Identification and characterization of all-trans-retinol dehydrogenase from photoreceptor outer segments, the visual cycle enzyme that reduces all-trans-retinal to all-trans-retinol

Limited roles of Rdh8, Rdh12, and Abca4 in all-trans-retinal clearance in mouse retina

PURPOSE

Although the retinoid cycle is essential for vision, all-trans-retinal and the side products of this cycle are toxic. Delayed clearance of all-trans-retinal causes accumulation of its condensation products, A2E, and all-trans-retinal dimer (RALdi), both associated with human macular degeneration. The protective roles were examined of the all-trans-RDHs, Rdh8 and Rdh12, and the ATP-binding cassette transporter Abca4, retinoid cycle enzymes involved in all-trans-retinal clearance.

METHODS

Mice genetically engineered to lack Rdh8, Rdh12, and Abca4, either singly or in various combinations, were investigated because all-trans-retinal clearance is achieved by all-trans-RDHs and Abca4. Knockout mice were evaluated by spectral-domain optical coherence tomography (SD-OCT), electroretinography, retinal morphology, and visual retinoid profiling with HPLC and MS. ARPE19 cells were examined to evaluate A2E and RALdi oxidation and toxicity induced by exposure to UV and blue light.

RESULTS

Rdh8(-/-)Abca4(-/-) and Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice displayed slowly progressive, severe retinal degeneration under room light conditions. Intense light-induced acute retinal degeneration was detected by SD-OCT in Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice. Amounts of A2E in the RPE correlated with diminished all-trans-retinal clearance, and the highest A2E amounts were found in Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice. However oxidized A2E was not found in any of these mice, and A2E oxidation was not induced by blue light and UV illumination of A2E-loaded ARPE19 cells. Of interest, addition of all-trans-retinal did activate retinoic acid receptors in cultured cells.

CONCLUSIONS

Rdh8, Rdh12, and Abca4 all protect the retina and reduce A2E production by facilitating all-trans-retinal clearance. Delayed all-trans-retinal clearance contributes more than A2E oxidation to light-induced cellular toxicity.

Identification and characterization of retinoid-active short-chain dehydrogenases/reductases in Drosophila melanogaster

BACKGROUND

In chordates, retinoid metabolism is an important target of short-chain dehydrogenases/reductases (SDRs). It is not known whether SDRs play a role in retinoid metabolism of protostomes, such as Drosophila melanogaster.

METHODS

Drosophila genome was searched for genes encoding proteins with approximately 50% identity to human retinol dehydrogenase 12 (RDH12). The corresponding proteins were expressed in Sf9 cells and biochemically characterized. Their phylogenetic relationships were analyzed using PHYLIP software.

RESULTS

A total of six Drosophila SDR genes were identified. Five of these genes are clustered on chromosome 2 and one is located on chromosome X. The deduced proteins are 300 to 406 amino acids long and are associated with microsomal membranes. They recognize all-trans-retinaldehyde and all-trans-3-hydroxyretinaldehyde as substrates and prefer NADPH as a cofactor. Phylogenetically, Drosophila SDRs belong to the same branch of the SDR superfamily as human RDH12, indicating a common ancestry early in bilaterian evolution, before a protostome-deuterostome split.

CONCLUSIONS

Similarities in the substrate and cofactor specificities of Drosophila versus human SDRs suggest conservation of their function in retinoid metabolism throughout protostome and deuterostome phyla.

GENERAL SIGNIFICANCE

The discovery of Drosophila retinaldehyde reductases sheds new light on the conversion of beta-carotene and zeaxantine to visual pigment and provides a better understanding of the evolutionary roots of retinoid-active SDRs.

Evaluation of potential therapies for a mouse model of human age-related macular degeneration caused by delayed all-trans-retinal clearance

PURPOSE

Evaluate the efficacy of potential therapeutics in Rdh8(-/-)Abca4(-/-) mice, a rodent model of human age-related macular degeneration (AMD).

METHODS

Therapeutic efficacy of several antioxidant agents (ascorbic acid, alpha-lipoic acid, alpha-tocopherol, Mn(III)-tetrakis(4-benzoic acid)-porphyrin, and butylated hydroxytoluene), an immunosuppressive agent with antivascular endothelial growth factor (VEGF) activity (sirolimus, also known as rapamycin), a retinoid cycle inhibitor (retinylamine), and an artificial chromophore (9-cis-retinyl acetate) were evaluated side by side in a recently described murine model of AMD, the Rdh8(-/-)Abca4(-/-) mouse. This animal exhibits a retinopathy caused by delayed all-trans-retinal clearance resulting from the absence of both ATP-binding cassette transporter 4 (Abca4) and retinol dehydrogenase 8 (Rdh8) activities. Drug efficacy was evaluated by retinal histologic analyses and electroretinograms (ERGs).

RESULTS

All tested agents partially prevented atrophic changes in the Rdh8(-/-)Abca4(-/-) retina with retinylamine demonstrating the greatest efficacy. A significant reduction of complement deposition on Bruch's membrane was observed in sirolimus-treated mice, although the severity of retinal degeneration was similar to that observed in antioxidant- and 9-cis-retinyl acetate-treated mice. Sirolimus treatment of 6-month-old Rdh8(-/-)Abca4(-/-) mice for 4 months prevented choroidal neovascularization without changing retinal VEGF levels.

CONCLUSIONS

Mechanism-based therapy with retinylamine markedly attenuated degenerative retinopathy in Rdh8(-/-)Abca4(-/-) mice. Further understanding of pathogenic mechanisms involved in AMD is needed to develop more effective therapeutics.

Purified RPE65 shows isomerohydrolase activity after reassociation with a phospholipid membrane

Generation of 11-cis-retinol from all-trans-retinyl ester in the retinal pigment epithelium is a critical step in the visual cycle and is essential for perception of light. Recent findings from cell culture models suggest that protein RPE65 is the retinoid isomerohydrolase that catalyzes the reaction. However, previous attempts to detect the enzymatic activity of purified RPE65 were unsuccessful, and thus its enzymatic function remains controversial. Here, we developed a novel liposome-based assay for isomerohydrolase activity. The results showed that purified recombinant chicken RPE65 had a high affinity for all-trans-retinyl palmitate-containing liposomes and demonstrated a robust isomerohydrolase activity. Furthermore, we found that all-trans-retinyl ester must be incorporated into the phospholipid membrane to serve as a substrate for isomerohydrolase. This assay system using purified RPE65 enabled us to measure kinetic parameters for the enzymatic reaction catalyzed by RPE65. These results provide conclusive evidence that RPE65 is the isomerohydrolase of the visual cycle.

