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Hematopoietic cells (HPCs) develop from hemogenic endothelial cells (ECs), a specialized type of ECs undergoing hematopoietic transition. However, the mesoderm origin for hemogenic ECs or HPCs has not been clarified. To examine the origin for hemogenic mesoderm, we inactivated Etv2, a master regulator for EC/HPC commitment, in specific regions. Region-specific Etv2 ablation in early mesoderm caused local EC differentiation block, resulting in the loss of specific vascular beds without compensatory migration of residual ECs into avascular area. This feature of local EC/HPC differentiation block was correlated to the hemogenic potential of each mesoderm subset. We found that caudal-lateral mesoderm of E7.5-8.5 embryos represent the pre-committed population critical for generating hemogenic ECs. Etv2 ablation in caudal-lateral mesoderm by Hoxb6 Cre or Hoxb6CreER transgene affected vitelline plexus formation and intra-aortic hematopoietic clusters. In differentiated embryonic stem cells, this mesoderm subset marked by Hoxb6-lateral mesoderm promoter showed enriched T lymphopoietic potential among Flk-1(+) cells, which could be regarded as a characteristic for definitive HPCs. These findings indicate that critical mesoderm precursors possibly for definitive type hemogenic ECs are regionally specified in primitive mesoderm, suggesting that Hoxb6(+) caudal-lateral mesoderm represents the critical source of HPCs, which are potentially useful to enrich definitive HPCs from embryonic stem cells., (Copyright © 2013 ISEH - Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved.)

Etv2 is a master gene for the commitment of hematopoietic/endothelial cells and is a potent inducer of endothelial/hematopoietic cells from embryonic stem cells. Etv2 is highly expressed in endothelial/hematopoietic precursors but is downregulated when they are differentiated, indicating that Etv2 should have transient but not constitutive function. However, relatively little attention has been paid to the importance of transient Etv2 expression. To determine whether transient Etv2 expression is essential to normal development and cell differentiation, we generated mice that constitutively express Etv2 from a Cre-activatable ROSA26 locus in endothelial/hematopoietic, somite, or neuronal lineages. Constitutive Etv2 expression caused profound phenotypes in hematopoietic/endothelial cells, with little effect on somite or neuronal lineages. In hematopoietic/endothelial lineages, constitutive Etv2 expression induced by Tie-2 Cre transgene caused abnormal yolk sac vasculature. Prolonged vascular endothelial cadherin expression and decreased B lymphopoiesis were observed in Etv2 expressing vascular endothelial cadherin(+)/CD45(+) cells, indicating that Etv2 forces endothelial program on hematopoietic cells. Etv2 expression in adult hematopoietic cells by Vav-iCre transgene also conferred an endothelial phenotype on hematopoietic stem cells and suppressed hematopoiesis, with erythropoiesis severely affected. We conclude that constitutive Etv2 expression perturbs vascular development and hematopoiesis. While promoting hematopoiesis/vasculogenesis, Etv2 expression should be tightly regulated to achieve normal vascular development and hematopoiesis., (Copyright © 2012 ISEH - Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved.)

Hematopoietic stem cell (HSC) proliferation is tightly regulated by a poorly understood complex of positive and negative cell-cycle regulatory mechanisms. Necdin (Ndn) is an evolutionally conserved multifunctional protein that has been implicated in cell-cycle regulation of neuronal cells. Here, we provide evidence that necdin plays an important role in restricting excessive HSC proliferation during hematopoietic regeneration. We identify Ndn as being preferentially expressed in the HSC population on the basis of gene expression profiling and demonstrate that mice deficient in Ndn show accelerated recovery of the hematopoietic system after myelosuppressive injury, whereas no overt abnormality is seen in steady-state hematopoiesis. In parallel, after myelosuppression, Ndn-deficient mice exhibit an enhanced number of proliferating HSCs. Based on these findings, we propose that necdin functions in a negative feedback loop that prevents excessive proliferation of HSCs during hematopoietic regeneration. These data suggest that the inhibition of necdin after clinical myelosuppressive treatment (eg, chemotherapy, HSC transplantation) may provide therapeutic benefits by accelerating hematologic recovery.

