Light-activated retinoid transport in cephalopod photoreceptors

Rho GTPases regulate rhabdom morphology in octopus photoreceptors

In the cephalopod retina, light/dark adaptation is accompanied by a decrease/increase in rhabdom size and redistribution of rhodopsin and retinochrome. Rearrangements in the actin cytoskeleton probably govern changes in rhabdom size by regulating the degradation/formation of rhabdomere microvilli. Photopigment movements may be directed by microtubules present in the outer segment core cytoplasm. We believe that rhodopsin activation by light stimulates Rho and Rac signaling pathways, affecting these cytoskeletal systems and their possible functions in controlling rhabdom morphology and protein movements. In this study, we localized cytoskeletal and signaling proteins in octopus photoreceptors to determine their concurrence between the lighting conditions. We used toxin B from Clostridium difficile to inhibit the activity of Rho/Rac and observed its effect on the location of signaling proteins and actin and tubulin. In both lighting conditions, we found Rho in specific sets of juxtaposed rhabdomeres in embryonic and adult retinas. In the light, Rho and actin were localized along the length of the rhabdomere, but, in the dark, both proteins were absent from a space beneath the inner limiting membrane. Rac colocalized with tubulin in the outer segment core cytoplasm and, like Rho, the two proteins were also absent beneath the inner limiting membrane in the dark. The distribution of actin and Rho was affected by toxin B and, in dark-adapted retinas, actin and Rho distribution was similar to that observed in the light. Our results suggest that the Rho/Rac GTPases are candidates for the regulation of rhabdomere size and protein movements in light-dark-adapted octopus photoreceptors.

Retinoid cycling proteins redistribute in light-/dark-adapted octopus retinas

In cephalopods, the complex rhodopsin-retinochrome system serves to regenerate metarhodopsin and metaretinochrome after illumination. In the dark, a soluble protein, retinal-binding protein (RALBP), shuttles 11-cis retinal released from metaretinochrome located in the photoreceptor inner segments to metarhodopsin present in the rhabdoms. While in the rhabdoms, RALBP delivers 11-cis retinal to regenerate rhodopsin and in turn binds the all-trans isomer released by metarhodopsin. RALBP then returns all-trans retinal to the inner segments to restore retinochrome. The conventional interpretation of retinoid cycling is contradicted by immunocytochemical studies showing that, in addition to rhodopsin, retinochrome is present in the rhabdomal compartment, making possible the direct exchange of chromophores between the metapigments with the potential exclusion of RALBP. By using immunofluorescence and laser scanning confocal microscopy, we have precisely located opsin, aporetinochrome, and RALBP in light-/dark-adapted octopus retinas. We found differences in the distribution of all three proteins throughout the retina. Most significantly, comparison of cross sections though light- and dark-adapted rhabdoms showed a dramatic shift in position of the proteins. In the dark, opsin and retinochrome colocalized at the base of the rhabdomal microvilli. In the light, opsin redistributed along the length of the microvillar membranes, and retinochrome retreated to a location that is perhaps extracellular. RALBP was present in the core cytoplasm of the photoreceptor outer segments in the dark, and RALBP moved to the periphery in the light. Because of the colocalization of opsin and retinochrome in the dark, we believe that the two metapigments participate directly in chromophore exchange. RALBP may serve to transport additional chromophore from the inner segments to the rhabdoms and may not be immediately involved in the exchange process.

Distribution of three retinal proteins in developing octopus photoreceptors

The expression of proteins unique to plasma membrane domains of developing photoreceptors is used as a marker for retinal differentiation in vertebrates. Invertebrate photoreceptors are also compartmentalized, but little information is available on the development of these compartments or the expression of retinal proteins specific to these cellular regions. Using routine electron microscopy techniques, we have made observations on the formation of photoreceptor organelles, including myeloid bodies and rhabdomeres, in embryonic octopus eyes from an early stage in development through hatching. Immunocytochemical experiments on the embryos demonstrate a timed expression of three retinal proteins during development, and the early separation of the octopus photoreceptor plasma membrane into distinct domains. Using polyclonal antibodies for opsin, retinochrome and retinal binding protein we have shown that opsin appears first and is confined to the distal end of the photoreceptor that will eventually differentiate into rhabdomeres. This membrane domain is separated from the proximal/inner segment plasma membrane by a septate junction. Retinochrome is expressed later when the myeloid bodies appear in the inner segments, and retinal binding protein is apparently not synthesized until sometime after hatching. These results suggest that, in the cephalopod retina, protein components of the retinoid cycling apparatus appear in a specific developmental sequence during the differentiation of this tissue.