Involvement of all-trans-retinal in acute light-induced retinopathy of mice

Exposure to bright light can cause visual dysfunction and retinal photoreceptor damage in humans and experimental animals, but the mechanism(s) remain unclear. We investigated whether the retinoid cycle (i.e. the series of biochemical reactions required for vision through continuous generation of 11-cis-retinal and clearance of all-trans-retinal, respectively) might be involved. Previously, we reported that mice lacking two enzymes responsible for clearing all-trans-retinal, namely photoreceptor-specific ABCA4 (ATP-binding cassette transporter 4) and RDH8 (retinol dehydrogenase 8), manifested retinal abnormalities exacerbated by light and associated with accumulation of diretinoid-pyridinium-ethanolamine (A2E), a condensation product of all-trans-retinal and a surrogate marker for toxic retinoids. Now we show that these mice develop an acute, light-induced retinopathy. However, cross-breeding these animals with lecithin:retinol acyltransferase knock-out mice lacking retinoids within the eye produced progeny that did not exhibit such light-induced retinopathy until gavaged with the artificial chromophore, 9-cis-retinal. No significant ocular accumulation of A2E occurred under these conditions. These results indicate that this acute light-induced retinopathy requires the presence of free all-trans-retinal and not, as generally believed, A2E or other retinoid condensation products. Evidence is presented that the mechanism of toxicity may include plasma membrane permeability and mitochondrial poisoning that lead to caspase activation and mitochondria-associated cell death. These findings further understanding of the mechanisms involved in light-induced retinal degeneration.

Medium- and short-chain dehydrogenase/reductase gene and protein families : Medium-chain and short-chain dehydrogenases/reductases in retinoid metabolism

Retinoic acid (RA), the most active retinoid, is synthesized in two steps from retinol. The first step, oxidation of retinol to retinaldehyde, is catalyzed by cytosolic alcohol dehydrogenases (ADHs) of the medium-chain dehydrogenase/reductase (MDR) superfamily and microsomal retinol dehydrogenases (RDHs) of the short-chain dehydrogenase/reductase (SDR) superfamily. The second step, oxidation of retinaldehyde to RA, is catalyzed by several aldehyde dehydrogenases. ADH1 and ADH2 are the major MDR enzymes in liver retinol detoxification, while ADH3 (less active) and ADH4 (most active) participate in RA generation in tissues. Several NAD(+)- and NADP(+)-dependent SDRs are retinoid active. Their in vivo contribution has been demonstrated in the visual cycle (RDH5, RDH12), adult retinoid homeostasis (RDH1) and embryogenesis (RDH10). K(m) values for most retinoid-active ADHs and RDHs are close to 1 microM or lower, suggesting that they participate physiologically in retinol/retinaldehyde interconversion. Probably none of these enzymes uses retinoids bound to cellular retinol-binding protein, but only free retinoids. The large number of enzymes involved in the two directions of this step, also including aldo-keto reductases, suggests that retinaldehyde levels are strictly regulated.

Highly efficient retinal metabolism in cones

After bleaching of visual pigment in vertebrate photoreceptors, all-trans retinal is reduced to all-trans retinol by retinol dehydrogenases (RDHs). We investigated this reaction in purified carp rods and cones, and we found that the reducing activity toward all-trans retinal in the outer segment (OS) of cones is >30 times higher than that of rods. The high activity of RDHs was attributed to high content of RDH8 in cones. In the inner segment (IS) in both rods and cones, RDH8L2 and RDH13 were found to be the major enzymes among RDH family proteins. We further found a previously undescribed and effective pathway to convert 11-cis retinol to 11-cis retinal in cones: this oxidative conversion did not require NADP(+) and instead was coupled with reduction of all-trans retinal to all-trans retinol. The activity was >50 times effective than the oxidizing activity of RDHs that require NADP(+). These highly effective reactions of removal of all-trans retinal by RDH8 and production of 11-cis retinal by the coupling reaction are probably the underlying mechanisms that ensure effective visual pigment regeneration in cones that function under much brighter light conditions than rods.

Retinopathy in mice induced by disrupted all-trans-retinal clearance

The visual (retinoid) cycle is a fundamental metabolic process in vertebrate retina responsible for production of 11-cis-retinal, the chromophore of rhodopsin and cone pigments. 11-cis-Retinal is bound to opsins, forming visual pigments, and when the resulting visual chromophore 11-cis-retinylidene is photoisomerized to all-trans-retinylidene, all-trans-retinal is released from these receptors. Toxic byproducts of the visual cycle formed from all-trans-retinal often are associated with lipofuscin deposits in the retinal pigmented epithelium (RPE), but it is not clear whether aberrant reactions of the visual cycle participate in RPE atrophy, leading to a rapid onset of retinopathy. Here we report that mice lacking both the ATP-binding cassette transporter 4 (Abca4) and enzyme retinol dehydrogenase 8 (Rdh8), proteins critical for all-trans-retinal clearance from photoreceptors, developed severe RPE/photoreceptor dystrophy at an early age. This phenotype includes lipofuscin, drusen, and basal laminar deposits, Bruch's membrane thickening, and choroidal neovascularization. Importantly, the severity of visual dysfunction and retinopathy was exacerbated by light but attenuated by treatment with retinylamine, a visual cycle inhibitor that slows the flow of all-trans-retinal through the visual cycle. These findings provide direct evidence that aberrant production of toxic condensation byproducts of the visual cycle in mice can lead to rapid, progressive retinal degeneration.

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.

