The δ-Subunit of the Epithelial Sodium Channel (ENaC) Enhances Channel Activity and Alters Proteolytic ENaC Activation*

摘要

The epithelial sodium channel (ENaC) is probably a heterotrimer with three well characterized subunits (αβγ). In humans an additional δ-subunit (δ-hENaC) exists but little is known about its function. Using the Xenopus laevis oocyte expression system, we compared the functional properties of αβγ- and δβγ-hENaC and investigated whether δβγ-hENaC can be proteolytically activated. The amiloride-sensitive ENaC whole-cell current (ΔIami) was about 11-fold larger in oocytes expressing δβγ-hENaC than in oocytes expressing αβγ-hENaC. The 2-fold larger single-channel Na+ conductance of δβγ-hENaC cannot explain this difference. Using a chemiluminescence assay, we demonstrated that an increased channel surface expression is also not the cause. Thus, overall channel activity of δβγ-hENaC must be higher than that of αβγ-hENaC. Experiments exploiting the properties of the known βS520C mutant ENaC confirmed this conclusion. Moreover, chymotrypsin had a reduced stimulatory effect on δβγ-hENaC whole-cell currents compared with its effect on αβγ-hENaC whole-cell currents (2-fold versus 5-fold). This suggests that the cell surface pool of so-called near-silent channels that can be proteolytically activated is smaller for δβγ-hENaC than for αβγ-hENaC. Proteolytic activation of δβγ-hENaC was associated with the appearance of a δ-hENaC cleavage product at the cell surface. Finally, we demonstrated that a short inhibitory 13-mer peptide corresponding to a region of the extracellular loop of human α-ENaC inhibited ΔIami in oocytes expressing αβγ-hENaC but not in those expressing δβγ-hENaC. We conclude that the δ-subunit of ENaC alters proteolytic channel activation and enhances base-line channel activity.