Initiation of mammalian liver development from endoderm by fibroblast growth factors

Effects of growth factors on the growth and differentiation of mouse fetal liver epithelial cells in primary cultures

BACKGROUND AND AIMS

Growth factors (GF) are thought to affect the growth and differentiation of hepatocytes during liver development. However, in the midfetal liver, little is known concerning the role of GF.

METHODS

The DNA synthesis of fetal liver epithelial cells (FLEC) in monolayer culture and the liver-specific gene expressions of FLEC in 3-D culture were examined in medium supplemented with various GF.

RESULTS

DNA synthesis of FLEC was higher than that of adult hepatocytes without GF, and was increased by hepatocyte growth factor (HGF), heparin-binding epidermal growth factor-like growth factor (HB-EGF), basic fibroblast growth factor (bFGF), epidermal growth factor (EGF) or transforming growth factor-alpha (TGF-alpha). However, FLEC responded less to GF in terms of DNA synthesis than adult hepatocytes. The liver-specific gene expressions were increased in the presence of HGF, HB-EGF, bFGF and EGF. In embryonic day (E) 13.5 FLEC, this increase was more apparent in the presence of HB-EGF, whereas in E14.5 FLEC, it was more apparent in the presence of HGF.

CONCLUSIONS

Hepatocyte growth factor, HB-EGF, bFGF, EGF and TGF-alpha increased DNA synthesis of FLEC. HGF, HB-EGF, bFGF and EGF led to an increase in liver-specific gene expressions; and their effects on differentiation differ as a function of gestation age.

Distinct populations of endoderm cells converge to generate the embryonic liver bud and ventral foregut tissues

The location and movement of mammalian gut tissue progenitors, prior to the expression of tissue-specific genes, has been unknown, but this knowledge is essential to identify transitions that lead to cell type specification. To address this, we used vital dyes to label exposed anterior endoderm cells of early somite stage mouse embryos, cultured the embryos into the tissue bud phase of development, and determined the tissue fate of the dye labeled cells. This approach was performed at three embryonic stages that are prior to, or coincident with, foregut tissue patterning (1-3 somites, 4-6 somites, and 7-10 somites). Short-term labeling experiments tracked the movement of tissue progenitor cells during foregut closure. Surprisingly, we found that two distinct types of endoderm-progenitor cells, lateral and medial, arising from three spatially separated embryonic domains, converge to generate the epithelial cells of the liver bud. Whereas the lateral endoderm-progenitors give rise to descendants that are constrained in tissue fate and position along the anterior-posterior axis of the gut, the medial gut endoderm-progenitors give rise to descendants that stream along the anterior-posterior axis at the ventral midline and contribute to multiple gut tissues. The fate map reveals extensive morphogenetic movement of progenitors prior to tissue specification, it permits a detailed analysis of endoderm tissue patterning, and it illustrates that diverse progenitor domains can give rise to individual tissue cell types.

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.

Differential expression of heparan sulfate 6-O-sulfotransferase isoforms in the mouse embryo suggests distinctive roles during organogenesis

Heparan sulfate (HS) interactions with secreted morphogens such as fibroblast growth factors, hedgehogs, and Wnts are essential for embryonic development. Formation of biologically relevant HS structures is a result of the coordinated action of various biosynthetic enzymes. HS 6-O-sulfotransferases (6OST) catalyze the transfer of sulfate groups to the 6-O position of glucosamine residues in HS. Three 6OST isoforms have been described in the mouse; however, little is known about their role in generating specific HS protein-binding sequences, expression pattern, and function in vivo. To gain insights into the distribution of these isoforms and their potential role in development, we mapped 6OST1-3 gene expression during mouse organogenesis. We report dynamic expression of these isoforms with striking differences in tissue distribution in many developing organs. We show that 6OST transcripts are differentially expressed in several sites where heparin-binding growth factors are critical for development. 6OST1 is predominantly transcribed in epithelial and neural-derived tissues, whereas 6OST2 is more mesenchymal. 6OST3 appears at later stages and in a more restricted manner. The patterns reported here strongly suggest that the HS structures modified by these enzymes have different roles in growth factor-induced developmental processes.

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.

Human amniotic epithelial cells possess hepatocyte-like characteristics and functions

Hepatocyte transplantation is expected to become a novel method for treatment of liver disease. However, many questions remain regarding this approach, especially concerning donor cells. To evaluate whether human amniotic epithelial cells can be used as a cell source for hepatocyte transplantation, hepatic gene expression and functions of human amniotic epithelial cells were analyzed. Reverse transcription-polymerase chain reaction analysis demonstrated that human amniotic epithelial cells expressed albumin, alpha(1)-antitrypsin, and other hepatocyte-related genes. Cultivated human amniotic epithelial cells demonstrated albumin production, glycogen storage, and albumin secretion consistent with the hepatocyte gene expression profile. In organ culture, the amnion secreted 30-fold larger amounts of albumin than human amniotic epithelial cells in monolayer culture. Moreover, in organ culture the amnion also secreted alpha(1)-antitrypsin. Following transplantation into mice, the amnion survived and secreted albumin. These observations suggest that transplantation of human amniotic epithelial cells and/or amnion could be novel therapeutic strategy for treatment of hepatic diseases, including alpha(1)-antitrypsin deficiency.

Differentiation and isolation of hepatic-like cells from human embryonic stem cells

Human embryonic stem cells are pluripotent cells that can serve as a cell source for transplantation medicine, and as a tool to study human embryogenesis. We investigate here the potential of human embryonic stem cells to differentiate into hepatic cells. We have characterized the expression level of liver-enriched genes in undifferentiated and differentiated human embryonic stem cells by DNA microarrays. Our analysis revealed a subset of fetal hepatic enriched genes that are expressed in human embryonic stem cells upon differentiation into embryoid bodies. In order to isolate the hepatic-like cells, we introduced a reporter gene regulated by a hepatocyte-specific promoter into human embryonic stem cells. We isolated clones of human embryonic stem cells that express enhanced green fluorescent protein upon in vitro differentiation. Through immunostaining, we showed that most of these cells express albumin, while some cells still express the earlier expressed protein alpha-fetoprotein. Using fluorescence activated cell sorter, we were able to sort out the fluorescent differentiated cells and expand them for a few more weeks. This is the first report to demonstrate the possibility of purifying differentiated derivatives of human embryonic stem cells and culturing them further. Through confocal microscopy, we detected clusters of hepatic-like cells in 20-day-old embryoid bodies and in teratomas. As observed during embryonic development, we showed that in teratomas, the hepatic-like endodermal cells develop next to cardiac mesodermal cells. In order to examine the secreted factors involved in the induction of hepatic differentiation, human embryonic stem cells were grown in the presence of various growth factors, demonstrating the potential involvement of acidic fibroblast growth factor in the differentiation. In conclusion, given certain growth conditions and genetic manipulation, we can now differentiate and isolate hepatic-like cells from human embryonic stem cells.

