An evolutionary-conserved Wnt3/β-catenin/Sp5 feedback loop restricts head organizer activity in Hydra

The Hydra polyp regenerates its head by transforming the gastric tissue below the wound into a head organizer made of two antagonistic cross-reacting components. The activator, previously characterized as Wnt3, drives apical differentiation by acting locally and auto-catalytically. The uncharacterized inhibitor, produced under the control of the activator, prevents ectopic head formation. By crossing RNA-seq data obtained in a {beta}-catenin(RNAi) screen performed in planarians and a quantitative analysis of positional and temporal gene expression in Hydra, we identified Sp5 as a transcription factor that fulfills the head inhibitor properties: a Wnt/{beta}-catenin inducible expression, a graded apical-to-basal expression, a sustained up-regulation during head regeneration, a multi-headed phenotype when knocked-down, a repressing activity on Wnt3 expression. In mammalian cells, Hydra and zebrafish Sp5 repress Wnt3 promoter activity while Hydra Sp5 also auto-activates its expression, possibly via {beta}-catenin and/or Tcf/Lef1 interaction. This work identifies Sp5 as a novel potent feedback loop inhibitor of Wnt/{beta}-catenin signaling across eumetazoans.