Nedd4-2 catalyzes ubiquitination and degradation of cell surface ENaC

Epithelial Na+ absorption is regulated by Nedd4-2, an E3 ubiquitin-protein ligase that reduces expression of the epithelial Na+ channel ENaC at the cell surface. Defects in this regulation cause Liddle syndrome, an inherited form of hypertension. Previous work found that Nedd4-2 binds to ENaC via PY motifs located in the C termini of alpha-, beta-, and gamma ENaC. However, little is known about the mechanism by which Nedd4-2 regulates ENaC surface expression. Here we found that Nedd4-2 catalyzes ubiquitination of alpha-, beta-, and gamma ENaC; Nedd4-2 overexpression increased ubiquitination, whereas Nedd4-2 silencing decreased ubiquitination. Although Nedd4-2 increased both mono/oligoubiquitinated and multiubiquitinated forms of ENaC, monoubiquitination was sufficient for Nedd4-2 to reduce ENaC surface expression and reduce ENaC current. Ubiquitination was disrupted by Liddle syndrome-associated mutations in ENaC or mutation of the catalytic HECT domain in Nedd4-2. Several findings suggest that the interaction between Nedd4-2 and ENaC is localized to the cell surface. First, Nedd4-2 bound to a population of ENaC at the cell surface. Second, Nedd4-2 catalyzed ubiquitination of cell surface ENaC. Third, Nedd4-2 selectively reduced ENaC expression at the cell surface but did not alter the quantity of immature ENaC in the biosynthetic pathway. Finally, Nedd4-2 induced degradation of the cell surface pool of ENaC. Together, the data suggest a model in which Nedd4-2 binds to and ubiquitinates ENaC at the cell surface, which targets surface ENaC for degradation, and thus, reduces epithelial Na+ transport.