Analysis of the NADH-dependent retinaldehyde reductase activity of amphioxus retinol dehydrogenase enzymes enhances our understanding of the evolution of the retinol dehydrogenase family

In vertebrates, multiple microsomal retinol dehydrogenases are involved in reversible retinol/retinal interconversion, thereby controlling retinoid metabolism and retinoic acid availability. The physiologic functions of these enzymes are not, however, fully understood, as each vertebrate form has several, usually overlapping, biochemical roles. Within this context, amphioxus, a group of chordates that are simpler, at both the functional and genomic levels, than vertebrates, provides a suitable evolutionary model for comparative studies of retinol dehydrogenase enzymes. In a previous study, we identified two amphioxus enzymes, Branchiostoma floridae retinol dehydrogenase 1 and retinol dehydrogenase 2, both candidates to be the cephalochordate orthologs of the vertebrate retinol dehydrogenase enzymes. We have now proceeded to characterize these amphioxus enzymes. Kinetic studies have revealed that retinol dehydrogenase 1 and retinol dehydrogenase 2 are microsomal proteins that catalyze the reduction of all-trans-retinaldehyde using NADH as cofactor, a remarkable combination of substrate and cofactor preferences. Moreover, evolutionary analysis, including the amphioxus sequences, indicates that Rdh genes were extensively duplicated after cephalochordate divergence, leading to the gene cluster organization found in several mammalian species. Overall, our data provide an evolutionary reference with which to better understand the origin, activity and evolution of retinol dehydrogenase enzymes.

A novel cone visual cycle in the cone-dominated retina

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

Functional aspects of 17beta-hydroxysteroid dehydrogenase 1 determined by comparison to a closely related retinol dehydrogenase

Determining the functional aspects of a gene or protein is a difficult and time-consuming process. De novo analysis is surely the hardest and so it is often quite useful to start with a comparison to functionally or structurally related proteins. Although 17beta-hydroxysteroid dehydrogenase type 1 (17beta-HSD 1) can hardly be called a new protein but rather the best characterized among the family of 17beta-HSDs some aspects of structure-function relationships remain unclear. We have sought new aspects of 17beta-HSD 1 function through a comparison with its closest homolog, a photoreceptor-associated retinol dehydrogenase (prRDH). Overall amino acid identity and size of the proteins are highly conserved, but major differences occur in the C-termini, where prRDH, but not 17beta-HSD 1, harbors motifs indicative of membrane localization. To gain insight into substrate discrimination by prRDH and 17beta-HSD 1, we constructed 3D-structure models of the corresponding zebrafish enzymes. Investigation of the substrate binding site revealed a few identical amino acids, and suggested a role for G143 in zebrafish 17beta-HSD 1 and M146 and M147 in the two zebrafish paralogs prRDH 1 and prRDH 2, respectively, in substrate specificity. Activity measurements of modified proteins in transiently transfected intact HEK 293 cells hint at a putative role of these amino acids in discrimination between steroid and retinoid substrates.

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.

Targeted disruption of the murine retinal dehydrogenase gene Rdh12 does not limit visual cycle function

RDH12 codes for a member of the family of short-chain alcohol dehydrogenases/reductases proposed to function in the visual cycle that supplies the chromophore 11-cis retinal to photoreceptor cells. Mutations in RDH12 cause severe and progressive childhood onset autosomal-recessive retinal dystrophy, including Leber congenital amaurosis. We generated Rdh12 knockout mice, which exhibited grossly normal retinal histology at 10 months of age. Levels of all-trans and 11-cis retinoids in dark- and light-adapted animals and scotopic and photopic electroretinogram (ERG) responses were similar to those for the wild type, as was recovery of the ERG response following bleaching, for animals matched for an Rpe65 polymorphism (p.L450M). Lipid peroxidation products and other measures of oxidative stress did not appear to be elevated in Rdh12(-/-) animals. RDH12 was localized to photoreceptor inner segments and the outer nuclear layer in both mouse and human retinas by immunohistochemistry. The present findings, together with those of earlier studies showing only minor functional deficits in mice deficient for Rdh5, Rdh8, or Rdh11, suggest that the activity of any one isoform is not rate limiting in the visual response.

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).

Visual cycle and its metabolic support in gecko photoreceptors

Photoreceptors of nocturnal geckos are transmuted cones that acquired rod morphological and physiological properties but retained cone-type phototransduction proteins. We have used microspectrophotometry and microfluorometry of solitary isolated green-sensitive photoreceptors of Tokay gecko to study the initial stages of the visual cycle within these cells. These stages are the photolysis of the visual pigment, the reduction of all-trans retinal to all-trans retinol, and the clearance of all-trans retinol from the outer segment (OS) into the interphotoreceptor space. We show that the rates of decay of metaproducts (all-trans retinal release) and retinal-to-retinol reduction are intermediate between those of typical rods and cones. Clearance of retinol from the OS proceeds at a rate that is typical of rods and is greatly accelerated by exposure to interphotoreceptor retinoid-binding protein, IRBP. The rate of retinal release from metaproducts is independent of the position within the OS, while its conversion to retinol is strongly spatially non-uniform, being the fastest at the OS base and slowest at the tip. This spatial gradient of retinol production is abolished by dialysis of saponin-permeabilized OSs with exogenous NADPH or substrates for its production by the hexose monophosphate pathway (NADP+glucose-6-phosphate or 6-phosphogluconate, glucose-6-phosphate alone). Following dialysis by these agents, retinol production is accelerated by several-fold compared to the fastest rates observed in intact cells in standard Ringer solution. We propose that the speed of retinol production is set by the availability of NADPH which in turn depends on ATP supply within the outer segment. We also suggest that principal source of this ATP is from mitochondria located within the ellipsoid region of the inner segment.

