Bone matrix proteins and extracellular acidification: potential co-regulators of osteoclast morphology

Osteoclasts are signaled by the bone matrix proteins fibronectin (FN), vitronectin (VN), and osteopontin (OPN) via integrins. To perform their resorptive function, osteoclasts cycle between compact (polarized), spread (non-resorbing) and migratory morphologies. Here we investigate the effects of matrix proteins on osteoclast morphology and how those effects are mediated using RAW 264.7 cells differentiated into osteoclasts on FN, VN, and OPN-coated culture dishes. After 96 h, 80% of osteoclasts on FN were compact while 25% and 16% on VN were in compact and migratory states respectively. In contrast, OPN induced osteoclast spreading. Furthermore, osteoclasts formed on VN and FN were two- to fourfold smaller than those formed on OPN in the 21-30 nuclei/osteoclast group. These effects were not due to defects in cytoskeletal reorganization of osteoclasts on VN and FN, demonstrated by the ability of these cells to spread in response to 35 ng/ml macrophage colony stimulating factor (M-CSF). Conversely, osteoclasts on OPN failed to spread when induced by M-CSF. Moreover, the extracellular pH on FN and VN (7.25 and 7.3, respectively) was significantly lower than that on OPN (∼7.4). We further investigated the role of extracellular pH and found that at pH 7.5 the duration of an osteoclast's compact phase was 25.6 min and that of the spread phase was 62.5 min. Reducing the pH to 7.0 increased the frequency of osteoclast cycling by threefold. These results show that matrix proteins play a role in regulating osteoclast morphology, possibly via altering extracellular and intracellular pH.