Generation of Functional Liver Sinusoidal Endothelial Cells from Human Pluripotent Stem-Cell-Derived Venous Angioblasts

Identification of the biological functions and chemo-therapeutic responses of ITGB superfamily in ovarian cancer

BACKGROUND

Patients with ovarian cancer (OC) tend to face a poor prognosis due to a lack of typical symptoms and a high rate of recurrence and chemo-resistance. Therefore, identifying representative and reliable biomarkers for early diagnosis and prediction of chemo-therapeutic responses is vital for improving the prognosis of OC.

METHODS

Expression levels, IHC staining, and subcellular distribution of eight ITGBs were analyzed using The Cancer Genome Atlas (TCGA)-Ovarian Serous Cystadenocarcinoma (OV) database, GEO DataSets, and the HPA website. PrognoScan and Univariate Cox were used for prognostic analysis. TIDE database, TIMER database, and GSCA database were used to analyze the correlation between immune functions and ITGBs. Consensus clustering analysis was performed to subtype OC patients in the TCGA database. LASSO regression was used to construct the predictive model. The Cytoscape software was used for identifying hub genes. The 'pRRophetic' R package was applied to predict chemo-therapeutic responses of ITGBs.

RESULTS

ITGBs were upregulated in OC tissues except ITGB1 and ITGB3. High expression of ITGBs correlated with an unfavorable prognosis of OC except ITGB2. In OC, there was a strong correlation between immune responses and ITGB2, 6, and 7. In addition, the expression matrix of eight ITGBs divided the TCGA-OV database into two subgroups. Subgroup A showed upregulation of eight ITGBs. The predictive model distinguishes OC patients from favorable prognosis to poor prognosis. Chemo-therapeutic responses showed that ITGBs were able to predict responses of common chemo-therapeutic drugs for patients with OC.

CONCLUSIONS

This article provides evidence for predicting prognosis, immuno-, and chemo-therapeutic responses of ITGBs in OC and reveals related biological functions of ITGBs in OC.

Liver specification of human iPSC-derived endothelial cells transplanted into mouse liver

Background & Aims

Liver sinusoidal endothelial cells (LSECs) are important in liver development, regeneration, and pathophysiology, but the differentiation process underlying their tissue-specific phenotype is poorly understood and difficult to study because primary human cells are scarce. The aim of this study was to use human induced pluripotent stem cell (hiPSC)-derived LSEC-like cells to investigate the differentiation process of LSECs.

Methods

hiPSC-derived endothelial cells (iECs) were transplanted into the livers of Fah-/-/Rag2-/-/Il2rg-/- mice and assessed over a 12-week period. Lineage tracing, immunofluorescence, flow cytometry, plasma human factor VIII measurement, and bulk and single cell transcriptomic analysis were used to assess the molecular and functional changes that occurred following transplantation.

Results

Progressive and long-term repopulation of the liver vasculature occurred as iECs expanded along the sinusoids between hepatocytes and increasingly produced human factor VIII, indicating differentiation into LSEC-like cells. To chart the developmental profile associated with LSEC specification, the bulk transcriptomes of transplanted cells between 1 and 12 weeks after transplantation were compared against primary human adult LSECs. This demonstrated a chronological increase in LSEC markers, LSEC differentiation pathways, and zonation. Bulk transcriptome analysis suggested that the transcription factors NOTCH1, GATA4, and FOS have a central role in LSEC specification, interacting with a network of 27 transcription factors. Novel markers associated with this process included EMCN and CLEC14A. Additionally, single cell transcriptomic analysis demonstrated that transplanted iECs at 4 weeks contained zonal subpopulations with a region-specific phenotype.

Conclusions

Collectively, this study confirms that hiPSCs can adopt LSEC-like features and provides insight into LSEC specification. This humanised xenograft system can be applied to further interrogate LSEC developmental biology and pathophysiology, bypassing current logistical obstacles associated with primary human LSECs.

Impact and implications

Liver sinusoidal endothelial cells (LSECs) are important cells for liver biology, but better model systems are required to study them. We present a pluripotent stem cell xenografting model that produces human LSEC-like cells. A detailed and longitudinal transcriptomic analysis of the development of LSEC-like cells is included, which will guide future studies to interrogate LSEC biology and produce LSEC-like cells that could be used for regenerative medicine.

Organoid-guided precision hepatology for metabolic liver disease

Metabolic dysfunction-associated steatotic liver disease affects millions of people worldwide. Progress towards a definitive cure has been incremental and treatment is currently limited to lifestyle modification. Hepatocyte-specific lipid accumulation is the main trigger of lipotoxic events, driving inflammation and fibrosis. The underlying pathology is extraordinarily heterogenous, and the manifestations of steatohepatitis are markedly influenced by metabolic communications across non-hepatic organs. Synthetic human tissue models have emerged as powerful platforms to better capture the mechanistic diversity in disease progression, while preserving person-specific genetic traits. In this review, we will outline current research efforts focused on integrating multiple synthetic tissue models of key metabolic organs, with an emphasis on organoid-based systems. By combining functional genomics and population-scale en masse profiling methodologies, human tissues derived from patients can provide insights into personalised genetic, transcriptional, biochemical, and metabolic states. These collective efforts will advance our understanding of steatohepatitis and guide the development of rational solutions for mechanism-directed diagnostic and therapeutic investigation.

