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

Advances and Therapeutic Opportunities in Visual Cycle Modulation

The visual cycle is a metabolic pathway that enables continuous vision by regenerating the 11-cis-retinal chromophore for photoreceptors opsins. Although integral to normal visual function, the flux of retinoids through this cycle can contribute to a range of retinal pathologies, including Stargardt disease, age-related macular degeneration, and diabetic retinopathy. In such conditions, intermediates and byproducts of the visual cycle, such as bisretinoid components of lipofuscin, can accumulate, concomitant with cellular damage and eventual photoreceptor loss. This has inspired efforts to modulate the visual cycle, aiming to slow or prevent the formation of these toxic intermediates and thus preserve retinal structure and function. Over the past two decades, multiple strategies to modulate the visual cycle have emerged. These include both intrinsic approaches, targeting key enzymes, retinoid-binding proteins, or receptors within the pigment epithelium or photoreceptors (e.g., RPE65, CRBP1, and rhodopsin inhibitors/antagonists) and extrinsic strategies that indirectly alter retinoid availability within the retina (e.g., RBP4 antagonists). Many of these agents have shown promise in animal models of visual cycle-associated retinal diseases, reducing pathological changes, and improving retinal survival. Several have advanced into clinical studies, although none are currently FDA-approved. Challenges remain in optimizing drug specificity and duration of action while minimizing side effects such as nyctalopia. In this review, we comprehensively examine current and emerging visual cycle modulators, discuss their medicinal chemistry, mechanisms of action, efficacy in preclinical and clinical studies, and highlight future opportunities for drug discovery aimed at safely and effectively preserving vision through modulation of this biochemical pathway.

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.

The Retina-Based Visual Cycle

The continuous function of vertebrate photoreceptors requires regeneration of their visual pigment following its destruction upon activation by light (photobleaching). For rods, the chromophore required for the regeneration of rhodopsin is derived from the adjacent retinal pigmented epithelium (RPE) cells through a series of reactions collectively known as the RPE visual cycle. Mounting biochemical and functional evidence demonstrates that, for cones, pigment regeneration is supported by the parallel supply with chromophore by two pathways-the canonical RPE visual cycle and a second, cone-specific retina visual cycle that involves the Müller glial cells in the neural retina. In this article, we review historical information that led to the discovery of the retina visual cycle and discuss what is currently known about the reactions and molecular components of this pathway and its functional role in supporting cone-mediated vision.

The First Steps of the Visual Cycle in Human Rod and Cone Photoreceptors

Purpose

Light detection destroys the visual pigment. Its regeneration, necessary for the recovery of light sensitivity, is accomplished through the visual cycle. Release of all-trans retinal by the light-activated visual pigment and its reduction to all-trans retinol comprise the first steps of the visual cycle. In this study, we determined the kinetics of all-trans retinol formation in human rod and cone photoreceptors.

Methods

Single living rod and cone photoreceptors were isolated from the retinas of human cadaver eyes (ages 21 to 90 years). Formation of all-trans retinol was measured by imaging its outer segment fluorescence (excitation, 360 nm; emission, >420 nm). The extent of conversion of released all-trans retinal to all-trans retinol was determined by measuring the fluorescence excited by 340 and 380 nm. Measurements were repeated with photoreceptors isolated from Macaca fascicularis retinas. Experiments were carried out at 37°C.

Results

We found that ∼80% to 90% of all-trans retinal released by the light-activated pigment is converted to all-trans retinol, with a rate constant of 0.24 to 0.55 min-1 in human rods and ∼1.8 min-1 in human cones. In M. fascicularis rods and cones, the rate constants were 0.38 ± 0.08 min-1 and 4.0 ± 1.1 min-1, respectively. These kinetics are several times faster than those measured in other vertebrates. Interphotoreceptor retinoid-binding protein facilitated the removal of all-trans retinol from human rods.

