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Mechanical stress to bone plays a crucial role in the maintenance of bone homeostasis. It causes the deformation of bone matrix and generates strain force, which could initiate the mechano-transduction pathway. The presence of osteopontin (OPN), which is one of the abundant proteins in bone matrix, is required for the effects of mechanical stress on bone, as we have reported that OPN-null (OPN−/−) mice showed resistance to unloading-induced bone loss. However, cellular mechanisms underlying the phenomenon have not been completely elucidated. To obtain further insight into the role of OPN in mediating mechanical stress effect on bone, we examined  in vitro  mineralization and osteoclast-like cell formation in bone marrow cells obtained from hind limb bones of OPN−/− mice after tail suspension. The levels of mineralized nodule formation of bone marrow cells derived from the femora subjected to unloading were decreased compared with that from loaded control in wild-type mice. However, these were not decreased in cells from OPN−/− mice after tail suspension compared with that from loaded OPN−/− mice. Moreover, while spreading of osteoclast-like cells derived from bone marrow cells of the femora subjected to unloading was enhanced compared with that from loaded control in wild-type mice, this enhancement of spreading of these cells derived from the femora subjected to unloading was not recognized compared with those from loaded control in OPN−/− mice. These data provided cellular bases for the effect of the OPN deficiency on  in vitro  reduced mineralized nodule formation by osteoblasts and on enhancement of osteoclast spreading  in vitro  induced by the absence of mechanical stress. These  in vitro  results correlate well with the resistance to unloading-induced bone loss in OPN−/− mice  in vivo , suggesting that OPN has an important role in the effects of unloading-induced alterations of differentiation of bone marrow into osteoblasts and osteoclasts.

Osteopontin is required for mechanical stress-dependent signals to bone marrow cells