The first haematopoietic stem cells (HSCs) appear in the aorta-gonad-mesonephros (AGM) region, major vitelline and umbilical vessels, and placenta; however, whether they arise locally or from immigrant yolk sac precursor cells remains unclear. This issue is best addressed by measuring cell-lineage relationships rather than cell potentials. To undertake long-term in vivo tracing of yolk sac cells, we designed a non-invasive pulse-labelling system based on Cre/loxP recombination. Here we show that in Runx1(+/-) (runt-related transcription factor 1) heterozygous mice, yolk sac cells expressing Runx1 at embryonic day 7.5 develop into fetal lymphoid progenitors and adult HSCs. During mid-gestation the labelled (embryonic day 7.5) yolk sac cells colonize the umbilical cord, the AGM region and subsequently the embryonic liver. This raises the possibility that some HSCs associated with major embryonic vasculature are derived from yolk sac precursors. We observed virtually no contribution of the labelled cells towards the yolk sac vasculature, indicating early segregation of endothelial and haematopoietic lineages.

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.

Hematopoiesis in most vertebrate species occurs in two distinct phases, primitive and definitive, which diverge from FLK1(+)VE-cadherin(-) mesoderm and FLK1(+)VE-cadherin(+) endothelial cells (EC), respectively. This study aimed at determining the stage at which hematopoietic lineage fate is determined by manipulating the SCL/tal-1 expression that is known to be essential for the early development of the primitive and definitive hematopoietic systems. We established SCL-null ES cell lines in which SCL expression is rescued by tamoxifen-inducible Cre recombinase-loxP site-mediated recombination. While no hematopoietic cells (HPC) were detected in SCL-null ES cell differentiation cultures, SCL gene reactivation from day 2 to day 4 after initiation of differentiation could rescue both primitive and definitive hematopoiesis. SCL reactivation at later phases was ineffective. Moreover, generation of VE-cadherin(+) EC that can give rise to definitive HPC required SCL reactivation prior to VE-cadherin expression. These results indicated that the competence to become HPC is acquired at the mesodermal stage by a SCL-dependent process that takes place independently of determination of endothelial fate.

Objective: The aim of this study was to assess the potential of FLK1(+) and vascular endothelial (VE)-cadherin(+) populations from different stages of embryonic development to generate hematopoietic cells ex vivo and to contribute to the hematopoietic systems of recipient mice., Materials and Methods: FLK1(+) of VE-cadherin(+) cells were isolated from 7.5- to 9.5-dpc concepti and cultured ex vivo on OP9 stromal cells and hematopoietic development examined. VE-cadherin(+)CD45(-) cells from 8.5- and 9.5-dpc concepti were injected intrahepatically into newborn busulfan-treated SCID recipients and engraftment monitored., Results: VE-cadherin(+) cells from 7.5- and 8.5-dpc concepti can readily generate hematopoi-etic cells ex vivo compared to FLK1(+) VE-cadherin(-) cells. Similar hematopoietic potential can be found in the VE-cadherin(+) cells from the 8.5-dpc yolk sac. When VE-cadherin(+)CD45(-) cells were injected into SCID recipients, long-term engraftment, particularly within the lymphoid system, was observed. This potential was observed in VE-cadherin(+)CD45(-) cells from 9.5-dpc embryo or yolk sac but from tissues from younger concepti., Conclusions: FLK1(+)VE-cadherin(-) cells, possibly representing the lateral plate mesoderm, are not as effective at generating hematopoietic cells compared to similarly staged VE-cadherin(+) cells. VE-cadherin(+)CD45(-) cells can also contribute to the hematolymphoid system of intrahepatically injected newborn SCID recipients, suggesting that cells bearing an endothelial phenotype are capable of generating long-term hematopoietic precursors.

