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PubMed2003

Assessing structural elements that influence Schiff base stability: mutants E113Q and D190N destabilize rhodopsin through different mechanisms.

The stability of the retinal chromophore attachment varies between different visual pigments and may factor in some retinal disease states. Opsin appears to stabilize this Schiff base linkage by: (i) affecting the hydrolysis chemistry, (ii) shielding the retinal linkage from solvent, or (iii) acting as a kinetic trap to slow retinal release. Here we describe methods to determine Schiff base stability in rhodopsin, present examples of dark state and MII rhodopsin stability differences, and show that studies of mutants E113Q and D190N demonstrate different parts of rhodopsin influence Schiff base stability in different ways.

Genetic Linkage↗

PubMed2002

Environmental stimulus perception and control of circadian clocks.

Circadian rhythms are regulated by clocks located in specific structures of the central nervous system, such as the suprachiasmatic nucleus (SCN) in mammals, and by peripheral oscillators present in various other tissues. Recent discoveries have elucidated the control of central and peripheral clocks by environmental signals. The major synchroniser in animals is light. In mammals, a subset of retinal ganglion cells receive light signals that are transmitted to the SCN via the retinohypothalamic tract. Photoreception is probably elicited by a novel opsin, melanopsin, although cryptochromes may also play a role. These signals feed directly to the SCN master clock, which then provides timing cues to peripheral clocks. In contrast to mammals, peripheral tissues in the fly and in the fish are directly photoreceptive. However, alternative routes exist. Some peripheral clocks in mammals can be specifically entrained in an SCN-independent manner by restricting food during the light period.

Animals↗

PubMed1998

Mechanism of spectral tuning in the dolphin visual pigments.

The absorption maxima of both rod and cone visual pigments of the bottlenose dolphin (Tursiops truncatus) are blue-shifted relative to those of terrestrial mammals. A comparison of the sequence of the dolphin rod photopigment gene with that of the bovine rod suggests that, fo the 28 nonidentical amino acids, three amino acid substitutions at positions 83, 292, and 299 in the dolphin rod pigment are responsible for the 10 nm blue shift in absorption maxima. A similar comparison of the dolphin long-wavelength sensitive (LWS) cone photopigment gene with those of the human LWS cones suggests that a single substitution at position 292 (using the convention of rhodopsin numbering) in the dolphin LWS cone pigment results in a blue shift in absorption maxima. A mutagenesis study reveals that the combination of the three dolphin specific substitutions in the bovine rod pigment (83D to 83N, 292A to 292S, and 299A to 299S) causes a blue shift from the wild-type lambdamax of 499 nm to 389 nm. The single substitution in the dolphin LWS cone pigment (292S to 292A) causes a red shift from the wild-type lambdamax of 524 nm to 552 nm. The interactions of the three amino acids identified in the rod pigment with the chromophore may be a general mechanism for blue shifting in rod visual pigments. Furthermore, the single substitution in the dolphin LWS opsin gene is a novel mechanism of wavelength modulation in mammalian LWS pigments.

Animals↗

PubMed1968

Studies on vision. The nature of the retinal-opsin linkage.

1. Convenient methods for the preparation of tritiated 11-cis-retinol, 11-cis-retinal and rhodopsin are described. Irradiation of labelled rhodopsin in the presence of sodium borohydride resulted in the irreversible binding of the retinyl moiety to the active site. Degradative studies established that the retinyl moiety in this reduced derivative of rhodopsin was attached to the in-amino group of lysine. In connexion with this investigation the synthesis of a number of N-retinylidene- and N-retinyl-amino acids was achieved. Derivatives of lysine with the retinyl moiety attached either to the alpha-amino group or to the in-amino group were also synthesized and characterized. 2. It is suggested that the involvement of a charge-transfer interaction between the retinylidene chromophore and a suitable group -X or -X.H on the opsin best explains the spectroscopic properties of rhodopsin and other visual proteins.

Amino Acids↗

PubMed1999

A synthetic gene coding for the green fluorescent protein (GFP) is a versatile reporter in Chlamydomonas reinhardtii.

