Wnt signaling through the canonical $-catenin pathway controls cell fate determination and cell proliferation, and is essential for animal development. Defective Wnt/β-catenin signaling has been associated with human colorectal cancers, familial osteoporosis, and other diseases. Thus understanding of Wnt/β-catenin signaling is high relevant to human health. Wnt/β-catenin signaling is initiated by two distinct families of cell surface receptors. One is a member of the Frizzled (Fz) family of serpentine receptors, and the other is a single transmembrane receptor belonging to the LDL receptor related protein family, LRP5 or LRP6. How Wnt leads to the activation of these two classes of receptors is a critical but poorly understood issue. We showed that Fz and LRP5/6 forms a Wnt-inducible co-receptor complex in vitro, and that LRP6 plays a key role in the signaling process. We recently discovered that phosphorylation likely underlies LRP6 activation. We found that a PPP(S/T)P motif, which is reiterated five times in the intracellular domain of LRP5/6, is essential for LRP6 signaling. We demonstrated that phosphorylation of this PPP(S/T)P motif is required for signaling and for binding/recognition by Axin, a key scaffolding protein that regulates $-catenin stability. We further showed that Wnt stimulation induces LRP6 phosphorylation. We propose that Wnt activates transmembrane signaling via inducing LRP6 phosphorylation. In this application, we outline three specific aims to test this working hypothesis. (1) To characterize fully all five PPP(S/T)P motifs in LRP6 intracellular domain. We will investigate the function of these PPP(S/T)P motifs, and examine whether their differential phosphorylation controls LRP6 signaling. (2) To investigate the interaction of Axin with the phosphorylated PPP(S/T)P motifs. We will identify the Axin domain/module that recognizes PPP(S/T)P phosphorylation, and examine whether LRP6-Axin association alters the composition of the Axin complex, thereby governing $-catenin stability. These experiments will significantly enhance our understanding of Wnt receptor activation, and provide novel insights into Wnt signal transduction.
