Selective cleavage of SNAREs in sensory neurons unveils protein complexes mediating peptide exocytosis triggered by different stimuli

Abstract

Oligomerisation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes is required for synaptic vesicle fusion and neurotransmitter release. How these regulate the release of pain peptides elicited by different stimuli from sensory neurons has not been established. Herein, K(+) depolarization was found to induce multiple sodium dodecyl sulfate (SDS)-resistant SNARE complexes in sensory neurons exposed to botulinum neurotoxins (BoNTs), with molecular weights ranging from 104-288 k (large) to 38-104 k (small). Isoform 1 of vesicle-associated membrane protein 1 (VAMP 1) assembled into stable complexes upon depolarisation and was required for the participation of intact synaptosome-associated protein of relative molecular mass 25 k (SNAP-25) or BoNT/A-truncated form (SNAP-25A) in the large functional and small inactive SDS-resistant SNARE complexes. Cleaving VAMP 1 decreased SNAP-25A in the functional complexes to a much greater extent than the remaining intact SNAP-25. Syntaxin 1 proved essential for the incorporation of intact and SNAP-25A into the large complexes. Truncation of syntaxin 1 by BoNT/C1 caused /A- and/or /C1-truncated SNAP-25 to appear in non-functional complexes and blocked the release of calcitonin gene-related peptide (CGRP) elicited by capsaicin, ionomycin, thapsigargin or K(+) depolarization. Only the latter two were susceptible to /A. Inhibition of CGRP release by BoNT/A was reversed by capsaicin and/or ionomycin, an effect overcome by BoNT/C1. Unlike BoNT/B, BoNT/D cleaved VAMP 1 in addition to 2 and 3 in rat sensory neurons and blocked both CGRP and substance P release. Thus, unlike SNAP-25, syntaxin 1 and VAMP 1 are more suitable targets to abolish functional SNARE complexes and pain peptide release evoked by any stimuli.