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PubMedDate not supplied

Packaging of rhodopsin and porphyropsin in the compound eye of the crayfish.

The distribution of 3-dehydroretinal (Ral2) in dorsal, middle, and ventral slices of eyes of the crayfish Procambarus clarkii was examined by HPLC. No pronounced differences were found. Similar results were obtained when the eyes were cut into anterior, intermediate, and posterior portions. Dichroic difference spectra were measured in single halves of microvillar layers of isolated rhabdoms and the proportions of rhodopsin (P1) and porphyropsin (P2) estimated by comparison with computer-generated mixtures of these pigments, whose spectra are known from previous work. The fraction of visual pigment that is porphyropsin appears to be uniform throughout individual retinular cells and among the retinular cells of individual rhabdoms, but various substantially among different rhabdoms from the same eye. The interommatidial variation in the amount of P2 greatly exceeds the gross regional variation in Ral2. This means there is an intermingling of ommatidia with different levels of P2. The variability in P2 among ommatidia is not likely to have important implications for the vision of the crayfish but suggests that in the metabolism of retinoids, individual ommatidia are quasi-independent metabolic units. The results are compatible with a single opsin for both crayfish rhodopsin and porphyropsin.

Animals↗
PubMed1993

Zebrafish ultraviolet visual pigment: absorption spectrum, sequence, and localization.

In many vertebrates, UV-sensitive photoreceptors have been identified by microspectrophotometry and UV-visual sensitivity has been identified by behavioral studies, but as yet no vertebrate UV-sensitive pigment gene has been isolated. We have sequenced a cDNA clone that hybridizes to short single cone cells in the zebrafish (Brachydanio rerio). These cells, which make up 25% of the cone population in zebrafish retinae, are UV-sensitive (lambda max approximately 360 nm). The visual pigment encoded by this gene is unusual in that its amino acid sequence is more homologous to the rod pigment rhodopsin (up to 89%) than to other cone pigments (35-83%). Like all other vertebrate visual pigments, it contains a lysine residue at position 296, the presumptive retinal binding site, and a glutamate residue at position 113. However, it is unique in possessing a lysine residue at position 126, which may account for the UV-sensitivity of the pigment.

Amino Acid Sequence↗
PubMed1990

Functional expression in vitro of bovine visual rhodopsin.

Expression of functionally active bovine visual rhodopsin was achieved by sequential transcription and translation in vitro of rhodopsin gene cDNA with co-translational insertion of the protein into phosphatidylcholine liposomes. The recombinant rhodopsin has functional, spectral and immunochemical properties similar to those of natural rhodopsin from bovine retina. Two mutant rhodopsins, Cys316----Ser and Asp330----Asn, Asp331----Asn, were produced by oligonucleotide-directed mutagenesis. The first mutation does not affect rhodopsin's ability to stimulate transducin GTPase and visual cGMP phosphodiesterase activities, while the second double mutation leads to a sharp decrease in rhodopsin activity.

Animals↗
PubMed1994

Rod outer segment retinol dehydrogenase: substrate specificity and role in phototransduction.

The reaction catalyzed by all-trans-retinol dehydrogenase of rod outer segments completes the quenching of photoactivated rhodopsin and initiates the cycle of reactions leading to regeneration of visual pigment. The goal of this study was to determine the kinetic parameters of the dehydrogenase at physiological levels of bleaching, to investigate its specificity, and to determine its possible role in modulating phototransduction. Reduction of all-trans-retinal could be measured after bleaching < 0.15% rhodopsin. Kinetic parameters for the forward reaction determined with endogenous all-trans-retinal were Km = 1.1 microM; Vmax = 7 nmol/min/mg rhodopsin. The low enzymatic activity suggests that at high bleach rates, all-trans-retinal could accumulate, increasing the steady state level of bleaching intermediates or promoting formation of pseudophotoproducts. Active pseudophotoproducts, which stimulate Gt activation and opsin phosphorylation by rhodopsin kinase, are formed with opsin and all-trans-retinal as well as retinal analogues lacking the 13 methyl or the terminal two carbons of the polyene chain. Addition of all-trans-retinol, NADP, and [32P]ATP to rod outer segments increased rhodopsin phosphorylation. Kinetic parameters for the reverse reaction determined with exogenous all-trans-retinol were Km = 10 microM; Vmax = 11 nmol/min/mg rhodopsin. Our results support the hypothesis that all-trans-retinol dehydrogenase could influence the phototransduction cascade, including activities of Gt, rhodopsin kinase, and binding of arrestin, by impeding the recycling of rhodopsin at high bleach levels.

