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

LATS-YAP/TAZ controls lineage specification by regulating TGFβ signaling and Hnf4α expression during liver development

The Hippo pathway regulates the self-renewal and differentiation of various adult stem cells, but its role in cell fate determination and differentiation during liver development remains unclear. Here we report that the Hippo pathway controls liver cell lineage specification and proliferation separately from Notch signalling, using mice and primary hepatoblasts with liver-specific knockout of Lats1 and Lats2 kinase, the direct upstream regulators of YAP and TAZ. During and after liver development, the activation of YAP/TAZ induced by loss of Lats1/2 forces hepatoblasts or hepatocytes to commit to the biliary epithelial cell (BEC) lineage. It increases BEC and fibroblast proliferation by up-regulating TGFβ signalling, but suppresses hepatoblast to hepatocyte differentiation by repressing Hnf4α expression. Notably, oncogenic YAP/TAZ activation in hepatocytes induces massive p53-dependent cell senescence/death. Together, our results reveal that YAP/TAZ activity levels govern liver cell differentiation and proliferation in a context-dependent manner.

The Hippo pathway regulates the differentiation of stem and progenitor cells, but it is unclear how it acts in liver development. Here, the authors knockout Hippo pathway components Lats1 and 2 in the liver, causing suppression of hepatocyte differentiation but promoting biliary cell differentiation.

The Involving Roles of Intrahepatic and Extrahepatic Stem/Progenitor Cells (SPCs) to Liver Regeneration

Liver regeneration is usually attributed to mature hepatocytes, which possess a remarkable potential to proliferate under mild to moderate injury. However, when the liver is severely damaged or hepatocyte proliferation is greatly inhibited, liver stem/progenitor cells (LSPCs) will contribute to the liver regeneration process. LSPCs in the developing liver have been extensively characterized, however, their contributing role to liver regeneration has not been completely understood. In addition to the restoration of the liver parenchymal tissue by hepatocytes or/and LSPCs, or in some cases bone marrow (BM) derived cells, such as hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs), the wound healing after injury in terms of angiopoiesis by liver sinusoidal endothelial cells (LSECs) or/and sinusoidal endothelial progenitor cells (SEPCs) is another important aspect taking place during regeneration. To conclude, liver regeneration can be mainly divided into three distinct restoring levels according to the cause and severity of injury: hepatocyte dominant regeneration, LSPCs mediated regeneration, extrahepatic stem cells participative regeneration. In this review, we focus on the recent findings of liver regeneration, especially on those related to stem/progenitor cells (SPCs)-mediated regeneration and their potential clinical applications and challenges.

Hybrid Periportal Hepatocytes Regenerate the Injured Liver without Giving Rise to Cancer

Compensatory proliferation triggered by hepatocyte loss is required for liver regeneration and maintenance but also promotes development of hepatocellular carcinoma (HCC). Despite extensive investigation, the cells responsible for hepatocyte restoration or HCC development remain poorly characterized. We used genetic lineage tracing to identify cells responsible for hepatocyte replenishment following chronic liver injury and queried their roles in three distinct HCC models. We found that a pre-existing population of periportal hepatocytes, located in the portal triads of healthy livers and expressing low amounts of Sox9 and other bile-duct-enriched genes, undergo extensive proliferation and replenish liver mass after chronic hepatocyte-depleting injuries. Despite their high regenerative potential, these so-called hybrid hepatocytes do not give rise to HCC in chronically injured livers and thus represent a unique way to restore tissue function and avoid tumorigenesis. This specialized set of pre-existing differentiated cells may be highly suitable for cell-based therapy of chronic hepatocyte-depleting disorders.

Human ductal plate and its derivatives express antigens of cholangiocellular, hepatocellular, hepatic stellate/progenitor cell, stem cell, and neuroendocrine lineages, and proliferative antigens

Molecular mechanisms of human ductal plate (DP) development and differentiation (DD) are unclear. The author immunohistochemically investigated expressions of cholangiocellular antigens (CEA, CA19-9, EMA, MUC1, MUC2, MUC5AC, MUC6, mucins, CK7, and CK19), hepatocellular antigens (HepPar1, AFP, CK8, and CK18), hepatic stellate/progenitor cell (HSC) antigens or stem cell (SC) antigens (C-erbB2, CD56, chromogranin, synaptophysin, bcl2, NSE, NCAM, KIT, and PDGFRA), and proliferating antigen (Ki67) in 32 human fetal livers (HFL). The DD of human intrahepatic bile duct (IBD) could be categorized into four stages: DP, remodeling DP, remodeled DP, and immature IBD. All the molecules examined were expressed in the DP and DP derivatives. These results suggest that human DP or DP derivatives have capacities to differentiate into cholangiocellular, hepatocellular, HSC, SC, and neuroendocrine lineages. The data also suggest that NCAM, KIT/SC factor-signaling, NSE, HGF/MET signaling, PDGFa/PDGFRA signaling, chromogranin, synaptophysin, and CD56 play important roles in DD of DP and biliary cells of HFL. DP, DP derivatives, and IBD in HFL have proliferative capacity.

Combinatorial microenvironmental regulation of liver progenitor differentiation by Notch ligands, TGFβ, and extracellular matrix

The bipotential differentiation of liver progenitor cells underlies liver development and bile duct formation as well as liver regeneration and disease. TGFβ and Notch signaling are known to play important roles in the liver progenitor specification process and tissue morphogenesis. However, the complexity of these signaling pathways and their currently undefined interactions with other microenvironmental factors, including extracellular matrix (ECM), remain barriers to complete mechanistic understanding. Utilizing a series of strategies, including co-cultures and cellular microarrays, we identified distinct contributions of different Notch ligands and ECM proteins in the fate decisions of bipotential mouse embryonic liver (BMEL) progenitor cells. In particular, we demonstrated a cooperative influence of Jagged-1 and TGFβ1 on cholangiocytic differentiation. We established ECM-specific effects using cellular microarrays consisting of 32 distinct combinations of collagen I, collagen III, collagen IV, fibronectin, and laminin. In addition, we demonstrated that exogenous Jagged-1, Delta-like 1, and Delta-like 4 within the cellular microarray format was sufficient for enhancing cholangiocytic differentiation. Further, by combining Notch ligand microarrays with shRNA-based knockdown of Notch ligands, we systematically examined the effects of both cell-extrinsic and cell-intrinsic ligand. Our results highlight the importance of divergent Notch ligand function and combinatorial microenvironmental regulation in liver progenitor fate specification.

Mechanism of pancreatic and liver malformations in human fetuses with short-rib polydactyly syndrome

BACKGROUND

The short-rib polydactyly (SRP) syndromes are rare skeletal dysplasias caused by abnormalities in primary cilia, sometimes associated with visceral malformations.

METHODS

The pathogenesis of ductal plate malformation (DPM) varies in different syndromes and has not been investigated in SRP. We have studied liver development in five SRP fetuses and pancreatic development in one SRP fetus, with genetically confirmed mutations in cilia related genes, with and without DPMs, using the immunoperoxidase technique, and compared these to other syndromes with DPM.

