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PubMed1963

CHANGES IN ELECTRICAL CONDUCTANCE OF RHODOPSIN ON PHOTOLYSIS.

The change in electrical conductance of rhodopsin solutions was studied with flash-photolysis techniques. The whole pattern of the conductance change on illumination consists of three different processes: (I) the initial decrease, (II) the increase, and (III) the slow decrease, which are in decreasing order of reaction rate. The processes I, II, and III can be most distinctly recognized on flash illumination of acid, slightly acid, and alkaline rhodopsins, respectively. On the other hand, the bleaching of rhodopsin also shows at least three successive phases of different rates, but none of them corresponds in reaction rate to any of the processes of the conductance change. The conductance change may be related to conformational changes of opsin following photoisomerization of retinene, being due to hydrogen or hydroxyl ions and some other inorganic electrolytes. The amount of the change, especially the initial decrease, is proportional to the amount of rhodopsin bleached, even when the photochemical back reaction towards rhodopsin and isorhodopsin occurs in the chromophore depending on the intensity of illumination. Of the three processes, the slow decrease is most severely affected by aging, but the initial decrease and increase are slightly affected. These two processes promptly caused by illumination are connected closely to the conformational changes during the conversion of rhodopsin to metarhodopsin, and perhaps to the initial stage of excitation of rod cells.

Chemical Phenomena↗
PubMed2002

Multiple mechanisms of rhabdom shedding in the lateral eye of Limulus polyphemus.

Rhabdom shedding in horseshoe crab lateral eye photoreceptors was studied with anti-opsin and anti-arrestin immunocytochemistry. Two, possibly three, distinct shedding mechanisms were revealed in animals maintained in natural lighting. Transient rhabdom shedding, triggered by dawn, is a brief, synchronous event that removes up to 10% of the rhabdom membrane. Whorls of rhabdomeral membrane break into vesicles and form compact multivesicular bodies. These debris particles are immunoreactive for opsin and are of a relatively uniform size, averaging approximately 2 microm(2) in area. Transient shedding requires that input from circadian efferent fibers to the retina precedes the light trigger, and cutting the optic nerve blocks efferent input and transient shedding. Light-driven rhabdom shedding is a progressive process. Rhabdomeral membrane is removed by coated vesicles that accumulate into loosely packed multivesicular bodies. These debris particles label with antibodies directed against opsin, arrestin, and adaptin, and they have a large distribution of sizes, averaging almost 6 microm(2) in area and ranging up to 25 microm(2) or more. The amount of rhabdomeral membrane removed by light-driven shedding has seasonal variation and depends on latitude. Light-driven shedding does not require circadian efferent input. A possible third shedding mechanism, light-independent shedding, is observed when transient shedding is blocked either by 48 hours of darkness or by cutting the optic nerve. Small particles, averaging 1.8 microm(2) in area, exhibiting opsin but not arrestin immunoreactivity can then be found in the cytoplasm surrounding the rhabdom. The nature of light-independent shedding is not yet clear.

Amino Acid Sequence↗
PubMed2007

Role of a novel photopigment, melanopsin, in behavioral adaptation to light.

Adaptation to changes in the ambient light is of critical importance to life. In mammals, three principal photoadaptation mechanisms depend on ocular photoreception and exhibit spectral sensitivity suggestive of the opsin class of photopigment(s). These include rapid adaptation of the visual system to the ambient light by pupil constriction, direct modulation of neuroendocrine function and entrainment of the circadian clock to the day:night cycle. Surprisingly, these processes can largely function independent of classical rod/cone photoreceptors, suggesting a novel opsin-based signaling mechanism. They appear to involve a recently discovered network of intrinsically photosensitive retinal ganglion cells that make direct or indirect axonal connections to brain centers regulating photoadaptive behaviors. The discovery of a novel opsin, melanopsin, in these cells has offered an exciting entry point to explore, at the molecular level, how mammals adapt to their light environment. There is now genetic proof of a principal role for melanopsin in all three major photoadaptation processes.

Adaptation, Biological↗
PubMed1994

What makes red visual pigments red? A resonance Raman microprobe study of retinal chromophore structure in iodopsin.

