Initiation of mammalian liver development from endoderm by fibroblast growth factors

Liver-Enriched Gene 1, a Glycosylated Secretory Protein, Binds to FGFR and Mediates an Anti-stress Pathway to Protect Liver Development in Zebrafish

Unlike mammals and birds, teleost fish undergo external embryogenesis, and therefore their embryos are constantly challenged by stresses from their living environment. These stresses, when becoming too harsh, will cause arrest of cell proliferation, abnormal cell death or senescence. Such organisms have to evolve a sophisticated anti-stress mechanism to protect the process of embryogenesis/organogenesis. However, very few signaling molecule(s) mediating such activity have been identified. liver-enriched gene 1 (leg1) is an uncharacterized gene that encodes a novel secretory protein containing a single domain DUF781 (domain of unknown function 781) that is well conserved in vertebrates. In the zebrafish genome, there are two copies of leg1, namely leg1a and leg1b. leg1a and leg1b are closely linked on chromosome 20 and share high homology, but are differentially expressed. In this report, we generated two leg1a mutant alleles using the TALEN technique, then characterized liver development in the mutants. We show that a leg1a mutant exhibits a stress-dependent small liver phenotype that can be prevented by chemicals blocking the production of reactive oxygen species. Further studies reveal that Leg1a binds to FGFR3 and mediates a novel anti-stress pathway to protect liver development through enhancing Erk activity. More importantly, we show that the binding of Leg1a to FGFR relies on the glycosylation at the 70^th asparagine (Asn⁷⁰ or N⁷⁰), and mutating the Asn⁷⁰ to Ala⁷⁰ compromised Leg1’s function in liver development. Therefore, Leg1 plays a unique role in protecting liver development under different stress conditions by serving as a secreted signaling molecule/modulator.

Although being challenged by stresses from their living environment during embryogenesis, teleost fish harbor a robust genetic program dictating liver development as long as any environmental change, including temperature or natural UV irradiation, is not detrimental. It is therefore of interest to explore the mechanism(s) behind this phenomenon. We showed that Liver-enriched gene 1 (Leg1) plays a unique role in protecting liver development under different stress conditions by serving as a secretory signaling molecule/modulator that binds to FGF receptor and activates the Erk signaling pathway. This finding may explain the adaption of teleost fish in coping with environmental changes.

Progressive developmental restriction, acquisition of left-right identity and cell growth behavior during lobe formation in mouse liver development

To identify cell-based decisions implicated in morphogenesis of the mammalian liver, we performed clonal analysis of hepatocytes/hepatoblasts in mouse liver development, using a knock-in allele of Hnf4a/laacZ This transgene randomly undergoes a low frequency of recombination that generates a functional lacZ gene that produces β-galactosidase in tissues in which Hnf4a is expressed. Two types of β-galactosidase-positive clones were found. Most have undergone three to eight cell divisions and result from independent events (Luria-Delbrück fluctuation test); we calculate that they arose between E8.5 and E13.5. A second class was mega-clones derived from early endoderm progenitors, generating many descendants. Some originated from multi-potential founder cells, with labeled cells in the liver, pancreas and/or intestine. A few mega-clones populate only one side of the liver, indicating hepatic cell chirality. The patterns of labeled cells indicate cohesive and often oriented growth, notably in broad radial stripes, potentially implicated in the formation of liver lobes. This retrospective clonal analysis gives novel insights into clonal origins, cell behavior of progenitors and distinct properties of endoderm cells that underlie the formation and morphogenesis of the liver.

From Endoderm to Liver Bud: Paradigms of Cell Type Specification and Tissue Morphogenesis

The early specification, rapid growth and morphogenesis, and conserved functions of the embryonic liver across diverse model organisms have made the system an experimentally facile paradigm for understanding basic regulatory mechanisms that govern cell differentiation and organogenesis. This essay highlights concepts that have emerged from studies of the discrete steps of foregut endoderm development into the liver bud, as well as from modeling the steps via embryonic stem cell differentiation. Such concepts include understanding the chromatin basis for the competence of progenitor cells to develop into specific lineages; the importance of combinatorial signaling from different sources to induce cell fates; the impact of inductive signaling on preexisting chromatin states; the ability of separately specified domains of cells to merge into a common tissue; and the marked cell biological dynamics, including interactions with the developing vasculature, which establish the initial morphogenesis and patterning of a tissue. The principles gleaned from these studies, focusing on the 2 days it takes for the endoderm to develop into a liver bud, should be instructive for many other organogenic systems and for manipulating tissues in regenerative contexts for biomedical purposes.

Enhancement of hepatocyte differentiation from human embryonic stem cells by Chinese medicine Fuzhenghuayu

Chinese medicine, Fuzhenghuayu (FZHY), appears to prevent fibrosis progression and improve liver function in humans. Here we found that FZHY enhanced hepatocyte differentiation from human embryonic stem cells (hESC). After treatment with FZHY, albumin expression was consistently increased during differentiation and maturation process, and expression of metabolizing enzymes and transporter were also increased. Importantly, expression of mesenchymal cell and cholangiocyte marker was significantly reduced by treatment with FZHY, indicating that one possible mechanism of FZHY’s role is to inhibit the formation of mesenchymal cells and cholangiocytes. Edu-labelled flow cytometric analysis showed that the percentage of the Edu positive cells was increased in the treated cells. These results indicate that the enhanced proliferation involved hepatocytes rather than another cell type. Our investigations further revealed that these enhancements by FZHY are mediated through activation of canonical Wnt and ERK pathways and inhibition of Notch pathway. Thus, FZHY not only promoted hepatocyte differentiation and maturation, but also enhanced hepatocyte proliferation. These results demonstrate that FZHY appears to represent an excellent therapeutic agent for the treatment of liver fibrosis, and that FZHY treatment can enhance our efforts to generate mature hepatocytes with proliferative capacity for cell-based therapeutics and for pharmacological and toxicological studies.

Cell-based therapies of liver diseases: age-related challenges

The scope of this review is to revise recent advances of the cell-based therapies of liver diseases with an emphasis on cell donor's and patient's age. Regenerative medicine with cell-based technologies as its integral part is focused on the structural and functional restoration of tissues impaired by sickness or aging. Unlike drug-based medicine directed primarily at alleviation of symptoms, regenerative medicine offers a more holistic approach to disease and senescence management aimed to achieve restoration of homeostasis. Hepatocyte transplantation and organ engineering are very probable forthcoming options of liver disease treatment in people of different ages and vigorous research and technological innovations in this area are in progress. Accordingly, availability of sufficient amounts of functional human hepatocytes is crucial. Direct isolation of autologous hepatocytes from liver biopsy is problematic due to related discomfort and difficulties with further expansion of cells, particularly those derived from aging people. Allogeneic primary human hepatocytes meeting quality standards are also in short supply. Alternatively, autologous hepatocytes can be produced by reprogramming of differentiated cells through the stage of induced pluripotent stem cells. In addition, fibroblasts and mesenchymal stromal cells can be directly induced to undergo advanced stage hepatogenic differentiation. Reprogramming of cells derived from elderly people is accompanied by the reversal of age-associated changes at the cellular level manifesting itself by telomere elongation and the U-turn of DNA methylation. Cell reprogramming can provide high quality rejuvenated hepatocytes for cell therapy and liver tissue engineering. Further technological advancements and establishment of national and global registries of induced pluripotent stem cell lines homozygous for HLA haplotypes can allow industry-style production of livers for immunosuppression-free transplantation.