Conclusions

The first steps of the visual cycle in human photoreceptors are several times faster than in other vertebrates and in line with the rapid recovery of light sensitivity exhibited by the human visual system.

Scavenging of Cation Radicals of the Visual Cycle Retinoids by Lutein, Zeaxanthin, Taurine, and Melanin

In the retina, retinoids involved in vision are under constant threat of oxidation, and their oxidation products exhibit deleterious properties. Using pulse radiolysis, this study determined that the bimolecular rate constants of scavenging cation radicals of retinoids by taurine are smaller than 2 × 107 M-1s-1 whereas lutein scavenges cation radicals of all three retinoids with the bimolecular rate constants approach the diffusion-controlled limits, while zeaxanthin is only 1.4-1.6-fold less effective. Despite that lutein exhibits greater scavenging rate constants of retinoid cation radicals than other antioxidants, the greater concentrations of ascorbate in the retina suggest that ascorbate may be the main protectant of all visual cycle retinoids from oxidative degradation, while α-tocopherol may play a substantial role in the protection of retinaldehyde but is relatively inefficient in the protection of retinol or retinyl palmitate. While the protection of retinoids by lutein and zeaxanthin appears inefficient in the retinal periphery, it can be quite substantial in the macula. Although the determined rate constants of scavenging the cation radicals of retinol and retinaldehyde by dopa-melanin are relatively small, the high concentration of melanin in the RPE melanosomes suggests they can be scavenged if they are in proximity to melanin-containing pigment granules.

Obtaining absorbance spectra from turbid retinal cell and tissue suspensions - Beating the light-scatter problem

Light scattering and inability to uniformly expose the cuvette contents to an incident light beam are significant limitations of traditional spectrophotometers. The first of these drawbacks limits their usefulness in studies of turbid cellular and tissue suspensions; the second limits their use in photodecomposition studies. Our strategy circumvents both problems. Although we describe its potential usefulness in vision sciences, application of spherical integrating cuvettes has broad application. Absorbance spectra of turbid bovine rod outer segments and dispersed living frog retina were studied using a standard single-pass 1 cm cuvettes, or a spherical integrating cuvette (DeSa Presentation Chamber, DSPC). The DSPC was mounted on an OLIS Rapid Scanning Spectrophotometer configured to generate 100 spectral scans/sec. To follow rhodopsin bleaching kinetics in living photoreceptors, portions of dark-adapted frog retina were suspended in the DSPC. The incoming spectral beam at 2 scans/sec entered the chamber through a single port. Separate ports contained a 519 nm light emitting diode (LED), or window to the photomultiplier tube. The surface of the DSPC was coated with a highly reflective coating allowing the chamber to act as a multi-pass cuvette. The LED is triggered to flash and the PMT shutter temporarily closed during a "Dark-Interval" between each spectral scan. By interleafing scans with LED pulses, spectra changes can be followed in real time. Kinetic analysis of the 3-dimensional data was performed by Singular Value Decomposition. For crude bovine rod outer segment suspensions, the 1 cm single-pass traditional cuvette gave non-informative spectra dominated by high absorbances and Rayleigh scattering. In contrast, spectra generated using the DSPC showed low overall absorbance with peaks at 405 and 503 nm. The later peak disappeared with exposure to white light in presence of 100 mM hydroxylamine. For the dispersed living retinal, the sample was pulsed at 519 nm between the spectra. The 495 nm rhodopsin peak gradually reduced in size concomitant with the emergence of a 400 nm peak, probably representing Meta II. A conversion mechanism of two species, A → B with rate constant of 0.132 sec^-1 was fit to the data. To our knowledge this is the first application of integrating sphere technology to retinal spectroscopy. Remarkably, the spherical cuvette designed for total internal reflectance to produce diffused light was efffectively immune to light scattering. Furthermore, the higher effective path length enhanced sensitivity and could be accounted for mathematically allowing determination of absorbance/cm. The approach, which complements the use of the CLARiTy RSM 1000 for photodecomposition studies (Gonzalez-Fernandez et al. Mol Vis 2016, 22:953), may facilitate studies of metabolically active photoreceptor suspensions or whole retinas in physiological assays.

