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

Alexandr V Ilyaskin,Alexei Diakov,Christoph Korbmacher,Silke Haerteis

doi:10.1074/jbc.m116.726471

Alexandr V Ilyaskin, Alexei Diakov + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m116.726471

Copy DOI

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 (αβγ). 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 αβγ- and δβγ-configuration. We demonstrated that tauro-conjugated bile acids significantly stimulate ENaC in the αβγ- and in the δβγ-configuration. In contrast, non-conjugated bile acids have a robust stimulatory effect only on δβγENaC. 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 βENaC (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.

Highlights

Acid-sensing ion channel 1 (ASIC1) belonging to the same channel family suggests that ENaC is a heterotrimer composed of three homologous subunits: ␣, ␤, and ␥ [2,3,4]
Bile Acids Are More Potent Activators of ENaC in the ␦␤␥than the ␣␤␥-Configuration—To test the effect of bile acids on ENaC function, human ␣␤␥- or ␦␤␥ENaC was heterologously expressed in Xenopus laevis oocytes
Control experiments demonstrated that bile acids had no effect on whole-cell currents in non-injected oocytes and that in ENaCexpressing oocytes the current stimulated by tauro-deoxycholic acid (t-DCA) was fully blocked by amiloride

Summary

Introduction

Acid-sensing ion channel 1 (ASIC1) belonging to the same channel family suggests that ENaC is a heterotrimer composed of three homologous subunits: ␣, ␤, and ␥ [2,3,4]. Local factors affecting ENaC function are likely to be important for tissue-specific ENaC regulation. Bile acids have previously been shown to activate another member of the ENaC/degenerin ion channel family, the bile acid-sensitive ion channel (BASIC) [35, 36], previously named intestine Naϩ channel (INaC) in humans [37], and brain liver intestine Naϩ channel (BLINaC) in mouse and rat [38]. The physiological role and mechanism of ENaC activation by bile acids remain to be elucidated. It has not yet been shown whether bile acids affect the function of human ENaC, which can occur in at least two configurations (␣␤␥ or ␦␤␥) in different tissues. Activation of Human ENaC by Bile Acids bile acids can activate human ENaC, probably by interacting with the degenerin region

Objectives

Methods

Results

Conclusion