Epithelial Sodium Channels (ENaCs)

Abstract

Amiloride-sensitive epithelial sodium (Na+) channels (ENaCs) play a crucial role in Na+ transport and fluid reabsorption in the kidney, lung, and colon. ENaC is made up of three homologous subunits, α, β, and γ. The magnitude of ENaC-mediated Na+ transport in epithelial cells depends on the average open probability of the channels and the number of channels on the apical surface of epithelial cells. The open probability of ENaC depends upon the association of ENaCs with inositol lipid phosphates and association with a collection of chaperone proteins including calmodulin and MARCKS (myristoylated alanine-rich C-kinase substrate). MARCKS when associated with the membrane sequesters PIP2 and increases open probability and channel stability. The number of channels in the apical membrane, in turn, depends upon a balance between the rate of ENaC insertion and the rate of removal from the apical membrane. To this end, ENaC is associated with an additional collection of proteins, the ENaC regulatory complex (ERC) that contains proteins including SGK1, GILZ1, ERK, and Nedd4-2 that regulate channel stability in the membrane. The ERC under basal conditions contains several molecules including ERK and Nedd4-2 that inhibit ENaC activity. The C-terminal domain of all three subunits is intracellular and contains a proline-rich motif (PPxY) to which one isoform of the ubiquitin ligase, Nedd4-2 (neural precursor cell-expressed developmentally downregulated protein), binds, thereby, promoting ubiquitin conjugation of the channel subunits followed by internalization and degradation or recycling. In the presence of aldosterone, several proteins including SGK1, GILZ1, and MARCKS are recruited to the complex and stimulate ENaC activity. In addition, aldosterone promotes insertion of channels from intracellular pools and transcription/translation of new ENaC. Other agents that alter ENaC activity work by modulating PIP2 presentation to the channel or by altering the rate of retrieval, recycling, insertion, or transcription/translation of ENaC.