Retinoic acid signaling positively regulates liver specification by inducing wnt2bb gene expression in medaka

Generation of in vivo-like multicellular liver organoids by mimicking developmental processes: A review

Liver is involved in metabolic reactions, ammonia detoxification, and immunity. Multicellular liver tissue cultures are more desirable for drug screening, disease modeling, and researching transplantation therapy, than hepatocytes monocultures. Hepatocytes monocultures are not stable for long. Further, hepatocyte-like cells induced from pluripotent stem cells and in vivo hepatocytes are functionally dissimilar. Organoid technology circumvents these issues by generating functional ex vivo liver tissue from intrinsic liver progenitor cells and extrinsic stem cells, including pluripotent stem cells. To function as in vivo liver tissue, the liver organoid cells must be arranged precisely in the 3-dimensional space, closely mimicking in vivo liver tissue. Moreover, for long term functioning, liver organoids must be appropriately vascularized and in contact with neighboring epithelial tissues (e.g., bile canaliculi and intrahepatic bile duct, or intrahepatic and extrahepatic bile ducts). Recent discoveries in liver developmental biology allows one to successfully induce liver component cells and generate organoids. Thus, here, in this review, we summarize the current state of knowledge on liver development with a focus on its application in generating different liver organoids. We also cover the future prospects in creating (functionally and structurally) in vivo-like liver organoids using the current knowledge on liver development.

Deciphering the role of retinoic acid in hepatic patterning and induction in the mouse

Retinoic acid (RA), a metabolite of vitamin A, is a small molecule and morphogen that is required for embryonic development. While normal RA signals are required for hepatic development in a variety of vertebrates, a role for RA during mammalian hepatic specification has yet to be defined. To examine the requirement for RA in murine liver induction, we performed whole embryo culture with the small molecule RA inhibitor, BMS493, to attenuate RA signaling immediately prior to hepatic induction and through liver bud formation. BMS493 treated embryos demonstrated a significant loss of hepatic specification that was confined to the prospective dorsal anterior liver bud. Examination of RA attenuated embryos demonstrates that while the liver bud displays normal expression of foregut endoderm markers and the hepato-pancreatobiliary domain marker, PROX1, the dorsal/anterior liver bud excludes the critical hepatic marker, HNF4α, indicating that RA signals are required for dorsal/anterior hepatic induction. These results were confirmed and extended by careful examination of Rdh10^trex/trex embryos, which carry a genetic perturbation in RA synthesis. At E9.5 Rdh10^trex/trex embryos display a similar yet more significant loss of the anterior/dorsal liver bud. Notably the anterior/dorsal liver bud loss correlates with the known dorsal-ventral gradient of the RA synthesis enzyme, Aldh1a2. In addition to altered hepatic specification, the mesoderm surrounding the liver bud is disorganized in RA abrogated embryos. Analysis of E10.5 Rdh10^trex/trex embryos reveals small livers that appear to lack the dorsal/caudal lobes. Finally, addition of exogenous RA prior to hepatic induction results in a liver bud that has failed to thicken and is largely unspecified. Taken together our ex vivo and in vivo evidence demonstrate that the generation of normal RA gradients is required for hepatic patterning, specification, and growth.

Wnt8a is one of the candidate genes that play essential roles in the elongation of the seahorse prehensile tail

Seahorses are a hallmark of specialized morphological features due to their elongated prehensile tail. However, the underlying genomic grounds of seahorse tail development remain elusive. Herein, we evaluated the roles of essential genes from the Wnt gene family for the tail developmental process in the lined seahorse (Hippocampus erectus). Comparative genomic analysis revealed that the Wnt gene family is conserved in seahorses. The expression profiles and in situ hybridization suggested that Wnt5a, Wnt8a, and Wnt11 may participate in seahorse tail development. Like in other teleosts, Wnt5a and Wnt11 were found to regulate the development of the tail axial mesoderm and tail somitic mesoderm, respectively. However, a significantly extended expression period of Wnt8a during seahorse tail development was observed. Signaling pathway analysis further showed that Wnt8a up-regulated the expression of the tail axial mesoderm gene (Shh), while interaction analysis indicated that Wnt8a could promote the expression of Wnt11. In summary, our results indicate that the special extended expression period of Wnt8a might promote caudal tail axis formation, which contributes to the formation of the elongated tail of the seahorse.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00099-7.