Alcohol Oxidoreductases↗
PubMed1992

Rhodopsin regeneration in the normal and in the detached/replaced retina of the skate.

The bleaching and regeneration of rhodopsin in the skate retina was studied by means of fundus reflectometry, both in the normal eyecup preparation and after the retina had been detached and then replaced on the surface of the pigment epithelium (RPE). After bleaching virtually all the rhodopsin in the retinal test area of the normal eyecup, more than 90% of the photopigment was reformed after about 2 hr in darkness; over most of this time course, rhodopsin density rose linearly at a rate of 0.875% min-1 with a half-time of 55 min. Detaching the retina from its pigment epithelium resulted in a number of abnormalities, both structural and functional. Histological examination of the detached/replaced (D/R) retina showed striking alterations in the structural integrity of the RPE cells at their interface with the neural retina. The cells appeared vacuolated and misshapen, and the apical processes of the RPE, which normally ensheath the receptor outer segments, were shredded and free of their association with the visual cells. These morphological changes, as well as dilution of the IRBP content of the subretinal space caused by separation of the tissues, appear to be the main factors contributing to the functional abnormalities in rhodopsin kinetics. But despite these abnormalities and the persistent detachment, the rate of regeneration and the amount of rhodopsin reformed after bleaching were reduced by less than 50% of their normal values. The fact that a significant fraction of the bleached rhodopsin was regenerated under these conditions indicates that 11-cis retinal formed in the RPE was able to traverse a much greater than normal subretinal space to reach the opsin-bearing photoreceptor membranes.

Animals↗
PubMed2003

Homothorax switches function of Drosophila photoreceptors from color to polarized light sensors.

Different classes of photoreceptors (PRs) allow animals to perceive various types of visual information. In the Drosophila eye, the outer PRs of each ommatidium are involved in motion detection while the inner PRs mediate color vision. In addition, flies use a specialized class of inner PRs in the "dorsal rim area" of the eye (DRA) to detect the e-vector of polarized light, allowing them to exploit skylight polarization for orientation. We show that homothorax is both necessary and sufficient for inner PRs to adopt the polarization-sensitive DRA fate instead of the color-sensitive default state. Homothorax increases rhabdomere size and uncouples R7-R8 communication to allow both cells to express the same opsin rather than different ones as required for color vision. Homothorax expression is induced by the iroquois complex and the wingless (wg) pathway. However, crucial wg pathway components are not required, suggesting that additional signals are involved.

Animals↗
PubMed2004

Recombinant adeno-associated virus type 2-mediated gene delivery into the Rpe65-/- knockout mouse eye results in limited rescue.

BACKGROUND: Leber's congenital amaurosis (LCA) is a severe form of retinal dystrophy. Mutations in the RPE65 gene, which is abundantly expressed in retinal pigment epithelial (RPE) cells, account for approximately 10-15% of LCA cases. In this study we used the high turnover, and rapid breeding and maturation time of the Rpe65-/- knockout mice to assess the efficacy of using rAAV-mediated gene therapy to replace the disrupted RPE65 gene. The potential for rAAV-mediated gene treatment of LCA was then analyzed by determining the pattern of RPE65 expression, the physiological and histological effects that it produced, and any improvement in visual function. METHODS: rAAV.RPE65 was injected into the subretinal space of Rpe65-/- knockout mice and control mice. Histological and immunohistological analyses were performed to evaluate any rescue of photoreceptors and to determine longevity and pattern of transgene expression. Electron microscopy was used to examine ultrastructural changes, and electroretinography was used to measure changes in visual function following rAAV.RPE65 injection. RESULTS: rAAV-mediated RPE65 expression was detected for up to 18 months post injection. The delivery of rAAV.RPE65 to Rpe65-/- mouse retinas resulted in a transient improvement in the maximum b-wave amplitude under both scotopic and photopic conditions (76% and 59% increase above uninjected controls, respectively) but no changes were observed in a-wave amplitude. However, this increase in b-wave amplitude was not accompanied by any slow down in photoreceptor degeneration or apoptotic cell death. Delivery of rAAV.RPE65 also resulted in a decrease in retinyl ester lipid droplets and an increase in short wavelength cone opsin-positive cells, suggesting that the recovery of RPE65 expression has long-term benefits for retinal health. CONCLUSION: This work demonstrated the potential benefits of using the Rpe65-/- mice to study the effects and mechanism of rAAV.RPE65-mediated gene delivery into the retina. Although the functional recovery in this model was not as robust as in the dog model, these experiments provided important clues about the long-term physiological benefits of restoration of RPE65 expression in the retina.

