Yiwei Zong

Notch signaling controls liver development by regulating biliary differentiation

In the mammalian liver, bile is transported to the intestine through an intricate network of bile ducts. Notch signaling is required for normal duct formation, but its mode of action has been unclear. Here, we show in mice that bile ducts arise through a novel mechanism of tubulogenesis involving sequential radial differentiation. Notch signaling is activated in a subset of liver progenitor cells fated to become ductal cells, and pathway activation is necessary for biliary fate. Notch signals are also required for bile duct morphogenesis, and activation of Notch signaling in the hepatic lobule promotes ectopic biliary differentiation and tubule formation in a dose-dependent manner. Remarkably, activation of Notch signaling in postnatal hepatocytes causes them to adopt a biliary fate through a process of reprogramming that recapitulates normal bile duct development. These results reconcile previous conflicting reports about the role of Notch during liver development and suggest that Notch acts by coordinating biliary differentiation and morphogenesis.

Intrahepatic bile ducts develop according to a new mode of tubulogenesis regulated by the transcription factor SOX9.

BACKGROUND & AIMS A number of diseases are characterized by defective formation of the intrahepatic bile ducts. In the embryo, hepatoblasts differentiate to cholangiocytes, which give rise to the bile ducts. Here, we investigated duct development in mouse liver and characterized the role of the SRY-related HMG box transcription factor 9 (SOX9). METHODS We identified SOX9 as a new biliary marker and used it in immunostaining experiments to characterize bile duct morphogenesis. The expression of growth factors was determined by in situ hybridization and immunostaining, and their role was studied on cultured hepatoblasts. SOX9 function was investigated by phenotyping mice with a liver-specific inactivation of Sox9. RESULTS Biliary tubulogenesis started with formation of asymmetrical ductal structures, lined on the portal side by cholangiocytes and on the parenchymal side by hepatoblasts. When the ducts grew from the hilum to the periphery, the hepatoblasts lining the asymmetrical structures differentiated to cholangiocytes, thereby allowing formation of symmetrical ducts lined only by cholangiocytes. We also provide evidence that transforming growth factor-beta promotes differentiation of the hepatoblasts lining the asymmetrical structures. In the absence of SOX9, the maturation of asymmetrical structures into symmetrical ducts was delayed. This was associated with abnormal expression of CCAAT/Enhancer Binding Protein alpha and Homolog of Hairy/Enhancer of Split-1, as well as of the transforming growth factor-beta receptor type II, which are regulators of biliary development. CONCLUSIONS Our results suggest that biliary development proceeds according to a new mode of tubulogenesis characterized by transient asymmetry and whose timing is controlled by SOX9.

Robust cellular reprogramming occurs spontaneously during liver regeneration

Cellular reprogramming-the ability to interconvert distinct cell types with defined factors-is transforming the field of regenerative medicine. However, this phenomenon has rarely been observed in vivo without exogenous factors. Here, we report that activation of Notch, a signaling pathway that mediates lineage segregation during liver development, is sufficient to reprogram hepatocytes into biliary epithelial cells (BECs). Moreover, using lineage tracing, we show that hepatocytes undergo widespread hepatocyte-to-BEC reprogramming following injuries that provoke a biliary response, a process requiring Notch. These results provide direct evidence that mammalian regeneration prompts extensive and dramatic changes in cellular identity under injury conditions.

Notch Signaling Is Activated in Human Hepatocellular Carcinoma and Induces Tumor Formation in Mice

BACKGROUND & AIMS The Notch signaling pathway is activated in leukemia and solid tumors (such as lung cancer), but little is known about its role in liver cancer. METHODS The intracellular domain of Notch was conditionally expressed in hepatoblasts and their progeny (hepatocytes and cholangiocytes) in mice. This was achieved through Cre expression under the control of an albumin and α-fetoprotein (AFP) enhancer and promoter (AFP-Notch intracellular domain [NICD]). We used comparative functional genomics to integrate transcriptome data from AFP-NICD mice and human hepatocellular carcinoma (HCC) samples (n = 683). A Notch gene signature was generated using the nearest template prediction method. RESULTS AFP-NICD mice developed HCC with 100% penetrance when they were 12 months old. Activation of Notch signaling correlated with activation of 3 promoters of insulin-like growth factor 2; these processes appeared to contribute to hepatocarcinogenesis. Comparative functional genomic analysis identified a signature of Notch activation in 30% of HCC samples from patients. These samples had altered expression in Notch pathway genes and activation of insulin-like growth factor signaling, despite a low frequency of mutations in regions of NOTCH1 associated with cancer. Blocking Notch signaling in liver cancer cells with the Notch activation signature using γ-secretase inhibitors or by expressing a dominant negative form of mastermind-like 1 reduced their proliferation in vitro. CONCLUSIONS Notch signaling is activated in human HCC samples and promotes formation of liver tumors in mice. The Notch signature is a biomarker of response to Notch inhibition in vitro.

