Endoplasmic reticulum membrane contact sites coordinate exocytic site assembly and activity in neuroendocrine cells

Membrane contact sites (MCS) between intracellular organelles regulate lipid exchange, organelles dynamics and spatial organization of signaling pathways, yet their contribution to regulated exocytosis remains poorly understood. Here, we investigated the role of endoplasmic reticulum (ER) MCS in calcium-regulated exocytosis using primary bovine chromaffin cells. Combining electron microscopy on plasma membrane (PM) sheets with immunogold labeling, we identified ER structures contacting docked secretory granules and classified three types of MCS: ER-PM, ER-granule and tripartite ER-PM-granule contacts. These contacts are enriched at exocytic sites and contain Orai1 and STIM1, both known for mediating store-operated calcium release. Functional perturbation of the Orai/STIM pathway revealed that constitutive STIM activation or pharmacological inhibition of Orai1 reduced the number of exocytotic events, slowed catecholamine release and disrupted actin organization at granule docking sites. Together, our findings revealed a previously unrecognized role for ER MCS in organizing exocytic sites and controlling secretion efficiency in neuroendocrine cells.