Advanced Techniques and Awaited Clinical Applications for Human Pluripotent Stem Cell Differentiation into Hepatocytes

Liver transplantation is currently the only curative treatment for several liver diseases such as acute liver failure, end-stage liver disorders, primary liver cancers, and certain genetic conditions. Unfortunately, despite improvements to transplantation techniques, including live donor transplantation, the number of organs available remains insufficient to meet patient needs. Hepatocyte transplantation has enabled some encouraging results as an alternative to organ transplantation, but primary hepatocytes are little available and cannot be amplified using traditional two-dimensional culture systems. Indeed, although recent studies have tended to show that three-dimensional culture enables long-term hepatocyte culture, it is still agreed that, like most adult primary cell types, hepatocytes remain refractory to in vitro expansion. Because of their exceptional properties, human pluripotent stem cells (hPSCs) can be amplified indefinitely and differentiated into any cell type, including liver cells. While many teams have worked on hepatocyte differentiation, there has been a consensus that cells obtained after hPSC differentiation have more fetal than adult hepatocyte characteristics. New technologies have been used to improve the differentiation process in recent years. This review discusses the technical improvements made to hepatocyte differentiation protocols and the clinical approaches developed to date and anticipated in the near future.

Global DNA hypermethylation pattern and unique gene expression signature in liver cancer from patients with Indigenous American ancestry

Hepatocellular carcinoma (HCC) usually afflicts individuals in their maturity after a protracted liver disease. Contrasting with this pattern, the age structure of HCC in Andean people displays a bimodal distribution with half of the patients developing HCC in adolescence and early adulthood. To deepen our understanding of the molecular determinants of the disease in this population, we conducted an integrative analysis of gene expression and DNA methylation in HCC developed by 74 Peruvian patients, including 39 adolescents and young adults. While genome-wide hypomethylation is considered as a paradigm in human HCCs, our analysis revealed that Peruvian tumors are associated with a global DNA hypermethylation. Moreover, pathway enrichment analysis of transcriptome data characterized an original combination of signatures. Peruvian HCC forgoes canonical activations of IGF2, Notch, Ras/MAPK, and TGF-β signals to depend instead on Hippo/YAP1, MYC, and Wnt/β-catenin pathways. These signatures delineate a homogeneous subtype of liver tumors at the interface of the proliferative and non-proliferative classes of HCCs. Remarkably, the development of this HCC subtype occurs in patients with one of the four Native American mitochondrial haplogroups A-D. Finally, integrative characterization revealed that Peruvian HCC is apparently controlled by the PRC2 complex that mediates cell reprogramming with massive DNA methylation modulating gene expression and pinpointed retinoid signaling as a potential target for epigenetic therapy.

Zebrafish miR-462-731 is required for digestive organ development

MicroRNAs (miRNAs), as important regulators of post-transcriptional gene expression, play important roles in the occurrence and function of organs. In this study, morpholino (MO) knockdown of miR-462/miR-731 was used to investigate the potential mechanisms of the miR-462-731 cluster during zebrafish liver development. The results showed significant reduction of digestive organs, especially liver and exocrine pancreas after the miR-462/miR-731 knockdown, and those phenotypes could be partially rescued by corresponding miRNA duplex. Acinar cells of the exocrine pancreas were also severely affected with pancreatic insufficiency. In particular, knockdown of miR-462 caused pancreas morphogenesis abnormity with specific bilateral exocrine pancreas. Additionally, it was found that miR-731 played a role in liver and exocrine pancreas development by directly targeting dkk3b, instead of the down-regulation of Wnt/β-catenin signaling. These findings contribute significantly to our understanding of molecular mechanisms of miR-462-731 cluster in development of digestive organs.

Epithelial Morphogenesis during Liver Development

Tissue stem/progenitor cells supply multiple types of epithelial cells that eventually acquire specialized functions during organ development. In addition, three-dimensional (3D) tissue structures need to be established for organs to perform their physiological functions. The liver contains two types of epithelial cells, namely, hepatocytes and cholangiocytes, which are derived from hepatoblasts, fetal liver stem/progenitor cells (LPCs), in mid-gestation. Hepatocytes performing many metabolic reactions form cord-like structures, whereas cholangiocytes, biliary epithelial cells, form tubular structures called intrahepatic bile ducts. Analyses for human genetic diseases and mutant mice have identified crucial molecules for liver organogenesis. Functions of those molecules can be examined in in vitro culture systems where LPCs are induced to differentiate into hepatocytes or cholangiocytes. Recent technical advances have revealed 3D epithelial morphogenesis during liver organogenesis. Therefore, the liver is a good model to understand how tissue stem/progenitor cells differentiate and establish 3D tissue architectures during organ development.

