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

Cell and gene therapeutic approaches in non-alcoholic fatty liver disease

Non-Alcoholic Fatty Liver Disease (NAFLD) refers to a range of conditions marked by the buildup of triglycerides in liver cells, accompanied by inflammation, which contributes to liver damage, clinical symptoms, and histopathological alterations. Multiple molecular pathways contribute to NAFLD pathogenesis, including immune dysregulation, endoplasmic reticulum stress, and tissue injury. Both the innate and adaptive immune systems play crucial roles in disease progression, with intricate crosstalk between liver and immune cells driving NAFLD development. Among emerging therapeutic strategies, cell and gene-based therapies have shown promise. This study reviews the pathophysiological mechanisms of NAFLD and explores the therapeutic potential of cell-based interventions, highlighting their immunomodulatory effects, inhibition of hepatic stellate cells, promotion of hepatocyte regeneration, and potential for hepatocyte differentiation. Additionally, we examine gene delivery vectors designed to target NAFLD, focusing on their role in engineering hepatocytes through gene addition or editing to enhance therapeutic efficacy.

3D organoid cultivation improves the maturation and functional differentiation of cholangiocytes from human pluripotent stem cells

Idiopathic cholangiopathies are diseases that affect cholangiocytes, and they have unknown etiologies. Currently, orthotopic liver transplantation is the only treatment available for end-stage liver disease. Limited access to the bile duct makes it difficult to model cholangiocyte diseases. In this study, by mimicking the embryonic development of cholangiocytes and using a robust, feeder- and serum-free protocol, we first demonstrate the generation of unique functional 3D organoids consisting of small and large cholangiocytes derived from human pluripotent stem cells (PSCs), as opposed to traditional 2D culture systems. At day 28 of differentiation, the human PSC-derived cholangiocytes expressed markers of mature cholangiocytes, such as CK7, CK19, and cystic fibrosis transmembrane conductance regulator (CFTR). Compared with the 2D culture system-generated cholangiocytes, the 3D cholangiocyte organoids (COs) showed higher expression of the region-specific markers of intrahepatic cholangiocytes YAP1 and JAG1 and extrahepatic cholangiocytes AQP1 and MUC1. Furthermore, the COs had small-large tube-like structures and functional assays revealed that they exhibited characteristics of mature cholangiocytes, such as multidrug resistance protein 1 transporter function and CFTR channel activity. In addition to the extracellular matrix supports, the epidermal growth factor receptor (EGFR)-mediated signaling regulation might be involved in this cholangiocyte maturation and differentiation. These results indicated the successful generation of intrahepatic and extrahepatic cholangiocytes by using our 3D organoid protocol. The results highlight the advantages of our 3D culture system over the 2D culture system in promoting the functional differentiation and maturation of cholangiocytes. In summary, in advance of the previous works, our study provides a possible concept of small-large cholangiocyte transdifferentiation of human PSCs under cost-effective 3D culture conditions. The study findings have implications for the development of effective cell-based therapy using COs for patients with cholangiopathies.

Reversal of liver failure using a bioartificial liver device implanted with clinical-grade human-induced hepatocytes

Liver resection is the first-line treatment for primary liver cancers, providing the potential for a cure. However, concerns about post-hepatectomy liver failure (PHLF), a leading cause of death following extended liver resection, have restricted the population of eligible patients. Here, we engineered a clinical-grade bioartificial liver (BAL) device employing human-induced hepatocytes (hiHeps) manufactured under GMP conditions. In a porcine PHLF model, the hiHep-BAL treatment showed a remarkable survival benefit. On top of the supportive function, hiHep-BAL treatment restored functions, specifically ammonia detoxification, of the remnant liver and facilitated liver regeneration. Notably, an investigator-initiated study in seven patients with extended liver resection demonstrated that hiHep-BAL treatment was well tolerated and associated with improved liver function and liver regeneration, meeting the primary outcome of safety and feasibility. These encouraging results warrant further testing of hiHep-BAL for PHLF, the success of which would broaden the population of patients eligible for liver resection.

Modeling Nonalcoholic Fatty Liver Disease in the Dish Using Human-Specific Platforms: Strategies and Limitations

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease affecting multiple cell types of the human liver. The high prevalence of NAFLD and the lack of approved therapies increase the demand for reliable models for the preclinical discovery of drug targets. In the last decade, multiple proof-of-principle studies have demonstrated human-specific NAFLD modeling in the dish. These systems have included technologies based on human induced pluripotent stem cell derivatives, liver tissue section cultures, intrahepatic cholangiocyte organoids, and liver-on-a-chip. These platforms differ in functional maturity, multicellularity, scalability, and spatial organization. Identifying an appropriate model for a specific NAFLD-related research question is challenging. Therefore, we review different platforms for their strengths and limitations in modeling NAFLD. To define the fidelity of the current human in vitro NAFLD models in depth, we define disease hallmarks within the NAFLD spectrum that range from steatosis to severe fibroinflammatory tissue injury. We discuss how the most common methods are efficacious in modeling genetic contributions and aspects of the early NAFLD-related tissue response. We also highlight the shortcoming of current models to recapitulate the complexity of inter-organ crosstalk and the chronic process of liver fibrosis-to-cirrhosis that usually takes decades in patients. Importantly, we provide methodological overviews and discuss implementation hurdles (eg, reproducibility or costs) to help choose the most appropriate NAFLD model for the individual research focus: hepatocyte injury, ductular reaction, cellular crosstalk, or other applications. In sum, we highlight current strategies and deficiencies to model NAFLD in the dish and propose a framework for the next generation of human-specific investigations.

