Runx1 is required for progression of CD41+ embryonic precursors into HSCs but not prior to this

Engineered hematopoietic and immune cells derived from human pluripotent stem cells

For the past decade, significant advances have been achieved in human hematopoietic stem cell (HSC) transplantation for treating various blood diseases and cancers. However, challenges remain with the quality control, amount, and cost of HSCs and HSC-derived immune cells. The advent of human pluripotent stem cells (hPSCs) may transform HSC transplantation and cancer immunotherapy by providing a cost-effective and scalable cell source for fundamental studies and translational applications. In this review, we discuss the current developments in the field of stem cell engineering for hematopoietic stem and progenitor cell (HSPC) differentiation and further differentiation of HSPCs into functional immune cells. The key advances in stem cell engineering include the generation of HSPCs from hPSCs, genetic modification of hPSCs, and hPSC-derived HSPCs for improved function, further differentiation of HPSCs into functional immune cells, and applications of cell culture platforms for hematopoietic cell manufacturing. Current challenges impeding the translation of hPSC-HSPCs and immune cells as well as further directions to address these challenges are also discussed.

The (intra-aortic) hematopoietic cluster cocktail: what is in the mix?

The adult-definitive hematopoietic hierarchy and hematopoietic stem cells (HSCs) residing in the bone marrow are established during embryonic development. In mouse, human, and many other mammals, it is the sudden formation of so-called intra-aortic/arterial hematopoietic clusters (IAHCs) that best signifies and visualizes this de novo generation of HSCs and hematopoietic progenitor cells (HPCs). Cluster cells arise through an endothelial-to-hematopoietic transition and, for some time, express markers/genes of both tissue types, whilst acquiring more hematopoietic features and losing endothelial ones. Among several hundreds of IAHC cells, the midgestation mouse embryo contains only very few bona fide adult-repopulating HSCs, suggestive of a challenging cell fate to achieve. Most others are HPCs of various types, some of which have the potential to mature into HSCs in vitro. Based on the number of cells that reveal hematopoietic function, a fraction of IAHC cells is uncharacterized. This review aims to explore the current state of knowledge on IAHC cells. We will describe markers useful for isolation and characterization of these fleetingly produced, yet vitally important, cells and for the refined enrichment of the HSCs they contain, and speculate on the role of some IAHC cells that are as-yet functionally uncharacterized.

De Novo Generation of Human Hematopoietic Stem Cells from Pluripotent Stem Cells for Cellular Therapy

The ability to manufacture human hematopoietic stem cells (HSCs) in the laboratory holds enormous promise for cellular therapy of human blood diseases. Several differentiation protocols have been developed to facilitate the emergence of HSCs from human pluripotent stem cells (PSCs). Most approaches employ a stepwise addition of cytokines and morphogens to recapitulate the natural developmental process. However, these protocols globally lack clinical relevance and uniformly induce PSCs to produce hematopoietic progenitors with embryonic features and limited engraftment and differentiation capabilities. This review examines how key intrinsic cues and extrinsic environmental inputs have been integrated within human PSC differentiation protocols to enhance the emergence of definitive hematopoiesis and how advances in genomics set the stage for imminent breakthroughs in this field.

Redundant mechanisms driven independently by RUNX1 and GATA2 for hematopoietic development

