Characterization of a dehydrogenase activity responsible for oxidation of 11-cis-retinol in the retinal pigment epithelium of mice with a disrupted RDH5 gene. A model for the human hereditary disease fundus albipunctatus

A Molecular Genetic Analysis of RPE65-Associated Forms of Inherited Retinal Degenerations in the Russian Federation

Pathogenic variants in the RPE65 gene cause the only known form of inherited retinal degenerations (IRDs) that are prone to gene therapy. The current study is aimed at the evaluation of the prevalence of RPE65-associated retinopathy in the Russian Federation, the characterization of known variants in the RPE65 gene, and the establishment of the specificities of the mutation spectrum in Russian patients.

METHODS

The analysis was carried out on blood samples obtained from 1053 non-related IRDs patients. The analysis, which consisted of 211 genes, was carried out based on the method of massive parallel sequencing (MPS) for all probands. Variant validation, as well as biallelic status verification, were carried out using direct automated Sanger sequencing. The number of copies of RPE65 exons 1-14 was analyzed with quantitative MLPA using an MRC-Holland SALSA MLPA probemix.

RESULTS

Out of 1053 non-related patients, a molecular genetic diagnosis of IRDs has been confirmed in 474 cases, including 25 (5.3%) patients with RPE65-associated retinopathy. We detected 26 variants in the RPE65 gene, nine of which have not been previously described in the literature. The most common mutations in the Russian population were c.304G>T/p.(Glu102*), c.370C>T/p.(Arg124*), and c.272G>A/p.(Arg91Gln), which comprised 41.8% of all affected chromosomes.

CONCLUSIONS

The current study shows that pathogenic variants in the RPE65 gene contribute significantly to the pathogenesis of IRDs and comprise 5.3% of all patients with a confirmed molecular genetic diagnosis. This study allowed for the formation of a cohort for target therapy of the disorder; such therapy has already been carried out for some patients.

Fundus Albipunctatus Associated with Biallelic LRAT Gene Mutation: A Case Report with Long-Term Follow-Up

This case report presents a 26-year-old female patient diagnosed with fundus albipunctatus (FAP), a rare form of congenital stationary night blindness. The patient's clinical history and retinal findings spanning 23 years are consistent with FAP. The patient has profound night blindness, photophobia, and mild color vision changes with preserved best-corrected visual acuity (BCVA). Small white dots are present throughout the fundus, sparing the central macula. Electroretinograms (ERG) are consistent with congenital stationary night blindness (CSNB) and suggest a lack of rod response. Ophthalmic imaging has remained stable over time. Genetic testing revealed two biallelic missense mutations in the LRAT gene, c.197G>A (p.Gly66Glu) and c.557A>C (p.Lys186Thr). LRAT mutations are known to contribute to other retinal conditions but have not been previously associated with FAP. While there are currently no available treatments for FAP, this report expands our understanding of the genetic landscape of FAP to include LRAT and provides clinical data to support this finding.

Large Benefit from Simple Things: High-Dose Vitamin A Improves RBP4-Related Retinal Dystrophy

Inherited retinal diseases (IRD) are a group of heterogeneous disorders, most of which lead to blindness with limited therapeutic options. Pathogenic variants in RBP4, coding for a major blood carrier of retinol, retinol-binding protein 4, are responsible for a peculiar form of IRD. The aim of this study was to investigate if retinal function of an RBP4-related IRD patient can be improved by retinol administration. Our patient presented a peculiar white-dot retinopathy, reminiscent of vitamin A deficient retinopathy. Using a customized next generation sequencing (NGS) IRD panel we discovered a novel loss-of-function homozygous pathogenic variant in RBP4: c.255G >A, p.(Trp85*). Western blotting revealed the absence of RBP4 protein in the patient’s serum. Blood retinol levels were undetectable. The patient was put on a high-dose oral retinol regimen (50,000 UI twice a week). Subjective symptoms and retinal function markedly and sustainably improved at 5-months and 1-year follow-up. Here we show that this novel IRD case can be treated by oral retinol administration.

A large animal model of RDH5-associated retinopathy recapitulates important features of the human phenotype

Pathogenic variants in retinol dehydrogenase 5 (RDH5) attenuate supply of 11-cis-retinal to photoreceptors leading to a range of clinical phenotypes including night blindness because of markedly slowed rod dark adaptation and in some patients, macular atrophy. Current animal models (such as Rdh5-/- mice) fail to recapitulate the functional or degenerative phenotype. Addressing this need for a relevant animal model we present a new domestic cat model with a loss-of-function missense mutation in RDH5 (c.542G > T; p.Gly181Val). As with patients, affected cats have a marked delay in recovery of dark adaptation. In addition, the cats develop a degeneration of the area centralis (equivalent to the human macula). This recapitulates the development of macular atrophy that is reported in a subset of patients with RDH5 mutations and is shown in this paper in seven patients with biallelic RDH5 mutations. There is notable variability in the age at onset of the area centralis changes in the cat, with most developing changes as juveniles but some not showing changes over the first few years of age. There is similar variability in development of macular atrophy in patients and while age is a risk factor, it is hypothesized that genetic modifying loci influence disease severity, and we suspect the same is true in the cat model. This novel cat model provides opportunities to improve molecular understanding of macular atrophy and test therapeutic interventions for RDH5-associated retinopathies.

Novel variants in the RDH5 Gene in a Chinese Han family with fundus albipunctatus

Background

The aim of this study is to identify the genetic defects in a Chinese family with fundus albipunctatus.

Methods

Complete ophthalmic examinations, including slit-lamp biomicroscopy, dilated indirect ophthalmoscopy, fundus photography, autofluorescence, swept source optical coherence tomography (SS-OCT) and full-field electroretinography (ffERG) were performed. Genomic DNA was extracted from blood samples and whole genome sequencing was performed. Variants were validated with Sanger sequencing.

Results

Six members in this Chinese family, including three affected individuals and three controls, were recruited in this study. The ophthalmic examination of three recruited patients was consistent with fundus albipunctatus. Three variants, a novel frameshift deletion c.39delA [p.(Val14CysfsX47] and a haplotype of two rare missense variants, c.683G > A [p.(Arg228Gln)] along with c.710A > G [p.(Tyr237Cys], within the retinal dehydrogenase 5 (RDH5) gene were found to segregate with fundus albipunctatus in this family in an autosomal recessive matter.

Conclusion

We identified novel compound heterozygous variants in RDH5 responsible for fundus albipunctatus in a large Chinese family. The results of our study further broaden the genetic defects of RDH5 associated with fundus albipunctatus.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-022-02301-5.

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.

A spectral-domain optical coherence tomographic analysis of Rdh5-/- mice retina

PURPOSE

To investigate the longitudinal findings of spectral-domain optical coherence tomography (SD-OCT) in relation to the morphologic features in Rdh5 knockout (Rdh5-/-) mice.

