Mitf is expressed in osteoclast progenitors in vitro

Microphthalmia mutant (mi/mi) mice reveal defects in osteoclastogenesis and exhibit osteopetrosis. However, there have been no studies to test the importance of Mitf in in vitro osteoclastogenesis using the cells derived from mi/mi mice. Therefore, we investigated in vitro osteoclastogenesis using the cells derived from mi/mi mice. We cocultured spleen cells prepared from either wild-type or mi/mi mice with ST2 or TM8 stromal cells and found that formation of TRAP-positive cells was significantly reduced in the cocultures of mi/mi spleen cells compared to wild-type spleen cells in the presence of 1,25(OH)(2) vitamin D(3) (vitamin D). We further investigated Mitf expression by Northern blot analysis in relation to the differentiation of osteoclasts using the cocultures of bone marrow cells with stromal/osteoblastic cells and found positive correlation in expression levels of c-fms and Mitf. Moreover, osteoclast-progenitor-like C7 cells expressed c-fms as well as Mitf mRNAs when cultured alone. C7 cells also expressed Mitf protein in their nuclei. Similar results were obtained when we used primary spleen cells, which differentiate into osteoclasts cultured in the presence of M-CSF and RANKL/ODF. Mitf expression levels in the cocultures of C7 cells and ST2 cells were not changed by treatment with vitamin D in the presence or absence of dexthamethasone. These results suggest that Mitf is expressed in osteoclast progenitors and its presence facilitates osteoclastogenesis.