Journal Article↗
PubMed1997

Photic regulation of pineal function. Analogies between retinal and pineal photoreception.

Light absorbed by a photopigment in a photoreceptor cell causes a photochemical reaction converting the 11-cis retinal chromophore into the all-trans configuration. These changes lead to a series of events that causes cGMP hydrolysis, a following decrease of cGMP in the cytoplasm of the photoreceptor outer segment and a closure of cGMP-gated cationic channels. As a consequence of these processes the membrane hyperpolarizes. In pineal photoreceptor cells of lower vertebrates these processes are only partly investigated. Molecules involved in the phototransduction process and the desensitization, like opsin, vitamin A, alpha-transducin and arrestin, have been immunocytochemically localized in pineal photoreceptors and also electrophysiological studies have shown that phototransduction mechanisms in pineal photoreceptors might be very similar to those found in retinal photoreceptors. This review will summarize some of the current knowledge on pineal photoreception and compare it with retinal processes.

Animals↗
PubMed1989

The Gecko visual pigment: the dark exchange reaction and the effects of anions.

A dark reaction is known to occur in retinal extracts of the gecko (Gekko gekko), in which the natural 11-cis-chromophore of the 521-pigment is apparently replaced by adding 9-cis-retinal to form the 9-cis-photopigment. With chloride-deficient extracts the reaction involves some 70% of the 521-pigment. Anions like nitrate, azide, thiocyanate and cyanate that shift the spectrum toward the blue do not affect this 70% exchange. Anions like fluoride, iodide and sulfate likewise do not alter this magnitude of reaction. In contrast, chloride and bromide that induce a bathochromic spectral shift lead to a decrease in this dark replacement of the 11-cis chromophore. This protection is similar to the action of these two anions in antagonizing the pigment loss by NH2OH and by temperature, both occurring in the dark. Apparently, chloride and bromide alter the opsin conformation so as to stabilize and/or protect the Schiff's base linkage but nitrate, azide, thiocyanate and cyanate act at a different opsin site or by a different mechanism.

Animals↗
PubMed2003

Melanopsin is required for non-image-forming photic responses in blind mice.

Although mice lacking rod and cone photoreceptors are blind, they retain many eye-mediated responses to light, possibly through photosensitive retinal ganglion cells. These cells express melanopsin, a photopigment that confers this photosensitivity. Mice lacking melanopsin still retain nonvisual photoreception, suggesting that rods and cones could operate in this capacity. We observed that mice with both outer-retinal degeneration and a deficiency in melanopsin exhibited complete loss of photoentrainment of the circadian oscillator, pupillary light responses, photic suppression of arylalkylamine-N-acetyltransferase transcript, and acute suppression of locomotor activity by light. This indicates the importance of both nonvisual and classical visual photoreceptor systems for nonvisual photic responses in mammals.

Animals↗
PubMedDate not supplied

Circadian organization in quail retina: differential regulation of melatonin synthesis and iodopsin gene expression in vitro.

