HoxA3 is an apical regulator of hemogenic endothelium

Lillehei Heart Institute and Department of Pediatrics, University of Minnesota, 312 Church St. SE, Minneapolis, MN 55455, USA.
Nature Cell Biology (Impact Factor: 19.68). 01/2011; 13(1):72-8. DOI: 10.1038/ncb2137
Source: PubMed


During development, haemogenesis occurs invariably at sites of vasculogenesis. Between embryonic day (E) 9.5 and E10.5 in mice, endothelial cells in the caudal part of the dorsal aorta generate haematopoietic stem cells and are referred to as haemogenic endothelium. The mechanisms by which haematopoiesis is restricted to this domain, and how the morphological transformation from endothelial to haematopoietic is controlled are unknown. We show here that HoxA3, a gene uniquely expressed in the embryonic but not yolk sac vasculature, restrains haematopoietic differentiation of the earliest endothelial progenitors, and induces reversion of the earliest haematopoietic progenitors into CD41-negative endothelial cells. This reversible modulation of endothelial-haematopoietic state is accomplished by targeting key haematopoietic transcription factors for downregulation, including Runx1, Gata1, Gfi1B, Ikaros, and PU.1. Through loss-of-function, and gain-of-function epistasis experiments, and the identification of antipodally regulated targets, we show that among these factors, Runx1 is uniquely able to erase the endothelial program set up by HoxA3. These results suggest both why a frank endothelium does not precede haematopoiesis in the yolk sac, and why haematopoietic stem cell generation requires Runx1 expression only in endothelial cells.

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Available from: Michelina Iacovino, Feb 18, 2014
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    • "LacZ/+ embryos showed that there were no LacZ + cells in the intraembryonic region, consistent with previous studies that showed that Runx1 remains undetectable in the embryo proper until E8.25 (4–7 somite pairs [sp] stage) (Iacovino et al., 2011; North et al., 1999; Zeigler et al., 2006). Runx1 expression in the embryo proper became detectable at $E9.0 in the posterior vascular complex, including the DA (Figure S3B). "
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    ABSTRACT: A large gap exists in our understanding of the course of differentiation from mesoderm to definitive hematopoietic stem cells (HSCs). Previously, we reported that Runx1(+) cells in embryonic day 7.5 (E7.5) embryos contribute to the hemogenic endothelium in the E10.5 aorta-gonad-mesonephros (AGM) region and HSCs in the adult bone marrow. Here, we show that two Runx1(+) populations subdivided by Gata1 expression exist in E7.5 embryos. The hemogenic endothelium and the HSCs are derived only from the Runx1(+)Gata1(-) population. A subset of this population moves from the extra- to intraembryonic region during E7.5-E8.0, where it contributes to the hemogenic endothelium of the dorsal aorta (DA). Migration occurs before the heartbeat is initiated, and it is independent of circulation. This suggests a developmental trajectory from Runx1(+) cells in the E7.5 extraembryonic region to definitive HSCs via the hemogenic endothelium.
    Cell Reports 06/2014; 8(1). DOI:10.1016/j.celrep.2014.05.055 · 8.36 Impact Factor
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    • "Runx1 is prematurely upregulated at the first stage of the hemogenic endothelium (CD41− cells), together with RUNX1-responsive targets Gfi1 and Spi1 (supplementary material Table S1) (Lancrin et al., 2012; Lichtinger et al., 2012) which is one of the first signs that hematopoietic differentiation is going astray. A reason for this could be the fact that HoxA3, which is specifically expressed at this stage and has been shown to be required for the repression of Runx1 in the hemogenic endothelium (Iacovino et al., 2011) is 1.6-fold downregulated (supplementary material Table S1). At the next stage, after the endothelial-hematopoietic transition (CD41+ cells), the expression of the erythroid regulators Klf1 and Gata1, as well as expression of embryonic globin genes, increases almost fourfold, whereas expression of genes specifying the myeloid lineage, such as Spi1 and Cebpe, is reduced (although normally expression would increase) (Lichtinger et al., 2012) (supplementary material Table S1). "
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    ABSTRACT: Mammalian development is regulated by the interplay of tissue-specific and ubiquitously expressed transcription factors, such as Sp1. Sp1 knockout mice die in utero with multiple phenotypic aberrations, but the underlying molecular mechanism of this differentiation failure has been elusive. Here, we have used conditional knockout mice as well as the differentiation of mouse ES cells as a model with which to address this issue. To this end, we examined differentiation potential, global gene expression patterns and Sp1 target regions in Sp1 wild-type and Sp1-deficient cells representing different stages of hematopoiesis. Sp1(-/-) cells progress through most embryonic stages of blood cell development but cannot complete terminal differentiation. This failure to fully differentiate is not seen when Sp1 is knocked out at later developmental stages. For most Sp1 target and non-target genes, gene expression is unaffected by Sp1 inactivation. However, Cdx genes and multiple Hox genes are stage-specific targets of Sp1 and are downregulated at an early stage. As a consequence, expression of genes involved in hematopoietic specification is progressively deregulated. Our work demonstrates that the early absence of active Sp1 sets a cascade in motion that culminates in a failure of terminal hematopoietic differentiation and emphasizes the role of ubiquitously expressed transcription factors for tissue-specific gene regulation. In addition, our global side-by-side analysis of the response of the transcriptional network to perturbation sheds a new light on the regulatory hierarchy of hematopoietic specification.
    Development 06/2014; 141(12):2391-2401. DOI:10.1242/dev.106054 · 6.46 Impact Factor
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    • "Here, Hoxa3 domain of expression determines when and where hematopoietic cells emerge from the endothelium. Hoxa3 represses a cascade of transcription factors that promote hemo- genesis, including Runx1, and induces a set of genes that maintain endothelial character, acting as a gatekeeper for the entry into the hematopoietic program (Iacovino et al., 2011). Anterior Hox genes have been suggested to play an important role also in mammalian cardiac mesoderm. "
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    ABSTRACT: During embryonic development, cells become gradually restricted in their developmental potential and start elaborating lineage-specific transcriptional networks to ultimately acquire a unique differentiated state. Hox genes play a central role in specifying regional identities, thereby providing the cell with critical information on positional value along its differentiation path. The exquisite DNA-binding specificity of the Hox proteins is frequently dependent upon their interaction with members of the TALE family of homeodomain proteins. In addition to their function as Hox-cofactors, TALE homeoproteins control multiple crucial developmental processes through Hox-independent mechanisms. Here, we will review recent findings on the function of both Hox and TALE proteins in cell differentiation, referring mostly to vertebrate species. In addition, we will discuss the direct implications of this knowledge on cell plasticity and cell reprogramming. Developmental Dynamics, 2013. © 2013 Wiley Periodicals, Inc.
    Developmental Dynamics 01/2014; 243(1). DOI:10.1002/dvdy.24075 · 2.38 Impact Factor
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