Rhodopsin and phototransduction

A light in the dark: state of the art and perspectives in optogenetics and optopharmacology for restoring vision

In the last decade, innovative therapeutic strategies against inherited retinal degenerations (IRDs) have emerged. In particular, chemical- and opto-genetics approaches or a combination of them have been identified for modulating neuronal/optical activity in order to restore vision in blinding diseases. The ‘chemical-genetics approach’ (optopharmacology) uses small molecules (exogenous photoswitches) for restoring light sensitivity by activating ion channels. The ‘opto-genetics approach’ employs light-activated photosensitive proteins (exogenous opsins), introduced by viral vectors in injured tissues, to restore light response. These approaches offer control of neuronal activities with spatial precision and limited invasiveness, although with some drawbacks. Currently, a combined therapeutic strategy (optogenetic pharmacology) is emerging. This review describes the state of the art and provides an overview of the future perspectives in vision restoration.

Are Anti-Retinal Autoantibodies a Cause or a Consequence of Retinal Degeneration in Autoimmune Retinopathies?

Autoantibodies (AAbs) against various retinal proteins have been associated with vision loss in paraneoplastic and non-paraneoplastic autoimmune retinopathies (AR). There are two major paraneoplastic syndromes associated anti-retinal AAbs, cancer-associated retinopathy (CAR), and melanoma-associated retinopathy. Some people without a cancer diagnosis may present symptoms of CAR and have anti-retinal AAbs. The etiology and pathogenesis of those entities are not fully understood. In this review, we provide evidence for the role of AAbs in retinal death and degeneration. Studies of epitope mapping for anti-recoverin, anti-enolase, and anti-carbonic anhydrase II revealed that although patients' AAbs may recognize the same retinal protein as normal individuals they bind to different molecular domains, which allows distinguishing between normal and diseased AAbs. Given the great diversity of anti-retinal AAbs, it is likely some antibodies have greater pathogenic potential than others. Pathogenic, but not normal antibodies penetrate the target cell, reach their specific antigen, induce apoptosis, and impact retinal pathophysiology. Photoreceptors, dying by apoptosis, induced by other than immunologic mechanisms produce substantial amounts of metabolic debris, which consequently leads to autoimmunization and enhanced permeability of the blood-retinal barrier. AAbs that were made as a part of anti-cancer response are likely to be the cause of retinal degeneration, whereas others, generated against released antigens from damaged retina, contribute to the progression of retinopathy. Altogether, AAbs may trigger retinal degeneration and may also exacerbate the degenerative process in response to the release of sequestered antigens and influence disease progression.

Neural heterogeneities determine response characteristics to second-, but not first-order stimulus features

Neural heterogeneities are seen ubiquitously, but how they determine neural response properties remains unclear. Here we show that heterogeneities can either strongly, or not at all, influence neural responses to a given stimulus feature. Specifically, we recorded from peripheral electroreceptor neurons, which display strong heterogeneities in their resting discharge activity, in response to naturalistic stimuli consisting of a fast time-varying waveform (i.e., first-order) whose amplitude (i.e., second-order or envelope) varied slowly in the weakly electric fish Apteronotus leptorhynchus. Although electroreceptors displayed relatively homogeneous responses to first-order stimulus features, further analysis revealed two subpopulations with similar sensitivities that were excited or inhibited by increases in the envelope, respectively, for stimuli whose frequency content spanned the natural range. We further found that a linear-nonlinear cascade model incorporating the known linear response characteristics to first-order features and a static nonlinearity accurately reproduced experimentally observed responses to both first- and second-order features for all stimuli tested. Importantly, this model correctly predicted that the response magnitude is independent of either the stimulus waveform's or the envelope's frequency content. Further analysis of our model led to the surprising prediction that the mean discharge activity can be used to determine whether a given neuron is excited or inhibited by increases in the envelope. This prediction was validated by our experimental data. Thus, our results provide key insight as to how neural heterogeneities can determine response characteristics to some, but not other, behaviorally relevant stimulus features.

Proliferative vitreoretinopathy may be a risk factor in combined macular hole retinal detachment cases

OBJECTIVE

To review the incidence and closure rate of full-thickness macular holes (MH) in cases associated with concomitant rhegmatogenous retinal detachment (RRD).

