Hdac1 Regulates Differentiation of Bipotent Liver Progenitor Cells During Regeneration via Sox9b and Cdk8

Mdka produced by the activated HSCs drives bipotential progenitor cell redifferentiation during zebrafish biliary-mediated liver regeneration

BACKGROUND AND AIMS

After extensive hepatocyte loss or impaired hepatocyte proliferation, liver regeneration occurs through trans-differentiation of biliary epithelial cells (BECs), which involves dedifferentiation of biliary epithelial cells into bipotential progenitor cells (BP-PCs) and subsequent redifferentiation of BP-PCs into nascent hepatocytes and biliary epithelial cells. Despite several studies on the redifferentiation process of BP-PCs into nascent hepatocytes, the contributions of nonparenchymal cells in this process remain poorly understood.

APPROACH AND RESULTS

Using the zebrafish severe liver injury model, we observed specific expression of midkine a (Mdka) in the activated HSCs through single-cell analyses and fluorescence in situ hybridization. Genetic mutation, pharmacological inhibition, whole-mount in situ hybridizations, and antibody staining demonstrated an essential role of mdka in the redifferentiation of BP-PCs during liver regeneration. Notably, we identified Nucleolin (Ncl), the potential receptor for Mdka, specifically expressed in BP-PCs, and its mutant recapitulated the mdka mutant phenotypes with impaired BP-PC redifferentiation. Mechanistically, the Mdka-Ncl axis drove Erk1 activation in BP-PCs during liver regeneration. Furthermore, overexpression of activated Erk1 partially rescued the defective liver regeneration in the mdka mutant.

CONCLUSIONS

The activated HSCs produce Mdka to drive the redifferentiation process of BP-PCs through activating Erk1 during the biliary-mediated liver regeneration, implying previously unappreciated contributions of nonparenchymal cells to this regeneration process.

Protein modifications in hepatic ischemia-reperfusion injury: molecular mechanisms and targeted therapy

Ischemia-reperfusion injury refers to the damage that occurs when blood supply is restored to organs or tissues after a period of ischemia. This phenomenon is commonly observed in clinical contexts such as organ transplantation and cardiac arrest resuscitation. Among these, hepatic ischemia-reperfusion injury is a prevalent complication in liver transplantation, significantly impacting the functional recovery of the transplanted liver and potentially leading to primary graft dysfunction. With the growing demand for organ transplants and the limited availability of donor organs, effectively addressing hepatic ischemia-reperfusion injury is essential for enhancing transplantation success rates, minimizing complications, and improving graft survival. The pathogenesis of hepatic ischemia-reperfusion injury is multifaceted, involving factors such as oxidative stress and inflammatory responses. This article focuses on the role of protein post-translational modifications in hepatic ischemia-reperfusion injury, including phosphorylation, ubiquitination, acetylation, ADP-ribosylation, SUMOylation, crotonylation, palmitoylation, and S-nitrosylation. Initially, we examined the historical discovery of these protein post-translational modifications and subsequently investigated their impact on cellular signal transduction, enzymatic activity, protein stability, and protein-protein interactions. The emphasis of this study is on the pivotal role of protein post-translational modifications in the progression of hepatic ischemia-reperfusion injury and their potential as therapeutic targets. This study aims to conduct a comprehensive analysis of recent advancements in research on protein modifications in hepatic ischemia-reperfusion injury, investigate the underlying molecular mechanisms, and explore future research trajectories. Additionally, future research directions are proposed, including the exploration of interactions between various protein modifications, the identification of specific modification sites, and the development of drugs targeting these modifications. These efforts aim to deepen our understanding of protein post-translational modifications in hepatic ischemia-reperfusion injury and pave the way for innovative therapeutic interventions.

Molecular mechanisms in liver repair and regeneration: from physiology to therapeutics

Liver repair and regeneration are crucial physiological responses to hepatic injury and are orchestrated through intricate cellular and molecular networks. This review systematically delineates advancements in the field, emphasizing the essential roles played by diverse liver cell types. Their coordinated actions, supported by complex crosstalk within the liver microenvironment, are pivotal to enhancing regenerative outcomes. Recent molecular investigations have elucidated key signaling pathways involved in liver injury and regeneration. Viewed through the lens of metabolic reprogramming, these pathways highlight how shifts in glucose, lipid, and amino acid metabolism support the cellular functions essential for liver repair and regeneration. An analysis of regenerative variability across pathological states reveals how disease conditions influence these dynamics, guiding the development of novel therapeutic strategies and advanced techniques to enhance liver repair and regeneration. Bridging laboratory findings with practical applications, recent clinical trials highlight the potential of optimizing liver regeneration strategies. These trials offer valuable insights into the effectiveness of novel therapies and underscore significant progress in translational research. In conclusion, this review intricately links molecular insights to therapeutic frontiers, systematically charting the trajectory from fundamental physiological mechanisms to innovative clinical applications in liver repair and regeneration.

Chemically Defined Organoid Culture System for Cholangiocyte Differentiation

Cholangiocyte organoids provide a powerful platform for applications ranging from in vitro modeling to tissue engineering for regenerative medicine. However, their expansion and differentiation are typically conducted in animal-derived hydrogels, which impede the full maturation of organoids into functional cholangiocytes. In addition, these hydrogels are poorly defined and complex, limiting the clinical applicability of organoids. In this study, a novel medium composition combined with synthetic polyisocyanopeptide (PIC) hydrogels to enhance the maturation of intrahepatic cholangiocyte organoids (ICOs) into functional cholangiocytes is utilized. ICOs cultured in the presence of sodium butyrate and valproic acid, a histone deacetylase inhibitor, and a Notch signaling activator, respectively, in PIC hydrogel exhibit a more mature phenotype, as evidenced by increased expression of key cholangiocyte markers, crucial for biliary function. Notably, mature cholangiocyte organoids in PIC hydrogel display apical-out polarity, in contrast to the traditional basal-out polarization of ICOs cultured in Matrigel. Moreover, these mature cholangiocyte organoids effectively model the biliary pro-fibrotic response induced by transforming growth factor beta. Taken together, an animal-free, chemically defined culture system that promotes the ICOs into mature cholangiocytes with apical-out polarity, facilitating regenerative medicine applications and in vitro studies that require access to the apical membrane, is developed.

HDAC1: a promising target for cancer treatment: insights from a thorough analysis of tumor functions

Background

Many significant findings from recent studies have revealed the significance of histone deacetylase 1 (HDAC1) in the development of tumors and its strong association with tumor prognosis; these studies have mainly focused on one single cancer such as in lung cancer, breast cancer, and hepatocellular carcinoma (HCC). To date, there has been no comprehensive analysis and pan-analysis conducted from the overall perspective of cancer across all types. Hence, we analyzed public databases, conducted tube formation assay, and immunohistochemistry (IHC) staining of HDAC1 on six kinds of clinical samples to explore the prognostic and oncogenic effects of HDAC1 on 33 tumors for the first time. There currently remains a lack of efficient testing methods, therapies, and diagnostic and prognostic markers of tumor formation and development in different tumors.

