Activation of the Human Epithelial Sodium Channel (ENaC) by Bile Acids Involves the Degenerin Site.

ABSTRACT

The epithelial sodium channel (ENaC) is a member of the ENaC/degenerin ion channel family, which also includes the bile acid-sensitive ion channel (BASIC). So far little is known about the effects of bile acids on ENaC function. ENaC is probably a heterotrimer consisting of three well characterized subunits (αßγ). In humans, but not in mice and rats, an additional δ-subunit exists. The aim of this study was to investigate the effects of chenodeoxycholic, cholic, and deoxycholic acid in unconjugated (CDCA, CA, and DCA) and tauro-conjugated (t-CDCA, t-CA, t-DCA) form on human ENaC in its αßγ- and δßγ-configuration. We demonstrated that tauro-conjugated bile acids significantly stimulate ENaC in the αßγ- and in the δßγ-configuration. In contrast, non-conjugated bile acids have a robust stimulatory effect only on δßγENaC. Bile acids stimulate ENaC-mediated currents by increasing the open probability of active channels without recruiting additional near-silent channels known to be activated by proteases. Stimulation of ENaC activity by bile acids is accompanied by a significant reduction of the single-channel current amplitude, indicating an interaction of bile acids with a region close to the channel pore. Analysis of the known ASIC1 (acid-sensing ion channel) crystal structure suggested that bile acids may bind to the pore region at the degenerin site of ENaC. Substitution of a single amino acid residue within the degenerin region of ßENaC (N521C or N521A) significantly reduced the stimulatory effect of bile acids on ENaC, suggesting that this site is critical for the functional interaction of bile acids with the channel.