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

Rhodopsin, light-sensor of vision

The light sensor of vertebrate scotopic (low-light) vision, rhodopsin, is a G-protein-coupled receptor comprising a polypeptide chain with bound chromophore, 11-cis-retinal, that exhibits remarkable physicochemical properties. This photopigment is extremely stable in the dark, yet its chromophore isomerises upon photon absorption with 70% efficiency, enabling the activation of its G-protein, transducin, with high efficiency. Rhodopsin's photochemical and biochemical activities occur over very different time-scales: the energy of retinaldehyde's excited state is stored in <1 ps in retinal-protein interactions, but it takes milliseconds for the catalytically active state to form, and many tens of minutes for the resting state to be restored. In this review, we describe the properties of rhodopsin and its role in rod phototransduction. We first introduce rhodopsin's gross structural features, its evolution, and the basic mechanisms of its activation. We then discuss light absorption and spectral sensitivity, photoreceptor electrical responses that result from the activity of individual rhodopsin molecules, and recovery of rhodopsin and the visual system from intense bleaching exposures. We then provide a detailed examination of rhodopsin's molecular structure and function, first in its dark state, and then in the active Meta states that govern its interactions with transducin, rhodopsin kinase and arrestin. While it is clear that rhodopsin's molecular properties are exquisitely honed for phototransduction, from starlight to dawn/dusk intensity levels, our understanding of how its molecular interactions determine the properties of scotopic vision remains incomplete. We describe potential future directions of research, and outline several major problems that remain to be solved.

Deep Diversity: Extensive Variation in the Components of Complex Visual Systems across Animals

Understanding the molecular underpinnings of the evolution of complex (multi-part) systems is a fundamental topic in biology. One unanswered question is to what the extent do similar or different genes and regulatory interactions underlie similar complex systems across species? Animal eyes and phototransduction (light detection) are outstanding systems to investigate this question because some of the genetics underlying these traits are well characterized in model organisms. However, comparative studies using non-model organisms are also necessary to understand the diversity and evolution of these traits. Here, we compare the characteristics of photoreceptor cells, opsins, and phototransduction cascades in diverse taxa, with a particular focus on cnidarians. In contrast to the common theme of deep homology, whereby similar traits develop mainly using homologous genes, comparisons of visual systems, especially in non-model organisms, are beginning to highlight a "deep diversity" of underlying components, illustrating how variation can underlie similar complex systems across taxa. Although using candidate genes from model organisms across diversity was a good starting point to understand the evolution of complex systems, unbiased genome-wide comparisons and subsequent functional validation will be necessary to uncover unique genes that comprise the complex systems of non-model groups to better understand biodiversity and its evolution.

Microbiota mitochondria disorders as hubs for early age-related macular degeneration

Age-related macular degeneration (AMD) is a progressive neurodegenerative disease affecting the central area (macula lutea) of the retina. Research on the pathogenic mechanism of AMD showed complex cellular contribution governed by such risk factors as aging, genetic predisposition, diet, and lifestyle. Recent studies suggested that microbiota is a transducer and a modifier of risk factors for neurodegenerative diseases, and mitochondria may be one of the intracellular targets of microbial signaling molecules. This review explores studies supporting a new concept on the contribution of microbiota-mitochondria disorders to AMD. We discuss metabolic, vascular, immune, and neuronal mechanism in AMD as well as key alterations of photoreceptor cells, retinal pigment epithelium (RPE), Bruch's membrane, choriocapillaris endothelial, immune, and neuronal cells. Special attention was paid to alterations of mitochondria contact sites (MCSs), an organelle network of mitochondria, endoplasmic reticulum, lipid droplets (LDs), and peroxisomes being documented based on our own electron microscopic findings from surgically removed human eyes. Morphometry of Bruch's membrane lipids and proteoglycans has also been performed in early AMD and aged controls. Microbial metabolites (short-chain fatty acids, polyphenols, and secondary bile acids) and microbial compounds (lipopolysaccharide, peptidoglycan, and bacterial DNA)-now called postbiotics-in addition to local effects on resident microbiota and mucous membrane, regulate systemic metabolic, vascular, immune, and neuronal mechanisms in normal conditions and in various common diseases. We also discuss their antioxidant, anti-inflammatory, and metabolic effects as well as experimental and clinical observations on regulating the main processes of photoreceptor renewal, mitophagy, and autophagy in early AMD. These findings support an emerging concept that microbiota-mitochondria disorders may be a crucial pathogenic mechanism of early AMD; and similarly, to other age-related neurodegenerative diseases, new treatment approaches should be targeted at these disorders.

In situ autofluorescence lifetime assay of a photoreceptor stimulus response in mouse retina and human retinal organoids

Photoreceptors are the key functional cell types responsible for the initiation of vision in the retina. Phototransduction involves isomerization and conversion of vitamin A compounds, known as retinoids, and their recycling through the visual cycle. We demonstrate a functional readout of the visual cycle in photoreceptors within stem cell-derived retinal organoids and mouse retinal explants based on spectral and lifetime changes in autofluorescence of the visual cycle retinoids after exposure to light or chemical stimuli. We also apply a simultaneous two- and three-photon excitation method that provides specific signals and increases contrast between these retinoids, allowing for reliable detection of their presence and conversion within photoreceptors. This multiphoton imaging technique resolves the slow dynamics of visual cycle reactions and can enable high-throughput functional screening of retinal tissues and organoid cultures with single-cell resolution.

Spectroscopy and photoisomerization of protonated Schiff-base retinal derivatives in vacuo

The protonated Schiff-base retinal acts as the chromophore in bacteriorhodopsin as well as in rhodopsin. In both cases, photoexcitation initializes fast isomerization which eventually results in storage of chemical energy or signaling. The details of the photophysics for this important chromophore is still not fully understood. In this study, action-absorption spectra and photoisomerization dynamics of three retinal derivatives are measured in the gas phase and compared to that of the protonated Schiff-base retinal. The retinal derivatives include C₉C₁₀trans-locked, C₁₃C₁₄trans-locked and a retinal derivative without the β-ionone ring. The spectroscopy as well as the isomerization speed of the chromophores are altered significantly as a consequence of the steric constraints.

