Genetic interactions between hepatocyte nuclear factor-6 and Notch signaling regulate mouse intrahepatic bile duct development in vivo

Spatially segregated defects and IGF1-responsiveness of hilar and peripheral biliary organoids from a model of Alagille syndrome

BACKGROUND & AIMS

Alagille syndrome (ALGS) manifests with peripheral intrahepatic bile duct (IHBD) paucity, which can spontaneously resolve. In a model for ALGS, Jag1Ndr/Ndr mice, this occurs with distinct architectural mechanisms in hilar and peripheral IHBDs. Here, we investigated region-specific IHBD characteristics and addressed whether IGF1, a cholangiocyte mitogen that is downregulated in ALGS and in Jag1Ndr/Ndr mice, can improve biliary outcomes.

METHODS

Intrahepatic cholangiocyte organoids (ICOs) were derived from hilar and peripheral adult Jag1+/+ and Jag1Ndr/Ndr livers (hICOs and pICOs, respectively). ICOs were grown in Matrigel or microwell arrays, and characterized using bulk RNA sequencing, immunofluorescence, and high throughput analyses of nuclear sizes. ICOs were treated with IGF1, followed by analyses of growth, proliferation, and death. CellProfiler and Python scripts were custom written for image analyses. Key results were validated in vivo by immunostaining.

RESULTS

Cell growth assays and transcriptomics demonstrated that Jag1Ndr/Ndr ICOs were less proliferative than Jag1+/+ ICOs. IGF1 specifically rescued survival and growth of Jag1Ndr/Ndr pICOs. Jag1Ndr/Ndr hICOs were the least proliferative, with lower Notch signalling and an enrichment of hepatocyte signatures and IGF uptake/transport pathways. In vitro (Jag1Ndr/Ndr hICOs) and in vivo (Jag1Ndr/Ndr hilar portal tracts) analyses revealed ectopic HNF4a+ hepatocytes.

CONCLUSIONS

Hilar and peripheral Jag1Ndr/Ndr ICOs exhibit differences in Notch signalling status, proliferation, and cholangiocyte commitment which may result in cholangiocyte-to-hepatocyte transdifferentiation. While Jag1Ndr/Ndr pICOs can be rescued by IGF1, hICOs are unresponsive, perhaps due to their hepatocyte-like state and/or expression of IGF transport components. IGF1 represents a potential therapeutic for peripheral bile ducts.

Biliary atresia: the development, pathological features, and classification of the bile duct

Biliary atresia is an occlusive biliary disease involving intrahepatic and extrahepatic bile ducts. Its etiology and pathogenesis are unclear. There are many manifestations of bile duct involvement in biliary atresia, but little is known about its occurrence and development. In addition, different classification methods have been proposed in different periods of biliary atresia, each with its advantages and disadvantages. The combined application of biliary atresia classification will help to improve the survival rate of patients with native liver. Therefore, this article reviews the development, pathological features, and classification of intrahepatic and extrahepatic bile ducts in biliary atresia, to provide a reference for the study of the pathogenesis and the choice of treatment methods.

Hepatic ribosomal protein S6 (Rps6) insufficiency results in failed bile duct development and loss of hepatocyte viability; a ribosomopathy-like phenotype that is partially p53-dependent

Defective ribosome biogenesis (RiBi) underlies a group of clinically diverse human diseases collectively known as the ribosomopathies, core manifestations of which include cytopenias and developmental abnormalities that are believed to stem primarily from an inability to synthesize adequate numbers of ribosomes and concomitant activation of p53. The importance of a correctly functioning RiBi machinery for maintaining tissue homeostasis is illustrated by the observation that, despite having a paucity of certain cell types in early life, ribosomopathy patients have an increased risk for developing cancer later in life. This suggests that hypoproliferative states trigger adaptive responses that can, over time, become maladaptive and inadvertently drive unchecked hyperproliferation and predispose to cancer. Here we describe an experimentally induced ribosomopathy in the mouse and show that a normal level of hepatic ribosomal protein S6 (Rps6) is required for proper bile duct development and preservation of hepatocyte viability and that its insufficiency later promotes overgrowth and predisposes to liver cancer which is accelerated in the absence of the tumor-suppressor PTEN. We also show that the overexpression of c-Myc in the liver ameliorates, while expression of a mutant hyperstable form of p53 partially recapitulates specific aspects of the hepatopathies induced by Rps6 deletion. Surprisingly, co-deletion of p53 in the Rps6-deficient background fails to restore biliary development or significantly improve hepatic function. This study not only reveals a previously unappreciated dependence of the developing liver on adequate levels of Rps6 and exquisitely controlled p53 signaling, but suggests that the increased cancer risk in ribosomopathy patients may, in part, stem from an inability to preserve normal tissue homeostasis in the face of chronic injury and regeneration.

