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Blocking the Wnt inhibitor, sclerostin, increases the rate of bone formation in rodents and in humans. On a cellular level, the antibody against sclerostin acts by increasing osteoblast numbers partly by activating the quiescent bone-lining cells in vivo. No evidence currently exists, to determine whether blocking sclerostin affects early cells of the osteoblast lineage. Here we use a lineage-tracing strategy that uses a tamoxifen-dependent cre recombinase, driven by the Sox9 promoter to mark early cells of the osteoblast lineage. We show that, when adult mice are treated with the rat-13C7, an antibody that blocks sclerostin action in rodents, it increases the numbers of osteoblast precursors and their differentiation into mature osteoblasts in vivo. We also show that rat-13C7 administration suppresses adipogenesis by suppressing the differentiation of Sox9creER+ skeletal precursors into bone marrow adipocytes in vivo. Using floxed alleles of the CTNNB1 gene encoding β-catenin, we show that these precursor cells express the canonical Wnt signaling mediator, β-catenin, and that the actions of the rat-13C7 antibody to increase the number of early precursors is dependent on direct stimulation of Wnt signaling. The increase in osteoblast precursors and their progeny after the administration of the antibody leads to a robust suppression of apoptosis without affecting the rate of their proliferation. Thus, neutralizing the Wnt-inhibitor sclerostin increases the numbers of early cells of the osteoblast lineage osteoblasts and suppresses their differentiation into adipocytes in vivo. © 2021 American Society for Bone and Mineral Research (ASBMR)., (© 2021 American Society for Bone and Mineral Research (ASBMR).)

Sclerostin antibody (Scl-Ab) increases osteoblast activity, in part through increasing modeling-based bone formation on previously quiescent surfaces. Histomorphometric studies have suggested that this might occur through conversion of bone lining cells into active osteoblasts. However, direct data demonstrating Scl-Ab-induced conversion of lining cells into active osteoblasts are lacking. Here, we used in vivo lineage tracing to determine if Scl-Ab promotes the conversion of lining cells into osteoblasts on periosteal and endocortical bone surfaces in mice. Two independent, tamoxifen-inducible lineage-tracing strategies were used to label mature osteoblasts and their progeny using the DMP1 and osteocalcin promoters. After a prolonged "chase" period, the majority of labeled cells on bone surfaces assumed a thin, quiescent morphology. Then, mice were treated with either vehicle or Scl-Ab (25 mg/kg) twice over the course of the subsequent week. After euthanization, marked cells were enumerated, their thickness quantified, and proliferation and apoptosis examined. Scl-Ab led to a significant increase in the average thickness of labeled cells on periosteal and endocortical bone surfaces, consistent with osteoblast activation. Scl-Ab did not induce proliferation of labeled cells, and Scl-Ab did not regulate apoptosis of labeled cells. Therefore, direct reactivation of quiescent bone lining cells contributes to the acute increase in osteoblast numbers after Scl-Ab treatment in mice. © 2016 American Society for Bone and Mineral Research., (© 2016 American Society for Bone and Mineral Research.)