Isolated retinas synthesize visual pigments from tetinol congeners delivered by liposomes

Vitamin A dimers trigger the protracted death of retinal pigment epithelium cells

Cellular events responsible for the initiation of major neurodegenerative disorders of the eye leading to blindness, including age-related macular degeneration, Stargardt and Best diseases, are poorly understood. Accumulation of vitamin A dimers, such as N-retinylidene-N-retinylethanolamine (A2E) in the retinal pigment epithelium (RPE), is one of the earliest measurable events preceding retinal degeneration. However, the extent to which these dimers contribute to tissue degeneration is not clear. To determine if A2E could trigger morphological changes associated with the degenerating RPE and subsequent cell death, we evaluated its toxicity to cultured human RPE cells (ARPE-19). We show that A2E triggered the accumulation of debris followed by a protracted death. A2E was up to ≈ 14-fold more toxic than its precursor, retinaldehyde. Measurements reveal that the concentration of A2E in the aged human eye could exceed the concentration of all other retinoids, opening the possibility of A2E-triggered cell death by several reported mechanisms. Findings suggest that accumulation of vitamin A dimers such as A2E in the human eye might be responsible for the formation of ubiquitous RPE debris, an early indication of retinal degeneration, and that preventing or reducing the accumulation of vitamin A dimers is a prudent strategy to prevent blindness.

Interaction of 11-cis-retinol dehydrogenase with the chromophore of retinal g protein-coupled receptor opsin

Vertebrate opsins in both photoreceptors and the retinal pigment epithelium (RPE) have fundamental roles in the visual process. The visual pigments in photoreceptors are bound to 11-cis-retinal and are responsible for the initiation of visual excitation. Retinochrome-like opsins in the RPE are bound to all-trans-retinal and play an important role in chromophore metabolism. The retinal G protein-coupled receptor (RGR) of the RPE and Müller cells is an abundant opsin that generates 11-cis-retinal by stereospecific photoisomerization of its bound all-trans-retinal chromophore. We have analyzed a 32-kDa protein (p32) that co-purifies with bovine RGR from RPE microsomes. The co-purified p32 was identified by mass spectrometric analysis as 11-cis-retinol dehydrogenase (cRDH), and enzymatic assays have confirmed the isolation of an active cRDH. The co-purified cRDH showed marked substrate preference to 11-cis-retinal and preferred NADH rather than NADPH as the cofactor in reduction reactions. cRDH did not react with endogenous all-trans-retinal bound to RGR but reacted specifically with 11-cis-retinal that was generated by photoisomerization after irradiation of RGR. The reduction of 11-cis-retinal to 11-cis-retinol by cRDH enhanced the net photoisomerization of all-trans-retinal bound to RGR. These results indicate that cRDH is involved in the processing of 11-cis-retinal after irradiation of RGR opsin and suggest that cRDH has a novel role in the visual cycle.

Visual pigment reconstitution in intact goldfish retina using synthetic retinaldehyde isomers

A protocol has been developed for reconstituting visual pigments in intact retinae by delivering synthetic isomers of retinal incorporated in phospholipid vesicles. Calibration curves have been constructed relating the lambda(max) of the native porphyropsins (visual pigments based on 11-cis 3-dehydroretinal) of the rods and four spectral classes of cone in the goldfish, and the equivalent photosensitive pigments regenerated from 11-cis retinal (rhodopsins) and the commercially available isomer, 9-cis retinal (isorhodopsins). The relationship between the lambda(max) of rhodopsins and isorhodopsins appears to be linear, such that the difference in lambda(max) changes sign at about 380 nm. We therefore conclude that the protocol for reconstituting visual pigments with 9-cis retinal is suitable for all classes of vertebrate opsin-based photopigments.

Bleached pigment activates transduction in isolated rods of the salamander retina

1. We have used suction electrode recording together with rapid steps into Li+ solution and 0.5 mM IBMX solution to estimate the rates of the guanylyl phosphodiesterase (PDE) and guanylyl cyclase in isolated rods of the salamander, Ambystoma tigrinum. 2. We show that both the PDE and cyclase velocities are accelerated by steady background light. The steady velocities of both enzymes appear to be saturating functions of background intensity. 3. Bleaching also accelerates both the PDE and cyclase. This effect is maintained long after the bleaching stimulus is removed (up to 2 h) and is reversed only if the photopigment is regenerated with exogenous chromophore. 4. The estimated steady-state PDE and cyclase velocities appear to be linear functions of the amount of pigment bleached, as if each bleached pigment molecule activated the transduction cascade with the same probability and gain. 5. The effectiveness of bleached pigment in activating transduction is only 10(-6) to 10(-7) times that of activated rhodopsin (Rh*), but this is sufficient after large bleaches to produce an 'equivalent background' excitation of the rod, which is probably responsible, at least in part, for bleaching desensitization.

