Hepatocyte-Specific β-Catenin Deletion During Severe Liver Injury Provokes Cholangiocytes to Differentiate Into Hepatocytes

Liver and bile duct organoids and tumoroids

Organoids refer to 3D cultures established to recapitulate histology, pathology, architecture, and genetic traits of various organs and tissues in the body, thereby replacing 2D cell cultures, xenograft, and animal models. Organoids form a 3D in vitro mimic of original tissues like the liver and are derived from embryonic or adult tissue stem cells. Liver and bile duct tumor organoids, also called, tumoroids capture genetic diversity, cellular, and pathophysiological properties of original tumors. Moreover, co-culture techniques along with genetic modulation of organoids allow for using tumoroids in liver and bile duct cancer research and drug screening/testing. Therefore, tumoroids are promising platforms for studying liver and bile duct cancer, which paves the way for the new era of personalized therapies. In the current review, we aimed to discuss liver and bile duct organoids with special emphasis on tumoroids and their applications, advantages, and shortcomings.

Aberrant differentiation and proliferation of hepatocytes in chronic liver injury and liver tumors

Chronic liver injury induces liver cirrhosis and facilitates hepatocarcinogenesis. However, the effects of this condition on hepatocyte proliferation and differentiation are unclear. We showed that rodent hepatocytes display a ductular phenotype when they are cultured within a collagenous matrix. This process involves transdifferentiation without the emergence of hepatoblastic features and is at least partially reversible. During the ductular reaction in chronic liver diseases with progressive fibrosis, some hepatocytes, especially those adjacent to ectopic ductules, demonstrate ductular transdifferentiation, but the majority of increased ductules originate from the existing bile ductular system that undergoes extensive remodeling. In chronic injury, hepatocyte proliferation is weak but sustained, and most regenerative nodules in liver cirrhosis are composed of clonally proliferating hepatocytes, suggesting that a small fraction of hepatocytes maintain their proliferative capacity in chronic injury. In mouse hepatocarcinogenesis models, hepatocytes activate the expression of various fetal/neonatal genes, indicating that these cells undergo dedifferentiation. Hepatocyte-specific somatic integration of various oncogenes in mice demonstrated that hepatocytes may be the cells of origin for a broad spectrum of liver tumors through transdifferentiation and dedifferentiation. In conclusion, the phenotypic plasticity and heterogeneity of mature hepatocytes are important for understanding the pathogenesis of chronic liver diseases and liver tumors.

Smart Microneedle Arrays Integrating Cell-Free Therapy and Nanocatalysis to Treat Liver Fibrosis

Liver fibrosis is a chronic pathological condition lacking specific clinical treatments. Stem cells, with notable potential in regenerative medicine, offer promise in treating liver fibrosis. However, stem cell therapy is hindered by potential immunological rejection, carcinogenesis risk, efficacy variation, and high cost. Stem cell secretome-based cell-free therapy offers potential solutions to address these challenges, but it is limited by low delivery efficiency and rapid clearance. Herein, an innovative approach for in situ implantation of smart microneedle (MN) arrays enabling precisely controlled delivery of multiple therapeutic agents directly into fibrotic liver tissues is developed. By integrating cell-free and platinum-based nanocatalytic combination therapy, the MN arrays can deactivate hepatic stellate cells. Moreover, they promote excessive extracellular matrix degradation by more than 75%, approaching normal levels. Additionally, the smart MN arrays can provide hepatocyte protection while reducing inflammation levels by ≈70-90%. They can also exhibit remarkable capability in scavenging almost 100% of reactive oxygen species and alleviating hypoxia. Ultimately, this treatment strategy can effectively restrain fibrosis progression. The comprehensive in vitro and in vivo experiments, supplemented by proteome and transcriptome analyses, substantiate the effectiveness of the approach in treating liver fibrosis, holding immense promise for clinical applications.

Acquisition of epithelial plasticity in human chronic liver disease

For many adult human organs, tissue regeneration during chronic disease remains a controversial subject. Regenerative processes are easily observed in animal models, and their underlying mechanisms are becoming well characterized1-4, but technical challenges and ethical aspects are limiting the validation of these results in humans. We decided to address this difficulty with respect to the liver. This organ displays the remarkable ability to regenerate after acute injury, although liver regeneration in the context of recurring injury remains to be fully demonstrated. Here we performed single-nucleus RNA sequencing (snRNA-seq) on 47 liver biopsies from patients with different stages of metabolic dysfunction-associated steatotic liver disease to establish a cellular map of the liver during disease progression. We then combined these single-cell-level data with advanced 3D imaging to reveal profound changes in the liver architecture. Hepatocytes lose their zonation and considerable reorganization of the biliary tree takes place. More importantly, our study uncovers transdifferentiation events that occur between hepatocytes and cholangiocytes without the presence of adult stem cells or developmental progenitor activation. Detailed analyses and functional validations using cholangiocyte organoids confirm the importance of the PI3K-AKT-mTOR pathway in this process, thereby connecting this acquisition of plasticity to insulin signalling. Together, our data indicate that chronic injury creates an environment that induces cellular plasticity in human organs, and understanding the underlying mechanisms of this process could open new therapeutic avenues in the management of chronic diseases.

ILKAP Promotes the Metastasis of Hepatocellular Carcinoma Cells by Inhibiting β-Catenin Degradation and Enhancing the WNT Signaling Pathway

The incidence of Hepatocellular carcinoma (HCC) and HCC-related deaths have remarkably increased over the recent decades. It has been reported that β-catenin activation can be frequently observed in HCC cases. This study identified the integrin-linked kinase-associated phosphatase (ILKAP) as a novel β-catenin-interacting protein. ILKAP is localized both in the nucleus and cytoplasm and regulates the WNT pathway in different ways. First, it is demonstrated that ILKAP activates the WNT pathway in HCC cells by increasing the protein level of β-catenin and other proteins associated with the WNT signaling, such as c-Myc and CyclinD1. Next, it is shown that ILKAP promotes the metastasis of HCC both in vitro and in vivo in a zebrafish xenograft model. It is also found that ILKAP dephosphorylates the GSK3β and CK1, contributing to the reduced ubiquitination of β-catenin. Furthermore, it is identified that ILKAP functions by mediating binding between TCF4 and β-catenin to enhance expression of WNT target genes. Taken together, the study demonstrates a critical function of ILKAP in metastasis of HCC, since ILKAP is crucial for the activation of the WNT pathway via stabilization of β-catenin and increased binding between TCF4 and β-catenin.

Iron-saturated bovine lactoferrin: a promising chemopreventive agent for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a tumor with minimal chance of cure due to underlying liver diseases, late diagnosis, and inefficient treatments. Thus, HCC treatment warrants the development of additional strategies. Lactoferrin (Lf) is a mammalian multifunctional iron-binding glycoprotein of the innate immune response and can be found as either a native low iron form (native-Lf) or a high iron form (holo-Lf). Bovine Lf (bLf), which shares many functions with human Lf (hLf), is safe for humans and has several anticancer activities, including chemotherapy boost in cancer. We found endogenous hLf is downregulated in HCC tumors compared with normal liver, and decreased hLf levels in HCC tumors are associated with shorter survival of HCC patients. However, the chemoprotective effect of 100% iron saturated holo-bLf on experimental hepatocarcinogenesis has not yet been determined. We aimed to evaluate the chemopreventive effects of holo-bLf in different HCC models. Remarkably, a single dose (200 mg kg-1) of holo-bLf was effective in preventing early carcinogenic events in a diethylnitrosamine induced HCC in vivo model, such as necrosis, ROS production, and the surge of facultative liver stem cells, and eventually, holo-bLf reduced the number of preneoplastic lesions. For an established HCC model, holo-bLf treatment significantly reduced HepG2 tumor burden in xenotransplanted mice. Finally, holo-bLf in combination with sorafenib, the advanced HCC first-line treatment, synergistically decreased HepG2 viability by arresting cells in the G0/G1 phase of the cell cycle. Our findings provide the first evidence suggesting that holo-bLf has the potential to prevent HCC or to be used in combination with treatments for established HCC.