FGF2 mediates hepatic progenitor cell formation during human pluripotent stem cell differentiation by inducing the WNT antagonist NKD1

Fibroblast growth factors (FGFs) are required to specify hepatic fate within the definitive endoderm through activation of the FGF receptors (FGFRs). While the signaling pathways involved in hepatic specification are well understood, the mechanisms through which FGFs induce hepatic character within the endoderm are ill defined. Here we report the identification of genes whose expression is directly regulated by FGFR activity during the transition from endoderm to hepatic progenitor cell. The FGFR immediate early genes that were identified include those encoding transcription factors, growth factors, and signaling molecules. One of these immediate early genes encodes naked cuticle homolog 1 (NKD1), which is a repressor of canonical WNT (wingless-type MMTV integration site) signaling. We show that loss of NKD1 suppresses the formation of hepatic progenitor cells from human induced pluripotent stem cells and that this phenotype can be rescued by using a pharmacological antagonist of canonical WNT signaling. We conclude that FGF specifies hepatic fate at least in large part by inducing expression of NKD1 to transiently suppress the canonical WNT pathway.

Orchestrating liver development

The liver is a central regulator of metabolism, and liver failure thus constitutes a major health burden. Understanding how this complex organ develops during embryogenesis will yield insights into how liver regeneration can be promoted and how functional liver replacement tissue can be engineered. Recent studies of animal models have identified key signaling pathways and complex tissue interactions that progressively generate liver progenitor cells, differentiated lineages and functional tissues. In addition, progress in understanding how these cells interact, and how transcriptional and signaling programs precisely coordinate liver development, has begun to elucidate the molecular mechanisms underlying this complexity. Here, we review the lineage relationships, signaling pathways and transcriptional programs that orchestrate hepatogenesis.

Differentiation of Bone Marrow: Derived Mesenchymal Stem Cells into Hepatocyte-like Cells

Cirrhosis is the end-stage liver fibrosis, whereby normal liver architecture is disrupted by fibrotic bands, parenchymal nodules and vascular distortion. Portal hypertension and hepatocyte dysfunction are the end results and give rise to major systemic complications and premature death. Mesenchymal stem cells (MSC) have the capacity of self-renew and to give rise to cells of various lineages, so MSC can be isolated from bone marrow (BM) and induced to differentiate into hepatocyte-like cells. MSC were induced to differentiate into hepatocyte-like cells by hepatotic growth factor (HGF) and fibroblast growth factor-4 (FGF-4). Differentiated cells were examined for the expression of hepatocyte-specific markers and hepatocyte functions. MSC were isolated. Flow cytometry analysis showed that they expressed the MSC-specific markers, reverse transcriptase-polymerase chain reaction (RT-PCR) demonstrated that MSC expressed the hepatocyte-specific marker cytokeratin 18 (CK-18) following hepatocyte induction. This study demonstrates that BM-derived-MSC can differentiate into functional hepatocyte-like cells following the induction of HGF and FGF-4. MSC can serve as a favorable cell source for tissue engineering in the treatment of liver disease.

A Paracrine Mechanism Accelerating Expansion of Human Induced Pluripotent Stem Cell-Derived Hepatic Progenitor-Like Cells

Hepatic stem/progenitor cells in liver development have a high proliferative potential and the ability to differentiate into both hepatocytes and cholangiocytes. In this study, we focused on the cell surface molecules of human induced pluripotent stem (iPS) cell-derived hepatic progenitor-like cells (HPCs) and analyzed how these molecules modulate expansion of these cells. Human iPS cells were differentiated into immature hepatic lineage cells by cytokines. In addition to hepatic progenitor markers (CD13 and CD133), the cells were coimmunostained for various cell surface markers (116 types). The cells were analyzed by flow cytometry and in vitro colony formation culture with feeder cells. Twenty types of cell surface molecules were highly expressed in CD13(+)CD133(+) cells derived from human iPS cells. Of these molecules, CD221 (insulin-like growth factor receptor), which was expressed in CD13(+)CD133(+) cells, was quickly downregulated after in vitro expansion. The proliferative ability was suppressed by a neutralizing antibody and specific inhibitor of CD221. Overexpression of CD221 increased colony-forming ability. We also found that inhibition of CD340 (erbB2) and CD266 (fibroblast growth factor-inducible 14) signals suppressed proliferation. In addition, both insulin-like growth factor (a ligand of CD221) and tumor necrosis factor-like weak inducer of apoptosis (a ligand of CD266) were provided by feeder cells in our culture system. This study revealed the expression profiles of cell surface molecules in human iPS cell-derived HPCs and that the paracrine interactions between HPCs and other cells through specific receptors are important for proliferation.

Pleiotropy as the Mechanism for Evolving Novelty: Same Signal, Different Result

In contrast to the probabilistic way of thinking about pleiotropy as the random expression of a single gene that generates two or more distinct phenotypic traits, it is actually a deterministic consequence of the evolution of complex physiology from the unicellular state. Pleiotropic novelties emerge through recombinations and permutations of cell-cell signaling exercised during reproduction based on both past and present physical and physiologic conditions, in service to the future needs of the organism for its continued survival. Functional homologies ranging from the lung to the kidney, skin, brain, thyroid and pituitary exemplify the evolutionary mechanistic strategy of pleiotropy. The power of this perspective is exemplified by the resolution of evolutionary gradualism and punctuated equilibrium in much the same way that Niels Bohr resolved the paradoxical duality of light as Complementarity.

Id2a is required for hepatic outgrowth during liver development in zebrafish

During development, inhibitor of DNA binding (Id) proteins, a subclass of the helix-loop-helix family of proteins, regulate cellular proliferation, differentiation, and apoptosis in various organs. However, a functional role of Id2a in liver development has not yet been reported. Here, using zebrafish as a model organism, we provide in vivo evidence that Id2a regulates hepatoblast proliferation and cell death during liver development. Initially, in the liver, id2a is expressed in hepatoblasts and after their differentiation, id2a expression is restricted to biliary epithelial cells. id2a knockdown in zebrafish embryos had no effect on hepatoblast specification or hepatocyte differentiation. However, liver size was greatly reduced in id2a morpholino-injected embryos, indicative of a hepatic outgrowth defect attributable to the significant decrease in proliferating hepatoblasts concomitant with the significant increase in hepatoblast cell death. Altogether, these data support the role of Id2a as an important regulator of hepatic outgrowth via modulation of hepatoblast proliferation and survival during liver development in zebrafish.

Human pluripotent stem cell-derived cholangiocytes: current status and future applications

PURPOSE OF REVIEW

Pluripotent stem cells, such as embryonic stem cells and inducible pluripotent stem (iPS) cells, have high proliferative multipotency for differentiation into mature functional cells that are useful for treatment and basic research on several diseases. Cholangiocytes are differentiated from fetal hepatic progenitor cells (hepatoblasts) and are important for transport of bile acids that are synthesized by mature hepatocytes in the liver. However, the molecular mechanisms of development and function of human cholangiocytes remain unknown. This review mentions the potential of human cholangiocytic culture from pluripotent stem cells to contribute to the analyses of the human bile duct system and diseases.

RECENT FINDINGS

Recent studies found that human hepatic cholangiocytic cells can be differentiated from human embryonic stem and iPS cells in a suitable culture condition. Cholangiocytic cysts have epithelial cell polarity formed in a three-dimensional cell culture system using extracellular matrices.

SUMMARY

Disease pathogenesis was elucidated in vitro using differentiated cells from disease-related iPS cells. Using genome-editing enzymes, iPS cells with disease-specific gene mutations can be easily and rapidly established. These disease-related iPS cells and cholangiocytic culture system may be useful for analyses and drug screening of human bile duct diseases.