Endothelial Slc35a1 Deficiency Causes Loss of LSEC Identity and Exacerbates Neonatal Lipid Deposition in the Liver in Mice

BACKGROUND & AIMS

The functional maturation of the liver largely occurs after birth. In the early stages of life, the liver of a newborn encounters enormous high-fat metabolic stress caused by the consumption of breast milk. It is unclear how the maturing liver adapts to high lipid metabolism. Liver sinusoidal endothelial cells (LSECs) play a fundamental role in establishing liver vasculature and are decorated with many glycoproteins on their surface. The Slc35a1 gene encodes a cytidine-5'-monophosphate (CMP)-sialic acid transporter responsible for transporting CMP-sialic acids between the cytoplasm and the Golgi apparatus for protein sialylation. This study aimed to determine whether endothelial sialylation plays a role in hepatic vasculogenesis and functional maturation.

METHODS

Endothelial-specific Slc35a1 knockout mice were generated. Liver tissues were collected for histologic analysis, lipidomic profiling, RNA sequencing, confocal immunofluorescence, and immunoblot analyses.

RESULTS

Endothelial Slc35a1-deficient mice exhibited excessive neonatal hepatic lipid deposition, severe liver damage, and high mortality. Endothelial deletion of Slc35a1 led to sinusoidal capillarization and disrupted hepatic zonation. Mechanistically, vascular endothelial growth factor receptor 2 (VEGFR2) in LSECs was desialylated and VEGFR2 signaling was enhanced in Slc35a1-deficient mice. Inhibition of VEGFR2 signaling by SU5416 alleviated lipid deposition and restored hepatic vasculature in Slc35a1-deficient mice.

CONCLUSIONS

Our findings suggest that sialylation of LSECs is critical for maintaining hepatic vascular development and lipid homeostasis. Targeting VEGFR2 signaling may be a new strategy to prevent liver disorders associated with abnormal vasculature and lipid deposition.

Building human artery and vein endothelial cells from pluripotent stem cells, and enduring mysteries surrounding arteriovenous development

Owing to their manifold roles in health and disease, there have been intense efforts to synthetically generate blood vessels in vitro from human pluripotent stem cells (hPSCs). However, there are multiple types of blood vessel, including arteries and veins, which are molecularly and functionally different. How can we specifically generate either arterial or venous endothelial cells (ECs) from hPSCs in vitro? Here, we summarize how arterial or venous ECs arise during embryonic development. VEGF and NOTCH arbitrate the bifurcation of arterial vs. venous ECs in vivo. While manipulating these two signaling pathways biases hPSC differentiation towards arterial and venous identities, efficiently generating these two subtypes of ECs has remained challenging until recently. Numerous questions remain to be fully addressed. What is the complete identity, timing and combination of extracellular signals that specify arterial vs. venous identities? How do these extracellular signals intersect with fluid flow to modulate arteriovenous fate? What is a unified definition for endothelial progenitors or angioblasts, and when do arterial vs. venous potentials segregate? How can we regulate hPSC-derived arterial and venous ECs in vitro, and generate organ-specific ECs? In turn, answers to these questions could avail the production of arterial and venous ECs from hPSCs, accelerating vascular research, tissue engineering, and regenerative medicine.

iPSC-derived cells for whole liver bioengineering

Liver bioengineering stands as a prominent alternative to conventional hepatic transplantation. Through liver decellularization and/or bioprinting, researchers can generate acellular scaffolds to overcome immune rejection, genetic manipulation, and ethical concerns that often accompany traditional transplantation methods, in vivo regeneration, and xenotransplantation. Hepatic cell lines derived from induced pluripotent stem cells (iPSCs) can repopulate decellularized and bioprinted scaffolds, producing an increasingly functional organ potentially suitable for autologous use. In this mini-review, we overview recent advancements in vitro hepatocyte differentiation protocols, shedding light on their pivotal role in liver recellularization and bioprinting, thereby offering a novel source for hepatic transplantation. Finally, we identify future directions for liver bioengineering research that may allow the implementation of these systems for diverse applications, including drug screening and liver disease modeling.

Vascularized liver-on-a-chip model to investigate nicotine-induced dysfunction

The development of physiologically relevant in vitro systems for simulating disease onset and progression and predicting drug metabolism holds tremendous value in reducing drug discovery time and cost. However, many of these platforms lack accuracy in replicating the tissue architecture and multicellular interactions. By leveraging three-dimensional cell culture, biomimetic soft hydrogels, and engineered stimuli, in vitro models have continued to progress. Nonetheless, the incorporation of the microvasculature has been met with many challenges, specifically with the addition of parenchymal cell types. Here, a systematic approach to investigating the initial seeding density of endothelial cells and its effects on interconnected networks was taken and combined with hepatic spheroids to form a liver-on-a-chip model. Leveraging this system, nicotine's effects on microvasculature and hepatic function were investigated. The findings indicated that nicotine led to interrupted adherens junctions, decreased guanosine triphosphate cyclohydrolase 1 expression, impaired angiogenesis, and lowered barrier function, all key factors in endothelial dysfunction. With the combination of the optimized microvascular networks, a vascularized liver-on-a-chip was formed, providing functional xenobiotic metabolism and synthesis of both albumin and urea. This system provides insight into potential hepatotoxicity caused by various drugs and allows for assessing vascular dysfunction in a high throughput manner.

Generation of functional liver sinusoidal endothelial-like cells from human bone marrow-derived mesenchymal stem cells

Introduction

Liver sinusoidal endothelial cells (LSECs) are specialized vascular endothelial cells that play an important role in the maintenance of biological homeostasis. However, the lack of versatile human LSECs has hindered research on LSECs and development of medical technologies for liver diseases including hemophilia A. In this study, we developed a technique to induce LSEC differentiation from human bone marrow-derived mesenchymal stem cells (BM-MSCs).

Methods

To induce LSECs from human BM-MSCs, cytokines and chemical compounds associated with signaling implicated in LSEC differentiation and liver development were screened. Then LSEC-related genes and proteins expression in the differentiated cells were analyzed by qPCR and flow cytometry analysis, respectively. LSEC-related functions of the differentiated cells were also examined.

