Channels Inhibiting ENaC Activation

Elizabeth M. Adler

Science Signaling, AAAS, Washington, DC 20005, USA

Na⁺ movement across various transporting epithelia depends on its entry through apical epithelial Na⁺ channels (ENaC) and its exit by means of a basolateral Na⁺-K⁺-ATPase. ENaC activity is inhibited by extracellular and intracellular Na⁺, helping maintain Na⁺ homeostasis through negative feedback. Noting that ENaC is activated by proteolytic cleavage, and is present in both cleaved and uncleaved forms at the cell surface, Knight et al. investigated Na⁺’s effects on its proteolytic activation. When human embryonic kidney (HEK) 293T cells expressing the , β, and subunits of ENaC were incubated in medium containing 135 mM Na⁺, more of the cell surface -subunit (ENaC) was in the full-length than in the cleaved form, whereas this ratio was reversed in 0 mM Na⁺. The ratio of full-length to cleaved ENaC increased as Na⁺ concentration was increased to 34 mM, but not thereafter. When Fischer rat thyroid cells expressing ENaC were used to create a barrier between compartments containing 135 mM Na⁺, the initial current sensitive to amiloride (an ENaC blocker) was greater in cells preincubated in 0 mM Na⁺ than in 135 mM Na⁺. Moreover, trypsin increased current in cells preincubated in 135 mM Na⁺ but had little effect on current in those preincubated in 0 mM Na⁺. A combination of pharmacological and mutational analysis indicated that proteolytic cleavage of ENaC was regulated by intracellular Na⁺. The 135 mM Na⁺ inhibited proteolysis of ENaC by trypsin without inhibiting trypsin’s enzymatic activity. The authors propose that increased intracellular Na⁺ inhibits ENaC activation through Na⁺-dependent regulation of cleavage site accessibility.

K. K. Knight, D. M. Wentzlaff, P. M. Snyder, Intracellular sodium regulates proteolytic activation of the epithelial sodium channel. J. Biol. Chem. 283, 27477-27482 (2008). [Abstract] [Full Text]

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