In retinal cones, membrane depolarization in darkness activates the cGMP-dependent conductance. A model of Ca homeostasis and the regulation of guanylate cyclase

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.

Permeability and interaction of Ca2+ with cGMP-gated ion channels differ in retinal rod and cone photoreceptors

We studied the ionic permeability of cGMP-dependent currents in membrane patches detached from the outer segment of retinal cone and rod photoreceptors. Reversal potentials measured in membranes exposed to symmetric Na+ but with varying cytoplasmic Ca2+ concentrations reveal that the permeability ratio, PCa/PNa, is higher in the cGMP-gated channels of cones (7.6 +/- 0.8) than in those of rods (3.1 +/- 1.0). Ca2+ blocks both channels in a voltage-dependent manner. At any Ca2+ concentration, the channel block is maximal near the ionic reversal potential. The maximal block is essentially identical in channels of cones and rods with respect to its extent and voltage and Ca2+ dependence. The Ca2+ block is relieved by voltage, but the features of this relief differ markedly between rods and cones. Whereas the Boltzmann distribution function describes the relief of block by hyperpolarizing voltages, any given voltage is more effective in relieving the Ca2+ block in cones than in rods. Similarly, depolarizing voltages more effectively relieve Ca2+ block in cones than in rods. Our results suggest that channels contain two binding sites for Ca2+, one of which is similar in the two receptor types. The second site either interacts more strongly with Ca2+ than the first one or it is located differently in the membrane, so as to be less sensitive to membrane voltage. The channels in rods and cones differ in the features of this second site. The difference in Ca2+ permeability between the channels is likely to result in light-dependent changes in cytoplasmic Ca2+ concentration that are larger and faster in cones than in rods. The functional differences between channels, therefore, may be critically important in explaining the differences in the phototransduction signal of the two photoreceptor types.

Spontaneous, ligand-independent activity of the cGMP-gated ion channels in cone photoreceptors of fish

1. We studied the electrical conductance of membrane patches detached from the outer segment of single cone photoreceptors isolated from striped bass retina. 2. Only a single class of ion channels exists in the plasma membrane of the cone outer segments; they are gated by cytoplasmic cGMP and select cations over anions, but distinguish poorly among cations. In the absence of added cGMP and of divalent cations, however, membrane patches detached from the outer segments exhibit a small conductance that ideally selects cations over anions, but distinguishes poorly between Na+ and Li+. 3. The cGMP-independent conductance does not arise from the effect of residual cGMP that may remain associated with the detached membrane, because treatment of the patch with cGMP-specific phosphodiesterase does not affect this conductance. 4. The cGMP-independent conductance is pharmacologically indistinguishable from that activated by cGMP. Ca2+ and L-cis-diltiazem block both conductances at comparable concentrations and with similar quantitative characteristics. 5. We analysed the noise of Ca(2+)- or L-cis-diltiazem-dependent macroscopic currents both in the presence and in the absence of cGMP. In the presence of cGMP, the power density spectrum of the noise is well fitted by the sum of two Lorentzian components. The same function with similar corner frequencies fits the noise of the cGMP-independent currents. However, the total power in the current fluctuations is smaller in the absence of cGMP than in its presence; also, the ratio of the zero frequency asymptotes of the low over the high frequency components, S1(0)/Sh(0), is larger in the absence of cGMP than in its presence.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in transduction between rod and cone photoreceptors: an exploration of the role of calcium homeostasis

Rod and cone photoreceptors respond to light with distinct sensitivity and kinetics. Recent biochemical and electrophysiological studies demonstrate that the enzymes of the phototransduction cascade are similar, but not identical, in these two photoreceptor types. In contrast, light or voltage stimulation generates changes in the cytoplasmic concentration of Ca2+ in the outer segment that are far larger and faster in cones than in rods. This distinction reflects rod-cone differences in each of the elements that control Ca2+ homeostasis: cell volume, the rate of Ca2+ clearance from the outer segment, the cytoplasmic Ca2+ buffering, and the Ca2+ influx through cGMP-gated ion channels.