Immunocytochemical localization of retinal binding protein in the octopus retina: a shuttle protein for 11-cis retinal

The cephalopod retina contains two photopigments that are spatially separated within the photoreceptors; rhodopsin, localized in the light-sensitive rhabdoms, and retinochrome, present in the myeloid bodies of the photoreceptor inner segments. In the light, the chromophore of retinochrome, all-trans retinal, is photoisomerized to 11-cis to form metaretinochrome. Metaretinochrome is believed to serve as a store for 11-cis retinal used in the regeneration or biosynthesis of rhodopsin. Previous studies suggest that a soluble retinal binding protein (RALBP) serves as a shuttle between retinochrome and rhodopsin, and, in the dark, may transport chromophore from the myeloid bodies to the rhabdoms. Our study supports this hypothesis and demonstrates that RALBP is in the correct cellular locations to function as a shuttle. Dark- and light-adapted octopus retinas were labeled with anti-RALBP using immunofluorescence and immunogold techniques. Our results showed that RALBP was distributed differently in the dark- and light-adapted retinas. Our most significant observation was that myeloid bodies from light-adapted retinas were more heavily labeled by anti-RALBP than myeloid bodies in dark-adapted retinas. The rhabdomeres, interphotoreceptor matrix, and inner limiting membrane were also labeled in both light and dark conditions. Based on these results and evidence from previous biochemical studies, we conclude that in the dark RALBP leaves the myeloid bodies and transports 11-cis retinal to the rhabdoms where chromophore exchange with metarhodopsin may occur.

Microscopic and biochemical characterization of lectin binding sites in the cephalopod retina

Using light and electron microscope cytochemistry and lectin blotting techniques, we have shown that the lectins concanavalin A (Con A), Ricinus communis agglutinin (RCA), and peanut agglutinin (PNA) bind to specific glycoconjugants in the adult cephalopod retina. For light microscope lectin cytochemistry, aldehyde-fixed, frozen, or Araldite-embedded, etched sections of cephalopod retinas were incubated with FITC- or TRITC-conjugated lectins and examined by using epifluorescence microscopy. Con A labeled structures in the entire retina including the inner limiting membrane (ILM), rhabdomeric membranes, interphotoreceptor matrix (IPM), and structures in the photoreceptor inner segments. RCA labeling was similar to that of Con A except that there was a decrease in the staining of the rhabdom tips near the ILM. PNA labeled only the interphotoreceptor matrix between apposing rhabdomeres. The intensity of staining of the IPM by PNA also decreased or was absent toward the rhabdom tips. None of the lectins labeled the myeloid bodies located in the photoreceptor inner segments. Electron microscope (EM) lectin cytochemistry was performed on aldehyde-fixed, LR White-embedded tissue or on Araldite-embedded, periodate-etched sections by using gold-conjugated lectins. EM results confirmed the observations made by light microscopy. Lectin blots with a retinal extract or light-sensitive membrane fraction revealed a variety of protein bands labeled by all three lectins. Con A and RCA labeled opsin and its aggregates whereas PNA did not. None of the lectins labeled retinochrome. The labeling of the cephalopod IPM by PNA suggests a structural similarity between the IPM of vertebrates and invertebrates. In other studies, we have demonstrated the presence of a retinoid binding protein in the IPM of cephalopods.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinal-binding protein as a shuttle for retinal in the rhodopsin-retinochrome system of the squid visual cells

The molluscan visual cell is characterized by having two photopigment systems, rhodopsin and retinochrome. In connection with these systems, located separately in the rhabdomal microvilli and in the nucleated cell bodies, the physiological role of retinal-binding protein (RALBP) was investigated in the squid (Todarodes pacificus) by using 3-dehydroretinal (retinal 2) as a tracer for retinal chromophore. In dark-adapted eyes, squid RALBP is combined abundantly with 11-cis-retinal. However, upon incubation with an excess of all-trans-retinal or retinol, RALBP took up great amounts of each of them, releasing its native retinoid ligands. When an all-trans-retinal-rich RALBP thus produced was incubated in the dark with metaretinochrome 2-carrying membranes, the RALBP released all-trans-retinal to the membranes to regenerate retinochrome, taking up 11-cis-retinal 2 from metaretinochrome 2. Upon further incubation of this 11-cis-retinal 2-rich RALBP with metarhodopsin-carrying membranes, the RALBP released the 11-cis-retinal 2 to the membranes to form rhodopsin 2, receiving all-trans-retinal from metarhodopsin. These findings show that squid RALBP is capable of serving as a shuttle during the recycling of retinal in the rhodopsin-retinochrome conjugate system to maintain the photoreceptive function of the visual cells.

