Cbfa2 is required for the formation of intra-aortic hematopoietic clusters

Roles of RUNX in Hypoxia-Induced Responses and Angiogenesis

During the past two decades, Runt domain transcription factors (RUNX1, 2, and 3) have been investigated in regard to their function, structural elements, genetic variants, and roles in normal development and pathological conditions. The Runt family proteins are evolutionarily conserved from Drosophila to mammals, emphasizing their physiological importance. A hypoxic microenvironment caused by insufficient blood supply is frequently observed in developing organs, growing tumors, and tissues that become ischemic due to impairment or blockage of blood vessels. During embryonic development and tumor growth, hypoxia triggers a stress response that overcomes low-oxygen conditions by increasing erythropoiesis and angiogenesis and triggering metabolic changes. This review briefly introduces hypoxic conditions and cellular responses, as well as angiogenesis and its related signaling pathways, and then describes our current knowledge on the functions and molecular mechanisms of Runx family proteins in hypoxic responses, especially in angiogenesis.

Roles of the RUNX1 Enhancer in Normal Hematopoiesis and Leukemogenesis

Enhancers are regulatory elements in genomic DNA that contain specific sequence motifs that are bound by DNA-binding transcription factors. The activity of enhancers is tightly regulated in an integrated and combinatorial manner, thus yielding complex patterns of transcription in different tissues. Identifying enhancers is crucial to understanding the physiological and pathogenic roles of their target genes. The RUNX1 intronic enhancer, eR1, acts in cis to regulate RUNX1 gene expression in hematopoietic stem cells (HSCs) and hemogenic endothelial cells. RUNX1 and other hematopoietic transcription factors TAL1/SCL, GATA2, PU.1, LMO2 and LDB1 bind at this region. Interestingly, recent studies have revealed that this region is involved in a large cluster of enhancers termed a super-enhancer. The RUNX1 super-enhancer is observed in normal HSCs and T-cell acute lymphoblastic leukemia cells. In this review, we describe the discovery of eR1 and its roles in normal development and leukemogenesis, as well as its potential applications in stem cell research.

Interplay between SOX7 and RUNX1 regulates hemogenic endothelial fate in the yolk sac

Endothelial to hematopoietic transition (EHT) is a dynamic process involving the shutting down of endothelial gene expression and switching on of hematopoietic gene transcription. Although the factors regulating EHT in hemogenic endothelium (HE) of the dorsal aorta have been relatively well studied, the molecular regulation of yolk sac HE remains poorly understood. Here, we show that SOX7 inhibits the expression of RUNX1 target genes in HE, while having no effect on RUNX1 expression itself. We establish that SOX7 directly interacts with RUNX1 and inhibits its transcriptional activity. Through this interaction we demonstrate that SOX7 hinders RUNX1 DNA binding as well as the interaction between RUNX1 and its co-factor CBFβ. Finally, we show by single-cell expression profiling and immunofluorescence that SOX7 is broadly expressed across the RUNX1+ yolk sac HE population compared with SOX17. Collectively, these data demonstrate for the first time how direct protein-protein interactions between endothelial and hematopoietic transcription factors regulate contrasting transcriptional programs during HE differentiation and EHT.

A view of human haematopoietic development from the Petri dish

Human pluripotent stem cells (hPSCs) provide an unparalleled opportunity to establish in vitro differentiation models that will transform our approach to the study of human development. In the case of the blood system, these models will enable investigation of the earliest stages of human embryonic haematopoiesis that was previously not possible. In addition, they will provide platforms for studying the origins of human blood cell diseases and for generating de novo haematopoietic stem and progenitor cell populations for cell-based regenerative therapies.

PRDM1/BLIMP1 is widely distributed to the nascent fetal-placental interface in the mouse gastrula

BACKGROUND

PRDM1 is a transcriptional repressor that contributes to primordial germ cell (PGC) development. During early gastrulation, epiblast-derived PRDM1 is thought to be restricted to a lineage-segregated germ line in the allantois. However, given recent findings that PGCs overlap an allantoic progenitor pool that contributes widely to the fetal-umbilical interface, posterior PRDM1 may also contribute to soma.

RESULTS

Within the posterior mouse gastrula (early streak, 12-s stages, embryonic days ∼6.75-9.0), PRDM1 localized to all tissues containing putative PGCs; however, PRDM1 was also found in all three primary germ layers, their derivatives, and two presumptive growth centers, the allantoic core domain and ventral ectodermal ridge. While PRDM1 and STELLA colocalized predominantly within the hindgut, where putative PGCs reside, other colocalizing cells were found in non-PGC sites. Additional PRDM1 and STELLA cells were found independent of each other throughout the posterior region, including the hindgut. The Prdm1-Cre-driven reporter supported PRDM1 localization in the majority of sites; however, some Prdm1 descendants were found in sites independent of PRDM1 protein, including allantoic mesothelium and hindgut endoderm.

CONCLUSIONS

Posterior PRDM1 contributes more broadly to the developing fetal-maternal connection than previously recognized, and PRDM1 and STELLA, while overlapping in putative PGCs, also co-localize in several other tissues. Developmental Dynamics 246:50-71, 2017. © 2016 Wiley Periodicals, Inc.