Organoids and regenerative hepatology

The burden of liver diseases is increasing worldwide, with liver transplantation remaining the only treatment option for end-stage liver disease. Regenerative medicine holds great potential as a therapeutic alternative, aiming to repair or replace damaged liver tissue with healthy functional cells. The properties of the cells used are critical for the efficacy of this approach. The advent of liver organoids has not only offered new insights into human physiology and pathophysiology, but also provided an optimal source of cells for regenerative medicine and translational applications. Here, we discuss various historical aspects of 3D organoid culture, how it has been applied to the hepatobiliary system, and how organoid technology intersects with the emerging global field of liver regenerative medicine. We outline the hepatocyte, cholangiocyte, and nonparenchymal organoids systems available and discuss their advantages and limitations for regenerative medicine as well as future directions.

Modelling urea cycle disorders using iPSCs

The urea cycle is a liver-based pathway enabling disposal of nitrogen waste. Urea cycle disorders (UCDs) are inherited metabolic diseases caused by deficiency of enzymes or transporters involved in the urea cycle and have a prevalence of 1:35,000 live births. Patients present recurrent acute hyperammonaemia, which causes high rate of death and neurological sequelae. Long-term therapy relies on a protein-restricted diet and ammonia scavenger drugs. Currently, liver transplantation is the only cure. Hence, high unmet needs require the identification of effective methods to model these diseases to generate innovative therapeutics. Advances in both induced pluripotent stem cells (iPSCs) and genome editing technologies have provided an invaluable opportunity to model patient-specific phenotypes in vitro by creating patients' avatar models, to investigate the pathophysiology, uncover novel therapeutic targets and provide a platform for drug discovery. This review summarises the progress made thus far in generating 2- and 3-dimensional iPSCs models for UCDs, the challenges encountered and how iPSCs offer future avenues for innovation in developing the next-generation of therapies for UCDs.

Hepatic Models in Precision Medicine: An African Perspective on Pharmacovigilance

Pharmaceuticals are indispensable to healthcare as the burgeoning global population is challenged by diseases. The African continent harbors unparalleled genetic diversity, yet remains largely underrepresented in pharmaceutical research and development, which has serious implications for pharmaceuticals approved for use within the African population. Adverse drug reactions (ADRs) are often underpinned by unique variations in genes encoding the enzymes responsible for their uptake, metabolism, and clearance. As an example, individuals of African descent (14-34%) harbor an exclusive genetic variant in the gene encoding a liver metabolizing enzyme (CYP2D6) which reduces the efficacy of the breast cancer chemotherapeutic Tamoxifen. However, CYP2D6 genotyping is not required prior to dispensing Tamoxifen in sub-Saharan Africa. Pharmacogenomics is fundamental to precision medicine and the absence of its implementation suggests that Africa has, to date, been largely excluded from the global narrative around stratified healthcare. Models which could address this need, include primary human hepatocytes, immortalized hepatic cell lines, and induced pluripotent stem cell (iPSC) derived hepatocyte-like cells. Of these, iPSCs, are promising as a functional in vitro model for the empirical evaluation of drug metabolism. The scale with which pharmaceutically relevant African genetic variants can be stratified, the expediency with which these platforms can be established, and their subsequent sustainability suggest that they will have an important role to play in the democratization of stratified healthcare in Africa. Here we discuss the requirement for African hepatic models, and their implications for the future of pharmacovigilance on the African continent.

Stem cell therapy in liver regeneration: Focus on mesenchymal stem cells and induced pluripotent stem cells

The liver has the ability to repair itself after injury; however, a variety of pathological changes in the liver can affect its ability to regenerate, and this could lead to liver failure. Mesenchymal stem cells (MSCs) are considered a good source of cells for regenerative medicine, as they regulate liver regeneration through different mechanisms, and their efficacy has been demonstrated by many animal experiments and clinical studies. Induced pluripotent stem cells, another good source of MSCs, have also made great progress in the establishment of organoids, such as liver disease models, and in drug screening. Owing to the recent developments in MSCs and induced pluripotent stem cells, combined with emerging technologies including graphene, nano-biomaterials, and gene editing, precision medicine and individualized clinical treatment may be realized in the near future.

