Abstract
Synaptic transmission requires a stable pool of release-ready (primed) vesicles. Here we show that two molecules involved in SNARE-complex assembly, Munc13-1 and Munc18-1, together stabilize release-ready vesicles by preventing de-priming. Replacing neuronal Munc18-1 by a non-neuronal isoform Munc18-2 (Munc18-1/2SWAP) supports activity-dependent priming, but primed vesicles fall back into a non-releasable state (de-prime) within seconds. Munc13-1 deficiency produces a similar defect. Inhibitors of N-ethylmaleimide sensitive factor (NSF), N-ethylmaleimide (NEM) or interfering peptides, prevent de-priming in munc18-1/2SWAP or munc13-1 null synapses, but not in CAPS-1/2 null, another priming-deficient mutant. NEM rescues synaptic transmission in munc13-1 null and munc18-1/2SWAP synapses, in acute munc13-1 null slices and even partially in munc13-1/2 double null synapses. Together these data indicate that Munc13-1 and Munc18-1, but not CAPS-1/2, stabilize primed synaptic vesicles by preventing NSF-dependent de-priming.
Highlights
Synaptic transmission requires a stable pool of release-ready vesicles
This property depends critically on the availability of a pool of synaptic vesicles that is immediately available for exocytosis, ‘the readily releasable pool (RRP)’ or the ‘primed’ vesicles
The mammalian SNARE complexes that drive synaptic exocytosis consist of Syntaxin[1], Synaptobrevin[2] and SNAP25
Summary
We show that two molecules involved in SNARE-complex assembly, Munc[] and Munc[], together stabilize release-ready vesicles by preventing de-priming. NEM rescues synaptic transmission in munc[] null and munc18-1/2SWAP synapses, in acute munc[] null slices and even partially in munc13-1/2 double null synapses Together these data indicate that Munc[] and Munc[], but not CAPS-1/2, stabilize primed synaptic vesicles by preventing NSF-dependent de-priming. Plausible alternatives for this model have been suggested[3,6] It is not clear which molecular processes stabilize this proposed partially assembled state, that is, prevent their instant full assembly or their complete disassembly. The molecular factors that prevent disassembly of partially assembled SNARE complexes in primed vesicles (de-priming) are unknown and de-priming remains a poorly characterized process. Hereby, preventing de-priming rescued synaptic transmission in munc[] null, in synapses where Munc[] had been replaced by Munc[] and even partially in munc13-1/2 double null synapses
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