Effect of light-adaptation on the binding of 48-kDa protein (S-antigen) to photoreceptor cell membranes

Light induces translocation of NF-κB p65 to the mitochondria and suppresses expression of cytochrome c oxidase subunit III (COX III) in the rat retina

The transcription factor nuclear factor kappaB (NF-κB) plays various roles in cell survival, apoptosis, and inflammation. In the rat retina, NF-κB activity increases after exposure to damaging light, resulting in degeneration of photoreceptors. Here, we report that in dark-adapted rats exposed for 6 h to bright white light, the p65 subunit of retinal NF-κB translocates to the mitochondria, an event associated with a decrease in expression of cytochrome c oxidase subunit III (COX III). However, sustained exposure for 12 h depleted p65 from the mitochondria, and enhanced COX III expression. Treatment with the protective antioxidant PBN prior to light exposure prevents p65 depletion in the mitochondria and COX III upregulation during prolonged exposure, and apoptosis in photoreceptor cells. These results indicate that COX III expression is sensitive to the abundance of NF-κB p65 in the mitochondria, which, in turn, is affected by exposure to damaging light.

Diacylglyceride lipase activity in rod outer segments depends on the illumination state of the retina

We have demonstrated that the competition between phosphatidic acid (PA) and lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P) and ceramide 1-phosphate (C1P) for lipid phosphate phosphatases (LPP) generates different levels of diacylglycerol (DAG) depending on the illumination state of the retina. The aim of the present research was to determine the diacylglyceride lipase (DAGL) activity in purified rod outer segments (ROS) obtained from dark-adapted retinas (DROS) or light-adapted retinas (BLROS) as well as in ROS membrane preparations depleted of soluble and peripheral proteins. [2-(3)H]monoacylglycerol (MAG), the product of DAGL, was evaluated from [2-(3)H]DAG generated by LPP action on [2-(3)H]PA in the presence of either LPA, S1P or C1P. MAG production was inhibited by 55% in BLROS and by 25% when the enzymatic assay was carried out in ROS obtained from dark-adapted retinas and incubated under room light (LROS). The most important events occurred in DROS where co-incubation of [2-(3)H]PA with LPA, S1P or C1P diminished MAG production. A higher level of DAGL activity was observed in LROS than in BLROS, though this difference was not apparent in the presence of LPA, S1P or C1P. DAGL activity in depleted DROS was diminished with respect to that in entire DROS. LPA, S1P and C1P produced a similar decrease in MAG production in depleted DROS whereas only C1P significantly diminished MAG generation in depleted BLROS. Sphingosine and ceramide inhibited MAG production in entire DROS and stimulated its generation in BLROS. Sphingosine and ceramide stimulated MAG generation in both depleted DROS and BLROS. Under our experimental conditions the degree of MAG production depended on the illumination state of the retina. We therefore suggest that proteins related to phototransduction phenomena are involved in the effects observed in the presence of S1P/sphingosine or C1P/ceramide.

Functional comparisons of visual arrestins in rod photoreceptors of transgenic mice

PURPOSE

To examine the biochemical characteristics of rod and cone arrestin with respect to their ability to quench the activity of light-activated rhodopsin in transgenic mice.

METHODS

The mouse rod opsin promoter was used to drive expression of mouse cone arrestin in rod photoreceptor cells of rod arrestin knockout (arr1-/-) mice. Suction electrode recordings from single rods were performed to investigate cone arrestin's ability to quench the catalytic activity of light-activated rhodopsin. In addition, the ability of cone arrestin to prevent light-induced retinal damage caused by prolonged activation of the phototransduction cascade was assessed.

RESULTS

Two independent lines of transgenic mice were obtained that expressed cone arrestin in rod photoreceptors, and each was bred into the arr1-/- background. Flash responses measured by suction electrode recordings showed that cone arrestin reduced signaling from photolyzed rhodopsin but was unable to quench its activity completely. Consistent with this observation, expression of mouse cone arrestin conferred dose-dependent protection against photoreceptor cell death caused by low light exposure to arr1-/- retinas, but did not appear to be as effective as rod arrestin.

