Generating human hematopoietic stem cells in vitro -exploring endothelial to hematopoietic transition as a portal for stemness acquisition

IκBα controls dormancy in hematopoietic stem cells via retinoic acid during embryonic development

Recent findings suggest that Hematopoietic Stem Cells (HSC) and progenitors arise simultaneously and independently of each other already in the embryonic aorta-gonad mesonephros region, but it is still unknown how their different features are established. Here, we uncover IκBα (Nfkbia, the inhibitor of NF-κB) as a critical regulator of HSC proliferation throughout development. IκBα balances retinoic acid signaling levels together with the epigenetic silencer, PRC2, specifically in HSCs. Loss of IκBα decreases proliferation of HSC and induces a dormancy related gene expression signature instead. Also, IκBα deficient HSCs respond with superior activation to in vitro culture and in serial transplantation. At the molecular level, chromatin regions harboring binding motifs for retinoic acid signaling are hypo-methylated for the PRC2 dependent H3K27me3 mark in IκBα deficient HSCs. Overall, we show that the proliferation index in the developing HSCs is regulated by a IκBα-PRC2 axis, which controls retinoic acid signaling.

Cis inhibition of NOTCH1 through JAGGED1 sustains embryonic hematopoietic stem cell fate

Hematopoietic stem cells (HSCs) develop from the hemogenic endothelium (HE) in the aorta- gonads-and mesonephros (AGM) region and reside within Intra-aortic hematopoietic clusters (IAHC) along with hematopoietic progenitors (HPC). The signalling mechanisms that distinguish HSCs from HPCs are unknown. Notch signaling is essential for arterial specification, IAHC formation and HSC activity, but current studies on how Notch segregates these different fates are inconsistent. We now demonstrate that Notch activity is highest in a subset of, GFI1 + , HSC-primed HE cells, and is gradually lost with HSC maturation. We uncover that the HSC phenotype is maintained due to increasing levels of NOTCH1 and JAG1 interactions on the surface of the same cell (cis) that renders the NOTCH1 receptor from being activated. Forced activation of the NOTCH1 receptor in IAHC activates a hematopoietic differentiation program. Our results indicate that NOTCH1-JAG1 cis-inhibition preserves the HSC phenotype in the hematopoietic clusters of the embryonic aorta.

The genesis of human hematopoietic stem cells

Developmental hematopoiesis consists of multiple, partially overlapping hematopoietic waves that generate the differentiated blood cells required for embryonic development while establishing a pool of undifferentiated hematopoietic stem cells (HSCs) for postnatal life. This multilayered design in which active hematopoiesis migrates through diverse extra and intraembryonic tissues has made it difficult to define a roadmap for generating HSCs vs non-self-renewing progenitors, especially in humans. Recent single-cell studies have helped in identifying the rare human HSCs at stages when functional assays are unsuitable for distinguishing them from progenitors. This approach has made it possible to track the origin of human HSCs to the unique type of arterial endothelium in the aorta-gonad-mesonephros region and document novel benchmarks for HSC migration and maturation in the conceptus. These studies have delivered new insights into the intricate process of HSC generation and provided tools to inform the in vitro efforts to replicate the physiological developmental journey from pluripotent stem cells via distinct mesodermal and endothelial intermediates to HSCs.

PEAR1 is a potential regulator of early hematopoiesis of human pluripotent stem cells

Hemogenic endothelial (HE) cells are specialized endothelial cells to give rise to hematopoietic stem/progenitor cells during hematopoietic development. The underlying mechanisms that regulate endothelial-to-hematopoietic transition (EHT) of human HE cells are not fully understand. Here, we identified platelet endothelial aggregation receptor-1 (PEAR1) as a novel regulator of early hematopoietic development in human pluripotent stem cells (hPSCs). We found that the expression of PEAP1 was elevated during hematopoietic development. A subpopulation of PEAR1⁺ cells overlapped with CD34⁺ CD144⁺ CD184⁺ CD73^- arterial-type HE cells. Transcriptome analysis by RNA sequencing indicated that TAL1/SCL, GATA2, MYB, RUNX1 and other key transcription factors for hematopoietic development were mainly expressed in PEAR1⁺ cells, whereas the genes encoding for niche-related signals, such as fibronectin, vitronectin, bone morphogenetic proteins and jagged1, were highly expressed in PEAR1^- cells. The isolated PEAR1⁺ cells exhibited significantly greater EHT capacity on endothelial niche, compared with the PEAR1^- cells. Colony-forming unit (CFU) assays demonstrated the multilineage hematopoietic potential of PEAR1⁺ -derived hematopoietic cells. Furthermore, PEAR1 knockout in hPSCs by CRISPR/Cas9 technology revealed that the hematopoietic differentiation was impaired, resulting in decreased EHT capacity, decreased expression of hematopoietic-related transcription factors, and increased expression of niche-related signals. In summary, this study revealed a novel role of PEAR1 in balancing intrinsic and extrinsic signals for early hematopoietic fate decision.

SAHA Enhances Differentiation of CD34+CD45+ Hematopoietic Stem and Progenitor Cells from Pluripotent Stem Cells Concomitant with an Increase in Hemogenic Endothelium

Epigenetic modification is an important process during hematopoietic cell differentiation. Histone deacetylase (HDAC) inhibitors have previously been shown to enhance expansion of umbilical cord blood-derived hematopoietic stem cells (HSCs). However, the effect of HDAC inhibitors on pluripotent stem cells (PSCs) in this context is less understood. For years, investigators have considered PSC-derived natural killer (NK) and T-cell therapies. These "off-the-shelf" cellular therapies are now entering the clinic. However, the in vitro commitment of PSCs to the hematopoietic lineage is inefficient and represents a major bottleneck. We investigated whether HDAC inhibitors (HDACi) influence human PSC differentiation into CD34+CD45+ hematopoietic stem and progenitor cells (HSPCs), focusing on hemogenic endothelium (HE). Pluripotent stem cells cultured in the presence of HDACi showed a 2-5 times increase in HSPCs. Concurrent with this, HDACi-treated PSCs increased expression of 7 transcription factors (HOXA5, HOXA9, HOXA10, RUNX1, ERG, SPI1, and LCOR) recently shown to convert HE to HSPCs. ChIP-qPCR showed that SAHA upregulated acetylated-H3 at the promoter region of the above key genes. SAHA-treated human PSC-derived CD34+CD45+ cells showed primary engraftment in immunodeficient mice, but not serial transplantation. We further demonstrate that SAHA-derived HSPCs could differentiate into functional NK cells in vitro. The addition of SAHA is an easy and effective approach to overcoming the bottleneck in the transition from PSC to HSPCs for "off-the-shelf" cellular immunotherapy.