Ductular Reactions in Liver Injury, Regeneration, and Disease Progression-An Overview

Ductular reaction (DR) is a complex cellular response that occurs in the liver during chronic injuries. DR mainly consists of hyper-proliferative or reactive cholangiocytes and, to a lesser extent, de-differentiated hepatocytes and liver progenitors presenting a close spatial interaction with periportal mesenchyme and immune cells. The underlying pathology of DRs leads to extensive tissue remodeling in chronic liver diseases. DR initiates as a tissue-regeneration mechanism in the liver; however, its close association with progressive fibrosis and inflammation in many chronic liver diseases makes it a more complicated pathological response than a simple regenerative process. An in-depth understanding of the cellular physiology of DRs and their contribution to tissue repair, inflammation, and progressive fibrosis can help scientists develop cell-type specific targeted therapies to manage liver fibrosis and chronic liver diseases effectively.

Interrogations of single-cell RNA splicing landscapes with SCASL define new cell identities with physiological relevance

RNA splicing shapes the gene regulatory programs that underlie various physiological and disease processes. Here, we present the SCASL (single-cell clustering based on alternative splicing landscapes) method for interrogating the heterogeneity of RNA splicing with single-cell RNA-seq data. SCASL resolves the issue of biased and sparse data coverage on single-cell RNA splicing and provides a new scheme for classifications of cell identities. With previously published datasets as examples, SCASL identifies new cell clusters indicating potentially precancerous and early-tumor stages in triple-negative breast cancer, illustrates cell lineages of embryonic liver development, and provides fine clusters of highly heterogeneous tumor-associated CD4 and CD8 T cells with functional and physiological relevance. Most of these findings are not readily available via conventional cell clustering based on single-cell gene expression data. Our study shows the potential of SCASL in revealing the intrinsic RNA splicing heterogeneity and generating biological insights into the dynamic and functional cell landscapes in complex tissues.

VEGFA mRNA-LNP promotes biliary epithelial cell-to-hepatocyte conversion in acute and chronic liver diseases and reverses steatosis and fibrosis

The liver is known for its remarkable regenerative ability through proliferation of hepatocytes. Yet, during chronic injury or severe hepatocyte death, proliferation of hepatocytes is exhausted. To overcome this hurdle, we propose vascular-endothelial-growth-factor A (VEGFA) as a therapeutic means to accelerate biliary epithelial-cell (BEC)-to-hepatocyte conversion. Investigation in zebrafish establishes that blocking VEGF receptors abrogates BEC-driven liver repair, while VEGFA overexpression promotes it. Delivery of VEGFA via nonintegrative and safe nucleoside-modified mRNA encapsulated into lipid nanoparticles (mRNA-LNPs) in acutely or chronically injured mouse livers induces robust BEC-to-hepatocyte conversion and elimination of steatosis and fibrosis. In human and murine diseased livers, we further identified VEGFA-receptor KDR-expressing BECs associated with KDR-expressing cell-derived hepatocytes. This work defines KDR-expressing cells, most likely being BECs, as facultative progenitors. This study reveals unexpected therapeutic benefits of VEGFA delivered via nucleoside-modified mRNA-LNP, whose safety is widely validated with COVID-19 vaccines, for harnessing BEC-driven repair to potentially treat liver diseases.

Modeling Liver Development and Disease in a Dish

Historically, biological research has relied primarily on animal models. While this led to the understanding of numerous human biological processes, inherent species-specific differences make it difficult to answer certain liver-related developmental and disease-specific questions. The advent of 3D organoid models that are either derived from pluripotent stem cells or generated from healthy or diseased tissue-derived stem cells have made it possible to recapitulate the biological aspects of human organs. Organoid technology has been instrumental in understanding the disease mechanism and complements animal models. This review underscores the advances in organoid technology and specifically how liver organoids are used to better understand human-specific biological processes in development and disease. We also discuss advances made in the application of organoid models in drug screening and personalized medicine.

Intrahepatic osteopontin signaling by CREBZF defines a checkpoint for steatosis-to-NASH progression

BACKGROUND AND AIMS

NASH has emerged as a leading cause of chronic liver disease. However, the mechanisms that govern NASH fibrosis remain largely unknown. CREBZF is a CREB/ATF bZIP transcription factor that causes hepatic steatosis and metabolic defects in obesity.

APPROACH AND RESULTS

Here, we show that CREBZF is a key mechanism of liver fibrosis checkpoint that promotes hepatocyte injury and exacerbates diet-induced NASH in mice. CREBZF deficiency attenuated liver injury, fibrosis, and inflammation in diet-induced mouse models of NASH. CREBZF increases HSC activation and fibrosis in a hepatocyte-autonomous manner by stimulating an extracellular matrix protein osteopontin, a key regulator of fibrosis. The inhibition of miR-6964-3p mediates CREBZF-induced production and secretion of osteopontin in hepatocytes. Adeno-associated virus -mediated rescue of osteopontin restored HSC activation, liver fibrosis, and NASH progression in CREBZF-deficient mice. Importantly, expression levels of CREBZF are increased in livers of diet-induced NASH mouse models and humans with NASH.

CONCLUSIONS

Osteopontin signaling by CREBZF represents a previously unrecognized intrahepatic mechanism that triggers liver fibrosis and contributes to the severity of NASH.

PPARα activation promotes liver progenitor cell-mediated liver regeneration by suppressing YAP signaling in zebrafish

Despite the robust regenerative capacity of the liver, prolonged and severe liver damage impairs liver regeneration, leading to liver failure. Since the liver co-opts the differentiation of liver progenitor cells (LPCs) into hepatocytes to restore functional hepatocytes, augmenting LPC-mediated liver regeneration may be beneficial to patients with chronic liver diseases. However, the molecular mechanisms underlying LPC-to-hepatocyte differentiation have remained largely unknown. Using the zebrafish model of LPC-mediated liver regeneration, Tg(fabp10a:pt-β-catenin), we present that peroxisome proliferator-activated receptor-alpha (PPARα) activation augments LPC-to-hepatocyte differentiation. We found that treating Tg(fabp10a:pt-β-catenin) larvae with GW7647, a potent PPARα agonist, enhanced the expression of hepatocyte markers and simultaneously reduced the expression of biliary epithelial cell (BEC)/LPC markers in the regenerating livers, indicating enhanced LPC-to-hepatocyte differentiation. Mechanistically, PPARα activation augments the differentiation by suppressing YAP signaling. The differentiation phenotypes resulting from GW7647 treatment were rescued by expressing a constitutively active form of Yap1. Moreover, we found that suppression of YAP signaling was sufficient to promote LPC-to-hepatocyte differentiation. Treating Tg(fabp10a:pt-β-catenin) larvae with the TEAD inhibitor K-975, which suppresses YAP signaling, phenocopied the effect of GW7647 on LPC differentiation. Altogether, our findings provide insights into augmenting LPC-mediated liver regeneration as a regenerative therapy for chronic liver diseases.

