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PubMed1979

Biochemical aspects of the visual process. XL. Spectral and chemical analysis of metarhodopsin III in photoreceptor membrane suspensions.

The late photointermediates of rhodopsin photolysis have been analyzed spectrally and chemically in bovine rod outer segment membrane suspension at 25 degrees C and pH 6.5. The decay of metarhodopsin II follows two spectrally distinct routes, resulting 40 min after illumination in a stable mixture of photo-products with absorbance maxima around 380 and 452 nm, free retinal and metarhodopsin III, respectively. Chemical analysis shows that three different products are involved: free retinal (approx. 34%), protein-bound retinal (approx. 51%) and lipid-bound retinal (approx. 15%). The latter fraction consists of retinylidene-phosphatidylethanolamine exclusively. Photolysis of membranes reconstituted with various phospholipids gives a qualitatively normal spectral picture, but the production of metarhodopsin III may vary with the phospholipid composition, i.e. with the percent of phosphatidylethanolamine present. Chemical analysis shows that with increasing phosphaatidylethanolamine content of the membrane, the retinylidene phosphatidylethanolamine fraction increases proportionally at the expense of free retinal, while the fraction of protein-bound retinal remains unaffected. The results indicate that under these conditions metarhodopsin III (defined as a long wavelength product of metarhodopsin II decay) is composed of two chemically distinct components: opsin-bound retinal and retinylidene phosphatidylethanolamine.

Animals↗
PubMed1983

The gecko visual pigment: its photosensitivity and the effects of chloride and nitrate ions.

By use of the method of photometric curves, the photosensitivity of the major and ion-sensitive pigment of Gekko gekko has been determined and compared with that of rhodopsins of the frog (Rana pipiens) and of the fish (Porichthys notatus). In the presence of Cl- (or Br-), the gecko pigment has the same photosensitivity as the other A1 rod pigments, but unlike these, the addition of NH2OH does not lead to a Dartnall effect, i.e. an enhancement in the measured rate of photic bleaching. This is because the gecko pigment has no meta-III intermediate. In the Cl- -deficient state the gecko pigment has a photosensitivity 0.8 times that of the Cl- -provided system. The increase in photosensitivity brought on by Cl- is quantitatively accounted for by the Cl- -induced hyperchromic effect. The addition of NH2OH to the system without added Cl- leads to a small increase in measured rate of photic bleaching with an apparent 13% increment in photosensitivity. This is not a classical Dartnall effect for here again no meta-III is involved. The possibility is raised of an additional, yet undiscovered, action of NH2OH on the opsin moiety. Nitrate ions (NO3-) are known to produce an increase in extinction coefficient similar to that of Cl- and a hypochromic shift in the spectral absorbance. Despite the hyperchromic action, NO3- produces a reduction in the measured rate of photic bleaching, an effect explained by the appearance of a meta-III type intermediate absorbing at about 470 nm. While Cl- is able to antagonize the NO3- -induced hypochromic shift, it is unable to reverse the NO3- -induction of meta-III. This, along with other differences in responses of the gecko pigment to these two ions, suggests that Cl- and NO3- act at two different sites and produce unique conformational changes in the protein molecule.

Animals↗
PubMedDate not supplied

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.

Humans↗
PubMedDate not supplied

Rod and cone function in coneless mice.

Transgenic coneless mice were initially developed to study retinal function in the absence of cones. In coneless mice created by expressing an attenuated diphtheria toxin under the control of flanking sequences from the human L-cone opsin gene, a small number of cones (3-5% of the normal complement) survive in a retina that otherwise appears structurally quite normal. These cones predominantly ( approximately 87% of the total) contain UV-sensitive photopigment. ERG recordings, photoreceptor labeling, and behavioral measurements were conducted on coneless and wild-type mice to better understand how the nature of this alteration in receptor complement impacts vision. Signals from the small residual population of UV cones are readily detected in the flicker ERG where they yield signal amplitudes at saturation that are roughly proportional to the number of surviving cones. Behavioral measurements show that rod-based vision in coneless mice does not differ significantly from that of wild-type mice, nor does their rod system show any evidence of age-related deterioration. Coneless mice are able to make accurate rod-based visual discriminations at light levels well in excess of those required to reach cone threshold in wild-type mice.