Notch Signaling in HSC Emergence: When, Why and How

The hematopoietic stem cell (HSC) sustains blood homeostasis throughout life in vertebrates. During embryonic development, HSCs emerge from the aorta-gonads and mesonephros (AGM) region along with hematopoietic progenitors within hematopoietic clusters which are found in the dorsal aorta, the main arterial vessel. Notch signaling, which is essential for arterial specification of the aorta, is also crucial in hematopoietic development and HSC activity. In this review, we will present and discuss the evidence that we have for Notch activity in hematopoietic cell fate specification and the crosstalk with the endothelial and arterial lineage. The core hematopoietic program is conserved across vertebrates and here we review studies conducted using different models of vertebrate hematopoiesis, including zebrafish, mouse and in vitro differentiated Embryonic stem cells. To fulfill the goal of engineering HSCs in vitro, we need to understand the molecular processes that modulate Notch signaling during HSC emergence in a temporal and spatial context. Here, we review relevant contributions from different model systems that are required to specify precursors of HSC and HSC activity through Notch interactions at different stages of development.

Assessment of Endothelial-to-Hematopoietic Transition of Individual Hemogenic Endothelium and Bulk Populations in Defined Conditions

Endothelial-to-hematopoietic transition (EHT) is a unique morphogenic event in which flat, adherent hemogenic endothelial (HE) cells acquire round, non-adherent blood cell morphology. Investigating the mechanisms of EHT is critical for understanding the development of hematopoietic stem cells (HSCs) and the entirety of the adult immune system, and advancing technologies for manufacturing blood cells from human pluripotent stem cells (hPSCs). Here we describe a protocol to (a) generate and isolate subsets of HE from hPSCs, (b) assess EHT and hematopoietic potential of HE subsets in bulk cultures and at the single-cell level, and (c) evaluate the role of NOTCH signaling during HE specification and EHT. The generation of HE from hPSCs and EHT bulk cultures are performed in xenogen- and feeder-free system, providing the unique advantage of being able to investigate the role of individual signaling factors during EHT and the definitive lympho-myeloid cell specification from hPSCs.

HSC and miRNA Regulation with Implication for Foetal Haemoglobin Induction in Beta Haemoglobinopathies

Sickle cell disease (SCD) is one of the most common haemoglobinopathies worldwide, with up to 70 % of global SCD annual births occurring in sub-Saharan Africa. Reports have shown that 50 to 80 % of affected children in these countries die annually. Efforts geared towards understanding and controlling HbF production in SCD patients could lead to strategies for effective control of globin gene expression and therapeutic approaches that could be beneficial to individuals with haemoglobinopathies. Hemopoietic stem cells (HSCs) are characterized by a specific miRNA signature in every state of differentiation. The role of miRNAs has become evident both in the maintenance of the "stemness" and in the early induction of differentiation by modulation of the expression of the master pluripotency genes and during early organogenesis. miRNAs are extra regulatory mechanisms in hematopoietic stem cells (HSCs) via influencing transcription profiles together with transcript stability. miRNAs have been reported to be used to reprogram primary somatic cells toward pluripotency. Their involvement in cell editing holds the potential for therapy for many genetic diseases. This review provides a snapshot of miRNA involvement in cell fate decisions, haemoglobin induction pathway, and their journey as some emerge prime targets for therapy in beta haemoglobinopathies.

Murine AGM single-cell profiling identifies a continuum of hemogenic endothelium differentiation marked by ACE

In vitro generation and expansion of hematopoietic stem cells (HSCs) holds great promise for the treatment of any ailment that relies on bone marrow or blood transplantation. To achieve this, it is essential to resolve the molecular and cellular pathways that govern HSC formation in the embryo. HSCs first emerge in the aorta-gonad-mesonephros region (AGM) where a rare subset of endothelial cells, hemogenic endothelium (HE), undergoes an endothelial-to-hematopoietic transition (EHT). Here, we present full-length single-cell-RNA-sequencing of the EHT process with a focus on HE and dorsal aorta niche cells. By using Runx1b and Gfi1/1b transgenic reporter mouse models to isolate HE, we uncovered that the pre-HE to HE continuum is specifically marked by Angiotensin-I converting enzyme (ACE) expression. We established that HE cells begin to enter the cell cycle near the time of EHT initiation when their morphology still resembles endothelial cells. We further demonstrated that RUNX1 AGM niche cells consist of vascular smooth muscle cells and PDGFRa+ mesenchymal cells and can functionally support hematopoiesis. Overall, our study provides new insights into HE differentiation towards HSC and the role of AGM RUNX1+ niche cells in this process. Our expansive scRNA-seq datasets represents a powerful resource to investigate these processes further.

Industrially Compatible Transfusable iPSC-Derived RBCs: Progress, Challenges and Prospective Solutions

Amidst the global shortfalls in blood supply, storage limitations of donor blood and the availability of potential blood substitutes for transfusion applications, society has pivoted towards in vitro generation of red blood cells (RBCs) as a means to solve these issues. Many conventional research studies over the past few decades have found success in differentiating hematopoietic stem and progenitor cells (HSPCs) from cord blood, adult bone marrow and peripheral blood sources. More recently, techniques that involve immortalization of erythroblast sources have also gained traction in tackling this problem. However, the RBCs generated from human induced pluripotent stem cells (hiPSCs) still remain as the most favorable solution due to many of its added advantages. In this review, we focus on the breakthroughs for high-density cultures of hiPSC-derived RBCs, and highlight the major challenges and prospective solutions throughout the whole process of erythropoiesis for hiPSC-derived RBCs. Furthermore, we elaborate on the recent advances and techniques used to achieve cost-effective, high-density cultures of GMP-compliant RBCs, and on their relevant novel applications after downstream processing and purification.

