The PY Motif of ENaC, Mutated in Liddle Syndrome, Regulates Channel Internalization, Sorting and Mobilization from Subapical Pool

Abstract

The epithelial-Na(+)-channel (alphabetagammaENaC) regulates kidney salt-transport and blood pressure. Each ENaC subunit contains a PY motif (PPxY) and its mutation in beta/gammaENaC causes Liddle syndrome, a hereditary hypertension. These (extended) PY motifs (PP(616)xY(618)xxL(621)) serve as binding sites for the ubiquitin ligase Nedd4-2, which decreases cell-surface expression of ENaC by unknown route(s). Using polarized kidney epithelia [Madin-Darby canine kidney I (MDCK-I)] cells stably expressing extracellularly myc-tagged wild type (WT) or PY-motif mutants of betaENaC (P616A, Y618A or L621A, with WT-alphagammaENaC), and live-imaging plus enzyme-linked immunosorbent assay (ELISA)-type assays to analyze routes/rates of ENaC internalization/recycling, we show here that cell-surface half-life of all PY mutants was fourfold longer than WT-ENaC (approximately 120 versus 30 minutes), reflecting primarily reduced channel internalization but also attenuated replenishment of cell-surface ENaC from a large subapical pool. The Y618A mutant revealed more severe internalization and replenishment defects than the other PY mutants. Internalized WT-ENaC was detected in sorting/recycling and late endosomes/lysosomes, while the Y618A mutant accumulated in the former. Nedd4-2 ubiquitinated ENaC at the apical membrane causing channel internalization and degradation. Cyclic AMP (cAMP) accelerated mobilization of subapical ENaC to the cell surface and long-term ENaC recycling, but only mobilization, not recycling, was inhibited in the PY mutants. These results suggest that the ENaC PY motifs (and Nedd4-2) primarily regulate channel internalization but also affect cAMP-dependent replenishment, providing important insight into the Liddle syndrome defects.