Movement of retinal along cone and rod photoreceptors

Single isolated photoreceptors can be taken through a visual cycle of light adaptation by bleaching visual pigment, followed by dark adaptation when supplied with 11-cis retinal. Light adaptation after bleaching is manifested by faster response kinetics and a permanent reduction in sensitivity to light flashes, presumed to be due to the presence of bleached visual pigment. The recovery of flash sensitivity during dark adaptation is assumed to be due to regeneration of visual pigment to pre-bleach levels. In previous work, the outer segments of bleached, light-adapted cells were exposed to 11-cis retinal. In the present work, the cell bodies of bleached photoreceptors were exposed. We report a marked difference between rods and cones. Bleached cones recover sensitivity when their cell bodies are exposed to 11-cis retinal. Bleached rods do not. These results imply that retinal can move freely along the cone photoreceptor, but retinal either is not taken up by the rod cell body or retinal cannot move from the rod cell body to the rod outer segment. The free transfer of retinal along cone but not along rod photoreceptors could explain why, during dark adaptation in the retina, cones have access to a store of 11-cis retinal which is not available to rods. Additional experiments investigated the movement of retinal along bleached rod outer segments. The results indicate that retinal can move along the rod outer segment, but that this movement is slow, occurring at about the same rate as the regeneration of visual pigment.

Rod-opsin immunoreaction in the pineal organ of the pigmented mouse does not indicate the presence of a functional photopigment

The aim of the present study was to characterize the rod-opsin immunoreaction in the mammalian pineal organ. Pigmented mice (strain C57BL) were selected as the animal model. Immunocytochemical investigations involving the use of highly specific polyclonal and monoclonal antibodies against bovine rod-opsin (the apoprotein of the photopigment rhodopsin) showed that approximately 25% of all pinealocytes were rod-opsin immunoreactive. Immunoblotting techniques revealed three protein bands of approximately 40, 75, and 110 kDa; these were detected by the monoclonal antibody and the polyclonal antiserum in retinal and pineal extracts. These protein bands presumably represented the monomeric, dimeric and trimeric forms of rod-opsin. The amount of rod-opsin in retina and pineal organ was quantified by means of an enzyme-linked immunosorbent assay. This yielded 570 +/- 30 pmoles rod-opsin per eye and 0.3 +/- 0.05 pmoles rod-opsin per pineal organ. High pressure liquid chromatography analysis of whole eye extracts demonstrated the chromophoric group of the photopigment rhodopsin, 11-cis retinal, and its isomer, all-trans-retinal. A shift from 11-cis retinal to all-trans-retinal was found upon light adaptation. No retinals were detected in the pineal organ. Autoradiographic investigations showed that 3H-retinol, intraperitoneally injected into the animals, was incorporated into the outer and inner segments of retinal photoreceptors, but not into the pineal organ. It is concluded that the mouse pineal organ contains the authentic apoprotein of rhodopsin but that it lacks retinal derivatives as essential components of all known vertebrate photopigments.(ABSTRACT TRUNCATED AT 250 WORDS)

Noncovalent occupancy of the retinal-binding pocket of opsin diminishes bleaching adaptation of retinal cones

Bright light bleaches visual pigment and leads to a persistent desensitization of isolated rod and cone photoreceptors called bleaching adaptation. Bleaching adaptation results from the combined effects of pigment depletion and adaptational modulation of certain cellular reactions in the visual transduction cascade. Here, we present evidence that in solitary cone photoreceptors isolated from the salamander retina, the latter effect is due to the presence of free opsin in the outer segment. Also, we demonstrate that this "opsin adaptation" can be reversed by treating the cells with synthetic retinoids similar to 11-cis retinal but having polyene chains too short to form protonated Schiff base attachments to opsin.