A reference single-cell regulomic and transcriptomic map of cynomolgus monkeys

Non-human primates are attractive laboratory animal models that accurately reflect both developmental and pathological features of humans. Here we present a compendium of cell types across multiple organs in cynomolgus monkeys (Macaca fascicularis) using both single-cell chromatin accessibility and RNA sequencing data. The integrated cell map enables in-depth dissection and comparison of molecular dynamics, cell-type compositions and cellular heterogeneity across multiple tissues and organs. Using single-cell transcriptomic data, we infer pseudotime cell trajectories and cell-cell communications to uncover key molecular signatures underlying their cellular processes. Furthermore, we identify various cell-specific cis-regulatory elements and construct organ-specific gene regulatory networks at the single-cell level. Finally, we perform comparative analyses of single-cell landscapes among mouse, monkey and human. We show that cynomolgus monkey has strikingly higher degree of similarities in terms of immune-associated gene expression patterns and cellular communications to human than mouse. Taken together, our study provides a valuable resource for non-human primate cell biology.

Non-human primates are attractive laboratory animal models that can accurately reflect some developmental and pathological features of humans. Here the authors chart a reference cell map of cynomolgus monkeys using both scATAC-seq and scRNA-seq data across multiple organs, providing insights into the molecular dynamics and cellular heterogeneity of this organism.

Fine-scale visualizing the hierarchical structure of mouse biliary tree with fluorescence microscopy method

The liver is a vital organ and the hepatic lobule serves as the most basic structural and functional unit which is mainly assembled with parenchymal cells including hepatocytes and biliary epithelial cells. The continuous tubular arrangement of biliary cells which constitutes the biliary tracts is critical for liver function, however, the biliary tracts are often disrupted in many liver diseases such as cirrhosis and some congenital disorders. Visualization of the biliary tracts in fine-scale and three-dimension will help to understanding the structure basis of these liver diseases. In the present study, we established several biliary tract injury mouse models by diet feeding, surgery or genetic modification. The cytoplasm and nuclei of the parenchymal cells were marked by active uptake of fluorescent dyes Rhodamine B (red) and Hoechst (blue), respectively. After the removal of liver en bloc, the biliary tracts were retrogradely perfused with green fluorescent dye, fluorescein isothiocyanate (FITC). The liver was then observed under confocal microscopy. The fine-scale and three-dimensional (3D) structure of the whole biliary tree, particularly the network of the end-terminal bile canaliculi and neighboring hepatocytes were clearly visualized. The biliary tracts displayed clear distinct characteristics in normal liver and diseased liver models. Taken together, we have developed a simple and repeatable imaging method to visualize the fine-scale and hierarchical architecture of the biliary tracts spreading in the mouse liver.

Cellular plasticity at the nexus of development and disease

In October 2020, the Keystone Symposia Global Health Series hosted a Keystone eSymposia entitled 'Tissue Plasticity: Preservation and Alteration of Cellular Identity'. The event synthesized groundbreaking research from unusually diverse fields of study, presented in various formats, including live and virtual talks, panel discussions and interactive e-poster sessions. The meeting focused on cell identity changes and plasticity in multiple tissues, species and developmental contexts, both in homeostasis and during injury. Here, we review the key themes of the meeting: (1) cell-extrinsic drivers of plasticity; (2) epigenomic regulation of cell plasticity; and (3) conserved mechanisms governing plasticity. A salient take-home conclusion was that there may be conserved mechanisms used by cells to execute plasticity, with autodegradative activity (autophagy and lysosomes) playing a crucial initial step in diverse organs and organisms.

