Light-modulated ADP-ribosylation, protein phosphorylation and protein binding in isolated fly photoreceptor membranes

Chemical excitation and inactivation in photoreceptors of the fly mutants trp and nss

The Drosophila and Lucilia photoreceptor mutants, trp and nss, respond like wild-type flies to a short pulse of intense light or prolonged dim light; however, upon continuous intense illumination, the trp and nss mutants are unable to maintain persistent excitation. This defect manifests itself by a decline of the receptor potential toward baseline during prolonged intense illumination with little change in the shape or amplitude of the quantal responses to single photons (quantum bumps). Previous work on the trp and nss mutants suggests that a negative feedback loop may control the rate of bump production. Chemical agents affecting different steps of the phototransduction cascade were used in conjunction with light to identify a possible branching point of the feedback loop and molecular stages which are affected by the mutation. Fluoride ions, which in the dark both excite and adapt the photoreceptors of wild-type flies, neither excite nor adapt the photoreceptors of the trp and nss mutants. The hydrolysis-resistant analogue, GTP gamma S, which excites the photoreceptors of wild-type flies, resulting in noisy depolarization, markedly reduces the light response of both mutant flies. Intracellular recordings revealed, however, that the inhibitory effect of GTP gamma S on the nss mutant was accompanied neither by any significant depolarization nor by an increase in the noise, and thus was very different from the effect of a dim background light. The combination of inositol trisphosphate and diphosphoglycerate (InsP3 + DPG), which efficiently excites the photoreceptors of wild-type Lucilia, also excites the photoreceptors of nss Lucilia mutant. The InsP3 + DPG together act synergistically with light to accelerate the decline of the response to light in the mutant flies. These results suggest that the fly phototransduction pathway involves a feedback regulatory loop, which branches subsequent to InsP3 production and regulates guanine nucleotide-binding protein (G protein)-phospholipase C activity. A defect in this regulatory loop, which may cause an unusually low level of intracellular Ca2+, severely reduces the triggering of bumps in the mutants during intense prolonged illumination.

Chemically induced retinal degeneration in the rdgB (retinal degeneration B) mutant of Drosophila

Chemicals which affect different steps of the phototransduction cascade were used to identify the site of action of the rdgB gene product of Drosophila. In the rdgB mutant, the photoreceptor cells degenerate after several days of exposure to light, whereas raising the flies in the dark largely prevents the degeneration. In the rdgBKS222 mutant, which was used in the present studies, the light-induced degeneration is characterized by (1) selective degeneration of the peripheral but not the central photoreceptor cells; (2) random distribution of degenerated cells among ommatidia; and (3) the degeneration is specific to the rdgB but not the wild-type photoreceptor cells. In the present study, we show that application of specific chemical agents to the eyes of rdgBKS222 flies in the dark mimics the effects of light and causes retinal degeneration indistinguishable from light. The agents used in these studies are the metabolically stable GTP analogs GTP gamma S and Gpp(NH)p as well as fluoride ions, which are known to activate the transducing guanine nucleotide binding protein (G-protein of fly photoreceptors). It is unlikely that the chemically induced retinal degeneration is mediated by effects on energy metabolism, since application of the metabolic inhibitors CN- and 2-deoxy-D-glucose did not increase the extent of retinal degeneration over that observed in control flies treated with Ringer solution. The GDP analog GDP beta S, which inhibits G-protein activity, greatly reduced the extent of retinal degeneration in the dark, over that observed in control flies treated with Ringer solution. These results suggest that activation of the G-protein precedes the step in the transduction cascade that leads to retinal degeneration and provides a powerful tool to investigate the molecular mechanism of light-induced degeneration in the rdgB mutant.

The localization of calcium release by inositol trisphosphate in Limulus photoreceptors and its control by negative feedback

Microvillar photoreceptors of invertebrates exhibit a light-induced rise in the intracellular concentration of free calcium (Cai) that results in part from release of calcium from an intracellular compartment. This light-induced release of calcium appears to result from a cascade of reactions that involve rhodopsin, a GTP-binding protein and a phospholipase-C which releases inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) from the plasma membrane; the Ins(1,4,5)P3 acts to release calcium from smooth endoplasmic reticulum. In the ventral photoreceptor of the horseshoe crab Limulus polyphemus not all of the endoplasmic reticulum is subject to calcium release by Ins(1,4,5)P3. Only endoplasmic reticulum in the light-sensitive region of the cell is competent to release calcium in response to Ins(1,4,5)P3. The release of calcium by Ins(1,4,5)P3 in ventral photoreceptors appears to be subject to feedback inhibition through elevated Cai. We suggest that this feedback inhibition contributes to sensory adaptation in the photoreceptor and may account for oscillatory membrane responses sometimes observed with large injections of Ins(1,4,5)P3.

Light-induced changes in the content of inositol phosphates in squid (Loligo pealei) retina

Illumination induced an increase in inositol polyphosphates, inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate, in the photoreceptors of the squid, Loligo pealei. There was a concomitant decrease in phosphatidylinositol 4,5-bisphosphate, but no light-induced change in any other phosphoinositide. None of these stimulus-induced changes were altered by treatment in vivo with pertussis toxin, which ADP-ribosylated a Mr-39000-peptide. These findings support the hypothesis that inositol 1,4,5-trisphosphate participates as either a messenger or a modulator in transduction in invertebrate photoreceptors.

Coupling of photoexcited rhodopsin to inositol phospholipid hydrolysis in fly photoreceptors

Fly photoreceptor membranes were used to test the effect on defined biochemical reactions of light and of compounds causing photoreceptor excitation. Complementary electrophysiological studies examined whether putative second messengers excite the fly photoreceptor cells. This analysis revealed the following sequence of events: photoexcited rhodopsin activates a G protein by facilitating GTP binding. The G protein then activates a phospholipase C that generates inositol trisphosphate, which in turn acts as an internal messenger to bring about depolarization of the photoreceptor cell. Binding assays of GTP analogs and measurements of GTPase activity showed that there are 1.6 million copies of G protein per photoreceptor cell. The GTP binding component is a 41-kDa protein, and the light-activated GTPase is dependent on photoconversion of rhodopsin to metarhodopsin. Analysis of phospholipase C activity revealed that this enzyme is under stringent control of the G protein, that the major product formed is inositol trisphosphate, and that this product is rapidly hydrolyzed by a specific phosphomonoesterase. Introduction of inositol trisphosphate to the intact photoreceptor cell mimics the effect of light, and bisphosphoglycerate, which inhibits inositol trisphosphate hydrolysis, enhances the effects of inositol trisphosphate and of dim light. The interaction of photoexcited rhodopsin with a G protein is thus similar in both vertebrate and invertebrate photoreceptors. These G proteins, however, activate different photoreceptor enzymes: phospholipase C in invertebrates and cGMP phosphodiesterase in vertebrates.