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

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

Solubility of retinoids in water

Spectrophotometric and radioactive techniques were used to measure the water solubility of retinaldehyde, retinol (vitamin A), and retinoic acid under physiological conditions. Hydration decreases the molar extinction coefficient of these substances and shifts their absorption peak bathochromically (10 nm for retinal and approximately 1 nm for the rest). We find their solubility to be about 0.1 microM at room temperature, pH 7.3 (with experimental values being 0.11 microM for retinaldehyde, 0.06 microM for retinol, and 0.21 microM for retinoic acid). To prevent oxidative degradation of retinol, which is the most labile retinoid, our argon-saturated buffer solutions contained physiological levels of ascorbate or alpha-tocopherol. To the best of our knowledge, water solubility of these compounds has not yet been previously reported. Although the measured solubilities are relatively low, they are significant and may account for the movement of retinoids through the aqueous phase as observed by others during exchange with binding proteins and during intervesicular transfer in the absence of binding proteins. Diffusion of uncomplexed retinoids through the aqueous phase can be a major pathway for transport over subcellular distances.

Photoreceptor differentiation in cerebellar medulloblastoma: evidence for a functional photopigment and authentic S-antigen (arrestin)

The aim of the present study was to evaluate the putative photoreceptor differentiation found in certain cerebellar medulloblastomas. The analyses were focussed on S-antigen, rod-opsin (the apoprotein of the visual pigment rhodopsin) and 11-cis retinal (the prosthetic group of rhodopsin). Fresh frozen and paraffin-embedded biopsy specimens of three medulloblastomas were investigated by means of immunocytochemistry, enzyme-linked immunosorbent assay (ELISA), high-pressure liquid chromatography (HPLC), and immunoblotting. As shown in paraffin sections, one out of the three tumors (tumor A) contained S-antigen- and rod-opsin-immunoreactive tumor cells. The immunoblotting technique revealed in this tumor a single protein band of approximately 48-50 kDa that reacted with the S-antigen antibody and three protein bands of approximately 40, 75 and 110 kDa recognized by the rod-opsin antibody. These bands could not be detected in the two remaining tumors (tumor B and C). The rod-opsin content of tumor A was quantified by the ELISA; 11.7 pmol rod-opsin were calculated for the biopsy. The HPLC demonstrated the presence of 11-cis- and all-trans-retinal in tumor A, but not in tumors B and C. Furthermore, it was shown that 11-cis-retinal was converted to all-trans-retinal upon illumination of the tumor extract. The ratio between 11-cis- and all-trans-retinal was approximately 1:1 before illumination and 3:5 after illumination. A total of 2-3 pmol of retinal was found in the biopsy of tumor A. In addition all-trans-retinol was present in this tumor. The results indicate that certain medulloblastomas express a functional photopigment and S-antigen, another protein of the phototransduction cascade. They strongly support the concept that medulloblastoma cells may differentiate along the photoreceptor cell lineage.

Sensitization of bleached rod photoreceptors by 11-cis-locked analogues of retinal

Photoactivation of rhodopsin initiates both excitation and adaptation in vertebrate rod photoreceptors. Bleaching of rhodopsin to free opsin and all-trans-retinal in isolated rods produces a stable desensitization (bleaching adaptation) that is much larger than expected from pigment depletion alone. In our experiments, a 93% bleach produced a 500-fold increase in the light intensity required for saturation of the light response. This component of adaptation was 32-fold larger than the 16-fold increase expected from pigment depletion alone. 11-cis-Retinal, when delivered to isolated rods from liposomes, combines with free opsin to form a bleachable photopigment that fully restores sensitivity. 11-cis-Locked analogues of retinal combine with opsin to form unbleachable pigments in isolated bleached rods from the tiger salamander. They restore sensitivity to a substantial (16- to 25-fold) but incomplete extent. The analogues apparently relieve a stable component of adaptation when they interact with opsin. Because these analogues do not detectably excite rods, the structural requirements of both retinal and opsin for the relief of adaptation are different from those of excitation. The biochemical basis of light adaptation resulting from pigment bleaching and the minimum structural requirements of retinal for its relief remain to be determined.

