Mechanically strained cells of the osteoblast lineage organize their extracellular matrix through unique sites of α(v)β₃-integrin expression

Bone cells transduce mechanical signals into anabolic biochemical responses. However, the mechanisms of mechanotransduction are unknown. To address this issue, we performed studies in primary cells of the human osteoblast lineage grown on collagen/vitronectin-coated supports. We discovered that mechanical strain stimulated a redistribution of the α(v)β₃-integrin to irregular plaque-like areas at the cell-extracellular matrix surface. Proteins involved in integrin-matrix interactions in focal adhesions, vinculin and talin, did not localize to the plaque-like areas of α(v)β₃-expression, but signaling molecules such as focal adhesion kinase (FAK) did. Mechanical strain increased the number and size of the plaques defined by surface expression of α(v)β₃-integrin. Osteopontin was secreted as a cross-linked macromolecular complex, likely through the action of tissue transglutaminase that also was found in the plaques of α(v)β₃-integrin cell-matrix interaction. Mechanical strain increased mineralization of the extracellular matrix that developed in these plaques in α(v)β₃-integrin-dependent manner. Because the plaque-like areas of cell-matrix interaction exhibit macromolecular assembly and mineralization, we conclude that they may represent subcellular domains of bone formation and that α(v)β₃-integrin activation represents one mechanism by which mechanical strain stimulates bone formation.