Evi1 regulates Notch activation to induce zebrafish hematopoietic stem cell emergence

During development, hematopoietic stem cells (HSCs) emerge from aortic endothelial cells (ECs) through an intermediate stage called hemogenic endothelium by a process known as endothelial-to-hematopoietic transition (EHT). While Notch signaling, including its upstream regulator Vegf, is known to regulate this process, the precise molecular control and temporal specificity of Notch activity remain unclear. Here, we identify the zebrafish transcriptional regulator evi1 as critically required for Notch-mediated EHT In vivo live imaging studies indicate that evi1 suppression impairs EC progression to hematopoietic fate and therefore HSC emergence. evi1 is expressed in ECs and induces these effects cell autonomously by activating Notch via pAKT Global or endothelial-specific induction of notch, vegf, or pAKT can restore endothelial Notch and HSC formations in evi1 morphants. Significantly, evi1 overexpression induces Notch independently of Vegf and rescues HSC numbers in embryos treated with a Vegf inhibitor. In sum, our results unravel evi1-pAKT as a novel molecular pathway that, in conjunction with the shh-vegf axis, is essential for activation of Notch signaling in VDA endothelial cells and their subsequent conversion to HSCs.

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.

New insights into the regulation by RUNX1 and GFI1(s) proteins of the endothelial to hematopoietic transition generating primordial hematopoietic cells

The first hematopoietic cells are generated very early in ontogeny to support the growth of the embryo and to provide the foundation to the adult hematopoietic system. There is a considerable therapeutic interest in understanding how these first blood cells are generated in order to try to reproduce this process in vitro. This would allow generating blood products, or hematopoietic cell populations from embryonic stem (ES) cells, induced pluripotent stem cells or through directed reprogramming. Recent studies have clearly established that the first hematopoietic cells originate from a hemogenic endothelium (HE) through an endothelial to hematopoietic transition (EHT). The molecular mechanisms underlining this transition remain largely unknown with the exception that the transcription factor RUNX1 is critical for this process. In this Extra Views report, we discuss our recent studies demonstrating that the transcriptional repressors GFI1 and GFI1B have a critical role in the EHT. We established that these RUNX1 transcriptional targets are actively implicated in the downregulation of the endothelial program and the loss of endothelial identity during the formation of the first blood cells. In addition, our results suggest that GFI1 expression provides an ideal novel marker to identify, isolate and study the HE cell population.

Insights into blood cell formation from hemogenic endothelium in lesser-known anatomic sites

BACKGROUND

Hematopoietic stem and progenitor cells (HSPCs) are generated de novo in the embryo in a process termed the endothelial to hematopoietic transition (EHT). EHT is most extensively studied in the yolk sac and dorsal aorta. Recently new sites of hematopoiesis have been described, including the heart, somites, head, and venous plexus of the yolk sac.

RESULTS

We examined sites of HSPC formation in well-studied and in less well-known sites by mapping the expression of the key EHT factor Runx1 along with several other markers by means of confocal microscopy. We identified sites of HSPC formation in the head, heart and somites. We also identified sites of HSPC formation in both the arterial and venous plexuses of the yolk sac, and show that progenitors with lymphoid potential are enriched in hematopoietic clusters in close proximity to arteries. Furthermore, we demonstrate that many of the cells in hematopoietic clusters resemble monocytes or granulocytes based on nuclear shape.

CONCLUSIONS

We identified sites of HSPC formation in the head, heart, and somites, confirming that embryonic hematopoiesis is less spatially restricted than previously thought. Furthermore, we show that HSPCs in the yolk sac with lymphoid potential are located in closer proximity to arteries than to veins. Developmental Dynamics 245:1011-1028, 2016. © 2016 Wiley Periodicals, Inc.

Expression of hematopoietic transcription factors Runt, CBFβ and GATA during ontogenesis of scallop Chlamys farreri

Transcription factors Runx1, CBFβ and GATA1/2/3 play essential roles in regulating hematopoietic development during embryogenesis of vertebrate. In previous study, the orthologous genes of Runt, CBFβ and GATA1/2/3 have been identified from scallop Chlamys farreri and proved to have conserved function in regulating hemocyte production. Here, these three transcription factors were selected as hematopoietic markers to explore potential developmental events of hematopoiesis during ontogenesis of scallop. The transcripts of CfRunt, CfCBFβ and CfGATA were detected abundantly after 32-cell embryo, trochophore and morula stage, and reached to a peak level in 32-cell embryos and D-shaped veligers, pediveligers or gastrula respectively. Further whole-mount immunofluorescence assay showed that the immunoreactivity of CfRunt was firstly observed at 32-cell stage and then its distribution was specialized gradually to the mesoderm during gastrulation. By trochophore, the expression of CfRunt, CfCBFβ and CfGATA proteins occurred coincidently in two specific symmetry cell mass located bilaterally on prototroch, and then disappeared rapidly in D-shaped or umbonal vliger, respectively. However, remarkable expressions of the three transcription factors were observed consistently in a new sinus structure appeared at the dorsal anterior side of D-shaped and umbonal veliger. After bacterial challenge, the mRNA expression levels of the three transcription factors were up-regulated or down-regulated significantly in trochophore, D-shaped veliger and pediveliger, indicating the available hematopoietic regulation in scallop larvae. The results revealed that scallop might experience two waves of hematopoiesis during early development, which occurred in the bilateral symmetry cell mass of trochophore and the sinus structure of veliger.

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

Advances in cellular reprogramming technologies have created alternative platforms for the production of blood cells, either through inducing pluripotency in somatic cells or by way of direct conversion of nonhematopoietic cells into blood cells. However, de novo generation of hematopoietic stem cells (HSCs) with robust and sustained multilineage engraftment potential remains a significant challenge. Hemogenic endothelium (HE) has been recognized as a unique transitional stage of blood development from mesoderm at which HSCs arise in certain embryonic locations. The major aim of this review is to summarize historical perspectives and recent advances in the investigation of endothelial to hematopoietic transition (EHT) and HSC formation in the context of aiding in vitro approaches to instruct HSC fate from human pluripotent stem cells. In addition, direct conversion of somatic cells to blood and HSCs and progression of this conversion through HE stage are discussed. A thorough understanding of the intrinsic and microenvironmental regulators of EHT that lead to the acquisition of self-renewal potential by emerging blood cells is essential to advance the technologies for HSC production and expansion.