Effect of visible light on normal and P23H-3 transgenic rat retinas: characterization of a novel retinoic acid derivative present in the P23H-3 retina

Transgenic rats with the P23H mutation in rhodopsin exhibit increased susceptibility to light damage, compared with normal animals. It is known that light-induced retinal damage requires repetitive bleaching of rhodopsin and that photoreceptor cell loss is by apoptosis; however, the underlying molecular mechanism(s) leading to photoreceptor cell death are still unknown. Photoproducts, such as all-trans retinal or other retinoid metabolites, released by the extensive bleaching of rhodopsin could lead to activation of degenerative processes, especially in animals genetically predisposed to retinal degenerations. Using wild-type and transgenic rats carrying the P23H opsin mutation, we evaluated the effects of acute intense visible light on retinoid content, type and distribution in ocular tissues. Rats were exposed to green light (480-590 nm) for 0, 5, 10, 30 and 120 min. Following light treatment, rats were sacrificed and neural retinas were dissected free of the retinal pigment epithelium. Retinoids were extracted from retinal tissues and then subjected to HPLC and mass spectral analysis. We found that the light exposure affected relative levels of retinoids in the neural retina and retinal pigment epithelium of wild-type and P23H rat eyes similarly. In the P23H rat retina but not the wild-type rat retina, we found a retinoic acid-like compound with an absorbance maximum of 357 nm and a mass of 304 daltons. Production of this retinoic acid-like compound in transgenic rats is influenced by the age of the animals and the duration of light exposure. It is possible that this unique retinoid may be involved in the process of light-induced retinal degeneration.

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.

Visual cycle: Dependence of retinol production and removal on photoproduct decay and cell morphology

The visual cycle is a chain of biochemical reactions that regenerate visual pigment following exposure to light. Initial steps, the liberation of all-trans retinal and its reduction to all-trans retinol by retinol dehydrogenase (RDH), take place in photoreceptors. We performed comparative microspectrophotometric and microfluorometric measurements on a variety of rod and cone photoreceptors isolated from salamander retinae to correlate the rates of photoproduct decay and retinol production. Metapigment decay rate was spatially uniform within outer segments and 50-70 times faster in the cells that contained cone-type pigment (SWS2 and M/LWS) compared to cells with rod-type pigment (RH1). Retinol production rate was strongly position dependent, fastest at the base of outer segments. Retinol production rate was 10-40 times faster in cones with cone pigments (SWS2 and M/LWS) than in the basal OS of rods containing rod pigment (RH1). Production rate was approximately five times faster in rods containing cone pigment (SWS2) than the rate in basal OS of rods containing the rod pigment (RH1). We show that retinol production is defined either by metapigment decay rate or RDH reaction rate, depending on cell type or outer segment region, whereas retinol removal is defined by the surface-to-volume ratio of the outer segment and the availability of retinoid binding protein (IRBP). The more rapid rates of retinol production in cones compared to rods are consistent with the more rapid operation of the visual cycle in these cells.

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.

Gene Therapy Restores Vision-Dependent Behavior as Well as Retinal Structure and Function in a Mouse Model of RPE65 Leber Congenital Amaurosis

Retinal pigment epithelium-specific protein 65 kDa (RPE65) is a protein responsible for isomerization of all-trans-retinaldehyde to its photoactive 11-cis-retinaldehyde and is essential for the visual cycle. RPE65 mutations can cause severe, early onset retinal diseases such as Leber congenital amaurosis (LCA). A naturally occurring rodent model of LCA with a recessive nonsense Rpe65 mutation, the rd12 mouse, displays a profoundly diminished rod electroretinogram (ERG), an absence of 11-cis-retinaldehyde and rhodopsin, an overaccumulation of retinyl esters in retinal pigmented epithelial (RPE) cells, and photoreceptor degeneration. rd12 mice were injected subretinally at postnatal day 14 with rAAV5-CBA-hRPE65 vector. RPE65 expression was found over large areas of RPE soon after treatment. This led to improved rhodopsin levels with ERG signals restored to near normal. Retinyl ester levels were maintained at near normal, and fundus and retinal morphology remained normal. All parameters of restored retinal health remained stable for at least 7 months. The Morris water maze behavioral test was modified to test rod function under very dim light; rd12 mice treated in one eye performed similar to normally sighted C57BL/6J mice, while untreated rd12 mice performed very poorly, demonstrating that gene therapy can restore normal vision-dependent behavior in a congenitally blind animal.

Transformation of adult retina from the regenerative to the axonogenesis state activates specific genes in various subsets of neurons and glial cells

The purpose of this study was to identify the gene expression profile of the regenerating retina in vitro. To achieve this goal, three experimental groups were studied: (1) an injury control group (OC-LI group) that underwent open crush (OC) of the optic nerve and lens injury (LI) in vivo; (2) an experimental group (OC-LI-R group) that comprised animals treated like those in the OC-LI group except that retinal axons were allowed to regenerate (R) in vitro; and (3) an experimental group (OC-LI-NR group) that comprised animals treated as those in the OC-LI group, except that the retinas were cultured in vitro with the retinal ganglion cell (RGC) layer facing upwards to prevent axonal regeneration (NR). Gene expression in each treatment group was compared to that of untreated controls. Immunohistochemistry was used to examine whether expression of differentially regulated genes also occurred at the protein level and to localize these proteins to the respective retinal cells. Genes that were regulated belonged to different functional categories such as antioxidants, antiapoptotic molecules, transcription factors, secreted signaling molecules, inflammation-related genes, and others. Comparison of changes in gene expression among the various treatment groups revealed a relatively small cohort of genes that was expressed in different subsets of cells only in the OC-LI-R group; these genes can be considered to be regeneration-specific. Our findings demonstrate that axonal regeneration of RGC involves an orchestrated response of all retinal neurons and glia, and could provide a platform for the development of therapeutic strategies for the regeneration of injured ganglion cells.

Retinal degeneration associated with RDH12 mutations results from decreased 11-cis retinal synthesis due to disruption of the visual cycle

Retinoid dehydrogenases/reductases catalyze key oxidation-reduction reactions in the visual cycle that converts vitamin A to 11-cis retinal, the chromophore of the rod and cone photoreceptors. It has recently been shown that mutations in RDH12, encoding a retinol dehydrogenase, result in severe and early-onset autosomal recessive retinal dystrophy (arRD). In a cohort of 1011 individuals diagnosed with arRD, we have now identified 20 different disease-associated RDH12 mutations, of which 16 are novel, in a total of 22 individuals (2.2%). Haplotype analysis suggested a founder mutation for each of the three common mutations: p.L99I, p.T155I and c.806_810delCCCTG. Patients typically presented with early disease that affected the function of both rods and cones and progressed to legal blindness in early adulthood. Eleven of the missense variants identified in our study exhibited profound loss of catalytic activity when expressed in transiently transfected COS-7 cells and assayed for ability to convert all-trans retinal to all-trans retinol. Loss-of-function appeared to result from decreased protein stability, as expression levels were significantly reduced. For the p.T49M variant, differing activity profiles were associated with each of the alleles of the common p.R161Q RDH12 polymorphism, suggesting that genetic background may act as a modifier of mutation effect. A locus (LCA3) for Leber congenital amaurosis, a severe, early-onset form of arRD, maps close to RDH12 on chromosome 14q24. Haplotype analysis in the family in which LCA3 was mapped excluded RDH12 as the LCA3 gene and thus suggests the presence of a novel arRD gene in this region.

