Role of metalloproteinases at the onset of liver development

At the onset of liver development, the hepatic precursor cells, namely, the hepatoblasts, derive from the ventral foregut endoderm and form a bud surrounded by a basement membrane (BM). To initiate liver growth, the hepatoblasts migrate across the BM and invade the neighboring septum transversum mesenchyme. In the present study, carried out in the mouse embryo, we searched for effectors involved in this process and we examined the role of matrix metalloproteinases (MMPs). We found expression of a broad range of MMPs, among which MMP-2 was predominantly expressed in the septum transversum and MMP-14 in the hepatoblasts. Using a new liver explant culture system we showed that inhibition of MMP activity represses migration of the hepatoblasts. We conclude that MMPs are required to initiate expansion of the liver during development and that our culture system provides a new model to study hepatoblast migration.     

Lhx2 is expressed in the septum transversum mesenchyme that becomes an integral part of the liver and the formation of these cells is independent of functional Lhx2

Liver development is based on reciprocal interactions between ventral foregut endoderm and adjacent mesenchymal tissues. Targeted disruption of the LIM-homeobox gene Lhx2 has revealed that it is important for the expansion of the liver during embryonic development, whereas it appears not to be involved in the induction of hepatic fate. It is not known whether Lhx2 is expressed in the endodermal or mesenchymal portion of the liver, or if the cells normally expressing Lhx2 are absent or present in the liver of Lhx2(-/-) embryos. To address this we have analyzed gene expression from the Lhx2 locus during hepatic development in wild type and Lhx2(-/-) mice. Lhx2 is expressed in cells of the septum transversum mesenchyme adjacent to the liver bud from embryonic day 9. The hepatic cords subsequently migrate into and intermingle with the Lhx2+ cells of the septum transversum mesenchyme. Lhx2 expression is thereafter maintained in a subpopulation of mesenchymal cells in the liver until adult life. In adult liver the Lhx2+ mesenchymal cells co-express desmin, a marker associated with stellate cells. At embryonic day 10.5, cells expressing the mutant Lhx2 allel are present in Lhx2(-/-) livers, and expression of Hlx, hepatocyte growth factor, Hex and Prox1, genes known to be important in liver development, is independent of functional Lhx2 expression. Thus, Lhx2 is specifically expressed in the liver-associated septum transversum mesenchyme that subsequently becomes an integral part of the liver and the formation of these mesenchymal cells does not require functional Lhx2.     

Neurturin-GFRalpha2 signaling controls liver bud migration along the ductus venosus in the chick embryo

During chick liver development, the liver bud arises from the foregut, invaginates into the septum transversum, and elongates along and envelops the ductus venosus. However, the mechanism of liver bud migration is only poorly understood. Here, we demonstrate that a GDNF family ligand involved in neuronal outgrowth and migration, neurturin (NRTN), and its receptor, GFRalpha2, are essential for liver bud migration. In the chick embryo, we found that GFRalpha2 was expressed in the liver bud and that NRTN was expressed in the endothelial cells of the ductus venosus. Inhibition of GFRalpha2 signaling suppressed liver bud elongation along the ductus venous without affecting cell proliferation and apoptosis. Moreover, ectopic expression of NRTN perturbed the directional migration along the ductus venosus, leading to splitting or ectopic branching of the liver. We showed that liver buds selectively migrated toward an NRTN-soaked bead in vitro. These data represent a new model for liver bud migration: NRTN secreted from endothelial cells functions as a chemoattractant to direct the migration of the GFRalpha2-expressing liver bud in early liver development.     

Wnt9a secreted from the walls of hepatic sinusoids is essential for morphogenesis, proliferation, and glycogen accumulation of chick hepatic epithelium

