Synaptotagmin 9 modulates spontaneous neurotransmitter release in striatal neurons by regulating substance P secretion: SYT9 regulates SP release from striatal neurons

Synaptotagmin 9 (SYT9) is a tandem C2-domain Ca 2+ sensor for exocytosis in neuroendocrine cells; its function in neurons remains unclear. Here, we show that, in mixed-sex cultures, SYT9 does not trigger rapid synaptic vesicle exocytosis in mouse cortical, hippocampal, or striatal neurons, unless it is massively overexpressed. In striatal neurons, loss of SYT9 reduced the frequency of spontaneous neurotransmitter release events (minis). We delved into the underlying mechanism and discovered that SYT9 was localized to dense core vesicles that contain substance P (SP). Loss of SYT9 impaired SP release, causing the observed decrease in mini frequency. This model is further supported by loss-of-function mutants. Namely, Ca 2+ binding to the C2A domain of SYT9 triggered membrane fusion in vitro , and mutations that disrupted this activity abolished the ability of SYT9 to regulate both SP release and mini frequency. We conclude that SYT9 indirectly regulates synaptic transmission in striatal neurons by controlling SP release.