Osteoclasts are macrophage-derived polykaryons that degrade bone in an acidic extracellular space. This differentiation includes expression of proteinases and acid transport proteins, cell fusion, and bone attachment, but the sequence of events is unclear. We studied two proteins expressed at high levels only in the osteoclast, cathepsin K, a thiol proteinase, and tartrate-resistant acid phosphatase (TRAP), and compared this expression with acid transport and bone degradation. Osteoclastic differentiation was studied using human apheresis macrophages cocultured with MG63 osteosarcoma cells, which produce cytokines including RANKL and CSF-1 that mediate efficient osteoclast formation. Immunoreactive cathepsin K appeared at 3-5 days. Cathepsin K activity was seen on bone substrate but not within cells, and cathepsin K increased severalfold during further differentiation and multinucleation from 7 to 14 days. TRAP also appeared at 3-5 d, independently of cell fusion or bone attachment, and TRAP activity reached much higher levels in osteoclasts attached to bone fragments. Two proteinases that occur in the precursor macrophages, cathepsin B, a thiol proteinase related to cathepsin K, and an unrelated lysosomal aspartate proteinase, cathepsin D, were also studied to determine the specificity of the differentiation events. Cathepsin B occurred at all times, but increased two- to threefold in parallel with cathepsin K. Cathepsin D activity did not change with differentiation, and secreted activity was not significant. In situ acid transport measurements showed increased acid accumulation after 7 days either in cells on osteosarcoma matrix or attached to bone, but bone pit activity and maximal acid uptake required 10-14 days. We conclude that TRAP and thiol proteinase expression begin at essentially the same time, and precede cell fusion and bone attachment. However, major increases in acid secretion and proteinases expression continue during cell fusion and bone attachment from 7 to 14 days.
Copyright 2000 Wiley-Liss, Inc.