Inhibition of the visual cycle in vivo by 13-cis retinoic acid protects from light damage and provides a mechanism for night blindness in isotretinoin therapy

Unveiling the hidden ocular risks of isotretinoin: a comprehensive FAERS-Based analysis

OBJECTIVE

While extensive research has been conducted on isotretinoin's systemic side effects, studies focusing on its ocular side effects remain limited and often lack substantial sample sizes. To address this gap, we conducted a comprehensive investigation of isotretinoin-related ocular toxicity using data from the FAERS spanning 2004 to 2024.

METHODS

After excluding duplicate and incomplete records from the FAERS database, we identified 760 eye-related adverse event reports from a total of 45,258 isotretinoin-related entries. We employed the Reporting Odds Ratio (ROR) method to assess the risk of ocular problems. Additionally, we examined the onset timing of eye toxicity.

RESULTS

Among the 760 reports analyzed, dry eye emerged as the most frequently reported condition (n = 222), although it did not exhibit the strongest association. The ROR was observed for night blindness (ROR = 35.8, 95% CI = 29.66-43.21), indicating a significant risk. This finding underscores the need to focus on isotretinoin's impact on the retina and fundus, especially since night blindness and vision loss can manifest as early as the first day of treatment.

CONCLUSION

Our study has identified various ocular Preferred Terms (PTs) associated with isotretinoin, highlighting a significant link to retinal effects. Notably, some individuals reported blindness and night blindness following short-term use of isotretinoin. These findings prompt new recommendations for safety monitoring by clinicians. However, additional clinical and fundamental research is essential to substantiate these observations and further elucidate the effects of isotretinoin on ocular health.

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.

Gene Therapy for Inherited Retinal Diseases: From Laboratory Bench to Patient Bedside and Beyond

This comprehensive review provides a thorough examination of inherited retinal diseases (IRDs), encompassing their classification, genetic underpinnings, and the promising landscape of gene therapy trials. IRDs, a diverse group of genetic conditions causing vision loss through photoreceptor cell death, are explored through various angles, including inheritance patterns, gene involvement, and associated systemic disorders. The focal point is gene therapy, which offers hope for halting or even reversing the progression of IRDs. The review highlights ongoing clinical trials spanning retinal cell replacement, neuroprotection, pharmacological interventions, and optogenetics. While these therapies hold tremendous potential, they face challenges like timing optimization, standardized assessment criteria, inflammation management, vector refinement, and raising awareness among vision scientists. Additionally, translating gene therapy success into widespread adoption and addressing cost-effectiveness are crucial challenges to address. Continued research and clinical trials are essential to fully harness gene therapy's potential in treating IRDs and enhancing the lives of affected individuals.

Rapamycin Inhibits Light-Induced Necrosome Activation Occurring in Wild-Type, but not RPE65-Null, Mouse Retina

Purpose

Both photodamage and aberrant visual cycle contribute to disease progress of many retinal degenerative disorders, whereas the signaling pathways causing photoreceptor death remain unclear. Here we investigated the effects of intense photo-stress on (1) necrosome activation in wild-type and RPE65-null mice, (2) interaction of p62/Sequestosome-1 with the necrosome proteins, and (3) the effects of rapamycin on photodamage-induced necrosome activation and retinal degeneration in wild-type mice.

Methods

Dark-adapted rd12 mice and 129S2/Sv mice with or without rapamycin treatment were exposed to 15,000 lux light for different times. Expression levels and subcellular localization of proteins were determined through immunoblot and immunohistochemical analyses. Cone sheaths were stained with peanut agglutinin. Correlation between photoreceptor degeneration and receptor-interacting protein kinase-1 (RIPK1) expression was assessed with Spearman's correlation analysis. Protein-protein interaction was analyzed by immunoprecipitation.

Results

Intense light caused rod and cone degeneration accompanied by a significant increase in RIPK1-RIPK3 expressions, mixed lineage kinase domain-like protein phosphorylation, damage-associated molecular patterns protein release, and inflammatory responses in wild-type mouse retina. The same intense light did not induce the necrosome activation in rd12 retina, but it did in rd12 mice that received 9-cis-retinal supply. RIPK1 expression levels are positively correlated with the degrees of rod and cone degeneration. Photodamage upregulated expression and interaction of the p62 autophagosome cargo protein with the necrosome proteins, whereas rapamycin treatment attenuated the light-induced necrosome activation and photoreceptor degeneration.

Conclusions

Necrosome activation contributed to photodamage-induced rod and cone degeneration. The visual cycle and autophagy are the important therapeutic targets to alleviate light-induced retinal necroptosis.

Cross-sectional assessment of the ellipsoid zone and the retinal pigment epithelium-Bruch membrane complex after systemic isotretinotin use

PURPOSE

To evaluate the cross-sectional areas of the retinal pigment epithelium-Bruch membrane complex (RPE-B) and ellipsoid zon (EZ) and the thickness of the macula, retinal nerve fibre layer (RNFL), and ganglion cell-inner plexiform layer (GC-IPL) in patients using short-term systemic isotretinoin.

METHOD

A total of 43 right eyes of 43 patients treated with systemic isotretinoin for acne vulgaris were included in this prospective study. Macula, GC-IPL, RNFL thicknesses and central cross-sectional EZ and RPE-B areas were evaluated with optical coherence tomography (Zeiss, Cirrus HD OCT 5000) prior to treatment and in first, second and third months after the onset of isotretinoin treatment. For the measurement of EZ and RPE-B area, foveal EDI-OCT scans were binarized by using the public domain software ImageJ 1.51 s.

RESULTS

Mean duration of isotretinoin treatment was 77 ± 15 days and mean dose was 2228 ± 574 milligrams. There was a statistically significant increment in central cross-sectional EZ and RPE-B areas in each follow-up examination, when analysed by repeated measurement analysis (p:0.002 and p:0.006, respectively). There was no correlation between total isotretinoin dose and the difference between final and basal EZ and RPE-B areas (p > 0.05, for both). When repeated measurements in follow-up examinations were compared, GC-IPL thicknesses except the superotemporal region (p:0.040) and RNFL thicknesses did not show a significant difference (p > 0.05). There was not any significant relation between total isotretinoin dose and 3rd month and basal measurement differences in macula, GC-IPL and RNFL thicknesses in any area (p > 0.05, for all).

CONCLUSION

There has been an increase in the area of RPE-B and EZ with short-term use of isotretinoin therapy. Future studies examining the relationship between functional tests and the RPE-B and EZ areas may provide more in-depth information on the effects of isotretinoin in the eye.