We have obtained resonance Raman spectra of iodopsin, a red-sensitive (lambda max 571 nm) pigment from chicken cone cells, to investigate the molecular mechanism of the opsin shift in visual pigments. Detergent-solubilized iodopsin samples were examined with a Raman microprobe to obtain spectra from a 77-K photostationary steady-state mixture composed of 11-cis-iodopsin and its 9-cis-isoiodopsin and all-trans-bathoiodopsin photoproducts. The vibrational modes of these species have been assigned by comparison with spectra of the corresponding bovine pigments. The single bond stretching frequencies of the bovine, toad, and chicken pigments are found to exhibit a regular correlation as a function of the pigment absorption maxima that is consistent with the expected effects of increased electron delocalization. The C = NH stretching frequencies of iodopsin and bathoiodopsin are at 1644 and 1638 cm-1, respectively, and shift down to 1621 and 1617 cm-1, respectively, when the nitrogen is deuterated. The C = ND stretching frequencies of the various pigments are found to decrease linearly with increasing absorption maxima, suggesting that at least part of the opsin shift in visual pigments results from weakened electrostatic interaction between the retinal chromophore and its protein counterion. The Raman data are inconsistent with the idea that a charged protein residue is shifted along the chromophore to regulate the opsin shift. Taken together with the mutagenesis and model compound results, these resonance Raman data suggest that the opsin shift between the green and red cone visual pigment arises from two effects. First, Tyr-274 provides increased electrostatic stabilization of the Schiff base-counterion ion pair. Second, the opsin shift is enhanced by the dipolar residues Ser-177 and Thr-282 that interact with the chromophore near the ionone ring to preferentially stabilize the highly dipolar charge distribution of the electronically excited retinal chromophore [Mathies, R., & Stryer, L. (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 2169-2173].

Animals↗
PubMed2003

Spontaneous activity of opsin apoprotein is a cause of Leber congenital amaurosis.

Mutations in Rpe65 disrupt synthesis of the opsin chromophore ligand 11-cis-retinal and cause Leber congenital amaurosis (LCA), a severe, early-onset retinal dystrophy. To test whether light-independent signaling by unliganded opsin causes the degeneration, we used Rpe65-null mice, a model of LCA. Dark-adapted Rpe65-/- mice behaved as if light adapted, exhibiting reduced circulating current, accelerated response turn-off, and diminished intracellular calcium. A genetic block of transducin signaling completely rescued degeneration irrespective of an elevated level of retinyl ester. These studies clearly show that activation of sensory transduction by unliganded opsin, and not the accumulation of retinyl esters, causes light-independent retinal degeneration in LCA. A similar mechanism may also be responsible for degeneration induced by vitamin A deprivation.

Animals↗
PubMed2004

A misassembled transmembrane domain of a polytopic protein associates with signal peptide peptidase.

The endoplasmic reticulum (ER) exerts a quality control over newly synthesized proteins and a variety of components have been implicated in the specific recognition of aberrant or misfolded polypeptides. We have exploited a site-specific cross-linking approach to search for novel ER components that may specifically recognize the misassembled transmembrane domains present in truncated polytopic proteins. We find that a single probe located in the transmembrane domain of a truncated opsin fragment is cross-linked to several ER proteins. These components are distinct from subunits of the Sec61 complex and represent a 'post-translocon' environment. In this study, we identify one of these post-translocon cross-linking partners as the signal peptide peptidase (SPP). We find that the interaction of truncated opsin chains with SPP is mediated by its second transmembrane domain, and propose that this interaction may contribute to the recognition of misassembled transmembrane domains during membrane protein quality control at the ER.

Alternative Splicing↗
PubMed1996

Cloning and expression analysis of Aedes aegypti opsin: adaptation of an in situ hybridization protocol for mosquitoes.

