Feeder-free and serum-free production of hepatocytes, cholangiocytes, and their proliferating progenitors from human pluripotent stem cells: application to liver-specific functional and cytotoxic assays

Preclinical Research of Stem Cells: Challenges and Progress

In recent years, great breakthroughs have been made in basic research and clinical applications of stem cells in regenerative medicine and other fields, which continue to inspire people to explore the field of stem cells. With nearly unlimited self-renewal ability, stem cells can generate at least one type of highly differentiated daughter cell, which provides broad development prospects for the treatment of human organ damage and other diseases. In the field of stem cell research, related technologies for inducing or isolating stem cells are relatively mature, and a variety of stable stem cell lines have been successfully constructed. To realize the full clinical application of stem cells as soon as possible, it is more and more important to further optimize each stage of stem cell research while conforming to Current Good Manufacture Practices (cGMP) standards. Here, we synthesized recent developments in stem cell research and focus on the introduction of xenogenicity in the preclinical research process and the remaining problems of various cell bioreactors. Our goal is to promote the development of technologies for xeno-free culture and clinical expansion of stem cells through in-depth discussion of current research. This review will provide new insight into stem cell research protocols and will contribute to the creation of efficient and stable stem cell expansion systems.

A dataset of definitive endoderm and hepatocyte differentiations from human induced pluripotent stem cells

Hepatocytes are a major parenchymal cell type in the liver and play an essential role in liver function. Hepatocyte-like cells can be differentiated in vitro from induced pluripotent stem cells (iPSCs) via definitive endoderm (DE)-like cells and hepatoblast-like cells. Here, we explored the in vitro differentiation time-course of hepatocyte-like cells. We performed methylome and transcriptome analyses for hepatocyte-like cell differentiation. We also analyzed DE-like cell differentiation by methylome, transcriptome, chromatin accessibility, and GATA6 binding profiles, using finer time-course samples. In this manuscript, we provide a detailed description of the dataset and the technical validations. Our data may be valuable for the analysis of the molecular mechanisms underlying hepatocyte and DE differentiations.

Dialysis based-culture medium conditioning improved the generation of human induced pluripotent stem cell derived-liver organoid in a high cell density

Human pluripotent stem cell-derived liver organoids (HLOs) have recently become a promising alternative for liver regenerative therapy. To realize this application, a large amount of human-induced pluripotent stem cells (hiPSCs) derived-liver cells are required for partial liver replacement during transplantation. This method requires stepwise induction using costly growth factors to direct the hiPSCs into the hepatic lineage. Therefore, we developed a simple dialysis-based medium conditioning that fully utilized growth factors accumulation to improve hepatic differentiation of hiPSCs at a high cell density. The results demonstrated that the dialysis culture system could accumulate the four essential growth factors required in each differentiation stage: activin A, bone morphogenetic protein 4 (BMP4), hepatocyte growth factor (HGF), and oncostatin M (OSM). As a result, this low lactate culture environment allowed high-density bipotential hepatic differentiation of up to 4.5 × 10⁷ cells/mL of human liver organoids (HLOs), consisting of hiPSC derived-hepatocyte like cells (HLCs) and cholangiocyte like-cells (CLCs). The differentiated HLOs presented a better or comparable hepatic marker and hepatobiliary physiology to the one that differentiated in suspension culture with routine daily medium replacement at a lower cell density. This simple miniaturized dialysis culture system demonstrated the feasibility of cost-effective high-density hepatic differentiation with minimum growth factor usage.

Induced hepatic stem cells are suitable for human hepatocyte production

Human hepatocytes were transfected with Sendai virus vectors (SeV) expressing OCT3/4, SOX2, KLF4, and C-MYC to produce hepatocyte-derived induced pluripotent stem cells (iPSCs). The messenger RNA (mRNA) expression of undifferentiated markers (passage 19-21) and hepatocyte-specific markers (HSMs) (passage 0-20) in 48 established hepatocyte-derived iPSC-like colonies was examined. Among the 48 clones, 10 clones continuously expressed HSM mRNA (HNF1β and HNF4α) in passage 0-20. The colonies which expressed HSMs (iTS-L cells: induced tissue-specific stem cells from liver) showed a different tendency in microarray and methylation analyses to fibroblast-derived iPSCs (strain: 201B7). iTS-L cells were less likely to form teratomas in mice than iPSCs (He). The iTS-L cells were differentiated into hepatocyte-like cells more efficiently than iPSCs (He) or iPSCs (201B7). These data suggest that SeV expressing OCT3/4, SOX2, KLF4, and C-MYC induce the generation of iPSCs and iTS-L cells.

