Nodal andchurchill1position the expression of a notch ligand duringXenopusgerm layer segregation

Abstract

In vertebrates, Nodal signaling plays a major role in endomesoderm induction, but germ layer delimitation is poorly understood. In avian embryos, the neural/mesoderm boundary is controlled by the transcription factor CHURCHILL1, presumably through the repressor ZEB2, but there is scarce knowledge about its role in other vertebrates. During amphibian gastrulation, Delta/Notch signaling refines germ layer boundaries in the marginal zone, but it is unknown the place this pathway occupies in the network comprising Churchill1 and Nodal. Here, we show thatXenopus churchill1is expressed in the presumptive neuroectoderm at mid-blastula transition and during gastrulation, upregulateszeb2, preventsdll1expression in the neuroectoderm, and favors neuroectoderm over endomesoderm development. Nodal signaling preventsdll1expression in the endoderm but induces it in the presumptive mesoderm, from where it activates Notch1 and its target genehes4in the non-involuting marginal zone. We propose a model where Nodal and Churchill1 position Dll1/Notch1/Hes4 domains in the marginal zone, ensuring the delimitation between mesoderm and neuroectoderm.