METHODS

A retrospective consecutive case series was performed from patients undergoing surgical repair of RRD and simultaneous closure of MH. The presence of proliferative vitreoretinopathy (PVR), rates of hole closure and reattachment, and visual acuity outcomes were evaluated.

RESULTS

There were a total of 607 RRDs during the study period. The incidence of concomitant MH in RRD cases was 2.3% (14 of 607), and the overall incidence of PVR was 15.8% (96 of 607). All eyes with a MH had a primary break that was distinct from the MH. Five patients did not meet the inclusion criteria for review of the postoperative outcomes. In the remaining 9 patients, the retinal reattachment rate was 100%, and MH closure was achieved in 8 of 9 (89%) eyes after a single surgery. At the time of primary repair, PVR was present in 6 of these 9 cases (66.7%). There was a significant association between the presence of PVR and a concomitant MH (P = 0.0027). The mean preoperative visual acuity was 2.59 ± 0.649 logarithm of the minimum angle of resolution units and significantly improved to 1.23 ± 1.01 logarithm of the minimum angle of resolution units (P = 0.00124). Overall, 88.8% of patients showed an improvement in visual acuity at the final postoperative visit, and a visual acuity of 20/125 or better was achieved in 66.7% of cases.

CONCLUSION

Macular holes combined with a RRD are infrequent, and good anatomical results can be achieved after a simultaneous repair. Also, PVR may be more frequently encountered in this particular subset of RRDs.

Expression of G-protein-coupled receptor kinase 6 (GRK6) after acute spinal cord injury in adult rat

Spinal cord injury frequently results in permanent loss of neurological function. It includes many complex molecular and biochemical mechanisms. G-protein-coupled receptor kinase 6 (GRK6) is an intracellular kinase that regulates the sensitivity of certain G-protein-coupled receptors. Some studies reported GRK2 and GRK5 modulate the NFκB pathway in macrophages. Additionally, GRK2 is referred to as regulating activation of spinal cord microglia and GRK6 expression is significantly elevated in most brain regions in the MPTP-lesioned parkinsonian monkeys. However, the expression and function of GRK6 in nervous system lesion and repair are not well understood. In this study, we performed an acute spinal cord injury (SCI) model in adult rats. Western blot analysis showed the expression of GRK6 was upregulated significantly at protein level in spinal cord after SCI. Immunohistochemistry and immunofluorescence revealed wide expression of GRK6 in the normal spinal cord. After injury, GRK6 expression was increased predominantly in microglia, which expressed F4/80 (marker of macrophages and activated microglia) strongly. To understand whether GRK6 played a role in microglia activation, we applied lipopolysaccharide (LPS) to induce microglia activation in vitro. Western blot analysis demonstrated up-regulation in GRK6 protein expression after LPS stimulation was time- and dose-dependent and that up-regulation in F4/80 expression was concomitant with GRK6. These data suggested that GRK6 might be involved in the pathophysiology of SCI.

Conservation and divergence of Grb7 family of Ras-binding domains

Ras proteins are signal-transducing GTPases that cycle between inactive GDP-bound and active GTP-bound forms. Ras is a prolific signaling molecule interacting with a spectrum of effector molecules and acting through more than one signaling pathway. The Ras-effector proteins contain a Ras-associating (RA) domain through which these associate with Ras in a GTP-dependent manner. The RA domain is highly conserved among the members of the growth factor receptor-bound (Grb) 7 family of proteins which includes Grb7, Grb10 and Grb14. Our laboratory has reported an unusual observation that RA domain of Grb14 binds to the C-terminal nucleotide binding site of cyclic nucleotide gated channel (CTRCNGA1) and inhibits the channel activity. Molecular modeling of the CTR-CNGA1 displays 50%-70% tertiary structural similarity towards Ras proteins. We named this region as Ras-like domain (RLD). The interaction between RA-Grb14 and RLD-CNGA1 is mediated through a simple protein-protein interaction temporally and spatially regulated by light and cGMP. It is interesting to note that Grb14 binds to GTPase-mutant Rab5, a Ras-related small GTPase whereas Grb10 binds only to GTP-bound form of active Rab5 but not to GTPase-defective mutant Rab5. These results suggest that Grb14 might have been evolved later in the evolution that binds to both Ras and nucleotide binding proteins such as CNGA1. Our studies also suggest that eukaryotic CNG channels could be evolved through a gene fusion between prokaryotic ion channels and cyclic nucleotide binding proteins, both of which might have undergone several sequence variations for functional adaptation during evolution.

