VEGFA-dependent and -independent pathways synergise to drive Scl expression and initiate programming of the blood stem cell lineage in Xenopus.
ABSTRACT: The first haematopoietic stem cells share a common origin with the dorsal aorta and derive from putative adult haemangioblasts in the dorsal lateral plate (DLP) mesoderm. Here we show that the transcription factor (TF) stem cell leukaemia (Scl/Tal1) is crucial for development of these adult haemangioblasts in Xenopus and establish the regulatory cascade controlling its expression. We show that VEGFA produced in the somites is required to initiate adult haemangioblast programming in the adjacent DLP by establishing endogenous VEGFA signalling. This response depends on expression of the VEGF receptor Flk1, driven by Fli1 and Gata2. Scl activation requires synergy between this VEGFA-controlled pathway and a VEGFA-independent pathway controlled by Fli1, Gata2 and Etv2/Etsrp/ER71, which also drives expression of the Scl partner Lmo2. Thus, the two ETS factors Fli1 and Etv6, which drives the VEGFA expression in both somites and the DLP, sit at the top of the adult haemangioblast gene regulatory network (GRN). Furthermore, Gata2 is initially activated by Fli1 but later maintained by another ETS factor, Etv2. We also establish that Flk1 and Etv2 act independently in the two pathways to Scl activation. Thus, detailed temporal, epistatic measurements of key TFs and VEGFA plus its receptor have enabled us to build a Xenopus adult haemangioblast GRN.
Project description: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.
Project description:Conservation of the vertebrate body plan has been attributed to the evolutionary stability of gene-regulatory networks (GRNs). We describe a regulatory circuit made up of Gata2, Fli1, and Scl/Tal1 and their enhancers, Gata2-3, Fli1+12, and Scl+19, that operates during specification of hematopoiesis in the mouse embryo. We show that the Fli1+12 enhancer, like the Gata2-3 and Scl+19 enhancers, targets hematopoietic stem cells (HSCs) and relies on a combination of Ets, Gata, and E-Box motifs. We show that the Gata2-3 enhancer also uses a similar cluster of motifs and that Gata2, Fli1, and Scl are expressed in embryonic day-11.5 dorsal aorta where HSCs originate and in fetal liver where they multiply. The three HSC enhancers in these tissues and in ES cell-derived hemangioblast equivalents are bound by each of these transcription factors (TFs) and form a fully connected triad that constitutes a previously undescribed example of both this network motif in mammalian development and a GRN kernel operating during the specification of a mammalian stem cell.
Project description:The ETS transcription factor Etv2 is necessary and sufficient for the generation of hematopoietic and endothelial cells. However, upstream regulators of Etv2 in hemangiogenesis, generation of hematopoietic and endothelial cells, have not been clearly addressed. Here we track the developmental route of hemangiogenic progenitors from mouse embryonic stem cells, perform genome-wide CRISPR screening, and transcriptome analysis of en route cell populations by utilizing Brachyury, Etv2, or Scl reporter embryonic stem cell lines to further understand the mechanisms that control hemangiogenesis. We identify the forkhead transcription factor Foxh1, in part through Eomes, to be critical for the formation of FLK1+ mesoderm, from which the hemangiogenic fate is specified. Importantly, hemangiogenic fate is specified not simply by the onset of Etv2 expression, but by a threshold-dependent mechanism, in which VEGF-FLK1 signaling plays an instructive role by promoting Etv2 threshold expression. These studies reveal comprehensive cellular and molecular pathways governing the hemangiogenic cell lineage development.How haematopoietic and endothelial cell lineages are specified is unclear. Here, the authors identify the forkhead transcription factor Foxh1 as regulating FLK1+ mesoderm formation in mouse embryonic stem cells, which in turn specifies hemangiogenic fate via Etv2.