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.

Biochemical properties of purified human retinol dehydrogenase 12 (RDH12): catalytic efficiency toward retinoids and C9 aldehydes and effects of cellular retinol-binding protein type I (CRBPI) and cellular retinaldehyde-binding protein (CRALBP) on the oxidation and reduction of retinoids

Retinol dehydrogenase 12 (RDH12) is a novel member of the short-chain dehydrogenase/reductase superfamily of proteins that was recently linked to Leber's congenital amaurosis 3 (LCA). We report the first biochemical characterization of purified human RDH12 and analysis of its expression in human tissues. RDH12 exhibits approximately 2000-fold lower K(m) values for NADP(+) and NADPH than for NAD(+) and NADH and recognizes both retinoids and lipid peroxidation products (C(9) aldehydes) as substrates. The k(cat) values of RDH12 for retinaldehydes and C(9) aldehydes are similar, but the K(m) values are, in general, lower for retinoids. The enzyme exhibits the highest catalytic efficiency for all-trans-retinal (k(cat)/K(m) approximately 900 min(-)(1) microM(-)(1)), followed by 11-cis-retinal (450 min(-)(1) mM(-)(1)) and 9-cis-retinal (100 min(-)(1) mM(-)(1)). Analysis of RDH12 activity toward retinoids in the presence of cellular retinol-binding protein (CRBP) type I or cellular retinaldehyde-binding protein (CRALBP) suggests that RDH12 utilizes the unbound forms of all-trans- and 11-cis-retinoids. As a result, the widely expressed CRBPI, which binds all-trans-retinol with much higher affinity than all-trans-retinaldehyde, restricts the oxidation of all-trans-retinol by RDH12, but has little effect on the reduction of all-trans-retinaldehyde, and CRALBP inhibits the reduction of 11-cis-retinal stronger than the oxidation of 11-cis-retinol, in accord with its higher affinity for 11-cis-retinal. Together, the tissue distribution of RDH12 and its catalytic properties suggest that, in most tissues, RDH12 primarily contributes to the reduction of all-trans-retinaldehyde; however, at saturating concentrations of peroxidic aldehydes in the cells undergoing oxidative stress, for example, photoreceptors, RDH12 might also play a role in detoxification of lipid peroxidation products.

Functional Characterization of Mouse RDH11 as a Retinol Dehydrogenase Involved in Dark Adaptation in Vivo

We previously cloned mouse RDH11 (mRDH11) as a gene regulated by the transcription factor sterol regulatory element-binding proteins and showed that it is a retinol dehydrogenase expressed in non-ocular tissues such as the liver and testis and in the retina (Kasus-Jacobi, A., Ou, J., Bashmakov, Y. K., Shelton, J. M., Richardson, J. A., Goldstein, J. L., and Brown, M. S. (2003) J. Biol. Chem. 278, 32380-32389). It was proposed to function in the recycling of the visual chromophore 11-cis-retinal after photoisomerization by a bleaching light, a pathway referred to as the visual cycle. In this work, we describe our studies on the ocular function of mRDH11. We created a knockout mouse by replacing the mrdh11 coding sequence with the lacZ reporter gene for expression profiling. 5-Bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining demonstrated active transcription of this gene in photoreceptor cells. We show by immunoblot analysis that mRDH11 is associated with retinal membranes purified from a non-outer segment fraction of the retina. No obvious retinal defect was found during development and aging of RDH11-deficient mice. The functional consequences of mRDH11 disruption were investigated by electroretinography. Dark adaptation was delayed by a factor of 2.5-3 compared with wild-type mice. However, the kinetics of 11-cis-retinal recycling during dark adaptation was not affected, suggesting that mRDH11 is not involved in the visual cycle. We propose that mRDH11 disruption affects retinoid metabolism in photoreceptor inner segments and delays the kinetics of dark adaptation through modulation of calcium homeostasis.

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.

Transcriptional Profiling of Circadian Patterns of mRNA Expression in the Chick Retina

Previous transcriptome analyses have identified candidate molecular components of the avian pineal clock, and herein we employ high density cDNA microarrays of pineal gland transcripts to determine oscillating transcripts in the chick retina under daily and constant darkness conditions. Subsequent comparative transcriptome analysis of the pineal and retinal oscillators distinguished several transcriptional similarities between the two as well as significant differences. Rhythmic retinal transcripts were classified according to functional categories including phototransductive elements, transcription/translation factors, carrier proteins, cell signaling molecules, and stress response genes. Candidate retinal clock transcripts were also organized relative to time of day mRNA abundance, revealing groups accumulating peak mRNA levels across the circadian day but primarily reaching peak values at subjective dawn or subjective dusk. Comparison of the chick retina transcriptome to the pineal transcriptome under constant conditions yields an interesting group of conserved genes. This group includes putative clock elements cry1 and per3 in addition to several previously unidentified and uninvestigated genes exhibiting profiles of mRNA abundance that varied markedly under daily and constant conditions. In contrast, many transcripts were differentially regulated, including those believed to be involved in both melatonin biosynthesis and circadian clock mechanisms. Our results indicate an intimate transcriptional relationship between the avian pineal and retina in addition to providing previously uncharacterized molecular elements that we hypothesize to be involved in circadian rhythm generation.

