Hematopoietic cells in cultures of the murine embryonic aorta-gonad-mesonephros region are induced by c-Myb

Macrophage Development and Function

Macrophages were first described over a hundred years ago. Throughout the years, they were shown to be essential players in their tissue-specific environment, performing various functions during homeostatic and disease conditions. Recent reports shed more light on their ontogeny as long-lived, self-maintained cells with embryonic origin in most tissues. They populate the different tissues early during development, where they help to establish and maintain homeostasis. In this chapter, the history of macrophages is discussed. Furthermore, macrophage ontogeny and core functions in the different tissues are described.

Mast cell ontogeny: From fetal development to life-long health and disease

Mast cells (MCs) are evolutionarily ancient innate immune cells with important roles in protective immunity against bacteria, parasites, and venomous animals. They can be found in most organs of the body, where they also contribute to normal tissue functioning, for example by engaging in crosstalk with nerves. Despite this, they are most widely known for their detrimental roles in allergy, anaphylaxis, and atopic disease. Just like macrophages, mast cells were conventionally thought to originate from the bone marrow. However, they are already present in fetal tissues before the onset of bone marrow hematopoiesis, questioning this dogma. In recent years, our view of myeloid cell ontogeny has been revised. We now know that the first mast cells originate from progenitors made in the extra-embryonic yolk sac, and later get supplemented with mast cells produced from subsequent waves of hematopoiesis. In most connective tissues, sizeable populations of fetal-derived mast cells persist into adulthood, where they self-maintain largely independently from the bone marrow. These developmental origins are highly reminiscent of macrophages, which are known to have critical functions in development. Mast cells too may thus support healthy development. Their fetal origins and longevity also make mast cells susceptible to genetic and environmental perturbations, which may render them pathological. Here, we review our current understanding of mast cell biology from a developmental perspective. We first summarize how mast cell populations are established from distinct hematopoietic progenitor waves, and how they are subsequently maintained throughout life. We then discuss what functions mast cells may normally have at early life stages, and how they may be co-opted to cause, worsen, or increase susceptibility to disease.

Defining the Hoxb8 cell lineage during murine definitive hematopoiesis

Previously, we have demonstrated that a subpopulation of microglia, known as Hoxb8 microglia, is derived from the Hoxb8 lineage during the second wave (E8.5) of yolk sac hematopoiesis, whereas canonical non-Hoxb8 microglia arise from the first wave (E7.5). Hoxb8 microglia have an ontogeny distinct from non-Hoxb8 microglia. Dysfunctional Hoxb8 microglia cause the acquisition of chronic anxiety and an obsessive-compulsive spectrum-like behavior, trichotillomania, in mice. The nature and fate of the progenitors generated during E8.5 yolk sac hematopoiesis have been controversial. Herein, we use the Hoxb8 cell lineage reporter to define the ontogeny of hematopoietic cells arising during the definitive waves of hematopoiesis initiated in the E8.5 yolk sac and aorta-gonad-mesonephros (AGM) region. Our murine cell lineage analysis shows that the Hoxb8 cell lineage reporter robustly marks erythromyeloid progenitors, hematopoietic stem cells and their progeny, particularly monocytes. Hoxb8 progenitors and microglia require Myb function, a hallmark transcription factor for definitive hematopoiesis, for propagation and maturation. During adulthood, all immune lineages and, interestingly, resident macrophages in only hematopoietic/lymphoid tissues are derived from Hoxb8 precursors. These results illustrate that the Hoxb8 lineage exclusively mirrors murine definitive hematopoiesis.

Epigenetics and tissue immunity-Translating environmental cues into functional adaptations

There is an increasing appreciation that many innate and adaptive immune cell subsets permanently reside within non-lymphoid organs, playing a critical role in tissue homeostasis and defense. The best characterized are macrophages and tissue-resident T lymphocytes that work in concert with organ structural cells to generate appropriate immune responses and are functionally shaped by organ-specific environmental cues. The interaction of tissue epithelial, endothelial and stromal cells is also required to attract, differentiate, polarize and maintain organ immune cells in their tissue niche. All of these processes require dynamic regulation of cellular transcriptional programmes, with epigenetic mechanisms playing a critical role, including DNA methylation and post-translational histone modifications. A failure to appropriately regulate immune cell transcription inevitably results in inadequate or inappropriate immune responses and organ pathology. Here, with a focus on the mammalian kidney, an organ which generates differing regional environmental cues (including hypersalinity and hypoxia) due to its physiological functions, we will review the basic concepts of tissue immunity, discuss the technologies available to profile epigenetic modifications in tissue immune cells, including those that enable single-cell profiling, and consider how these mechanisms influence the development, phenotype, activation and function of different tissue immune cell subsets, as well as the immunological function of structural cells.

Megakaryocyte production is sustained by direct differentiation from erythromyeloid progenitors in the yolk sac until midgestation

The extra-embryonic yolk sac contains the first definitive multipotent hematopoietic cells, denominated erythromyeloid progenitors. They originate in situ prior to the emergence of hematopoietic stem cells and give rise to erythroid, monocytes, granulocytes, mast cells and macrophages, the latter in a Myb transcription factor-independent manner. We uncovered here the heterogeneity of yolk sac erythromyeloid progenitors, at the single cell level, and discriminated multipotent from committed progenitors, prior to fetal liver colonization. We identified two temporally distinct megakaryocyte differentiation pathways. The first occurs in the yolk sac, bypasses intermediate bipotent megakaryocyte-erythroid progenitors and, similar to the differentiation of macrophages, is Myb-independent. By contrast, the second originates later, from Myb-dependent bipotent progenitors expressing Csf2rb and colonize the fetal liver, where they give rise to megakaryocytes and to large numbers of erythrocytes. Understanding megakaryocyte development is crucial as they play key functions during vascular development, in particular in separating blood and lymphatic networks.

