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Compensatory endocytosis keeps the surface area of secretory cells constant following exocytosis. At chemical synapses, clathrin-independent ultrafast endocytosis maintains such homeostasis. This endocytic pathway is temporally and spatially coupled to exocytosis, initiating within 50 ms at the region immediately next to where vesicles fuse: the active zone. How synaptic vesicle exocytosis induces ultrafast endocytosis is unknown. Here, we demonstrate that actin filaments are enriched in the region surrounding active zone at mouse hippocampal synapses and that the membrane area conservation due to this actin corral is necessary for exo-endocytic coupling. Simulations suggest that flattening of fused vesicles exerts lateral membrane pressure in the plasma membrane against the actin corral, resulting in rapid formation of endocytic pits at the border between the active zone and the surrounding actin-enriched region. Consistent with our simulations, ultrafast endocytosis does not initiate when actin organization is disrupted, either pharmacologically or by ablation of the actin-binding protein Epsin1. These data suggest that endocytosis is mechanically coupled to exocytosis at synapses.

Synaptotagmin (SYT) 7 has emerged as key regulator of presynaptic function, but its localization and precise function in the synaptic vesicle cycle remain unclear. Here, we used iGluSnFR to optically and directly interrogate glutamate release, at the single bouton level, in SYT7 KO dissociated mouse hippocampal neurons. We analyzed asynchronous release, paired pulse facilitation, and synaptic vesicle replenishment, and found that SYT7 contributes to each of these processes to different degrees. ‘Zap-and-freeze’ electron microscopy revealed that loss of SYT7 impairs the docking of synaptic vesicles after a stimulus and the recovery of depleted synaptic vesicles after a stimulus train. To execute these functions, SYT7 must be targeted to the plasma membrane via γ-secretase-mediated cleavage of the amino terminus, followed by palmitoylation. The complex sorting itinerary of SYT7 endows this Ca2+-sensor with the ability to control crucial forms of synaptic function and plasticity. SYT7 mediated asynchronous release, paired pulse facilitation, and synaptic vesicle replenishment was observed optically at individual hippocampal synapses Localization, trafficking, and stability of SYT7 is dependent on processing by γ-secretase Short term plasticity defects arise in SYT7KOs due to decreased docking of synaptic vesicles after stimulation SYT7 promotes paired-pulse facilitation and asynchronous release via distinct mechanisms

Neurotransmitter is released synchronously and asynchronously following an action potential. The release sites of these two phases are segregated within an active zone, with asynchronous release sites enriched near the center. Here we demonstrate that synchronous and asynchronous release sites are aligned with AMPA receptor and NMDA receptor clusters, respectively. Computational simulations indicate that this spatial and temporal arrangement of release ensures maximal membrane depolarization through AMPA receptors, alleviating the pore-blocking magnesium leading to greater activation of NMDA receptors. Together, these results suggest that release sites are organized to efficiently activate NMDA receptors.

Synaptic vesicles fuse with the plasma membrane to release neurotransmitter following an action potential, after which new vesicles must refill vacated release sites. How many vesicles can fuse at a single active zone, where they fuse within the active zone, and how quickly they are replaced with new vesicles is not well-established. To capture synaptic vesicle exocytosis at cultured mouse hippocampal synapses, we induced single action potentials by electrical field stimulation then subjected neurons to high-pressure freezing to examine their morphology by electron microscopy. During synchronous release, multiple vesicles can fuse at a single active zone; this multivesicular release is augmented by increasing the extracellular calcium concentration. Synchronous fusions are distributed throughout the active zone, whereas asynchronous fusions are biased toward the center of the active zone. Immediately after stimulation a large fraction of vesicles become undocked. Between 8 and 14 ms, new vesicles are recruited to the plasma membrane and fully replenish the docked pool, but docking of these vesicles is transient and they either undock or fuse within 100 ms. These results demonstrate that recruitment of synaptic vesicles to release sites is rapid and reversible.