MATERIALS AND METHODS

The mouse retina was segmented into four layers; the inner retinal (A), outer plexiform and outer nuclear (B), rod/cone (C), and retinal pigment epithelium (RPE)/choroid (D) layers. The thickness of each retinal layer of Rdh5-/- mice was longitudinally and quantitatively measured at six time points from postnatal months (PM) 1 to PM6 using SD-OCT. Age-matched C57BL/6J mice were employed as wild-type controls. The data were statistically compared using Student's t-test. The fundus appearance was assessed, histologic and ultrastructural examinations were performed in both groups.

RESULTS

Layers A and B were significantly thinner in the Rdh5-/- mice than in the wild-type C57BL/6J mice during the observation periods. Layers C and D became thinner in the Rdh5-/- mice than in the wild-type mice after PM6. Although no abnormalities corresponding to whitish fundus dots were detected by SD-OCT or histologic examinations, the intracellular accumulation of low-density vacuoles was noted in the RPE of the Rdh5-/- mice by electron microscopy. The photoreceptor nuclei appeared less dense in the Rdh5-/- mice than in the wild-type mice.

DISCUSSION

The results from the present study suggest that although it is difficult to detect qualitative abnormalities, SD-OCT can detect quantitative changes in photoreceptors even in the early stage of retinal degeneration induced by the Rdh5 gene mutation in mice.

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.

Retinol Dehydrogenases Regulate Vitamin A Metabolism for Visual Function

The visual system produces visual chromophore, 11-cis-retinal from dietary vitamin A, all-trans-retinol making this vitamin essential for retinal health and function. These metabolic events are mediated by a sequential biochemical process called the visual cycle. Retinol dehydrogenases (RDHs) are responsible for two reactions in the visual cycle performed in retinal pigmented epithelial (RPE) cells, photoreceptor cells and Müller cells in the retina. RDHs in the RPE function as 11-cis-RDHs, which oxidize 11-cis-retinol to 11-cis-retinal in vivo. RDHs in rod photoreceptor cells in the retina work as all-trans-RDHs, which reduce all-trans-retinal to all-trans-retinol. Dysfunction of RDHs can cause inherited retinal diseases in humans. To facilitate further understanding of human diseases, mouse models of RDHs-related diseases have been carefully examined and have revealed the physiological contribution of specific RDHs to visual cycle function and overall retinal health. Herein we describe the function of RDHs in the RPE and the retina, particularly in rod photoreceptor cells, their regulatory properties for retinoid homeostasis and future therapeutic strategy for treatment of retinal diseases.

Conditional Ablation of Retinol Dehydrogenase 10 in the Retinal Pigmented Epithelium Causes Delayed Dark Adaption in Mice

Regeneration of the visual chromophore, 11-cis-retinal, is a crucial step in the visual cycle required to sustain vision. This cycle consists of sequential biochemical reactions that occur in photoreceptor cells and the retinal pigmented epithelium (RPE). Oxidation of 11-cis-retinol to 11-cis-retinal is accomplished by a family of enzymes termed 11-cis-retinol dehydrogenases, including RDH5 and RDH11. Double deletion of Rdh5 and Rdh11 does not limit the production of 11-cis-retinal in mice. Here we describe a third retinol dehydrogenase in the RPE, RDH10, which can produce 11-cis-retinal. Mice with a conditional knock-out of Rdh10 in RPE cells (Rdh10 cKO) displayed delayed 11-cis-retinal regeneration and dark adaption after bright light illumination. Retinal function measured by electroretinogram after light exposure was also delayed in Rdh10 cKO mice as compared with controls. Double deletion of Rdh5 and Rdh10 (cDKO) in mice caused elevated 11/13-cis-retinyl ester content also seen in Rdh5(-/-)Rdh11(-/-) mice as compared with Rdh5(-/-) mice. Normal retinal morphology was observed in 6-month-old Rdh10 cKO and cDKO mice, suggesting that loss of Rdh10 in the RPE does not negatively affect the health of the retina. Compensatory expression of other retinol dehydrogenases was observed in both Rdh5(-/-) and Rdh10 cKO mice. These results indicate that RDH10 acts in cooperation with other RDH isoforms to produce the 11-cis-retinal chromophore needed for vision.

Retinol dehydrogenases: membrane-bound enzymes for the visual function

Retinoid metabolism is important for many physiological functions, such as differenciation, growth, and vision. In the visual context, after the absorption of light in rod photoreceptors by the visual pigment rhodopsin, 11-cis retinal is isomerized to all-trans retinal. This retinoid subsequently undergoes a series of modifications during the visual cycle through a cascade of reactions occurring in photoreceptors and in the retinal pigment epithelium. Retinol dehydrogenases (RDHs) are enzymes responsible for crucial steps of this visual cycle. They belong to a large family of proteins designated as short-chain dehydrogenases/reductases. The structure of these RDHs has been predicted using modern bioinformatics tools, which allowed to propose models with similar structures including a common Rossman fold. These enzymes undergo oxidoreduction reactions, whose direction is dictated by the preference and concentration of their individual cofactor (NAD(H)/NADP(H)). This review presents the current state of knowledge on functional and structural features of RDHs involved in the visual cycle as well as knockout models. RDHs are described as integral or peripheral enzymes. A topology model of the membrane binding of these RDHs via their N- and (or) C-terminal domain has been proposed on the basis of their individual properties. Membrane binding is a crucial issue for these enzymes because of the high hydrophobicity of their retinoid substrates.

Fundus albipunctatus: novel mutations and phenotypic description of Israeli patients

PURPOSE

To characterize the genetic defects associated with fundus albipunctatus (FAP) in patients in Israel.

METHODS

Twenty patients with FAP from diverse ethnicities underwent ophthalmic and electroretinogram tests following the International Society for Clinical Electrophysiology of Vision protocol. Genomic DNA was extracted from peripheral blood. Mutation analysis of the 11-cis retinol dehydrogenase (RDH5) gene was performed with direct sequencing of PCR-amplified exons.

RESULTS

Four novel RDH5 gene mutations were identified. Of them, the null mutations c.343C>T (p.R54X) and c.242delTGCC were most prevalent. Macular involvement was present in two patients who carry different mutation types.

CONCLUSIONS

Mutation analysis of the RDH5 gene in the present series revealed four novel mutations and a previously reported one. No significant genotype-phenotype correlation was found.

Novel mutations in RDH5 cause fundus albipunctatus in two consanguineous Pakistani families

PURPOSE

To identify the underlying genetic causes of fundus albipunctatus (FA), a rare form of congenital stationary night blindness that is characterized by the presence of white dots in the midperiphery of the retina and delayed dark adaptation, in Pakistan.

METHODS

Two families with FA were identified by fundus examination, and genome-wide single nucleotide polymorphism genotyping was performed for two individuals from family A and six individuals from family B. Genotyping data were subsequently used to identify the identical homozygous regions present in the affected individuals of both families using the online homozygosity mapping tool Homozygosity Mapper. Candidate genes selected from the homozygous regions were sequenced.

