Localization and functional expression of the calcium-activated chloride channel TMEM16A in taste bud cells

Taste is a specialized chemosensory system dedicated to the evaluation and discrimination of different types of chemicals entering the mouth. On the tongue, different types of papillae contain several taste buds, onion-shaped structures composed of up 100 taste bud cells. These cells express specific receptors and ion channels and can be divided into three different types. Type I are the most abundant cells in the taste buds and are considered to have a glial-like function. Type II and III can generate action potentials upon stimulation with different types of tastants. The Ca2+-activated Cl- channels TMEM16A and TMEM16B have relevant roles in many physiological processes including neuronal excitability and regulation of Cl- homeostasis and a recent study reported the expression of both channels in various types of taste cells. Here, we re-examined the functional expression of Ca2+-activated Cl- channels in taste cells of mouse vallate papillae by combining immunohistochemical and electrophysiological approaches. By using specific markers and knockout mice, we found that only type I cells expressed TMEM16A, whereas TMEM16B was not expressed in taste cells. Furthermore, TMEM16A was found to largely co-localize with the inwardly rectifying K+ channel KNCJ1 in the apical part of type I cells. Whole-cell patch-clamp recordings on dissociated taste cells showed Ca2+-activated Cl- currents in type I cells have properties similar to those of TMEM16A. Indeed, blockage by Ani9, a specific TMEM16A channel blocker, indicated that Ca2+-activated Cl- currents are due to TMEM16A channels. On the other hand, we did not detect any Ca2+-activated Cl- current in type II and type III cells. ATP is released by type II cells in response to various tastants and reaches type I cells where it is hydrolyzed by ecto-ATPases. Type I cells also express P2Y purinergic receptors and stimulation with ATP can activate a strong Cl- conductance in these cells. We found that ATP-evoked Cl- current was blocked by Ani9, indicating a possible role of TMEM16A in ATP-mediated signaling. Altogether, our results establish that TMEM16A-mediated currents are functional in type I taste cells and provide a foundation for future work investigating the precise physiological role of TMEM16A in the taste system.