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PubMed2004

Opsin photoisomerases in the chick retina and pineal gland: characterization, localization, and circadian regulation.

PURPOSE: The chick retina and pineal gland exhibit circadian oscillations in biochemical and physiological processes in vivo and in vitro, which entrain to 24-hour light-dark cycles. However, the phototransduction mechanisms responsible for entrainment are largely unknown. The present study characterizes two candidate opsinlike genes that may play a role in entrainment of the clocks in these tissues. METHODS: Bioinformatics, cladistic techniques, and in situ hybridization and Northern blot analyses were conducted to characterize, localize, and determine the circadian expression of the candidate opsinlike genes in the retina and pineal gland. RESULTS: Two candidate photosensors and/or photoisomerases were predominantly distributed within the pineal gland and retina: the retinal pigmented epithelium-derived rhodopsin homologue (peropsin, gRrh) and retinal G-protein-coupled receptor opsin (RGR opsin, gRgr). Northern blot and in situ analyses revealed mRNA expression for both opsins in the pineal gland, retina, and brain tissue. The mRNA for both genes within the pineal gland and retina is regulated on a circadian basis such that they are highest in late subjective day. Digoxigenin in situ analyses showed retinal gRgr message within the inner nuclear layer (INL) and retinal ganglion cell layer (RGL), whereas gRrh message was distributed predominantly in the RGL. In the pineal gland, gRgr message was sparsely distributed among pinealocytes in follicles, but not within the follicles themselves, whereas gRrh was localized in interstitial areas indicative of astrocytic and/or endothelial origin. CONCLUSIONS: The presence of two putative photoisomerases within the pineal gland and in retinal layers associated with biological clock function provides two candidate opsinlike genes that may serve in the visual cycle regulation of the circadian clock.

Amino Acid Sequence↗
PubMed2003

Molecular evolution of bat color vision genes.

The two suborders of bats, Megachiroptera (megabats) and Microchiroptera (microbats), use different sensory modalities for perceiving their environment. Megabats are crepuscular and rely on a well-developed eyes and visual pathway, whereas microbats occupy a nocturnal niche and use acoustic orientation or echolocation more than vision as the major means of perceiving their environment. In view of the differences associated with their sensory systems, we decided to investigate the function and evolution of color vision (opsin genes) in these two suborders of bats. The middle/long wavelength (M/L) and short wavelength (S) opsin genes were sequenced from two frugivorous species of megabats, Haplonycteris fischeri and Pteropus dasymallus formosus, and one insectivorous species of microbat, Myotis velifer. Contrary to the situation in primates, where many nocturnal species have lost the functional S opsin gene, both crepuscular and strictly nocturnal species of bats that we examined have functional M/L and S opsin genes. Surprisingly, the S opsin in these bats may be sensitive to UV light, which is relatively more abundant at dawn and at dusk. The M/L opsin in these bats appears to be the L type, which is sensitive to red and may be helpful for identifying fruits among leaves or for other purposes. Most interestingly, H. fischeri has a recent duplication of the M/L opsin gene, representing to date the only known case of opsin gene duplication in non-primate mammals. Some of these observations are unexpected and may provide insights into the effect of nocturnal life on the evolution of opsin genes in mammals and the evolution of the life history traits of bats in general.

Amino Acid Sequence↗
PubMed2005

Polymorphic New World monkeys with more than three M/L cone types.

Most New World (platyrrhine) monkeys have M/L cone photopigment polymorphisms that map directly into individual variations in visual sensitivity and color vision. We used electroretinogram flicker photometry to examine M/L cone photopigments in the New World monkey Callicebus moloch (the dusky Titi). Like other New World monkeys, this species has an M/L cone photopigment polymorphism that reflects the presence of X-chromosome opsin gene alleles. However, unlike other platyrrhines in which three M/L photopigments are typical, Callicebus has a total of five M/L cone photopigments. The peak sensitivity values for these pigments extend across the range from 530 to 562 nm. The result is an enhanced array of potential color vision phenotypes in this species.

Animals↗
PubMed1996

Characterization of the mutant visual pigment responsible for congenital night blindness: a biochemical and Fourier-transform infrared spectroscopy study.

