In vitro differentiation of pluripotent stem cells into hepatocyte like cells - basic principles and current progress

Engineered microtissues to model the effects of dynamic heterotypic cell signaling on iPSC-derived human hepatocyte maturation

In vitro human liver models are indispensable for compound metabolism/toxicity screening, disease modeling, and regenerative medicine. While induced pluripotent stem cell-derived human hepatocyte-like cells (iHeps) mitigate the sourcing limitations with primary human hepatocytes (PHHs), their functional maturity is rate-limiting for application use. During development, immature hepatoblasts interact with different non-parenchymal cell (NPC) types, such as mesenchyme and endothelia, in a spatiotemporal manner to progress through functional maturation. Modeling such interactions in vitro is critical to elucidate the key regulators of iHep maturation. Here, we utilized high-throughput droplet microfluidics to encapsulate iHeps within monodisperse collagen I microgels (Ø ∼ 250 µm), which were coated with NPCs to generate 'microtissues' placed within microwells in multiwell plates. Embryonic fibroblasts and liver sinusoidal endothelial cells (LSECs) induced the highest level of iHep maturation over 4+ weeks of culture compared to adult hepatic stellate cells (myofibroblastic), liver portal fibroblasts, dermal fibroblasts, and human umbilical vein endothelial cells. Combining iHep microtissues in plates with Transwell inserts containing different NPC types enabled the modeling of dynamic heterotypic signaling on iHep maturation; introducing embryonic fibroblast signaling first, followed by LSECs, led to the highest iHep maturation. Unique cytokine secretion profiles were detected across the top-performing microtissue configurations; stromal-derived factor-1 alpha was validated as one factor that enhanced iHep maturation. Lastly, gene expression patterns and regulatory networks showed adult PHH-like maturation in LSEC/iHep microtissues compared to iHep-only microtissues. Overall, microtissues are useful for elucidating the microenvironmental determinants of iHep maturation and for future use in downstream applications. STATEMENT OF SIGNIFICANCE: Induced pluripotent stem cell-derived hepatocyte-like cells (iHeps) hold great promise for drug screening, disease modeling, and regenerative medicine but often exhibit immature phenotypes. We utilized high-throughput droplet microfluidics to generate 3D microtissues containing iHeps and non-parenchymal cell (NPC) types to elucidate the effects of dynamic NPC signaling on iHep maturation. We observed that iHep maturation is significantly enhanced with embryonic fibroblasts and liver sinusoidal endothelial cells (LSEC) compared to adult liver fibroblasts and non-liver endothelia; the LSEC/iHep microtissues showed adult liver-like gene expression signatures. The highest iHep maturation in microtissues was achieved when mesenchymal stimulation was introduced first, followed by LSEC stimulation. Our platform provides a robust framework to elucidate cellular and molecular mediators of iHep maturation and biomedical applications.

Standard Protocols for Characterising Primary and In Vitro-Generated Human Hepatocytes

Hepatocyte-like cells (HLCs) derived from pluripotent stem cells (PSCs) or direct reprogramming are an unlimited source of human hepatocytes for biomedical applications. HLCs are used to model human diseases, develop precise drugs and establish groundbreaking regenerative cell-based therapies. Primary human hepatocytes are the gold standard for studying human liver biology and pathology. However, their widespread use is limited by their rapid dedifferentiation in vitro, reliance on transplant-rejected donor organs, poor scalability and significant batch-to-batch variations. Therefore, high-quality 'off-the-shelf' HLCs are needed to overcome those limitations. Basic stepwise differentiation protocols have been developed to generate HLCs from PSCs. To evaluate the quality of the in vitro generated products, HLCs have been phenotyped using various methods. This review discusses various biological assays and methods available for the robust evaluation of HLC quality, emphasising the importance of using 24-h cultured primary human hepatocytes (PHHs) as a reference standard for comparison.

A Study on iPSC-Associated Factors in the Generation of Hepatocytes

BACKGROUND

Hepatocytes are an attractive cell source in hepatic tissue engineering because they are the primary cells of the liver, maintaining liver homeostasis through their intrinsic function. Due to the increasing demand for liver donors, a wide range of methods are being studied to obtain functionally active hepatocytes. iPSCs are one of the alternative cell sources, which shows great promise as a tool for generating hepatocytes.

METHODS

This study determined whether factors associated with iPSCs contributed to variation in hepatocyte-like cells derived from iPSCs. The factors of concern for the iPSCs included the culture system, the source of iPSCs, and cell seeding density for initiating the differentiation.