Results

We found that the gene expression of LSEC markers, such as LYVE1, was considerably increased by culturing human BM-MSCs with bone morphogenetic protein 4, fibroblast growth factor 8b, transforming growth factor-β signal inhibitor, and cyclic AMP. Furthermore, the differentiated cells expressed LSEC marker proteins and clearly demonstrated LSEC-specific functions, such as the uptake of hyaluronic acid.

Conclusions

Our result indicate that the functional LSEC-like cells were successfully generated from human BM-MSCs using our established protocol.

Combined Treatment of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes and Endothelial Cells Regenerate the Infarcted Heart in Mice and Non-Human Primates

BACKGROUND

Remuscularization of the mammalian heart can be achieved after cell transplantation of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CMs). However, several hurdles remain before implementation into clinical practice. Poor survival of the implanted cells is related to insufficient vascularization, and the potential for fatal arrhythmogenesis is associated with the fetal cell-like nature of immature CMs.

METHODS

We generated 3 lines of hiPSC-derived endothelial cells (ECs) and hiPSC-CMs from 3 independent donors and tested hiPSC-CM sarcomeric length, gap junction protein, and calcium-handling ability in coculture with ECs. Next, we examined the therapeutic effect of the cotransplantation of hiPSC-ECs and hiPSC-CMs in nonobese diabetic-severe combined immunodeficiency (NOD-SCID) mice undergoing myocardial infarction (n≥4). Cardiac function was assessed by echocardiography, whereas arrhythmic events were recorded using 3-lead ECGs. We further used healthy non-human primates (n=4) with cell injection to study the cell engraftment, maturation, and integration of transplanted hiPSC-CMs, alone or along with hiPSC-ECs, by histological analysis. Last, we tested the cell therapy in ischemic reperfusion injury in non-human primates (n=4, 3, and 4 for EC+CM, CM, and control, respectively). Cardiac function was evaluated by echocardiography and cardiac MRI, whereas arrhythmic events were monitored by telemetric ECG recorders. Cell engraftment, angiogenesis, and host-graft integration of human grafts were also investigated.

RESULTS

We demonstrated that human iPSC-ECs promote the maturity and function of hiPSC-CMs in vitro and in vivo. When cocultured with ECs, CMs showed more mature phenotypes in cellular structure and function. In the mouse model, cotransplantation augmented the EC-accompanied vascularization in the grafts, promoted the maturity of CMs at the infarct area, and improved cardiac function after myocardial infarction. Furthermore, in non-human primates, transplantation of ECs and CMs significantly enhanced graft size and vasculature and improved cardiac function after ischemic reperfusion.

CONCLUSIONS

These results demonstrate the synergistic effect of combining iPSC-derived ECs and CMs for therapy in the postmyocardial infarction heart, enabling a promising strategy toward clinical translation.

Oit3, a promising hallmark gene for targeting liver sinusoidal endothelial cells

Liver sinusoidal endothelial cells (LSECs) play a pivotal role in maintaining liver homeostasis and influencing the pathological processes of various liver diseases. However, neither LSEC-specific hallmark genes nor a LSEC promoter-driven Cre mouse line has been introduced before, which largely restricts the study of liver diseases with vascular disorders. To explore LSEC-specific hallmark genes, we compared the top 50 marker genes between liver endothelial cells (ECs) and liver capillary ECs and identified 18 overlapping genes. After excluding globally expressed genes and those with low expression percentages, we narrowed our focus to two final candidates: Oit3 and Dnase1l3. Through single-cell RNA sequencing (scRNA-seq) and analysis of the NCBI database, we confirmed the extrahepatic expression of Dnase1l3. The paired-cell sequencing data further demonstrated that Oit3 was predominantly expressed in the midlobular liver ECs. Subsequently, we constructed inducible Oit3-CreERT2 transgenic mice, which were further crossed with ROSA26-tdTomato mice. Microscopy validated that the established Oit3-CreERT2-tdTomato mice exhibited significant fluorescence in the liver rather than in other organs. The staining analysis confirmed the colocalization of tdTomato and EC markers. Ex-vivo experiments further confirmed that isolated tdTomato+ cells exhibited well-differentiated fenestrae and highly expressed EC markers, confirming their identity as LSECs. Overall, Oit3 is a promising hallmark gene for tracing LSECs. The establishment of Oit3-CreERT2-tdTomato mice provides a valuable model for studying the complexities of LSECs in liver diseases.

Scalable production of tissue-like vascularized liver organoids from human PSCs

The lack of physiological parity between 2D cell culture and in vivo culture has led to the development of more organotypic models, such as organoids. Organoid models have been developed for a number of tissues, including the liver. Current organoid protocols are characterized by a reliance on extracellular matrices (ECMs), patterning in 2D culture, costly growth factors and a lack of cellular diversity, structure, and organization. Current hepatic organoid models are generally simplistic and composed of hepatocytes or cholangiocytes, rendering them less physiologically relevant compared to native tissue. We have developed an approach that does not require 2D patterning, is ECM independent, and employs small molecules to mimic embryonic liver development that produces large quantities of liver-like organoids. Using single-cell RNA sequencing and immunofluorescence, we demonstrate a liver-like cellular repertoire, a higher order cellular complexity, presenting with vascular luminal structures, and a population of resident macrophages: Kupffer cells. The organoids exhibit key liver functions, including drug metabolism, serum protein production, urea synthesis and coagulation factor production, with preserved post-translational modifications such as N-glycosylation and functionality. The organoids can be transplanted and maintained long term in mice producing human albumin. The organoids exhibit a complex cellular repertoire reflective of the organ and have de novo vascularization and liver-like function. These characteristics are a prerequisite for many applications from cellular therapy, tissue engineering, drug toxicity assessment, and disease modeling to basic developmental biology.

