Hemogenic endothelium: origins, regulation, and implications for vascular biology

The study of endothelial development has been intertwined with hematopoiesis since the early 20th century when a bi-potential cell (hemangioblast) was noted to produce both endothelial and hematopoietic cells. Since then, ideas regarding the nature of connection between the vascular and hematopoietic systems have ranged from a tenuous association to direct lineage origination. In this review, historical data that spans hematopoietic development is examined within the context of hemogenic endothelium. Hemogenic endothelium, a specialized endothelial population capable of hematopoiesis, is an emerging theory that has recently gained momentum. Evidence across species and decades are reviewed, as are the possible modulators of the phenomenon, which include pathways that specify definitive hematopoiesis (Runx1), arterial identity (Notch1), as well as physiological and developmental factors.     

Rumba and Haus3 are essential factors for the maintenance of hematopoietic stem/progenitor cells during zebrafish hematopoiesis

The hallmark of vertebrate definitive hematopoiesis is the establishment of the hematopoietic stem/progenitor cell (HSPC) pool during embryogenesis. This process involves a defined ontogenic switching of HSPCs in successive hematopoietic compartments and is evolutionarily conserved from teleost fish to human. In zebrafish, HSPCs originate from the ventral wall of the dorsal aorta (VDA), from which they subsequently mobilize to an intermediate hematopoietic site known as the caudal hematopoietic tissue (CHT) and finally colonize the kidney for adult hematopoiesis. Despite substantial understanding of the ontogeny of HSPCs, the molecular basis governing migration, colonization and maintenance of HSPCs remains to be explored fully. Here, we report the isolation and characterization of two zebrafish mutants, rumba(hkz1) and samba(hkz2), that are defective in generating definitive hematopoiesis. We find that HSPC initiation in the VDA and subsequent homing to the CHT are not affected in these two mutants. However, the further development of HSPCs in the CHT is compromised in both mutants. Positional cloning reveals that Rumba is a novel nuclear C2H2 zinc-finger factor with unknown function and samba encodes an evolutionarily conserved protein that is homologous to human augmin complex subunit 3 (HAUS3). Furthermore, we show that these two factors independently regulate cell cycle progression of HSPCs and are cell autonomously required for HPSC development in the CHT. Our study identifies Rumba and Haus3 as two essential regulators of HSPC maintenance during zebrafish fetal hematopoiesis.     

Induction of embryonic hematopoietic and endothelial stem/progenitor cells by hedgehog-mediated signals

Blood and vascular endothelial cells form in all vertebrates during gastrulation, a process in which the mesoderm of the embryo is induced and then patterned by molecules whose identity is still largely unknown. Blood islands' of primitive hematopoietic cell clusters surrounded by a layer of endothelial cells form in the yolk sac, external to the developing embryo proper. These lineages arise from a layer of extraembryonic mesoderm that is closely apposed with a layer of primitive (visceral) endoderm. Despite the identification of genes such as Flk1, SCL/tal-1, Cbfa2/Runx1/AML1 and CD34 that are expressed during the induction of primitive hematopoiesis and vasculogenesis, the early molecular and cellular events involved in these processes are not well understood. Recent work has demonstrated that extracellular signals secreted by visceral endoderm surrounding the embryo are essential for the initiation of these events. A member of the Hedgehog family of signaling molecules (Indian hedgehog) is produced by visceral endoderm, can induce formation of blood and endothelial cells in explant cultures and can reprogram prospective neurectoderm along hematopoietic and endothelial cell lineages. Hedgehog proteins also stimulate proliferation of definitive hematopoietic stem/progenitor cells. These findings may have important implications for regulating hematopoiesis and vascular development for therapeutic purposes in humans and for the development of new sources of stem cells for transplantation and gene therapy.     

Runx1 modulates adult hair follicle stem cell emergence and maintenance from distinct embryonic skin compartments

Runx1 controls hematopoietic stem cell emergence and hair follicle stem cell (HFSC) activation and proliferation in adult skin. Here we use lineage tracing and mouse genetic manipulation to address the role of Runx1 in the embryonic development of HFSCs. We find Runx1 is expressed in distinct classes of embryonic skin precursors for short-term HF progenitors, adult HFSCs, and mesenchymal progenitors. Runx1 acts in the embryonic epithelium for timely emergence of adult HFSCs and short-term progenitors, but is dispensable for both of them. In contrast, Runx1 is strictly needed in the embryonic mesenchyme for proper adult HFSC differentiation and long-term skin integrity. Our data implicate Runx1 in epithelial cell adhesion and migration and in regulation of paracrine epithelial-mesenchymal cross talk. The latter involves Lef1 and Wnt signaling modulation in opposing directions from two distinct skin compartments. Thus, a master regulator of hematopoiesis also controls HFSC emergence and maintenance via modulation of bidirectional cross talking between nascent stem cells and their niche.     

Characterization of GATA-1(+) hemangioblastic cells in the mouse embryo

Hemangioblasts are thought to be one of the sources of hematopoietic progenitors, yet little is known about their localization and fate in the mouse embryo. We show here that a subset of cells co-expressing the hematopoietic marker GATA-1 and the endothelial marker VE-cadherin localize on the yolk sac blood islands at embryonic day 7.5. Clonal analysis demonstrated that GATA-1(+) cells isolated from E7.0-7.5 embryos include a common precursor for hematopoietic and endothelial cells. Moreover, this precursor possesses primitive and definitive hematopoietic bipotential. By using a transgenic complementation rescue approach, GATA-1(+) cell-derived progenitors were selectively restored in Runx1-deficient mice. In the rescued mice, definitive erythropoiesis was recovered but the rescued progenitors did not display multilineage hematopoiesis or intra-aortic hematopoietic clusters. These results provide evidence of the presence of GATA-1(+) hemangioblastic cells in the extra-embryonic region and also their functional contribution to hematopoiesis in the embryo.     

Co‐activator activator (CoAA) prevents the transcriptional activity of Runt domain transcription factors

Runx proteins are essential for a number of developmental processes and are aberrantly expressed in many human cancers. Runx factors bind DNA and co‐factors to activate or repress genes crucial for bone formation, hematopoiesis, and neuronal development. Co‐activator activator (CoAA) is a nuclear protein that regulates gene expression, RNA splicing and is overexpressed in many human tumors. In this study, we identified CoAA as a Runx2 binding protein. CoAA repressed Runx factor‐dependent activation of reporter genes in a histone deacetylase‐independent manner. CoAA also blocked Runx2‐mediated repression of the Axin2 promoter, a novel Runx target gene. The carboxy‐terminus of CoAA is essential for binding the Runt domains of Runx1 and Runx2. In electophoretic mobility shift assays, CoAA inhibited Runx2 interactions with DNA. These data indicate that CoAA is an inhibitor of Runx factors and can negate Runx factor regulation of gene expression. CoAA is expressed at high levels in human fetal osteoblasts and osteosarcoma cell lines. Suppression of CoAA expression by RNA interference reduced osteosarcoma cell viability in vitro, suggesting that it contributes to the proliferation and/or survival of osteoblast lineage cells. J. Cell. Biochem. 108: 378–387, 2009. © 2009 Wiley‐Liss, Inc.