Physiological and microfluorometric studies of reduction and clearance of retinal in bleached rod photoreceptors

The visual cycle comprises a sequence of reactions that regenerate the visual pigment in photoreceptors during dark adaptation, starting with the reduction of all-trans retinal to all-trans retinol and its clearance from photoreceptors. We have followed the reduction of retinal and clearance of retinol within bleached outer segments of red rods isolated from salamander retina by measuring its intrinsic fluorescence. Following exposure to a bright light (bleach), increasing fluorescence intensity was observed to propagate along the outer segments in a direction from the proximal region adjacent to the inner segment toward the distal tip. Peak retinol fluorescence was achieved after approximately 30 min, after which it declined very slowly. Clearance of retinol fluorescence is considerably accelerated by the presence of the exogenous lipophilic substances IRBP (interphotoreceptor retinoid binding protein) and serum albumin. We have used simultaneous fluorometric and electrophysiological measurements to compare the rate of reduction of all-trans retinal to all-trans retinol to the rate of recovery of flash response amplitude in these cells in the presence and absence of IRBP. We find that flash response recovery in rods is modestly accelerated in the presence of extracellular IRBP. These results suggest such substances may participate in the clearance of retinoids from rod photoreceptors, and that this clearance, at least in rods, may facilitate dark adaptation by accelerating the clearance of photoproducts of bleaching.

Global analysis of genes differentially expressed in branching and non-branching regions of the mouse embryonic lung

During development, the proximal and distal regions of respiratory tract undergo distinct processes that ultimately give rise to conducting airways and alveoli. To gain insights into the genetic pathways differentially activated in these regions when branching morphogenesis is initiating, we characterized their transcriptional profiles in murine rudiments isolated at embryonic (E) day 11.5. By using oligonucleotide microarrays, we identified 83 and 128 genes preferentially expressed in branching and non-branching regions, respectively. The majority of these genes (85%) had not been previously described in the lung, or in other organs. We report restricted expression patterns of 22 of these genes were by in situ hybridization. Among them in the lung potential components of the Wnt, TGF beta, FGF and retinoid pathways identified in other systems, and uncharacterized genes, such as translocases, small GTPases and splicing factors. In addition, we provide a more detailed analysis of the expression pattern and regulation of a representative gene from the distal (transforming growth factor, beta induced) and proximal (WW domain-containing protein 2) regions. Our data suggest that these genes may regulate focal developmental events specific of each of these regions during respiratory tract formation.

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.

Structure and function of retinol dehydrogenases of the short chain dehydrogenase/reductase family

All-trans-retinol is the common precursor of the active retinoids 11-cis-retinal, all-trans-retinoic acid (atRA) and 9-cis-retinoic acid (9cRA). Genetic and biochemical data supports an important role of the microsomal members of the short chain dehydrogenases/reductases (SDRs) in the first oxidative conversion of retinol into retinal. Several retinol dehydrogenases of this family have been reported in recent years. However, the structural and functional data on these enzymes is limited. The prototypic enzyme RDH5 and the related enzyme CRAD1 have been shown to face the lumen of the endoplasmic reticulum (ER), suggesting a compartmentalized synthesis of retinal. This is a matter of debate as a related enzyme has been proposed to have the opposite membrane topology. Recent data indicates that RDH5, and presumably other members of the SDRs, occur as functional homodimers, and need to interact with other proteins for proper intracellular localization and catalytic activity. Further analyses on the compartmentalization, membrane topology, and functional properties of microsomal retinol dehydrogenases, will give important clues about how retinoids are processed.

Mutations in RDH12 encoding a photoreceptor cell retinol dehydrogenase cause childhood-onset severe retinal dystrophy

We identified three consanguineous Austrian kindreds with 15 members affected by autosomal recessive childhood-onset severe retinal dystrophy, a genetically heterogeneous group of disorders characterized by degeneration of the photoreceptor cells. A whole-genome scan by microarray analysis of single-nucleotide polymorphisms (ref. 2) identified a founder haplotype and defined a critical interval of 1.53 cM on chromosome 14q23.3-q24.1 that contains the gene associated with this form of retinal dystrophy. RDH12 maps in this region and encodes a retinol dehydrogenase proposed to function in the visual cycle. A homozygous 677A-->G transition (resulting in Y226C) in RDH12 was present in all affected family members studied, as well as in two Austrian individuals with sporadic retinal dystrophy. We identified additional mutations in RDH12 in 3 of 89 non-Austrian individuals with retinal dystrophy: a 5-nucleotide deletion (806delCCCTG) and the transition 565C-->T (resulting in Q189X), each in the homozygous state, and 146C-->T (resulting in T49M) and 184C-->T (resulting in R62X) in compound heterozygosity. When expressed in COS-7 cells, Cys226 and Met49 variants had diminished and aberrant activity, respectively, in interconverting isomers of retinol and retinal. The severe visual impairment of individuals with mutations in RDH12 is in marked contrast to the mild visual deficiency in individuals with fundus albipunctatus caused by mutations in RDH5, encoding another retinal dehydrogenase. Our studies show that RDH12 is associated with retinal dystrophy and encodes an enzyme with a unique, nonredundant role in the photoreceptor cells.

An outer segment localization signal at the C terminus of the photoreceptor-specific retinol dehydrogenase

Photoreceptor retinol dehydrogenase (prRDH) is a membrane-associated cytosolic protein that localizes to the outer segments (OS) of rods and cones. Here, we demonstrate that the C-terminal 16 amino acids of prRDH confer membrane association as well as cone and rod OS targeting on a linked green fluorescent protein. Membrane association in transfected 293 cells and in transgenic Xenopus photoreceptors is mediated by fatty acylation at one or more evolutionarily conserved cysteines within the prRDH C-terminal tail. In bovine OS, native prRDH is similarly acylated, and hydrolysis of this linkage releases the protein from the membrane. Efficient OS localization requires both membrane association and the prRDH sequence. (V/I)XPX at the extreme C terminus, which closely resembles the C-terminal sequence that targets opsin/rhodopsin to the OS. Taken together, these data imply that the C-terminal. (V/I)XPX sequence is a general OS localization signal that can function in the context of both integral and peripheral membrane proteins. This strategy for OS localization resembles those used for protein localization to mitochondria, peroxisomes, endosomes, and endoplasmic reticulum; in each case, a short N- or C-terminal sequence is shared among structurally diverse proteins that are targeted to the same subcellular destination.

