Background: Toxic components of lipofuscin in the retina are proposed to arise from all-trans-retinal, a by-product of light detection. Results: Lipofuscin precursors form from 11-cis-retinal; lipofuscin accumulation is independent of light exposure. Conclusion: 11-cis-Retinal is the primary source of lipofuscin components. Significance: 11-cis-Retinal may play a major role in the pathogenesis of macular degenerations.

Light detection by vertebrate rod photoreceptor outer segments results in the destruction of the visual pigment, rhodopsin, as its retinyl moiety is photoisomerized from 11-cis to all-trans. The regeneration of rhodopsin is necessary for vision and begins with the release of the all-trans retinal and its reduction to all-trans retinol. Retinol is then transported out of the rod outer segment for further processing. We used fluorescence imaging to monitor retinol fluorescence and quantify the kinetics of its formation and clearance after rhodopsin bleaching in the outer segments of living isolated frog (Rana pipiens) rod photoreceptors. We independently measured the release of all-trans retinal from bleached rhodopsin in frog rod outer segment membranes and the rate of all-trans retinol removal by the lipophilic carriers interphotoreceptor retinoid binding protein (IRBP) and serum albumin. We find that the kinetics of all-trans retinol formation in frog rod outer segments after rhodopsin bleaching are to a good first approximation determined by the kinetics of all-trans retinal release from the bleached pigment. For the physiological concentrations of carriers, the rate of retinol removal from the outer segment is determined by the IRBP concentration, while the effect of serum albumin is negligible. The results indicate the presence of a specific interaction between IRBP and rod outer segment, probably mediated by a receptor. The effect of different concentrations of IRBP on the rate of retinol removal shows no cooperativity and has an EC 50 of 40 μmol/L.The vertebrate cells responsible for vision are the rod and cone photoreceptors of the retina that convert incoming light to an electrical signal. This conversion takes place in the photoreceptor outer segments, which are full of membrane disks containing the visual pigment, and, in a physiologically important arrangement, are enveloped by the retinal pigment epithelium (RPE). The visual pigment is composed of a chromophore, 11-cis retinal, attached to an integral membrane protein, opsin. The detection of light begins with the absorption of incoming photons by the visual pigment. An absorbed photon isomerizes the chromophore † Supported by NIH/NEI grants EY14850 (YK), EY04939 (RKC) , NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript moiety from 11-cis to all-trans bringing about a conformational change that initiates a cascade of reactions culminating in membrane potential change. The recovery of the cell from light involves the deactivation of the intermediates activated by light, and the reestablishment of membrane potential (1,2). However, the isomerized chromophore, all-trans retinal, remains. For vision to be possible, it is essential that the visual pigment regenerate: that is, the alltrans retinal has to be removed, and fresh 11-cis retinal has to be provided to combine with opsin and reform the visual pigment. The reactions regenerating the pigment are known as the Visual Cycle (3-5).In the case of the rod photoreceptors, t...

PURPOSE To test whether the formation of all-trans retinol limits the regeneration of the visual pigment. all-trans retinol is formed after visual pigment bleaching through the reduction of all-trans retinal in a reaction involving NADPH. This reduction begins the recycling of the chromophore for the regeneration of the visual pigment. METHODS Experiments were performed with dark-adapted, isolated retinas and isolated photoreceptor cells from wild-type and Nrl−/− mice. The photoreceptors of Nrl−/− mice are conelike and contain only cone pigments. The formation of all-trans retinol after pigment bleaching was measured by quantitative HPLC of retinoids extracted from isolated retinas and by imaging the fluorescence of retinol in photoreceptor outer segments. Experiments were performed at 37°C. RESULTS In rods, the formation of all-trans retinol proceeded with first-order kinetics, with a rate constant of 0.06 ± 0.02 minute−1, significantly faster than the reported rate constant for rhodopsin regeneration. In Nrl−/− photoreceptors, the formation of all-trans retinol occurred at least 100 times faster than in rods. For both cell types, the fraction of all-trans retinal converted to all-trans retinol at equilibrium is ∼0.8, indicating the presence of a similar fraction of reduced NADPH. CONCLUSIONS Formation of all-trans retinol does not limit the regeneration of bleached visual pigment. Formation of all-trans retinol in the cone-like Nrl−/− photoreceptors is much faster than in rods, consistent with a faster regeneration of the visual pigment after bleaching. Different types of photoreceptors contain a comparable fraction of reduced NADPH to drive the reduction of all-trans retinal.