Retinoid requirements for recovery of sensitivity after visual-pigment bleaching in isolated photoreceptors

After visual-pigment bleaching, single isolated rod photoreceptors of Ambystoma tigrinum recover their sensitivity to light when supplied with 11-cis-retinal from liposomes or with 11-cis-retinal bound to interphotoreceptor retinoid-binding protein. Bleached rods do not recover sensitivity, or do so only very slowly, after exposure to 11-cis-retinol. The latter retinoid is "toxic" in that rods actually lose sensitivity in its presence. In contrast, bleached isolated cone cells recover sensitivity when either retinoid is supplied. It is suggested that the major pathway for rhodopsin regeneration during dark adaptation in the intact eye is transport of 11-cis-retinal from the pigment epithelium to the retina. The results also suggest that there may be separate pathways for visual-pigment regeneration in rods and cones during dark adaptation.

Studies of the spontaneous transfer of retinol from the retinol:retinol-binding protein complex to unilamellar liposomes

The transfer of retinol from its complex with the retinol-binding protein to cell surfaces was studied using unilamellar liposomes as a cell surface model. The transfer of retinol to liposomes at 37 degrees C was rapid and reached an apparent equilibrium within 60 min. The amount of retinol transferred to the liposomes at equilibrium was directly proportional to the starting concentration of retinol:retinol-binding protein over a wide range of retinol:retinol-binding protein concentrations and also directly proportional to the concentration of liposomal phospholipid in the system, when the concentration of retinol:retinol-binding protein was held constant. The transfer increased slightly with temperature. Transfer was increased by a factor of 1.8 at pH 4.5 compared to pH around 7. Prealbumin in amounts sufficient to complex all retinol:retinol-binding protein, decreased retinol transfer to liposomes indicating that prealbumin increases the affinity of retinol-binding protein for retinol. Addition of apo retinol-binding protein to the system decreased the transfer of retinol to liposomes considerably probably through competition with the liposomes for retinol. In similarly designed experiments delipidated bovine serum albumin competed much less with liposomes for retinol. The results show that spontaneous transfer of retinol from the retinol:retinol-binding protein complex to liposomal membranes occurs in vitro and suggests that a similar transfer may occur in vivo from retinol:retinol-binding protein to cell surface membranes.

Distribution and axial diffusion of retinol in bleached rod outer segments of frogs (Rana pipiens)

Isolated retinas and rod outer segments from frogs (Rana pipiens) were exposed to light that produced axially uniform total bleaches of rhodopsin. Using fluorescence video microscopy, it was shown that the formation and equilibrium distribution of all-trans-retinol, the final chromophore product of rhodopsin bleaching is axially uniform. This result shows that the rate and amount of oxidoreductase-mediated reduction of all-trans-retinal to all-trans-retinol is not affected by the relative age of the disk membranes to which the enzymes are bound. Therefore previously reported axial differences in regeneration of rhodopsin and recovery of photocurrent after exposure to bright light probably are not due to axial differences in the formation of rhodopsin photoproducts. In addition, measurements on individual rod cells show that there is no significant redistribution of retinol for up to 2 hr following localized partial bleaches of rhodopsin. This raises the perplexing question of how retinol is shuttled between disk membranes and the pigment epithelium during visual pigment regeneration following substantial bleaches.

Factors affecting the concentration of free holo retinol-binding protein in human plasma

The total concentrations of retinol, retinol-binding protein and prealbumin were determined in plasma from twenty healthy men and sixty patients with various inflammatory conditions. These concentrations were all strongly correlated to each other and lower in the patient group. The concentration of free (not prealbumin-bound) holo retinol-binding protein, the presumed 'active' supplier of retinol to the tissues, was calculated. It was found not to be decreased in the patient group. Of the measured total concentrations and their possible ratios in the whole material, the retinol/prealbumin ratio showed the strongest correlation to the concentration of free holo retinol-binding protein. The importance of the concentration of free holo retinol-binding protein for the vitamin A supply to the cells was supported by calculations on data from the literature showing that this concentration better than the above-mentioned total concentrations distinguished between patients with normal and abnormal dark adaptation ability.

Rod outer segment membranes: good acceptors for retinoids?

The exchange of all-trans retinoids (retinal, retinol, retinylpalmitate) between PC-vesicles, PC-vesicles and liver microsomes or PC-vesicles and rod outer segment membranes is investigated using 11,12(3)H labeled compounds. In the first two systems, retinal and retinol exchange rapidly, retinyl acetate slowly and retinyl palmitate not at all. Rod outer segment membranes however take up relatively small amounts of retinoids (retinylpalmitate less than retinol less than retinal) and rapidly lose 60-90% of their label in the presence of PC-vesicles. E.G. retinoids clearly favour the PC-vesicle membrane. Apparently, rod outer segment membranes have a much lower affinity for retinoids than other artificial or natural membranes investigated so far.

