Animal body plan evolution

Canonical Wnt and Bmp/Chordin signalling in axis formation and regeneration

Cnidarians exhibit a highly conserved canonical Wnt pathway. In the hydrozoan polyp Hydra, this pathway represents a key element in the head organizer, which controls positional information along the major oral-aboral body axis and during regeneration. In the anthozoan polyp Nematostella, Wnts are activated at the blastoporal organizer during gastrulation, which later develops into the head of the polyps. In both species, we can induce ectopic head structures or inhibit head formation by using small molecule inhibitors against GSK3 and beta-Catenin, respectively. In the loerm, our studies aim at a more complete understanding of the molecular network acting in these organizers. Our previous results also suggested that the Bmp/Chordin signalling system acts in patterning the cnidarian oral-aboral axis with Chordin being activated in the oral organizers. In bilaterians, this signalling system regulates dorsal-ventral axis formation. In order to trace the origin of the bilaterian body axes from more radially symmetric ancestors, we have started to study Wnt and Bmp/Chordin pathways in the basal bilaterian flatworms Macrostomum lignano (Rhabditophora) and Isodiametra pulchra (Acoelomorpha).

Non-canonical Wnt signalling and actin dynamics during tissue evagination

Basal eumetazoans were the first animals to invent complex morphogenetic tissue movements resulting in body outgrowths and appendages. To approach the molecular basis of tissue evagination in Hydra, a basal eumetazoan, we characterized a noncanonical Wnt signalling pathway including Wnt5, Wnt8, Frizzled2, JNK, Rho-kinase, and Strabismus. This pathway acts in convergent extension-like tissue movements during bud and tentacle evagination. Interestingly, non-canonical Wnt signalling is directly activated by the canonical Wnt pathway in Hydra. This coordination seems to be essential to define the correct polyp body plan with tentacles and buds evaginating at the appropriate positions along the oral-aboral body axis. In the future, we aim at a more detailed analysis of the molecular interaction between canonical and non-canonical Wnt signalling in cnidarians. Furthermore, we plan to characterize those actin-binding proteins involved in cell polarization during bud and tentacle formation in order to gain more insight in the mechanics of tissue evagination.

Cadherin-catenin based cell adhesion and the establishment of true epithelia

Evolution of true epithelia was a major invention in eumetazoan phylogeny (Hyman: "animals of tissue grade"). Epithelial cells exhibit apical-basal polarity, apical cell-cell junctions, and cell contact sites to a basal extracellular matrix. Hydra is an excellent experimental system to approach the evolution of cell adhesion and epithelial tissues, because it offers access to complete self-organization in reaggregation assays starting from single cell suspensions. In the recent past, we have characterized the core members of a Cadherin-Catenin cell adhesion complex in Hydra. We also identified desmosomal junctions in epithelial cells in Hydra. All these data support the view that cnidarians have a functional classic Cadherin-Catenin adhesion complex that is involved in the establishment of cell-cell contact sites. Currently, we aim at the precise intracellular localization of these proteins by using immunogold-stainings in transmission electron microscopy. As a long term perspective, we want to understand how cell adhesion and inductive signalling cooperate during self-organization.

Members of this workgroup are

• Bert Hobmayer (head)