RESULTS

Acetylated tubulin expression was abnormal in DPM in SRP, Meckel syndrome, and autosomal recessive polycystic kidney disease (ARPKD), confirming ciliary anomalies. SDF-1 was abnormally expressed in SRP and two of three cases of autosomal dominant polycystic kidney disease (ADPKD) but not ARPKD or Meckel. Increased density of quiescent hepatic stellate cells was seen in SRP, Meckel, one of three cases of ARPKD, and two of three cases of ADPKD with aberrant hepatocyte expression of keratin 19 in SRP and ADPKD. Immunophenotypic abnormalities were present even in fetal liver without fully developed DPMs. The SRP case with DPM and pancreatic malformations showed abnormalities in the pancreatic head (influenced by mesenchyme from the septum transversum, similar to liver) but not pancreatic body (influenced by mesenchyme adjacent to the notochord).

CONCLUSION

In SRP, there are differentiation defects of hepatocytes, cholangiocytes, and liver mesenchyme and, in rare cases, pancreatic mesenchymal anomalies. The morphological changes were subtle in early gestation but immunophenotypic abnormalities were present. Mesenchymal-epithelial interactions may contribute to the malformations. Birth Defects Research (Part A) 106:549-562, 2016. © 2016 Wiley Periodicals, Inc.

The role of IL-17 signaling in regulation of the liver-brain axis and intestinal permeability in Alcoholic Liver Disease

Alcoholic liver disease (ALD) progresses from a normal liver, to steatosis, steatohepatitis, fibrosis and hepatocellular carcinoma (HCC). Despite intensive studies, the pathogenesis of ALD is poorly understood, in part due to a lack of suitable animal models which mimic the stages of ALD progression. Furthermore, the role of IL-17 in ALD has not been evaluated. We and others have recently demonstrated that IL-17 signaling plays a critical role in development of liver fibrosis and cancer. Here we summarize the most recent evidence supporting the role of IL-17 in ALD. As a result of a collaborative effort of Drs. Karin, Gao, Tsukamoto and Kisseleva, we developed several improved models of ALD in mice: 1) chronic-plus-binge model that mimics early stages of steatohepatitis, 2) intragastric ethanol feeding model that mimics alcoholic steatohepatitis and fibrosis, and 3) diethylnitrosamine (DEN)+alcohol model that mimics alcoholic liver cancer. These models might provide new insights into the mechanism of IL-17 signaling in ALD and help identify novel therapeutic targets.

Role of β-catenin in development of bile ducts

Beta-catenin is known to play stage- and cell-specific functions during liver development. However, its role in development of bile ducts has not yet been addressed. Here we used stage-specific in vivo gain- and loss-of-function approaches, as well as lineage tracing experiments in the mouse, to first demonstrate that β-catenin is dispensable for differentiation of liver precursor cells (hepatoblasts) to cholangiocyte precursors. Second, when β-catenin was depleted in the latter, maturation of cholangiocytes, bile duct morphogenesis and differentiation of periportal hepatocytes from cholangiocyte precursors was normal. In contrast, stabilization of β-catenin in cholangiocyte precursors perturbed duct development and cholangiocyte differentiation. We conclude that β-catenin is dispensable for biliary development but that its activity must be kept within tight limits. Our work is expected to significantly impact on in vitro differentiation of stem cells to cholangiocytes for toxicology studies and disease modeling.

The role of liver progenitor cells during liver regeneration, fibrogenesis, and carcinogenesis

The growing worldwide challenge of cirrhosis and hepatocellular carcinoma due to increasing prevalence of excessive alcohol consumption, viral hepatitis, obesity, and the metabolic syndrome has sparked interest in stem cell-like liver progenitor cells (LPCs) as potential candidates for cell therapy and tissue engineering, as an alternative approach to whole organ transplantation. However, LPCs always proliferate in chronic liver diseases with a predisposition to cancer; they have been suggested to play major roles in driving fibrosis, disease progression, and may even represent tumor-initiating cells. Hence, a greater understanding of the factors that govern their activation, communication with other hepatic cell types, and bipotential differentiation as opposed to their potential transformation is needed before their therapeutic potential can be harnessed.

Ductal Plate Malformation in the Liver of Boxer Dogs: Clinical and Histological Features

Ductal plate malformations (DPMs) represent developmental biliary disorders with a wide phenotypic spectrum. This study characterizes DPM in 30 Boxer dogs. Median age was 1.5 (range, 0.3-10.0) years, with 12 dogs <1 year. Clinical features included increased serum levels of liver enzymes (28), gastrointestinal signs (16), poor body condition (14), abdominal effusion (9), and hepatic encephalopathy (2). Additional malformations included gallbladder atresia (8), atrophied left liver (2), absent quadrate lobe with left-displaced gallbladder (1), portal vasculature atresia (left liver, 1), intrahepatic portosystemic shunt (1), and complex intrahepatic arteriovenous malformation (1). All dogs had portal tracts dimensionally expanded by a moderate-to-severe multiple small bile duct phenotype embedded in abundant extracellular matrix; 80% displayed variable portal-to-portal bridging. Quantitative analysis confirmed significantly increased fibrillar collagen and a 3-fold increased portal tract area relative to 6 Boxer and 10 non-Boxer controls. Biliary phenotype was dominated by tightly formed CK19-positive ductules, typically 10 to 15 μm in diameter, with 3 to >30 profiles per portal tract, reduced luminal apertures, and negative Ki-67 immunoreactivity. CK19-positive biliary epithelium intersected directly with zone 1 hepatocytes as a signature feature when considered with other DPM characteristics. Phenotypic variation included a multiple small bile duct phenotype (all dogs), predominantly thin-walled sacculated ducts (4), well-formed saccular ducts (4), and sacculated segmental, interlobular, and intralobular ducts (Caroli malformation, 2 dogs, one with bridging portal fibrosis). Histologic evidence of portal venous hypoperfusion accompanied increased biliary profiles in every case. We propose that this spectrum of disorders be referred to as DPM with appropriate modifiers to characterize the unique phenotypes.

Stem/Progenitor Cell Niches Involved in Hepatic and Biliary Regeneration

Niches containing stem/progenitor cells are present in different anatomical locations along the human biliary tree and within liver acini. The most primitive stem/progenitors, biliary tree stem/progenitor cells (BTSCs), reside within peribiliary glands located throughout large extrahepatic and intrahepatic bile ducts. BTSCs are multipotent and can differentiate towards hepatic and pancreatic cell fates. These niches' matrix chemistry and other characteristics are undefined. Canals of Hering (bile ductules) are found periportally and contain hepatic stem/progenitor cells (HpSCs), participating in the renewal of small intrahepatic bile ducts and being precursors to hepatocytes and cholangiocytes. The niches also contain precursors to hepatic stellate cells and endothelia, macrophages, and have a matrix chemistry rich in hyaluronans, minimally sulfated proteoglycans, fetal collagens, and laminin. The microenvironment furnishes key signals driving HpSC activation and differentiation. Newly discovered third niches are pericentral within hepatic acini, contain Axin2+ unipotent hepatocytic progenitors linked on their lateral borders to endothelia forming the central vein, and contribute to normal turnover of mature hepatocytes. Their relationship to the other stem/progenitors is undefined. Stem/progenitor niches have important implications in regenerative medicine for the liver and biliary tree and in pathogenic processes leading to diseases of these tissues.