RESULTS

Our results found iPSC-dependent variances among differentiated hepatocyte-like cells. The matrix used in culturing iPSCs significantly impacts cell morphologies, characteristics, and the expression of pluripotent genes, such as OCT4 and SOX2, varied in iPSCs derived from different sources. These characteristics, in turn, play a consequential role in determining the functional activity of the iPSC-derived hepatocyte-like cells. In addition, cell seeding density was observed to be an essential factor for the efficient generation of iPSC-derived hepatocyte-like cells, with 2- 4 × 10 cells/cm of seeding density resulting in good morphology and functionality.

CONCLUSION

This study provides the baseline of effective differentiation protocols for iPSC-derived hepatocyte-like cells with the appropriate conditions, including cell culture media, iPSC source, and the seeding density of iPSCs.

The therapeutic landscape of citrin deficiency

Citrin deficiency (CD) is a recessive, liver disease caused by sequence variants in the SLC25A13 gene encoding a mitochondrial aspartate-glutamate transporter. CD manifests as different age-dependent phenotypes and affects crucial hepatic metabolic pathways including malate-aspartate-shuttle, glycolysis, gluconeogenesis, de novo lipogenesis and the tricarboxylic acid and urea cycles. Although the exact pathophysiology of CD remains unclear, impaired use of glucose and fatty acids as energy sources due to NADH shuttle defects and PPARα downregulation, respectively, indicates evident energy deficit in CD hepatocytes. The present review summarizes current trends on available and potential treatments for CD. Baseline recommendation for CD patients is dietary management, often already present as a self-selected food preference, that includes protein and fat-rich food, and avoidance of excess carbohydrates. At present, liver transplantation remains the sole curative option for severe CD cases. Our extensive literature review indicated medium-chain triglycerides (MCT) as the most widely used CD treatment in all age groups. MCT can effectively improve symptoms across disease phenotypes by rapidly supplying energy to the liver, restoring redox balance and inducing lipogenesis. In contrast, sodium pyruvate restored glycolysis and displayed initial preclinical promise, with however limited efficacy in adult CD patients. Ursodeoxycholic acid, nitrogen scavengers and L-arginine treatments effectively address specific pathophysiological aspects such as cholestasis and hyperammonemia and are commonly administered in combination with other drugs. Finally, future possibilities including restoring redox balance, amino acid supplementation, enhancing bioenergetics, improving ureagenesis and mRNA/DNA-based gene therapy are also discussed.

Small Molecule-Mediated Stage-Specific Reprogramming of MSCs to Hepatocyte-Like Cells and Hepatic Tissue for Liver Injury Treatment

BACKGROUND

Derivation of hepatocytes from stem cells has been established through various protocols involving growth factor (GF) and small molecule (SM) agents, among others. However, mesenchymal stem cell-based derivation of hepatocytes still remains expensive due to the use of a cocktail of growth factors, and a long duration of differentiation is needed, thus limiting its potential clinical application.

METHODS

In this study, we developed a chemically defined differentiation strategy that is exclusively based on SM and takes 14 days, while the GF-based protocol requires 23-28 days.

RESULTS

We optimized a stage-specific differentiation protocol for the differentiation of rat bone marrow-derived mesenchymal stem cells (MSCs) into functional hepatocyte-like cells (dHeps) that involved four stages, i.e., definitive endoderm (DE), hepatic competence (HC), hepatic specification (HS) and hepatic differentiation and growth. We further generated hepatic tissue using human decellularized liver extracellular matrix and compared it with hepatic tissue derived from the growth factor-based protocol at the transcriptional level. dHep, upon transplantation in a rat model of acute liver injury (ALI), was capable of ameliorating liver injury in rats and improving liver function and tissue damage compared to those in the ALI model.

CONCLUSIONS

In summary, this is the first study in which hepatocytes and hepatic tissue were derived from MSCs utilizing a stage-specific strategy by exclusively using SM as a differentiation factor.

Toolbox for creating three-dimensional liver models

Research within the hepato-biliary system and hepatic function is currently experiencing heightened interest, this is due to the high frequency of relapse rates observed in chronic conditions, as well as the imperative for the development of innovative therapeutic strategies to address both inherited and acquired diseases within this domain. The most commonly used sources for studying hepatocytes include primary human hepatocytes, human hepatic cancer cell lines, and hepatic-like cells derived from induced pluripotent stem cells. However, a significant challenge in primary hepatic cell culture is the rapid decline in their phenotypic characteristics, dedifferentiation and short cultivation time. This limitation creates various problems, including the inability to maintain long-term cell cultures, which can lead to failed experiments in drug development and the creation of relevant disease models for researchers' purposes. To address these issues, the creation of a powerful 3D cell model could play a pivotal role as a personalized disease model and help reduce the use of animal models during certain stages of research. Such a cell model could be used for disease modelling, genome editing, and drug discovery purposes. This review provides an overview of the main methods of 3D-culturing liver cells, including a discussion of their characteristics, advantages, and disadvantages.