Visual cycle in the mammalian eye

This work was designed to provide an insight into the mammalian visual cycle by investigating the possible function of retinoid-binding proteins in this system, and the distribution and type of 11-cis retinoids present in the interphotoreceptor matrix and the cytosols of the retinal pigment epithelium and retina. The total retinol and retinal in the soluble fractions from these three compartments was 8% (3.31 nmol/eye) of the retinyl palmitate and stearate stored in the pigment epithelium membrane fractions (39 nmol/eye). Only small amounts of retinoids were detected in the rod outer segment cytosol. The insoluble fractions also contained retinol, nearly all of which was found in the retina. The retinoids in the soluble fractions appeared to be bound to cellular retinol-binding protein (CRBP), cellular retinal-binding protein (CRA1BP) and interstitial retinol-binding protein (IRBP, a high-Mr glycoprotein). Using immunospecific precipitation, immunoblot and immunocytochemical techniques it was demonstrated that IRBP was localized in the interphotoreceptor matrix and was synthesized and secreted by the retina, a process that did not require the protein to be glycosylated. The amount of retinol bound to IRBP increased if the eyes were exposed to light, when it was estimated that the protein carried up to 30% of its full capacity for all-trans retinol. In addition to all-trans retinol, IRBP carried smaller amounts of 11-cis retinol. The proportion of 11-cis retinol was frequently higher in eyes that had been protected from illumination, suggesting that IRBP plays a role in rhodopsin regeneration during dark-adaptation. Additionally, endogenous 11-cis retinoids in the retina and RPE cytosols were bound to an Mr 33,000 protein tentatively identified as CRA1BP. The 11-cis retinoid in the retina cytosol was mainly in the form of retinol, while in the RPE cytosol it was mainly in the form of retinal. Substantial amounts of 11-cis retinol were also found in the insoluble (membrane) fraction from the retina. It is suggested that in the mammalian retina 11-cis retinol is generated from all-trans retinol (possibly in the Muller cells). Lack of an 11-cis retinol oxidoreductase in the retina prevents it from being utilized for rhodopsin regeneration until it has been transported to the pigment epithelium, where it is converted to 11-cis retinal and returned to the rod outer segments. It is also suggested that IRBP may be implicated in the transport of retinoids between the rod outer segments, the Muller cells and the pigment epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)

Suitability of retinol, retinal and retinyl palmitate for the regeneration of bleached rhodopsin in the isolated frog retina

The all-trans, 9-cis and 11-cis isomers of retinol, retinal and retinyl palmitate were incorporated into unilamellar dioleoyl-lecithin vesicles (liposomes). Isolated frog retinas were inserted into a perfusion chamber and their absorption spectra were recorded (i) in the dark-adapted state, (ii) after exposure to intense light that bleached greater than 95% of rhodopsin and (iii) after liposomes had been added to the perfusate of the bleached preparations. In each experiment, one of the above isomers of the retinol analogs was tested. Regeneration was promoted by the 9-cis and the 11-cis isomer of retinol and of retinal whereas the trans isomers of all compounds and the cis isomers of retinyl palmitate were inactive.

On the evaluation of photoreceptor properties by micro-fluorimetric measurements of fluorochrome diffusion

By use of the microfluorimetric technique it is possible to study the diffusion of the fluorochrome di-dansylcystine (DDC) within isolated frog rod outer segments (ros) which are immobilysed in agarose gel. For this purpose, by a short hypotonic shock a leak is applied to one end of the ros. By this open end the DDC enters the rod and migrates through the whole outer segment. Following the propagation of the fluorescence boundary with time the cytoplasmatic diffusion constant can be determined if a chromatographic model is used to allow for the considerable binding of DDC to the inner membrane surface. With a binding constant K = 5 . 10(-4) cm the cytoplasmatic diffusion constant was found to be D = 1.3 . 10(-6) cm2/s whereas Dg = 2 . 10(-6 cm2/s and Dr = 3.5 . 10(-6) cm2/s were found in agarose gel or ringer solution, respectively. Using the mobility reduction factor given by D/Dr approximately equal to 0.4 to calculate the cytoplasmatic conductivity an inner resistance per length of 1.7 M omega/mu could be calculated for a frog rod which is in good agreement with corresponding data obtained from electrophysiological measurements.