Design by Nature: Emerging Applications of Native Liver Extracellular Matrix for Cholangiocyte Organoid-Based Regenerative Medicine

Organoid technology holds great promise for regenerative medicine. Recent studies show feasibility for bile duct tissue repair in humans by successfully transplanting cholangiocyte organoids in liver grafts during perfusion. Large-scale expansion of cholangiocytes is essential for extending these regenerative medicine applications. Human cholangiocyte organoids have a high and stable proliferation capacity, making them an attractive source of cholangiocytes. Commercially available basement membrane extract (BME) is used to expand the organoids. BME allows the cells to self-organize into 3D structures and stimulates cell proliferation. However, the use of BME is limiting the clinical applications of the organoids. There is a need for alternative tissue-specific and clinically relevant culture substrates capable of supporting organoid proliferation. Hydrogels prepared from decellularized and solubilized native livers are an attractive alternative for BME. These hydrogels can be used for the culture and expansion of cholangiocyte organoids in a clinically relevant manner. Moreover, the liver-derived hydrogels retain tissue-specific aspects of the extracellular microenvironment. They are composed of a complex mixture of bioactive and biodegradable extracellular matrix (ECM) components and can support the growth of various hepatobiliary cells. In this review, we provide an overview of the clinical potential of native liver ECM-based hydrogels for applications with human cholangiocyte organoids. We discuss the current limitations of BME for the clinical applications of organoids and how native ECM hydrogels can potentially overcome these problems in an effort to unlock the full regenerative clinical potential of the organoids.

Future generation of combined multimodal approach to treat brain glioblastoma multiforme and potential impact on micturition control

Glioblastoma remains lethal even when treated with standard therapy. This review aims to outline the recent development of various advanced therapeutics for glioblastoma and briefly discuss the potential impact of glioblastoma and some of its therapeutic approaches on the neurological function micturition control. Although immunotherapy led to success in treating hematological malignancies, but no similar success occurred in treatment for brain glioblastoma. Neither regenerative medicine nor stem cell therapy led to astounding success in glioblastoma. However, CRISPR Cas system holds potential in multiple applications due to its capacity to knock-in and knock-out genes, modify immune cells and cell receptors, which will enable it to address clinical challenges in immunotherapy such as CAR-T and regenerative therapy for brain glioblastoma, improving the precision and safety of these approaches. The studies mentioned in this review could indicate that glioblastoma is a malignant disease with multiple sophisticated barriers to be overcome and more challenges might arise in the attempt of researchers to yield a successful cure. A multimodal approach of future generation of refined and safe therapeutics derived from CRISPR Cas therapeutics, immunotherapy, and regenerative therapeutics mentioned in this review might prolong survival or even contribute towards a potential cure for glioblastoma.

The Application of Induced Pluripotent Stem Cells Against Liver Diseases: An Update and a Review

Liver diseases are a major health concern globally, and are associated with poor survival and prognosis of patients. This creates the need for patients to accept the main alternative treatment of liver transplantation to prevent progression to end-stage liver disease. Investigation of the molecular mechanisms underpinning complex liver diseases and their pathology is an emerging goal of stem cell scope. Human induced pluripotent stem cells (hiPSCs) derived from somatic cells are a promising alternative approach to the treatment of liver disease, and a prospective model for studying complex liver diseases. Here, we review hiPSC technology of cell reprogramming and differentiation, and discuss the potential application of hiPSC-derived liver cells, such as hepatocytes and cholangiocytes, in refractory liver-disease modeling and treatment, and drug screening and toxicity testing. We also consider hiPSC safety in clinical applications, based on genomic and epigenetic alterations, tumorigenicity, and immunogenicity.

Continual proteomic divergence of HepG2 cells as a consequence of long-term spheroid culture

Three-dimensional models are considered a powerful tool for improving the concordance between in vitro and in vivo phenotypes. However, the duration of spheroid culture may influence the degree of correlation between these counterparts. When using immortalised cell lines as model systems, the assumption for consistency and reproducibility is often made without adequate characterization or validation. It is therefore essential to define the biology of each spheroid model by investigating proteomic dynamics, which may be altered relative to culture duration. As an example, we assessed the influence of culture duration on the relative proteome abundance of HepG2 cells cultured as spheroids, which are routinely used to model aspects of the liver. Quantitative proteomic profiling of whole cell lysates labelled with tandem-mass tags was conducted using liquid chromatography-tandem mass spectrometry (LC-MS/MS). In excess of 4800 proteins were confidently identified, which were shared across three consecutive time points over 28 days. The HepG2 spheroid proteome was divergent from the monolayer proteome after 14 days in culture and continued to change over the successive culture time points. Proteins representing the recognised core hepatic proteome, cell junction, extracellular matrix, and cell adhesion proteins were found to be continually modulated.

