CoA- and non-CoA-dependent retinol esterification in retinal pigment epithelium

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

Vitamin A and Vision

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

Genetic dissection of retinoid esterification and accumulation in the liver and adipose tissue

Approximately 80-90% of all retinoids in the body are stored as retinyl esters (REs) in the liver. Adipose tissue also contributes significantly to RE storage. The present studies, employing genetic and nutritional interventions, explored factors that are responsible for regulating RE accumulation in the liver and adipose tissue and how these influence levels of retinoic acid (RA) and RA-responsive gene expression. Our data establish that acyl-CoA:retinol acyltransferase (ARAT) activity is not involved in RE synthesis in the liver, even when mice are nutritionally stressed by feeding a 25-fold excess retinol diet or upon ablation of cellular retinol-binding protein type I (CRBPI), which is proposed to limit retinol availability to ARATs. Unlike the liver, where lecithin:retinol acyltransferase (LRAT) is responsible for all RE synthesis, this is not true for adipose tissue where Lrat-deficient mice display significantly elevated RE concentrations. However, when CrbpI is also absent, RE levels resemble wild-type levels, suggesting a role for CrbpI in RE accumulation in adipose tissue. Although expression of several RA-responsive genes is elevated in Lrat-deficient liver, employing a sensitive liquid chromatography tandem mass spectrometry protocol and contrary to what has been assumed for many years, we did not detect elevated concentrations of all-trans-RA. The elevated RA-responsive gene expression was associated with elevated hepatic triglyceride levels and decreased expression of Pparδ and its downstream Pdk4 target, suggesting a role for RA in these processes in vivo.

Vitamin A metabolism in rod and cone visual cycles

The chromophore of all known visual pigments consists of 11-cis-retinal (derived from either vitamin A1 or A2) or a hydroxylated derivative, bound to a protein (opsin) via a Schiff base. Absorption of a photon results in photoisomerization of the chromophore to all-trans-retinal and conversion of the visual pigment to the signaling form. Regeneration of the 11-cis-retinal occurs in an adjacent tissue and involves several enzymes, several water-soluble retinoid-binding proteins, and intra- and intercellular diffusional processes. Rod photoreceptor cells depend completely on the output of 11-cis-retinal from adjacent retinal pigment epithelial (RPE) cells. Cone photoreceptors cells can use 11-cis-retinal from the RPE and from a second more poorly characterized cycle, which appears to involve adjacent Müller (glial) cells. Recent progress in the characterization of rod and cone visual cycle components and reactions will result in the development of approaches to the amelioration of blinding eye diseases associated with visual cycle defects.

Metabolism of carotenoids and retinoids related to vision

All animals endowed with the ability to detect light through visual pigments must have evolved pathways in which dietary precursors for the involved chromophore are absorbed, transported, and metabolized. Knowledge about this metabolism has exponentially increased over the past decade. Genetic manipulation of animal models provided insights into the metabolic flow of these compounds through the body and in the eyes, unraveling their regulatory aspects and aberrant side reactions. The scheme that emerges reveals a common origin of key components for chromophore metabolism that have been adapted to the specific requirements of retinoid biology in different animal classes.

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.

The cone-specific visual cycle

Cone photoreceptors mediate our daytime vision and function under bright and rapidly-changing light conditions. As their visual pigment is destroyed in the process of photoactivation, the continuous function of cones imposes the need for rapid recycling of their chromophore and regeneration of their pigment. The canonical retinoid visual cycle through the retinal pigment epithelium cells recycles chromophore and supplies it to both rods and cones. However, shortcomings of this pathway, including its slow rate and competition with rods for chromophore, have led to the suggestion that cones might use a separate mechanism for recycling of chromophore. In the past four decades biochemical studies have identified enzymatic activities consistent with recycling chromophore in the retinas of cone-dominant animals, such as chicken and ground squirrel. These studies have led to the hypothesis of a cone-specific retina visual cycle. The physiological relevance of these studies was controversial for a long time and evidence for the function of this visual cycle emerged only in very recent studies and will be the focus of this review. The retina visual cycle supplies chromophore and promotes pigment regeneration only in cones but not in rods. This pathway is independent of the pigment epithelium and instead involves the Müller cells in the retina, where chromophore is recycled and supplied selectively to cones. The rapid supply of chromophore through the retina visual cycle is critical for extending the dynamic range of cones to bright light and for their rapid dark adaptation following exposure to light. The importance of the retina visual cycle is emphasized also by its preservation through evolution as its function has now been demonstrated in species ranging from salamander to zebrafish, mouse, primate, and human.

