Synaptotagmin-1 functions as the Ca(2+)-sensor for spontaneous release

Spontaneous ‘mini’ release occurs at all synapses, but its nature remains enigmatic. Here, we show that in murine cortical neurons, >95% of spontaneous release is induced by Ca(2+)-binding to synaptotagmin-1, the Ca(2+)-sensor for fast synchronous neurotransmitter release. Thus, spontaneous and evoked release use the same Ca(2+)-dependent release mechanism. As a consequence, synaptotagmin-1 mutations that alter its Ca(2+)-affinity alter spontaneous and evoked release correspondingly. Paradoxically, synaptotagmin-1 deletions (as opposed to point mutations) massively increase spontaneous release. This increased spontaneous release remains Ca(2+)-dependent, but is activated at lower Ca(2+)-concentrations, and with a lower Ca(2+)-cooperativity, than synaptotagmin-driven spontaneous release. Thus, in addition to serving as Ca(2+)-sensor for spontaneous and evoked release, synaptotagmin-1 clamps a second, more sensitive Ca(2+)-sensor for spontaneous release which resembles the Ca(2+)-sensor for evoked asynchronous release. Viewed together, these data suggest that synaptotagmin-1 controls both evoked and spontaneous release at a synapse as a simultaneous Ca(2+)-dependent activator and clamp of exocytosis.