Abstract
The cysteine knot protein sclerostin is an osteocyte-derived secreted inhibitor of the Wnt co-receptors LRP5 and LRP6. LRP5 plays a dominant role in bone homeostasis, but we previously reported that Sost/sclerostin suppression significantly increased osteogenesis regardless of Lrp5 presence or absence. Those observations suggested that the bone forming effects of sclerostin inhibition can occur through Lrp6 (when Lrp5 is suppressed), or through other yet undiscovered mechanisms independent of Lrp5/6. To distinguish between these two possibilities, we generated mice with compound deletion of Lrp5 and Lrp6 selectively in bone, and treated them with sclerostin monoclonal antibody (Scl-mAb). All mice were homozygous flox for both Lrp5 and Lrp6 (Lrp5(f/f); Lrp6(f/f)), and varied only in whether or not they carried the Dmp1-Cre transgene. Positive (Cre+) and negative (Cre-) mice were injected with Scl-mAb or vehicle from 4.5 to 14 weeks of age. Vehicle-treated Cre+ mice exhibited significantly reduced skeletal properties compared to vehicle-treated Cre- mice, as assessed by DXA, μCT, pQCT, and histology, indicating that Lrp5/6 deletions were effective and efficient. Scl-mAb treatment improved nearly every bone-related parameter among Cre- mice, but the same treatment in Cre+ mice resulted in little to no improvement in skeletal properties. For the few endpoints where Cre+ mice responded to Scl-mAb, it is likely that antibody-induced promotion of Wnt signaling occurred in cell types earlier in the mesenchymal/osteoblast differentiation pathway than the Dmp1-expressing stage. This latter conclusion was supported by changes in some histomorphometric parameters. In conclusion, unlike with the deletion of Lrp5 alone, the bone-selective late-stage co-deletion of Lrp5 and Lrp6 significantly impairs or completely nullifies the osteogenic action of Scl-mAb, and highlights a major role for both Lrp5 and Lrp6 in the mechanism of action for the bone-building effects of sclerostin antibody.