Podosomes Display Actin Turnover and Dynamic Self-Organization in Osteoclasts Expressing Actin-Green Fluorescent Protein

Podosomes, small actin-based adhesion structures, differ from focal adhesions in two aspects: their core structure and their ability to organize into large patterns in osteoclasts. To address the mechanisms underlying these features, we imaged live preosteoclasts expressing green fluorescent protein-actin during their differentiation. We observe that podosomes always form inside or close to podosome groups, which are surrounded by an actin cloud. Fluorescence recovery after photobleaching shows that actin turns over in individual podosomes in contrast to cortactin, suggesting a continuous actin polymerization in the podosome core. The observation of podosome assemblies during osteoclast differentiation reveals that they evolve from simple clusters into rings that expand by the continuous formation of new podosomes at their outer ridge and inhibition of podosome formation inside the rings. This self-organization of podosomes into dynamic rings is the mechanism that drives podosomes at the periphery of the cell in large circular patterns. We also show that an additional step of differentiation, requiring microtubule integrity, stabilizes the podosome circles at the cell periphery to form the characteristic podosome belt pattern of mature osteoclasts. These results therefore provide a mechanism for the patterning of podosomes in osteoclasts and reveal a turnover of actin inside the podosome.