RESULTS

Three identical homozygous regions were identified in affected persons of family A (on chromosomes 8, 10, and 12), whereas a single shared homozygous region on chromosome 12 was found in family B. In both families, the homozygous region on chromosome 12 harbored the retinol dehydrogenase 5 (RDH5) gene, in which mutations are known to be causative of FA. RDH5 sequence analysis revealed a novel five base pair deletion, c.913_917delGTGCT (p.Val305Hisfs*29), in family A, and a novel missense mutation, c.758T>G (p.Met253Arg), in family B.

CONCLUSIONS

We identified two novel disease-causing RDH5 mutations in Pakistani families with FA, which will improve diagnosis and genetic counseling, and may even lead to treatment of this disease in these families.

Key enzymes of the retinoid (visual) cycle in vertebrate retina

A major goal in vision research over the past few decades has been to understand the molecular details of retinoid processing within the retinoid (visual) cycle. This includes the consequences of side reactions that result from delayed all-trans-retinal clearance and condensation with phospholipids that characterize a variety of serious retinal diseases. Knowledge of the basic retinoid biochemistry involved in these diseases is essential for development of effective therapeutics. Photoisomerization of the 11-cis-retinal chromophore of rhodopsin triggers a complex set of metabolic transformations collectively termed phototransduction that ultimately lead to light perception. Continuity of vision depends on continuous conversion of all-trans-retinal back to the 11-cis-retinal isomer. This process takes place in a series of reactions known as the retinoid cycle, which occur in photoreceptor and RPE cells. All-trans-retinal, the initial substrate of this cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids. Therefore, much experimental effort has been devoted to elucidate molecular mechanisms of the retinoid cycle and all-trans-retinal-mediated retinal degeneration, resulting in delineation of many key steps involved in regenerating 11-cis-retinal. Three particularly important reactions are catalyzed by enzymes broadly classified as acyltransferases, short-chain dehydrogenases/reductases and carotenoid/retinoid isomerases/oxygenases. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Retinol dehydrogenases (RDHs) in the visual cycle

The isomerization of 11-cis retinal to all-trans retinal in photoreceptors is the first step in vision. For photoreceptors to function in constant light, the all-trans retinal must be converted back to 11-cis retinal via the enzymatic steps of the visual cycle. Within this cycle, all-trans retinal is reduced to all-trans retinol in photoreceptors and transported to the retinal pigment epithelium (RPE). In the RPE, all-trans retinol is converted to 11-cis retinol, and in the final enzymatic step, 11-cis retinol is oxidized to 11-cis retinal. The first and last steps of the classical visual cycle are reduction and oxidation reactions, respectively, that utilize retinol dehydrogenase (RDH) enzymes. The visual cycle RDHs have been extensively studied, but because multiple RDHs are capable of catalyzing each step, the exact RDHs responsible for each reaction remain unknown. Within rods, RDH8 is largely responsible for the reduction of all-trans retinal with possible assistance from RDH12. retSDR1 is thought to reduce all-trans retinal in cones. In the RPE, the oxidation of 11-cis retinol is carried out by RDH5 with possible help from RDH11 and RDH10. Here, we review the characteristics of each RDH in vitro and the findings from knockout models that suggest the roles for each in the visual cycle.

Importance of membrane structural integrity for RPE65 retinoid isomerization activity

Regeneration of visual chromophore in the vertebrate visual cycle involves the retinal pigment epithelium-specific protein RPE65, the key enzyme catalyzing the cleavage and isomerization of all-trans-retinyl fatty acid esters to 11-cis-retinol. Although RPE65 has no predicted membrane spanning domains, this protein predominantly associates with microsomal fractions isolated from bovine retinal pigment epithelium (RPE). We have re-examined the nature of RPE65 interactions with native microsomal membranes by using extraction and phase separation experiments. We observe that hydrophobic interactions are the dominant forces that promote RPE65 association with these membranes. These results are consistent with the crystallographic model of RPE65, which features a large lipophilic surface that surrounds the entrance to the catalytic site of this enzyme and likely interacts with the hydrophobic core of the endoplasmic reticulum membrane. Moreover, we report a critical role for phospholipid membranes in preserving the retinoid isomerization activity and physical properties of RPE65. Isomerase activity measured in bovine RPE was highly sensitive to phospholipase A(2) treatment, but the observed decline in 11-cis-retinol production did not directly reflect inhibition by products of lipid hydrolysis. Instead, a direct correlation between the kinetics of phospholipid hydrolysis and retinoid isomerization suggests that the lipid membrane structure is critical for RPE65 enzymatic activity. We also provide evidence that RPE65 operates in a multiprotein complex with retinol dehydrogenase 5 and retinal G protein-coupled receptor in RPE microsomes. Modifications in the phospholipid environment affecting interactions with these protein components may be responsible for the alterations in retinoid metabolism observed in phospholipid-depleted RPE microsomes. Thus, our results indicate that the enzymatic activity of native RPE65 strongly depends on its membrane binding and phospholipid environment.

Membrane-binding and enzymatic properties of RPE65

Regeneration of visual pigments is essential for sustained visual function. Although the requirement for non-photochemical regeneration of the visual chromophore, 11-cis-retinal, was recognized early on, it was only recently that the trans to cis retinoid isomerase activity required for this process was assigned to a specific protein, a microsomal membrane enzyme called RPE65. In this review, we outline progress that has been made in the functional characterization of RPE65. We then discuss general concepts related to protein-membrane interactions and the mechanism of the retinoid isomerization reaction and describe some of the important biochemical and structural features of RPE65 with respect to its membrane-binding and enzymatic properties.

The biochemical and structural basis for trans-to-cis isomerization of retinoids in the chemistry of vision

Recently, much progress has been made in elucidating the chemistry and metabolism of retinoids and carotenoids, as well as the structures of processing proteins related to vision. Carotenoids and their retinoid metabolites are isoprenoids, so only a limited number of chemical transformations are possible, and just a few of these occur naturally. Although there is an intriguing evolutionary conservation of the key components involved in the production and recycling of chromophores, these genes have also adapted to the specific requirements of insect and vertebrate vision. These 'ancestral footprints' in animal genomes bear witness to the common origin of the chemistry of vision, and will further stimulate research across evolutionary boundaries.

Phospholipid meets all-trans-retinal: the making of RPE bisretinoids

The lipid phase of the photoreceptor outer segment membrane is essential to the photon capturing and signaling functions of rhodopsin. Rearrangement of phospholipids in the bilayer accompanies the formation of the active intermediates of rhodopsin following photon absorption. Furthermore, evidence for the formation of a condensation product between the photolyzed chromophore all-trans-retinal and phosphatidylethanolamine indicates that phospholipid may also participate in the movement of the retinoid in the membrane. The downside of these interactions is the formation of bisretinoid-phosphatidylethanolamine compounds that accumulate in retinal pigment epithelial cells with age and that are particularly abundant in some retinal disorders. The propensity of these compounds to negatively impact on the cells has been linked to the pathogenesis of some retinal disorders including juvenile onset recessive Stargardt disease and age-related macular degeneration.