Mouse Models for Diseases in the Cholangiocyte Lineage

Cholangiopathies are an important group of liver diseases affecting the biliary system, and the purpose of this review is to describe how diseases in the biliary system can be studied in mouse models. A particular focus is placed on mouse models for Alagille syndrome, a cholangiopathy with a strong genetic link to dysfunctional Notch signaling. Recently, a number of different genetic mouse models based on various manipulations of the Notch signaling pathway have been generated to study Alagille syndrome, and we discuss the resulting phenotypes, and possible causes for the phenotypic heterogeneity among the various models. In the final section, we provide a more general discussion on how well mouse models can be expected to mimic human liver disease, as well as an outlook toward the need for new technologies that can help us to gain new insights from mouse models for liver disease.

Development of the Intrahepatic and Extrahepatic Biliary Tract: A Framework for Understanding Congenital Diseases

The involvement of the biliary tract in the pathophysiology of liver diseases and the increased attention paid to bile ducts in the bioconstruction of liver tissue for regenerative therapy have fueled intense research into the fundamental mechanisms of biliary development. Here, I review the molecular, cellular and tissular mechanisms driving differentiation and morphogenesis of the intrahepatic and extrahepatic bile ducts. This review focuses on the dynamics of the transcriptional and signaling modules that promote biliary development in human and mouse liver and discusses studies in which the use of zebrafish uncovered unexplored processes in mammalian biliary development. The review concludes by providing a framework for interpreting the mechanisms that may help us understand the origin of congenital biliary diseases.

Molecular regulation of mammalian hepatic architecture

The essential liver exocrine and endocrine functions require a precise spatial arrangement of the hepatic lobule consisting of the central vein, portal vein, hepatic artery, intrahepatic bile duct system, and hepatocyte zonation. This allows blood to be carried through the liver parenchyma sampled by all hepatocytes and bile produced by the hepatocytes to be carried out of the liver through the intrahepatic bile duct system composed of cholangiocytes. The molecular orchestration of multiple signaling pathways and epigenetic factors is required to set up lineage restriction of the bipotential hepatoblast progenitor into the hepatocyte and cholangiocyte cell lineages, and to further refine cell fate heterogeneity within each cell lineage reflected in the functional heterogeneity of hepatocytes and cholangiocytes. In addition to the complex molecular regulation, there is a complicated morphogenetic choreography observed in building the refined hepatic epithelial architecture. Given the multifaceted molecular and cellular regulation, it is not surprising that impairment of any of these processes can result in acute and chronic hepatobiliary diseases. To enlighten the development of potential molecular and cellular targets for therapeutic options, an understanding of how the intricate hepatic molecular and cellular interactions are regulated is imperative. Here, we review the signaling pathways and epigenetic factors regulating hepatic cell lineages, fates, and epithelial architecture.

YAP Activation Drives Liver Regeneration after Cholestatic Damage Induced by Rbpj Deletion

Liver cholestasis is a chronic liver disease and a major health problem worldwide. Cholestasis is characterised by a decrease in bile flow due to impaired secretion by hepatocytes or by obstruction of bile flow through intra- or extrahepatic bile ducts. Thereby cholestasis can induce ductal proliferation, hepatocyte injury and liver fibrosis. Notch signalling promotes the formation and maturation of bile duct structures. Here we investigated the liver regeneration process in the context of cholestasis induced by disruption of the Notch signalling pathway. Liver-specific deletion of recombination signal binding protein for immunoglobulin kappa j region (Rbpj), which represents a key regulator of Notch signalling, induces severe cholestasis through impaired intra-hepatic bile duct (IHBD) maturation, severe necrosis and increased lethality. Deregulation of the biliary compartment and cholestasis are associated with the change of several signalling pathways including a Kyoto Encyclopedia of Genes and Genomes (KEGG) gene set representing the Hippo pathway, further yes-associated protein (YAP) activation and upregulation of SRY (sex determining region Y)-box 9 (SOX9), which is associated with transdifferentiation of hepatocytes. SOX9 upregulation in cholestatic liver injury in vitro is independent of Notch signalling. We could comprehensively address that in vivo Rbpj depletion is followed by YAP activation, which influences the transdifferentiation of hepatocytes and thereby contributing to liver regeneration.

