UNC-18 and Tomosyn Antagonistically Control Synaptic Vesicle Priming Downstream of UNC-13 in <i>Caenorhabditis elegans</i>

Abstract

Munc18-1/UNC-18 is believed to prime SNARE-mediated membrane fusion, yet the underlying mechanisms remain enigmatic. Here, we examine how potential gain-of-function mutations of Munc18-1/UNC-18 affect locomotory behavior and synaptic transmission, and how Munc18-1-mediated priming is related to Munc13-1/UNC-13 and Tomosyn/TOM-1, positive and negative SNARE regulators, respectively. We show that a Munc18-1(P335A)/UNC-18(P334A) mutation leads to significantly increased locomotory activity and acetylcholine release in <i>Caenorhabditis elegans</i>, as well as enhanced synaptic neurotransmission in cultured mammalian neurons. Importantly, similar to <i>tom-1</i> null mutants, <i>unc-18</i>(<i>P334A</i>) mutants partially bypass the requirement of UNC-13. Moreover, <i>unc-18</i>(<i>P334A</i>) and <i>tom-1</i> null mutations confer a strong synergy in suppressing the phenotypes of <i>unc-13</i> mutants. Through biochemical experiments, we demonstrate that Munc18-1(P335A) exhibits enhanced activity in SNARE complex formation as well as in binding to the preformed SNARE complex, and partially bypasses the Munc13-1 requirement in liposome fusion assays. Our results indicate that Munc18-1/UNC-18 primes vesicle fusion downstream of Munc13-1/UNC-13 by templating SNARE complex assembly and acts antagonistically with Tomosyn/TOM-1.<b>SIGNIFICANCE STATEMENT</b> At presynaptic sites, SNARE-mediated membrane fusion is tightly regulated by several key proteins including Munc18/UNC-18, Munc13/UNC-13, and Tomosyn/TOM-1. However, how these proteins interact with each other to achieve the precise regulation of neurotransmitter release remains largely unclear. Using <i>Caenorhabditis elegans</i> as an <i>in vivo</i> model, we found that a gain-of-function mutant of UNC-18 increases locomotory activity and synaptic acetylcholine release, that it partially bypasses the requirement of UNC-13 for release, and that this bypass is synergistically augmented by the lack of TOM-1. We also elucidated the biochemical basis for the gain-of-function caused by this mutation. Thus, our study provides novel mechanistic insights into how Munc18/UNC-18 primes synaptic vesicle release and how this protein interacts functionally with Munc13/UNC-13 and Tomosyn/TOM-1.