A mutation in the gene for the rod photoreceptor molecule rhodopsin causes congenital night blindness. The mutation results in a replacement of Gly90 by an aspartic acid residue. Two molecular mechanisms have been proposed to explain the physiology of affected rod cells. One involves constitutive activity of the G90D mutant opsin [Rao, V. R., Cohen, G. B., & Oprian, D. D. (1994) Nature 367, 639-642]. A second involves increased photoreceptor noise caused by thermal isomerization of the G90D pigment chromophore [Sieving, P. A., Richards, J. E., Naarendorp F., Bingham, E. L., Scott, K., & Alpern, M. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 880-884]. Based on existing models of rhodopsin and in vitro biochemical studies of site-directed mutants, it appears likely that Gly90 is in the immediate proximity of the Schiff base chromophore linkage. We have studied in detail the mutant pigments G90D and G90D/E113A using biochemical and Fourier-transform infrared (FTIR) spectroscopic methods. The photoproduct of mutant pigment G90D, which absorbs maximally at 468 nm and contains a protonated Schiff base linkage, can activate transducin. However, the active photoproduct decays rapidly to opsin and free all-trans-retinal. FTIR studies of mutant G90D show that the dark state of the pigment has several structural features of metarhodopsin II, the active form of rhodopsin. These include a protonated carboxylic acid group at position Glu113 and increased hydrogen-bond strength of Asp83. Additional results, which relate to the structure of the active G90D photoproduct, are also reported. Taken together, these results may be relevant to understanding the molecular mechanism of congenital night blindness caused by the G90D mutation in human rhodopsin.

Aspartic Acid↗
PubMed2006

Polymorphism of visual pigment genes in the muriqui (Primates, Atelidae).

Colour vision varies within the family Atelidae (Primates, Platyrrhini), which consists of four genera with the following cladistic relationship: {Alouatta[Ateles (Lagothrix and Brachyteles)]}. Spider monkeys (Ateles) and woolly monkeys (Lagothrix) are characteristic of platyrrhine monkeys in possessing a colour vision polymorphism. The polymorphism results from allelic variation of the single-locus middle-to-long wavelength (M/L) cone opsin gene on the X-chromosome. The presence in the population of alleles coding for different M/L photopigments results in a variety of colour vision phenotypes. Such a polymorphism is absent in howling monkeys (Alouatta), which, alone among platyrrhines, acquired uniform trichromatic vision similar to that of Old World monkeys, apes, and humans through opsin gene duplication. Dietary and morphological similarities between howling monkeys and muriquis (Brachyteles) raise the possibility that the two genera share a similar form of colour vision, uniform trichromacy. Yet parsimony predicts that the colour vision of Brachyteles will resemble the polymorphism present in Lagothrix and Ateles. Here we test this assumption. We obtained DNA from the blood or faeces of 18 muriquis and sequenced exons 3 and 5 of the M/L opsin gene. Our results affirm the existence of a single M/L cone opsin gene in the genus Brachyteles. We detected three alleles with predicted lambdamax values of 530, 550, and 562 nm. Two females were heterozygous and are thus predicted to have different types of M/L cone pigment. We discuss the implication of this result towards understanding the evolutionary ecology of trichromatic vision.

Animals↗
PubMed2000

Electrical approach to study rhodopsin activation in single cells with early receptor current assay.

The ERC is a conformation-dependent charge motion similar to the gating currents of ionic channels. Both the waveforms and bandwidth of ERCs and ionic channel gating currents are similar, providing support to the initial suggestion that the ERP was a kind of gating current. In ionic channels the electrostatic field promotes motion of alpha-helical elements that stimulate large-scale molecular events that promote opening of the ionic pore. In ionic channels gating currents of expressed channel mutants has contributed significantly to understanding the mechanism of activation. Given the known role of electrical processes to rhodopsin activation, the ERC approach applied to mutant and wild-type visual pigments is likely to lead to a fuller understanding of the mechanism of conformational activation. This method is currently well suited to investigate the later phases of rhodopsin activation that are thought to be electrostatic in nature. We anticipate that ERC studies will make significant contributions to understanding how the breakdown of the electrostatic interaction between the PSB and its counterion is initiated and propagated to induce the proton uptake on the cytoplasmic surface of the pigment and the shaping of the transducin docking domain. We encourage collaboration to apply the ERC methodology to interesting mutant pigments and retinal analogs. We expect that the ERC methodology can soon be applied to understand rapid charge displacements associated with photochemistry (i.e., R1), the effects of transduction proteins on R2, and the measurement of electrical processes during cone visual pigment activation.

