Synaptotagmin-1 functions as a Ca2+ sensor for spontaneous release

Abstract

The authors show that spontaneous and evoked neurotransmitter release in cortical neurons use the same Ca2+-dependent release mechanism. Their data suggest that synaptotagmin-1 controls both evoked and spontaneous release, acting as a simultaneous Ca2+-dependent activator and clamp of exocytosis. Spontaneous 'mini' release occurs at all synapses, but its nature remains enigmatic. We found that >95% of spontaneous release in murine cortical neurons was induced by Ca2+-binding to synaptotagmin-1 (Syt1), the Ca2+ sensor for fast synchronous neurotransmitter release. Thus, spontaneous and evoked release used the same Ca2+-dependent release mechanism. As a consequence, Syt1 mutations that altered its Ca2+ affinity altered spontaneous and evoked release correspondingly. Paradoxically, Syt1 deletions (as opposed to point mutations) massively increased spontaneous release. This increased spontaneous release remained Ca2+ dependent but was activated at lower Ca2+ concentrations and with a lower Ca2+ cooperativity than synaptotagmin-driven spontaneous release. Thus, in addition to serving as a Ca2+ sensor for spontaneous and evoked release, Syt1 clamped a second, more sensitive Ca2+ sensor for spontaneous release that resembles the Ca2+ sensor for evoked asynchronous release. These data suggest that Syt1 controls both evoked and spontaneous release at a synapse as a simultaneous Ca2+-dependent activator and clamp of exocytosis.