Pathogenesis of Type 2 Epithelial to Mesenchymal Transition (EMT) in Renal and Hepatic Fibrosis

Epithelial to mesenchymal transition (EMT), particularly, type 2 EMT, is important in progressive renal and hepatic fibrosis. In this process, incompletely regenerated renal epithelia lose their epithelial characteristics and gain migratory mesenchymal qualities as myofibroblasts. In hepatic fibrosis (importantly, cirrhosis), the process also occurs in injured hepatocytes and hepatic progenitor cells (HPCs), as well as ductular reaction-related bile epithelia. Interestingly, the ductular reaction contributes partly to hepatocarcinogenesis of HPCs, and further, regenerating cholangiocytes after injury may be derived from hepatic stellate cells via mesenchymal to epithelia transition, a reverse phenomenon of type 2 EMT. Possible pathogenesis of type 2 EMT and its differences between renal and hepatic fibrosis are reviewed based on our experimental data.

LKB1 and Notch Pathways Interact and Control Biliary Morphogenesis

Background

LKB1 is an evolutionary conserved kinase implicated in a wide range of cellular functions including inhibition of cell proliferation, regulation of cell polarity and metabolism. When Lkb1 is inactivated in the liver, glucose homeostasis is perturbed, cellular polarity is affected and cholestasis develops. Cholestasis occurs as a result from deficient bile duct development, yet how LKB1 impacts on biliary morphogenesis is unknown.

Methodology/Principal Findings

We characterized the phenotype of mice in which deletion of the Lkb1 gene has been specifically targeted to the hepatoblasts. Our results confirmed that lack of LKB1 in the liver results in bile duct paucity leading to cholestasis. Immunostaining analysis at a prenatal stage showed that LKB1 is not required for differentiation of hepatoblasts to cholangiocyte precursors but promotes maturation of the primitive ductal structures to mature bile ducts. This phenotype is similar to that obtained upon inactivation of Notch signaling in the liver. We tested the hypothesis of a functional overlap between the LKB1 and Notch pathways by gene expression profiling of livers deficient in Lkb1 or in the Notch mediator RbpJκ and identified a mutual cross-talk between LKB1 and Notch signaling. In vitro experiments confirmed that Notch activity was deficient upon LKB1 loss.

Conclusion

LKB1 and Notch share a common genetic program in the liver, and regulate bile duct morphogenesis.

Advances in Liver Regeneration: Revisiting Hepatic Stem/Progenitor Cells and Their Origin

The liver has evolved to become a highly plastic organ with extraordinary regenerative capabilities. What drives liver regeneration is still being debated. Adult liver stem/progenitor cells have been characterized and used to produce functional hepatocytes and biliary cells in vitro. However, in vivo, numerous studies have questioned whether hepatic progenitor cells have a significant role in liver regeneration. Mature hepatocytes have recently been shown to be more plastic than previously believed and give rise to new hepatocytes after acute and chronic injury. In this review, we discuss current knowledge in the field of liver regeneration and the importance of the serotonin pathway as a clinical target for patients with liver dysfunction.

Diverse routes to liver regeneration

The liver's ability to regenerate is indisputable; for example, after a two-thirds partial hepatectomy in rats all residual hepatocytes can divide, questioning the need for a specific stem cell population. On the other hand, there is a potential stem cell compartment in the canals of Hering, giving rise to ductular reactions composed of hepatic progenitor cells (HPCs) when the liver's ability to regenerate is hindered by replicative senescence, but the functional relevance of this response has been questioned. Several papers have now clarified regenerative mechanisms operative in the mouse liver, suggesting that the liver is possibly unrivalled in its versatility to replace lost tissue. Under homeostatic conditions a perivenous population of clonogenic hepatocytes operates, whereas during chronic damage a minor population of periportal clonogenic hepatocytes come to the fore, while the ability of HPCs to completely replace the liver parenchyma has now been shown.

Two Fresh Streams to Fill the Liver's Hepatocyte Pool

Differentiated hepatocytes are known for their capacity to regenerate the adult liver. Two papers discussed here show that distinct subpopulations of adult hepatocytes may be the crucial populations underpinning such regeneration.

Liver Stem Cells: Experimental Findings and Implications for Human Liver Disease

Evidence from human histopathology and experimental studies with rodents and zebrafish has shown that hepatocytes and cholangiocytes may function as facultative stem cells for each other in conditions of impaired regeneration. The interpretation of the findings derived from these studies has generated considerable discussion and some controversies. This review examines the evidence obtained from the different experimental models and considers implications that these studies may have for human liver disease.

Modeling mouse and human development using organoid cultures

In vitro three-dimensional (3D) cultures are emerging as novel systems with which to study tissue development, organogenesis and stem cell behavior ex vivo. When grown in a 3D environment, embryonic stem cells (ESCs) self-organize into organoids and acquire the right tissue patterning to develop into several endoderm- and ectoderm-derived tissues, mimicking their in vivo counterparts. Tissue-resident adult stem cells (AdSCs) also form organoids when grown in 3D and can be propagated in vitro for long periods of time. In this Review, we discuss recent advances in the generation of pluripotent stem cell- and AdSC-derived organoids, highlighting their potential for enhancing our understanding of human development. We will also explore how this new culture system allows disease modeling and gene repair for a personalized regenerative medicine approach.

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.

Hepatic oval cells and liver diseases

In recent years, hepatic oval cells (HOC) have gradually become a research hotspot, and their participation in the reconstruction of liver structure and function has been preliminarily confirmed. This provides a new direction for the study of the pathogenesis and treatment of liver injury, hepatitis, liver fibrosis, cirrhosis, liver neoplasms and other liver diseases. This paper will discuss the relationship between hepatic oval cells and liver diseases.

