilibrium is approximately 0.8, indicating the presence of a similar fraction of reduced NADPH.

CONCLUSIONS

Formation of all-trans retinol does not limit the regeneration of bleached visual pigment. Formation of all-trans retinol in the cone-like Nrl(-/-) photoreceptors is much faster than in rods, consistent with a faster regeneration of the visual pigment after bleaching. Different types of photoreceptors contain a comparable fraction of reduced NADPH to drive the reduction of all-trans retinal.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Two-photon microscopy: shedding light on the chemistry of vision

Two-photon microscopy (TPM) has come to occupy a prominent place in modern biological research with its ability to resolve the three-dimensional distribution of molecules deep inside living tissue. TPM can employ two different types of signals, fluorescence and second harmonic generation, to image biological structures with subcellular resolution. Two-photon excited fluorescence imaging is a powerful technique with which to monitor the dynamic behavior of the chemical components of tissues, whereas second harmonic imaging provides novel ways to study their spatial organization. Using TPM, great strides have been made toward understanding the metabolism, structure, signal transduction, and signal transmission in the eye. These include the characterization of the spatial distribution, transport, and metabolism of the endogenous retinoids, molecules essential for the detection of light, as well as the elucidation of the architecture of the living cornea. In this review, we present and discuss the current applications of TPM for the chemical and structural imaging of the eye. In addition, we address what we see as the future potential of TPM for eye research. This relatively new method of microscopy has been the subject of numerous technical improvements in terms of the optics and indicators used, improvements that should lead to more detailed biochemical characterizations of the eyes of live animals and even to imaging of the human eye in vivo.

Interphotoreceptor retinoid-binding protein is the physiologically relevant carrier that removes retinol from rod photoreceptor outer segments

Light detection by vertebrate rod photoreceptor outer segments results in the destruction of the visual pigment, rhodopsin, as its retinyl moiety is photoisomerized from 11-cis to all-trans. The regeneration of rhodopsin is necessary for vision and begins with the release of the all-trans retinal and its reduction to all-trans retinol. Retinol is then transported out of the rod outer segment for further processing. We used fluorescence imaging to monitor retinol fluorescence and quantify the kinetics of its formation and clearance after rhodopsin bleaching in the outer segments of living isolated frog (Rana pipiens) rod photoreceptors. We independently measured the release of all-trans retinal from bleached rhodopsin in frog rod outer segment membranes and the rate of all-trans retinol removal by the lipophilic carriers interphotoreceptor retinoid binding protein (IRBP) and serum albumin. We find that the kinetics of all-trans retinol formation in frog rod outer segments after rhodopsin bleaching are to a good first approximation determined by the kinetics of all-trans retinal release from the bleached pigment. For the physiological concentrations of carriers, the rate of retinol removal from the outer segment is determined by IRBP concentration, whereas the effect of serum albumin is negligible. The results indicate the presence of a specific interaction between IRBP and the rod outer segment, probably mediated by a receptor. The effect of different concentrations of IRBP on the rate of retinol removal shows no cooperativity and has an EC50 of 40 micromol/L.

A novel cone visual cycle in the cone-dominated retina

The visual processing of humans is primarily reliant upon the sensitivity of cone photoreceptors to light during daylight conditions. This underscores the importance of understanding how cone photoreceptors maintain the ability to detect light. The vertebrate retina consists of a combination of both rod and cone photoreceptors. Subsequent to light exposure, both rod and cone photoreceptors are dependent upon the recycling of vitamin A to regenerate photopigments, the proteins responsible for detecting light. Metabolic processing of vitamin A in support of rod photopigment renewal, the so-called "rod visual cycle", is well established. However, the metabolic processing of vitamin A in support of cone photopigment renewal remains a challenge for characterization in the recently discovered "cone visual cycle". In this review we summarize the research that has defined the rod visual cycle and our current concept of the novel cone visual cycle. Here, we highlight the research that supports the existence of a functional cone-specific visual cycle: the identification of novel enzymatic activities that contribute to retinoid recycling, the observation of vitamin A recycling in cone-dominated retinas, and the localization of some of these activities to the Müller cell. In the opinions of the authors, additional research on the possible interactions between these two visual cycles in the duplex retina is needed to understand visual detection in the human retina.

