A proton current associated with sour taste: distribution and functional properties

Abstract

Sour taste is detected by taste receptor cells that respond to acids through yet poorly understood mechanisms. The cells that detect sour express the protein PKD2L1, which is not the sour receptor but nonetheless serves as a useful marker for sour cells. By use of mice in which the PKD2L1 promoter drives expression of yellow fluorescent protein, we previously reported that sour taste cells from circumvallate papillae in the posterior tongue express a proton current. To establish a correlation between this current and sour transduction, we examined its distribution by patch-clamp recording. We find that the current is present in PKD2L1-expressing taste cells from mouse circumvallate, foliate, and fungiform papillae but not in a variety of other cells, including spinal cord neurons that express PKD2L1. We describe biophysical properties of the current, including pH-dependent Zn 2+ inhibition, lack of voltage-dependent gating, and activation at modest pH values (6.5) that elicit action potentials in isolated cells. Consistent with a channel that is constitutively open, the cytosol of sour taste cells is acidified. These data define a functional signature for the taste cell proton current and indicate that its expression is mostly restricted to the subset of taste cells that detect sour.-Bushman, J. D., Ye, W., Liman, E. R. A proton current associated with sour taste: distribution and functional properties. FASEB J. 29, 3014-3026 (2015). www.fasebj.org

The K + channel K IR 2.1 functions in tandem with proton influx to mediate sour taste transduction

“…A similar block of the K + current in TRPM5 cells was not observed (Fig. 9A), consistent with the observation that these cells do not have a proton current (16,21).…”

“…9 C and D). At more acidic pH (6.6), action potentials were evoked even in the absence of the weak acid as we previously reported (21), and addition of the weak acid did not produce a significant change in firing rate.…”

“…Freshly isolated cells were loaded with the pH-sensitive dye phrodo red, and fluorescent intensity was monitored under the same conditions used to measure action potentials in cellattached recording. As we previously reported, PKD2L1 cells had an initial starting pH that was acidified compared with that of TRPM5, a phenomenon that can be attributed to proton leak through ungated proton channels in these cells (21). There was, however, no difference in the final pH reached in response to the weak acid solutions between cell types (pH 6.47 ± 0.08 in TRPM5 cells versus 6.34 ± 0.05 in PKD2L1 cells, P = 0.43 by two-way ANOVA followed by Sidak's multiple-comparison test; Fig.…”

“…Similarly, calcium responses from sour-sensitive cells in slice recording can be evoked with weak acids at a higher pH compared with strong acids (20). Moreover, we previously reported that action potentials can be elicited in sour taste cells in response to extracellular pH of 6.5-6.7, where the current carried by protons (2-3 pA) is unlikely to be sufficient to depolarize the cell (21). All of these phenomena can be explained if intracellular acidification increases membrane excitability of sour taste cells.…”

“…In the previous experiments, we took safeguards to prevent proton entry through the proton conductance, which would have generated an increase in the inward current and confounded interpretation of the results. Thus, whenever the extracellular solution was acidified, we added Zn 2+ (2 mM) to block the proton current (16,21). To determine whether entry of protons through the proton conductance could block the resting K + current in PKD2L1 cells, we simply measured the response to extracellular acidification in the absence of Zn 2+ .…”