The Tec Family Tyrosine Kinase Btk Regulates RANKL-induced Osteoclast Maturation

Abstract

A spontaneous mutation in Bruton's tyrosine kinase (Btk) induces a defect in B-cell development that results in the immunodeficiency diseases X-linked agammaglobulinemia in humans and X-linked immunodeficiency (Xid) in mice. Here we show an unexpected role of Btk in osteoclast formation. When bone marrow cells derived from Xid mice were stimulated with receptor activator of NF-B ligand, an osteoclast differentiation factor, they did not completely differentiate into mature multinucleated osteoclasts. Moreover, we found that the defects appeared to occur at the stage in which mononuclear preosteoclasts fuse to generate multinucleated cells. Supporting this notion, macrophages from Xid mice also failed to form multinucleated foreign body giant cells. The fusion defect of the Xid mutant osteoclasts was caused by decreased expression of nuclear factor of activated T cells c1 (NFATc1), a master regulator of osteoclast differentiation, as well as reduced expression of various osteoclast fusion-related molecules, such as the d2 isoform of vacuolar H ؉ -ATPase V0 domain and the dendritic cell-specific transmembrane protein. This deficiency was completely rescued by the introduction of a constitutively active form of NFATc1 into bone marrow-derived macrophages. Our data provide strong evidence that Btk plays a critical role in osteoclast multinucleation by modulating the activity of NFATc1.The processes underlying bone remodeling, which are dependent on a balance between osteoblast-driven bone formation and osteoclast-mediated bone resorption, control homeostasis in the skeletal system (1, 2). For efficient bone resorption, the fusion of mononuclear preosteoclasts from the myeloid lineage is critical for the formation of functional multinucleated osteoclasts (2-5). Recently, it was reported that mice genetically deficient for dendritic cell-specific transmembrane protein (DC-STAMP) or the d2 isoform of the vacuolar H ϩ -ATPase V0 domain (Atp6v0d2) exhibit osteopetrotic phenotypes due to defects in osteoclast fusion during osteoclastogenesis (4, 5). Interestingly, these studies also showed that cellcell fusion is essential for the formation of giant macrophages (4, 5).In concert with macrophage colony-stimulating factor (M-CSF), 3 receptor activator of NF-B ligand (RANKL), a member of the tumor necrosis factor superfamily, regulates the differentiation of osteoclasts as well as cell survival, fusion, and activation (3, 6, 7). The binding of RANKL to receptor activator of NF-B induces the expression of transcription factors, including NFATc1, NF-B, c-Fos, Mitf, and PU.1; these transcription factors have been shown to be important for osteoclastogenesis in vitro and in vivo (2, 3, 6). For example, NFATc1-null cells do not differentiate into osteoclasts and overexpression of intact NFATc1 stimulates osteoclast development from monocyte lineage precursors in a RANKL-independent manner (8). In addition to its important role in osteoclast differentiation, NFATc1 appears to be involved in bone resorption by osteoclasts (9). Rece...

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