Animals↗
PubMed2002

Allelic variation in the squirrel monkey x-linked color vision gene: biogeographical and behavioral correlates.

Most Neotropical primate species possess a polymorphic X-linked and a monomorphic autosomal color vision gene. Consequently, populations are composed of both dichromatics and trichromatics. Most theories on the maintenance of this genetic system revolve around possible advantages for foraging ecology. To examine the issue from a different angle, we compared the numbers and relative frequencies of alleles at the X-linked locus among three species of Saimiri representing a wide range of geographical and behavioral variation in the genus. Exons 3, 4, and 5 of the X-linked opsin gene were sequenced for a large number of X chromosomes for all three species. Several synonymous mutations were detected in exons 4 and 5 for the originally reported alleles but only a single nonsynonymous change was detected. Two alleles were found that appeared to be the result of recombination events. The low occurrence of recombinant alleles and absence of mutations in the amino acids critical for spectral tuning indicates that stabilizing selection acts to maintain the combinations of critical sites specific to each allele. Allele frequencies were approximately the same for all Saimiri species, with a slight but significant difference between S. boliviensis and S. oerstedii. No apparent correlation exists between allele frequencies and behavioral or biogeographical differences between species, casting doubt on the speculation that the spectral sensitivities of the alleles have been maintained because they are specifically well-tuned to Saimiri visual ecology. Rather, the spectral tuning peaks might have been maintained because they are as widely spaced as possible within the limited range of middlewave to longwave spectra useful to all primates. This arrangement creates a balance between maximizing the distance between spectral tuning peaks (allowing the color opponency of the visual system to distinguish between peaks) and maximizing the number of alleles within a limited range (yielding the greatest possible frequency of heterozygotes).

Alleles↗
PubMed1989

The transitory complex between photoexcited rhodopsin and transducin. Reciprocal interaction between the retinal site in rhodopsin and the nucleotide site in transducin.

In the first step of the visual transduction cascade a photoexcited rhodopsin molecule, R*ret, binds to a GDP-carrying transducin molecule, TGDP. The R*-T interaction causes the opening of the nucleotide site in T and catalyzes the GDP/GTP exchange by allowing the release of the GDP. We have studied the influences on this R*-T transitory complex of the occupancies of the nucleotide site in T and the retinal site in rhodopsin. After elimination of the GDP released from the bound transducin, the complex, named R*ret-te (ret for retinal present, e for nucleotide site empty) remains stabilized almost indefinitely in a medium whose ionic composition is close to physiological. In this complex the bound Te retains a lasting ability to interact with GDP or GTP, and R*ret remains spectroscopically in the meta-II state, by contrast with free R*ret which decays to opsin and free retinal. Hence the R*-T interaction which opens the nucleotide site in T conversely blocks the retinal site in R*ret. Upon prolonged incubation in a low-ionic-strength medium the R*ret-Tc complex dissociates partially, but the liberated Te is then unable to rebind GDP or GTP, even in the presence of R*ret, it is probably denaturated. Upon treatment of the R*ret-Te complex by a high concentration of hydroxylamine, the retinal can be removed from the rhodopsin. The Re-Te complex remains stable and the complexed transducin keeps its capacity to bind GTP. TGTP then dissociates from Re. The liberated Re loses its capacity to interact with a new transducin. These data are integrated into a discussion of the development of the cascade. We stress that affinities, i.e. dissociation equilibrium constants, are insufficient to describe the flow of reactions triggered by one R*ret molecule. It depends on a few critical rapid binding and dissociation processes, and is practically insensitive to other slow ones, hence to the values of affinities that express only the ratio of kinetics constants. The effect of the R*-T interaction on the retinal site in rhodopsin is analogous to the effect of the binding of a G-protein on the apparent affinity of a receptor for its agonist.

Animals↗
PubMed1989

Rhodopsin levels and retinal function in cats during recovery from vitamin A deficiency.

