STIM1 and Endoplasmic Reticulum-Plasma Membrane Contact Sites Oscillate Independently of Calcium-Induced Calcium Release

Calcium (Ca{superscript 2}) release from intracellular stores, Ca{superscript 2} entry across the plasma membrane, and their coordination via store-operated Ca{superscript 2} entry (SOCE) are critical for receptor-activated Ca{superscript 2} oscillations. However, the precise mechanism of Ca{superscript 2} oscillations and whether their control loop resides at the plasma membrane or intracellularly remain unresolved. By examining the dynamics of stromal interaction molecule 1 (STIM1)--an endoplasmic reticulum (ER)-localized Ca{superscript 2} sensor that activates the Orai1 channel on the plasma membrane for SOCE--and in mast cells, we found that a significant proportion of cells exhibited STIM1 oscillations with the same periodicity as Ca{superscript 2} oscillations. These cortical oscillations, occurring in the cells cortical region and shared with ER-plasma membrane (ER-PM) contact sites proteins, were only detectable using total internal reflection fluorescence microscopy (TIRFM). Notably, STIM1 oscillations could occur independently of Ca{superscript 2} oscillations. Simultaneous imaging of cytoplasmic Ca{superscript 2} and ER Ca{superscript 2} with SEPIA-ER revealed that receptor activation does not deplete ER Ca{superscript 2}, whereas receptor activation without extracellular Ca{superscript 2} influx induces cyclic ER Ca{superscript 2} depletion. However, under such nonphysiological conditions, cyclic ER Ca{superscript 2} oscillations lead to sustained STIM1 recruitment, indicating that oscillatory Ca{superscript 2} release is neither necessary nor sufficient for STIM1 oscillations. Using optogenetic tools to manipulate ER-PM contact site dynamics, we found that persistent ER-PM contact sites reduced the amplitude of Ca{superscript 2} oscillations without alteration of oscillation frequency. Together, these findings suggest an active cortical mechanism governs the rapid dissociation of ER-PM contact sites, thereby control amplitude of oscillatory Ca{superscript 2} dynamics during receptor-induced Ca{superscript 2} oscillations.