Analysis of Endogenous LRP6 Function Reveals a Novel Feedback Mechanism by Which Wnt Negatively Regulates Its Receptor

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Abstract
The canonical Wnt pathway plays a crucial role in embryonic development, and its deregulation is involved in human diseases. The LRP6 single-span transmembrane coreceptor is essential for transmission of canonical Wnt signaling. However, due to the lack of immunological reagents, our understanding of LRP6 structure and function has relied on studies involving its overexpression, and regulation of the endogenous receptor by the Wnt ligand has remained unexplored. Using a highly sensitive and specific antibody to LRP6, we demonstrate that the endogenous receptor is modified by N-glycosylation and is phosphorylated in response to Wnt stimulation in a sustained yet ligand-dependent manner. Moreover, following triggering by Wnt, endogenous LRP6 is internalized and recycled back to the cellular membrane within hours of the initial stimulus. Finally, we have identified a novel feedback mechanism by which Wnt, acting through β-catenin, negatively regulates LRP6 at the mRNA level. Together, these findings contribute significantly to our understanding of LRP6 function and uncover a new level of regulation of Wnt signaling. In light of the direct role that the Wnt pathway plays in human bone diseases and malignancies, our findings may support the development of novel therapeutic approaches that target Wnt signaling through LRP6.