Dark-adaptation in the eyes of a lake and a sea population of opossum shrimp (Mysis relicta): retinoid isomer dynamics, rhodopsin regeneration, and recovery of light sensitivity

We have studied dark-adaptation at three levels in the eyes of the crustacean Mysis relicta over 2-3 weeks after exposing initially dark-adapted animals to strong white light: regeneration of 11-cis retinal through the retinoid cycle (by HPLC), restoration of native rhodopsin in photoreceptor membranes (by MSP), and recovery of eye photosensitivity (by ERG). We compare two model populations ("Sea", S_p, and "Lake", L_p) inhabiting, respectively, a low light and an extremely dark environment. 11-cis retinal reached 60-70% of the pre-exposure levels after 2 weeks in darkness in both populations. The only significant L_p/S_p difference in the retinoid cycle was that L_p had much higher levels of retinol, both basal and light-released. In S_p, rhodopsin restoration and eye photoresponse recovery parallelled 11-cis retinal regeneration. In L_p, however, even after 3 weeks only ca. 25% of the rhabdoms studied had incorporated new rhodopsin, and eye photosensitivity showed only incipient recovery from severe depression. The absorbance spectra of the majority of the L_p rhabdoms stayed constant around 490-500 nm, consistent with metarhodopsin II dominance. We conclude that sensitivity recovery of S_p eyes was rate-limited by the regeneration of 11-cis retinal, whilst that of L_p eyes was limited by inertia in photoreceptor membrane turnover.

The Symbiotic Relationship between the Neural Retina and Retinal Pigment Epithelium Is Supported by Utilizing Differential Metabolic Pathways

The neural retina and retinal pigment epithelium (RPE) maintain a symbiotic metabolic relationship, disruption of which leads to debilitating vision loss. The current study was undertaken to identify the differences in the steady-state metabolite levels and the pathways functioning between bona fide neural retina and RPE. Global metabolomics and cluster analyses identified 650 metabolites differentially modulated between the murine neural retina and RPE. Of these, 387 and 163 were higher in the RPE and the neural retina, respectively. Further analysis coupled with transcript and protein level investigations revealed that under normal physiological conditions, the RPE utilizes the pentose phosphate (>3-fold in RPE), serine (>10-fold in RPE), and sphingomyelin biosynthesis (>5-fold in RPE) pathways. Conversely, the neural retina relied mostly on glycolysis. These results show how the RPE and the neural retina have acquired an efficient, complementary and metabolically diverse symbiotic niche to support each other's distinct functions.

Engineering aldo-keto reductase 1B10 to mimic the distinct 1B15 topology and specificity towards inhibitors and substrates, including retinoids and steroids

The aldo-keto reductase (AKR) superfamily comprises NAD(P)H-dependent enzymes that catalyze the reduction of a variety of carbonyl compounds. AKRs are classified in families and subfamilies. Humans exhibit three members of the AKR1B subfamily: AKR1B1 (aldose reductase, participates in diabetes complications), AKR1B10 (overexpressed in several cancer types), and the recently described AKR1B15. AKR1B10 and AKR1B15 share 92% sequence identity, as well as the capability of being active towards retinaldehyde. However, AKR1B10 and AKR1B15 exhibit strong differences in substrate specificity and inhibitor selectivity. Remarkably, their substrate-binding sites are the most divergent parts between them. Out of 27 residue substitutions, six are changes to Phe residues in AKR1B15. To investigate the participation of these structural changes, especially the Phe substitutions, in the functional features of each enzyme, we prepared two AKR1B10 mutants. The AKR1B10 m mutant carries a segment of six AKR1B15 residues (299-304, including three Phe residues) in the respective AKR1B10 region. An additional substitution (Val48Phe) was incorporated in the second mutant, AKR1B10mF48. This resulted in structures with smaller and more hydrophobic binding pockets, more similar to that of AKR1B15. In general, the AKR1B10 mutants mirrored well the specific functional features of AKR1B15, i.e., the different preferences towards the retinaldehyde isomers, the much higher activity with steroids and ketones, and the unique behavior with inhibitors. It can be concluded that the Phe residues of loop C (299-304) contouring the substrate-binding site, in addition to Phe at position 48, strongly contribute to a narrower and more hydrophobic site in AKR1B15, which would account for its functional uniqueness. In addition, we have investigated the AKR1B10 and AKR1B15 activity toward steroids. While AKR1B10 only exhibits residual activity, AKR1B15 is an efficient 17-ketosteroid reductase. Finally, the functional role of AKR1B15 in steroid and retinaldehyde metabolism is discussed.

A cell culture condition that induces the mesenchymal-epithelial transition of dedifferentiated porcine retinal pigment epithelial cells