In vitro transdifferentiation of hepatoma cells into functional pancreatic cells

We have characterised the transdifferentiation of human HepG2 (hepatoma) cells to pancreatic cells following introduction of an activated version of the pancreatic transcription factor Pdx1 (XlHbox8-VP16). The following questions are addressed: (1) are all types of pancreatic cells produced? (2) is the requirement for expression of the transgene temporary or permanent? (3) are the transdifferentiated beta-cells responsive to physiological stimuli? The results showed that both pancreatic exocrine cells (by detection of amylase protein), and endocrine cells (by detecting insulin, glucagon and somatostatin proteins) are induced after XlHbox8VP16 transfection. Moreover, the hepatic phenotype becomes suppressed during transdifferentiation of hepatocytes to pancreatic cells. Requirement for the transgene is only temporary and it is no longer required once the pancreatic differentiation program is activated. Finally, we provided results to suggest that the transdifferentiated cells are functional by detecting: (1) functional markers for pancreatic beta-cells including prohormone convertase 1/3 (PC1/3), insulin C-peptide and glucagon-like peptide 1 receptor (GLP-1R), (2) increased insulin mRNA expression after treatment of cells with GLP-1 and betacellulin, physiological stimuli that regulate pancreatic function and (3) elevated insulin secretion after glucose challenge. The transdifferentiation of hepatic to pancreatic cells represents one possible source of beta-cells for human islet transplantation and this study shows that such a transdifferentiation can be achieved in vitro.

Liver development update: new embryo models, cell lineage control, and morphogenesis

The three phases of liver development that are the focus of this review are: the specification of hepatoblasts within the endoderm, the lineage split of hepatoblasts into hepatocytes and biliary cells, and the interaction of these cells with different mesodermal cell derivatives during liver morphogenesis. Advances in these areas include new genes and experimental models for studying liver development, the role of HNF6 and HNF1beta transcription factors and notch signaling in the hepatocyte-biliary cell lineage decision, the identification of genomic targets for HNF4, and HNF4's role in controlling hepatic epithelial structure and the sinusoidal organization of the liver.

Different thresholds of fibroblast growth factors pattern the ventral foregut into liver and lung

Cell fate and morphogenesis within the embryo is dependent upon secreted molecules that transduce signals between neighboring tissues. Reciprocal mesenchymal-epithelial interactions have proven essential during branching morphogenesis and cell differentiation within the lung; however, the interactions that result in lung specification from the foregut endoderm, prior to lung bud formation, are poorly understood. In this study, we investigate the tissue requirements and signals necessary for specification of a pulmonary cell fate using embryo tissue explants. We show that NKX2.1, an early transcription factor crucial for lung development, is expressed in the ventral foregut endoderm shortly after albumin and Pdx1, early markers of the liver and pancreas lineages, respectively. Similar to hepatic specification, direct contact of cardiac mesoderm with ventral endoderm is required to induce in vitro expression of NKX2.1 and downstream lung target genes including surfactant protein C and Clara cell secretory protein. In the absence of cardiac mesoderm, ventral foregut endoderm explants respond to exogenous fibroblast growth factor (FGF) 1 and FGF2 in a dose-dependent manner, with lower concentrations activating liver specific genes and higher concentrations activating lung specific genes. This signaling appears to be instructive, as the prospective dorsal midgut endoderm, which predominantly gives rise to the intestinal tract, is competent to respond to FGFs by inducing NKX2.1. Furthermore, the temporal expression and selective inhibition of FGF receptors 1 and 4 present within the endoderm implies that signaling through FGFR4 is involved in specifying lung versus liver. Together, the findings suggest that a concentration threshold of FGFs emanating from the cardiac mesoderm are involved in patterning the foregut endoderm.

Transcription factor HNF-6/OC-1 inhibits the stimulation of the HNF-3alpha/Foxa1 gene by TGF-beta in mouse liver

A network of liver-enriched transcription factors controls differentiation and morphogenesis of the liver. These factors interact via direct, feedback, and autoregulatory loops. Previous work has suggested that hepatocyte nuclear factor (HNF)-6/OC-1 and HNF-3alpha/FoxA1 participate coordinately in this hepatic network. We investigated how HNF-6 controls the expression of Foxa1. We observed that Foxa1 expression was upregulated in the liver of Hnf6(-/-) mouse embryos and in bipotential mouse embryonic liver (BMEL) cell lines derived from embryonic Hnf6(-/-) liver, suggesting that HNF-6 inhibits the expression of Foxa1. Because no evidence for a direct repression of Foxa1 by HNF-6 was found, we postulated the existence of an indirect mechanism. We found that the expression of a mediator and targets of the transforming growth factor beta (TGF-beta) signaling was increased both in Hnf6(-/-) liver and in Hnf6(-/-) BMEL cell lines. Using these cell lines, we demonstrated that TGF-beta signaling was increased in the absence of HNF-6, and that this resulted from upregulation of TGF-beta receptor II expression. We also found that TGF-beta can stimulate the expression of Foxa1 in Hnf6(+/+) cells and that inhibition of TGF-beta signaling in Hnf6(-/-) cells down-regulates the expression of Foxa1. In conclusion, we propose that Foxa1 upregulation in the absence of HNF-6 results from increased TGF-beta signaling via increased expression of the TGF-beta receptor II. We further conclude that HNF-6 inhibits Foxa1 by inhibiting the activity of the TGF-beta signaling pathway. This identifies a new mechanism of interaction between liver-enriched transcription factors whereby one factor indirectly controls another by modulating the activity of a signaling pathway.

In vitro hepatic differentiation of human mesenchymal stem cells

This study examined whether mesenchymal stem cells (MSCs), which are stem cells originated from embryonic mesoderm, are able to differentiate into functional hepatocyte-like cells in vitro. MSCs were isolated from human bone marrow and umbilical cord blood, and the surface phenotype and the mesodermal multilineage differentiation potentials of these cells were characterized and tested. To effectively induce hepatic differentiation, we designed a novel 2-step protocol with the use of hepatocyte growth factor and oncostatin M. After 4 weeks of induction, cuboidal morphology, which is characteristic of hepatocytes, was observed, and cells also expressed marker genes specific of liver cells in a time-dependent manner. Differentiated cells further demonstrated in vitro functions characteristic of liver cells, including albumin production, glycogen storage, urea secretion, uptake of low-density lipoprotein, and phenobarbital-inducible cytochrome P450 activity. In conclusion, human MSCs from different sources are able to differentiate into functional hepatocyte-like cells and, hence, may serve as a cell source for tissue engineering and cell therapy of hepatic tissues. Furthermore, the broad differentiation potential of MSCs indicates that a revision of the definition may be required.

Basic fibroblast growth factor promotes the trans-differentiation of mouse bone marrow cells into hepatic lineage cells via multiple liver-enriched transcription factors

BACKGROUND/AIMS

Evidence that bone marrow cells have trans-differentiating potential to hepatocytes has been described in recent reports. However, the molecular mechanism underlying this phenomenon is unclear. To address this issue, we investigated the parameters involved in the trans-differentiation of bone marrow cells into a hepatic lineage.

METHODS

Mouse BM cells were cultured in a collagen gel without or with growth factors including basic fibroblast growth factor. The expression of hepatocyte-specific markers, cholangiocyte-specific marker and liver-enriched transcription factors was identified by RT-PCR and immunohistochemistry.

RESULTS

Basic fibroblast growth factor was found to be the most effective for inducing albumin in cultured BM cells. Furthermore, on stimulation of basic fibroblast growth factor, BM cells were found to express other hepatocyte-specific markers and a cholangiocyte-specific marker. This conversion was found to be associated with the induction of transcription factors including hepatocyte nuclear factors and GATA family proteins.

