Aberrant CFTR-dependent HCO3- transport in mutations associated with cystic fibrosis

Cystic fibrosis (CF) is a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). Initially, Cl- conductance in the sweat duct was discovered to be impaired in CF1, a finding that has been extended to all CFTR-expressing cells(2-4). Subsequent cloning of the gene(5,6) showed that CFTR functions as a cyclic-AMP-regulated Cl- channel(7); and some CF-causing mutations inhibit CFTR Cl- channel activity(2-4,8). The identification of additional C-causing mutants with normal Cl- channel activity indicates, however, that other CFTR-dependent processes contribute to the disease. Indeed, CFTR regulates other transporters(3,4), including Cl- -coupled HCO3- transport(9,10). Alkaline fluids are secreted by normal tissues, whereas acidic fluids are secreted by mutant CFTR-expressing tissues(11), indicating the importance of this activity. HCO3- and pH affect mucin viscosity(12,13) and bacterial binding(14,15). We have examined Cl- -coupled HCO3- transport by CFTR mutants that retain substantial or normal Cl- channel activity. Here we show that mutants reported to be associated with CF with pancreatic insufficiency do not support HCO3- transport, and those associated with pancreatic sufficiency show reduced HCO3- transport. Our findings demonstrate the importance of HCO3- transport in the function of secretory epithelia and in CF.