Dynamics and function of vitamin A compounds in rat retina after a small bleach of rhodopsin

Relationships among visual cycle retinoids, rhodopsin phosphorylation, and phototransduction in mouse eyes during light and dark adaptation

Phosphorylation and regeneration of rhodopsin, the prototypical G-protein-coupled receptor, each can influence light and dark adaptation. To evaluate their relative contributions, we quantified rhodopsin, retinoids, phosphorylation, and photosensitivity in mice during a 90 min illumination followed by dark adaptation. During illumination, all-trans-retinyl esters and, to a lesser extent, all-trans-retinal accumulate and reach the steady state in <1 h. Each major phosphorylation site on rhodopsin reaches a steady state level of phosphorylation at a different time during illumination. The dominant factor that limits dark adaptation is isomerization of retinal. During dark adaptation, dephosphorylation of rhodopsin occurs in two phases. The faster phase corresponds to rapid dephosphorylation of regenerated rhodopsin present at the end of the illumination period. The slower phase corresponds to dephosphorylation of rhodopsin as it forms by regeneration. We conclude that rhodopsin phosphorylation has three physiological functions: it quenches phototransduction, reduces sensitivity during light adaptation, and suppresses bleached rhodopsin activity during dark adaptation.

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.

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.

What is lipofuscin? Defining characteristics and differentiation from other autofluorescent lysosomal storage bodies

Lipofuscins, also known as age-pigments, have three defining characteristics: (1) they consist of intracellular secondary lysosomes; (2) they have a yellow autofluorescent emission when excited by near ultraviolet or blue light; and (3) they accumulate during normal senescence. Lysosomal storage bodies with similar fluorescence properties accumulate in various cell types as a result of specific pathological conditions or experimental manipulations. As a class, the latter are often referred to as ceroid pigments. In general, the mechanisms involved in the formation of ceroid pigments cannot be assumed to be closely similar to those involved in lipofuscin formation. In fact, the mechanisms of formation almost certainly differ, not only between lipofuscins and ceroids, but also among different lipofuscins and different ceroids. Presently, the most detailed knowledge about the mechanisms involved in lipofuscin formation come from studies on the retinal pigment epithelium (RPE) of the eye. These studies indicate that at least the autofluorescent constituents of RPE lipofuscin are generated from derivatives of vitamin A that occur in the retina. Oxidative stress to the retina appears to promote the formation of these RPE fluorophores. Whether similar mechanisms are involved in the formation of the lipofuscins that occur in other tissues remains to be determined. The mechanisms involved in RPE lipofuscin fluorophore formation are closely related to metabolic pathways that are specific to the retina. Thus, it appears likely that the mechanisms by which lipofuscins form in other tissues differ fundamentally from those that underlie RPE lipofuscin formation.

Bernard Strehler-inspiration for basic research into the mechanisms of aging

Bernard Strehler, who passed away recently, has provided inspiration and intellectual guidance for a generation of scientists interested in the biology of aging. My own career in this field was launched in large part by the ideas and concepts discussed by Dr Strehler in his book, Time, Cells, and Aging. Much of my scientific career has been devoted to studying one aspect of the aging process-the intracellular accumulation of autofluorescent lysosomal storage bodies (lipofuscin) during senescence. Work in my laboratory has contributed to the elucidation of the molecular mechanisms that underlie formation of lipofuscin in the retinal pigment epithelium of the eye. The challenge for the work on lipofuscin, and for much of the current research on aging, is to determine whether specific age-related changes such as lipofuscin accumulation, are involved in determining maximum life span. Bernard Strehler's eloquent statement of this challenge will hopefully continue to inspire new research to further our understanding of the aging phenomenon.

Mechanistic studies of ABCR, the ABC transporter in photoreceptor outer segments responsible for autosomal recessive Stargardt disease

ABCR is an ABC transporter that is found exclusively in vertebrate photoreceptor outer segments. Mutations in the human ABCR gene are responsible for autosomal recessive Stargardt disease, the most common cause of early onset macular degeneration. In this paper we review our recent work with purified and reconstituted ABCR derived from bovine retina and from cultured cells expressing wild type or site-directed mutants of human ABCR. These experiments implicate all-trans-retinal (or Schiff base adducts between all-trans-retinal and phosphatidylethanolamine) as the transport substrate, and they reveal asymmetric roles for the two nucleotide binding domains in the transport reaction. A model for the retinal transport reaction is presented which accounts for these experimental observations.