CONCLUSIONS

We established an in vitro culture system in which mouse bone marrow cells could trans-differentiate to hepatic lineage cells in response to growth factors, without cell fusion. In particular, basic fibroblast growth factor has the ability to induce the trans-differentiation into hepatic lineage cells from BM cells.

Exploring interactions between rat hepatocytes and nonparenchymal cells using gene expression profiling

Cocultivation of primary hepatocytes with a plethora of nonparenchymal cells (from within and outside the liver) has been shown to support hepatic functions in vitro. Despite significant investigation into this phenomenon, the molecular mechanism underlying epithelial-nonparenchymal interactions in hepatocyte cocultures remains poorly understood. In this study, we present a functional genomic approach utilizing gene expression profiling to isolate molecular mediators potentially involved in induction of liver-specific functions by nonparenchymal cells. Specifically, primary rat hepatocytes were cocultivated with closely related murine fibroblast cell types (3T3-J2, NIH-3T3, mouse embryonic fibroblasts) to allow their classification as "high," "medium," or "low" inducers of hepatic functions. These functional responses were correlated with fibroblast gene expression profiles obtained using Affymetrix GeneChips. Microarray data analysis provided us with 17 functionally characterized candidate genes in the cell communication category (cell surface, extracellular matrix, secreted factors) that may be involved in induction of hepatic functions. Further analysis using various databases (i.e., PubMed, GenBank) facilitated prioritization of the candidates for functional characterization. We experimentally validated the potential role of two candidates in our coculture model. The cell surface protein, neural cadherin (N-cadherin), was localized to hepatocyte-fibroblast junctions, while adsorbed decorin up-regulated hepatic functions in pure cultures as well as cocultures with low-inducing fibroblasts. In the future, identifying mediators of hepatocyte differentiation may have implications for both fundamental hepatology and cell-based therapies (e.g., bioartificial liver devices). In conclusion, the functional genomic approach presented in this study may be utilized to investigate mechanisms of cell-cell interaction in a variety of tissues and disease states.

Immunolocalization of extracellular matrix components and integrins during mouse liver development

Intrahepatic biliary cell differentiation takes place in periportal hepatoblasts under the influence of the subjacent connective tissue, the mechanism of which is still unclear. This study was undertaken to analyze the immunolocalization of extracellular matrix components and their cellular receptors during mouse liver development, with special attention given to biliary differentiation and vascular development. In young fetal mouse liver, primitive structures of sinusoids were developed between hepatic cords associated with hematopoietic cells demonstrated by immunohistochemistry of basal laminar components, the alpha6 integrin subunit, and PECAM-1. Portal veins and hepatic veins showed different staining intensities of alpha2, alpha3, and alpha6 integrin subunits from early stages of development. Anti-beta4 integrin subunit antibodies reacted with portal veins, but not with hepatic veins after perinatal stages. Their different phenotypes may be related to the preferential differentiation of periportal bile ducts. In intrahepatic bile duct development, periportal hepatoblasts adjacent to the connective tissue were immunostained for each basal laminar component on the basal side at almost the same time; alpha3, alpha5, alpha6, and beta4 integrin subunits were immunohistochemically detectable later than the basal laminar components. These staining patterns of intrahepatic bile duct cells clearly differed from those of extrahepatic bile duct cells from the beginning of their development, suggesting that these ducts are of different origins. In conclusion, the vascular structures, including sinusoids, portal veins, and hepatic veins, develop from early stages of liver development, and the extracellular matrix components may play important roles in biliary differentiation and vascular development. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Fibroblast growth factor enriches the embryonic liver cultures for hepatic progenitors

Fibroblast growth factors (FGFs) play an important role in hepatic induction during development. The aim of our study was to investigate the effect of exogenous FGFs on ex vivo liver development. We begin our analysis by examining FGF signaling during early mouse liver development. Phospho-FGF receptor (Tyr653/654) was detected in embryonic day 10 (E10) to E12 livers only. Next, E10 livers were cultured in the presence of FGF1, FGF4, or FGF8 for 72 hours and examined for histology, proliferation, apoptosis, and differentiation. FGFs especially FGF8 promoted sheet-like architecture, cell proliferation, and survival as compared to the control. All FGFs induced a striking increase in the number of c-kit and alpha-fetoprotein-positive progenitors, without altering albumin staining. However these progenitors were CK-19-positive (biliary and bipotential progenitor marker) only in the presence of FGF1 or FGF4 and not FGF8. FGFs also induced beta-catenin, a stem cell renewal factor in these cultures. In conclusion, the presence of activated FGFR indicates a physiological role of FGF during early liver development. FGF1 and FGF4 enrich the embryonic liver cultures for bipotential hepatic progenitors. FGF8 promotes such enrichment and induces a one-step differentiation toward a unipotential hepatocyte progenitor. Thus, FGFs might be useful for enrichment and propagation of developmental hepatic progenitors.

Mutations affecting liver development and function in Medaka, Oryzias latipes, screened by multiple criteria

We report here mutations affecting various aspects of liver development and function identified by multiple assays in a systematic mutagenesis screen in Medaka. The 22 identified recessive mutations assigned to 19 complementation groups fell into five phenotypic groups. Group 1, showing defective liver morphogenesis, comprises mutations in four genes, which may be involved in the regulation of growth or patterning of the gut endoderm. Group 2 comprises mutations in three genes that affect the laterality of the liver; in kendama mutants of this group, the laterality of the heart and liver is uncoupled and randomized. Group 3 includes mutations in three genes altering bile color, indicative of defects in hemoglobin-bilirubin metabolism and globin synthesis. Group 4 consists of mutations in three genes, characterized by a decrease in the accumulation of fluorescent metabolite of a phospholipase A(2) substrate, PED6, in the gall bladder. Lipid metabolism or the transport of lipid metabolites may be affected by these mutations. Mutations in Groups 3 and 4 may provide animal models for relevant human diseases. Group 5 mutations in six genes affect the formation of endoderm, endodermal rods and hepatic bud from which the liver develops. These Medaka mutations, identified by morphological and metabolite marker screens, should provide clues to understanding molecular mechanisms underlying formation of a functional liver.

High-ratio differentiation of embryonic stem cells into hepatocytes in vitro

BACKGROUND/PURPOSE

To date, in differentiating system of embryonic stem (ES) cells into hepatocytes, hepatic differentiation ratio was still not shown. Here, after investigating hepatic differentiation from ES cells, we determined the differentiation ratios of hepatocytes and studied how to improve the ratios in ES cell differentiating system.

METHODS

Embryonic bodies (EBs) formed from ES cells for 5 days were plated onto culture dishes and some growth factors were added into medium for hepatic differentiation. Expressions of hepatic genes and proteins were analysed using reverse transcriptase-polymerase chain reaction, immunocytochemistry (ICC) and radioimmunoassay. The relative counts of hepatocyte-like cells among all EBs cells were analysed by flow cytometry by which hepatic differentiation ratios were determined. Then, we observed the spatial distribution of ICC-positive cells in EB cells cluster and isolated the cells of positive areas in other EBs clusters without ICC examined. At last, isolated cells were re-cultured with previous condition and hepatic differentiation ratios were also determined.

RESULTS

The hepatic genes and proteins were, respectively, expressed in cytoplasm. Hepatic differentiation ratio was first determined at day 11 to be 12.1% and the level reached maximum to be 33.4% at day 21. In isolated cells culture system, hepatic genes and proteins expressed stronger than that expressed in EBs cluster and hepatic differentiation ratio was got to 72.6% at day 21.