The use of Chlamydomonas reinhardtii as a model system has been hindered by difficulties encountered in expressing foreign genes. We have synthesised a gene encoding green fluorescent protein (GFP) adapted to the codon usage of C. reinhardtii (cgfp). After verifying the gene was functional in Escherichia coli, the cgfp was fused in frame to the phleomycin resistance gene ble from Streptoalloteichus hindustanus and expressed in C. reinhardtii under control of the rbcS2 promoter and intron sequences. The GFP-fluorescence was seen only in the nucleus demonstrating the nuclear accumulation of the Ble-GFP fusion protein. The cgfp was also fused to the chlamyopsin gene, cop, and expressed in C. reinhardtii under control of the cop promoter. The eyespot became fluorescent indicating that the opsin-GFP fusion protein was correctly directed into the eyespot along with the endogenous unmodified opsin. We conclude that cgfp provides a useful tool to visualize protein synthesis and localisation in vivo in C. reinhardtii and possibly in related green algal species.

Algal Proteins↗

PubMed2002

Loss of photic entrainment and altered free-running circadian rhythms in math5-/- mice.

Mammalian free-running circadian rhythms are entrained to the external light/dark cycle by photic signaling to the suprachiasmatic nuclei via the retinohypothalamic tract (RHT). We investigated the circadian entrainment and clock properties of math5-/- mutant mice. math5 is a critical regulator of retinal ganglion cell development; math5-/- mice show severe optic nerve hypoplasia. By anterograde cholera toxin B tracing, we find that math5-/- mice do not develop an identifiable RHT pathway. This appears to be attributable to agenesis or dysgenesis of the majority of RHT-projecting retinal ganglion cells. math5-/- mice display free-running circadian rhythms with a period approximately 1 hr longer than B6/129 controls (24.43 +/- 0.10 vs 23.62 +/- 0.19 hr; p < 0.00001). The free-running period of heterozygote mice is indistinguishable from that of controls. math5-/- mice show no entrainment to light/dark cycles, whereas heterozygote mice show normal entrainment to both 12 hr light/dark cycles and to a 1 hr skeletal photoperiod. math5-/- mice show reduced ability to entrain their rhythms to the nonphotic time cue of restricted running wheel access but demonstrate both free-running behavior and entrained anticipation of wheel unlocking in these conditions, suggesting the presence of a second diurnal oscillatory system in math5-/- animals. These results demonstrate that retinal ganglion cell input is not necessary for the development of a free-running circadian timekeeping system in the suprachiasmatic nucleus but is important for both photic entrainment and determination of the free-running period.

Animals↗

PubMed1985

Proteins of the bovine interphotoreceptor matrix: retinoid binding and other functions.

Bovine interphotoreceptor matrix (IPM) is rich in protein, containing a limited and specific set of these macromolecules. The matrix proteins are synthesized by adjacent tissues (neural retina and pigment epithelium), but are not, themselves, major constituents of these tissues. Of possible functional importance are the presence of some lysosomal hydrolases and of glycoprotein components that foster the aggregation of RPE cells. The IPM contains also a unique retinoid-binding protein - IRBP, a large glycoprotein located only in the IPM. IRBP carries more all-trans-retinol in light- than in dark-adapted eyes; this finding, coupled with IRBP's strategic position, indicates that IRBP may be a shuttle for vitamin A in the visual cycle. Furthermore, IRBP may mediate exchange of retinal among opsin molecules in the dark. The molecular weight of bovine IRBP is 133,000; the molecular radius is 56A. These measurements show that IRBP is probably an elongated molecule. Retinol binds relatively loosely to IRBP (with dissociation constant 1.3 X 10(-6) M), implying that IRBP should be able to transfer its vitamin A to cellular retinoid carriers in the retina and RPE. The absorbance, fluorescence, and circular dichroic characteristics of IRBP are compared to those of other retinol-binding proteins.

Acetylglucosaminidase↗

PubMed2004

Blue cone monochromatism: clinical findings in patients with mutations in the red/green opsin gene cluster.

