The Molecular Basis of Chloride Channel Dysregulation in Cystic Fibrosis

The opening and closing of chloride (Cl⁻) channels in the apical membrane of epithelial cells is regulated by hormones, neurotransmitters and enterotoxins (intestine) acting through a variety of intracellular messengers, including cyclic nucleotides (cAMP, cGMP), calcium (Ca) and diacylglycerol (DAG). The chloride impermeability of epithelial membranes observed in cystic fibrosis (CF) patients does not result from a defect in the Cl⁻ conducting properties of the channel or in channel recruitment but stems either from a defect in a key regulator of the channel, presumably a phosphoprotein, or from the hyperactivation of a channel closing mechanism, presumably a protein phosphatase or a down‐regulating protein kinase (i.e. protein kinase C). In vitro phosphorylation of isolated intestinal brush border membranes has revealed the existence of a 25000 molecular weight proteolipid (p25) acting as a cosubstrate for both cGMP‐ and cAMP‐dependent protein kinases and cross‐reacting with antibodies directed against the cytoplasmic tail of the band 3 anion exchanger from erythrocytes. The putative role of p25 in Cl⁻ channel regulation and its relationship to an unidentified GTP‐binding protein recently implicated in Cl⁻ channel activation is discussed on the basis of a regulatory model indicating potential sites of the CF defect at a molecular level.