The mevalonate pathway regulates primitive streak formation via protein farnesylation

The primitive streak in peri-implantation embryos forms the mesoderm and endoderm and controls cell differentiation. The metabolic cues regulating primitive streak formation remain largely unknown. Here we utilised a mouse embryonic stem (ES) cell differentiation system and a library of well-characterised drugs to identify these metabolic factors. We found that statins, which inhibit the mevalonate metabolic pathway, suppressed primitive streak formation in vitro and in vivo. Using metabolomics and pharmacologic approaches we identified the downstream signalling pathway of mevalonate and revealed that primitive streak formation requires protein farnesylation but not cholesterol synthesis. A tagging-via-substrate approach revealed that nuclear lamin B1 and small G proteins were farnesylated in embryoid bodies and important for primitive streak gene expression. In conclusion, protein farnesylation driven by the mevalonate pathway is a metabolic cue essential for primitive streak formation.

Morphogenesis of liver epithelial cells

The mammalian liver is a physiologically important organ performing various types of metabolism, producing serum proteins, detoxifying bilirubin and ammonia, and protecting the body from infection. Those physiological functions are achieved with the 3D tissue architecture of liver epithelial cells. The liver contains two types of epithelial cells, namely, hepatocytes and cholangiocytes. They split from hepatoblasts (embryonic liver stem cells) in mid-gestation and differentiate into structurally and functionally mature cells. Analyses of mutant mice showing abnormal liver organogenesis have identified genes involved in liver development. In vitro culture systems have been used to examine the mechanism in which each molecule or signaling pathway regulates the morphogenesis and functional differentiation of hepatocytes and cholangiocytes. In addition, liver epithelial cells as well as mesenchymal, sinusoidal endothelial and hematopoietic cells can be purified from developing livers, which enables us to perform genome-wide screening to identify novel genes regulating epithelial morphogenesis in the liver. By combining these in vivo and in vitro systems, the liver could be a unique and suitable model for revealing a principle, governing epithelial morphogenesis. In this review, we summarize recent progress in the understanding of the development of liver epithelial tissue structures.

Endoderm specification and liver development

The endoderm is the innermost embryonic germ layer, and in zebrafish, it gives rise to the lining of the gut, the gills, liver, pancreas, gallbladder, and derivatives of the pharyngeal pouch. These organs form the gastrointestinal tract and are involved with the absorption, delivery, and metabolism of nutrients. The liver has a central role in regulating these processes because it controls carbohydrate and lipid metabolism, protein synthesis, and breakdown of endogenous and xenobiotic products. Liver dysfunction frequently leads to significant morbidity and mortality; however, in most settings of organ injury, the liver exhibits remarkable regenerative capacity. In this chapter, we review the principal mechanisms of endoderm and liver formation and provide protocols to assess liver formation and liver regeneration.

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

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

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

Orchestrating liver development

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

The lure of zebrafish in liver research: regulation of hepatic growth in development and regeneration

The liver is an essential organ that plays a pivotal role in metabolism, digestion and nutrient storage. Major efforts have been made to develop zebrafish (Danio rerio) as a model system to study the pathways regulating hepatic growth during liver development and regeneration. Zebrafish offer unique advantages over other vertebrates including in vivo imaging at cellular resolution and the capacity for large-scale chemical and genetic screens. Here, we review the cellular and molecular mechanisms that regulate hepatic growth during liver development in zebrafish. We also highlight emerging evidence that developmental pathways are reactivated following liver injury to facilitate regeneration. Finally, we discuss how zebrafish have transformed drug discovery efforts and enabled the identification of drugs that stimulate hepatic growth and provide hepatoprotection in pre-clinical models of liver injury, with the ultimate goal of identifying novel therapeutic approaches to treat liver disease.