Opsin is a G protein coupled photoreceptor that activates a signal transduction cascade in the ommatidia. Its primary and secondary structure is conserved both in insects and vertebrates as exemplified by the Drosophila opsins. Through serendiplious cloning of a PCR fragment, we have identified an opsin cDNA. The latter was used to clone full length cDNAs from a mosquito head library. The main purpose of cloning was to have a positive control probe to establish an in situ hybridization protocol for less abundant probes. Opsin-mRNA is localized specifically to the visual receptor cells in the ommatida. No other cells in the brain or the remainder of the body are positive. This is confirmed by Northern blot analysis. The sequence of the receptor, of which we have found two different transcripts, confirms its typical topology, including the seven transmembrane spanning regions and the intracellular carboxy terminus that has potential phosphorylation sites. Our in situ hybridization protocol combines several procedures: the most important points are: (a) the immediate processing of sections after cutting, and (b) the sections are never allowed to dry out once the procedure was started. Our protocol has a much higher sensitivity, using approximately 50 x lower concentrations of probe compared to published protocols. In addition to the detection of opsin-mRNA, it has been successfully applied to the detection of the low abundant insulin receptor homologue. Furthermore, Aedes aegypti probes were visualizing a similar tissue specificity when applied to the malaria mosquito Anopheles albimanus.

Aedes↗
PubMed2001

Direct innervation of GnRH neurons by encephalic photoreceptors in birds.

In nonmammalian vertebrates, photic cues that regulate the timing of seasonal reproductive cyclicity are detected by nonretinal, nonpineal deep brain photoreceptors. It has long been assumed that the underlying mechanism involves the transmission of photic information from the photoreceptor to a circadian system, and thence to the reproductive axis. An alternative hypothesis is that there is direct communication between the brain photoreceptor and the reproductive axis. In the present study, light and confocal microscopy reveal that gonadotropin releasing hormone (GnRH) neurons and processes are scattered among photoreceptor cells (identified by their opsin-immunoreactivity) in the lateral septum (SL). In the median eminence (ME), opsin and GnRH immunoreactive fibers overlap extensively. Single and double label ultrastructural immunocytochemistry indicate that in the SL and preoptic area (POA), opsin positive terminals form axo-dendritic synapses onto GnRH dendrites. In the ME, opsin and GnRH terminals lie adjacent to each other, make contact with tanycytes, or terminate on the hypophyseal portal capillaries. These results reveal thatbrain photoreceptors communicate directly with GnRH-neurons; this represents a means by which photoperiodic information reaches the reproductive axis.

Animals↗
PubMed2001

Effect of GDNF on neuroblast proliferation and photoreceptor survival: additive protection with docosahexaenoic acid.

PURPOSE: In a previous study, it was reported that docosahexaenoic acid (DHA) is essential to postpone apoptosis and to promote differentiation of rat retina photoreceptors in vitro. In the current study, the protective effects of GDNF on photoreceptor cells during development in vitro and its action when combined with DHA were investigated. METHODS: Rat retina neuronal cultures were incubated in a chemically defined medium, either without photoreceptor survival factors or supplemented with GDNF, DHA, or GDNF plus DHA. Evolution of survival, apoptosis, opsin expression, mitochondrial functioning, and cell proliferation were investigated at different times of development in vitro. RESULTS: Incubation with GDNF selectively increased the number of surviving photoreceptors, reduced their apoptosis, and augmented opsin expression. Proliferative cell nuclei antigen (PCNA) determination and addition of [(3)H]-thymidine or bromodeoxyuridine showed that GDNF promoted neuroblast proliferation during the first hours of development in vitro. The combined addition of GDNF and DHA enhanced opsin expression and photoreceptor survival in an additive manner. The advance of photoreceptor apoptosis in cultures without trophic factors correlated with an increased impairment in mitochondrial functionality. Addition of GDNF and DHA significantly diminished the loss of mitochondrial activity. CONCLUSIONS: These results show that GDNF stimulated the cell cycle progression, leading to neuroblast proliferation at early stages of development, and delayed the onset of apoptosis later on, improving differentiation and acting as a trophic factor for photoreceptors. The combination of GDNF with DHA had an additive effect both on photoreceptor survival and on opsin expression. Preservation of mitochondrial function may be involved in the antiapoptotic effect of both factors.

Animals↗
PubMed1986

Modification of ovine opsin with the photosensitive hydrophobic probe 1-azido-4-[125I]iodobenzene. Labelling of the chromophore-attachment domain.