CONCLUSIONS

Cone arrestin can partially substitute for rod arrestin in arr1-/- rods, offering a degree of protection from light-induced damage and increasing the extent of rhodopsin deactivation in response to flashes of light. Although earlier work has shown that rod arrestin can bind and deactivate cone pigments efficiently, the results suggest that cone arrestin binds light-activated, phosphorylated rhodopsin less efficiently than does rod arrestin in vivo. These results suggest that the structural requirements for high-affinity binding are fundamentally distinct for rod and cone arrestins.

Stable rhodopsin/arrestin complex leads to retinal degeneration in a transgenic mouse model of autosomal dominant retinitis pigmentosa

Over 100 rhodopsin mutation alleles have been associated with autosomal dominant retinitis pigmentosa (ADRP). These mutations appear to cause photoreceptor cell death through diverse molecular mechanisms. We show that K296E, a rhodopsin mutation associated with ADRP, forms a stable complex with arrestin that is toxic to mouse rod photoreceptors. This cell death pathway appears to be conserved from flies to mammals. A genetics approach to eliminate arrestin unmasked the constitutive activity of K296E and caused photoreceptor cell death through a transducin-dependent mechanism that is similar to light damage. Expressing K296E in the arrestin/transducin double knock-out background prevented transducin signaling and led to substantially improved retinal morphology but did not fully prevent cell death caused by K296E. The adverse effect of K296E in the arrestin/transducin knock-out background can be mimicked by constant exposure to low light. Furthermore, we found that arrestin binding causes K296E to mislocalize to the wrong cellular compartment. Accumulation of stable rhodopsin/arrestin complex in the inner segment may be an important mechanism for triggering the cell death pathway in the mammalian photoreceptor cell.

Two stages of light-dependent TRPL-channel translocation in Drosophila photoreceptors

Transient receptor potential (TRP) channels across species are expressed in sensory receptor cells, and often localized to specialized subcellular sites. In Drosophila photoreceptors, TRP-like (TRPL) channels are localized to the signaling compartment, the rhabdomere, in the dark, and undergo light-induced translocation into the cell body as a mechanism for long-term light-adaptation. We show that translocation of TRPL channels occurs in two distinct stages, first to the neighboring stalk membrane then to the basolateral membrane. In the first stage, light-induced translocation occurs within 5 minutes, whereas the second stage takes over 6 hours. The exclusive apical localization of TRPL channels in the first stage of translocation suggests that channels are released from the rhabdomere and diffuse laterally through the membrane into the adjoining stalk membrane. In the second stage, TRPL channels are localized in the basolateral membrane, implicating a different transport mechanism. Genetic analyses suggest that activation of the other light-activated TRP channel and eye-protein-kinase C (eye-PKC) are both required for the second stage of TRPL translocation in R1 to R6 photoreceptor cells, whereas only phospholipase C (PLC) is required for the first stage. Finally, we show that arrestin2 is required for the rhabdomeric localization and stability of TRPL channels.

Quantification of the cytoplasmic spaces of living cells with EGFP reveals arrestin-EGFP to be in disequilibrium in dark adapted rod photoreceptors

The hypothesis is tested that enhanced green fluorescent protein (EGFP) can be used to quantify the aqueous spaces of living cells, using as a model transgenic Xenopus rods. Consistent with the hypothesis, regions of rods having structures that exclude EGFP, such as the mitochondrial-rich ellipsoid and the outer segments, have highly reduced EGFP fluorescence. Over a 300-fold range of expression the average EGFP concentration in the outer segment was approximately half that in the most intensely fluorescent regions of the inner segment, in quantitative agreement with prior X-ray diffraction estimates of outer segment cytoplasmic volume. In contrast, the fluorescence of soluble arrestin-EGFP fusion protein in the dark adapted rod outer segment was approximately threefold lower than predicted by the EGFP distribution, establishing that the fusion protein is not equilibrated with the cytoplasm. Arrestin-EGFP mass was conserved during a large-scale, light-driven redistribution in which approximately 40% of the protein in the inner segment moved to the outer segment in less than 30 minutes.

