SNAP-25 disease variants affect synaptic transmission by destabilizing SNARE complexes within a multimeric SNARE ring

SNAREopathies are neurodevelopmental syndromes arising from de novo mutations in the synaptic machinery. Two variants are SNAP-25 I192N and I192T, which cause early lethality and moderate developmental delay, respectively. Both variants lead to destabilization of the C-terminal end of the SNARE complex and reduced binding to the Munc18-1:VAMP2:syntaxin-1 acceptor complex while reducing spontaneous and evoked synaptic release in autaptic glutamatergic neurons (severity: I192N > I192T). I192N, but not I192T, negatively affects neuronal survival and dendritic branching. I192N is strongly dominant negative in the presence of wild-type SNAP-25, leading to impaired survival and reduced synaptic release. The dominant-negative effect is stronger for spontaneous than for evoked release and mild for sustained release. We consider how release is affected upon random incorporation of variant and wild-type proteins into a ring of SNARE complexes. We suggest that only a subset (approximately three) of the pre-assembled SNAREs (approximately six) mediates evoked release, rendering evoked release less susceptible to variants.