Dual functions of SNAP25 in mouse taste buds.

Type III cells in mouse taste buds are considered to transmit aversive stimuli, such as sourness, to the gustatory nerve through vesicular synapses. Synaptosome-associated protein 25 (SNAP25) might contribute to synaptic vesicular release in sour sensation, although direct evidence has been lacking. Here, we demonstrated that epithelia-specific Snap25 conditional knockout (cKO) mice exhibited a significant reduction in the number of type III cells. Notably, the proportion of 5-ethynyl 2'-deoxyuridine-positive post-mitotic type III cells in Snap25 cKO mice was significantly lower on tracing day 14, but not at day 7, which suggests that SNAP25 contributes to the maintenance of type III cells. In a short-term lick test, Snap25 cKO (sour taste absent) and Snap25/ transient receptor potential vanilloid 1 double KO (sour taste and somatosensory absent) mice exhibit a significantly higher lick response to sour tastants, confirming the role of SNAP25 for sour sensation. Electrophysiological recordings of the chorda tympani nerve reveal nearly abolished ammonium and sour taste responses in Snap25 cKO mice, which concludes sour-dependent synapse transmission in type III cells. Overall, these data suggest that vesicular synapses in taste buds are indispensable for transmission of information from, and the replenishment of, sour-sensitive type III taste cells. KEY POINTS: The lack of SNAP25 in sour taste cells (Snap25 conditional knockout) significantly reduces the number of sour taste cells in the fungiform and circumvallate papillae of mice. Snap25/Trpv1 double knockout mice exhibit increased lick responses to sour tastants in the short-term lick test. The chorda tympani nerve of Snap25 conditional knockout mice shows reduced electrophysiological responses to sour tastants. These findings demonstrate that SNAP25 contributes not only to sour taste signal transduction, but also to the maintenance of sour taste cells, underscoring the dual functions of SNAP25 in taste cells.