CONCLUSIONS

Isolating hepatocyte-like cells from EBs cell cluster and re-culturing them could produce hepatocytes with high differentiation ratio. This culture system may produce a new source of cell types for hepatocytes replacement therapies in hepatic failure.

Regulation of Hex gene expression and initial stages of avian hepatogenesis by Bmp and Fgf signaling

The vertebrate liver and heart arise from adjacent cell layers in the anterior lateral (AL) endoderm and mesoderm of late gastrula embryos, and the earliest stages of liver and heart development are interrelated through reciprocal tissue interactions. Although classical embryological studies performed several decades ago in chick and quail defined the timing of hepatogenic induction in birds and the important role for cardiogenic mesoderm in this process, almost nothing is known about the molecular aspects of avian liver development. Here we use in vivo and explantation assays to investigate tissue interactions and signaling pathways regulating Hex, a homeobox gene required for liver development, and the earliest stages of hepatogenesis in the chick embryo. We find that explants of late gastrula anterior lateral endoderm plus mesoderm, which have been used extensively for studies relating to heart development, also produce albumin-expressing hepatoblasts. Expression of Hex, the earliest known molecular marker for the hepatogenic endoderm, and albumin, indicative of early committed hepatoblasts, requires both autocrine Bmp signaling and a specific paracrine signal from the cardiogenic (anterior lateral) mesoderm. Endodermal expression of Fox2a, in contrast, requires the mesoderm but is independent of Bmp signaling. In vivo induction assays show that the ability of BMP2 to activate Hex expression in the endoderm is restricted to a region that is only slightly larger than the endogenous domain of Hex expression. Although Fgfs can substitute for the cardiogenic mesoderm to support the expression of Hex and albumin in the endoderm, several Fgf genes are expressed in the anterior lateral endoderm but an Fgf expressed predominantly in the mesoderm was not identified. Studies also showed that Fgf gene expression in the endoderm does not require a signal from the mesoderm. Mechanisms regulating endodermal signaling pathways activated by Fgfs may therefore be more complex than previously appreciated.

Liver regeneration in FGF-2-deficient mice: VEGF acts as potential functional substitute for FGF-2

BACKGROUND/AIMS

The angiogenic properties, its role in mesoderm differentiation and cell culture studies implicate an important role of fibroblast growth factor (FGF-2) in liver regeneration. The aim of the study was to evaluate this role in a FGF-2 knockout mouse model.

METHODS

Liver regeneration after left hemihepatectomy (partial hepatectomy, PH) was evaluated in homozygous FGF-2 deficient (-/-) mice (male C57BL/6J) and their FGF-2 competent (+/+) littermates (controls) (day 0-10).

RESULTS

FGF-2-(-/-) mice displayed normal dynamics in liver regeneration. FGF-2 protein was overexpressed 4 days post PH in controls. BrdU incorporation showed a biphasic pattern in FGF-2-(-/-) mice, whereas it decreased continuously after one peak (day 2) in controls. In FGF-2-(-/-) livers hepatic growth factor mRNA post PH was 1 day longer decreased and markedly less elevated thereafter compared with control. Vascular endothelial growth factor (VEGF) mRNA levels were clearly increased in FGF-2-(-/-) mice pre- and postoperatively in contrast to controls. VEGF protein levels in livers of FGF-2-(-/-) mice were elevated preoperatively, but similar in both groups after PH. With SU5416, a VEGF-receptor inhibitor, liver regeneration in FGF-2-(-/-) mice was reduced significantly, whereas it remained unchanged in controls.

CONCLUSIONS

Liver regeneration dynamics in FGF-2-(-/-) mice were comparable with controls, potentially due to a functional substitution of FGF-2 by VEGF.

Morphogenesis of chicken liver: identification of localized growth zones and the role of beta-catenin/Wnt in size regulation

During development and regeneration, new cells are added and incorporated to the liver parenchyma. Regulation of this process contributes to the final size and shape of the particular organs, including the liver. We identified the distribution of liver growth zones using an embryonic chicken model because of its accessibility to experimentation. Hepatocyte precursors were first generated all over the primordia surrounding the vitelline blood vessel at embryonic day 2 (E2), then became limited to the peripheral growth zones around E6. Differentiating daughter cells of the peripheral hepatocyte precursors were shown by DiI microinjection to be laid inward and were subsequently organized to form the hepatic architecture. At E8, hepatocyte precursor cells were further restricted to limited segments of the periphery, called localized growth zones (LoGZ). Adhesion and signaling molecules in the growth zone were studied. Among them, beta-catenin and Wnt 3a were highly enriched. We overexpressed constitutively active beta-catenin using replication competent avian sarcoma (RCAS) virus. Liver size increased about 3-fold with an expanded hepatocyte precursor cell population. In addition, blocking beta-catenin activity by either overexpression of dominant-negative LEF1 or overexpression of a secreted Wnt inhibitor Dickkopf (DKK) resulted in decreased liver size with altered liver shape. Our data suggest that (1) the duration of active growth zone activity modulates the size of the liver; (2) a shift in the position of the localized growth zone helps to shape the liver; and (3) beta-catenin/Wnt are involved in regulating growth zone activities during liver development.

Endothelial cell interactions initiate dorsal pancreas development by selectively inducing the transcription factor Ptf1a

Dorsal and ventral pancreatic bud development from the endoderm requires inductive interactions with diverse mesodermal cell types and the action of transcription factors expressed within the endoderm. Presently it is unclear which mesodermal interactions activate which pancreatic transcription factors, and whether such inductions are common for initiating dorsal and ventral pancreas development. Previous studies of Lammert et al. showed that signaling from embryonic blood vessel cells, derived from the mesoderm, promotes pancreatic bud development. Using a combination of mouse Flk1(-/-) embryos lacking endothelial cells and tissue recombination experiments, we discovered that the initial induction of dorsal endoderm cells positive for the pancreatic and duodenal transcription factor Pdx1 does not require aorta or endothelial cell interactions, but dorsal pancreatic bud emergence and the maintenance of Pdx1 expression does. Aortal endothelial cells induce the crucial pancreatic transcription factor Ptf1a in the dorsal pancreatic endoderm; whereas the vitelline veins, which are normally adjacent to the emerging ventral pancreatic bud, are unnecessary for ventral Ptf1a induction or for ventral pancreatic bud initiation. We find that the aorta cells themselves, apart from the blood supply, cause the induction of Ptf1a in dorsal endoderm explants. Thus, endothelial cell interactions specifically promote early dorsal pancreatic development, at least in part, by inducing Ptf1a(+) pancreatic progenitors. Additionally, we find that endothelial cells are necessary for the induction of both the insulin and glucagon genes.