The NIPBL-gene mutation of a Cornelia de Lange Syndrome patient causes deficits in the hepatocyte differentiation of induced Pluripotent Stem Cells via altered chromatin-accessibility

The Cornelia de Lange syndrome (CdLS) is a rare genetic disease, which is characterized by a cohesinopathy. Mutations of the NIPBL gene are observed in 65% of CdLS patients. A novel iPSC (induced Pluripotent Stem Cell) line was reprogrammed from the leukocytes of a CdLS patient carrying a missense mutation of the NIPBL gene. A mutation-corrected isogenic iPSC-line and two iPSC-lines generated from the healthy parents were used as controls. The iPSC lines were differentiated along the hepatocyte-lineage. Comparative immunofluorescence, RNA-seq and ATAC-seq analyses were performed on undifferentiated and differentiated iPSCs. In addition, chromatin organization was studied by ChIP-Seq analysis on the patient derived iPSCs as well as the respective controls. Relative to the mutation-corrected and the healthy-parents iPSCs, the patient-derived counterparts are defective in terms of differentiation along the hepatocyte-lineage. One-third of the genes selectively up-regulated in CdLS-derived iPSCs and hepatic cells are non-protein-coding genes. By converse, most of the selectively down-regulated genes code for transcription factors and proteins regulating neural differentiation. Some of the transcriptionally silenced loci, such as the DPP6 gene on chromosome 7q36.2 and the ZNF gene cluster on chromosome 19p12, are located in closed-chromatin regions. Relative to the corresponding controls, the global transcriptomic differences observed in CdLS undifferentiated iPSCs are associated with altered chromatin accessibility, which was confirmed by ChIP-Seq analysis. Thus, the deficits in the differentiation along the hepatocyte lineage observed in our CdLS patient is likely to be due to a transcriptional dysregulation resulting from a cohesin-dependent alteration of chromatin accessibility.

The future of liver transplantation

Over the last 50 years, liver transplantation has evolved into a procedure routinely performed in many countries worldwide. Those able to access this therapy frequently experience a miraculous risk-benefit ratio, particularly if they face the imminently life-threatening disease. Over the decades, the success of liver transplantation, with dramatic improvements in early posttransplant survival, has aggressively driven demand. However, despite the emergence of living donors to augment deceased donors as a source of organs, supply has lagged far behind demand. As a result, rationing has been an unfortunate focus in recent decades. Recent shifts in the epidemiology of liver disease combined with transformative innovations in liver preservation suggest that the underlying premise of organ shortage may erode in the foreseeable future. The focus will sharpen on improving equitable access while mitigating constraints related to workforce training, infrastructure for organ recovery and rehabilitation, and their associated costs. Research efforts in liver preservation will undoubtedly blossom with the aim of optimizing both the timing and conditions of transplantation. Coupled with advances in genetic engineering, regenerative biology, and cellular therapies, the portfolio of innovation, both broad and deep, offers the promise that, in the future, liver transplantation will not only be broadly available to those in need but also represent a highly durable life-saving therapy.

Forskolin induces FXR expression and enhances maturation of iPSC-derived hepatocyte-like cells

The generation of iPSC-derived hepatocyte-like cells (HLCs) is a powerful tool for studying liver diseases, their therapy as well as drug development. iPSC-derived disease models benefit from their diverse origin of patients, enabling the study of disease-associated mutations and, when considering more than one iPSC line to reflect a more diverse genetic background compared to immortalized cell lines. Unfortunately, the use of iPSC-derived HLCs is limited due to their lack of maturity and a rather fetal phenotype. Commercial kits and complicated 3D-protocols are cost- and time-intensive and hardly useable for smaller working groups. In this study, we optimized our previously published protocol by fine-tuning the initial cell number, exchanging antibiotics and basal medium composition and introducing the small molecule forskolin during the HLC maturation step. We thereby contribute to the liver research field by providing a simple, cost- and time-effective 2D differentiation protocol. We generate functional HLCs with significantly increased HLC hallmark gene (ALB, HNF4α, and CYP3A4) and protein (ALB) expression, as well as significantly elevated inducible CYP3A4 activity.