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.

Resolving early mesoderm diversification through single-cell expression profiling

In mammals, specification of the three major germ layers occurs during gastrulation, when cells ingressing through the primitive streak differentiate into the precursor cells of major organ systems. However, the molecular mechanisms underlying this process remain unclear, as numbers of gastrulating cells are very limited. In the mouse embryo at embryonic day 6.5, cells located at the junction between the extra-embryonic region and the epiblast on the posterior side of the embryo undergo an epithelial-to-mesenchymal transition and ingress through the primitive streak. Subsequently, cells migrate, either surrounding the prospective ectoderm contributing to the embryo proper, or into the extra-embryonic region to form the yolk sac, umbilical cord and placenta. Fate mapping has shown that mature tissues such as blood and heart originate from specific regions of the pre-gastrula epiblast, but the plasticity of cells within the embryo and the function of key cell-type-specific transcription factors remain unclear. Here we analyse 1,205 cells from the epiblast and nascent Flk1(+) mesoderm of gastrulating mouse embryos using single-cell RNA sequencing, representing the first transcriptome-wide in vivo view of early mesoderm formation during mammalian gastrulation. Additionally, using knockout mice, we study the function of Tal1, a key haematopoietic transcription factor, and demonstrate, contrary to previous studies performed using retrospective assays, that Tal1 knockout does not immediately bias precursor cells towards a cardiac fate.

The characterization of hematopoietic tissue in adult Chinese mitten crab Eriocheir sinensis

Invertebrates rely on the efficient innate immune mechanisms against invaders, in which the continuous production of hemocytes (hematopoiesis) is indispensable. In the present study, the hematopoietic tissue (HPT) from Chinese mitten crab Eriocheir sinensis was identified and characterized. It was a thin and non-transparent sheet located at the dorsolateral side of the stomach, which was composed of a series of ovoid lobules. Each lobule was surrounded by connective tissue containing a large amount of spherical cells with big nucleus. In HPT, the cells were full of mitochondria and granules, and DNA replication was detected in some cells by EdU labeling technique. Cell proliferation was observed in HPT by transmission electron microscope (TEM). The distribution of two transcription factors, GATA1 and RUNX1, were examined by human GATA1 and RUNX1 antibodies, respectively. Three homologues of RUNX1 were detected in the HPT while no signal of RUNX1 was observed in hemocytes, and GATA1 was detected in both HPT and some hemocytes. The mRNA transcript of a novel hematopoiesis related cytokine EsAst was detected in hepatopancreas and hemocytes, but it was no detectable in HPT. The mRNA expression level of EsAst in hepatopancreas was 1.38-fold higher than that in hemocytes. Total hemocytes counts were related to the mRNA expression level of EsAst post Aeromonas hydrophila challenge. The results suggested that the stem cells in the hematopoietic tissue of Chinese mitten crab E. sinensis were regulated by transcriptional and humoral factors to generate hemocytes.

Lack of Phenotypical and Morphological Evidences of Endothelial to Hematopoietic Transition in the Murine Embryonic Head during Hematopoietic Stem Cell Emergence

During mouse ontogeny, hematopoietic cells arise from specialized endothelial cells, i.e., the hemogenic endothelium, and form clusters in the lumen of arterial vessels. Hemogenic endothelial cells have been observed in several embryonic tissues, such as the dorsal aorta, the placenta and the yolk sac. Recent work suggests that the mouse embryonic head also produces hematopoietic stem cells (HSCs)/progenitors. However, a histological basis for HSC generation in the head has not yet been determined because the hematopoietic clusters and hemogenic endothelium in the head region have not been well characterized. In this study, we used whole-mount immunostaining and 3D confocal reconstruction techniques to analyze both c-Kit⁺ hematopoietic clusters and Runx1⁺ hemogenic endothelium in the whole-head vasculature. The number of c-Kit⁺ hematopoietic cells was 20-fold less in the head arteries than in the dorsal aorta. In addition, apparent nascent hematopoietic cells, which are characterized by a “budding” structure and a Runx1⁺ hemogenic endothelium, were not observed in the head. These results suggest that head HSCs may not be or are rarely generated from the endothelium in the same manner as aortic HSCs.

Extravascular endothelial and hematopoietic islands form through multiple pathways in midgestation mouse embryos

The de novo generation of hematopoietic cells occurs during midgestation when a population of endothelial cells called hemogenic endothelium transitions into hematopoietic progenitors and stem cells. In mammalian embryos, the newly formed hematopoietic cells form clusters in the lumens of the major arteries in the embryo proper and in the vascular plexus of the yolk sac. Small clusters of hematopoietic cells that are independent of the vasculature (referred to here as extravascular islands) were shown to form in the mesentery during vascular remodeling of the vitelline artery. Using three-dimensional imaging of whole mouse embryos we demonstrate that extravascular budding of hematopoietic clusters is a more widespread phenomenon that occurs from the vitelline and the umbilical arteries both proximal to the embryo proper and distal in the extraembryonic yolk sac and placenta. Furthermore, we show that there are several mechanisms by which hematopoietic clusters leave the arteries, including vascular remodeling and extrusion. Lastly, we provide static images suggesting that extravascular islands contribute to the formation of new blood vessels. Thus, extravascular islands may represent a novel mechanism of vasculogenesis whereby established vessels contribute endothelial and hematopoietic cells to developing vascular beds.

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.

