Control of epithelial Na⁺ conductance by the cystic fibrosis transmembrane conductance regulator

Abstract

Cystic fibrosis transmembrane conductance regulator (CFTR) is an epithelial Cl^– channel expressed in luminal membranes of secretory and reabsorptive epithelia. CFTR plays a predominant role in both cAMP- and Ca²⁺-activated secretion of electrolytes. Although Ca²⁺-dependent Cl^– channels exist independent of CFTR in the airway epithelium, their physiological significance remains to be determined. However, CFTR seems to be the only relevant Cl^– conductance in the colonic epithelium. Apart from its secretory function, CFTR also has a task in regulating the reabsorption of electrolytes by controlling the activity of the epithelial Na⁺ channel, ENaC. Accordingly, defects in CFTR causing the disease cystic fibrosis (CF) lead to disturbances of both the secretion and absorption of electrolytes. Therefore, it is unclear what is pathophysiologically more important for the development of CF lung disease, the impaired secretion of Cl^– or the enhanced reabsorption of Na⁺ and consecutive hyperabsorption of electrolytes. The mechanisms of how CFTR and ENaC interact are unknown. Previous work has given rise to several interesting working hypothesis, such as direct protein interaction or interaction via cytoskeletal proteins. Recent studies demonstrate the importance of the first nucleotide binding fold of CFTR, not only for the inhibition of ENaC but also for the interaction with other ion channels. Further studies are required to demonstrate whether regulation of other ion channels and membrane transport by CFTR occur by a common mechanism.