Delineating the origins, developmental programs and homeostatic functions of tissue-resident macrophages

A literature covering 150 years of research indicates that macrophages are a diverse family of professional phagocytes that continuously explore their environment, recognize and scavenge pathogens, unfit cells, cell debris as well as metabolites, and produce a large range of bioactive molecules and growth factors. A new paradigm suggests that most tissue-resident macrophages originate from fetal precursors that colonize developing organs and self-maintain independently of bone marrow-derived cells throughout life. The differentiation of these precursors is driven by a core macrophage transcriptional program and immediately followed by their specification through expression of tissue-specific transcriptional regulators early during embryogenesis. Despite our increasing understanding of ontogeny and genetic programs that shape differentiation processes and functions of macrophages, the precise developmental trajectories of tissue-resident macrophages remain undefined. Here, I review current models of fetal hematopoietic waves, possible routes of macrophage development and their roles during homeostasis. Further, transgenic mouse models are discussed providing a toolset to study the developmentally and functionally distinct arms of the phagocyte system in vivo.

[The role of PDK1 in the transition of endothelial to hematopoietic cells]

目的

研究磷酸肌醇依赖性激酶1（PDK1）在内皮细胞向造血细胞转化阶段对造血干细胞（HSC）发生的影响。

方法

应用Vec-Cre在内皮细胞中特异性敲除PDK1基因，取对照组PDK1^fl/fl、PDK1^fl/+小鼠及敲除组Vec-Cre；PDK1^fl/fl小鼠胚胎的主动脉-性腺-中肾区（AGM区）细胞进行集落形成实验，检测PDK1基因对造血祖细胞功能的影响；取对照组和敲除组AGM区细胞行移植实验，检测PDK1对HSC功能的影响；取对照组和敲除组AGM区细胞，通过流式细胞术检测PDK1对能够向造血转化的CD31⁺c-Kit^high细胞群比例、细胞周期及细胞凋亡的影响；分选对照组和敲除组AGM区CD31⁺c-Kit^high细胞群，通过Real-time PCR检测PDK1对内皮向造血转换相关的转录因子（RUNX1、P2-RUNX1、GATA2）的影响。

结果

PDK1敲除后，造血祖细胞形成的克隆形态变小，数目减少[敲除组CFU-GM为（24±5）个/ee，对照组为（62±1）个/ee，P=0.001]；破坏了造血干细胞重建造血及多向分化的能力（敲除组移植5只，0只重建，对照组移植7只，5只重建，P=0.001）；AGM区CD31⁺c-Kit^high比例降低[敲除组CD31⁺c-Kit^high比例为（0.145±0.017）%，对照组比例为（0.385±0.04）%，P=0.001]；并且AGM区由内皮细胞向造血细胞转换的关键转录因子表达下降，但对CD31⁺c-Kit^high细胞的增殖和凋亡无明显影响。

结论

在内皮细胞中特异敲除PDK1基因，导致具有向造血转化的内皮细胞群比例降低，影响了HSC的发生，破坏了HSC重建造血的能力。

Adult zebrafish Langerhans cells arise from hematopoietic stem/progenitor cells

The origin of Langerhans cells (LCs), which are skin epidermis-resident macrophages, remains unclear. Current lineage tracing of LCs largely relies on the promoter-Cre-LoxP system, which often gives rise to contradictory conclusions with different promoters. Thus, reinvestigation with an improved tracing method is necessary. Here, using a laser-mediated temporal-spatial resolved cell labeling method, we demonstrated that most adult LCs originated from the ventral wall of the dorsal aorta (VDA), an equivalent to the mouse aorta, gonads, and mesonephros (AGM), where both hematopoietic stem cells (HSCs) and non-HSC progenitors are generated. Further fine-fate mapping analysis revealed that the appearance of LCs in adult zebrafish was correlated with the development of HSCs, but not T cell progenitors. Finally, we showed that the appearance of tissue-resident macrophages in the brain, liver, heart, and gut of adult zebrafish was also correlated with HSCs. Thus, the results of our study challenged the EMP-origin theory for LCs.

Zebrafish In-Vivo Screening for Compounds Amplifying Hematopoietic Stem and Progenitor Cells: - Preclinical Validation in Human CD34+ Stem and Progenitor Cells

The identification of small molecules that either increase the number and/or enhance the activity of human hematopoietic stem and progenitor cells (hHSPCs) during ex vivo expansion remains challenging. We used an unbiased in vivo chemical screen in a transgenic (c-myb:EGFP) zebrafish embryo model and identified histone deacetylase inhibitors (HDACIs), particularly valproic acid (VPA), as significant enhancers of the number of phenotypic HSPCs, both in vivo and during ex vivo expansion. The long-term functionality of these expanded hHSPCs was verified in a xenotransplantation model with NSG mice. Interestingly, VPA increased CD34⁺ cell adhesion to primary mesenchymal stromal cells and reduced their in vitro chemokine-mediated migration capacity. In line with this, VPA-treated human CD34⁺ cells showed reduced homing and early engraftment in a xenograft transplant model, but retained their long-term engraftment potential in vivo, and maintained their differentiation ability both in vitro and in vivo. In summary, our data demonstrate that certain HDACIs lead to a net expansion of hHSPCs with retained long-term engraftment potential and could be further explored as candidate compounds to amplify ex-vivo engineered peripheral blood stem cells.

Human Induced Pluripotent Stem Cell-Derived Macrophages Share Ontogeny with MYB-Independent Tissue-Resident Macrophages

Tissue-resident macrophages, such as microglia, Kupffer cells, and Langerhans cells, derive from Myb-independent yolk sac (YS) progenitors generated before the emergence of hematopoietic stem cells (HSCs). Myb-independent YS-derived resident macrophages self-renew locally, independently of circulating monocytes and HSCs. In contrast, adult blood monocytes, as well as infiltrating, gut, and dermal macrophages, derive from Myb-dependent HSCs. These findings are derived from the mouse, using gene knockouts and lineage tracing, but their applicability to human development has not been formally demonstrated. Here, we use human induced pluripotent stem cells (iPSCs) as a tool to model human hematopoietic development. By using a CRISPR-Cas9 knockout strategy, we show that human iPSC-derived monocytes/macrophages develop in an MYB-independent, RUNX1-, and SPI1 (PU.1)-dependent fashion. This result makes human iPSC-derived macrophages developmentally related to and a good model for MYB-independent tissue-resident macrophages, such as alveolar and kidney macrophages, microglia, Kupffer cells, and Langerhans cells.