Methods

Our initial objective was to investigate the possible cancer-causing functions of HDAC1 in 33 different types of tumors by utilizing The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases, and many different online websites, such as Tumor IMmune Estimation Resource 2 (TIMER2), Gene Expression Profiling Interactive Analysis 2 (GEPIA2), Genotype Tissue Expression (GTEx) database, Clinical Proteomic Tumor Analysis Consortium (CPTAC) dataset, and University of ALabama at Brimingham CANcer data analysis portal (UALCAN) tool, and so on. We even used small interfering RNA (siRNA) to knock down HDAC2 in HCC cell lines. IHC of HDAC1 was performed.

Results

HDAC1 exhibited high expression in numerous tumors, and strong correlations were observed between the messenger RNA (mRNA) levels of HDAC1 and the prognosis of individuals diagnosed with tumors. Human umbilical vein endothelial cells (HUVECs) tube formation and migration were significantly inhibited by conditioned media from HCC cells treated with siRNA of HDAC1. Several types of cancer have been found to exhibit elevated levels of phosphorylation at S421. Furthermore, as in bladder urothelial carcinoma (BLCA), breast invasive carcinoma (BRCA), and kidney renal papillary cell carcinoma (KIRP), HDAC1 expression was found to be correlated with inflammatory cell infiltration.

Conclusions

The levels of HDAC1 are expected to adapt to clinical adjuvant targeted therapy in most types of solid cancer.

Using different zebrafish models to explore liver regeneration

The liver possesses an impressive capability to regenerate following various injuries. Given its profound implications for the treatment of liver diseases, which afflict millions globally, liver regeneration stands as a pivotal area of digestive organ research. Zebrafish (Danio rerio) has emerged as an ideal model organism in regenerative medicine, attributed to their remarkable ability to regenerate tissues and organs, including the liver. Many fantastic studies have been performed to explore the process of liver regeneration using zebrafish, especially the extreme hepatocyte injury model. Biliary-mediated liver regeneration was first discovered in the zebrafish model and then validated in mammalian models and human patients. Considering the notable expansion of biliary epithelial cells in many end-stage liver diseases, the promotion of biliary-mediated liver regeneration might be another way to treat these refractory liver diseases. To date, a comprehensive review discussing the current advancements in zebrafish liver regeneration models is lacking. Therefore, this review aims to investigate the utility of different zebrafish models in exploring liver regeneration, highlighting the genetic and cellular insights gained and discussing the potential translational impact on human health.

Epigenetic regulation in liver regeneration

The liver is considered unique in its enormous capacity for regeneration and self-repair. In contrast to other regenerative organs (i.e., skin, skeletal muscle, and intestine), whether the adult liver contains a defined department of stem cells is still controversial. In order to compensate for the massive loss of hepatocytes following liver injury, the liver processes a precisely controlled transcriptional reprogram that can trigger cell proliferation and cell-fate switch. Epigenetic events are thought to regulate the organization of chromatin architecture and gene transcription during the liver regenerative process. In this review, we will summarize how changes to the chromatin by epigenetic modifiers are translated into cell fate transitions to restore liver homeostasis during liver regeneration.

HDAC inhibitors support long-term expansion of porcine hepatocytes in vitro

Hepatocyte transplantation is an effective treatment for end-stage liver disease. However, due to the limited supply of human hepatocytes, porcine hepatocytes have garnered attention as a potential alternative source. Nonetheless, traditional primary porcine hepatocytes exhibit certain limitations in function maintenance and in vitro proliferation. This study has discovered that by using histone deacetylase inhibitors (HDACi), primary porcine hepatocytes can be successfully reprogrammed into liver progenitor cells with high proliferative potential. This method enables porcine hepatocytes to proliferate over an extended period in vitro and exhibit increased susceptibility in lentivirus-mediated gene modification. These liver progenitor cells can readily differentiate into mature hepatocytes and, upon microencapsulation transplantation into mice with acute liver failure, significantly improve the survival rate. This research provides new possibilities for the application of porcine hepatocytes in the treatment of end-stage liver disease.

Liver regeneration after injury: Mechanisms, cellular interactions and therapeutic innovations

The liver possesses a distinctive capacity for regeneration within the human body. Under normal circumstances, liver cells replicate themselves to maintain liver function. Compensatory replication of healthy hepatocytes is sufficient for the regeneration after acute liver injuries. In the late stage of chronic liver damage, a large number of hepatocytes die and hepatocyte replication is blocked. Liver regeneration has more complex mechanisms, such as the transdifferentiation between cell types or hepatic progenitor cells mediated. Dysregulation of liver regeneration causes severe chronic liver disease. Gaining a more comprehensive understanding of liver regeneration mechanisms would facilitate the advancement of efficient therapeutic approaches. This review provides an overview of the signalling pathways linked to different aspects of liver regeneration in various liver diseases. Moreover, new knowledge on cellular interactions during the regenerative process is also presented. Finally, this paper explores the potential applications of new technologies, such as nanotechnology, stem cell transplantation and organoids, in liver regeneration after injury, offering fresh perspectives on treating liver disease.

Diverse functions of SOX9 in liver development and homeostasis and hepatobiliary diseases

The liver is the central organ for digestion and detoxification and has unique metabolic and regenerative capacities. The hepatobiliary system originates from the foregut endoderm, in which cells undergo multiple events of cell proliferation, migration, and differentiation to form the liver parenchyma and ductal system under the hierarchical regulation of transcription factors. Studies on liver development and diseases have revealed that SRY-related high-mobility group box 9 (SOX9) plays an important role in liver embryogenesis and the progression of hepatobiliary diseases. SOX9 is not only a master regulator of cell fate determination and tissue morphogenesis, but also regulates various biological features of cancer, including cancer stemness, invasion, and drug resistance, making SOX9 a potential biomarker for tumor prognosis and progression. This review systematically summarizes the latest findings of SOX9 in hepatobiliary development, homeostasis, and disease. We also highlight the value of SOX9 as a novel biomarker and potential target for the clinical treatment of major liver diseases.