Fetal liver development and implications for liver disease pathogenesis

The metabolic, digestive and homeostatic roles of the liver are dependent on proper crosstalk and organization of hepatic cell lineages. These hepatic cell lineages are derived from their respective progenitors early in organogenesis in a spatiotemporally controlled manner, contributing to the liver's specialized and diverse microarchitecture. Advances in genomics, lineage tracing and microscopy have led to seminal discoveries in the past decade that have elucidated liver cell lineage hierarchies. In particular, single-cell genomics has enabled researchers to explore diversity within the liver, especially early in development when the application of bulk genomics was previously constrained due to the organ's small scale, resulting in low cell numbers. These discoveries have substantially advanced our understanding of cell differentiation trajectories, cell fate decisions, cell lineage plasticity and the signalling microenvironment underlying the formation of the liver. In addition, they have provided insights into the pathogenesis of liver disease and cancer, in which developmental processes participate in disease emergence and regeneration. Future work will focus on the translation of this knowledge to optimize in vitro models of liver development and fine-tune regenerative medicine strategies to treat liver disease. In this Review, we discuss the emergence of hepatic parenchymal and non-parenchymal cells, advances that have been made in in vitro modelling of liver development and draw parallels between developmental and pathological processes.

Characterization of human stem cell-derived hepatic stellate cells and liver sinusoidal endothelial cells during extended in vitro culture

Background: There is a significant need for predictive and stable in vitro human liver representations for disease modeling and drug testing. Hepatic stellate cells (HSCs) and liver sinusoidal endothelial cells (LSECs) are important non-parenchymal cell components of the liver and are hence of relevance in a variety of disease models, including hepatic fibrosis. Pluripotent stem cell- (PSC-) derived HSCs (scHSCs) and LSECs (scLSECs) offer an attractive alternative to primary human material; yet, the suitability of scHSCs and scLSECs for extended in vitro modeling has not been characterized. Methods: In this study, we describe the phenotypic and functional development of scHSCs and scLSECs during 14 days of 2D in vitro culture. Cell-specific phenotypes were evaluated by cell morphology, immunofluorescence, and gene- and protein expression. Functionality was assessed in scHSCs by their capacity for intracellular storage of vitamin A and response to pro-fibrotic stimuli induced by TGF-β. scLSECs were evaluated by nitric oxide- and factor VIII secretion as well as endocytic uptake of bioparticles and acetylated low-density lipoprotein. Notch pathway inhibition and co-culturing scHSCs and scLSECs were separately tested as options for enhancing long-term stability and maturation of the cells. Results and Conclusion: Both scHSCs and scLSECs exhibited a post-differentiation cell type-specific phenotype and functionality but deteriorated during extended culture with PSC line-dependent variability. Therefore, the choice of PSC line and experimental timeframe is crucial when designing in vitro platforms involving scHSCs and scLSECs. Notch inhibition modestly improved long-term monoculture in a cell line-dependent manner, while co-culturing scHSCs and scLSECs provides a strategy to enhance phenotypic and functional stability.

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.

Scar-Degrading Endothelial Cells as a Treatment for Advanced Liver Fibrosis

Deposition of extracellular matrix (ECM) in the liver is an important feature of liver cirrhosis. Recovery from liver cirrhosis is physiologically challenging, partially due to the ECM in scar tissue showing resistance to cell-mediated degradation by secreted matrix metalloproteinases (MMPs). Here, a cell-mediated ECM-degradation screening system (CEDSS) in vitro is constructed for high-throughput searching for cells with tremendous degradation ability. ECM-degrading liver sinusoidal endothelial cells (dLSECs) are screened using CEDSS, which exhibit 17 times the ability to degrade collagen when compared to other cells. The degradation ability of dLSECs is mediated by the upregulation of MMP9. In particular, mRNA expression of MMP9 shows an 833-fold increase in dLSECs compared to normal endothelial cells (nLSECs), and MMP9 is regulated by transcription factor c-Fos. In vivo, single intrasplenic injection of dLSECs alleviates advanced liver fibrosis in mice, while intraperitoneal administration of liver-targeting peptide-modified dLSECs shows enhanced fibrosis-targeting effects. Degradative human umbilical vein endothelial cells (dHUVECs) prove their enhanced potential of clinical translation. Together, these results highlight the potential of ECM-degrading endothelial cells in alleviating advanced liver fibrosis, thus providing remarkable insights in the development of ECM-targeting therapeutics.

Advances in Recellularization of Decellularized Liver Grafts with Different Liver (Stem) Cells: Towards Clinical Applications

Liver transplantation is currently the only curative therapy for patients with acute or chronic liver failure. However, a dramatic gap between the number of available liver grafts and the number of patients on the transplantation waiting list emphasizes the need for valid liver substitutes. Whole-organ engineering is an emerging field of tissue engineering and regenerative medicine. It aims to generate transplantable and functional organs to support patients on transplantation waiting lists until a graft becomes available. It comprises two base technologies developed in the last decade; (1) organ decellularization to generate a three-dimensional (3D) extracellular matrix scaffold of an organ, and (2) scaffold recellularization to repopulate both the parenchymal and vascular compartments of a decellularized organ. In this review article, recent advancements in both technologies, in relation to liver whole-organ engineering, are presented. We address the potential sources of hepatocytes and non-parenchymal liver cells for repopulation studies, and the role of stem-cell-derived liver progeny is discussed. In addition, different cell seeding strategies, possible graft modifications, and methods used to evaluate the functionality of recellularized liver grafts are outlined. Based on the knowledge gathered from recent transplantation studies, future directions are summarized.