Light-dependent subcellular translocation of Gqα in Drosophila photoreceptors is facilitated by the photoreceptor-specific myosin III NINAC

We examine the light-dependent subcellular translocation of the visual Gqα protein between the signaling compartment, the rhabdomere and the cell body in Drosophila photoreceptors. We characterize the translocation of Gqα and provide the first evidence implicating the involvement of the photoreceptor-specific myosin III NINAC in Gqα transport. Translocation of Gqα from the rhabdomere to the cell body is rapid, taking less than 5 minutes. Higher light intensities increased the quantity of Gqα translocated out of the rhabdomeres from 20% to 75%, consistent with a mechanism for light adaptation. We demonstrate that translocation of Gqα requires rhodopsin, but none of the known downstream phototransduction components, suggesting that the signaling pathway triggering translocation occurs upstream of Gqα. Finally, we show that ninaC mutants display a significantly reduced rate of Gqα transport from the cell body to the rhabdomere, suggesting that NINAC might function as a light-dependent plus-end motor involved in the transport of Gqα.

Light-dependent translocation of arrestin in the absence of rhodopsin phosphorylation and transducin signaling

Visual arrestin plays a crucial role in the termination of the light response in vertebrate photoreceptors by binding selectively to light-activated, phosphorylated rhodopsin. Arrestin localizes predominantly to the inner segments and perinuclear region of dark-adapted rod photoreceptors, whereas light induces redistribution of arrestin to the rod outer segments. The mechanism by which arrestin redistributes in response to light is not known, but it is thought to be associated with the ability of arrestin to bind photolyzed, phosphorylated rhodopsin in the outer segment. In this study, we show that light-driven translocation of arrestin is unaffected in two different mouse models in which rhodopsin phosphorylation is lacking. We further show that arrestin movement is initiated by rhodopsin but does not require transducin signaling. These results exclude passive diffusion and point toward active transport as the mechanism for light-dependent arrestin movement in rod photoreceptor cells.

Crystallization and preliminary X-ray analysis of arrestin from bovine rod outer segment

We present the first X-ray study of a member of the arrestin family, the bovine retinal arrestin. Arrestin is essential for the fine regulation and termination of the light-induced enzyme cascade in vertebrate rod outer segments. It plays an important role in quenching phototransduction by its ability to preferentially bind to phosphorylated light-activated rhodopsin. The crystals diffract between 3 angstroms and 3.5 angstroms (space group P2(1)2(1)2, cell dimensions a = 169.17 angstroms, b = 185.53 angstroms, c = 90.93 angstroms, T = 100 K). The asymmetric unit contains four molecules with a solvent content of 68.5% by volume.

Circadian aqueous flow mediated by beta-arrestin induced homologous desensitization

It was hypothesized that homologous desensitization regulates signal transduction from the beta-adrenergic receptor in the ocular ciliary epithelium to affect the circadian rhythm of aqueous humor secretion. beta-arrestin-1 was cloned from the rabbit ciliary epithelium, and the full length cDNA used as a probe for Northern blot analysis to examine the diurnal expression of beta-arrestin mRNA. Protein expression of beta-arrestin-1 at intervals during the circadian cycle of aqueous secretion showed a decrease in beta-arrestin expression when maximal activation of the beta-adrenergic receptor is known to increase secretion. Diurnal expression of beta-arrestin suggests that homologous desensitization can regulate the circadian rhythm of aqueous flow.

