Liddle’s syndrome mutations increase Na + transport through dual effects on epithelial Na + channel surface expression and proteolytic cleavage

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Abstract
Liddle's syndrome, an inherited form of hypertension, is caused by mutations that delete or disrupt a C-terminal PY motif in the epithelial Na+ channel (ENaC). Previous work indicates that these mutations increase expression of ENaC at the cell surface by disrupting its binding to Nedd4-2, an E3 ubiquitin-protein ligase that targets ENaC for degradation. However, it remains uncertain whether this mechanism alone is responsible; increased activity of ENaC channels could also contribute to excessive Na+ transport in Liddle's syndrome. ENaC activity is controlled in part by its cleavage state; proteolytic cleavage produces channels with a high open-state probability, whereas uncleaved channels are inactive. Here, we found that Liddle's syndrome mutations have two distinct effects of ENaC surface expression, both of which contribute to increased Na+ transport. First, these mutations increased ENaC expression at the cell surface; second, they increased the fraction of ENaC at the cell surface that was cleaved (active). This disproportionate increase in cleavage was reproduced by expression of a dominant-negative Nedd4-2 or mutation of ENaC ubiquitination sites, interventions that disrupt ENaC endocytosis and lysosomal degradation. Conversely, overexpression of Nedd4-2 had the opposite effect, decreasing the fraction of cleaved ENaC at the cell surface. Thus, the data not only suggest that Nedd4-2 regulates epithelial Na+ transport in part by controlling the relative expression of cleaved and uncleaved ENaC at the cell surface but also provide a mechanism by which Liddle's syndrome mutations alter ENaC activity.