Notch-signaling is required for mediating between two pattern forming processes during head regeneration inHydrapolyps

Hydrapolyps regenerate lost body parts, including the head. In addition,Hydrahead tissue has organizer properties thus being able to recruit body column tissue from a host polyp to produce ectopic hydranths after transplantation. These pattern forming processes involve Notch- and Wnt/β-catenin-signaling.Hydrahead regeneration consists of two parts, hypostome/organizer and tentacle development. Previous work had shown that the Notch inhibitor DAPT blocks hypostome regeneration and organizer formation, but not the appearance of tentacle genes and tentacle tissue. Here we show that the β-catenin inhibitor iCRT14 blocks tentacle regeneration, but not regeneration of hypostome and organizer tissue. Using RT-qPCR gene expression analyzes during head regeneration we found that DAPT inhibitsHyWnt3- andHyBMP2/4expression and expression of transcriptional repressor genes includingCnGsc,Sp5andHyHes,while increasing expression ofHyBMP5/8band thec-fos-related geneHyKayak.ICRT14 blocks expression of the tentacle specification factorHyAlx,but not expression ofHyWnt3. Thus, in accordance with regeneration of two head structures we find two signaling and gene expression modules withHyWnt3andHyBMP4part of a hypostome/organizer module, andBMP5/8,HyAlxand β-catenin part of a tentacle module. We conclude that Notch functions as an inhibitor of tentacle production to allow regeneration of hypostome/head organizer. Furthermore, withHyKayakwe present a candidate target gene for HvNotch induced repressor genes. Using siRNA and the Fos/Jun-inhibitor T5224 we show that HyKayak attenuates the expression ofHyWnt3.Finally, Notch signaling was not required for head regeneration of fresh water polyps ofCraspedacusta. Polyps ofCraspedacustado not have tentacles and thus, after head removal only regenerate a hypostome with a crescent of nematocytes around the mouth opening. This corroborates the idea that Notch-signaling mediates between two pattern forming processes duringHydrahead regeneration.