All-trans retinol in rod photoreceptor outer segments moves unrestrictedly by passive diffusion

The visual pigment protein of vertebrate rod photoreceptors, rhodopsin, contains an 11-cis retinyl moiety that is isomerized to all-trans upon light absorption. Subsequently, all-trans retinal is released from the protein and reduced to all-trans retinol, the first step in the recycling of rhodopsin's chromophore group through the series of reactions that constitute the visual cycle. The concentration of all-trans retinol in photoreceptor outer segments can be monitored from its fluorescence. We have used two-photon excitation (720 nm) of retinol fluorescence and fluorescence recovery after photobleaching to characterize the mobility of all-trans retinol in frog photoreceptor outer segments. Retinol produced after rhodopsin bleaching moved laterally in the disk membrane bilayer with an apparent diffusion coefficient of 2.5 +/- 0.3 micro m(2) s(-1). The diffusion coefficient of exogenously added retinol was 3.2 +/- 0.5 micro m(2) s(-1). These diffusion coefficients are in close agreement with those reported for lipids, suggesting that retinol is not tightly bound to protein sites that would be diffusing much more slowly in the plane of the membrane. In agreement with this interpretation, a fluorescent-labeled C-16 fatty acid diffused laterally with a similar diffusion coefficient, 2.2 +/- 0.2 micro m(2) s(-1). Retinol also moved along the length of the rod outer segment, with an apparent diffusion coefficient of 0.07 +/- 0.01 micro m(2) s(-1), again suggesting that it is not tightly bound to proteins that would confine it to the disks. The axial diffusion coefficient of exogenously added retinol was 0.05 +/- 0.01 micro m(2) s(-1). In agreement with passive diffusion, the rate of axial movement was inversely proportional to the square of the length of the rod outer segment. Diffusion of retinol on the plasma membrane of the outer segment can readily account for the measured value of the axial diffusion coefficient, as the plasma membrane comprises approximately 1% of the total outer-segment membrane. The values of both the lateral and axial diffusion coefficients are consistent with most of the all-trans retinol in the outer segments moving unrestricted and not being bound to carrier proteins. Therefore, and in contrast to other steps of the visual cycle, there does not appear to be any specialized processing for all-trans retinol within the rod outer segment.

The retinal pigment epithelium in visual function

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

Interphotoreceptor retinoid-binding protein (IRBP) promotes the release of all-trans retinol from the isolated retina following rhodopsin bleaching illumination

All-trans retinol generated in rod photoreceptors upon the bleaching of rhodopsin is known to move from the rods to the retinal pigment epithelium (RPE), where it is enzymatically converted to 11-cis retinal in the retinoid visual cycle. Interphotoreceptor retinoid-binding protein (IRBP) contained in the extracellular compartment (interphotoreceptor matrix) that separates the retina and RPE has been hypothesized to facilitate this movement of all-trans retinol, but the precise role of IRBP in this process remains unclear. To examine the activity of IRBP in the release of all-trans retinol from the rods, initially dark-adapted isolated retinas obtained from toad (Bufo marinus) eyes were bleached and then incubated in darkness for defined periods (5-180 min) in physiological saline (Ringer solution) supplemented with IRBP (here termed 'IRBP I') at defined concentrations (2-90 microm). Retinoids present in the retina and extracellular medium were then determined by extraction and HPLC analysis. Preparations incubated with > or =10 microm IRBP I showed a pronounced release of all-trans retinol with increasing period of incubation. As determined with 25 microm IRBP I, the increase of all-trans retinol in the extracellular medium was accompanied by a significant decrease in the combined amount of all-trans retinal and all-trans retinol contained in the retina. This effect was not mimicked by unsupplemented Ringer solution or by Ringer solution containing 25 or 90 microm bovine serum albumin. However, incubation with 'IRBP II', a previously described variant of IRBP with altered lectin-binding properties, led to the appearance of substantial all-trans retinol in the extracellular medium. The results suggest that in vivo, IRBP plays a direct role in the release of all-trans retinol from the rods during operation of the visual cycle.

