Fibroblast growth factor controls the timing of Scl, Lmo2, and Runx1 expression during embryonic blood development

Abstract

To program pluripotent cells into blood, a knowledge of the locations of precursors during their journey through the embryo and the signals they experience would be informative. The anterior (a) and posterior (p) ventral blood islands (VBIs) in Xenopus are derived from opposite sides of the pregastrula embryo. The aVBI goes through a "hemangioblast" state, characterized by coexpression of blood and endothelial genes at neurula stages, whereas the pVBI expresses these genes in a nonoverlapping fashion several hours later, after commitment to either a blood or an endothelial fate. We describe a novel role for fibroblast growth factor (FGF) in controlling the timing of Scl, Lmo2, and Runx1 expression in the 2 VBI compartments. Blocking FGF signaling during gastrulation expands expression at neurula stages into posterior regions. We show, by lineage labeling, explant analysis, and targeted blocking of FGF signaling, that this is due to the pVBI prematurely expressing these genes with the timing of the aVBI. In contrast, overex- IntroductionDuring early vertebrate embryogenesis, the first tissues to develop are blood and endothelium located in the blood islands of yolk sac mesoderm. 1 The blood islands are the source of primitive erythropoiesis and vasculogenesis, which supply the early embryo with a circulatory system carrying primarily erythroid and some myeloid cells. 2 As a result of early observations in chick, a common precursor to blood and endothelium, called the hemangioblast, was proposed. 3,4 Evidence for the existence of the hemangioblast comes from the observations that many genes are coexpressed and/or required for the very early stages of blood and endothelial development. 1,5 In addition, transient mouse embryonic stem cell (ESC)-derived blast colony forming cells (BL-CFCs) and more recently equivalent human colonies, give rise to both lineages and are considered to be the in vitro equivalent of hemangioblasts. 6,7 Comparable colonies have been detected in cultures from mouse embryos, suggesting that such bipotential precursors may also exist in vivo. 8,9 Very recently, clonal analysis of progenitor contributions to yolk sac populations using mouse ESCs, 10 and single cell resolution fate mapping in zebrafish late blastula/gastrula embryos 11 have arrived at contradictory conclusions on the existence of the hemangioblast. The fish study verifies a common progenitor for blood and endothelium, whereas the mouse study claims that few, if any, hemangioblasts exist. Further fate mapping may be required to resolve the issue.In Xenopus, blood and endothelium of the primitive lineage are located in the ventral blood island (VBI) of the tailbud stage embryo (approximately 26-36 hours after fertilization [hpf]). Fate mapping has demonstrated that the VBI is composed of 2 compartments, the anterior (a) and posterior (p) VBI, which are derived from opposite sides of the 32-cell stage embryo. 12 The precursors of the anterior VBI (aVBI) and posterior VBI (pVBI), therefore, take very different routes through the e...

Evolution of the germ line–soma relationship in vertebrate embryos

“…The VMZ of axolotl embryos is patterned by the reciprocal effects of Nodal and FGF; Nodal induces blood in the VMZ, while FGF represses blood specification and induces PGCs. The effect of FGF on blood specification is conserved in the VMZ of Xenopus embryos (Walmsley et al 2008), and at least in part stems from Nodal signaling inhibition. Nevertheless, excess SMAD signaling can override the effects of FGF in the axolotl VMZ (A D Johnson, unpublished observations), driving potential PGCs towards endoderm specification.…”

Serum- and Stromal Cell-Free Hypoxic Generation of Embryonic Stem Cell-Derived Hematopoietic Cells In Vitro, Capable of Multilineage Repopulation of Immunocompetent Mice

“…Mesoderm specification and its maintenance are known to require active FGF signaling [46]. However, continuation of FGF activity suppresses blood specification later on during development, presumably by antagonizing the BMP4 signaling pathway [47,48]. Thus, FGFs present in serum may promote mesoderm specification, but because of their constant presence, they may also be responsible for the lower content of earliest CD41 ϩ hematopoietic cells compared with serum-free, defined conditions.…”

Dissecting BMP signaling input into the gene regulatory networks driving specification of the blood stem cell lineage

“…Whereas BMP ligands such as bmp4 are expressed in both populations (A.C.-U., direct submission to Xenbase), vegfa is only expressed in adult hemangioblasts (13) [see extensive profile in Xenbase (36)]. Previously, Walmsley et al (38) and Myers and Krieg (34) proposed that in the ventral blood island, sustained BMP signaling to hemangioblast precursors ensures commitment to the erythroid lineage and inhibition of endothelial cell fate. Conversely, VEGFA ligand is shown to be inhibitory to blood formation across species and, at least in the murine system, this act is through inhibition of gata1 expression (59)(60)(61).…”

“…], and the second one from posterior ventral tissues around stage 30 in close association with the primitive blood islands (37). Embryonic hemangioblasts are first exposed to BMP ligands at the end of gastrulation by stage 13 (38). To probe the BMP requirement of the first myeloid wave, embryos were treated from stage 12.…”