Early hematopoiesis and macrophage development

The paradigm that all blood cells are derived from hematopoietic stem cells (HSCs) has been challenged by two findings. First, there are tissue-resident hematopoietic cells, including subsets of macrophages that are not replenished by adult HSCs, but instead are maintained by self-renewal of fetal-derived cells. Second, during embryogenesis, there is a conserved program of HSC-independent hematopoiesis that precedes HSC function and is required for embryonic survival. The presence of waves of HSC-independent hematopoiesis as well as fetal HSCs raises questions about the origin of fetal-derived adult tissue-resident macrophages. In the murine embryo, historical examination of embryonic macrophage and monocyte populations combined with recent reports utilizing genetic lineage-tracing approaches has led to a model of macrophage ontogeny that can be integrated with existing models of hematopoietic ontogeny. The first wave of hematopoiesis contains primitive erythroid, megakaryocyte and macrophage progenitors that arise in the yolk sac, and these macrophage progenitors are the source of early macrophages throughout the embryo, including the liver. A second wave of multipotential erythro-myeloid progenitors (EMPs) also arises in the yolk sac. EMPs colonize the fetal liver, initiating myelopoiesis and forming macrophages. Lineage tracing indicates that this second wave of macrophages are distributed in most fetal tissues, although not appreciably in the brain. Thus, fetal-derived adult tissue-resident macrophages, other than microglia, appear to predominately derive from EMPs. While HSCs emerge at midgestation and colonize the fetal liver, the relative contribution of fetal HSCs to tissue macrophages at later stages of development is unclear. The inclusion of macrophage potential in multiple waves of hematopoiesis is consistent with reports of their functional roles throughout development in innate immunity, phagocytosis, and tissue morphogenesis and remodeling. Understanding the influences of developmental origin, as well as local tissue-specific signals, will be necessary to fully decode the diverse functions and responses of tissue-resident macrophages.

Preeclampsia and Inflammatory Preterm Labor Alter the Human Placental Hematopoietic Niche

BACKGROUND

The human placenta is a source of hematopoietic stem and progenitor cells (HSPCs). The RUNX1 transcription factor is required for the formation of functional HSPCs. The impact of preeclampsia (PE) and preterm labor (PTL, spontaneous preterm labor [sPTL] and inflammatory preterm labor [iPTL]) on HSPC localization and RUNX1 expression in the human placenta is unknown.

METHODS

We compared the frequency and density of HSPC in control samples from sPTL (n = 6) versus PE (n = 6) and iPTL (n = 6). We examined RUNX1 protein and RNA expression in placentas from normal pregnancies (5-22 weeks, n = 8 total) and in placentas from the aforementioned pregnancy complications (n = 5/group).

RESULTS

Hematopoietic stem and progenitor cells were rare cell types, associated predominantly with the vasculature of placental villi. The HSPC density was greater in the chorionic plate (CP) compared to the villi (P < .001) and greater in PE and iPTL samples as compared to controls within the CP (not significant) and overall (P < .05). During the fetal period, RUNX1 was expressed in the mesenchyme of the CP and villi. Inflammatory PTL samples were more likely to exhibit intraluminal RUNX1(+) cell populations (P < .001) and RUNX1(+) cell clusters attached to arterial endothelial cells.

CONCLUSION

Placental HSPCs likely arise from hematopoietic niches comprised RUNX1(+) mesenchyme and vascular endothelium. Pregnancy complications that result in preterm birth differentially affect placental HSPC localization and RUNX1 expression. Our results support previous findings that inflammation positively regulates hematopoiesis. We present new evidence that hemogenic endothelium may be active at later stages of human fetal development in the context of inflammation.

Transcriptional Auto-Regulation of RUNX1 P1 Promoter

RUNX1 a member of the family of runt related transcription factors (RUNX), is essential for hematopoiesis. The expression of RUNX1 gene is controlled by two promoters; the distal P1 promoter and the proximal P2 promoter. Several isoforms of RUNX1 mRNA are generated through the use of both promoters and alternative splicing. These isoforms not only differs in their temporal expression pattern but also exhibit differences in tissue specificity. The RUNX1 isoforms derived from P2 are expressed in a variety of tissues, but expression of P1-derived isoform is restricted to cells of hematopoietic lineage. However, the control of hematopoietic-cell specific expression is poorly understood. Here we report regulation of P1-derived RUNX1 mRNA by RUNX1 protein. In silico analysis of P1 promoter revealed presence of two evolutionary conserved RUNX motifs, 0.6kb upstream of the transcription start site, and three RUNX motifs within 170bp of the 5'UTR. Transcriptional contribution of these RUNX motifs was studied in myeloid and T-cells. RUNX1 genomic fragment containing all sites show very low basal activity in both cell types. Mutation or deletion of RUNX motifs in the UTR enhances basal activity of the RUNX1 promoter. Chromatin immunoprecipitation revealed that RUNX1 protein is recruited to these sites. Overexpression of RUNX1 in non-hematopoietic cells results in a dose dependent activation of the RUNX1 P1 promoter. We also demonstrate that RUNX1 protein regulates transcription of endogenous RUNX1 mRNA in T-cell. Finally we show that SCL transcription factor is recruited to regions containing RUNX motifs in the promoter and the UTR and regulates activity of the RUNX1 P1 promoter in vitro. Thus, multiple lines of evidence show that RUNX1 protein regulates its own gene transcription.

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.

