Integration of Wnt and FGF signaling in the Xenopus gastrula at TCF and Ets binding sites shows the importance of short-range repression by TCF in patterning the marginal zone.

Abstract

During Xenopus gastrulation, Wnt and FGF signaling pathways cooperate to induce posterior structures. Wnt target expression around the blastopore falls into two main categories: a horseshoe shape with a dorsal gap, as in Wnt8 expression; or a ring, as in FGF8 expression. Using ChIP-seq, we show, surprisingly, that the FGF signaling mediator Ets2 binds near all Wnt target genes. However, β-catenin preferentially binds at the promoters of genes with horseshoe patterns, but further from the promoters of genes with ring patterns. Manipulation of FGF or Wnt signaling demonstrated that 'ring' genes are responsive to FGF signaling at the dorsal midline, whereas 'horseshoe' genes are predominantly regulated by Wnt signaling. We suggest that, in the absence of active β-catenin at the dorsal midline, the DNA-binding protein TCF binds and actively represses gene activity only when close to the promoter. In contrast, genes without functional TCF sites at the promoter may be predominantly regulated by Ets at the dorsal midline and are expressed in a ring. These results suggest recruitment of only short-range repressors to potential Wnt targets in the Xenopus gastrula.