FGFR-ERK signaling is an essential component of tissue separation.

Formation of a constriction and tissue separation between parent and young polyp is a hallmark of the Hydra budding process and controlled by fibroblast growth factor receptor (FGFR) signaling. Appearance of a cluster of cells positive for double phosphorylated ERK (dpERK) at the late separation site indicated that the RAS/MEK/ERK pathway might be a downstream target of the Hydra Kringelchen FGFR. In fact, inhibition of ERK phosphorylation by the MEK inhibitor U0126 reversibly delayed bud detachment and prevented formation of the dpERK-positive cell cluster indicating de novo-phosphorylation of ERK at the late bud base. In functional studies, a dominant-negative Kringelchen FGFR prevented bud detachment as well as appearance of the dpERK-positive cell cluster. Ectopic expression of full length Kringelchen, on the other hand, induced a localized rearrangement of the actin cytoskeleton at sites of constriction, localized ERK-phosphorylation and autotomy of the body column. Our data suggest a model in which (i) the Hydra FGFR targets, via an unknown pathway, the actin cytoskeleton to induce a constriction and (ii) FGFR activates MEK/ERK signaling at the late separation site to allow tissue separation.