A Wnt-specific astacin proteinase controls head formation inHydra

AbstractTheHydrahead organizer acts as a signaling center that initiates and maintains the primary body axis in steady state polyps and during budding or regeneration. Wnt/beta-Catenin signaling functions as a primary cue controlling this process, but how Wnt ligand activity is locally restricted at the protein level is poorly understood.Here we report the identification of an astacin family proteinase as a Wnt processing factor.Hydraastacin-7 (HAS-7) is expressed from gland cells as an apical-distal gradient in the body column, peaking close beneath the tentacle zone.HAS-7siRNA knockdown abrogates HyWnt3 proteolysis in the head tissue and induces a robust double axis phenotype, which is rescued by simultaneousHyWnt3knockdown. Accordingly, double axes are also observed in conditions of increased Wnt levels as in transgenic actin::HyWnt3 and HyDkk1/2/4 siRNA treated animals.HyWnt3-induced double axes inXenopusembryos could be rescued by co-injection ofHAS-7mRNA. Mathematical modelling combined with experimental promotor analysis indicate an indirect regulation ofHAS-7by beta-Catenin, expanding the classical Turing-type activator-inhibitor model.Our data suggest a negative regulatory function of Wnt processing astacin proteinases in the global patterning of the oral-aboral axis inHydra.