Background: Early mesoderm can be classified into Flk-1+ or PDGF receptor alpha (PDGFRα)+ population, grossly representing lateral and paraxial mesoderm, respectively. It has been demonstrated that all endothelial (EC) and hematopoietic (HPC) cells are derived from Flk-1+ cells. Although PDGFRα+ cells give rise to ECs/HPCs in in vitro ES differentiation, whether PDGFRα+ population can become hemato-endothelial lineages has not been proved in mouse embryos. Results: Using PDGFRαMerCreMer mice, PDGFRα+ early mesoderm was shown to contribute to endothelial cells including hemogenic ECs, fetal liver B lymphocytes, and Lin-Kit+Sca-1+ (KSL) cells. Contribution of PDGFRα+ mesoderm into ECs and HPCs was limited until E8.5, indicating that PDGFRα+/Flk-1+ population that exists until E8.5 may be the source for hemato-endothelial lineages from PDGFRα+ population. The functional significance of PDGFRα+ mesoderm in vascular development and hematopoiesis was confirmed by genetic deletion of Etv2 or restoration of Runx1 in PDGFRα+ cells. Etv2 deletion and Runx1 restoration in PDGFRα+ cells resulted in abnormal vascular remodeling and rescue of fetal liver CD45+ and Lin-Kit+Sca-1+ (KSL) cells, respectively. Conclusions: Endothelial and hematopoietic cells can be derived from PDGFRα+ early mesoderm in mice. PDGFRα+ mesoderm is functionally significant in vascular development and hematopoiesis from phenotype analysis of genetically modified embryos. Developmental Dynamics 242:254-268, 2013. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The mouse Flk1 gene is expressed in various mesodermal progenitor cells of developing embryos. Recent studies have shown that Flk1 expression marks multipotent mesodermal progenitors, giving rise to various hemato-cardiovascular cell lineages during development. Flk1 expression also marks hemato-cardiovascular cell lineages in differentiating embryonic stem (ES) cells, which may be used in transplantation decisions to treat cardiovascular diseases. Despite its developmental and clinical importance in cardiovascular tissues, the transcriptional regulatory system of Flk1 has remained unclear. Here, we report a novel enhancer of the mouse Flk1 gene directing early mesodermal expression during development as well as ES differentiation. The enhancer enriches various mesodermal progenitors, such as primitive erythropoietic progenitors, hemangioblast (BL-CFC) and cardiovascular progenitors (CV-CFC). The enhancer is activated by Bmp, Wnt and Fgf, and it contains Gata-, Cdx-, Tcf/Lef-, ER71/Etv2- and Fox-binding sites, some of which are bound specifically by each of these transcription factors. As these transcription factors are known to act under the control of the Bmp, Wnt and Fgf families, early Flk1 expression may be induced by cooperative interactions between Gata, Tcf/Lef, Cdx and ER71/Etv2 under the control of Bmp, Wnt and Fgf signaling. The enhancer is required for early Flk1 expression and for hemangioblast development during ES differentiation. [ABSTRACT FROM AUTHOR]

Hemangioblasts are thought to be one of the sources of hematopoietic progenitors, yet little is known about their localization and fate in the mouse embryo. We show here that a subset of cells co-expressing the hematopoietic marker GATA-1 and the endothelial marker VE-cadherin localize on the yolk sac blood islands at embryonic day 7.5. Clonal analysis demonstrated that GATA-1+ cells isolated from E7.0–7.5 embryos include a common precursor for hematopoietic and endothelial cells. Moreover, this precursor possesses primitive and definitive hematopoietic bipotential. By using a transgenic complementation rescue approach, GATA-1+ cell-derived progenitors were selectively restored in Runx1-deficient mice. In the rescued mice, definitive erythropoiesis was recovered but the rescued progenitors did not display multilineage hematopoiesis or intra-aortic hematopoietic clusters. These results provide evidence of the presence of GATA-1+ hemangioblastic cells in the extra-embryonic region and also their functional contribution to hematopoiesis in the embryo. [ABSTRACT FROM AUTHOR]

The hematopoietic system undergoes a qualitative change during the embryogenesis of most vertebrates. It is designated as the shift of primitive to definitive hematopoiesis and suitable microenvironment must be established to support this shift. While studying the role of platelet derived growth factor receptor α (PDGFR α) in embryonic hematopoiesis, we found that it was expressed in a stromal cell component of liver, a major site of this shift, but not in the yolk sac, the site of primitive hematopoiesis. Thus, we considered that development of PDGFRα positive stromal cells is an essential requirement for this shift. Without PDFGRα positive cell component, erythropoiesis was suppressed in the culture of fetal liver. Moreover, injection of an antagonistic anti-PDGFRα monoclonal antibody during embryogenesis suppressed the production of definitive erythrocytes. These indicated that PDGF exerts its effect on a subset of stromal components to prepare a microenvironment that can support the definitive erythropoiesis. [ABSTRACT FROM PUBLISHER]