Sourcing cells for in vitro models of human vascular barriers of inflammation

The vascular system plays a critical role in the progression and resolution of inflammation. The contributions of the vascular endothelium to these processes, however, vary with tissue and disease state. Recently, tissue chip models have emerged as promising tools to understand human disease and for the development of personalized medicine approaches. Inclusion of a vascular component within these platforms is critical for properly evaluating most diseases, but many models to date use "generic" endothelial cells, which can preclude the identification of biomedically meaningful pathways and mechanisms. As the knowledge of vascular heterogeneity and immune cell trafficking throughout the body advances, tissue chip models should also advance to incorporate tissue-specific cells where possible. Here, we discuss the known heterogeneity of leukocyte trafficking in vascular beds of some commonly modeled tissues. We comment on the availability of different tissue-specific cell sources for endothelial cells and pericytes, with a focus on stem cell sources for the full realization of personalized medicine. We discuss sources available for the immune cells needed to model inflammatory processes and the findings of tissue chip models that have used the cells to studying transmigration.

Liver-on-a-chip devices: the pros and cons of complexity

Physiologically relevant and broadly applicable liver cell culture platforms are of great importance in both drug development and disease modeling. Organ-on-a-chip systems offer a promising alternative to conventional, static two-dimensional (2-D) cultures, providing much-needed cues such as perfusion, shear stress, and three-dimensional (3-D) cell-cell communication. However, such devices cover a broad range of complexity both in manufacture and in implementation. In this review, we summarize the key features of the human liver that should be reflected in a physiologically relevant liver-on-a-chip model. We also discuss different material properties of importance in producing liver-on-a-chip devices and summarize recent and current progress in the field, highlighting different types of devices at different levels of complexity.

Unraveling the Complexity of Liver Disease One Cell at a Time

The human liver is a complex organ made up of multiple specialized cell types that carry out key physiological functions. An incomplete understanding of liver biology limits our ability to develop therapeutics to prevent chronic liver diseases, liver cancers, and death as a result of organ failure. Recently, single-cell modalities have expanded our understanding of the cellular phenotypic heterogeneity and intercellular cross-talk in liver health and disease. This review summarizes these findings and looks forward to highlighting new avenues for the application of single-cell genomics to unravel unknown pathogenic pathways and disease mechanisms for the development of new therapeutics targeting liver pathology. As these technologies mature, their integration into clinical data analysis will aid in patient stratification and in developing treatment plans for patients suffering from liver disease.

Generating human artery and vein cells from pluripotent stem cells highlights the arterial tropism of Nipah and Hendra viruses

Stem cell research endeavors to generate specific subtypes of classically defined "cell types." Here, we generate >90% pure human artery or vein endothelial cells from pluripotent stem cells within 3-4 days. We specified artery cells by inhibiting vein-specifying signals and vice versa. These cells modeled viral infection of human vasculature by Nipah and Hendra viruses, which are extraordinarily deadly (∼57%-59% fatality rate) and require biosafety-level-4 containment. Generating pure populations of artery and vein cells highlighted that Nipah and Hendra viruses preferentially infected arteries; arteries expressed higher levels of their viral-entry receptor. Virally infected artery cells fused into syncytia containing up to 23 nuclei, which rapidly died. Despite infecting arteries and occupying ∼6%-17% of their transcriptome, Nipah and Hendra largely eluded innate immune detection, minimally eliciting interferon signaling. We thus efficiently generate artery and vein cells, introduce stem-cell-based toolkits for biosafety-level-4 virology, and explore the arterial tropism and cellular effects of Nipah and Hendra viruses.

FcγR-Mediated Trogocytosis 2.0: Revisiting History Gives Rise to a Unifying Hypothesis

There is increasing interest in the clinical implications and immunology of trogocytosis, a process in which the receptors on acceptor cells remove and internalize cognate ligands from donor cells. We have reported that this phenomenon occurs in cancer immunotherapy, in which cells that express FcγR remove and internalize CD20 and bound mAbs from malignant B cells. This process can be generalized to include other reactions including the immune adherence phenomenon and antibody-induced immunosuppression. We discuss in detail FcγR-mediated trogocytosis and the evidence supporting a proposed predominant role for liver sinusoidal endothelial cells via the action of the inhibitory receptor FcγRIIb2. We describe experiments to test the validity of this hypothesis. The elucidation of the details of FcγR-mediated trogocytosis has the potential to allow for the development of novel therapies that can potentially block or enhance this reaction, depending upon whether the process leads to unfavorable or positive biological effects.

Strategies for Regenerative Vascular Tissue Engineering

Vascularization remains one of the key challenges in creating functional tissue-engineered constructs for therapeutic applications. This review aims to provide a developmental lens on the necessity of blood vessels in defining tissue function while exploring stem cells as a suitable source for vascular tissue engineering applications. The intersections of stem cell biology, material science, and engineering are explored as potential solutions for directing vascular assembly.

Specification of fetal liver endothelial progenitors to functional zonated adult sinusoids requires c-Maf induction

The liver vascular network is patterned by sinusoidal and hepatocyte co-zonation. How intra-liver vessels acquire their hierarchical specialized functions is unknown. We study heterogeneity of hepatic vascular cells during mouse development through functional and single-cell RNA-sequencing. The acquisition of sinusoidal endothelial cell identity is initiated during early development and completed postnatally, originating from a pool of undifferentiated vascular progenitors at E12. The peri-natal induction of the transcription factor c-Maf is a critical switch for the sinusoidal identity determination. Endothelium-restricted deletion of c-Maf disrupts liver sinusoidal development, aberrantly expands postnatal liver hematopoiesis, promotes excessive postnatal sinusoidal proliferation, and aggravates liver pro-fibrotic sensitivity to chemical insult. Enforced c-Maf overexpression in generic human endothelial cells switches on a liver sinusoidal transcriptional program that maintains hepatocyte function. c-Maf represents an inducible intra-organotypic and niche-responsive molecular determinant of hepatic sinusoidal cell identity and lays the foundation for the strategies for vasculature-driven liver repair.