RUNX1 is essential for the generation of hematopoietic stem cells (HSCs). Runx1-null mouse embryos lack definitive hematopoiesis and die in mid-gestation. However, although zebrafish embryos with a runx1 W84X mutation have defects in early definitive hematopoiesis, some runx1W84X/W84X embryos can develop to fertile adults with blood cells of multilineages, raising the possibility that HSCs can emerge without RUNX1. Here, using 3 new zebrafish runx1-/- lines, we uncovered the compensatory mechanism for runx1-independent hematopoiesis. We show that, in the absence of a functional runx1, a cd41-green fluorescent protein (GFP)+ population of hematopoietic precursors still emerge from the hemogenic endothelium and can colonize the hematopoietic tissues of the mutant embryos. Single-cell RNA sequencing of the cd41-GFP+ cells identified a set of runx1-/--specific signature genes during hematopoiesis. Significantly, gata2b, which normally acts upstream of runx1 for the generation of HSCs, was increased in the cd41-GFP+ cells in runx1-/- embryos. Interestingly, genetic inactivation of both gata2b and its paralog gata2a did not affect hematopoiesis. However, knocking out runx1 and any 3 of the 4 alleles of gata2a and gata2b abolished definitive hematopoiesis. Gata2 expression was also upregulated in hematopoietic cells in Runx1-/- mice, suggesting the compensatory mechanism is conserved. Our findings indicate that RUNX1 and GATA2 serve redundant roles for HSC production, acting as each other's safeguard.

Recent Advances in Developmental Hematopoiesis: Diving Deeper With New Technologies

The journey of a hematopoietic stem cell (HSC) involves the passage through successive anatomical sites where HSCs are in direct contact with their surrounding microenvironment, also known as niche. These spatial and temporal cellular interactions throughout development are required for the acquisition of stem cell properties, and for maintaining the HSC pool through balancing self-renewal, quiescence and lineage commitment. Understanding the context and consequences of these interactions will be imperative for our understanding of HSC biology and will lead to the improvement of in vitro production of HSCs for clinical purposes. The aorta-gonad-mesonephros (AGM) region is in this light of particular interest since this is the cradle of HSC emergence during the embryonic development of all vertebrate species. In this review, we will focus on the developmental origin of HSCs and will discuss the novel technological approaches and recent progress made to identify the cellular composition of the HSC supportive niche and the underlying molecular events occurring in the AGM region.

Analysis of Runx1 Using Induced Gene Ablation Reveals Its Essential Role in Pre-liver HSC Development and Limitations of an In Vivo Approach

Hematopoietic stem cells (HSCs) develop in the embryonic aorta-gonad-mesonephros (AGM) region and subsequently relocate to fetal liver. Runx1 transcription factor is essential for HSC development, but is largely dispensable for adult HSCs. Here, we studied tamoxifen-inducible Runx1 inactivation in vivo. Induction at pre-liver stages (up to embryonic day 10.5) reduced erythromyeloid progenitor numbers, but surprisingly did not block the appearance of Runx1-null HSCs in liver. By contrast, ex vivo analysis showed an absolute Runx1 dependency of HSC development in the AGM region. We found that, contrary to current beliefs, significant Cre-inducing tamoxifen activity persists in mouse blood for at least 72 hr after injection. This deferred recombination can hit healthy HSCs, which escaped early Runx1 ablation and result in appearance of Runx1-null HSCs in liver. Such extended recombination activity in vivo is a potential source of misinterpretation, particularly in analysis of dynamic developmental processes during embryogenesis.

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Runx1 ablation induced in vivo at the AGM stage yields null HSCs in fetal liver

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Controlled Runx1 ablation in cultured AGM region blocks HSC development

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Discrepancy is explained by persistence of Cre activity in vivo for at least 3 days

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Runx1 is essential at pre-liver stage of HSC development

RUNX transcription factors: orchestrators of development

RUNX transcription factors orchestrate many different aspects of biology, including basic cellular and developmental processes, stem cell biology and tumorigenesis. In this Primer, we introduce the molecular hallmarks of the three mammalian RUNX genes, RUNX1, RUNX2 and RUNX3, and discuss the regulation of their activities and their mechanisms of action. We then review their crucial roles in the specification and maintenance of a wide array of tissues during embryonic development and adult homeostasis.