Promotion of mesenchymal-to-epithelial transition by Rac1 inhibition with small molecules accelerates hepatic differentiation of mesenchymal stromal cells

In vitro differentiation of stem cells into specific cell lineages provides a stable cell supply for cell therapy and tissue engineering. Therefore, understanding the mechanisms underlying such differentiation processes is critical for generating committed lineage-specific cell progenies effectively. We previously developed a two-step protocol to differentiate mesenchymal stromal cells (MSCs) into hepatocyte-like cells. Since hepatic differentiation involves mesenchymal-epithelial transition (MET), we hypothesize that promoting MET could further accelerate the differentiation process. Ras-related C3 botulinum toxin substrate 1 (Rac1) is involved in actin polymerization and its role in MET was investigated in the study. Our results showed that inhibition of Rac1 activation by Rac1-specific inhibitor, NSC23766, led to cells favoring epithelial morphology and being more packed during hepatic differentiation. In addition, Rac1 inhibition accelerated the upregulation of hepatic marker genes accompanied by more mature hepatic functions. Taken together, promotion of MET by inhibiting Rac1 accelerates the hepatic differentiation of MSCs. Our findings open a new prospect of directing the commitment of MSCs by manipulating cell morphology and cytoskeleton arrangement through small molecules. The results provide further insight into scaffold design for rapid production of MSC-differentiated hepatocytes.

Derivation of Endodermal Progenitors From Pluripotent Stem Cells

Stem and progenitor cells play important roles in organogenesis during development and in tissue homeostasis and response to injury postnatally. As the regenerative capacity of many human tissues is limited, cell replacement therapies hold great promise for human disease management. Pluripotent stem cells such as embryonic stem (ES) cells and induced pluripotent stem (iPS) cells are prime candidates for the derivation of unlimited quantities of clinically relevant cell types through development of directed differentiation protocols, that is, the recapitulation of developmental milestones in in vitro cell culture. Tissue-specific progenitors, including progenitors of endodermal origin, are important intermediates in such protocols since they give rise to all mature parenchymal cells. In this review, we focus on the in vivo biology of embryonic endodermal progenitors in terms of key transcription factors and signaling pathways. We critically review the emerging literature aiming to apply this basic knowledge to achieve the efficient and reproducible in vitro derivation of endodermal progenitors such as pancreas, liver and lung precursor cells.

Wnt and FGF mediated epithelial-mesenchymal crosstalk during lung development

BACKGROUND

The adaptation to terrestrial life required the development of an organ capable of efficient air-blood gas exchange. To meet the metabolic load of cellular respiration, the mammalian respiratory system has evolved from a relatively simple structure, similar to the two-tube amphibian lung, to a highly complex tree-like system of branched epithelial airways connected to a vast network of gas exchanging units called alveoli. The development of such an elaborate organ in a relatively short time window is therefore an extraordinary feat and involves an intimate crosstalk between mesodermal and endodermal cell lineages.

RESULTS

This review describes the molecular processes governing lung development with an emphasis on the current knowledge on the role of Wnt and FGF signaling in lung epithelial differentiation.

CONCLUSIONS

The Wnt and FGF signaling pathways are crucial for the dynamic and reciprocal communication between epithelium and mesenchyme during lung development. In addition, some of this developmental crosstalk is reemployed in the adult lung after injury to drive regeneration, and may, when aberrantly or chronically activated, result in chronic lung diseases. Novel insights into how the Wnt and FGF pathways interact and are integrated into a complex gene regulatory network will not only provide us with essential information about how the lung regenerates itself, but also enhance our understanding of the pathogenesis of chronic lung diseases, as well as improve the controlled differentiation of lung epithelium from pluripotent stem cells.

Promoting the recovery of injured liver with poly (3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) scaffolds loaded with umbilical cord-derived mesenchymal stem cells

Cell-based therapies are major focus of current research for treatment of liver diseases. In this study, mesenchymal stem cells were isolated from human umbilical cord Wharton's jelly (WJ-MSCs). Results confirmed that WJ-MSCs isolated in this study could express the typical MSC-specific markers and be induced to differentiate into adipocytes, osteoblasts, and chondrocytes. They could also be induced to differentiate into hepatocyte-like cells. Poly (3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) (PHBVHHx) is a new member of polyhydroxyalkanoate family and biodegradable polyester produced by bacteria. PHBVHHx scaffolds showed much higher cell attachment and viability than the other polymers tested. PHBVHHx scaffolds loaded with WJ-MSCs were transplanted into liver-injured mice. Liver morphology improved after 30 days of transplantation and looked similar to normal liver. Concentrations of serum alanine aminotransferase and total bilirubin were significantly lower, and albumin was significantly higher on days 14 and 30 in the WJ-MSCs+scaffold group than in the carbon tetrachloride (CCl4) group. Hematoxylin-eosin staining showed that liver had similar structure of normal liver lobules and similar size and shape of normal hepatic cells, and Masson staining demonstrated that liver had less blue staining for collagen after 30 days of transplantation. Real-time reverse transcription-polymerase chain reaction (RT-PCR) showed that the expression of the bile duct epithelial cell gene CK-19 in mouse liver is significantly lower on days 14 and 30 in the WJ-MSCs+scaffold group than in the CCl4 group. Real-time RT-PCR, immunocytochemistry, and periodic acid-Schiff staining showed that WJ-MSCs in scaffolds differentiated into hepatocyte-like cells on days 14 and 30 in the WJ-MSCs+scaffold group. Real-time RT-PCR also demonstrated that WJ-MSCs in scaffolds expressed endothelial cell genes Flk-1, vWF, and VE-cadherin on days 14 and 30 in the WJ-MSCs+scaffold group, indicating that WJ-MSCs also differentiated into endothelial-like cells. These results demonstrated that PHBVHHx scaffolds loaded with WJ-MSCs significantly promoted the recovery of injured liver and could be further studied for liver tissue engineering.

Ethanol negatively regulates hepatic differentiation of hESC by inhibition of the MAPK/ERK signaling pathway in vitro

BACKGROUND

Alcohol insult triggers complex events in the liver, promoting fibrogenic/inflammatory signals and in more advanced cases, aberrant matrix deposition. It is well accepted that the regenerative capacity of the adult liver is impaired during alcohol injury. The liver progenitor/stem cells have been shown to play an important role in liver regeneration -in response to various chronic injuries; however, the effects of alcohol on stem cell differentiation in the liver are not well understood.

METHODS

We employed hepatic progenitor cells derived from hESCs to study the impact of ethanol on hepatocyte differentiation by exposure of these progenitor cells to ethanol during hepatocyte differentiation.

RESULTS

We found that ethanol negatively regulated hepatic differentiation of hESC-derived hepatic progenitor cells in a dose-dependent manner. There was also a moderate cell cycle arrest at G1/S checkpoint in the ethanol treated cells, which is associated with a reduced level of cyclin D1 in these cells. Ethanol treatment specifically inhibited the activation of the ERK but not JNK nor the p38 MAP signaling pathway. At the same time, the WNT signaling pathway was also reduced in the cells exposed to ethanol. Upon evaluating the effects of the inhibitors of these two signaling pathways, we determined that the Erk inhibitor replicated the effects of ethanol on the hepatocyte differentiation and attenuated the WNT/β-catenin signaling, however, inhibitors of WNT only partially replicated the effects of ethanol on the hepatocyte differentiation.

CONCLUSION

Our results demonstrated that ethanol negatively regulated hepatic differentiation of hESC-derived hepatic progenitors through inhibiting the MAPK/ERK signaling pathway, and subsequently attenuating the WNT signaling pathway. Thus, our finding provides a novel insight into the mechanism by which alcohol regulates cell fate selection of hESC-derived hepatic progenitor cells, and the identified pathways may provide therapeutic targets aimed at promoting liver repair and regeneration during alcoholic injury.

FGF signaling is required for anterior but not posterior specification of the murine liver bud

BACKGROUND

The definitive endoderm arises as a naive epithelial sheet that produces the entire gut tube and associated organs including the liver, pancreas and lungs. Murine explant studies demonstrate that fibroblast growth factor (FGF) signaling from adjacent tissues is required to induce hepatic gene expression from isolated foregut endoderm. The requirement of FGF signaling during liver development is examined by means of small molecule inhibition during whole embryo culture.

