The cystic fibrosis transmembrane conductance regulator and acid-base transporters of the murine duodenum
Abstract
The alkaline mucus barrier of the duodenum plays an important role in protecting the epithelium from acidic chyme entering from the stomach. Active HCO₃⁻ secretion involves the apical membrane activities of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel, the protein that is defective in cystic fibrosis (CF), and Cl⁻/HCO₃⁻ exchangers. Under basal conditions, studies of CF patients and mouse models indicate that HCO₃⁻ secretion by anion exchange predominates. In addition, basal HCO₃⁻ secretion is reduced in the CF duodenum, but the specific pathophysiology for this deficiency has yet to be elucidated. Our studies reveal that Cl⁻ channel activity by CFTR facilitates apical membrane Cl⁻in/HCO₃⁻out exchange by providing a Cl⁻ 'leak' and is responsible for the reduced rate of Cl⁻/HCO₃⁻ exchange in the murine CF intestine. Using mice with gene-targeted deletions of the apical membrane Cl⁻/HCO₃⁻ exchangers PAT-1, DRA, and AE4, PAT-1 was found to be the major Cl⁻/HCO₃⁻ exchanger of the upper villus of the duodenum. Interestingly, these studies also revealed a novel role for PAT-1 as a base-importer (i.e., Cl⁻out/HCO₃⁻in) whereby it interacts with carbonic anhydrase II (CAII), the most widely expressed isozyme of the small intestine, during H⁺/peptide transport to minimize intracellular acidification and sustain nutrient absorption.
Degree
Ph. D.