Human Bile Contains Cholangiocyte Organoid-Initiating Cells Which Expand as Functional Cholangiocytes in Non-canonical Wnt Stimulating Conditions

Diseases of the bile duct (cholangiopathies) remain a common indication for liver transplantation, while little progress has been made over the last decade in understanding the underlying pathophysiology. This is largely due to lack of proper in vitro model systems to study cholangiopathies. Recently, a culture method has been developed that allows for expansion of human bile duct epithelial cells grown as extrahepatic cholangiocyte organoids (ncECOs) in non-canonical Wnt-stimulating conditions. These ncECOs closely resemble cholangiocytes in culture and have shown to efficiently repopulate collagen scaffolds that could act as functional biliary tissue in mice. Thus far, initiation of ncECOs required tissue samples, thereby limiting broad patient-specific applications. Here, we report that bile fluid, which can be less invasively obtained and with low risk for the patients, is an alternative source for culturing ncECOs. Further characterization showed that bile-derived cholangiocyte organoids (ncBCOs) are highly similar to ncECOs obtained from bile duct tissue biopsies. Compared to the previously reported bile-cholangiocyte organoids cultured in canonical Wnt-stimulation conditions, ncBCOs have superior function of cholangiocyte ion channels and are able to respond to secretin and somatostatin. In conclusion, bile is a new, less invasive, source for patient-derived cholangiocyte organoids and makes their regenerative medicine applications more safe and feasible.

Immortalized stem cell-derived hepatocyte-like cells: An alternative model for studying dengue pathogenesis and therapy

Suitable cell models are essential to advance our understanding of the pathogenesis of liver diseases and the development of therapeutic strategies. Primary human hepatocytes (PHHs), the most ideal hepatic model, are commercially available, but they are expensive and vary from lot-to-lot which confounds their utility. We have recently developed an immortalized hepatocyte-like cell line (imHC) from human mesenchymal stem cells, and tested it for use as a substitute model for hepatotropic infectious diseases. With a special interest in liver pathogenesis of viral infection, herein we determined the suitability of imHC as a host cell target for dengue virus (DENV) and as a model for anti-viral drug testing. We characterized the kinetics of DENV production, cellular responses to DENV infection (apoptosis, cytokine production and lipid droplet metabolism), and examined anti-viral drug effects in imHC cells with comparisons to the commonly used hepatoma cell lines (HepG2 and Huh-7) and PHHs. Our results showed that imHC cells had higher efficiencies in DENV replication and NS1 secretion as compared to HepG2 and Huh-7 cells. The kinetics of DENV infection in imHC cells showed a slower rate of apoptosis than the hepatoma cell lines and a certain similarity of cytokine profiles to PHHs. In imHC, DENV-induced alterations in levels of lipid droplets and triacylglycerols, a major component of lipid droplets, were more apparent than in hepatoma cell lines, suggesting active lipid metabolism in imHC. Significantly, responses to drugs with DENV inhibitory effects were greater in imHC cells than in HepG2 and Huh-7 cells. In conclusion, our findings suggest superior suitability of imHC as a new hepatocyte model for studying mechanisms underlying viral pathogenesis, liver diseases and drug effects.

A model system resembling normal human liver cells is needed for advancement of hepatotropic infectious disease research. Here we show that immortalized cells (imHC) derived from human stem cells have a higher efficiency of DENV replication and a lower rate of cell death in response to DENV infection than the cancer cell-derived model systems currently used. The imHC also have active fat metabolism and respond well to anti-viral drug treatment, making them an attractive model for the initial stage of drug discovery and testing.

Tissue engineering of the biliary tract and modelling of cholestatic disorders

Our insight into the pathogenesis of cholestatic liver disease remains limited, partly owing to challenges in capturing the multitude of factors that contribute to disease pathogenesis in vitro. Tissue engineering could address this challenge by combining cells, materials and fabrication strategies into dynamic modelling platforms, recapitulating the multifaceted aetiology of cholangiopathies. Herein, we review the advantages and limitations of platforms for bioengineering the biliary tree, looking at how these can be applied to model biliary disorders, as well as exploring future directions for the field.

What gastroenterologists and hepatologists should know about organoids in 2019

Most of the research behind new medical advances is carried out using either animal models or cancer cells, which both have their disadvantage in particular with regard to medical applications such as personalized medicine and novel therapeutic approaches. However, recent advances in stem cell biology have enabled long-term culturing of organotypic intestinal or hepatic tissues derived from tissue resident or pluripotent stem cells. These 3D structures, denoted as organoids, represent a substantial advance in structural and functional complexity over traditional in vitro cell culture models that are often non-physiological and transformed. They can recapitulate the in vivo architecture, functionality and genetic signature of the corresponding tissue. The opportunity to model epithelial cell biology, epithelial turnover, barrier dynamics, immune-epithelial communication and host-microbe interaction more efficiently than previous culture systems, greatly enhance the translational potential of organotypic hepato-gastrointestinal culture systems. Thus there is increasing interest in using such cultured cells as a source for tissue engineering, regenerative medicine and personalized medicine. This review will highlight some of the established and also some exciting novel perspectives on organoids in the fields of gastroenterology and hepatology.