Definitive hematopoiesis has been proposed to arise from hemogenic endothelial cells during mouse embryogenesis. The c- myb proto-oncogene is essential for the development of definitive hematopoiesis and was reported to be activated in hemogenic endothelial cells. To investigate whether c-Myb is involved in regulating the development of hemogenic endothelial cells, we conditionally induced c- myb over-expression during the in vitro differentiation of embryonic stem cells. VE-cadherin+ CD45− cells inducibly expressing c-Myb showed an increase in multilineage colony formation as well as an augmented capacity of the colony forming cells to self-renew in vitro under the condition that only the endogenous c- myb gene was expressed during differentiation of hematopoietic cells. Over-expression of c-Myb in the endothelial population led to activation of genes associated with definitive hematopoiesis such as Runx1, Hoxb4, Mll and Etv6. Our data provide evidence that c-Myb is able to exert an effect in endothelial cells which fosters the establishment of their hemogenic potential. [ABSTRACT FROM AUTHOR]

We present here a speculative view of embryonic hematopoiesis. We do this with the hope of finding directions for future study, keeping in mind that our model may diverge from the real situation. However, we want to emphasize that previous models have neglected the possibility that endothelial cells (EC) represent a progenitor of hematopoeitic cells (HPC). Emerging evidence, including our own,a nd previous histological studies argue for the presence of hemogenic EC. We discuss seemingly contradictory points of view of embryonic hematopoiesis, such as the origin of lymphogenic progenitors, and have shown that they may be resolved more satisfactory by introducing this notion. Obviously, a good model should be a testable one, so we are currently developing experimental systems to demonstrate that all B cells are, indeed, the progeny of EC. [ABSTRACT FROM AUTHOR]

The authors discuss a study on the process of developing a new bone marrow (BM) culture condition by using OP9 stromal cell line with insufficient macrophage-colony stimulating factor (M-CSF) production. They also applied epidermal growth factor (EGF) for survival of such stromal cells under chemically defined medium called mSFO2 and stem cell factor (SCF) for BM cell proliferation. They found that the molecules that act on hematopoietic progenitors and those that promote stromal cell components both regulate hematopoiesis.

Abstract: The ontogenic source of definitive hematopoietic system has been identified in non-mammalian vertebrates such as birds and amphibians by orthotopic embryo grafting, but remains unclear for mammals because of technical difficulties. Here, we successfully generated mouse chimeras by grafting yolk sac (YS) on YS of the host embryos before establishing circulation between YS and embryo proper and cultured the whole embryo for 66 h. Donor YS were isolated from C57BL/6 Ly-5.1 and EGFP-transgenic mouse embryos, and recipient embryos from C57BL/6 Ly-5.2 mouse. Almost one-half of the grafts in YS–YS chimeras survived and had obvious blood flow; graft-derived cells comprised 12.7±0.9% of the blood cells in the circulation. These graft-derived blood cells consisted mainly of erythroid cells, some myeloid cells and a few blastic cells. In addition, CD19+ B cells were generated from the graft-derived cells isolated from aorta–gonad–mesonephros (AGM) regions of the YS–YS chimeras; however, the frequency of the YS-derived B cell was low (1.0±0.6%) when co-cultured with OP9 stromal cells. These results demonstrate that B cell potential exists in YS before the circulation. Although the major source for B cell is intra-embryonic AGM region, YS may contribute to definitive lymphopoiesis in vivo in mice. [Copyright &y& Elsevier]

Abstract: Signals of Notch transmembrane receptors function to regulate a wide variety of developmental cell fates. Here we investigate the role of Notch signaling in the development of mesodermal cell types by expressing a tamoxifen-inducible, activated form of Notch1 in embryonic stem cells (ESC). For differentiation of ESC into first mesodermal progenitor cells and then endothelial, mural, cardiac muscle and hematopoietic cells, the OP9 stroma co-culture system was used. Timed activation of Notch signaling by the addition of tamoxifen at various stages during differentiation of ESC into mesodermal cell lineages results in profound alterations in the generation of all of these cells. Differentiation of ESC into Flk1+ mesodermal cells is inhibited by activated Notch. When Notch signaling is activated in mesodermal cells, generation of cardiac muscle, endothelial and hematopoietic cells is inhibited, favoring the generation of mural cells. Activation of Notch signaling in hematopoietic cells reduces colony formation and maintenance of hematopoiesis. These data suggest that Notch signaling plays a regulatory role in mesodermal development, cardiomyogenesis, the balanced generation of endothelial versus mural cells of blood vessels and hematopoietic development. [Copyright &y& Elsevier]