Retinal pigment epithelium 65 kDa protein (RPE65): An update

Vertebrate vision critically depends on an 11-cis-retinoid renewal system known as the visual cycle. At the heart of this metabolic pathway is an enzyme known as retinal pigment epithelium 65 kDa protein (RPE65), which catalyzes an unusual, possibly biochemically unique, reaction consisting of a coupled all-trans-retinyl ester hydrolysis and alkene geometric isomerization to produce 11-cis-retinol. Early work on this isomerohydrolase demonstrated its membership to the carotenoid cleavage dioxygenase superfamily and its essentiality for 11-cis-retinal production in the vertebrate retina. Three independent studies published in 2005 established RPE65 as the actual isomerohydrolase instead of a retinoid-binding protein as previously believed. Since the last devoted review of RPE65 enzymology appeared in this journal, major advances have been made in a number of areas including our understanding of the mechanistic details of RPE65 isomerohydrolase activity, its phylogenetic origins, the relationship of its membrane binding affinity to its catalytic activity, its role in visual chromophore production for rods and cones, its modulation by macromolecules and small molecules, and the involvement of RPE65 mutations in the development of retinal diseases. In this article, I will review these areas of progress with the goal of integrating results from the varied experimental approaches to provide a comprehensive picture of RPE65 biochemistry. Key outstanding questions that may prove to be fruitful future research pursuits will also be highlighted.

An Overview of the Genetics of ABCA4 Retinopathies, an Evolving Story

Stargardt disease (STGD1) and ABCA4 retinopathies (ABCA4R) are caused by pathogenic variants in the ABCA4 gene inherited in an autosomal recessive manner. The gene encodes an importer flippase protein that prevents the build-up of vitamin A derivatives that are toxic to the RPE. Diagnosing ABCA4R is complex due to its phenotypic variability and the presence of other inherited retinal dystrophy phenocopies. ABCA4 is a large gene, comprising 50 exons; to date > 2000 variants have been described. These include missense, nonsense, splicing, structural, and deep intronic variants. Missense variants account for the majority of variants in ABCA4. However, in a significant proportion of patients with an ABCA4R phenotype, a second variant in ABCA4 is not identified. This could be due to the presence of yet unknown variants, or hypomorphic alleles being incorrectly classified as benign, or the possibility that the disease is caused by a variant in another gene. This underlines the importance of accurate genetic testing. The pathogenicity of novel variants can be predicted using in silico programs, but these rely on databases that are not ethnically diverse, thus highlighting the need for studies in differing populations. Functional studies in vitro are useful towards assessing protein function but do not directly measure the flippase activity. Obtaining an accurate molecular diagnosis is becoming increasingly more important as targeted therapeutic options become available; these include pharmacological, gene-based, and cell replacement-based therapies. The aim of this review is to provide an update on the current status of genotyping in ABCA4 and the status of the therapeutic approaches being investigated.

The role of multimodal imaging and vision function testing in ABCA4-related retinopathies and their relevance to future therapeutic interventions

The aim of this review article is to describe the specific features of Stargardt disease and ABCA4 retinopathies (ABCA4R) using multimodal imaging and functional testing and to highlight their relevance to potential therapeutic interventions. Standardised measures of tissue loss, tissue function and rate of change over time using formal structured deep phenotyping in Stargardt disease and ABCA4R are key in diagnosis, and prognosis as well as when selecting cohorts for therapeutic intervention. In addition, a meticulous documentation of natural history will be invaluable in the future to compare treated with untreated retinas. Despite the familiarity with the term Stargardt disease, this eponymous classification alone is unhelpful when evaluating ABCA4R, as the ABCA4 gene is associated with a number of phenotypes, and a range of severity. Multimodal imaging, psychophysical and electrophysiologic measurements are necessary in diagnosing and characterising these differing retinopathies. A wide range of retinal dystrophy phenotypes are seen in association with ABCA4 mutations. In this article, these will be referred to as ABCA4R. These different phenotypes and the existence of phenocopies present a significant challenge to the clinician. Careful phenotypic characterisation coupled with the genotype enables the clinician to provide an accurate diagnosis, associated inheritance pattern and information regarding prognosis and management. This is particularly relevant now for recruiting to therapeutic trials, and in the future when therapies become available. The importance of accurate genotype-phenotype correlation studies cannot be overemphasised. This approach together with segregation studies can be vital in the identification of causal mutations when variants in more than one gene are being considered as possible. In this article, we give an overview of the current imaging, psychophysical and electrophysiological investigations, as well as current therapeutic research trials for retinopathies associated with the ABCA4 gene.

Prolonged Melanopsin-based Photoresponses Depend in Part on RPE65 and Cellular Retinaldehyde-binding Protein (CRALBP)

PURPOSE

Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin and can signal light continuously for many hours. Melanopsin is excited when its chromophore 11-cis-retinal absorbs a photon and becomes all-trans-retinal, which must be reisomerized to 11-cis-retinal to regenerate photoexcitable melanopsin. Due to the great distance separating ipRGCs from the retinal pigment epithelium (RPE) whose retinoid cycle produces 11-cis-retinal, ipRGCs had been assumed to regenerate all melanopsin molecules autonomously. Surprisingly, we previously found that pharmacologically inhibiting the retinoid cycle rendered melanopsin-based responses to prolonged illumination less sustained, suggesting that the RPE may supply retinoids to help ipRGCs regenerate melanopsin during extended photostimulation. However, the specificity of those drugs is unclear. Here, we reexamined the role of the retinoid cycle, and tested whether the RPE-to-ipRGC transport of retinoids utilizes cellular retinaldehyde-binding protein (CRALBP), present throughout the RPE and Müller glia.

METHODS

To measure melanopsin-mediated photoresponses in isolation, all animals were 8- to 12-month-old rod/cone-degenerate mice. We genetically knocked out RPE-specific 65 kDa protein (RPE65), a critical enzyme in the retinoid cycle. We also knocked out the CRALBP gene rlbp1 mainly in Foxg1-expressing Müller cells. We obtained multielectrode-array recordings from ipRGCs in a novel RPE-attached mouse retina preparation, and imaged pupillary light reflexes in vivo.

RESULTS

Melanopsin-based ipRGC responses to prolonged light became less tonic in both knockout lines, and pupillary light reflexes were also less sustained in RPE65-knockout than control mice.

CONCLUSIONS

These results confirm that ipRGCs rely partly on the retinoid cycle to continuously regenerate melanopsin during prolonged photostimulation, and suggest that CRALBP in Müller glia likely transports 11-cis-retinal from the RPE to ipRGCs - this is the first proposed functional role for CRALBP in the inner retina.

