Activation of the epithelial sodium channel by the metalloprotease meprin beta subunit

The Epithelial Na+ Channel (ENaC) is an apical heteromeric channel that mediates Na+ entry into epithelial cells from the luminal cell surface. ENaC is activated by proteases that interact with the channel during biosynthesis or at the extracellular surface. Meprins are cell surface and secreted metalloproteinases of the kidney and intestine. We discovered by affinity chromatography that meprins bind gamma-ENaC, a subunit of the ENaC hetero-oligomer. The physical interaction involves NH2-terminal cytoplasmic residues 37-54 of gamma-ENaC, containing a critical gating domain immediately before the first transmembrane domain, and the cytoplasmic COOH-terminal tail of meprin beta (residues 679-704). This potential association was confirmed by co-expression and co-immunoprecipitation studies. Functional assays revealed that meprins stimulate ENaC expressed exogenously in Xenopus oocytes and endogenously in epithelial cells. Co-expression of ENaC subunits and meprin beta or alpha/beta in Xenopus oocytes increased amiloride-sensitive Na+ currents approximately twofold. This increase was blocked by preincubation with an inhibitor of meprin activity, actinonin. The meprin-mediated increase in ENaC currents in oocytes and epithelial cell monolayers required meprin beta, but not the alpha subunit. Meprin beta promoted cleavage of a and gamma-ENaC subunits at sites close to the second transmembrane domain in the extracellular domain of each channel subunit. Thus, meprin beta regulates the activity of ENaC in a metalloprotease-dependent fashion.