The pathological change of retinal pigment epithelial (RPE) cells is one of the main reasons for the development of age-related macular degeneration (AMD). Thus, cultured RPE cells are a proper cell model for studying the etiology of AMD in vitro. However, such cultured RPE cells easily undergo epithelial-mesenchymal transition (EMT) that results in changes of cellular morphology and functions of the cells. To restore and maintain the mesenchymal-epithelial transition (MET) of the cultured RPE cells, we cultivated dedifferentiated porcine RPE (pRPE) cells and compared their behaviors in four conditions: 1) in cell culture dishes with DMEM/F12 containing FBS (CC dish-FBS), 2) in petri dishes with DMEM/F12 containing FBS (Petri dish-FBS), 3) in cell culture dishes with DMEM/F12 containing N2 and B27 supplements (CC dish-N2B27), and 4) in petri dishes with DMEM/F12 containing N2 and B27 (Petri dish-N2B27). In addition to observing the cell morphology and behavior, RPE specific markers, as well as EMT-related genes and proteins, were examined by immunostaining, quantitative real-time PCR and Western blotting. The results showed that dedifferentiated pRPE cells maintained EMT in CC dish-FBS, Petri dish-FBS and CC dish-N2B27 groups, whereas MET was induced when the dedifferentiated pRPE cells were cultured in Petri dish-N2B27. Such induced pRPE cells showed polygonal morphology with increased expression of RPE-specific markers and decreased EMT-associated markers. Similar results were observed in induced pluripotent stem cell-derived RPE cells. Furthermore, during the re-differentiation of those dedifferentiated pRPE cells, Petri dish-N2B27 reduced the activity of RhoA and induced F-actin rearrangement, which promoted the nuclear exclusion of transcriptional co-activator with PDZ-binding motif (TAZ) and TAZ target molecule zinc finger E-box binding protein (ZEB1), both of which are EMT inducing factors. This study provides a simple and reliable method to reverse dedifferentiated phenotype of pRPE cells into epithelialized phenotype, which is more appropriate for studying AMD in vitro, and suggests that MET of other cell types might be induced by a similar approach.

Characterization of AKR1B16, a novel mouse aldo-keto reductase

Aldo-keto reductases (AKRs) are distributed in three families and multiple subfamilies in mammals. The mouse Akr1b3 gene is clearly orthologous to human AKR1B1, both coding for aldose reductase, and their gene products show similar tissue distribution, regulation by osmotic stress and kinetic properties. In contrast, no unambiguous orthologs of human AKR1B10 and AKR1B15.1 have been identified in rodents. Although two more AKRs, AKR1B7 and AKR1B8, have been identified and characterized in mouse, none of them seems to exhibit properties similar to the human AKRs. Recently, a novel mouse AKR gene, Akr1b16, was annotated and the respective gene product, AKR1B16 (sharing 83% and 80% amino acid sequence identity with AKR1B10 and AKR1B15.1, respectively), was expressed as insoluble and inactive protein in a bacterial expression system. Here we describe the expression and purification of a soluble and enzymatically active AKR1B16 from E. coli using three chaperone systems. A structural model of AKR1B16 allowed the estimation of its active-site pocket volume, which was much wider (402 Å³) than those of AKR1B10 (279 Å³) and AKR1B15.1 (60 Å³). AKR1B16 reduced aliphatic and aromatic carbonyl compounds, using NADPH as a cofactor, with moderate or low activity (highest k_cat values around 5 min^-1). The best substrate for the enzyme was pyridine-3-aldehyde. AKR1B16 showed poor inhibition with classical AKR inhibitors, tolrestat being the most potent. Kinetics and inhibition properties resemble those of rat AKR1B17 but differ from those of the human enzymes. In addition, AKR1B16 catalyzed the oxidation of 17β-hydroxysteroids in a NADP⁺-dependent manner. These results, together with a phylogenetic analysis, suggest that mouse AKR1B16 is an ortholog of rat AKR1B17, but not of human AKR1B10 or AKR1B15.1. These human enzymes have no counterpart in the murine species, which is evidenced by forming a separate cluster in the phylogenetic tree and by their unique activity with retinaldehyde.

Substrate Specificity, Inhibitor Selectivity and Structure-Function Relationships of Aldo-Keto Reductase 1B15: A Novel Human Retinaldehyde Reductase

Human aldo-keto reductase 1B15 (AKR1B15) is a newly discovered enzyme which shares 92% amino acid sequence identity with AKR1B10. While AKR1B10 is a well characterized enzyme with high retinaldehyde reductase activity, involved in the development of several cancer types, the enzymatic activity and physiological role of AKR1B15 are still poorly known. Here, the purified recombinant enzyme has been subjected to substrate specificity characterization, kinetic analysis and inhibitor screening, combined with structural modeling. AKR1B15 is active towards a variety of carbonyl substrates, including retinoids, with lower kcat and Km values than AKR1B10. In contrast to AKR1B10, which strongly prefers all-trans-retinaldehyde, AKR1B15 exhibits superior catalytic efficiency with 9-cis-retinaldehyde, the best substrate found for this enzyme. With ketone and dicarbonyl substrates, AKR1B15 also shows higher catalytic activity than AKR1B10. Several typical AKR inhibitors do not significantly affect AKR1B15 activity. Amino acid substitutions clustered in loops A and C result in a smaller, more hydrophobic and more rigid active site in AKR1B15 compared with the AKR1B10 pocket, consistent with distinct substrate specificity and narrower inhibitor selectivity for AKR1B15.

Retinal pigment epithelium, age-related macular degeneration and neurotrophic keratouveitis

Age-related macular degeneration (AMD) is the leading cause of impaired vision and blindness in the aging population. The aims of our studies were to identify qualitative and quantitative alterations in mitochondria in human retinal pigment epithelium (RPE) from AMD patients and controls and to test the protective effects of pigment epithelium-derived factor (PEDF), a known neurotrophic and antiangiogenic substance, against neurotrophic keratouveitis. Histopathological alterations were studied by means of morphometry, light and electron microscopy. Unexpectedly, morphometric data showed that the RPE alterations noted in AMD may also develop in normal aging, 10-15 years later than appearing in AMD patients. Reduced tear secretion, corneal ulceration and leukocytic infiltration were found in capsaicin (CAP)-treated rats, but this effect was significantly attenuated by PEDF. These findings suggest that PEDF accelerated the recovery of tear secretion and also prevented neurotrophic keratouveitis and vitreoretinal inflammation. PEDF may have a clinical application in inflammatory and neovascular diseases of the eye.