Type of fatty acids in maternal diets during pregnancy and/or lactation and metabolic consequences of the offspring

During pregnancy and/or lactation, maternal nutrition is related to the adequate development of the fetus, newborn and future adult, likely by modifications in fetal programming and epigenetic regulation. Fetal programming is characterized by adaptive responses to specific environmental conditions during early life stages, which may alter gene expression and permanently affect the structure and function of several organs and tissues, thus influencing the susceptibility to metabolic disorders. Regarding lipid metabolism during the first two trimesters of pregnancy, the maternal body accumulates fat, whereas in late pregnancy, the lipolytic activity in the maternal adipose tissue is increased. However, an excess or deficiency of certain fatty acids may lead to adverse consequences to the fetuses and newborns. Fetal exposure to trans fatty acids appears to promote early deleterious effects in the offspring's health, thereby increasing the individual risk for developing metabolic diseases throughout life. Similarly, the maternal intake of saturated fatty acids seems to trigger alterations in the liver and adipose tissue function associated with insulin resistance and diabetes. The polyunsaturated fatty acids (PUFAs), particularly long-chain PUFAs (long-chain PUFA-arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid), play an important and beneficial physiologic role in the offspring who receive this fatty acid during critical periods of development. Therefore, the maternal nutritional condition and fatty acid intake during pregnancy and/or lactation are critical factors that are strongly associated with normal fetal and postnatal development, which influence the modifications in fetal programming and in the individual risk for developing metabolic diseases throughout life.

Rargb regulates organ laterality in a zebrafish model of right atrial isomerism

Developmental signals determine organ morphology and position during embryogenesis. To discover novel modifiers of liver development, we performed a chemical genetic screen in zebrafish and identified retinoic acid as a positive regulator of hepatogenesis. Knockdown of the four RA receptors revealed that all receptors affect liver formation, however specific receptors exert differential effects. Rargb knockdown results in bilateral livers but does not impact organ size, revealing a unique role for Rargb in conferring left-right positional information. Bilateral populations of hepatoblasts are detectable in rargb morphants, indicating Rargb acts during hepatic specification to position the liver, and primitive endoderm is competent to form liver on both sides. Hearts remain at the midline and gut looping is perturbed in rargb morphants, suggesting Rargb affects lateral plate mesoderm migration. Overexpression of Bmp during somitogenesis similarly results in bilateral livers and midline hearts, and inhibition of Bmp signaling rescues the rargb morphant phenotype, indicating Rargb functions upstream of Bmp to regulate organ sidedness. Loss of rargb causes biliary and organ laterality defects as well as asplenia, paralleling symptoms of the human condition right atrial isomerism. Our findings uncover a novel role for RA in regulating organ laterality and provide an animal model of one form of human heterotaxia.

An antiapoptotic role of sorting nexin 7 is required for liver development in zebrafish

Sorting nexin (SNX) family proteins are best characterized for their abilities to regulate protein trafficking during processes such as endocytosis of membrane receptors, endosomal sorting, and protein degradation, but their in vivo functions remain largely unknown. We started to investigate the biological functions of SNXs using the zebrafish model. In this study, we demonstrated that SNX7 was essential for embryonic liver development. Hepatoblasts were specified normally, and the proliferation of these cells was not affected when SNX7 was knocked down by gene-specific morpholinos; however, they underwent massive apoptosis during the early budding stage. SNX7 mainly regulated the survival of cells in the embryonic liver and did not affect the viability of cells in other endoderm-derived organs. We further demonstrated that down-regulation of SNX7 by short interfering RNAs induced apoptosis in cell culture. At the molecular level, the cellular FLICE-like inhibitory protein (c-FLIP)/caspase 8 pathway was activated when SNX7 was down-regulated. Furthermore, overexpression of c-FLIP(S) was able to rescue the SNX7 knockdown-induced liver defect.

CONCLUSION

SNX7 is a liver-enriched antiapoptotic protein that is indispensable for the survival of hepatoblasts during zebrafish early embryogenesis.