De novo formation of the biliary system by TGFβ-mediated hepatocyte transdifferentiation

Transdifferentiation is a complete and stable change in cell identity that serves as an alternative to stem-cell-mediated organ regeneration. In adult mammals, findings of transdifferentiation have been limited to the replenishment of cells lost from preexisting structures, in the presence of a fully developed scaffold and niche¹. Here we show that transdifferentiation of hepatocytes in the mouse liver can build a structure that failed to form in development-the biliary system in a mouse model that mimics the hepatic phenotype of human Alagille syndrome (ALGS)². In these mice, hepatocytes convert into mature cholangiocytes and form bile ducts that are effective in draining bile and persist after the cholestatic liver injury is reversed, consistent with transdifferentiation. These findings redefine hepatocyte plasticity, which appeared to be limited to metaplasia, that is, incomplete and transient biliary differentiation as an adaptation to cell injury, based on previous studies in mice with a fully developed biliary system^3-6. In contrast to bile duct development^7-9, we show that de novo bile duct formation by hepatocyte transdifferentiation is independent of NOTCH signalling. We identify TGFβ signalling as the driver of this compensatory mechanism and show that it is active in some patients with ALGS. Furthermore, we show that TGFβ signalling can be targeted to enhance the formation of the biliary system from hepatocytes, and that the transdifferentiation-inducing signals and remodelling capacity of the bile-duct-deficient liver can be harnessed with transplanted hepatocytes. Our results define the regenerative potential of mammalian transdifferentiation and reveal opportunities for the treatment of ALGS and other cholestatic liver diseases.

Transcription factor GATA6: a novel marker and putative inducer of ductal metaplasia in biliary atresia

Biliary atresia (BA), a neonatal liver disease, is characterized by obstruction of extrahepatic bile ducts with subsequent cholestasis, inflammation, and progressive liver fibrosis. To gain insights into the pathophysiology of BA, we focused attention on GATA6, a transcription factor implicated in biliary development. Early in fetal development GATA6 expression is evident in cholangiocytes and hepatocytes, but by late gestation it is extinguished in hepatocytes. Utilizing a unique set of BA liver samples collected before and after successful portoenterostomy (PE), we found that GATA6 expression is markedly upregulated in hepatocytes of patients with BA compared with healthy and cholestatic disease controls. This upregulation is recapitulated in two murine models simulating bile duct obstruction and intrahepatic bile ductule expansion. GATA6 expression in BA livers correlates with two established negative prognostic indicators (age at PE, degree of intrahepatic bile ductule expansion) and decreases after normalization of serum bilirubin by PE. GATA6 expression in BA livers correlates with expression of known regulators of cholangiocyte differentiation ( JAGGED1, HNF1β, and HNF6). These same genes are upregulated after enforced expression of GATA6 in human hepatocyte cell models. In conclusion, GATA6 is a novel marker and a putative driver of hepatocyte-cholangiocyte metaplasia in BA, and its expression in hepatocytes is downregulated after successful PE. NEW & NOTEWORTHY A pathological hallmark in the liver of patients with biliary atresia is ductular reaction, an expansion of new bile ductules that are thought to arise from conversion of mature hepatocytes. Here, we show that transcription factor GATA6 is a marker and potential driver of hepatocyte ductal metaplasia in biliary atresia. Hepatocyte GATA6 expression is elevated in biliary atresia, correlates with bile duct expansion, and decreases after successful portoenterostomy.

The molecular functions of hepatocyte nuclear factors - In and beyond the liver

The hepatocyte nuclear factors (HNFs) namely HNF1α/β, FOXA1/2/3, HNF4α/γ and ONECUT1/2 are expressed in a variety of tissues and organs, including the liver, pancreas and kidney. The spatial and temporal manner of HNF expression regulates embryonic development and subsequently the development of multiple tissues during adulthood. Though the HNFs were initially identified individually based on their roles in the liver, numerous studies have now revealed that the HNFs cross-regulate one another and exhibit synergistic relationships in the regulation of tissue development and function. The complex HNF transcriptional regulatory networks have largely been elucidated in rodent models, but less so in human biological systems. Several heterozygous mutations in these HNFs were found to cause diseases in humans but not in rodents, suggesting clear species-specific differences in mutational mechanisms that remain to be uncovered. In this review, we compare and contrast the expression patterns of the HNFs, the HNF cross-regulatory networks and how these liver-enriched transcription factors serve multiple functions in the liver and beyond, extending our focus to the pancreas and kidney. We also summarise the insights gained from both human and rodent studies of mutations in several HNFs that are known to lead to different disease conditions.

