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 25,000 molecular weight proteolipid (p25) acting as 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.