Tensile stress induces α-adaptin C production in mouse calvariae in an organ culture: Possible involvement of endocytosis in mechanical stress-stimulated osteoblast differentiation

We previously demonstrated that tensile stress (TS)-induced osteoblast differentiation eventually led to osteogenesis in an organ culture of mouse calvarial sutures. In the present study, we employed RNA-fingerprinting using an arbitrarily primed polymerase chain reaction (RAP-PCR) to identify α-adaptin C, a component of the endocytosis machinery AP2, as a TS-inducible gene. Protein production, as well as the gene expression of α-adaptin C, was induced by TS as early as 3 h following the initiation of loading. In situ hybridization and immunohistochemical analysis revealed that the induction of α-adaptin C mostly occurred in fibroblastic cells in the sutures, suggesting that it precedes TS-induced osteoblast differentiation. Consistent with this result, TS significantly increased the number of coated pits (CPs) and coated vesicles (CVs) in the undifferentiated fibroblastic cells but not in the osteoblastic cells around calvarial bones. Further, TS-induced osteoblast differentiation was suppressed when endocytosis was inhibited by potassium depletion. These results, taken together, suggest that TS accelerates osteoblast differentiation and osteogenesis, possibly through the induction of the α-adaptin C expression and consequent activation of receptor-mediated endocytosis.