β-Catenin-Dependent Control of Positional Information along the AP Body Axis in Planarians Involves a Teashirt Family Member

Wnt/beta-catenin signaling regulates tissue homeostasis and regeneration in metazoans. In planarians-flatworms with high regenerative potential-Wnt ligands are thought to control tissue polarity by shaping a beta-catenin activity gradient along the anterior-posterior axis, yet the downstream mechanisms are poorly understood. We performed an RNA sequencing (RNA-seq)-based screen and identified hundreds of beta-catenin-dependent transcripts, of which several were expressed in muscle tissue and stem cells in a graded fashion. In particular, a teashirt (tsh) ortholog was induced in a beta-catenin-dependent manner during regeneration in planarians and zebrafish, and RNAi resulted in two-headed planarians. Strikingly, intact planarians depleted of tsh induced anterior markers and slowly transformed their tail into a head, reminiscent of beta-catenin RNAi phenotypes. Given that beta-catenin RNAi enhanced the formation of muscle cells expressing anterior determinants in tail regions, our study suggests that this pathway controls tissue polarity through regulating the identity of differentiating cells during homeostasis and regeneration.