Calcium entry-calcium refilling (CECR) coupling between store-operated Ca2+ entry and sarco/endoplasmic reticulum Ca2+-ATPase

Cross-talk between subcellular organelles is essential for cellular Ca2+ homeostasis. We have studied the effects of knocking down STIM1, the Ca2+ sensor of the endoplasmic reticulum (ER), on several homeostatic Ca2+-handling mechanisms, including plasma membrane Ca2+ entry and transport by ER, mitochondria and nucleus. We have used targeted aequorins to selectively measure calcium fluxes in different organelles. Actions of STIM1 were extremely selective, restricted to store operated Ca2+ channels (SOC) and Ca2+ uptake by the ER. No interactions with uptake or release of Ca2+ by mitochondria or nucleus were detected. Ca2+ exit from the ER, including passive leak, release via inositol 1,4,5-trisphosphate and ryanodine receptors, was unaffected. STIM1 knock-down inhibited ER Ca2+ uptake in intact but not in permeabilized cells, suggesting a privileged calcium entry-calcium refilling (CECR) coupling between plasma membrane SOC and ER calcium pump in the intact cell. As a result a large part of the entering Ca2+ is taken up into the ER without reaching the bulk cytosol. The tightness of CECR, as measured by the slope of the stimulus-signal strength function, was comparable to classic excitation–response coupling mechanisms, such as excitation–contraction, excitation–secretion or excitation–transcription coupling.