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iTS cells have self-renewal and multipotency

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iTS cells express tissue-specific markers

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iTS-L cells are less prone to teratoma formation than iPSCs

Preclinical models of idiosyncratic drug-induced liver injury (iDILI): Moving towards prediction

Idiosyncratic drug-induced liver injury (iDILI) encompasses the unexpected harms that prescription and non-prescription drugs, herbal and dietary supplements can cause to the liver. iDILI remains a major public health problem and a major cause of drug attrition. Given the lack of biomarkers for iDILI prediction, diagnosis and prognosis, searching new models to predict and study mechanisms of iDILI is necessary. One of the major limitations of iDILI preclinical assessment has been the lack of correlation between the markers of hepatotoxicity in animal toxicological studies and clinically significant iDILI. Thus, major advances in the understanding of iDILI susceptibility and pathogenesis have come from the study of well-phenotyped iDILI patients. However, there are many gaps for explaining all the complexity of iDILI susceptibility and mechanisms. Therefore, there is a need to optimize preclinical human in vitro models to reduce the risk of iDILI during drug development. Here, the current experimental models and the future directions in iDILI modelling are thoroughly discussed, focusing on the human cellular models available to study the pathophysiological mechanisms of the disease and the most used in vivo animal iDILI models. We also comment about in silico approaches and the increasing relevance of patient-derived cellular models.

Ammonia-based enrichment and long-term propagation of zone I hepatocyte-like cells

Ammonia has a cytotoxic effect and can therefore be used as a selection agent for enrichment of zone I hepatocytes. However, it has not yet been determined whether ammonia-treated hepatocyte-like cells are able to proliferate in vitro. In this study, we employed an ammonia selection strategy to purify hepatocyte-like cells that were differentiated from human embryonic stem cells (ESCs) and from induced pluripotent stem cells (iPSCs). The resistance to cytotoxicity or cell death by ammonia is likely attributable to the metabolism of ammonia in the cells. In addition to ammonia metabolism-related genes, ammonia-selected hepatocytes showed increased expression of the cytochrome P450 genes. Additionally, the ammonia-selected cells achieved immortality or at least an equivalent life span to human pluripotent stem cells without affecting expression of the liver-associated genes. Ammonia treatment in combination with in vitro propagation is useful for obtaining large quantities of hepatocytes.

Directed differentiation of human induced pluripotent stem cells to hepatic stellate cells

Hepatic stellate cells (HSCs) are nonparenchymal liver cells responsible for extracellular matrix homeostasis and are the main cells involved in the development of liver fibrosis following injury. The lack of reliable sources of HSCs has hence limited the development of complex in vitro systems to model liver diseases and toxicity. Here we describe a protocol to differentiate human induced pluripotent stem cells (iPSCs) into hepatic stellate cells (iPSC-HSCs). The protocol is based on the addition of several growth factors important for liver development sequentially over 12 d. iPSC-HSCs present phenotypic and functional characteristics of primary HSCs and can be expanded or frozen and used to perform high-throughput in vitro studies. We also describe how to coculture iPSC-HSCs with hepatocytes, which self-assemble into three-dimensional (3D) hepatic spheroids. This protocol enables the generation of HSC-like cells for in vitro modeling and drug screening studies.