Colors with functions: elucidating the biochemical and molecular basis of carotenoid metabolism

Carotenoids affect a rich variety of physiological functions in nature and are beneficial for human health, serving as antioxidants in lipophilic environments and blue light filters in the macula of human retina. These dietary compounds also serve as precursors of a unique set of apo-carotenoid cleavage products, including retinoids. Although knowledge about retinoid biology has tremendously increased, the metabolism of retinoids' parent precursors remains poorly understood. Recently, molecular players in carotenoid metabolism have been identified and biochemically characterized. Moreover, mutations in their corresponding genes impair carotenoid metabolism and induce various pathologies in animal models. Polymorphisms in these genes alter carotenoid and retinoid homeostasis in humans as well. This review summarizes our current knowledge about the molecular/biochemical basis of carotenoid metabolism and particularly the physiological role of carotenoids in retinoid-dependent physiological processes.

Hepatic stellate cell lipid droplets: a specialized lipid droplet for retinoid storage

The majority of retinoid (vitamin A and its metabolites) present in the body of a healthy vertebrate is contained within lipid droplets present in the cytoplasm of hepatic stellate cells (HSCs). Two types of lipid droplets have been identified through histological analysis of HSCs within the liver: smaller droplets bounded by a unit membrane and larger membrane-free droplets. Dietary retinoid intake but not triglyceride intake markedly influences the number and size of HSC lipid droplets. The lipids present in rat HSC lipid droplets include retinyl ester, triglyceride, cholesteryl ester, cholesterol, phospholipids and free fatty acids. Retinyl ester and triglyceride are present at similar concentrations, and together these two classes of lipid account for approximately three-quarters of the total lipid in HSC lipid droplets. Both adipocyte-differentiation related protein and TIP47 have been identified by immunohistochemical analysis to be present in HSC lipid droplets. Lecithin:retinol acyltransferase (LRAT), an enzyme responsible for all retinyl ester synthesis within the liver, is required for HSC lipid droplet formation, since Lrat-deficient mice completely lack HSC lipid droplets. When HSCs become activated in response to hepatic injury, the lipid droplets and their retinoid contents are rapidly lost. Although loss of HSC lipid droplets is a hallmark of developing liver disease, it is not known whether this contributes to disease development or occurs simply as a consequence of disease progression. Collectively, the available information suggests that HSC lipid droplets are specialized organelles for hepatic retinoid storage and that loss of HSC lipid droplets may contribute to the development of hepatic disease.

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.

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.

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.

11-cis-Acyl-CoA:retinol O-acyltransferase activity in the primary culture of chicken Muller cells

A novel retinoid cycle has recently been identified in the cone-dominated chicken retina, and this cone cycle accumulates 11-cis-retinyl esters upon light adaptation. The purpose of this study is to investigate how 11-cis-retinyl esters are formed in the retina. Primary cultures of chicken Muller cells and cell membrane were incubated with all-trans- or 11-cis-retinol to study retinyl ester synthesis. In Muller cells, esterification of 11-cis-retinol was four times greater than esterification of all-trans-retinol. In the presence of palmitoyl-CoA and CRALBP, Muller cell membranes synthesized 11-cis-retinyl ester from 11-cis-retinol at a rate which was 20-fold higher than that of all-trans-retinyl ester. In the absence of CRALBP, 11-cis-retinyl ester synthesis was greatly reduced (by 7-fold). In the absence of palmitoyl-CoA, retinyl ester synthesis was not observed. Muller cell membranes incubated with radiolabeled palmitoyl-CoA resulted in the transfer of the labeled acyl group to retinol. This acyl transfer was greatly reduced in the presence of progesterone, a known ARAT inhibitor. 11-cis-ARAT activity remained unchanged when assayed in the presence of all-trans-retinol, suggesting a distinct catalytic activity from that of all-trans-ARAT. Apparent kinetic rates for 11-cis-ARAT were 0.135 nmol min(-)(1) mg(-)(1) (V(max)) and 11.25 microM (K(M)) and for all-trans-ARAT were 0.0065 nmol min(-)(1) mg(-)(1) (V(max)) and 28.88 microM (K(M)). Our data indicate that Muller cells in the chicken retina possess 11-cis-ARAT activity, thus providing an explanation for the accumulation of 11-cis-retinyl esters in the cone cycle.