Oligomerization of G protein-coupled receptors: past, present, and future

G protein-coupled receptor (GPCR)-mediated signal transduction has been studied for more than a century. Despite the intense focus on this class of proteins, a molecular understanding of what constitutes the functional form of the receptor is still uncertain. GPCRs have traditionally been conceptualized as monomeric proteins, and this view has changed little over the years until relatively recently. Recent biochemical and biophysical studies have challenged this traditional concept, and point instead to a mechanistic view of signal transduction wherein the receptor functions as an oligomer. Cooperative interactions within such an oligomeric array may be critical for the propagation of an external signal across the cell membrane and to the G protein, and may therefore underlie the mechanistic basis of signaling.

The three-dimensional structure of bovine rhodopsin determined by electron cryomicroscopy

G-protein-coupled receptors are integral membrane proteins that respond to environmental signals and initiate signal transduction pathways, which activate cellular processes. Rhodopsin, a well known member of the G-protein-coupled receptor family, is located in the disk membranes of the rod outer segment, where it is responsible for the visualization of dim light. Rhodopsin is the most extensively studied G-protein-coupled receptor, and knowledge about its structure serves as a template for other related receptors. We have gained detailed structural knowledge from the crystal structure (1), which was solved by x-ray crystallography in 2000 using three-dimensional crystals. Here we report a three-dimensional density map of bovine rhodopsin determined by electron cryomicroscopy of two-dimensional crystals with p22(1)2(1) symmetry. The usage of relatively small and disordered crystals made the process of structure determination challenging. Special attention was paid to the extraction of amplitudes and phases, since usable raw data were limited to a maximum tilt of 45 degrees. In the refinement process, an improved unbending procedure was applied. This led to a final resolution of 5.5 A in the membrane plane and approximately 13 A perpendicular to it, making our electron density map the most accurate map of a G-protein-coupled receptor currently available by electron microscopy. Most important is the information we gain about the center of the membrane plane and the orientation of the molecule relative to the bilayer. This information cannot be retrieved from the three-dimensional crystals. In our electron density map, all seven transmembrane helices were identified, and their arrangement is in agreement with the arrangement known from the crystal structure (1). In the retinal binding pocket, a density peak adjacent to helix 3 suggests the position of the beta-ionine ring of the chromophore, and in its vicinity several of the bigger amino acids can be identified.

Three-dimensional structure of an invertebrate rhodopsin and basis for ordered alignment in the photoreceptor membrane

Invertebrate rhodopsins activate a G-protein signalling pathway in microvillar photoreceptors. In contrast to the transducin-cyclic GMP phosphodiesterase pathway found in vertebrate rods and cones, visual transduction in cephalopod (squid, octopus, cuttlefish) invertebrates is signalled via Gq and phospholipase C. Squid rhodopsin contains the conserved residues of the G-protein coupled receptor (GPCR) family, but has only 35% identity with mammalian rhodopsins. Unlike vertebrate rhodopsins, cephalopod rhodopsin is arranged in an ordered lattice in the photoreceptor membranes. This organization confers sensitivity to the plane of polarized light and also provides the optimal orientation of the linear retinal chromophores in the cylindrical microvillar membranes for light capture. Two-dimensional crystals of squid rhodopsin show a rectilinear arrangement that is likely to be related to the alignment of rhodopsins in vivo.Here, we present a three-dimensional structure of squid rhodopsin determined by cryo-electron microscopy of two-dimensional crystals. Docking the atomic structure of bovine rhodopsin into the squid density map shows that the helix packing and extracellular plug structure are conserved. In addition, there are two novel structural features revealed by our map. The linear lattice contact appears to be made by the transverse C-terminal helix lying on the cytoplasmic surface of the membrane. Also at the cytoplasmic surface, additional density may correspond to a helix 5-6 loop insertion found in most GPCRs relative to vertebrate rhodopsins. The similarity supports the conservation in structure of rhodopsins (and other G-protein-coupled receptors) from phylogenetically distant organisms. The map provides the first indication of the structural basis for rhodopsin alignment in the microvillar membrane.

