HOLSTEIN, TW; University of Darmstadt: Origins of axial patterning

The molecular nature of signaling centers (organizers) plays a pivotal role in the origin and evolution of metazoan body axes. At present it is unclear, which signaling cascades actually constitute the basal regulatory network required for a robust patterning of the metazoan body axes. We have shown that members of the Wnt/wg signaling pathway (Wnt, Dishevelled, GSK3, beta-Catenin, Tcf/Lef) act in the Hydra head organizer (Hobmayer et al., 2000, Technau et al 2000)). Wnt genes also have an important function in early embryogenesis of Nematostalla, a member of the basal cnidarian class Anthozoa. There is now additional evidence for a function of extracellular Wnt and BMP2/4 antagonists in cnidarian axis formation. Chordin, the BMP2/4 antagonist in vertebrates. and a Dickkopf-related protein are transcriptionally up regulated early during Hydra head regeneration and during bud formation. Both genes are also up regulated during de novo pattern formation in re-aggregates from single cells. This indicates that, Wnt and TGF-beta secreted proteins and their extracellular antagonists act as short-range inducers and long-range organizers in cnidarian axis formation. Our data support the idea that both signaling pathways constituted the core network role in organizer formation leading to the evolution of axial differentiation in early multi-cellular animals.