Development of Hematopoietic Stem Cells in the Early Mammalian Embryo

Hematopoietic stem cells (HSCs) were the first stem cells discovered in humans. A. A. Maximov proposed an idea of blood stem cells that was confirmed later by McCulloch and Till experimentally. HSCs were the first type of stem cells to be used in clinics and ever since are being continually used. Indeed, a single HSC transplanted intravenously is capable of giving rise to all types of blood cells. In recent decades, human and animal HSC origin, development, hierarchy, and gene signature have been extensively investigated. Due to the constant need for donor blood and HSCs suitable for therapeutic transplants, the experimental possibility of obtaining HSCs in vitro by directed differentiation of pluripotent stem cells (PSCs) has been considered in recent years. However, despite all efforts, it is not yet possible to reproduce in vitro the ontogenesis of HSCs and obtain cells capable of long-term maintenance of hematopoiesis. The study of hematopoiesis in embryonic development facilitates the establishment and improvement of protocols for deriving blood cells from PCSs and allows a better understanding of the pathogenesis of various types of proliferative blood diseases, anemia, and immunodeficiency. This review focuses on the development of hematopoiesis in mammalian ontogenesis.

MAPK p38alpha Kinase Influences Haematopoiesis in Embryonic Stem Cells

The activation of p38alpha kinase mediates cell response to various extracellular factors including many interleukins and growth factors important for haematopoiesis. The role of p38alpha kinase was previously analysed in particular haematopoietic cells. In this study and for the first time, the role of p38alpha kinase in haematopoiesis was studied using a model of continuous haematopoietic development in pluripotent embryonic stem cells in vitro. The expression of transcripts associated with haematopoiesis and the potential for the formation of specific haematopoietic cell colonies were compared between wild-type and mutant p38alpha gene-depleted cells. The absence of p38alpha kinase led to the inhibition of hemangioblast formation during the first step of haematopoiesis. Later, during differentiation, due to the lack of p38alpha kinase, erythrocyte maturation was impaired. Mutant p38α−/− cells also exhibited decreased potential with respect to the expansion of granulocyte colony-forming units. This effect was reversed in the absence of erythropoietin as shown by colony-forming unit assay in media for colony-forming unit granulocytes/macrophages. p38alpha kinase thus plays an important role in the differentiation of common myeloid precursor cells into granulocyte lineages.

Transcription factor mutations as a cause of familial myeloid neoplasms

The initiation and evolution of myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) are driven by genomic events that disrupt multiple genes controlling hematopoiesis. Human genetic studies have discovered germline mutations in single genes that instigate familial MDS/AML. The best understood of these genes encode transcription factors, such as GATA-2, RUNX1, ETV6, and C/EBPα, which establish and maintain genetic networks governing the genesis and function of blood stem and progenitor cells. Many questions remain unanswered regarding how genes and circuits within these networks function in physiology and disease and whether network integrity is exquisitely sensitive to or efficiently buffered from perturbations. In familial MDS/AML, mutations change the coding sequence of a gene to generate a mutant protein with altered activity or introduce frameshifts or stop codons or disrupt regulatory elements to alter protein expression. Each mutation has the potential to exert quantitatively and qualitatively distinct influences on networks. Consistent with this mechanistic diversity, disease onset is unpredictable and phenotypic variability can be considerable. Efforts to elucidate mechanisms and forge prognostic and therapeutic strategies must therefore contend with a spectrum of patient-specific leukemogenic scenarios. Here we illustrate mechanistic advances in our understanding of familial MDS/AML syndromes caused by germline mutations of hematopoietic transcription factors.

Murine hematopoietic stem cell activity is derived from pre-circulation embryos but not yolk sacs

The embryonic site of definitive hematopoietic stem cell (dHSC) origination has been debated for decades. Although an intra-embryonic origin is well supported, the yolk sac (YS) contribution to adult hematopoiesis remains controversial. The same developmental origin makes it difficult to identify specific markers that discern between an intraembryonic versus YS-origin using a lineage trace approach. Additionally, the highly migratory nature of blood cells and the inability of pre-circulatory embryonic cells (i.e., 5-7 somite pairs (sp)) to robustly engraft in transplantation, even after culture, has precluded scientists from properly answering these questions. Here we report robust, multi-lineage and serially transplantable dHSC activity from cultured 2-7sp murine embryonic explants (Em-Ex). dHSC are undetectable in 2-7sp YS explants. Additionally, the engraftment from Em-Ex is confined to an emerging CD31⁺CD45⁺c-Kit⁺CD41^- population. In sum, our work supports a model in which the embryo, not the YS, is the major source of lifelong definitive hematopoiesis.

