Fli1 acts at the top of the transcriptional network driving blood and endothelial development

Genome-wide analysis shows that Ldb1 controls essential hematopoietic genes/pathways in mouse early development and reveals novel players in hematopoiesis

The first site exhibiting hematopoietic activity in mammalian development is the yolk-sac blood island, which originates from the hemangioblast. Here we performed differentiation assays, as well as genome-wide molecular and functional studies in blast colony-forming cells to gain insight into the function of the essential Ldb1 factor in early primitive hematopoietic development. We show that the previously reported lack of yolk-sac hematopoiesis and vascular development in Ldb1(-/-) mouse result from a decreased number of hemangioblasts and a block in their ability to differentiate into erythroid and endothelial progenitor cells. Transcriptome analysis and correlation with the genome-wide binding pattern of Ldb1 in hemangioblasts revealed a number of direct-target genes and pathways misregulated in the absence of Ldb1. The regulation of essential developmental factors by Ldb1 defines it as an upstream transcriptional regulator of hematopoietic/endothelial development. We show the complex interplay that exists between transcription factors and signaling pathways during the very early stages of hematopoietic/endothelial development and the specific signaling occurring in hemangioblasts in contrast to more advanced hematopoietic developmental stages. Finally, by revealing novel genes and pathways not previously associated with early development, our study provides novel candidate targets to manipulate the differentiation of hematopoietic and/or endothelial cells.

Efficient direct reprogramming of mature amniotic cells into endothelial cells by ETS factors and TGFβ suppression

ETS transcription factors ETV2, FLI1, and ERG1 specify pluripotent stem cells into induced vascular endothelial cells (iVECs). However, iVECs are unstable and drift toward nonvascular cells. We show that human midgestation c-Kit(-) lineage-committed amniotic cells (ACs) can be reprogrammed into vascular endothelial cells (rAC-VECs) without transitioning through a pluripotent state. Transient ETV2 expression in ACs generates immature rAC-VECs, whereas coexpression with FLI1/ERG1 endows rAC-VECs with a vascular repertoire and morphology matching mature endothelial cells (ECs). Brief TGFβ-inhibition functionalizes VEGFR2 signaling, augmenting specification of ACs into rAC-VECs. Genome-wide transcriptional analyses showed that rAC-VECs are similar to adult ECs in which vascular-specific genes are expressed and nonvascular genes are silenced. Functionally, rAC-VECs form stable vasculature in Matrigel plugs and regenerating livers. Therefore, short-term ETV2 expression and TGFβ inhibition with constitutive ERG1/FLI1 coexpression reprogram mature ACs into durable rAC-VECs with clinical-scale expansion potential. Banking of HLA-typed rAC-VECs establishes a vascular inventory for treatment of diverse disorders.

Global analysis of the haematopoietic and endothelial transcriptome during zebrafish development

In this paper, we use zebrafish embryos to characterise the transcriptome of the developing blood and endothelium, two cell types that are closely associated during development. High-throughput sequencing identified 754 genes whose transcripts are enriched threefold or more in blood and/or vascular endothelial cells compared with the rest of the embryo at 26–28 h post fertilisation. Of these genes, 388 were classified as novel to these cell types after cross-reference with PubMed and the zebrafish information network (ZFIN). Analysis by quantitative PCR and in situ hybridisation showed that 83% (n = 41) of these novel genes are expressed in blood or vascular endothelium. Of 10 novel genes selected for knockdown by antisense morpholino oligonucleotides, we confirmed that two, tmem88a and trim2a, are required for primitive erythropoiesis and myelopoiesis. Our results provide a catalogue of genes whose expression is enriched in the developing blood and endothelium in zebrafish, many of which will be required for the development of those cell types, both in fish and in mammals.

The biochemistry of hematopoietic stem cell development

BACKGROUND

The cornerstone of the adult hematopoietic system and clinical treatments for blood-related disease is the cohort of hematopoietic stem cells (HSC) that is harbored in the adult bone marrow microenvironment. Interestingly, this cohort of HSCs is generated only during a short window of developmental time. In mammalian embryos, hematopoietic progenitor and HSC generation occurs within several extra- and intraembryonic microenvironments, most notably from 'hemogenic' endothelial cells lining the major vasculature. HSCs are made through a remarkable transdifferentiation of endothelial cells to a hematopoietic fate that is long-lived and self-renewable. Recent studies are beginning to provide an understanding of the biochemical signaling pathways and transcription factors/complexes that promote their generation.

SCOPE OF REVIEW

The focus of this review is on the biochemistry behind the generation of these potent long-lived self-renewing stem cells of the blood system. Both the intrinsic (master transcription factors) and extrinsic regulators (morphogens and growth factors) that affect the generation, maintenance and expansion of HSCs in the embryo will be discussed.

MAJOR CONCLUSIONS

The generation of HSCs is a stepwise process involving many developmental signaling pathways, morphogens and cytokines. Pivotal hematopoietic transcription factors are required for their generation. Interestingly, whereas these factors are necessary for HSC generation, their expression in adult bone marrow HSCs is oftentimes not required. Thus, the biochemistry and molecular regulation of HSC development in the embryo are overlapping, but differ significantly from the regulation of HSCs in the adult.