Development of the liver: Insights into organ and tissue morphogenesis

Recent development of improved tools and methods to analyse tissues at the three-dimensional level has expanded our capacity to investigate morphogenesis of foetal liver. Here, we review the key morphogenetic steps during liver development, from the prehepatic endoderm stage to the postnatal period, and consider several model organisms while focussing on the mammalian liver. We first discuss how the liver buds out of the endoderm and gives rise to an asymmetric liver. We next outline the mechanisms driving liver and lobe growth, and review morphogenesis of the intra- and extrahepatic bile ducts; morphogenetic responses of the biliary tract to liver injury are discussed. Finally, we describe the mechanisms driving formation of the vasculature, namely venous and arterial vessels, as well as sinusoids.

MicroRNA-337-3p controls hepatobiliary gene expression and transcriptional dynamics during hepatic cell differentiation

Transcriptional networks control the differentiation of the hepatocyte and cholangiocyte lineages from embryonic liver progenitor cells and their subsequent maturation to the adult phenotype. However, how relative levels of hepatocyte and cholangiocyte gene expression are determined during differentiation remains poorly understood. Here, we identify microRNA (miR)-337-3p as a regulator of liver development. miR-337-3p stimulates expression of cholangiocyte genes and represses hepatocyte genes in undifferentiated progenitor cells in vitro and in embryonic mouse livers. Beyond the stage of lineage segregation, miR-337-3p controls the transcriptional network dynamics of developing hepatocytes and balances both cholangiocyte populations that constitute the ductal plate. miR-337-3p requires Notch and transforming growth factor-β signaling and exerts a biphasic control on the hepatocyte transcription factor hepatocyte nuclear factor 4α by modulating its activation and repression. With the help of an experimentally validated mathematical model, we show that this biphasic control results from an incoherent feedforward loop between miR-337-3p and hepatocyte nuclear factor 4α.

CONCLUSION

Our results identify miR-337-3p as a regulator of liver development and highlight how tight quantitative control of hepatic cell differentiation is exerted through specific gene regulatory network motifs. (Hepatology 2018;67:313-327).

Footprint-free human fetal foreskin derived iPSCs: A tool for modeling hepatogenesis associated gene regulatory networks

Induced pluripotent stem cells (iPSCs) are similar to embryonic stem cells and can be generated from somatic cells. We have generated episomal plasmid-based and integration-free iPSCs (E-iPSCs) from human fetal foreskin fibroblast cells (HFF1). We used an E-iPSC-line to model hepatogenesis in vitro. The HLCs were characterized biochemically, i.e. glycogen storage, ICG uptake and release, UREA and bile acid production, as well as CYP3A4 activity. Ultra-structure analysis by electron microscopy revealed the presence of lipid and glycogen storage, tight junctions and bile canaliculi- all typical features of hepatocytes. Furthermore, the transcriptome of undifferentiated E-iPSC, DE, HE and HLCs were compared to that of fetal liver and primary human hepatocytes (PHH). K-means clustering identified 100 clusters which include developmental stage-specific groups of genes, e.g. OCT4 expression at the undifferentiated stage, SOX17 marking the DE stage, DLK and HNF6 the HE stage, HNF4α and Albumin is specific to HLCs, fetal liver and adult liver (PHH) stage. We use E-iPSCs for modeling gene regulatory networks associated with human hepatogenesis and gastrulation in general.

Longitudinal in vivo bioimaging of hepatocyte transcription factor activity following cholestatic liver injury in mice

Molecular mechanisms regulating liver repair following cholestatic injury remain largely unknown. We have combined a mouse model of acute cholestatic liver injury, partial bile duct ligation (pBDL), with a novel longitudinal bioimaging methodology to quantify transcription factor activity during hepatic injury and repair. We administered lentiviral transcription factor activated luciferase/eGFP reporter (TFAR) cassettes to neonatal mice enabling longitudinal TFAR profiling by continued bioimaging throughout the lives of the animals and following pBDL in adulthood. Neonatal intravascular injection of VSV-G pseudotyped lentivirus resulted in almost exclusive transduction of hepatocytes allowing analysis of hepatocyte-specific transcription factor activity. We recorded acute but transient responses with NF-κB and Smad2/3 TFAR whilst our Notch reporter was repressed over the 40 days of evaluation post-pBDL. The bipotent hepatic progenitor cell line, HepaRG, can be directed to differentiate into hepatocytes and biliary epithelia. We found that forced expression of the Notch inhibitor NUMB in HepaRG resulted in enhanced hepatocyte differentiation and proliferation whereas over-expressing the Notch agonist JAG1 resulted in biliary epithelial differentiation. In conclusion, our data demonstrates that hepatocytes rapidly upregulate NF-κB and Smad2/3 activity, whilst repressing Notch signalling. This transcriptional response to cholestatic liver injury likely promotes partial de-differentiation to allow pro-regenerative proliferation of hepatocytes.