The hydrophobic photosensitive probe 1-azido-4-[125I]iodobenzene (AIB) partitioned preferentially into photoreceptor disc membranes and, upon u.v. irradiation, became covalently bound to opsin and phospholipid. The labelling of both protein and phospholipid was linearly related to AIB concentration. The amount of probe incorporated into protein was not significantly different when membranes were irradiated at -100 degrees, 4 degrees or 25 degrees C, but irreversible aggregation of monomeric opsin was dramatically reduced by performing the photolysis at -100 degrees C. Labelling of opsin after irradiation at -100 degrees or 4 degrees was not significantly reduced by the presence of lysine in the aqueous buffer, indicating that significant amounts of reactive species did not enter the aqueous phase. The incorporation into phospholipid, unlike that into opsin, decreased as the temperature of irradiation increased. Some labelling of opsin occurred on incubation with pre-photoactivated AIB, indicating that reaction may also occur with reactive species of longer lifetimes than the nitrene. Proteolysis of labelled opsin with Staphylococcus aureus V8 proteinase yielded two radiolabelled membrane-bound fragments. The location of the modified sites (cysteine, tryptophan, tyrosine, lysine and histidine residues: all nucleophiles) in the smaller fragment was entirely consistent with putative models for the protein derived from other studies.

Affinity Labels↗
PubMed2002

Structure and function in rhodopsin: high-level expression of rhodopsin with restricted and homogeneous N-glycosylation by a tetracycline-inducible N-acetylglucosaminyltransferase I-negative HEK293S stable mammalian cell line.

An HEK293S cell line resistant to ricin was prepared by mutagenesis by using ethyl methanesulfonate. It was shown to lack N-acetylglucosaminyltransferase I (GnTI) activity, and consequently unable to synthesize complex N-glycans. The tetracycline-inducible opsin expression system was assembled into this GnTI(-) HEK293S cell line. Stable cell lines were isolated that gave tetracycline/sodium butyrate-inducible expression of the WT opsin gene at levels comparable with those observed in the parent tetracycline-inducible HEK293S cell line. Analysis of the N-glycan in rhodopsin expressed by the HEK293S GnTI(-) stable cell line showed it to be Man(5)GlcNAc(2). In a larger-scale expression experiment (1.1 liter) a WT opsin production level of 6 mg/liter was obtained. Further, the toxic constitutively active rhodopsin mutant, E113Q/E134Q/M257Y, previously shown to require inducible expression, has now been expressed in an HEK293S GNTI(-)-inducible cell line at levels comparable with those obtained with WT rhodopsin.

Animals↗
PubMed1952

The role of sulfhydryl groups in the bleaching and synthesis of rhodopsin.

The condensation of retinene(1) with opsin to form rhodopsin is optimal at pH about 6, a pH which favors the condensation of retinene(1) with sulfhydryl rather than with amino groups. The synthesis of rhodopsin, though unaffected by the less powerful sulfhydryl reagents, monoiodoacetic acid and its amide, is inhibited completely by p-chloromercuribenzoate (PCMB). This inhibition is reversed in part by the addition of glutathione. PCMB does not attack rhodopsin itself, nor does it react with retinene(1). Its action in this system is confined to the -SH groups of opsin. Under some conditions the synthesis of rhodopsin is aided by the presence of such a sulfhydryl compound as glutathione, which helps to keep the -SH groups of opsin free and reduced. By means of the amperometric silver titration of Kolthoff and Harris, it is shown that sulfhydryl groups are liberated in the bleaching of rhodopsin, two such groups for each retinene(1) molecule that appears. This is true equally of rhodopsin from the retinas of cattle, frogs) and squid. The exposure of new sulfhydryl groups adds an important element to the growing evidence that relates the bleaching of rhodopsin to protein denaturation. The place of sulfhydryl groups in the structure of rhodopsin is still uncertain. They may be concerned directly in binding the chromophore to opsin; or alternatively they may furnish hydrogen atoms for some reductive change by which the chromophore is formed from retinene(1). In the amperometric silver titration, the bleaching of rhodopsin yields directly an electrical variation. This phenomenon may have some fundamental connection with the role of rhodopsin in visual excitation, and may provide a model of the excitation process in general.