Transcriptional Control: A Directional Sign at the Crossroads of Adult Hepatic Progenitor Cells' Fates

Hepatic progenitor cells (HPCs) have a bidirectional potential to differentiate into hepatocytes and bile duct epithelial cells and constitute a second barrier to liver regeneration in the adult liver. They are usually located in the Hering duct in the portal vein region where various cells, extracellular matrix, cytokines, and communication signals together constitute the niche of HPCs in homeostasis to maintain cellular plasticity. In various types of liver injury, different cellular signaling streams crosstalk with each other and point to the inducible transcription factor set, including FoxA1/2/3, YB-1, Foxl1, Sox9, HNF4α, HNF1α, and HNF1β. These transcription factors exert different functions by binding to specific target genes, and their products often interact with each other, with diverse cascades of regulation in different molecular events that are essential for homeostatic regulation, self-renewal, proliferation, and selective differentiation of HPCs. Furthermore, the tumor predisposition of adult HPCs is found to be significantly increased under transcriptional factor dysregulation in transcriptional analysis, and the altered initial commitment of the differentiation pathway of HPCs may be one of the sources of intrahepatic tumors. Related transcription factors such as HNF4α and HNF1 are expected to be future targets for tumor treatment.

Zebrafish-A Suitable Model for Rapid Translation of Effective Therapies for Pediatric Cancers

Pediatric cancers are the leading cause of disease-related deaths in children and adolescents. Most of these tumors are difficult to treat and have poor overall survival. Concerns have also been raised about drug toxicity and long-term detrimental side effects of therapies. In this review, we discuss the advantages and unique attributes of zebrafish as pediatric cancer models and their importance in targeted drug discovery and toxicity assays. We have also placed a special focus on zebrafish models of pediatric brain cancers-the most common and difficult solid tumor to treat.

Epigenetic modification in liver fibrosis: Promising therapeutic direction with significant challenges ahead

Liver fibrosis, characterized by scar tissue formation, can ultimately result in liver failure. It's a major cause of morbidity and mortality globally, often associated with chronic liver diseases like hepatitis or alcoholic and non-alcoholic fatty liver diseases. However, current treatment options are limited, highlighting the urgent need for the development of new therapies. As a reversible regulatory mechanism, epigenetic modification is implicated in many biological processes, including liver fibrosis. Exploring the epigenetic mechanisms involved in liver fibrosis could provide valuable insights into developing new treatments for chronic liver diseases, although the current evidence is still controversial. This review provides a comprehensive summary of the regulatory mechanisms and critical targets of epigenetic modifications, including DNA methylation, histone modification, and RNA modification, in liver fibrotic diseases. The potential cooperation of different epigenetic modifications in promoting fibrogenesis was also highlighted. Finally, available agonists or inhibitors regulating these epigenetic mechanisms and their potential application in preventing liver fibrosis were discussed. In summary, elucidating specific druggable epigenetic targets and developing more selective and specific candidate medicines may represent a promising approach with bright prospects for the treatment of chronic liver diseases.

Macrophage cytotherapy on liver cirrhosis

Macrophages, an essential cell population involved in mediating innate immunity in the host, play a crucial role on the development of hepatic cirrhosis. Extensive studies have highlighted the potential therapeutic benefits of macrophage therapy in treating hepatic cirrhosis. This review aims to provide a comprehensive overview of the various effects and underlying mechanisms associated with macrophage therapy in the context of hepatic cirrhosis.

Hepatocyte reprogramming in liver regeneration: Biological mechanisms and applications

The liver is one of the few organs that retain the capability to regenerate in adult mammals. This regeneration process is mainly facilitated by the dynamic behavior of hepatocytes, which are the major functional constituents in the liver. In response to liver injury, hepatocytes undergo remarkable alterations, such as reprogramming, wherein they lose their original identity and acquire properties from other cells. This phenomenon of hepatocyte reprogramming, coupled with hepatocyte expansion, plays a central role in liver regeneration, and its underlying mechanisms are complex and multifaceted. Understanding the fate of reprogrammed hepatocytes and the mechanisms of their conversion has significant implications for the development of innovative therapeutics for liver diseases. Herein, we review the plasticity of hepatocytes in response to various forms of liver injury, with a focus on injury-induced hepatocyte reprogramming. We provide a comprehensive summary of current knowledge on the molecular and cellular mechanisms governing hepatocyte reprogramming, specifically in the context of liver regeneration, providing insight into potential applications of this process in the treatment of liver disorders, including chronic liver diseases and liver cancer.

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.

Arouse potential stemness: Intrinsic and acquired stem cell therapeutic strategies for advanced liver diseases

Liver diseases are a major health issue, and prolonged liver injury always progresses. Advanced liver disorders impair liver regeneration. Millions of patients die yearly worldwide, even with the available treatments of liver transplantation and artificial liver support system. With its abundant cell resources and significant differentiative potential, stem cell therapy is a viable treatment for various disorders and offers hope to patients waiting for orthotopic liver transplantation. Considering such plight, stem cell therapeutic strategies deliver hope to the patients. Moreover, we conclude intrinsic and acquired perspectives based on stem cell sources. The properties and therapeutic uses of these stem cells' specific types or sources were then reviewed. Owing to the recent investigations of the above cells, a safe and effective therapy will emerge for advanced liver diseases soon.

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.

T, Mishoe Hernandez L, DeLaurier A. Exposure to valproic acid (VPA) reproduces hdac1 loss of function phenotypes in zebrafish

Histone deacetylases are enzymes that remove acetyl groups from histone tails and are understood to act as repressors of transcriptional activity. Hdac1 has been previously shown to function in eye, pectoral fin, heart, liver, and pharyngeal skeletal development. We show that high doses of Valproic Acid (VPA) reproduce the hdac1 phenotype. We identify tbx5 genes as potential targets of Hdac1 in eye, pectoral fin, and heart development. Using timed exposures, we show that skeletal structures in the pharyngeal arches are impacted by VPA between 24-36 hours post-fertilization, indicating a role for Hdac1 during post-migration patterning, differentiation, or proliferation of cranial neural crest cells.

LSD1 promotes the egress of hematopoietic stem and progenitor cells into the bloodstream during the endothelial-to-hematopoietic transition

During embryonic development, primitive and definitive waves of hematopoiesis take place to provide proper blood cells for each developmental stage, with the possible involvement of epigenetic factors. We previously found that lysine-specific demethylase 1 (LSD1/KDM1A) promotes primitive hematopoietic differentiation by shutting down the gene expression program of hemangioblasts in an Etv2/Etsrp-dependent manner. In the present study, we demonstrated that zebrafish LSD1 also plays important roles in definitive hematopoiesis in the development of hematopoietic stem and progenitor cells. A combination of genetic approaches and imaging analyses allowed us to show that LSD1 promotes the egress of hematopoietic stem and progenitor cells into the bloodstream during the endothelial-to-hematopoietic transition. Analysis of compound mutant lines with Etv2/Etsrp mutant zebrafish revealed that, unlike in primitive hematopoiesis, this function of LSD1 was independent of Etv2/Etsrp. The phenotype of LSD1 mutant zebrafish during the endothelial-to-hematopoietic transition was similar to that of previously reported compound knockout mice of Gfi1/Gfi1b, which forms a complex with LSD1 and represses endothelial genes. Moreover, co-knockdown of zebrafish Gfi1/Gfi1b genes inhibited the development of hematopoietic stem and progenitor cells. We therefore hypothesize that the shutdown of the Gfi1/Gfi1b-target genes during the endothelial-to-hematopoietic transition is one of the key evolutionarily conserved functions of LSD1 in definitive hematopoiesis.