Adult Japanese quail have an endogenous circadian clock located in their eyes that has been shown to regulate melatonin biosynthesis. We investigated if a circadian oscillator is present in cultures of dispersed embryonic quail retina. Melatonin release in retinal cell culture is modulated by the light cycle, indicating that there are functional photoreceptors in culture. However, when cultures were placed in constant darkness no rhythm of melatonin was observed, indicating that at this period of development the circadian oscillator does not influence melatonin release. To explore further the question of whether a circadian oscillator is present in embryonic cell culture, we examined expression of iodopsin, the red visual pigment. Iodopsin mRNA is expressed in a circadian rhythm with peak levels occurring late in the afternoon (ZT 9). Analysis indicates that the clock influence is at the level of gene transcription. These results suggest that a clock is not "hooked up" to melatonin release embryonically or that a different oscillator regulates photopigment expression versus melatonin release.

Animals↗
PubMed2005

Does the chromophore's ring move after photoexcitation of rhodopsin?

By comparing the shift of the absorption maxima when a visual pigment is converted to its lumirhodopsin photointermediate for two classes of pigments, we can infer whether or not the pigment's beta-ionone ring has left its binding site. We compare this shift for the long-wavelength sensitive visual pigment of chicken iodopsin (lambdamax = 571 nm), which has polar residues in the ring binding site that interact with the ring, with that for three pigments, which do not. We conclude that by the time the Lumi product of the pigment is formed, the ring has moved away from the ring binding site.

Animals↗
PubMed1999

The ecology of visual pigments.

The visual systems of vertebrates have adapted to function in photic environments ranging from the broad spectrum of full sunlight to almost total darkness, including the restricted spectral ranges found in different coloured aquatic environments. Such adaptations are immediately obvious at the level of retinal photoreceptors. The basic vertebrate photoreceptor pattern consists of rods and four spectrally distinct classes of cone that span the spectrum from the near ultraviolet to the far red. This arrangement is found in many diurnal species including shallow-living teleosts, reptiles and birds, but is noticeably absent in mammals. In freshwater teleosts the visual pigments may be porphyropsins which have maximum sensitivities displaced to longer wavelengths than their equivalent rhodopsins. Water acts as a monochromator, so that with increasing depth the spectral range of the ambient light is restricted, primarily at long wavelengths. Therefore, at depth the down-welling daylight is not only attenuated in intensity, but is restricted to a narrow spectral band centred around 470 nm. Closely related species that live at increasing depths show a loss of long-wave-sensitive cones and a displacement of the maximum sensitivities of middle-wave-sensitive cones and rods to shorter wavelengths. Such species offer a natural model for determining specific amino acids in opsin responsible for the spectral tuning of these middle-wave-sensitive pigments.

Animals↗
PubMed2004

Dark adaptation and the retinoid cycle of vision.

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.

Animals↗
PubMed2001

A null mutation in guanylate cyclase-1 alters the temporal dynamics and light entrainment properties of the iodopsin rhythm in cone photoreceptor cells.

Guanylate cyclase-1 (GC1) plays a critical role in visual phototransduction and its absence severely compromises the ability of the photoreceptor cells to transduce light for vision. In this study we sought to determine if the absence of GC1 has any effect on light entrainment of the circadian oscillators located in these cells. We compared the rhythmic changes in transcript levels of iodopsin, a photoreceptor-specific gene whose expression is regulated by circadian oscillators, in retinas of normal chickens and GUCY1*B (*B) chickens that carry a null mutation in GC1. Our results show that iodopsin rhythms are present in *B retinas and that they can be entrained to light; however, the rise and fall of iodopsin transcript levels in *B retina under cyclic light conditions is significantly more rapid than that observed in normal retina, and under constant dark conditions, the phase of the iodopsin rhythm in *B retina is advanced by 6 h relative to that observed in normal retina. In addition, the rate of entrainment of the iodopsin rhythm in *B retina to a reversal of the light cycle is significantly slower than normal. The results of our study show that a functioning visual phototransduction cascade is not essential for light entrainment of the oscillators that drive the iodopsin rhythm in photoreceptor cells. We propose that the abnormal synthesis of cGMP in *B photoreceptors underlies the irregular iodopsin rhythms observed in post-hatch *B retina.

Animals↗
PubMed2002

The nuclear receptor NR2E3 plays a role in human retinal photoreceptor differentiation and degeneration.

