Xenopus Egg Wnt/β-Catenin Pathway

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Abstract
In embryonic development in vertebrates, β-catenin signaling promotes polarization of the embryo to establish the dorsoventral axis, and it is this process that is highlighted by the Xenopus Egg Wnt/β-Catenin Pathway. In the amphibian Xenopus, fertilization of the egg results in the establishment of a parallel array of microtubules with the plus end pointing away from the sperm entry point. Concurrent with a process of cortical rotation, in which the cortex of the egg utilizes these microtubules to rotate relative to the inner cytoplasm, there is a movement of small vesicles toward the plus end of the microtubules. Cells inheriting these vesicles are destined to give rise to dorsal and anterior structures of the embryo. Three components of the Wnt/β-catenin pathway—Dishevelled, glycogen synthase kinase 3 (GSK-3)-binding protein (GBP), and β-catenin—accumulate on the side of the egg and early embryo that receive these small vesicles. It is likely that Dishevelled and GBP are associated with the vesicles that move along the microtubules and that they promote the stabilization and dorsal accumulation of β-catenin. Although Wnts and Frizzled are present in the egg, their roles in axis specification remain unclear. Dorsal β-catenin then activates direct target genes, including the homeobox genes siamois and twinned. Activation of these genes on the prospective dorsal side is necessary and sufficient for formation of the gastrula organizer, which organizes the embryonic germ layers and axes.