A proposed role for all-trans retinal in regulation of rhodopsin regeneration in human rods

In order to account for the multi-phasic dynamics of photopigment regeneration in human rods, we developed a new model of the retinoid cycle. We first examined the relative roles of the classical and channeling mechanisms of metarhodopsin decay in establishing these dynamics. We showed that neither of these mechanisms alone, nor a linear combination of the two, can adequately account for the dynamics of rhodopsin regeneration at all bleach levels. Our new model adds novel inhibitory interactions in the cycle of regeneration of rhodopsin that are consistent with the 3D structure of rhodopsin. Our analyses show that the dynamics of human rod photopigment regeneration can be accounted for by end-product regulation of the channeling mechanism where all-trans retinal (tral) inhibits the binding of 11-cis retinal to the opsin.tral complex.

Rpe65 is the retinoid isomerase in bovine retinal pigment epithelium

The first event in light perception is absorption of a photon by an opsin pigment, which induces isomerization of its 11-cis-retinaldehyde chromophore. Restoration of light sensitivity to the bleached opsin requires chemical regeneration of 11-cis-retinaldehyde through an enzymatic pathway called the visual cycle. The isomerase, which converts an all-trans-retinyl ester to 11-cis-retinol, has never been identified. Here, we performed an unbiased cDNA expression screen to identify this isomerase. We discovered that the isomerase is a previously characterized protein called Rpe65. We confirmed our identification of the isomerase by demonstrating catalytic activity in mammalian and insect cells that express Rpe65. Mutations in the human RPE65 gene cause a blinding disease of infancy called Leber congenital amaurosis. Rpe65 with the Leber-associated C330Y and Y368H substitutions had no isomerase activity. Identification of Rpe65 as the isomerase explains the phenotypes in rpe65-/- knockout mice and in humans with Leber congenital amaurosis.

Lecithin:retinol acyltransferase in ARPE-19

The purpose of this study is to investigate if a readily available cell line (APRE-19) may be used to study in vitro function of visual cycle enzymes such as lecithin:retinol acyltransferase (LRAT). Cells incubated with exogenous retinol accumulated intracellular all-trans retinol and all-trans retinyl ester. Membrane proteins from ARPE-19 exhibited LRAT activity, which was inhibited by an LRAT inhibitor, retinyl bromoacetate (RBA). Gene microarray and Western blot results indicated that ARPE-19 cells expressed LRAT transcript and the LRAT protein. Therefore, our data show that ARPE-19 contains an active LRAT enzyme and suggest that it is an appropriate cell system to study visual cycle enzymes.

Acyl CoA:retinol acyltransferase (ARAT) activity is present in bovine retinal pigment epithelium

Visual perception is mediated by a family of G protein-coupled receptors called the opsins. The light-absorbing chromophore in most opsins is 11-cis-retinaldehyde, which is isomerized to all-trans-retinaldehyde upon absorption of a photon. Restoration of light sensitivity to the photobleached opsin requires chemical re-isomerization of the chromophore. This is carried out by an enzymatic pathway called the visual cycle in retinal pigment epithelial cells. The isomerase in this pathway uses fatty-acyl esters of all-trans-retinol as substrate. A retinyl-ester synthase that produces these esters, called lecithin:retinol acyltransferase (LRAT), has been extensively characterized. Based on prior biochemical studies and the phenotype in lrat(-/-) knockout mice, it has been assumed that LRAT is the sole or dominant retinyl-ester synthase in the retinal pigment epithelium. Here we demonstrate the presence of a second ester synthase activity in these cells called acyl CoA:retinol acyltransferase (ARAT). We show that this activity uses palmitoyl coenzyme A as an acyl donor, unlike LRAT which uses phosphatidylcholine. Similar to LRAT, ARAT esterifies both all-trans-retinol and 11-cis-retinol. LRAT and ARAT are both potently inhibited by the retinyl-ester analog, all-trans-retinylbromoacetate, but only ARAT is inhibited by progesterone. Unexpectedly, the maximum turnover rate (V(max)) of ARAT was similar to that of LRAT. However, the Michaelis constant (K(M)) of ARAT was 10-fold higher than the K(M) of LRAT for all-trans-retinol. These observations suggest that ARAT may complement LRAT to provide additional retinyl-ester synthase activity under conditions of high all-trans-retinol. These conditions occur in the retina following exposure to bright light.