Hex homeobox gene-dependent tissue positioning is required for organogenesis of the ventral pancreas

In animal development, digestive tissues emerge from different positions of the endoderm as a result of patterning signals from overlying mesoderm. Although embryonic tissue movement during gastrulation generates an initial positional relationship between the endoderm and mesoderm, the role of subsequent endoderm movement against the mesoderm in patterning is unknown. At embryonic day 8.5 in the mouse, proliferation of cells at the leading edge of ventral-lateral endoderm, where the liver and ventral pancreas emerge, helps close off the foregut. During this time, the endoderm grows adjacent to and beyond the cardiogenic mesoderm, an inducer of the liver program and an inhibitor of the pancreas program. The homeobox gene Hex is expressed in this endoderm cell domain and in the liver and ventral pancreas buds, after organogenesis. We have found that in Hex(-/-) embryos, there is a complete failure in ventral pancreatic specification, while the liver program is still induced. However, when Hex-null ventral endoderm is isolated prior to its interaction with cardiogenic mesoderm and is cultured in vitro, it activates early pancreas genes. We found that Hex controls the proliferation rate, and thus the positioning, of the leading edge of endoderm cells that grow beyond the cardiogenic mesoderm, during gut tube closure. Thus, Hex-controlled positioning of endoderm cells beyond cardiogenic mesoderm dictates ventral pancreas specification. Other endodermal transcription factors may also function morphogenetically rather than by directly regulating tissue-specific programs.

Ectopically expressed PDX-1 in liver initiates endocrine and exocrine pancreas differentiation but causes dysmorphogenesis

To date, the potency of pancreatic and duodenal homeobox gene 1 (PDX-1) in inducing differentiation into insulin-producing cells has been demonstrated in some cells and tissues. In order to carry out efficient screening of somatic tissues and cells that can transdifferentiate into beta-cell-like cells in response to PDX-1, we generated CAG-CAT-PDX1 transgenic mice carrying a transgene cassette composed of the chicken beta-actin gene (CAG) promoter and a floxed stuffer DNA sequence (CAT) linked to PDX-1 cDNA. When the mice were crossed with Alb-Cre mice, which express the Cre recombinase driven by the rat albumin gene promoter, PDX-1 was expressed in more than 50% of hepatocytes and cholangiocytes. The PDX-1 (+) livers expressed a variety of endocrine hormone genes such as insulin, glucagon, somatostatin, and pancreatic polypeptide. In addition, they expressed exocrine genes such as elastase-1 and chymotrypsinogen 1B. However, the mice exhibited marked jaundice due to conjugated hyperbilirubinemia, and the liver tissue displayed abnormal lobe structures and multiple cystic lesions. Thus, the in vivo ectopic expression of PDX-1 in albumin-producing cells was able to initiate but not complete the differentiation of liver cells into pancreatic cells. The conditional PDX-1 transgenic mouse system developed in this study appeared to be useful for efficient screening of PDX-1 responsive somatic tissues and cells.

A population of c-Kit(low)(CD45/TER119)- hepatic cell progenitors of 11-day postcoitus mouse embryo liver reconstitutes cell-depleted liver organoids

Embryo liver morphogenesis takes place after gastrulation and starts with a ventral foregut evagination that reacts to factor signaling from both cardiac mesoderm and septum transversum mesenchyme. Current knowledge of the progenitor stem cell populations involved in this early embryo liver development is scarce. We describe here a population of 11-day postcoitus c-Kit(low)(CD45/TER119)- liver progenitors that selectively expressed hepatospecific genes and proteins in vivo, was self-maintained in vitro by long-term proliferation, and simultaneously differentiated into functional hepatocytes and bile duct cells. Purified c-Kit(low)(CD45/TER119)- liver cells cocultured with cell-depleted fetal liver fragments engrafted and repopulated the hepatic cell compartments of the latter organoids, suggesting that they may include the embryonic stem cells responsible for liver development.

Establishment, characterization, and long-term maintenance of cultures of human fetal hepatocytes

Cultured human hepatocytes have broad research and clinical applications; however, the difficulties in culturing rodent and human hepatocytes are well known. These problems include the rapid loss of the hepatocytic phenotype in primary culture and the limited replicating capacity of the cultured cells. We describe the establishment of serum-free primary cultures of human fetal hepatocytes (HFHs) that retain hepatocytic morphology and gene expression patterns for several months and maintain sufficient proliferative activity to permit subculturing for at least 2 passages. Initially, HFH cultures contained 2 main cell types that morphologically resembled large and small hepatocytes. The fetal hepatocytes expressed alpha-fetoprotein (AFP), cytokeratin (CK) 19, albumin, and other hepatic proteins. Treatment of the cultures with oncostatin M (OSM) increased cell size and enhanced cell differentiation and formation of bile canaliculi, probably through an effect on hepatocyte nuclear factor (HNF) 4alpha. Approximately 1 month after plating, multiple clusters of very small cells became apparent in the cultures. These cells had very few organelles and are referred to as blast-like cells. Flow cytometric analysis of these cells showed that they express oval cell/stem cell markers such as CD90 (Thy-1), CD34, and OV-6 but do not stain with antibodies to beta(2)-microglobulin. HFH cultures maintained for 9 to 12 months produced grossly visible organoids containing ductular structures that stained for CK18, CK19, and AFP. In conclusion, HFH cultures, which might contain a population of hepatic stem cells, constitute an excellent tool for a variety of studies with human hepatocytes, including the mechanisms of viral infection.

Role of fibroblast growth factor type 1 and 2 in carbon tetrachloride-induced hepatic injury and fibrogenesis

Genomic ablation of hepatocyte-specific fibroblast growth factor receptor (FGFR)4 in mice revealed a role of FGF signaling in cholesterol and bile acid metabolism and hepatolobular restoration in response to injury without effect on liver development or hepatocyte proliferation. Although the potential role of all 23 FGF polypeptides in the liver is still unclear, the most widely studied prototypes, FGF1 and FGF2, are present and have been implicated in liver cell growth and function in vitro. To determine whether FGF1 and FGF2 play a role in response to injury and fibrosis, we examined the impact of both acute and chronic exposure to carbon tetrachloride (CCl(4)) in the livers of FGF1- and FGF2-deficient mice. After acute CCl(4) exposure, FGF1(-/-)FGF2(-/-) mice exhibited an accelerated release of serum alanine aminotransferase similar to FGFR4 deficiency, but no effect on overall hepatolobular restoration or bile acid metabolism. FGF1(-/-)FGF2(-/-) mice exhibited a normal increase in alpha-smooth muscle actin and desmin associated with activation and migration of hepatic stellate cells to damage, but a reduced level of hepatic stellate cell-derived matrix collagen alpha1(I) synthesis. Liver fibrosis resulting from chronic CCl(4) exposure was markedly decreased in the livers of FGF1/FGF2-deficient mice. These results suggest an agonist role for FGF1 and FGF2 in specifically insult-induced liver matrix deposition and hepatic fibrogenesis and a potential target for the prevention of hepatic fibrosis.

Spatial and temporal patterns of ERK signaling during mouse embryogenesis

Signaling between tissues is essential to form the complex, three-dimensional organization of an embryo. Because many receptor tyrosine kinases signal through the RAS-MAPK pathway, phosphorylated ERK can be used as an indicator of when and where signaling is active during development. Using whole-mount immunohistochemistry with antibodies specific to phosphorylated ERK1 and ERK2, we analyzed the location, timing, distribution, duration and intensity of ERK signaling during mouse embryogenesis (5-10.5 days postcoitum). Spatial and temporal domains of ERK activation were discrete with well-defined boundaries, indicating specific regulation of signaling in vivo. Prominent, sustained domains of ERK activation were seen in the ectoplacental cone, extra-embryonic ectoderm, limb buds, branchial arches, frontonasal process, forebrain, midbrain-hindbrain boundary, tailbud, foregut and liver. Transient activation was seen in neural crest, peripheral nervous system, nascent blood vessels, and anlagen of the eye, ear and heart. In the contiguous domains of ERK signaling, phospho-ERK staining was cytoplasmic with no sign of nuclear translocation. With few exceptions, the strongest domains of ERK activation correlated with regions of known or suspected fibroblast growth factor (FGF) signaling, and brief incubation with an inhibitor of the fibroblast growth factor receptor (FGFR) specifically diminished the phospho-ERK staining in these regions. Although many domains of ERK activation were FGFR-dependent, not all domains of FGF signaling were phospho-ERK positive. These studies identify key domains of sustained ERK signaling in the intact mouse embryo, give significant insight into the regulation of this signaling in vivo and pinpoint regions where downstream target genes can be sought.

