POU-V factors antagonize maternal VegT activity and β-Catenin signaling in Xenopus embryos

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Abstract
VegT and β‐Catenin are key players in the hierarchy of factors that are required for induction and patterning of mesendoderm in Xenopus embryogenesis. By descending the genetic cascades, cells lose their pluripotent status and are determined to differentiate into distinct tissues. Mammalian Oct‐3/4, a POU factor of subclass V (POU‐V), is required for the maintenance of pluripotency of embryonic stem cells. However, its molecular function within the early embryo is yet poorly understood. We here show that the two maternal Xenopus POU‐V factors, Oct‐60 and Oct‐25, inhibit transcription of genes activated by VegT and β‐Catenin. Maternal POU‐V factors and maternal VegT show an opposite distribution along the animal/vegetal axis. Oct‐25, VegT and Tcf3 interact with each other and form repression complexes on promoters of VegT and β‐Catenin target genes. We suggest that POU‐V factors antagonize primary inducers to allow germ layer specification in a temporally and spatially coordinated manner.