Cross-talk between subcellular organelles is essential for cellular Ca(2+) homeostasis. We have studied the effects of knocking down STIM1, the Ca(2+) sensor of the endoplasmic reticulum (ER), on several homeostatic Ca(2+)-handling mechanisms, including plasma membrane Ca(2+) 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 Ca(2+) channels (SOC) and Ca(2+) uptake by the ER. No interactions with uptake or release of Ca(2+) by mitochondria or nucleus were detected. Ca(2+) exit from the ER, including passive leak, release via inositol 1,4,5-trisphosphate and ryanodine receptors, was unaffected. STIM1 knock-down inhibited ER Ca(2+) 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 Ca(2+) 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.
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