Peggy Raynaud

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.

Embryonic ductal plate cells give rise to cholangiocytes, periportal hepatocytes, and adult liver progenitor cells.

UNLABELLED BACKGROUND& AIMS: Embryonic biliary precursor cells form a periportal sheet called the ductal plate, which is progressively remodeled to generate intrahepatic bile ducts. A limited number of ductal plate cells participate in duct formation; those not involved in duct development are believed to involute by apoptosis. Moreover, cells that express the SRY-related HMG box transcription factor 9 (SOX9), which include the embryonic ductal plate cells, were proposed to continuously supply the liver with hepatic cells. We investigated the role of the ductal plate in hepatic morphogenesis. METHODS Apoptosis and proliferation were investigated by immunostaining of mouse and human fetal liver tissue. The postnatal progeny of SOX9-expressing ductal plate cells was analyzed after genetic labeling, at the ductal plate stage, by Cre-mediated recombination of a ROSA26RYFP reporter allele. Inducible Cre expression was induced by SOX9 regulatory regions, inserted in a bacterial artificial chromosome. Livers were studied from mice under normal conditions and during diet-induced regeneration. RESULTS Ductal plate cells did not undergo apoptosis and showed limited proliferation. They generated cholangiocytes lining interlobular bile ducts, bile ductules, and canals of Hering, as well as periportal hepatocytes. Oval cells that appeared during regeneration also derived from the ductal plate. We did not find that liver homeostasis required a continuous supply of cells from SOX9-expressing progenitors. CONCLUSIONS The ductal plate gives rise to cholangiocytes lining the intrahepatic bile ducts, including its most proximal segments. It also generates periportal hepatocytes and adult hepatic progenitor cells.

Biliary differentiation and bile duct morphogenesis in development and disease.

The biliary tract consists of a network of intrahepatic and extrahepatic ducts that collect and drain the bile produced by hepatocytes to the gut. The bile ducts are lined by cholangiocytes, a specialized epithelial cell type that has a dual origin. Intrahepatic cholangiocytes derive from the liver precursor cells, whereas extrahepatic cholangiocytes are generated directly from the endoderm. In this review we discuss the mechanisms of cholangiocyte differentiation and bile duct morphogenesis, and describe how developing ducts interact with the hepatic artery. We also present an overview of the mechanisms of biliary dysgenesis in humans.

A classification of ductal plate malformations based on distinct pathogenic mechanisms of biliary dysmorphogenesis

Ductal plate malformations (DPMs) are developmental anomalies considered to result from lack of ductal plate remodeling during bile duct morphogenesis. In mice, bile duct development is initiated by the formation of primitive ductal structures lined by two cell types, namely ductal plate cells and hepatoblasts. During ductal plate remodeling, the primitive ductal structures mature to ducts as a result from differentiation of the ductal plate cells and hepatoblasts to cholangiocytes. Here, we report this process is conserved in human fetal liver. These findings prompted us to evaluate how DPMs develop in three mouse models, namely mice with livers deficient in hepatocyte nuclear factor 6 (HNF6), HNF1β, or cystin‐1 (cpk [congenital polycystic kidney] mice). Human liver from a patient with a HNF1B/TCF2 mutation, and from fetuses affected with autosomal recessive polycystic kidney disease (ARPKD) were also analyzed. Despite the epistatic relationship between HNF6, HNF1β, and cystin‐1, the three mouse models displayed distinct morphogenic mechanisms of DPM. They all developed biliary cysts lined by cells with abnormal apicobasal polarity. However, the absence of HNF6 led to an early defect in ductal plate cell differentiation. In HNF1β‐deficient liver, maturation of the primitive ductal structures was impaired. Normal differentiation and maturation but abnormal duct expansion was apparent in cpk mouse livers and in human fetal ARPKD. Conclusion: DPM is the common endpoint of distinct defects initiated at distinct stages of bile duct morphogenesis. Our observations provide a new pathogenic classification of DPM. (HEPATOLOGY 2011;)

Epithelial expression of angiogenic growth factors modulate arterial vasculogenesis in human liver development

Intrahepatic bile ducts maintain a close anatomical relationship with hepatic arteries. During liver ontogenesis, the development of the hepatic artery appears to be modulated by unknown signals originating from the bile duct. Given the capability of cholangiocytes to produce angiogenic growth factors and influence peribiliary vascularization, we studied the immunohistochemical expression of vascular endothelial growth factor (VEGF), angiopoietin‐1, angiopoietin‐2, and their cognate receptors (VEGFR‐1, VEGFR‐2, Tie‐2) in fetal human livers at different gestational ages and in mice characterized by defective biliary morphogenesis (Hnf6−/−). The results showed that throughout the different developmental stages, VEGF was expressed by developing bile ducts and angiopoietin‐1 by hepatoblasts, whereas their cognate receptors were variably expressed by vascular cells according to the different maturational stages. Precursors of endothelial and mural cells expressed VEGFR‐2 and Tie‐2, respectively. In immature hepatic arteries, endothelial cells expressed VEGFR‐1, whereas mural cells expressed both Tie‐2 and Angiopoietin‐2. In mature hepatic arteries, endothelial cells expressed Tie‐2 along with VEGFR‐1. In early postnatal Hnf6−/− mice, VEGF‐expressing ductal plates failed to incorporate into the portal mesenchyma, resulting in severely altered arterial vasculogenesis. Conclusion: The reciprocal expression of angiogenic growth factors and receptors during development supports their involvement in the cross talk between liver epithelial cells and the portal vasculature. Cholangiocytes generate a VEGF gradient that is crucial during the migratory stage, when it determines arterial vasculogenesis in their vicinity, whereas angiopoietin‐1 signaling from hepatoblasts contributes to the remodeling of the hepatic artery necessary to meet the demands of the developing epithelium. (HEPATOLOGY 2008.)