Definitive Hematopoiesis in the Yolk Sac Emerges from Wnt-Responsive Hemogenic Endothelium Independently of Circulation and Arterial Identity

Adult-repopulating hematopoietic stem cells (HSCs) emerge in low numbers in the midgestation mouse embryo from a subset of arterial endothelium, through an endothelial-to-hematopoietic transition. HSC-producing arterial hemogenic endothelium relies on the establishment of embryonic blood flow and arterial identity, and requires β-catenin signaling. Specified prior to and during the formation of these initial HSCs are thousands of yolk sac-derived erythro-myeloid progenitors (EMPs). EMPs ensure embryonic survival prior to the establishment of a permanent hematopoietic system, and provide subsets of long-lived tissue macrophages. While an endothelial origin for these HSC-independent definitive progenitors is also accepted, the spatial location and temporal output of yolk sac hemogenic endothelium over developmental time remain undefined. We performed a spatiotemporal analysis of EMP emergence, and document the morphological steps of the endothelial-to-hematopoietic transition. Emergence of rounded EMPs from polygonal clusters of Kit(+) cells initiates prior to the establishment of arborized arterial and venous vasculature in the yolk sac. Interestingly, Kit(+) polygonal clusters are detected in both arterial and venous vessels after remodeling. To determine whether there are similar mechanisms regulating the specification of EMPs with other angiogenic signals regulating adult-repopulating HSCs, we investigated the role of embryonic blood flow and Wnt/β-catenin signaling during EMP emergence. In embryos lacking a functional circulation, rounded Kit(+) EMPs still fully emerge from unremodeled yolk sac vasculature. In contrast, canonical Wnt signaling appears to be a common mechanism regulating hematopoietic emergence from hemogenic endothelium. These data illustrate the heterogeneity in hematopoietic output and spatiotemporal regulation of primary embryonic hemogenic endothelium.

Development of hematopoietic stem and progenitor cells from human pluripotent stem cells

Human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), provide a new cell source for regenerative medicine, disease modeling, drug discovery, and preclinical toxicity screening. Understanding of the onset and the sequential process of hematopoietic cells from differentiated hPSCs will enable the achievement of personalized medicine and provide an in vitro platform for studying of human hematopoietic development and disease. During embryogenesis, hemogenic endothelial cells, a specified subset of endothelial cells in embryonic endothelium, are the primary source of multipotent hematopoietic stem cells. In this review, we discuss current status in the generation of multipotent hematopoietic stem and progenitor cells from hPSCs via hemogenic endothelial cells. We also review the achievements in direct reprogramming from non-hematopoietic cells to hematopoietic stem and progenitor cells. Further characterization of hematopoietic differentiation in hPSCs will improve our understanding of blood development and expedite the development of hPSC-derived blood products for therapeutic purpose.

GFI1 proteins orchestrate the emergence of haematopoietic stem cells through recruitment of LSD1

In vertebrates, the first haematopoietic stem cells (HSCs) with multi-lineage and long-term repopulating potential arise in the AGM (aorta-gonad-mesonephros) region. These HSCs are generated from a rare and transient subset of endothelial cells, called haemogenic endothelium (HE), through an endothelial-to-haematopoietic transition (EHT). Here, we establish the absolute requirement of the transcriptional repressors GFI1 and GFI1B (growth factor independence 1 and 1B) in this unique trans-differentiation process. We first demonstrate that Gfi1 expression specifically defines the rare population of HE that generates emerging HSCs. We further establish that in the absence of GFI1 proteins, HSCs and haematopoietic progenitor cells are not produced in the AGM, revealing the critical requirement for GFI1 proteins in intra-embryonic EHT. Finally, we demonstrate that GFI1 proteins recruit the chromatin-modifying protein LSD1, a member of the CoREST repressive complex, to epigenetically silence the endothelial program in HE and allow the emergence of blood cells.

The RUNX complex: reaching beyond haematopoiesis into immunity

Among their diverse roles as transcriptional regulators during development and cell fate specification, the RUNX transcription factors are best known for the parts they play in haematopoiesis. RUNX proteins are expressed throughout all haematopoietic lineages, being necessary for the emergence of the first haematopoietic stem cells to their terminal differentiation. Although much progress has been made since their discoveries almost two decades ago, current appreciation of RUNX in haematopoiesis is largely grounded in their lineage-specifying roles. In contrast, the importance of RUNX to immunity has been mostly obscured for historic, technical and conceptual reasons. However, this paradigm is likely to shift over time, as a primary purpose of haematopoiesis is to resource the immune system. Furthermore, recent evidence suggests a role for RUNX in the innate immunity of non-haematopoietic cells. This review takes a haematopoiesis-centric approach to collate what is known of RUNX's contribution to the overall mammalian immune system and discuss their growing prominence in areas such as autoimmunity, inflammatory diseases and mucosal immunity.

Single-cell resolution of morphological changes in hemogenic endothelium

Endothelial-to-hematopoietic transition (EHT) occurs within a population of hemogenic endothelial cells during embryogenesis, and leads to the formation of the adult hematopoietic system. Currently, the prospective identification of specific endothelial cells that will undergo EHT, and the cellular events enabling this transition, are not known. We set out to define precisely the morphological events of EHT, and to correlate cellular morphology with the expression of the transcription factors RUNX1 and SOX17. A novel strategy was developed to allow for correlation of immunofluorescence data with the ultrastructural resolution of scanning electron microscopy. The approach can identify single endothelial cells undergoing EHT, as identified by the ratio of RUNX1 to SOX17 immunofluorescence levels, and the morphological changes associated with the transition. Furthermore, this work details a new technical resource that is widely applicable for correlative analyses of single cells in their native tissue environments.

Hematopoietic progenitors are required for proper development of coronary vasculature

RATIONALE

During embryogenesis, hematopoietic cells appear in the myocardium prior to the initiation of coronary formation. However, their role is unknown.

OBJECTIVE

Here we investigate whether pre-existing hematopoietic cells are required for the formation of coronary vasculature.

