Emma Hickman

Novel Role for Pendrin in Orchestrating Bicarbonate Secretion in Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)-expressing Airway Serous Cells

In most HCO3−-secreting epithelial tissues, SLC26 Cl−/HCO3− transporters work in concert with the cystic fibrosis transmembrane conductance regulator (CFTR) to regulate the magnitude and composition of the secreted fluid, a process that is vital for normal tissue function. By contrast, CFTR is regarded as the only exit pathway for HCO3− in the airways. Here we show that Cl−/HCO3− anion exchange makes a major contribution to transcellular HCO3− transport in airway serous cells. Real-time measurement of intracellular pH from polarized cultures of human Calu-3 cells demonstrated cAMP/PKA-activated Cl−-dependent HCO3− transport across the luminal membrane via CFTR-dependent coupled Cl−/HCO3− anion exchange. The pharmacological and functional profile of the luminal anion exchanger was consistent with SLC26A4 (pendrin), which was shown to be expressed by quantitative RT-PCR, Western blot, and immunofluorescence. Pendrin-mediated anion exchange activity was confirmed by shRNA pendrin knockdown (KD), which markedly reduced cAMP-activated Cl−/HCO3− exchange. To establish the relative roles of CFTR and pendrin in net HCO3− secretion, transepithelial liquid secretion rate and liquid pH were measured in wild type, pendrin KD, and CFTR KD cells. cAMP/PKA increased the rate and pH of the secreted fluid. Inhibiting CFTR reduced the rate of liquid secretion but not the pH, whereas decreasing pendrin activity lowered pH with little effect on volume. These results establish that CFTR predominately controls the rate of liquid secretion, whereas pendrin regulates the composition of the secreted fluid and identifies a critical role for this anion exchanger in transcellular HCO3− secretion in airway serous cells.

Protein phosphatase 1 coordinates CFTR-dependent airway epithelial HCO3− secretion by reciprocal regulation of apical and basolateral membrane Cl−-HCO3− exchangers

Our recent studies on human airway serous‐like Calu‐3 cells showed that cAMP agonists stimulated a HCO3− rich secretion containing up to 80 mM HCO3−. This alkaline secretion relied on a coordinated switch in the activity of distinct Cl−‐HCO3− anion exchangers (AE) located at different regions of the cell. At the apical membrane, cAMP agonists activated the electroneutral AE pendrin (SLC26A4), together with cystic fibrosis transmembrane conductance regulator (CFTR), while at the basolateral membrane the agonists inhibited AE2 (SLC4A2). However, the underlying mechanism(s) that orchestrates this cAMP‐dependent switch in AE activity has not been elucidated.