RESUMO
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 that <i>Xenopus churchill1</i> is expressed in the presumptive neuroectoderm at mid-blastula transition and during gastrulation, upregulates <i>zeb2</i>, prevents <i>dll1</i> expression in the neuroectoderm, and favors neuroectoderm over endomesoderm development. Nodal signaling prevents <i>dll1</i> expression in the endoderm but induces it in the presumptive mesoderm, from where it activates Notch1 and its target gene <i>hes4</i> in 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.
Assuntos
Endoderma , Regulação da Expressão Gênica no Desenvolvimento , Animais , Endoderma/metabolismo , Ligantes , Fatores de Transcrição/genética , Fator de Crescimento Transformador beta/metabolismo , Xenopus laevis/genéticaRESUMO
Bilaterian embryos are triploblastic organisms which develop three complete germ layers (ectoderm, mesoderm, and endoderm). While the ectoderm develops mainly from the animal hemisphere, there is diversity in the location from where the endoderm and the mesoderm arise in relation to the animal-vegetal axis, ranging from endoderm being specified between the ectoderm and mesoderm in echinoderms, and the mesoderm being specified between the ectoderm and the endoderm in vertebrates. A common feature is that part of the mesoderm segregates from an ancient bipotential endomesodermal domain. The process of segregation is noisy during the initial steps but it is gradually refined. In this review, we discuss the role of the Notch pathway in the establishment and refinement of boundaries between germ layers in bilaterians, with special focus on its interaction with the Wnt/ß-catenin pathway.