Osteoclast size heterogeneity in rat long bones is associated with differences in adhesive ligand specificity

Prothrombin (PT) is an RGD-containing bone-residing precursor to the serine protease thrombin (TH), which acts as an agonist for a variety of cellular responses in osteoblasts and osteoclasts. We show here that PT, TH, osteopontin (OPN) and fibronectin (FN) promoted adhesion of isolated neonatal rat long bone osteoclasts. However, the cells that adhered to PT and TH were smaller in size, rounded and contained 3–4 nuclei, in comparison to the cells adhering to OPN and FN, which were larger with extended cytoplasmic processes and 6–7 nuclei. Attachment of the larger osteoclasts to OPN and FN was inhibited by antibodies towards β3 and β1 integrin subunits, respectively. Whereas an RGD-containing peptide inhibited adhesion of the smaller osteoclasts to PT and TH, this was not seen with the β3 or β1 antibodies. In contrast, the β1 antibody augmented osteoclast adhesion to PT and TH in an RGD-dependent manner. Small osteoclasts were less efficient in resorbing mineralized bovine bone slices, as well as expressed lower mRNA levels of MMP-9 and the cathepsins K and L compared to large osteoclasts. The small osteoclast adhering to PT and TH may represent either an immature, less functional precursor to the large osteoclast or alternatively constitute a distinct osteoclast population with a specific role in bone.