A proteomic time course through the differentiation of human induced pluripotent stem cells into hepatocyte-like cells

Numerous in vitro models endeavour to mimic the characteristics of primary human hepatocytes for applications in regenerative medicine and pharmaceutical science. Mature hepatocyte-like cells (HLCs) derived from human induced pluripotent stem cells (hiPSCs) are one such in vitro model. Due to insufficiencies in transcriptome to proteome correlation, characterising the proteome of HLCs is essential to provide a suitable framework for their continual optimization. Here we interrogated the proteome during stepwise differentiation of hiPSCs into HLCs over 40 days. Whole cell protein lysates were collected and analysed using stabled isotope labelled mass spectrometry based proteomics. Quantitative proteomics identified over 6,000 proteins in duplicate multiplexed labelling experiments across two different time course series. Inductive cues in differentiation promoted sequential acquisition of hepatocyte specific markers. Analysis of proteins classically assigned as hepatic markers demonstrated trends towards maximum relative abundance between differentiation day 30 and 32. Characterisation of abundant proteins in whole cells provided evidence of the time dependent transition towards proteins corresponding with the functional repertoire of the liver. This data highlights how far the proteome of undifferentiated precursors have progressed to acquire a hepatic phenotype and constructs a platform for optimisation and improved maturation of HLC differentiation.

In situ differentiation and generation of functional liver organoids from human iPSCs in a 3D perfusable chip system

Liver organoids derived from human pluripotent stem cells (PSCs) represent a new type of in vitro liver model for understanding organ development, disease mechanism and drug testing. However, engineering liver organoids with favorable functions in a controlled cellular microenvironment remains challenging. In this work, we present a new strategy for engineering liver organoids derived from human induced PSCs (hiPSCs) in a 3D perfusable chip system by combining stem cell biology with microengineering technology. This approach enabled formation of hiPSC-based embryoid bodies (EBs), in situ hepatic differentiation, long-term 3D culture and generation of liver organoids in a perfusable micropillar chip. The generated liver organoids exhibited favorable growth and differentiation of hepatocytes and cholangiocytes, recapitulating the key features of human liver formation with cellular heterogeneity. The liver organoids in perfused cultures displayed improved cell viability and higher expression of endodermal genes (SOX17 and FOXA2) and mature hepatic genes (ALB and CYP3A4) under perfused culture conditions. In addition, the liver organoids showed a marked enhancement of hepatic-specific functions, including albumin and urea production and metabolic capabilities, indicating the role of mechanical fluid flow in promoting the functions of the liver organoids. Moreover, the liver organoids exhibited hepatotoxic response after exposure to acetaminophen (APAP) in a dose- and time-dependent manner. The established liver organoid-on-a-chip system may provide a promising platform for engineering stem cell-based organoids with applications in regenerative medicine, disease modeling and drug testing.

In Vitro Expansion of Primary Human Hepatocytes with Efficient Liver Repopulation Capacity

Transplantation of human hepatocytes (HHs) holds significant potential for treating liver diseases. However, the supply of transplantable HHs is severely constrained by limited donor availability and compromised capacity for in vitro expansion. In response to chronic injury, some HHs are reprogrammed into proliferative cells that express both hepatocyte and progenitor markers, suggesting exploitable strategies for expanding HHs in vitro. Here, we report defined medium conditions that allow 10,000-fold expansion of HHs. These proliferating HHs are bi-phenotypic, partially retaining hepatic features while gaining expression of progenitor-associated genes. Importantly, these cells engraft into injured mouse liver at a level comparable to primary HHs, and they undergo maturation following transplantation in vivo or differentiation in vitro. Thus, this study provides a protocol that enables large-scale expansion of transplantable HHs, which could be further developed for modeling and treating human liver disease.

Bioengineering Organs for Blood Detoxification

For patients with severe kidney or liver failure the best solution is currently organ transplantation. However, not all patients are eligible for transplantation and due to limited organ availability, most patients are currently treated with therapies using artificial kidney and artificial liver devices. These therapies, despite their relative success in preserving the patients' life, have important limitations since they can only replace part of the natural kidney or liver functions. As blood detoxification (and other functions) in these highly perfused organs is achieved by specialized cells, it seems relevant to review the approaches leading to bioengineered organs fulfilling most of the native organ functions. There, the culture of cells of specific phenotypes on adapted scaffolds that can be perfused takes place. In this review paper, first the functions of kidney and liver organs are briefly described. Then artificial kidney/liver devices, bioartificial kidney devices, and bioartificial liver devices are focused on, as well as biohybrid constructs obtained by decellularization and recellularization of animal organs. For all organs, a thorough overview of the literature is given and the perspectives for their application in the clinic are discussed.