Effect of hydroxylamine on the subcellular distribution of arrestin (S-antigen) in rod photoreceptors

The immunocytochemical labeling of arrestin (S-antigen) in photoreceptors of the ovine retina was examined following incubation of the retina with hydroxylamine (NH2OH), an agent known to inhibit the phosphorylation of photoactivated rhodopsin. Intact, isolated retinas bathed in medium containing 20 mM NH2OH, or in control medium lacking NH2OH, were maintained in darkness or exposed to bright light for 3 min (dark-adapted and light-adapted conditions, respectively); further incubated in darkness for 10 min; and then fixed and prepared for cryosectioning. Cryosections were incubated with anti-S-antigen monoclonal antibody MAb A2G5; with secondary antibodies that were conjugated with horseradish peroxidase; and with either 3-amino-9-ethyl carbazole or diaminobenzidine as chromogen. Anti-arrestin labeling in cryosections was then analyzed densitometrically using a light-microscopic image processing system. In dark-adapted control retinas, labeling density of the photoreceptor outer segment (OS) layer (0.061 +/- 0.004; average +/- S.E.M.) was less than that of the inner segment (IS) layer (0.138 +/- 0.011). In light-adapted control retinas, OS labeling density (0.139 +/- 0.007) exceeded IS labeling density (0.095 +/- 0.005). Incubation with NH2OH eliminated this light-dependent increase in labeling of the OS relative to that of the IS, i.e. eliminated the increase in relative OS/IS labeling. Densities of labeling were 0.110 +/- 0.006 (OS) and 0.183 +/- 0.006 (IS) in NH2OH-treated dark-adapted retinas vs. 0.078 +/- 0.004 (OS) and 0.182 +/- 0.008 (IS) in NH2OH-treated light-adapted retinas. Anti-arrestin labeling was also examined in retinas that were exposed to 3 min or 13 min of bright light and then immediately fixed.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of arrestin (48K protein) and rhodopsin kinase on visual transduction

The shutoff of the phototransduction cascade in retinal rods requires the inactivation of light-activated rhodopsin. The underlying mechanisms were studied in functionally intact detached rod outer segments by testing the effect of either sangivamycin, an inhibitor of rhodopsin kinase, or phytic acid, an inhibitor of 48K protein binding to phosphorylated rhodopsin, on light responses recorded in whole-cell voltage clamp. The results suggest that isomerized rhodopsin is inactivated fully by multiple phosphorylation and that the binding of 48K protein accelerates recovery by quenching partially phosphorylated rhodopsin. Higher concentrations of sangivamycin cause changes in the light response that cannot be explained by selective inhibition of rhodopsin kinase and suggest that other protein kinases are needed for normal rod function.

Levels of mRNA encoding proteins of the cGMP cascade as a function of light environment

The levels of the retinal mRNAs encoding opsin, the alpha subunit of rod transducin (T alpha), S-antigen (S-ag) and the gamma subunit of rod-specific cGMP-phosphodiesterase (cGMP-PDE) were measured in rats reared in cyclic light or darkness and after adaptation for different periods of time to the opposite light environment. We found that rats changed from cyclic light to darkness, gradually increased their retinal content of opsin and T alpha mRNAs but decreased their levels of S-ag mRNA. The reverse results were obtained when rats were changed from darkness to cyclic light. In contrast, the levels of retinal cGMP-PDE gamma mRNA remained unchanged in animals adapted to either of the two rearing light conditions. Our results indicate that some mRNAs encoding proteins associated with the cGMP cascade are responsive to environmental lighting and may be involved in the long term light or dark adaptive processes.