GENERAL SIGNIFICANCE

HSC numbers for clinical use are limiting, and despite much research into the molecular basis of HSC regulation in the adult bone marrow, no panel of growth factors, interleukins and/or morphogens has been found to sufficiently increase the number of these important stem cells. An understanding of the biochemistry of HSC generation in the developing embryo provides important new knowledge on how these complex stem cells are made, sustained and expanded in the embryo to give rise to the complete adult hematopoietic system, thus stimulating novel strategies for producing increased numbers of clinically useful HSCs. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

The biochemistry of hematopoietic stem cell development

BACKGROUND

The cornerstone of the adult hematopoietic system and clinical treatments for blood-related disease is the cohort of hematopoietic stem cells (HSC) that is harbored in the adult bone marrow microenvironment. Interestingly, this cohort of HSCs is generated only during a short window of developmental time. In mammalian embryos, hematopoietic progenitor and HSC generation occurs within several extra- and intraembryonic microenvironments, most notably from 'hemogenic' endothelial cells lining the major vasculature. HSCs are made through a remarkable transdifferentiation of endothelial cells to a hematopoietic fate that is long-lived and self-renewable. Recent studies are beginning to provide an understanding of the biochemical signaling pathways and transcription factors/complexes that promote their generation.

SCOPE OF REVIEW

The focus of this review is on the biochemistry behind the generation of these potent long-lived self-renewing stem cells of the blood system. Both the intrinsic (master transcription factors) and extrinsic regulators (morphogens and growth factors) that affect the generation, maintenance and expansion of HSCs in the embryo will be discussed.

MAJOR CONCLUSIONS

The generation of HSCs is a stepwise process involving many developmental signaling pathways, morphogens and cytokines. Pivotal hematopoietic transcription factors are required for their generation. Interestingly, whereas these factors are necessary for HSC generation, their expression in adult bone marrow HSCs is oftentimes not required. Thus, the biochemistry and molecular regulation of HSC development in the embryo are overlapping, but differ significantly from the regulation of HSCs in the adult.

GENERAL SIGNIFICANCE

HSC numbers for clinical use are limiting, and despite much research into the molecular basis of HSC regulation in the adult bone marrow, no panel of growth factors, interleukins and/or morphogens has been found to sufficiently increase the number of these important stem cells. An understanding of the biochemistry of HSC generation in the developing embryo provides important new knowledge on how these complex stem cells are made, sustained and expanded in the embryo to give rise to the complete adult hematopoietic system, thus stimulating novel strategies for producing increased numbers of clinically useful HSCs. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Maternal pak4 expression is required for primitive myelopoiesis in zebrafish

Transcripts of pak4, the zebrafish ortholog of p21-activated kinase 4 (PAK4), are most abundant in the egg and fall to low levels by the end of gastrulation, after which expression is essentially ubiquitous. Translation of maternal mRNA into pak4 protein is first detectable at high stage (3.3hpf). Splice-blocking morpholino oligonucleotides (MOs) were used to prevent zygotic pak4 expression. This had no discernable effect on development through larval stages. In contrast, a translation-blocking MO, alone or in combination with the splice MOs, resulted in a complex lethal phenotype. In addition to disrupted somite development and other morphogenetic abnormalities, the knockdown of maternal pak4 expression led to alterations in regulatory gene expression in the primitive hematopoietic domains, leading to deficiencies in granulocyte and leukocyte lineages. At least some of the effects of pak4 knockdown on gene expression could be mimicked by treatment with actin depolymerization agents, suggesting a mechanistic link between regulation of microfilament dynamics by pak4 and regulation of gene expression in primitive myeloid cell differentiation.

Constitutive activation of c-Abl/protein kinase C-δ/Fli1 pathway in dermal fibroblasts derived from patients with localized scleroderma

BACKGROUND

A noncanonical pathway of transforming growth factor-β signalling, the c-Abl/protein kinase C-δ (PKC-δ)/Friend leukemia virus integration 1 (Fli1) axis, is a powerful regulator of collagen synthesis in dermal fibroblasts.

OBJECTIVES

To investigate the significance of the c-Abl/PKC-δ/Fli1 pathway for the establishment of the profibrotic phenotype in lesional dermal fibroblasts from patients with localized scleroderma (LSc).

METHODS

The activation status of the c-Abl/PKC-δ/Fli1 pathway was evaluated by immunoblotting and chromatin immunoprecipitation using cultured dermal fibroblasts from patients with LSc and closely matched healthy controls and by immunostaining on skin sections. The effects of a platelet-derived growth factor receptor inhibitor AG1296 and gene silencing of c-Abl on the expression levels of type I collagen were evaluated by immunoblotting.

RESULTS

The phosphorylation levels of Fli1 at threonine 312 were increased, while the total Fli1 levels and the binding of Fli1 to the COL1A2 promoter were decreased, in cultured LSc fibroblasts compared with cultured normal fibroblasts. Furthermore, in cultured LSc fibroblasts, the expression levels of c-Abl were elevated compared with cultured normal fibroblasts and PKC-δ was preferentially localized in the nucleus. These findings were also confirmed in vivo by immunohistochemistry using skin sections. Moreover, gene silencing of c-Abl, but not AG1296, significantly suppressed the expression of type I collagen in cultured LSc fibroblasts.

CONCLUSIONS

Constitutive activation of the c-Abl/PKC-δ/Fli1 pathway at least partially contributes to the establishment of the profibrotic phenotype in LSc dermal fibroblasts, which provides a novel molecular basis to explain the efficacy of imatinib against skin sclerosis in a certain subset of LSc.