Normal human retinal development involves orderly generation of rods and cones by complex mechanisms. Cell-fate specification involves progenitor cell lineage and external signals such as soluble factors and cell-cell interactions. In most inherited human retinal degenerations, including retinitis pigmentosa, a mutant gene causes loss of visual function, death of mature rods, and eventually death of all cone subtypes. Only one inherited retinal disorder, the enhanced S cone syndrome (ESCS), shows increased visual function, involving the minority S (blue) cones, and decreased rod and L/M (red/green) cone function. This autosomal recessive disease is caused by mutations in NR2E3, a photoreceptor nuclear receptor transcription factor, and may result from abnormal cell-fate determination, leading to excess S cones at the expense of other photoreceptor subtypes. In 16 ESCS patients with the most common NR2E3 mutation, R311Q, we documented an abnormal ratio of S to L/M cone function and progressive retinal degeneration. We studied the postmortem retina of an ESCS patient homozygous for NR2E3 R311Q. No rods were identified, but cones were increased approximately 2-fold, and 92% were S cones. Only 15% of the cones expressed L/M cone opsin, and some coexpressed S cone opsin. The retina was disorganized, with densely packed cones intermixed with inner retinal neurons. The retina was also degenerate, retaining photoreceptors in only the central and far peripheral regions. These observations suggest a key role for NR2E3 in regulation of human photoreceptor development. Degeneration of the NR2E3 retina may result from defective development, known S cone fragility, or abnormal maintenance of mature photoreceptors.

Adult↗
PubMed2003

(9Z)- and (11Z)-8-methylretinals for artificial visual pigment studies: stereoselective synthesis, structure, and binding models.

Artificial visual pigment formation was studied by using 8-methyl-substituted retinals in an effort to understand the effect that alkyl substitution of the chromophore side chain has on the visual cycle. The stereoselective synthesis of the 9-cis and 11-cis isomers of 8-methylretinal, as well as the 5-demethylated analogues is also described. The key bond formations consist of a thallium-accelerated Suzuki cross-coupling reaction between cyclohexenylboronic acids and dienyliodides (C6-C7), and a highly stereocontrolled Horner-Wadsworth-Emmons or Wittig condensation (C11-C12). The cyclohexenylboronic acid was prepared by trapping the precursor cyclohexenyllithium species with B(OiPr)(3) or B(OMe)(3). The cyclohexenyllithium species is itself obtained by nBuLi-induced elimination of a trisylhydrazone (Shapiro reaction), or depending upon the steric hindrance of the ring, by iodine-metal exchange. In binding experiments with the apoprotein opsin, only 9-cis-5-demethyl-8-methylretinal yielded an artificial pigment; 9-cis-8-methylretinal simply provided residual binding, while evidence of artificial pigment formation was not found for the 11-cis analogues. Molecular-mechanics-based docking simulations with the crystal structure of rhodopsin have allowed us to rationalize the lack of binding displayed by the 11-cis analogues. Our results indicate that these isomers are highly strained, especially when bound, due to steric clashes with the receptor, and that these interactions are undoubtedly alleviated when 9-cis-5-demethyl-8-methylretinal binds opsin.

Binding Sites↗
PubMed1982

Pathogenesis of progressive rod-cone degeneration in miniature poodles.

Visual cell pathologic changes and outer segment renewal were investigated in miniature poodles with progressive rod-cone degeneration. Early in this disease, visual cells in the posterior pole and equatorial regions show outer segment lamellar disorientation and vesicular profiles. Visual cells are normal in the periphery. Outer segment renewal determined after intravitreal injection of 3H-leucine was abnormally slower in affected animals than in controls. This renewal abnormality was similar in structurally normal and diseased photoreceptors, suggesting that the renewal defect is the earliest recognizable abnormality in the disease. The pigment epithelium was normal; the presence and density of pigment did not appear to affect the extent and severity of the disease or modify the abnormal renewal rate. As the disease progressed, photoreceptor outer segments were lost, and the remaining diminutive photoreceptors accumulated label in the inner segment and perinuclear zones. Sodium dodecyl sulfate gel electrophoresis of crude rod outer segment preparations showed no differences in opsin synthesis between normal and affected retinas early in the disease, but opsin synthesis decreased in the late stage of the disease.

Animals↗