Structure of rhodopsin

Two-dimensional crystals of rhodopsin were studied to determine the arrangement of the transmembrane alpha helices. A combination of electron cryo-microscopy, image processing and electron crystallography was used to extract amplitudes and phases from images, and a three-dimensional map to a resolution of 7.5 A was calculated. Density peaks for all seven transmembrane helices were observed and the helix axes for all seven helices could be estimated. Near the intracellular side, which interacts with the G protein transducin, we observed three layers of helices arranged differently from bacteriorhodopsin. The arrangement opens up towards the extracellular side forming a cavity that serves as the binding pocket for the retinal. This cavity is closed towards the intracellular side by the long and highly tilted helix 3, and must be closed towards the extracellular side by the loop linking helices 4 and 5 that is linked by a disulphide bridge to the extracellular end of helix 3.

Regulation of sorting and post-Golgi trafficking of rhodopsin by its C-terminal sequence QVS(A)PA

Several mutations that cause severe forms of the human disease autosomal dominant retinitis pigmentosa cluster in the C-terminal region of rhodopsin. Recent studies have implicated the C-terminal domain of rhodopsin in its trafficking on specialized post-Golgi membranes to the rod outer segment of the photoreceptor cell. Here we used synthetic peptides as competitive inhibitors of rhodopsin trafficking in the frog retinal cell-free system to delineate the potential regulatory sequence within the C terminus of rhodopsin and model the effects of severe retinitis pigmentosa alleles on rhodopsin sorting. The rhodopsin C-terminal sequence QVS(A)PA is highly conserved among different species. Peptides that correspond to the C terminus of bovine (amino acids 324-348) and frog (amino acids 330-354) rhodopsin inhibited post-Golgi trafficking by 50% and 60%, respectively, and arrested newly synthesized rhodopsin in the trans-Golgi network. Peptides corresponding to the cytoplasmic loops of rhodopsin and other control peptides had no effect. When three naturally occurring mutations: Q344ter (lacking the last five amino acids QVAPA), V345M, and P347S were introduced into the frog C-terminal peptide, the inhibitory activity of the peptides was no longer detectable. These observations suggest that the amino acids QVS(A)PA comprise a signal that is recognized by specific factors in the trans-Golgi network. A lack of recognition of this sequence, because of mutations in the last five amino acids causing autosomal dominant retinitis pigmentosa, most likely results in abnormal post-Golgi membrane formation and in an aberrant subcellular localization of rhodopsin.

Light, sight and fight for insight

The visual processes in the human retina begin with absorption of photons from light. This light energy is converted into electrical stimuli in a series of enzymatic steps which initiates neural responses to light. Therefore, any structural and functional abnormalities in these molecules will likely interfere with signal transduction which may ultimately lead to blindness. A new era began in 1990 with an ultimate goal of redefining and developing new treatments for the photoreceptor disorders by identifying mutations in the genes encoding phototransduction cascade enzymes. As a result of this intense investigation around the world, mutations have now been identified in eight genes, in several retinal dystrophies. Almost all of these genes encode signal transduction enzymes and all are highly expressed in photoreceptor cells. This effort has been further aided by gene disruption technology. Although there are many puzzles that need to be solved, these approaches have given some insight into the genetic eye disorders and will undoubtedly improve our understanding of inherited eye disorders in the future. This improved knowledge may eventually lead to prevention or a cure.