Melanopsin is highly resistant to light and chemical bleaching in vivo

Melanopsin is the photopigment of mammalian intrinsically photosensitive retinal ganglion cells, where it contributes to light entrainment of circadian rhythms, and to the pupillary light response. Previous work has shown that the melanopsin photocycle is independent of that used by rhodopsin (Tu, D. C., Owens, L. A., Anderson, L., Golczak, M., Doyle, S. E., McCall, M., Menaker, M., Palczewski, K., and Van Gelder, R. N. (2006) Inner retinal photoreception independent of the visual retinoid cycle. Proc. Natl. Acad. Sci. U.S.A. 103, 10426-10431). Here we determined the ability of apo-melanopsin, formed by ex vivo UV light bleaching, to use selected chromophores. We found that 9-cis-retinal, but not all-trans-retinal or 9-cis-retinol, is able to restore light-dependent ipRGC activity after bleaching. Melanopsin was highly resistant to both visible-spectrum photic bleaching and chemical bleaching with hydroxylamine under conditions that fully bleach rod and cone photoreceptor cells. These results suggest that the melanopsin photocycle can function independently of both rod and cone photocycles, and that apo-melanopsin has a strong preference for binding cis-retinal to generate functional pigment. The data support a model in which retinal is continuously covalently bound to melanopsin and may function through a reversible, bistable mechanism.

Analysis, occurrence, and function of 9-cis-retinoic acid

Metabolic conversion of vitamin A (retinol) into retinoic acid (RA) controls numerous physiological processes. 9-cis-retinoic acid (9cRA), an active metabolite of vitamin A, is a high affinity ligand for retinoid X receptor (RXR) and also activates retinoic acid receptor (RAR). Despite the identification of candidate enzymes that produce 9cRA and the importance of RXRs as established by knockout experiments, in vivo detection of 9cRA in tissue was elusive until recently when 9cRA was identified as an endogenous pancreas retinoid by validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) methodology. This review will discuss the current status of the analysis, occurrence, and function of 9cRA. Understanding both the nuclear receptor-mediated and non-genomic mechanisms of 9cRA will aid in the elucidation of disease physiology and possibly lead to the development of new retinoid-based therapeutics. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Loss of mitochondrial complex I activity potentiates dopamine neuron death induced by microtubule dysfunction in a Parkinson's disease model

The combination of microtubule depolymerization and the accumulation of cytosolic dopamine and reactive oxygen species selectively affects survival of dopaminergic neurons.

Mitochondrial complex I dysfunction is regarded as underlying dopamine neuron death in Parkinson’s disease models. However, inactivation of the Ndufs4 gene, which compromises complex I activity, does not affect the survival of dopamine neurons in culture or in the substantia nigra pars compacta of 5-wk-old mice. Treatment with piericidin A, a complex I inhibitor, does not induce selective dopamine neuron death in either Ndufs4^+/+ or Ndufs4^−/− mesencephalic cultures. In contrast, rotenone, another complex I inhibitor, causes selective toxicity to dopamine neurons, and Ndufs4 inactivation potentiates this toxicity. We identify microtubule depolymerization and the accumulation of cytosolic dopamine and reactive oxygen species as alternative mechanisms underlying rotenone-induced dopamine neuron death. Enhanced rotenone toxicity to dopamine neurons from Ndufs4 knockout mice may involve enhanced dopamine synthesis caused by the accumulation of nicotinamide adenine dinucleotide reduced. Our results suggest that the combination of disrupting microtubule dynamics and inhibiting complex I, either by mutations or exposure to toxicants, may be a risk factor for Parkinson’s disease.

NBHA reduces acrolein-induced changes in ARPE-19 cells: possible involvement of TGFβ

PURPOSE

Acrolein, a toxic, reactive aldehyde formed metabolically and environmentally, has been implicated in the damage to and dysfunction of the retinal pigment epithelium (RPE) that accompanies age-related macular degeneration (AMD). Our purpose was to investigate the potential of acrolein to influence the release of transforming growth factor beta-2 (TGFβ2) and vascular endothelial growth factor (VEGF), to assess the ability of N-benzylhydroxylamine (NBHA) to prevent the effect of acrolein on cytokine release and reduction of viable cells, and to explore the pathway by which acrolein might be causing the increase of VEGF.

MATERIALS AND METHODS

Confluent ARPE-19 cells were treated with acrolein and/or NBHA. They were also pretreated with SIS3, a specific inhibitor of SMAD 3, and ZM39923, a JAK3 inhibitor, before being treated with acrolein. Viable cells were counted; ELISA was used to measure the TGFβ2 and/or VEGF in the conditioned media.

RESULTS

Acrolein was shown to reduce the number of viable ARPE-19 cells and to upregulate the release of the proangiogenic cytokines TGFβ2 and VEGF. Co-treatment with 200 μM NBHA significantly reduced the effects of acrolein on viable cell number and TGFβ2 release. Pretreatment of the cells with SIS3 partially blocked the action of acrolein on decreased viable cell number and VEGF upregulation, suggesting that part of the effects of acrolein are mediated by the increased levels of TGFβ and its signaling.

CONCLUSIONS

Our results suggest that the action of acrolein on the reduction of viability and VEGF increase by ARPE-19 cells is partially mediated by TGFβ2. By reducing the effects of acrolein, NBHA and SIS3 could be potential pharmacological agents in the prevention and progression of acrolein-induced damage to the RPE that relates to AMD.

