Hypothesis – buttressed rings assemble, clamp, and release SNAREpins for synaptic transmission

Abstract

Neural networks are optimized to detect temporal coincidence on the millisecond timescale. Here, we offer a synthetic hypothesis based on recent structural insights into SNAREs and the C2 domain proteins to explain how synaptic transmission can keep this pace. We suggest that an outer ring of up to six curved Munc13 ‘MUN’ domains transiently anchored to the plasma membrane via its flanking domains surrounds a stable inner ring comprised of synaptotagmin C2 domains to serve as a work‐bench on which SNAREpins are templated. This ‘buttressed‐ring hypothesis’ affords straightforward answers to many principal and long‐standing questions concerning how SNAREpins can be assembled, clamped, and then released synchronously with an action potential.

Rearrangements under confinement lead to increased binding energy of Synaptotagmin‐1 with anionic membranes in Mg2+ and Ca2+

“…Nonetheless, if all Syt1 are bound to the plasma membrane, e.g. as in the recently proposed ring‐shaped oligomers model , their local concentration is increased up to the same order of magnitude as the concentration in the current SFA experiments. In any case, using 10× smaller Syt1 concentration in the present measurements would decrease the magnitudes of Γ and W ad accordingly, making accurate measurement of E Syt1 impossible.…”

Synaptotagmin oligomers are necessary and can be sufficient to form a Ca2+‐sensitive fusion clamp

“…One possibility is that Syt1 oligomers act as a ‘washer’ (or spacer) to sterically block fusion . Additionally but perhaps alternatively, Syt1 oligomers could bind and organize the SNAREs in a stable, partially assembled state as outlined in the buttressed ring model . Further experiments, including detailed mutational analysis, is required to dissect the precise molecular mechanism of the fusion clamp observed in this system.…”

“…As the Syt1 rings assembled on phospholipid surfaces are disassembled by Ca 2+ , we hypothesized that the assembly of a Syt ring provides the core mechanism of this fusion clamp, and its disassembly by Ca 2+ enables synchronous vesicle release . Indeed, expressing the ring‐destabilizing F349A mutant in neuroendocrine (PC12) cells dominantly and dramatically increases spontaneous release , suggesting that Syt1 oligomers play a necessary role in providing a reversible fusion clamp.…”

Mechanism of neurotransmitter release coming into focus

“…Third, Syt1 was shown to form ring‐like oligomers that were proposed to inhibit neurotransmitter release before Ca 2+ influx and to be disassembled upon Ca 2+ ‐dependent membrane binding to the C 2 domains, but it is unclear whether such oligomers are compatible with the crystal structures of Syt1‐SNARE complex assemblies. It is worth noting that the observed rings led to a hypothesis whereby a buttressed ring of Syt1s is surrounded by a ring of Munc13s and, interestingly, supramolecular assemblies containing multiple Munc13‐1 molecules have been observed by super‐resolution imaging of synapses . Fourth, and more generally, it is unclear how this model can be integrated with the notion that Munc18‐1 and/or Munc13‐1 may also bind to the SNARE complex as intrinsic components of the complex that triggers membrane fusion (Fig.…”