Relative Contributions of All-Trans and 11-Cis Retinal to Formation of Lipofuscin and A2E Accumulating in Mouse Retinal Pigment Epithelium

Purpose

Bis-retinoids are a major component of lipofuscin that accumulates in the retinal pigment epithelium (RPE) in aging and age-related macular degeneration (AMD). Although bis-retinoids are known to originate from retinaldehydes required for the light response of photoreceptor cells, the relative contributions of the chromophore, 11-cis retinal, and photoisomerization product, all-trans retinal, are unknown. In photoreceptor outer segments, all-trans retinal, but not 11-cis retinal, is reduced by retinol dehydrogenase 8 (RDH8). Using Rdh8^−/− mice, we evaluated the contribution of increased all-trans retinal to the formation and stability of RPE lipofuscin.

Methods

Rdh8^−/− mice were reared in cyclic-light or darkness for up to 6 months, with selected light-reared cohorts switched to dark-rearing for the final 1 to 8 weeks. The bis-retinoid A2E was measured from chloroform-methanol extracts of RPE-choroid using HPLC-UV/VIS spectroscopy. Lipofuscin fluorescence was measured from whole flattened eyecups (excitation, 488 nm; emission, 565–725 nm).

Results

Cyclic-light-reared Rdh8^−/− mice accumulated A2E and RPE lipofuscin approximately 1.5 times and approximately 2 times faster, respectively, than dark-reared mice. Moving Rdh8^−/− mice from cyclic-light to darkness resulted in A2E levels less than expected to have accumulated before the move.

Conclusions

Our findings establish that elevated levels of all-trans retinal present in cyclic-light-reared Rdh8^−/− mice, which remain low in wild-type mice, contribute only modestly to RPE lipofuscin formation and accumulation. Furthermore, decreases in A2E levels occurring after moving cyclic-light-reared Rdh8^−/− mice to darkness are consistent with processing of A2E within the RPE and the existence of a mechanism that could be a therapeutic target for controlling A2E cytotoxicity.

Correcting QUEST Magnetic Resonance Imaging-Sensitive Free Radical Production in the Outer Retina In Vivo Does Not Correct Reduced Visual Performance in 24-Month-Old C57BL/6J Mice

Purpose

To test the hypothesis that acutely correcting a sustained presence of outer retina free radicals measured in vivo in 24-month-old mice corrects their reduced visual performance.

Methods

Male C57BL/6J mice two and 24 months old were noninvasively evaluated for unremitted production of paramagnetic free radicals based on whether 1/T1 in retinal laminae are reduced after acute antioxidant administration (QUEnch-assiSTed [QUEST] magnetic resonance imaging [MRI]). Superoxide production was measured in freshly excised retina (lucigenin assay). Combining acute antioxidant administration with optical coherence tomography (i.e., QUEST OCT) tested for excessive free radical–induced shrinkage of the subretinal space volume. Combining antioxidant administration with optokinetic tracking tested for a contribution of uncontrolled free radical production to cone-based visual performance declines.

Results

At two months, antioxidants had no effect on 1/T1 in vivo in any retinal layer. At 24 months, antioxidants reduced 1/T1 only in superior outer retina. No age-related change in retinal superoxide production was measured ex vivo, suggesting that free radical species other than superoxide contributed to the positive QUEST MRI signal at 24 months. Also, subretinal space volume did not show evidence for age-related shrinkage and was unresponsive to antioxidants. Finally, visual performance declined with age and was not restored by antioxidants that were effective per QUEST MRI.