Visual cycle modulation in neurovascular retinopathy

Rats with oxygen-induced retinopathy (OIR) model the pediatric retinal disease retinopathy of prematurity (ROP). Recent findings in OIR rats imply a causal role for the rods in the ROP disease process, although only experimental manipulation of rod function can establish this role conclusively. Accordingly, a visual cycle modulator (VCM) - with no known direct effect on retinal vasculature - was administered to "50/10 model" OIR Sprague-Dawley rats to test the hypotheses that it would 1) alter rod function and 2) consequently alter vascular outcome. Four litters of pups (N=46) were studied. For two weeks, beginning on postnatal day (P) 7, the first and fourth litters were administered 6 mg kg(-1) N-retinylacetamide (the VCM) intraperitoneally; the second and third litters received vehicle (DMSO) alone. Following a longitudinal design, retinal function was assessed by electroretinography (ERG) and the status of the retinal vessels was monitored using computerized fundus photograph analysis. Rod photoreceptor and post-receptor response amplitudes were significantly higher in VCM-treated than in vehicle-treated rats; deactivation of phototransduction was also significantly more rapid. Notably, the arterioles of VCM-treated rats showed significantly greater recovery from OIR. Presuming that the VCM did not directly affect the retinal vessels, a causal role for the neural retina - particularly the rod photoreceptors - in OIR was confirmed. There was no evidence of negative alteration of photoreceptor function consequent to VCM treatment. This finding implicates the rods as a possible therapeutic target in neurovascular diseases such as ROP.

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

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

RPE65, visual cycle retinol isomerase, is not inherently 11-cis-specific: support for a carbocation mechanism of retinol isomerization

The mechanism of retinol isomerization in the vertebrate retina visual cycle remains controversial. Does the isomerase enzyme RPE65 operate via nucleophilic addition at C(11) of the all-trans substrate, or via a carbocation mechanism? To determine this, we modeled the RPE65 substrate cleft to identify residues interacting with substrate and/or intermediate. We find that wild-type RPE65 in vitro produces 13-cis and 11-cis isomers equally robustly. All Tyr-239 mutations abolish activity. Trp-331 mutations reduce activity (W331Y to approximately 75% of wild type, W331F to approximately 50%, and W331L and W331Q to 0%) establishing a requirement for aromaticity, consistent with cation-pi carbocation stabilization. Two cleft residues modulate isomerization specificity: Thr-147 is important, because replacement by Ser increases 11-cis relative to 13-cis by 40% compared with wild type. Phe-103 mutations are opposite in action: F103L and F103I dramatically reduce 11-cis synthesis relative to 13-cis synthesis compared with wild type. Thr-147 and Phe-103 thus may be pivotal in controlling RPE65 specificity. Also, mutations affecting RPE65 activity coordinately depress 11-cis and 13-cis isomer production but diverge as 11-cis decreases to zero, whereas 13-cis reaches a plateau consistent with thermal isomerization. Lastly, experiments using labeled retinol showed exchange at 13-cis-retinol C(15) oxygen, thus confirming enzymatic isomerization for both isomers. Thus, RPE65 is not inherently 11-cis-specific and can produce both 11- and 13-cis isomers, supporting a carbocation (or radical cation) mechanism for isomerization. Specific visual cycle selectivity for 11-cis isomers instead resides downstream, attributable to mass action by CRALBP, retinol dehydrogenase 5, and high affinity of opsin apoproteins for 11-cis-retinal.

Functional expression, targeting and Ca2+ signaling of a mouse melanopsin-eYFP fusion protein in a retinal pigment epithelium cell line

Melanopsin, first discovered in Xenopus melanophores, is now established as a functional sensory photopigment of the intrinsically photosensitive retinal ganglion cells. These ganglion cells drive circadian rhythm and pupillary adjustments through projection to the brain. Melanopsin shares structural similarities with all known opsins. Comprehensive characterization of melanopsin with respect to its spectral properties, photochemical cascade and signaling partners requires a suitable recombinant system and high expression levels. This combination has not yet been described. To address this issue, we have expressed recombinant mouse melanopsin in several cell lines. Using enhanced yellow fluorescent protein (eYFP) as a visualization tag, expression was observed in all cell lines. Confocal microscopy revealed that melanopsin was properly routed to the plasma membrane only in retinal pigment epithelium (RPE)-derived D407 cells and in human embryonic kidney (HEK) cells. Further, we performed intracellular calcium measurements in order to probe the melanopsin signaling activity of this fusion protein. Transfected cells were loaded with the calcium indicator Fura2-AM. Upon illumination, an immediate but transient calcium response was observed in HEK as well as in D407 cells, while mock-transfected cells showed no calcium response under identical conditions. Supplementation with 11-cis retinal or all-trans retinal enhanced the response. After prolonged illumination the cells became desensitized. Thus, RPE-derived cells expressing recombinant melanopsin may constitute a suitable system for the study of the structural and functional characteristics of melanopsin.

Metabolic basis of visual cycle inhibition by retinoid and nonretinoid compounds in the vertebrate retina

In vertebrate retinal photoreceptors, the absorption of light by rhodopsin leads to photoisomerization of 11-cis-retinal to its all-trans isomer. To sustain vision, a metabolic system evolved that recycles all-trans-retinal back to 11-cis-retinal. The importance of this visual (retinoid) cycle is underscored by the fact that mutations in genes encoding visual cycle components induce a wide spectrum of diseases characterized by abnormal levels of specific retinoid cycle intermediates. In addition, intense illumination can produce retinoid cycle by-products that are toxic to the retina. Thus, inhibition of the retinoid cycle has therapeutic potential in physiological and pathological states. Four classes of inhibitors that include retinoid and nonretinoid compounds have been identified. We investigated the modes of action of these inhibitors by using purified visual cycle components and in vivo systems. We report that retinylamine was the most potent and specific inhibitor of the retinoid cycle among the tested compounds and that it targets the retinoid isomerase, RPE65. Hydrophobic primary amines like farnesylamine also showed inhibitory potency but a short duration of action, probably due to rapid metabolism. These compounds also are reactive nucleophiles with potentially high cellular toxicity. We also evaluated the role of a specific protein-mediated mechanism on retinoid cycle inhibitor uptake by the eye. Our results show that retinylamine is transported to and taken up by the eye by retinol-binding protein-independent and retinoic acid-responsive gene product 6-independent mechanisms. Finally, we provide evidence for a crucial role of lecithin: retinol acyltransferase activity in mediating tissue specific absorption and long lasting therapeutic effects of retinoid-based visual cycle inhibitors.