Impact of Three-Dimentional Culture Systems on Hepatic Differentiation of Puripotent Stem Cells and Beyond

Generation of functional hepatocytes from human pluripotent stem cells (hPSCs) is a vital tool to produce large amounts of human hepatocytes, which hold a great promise for biomedical and regenerative medicine applications. Despite a tremendous progress in developing the differentiation protocols recapitulating the developmental signalling and stages, these resulting hepatocytes from hPSCs yet achieve maturation and functionality comparable to those primary hepatocytes. The absence of 3D milieu in the culture and differentiation of these hepatocytes may account for this, at least partly, thus developing an optimal 3D culture could be a step forward to achieve this aim. Hence, review focuses on current development of 3D culture systems for hepatic differentiation and maturation and the future perspectives of its application.

Modeling Disease with Human Inducible Pluripotent Stem Cells

Understanding the physiopathology of disease remains an essential step in developing novel therapeutics. Although animal models have certainly contributed to advancing this enterprise, their limitation in modeling all the aspects of complex human disorders is one of the major challenges faced by the biomedical research field. Human induced pluripotent stem cells (hiPSCs) derived from patients represent a great opportunity to overcome this deficiency because these cells cover the genetic diversity needed to fully model human diseases. Here, we provide an overview of the history of hiPSC technology and discuss common challenges and approaches that we and others have faced when using hiPSCs to model disease. Our emphasis is on liver disease, and consequently, we review the progress made using this technology to produce functional liver cells in vitro and how these systems are being used to recapitulate a diversity of developmental, metabolic, genetic, and infectious liver disorders.

Liver cell therapy: is this the end of the beginning?

The prevalence of liver diseases is increasing globally. Orthotopic liver transplantation is widely used to treat liver disease upon organ failure. The complexity of this procedure and finite numbers of healthy organ donors have prompted research into alternative therapeutic options to treat liver disease. This includes the transplantation of liver cells to promote regeneration. While successful, the routine supply of good quality human liver cells is limited. Therefore, renewable and scalable sources of these cells are sought. Liver progenitor and pluripotent stem cells offer potential cell sources that could be used clinically. This review discusses recent approaches in liver cell transplantation and requirements to improve the process, with the ultimate goal being efficient organ regeneration. We also discuss the potential off-target effects of cell-based therapies, and the advantages and drawbacks of current pre-clinical animal models used to study organ senescence, repopulation and regeneration.

Advances in the generation of bioengineered bile ducts

The generation of bioengineered biliary tissue could contribute to the management of some of the most impactful cholangiopathies associated with liver transplantation, such as biliary atresia or ischemic cholangiopathy. Recent advances in tissue engineering and in vitro cholangiocyte culture have made the achievement of this goal possible. Here we provide an overview of these developments and review the progress towards the generation and transplantation of bioengineered bile ducts. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni and Peter Jansen.

Reconstruction of the mouse extrahepatic biliary tree using primary human extrahepatic cholangiocyte organoids

The treatment of common bile duct (CBD) disorders, such as biliary atresia or ischemic strictures, is restricted by the lack of biliary tissue from healthy donors suitable for surgical reconstruction. Here we report a new method for the isolation and propagation of human cholangiocytes from the extrahepatic biliary tree in the form of extrahepatic cholangiocyte organoids (ECOs) for regenerative medicine applications. The resulting ECOs closely resemble primary cholangiocytes in terms of their transcriptomic profile and functional properties. We explore the regenerative potential of these organoids in vivo and demonstrate that ECOs self-organize into bile duct-like tubes expressing biliary markers following transplantation under the kidney capsule of immunocompromised mice. In addition, when seeded on biodegradable scaffolds, ECOs form tissue-like structures retaining biliary characteristics. The resulting bioengineered tissue can reconstruct the gallbladder wall and repair the biliary epithelium following transplantation into a mouse model of injury. Furthermore, bioengineered artificial ducts can replace the native CBD, with no evidence of cholestasis or occlusion of the lumen. In conclusion, ECOs can successfully reconstruct the biliary tree, providing proof of principle for organ regeneration using human primary cholangiocytes expanded in vitro.

Directed differentiation of human induced pluripotent stem cells into functional cholangiocyte-like cells

The difficulty in isolating and propagating functional primary cholangiocytes is a major limitation in the study of biliary disorders and the testing of novel therapeutic agents. To overcome this problem, we have developed a platform for the differentiation of human pluripotent stem cells (hPSCs) into functional cholangiocyte-like cells (CLCs). We have previously reported that our 26-d protocol closely recapitulates key stages of biliary development, starting with the differentiation of hPSCs into endoderm and subsequently into foregut progenitor (FP) cells, followed by the generation of hepatoblasts (HBs), cholangiocyte progenitors (CPs) expressing early biliary markers and mature CLCs displaying cholangiocyte functionality. Compared with alternative protocols for biliary differentiation of hPSCs, our system does not require coculture with other cell types and relies on chemically defined conditions up to and including the generation of CPs. A complex extracellular matrix is used for the maturation of CLCs; therefore, experience in hPSC culture and 3D organoid systems may be necessary for optimal results. Finally, the capacity of our platform for generating large amounts of disease-specific functional cholangiocytes will have broad applications for cholangiopathies, in disease modeling and for screening of therapeutic compounds.

