Differential Assembly of Native ENaC Complexes Across Mouse Epithelial Tissues

The epithelial sodium channel governs sodium and fluid absorption in kidney, lung, and colon epithelia, but the molecular organization of native epithelial sodium channel complexes in vivo remains poorly defined. Low abundance and biochemical instability have limited direct analysis of epithelial sodium channel assembly, composition, and regulatory associations in native tissues.

We generated a knock-in mouse in which the endogenous epithelial sodium channel γ subunit was fused to a fluorescent protein and affinity tag while preserving physiologic channel function. We validated channel activity in vivo using electrolyte measurements and pharmacologic inhibition, and assessed channel pharmacology by radioligand binding. Native epithelial sodium channel complexes were analyzed from lung, kidney, and colon using fluorescence-detection size-exclusion chromatography, single-molecule pull-down, fluorescent antibody fragments, and mass spectrometry.

The tagged γ subunit preserved normal epithelial sodium channel function in vivo. Native epithelial sodium channel complexes directly isolated from lung, kidney, and colon, revealed marked tissue-to-tissue differences in channel abundance and apparent complex size. Dual-color fluorescence analyses distinguished fully assembled channels from broader γ-containing assemblies. Proteomic analysis identified regulatory proteins associated with γ-containing complexes, indicating that epithelial sodium channel exists in multiple assembly and regulatory states in vivo.

Native epithelial sodium channel architecture is heterogeneous across epithelial tissues. Endogenous tagging enables direct molecular interrogation of epithelial sodium channel assembly and regulation in vivo.