CD45 expression discriminates waves of embryonic megakaryocytes in the mouse

Embryonic megakaryopoiesis starts in the yolk sac on gestational day 7.5 as part of the primitive wave of hematopoiesis, and it continues in the fetal liver when this organ is colonized by hematopoietic progenitors between day 9.5 and 10.5, as the definitive hematopoiesis wave. We characterized the precise phenotype of embryo megakaryocytes in the liver at gestational day 11.5, identifying them as CD41⁺⁺CD45-CD9⁺⁺CD61⁺MPL⁺CD42c⁺ tetraploid cells that express megakaryocyte-specific transcripts and display differential traits when compared to those present in the yolk sac at the same age. In contrast to megakaryocytes from adult bone marrow, embryo megakaryocytes are CD45⁻ until day 13.5 of gestation, as are both the megakaryocyte progenitors and megakaryocyte/erythroid-committed progenitors. At gestational day 11.5, liver and yolk sac also contain CD41⁺CD45⁺ and CD41⁺CD45⁻ cells. These populations, and that of CD41⁺⁺CD45⁻CD42c⁺ cells, isolated from liver, differentiate in culture into CD41⁺⁺CD45⁻CD42c⁺ proplatelet-bearing megakaryocytes. Also present at this time are CD41⁻CD45⁺⁺CD11b⁺ cells, which produce low numbers of CD41⁺⁺CD45⁻CD42c⁺ megakaryocytes in vitro, as do fetal liver cells expressing the macrophage-specific Csf receptor-1 (Csf1r/CD115) from MaFIA transgenic mice, which give rise poorly to CD41⁺⁺CD45⁻CD42c⁺ embryo megakaryocytes both in vivo and in vitro. In contrast, around 30% of adult megakaryocytes (CD41⁺⁺CD45⁺⁺CD9⁺⁺CD42c⁺) from C57BL/6 and MaFIA mice express CD115. We propose that differential pathways operating in the mouse embryo liver at gestational day 11.5 beget CD41⁺⁺CD45⁻CD42c⁺ embryo megakaryocytes that can be produced from CD41⁺CD45⁻ or from CD41⁺CD45⁺ cells, at difference from those from bone marrow.

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.

HOXB4 Increases Runx1 Expression to Promote the de novo Formation of Multipotent Hematopoietic Cells

BACKGROUND

The de novo generation of patient-specific hematopoietic stem and progenitor cells from induced pluripotent stem cells (iPSCs) has become a promising approach for cell replacement therapies in the future. However, efficient differentiation protocols for producing fully functional human hematopoietic stem cells are still missing. In the mouse model, ectopic expression of the human homeotic selector protein HOXB4 has been shown to enforce the development of hematopoietic stem cells (HSCs) in differentiating pluripotent stem cell cultures. However, the mechanism how HOXB4 mediates the formation of HSCs capable of long-term, multilineage repopulation after transplantation is not well understood yet.

METHODS

Using a mouse embryonic stem (ES) cell-based differentiation model, we asked whether retrovirally expressed HOXB4 induces the expression of Runx1/AML1, a gene whose expression is absolutely necessary for the formation of definitive, adult HSCs during embryonic development.

RESULTS

During ES cell differentiation, basal expression of Runx1 was observed in all cultures, irrespective of ectopic HOXB4 expression. However, only in those cultures ectopically expressing HOXB4, substantial amounts of hematopoietic progenitors were generated which exclusively displayed increased Runx1 expression.

CONCLUSIONS

Our results strongly suggest that HOXB4 does not induce basal Runx1 expression but, instead, mediates an increase of Runx1 expression which appears to be a prerequisite for the formation of hematopoietic stem and progenitor cells.

