Rhodopsin and phototransduction

Linking genotypes with phenotypes in human retinal degenerations: Implications for future research and treatment

Although undoubtedly it will be incomplete by the time it is published, the target article by Daiger et al. organizes mutations in genes that produce retinal degenerations in humans into categories of clinically relevant phenotypes. Such classifications should help us understand the link between altered photoreceptor cell proteins and subsequent cell death, and they may yield insight into methods for preventing consequent blindness.

Genetic and clinical heterogeneity in tapetal retinal dystrophies

Large scale DNA-mutation screening in patients with hereditary retinal diseases greatly enhances our knowledge about retinal function and diseases. Scientists, clinicians, patients, and families involved with retinal disorders may directly benefit from these developments. However, certain aspects of this expanding knowledge, such as the correlation between genotype and phenotype, may be much more complicated than we expect at present.

The determination of rhodopsin structure may require alternative approaches

The structure of rhodopsin is a subject of intense interest. Solving the structure by traditional methods has proved exceedingly challenging. It may therefore be useful to confront the problem by a combination of alternate techniques. These include FTIR (Fourier transform infrared spectroscopy) and AFM (atomic force microscopy) on the intact protein. Furthermore, additional insights may be gained through structural investigations of discrete rhodopsin domains.

Na-Ca + K exchanger and Ca2+ homeostasis in retinal rod outer segments: Inactivation of the Ca2+ efflux mode and possible involvement of intracellular Ca2+ stores in Ca2+ homeostasis

Inactivation of the Ca2+ extrusion mode of the retinal rod Na- Ca + K exchanger is suggested to be the mechanism that prevents lowering of cytosolic free Ca2+ to < 1 nM when rod cells are saturated for a prolonged time under bright light conditions. Under these conditions, Ca2+ fluxes across disk membranes can contribute significantly to Ca2+ homeostasis in rods.

Nuclear magnetic resonance studies on the structure and function of rhodopsin

Magic angle spinning (MAS) NMR methods provide a means of obtaining high resolution structural data on rhodopsin and its photoin termediates. Current work has focused on the structure of the retinal chromophore and its interactions with surrounding protein charges. The recent development of MAS NMR methods for measuring internuclear distances with a resolution of ∼0.2 will complement diffraction methods for addressing key mechanistic questions.

Glutamate accumulation in the photoreceptor-presumed final common path of photoreceptor cell death

Genetic abnormalities of three factors related to the photoreceptor mechanism have been reported in both animal models and humans. Apoptotic mechanism has also been suggested as a final common pathway of photoreceptor cell death. Our findings of increased level of glutamate in photoreceptor cells in rds mice suggest that amino acid might mediate between these two pathological mechanisms.

Unique lipids and unique properties of retinal proteins

Amino-terminal heteroacylation has been identified in retinal proteins including recoverin and α subunit of G-protein, transducin. The tissue-specific modification seems to mediate not only a proteinmembrane interaction but also a specific protein-protein interaction. The mechanism generating the heterogeneity and its physiological role are still unclear, but an interesting idea for the latter postulates a fine regulation of the signal transduction pathway by distinct N-acyl groups.

Further insight into the structural and regulatory properties of the cGMP-gated channel

Recent studies from several different laboratories have provided further insight into structure-function relationships of cyclic nucleotide-gated channel and in particular the cCMPgated channel of rod photoreceptors. Site-directed mutagenesis and rod-olfactory chimeria constructs have defined important amino acids and peptide segments of the channel that are important in ion blockage, ligand specificity, and gating properties. Molecular cloning studies have indicated that cyclic nucleotide-gated channels consist of two subunits that are required to reproduce the properties of the native channels. Biochemical analysis of the cGMP-gated channel of rodcells have indicated that the 240 kDa protein that co-purifies with the 63 kDa channel subunit contains both the previously cloned second subunit of the channel and a glutamic acid-rich protein. The regulatory properties of the cGMP-gated channel from rod cells has also been studied in more detail. Studies indicate that the beta subunit of the cGMP-gated channel of rod cells contains the binding site for calmodulin. Interaction of calmodulin with the channel alters the apparent affinity of the channel for cGMP in all in vitro systems that have been studied. The significance of these recent studies are discussed in relation to the commentaries on the target article.

Unsolved issues in S-modulin/recoverin study

S-Modulin is a frog homolog of recoverin. The function and the underlying mechanism of the action of these proteins are now understood in general. However, there remain some unsolved issues including; two distinct effects of S-modulin; Ca2+-dependent binding of S-modulin to membranes and a possible target protein; S-modulin-like proteins in other neurons. These issues are considered in this commentary.