Tissue-Engineered Bile Ducts for Disease Modeling and Therapy

Recent biotechnical advances in the in vitro culture of cholangiocytes and generation of bioengineered biliary tissue have a high potential for creating biliary tissue to be used for disease modeling, drug screening, and transplantation. For the past few decades, scientists have searched for a source of cholangiocytes, focused on primary cholangiocytes or cholangiocytes derived from hepatocytes or stem cells. At the same time, the development of scaffolds for biliary tissue engineering for transplantation and modeling of cholangiopathies has been explored. In this review, we provide an overview on the current understanding of cholangiocytes sources, the effect of signaling molecules, and transcription factors on cell differentiation, along with the effects of extracellular matrix molecules and scaffolds on bioengineered biliary tissues, and their application in disease modeling and drug screening. Impact statement Over the past few decades, biliary tissue engineering has acquired significant attention, but currently a number of factors hinder this field to eventually generate bioengineered bile ducts that mimic in vivo physiology and are suitable for transplantation. In this review, we present the latest advances with respect to cell source selection, influence of growth factors and scaffolds, and functional characterization, as well as applications in cholangiopathy modeling and drug screening. This review is suited for a broad spectrum of readers, including fundamental liver researchers and clinicians with interest in the current state and application of bile duct engineering and disease modeling.

Immortalization of human hepatocytes from biliary atresia with CDK4R24C, cyclin D1, and TERT for cytochrome P450 induction testing

Hepatocytes are an important tool for in vitro toxicology testing. In addition to primary cultures, a limited number of immortalized cell lines have been developed. We here describe a new cell line, designated as HepaMN, which has been established from a liver associated with biliary atresia. Hepatocytes were isolated from a liver of 4-year-old girl with biliary atresia and immortalized by inoculation with CSII-CMV-TERT, CSII-CMV-Tet-Off, CSII-TRE-Tight-cyclin D1 and CSII-TRE-Tight-CDK4^R24C (mutant CDK4: an INK4a-resistant form of CDK4) lentiviruses at the multiplicity of infection of 3 to 10. HepaMN cells exhibited morphological homogeneity, displaying hepatocyte-like phenotypes. Phenotypic studies in vivo and in vitro revealed that HepaMN cells showed polarized and functional hepatocyte features along with a canalicular cell phenotype under defined conditions, and constitutively expressed albumin and carbamoyl phosphate synthetase I in addition to epithelial markers. Since HepaMN cells are immortal and subcloned, kinetics and expression profiles were independent of population doublings. HepaMN cells showed increased CYP3A4 expression after exposure to rifampicin, implying that their close resemblance to normal human hepatocytes makes them suitable for research applications including drug metabolism studies.

Utility of Common Marmoset (Callithrix jacchus) Embryonic Stem Cells in Liver Disease Modeling, Tissue Engineering and Drug Metabolism

The incidence of liver disease is increasing significantly worldwide and, as a result, there is a pressing need to develop new technologies and applications for end-stage liver diseases. For many of them, orthotopic liver transplantation is the only viable therapeutic option. Stem cells that are capable of differentiating into all liver cell types and could closely mimic human liver disease are extremely valuable for disease modeling, tissue regeneration and repair, and for drug metabolism studies to develop novel therapeutic treatments. Despite the extensive research efforts, positive results from rodent models have not translated meaningfully into realistic preclinical models and therapies. The common marmoset Callithrix jacchus has emerged as a viable non-human primate model to study various human diseases because of its distinct features and close physiologic, genetic and metabolic similarities to humans. C. jacchus embryonic stem cells (cjESC) and recently generated cjESC-derived hepatocyte-like cells (cjESC-HLCs) could fill the gaps in disease modeling, liver regeneration and metabolic studies. They are extremely useful for cell therapy to regenerate and repair damaged liver tissues in vivo as they could efficiently engraft into the liver parenchyma. For in vitro studies, they would be advantageous for drug design and metabolism in developing novel drugs and cell-based therapies. Specifically, they express both phase I and II metabolic enzymes that share similar substrate specificities, inhibition and induction characteristics, and drug metabolism as their human counterparts. In addition, cjESCs and cjESC-HLCs are advantageous for investigations on emerging research areas, including blastocyst complementation to generate entire livers, and bioengineering of discarded livers to regenerate whole livers for transplantation.