Cell lineage tracing in human epithelial tissues using mitochondrial DNA mutations as clonal markers

The study of cell lineages through heritable genetic lineage tracing is well established in experimental animals, particularly mice. While such techniques are not feasible in humans, we have taken advantage of the fact that the mitochondrial genome is highly prone to nonpathogenic mutations and such mutations can be used as clonal markers to identify stem cell derived clonal populations in human tissue sections. A mitochondrial DNA (mtDNA) mutation can spread by a stochastic process through the several copies of the circular genome in a single mitochondrion, and then through the many mitochondria in a single cell, a process called 'genetic drift.' This process takes many years and so is likely to occur only in stem cells, but once established, the fate of stem cell progeny can be followed. A cell having at least 80% of its mtDNA genomes bearing the mutation results in a demonstrable deficiency in mtDNA-encoded cytochrome c oxidase (CCO), optimally detected in frozen tissue sections by dual-color histochemistry, whereby CCO activity stains brown and CCO deficiency is highlighted by subsequent succinate dehydrogenase activity, staining the CCO-deficient areas blue. Cells with CCO deficiency can be laser captured and subsequent mtDNA sequencing can ascertain the nature of the mutation. If all cells in a CCO-deficient area have an identical mutation, then a clonal population has been identified; the chances of the same mutation initially arising in separate cells are highly improbable. The technique lends itself to the study of both normal epithelia and can answer several questions in tumor biology. WIREs Dev Biol 2016, 5:103-117. doi: 10.1002/wdev.203 For further resources related to this article, please visit the WIREs website.

Proliferation-Independent Initiation of Biliary Cysts in Polycystic Liver Diseases

Biliary cysts in adult patients affected by polycystic liver disease are lined by cholangiocytes that proliferate, suggesting that initiation of cyst formation depends on proliferation. Here, we challenge this view by analyzing cyst-lining cell proliferation and differentiation in Cpk mouse embryos and in livers from human fetuses affected by Autosomal Recessive Polycystic Kidney Disease (ARPKD), at early stages of cyst formation. Proliferation of fetal cholangiocyte precursors, measured by immunostaining in human and mouse livers, was low and did not differ between normal and ARPKD or Cpk livers, excluding excessive proliferation as an initiating cause of liver cysts. Instead, our analyses provide evidence that the polycystic livers exhibit increased and accelerated differentiation of hepatoblasts into cholangiocyte precursors, eventually coalescing into large biliary cysts. Lineage tracing experiments, performed in mouse embryos, indicated that the cholangiocyte precursors in Cpk mice generate cholangiocytes and periportal hepatocytes, like in wild-type animals. Therefore, contrary to current belief, cyst formation in polycystic liver disease does not necessarily depend on overproliferation. Combining our prenatal data with available data from adult livers, we propose that polycystic liver can be initiated by proliferation-independent mechanisms at a fetal stage, followed by postnatal proliferation-dependent cyst expansion.

Determining the fate of hepatic cells by lineage tracing: facts and pitfalls

Slow renewal of the epithelial cells by proliferation ensures homeostasis of the liver, but extensive proliferation may occur upon injury. When proliferation is impaired, transdifferentiation of mature cells or differentiation of stem cells allows production of new hepatocytes and cholangiocytes. While lineage tracings using cyclization recombinase (Cre) recombinase-mediated cell labeling represent the gold standard for defining cell fate, there are more variables than was initially realized. This led to controversies about the capacity of liver cells to switch their fate. Here, I review how cells are traced in the liver and highlight the experimental pitfalls that may cause misinterpretations and controversies.

Transcription factors SOX4 and SOX9 cooperatively control development of bile ducts

In developing liver, cholangiocytes derive from the hepatoblasts and organize to form the bile ducts. Earlier work has shown that the SRY-related High Mobility Group box transcription factor 9 (SOX9) is transiently required for bile duct development, raising the question of the potential involvement of other SOX family members in biliary morphogenesis. Here we identify SOX4 as a new regulator of cholangiocyte development. Liver-specific inactivation of SOX4, combined or not with inactivation of SOX9, affects cholangiocyte differentiation, apico-basal polarity and bile duct formation. Both factors cooperate to control the expression of mediators of the Transforming Growth Factor-β, Notch, and Hippo-Yap signaling pathways, which are required for normal development of the bile ducts. In addition, SOX4 and SOX9 control formation of primary cilia, which are known signaling regulators. The two factors also stimulate secretion of laminin α5, an extracellular matrix component promoting bile duct maturation. We conclude that SOX4 is a new regulator of liver development and that it exerts a pleiotropic control on bile duct development in cooperation with SOX9.

Lineage fate of ductular reactions in liver injury and carcinogenesis

Ductular reactions (DRs) are observed in virtually all forms of human liver disease; however, the histogenesis and function of DRs in liver injury are not entirely understood. It is widely believed that DRs contain bipotential liver progenitor cells (LPCs) that serve as an emergency cell pool to regenerate both cholangiocytes and hepatocytes and may eventually give rise to hepatocellular carcinoma (HCC). Here, we used a murine model that allows highly efficient and specific lineage labeling of the biliary compartment to analyze the histogenesis of DRs and their potential contribution to liver regeneration and carcinogenesis. In multiple experimental and genetic liver injury models, biliary cells were the predominant precursors of DRs but lacked substantial capacity to produce new hepatocytes, even when liver injuries were prolonged up to 12 months. Genetic modulation of NOTCH and/or WNT/β-catenin signaling within lineage-tagged DRs impaired DR expansion but failed to redirect DRs from biliary differentiation toward the hepatocyte lineage. Further, lineage-labeled DRs did not produce tumors in genetic and chemical HCC mouse models. In summary, we found no evidence in our system to support mouse biliary-derived DRs as an LPC pool to replenish hepatocytes in a quantitatively relevant way in injury or evidence that DRs give rise to HCCs.

Dynamic three-dimensional morphogenesis of intrahepatic bile ducts in mouse liver development

During liver development, biliary epithelial cells differentiated from bipotential hepatic progenitor cells (hepatoblasts) form a cell layer, called the ductal plate surrounding portal veins (PVs), and develop into intrahepatic bile ducts (IBDs) following developmental programs. Because IBDs make duct structures in the liver, it is necessary to perform sequential and three-dimensional (3D) analyses from the early stages of liver development to address the process of morphogenesis in detail. However, to date, the development of IBDs has mainly been investigated using tissue sections in two-dimensional planes, and examinations of the 3D morphogenesis and quantitative analyses based on morphometrics have not been performed. Therefore, in this study, we simulated the solid structures of IBDs from mouse embryos to adults in silico, analyzed the subjects for the length and number of developing duct structures, number of predicted connections, and discrete distance from the PV, and examined the developmental process of the IBD in detail in a quantitative manner.

CONCLUSIONS

Through quantitative analyses with spatiotemporal observations using a 3D structural reconstruction model and morphometrics, we succeeded in constructing a 3D dynamic model of bile duct formation. Because the 3D reconstruction technique used in this study is available for analyzing solid structures in tissues that are difficult to approach, it shows promise for wide use in the fields of biology and medicine.

