Expression on outer membranes of mannose residues, which are involved in osteoclast formation via cellular fusion events

Osteoclast, the bone-resorbing cell, is formed from hematopoietic precursors via cell-cell fusion. To evaluate the possibility that under certain specific conditions mannose residues may be expressed on the mammalian cell surface, we examined the action of pradimicin derivatives, which bind specific sugars such as the mannose residue, on the formation of osteoclast induced in the coculture of mouse spleen cells with mouse stromal cells, a process in which cell-cell fusion is involved. Osteoclast formation was inhibited by treatment of this coculture system with pradimicin at the later stage (day 4-7), and this inhibition was specifically abrogated by mannose-rich yeast mannan. During the 8-day cocultivation, osteoclast formation was blocked by the pradimicin on days 6 and 7, when mononuclear preosteoclasts fused into multinucleated osteoclasts. With an interactive laser cytometer ACAS570, fluorescein isothiocyanate-labeled pradimicin was observed to bind osteoclast progenitors at the fusion stage and to have no binding affinity for osteoclast progenitors at the early stage (day 0-3) or for osteoclasts, which were formed after performing fusion between mononuclear preosteoclasts. These results suggest that mannose residues were expressed on outer membranes of monocytes under pathophysiological conditions and that they were involved in the osteoclast formation via cellular membrane fusion events.