Retinal pigment epithelium-retinal G protein receptor-opsin mediates light-dependent translocation of all-trans-retinyl esters for synthesis of visual chromophore in retinal pigment epithelial cells

Visual perception begins with the absorption of a photon by an opsin pigment, inducing isomerization of its 11-cis-retinaldehyde chromophore. After a brief period of activation, the resulting all-trans-retinaldehyde dissociates from the opsin apoprotein rendering it insensitive to light. Restoring light sensitivity to apo-opsin requires thermal re-isomerization of all-trans-retinaldehyde to 11-cis-retinaldehyde via an enzyme pathway called the visual cycle in retinal pigment epithelial (RPE) cells. Vertebrates can see over a 10(8)-fold range of background illumination. This implies that the visual cycle can regenerate a visual chromophore over a similarly broad range. However, nothing is known about how the visual cycle is regulated. Here we show that RPE cells, functionally or physically separated from photoreceptors, respond to light by mobilizing all-trans-retinyl esters. These retinyl esters are substrates for the retinoid isomerase and hence critical for regenerating visual chromophore. We show in knock-out mice and by RNA interference in human RPE cells that this mobilization is mediated by a protein called "RPE-retinal G protein receptor" (RGR) opsin. These data establish that RPE cells are intrinsically sensitive to light. Finally, we show that in the dark, RGR-opsin inhibits lecithin:retinol acyltransferase and all-trans-retinyl ester hydrolase in vitro and that this inhibition is released upon exposure to light. The results of this study suggest that RGR-opsin mediates light-dependent translocation of all-trans-retinyl esters from a storage pool in lipid droplets to an "isomerase pool" in membranes of the endoplasmic reticulum. This translocation permits insoluble all-trans-retinyl esters to be utilized as substrate for the synthesis of a new visual chromophore.

Redundant and unique roles of retinol dehydrogenases in the mouse retina

Highly abundant short-chain alcohol dehydrogenases (RDHs) in the retina were assumed to be involved in the recycling of 11-cis-retinal chromophore in the visual cycle. Mutations in human RDH genes are associated with Fundus albipunctatus, a mild form of night blindness (RDH5) and an autosomal recessive, childhood-onset severe retinal dystrophy (RDH12). Rdh12 knockout mice were found to be susceptible to light-induced photoreceptor apoptosis, whereas Rdh5 and Rdh8 knockout mice displayed only delayed dark adaptation. However, each knockout mouse eventually regenerated normal levels of visual pigments, suggesting that RDHs compensate for each other in the visual cycle. Here, we established RDH double knockout (Rdh8(-/-)Rdh12(-/-)) and triple knockout (Rdh5(-/-)Rdh8(-/-)Rdh12(-/-)) mice generated on various genetic backgrounds including a rod alpha-transducin knockout to test cone function. RDH activity was severely reduced in Rdh8(-/-)Rdh12(-/-) retina extracts, whereas Rdh8(-/-) RDH activity was intermediate and Rdh12(-/-) RDH activity was reduced only slightly. Surprisingly, all multiple knockout mice produced sufficient amounts of the chromophore to regenerate rhodopsin and cone pigments in vivo. Three-month-old Rdh8(-/-)Rdh12(-/-) mice characteristically displayed a slowly progressing rod-cone dystrophy accompanied by accumulation of N-retinylidene-N-retinylethanolamine (A2E), a toxic substance known to contribute to retinal degeneration. A2E accumulation and retinal degeneration were prevented by application of retinylamine, a potent retinoid cycle inhibitor. The results suggest that RDH8 and RDH12 are dispensable in support of the visual cycle but appear to be key components in clearance of free all-trans-retinal, thereby preventing A2E accumulation and photoreceptor cell death.

Retinoid processing in cone and Müller cell lines

To determine whether cones and Müller cells in the rod dominated retina cooperate to regenerate the 11-cis retinal chromophore via the retinoid cycle, two cell lines from the rod dominated retinas of Murine were used for this study: 661W, a mouse cell line derived from cones, and rMC-1, a rat Müller cell line. Retinoid cycle enzymes were analyzed by RT-PCR, and their catalytic activity was detected by incubation with retinoids and analyzed by HPLC. We found that 661W cells are capable of reducing all-trans retinal to all-trans retinol due to the presence of multiple dehydrogenases and to generate minor amounts of retinyl-ester. The rMC-1 cells take up all-trans retinol and oxidize it to all-trans retinal or esterify it to retinyl-ester, but are incapable of isomerizing all-trans retinoids to 11-cis retinoids. This could be a reflection of lack of necessary activities in Müller cells in vivo, which suggests that Müller cells do not contribute to retinoid cycling by regenerating 11-cis retinoids. Alternatively, this could be due to the potential that rMC-1, as a transformed cell line, has stopped expressing the proteins needed for the regeneration of 11-cis retinoids.

Novel RDH12 mutations associated with Leber congenital amaurosis and cone-rod dystrophy: biochemical and clinical evaluations

The purpose of this study was to determine the role of the retinol dehydrogenase 12 (RDH12) gene in patients affected with Leber congenital amaurosis (LCA), autosomal recessive retinitis pigmentosa (arRP) and autosomal dominant/recessive cone-rod dystrophies (CORD). Changes in the promoter region, coding regions and exon/intron junctions of the RDH12 gene were evaluated using direct DNA sequencing of patients affected with LCA (n=36 cases), RP (n=62) and CORD (n=21). The allele frequency of changes observed was assessed in a multiethnic control population (n=159 individuals). Detailed biochemical and structural modeling analysis of the observed mutations were performed to assess their biological role in the inactivation of Rdh12. A comprehensive clinical assessment of retinal structure and function in LCA patients carrying mutations in the RDH12 gene was completed. Of the six changes identified, three were novel including a homozygous C201R change in a patient affected with LCA, a heterozygous A177V change in patients affected with CORD and a heterozygous G46G change in a patient affected with LCA. A novel compound heterozygote T49M/A269fsX270 mutation was also found in a patient with LCA, and both homozygous and heterozygous R161Q changes were seen in 26 patients affected with LCA, CORD or RP. These R161Q, G46G and the A177V sequence changes were shown to be polymorphic. We found that Rdh12 mutant proteins associated with LCA were inactive or displayed only residual activity when expressed in COS-7 and Sf9 cells, whereas those mutants that were considered polymorphisms were fully active. Thus, impairment of retinal structure and function for patients carrying these mutations correlated with the biochemical properties of the mutants.