BACKGROUND: X-linked blue cone monochromatism (BCM) has to be differentiated from x-linked cone dystrophy and autosomal recessive rod monochromatism. METHODS: In nine male patients with congenital cone dysfunction (one family, six single cases; age range: 9-55 years), mutations in the red/green opsin gene cluster were confirmed. Clinical findings were analyzed retrospectively. RESULTS: In one family and three single cases, a single red-green hybrid gene was found carrying a Cys203Arg mutation. Two patients had multiple opsin genes, a red/green hybrid gene and at least one green pigment gene, all carrying the Cys203Arg mutation. In one patient, a large deletion of the locus control region and parts of the red pigment gene were detected. Two patients (ages: 45 and 55 years) complained about progression. Two patients presented with nystagmus. Refractive errors (+8.0 and -11.0 D) and visual acuity were variable (0.05-0.3). Only four patients had a visual acuity > or = 0.1. In two patients, visual acuity could be improved using blue filter glasses. Four of five patients > or = 25 years had dystrophic alterations in the macula. Severe color vision defects and relative central scotoma were present in all patients. In the electroretinogram, residual cone responses were detected in 2/8 patients. CONCLUSIONS: Hybrid red/green opsin genes carrying the Cys203Arg mutation are a frequent cause of BCM in German patients. Molecular genetic evaluation is mandatory for adequate diagnosis of patients since from the clinical data only two patients were diagnosed as having BCM. In the other patients, either rod monochromatism or cone-rod dystrophy could not be excluded with certainty. The patients should be cautioned that macular dystrophy may develop in adults older than 30 years.

Adolescent↗

PubMed2006

Late stages of visual pigment photolysis in situ: cones vs. rods.

Slow photolysis reactions and the regeneration of the dark pigment constitute the mechanisms of dark adaptation whereby photoreceptor cells restore their sensitivity after bright illumination. We present data on the kinetics of the late stages of the photolysis of the visual pigment in intact rods and red- and green-sensitive cones of the goldfish retina. Measurements were made on single photoreceptors by means of a fast-scanning dichroic microspectrophotometer. We show that in cones the hydrolysis of the opsin-all-trans 3-dehydroretinal linkage proceeds with a half-time of approximately 5s at 20 degrees C that is almost two orders of magnitude faster than in rods. 3-Dehydroretinol in cones is produced approximately 3-fold faster than retinol in amphibian rhodopsin rods; the rate of the reaction is limited by the speed of retinal reduction catalyzed by retinoldehydrogenase. The fast hydrolysis of the 3-dehydroretinal/opsin Schiff base and the correspondingly fast appearance of the substrates for dark visual pigment regeneration (free opsin and 3-dehydroretinol) provide essential conditions for faster dark adaptation of cone (diurnal) as compared to rod (nocturnal) vision.

Animals↗

PubMed2001

Topography of different photoreceptor cell types in the larval retina of Atlantic halibut (Hippoglossus hippoglossus).

The identities of single cone cells in the retina of Atlantic halibut (Hippoglossus hippoglossus) larvae were studied by in situ hybridisation using RNA probes for the five different halibut opsins. Four different cone opsins (ultraviolet-, blue-, green- and red-sensitive) are expressed in Atlantic halibut at the end of the yolk-sac period, whereas rod opsin is expressed later in development. Photoreceptor cells expressing ultraviolet-sensitive opsin are found only in the ventral retina, presumably to optimise detection of the downwelling ultraviolet light. The majority of the photoreceptors (approximately 90%) in the retina express green-sensitive opsin and its distribution shows no regional differences. In contrast, blue- and red-sensitive opsins are expressed much less frequently (in approximately 10% of photoreceptors), although these two opsins are also found over the entire retina. The expression patterns of the different visual pigments indicate some form of mosaic expression in the single-coned larval retina, and this is reminiscent of the square mosaic expression found in post-metamorphic Atlantic halibut. These findings suggest plasticity in green-opsin-expressing cells during development, resulting in a square mosaic expression pattern.

Animals↗

PubMed2000

Protection of Rpe65-deficient mice identifies rhodopsin as a mediator of light-induced retinal degeneration.

Light-induced apoptosis of photoreceptors represents an animal model for retinal degeneration. Major human diseases that affect vision, such as age-related macular degeneration (AMD) and some forms of retinitis pigmentosa (RP), may be promoted by light. The receptor mediating light damage, however, has not yet been conclusively identified; candidate molecules include prostaglandin synthase, cytochrome oxidase, rhodopsin, and opsins of the cones and the retinal pigment epithelium (PE). We exposed to bright light two groups of genetically altered mice that lack the visual pigment rhodopsin (Rpe65-/- and Rho-/-). The gene Rpe65 is specifically expressed in the PE and essential for the re-isomerization of all-trans retinol in the visual cycle and thus for the regeneration of rhodopsin after bleaching. Rho-/- mice do not express the apoprotein opsin in photoreceptors, which, consequently, do not contain rhodopsin. We show that photoreceptors lacking rhodopsin in these mice are completely protected against light-induced apoptosis. The transcription factor AP-1, a central element in the apoptotic response to light, is not activated in the absence of rhodopsin, indicating that rhodopsin is essential for the generation or transduction of the intracellular death signal induced by light.