RESULTS

Loss of FGF signaling before hepatic induction results in morphological defects and gene expression changes that are confined to the anterior liver bud. In contrast the posterior portion of the liver bud remains relatively unaffected. Because FGF is thought to act as a morphogen during endoderm organogenesis, the ventral pancreas was also examined after FGF inhibition. Although the size of the ventral pancreas is not affected, loss of FGF signaling results in a significantly higher density of ventral pancreas cells.

CONCLUSIONS

The requirement for FGF-mediated induction of hepatic gene expression differs across the anterior/posterior axis of the developing liver bud. These results underscore the importance of studying tissue differentiation in the context of the whole embryo.

Mesenchymal progenitor cells in mouse foetal liver regulate differentiation and proliferation of hepatoblasts

BACKGROUND & AIMS

Hepatoblasts are somatic progenitor cells of the foetal liver that possess high proliferative capacity and bi-potency for differentiation into both hepatocytes and cholangiocytes. Although mesenchymal cells are known to be important for liver ontogeny, current understanding of their interaction with hepatoblasts remains obscure. Mesenchymal cell populations in the developing liver were purified and their potential to support proliferation and differentiation of hepatoblasts was examined.

METHODS

Foetal liver cells were fractionated with a flow cytometer using antibodies against cell surface markers. Gene expression of mesenchymal-specific transcripts and morphological characteristics were analysed. The ability of the mesenchymal cells to support hepatoblast function was analysed using a transwell and direct coculture system.

RESULTS

CD45(-) Ter119(-) CD71(-) Dlk1(mid) PDGFRα(+) cells from the mid-foetal stage liver expressed the mesenchymal cell-specific transcription factors H2.0-like homeobox 1 and LIM homeobox 2 at high levels. Foetal mesenchymal cells make contact with hepatoblasts in vivo and possess the potential to differentiate into chondrocytes, osteocytes and adipocytes under appropriate cell culture conditions, indicating that these cells are possible candidates for mesenchymal stem/progenitor cells. Foetal mesenchymal cells expressed pleiotrophin, hepatocyte growth factor and midkine 1, which are involved in the growth of hepatoblasts. Using the coculture system with hepatoblasts and foetal mesenchymal cells, these cells were shown to support proliferation and maturation of hepatoblasts through indirect and direct interactions respectively.

CONCLUSIONS

Dlk1(mid) PDGFRα(+) cells in non-haematopoetic fraction derived from the foetal liver exhibit mesenchymal stem/progenitor cell characteristics and have abilities to support proliferation and differentiation of hepatoblasts.

Differential expression of natural antisense transcripts during liver development in embryonic mice

The mature adult liver is a metabolizing and synthesizing organ consisting of hepatic lobules. The liver development during fetal stages is induced by the interaction between hepatoblast cells derived from the endoderm and stellate cells and/or endothelial cells derived from the mesoderm. Recently, the association of non-coding RNAs, such as microRNAs, in the liver development process has been reported. In the present study, the gene expression changes were investigated of natural antisense transcripts (NATs), a type of non-coding RNA, during the liver development using livers from embryonic day (E) 14, E17, E19 and newborn (NB). The upregulated NATs of 87 genes and the downregulated NATs of 26 genes were identified when the expression was >5.0-fold up/downregulated NATs at E17, E19 or NB compared to those at E14, as examined by microarray analysis. Among these NATs, strand-specific reverse transcription-quantitative polymerase chain reaction analyses validated that the NAT expression of Fga, Fgb, Fgg, F2, Apoa1, Fabp1, C3 and Proc, which have metabolic and synthesizing functions in the adult liver, increased, whereas the NAT expression of Ermap and Tfrc, which function in the hematopoiesis, decreased during the developmental stages. These results indicate that the differential expression of several NATs is induced during the process of liver development, suggesting that these NATs may be involved in the regulation of gene expression and functional conversion from a hematopoietic to a metabolic organ in the developmental stages of the liver.

Hypoxia-inducible factor 2 alpha is essential for hepatic outgrowth and functions via the regulation of leg1 transcription in the zebrafish embryo

The liver plays a vital role in metabolism, detoxification, digestion, and the maintenance of homeostasis. During development, the vertebrate embryonic liver undergoes a series of morphogenic processes known as hepatogenesis. Hepatogenesis can be separated into three interrelated processes: endoderm specification, hepatoblast differentiation, and hepatic outgrowth. Throughout this process, signaling molecules and transcription factors initiate and regulate the coordination of cell proliferation, apoptosis, differentiation, intercellular adhesion, and cell migration. Hifs are already recognized to be essential in embryonic development, but their role in hepatogenesis remains unknown. Using the zebrafish embryo as a model organism, we report that the lack of Hif2-alpha but not Hif1-alpha blocks hepatic outgrowth. While Hif2-alpha is not involved in hepatoblast specification, this transcription factor regulates hepatocyte cell proliferation during hepatic outgrowth. Furthermore, we demonstrated that the lack of Hif2-alpha can reduce the expression of liver-enriched gene 1 (leg1), which encodes a secretory protein essential for hepatic outgrowth. Additionally, exogenous mRNA expression of leg1 can rescue the small liver phenotype of hif2-alpha morphants. We also showed that Hif2-alpha directly binds to the promoter region of leg1 to control leg1 expression. Interestingly, we discovered overrepresented, high-density Hif-binding sites in the potential upstream regulatory sequences of leg1 in teleosts but not in terrestrial mammals. We concluded that hif2-alpha is a key factor required for hepatic outgrowth and regulates leg1 expression in zebrafish embryos. We also proposed that the hif2-alpha-leg1 axis in liver development may have resulted from the adaptation of teleosts to their environment.

Microarray comparison of the gene expression profiles in the adult vs. embryonic day 14 rat liver

The aim of the present study was to identify the differentially-expressed genes of embryonic day 14 (ED 14) rat liver in comparison to adult rat liver, which may provide specific information for the investigation of the hepatogenesis mechanism. The gene expression profiles of ED 14 and adult rat livers were investigated using microarray analysis (the Illumina RatRef-12 Expression BeadChip). Quantitative polymerase chain reaction (qPCR) analyses were conducted to confirm the gene expression. There were 787 genes upregulated in the embryonic liver. Based on the gene ontology classification system, which was analyzed by the database for annotation, visualization and integrated discovery software, a number of the upregulated genes were categorized into the distinct and differentially-expressed functional groups, including metabolism pathway, cell cycle, transcription, signal transduction, purine metabolism, cell structure, transportation and apoptosis. qPCR analyses confirmed the gene expression. Eleven upregulated genes were found in the ED 14 rat liver, which may provide specific information for the understanding of the molecular mechanisms that control hepatogenesis. These overexpressed genes are potential markers for identifying hepatic progenitor cells.

Fibroblast growth factor family as a potential target in the treatment of hepatocellular carcinoma

Hepatocellular cancer (HCC) is currently the third leading cause of cancer death worldwide. The prognosis of patients diagnosed with late-stage disease is dismal due to high resistance to conventional systemic therapies. The introduction of sorafenib, despite its limited efficacy, as the standard systemic therapy for advanced HCC has paved a way for targeted molecular therapies for HCC. Fibroblast growth factor (FGF) signaling plays an important role in the developing embryo and the adult. The FGF signaling pathway is often hijacked by cancer cells, including HCC. Several alterations in FGF signaling correlate with poor outcome in HCC patients, suggesting that this family of signaling molecules plays an important role in the development of HCC. Multikinase inhibitors targeting FGF signaling are currently under investigation in clinical trials. This review discusses the current understanding of the biological and clinical implications of aberrant FGF signaling in the prognosis, diagnosis, and treatment of HCC.