DNA methylation alters transcriptional rates of differentially expressed genes and contributes to pathophysiology in mice fed a high fat diet

OBJECTIVE

Overnutrition can alter gene expression patterns through epigenetic mechanisms that may persist through generations. However, it is less clear if overnutrition, for example a high fat diet, modifies epigenetic control of gene expression in adults, or by what molecular mechanisms, or if such mechanisms contribute to the pathology of the metabolic syndrome. Here we test the hypothesis that a high fat diet alters hepatic DNA methylation, transcription and gene expression patterns, and explore the contribution of such changes to the pathophysiology of obesity.

METHODS

RNA-seq and targeted high-throughput bisulfite DNA sequencing were used to undertake a systematic analysis of the hepatic response to a high fat diet. RT-PCR, chromatin immunoprecipitation and in vivo knockdown of an identified driver gene, Phlda1, were used to validate the results.

RESULTS

A high fat diet resulted in the hypermethylation and decreased transcription and expression of Phlda1 and several other genes. A subnetwork of genes associated with Phlda1 was identified from an existing Bayesian gene network that contained numerous hepatic regulatory genes involved in lipid and body weight homeostasis. Hepatic-specific depletion of Phlda1 in mice decreased expression of the genes in the subnetwork, and led to increased oil droplet size in standard chow-fed mice, an early indicator of steatosis, validating the contribution of this gene to the phenotype.

CONCLUSIONS

We conclude that a high fat diet alters the epigenetics and transcriptional activity of key hepatic genes controlling lipid homeostasis, contributing to the pathophysiology of obesity.

Growth Hormone Mediates Its Protective Effect in Hepatic Apoptosis through Hnf6

BACKGROUND AND AIMS

Growth hormone (GH) not only supports hepatic metabolism but also protects against hepatocyte cell death. Hnf6 (or Oc1) belonging to the Onecut family of hepatocyte transcription factors known to regulate differentiated hepatic function, is a GH-responsive gene. We evaluate if GH mediates Hnf6 activity to attenuate hepatic apoptotic injury.

METHODS

We used an animal model of hepatic apoptosis by bile duct ligation (BDL) with Hnf6 -/- (KO) mice in which hepatic Hnf6 was conditionally inactivated. GH was administered to adult wild type WT and KO mice for the 7 days of BDL to enhance Hnf6 expression. In vitro, primary hepatocytes derived from KO and WT liver were treated with LPS and hepatocyte apoptosis was assessed with and without GH treatment.

RESULTS

In WT mice, GH treatment enhanced Hnf6 expression during BDL, inhibited Caspase -3, -8 and -9 responses and diminished hepatic apoptotic and fibrotic injury. GH-mediated upregulation of Hnf6 expression and parallel suppression of apoptosis and fibrosis in WT BDL liver were abrogated in KO mice. LPS activated apoptosis and suppressed Hnf6 expression in primary hepatocytes. GH/LPS co-treatment enhanced Hnf6 expression with corresponding attenuation of apoptosis in WT-derived hepatocytes, but not in KO hepatocytes. ChiP-on-ChiP and electromobility shift assays of KO and WT liver nuclear extracts identified Ciap1 (or Birc2) as an Hnf6-bound target gene. Ciap1 expression patterns closely follow Hnf6 expression in the liver and in hepatocytes.

CONCLUSION

GH broad protective actions on hepatocytes during liver injury are effected through Hnf6, with Hnf6 transcriptional activation of Ciap1 as an underlying molecular mediator.

