ER Ca2+-levels control neuromodulator secretion by regulating L-type Ca2+-channel activity via its STIM1-interacting domain

Abstract Regulated secretion is typically triggered by (local) increases in intracellular Ca 2+ , but the source of Ca 2+ , influx through voltage gated Ca 2+ channels or release from the endoplasmic reticulum (ER), has distinct effects, particularly for neuropeptide secretion from dense-core vesicles (DCVs). Here, we show that in primary mouse neurons acute ER Ca 2+ depletion by caffeine, cyclopiazonic acid or thapsigargin resulted in minute increases in bulk cytosolic free Ca 2+ ([Ca 2+ ]bulk) that did not trigger significant DCV exocytosis. Remarkably, following acute ER Ca 2+ depletion, action potential (AP) trains triggered 50-90% less DCV exocytosis as compared to naïve neurons. In contrast, synaptic vesicle (SV) exocytosis was similar with/without acute ER Ca 2+ depletion. Unexpectedly, acute ER Ca 2+ depletion also reduced AP-induced [Ca 2+ ]bulk-increases. L-type Ca 2+ -channel inhibitor nimodipine produced similar effects: reduced [Ca 2+ ]bulk-increase, DCV-exocytosis but not SV exocytosis, i.e., for all three parameters a phenocopy of ER depletion. Finally, introducing L-type channels lacking STIM1 interaction sites restored DCV exocytosis following ER store depletion. We conclude that in mouse neurons, acute ER Ca 2+ release is not effective in releasing neuromodulators. Instead, ER depletion activates a STIM1-dependent negative feedback loop that inhibits L-type Ca 2+ channel activity, essential for DCV- but not SV-exocytosis.