Oral isotretinoin and topical retinoid use in a series of young patients with ocular melanoma

Purpose

To describe the first series of six young uveal melanoma (UM) patients with oral isotretinoin and/or topical retinoid therapy prior to diagnosis.

Observations

The case series is based on clinical observations at our UM quaternary referral center. Six UM patient cases are reported, ages 16–44 years old. All had been using either oral (isotretinoin) and/or topical (tretinoin or tazarotene) retinoid treatment (3 months–~10 years) prior to or at the time of diagnosis (3 of 6 cases). All patients had ocular complaints on presentation, and the onset of certain symptoms corresponded with the course of retinoids. Other potential risk factors or relevant history included Caucasian background, cone-rod dystrophy and active smoker status (Case 2), family history of UM and pregnancy at time of diagnosis (Case 3), past smoking and possible secondary Chernobyl exposure as a baby (Case 5). All patients were treated with proton beam radiotherapy and currently have no sign of recurrent or metastatic disease.

Conclusions and importance

Retinoid therapy has been linked to various benign and/or reversible effects on the anterior and posterior eye, though pathophysiology remains not well understood. Uveal melanoma (UM) is a rare cancer diagnosis in young adults. We report here the first case series of young UM patients with a history of retinoid use and ocular complaints. No causal link is claimed and further systematic epidemiologic and biologic study of retinoid therapy and ocular impact may provide additional relevant data, particularly in young ocular melanoma patients.

Pharmacotherapy for metabolic and cellular stress in degenerative retinal diseases

Retinal photoreceptors continually endure stresses associated with prolonged light exposure and the metabolic demands of dark adaptation. Although healthy photoreceptors are able to withstand these stresses for several decades, the disease-affected retina functions at a reduced capacity and is at an increased risk for dysfunction. To alleviate cellular and metabolic stressors in degenerative retinal diseases, a new class of drugs that modulate the metabolic activity of the retina have been developed. A clinical candidate in this class (emixustat) has been shown to reduce retinal pathology in various animal models of human retinal disease and is currently under clinical study. Here, we describe the pharmacological properties of emixustat, its mechanisms of action, and potential for use in the treatment of specific retinal diseases.

Impacts of deletion and ichthyosis prematurity syndrome-associated mutations in fatty acid transport protein 4 on the function of RPE65

The retinal pigment epithelium-specific 65 kDa (RPE65) isomerase plays a pivotal role in photoreceptor survival and function. RPE65-catalyzed synthesis of 11-cis-retinol from all-trans-retinyl esters in the visual cycle is negatively regulated, through a heretofore unknown mechanism, by the fatty acid transport protein FATP4, mutations in which are associated with ichthyosis prematurity syndrome (IPS). Here, we analyzed the interaction between deletion mutants of FATP4 and RPE65 and the impacts of IPS-associated FATP4 mutations on RPE65 expression, 11-cis-retinol synthesis, and all-trans-retinyl ester synthesis. Our results suggest that the interaction between FATP4 and RPE65 contributes to the inhibition of RPE65 function and that IPS-associated nonsense and missense mutations in FATP4 have different effects on the visual cycle.

Functional trade-offs and environmental variation shaped ancient trajectories in the evolution of dim-light vision

Trade-offs between protein stability and activity can restrict access to evolutionary trajectories, but widespread epistasis may facilitate indirect routes to adaptation. This may be enhanced by natural environmental variation, but in multicellular organisms this process is poorly understood. We investigated a paradoxical trajectory taken during the evolution of tetrapod dim-light vision, where in the rod visual pigment rhodopsin, E122 was fixed 350 million years ago, a residue associated with increased active-state (MII) stability but greatly diminished rod photosensitivity. Here, we demonstrate that high MII stability could have likely evolved without E122, but instead, selection appears to have entrenched E122 in tetrapods via epistatic interactions with nearby coevolving sites. In fishes by contrast, selection may have exploited these epistatic effects to explore alternative trajectories, but via indirect routes with low MII stability. Our results suggest that within tetrapods, E122 and high MII stability cannot be sacrificed-not even for improvements to rod photosensitivity.

NDRG2 suppression as a molecular hallmark of photoreceptor-specific cell death in the mouse retina

Photoreceptor cell death is recognized as the key pathogenesis of retinal degeneration, but the molecular basis underlying photoreceptor-specific cell loss in retinal damaging conditions is virtually unknown. The N-myc downstream regulated gene (NDRG) family has recently been reported to regulate cell viability, in particular NDRG1 has been uncovered expression in photoreceptor cells. Accordingly, we herein examined the potential roles of NDRGs in mediating photoreceptor-specific cell loss in retinal damages. By using mouse models of retinal degeneration and the 661 W photoreceptor cell line, we showed that photoreceptor cells are indeed highly sensitive to light exposure and the related oxidative stress, and that photoreceptor cells are even selectively diminished by phototoxins of the alkylating agent N-Methyl-N-nitrosourea (MNU). Unexpectedly, we discovered that of all the NDRG family members, NDRG2, but not the originally hypothesized NDRG1 or other NDRG subtypes, was selectively expressed and specifically responded to retinal damaging conditions in photoreceptor cells. Furthermore, functional experiments proved that NDRG2 was essential for photoreceptor cell viability, which could be attributed to NDRG2 control of the photo-oxidative stress, and that it was the suppression of NDRG2 which led to photoreceptor cell loss in damaging conditions. More importantly, NDRG2 preservation contributed to photoreceptor-specific cell maintenance and retinal protection both in vitro and in vivo. Our findings revealed a previously unrecognized role of NDRG2 in mediating photoreceptor cell homeostasis and established for the first time the molecular hallmark of photoreceptor-specific cell death as NDRG2 suppression, shedding light on improved understanding and therapy of retinal degeneration.

Stargardt macular dystrophy and evolving therapies

INTRODUCTION

Stargardt macular dystrophy (STGD1) is a hereditary retinal degeneration that lacks effective treatment options. Gene therapy, stem cell therapy, and pharmacotherapy with visual cycle modulators (VCMs) and complement inhibitors are discussed as potential treatments.

