Establishment of the dorso-ventral axis in Xenopus embryos is presaged by early asymmetries in β-catenin that are modulated by the Wnt signaling pathway

Abstract

Eggs of Xenopus laevis undergo a postfertilization cortical rotation that specifics the position of the dorso-ventral axis and activates a transplantable dorsal-determining activity in dorsal blastomeres by the 32- cell stage. There have heretofore been no reported dorso-ventral asymmetries in endogenous signaling proteins that may be involved in this dorsal- determining activity during early cleavage stages. We focused on β-catenin as a candidate for an asymmetrically localized dorsal-determining factor since it is both necessary and sufficient for dorsal axis formation. We report that β-catenin displays greater cytoplasmic accumulation on the future dorsal side of the Xenopus embryo by the two-cell stage. This asymmetry persists and increases through early cleavage stages, with β- catenin accumulating in dorsal but not ventral nuclei by the 16- to 32-cell stages. We then investigated which potential signaling factors and pathways are capable of modulating the steady-state levels of endogenous β-catenin. Steady state levels and nuclear accumulation of β-catenin increased in response to ectopic Xenopus Wnt-8 (Xwnt-8) and to the inhibition of glycogen synthase kinase-3, whereas neither Xwnt-5A, BVgl, nor noggin increased β- catenin levels before the mid-blastula stage. As greater levels and nuclear accumulation of β-catenin on the future dorsal side of the embryo correlate with the induction of specific dorsal genes, our data suggest that early asymmetries in β-catenin presage and may specify dorso-ventral differences in gene expression and cell fate. Our data further support the hypothesis that these dorso-ventral differences in β-catenin arise in response to the post fertilization activation of a signaling pathway that involves Xenopus glycogen synthase kinase-3.