Functional expression of TMEM16A in taste bud cells.

The Ca2+ -activated Cl- channels TMEM16A and TMEM16B have relevant roles in many physiological processes including neuronal excitability and regulation of Cl- homeostasis. Here, we examined the presence of Ca2+ -activated Cl- channels in taste cells of mouse vallate papillae by using immunohistochemistry and electrophysiological recordings. By using immunohistochemistry we showed that only TMEM16A, and not TMEM16B, was expressed in taste bud cells where it largely co-localized with the inwardly rectifying K+ channel KNCJ1 in the apical part of type I cells. By using whole-cell patch-clamp recordings in isolated cells from taste buds, we measured an average current of -1083 pA at -100 mV in 1.5 μm Ca2+ and symmetrical Cl- in type I cells. Ion substitution experiments and blockage by Ani-9, a specific TMEM16A channel blocker, indicated that Ca2+ activated anionic currents through TMEM16A channels. We did not detect any Ca2+ -activated Cl- currents in type II or III taste cells. ATP is released by type II cells in response to various tastants and reaches type I cells where it is hydrolysed by ecto-ATPases. Type I cells also express P2Y purinergic receptors and stimulation of type I cells with extracellular ATP produced large Ca2+ -activated Cl- currents blocked by Ani-9, indicating a possible role of TMEM16A in ATP-mediated signalling. These results provide a definitive demonstration that TMEM16A-mediated currents are functional in type I taste cells and provide a foundation for future studies investigating physiological roles for these often-neglected taste cells.