Subcellular Elevation of Cytosolic Free Calcium Is Required for Osteoclast Migration

Abstract

Osteoclasts are multinucleated cells responsible for the resorption of bone and other mineralized tissues during development, physiological remodeling, and pathological bone loss. Osteoclasts have the ability to resorb substrate while concurrently migrating. However, the subcellular processes underlying migration are not well understood. It has been proposed that, in other cell types, cytosolic free Ca(2+) concentration ([Ca(2+) ]i ) regulates cell protrusion as well as retraction. Integration of these distinct events would require precise spatiotemporal patterning of subcellular Ca(2+) . The large size of osteoclasts offers a unique opportunity to monitor patterns of Ca(2+) during cell migration. We used ratiometric imaging to map [Ca(2+) ]i within rat and mouse osteoclasts. Migration was characterized by lamellipodial outgrowth at the leading edge, along with intermittent retraction of the uropod. Migrating osteoclasts displayed elevation of [Ca(2+) ]i in the uropod, that began prior to retraction. Dissipation of this [Ca(2+) ]i gradient by loading osteoclasts with the Ca(2+) chelator BAPTA abolished uropod retraction, on both glass and mineralized substrates. In contrast, elevation of [Ca(2+) ]i using ionomycin initiated prompt uropod retraction. To investigate downstream effectors, we treated cells with calpain inhibitor-1, which impaired uropod retraction. In contrast, lamellipodial outgrowth at the leading edge of osteoclasts was unaffected by any of these interventions, indicating that the signals regulating outgrowth are distinct from those triggering retraction. The large size of mature, multinucleated osteoclasts allowed us to discern a novel spatiotemporal pattern of Ca(2+) involved in cell migration. Whereas localized elevation of Ca(2+) is necessary for uropod retraction, lamellipod outgrowth is independent of Ca(2+) -a heretofore unrecognized degree of specificity underlying the regulation of osteoclast migration.