VegT Activation of Sox17 at the Midblastula Transition Alters the Response to Nodal Signals in the Vegetal Endoderm Domain

Abstract

In Xenopus, the prospective endoderm and mesoderm are localized to discrete, adjacent domains at the beginning of gastrulation, and this is made evident by the expression of Sox17 in vegetal blastomeres and Brachyury (Xbra) in marginal blastomeres. Here, we examine the regulation of Sox17alpha expression and the role of Sox17alpha in establishing the vegetal endodermal gene expression domain. Injection of specific inhibitors of VegT or Nodal resulted in a loss of Sox17alpha expression in the gastrula. However, the onset of Sox17alpha expression at the midblastula transition was dependent on VegT, but not on Nodal function, indicating that Sox17alpha expression is initiated by VegT and then maintained by Nodal signals. Consistent with these results, VegT, but not Xenopus Nodal-related-1 (Xnr1), can activate Sox17alpha expression at the midblastula stage in animal explants. In addition, VegT activates Sox17alpha in the presence of cycloheximide or a Nodal antagonist, suggesting that Sox17alpha is an immediate-early target of VegT in vegetal blastomeres. Given that Nodal signals are necessary and sufficient for both mesodermal and endodermal gene expression, we propose that VegT activation of Sox17alpha at the midblastula transition prevents mesodermal gene expression in response to Nodal signals, thus establishing the vegetal endodermal gene expression domain. Supporting this idea, Sox17alpha misexpression in the marginal zone inhibits the expression of multiple mesodermal genes. Furthermore, in animal explants, Sox17alpha prevents the induction of Xbra and MyoD, but not Sox17beta or Mixer, in response to Xnr1. Therefore, VegT activation of Sox17alpha plays an important role in establishing a region of endoderm-specific gene expression in vegetal blastomeres.

Cephalic hedgehog expression is regulated directly by Sox17 in endoderm development of Xenopus laevis

“…6a). This phenomenon coincides with the reports in which the repression of Xsox17 activity in endoderm cells caused to recovery of mesodermal factor (Engleka et al 2001). It was reported that HNF1b, endodermin, and HNF3a/bin endoderm cells are directly controlled by Xsox17 ).…”

Genomic profiling of Mixer and Sox17β targets during Xenopus endoderm development

“…WE, whole embryo; ϪRT, minus reverse transcriptase. Henry and Melton, 1998;Clements et al, 1999;Yasuo and Lemaire, 1999;Engleka et al, 2001;Shivdasani, 2002;Stainier, 2002;Loose and Patient, 2004). It is this collection of unknown target genes that ultimately defines the early endoderm cell.…”

QSulf1, a heparan sulfate 6- O -endosulfatase, inhibits fibroblast growth factor signaling in mesoderm induction and angiogenesis

“…Total RNA was purified from stage 11 cultured animal caps and embryos by using an RNAqueous kit (Ambion) and quantified by spectrometer. RT-PCR for EF1␣, Brachyury, and MyoD expression was performed as described (36). Data shown are representative of five independent assays.…”