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⁺ [ΔCa²⁺]_o in Drosophila retina using Ca²⁺-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 wildtype, the light-induced decrease in [Ca²⁺J_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.