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 (InsP 3 R) of Drosophila has no effect on phototransduction. This led to the proposal that Drosophila photoreceptors do not use the InsP 3 branch of phospholipase C (PLC)-mediated signaling for phototransduction, unlike most other inositol lipidsignaling systems. To examine this hypothesis we applied the membrane-permeant InsP 3 R antagonist 2-aminoethoxydiphenyl borate (2-APB), which has proved to be an important probe for assessing InsP 3 R 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 InsP 3 -mediated Ca 2ϩ release and store-operated Ca 2ϩ entry in Xenopus oocytes at a stage operating after production of InsP 3 but before the opening of the surface membrane Cl Ϫ channels by Ca 2ϩ . 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 InsP 3 branch of the inositol lipid-signaling pathway.

Recent studies in Drosophila melanogaster retina indicate that absorption of light causes the translocation of signaling molecules and actin from the photoreceptor's signaling membrane to the cytosol, but the underlying mechanisms are not fully understood. As ezrin-radixin-moesin (ERM) proteins are known to regulate actin–membrane interactions in a signal-dependent manner, we analyzed the role of Dmoesin, the unique D. melanogaster ERM, in response to light. We report that the illumination of dark-raised flies triggers the dissociation of Dmoesin from the light-sensitive transient receptor potential (TRP) and TRP-like channels, followed by the migration of Dmoesin from the membrane to the cytoplasm. Furthermore, we show that light-activated migration of Dmoesin results from the dephosphorylation of a conserved threonine in Dmoesin. The expression of a Dmoesin mutant form that impairs this phosphorylation inhibits Dmoesin movement and leads to light-induced retinal degeneration. Thus, our data strongly suggest that the light- and phosphorylation-dependent dynamic association of Dmoesin to membrane channels is involved in maintenance of the photoreceptor cells.

A variety of rod opsin mutations result in autosomal dominant retinitis pigmentosa and congenital night blindness in humans. One subset of these mutations encodes constitutively active forms of the rod opsin protein. Some of these dominant rod opsin mutant proteins, which desensitize transgenic Xenopus rods, provide an animal model for congenital night blindness. In a genetic screen to identify retinal degeneration mutants in Drosophila, we identified a dominant mutation in the ninaE gene (NinaE pp100 ) that encodes the rhodopsin that is expressed in photoreceptors R1-R6. Deep pseudopupil analysis and histology showed that the degeneration was attributable to a light-independent apoptosis. Whole-cell recordings revealed that the NinaE pp100 mutant photoreceptor cells were strongly desensitized, which partially masked their constitutive activity. This desensitization primarily resulted from both the persistent binding of arrestin (ARR2) to the NINAE pp100 mutant opsin and the constitutive activity of the phototransduction cascade. Whereas mutations in several Drosophila genes other than ninaE were shown to induce photoreceptor cell apoptosis by stabilizing a rhodopsinarrestin complex, NinaE pp100 represented the first rhodopsin mutation that stabilized this protein complex. Additionally, the NinaE pp100 mutation led to elevated levels of G q ␣ in the cytosol, which mediated a novel retinal degeneration pathway. Eliminating both G q ␣ and arrestin completely rescued the NinaE pp100 -dependent photoreceptor cell death, which indicated that the degeneration is entirely dependent on both G q ␣ and arrestin. Such a combination of multiple pathological pathways resulting from a single mutation may underlie several dominant retinal diseases in humans.