Panic at the Bile Duct: How Intrahepatic Cholangiocytes Respond to Stress and Injury

In the liver, biliary epithelial cells (BECs) line intrahepatic bile ducts (IHBDs) and are primarily responsible for modifying and transporting hepatocyte-produced bile to the digestive tract. BECs comprise only 3% to 5% of the liver by cell number but are critical for maintaining choleresis through homeostasis and disease. To this end, BECs drive an extensive morphologic remodeling of the IHBD network termed ductular reaction (DR) in response to direct injury or injury to the hepatic parenchyma. BECs are also the target of a broad and heterogenous class of diseases termed cholangiopathies, which can present with phenotypes ranging from defective IHBD development in pediatric patients to progressive periductal fibrosis and cancer. DR is observed in many cholangiopathies, highlighting overlapping similarities between cell- and tissue-level responses by BECs across a spectrum of injury and disease. The following core set of cell biological BEC responses to stress and injury may moderate, initiate, or exacerbate liver pathophysiology in a context-dependent manner: cell death, proliferation, transdifferentiation, senescence, and acquisition of neuroendocrine phenotype. By reviewing how IHBDs respond to stress, this review seeks to highlight fundamental processes with potentially adaptive or maladaptive consequences. A deeper understanding of how these common responses contribute to DR and cholangiopathies may identify novel therapeutic targets in liver disease.

Intact mitochondrial function in the setting of telomere-induced senescence

Mitochondria play essential roles in metabolic support and signaling within all cells. Congenital and acquired defects in mitochondria are responsible for several pathologies, including premature entrance to cellar senescence. Conversely, we examined the consequences of dysfunctional telomere-driven cellular senescence on mitochondrial biogenesis and function. We drove senescence in vitro and in vivo by deleting the telomere-binding protein TRF2 in fibroblasts and hepatocytes, respectively. Deletion of TRF2 led to a robust DNA damage response, global changes in transcription, and induction of cellular senescence. In vitro, senescent cells had significant increases in mitochondrial respiratory capacity driven by increased cellular and mitochondrial volume. Hepatocytes with dysfunctional telomeres maintained their mitochondrial respiratory capacity in vivo, whether measured in intact cells or purified mitochondria. Induction of senescence led to the upregulation of overlapping and distinct genes in fibroblasts and hepatocytes, but transcripts related to mitochondria were preserved. Our results support that mitochondrial function and activity are preserved in telomere dysfunction-induced senescence, which may facilitate continued cellular functions.

VEGF signaling governs the initiation of biliary-mediated liver regeneration through the PI3K-mTORC1 axis

Biliary epithelial cells (BECs) are a potential source to repair the damaged liver when hepatocyte proliferation is compromised. Promotion of BEC-to-hepatocyte transdifferentiation could be beneficial to the clinical therapeutics of patients with end-stage liver diseases. However, mechanisms underlying the initiation of BEC transdifferentiation remain largely unknown. Here, we show that upon extreme hepatocyte injury, vegfaa and vegfr2/kdrl are notably induced in hepatic stellate cells and BECs, respectively. Pharmacological and genetic inactivation of vascular endothelial growth factor (VEGF) signaling would disrupt BEC dedifferentiation and proliferation, thus restraining hepatocyte regeneration. Mechanically, VEGF signaling regulates the activation of the PI3K-mammalian target of rapamycin complex 1 (mTORC1) axis, which is essential for BEC-to-hepatocyte transdifferentiation. In mice, VEGF signaling exerts conserved roles in oval cell activation and BEC-to-hepatocyte differentiation. Taken together, this study shows VEGF signaling as an initiator of biliary-mediated liver regeneration through activating the PI3K-mTORC1 axis. Modulation of VEGF signaling in BECs could be a therapeutic approach for patients with end-stage liver diseases.

Hepatocyte dedifferentiation profiling in alcohol-related liver disease identifies CXCR4 as a driver of cell reprogramming

BACKGROUND & AIMS

Loss of hepatocyte identity is associated with impaired liver function in alcohol-related hepatitis (AH). In this context, hepatocyte dedifferentiation gives rise to cells with a hepatobiliary (HB) phenotype expressing biliary and hepatocyte markers and showing immature features. However, the mechanisms and impact of hepatocyte dedifferentiation in liver disease are poorly understood.

METHODS

HB cells and ductular reaction (DR) cells were quantified and microdissected from liver biopsies from patients with alcohol-related liver disease (ArLD). Hepatocyte-specific overexpression or deletion of C-X-C motif chemokine receptor 4 (CXCR4), and CXCR4 pharmacological inhibition were assessed in mouse liver injury. Patient-derived and mouse organoids were generated to assess plasticity.

RESULTS

Here, we show that HB and DR cells are increased in patients with decompensated cirrhosis and AH, but only HB cells correlate with poor liver function and patients' outcome. Transcriptomic profiling of HB cells revealed the expression of biliary-specific genes and a mild reduction of hepatocyte metabolism. Functional analysis identified pathways involved in hepatocyte reprogramming, inflammation, stemness, and cancer gene programs. The CXCR4 pathway was highly enriched in HB cells and correlated with disease severity and hepatocyte dedifferentiation. In vitro, CXCR4 was associated with a biliary phenotype and loss of hepatocyte features. Liver overexpression of CXCR4 in chronic liver injury decreased the hepatocyte-specific gene expression profile and promoted liver injury. CXCR4 deletion or its pharmacological inhibition ameliorated hepatocyte dedifferentiation and reduced DR and fibrosis progression.

CONCLUSIONS

This study shows the association of hepatocyte dedifferentiation with disease progression and poor outcome in AH. Moreover, the transcriptomic profiling of HB cells revealed CXCR4 as a new driver of hepatocyte-to-biliary reprogramming and as a potential therapeutic target to halt hepatocyte dedifferentiation in AH.

IMPACT AND IMPLICATIONS

Here, we show that hepatocyte dedifferentiation is associated with disease severity and a reduced synthetic capacity of the liver. Moreover, we identify the CXCR4 pathway as a driver of hepatocyte dedifferentiation and as a therapeutic target in alcohol-related hepatitis. Therefore, this study reveals the importance of preserving strict control over hepatocyte plasticity in order to preserve liver function and promote tissue repair.

Hepatocytes undergo punctuated expansion dynamics from a periportal stem cell niche in normal human liver

BACKGROUND & AIMS

While normal human liver is thought to be generally quiescent, clonal hepatocyte expansions have been observed, though neither their cellular source nor their expansion dynamics have been determined. Knowing the hepatocyte cell of origin, and their subsequent dynamics and trajectory within the human liver will provide an important basis to understand disease-associated dysregulation.

METHODS

Herein, we use in vivo lineage tracing and methylation sequence analysis to demonstrate normal human hepatocyte ancestry. We exploit next-generation mitochondrial sequencing to determine hepatocyte clonal expansion dynamics across spatially distinct areas of laser-captured, microdissected, clones, in tandem with computational modelling in morphologically normal human liver.