Involvement of hepatic stimulator substance in the regulation of hepatoblast maturation into hepatocytes in vitro

Hepatic stimulator substance (HSS), also known as augmenter of liver regeneration (ALR), acts as a hepatotrophic growth factor to promote liver regeneration after liver damage or partial hepatectomy. However, the expression and function of HSS during liver development in mammals remain largely unknown. In this work, the hepatoblasts were isolated from mice at embryonic day 13.5 (E13.5), and HSS expression and its role during hepatoblast maturation were investigated. The results showed that HSS expression was enhanced in the hepatoblasts compared with mouse primary hepatocytes. HSS expression (23 kDa) was significantly decreased if the hepatoblast maturation was induced by a combination of oncostatin M (OSM), dexamethasone (DEX), and hepatocyte growth factor (HGF). We also found that knockdown of HSS expression (mainly 23-kDa isoform) by siRNA promoted hepatoblast maturation and also activated the signal transducer and activator of transcription 3 (STAT3) phosphorylation levels. However, if STAT3 activity was blocked by a small-molecule inhibitor Stattic, then hepatocyte maturation could be abolished, suggesting that STAT3 was most likely a potential molecule responsible for HSS signaling. In summary, our results demonstrated for the first time that HSS might be an active factor participating in the regulation of liver development and hepatocyte maturation.

Fibroblast growth factor signaling regulates the expansion of A6-expressing hepatocytes in association with AKT-dependent β-catenin activation

BACKGROUND & AIMS

Fibroblast Growth Factors (FGFs) promote the proliferation and survival of hepatic progenitor cells (HPCs) via AKT-dependent β-catenin activation. Moreover, the emergence of hepatocytes expressing the HPC marker A6 during 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced liver injury is mediated partly by FGF and β-catenin signaling. Herein, we investigate the role of FGF signaling and AKT-mediated β-catenin activation in acute DDC liver injury.

METHODS

Transgenic mice were fed DDC chow for 14days concurrent with either Fgf10 over-expression or inhibition of FGF signaling via expression of soluble dominant-negative FGF Receptor (R)-2IIIb.

RESULTS

After 14days of DDC treatment, there was an increase in periportal cells expressing FGFR1, FGFR2, and AKT-activated phospho-Serine 552 (pSer552) β-Catenin in association with up-regulation of genes encoding the FGFR2IIIb ligands, Fgf7, Fgf10, and Fgf22. In response to Fgf10 over-expression, there was an increase in the number of pSer552-β-Catenin((positive)+ive) periportal cells as well as cells co-positive for A6 and hepatocyte marker, Hepatocyte Nuclear Factor-4α (HNF4α). A similar expansion of A6(+ive) cells was observed after Fgf10 over-expression with regular chow and after partial hepatectomy during ethanol toxicity. Inhibition of FGF signaling increased the periportal A6(+ive)HNF4α(+ive) cell population while reducing centrolobular A6(+ive) HNF4α(+ive) cells. AKT inhibition with Wortmannin attenuated FGF10-mediated A6(+ive)HNF4α(+ive) cell expansion. In vitro analyses using FGF10 treated HepG2 cells demonstrated AKT-mediated β-Catenin activation but not enhanced cell migration.

CONCLUSIONS

During acute DDC treatment, FGF signaling promotes the expansion of A6-expressing liver cells partly via AKT-dependent activation of β-Catenin expansion of A6(+ive) periportal cells and possibly by reprogramming of centrolobular hepatocytes.

Development of a chemically defined medium and discovery of new mitogenic growth factors for mouse hepatocytes: mitogenic effects of FGF1/2 and PDGF

Chemically defined serum-free media for rat hepatocytes have been useful in identifying EGFR ligands and HGF/MET signaling as direct mitogenic factors for rat hepatocytes. The absence of such media for mouse hepatocytes has prevented screening for discovery of such mitogens for mouse hepatocytes. We present results obtained by designing such a chemically defined medium for mouse hepatocytes and demonstrate that in addition to EGFR ligands and HGF, the growth factors FGF1 and FGF2 are also important mitogenic factors for mouse hepatocytes. Smaller mitogenic response was also noticed for PDGF AB. Mouse hepatocytes are more likely to enter into spontaneous proliferation in primary culture due to activation of cell cycle pathways resulting from collagenase perfusion. These results demonstrate unanticipated fundamental differences in growth biology of hepatocytes between the two rodent species.

A Network Map of FGF-1/FGFR Signaling System

Fibroblast growth factor-1 (FGF-1) is a well characterized growth factor among the 22 members of the FGF superfamily in humans. It binds to all the four known FGF receptors and regulates a plethora of functions including cell growth, proliferation, migration, differentiation, and survival in different cell types. FGF-1 is involved in the regulation of diverse physiological processes such as development, angiogenesis, wound healing, adipogenesis, and neurogenesis. Deregulation of FGF-1 signaling is not only implicated in tumorigenesis but also is associated with tumor invasion and metastasis. Given the biomedical significance of FGFs and the fact that individual FGFs have different roles in diverse physiological processes, the analysis of signaling pathways induced by the binding of specific FGFs to their cognate receptors demands more focused efforts. Currently, there are no resources in the public domain that facilitate the analysis of signaling pathways induced by individual FGFs in the FGF/FGFR signaling system. Towards this, we have developed a resource of signaling reactions triggered by FGF-1/FGFR system in various cell types/tissues. The pathway data and the reaction map are made available for download in different community standard data exchange formats through NetPath and NetSlim signaling pathway resources.

Re-evaluation of liver stem/progenitor cells

Liver stem/progenitor cells (LPCs) are defined as cells that supply two types of liver epithelial cells, hepatocytes and cholangiocytes, during development, cellular turnover, and regeneration. Hepatoblasts, which are fetal LPCs derived from endoderm stem cells, robustly proliferate and differentiate into hepatocytes and cholangiocytes during fetal life. Between mid-gestation and the neonatal period, some cholangiocytes function as LPCs. Although LPCs in adult livers can be enriched in cells positive for cholangiocyte markers, their tissue localization and functions in cellular turnover remain obscure. On the other hand, it is well known that liver regeneration under conditions suppressing hepatocyte proliferation is supported by LPCs, though their origin has not been clearly identified. Recently many groups took advantage of new techniques including prospective isolation of LPCs by fluorescence-activated cell sorting and genetic lineage tracing to facilitate our understanding of epithelial supply in normal and injured livers. Those works suggest that, in normal livers, the turnover of hepatocytes mostly depends on duplication of hepatocytes. It is also demonstrated that liver epithelial cells as well as LPCs have great plasticity and flexible differentiation capability to respond to various types of injuries by protecting or repairing liver tissues.

Differentiation of nonhuman primate embryonic stem cells into hepatocyte-like cells

OBJECTIVE

To investigate whether cells derived from rhesus monkey embryonic stem cells (ESC) had hepatocyte characteristics after the differentiation.

METHODS

Rhesus monkey ESC were induced towards hepatocyte-like cells via a four-step differentiation process: the formation of embryoid bodies (EB), EB in activin A and insulin-transferrin-selenium medium for 4 days, in fibroblast growth factor (FGF)-4 and bone morphogenetic protein-2 (BMP2) medium for 8 days, in hepatocyte culture medium containing hepatocyte growth factor for 3 days and then with oncostatin M and dexamethasone for another 5 days. Expression of albumin (ALB), glucose-6-phosphatase, α-fetoprotein (AFP) and α-1 antitrypsin (α1-AT) at the mRNA level in differentiated cells were detected by reverse transcription-polymerase chain reaction. The expression of hepatocyte markers AFP, ALB, hepatocyte nuclear factor 4 (HNF4), cytokeratin 8 (CK8), CK19 and cell proliferation marker, Ki67, in the differentiated cells were determined by immunocytochemistry. The ultrastructure of the differentiated cells was examined by electron microscopy. Indocyanine green (ICG) uptake was also explored.

