beta-Catenin target genes in axis formation and regeneration

Most of the more than 30 major animal body plans share bilateral body symmetry. They likely evolved in the early Cambrian from more radially symmetric ancestors, which are nowadays represented by only a few metazoan phyla. Extant cnidarians are representatives of ancestral pre-Bilateria and form the sister group to the Bilateria. Their major oral-aboral body axis is organized by an oral signalling centre. This signalling centre forms at the blastopore during early embryogenesis, and the blastoporal tissue will later give rise to the head of the developing polyp. In adult Hydra polyps, the head has maintained organizer activity to regulate oral-aboral patterning during steady-state tissue turnover. A large number of recent data shows that Wnt/beta-Catenin signalling is a core player in axis formation in higher animals as well as in cnidarians. Wnt/beta-Catenin signalling acts in the head and blastoporal organizers of hydrozoan and anthozoan polyps to establish oral cell fates. Interestingly, members of the sFRP family of Wnt antagonists seem to act in the specification of the aboral pole in hydrozoan and anthozoan polyps. Hence, major determinants for positional information along the oral-aboral axis seem to be conserved within the Cnidaria. How this positional information is translated into cellular behaviour, and to what extent these downstream events are conserved within the Cnidaria and between the Cnidaria and Bilateria is totally unknown. The general aim in this proposal is to use systematic expression profiling to characterize critical target genes regulated by nuclear beta-Catenin during cnidarian axis formation and regeneration. This will provide a more detailed understanding of the molecular mechanisms underlying the instructive effects of Wnt/beta-Catenin signalling on axis formation and regeneration. In order to define the degree of evolutionary conservation of these mechanisms and to use the complementary strength of the two organisms, the hydrozoan polyp Hydra magnipapillata and the anthozoan polyp Nematostella vectensis will be used. Due to the possibility to produce tissues with highly activated as well as inhibited nuclear beta-Catenin activity, the expression profiling will be done in Hydra. Orthologs of beta-Catenin target genes will then be retrieved from the Hydra and Nematostella genome sequences and studied in more detailed. Hydra offers the analysis of axis formation during asexual reproduction and regeneration; Nematostella offers perfect access to axis formation during embryonic development.

Most of the more than 30 major animal body plans exhibit bilateral body symmetry and they evolved from more radially symmetrical ancestors during the Cambrian radiation. Today, these radially symmetrical ancestors are represented by members of the phylum Cnidaria. Accumulating data show that Wnt/beta-Catenin signalling is a decisive player in the formation of the primary body axis in cnidarians and bilaterians. Wnt/beta-Catenin signalling acts in the head and blastoporal organizers of cnidarian polyps to establish positional values along the developing primary body axis. How this positional information is translated into cellular behaviour, and to what extent these downstream events are conserved within the Cnidaria and between the Cnidaria and Bilateria is totally unknown. Primary aim in this proposal was to characterize critical target genes regulated by nuclear ?-Catenin-Tcf complexes during cnidarian axis formation and regeneration. Core project was an expression profiling screening using polyp tissues, in which beta-Catenin activity was either strongly enhanced or strongly inhibited. From an initial set of about 200 responsive genes from the expression profiling data, we selected over two steps a final number of 10 genes, including three already known target genes and seven target genes, which are so far not described. We have now started to study these candidates in detail including functional manipulation by transgenic transformation and experiments to demonstrate Tcf-binding at corresponding promoter elements. Our preliminary results emphasize a role of beta-Catenin in position-dependent cell fate regulation in the head (hypostome plus tentacles) and the upper body column. To this unbiased profiling approach we added a set of defined candidate genes, which suggest broader implications of Wnt/beta-Catenin signaling in tissue morphogenesis, cellular affinities and cell proliferation. In summary, our results provide a more detailed understanding of the molecular mechanisms underlying the instructive effects of Wnt/beta-Catenin signalling on axis formation in the ancestral model systems Hydra and Nematostella. By using the complementary strength of both systems, we have extended our scope from asexual reproduction and regeneration to early embryonic development.