Teleost growth factor independence (gfi) genes differentially regulate successive waves of hematopoiesis

Growth Factor Independence (Gfi) transcription factors play essential roles in hematopoiesis, differentially activating and repressing transcriptional programs required for hematopoietic stem/progenitor cell (HSPC) development and lineage specification. In mammals, Gfi1a regulates hematopoietic stem cells (HSC), myeloid and lymphoid populations, while its paralog, Gfi1b, regulates HSC, megakaryocyte and erythroid development. In zebrafish, gfi1aa is essential for primitive hematopoiesis; however, little is known about the role of gfi1aa in definitive hematopoiesis or about additional gfi factors in zebrafish. Here, we report the isolation and characterization of an additional hematopoietic gfi factor, gfi1b. We show that gfi1aa and gfi1b are expressed in the primitive and definitive sites of hematopoiesis in zebrafish. Our functional analyses demonstrate that gfi1aa and gfi1b have distinct roles in regulating primitive and definitive hematopoietic progenitors, respectively. Loss of gfi1aa silences markers of early primitive progenitors, scl and gata1. Conversely, loss of gfi1b silences runx-1, c-myb, ikaros and cd41, indicating that gfi1b is required for definitive hematopoiesis. We determine the epistatic relationships between the gfi factors and key hematopoietic transcription factors, demonstrating that gfi1aa and gfi1b join lmo2, scl, runx-1 and c-myb as critical regulators of teleost HSPC. Our studies establish a comparative paradigm for the regulation of hematopoietic lineages by gfi transcription factors.

Regulation of endothelial and hematopoietic development by the ETS transcription factor Etv2

PURPOSE OF REVIEW

Vasculogenesis and hematopoiesis are essential for development. Recently, the ETS domain transcription factor Etv2 has been identified as an essential regulator of vasculogenesis and hematopoiesis. Here, we review the recent studies that have established the critical role of Etv2 in the specification of mesoderm to blood and endothelial cells.

RECENT FINDINGS

Loss and gain-of-function studies have demonstrated the conserved role of Etv2 in endothelial and hematopoietic development. Recent studies have placed Etv2 at or near the top of the hierarchy in specification of these lineages and have begun to dissect the upstream regulators and downstream effectors of Etv2 function using multiple model organisms and experimental systems.

SUMMARY

Etv2 is essential for the specification of endothelial and hematopoietic lineages. Understanding the mechanisms through which Etv2 specifies endothelial and blood cells by defining upstream transcriptional regulators and cofactors will lead to greater insight into vasculogenesis and hematopoiesis, and may help to identify therapeutic targets to treat vascular disorders or to promote or inhibit vessel growth.

The Flk1-Cre-mediated deletion of ETV2 defines its narrow temporal requirement during embryonic hematopoietic development

During embryonic development, the emergence of hematopoiesis and vasculogenesis is tightly associated, with many transcription factors implicated in both developmental processes. Among those factors, ETV2 acts at the top of the hierarchy and controls the formation of both lineages. However, it is not known at which stage of mesoderm development ETV2 is acting and whether ETV2 activity is further required once mesodermal precursors have been specified to the hematopoietic and endothelial fates. In this study, we characterize the developmental window during which ETV2 expression is required for hematopoietic and endothelial development. Using cre-mediated deletion of ETV2, we demonstrate that ETV2 is acting prior to or at the time of FLK1 expression in mesodermal precursors to initiate the hematopoietic and endothelial program. Using the in vitro differentiation of embryonic stem cells as a model system, we further show that ETV2 re-expression in Etv2(-/-) Flk1-negative precursors drives hematopoiesis specification and switches on the expression of most genes known to be implicated in hematopoietic and endothelial development. Among the downstream targets of ETV2, we identify the transcription factors SCL, GATA2, and FLI1 known to operate a recursive loop controlling hematopoietic development. Surprisingly, SCL re-expression in Etv2(-/-) cells fully rescues hematopoiesis, while the re-expression of FLI1 or GATA2 promotes only a very limited rescue. Altogether, our data establish that ETV2 is required very transiently to specify mesodermal precursors to hematopoiesis and vasculogenesis and that SCL is one of the key downstream targets of ETV2 in controlling hematopoietic specification.

Hematopoietic stem cell development and regulatory signaling in zebrafish

BACKGROUND

Hematopoietic stem cells (HSCs) are a population of multipotent cells that can self-renew and differentiate into all blood lineages. HSC development must be tightly controlled from cell fate determination to self-maintenance during adulthood. This involves a panel of important developmental signaling pathways and other factors which act synergistically within the HSC population and/or in the HSC niche. Genetically conserved processes of HSC development plus many other developmental advantages make the zebrafish an ideal model organism to elucidate the regulatory mechanisms underlying HSC programming.

SCOPE OF REVIEW

This review summarizes recent progress on zebrafish HSCs with particular focus on how developmental signaling controls hemogenic endothelium-derived HSC development. We also describe the interaction of different signaling pathways during these processes.

MAJOR CONCLUSIONS

The hematopoietic stem cell system is a paradigm for stem cell studies. Use of the zebrafish model to study signaling regulation of HSCs in vivo has resulted in a great deal of information concerning HSC biology in vertebrates.

GENERAL SIGNIFICANCE

These new findings facilitate a better understanding of molecular mechanisms of HSC programming, and will provide possible new strategies for the treatment of HSC-related hematological diseases, such as leukemia. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Tumor-specific retargeting of an oncogenic transcription factor chimera results in dysregulation of chromatin and transcription

Chromosomal translocations involving transcription factor genes have been identified in an increasingly wide range of cancers. Some translocations can create a protein "chimera" that is composed of parts from different proteins. How such chimeras cause cancer, and why they cause cancer in some cell types but not others, is not understood. One such chimera is EWS-FLI, the most frequently occurring translocation in Ewing Sarcoma, a malignant bone and soft tissue tumor of children and young adults. Using EWS-FLI and its parental transcription factor, FLI1, we created a unique experimental system to address questions regarding the genomic mechanisms by which chimeric transcription factors cause cancer. We found that in tumor cells, EWS-FLI targets regions of the genome distinct from FLI1, despite identical DNA-binding domains. In primary endothelial cells, however, EWS-FLI and FLI1 demonstrate similar targeting. To understand this mistargeting, we examined chromatin organization. Regions targeted by EWS-FLI are normally repressed and nucleosomal in primary endothelial cells. In tumor cells, however, bound regions are nucleosome depleted and harbor the chromatin signature of enhancers. We next demonstrated that through chimerism, EWS-FLI acquired the ability to alter chromatin. Expression of EWS-FLI results in nucleosome depletion at targeted sites, whereas silencing of EWS-FLI in tumor cells restored nucleosome occupancy. Thus, the EWS-FLI chimera acquired chromatin-altering activity, leading to mistargeting, chromatin disruption, and ultimately, transcriptional dysregulation.