Liver fibrosis in fish research: From an immunological perspective

Liver fibrosis is a pathological process whereby the liver is subjected to various acute and chronic injuries, resulting in the activation of hepatic stellate cells (HSCs), an imbalance of extracellular matrix generation and degradation, and deposition in the liver. This review article summarizes the current understanding of liver fibrosis in fish research. Liver fibrosis is a common pathological condition that occurs in fish raised in aquaculture. It is often associated with poor water quality, stressful conditions, and the presence of pathogens. The review describes the pathophysiology of liver fibrosis in fish, including the roles of various cells and molecules involved in the development and progression of the disease. The review also covers the various methods used to diagnose and assess the severity of liver fibrosis in fish, including histological analysis, biochemical markers, and imaging techniques. In addition, the article discusses the current treatment options for liver fibrosis in fish, including dietary interventions, pharmaceuticals, and probiotics. This review highlights the need for more in-depth research in this area to better understand the mechanisms by which liver fibrosis in fish occurs and to develop effective prevention and treatment strategies. Finally, improved management practices and the development of new treatments will be critical to the sustainability of aquaculture and the health of farmed fish.

Tris(2-chloroethyl) Phosphate Exerts Hepatotoxic Impacts on Zebrafish by Disrupting Hypothalamic-Pituitary-Thyroid and Gut-Liver Axes

The ubiquitous environmental presence of tris(2-chloroethyl) phosphate (TCEP) poses a potential threat to animals; however, little is known about its hepatotoxicity. In this study, the effects of TCEP exposure (0.5 and 5.0 μg/L for 28 days) on liver health and the potential underlying toxification mechanisms were investigated in zebrafish. Our results demonstrated that TCEP exposure led to hepatic tissue lesions and resulted in significant alterations in liver-injury-specific markers. Moreover, TCEP-exposed fish had significantly lower levels of thyrotropin-releasing hormone and thyroid-stimulating hormone in the brain, evidently less triiodothyronine whereas more thyroxine in plasma, and markedly altered expressions of genes from the hypothalamic-pituitary-thyroid (HPT) axis in the brain or liver. In addition, a significantly higher proportion of Bacteroidetes in the gut microbiota, an elevated bacterial source endotoxin lipopolysaccharide (LPS) in the plasma, upregulated expression of LPS-binding protein and Toll-like receptor 4 in the liver, and higher levels of proinflammatory cytokines in the liver were detected in TCEP-exposed zebrafish. Furthermore, TCEP-exposed fish also suffered severe oxidative damage, possibly due to disruption of the antioxidant system. These findings suggest that TCEP may exert hepatotoxic effects on zebrafish by disrupting the HPT and gut-liver axes and thereafter inducing hepatic inflammation and oxidative stress.

Hdac1-deficiency affects the cell cycle axis Cdc25-Cdk1 causing impaired G2/M phase progression and reduced cardiomyocyte proliferation in zebrafish

Zebrafish have the ability to fully regenerate their hearts after injury since cardiomyocytes subsequently dedifferentiate, re-enter cell cycle, and proliferate to replace damaged myocardial tissue. Recent research identified the reactivation of dormant developmental pathways during cardiac regeneration in adult zebrafish, suggesting pro-proliferative pathways important for developmental heart growth to be also critical for regenerative heart growth after injury. Histone deacetylase 1 (Hdac1) was recently shown to control both, embryonic as well as adult regenerative cardiomyocyte proliferation in the zebrafish model. Nevertheless, regulatory pathways controlled by Hdac1 are not defined yet. By analyzing RNA-seq-derived transcriptional profiles of the Hdac1-deficient zebrafish mutant baldrian, we here identified DNA damage response (DDR) pathways activated in baldrian mutant embryos. Surprisingly, although the DDR signaling pathway was transcriptionally activated, we found the complete loss of protein expression of the known DDR effector and cell cycle inhibitor p21. Consequently, we observed an upregulation of the p21-downstream target Cdk2, implying elevated G1/S phase transition in Hdac1-deficient zebrafish hearts. Remarkably, Cdk1, another p21-but also Cdc25-downstream target was downregulated. Here, we found the significant downregulation of Cdc25 protein expression, explaining reduced Cdk1 levels and suggesting impaired G2/M phase progression in Hdac1-deficient zebrafish embryos. To finally prove defective cell cycle progression due to Hdac1 loss, we conducted Cytometer-based cell cycle analyses in HDAC1-deficient murine HL-1 cardiomyocytes and indeed found impaired G2/M phase transition resulting in defective cardiomyocyte proliferation. In conclusion, our results suggest a critical role of Hdac1 in maintaining both, regular G1/S and G2/M phase transition in cardiomyocytes by controlling the expression of essential cell cycle regulators such as p21 and Cdc25.

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-to-cholangiocyte conversion occurs through transdifferentiation independently of proliferation in zebrafish

BACKGROUND AND AIMS

Injury to biliary epithelial cells (BECs) lining the hepatic bile ducts leads to cholestatic liver diseases. Upon severe biliary damage, hepatocytes can convert to BECs, thereby contributing to liver recovery. Given a potential of augmenting this hepatocyte-to-BEC conversion as a therapeutic option for cholestatic liver diseases, it will be important to thoroughly understand the cellular and molecular mechanisms of the conversion process.

APPROACH AND RESULTS

Towards this aim, we have established a zebrafish model for hepatocyte-to-BEC conversion by employing Tg(fabp10a:CFP-NTR) zebrafish with a temporal inhibition of Notch signaling during regeneration. Cre/loxP-mediated permanent and H2B-mCherry-mediated short-term lineage tracing revealed that in the model, all BECs originate from hepatocytes. During the conversion, BEC markers are sequentially induced in the order of Sox9b, Yap/Taz, Notch activity/ epcam , and Alcama/ krt18 ; the expression of the hepatocyte marker Bhmt disappears between the Sox9b and Yap/Taz induction. Importantly, live time-lapse imaging unambiguously revealed transdifferentiation of hepatocytes into BECs: hepatocytes convert to BECs without transitioning through a proliferative intermediate state. In addition, using compounds and transgenic and mutant lines that modulate Notch and Yap signaling, we found that both Notch and Yap signaling are required for the conversion even in Notch- and Yap-overactivating settings.

CONCLUSIONS

Hepatocyte-to-BEC conversion occurs through transdifferentiation independently of proliferation, and Notch and Yap signaling control the process in parallel with a mutually positive interaction. The new zebrafish model will further contribute to a thorough understanding of the mechanisms of the conversion process.