Lithium Directs Embryonic Stem Cell Differentiation Into Hemangioblast-Like Cells

Definitive hematopoietic stem cells (HSCs) derive from specialized regions of the endothelium known as the hemogenic endothelium (HE) during embryonic developmental processes. This knowledge opens up new possibilities for designing new strategies to obtain HSCs in vitro from pluripotent stem cells (PSCs). Previous advances in this field show that the Wnt/β-catenin signaling pathway plays a crucial role in PSC-derived HSC formation. In this work, lithium, a GSK3 inhibitor, is identified as an element capable of stabilizing β-catenin and inducing embryonic stem cells (ESCs) differentiation in hemangioblast-like cells, highly consistent with the role of Wnt agonists on ESC differentiation. ESCs treated with 10 mm lithium express CD31+, SCA-1+, Nkx2-5+, CD34+, and FLK1+ cells characteristic of the hemangioblast cells that precede HE development. However, 10 mm Li treated cells remain arrested in a hemangioblast-like phase, which switched into the expression of HE markers after stimulation with maturation medium. The ability of lithium-treated ESCs to further derive into HE is confirmed after defined maturation, resulting in a rapid increase in cells positive for the HE markers RUNX1 and SOX17. The results represent a novel strategy for generating HSC precursors in vitro as a multipotent source of stem cells for blood disease therapies.

A 3D iPSC-differentiation model identifies interleukin-3 as a regulator of early human hematopoietic specification

Hematopoietic development is spatiotemporally tightly regulated by defined cell-intrinsic and extrinsic modifiers. The role of cytokines has been intensively studied in adult hematopoiesis; however, their role in embryonic hematopoietic specification remains largely unexplored. Here, we used induced pluripotent stem cell (iPSC) technology and established a 3-dimensional (3D), organoid-like differentiation system (“hemanoid”) maintaining the structural cellular integrity to evaluate the effect of cytokines on embryonic hematopoietic development. We show that defined stages of early human hematopoietic development were recapitulated within the generated hemanoids. We identified KDR+/CD34^high/CD144+/CD43^–/CD45^– hemato-endothelial progenitors (HEP) forming organized, vasculature-like structures and giving rise to CD34^low/CD144^–/CD43⁺/CD45⁺ hematopoietic progenitor cells. We demonstrate that the endothelial to hematopoietic transition of HEP is dependent on the presence of interleukin 3 (IL-3). Inhibition of IL-3 signaling blocked hematopoietic differentiation and arrested the cells in the HEP stage. Thus, our data suggest an important role for IL-3 in early human hematopoiesis by supporting the endothelial to hematopoietic transition of HEP and highlight the potential of a hemanoid-based model to study human hematopoietic development.

Toward a better definition of hematopoietic progenitors suitable for B cell differentiation

The success of inducing human pluripotent stem cells (hIPSC) offers new opportunities for cell-based therapy. Since B cells exert roles as effector and as regulator of immune responses in different clinical settings, we were interested in generating B cells from hIPSC. We differentiated human embryonic stem cells (hESC) and hIPSC into B cells onto OP9 and MS-5 stromal cells successively. We overcame issues in generating CD34+CD43+ hematopoietic progenitors with appropriate cytokine conditions and emphasized the difficulties to generate proper hematopoietic progenitors. We highlight CD31intCD45int phenotype as a possible marker of hematopoietic progenitors suitable for B cell differentiation. Defining precisely proper lymphoid progenitors will improve the study of their lineage commitment and the signals needed during the in vitro process.

Early development and functional properties of tryptase/chymase double-positive mast cells from human pluripotent stem cells

Mast cells (MCs) play a pivotal role in the hypersensitivity reaction by regulating the innate and adaptive immune responses. Humans have two types of MCs. The first type, termed MC_TC, is found in the skin and other connective tissues and expresses both tryptase and chymase, while the second, termed MC_T, which only expresses tryptase, is found primarily in the mucosa. MCs induced from human adult-type CD34⁺ cells are reported to be of the MC_T type, but the development of MCs during embryonic/fetal stages is largely unknown. Using an efficient coculture system, we identified that a CD34⁺c-kit⁺ cell population, which appeared prior to the emergence of CD34⁺CD45⁺ hematopoietic stem and progenitor cells (HSPCs), stimulated robust production of pure Tryptase⁺Chymase⁺ MCs (MC_TCs). Single-cell analysis revealed dual development directions of CD34⁺c-kit⁺ progenitors, with one lineage developing into erythro-myeloid progenitors (EMP) and the other lineage developing into HSPC. Interestingly, MC_TCs derived from early CD34⁺c-kit⁺ cells exhibited strong histamine release and immune response functions. Particularly, robust release of IL-17 suggested that these early developing tissue-type MC_TCs could play a central role in tumor immunity. These findings could help elucidate the mechanisms controlling early development of MC_TCs and have significant therapeutic implications.

Biphasic Regulation of Mesenchymal Genes Controls Fate Switches During Hematopoietic Differentiation of Human Pluripotent Stem Cells

Epithelial-mesenchymal transition (EMT) or its reverse process mesenchymal-epithelial transition (MET) occurs in multiple physiological and pathological processes. However, whether an entire EMT-MET process exists and the potential function during human hematopoiesis remain largely elusive. Utilizing human pluripotent stem cell (hPSC)-based systems, it is discovered that while EMT occurs at the onset of human hematopoietic differentiation, MET is not detected subsequently during differentiation. Instead, a biphasic activation of mesenchymal genes during hematopoietic differentiation of hPSCs is observed. The expression of mesenchymal genes is upregulated during the fate switch from pluripotency to the mesoderm, sustained at the hemogenic endothelium (HE) stage, and attenuated during hemogenic endothelial cell (HEP) differentiation to hematopoietic progenitor cells (HPCs). A similar expression pattern of mesenchymal genes is also observed during human and murine hematopoietic development in vivo. Wnt signaling and its downstream gene SNAI1 mediate the up-regulation of mesenchymal genes and initiation of mesoderm induction from pluripotency. Inhibition of transforming growth factor-β (TGF-β) signaling and downregulation of HAND1, a downstream gene of TGF-β, are required for the downregulation of mesenchymal genes and the capacity of HEPs to generate HPCs. These results suggest that the biphasic regulation of mesenchymal genes is an essential mechanism during human hematopoiesis.