RESULTS

Hepatocyte clones and rare SOX9+ hepatocyte progenitors commonly associate with portal tracts and we present evidence that clones can lineage-trace with cholangiocytes, indicating the presence of a bipotential common ancestor at this niche. Within clones, we demonstrate methylation CpG sequence diversity patterns indicative of periportal not pericentral ancestral origins, indicating a portal to central vein expansion trajectory. Using spatial analysis of mitochondrial DNA variants by next-generation sequencing coupled with mathematical modelling and Bayesian inference across the portal-central axis, we demonstrate that patterns of mitochondrial DNA variants reveal large numbers of spatially restricted mutations in conjunction with limited numbers of clonal mutations.

CONCLUSIONS

These datasets support the existence of a periportal progenitor niche and indicate that clonal patches exhibit punctuated but slow growth, then quiesce, likely due to acute environmental stimuli. These findings crucially contribute to our understanding of hepatocyte dynamics in the normal human liver.

IMPACT AND IMPLICATIONS

The liver is mainly composed of hepatocytes, but we know little regarding the source of these cells or how they multiply over time within the disease-free human liver. In this study, we determine a source of new hepatocytes by combining many different lab-based methods and computational predictions to show that hepatocytes share a common cell of origin with bile ducts. Both our experimental and computational data also demonstrate hepatocyte clones are likely to expand in slow waves across the liver in a specific trajectory, but often lie dormant for many years. These data show for the first time the expansion dynamics of hepatocytes in normal liver and their cell of origin enabling the accurate measurment of changes to their dynamics that may lead to liver disease. These findings are important for researchers determining cancer risk in human liver.

Distinct hepatocyte identities in liver homeostasis and regeneration

The process of metabolic liver zonation is spontaneously established by assigning distributed tasks to hepatocytes along the porto-central blood flow. Hepatocytes fulfil critical metabolic functions, while also maintaining hepatocyte mass by replication when needed. Recent technological advances have enabled us to fine-tune our understanding of hepatocyte identity during homeostasis and regeneration. Subsets of hepatocytes have been identified to be more regenerative and some have even been proposed to function like stem cells, challenging the long-standing view that all hepatocytes are similarly capable of regeneration. The latest data show that hepatocyte renewal during homeostasis and regeneration after liver injury is not limited to rare hepatocytes; however, hepatocytes are not exactly the same. Herein, we review the known differences that give individual hepatocytes distinct identities, recent findings demonstrating how these distinct identities correspond to differences in hepatocyte regenerative capacity, and how the plasticity of hepatocyte identity allows for division of labour among hepatocytes. We further discuss how these distinct hepatocyte identities may play a role during liver disease.

Rngtt governs biliary-derived liver regeneration initiation by transcriptional regulation of mTORC1 and Dnmt1 in zebrafish

In cases of end-stage liver diseases, the proliferation of existing hepatocytes is compromised, a feature of human chronic liver disease, in which most hepatocytes are dysfunctional. So far, liver transplantation represents the only curative therapeutic solution for advanced liver diseases, and the shortage of donor organs leads to high morbidity and mortality worldwide. The promising treatment is to prompt the biliary epithelial cells (BECs) transdifferentiation. However, the critical factors governing the initiation of BEC-derived liver regeneration are largely unknown. The zebrafish has advantages in large-scale genetic screens to identify the critical factors involved in liver regeneration. Here, we combined N-ethyl-N-nitrosourea screen, positional cloning, transgenic lines, antibody staining, and in situ hybridization methods and identified a liver regeneration defect mutant ( lrd ) using the zebrafish extensive liver injury model. Through positional cloning and genomic sequencing, we mapped the mutation site to rngtt . Loss of rngtt leads to the defects of BEC dedifferentiation, bipotential progenitor cell activation, and cell proliferation in the initiation stage of liver regeneration. The transdifferentiation from BECs to hepatocytes did not occur even at the late stage of liver regeneration. Mechanically, Rngtt transcriptionally regulates the attachment of mRNA cap to mTOR complex 1 (mTORC1) components and dnmt1 to maintain the activation of mTORC1 and DNA methylation in BECs after severe liver injury and prompt BEC to hepatocyte conversion. Furthermore, rptor and dnmt1 mutants displayed the same liver regeneration defects as rngtt mutation. In conclusion, our results suggest Rngtt is a new factor that initiates BEC-derived liver regeneration.

Ductular reaction in non-alcoholic fatty liver disease: When Macbeth is perverted

Non-alcoholic fatty liver disease (NAFLD) or metabolic (dysfunction)-associated fatty liver disease is the leading cause of chronic liver diseases defined as a disease spectrum comprising hepatic steatosis, non-alcoholic steatohepatitis (NASH), liver fibrosis, cirrhosis, and hepatic carcinoma. NASH, characterized by hepatocyte injury, steatosis, inflammation, and fibrosis, is associated with NAFLD prognosis. Ductular reaction (DR) is a common compensatory reaction associated with liver injury, which involves the hepatic progenitor cells (HPCs), hepatic stellate cells, myofibroblasts, inflammatory cells (such as macrophages), and their secreted substances. Recently, several studies have shown that the extent of DR parallels the stage of NASH and fibrosis. This review summarizes previous research on the correlation between DR and NASH, the potential interplay mechanism driving HPC differentiation, and NASH progression.

Cholangiokines: undervalued modulators in the hepatic microenvironment

The biliary epithelial cells, also known as cholangiocytes, line the intra- and extrahepatic bile ducts, forming a barrier between intra- and extra-ductal environments. Cholangiocytes are mostly known to modulate bile composition and transportation. In hepatobiliary diseases, bile duct injury leads to drastic alterations in cholangiocyte phenotypes and their release of soluble mediators, which can vary depending on the original insult and cellular states (quiescence, senescence, or proliferation). The cholangiocyte-secreted cytokines (also termed cholangiokines) drive ductular cell proliferation, portal inflammation and fibrosis, and carcinogenesis. Hence, despite the previous consensus that cholangiocytes are bystanders in liver diseases, their diverse secretome plays critical roles in modulating the intrahepatic microenvironment. This review summarizes recent insights into the cholangiokines under both physiological and pathological conditions, especially as they occur during liver injury-regeneration, inflammation, fibrosis and malignant transformation processes.

VEGFA mRNA-LNP promotes biliary epithelial cell-to-hepatocyte conversion in acute and chronic liver diseases and reverses steatosis and fibrosis

The liver is known for its remarkable regenerative ability through proliferation of hepatocytes. Yet, during chronic injury or severe hepatocyte death, proliferation of hepatocytes is exhausted. To overcome this hurdle, we propose vascular-endothelial-growth-factor A (VEGFA) as a therapeutic means to accelerate biliary epithelial cell (BEC)-to-hepatocyte conversion. Investigation in zebrafish establishes that blocking VEGF receptors abrogates BEC-driven liver repair, while VEGFA overexpression promotes it. Delivery of VEGFA via non-integrative and safe nucleoside-modified mRNA encapsulated into lipid-nanoparticles (mRNA-LNP) in acutely or chronically injured mouse livers induces robust BEC-to-hepatocyte conversion and reversion of steatosis and fibrosis. In human and murine diseased livers, we further identified VEGFA-receptor KDR-expressing BECs associated with KDR-expressing cell-derived hepatocytes. This defines KDR-expressing cells, most likely being BECs, as facultative progenitors. This study reveals novel therapeutic benefits of VEGFA delivered via nucleoside-modified mRNA-LNP, whose safety is widely validated with COVID-19 vaccines, for harnessing BEC-driven repair to potentially treat liver diseases.