Identification and characterization of 17 beta-hydroxysteroid dehydrogenases in the zebrafish, Danio rerio

The 17 beta-hydroxysteroid dehydrogenases (17 beta-HSDs) are key enzymes in the final steps of steroid hormone synthesis. 17beta-HSD type 1 (HSD17B1) catalyzes the reduction of estrone to estradiol, while type 3 (HSD17B3) performs the conversion of androstenedione to testosterone. Here we present a functional genomics study of putative candidates of these enzymes in the zebrafish. By an in silico screen of zebrafish EST databases we identified three candidate homologs for both HSD17B1 and HSD17B3. Phylogenetic analysis, unique expression patterns (RT-PCR) during embryogenesis and adulthood, as well as activity measurements revealed that one of the HSD17B1 candidates is the zebrafish homolog, while the other two are paralogous photoreceptor-associated retinol dehydrogenases. All three HSD17B3 candidate genes showed nearly identical, ubiquitous expressions in embryogenesis and adult tissues and were identified to be paralogs of HSD17B12 and a yet uncharacterized putative steroid dehydrogenase. Phylogenetic analysis shows that HSD17B3 and HSD17B12 are descendants from a common ancestor.

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.

Using denaturing HPLC for SNP discovery and genotyping, and establishing the linkage disequilibrium pattern for the all-trans-retinol dehydrogenase (RDH8) gene

All-trans-retinol dehydrogenase (RDH8) is a visual cycle enzyme that reduces all-trans-retinal to all-trans-retinol. As part of an on-going effort to map genes involved in complex eye diseases, myopia in particular, using association studies, single nucleotide polymorphisms (SNPs) were identified and linkage disequilibrium (LD) pattern was established within and around the RDH8 gene. We used denaturing high-performance liquid chromatography (DHPLC) to screen SNPs in four DNA pools each consisting of DNA from five individuals and genotyped the identified SNPs in 150 Chinese subjects from Hong Kong. Fifteen SNPs were identified: seven were common with the minor allele frequency >0.05 and ten were novel. Common SNPs were included in LD and haplotype analysis using the ASSOCIATE and EH programmes. Four SNPs in the 3' region exhibited significant LD and formed a haplotype block, while three common SNPs in the 5' region did not exhibit useful LD. The LD pattern around the RDH8 gene suggested that one SNP from the 3'region and two to three SNPs from the 5' region were needed in association studies involving RDH8. Our results demonstrated the efficiency of DHPLC in screening SNPs when coupled with DNA pooling strategy and in genotyping SNPs.

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.

Transcriptional profiling of the chick pineal gland, a photoreceptive circadian oscillator and pacemaker

The avian pineal gland contains both circadian oscillators and photoreceptors to produce rhythms in biosynthesis of the hormone melatonin in vivo and in vitro. The molecular mechanisms for melatonin biosynthesis are largely understood, but the mechanisms driving the rhythm itself or the photoreceptive processes that entrain the rhythm are unknown. We have produced cDNA microarrays of pineal gland transcripts under light-dark and constant darkness conditions. Rhythmic transcripts were classified according to function, representing diverse functional groups, including phototransduction pathways, transcription/translation factors, ion channel proteins, cell signaling molecules, and immune function genes. These were also organized relative to time of day mRNA abundance in light-dark and constant darkness. The transcriptional profile of the chick pineal gland reveals a more complex form of gene regulation than one might expect from a gland whose sole apparent function is the rhythmic biosynthesis of melatonin. The mRNAs encoding melatonin biosynthesis are rhythmic as are many orthologs of mammalian "clock genes." However, the oscillation of phototransductive, immune, stress response, hormone binding, and other important processes in the transcriptome of the pineal gland, raises new questions regarding the role of the pineal gland in circadian rhythm generation, organization, and avian physiology.

Vitamin A metabolism in the retinal pigment epithelium: genes, mutations, and diseases

Mutations in the genes necessary for the metabolism of vitamin A (all-trans retinol) and cycling of retinoids between the photoreceptors and retinal pigment epithelium (RPE) (the visual cycle) have recently emerged as an important class of genetic defects responsible for retinal dystrophies and dysfunctions. Research into the causes and treatment of diseases resulting from defects in retinal vitamin A metabolism is currently the subject of intense interest, since disorders affecting the RPE are, in principle, more accessible to therapeutic intervention than those affecting the proteins of photoreceptor cells. This chapter presents an overview of the visual cycle, as well as the function of the RPE genes involved in the conversion of vitamin A to 11-cis retinal, the chromophore of the visual pigments. The identification of disease-causing mutations in this group of genes is described as well as the associated phenotypes that range from stationary night blindness to childhood-onset severe visual handicap. Consideration is also given to alternative genetic paradigms potentially relevant to defects in vitamin A metabolism, including a discussion of the relationship of this pathway to age-related macular degeneration, a non-Mendelian disease of late onset. Finally, progress and prospects for targeted therapeutic intervention in vitamin A metabolism are presented, including retinoid and gene replacement therapy. On the basis of early successes in animal models, and plans underway for Phase I/II clinical trials, it is hoped that the near future will bring effective therapies for many retinal dystrophy patients with defects in vitamin A metabolism.

Characterization of mouse short-chain aldehyde reductase (SCALD), an enzyme regulated by sterol regulatory element-binding proteins

Sterol regulatory element-binding proteins (SREBPs) enhance transcription of genes encoding all of the proteins required for the cellular synthesis and uptake of cholesterol and unsaturated fatty acids. Here, we use suppression subtractive hybridization to identify a previously unrecognized SREBP-enhanced gene in mice. The gene encodes a membrane-bound enzyme that we designate SCALD, for short-chain aldehyde reductase. We expressed SCALD in bacteria, purified it extensively, and studied its catalytic properties in detergent solution. The enzyme specifically uses NADPH to reduce a variety of short-chain aldehydes, including nonanal and 4-hydroxy-2-nonenal. The enzyme also reduces retinaldehydes, showing equal activity for all-trans-retinal and 9-cis-retinal. Northern blot analysis indicates that SCALD is expressed most abundantly in mouse liver and testis. In the liver of mice, SCALD is suppressed by fasting and induced by refeeding, consistent with regulation by SREBPs. In testis, SCALD expression is restricted to pachytene spermatocytes, as revealed by visualization of mRNA and protein. SCALD is also expressed in four layers of the retina, including the outer segment of rods and cones, as revealed by immunohistochemistry. SCALD appears to be the mouse ortholog of the human protein that has been designated variously as prostate short-chain dehydrogenase/reductase 1, retinal reductase 1, and retinol dehydrogenase 11. In view of its ability to reduce short-chain aldehydes in addition to retinals, we propose that SCALD may be induced by SREBP in liver and other tissues to prevent toxicity from fatty aldehydes that are generated from oxidation of unsaturated fatty acids that are synthesized as a result of SREBP activity.