Fundus albipunctatus in a 6-year old girl due to compound heterozygous mutations in the RDH5 gene

We report a case study of Fundus Albipunctatus (FA) due to compound heterozygous mutations in RDH5, the gene encoding for the 11-cis-retinal dehydrogenase (RDH). A 6-year old Hispanic American female with a clinical presentation suggestive of FA underwent dark-adapted full-field flash electroretinography (ERG) at 30 and 120 min. The pattern of ERG abnormalities was consistent with the working diagnosis FA. However, only ERG responses to dim stimuli were profoundly affected, and maximal ERG responses were already near normal after only 30 min of dark adaptation. The patient also demonstrated a subnormal maximal ERG response b/a-wave ratio at 30 min that resolved after 120 min of dark adaptation. When measurable, dark-adapted post-receptoral responses were normal in timing under all circumstances, and became increasingly faster after prolonged dark adaptation. Cone-driven responses were completely normal at this young age. Sequencing of the RDH5 gene revealed two distinct missense mutations, a G238W mutation, previously reported in patients with FA, and a D128N mutation, which has not been reported before but is known to cause reduced 11-cis-RDH activity. These findings confirmed the clinical and functional diagnosis of FA and excluded that of retinitis punctata albescens (RPA). The behavior of dark-adapted ERG responses in FA displays characteristics that differ from those of RPA patients, which may be useful to differentiate functionally these two conditions at their common albipunctate stages.

Diseases caused by defects in the visual cycle: retinoids as potential therapeutic agents

Absorption of a photon by an opsin pigment causes isomerization of the chromophore from 11-cis-retinaldehyde to all-trans-retinaldehyde. Regeneration of visual chromophore following light exposure is dependent on an enzyme pathway called the retinoid or visual cycle. Our understanding of this pathway has been greatly facilitated by the identification of disease-causing mutations in the genes coding for visual cycle enzymes. Defects in nearly every step of this pathway are responsible for human-inherited retinal dystrophies. These retinal dystrophies can be divided into two etiologic groups. One involves the impaired synthesis of visual chromophore. The second involves accumulation of cytotoxic products derived from all-trans-retinaldehyde. Gene therapy has been successfully used in animal models of these diseases to rescue the function of enzymes involved in chromophore regeneration, restoring vision. Dystrophies resulting from impaired chromophore synthesis can also be treated by supplementation with a chromophore analog. Dystrophies resulting from the accumulation of toxic pigments can be treated pharmacologically by inhibiting the visual cycle, or limiting the supply of vitamin A to the eyes. Recent progress in both areas provides hope that multiple inherited retinal diseases will soon be treated by pharmaceutical intervention.

Ontogeny of rdh9 (Crad3) expression: ablation causes changes in retinoid and steroid metabolizing enzymes, but RXR and androgen signaling seem normal

Crad3 (cis-retinol/androgen dehydrogenase 3), a short-chain dehydrogenase/reductase, converts 9-cis-retinol into 9-cis-retinal and 3alpha-androstanediol into dihydrotestosterone. Crad3 may serve in biosynthesis of 9-cis-retinoic acid, a putative RXR ligand, and/or regeneration of potent androgens. RT-PCR showed that expression of the gene that encodes Crad3, rdh9, begins in liver by e11.5, and in kidney, testis, brain and intestine during e15.5-e16.5. In situ hybridization showed rdh9 expression in embryonic liver, ganglia, small intestine, lung, skin and vertebral cartilage. In adult, in situ hybridization revealed rdh9 expression intensely in hepatocytes, weakly in kidney glomerulus, and intensely in collecting tubules. In intestine, undifferentiated epithelia had greater expression than differentiated epithelia at the distal villus end. Testes expressed rdh9 in spermatogonia, and weakly in Leydig cells. Adult brain expressed rdh9 in the dentate gyrus and CA regions of the hippocampus, the cerebellum Purkinje cells, and the glomerular and mitral cell layers of the olfactory bulb. Rdh9-null mice, backcrossed against C57BL/6J mice, were born in Mendelian frequency, were healthy and fertile, and had normal tissue retinoid and serum dihydrotestosterone levels. Expression of rdh1, a gene that encodes an efficient retinol dehydrogenase, decreased 3- to 8-fold in rdh9-null mice, depending on dietary vitamin A. Microarray analysis and quantitative PCR revealed 2- to 4-fold increases in mRNA of enzymes that catalyze xenobiotic and steroid metabolism, including Cyp2, Cyp3, 11beta-hydroxysteroid dehydrogenase type 2, and 17beta-hydroxsteroid dehydrogenases types 4 and 5. These data indicate widespread Crad3 function(s) in steroid and/or retinoid metabolism starting mid embryogenesis.

Retinol dehydrogenase (RDH12) protects photoreceptors from light-induced degeneration in mice

RDH12 has been suggested to be one of the retinol dehydrogenases (RDH) involved in the vitamin A recycling system (visual cycle) in the eye. Loss of function mutations in the RDH12 gene were recently reported to be associated with autosomal recessive childhood-onset severe retinal dystrophy. Here we show that RDH12 localizes to the photoreceptor inner segments and that deletion of this gene in mice slows the kinetics of all-trans-retinal reduction, delaying dark adaptation. However, accelerated 11-cis-retinal production and increased susceptibility to light-induced photoreceptor apoptosis were also observed in Rdh12(-/-) mice, suggesting that RDH12 plays a unique, nonredundant role in the photoreceptor inner segments to regulate the flow of retinoids in the eye. Thus, severe visual impairments of individuals with null mutations in RDH12 may likely be caused by light damage(1).

Improvement in rod and cone function in mouse model of Fundus albipunctatus after pharmacologic treatment with 9-cis-retinal

PURPOSE

To assess changes in rod and cone visual functions in a mouse model of Fundus albipunctatus with disrupted 11-cis-retinol dehydrogenase (RDH) genes after pharmacologic treatment with an artificial retinal chromophore.

METHODS

Retinoid levels and photoreceptor functions of Rdh5-/-Rdh11-/- mice at a variety of light intensities were analyzed with normal-phase HPLC and ERG techniques. Production of 11-cis-retinal, the visual pigment chromophore, was suppressed with a potent inhibitor of the retinoid cycle, all-trans-retinylamine (Ret-NH2). The chromophore was replaced by a functional geometric isomer, 9-cis-retinal, delivered by oral gavage.

RESULTS

Aberrant cone responses were detected in 12-month-old Rdh5-/-Rdh11-/- mice raised in a 12-hour light/12-hour dark cycle. This cone defect was exacerbated in conditions of low levels of 11-cis-retinal. Administration of 9-cis-retinal increased the rate of dark adaptation and improved cone function in Rdh5-/-Rdh11-/- mice.

CONCLUSIONS

Disruption of 11-cis-RDHs causes a slowly developing cone dystrophy caused by inefficient cone pigment regeneration. Rod and cone visual function improved significantly in the mouse model of F. albipunctatus after treatment with 9-cis-retinal, suggesting a potential approach to slow the progression of cone dystrophy in affected humans.

Comparative genomic and phylogenetic analysis of short-chain dehydrogenases/reductases with dual retinol/sterol substrate specificity

Human short-chain dehydrogenases/reductases with dual retinol/sterol substrate specificity (RODH-like enzymes) are thought to contribute to the oxidation of retinol for retinoic acid biosynthesis and to the metabolism of androgenic and neuroactive 3alpha-hydroxysteroids. Here, we investigated the phylogeny and orthology of these proteins to understand better their origins and physiological roles. Phylogenetic and genomic analysis showed that two proteins (11-cis-RDH and RDHL) are highly conserved, and their orthologs can be identified in the lower taxa, such as amphibians and fish. Two other proteins (RODH-4 and 3alpha-HSD) are significantly less conserved. Orthologs for 3alpha-HSD are present in all mammals analyzed, whereas orthologs for RODH-4 can be identified in some mammalian species but not in others due to species-specific gene duplications. Understanding the evolution and divergence of RODH-like enzymes in various vertebrate species should facilitate further investigation of their in vivo functions using animal models.