The triacylglycerol synthesis enzyme DGAT1 also catalyzes the synthesis of diacylglycerols, waxes, and retinyl esters

The final step of triacylglycerol biosynthesis is catalyzed by acyl CoA:diacylglycerol acyltransferase (DGAT) enzymes. The two known DGATs, DGAT1 and DGAT2, are encoded by unrelated genes. Although both DGAT1 and DGAT2 knockout mice have reduced tissue triacylglycerol contents, they have disparate phenotypes, prompting us to investigate whether the two enzymes have unrecognized functional differences. We now report that DGAT1 exhibits additional acyltransferase activities in vitro, including those of acyl CoA:monoacylglycerol acyltransferase (MGAT), wax monoester and wax diester synthases, and acyl CoA:retinol acyltransferase (ARAT), which catalyze the synthesis of diacylglycerols, wax esters, and retinyl esters, respectively. These activities were demonstrated in in vitro assays with membranes from insect cells or homogenates from COS7 cells overexpressing DGAT1. Wax synthase and ARAT activities were also demonstrated in intact COS7 cells expressing DGAT1. Additionally, cells and tissues from DGAT1-deficient mice exhibited reduced ARAT activity, and the mice had increased levels of unesterified retinol in their livers on a high-retinol diet. Our findings indicate that DGAT1 can utilize a variety of acyl acceptors as substrates in vitro and suggest that these activities may be relevant to the in vivo functions of DGAT1.

Cloning, gene organization and identification of an alternative splicing process in lecithin:retinol acyltransferase cDNA from human liver

Lecithin:retinol acyltransferase (LRAT) catalyzes the synthesis of retinyl esters in many tissues and is crucial for the transport and intracellular storage of vitamin A. LRAT expression is highly regulated in the liver. In this study, we have cloned and sequenced the full-length LRAT mRNA from human liver and identified its 5'- and 3'-ends. Full-length LRAT mRNA comprises 5023 nt with a predicted ORF of 230 amino acids, a short 5'UTR, and a relatively long 3'UTR of 4 kb containing several polyadenylation signals and AU-rich regions. Based on alignment of this mRNA with human genomic DNA in the GenBank database, the human LRAT gene spans about 9.1 kbp and consists of two exons and a relatively long 4-kbp intron. Further analysis of normal liver revealed a minor alternative splicing variant which lacks a 103 nt polynucleotide contained in the 5'UTR of the full-length LRAT transcript. This variant predicts that the LRAT gene is organized into three exons and two introns, as reported for LRAT cloned from retinal pigment epithelium (RPE) cells. These two LRAT mRNA variants are also present in testis, which is known to express LRAT and contain retinyl esters. Major and minor transcription start sites for human liver LRAT mRNA were identified and the sequence of the upstream proximal promoter region was retrieved from the GenBank database and physically analyzed for the presence of putative cis-acting elements essential for basal transcription. This region contains a TATA box, CCAAT box and Sp1 site, which are apparently conserved in mouse and rat LRAT genes. Our results provide evidence that multiple LRAT mRNA transcripts, which are expressed in a tissue-specific manner, may result from several mechanisms including differential splicing of the 5'UTR region and the use of multiple polyadenylation signals in the 3'UTR.

Uptake and esterification of vitamin A by RCS rat retinal pigment epithelial cells in primary culture

We investigated the capacity of Royal College of Surgeons (RCS) rat retinal pigment epithelial (RPE) cells to take up all-trans-retinol (ROL) (vitamin A) and to metabolize it into retinyl esters (RE). Cultures of RPE cells were established from RCS and control newborn rats. All-trans-ROL was delivered to the apical surface of the RPE monolayer. Retinoids were analyzed by high-performance liquid chromatography. The cellular retinol-binding protein type I (CRBP-I) was assessed by Western blotting. Before supplementation with ROL, RE were lower in RCS rats. After ROL supplementation, esters increased and reached values that were similar in the two strains, but the increase, expressed relative to the initial value, was higher in RCS rats. The uptake of ROL and the level of CRBP-I were greater in RCS rats. Our results provide evidence of a functional retinol esterifying enzyme in cultured RCS RPE cells and suggest that CRBP-I could play a role in the uptake and esterification of ROL in the RPE cells.

Protein database, human retinal pigment epithelium

The retinal pigment epithelium (RPE) is a single cell layer adjacent to the rod and cone photoreceptors that plays key roles in retinal physiology and the biochemistry of vision. RPE cells were isolated from normal adult human donor eyes, subcellular fractions were prepared, and proteins were fractionated by electrophoresis. Following in-gel proteolysis, proteins were identified by peptide sequencing using liquid chromatography tandem electrospray mass spectrometry and/or by peptide mass mapping using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Preliminary analyses have identified 278 proteins and provide a starting point for building a database of the human RPE proteome.