METHODS AND RESULTS

As a model of for hematopoietic cell deficient animals, we used Runx1 knockout embryos and Vav1-cre; R26-DTA embryos, latter of which genetically ablates 2/3 of CD45(+) hematopoietic cells. Both Runx1 knockout embryos and Vav1-cre; R26-DTA embryos revealed disorganized, hypoplastic microvasculature of coronary vessels on section and whole-mount stainings. Furthermore, coronary explant experiments showed that the mouse heart explants from Runx1 and Vav1-cre; R26-DTA embryos exhibited impaired coronary formation ex vivo. Interestingly, in both models it appears that epicardial to mesenchymal transition is adversely affected in the absence of hematopoietic progenitors.

CONCLUSION

Hematopoietic cells are not merely passively transported via coronary vessel, but substantially involved in the induction of the coronary growth. Our findings suggest a novel mechanism of coronary growth.

Repression of arterial genes in hemogenic endothelium is sufficient for haematopoietic fate acquisition

Changes in cell fate and identity are essential for endothelial-to-haematopoietic transition (EHT), an embryonic process that generates the first adult populations of haematopoietic stem cells (HSCs) from hemogenic endothelial cells. Dissecting EHT regulation is a critical step towards the production of in vitro derived HSCs. Yet, we do not know how distinct endothelial and haematopoietic fates are parsed during the transition. Here we show that genes required for arterial identity function later to repress haematopoietic fate. Tissue-specific, temporally controlled, genetic loss of arterial genes (Sox17 and Notch1) during EHT results in increased production of haematopoietic cells due to loss of Sox17-mediated repression of haematopoietic transcription factors (Runx1 and Gata2). However, the increase in EHT can be abrogated by increased Notch signalling. These findings demonstrate that the endothelial haematopoietic fate switch is actively repressed in a population of endothelial cells, and that derepression of these programs augments haematopoietic output.

The first haematopoietic stem and progenitor cells arise from the hemogenic endothelium of arterial vascular beds. Here the authors describe the mechanism that regulates the endothelial-to-haematopoietic transition and show that Sox17 and Notch1, genes critical to arterial endothelium identity, are also crucial repressors of haematopoietic fate.

MicroRNA-9 Regulates the Differentiation and Function of Myeloid-Derived Suppressor Cells via Targeting Runx1

Myeloid-derived suppressor cells (MDSCs) play a critical role in tumor-associated immunosuppression, thus affecting effective immunotherapies for cancers. However, the molecular mechanisms involved in regulating the differentiation and function of MDSCs remain largely unclear. In this study, we found that inhibition of microRNA (miR)-9 promoted the differentiation of MDSCs with significantly reduced immunosuppressive function whereas overexpression of miR-9 markedly enhanced the function of MDSCs. Notably, knockdown of miR-9 significantly impaired the activity of MDSCs and inhibited the tumor growth of Lewis lung carcinoma in mice. Moreover, miR-9 regulated MDSCs differentiation by targeting the runt-related transcription factor 1, an essential transcription factor in regulating MDSC differentiation and function. Furthermore, the CREB was found to regulate miR-9 expression in MDSCs. Taken together, our findings have identified a critical role of miR-9 in regulating the differentiation and function of MDSCs.

Making Blood: The Haematopoietic Niche throughout Ontogeny

Approximately one-quarter of all cells in the adult human body are blood cells. The haematopoietic system is therefore massive in scale and requires exquisite regulation to be maintained under homeostatic conditions. It must also be able to respond when needed, such as during infection or following blood loss, to produce more blood cells. Supporting cells serve to maintain haematopoietic stem and progenitor cells during homeostatic and pathological conditions. This coalition of supportive cell types, organised in specific tissues, is termed the haematopoietic niche. Haematopoietic stem and progenitor cells are generated in a number of distinct locations during mammalian embryogenesis. These stem and progenitor cells migrate to a variety of anatomical locations through the conceptus until finally homing to the bone marrow shortly before birth. Under stress, extramedullary haematopoiesis can take place in regions that are typically lacking in blood-producing activity. Our aim in this review is to examine blood production throughout the embryo and adult, under normal and pathological conditions, to identify commonalities and distinctions between each niche. A clearer understanding of the mechanism underlying each haematopoietic niche can be applied to improving ex vivo cultures of haematopoietic stem cells and potentially lead to new directions for transplantation medicine.

Loss of RUNX1/AML1 arginine-methylation impairs peripheral T cell homeostasis

RUNX1 (previously termed AML1) is a frequent target of human leukaemia-associated gene aberrations, and it encodes the DNA-binding subunit of the Core-Binding Factor transcription factor complex. RUNX1 expression is essential for the initiation of definitive haematopoiesis, for steady-state thrombopoiesis, and for normal lymphocytes development. Recent studies revealed that protein arginine methyltransferase 1 (PRMT1), which accounts for the majority of the type I PRMT activity in cells, methylates two arginine residues in RUNX1 (R206 and R210), and these modifications inhibit corepressor-binding to RUNX1 thereby enhancing its transcriptional activity. In order to elucidate the biological significance of these methylations, we established novel knock-in mouse lines with non-methylable, double arginine-to-lysine (RTAMR-to-KTAMK) mutations in RUNX1. Homozygous Runx1(KTAMK) (/) (KTAMK) mice are born alive and appear normal during adulthood. However, Runx1(KTAMK) (/) (KTAMK) mice showed a reduction in CD3(+) T lymphoid cells and a decrease in CD4(+) T cells in peripheral lymphoid organs, in comparison to their wild-type littermates, leading to a reduction in the CD4(+) to CD8(+) T-cell ratio. These findings suggest that arginine-methylation of RUNX1 in the RTAMR-motif is dispensable for the development of definitive haematopoiesis and for steady-state platelet production, however this modification affects the role of RUNX1 in the maintenance of the peripheral CD4(+) T-cell population.