Interphotoreceptor retinoid-binding protein (IRBP)-deficient C57BL/6 mice have enhanced immunological and immunopathogenic responses to IRBP and an altered recognition of IRBP epitopes

Experimental autoimmune uveitis (EAU) and pinealitis (EAP) can be induced in susceptible mice by immunization with immunologically privileged retinal antigens. In the present study, we analyzed the immunologic and immunopathologic responses of mice deficient in the retinal autoantigen interphotoreceptor retinoid-binding protein (IRBP). The consequences of IRBP deficiency on the T-cell repertoire were also investigated. IRBP+/+, IRBP+/- and IRBP-/- mice on the C57BL/6 background were immunized with IRBP or with a pathogenic epitope, IRBP(1-20) peptide in adjuvant, and were evaluated for disease severity and immunological responses. C57BL/6 IRBP-/- mice were completely resistant to EAU and EAP, and had enhanced immunological responses to IRBP and to its pathogenic peptide 1-20, as compared to their IRBP+/+ counterparts. IRBP-/- mice exhibited an altered IRBP epitope recognition. T cell epitope mapping revealed a response to IRBP peptide 271-290 in IRBP-/- mice, that was absent in the wild type. Primed T cells of IRBP-/- mice transferred an exacerbated form of EAU to nai;ve wild type recipients. A gene-dose effect was evident in that C57BL/6 IRBP+/- mice, exhibited intermediate immunological responses and lower disease scores compared to wild type. We conclude that expression of IRBP in target tissues is a necessary prerequisite for disease induction, excluding other retinoid-binding or vision-related proteins as surrogate targets. Furthermore, endogenous expression of IRBP is directly responsible for lowering the threshold of susceptibility to uveitic disease.

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.

HEK293S cells have functional retinoid processing machinery

Rhodopsin activation is measured by the early receptor current (ERC), a conformation-associated charge motion, in human embryonic kidney cells (HEK293S) expressing opsins. After rhodopsin bleaching in cells loaded with 11-cis-retinal, ERC signals recover in minutes and recurrently over a period of hours by simple dark adaptation, with no added chromophore. The purpose of this study is to investigate the source of ERC signal recovery in these cells. Giant HEK293S cells expressing normal wild-type (WT)-human rod opsin (HEK293S) were regenerated by solubilized 11-cis-retinal, all-trans-retinal, or Vitamin A in darkness. ERCs were elicited by flash photolysis and measured by whole-cell recording. Visible flashes initially elicit bimodal (R(1), R(2)) ERC signals in WT-HEK293S cells loaded with 11-cis-retinal for 40 min or overnight. In contrast, cells regenerated for 40 min with all-trans-retinal or Vitamin A had negative ERCs (R(1)-like) or none at all. After these were placed in the dark overnight, ERCs with outward R(2) signals were recorded the following day. This indicates conversion of loaded Vitamin A or all-trans-retinal into cis-retinaldehyde that regenerated ground-state pigment. 4-butylaniline, an inhibitor of the mammalian retinoid cycle, reversibly suppressed recovery of the outward R(2) component from Vitamin A and 11-cis-retinal-loaded cells. These physiological findings are evidence for the presence of intrinsic retinoid processing machinery in WT-HEK293S cells similar to what occurs in the mammalian eye.

The effect of retinoids and butyrate on the expression of CRX and IRBP in retinoblastoma cells

We sought to determine whether differentiation agents such as retinoids and butyrate regulate transcript levels of interphotoreceptor retinoid binding protein (IRBP) and cone rod homeobox (CRX), a homeodomain transcription factor that regulates IRBP promoter activity. WERI-Rb1 retinoblastoma cells were treated with all-trans retinol, all-trans retinoic acid, or butyrate. IRBP and CRX mRNA levels were determined by quantitative RT-PCR. Butyrate at low concentrations increased both mRNA levels but suppressed them at higher concentrations. Retinoic acid had minimal effects. Retinol increased CRX mRNA over four fold. IRBP and CRX transcript levels are sensitive to butyrate and CRX expression is sensitive to retinol.