Suppressing Pitx2 inhibits proliferation and promotes differentiation of iHepSCs

Induced hepatic stem cells (iHepSCs) have great potential as donors for liver cell therapy due to their abilities for self-renewal and bi-potential differentiation. However, the molecular mechanism regulating proliferation and differentiation of iHepSCs is poorly understood. In this study, we provide evidence that the homeodomain transcription factor, Pitx2, is essential to maintain iHepSCs stem cell characteristics. Suppressing Pitx2 expression in iHepSCs by lentivirus mediated specific shRNA markedly reduced the expression of the hepatic stem cell-associated genes (Lgr5, EpCAM, and Sox9) with concomitant inhibition of proliferation by blocking the G1/S phase transition, and these phenotypic changes were reversed upon re-expression of Pitx2. Pitx2 knockdown also resulted in up-regulation of the p53-induced Cdk inhibitor p21, and down-regulation of its downstream effector CDK2-Cyclin E kinase complex. Furthermore, we observed that iHepSCs were more efficiently induced to differentiate into both hepatocytes and cholangiocytes when Pitx2 expression was suppressed, as compared to unmanipulated iHepSCs. These findings reveal that Pitx2 expression may be leveraged to control the status of iHepSCs during expansion in vitro to provide a strategy for further application of iHepSCs in liver cell therapy.

Notch in fibrosis and as a target of anti-fibrotic therapy

The Notch pathway represents a highly conserved signaling network with essential roles in regulation of key cellular processes and functions, many of which are critical for development. Accumulating evidence indicates that it is also essential for fibrosis and thus the pathogenesis of chronic fibroproliferative diseases in diverse organs and tissues. Different effects of Notch activation are observed depending on cellular and tissue context as well as in both physiologic and pathologic states. Close interactions of Notch signaling pathway with other signaling pathways have been identified. In this review, current knowledge on the role of the Notch signaling with special focus on fibrosis and its potential as a therapeutic target is summarized.

Notch Signaling Coordinates Progenitor Cell-Mediated Biliary Regeneration Following Partial Hepatectomy

Aberrant transcriptional regulation contributes to the pathogenesis of both congenital and adult forms of liver disease. Although the transcription factor RBPJ is essential for liver morphogenesis and biliary development, its specific function in the differentiation of hepatic progenitor cells (HPC) has not been investigated, and little is known about its role in adult liver regeneration. HPCs are bipotent liver stem cells that can self-replicate and differentiate into hepatocytes or cholangiocytes in vitro. HPCs are thought to play an important role in liver regeneration and repair responses. While the coordinated repopulation of both hepatocyte and cholangiocyte compartment is pivotal to the structure and function of the liver after regeneration, the mechanisms coordinating biliary regeneration remain vastly understudied. Here, we utilized complex genetic manipulations to drive liver-specific deletion of the Rbpj gene in conjunction with lineage tracing techniques to delineate the precise functions of RBPJ during biliary development and HPC-associated biliary regeneration after hepatectomy. Furthermore, we demonstrate that RBPJ promotes HPC differentiation toward cholangiocytes in vitro and blocks hepatocyte differentiation through mechanisms involving Hippo-Notch crosstalk. Overall, this study demonstrates that the Notch-RBPJ signaling axis critically regulates biliary regeneration by coordinating the fate decision of HPC and clarifies the molecular mechanisms involved.

Onecut transcription factors in development and disease

Developmental processes are remarkably well conserved among species, and among the most highly conserved developmental regulators are transcription factor families. The Onecut transcription factor family consists of three members known for their single "cut" DNA-binding domain and an aberrant homeodomain. The three members of the Onecut family are highly conserved from Drosophila to humans and have significant roles in regulating the development of diverse tissues derived from the ectoderm or endoderm, where they activate a number of gene families. Of note, the genetic interaction between Onecut family members and Neurogenin genes appears to be essential in multiple tissues for proper specification and development of unique cell types. This review highlights the importance of the Onecut factors in cell fate specification and organogenesis, highlighting their role in vertebrates, and discusses their role in the maintenance of cell fate and prevention of disease. We cover the essential spatial and temporal control of Onecut factor expression and how this tight regulation is required for proper specification and subsequent terminal differentiation of multiple tissue types including those within the retina, central nervous system, liver and pancreas. Beyond development, Onecut factors perform necessary functions in mature cell types; their misregulation can contribute to diseases such as pancreatic cancer. Given the importance of this family of transcription factors in development and disease, their consideration in essential transcription factor networks is underappreciated.

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.

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.