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Human iPSC-derived macrophages are MYB independent but RUNX1 and SPI1 dependent

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Transcription factor dependence genetically uncovers ontogeny of hiPSC macrophages

The development and maintenance of resident macrophages

The molecular and cellular mechanisms that underlie the many roles of macrophages in health and disease states in vivo remain poorly understood. The purpose of this Review is to present and discuss current knowledge on the developmental biology of macrophages, as it underlies the concept of a layered myeloid system composed of 'resident' macrophages that originate mainly from progenitor cells generated in the yolk sac and of 'passenger' or 'transitory' myeloid cells that originate and renew from bone marrow hematopoietic stem cells, and to provide a framework for investigating the functions of macrophages in vivo.

Epigenetic regulation of hematopoiesis by DNA methylation

During embryonic development, cell type-specific transcription factors promote cell identities, while epigenetic modifications are thought to contribute to maintain these cell fates. Our understanding of how genetic and epigenetic modes of regulation work together to establish and maintain cellular identity is still limited, however. Here, we show that DNA methyltransferase 3bb.1 (dnmt3bb.1) is essential for maintenance of hematopoietic stem and progenitor cell (HSPC) fate as part of an early Notch-runx1-cmyb HSPC specification pathway in the zebrafish. Dnmt3bb.1 is expressed in HSPC downstream from Notch1 and runx1, and loss of Dnmt3bb.1 activity leads to reduced cmyb locus methylation, reduced cmyb expression, and gradual reduction in HSPCs. Ectopic overexpression of dnmt3bb.1 in non-hematopoietic cells is sufficient to methylate the cmyb locus, promote cmyb expression, and promote hematopoietic development. Our results reveal an epigenetic mechanism supporting the maintenance of hematopoietic cell fate via DNA methylation-mediated perdurance of a key transcription factor in HSPCs.

DOI:http://dx.doi.org/10.7554/eLife.11813.001

The cells in our blood are constantly being replaced with new cells that are produced by stem cells called hematopoietic stem and progenitor cells (or HSPCs for short). The HSPCs form early on in the development of the embryo and continue in the same role throughout the life of the animal.

A gene called runx1 is required for HSPCs to form, but is not required for these cells to maintain their role (cell identity) in the long term. In mice, this gene is only expressed for a brief period of time as the HSPCs form, and is switched off in the mature stem cells. Another gene called cmyb – which is switched on by runx1 – is also required for HSPCs to form. However, unlike runx1, cmyb continues to be expressed in mature HSPCs and is required to maintain HSPC identity. It is not known how the temporary activation of runx1 causes the long-term expression of cmyb.

One possible explanation is that the cmyb gene may be subject to a process called DNA methylation. This process is carried out by enzymes called DNA methyltransferases and can have long-term effects on the expression of genes by modifying the structure of the DNA that encodes them. Here, Gore et al. investigate the role of a particular DNA methyltransferase in the formation of HSPCs in zebrafish embryos. The experiments show that this enzyme is activated in developing HSPCs in response to an increase in runx1 expression. The loss of this enzyme’s activity reduces both the amount that cmyb is methylated and its level of expression, which results in a gradual decline in the number of HSPCs in zebrafish.

Further experiments show that if the DNA methyltransferase is artificially activated in cells that don’t normally form blood cells, these cells change their identity to do so. This switch is accompanied by methylation of cmyb and an increase in its expression. Gore et al.’s findings reveal that the temporary activation of runx1 triggers the production of an enzyme that methylates cmyb to maintain the identity of HSPCs. Future studies should help to reveal exactly how runx1 promotes DNA methylation, and whether this process can be harnessed to promote HSPC formation for research or medical treatments.

DOI:http://dx.doi.org/10.7554/eLife.11813.002

Temporal-Spatial Resolution Fate Mapping Reveals Distinct Origins for Embryonic and Adult Microglia in Zebrafish

Microglia are CNS resident macrophages, and they play important roles in neural development and function. Recent studies have suggested that murine microglia arise from a single source, the yolk sac (YS), yet these studies lack spatial resolution to define the bona fide source(s) for microglia. Here, using light-induced high temporal-spatial resolution fate mapping, we challenge this single-source view by showing that microglia in zebrafish arise from multiple sources. The embryonic/larval microglia originate from the rostral blood island (RBI) region, the equivalent of mouse YS for myelopoiesis, whereas the adult microglia arise from the ventral wall of dorsal aorta (VDA) region, a tissue also producing definitive hematopoiesis in mouse. We further show that the VDA-region-derived microglia are Runx1 dependent, but cMyb independent, and developmentally regulated differently from the RBI region-derived microglia. Our study establishes a new paradigm for investigating the development and function of distinct microglia populations.

Monocyte homeostasis and the plasticity of inflammatory monocytes

Monocytes are mononuclear myeloid cells that develop in the bone marrow and circulate within the bloodstream. Although they have long been argued to play a role in the repopulation of tissue-resident macrophages, this has been questioned by numerous recent studies, which has forced a reappraisal of their biology. Here we discuss monocyte development, as well as the homeostatic control of monocyte subpopulations within the blood. We also outline the known functions of monocyte subsets. Finally, we highlight the plastic nature of monocytes, which are capable of a remarkable range of phenotypic and functional changes that depend on signals from local microenvironments.

Development and homeostasis of "resident" myeloid cells: the case of the microglia

Microglia, macrophages of the central nervous system, play an important role in brain homeostasis. Their origin has been unclear. Recent fate-mapping experiments have established that microglia mostly originate from Myb-independent, FLT3-independent, but PU.1-dependent precursors that express the CSF1-receptor at E8.5 of embryonic development. These precursors are presumably located in the yolk sac (YS) at this time before invading the embryo between E9.5 and E10.5 and colonizing the fetal liver. Indeed, the E14.5 fetal liver contains a large population of Myb-independent YS-derived myeloid cells. This myeloid lineage is distinct from hematopoietic stem cells (HSCs), which require the transcription factor Myb for their development and maintenance. This "yolky" beginning and the independence from conventional HSCs are not unique to microglia. Indeed, several other populations of F4/80-positive macrophages develop also from YS Myb-independent precursors, such as Kupffer cells in the liver, Langerhans cells in the epidermis, and macrophages in the spleen, kidney, pancreas, and lung. Importantly, microglia and the other Myb-independent macrophages persist, at least in part, in adult mice and likely self-renew within their respective tissues of residence, independently of bone marrow HSCs. This suggests the existence of tissue resident macrophage "stem cells" within tissues such as the brain, and opens a new era for the molecular and cellular understanding of myeloid cells responses during acute and chronic inflammation.

