TheWnt/β-catenin/TCF/Sp5/Zic4gene network that regulates head organizer activity inHydrais differentially regulated in epidermis and gastrodermis

Abstract

AbstractInHydra, head formation depends on Wnt/β-catenin signaling, which positively regulatesSp5andZic4, with Sp5 limitingWnt3/β-cateninexpression and Zic4 triggering tentacle formation. Using transgenic lines in which theHySp5promoter drives eGFP expression in the epidermis or gastrodermis, we show that in intact animals, epidermalHySp5:GFP is expressed strongly apically and weakly along the body column, while gastrodermalHySp5:GFP is also maximally expressed apically but absent from the oral region, and remains high along the upper body column. During apical regeneration, gastrodermalHySp5:GFP appears early and diffusely, epidermalHySp5:GFP later. Upon alsterpaullone treatment, apicalHySp5:GFPexpression is shifted to the body column where epidermalHySp5:GFP transiently forms ectopic circular figures. Afterβ-catenin(RNAi), only epidermalHySp5:GFP is down-regulated, while pseudo-bud structures expressing gastrodermalHySp5:GFP develop.Sp5(RNAi) highlights the negative autoregulation ofSp5in epidermis, involving direct binding of Sp5 to its own promoter as observed in human HEK293T cells. In these cells, HyZic4, which can interact with huTCF1, regulatesWnt3negatively andSp5positively. This differential regulation of theWnt/β-catenin/TCF/Sp5/Zic4network in epidermis and gastrodermis highlights distinct architectures and patterning roles in the hypostome, tentacle and body column, as well as distinct regulations in homeostatic and developmental organizers.