Human aldose reductase and human small intestine aldose reductase are efficient retinal reductases: consequences for retinoid metabolism

Aldo-keto reductases (AKRs) are NAD(P)H-dependent oxidoreductases that catalyse the reduction of a variety of carbonyl compounds, such as carbohydrates, aliphatic and aromatic aldehydes and steroids. We have studied the retinal reductase activity of human aldose reductase (AR), human small-intestine (HSI) AR and pig aldehyde reductase. Human AR and HSI AR were very efficient in the reduction of all- trans -, 9- cis - and 13- cis -retinal ( k (cat)/ K (m)=1100-10300 mM(-1).min(-1)), constituting the first cytosolic NADP(H)-dependent retinal reductases described in humans. Aldehyde reductase showed no activity with these retinal isomers. Glucose was a poor inhibitor ( K (i)=80 mM) of retinal reductase activity of human AR, whereas tolrestat, a classical AKR inhibitor used pharmacologically to treat diabetes, inhibited retinal reduction by human AR and HSI AR. All- trans -retinoic acid failed to inhibit both enzymes. In this paper we present the AKRs as an emergent superfamily of retinal-active enzymes, putatively involved in the regulation of retinoid biological activity through the assimilation of retinoids from beta-carotene and the control of retinal bioavailability.

A cleavable affinity biotinylating agent reveals a retinoid binding role for RPE65

Retinal pigment epithelial (RPE) membranes contain the full biochemical apparatus capable of processing all-trans-retinol (vitamin A) into 11-cis-retinal, the visual chromophore. As many of these proteins are integral membrane proteins and resistant to traditional methods of identification, alternate methods of identifying these proteins are sought. The approach described here involves affinity biotinylation with alkali cleavable linkers. A vitamin A containing affinity-labeling haloacetate is described which facilitates the identification of retinoid binding proteins (RBPs). Treatment of crude bovine RPE membranes with (3R)-3-[boc-lys(biotinyl)-O]-all-trans-retinol chloroacetate 1 in the low micromolar range led to the specific labeling of RPE65 and lecithin retinol acyltransferase (LRAT). Only RPE65 is labeled at 5 microM 1 at 4 degrees C. Labeled RPE65 was readily isolated by binding the labeled protein to avidin-containing beads, followed by cleavage of the protein from the beads at pH 11. Trypsin digestion of RPE65 modified by 1, followed by mass spectrometry, demonstrates that C231 and C448 are alkylated by 1. These studies validate the approach that was used, and furthermore demonstrate that RPE65, a major membrane-associated protein of the RPE, is a RBP.

All-trans-retinyl esters are the substrates for isomerization in the vertebrate visual cycle

The identification of the critical enzyme(s) that carries out the trans to cis isomerization producing 11-cis-retinol during the operation of the visual cycle remains elusive. Confusion exists in the literature as to the exact nature of the isomerization substrate. At issue is whether it is an all-trans-retinyl ester or all-trans-retinol (vitamin A). As both putative substrates interconvert rapidly in retinal pigment epithelial membranes, the choice of substrate can be ambiguous. The two enzymes that effect interconversion of all-trans-retinol and all-trans-retinyl esters are lecithin retinol acyl transferase (LRAT) and retinyl ester hydrolase (REH). The retinyl ester or all-trans-retinol pools are radioactively labeled separately in the presence of inhibitors of LRAT and REH, effectively preventing their interconversion. Pulse-chase experiments unambiguously demonstrate that all-trans-retinyl esters, and not all-trans-retinol, are the precursors of 11-cis-retinol. When the all-trans-retinyl ester pool is radioactively labeled, the resulting 11-cis-retinol is labeled with the same specific activity as the precursor ester. The converse is true with vitamin A. These data unambiguously establish all-trans-retinyl esters as the precursors of 11-cis-retinol.

Retinyl esters are the substrate for isomerohydrolase

Regeneration of 11-cis retinal from all-trans retinol in the retinal pigment epithelium (RPE) is a critical step in the visual cycle. The enzyme(s) involved in this isomerization process has not been identified and both all-trans retinol and all-trans retinyl esters have been proposed as the substrate. This study is to determine the substrate of the isomerase enzyme or enzymatic complex. Incubation of bovine RPE microsomes with all-trans [(3)H]-retinol generated both retinyl esters and 11-cis retinol. Inhibition of lecithin retinol acyltransferase (LRAT) with 10-N-acetamidodecyl chloromethyl ketone (AcDCMK) or cellular retinol-binding protein I (CRBP) diminished the generation of both retinyl esters and 11-cis retinol from all-trans retinol. The 11-cis retinol production correlated with the retinyl ester levels, but not with the all-trans retinol levels in the reaction mixture. When retinyl esters were allowed to form prior to the addition of the LRAT inhibitors, a significant amount of isomerization product was generated. Incubation of all-trans [(3)H]-retinyl palmitate with RPE microsomes generated 11-cis retinol without any detectable production of all-trans retinol. The RPE65 knockout (Rpe65(-/-)) mouse eyecup lacks the isomerase activity, but LRAT activity remains the same as that in the wild-type (WT) mice. Retinyl esters in WT mice plateau at 8 weeks-of-age, but Rpe65(-/-) mice continue to accumulate retinyl esters with age (e.g., at 36 weeks, the levels are 20x that of WT). Our data indicate that the retinyl esters are the substrate of the isomerization reaction.