Meltrin-alpha, a fusion protein involved in multinucleated giant cell and osteoclast formation

The formation of multinucleated cells such as myotubes, macrophage-derived giant cells (MGC) and osteoclasts is the result of cell-cell fusion of mono nuclear precursors. Meltrin-alpha, -beta, and -gamma are members of a recen tly discovered family of proteins that contain disintegrin and metalloprote ase domains and are related to fertilin, a protein involved in egg-sperm fu sion. Based on this and evidence implicating meltrin-alpha in myoblast form ation, we have investigated the possibility that meltrins may also play a r ole in the formation of MGC and osteoclasts. Using in situ RT-PCR, we have determined that murine mononuclear alveolar macrophages cultured under basa l conditions express the transcript for meltrin-beta, but not for meltrin-a lpha. However, meltrin-alpha mRNA appeared in mononuclear cells before cell fusion after treatment with 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3], a potent inducer of giant cell and osteoclast formation. Moreover, addition o f meltrin-alpha antisense oligonucleotides to the cultures caused a 50% inh ibition of giant cell formation. Similarly, meltrin-alpha antisense oligonu cleotides inhibited by 70% the formation of multinucleated osteoclast-like cells expressing tartrate-resistant acid phosphatase (TRAP) in co-cultures of bone marrow cells and osteoblastic cells (2107) in the presence of 1,25( OH)(2)D-3. Mononucleated TRAP-positive cells, induced by 1,25(OH)(2)D-3 in the co-cultures, also expressed meltrin-a mRNA, but their number was not ch anged in the presence of meltrin-alpha antisense oligonucleotide. In contra st to mononuclear macrophages and osteoclast-like cells, murine bone marrow stroma and calvaria derived-cell lines (+/+ LDA.11 and 2107), primary cult ures of calvaria cells, and primary cultures of bone marrow cells expressed both meltrin-alpha and -beta mRNA under basal conditions; whereas embryoni c fibroblasts (NIH3T3) expressed only the meltrin-beta transcript. Upregula tion of meltrin-alpha protein expression during cell fusion in alveolar mac rophages and expression in osteoblastic cell lines were confirmed by Wester n blot analysis. These observations demonstrate that meltrins play a role i n MGC and osteoclast formation from mononuclear precursors, as in the case with myotubes.