β-Catenin acts in a position-independent regeneration response in the simple eumetazoan Hydra

Wnt/beta-Catenin signaling plays crucial roles in regenerative processes in eumetazoans. It also acts in regeneration and axial patterning in the simple freshwater polyp Hydra, whose morphallactic regenerative capacity is unparalleled in the animal kingdom. Previous studies have identified fl-catenin as an early response gene activated within the first 30 min in Hydra head regeneration. Here, we have studied the role of beta-Catenin in more detail. First, we show that nuclear beta-Catenin signaling is required for head and foot regeneration. Loss of nuclear beta-Catenin function blocks head and foot regeneration. Transgenic Hydra tissue, in which beta-Catenin is over-expressed, regenerates more heads and feet. In addition, we have identified a set of putative beta-Catenin target genes by transcriptional profiling, and these genes exhibit distinct expression patterns in the hypostome, in the tentacles, or in an apical gradient in the body column. All of them are transcriptionally up-regulated in the tips of early head and foot regenerates. In foot regenerates, this is a transient response, and expression starts to disappear after 12-36 h. ChIP experiments using an anti-HydraTcf antibody show Tcf binding at promoters of these targets. We propose that gene regulatory P-Catenin activity in the pre-patterning phase is generally required as an early regeneration response. When regenerates are blocked with iCRT14, initial local transcriptional activation of beta-eaten/a and the target genes occurs, and all these genes remain upregulated at the site of both head and foot regeneration for the following 2-3 days. This indicates that the initial regulatory network is followed by position-specific programs that inactivate fractions of this network in order to proceed to differentiation of head or foot structures. brachyuryl (hybral) has previously been described as early response gene in head and foot regeneration. The HyBral protein, however, appears in head regenerating tips not earlier than about twelve hours after decapitation, and HyBral translation does not occur in iCRT14-treated regenerates. Foot regenerates never show detectable levels of HyBral protein at all. These results suggest that translational control mechanisms may play a decisive role in the head- and foot-specific differentiation phase, and HyBral is an excellent candidate for such a key regulator of head specification.