Sin3a Associated Protein 130kDa, sap130, plays an evolutionary conserved role in zebrafish heart development

Hypoplastic left heart syndrome (HLHS) is a congenital heart disease where the left ventricle is reduced in size. A forward genetic screen in mice identified SIN3A associated protein 130kDa ( Sap130 ), a protein in the chromatin modifying SIN3A/HDAC1 complex, as a gene contributing to the digenic etiology of HLHS. Here, we report the role of zebrafish sap130 genes in heart development. Loss of sap130a, one of two Sap130 orthologs, resulted in smaller ventricle size, a phenotype reminiscent to the hypoplastic left ventricle in mice. While cardiac progenitors were normal during somitogenesis, diminution of the ventricle size suggest the Second Heart Field (SHF) was the source of the defect. To explore the role of sap130a in gene regulation, transcriptome profiling was performed after the heart tube formation to identify candidate pathways and genes responsible for the small ventricle phenotype. Genes involved in cardiac differentiation and cell communication were dysregulated in sap130a , but not in sap130b mutants. Confocal light sheet analysis measured deficits in cardiac output in MZsap130a supporting the notion that cardiomyocyte maturation was disrupted. Lineage tracing experiments revealed a significant reduction of SHF cells in the ventricle that resulted in increased outflow tract size. These data suggest that sap130a is involved in cardiogenesis via regulating the accretion of SHF cells to the growing ventricle and in their subsequent maturation for cardiac function. Further, genetic studies revealed an interaction between hdac1 and sap130a , in the incidence of small ventricles. These studies highlight the conserved role of Sap130a and Hdac1 in zebrafish cardiogenesis.

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.

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.

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.

Preclinical-to-clinical innovations in stem cell therapies for liver regeneration

Acute and chronic liver diseases are the major cause of high morbidity and mortality globally. Liver transplantation is a widely used therapeutic option for liver failure. However, the shortage of availability of liver donors has encouraged research on the alternative approach to liver regeneration. Cell-based regenerative medicine is the best alternative therapy to cater to this need. To date, advanced preclinical approaches have been undertaken on stem cell differentiation and their use in liver tissue engineering for generating efficacious and promising regenerative therapies. Advancements in the bioengineering of stem cells, and organoid generation are the way forward to efficient therapies against liver injury. This review summarizes the recent approaches for stem cell therapy-based liver regeneration and their proof of concepts for clinical application, bioengineering liver organoids to alleviate the liver failure caused due to chronic liver diseases.

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.

NOTCH-YAP1/TEAD-DNMT1 Axis Drives Hepatocyte Reprogramming Into Intrahepatic Cholangiocarcinoma

BACKGROUND & AIMS

Intrahepatic cholangiocarcinoma (ICC) is a devastating liver cancer with extremely high intra- and inter-tumoral molecular heterogeneity, partly due to its diverse cellular origins. We investigated clinical relevance and the molecular mechanisms underlying hepatocyte (HC)-driven ICC development.

METHODS

Expression of ICC driver genes in human diseased livers at risk for ICC development were examined. The sleeping beauty and hydrodynamic tail vein injection based Akt-NICD/YAP1 ICC model was used to investigate pathogenetic roles of SRY-box transcription factor 9 (SOX9) and yes-associated protein 1 (YAP1) in HC-driven ICC. We identified DNA methyltransferase 1 (DNMT1) as a YAP1 target, which was validated by loss- and gain-of-function studies, and its mechanism addressed by chromatin immunoprecipitation sequencing.

RESULTS

Co-expression of AKT and Notch intracellular domain (NICD)/YAP1 in HC yielded ICC that represents 13% to 29% of clinical ICC. NICD independently regulates SOX9 and YAP1 and deletion of either, significantly delays ICC development. Yap1 or TEAD inhibition, but not Sox9 deletion, impairs HC-to-biliary epithelial cell (BEC) reprogramming. DNMT1 was discovered as a novel downstream effector of YAP1-TEAD complex that directs HC-to-BEC/ICC fate switch through the repression of HC-specific genes regulated by master regulators for HC differentiation, including hepatocyte nuclear factor 4 alpha, hepatocyte nuclear factor 1 alpha, and CCAAT/enhancer-binding protein alpha/beta. DNMT1 loss prevented NOTCH/YAP1-dependent HC-driven cholangiocarcinogenesis, and DNMT1 re-expression restored ICC development following TEAD repression. Co-expression of DNMT1 with AKT was sufficient to induce tumor development including ICC. DNMT1 was detected in a subset of HCs and dysplastic BECs in cholestatic human livers prone to ICC development.

CONCLUSION

We identified a novel NOTCH-YAP1/TEAD-DNMT1 axis essential for HC-to-BEC/ICC conversion, which may be relevant in cholestasis-to-ICC pathogenesis in the clinic.

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.

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.

Hippo-Yap/Taz signalling in zebrafish regeneration

The extent of tissue regeneration varies widely between species. Mammals have a limited regenerative capacity whilst lower vertebrates such as the zebrafish (Danio rerio), a freshwater teleost, can robustly regenerate a range of tissues, including the spinal cord, heart, and fin. The molecular and cellular basis of this altered response is one of intense investigation. In this review, we summarise the current understanding of the association between zebrafish regeneration and Hippo pathway function, a phosphorylation cascade that regulates cell proliferation, mechanotransduction, stem cell fate, and tumorigenesis, amongst others. We also compare this function to Hippo pathway activity in the regenerative response of other species. We find that the Hippo pathway effectors Yap/Taz facilitate zebrafish regeneration and that this appears to be latent in mammals, suggesting that therapeutically promoting precise and temporal YAP/TAZ signalling in humans may enhance regeneration and hence reduce morbidity.

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 biology: Implications for liver tumour therapies

The liver has remarkable regenerative potential, with the capacity to regenerate after 75% hepatectomy in humans and up to 90% hepatectomy in some rodent models, enabling it to meet the challenge of diverse injury types, including physical trauma, infection, inflammatory processes, direct toxicity, and immunological insults. Current understanding of liver regeneration is based largely on animal research, historically in large animals, and more recently in rodents and zebrafish, which provide powerful genetic manipulation experimental tools. Whilst immensely valuable, these models have limitations in extrapolation to the human situation. In vitro models have evolved from 2-dimensional culture to complex 3 dimensional organoids, but also have shortcomings in replicating the complex hepatic micro-anatomical and physiological milieu. The process of liver regeneration is only partially understood and characterized by layers of complexity. Liver regeneration is triggered and controlled by a multitude of mitogens acting in autocrine, paracrine, and endocrine ways, with much redundancy and cross-talk between biochemical pathways. The regenerative response is variable, involving both hypertrophy and true proliferative hyperplasia, which is itself variable, including both cellular phenotypic fidelity and cellular trans-differentiation, according to the type of injury. Complex interactions occur between parenchymal and non-parenchymal cells, and regeneration is affected by the status of the liver parenchyma, with differences between healthy and diseased liver. Finally, the process of termination of liver regeneration is even less well understood than its triggers. The complexity of liver regeneration biology combined with limited understanding has restricted specific clinical interventions to enhance liver regeneration. Moreover, manipulating the fundamental biochemical pathways involved would require cautious assessment, for fear of unintended consequences. Nevertheless, current knowledge provides guiding principles for strategies to optimise liver regeneration potential.

