Mutual antagonism between dickkopf1 and dickkopf2 regulates Wnt/beta-catenin signalling.

Wnts are secreted glycoproteins implicated in diverse processes during embryonic patterning in metazoans. They signal through seven-transmembrane receptors of the Frizzled (Fz) family [1] to stabilise beta-catenin [2]. Wnts are antagonised by several extracellular inhibitors including the product of the dickkopf1 (dkk1) gene, which was identified in Xenopus embryos and is a member of a multigene family. The dkk1 gene acts upstream of the Wnt pathway component dishevelled but its mechanism of action is unknown [3]. Although the function of Dkk1 as a Wnt inhibitor in vertebrates is well established [3-6], the effect of other Dkks on the Wnt/beta-catenin pathway is unclear. Here, we report that a related family member, Dkk2, activates rather than inhibits the Wnt/beta-catenin signalling pathway in Xenopus embryos. Dkk2 strongly synergised with Wnt receptors of the Fz family to induce Wnt signalling responses. The study identifies Dkk2 as a secreted molecule that is able to activate Wnt/beta-catenin signalling. The results suggest that a coordinated interplay between inhibiting dkk1 and activating dkk2 can modulate Fz signalling.