Mitochondrial Ca ²⁺ flickers on endoplasmic reticulum (ER)-mitochondrial contact sites to suppress store-operated Ca ²⁺ entry

Successful Ca ²⁺ signaling requires appropriate Ca ²⁺ storage and buffering by endoplasmic reticulum (ER) and mitochondria. Recent research has elucidated how Ca ²⁺ storage in ER is controlled by STIM1-mediated store-operated Ca ²⁺ entry (SOCE). However, how cells employ mitochondrial Ca ²⁺ buffering to maintain Ca ²⁺ homeostasis has remained elusive. Here, with the use of mitochondria-tethered Ca ²⁺ sensor, we noticed local Ca ²⁺ flickering within individual mitochondria. Those Ca ²⁺ 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 IP ₃ R-VDAC1-driven mitochondrial Ca ²⁺ flickers on EMC. Those flickers depleted peri-EMC Ca ²⁺ 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 Ca ²⁺ homeostasis, driven by mitochondrial Ca ²⁺ flickers to reduce SOCE and to prevent intracellular Ca ²⁺ overload.