Highlights

Complementary mouse and zebrafish models of liver injury demonstrate the therapeutic impact of VEGFA-KDR axis activation to harness BEC-driven liver regeneration.VEGFA mRNA LNPs restore two key features of the chronic liver disease in humans such as steatosis and fibrosis.Identification in human cirrhotic ESLD livers of KDR-expressing BECs adjacent to clusters of KDR+ hepatocytes suggesting their BEC origin.KDR-expressing BECs may represent facultative adult progenitor cells, a unique BEC population that has yet been uncovered.

The MRN complex maintains the biliary-derived hepatocytes in liver regeneration through ATR-Chk1 pathway

When the proliferation of residual hepatocytes is prohibited, biliary epithelial cells (BECs) transdifferentiate into nascent hepatocytes to accomplish liver regeneration. Despite significant interest in transdifferentiation, little is known about the maintenance of nascent hepatocytes in post-injured environments. Here, we perform an N-ethyl-N-nitrosourea (ENU) forward genetic screen and identify a mutant containing a nonsense mutation in the gene nibrin (nbn), which encodes a component of the Mre11-Rad50-Nbn (MRN) complex that activates DNA damage response (DDR). The regenerated hepatocytes cannot be maintained and exhibit apoptosis in the mutant. Mechanistically, the nbn mutation results in the abrogation of ATR-Chk1 signaling and accumulations of DNA damage in nascent hepatocytes, which eventually induces p53-mediated apoptosis. Furthermore, loss of rad50 or mre11a shows similar phenotypes. This study reveals that the activation of DDR by the MRN complex is essential for the survival of BEC-derived hepatocytes, addressing how to maintain nascent hepatocytes in the post-injured environments.

Hepatocyte Dedifferentiation Profiling In Alcohol-Related Liver Disease Identifies CXCR4 As A Driver Of Cell Reprogramming

Background and Aims

Loss of hepatocyte identity is associated with impaired liver function in alcohol-related hepatitis (AH). In this context, hepatocyte dedifferentiation gives rise to cells with a hepatobiliary (HB) phenotype expressing biliary and hepatocytes markers and showing immature features. However, the mechanisms and the impact of hepatocyte dedifferentiation in liver disease are poorly understood.

Methods

HB cells and ductular reaction (DR) cells were quantified and microdissected from liver biopsies from patients with alcohol-related liver disease (ALD). Hepatocyte- specific overexpression or deletion of CXCR4, and CXCR4 pharmacological inhibition were assessed in mouse liver injury. Patient-derived and mouse organoids were generated to assess plasticity.

Results

Here we show that HB and DR cells are increased in patients with decompensated cirrhosis and AH, but only HB cells correlate with poor liver function and patients' outcome. Transcriptomic profiling of HB cells revealed the expression of biliary-specific genes and a mild reduction of hepatocyte metabolism. Functional analysis identified pathways involved in hepatocyte reprogramming, inflammation, stemness and cancer gene programs. CXCR4 pathway was highly enriched in HB cells, and correlated with disease severity and hepatocyte dedifferentiation. In vitro , CXCR4 was associated with biliary phenotype and loss of hepatocyte features. Liver overexpression of CXCR4 in chronic liver injury decreased hepatocyte specific gene expression profile and promoted liver injury. CXCR4 deletion or its pharmacological inhibition ameliorated hepatocyte dedifferentiation and reduced DR and fibrosis progression.

Conclusions

This study shows the association of hepatocyte dedifferentiation with disease progression and poor outcome in AH. Moreover, the transcriptomic profiling of HB cells revealed CXCR4 as a new driver of hepatocyte-to-biliary reprogramming and as a potential therapeutic target to halt hepatocyte dedifferentiation in AH.

Lay summary

Here we describe that hepatocyte dedifferentiation is associated with disease severity and a reduced synthetic capacity of the liver. Moreover, we identify the CXCR4 pathway as a driver of hepatocyte dedifferentiation and as a therapeutic target in alcohol-related hepatitis.

Bipotent transitional liver progenitor cells contribute to liver regeneration

Following severe liver injury, when hepatocyte-mediated regeneration is impaired, biliary epithelial cells (BECs) can transdifferentiate into functional hepatocytes. However, the subset of BECs with such facultative tissue stem cell potential, as well as the mechanisms enabling transdifferentiation, remains elusive. Here we identify a transitional liver progenitor cell (TLPC), which originates from BECs and differentiates into hepatocytes during regeneration from severe liver injury. By applying a dual genetic lineage tracing approach, we specifically labeled TLPCs and found that they are bipotent, as they either differentiate into hepatocytes or re-adopt BEC fate. Mechanistically, Notch and Wnt/β-catenin signaling orchestrate BEC-to-TLPC and TLPC-to-hepatocyte conversions, respectively. Together, our study provides functional and mechanistic insights into transdifferentiation-assisted liver regeneration.

Hepatic lipid overload triggers biliary epithelial cell activation via E2Fs

During severe or chronic hepatic injury, biliary epithelial cells (BECs) undergo rapid activation into proliferating progenitors, a crucial step required to establish a regenerative process known as ductular reaction (DR). While DR is a hallmark of chronic liver diseases, including advanced stages of non-alcoholic fatty liver disease (NAFLD), the early events underlying BEC activation are largely unknown. Here, we demonstrate that BECs readily accumulate lipids during high-fat diet feeding in mice and upon fatty acid treatment in BEC-derived organoids. Lipid overload induces metabolic rewiring to support the conversion of adult cholangiocytes into reactive BECs. Mechanistically, we found that lipid overload activates the E2F transcription factors in BECs, which drive cell cycle progression while promoting glycolytic metabolism. These findings demonstrate that fat overload is sufficient to reprogram BECs into progenitor cells in the early stages of NAFLD and provide new insights into the mechanistic basis of this process, revealing unexpected connections between lipid metabolism, stemness, and regeneration.

Multiple Facets of Cellular Homeostasis and Regeneration of the Mammalian Liver

Liver regeneration occurs in response to diverse injuries and is capable of functionally reestablishing the lost parenchyma. This phenomenon has been known since antiquity, encapsulated in the Greek myth where Prometheus was to be punished by Zeus for sharing the gift of fire with humanity by having an eagle eat his liver daily, only to have the liver regrow back, thus ensuring eternal suffering and punishment. Today, this process is actively leveraged clinically during living donor liver transplantation whereby up to a two-thirds hepatectomy (resection or removal of part of the liver) on a donor is used for transplant to a recipient. The donor liver rapidly regenerates to recover the lost parenchymal mass to form a functional tissue. This astonishing regenerative process and unique capacity of the liver are examined in further detail in this review.

Is liver regeneration key in hepatocellular carcinoma development?

The liver is the largest organ of the mammalian body and has the remarkable ability to fully regenerate in order to maintain tissue homeostasis. The adult liver consists of hexagonal lobules, each with a central vein surrounded by six portal triads localized in the lobule border containing distinct parenchymal and nonparenchymal cells. Because the liver is continuously exposed to diverse stress signals, several sophisticated regenerative processes exist to restore its functional status following impairment. However, these stress signals can affect the liver's capacity to regenerate and may lead to the development of hepatocellular carcinoma (HCC), one of the most aggressive liver cancers. Here, we review the mechanisms of hepatic regeneration and their potential to influence HCC development.

