Characterization of a novel splice variant of δ ENaC subunit in human lungs

Zhao R-Z, Nie H-G, Su X-F, Han D-Y, Lee A, Huang Y, Chang Y, Matalon S, Ji H-L. Characterization of a novel splice variant of delta ENaC subunit in human lungs. Am J Physiol Lung Cell Mol Physiol 302: L1262-L1272, 2012. First published April 13, 2012; doi:10.1152/ajplung.00331.2011.-Salt absorption via apical epithelial sodium channels ( ENaC) is a critical rate-limiting process in maintaining airway and lung lining fluid at the physiological level. delta ENaC ( termed delta 1 in this article) has been detected in human lung epithelial cells in addition to alpha, beta, and gamma subunits ( Ji HL, Su XF, Kedar S, Li J, Barbry P, Smith PR, Matalon S, Benos DJ. J Biol Chem 281: 8233-8241, 2006; Nie HG, Chen L, Han DY, Li J, Song WF, Wei SP, Fang XH, Gu X, Matalon S, Ji HL, J Physiol 587: 2663-2676, 2009) and may contribute to the differences in the biophysical properties of amiloride-inhibitable cation channels in pulmonary epithelial cells. Here we cloned a splicing variant of the delta 1 ENaC, namely, delta 2 ENaC in human bronchoalveolar epithelial cells (16HBEo). delta 2 ENaC possesses 66 extra amino acids attached to the distal amino terminal tail of the delta 1 ENaC. delta 2 ENaC was expressed in both alveolar type I and II cells of human lungs as revealed by in situ hybridization and real-time RT-PCR. To characterize the biophysical and pharmacological features of the splicing variant, we injected Xenopus oocytes with human ENaC cRNAs and measured whole cell and single channel currents of delta 1 beta gamma, delta 2 beta gamma, and alpha beta gamma channels. Oocytes injected with delta 2 beta gamma cRNAs exhibited whole cell currents significantly greater than those expressing delta 1 beta gamma and alpha beta gamma channels. Single channel activity, unitary conductance, and open probability of delta 2 beta gamma channels were significantly greater compared with delta 1 beta gamma and alpha beta gamma channels. In addition, delta 2 beta gamma and delta 1 beta gamma channels displayed significant differences in apparent Na+ affinity, dissociation constant for amiloride ( K-i(amil)), the EC50 for capsazepine activation, and gating kinetics by protons. Channels comprising of this novel splice variant may contribute to the diversities of native epithelial Na+ channels.