Adaptive changes in visual cell transduction protein levels: effect of light

Long-term environmental light-mediated changes in visual cell transduction proteins were studied to assess the influence of rearing environment on their levels and their potential effects on intense light-induced retinal damage. The levels of rhodopsin, S-antigen and the alpha subunit of transducin were measured in whole eye detergent extracts, retinal homogenates or rod outer segments isolated from rats reared in weak cyclic light or darkness, and following a change in rearing environment. Rats changed from weak cyclic light to darkness had 22% more rhodopsin per eye than cyclic-light rats after 12-14 days in the new environment. Western trans-blot analysis of retinal proteins from these dark-maintained animals contained 65% higher levels of immunologically detectable alpha transducin; S-antigen levels were approximately 45% lower than in cyclic-light rats. In rats changed from the dark environment to weak cyclic light, rhodopsin levels decreased by 18% during a comparable period; retinal alpha transducin was 35% lower, S-antigen was 30% higher. At various times after the change in rearing environment, some rats were exposed to intense visible light to determine their susceptibility to retinal damage. Two weeks after an 8-hr exposure, cyclic-light reared rats had rhodopsin levels only 10% lower than control (2.1 nmol per eye). However, rhodopsin was 75% lower when cyclic-light rats were maintained in darkness for 2 weeks before intense light. For animals originally reared in darkness, rhodopsin was 78% lower following 8 hr of intense light, whereas only 30% rhodopsin loss occurred in dark-reared rats after previous maintenance for 2 weeks in weak cyclic-light.(ABSTRACT TRUNCATED AT 250 WORDS)

Calbindin and calretinin localization in retina from different species

Calbindin-D28K and calretinin are homologous calcium-binding proteins localized in many neurons of the central nervous systems. We have compared polyclonal antibodies against calbindin and calretinin and have shown by western blots using purified calbindin and calretinin from rat that (1) anti-calretinin does not recognize calbindin and (2) anti-calbindin presents some cross-reactivity with calretinin. In this report, we have compared by immunohistochemistry the localization of both calcium-binding proteins in the retina of monkey, pig, sheep, rat, cat, pigeon, and salamander. These results are compared with previous data for chick. There are many differences between species and not within species, but some aspects of the distribution are conserved. All species, except rat and monkey, have some cones which contain calbindin only. Most species also have some bipolar cells containing calbindin only. Calretinin is rarely seen in photoreceptors or bipolar cells. All species have horizontal cells which contain calretinin or calbindin or both. All species have amacrine cells and ganglion cells containing one or other protein. In the cat ganglion cell layer, the calretinin antisera define a new, asymmetric, type of cell.

Kinetics, binding constant, and activation energy of the 48-kDa protein-rhodopsin complex by extra-metarhodopsin II

We have found that the 48-kDa protein (or S-antigen 48k) of the rod photoreceptor enhances the light-induced formation of the photoproduct metarhodopsin II (MII) from prephosphorylated rhodopsin. The effect is analogous to the known enhancement of MII (extra-MII) that results from selective interaction of MII with G-protein. We have determined some parameters of the MII-48k interaction by measuring the extra-MII absorption change induced by the 48-kDa protein. The amplitude saturation yields a dissociation constant for the MII-48k complex on the order of 50 nM. At the technical limit of these measurements, 13.7 degrees C and 12 microM 48-kDa protein, we find a rate of 2.3 s-1 for formation of the 48k-MII complex. Extrapolation of these values to cellular conditions yields an occupation time of phosphorylated MII by 48k less than 200 ms. This is short compared to estimated rates of phosphorylation. The temperature dependence of the MII-48k formation rate is very high (Q10 for 5 degrees C/15 degrees C = 9-10). The related Arrhenius activation energy (165 kJ mol-1) is correspondingly high and indicates a considerable transient chemical change during the binding process.

Changes in immunoreactivity in S-antigen-induced pinealitis

Changes in immunoreactivity in rat pineal gland were investigated in S-antigen (S-Ag)-induced pinealitis. Anti-S antibodies were used to study the accessibility of pathogenic epitopes in the organ, and phosphatidylinositol 4,5-bisphospate (PI[4,5]P2) antibodies were chosen to study changes in functional phospholipid in membranes. Control pineal gland (without inflammation) did not show much reactivity toward these antibodies. Immunoreactivity toward both antibodies was markedly increased throughout the organ at the peak of inflammation and decreased almost to control level in the postinflammatory stage. The rise and fall of immunoreactivity were attributed to changes in the accessibility of antigenic sites rather than changes in the amounts of S-Ag and P(4,5)P2. The transient enhancement in immunoreactivity, probably consequential to tissue damage, suggests that the pineal gland, unlike the retinal photoreceptors, is capable of quick repair of tissue.