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: 10.45). 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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    • "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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    • "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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    • "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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