Evidence against the acidification hypothesis in cystic fibrosis

The pleiotropic effects of cystic fibrosis (CF) result from the mislocaliza tion or inactivity of an apical membrane chloride channel, the cystic fibro sis transmembrane conductance regulator (CFTR). CFTR may also modulate intr acellular chloride conductances and thus affect organelle pH. To test the r ole of CFTR in organelle pH regulation, we developed a model system to sele ctively perturb the pH of a subset of acidified compartments in polarized c ells and determined the effects on various protein trafficking steps. We th en tested whether these effects were observed in cells lacking wild-type CF TR and whether reintroduction of CFTR affected trafficking in these cells. Our model system involves adenovirus-mediated expression of the influenza v irus M2 protein, an acid-activated ion channel. M2 expression selectively s lows traffic through the trans-Golgi network (TGN) and apical endocytic com partments in polarized Madin-Darby canine kidney (MDCK) cells. Expression o f M2 or treatment with other pH perturbants also slowed protein traffic in the CF cell line CFPAC, suggesting that the TGN in this cell line is normal ly acidified. Expression of functional CFTR had no effect on traffic and fa iled to rescue the effect of M2. Our results argue against a role for CFTR in the regulation of organelle pH and protein trafficking in epithelial cel ls.