Synaptotagmin 1 clamps synaptic vesicle fusion in mammalian neurons independent of complexin

Abstract

Synaptic vesicle (SV) exocytosis is mediated by SNARE proteins. Reconstituted SNAREs are constitutively active, so a major focus has been to identify fusion clamps that regulate their activity in synapses: the primary candidates are synaptotagmin (syt) 1 and complexin I/II. Syt1 is a Ca2+ sensor for SV release that binds Ca2+ via tandem C2-domains, C2A and C2B. Here, we first determined whether these C2-domains execute distinct functions. Remarkably, the C2B domain profoundly clamped all forms of SV fusion, despite synchronizing residual evoked release and rescuing the readily-releasable pool. Release was strongly enhanced by an adjacent C2A domain, and by the concurrent binding of complexin to trans-SNARE complexes. Knockdown of complexin had no impact on C2B-mediated clamping of fusion. We postulate that the C2B domain of syt1, independent of complexin, is the molecular clamp that arrests SVs prior to Ca2+-triggered fusion.

Resolving kinetic intermediates during the regulated assembly and disassembly of fusion pores

“…Then addition of Ca 2+ drives further assembly and thus fusion pore opening. These findings are consistent with cell-based studies which indicate that the C2B-domain of apo-syt1 is a potent fusion clamp 26,49 . This clamping activity appears to be controlled by conformational changes that determine the relative disposition of the tandem C2-domains.…”

“…The first goal was to address the impact of syt1 on recombinant fusion pores. Under resting conditions, it has been proposed that apo-syt1 serves as a fusion clamp that prevents SV exocytosis until the arrival of an action potential and a concomitant increase in [Ca 2+ ] i 26,28,29,49 . We observed that a clamping function for syt1 emerged in the ND-BLM system and was most prominent when fusion pores were formed using physiological densities of syb2 30 (Figs.…”

Adipose Tissue and Brain Metabolic Responses to Western Diet—Is There a Similarity between the Two?

“…Accordingly, we found a significant decrease in the amount of synaptotagmin I in both hippocampus and frontal cortex of middle aged HFF rats. As known, synaptotagmin I participates in the regulation of synaptic vesicle exocytosis, acting for clamping synaptic vesicle fusion in mammalian neurons [22]. Despite a lower level of BDNF in both hippocampus and frontal cortex of the HFF group, no differences in the other markers, synaptophysin and synapsin I, were detected.…”

“…Synaptophysin and synapsin I are involved in synaptic growth, as they play key roles in synapse formation, maturation, and maintenance [21]. Synaptotagmin I is a major calcium sensor for transmitter release at central synapse and is also crucial for clamping synaptic vesicle fusion in mammalian neurons [22]. HFF diet did not affect synapsin I or synaptophysin level, while it was associated with a marked decrease of synaptotagmin both in the hippocampus ( Figure 10) and the frontal cortex ( Figure 11).…”

Independent Yet Synergistic Roles of Synaptotagmin-1 and Complexin in Calcium Regulated Neuronal Exocytosis

“…Biochemical analyses show that Cpx catalyzes the initial stages of SNARE assembly, but then blocks complete assembly (Giraudo et al 2009;Kummel et al 2011;Li et al 2011). Physiological studies show that Cpx indeed clamps spontaneous fusion in invertebrate neurons (Huntwork and Littleton 2007;Wragg et al 2013;Cho et al 2014), but its importance as a fusion clamp in mammalian neurons is still under debate (Yang, Cao, and Sudhof 2013;Lopez-Murcia et al 2019;Courtney et al 2019).…”