beta-Catenin translocation into nuclei demarcates the dorsalizing centers in frog and fish embryos

The question of how dorsal-ventral polarity is established in vertebrates is central to our understanding of their early development. Several lines of evidence suggest that wnt-signaling is involved in the induction of dorsal-specific gene expression in the Spemann Organizer of amphibians. Here, we show that beta-catenin, acting as a component of the wnt-pathway, transiently accumulates in nuclei on the dorsal side of Xenopus and zebrafish blastulae. The spatially restricted nuclear translocation of beta-catenin precedes the expression of dorsal-specific genes. In experimentally ventralized frog embryos the dorsal-ventral pattern of beta-catenin nuclear staining is abolished; in contrast, embryos hyperdorsalized by Li-ions or by injection of Xwnt8-mRNA exhibit an enhanced nuclear accumulation of beta-catenin. The results show that translocation of beta-catenin into nuclei in the wake of wnt-signaling is an early step in the establishment of the dorsal-ventral axis in frog and fish embryos.