Light-induced damage to the retina: role of rhodopsin chromophore revisited

The presence of the regenerable visual pigment rhodopsin has been shown to be primarily responsible for the acute photodamage to the retina. The photoexcitation of rhodopsin leads to isomerization of its chromophore 11-cis-retinal to all-trans-retinal (ATR). ATR is a potent photosensitizer and its role in mediating photodamage has been suspected for over two decades. However, there was lack of experimental evidence that free ATR exists in the retina in sufficient concentrations to impose a risk of photosensitized damage. Identification in the retina of a retinal dimer and a pyridinium bisretinoid, so called A2E, and determination of its biosynthetic pathway indicate that substantial amounts of ATR do accumulate in the retina. Both light damage and A2E accumulation are facilitated under conditions where efficient retinoid cycle operates. Efficient retinoid cycle leads to rapid regeneration of rhodopsin, which may result in ATR release from the opsin "exit site" before its enzymatic reduction to all-trans-retinol. Here we discuss photodamage to the retina where ATR could play a role as the main toxic and/or phototoxic agent. Moreover, we discuss secondary products of (photo)toxic properties accumulating within retinal lipofuscin as a result of ATR accumulation.

Aberrant metabolites in mouse models of congenital blinding diseases: formation and storage of retinyl esters

Regeneration of the visual chromophore, 11-cis-retinal, is a critical step in restoring photoreceptors to their dark-adapted conditions. This regeneration process, called the retinoid cycle, takes place in the photoreceptor outer segments and the retinal pigment epithelium (RPE). Disabling mutations in nearly all of the retinoid cycle genes are linked to human conditions that cause congenital or progressive defects in vision. Several mouse models with disrupted genes related to this cycle contain abnormal fatty acid retinyl ester levels in the RPE. To investigate the mechanisms of retinyl ester accumulation, we generated single or double knockout mice lacking retinoid cycle genes. All-trans-retinyl esters accumulated in mice lacking RPE65, but they are reduced in double knockout mice also lacking opsin, suggesting a connection between visual pigment regeneration and the retinoid cycle. Only Rdh5-deficient mice accumulate cis-retinyl esters, regardless of the simultaneous disruption of RPE65, opsin, and prRDH. 13-cis-Retinoids are produced at higher levels when the flow of retinoid through the cycle was increased, and these esters are stored in specific structures called retinosomes. Most importantly, retinylamine, a specific and effective inhibitor of the 11-cis-retinol formation, also inhibits the production of 13-cis-retinyl esters. The data presented here support the idea that 13-cis-retinyl esters are formed through an aberrant enzymatic isomerization process.

The retinal pigment epithelium in visual function

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

Role of photoreceptor-specific retinol dehydrogenase in the retinoid cycle in vivo

The retinoid cycle is a recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. Photoreceptor-specific retinol dehydrogenase (prRDH) catalyzes reduction of all-trans-retinal to all-trans-retinol and is thought to be a key enzyme in the retinoid cycle. We disrupted mouse prRDH (human gene symbol RDH8) gene expression by targeted recombination and generated a homozygous prRDH knock-out (prRDH-/-) mouse. Histological analysis and electron microscopy of retinas from 6- to 8-week-old prRDH-/- mice revealed no structural differences of the photoreceptors or inner retina. For brief light exposure, absence of prRDH did not affect the rate of 11-cis-retinal regeneration or the decay of Meta II, the activated form of rhodopsin. Absence of prRDH, however, caused significant accumulation of all-trans-retinal following exposure to bright lights and delayed recovery of rod function as measured by electroretinograms and single cell recordings. Retention of all-trans-retinal resulted in slight overproduction of A2E, a condensation product of all-trans-retinal and phosphatidylethanolamine. We conclude that prRDH is an enzyme that catalyzes reduction of all-trans-retinal in the rod outer segment, most noticeably at higher light intensities and prolonged illumination, but is not an essential enzyme of the retinoid cycle.

Delayed dark adaptation in 11-cis-retinol dehydrogenase-deficient mice: a role of RDH11 in visual processes in vivo

The oxidation of 11-cis-retinol to 11-cis-retinal in the retinal pigment epithelium (RPE) represents the final step in a metabolic cycle that culminates in visual pigment regeneration. Retinol dehydrogenase 5 (RDH5) is responsible for a majority of the 11-cis-RDH activity in the RPE, but the formation of 11-cis-retinal in rdh5-/- mice suggests another enzyme(s) is present. We have previously shown that RDH11 is also highly expressed in RPE cells and has dual specificity for both cis- and trans-retinoid substrates. To investigate the role of RDH11 in the retinoid cycle, we generated rdh11-/- and rdh5-/-rdh11-/- mice and examined their electrophysiological responses to various intensities of illumination and during dark adaptation. Retinoid profiles of darkadapted rdh11-/- mice did not show significant differences compared with wild-type mice, whereas an accumulation of cis-esters was detected in rdh5-/- and rdh5-/-rdh11-/- mice. Following light stimulation, 73% more cis-retinyl esters were stored in rdh5-/-rdh11-/- mice compared with rdh5-/- mice. Single-flash ERGs of rdh11-/- showed normal responses under dark- and light-adapted conditions, but exhibited delayed dark adaptation following high bleaching levels. Double knockout mice also had normal ERG responses in dark- and light-adapted conditions, but had a further delay in dark adaptation relative to either rdh11-/- or rdh5-/- mice. Taken together, these results suggest that RDH11 has a measurable role in regenerating the visual pigment by complementing RDH5 as an 11-cis-RDH in RPE cells, and indicate that an additional unidentified enzyme(s) oxidizes 11-cis-retinol or that an alternative pathway contributes to the retinoid cycle.

Rhodopsin signaling and organization in heterozygote rhodopsin knockout mice

Rhodopsin (Rho) resides within internal membrane structures called disc membranes that are found in the rod outer segments (ROS) of photoreceptors in the retina. Rho expression is essential for formation of ROS, which are absent in knockout Rho-/- mice. ROS of mice heterozygous for the Rho gene deletion (Rho+/-) may have a lower Rho density than wild type (WT) membranes, or the ROS structure may be reduced in size due to lower Rho expression. Here, we present evidence that the smaller volume of ROS from heterozygous mice is most likely responsible for observed electrophysiological response differences. In Rho+/- mice as compared with age-matched WT mice, the length of ROS was shorter by 30-40%, and the average diameter of ROS was reduced by approximately 20%, as demonstrated by transmission and scanning electron microscopy. Together, the reduction of the volume of ROS was approximately 60% in Rho+/- mice. Rho content in the eyes was reduced by approximately 43% and 11-cis-retinal content in the eye was reduced by approximately 38%, as determined by UV-visible spectroscopy and retinoid analysis, respectively. Transmission electron microscopy of negatively stained disc membranes from Rho+/- mice indicated a typical morphology apart from the reduced size of disc diameter. Power spectra calculated from disc membrane regions on such electron micrographs displayed a diffuse ring at approximately 4.5 nm(-1), indicating paracrystallinity of Rho. Atomic force microscopy of WT and Rho+/- disc membranes revealed, in both cases, Rho organized in paracrystalline and raftlike structures. From these data, we conclude that the differences in physiological responses measured in WT and Rho+/- mice are due to structural changes of the whole ROS and not due to a lower density of Rho.