The regeneration of rhodopsin and the recycling of its chromophore are not strongly coupled. Neither the activities of Abca4, rhodopsin kinase, and arrestin, nor the palmitylation of rhodopsin affects the formation of all-trans retinol.

Summary Orange autofluorescence from lipofuscin in the lysosomes of the retinal pigment epithelium (RPE) is a hallmark of aging in the eye. One of the major components of lipofuscin is A2E, the levels of which increase with age and in pathologic conditions, such as Stargardt disease or age-related macular degeneration. In vitro studies have suggested that A2E is highly phototoxic and, more specifically, that A2E and its oxidized derivatives contribute to RPE damage and subsequent photoreceptor cell death. To date, absorption spectroscopy has been the primary method to identify and quantitate A2E. Here, a new mass spectrometric method was developed for the specific detection of low levels of A2E and compared to a traditional method of analysis. The new mass spectrometry method allows the detection and quantitation of approximately 10,000-fold less A2E than absorption spectroscopy and the detection and quantitation of low levels of oxidized A2E, with localization of the oxidation sites. This study suggests that identification and quantitation of A2E from tissue extracts by chromatographic absorption spectroscopyoverestimates the amount of A2E. This mass spectrometry approach makes it possible to detect low levels of A2E and its oxidized metabolites with greater accuracy than traditional methods, thereby facilitating a more exact analysis of bis-retinoids in animal models of inherited retinal degeneration as well as in normal and diseased human eyes.

ABCA4 is a member of the superfamily of ATP binding cassette (ABC) proteins that is localized in outer segment disc membranes of rod and cone photoreceptor cells. Mutations in the ABCA4 gene are responsible for Stargardt macular dystrophy, cone-rod dystrophy and retinitis pigmentosa. Biochemical studies together with analysis of abca4 knockout mice implicate ABCA4 in the transport of N-retinylidene-phosphatidylethanolamine across disk membranes. This transport process facilitates the complete removal of retinal derivatives from photoreceptors following the photobleaching of rhodopsin and cone opsin as part of the visual cycle. Loss in the activity of ABCA4 leads to the production of diretinal derivatives in disc membranes which accumulate in adjacent retinal pigment epithelial (RPE) cells as lipofuscin deposits following phagocytosis of outer segments. Progressive buildup of these toxic diretinal compounds causes the degeneration of RPE and photoreceptors and a loss in vision. Recently, we have investigated the effect of C-terminal deletion, including several disease related mutations, on the structural and functional properties of ABCA4. Our studies indicate that ABCA4 contains a conserved motif near the C-terminus that is crucial for proper protein folding and functional activity of ABCA4. Individuals missing this motif due to C-terminal truncation of ABCA4 exhibit a severe form of retinal degeneration known as cone-rod dystrophy. Light detection destroys the visual pigment of vertebrate rod photoreceptors, rhodopsin, as its retinyl moiety is photoisomerized from 11-cis to all-trans. Rhodopsin is regenerated through a series of reactions that begin in the rod outer segment with the release of the all-trans retinal and its reduction to all-trans retinol. All-trans retinol is then transported to the neighboring retinal pigment epithelial cells where it is used to remake 11-cis retinal. The reduction of all-trans retinal to all-trans retinol is catalyzed by retinol dehydrogenase and requires metabolic input in the form of NADPH. We have used the fluorescence of all-trans retinol to monitor its concentration in isolated mouse rod photoreceptors. After the bleaching of rhodopsin, all-trans retinol formation proceeds with a rate of~0.06 min -1 , which is faster than the rate of rhodopsin regeneration in whole animals; this would allow recycled chromophore to contribute to the 11-cis retinal used for regeneration. Inner segment metabolic pathways appear to make a significant contribution to the pool of NADPH needed for the reduction of all-trans retinal, as formation of all-trans retinol is suppressed in rod outer segments separated from the cell body. Finally, generation of all-trans retinol is suppressed in the absence of glucose, indicating a critical dependence of all-trans retinol formation on the level of metabolic activity. The recovery of sensitivity following photopigment bleaching requires the quenching of phototransduction, and the reduction of all-trans retinal is key. Retinol fluorescence increases after ...