Fast resupply of synaptic vesicles requires Synaptotagmin-3

Sustained neuronal activity demands quick resupply of synaptic vesicles in order to maintain reliable synaptic transmission. Such vesicle replenishment is accelerated by sub-micromolar presynaptic Ca2+ signals by an as yet unidentified high-affinity Ca2+ sensor1-4. Here we identify a novel presynaptic role for the high-affinity Ca2+ sensor Synaptotagmin-3 (SYT3)5 in driving vesicle replenishment and short-term synaptic plasticity. Synapses in Syt3 knockout mice exhibit enhanced short-term depression, and recovery is slower and insensitive to presynaptic residual Ca2+. During sustained neuronal firing, SYT3 speeds vesicle replenishment and increases the size of the readily releasable pool of vesicles. SYT3 also mediates a second form of short-term enhancement called facilitation, under conditions of low vesicle release probability. Models of vesicle trafficking suggest that SYT3 could combat synaptic depression by accelerating vesicle docking at active zones. Our results reveal a critical role for presynaptic SYT3 in maintaining reliable high-frequency synaptic transmission in neural circuits.