AREAS COVERED

Investigational therapies for STGD1 aim to reduce toxic bisretinoids and lipofuscin in the retina and retinal pigment epithelium (RPE). These agents include C20-D3-vitamin A (ALK-001), isotretinoin, VM200, emixustat, and A1120. Avacincaptad pegol is a C5 complement inhibitor that may reduce inflammation-related RPE damage. Animal models of STGD1 show promising data for these treatments, though proof of efficacy in humans is lacking. Fenretinide and emixustat are VCMs for dry AMD and STGD1 that failed to halt geographic atrophy progression or improve vision in trials for AMD. A1120 prevents retinol transport into RPE and may spare side effects typically seen with VCMs (nyctalopia and chromatopsia). Stem cell transplantation suggests potential biologic plausibility in a phase I/II trial. Gene therapy aims to augment the mutated ABCA4 gene, though results of a phase I/II trial are pending.

EXPERT OPINION

Stem cell transplantation, ABCA4 gene therapy, VCMs, and complement inhibitors offer biologically plausible treatment mechanisms for treatment of STGD1. Further trials are warranted to assess efficacy and safety in humans.

Effect of 60 and 90 days of isotretinoin treatment on the structure of the small intestine mucosa in young male Wistar rats

Isotretinoin is a substance used in cases of severe acne and acne resistant to other treatments. This skin disease affects patients of all ages and can interfere with social life, especially in adolescents. The drug acts by suppressing sebaceous gland activity and creating an inhospitable environment for Propionibacterium acne. The integrity of the small intestine is important for correct nutrition and patient treatment. We intended to assess the small intestine structure after treatment with 5 mg/kg isotretinoin solution and after a period without the drug, which could be considered a rest period. Young male Wistar rats (n=24) were separated into 4 groups (n=6): C: water; D0: soybean oil; D5a: 5 mg/kg; D5b: 5 mg/kg for 60 days followed by 30 days of rest period. Soybean oil was used to dilute the drug and it was offered daily by gavage. The animals were euthanized and the duodenum, jejunum and ileum were collected for analysis with light and scanning electron microscopy. The treatment stimulated tissue proliferation in the jejunum and ileum but had no significant effect in the duodenum. The results also showed a modification in goblet cell frequency in the duodenum and ileum. A further finding was that some modifications disappeared during the rest period. The protocol showed that the small intestine was somewhat altered by the treatment yet no lasting damage was caused.

Ciliary neurotrophic factor (CNTF) protects retinal cone and rod photoreceptors by suppressing excessive formation of the visual pigments

The retinal pigment epithelium (RPE)-dependent visual cycle provides 11-cis-retinal to opsins in the photoreceptor outer segments to generate functional visual pigments that initiate phototransduction in response to light stimuli. Both RPE65 isomerase of the visual cycle and the rhodopsin visual pigment have recently been identified as critical players in mediating light-induced retinal degeneration. These findings suggest that the expression and function of RPE65 and rhodopsin need to be coordinately controlled to sustain normal vision and to protect the retina from photodamage. However, the mechanism controlling the development of the retinal visual system remains poorly understood. Here, we show that deficiency in ciliary neurotrophic factor (CNTF) up-regulates the levels of rod and cone opsins accompanied by an increase in the thickness of the outer nuclear layers and the lengths of cone and rod outer segments in the mouse retina. Moreover, retinoid isomerase activity, expression levels of RPE65 and lecithin:retinol acyltransferase (LRAT), which synthesizes the RPE65 substrate, were also significantly increased in the Cntf ^-/- RPE. Rod a-wave and cone b-wave amplitudes of electroretinograms were increased in Cntf ^-/- mice, but rod b-wave amplitudes were unchanged compared with those in WT mice. Up-regulated RPE65 and LRAT levels accelerated both the visual cycle rate and recovery rate of rod light sensitivity in Cntf ^-/- mice. Of note, rods and cones in Cntf ^-/- mice exhibited hypersusceptibility to light-induced degeneration. These results indicate that CNTF is a common extracellular factor that prevents excessive production of opsins, the photoreceptor outer segments, and 11-cis-retinal to protect rods and cones from photodamage.

A non-retinoid antagonist of retinol-binding protein 4 rescues phenotype in a model of Stargardt disease without inhibiting the visual cycle

A primary pathological defect in the heritable eye disorder Stargardt disease is excessive accumulation of cytotoxic lipofuscin bisretinoids in the retina. Age-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE) matches the age-dependent increase in the incidence of the atrophic (dry) form of age-related macular degeneration (AMD) and therefore may be one of several pathogenic factors contributing to AMD progression. Lipofuscin bisretinoid synthesis in the retina depends on the influx of serum retinol from the circulation into the RPE. Formation of the tertiary retinol-binding protein 4 (RBP4)-transthyretin-retinol complex in the serum is required for this influx. Herein, we report the pharmacological effects of the non-retinoid RBP4 antagonist, BPN-14136. BPN-14136 dosing in the Abca4^-/- mouse model of increased lipofuscinogenesis significantly reduced serum RBP4 levels and inhibited bisretinoid synthesis, and this inhibition correlated with a partial reduction in visual cycle retinoids such as retinaldehydes serving as bisretinoid precursors. BPN-14136 administration at doses inducing maximal serum RBP4 reduction did not produce changes in the rate of the visual cycle, consistent with minimal changes in dark adaptation. Abca4^-/- mice exhibited dysregulation of the complement system in the retina, and BPN-14136 administration normalized the retinal levels of proinflammatory complement cascade components such as complement factors D and H, C-reactive protein, and C3. We conclude that BPN-14136 has several beneficial characteristics, combining inhibition of bisretinoid synthesis and reduction in retinaldehydes with normalization of the retinal complement system. BPN-14136, or a similar compound, may be a promising drug candidate to manage Stargardt disease and dry AMD.

A novel RPE65 inhibitor CU239 suppresses visual cycle and prevents retinal degeneration

The retinoid visual cycle is an ocular retinoid metabolism specifically dedicated to support vertebrate vision. The visual cycle serves not only to generate light-sensitive visual chromophore 11-cis-retinal, but also to clear toxic byproducts of normal visual cycle (i.e. all-trans-retinal and its condensation products) from the retina, ensuring both the visual function and the retinal health. Unfortunately, various conditions including genetic predisposition, environment and aging may attribute to a functional decline of the all-trans-retinal clearance. To combat all-trans-retinal mediated retinal degeneration, we sought to slow down the retinoid influx from the RPE by inhibiting the visual cycle with a small molecule. The present study describes identification of CU239, a novel non-retinoid inhibitor of RPE65, a key enzyme in the visual cycle. Our data demonstrated that CU239 selectively inhibited isomerase activity of RPE65, with IC₅₀ of 6 μM. Further, our results indicated that CU239 inhibited RPE65 via competition with its substrate all-trans-retinyl ester. Mice with systemic injection of CU239 exhibited delayed chromophore regeneration after light bleach, and conferred a partial protection of the retina against injury from high intensity light. Taken together, CU239 is a potent visual cycle modulator and may have a therapeutic potential for retinal degeneration.