Current Trends on Hemopoietic Stem Cells

Advances in single-cell technology and genetic mouse models have resulted in the identification of new types of hemopoietic stem cells (HSC), resulting in baffling observations, suggesting a reconsideration of the long-held notion that all hematopoietic cells in the adult are derived from HSCs. The existence of long-lived HSC-independent hematopoiesis has led to the conclusion that despite the single hierarchical differentiation route that generates functional blood types, other differentiation routes exist in-vivo. Heterogeneity in the HSC population and the evolving knowledge around HSC has translated to it's improved application as a therapeutic tool for various blood disorders. The reprogramming of non-hematopoietic somatic and mature blood cells to pluripotency with their subsequent differentiation into hematopoietic stem cells/progenitor cells and the introduction of new generation sequencing holds the potential for the resolution of ambiguities involved in HSC bone marrow transplantation. There is a change in the paradigm for HSC transplantation donor selection. Donor choice favors haploidentical HCT than cord blood. This review provides a general overview of the current events around hemopoietic stem cells, with emphasis on the rising trend of HSC transplantation, especially haploidentical stem cell transplantation.

Biomimetic aorta-gonad-Mesonephros-on-a-Chip to study human developmental hematopoiesis

A fundamental limitation in the derivation of hematopoietic stem and progenitor cells is the imprecise understanding of human developmental hematopoiesis. Herein we established a multilayer microfluidic Aorta-Gonad-Mesonephros (AGM)-on-a-chip to emulate developmental hematopoiesis from pluripotent stem cells. The device consists of two layers of microchannels separated by a semipermeable membrane, which allows the co-culture of human hemogenic endothelial (HE) cells and stromal cells in a physiological relevant spatial arrangement to replicate the structure of the AGM. HE cells derived from human induced pluripotent stem cells (hiPSCs) were cultured on a layer of mesenchymal stromal cells in the top channel while vascular endothelial cells were co-cultured on the bottom side of the membrane within the microfluidic device. We show that this AGM-on-a-chip efficiently derives endothelial-to-hematopoietic transition (EHT) from hiPSCs compared with regular suspension culture. The presence of mesenchymal stroma and endothelial cells renders functional HPCs in vitro. We propose that the AGM-on-a-chip could serve as a platform to dissect the cellular and molecular mechanisms of human developmental hematopoiesis.

MSX2 suppression through inhibition of TGFβ signaling enhances hematopoietic differentiation of human embryonic stem cells

Background

Strategies of generating functional blood cells from human pluripotent stem cells (hPSCs) remain largely unsuccessful due to the lack of a comprehensive understanding of hematopoietic development. Endothelial-to-hematopoietic transition (EHT) serves as the pivotal mechanism for the onset of hematopoiesis and is negatively regulated by TGF-β signaling. However, little is known about the underlying details of TGF-β signaling during EHT.

Methods

In this study, by applying genome-wide gene profiling, we identified muscle segment homeobox2 (MSX2) as a potential mediator of TGF-β signaling during EHT. We generated MSX2-deleted human embryonic stem cell (hESC) lines using the CRISPR/Cas9 technology and induced them to undergo hematopoietic differentiation. The role of MSX2 in hematopoiesis and functional regulation of TGFβ signaling in EHT was studied.

Results

We identified MSX2 as a novel regulator of human hematopoiesis. MSX2 deletion promotes the production of hematopoietic cells from hESCs. Functional and bioinformatics studies further demonstrated that MSX2 deletion augments hematopoietic differentiation of hESCs by facilitating EHT. Mechanistically, MSX2 acts as a downstream target of TGFβ signaling to mediate its function during EHT.

Conclusions

Our results not only improve the understanding of EHT, but may also provide novel insight into the efficient production of functional blood cells from hPSCs for regenerative medicine.

Benzene and its effects on cell signaling pathways related to hematopoiesis and leukemia

Benzene is an environmental toxicant found in many consumer products. It is an established human carcinogen and is known to cause acute myeloid leukemia in adults. Epidemiological evidence has since shown that benzene can cross the placenta and affect the fetal liver. Animal studies have shown that in utero exposure to benzene can increase tumor incidence in offspring. Although there have been risk factors established for acute myeloid leukemia, they still do not account for many of the cases. Clearly then, current efforts to elucidate the mechanism by which benzene exerts its carcinogenic properties have been superficial. Owing to the critical role of cell signaling pathways in the development of an organism and its various organ systems, it seems plausible to suspect that these pathways may have a role in leukemogenesis. This review article assesses current evidence of the effects of benzene on critical hematopoietic signaling pathways. Pathways discussed included Hedgehog, Notch/Delta, Wingless/Integrated, nuclear factor-kappaB and others. Following a review of the literature, it seems that current evidence about the effects of benzene on these critical signaling pathways remains limited. Given the important role of these pathways in hematopoiesis, more attention should be given to them.

Transcriptional control of blood cell emergence

The haematopoietic system is established during embryonic life through a series of developmental steps that culminates with the generation of haematopoietic stem cells. Characterisation of the transcriptional network that regulates blood cell emergence has led to the identification of transcription factors essential for this process. Among the many factors wired within this complex regulatory network, ETV2, SCL and RUNX1 are the central components. All three factors are absolutely required for blood cell generation, each one controlling a precise step of specification from the mesoderm germ layer to fully functional blood progenitors. Insight into the transcriptional control of blood cell emergence has been used for devising protocols to generate blood cells de novo, either through reprogramming of somatic cells or through forward programming of pluripotent stem cells. Interestingly, the physiological process of blood cell generation and its laboratory-engineered counterpart have very little in common.