Lineage tracing demonstrates no evidence of cholangiocyte epithelial‐to‐mesenchymal transition in murine models of hepatic fibrosis

Whether or not cholangiocytes or their hepatic progenitors undergo an epithelial‐to‐mesenchymal transition (EMT) to become matrix‐producing myofibroblasts during biliary fibrosis is a significant ongoing controversy. To assess whether EMT is active during biliary fibrosis, we used Alfp‐Cre × Rosa26‐YFP mice, in which the epithelial cells of the liver (hepatocytes, cholangiocytes, and their bipotential progenitors) are heritably labeled at high efficiency with yellow fluorescent protein (YFP). Primary cholangiocytes isolated from our reporter strain were able to undergo EMT in vitro when treated with transforming growth factor‐β1 alone or in combination with tumor necrosis factor‐α, as indicated by adoption of fibroblastoid morphology, intracellular relocalization of E‐cadherin, and expression of α‐smooth muscle actin (α‐SMA). To determine whether EMT occurs in vivo, we induced liver fibrosis in Alfp‐Cre × Rosa26‐YFP mice using the bile duct ligation (BDL) (2, 4, and 8 weeks), carbon tetrachloride (CCl4) (3 weeks), and 3,5‐diethoxycarbonyl‐1,4‐dihydrocollidine (DDC; 2 and 3 weeks) models. In no case did we find evidence of colocalization of YFP with the mesenchymal markers S100A4, vimentin, α‐SMA, or procollagen 1α2, although these proteins were abundant in the peribiliary regions. Conclusion: Hepatocytes and cholangiocytes do not undergo EMT in murine models of hepatic fibrosis. (Hepatology 2011;)

Molecular mechanisms of bile duct development

The mammalian biliary system, consisting of the intrahepatic and extrahepatic bile ducts, is responsible for transporting bile from the liver to the intestine. Bile duct dysfunction, as is seen in some congenital biliary diseases such as Alagille syndrome and biliary atresia, can lead to the accumulation of bile in the liver, preventing the excretion of detoxification products and ultimately leading to liver damage. Bile duct formation requires coordinated cell-cell interactions, resulting in the regulation of cell differentiation and morphogenesis. Multiple signaling molecules and transcription factors have been identified as important regulators of bile duct development. This review summarizes recent progress in the field. Insights gained from studies of the molecular mechanisms of bile duct development have the potential to reveal novel mechanisms of differentiation and morphogenesis in addition to potential targets for therapy of bile duct disorders.

Function of GATA Factors in the Adult Mouse Liver

GATA transcription factors and their Friend of Gata (FOG) cofactors control the development of diverse tissues. GATA4 and GATA6 are essential for the expansion of the embryonic liver bud, but their expression patterns and functions in the adult liver are unclear. We characterized the expression of GATA and FOG factors in whole mouse liver and purified hepatocytes. GATA4, GATA6, and FOG1 are the most prominently expressed family members in whole liver and hepatocytes. GATA4 chromatin immunoprecipitation followed by high throughput sequencing (ChIP-seq) identified 4409 occupied sites, associated with genes enriched in ontologies related to liver function, including lipid and glucose metabolism. However, hepatocyte-specific excision of Gata4 had little impact on gross liver architecture and function, even under conditions of regenerative stress, and, despite the large number of GATA4 occupied genes, resulted in relatively few changes in gene expression. To address possible redundancy between GATA4 and GATA6, both factors were conditionally excised. Surprisingly, combined Gata4,6 loss did not exacerbate the phenotype resulting from Gata4 loss alone. This points to the presence of an unusually robust transcriptional network in adult hepatocytes that ensures the maintenance of liver function.

Molecular mechanisms of liver and bile duct development

The liver is derived from the ventral foregut endoderm. After hepatic specification, liver progenitor cells delaminate from the endoderm and invade the septum transversum mesenchyme to form the liver bud. In addition to proliferation and expansion, liver progenitor cells differentiate into two epithelial cell types, each arranged into unique structures with distinctive function. Growth, morphogenesis, and differentiation during liver development are regulated by a variety of factors that are expressed in a spatially and temporally specific manner. A comprehensive understanding of the regulatory mechanisms underlying the liver development has influenced the diagnosis of liver diseases and further progress will be critical for future advances in therapy. This review highlights some of the best understood steps of liver development, summarizing progress in our understanding of the molecular mechanisms that underlie differentiation, morphogenesis, and functional integration of the liver. WIREs Dev Biol 2012 doi: 10.1002/wdev.47