Understanding Hematopoietic Stem Cell Development through Functional Correlation of Their Proliferative Status with the Intra-aortic Cluster Architecture

During development, hematopoietic stem cells (HSCs) emerge in the aorta-gonad-mesonephros (AGM) region through a process of multi-step maturation and expansion. While proliferation of adult HSCs is implicated in the balance between self-renewal and differentiation, very little is known about the proliferation status of nascent HSCs in the AGM region. Using Fucci reporter mice that enable in vivo visualization of cell-cycle status, we detect increased proliferation during pre-HSC expansion followed by a slowing down of cycling once cells start to acquire a definitive HSC state, similar to fetal liver HSCs. We observe time-specific changes in intra-aortic hematopoietic clusters corresponding to HSC maturation stages. The proliferative architecture of the clusters is maintained in an orderly anatomical manner with slowly cycling cells at the base and more actively proliferating cells at the more apical part of the cluster, which correlates with c-KIT expression levels, thus providing an anatomical basis for the role of SCF in HSC maturation.

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Expansion of HSC precursors is accompanied by increased proliferation

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Final steps of HSC maturation are accompanied by decelerating proliferation

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Proliferative architecture of intra-aortic clusters is maintained during HSC development

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c-Kit expression levels correlate with the proliferative status of HSC precursors

RUNX1 deficiency (familial platelet disorder with predisposition to myeloid leukemia, FPDMM)

In this review, we discuss disease-causing alterations of RUNT-related transcription factor 1 (RUNX1), a master regulator of hematopoietic differentiation. Familial platelet disorder with predisposition to myeloid leukemia (FPDMM) typically presents with (1) mild to moderate thrombocytopenia with normal-sized platelets; (2) functional platelets defects leading to prolonged bleeding; and (3) an increased risk to develop myelodysplastic syndromes (MDS), acute myeloid leukemia (AML), or T-cell acute lymphoblastic leukemia (T-ALL). Hematological neoplasms in carriers of a germline RUNX1 mutation need additional secondary mutations or chromosome aberrations to develop. If a disease-causing mutation is known in the family, it is important to prevent hematopoietic stem cell transplantation from a sibling or other relative carrying the familial mutation. First experiments introducing a wild-type copy of RUNX1 into induce pluripotent stem cells (iPSC) lines from patients with FPDMM appear to demonstrate that by gene correction reversal of the phenotype may be possible.

The molecular basis of endothelial cell plasticity

The endothelium is capable of remarkable plasticity. In the embryo, primitive endothelial cells differentiate to acquire arterial, venous or lymphatic fates. Certain endothelial cells also undergo hematopoietic transition giving rise to multi-lineage hematopoietic stem and progenitors while others acquire mesenchymal properties necessary for heart development. In the adult, maintenance of differentiated endothelial state is an active process requiring constant signalling input. The failure to do so leads to the development of endothelial-to-mesenchymal transition that plays an important role in pathogenesis of a number of diseases. A better understanding of these phenotypic changes may lead to development of new therapeutic interventions.

Vascular endothelium possesses remarkable plasticity in response to cues from its surroundings, leading to great heterogeneity of endothelial cells in different vascular beds. Here the authors explain the molecular basis of endothelial plasticity during embryogenesis and in various diseases.

Runx transcription factors in the development and function of the definitive hematopoietic system

The Runx family of transcription factors (Runx1, Runx2, and Runx3) are highly conserved and encode proteins involved in a variety of cell lineages, including blood and blood-related cell lineages, during developmental and adult stages of life. They perform activation and repressive functions in the regulation of gene expression. The requirement for Runx1 in the normal hematopoietic development and its dysregulation through chromosomal translocations and loss-of-function mutations as found in acute myeloid leukemias highlight the importance of this transcription factor in the healthy blood system. Whereas another review will focus on the role of Runx factors in leukemias, this review will provide an overview of the normal regulation and function of Runx factors in hematopoiesis and focus particularly on the biological effects of Runx1 in the generation of hematopoietic stem cells. We will present the current knowledge of the structure and regulatory features directing lineage-specific expression of Runx genes, the models of embryonic and adult hematopoietic development that provide information on their function, and some of the mechanisms by which they affect hematopoietic function.