Project description:Comprehensive understanding of the mechanisms regulating angiogenesis might provide new strategies for angiogenic therapies for treating diverse physiological and pathological ischemic conditions. The E-twenty six (ETS) factor Ets variant 2 (ETV2; aka Ets-related protein 71) is essential for the formation of hematopoietic and vascular systems. Despite its indispensable function in vessel development, ETV2 role in adult angiogenesis has not yet been addressed. We have therefore investigated the role of ETV2 in vascular regeneration.We used endothelial Etv2 conditional knockout mice and ischemic injury models to assess the role of ETV2 in vascular regeneration. Although Etv2 expression was not detectable under steady-state conditions, its expression was readily observed in endothelial cells after injury. Mice lacking endothelial Etv2 displayed impaired neovascularization in response to eye injury, wounding, or hindlimb ischemic injury. Lentiviral Etv2 expression in ischemic hindlimbs led to improved recovery of blood perfusion with enhanced vessel formation. After injury, fetal liver kinase 1 (Flk1), aka VEGFR2, expression and neovascularization were significantly upregulated by Etv2, whereas Flk1 expression and vascular endothelial growth factor response were significantly blunted in Etv2-deficient endothelial cells. Conversely, enforced Etv2 expression enhanced vascular endothelial growth factor-mediated endothelial sprouting from embryoid bodies. Lentiviral Flk1 expression rescued angiogenesis defects in endothelial Etv2 conditional knockout mice after hindlimb ischemic injury. Furthermore, Etv2(+/-); Flk1(+/-) double heterozygous mice displayed a more severe hindlimb ischemic injury response compared with Etv2(+/-) or Flk1(+/-) heterozygous mice, revealing an epistatic interaction between ETV2 and FLK1 in vascular regeneration.Our study demonstrates a novel obligatory role for the ETV2 in postnatal vascular repair and regeneration.
Project description:Independent mouse knockouts of Etv2 and Flk1 are embryonic lethal and lack hematopoietic and endothelial lineages. We previously reported that Flk1 activates Etv2 in the initiation of hematopoiesis and vasculogenesis. However, Flk1 and its ligand VEGF are expressed throughout development, from E7.0 to adulthood, whereas Etv2 is expressed only transiently during embryogenesis. These observations suggest a complex regulatory interaction between Flk1 and Etv2. To further examine the Flk1 and Etv2 regulatory interaction, we transduced Etv2 and Flk1 mutant ES cells with viral integrants that inducibly overexpress Flk1 or Etv2. We demonstrated that forced expression of Etv2 rescued the hematopoietic and endothelial potential of differentiating Flk1 and Etv2 mutant cells. We further discovered that forced expression of Flk1 can rescue that of the Flk1, but not Etv2 mutant cells. Therefore, we conclude that the requirement for Flk1 can be bypassed by expressing Etv2, supporting the notion that disruption of Etv2 expression is responsible for the early phenotypes of the Etv2 and Flk1 mutant embryos.
Project description:Previous reports regarding the genetic hierarchy between Ets related protein 71 (Er71/Etv2) and Flk1 is unclear. In the present study, we pursued a genetic approach to define the molecular cascade between Etv2 and Flk1. Using a transgenic Etv2-EYFP reporter mouse, we examined the expression pattern of Etv2 relative to Flk1 in the early conceptus. Etv2-EYFP was expressed in subset of Flk1 positive cells during primitive streak stages, suggesting that Flk1 is upstream of Etv2 during gastrulation. Analysis of reporter gene expression in Flk1 and Etv2 mutant mice further supports the hypothesis that Flk1 is necessary for Etv2 expression. The frequency of cells expressing Flk1 in Etv2 mutants is only modestly altered (21% decrease), whereas expression of the Etv2-EYFP transgenic reporter was severely reduced in the Flk1 null background. We further demonstrate using transcriptional assays that, in the presence of Flk1, the Etv2 promoter is activated by VEGF, the Flk1 ligand. Pharmacological inhibition studies demonstrate that VEGF mediated activation is dependent on p38 MAPK, which activates Creb. We identify the VEGF response element in the Etv2 promoter and demonstrate that Creb binds to this motif by EMSA and ChIP assays. In summary, we provide new evidence that VEGF activates Etv2 by signaling through Flk1, which activates Creb through the p38 MAPK signaling cascade.