Therapeutic correction of hemophilia A by transplantation of hPSC-derived liver sinusoidal endothelial cell progenitors

Liver sinusoidal endothelial cells (LSECs) form the predominant microvasculature in the liver where they carry out many functions including the secretion of coagulation factor VIII (FVIII). To investigate the early origins of this lineage, we develop an efficient and scalable protocol to produce human pluripotent stem cell (hPSC)-derived LSEC progenitors characterized as venous endothelial cells (VECs) from different mesoderm subpopulations. Using a sensitive and quantitative vascular competitive transplantation assay, we demonstrate that VECs generated from BMP4 and activin A-induced KDR⁺CD235a/b⁺ mesoderm are 50-fold more efficient at LSEC engraftment than venous cells from BMP4 and WNT-induced KDR⁺CD235a/b^- mesoderm. When transplanted into immunocompromised hemophilia A mice (NSG-HA), these VECs engraft the liver, proliferate, and mature to functional LSECs that secrete bioactive FVIII capable of correcting the bleeding phenotype. Together, these findings highlight the importance of appropriate mesoderm induction for generating hPSC-derived LSECs capable of functioning in a preclinical model of hemophilia A.

Organ-Specific Endothelial Cell Differentiation and Impact of Microenvironmental Cues on Endothelial Heterogeneity

Endothelial cells throughout the body are heterogeneous, and this is tightly linked to the specific functions of organs and tissues. Heterogeneity is already determined from development onwards and ranges from arterial/venous specification to microvascular fate determination in organ-specific differentiation. Acknowledging the different phenotypes of endothelial cells and the implications of this diversity is key for the development of more specialized tissue engineering and vascular repair approaches. However, although novel technologies in transcriptomics and proteomics are facilitating the unraveling of vascular bed-specific endothelial cell signatures, still much research is based on the use of insufficiently specialized endothelial cells. Endothelial cells are not only heterogeneous, but their specialized phenotypes are also dynamic and adapt to changes in their microenvironment. During the last decades, strong collaborations between molecular biology, mechanobiology, and computational disciplines have led to a better understanding of how endothelial cells are modulated by their mechanical and biochemical contexts. Yet, because of the use of insufficiently specialized endothelial cells, there is still a huge lack of knowledge in how tissue-specific biomechanical factors determine organ-specific phenotypes. With this review, we want to put the focus on how organ-specific endothelial cell signatures are determined from development onwards and conditioned by their microenvironments during adulthood. We discuss the latest research performed on endothelial cells, pointing out the important implications of mimicking tissue-specific biomechanical cues in culture.

Current Status and Challenges of Human Induced Pluripotent Stem Cell-Derived Liver Models in Drug Discovery

The pharmaceutical industry is in high need of efficient and relevant in vitro liver models, which can be incorporated in their drug discovery pipelines to identify potential drugs and their toxicity profiles. Current liver models often rely on cancer cell lines or primary cells, which both have major limitations. However, the development of human induced pluripotent stem cells (hiPSCs) has created a new opportunity for liver disease modeling, drug discovery and liver toxicity research. hiPSCs can be differentiated to any cell of interest, which makes them good candidates for disease modeling and drug discovery. Moreover, hiPSCs, unlike primary cells, can be easily genome-edited, allowing the creation of reporter lines or isogenic controls for patient-derived hiPSCs. Unfortunately, even though liver progeny from hiPSCs has characteristics similar to their in vivo counterparts, the differentiation of iPSCs to fully mature progeny remains highly challenging and is a major obstacle for the full exploitation of these models by pharmaceutical industries. In this review, we discuss current liver-cell differentiation protocols and in vitro iPSC-based liver models that could be used for disease modeling and drug discovery. Furthermore, we will discuss the challenges that still need to be overcome to allow for the successful implementation of these models into pharmaceutical drug discovery platforms.

Organogenesis in vitro

Organoids are three-dimensional structures that self-organize from human pluripotent stem cells or primary tissue, potentially serving as a traceable and manipulatable platform to facilitate our understanding of organogenesis. Despite the ongoing advancement in generating organoids of diverse systems, biological applications of in vitro generated organoids remain as a major challenge in part due to a substantial lack of intricate complexity. The studies of development and regeneration enumerate the essential roles of highly diversified nonepithelial populations such as mesenchyme and endothelium in directing fate specification, morphogenesis, and maturation. Furthermore, organoids with physiological and homeostatic functions require direct and indirect inter-organ crosstalk recapitulating what is seen in organogenesis. We herein review the evolving organoid technology at the cell, tissue, organ, and system level with a main emphasis on endoderm derivatives.

Signalling pathways and transcriptional regulators orchestrating liver development and cancer

Liver development is controlled by key signals and transcription factors that drive cell proliferation, migration, differentiation and functional maturation. In the adult liver, cell maturity can be perturbed by genetic and environmental factors that disrupt hepatic identity and function. Developmental signals and fetal genetic programmes are often dysregulated or reactivated, leading to dedifferentiation and disease. Here, we highlight signalling pathways and transcriptional regulators that drive liver cell development and primary liver cancers. We also discuss emerging models derived from pluripotent stem cells, 3D organoids and bioengineering for improved studies of signalling pathways in liver cancer and regenerative medicine.

