Abstract
Synaptotagmin 7 (SYT7) has emerged as a key regulator of presynaptic function, but its localization and precise role in the synaptic vesicle cycle remain the subject of debate. Here, we used iGluSnFR to optically interrogate glutamate release, at the single-bouton level, in SYT7KO-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 a loss of SYT7 diminishes docking of synaptic vesicles after a stimulus and inhibits the recovery of depleted synaptic vesicles after a stimulus train. SYT7 supports these functions from the axonal plasma membrane, where its localization and stability require both γ-secretase-mediated cleavage and palmitoylation. In summary, SYT7 is a peripheral membrane protein that controls multiple modes of synaptic vesicle (SV) exocytosis and plasticity, in part, through enhancing activity-dependent docking of SVs.
Highlights
Calcium affords remarkable control over myriad membrane trafficking events in cells
Jackman et al, 2016 demonstrated that Synaptotagmin 7 (SYT7) was required for paired-pulse facilitation (PPF), a form of plasticity in which release is enhanced in response to a second stimulus when applied shortly after a conditioning stimulus (Regehr, 2012)
We found a small (3% difference in medians or 1.8% according to the Hodges-Lehmann estimate), but statistically significant, decrease in asynchronous release from SYT7KO neurons in response to the single stimulus (Figure 1c)
Summary
Calcium affords remarkable control over myriad membrane trafficking events in cells. In presynaptic nerve terminals, Ca2+ is important as it regulates numerous aspects of the synaptic vesicle (SV) cycle, including modes of exocytosis, endocytosis, and several forms of synaptic plasticity. Two years later, Jackman et al, 2016 demonstrated that SYT7 was required for paired-pulse facilitation (PPF), a form of plasticity in which release is enhanced in response to a second stimulus when applied shortly after a conditioning stimulus (Regehr, 2012) These authors found that facilitation supported frequency-invariant transmission at Purkinje cell to deep cerebellar nuclei and at vestibular synapses in mice (Turecek et al, 2017). Found to have no influence on the SV cycle in KO studies, SYT7 has been reported to fulfill several different functions at various types of synapses To reconcile these phenotypes, and to gain insights into the underlying mechanisms, we examined SV exocytosis in wild-type (WT) and SYT7KO hippocampal synapses in dissociated cultures using an optical biosensor for glutamate (iGluSnFR) (Marvin et al, 2018). By retargeting and restricting SYT7 to various membranes in the synapse, we show for the first time that SYT7 must localize to the PM to support asynchronous release, PPF, and SV replenishment
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