Cell-cell fusion: human multinucleated osteoclasts

Osteoclasts are known to be formed by fusion of circulating mononuclear precursor cells which originate from haematopoietic stem cells. The precise mechanisms regulating the cell-cell fusion of these circulating cells to multinucleated osteoclasts remain unclear. In the present study, human peripheral blood mononuclear cells (PBMNCs) from healthy donors were treated with the macrophagecolony stimulating factor (M-CSF) and receptor activator of nuclear factor (NF)-κB ligand (RANKL) to induce osteoclast differentiation. Osteoclast formation and resorption activity were investigated through the use of tartrate-resistant acid phosphatase (TRAP) staining and lacunar resorption on dentine slices respectively. Real-time reverse-transcription polymerase chain reaction (PCR) was used to detect expression of dendritic cell-specific transmembrane protein (DC-STAMP) in these cells. The results showed that under the treatment of M-CSF and RANKL, PBMNCs differentiated into multinucleated osteoclasts through cell-cell fusion of mononucleated cells. These osteoclasts were TRAP positive and capable of resorbing the bone. Expression of DC-STAMP was much higher in the cells treated with both M-CSF and RANKL than those treated with M-CSF alone. We concluded that human PBMNCs might differentiate into active osteoclasts under certain conditions and the DC-STAMP, which is believed critical for osteoclast development, will be a possible therapeutic target for osteoclast related diseases in future.