Developmental programming of adult haematopoiesis system

The Barker hypothesis of 'foetal origin of adult diseases' has led to emphasize the concept of 'developmental programming', based on the crucial role of epigenetic factors. Accordingly, it has been demonstrated that parental adversity (before conception and during pregnancy) and foetal factors (i.e., hypoxia, malnutrition and placental insufficiency) permanently modify the physiological systems of the progeny, predisposing them to premature ageing and chronic disease during adulthood. Thus, an altered functionality of the endocrine, immune, nervous and cardiovascular systems is observed in the progeny. However, it remains to be understood whether the haematopoietic system itself also represents a portrait of foetal programming. Here, we provide evidence, reporting and discussing related theories, and results of studies described in the literature. In addition, we have outlined our opinions and suggest how it is possible to intervene to correct foetal mal-programming. Some pro-health interventions and recommendations are proposed, with the hope of guarantee the health of future generations and trying to combat the continuous increase in age-related diseases in human populations.

Overexpression of GATA2 Enhances Development and Maintenance of Human Embryonic Stem Cell-Derived Hematopoietic Stem Cell-like Progenitors

GATA2 is essential for the endothelial-to-hematopoietic transition (EHT) and generation of hematopoietic stem cells (HSCs). It is poorly understood how GATA2 controls the development of human pluripotent stem cell (hPSC)-derived HS-like cells. Here, using human embryonic stem cells (hESCs) in which GATA2 overexpression was induced by doxycycline (Dox), we elucidated the dual functions of GATA2 in definitive hematopoiesis before and after the emergence of CD34⁺CD45⁺CD90⁺CD38^- HS-like cells. Specifically, GATA2 promoted expansion of hemogenic precursors via the EHT and then helped to maintain HS-like cells in a quiescent state by regulating cell cycle. RNA sequencing showed that hPSC-derived HS-like cells were very similar to human fetal liver-derived HSCs. Our findings will help to elucidate the mechanism that controls the early stages of human definitive hematopoiesis and may help to develop a strategy to generate hPSC-derived HSCs.

R-spondin2 promotes hematopoietic differentiation of human pluripotent stem cells by activating TGF beta signaling

BACKGROUND

Human pluripotent stem cells (hPSCs) provide supplies of potential functional blood cells to suffice the clinical needs. However, the underlying mechanism of generating genuine hematopoietic stem cells (HSCs) and functional blood cells from hPSCs remains largely elusive.

METHOD

In this study, we supplied R-spondin2 exogenously during hematopoietic differentiation of hPSCs under various culture conditions and analyzed the production of hematopoietic progenitor cells (HPCs). We further added R-spondin2 at different temporal window to pin down the stage at which R-spondin2 conferred its effects. RNA-SEQ-based gene profiling was applied to analyze genes with significantly altered expression and altered signaling pathways. Finally, megakaryocytic differentiation and platelet generation were determined using HPCs with R-spondin2 treatment.

RESULTS

We found that R-spondin2 generated by hematopoiesis-supporting stromal cells significantly enhances hematopoietic differentiation of hPSCs. Supply of R-spondin2 exogenously at the early stage of mesoderm differentiation elevates the generation of APLNR⁺ cells. Furthermore, early treatment of cells with R-spondin2 enables us to increase the output of hPSC-derived platelet-like particles (PLPs) with intact function. At the mechanistic level, R-spondin2 activates TGF-β signaling to promote the hematopoietic differentiation.

CONCLUSIONS

Our results demonstrate that a transient supply of R-spondin2 can efficiently promote hematopoietic development by activating both WNT and TGF-β signaling. R-spondin2 can be therefore used as a powerful tool for large-scale generation of functional hematopoietic progenitors and platelets for translational medicine.

Understanding the Journey of Human Hematopoietic Stem Cell Development

Hematopoietic stem cells (HSCs) surface during embryogenesis leading to the genesis of the hematopoietic system, which is vital for immune function, homeostasis balance, and inflammatory responses in the human body. Hematopoiesis is the process of blood cell formation, which initiates from hematopoietic stem/progenitor cells (HSPCs) and is responsible for the generation of all adult blood cells. With their self-renewing and pluripotent properties, human pluripotent stem cells (hPSCs) provide an unprecedented opportunity to create in vitro models of differentiation that will revolutionize our understanding of human development, especially of the human blood system. The utilization of hPSCs provides newfound approaches for studying the origins of human blood cell diseases and generating progenitor populations for cell-based treatments. Current shortages in our knowledge of adult HSCs and the molecular mechanisms that control hematopoietic development in physiological and pathological conditions can be resolved with better understanding of the regulatory networks involved in hematopoiesis, their impact on gene expression, and further enhance our ability to develop novel strategies of clinical importance. In this review, we delve into the recent advances in the understanding of the various cellular and molecular pathways that lead to blood development from hPSCs and examine the current knowledge of human hematopoietic development. We also review how in vitro differentiation of hPSCs can undergo hematopoietic transition and specification, including major subtypes, and consider techniques and protocols that facilitate the generation of hematopoietic stem cells.

Endothelial-to-haematopoietic transition: an update on the process of making blood

The first definitive blood cells during embryogenesis are derived from endothelial cells in a highly conserved process known as endothelial-to-haematopoietic transition (EHT). This conversion involves activation of a haematopoietic transcriptional programme in a subset of endothelial cells in the major vasculature of the embryo, followed by major morphological changes that result in transitioning cells rounding up, breaking the tight junctions to neighbouring endothelial cells and adopting a haematopoietic fate. The whole process is co-ordinated by a complex interplay of key transcription factors and signalling pathways, with additional input from surrounding tissues. Diverse model systems, including mouse, chick and zebrafish embryos as well as differentiation of pluripotent cells in vitro, have contributed to the elucidation of the details of the EHT, which was greatly accelerated in recent years by sophisticated live imaging techniques and advances in transcriptional profiling, such as single-cell RNA-Seq. A detailed knowledge of these developmental events is required in order to be able to apply it to the generation of haematopoietic stem cells from pluripotent stem cells in vitro - an achievement which is of obvious clinical importance. The aim of this review is to summarise the latest findings and describe how these may have contributed towards achieving this goal.