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.

Synthesis of the all-trans-retinal chromophore of retinal G protein-coupled receptor opsin in cultured pigment epithelial cells

Light-dependent production of 11-cis-retinal by the retinal pigment epithelium (RPE) and normal regeneration of rhodopsin under photic conditions involve the RPE retinal G protein-coupled receptor (RGR) opsin. This microsomal opsin is bound to all-trans-retinal which, upon illumination, isomerizes stereospecifically to the 11-cis isomer. In this paper, we investigate the synthesis of the all-trans-retinal chromophore of RGR in cultured ARPE-hRGR and freshly isolated bovine RPE cells. Exogenous all-trans-[(3)H]retinol is incorporated into intact RPE cells and converted mainly into retinyl esters and all-trans-retinal. The intracellular processing of all-trans-[(3)H]retinol results in physiological binding to RGR of a radiolabeled retinoid, identified as all-trans-[(3)H]retinal. The ARPE-hRGR cells contain a membrane-bound NADPH-dependent retinol dehydrogenase that reacts efficiently with all-trans-retinol but not the 11-cis isomer. The NADPH-dependent all-trans-retinol dehydrogenase activity in isolated RPE microsomal membranes can be linked in vitro to specific binding of the chromophore to RGR. These findings provide confirmation that RGR opsin binds the chromophore, all-trans-retinal, in the dark. A novel all-trans-retinol dehydrogenase exists in the RPE and performs a critical function in chromophore biosynthesis.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

The upstream region of the Rpe65 gene confers retinal pigment epithelium-specific expression in vivo and in vitro and contains critical octamer and E-box binding sites

RPE65 is essential for all-trans- to 11-cis-retinoid isomerization, the hallmark reaction of the retinal pigment epithelium (RPE). Here, we identify regulatory elements in the Rpe65 gene and demonstrate their functional relevance to Rpe65 gene expression. We show that the 5' flanking region of the mouse Rpe65 gene, like the human gene, lacks a canonical TATA box and consensus GC and CAAT boxes. The mouse and human genes do share several cis-acting elements, including an octamer, a nuclear factor one (NFI) site, and two E-box sites, suggesting a conserved mode of regulation. A mouse Rpe65 promoter/beta-galactosidase transgene containing bases -655 to +52 (TR4) of the mouse 5' flanking region was sufficient to direct high RPE-specific expression in transgenic mice, whereas shorter fragments (-297 to +52 or -188 to +52) generated only background activity. Furthermore, transient transfection of analogous TR4/luciferase constructs also directed high reporter activity in the human RPE cell line D407 but weak activity in the non-RPE cell lines HeLa, HepG2, and HS27. Functional binding of potential transcription factors to the octamer sequence, AP-4, and NFI sites was demonstrated by directed mutagenesis, electrophoretic mobility shift assay, and cross-linking. Mutations of these sites abolished binding and corresponding transcriptional activity and indicated that octamer and E-box transcription factors synergistically regulate the RPE65 promoter function. Thus, we have identified the regulatory region in the Rpe65 gene that accounts for tissue-specific expression in the RPE and found that octamer and E-box transcription factors play a critical role in the transcriptional regulation of the Rpe65 gene.

Intracellular localization and membrane topology of 11-cis retinol dehydrogenase in the retinal pigment epithelium suggest a compartmentalized synthesis of 11-cis retinaldehyde

11-cis retinol dehydrogenase (EC 1.1.1.105) catalyses the last step in the biosynthetic pathway generating 11-cis retinaldehyde, the common chromophore of all visual pigments in higher animals. The enzyme is abundantly expressed in retinal pigment epithelium of the eye and is a member of the short chain dehydrogenase/reductase superfamily. In this work we demonstrate that a majority of 11-cis retinol dehydrogenase is associated with the smooth ER in retinal pigment epithelial cells and that the enzyme is an integral membrane protein, anchored to membranes by two hydrophobic peptide segments. The catalytic domain of the enzyme is confined to a lumenal compartment and is not present on the cytosolic aspect of membranes. Thus, the subcellular localization and the membrane topology of 11-cis retinol dehydrogenase suggest that generation of 11-cis retinaldehyde is a compartmentalized process.