Hepatic epithelial morphogenesis, including hepatoblast migration and proliferation in the septum transversum, requires the interaction of hepatic epithelium with the embryonic sinusoidal wall. No factors that mediate this interaction have yet been identified. As the β-catenin pathway is active in hepatoblast proliferation, then Wnt ligands might activate the canonical Wnt pathway during liver development. Here, we investigated the role of Wnts in mediating epithelial vessel interactions in the developing chick liver. We found that Wnt9a was specifically expressed in both endothelial and stellate cells of the embryonic sinusoidal wall. Induced overexpression of Wnt9a resulted in hepatomegaly with hyperplasia of the hepatocellular cords, and in hyperproliferation of hepatocytes. Knockdown of Wnt9a caused a reduction in liver size, with hypoplasia of hepatocellular cord branching, and hypoproliferation of hepatoblasts, and also inhibited glycogen accumulation at later developmental stages. Wnt9a promoted in vivo stabilization of β-catenin through binding with Frizzled 4, 7, and 9, and activated TOPflash reporter expression in vitro via Frizzled 7 and 9. Our results demonstrate that Wnt9a from the embryonic sinusoidal wall is required for the proper morphogenesis of chick hepatocellular cords, proliferation of hepatoblasts/hepatocytes, and glycogen accumulation in hepatocytes. Wnt9a signaling appears to be mediated by an Fzd7/9-β-catenin pathway.     

GATA6 is essential for embryonic development of the liver but dispensable for early heart formation

Several lines of evidence suggest that GATA6 has an integral role in controlling development of the mammalian liver. Unfortunately, this proposal has been impossible to address directly because mouse embryos lacking GATA6 die during gastrulation. Here we show that the early embryonic deficiency associated with GATA6-knockout mice can be overcome by providing GATA6-null embryos with a wild-type extraembryonic endoderm with the use of tetraploid embryo complementation. Analysis of rescued Gata6-/- embryos revealed that, although hepatic specification occurs normally, the specified cells fail to differentiate and the liver bud does not expand. Although GATA6 is expressed in multiple tissues that impact development of the liver, including the heart, septum transversum mesenchyme, and vasculature, all are relatively unaffected by loss of GATA6, which is consistent with a cell-autonomous requirement for GATA6 during hepatogenesis. We also demonstrate that a closely related GATA factor, GATA4, is expressed transiently in the prehepatic endoderm during hepatic specification and then lost during expansion of the hepatic primordium. Our data support the proposal that GATA4 and GATA6 are functionally redundant during hepatic specification but that GATA6 alone is available for liver bud growth and commitment of the endoderm to a hepatic cell fate.     

GATA6在肝脏发育中的作用及调控机制

GATA6 (GATA binding protein 6)是GATA锌指转录因子家族成员之一,以其保守的结合基序(G/A)GATA(A/T) 而得名。GATA家族在脊椎动物细胞命运决定与分化、增殖和迁移以及内胚层和中胚层来源的器官发育中具有重要作用。GATA6作为谱系特化因子、染色质重塑因子、多能性因子和“先锋因子”,在内胚层肝脏谱系决定、肝脏特化、肝芽生长以及肝母细胞增殖分化等阶段发挥关键的调控作用。本文综述了GATA6在肝脏发育中的作用及其研究进展,以期为进一步研究 GATA6 等发育关键转录因子的功能及调控机制提供参考。     

Mab21l2 is essential for embryonic heart and liver development

During mouse embryogenesis, proper formation of the heart and liver is especially important and is crucial for embryonic viability. In this study, we showed that Mab21l2 was expressed in the trabecular and compact myocardium, and that deletion of Mab21l2 resulted in a reduction of the trabecular myocardium and thinning of the compact myocardium. Mab21l2-deficient embryonic hearts had decreased expression of genes that regulate cell proliferation and apoptosis of cardiomyocytes. These results show that Mab21l2 functions during heart development by regulating the expression of such genes. Mab21l2 was also expressed in the septum transversum mesenchyme (STM). Epicardial progenitor cells are localized to the anterior surface of the STM (proepicardium), and proepicardial cells migrate onto the surface of the heart and form the epicardium, which plays an important role in heart development. The rest of the STM is essential for the growth and survival of hepatoblasts, which are bipotential progenitors for hepatocytes and cholangiocytes. Proepicardial cells in Mab21l2-deficient embryos had defects in cell proliferation, which led to a small proepicardium, in which α4 integrin expression, which is essential for the migration of proepicardial cells, was down-regulated, suggesting that defects occurred in its migration. In Mab21l2-deficient embryos, epicardial formation was defective, suggesting that Mab21l2 plays important roles in epicardial formation through the regulation of the cell proliferation of proepicardial cells and the migratory process of proepicardial cells. Mab21l2-deficient embryos also exhibited hypoplasia of the STM surrounding hepatoblasts and decreased hepatoblast proliferation with a resultant loss of defective morphogenesis of the liver. These findings demonstrate that Mab21l2 plays a crucial role in both heart and liver development through STM formation.