Article

Live imaging of Runx1 expression in the dorsal aorta tracks the emergence of blood progenitors from endothelial cells

Department of Molecular Medicine & Pathology, School of Medical Sciences, University of Auckland, Auckland, New Zealand.
Blood (Impact Factor: 10.43). 05/2010; 116(6):909-14. DOI: 10.1182/blood-2010-01-264382
Source: PubMed

ABSTRACT Blood cells of an adult vertebrate are continuously generated by hematopoietic stem cells (HSCs) that originate during embryonic life within the aorta-gonad-mesonephros region. There is now compelling in vivo evidence that HSCs are generated from aortic endothelial cells and that this process is critically regulated by the transcription factor Runx1. By time-lapse microscopy of Runx1-enhanced green fluorescent protein transgenic zebrafish embryos, we were able to capture a subset of cells within the ventral endothelium of the dorsal aorta, as they acquire hemogenic properties and directly emerge as presumptive HSCs. These nascent hematopoietic cells assume a rounded morphology, transiently occupy the subaortic space, and eventually enter the circulation via the caudal vein. Cell tracing showed that these cells subsequently populated the sites of definitive hematopoiesis (thymus and kidney), consistent with an HSC identity. HSC numbers depended on activity of the transcription factor Runx1, on blood flow, and on proper development of the dorsal aorta (features in common with mammals). This study captures the earliest events of the transition of endothelial cells to a hemogenic endothelium and demonstrates that embryonic hematopoietic progenitors directly differentiate from endothelial cells within a living organism.

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    • "Finally, HSCs colonize the kidney in zebrafish and the bone marrow in mammals, where they establish residence for the remainder of life. The zebrafish model has proven valuable to our understanding of HSPC development, including the first direct in vivo visualization of their emergence (Bertrand et al., 2010; Kissa and Herbomel, 2010; Lam et al., 2010). The transcription factor Runx1 is required for EHT in both mice and zebrafish (Chen et al., 2009; Kissa and Herbomel, 2010; Lancrin et al., 2009). "
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    ABSTRACT: The adult blood system is established by hematopoietic stem cells (HSCs), which arise during development from an endothelial-to-hematopoietic transition of cells comprising the floor of the dorsal aorta. Expression of aortic runx1 has served as an early marker of HSC commitment in the zebrafish embryo, but recent studies have suggested that HSC specification begins during the convergence of posterior lateral plate mesoderm (PLM), well before aorta formation and runx1 transcription. Further understanding of the earliest stages of HSC specification necessitates an earlier marker of hemogenic endothelium. Studies in mice have suggested that GATA2 might function at early stages within hemogenic endothelium. Two orthologs of Gata2 exist in zebrafish: gata2a and gata2b. Here, we report that gata2b expression initiates during the convergence of PLM, becoming restricted to emerging HSCs. We observe Notch-dependent gata2b expression within the hemogenic subcompartment of the dorsal aorta that is in turn required to initiate runx1 expression. Our results indicate that Gata2b functions within hemogenic endothelium from an early stage, whereas Gata2a functions more broadly throughout the vascular system. © 2015. Published by The Company of Biologists Ltd.
    Development 03/2015; 142(6):1050-61. DOI:10.1242/dev.119180 · 6.27 Impact Factor
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    • "Definitive hematopoietic stem cells (HSCs) replenish the blood supply throughout life, and their precursors are known to arise during embryogenesis at E10.5 in the region of the embryo containing the dorsal aorta, gonads, and mesonephroi (AGM) (Medvinsky and Dzierzak, 1996; Muller et al., 1994). Evidence from both zebrafish and mouse now implicate a subpopulation of endothelial cells with hemogenic potential as a de novo source of HSCs that contribute to life-long hematopoiesis (Bertrand et al., 2010; Boisset et al., 2010; Chen, M.J. et al., 2009; Eilken et al., 2009; Kissa and Herbomel, 2010; Lam et al., 2010; Lancrin et al., 2009; Zovein et al., 2008). Indeed, endothelial cells and nascent hematopoietic cells share several of the same cell surface markers, and both are derived from an angioblast population that expresses Brachyury and Flk-1 (Huber et al., 2004; Kabrun et al., 1997; Millauer et al., 1993; Shalaby et al., 1995, 1997; Yamaguchi et al., 1993). "
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    ABSTRACT: The hematopoietic system is dynamic during development and in adulthood, undergoing countless spatial and temporal transitions during the course of one's life. Microenvironmental cues in the many unique hematopoietic niches differ, characterized by distinct soluble molecules, membrane-bound factors, and biophysical features that meet the changing needs of the blood system. Research from the last decade has revealed the importance of substrate elasticity and biomechanical force in determination of stem cell fate. Our understanding of the role of these factors in hematopoiesis is still relatively poor; however, the developmental origin of blood cells from the endothelium provides a model for comparison. Many endothelial mechanical sensors and second messenger systems may also determine hematopoietic stem cell fate, self renewal, and homing behaviors. Further, the intimate contact of hematopoietic cells with mechanosensitive cell types, including osteoblasts, endothelial cells, mesenchymal stem cells, and pericytes, places them in close proximity to paracrine signaling downstream of mechanical signals. The objective of this review is to present an overview of the sensors and intracellular signaling pathways activated by mechanical cues and highlight the role of mechanotransductive pathways in hematopoiesis.
    Differentiation 07/2013; DOI:10.1016/j.diff.2013.06.004 · 2.84 Impact Factor
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    • "Live-imaging techniques showed that embryonic stem cells generated ECs that, in turn, produced HCs (Eilken et al., 2009; Lancrin et al., 2009). Finally, time-lapse approaches showed that this production occurs in vivo in mouse aortic explants (Boisset et al., 2010) and in whole zebrafish embryos (Bertrand et al., 2010a; Kissa and Herbomel, 2010; Lam et al., 2010). When and how the hemogenic program is induced are yet to be discovered. "
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    ABSTRACT: Hematopoietic stem cells (HSCs) are produced by a small cohort of hemogenic endothelial cells (ECs) during development through the formation of intra-aortic hematopoietic cell (HC) clusters. The Runx1 transcription factor plays a key role in the EC-to-HC and -HSC transition. We show that Runx1 expression in hemogenic ECs and the subsequent initiation of HC formation are tightly controlled by the subaortic mesenchyme, although the mesenchyme is not a source of HCs. Runx1 and Notch signaling are involved in this process, with Notch signaling decreasing with time in HCs. Inhibiting Notch signaling readily increases HC production in mouse and chicken embryos. In the mouse, however, this increase is transient. Collectively, we show complementary roles of hemogenic ECs and mesenchymal compartments in triggering aortic hematopoiesis. The subaortic mesenchyme induces Runx1 expression in hemogenic-primed ECs and collaborates with Notch dynamics to control aortic hematopoiesis.
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