Organization, structure and activity of proteins in monolayers

Many different processes take place at the cell membrane interface. Indeed, for instance, ligands bind membrane proteins which in turn activate peripheral membrane proteins, some of which are enzymes whose action is also located at the membrane interface. Native cell membranes are difficult to use to gain information on the activity of individual proteins at the membrane interface because of the large number of different proteins involved in membranous processes. Model membrane systems, such as monolayers at the air-water interface, have thus been extensively used during the last 50 years to reconstitute proteins and to gain information on their organization, structure and activity in membranes. In the present paper, we review the recent work we have performed with membrane and peripheral proteins as well as enzymes in monolayers at the air-water interface. We show that the structure and orientation of gramicidin has been determined by combining different methods. Furthermore, we demonstrate that the secondary structure of rhodopsin and bacteriorhodopsin is indistinguishable from that in native membranes when appropriate conditions are used. We also show that the kinetics and extent of monolayer binding of myristoylated recoverin is much faster than that of the nonmyristoylated form and that this binding is highly favored by the presence polyunsaturated phospholipids. Moreover, we show that the use of fragments of RPE65 allow determine which region of this protein is most likely involved in membrane binding. Monomolecular films were also used to further understand the hydrolysis of organized phospholipids by phospholipases A2 and C.

Vitamins A and E: metabolism, roles and transfer to offspring

Vitamins A and E are essential, naturally occurring, fat-soluble nutrients that are involved in several important biological processes such as immunity, protection against tissue damage, reproduction, growth and development. They are extremely important during the early stages of life and must be transferred adequately to the young during gestation and lactation. The present article presents an overview of their biological functions, metabolism and dynamics of transfer to offspring in mammals. Among other topics, the review focuses on the biochemical aspects of their intestinal absorption, blood transport, tissue uptake, storage and catabolism. It also describes their different roles as well as their use as preventive and therapeutic agents. Finally, the mechanisms involved in their transfer during gestation and lactation are discussed.

Light at the end of the tunnel? Advances in the understanding and treatment of glaucoma and inherited retinal degeneration

Glaucoma and inherited retinal degeneration/dystrophy are leading causes of blindness in veterinary patients. Currently, there is no treatment for the loss of vision that characterizes both groups of diseases. However, this reality may soon change as recent advances in understanding of the disease processes allow researchers to develop new therapies aimed at preventing blindness and restoring vision to blind patients. Elucidating the molecular mechanisms of retinal ganglion cell death in glaucoma patients has led to the development of neuroprotective drugs which protect retinal cells and their function from the disastrous effects of elevated pressure. Identification of the genetic mutation responsible for inherited degenerations and dystrophies of the outer retina has enabled researchers using gene therapy to restore vision to blind dogs. Other patients may benefit from retinal transplantation, stem cell therapy, neuroprotective drugs, nutritional supplementation and even retinal prostheses. It is possible that soon it will be possible to restore sight to some blind patients.

G protein-coupled receptor rhodopsin

The rhodopsin crystal structure provides a structural basis for understanding the function of this and other G protein-coupled receptors (GPCRs). The major structural motifs observed for rhodopsin are expected to carry over to other GPCRs, and the mechanism of transformation of the receptor from inactive to active forms is thus likely conserved. Moreover, the high expression level of rhodopsin in the retina, its specific localization in the internal disks of the photoreceptor structures [termed rod outer segments (ROS)], and the lack of other highly abundant membrane proteins allow rhodopsin to be examined in the native disk membranes by a number of methods. The results of these investigations provide evidence of the propensity of rhodopsin and, most likely, other GPCRs to dimerize, a property that may be pertinent to their function.

ARPE-19 cell growth and cell functions in euglycemic culture media

Human retinal pigmented epithelial cells (ARPE-19) grown in euglycemic media (5.5 mM) had lower cell number, significantly different cell morphology, and lower levels of vascular endothelial growth factor (VEGF) in the culture media than those grown in hyperglycemic media (18 mM) customarily used for culturing ARPE-19 cells. Although it has been shown that within a 24-hour period, all-trans retinoic acid significantly reduces VEGF secretion by retinal pigmented epithelial cells (grown in 18 mM glucose), such an inhibitory effect was not observed in cells grown in 5.5 mM glucose. Our results suggest that ARPE-19 cells grown in euglycemic media exhibit distinctly different cell growth, cell differentiation, and cell functions in comparison to cells grown in hyperglycemic media. Because euglycemic conditions provide a physiological glucose environment, this glucose concentration must be included in all future investigations of the mechanism of diabetic retinopathy when studying VEGF secretion by ARPE-19 cells.

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.

Confluent monolayers of cultured human fetal retinal pigment epithelium exhibit morphology and physiology of native tissue

PURPOSE

Provide a reproducible method for culturing confluent monolayers of hfRPE cells that exhibit morphology, physiology, polarity, and protein expression patterns similar to native tissue.

METHODS

Human fetal eyes were dissected on arrival, and RPE cell sheets were mechanically separated from the choroid and cultured in a specifically designed medium comprised entirely of commercially available components. Physiology experiments were performed with previously described techniques. Standard techniques were used for immunohistochemistry, electron microscopy, and cytokine measurement by ELISA.

RESULTS

Confluent monolayers of RPE cell cultures exhibited epithelial morphology and heavy pigmentation, and electron microscopy showed extensive apical membrane microvilli. The junctional complexes were identified with immunofluorescence labeling of various tight junction proteins. The mean transepithelial potential (TEP) was 2.6 +/- 0.8 mV, apical positive, and the mean transepithelial resistance (R(T)) was 501 +/- 138 Omega . cm(2) (mean +/- SD; n = 35). Addition of 100 microM adenosine triphosphate (ATP) to the apical bath increased net fluid absorption from 13.6 +/- 2.6 to 18.8 +/- 4.6 microL . cm(-2) per hour (mean +/- SD; n = 4). In other experiments, VEGF was mainly secreted into the basal bath (n = 10), whereas PEDF was mainly secreted into the apical bath (n = 10).