IRBP-like proteins in the eyes of six cephalopod species--immunochemical relationship to vertebrate interstitial retinol-binding protein (IRBP) and cephalopod retinal-binding protein

SDS polyacrylamide gel electrophoresis and immunoblotting were used to examine soluble proteins from the eyes of six species of cephalopods i.e. Lolliguncula brevis, Sepia officinalis, Octopus maya, Octopus bimaculoides, Rossia pacifica and Loligo opalescens. All species had a protein ("IRBP") with molecular weight virtually identical with vertebrate interstitial retinol-binding protein (IRBP) averaging 132,400 +/- 700 (n = 6). "IRBP" reacted on nitrocellulose blot transfers with rabbit antibovine IRBP and rabbit antifrog IRBP antibodies. Unlike vertebrate IRBP, cephalopod "IRBP" (from L. brevis) did not bind exogenous retinol or concanavalin A. The N-terminal amino acid appeared to be blocked in samples electroeluted from SDS gels. The antifrog IRBP antibodies also reacted with a series of proteins with molecular weights between 46,000 and 47,000, identified as retinal-binding protein (RALBP) with anti-RALBP antibodies. Anti-IRBP also reacted with pure RALBP prepared from Todarodes pacificus. Occasionally, anti-RALBP antibodies were seen to react weakly with "IRBP" in some cephalopods. We conclude that RALBP, cephalopod "IRBP" and vertebrate IRBP share a common but distant ancestry, and that a protein resembling IRBP appeared before the vertebrates diverged from the invertebrates. Both RALBP and IRBP appear to have analogous functions in shuttling retinoids between rhodopsin and the corresponding isomerizing system, retinochrome in the cephalopods and retinol isomerase in the vertebrates. The function of cephalopod "IRBP" is unknown.

Immunocytochemical localization of photopigments in cephalopod retinae

The photopigments, rhodopsin and retinochrome, have been localized in cephalopod retinae using light and electron microscopic immunocytochemical methods. Polyclonal antibodies prepared against squid opsin demonstrated the presence of this protein in the photoreceptor rhabdomes, Golgi zone, Golgi-associated vesicles, plasma membrane, large cytoplasmic vesicles, and axonal membranes of octopus retinae. Monoclonal anti-opsin immunostained the rhabdomes and multivesicular bodies in the photoreceptor inner segments of squid. We believe the multivesicular bodies are involved in rhabdome turnover. Polyclonal anti-retinochrome localized this photopigment to the myeloid bodies of the photoreceptor inner segments, the rhabdomes, and to the extracellular space between opposing rhabdomeres in octopus retina. The results suggest some interesting functional relationships between rhodopsin and retinochrome with regard to chromophore exchange between illuminated forms of these photopigments and chromophore addition to newly synthesized opsin.

Isolation and characterization of a retinal-binding protein from the squid retina

A retinal-binding protein (RALBP) was isolated from the squid retina, and purified by anion-exchange and size-exclusion chromatography. The molecular weight was determined to be 51,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by gel filtration. The purified sample showed absorption maxima at about 330 and 400 nm in addition to a protein band, indicating the occurrence of retinol and retinal, respectively. The relative heights of these two peaks varied from preparation to preparation, depending on retinoid ligands. Irradiation of RALBP caused no marked change in absorption, but the amount of 11-cis-retinal decreased to form a photosteady state mixture with all-trans- and 13-cis-retinals. RALBP was fairly stable even in the presence of hydroxylamine (100 mM), but was affected by sodium borohydride (30 mM) or borane dimethylamine (400 mM), with the retinal reduced to retinol. When incubated with metaretinochrome-carrying membranes in the dark, RALBP specifically took up 11-cis-retinal and lost all-trans-retinol. Upon further incubation of this RALBP with opsin-containing membranes, rhodopsin was progressively formed in the dark. Squid RALBP may act as a shuttle in transferring the 11-cis-retinal from metaretinochrome to opsin in the visual cells.