Xenopus POU factors of subclass V inhibit activin/nodal signaling during gastrulation

Three POU factors of subclass V, Oct-25, Oct-60 and Oct-91 are expressed in Xenopus oocytes and early embryos. We here demonstrate that vegetal overexpression of Oct-25, Oct-60, Oct-91 or mammalian Oct-3/4 suppresses mesendoderm, formation in Xenopus embryos. Oct-25 and Oct-60 are shown to inhibit activin/nodal and FGF signaling pathways. Loss of Oct-25 and Oct-60 function results in elevated transcription of mesendodermal marker genes and ectopic formation of endoderm in the equatorial region of gastrula stage embryos. Within the ectoderm, Oct-25 promotes neural fate by upregulating neuroectodermal genes, such as Xsox2, which prevent differentiation of neural progenitors into neurons. We also show that mouse Oct-3/4 and Xenopus Oct-25 or Oct-60 behave as functional homologues. We conclude that Xenopus Oct proteins are required to control the levels of embryonic signaling pathways, thereby ensuring the correct specification of germ layers. (c) 2006 Elsevier Ireland Ltd. All rights reserved.