Peg3/PW1 Is a Marker of a Subset of Vessel Associated Endothelial Progenitors

Vascular associated endothelial cell (ECs) progenitors are still poorly studied and their role in the newly forming vasculature at embryonic or postnatal stage remains elusive. In the present work, we first defined a set of genes highly expressed during embryo development and strongly downregulated in the adult mouse. In this group, we then concentrated on the progenitor cell marker Peg3/PW1. By in vivo staining of the vasculature we found that only a subset of cells coexpressed endothelial markers and PW1. These cells were quite abundant in the embryo vasculature but declined in number at postnatal and adult stages. Using a reporter mouse for PW1 expression, we have been able to isolate PW1-positive (PW1posECs) and negative endothelial cells (PW1negECs). PW1-positive cells were highly proliferative in comparison to PW1negECs and were able to form colonies when seeded at clonal dilution. Furthermore, by RNAseq analysis, PW1posECs expressed endothelial cell markers together with mesenchymal and stem cell markers. When challenged by endothelial growth factors in vitro, PW1posECs were able to proliferate more than PW1negECs and to efficiently form new vessels in vivo. Taken together these data identify a subset of vessel associated endothelial cells with characteristics of progenitor cells. Considering their high proliferative potential these cells may be of particular importance to design therapies to improve the perfusion of ischemic tissues or to promote vascular repair. Stem Cells 2017;35:1328-1340.

Runx Family Genes in Tissue Stem Cell Dynamics

The Runx family genes play important roles in development and cancer, largely via their regulation of tissue stem cell behavior. Their involvement in two organs, blood and skin, is well documented. This review summarizes currently known Runx functions in the stem cells of these tissues. The fundamental core mechanism(s) mediated by Runx proteins has been sought; however, it appears that there does not exist one single common machinery that governs both tissue stem cells. Instead, Runx family genes employ multiple spatiotemporal mechanisms in regulating individual tissue stem cell populations. Such specific Runx requirements have been unveiled by a series of cell type-, developmental stage- or age-specific gene targeting studies in mice. Observations from these experiments revealed that the regulation of stem cells by Runx family genes turned out to be far more complex than previously thought. For instance, although it has been reported that Runx1 is required for the endothelial-to-hematopoietic cell transition (EHT) but not thereafter, recent studies clearly demonstrated that Runx1 is also needed during the period subsequent to EHT, namely at perinatal stage. In addition, Runx1 ablation in the embryonic skin mesenchyme eventually leads to complete loss of hair follicle stem cells (HFSCs) in the adult epithelium, suggesting that Runx1 facilitates the specification of skin epithelial stem cells in a cell extrinsic manner. Further in-depth investigation into how Runx family genes are involved in stem cell regulation is warranted.

Hematopoietic (stem) cell development - how divergent are the roads taken?

The development of the hematopoietic system during early embryonic stages occurs in spatially and temporally distinct waves. Hematopoietic stem cells (HSC), the most potent and self‐renewing cells of this system, are produced in the final ‘definitive’ wave of hematopoietic cell generation. In contrast to HSCs in the adult, which differentiate via intermediate progenitor populations to produce functional blood cells, the generation of hematopoietic cells in the embryo prior to HSC generation occurs in the early waves by producing blood cells without intermediate progenitors (such as the ‘primitive’ hematopoietic cells). The lineage relationship between the early hematopoietic cells and the cells giving rise to HSCs, the genetic networks controlling their emergence, and the precise temporal determination of HSC fate remain topics of intense research and debate. This Review article discusses the current knowledge on the step‐wise embryonic establishment of the adult hematopoietic system, examines the roles of pivotal intrinsic regulators in this process, and raises questions concerning the temporal onset of HSC fate determination.