A lineage of myeloid cells independent of Myb and hematopoietic stem cells

Macrophages and dendritic cells (DCs) are key components of cellular immunity and are thought to originate and renew from hematopoietic stem cells (HSCs). However, some macrophages develop in the embryo before the appearance of definitive HSCs. We thus reinvestigated macrophage development. We found that the transcription factor Myb was required for development of HSCs and all CD11b(high) monocytes and macrophages, but was dispensable for yolk sac (YS) macrophages and for the development of YS-derived F4/80(bright) macrophages in several tissues, such as liver Kupffer cells, epidermal Langerhans cells, and microglia--cell populations that all can persist in adult mice independently of HSCs. These results define a lineage of tissue macrophages that derive from the YS and are genetically distinct from HSC progeny.

Regulated expression and role of c-Myb in the cardiovascular-directed differentiation of mouse embryonic stem cells

RATIONALE

c-myb null (knockout) embryonic stem cells (ESC) can differentiate into cardiomyocytes but not contractile smooth muscle cells (SMC) in embryoid bodies (EB).

OBJECTIVE

To define the role of c-Myb in SMC differentiation from ESC.

METHODS AND RESULTS

In wild-type (WT) EB, high c-Myb levels on days 0-2 of differentiation undergo ubiquitin-mediated proteosomal degradation on days 2.5-3, resurging on days 4-6, without changing c-myb mRNA levels. Activin-A and bone morphogenetic protein 4-induced cardiovascular progenitors were isolated by FACS for expression of vascular endothelial growth factor receptor (VEGFR)2 and platelet-derived growth factor receptor (PDGFR)α. By day 3.75, hematopoesis-capable VEGFR2+ cells were fewer, whereas cardiomyocyte-directed VEGFR2+/PDGFRα+ cells did not differ in abundance in knockout versus WT EB. Importantly, highest and lowest levels of c-Myb were observed in VEGFR2+ and VEGFR2+/PDGFRα+ cells, respectively. Proteosome inhibitor MG132 and lentiviruses enabling inducible expression or knockdown of c-myb were used to regulate c-Myb in WT and knockout EB. These experiments showed that c-Myb promotes expression of VEGFR2 over PDGFRα, with chromatin immunopreciptation and promoter-reporter assays defining specific c-Myb-responsive binding sites in the VEGFR2 promoter. Next, FACS-sorted VEGFR2+ cells expressed highest and lowest levels of SMC- and fibroblast-specific markers, respectively, at days 7-14 after retinoic acid (RA) as compared with VEGFR2+/PDGFRα+ cells. By contrast, VEGFR2+/PDGFRα+ cells cultured without RA beat spontaneously, like cardiomyocytes between days 7 and 14, and expressed cardiac troponin. Notably, RA was required to more fully differentiate SMC from VEGFR2+ cells and completely blocked differentiation of cardiomyocytes from VEGFR2+/PDGFRα+ cells.

CONCLUSIONS

c-Myb is tightly regulated by proteosomal degradation during cardiovascular-directed differentiation of ESC, expanding early-stage VEGFR2+ progenitors capable of RA-responsive SMC formation.

The vascular origin of hematopoietic cells

More than a century ago, several embryologists described sites of hematopoietic activity in the vascular wall of mid-gestation vertebrate embryos, and postulated the transient existence of a blood generating endothelium during ontogeny. This hypothesis gained significant attention in the 1970s when orthotopic transplantation experiments between quail and chick embryos revealed specific vascular areas as the site of the origin of definitive hematopoiesis. However, the vascular origin of hematopoietic precursors remained elusive and controversial for decades. Only recently, multiple experimental approaches have clearly documented that during vertebrate development definitive hematopoietic precursors arise from a subset of vascular endothelial cells. Interestingly, this differentiation is promoted by the intravascular fluid mechanical forces generated by the establishment of blood flow upon the initiation of heartbeat, and it is therefore connected with cardiovascular development in several critical aspects. In this review we present our current understanding of the relationship between vascular and definitive hematopoietic development through an historical analysis of the scientific evidence produced in this area of investigation.

Hematopoietic stem cell emergence in the conceptus and the role of Runx1

Hematopoietic stem cells (HSCs) are functionally defined as cells that upon transplantation into irradiated or otherwise immunocompromised adult organisms provide long-term reconstitution of the entire hematopoietic system. They emerge in the vertebrate conceptus around midgestation. Genetic studies have identified a number of transcription factors and signaling molecules that act at the onset of hematopoiesis, and have begun to delineate the molecular mechanisms underlying the formation of HSCs. One molecule that has been a particularly useful marker of this developmental event in multiple species is Runx1 (also known as AML1, Pebp2alpha). Runx1 is a sequence-specific DNA-binding protein, that along with its homologues Runx2 and Runx3 and their shared non-DNA binding subunit CBFbeta, constitute a small family of transcription factors called core-binding factors (CBFs). Runx1 is famous for its role in HSC emergence, and notorious for its involvement in leukemia, as chromosomal rearrangements and inactivating mutations in the human RUNX1 gene are some of the most common events in de novo and therapy-related acute myelogenous leukemia, myelodysplastic syndrome and acute lymphocytic leukemia. Here we will review the role of Runx1 in HSC emergence in the mouse conceptus and describe some of the genetic pathways that operate upstream and downstream of this gene. Where relevant, we will include data obtained from other species and embryonic stem (ES) cell differentiation cultures.