A comparison between different human hepatocyte models reveals profound differences in net glucose production, lipid composition and metabolism in vitro

Hepatocytes are responsible for maintaining a stable blood glucose concentration during periods of nutrient scarcity. The breakdown of glycogen and de novo synthesis of glucose are crucial metabolic pathways deeply interlinked with lipid metabolism. Alterations in these pathways are often associated with metabolic diseases with serious clinical implications. Studying energy metabolism in human cells is challenging. Primary hepatocytes are still considered the golden standard for in vitro studies and have been instrumental in elucidating key aspects of energy metabolism found in vivo. As a result of several limitations posed by using primary cells, a multitude of alternative hepatocyte cellular models emerged as potential substitutes. Yet, there remains a lack of clarity regarding the precise applications for which these models accurately reflect the metabolic competence of primary hepatocytes. In this study, we compared primary hepatocytes, stem cell-derived hepatocytes, adult donor-derived liver organoids, immortalized Upcyte-hepatocytes and the hepatoma cell line HepG2s in their response to a glucose production challenge. We observed the highest net glucose production in primary hepatocytes, followed by organoids, stem-cell derived hepatocytes, Upcyte-hepatocytes and HepG2s. Glucogenic gene induction was observed in all tested models, as indicated by an increase in G6PC and PCK1 expression. Lipidomic analysis revealed considerable differences across the models, with organoids showing the closest similarity to primary hepatocytes in the common lipidome, comprising 347 lipid species across 19 classes. Changes in lipid profiles as a result of the glucose production challenge showed a variety of, and in some cases opposite, trends when compared to primary hepatocytes.

Cryopreserved cGMP-compliant human pluripotent stem cell-derived hepatic progenitors rescue mice from acute liver failure through rapid paracrine effects on liver cells

BACKGROUND

Liver transplantation remains the only curative treatment for end-stage liver diseases. Unfortunately, there is a drastic organ donor shortage. Hepatocyte transplantation emerged as a viable alternative to liver transplantation. Considering their unique expansion capabilities and their potency to be driven toward a chosen cell fate, pluripotent stem cells are extensively studied as an unlimited cell source of hepatocytes for cell therapy. It has been previously shown that freshly prepared hepatocyte-like cells can cure mice from acute and chronic liver failure and restore liver function.

METHODS

Human PSC-derived immature hepatic progenitors (GStemHep) were generated using a new protocol with current good manufacturing practice compliant conditions from PSC amplification and hepatic differentiation to cell cryopreservation. The therapeutic potential of these cryopreserved cells was assessed in two clinically relevant models of acute liver failure, and the mode of action was studied by several analytical methods, including unbiased proteomic analyses.

RESULTS

GStemHep cells present an immature hepatic phenotype (alpha-fetoprotein positive, albumin negative), secrete hepatocyte growth factor and do not express major histocompatibility complex. A single dose of thawed GStemHep rescue mice from sudden death caused by acetaminophen and thioacetamide-induced acute liver failure, both in immunodeficient and immunocompetent animals in the absence of immunosuppression. Therapeutic biological effects were observed as soon as 3 h post-cell transplantation with a reduction in serum transaminases and in liver necrosis. The swiftness of the therapeutic effect suggests a paracrine mechanism of action of GStemHep leading to a rapid reduction of inflammation as well as a rapid cytoprotective effect with as a result a proteome reprograming of the host hepatocytes. The mode of action of GStemHep relie on the alleviation of inhibitory factors of liver regeneration, an increase in proliferation-promoting factors and a decrease in liver inflammation.

CONCLUSIONS

We generated cryopreserved and current good manufacturing practice-compliant human pluripotent stem cell-derived immature hepatic progenitors that were highly effective in treating acute liver failure through rapid paracrine effects reprogramming endogenous hepatocytes. This is also the first report highlighting that human allogeneic cells could be used as cryopreserved cells and in the absence of immunosuppression for human PSC-based regenerative medicine for acute liver failure.

