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Bone remodeling becomes disturbed in a variety of pathological conditions that affect the skeleton, including RA, in which there is local and/or systemic alteration in the levels of proinflammatory cytokines [...], Myostatin (also known as growth and differentiation factor 8) is a secreted member of the transforming growth factor-β (TGF-β) family that is mainly expressed in skeletal muscle, which is also its primary target tissue. Deletion of the myostatin gene (Mstn) in mice leads to muscle hypertrophy, and animal studies support the concept that myostatin is a negative regulator of muscle growth and regeneration (1-5). However, myostatin deficiency also increases bone formation, mainly through loading-associated effects on bone (6-11). Here we report a previously unknown direct role for myostatin in osteoclastogenesis and in the progressive loss of articular bone in rheumatoid arthritis (RA). We demonstrate that myostatin is highly expressed in the synovial tissues of RA subjects and of human tumor necrosis factor (TNF)-α transgenic (hTNFtg) mice, a model for human RA (12). Myostatin strongly accelerates receptor activator of nuclear factor κB ligand (RANKL)-mediated osteoclast formation in vitro through transcription factor SMAD2-dependent regulation of nuclear factor of activated T-cells (NFATC1). Myostatin deficiency or antibody-mediated inhibition leads to an amelioration of arthritis severity in hTNFtg mice, chiefly reflected by less bone destruction. Consistent with these effects in hTNFtg mice, the lack of myostatin leads to increased grip strength and less bone erosion in the K/BxN serum-induced arthritis model in mice. The results strongly suggest that myostatin is a potent therapeutic target for interfering with osteoclast formation and joint destruction in RA.

Osteoporosis is a major health problem particularly affecting the aging population. In industrialized societies, it is an endemic disease characterized by an imbalance in skeletal turnover (1). This leads to [...], Understanding the mechanisms of osteoclastogenesis is crucial for developing new drugs to treat diseases associated with bone loss, such as osteoporosis. Here we report that the C-C chemokine receptor-2 (CCR2) is crucially involved in balancing bone mass. CCR2-knockout mice have high bone mass owing to a decrease in number, size and function of osteoclasts. In normal mice, activation of CCR2 in osteoclast progenitor cells results in both nuclear factor-κB (NF-κB) and extracellular signal-related kinase 1 and 2 (ERK1/2) signaling but not that of p38 mitogen-activated protein kinase or c-Jun N-terminal kinase. The induction of NF-κB and ERK1/2 signaling in turn leads to increased surface expression of receptor activator of NF-κB (RANK, encoded by Tnfrsf11a), making the progenitor cells more susceptible to RANK ligand-induced osteoclastogenesis. In ovariectomized mice, a model of postmenopausal osteoporosis, CCR2 is upregulated on wild-type preosteoclasts, thus increasing the surface expression of RANK on these cells and their osteoclastogenic potential, whereas CCR2-knockout mice are resistant to ovariectomy- induced bone loss. These data reveal a previously undescribed pathway by which RANK, osteoclasts and bone homeostasis are regulated in health and disease.