Side effects of retinoid therapy on the quality of vision

Retinoids are compounds chemically related to vitamin A, which are frequently used in dermatological practice (1). They are characterized by numerous mechanisms of action leading to normalization of keratinocyte proliferation and maturation. They have anti-seborrhoeic, immunomodulatory and anti-inflammatory effects (1, 2). A number of side effects to retinoid treatment have been recorded; one group of such side effects relates to eyes and vision. Dry eye syndrome and blepharoconjunctivitis are the most common side effects, appearing in 20-50 % of patients treated with retinoids. They often contribute to the occurrence of other side-effects such as eye discomfort and contact lens intolerance. Due to the widespread use in clinical practice, the adverse effects, including ocular side effects, should be studied. To confirm the variety of adverse effects of retinoids, several case reports of rare side-effects are presented.

Retinal Histopathology in Eyes from a Patient with Stargardt disease caused by Compound Heterozygous ABCA4 Mutations

BACKGROUND

The goal of this study was to define the histopathology of the retina in donor eyes from a patient with Stargardt disease (STGD1) due to compound mutations in the ABCA4 gene.

MATERIALS AND METHODS

Eyes were obtained from a 66-year-old female and fixed within 18 hours postmortem. The fundi of the posterior globes were evaluated with macroscopic, SLO and OCT imaging. The perifoveal and peripheral regions were processed for electron microscopy and immunocytochemistry using cell specific antibodies. Two age-similar normal eyes were used as controls. Prior ophthalmic examinations and genetic test results were also reviewed.

RESULTS

All imaging modalities showed scattered bone spicules in the peripheral retina. Atrophy of the RPE was present around the optic nerve as evidenced by the absence of SLO autofluorescence. Histology analysis showed a severely degenerated fovea with little evidence of any retinal layering or remaining RPE. The fovea was severely degenerated, with little evidence of any retinal cell layer, including the RPE. In contrast, retinal nuclear layers were present in the periphery. The perifoveal region contained few cones labeled with cone-specific antibodies; some rhodopsin-labeled cells, reactive glia labeled with GFAP; and decreased autofluorescence of the RPE. The fovea was free of cone-specific labeling, contained a few disorganized rhodopsin-labeled cells and showed substantial GFAP labeling and no autofluorescent material in the retina. The periphery displayed stubby cells labeled with cone-specific antibodies, decreased rhodopsin-labeled cells, increased GFAP staining, and autofluorescent granules in the RPE.

CONCLUSIONS

The histopathology of the retina in this patient with Stargardt disease displayed a highly degenerated fovea. In all retinal locations studied, cones were more severely affected than rods.

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.

PBN (Phenyl-N-Tert-Butylnitrone)-Derivatives Are Effective in Slowing the Visual Cycle and Rhodopsin Regeneration and in Protecting the Retina from Light-Induced Damage

A2E and related toxic molecules are part of lipofuscin found in the retinal pigment epithelial (RPE) cells in eyes affected by Stargardt's disease, age-related macular degeneration (AMD), and other retinal degenerations. A novel therapeutic approach for treating such degenerations involves slowing down the visual cycle, which could reduce the amount of A2E in the RPE. This can be accomplished by inhibiting RPE65, which produces 11-cis-retinol from all-trans-retinyl esters. We recently showed that phenyl-N-tert-butylnitrone (PBN) inhibits RPE65 enzyme activity in RPE cells. In this study we show that like PBN, certain PBN-derivatives (PBNDs) such as 4-F-PBN, 4-CF3-PBN, 3,4-di-F-PBN, and 4-CH3-PBN can inhibit RPE65 and synthesis of 11-cis-retinol in in vitro assays using bovine RPE microsomes. We further demonstrate that systemic (intraperitoneal, IP) administration of these PBNDs protect the rat retina from light damage. Electroretinography (ERG) and histological analysis showed that rats treated with PBNDs retained ~90% of their photoreceptor cells compared to a complete loss of function and 90% loss of photoreceptors in the central retina in rats treated with vehicle/control injections. Topically applied PBN and PBNDs also significantly slowed the rate of the visual cycle in mouse and baboon eyes. One hour dark adaptation resulted in 75-80% recovery of bleachable rhodopsin in control/vehicle treated mice. Eye drops of 5% 4-CH3-PBN were most effective, inhibiting the regeneration of bleachable rhodopsin significantly (60% compared to vehicle control). In addition, a 10% concentration of PBN and 5% concentration of 4-CH3-PBN in baboon eyes inhibited the visual cycle by 60% and by 30%, respectively. We have identified a group of PBN related nitrones that can reach the target tissue (RPE) by systemic and topical application and slow the rate of rhodopsin regeneration and therefore the visual cycle in mouse and baboon eyes. PBNDs can also protect the rat retina from light damage. There is potential in developing these compounds as preventative therapeutics for the treatment of human retinal degenerations in which the accumulation of lipofuscin may be pathogenic.

Graded gene expression changes determine phenotype severity in mouse models of CRX-associated retinopathies

Background

Mutations in the cone-rod-homeobox protein CRX are typically associated with dominant blinding retinopathies with variable age of onset and severity. Five well-characterized mouse models carrying different Crx mutations show a wide range of disease phenotypes. To determine if the phenotype variability correlates with distinct changes in CRX target gene expression, we perform RNA-seq analyses on three of these models and compare the results with published data.

Results

Despite dramatic phenotypic differences between the three models tested, graded expression changes in shared sets of genes are detected. Phenotype severity correlates with the down-regulation of genes encoding key rod and cone phototransduction proteins. Interestingly, in increasingly severe mouse models, the transcription of many rod-enriched genes decreases decrementally, whereas that of cone-enriched genes increases incrementally. Unlike down-regulated genes, which show a high degree of CRX binding and dynamic epigenetic profiles in normal retinas, the up-regulated cone-enriched genes do not correlate with direct activity of CRX, but instead likely reflect a change in rod cell-fate integrity. Furthermore, these analyses describe the impact of minor gene expression changes on the phenotype, as two mutants showed marginally distinguishable expression patterns but huge phenotypic differences, including distinct mechanisms of retinal degeneration.

Conclusions

Our results implicate a threshold effect of gene expression level on photoreceptor function and survival, highlight the importance of CRX in photoreceptor subtype development and maintenance, and provide a molecular basis for phenotype variability in CRX-associated retinopathies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0732-z) contains supplementary material, which is available to authorized users.