Enhanced Hepatic Differentiation of Human Induced Pluripotent Stem Cells Using Gas-Permeable Membrane

IMPACT STATEMENT

Although oxygen is a vital nutrient for the hepatocytes in vitro, few reports have focused on its effect during hepatic differentiation of induced pluripotent stem cells (iPSCs). In this report, we performed the hepatic differentiation of human iPSCs (hiPSCs) under different atmospheric oxygen concentrations and oxygen supply fluxes to investigate the effects of oxygen in terms of both the concentration and the supply flux. Results demonstrate that direct oxygenation through a polydimethylsiloxane (PDMS) membrane enhances the maturation and efficient production of hiPSC-derived hepatocyte-like cells (iHeps). Thus, direct oxygenation through a PDMS membrane is a better alternative culture method over conventional tissue culture-treated polystyrene (TCPS) plates for the maturation of hiPSC-derived hepatocytes in vitro.

Innovation for hepatotoxicity in vitro research models: A review

Many categories of drugs can induce hepatotoxicity, so improving the prediction of toxic drugs is important. In vitro models using human hepatocytes are more accurate than in vivo animal models. Good in vitro models require an abundance of metabolic enzyme activities and normal cellular polarity. However, none of the in vitro models can completely simulate hepatocytes in the human body. There are two ways to overcome this limitation: enhancing the metabolic function of hepatocytes and changing the cultural environment. In this review, we summarize the current state of research, including the main characteristics of in vitro models and their limitations, as well as improved technology and developmental prospects. We hope that this review provides some new ideas for hepatotoxicity research.

Hedgehog Signaling Overcomes an EZH2-Dependent Epigenetic Barrier to Promote Cholangiocyte Expansion

BACKGROUND & AIMS

Developmental morphogens play an important role in coordinating the ductular reaction and portal fibrosis occurring in the setting of cholangiopathies. However, little is known about how membrane signaling events in ductular reactive cells (DRCs) are transduced into nuclear transcriptional changes to drive cholangiocyte maturation and matrix deposition. Therefore, the aim of this study was to investigate potential mechanistic links between cell signaling events and epigenetic regulators in DRCs.

METHODS

Using directed differentiation of induced pluripotent stem cells (iPSC), isolated DRCs, and in vivo models, we examine the mechanisms whereby sonic hedgehog (Shh) overcomes an epigenetic barrier in biliary precursors and promotes both cholangiocyte maturation and deposition of fibronectin (FN).

RESULTS

We demonstrate, for the first time, that Gli1 influences the differentiation state and fibrogenic capacity of iPSC-derived hepatic progenitors and isolated DRCs. We outline a novel pathway wherein Shh-mediated Gli1 binding in key cholangiocyte gene promoters overcomes an epigenetic barrier conferred by the polycomb protein, enhancer of zeste homolog 2 (EZH2) and initiates the transcriptional program of cholangiocyte maturation. We also define previously unknown functional Gli1 binding sites in the promoters of cytokeratin (CK)7, CK19, and FN. Our in vivo results show that EZH2 KO mice fed the choline-deficient, ethanolamine supplemented (CDE) diet have an exaggerated cholangiocyte expansion associated with more robust ductular reaction and increased peri-portal fibrosis.

CONCLUSION

We conclude that Shh/Gli1 signaling plays an integral role in cholangiocyte maturation in vitro by overcoming an EZH2-dependent epigenetic barrier and this mechanism also promotes biliary expansion in vivo.

Ductular reaction-on-a-chip: Microfluidic co-cultures to study stem cell fate selection during liver injury

Liver injury modulates local microenvironment, triggering production of signals that instruct stem cell fate choices. In this study, we employed a microfluidic co-culture system to recreate important interactions in the liver stem cell niche, those between adult hepatocytes and liver progenitor cells (LPCs). We demonstrate that pluripotent stem cell-derived LPCs choose hepatic fate when cultured next to healthy hepatocytes but begin biliary differentiation program when co-cultured with injured hepatocytes. We connect this fate selection to skewing in production of hepatocyte growth factor (HGF) and transforming growth factor (TGF)-β1 caused by injury. Significantly, biliary fate selection of LPCs was not observed in the absence of hepatocytes nor did it happen in the presence of TGF-β inhibitors. Our study demonstrates that microfluidic culture systems may offer an interesting new tool for dissecting cellular interactions leading to aberrant stem cell differentiation during injury.