Distribution of 11-cis LRAT, 11-cis RD and 11-cis REH in bovine retinal pigment epithelium membranes

Our recent finding of the co-localization of 11-cis retinyl esters and 11-cis retinyl ester hydrolase (11-cis REH) activity in bovine retinal pigment epithelium (RPE) plasma membrane (PM) has led us to explore the possibility that the PM may provide 11-cis retinal for rhodopsin regeneration. In the RPE, visual chromophore is synthesized via a membrane associated 11-cis retinol dehydrogenase (11-cis RD). Accordingly, bovine RPE membranes enriched with either endoplasmic reticulum (ER) or plasma membrane (PM) enzyme markers were prepared and assayed for visual cycle enzyme activities. Pronounced 11-cis RD activity was associated with both ER- and PM-enriched membrane fractions. In contrast, 11-cis REH activity was mostly recovered in PM-enriched fractions while LRAT activity was found only in ER-enriched membranes. The finding that both 11-cis retinol and 11-cis retinal can be produced at the PM of the bovine RPE strongly suggests that 11-cis retinyl esters at this subcellular locale serve as a precursor of visual chromophore for pigment regeneration.

Localization of cellular retinoid-binding proteins suggests specific roles for retinoids in the adult central nervous system

Retinoic acid, the active metabolite of retinoids (vitamin A compounds), is thought to act as a gene regulator via ligand-activated transcription factors. In order to investigate possible roles of retinoids and retinoid-controlled gene expression in brain function, we have used immunohistochemistry to localize the possible presence of two intracellular retinoid-binding proteins, cellular retinol-binding protein type I and cellular retinoic acid-binding protein type I, in the adult rat central nervous system. We find a widespread, yet distinct, presence of these two binding proteins in the brain and spinal cord. Most of the immunoreactivity is neuronal, including cell somata, as well as dendritic and axonal processes and axon terminals. Cellular retinol-binding protein type I-immunoreactivity is also found in the walls of cerebral blood vessels, the meninges, the choroid plexus, certain ependymal cells, tanocytes and certain other glial elements. The cellular retinol-binding protein type I- and cellular retinoic acid-binding protein type I-immunoreactivity patterns appear to be almost exclusively non-overlapping. Very strong cellular retinol-binding protein type I-immunoreactivity is found in the dendritic layers of the hippocampal formation and dentate gyrus. Cellular retinol-binding protein type I-immunoreactivity is also present in layer 5 cortical pyramidal neurons and neurons in the glomerular layer of the olfactory bulb. Many other areas, e.g. hypothalamic nuclei and amygdala areas, contain networks of varicose cellular retinol-binding protein type I-immunoreactive nerve fibers. The medial amygdaloid nucleus contains strongly cellular retinol-binding protein type I-positive neurons. Cellular retinoic acid-binding protein type I-immunoreactivity is more restricted in the adult brain. Strong cellular retinoic acid-binding protein type I-immunoreactivity is, however, found in a population of medium-sized neurons scattered throughout the striatum, in neurons in the glomerular layer of the olfactory bulb, the olfactory nerve and in a group of nerve cells close to the third ventricle in hypothalamus. The remarkably selective patterns of cellular retinol-binding protein type I- and cellular retinoic acid-binding protein type I-immunoreactivity discovered in the adult rat brain suggest that retinoids have important roles as regulators of gene expression in normal brain function. The high levels of cellular retinol-binding protein type I-immunoreactivity found in hippocampus suggest that one such role might relate to brain plasticity.

Identification and immunohistochemistry of retinol dehydrogenase from bovine retinal pigment epithelium

We studied the properties of retinol dehydrogenase (11-cis-specific) from bovine retinal pigment epithelium. Detergents caused a loss of retinol dehydrogenase activity; therefore, we added 3 mM NADH as a stabilizer to solubilize this enzyme and partially purified this enzyme using Sepharose CL-6B and hydroxyapatite column chromatography. The partially-purified sample, which contained two major proteins (66 kDa, 33 kDa), had substrate preference to 11-cis and 13-cis-retinal but not to all-trans and 9-cis isomers. Monoclonal anti-33 kDa protein of retinal pigment epithelial crude extract by Western blotting. In addition, we found that monoclonal anti-retinol dehydrogenase antibody bound specifically to retinal pigment epithelium and not to Müller cells or to rod outer segments by immunohistochemical methods.

