PROJECT SUMMARY Wnt signaling and mechanical loading are key regulators of skeletal health. Wnt signaling initiates when a Wnt ligand binds to frizzled (FZD) and LRP5/6 co-receptors. Several Wnt pathway factors (Lrp5, Sost) have known roles in bone’s anabolic response to loading. In the past funding period, we focused on the Wnt ligands (‘Wnts’), which have been relatively understudied. We discovered that Wnt ligand secretion by Osx+ bone cells (osteoblasts/osteocytes) is essential for the anabolic response to skeletal loading, and that Wnt1 upregulation in particular is required. But with aging, loading induces less Wnt1 upregulation and less bone formation. We now shift focus to intervention, and to test the overall hypothesis that molecules that mimic Wnts can rescue the loss of bone mass and mechanoresponsiveness that are hallmarks of aging. The ability to test this hypothesis is enabled by the recent development of Wnt surrogates – antibody-based molecules that function like Wnt ligands by directly binding FZD and LRP5/6. In contrast to sclerostin antibody-based treatments that work by “inhibiting the inhibitor”, Wnt surrogates directly activate Wnt signaling. Our pilot data show that a novel tetravalent molecule that targets several FZD receptors and LRP5 potently increases bone formation, bone mass and strength in young-adult mice. We propose to test whether this LRP5-binding Wnt surrogate is effective in increasing bone mass in two mouse models of osteoporosis – aging and Wnt1 conditional deletion (which models the early-onset osteoporosis that afflicts people with WNT1 mutations). We also will evaluate the possible synergy between Wnt surrogate treatment and mechanical loading, and test whether the surrogate rescues the impaired response to loading seen in aged mice and in Wnt1 mutant mice. Next, because LRP5 and LRP6 are each required for skeletal health, we will take advantage of the modularity of the Wnt surrogate platform to compare cell and bone responses to LRP5-binding and LRP6-binding surrogates. Finally, we will test whether the LRP6-binding surrogate can rescue osteoporosis in mice with conditional deletion of Lrp5, which models human osteoporosis-pseudoglioma (OPPG) syndrome. Development of Wnt surrogates addresses a need for new, targeted anabolic therapies that can be used for a personalized approach to the treatment of osteoporosis.