Regulation of the epithelial Na+ channel by Nedd4 and ubiquitination

The epithelial Na+ channel (ENaC) is comprised of three subunits, alpha, be ta and gamma, and plays an essential role in Na+ and fluid absorption in th e kidney, colon and lung. We had identified proline-rich sequences at the C termini of alpha beta gamma ENaC, which include the sequence PPxY, the PY motif. This sequence in beta or gamma ENaC is deleted or mutated in Liddle' s syndrome, a hereditary form of arterial hypertension. Our previous work d emonstrated that these PY motifs bind to the WW domains of Nedd4. a ubiquit in protein ligase containing a C2 domain, three or four WW domains and a ub iquitin protein ligrase Hect domain. Accordingly, we have recently demonstr ated that Nedd4, regulates ENaC function by controlling the number of chann els at the cell surface, that this regulation is impaired in ENaC bearing L iddle's syndrome mutations, and that ENaC stability and function are regula ted by ubiquitination. The C2 domain is responsible for localizing Nedd4 to the plasma membrane in a Ca2+-dependent manner, and in polarized epithelia l MDCK cells this localization is primarily apical. In accordance, electrop hysiological characterization of ENaC expressed in MDCK cells revealed inhi bition of channel activity by elevated intracellular Ca2+ levels. Thus, in response to Ca2+, Nedd4 may be mobilized to the apical membrane via its C2 domain, where it binds ENaC via Nedd4-WW:ENaC-PY motifs' interactions, lead ing to ubiquitination of the channel by the Nedd4-Hect domain and subsequen t channel endocytosis and lysosomal degradation. This process may be at lea st partially impaired in Liddle's syndrome due to reduced Nedd4 binding, le ading to increased retention of ENaC at the cell surface.