Mass spectrometry reveals that oxysterols are secreted from non-alcoholic fatty liver disease induced organoids

Oxysterols are potential biomarkers for liver metabolism that are altered under disease conditions such as non-alcoholic fatty liver disease (NAFLD). We here apply sterolomics to organoids used for disease modeling of NAFLD. Using liquid chromatography-mass spectrometry with on-line sample clean-up and enrichment, we establish that liver organoids produce and secrete oxysterols. We find elevated levels of 26-hydroxycholesterol, an LXR agonist and the first oxysterol in the acidic bile acid synthesis, in medium from steatotic liver organoids compared to untreated organoids. Other upregulated sterols in medium from steatotic liver organoids are dihydroxycholesterols, such as 7α,26-dihydroxycholesterol, and 7α,25-dihydroxycholesterol. Through 26-hydroxycholesterol exposure to human stem cell-derived hepatic stellate cells, we observe a trend of expressional downregulation of the pro-inflammatory cytokine CCL2, suggesting a protective role of 26-hydroxycholesterol during early-phased NAFLD disease development. Our findings support the possibility of oxysterols serving as NAFLD indicators, demonstrating the usefulness of combining organoids and mass spectrometry for disease modeling and biomarker studies.

Therapeutic Cell Repopulation of the Liver: From Fetal Rat Cells to Synthetic Human Tissues

Progenitor cells isolated from the fetal liver can provide a unique cell source to generate new healthy tissue mass. Almost 20 years ago, it was demonstrated that rat fetal liver cells repopulate the normal host liver environment via a mechanism akin to cell competition. Activin A, which is produced by hepatocytes, was identified as an important player during cell competition. Because of reduced activin receptor expression, highly proliferative fetal liver stem/progenitor cells are resistant to activin A and therefore exhibit a growth advantage compared to hepatocytes. As a result, transplanted fetal liver cells are capable of repopulating normal livers. Important for cell-based therapies, hepatic stem/progenitor cells containing repopulation potential can be separated from fetal hematopoietic cells using the cell surface marker δ-like 1 (Dlk-1). In livers with advanced fibrosis, fetal epithelial stem/progenitor cells differentiate into functional hepatic cells and out-compete injured endogenous hepatocytes, which cause anti-fibrotic effects. Although fetal liver cells efficiently repopulate the liver, they will likely not be used for human cell transplantation. Thus, utilizing the underlying mechanism of repopulation and developed methods to produce similar growth-advantaged cells in vitro, e.g., human induced pluripotent stem cells (iPSCs), this approach has great potential for developing novel cell-based therapies in patients with liver disease. The present review gives a brief overview of the classic cell transplantation models and various cell sources studied as donor cell candidates. The advantages of fetal liver-derived stem/progenitor cells are discussed, as well as the mechanism of liver repopulation. Moreover, this article reviews the potential of in vitro developed synthetic human fetal livers from iPSCs and their therapeutic benefits.

Advancements in MAFLD Modeling with Human Cell and Organoid Models

Metabolic (dysfunction) associated fatty liver disease (MAFLD) is one of the most prevalent liver diseases and has no approved therapeutics. The high failure rates witnessed in late-phase MAFLD drug trials reflect the complexity of the disease, and how the disease develops and progresses remains to be fully understood. In vitro, human disease models play a pivotal role in mechanistic studies to unravel novel disease drivers and in drug testing studies to evaluate human-specific responses. This review focuses on MAFLD disease modeling using human cell and organoid models. The spectrum of patient-derived primary cells and immortalized cell lines employed to model various liver parenchymal and non-parenchymal cell types essential for MAFLD development and progression is discussed. Diverse forms of cell culture platforms utilized to recapitulate tissue-level pathophysiology in different stages of the disease are also reviewed.

Can next-generation humanized mice that reconstituted with both functional human immune system and hepatocytes model the progression of viral hepatitis to hepatocarcinogenesis?

Hepatitis B virus (HBV) and Hepatitis C virus (HCV) chronic infections cause liver immunopathological diseases such as hepatitis, fibrosis, cirrhosis, and hepatocellular carcinomas, which are difficult to treat and continue to be major health problems globally. Due to the species-specific hepato-tropism of HBV and HCV, conventional rodent models are limited in their utility for studying the infection and associated liver immunopathogenesis. Humanized mice reconstituted with both functional human immune system and hepatocytes (HIS-HuHEP mice) have been extremely instrumental for in vivo studies of HBV or HCV infection and human-specific aspects of the progression of liver immunopathogenesis. However, none of the current HIS-HuHEP mice can model the progression of viral hepatitis to hepatocarcinogenesis which may be a notorious result of HBV or HCV chronic infection in patients, suggesting that they were functionally compromised and that there is still significant space to improve and establish next-generation of HIS-HuHEP mice with more sophisticated functions. In this review, we first summarize the principal requirements to establish HIS-HuHEP mice. We then discuss the respective protocols for current HIS-HuHEP mice and their applications, as well as their advantages and disadvantages. We also raise perspectives for further improving and establishing next-generation HIS-HuHEP mice.