Identification and functional characterization of TMEM16A, a Ca ²⁺-activated Cl^- channel activated by extracellular nucleotides, in biliary epithelium

Cl^- channels in the apical membrane of biliary epithelial cells (BECs) provide the driving force for ductular bile formation. Although a cystic fibrosis transmembrane conductance regulator has been identified in BECs and contributes to secretion via secretin binding basolateral receptors and increasing [cAMP]_i, an alternate Cl^- secretory pathway has been identified that is activated via nucleotides (ATP, UTP) binding apical P2 receptors and increasing [Ca²⁺]i. The molecular identity of this Ca²⁺-activated Cl^- channel is unknown. The present studies in human, mouse, and rat BECs provide evidence that. TMEM16A is the operative channel and contributes to Ca²⁺-activated Cl^- secretion in response to extracellular nucleotides. Furthermore, Cl^- currents measured from BECs isolated from distinct areas of intrahepatic bile ducts revealed important functional differences. Large BECs, but not small BECs, exhibit cAMP-stimulated Cl^- currents. However, both large and small BECs express TMEM16A and exhibit Ca²⁺-activated Cl^- efflux in response to extracellular nucleotides. Incubation of polarized BEC monolayers with IL-4 increased TMEM16A protein expression, membrane localization, and transepithelial secretion (I_sc). These studies represent the first molecular identification of an alternate, noncystic fibrosis transmembrane conductance regulator, Cl^- channel in BECs and suggest that TMEM16A may be a potential target to modulate bile formation in the treatment of cholestatic liver disorders.