A long form of the alpha subunit of the human epithelial Na+ channel forms hyperactivated channels (1109.12)

Abstract

The Alpha subunit of ENaC is critical to the ability to form a channel. Alpha‐like subunits have been described and have been shown to affect channel activity and/or regulation. A new isoform of the alpha subunit containing an extended N‐terminus resulting in a 728 a.a. subunit was recently identified. We examined the electrophysiological properties of channels formed by this isoform. Channels formed by α728βγ exhibited much higher activity than those formed by α669βγ in the Xenopus oocyte expression system. These differences were much larger under conditions which elevated the intracellular [Na+]. α728βγ also exhibited reduced current/voltage rectification. These differences occurred despite similar single channel conductances (~5.5 pS). The role of intracellular Na+ and Na+ self‐inhibition in these differences was examined. We find that α728βγ sustained much higher activity under elevated [Na+] than α669βγ and that intracellular and acute increase of [Na+] by injection of Na2SO4 led to further increase of conductance. α728βγ channels were further activated by extracellular cleavage by trypsin or subtilisin; with 2 notable differences: 1) fold activation was smaller than that observed with α669βγ, and 2) the large baseline activity of α728βγ channels was observed in the absence of detectable cleavage at the plasma membrane. These data indicate that α728 forms the structural basis of an ENaC with differing activity and regulation than α669.Grant Funding Source: National Institute of Diabetes and Digestive and Kidney Diseases Grant DK‐55626