Update on Hepatobiliary Plasticity

The liver field has been debating for decades the contribution of the plasticity of the two epithelial compartments in the liver, hepatocytes and biliary epithelial cells (BECs), to derive each other as a repair mechanism. The hepatobiliary plasticity has been first observed in diseased human livers by the presence of biphenotypic cells expressing hepatocyte and BEC markers within bile ducts and regenerative nodules or budding from strings of proliferative BECs in septa. These observations are not surprising as hepatocytes and BECs derive from a common fetal progenitor, the hepatoblast, and, as such, they are expected to compensate for each other's loss in adults. To investigate the cell origin of regenerated cell compartments and associated molecular mechanisms, numerous murine and zebrafish models with ability to trace cell fates have been extensively developed. This short review summarizes the clinical and preclinical studies illustrating the hepatobiliary plasticity and its potential therapeutic application.

Biliary epithelial cells are facultative liver stem cells during liver regeneration in adult zebrafish

The liver is a highly regenerative organ, yet the presence of a dedicated stem cell population remains controversial. Here, we interrogate a severe hepatocyte injury model in adult zebrafish to define that regeneration involves a stem cell population. After near-total hepatocyte ablation, single-cell transcriptomic and high-resolution imaging analyses throughout the entire regenerative timeline reveal that biliary epithelial cells undergo transcriptional and morphological changes to become hepatocytes. As a population, biliary epithelial cells give rise to both hepatocytes and biliary epithelial cells. Biliary epithelial cells proliferate and dedifferentiate to express hepatoblast transcription factors prior to hepatocyte differentiation. This process is characterized by increased MAPK, PI3K, and mTOR signaling, and chemical inhibition of these pathways impairs biliary epithelial cell proliferation and fate conversion. We conclude that, upon severe hepatocyte ablation in the adult liver, biliary epithelial cells act as facultative liver stem cells in an EGFR-PI3K-mTOR-dependent manner.

DNA methylation in cell plasticity and malignant transformation in liver diseases

The liver possesses extraordinary regenerative capacity mainly attributable to the ability of hepatocytes (HCs) and biliary epithelial cells (BECs) to self-replicate. This ability is left over from their bipotent parent cell, the hepatoblast, during development. When this innate regeneration is compromised due to the absence of proliferative parenchymal cells, such as during cirrhosis, HCs and BEC can transdifferentiate; thus, adding another layer of complexity to the process of liver repair. In addition, dysregulated lineage maintenance in these two cell populations has been shown to promote malignant growth in experimental conditions. Here, malignant transformation, driven in part by insufficient maintenance of lineage reprogramming, contributes to end-stage liver disease. Epigenetic changes are key drivers for cell fate decisions as well as transformation by finetuning overall transcription and gene expression. In this review, we address how altered DNA methylation contributes to the initiation and progression of hepatic cell fate conversion and cancer formation. We also discussed the diagnostic and therapeutic potential of targeting DNA methylation in liver cancer, its current limitations, and what future research is necessary to facilitate its contribution to clinical translation.

Hepatocyte-derived GDF15 suppresses feeding and improves insulin sensitivity in obese mice

Growth differentiation factor 15 (GDF15) is a stress-induced secreted protein whose circulating levels are increased in the context of obesity. Recombinant GDF15 reduces body weight and improves glycemia in obese models, which is largely attributed to the central action of GDF15 to suppress feeding and reduce body weight. Despite these advances in knowledge, the tissue-specific sites of GDF15 production during obesity are unknown, and the effects of modulating circulating GDF15 levels on insulin sensitivity have not been evaluated directly. Here, we demonstrate that hepatocyte Gdf15 expression is sufficient for changes in circulating levels of GDF15 during obesity and that restoring Gdf15 expression specifically in hepatocytes of Gdf15 knockout mice results in marked improvements in hyperinsulinemia, hepatic insulin sensitivity, and to a lesser extent peripheral insulin sensitivity. These data support that liver hepatocytes are the primary source of circulating GDF15 in obesity.

The RSPO-LGR4/5-ZNRF3/RNF43 module in liver homeostasis, regeneration, and disease

WNT/β-catenin signaling plays pivotal roles during liver development, homeostasis, and regeneration. Likewise, its deregulation disturbs metabolic liver zonation and is responsible for the development of a large number of hepatic tumors. Liver fibrosis, which has become a major health burden for society and a hallmark of NASH, can also be promoted by WNT/β-catenin signaling. Upstream regulatory mechanisms controlling hepatic WNT/β-catenin activity may constitute targets for the development of novel therapies addressing these life-threatening conditions. The R-spondin (RSPO)-leucine-rich repeat-containing G protein-coupled receptor (LGR) 4/5-zinc and ring finger (ZNRF) 3/ring finger 43 (RNF43) module is fine-tuning WNT/β-catenin signaling in several tissues and is essential for hepatic WNT/β-catenin activity. In this review article, we recapitulate the role of the RSPO-LGR4/5-ZNRF3/RNF43 module during liver development, homeostasis, metabolic zonation, regeneration, and disease. We further discuss the controversy around LGR5 as a liver stem cell marker.

Cellular crosstalk during liver regeneration: unity in diversity

The liver is unique in its ability to regenerate from a wide range of injuries and diseases. Liver regeneration centers around hepatocyte proliferation and requires the coordinated actions of nonparenchymal cells, including biliary epithelial cells, liver sinusoidal endothelial cells, hepatic stellate cells and kupffer cells. Interactions among various hepatocyte and nonparenchymal cells populations constitute a sophisticated regulatory network that restores liver mass and function. In addition, there are two different ways of liver regeneration, self-replication of liver epithelial cells and transdifferentiation between liver epithelial cells. The interactions among cell populations and regenerative microenvironment in the two modes are distinct. Herein, we first review recent advances in the interactions between hepatocytes and surrounding cells and among nonparenchymal cells in the context of liver epithelial cell self-replication. Next, we discuss the crosstalk of several cell types in the context of liver epithelial transdifferentiation, which is also crucial for liver regeneration.

Cellular Homeostasis and Repair in the Biliary Tree

During biliary tree homeostasis, BECs are largely in a quiescent state and their turnover is slow for maintaining normal tissue homeostasis. BTSCs continually replenish new BECs in the luminal surface of EHBDs. In response to various types of biliary injuries, distinct cellular sources, including HPCs, BTSCs, hepatocytes, and BECs, repair or regenerate the injured bile duct. BEC, biliary epithelial cell; BTSC, biliary tree stem/progenitor cell; EHBD, extrahepatic bile ducts; HPC, hepatic progenitor cell.The biliary tree comprises intrahepatic bile ducts and extrahepatic bile ducts lined with epithelial cells known as biliary epithelial cells (BECs). BECs are a common target of various cholangiopathies for which there is an unmet therapeutic need in clinical hepatology. The repair and regeneration of biliary tissue may potentially restore the normal architecture and function of the biliary tree. Hence, the repair and regeneration process in detail, including the replication of existing BECs, expansion and differentiation of the hepatic progenitor cells and biliary tree stem/progenitor cells, and transdifferentiation of the hepatocytes, should be understood. In this paper, we review biliary tree homeostasis, repair, and regeneration and discuss the feasibility of regenerative therapy strategies for cholangiopathy treatment.

