UNC-13-Munc13s play a central function in synaptic vesicle priming through their MUN domains. However, it is unclear whether this function arises from the ability of the MUN domain to mediate the transition from the Munc18-1–closed syntaxin-1 complex to the SNARE complex in vitro. The crystal structure of rat Munc13-1 MUN domain now reveals an elongated, arch-shaped architecture formed by α-helical bundles, with a highly conserved hydrophobic pocket in the middle. Mutation of two residues (NF) in this pocket abolishes the stimulation caused by the Munc13-1 MUN domain on SNARE complex assembly and on SNARE-dependent proteoliposome fusion in vitro. Moreover, the same mutation in UNC-13 abrogates synaptic vesicle priming in C. elegans neuromuscular junctions. These results strongly support the notion that orchestration of syntaxin-1 opening and SNARE complex assembly underlies the central role of UNC-13-Munc13s in synaptic vesicle priming.

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Animals utilize specialized sensory neurons enabling the detection of a wide range of environmental stimuli from the presence of toxic chemicals to that of touch. However, how these neurons discriminate between different kinds of stimuli remains poorly understood. By combining in vivo calcium imaging and molecular genetic manipulation, here we investigate the response patterns and the underlying mechanisms of the C. elegans phasmid neurons PHA/PHB to a variety of sensory stimuli. Our observations demonstrate that PHA/PHB neurons are polymodal sensory neurons which sense harmful chemicals, hyperosmotic solutions and mechanical stimulation. A repulsive concentration of IAA induces calcium elevations in PHA/PHB and both OSM-9 and TAX-4 are essential for IAA-sensing in PHA/PHB. Nevertheless, the PHA/PHB neurons are inhibited by copper and post-synaptically activated by copper removal. Neuropeptide is likely involved in copper removal-induced calcium elevations in PHA/PHB. Furthermore, mechanical stimulation activates PHA/PHB in an OSM-9-dependent manner. Our work demonstrates how PHA/PHB neurons respond to multiple environmental stimuli and lays a foundation for the further understanding of the mechanisms of polymodal signaling, such as nociception, in more complex organisms.

Membrane excitability is a fundamentally important feature for all excitable cells including both neurons and muscle cells. However, the background depolarizing conductances in excitable cells, especially in muscle cells, are not well characterized. Although mutations in transmembrane channel-like (TMC) proteins TMC1 and TMC2 cause deafness and vestibular defects in mammals, their precise action modes are elusive. Here, we discover that both TMC-1 and TMC-2 are required for normal egg laying in C. elegans. Mutations in these TMC proteins cause membrane hyperpolarization and disrupt the rhythmic calcium activities in both neurons and muscles involved in egg laying. Mechanistically, TMC proteins enhance membrane depolarization through background leak currents and ectopic expression of both C. elegans and mammalian TMC proteins results in membrane depolarization. Therefore, we have identified an unexpected role of TMC proteins in modulating membrane excitability. Our results may provide mechanistic insights into the functions of TMC proteins in hearing loss and other diseases.