Developing HSCs become Notch independent by the end of maturation in the AGM region

The first definitive hematopoietic stem cells (dHSCs) in the mouse emerge in the dorsal aorta of the embryonic day (E) 10.5 to 11 aorta-gonad-mesonephros (AGM) region. Notch signaling is essential for early HSC development but is dispensable for the maintenance of adult bone marrow HSCs. How Notch signaling regulates HSC formation in the embryo is poorly understood. We demonstrate here that Notch signaling is active in E10.5 HSC precursors and involves both Notch1 and Notch2 receptors, but is gradually downregulated while they progress toward dHSCs at E11.5. This downregulation is accompanied by gradual functional loss of Notch dependency. Thus, as early as at final steps in the AGM region, HSCs begin acquiring the Notch independency characteristic of adult bone marrow HSCs as part of the maturation program. Our data indicate that fine stage-dependent tuning of Notch signaling may be required for the generation of definitive HSCs from pluripotent cells.

Concealed expansion of immature precursors underpins acute burst of adult HSC activity in foetal liver

One day prior to mass emergence of haematopoietic stem cells (HSCs) in the foetal liver at E12.5, the embryo contains only a few definitive HSCs. It is thought that the burst of HSC activity in the foetal liver is underpinned by rapid maturation of immature embryonic precursors of definitive HSCs, termed pre-HSCs. However, because pre-HSCs are not detectable by direct transplantations into adult irradiated recipients, the size and growth of this population, which represents the embryonic rudiment of the adult haematopoietic system, remains uncertain. Using a novel quantitative assay, we demonstrate that from E9.5 the pre-HSC pool undergoes dramatic growth in the aorta-gonad-mesonephros region and by E11.5 reaches the size that matches the number of definitive HSCs in the E12.5 foetal liver. Thus, this study provides for the first time a quantitative basis for our understanding of how the large population of definitive HSCs emerges in the foetal liver.

Specification and function of hemogenic endothelium during embryogenesis

Hemogenic endothelium is a specialized subset of developing vascular endothelium that acquires hematopoietic potential and can give rise to multilineage hematopoietic stem and progenitor cells during a narrow developmental window in tissues such as the extraembryonic yolk sac and embryonic aorta-gonad-mesonephros. Herein, we review current knowledge about the historical and developmental origins of hemogenic endothelium, the molecular events that govern hemogenic specification of vascular endothelial cells, the generation of multilineage hematopoietic stem and progenitor cells from hemogenic endothelium, and the potential for translational applications of knowledge gained from further study of these processes.

Concise review: programming human pluripotent stem cells into blood

Blood disorders are treated with cell therapies including haematopoietic stem cell (HSC) transplantation as well as platelet and red blood cell transfusions. However the source of cells is entirely dependent on donors, procedures are susceptible to transfusion-transmitted infections and serious complications can arise in recipients due to immunological incompatibility. These problems could be alleviated if it was possible to produce haematopoietic cells in vitro from an autologous and renewable cell source. The production of haematopoietic cells in the laboratory from human induced pluripotent stem cells (iPSCs) may provide a route to realize this goal but it has proven challenging to generate long-term reconstituting HSCs. To date, the optimization of differentiation protocols has mostly relied on the manipulation of extrinsic signals to mimic the in vivo environment. We review studies that have taken an alternative approach to modulate intrinsic signals by enforced expression of transcription factors. Single and combinations of multiple transcription factors have been used in a variety of contexts to enhance the production of haematopoietic cells from human pluripotent stem cells. This programming approach, together with the recent advances in the production and use of synthetic transcription factors, holds great promise for the production of fully functional HSCs in the future.