Human umbilical cord blood as a source of transplantable hepatic progenitor cells

Human umbilical cord blood (UCB) cells have many advantages as grafts for cell transplantation because of the immaturity of newborn cells compared with adult cells. In contrast to their hematopoietic and mesenchymal potential, it remains unclear whether UCB cells have endodermal competence. Here, with a view to utilize UCB cells for cell transplantation into injured liver, we investigated the hepatic potential of UCB cells both in vitro and in vivo. We determined the most efficient conditions leading UCB cells to produce albumin (ALB). In a novel primary culture system supplemented with a combination of growth/differentiation factors, about 50% of UCB cells in 21-day cultures expressed ALB, and the ALB(+) cells coexpressed hepatocyte lineage markers. The ALB-expressing cells were able to proliferate in the culture system. Moreover, in the cell-transplantation model into liver-injured severe combined immunodeficient mice, inoculated UCB cells developed into functional hepatocytes in the liver, which released human ALB into the sera of the recipient mice. In conclusion, this study demonstrates that human UCB is a source of transplantable hepatic progenitor cells. Our findings may have relevance to clinical application of UCB-derived cell transplantation as a novel therapeutic option for liver failure.

Induction of hepatic differentiation in embryonic stem cells by co-culture with embryonic cardiac mesoderm

BACKGROUND

Modifications in vitro have been used to direct embryonic stem (ES) cells toward endodermal phenotypes including hepatocytes; however, developmental correlates and evidence of biologic activity is lacking, and critical cell-cell interactions have not been investigated. In this study, we hypothesized that cardiac mesoderm (CM) signals ES cells in co-culture to undergo differentiation toward early hepatocyte lineage as determined by morphology and induction of genes essential for endodermal competence and hepatocyte development.

METHODS

Green fluorescent protein ES derived from A129 mice were cultured with or without embryonic chick cardiac mesoderm. Cultures from day 1, 2, and 4 were analyzed for colony formation and ES morphology and 10(6) ES-derived cells were isolated for mRNA analysis.

RESULTS

ES in co-culture with CM displayed colony formation, polymorphic appearance, and definitive interface with CM. In addition, ES + CM co-culture activated crucial transcription factors (sox 17alpha, HNF3beta, and GATA 4) required for hepatocyte development by day 1. mRNA for albumin and especially a-fetoprotein were also increased by culture days 2 and 4.

CONCLUSIONS

ES cells co-cultured with CM display morphology and gene expression pattern required for hepatocyte differentiation and appear to recapitulate the molecular events of hepatogenesis.

Fibroblast growth factor receptor signalling is crucial for liver homeostasis and regeneration

Several growth factors have been suggested to play a crucial role in liver regeneration, but a functional proof is still missing. Since fibroblast growth factors are important for the initiation of mammalian liver development, we determined the roles of these mitogens in liver repair by targeted expression of a dominant-negative fibroblast growth factor receptor (FGFR) in hepatocytes of transgenic mice. The liver of young animals appeared histologically normal, and liver function was not obviously impaired. In aged transgenic mice, the frequency of fatty liver development was strongly increased compared to control animals. Following partial hepatectomy, transgenic mice showed markedly reduced hepatocyte proliferation because of an arrest in the late G(1) phase of the cell cycle. These data demonstrate a key role of FGFR signalling in repair after liver injury.

Sera from liver failure patients and a demethylating agent stimulate transdifferentiation of murine bone marrow cells into hepatocytes in coculture with nonparenchymal liver cells

BACKGROUND/AIMS

The plasticity of bone marrow cells (BMCs) is shown by their ability to differentiate into mesenchymal as well as endodermal and ectodermal lineages. Transdifferentiation of BMCs into hepatocytes has also been demonstrated, both in vitro and in vivo. In the present study we investigated the effects of liver nonparenchymal cells (NPCs) and sera from liver failure patients (HSLF) on the in vitro transdifferentiation of murine BMCs into hepatocytes.

METHODS

Liver NPCs from wild-type mice, and 5-azacytidine-treated BMCs from green fluorescence protein transgenic mice, were cocultured in medium containing HSLF in combination with several cytokines. Hepatocyte-specific gene expression in BMCs was identified by immunocytochemistry and reverse transcription-polymerase chain reaction.

RESULTS

Bone marrow cell-derived hepatocyte-like colonies appeared after several days of coculture in medium containing HSLF, oncostatin M (OSM) and hepatocyte growth factor (HGF). These colonies expressed hepatocyte-specific genes. Transdifferentiation was enhanced by 5-azacytidine treatment, and by HSLF, OSM and HGF. It did not take place when the BMCs were separated from the NPCs in a dual chamber dish, or cultured with other mesenchymal cells.

CONCLUSIONS

Direct interaction of murine BMCs with liver NPCs, as well as soluble factors in the HSLF and a demethylating agent, strongly stimulate transdifferentiation into hepatocytes.

Signals from lateral plate mesoderm instruct endoderm toward a pancreatic fate

During embryonic development, organs arise along the gut tube as a series of buds in a stereotyped anterior-posterior (A-P) pattern. Using chick-quail chimeras and in vitro tissue recombination, we studied the interactions governing the induction and maintenance of endodermal organ identify focusing on the pancreas. Though several permissive signals in pancreatic development have been previously identified, here we provide evidence that lateral plate mesoderm sends instructive signals to the endoderm, signals that induce expression of the pancreatic genes Pdx1, p48, Nkx6.1, glucagon, and insulin. Moreover, this instructive signal directs cells to form ectopic insulin-positive islet-like clusters in endoderm that would otherwise form more rostral organs. Once generated, endocrine cells no longer require interaction with mesoderm, but nonendocrine cells continue to require permissive signals from the mesoderm. Stimulation of activin, BMP, or retinoic acid signaling is sufficient to induce Pdx1 expression in endoderm anterior to the pancreas. Lateral plate mesoderm appears to pattern the endoderm in a posterior-dominant fashion as first noted in the patterning of the neural tube at the same embryonic stage. These findings argue for a central role of the mesoderm in coordinating the A-P pattern of all three primary germ layers.

Impaired differentiation of fetal hepatocytes in homozygous jumonji mice

Homozygous jumonji (jmj(-)/jmj(-)) mice were previously shown to exhibit hepatic hypoplasia and defective hematopoiesis in the liver and die at around embryonic day 15.5 (E15.5), suggesting that jmj is essential for liver development. In order to gain insight into the mechanism of liver development, we analyzed the expression and function of jmj in fetal hepatocytes. The number of hepatocytes in jmj(-)/jmj(-) mice was markedly reduced in comparison with control mice and the expression of jmj in hepatocytes increased along with development. As jmj(-)/jmj(-) embryos die by E15.5, we employed an in vitro culture system in which fetal hepatocytes differentiate in response to oncostatin M. The proliferation potential of jmj(-)/jmj(-) hepatocytes was comparable to that of wild type cells in vitro, however maturation of hepatocytes as evidenced by the expression of liver enzymes such as tyrosine amino transferase was severely impaired by the jmj gene inactivation. These results suggested that jmj plays a pivotal role in the development of mid-fetal hepatocytes to the neonatal stage.