Animals↗
PubMed2002

Developmental changes in the cone visual pigments of black bream Acanthopagrus butcheri.

The spectral absorption characteristics of the visual pigments in the photoreceptors of the black bream Acanthopagrus butcheri Munro (Sparidae, Teleostei), were measured using microspectrophotometry. A single cohort of fish aged 5-172 days post-hatch (dph), aquarium-reared adults and wild-caught juveniles were investigated. During the larval stage and in juveniles younger than 100 dph, two classes of visual pigment were found, with wavelengths of maximum absorbance (lambda(max)) at approximately 425 nm and 535 nm. Following double cone formation, from 40 dph onwards, the short wavelength-sensitive pigment was recorded in single cones and the longer wavelength-sensitive pigment in double cones. From 100 dph, a gradual shift in the lambda(max) towards longer wavelengths was observed in both cone types. By 160 dph, and in adults, all single cones had a lambda(max) at approximately 475 nm while the lambda(max) in double cones ranged from 545 to 575 nm. The relationships between the lambda(max) and the ratio of bandwidth:lambda(max), for changes in either chromophore or opsin, were modelled mathematically for the long-wavelength-sensitive visual pigments. Comparing our data with the models indicated that changes in lambda(max) were not mediated by a switch from an A(1) to A(2) chromophore, rather a change in opsin expression was most likely. The shifts in the lambda(max) of the visual pigments occur at a stage when the juvenile fish begin feeding in deeper, tanninstained estuarine waters, which transmit predominantly longer wavelengths, so the spectral sensitivity changes may represent an adaptation by the fish to the changing light environment.

Animals↗
PubMed2006

Light-sensitive motile iridophores and visual pigments in the neon tetra, Paracheirodon innesi.

Although motile iridophores in the longitudinal stripes of neon tetra skin are under control of the sympathetic nervous system, they also respond to light directly and show circadian color changes. Using neon tetra skin, we found that the photoresponse of iridophores depends on light intensity, and that light near 500 nm is most effective. RT-PCR demonstrated the expression of mRNAs encoding rhodopsin and two kinds of cone opsins (Pi-green1 and Pi-green2) in neon tetra skin where the light-sensitive iridophores exist. These mRNAs are also expressed in the lateral eyes. The cone opsin genes, Pi-green1 and Pi-green2, show high similarity with the g101 and g103 genes of unique green cone opsins (belonging to the MWS/LWS group) of the blind Mexican cavefish. These results show that Pi-green1, Pi-green2, and/or rhodopsin may play important roles in the photoresponse of neon tetra iridophores, which are most sensitive to light near 500 nm.

Animals↗
PubMedDate not supplied

Studies on retinotoxic potential of a novel antitumor antibiotic--sparsomycin--in rats.

Sparsomycin (Sm) is a potent inhibitor of protein synthesis with an anticancer potential. Two years after its discovery in 1962 a phase I clinical trial revealed serious drug-induced retinotoxicity. The mechanism of this toxicity still remains unresolved; however, its understanding is important for the reintroduction of Sm or one of its analogues in clinical practice. If Sm penetrates the retina, its toxic effect should be seen as inhibition of a protein(s) vital for the visual function. Treatment of healthy albino rats and Royal College of Surgeon (RCS) rats with subtoxic doses of Sm was unable to produce ocular toxic effects. Disruption of the blood-retina barrier with sodium iodate allowed Sm to decrease opsin content by only 27%. These results strongly indicate that Sm might become retinotoxic solely upon extreme conditions such as permeabilization of the blood-retina barrier which may happen only in some rare pathological situations.

Animals↗
PubMed1997

Mutation of a conserved proline disrupts the retinal-binding pocket of the X-linked cone opsins.