Gene-based approach to human gene-phenotype correlations

Elucidating the genetic basis of human phenotypes is a major goal of contemporary geneticists. Logically, two fundamental and contrasting approaches are available, one that begins with a phenotype and concludes with the identification of a responsible gene or genes; the other that begins with a gene and works toward identifying one or more phenotypes resulting from allelic variation of it. This paper provides a conceptual overview of phenotype-based vs. gene-based procedures with emphasis on gene-based methods. A key feature of a gene-based approach is that laboratory effort first is devoted to developing an assay for mutations in the gene under regard; the assay then is applied to the evaluation of large numbers of unrelated individuals with a variety of phenotypes that are deemed potentially resulting from alleles at the gene. No effort is directed toward chromosomally mapping the loci responsible for the phenotypes scanned. Example is made of my laboratory's successful use of a gene-based approach to identify genes causing hereditary diseases of the retina such as retinitis pigmentosa. Reductions in the cost and improvements in the speed of scanning individuals for DNA sequence anomalies may make a gene-based approach an efficient alternative to phenotype-based approaches to correlating genes with phenotypes.

Arrangement of rhodopsin transmembrane alpha-helices

Rhodopsins, the photoreceptors in rod cells, are G-protein-coupled receptors with seven hydrophobic segments containing characteristic conserved sequence patterns that define a large family. Members of the family are expected to share a conserved transmembrane structure. Direct evidence for the arrangement of seven alpha-helices was obtained from a 9A projection map of bovine rhodopsin. Structural constraints inferred from a comparison of G-protein-coupled receptor sequences were used to assign the seven hydrophobic stretches in the sequence to features in the projection map. A low-resolution three-dimensional structure of bovine rhodopsin and two projection structures of frog rhodopsin confirmed the position of the three least tilted helices, 4, 6 and 7. A more elongated peak of density for helix 5 indicated that it is tilted or bent, but helices 1, 2 and 3 were not resolved. Here we have used electron micrographs of frozen-hydrated two-dimensional frog rhodopsin crystals to determine the structure of frog rhodopsin. Seven rods of density in the map are used to estimate tilt angles for the seven helices. Density visible on the extracellular side of the membrane suggests a folded domain. Density extends from helix 6 on the intracellular side, and a short connection between helices 1 and 2, and possibly a part of the carboxy terminus, are visible.

A G protein-coupled receptor phosphatase required for rhodopsin function

Heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors are phosphorylated by kinases that mediate agonist-dependent receptor deactivation. Although many receptor kinases have been isolated, the corresponding phosphatases, necessary for restoring the ground state of the receptor, have not been identified. Drosophila RDGC (retinal degeneration C) is a phosphatase required for rhodopsin dephosphorylation in vivo. Loss of RDGC caused severe defects in the termination of the light response as well as extensive light-dependent retinal degeneration. These phenotypes resulted from the hyperphosphorylation of rhodopsin because expression of a truncated rhodopsin lacking the phosphorylation sites restored normal photoreceptor function. These results suggest the existence of a family of receptor phosphatases involved in the regulation of G protein-coupled signaling cascades.

Residues within the polycationic region of cGMP phosphodiesterase gamma subunit crucial for the interaction with transducin alpha subunit. Identification by endogenous ADP-ribosylation and site-directed mutagenesis

Interaction between the gamma subunit (Pgamma) of cGMP phosphodiesterase and the alpha subunit (Talpha) of transducin is a key step for the regulation of cGMP phosphodiesterase in retinal rod outer segments. Here we have utilized a combination of specific modification by an endogenous enzyme and site-directed mutagenesis of the Pgamma polycationic region to identify residues required for the interaction with Talpha. Pgamma, free or complexed with the alphabeta subunit (Palphabeta) of cGMP phosphodiesterase, was specifically radiolabeled by prewashed rod membranes in the presence of [adenylate-32P]NAD. Identification of ADP-ribose in the radiolabeled Pgamma and radiolabeling of arginine-replaced mutant forms of Pgamma indicate that both arginine 33 and arginine 36 are similarly ADP-ribosylated by endogenous ADP-ribosyltransferase, but only one arginine is modified at a time. Pgamma complexed with Talpha (both GTP- and GDP-bound forms) was not ADP-ribosylated; however, agmatine, which cannot interact with Talpha, was ADP-ribosylated in the presence of Talpha, suggesting that a Pgamma domain containing these arginines is masked by Talpha. A Pgamma mutant (R33,36K), as well as wild type Pgamma, inhibited both GTP hydrolysis of Talpha and GTP binding to Talpha. Moreover, GTP-bound Talpha activated Palphabeta that had been inhibited by R33,36K. However, another Pgamma mutant (R33,36L) could not inhibit these Talpha functions. In addition, GTP-bound Talpha could not activate Palphabeta inhibited by R33,36L. These results indicate that a Pgamma domain containing these arginines is required for its interaction with Talpha, but not with Palphabeta, and that positive charges in these arginines are crucial for the interaction.

