Differential Localization of Myosin II Isoforms in Resting and Activated Osteoclasts

Osteoclasts resorb bone through a cyclical process of attachment to matrix, polarization, retrac- tion, and migration. Although this process requires major alterations in the organization of actin structures, little is known about roles that myosins play in osteo- clast cytoskeletal dynamics. We performed immunolo- calization of myosin II using antibodies against heavy chain isoforms IIAand IIB and found that osteoclasts expressed the isoforms in distinct subcellular locations. Myosin IIAwas enriched in dynamic cytoskeletal compartments, including the sealing zones of polarized and unpolarized osteoclasts. In contrast, myosin IIB was generally absent from these regions and maintained a comparatively static distribution during different phases of the osteoclast activation cycle. Inhibition of myosin II in osteoclasts by treatment with 2,3-butanedione mon- oxime caused detachment of unpolarized, but not po- larized, cells from the bone matrix. These results suggest that myosin IIAis critical to development of an acti- vated osteoclast phenotype.