Activation of TAF9 via Danshensu-Induced Upregulation of HDAC1 Expression Alleviates Non-alcoholic Fatty Liver Disease

Fatty acid β-oxidation is an essential pathogenic mechanism in nonalcoholic fatty liver disease (NAFLD), and TATA-box binding protein associated factor 9 (TAF9) has been reported to be involved in the regulation of fatty acid β-oxidation. However, the function of TAF9 in NAFLD, as well as the mechanism by which TAF9 is regulated, remains unclear. In this study, we aimed to investigate the signaling mechanism underlying the involvement of TAF9 in NAFLD and the protective effect of the natural phenolic compound Danshensu (DSS) against NAFLD via the HDAC1/TAF9 pathway. An in vivo model of high-fat diet (HFD)-induced NAFLD and a palmitic acid (PA)-treated AML-12 cell model were developed. Pharmacological treatment with DSS significantly increased fatty acid β-oxidation and reduced lipid droplet (LD) accumulation in NAFLD. TAF9 overexpression had the same effects on these processes both in vivo and in vitro. Interestingly, the protective effect of DSS was markedly blocked by TAF9 knockdown. Mechanistically, TAF9 was shown to be deacetylated by HDAC1, which regulates the capacity of TAF9 to mediate fatty acid β-oxidation and LD accumulation during NAFLD. In conclusion, TAF9 is a key regulator in the treatment of NAFLD that acts by increasing fatty acid β-oxidation and reducing LD accumulation, and DSS confers protection against NAFLD through the HDAC1/TAF9 pathway.

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.

Farnesoid X Receptor Is Required for the Redifferentiation of Bipotential Progenitor Cells During Biliary-Mediated Zebrafish Liver Regeneration

BACKGROUND AND AIMS

Liver regeneration after extreme hepatocyte loss occurs through transdifferentiation of biliary epithelial cells (BECs), which includes dedifferentiation of BECs into bipotential progenitor cells (BPPCs) and subsequent redifferentiation into nascent hepatocytes and BECs. Although multiple molecules and signaling pathways have been implicated to play roles in the BEC-mediated liver regeneration, mechanisms underlying the dedifferentiation-redifferentiation transition and the early phase of BPPC redifferentiation that is pivotal for both hepatocyte and BEC directions remain largely unknown.

APPROACH AND RESULTS

The zebrafish extreme liver damage model, genetic mutation, pharmacological inhibition, transgenic lines, whole-mount and fluorescent in situ hybridizations and antibody staining, single-cell RNA sequencing, quantitative real-time PCR, and heat shock-inducible overexpression were used to investigate roles and mechanisms of farnesoid X receptor (FXR; encoded by nuclear receptor subfamily 1, group H, member 4 [nr1h4]) in regulating BPPC redifferentiation. The nr1h4 expression was significantly up-regulated in response to extreme liver injury. Genetic mutation or pharmacological inhibition of FXR was ineffective to BEC-to-BPPC dedifferentiation but blocked the redifferentiation of BPPCs to both hepatocytes and BECs, leading to accumulation of undifferentiated or less-differentiated BPPCs. Mechanistically, induced overexpression of extracellular signal-related kinase (ERK) 1 (encoded by mitogen-activated protein kinase 3) rescued the defective BPPC-to-hepatocyte redifferentiation in the nr1h4 mutant, and ERK1 itself was necessary for the BPPC-to-hepatocyte redifferentiation. The Notch activities in the regenerating liver of nr1h4 mutant attenuated, and induced Notch activation rescued the defective BPPC-to-BEC redifferentiation in the nr1h4 mutant.

CONCLUSIONS

FXR regulates BPPC-to-hepatocyte and BPPC-to-BEC redifferentiations through ERK1 and Notch, respectively. Given recent applications of FXR agonists in the clinical trials for liver diseases, this study proposes potential underpinning mechanisms by characterizing roles of FXR in the stimulation of dedifferentiation-redifferentiation transition and BPPC redifferentiation.

Social Deficits and Repetitive Behaviors Are Improved by Early Postnatal Low-Dose VPA Intervention in a Novel shank3-Deficient Zebrafish Model

Mutations of the SHANK3 gene are found in some autism spectrum disorder (ASD) patients, and animal models harboring SHANK3 mutations exhibit a variety of ASD-like behaviors, presenting a unique opportunity to explore the underlying neuropathological mechanisms and potential pharmacological treatments. The histone deacetylase (HDAC) valproic acid (VPA) has demonstrated neuroprotective and neuroregenerative properties, suggesting possible therapeutic utility for ASD. Therefore, SHANK3-associated ASD-like symptoms present a convenient model to evaluate the potential benefits, therapeutic window, and optimal dose of VPA. We constructed a novel shank3-deficient (shank3ab^–/–) zebrafish model through CRISPR/Cas9 editing and conducted comprehensive morphological and neurobehavioral evaluations, including of core ASD-like behaviors, as well as molecular analyses of synaptic proteins expression levels. Furthermore, different VPA doses and treatment durations were examined for effects on ASD-like phenotypes. Compared to wild types (WTs), shank3ab^–/– zebrafish exhibited greater developmental mortality, more frequent abnormal tail bending, pervasive developmental delay, impaired social preference, repetitive swimming behaviors, and generally reduced locomotor activity. The expression levels of synaptic proteins were also dramatically reduced in shank3ab^–/– zebrafish. These ASD-like behaviors were attenuated by low-dose (5 μM) VPA administered from 4 to 8 days post-fertilization (dpf), and the effects persisted to adulthood. In addition, the observed underexpression of grm5, encoding glutamate metabotropic receptor 5, was significantly improved in VPA-treated shank3ab^–/– zebrafish. We report for the first time that low-dose VPA administered after neural tube closure has lasting beneficial effects on the social deficits and repetitive behavioral patterns in shank3-deficient ASD model zebrafish. These findings provide a promising strategy for ASD clinical drug development.