UM171 expands distinct types of myeloid and NK progenitors from human pluripotent stem cells

Scaling up blood cell production from hPSCs is critical to advancing hPSC technologies for blood transfusion, immunotherapy, and transplantation. Here we explored the potential of the HSC agonist pyrimido-indole derivative UM171, to expand hematopoietic progenitors (HPs) derived from hPSCs in chemically defined conditions. We revealed that culture of hPSC-HPs in HSC expansion conditions (SFEM with added TPO, SCF, FLT3L, IL3 and IL6) in the presence of UM171 predominantly expanded HPs with a unique CD34⁺CD41a^loCD45⁺ phenotype that were enriched in granulocytic progenitors (G-CFCs). In contrast, in lymphoid cultures on OP9-DLL4, in the presence of SCF, FLT3L, and IL7, UM171 selectively expanded CD34⁺CD45⁺CD7⁺ lymphoid progenitors with NK cell potential, and increased NK cell output up to 10-fold. These studies should improve our understanding of the effect of UM171 on de novo generated HPs, and facilitate development of protocols for robust granulocyte and lymphoid cell production from hPSCs, for adoptive immunotherapies.

MEIS2 regulates endothelial to hematopoietic transition of human embryonic stem cells by targeting TAL1

Background

Despite considerable progress in the development of methods for hematopoietic differentiation, efficient generation of transplantable hematopoietic stem cells (HSCs) and other genuine functional blood cells from human embryonic stem cells (hESCs) is still unsuccessful. Therefore, a better understanding of the molecular mechanism underlying hematopoietic differentiation of hESCs is highly demanded.

Methods

In this study, by using whole-genome gene profiling, we identified Myeloid Ectopic Viral Integration Site 2 homolog (MEIS2) as a potential regulator of hESC early hematopoietic differentiation. We deleted MEIS2 gene in hESCs using the CRISPR/CAS9 technology and induced them to hematopoietic differentiation, megakaryocytic differentiation.

Results

In this study, we found that MEIS2 deletion impairs early hematopoietic differentiation from hESCs. Furthermore, MEIS2 deletion suppresses hemogenic endothelial specification and endothelial to hematopoietic transition (EHT), leading to the impairment of hematopoietic differentiation. Mechanistically, TAL1 acts as a downstream gene mediating the function of MEIS2 during early hematopoiesis. Interestingly, unlike MEIS1, MEIS2 deletion exerts minimal effects on megakaryocytic differentiation and platelet generation from hESCs.

Conclusions

Our findings advance the understanding of human hematopoietic development and may provide new insights for large-scale generation of functional blood cells for clinical applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1074-z) contains supplementary material, which is available to authorized users.

Arterial identity of hemogenic endothelium: a key to unlock definitive hematopoietic commitment in human pluripotent stem cell cultures

Human pluripotent stem cells (hPSCs) have been suggested as a potential source for the de novo production of blood cells for transfusion, immunotherapies, and transplantation. However, even with advanced hematopoietic differentiation methods, the primitive and myeloid-restricted waves of hematopoiesis dominate in hPSC differentiation cultures, whereas cell surface markers to distinguish these waves of hematopoiesis from lympho-myeloid hematopoiesis remain unknown. In the embryo, hematopoietic stem cells (HSCs) arise from hemogenic endothelium (HE) lining arteries, but not veins. This observation led to a long-standing hypothesis that arterial specification is an essential prerequisite to initiate the HSC program. It has also been established that lymphoid potential in the yolk sac and extraembryonic vasculature is mostly confined to arteries, whereas myeloid-restricted hematopoiesis is not specific to arterial vessels. Here, we review how the link between arterialization and the subsequent definitive multilineage hematopoietic program can be exploited to identify HE enriched in lymphoid progenitors and aid in in vitro approaches to enhance the production of lymphoid cells and potentially HSCs from hPSCs. We also discuss alternative models of hematopoietic specification at arterial sites and recent advances in our understanding of hematopoietic development and the production of engraftable hematopoietic cells from hPSCs.

GATA2 Is Dispensable for Specification of Hemogenic Endothelium but Promotes Endothelial-to-Hematopoietic Transition

The transcriptional factor GATA2 is required for blood and hematopoietic stem cell formation during the hemogenic endothelium (HE) stage of development in the embryo. However, it is unclear if GATA2 controls HE lineage specification or if it solely regulates endothelial-to-hematopoietic transition (EHT). To address this problem, we innovated a unique system, which involved generating GATA2 knockout human embryonic stem cell (hESC) lines with conditional GATA2 expression (iG2-/- hESCs). We demonstrated that GATA2 activity is not required for VE-cadherin+CD43-CD73+ non-HE or VE-cadherin+CD43-CD73- HE generation and subsequent HE diversification into DLL4+ arterial and DLL4- non-arterial lineages. However, GATA2 is primarily needed for HE to undergo EHT. Forced expression of GATA2 in non-HE failed to induce blood formation. The lack of GATA2 requirement for generation of HE and non-HE indicates the critical role of GATA2-independent pathways in specification of these two distinct endothelial lineages.

Application of small molecule CHIR99021 leads to the loss of hemangioblast progenitor and increased hematopoiesis of human pluripotent stem cells

Improving our understanding of the intricacies of hematopoietic specification of induced or embryonic human pluripotent stem cells is beneficial for many areas of research and translational medicine. Currently, it is not clear whether, during human pluripotent stem cells hematopoietic differentiation in vitro, the maturation of definitive progenitors proceeds through a primitive progenitor (hemangioblast) intermediate or if it develops independently. The objective of this study was to investigate the early stages of hematopoietic specification of pluripotent stem cells in vitro. By implementing an adherent culture, serum-free differentiation system that utilizes a small molecule, CHIR99021, to induce human pluripotent stem cells toward various hematopoietic lineages, we established that, compared with the OP9 coculture hematopoietic induction system, the application of CHIR99021 alters the early steps of hematopoiesis such as hemangioblasts, angiogenic hematopoietic progenitors, and hemogenic endothelium. Importantly, it is associated with the loss of hemangioblast progenitors, loss of CD43⁺ (primitive hematopoietic marker) expression, and predominant development of blast-forming unit erythroid colonies in semisolid medium. These data support the hypothesis that the divergence of primitive and definitive programs during human pluripotent stem cells differentiation precedes the hemangioblast stage. Furthermore, we have shown that the inhibition of primitive hematopoiesis is associated with an increase in hematopoietic potential, which is a fruitful finding due to the growing need for lymphoid and myeloid cells in translational applications.