Early arterial differentiation and patterning in the avian embryo model

Of the many models to study vascular biology the avian embryo remains an informative and powerful model system that has provided important insights into endothelial cell recruitment, assembly and remodeling during development of the circulatory system. This review highlights several discoveries in the avian system that show how arterial patterning is regulated using the model of dorsal aortae development along the embryo midline during gastrulation and neurulation. These discoveries were made possible through spatially and temporally controlled gain-of-function experiments that provided direct evidence that BMP signaling plays a pivotal role in vascular recruitment, patterning and remodeling and that Notch-signaling recruits vascular precursor cells to the dorsal aortae. Importantly, BMP ligands are broadly expressed throughout embryos but BMP signaling activation region is spatially defined by precisely regulated expression of BMP antagonists. These discoveries provide insight into how signaling, both positive and negative, regulate vascular patterning. This review also illustrates similarities of early arterial patterning along the embryonic midline in amniotes both avian and mammalians including human, evolutionarily specialized from non-amniotes such as fish and frog.

Fgf differentially controls cross-antagonism between cardiac and haemangioblast regulators

Fibroblast growth factor (Fgf) has been implicated in the control of heart size during development, although whether this is by controlling cell fate, survival or proliferation has not been clear. Here, we show that Fgf, without affecting survival or proliferation, acts during gastrulation to drive cardiac fate and restrict anterior haemangioblast fate in zebrafish embryos. The haemangioblast programme was thought to be activated before the cardiac programme and is repressive towards it, suggesting that activation by Fgf of the cardiac programme might be via suppression of the haemangioblast programme. However, we show that the cardiac regulator nkx2.5 can also repress the haemangioblast programme and, furthermore, that cardiac specification still requires Fgf signalling even when haemangioblast regulators are independently suppressed. We further show that nkx2.5 and the cloche candidate gene lycat are expressed during gastrulation and regulated by Fgf, and that nkx2.5 overexpression, together with loss of the lycat targets etsrp and scl can stably induce expansion of the heart. We conclude that Fgf controls cardiac and haemangioblast fates by the simultaneous regulation of haemangioblast and cardiac regulators. We propose that elevation of Fgf signalling in the anterior haemangioblast territory could have led to its recruitment into the heart field during evolution, increasing the size of the heart.

Regulation of endothelial cell development by ETS transcription factors

The ETS family of transcription factors plays an essential role in controlling endothelial gene expression. Multiple members of the ETS family are expressed in the developing endothelium and evidence suggests that the proteins function, to some extent, redundantly. However, recent studies have demonstrated a crucial non-redundant role for ETV2, as a primary player in specification and differentiation of the endothelial lineage. Here, we review the contribution of ETS factors, and their partner proteins, to the regulation of embryonic vascular development.

Abnormal expression of FLI1 protein is an adverse prognostic factor in acute myeloid leukemia

Friend leukemia virus integration 1 (FLI1), an Ets transcription factor family member, is linked to acute myelogenous leukemia (AML) by chromosomal events at the FLI1 locus, but the biologic impact of FLI1 expression on AML is unknown. FLI1 protein expression was measured in 511 newly diagnosed AML patients. Expression was similar in peripheral blood (PB) and BM and higher at diagnosis than at relapse (P = .02). Compared with normal CD34(+) cells, expression in AML was above or below normal in 32% and 5% of patients, respectively. Levels were negatively correlated with an antecedent hematologic disorder (P = .002) but not with age or cytogenetics. Mutated NPM1 (P = .0007) or FLT3-ITD (P < .02) had higher expression. FLI1 levels were negatively correlated with 10 of 195 proteins associated with proliferation and stromal interaction, and positively correlated (R > 0.3) with 19 others. The FLI1 level was not predictive of remission attainment, but patients with low or high FLI1 expression had shorter remission duration (22.6 and 40.3 vs 51.1 weeks, respectively; P = .01) and overall survival (45.2 and 35.4 vs 59.4 weeks, respectively; P = .03). High FLI1 levels were adverse in univariate and multivariate analysis. FLI1 expression is frequently abnormal and prognostically adverse in AML. FLI1 and/or its response genes may be therapeutically targetable to interfere with AML cell biology.

