Mitochondrial Ca2+ flickers on endoplasmic reticulum (ER)-mitochondrial contact sites to suppress store-operated Ca2+ entry

Successful Ca2+ signaling requires appropriate Ca2+ storage and buffering by endoplasmic reticulum (ER) and mitochondria. Recent research has elucidated how Ca2+ storage in ER is controlled by STIM1-mediated store-operated Ca2+ entry (SOCE). However, how cells employ mitochondrial Ca2+ buffering to maintain Ca2+ homeostasis has remained elusive. Here, with the use of mitochondria-tethered Ca2+ sensor, we noticed local Ca2+ flickering within individual mitochondria. Those Ca2+ flickers were generated on ER-mitochondrial contact sites (EMC), as indicated by their downregulation under EMC breakdown and upregulation under EMC induction. Surprisingly, EMC breakdown increased SOCE while EMC induction reduced SOCE. Further investigations revealed that EMC effect on SOCE was not through biological functions of mitochondria or through STIM1 regulators, but via IP3R-VDAC1-driven mitochondrial Ca2+ flickers on EMC. Those flickers depleted peri-EMC Ca2+ inside ER, resulting in STIM1 sequestration around EMC to cause SOCE reduction. Moreover, EMC breakdown also increased availability of STIM1 to bind with microtubule plus ends, preventing STIM1 over-activation and SOCE upregulation. Overall, ER, mitochondria and microtubules constitute a self-sufficient system to control Ca2+ homeostasis, driven by mitochondrial Ca2+ flickers to reduce SOCE and to prevent intracellular Ca2+ overload.