Vitamin A Transport Mechanism of the Multitransmembrane Cell-Surface Receptor STRA6

Vitamin A has biological functions as diverse as sensing light for vision, regulating stem cell differentiation, maintaining epithelial integrity, promoting immune competency, regulating learning and memory, and acting as a key developmental morphogen. Vitamin A derivatives have also been used in treating human diseases. If vitamin A is considered a drug that everyone needs to take to survive, evolution has come up with a natural drug delivery system that combines sustained release with precise and controlled delivery to the cells or tissues that depend on it. This "drug delivery system" is mediated by plasma retinol binding protein (RBP), the principle and specific vitamin A carrier protein in the blood, and STRA6, the cell-surface receptor for RBP that mediates cellular vitamin A uptake. The mechanism by which the RBP receptor absorbs vitamin A from the blood is distinct from other known cellular uptake mechanisms. This review summarizes recent progress in elucidating the fundamental molecular mechanism mediated by the RBP receptor and multiple newly discovered catalytic activities of this receptor, and compares this transport system with retinoid transport independent of RBP/STRA6. How to target this new type of transmembrane receptor using small molecules in treating diseases is also discussed.

Molecular pharmacodynamics of emixustat in protection against retinal degeneration

Emixustat is a visual cycle modulator that has entered clinical trials as a treatment for age-related macular degeneration (AMD). This molecule has been proposed to inhibit the visual cycle isomerase RPE65, thereby slowing regeneration of 11-cis-retinal and reducing production of retinaldehyde condensation byproducts that may be involved in AMD pathology. Previously, we reported that all-trans-retinal (atRAL) is directly cytotoxic and that certain primary amine compounds that transiently sequester atRAL via Schiff base formation ameliorate retinal degeneration. Here, we have shown that emixustat stereoselectively inhibits RPE65 by direct active site binding. However, we detected the presence of emixustat-atRAL Schiff base conjugates, indicating that emixustat also acts as a retinal scavenger, which may contribute to its therapeutic effects. Using agents that lack either RPE65 inhibitory activity or the capacity to sequester atRAL, we assessed the relative importance of these 2 modes of action in protection against retinal phototoxicity in mice. The atRAL sequestrant QEA-B-001-NH2 conferred protection against phototoxicity without inhibiting RPE65, whereas an emixustat derivative incapable of atRAL sequestration was minimally protective, despite direct inhibition of RPE65. These data indicate that atRAL sequestration is an essential mechanism underlying the protective effects of emixustat and related compounds against retinal phototoxicity. Moreover, atRAL sequestration should be considered in the design of next-generation visual cycle modulators.

Effects of organic solvents on two retinal pigment epithelial lipofuscin fluorophores, A2E and all-trans-retinal dimer

Gene and drug therapies are being developed to alleviate vision loss in patients with Stargardt's disease and age-related macular degeneration (AMD). To evaluate the therapeutic effects of these treatments, organic solvents are routinely used to extract and quantify bisretinoid lipofuscin constituents, such as N-retinylidene-N-retinyl-ethanolamine (A2E) and all-trans-retinal dimer (ATR-dimer). By high-performance liquid chromatography (HPLC), we found that A2E and ATR-dimer were both altered by tetrahydrofuran (THF) and chloroform, but were stable in dimethyl sulfoxide (DMSO) or methanol (MeOH). In addition, cyclohexane and ethanol (EtOH) did not alter ATR-dimer, whereas an alteration of A2E occurred in EtOH. On the basis of these findings, we designed processes II-IV, generated by modifications of process I, a routine method to measure bisretinoid compounds in vivo. Extra amounts of either ATR-dimer or A2E in mouse eyecups were released by processes II-IV versus process I. Efforts to clarify the effects of organic solvents on lipofuscin pigments are important because such studies can guide the handling of these fluorophores in related experiments.

Angiotensin II type 1 receptor blockade suppresses light-induced neural damage in the mouse retina

Exposure to light contributes to the development and progression of retinal degenerative diseases. However, the mechanisms underlying light-induced tissue damage are not fully understood. Here, we examined the role of angiotensin II type 1 receptor (AT1R) signaling, which is part of the renin-angiotensin system, in light-induced retinal damage. Light-exposed Balb/c mice that were treated with the AT1R blockers (angiotensin II receptor blockers; ARBs) valsartan, losartan, and candesartan before and after the light exposure exhibited attenuated visual function impairment, compared to vehicle-treated mice. This effect was dose-dependent and observed across the ARB class of inhibitors. Further evaluation of valsartan showed that it suppressed a number of light-induced retinal effects, including thinning of the photoreceptor cell layer caused by apoptosis, shortening of the photoreceptor cell outer segment, and increased levels of reactive oxygen species (ROS). The role of ROS in retinal pathogenesis was investigated further using the antioxidant N-acetyl-l-cysteine (NAC). Treatment of light-exposed mice with NAC before the light exposure suppressed the visual function impairment and photoreceptor cell histological changes due to apoptosis. Moreover, treatment with valsartan or NAC suppressed the induction of c-fos (a component of the AP-1 transcription factor) and the upregulation of fasl (a proapoptotic molecule whose transcript is regulated downstream of AP-1). Our results suggest that AT1R signaling mediates light-induced apoptosis, by increasing the levels of ROS and proapoptotic molecules in the retina. Thus, AT1R blockade may represent a new therapeutic approach for preventing light-induced retinal neural tissue damage.

Photoreceptor cell death and rescue in retinal detachment and degenerations

Photoreceptor cell death is the ultimate cause of vision loss in various retinal disorders, including retinal detachment (RD). Photoreceptor cell death has been thought to occur mainly through apoptosis, which is the most characterized form of programmed cell death. The caspase family of cysteine proteases plays a central role for inducing apoptosis, and in experimental models of RD, dying photoreceptor cells exhibit caspase activation; however, there is a paradox that caspase inhibition alone does not provide a sufficient protection against photoreceptor cell loss, suggesting that other mechanisms of cell death are involved. Recent accumulating evidence demonstrates that non-apoptotic forms of cell death, such as autophagy and necrosis, are also regulated by specific molecular machinery, such as those mediated by autophagy-related proteins and receptor-interacting protein kinases, respectively. Here we summarize the current knowledge of cell death signaling and its roles in photoreceptor cell death after RD and other retinal degenerative diseases. A body of studies indicate that not only apoptotic but also autophagic and necrotic signaling are involved in photoreceptor cell death, and that combined targeting of these pathways may be an effective neuroprotective strategy for retinal diseases associated with photoreceptor cell loss.