CONCLUSIONS

A new cell culture procedure has been developed that produces confluent primary hfRPE cultures with morphological and physiological characteristics of the native tissue. Epithelial polarity and function of these easily reproducible primary cultures closely resemble previously studied native human fetal and bovine RPE-choroid explants.

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

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

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

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

Recombination reaction of rhodopsin in situ studied by photoconversion of “indicator yellow”

We measured the kinetics of recombination of 11-cis-retinal with opsin in intact frog rod outer segment (ROS). The rhodopsin in ROS was bleached and allowed to decay to "indicator yellow," a photoproduct where all-trans-retinal is partly free, and partly bound to non-specific amino groups of disk membranes. By briefly illuminating the "indicator yellow" by an intense 465 or 380-nm flash, we then photoconverted all-trans-retinal to (mostly) the 11-cis- form thus introducing into ROS a certain amount of cis-chromophore. The recombination of cis-retinal with opsin and the formation of rhodopsin were followed by fast single-cell microspectrophotometry. Regeneration proceeded with a time constant of approximately 3.5 min; up to 27% of bleached visual pigment was restored. The regenerated pigment consisted of 91% rhodopsin (11-cis-chromophore) and 9% of presumably isorhodopsin (9-cis-chromophore). The recombination of 11-cis-retinal with opsin inside the ROS proceeds substantially faster than rhodopsin regeneration in the intact eye and, hence, is not the rate-limiting step in the visual cycle.

Beyond counting photons: trials and trends in vertebrate visual transduction

For over 30 years, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and G protein signaling. Recently, photoreceptors have become an equally attractive model for studying many facets of neuronal cell biology. The primary goal of this review is to illustrate this rapidly growing trend. We will highlight the areas of active research in photoreceptor biology that reveal how different specialized compartments of the cell cooperate in fulfilling its overall function: converting photon absorption into changes in neurotransmitter release. The same trend brings us closer to understanding how defects in photoreceptor signaling can lead to cell death and retinal degeneration.

Lecithin:retinol acyltransferase is responsible for amidation of retinylamine, a potent inhibitor of the retinoid cycle

Lecithin:retinol acyltransferase (LRAT) catalyzes the transfer of an acyl group from the sn-1 position of phosphatidylcholine to all-trans-retinol (vitamin A) and plays an essential role in the regeneration of visual chromophore as well as in the metabolism of vitamin A. Here we demonstrate that retinylamine (Ret-NH2), a potent and selective inhibitor of 11-cis-retinal biosynthesis (Golczak, M., Kuksa, V., Maeda, T., Moise, A. R., and Palczewski, K. (2005) Proc. Natl. Acad. Sci. U. S. A. 102, 8162-8167), is a substrate for LRAT. LRAT catalyzes the transfer of the acyl group onto Ret-NH2 leading to the formation of N-retinylpalmitamide, N-retinylstearamide, and N-retinylmyristamide with a ratio of 15:6:2, respectively. The presence of N-retinylamides was detected in vivo in mice supplemented with Ret-NH2. N-Retinylamides are thus the main metabolites of Ret-NH2 in the liver and the eye and can be mobilized by hydrolysis/deamidation back to Ret-NH2. Using two-photon microscopy and the intrinsic fluorescence of N-retinylamides, we showed that newly formed amides colocalize with the retinyl ester storage particles (retinosomes) in the retinal pigment epithelium. These observations provide new information concerning the substrate specificity of LRAT and explain the prolonged effect of Ret-NH2 on the rate of 11-cis-retinal recovery in vivo.

Adeno-associated virus-vectored gene therapy for retinal disease

Recombinant adeno-associated viral (AAV) vectors have become powerful gene delivery tools for the treatment of retinal degeneration in a variety of animal models that mimic corresponding human diseases. AAV vectors possess a number of features that render them ideally suited for retinal gene therapy, including a lack of pathogenicity, minimal immunogenicity, and the ability to transduce postmitotic cells in a stable and efficient manner. In the sheltered environment of the retina, AAV vectors are able to maintain high levels of transgene expression in the retinal pigmented epithelium (RPE), photoreceptors, or ganglion cells for long periods of time after a single treatment. Each cell type can be specifically targeted by choosing the appropriate combination of AAV serotype, promoter, and intraocular injection site. The focus of this review is on examples of AAV-mediated gene therapy in those animal models of inherited retinal degeneration caused by mutations directly affecting the interacting unit formed by photoreceptors and the RPE. In each case discussed, expression of the therapeutic gene resulted in significant recovery of retinal structure and/or visual function. Because of the key role of the vasculature in maintaining a healthy retina, a summary of AAV gene therapy applications in animal models of retinal neovascular diseases is also included.

Mitochondrial alterations of retinal pigment epithelium in age-related macular degeneration

Mitochondrial dysfunctions have been implicated in the pathophysiology of several age-related diseases including age-related macular degeneration (AMD), a progressive neurodegenerative disease affecting primarily the retinal pigment epithelium (RPE). The aims of our electron microscopic and morphometric studies were to reveal qualitative and quantitative alterations of mitochondria in human RPE from AMD and from age- and sex-matched controls. With increasing age a significant decrease in number and area of mitochondria, as well as loss of cristae and matrix density were found in both AMD and control specimens. These decreases were significantly greater in AMD than in normal aging. Alterations of mitochondria were accompanied by proliferation of peroxisomes and lipofuscin granules in both AMD and control specimens, although the difference between groups was significant only for peroxisomes. Unexpectedly, morphometric data showed that the RPE alterations seen in AMD may also develop in normal aging, 10-15 years after appearing in AMD patients. These findings suggest that (i) the severity of mitochondrial and peroxisomal alterations are different between AMD and normal aging, and (ii) the timing of damage to RPE may be critical for the development of AMD. We conclude that besides the well-documented age-related changes in mitochondrial DNA, alterations of mitochondrial membranes may also play a role in the pathogenesis of AMD. These membranes could be a new target for treatment of AMD and other age-related diseases.