Regulation of hematopoietic cell clusters in the placental niche through SCF/Kit signaling in embryonic mouse

Hematopoietic stem cells (HSCs) emerge from and expand in the mouse placenta at mid-gestation. To determine their compartment of origin and define extrinsic signals governing their commitment to this lineage, we identified hematopoietic cell (HC) clusters in mouse placenta, defined as cells expressing the embryonic HSC markers CD31, CD34 and Kit, by immunohistochemistry. HC clusters were first observed in the placenta at 9.5 days post coitum (dpc). To determine their origin, we tagged the allantoic region with CM-DiI at 8.25 dpc, prior to placenta formation, and cultured embryos in a whole embryo culture (WEC) system. CM-DiI-positive HC clusters were observed 42 hours later. To determine how clusters are extrinsically regulated, we isolated niche cells using laser capture micro-dissection and assayed them for expression of genes encoding hematopoietic cytokines. Among a panel of candidates assayed, only stem cell factor (SCF) was expressed in niche cells. To define niche cells, endothelial and mesenchymal cells were sorted by flow cytometry from dissociated placenta and hematopoietic cytokine gene expression was investigated. The endothelial cell compartment predominantly expressed SCF mRNA and protein. To determine whether SCF/Kit signaling regulates placental HC cluster proliferation, we injected anti-Kit neutralizing antibody into 10.25 dpc embryos and assayed cultured embryos for expression of hematopoietic transcription factors. Runx1, Myb and Gata2 were downregulated in the placental HC cluster fraction relative to controls. These observations demonstrate that placental HC clusters originate from the allantois and are regulated by endothelial niche cells through SCF/Kit signaling.

Newly emerging roles for prostaglandin E2 regulation of hematopoiesis and hematopoietic stem cell engraftment

PURPOSE OF REVIEW

Hematopoietic stem cell (HSC) transplantation is an effective treatment for leukemia, lymphoma, blood disorders, and autoimmune diseases. Successful transplantation is dependent upon efficient homing and engraftment of HSCs. Recently, prostaglandin E2 (PGE2) exposure, either in vivo or ex vivo, has been shown to increase engraftment. These results establish PGE2 as a regulator of hematopoietic development.

RECENT FINDINGS

The underlying mechanisms of PGE2 regulation of HSC development were poorly understood until recently. Ex-vivo exposure of LSK cells to PGE2 results in increased homing efficiency of HSCs to the murine bone marrow compartment. In addition, in-vivo treatment with PGE2 preferentially expands short-term HSCs without affecting long-term HSC number and engraftment in murine bone marrow. PGE2 acts through EP4 receptors to mediate lymphoid precursor development in the zebrafish. An in-vivo interaction between PGE2 and the Wnt signaling pathways controls HSC engraftment.

SUMMARY

PGE2 has a new role in HSC homing and survival, as well as short-term-HSC engraftment. PGE2 is currently being tested in clinical trials as a potential therapy to enhance HSC engraftment following a transplantation procedure.

Prostaglandin E2 regulates vertebrate haematopoietic stem cell homeostasis

Haematopoietic stem cell (HSC) homeostasis is tightly controlled by growth factors, signalling molecules and transcription factors. Definitive HSCs derived during embryogenesis in the aorta-gonad-mesonephros region subsequently colonize fetal and adult haematopoietic organs. To identify new modulators of HSC formation and homeostasis, a panel of biologically active compounds was screened for effects on stem cell induction in the zebrafish aorta-gonad-mesonephros region. Here, we show that chemicals that enhance prostaglandin (PG) E2 synthesis increased HSC numbers, and those that block prostaglandin synthesis decreased stem cell numbers. The cyclooxygenases responsible for PGE2 synthesis were required for HSC formation. A stable derivative of PGE2 improved kidney marrow recovery following irradiation injury in the adult zebrafish. In murine embryonic stem cell differentiation assays, PGE2 caused amplification of multipotent progenitors. Furthermore, ex vivo exposure to stabilized PGE2 enhanced spleen colony forming units at day 12 post transplant and increased the frequency of long-term repopulating HSCs present in murine bone marrow after limiting dilution competitive transplantation. The conserved role for PGE2 in the regulation of vertebrate HSC homeostasis indicates that modulation of the prostaglandin pathway may facilitate expansion of HSC number for therapeutic purposes.

Inhibitory effects of homeodomain-interacting protein kinase 2 on the aorta-gonad-mesonephros hematopoiesis

Definitive hematopoiesis starts in the aorta-gonad-mesonephros (AGM) region of the mouse embryo. Our previous studies revealed that STAT3, a gp130 downstream transcription factor, is required for AGM hematopoiesis and that homeodomain-interacting protein kinase 2 (HIPK2) phosphorylates serine-727 of STAT3. HIPK2 is a serine/threonine kinase known to be involved in transcriptional repression and apoptosis. In the present study, we examined the role of HIPK2 in hematopoiesis in mouse embryo. HIPK2 transcripts were found in fetal hematopoietic tissues such as the mouse AGM region and fetal liver. In cultured AGM cells, HIPK2 protein was detected in adherent cells. Functional analyses of HIPK2 were carried out by introducing wild-type and mutant HIPK2 constructs into AGM cultures. Production of CD45(+) hematopoietic cells was suppressed by forced expression of HIPK2 in AGM cultures. This suppression required the kinase domain and nuclear localization signals of HIPK2, but the kinase activity was dispensable. HIPK2-overexpressing AGM-derived nonadherent cells did not form cobblestone-like colonies in cultures with stromal cells. Furthermore, overexpression of HIPK2 in AGM cultures impeded the expansion of CD45(low)c-Kit(+) cells, which exhibit the immature hematopoietic progenitor phenotype. These data indicate that HIPK2 plays a negative regulatory role in AGM hematopoiesis in the mouse embryo.

Proper levels of c-Myb are discretely defined at distinct steps of hematopoietic cell development

The definitive hematopoietic cell lineages have been proposed to originate from hemogenic endothelial cells during mouse embryogenesis. c-Myb is a transcription factor that is essential for the development of definitive hematopoiesis. To investigate the functional role of c-Myb in hematopoietic cell development from endothelial cells, we introduced a c-myb transgene expressed under the control of a tetracycline-regulated promoter into the c-myb(-/-) embryonic stem (ES) cell line, with the aim of inducing c-Myb expression at any stage and at any level. Induction of c-Myb expression after replating c-myb(-)(/)(-) endothelial cells rescued the generation and proliferation of definitive hematopoietic progenitor cells, suggesting that c-Myb expression in developing endothelial cells is not a prerequisite for their hematogenic potential. Overexpression of c-Myb, however, prevented the terminal differentiation of erythrocytes and megakaryocytes and completely abolished B-lymphocyte development. Our results indicate that c-Myb is a major factor that controls differentiation as well as proliferation of hematopoietic progenitor cells derived from hemogenic endothelial cells, and that appropriate levels of c-Myb protein are strictly defined at distinct differentiation steps of each hematopoietic cell lineage.

