Steichele, M.; Sauermann, L.; Pan, Q.; Moneer, J.; de la Porte, A.; Hess, M.; Mercker, M.; Strube, C.; Jenewein, M.; Boettger, A.

Wnt- and Notch signaling pathways are involved in regulating Hydra head regeneration. The Notch-inhibitor DAPT blocks expression of transcriptional repressor genes including CnGsc and HyHes and attenuates hypostomal HyWnt3- and HyBMP2/4 expression in regenerating Hydra heads while increasing expression of HyBMP5/8b and the c-fos-related gene HyKayak. Inhibition of the transcriptional activity of beta-catenin blocks expression of HyAlx during Hydra head regeneration and prevents formation of tentacles, however hypostome regeneration and Wnt3 expression patterns remain intact. This indicates competing pathways for hypostome and tentacle regeneration. We tested this hypothesis on fresh water polyps of Craspedacusta, which do not have tentacles and thus after head removal only regenerate a hypostome with a crescent of nematocytes around the mouth opening. We found that head regeneration in Craspedacusta was inhibited by the beta-catenin-inhibitor iCRT14, but not by the Notch-inhibitor DAPT, indirectly confirming that in Hydra Notch-signaling maybe needed to balance separate signaling modules for hypostome and tentacle formation, involving Wnt3/BMP2/4 or HyAlx/HyBMP5/8b expression, respectively. We suggest that during head regeneration, Notch works by inhibiting HyBMP5/8b expression and HyKayak mediated repression of HyWnt3, rather than directly activating transcription at the HyWnt3 promoter. We thus conclude that HvNotch mediates between two pattern forming systems in Hydra by coordinating a lateral inhibition process restricting the tentacle system.