The Total Synthesis of Spectinabilin and Its Biomimetic Conversion to SNF4435C and SNF4435D

[reaction: see text] A short synthesis of (+/-)-spectinabilin via a trans-selective Suzuki coupling and subsequent Negishi-type methylation, and its biomimetic conversion to (+/-)-SNF4435C and (+/-)-SNF4435D is described.

Positively charged retinoids are potent and selective inhibitors of the trans-cis isomerization in the retinoid (visual) cycle

In vertebrate retinal photoreceptors, photoisomerization of opsin-bound visual chromophore 11-cis-retinal to all-trans-retinal triggers phototransduction events. Regeneration of the chromophore is a critical step in restoring photoreceptors to their dark-adapted state. This regeneration process, called the retinoid cycle, takes place in the photoreceptor outer segments and in the retinal pigmented epithelium (RPE). We have suggested that the regeneration of the chromophore might occur through a retinyl carbocation intermediate. Here, we provide evidence that isomerization is inhibited by positively charged retinoids, which could act as transition state analogs of the isomerization process. We demonstrate that retinylamine (Ret-NH2) potently and selectively inhibits the isomerization step of the retinoid cycle in vitro and in vivo. Ret-NH2 binds a protein(s) in the RPE microsomes, but it does not bind RPE65, a protein implicated in the isomerization reaction. Although Ret-NH2 inhibits the regeneration of visual chromophore in rods and, in turn, severely attenuates rod responses, it has a much smaller effect on cone function in mice. Ret-NH2 interacts only at micromolar concentrations with retinoic acid receptor, does not activate retinoid-X receptor, and is not a substrate for CYP26s, the retinoic acid-metabolizing cytochrome P450 enzymes. Ret-NH2 can be a significant investigational tool to study the mechanism of regeneration of visual chromophore.

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.

Retinal dehydrogenase 12 (RDH12) mutations in leber congenital amaurosis

Leber congenital amaurosis (LCA), the most early-onset and severe form of all inherited retinal dystrophies, is responsible for congenital blindness. Ten LCA genes have been mapped, and seven of these have been identified. Because some of these genes are involved in the visual cycle, we regarded the retinal pigment epithelium and photoreceptor-specific retinal dehydrogenase (RDH) genes as candidate genes in LCA. Studying a series of 110 unrelated patients with LCA, we found mutations in the photoreceptor-specific RDH12 gene in a significant subset of patients (4.1%). Interestingly, all patients harboring RDH12 mutations had a severe yet progressive rod-cone dystrophy with severe macular atrophy but no or mild hyperopia.

Identification of all-trans-retinol:all-trans-13,14-dihydroretinol saturase

Retinoids carry out essential functions in vertebrate development and vision. Many of the retinoid processing enzymes remain to be identified at the molecular level. To expand the knowledge of retinoid biochemistry in vertebrates, we studied the enzymes involved in plant metabolism of carotenoids, a related group of compounds. We identified a family of vertebrate enzymes that share significant similarity and a putative phytoene desaturase domain with a recently described plant carotenoid isomerase (CRTISO), which isomerizes prolycopene to all-trans-lycopene. Comparison of heterologously expressed mouse and plant enzymes indicates that unlike plant CRTISO, the CRTISO-related mouse enzyme is inactive toward prolycopene. Instead, the CRTISO-related mouse enzyme is a retinol saturase carrying out the saturation of the 13-14 double bond of all-trans-retinol to produce all-trans-13,14-dihydroretinol. The product of mouse retinol saturase (RetSat) has a shifted UV absorbance maximum, lambda(max) = 290 nm, compared with the parent compound, all-trans-retinol (lambda(max) = 325 nm), and its MS analysis (m/z = 288) indicates saturation of a double bond. The product was further identified as all-trans-13,14-dihydroretinol, since its characteristics were identical to those of a synthetic standard. Mouse RetSat is membrane-associated and expressed in many tissues, with the highest levels in liver, kidney, and intestine. All-trans-13,14-dihydroretinol was also detected in several tissues of animals maintained on a normal diet. Thus, saturation of all-trans-retinol to all-trans-13,14-dihydroretinol by RetSat produces a new metabolite of yet unknown biological function.

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.

Noninvasive two-photon imaging reveals retinyl ester storage structures in the eye

Visual sensation in vertebrates is triggered when light strikes retinal photoreceptor cells causing photoisomerization of the rhodopsin chromophore 11-cis-retinal to all-trans-retinal. The regeneration of preillumination conditions of the photoreceptor cells requires formation of 11-cis-retinal in the adjacent retinal pigment epithelium (RPE). Using the intrinsic fluorescence of all-trans-retinyl esters, noninvasive two-photon microscopy revealed previously uncharacterized structures (6.9 +/- 1.1 microm in length and 0.8 +/- 0.2 microm in diameter) distinct from other cellular organelles, termed the retinyl ester storage particles (RESTs), or retinosomes. These structures form autonomous all-trans-retinyl ester-rich intracellular compartments distinct from other organelles and colocalize with adipose differentiation-related protein. As demonstrated by in vivo experiments using wild-type mice, the RESTs participate in 11-cis-retinal formation. RESTs accumulate in Rpe65-/- mice incapable of carrying out the enzymatic isomerization, and correspondingly, are absent in the eyes of Lrat-/- mice deficient in retinyl ester synthesis. These results indicate that RESTs located close to the RPE plasma membrane are essential components in 11-cis-retinal production.