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.

Isomerization of all-trans-retinol to cis-retinols in bovine retinal pigment epithelial cells: dependence on the specificity of retinoid-binding proteins

In the retinal rod and cone photoreceptors, light photoactivates rhodopsin or cone visual pigments by converting 11-cis-retinal to all-trans-retinal, the process that ultimately results in phototransduction and visual sensation. The production of 11-cis-retinal in adjacent retinal pigment epithelial (RPE) cells is a fundamental process that allows regeneration of the vertebrate visual system. Here, we present evidence that all-trans-retinol is unstable in the presence of H(+) and rearranges to anhydroretinol through a carbocation intermediate, which can be trapped by alcohols to form retro-retinyl ethers. This ability of all-trans-retinol to form a carbocation could be relevant for isomerization. The calculated activation energy of isomerization of all-trans-retinyl carbocation to the 11-cis-isomer was only approximately 18 kcal/mol, as compared to approximately 36 kcal/mol for all-trans-retinol. This activation energy is similar to approximately 17 kcal/mol obtained experimentally for the isomerization reaction in RPE microsomes. Mass spectrometric (MS) analysis of isotopically labeled retinoids showed that isomerization proceeds via alkyl cleavage mechanism, but the product of isomerization depended on the specificity of the retinoid-binding protein(s) as evidenced by the production of 13-cis-retinol in the presence of cellular retinoid-binding protein (CRBP). To test the influence of an electron-withdrawing group on the polyene chain, which would inhibit carbocation formation, 11-fluoro-all-trans-retinol was used in the isomerization assay and was shown to be inactive. Together, these results strengthen the idea that the isomerization reaction is driven by mass action and may occur via carbocation intermediate.

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

Retinol dehydrogenase (RDH), the enzyme that catalyzes the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment, was the first visual cycle enzymatic activity to be identified. Previous work has shown that this enzyme utilizes NADPH, shows a marked preference for all-trans-retinal over 11-cis-retinal, and is tightly associated with the outer segment membrane. This paper reports the identification of a novel member of the short chain dehydrogenase/reductase family, photoreceptor RDH (prRDH), using subtraction and normalization of retina cDNA, high throughput sequencing, and data base homology searches to detect retina-specific genes. Bovine and human prRDH are highly homologous and are most closely related to 17-beta-hydroxysteroid dehydrogenase 1. The enzymatic properties of recombinant bovine prRDH closely match those previously reported for RDH activity in crude bovine rod outer segment preparations. In situ hybridization and RNA blotting show that the PRRDH gene is expressed specifically in photoreceptor cells, and protein blotting and immunocytochemistry show that prRDH localizes exclusively to both rod and cone outer segments and that prRDH is tightly associated with outer segment membranes. Taken together, these data indicate that prRDH is the enzyme responsible for the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment.

Molecular characterization of a novel short-chain dehydrogenase/reductase that reduces all-trans-retinal

The reduction of all-trans-retinal in photoreceptor outer segments is the first step in the regeneration of bleached visual pigments. We report here the cloning of a dehydrogenase, retSDR1, that belongs to the short-chain dehydrogenase/reductase superfamily and localizes predominantly in cone photoreceptors. retSDR1 expressed in insect cells displayed substrate specificities of the photoreceptor all-trans-retinol dehydrogenase. Homology modeling of retSDR1 using the carbonyl reductase structure as a scaffold predicted a classical Rossmann fold for the nucleotide binding, and an N-terminal extension that could facilitate binding of the enzyme to the cell membranes. The presence of retSDR1 in a subset of inner retinal neurons and in other tissues suggests that the enzyme may also be involved in retinol metabolism outside of photoreceptors.