Cannabinoid Receptor-2 Regulates Embryonic Hematopoietic Stem Cell Development via Prostaglandin E2 and P-Selectin Activity

Cannabinoids (CB) modulate adult hematopoietic stem and progenitor cell (HSPCs) function, however, impact on the production, expansion, or migration of embryonic HSCs is currently uncharacterized. Here, using chemical and genetic approaches targeting CB-signaling in zebrafish, we show that CB receptor (CNR) 2, but not CNR1, regulates embryonic HSC development. During HSC specification in the aorta-gonad-mesonephros (AGM) region, CNR2 stimulation by AM1241 increased runx1;cmyb(+) HSPCs, through heightened proliferation, whereas CNR2 antagonism decreased HSPC number; FACS analysis and absolute HSC counts confirmed and quantified these effects. Epistatic investigations showed AM1241 significantly upregulated PGE2 synthesis in a Ptgs2-dependent manner to increase AGM HSCs. During the phases of HSC production and colonization of secondary niches, AM1241 accelerated migration to the caudal hematopoietic tissue (CHT), the site of embryonic HSC expansion, and the thymus; however these effects occurred independently of PGE2. Using a candidate approach for HSC migration and retention factors, P-selectin was identified as the functional target of CNR2 regulation. Epistatic analyses confirmed migration of HSCs into the CHT and thymus was dependent on CNR2-regulated P-selectin activity. Together, these data suggest CNR2-signaling optimizes the production, expansion, and migration of embryonic HSCs by modulating multiple downstream signaling pathways.

Endothelium and NOTCH specify and amplify aorta-gonad-mesonephros-derived hematopoietic stem cells

Hematopoietic stem cells (HSCs) first emerge during embryonic development within vessels such as the dorsal aorta of the aorta-gonad-mesonephros (AGM) region, suggesting that signals from the vascular microenvironment are critical for HSC development. Here, we demonstrated that AGM-derived endothelial cells (ECs) engineered to constitutively express AKT (AGM AKT-ECs) can provide an in vitro niche that recapitulates embryonic HSC specification and amplification. Specifically, nonengrafting embryonic precursors, including the VE-cadherin-expressing population that lacks hematopoietic surface markers, cocultured with AGM AKT-ECs specified into long-term, adult-engrafting HSCs, establishing that a vascular niche is sufficient to induce the endothelial-to-HSC transition in vitro. Subsequent to hematopoietic induction, coculture with AGM AKT-ECs also substantially increased the numbers of HSCs derived from VE-cadherin⁺CD45⁺ AGM hematopoietic cells, consistent with a role in supporting further HSC maturation and self-renewal. We also identified conditions that included NOTCH activation with an immobilized NOTCH ligand that were sufficient to amplify AGM-derived HSCs following their specification in the absence of AGM AKT-ECs. Together, these studies begin to define the critical niche components and resident signals required for HSC induction and self-renewal ex vivo, and thus provide insight for development of defined in vitro systems targeted toward HSC generation for therapeutic applications.

Gata2b is a restricted early regulator of hemogenic endothelium in the zebrafish embryo

The adult blood system is established by hematopoietic stem cells (HSCs), which arise during development from an endothelial-to-hematopoietic transition of cells comprising the floor of the dorsal aorta. Expression of aortic runx1 has served as an early marker of HSC commitment in the zebrafish embryo, but recent studies have suggested that HSC specification begins during the convergence of posterior lateral plate mesoderm (PLM), well before aorta formation and runx1 transcription. Further understanding of the earliest stages of HSC specification necessitates an earlier marker of hemogenic endothelium. Studies in mice have suggested that GATA2 might function at early stages within hemogenic endothelium. Two orthologs of Gata2 exist in zebrafish: gata2a and gata2b. Here, we report that gata2b expression initiates during the convergence of PLM, becoming restricted to emerging HSCs. We observe Notch-dependent gata2b expression within the hemogenic subcompartment of the dorsal aorta that is in turn required to initiate runx1 expression. Our results indicate that Gata2b functions within hemogenic endothelium from an early stage, whereas Gata2a functions more broadly throughout the vascular system.

Runx1 is required for hematopoietic defects and leukemogenesis in Cbfb-MYH11 knock-in mice

CBFβ-SMMHC, the fusion protein generated by the chromosome 16 inversion fusion gene, CBFB-MYH11, is known to initiate leukemogenesis. However, the mechanism through which CBFβ-SMMHC contributes to leukemia development is not well understood. Previously it was proposed that CBFβ-SMMHC acts by dominantly repressing the transcription factor RUNX1, but we recently showed that CBFβ-SMMHC has activities that are independent of RUNX1 repression. In addition, we showed that a modified CBFβ-SMMHC with decreased RUNX1 binding activity accelerates leukemogenesis. These results raise questions about the importance of RUNX1 in leukemogenesis by CBFβ-SMMHC. To test this, we generated mice expressing Cbfb-MYH11 in a Runx1 deficient background, resulting from either homozygous Runx1 null alleles (Runx1^−/−) or a single dominant negative Runx1 allele (Runx1^+/lz). We found that loss of Runx1 activity rescued the differentiation defects induced by Cbfb-MYH11 during primitive hematopoiesis. During definitive hematopoiesis, RUNX1 loss also significantly reduced the proliferation and differentiation defects induced by Cbfb-MYH11. Importantly, Cbfb-MYH11 induced leukemia had much longer latency in Runx1^+/lz mice than in Runx1 sufficient mice. These data indicate that Runx1 activity is critical for Cbfb-MYH11 induced hematopoietic defects and leukemogenesis.