EpCAM and the biology of hepatic stem/progenitor cells

Epithelial cell adhesion molecule (EpCAM) is a transmembrane glycoprotein, which is frequently and highly expressed on carcinomas, tumor-initiating cells, selected tissue progenitors, and embryonic and adult stem cells. During liver development, EpCAM demonstrates a dynamic expression, since it can be detected in fetal liver, including cells of the parenchyma, whereas mature hepatocytes are devoid of EpCAM. Liver regeneration is associated with a population of EpCAM-positive cells within ductular reactions, which gradually lose the expression of EpCAM along with maturation into hepatocytes. EpCAM can be switched on and off through a wide panel of strategies to fine-tune EpCAM-dependent functional and differentiative traits. EpCAM-associated functions relate to cell-cell adhesion, proliferation, maintenance of a pluripotent state, regulation of differentiation, migration, and invasion. These functions can be conferred by the full-length protein and/or EpCAM-derived fragments, which are generated upon regulated intramembrane proteolysis. Control by EpCAM therefore not only depends on the presence of full-length EpCAM at cellular membranes but also on varying rates of the formation of EpCAM-derived fragments that have their own regulatory properties and on changes in the association of EpCAM with interaction partners. Thus spatiotemporal localization of EpCAM in immature liver progenitors, transit-amplifying cells, and mature liver cells will decisively impact the regulation of EpCAM functions and might be one of the triggers that contributes to the adaptive processes in stem/progenitor cell lineages. This review will summarize EpCAM-related molecular events and how they relate to hepatobiliary differentiation and regeneration.

Advances in hepatic stem/progenitor cell biology

The liver is famous for its strong regenerative capacity, employing different modes of regeneration according to type and extent of injury. Mature liver cells are able to proliferate in order to replace the damaged tissue allowing the recovery of the parenchymal function. In more severe scenarios hepatocytes are believed to arise also from a facultative liver progenitor cell compartment. In human, severe acute liver failure and liver cirrhosis are also both important clinical targets in which regeneration is impaired, where the role of this stem cell compartment seems more convincing. In animal models, the current state of ambiguity regarding the identity and role of liver progenitor cells in liver physiology dampens the enthusiasm for the potential use of these cells in regenerative medicine. The aim of this review is to give the basics of liver progenitor cell biology and discuss recent results vis-à-vis their identity and contribution to liver regeneration.

MBSJ MCC Young Scientist Award 2012 Liver regeneration: a unique and flexible reaction depending on the type of injury

The liver can be thought of as a mysterious organ, because it has an elegant regenerative capability. This phenomenon has been well known since ancient times and is already applied to medical treatments for severe hepatic disorders by transplanting portions of liver received from living donors. However, it was not until quite recently that the mechanism underlying the principle of liver regeneration was investigated more deeply. Recent advances in the technologies for characterizing cell properties and examining the molecular nature of cells are enabling us to understand what occurs in the regenerating liver. After acute liver damage, hepatocytes actively proliferate in response to external stimulation by humoral factors. However, in the chronically injured liver, hepatocytes cannot proliferate well, but biliary cells appearing after chronic liver damage form primitive ductules around portal veins of the liver. These biliary cells may have a multiple origin, including hepatocytes, and contain progenitor cells giving rise to both hepatocytes and biliary cells, or represent cells that can be directly converted into hepatocytes. Although liver regeneration is more complicated than we had thought, unremitting efforts by researchers will certainly connect the numerous findings obtained in basic research with the development of new therapeutic strategies for liver diseases.

The versatile functions of Sox9 in development, stem cells, and human diseases

The transcription factor Sox9 was first discovered in patients with campomelic dysplasia, a haploinsufficiency disorder with skeletal deformities caused by dysregulation of Sox9 expression during chondrogenesis. Since then, its role as a cell fate determiner during embryonic development has been well characterized; Sox9 expression differentiates cells derived from all three germ layers into a large variety of specialized tissues and organs. However, recent data has shown that ectoderm- and endoderm-derived tissues continue to express Sox9 in mature organs and stem cell pools, suggesting its role in cell maintenance and specification during adult life. The versatility of Sox9 may be explained by a combination of post-transcriptional modifications, binding partners, and the tissue type in which it is expressed. Considering its importance during both development and adult life, it follows that dysregulation of Sox9 has been implicated in various congenital and acquired diseases, including fibrosis and cancer. This review provides a summary of the various roles of Sox9 in cell fate specification, stem cell biology, and related human diseases. Ultimately, understanding the mechanisms that regulate Sox9 will be crucial for developing effective therapies to treat disease caused by stem cell dysregulation or even reverse organ damage.

Polycystic liver diseases: advanced insights into the molecular mechanisms

Polycystic liver diseases are genetic disorders characterized by progressive bile duct dilatation and/or cyst development. The large volume of hepatic cysts causes different symptoms and complications such as abdominal distension, local pressure with back pain, hypertension, gastro-oesophageal reflux and dyspnea as well as bleeding, infection and rupture of the cysts. Current therapeutic strategies are based on surgical procedures and pharmacological management, which partially prevent or ameliorate the disease. However, as these treatments only show short-term and/or modest beneficial effects, liver transplantation is the only definitive therapy. Therefore, interest in understanding the molecular mechanisms involved in disease pathogenesis is increasing so that new targets for therapy can be identified. In this Review, the genetic mechanisms underlying polycystic liver diseases and the most relevant molecular pathways of hepatic cystogenesis are discussed. Moreover, the main clinical and preclinical studies are highlighted and future directions in basic as well as clinical research are indicated.

Downregulation of miR122 by grainyhead-like 2 restricts the hepatocytic differentiation potential of adult liver progenitor cells

Late fetal and adult livers are reported to contain bipotential liver stem/progenitor cells (LPCs), which share surface markers, including epithelial cell adhesion molecule (EpCAM), with cholangiocytes and differentiate into both hepatocytes and cholangiocytes. However, recent results do not necessarily support the idea that LPCs contribute significantly to cellular turnover and regeneration by supplying new hepatocytes. Here, we examined the colony-forming capability of EpCAM(+) cells isolated from mouse livers between E17 and 11 weeks of age. We found that the number of bipotential colonies was greatly reduced between 1 and 6 weeks, indicating that the number of LPCs decreases during postnatal development. Moreover, bipotential colonies derived from adult LPCs contained substantially fewer albumin(+) cells than those from neonatal LPCs. We further examined the differentiation potential of neonatal and adult LPCs by transplantation and found that neonatal cells differentiated into mature hepatocytes in recipient livers more frequently than adult LPCs. Since we previously reported that the transcription factor grainyhead-like 2 (GRHL2) expressed in EpCAM(+) cells inhibits hepatocytic differentiation, we examined whether targets of GRHL2 might block hepatocytic differentiation. DNA and microRNA microarrays revealed that miR122, the expression of which correlates with hepatocytic differentiation, was greatly reduced in adult as compared with neonatal EpCAM(+) cells. Indeed, GRHL2 negatively regulates the promoter/enhancer activity of the Mir122 gene. Our results indicate that neonatal but not adult EpCAM(+) LPCs have great potential to produce albumin(+) hepatocytes. GRHL2 suppresses transcription of miR122 and thereby restricts the differentiation potential of adult LPCs.