Mechanisms of photoreceptor degenerations

The candidate gene approach has identified many causes of photoreceptor rod cell death in retinitis pigmentosa. Some mutations lead to increased cyclicGMP concentrations in rods. Rod photoreceptors are also particularly susceptible to some mutations in housekeeping genes. Although many more cases of macular degeneration than retinitis pigmentosa occur each year, there is much less known about both genetic and sporadic forms of this disease.

Reduced cytoplasmic calcium concentration may be both necessary and sufficient for photoreceptor light adaptation

Light adaptation is modulated almost exclusively by changes in intracellular Ca2+ concentration, and other Ca2+-independent mechanisms are likely to play only a minor role. Changes in Ca2+i may be not only necessary for light adaptation to take place but sufficient to cause it.

The genetic kaleidoscope of vision

Site-specific phenotypic effects of the 73 known alleles in the rhodopsin gene that cause retinal degeneration are difficult to interpret because most alleles are documented in only one case or one family, which means variation in effects could actually arise from interactions with other loci. However, sample sizes necessary to detect epistatic interaction may place an answer to this question beyond our grasp.

Evidence that the type I adenylyl cyclase may be important for neuroplasticity: Mutant mice deficient in the gene for type I adenylyl cyclase show altered behavior and LTP

The regulatory properties of the neurospecific, type I adenylyl cyclase and its distribution within brain have suggested that this enzyme may be important for neuroplasticity. To address this issue, the murine, Ca2+ -stimulated adenylyl cyclase (type I), was inactivated by targeted mutagenesis. Ca2+ -stimulated adenylyl cyclase activity was reduced 40% to 60% in the hippocampus, neocortex, and cerebellum. Long term potentiation in the CA1 region of the hippocampus from mutants was perturbed relative to controls. Both the initial slope and maxim um extent of changes in synaptic response were reduced. Although mutant mice learned to find a hidden platform normally in the Morris water task, they did not display a preference for the region where the platform had been when it was removed. The behavioral phenotype of these mice is very similar to that exhibited by mice which have been surgically lesioned in the hippocampus. These results indicate that disruption of the gene for the type I adenylyl cyclase produces changes in spatial memory and indicate that the cAMP signal transduction pathway may play an important role for synaptic plasticity.

Calcium/calmodulin-sensitive adenylyl cyclase as an example of a molecular associative integrator

Evidence suggests that the Ca2+/calmodulin-sensitive adenylyl cyclase may play a key role in neural plasticity and learning in Aplysia, Drosophila, and mammals. This dually-regulated enzyme has been proposed as a possible site of stimulus convergence during associative learning. This commentary discusses the evidence that is required to demonstrate that a protein in a second messenger cascade actually functions as a molecular site of associative integration. It also addresses the issue of how a dually-regulated protein could contribute to the temporal pairing requirements of classical conditioning: that relationship between stimuli display both temporal contiguity and predictability.

The key to rhodopsin function lies in the structure of its interface with transducin

Light activated rhodopsin functions by catalyzing the exchange of GTP for GDP on numerous copies of transducin. Peptide mapping has shown that at least six regions, three on rhodopsin and three on the transducin alpha subunit, are involved in the active interface between the two proteins. The most informative structural studies of rhodopsin should include focus on the transducin interaction.

Genetic approaches to mechanosensory transduction

Genetic approaches in several organisms provide the means of solving a previously intractable problem: characterizing the molecular foundations of the mechanical senses. In nematode mechanosensory cells, members of a novel class of epithelial ion channel subunits have been implicated as components of a mechanically gated channel. In insect mechanosensory bristles, mutations specifically defective in mechanoreceptor potentials have been identified. And in bacteria, a stretch-activated channel has been molecularly characterized for the first time. Although mechanosensitivity can be a property of an isolated channel, sensory transduction in eukaryotic mechanosensory cells probably requires the interaction of several membrane and cytoskeletal components.