Human hepatocytes derived from pluripotent stem cells: a promising cell model for drug hepatotoxicity screening

Drug-induced liver injury (DILI) is a frequent cause of failure in both clinical and post-approval stages of drug development, and poses a key challenge to the pharmaceutical industry. Current animal models offer poor prediction of human DILI. Although several human cell-based models have been proposed for the detection of human DILI, human primary hepatocytes remain the gold standard for preclinical toxicological screening. However, their use is hindered by their limited availability, variability and phenotypic instability. In contrast, pluripotent stem cells, which include embryonic and induced pluripotent stem cells (iPSCs), proliferate extensively in vitro and can be differentiated into hepatocytes by the addition of soluble factors. This provides a stable source of hepatocytes for multiple applications, including early preclinical hepatotoxicity screening. In addition, iPSCs also have the potential to establish genotype-specific cells from different individuals, which would increase the predictivity of toxicity assays allowing more successful clinical trials. Therefore, the generation of human hepatocyte-like cells derived from pluripotent stem cells seems to be promising for overcoming limitations of hepatocyte preparations, and it is expected to have a substantial repercussion in preclinical hepatotoxicity risk assessment in early drug development stages.

New categorization of human vascular endothelial cells by pro- vs anti-proliferative phenotypes

AIM: To integrally understand the effects of human vascular endothelial cells (VECs) on the proliferation of vascular smooth muscle cells (VSMCs).

METHODS: Various kinds of human VECs of different origins were co-cultured with human aortic smooth muscle cells, a representative of human VSMCs. To exclude the irrelevant effects due to growth competition between VECs and VSMCs, the proliferation of VECs had previously been arrested via a low-dose gamma ray irradiation. To discriminately analyze the proliferation of VSMCs from that of VECs, the former cells were labeled with red fluorescent dye while the latter cells were labeled with green fluorescent dye before performing co-culture experiments. After 4 d, total cells were harvested and subjected to flow cytometric analyses. Decrements in red fluorescence intensities due to proliferation-mediated dilutions were measured and mathematically processed using a specific software to quantitatively evaluate the proliferation of VSMCs. The findings obtained from the flow cytometry-based analyses were further validated by microscopic observations.

RESULTS: Commercially available primary cultured human VECs exclusively promoted VSMC proliferation regardless of their tissue origins and we termed these pro-proliferative VECs as “type-I”. By contrast, VECs freshly generated from human bone marrow-derived endothelial progenitors cells or human pluripotent stem cells including embryonic stem cells and induced pluripotent stem cells suppressed VSMC proliferation and we termed these anti-proliferative VECs as “type-II”. Repetitive subcultures as well as oxidative stress induced “type-II VECs to type-I” conversion along with an induction of Regulator of G-protein signaling 5 (RGS5). Compatibly, anti-oxidant treatments suppressed both the subculture-dependent “type-II to type-I” conversion and an induction of RGS5 gene. Immunostaining studies of clinical specimens indicated that RGS5 protein expressions in endothelial layers were low in normal arteries but they were up-regulated in pathological arteries including hypertension, atherosclerosis and autoimmune vasculitis in a dose-dependent manner. Overexpression and knockdown of RGS5 caused that “type-II to type-I” and “type-I to type-II” phenotype conversions of VECs, respectively.

CONCLUSION: Human VECs are categorized into two types: pro-proliferative RGS5^high VECs (type-I) and anti-proliferative RGS5^low VECs (type-II).

Potential of human induced pluripotent stem cells in studies of liver disease

Liver disease is a leading cause of death in the Western world. However, our insight into the underlying disease mechanisms and the development of novel therapeutic agents has been hindered by limited availability of primary tissue, intraspecies variability associated with the use of animal models, and reduced long-term viability of isolated and diseased liver cells. The emergence of human induced pluripotent stem cells and differentiation protocols to generate hepatocyte-like cells has opened the possibility of addressing these issues. Here, we discuss the recent progress and potential in the production of various cell types constituting the liver and their applications to model liver diseases and test drug toxicity in vitro.

