Constitutive, calcium-independent endoplasmic reticulum-plasma membrane contact site oscillations and its implications in store-operated calcium entry

The endoplasmic reticulum and plasma membrane (ER-PM) contact site dynamics plays a central role for store-operated calcium entry (SOCE). ER localized calcium sensor STIM1 translocates to the contact sites, interacts with Orai and mediates calcium influx from the extracellular environment. Different species of phosphoinositides (PIPs) have been reported to be involved in contact site dynamics as well as STIM1 translocation. However, most of the studies were based on loss-of-function experiments or conditions that generate massive calcium store depletion. The kinetics of ER-PM contact site dynamics during physiological stimuli -induced calcium oscillations are not well understood. Using total internal reflection fluorescence microscopy (TIRFM), we investigated the relationship between dynamics of STIM1 as well as cortical ER (cER) proteins and calcium oscillations in rat basophilic leukemia (RBL) mast cells. Surprisingly, a significant percentage of cells displayed cyclic STIM1 and cER dynamics that were calcium-independent. Using specific lipid sensors, we showed that cyclic ER-PM contact site assembly was in phase with PI(4)P oscillation, but preceded phases of PI(4,5)P2 or PI(3,4,5)P3 oscillation. Optogenetic recruitment of the phosphoinositide 5-phosphatase from INPP5E, which decreased PI(4,5)P2 and increased PI(4)P levels on the plasma membrane, stimulated the translocation of STIM1 and inhibited calcium oscillations. Interestingly, prolonged stable translocation of STIM1 to the plasma membrane had an inhibitory effect on calcium oscillations. Collectively, our findings suggest that ER-PM contact sites formation is PI(4)P-dependent. In addition, reversibility of ER-PM contact sites dynamics and intermediate strength of ER-PM contact are needed for calcium oscillation.