Etv2/ER71 induces vascular mesoderm from Flk1+PDGFRα+ primitive mesoderm

Etv2 (Ets Variant 2) has been shown to be an indispensable gene for the development of hematopoietic cells (HPCs)/endothelial cells (ECs). However, how Etv2 specifies the mesoderm-generating HPCs/ECs remains incompletely understood. In embryonic stem cell (ESC) differentiation culture and Etv2-null embryos, we show that Etv2 is dispensable for generating primitive Flk-1(+)/PDGFRα(+) mesoderm but is required for the progression of Flk-1(+)/PDGFRα(+) cells into vascular/hematopoietic mesoderm. Etv2-null ESCs and embryonic cells were arrested as Flk-1(+)/PDGFRα(+) and failed to generate Flk-1(+)/PDGFRα(-) mesoderm. Flk-1(+)/Etv2(+) early embryonic cells showed significantly higher hemato-endothelial potential than the Flk-1(+)/Etv2(-) population, suggesting that Etv2 specifies a hemato-endothelial subset of Flk-1(+) mesoderm. Critical hemato-endothelial genes were severely down-regulated in Etv2-null Flk-1(+) cells. Among those genes Scl, Fli1, and GATA2 were expressed simultaneously with Etv2 in early embryos and seemed to be critical targets. Etv2 reexpression in Etv2-null cells restored the development of CD41(+), CD45(+), and VE-cadherin(+) cells. Expression of Scl or Fli1 alone could also restore HPCs/ECs in the Etv2-null background, indicating that these 2 genes are critical downstream targets. Furthermore, VEGF induced Etv2 potently and rapidly in Flk-1(+) mesoderm. We propose that Flk-1(+)/PDGFRα(+) primitive mesoderm is committed into Flk-1(+)/PDGFRα(-) vascular mesoderm through Etv2 and that up-regulation of Etv2 by VEGF promotes this commitment.

Key transcriptional regulators of early vascular development

The formation of the vasculature depends on the precise spatial and temporal control of gene expression to define endothelial cell identity and to ensure the correct distribution and structure of the forming vessel network. This review provides an overview of the establishment of the vascular system, accompanied by a detailed discussion of the transcription factors involved in regulating endothelial gene expression during vasculogenesis and early vessel formation in both fish and mammalian systems. We also review the transcriptional pathways lying both upstream and downstream of key vascular transcription factors.

Transcription factor networks in erythroid cell and megakaryocyte development

Erythroid cells and megakaryocytes are derived from a common precursor, the megakaryocyte-erythroid progenitor. Although these 2 closely related hematopoietic cell types share many transcription factors, there are several key differences in their regulatory networks that lead to differential gene expression downstream of the megakaryocyte-erythroid progenitor. With the advent of next-generation sequencing and our ability to precisely define transcription factor chromatin occupancy in vivo on a global scale, we are much closer to understanding how these 2 lineages are specified and in general how transcription factor complexes govern hematopoiesis.

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.

Fli+ etsrp+ hemato-vascular progenitor cells proliferate at the lateral plate mesoderm during vasculogenesis in zebrafish

BACKGROUND

Vasculogenesis, the de novo formation of blood vessels from precursor cells is critical for a developing embryo. However, the signals and events that dictate the formation of primary axial vessels remain poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we use ets-related protein-1 (etsrp), which is essential for vascular development, to analyze the early stages of vasculogenesis in zebrafish. We found etsrp(+) cells of the head, trunk and tail follow distinct developmental sequences. Using a combination of genetic, molecular and chemical approaches, we demonstrate that fli(+)etsrp(+) hemato-vascular progenitors (FEVPs) are proliferating at the lateral plate mesoderm (LPM). The Shh-VEGF-Notch-Hey2 signaling pathway controls the proliferation process, and experimental modulation of single components of this pathway alters etsrp(+) cell numbers at the LPM.

CONCLUSIONS/SIGNIFICANCE

This study for the first time defines factors controlling proliferation, and cell numbers of pre-migratory FEVPs in zebrafish.

Promotion of avian endothelial cell differentiation by GATA transcription factors

In the avian embryo, endothelial cells originate from several sources, including the lateral plate and somite mesoderm. In this study, we show that Gata transcription factors are expressed in the lateral plate and in vasculogenic regions of the avian somite and are able to promote a vascular endothelial fate when ectopically expressed in somite precursors. A fusion of GATA4 to the transcriptional activator VP16 promoted endothelium formation, indicating that GATA transcription factors promote vasculogenesis via activation of downstream targets, while a fusion of GATA4 to the transcriptional repressor engrailed repressed expression of Vascular Endothelial Growth Factor Receptor 2, a marker of endothelial precursors. These findings indicate a role for GATA transcription factors in the differentiation of the endothelium.

In vitro organogenesis using multipotent cells

Abstract The establishment of efficient methods for promoting stem cell differentiation into target cells is important not only in regenerative medicine, but also in drug discovery. In addition to embryonic stem (ES) cells and various somatic stem cells, such as mesenchymal stem cells derived from bone marrow, adipose tissue, and umbilical cord blood, a novel dedifferentiation technology that allows the generation of induced pluripotent stem (iPS) cells has been recently developed. Although an increasing number of stem cell populations are being described, there remains a lack of protocols for driving the differentiation of these cells. Regeneration of organs from stem cells in vitro requires precise blueprints for each differentiation step. To date, studies using various model organisms, such as zebrafish, Xenopus laevis, and gene-targeted mice, have uncovered several factors that are critical for the development of organs. We have been using X. laevis, the African clawed frog, which has developmental patterns similar to those seen in humans. Moreover, Xenopus embryos are excellent research tools for the development of differentiation protocols, since they are available in high numbers and are sufficiently large and robust for culturing after simple microsurgery. In addition, Xenopus eggs are fertilized externally, and all stages of the embryo are easily accessible, making it relatively easy to study the functions of individual gene products during organogenesis using microinjection into embryonic cells. In the present review, we provide examples of methods for in vitro organ formation that use undifferentiated Xenopus cells. We also describe the application of amphibian differentiation protocols to mammalian stem cells, so as to facilitate the development of efficient methodologies for in vitro differentiation.