Decoding the regulatory network of early blood development from single-cell gene expression measurements

Reconstruction of the molecular pathways controlling organ development has been hampered by a lack of methods to resolve embryonic progenitor cells. Here we describe a strategy to address this problem that combines gene expression profiling of large numbers of single cells with data analysis based on diffusion maps for dimensionality reduction and network synthesis from state transition graphs. Applying the approach to hematopoietic development in the mouse embryo, we map the progression of mesoderm toward blood using single-cell gene expression analysis of 3,934 cells with blood-forming potential captured at four time points between E7.0 and E8.5. Transitions between individual cellular states are then used as input to develop a single-cell network synthesis toolkit to generate a computationally executable transcriptional regulatory network model of blood development. Several model predictions concerning the roles of Sox and Hox factors are validated experimentally. Our results demonstrate that single-cell analysis of a developing organ coupled with computational approaches can reveal the transcriptional programs that underpin organogenesis.

The first wave of B lymphopoiesis develops independently of stem cells in the murine embryo

In the developing mouse embryo, there are several waves of hematopoiesis. Primitive and definitive erythromyeloid lineages appear prior to hematopoietic stem cell (HSC) emergence, and these waves are considered to be transient and support embryonic homeostasis until HSC-derived hematopoiesis is established. However, recent evidence strongly suggests that HSC-independent immune cells, such as tissue macrophages and some innate lymphoid cells, develop in the mouse embryo and persist into postnatal life. Innate type B-1 cells have also been reported to emerge from hemogenic endothelial cells in the extraembryonic yolk sac and para-aortic splanchnopleura, and continue to develop in the fetal liver, even in HSC-deficient mouse embryos. Here, this review discusses B-1 cell development in the context of the layered immune system hypothesis of B lymphopoiesis and the emergence of B-1 cells independent of HSCs.

Characterization of putative haematopoietic cells from bovine yolk sac

The yolk sac is an extra-embryonic membrane that plays an important role in early embryonic survival. It is the production site for blood cells during embryonic mammalian development and is a likely source of stem cells. The aim of this study was to identify and characterize the putative haematopoietic cells from the yolk sac of bovine embryos at different stages of gestation. The yolk sac regresses according to gestational age and embryos are characterized into groups (I-V) according to the crown-rump measurement. Groups I-III survived in culture longer and exhibited the formation of cell clusters, whereas groups IV and V could not be maintained in culture for an extended period of time. Flow-cytometry analysis revealed that groups I-III had similar characteristics, including high expression levels of the haematopoietic markers CD34, CD90 and CD117. In groups IV and V, decreases were observed in the expression levels of CD117 and CD34. Cells were found to be capable of survival post-cryopreservation and exhibited varying abilities to form colonies in a methylcellulose matrix, depending on gestational age. Cytological analysis revealed the presence of blood cells (lymphocytes and monocytes). Quantitative PCR analysis demonstrated the presence of the haematopoietic progenitor genes GATA3 and LMO2, but not RUNX1. Thus, we have successfully isolated and characterized haematopoietic cells from the bovine embryo yolk sac at varying gestational ages. This study is crucial for the understanding of the development of the haematopoietic system and the embryonic function of this organ. Copyright © 2015 John Wiley & Sons, Ltd.

Scl binds to primed enhancers in mesoderm to regulate hematopoietic and cardiac fate divergence

Scl/Tal1 confers hemogenic competence and prevents ectopic cardiomyogenesis in embryonic endothelium by unknown mechanisms. We discovered that Scl binds to hematopoietic and cardiac enhancers that become epigenetically primed in multipotent cardiovascular mesoderm, to regulate the divergence of hematopoietic and cardiac lineages. Scl does not act as a pioneer factor but rather exploits a pre-established epigenetic landscape. As the blood lineage emerges, Scl binding and active epigenetic modifications are sustained in hematopoietic enhancers, whereas cardiac enhancers are decommissioned by removal of active epigenetic marks. Our data suggest that, rather than recruiting corepressors to enhancers, Scl prevents ectopic cardiogenesis by occupying enhancers that cardiac factors, such as Gata4 and Hand1, use for gene activation. Although hematopoietic Gata factors bind with Scl to both activated and repressed genes, they are dispensable for cardiac repression, but necessary for activating genes that enable hematopoietic stem/progenitor cell development. These results suggest that a unique subset of enhancers in lineage-specific genes that are accessible for regulators of opposing fates during the time of the fate decision provide a platform where the divergence of mutually exclusive fates is orchestrated.

Whole-transcriptome analysis of endothelial to hematopoietic stem cell transition reveals a requirement for Gpr56 in HSC generation

Using highly sensitive RNAseq to examine the whole transcriptome of enriched aortic hematopoietic stem cells and endothelial cells, the authors find G-protein–coupled receptor, Gpr56, is required to generate the first HSCs during endothelial to hematopoietic cell transition.

Hematopoietic stem cells (HSCs) are generated via a natural transdifferentiation process known as endothelial to hematopoietic cell transition (EHT). Because of small numbers of embryonal arterial cells undergoing EHT and the paucity of markers to enrich for hemogenic endothelial cells (ECs [HECs]), the genetic program driving HSC emergence is largely unknown. Here, we use a highly sensitive RNAseq method to examine the whole transcriptome of small numbers of enriched aortic HSCs, HECs, and ECs. Gpr56, a G-coupled protein receptor, is one of the most highly up-regulated of the 530 differentially expressed genes. Also, highly up-regulated are hematopoietic transcription factors, including the “heptad” complex of factors. We show that Gpr56 (mouse and human) is a target of the heptad complex and is required for hematopoietic cluster formation during EHT. Our results identify the processes and regulators involved in EHT and reveal the surprising requirement for Gpr56 in generating the first HSCs.