Cell Fusion↗
PubMed1988

Fenretinide does not block visual pigment formation in the rat.

Abnormal retinal function has been reported in patients receiving the synthetic retinoid fenretinide. In this study, the vitamin A deprived rat was used as a model to test the possibility that fenretinide is interfering with the vitamin A transport system and/or blocking the normal uptake of 11-cis retinal into the visual pigment. Assessing retinal function by electroretinogram measurements and visual pigment formation by competition studies, our data demonstrate normal uptake of vitamin A and normal visual pigment formation in the rat after treatment with the drug.

Animals↗
PubMed2002

Structural basis for sensory rhodopsin function.

The crystal structure of sensory rhodopsin II from Natronobacterium pharaonis was recently solved at 2.1 A resolution from lipidic cubic phase-grown crystals. A critical analysis of previous structure-function studies is possible within the framework of the high-resolution structure of this photoreceptor. Based on the structure, a molecular understanding emerges of the efficiency and selectivity of the photoisomerization reaction, of the interaction of the sensory receptor and its cognate transducer protein HtrII, and of the mechanism of spectral tuning in photoreceptors. The architecture of the retinal binding pocket is compact, representing a major determinant for the selective binding of the chromophore, all-trans retinal to the apoprotein, opsin. Several chromophore-protein interactions revealed by the structure were not predicted by previous mutagenesis and spectroscopic analyses. The structure suggests likely mechanisms by which photoisomerization triggers the activation of sensory rhodopsin II, and highlights the possibility of a unified mechanism of signaling mediated by sensory receptors, including visual rhodopsins. Future investigations using time-resolved crystallography, structural dynamics, and computational studies will provide the basis to unveil the molecular mechanisms of sensory receptors-mediated transmembrane signaling.

Amino Acid Sequence↗
PubMed1996

Synthesis of isotopically labelled retinal. Structural and functional studies at the atomic level of the chromophore in visual pigments.

OBJECTIVE: To describe the strategy followed in studying the structure and function of the chromophore in visual pigments at the atomic level. METHODS: A three-step strategy was followed. First, 11-cis retinal with high enrichment (99% 13C or 99% 2H) at pre-determined positions or combinations of positions was synthesized. Second, the rhodopsins with isotope label in the chromophore were prepared combining opsin with the isotopically labelled 11-cis retinal. Third, these systems were studied with isotope-sensitive physical techniques. CONCLUSIONS: Isotope labelling together with isotope-sensitive spectroscopy is a highly promising method for obtaining information at the atomic level of biologically active systems. Strategies based on specific stable isotope labelling are likely to be applicable to many research areas.

Carbon Isotopes↗
PubMed1992

Photoreceptor-specific mRNAs in mice carrying different allelic combinations at the rd and rds loci.

Several retinal mRNAs encoding photoreceptor-specific proteins have been examined in congenic lines of mice carrying different allelic combinations at the rd and rds loci, to determine how mRNA expression is affected by the presence of both the rd and rds genes together or by the presence of one or two rd and rds alleles in the visual cells. Slot blots with retinal RNA from 9 to 30 days old C3H +/+, +/+; rd/+, +/+; rd/rd, +/+; +/+, +/rds; +/+, rds/rds; and the rd/rd, rds/rds double homozygous mutant mice were hybridized successively to several [32P]cDNA probes encoding proteins involved in phototransduction. The increases and decreases in the levels of mRNA coding for opsin, the alpha-subunit of transducin, 48 kDa protein and the beta-subunit of cGMP-phosphodiesterase in the heterozygous and homozygous rds and rd mouse retinas reflected the growth and degeneration of the photoreceptor cells. The heterozygous rd mouse (rd/+, +/+) retina expressed cGMP-phosphodiesterase beta mRNA levels halfway between those in the control (+/+, +/+) and homozygous (rd/rd, +/+) mice, indicating a possible dosage effect. The double homozygous mutant (rd/rd, rds/rds) showed intermediate levels between those observed in the two homozygotes for all mRNAs studied, suggesting a possible interaction between the rd and rds genes.

