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.