A common mechanism underlies vertebrate calcium signaling and Drosophila phototransduction

Drosophila phototransduction is an important model system for studies of inositol lipid signaling. Light excitation in Drosophila photoreceptors depends on phospholipase C, because null mutants of this enzyme do not respond to light. Surprisingly, genetic elimination of the apparently single inositol trisphosphate receptor (InsP₃R) of Drosophila has no effect on phototransduction. This led to the proposal that Drosophila photoreceptors do not use the InsP₃ branch of phospholipase C (PLC)-mediated signaling for phototransduction, unlike most other inositol lipid-signaling systems. To examine this hypothesis we applied the membrane-permeant InsP₃R antagonist 2-aminoethoxydiphenyl borate (2-APB), which has proved to be an important probe for assessing InsP₃R involvement in various signaling systems. We first examined the effects of 2-APB on Xenopus oocytes. We found that 2-APB is efficient at reversibly blocking the robust InsP₃-mediated Ca²⁺ release and store-operated Ca²⁺ entry in Xenopus oocytes at a stage operating after production of InsP₃ but before the opening of the surface membrane Cl^- channels by Ca²⁺. We next demonstrated that 2-APB is effective at reversibly blocking the response to light of Drosophila photoreceptors in a light-dependent manner at a concentration range similar to that effective in Xenopus oocytes and other cells. We show furthermore that 2-APB does not directly block the light-sensitive channels, indicating that it operates upstream in the activation of these channels. The results indicate an important link in the coupling mechanism of vertebrate store-operated channels and Drosophila TRP channels, which involves the InsP₃ branch of the inositol lipid-signaling pathway.