Ectodermally derived steel/stem cell factor functions non–cell autonomously during primitive erythropoiesis in Xenopus

Abstract

Signals derived from nonhematopoietic tissues are essential for normal primitive erythropoiesis in vertebrates, but little is known about the nature of these signals. In Xenopus, unidentified factors secreted by ectodermal cells during gastrulation are required to enable the underlying ventral mesoderm to form blood. Steel is expressed in the ectoderm of early Xenopus embryos and is known to regulate definitive erythroid progenitor survival and differentiation in other organisms, making it an excellent candidate regulator of primitive erythropoiesis. In this study, we tested whether steel signaling is required for primitive red blood cell differentiation in mice and frogs. We show that Xsl is expressed in the ectoderm in Xenopus gastrulae and that c-kit homologs are expressed in the underlying mesoderm at the same stages of development. We present loss of function data in whole Xenopus embryos and explants that demonstrate a requirement for ectodermally derived steel to signal through c-kit in the mesoderm to support early steps in the differentiation of primitive erythroid but not myeloid cells. Finally, we show that primitive erythropoiesis is not disrupted in mouse embryos that lack c-kit function. Our data suggest a previously unrecognized and unique function of steel/c-kit during primitive erythropoiesis in Xenopus. IntroductionThe first phase of vertebrate blood development, termed primitive hematopoiesis, gives rise primarily to embryonic erythrocytes and some leukocytes. In Xenopus laevis, primitive hematopoiesis occurs intraembryonically in a ventral mesodermal cell population known as the ventral blood island (VBI), which is the functional equivalent of the mammalian yolk sac. 1,2 The VBI has 2 distinct early embryonic origins: the anterior portion of the VBI is derived from dorsal blastomeres, whereas the posterior VBI is derived from ventral blastomeres. [3][4][5] Primitive blood cells begin to differentiate within the VBI in an anterior-to-posterior wave at the tail bud stage of development, and subsequently begin to circulate after stage 33/34 when the heart starts to beat. In Xenopus, definitive hematopoiesis initiates in a separate mesodermal population located near the dorsal lateral plate and contributes adult hematopoietic stem cells. 3,6,7 This compartment is equivalent to the murine aorta, gonad mesonephros region, which is the first site of definitive hematopoiesis. [8][9][10][11] Embryologic experiments have shown that signals derived from nonhematopoietic tissues are essential for normal primitive erythropoiesis in vertebrates. [12][13][14] Numerous lines of evidence suggest that ectodermal cells, which come into contact with underlying ventral mesoderm during gastrulation, serve as an important source of such signals in Xenopus. For instance, when embryos are induced to exogastrulate, which prevents the prospective ectoderm from coming into apposition with the mesoderm, primitive erythrocytes fail to differentiate although other mesodermal cell types develop normally. 15 Tissue recombin...

GATA2 regulates Wnt signaling to promote primitive red blood cell fate

“…Embryos were staged according to Nieuwkoop and Faber (Nieuwkoop and Faber, 1967). Ectoderm or mesoderm explants were dissected with watchmakers’ forceps and cultured independently or as recombinants as described previously (Goldman et al, 2006). Ectoderm removal assays were performed by removing the vitelline coat with watchmakers' forceps, dissecting away the dorsal half of the embryo and then peeling off the ectoderm using tungsten needles or an eyebrow knife.…”

Dimerization of Kit‐ligand and efficient cell‐surface presentation requires a conserved Ser‐Gly‐Gly‐Tyr motif in its transmembrane domain

“…Whether this difference in TMD accounts for the exclusive role of Kitl/c-kit in melanocyte migration and survival can only be speculated (51). In zebrafish, cells of the hematopoietic or germ cell lineage do not depend on either of the two Kitl orthologs (50,51), which is in contrast to Xenopus leavis, where the development of the mesenchyme-derived hematopoietic precursor cells depend on epithelially expressed Kitl, strongly resembling paracrine signaling frequently observed for Kitl in mammals (52).…”

Distinct mechanisms control the timing of differentiation of two myeloid populations in Xenopus ventral blood islands

“…This suggests that the endogenous FGF signal regulates endothelial cell differentiation in the pVBI, but not in the aVBI. The recent studies have shown that formation of VBI is mediated by complex signals derived from the ectoderm and mesoderm layers and interaction of these two layers is essential (Maéno 2003; Goldman et al. 2006).…”

“…This suggests that the endogenous FGF signal regulates endothelial cell differentiation in the pVBI, but not in the aVBI. The recent studies have shown that formation of VBI is mediated by complex signals derived from the ectoderm and mesoderm layers and interaction of these two layers is essential (Maé no 2003;Goldman et al 2006). It is likely that VEGF and FGF signals in different portions of the pVBI are activated under the control of Wnt (Tran et al 2010) and BMP (Maé no 2003) signals and that they determine the distributions of endothelial cells, erythroid cells and myeloid cells.…”