Alleles↗
PubMed1999

Time-resolved rhodopsin activation currents in a unicellular expression system.

The early receptor current (ERC) is the charge redistribution occurring in plasma membrane rhodopsin during light activation of photoreceptors. Both the molecular mechanism of the ERC and its relationship to rhodopsin conformational activation are unknown. To investigate whether the ERC could be a time-resolved assay of rhodopsin structure-function relationships, the distinct sensitivity of modern electrophysiological tools was employed to test for flash-activated ERC signals in cells stably expressing normal human rod opsin after regeneration with 11-cis-retinal. ERCs are similar in waveform and kinetics to those found in photoreceptors. The action spectrum of the major R(2) charge motion is consistent with a rhodopsin photopigment. The R(1) phase is not kinetically resolvable and the R(2) phase, which overlaps metarhodopsin-II formation, has a rapid risetime and complex multiexponential decay. These experiments demonstrate, for the first time, kinetically resolved electrical state transitions during activation of expressed visual pigment in a unicellular environment (single or fused giant cells) containing only 6 x 10(6)-8 x 10(7) molecules of rhodopsin. This method improves measurement sensitivity 7 to 8 orders of magnitude compared to other time-resolved techniques applied to rhodopsin to study the role particular amino acids play in conformational activation and the forces that govern those transitions.

Cell Fusion↗
PubMed1995

Galactosylation of rhodopsin by the human retina.

The major oligosaccharide chains of bovine, frog and human rhodopsins are abridged, hybrid, asparagine-linked structures that contain only mannose and N-acetylglucosamine as the constituent sugars. Isomers that also contain galactose have been detected in varying amounts in rhodopsins from different species. As part of studies to examine the mechanisms used by the human retina to control the glycosylation of rhodopsin, the kinetics of the retinal galactosyltransferase was examined using Golgi-enriched fractions of human retina as the enzyme source and rhodopsin, opsin, and the oligosaccharide isolated from rhodopsin as acceptors. These reactions were compared to those using bovine retina and rat liver Golgi. A comparison of the Vmax/Km ratios revealed that the efficiency of the human retinal galactosyltransferase was some 20-fold lower than that of the rat liver. In addition, consistent with the higher state of galactosylation of human as compared to bovine rhodopsin, greater efficiency was observed with the human preparation. While the conformation of the visual pigment exerted an effect, its low galactosylation was not due to a major directing influence on glycosylation by the polypeptide matrix as indicated by the even lower activity toward the isolated oligosaccharide. The galactosylated oligosaccharides obtained by these procedures were identified by chromatographic methods. The relatively low activity of the retinal galactosyltransferases observed in this study may help explain the limited glycosylation that is typical of rhodopsin.

Animals↗
PubMedDate not supplied

Topography of ganglion cells and photoreceptors in the retina of a New World monkey: the marmoset Callithrix jacchus.

We studied the anatomical substrates of spatial vision in a New World monkey, the marmoset Callithrix jacchus. This species has good visual acuity and a foveal specialization which is qualitatively similar to that of humans and other Old World primates. We measured the spatial density of retinal ganglion cells and photoreceptors, and calculated the relative numbers of these cell populations. We find that ganglion cells outnumber photoreceptors by between 2.4:1 and 4.2:1 in the fovea. The peak sampling density of ganglion cells is close to 550,000 cells/mm2. This value falls by almost 1000-fold between the fovea and peripheral retina; a value which approaches recent estimates of the centroperipheral ganglion cell gradient for human and macaque monkey retina and primary visual cortex. The marmoset shows a sex-linked polymorphism of color vision: all male and some female marmosets are dichromats. Six of the retinas used in the present study came from animals whose chromatic phenotype was identified in electrophysiological experiments and confirmed by polymerase chain reaction (PCR) amplification of cone opsin encoding genes. One animal was a trichromat and the others were dichromats. Antibodies against short wavelength-sensitive (SWS) cones labeled close to 8% of all cones near the fovea of one dichromat animal, consistent with electrophysiological evidence that the SWS system is present in all marmosets. The topography and spatial density of cone photoreceptors and ganglion cells was similar to that reported for macaque retina, and we found no obvious difference between dichromatic and trichromatic marmoset retinas. These results reinforce the view that the main determinate of primate foveal topography is the requirement for maximal spatial resolution.