Extended vitamin A deficiency in the cat led to an abnormal appearance in the tapetal fundus with the formation of a dark brown streak centered on the area centralis. At this time rod sensitivity, as measured by the b-wave of the electroretinogram, was reduced by more than two log units; the level of rod visual pigment was reduced by about 90% throughout the paracentral retinal region and was essentially absent from the area centralis. Following oral supplementation with vitamin A there was a rapid partial recovery of both rhodopsin levels and rod sensitivity. Further recovery continued over more than 18 days to levels that were not substantially below normal. This recovery was absent from the area centralis, in which measured visual pigment levels remained very low. In supplemented cats, the brown color in the fundus faded but there remained a small hyper-reflective zone at the area centralis. Morphological examination of the central retina in a supplemented cat showed an outer nuclear layer reduced to one or two rows in the small zone with low rhodopsin levels. Cone but not rod photoreceptors were present in this zone and they appeared to lack outer segments. During recovery, the increase in rod sensitivity was approximately linearly related to the recovery of rhodopsin levels. Thus, in these conditions reduction in sensitivity resulting from previous vitamin A deficiency was limited by the ability of the photoreceptors to absorb incident quanta. The time course of the recovery of rhodopsin and sensitivity suggests that at least two processes were involved. The faster of these may be the regeneration of rhodopsin from existing opsin molecules in the outer segments, while the slower may depend on the renewal of the outer segments themselves.

Animals↗
PubMed2002

Vibrational polarization and opsin shift of retinal schiff bases: theoretical study.

The changes in the electronic excitation energy arising from molecular structural displacement induced by external electric field (so-called vibrational polarization) are examined theoretically for the protonated and neutral 11-cis retinal Schiff bases. It is shown that the magnitude of the field-induced structural displacement is significantly large for the protonated species, so that the change in the electronic excitation energy arising from this structural displacement is of the same order of magnitude as that arising from the direct effect of electric field on the electronic wave function. These two effects contribute additively to the field-induced spectral shift. The intensity-carrying mode (ICM) theory is employed to extract a single vibrational mode (called primary infrared ICM) that is most important for the field-induced structural displacement. A simple one-dimensional model is constructed, and the extent to which we can interpret the field-induced spectral shift by such a model is examined. In the case of the neutral species, only a small change in the electronic excitation energy is induced by external electric field, mainly because the vibrational polarizability of this species is small. The meaning of these results in the spectral tuning of visual pigments is discussed.

Electrochemistry↗
PubMed2002

Different structural organization of the encephalopsin gene in man and mouse.

Encephalopsin, also called Panopsin, is a recently discovered extraretinal photoreceptor, which may play a role in non-visual photic processes such as the entrainment of circadian rhythm or the regulation of pineal melatonin production. Based on RT-PCR data and comparative genomic sequence analysis, we show that the human OPN3 gene consists of six exons and expresses various splice variants, while the murine homologue contains four exons and produces just one splice form. Furthermore, the human OPN3 gene overlaps with the neighboring KMO gene on a genomic as well as on an RNA level, whereas the corresponding genes in mouse lie close together but do not overlap. This finding is of particular interest, since differences in gene organization between man and mouse, that have been reported so far, occur within gene clusters, i.e. the number of genes within a certain cluster may differ between man and mouse. OPN3 provides an exception to this rule, since it is positionally uncoupled from other genes of the opsin family.

3' Flanking Region↗
PubMed2004

Photoreceptors and photopigments in a subterranean rodent, the pocket gopher (Thomomys bottae).

Pocket gophers (Thomomys bottae) are rodents that spend much of their lives in near-lightless subterranean burrows. The visual adaptations associated with this extreme environment were investigated by making anatomical observations of retinal organization and by recording retinal responses to photic stimulation. The size of the eye is within the normal range for rodents, the lens transmits light well down into the ultraviolet, and the retina conforms to the normal mammalian plan. Electroretinogram recording revealed the presence of three types of photopigments, a rod pigment with a spectral peak of about 495 nm and two types of cone pigment with respective peak values of about 367 nm (UV) and 505 nm (medium-wavelength sensitive). Both in terms of responsivity to lights varying in temporal frequency and in response recovery following intense light adaptation, the cone responses of the pocket gopher are similar to those of other rodents. Labeling experiments indicate an abundance of cones that reach densities in excess of 30,000 mm-2. Cones containing UV opsin are found throughout the retina, but those containing medium-wavelength sensitive opsin are mostly restricted to the dorsal retina where coexpression of the two photopigments is apparently the rule. Rod densities are lower than those typical for nocturnal mammals.

Adaptation, Ocular↗
PubMed2002

The molecular mechanism for the spectral shifts between vertebrate ultraviolet- and violet-sensitive cone visual pigments.

