Specification and patterning of the animal‐vegetal axis in sea urchins by the canonical Wnt signaling pathway

Abstract

Pattern formation along the sea urchin A-V axis is initiated by the selective activation of the canonical Wnt signaling pathway in vegetal blastomeres. Activation of this pathway is essential for deployment of the endomesoderm gene regulatory network (EGRN), and for pattern formation along the entire A-V axis. During early embryogenesis the canonical Wnt signaling pathway is selectively activated by Dishevelled (Dsh), a critical activator of the Wnt pathway. Dsh is highly enriched in vesicular structures at the vegetal pole in eggs and early embryos, and selective activation of this protein leads to the nuclearization of β-catenin in the endomesoderm. Following activation of canonical Wnt signaling by Dsh, signaling by β-catenin and the Lef/Tcf transcription factors regulates endomesoderm specification by activating the EGRN. One critical early target of nuclear β-catenin is Wnt8, which is selectively expressed in the micromeres at the 16-cell stage and in the macromeres one cleavage division later. Wnt8 signaling is not required for the endomesoderm-inducing activity of the micromeres, but this protein regulates primary mesenchyme cell differentiation. Within the endomesodermal domain Wnt8 regulates the later specification of endoderm and mesoderm. These results have highlighted the important role of the canonical Wnt signaling pathway in patterning the A-V axis in the sea urchin embryo, and have strongly suggested that this axis is initially specified by a cytoplasmic/cytoarchitectural mechanism to activate Dsh in vegetal blastomeres. Additionally, this work along with work in vertebrates and cnidarians has shown that the canonical Wnt pathway plays a conserved role in early pattern formation in metazoan embryos.