HvNotch coordinates two independent pattern forming systems during head regeneration in Hydra by supporting a lateral inhibition process restricting the tentacle system
Hydra polyps regenerate lost body parts, including the head. In addition, Hydra head 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/{beta}-catenin-signaling. Hydra head 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 {beta}-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 inhibits HyWnt3- and HyBMP2/4 expression and expression of transcriptional repressor genes including CnGsc, Sp5 and HyHes, while increasing expression of HyBMP5/8b and the c-fos-related gene HyKayak. ICRT14 blocks expression of the tentacle specification factor HyAlx, but not expression of HyWnt3. Thus, in accordance with regeneration of two head structures we find two signaling and gene expression modules with HyWnt3 and HyBMP4 part of a hypostome/organizer module, and BMP5/8, HyAlx and {beta}-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, with HyKayak we 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 of HyWnt3. Finally, Notch signaling was not required for head regeneration of fresh water polyps of Craspedacusta. Polyps of Craspedacusta do 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 during Hydra head regeneration.