Activation of TRP Channels by Protons and Phosphoinositide Depletion in Drosophila Photoreceptors

SummaryBackgroundPhototransduction in microvillar photoreceptors is mediated via G protein-coupled phospholipase C (PLC), but how PLC activation leads to the opening of the light-sensitive TRPC channels (TRP and TRPL) remains unresolved. In Drosophila, InsP3 appears not to be involved, and recent studies have implicated lipid products of PLC activity, e.g., diacylglycerol, its metabolites, or the reduction in PIP2. The fact that hydrolysis of the phosphodiester bond in PIP2 by PLC also releases a proton is seldom recognized and has neither been measured in vivo nor implicated previously in a signaling context.ResultsFollowing depletion of PIP2 and other phosphoinositides by a variety of experimental manipulations, the light-sensitive channels in Drosophila photoreceptors become remarkably sensitive to rapid and reversible activation by the lipophilic protonophore 2-4 dinitrophenol in a pH-dependent manner. We further show that light induces a rapid (<10 ms) acidification originating in the microvilli, which is eliminated in mutants of PLC, and that heterologously expressed TRPL channels are activated by acidification of the cytosolic surface of inside-out patches.ConclusionsOur results indicate that a combination of phosphoinositide depletion and acidification of the membrane/boundary layer is sufficient to activate the light-sensitive channels. Together with the demonstration of light-induced, PLC-dependent acidification, this suggests that excitation in Drosophila photoreceptors may be mediated by PLC's dual action of phosphoinositide depletion and proton release.

► Light induces rapid phospholipase C-mediated acidification in Drosophila photoreceptors
► The light-sensitive channels (TRP/TRPL) can be rapidly activated by protonophores
► But only after PIP2 and other phosphoinositides (PI) have been depleted by PLC
► Combinatorial activation of TRP channels by PI depletion and proton release