Wnt signaling in hydroid development: ectopic heads and giant buds induced by GSK-3 beta inhibitors: Ectopic heads and giant buds induced by GSK-3β inhibitors

In Hydractinia, a colonial marine hydroid representing the basal phylum Cnidaria, Writ signaling plays a major role in the specification of the primary body axis in embryogenesis and in the establishment of the oral pole during metamorphosis. Here we report supplementing investigations on head regeneration and bud formation in post-metamorphic development. Head and bud formation were accompanied by the expression of Wnt, frizzled and Tcf. Activation of Writ signaling by blocking GSK-3 beta affected regeneration, the patterning of growing polyps and the asexual formation of new polyps in the colony. In the presence of lithium ions or paullones, gastric segments excised from adult polyps showed reversal of tissue polarity as they frequently regenerated heads at both ends. Phorbol myristate acetate, a known activator of protein kinase C increased this effect. Global activation of the Wnt pathway caused growing polyps to form ectopic tentacles and additional heads along their body column. Repeated treatment of colonies evoked the emergence of many and dramatically oversized bud fields along the circumference of the colony. These giant fields fell apart into smaller sub-fields, which gave rise to arrays of multiheaded polyps. We interpret the morphogenetic effects of blocking GSK-3 beta as reflecting increase in positional value in terms of positional information and activation of Writ target genes in molecular terms.