Engineered in vitro disease models

The ultimate goal of most biomedical research is to gain greater insight into mechanisms of human disease or to develop new and improved therapies or diagnostics. Although great advances have been made in terms of developing disease models in animals, such as transgenic mice, many of these models fail to faithfully recapitulate the human condition. In addition, it is difficult to identify critical cellular and molecular contributors to disease or to vary them independently in whole-animal models. This challenge has attracted the interest of engineers, who have begun to collaborate with biologists to leverage recent advances in tissue engineering and microfabrication to develop novel in vitro models of disease. As these models are synthetic systems, specific molecular factors and individual cell types, including parenchymal cells, vascular cells, and immune cells, can be varied independently while simultaneously measuring system-level responses in real time. In this article, we provide some examples of these efforts, including engineered models of diseases of the heart, lung, intestine, liver, kidney, cartilage, skin and vascular, endocrine, musculoskeletal, and nervous systems, as well as models of infectious diseases and cancer. We also describe how engineered in vitro models can be combined with human inducible pluripotent stem cells to enable new insights into a broad variety of disease mechanisms, as well as provide a test bed for screening new therapies.

Development and characterization of human-induced pluripotent stem cell-derived cholangiocytes

Cholangiocytes are the target of a heterogeneous group of liver diseases known as the cholangiopathies. An evolving understanding of the mechanisms driving biliary development provides the theoretical underpinnings for rational development of induced pluripotent stem cell (iPSC)-derived cholangiocytes (iDCs). Therefore, the aims of this study were to develop an approach to generate iDCs and to fully characterize the cells in vitro and in vivo. Human iPSC lines were generated by forced expression of the Yamanaka pluripotency factors. We then pursued a stepwise differentiation strategy toward iDCs, using precise temporal exposure to key biliary morphogens, and we characterized the cells, using a variety of morphologic, molecular, cell biologic, functional, and in vivo approaches. Morphology shows a stepwise phenotypic change toward an epithelial monolayer. Molecular analysis during differentiation shows appropriate enrichment in markers of iPSC, definitive endoderm, hepatic specification, hepatic progenitors, and ultimately cholangiocytes. Immunostaining, western blotting, and flow cytometry demonstrate enrichment of multiple functionally relevant biliary proteins. RNA sequencing reveals that the transcriptome moves progressively toward that of human cholangiocytes. iDCs generate intracellular calcium signaling in response to ATP, form intact primary cilia, and self-assemble into duct-like structures in three-dimensional culture. In vivo, the cells engraft within mouse liver, following retrograde intrabiliary infusion. In summary, we have developed a novel approach to generate mature cholangiocytes from iPSCs. In addition to providing a model of biliary differentiation, iDCs represent a platform for in vitro disease modeling, pharmacologic testing, and individualized, cell-based, regenerative therapies for the cholangiopathies.

Stem Cell Strategies to Evaluate Idiosyncratic Drug-induced Liver Injury

The host-dependent nature of idiosyncratic drug-induced liver injury (iDILI) suggests that rare genetic polymorphisms may contribute to the disease. Indeed, a few mutations in key genes have already been identified using conventional human genetics approaches. Over 50 commonly used drugs can precipitate iDILI, making this a substantial medical problem. Only recently have human induced pluripotent stem cells been used as a research tool to discover novel iDILI genes and to study the mechanisms of iDILI in vitro. Here we review the current state of stem cell use in the investigation of iDILI, with a special focus on genetics. In addition, the concerns and difficulties associated with genetics and animal model research are discussed. We then present the features of patient-specific pluripotent stem cells (which may be derived from iDILI patients themselves), and explain why these cells may be of great utility. A variety of recent approaches to produce hepatocyte-like cells from pluripotent cells and the associated advantages and limitations of such cells are discussed. Future directions for the use of stem cell science to investigate iDILI include novel ways to identify new iDILI genes, a consideration of epigenetic impacts on iDILI, and the development of new and improved strategies for the production of hepatocytes from human pluripotent cells.

Hepatectomised patient sera promote hepatocyte differentiation of human-induced pluripotent stem cells

BACKGROUND

Human induced pluripotent stem cells, which can be differentiated into hepatocyte-like cells, could provide a source for liver regeneration and bio-artificial liver devices. However, the functionality of hepatocyte-like cells is significantly lower than that of primary hepatocytes.

AIMS

To investigate whether serum from patients undergoing hepatectomy might promote differentiation from human induced pluripotent stem cells to hepatocyte-like cells.