Reduction of all-trans-retinal limits regeneration of visual pigment in mice

Absorption of photons by pigments in photoreceptor cells results in photoisomerization of the chromophore, 11-cis-retinal, to all-trans-retinal and activation of opsin. Photolysed chromophore is converted back to the 11-cis-configuration via several enzymatic steps in photoreceptor and retinal pigment epithelial cells. We investigated the levels of retinoids in mouse retina during constant illumination and regeneration in the dark as a means of obtaining more information about the rate-limiting step of the visual cycle and about cycle intermediates that could be responsible for desensitization of the visual system. All-trans-retinal accumulated in the retinas during constant illumination and following flash illumination. Decay of all-trans-retinal in the dark following constant illumination occurred without substantial accumulation of all-trans-retinal, generated by constant approximately equal to visual pigment regeneration (t1/2 approximately 5 and t1/2 approximately 7 min, respectively). All-trans-retinal, generated by constant illumination, decayed approximately 3 times more rapidly than that generated by a flash and, as shown previously, the rate of rhodopsin regeneration following a flash was approximately 4 times slower than after constant illumination. The retinyl ester pool (> 95% all-trans-retinyl ester) did not show a statistically significant change in size or composition during illumination. In addition, constant illumination increased the amount of photoreceptor membrane-associated arrestin. The results suggest that the rate-limiting step of the visual cycle is the reduction of all-trans-retinal to all-trans-retinol by all-trans-retinol dehydrogenase. The accumulation of all-trans-retinal during illumination may be responsible, in part, for the reduction in sensitivity of the visual system that accompanies photobleaching and may contribute to the development of retinal pathology associated with light damage and aging.

Serotonergic, 5-HT2, receptor-mediated phosphoinositide turnover and mobilization of calcium in cultured rat retinal pigment epithelium cells

Cultured rat retinal pigment epithelium cells are shown to contain serotonergic, 5-HT2, receptors associated with phosphoinositide turnover and mobilization of intracellular calcium. Serotonin at a concentration of 10 microM induced a 2.5-fold increase in [3H]-inositol phosphates (more than 75% is in the form of [3H]-inositol-1-phosphate) accumulation within 30 min in cells preincubated in [3H]-myo-inositol and exposed to 5 mM lithium chloride. The EC50 value of serotonin was approx. 0.9 microM and the saturation concentration was 100 microM. Serotonin analogues like tryptamine, 5-methoxytryptamine, alpha-methyl-serotonin and the 5-HT2 agonists quipazine and DOI (1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane) all stimulated InsPs accumulation to some degree. Carbachol, noradrenaline, isoproterenol, dopamine, tryptophan, 5-hydroxytryptophan, 8-hydroxy-2(di-n-propyl-amino) tetralin, 2-methyl-serotonin and NECA (5'-[N-ethyl]-carboxamidoadenosine) were inactive. The serotonin-induced response was blocked most effectively by ketanserin and methysergide but not by 5-HT3 or 5-HT1 antagonists. The serotonin response was attenuated by the active phorbol ester, 4 beta-phorbol 12-myristate 13-acetate and this was attenuated by the non-selective protein kinase C inhibitor, staurosporine. Pertussis toxin failed to influence the serotonin-mediated phosphoinositide turnover. Addition of serotonin to cultures loaded with Fura-2 showed a transient increase in calcium concentrations in most of the cells. This change in calcium was independent of external calcium and the serotonin response was attenuated by ketanserin but not by the 5-HT3 antagonist granisetron.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide-induced cytosolic Ca2+ transients and phosphatidylinositol turnover in cultured human retinal pigment epithelial cells

Neuropeptide-induced mobilization of cytosolic free Ca2+ concentration ([Ca2+]i) and phosphatidylinositol (PI) turnover in cultured human retinal pigment epithelial (RPE) cells were studied and their temporal relationship was compared. After RPE cells were loaded with fura-2/AM, [Ca2+]i was analyzed using a digital imaging microscopy system. Bombesin-related peptides which include bombesin, neuromedin B, and neuromedin C induced significant [Ca2+]i transients in RPE cells, whereas other neuropeptides, neuropeptide Y, vasoactive intestinal polypeptide (VIP), and substance P were not effective to produce [Ca2+]i transients. The percentage of reactive cells which showed positive [Ca2+]i transients induced by bombesin-related peptides was around 50%. Bombesin (1 microM) showed a peak concentration of 663 +/- 27.0 nM (mean +/- S.E.M., n = 61), neuromedin B (1 microM), 327 +/- 28.7 nM (mean +/- S.E.M., n = 38), and neuromedin C (1 microM), 357 +/- 22.7 nM (mean +/- S.E.M., n = 32). Ca2+ transients occurred within 30 s and lasted less than 5 min after the application of the neuropeptides. Chelation of the extracellular Ca2+ by EGTA significantly shortened the total time of [Ca2+]i transients induced by the above. The measurements of phosphoinositides in RPE cells revealed that neuropeptide-induced PI turnover was as quick as [Ca2+]i transients. Inositol biphosphate (IP2) and inositol triphosphate (IP3) in RPE cells showed transient increases at 15 s after the stimulation by bombesin-related peptides. These data show that changes in [Ca2+]i and PI turnover are directly linked and both are important in the signal transduction system of bombesin-related peptides in RPE cells. The data also suggest that bombesin-related peptides may play some possible roles in RPE cells.

