Intracellular Na+ Regulates Epithelial Na+ Channel Maturation

Epithelial Na⁺ channel (ENaC) function is regulated by the intracellular Na⁺ concentration (Na⁺]_i) through a process known as Na⁺ feedback inhibition. Although this process is known to decrease the expression of proteolytically processed active channels on the cell surface, it is unknown how Na⁺]_i alters ENaC cleavage. We show here that Na⁺]_i regulates the posttranslational processing of ENaC subunits during channel biogenesis. At times when Na⁺]_i is low, ENaC subunits develop mature N-glycans and are processed by proteases. Conversely, glycan maturation and sensitivity to proteolysis are reduced when Na⁺]_i is relatively high. Surface channels with immature N-glycans were not processed by endogenous channel activating proteases, nor were they sensitive to cleavage by exogenous trypsin. Biotin chase experiments revealed that the immature surface channels were not converted into mature cleaved channels following a reduction in Na⁺]_i. The hypothesis that Na⁺]_i regulates ENaC maturation within the biosynthetic pathways is further supported by the finding that Brefeldin A prevented the accumulation of processed surface channels following a reduction in Na⁺]_i. Therefore, increased Na⁺]_i interferes with ENaC N-glycan maturation and prevents the channel from entering a state that allows proteolytic processing.