Immune-related zinc finger gene ZFAT is an essential transcriptional regulator for hematopoietic differentiation in blood islands

TAL1 plays pivotal roles in vascular and hematopoietic developments through the complex with LMO2 and GATA1. Hemangioblasts, which have a differentiation potential for both endothelial and hematopoietic lineages, arise in the primitive streak and migrate into the yolk sac to form blood islands, where primitive hematopoiesis occurs. ZFAT (a zinc-finger gene in autoimmune thyroid disease susceptibility region/an immune-related transcriptional regulator containing 18 C(2)H(2)-type zinc-finger domains and one AT-hook) was originally identified as an immune-related transcriptional regulator containing 18 C(2)H(2)-type zinc-finger domains and one AT-hook, and is highly conserved among species. ZFAT is thought to be a critical transcription factor involved in immune-regulation and apoptosis; however, developmental roles for ZFAT remain unknown. Here we show that Zfat-deficient (Zfat(-/-)) mice are embryonic-lethal, with impaired differentiation of hematopoietic progenitor cells in blood islands, where ZFAT is exactly expressed. Expression levels of Tal1, Lmo2, and Gata1 in Zfat(-/-) yolk sacs are much reduced compared with those of wild-type mice, and ChIP-PCR analysis revealed that ZFAT binds promoter regions for these genes in vivo. Furthermore, profound reduction in TAL1, LMO2, and GATA1 protein expressions are observed in Zfat(-/-) blood islands. Taken together, these results suggest that ZFAT is indispensable for mouse embryonic development and functions as a critical transcription factor for primitive hematopoiesis through direct-regulation of Tal1, Lmo2, and Gata1. Elucidation of ZFAT functions in hematopoiesis might lead to a better understanding of transcriptional networks in differentiation and cellular programs of hematopoietic lineage and provide useful information for applied medicine in stem cell therapy.

The impact of Fli1 deficiency on the pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is an autoimmune inflammatory disease with unknown etiology characterized by microvascular injury and fibrosis of the skin and internal organs. A growing body of evidence suggests that deficiency of the transcription factor Fli1 (Friend leukemia integration-1) has a pivotal role in the pathogenesis of SSc. Fli1 is expressed in fibroblasts, endothelial cells, and immune cells, and has important roles in the activation, differentiation, development, and survival of these cells. Previous studies demonstrated that Fli1 is downregulated in SSc fibroblasts by an epigenetic mechanism and a series of experiments with Fli1-deficient animal models revealed that Fli1 deficiency in fibroblasts and endothelial cells reproduces the histopathologic features of fibrosis and vasculopathy in SSc, respectively. In this article, we review the impact of Fli1 deficiency on the pathogenesis of SSc and discuss a new therapeutic strategy for SSc by targeting the transcription factor Fli1.

The sequential expression of CD40 and Icam2 defines progressive steps in the formation of blood precursors from the mesoderm germ layer

During embryogenesis, the hematopoietic program is specified from the mesodermal germ layer through the formation of hemangioblast. This precursor gives rise to a hemogenic endothelium that later on matures to generate primitive and definitive hematopoietic precursors. A lack of specific cell surface markers to identify cells with discrete developmental potential is a major hurdle in the quest to further understand the cellular and molecular program governing blood formation. In the present study, we identify CD40 and Icam2, two markers typically associated with the adult immunological compartment, as expressed at the earliest stages of blood specification both in vitro and in vivo. Using in vitro serum-free culture conditions that support the efficient and directed differentiation of embryonic stem cells, we show that the sequential expression of CD40 and Icam2 delineate a transition in the acquisition of the blood potential from hemangioblast to hemogenic endothelium leading to the formation of primitive and definitive hematopoietic progenitors. CD40 is transiently expressed at the onset of blood development and marks first the hemangioblast then the hemogenic endothelium but is no longer expressed on fully committed hematopoietic precursors within the fetal liver. In contrast, Icam2 is first expressed on the hemogenic endothelium and its expression persists on fetal liver hematopoietic progenitors. Taken together, our data identify novel cell surface markers allowing us to further refine our understanding of the events marking progressive hematopoietic commitment from the mesoderm germ layer.

The inositol phosphatase SHIP-1 is negatively regulated by Fli-1 and its loss accelerates leukemogenesis

The activation of Fli-1, an Ets transcription factor, is the critical genetic event in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. Fli-1 overexpression leads to erythropoietin-dependent erythroblast proliferation, enhanced survival, and inhibition of terminal differentiation, through activation of the Ras pathway. However, the mechanism by which Fli-1 activates this signal transduction pathway has yet to be identified. Down-regulation of the Src homology 2 (SH2) domain-containing inositol-5-phosphatase-1 (SHIP-1) is associated with erythropoietin-stimulated erythroleukemic cells and correlates with increased proliferation of transformed cells. In this study, we have shown that F-MuLV-infected SHIP-1 knockout mice display accelerated erythroleukemia progression. In addition, RNA interference (RNAi)-mediated suppression of SHIP-1 in erythroleukemia cells activates the phosphatidylinositol 3-kinase (PI 3-K) and extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathways, blocks erythroid differentiation, accelerates erythropoietin-induced proliferation, and leads to PI 3-K-dependent Fli-1 up-regulation. Chromatin immunoprecipitation and luciferase assays confirmed that Fli-1 binds directly to an Ets DNA binding site within the SHIP-1 promoter and suppresses SHIP-1 transcription. These data provide evidence to suggest that SHIP-1 is a direct Fli-1 target, SHIP-1 and Fli-1 regulate each other in a negative feedback loop, and the suppression of SHIP-1 by Fli-1 plays an important role in the transformation of erythroid progenitors by F-MuLV.