Inhibition of de novo ceramide biosynthesis by FTY720 protects rat retina from light-induced degeneration

Light-induced retinal degeneration (LIRD) in albino rats causes apoptotic photoreceptor cell death. Ceramide is a second messenger for apoptosis. We tested whether increases in ceramide mediate photoreceptor apoptosis in LIRD and if inhibition of ceramide synthesis protects the retina. Sprague-Dawley rats were exposed to 2,700 lux white light for 6 h, and the retinal levels of ceramide and its intermediary metabolites were measured by GC-MS or electrospray ionization tandem mass spectrometry. Enzymes of the de novo biosynthetic and sphingomyelinase pathways of ceramide generation were assayed, and gene expression was measured. The dosage and temporal effect of the ceramide synthase inhibitor FTY720 on the LIRD retina were measured by histological and functional analyses. Retinal ceramide levels increased coincident with the increase of dihydroceramide at various time points after light stress. Light stress in retina induces ceramide generation predominantly through the de novo pathway, which was prevented by systemic administration of FTY720 (10 mg/kg) leading to the protection of retinal structure and function. The neuroprotection of FTY720 was independent of its immunosuppressive action. We conclude that ceramide increase by de novo biosynthesis mediates photoreceptor apoptosis in the LIRD model and that inhibition of ceramide production protects the retina against light stress.

Pharmacological inhibition of lipofuscin accumulation in the retina as a therapeutic strategy for dry AMD treatment

Age-related macular degeneration (AMD) is the leading cause of blindness in the western world. There is no FDA-approved treatment for the most prevalent dry (atrophic) form of AMD. Photoreceptor degeneration in dry AMD is triggered by abnormalities in the retinal pigment epithelium (RPE). It has been suggested that excessive accumulation of fluorescent lipofuscin pigment in the RPE represents an important pathogenic factor in etiology and progression of dry AMD. Cytotoxic lipofuscin bisretinoids, such as A2E, are formed in the retina in a non-enzymatic way from visual cycle retinoids. Inhibition of toxic bisretinoid production in the retina seems to be a sound treatment strategy for dry AMD. In this review we discuss the following classes of pharmacological treatments inhibiting lipofuscin bisretinoid formation in the retina: direct inhibitors of key visual cycle enzymes, RBP4 antagonists, primary amine-containing aldehyde traps, and deuterated analogs of vitamin A.

Retina, retinol, retinal and the natural history of vitamin A as a light sensor

Light is both the ultimate energy source for most organisms and a rich information source. Vitamin A-based chromophore was initially used in harvesting light energy, but has become the most widely used light sensor throughout evolution from unicellular to multicellular organisms. Vitamin A-based photoreceptor proteins are called opsins and have been used for billions of years for sensing light for vision or the equivalent of vision. All vitamin A-based light sensors for vision in the animal kingdom are G-protein coupled receptors, while those in unicellular organisms are light-gated channels. This first major switch in evolution was followed by two other major changes: the switch from bistable to monostable pigments for vision and the expansion of vitamin A's biological functions. Vitamin A's new functions such as regulating cell growth and differentiation from embryogenesis to adult are associated with increased toxicity with its random diffusion. In contrast to bistable pigments which can be regenerated by light, monostable pigments depend on complex enzymatic cycles for regeneration after every photoisomerization event. Here we discuss vitamin A functions and transport in the context of the natural history of vitamin A-based light sensors and propose that the expanding functions of vitamin A and the choice of monostable pigments are the likely evolutionary driving forces for precise, efficient, and sustained vitamin A transport.

Exome analysis identified a novel mutation in the RBP4 gene in a consanguineous pedigree with retinal dystrophy and developmental abnormalities

Retinitis Pigmentosa (RP) is a common form of retinal degeneration characterized by photoreceptor degeneration and retinal pigment epithelium (RPE) atrophy causing loss of visual field and acuities. Exome sequencing identified a novel homozygous splice site variant (c.111+1G>A) in the gene encoding retinol binding protein 4 (RBP4). This change segregated with early onset, progressive, and severe autosomal recessive retinitis pigmentosa (arRP) in an eight member consanguineous pedigree of European ancestry. Additionally, one patient exhibited developmental abnormalities including patent ductus arteriosus and chorioretinal and iris colobomas. The second patient developed acne from young age and extending into the 5^th decade. Both patients had undetectable levels of RBP4 in the serum suggesting that this mutation led to either mRNA or protein instability resulting in a null phenotype. In addition, the patients exhibited severe vitamin A deficiency, and diminished serum retinol levels. Circulating transthyretin levels were normal. This study identifies the RBP4 splice site change as the cause of RP in this pedigree. The presence of developmental abnormalities and severe acne in patients with retinal degeneration may indicate the involvement of genes that regulate vitamin A absorption, transport and metabolism.

Retinal photodamage mediated by all-trans-retinal

Accumulation of all-trans-retinal (all-trans-RAL), reactive vitamin A aldehyde, is one of the key factors in initiating retinal photodamage. This photodamage is characterized by progressive retinal cell death evoked by light exposure in both an acute and chronic fashion. Photoactivated rhodopsin releases all-trans-RAL, which is subsequently transported by ATP-binding cassette transporter 4 and reduced to all-trans-retinol by all-trans-retinol dehydrogenases located in photoreceptor cells. Any interruptions in the clearing of all-trans-RAL in the photoreceptors can cause an accumulation of this reactive aldehyde and its toxic condensation products. This accumulation may result in the manifestation of retinal dystrophy including human retinal degenerative diseases such as Stargardt's disease and age-related macular degeneration. Herein, we discuss the mechanisms of all-trans-RAL clearance in photoreceptor cells by sequential enzymatic reactions, the visual (retinoid) cycle, and potential molecular pathways of retinal photodamage. We also review recent imaging technologies to monitor retinal health status as well as novel therapeutic strategies preventing all-trans-RAL-associated retinal photodamage.

Safety and effect on rod function of ACU-4429, a novel small-molecule visual cycle modulator

BACKGROUND

ACU-4429 is a first in class small-molecule visual cycle modulator that inhibits the isomerase complex and, in mouse models of retinal degeneration, prevents the accumulation of A2E. The purpose of this study was to assess the tolerability, pharmacokinetics, pharmacodynamics, and safety of a single, orally administered dose of ACU-4429 in healthy subjects.

