Notch2 Signaling Promotes Biliary Epithelial Cell Fate Specification and Tubulogenesis During Bile Duct Development in Mice

Intrahepatic bile duct (IHBD) development begins with the differentiation of hepatoblasts into a single continuous biliary epithelial cell (BEC) layer, called the ductal plate. During ductal plate remodeling, tubular structures arise at distinct sites of the ductal plate, forming bile ducts that dilate into the biliary tree. Alagille syndrome patients, who suffer from bile duct paucity, carry Jagged1 and Notch2 mutations, indicating that Notch2 signaling is important for IHBD development. To clarify the role of Notch2 in BEC differentiation, tubulogenesis, and BEC survival, we developed a mouse model for conditional expression of activated Notch2 in the liver. We show that expression of the intracellular domain of Notch2 (Notch2ICD) differentiates hepatoblasts into BECs, which form additional bile ducts in periportal regions and ectopic ducts in lobular regions. Additional ducts in periportal regions are maintained into adulthood and connect to the biliary tight junction network, resulting in an increased number of bile ducts per portal tract. Remarkably, Notch2ICD-expressing ductal plate remnants were not eliminated during postnatal development, implicating Notch2 signaling in BEC survival. Ectopic ducts in lobular regions did not persist into adulthood, indicating that local signals in the portal environment are important for maintaining bile ducts. Conclusion: Notch2 signaling regulates BEC differentiation, the induction of tubulogenesis during IHBD development, and BEC survival. (HEPATOLOGY 2009;50: 871-879.)