Short wavelength-sensitive opsins from the Saharan silver and carpenter ants

We have previously cloned the opsins coding for the long-wavelength visual pigments from the Saharan silver ant and carpenter ant. Here we report two new cDNA clones isolated from cDNA libraries which also code for opsin proteins. These cDNAs code for deduced proteins with 369 amino acids which are 91% identical to each other, but only 38% identical to the previously cloned opsins. Phyletic comparisons suggest that these opsins are likely the ultraviolet sensitive visual pigments, a conclusion that is supported by the presence of a phenylalanine at the counterion position in the third transmembrane segment.

Steric hindrance between chromophore substituents as the driving force of rhodopsin photoisomerization: 10-methyl-13-demethyl retinal containing rhodopsin

A visual chromophore analogue, 10-methyl-13-demethyl (dm) retinal, was synthesized and reconstituted with bleached bovine rhodopsin to form a visual pigment derivative with absorbance maximum at 505 nm. The investigations with this new compound were stimulated from recent results using 13-dm retinal as a chromophore that revealed a remarkable loss in quantum efficiency (phi of 13-dm retinal-containing rhodopsin: 0.30, Ternieden and Gärtner, J. Photochem. Photobiol. B Biol, 33, 83-86, 1996). The quantum efficiency of the new pigment was determined as 0.59 by quantitative bleaching using reconstituted rhodopsin as a reference. The very similar quantum efficiencies of rhodopsin and the new pigment give experimental support for the recently presented hypothesis that a steric hindrance between the substituents at positions 10 and 13 in 11-cis-retinal is elevated during the photoisomerization and thus facilitates the rapid photoisomerization of the visual chromophore (Peteanu et al., Proc. Natl. Acad. Sci. USA 90, 11762-11766, 1993). Such steric hindrance is removed from the molecule by the elimination of the methyl group from position 13 and can be re-established via a rearrangement of the substitution pattern by introducing a methyl group at position 10 of 13-dm retinal.

Ant opsins: sequences from the Saharan silver ant and the carpenter ant

cDNA clones encoding opsins from compound eyes of carpenter ant, Camponotus abdominalis, and Saharan silver ant, Cataglyphis bombycina, were isolated from cDNA libraries. The opsin cDNAs from each species code for deduced proteins with 378 amino acids which are 92% identical. Of the 30 amino acid differences between the two proteins, 13 are non-conservative. Eight of these non-conservative substitutions are within the membrane spanning domain. The presence of a potential Schiff-base counterion in helix III in both species suggests that these opsins are the protein moiety of the visible range pigments. When compared to all known opsins, these opsins are most similar to the opsin from preying mantis (76% identity at the amino acid level). Phyletic comparisons group the two ant opsins with the other arthropod long wavelength opsins.

Projection structure of frog rhodopsin in two crystal forms

Rhodopsin is the G protein-coupled receptor that upon light activation triggers the visual transduction cascade. Rod cell outer segment disc membranes were isolated from dark-adapted frog retinas and were extracted with Tween detergents to obtain two-dimensional rhodopsin crystals for electron crystallography. When Tween 80 was used, tubular structures with a p2 lattice (a = 32 A, b = 83 A, gamma = 91 degrees) were formed. The use of a Tween 80/Tween 20 mixture favored the formation of larger p22(1)2(1) lattices (a = 40 A, b = 146 A, gamma = 90 degrees). Micrographs from frozen hydrated frog rhodopsin crystals were processed, and projection structures to 7-A resolution for the p22(1)2(1) form and to 6-A resolution for the p2 form were calculated. The maps of frog rhodopsin in both crystal forms are very similar to the 9-A map obtained previously for bovine rhodopsin and show that the arrangement of the helices is the same. In a tentative topographic model, helices 4, 6, and 7 are nearly perpendicular to the plane of the membrane. In the higher-resolution projection maps of frog rhodopsin, helix 5 looks more tilted than it appeared previously. The quality of the two frog rhodopsin crystals suggests that they would be suitable to obtain a three-dimensional structure in which all helices would be resolved.

