β-Catenin deletion in hepatoblasts disrupts hepatic morphogenesis and survival during mouse development

Abstract

␤-Catenin, the central component of the canonical Wnt pathway, plays important roles in the processes of liver regeneration, growth, and cancer. Previously, we identified temporal expression of ␤-catenin during liver development. Here, we characterize the hepatic phenotype, resulting from the successful deletion of ␤-catenin in the developing hepatoblasts utilizing Foxa3-cyclization recombination and floxed-␤-catenin (exons 2 through 6) transgenic mice. ␤-Catenin loss in developing livers resulted in significantly underdeveloped livers after embryonic day 12 (E12) with lethality occurring at around E17 stages. Histology revealed an overall deficient hepatocyte compartment due to (1) increased cell death due to oxidative stress and apoptosis, and (2) diminished expansion secondary to decreased cyclin-D1 and impaired proliferation. Also, the remnant hepatocytes demonstrated an immature phenotype as indicated by high nuclear to cytoplasmic ratio, poor cell polarity, absent glycogen, and decreased expression of key liver-enriched transcription factors: CCAATenhancer binding protein-␣ and hepatocyte nuclear factor-4␣. A paucity of primitive bile ducts was also observed. While the stem cell assays demonstrated no intrinsic defect in hematopoiesis, distorted hepatic architecture and deficient hepatocyte compartments resulted in defective endothelial cell organization leading to overall fetal pallor. Conclusion: ␤-Catenin regulates multiple, critical events during the process of hepatic morphogenesis, including hepatoblast maturation, expansion, and survival, making it indispensable to survival. (HEPATOLOGY 2008;47:1667-1679

Calpain Induces N-terminal Truncation of β-Catenin in Normal Murine Liver Development

“…5). Reports that both stabilization of (full-length) ␤-catenin and its deletion lead to a defect in hepatocyte differentiation (12,29) we believe can be explained by the necessity of the ⌬95-␤-catenin in this process.…”

“…Wnt/␤-catenin has a role in patterning the foregut endoderm from which the liver arises, in producing a gradient involved in hepatic induction that patterns the gut along the anterior-posterior axis, and in promoting expansion of bipotential hepatoblasts that form the nascent liver bud (3)(4)(5)(6)(7)(8)(9)(10)(11)(12). The importance of the Wnt/␤-catenin pathway in liver persists throughout the life of an adult organism, as it plays roles in zonal metabolism (13)(14)(15)(16), hepatic progenitor maintenance (17)(18)(19)(20)(21), and liver regeneration (22)(23)(24)(25)(26)(27), and its dysregulation is observed in many liver cancers (28).…”

“…Premature activation of ␤-catenin via targeted deletion of Adenomatous Polyposis Coli gene product (APC) in the livers of developing mice results in pronounced biliary differentiation of hepatoblasts at the expense of hepatocytes (29) and is consistent with a role for ␤-catenin activity in promoting hepatoblast differentiation into biliary epithelial cells (29). Hepatoblast-specific, FoxA3-Cre driven ␤-catenin deletion, however, leads to not only defects in biliary specification of hepatoblasts but also maturation of hepatocytes (12). Embryos possessing the ␤-catenin deletion die late in gestation, with livers exhibiting abnormalities beginning at approximately embryonic day 13, when hepatoblast differentiation starts to occur.…”

“…Knock-out livers appear to arrest at this stage, composed of cells exhibiting the high nuclear-to-cytoplasmic ratio and unpolarized morphology reminiscent of uncommitted E13/14 stage hepatoblasts. Knock-out livers show an absence of bile ducts and also expression of the hepatocyte-specific transcription factors C/ebp␣ and HNF4␣ as well as reduced expression of several other hepatocyte markers (12). This suggests that ␤-catenin activity is necessary for both biliary epithelial cell and hepatocyte differentiation, but it is unclear what mechanism could account for both of these observations.…”

Depletion of β-catenin from mature hepatocytes of mice promotes expansion of hepatic progenitor cells and tumor development

“…Specific deletion of β-catenin in hepatoblasts was lethal for embryos at the embryonic day 17 stage, showing an overall deficient of parenchymal hepatocytes (13), These results indicate the critical role that β-catenin in the proliferation and differentiation of embryonic hepatoblasts. However, when β-catenin was conditionally knocked out in hepatocytes after birth, the mice showed only a 10% to 20% reduction in liver mass during the 12-mo followup period (14,15).…”