Addition of the chromophore to rat rhodopsin is an early post-translational event

Rat retinas were labeled either by intravitreal injection of [14C]leucine or by incubation with [3H]-leucine or [35S]-methionine. Subcellular fractions were prepared on linear sucrose gradients and rhodopsin was extracted with detergent and purified by chromatography on ConA-Sepharose. A fraction enriched in rough endoplasmic reticulum (RER) and substantially free of rod outer segments (ROS) was found to contain a light-sensitive protein exhibiting the properties of rhodopsin on ConA-Sepharose or Agarose chromatography and on SDS-polyacrylamide gel electrophoresis, as well as immunologically. Intravitreal injection of [3H]-retinol also labeled the rhodopsin in the RER under conditions in which the rhodopsin in the ROS was not heavily labeled. Thus the chromophore appears to be attached to opsin shortly after the apoprotein is translated in the RER.

Rapid analysis of the major classes of retinoids by step gradient reversed-phase high-performance liquid chromatography using retinal (O-ethyl) oxime derivatives

A rapid step-gradient reversed-phase high-performance liquid chromatography (HPLC) method is presented for analysis of the major classes of retinoids in tissues. Retinal was converted into a new derivative, retinal (O-ethyl) oxime, since the standard derivative, retinaloxime, co-elutes with retinol on reversed-phase HPLC. The most abundant naturally occurring retinyl esters, retinyl palmitate and retinyl stearate, were eluted within 12 min to complete the separation. Retinoids were extracted in the presence of an antioxidant, butylated hydroxytoluene, and a lipid carrier, cholesterol. Recoveries of 98-100% were obtained from tissue samples by internal addition for the retinoids tested (retinol, retinal and retinyl palmitate); and the absolute recovery of endogenous retinal from rat eyecups was confirmed by spectrophotometric measurements of rhodopsin. Extraction was carried out in an air atmosphere and under subdued incandescent light rather than requiring inert atmosphere and safe-light conditions used in most methods. Cis-trans isomers were not separated under the reversed-phase HPLC conditions employed. Quantitation was carried out using retinyl acetate as internal standard and the day to day precision was better than 3.5%. A sensitivity of about 1 ng is obtained for all retinoids using absorbance monitoring at 325 nm and a C18 5 micrometers column with 12% reversed-phase loading. The tocopherols can also be separated and detected simultaneously with similar sensitivity by this method using a fluorescence detector in series [G. J. Handelman, L. J. Machlin, K. Fitch, J. J. Weiter and E. A. Dratz, J. Nutr., 115 (1985) 807].

100 years of the visual cycle

One--hundred years have passed since Franz Boll and Willy Kühne first characterized rhodopsin and discovered the visual pigment cycle. Some of the events and individuals of that period are described, and color photographs are presented to show the appearance of rhodopsin in the living retina.

Molecular aspects of photoreceptor function

The description of the molecular processes which underlie visual excitation is the fundamental problem in understanding vision at the level of a single photoreceptor. Thus far only a general outline of photoreceptor function has emerged with little known about actual biochemical and biophysical mechanisms.

Biochemical aspects of the visual process. XXVII. Stereospecificity of ocular retinol dehydrogenases and the visual cycle

A comparative study is made of the stereospecificity of two particulate retinol dehydrogenases from bovine eyes and of horse liver alcohol dehydrogenase. The particulate retinol dehydrogenase of outer segments reacts with the all-trans isomers of retinaldehyde and retinol but not with the 11-cis compounds. In contrast, a particulate retinol dehydrogenase present in pigment epithelium reacts preferentially with the 11-cis compounds. Horse liver alcohol dehydrogenase (EC 1.1.1.1.) can convert both isomers, but the all-trans isomers are clearly preferred. Differences with regard to cofactor preference and stability are also noted. The outer segment enzyme clearly functions in the rhodopsin cycle. It is unlikely that the 11-cis retinol dehydrogenase from pigment epithelium is directly involved in providing 11-cis retinaldehyde from rhodopsin regeneration, but it may serve to make available 11-cis retinaldehyde from rhodopdsin, digested in phagocytized rod sacs, for the synthesis of visual pigment by the visual cells.