Somatic Mutations in Circulating Cell-Free DNA and Risk for Hepatocellular Carcinoma in Hispanics

Hispanics are disproportionally affected by liver fibrosis and hepatocellular carcinoma (HCC). Advanced liver fibrosis is a major risk factor for HCC development. We aimed at identifying somatic mutations in plasma cell-free DNA (cfDNA) of Hispanics with HCC and Hispanics with advanced liver fibrosis but no HCC. Targeted sequencing of over 262 cancer-associated genes identified nonsynonymous mutations in 22 of the 27 HCC patients. Mutations were detected in known HCC-associated genes (e.g., CTNNB1, TP53, NFE2L2, and ARID1A). No difference in cfDNA concentrations was observed between patients with mutations and those without detectable mutations. HCC patients with higher cfDNA concentrations or higher number of mutations had a shorter overall survival (p < 0.001 and p = 0.045). Nonsynonymous mutations were also identified in 17 of the 51 subjects with advanced liver fibrosis. KMT2C was the most commonly mutated gene. Nine genes were mutated in both subjects with advanced fibrosis and HCC patients. Again, no significant difference in cfDNA concentrations was observed between subjects with mutations and those without detectable mutations. Furthermore, higher cfDNA concentrations and higher number of mutations correlated with a death outcome in subjects with advanced fibrosis. In conclusion, cfDNA features are promising non-invasive markers for HCC risk prediction and overall survival.

Farnesoid X Receptor Activation Impairs Liver Progenitor Cell-Mediated Liver Regeneration via the PTEN-PI3K-AKT-mTOR Axis in Zebrafish

BACKGROUND AND AIMS

Following mild liver injury, pre-existing hepatocytes replicate. However, if hepatocyte proliferation is compromised, such as in chronic liver diseases, biliary epithelial cells (BECs) contribute to hepatocytes through liver progenitor cells (LPCs), thereby restoring hepatic mass and function. Recently, augmenting innate BEC-driven liver regeneration has garnered attention as an alternative to liver transplantation, the only reliable treatment for patients with end-stage liver diseases. Despite this attention, the molecular basis of BEC-driven liver regeneration remains poorly understood.

APPROACH AND RESULTS

By performing a chemical screen with the zebrafish hepatocyte ablation model, in which BECs robustly contribute to hepatocytes, we identified farnesoid X receptor (FXR) agonists as inhibitors of BEC-driven liver regeneration. Here we show that FXR activation blocks the process through the FXR-PTEN (phosphatase and tensin homolog)-PI3K (phosphoinositide 3-kinase)-AKT-mTOR (mammalian target of rapamycin) axis. We found that FXR activation blocked LPC-to-hepatocyte differentiation, but not BEC-to-LPC dedifferentiation. FXR activation also suppressed LPC proliferation and increased its death. These defects were rescued by suppressing PTEN activity with its chemical inhibitor and ptena/b mutants, indicating PTEN as a critical downstream mediator of FXR signaling in BEC-driven liver regeneration. Consistent with the role of PTEN in inhibiting the PI3K-AKT-mTOR pathway, FXR activation reduced the expression of pS6, a marker of mTORC1 activation, in LPCs of regenerating livers. Importantly, suppressing PI3K and mTORC1 activities with their chemical inhibitors blocked BEC-driven liver regeneration, as did FXR activation.

CONCLUSIONS

FXR activation impairs BEC-driven liver regeneration by enhancing PTEN activity; the PI3K-AKT-mTOR pathway controls the regeneration process. Given the clinical trials and use of FXR agonists for multiple liver diseases due to their beneficial effects on steatosis and fibrosis, the detrimental effects of FXR activation on LPCs suggest a rather personalized use of the agonists in the clinic.

The FBXW7-NOTCH interactome: A ubiquitin proteasomal system-induced crosstalk modulating oncogenic transformation in human tissues

BACKGROUND

Ubiquitin ligases or E3 ligases are well programmed to regulate molecular interactions that operate at a post-translational level. Skp, Cullin, F-box containing complex (or SCF complex) is a multidomain E3 ligase known to mediate the degradation of a wide range of proteins through the proteasomal pathway. The three-dimensional domain architecture of SCF family proteins suggests that it operates through a novel and adaptable "super-enzymatic" process that might respond to targeted therapeutic modalities in cancer.

RECENT FINDINGS

Several F-box containing proteins have been characterized either as tumor suppressors (FBXW8, FBXL3, FBXW8, FBXL3, FBXO1, FBXO4, and FBXO18) or as oncogenes (FBXO5, FBXO9, and SKP2). Besides, F-box members like βTrcP1 and βTrcP2, the ones with context-dependent functionality, have also been studied and reported. FBXW7 is a well-studied F-box protein and is a tumor suppressor. FBXW7 regulates the activity of a range of substrates, such as c-Myc, cyclin E, mTOR, c-Jun, NOTCH, myeloid cell leukemia sequence-1 (MCL1), AURKA, NOTCH through the well-known ubiquitin-proteasome system (UPS)-mediated degradation pathway. NOTCH signaling is a primitive pathway that plays a crucial role in maintaining normal tissue homeostasis. FBXW7 regulates NOTCH protein activity by controlling its half-life, thereby maintaining optimum protein levels in tissue. However, aberrations in the FBXW7 or NOTCH expression levels can lead to poor prognosis and detrimental outcomes in patients. Therefore, the FBXW7-NOTCH axis has been a subject of intense study and research over the years, especially around the interactome's role in driving cancer development and progression. Several studies have reported the effect of FBXW7 and NOTCH mutations on normal tissue behavior. The current review attempts to critically analyze these mutations prognostic value in a wide range of tumors. Furthermore, the review summarizes the recent findings pertaining to the FBXW7 and NOTCH interactome and its involvement in phosphorylation-related events, cell cycle, proliferation, apoptosis, and metastasis.

CONCLUSION

The review concludes by positioning FBXW7 as an effective diagnostic marker in tumors and by listing out recent advancements made in cancer therapeutics in identifying protocols targeting the FBXW7-NOTCH aberrations in tumors.

Attenuating the Epidermal Growth Factor Receptor-Extracellular Signal-Regulated Kinase-Sex-Determining Region Y-Box 9 Axis Promotes Liver Progenitor Cell-Mediated Liver Regeneration in Zebrafish

BACKGROUND AND AIMS

The liver is a highly regenerative organ, but its regenerative capacity is compromised in severe liver injury settings. In chronic liver diseases, the number of liver progenitor cells (LPCs) correlates proportionally to disease severity, implying that their inefficient differentiation into hepatocytes exacerbates the disease. Moreover, LPCs secrete proinflammatory cytokines; thus, their prolonged presence worsens inflammation and induces fibrosis. Promoting LPC-to-hepatocyte differentiation in patients with advanced liver disease, for whom liver transplantation is currently the only therapeutic option, may be a feasible clinical approach because such promotion generates more functional hepatocytes and concomitantly reduces inflammation and fibrosis.

