SUMMARY Defective transepithelial electrolyte transport is thought to initiate cystic fibrosis (CF) lung disease. Yet, how loss of CFTR affects electrolyte transport remains uncertain. CFTR−/− pigs spontaneously develop lung disease resembling human CF. At birth, their airways exhibit a bacterial host defense defect, but are not inflamed. Therefore, we studied ion transport in newborn nasal and tracheal/bronchial epithelia in tissue, cultures, and in vivo. CFTR−/− epithelia showed markedly reduced Cl− and HCO3− transport. However, in contrast to a widely held view, lack of CFTR did not increase transepithelial Na+ or liquid absorption or reduce periciliary liquid depth. Like human CF, CFTR−/− pigs showed increased amiloride-sensitive voltage and current, but lack of apical Cl− conductance caused the change, not increased Na+ transport. These results indicate that CFTR provides the predominant transcellular pathway for Cl− and HCO3− in porcine airway epithelia, and reduced anion permeability may initiate CF airway disease.

In cystic fibrosis (CF), dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) ClThe ATP-binding cassette (ABC) 3 transporter cystic fibrosis transmembrane conductance regulator (CFTR) (1) is a multifunctional protein best known as a regulated Cl Ϫ channel (2). CFTR is assembled from five domains: two membrane-spanning domains (MSDs) that form an anion-selective pore, two nucleotide-binding domains (NBDs) that bind and hydrolyze ATP to control channel gating, and a unique regulatory domain (RD), whose phosphorylation by PKA is critical for CFTR activation (2, 3). CFTR is principally expressed in the apical membrane of epithelia throughout the body where it plays a fundamental role in fluid and electrolyte movements (4). Malfunction of CFTR causes the common genetic disease cystic fibrosis (CF) (4).Previous studies demonstrate that the regulation of intracellular pH (pH i ) is defective in CF epithelial cells (e.g. Ref. 5). They also reveal that expression of recombinant CFTR in heterologous cells modulates pH i (e.g. Ref. 6). Analysis of the literature suggests that CFTR modulates pH i in three main ways. First, CFTR itself directly transports HCO 3 Ϫ ions with a modest permeability (P HCO 3 Ϫ/P Cl Ϫ ϳ0.26 (7)). Second, CFTR regulates the Na ϩ /H ϩ exchanger isoform 3 (NHE3), which contributes to Na ϩ -dependent HCO 3 Ϫ reabsorption in pancreatic duct epithelia. CFTR stabilizes NHE3 expression at the cell surface and inhibits NHE3 activity by a cAMP-dependent mechanism when pancreatic HCO 3 Ϫ secretion is stimulated (8). Of note, the regulation of NHE3 by CFTR involves the association of CFTR and NHE3 with the scaffolding protein EBP50 to form a macromolecular complex (8). Third, CFTR regulates the Cl Ϫ /HCO 3 Ϫ (anion) exchanger (AE), which plays a central role in pancreatic HCO 3 Ϫ secretion. CFTR regulation of AE requires the cell surface expression and cAMP-dependent phosphorylation of CFTR, but not its transport of anions (6). Interestingly, Ko et al. (9) demonstrated that CFTR and members of the SLC26 family of AEs coordinate their activities through the interaction of the, phosphorylated RD of CFTR with the STAS (sulfate transporter and antisigma-factor antagonist) domain of SLC26 transporters. Thus, CFTR modulates pH i through its roles as an ion channel and regulator of transport proteins.A key unresolved question is how CFTR senses changes in pH i . As described above, CFTR might detect pH i changes indirectly through its interactions with NHE3 and SLC26 transporters. Consistent with this idea, Reddy et al. (10)

Peripheral nervous system abnormalities, including neuropathy, have been reported in people with cystic fibrosis. These abnormalities have largely been attributed to secondary manifestations of the disease. We tested the hypothesis that disruption of the cystic fibrosis transmembrane conductance regulator (CFTR) gene directly influences nervous system function by studying newborn CFTR −/− pigs. We discovered CFTR expression and activity in Schwann cells, and loss of CFTR caused ultrastructural myelin sheath abnormalities similar to those in known neuropathies. Consistent with neuropathic changes, we found increased transcripts for myelin protein zero, a gene that, when mutated, can cause axonal and/or demyelinating neuropathy. In addition, axon density was reduced and conduction velocities of the trigeminal and sciatic nerves were decreased. Moreover, in vivo auditory brainstem evoked potentials revealed delayed conduction of the vestibulocochlear nerve. Our data suggest that loss of CFTR directly alters Schwann cell function and that some nervous system defects in people with cystic fibrosis are likely primary.