Animals↗

PubMed1991

Quantum yield of CHAPSO-solubilized rhodopsin and 3-hydroxy retinal containing bovine opsin.

The quantum yields of bleaching for two artificial pigments, bovine opsin combined with (3R)-3-hydroxy retinal or (3R,S)-3-methoxy retinal, were determined in comparison to the value for regenerated bovine rhodopsin. Regeneration of the visual pigments was performed by incubation of 3-[(3-Cholamidopropyl)-dimethylammonio]-2-hydroxy-1- propanesulfonate (CHAPSO)-solubilized opsin with the 11-cis isomers of retinal and the respective retinal derivatives. The extinction coefficients of the pigments in CHAPSO were determined to 35,000 M-1 cm-1 (native rhodopsin), 35,300 M-1 cm-1 (regenerated rhodopsin) and 34,500 M-1 cm-1 (3-OH retinal opsin). With respect to rhodopsin (lambda max: 500 nm), the pigments carrying the substituted chromophores exhibit blue shifted absorbance maxima (3-hydroxy and 3-methoxy retinal opsin: 488 nm). In parallel experiments under absolutely identical conditions we find related to the value of CHAPSO solubilized rhodopsin (identical to 1) a quantum efficiency of bleaching for the 3-hydroxy pigment of 1.2.

Animals↗

PubMed2005

The signaling pathway in photoresponses that may be mediated by visual pigments in erythrophores of Nile tilapia.

The ability to increase the synthesis or vary the distribution of pigment in response to light is an important feature of many pigment cells. Unlike other light-sensitive pigment cells, erythrophores of Nile tilapia change the direction of pigment migration depending on the peak wavelength of incident light: light near 365, 400 or 600 nm induces pigment aggregation, while dispersion occurs in response to light at 500 nm. How these phenomena are achieved is currently unknown. In the present study, the phototransduction involved in the pigment dispersion caused by light at 500 nm or the aggregation by light at 600 nm was examined, using pertussis toxin, cholera toxin, blockers of ion channels, various chemicals affecting serial steps of signaling pathways and membrane-permeable cAMP analog. The results show that light-induced bidirectional movements in tilapia erythrophores may be controlled by cytosolic cAMP levels via Gi- or Gs-type G proteins. In addition, RT-PCR demonstrated for the first time the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist. Here, we suggest that multiple cone-type visual pigments may be present in the erythrophores, and that unique cascades in which such opsins couple to Gi or Gs-type G proteins are involved in the photoresponses in these pigment cells. Thus, tilapia erythrophore system seems to be a nice model for understanding the photoresponses of cells other than visual cells.

Animals↗

PubMed2004

Expression of melanopsin during development of the rat retina.

There is accumulating evidence that the new opsin-like protein, melanopsin, in adult rodents functions as non-visual photoreceptor. Here we report using immunohistochemistry and in situ hybridisation that melanopsin during rat retinal development is expressed already at prenatal day 18 in cells of the inner neuroblast layer. Perinatally the melanopsin positive cells increase in number and migrate towards the ganglion cell layer. During early postnatal development a melanopsin immunoreactive dendritic network is formed in the inner plexiform layer. Melanopsin is exclusively expressed in PACAP-containing cells which in adults become the retinal ganglion cells constituting the retinohypothalamic tract. The early expression of melanopsin argues for a photoreceptor role in the developing retinohypothalamic tract which is functional as early as the first day after birth.

Animals↗

PubMed1997

Mechanisms of wavelength tuning in the rod opsins of deep-sea fishes.

The main object of this study was to investigate the molecular basis for changes in the spectral sensitivity of the visual pigments of deep-sea fishes. The four teleost species studied, Hoplostethus mediterraneus, Cataetyx laticeps, Gonostoma elongatum and Histiobranchus bathybius, are phylogenetically distant from each other and live at depths ranging from 500 to almost 5000 m. A single fragment of the intronless rod opsin gene was PCR-amplified from each fish and sequenced. The wavelength of peak sensitivity for the rod visual pigments of the four deep-sea species varies from 483 nm in H. mediterraneus and G. elongatum to 468 nm in C. laticeps. Six amino acids at sites on the inner face of the chromophore-binding pocket formed by the seven transmembrane a-helices are identified as candidates for spectral tuning. Substitutions at these sites involve either a change of charge, or a gain or loss of a hydroxyl group. Two of these, at positions 83 and 292, are consistently substituted in the visual pigments of all four species and are likely to be responsible for the shortwave sensitivity of the pigments. Shifts to wavelengths shorter than 480 nm may involve substitution at one or more of the remaining four sites. None of the modifications found in the derived sequences of these opsins suggest functional adaptations, such as increased content of hydroxyl-bearing or proline residues, to resist denaturation by the elevated hydrostatic pressures of the deep sea. Phylogenetic evidence for the duplication of the rod opsin gene in the Anguilliform lineage is presented.