PURPOSE: To test the effects of disruption of a conserved proline in the green cone opsin molecule on light-activated isomerization, transducin activation, protein accumulation, glycosylation, and transport. METHODS: Stable cell lines were established by transfecting EBNA-293 cells with a plasmid containing wild-type or mutant (P307L) green opsin cDNA molecules. The proteins were induced by culturing the cells in the presence of CdCl2 and analyzed by spectra, transducin activation, Western blotting, and immunocytochemistry. RESULTS: The P307L mutation diminished ability of the visual pigment to absorb light at the appropriate wavelength and to activate transducin. Protein glycosylation and transport to the cell membrane were unaffected. Although there was some diminution in the accumulation of the opsin, this was insufficient to account for the observed effect. CONCLUSIONS: Like rhodopsin, the formation of the cone opsins visual pigments is dependent on the binding of retinal into a hydrophobic pocket that is formed by the second and fourth transmembranous loops. Disruption of a conserved proline near the retinal binding site represents a cause of color vision deficiency that is unrelated to spectral shifts of the photopigment.

Biological Transport↗
PubMed2002

Distinct functions of photoreceptor cell-specific nuclear receptor, thyroid hormone receptor beta2 and CRX in one photoreceptor development.

PURPOSE: To clarify the functions of a specific subtype of thyroid hormone receptor (TR), TRbeta2, and photoreceptor cell-specific nuclear receptor (PNR) in the development of cone photoreceptors. METHODS: The expression of short (S)- and medium (M)-wavelength cone opsins was analyzed by reverse transcription polymerase chain reaction (RT-PCR) and Northern blot analysis in mice without a functional PNR (rd7/rd7 mice), and levels of plasma thyroid hormones and expression of TRbeta2 were also examined. Concomitantly, by means of reporter assays, the roles of PNR and TRbeta2 in the S- and M-cone opsin expression were explored at the transcriptional level. RESULTS: In rd7/rd7 mice, an abnormal increase in cone photoreceptors was observed immediately before retinal maturation normally occurs. Although an increase in S-cone opsin in the retina was observed during and after retinal development, the expression of M-cone opsin expression was not perturbed during retinal maturation. Plasma concentrations of thyroid hormone and levels of TRbeta2 expression in the rd7/rd7 mouse retina over the developmental period were normal. Transcriptional studies demonstrated that TRbeta2, but not PNR, activated the M-cone opsin gene promoter function, while suppressing the S-cone opsin promoter function enhanced by CRX in a thyroid hormone-dependent manner. CONCLUSIONS: The results indicate that PNR may suppress proliferation of cone photoreceptor progenitor cells and that the regulation of S- and M-cone opsin gene expression is mediated by TRbeta2 and CRX, but not by PNR. Thus, our results partly disclosed the molecular mechanism of cone photoreceptor development, highlighting the distinct functions of PNR and TRbeta2.

Animals↗
PubMed1987

Changes in the localization and content of opsin during retinal development in the rds mutant mouse: immunocytochemistry and immunoassay.

Electron-microscope immunocytochemistry and antibody staining of nitrocellulose replicas of SDS gels (Western blots) were used in a developmental study to detect the presence and localization of opsin in the developing photoreceptors of rds (020/A) mutant mice and their BALB/c normal controls. Western blot analysis of isolated retinal membranes first detected opsin at 10 postnatal days in both strains. Opsin levels rose progressively with development in BALB/c normal retinas. In contrast, levels in the rds retina became undetectable by 30 days after peaking at 15 days. Specific binding of anti-opsin antibodies was first observed by immunocytochemistry at postnatal 5 days in the distal plasma membrane of the connecting cilium in both BALB/c and rds retinas. Thereafter, labeling intensity increased progressively with development in the BALB/c retina. Anti-opsin labeling remained localized primarily to the plasma membrane of the distal cilium and to the outer segment with the exception that light labeling of the inner-segment plasma membrane was observed from 5-15 postnatal days. Antibody binding to photoreceptors in the rds mouse retina predominated in the plasma membrane of the connecting cilium at 5 postnatal days, but opsin was present at higher density in the inner segment plasma membrane at 5-, 10-, 15- and 20 postnatal days, when compared with BALB/c photoreceptors. From 10-20 postnatal days opsin-rich vesicles were observed in the ventricular (subretinal) space of the rds retina. Maximum intensity of labeling was observed at 15 postnatal days. By 30 postnatal days, labeling of the ciliary and inner-segment plasma membrane decreased to near background levels.

Aging↗