Subunit composition critically determines pH sensitivity of epithelial sodium channels in Xenopus laevis

Abstract

The Epithelial Sodium Channel (ENaC) mediates the selective flux of sodium ions across the apical membrane of epithelial cells. ENaC consists of three subunits (α,β,γ) and its activity at the cell surface is subject to a multitude of regulatory mechanisms, including proteolytic cleavage, extracellular pH and Na+ ions. Channel regulation may also depend on ENaC subunit composition, as an additional δ‐subunit can replace α‐ENaC in the heterotrimeric configuration. As the physiological implications of ENaC containing the δ‐subunit are poorly understood, we characterized the regulation of Xenopus δβγ‐ENaC by extracellular pH and Na+.Regulation of Xenopus δβγ‐ENaC by extracellular Na+ and pH was investigated by two‐electrode voltage‐clamp and cell‐attached Patch‐Clamp electrophysiology using the Xenopus oocyte expression system. Successive reduction of extracellular pH from pH 8 – 6 increased amiloride‐sensitive transmembrane currents of oocytes expressing δβγ‐ENaC by > 7 fold. In cell‐attached Patch‐Clamp recordings the pH of the pipette solution (pHpip = extracellular) affected δβγ‐ENaC open probability and kinetics. Compared to pHpip at 7.4, alkaline conditions (pHpip 8) decreased open probability as well as the mean channel open time, whereas under acidic conditions (pHpip 6) an increased open probability was accompanied by a reduction of average channel closed times. Transmembrane currents mediated by Xenopus δβγ‐ENaC are sensitive to sodium self‐inhibition, a phenomenon describing the fast inhibition of ENaC in the presence of high extracellular Na+ concentrations. Channel activation under acidic conditions (pH 7 & 6) was accompanied by a decrease in magnitude and apparent Na+‐affinity of self‐inhibition, whereas these effects were reversed under alkaline conditions (pH 8). Hypothesizing mutual regulation of δβγ‐ENaC activity by extracellular Na+ and pH, we examined whether neutralization of pH‐sensitive aspartates in the δ‐subunit affected channel self‐inhibition and regulation by pH. Specific aspartates in a subunit domain corresponding to a putative Na+‐sensing site of mouse α‐ENaC (Kashlan et al., 2015, J Biol Chem 290:568–76) were replaced by asparagines via site‐directed mutagenesis (δD293N, δD296N, δD293,6N). All mutations decreased pH‐mediated channel activation. Notably, ENaC containing the δD296N mutation also displayed a decreased sodium self‐inhibition, which was not observed in channels bearing a single δD293N mutation. Furthermore, human δβγ‐ENaCs which have a reduced sodium self‐inhibition were not activated by acidic pH as observed with the Xenopus orthologue displaying a strong sodium self‐inhibition.In conclusion, these results demonstrate that activity of ENaC containing the δ‐subunit in Xenopus laevis is sensitive to changes in the extracellular pH, whereas ENaC containing the α‐subunit is not. Proton‐mediated activation of ENaC involves changes in single channel kinetics and a reduction of sodium self‐inhibition. Thus, sensitivity of this amphibian ENaC to extracellular pH is critically determined by its subunit composition.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.