Molecular insights into congenital disorders of the digestive system

Recent work is providing new insights into molecular mechanisms of digestive system development and their alteration in clinically significant disorders. An understanding of these mechanisms has largely been gained through the use of animal models, because many of the basic processes required in embryogenesis are functionally conserved among species. Such conserved factors include cell-cell signaling pathways and the regulation of gene expression. Disruption of these pathways have been implicated in several congenital disorders of the digestive system, including Hirschsprung disease, malrotation, altered sphincter development, Meckel diverticulum, biliary atresia, Alagille syndrome, pancreatic heterotopias, and pancreatic agenesis. In this review, we highlight recent studies in digestive system development, which elucidate mechanisms underlying congenital disorders of the human digestive system.

Role for growth factors and extracellular matrix in controlling differentiation of prospectively isolated hepatic stem cells

In liver development, a number of growth factors (GFs) and components of the extracellular matrix (ECMs) lead to differentiation of liver parenchymal cells. As the liver contains many cell types, specifically investigating their functional effects on hepatic stem cell populations is difficult. Prospective isolation and clonal assays for hepatic stem cells enable the examination of direct effects of GFs and ECMs on this rare cell fraction. Using previously purified cells that fulfill the criteria for hepatic stem cells, we examined how GFs and ECMs regulate differentiation in the developing liver. We show here that hepatocyte growth factor (HGF) induced early transition of albumin (ALB)-negative stem cells to ALB-positive hepatic precursors resembling hepatoblasts and then oncostatin M (OSM) promoted their differentiation to tryptophan-2, 3-dioxygenase (TO)-positive mature hepatocytes. During this transition, ECMs were necessary for the differentiation of stem cells and precursors, but their effects were only supportive. In the first step of stem cell differentiation induced by HGF, the expression of CCAAT/enhancer binding protein (C/EBP), a basic leucine zipper transcription factor, changed dramatically. When C/EBP function was inhibited in stem cells, they stopped differentiating to hepatocyte-lineage cells and proliferated actively. These are the first findings to illustrate the mechanism of hepatic stem cell differentiation in liver development.

Isolation of hepatoblasts based on the expression of Dlk/Pref-1

Hepatoblasts are common progenitors for hepatocytes and biliary epithelial cells, although their nature remains largely unknown. In order to isolate and to characterize hepatoblasts, we searched for cell surface antigens expressed in mouse fetal hepatic cells by the signal sequence trap method and found that Dlk, also known as Pref-1, was strongly expressed in fetal liver. Immunohistochemical as well as northern analysis indicated that Dlk was highly expressed in the E10.5 liver bud. The strong expression continued until the E16.5 stage and was significantly downregulated thereafter. Using a monoclonal antibody against Dlk, we isolated Dlk+ cells either by a fluorescence-activated cell sorter or by an automatic magnetic cell sorter. Dlk+ cells isolated from fetal livers expressed albumin and formed colonies when cultured at low density with HGF and EGF for 5 days. Over 60% of colonies derived from E14.5 Dlk+ cells contained both albumin+ and cytokeratin 19+ cells, indicating that a majority of colony-forming Dlk+ cells are able to differentiate into both hepatocyte and biliary epithelial cell lineages. In addition, numerous microvilli were observed by electronmicroscopic analysis in most of those cultured cells, also indicating differentiation of Dlk+ cells under this condition. Furthermore, 7% of the colony-forming Dlk+ cells were not only bipotential but also highly proliferative, forming a large colony containing more than 100 cells during 5 days of culture. By transplantation of Dlk+ cells into the spleen, donor-derived hepatocytes were found in the recipient liver, indicating that Dlk+ cells differentiated into hepatocytes in vivo. These results indicate that Dlk+ cells are hepatoblasts and that Dlk is a useful marker to enrich highly proliferative hepatoblasts from fetal liver.

A novel mitochondrial Ca2+-dependent solute carrier in the liver identified by mRNA differential display

Pancreatic AR42J cells have the feature of pluripotency of the precursor cells of the gut endoderm. Dexamethasone converts them to exocrine cells or liver cells. Using mRNA differential display techniques, we have identified a novel Ca2+-dependent member of the mitochondrial solute carrier superfamily, which is expressed during the course of differentiation, and have designated it MCSC. The corresponding cDNA comprises an open reading frame of 1407 base pairs encoding a polypeptide of 469 amino acids. The carboxyl-terminal-half of MCSC has high similarity with other mitochondrial carriers, and the amino-terminal-half has three canonical elongation factor-hand motifs and has calcium binding capacity. The deduced amino acid sequence revealed 79.1% homology to the rabbit peroxisomal Ca2+-dependent member of the mitochondrial superfamily, but the subcellular localization of the protein was exclusively mitochondrial, not peroxisomal. Northern blot and Western blot analyses revealed its predominant expression in the liver and the skeletal muscle. In the liver, the expression level of MCSC was higher in the adult stage than in the fetal stage, and MCSC was highly up-regulated in dexamethasone-treated AR42J cells before the expression of albumin. Taken together, MCSC may play an important role in regulating the function of hepatocytes rather than in differentiation in vivo.

In vivo studies of liver-type fatty acid binding protein (L-FABP) gene expression in liver of transgenic zebrafish (Danio rerio)

Mammalian liver fatty acid binding protein (L-FABP) is a small cytosolic protein in various tissues including liver, small intestine and kidney and is thought to play a crucial role in intracellular fatty acid trafficking and metabolism. To better understand its tissue-specific regulation during zebrafish hepatogenesis, we isolated 5'-flanking sequences of the zebrafish L-FABP gene and used a green fluorescent protein (GFP) transgenic strategy to generate liver-specific transgenic zebrafish. The 2.8-kb 5'-flanking sequence of zebrafish L-FABP gene was sufficient to direct GFP expression in liver primordia, first observed in 2 dpf embryos and then continuously to the adult stage. This pattern of transgenic expression is consistent with the expression pattern of the endogenous gene. F2 inheritance rates of 42-51% in all the seven transgenic lines were consistent with the ratio of Mendelian segregation. Further, hhex and zXbp-1 morphants displayed a visible liver defect, which suggests that it is possible to establish an in vivo system for screening genes required for liver development.

The potential of stem cells

Although stem cells have held the fascination of scientists for years, the attention of the general public has recently been captured by the derivation of human embryonic stem cells. In this review we describe the historical experiments leading up to the isolation of human embryonic stem cells and discuss recent advances in our understanding of both embryonic and somatic stem cells. Select examples are used to illustrate the potential of stem cells, both in the sense of their ability to differentiate into specific cell types and in the sense of their power to treat various diseases and conditions. Also discussed are recent studies describing current progress toward the treatment of Parkinson disease, spinal cord injuries, diabetes, and cardiac disease.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians

LEARNING OBJECTIVES

After completion of this article, the reader will be able to describe the various types of stem cells, outline potential clinical uses of stem cells, and summarize the somatic cell transdifferentiation debate.