Generation of a bile salt export pump deficiency model using patient-specific induced pluripotent stem cell-derived hepatocyte-like cells

Bile salt export pump (BSEP) plays an important role in hepatic secretion of bile acids and its deficiency results in severe cholestasis and liver failure. Mutation of the ABCB11 gene encoding BSEP induces BSEP deficiency and progressive familial intrahepatic cholestasis type 2 (PFIC2). Because liver transplantation remains standard treatment for PFIC2, the development of a novel therapeutic option is desired. However, a well reproducible model, which is essential for the new drug development for PFIC2, has not been established. Therefore, we attempted to establish a PFIC2 model by using iPSC technology. Human iPSCs were generated from patients with BSEP-deficiency (BD-iPSC), and were differentiated into hepatocyte-like cells (HLCs). In the BD-iPSC derived HLCs (BD-HLCs), BSEP was not expressed on the cell surface and the biliary excretion capacity was significantly impaired. We also identified a novel mutation in the 5'-untranslated region of the ABCB11 gene that led to aberrant RNA splicing in BD-HLCs. Furthermore, to evaluate the drug efficacy, BD-HLCs were treated with 4-phenylbutyrate (4PBA). The membrane BSEP expression level and the biliary excretion capacity in BD-HLCs were rescued by 4PBA treatment. In summary, we succeeded in establishing a PFIC2 model, which may be useful for its pathophysiological analysis and drug development.

Application of induced pluripotent stem cells in cholangiopathies

Induced pluripotent stem cells (iPSCs) are similar to embryonic stem cells (ESCs) in morphology, gene expression, cell self-renewal and differentiation potential. They avoid the problem of immune rejection and ethical issues associated with the application of ESCs. The application of iPSCs in a variety of diseases provides favorable experiences to the research of liver diseases. Cholangiopathies, such as primary biliary cirrhosis and primary sclerosing cholangitis, refer to a category of uncommon diseases that possess unclear pathogenesis, lack effective treatment and have a poor prognosis. Hence, investigating cholangiopathies-derived, individualized iPSCs and their differentiation into functional cells can mimic the disease phenotype and pathological process in vitro. The application of these cells has great significance for pathogenesis exploration, drug screening and therapeutic evaluation.

Enrichment of Pluripotent Stem Cell-Derived Hepatocyte-Like Cells by Ammonia Treatment

Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are potential resources for the regeneration of defective organs, including the liver. However, some obstacles must be overcome before this becomes reality. Undifferentiated cells that remain following differentiation have teratoma-forming potential. Additionally, practical applications require a large quantity of differentiated cells, so the differentiation process must be economical. Here we describe a DNA microarray-based global analysis of the gene expression profiles of differentiating human pluripotent stem cells. We identified differences and commonalities among six human pluripotent stem cell lines: the hESCs KhES1, KhES2, KhES3, and H1, and the iPSCs 201B7 and 243G1. Embryoid bodies (EBs) formed without requiring supplementation with inducing factors. EBs also expressed some liver-specific metabolic genes including the ammonia-metabolizing enzymes glutamine synthetase and carbamoyl-phosphate synthase 1. Real-time PCR analysis revealed hepatocyte-like differentiation of EBs treated with ammonia in Lanford medium. Analysis of DNA microarray data suggested that hepatocyte-like cells were the most abundant population in ammonia-treated cells. Furthermore, expression levels of undifferentiated pluripotent stem cell markers were drastically reduced, suggesting a reduced teratoma-forming capacity. These results indicate that treatment of EBs with ammonia in Lanford medium may be an effective inducer of hepatic differentiation in absence of expensive inducing factors.

Functional Human and Murine Tissue-Engineered Liver Is Generated from Adult Stem/Progenitor Cells

Liver disease affects large numbers of patients, yet there are limited treatments available to replace absent or ineffective cellular function of this crucial organ. Donor scarcity and the necessity for immunosuppression limit one effective therapy, orthotopic liver transplantation. But in some conditions such as inborn errors of metabolism or transient states of liver insufficiency, patients may be salvaged by providing partial quantities of functional liver tissue. After transplanting multicellular liver organoid units composed of a heterogeneous cellular population that includes adult stem and progenitor cells, both mouse and human tissue‐engineered liver (TELi) form in vivo. TELi contains normal liver components such as hepatocytes with albumin expression, CK19‐expressing bile ducts and vascular structures with α‐smooth muscle actin expression, desmin‐expressing stellate cells, and CD31‐expressing endothelial cells. At 4 weeks, TELi contains proliferating albumin‐expressing cells and identification of β2‐microglobulin‐expressing cells demonstrates that the majority of human TELi is composed of transplanted human cells. Human albumin is detected in the host mouse serum, indicating in vivo secretory function. Liquid chromatography/mass spectrometric analysis of mouse serum after debrisoquine administration is followed by a significant increase in the level of the human metabolite, 4‐OH‐debrisoquine, which supports the metabolic and xenobiotic capability of human TELi in vivo. Implanted TELi grew in a mouse model of inducible liver failure. Stem Cells Translational Medicine2017;6:238–248

