Direct Inhibition of GSK3β by the Phosphorylated Cytoplasmic Domain of LRP6 in Wnt/β-Catenin Signaling

Wnt/beta-catenin signaling plays a central role in development and is also involved in a diverse array of diseases. Binding of Wnts to the coreceptors Frizzled and LRP6/5 leads to phosphorylation of PPPSPxS motifs in the LRP6/5 intracellular region and the inhibition of GSK3 beta bound to the scaffold protein Axin. However, it remains unknown how GSK3 beta is specifically inhibited upon Wnt stimulation. Here, we show that overexpression of the intracellular region of LRP6 containing a Ser/Thr rich cluster and a PPPSPxS motif impairs the activity of GSK3 beta in cells. Synthetic peptides containing the PPPSPxS motif strongly inhibit GSK3b in vitro only when they are phosphorylated. Microinjection of these peptides into Xenopus embryos confirms that the phosphorylated PPPSPxS motif potentiates Wnt-induced second body axis formation. In addition, we show that the Ser/Thr rich cluster of LRP6 plays an important role in LRP6 binding to GSK3 beta. These observations demonstrate that phosphorylated LRP6/5 both recruits and directly inhibits GSK3 beta using two distinct portions of its cytoplasmic sequence, and suggest a novel mechanism of activation in this signaling pathway.