The short-wave-sensitive (SWS) visual pigments of vertebrate cone photoreceptors are divided into two classes on the basis of molecular identity, SWS1 and SWS2. Only the SWS1 class are present in mammals. The SWS1 pigments can be further subdivided into violet-sensitive (VS), with lambda(max) (the peak of maximal absorbance) values generally between 400 and 430 nm, and ultraviolet-sensitive (UVS), with a lambda(max)<380 nm. Phylogenetic evidence indicates that the ancestral pigment was UVS and that VS pigments have evolved separately from UVS pigments in the different vertebrate lineages. In this study, we have examined the mechanism of evolution of VS pigments in the mammalian lineage leading to present day ungulates (cow and pig). Amino acid sequence comparisons of the UVS pigments of teleost fish, amphibia, reptiles and rodents show that site 86 is invariably occupied by Phe but is replaced in bovine and porcine VS pigments by Tyr. Using site-directed mutagenesis of goldfish UVS opsin, we have shown that a Phe-86-->Tyr substitution is sufficient by itself to shift the lambda(max) of the goldfish pigment from a wild-type value of 360 nm to around 420 nm, and the reverse substitution of Tyr-86-Phe into bovine VS opsin produces a similar shift in the opposite direction. The substitution of this single amino acid is sufficient to account therefore for the evolution of bovine and porcine VS pigments. The replacement of Phe with polar Tyr at site 86 is consistent with the stabilization of Schiff-base protonation in VS pigments and the absence of protonation in UVS pigments.

Animals↗
PubMed1976

Tunable laser resonance Raman spectroscopic investigations of the transduction process in vertebrate rod cells.

Tunable laser resonance Raman spectroscopy has been applied to probe (in vivo) the role of rhodopsin in transducing light energy into the chemical necessary to generate a neural response. These in vivo experiments have suggested that the Schiff base linkage through which retinal is attached to opsin in rhodopsin is protonated. Furthermore, it appears that light eventually stimulates the deprotonation of the Schiff base linkage between the Meta I and Meta II steps in the intermediate sequence which is the result of light interacting with rhodopsin. Our data suggest that this deprotonation of the Schiff base occurs on the same time scale as overall proton release and uptake by the rhodopsin molecule. It is interesting to note that this series of protonations and deprotonations also occurs within the same time scale as the neural response generation in vertebrates and the generation of a proton gradient by bacteriorhodopsin, which is used by the bacterium, Halobacterium halobium, for ATP synthesis. If these data are analyzed within the context of the in vivo resonance Raman experiments (which seem to indicate that proton release is stimulated in the disc membrane during transduction) then there is a strong suggestion that the proton will assume an important role in any working hypothesis of visual transduction. In essence it appears that protons along with ATP and calcium ions must all be essential elements in the transduction process.

Adenosine Triphosphate↗
PubMed1990

Cloning of cDNA and amino acid sequence of one of chicken cone visual pigments.

The chicken has four kinds of color visual pigments, in addition to rhodopsin. A chicken genomic DNA library was screened with cDNA of human red-sensitive pigment and a chicken genomic DNA fragment including rhodopsin exons 2, 3 and 4, and then a genomic DNA fragment encoding a visual pigment, possibly an iodopsin, was cloned. A cDNA library, constructed from chicken retina mRNA, was screened with the genomic DNA fragment and the cDNA of human red-sensitive pigment, and the cDNA encoding the pigment was cloned. The nucleotide sequence of this cDNA was similar to that of the human red-sensitive pigment, with identities of 78% for the nucleotide sequence and 84% for the amino acid sequence with human red-sensitive pigment.

Amino Acid Sequence↗
PubMed1984

Preembedding labeling with biotinylated antibodies and subsequent visualization of the biotin groups exposed on thin sections.

The feasibility of labeling cell membranes with biotinylated ligands and detecting the biotin groups on thin sections was investigated. Fixed retinal tissue was incubated with biotinyl- antiopsin . Half of the biotinyl-antibody labeled retinal tissue was incubated with avidin-ferritin (AvF) and embedded in Epon (preembedding reaction). The second half was embedded in glutaraldehyde cross-linked bovine serum albumin (BSA). Thin sections of this preparation were incubated with AvF to detect biotinyl-antibodies exposed by the sectioning (postembedding reaction). Biotin groups on the thin section surface could be readily visualized with AvF. Stereoscopic images demonstrated that the ferritin particles were localized only on the exposed surface of the thin section. The labeling was highly specific, with a very low background. Quantitative analysis was employed in order to determine the optimal reaction conditions for maximizing the labeling density with minimizing nonspecific binding. The possibility of using biotinylated molecules in the study of dynamic cellular events and for the subsequent intracellular localization of biotin on thin sections is suggested.

