Wnt signaling through the canonical a-catenin pathway controls cell fate determination and cell proliferation, and is essential for animal development. Defective Wnt/b-catenin signaling has been associated with human colorectal cancer, familial osteoporosis, and other diseases. Thus understanding Wnt/b-catenin signaling is highly relevant to human health. Wnt/b-catenin signaling is initiated by 2 distinct families of cell surface receptors. One (1) is a member of the Frizzled (Fz) family of serpentine receptors, and the other is a single transmembrane receptor of the LDL receptor related protein family, LRP5 or LRP6. How Wnt leads to the activation of these receptors is a critical but poorly understood issue. Dr. He's group showed that Fz and LRP5/6 form a Wnt-inducible co-receptor complex in vitro, and that LRP6 plays a key role in the signaling process. They recently discovered that phosphorylation likely underlies LRP6 activation. They found that a PPP(S/T)P motif, which is reiterated 5 times in the intracellular domain of LRP5/6, is essential for LRP6 signaling function. They 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 b-catenin stability. They further showed that Wnt induces LRP6 phosphorylation in vivo. These results suggest that Wnt activates transmembrane signaling via inducing LRP6 phosphorylation. In this application, 3 specific aims are designed to substantiate/extend this working model. (1) To characterize fully all five PPP(S/T)P motifs in LRP6 intracellular domain. Issues to be addressed include the function of these PPP(S/T)P motifs and whether their differential phosphorylation controls LRP6 signaling. (2) To investigate Axin interaction with the phosphorylated PPP(S/T)P motifs.
The aim i s to 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 b-catenin stability. (3) To identify kinase (or kinases) that phosphorylates the PPP(S/T)P motif.
This aim i s to investigate known kinases implicated in Wnt/a-catenin signaling for their potential roles in PPP(S/T)P phosphorylation and to isolate the PPP(S/T)P kinase or kinases via several complementary approaches. These experiments will significantly enhance the understanding of Wnt receptor activation and Wnt signal transduction in development and disease.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Development - 1 Study Section (DEV)
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Anderson, Richard A
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Children's Hospital Boston
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