The Osr1 and Osr2 genes act in the pronephric anlage downstream of retinoic acid signaling and upstream of Wnt2b to maintain pectoral fin development

Vertebrate odd-skipped related genes (Osr) have an essential function during the formation of the intermediate mesoderm (IM) and the kidney structures derived from it. Here, we show that these genes are also crucial for limb bud formation in the adjacent lateral plate mesoderm (LPM). Reduction of zebrafish Osr function impairs fin development by the failure of tbx5a maintenance in the developing pectoral fin bud. Osr morphant embryos show reduced wnt2b expression, and increasing Wnt signaling in Osr morphant embryos partially rescues tbx5a expression. Thus, Osr genes control limb bud development in a non-cell-autonomous manner, probably through the activation of Wnt2b. Finally, we demonstrate that Osr genes are downstream targets of retinoic acid (RA) signaling. Therefore, Osr genes act as a relay within the genetic cascade of fin bud formation: by controlling the expression of the signaling molecule Wnt2ba in the IM they play an essential function transmitting the RA signaling originated in the somites to the LPM.

Inducing the liver: understanding the signals that promote murine liver budding

The endoderm emerges as an epithelial sheet that covers the surface of the developing murine embryo. This tissue will produce the entire gut tube as well as associated digestive and respiratory organs including the thyroid, thymus, lung, liver, and pancreas. The emergence of each endodermal organ occurs in a temporally distinct manner that is dependant upon reciprocal inductive interactions between the endoderm and the underlying mesoderm. The emergence of the hepatic endoderm, which occurs using a morphological process termed liver budding, initiates during early somitogenesis in the mouse at approximately 8.25 days post-coitum (dpc). Explant and transplant studies performed in chicken and mouse have demonstrated that secreted signals from adjacent mesodermal tissues initiate the hepatic gene program from ventral-fated endoderm. Here, we review the data in support of the roles of members of the fibroblast growth factor (FGF), bone morphogenetic protein (BMP), and Wnt signaling pathways in liver budding and discover that little is known about the precise endogenous signals involved in the molecular and morphological induction of liver budding in the mouse.

Endoderm specification, liver development, and regeneration

The endoderm is the innermost germ layer that gives rise to the lining of the gut, the gills, liver, pancreas, gallbladder, and derivatives of the pharyngeal pouch. These organs form the gastrointestinal tract and are involved with the absorption, delivery, and metabolism of nutrients. The liver has a central role in regulating these processes because it controls lipid metabolism, protein synthesis, and breakdown of endogenous and xenobiotic products. Liver dysfunction frequently leads to significant morbidity and mortality; however, in most settings of organ injury, the liver exhibits remarkable regenerative capacity.

Beta-catenin signaling in hepatic development and progenitors: which way does the WNT blow?

The Wnt/β-catenin pathway is an evolutionarily conserved signaling cascade that plays key roles in development and adult tissue homeostasis and is aberrantly activated in many tumors. Over a decade of work in mouse, chick, xenopus, and zebrafish models has uncovered multiple functions of this pathway in hepatic pathophysiology. Specifically, beta-catenin, the central component of the canonical Wnt pathway, is implicated in the regulation of liver regeneration, development, and carcinogenesis. Wnt-independent activation of beta-catenin by receptor tyrosine kinases has also been observed in the liver. In liver development across various species, through regulation of cell proliferation, differentiation, and maturation, beta-catenin directs foregut endoderm specification, hepatic specification of the foregut, and hepatic morphogenesis. Its role has also been defined in adult hepatic progenitors or oval cells especially in their expansion and differentiation. Thus, beta-catenin undergoes tight temporal regulation to exhibit pleiotropic effects during hepatic development and in hepatic progenitor biology.

The LIM protein Ajuba is required for ciliogenesis and left-right axis determination in medaka

Cilia are microtubule-based organelles that are present on the surfaces of almost all vertebrate cells. Most cilia function as sensory or molecular transport structures. Malfunctions of cilia have been implicated in several diseases of human development. The assembly of cilia is initiated by the centriole (or basal body), and several centrosomal proteins are involved in this process. The mammalian LIM protein Ajuba is a well-studied centrosomal protein that regulates cell division but its role in ciliogenesis is unknown. In this study, we isolated the medaka homolog of Ajuba and showed that Ajuba localizes to basal bodies of cilia in growth-arrested cells. Knockdown of Ajuba resulted in randomized left-right organ asymmetries and altered expression of early genes responsible for left-right body axis determination. At the cellular level, we found that Ajuba function was essential for ciliogenesis in the cells lining Kupffer's vesicle; it is these cells that induce the asymmetric fluid flow required for left-right axis determination. Taken together, our findings identify a novel role for Ajuba in the regulation of vertebrate ciliogenesis and left-right axis determination.