RESULTS

After induction, some differentiated cells were binucleate, which is typical of hepatocytes. Hepatocyte-specific genes ALB, glucose-6-phosphatase, AFP and α1-AT were expressed in the differentiated cells. The differentiated cells expressed hepatocyte markers AFP, ALB, HNF4, CK8 and CK19 at the protein level. The cells also expressed cell proliferation marker Ki67. Under electron microscopy, the ultrastructures of hepatocyte-like cells, such as mitochondrion and catalase-containing peroxisomes, were observed in the differentiated cells. ICG uptake test was positive in differentiated cells.

CONCLUSIONS

With cytokine induction, rhesus monkey ESC differentiated into cells displaying morphological features, gene expression patterns and metabolic activities characteristic of hepatocytes.

Efficient generation of hepatic cells from multipotent adult mouse germ-line stem cells using an OP9 co-culture system

On the basis of their self-renewal capacity and their ability to differentiate into derivatives of all three germ layers, germ line-derived multipotent adult stem cells (maGSCs) from mouse testis might serve as one of preferable sources for pluripotent stem cells in regenerative medicine. In our study, we aimed for an efficient hepatic differentiation protocol that is applicable for both maGSCs and embryonic stem cells (ESCs). We attempted to accomplish this goal by using a new established co-culture system with OP9 stroma cells for direct differentiation of maGSCs and ESCs into hepatic cells. We found that the hepatic differentiation of maGSCs was induced by the OP9 co-culture system in comparison to the gelatin culture. Furthermore, we showed that the combination of OP9 co-culture with activin A resulted in the increased expression of endodermal and early hepatic markers Gata4, Sox17, Foxa2, Hnf4, Afp, and Ttr compared to differentiated cells on gelatin or on OP9 alone. Moreover, the hepatic progenitors were capable of differentiating further into mature hepatic cells, demonstrated by the expression of liver-specific markers Aat, Alb, Tdo2, Krt18, Krt8, Krt19, Cps1, Sek, Cyp7a1, Otc, and Pah. A high percentage of maGSC-derived hepatic progenitors (51% AFP- and 61% DLK1-positive) and mature hepatic-like cells (26% ALB-positive) were achieved using this OP9 co-culture system. These generated hepatic cells successfully demonstrated in vitro functions associated with mature hepatocytes, including albumin and urea secretion, glycogen storage, and uptake of low-density lipoprotein. The established co-culture system for maGSCs into functional hepatic cells might serve as a suitable model to delineate the differentiation process for the generation of high numbers of mature hepatocytes in humans without genetic manipulations and make germ line-derived stem cells a potential autologous and alternative cell source for hepatic transplants in metabolic liver disorders.

Differentiation of Human Umbilical Cord Lining Membrane-Derived Mesenchymal Stem Cells into Hepatocyte-Like Cells

Background. Mesenchymal stem cells (MSCs), isolated from bone marrow, adipose tissue, and umbilical cord tissue, have been known to differentiate into hepatocyte-like cells. MSCs can also be easily obtained from umbilical cord lining membrane (CLMSCs). CLMSCs are more primitive MSCs than those isolated from other tissue sources. Objectives. The aim of this study was to investigate the in vitro differentiation of CLMSCs into hepatocyte lineage. Materials and Methods. In this study, CLMSCs were isolated through a tissue attachment method. Cells were characterized for expression of MSC-specific markers and differentiation potency. CLMSCs were induced to differentiate into hepatocytes by a simple two-step protocol. Differentiated cells were examined for the expression of hepatocyte-specific markers and hepatocyte functions. Results. CLMSCs expressed MSC-specific markers and differentiated into adipocytes and osteoblasts. RT-PCR, real-time qRT-PCR, Western blot, and immunocytochemistry analyses demonstrated that differentiated CLMSCs, having hepatocyte-like morphology, expressed several liver-specific markers, such as ALB, AFP, CK18, and CK19, at both mRNA and protein levels following hepatocyte differentiation. Furthermore, periodic acid-Schiff staining and low-density lipoprotein (LDL) uptake assay showed that differentiated cells could store glycogen and uptake LDL. Conclusion. This study demonstrated that CLMSCs can differentiate into functional hepatocyte-like cells. CLMSCs can serve as a favorable cell source for tissue engineering in the treatment of liver disease.

Setting appropriate boundaries: fate, patterning and competence at the neural plate border

The neural crest and craniofacial placodes are two distinct progenitor populations that arise at the border of the vertebrate neural plate. This border region develops through a series of inductive interactions that begins before gastrulation and progressively divide embryonic ectoderm into neural and non-neural regions, followed by the emergence of neural crest and placodal progenitors. In this review, we describe how a limited repertoire of inductive signals-principally FGFs, Wnts and BMPs-set up domains of transcription factors in the border region which establish these progenitor territories by both cross-inhibitory and cross-autoregulatory interactions. The gradual assembly of different cohorts of transcription factors that results from these interactions is one mechanism to provide the competence to respond to inductive signals in different ways, ultimately generating the neural crest and cranial placodes.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Zebrafish models of human liver development and disease

The liver performs a large number of essential synthetic and regulatory functions that are acquired during fetal development and persist throughout life. Their disruption underlies a diverse group of heritable and acquired diseases that affect both pediatric and adult patients. Although experimental analyses used to study liver development and disease are typically performed in cell culture models or rodents, the zebrafish is increasingly used to complement discoveries made in these systems. Forward and reverse genetic analyses over the past two decades have shown that the molecular program for liver development is largely conserved between zebrafish and mammals, and that the zebrafish can be used to model heritable human liver disorders. Recent work has demonstrated that zebrafish can also be used to study the mechanistic basis of acquired liver diseases. Here, we provide a comprehensive summary of how the zebrafish has contributed to our understanding of human liver development and disease.

Of plasticity and specificity: dialectics of the microenvironment and macroenvironment and the organ phenotype

The study of biological form and how it arises is the domain of the developmental biologists; but once the form is achieved, the organ poses a fascinating conundrum for all the life scientists: how are form and function maintained in adult organs throughout most of the life of the organism? That they do appears to contradict the inherently plastic nature of organogenesis during development. How do cells with the same genetic information arrive at, and maintain such different architectures and functions, and how do they keep remembering that they are different from each other? It is now clear that narratives based solely on genes and an irreversible regulatory dynamics cannot answer these questions satisfactorily, and the concept of microenvironmental signaling needs to be added to the equation. During development, cells rearrange and differentiate in response to diffusive morphogens, juxtacrine signals, and the extracellular matrix (ECM). These components, which constitute the modular microenvironment, are sensitive to cues from other tissues and organs of the developing embryo as well as from the external macroenvironment. On the other hand, once the organ is formed, these modular constituents integrate and constrain the organ architecture, which ensures structural and functional homeostasis and therefore, organ specificity. We argue here that a corollary of the above is that once the organ architecture is compromised in adults by mutations or by changes in the microenvironment such as aging or inflammation, that organ becomes subjected to the developmental and embryonic circuits in search of a new identity. But since the microenvironment is no longer embryonic, the confusion leads to cancer: hence as we have argued, tumors become new evolutionary organs perhaps in search of an elusive homeostasis.