Elements in the N-terminal signaling sequence that determine cytosolic topology of short-chain dehydrogenases/reductases. Studies with retinol dehydrogenase type 1 and cis-retinol/androgen dehydrogenase type 1

High affinity, retinoid-specific binding proteins chaperone retinoids to manage their transport and metabolism. Proposing mechanisms of retinoid transfer between these binding proteins and membrane-associated retinoid-metabolizing enzymes requires insight into enzyme topology. We therefore determined the topology of mouse retinol dehydrogenase type 1 (Rdh1) and cis-retinoid androgen dehydrogenase type 1 (Crad1) in the endoplasmic reticulum of intact mammalian cells. The properties of Rdh1 were compared with a chimera with a luminal signaling sequence (11beta-hydroxysteroid dehydrogenase (11beta-HSD1)(1-41)/Rdh1(23-317); the green fluorescent protein (GFP) fusion proteins Rdh1(1-22)/GFP, Crad1(1-22)/GFP, and 11beta-HSD1(1-41)/GFP; and signaling sequence charge difference mutants using confocal immunofluorescence, antibody access, proteinase K sensitivity, and deglycosylation assays. An N-terminal signaling sequence of 22 residues, consisting of a hydrophobic helix ending in a net positive charge, anchors Rdh1 and Crad1 in the endoplasmic reticulum facing the cytoplasm. Mutating arginine to glutamine in the signaling sequence did not affect topology. Inserting one or two arginine residues near the N terminus of the signaling sequence caused 28-95% inversion from cytoplasmic to luminal, depending on the net positive charge remaining at the C terminus of the signaling sequence; e.g. the mutant L3R,L5R,R16Q,R19Q,R21Q faced the lumen. Experiments with N- and C-terminal epitope-tagged Rdh1 and molecular modeling indicated that a hydrophobic helix-turn-helix near the C terminus of Rdh1 (residues 289-311) projects into the cytoplasm. These data provide insight into the features necessary to orient type III (reverse signal-anchor) proteins and demonstrate that Rdh1, Crad1, and other short-chain dehydrogenases/reductases, which share similar N-terminal signaling sequences such as human Rdh5 and mouse Rdh4, orient with their catalytic domains facing the cytoplasm.

Novel targeting strategy for generating mouse models with defects in the retinoid cycle

In addition to RDH5, other enzymes capable of oxidizing 11-cis-retinol are present within the retinal pigment epithelium, Müller cells and/or photoreceptors. Candidate proteins have meanwhile been identified. To study the physiological and pathological aspects of these enzymes, mice in which these genes are no longer functional are being generated. A fast-targeting strategy for the disruption of genes was developed. Generation of double and triple knockouts will aid in determining if these retinol dehydrogenases are responsible for the remaining 11-cis-retinol oxidation observed in RDH5 knockout animals.

Lecithin-retinol acyltransferase is essential for accumulation of all-trans-retinyl esters in the eye and in the liver

Lecithin-retinol acyltransferase (LRAT), an enzyme present mainly in the retinal pigmented epithelial cells and liver, converts all-trans-retinol into all-trans-retinyl esters. In the retinal pigmented epithelium, LRAT plays a key role in the retinoid cycle, a two-cell recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. We disrupted mouse Lrat gene expression by targeted recombination and generated a homozygous Lrat knock-out (Lrat-/-) mouse. Despite the expression of LRAT in multiple tissues, the Lrat-/- mouse develops normally. The histological analysis and electron microscopy of the retina for 6-8-week-old Lrat-/- mice revealed that the rod outer segments are approximately 35% shorter than those of Lrat+/+ mice, whereas other neuronal layers appear normal. Lrat-/- mice have trace levels of all-trans-retinyl esters in the liver, lung, eye, and blood, whereas the circulating all-trans-retinol is reduced only slightly. Scotopic and photopic electroretinograms as well as pupillary constriction analyses revealed that rod and cone visual functions are severely attenuated at an early age. We conclude that Lrat-/- mice may serve as an animal model with early onset severe retinal dystrophy and severe retinyl ester deprivation.

Retinoid cycle in the vertebrate retina: experimental approaches and mechanisms of isomerization

Retinoid cycle describes a set of chemical transformations that occur in the photoreceptor and retinal pigment epithelial cells. The hydrophobic and labile nature of the retinoid substrates and the two-cell chromophore utilization-regeneration system imposes significant constraints on the experimental biochemical approaches employed to understand this process. A brief description of the recent developments in the investigation of the retinoid cycle is the current topic, which includes a review of novel results and techniques pertaining to the retinoid cycle. The chemistry of the all-trans-retinol to 11-cis-retinol isomerization is also discussed.

Evaluation of the role of the retinal G protein-coupled receptor (RGR) in the vertebrate retina in vivo

The retinal G protein-coupled receptor (RGR) is a protein that structurally resembles visual pigments and other G protein-coupled receptors. RGR may play a role as a photoisomerase in the production of 11-cis-retinal, the chromophore of the visual pigments. As the proposed function of RGR, in a complex with 11-cis-retinol dehydrogenase (RDH5), is to regenerate 11-cis-retinal under light conditions and RDH5 is expected to function in the light-independent part of the retinoid cycle, we speculated that the simultaneous loss of function of both proteins should more severely affect the rhodopsin regeneration capacity. Here, we evaluated the role of RGR using rgr-/- single and rdh5-/-rgr-/- double knockout mice under a number of light conditions. The most striking phenotype of rgr-/- mice after a single flash of light includes light-dependent formation of 9-cis- and 13-cis-retinoid isomers. These isomers are not formed in wild-type mice because either all-trans-retinal is bound to RGR and protected from isomerization to 9-cis- or 13-cis-retinal or because RGR is able to eliminate these isomers directly or indirectly. After intense bleaching, a transient accumulation of all-trans-retinyl esters and an attenuated recovery of 11-cis-retinal were observed. Finally, even under conditions of prolonged light illumination, as investigated in vitro in biochemical assays or in vivo by electroretinogram (ERG) measurements, no evidence of catalytic-like photoisomerization-driven production of 11-cis-retinal could be attained. These and previous results suggest that RGR and RDH5 are likely to function in the retinoid cycle, although their role is not essential and regeneration of visual pigment is only mildly affected by the absence of both proteins in rod-dominated mice.

