APC mutant zebrafish uncover a changing temporal requirement for wnt signaling in liver development

Developmental signaling pathways hold the keys to unlocking the promise of adult tissue regeneration, and to inhibiting carcinogenesis. Patients with mutations in the Adenomatous Polyposis Colt (APC) gene are at increased risk of developing hepatoblastoma, an embryonal form of liver cancer, suggesting that Writ affects hepatic progenitor cells. To elucidate the role of APC loss and enhanced Writ activity in liver development, we examined APC mutant and writ inducible transgenic zebrafish. APC(+/-) embryos developed enlarged livers through biased induction of hepatic gene programs and increased proliferation. Conversely, APC(-/-) embryos formed no livers. Blastula transplantations determined that the effects of APC loss were cell autonomous. Induction of wnt modulators confirmed biphasic consequences of writ activation: endodermal pattern formation and gene expression required suppression of wnt signaling in early somitogenesis: later, increased writ activity altered endodermal fate by enhancing liver growth at the expense of pancreas formation; these effects persisted into the larval stage. In adult APC(+/-) zebrafish, increased writ activity significantly accelerated liver regeneration after partial hepatectomy. Similarly, liver regeneration was significantly enhanced in APC(Min/+) mice, indicating the conserved effect of Wnt pathway activation in liver regeneration across vertebrate species. These studies reveal an important and time-dependent role for wnt signaling during liver development and regeneration. (C) 2008 Published by Elsevier Inc.