Animals↗
PubMed1985

Light-induced conformational changes in the extradiscal regions of bovine rhodopsin.

Light-induced conformational changes occurring at the cytosolic surface of rhodopsin were investigated by performing limited digestions of native and illuminated visual pigment with thermolysin, Arg-C endoproteinase, papain and proteinase K. A higher susceptibility of the extradiscal regions of the bleached pigment to the proteases were observed together with altered capacities of the digested bleached rhodopsins to activate the cGMP phosphodiesterase. The overall results strongly suggest that light induces conformational changes not only in the C-terminal end but also in the second and the third extradiscal loop of rhodopsin.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1996

Light-mediated retinoic acid production.

Retinoids serve two main functions in biology: retinaldehyde forms the chromophore bound to opsins, and retinoic acid (RA) is the activating ligand of transcription factors. These two functions are linked in the vertebrate eye: we describe here that illumination of the retina results in an increase in RA synthesis, as detected with a RA bioassay and by HPLC. The synthesis is mediated by retinaldehyde dehydrogenases which convert some of the chromophore all-trans retinaldehyde, released from bleached rhodopsin, into RA. As the eye contains high levels of retinaldehyde dehydrogenases, and as the oxidation of retinaldehyde is an irreversible reaction, RA production has to be considered an unavoidable by-product of light. Through RA synthesis, light can thus directly influence gene transcription in the eye, which provides a plausible mechanism for light effects that cannot be explained by electric activity. Whereas the function of retinaldehyde as chromophore is conserved from bacteria to mammals, RA-mediated transcription is fully evolved only in vertebrates. Invertebrates differ from vertebrates in the mechanism of chromophore regeneration: while in the invertebrate visual cycle the chromophore remains bound, it is released as free all-trans retinaldehyde from illuminated vertebrate rhodopsin. RA synthesis occurring as corollary of dark regeneration in the vertebrate visual cycle may have given rise to the expansion of RA-mediated transcriptional regulation.

Animals↗
PubMed2004

Visual pigment phosphorylation but not transducin translocation can contribute to light adaptation in zebrafish cones.

The ability of cone photoreceptors to adapt to light is extraordinary. In this study we evaluated two biochemical processes, visual pigment phosphorylation and transducin translocation, for their ability to contribute to light adaptation in zebrafish cones. Since cytoplasmic Ca2+ regulates light adaptation, the sensitivities of these processes to both light and Ca2+ were examined. Cytoplasmic Ca2+ regulates the sites of light-stimulated phosphorylation. Unexpectedly, we found that Ca2+ also regulates the extent of phosphorylation of unbleached cone pigments. Immunocytochemical analyses revealed that neither light nor cytoplasmic Ca2+ influences the localization of transducin in zebrafish cones.

Adaptation, Ocular↗
PubMed1984

Squid m-retinochrome. Two forms of metaretinochrome.

When retinochrome absorbs light, it bleaches to m-retinochrome, which may act as a direct supplier of 11-cis-retinal to protein opsin to form rhodopsin. The present experiments were aimed at further elucidating the molecular state of m-retinochrome. Retinochrome and m-retinochome were mixed each with sodium borohydride, subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the resultant fluorescence bands were examined. The reduced retinochrome showed only one band due to N-retinyl protein, whereas the reduced m-retinochrome had two bands. When extracted with successive volumes of n-hexane, m-retinochrome released the 11-cis-retinal chromophore in a time-course consisting of fast and slow phases. These findings indicated that m-retinochrome exists in two forms, with loose and tight coupling of the chromophore to the protein moiety. Those forms were usually balanced in a molar ratio of about 1:2, and the proportion of the tight form of m-retinochrome was increased in the presence of excess 11-cis-retinal. Based upon these findings, the role of m-retinochrome in the visual cells is discussed.

Animals↗