APPROACH AND RESULTS

Here, using zebrafish models of LPC-mediated liver regeneration, we present a proof of principle of such therapeutics by demonstrating a role for the epidermal growth factor receptor (EGFR) signaling pathway in differentiation of LPCs into hepatocytes. We found that suppression of EGFR signaling promoted LPC-to-hepatocyte differentiation through the mitogen-activated ERK kinase (MEK)-extracellular signal-regulated kinase (ERK)-sex-determining region Y-box 9 (SOX9) cascade. Pharmacological inhibition of EGFR or MEK/ERK promoted LPC-to-hepatocyte differentiation as well as genetic suppression of the EGFR-ERK-SOX9 axis. Moreover, Sox9b overexpression in LPCs blocked their differentiation into hepatocytes. In the zebrafish liver injury model, both hepatocytes and biliary epithelial cells contributed to LPCs. EGFR inhibition promoted the differentiation of LPCs regardless of their origin. Notably, short-term treatment with EGFR inhibitors resulted in better liver recovery over the long term.

CONCLUSIONS

The EGFR-ERK-SOX9 axis suppresses LPC-to-hepatocyte differentiation during LPC-mediated liver regeneration. We suggest EGFR inhibitors as a proregenerative therapeutic drug for patients with advanced liver disease.

Contributions of biliary epithelial cells to hepatocyte homeostasis and regeneration in zebrafish

Whether transdifferentiation of the biliary epithelial cells (BECs) to hepatocytes occurs under physiological conditions and contributes to liver homeostasis remains under long-term debate. Similar questions have been raised under pathological circumstances if a fibrotic liver is suffered from severe injuries. To address these questions in zebrafish, we established a sensitive lineage tracing system specific for the detection of BEC-derived hepatocytes. The BEC-to-hepatocyte transdifferentiation occurred and became minor contributors to hepatocyte homeostasis in a portion of adult individuals. The BEC-derived hepatocytes distributed in clusters in the liver. When a fibrotic liver underwent extreme hepatocyte damages, BEC-to-hepatocyte transdifferentiation acted as the major origin of regenerating hepatocytes. In contrast, partial hepatectomy failed to induce the BEC-to-hepatocyte conversion. In conclusion, based on a sensitive lineage tracing system, our results suggest that BECs are able to transdifferentiate into hepatocytes and contribute to both physiological hepatocyte homeostasis and pathological regeneration.

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Developed sensitivity system to trace BECs derived hepatocytes in liver homeostasis

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BECs convert to hepatocytes in liver homeostasis but are individually heterogeneous

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BECs are the primary regeneration sources in the extreme injury of the fibrotic liver

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BECs fail to contribute to new hepatocytes after partial hepatectomy

Epigenetic Regulation of Cell-Fate Changes That Determine Adult Liver Regeneration After Injury

The adult liver has excellent regenerative potential following injury. In contrast to other organs of the body that have high cellular turnover during homeostasis (e.g., intestine, stomach, and skin), the adult liver is a slowly self-renewing organ and does not contain a defined stem-cell compartment that maintains homeostasis. However, tissue damage induces significant proliferation across the liver and can trigger cell-fate changes, such as trans-differentiation and de-differentiation into liver progenitors, which contribute to efficient tissue regeneration and restoration of liver functions. Epigenetic mechanisms have been shown to regulate cell-fate decisions in both embryonic and adult tissues in response to environmental cues. Underlying their relevance in liver biology, expression levels and epigenetic activity of chromatin modifiers are often altered in chronic liver disease and liver cancer. In this review, I examine the role of several chromatin modifiers in the regulation of cell-fate changes that determine efficient adult liver epithelial regeneration in response to tissue injury in mouse models. Specifically, I focus on epigenetic mechanisms such as chromatin remodelling, DNA methylation and hydroxymethylation, and histone methylation and deacetylation. Finally, I address how altered epigenetic mechanisms and the interplay between epigenetics and metabolism may contribute to the initiation and progression of liver disease and cancer.

Zebra-Fishing for Regenerative Awakening in Mammals

Regeneration is defined as the ability to regrow an organ or a tissue destroyed by degeneration or injury. Many human degenerative diseases and pathologies, currently incurable, could be cured if functional tissues or cells could be restored. Unfortunately, humans and more generally mammals have limited regenerative capabilities, capacities that are even further declining with age, contrary to simpler organisms. Initially thought to be lost during evolution, several studies have revealed that regenerative mechanisms are still present in mammals but are latent and thus they could be stimulated. To do so there is a pressing need to identify the fundamental mechanisms of regeneration in species able to efficiently regenerate. Thanks to its ability to regenerate most of its organs and tissues, the zebrafish has become a powerful model organism in regenerative biology and has recently engendered a number of studies attesting the validity of awakening the regenerative potential in mammals. In this review we highlight studies, particularly in the liver, pancreas, retina, heart, brain and spinal cord, which have identified conserved regenerative molecular events that proved to be beneficial to restore murine and even human cells and which helped clarify the real clinical translation potential of zebrafish research to mammals.

All routes lead to Rome: multifaceted origin of hepatocytes during liver regeneration

Liver is the largest internal organ that serves as the key site for various metabolic activities and maintenance of homeostasis. Liver diseases are great threats to human health. The capability of liver to regain its mass after partial hepatectomy has widely been applied in treating liver diseases either by removing the damaged part of a diseased liver in a patient or transplanting a part of healthy liver into a patient. Vast efforts have been made to study the biology of liver regeneration in different liver-damage models. Regarding the sources of hepatocytes during liver regeneration, convincing evidences have demonstrated that different liver-damage models mobilized different subtype hepatocytes in contributing to liver regeneration. Under extreme hepatocyte ablation, biliary epithelial cells can undergo dedifferentiation to liver progenitor cells (LPCs) and then LPCs differentiate to produce hepatocytes. Here we will focus on summarizing the progresses made in identifying cell types contributing to producing new hepatocytes during liver regeneration in mice and zebrafish.

Angel or Devil ? - CDK8 as the new drug target

Cyclin-dependent kinase 8 (CDK8) plays an momentous role in transcription regulation by forming kinase module or transcription factor phosphorylation. A large number of evidences have identified CDK8 as an important factor in cancer occurrence and development. In addition, CDK8 also participates in the regulation of cancer cell stress response to radiotherapy and chemotherapy, assists tumor cell invasion, metastasis, and drug resistance. Therefore, CDK8 is regarded as a promising target for cancer therapy. Most studies in recent years supported the role of CDK8 as a carcinogen, however, under certain conditions, CDK8 exists as a tumor suppressor. The functional diversity of CDK8 and its exceptional role in different types of cancer have aroused great interest from scientists but even more controversy during the discovery of CDK8 inhibitors. In addition, CDK8 appears to be an effective target for inflammation diseases and immune system disorders. Therefore, we summarized the research results of CDK8, involving physiological/pathogenic mechanisms and the development status of compounds targeting CDK8, provide a reference for the feasibility evaluation of CDK8 as a therapeutic target, and guidance for researchers who are involved in this field for the first time.