A Common Mechanism Underlies Vertebrate Calcium Signaling andDrosophilaPhototransduction

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Abstract
Drosophilaphototransduction is an important model system for studies of inositol lipid signaling. Light excitation inDrosophilaphotoreceptors 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) ofDrosophilahas no effect on phototransduction. This led to the proposal thatDrosophilaphotoreceptors do not use the InsP3branch 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 onXenopusoocytes. We found that 2-APB is efficient at reversibly blocking the robust InsP3-mediated Ca2+release and store-operated Ca2+entry inXenopusoocytes at a stage operating after production of InsP3but 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 ofDrosophilaphotoreceptors in a light-dependent manner at a concentration range similar to that effective inXenopusoocytes 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 andDrosophilaTRP channels, which involves the InsP3branch of the inositol lipid-signaling pathway.