Conclusions

An ongoing uncontrolled production of outer retina free radicals as measured in vivo in 24 mo C57BL/6J mice appears to be insufficient to explain reductions in visual performance.

Pathways and disease-causing alterations in visual chromophore production for vertebrate vision

All that we view of the world begins with an ultrafast cis to trans photoisomerization of the retinylidene chromophore associated with the visual pigments of rod and cone photoreceptors. The continual responsiveness of these photoreceptors is then sustained by regeneration processes that convert the trans-retinoid back to an 11-cis configuration. Recent biochemical and electrophysiological analyses of the retinal G-protein-coupled receptor (RGR) suggest that it could sustain the responsiveness of photoreceptor cells, particularly cones, even under bright light conditions. Thus, two mechanisms have evolved to accomplish the reisomerization: one involving the well-studied retinoid isomerase (RPE65) and a second photoisomerase reaction mediated by the RGR. Impairments to the pathways that transform all-trans-retinal back to 11-cis-retinal are associated with mild to severe forms of retinal dystrophy. Moreover, with age there also is a decline in the rate of chromophore regeneration. Both pharmacological and genetic approaches are being used to bypass visual cycle defects and consequently mitigate blinding diseases. Rapid progress in the use of genome editing also is paving the way for the treatment of disparate retinal diseases. In this review, we provide an update on visual cycle biochemistry and then discuss visual-cycle-related diseases and emerging therapeutics for these disorders. There is hope that these advances will be helpful in treating more complex diseases of the eye, including age-related macular degeneration (AMD).

Interphotoreceptor Retinoid-Binding Protein (IRBP) in Retinal Health and Disease

Interphotoreceptor retinoid-binding protein (IRBP), also known as retinol binding protein 3 (RBP3), is a lipophilic glycoprotein specifically secreted by photoreceptors. Enriched in the interphotoreceptor matrix (IPM) and recycled by the retinal pigment epithelium (RPE), IRBP is essential for the vision of all vertebrates as it facilitates the transfer of retinoids in the visual cycle. It also helps to transport lipids between the RPE and photoreceptors. The thiol-dependent antioxidant activity of IRBP maintains the delicate redox balance in the normal retina. Thus, its dysfunction is suspected to play a role in many retinal diseases. We have reviewed here the latest research on IRBP in both retinal health and disease, including the function and regulation of IRBP under retinal stress in both animal models and the human retina. We have also explored the therapeutic potential of targeting IRBP in retinal diseases. Although some technical barriers remain, it is possible that manipulating the expression of IRBP in the retina will rescue or prevent photoreceptor degeneration in many retinal diseases.

Novel fluorescent GPCR biosensor detects retinal equilibrium binding to opsin and active G protein and arrestin signaling conformations

Rhodopsin is a canonical class A photosensitive G protein-coupled receptor (GPCR), yet relatively few pharmaceutical agents targeting this visual receptor have been identified, in part due to the unique characteristics of its light-sensitive, covalently bound retinal ligands. Rhodopsin becomes activated when light isomerizes 11-cis-retinal into an agonist, all-trans-retinal (ATR), which enables the receptor to activate its G protein. We have previously demonstrated that, despite being covalently bound, ATR can display properties of equilibrium binding, yet how this is accomplished is unknown. Here, we describe a new approach for both identifying compounds that can activate and attenuate rhodopsin and testing the hypothesis that opsin binds retinal in equilibrium. Our method uses opsin-based fluorescent sensors, which directly report the formation of active receptor conformations by detecting the binding of G protein or arrestin fragments that have been fused onto the receptor's C terminus. We show that these biosensors can be used to monitor equilibrium binding of the agonist, ATR, as well as the noncovalent binding of β-ionone, an antagonist for G protein activation. Finally, we use these novel biosensors to observe ATR release from an activated, unlabeled receptor and its subsequent transfer to the sensor in real time. Taken together, these data support the retinal equilibrium binding hypothesis. The approach we describe should prove directly translatable to other GPCRs, providing a new tool for ligand discovery and mutant characterization.