Transcriptional regulation of hematopoietic stem cell development in zebrafish

The zebrafish (Danio rerio) is a well-established vertebrate model for studying hematopoiesis. The major advantages of this system include robust experimental techniques in both genetics and embryology, which have been utilized to model many aspects of human development and disease. Although much is known about the transcription factors involved in the terminal differentiation of peripheral blood lineages, little is known about the development and maintenance of the hematopoietic stem cell (HSC). This review will focus on the current knowledge of the transcriptional regulation of the HSC in the context of the zebrafish. Future studies using new technologies in the zebrafish model will enhance our understanding of the molecular networks regulating HSC pluripotency and differentiation.

Molecular genetics of T cell development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment.

Origins of lymphocyte developmental programs: transcription factor evidence

This review explores the evolutionary origins of lymphocyte development by focusing on the transcription factors that direct mammalian lymphocyte development today. Gene expression data suggest that the programs to make lymphocytes involve the same transcription factor ensembles in all animals with lymphocytes. Most of these factors, GATA, Runx, PU.1/Spi, EBF/Olf, Ikaros, and Pax-2/5/8 family members, are also encoded in the genomes of animals without lymphocytes. We consider the functions of these factors in animals without lymphocytes in terms of discrete program components, which could have been assembled in a new way to create the lymphocyte developmental program approximately 500 My ago.

Regulation of hematopoietic development in the aorta-gonad-mesonephros region mediated by Lnk adaptor protein

Development of hematopoietic cells in the aorta-gonad-mesonephros (AGM) region in the midgestation mouse embryo involves a multistep process, sequentially changing from endothelial cell-like cells, including hemangioblasts, into hematopoietic stem cells, progenitors, and/or lineage-committed cells. An adaptor molecule, Lnk, is known to negatively control the production of pro- and pre-B cells and hematopoietic progenitor cells in adult bone marrow. Here we show a role of Lnk in hematopoietic development in the AGM region. Lnk was predominantly expressed in the endothelial cells lining the dorsal aorta at embryonic day 11.5 (E11.5). Overexpression of Lnk in the primary culture of the AGM region at E11.5 suppressed the emergence of CD45+ hematopoietic cells. Point mutation in the SH2 domain of Lnk, which abolishes the binding capability of Lnk to c-Kit upon stimulation with stem cell factor (SCF), led to loss of Lnk-dependent inhibition of hematopoietic cell development in AGM cultures, suggesting Lnk-mediated inhibition of the SCF/c-Kit signaling pathway. In cultured AGM cells from Lnk homozygous mutant mouse embryos, the number of emerged CD45+ cells was 2.5-fold larger than that from heterozygous littermates. Furthermore, aorta cells of E11.5 Lnk homozygous mutant mice also showed enhanced hematopoietic colony-forming activity. Thus, Lnk is a negative regulator of hematopoiesis in the AGM region.

Progression through key stages of haemopoiesis is dependent on distinct threshold levels of c-Myb

The c-Myb transcription factor is expressed in immature haemopoietic cells and at key stages during differentiation. Loss of the c-myb gene results in embryonic lethality because mature blood cells fail to develop, although commitment to definitive haemopoiesis occurs. We have generated a knockdown allele of c-myb, expressing low levels of the protein, which has enabled us to investigate further the involvement of c-Myb in haemopoiesis. Low levels of c-Myb are sufficient to allow progenitor expansion but, importantly, we show that progression of progenitors towards terminal differentiation is significantly altered. Suboptimal levels of c-Myb favour differentiation of macrophage and megakaryocytes, while higher levels seem to be particularly important in the control of erythropoiesis and lymphopoiesis. We provide evidence that the transition from the CFU-E to erythroblasts is critically dependent on c-Myb levels. During thymopoiesis, c-Myb appears to regulate immature cell numbers and differentiation prior to expression of CD4 and CD8. Overall, our results point to a complex involvement of c-Myb in the regulation of proliferation and commitment within the haemopoietic hierarchy.

Expression and domain-specific function of GATA-2 during differentiation of the hematopoietic precursor cells in midgestation mouse embryos

The aorta-gonads-mesonephros (AGM) region of the mouse embryo has been assigned as the origin of definitive hematopoiesis. The transcription factor GATA-2 has specific but unclarified roles in early hematopoiesis. To elucidate the expression profile of GATA-2, we prepared transgenic mouse lines containing the green fluorescent protein (GFP) gene driven by GATA-2 gene regulatory elements. We also prepared a mouse line in which GFP reporter sequences were inserted into the endogenous GATA-2 gene. Both mouse mutants expressed GFP in the early hematopoietic tissues. The CD45 antigen, a marker of hematopoietic cells, was expressed in a small fraction of transgene (TG)-derived GFP+ cells. The remaining TG-GFP+/CD45- cells were adherent to plastic and produced CD45+ hematopoietic cells abundantly when cultured in vitro. Exogenous expression of GATA-2 in TG-GFP+/CD45- cells from the AGM region inhibited their differentiation into CD45+ cells. Loss of GATA-2 function through the disruption of the GATA-2 locus enhanced the earlier emergence of CD45+ cells in the yolk sac of the 9.5-day conceptus. These results demonstrated that GATA-2 is expressed in the precursor of hematopoietic cells and works as a gatekeeper to preserve their immaturity. A reduction of GATA-2 expression or activity is required for the differentiation of precursors to hematopoietic cells.

Ontogeny of hematopoiesis: examining the emergence of hematopoietic cells in the vertebrate embryo

Hematopoietic stem cells (HSCs) are responsible for generating all the lineages of the blood. During vertebrate development, waves of hematopoietic activity can be found in distinct anatomical sites, and they contribute to both embryonic and adult hematopoiesis. The origin of the HSCs that ultimately give rise to all the adult blood lineages has been a controversial issue in the field of hematopoiesis. Studies of amniotes have linked HSC activity to the aorta-gonad-mesonephros (AGM) region, whereas others suggest that the yolk sac is the true source of HSCs. This review describes both primitive and definitive hematopoiesis in mice, humans, chicks, frogs, and zebrafish and examines the current debate over the embryonic origins of HSCs.