Endothelial Fli1 deficiency impairs vascular homeostasis: a role in scleroderma vasculopathy

Systemic sclerosis or scleroderma (SSc) is a complex autoimmune connective tissue disease characterized by obliterative vasculopathy and tissue fibrosis. The molecular mechanisms underlying SSc vasculopathy are largely unknown. Friend leukemia integration factor 1 (Fli1), an important regulator of immune function and collagen fibrillogenesis, is expressed at reduced levels in endothelial cells in affected skin of patients with SSc. To develop a disease model and to investigate the function of Fli1 in the vasculature, we generated mice with a conditional deletion of Fli1 in endothelial cells (Fli1 CKO). Fli1 CKO mice showed a disorganized dermal vascular network with greatly compromised vessel integrity and markedly increased vessel permeability. We show that Fli1 regulates expression of genes involved in maintaining vascular homeostasis including VE-cadherin, platelet endothelial cell adhesion molecule 1, type IV collagen, matrix metalloproteinase 9, platelet-derived growth factor B, and S1P(1) receptor. Accordingly, Fli1 CKO mice are characterized by down-regulation of VE-cadherin and platelet endothelial cell adhesion molecule 1, impaired development of basement membrane, and a decreased presence of alpha-smooth muscle actin-positive cells in dermal microvessels. This phenotype is consistent with a role of Fli1 as a regulator of vessel maturation and stabilization. Importantly, vascular characteristics of Fli1 CKO mice are recapitulated by SSc microvasculature. Thus, persistently reduced levels of Fli1 in endothelial cells may play a critical role in the development of SSc vasculopathy.

Blood cell generation from the hemangioblast

Understanding how blood cells are generated is important from a biological perspective but also has potential implications in the treatment of blood diseases. Such knowledge could potentially lead to defining new conditions to amplify hematopoietic stem cells (HSCs) or could translate into new methods to produce HSCs, or other types of blood cells, from human embryonic stem cells or induced pluripotent stem cells. Additionally, as most key transcription factors regulating early hematopoietic development have also been implicated in various types of leukemia, understanding their function during normal development could result in a better comprehension of their roles during abnormal hematopoiesis in leukemia. In this review, we discuss our current understanding of the molecular and cellular mechanisms of blood development from the earliest hematopoietic precursor, the hemangioblast, a precursor for both endothelial and hematopoietic cell lineages.

Sox7-sustained expression alters the balance between proliferation and differentiation of hematopoietic progenitors at the onset of blood specification

The molecular mechanisms that regulate the balance between proliferation and differentiation of precursors at the onset of hematopoiesis specification are poorly understood. By using a global gene expression profiling approach during the course of embryonic stem cell differentiation, we identified Sox7 as a potential candidate gene involved in the regulation of blood lineage formation from the mesoderm germ layer. In the present study, we show that Sox7 is transiently expressed in mesodermal precursors as they undergo specification to the hematopoietic program. Sox7 knockdown in vitro significantly decreases the formation of both primitive erythroid and definitive hematopoietic progenitors as well as endothelial progenitors. In contrast, Sox7-sustained expression in the earliest committed hematopoietic precursors promotes the maintenance of their multipotent and self-renewing status. Removal of this differentiation block driven by Sox7-enforced expression leads to the efficient differentiation of hematopoietic progenitors to all erythroid and myeloid lineages. This study identifies Sox7 as a novel and important player in the molecular regulation of the first committed blood precursors. Furthermore, our data demonstrate that the mere sustained expression of Sox7 is sufficient to completely alter the balance between proliferation and differentiation at the onset of hematopoiesis.

Discovery of microvascular miRNAs using public gene expression data: miR-145 is expressed in pericytes and is a regulator of Fli1

Background

A function for the microRNA (miRNA) pathway in vascular development and angiogenesis has been firmly established. miRNAs with selective expression in the vasculature are attractive as possible targets in miRNA-based therapies. However, little is known about the expression of miRNAs in microvessels in vivo. Here, we identified candidate microvascular-selective miRNAs by screening public miRNA expression datasets.

Methods

Bioinformatics predictions of microvascular-selective expression were validated with real-time quantitative reverse transcription PCR on purified microvascular fragments from mouse. Pericyte expression was shown with in situ hybridization on tissue sections. Target sites were identified with 3' UTR luciferase assays, and migration was tested in a microfluid chemotaxis chamber.

Results

miR-145, miR-126, miR-24, and miR-23a were selectively expressed in microvascular fragments isolated from a range of tissues. In situ hybridization and analysis of Pdgfb retention motif mutant mice demonstrated predominant expression of miR-145 in pericytes. We identified the Ets transcription factor Friend leukemia virus integration 1 (Fli1) as a miR-145 target, and showed that elevated levels of miR-145 reduced migration of microvascular cells in response to growth factor gradients in vitro.

Conclusions

miR-126, miR-24 and miR-23a are selectively expressed in microvascular endothelial cells in vivo, whereas miR-145 is expressed in pericytes. miR-145 targets the hematopoietic transcription factor Fli1 and blocks migration in response to growth factor gradients. Our findings have implications for vascular disease and provide necessary information for future drug design against miRNAs with selective expression in the microvasculature.

Mta3-NuRD complex is a master regulator for initiation of primitive hematopoiesis in vertebrate embryos

Metastasis-associated antigens 1/2/3 (Mta1/2/3) are components of nucleosome remodeling and deacetylase (NuRD) complexes and have been found to play roles in embryonic development and homeostasis. However, their functions in primitive hematopoiesis are unknown. In this study, we demonstrate that knockdown of mta3 by antisense morpholinos abolishes primitive hematopoietic lineages and causes abnormal angiogenesis in zebrafish embryos. However, the expression of the pronephric duct and paraxial mesoderm markers is unaltered and the specification of angioblasts is unaffected in mta3 morphants. The results suggest that mta3 is specifically required for primitive hematopoiesis. Furthermore, inhibition of deacetylase activity with the inhibitors valproic acid (VPA) or trichostatin A (TSA) in zebrafish embryos completely blocks primitive hematopoiesis, resulting in hematopoietic defects almost identical to those seen in mta3 morphants. Importantly, overexpression of scl or scl and lmo2, 2 master genes for primitive hematopoiesis, is able to overturn effects of mta3 knockdown or VPA/TSA treatment; and overexpression of mta3, and human MBD3 or HDAC1, 2 other components of NuRD complex, enhances the expression of scl and lmo2 in the posterior lateral plate mesoderm during early primitive hematopoiesis. We conclude that Mta3-NuRD complex is essential for the initiation of primitive hematopoiesis. Thus, our findings provide new insight into the regulatory hierarchy of primitive hematopoiesis in vertebrates.

