Xenopus laevis POU91 protein, an Oct3/4 homologue, regulates competence transitions from mesoderm to neural cell fates

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Abstract
Cellular competence is defined as a cell's ability to respond to signaling cues as a function of time. In Xenopus laevis , cellular responsiveness to fibroblast growth factor (FGF) changes during development. At blastula stages, FGF induces mesoderm, but at gastrula stages FGF regulates neuroectoderm formation. A Xenopus Oct3/4 homologue gene, XLPOU91 , regulates mesoderm to neuroectoderm transitions. Ectopic XLPOU91 expression in Xenopus embryos inhibits FGF induction of Brachyury ( Xbra ), eliminating mesoderm, whereas neural induction is unaffected. XLPOU91 knockdown induces high levels of Xbra expression, with blastopore closure being delayed to later neurula stages. In morphant ectoderm explants, mesoderm responsiveness to FGF is extended from blastula to gastrula stages. The initial expression of mesoderm and endoderm markers is normal, but neural induction is abolished. Churchill ( chch ) and Sip1 , two genes regulating neural competence, are not expressed in XLPOU91 morphant embryos. Ectopic Sip1 or chch expression rescues the morphant phenotype. Thus, XLPOU91 epistatically lies upstream of chch / Sip1 gene expression, regulating the competence transition that is critical for neural induction. In the absence of XLPOU91 activity, the cues driving proper embryonic cell fates are lost.