Interphotoreceptor retinoid-binding protein (IRBP). Molecular biology and physiological role in the visual cycle of rhodopsin

The regeneration of visual pigment in rod photoreceptors of the vertebrate retina requires an exchange of retinoids between the neural retina and the retina pigment epithelium (RPE). It has been hypothesized that interphotoreceptor retinoid-binding protein (IRBP) functions as a two-way carrier of retinoid through the aqueous compartment (interphotoreceptor matrix) that separates the RPE and the photoreceptors. The first part of this review summarizes the cellular and molecular biology of IRBP. Work on the IRBP gene indicates that the protein contains a four-fold repeat structure that may be involved in binding multiple retinoid and fatty acid ligands. These repeats and other aspects of the gene structure indicate that the gene has had an active and complex evolutionary history. IRBP mRNA is detected only in retinal photoreceptors and in the pineal gland; expression is thus restricted to the two photosensitive tissues of vertebrate organisms. In the second part of this review, we consider the results obtained in experiments that have examined the activity of IRBP in the process of visual pigment regeneration. We also consider the results obtained on the bleaching and regeneration of rhodopsin in the acutely detached retina, as well as in experiments testing the ability of IRBP to protect its retinoid ligand from isomerization and oxidation. Taken together, the findings provide evidence that, in vivo, IRBP facilitates both the delivery of all-trans retinol to the RPE and the transfer of 11-cis retinal from the RPE to bleached rod photoreceptors, and thereby directly supports the regeneration of rhodopsin in the visual cycle.

Expression of cellular retinoid-binding proteins during normal and abnormal epidermal differentiation

Retinoids have important roles in growth and differentiation of epidermal cells. We have analyzed the expression of two intracellular retinoid-binding proteins, the cellular retinol-binding protein type I and the cellular retinoic acid-binding protein type I, during normal and abnormal epidermal differentiation. Both proteins were found to be expressed in normal epidermis with increasing expression from basal layer towards superficial layers. In psoriatic lesions, a hyperproliferative condition of the skin, the epidermal expression of cellular retinol-binding protein I was induced, whereas expression of cellular retinoic acid-binding protein I was sharply down-regulated. This and other features of psoriatic lesions indicate that down-regulation of cellular retinoic acid-binding protein I expression might cause aberrant retinoid-regulated gene expression in skin. In basal and squamous cell carcinomas, cellular retinoic acid-binding protein I expression was down-regulated, whereas cellular retinol-binding protein I was expressed. Apart from epidermal cells, a mesenchymal, dendritic cell-type, strongly expressing cellular retinoic acid-binding protein I, was identified in the dermis. In several hyperproliferative conditions of the skin, including psoriasis, and squamous and basal cell carcinomas, this cell type was abundant. These results have implications for the role of retinoids in normal and abnormal epidermal differentiation and suggest that part of the phenotype of psoriasis is due to inappropriate metabolism of retinoic acid in skin.

All-trans to 11-cis retinol isomerization in nuclear membrane fraction from bovine retinal pigment epithelium

Isomerization of all-trans to 11-cis retinol has been studied in a membrane preparation from the nuclear fraction of bovine retinal pigment epithelium. When the nuclear membrane preparation deprived of endogenous retinoids is incubated with 4.5 microM all-trans-retinol, the mean value calculated for the isomerase activity is 1.32 nmol 11-cis retinol formed hr-1 mg protein-1. Simultaneous formation of all-trans and 11-cis retinyl esters is also observed in the nuclear preparation. When assayed under the same experimental condition, RPE 150,000 g post-nuclear sediment shows about 70% of the isomerase activity found in the nuclear membrane fraction. Treatment of the nuclear membrane fraction with 0.5% (w/v) CHAPS produces a 200,000-g supernatant retaining 80% of the total isomerase activity and leads to a modest purification of the enzyme activity. Apparent values for Km and Vmax of the solubilized enzyme are 1.6 microM and 2.5 nmol 11-cis retinol formed h-1 mg protein-1, respectively. Bovine serum albumin and beta-lactoglobulin effectively stimulate the isomerization reaction. The mechanism underlying this activating effect remains unclarified at present. Some hypotheses are discussed.