Endogenous CRX expression and IRBP promoter activity in retinoblastoma cells

PURPOSE

To determine whether antisense oligonucleotides (AODNs) targeted against CRX, a photoreceptor-specific trans-acting factor, suppress CRX expression and interphotoreceptor retinoid binding protein (IRBP) promoter activity.

METHODS

Cultures of human retinoblastoma cells were transfected with chloramphenicol acetyltransferase (CAT) reporter plasmids containing a mouse IRBP promoter and AODNs directed against CRX. RT-PCR using primers specific to CRX, OTX2, GAPDH, or RNase H was conducted on total RNA isolated from retinoblastoma cells at various times following transfection with AODNs.

RESULTS

Transfection of retinoblastoma cells with IRBP promoter CAT constructs alone produced high activity. Co-transfection with AODNs suppressed IRBP promoter activity in a concentration-dependent manner, with half-maximal effect produced at about 2 nM AODN concentration. Transfection with CAT constructs containing an SV40 promoter produced high activity that was unaffected by co-transfection with AODNs. RT-PCR products were obtained for all target sequences. CRX RT-PCR product from cells transfected with AODNs was greatly diminished following transfection with an AODN whereas control transcripts, including that of OTX2, were relatively unaffected.

CONCLUSIONS

The CRX-specific AODNs specifically and potently suppressed CRX expression and IRBP promoter activity, as measured by RT-PCR and transient transfection assays, respectively. Little or no effect was seen on controls. These data suggest that endogenous CRX is required for IRBP promoter activity in retinoblastoma cells.

Evidence of a tissue-restricting DNA regulatory element in the mouse IRBP promoter

The expression of interphotoreceptor retinoid binding protein (IRBP) is limited to photoreceptor cells of the retina and pinealocytes of the pineal gland. We sought to define cis-elements of the mouse IRBP 5' flanking region that are required for this restricted activity. In vitro transient transfections of retinoblastoma and neuroblastoma cells and in vivo experiments with transgenic Xenopus laevis indicate that -1783/+101 and -156/+101 IRBP gene fragments directed expression predominantly to the retina and pineal, with minor neuronal expression elsewhere. In contrast, a -70/+101 fragment was less restrictive in controlling expression, exhibiting activity not only in retina, but also in forebrain, hindbrain, spinal cord, and motor neurons innervating gills.

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.

Photoaffinity labeling of human IRBP with all-trans-retinoic acid

Interphotoreceptor retinoid-binding protein (IRBP), found only in photosensitive tissues, is a large approximately 135-kDa glycoprotein that contains a fourfold repeat structure. IRBP may function as a buffer and prevent retinoid toxicity and retinoid degeneration. Here we asked (i) whether each repeat of IRBP possesses the capability of photo-crosslinking all-trans-retinoic acid (RA), (ii) within Repeat 1 whether a single retinoic acid-binding domain exists, and (iii) whether protease and CNBr digestion of Repeat 1 bound RA indicate the exact location of the binding site. 3H-RA cross-linked to all four repeats, consistent with the current model of multiple binding sites in IRBP. Acetone precipitation was effective in removing unbound 3H-RA. LysC and tryptic digestion of the RA-Repeat 1 detected 18- and 5-kDa bands, respectively. CNBr digestion showed two bands about 9 and 11 kDa in size. Our data suggests a single binding site near positions 151-160 in the center of Repeat 1.

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.

Site-specific DNA hypomethylation permits expression of the IRBP gene

Interphotoreceptor retinoid binding protein (IRBP), a putative component of the visual cycle, is expressed selectively in the retina and pineal gland. This study examined whether site-specific DNA hypomethylation plays a role in this expression regulation. Southern blotting of HpaII and MspI digests of DNA from various bovine and murine tissues (whole brain, retina, pineal gland, superior colliculus, cortex, thymus, habenular nucleus, cornea, liver, tail, and kidney) revealed that specific CpG dinucleotides in the IRBP gene promoter are hypomethylated in DNA from retinal photoreceptor cells and pineal gland compared to DNA from other tissues. These sites are methylated in DNA from non-photoreceptor retinal cells. Exogenous methylation of these sites diminished DNA:protein binding in electrophoretic mobility shift assays. HpaII methylation of chloramphenicol acetyltransferase reporter constructs suppressed IRBP but not SV40 promoter activity in transiently transfected primary cultures of embryonic chick retinal cells. These data indicate that specific cytosines in the bovine and murine IRBP promoters are unmethylated in photoreceptive cells but methylated in other tissues. This differential DNA methylation may modulate IRBP gene expression since exogenous methylation of the murine sites suppresses reporter gene transcription, apparently by inhibiting DNA:protein binding events.