Roles of Yes-associated protein and transcriptional coactivator with PDZ-binding motif in non-neoplastic liver diseases

The prevalence of liver disease has been increasing worldwide. Moreover, the burden of end-stage liver disease, including cirrhosis and liver cancer, is high because of high mortality and suboptimal treatment. The pathological process of liver disease includes steatosis, hepatocyte death, and fibrosis, which ultimately lead to cirrhosis and liver cancer. Clinical and preclinical evidence indicates that non-neoplastic liver diseases, particularly cirrhosis, are major risk factors for liver cancer, although the mechanism underlying this association remains unclear. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are transcriptional activators that regulate organ size and cancer development. YAP and TAZ play important roles in liver development, regeneration, and homeostasis. Abnormal YAP and TAZ levels have also been implicated in non-neoplastic liver diseases (e.g., non-alcoholic fatty liver disease, alcoholic liver disease, liver injury, and liver fibrosis). Here, we review recent findings on the roles of YAP and TAZ in non-neoplastic liver diseases and discuss directions for future research. This review provides a basis for the study of non-neoplastic liver diseases.

Liver regeneration: Cellular origin and molecular mechanisms

The liver is known as an organ with high proliferation potential. Clarifying the cellular origin and deepening the understanding of liver regeneration mechanisms will help provide new directions for the treatment of liver disease. With the development and application of lineage tracing technology, the specific distribution and dynamic changes of hepatocyte subpopulations in homeostasis and liver injury have been illustrated. Self-replication of hepatocytes is responsible for the maintenance of liver function and mass under homeostasis. The compensatory proliferation of remaining hepatocytes is the main mechanism of liver regeneration following acute and chronic liver injury. Transdifferentiation between hepatocytes and cholangiocytes has been recognized upon severe chronic liver injury. Wnt/β-catenin, Hippo/YAP and Notch signalling play essential roles in the maintenance of homeostatic liver and hepatocyte-to-cholangiocyte conversion under liver injury. In this review, we summarized the recent studies on cell origin of newly generated hepatocytes and the underlying mechanisms of liver regeneration in homeostasis and liver injury.

Cell identity conversion in liver regeneration after injury

Cell identity conversion in liver injury is the process that mature cells, specifically hepatocytes or cholangiocytes, convert into cells with other identities, which is found to play a pivotal role in liver regeneration. A better characterization of cell identity conversion will not only facilitate the understanding of liver tissue repair but also the development of novel regenerative therapies. In this review, we discuss the latest advances in cell identity conversion during liver regeneration, including conversions between hepatocytes and cholangiocytes and hepatocyte reprogramming to liver progenitor-like cells. To develop a unified description of cellular states in injury-related liver regeneration, we further propose the quantitative approach to explore cell identity conversion based on the Waddington's landscape.

Hippo signalling in the liver: role in development, regeneration and disease

The Hippo signalling pathway has emerged as a major player in many aspects of liver biology, such as development, cell fate determination, homeostatic function and regeneration from injury. The regulation of Hippo signalling is complex, with activation of the pathway by diverse upstream inputs including signals from cellular adhesion, mechanotransduction and crosstalk with other signalling pathways. Pathological activation of the downstream transcriptional co-activators yes-associated protein 1 (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ, encoded by WWTR1), which are negatively regulated by Hippo signalling, has been implicated in multiple aspects of chronic liver disease, such as the development of liver fibrosis and tumorigenesis. Thus, development of pharmacological inhibitors of YAP-TAZ signalling has been an area of great interest. In this Review, we summarize the diverse roles of Hippo signalling in liver biology and highlight areas where outstanding questions remain to be investigated. Greater understanding of the mechanisms of Hippo signalling in liver function should help facilitate the development of novel therapies for the treatment of liver disease.

Tel2 regulates redifferentiation of bipotential progenitor cells via Hhex during zebrafish liver regeneration

Upon extensive hepatocyte loss or impaired hepatocyte proliferation, liver regeneration occurs via biliary epithelial cell (BEC) transdifferentiation, which includes dedifferentiation of BECs into bipotential progenitor cells (BP-PCs) and then redifferentiation of BP-PCs to nascent hepatocytes and BECs. This BEC-driven liver regeneration involves reactivation of hepatoblast markers, but the underpinning mechanisms and their effects on liver regeneration remain largely unknown. Using a zebrafish extensive hepatocyte ablation model, we perform an N-ethyl-N-nitrosourea (ENU) forward genetic screen and identify a liver regeneration mutant, liver logan (lvl), in which the telomere maintenance 2 (tel2) gene is mutated. During liver regeneration, the tel2 mutation specifically inhibits transcriptional activation of a hepatoblast marker, hematopoietically expressed homeobox (hhex), in BEC-derived cells, which blocks BP-PC redifferentiation. Mechanistic studies show that Tel2 associates with the hhex promoter region and promotes hhex transcription. Our results reveal roles of Tel2 in the BP-PC redifferentiation process of liver regeneration by activating hhex.

Cellular origins of regenerating liver and hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the predominant primary cancer arising from the liver and is one of the major causes of cancer-related mortality worldwide. The cellular origin of HCC has been a topic of great interest due to conflicting findings regarding whether it originates in hepatocytes, biliary cells, or facultative stem cells. These cell types all undergo changes during liver injury, and there is controversy about their contribution to regenerative responses in the liver. Most HCCs emerge in the setting of chronic liver injury from viral hepatitis, fatty liver disease, alcohol, and environmental exposures. The injuries are marked by liver parenchymal changes such as hepatocyte regenerative nodules, biliary duct cellular changes, expansion of myofibroblasts that cause fibrosis and cirrhosis, and inflammatory cell infiltration, all of which may contribute to carcinogenesis. Addressing the cellular origin of HCC is the key to identifying the earliest events that trigger it. Herein, we review data on the cells of origin in regenerating liver and HCC and the implications of these findings for prevention and treatment. We also review the origins of childhood liver cancer and other rare cancers of the liver.

DNA methylation maintenance at the p53 locus initiates biliary-mediated liver regeneration

In cases of extensive liver injury, biliary epithelial cells (BECs) dedifferentiate into bipotential progenitor cells (BPPCs), then redifferentiate into hepatocytes and BECs to accomplish liver regeneration. Whether epigenetic regulations, particularly DNA methylation maintenance enzymes, play a role in this biliary-mediated liver regeneration remains unknown. Here we show that in response to extensive hepatocyte damages, expression of dnmt1 is upregulated in BECs to methylate DNA at the p53 locus, which represses p53 transcription, and in turn, derepresses mTORC1 signaling to activate BEC dedifferentiation. After BEC dedifferentiation and BPPC formation, DNA methylation at the p53 locus maintains in BPPCs to continue blocking p53 transcription, which derepresses Bmp signaling to induce BPPC redifferentiation. Thus, this study reveals promotive roles and mechanisms of DNA methylation at the p53 locus in both dedifferentiation and redifferentiation stages of biliary-mediated liver regeneration, implicating DNA methylation and p53 as potential targets to stimulate regeneration after extensive liver injury.

Hepatocyte generation in liver homeostasis, repair, and regeneration

The liver has remarkable capability to regenerate, employing mechanism to ensure the stable liver-to-bodyweight ratio for body homeostasis. The source of this regenerative capacity has received great attention over the past decade yet still remained controversial currently. Deciphering the sources for hepatocytes provides the basis for understanding tissue regeneration and repair, and also illustrates new potential therapeutic targets for treating liver diseases. In this review, we describe recent advances in genetic lineage tracing studies over liver stem cells, hepatocyte proliferation, and cell lineage conversions or cellular reprogramming. This review will also evaluate the technical strengths and limitations of methods used for studies on hepatocyte generation and cell fate plasticity in liver homeostasis, repair and regeneration.