Regulation of Blood Stem Cell Development

Understanding how the blood system is formed is an ongoing fundamental research challenge. Developmental biology has provided many insights into the molecules and processes that affect the formation of the blood tissues, both in health and disease. It is of particular interest for clinical transplantation therapies to understand how hematopoietic stem cells (HSCs)-the self-renewing purveyors of the adult blood system that produce over 10 different functionally specialized cell lineages and over 10(11) cells daily-are generated during embryonic stages. Recent successes to reprogram the fate of adult differentiated cells to pluripotency and to other cell lineages now highlight the importance of identifying the cells and molecules that affect the in vivo developmental initiation of rare and robust transplantable HSCs. The close association of the developing hematopoietic and vascular system, hematopoietic cell mobility through the circulation, and the essential role of the embryonic hematopoietic system in adult hematopoietic cell development make this a formidable study. This chapter reviews the advances, controversies, and current state of our knowledge of the growing field of hematopoietic development, with a special focus on the regulation of the natural transdifferentiation of endothelial cells to HSCs within the developing embryo.

Vascular niche promotes hematopoietic multipotent progenitor formation from pluripotent stem cells

Pluripotent stem cells (PSCs) represent an alternative hematopoietic stem cell (HSC) source for treating hematopoietic disease. The limited engraftment of human PSC-derived (hPSC-derived) multipotent progenitor cells (MPP) has hampered the clinical application of these cells and suggests that MPP require additional cues for definitive hematopoiesis. We hypothesized that the presence of a vascular niche that produces Notch ligands jagged-1 (JAG1) and delta-like ligand-4 (DLL4) drives definitive hematopoiesis. We differentiated hes2 human embryonic stem cells (hESC) and Macaca nemestrina-induced PSC (iPSC) line-7 with cytokines in the presence or absence of endothelial cells (ECs) that express JAG1 and DLL4. Cells cocultured with ECs generated substantially more CD34+CD45+ hematopoietic progenitors compared with cells cocultured without ECs or with ECs lacking JAG1 or DLL4. EC-induced cells exhibited Notch activation and expressed HSC-specific Notch targets RUNX1 and GATA2. EC-induced PSC-MPP engrafted at a markedly higher level in NOD/SCID/IL-2 receptor γ chain-null (NSG) mice compared with cytokine-induced cells, and low-dose chemotherapy-based selection further increased engraftment. Long-term engraftment and the myeloid-to-lymphoid ratio achieved with vascular niche induction were similar to levels achieved for cord blood-derived MPP and up to 20-fold higher than those achieved with hPSC-derived MPP engraftment. Our findings indicate that endothelial Notch ligands promote PSC-definitive hematopoiesis and production of long-term engrafting CD34+ cells, suggesting these ligands are critical for HSC emergence.

Improved hematopoietic differentiation efficiency of gene-corrected beta-thalassemia induced pluripotent stem cells by CRISPR/Cas9 system

The generation of beta-thalassemia (β-Thal) patient-specific induced pluripotent stem cells (iPSCs), subsequent homologous recombination-based gene correction of disease-causing mutations/deletions in the β-globin gene (HBB), and their derived hematopoietic stem cell (HSC) transplantation offers an ideal therapeutic solution for treating this disease. However, the hematopoietic differentiation efficiency of gene-corrected β-Thal iPSCs has not been well evaluated in the previous studies. In this study, we used the latest gene-editing tool, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9), to correct β-Thal iPSCs; gene-corrected cells exhibit normal karyotypes and full pluripotency as human embryonic stem cells (hESCs) showed no off-targeting effects. Then, we evaluated the differentiation efficiency of the gene-corrected β-Thal iPSCs. We found that during hematopoietic differentiation, gene-corrected β-Thal iPSCs showed an increased embryoid body ratio and various hematopoietic progenitor cell percentages. More importantly, the gene-corrected β-Thal iPSC lines restored HBB expression and reduced reactive oxygen species production compared with the uncorrected group. Our study suggested that hematopoietic differentiation efficiency of β-Thal iPSCs was greatly improved once corrected by the CRISPR/Cas9 system, and the information gained from our study would greatly promote the clinical application of β-Thal iPSC-derived HSCs in transplantation.