Cell-autonomous and signal-dependent expression of liver and intestine marker genes in pluripotent precursor cells from Xenopus embryos

Early regulatory events in respect to the embryonic development of the vertebrate liver are only poorly defined. A better understanding of the gene network that mediates the formation of hepatocytes from pluripotent embryonic precursor cells may help to establish in vitro protocols for hepatocyte differentiation. Here, we describe our first attempts to make use of early embryonic explants from the amphibian Xenopus laevis in order to address these questions. We have identified several novel embryonic liver and intestine marker genes in a random expression pattern screen with cDNA libraries derived from the embryonic liver anlage and from the adult liver of Xenopus laevis. Based on their embryonic expression characteristics, these genes, together with the previously known ones, can be categorized into four different groups: the liver specific group (LS), the liver and intestine group A (LIA), the liver and intestine group B (LIB), and the intestine specific group (IS). Dissociation of endodermal explants isolated from early neurula stage embryos reveals that all genes in the LIB and IS groups are expressed in a cell-autonomous manner. In contrast, expression of genes in the LS and LIA groups requires cell-cell interactions. The regular temporal expression profile of genes in all four groups is mimicked in ectodermal explants from early embryos, reprogrammed by co-injection of VegT and beta-catenin mRNAs. FGF signaling is found to be required for the induction of liver specific marker (LS group) gene expression in the same system.

Cloning, expression and genomic structure of a novel human GNB2L1 gene, which encodes a receptor of activated protein kinase C (RACK)

During a large-scale screen of a human fetal brain cDNA library, a novel human gene GNB2L1 encoding a novel RACK (receptor of activated protein kinase C) protein was isolated and sequenced. The cDNA is 1142 bp long and has a predicted open reading frame encoding 316 aa. The predicted protein shows higher similarity to rat RACK1 and many RACK proteins of different organisms including Drosophila, C. elegans, mouse, rat, human, C. fasciculata, zebrafish, A. thaliana, S. cerevisiae and so on, suggesting it is conserved during evolution. The gene was mapped to human chromosome 5q35.3, the telomer position of chromosome 5q, in which the disease gene for early-onset primary congenital lymphedema was mapped. Also, 5q35.3 is a frequently reported location for cytogenetic and molecular abnormalities in renal cell carcinomas. The gene has 8 exons and 7 introns. It is expressed ubiquitously in many human tissues detected by northern blot analysis and RT-PCR.

Experimental conversion of liver to pancreas

BACKGROUND

The liver and the pancreas arise from adjacent regions of endoderm in embryonic development. Pdx1 is a key transcription factor that is essential for the development of the pancreas and is not expressed in the liver. The aim of this study was to determine whether a gene overexpression protocol based on Pdx1 would be able to cause conversion of liver to pancreas.

RESULTS

We show that a modified form of Pdx1, carrying the VP16 transcriptional activation domain, can cause conversion of liver to pancreas, both in vivo and in vitro. Transgenic Xenopus tadpoles carrying the construct TTR-Xlhbox8-VP16:Elas-GFP were prepared. Xlhbox8 is the Xenopus homolog of Pdx1, the TTR (transthyretin) promoter directs expression to the liver, and the GFP is under the control of an elastase promoter and provides a real-time visible marker of pancreatic differentiation. In the transgenic tadpoles, part or all of the liver is converted to pancreas, containing both exocrine and endocrine cells, while liver differentiation products are lost from the regions converted to pancreas. The timing of events is such that the liver is differentiating by the time Xlhbox8-VP16 is expressed, so we consider this a transdifferentiation event rather than a reprogramming of embryonic development. Furthermore, this same construct will bring about transdifferentiation of human hepatocytes in culture, with formation of both exocrine and endocrine cells.

CONCLUSIONS

We consider that the conversion of liver to pancreas could be the basis of a new type of therapy for insulin-dependent diabetes. Although expression of the transgene is transient, once the ectopic pancreas is established, it persists thereafter.

Formation of the digestive system in zebrafish. I. Liver morphogenesis

Despite the essential functions of the digestive system, much remains to be learned about the cellular and molecular mechanisms responsible for digestive organ morphogenesis and patterning. We introduce a novel zebrafish transgenic line, the gutGFP line, that expresses GFP throughout the digestive system, and use this tool to analyze the development of the liver. Our studies reveal two phases of liver morphogenesis: budding and growth. The budding period, which can be further subdivided into three stages, starts when hepatocytes first aggregate, shortly after 24 h postfertilization (hpf), and ends with the formation of a hepatic duct at 50 hpf. The growth phase immediately follows and is responsible for a dramatic alteration of liver size and shape. We also analyze gene expression in the developing liver and find a correlation between the expression of certain transcription factor genes and the morphologically defined stages of liver budding. To further expand our understanding of budding morphogenesis, we use loss-of-function analyses to investigate factors potentially involved in this process. It had been reported that no tail mutant embryos appear to lack a liver primordium, as assessed by gata6 expression. However, analysis of gutGFP embryos lacking Ntl show that the liver is in fact present. We also find that, in these embryos, the direction of liver budding does not correlate with the direction of intestinal looping, indicating that the left/right behavior of these tissues can be uncoupled. In addition, we use the cloche mutation to analyze the role of endothelial cells in liver morphogenesis, and find that in zebrafish, unlike what has been reported in mouse, endothelial cells do not appear to be necessary for the budding of this organ.

From endoderm formation to liver and pancreas development in zebrafish

Recent studies in zebrafish have contributed to our understanding of early endoderm formation in vertebrates. Specifically, they have illustrated the importance of Nodal signaling as well as three transcription factors, Faust/Gata5, Bonnie and Clyde, and Casanova, in this process. Ongoing genetic and embryological studies in zebrafish are also contributing to our understanding of later aspects of endoderm development, including the formation of the gut and its associated organs, the liver and pancreas. The generation of transgenic lines expressing GFP in these organs promises to be particularly helpful in such studies.

Beta-catenin antisense studies in embryonic liver cultures: role in proliferation, apoptosis, and lineage specification

BACKGROUND & AIMS

Wnt/beta-catenin pathway activation occurs during liver growth in hepatoblastomas, hepatocellular cancers, and liver regeneration. The aim of this study was to investigate the role of beta-catenin, a key component of the Wnt pathway, in liver development as well as its normal distribution in developing liver.

METHODS

Embryonic liver cultures and beta-catenin antisense phosphorodiamidate morpholino oligomer (PMO) were used to elucidate the role of beta-catenin in liver development. Livers from embryos at 10 days of gestational development were cultured in the presence of antisense or control PMO for 72 hours and analyzed.

RESULTS

Beta-catenin shows stage-specific localization and distinct distribution compared with known markers in developing liver. A substantial decrease in beta-catenin protein was evident in the organs cultured in the presence of antisense. Beta-catenin inhibition decreased cell proliferation and increased apoptosis in these organ cultures. Presence of antisense resulted in loss of CK19 immunoreactivity of the bipotential stem cells. Beta-catenin inhibition also promoted c-kit immunoreactivity of the hepatocytes.

CONCLUSIONS

We conclude that the PMO antisense to beta-catenin effectively inhibits synthesis of its protein. Beta-catenin modulates cell proliferation and apoptosis in developing liver. It may play a significant role in early biliary lineage commitment of the bipotential stem cells and also seems to be important in hepatocyte maturation.