E-Syt1 Re-Arranges Stim1 Clusters To Stabilize Ring-Shaped Er-Pm Contact Sites And Accelerate Ca2+ Store Replenishment

In many non-excitable cells, the depletion of endoplasmic reticulum (ER) Ca2+ stores leads to the dynamic formation of membrane contact sites (MCSs) between the ER and the plasma membrane (PM), which activates the store-operated Ca2+ entry (SOCE) to refill the ER store. Two different Ca(2+)sensitive proteins, STIM1 and extended synaptotagmin-1 (E-syt1), are activated during this process. Due to the lack of live cell super-resolution imaging, how MCSs are dynamically regulated by STIM1 and E-syt1 coordinately during ER Ca2+ store depletion and replenishment remain unknown. With homebuilt super-resolution microscopes that provide superior axial and lateral resolution in live cells, we revealed that extracellular Ca2+ influx via SOCE activated E-syt1s to move towards the PM by similar to 12 nm. Unexpectedly, activated E-syt1s did not constitute the MCSs per se, but re-arranged neighboring ER structures into ring-shaped MCSs (230 similar to 280 nm in diameter) enclosing E-syt1 puncta, which helped to stabilize MCSs and accelerate local ER Ca2+ replenishment. Overall, we have demonstrated different roles of STIM1 and E-syt1 in MCS formation regulation, SOCE activation and ER Ca2+ store replenishment.