The cytosolic termini of the beta- and gamma-ENaC subunits are involved inthe functional interactions between cystic fibrosis transmembrane conductance regulator and epithelial sodium channel

Epithelial sodium channel (ENaC) and cystic fibrosis transmembrane conducta nce regulator (CFTR) are colocalized in the apical membrane of many epithel ia, These channels are essential for electrolyte and water secretion and/or reabsorption, In cystic fibrosis airway epithelia, a hyperactivated epithe lial Na+ conductance operates in parallel with defective Cl- secretion. Sev eral groups have shown that CFTR down-regulates ENaC activity, but the mech anisms and the regulation of CFTR by ENaC are unknown. To test the hypothes is that ENaC and CFTR regulate each other, and to identify the region(s) of ENaC involved in the interaction between CFTR and ENaC, rENaC and its muta nts were co-expressed with CFTR in Xenopus oocytes, Whole cell macroscopic sodium currents revealed that wild type (wt) alpha beta gamma-rENaC-induced Na+ current was inhibited by coexpression of CFTR, and further inhibited w hen CFTR was activated with a cAMP-raising mixture (CKT), Conversely, alpha beta gamma-rENaC stimulated CFTR-mediated Cl- currents up to similar to 6- fold, Deletion mutations in the intracellular tails of the three rENaC subu nits suggested that the carboxyl terminus of the beta subunit was required both for the down-regulation of ENaC by activated CFTR and the up-regulatio n of CFTR by ENaC, However, both the carboxyl terminus of the beta subunit and the amino terminus of the gamma subunit were essential for the down-reg ulation of rENaC by unstimulated CFTR, Interestingly, down-regulation of rE NaC by activated CFTR was Cl--dependent, while stimulation of CFTR by rENaC was not dependent on either cytoplasmic Na+ or a depolarized membrane pote ntial. In summary, there appear to be at least two different sites in ENaC involved in the intermolecular interaction between CFTR and ENaC.