A synthetic nanofibrillar matrix promotes in vitro hepatic differentiation of embryonic stem cells and induced pluripotent stem cells

Embryonic stem (ES) cells recapitulate normal developmental processes and serve as an attractive source for routine access to a large number of cells for research and therapies. We previously reported that ES cells cultured on M15 cells, or a synthesized basement membrane (sBM) substratum, efficiently differentiated into an endodermal fate and subsequently adopted fates of various digestive organs, such as the pancreas and liver. Here, we established a novel hepatic differentiation procedure using the synthetic nanofiber (sNF) as a cell culture scaffold. We first compared endoderm induction and hepatic differentiation between murine ES cells grown on sNF and several other substrata. The functional assays for hepatocytes reveal that the ES cells grown on sNF were directed into hepatic differentiation. To clarify the mechanisms for the promotion of ES cell differentiation in the sNF system, we focused on the function of Rac1, which is a Rho family member protein known to regulate the actin cytoskeleton. We observed the activation of Rac1 in undifferentiated and differentiated ES cells cultured on sNF plates, but not in those cultured on normal plastic plates. We also show that inhibition of Rac1 blocked the potentiating effects of sNF on endoderm and hepatic differentiation throughout the whole differentiation stages. Taken together, our results suggest that morphological changes result in cellular differentiation controlled by Rac1 activation, and that motility is not only the consequence, but is also able to trigger differentiation. In conclusion, we believe that sNF is a promising material that might contribute to tissue engineering and drug delivery.

Differentiation of hepatocytes from pluripotent stem cells

Differentiation of human embryonic stem (ES) and induced pluripotent stem (iPS) cells into hepatocyte-like cells provides a platform to study the molecular basis of human hepatocyte differentiation, to develop cell culture models of liver disease, and to potentially provide hepatocytes for treatment of end-stage liver disease. Additionally, hepatocyte-like cells generated from human pluripotent stem cells could serve as platforms for drug discovery, determination of pharmaceutical-induced hepatotoxicity, and evaluation of idiosyncratic drug-drug interactions. Here, we describe a step-wise protocol previously developed in our laboratory that facilitates the highly efficient and reproducible differentiation of human pluripotent stem cells into hepatocyte-like cells. Our protocol uses defined culture conditions and closely recapitulates key developmental events that are found to occur during hepatogenesis.

The road to regenerative liver therapies: the triumphs, trials and tribulations

The liver is one of the few organs that possess a high capacity to regenerate after liver failure or liver damage. The parenchymal cells of the liver, hepatocytes, contribute to the majority of the regeneration process. Thus, hepatocyte transplantation presents an alternative method to treating liver damage. However, shortage of hepatocytes and difficulties in maintaining primary hepatocytes still remain key obstacles that researchers must overcome before hepatocyte transplantation can be used in clinical practice. The unique properties of pluripotent stem cells (PSCs) and induced pluripotent stem cells (iPSCs) have provided an alternative approach to generating enough functional hepatocytes for cellular therapy. In this review, we will present a brief overview on the current state of hepatocyte differentiation from PSCs and iPSCs. Studies of liver regenerative processes using different cell sources (adult liver stem cells, hepatoblasts, hepatic progenitor cells, etc.) will be described in detail as well as how this knowledge can be applied towards optimizing culture conditions for the maintenance and differentiation of these cells towards hepatocytes. As the outlook of stem cell-derived therapy begins to look more plausible, researchers will need to address the challenges we must overcome in order to translate stem cell research to clinical applications.

Direct transdifferentiation of spermatogonial stem cells to morphological, phenotypic and functional hepatocyte-like cells via the ERK1/2 and Smad2/3 signaling pathways and the inactivation of cyclin A, cyclin B and cyclin E

Background

Severe shortage of liver donors and hepatocytes highlights urgent requirement of extra-liver and stem cell source of hepatocytes for treating liver-related diseases. Here we hypothesized that spermatogonial stem cells (SSCs) can directly transdifferentiate to hepatic stem-like cells capable of differentiating into mature hepatocyte-like cells in vitro without an intervening pluripotent state.

Results

SSCs first changed into hepatic stem-like cells since they resembled hepatic oval cells in morphology and expressed Ck8, Ck18, Ck7, Ck19, OV6, and albumin. Importantly, they co-expressed CK8 and CK19 but not ES cell markers. Hepatic stem-like cells derived from SSCs could differentiate into small hepatocytes based upon their morphological features and expression of numerous hepatic cell markers but lacking of bile epithelial cell hallmarks. Small hepatocytes were further coaxed to differentiate into mature hepatocyte-like cells, as identified by their morphological traits and strong expression of Ck8, Ck18, Cyp7a1, Hnf3b, Alb, Tat, Ttr, albumin, and CYP1A2 but not Ck7 or CK19. Notably, these differentiated cells acquired functional attributes of hepatocyte-like cells because they secreted albumin, synthesized urea, and uptake and released indocyanine green. Moreover, phosphorylation of ERK1/2 and Smad2/3 rather than Akt was activated in hepatic stem cells and mature hepatocytes. Additionally, cyclin A, cyclin B and cyclin E transcripts and proteins but not cyclin D1 or CDK1 and CDK2 transcripts or proteins were reduced in mature hepatocyte-like cells or hepatic stem-like cells derived from SSCs compared to SSCs.

Conclusions

SSCs can transdifferentiate to hepatic stem-like cells capable of differentiating into cells with morphological, phenotypic and functional characteristics of mature hepatocytes via the activation of ERK1/2 and Smad2/3 signaling pathways and the inactivation of cyclin A, cyclin B and cyclin E. This study thus provides an invaluable source of mature hepatocytes for treating liver-related diseases and drug toxicity screening and offers novel insights into mechanisms of liver development and cell reprogramming.

An in vitro expansion system for generation of human iPS cell-derived hepatic progenitor-like cells exhibiting a bipotent differentiation potential

Hepatoblasts, hepatic stem/progenitor cells in liver development, have a high proliferative potential and the ability to differentiate into both hepatocytes and cholangiocytes. In regenerative medicine and drug screening for the treatment of severe liver diseases, human induced pluripotent stem (iPS) cell-derived mature functional hepatocytes are considered to be a potentially good cell source. However, induction of proliferation of these cells is difficult ex vivo. To circumvent this problem, we generated hepatic progenitor-like cells from human iPS cells using serial cytokine treatments in vitro. Highly proliferative hepatic progenitor-like cells were purified by fluorescence-activated cell sorting using antibodies against CD13 and CD133 that are known cell surface markers of hepatic stem/progenitor cells in fetal and adult mouse livers. When the purified CD13^highCD133⁺ cells were cultured at a low density with feeder cells in the presence of suitable growth factors and signaling inhibitors (ALK inhibitor A-83-01 and ROCK inhibitor Y-27632), individual cells gave rise to relatively large colonies. These colonies consisted of two types of cells expressing hepatocytic marker genes (hepatocyte nuclear factor 4α and α-fetoprotein) and a cholangiocytic marker gene (cytokeratin 7), and continued to proliferate over long periods of time. In a spheroid formation assay, these cells were found to express genes required for mature liver function, such as cytochrome P450 enzymes, and secrete albumin. When these cells were cultured in a suitable extracellular matrix gel, they eventually formed a cholangiocytic cyst-like structure with epithelial polarity, suggesting that human iPS cell-derived hepatic progenitor-like cells have a bipotent differentiation ability. Collectively these data indicate that this novel procedure using an in vitro expansion system is useful for not only liver regeneration but also for the determination of molecular mechanisms that regulate liver development.

Vascularized and functional human liver from an iPSC-derived organ bud transplant

A critical shortage of donor organs for treating end-stage organ failure highlights the urgent need for generating organs from human induced pluripotent stem cells (iPSCs). Despite many reports describing functional cell differentiation, no studies have succeeded in generating a three-dimensional vascularized organ such as liver. Here we show the generation of vascularized and functional human liver from human iPSCs by transplantation of liver buds created in vitro (iPSC-LBs). Specified hepatic cells (immature endodermal cells destined to track the hepatic cell fate) self-organized into three-dimensional iPSC-LBs by recapitulating organogenetic interactions between endothelial and mesenchymal cells. Immunostaining and gene-expression analyses revealed a resemblance between in vitro grown iPSC-LBs and in vivo liver buds. Human vasculatures in iPSC-LB transplants became functional by connecting to the host vessels within 48 hours. The formation of functional vasculatures stimulated the maturation of iPSC-LBs into tissue resembling the adult liver. Highly metabolic iPSC-derived tissue performed liver-specific functions such as protein production and human-specific drug metabolism without recipient liver replacement. Furthermore, mesenteric transplantation of iPSC-LBs rescued the drug-induced lethal liver failure model. To our knowledge, this is the first report demonstrating the generation of a functional human organ from pluripotent stem cells. Although efforts must ensue to translate these techniques to treatments for patients, this proof-of-concept demonstration of organ-bud transplantation provides a promising new approach to study regenerative medicine.

