Genetic dissection of light-induced Ca2+ influx into Drosophila photoreceptors.

Abstract

Invertebrate photoreceptors use the inositol-lipid signaling cascade for phototransduction. A useful approach to dissect this pathway and its regulation has been provided by the isolation of Drosophila visual mutants. We measured extracellular changes of Ca 2+ [ACa2+]o in Drosophila retina using CaZ+-selective microelectrodes in both the transient receptor potential (trp) mutant, in which the calcium permeability of the light-sensitive channels is greatly diminished and in the inactivation-but-no-afterpotential C (inaC) mutant which lacks photoreceptor-specific protein kinase C (PKC). Illumination induced a decrease in extracellular [Ca 2+] with kinetics and magnitude that changed with light intensity. Compared to wildtype, the light-induced decrease in [Ca2+]o (the Ca 2+ signal) was diminished in trp but significantly enhanced in inaC. The enhanced Ca 2+ signal was diminished in the double mutant inaC;trp indicating that the effect of the trp mutation overrides the enhancement observed in the absence of eye-PKC. We suggest that the decrease in [CaZ+]o reflects light-induced Ca 2+ influx into the photoreceptors and that the trp mutation blocks a large fraction of this Ca 2+ influx, while the absence of eye specific PKC leads to enhancement of light-induced Ca 2 § influx. This suggestion was supported by Ca 2 § measurements in isolated ommatidia loaded with the fluorescent Ca 2 § indicator, Ca Green-5N, which indicated an approximately threefold larger light-induced increase in cellular Ca 2 § in inaC relative to WT. Our observations are consistent with the hypothesis that TRP is a light activated Ca 2+ channel and that the increased Ca 2 § influx observed in the absence of PKC is mediated mainly via the TRP channel.

Insect retinal pigments: Spectral characteristics and physiological functions

“…The pupillary response in the mutant inaC is protracted compared to that in wild-type (Hofstee and Stavenga, 1995), similar to the photometric measured time course of calcium in inaC (Peretz et al, 1994b). Remarkably, whereas the pupil in wild-type opens in the dark, i.e.…”

“…potential is greatly lengthened compared to that in wild type (Ranganathan et al, 1991;, and the light-induced calcium influx is much larger than that in a wild-type photoreceptor (Peretz et al, 1994b), clearly indicating that PKC serves to curtail the light-induced increase in cellular calcium. The visual defects of many more isolated photoreceptor mutants have been identified.…”

Depletion of PtdIns(4,5)P2 underlies retinal degeneration in Drosophila trp mutants

“…2+ -selective TRP channel is the major light-sensitive channel in Drosophila, and trp mutants have severely reduced light-induced Ca 2+ influx (Hardie, 1996;Hardie and Minke, 1992;Peretz et al, 1994a). Another mutant with compromised Ca 2+ influx is norpA, which lacks PLC and has no response to light.…”

Subcellular translocation of the eGFP-tagged TRPL channel inDrosophilaphotoreceptors requires activation of the phototransduction cascade

“…Electroretinograms (ERGs) were performed as described previously (Peretz et al, 1994a). ERG traces were recorded using Axon analog to digital converter operated by pClamp 8 software on a PC computer.…”