Conservation of intracellular Wnt signaling components in dorsal-ventral axis formation in zebrafish

The mechanism of early dorso-ventral axis specification in zebrafish embryos is not well understood. While beta-catenin has been clearly implicated as a determinant of the axis, the factors upstream and downstream of beta-catenin in this system are not defined. Unlike in Xenopus, where a sperm-induced cortical rotation is used to localize beta-catenin on the future dorsal side of the embryo, zebrafish do not have an obviously similar morphogenetic movement. Recently, a GSK-3 (Glycogen Synthase Kinase-3) binding protein (GBP) was identified as a novel member of the Wnt pathway required for maternal dorsal axis formation in Xenopus. GBP stabilizes beta-catenin levels by inhibiting GSK-3 and potentially provides a link between cortical rotation and beta-catenin regulation. Since zebrafish may use a different mechanism for regulating beta-catenin, we asked whether zebrafish also express a maternal GBP. We report the isolation of the zebrafish GBP gene and show that it is maternally expressed and is present as mRNA ubiquitously throughout early embryonic development. Over-expression of zebrafish GBP in frogs and fish leads to hyper-dorsalized phenotypes, similar to the effects resulting from over-expression of beta-catenin, indicating that components upstream of beta-catenin are conserved between amphibians and teleosts. We also examined whether Tcf (T cell factor) functions in zebrafish embryos. As in frogs, ectopic expression of a dominant negative form of XTcf-3 ventralizes zebrafish embryos. In addition, ectopic beta-catenin expression activates the promoter of the Tcf-dependent gene siamois, indicating that the step immediately downstream of beta-catenin is also conserved between fish and frogs.