Cell-based therapies for vascular regeneration: Past, present and future

Tissue vascularization remains one of the outstanding challenges in regenerative medicine. Beyond its role in circulating oxygen and nutrients, the vasculature is critical for organ development, function and homeostasis. Importantly, effective vascular regeneration is key in generating large 3D tissues for regenerative medicine applications to enable the survival of cells post-transplantation, organ growth, and integration into the host system. Therefore, the absence of clinically applicable means of (re)generating vessels is one of the main obstacles in cell replacement therapy. In this review, we highlight cell-based vascularization strategies which demonstrate clinical potential, discuss their strengths and limitations and highlight the main obstacles hindering cell-based therapeutic vascularization.

iPSC-endothelial cell phenotypic drug screening and in silico analyses identify tyrphostin-AG1296 for pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disorder leading to occlusive vascular remodeling. Current PAH therapies improve quality of life but do not reverse structural abnormalities in the pulmonary vasculature. Here, we used high-throughput drug screening combined with in silico analyses of existing transcriptomic datasets to identify a promising lead compound to reverse PAH. Induced pluripotent stem cell-derived endothelial cells generated from six patients with PAH were exposed to 4500 compounds and assayed for improved cell survival after serum withdrawal using a chemiluminescent caspase assay. Subsequent validation of caspase activity and improved angiogenesis combined with data analyses using the Gene Expression Omnibus and Library of Integrated Network-Based Cellular Signatures databases revealed that the lead compound AG1296 was positively associated with an anti-PAH gene signature. AG1296 increased abundance of bone morphogenetic protein receptors, downstream signaling, and gene expression and suppressed PAH smooth muscle cell proliferation. AG1296 induced regression of PA neointimal lesions in lung organ culture and PA occlusive changes in the Sugen/hypoxia rat model and reduced right ventricular systolic pressure. Moreover, AG1296 improved vascular function and BMPR2 signaling and showed better correlation with the anti-PAH gene signature than other tyrosine kinase inhibitors. Specifically, AG1296 up-regulated small mothers against decapentaplegic (SMAD) 1/5 coactivators, cAMP response element-binding protein 3 (CREB3), and CREB5: CREB3 induced inhibitor of DNA binding 1 and downstream genes that improved vascular function. Thus, drug discovery for PAH can be accelerated by combining phenotypic screening with in silico analyses of publicly available datasets.

Characterization of the proteome and metabolome of human liver sinusoidal endothelial-like cells derived from induced pluripotent stem cells

The liver is a complex organ composed of several cell types organized hierarchically. Among these, liver sinusoidal endothelial cells (LSECs) are specialized vascular cells known to interact with hepatocytes and hepatic stellate cells (HSCs), and to be involved in the regulation of important hepatic processes in healthy and pathological situations. Protocols for the differentiation of LSECs from human induced pluripotent stem cells, hiPSCs, have been proposed and in-depth analysis by transcriptomic profiling of those cells has been performed. In the present work, an extended analysis of those cells in terms of proteome and metabolome has been implemented. The proteomic analysis confirmed the expression of important endothelial markers and pathways. Among them, the expression of patterns typical of LSECs such as PECAM1, VWF, LYVE1, STAB1 (endothelial markers), CDH13, CDH5, CLDN5, ICAM1, MCAM-CD146, ICAM2, ESAM (endothelial cytoskeleton), NOSTRIN, NOS3 (Nitric Oxide endothelial ROS), ESM1, ENG, MMRN2, THBS1, ANGPT2 (angiogenesis), CD93, MRC1 (mannose receptor), CLEC14A (C-type lectin), CD40 (antigen), and ERG (transcription factor) was highlighted. Besides, the pathway analysis revealed the enrichment of the endocytosis, Toll-like receptor, Nod-like receptor, Wnt, Apelin, VEGF, cGMP-PCK, and PPAR related signaling pathways. Other important pathways such as vasopressin regulated water reabsorption, fluid shear stress, relaxin signaling, and renin secretion were also highlighted. At confluence, the metabolome profile appeared consistent with quiescent endothelial cell patterns. The integration of both proteome and metabolome datasets revealed a switch from fatty acid synthesis in undifferentiated hiPSCs to a fatty oxidation in LSECs and activation of the pentose phosphate pathway and polyamine metabolism in hiPSCs-derived LSECs. In conclusion, the comparison between the signature of LSECs differentiated following the protocol described in this work, and data found in the literature confirmed the particular relevance of these cells for future in vitro applications.

The next generation of endothelial differentiation: Tissue-specific ECs

Endothelial cells (ECs) sense and respond to fluid flow and regulate immune cell trafficking in all organs. Despite sharing the same mesodermal origin, ECs exhibit heterogeneous tissue-specific characteristics. Human pluripotent stem cells (hPSCs) can potentially be harnessed to capture this heterogeneity and further elucidate endothelium behavior to satisfy the need for increased accuracy and breadth of disease models and therapeutics. Here, we review current strategies for hPSC differentiation to blood vascular ECs and their maturation into continuous, fenestrated, and sinusoidal tissues. We then discuss the contribution of hPSC-derived ECs to recent advances in organoid development and organ-on-chip approaches.

One-Step Formation of Protein-Based Tubular Structures for Functional Devices and Tissues

Tubular biological structures consisting of extracellular matrix (ECM) proteins and cells are basic functional units of all organs in animals and humans. ECM protein solutions at low concentrations (5-10 milligrams per milliliter) are abundantly used in 3D cell culture. However, their poor "printability" and minute-long gelation time have made the direct extrusion of tubular structures in bioprinting applications challenging. Here, this limitation is overcome and the continuous, template-free conversion of low-concentration collagen, elastin, and fibrinogen solutions into tubular structures of tailored size and radial, circumferential and axial organization is demonstrated. The approach is enabled by a microfabricated printhead for the consistent circumferential distribution of ECM protein solutions and lends itself to scalable manufacture. The attached confinement accommodates minute-long residence times for pH, temperature, light, ionic and enzymatic gelation. Chip hosted ECM tubular structures are amenable to perfusion with aqueous solutions and air, and cyclic stretching. Predictive collapse and reopening in a crossed-tube configuration promote all-ECM valves and pumps. Tissue level function is demonstrated by factors secreted from cells embedded within the tube wall, as well as endothelial or epithelial barriers lining the lumen. The described approaches are anticipated to find applications in ECM-based organ-on-chip and biohybrid structures, hydraulic actuators, and soft machines.