The sequence of arrestins from rod and cone photoreceptors in the frogs Rana catesbeiana and Rana pipiens. Localization of gene transcripts by reverse-transcription polymerase chain reaction on isolated photoreceptors

Members of the arrestin protein family are known to participate in the inactivation of rhodopsin and other heptahelical receptors. Arrestins bind to the activated and phosphorylated state of these receptors, consequently blocking the ability of the receptors to activate the guanine-nucleotide-binding protein (G protein). We have determined the sequences of four retinal arrestins from two species of frog, Rana catesbeiana and Rana pipiens. Using polymerase chain reaction on reverse-transcribed mRNA isolated from single photoreceptor cells, we show that two of these arrestins are from rod photoreceptors and two rod photoreceptors and two are from cone photoreceptors. Comparison of these arrestins with the twenty known arrestin sequences identifies three regions of the protein that are well conserved across all phylogenetic groups. These regions may function in the binding of the arrestin to the heptahelical receptors. In addition, the Rana arrestins contain a uniquely acidic C-terminal sequence.

Ligands of steroid/thyroid receptors induce cone photoreceptors in vertebrate retina

The mechanisms by which multipotent progenitor cells are directed to alternative cell identities during the histogenesis of the vertebrate central nervous system are likely to involve several different types of signaling systems. Recent evidence indicates that 9-cis retinoic acid, which acts through members of the steroid/thyroid superfamily of receptors, directs progenitor cells to the rod photoreceptor cell fate. We now report that another effector of this family of receptors, thyroid hormone, induces an increase in the number of cone photoreceptors that develop in embryonic rat retinal cultures, and that combinations of 9-cis retinoic acid and triiodothyronine cause isolated progenitor cells to differentiate as either rods or cones, depending on the relative concentrations of the ligands. These results implicate thyroid hormone in CNS cell fate determination, and suggest that different photoreceptor phenotypes may be modulated through the formation of thyroid/retinoid receptor heterodimers.

Recoverin, a calcium-binding protein in photoreceptors

Recoverin is a Ca2+-binding protein found primarily in vertebrate photoreceptors. The proposed physiological function of recoverin is based on the finding that recoverin inhibits light-stimulated phosphorylation of rhodopsin. Recoverin interacts with rod outer segment membranes in a Ca2+-dependent manner. This interaction requires N-terminal acylation of recoverin. Four types of fatty acids have been detected on the N-terminus of recoverin, but the functional significance of this heterogeneous acylation is not yet clear.

Future directions for rhodopsin structure and function studies

NMR (nuclear magnetic resonance) may be useful for determining the structure of retinal and its environment in rhodopsin, but not for determining the complete protein structure. Aggregation and low yield of fragments of rhodopsin may make them difficult to study by NMR. A long-term multidisciplinary attack on rhodopsin structure is required.

More answers about cGMP-gated channels pose more questions

Our understanding of the molecular properties and cellular role of cGMP-gated channels in outer segments of vertebrate photo-receptors has come from over a decade of studies which have continuously altered and refined ideas about these channels. Further examination of this current view may lead to future surprises and further refine the understanding of cGMP-gated channels.

Cyclic nucleotides as regulators of light-adaptation in photoreceptors

Cyclic nucleotides can regulate the sensitivity of retinal rods to light through phosducin. The phosphorylation state of phosducin determines the amount of G available for activation by Rho*. Phosducin phosphorylation is regulated by cyclic nucleotides through their activation of cAMP-dependent protein kinase. The regulation of phosphodiesterase activity by the noncatalytic cGMP binding sites as well as Ca2+/calmodulin dependent regulation of cGMP binding to the cation channel are also discussed.

