Wnt signaling in vertebrate axis specification

TheWnt pathway is amajor embryonic signaling pathway that controls cell proliferation, cell fate, and body-axis determination in vertebrate embryos. Soon after egg fertilization, Wnt pathway components play a role in microtubule-dependent dorsoventral axis specification. Later in embryogenesis, another conserved function of the pathway is to specify the an-teroposterior axis. The dual role of Wnt signaling in Xenopus and zebrafish embryos is regulated at different developmental stages by distinct sets of Wnt target genes. This review highlights recent progress in the discrimination of different signaling branches and the identification of specific pathway targets during vertebrate axial development. Wnt pathways play major roles in cell-fatespecification, proliferation anddifferenti-ation, cell polarity, and morphogenesis (Clevers 2006; van Amerongen and Nusse 2009). Signal-ing is initiated in the responding cell by the in-teraction ofWnt ligandswith different receptors and coreceptors, including Frizzled, LRP5/6, ROR1/2, RYK, PTK7, and proteoglycans (An-gers and Moon 2009; Kikuchi et al. 2009; Mac-Donald et al. 2009). Receptor activation is ac-companied by the phosphorylation of Dishev-elled (Yanagawa et al. 1995), which appears to transduce the signal to both the cell membrane and the nucleus (Cliffe et al. 2003; Itoh et al. 2005; Bilic et al. 2007). Another common path-way component isb-catenin, an abundant com-ponent of adherens junctions (Nelson and Nusse 2004; Grigoryan et al. 2008). In response to signaling,b-cateninassociateswithT-cell fac-tors (TCFs) and translocates to the nucleus to stimulate Wnt target gene expression (Behren