Angiodiversity and organotypic functions of sinusoidal endothelial cells

‘Angiodiversity’ refers to the structural and functional heterogeneity of endothelial cells (EC) along the segments of the vascular tree and especially within the microvascular beds of different organs. Organotypically differentiated EC ranging from continuous, barrier-forming endothelium to discontinuous, fenestrated endothelium perform organ-specific functions such as the maintenance of the tightly sealed blood–brain barrier or the clearance of macromolecular waste products from the peripheral blood by liver EC-expressed scavenger receptors. The microvascular bed of the liver, composed of discontinuous, fenestrated liver sinusoidal endothelial cells (LSEC), is a prime example of organ-specific angiodiversity. Anatomy and development of LSEC have been extensively studied by electron microscopy as well as linage-tracing experiments. Recent advances in cell isolation and bulk transcriptomics or single-cell RNA sequencing techniques allowed the identification of distinct LSEC molecular programs and have led to the identification of LSEC subpopulations. LSEC execute homeostatic functions such as fine tuning the vascular tone, clearing noxious substances from the circulation, and modulating immunoregulatory mechanisms. In recent years, the identification and functional analysis of LSEC-derived angiocrine signals, which control liver homeostasis and disease pathogenesis in an instructive manner, marks a major change of paradigm in the understanding of liver function in health and disease. This review summarizes recent advances in the understanding of liver vascular angiodiversity and the functional consequences resulting thereof.

Development and Application of Endothelial Cells Derived From Pluripotent Stem Cells in Microphysiological Systems Models

The vascular endothelium is present in all organs and blood vessels, facilitates the exchange of nutrients and waste throughout different organ systems in the body, and sets the tone for healthy vessel function. Mechanosensitive in nature, the endothelium responds to the magnitude and temporal waveform of shear stress in the vessels. Endothelial dysfunction can lead to atherosclerosis and other diseases. Modeling endothelial function and dysfunction in organ systems in vitro, such as the blood–brain barrier and tissue-engineered blood vessels, requires sourcing endothelial cells (ECs) for these biomedical engineering applications. It can be difficult to source primary, easily renewable ECs that possess the function or dysfunction in question. In contrast, human pluripotent stem cells (hPSCs) can be sourced from donors of interest and renewed almost indefinitely. In this review, we highlight how knowledge of vascular EC development in vivo is used to differentiate induced pluripotent stem cells (iPSC) into ECs. We then describe how iPSC-derived ECs are being used currently in in vitro models of organ function and disease and in vivo applications.

Stem cell therapy and tissue engineering strategies using cell aggregates and decellularized scaffolds for the rescue of liver failure

Liver failure is a lethal condition with hepatocellular dysfunction, and liver transplantation is presently the only effective treatment. However, due to the limited availability of donors and the potential immune rejection, novel therapeutic strategies are actively sought to restore the normal hepatic architectures and functions, especially for livers with inherited metabolic dysfunctions or chronic diseases. Although the conventional cell therapy has shown promising results, the direct infusion of hepatocytes is hampered by limited hepatocyte sources, poor cell viability, and engraftment. Hence, this review mainly highlights the role of stem cells and progenitors as the alternative cell source and summarizes the potential approaches based on tissue engineering to improve the delivery efficiency of cells. Particularly, the underlying mechanisms for cell therapy using stem cells and progenitors are discussed in two main aspects: paracrine effect and cell differentiation. Moreover, tissue-engineering approaches using cell aggregates and decellularized liver scaffolds for bioengineering of functional hepatic constructs are discussed and compared in terms of the potential to replicate liver physiological structures. In the end, a potentially effective strategy combining the premium advantages of stem cell aggregates and decellularized liver scaffolds is proposed as the future direction of liver tissue engineering and regeneration.

PU.1 drives specification of pluripotent stem cell-derived endothelial cells to LSEC-like cells

To date, there is no representative in vitro model for liver sinusoidal endothelial cells (LSECs), as primary LSECs dedifferentiate very fast in culture and no combination of cytokines or growth factors can induce an LSEC fate in (pluripotent stem cell (PSC)-derived) endothelial cells (ECs). Furthermore, the transcriptional programmes driving an LSEC fate have not yet been described. Here, we first present a computational workflow (CenTFinder) that can identify transcription factors (TFs) that are crucial for modulating pathways involved in cell lineage specification. Using CenTFinder, we identified several novel LSEC-specific protein markers, such as FCN2 and FCN3, which were validated by analysis of previously published single-cell RNAseq data. We also identified PU.1 (encoded by the SPI1 gene) as a major regulator of LSEC-specific immune functions. We show that SPI1 overexpression (combined with the general EC TF ETV2) in human PSCs induces ECs with an LSEC-like phenotype. The ETV2-SPI1-ECs display increased expression of LSEC markers, such as CD32B and MRC1, as well as several of the proposed novel markers. More importantly, ETV2-SPI1-ECs acquire LSEC functions, including uptake of FSA-FITC, as well as labelled IgG. In conclusion, we present the CenTFinder computational tool to identify key regulatory TFs within specific pathways, in this work pathways of lineage specification, and we demonstrate its use by the identification and validation of PU.1 as a master regulator for LSEC fating.