IRBP enhances removal of 11- cis -retinaldehyde from isolated RPE membranes

Interphotoreceptor retinoid-binding protein (IRBP) greatly enhances the conversion of all- trans -retinol to 11- cis -retinal by the retinal pigment epithelium (RPE) and facilitates 11- cis -retinal release from the RPE. However, the mechanisms by which IRBP exerts these effects are not clear. Using a model system of purified bovine IRBP and isolated bovine RPE membranes, we investigated the possibility that IRBP may favor the delivery of all- trans -retinol to, or the release of 11- cis -retinal from, RPE membranes. As the interphotoreceptor space contains serum retinol-binding protein (RBP) and serum albumin in addition to IRBP, we similarly examined the exchange of retinoids between RPE membranes and human RBP or bovine serum albumin (BSA). Isolated RPE membranes were loaded with radioactive 11- cis -retinal and incubated with solutions of IRBP, RBP, BSA or with buffer alone. Membranes (pellet) and retinoid-binding protein or buffer (supernatant) were separated by centrifugation and analysed for radioactive 11- cis -retinal. Membranes incubated with buffer alone released only 4-5% of their 11- cis -retinal, while 25 microm IRBP removed 18-35%. More retinal was released as the membrane concentration was reduced. In contrast, RBP and BSA removed little retinal, even though both proteins are capable of binding this retinoid. Similar results were obtained with bovine liver membranes, consistent with the idea that the effects of IRBP do not depend on an RPE surface receptor for IRBP. IRBP was also markedly superior to RBP and BSA in removing all- trans -retinol from RPE membranes. In addition, IRBP efficiently delivered bound all- trans -retinol to membranes; however, in contrast to their differential removal of retinoids, all three binding proteins delivered comparable amounts of retinol to membranes. (This result supports the practice of using BSA as a retinoid carrier in in vitro experimental systems). We conclude that, whereas IRBP shares with other retinoid-binding proteins the ability to deliver retinol to membranes, IRBP is unique in its capacity to remove 11- cis -retinal from membranes. This may be the feature of IRBP that drives the vitamin A cycle to efficiently produce 11- cis -retinal.

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.

Visual sensitivity and interphotoreceptor retinoid binding protein in the mouse: regulation by vitamin A

Interphotoreceptor retinoid binding protein (IRBP) is a retinoid and fatty acid binding glycoprotein secreted by rod and cone photoreceptors in all vertebrates. IRBP is believed to serve as a carrier for retinoids in the bleaching and regeneration cycle of rhodopsin. IRBP protein has been found to be decreased in vitamin A-deprived rats; it is rapidly recovered after retinol repletion. To understand the mechanism for this recovery, we determined whether vitamin A affects transcription and translation of the IRBP gene. Wild-type and transgenic mice harboring the IRBP promoter-CAT reporter fusion gene were maintained on a retinol-deficient diet supplemented with retinoic acid (-A) or on a control diet (+A) for up to 60 wk postweaning. Some of the -A mice were given retinol repletion for 7 days (-A+A). Electroretinography analysis revealed alterations in waveform and a 2 log unit decrease in b-wave sensitivity in the -A mice over a broad range of stimulus wavelengths. Retinol repletion effected a full recovery. Immunochemistry showed a significant decrease in the immunogold-labeled IRBP between the retinal pigment epithelium and the outer segments of the -A mice compared with +A and -A+A mice. Northern blots showed no differences in the amounts of IRBP or CAT mRNA between these three treatment groups. These results suggest that the regulation of IRBP by retinol is not transcriptional.

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

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