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: 10.45). 05/2010; 116(6):909-14. DOI: 10.1182/blood-2010-01-264382
Source: PubMed


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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    • "Initially, hematopoietic commitment occurs at the 4-somite stage with the expression of the erythroidspecific transcription factor gata1 (Lieschke et al., 2002). Thereafter, HSC precursors are developed from a population expressing flk1 (Liao et al., 1997), named hemogenic endothelium, at 24 (hpf), and express runx1and c-myb at 36 hpf, in the evolutionarily conserved aorta–gonad–mesonephros (AGM) region (Bertrand et al., 2010; Kalev-Zylinska et al., 2002; Kissa and Herbomel, 2010; Lam et al., 2010). When HSC progenitors emerge from the AGM, they enter the circulation and seed in the caudal hematopoietic tissue (CHT), a secondary hematopoietic site equivalent to the mammalian fetal liver (Murayama et al., 2006). "
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    ABSTRACT: To grant survival against sterile or microbe induced inflammation, all animals rely on correct immune system functioning. The development of immunity occurs in vertebrates during embryogenesis in a process called hematopoiesis, which is characterized by the formation of blood cellular components such as embryonic erythrocytes and primitive macrophages. These cells are formed in a sterile environment from a rare subset of pluripotent hematopoietic stem cells (HSC) during a brief period of the primitive hematopoietic wave. Diverse signals, like Notch, are indispensable in HSC emergence and differentiation. However, to successfully replicate the process in vitro using pluripotent precursors, the full set of required signals is still a matter of debate. Among the latest findings, proinflammatory signals produced by transient primitive myelocites in zebrafish have been seen to act as essential mediators in establishing the HSC program of the adult vertebrate hematopoietic system. In this regard, the zebrafish immune model has emerged as a feasible live vertebrate model for examining developmental immunity and related host-microbe interactions, both at the molecular and cellular level. Thus, using the zebrafish embryo, this review summarizes recent findings, on the signals required for immune development and further maturation of the system, in a context where no adaptive immune response has yet been developed.
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    • "However, it was not until the advent of transgenic animal models that it became possible to reliably trace the lineage of individual cells and their labelled progeny over time (Kretzschmar and Watt, 2012). In recent years, pioneering studies using in vivo live-imaging platforms have begun to provide access to continuous-time lineage data (Bertrand et al., 2010;Boisset et al., 2010;Lam et al., 2010;Yaniv et al., 2006;Ritsma et al., 2013;Rompolas et al., 2012), whereas methods based on single-cell deep sequencing now offer the potential to resolve individual phylogenies, even in human tissues (Shapiro et al., 2013;Treutlein et al., 2014). By combining these lineage-tracing approaches with static marker-based assays, snapshots of clonal evolution over time can be integrated with population-level measures to reveal how stem and progenitor cells contribute to tissue maintenance. "
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    Full-text · Article · Apr 2015 · Development
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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.
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