NOTCH signaling specifies arterial-type definitive hemogenic endothelium from human pluripotent stem cells

NOTCH signaling is required for the arterial specification and formation of hematopoietic stem cells (HSCs) and lympho-myeloid progenitors in the embryonic aorta-gonad-mesonephros region and extraembryonic vasculature from a distinct lineage of vascular endothelial cells with hemogenic potential. However, the role of NOTCH signaling in hemogenic endothelium (HE) specification from human pluripotent stem cell (hPSC) has not been studied. Here, using a chemically defined hPSC differentiation system combined with the use of DLL1-Fc and DAPT to manipulate NOTCH, we discover that NOTCH activation in hPSC-derived immature HE progenitors leads to formation of CD144⁺CD43⁻CD73⁻DLL4⁺Runx1 + 23-GFP⁺ arterial-type HE, which requires NOTCH signaling to undergo endothelial-to-hematopoietic transition and produce definitive lympho-myeloid and erythroid cells. These findings demonstrate that NOTCH-mediated arterialization of HE is an essential prerequisite for establishing definitive lympho-myeloid program and suggest that exploring molecular pathways that lead to arterial specification may aid in vitro approaches to enhance definitive hematopoiesis from hPSCs.

It is unclear whether arterial specification is required for hematopoietic stem cell formation. Here, the authors use a chemically defined human pluripotent stem cell (hPSC) differentiation system to show the role of NOTCH signaling in forming arterial-type hemogenic endothelial cells.

Defining murine organogenesis at single-cell resolution reveals a role for the leukotriene pathway in regulating blood progenitor formation

During gastrulation, cell types from all three germ layers are specified and the basic body plan is established 1 . However, molecular analysis of this key developmental stage has been hampered by limited cell numbers and a paucity of markers. Single-cell RNA sequencing circumvents these problems, but has so far been limited to specific organ systems 2 . Here, we report single-cell transcriptomic characterization of >20,000 cells immediately following gastrulation at E8.25 of mouse development. We identify 20 major cell types, which frequently contain substructure, including three distinct signatures in early foregut cells. Pseudo-space ordering of somitic progenitor cells identifies dynamic waves of transcription and candidate regulators, which are validated by molecular characterization of spatially resolved regions of the embryo. Within the endothelial population, cells that transition from haemogenic endothelial to erythro-myeloid progenitors specifically express Alox5 and its co-factor Alox5ap, which control leukotriene production. Functional assays using mouse embryonic stem cells demonstrate that leukotrienes promote haematopoietic progenitor cell generation. Thus, this comprehensive single-cell map can be exploited to reveal previously unrecognized pathways that contribute to tissue development.

MEIS1 Regulates Hemogenic Endothelial Generation, Megakaryopoiesis, and Thrombopoiesis in Human Pluripotent Stem Cells by Targeting TAL1 and FLI1

Human pluripotent stem cells (hPSCs) provide an unlimited source for generating various kinds of functional blood cells. However, efficient strategies for generating large-scale functional blood cells from hPSCs are still lacking, and the mechanism underlying human hematopoiesis remains largely unknown. In this study, we identified myeloid ectopic viral integration site 1 homolog (MEIS1) as a crucial regulator of hPSC early hematopoietic differentiation. MEIS1 is vital for specification of APLNR⁺ mesoderm progenitors to functional hemogenic endothelial progenitors (HEPs), thereby controlling formation of hematopoietic progenitor cells (HPCs). TAL1 mediates the function of MEIS1 in HEP specification. In addition, MEIS1 is vital for megakaryopoiesis and thrombopoiesis from hPSCs. Mechanistically, FLI1 acts as a downstream gene necessary for the function of MEIS1 during megakaryopoiesis. Thus, MEIS1 controls human hematopoiesis in a stage-specific manner and can be potentially manipulated for large-scale generation of HPCs or platelets from hPSCs for therapeutic applications in regenerative medicine.

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MEIS1 knockout impairs hematopoiesis of hPSCs by suppressing HEP specification

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MEIS1^−/− megakaryocytes fail to undergo polyploidization and thrombopoiesis

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TAL1 mediates the function of MEIS1 in HEP specification

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FLI1 acts as a downstream target of MEIS1 during megakaryopoiesis and thrombopoiesis

The Generation of Human γδT Cell-Derived Induced Pluripotent Stem Cells from Whole Peripheral Blood Mononuclear Cell Culture

γδT cells constitute a small proportion of lymphocytes in peripheral blood. Unlike αβT cells, the anti‐tumor activities are exerted through several different pathways in a MHC‐unrestricted manner. Thus, immunotherapy using γδT cells is considered to be effective for various types of cancer. Occasionally, however, ex vivo expanded cells are not as effective as expected due to cell exhaustion. To overcome the issue of T‐cell exhaustion, researchers have generated induced pluripotent stem cells (iPSCs) that harbor the same T‐cell receptor (TCR) genes as their original T‐cells, which provide nearly limitless sources for antigen‐specific cytotoxic T lymphocytes (CTLs). However, these technologies have focused on αβT cells and require a population of antigen‐specific CTLs, which are purified by cell sorting with HLA‐peptide multimer, as the origin of iPS cells. In the present study, we aimed to develop an efficient and convenient system for generating iPSCs that harbor rearrangements of the TCRG and TCRD gene regions (γδT‐iPSCs) without cell‐sorting. We stimulated human whole peripheral blood mononuclear cell (PBMC) culture using Interleukin‐2 and Zoledronate to activate γδT cells. Gene transfer into those cells with the Sendai virus vector resulted in γδT cell‐dominant expression of exogenous genes. The introduction of reprogramming factors into the stimulated PBMC culture allowed us to establish iPSC lines. Around 70% of the established lines carried rearrangements at the TCRG and TCRD gene locus. The γδT‐iPSCs could differentiate into hematopoietic progenitors. Our technology will pave the way for new avenues toward novel immunotherapy that can be applied for various types of cancer. stemcellstranslationalmedicine2018;7:34–44

Pluripotent Stem Cell-Derived Hematopoietic Progenitors Are Unable to Downregulate Key Epithelial-Mesenchymal Transition-Associated miRNAs