Adaptation, Physiological↗

PubMed1998

Contrasting levels of DNA polymorphism at the autosomal and X-linked visual color pigment loci in humans and squirrel monkeys.

The X-linked color pigment (opsin) locus is known to be highly polymorphic in the squirrel monkey and other New World monkeys. To see whether this is also the case for the autosomal (blue) opsin locus, we obtained 32 squirrel monkey and 30 human blue opsin gene sequences. No amino acid polymorphism was found in either the squirrel monkey sample or the human sample, contrary to the situation at the X-linked opsin locus. This sharp contrast in the level of polymorphism might be due to differences in gene expression between the autosomal and the X-linked loci. At the X-linked locus, heterozygote advantage can occur because, owing to X-inactivation, the two alleles in a heterozygote are expressed in different cone cells, producing two types of cone cell, whereas at the autosomal locus, heterozygote advantage cannot occur because the two alleles in a heterozygote are expressed in the same cone cells, producing only one type of cone cell (i.e., phenotypically a homozygote). From the sequence data, the levels of nucleotide diversity (pi, i.e., the number of nucleotide differences per site) are estimated: for the human sample, pi = 0.00% per nondegenerate site, 0.00% per twofold degenerate site, and 0.04% per fourfold degenerate site in the coding regions and 0.01% per site in intron 4; for the squirrel monkey sample, pi = 0.00% per nondegenerate site, 0.00% per twofold degenerate site, and 0.15% per fourfold degenerate site in the coding regions and 0.17% per site in intron 4. The blue opsin genes from the common and pygmy chimpanzees, the gorilla, the capuchin, and the howler monkey were also sequenced. Features critical to the function of the opsin are well conserved in all known mammalian sequences. However, the interhelical loops are, on average, actually more conservative than the transmembrane helical regions. In addition, these sequence data and those from some other genes indicate that the common and pygmy chimpanzees are not closely related, their divergence data being from one third to one half the date of the human-chimpanzee divergence.

Amino Acid Sequence↗

PubMed2004

Nonvisual photoreception in the chick iris.

The embryonic chicken iris constricts to light ex vivo, but with characteristics atypical of visual phototransduction. The chick iris was most sensitive to short-wavelength light, demonstrating an action spectrum consistent with cryptochrome rather than with opsin pigments. Pupillary responses did not attenuate after saturating light exposure, but showed paradoxical potentiation. Iris photosensitivity was not affected by retinoid depletion or inhibitors of visual phototransduction. Knockdown of cryptochrome expression, but not of melanopsin expression, decreased iris photosensitivity. These data characterize a non-opsin photoreception mechanism in a vertebrate eye and suggest a conserved photoreceptive role for cryptochromes in vertebrates.

Animals↗

PubMed1991

Maintenance of opsin density in photoreceptor outer segments of retinoid-deprived rats.

Dietary deficiency in the retinoid precursors of the visual pigment chromophore 11-cis-retinal eventually results in selective degeneration of the photoreceptor cells of the vertebrate retina. Early effects of retinoid deficiency are depletion of rhodopsin from the retina and vesiculation of the photoreceptor outer segment disc membranes. Experiments were conducted to determine whether these early changes were accompanied by an alteration of the opsin content of the disc membranes. After being fed a retinoid-deficient diet containing retinoic acid for 26 weeks, the rhodopsin content of rat retinas was reduced by over 85%. Both the diameters and the lengths of the outer segments decreased significantly. However, immunocytochemical and freeze-fracture analyses indicated that retinoid deficiency did not lower opsin density in the outer-segment disc membranes. These findings indicate that in the rat, opsin synthesis and disc assembly are coordinated processes that remain coupled despite reduced availability of the vitamin A chromophore. The fact that disc size decreases and disc synthesis eventually ceases in retinoid-deprived rats indicates that specific retinoids are essential for disc morphogenesis. The mechanism by which these retinoids regulate disc assembly remains to be determined.

Analysis of Variance↗