Long term potentiation and CaM-sensitive adenylyl cyclase: Long-term prospects

The type I CaM-sensitive adenylyl cyclase is in a position to integrate signals from multiple inputs, consistent with the requirements for mediating long term potentiation (LTP). Biochemical and genetic evidence supports the idea that this enzyme plays an important role inc LTP. However, more work is needed before we will be certain of the role that CaM-sensitive adenylyl cyclases play in LTP.

Modulation of the cGMP-gated channel by calcium

Calcium acting through calmodulin has been shown to regulate the affinity of cyclic nucleotide-gated channels expressed in cell lines. But is calmodulin the Ca-sensor that normally regulates these channels?

How many light adaptation mechanisms are there?

The generally positive response to our target article indicates that most of the commentators accept our contention that light adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in adaptation. Finally, additional aspects of cGMP involvement in adaptation are considered. We discuss each of these points in turn.

Gene therapy, regulatory mechanisms, and protein function in vision

Hereditary retinal degeneration due to mutations in visual genes may be amenable to therapeutic interventions that modulate, either positively or negatively, the amount of protein product. Some of the proteins involved in phototransduction are rapidly moved by a lightdependent mechanism between the inner segment and the outer segment in rod photoreceptor cells, and this phenomenon is important in phototransduction.

A novel protein family of neuronal modulators

A number of proteins homologous to recoverin have been identified in the brains of the several vertebrate species. The brainderived members originally contain four EF-hand domains, but NH2- terminal domain is aberrant. Many of these proteins inhibited light-induced rhodopsin phosphorylation at high [Ca2+], suggesting that the brain-derived members may act as a Ca2+-sensitive modulator of receptor phosphorylation, as recoverin does.

The structure of rhodopsin and mechanisms of visual adaptation

Rapidly advancing studies on rhodopsin have focused on new strategies for crystallization of this integral membrane protein for x-ray analysis and on alternative methods for structural determination from nuclear magnetic resonance data. Functional studies of the interactions between the apoprotein and its chromophore have clarified the role of the chromophore in deactivation of opsin and in photoactivation of the pigment.

Crucial steps in photoreceptor adaptation: Regulation of phosphodiesterase and guanylate cyclase activities and Ca2+-buffering

This commentary discusses the balance of phosphodiesterase and guanylate cyclase activities in vertebrate photoreceptors at moderate light intensities. The rate of cGMP hydrolysis and synthesis seem to equal each other. Ca2+ as regulator of both enzyme activities is also effectively buffered in photoreceptor cells by cytoplasmic buffer components.

The atomic structure of visual rhodopsin: How and when?

Strong arguments are presented by Hargrave suggesting that the crystallization of visual rhodopsin for high resolution analysis by X-ray crystallography or electron microscopy is feasible. However, the effort needed to achieve this goal will most likely exceed the resources of a single laboratory and a concerted approach to the research is necessary.

Molecular insights gained from covalently tethering cGMP to the ligand-binding sites of retinal rod cGMP-gated channels

A photoaffinity analog of cGMP has been used to biochemically identify a new ligand-binding subunit of the retinal rod cGMP-activated ion channel, as well as amino acids in contact with cGMP in the original subunit. Covalent tethering of this probe to channels in excised menbrane patches has revealed a functional heteogeneity in the ligand-binding sites that may arise from the two biochemically identified subunits.

Genetic and functional complexity of inherited retinal degeneration

Recent findings emphasize the complexity, both genetic and functional, of the manifold genes and mutations causing inherited retinal degeneration in humans. Knowledge of the genetic bases of these diseases can contribute to design of rational therapy, as well as elucidating the function of each gene product in normal visual processes.

Channel structure and divalent cation regulation of phototransduction

The identification of additional subunits of the cGMP-gated cation channel suggests exciting questions about their regulatory roles and about structure/functional relationships. How do the different subunits interact? How is the complex assembled into the plasma membrane? Divalent cations have been implicated in the regulation of adaptation. One often overlooked cation is magnesium. Could this ion play a role in phototransduction?

Structure of the cGMP-gated channel

The subunit structure of the cGMP-gated cation channel of rod photoreceptors is rapidly being defined, and in the process the mode of regulation by Ca2+-calmodulin unraveled. Intriguingly, early results suggest that additional subunits of unknown function are associated with the channel and remain to be identified.