Photooxidation mediated by 11-cis and all-trans retinal in single isolated mouse rod photoreceptors

Retinal, the vitamin A aldehyde, is a potent photosensitizer that plays a major role in light-induced damage to vertebrate photoreceptors. 11-Cis retinal is the light-sensitive chromophore of rhodopsin, the photopigment of vertebrate rod photoreceptors. It is isomerized by light to all-trans, activating rhodopsin and beginning the process of light detection. All-trans retinal is released by activated rhodopsin, allowing its regeneration by fresh 11-cis retinal continually supplied to photoreceptors. The released all-trans retinal is reduced to all-trans retinol in a reaction using NADPH. We have examined the photooxidation mediated by 11-cis and all-trans retinal in single living rod photoreceptors isolated from mouse retinas. Photooxidation was measured with fluorescence imaging from the oxidation of internalized BODIPY C11, a fluorescent dye whose fluorescence changes upon oxidation. We found that photooxidation increased with the concentration of exogenously added 11-cis or all-trans retinal to metabolically compromised rod outer segments that lacked NADPH supply. In dark-adapted metabolically intact rod outer segments with access to NADPH, there was no significant increase in photooxidation following exposure of the cell to light, but there was significant increase following addition of exogenous 11-cis retinal. The results indicate that both 11-cis and all-trans retinal can mediate light-induced damage in rod photoreceptors. In metabolically intact cells, the removal of the all-trans retinal generated by light through its reduction to retinol minimizes all-trans retinal-mediated photooxidation. However, because the enzymatic machinery of the rod outer segment cannot remove 11-cis retinal, 11-cis-retinal-mediated photooxidation may play a significant role in light-induced damage to photoreceptor cells.

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.

Preventing diabetic retinopathy by mitigating subretinal space oxidative stress in vivo

Patients with diabetes continue to suffer from impaired visual performance before the appearance of overt damage to the retinal microvasculature and later sight-threatening complications. This diabetic retinopathy (DR) has long been thought to start with endothelial cell oxidative stress. Yet newer data surprisingly finds that the avascular outer retina is the primary site of oxidative stress before microvascular histopathology in experimental DR. Importantly, correcting this early oxidative stress is sufficient to restore vision and mitigate the histopathology in diabetic models. However, translating these promising results into the clinic has been stymied by an absence of methods that can measure and optimize anti-oxidant treatment efficacy in vivo. Here, we review imaging approaches that address this problem. In particular, diabetes-induced oxidative stress impairs dark-light regulation of subretinal space hydration, which regulates the distribution of interphotoreceptor binding protein (IRBP). IRBP is a vision-critical, anti-oxidant, lipid transporter, and pro-survival factor. We show how optical coherence tomography can measure subretinal space oxidative stress thus setting the stage for personalizing anti-oxidant treatment and prevention of impactful declines and loss of vision in patients with diabetes.

Coding Variants in HOOK2 and GTPBP3 May Contribute to Risk of Primary Angle Closure Glaucoma

Primary angle closure glaucoma (PACG) is a major cause of blindness worldwide. This study proposed to screen candidate PACG-associated variants in Chinese Han people. Whole exome sequencing was applied to five confirmed PACG patients and two primary angle closure suspect individuals within a PACG-enriched Chinese Han family. A series of bioinformatics analyses were implemented to obtain high-risk single nucleotide variant (SNV) loci for PACG, which were subsequently used for linkage analysis for identifying linkage genome regions. In addition, MassARRAY SNV genotyping was applied to high-risk PACG loci as well as those within linkage regions in another independent cohort including 251 PACG and 251 normal samples to further screen high-confidence SNVs. A total of 27 loci in 19 genes remained after linkage analysis. The 19 genes were significantly enriched in biological processes tightly related to PACG, including retinol metabolism and salmonella infection. Two nonsynonymous SNV loci, rs897804 in exon15 of HOOK2 and rs3745193 in exon7 of GTPBP3, were recognized with higher variant frequency in PACG samples than that in control samples after association analysis of MassARRAY SNV genotyping data. This study sheds new light on the understanding of PACG incidence among Chinese Han people.