METHODS

Serum from patients undergoing hepatectomy (acquired pre-hepatectomy and 3 hours, 1 day and 3 days post-hepatectomy) was used to replace foetal bovine serum when differentiating human induced pluripotent stem cells into hepatocyte-like cells. Properties of hepatocyte-like cells were assessed and compared with cells cultured in foetal bovine serum.

RESULTS

The differentiation efficiency and functionality of hepatocyte-like cells cultured in human serum 3 hours and 1 day post-hepatectomy were superior to those cultured in foetal bovine serum and human serum pre-hepatectomy. Human serum 3 days post-hepatectomy had an equal effect to that of human serum pre-hepatectomy. Some cytochrome P450 isozyme transcript levels of hepatocyte-like cells cultured in human serum were higher than those cultured in foetal bovine serum.

CONCLUSION

Human serum, particularly that acquired relatively soon after hepatectomy, can enhance the differentiation efficiency and functionality of hepatocyte-like cells derived from human induced pluripotent stem cells.

Cellular therapy for liver disease

Regenerative medicine is energizing and empowering basic science and has the potential to dramatically transform health care in the future. Given the remarkable intrinsic regenerative properties of the liver, as well as widespread adoption of regenerative strategies for liver disease (eg, liver transplant, partial hepatectomy, living donor transplant), hepatology has always been at the forefront of clinical regenerative medicine. However, an expanding pool of patients awaiting liver transplant, a limited pool of donor organs, and finite applicability of the current surgical approaches have created a need for more refined and widely available regenerative medicine strategies. Although cell-based therapies have been used extensively for hematologic malignant diseases and other conditions, the potential application of cellular therapy for acute and chronic liver diseases has only more recently been explored. New understanding of the mechanisms of liver regeneration and repair, including activation of local stem/progenitor cells and contributions from circulating bone marrow-derived stem cells, provide the theoretical underpinnings for the rational use of cell-based therapies in clinical trials. In this review, we dissect the scientific rationale for various modalities of cell therapy for liver diseases being explored in animal models and review those tested in human clinical trials. We also attempt to clarify some of the important ongoing questions that need to be addressed in order to bring these powerful therapies to clinical translation. Discussions will cover transplant of hepatocytes and liver stem/progenitor cells as well as infusion or stimulation of bone marrow-derived stem cells. We also highlight tremendous scientific advances on the horizon, including the potential use of induced pluripotent stem cells and their derivatives as individualized regenerative therapy for liver disease.

Stem cells and stem cell-derived tissues and their use in safety assessment

Toxicology has long relied on animal models in a tedious approach to understanding risk of exposure to an uncharacterized molecule. Stem cell-derived tissues can be made in high purity, quality, and quantity to enable a new approach to this problem. Currently, stem cell-derived tissues are primarily "generic" genetic backgrounds; the future will see the integration of various genetic backgrounds and complex three-dimensional models to create truly unique in vitro organoids. This minireview focuses on the state of the art of a number of stem cell-derived tissues and details their application in toxicology.

Human induced pluripotent stem cells in hepatology: beyond the proof of concept

The discovery of the wide plasticity of most cell types means that it is now possible to produce virtually any cell type in vitro. This concept, developed because of the possibility of reprogramming somatic cells toward induced pluripotent stem cells, provides the opportunity to produce specialized cells that harbor multiple phenotypical traits, thus integrating genetic interindividual variability. The field of hepatology has exploited this concept, and hepatocyte-like cells can now be differentiated from induced pluripotent stem cells. This review discusses the choice of somatic cells to be reprogrammed by emergent new and nonintegrative strategies, as well as the application of differentiated human induced pluripotent stem cells in hepatology, including liver development, disease modeling, host-pathogen interactions, and drug metabolism and toxicity. The actual consensus is that hepatocyte-like cells generated in vitro present an immature phenotype. Currently, developed strategies used to resolve this problem, such as overexpression of transcription factors, mimicking liver neonatal and postnatal modifications, and re-creating the three-dimensional hepatocyte environment in vitro and in vivo, are also discussed.