Project description:The fetal liver kinase 1 (FLK-1)(+) hemangioblast can generate hematopoietic, endothelial, and smooth muscle cells (SMCs). ER71/ETV2, GATA2, and SCL form a core transcriptional network in hemangioblast development. Transient coexpression of these three factors during mesoderm formation stage in mouse embryonic stem cells (ESCs) robustly enhanced hemangioblast generation by activating bone morphogenetic protein (BMP) and FLK-1 signaling while inhibiting phosphatidylinositol 3-kinase, WNT signaling, and cardiac output. Moreover, etsrp, gata2, and scl inhibition converted hematopoietic field of the zebrafish anterior lateral plate mesoderm to cardiac. FLK-1(+) hemangioblasts generated by transient coexpression of the three factors (ER71-GATA2-SCL [EGS]-induced FLK-1(+)) effectively produced hematopoietic, endothelial, and SMCs in culture and in vivo. Importantly, EGS-induced FLK-1(+) hemangioblasts, when codelivered with mesenchymal stem cells as spheroids, were protected from apoptosis and generated functional endothelial cells and SMCs in ischemic mouse hindlimbs, resulting in improved blood perfusion and limb salvage. ESC-derived, EGS-induced FLK-1(+) hemangioblasts could provide an attractive cell source for future hematopoietic and vascular repair and regeneration.
Project description:The phlda3 gene encodes a small, 127-amino acid protein with only a PH domain, and is involved in tumor suppression, proliferation of islet ?-cells, insulin secretion, glucose tolerance, and liver injury. However, the role of phlda3 in vascular development is unknown. Here, we show that phlda3 overexpression decreases the expression levels of hemangioblast markers scl, fli1, and etsrp and intersegmental vessel (ISV) markers flk1 and cdh5, and disrupts ISV development in tg(flk1:GFP) and tg(fli1:GFP) zebrafish. Moreover, phlda3 overexpression inhibits the activation of protein kinase B (AKT) in zebrafish embryos, and the developmental defects of ISVs by phlda3 overexpression were reversed by the expression of a constitutively active form of AKT. These data suggest that phlda3 is a negative regulator of hemangioblast specification and ISV development via AKT signaling.
Project description:The bone morphogenetic protein (BMP)/SMAD signaling pathway is a critical regulator of angiogenic sprouting and is involved in vascular development in the embryo. SMAD1 and SMAD5, the core mediators of BMP signaling, are vital for this activity, yet little is known about their transcriptional regulation in endothelial cells. Here, we have integrated multispecies sequence conservation, tissue-specific chromatin, in vitro reporter assay, and in vivo transgenic data to identify and validate Smad1+63 and the Smad5 promoter as tissue-specific cis-regulatory elements that are active in the developing endothelium. The activity of these elements in the endothelium was dependent on highly conserved ETS, GATA, and E-box motifs, and chromatin immunoprecipitation showed high levels of enrichment of FLI1, GATA2, and SCL at these sites in endothelial cell lines and E11 dorsal aortas in vivo. Knockdown of FLI1 and GATA2 but not SCL reduced the expression of SMAD1 and SMAD5 in endothelial cells in vitro. In contrast, CD31(+) cKit(-) endothelial cells harvested from embryonic day 9 (E9) aorta-gonad-mesonephros (AGM) regions of GATA2 null embryos showed reduced Smad1 but not Smad5 transcript levels. This is suggestive of a degree of in vivo selection where, in the case of reduced SMAD1 levels, endothelial cells with more robust SMAD5 expression have a selective advantage.
Project description:Transdifferentiation is the direct conversion from one somatic cell type into another desired somatic cell type. This reprogramming method offers an attractive approach for regenerative medicine. Here, we demonstrate that neonatal fibroblasts can be transdifferentiated into endothelial cells using only four endothelial transcription factors, namely, ETV2, FLI1, GATA2, and KLF4. We observed a significant up-regulation of endothelial genes including KDR, CD31, CD144, and vWF in human neonatal foreskin (BJ) fibroblasts infected with the lentiviral construct encoding the open reading frame of the four transcription factors. We observed morphological changes in BJ fibroblasts from the fibroblastic spindle shape into a more endothelial-like cobblestone structures. Fluorescence-activated cell sorting analysis revealed that ~16% of the infected cells with the lentiviral constructs encoding 4F expressed CD31. The sorted cells were allowed to expand for 2?weeks and these cells were immunostained and found to express endothelial markers CD31. The induced endothelial cells also incorporated fluorescence-labeled acetylated low-density lipoprotein and efficiently formed capillary-like networks when seeded on Matrigel. These results suggested that the induced endothelial cells were functional in vitro. Taken together, we successfully demonstrated the direct conversion of human neonatal fibroblasts into endothelial cells by transduction of lentiviral constructs encoding endothelial lineage-specific transcription factors ETV2, FLI1, GATA2, and KLF4. The directed differentiation of fibroblasts into endothelial cells may have significant utility in diseases characterized by fibrosis and loss of microvasculature.