The septum transversum mesenchyme induces gall bladder development

The liver, gall bladder, and ventral pancreas are formed from the posterior region of the ventral foregut. After hepatic induction, Sox17+/Pdx1+ pancreatobiliary common progenitor cells differentiate into Sox17+/Pdx1- gall bladder progenitors and Sox17-/Pdx1+ ventral pancreatic progenitors, but the cell-extrinsic signals that regulate this differentiation process are unknown. This study shows that the septum transversum mesenchyme (STM) grows in the posterior direction after E8.5, becoming adjacent to the presumptive gall bladder region, to induce gall bladder development. In this induction process, STM-derived BMP4 induces differentiation from common progenitor cells adjacent to the STM into gall bladder progenitor cells, by maintaining Sox17 expression and suppressing Pdx1 expression. Furthermore, the STM suppresses ectopic activation of the liver program in the posterior region of the ventral foregut following hepatic induction through an Fgf10/Fgfr2b/Sox9 signaling pathway. Thus, the STM plays pivotal roles in gall bladder development by both inductive and suppressive effects.

Reversal of hyperglycemia by insulin-secreting rat bone marrow- and blastocyst-derived hypoblast stem cell-like cells

β-cell replacement may efficiently cure type 1 diabetic (T1D) patients whose insulin-secreting β-cells have been selectively destroyed by autoantigen-reactive T cells. To generate insulin-secreting cells we used two cell sources: rat multipotent adult progenitor cells (rMAPC) and the highly similar rat extra-embryonic endoderm precursor (rXEN-P) cells isolated under rMAPC conditions from blastocysts (rHypoSC). rMAPC/rHypoSC were sequentially committed to definitive endoderm, pancreatic endoderm, and β-cell like cells. On day 21, 20% of rMAPC/rHypoSC progeny expressed Pdx1 and C-peptide. rMAPCr/HypoSC progeny secreted C-peptide under the stimulus of insulin agonist carbachol, and was inhibited by the L-type voltage-dependent calcium channel blocker nifedipine. When rMAPC or rHypoSC differentiated d21 progeny were grafted under the kidney capsule of streptozotocin-induced diabetic nude mice, hyperglycemia reversed after 4 weeks in 6/10 rMAPC- and 5/10 rHypoSC-transplanted mice. Hyperglycemia recurred within 24 hours of graft removal and the histological analysis of the retrieved grafts revealed presence of Pdx1-, Nkx6.1- and C-peptide-positive cells. The ability of both rMAPC and HypoSC to differentiate to functional β-cell like cells may serve to gain insight into signals that govern β-cell differentiation and aid in developing culture systems to commit other (pluripotent) stem cells to clinically useful β-cells for cell therapy of T1D.

EpCAM is an endoderm-specific Wnt derepressor that licenses hepatic development

Mechanisms underlying cell-type-specific response to morphogens or signaling molecules during embryonic development are poorly understood. To learn how response to the liver-inductive Wnt2bb signal is achieved, we identify an endoderm-enriched, single transmembrane protein, epithelial-cell-adhesion-molecule (EpCAM), as an endoderm-specific Wnt derepressor in zebrafish. hi2151/epcam mutants exhibit defective liver development similar to prt/wnt2bb mutants. EpCAM directly binds to Kremen1 and disrupts the Kremen1-Dickkopf2 (Dkk2) interaction, which prevents Kremen1-Dkk2-mediated removal of Lipoprotein-receptor-related protein 6 (Lrp6) from the cell surface. These data lead to a model in which EpCAM derepresses Lrp6 and cooperates with Wnt ligand to activate Wnt signaling through stabilizing membrane Lrp6 and allowing Lrp6 clustering into active signalosomes. Thus, EpCAM cell autonomously licenses and cooperatively activates Wnt2bb signaling in endodermal cells. Our results identify EpCAM as the key molecule and its functional mechanism to confer endodermal cells the competence to respond to the liver-inductive Wnt2bb signal.

Cancer stem cells in the development of liver cancer

Liver cancer is an aggressive disease with a poor outcome. Several hepatic stem/progenitor markers are useful for isolating a subset of liver cells with stem cell features, known as cancer stem cells (CSCs). These cells are responsible for tumor relapse, metastasis, and chemoresistance. Liver CSCs dictate a hierarchical organization that is shared in both organogenesis and tumorigenesis. An increased understanding of the molecular signaling events that regulate cellular hierarchy and stemness, and success in defining key CSC-specific genes, have opened up new avenues to accelerate the development of novel diagnostic and treatment strategies. This Review highlights recent advances in understanding the pathogenesis of liver CSCs and discusses unanswered questions about the concept of liver CSCs.

Cellular and molecular basis of liver development

Liver is a prime organ responsible for synthesis, metabolism, and detoxification. The organ is endodermal in origin and its development is regulated by temporal, complex, and finely balanced cellular and molecular interactions that dictate its origin, growth, and maturation. We discuss the relevance of endoderm patterning, which truly is the first step toward mapping of domains that will give rise to specific organs. Once foregut patterning is completed, certain cells within the foregut endoderm gain competence in the form of expression of certain transcription factors that allow them to respond to certain inductive signals. Hepatic specification is then a result of such inductive signals, which often emanate from the surrounding mesenchyme. During hepatic specification bipotential hepatic stem cells or hepatoblasts become apparent and undergo expansion, which results in a visible liver primordium during the stage of hepatic morphogenesis. Hepatoblasts next differentiate into either hepatocytes or cholangiocytes. The expansion and differentiation is regulated by cellular and molecular interactions between hepatoblasts and mesenchymal cells including sinusoidal endothelial cells, stellate cells, and also innate hematopoietic elements. Further maturation of hepatocytes and cholangiocytes continues during late hepatic development as a function of various growth factors. At this time, liver gains architectural novelty in the form of zonality and at cellular level acquires polarity. A comprehensive elucidation of such finely tuned developmental cues have been the basis of transdifferentiation of various types of stem cells to hepatocyte-like cells for purposes of understanding health and disease and for therapeutic applications.

Effect of different liver resection methods on liver damage and regeneration factors VEGF and FGF-2 in mice

BACKGROUND

Different approaches to study liver regeneration in murine models have been proposed. We investigated the effect of different liver resection models on liver damage and regeneration parameters in mice.

METHODS

We compared the technical aspect of the 2 most commonly used techniques of 50% and 70% liver resection. Liver damage, as determined by the change in serum alanine aminotransferase and aspartate aminotransferase, as well as the regeneration parameters VEGF and FGF-2 were analyzed at 6 time points. A postoperative vitality score was introduced.

RESULTS

Cholestasis was not observed for either technique. Both resection techniques resulted in full weight recovery of the liver after 240 hours, with no significant difference between sham and resection groups. Postoperative animal morbidity and total protein levels did not differ significantly for either method, indicating early and full functional recovery. However, comparing the mitogenic growth factors FGF-2 and VEGF, a significant increase in serum levels and, therefore, increased growth stimulus, was shown in the extended resection group.

CONCLUSION

Extended resection led to a greater response in growth factor expression. This finding is important since it shows that growth factor response differs acdording to the extent of resection. We have demonstrated the need to standardize murine hepatic resection models to adequately compare the resulting liver damage.