Spheroid culture for enhanced differentiation of human embryonic stem cells to hepatocyte-like cells

Stem cell-derived hepatocyte-like cells hold great potential for the treatment of liver disease and for drug toxicity screening. The success of these applications hinges on the generation of differentiated cells with high liver specific activities. Many protocols have been developed to guide human embryonic stem cells (hESCs) to differentiate to the hepatic lineage. Here we report cultivation of hESCs as three-dimensional aggregates that enhances their differentiation to hepatocyte-like cells. Differentiation was first carried out in monolayer culture for 20 days. Subsequently cells were allowed to self-aggregate into spheroids. Significantly higher expression of liver-specific transcripts and proteins, including Albumin, phosphoenolpyruvate carboxykinase, and asialoglycoprotein receptor 1 was observed. The differentiated phenotype was sustained for more than 2 weeks in the three-dimensional spheroid culture system, significantly longer than in monolayer culture. Cells in spheroids exhibit morphological and ultrastructural characteristics of primary hepatocytes by scanning and transmission electron microscopy in addition to mature functions, such as biliary excretion of metabolic products and cytochrome P450 activities. This three-dimensional spheroid culture system may be appropriate for generating high quality, functional hepatocyte-like cells from ESCs.

Cell fate conversion: direct induction of hepatocyte-like cells from fibroblasts

One of the essential features of stem cells is their cellular plasticity to differentiate into daughter cells with defined functions. Recently, induction of pluripotent stem cells from somatic cells by defined transcription factors led to the focus on cellular plasticity of terminally differentiated cells. This approach is adopted by other studies to demonstrate the cell fate conversion between different lineages of terminally differentiated cells. We and others showed that induced hepatocyte-like (iHep) cells are directly converted from mouse fibroblasts by overexpression of liver-enriched transcription factors. iHep cells as well as pluripotent stem cell- or mesenchymal stem cell-derived hepatocyte-like cells provide potential cell sources for disease modeling, transplantation, and tissue engineering independent of donor organs. Here, we review the latest advances in generating hepatocyte-like cells and summarize general criteria for evaluating these cells. In addition, we propose a possible role of the p19(Arf) /p53 pathway in cell fate maintenance, which apparently limits the formation of induced pluripotent stem (iPS) cells and iHep cells.

Hepatic differentiation of mouse iPS cells and analysis of liver engraftment potential of multistage iPS progeny

Hepatocyte transplantation is considered a promising therapy for patients with liver diseases. Induced pluripotent stem cells (iPSCs) are an unlimited source for the generation of functional hepatocytes. While several protocols that direct the differentiation of iPSCs into hepatocyte-like cells have already been reported, the liver engraftment potential of iPSC progeny obtained at each step of hepatic differentiation has not yet been thoroughly investigated. In this study, we present an efficient strategy to differentiate mouse iPSCs into hepatocyte-like cells and evaluate their liver engraftment potential at different time points of the protocol (5, 10, 15, and 20 days of differentiation). iPSCs were differentiated in the presence of cytokines, growth factors, and small molecules to finally generate hepatocyte-like cells. These iPSC-derived hepatocyte-like cells exhibited hepatocyte-associated functions, such as albumin secretion and urea synthesis. When we transplanted iPSC progeny into the spleen, we found that 15- and 20-day iPSC progeny engrafted into the livers and further acquired hepatocyte morphology. In contrast, 5- and 10-day iPSC progeny were also able to engraft but did not generate hepatocyte-like cells in vivo. Our data may aid in improving current protocols geared towards the use of iPSCs as a new source of liver-targeted cell therapies.

Organ engineering--combining stem cells, biomaterials, and bioreactors to produce bioengineered organs for transplantation

Often the only treatment available for patients suffering from diseased and injured organs is whole organ transplant. However, there is a severe shortage of donor organs for transplantation. The goal of organ engineering is to construct biological substitutes that will restore and maintain normal function in diseased and injured tissues. Recent progress in stem cell biology, biomaterials, and processes such as organ decellularization and electrospinning has resulted in the generation of bioengineered blood vessels, heart valves, livers, kidneys, bladders, and airways. Future advances that may have a significant impact for the field include safe methods to reprogram a patient's own cells to directly differentiate into functional replacement cell types. The subsequent combination of these cells with natural, synthetic and/or decellularized organ materials to generate functional tissue substitutes is a real possibility. This essay reviews the current progress, developments, and challenges facing researchers in their goal to create replacement tissues and organs for patients.