Distribution of vitamin A compounds in bovine eyes after bleaching adaptation

A seasonal increase in the amount of bleached rhodopsin caused, in living animals, by the seasonal increase of the intensity of sunlight in the early morning before the calves are killed, was verified in the bovine eyes subjected to the present study. This was used as a means of assaying distribution and isomer composition of esterified and unesterified retinol in eyes from animals light-adapted to a different extent under environmental conditions. The progressive increase of bleached rhodopsin results in a parallel increase of all-trans-retinol in retina and of both all-trans- and 11-cis-retinyl esters in pigment epithelium. Analytical subcellular fractionation of RPE homogenate reveals that retinyl esters accumulate without an exclusive subcellular localization in nuclear, mitochondrial/lysosomal and microsomal fractions. Whatever the amount of bleached rhodopsin, only small and constant amounts of retinyl esters are found in the soluble fraction of RPE, entirely under the all-trans configuration. When a considerable portion of rhodopsin is bleached (about 70%), substantial amounts of all-trans-retinol, along with minor amounts of 11-cis-retinol, accumulate in RPE subcellular organelles. The in vitro bleaching of bovine eyes results in a distribution of retinoids between retina and RPE which appears different from that detected in eyes naturally bleached to the same extent.

Requirement of insulin or IGF-1 for the maintenance of retinyl ester synthetase activity by cultured retinal pigment epithelial cells

Previous work from these laboratories showed that the retention of retinyl ester synthetase activity by cultured human retinal pigment epithelium is up to tenfold greater with PM medium (Medium 199 plus insulin, other added defined components, 1% serum and 1% retina extract) than with conventional culture media. The present work shows that insulin is the component of PM medium required for maintenance of ester synthetase activity and that insulin-like growth factor type 1 (IGF-1) also is effective at maintaining ester synthesis. In addition, insulin can maintain ester synthetase activity in cultured rat RPE. Preliminary dose-response measurements provide additional support for these findings and strongly suggest that both insulin and IGF-1 are maximally effective at physiological concentrations (1-10 ng ml-1).

Distribution of lecithin-retinol acyltransferase activity in different types of rat liver cells and subcellular fractions

It is now well documented that lecithin-retinol acyltransferase (LRAT) is the physiologically important enzyme activity involved in the esterification of retinol in the liver. However, no information regarding the cellular distribution of this enzyme in the liver is presently available. This study characterizes the distribution of LRAT activity in the different types of rat liver cells. Purified preparations of isolated parenchymal, fat-storing, and Kupffer + endothelial cells were isolated from rat livers and the LRAT activity present in microsomes prepared from each of these cell fractions was determined. The fat-storing cells were found to contain the highest level of LRAT specific activity (383 +/- 54 pmol retinyl ester formed min-1.mg-1 versus 163 +/- 22 pmol retinyl ester formed min-1.mg-1 for whole liver microsomes). The level of LRAT specific activity in parenchymal cell microsomes (158 +/- 53 pmol retinyl ester formed min-1.mg-1) was very similar to LRAT levels in whole liver microsomes. The Kuppfer + endothelial cell microsome fractions were found to contain LRAT, at low levels of activity. These results indicate that the fat-storing cells are very enriched in LRAT but the parenchymal cells also posses significant levels of LRAT activity.

Interaction of fatty acid binding protein with microsomes: removal of palmitic acid and retinyl esters

[14C] palmitic acid or [3H] retinyl esters incorporated in microsomal membranes were removed by a cytosolic fraction enriched in fatty acid binding protein. When mouse liver cytosol was fractionated by 70% ammonium sulphate, a precipitate and a soluble fraction were obtained. The soluble fraction containing the fatty acid binding protein was able to remove from microsomal membranes, [14C] palmitic acid or [3H] retinyl esters, whereas the precipitate fraction had no removal capacity. Retinoid analysis indicated that 70% ammonium sulphate soluble fraction was enriched in endogenous retinyl esters with regard to cytosol or 70% ammonium sulphate precipitate fraction.

Site of conversion of endogenous all-trans-retinoids to 11-cis-retinoids in the bovine eye

By use of a new high-resolution high-pressure liquid chromatographic method for the separation of isomeric forms of retinol, retinal, retinyl ester and retinal oxime, various retinoids were analyzed in separated retinal pigment epithelial tissue or neural retinal tissue from fresh bleached bovine eyes after incubation in the dark at either 30 or 4 degrees C for 90 min. 11-cis-Retinoids significantly increased during incubation at 30 degrees C, relative to those at 4 degrees C, in the retinal pigment epithelium, but not in the retina. The major forms of vitamin A in incubated retinal pigment epithelium and neural retina were retinyl esters (70%) and all-trans-retinol (69%), respectively. Thus, in keeping with observations on the isomerization of radioactive retinol in homogenates of eye tissues, the retinal pigment epithelium seems to be the primary site of 11-cis-retinoid formation from endogenous all-trans-retinoids in the bovine eye.