Hematopoietic stem cells derived from pluripotent stem cells could be used as an alternative to bone marrow transplants. Deriving these has been a long‐term goal for researchers. However, the success of these efforts has been limited with the cells produced able to engraft in the bone marrow of recipient animals only in very low numbers. There is evidence that defects in the migratory and homing capacity of the cells are due to mis‐regulation of miRNA expression and are responsible for their failure to engraft. We compared the miRNA expression profile of hematopoietic progenitors derived from pluripotent stem cells to those derived from bone marrow and found that numerous miRNAs are too highly expressed in hematopoietic progenitors derived from pluripotent stem cells, and that most of these are inhibitors of epithelial‐mesenchymal transition or metastasis (including miR‐200b, miR‐200c, miR‐205, miR‐148a, and miR‐424). We hypothesize that the high expression of these factors, which promote an adherent phenotype, may be causing the defect in hematopoietic differentiation. However, inhibiting these miRNAs, individually or in multiplex, was insufficient to improve hematopoietic differentiation in vitro, suggesting that other miRNAs and/or genes may be involved in this process. Stem Cells2018;36:55–64

Differentiation, Evaluation, and Application of Human Induced Pluripotent Stem Cell-Derived Endothelial Cells

The emergence of induced pluripotent stem cell (iPSC) technology paves the way to generate large numbers of patient-specific endothelial cells (ECs) that can be potentially delivered for regenerative medicine in patients with cardiovascular disease. In the last decade, numerous protocols that differentiate EC from iPSC have been developed by many groups. In this review, we will discuss several common strategies that have been optimized for human iPSC-EC differentiation and subsequent studies that have evaluated the potential of human iPSC-EC as a cell therapy or as a tool in disease modeling. In addition, we will emphasize the importance of using in vivo vessel-forming ability and in vitro clonogenic colony-forming potential as a gold standard with which to evaluate the quality of human iPSC-EC derived from various protocols.

Human haematopoietic stem cell development: from the embryo to the dish

Haematopoietic stem cells (HSCs) emerge during embryogenesis and give rise to the adult haematopoietic system. Understanding how early haematopoietic development occurs is of fundamental importance for basic biology and medical sciences, but our knowledge is still limited compared with what we know of adult HSCs and their microenvironment. This is particularly true for human haematopoiesis, and is reflected in our current inability to recapitulate the development of HSCs from pluripotent stem cells in vitro In this Review, we discuss what is known of human haematopoietic development: the anatomical sites at which it occurs, the different temporal waves of haematopoiesis, the emergence of the first HSCs and the signalling landscape of the haematopoietic niche. We also discuss the extent to which in vitro differentiation of human pluripotent stem cells recapitulates bona fide human developmental haematopoiesis, and outline some future directions in the field.

Conversion of adult endothelium to immunocompetent haematopoietic stem cells

Developmental pathways that orchestrate the fleeting transition of endothelial cells into haematopoietic stem cells remain undefined. Here we demonstrate a tractable approach for fully reprogramming adult mouse endothelial cells to haematopoietic stem cells (rEC-HSCs) through transient expression of the transcription-factor-encoding genes Fosb, Gfi1, Runx1, and Spi1 (collectively denoted hereafter as FGRS) and vascular-niche-derived angiocrine factors. The induction phase (days 0-8) of conversion is initiated by expression of FGRS in mature endothelial cells, which results in endogenous Runx1 expression. During the specification phase (days 8-20), RUNX1⁺ FGRS-transduced endothelial cells commit to a haematopoietic fate, yielding rEC-HSCs that no longer require FGRS expression. The vascular niche drives a robust self-renewal and expansion phase of rEC-HSCs (days 20-28). rEC-HSCs have a transcriptome and long-term self-renewal capacity similar to those of adult haematopoietic stem cells, and can be used for clonal engraftment and serial primary and secondary multi-lineage reconstitution, including antigen-dependent adaptive immune function. Inhibition of TGFβ and CXCR7 or activation of BMP and CXCR4 signalling enhanced generation of rEC-HSCs. Pluripotency-independent conversion of endothelial cells into autologous authentic engraftable haematopoietic stem cells could aid treatment of haematological disorders.

Cytokine-free directed differentiation of human pluripotent stem cells efficiently produces hemogenic endothelium with lymphoid potential

Background

The robust generation of human hematopoietic progenitor cells from induced or embryonic pluripotent stem cells would be beneficial for multiple areas of research, including mechanistic studies of hematopoiesis, the development of cellular therapies for autoimmune diseases, induced transplant tolerance, anticancer immunotherapies, disease modeling, and drug/toxicity screening. Over the past years, significant progress has been made in identifying effective protocols for hematopoietic differentiation from pluripotent stem cells and understanding stages of mesodermal, endothelial, and hematopoietic specification. Thus, it has been shown that variations in cytokine and inhibitory molecule treatments in the first few days of hematopoietic differentiation define primitive versus definitive potential of produced hematopoietic progenitor cells. The majority of current feeder-free, defined systems for hematopoietic induction from pluripotent stem cells include prolonged incubations with various cytokines that make the differentiation process complex and time consuming. We established that the application of Wnt agonist CHIR99021 efficiently promotes differentiation of human pluripotent stem cells in the absence of any hematopoietic cytokines to the stage of hemogenic endothelium capable of definitive hematopoiesis.

Methods

The hemogenic endothelium differentiation was accomplished in an adherent, serum-free culture system by applying CHIR99021. Hemogenic endothelium progenitor cells were isolated on day 5 of differentiation and evaluated for their endothelial, myeloid, and lymphoid potential.

Results

Monolayer induction based on GSK3 inhibition, described here, yielded a large number of CD31⁺CD34⁺ hemogenic endothelium cells. When isolated and propagated in adherent conditions, these progenitors gave rise to mature endothelium. When further cocultured with OP9 mouse stromal cells, these progenitors gave rise to various cells of myeloid lineages as well as natural killer lymphoid, T-lymphoid, and B-lymphoid cells.

Conclusion

The results of this study substantiate a method that significantly reduces the complexity of current protocols for hematopoietic induction, offers a defined system to study the factors that affect the early stages of hematopoiesis, and provides a new route of lymphoid and myeloid cell derivation from human pluripotent stem cells, thus enhancing their use in translational medicine.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0519-0) contains supplementary material, which is available to authorized users.