Increased activity of the epithelial sodium channel (ENaC) in the respiratory airways contributes to the pathophysiology of cystic fibrosis (CF), a genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. In some patients suffering from atypical CF a mutation can be identified in only one CFTR allele. We recently identified in this group of CF patients a heterozygous mutation (W493R) in the α-subunit of ENaC. Here, we investigate the functional effects of this mutation by expressing wild-type αβγENaC or mutant αW493RβγENaC in Xenopus oocytes. The αW493R mutation stimulated amiloride-sensitive whole-cell currents (ΔIami) by ∼4-fold without altering the single-channel conductance or surface expression of ENaC. As these data suggest that the open probability (Po) of the mutant channel is increased, we investigated the proteolytic activation of ENaC by chymotrypsin. Single-channel recordings revealed that chymotrypsin activated near-silent channels in outside-out membrane patches from oocytes expressing wild-type ENaC, but not in membrane patches from oocytes expressing the mutant channel. In addition, the αW493R mutation abolished Na+ self inhibition of ENaC, which might also contribute to its gain-of-function effects. We conclude that the αW493R mutation promotes constitutive activation of ENaC by reducing the inhibitory effect of extracellular Na+ and decreasing the pool of near-silent channels. The resulting gain-of-function phenotype of the mutant channel might contribute to the pathophysiology of CF in patients carrying this mutation.
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