Vascular patterning sets the stage for macro and micro hepatic architecture

Background The liver is a complex organ with a variety of tissue components that require a precise architecture for optimal function of metabolic and detoxification processes. As a result of the delicate orchestration required between the various hepatic tissues, it is not surprising that impairment of hepatic function can be caused by a variety of factors leading to chronic liver disease. Results Despite the growing rate of chronic liver disease, there are currently few effective treatment options besides orthotopic liver transplantation. Better therapeutic options reside in the potential for genetic and cellular therapies that promote progenitor cell activation aiding de novo epithelial and vascular regeneration, cell replacement, or population of bioartificial hepatic devices. In order to explore this area of new therapeutic potential, it is crucial to understand the factors that promote hepatic function through regulating cell identities and tissue architecture. Conclusions In this commentary, we review the signals regulating liver cell fates during development and regeneration and highlight the importance of patterning the hepatic vascular systems to set the groundwork for the macro and micro hepatic architecture of the epithelium.

Advances in liver regeneration

Liver regeneration after partial hepatectomy is the only example of a regenerative process in mammals in which the organ/body weight ratio returns to 100% of the original when the process is complete. The adjustment of liver weight to the needs of the body suggests a complicated set of control points, a 'hepatostat'. There has been much progress in elucidation of mechanisms involved in initiation of liver regeneration. More recent studies have focused on termination pathways, because these may be the underlying controls of the hepatostat and their elimination may be relevant to hepatic neoplasia. When the standard regenerative process is thwarted due to failure of either hepatocytes or biliary epithelial cells to proliferate, each of the two epithelial compartments can function as a source of facultative stem cells for the other.

Hepatic Notch2 deficiency leads to bile duct agenesis perinatally and secondary bile duct formation after weaning

Notch signaling plays an acknowledged role in bile-duct development, but its involvement in cholangiocyte-fate determination remains incompletely understood. We investigated the effects of early Notch2 deletion in Notch2(fl/fl)/Alfp-Cre(tg/-) ("Notch2-cKO") and Notch2(fl/fl)/Alfp-Cre(-/-) ("control") mice. Fetal and neonatal Notch2-cKO livers were devoid of cytokeratin19 (CK19)-, Dolichos-biflorus agglutinin (DBA)-, and SOX9-positive ductal structures, demonstrating absence of prenatal cholangiocyte differentiation. Despite extensive cholestatic hepatocyte necrosis and growth retardation, mortality was only ~15%. Unexpectedly, a slow process of secondary cholangiocyte differentiation and bile-duct formation was initiated around weaning that histologically resembled the ductular reaction. Newly formed ducts varied from rare and non-connected, to multiple, disorganized tubular structures that connected to the extrahepatic bile ducts. Jaundice had disappeared in ~30% of Notch2-cKO mice by 6 months. The absence of NOTCH2 protein in postnatally differentiating cholangiocyte nuclei of Notch2-cKO mice showed that these cells had not originated from non-recombined precursor cells. Notch2 and Hnf6 mRNA levels were permanently decreased in Notch2-cKO livers. Perinatally, Foxa1, Foxa2, Hhex, Hnf1β, Cebpα and Sox9 mRNA levels were all significantly lower in Notch2-cKO than control mice, but all except Foxa2 returned to normal or increased levels after weaning, coincident with the observed secondary bile-duct formation. Interestingly, Hhex and Sox9 mRNA levels remained elevated in icteric 6 months old Notch2-cKOs, but decreased to control levels in non-icteric Notch2-cKOs, implying a key role in secondary bile-duct formation.

CONCLUSION

Cholangiocyte differentiation becomes progressively less dependent on NOTCH2 signaling with age, suggesting that ductal-plate formation is dependent on NOTCH2, but subsequent cholangiocyte differentiation is not.

The biliary epithelium gives rise to liver progenitor cells

Severe liver diseases are characterized by expansion of liver progenitor cells (LPC), which correlates with disease severity. However, the origin and role of LPC in liver physiology and in hepatic injury remains a contentious topic. We found that ductular reaction cells in human cirrhotic livers express hepatocyte nuclear factor 1 homeobox B (HNF1β). However, HNF1β expression was not present in newly generated epithelial cell adhesion molecule (EpCAM)-positive hepatocytes. In order to investigate the role of HNF1β-expressing cells we used a tamoxifen-inducible Hnf1βCreER/R26R(Yfp/LacZ) mouse to lineage-trace Hnf1β(+) biliary duct cells and to assess their contribution to LPC expansion and hepatocyte generation. Lineage tracing demonstrated no contribution of HNF1β(+) cells to hepatocytes during liver homeostasis in healthy mice or after loss of liver mass. After acute acetaminophen or carbon tetrachloride injury no contribution of HNF1β(+) cells to hepatocyte was detected. We next assessed the contribution of Hnf1β(+) -derived cells following two liver injury models with LPC expansion, a diethoxycarbonyl-1,4-dihydro-collidin (DDC)-diet and a choline-deficient ethionine-supplemented (CDE)-diet. The contribution of Hnf1β(+) cells to liver regeneration was dependent on the liver injury model. While no contribution was observed after DDC-diet treatment, mice fed with a CDE-diet showed a small population of hepatocytes derived from Hnf1β(+) cells that were expanded to 1.86% of total hepatocytes after injury recovery. Genome-wide expression profile of Hnf1β(+) -derived cells from the DDC and CDE models indicated that no contribution of LPC to hepatocytes was associated with LPC expression of genes related to telomere maintenance, inflammation, and chemokine signaling pathways.

CONCLUSION

HNF1β(+) biliary duct cells are the origin of LPC. HNF1β(+) cells do not contribute to hepatocyte turnover in the healthy liver, but after certain liver injury, they can differentiate to hepatocytes contributing to liver regeneration.

Polycomb group protein Ezh2 regulates hepatic progenitor cell proliferation and differentiation in murine embryonic liver

In embryonic liver, hepatic progenitor cells are actively proliferating and generate a fundamental cellular pool for establishing parenchymal components. However, the molecular basis for the expansion of the progenitors maintaining their immature state remains elusive. Polycomb group proteins regulate gene expression throughout the genome by modulating of chromatin structure and play crucial roles in development. Enhancer of zeste homolog 2 (Ezh2), a key component of polycomb group proteins, catalyzes tri-methylation of lysine 27 of histone H3 (H3K27me3), which trigger the gene suppression. In the present study, we investigated a role of Ezh2 in the regulation of the expanding hepatic progenitor population in vivo. We found that Ezh2 is highly expressed in the actively proliferating cells at the early developmental stage. Using a conditional knockout mouse model, we show that the deletion of the SET domain of Ezh2, which is responsible for catalytic induction of H3K27me3, results in significant reduction of the total liver size, absolute number of liver parenchymal cells, and hepatic progenitor cell population in size. A clonal colony assay in the hepatic progenitor cells directly isolated from in vivo fetal livers revealed that the bi-potent clonogenicity was significantly attenuated by the Ezh2 loss of function. Moreover, a marker expression based analysis and a global gene expression analysis showed that the knockout of Ezh2 inhibited differentiation to hepatocyte with reduced expression of a number of liver-function related genes. Taken together, our results indicate that Ezh2 is required for the hepatic progenitor expansion in vivo, which is essential for the functional maturation of embryonic liver, through its activity for catalyzing H3K27me3.

