Genetic dissection of light-induced Ca2+ influx into Drosophila photoreceptors.
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 Ca2+ [delta Ca2+]o in Drosophila retina using Ca(2+)-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 [Ca2+] with kinetics and magnitude that changed with light intensity. Compared to wild-type, the light-induced decrease in [Ca2+]o (the Ca2+ signal) was diminished in trp but significantly enhanced in inaC. The enhanced Ca2+ 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 [Ca2+]o reflects light-induced Ca2+ influx into the photoreceptors and that the trp mutation blocks a large fraction of this Ca2+ influx, while the absence of eye specific PKC leads to enhancement of light-induced Ca2+ influx. This suggestion was supported by Ca2+ measurements in isolated ommatidia loaded with the fluorescent Ca2+ indicator, Ca Green-5N, which indicated an approximately threefold larger light-induced increase in cellular Ca2+ in inaC relative to WT. Our observations are consistent with the hypothesis that TRP is a light activated Ca2+ channel and that the increased Ca2+ influx observed in the absence of PKC is mediated mainly via the TRP channel.
Peretz
A
A
Department of Physiology, Hadassah Medical School, Hebrew University, Jerusalem, Israel.
Sandler
C
C
Kirschfeld
K
K
Hardie
R C
RC
Minke
B
B
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003956:Dialysis; D004330:Drosophila; D004594:Electrophysiology; D017736:Ion-Selective Electrodes; D009154:Mutation; D018408:Patch-Clamp Techniques; D010775:Photic Stimulation; D017956:Photoreceptor Cells, Invertebrate / Q000378:metabolism* / Q000528:radiation effects; D011493:Protein Kinase C / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology
D011493:Protein Kinase C; D002118:Calcium