Convergence of Wnt and FGF signals in the genesis of posterior neural plate through activation of the Sox2 enhancer N-1

The expression of the transcription factor gene Sox2 precisely marks the neural plate in various vertebrate species. We previously showed that the Sox2 expression prevailing in the neural plate of chicken embryos is actually regulated by the coordination of five phylogenetically conserved enhancers having discrete regional coverage, among which the 420-bp long enhancer N-1, active in the node-proximal region, is probably involved directly in the genesis of the posterior neural plate. We investigated the signaling systems regulating this enhancer, first identifying the 56-bp N-1 core enhancer (N-1c), which in a trimeric form recapitulates the activity of the enhancer N-1. Mutational analysis identified five blocks, A to E, that regulate the enhancer N-1c. Functional analysis of these blocks indicated that Writ and FGF signals synergistically activate the enhancer through Blocks A-B, bound by Lef1, and Block D, respectively. Fgf8b and Wnt8c expressed in the organizer-primitive streak region account for the activity in the embryo. Block E is essential for the repression of the enhancer N-1 c activity in the mesendodermal precursors. The enhancer N-1c is not affected by BMP signals. Thus, Wnt and FGF signals converge to activate Sox2 expression through the enhancer N-1 c, revealing the direct involvement of the Writ signal in the initiation of neural plate development.