Requirement of gp130 signaling for the AGM hematopoiesis

OBJECTIVE

Definitive hematopoiesis starts in the aorta-gonad-mesonephros (AGM) region during mouse development and remarkably expands in the liver at a later stage of ontogeny. gp130 is a signal transducing receptor component shared by all the IL-6 family cytokines, whose gene ablation in mouse results in the significant reduction in the fetal liver hematopoiesis. The present study aims to evaluate the role of gp130 signaling in the fetal mouse AGM hematopoiesis.

METHODS AND MATERIALS

Mouse AGM regions from the wild-type and gp130-deficient mice on embryonic day 11.5 were dissociated and cultured with a mixture of cytokines, including one which activates gp130. Wild-type human gp130 and its mutant constructs were introduced into cultured gp130-deficient AGM cells using retrovirus system. To further analyze gp130 downstream signaling, a dominant-negative mutant of STAT3 was also introduced.

RESULTS

The gp130 deficiency in the culture of fetal mouse AGM cells resulted in the failure of the expansion of the c-kit(+), Sca-1(+), and lineage markers(-) population. Such failure was rescued by introduction of a wild-type gp130 expression construct but not its mutant constructs having no ability to activate STAT3. In the normal AGM cell culture, introduction of a dominant-negative form of STAT3 in which Y(705) was changed to phenylalanine suppressed the expansion of hematopoietic cell colonies.

CONCLUSION

gp130 plays an indispensable role in the expansion of hematopoietic precursor cells in the fetal mouse AGM. In particular, the activation of STAT3 by gp130 is found to be important in this process.

Phosphorylation-dependent down-regulation of c-Myb DNA binding is abrogated by a point mutation in the v-myb oncogene

The viral Myb (v-Myb) oncoprotein of the avian myeloblastosis virus (AMV) is an activated form of the cellular transcription factor c-Myb causing acute monoblastic leukemia in chicken. Oncogenic v-Myb alterations include N- and C-terminal deletions as well as point mutations. Whereas truncations in Myb cause loss of various protein modifications, none of the point mutations in v-Myb has been directly linked to protein modifications. Here we show that the DNA-binding domain of c-Myb can be phosphorylated on serine 116 by the catalytic subunit of protein kinase A. Phosphorylation of Ser(116) differentially destabilizes a subtype of c-Myb-DNA complexes. The V117D mutation of the AMV v-Myb oncoprotein abolishes phosphorylation of the adjacent Ser(116) residue. Modification of Ser(116) was also detected in live cells in c-Myb, but not in AMV v-Myb. Phosphorylation-mimicking mutants of c-Myb failed to activate the resident mim-1 gene. Our data imply that protein kinase A or a kinase with similar specificity negatively regulates c-Myb function, including collaboration with C/EBP, and that the leukemogenic AMV v-Myb version evades inactivation by a point mutation that abolishes a phosphoacceptor consensus site. This suggests a novel link between Myb, a signal transduction pathway, cooperativity with C/EBP, and a point mutation in the myb oncogene.

Role of the microenvironment of the embryonic aorta-gonad-mesonephros region in hematopoiesis

Although various cytokines, growth factors, and chemokines are known to regulate hematopoiesis, expansion of hematopoietic stem cells (HSCs) in vitro with the use of such agents has proved problematic. Stromal cells are major components of the microenvironment that surrounds hematopoietic cells and are thought to play an important role in hematopoiesis in vivo. Co-culture of HSCs with stromal cells promotes hematopoiesis and self-renewal of HSCs. Definitive hematopoietic cells first appear during mammalian embryonic development in the aorta-gonad-mesonephros (AGM) region, and it is therefore thought that the microenvironment of this region plays an important role in HSC ontogeny. We have adopted two approaches to studying the contribution of the AGM microenvironment to hematopoiesis. In the first approach, we have developed an in vitro culture system for mouse AGM explants. Hematopoiesis is enhanced in such cultures by the presence of the combination of stem cell factor (SCF), basic fibroblast growth factor, leukemia inhibitory factor, and oncostatin M (SFLO culture). However, transplantation assays revealed that HSCs capable of long-term reconstitution of the hematopoietic compartment of irradiated mice (LTR-HSCs) do not expand in AGM-SFLO cultures; rather, these cultures appear to provide a favorable microenvironment for hematogenic angioblasts that are precursors of both endothelial and hematopoietic cells. In our second approach, we have established various stromal cell lines from the mouse AGM region. The AGM-S3 cell line supports human and mouse primitive hematopoietic cells as well as mouse LTR-HSCs. Maintenance of LTR-HSCs is mediated by a mechanism other than SCF signaling through its receptor (c-Kit). These two in vitro approaches should prove useful for further elucidation of the mechanisms that underlie hematopoiesis and HSC self-renewal.

Defective stem cell factor expression in c-myb null fetal liver stroma

High levels of c-Myb are observed in immature precursor myeloid and lymphoid cells, while downregulation of c-myb accompanies terminal differentiation to a mature phenotype. This has established c-Myb as a crucial transcription factor for hematopoiesis. Further evidence for this is the embryonic death of the c-myb homozygous mutant mouse at ED15 due to defective fetal liver erythropoiesis. Cells from fetal liver of wild-type and c-myb-/- embryos were examined in detail for their hematopoietic potential and the capacity of the stroma to support wild-type hematopoiesis. The c-myb-/- fetal liver was shown to harbor sevenfold fewer spleen focus-forming cells and a similarly lower number of cells with long-term repopulating capacity (high proliferative potential cells). However, shorter term repopulating cells were not substantially reduced. c-myb-/- stromal cells were unable to support the proliferation of wild-type bone marrow lineage-negative cells. This was found to be partly due to a decrease in stem cell factor (SCF) expression while partial rescue of the stromal cell cultures was achieved through the addition of exogenous SCF. DNA binding studies for two sites within the SCF promoter demonstrated an in vitro interaction between the SCF promoter and c-Myb and transient transfection studies demonstrated that c-Myb could substantially transactivate the SCF promoter in HEK293 cells. These data explain why the c-myb-/- embryos are so impaired in their ability to establish hematopoiesis.

