Rapid release of Ca21 from endoplasmic reticulum mediated by Na1/Ca21 exchange

Abstract

Phototransduction in Drosophila is mediated by phospholipase C (PLC) and Ca21-permeable TRP channels, but the function of endoplasmic reticulum (ER) Ca21 stores in this important model for Ca21 signaling remains obscure. We therefore expressed a low affinity Ca21 indicator (ER-GCaMP6-150) in the ER, and measured its fluorescence both in dissociated ommatidia and in vivo from intact flies of both sexes. Blue excitation light induced a rapid (tau;0.8 s), PLC-dependent decrease in fluorescence, representing depletion of ER Ca21 stores, followed by a slower decay, typically reaching;50% of initial dark-adapted levels, with significant depletion occurring under natural levels of illumination. The ER stores refilled in the dark within 100–200 s. Both rapid and slow store depletion were largely unaffected in InsP3 receptor mutants, but were much reduced in trp mutants. Strikingly, rapid (but not slow) depletion of ER stores was blocked by removing external Na1 and in mutants of the Na1/Ca21 exchanger, CalX, which we immuno-localized to ER membranes in addition to its established localization in the plasma membrane. Conversely, overexpression of calx greatly enhanced rapid depletion. These results indicate that rapid store depletion is mediated by Na1/Ca21 exchange across the ER membrane induced by Na1 influx via the light-sensitive channels. Although too slow to be involved in channel activation, this Na1/Ca21 exchange-dependent release explains the decades-old observation of a light-induced rise in cytosolic Ca21 in photoreceptors exposed to Ca21-free solutions.