Cross-talk between Notch and Hedgehog regulates hepatic stellate cell fate in mice

Liver repair involves phenotypic changes in hepatic stellate cells (HSCs) and reactivation of morphogenic signaling pathways that modulate epithelial-to-mesenchymal/mesenchymal-to-epithelial transitions, such as Notch and Hedgehog (Hh). Hh stimulates HSCs to become myofibroblasts (MFs). Recent lineage tracing studies in adult mice with injured livers showed that some MFs became multipotent progenitors to regenerate hepatocytes, cholangiocytes, and HSCs. We studied primary HSC cultures and two different animal models of fibrosis to evaluate the hypothesis that activating the Notch pathway in HSCs stimulates them to become (and remain) MFs through a mechanism that involves an epithelial-to-mesenchymal-like transition and requires cross-talk with the canonical Hh pathway. We found that when cultured HSCs transitioned into MFs, they activated Hh signaling, underwent an epithelial-to-mesenchymal-like transition, and increased Notch signaling. Blocking Notch signaling in MFs/HSCs suppressed Hh activity and caused a mesenchymal-to-epithelial-like transition. Inhibiting the Hh pathway suppressed Notch signaling and also induced a mesenchymal-to-epithelial-like transition. Manipulating Hh and Notch signaling in a mouse multipotent progenitor cell line evoked similar responses. In mice, liver injury increased Notch activity in MFs and Hh-responsive MF progeny (i.e., HSCs and ductular cells). Conditionally disrupting Hh signaling in MFs of bile-duct-ligated mice inhibited Notch signaling and blocked accumulation of both MF and ductular cells.

CONCLUSIONS

The Notch and Hedgehog pathways interact to control the fate of key cell types involved in adult liver repair by modulating epithelial-to-mesenchymal-like/mesenchymal-to-epithelial-like transitions.

Differentiation of progenitors in the liver: a matter of local choice

The liver is a complex organ that requires multiple rounds of cell fate decision for development and homeostasis throughout the lifetime. During the earliest phases of organogenesis, the liver acquires a separate lineage from the pancreas and the intestine, and subsequently, the liver bud must appropriately differentiate to form metabolic hepatocytes and cholangiocytes for proper hepatic physiology. In addition, throughout life, the liver is bombarded with chemical and pathological insults, which require the activation and correct differentiation of adult progenitor cells. This Review seeks to provide an overview of the complex signaling relationships that allow these tightly regulated processes to occur.

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.

3-dimensional resin casting and imaging of mouse portal vein or intrahepatic bile duct system

In organs, the correct architecture of vascular and ductal structures is indispensable for proper physiological function, and the formation and maintenance of these structures is a highly regulated process. The analysis of these complex, 3-dimensional structures has greatly depended on either 2-dimensional examination in section or on dye injection studies. These techniques, however, are not able to provide a complete and quantifiable representation of the ductal or vascular structures they are intended to elucidate. Alternatively, the nature of 3-dimensional plastic resin casts generates a permanent snapshot of the system and is a novel and widely useful technique for visualizing and quantifying 3-dimensional structures and networks. A crucial advantage of the resin casting system is the ability to determine the intact and connected, or communicating, structure of a blood vessel or duct. The structure of vascular and ductal networks are crucial for organ function, and this technique has the potential to aid study of vascular and ductal networks in several ways. Resin casting may be used to analyze normal morphology and functional architecture of a luminal structure, identify developmental morphogenetic changes, and uncover morphological differences in tissue architecture between normal and disease states. Previous work has utilized resin casting to study, for example, architectural and functional defects within the mouse intrahepatic bile duct system that were not reflected in 2-dimensional analysis of the structure(1,2), alterations in brain vasculature of a Alzheimer's disease mouse model(3), portal vein abnormalities in portal hypertensive and cirrhotic mice(4), developmental steps in rat lymphatic maturation between immature and adult lungs(5), immediate microvascular changes in the rat liver, pancreas, and kidney in response in to chemical injury(6). Here we present a method of generating a 3-dimensional resin cast of a mouse vascular or ductal network, focusing specifically on the portal vein and intrahepatic bile duct. These casts can be visualized by clearing or macerating the tissue and can then be analyzed. This technique can be applied to virtually any vascular or ductal system and would be directly applicable to any study inquiring into the development, function, maintenance, or injury of a 3-dimensional ductal or vascular structure.