Adult hepatocytes are generated by self-duplication rather than stem cell differentiation

The liver is thought to utilize facultative stem cells, also known as "oval cells" or "atypical ductal cells" (ADCs), for regeneration following various types of injury. However, this notion has been based largely on in vitro studies and transplantation models; where lineage tracing has been used, results have been conflicting and effect sizes have been small. Here, we used genetic and nucleoside analog-based tools to mark and track the origin and contribution of various cell populations to liver regeneration in vivo following several ADC-inducing insults. We report that, contrary to prevailing stem-cell-based models of regeneration, virtually all new hepatocytes come from preexisting hepatocytes.

Human fetal ductal plate revisited. I. ductal plate expresses NCAM, KIT, MET, PDGFRA, and neuroendocrine antigens (NSE, chromogranin, synaptophysin, and CD56)

BACKGROUND

The molecular mechanisms of ductal plate (DP) development and differentiation (DD) in human fetal livers (HFLs) are unclear.

MATERIALS AND METHODS

The author immunohistochemically investigated expressions of NCAM, KIT, KIT, PDGFRA, and neuroendocrine antigens in 32 HFLs.

RESULTS

The processes of human intrahepatic bile duct (IBD) DD could be categorized into four stages: DP, remodeling DP, remodeled DP, and mature IBD. NCAM was always expressed in DP and remodeling DP, but not in remodeled DP and mature IBD. The biliary elements were positive for cytokeratin (CK)7, 8, 18, and 19. The hepatoblasts were positive for CK8 and CD18, but negative for CK7 and CK19; however, periportal hepatoblasts showed biliary-type CKs (CK7 and CK19). NCAM was always expressed in DP and remodeling DP, but not in remodeled DP and mature IBD. KIT was occasionally (12/32 cases) expressed in DP and remodeling DP, but not in remodeled DP and mature IBD. NCAM expression was also seen in some hepatoblasts and hematopoietic cells and neurons. KIT was also expressed in some hepatoblasts, hematopoietic cells, and mast cells. MET and PDGFRA were strongly expressed in DP, remodeling DP, remodeled DP, and mature IBD. MET and PDGFRA were also strongly expressed in hepatoblasts and hematopoietic cells. MET and PDGFRA were not expressed in portal mesenchyme, portal veins, sinusoids, and hepatic veins. DP showed immunoreactive chromogranin, synaptophysin, neuron-specific enolase (NSE), and CD56. Expressions of chromogranin and CD56 were infrequently seen in remodeling DP. No expressions of these four neuroendocrine antigens were seen in remodeled DP and mature IBD. The nerve fibers were consistently positive for chromogranin, synaptophysin, NSE, and CD56 in the portal mesenchyme in the stages of remodeling DP, remodeled DP, and mature IBDs.

CONCLUSIONS

The data suggest that NCAM, KIT/stem cell factor-signaling, NSE, hepatocyte growth factor/MET signaling, PDGFα/PDGFRA signaling, chromogranin, synaptophysin, and CD56 play important roles in DD of biliary cells of HFL. They also suggest that the DP cells having neuroendocrine molecules give rise to hepatic stem/progenitor cells.

Clonal tracing of Sox9+ liver progenitors in mouse oval cell injury

Proliferating ducts, termed "oval cells," have long been thought to be bipotential, that is, produce both biliary ducts and hepatocytes during chronic liver injury. The precursor to oval cells is considered to be a facultative liver stem cell (LSC). Recent lineage tracing experiments indicated that the LSC is SRY-related HMG box transcription factor 9 positive (Sox9(+) ) and can replace the bulk of hepatocyte mass in several settings. However, no clonal relationship between Sox9(+) cells and the two epithelial liver lineages was established. We labeled Sox9(+) mouse liver cells at low density with a multicolor fluorescent confetti reporter. Organoid formation validated the progenitor activity of the labeled population. Sox9(+) cells were traced in multiple oval cell injury models using both histology and fluorescence-activated cell sorting. Surprisingly, only rare clones containing both hepatocytes and oval cells were found in any experiment. Quantitative analysis showed that Sox9(+) cells contributed only minimally (<1%) to the hepatocyte pool, even in classic oval cell injury models. In contrast, clonally marked mature hepatocytes demonstrated the ability to self-renew in all classic mouse oval cell activation injuries. A hepatocyte chimera model to trace hepatocytes and nonparenchymal cells also demonstrated the prevalence of hepatocyte-driven regeneration in mouse oval cell injury models.

CONCLUSION

Sox9(+) ductal progenitor cells give rise to clonal oval cell proliferation and bipotential organoids, but rarely produce hepatocytes in vivo. Hepatocytes themselves are the predominant source of new parenchyma cells in prototypical mouse models of oval cell activation.

Mesodermal mesenchymal cells give rise to myofibroblasts, but not epithelial cells, in mouse liver injury

Hepatic stellate cells (HSCs) and portal fibroblasts (PFs) are believed to be the major source of myofibroblasts that participate in fibrogenesis by way of synthesis of proinflammatory cytokines and extracellular matrices. Previous lineage tracing studies using MesP1(Cre) and Rosa26lacZ(flox) mice demonstrated that MesP1+ mesoderm gives rise to mesothelial cells (MCs), which differentiate into HSCs and PFs during liver development. In contrast, several in vivo and in vitro studies reported that HSCs can differentiate into other cell types, including hepatocytes, cholangiocytes, and progenitor cell types known as oval cells, thereby acting as stem cells in the liver. To test whether HSCs give rise to epithelial cells in adult liver, we determined the hepatic lineages of HSCs and PFs using MesP1(Cre) and Rosa26mTmG(flox) mice. Genetic cell lineage tracing revealed that the MesP1+ mesoderm gives rise to MCs, HSCs, and PFs, but not to hepatocytes or cholangiocytes, in the adult liver. Upon carbon tetrachloride injection or bile duct ligation surgery-mediated liver injury, mesodermal mesenchymal cells, including HSCs and PFs, differentiate into myofibroblasts but not into hepatocytes or cholangiocytes. Furthermore, differentiation of the mesodermal mesenchymal cells into oval cells was not observed. These results indicate that HSCs are not sufficiently multipotent to produce hepatocytes, cholangiocytes, or oval cells by way of mesenchymal-epithelial transition in vivo.

CONCLUSION

Cell lineage tracing demonstrated that mesodermal mesenchymal cells including HSCs are the major source of myofibroblasts but do not differentiate into epithelial cell types such as hepatocytes, cholangiocytes, and oval cells.

