We show that mice lacking β3 integrin are protected from OVX‐induced bone loss. Using a lentiviral‐based strategy to express β3 mutants in β3^−/− mice, we also show that β3^S752, but not β3^Y747/Y759, is important for osteoclastic bone resorption in vivo.

Introduction: Mice lacking the β3 integrin have dysfunctional osteoclasts and therefore accumulate bone mass with age. Thus, the αvβ3 integrin is a potential anti‐osteoporosis target. Identifying components of the β3 integrin that determine its function in vivo is essential for therapeutically exploiting the antiresorptive properties of αvβ3.

Materials and Methods: We used DXA and histomorphometry to assess bone loss after ovariectomy in wildtype and β3 integrin null mice. We used lentiviral vectors carrying various human β3 (hβ3) integrin constructs to transduce β3^−/− bone marrow and reconstituted lethally irradiated β3^−/− mice with the transduced marrow. The expressed constructs include the intact integrin and two mutants, namely hβ3^Y747F/Y759F and hβ3^S752P, each of which induces the bleeding dyscrasia, Glanzmann's thrombasthenia, in humans. Two months after transplantation, the expression of hβ3 was measured by flow cytometry of marrow‐derived macrophages. Osteoclast differentiation and function were assessed ex vivo by TRACP and actin‐ring staining, respectively. Reconstituted mice were ovariectomized, and bone loss was assessed by DXA, histomorphometry, and serum TRACP5b assay.

Results: β3^−/− mice are protected from ovariectomy‐induced bone loss, showing no difference in BMD compared with sham‐operated controls. We successfully expressed hβ3 integrins in β3^−/− hosts using lentiviral transduction of bone marrow. Two months after transplantation, 25–35% of marrow‐derived macrophages expressed the hβ3 constructs. Similar to its effect in vitro, hβ3^WT completely rescued the osteoclast and platelet phenotype of β3^−/− mice. Whereas platelet function remained deranged in β3^−/− mice overexpressing hβ3^Y747F/Y759F, osteoclast function was fully restored. In contrast, β3^−/− mice expressing hβ3^S752P continued to exhibit prolonged bleeding times and dysfunctional osteoclasts in vitro and ex vivo. Most importantly, hβ3^WT and hβ3^Y747F/Y759F transplanted mice underwent equivalent ovariectomy‐induced bone loss, whereas, like those bearing the control vector, hβ3^S752P transplanted mice were protected.

Conclusions: Functional β3 integrin is required for ovariectomy‐induced bone loss. β3^S752, but not β3^Y747/Y759, is critical for osteoclast function in vivo.