Molecular definition of the interaction between a tumor-specific tetrabranched peptide and LRP6 receptor

The tumor-specific tetrabranched peptide NT4 binds membrane sulfate glycosaminoglycans and receptors belonging to the low density lipoprotein receptor-related protein (LRP) family, like LRP6, which are overexpressed in cancer. The binding occurs through a multimeric positively-charged motif of NT4 that interacts with negatively charged motives in both glycosaminoglycans and LRP receptors. LRP6 has an essential function in canonical Wnt signaling, acting together with receptors of the Frizzled family as coreceptor for Wnt ligands. The extracellular domain of LRP6 contains four YWTD β-propellers, which are fundamental for interactions with ligands, such as Wnt and Wnt inhibitors. To investigate the molecular interactions between the NT4 peptide and LRP6 receptor, we synthesized a library of epitope mapping peptides reproducing the YWTD β-propeller 3 and 4 of LRP6. The peptides that showed to bind NT4 represented the portion of LRP6 located on the top face of β-propeller 3 and contained negatively charged residues, including glutamic acid-708 which is known to be involved in Wnt3a interaction. The results pave the way for a possible development of peptide inhibitors of Wnt3a pathway to be used as drugs in oncology.