[Genes that make the endothelial identity]

The endothelium is a tissue with a distinct identity due to the specific expression of molecular markers by endothelial cells. Further, the endothelium displays a structural heterogeneity illustrated by the expression of specific markers in arteries and in veins. Here, we present a review of the transcriptional and epigenetic mechanisms regulating the expression of the main markers of endothelial cells in man and mouse, demonstrating that there is no common and unique mechanism of specific expression of genes in these cells.

Common genetic control of haemangioblast and cardiac development in zebrafish

Over the past few years it has become clear that over half of the mammalian heart derives from outside the heart field as originally defined. Such a second heart field, however, has not been described in zebrafish, which could explain its smaller, two-chambered heart. Instead, zebrafish have a population of haemangioblasts, which is absent in mammalian embryos, raising the possibility that these cells represent the evolutionary ancestor of the second heart field. Here, we show for the first time that the genetic programmes of these anterior haemangioblasts and the adjacent heart field are co-regulated, by transcription factors previously associated with heart but not blood or endothelial development. We demonstrate that gata4, gata5 and gata6 are essential for anterior haemangioblast specification, and for subsequent myelopoiesis, acting as early as cloche and upstream of scl. The requirement for gata4, gata5 and gata6 in myeloid, endothelial and cardiac specification is in the mesoderm, but these factors also control, from within the endoderm and the yolk syncytial layer, the migration of the cardiac precursors as they differentiate. This genetic link between the blood/endothelial and cardiac programmes supports the notion that this haemangioblast population in zebrafish is an evolutionary antecedent of the second heart field, and has implications for the differentiation of haemangioblasts and cardiomyocytes from pluripotent cells, and for the origins of stem cells in the adult heart.

Discovery and characterization of novel vascular and hematopoietic genes downstream of etsrp in zebrafish

The transcription factor Etsrp is required for vasculogenesis and primitive myelopoiesis in zebrafish. When ectopically expressed, etsrp is sufficient to induce the expression of many vascular and myeloid genes in zebrafish. The mammalian homolog of etsrp, ER71/Etv2, is also essential for vascular and hematopoietic development. To identify genes downstream of etsrp, gain-of-function experiments were performed for etsrp in zebrafish embryos followed by transcription profile analysis by microarray. Subsequent in vivo expression studies resulted in the identification of fourteen genes with blood and/or vascular expression, six of these being completely novel. Regulation of these genes by etsrp was confirmed by ectopic induction in etsrp overexpressing embryos and decreased expression in etsrp deficient embryos. Additional functional analysis of two newly discovered genes, hapln1b and sh3gl3, demonstrates their importance in embryonic vascular development. The results described here identify a group of genes downstream of etsrp likely to be critical for vascular and/or myeloid development.

The role of the ETS factor erg in zebrafish vasculogenesis

Erg, a member of the ETS family of transcription factors, has been implicated by previous studies in endothelial and haematopoietic development. Deregulation of the human ERG locus is associated with acute myeloid leukaemia, prostate cancer and Ewing's sarcoma. To better understand the role of Erg during early development, we utilised the zebrafish as a model amenable to descriptive and functional studies in vivo. Zebrafish have a single erg gene that is expressed in mesoderm and its vascular derivatives during angioblast migration, vasculogenesis and early angiogenesis. Mutant and morphant expression analyses placed erg in a genetic pathway downstream of cloche, tal1/scl and etsrp during early angioblast migration. Furthermore, a combination of gain-of-function and loss-of-function studies suggested a redundant yet specific role for erg in both angioblast specification/proliferation and early angiogenesis, and a synergistic interaction with the critical ETS factor etsrp.

Transcriptional control of endothelial cell development

The transcription factors that regulate endothelial cell development have been a focus of active research for several years, and many players in the endothelial transcriptional program have been identified. This review discusses the function of several major regulators of endothelial transcription, including members of the Sox, Ets, Forkhead, GATA, and Kruppel-like families. This review also highlights recent developments aimed at unraveling the combinatorial mechanisms and transcription factor interactions that regulate endothelial cell specification and differentiation during vasculogenesis and angiogenesis.

Genome-wide analysis of the zebrafish ETS family identifies three genes required for hemangioblast differentiation or angiogenesis

ETS domain transcription factors have been linked to hematopoiesis, vasculogenesis, and angiogenesis. However, their biological functions and the mechanisms of action, remain incompletely understood. Here, we have performed a systematic analysis of zebrafish ETS domain genes and identified 31 in the genome. Detailed gene expression profiling revealed that 12 of them are expressed in blood and endothelial precursors during embryonic development. Combined with a phylogenetic tree assay, this suggests that some of the coexpressed genes may have redundant or additive functions in these cells. Loss-of-function analysis of 3 of them, erg, fli1, and etsrp, demonstrated that erg and fli1 act cooperatively and are required for angiogenesis possibly via direct regulation of an endothelial cell junction molecule, VE-cadherin, whereas etsrp is essential for primitive myeloid/endothelial progenitors (hemangioblasts) in zebrafish. Taken together, these results provide a global view of the ETS genes in the zebrafish genome during embryogenesis and provide new insights on the functions and biology of erg, fli1, and etsrp, which could be applicable to higher vertebrates, including mice and humans.