Diacylglycerol analogs inhibit the rod cGMP-gated channel by a phosphorylation-independent mechanism

The electrical response to light in retinal rods is mediated by cyclic nucleotide-gated, nonselective cation channels in the outer segment plasma membrane. Although cGMP appears to be the primary light-regulated second messenger, cellular levels of other substances, including Ca2+ and phosphatidylinositol-4,5-bisphosphate, are also sensitive to the level of illumination. We now show that diacylglycerol (DAG) analogs reversibly suppress the cGMP-activated conductance in excised patches from frog rod outer segments. This suppression did not require nucleoside triphosphates, indicating that a phosphorylation reaction was not involved. DAG was more effective at low than at high [cGMP]: with 50 microM 8-Br-cGMP, the DAG analog 1,2-dioctanoyl-sn-glycerol (1,2-DiC8) reduced the current with an IC50 of approximately 22 microM (Hill coefficient, 0.8), whereas with 1.2 microM 8-Br-cGMP, only approximately 1 microM 1,2-DiC8 was required to halve the current. DAG reduced the apparent affinity of the channels for cGMP: 4 microM 1,2-DiC8 produced a threefold increase in the K1/2 for channel activation by 8-Br-cGMP, as well as a threefold reduction in the maximum current, without changing the apparent stoichiometry or cooperativity of cGMP binding. Inhibition by 1,2-DiC8 was not relieved by supersaturating concentrations of 8-Br-cGMP, suggesting that DAG did not act by competitive inhibition of cGMP binding. Furthermore, DAG did not seem to significantly reduce single-channel conductance. A DAG analog similar to 1,2-DiC8--1,3-dioctanoyl-sn-glycerol (1,3-DiC8)--suppressed the current with the same potency as 1,2-DiC8, whereas an ethylene glycol of identical chain length (DiC8-EG) was much less effective. Our results suggest that DAG allosterically interferes with channel opening, and raise the question of whether DAG is involved in visual transduction.

Calcium pump in the disk membranes isolated from bovine retinal rod outer segments

The existence of a Ca2+ pump in rod outer segment disks of bovine retina is strongly suggested by the isolation on sodium dodecyl sulfate polyacrylamide gel electrophoresis of a hydroxylamine-sensitive phosphorylated intermediate (E-P) of molecular mass of about 100 kDa as well as by measurements of active calcium transport and adenosine 5'-triphosphate (ATP) hydrolysis. Active Ca2+ uptake by disks was dependent on the presence of Mg(2+)-ATP, was inhibited by vanadate or lanthanum and appeared poorly sensitive to calmodulin. ATP hydrolysis by disk membranes was a function of free Ca2+ concentration in the absence of exogenous Mg2+. The presence of a Ca2+ pump on disk membranes is discussed in terms of its possible role in Ca2+ ion buffering during photoreceptor cell functioning.

Photoactivation of rhodopsin involves alterations in cysteine side chains: detection of an S-H band in the Meta I-->Meta II FTIR difference spectrum

FTIR difference spectroscopy has been used to study the role of cysteine residues in the photoactivation of rhodopsin. A positive band near 2550 cm-1 with a low frequency shoulder is detected during rhodopsin photobleaching, which is assigned on the basis of its frequency and isotope shift to the S-H stretching mode of one or more cysteine residues. Time-resolved studies at low temperature show that the intensity of this band correlates with the formation and decay kinetics of the Meta II intermediate. Modification of rhodopsin with the reagent NEM, which selectively reacts with the SH groups of Cys-140 and Cys-316 on the cytoplasmic surface of rhodopsin, has no effect on the appearance of this band. Four other cysteine residues are also unlikely to contribute to this band because they are either thio-palmitylated (Cys-322 and Cys-323) or form a disulfide bond (Cys-110 and Cys-187). On this basis, it is likely that at least one of the four remaining cysteine residues in rhodopsin is structurally active during rhodopsin photoactivation. The possibility is also considered that this band arises from a transient cleavage of the disulfide bond between cysteine residues 110 and 187.

Phosphorylation sites in bovine rhodopsin

Bovine rhodopsin has been phosphorylated in rod outer segments by ATP and endogenous rhodopsin kinase. Mono-, di-, and triphosphorylated rhodopsins have been prepared by chromatofocusing. Nearly all of the phosphate is found in peptide 330-348, formed by digestion of phosphorhodopsins with endoproteinase Asp-N. Sequence analysis of the phosphopeptides shows that monophosphorylated rhodopsin consists of a mixture containing rhodopsins phosphorylated at 338Ser and 343Ser. Diphosphorylated rhodopsin is phosphorylated at both 338Ser and 343Ser. When rhodopsin becomes triphosphorylated it does not become phosphorylated on 334Ser but appears to become phosphorylated on one or more of the four threonine residues: 335Thr, 336Thr, 340Thr, and 342Thr.