Development of a therapeutic anti-HtrA1 antibody and the identification of DKK3 as a pharmacodynamic biomarker in geographic atrophy

Genetic polymorphisms in the region of the trimeric serine hydrolase high-temperature requirement 1 (HTRA1) are associated with increased risk of age-related macular degeneration (AMD) and disease progression, but the precise biological function of HtrA1 in the eye and its contribution to disease etiologies remain undefined. In this study, we have developed an HtrA1-blocking Fab fragment to test the therapeutic hypothesis that HtrA1 protease activity is involved in the progression of AMD. Next, we generated an activity-based small-molecule probe (ABP) to track target engagement in vivo. In addition, we used N-terminomic proteomic profiling in preclinical models to elucidate the in vivo repertoire of HtrA1-specific substrates, and identified substrates that can serve as robust pharmacodynamic biomarkers of HtrA1 activity. One of these HtrA1 substrates, Dickkopf-related protein 3 (DKK3), was successfully used as a biomarker to demonstrate the inhibition of HtrA1 activity in patients with AMD who were treated with the HtrA1-blocking Fab fragment. This pharmacodynamic biomarker provides important information on HtrA1 activity and pharmacological inhibition within the ocular compartment.

Retinal proteomic evaluation of rats following streptozotocin‑injection using shotgun proteomics

It is important to elucidate how retinal stimulation leads to retinal protection and dysfunction. The current study aimed to identify factors that are up‑ and downregulated in the retinas of streptozotocin (STZ)‑induced diabetic rats with acute retinal dysfunction. Retinal function was measured and changes in protein expressions were determined using electroretinograms (ERGs) and liquid chromatography/mass spectroscopy‑based shotgun proteomics, respectively. The results revealed that the plasma glucose levels of STZ rats were markedly higher when compared with normal rats. Furthermore, levels of a‑waves, b‑waves and oscillatory potential amplitudes on ERGs in STZ rats were decreased compared with healthy animals. With use of shotgun proteomics, 391 proteins were identified in the retinas of normal rats and 541 proteins were found in the retinas of STZ rats. Of the 560 proteins identified in rat retinas, 372 (66.4%) were present in both normal and STZ rats. Of these, 19 (3.39%) were unique to normal rats and 169 (30.1%) were unique to STZ rats. Gene Ontology analysis was performed on the candidate proteins that were differentially regulated in the retinas of STZ rats and focused on those classified as 'protein binding', which serve important roles in retinal neurodegeneration. The results revealed an excessive expression of retinol‑binding protein 1 (RBP1) and a negative expression of rod outer segment membrane protein 1 (Rom-1) in the retinas of STZ rats. Therefore, retinal function may be decreased with STZ‑induced injury, and expressions of Rom‑1 and RBP1 may be altered in the retinas of STZ rats.

Structural biology of 11-cis-retinaldehyde production in the classical visual cycle

The vitamin A derivative 11-cis-retinaldehyde plays a pivotal role in vertebrate vision by serving as the chromophore of rod and cone visual pigments. In the initial step of vision, a photon is absorbed by this chromophore resulting in its isomerization to an all-trans state and consequent activation of the visual pigment and phototransduction cascade. Spent chromophore is released from the pigments through hydrolysis. Subsequent photon detection requires the delivery of regenerated 11-cis-retinaldehyde to the visual pigment. This trans-cis conversion is achieved through a process known as the visual cycle. In this review, we will discuss the enzymes, binding proteins and transporters that enable the visual pigment renewal process with a focus on advances made during the past decade in our understanding of their structural biology.