Dual-substrate specificity short chain retinol dehydrogenases from the vertebrate retina

Retinoids are chromophores involved in vision, transcriptional regulation, and cellular differentiation. Members of the short chain alcohol dehydrogenase/reductase superfamily catalyze the transformation of retinol to retinal. Here, we describe the identification and properties of three enzymes from a novel subfamily of four retinol dehydrogenases (RDH11-14) that display dual-substrate specificity, uniquely metabolizing all-trans- and cis-retinols with C(15) pro-R specificity. RDH11-14 could be involved in the first step of all-trans- and 9-cis-retinoic acid production in many tissues. RDH11-14 fill the gap in our understanding of 11-cis-retinal and all-trans-retinal transformations in photoreceptor (RDH12) and retinal pigment epithelial cells (RDH11). The dual-substrate specificity of RDH11 explains the minor phenotype associated with mutations in 11-cis-retinol dehydrogenase (RDH5) causing fundus albipunctatus in humans and engineered mice lacking RDH5. Furthermore, photoreceptor RDH12 could be involved in the production of 11-cis-retinal from 11-cis-retinol during regeneration of the cone visual pigments. These newly identified enzymes add new elements to important retinoid metabolic pathways that have not been explained by previous genetic and biochemical studies.

Biochemical and physiological properties of rhodopsin regenerated with 11-cis-6-ring- and 7-ring-retinals

Phototransduction is initiated by the photoisomerization of rhodopsin (Rho) chromophore 11-cis-retinylidene to all-trans-retinylidene. Here, using Rho regenerated with retinal analogs with different ring sizes, which prevent isomerization around the C(11)=C(12) double bond, the activation mechanism of this G-protein-coupled receptor was investigated. We demonstrate that 11-cis-7-ring-Rho does not activate G-protein in vivo and in vitro, and that it does not isomerize along other double bonds, suggesting that it fits tightly into the binding site of opsin. In contrast, bleaching 11-cis-6-ring-Rho modestly activates phototransduction in vivo and at low pH in vitro. These results reveal that partial activation is caused by isomerization along other double bonds in more rigid 6-locked retinal isomers and protonation of key residues by lowering pH in 11-cis-6-ring-Rhos. Full activation is not achieved, because isomerization does not induce a complete set of conformational rearrangements of Rho. These results with 6- and 7-ring-constrained retinoids provide new insights into Rho activation and suggest a potential use of locked retinals, particularly 11-cis-7-ring-retinal, to inactivate opsin in some retinal degeneration diseases.

Isomerization and oxidation of vitamin a in cone-dominant retinas: a novel pathway for visual-pigment regeneration in daylight

The first step toward light perception is 11-cis to all-trans photoisomerization of the retinaldehyde chromophore in a rod or cone opsin-pigment molecule. Light sensitivity of the opsin pigment is restored through a multistep pathway called the visual cycle, which effects all-trans to 11-cis re-isomerization of the retinoid chromophore. The maximum throughput of the known visual cycle, however, is too slow to explain sustained photosensitivity in bright light. Here, we demonstrate three novel enzymatic activities in cone-dominant ground-squirrel and chicken retinas: an all-trans-retinol isomerase, an 11-cis-retinyl-ester synthase, and an 11-cis-retinol dehydrogenase. Together these activities comprise a novel pathway that regenerates opsin photopigments at a rate 20-fold faster than the known visual cycle. We suggest that this pathway is responsible for sustained daylight vision in vertebrates.

Recovery of visual functions in a mouse model of Leber congenital amaurosis

The visual process is initiated by the photoisomerization of 11-cis-retinal to all-trans-retinal. For sustained vision the 11-cis-chromophore must be regenerated from all-trans-retinal. This requires RPE65, a dominant retinal pigment epithelium protein. Disruption of the RPE65 gene results in massive accumulation of all-trans-retinyl esters in the retinal pigment epithelium, lack of 11-cis-retinal and therefore rhodopsin, and ultimately blindness. We reported previously (Van Hooser, J. P., Aleman, T. S., He, Y. G., Cideciyan, A. V., Kuksa, V., Pittler, S. J., Stone, E. M., Jacobson, S. G., and Palczewski, K. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 8623-8628) that in Rpe65-/- mice, oral administration of 9-cis-retinal generated isorhodopsin, a rod photopigment, and restored light sensitivity to the electroretinogram. Here, we provide evidence that early intervention by 9-cis-retinal administration significantly attenuated retinal ester accumulation and supported rod retinal function for more than 6 months post-treatment. In single cell recordings rod light sensitivity was shown to be a function of the amount of regenerated isorhodopsin; high doses restored rod responses with normal sensitivity and kinetics. Highly attenuated residual rod function was observed in untreated Rpe65-/- mice. This rod function is likely a consequence of low efficiency production of 11-cis-retinal by photo-conversion of all-trans-retinal in the retina as demonstrated by retinoid analysis. These studies show that pharmacological intervention produces long lasting preservation of visual function in dark-reared Rpe65-/- mice and may be a useful therapeutic strategy in recovering vision in humans diagnosed with Leber congenital amaurosis caused by mutations in the RPE65 gene, an inherited group of early onset blinding and retinal degenerations.

Isomerization of 11-cis-retinoids to all-trans-retinoids in vitro and in vivo

The regeneration of 11-cis-retinal, the universal chromophore of the vertebrate retina, is a complex process involving photoreceptors and adjacent retinal pigment epithelial cells (RPE). 11-cis-Retinal is coupled to opsins in both rod and cone photoreceptor cells and is photoisomerized to all-trans-retinal by light. Here, we show that RPE microsomes can catalyze the reverse isomerization of 11-cis-retinol to all-trans-retinol (and 13-cis-retinol), and membrane exposure to UV light further enhances the rate of this reaction. This conversion is inhibited when 11-cis-retinol is in a complex with cellular retinaldehyde-binding protein (CRALBP), providing a clear demonstration of the protective effect of retinoid-binding proteins in retinoid processes in the eye, a function that has been long suspected but never proven. The reverse isomerization is nonenzymatic and specific to alcohol forms of retinoids, and it displays stereospecific preference for 11-cis-retinol and 13-cis-retinol but is much less efficient for 9-cis-retinol. The mechanism of reverse isomerization was investigated using stable isotope-labeled retinoids and radioactive tracers to show that this reaction occurs with the retention of configuration of the C-15 carbon of retinol through a mechanism that does not eliminate the hydroxyl group, in contrast to the enzymatic all-trans-retinol to 11-cis-retinol reaction. The activation energy for the conversion of 11-cis-retinol to all-trans-retinol is 19.5 kcal/mol, and 20.1 kcal/mol for isomerization of 13-cis-retinol to all-trans-retinol. We also demonstrate that the reverse isomerization occurs in vivo using exogenous 11-cis-retinol injected into the intravitreal space of wild type and Rpe65-/- mice, which have defective forward isomerization. This study demonstrates an uncharacterized activity of RPE microsomes that could be important in the normal flow of retinoids in the eye in vivo during dark adaptation.