Liver Regeneration and Cell Transplantation for End-Stage Liver Disease

Liver transplantation is the only curative option for end-stage liver disease; however, the limitations of liver transplantation require further research into other alternatives. Considering that liver regeneration is prevalent in liver injury settings, regenerative medicine is suggested as a promising therapeutic strategy for end-stage liver disease. Upon the source of regenerating hepatocytes, liver regeneration could be divided into two categories: hepatocyte-driven liver regeneration (typical regeneration) and liver progenitor cell-driven liver regeneration (alternative regeneration). Due to the massive loss of hepatocytes, the alternative regeneration plays a vital role in end-stage liver disease. Advances in knowledge of liver regeneration and tissue engineering have accelerated the progress of regenerative medicine strategies for end-stage liver disease. In this article, we generally reviewed the recent findings and current knowledge of liver regeneration, mainly regarding aspects of the histological basis of regeneration, histogenesis and mechanisms of hepatocytes' regeneration. In addition, this review provides an update on the regenerative medicine strategies for end-stage liver disease. We conclude that regenerative medicine is a promising therapeutic strategy for end-stage liver disease. However, further studies are still required.

Recent advances in tissue stem cells

Stem cells are undifferentiated cells capable of self-renewal and differentiation, giving rise to specialized functional cells. Stem cells are of pivotal importance for organ and tissue development, homeostasis, and injury and disease repair. Tissue-specific stem cells are a rare population residing in specific tissues and present powerful potential for regeneration when required. They are usually named based on the resident tissue, such as hematopoietic stem cells and germline stem cells. This review discusses the recent advances in stem cells of various tissues, including neural stem cells, muscle stem cells, liver progenitors, pancreatic islet stem/progenitor cells, intestinal stem cells, and prostate stem cells, and the future perspectives for tissue stem cell research.

Genetic and Cellular Contributions to Liver Regeneration

The regenerative capabilities of the liver represent a paradigm for understanding tissue repair in solid organs. Regeneration after partial hepatectomy in rodent models is well understood, while regeneration in the context of clinically relevant chronic injuries is less studied. Given the growing incidence of fatty liver disease, cirrhosis, and liver cancer, interest in liver regeneration is increasing. Here, we will review the principles, genetics, and cell biology underlying liver regeneration, as well as new approaches being used to study heterogeneity in liver tissue maintenance and repair.

β-Catenin-NF-κB-CFTR interactions in cholangiocytes regulate inflammation and fibrosis during ductular reaction

Expansion of biliary epithelial cells (BECs) during ductular reaction (DR) is observed in liver diseases including cystic fibrosis (CF), and associated with inflammation and fibrosis, albeit without complete understanding of underlying mechanism. Using two different genetic mouse knockouts of β-catenin, one with β-catenin loss is hepatocytes and BECs (KO1), and another with loss in only hepatocytes (KO2), we demonstrate disparate long-term repair after an initial injury by 2-week choline-deficient ethionine-supplemented diet. KO2 show gradual liver repopulation with BEC-derived β-catenin-positive hepatocytes and resolution of injury. KO1 showed persistent loss of β-catenin, NF-κB activation in BECs, progressive DR and fibrosis, reminiscent of CF histology. We identify interactions of β-catenin, NFκB, and CF transmembranous conductance regulator (CFTR) in BECs. Loss of CFTR or β-catenin led to NF-κB activation, DR, and inflammation. Thus, we report a novel β-catenin-NFκB-CFTR interactome in BECs, and its disruption may contribute to hepatic pathology of CF.

The liver has an incredible capacity to repair itself or ‘regenerate’ – that is, it has the ability to replace damaged tissue with new tissue. In order to do this, the organ relies on hepatocytes (the cells that form the liver) and bile duct cells (the cells that form the biliary ducts) dividing and transforming into each other to repair and replace damaged tissue, in case the insult is dire.

During long-lasting or chronic liver injury, bile duct cells undergo a process called ‘ductular reaction’, which causes the cells to multiply and produce proteins that stimulate inflammation, and can lead to liver scarring (fibrosis). Ductular reaction is a hallmark of severe liver disease, and different diseases exhibit ductular reactions with distinct features. For example, in cystic fibrosis, a unique type of ductular reaction occurs at late stages, accompanied by both inflammation and fibrosis. Despite the role that ductular reaction plays in liver disease, it is not well understood how it works at the molecular level.

Hu et al. set out to investigate how a protein called β-catenin – which can cause many types of cells to proliferate – is involved in ductular reaction. They used three types of mice for their experiments: wild-type mice, which were not genetically modified; and two strains of genetically modified mice. One of these mutant mice did not produce β-catenin in biliary duct cells, while the other lacked β-catenin both in biliary duct cells and in hepatocytes.

After a short liver injury – which Hu et al. caused by feeding the mice a specific diet – the wild-type mice were able to regenerate and repair the liver without exhibiting any ductular reaction. The mutant mice that lacked β-catenin in hepatocytes showed a temporary ductular reaction, and ultimately repaired their livers by turning bile duct cells into hepatocytes. On the other hand, the mutant mice lacking β-catenin in both hepatocytes and bile duct cells displayed sustained ductular reactions, inflammation and fibrosis, which looked like that seen in patients with liver disease associated to cystic fibrosis. Further probing showed that β-catenin interacts with a protein called CTFR, which is involved in cystic fibrosis. When bile duct cells lack either of these proteins, another protein called NF-B gets activated, which causes the ductular reaction, leading to inflammation and fibrosis.

The findings of Hu et al. shed light on the role of β-catenin in ductular reaction. Further, the results show a previously unknown interaction between β-catenin, CTFR and NF-B, which could lead to better treatments for cystic fibrosis in the future.

The influence of tissue spatial geometry and functional organisation on liver regeneration

The adult liver exerts crucial functions, including nutrient metabolism and storage, bile production and drug detoxification. These complex functions expose the liver to constant damage induced by toxins, metabolic intermediates and oxidative stress. However, the adult liver exhibits an exceptional regenerative potential, which allows fast and efficient restoration of tissue architecture and function both after tissue resection and toxic damage. To accomplish its vital role, the liver shows a peculiar tissue architecture into functional units, which follow the gradient of oxygen and nutrients within the parenchyma. Much less is known about the influence of tissue spatial geometry and functional organisation on adult liver regeneration. Here I examine the experimental evidence in mouse models showing that the spatial organisation of the epithelial and mesenchymal compartments plays a key role in liver regeneration and favours the establishment of regenerative adult liver progenitors following liver injury. I also discuss the advantages and limitations of human and mouse 3D hepatic organoid systems, which recapitulate key aspects of liver function and architecture, as models of liver regeneration and disease. Finally, I analyse the role of the YAP/TAZ transcriptional co-activators as a central hub sensing the extra-cellular matrix (ECM), metabolic and epigenetic remodelling that regulate liver regeneration and promote liver disease, such as fibrosis, chronic liver disease and liver cancer. Together, the findings summarised here demonstrate that local physical and functional cellular interactions determined by the liver peculiar spatial geometry, play a crucial role in liver regeneration, and that their alterations have important implications for human liver disease.

Progress in human liver organoids

Understanding the development, regeneration, and disorders of the liver is the major goal in liver biology. Current mechanistic knowledge of human livers has been largely derived from mouse models and cell lines, which fall short in recapitulating the features of human liver cells or the structures and functions of human livers. Organoids as an in vitro system hold the promise to generate organ-like tissues in a dish. Recent advances in human liver organoids also facilitate the understanding of the biology and diseases in this complex organ. Here we review the progress in human liver organoids, mainly focusing on the methods to generate liver organoids, their applications, and possible future directions.