The potential of induced pluripotent stem cell derived hepatocytes

Orthotopic liver transplantation remains the only curative treatment for liver disease. However, the number of patients who die while on the waiting list (15%) has increased in recent years as a result of severe organ shortages; furthermore the incidence of liver disease is increasing worldwide. Clinical trials involving hepatocyte transplantation have provided encouraging results. However, transplanted cell function appears to often decline after several months, necessitating liver transplantation. The precise aetiology of the loss of cell function is not clear, but poor engraftment and immune-mediated loss appear to be important factors. Also, primary human hepatocytes (PHH) are not readily available, de-differentiate, and die rapidly in culture. Hepatocytes are available from other sources, such as tumour-derived human hepatocyte cell lines and immortalised human hepatocyte cell lines or porcine hepatocytes. However, all these cells suffer from various limitations such as reduced or differences in functions or risk of zoonotic infections. Due to their significant potential, one possible inexhaustible source of hepatocytes is through the directed differentiation of human induced pluripotent stem cells (hiPSCs). This review will discuss the potential applications and existing limitations of hiPSC-derived hepatocytes in regenerative medicine, drug screening, in vitro disease modelling and bioartificial livers.

Stem Cell Therapies for Treatment of Liver Disease

Cell therapy is an emerging form of treatment for several liver diseases, but is limited by the availability of donor livers. Stem cells hold promise as an alternative to the use of primary hepatocytes. We performed an exhaustive review of the literature, with a focus on the latest studies involving the use of stem cells for the treatment of liver disease. Stem cells can be harvested from a number of sources, or can be generated from somatic cells to create induced pluripotent stem cells (iPSCs). Different cell lines have been used experimentally to support liver function and treat inherited metabolic disorders, acute liver failure, cirrhosis, liver cancer, and small-for-size liver transplantations. Cell-based therapeutics may involve gene therapy, cell transplantation, bioartificial liver devices, or bioengineered organs. Research in this field is still very active. Stem cell therapy may, in the future, be used as a bridge to either liver transplantation or endogenous liver regeneration, but efficient differentiation and production protocols must be developed and safety must be demonstrated before it can be applied to clinical practice.

Cholangiocytes derived from human induced pluripotent stem cells for disease modeling and drug validation

The study of biliary disease has been constrained by a lack of primary human cholangiocytes. Here we present an efficient, serum-free protocol for directed differentiation of human induced pluripotent stem cells into cholangiocyte-like cells (CLCs). CLCs show functional characteristics of cholangiocytes, including bile acids transfer, alkaline phosphatase activity, γ-glutamyl-transpeptidase activity and physiological responses to secretin, somatostatin and vascular endothelial growth factor. We use CLCs to model in vitro key features of Alagille syndrome, polycystic liver disease and cystic fibrosis (CF)-associated cholangiopathy. Furthermore, we use CLCs generated from healthy individuals and patients with polycystic liver disease to reproduce the effects of the drugs verapamil and octreotide, and we show that the experimental CF drug VX809 rescues the disease phenotype of CF cholangiopathy in vitro. Our differentiation protocol will facilitate the study of biological mechanisms controlling biliary development, as well as disease modeling and drug screening.

Stem cell-derived liver cells for drug testing and disease modeling

Differences between animals and humans in liver pathways now necessitate the use of in vitro models of the human liver for several applications such as drug screening. However, isolated primary human hepatocytes (PHHs) are a limited resource for building such models given shortages of donor organs. In contrast, human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) can be propagated nearly indefinitely and differentiated into hepatocyte-like cells in vitro using soluble factors inspired from liver development. Additionally, iPSCs can be generated from patients with specific genetic backgrounds to study genotype-phenotype relationships. While current protocols to differentiate hESCs and iPSCs into human hepatocyte-like cells (hESC-HHs and iPSC-HHs) still need improvement to yield cells functionally similar to the adult liver, proof-of-concept studies have already shown utility of these cells in drug development and modeling liver diseases such as α1-antitrypsin deficiency, hepatitis B/C viral infections, and malaria. Here, we present an overview of hESC-HH and iPSC-HH culture platforms that have been utilized for the aforementioned applications. We also discuss the use of semiconductor-driven microfabrication tools to precisely control the microenvironment around these cells to enable higher and longer-term liver functions in vitro. Finally, we discuss areas for improvement in creating next generation stem cell-derived liver models. In the future, stem cell-derived hepatocyte-like cells could provide a sustainable cell source for high-throughput drug screening, enabling better mechanistic understanding of human liver diseases for the development of more efficacious and safer therapeutics, and personalized cell-based therapies in the clinic.