Identification and Functional Characterization of TMEM16A, a Ca2+-activated Cl− Channel Activated by Extracellular Nucleotides, in Biliary Epithelium

Amal K Dutta,Al-Karim Khimji,Charles Kresge,Abhijit Bugde,Michael Dougherty,Victoria Esser,Yoshiyuki Ueno,Shannon S Glaser,Gianfranco Alpini,Don C Rockey,Andrew P Feranchak

doi:10.1074/jbc.m110.164970

Abstract

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 [Ca(2+)](i). The molecular identity of this Ca(2+)-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 Ca(2+)-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 Ca(2+)-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.

Highlights

The formation of bile by the liver depends on the transport functions of two complimentary cell types: hepatocytes and intrahepatic bile duct epithelial cells, known as cholangiocytes
TMEM16A Mediates Cholangiocyte Secretion associated with these proteins are not consistent with the ATP-stimulated ClϪ currents described in biliary epithelium [8]
IL-4 increases cholangiocyte plasma membrane expression of TMEM16A and increases transepithelial secretion in response to extracellular nucleotides. In these studies of human, rat, and mouse biliary epithelial cells, we have identified TMEM16A as the molecular basis for Ca2ϩ-activated ClϪ channels stimulated by extracellular nucleotides

Summary

Introduction

The formation of bile by the liver depends on the transport functions of two complimentary cell types: hepatocytes and intrahepatic bile duct epithelial cells, known as cholangiocytes. TMEM16A Mediates Cholangiocyte Secretion associated with these proteins are not consistent with the ATP-stimulated ClϪ currents described in biliary epithelium [8]. The aim of the present studies was to identify the molecular basis for Ca2ϩ-activated ClϪ currents stimulated by extracellular nucleotides in different biliary cell models.

Results

Conclusion