Regulators of Osteoclast Differentiation and Cell-Cell Fusion

Osteoclasts are multinuclear giant cells derived from osteoclast/macrophage/dendritic cell common progenitor cells. The most characteristic feature of osteoclasts is multinucleation resulting from cell-cell fusion of mononuclear osteoclasts. Osteoclast cell-cell fusion is considered essential for re-organization of the cytoskeleton, such as the actin-ring and ruffled boarder to seal the resorbing area and to secret protons, respectively, to resorb bone; the fusion process is thus critical for osteoclast function. Various molecules, such as E-cadherin and macrophage fusion receptor (MFR), have been identified as regulators of osteoclast or macrophage cell-cell fusion. Laboratory production of osteoclasts used to be performed in a co-culture of osteoclast progenitors with osteoblastic cells, but recent advances in the identification of nuclear factor of kappa B ligand (RANKL) enabled the isolation of osteoclast-specific molecules involving osteoclast cell-cell fusion and differentiation regulators from purified osteoclast mRNA, since osteoclasts can be formed without osteoblasts. The essential cell-cell fusion regulator, dendritic cell-specific transmembrane protein (DC-STAMP), was isolated by a cDNA subtractive screen between mononuclear macrophages and RANKL-induced multinuclear osteoclasts. The cell-cell fusion of osteoclasts and foreign body giant cells (FBGCs) was completely abrogated in DC-STAMP-deficient mice in vivo and in vitro. Bone resorbing activity was significantly reduced but was still detected in DC-STAMP-deficient osteoclasts. DC-STAMP expression is positively regulated by two transcriptional factors: nuclear factor of activated T cells 1 (NFATc1) and c-Fos, both of which are essential for osteoclast differentiation. Furthermore, a novel osteoclastogenesis-regulating pathway involving two transcriptional repressors [B cell lymphoma 6 (Bcl6) and B lymphocyte-induced maturation protein 1 (Blimp1)] under RANKL stimulation has been discovered. The expression of osteoclastic genes such as DC-STAMP, NFATc1, and Cathepsin K, as well as osteoclast differentiation, was inhibited by Bcl6. Bcl6-deficient mice showed enhanced osteoclastogenesis and reduced bone mass, whereas osteoclast-specific Blimp1-conditional knockout mice showed elevated Bcl6 expression, osteoclastic gene expression, and osteoclast differentiation and increased bone mass. In this review, recent advances in our understanding of the regulators of osteoclast differentiation and cell-cell fusion are discussed.