METHODS

Sequential cohorts were administered single doses ranging from 2 mg to 75 mg. Full-field electroretinograms were recorded before and after exposure to full-field bleaching light. Pharmacokinetics samples were taken at predetermined times. Safety assessments included adverse events, vital signs, clinical laboratory assays, electrocardiograms, and ophthalmologic examination.

RESULTS

After 45-minute dark adaptation, electroretinographic findings demonstrated a dose-related slowing of the rate of recovery that reached its maximum on Day 2 and returned to baseline by Day 7. Mean area under the concentration curve and peak plasma concentration increased proportionally with increasing doses. Median time to peak concentration was 4 hours postdose. Mean elimination mean half-life was 4 hours to 6 hours. Adverse events were mild and visual in nature (dyschromatopsia and alteration in dark adaptation), transient, and resolved within a few days. Adverse event frequency was dose dependent.

CONCLUSION

Oral administration of ACU-4429 produced a dose-dependent inhibition of the b-wave of the electroretinograms, was well tolerated up to 75 mg, and demonstrated linear pharmacokinetics across doses.

Alpha-phenyl-N-tert-butylnitrone (PBN) prevents light-induced degeneration of the retina by inhibiting RPE65 protein isomerohydrolase activity

α-Phenyl-N-tert-butylnitrone (PBN), a free radical spin trap, has been shown previously to protect retinas against light-induced neurodegeneration, but the mechanism of protection is not known. Here we report that PBN-mediated retinal protection probably occurs by slowing down the rate of rhodopsin regeneration by inhibiting RPE65 activity. PBN (50 mg/kg) protected albino Sprague-Dawley rat retinas when injected 0.5-12 h before exposure to damaging light at 2,700 lux intensity for 6 h but had no effect when administered after the exposure. PBN injection significantly inhibited in vivo recovery of rod photoresponses and the rate of recovery of functional rhodopsin photopigment. Assays for visual cycle enzyme activities indicated that PBN inhibited one of the key enzymes of the visual cycle, RPE65, with an IC(50) = 0.1 mm. The inhibition type for RPE65 was found to be uncompetitive with K(i) = 53 μm. PBN had no effect on the activity of other visual cycle enzymes, lecithin retinol acyltransferase and retinol dehydrogenases. Interestingly, a more soluble form of PBN, N-tert-butyl-α-(2-sulfophenyl) nitrone, which has similar free radical trapping activity, did not protect the retina or inhibit RPE65 activity, providing some insight into the mechanism of PBN specificity and action. Slowing down the visual cycle is considered a treatment strategy for retinal diseases, such as Stargardt disease and dry age-related macular degeneration, in which toxic byproducts of the visual cycle accumulate in retinal cells. Thus, PBN inhibition of RPE65 catalytic action may provide therapeutic benefit for such retinal diseases.

The membrane receptor for plasma retinol-binding protein, a new type of cell-surface receptor

Vitamin A is essential for diverse aspects of life ranging from embryogenesis to the proper functioning of most adult organs. Its derivatives (retinoids) have potent biological activities such as regulating cell growth and differentiation. Plasma retinol-binding protein (RBP) is the specific vitamin A carrier protein in the blood that binds to vitamin A with high affinity and delivers it to target organs. A large amount of evidence has accumulated over the past decades supporting the existence of a cell-surface receptor for RBP that mediates cellular vitamin A uptake. Using an unbiased strategy, this specific cell-surface RBP receptor has been identified as STRA6, a multitransmembrane domain protein with previously unknown function. STRA6 is not homologous to any protein of known function and represents a new type of cell-surface receptor. Consistent with the diverse functions of vitamin A, STRA6 is widely expressed in embryonic development and in adult organ systems. Mutations in human STRA6 are associated with severe pathological phenotypes in many organs such as the eye, brain, heart, and lung. STRA6 binds to RBP with high affinity and mediates vitamin A uptake into cells. This review summarizes the history of the RBP receptor research, its expression in the context of known functions of vitamin A in distinct human organs, structure/function analysis of this new type of membrane receptor, pertinent questions regarding its very existence, and its potential implication in treating human diseases.

Novel lipofuscin bisretinoids prominent in human retina and in a model of recessive Stargardt disease

Bisretinoid adducts accumulate as lipofuscin in retinal pigment epithelial (RPE) cells of the eye and are implicated in the pathology of inherited and age-related macular degeneration. Characterization of the bisretinoids A2E and the all-trans-retinal dimer series has shown that these pigments form from reactions in photoreceptor cell outer segments that involve all-trans-retinal, the product of photoisomerization of the visual chromophore 11-cis-retinal. Here we have identified two related but previously unknown RPE lipofuscin compounds. By high performance liquid chromatography-electrospray ionization-tandem mass spectrometry, we determined that the first of these compounds is a phosphatidyl-dihydropyridine bisretinoid; to indicate this structure and its formation from two vitamin A-aldehyde (A2), we will refer to it as A2-dihydropyridine-phosphatidylethanolamine (A2-DHP-PE). The second pigment, A2-dihydropyridine-ethanolamine, forms from phosphate hydrolysis of A2-DHP-PE. The structure of A2-DHP-PE was corroborated by Fourier transform infrared spectroscopy, and density functional theory confirmed the presence of a dihydropyridine ring. This lipofuscin pigment is a fluorescent compound with absorbance maxima at approximately 490 and 330 nm, and it was identified in human, mouse, and bovine eyes. We found that A2-DHP-PE forms in reaction mixtures of all-trans-retinal and phosphatidylethanolamine, and in mouse eyecups we observed an age-related accumulation. As compared with wild-type mice, A2-DHP-PE is more abundant in mice with a null mutation in Abca4 (ATP-binding cassette transporter 4), the gene causative for recessive Stargardt macular degeneration. Efforts to clarify the composition of RPE lipofuscin are important because these compounds are targets of gene-based and drug therapies that aim to alleviate ABCA4-related retinal disease.

[Light exposition in vitreoretinal surgery. I. Basics]

Due to its function of light perception, the eye is exposed to high levels of radiation of the optical spectrum. Most of the ultraviolet and infrared radiation is absorbed in the cornea and lens, and mostly only radiation of the visible spectrum can reach the retina. Visible light can cause retinal damage by photomechanical, photothermal, and photochemical mechanisms. The most important mechanism of light damage to the retina under daily conditions or when using ophthalmologic light sources is the photochemical light toxicity caused by light-induced chemical reactions. The extent of damage depends on several factors, such as wavelength, exposure time, and irradiance. Particularly the shorter portion of the visible light spectrum (blue light) is responsible for photochemical damage to the retina.