Synaptotagmin 1 oligomers clamp and regulate different modes of neurotransmitter release

Synaptotagmin1 (Syt1) synchronises neurotransmitter release to action potentials acting as the fast Ca2+ release sensor and as the inhibitor (clamp) of spontaneous and delayed asynchronous release. Whilst the Syt1 Ca2+ activation mechanism has been well characterised, how Syt1 clamps transmitter release remains enigmatic. Here we show that C2B domain-dependent oligomerisation provides the molecular basis for the Syt1 clamping function. This follows from the investigation of a designed mutation (F349A), which selectively destabilises Syt1 oligomerisation. Using combination of fluorescence imaging and electrophysiology in neocortical synapses we show that Syt1F349A is more efficient than wild type Syt1 (Syt1WT) in triggering synchronous transmitter release but fails to clamp spontaneous and Synaptotagmin7 (Syt7)-mediated asynchronous release components both in rescue (Syt1-/- knock-out background) and dominant-interference (Syt1+/+ background) conditions. Thus we conclude that Ca2+-sensitive Syt1 oligomers, acting as an exocytosis clamp, are critical for maintaining the balance among the different modes of neurotransmitter release.