Animals↗
PubMed1989

The catalytic subunit of phosphatase 2A dephosphorylates phosphoopsin.

Rod cell outer segments were found to contain a protein phosphatase activity toward phosphoopsin with properties very similar to those of protein phosphatase 1 or 2A. The opsin phosphatase activity was stable to ethanol precipitation, had a Mr of 35,000-38,000 as determined by gel filtration, and was not dependent on divalent cations for activity. The chromatographic properties on DEAE-cellulose of the rod outer segment protein phosphatase were also similar to those reported for protein phosphatase 1 or 2A. In order to distinguish between these two protein phosphatases, we tested homogeneous preparations of protein phosphatases 1 and 2A from skeletal muscle for activity toward phosphoopsin. Protein phosphatase 2A dephosphorylated phosphoopsin at approximately 10% of its rate toward phosphorylase a, whereas protein phosphatase 1 had no activity toward phosphoopsin. We conclude that protein phosphatase 2A is present in the rod cell outer segment and that it is a likely candidate to perform the in vivo dephosphorylation of rhodopsin in the visual cycle.

Animals↗
PubMed1991

Circadian photoreception in the retinally degenerate mouse (rd/rd).

We have examined the effects of light on circadian locomotor rhythms in retinally degenerate mice (C57BL/6J mice homozygous for the rd allele: rd/rd). The sensitivity of circadian photoreception in these mice was determined by varying the irradiance of a 15 min light pulse (515 nm) given at circadian time 16 and measuring the magnitude of the phase shift of the locomotor rhythm. Experiments were performed on animals 80 days of age. Despite the loss of visual photoreceptors in the rd/rd retina, animals showed circadian responses to light that were indistinguishable from mice with normal retinas (rd/+ and +/+). While no photoreceptor outersegments were identified in the retina of rd/rd animals (80-100 days of age), we did identify a small number of perikarya that were immunoreactive for cone opsins, and even fewer cells that contained rod opsin. Using HPLC, we demonstrated the presence and photoisomerization of the rhodopsin chromophore 11-cis retinaldehyde. The rd/rd retinas contained about 2% of 11-cis retinaldehyde found in +/+ retinas. We have yet to determine whether the opsin immunoreactive perikarya or some other unidentified cell type mediate circadian light detection in the rd/rd retina.

Animals↗
PubMed1999

Enantioselective binding of an 11-cis-locked cyclopropyl retinal. The conformation of retinal in bovine rhodopsin.

[formula: see text] The conformation of the retinal chromophore in rhodopsin is central for understanding the visual transduction process. The absolute twist around the 12-s bond of the chromophore in rhodopsin has been determined by studies with 11-cis-locked 11,12-cyclopropylretinal analogues (11S,12R)-2 and (11R,12S)-3, enantioselectively synthesized with the aid of an enzyme. The finding that enantiomer 2 binds to opsin while the other 3 does not defines the absolute sense of twist around the 12-s bond.

Animals↗
PubMed1997

Visual pigment gene structure and expression in human retinae.

We determined the genotypes of the X-chromosome-linked red/green color vision genes by a novel PCR/SSCP-based method and assessed expression by mRNA analysis in retinae of 51 unselected post mortem eye specimens from Caucasian males of unknown color vision status. All individuals had a single red (long-wave) pigment gene and one or more (an average of two) green (middle-wave) pigment genes. Four males had 5'green-red3' hybrid genes in addition to normal red and green pigment genes. These findings are consistent with earlier studies on human visual pigment gene structure using Southern blotting and with a recent study using pulsed-field electrophoresis. We interpret claims of much larger numbers of red, green and green-red hybrid genes to be technical artifacts. The ratio of expressed red to green pigment retinal mRNA varied widely (1-10 with a mode of 4) and was not correlated with that of red to green pigment genes. In one individual with a green-red hybrid gene in addition to normal red and green pigment genes, the normal red pigment gene and the hybrid gene were both expressed, but the normal green gene was not. This person presumably had deuteranomalous color vision. Two with green-red hybrid genes expressed the normal red and green pigment genes, but not the hybrid genes. These two individuals presumably had normal color vision. We interpret the failure to express their green-red hybrid genes to be caused by their location at a more distal position in the visual pigment gene array.

Color Vision Defects↗