Role of Wnt/Beta-Catenin Signaling in Liver Development

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SUMMARY Wnt/¿-catenin signaling has come to the forefront in liver biology. Its role in liver development, regeneration & stem cells is beginning to be understood. We identified its role in liver regeneration & showed activation of the Wnt/¿-catenin signaling immediately after partial-hepatectomy (PHx). This was observed as nuclear translocation of ¿-catenin protein ensuring G1 to S transition mediated by factors such as Cyclin-D1. In addition, we identified highest ¿-catenin expression during early stages of hepatic morphogenesis in liver development & showed that its absence led to compromise in hepatoblast expansion & differentiation into bile ducts & failure of hepatocyte maturation. Recently, we have also identified the role of Wnt/¿-catenin in adult liver stem cells or oval cells, where this pathway regulates their emergence & expansion. As we have uncovered several key roles of this pathway, many new questions have arisen! Several of these are of high significance & have taken the form of the current proposal, which is a competing renewal of our previously funded application (1/1/2004-12/31/2008). In the current proposal we want to focus on three aspects of liver biology-development, regeneration & hepatocyte death. We have generated ¿-catenin-conditional null mice (KO1) with Foxa3-Cre driven deletion of ¿-catenin in hepatoblasts during development. This strategy unveiled the importance of ¿-catenin in regulating hepatoblast expansion & differentiation. We propose to identify the molecular basis by which ¿-catenin is regulating these two conceptually opposing events during development & hypothesize (based on stem cell paradigm) that differential interaction of ¿-catenin occurs temporally with cofactors enabling transactivation of distinct genes that regulate the two processes. We will elucidate the basis of failed biliary differentiation in absence of ¿-catenin & examine how lack of ¿-catenin retards hepatocyte maturation. Based on the controversy in the role of Wnt/¿-catenin signaling in hepatic specification in Zebrafish & Xenopus, we will utilize KO1 to address role of ¿-catenin in murine hepatic specification. We have also generated ¿-catenin-conditional-null mice (KO2) using Albumin-Cre & identified lack of proliferation in these mice at 40hrs (peak proliferation in controls) after PHx. We will address the molecular signaling in the absence of ¿-catenin that enables a dramatic rescue of hepatocyte proliferation at 72hrs in KO2 mice. While we are beginning to understand the role of canonical Wnt signaling, the role & extent of noncanonical pathways-Wnt/Ca2+ & planar cell polarity pathways; remain obscure & will be investigated in-depth in liver development & regeneration. Finally, based on enhanced apoptosis in hepatocytes lacking ¿-catenin, we investigated Fas-& TNF¿-mediated injury in the KO2. Interestingly, while KO2 mice were clearly more susceptible to Jo-2 (Fas-ligand) injury than controls, they were resistant to lipopolysaccharide (LPS)-injury. The molecular basis of these findings will be elucidated. Thus, this proposal will be a comprehensive analysis of canonical & noncanonical Wnt signaling in hepatic biology.