Role of Oncostatin M in hematopoiesis and liver development

Definitive hematopoietic stem cells (HSCs) first appear in the aorta/gonad/mesonephros (AGM) region and migrate to the fetal liver where they massively produce hematopoietic cells before establishing hematopoiesis in the bone marrow at a perinatal stage. In the AGM region, Oncostatin M (OSM) enhances the development of both hematopoietic and endothelial cells by possibly stimulating their common precursors, so-called hemangioblasts. During development of HSCs in the AGM region, the liver primodium is formed at the foregut and accepts HSCs. While fetal hepatic cells function as hematopoietic microenvironment for expansion of hematopoietic cells during mid to late gestation, they do not possess most of the metabolic functions of adult liver. Along with the expansion of hematopoietic cells in fetal liver, OSM is produced by hematopoietic cells and induces differentiation of fetal hepatic cells, conferring various metabolic activities of adult liver. Matured hepatic cells then lose the ability to support hematopoiesis. Thus, OSM appears to coordinate the development of liver and hematopoiesis in the fetus.

Initiation of adult myelopoiesis can occur in the absence of c-Myb whereas subsequent development is strictly dependent on the transcription factor

The c-Myb transcriptional regulator is crucial to the development and functioning of haemopoietic cells, so much so that mouse embryos homozygous for an inactivated c-myb allele die from anaemia at about day 15 of gestation. By analysing c-myb(-/-) chimaeras we show that no mature cells of any lymphoid or myeloid lineage can be detected in adult haemopoietic tissues. This demonstrates that the effects of c-myb ablation on haemopoiesis are cell autonomous and correlates with an absence in the c-myb(-/-) foetal liver of uni- and multilineage CFUs. Indeed, CFU assays performed on E8.5 yolk sac cells revealed that haemopoietic progenitors are already defective at this stage. However, although cells expressing high levels of c-Kit were absent, we could detect a high proportion of CD34+CD45+ cells in the c-myb(-/-) foetal liver. Examination of chimaeric embryos revealed that c-myb(-/-) donor-derived CD34+/Kit+ cells, representing committed definitive progenitors, initially populated the foetal liver, but are unable to expand like wild type progenitors. Our results showing no megakaryocytic CFUs and a reduction in the absolute numbers of megakaryocytes in the c-myb(-/-) foetal liver also refute early suggestions that megakaryopoiesis is unaffected by the absence of c-Myb.

In vitro differentiation of c-myb(-/-) ES cells reveals that the colony forming capacity of unilineage macrophage precursors and myeloid progenitor commitment are c-Myb independent

Mice homozygous for an inactivated c-myb allele exhibit embryonic (primitive) blood formation but die at about day 15 of gestation because of a failure to generate adult (definitive) haemopoiesis. Recently, it has been shown that commitment to definitive haemopoiesis does occur in vivo, but that some point in the subsequent development towards the differentiated lineages is compromised. Here we have asked whether it is possible to demonstrate this same distinction between the development of primitive and definitive haemopoiesis during the in vitro differentiation of c-myb null ES cells, and whether this can be used to define more precisely at which developmental stage the absence of c-Myb blocks the adult haemopoietic lineages. We investigated the kinetics of progenitor formation and commitment to differentiation using a combination of colony forming assays and analysis of RNA and surface antigen expression. Primitive unilineage macrophage and erythroid precursor commitment could develop in the absence of c-Myb. No precursors characteristic of definitive haemopoiesis were detected; nevertheless, we could show the expression of a programme of transcription and surface antigens which is consistent with the appearance of definitive progenitors blocked at an early multipotential stage.

The AML1 transcription factor functions to develop and maintain hematogenic precursor cells in the embryonic aorta-gonad-mesonephros region

We examined the role of the AML1 transcription factor in the development of hematopoiesis in the paraaortic splanchnopleural (P-Sp) and the aorta-gonad-mesonephros (AGM) regions of mouse embryos. The activity of colony-forming units of colonies from the P-Sp/AGM region was reduced severalfold by heterozygous disruption of the AML1 gene, indicating that AML1 functioned in a dosage-dependent manner to generate hematopoietic progenitors. In addition, no hematopoietic progenitor activity was detected in the P-Sp/AGM region of embryos with an AML1 null mutation. Similar results were obtained when a dispersed culture was first prepared from the P-Sp/AGM region before assay of the activity of the colony-forming units. In a culture of cells with the AML1(+/+) genotype, both hematopoietic and endothelial-like cell types emerged, but in a culture of cells with the AML1(-/-) genotype, only endothelial-like cells emerged. Interestingly, introduction of AML1 cDNA into the P-Sp/AGM culture with the AML1(-/-) genotype partially restored the production of hematopoietic cells. This restoration was observed for cultures prepared from 9.5-day postcoitum (dpc) embryos but not for cultures prepared from 11.5-dpc embryos. Therefore, the population of endothelial-like cells capable of growing in the AML1(-/-) culture would appear to contain inert but nonetheless competent hematogenic precursor cells up until at least the 9.5-dpc period. All these results support the notion that the AML1 transcription factor functions to develop and maintain hematogenic precursor cells in the embryonic P-Sp/AGM region.

Identification of podocalyxin-like protein 1 as a novel cell surface marker for hemangioblasts in the murine aorta-gonad-mesonephros region

Recent studies with avian embryos and murine embryonic stem cells have suggested that hematopoietic cells are derived from hemangioblasts, the common precursors of hematopoietic and endothelial cells. We molecularly cloned podocalyxin-like protein 1 (PCLP1) as a novel surface marker for endothelial-like cells in the aorta-gonad-mesonephros (AGM) region of mouse embryos, where long-term repopulating hematopoietic stem cells (LTR-HSCs) are known to arise. PCLP1+ CD45 cells in the AGM region incorporated acetylated low-density lipoprotein and produced both hematopoietic and endothelial cells when cocultured with OP9 stromal cells. Moreover, multiple lineages of hematopoietic cells were generated in vivo when PCLP1 +CD45-cells were injected into neonatal liver of busulfan-treated mice. Thus, PCLP1 can be used to separate hemangioblasts that give rise to LTR-HSCs.