Isolation of neonatal extrahepatic cholangiocytes

The intra and extrahepatic bile ducts of the liver are developmentally distinct, and may be differentially affected by certain diseases. However, differences between intra and extrahepatic cholangiocytes, and between neonatal and adult cells, are not well understood. Methods for the isolation of cholangiocytes from intrahepatic bile ducts are well established(1-4). Isolation of extrahepatic ductal cells, especially from the neonate, has not yet been described, although this would be of great benefit in understanding the differences between distinct cholangiocyte populations and in studying diseases such as biliary atresia that appear to target the extrahepatic ducts. Described here is an optimized technique to isolate both neonatal and adult mouse extrahepatic bile duct cells. This technique yields a pure cell population with minimal contamination from mesenchymal cells like fibroblasts. This method is based on the removal of the extrahepatic ducts and gallbladder, followed by meticulous dissection and scraping to remove fat and fibroblast layers. Structures are embedded in thick layers of collagen and cultured for approximately 3 weeks to allow outgrowth of cholangiocytes in monolayers, which can then be trypsinized and re plated for experimental use.

New insight into reactive ductular cells of biliary atresia provided by pathological assessment of SOX9

BACKGROUND

Biliary atresia (BA) patients may survive until adolescence after effective Kasai procedure (KP). If liver fibrosis progresses even after successful KP, liver transplantation (LTx) is inevitable. Elucidation of its cause and pathophysiology would open the possibility of treating these patients by non-invasive management. SOX9 is a transcription factor that regulates bile duct development and contributes to liver regeneration and fibrosis. To elucidate the role of SOX9 in BA liver, we investigated the SOX9 expression pattern.

METHOD

Immunostaining with anti-SOX9 antibody was done on hepatic specimens obtained at the time of KP or LTx. We analyzed the association of SOX9 expression with clinical data.

RESULTS

In BA livers, SOX9 was expressed in reactive ductular cells (RDCs), mostly with a nuclear-dominant pattern. SOX9 was also ectopically expressed in hepatocytes, which was more conspicuous at the timing of KP than LTx. SOX9 expression level was significantly correlated with age (days) at which KP was performed, AST and WBC count.

CONCLUSIONS

SOX9 may contribute to RDC formation in BA patients, by affecting both RDCs and hepatocytes. SOX9 could be a key molecule to understand the mechanism of RDC formation, and this understanding would provide a therapeutic strategy for effective treatment of BA.

Liver regeneration by stem/progenitor cells

The liver is renowned for its strong, robust regenerative capacity, employing different modes of regeneration according to type and extent of injury. The process of compensatory hypertrophy of the liver upon partial hepatectomy has been standing as a classical model for studying organ regeneration in mammals and a subject of exhaustive analyses. Meanwhile, in view of the physiological relevance for many of the human liver pathologies induced upon toxic insults or hepatitis, other injury models have recently drawn increasing attention. In those damaged livers where hepatocyte proliferation is compromised, adult liver stem/progenitor cells (LPCs) are activated and differentiate to hepatocytes and cholangiocytes, leading to functional recovery of the organ. Here, we summarize and discuss recent findings on the mechanisms underlying the regeneration process of the liver. Whereas the primary focus of this article is on those related to LPC-mediated regeneration, we also introduce topics on compensatory hypertrophy, where application of new technologies and molecular genetics approaches in mice has gained a paradigm shift. Identification of various markers for LPC populations has expedited their characterization and enabled us to examine their differentiation potential in vivo using genetic lineage-tracing approaches. Comprehensive studies regarding intercellular signaling pathways and their modes of action have succeeded in elucidating novel frameworks for the LPC-niche interaction functioning in the regenerating liver.

CONCLUSION

Advancing our understanding of the cellular and molecular mechanisms for liver regeneration should provide a basis for developing therapeutic strategies to treat patients with liver disease.

The liver is a peculiar organ when it comes to stem cells

This Commentary highlights the article by Sekiya and Suzuki, detailing genetic lineage tracing to determine the origin of cells that form primitive ductules in a mouse model of chronic liver injury.

Intrahepatic bile duct regeneration in mice does not require Hnf6 or Notch signaling through Rbpj

The potential for intrahepatic bile duct (IHBD) regeneration in patients with bile duct insufficiency diseases is poorly understood. Notch signaling and Hnf6 have each been shown to be important for the morphogenesis of IHBDs in mice. One congenital pediatric liver disease characterized by reduced numbers of IHBDs, Alagille syndrome, is associated with mutations in Notch signaling components. Therefore, we investigated whether liver cell plasticity could contribute to IHBD regeneration in mice with disruptions in Notch signaling and Hnf6. We studied a mouse model of bile duct insufficiency with liver epithelial cell-specific deficiencies in Hnf6 and Rbpj, a mediator of canonical Notch signaling. Albumin-Cre Hnf6(flox/flox)Rbpj(flox/flox) mice initially developed no peripheral bile ducts. The evolving postnatal liver phenotype was analyzed using IHBD resin casting, immunostaining, and serum chemistry. With age, Albumin-Cre Hnf6(flox/flox)Rbpj(flox/flox) mice mounted a ductular reaction extending through the hepatic tissue and then regenerated communicating peripheral IHBD branches. Rbpj and Hnf6 were determined to remain absent from biliary epithelial cells constituting the ductular reaction and the regenerated peripheral IHBDs. We report the expression of Sox9, a marker of biliary epithelial cells, in cells expressing hepatocyte markers. Tissue analysis indicates that reactive ductules did not arise directly from preexisting hilar IHBDs. We conclude that liver cell plasticity is competent for regeneration of IHBDs independent of Notch signaling via Rbpj and Hnf6.

Regeneration of liver after extreme hepatocyte loss occurs mainly via biliary transdifferentiation in zebrafish

BACKGROUND & AIMS

The liver has high regenerative capacity, but it is not clear whether most biliary cells (particularly larger cholangiocytes) transdifferentiate into hepatocytes in regenerating liver. We investigated how this process might contribute to liver regeneration in zebrafish.

METHODS

Zebrafish transgenic lines were generated using the standard I-SceI meganuclease transgenesis technique. Hepatocytes of the Tg(lfabp:mCherry-NTR)(cq2) animals were ablated by the administration of metronidazole. We investigated transdifferentiation of biliary cells to hepatocytes and expression of markers using whole mount antibody staining, fluorescent in situ hybridization, and Cre/loxP-based genetic lineage tracing analyses. The role of biliary cells in hepatocyte regeneration was explored using zebrafish larvae with defects in biliary cell development.

RESULTS

After extreme loss of hepatocytes, nearly all the biliary cells steadily lost their tubular morphology, proliferated, and expressed hepatocyte-specific markers. Cre/loxP-based inducible lineage tracing showed that new hepatocytes mainly arose from transdifferentiation of biliary cells; this process required Notch signaling and, in turn, activation of Sox9b in cholangiocytes. Activation of early endoderm and hepatoblast markers in most of the cholangiocytes indicated that biliary transdifferentiation includes a step of dedifferentiation into a bipotential intermediate. Defects in development of biliary cells impaired hepatocyte regeneration.

CONCLUSIONS

Using our zebrafish liver regeneration model, we found that biliary cells can transdifferentiate into hepatocytes and are the major contributors to hepatocyte regeneration after extreme hepatocyte loss.