Structure-based Mutation Analysis Shows the Importance of LRP5 Β-Propeller 1 in Modulating Dkk1-mediated Inhibition of Wnt Signaling

A single point mutation (G to T) in the low-density lipoprotein receptor related protein 5 (LRP5) gene results in a glycine to valine amino acid change (G171V) and is responsible for an autosomal dominant high bone mass trait (HBM) in two independent kindreds. LRP5 acts as a co-receptor to Wnts with Frizzled family members and transduces Wnt-canonical signals which can be antagonized by LRP5 ligand, Dickkopf 1 (Dkk1). In the presence of Wnt1, LRP5 or the HBM variant (LRP5–G171V) induces β-catenin nuclear translocation and activates T cell factor (TCF)-luciferase reporter activity. HBM variant suppresses Dkk1 function and this results in reduced inhibition of TCF activity as compared to that with LRP5. Structural analysis of LRP5 revealed that the HBM mutation lies in the 4th blade of the first β-propeller domain. To elucidate the functional significance and consequence of the LRP5–G171V mutation in vitro, we took a structure-based approach to design 15 specific LRP5 point mutations. These included (a) substitutions at the G171 in blade 4, (b) mutations in blades 2–6 of β-propeller 1, and (c) mutations in β-propellers 2, 3 and 4. Here we show that substitutions of glycine at 171 to K, F, I and Q also resulted in HBM-like activity in the presence of Wnt1 and Dkk1. This indicates the importance of the G171 site rather than the effect of specific amino acid modification to LRP5 receptor function. Interestingly, G171 equivalent residue mutations in other blades of β-propeller 1 (A65V, S127V, L200V, A214V and M282V) resulted in LRP5–G171V-like block of Dkk1 function. However G171V type mutations in other β-propellers of LRP5 did not result in resistance to Dkk1 function. These results indicate the importance of LRP5 β-propeller 1 for Dkk1 function and Wnt signaling. These data and additional comparative structural analysis of the LRP5 family member LDLR suggest a potential functional role of the first β-propeller domain through intramolecular interaction with other domains of LRP5 wherein Dkk1 can bind. Such studies may also lead to a better understanding of the mechanisms underlying the reduced function of Dkk1-like inhibitory ligands of LRP5 with HBM-like mutations and its relationship to increased bone density phenotypes.