It has been reported that soluble interleukin (IL)-6 receptor (sIL-6R) is detected in the serum of healthy individuals and its level is increased in patients with multiple myeloma and human immunodeficiency virus infection. Although several reports have suggested that sIL-6R potentiates 1L-6 action, its physiological role remains unclear. In this study, we examined the role of sIL-6R on osteoclast formation by IL-6, using a coculture of mouse osteoblasts and bone marrow cells. Neither recombinant mouse IL-6 (mIL-6) nor mouse sIL-6R (smIL-6R) induced osteoclast-like multinucleated cell (MNC) formation when they were added separately. In contrast, simultaneous treatment with mIL-6 and smIL-6R strikingly induced MNC formation. These MNCs satisfied major criteria of authentic osteoclasts, such as tartrateresistant acid phosphatase (TRAP) activity, calcitonin receptors, and pit formation on dentine slices. The MNC formation induced by mIL-6 and smIL-6R was dose-dependently inhibited by adding monoclonal anti-mouse IL-6R antibody (MR16-1). It is likely that osteoblasts and osteoclast progenitors are capable of transducing a signal from a complex of IL-6 and sIL-6R through gpl3O, even though they may have no or a very small number of IL-6Rs. Factors such as IL-1l, oncostatin M, and leukemia inhibitory factor, which are known to exert their functions through gpl3O (the signal-transducing chain of IL-6R), also induced MNC formation in our coculture system. These results suggest that increased circulating or locally produced sIL-6R induces osteoclast formation in the presence of IL-6 mediated by a mechanism involving gpl3O. This may play an important physiological or pathological role in conditions associated with increased osteoclastic bone resorption.

Collagenase-3 (MMP13), a member of the matrix metalloproteinase (MMP) family of neutral endopeptidases, is expressed in the skeleton during embryonic development and is highly overexpressed in human carcinomas and in chondrocytes and synovial cells in rheumatoid arthritis and osteoarthritis. To determine the functional roles of Mmp13, we generated Mmp13-null mice that showed profound defects in growth plate cartilage with markedly increased hypertrophic domains as well as delay in endochondral ossification and formation and vascularization of primary ossification centers. Absence of Mmp13 resulted in significant interstitial collagen accumulation due, in part, to the lack of appropriate collagenase-mediated cleavage that normally occurs in growth plates and primary ossification centers. Cartilaginous growth plate abnormalities persisted in adult mice and phenocopied defects observed in human hereditary chondrodysplasias. Our findings demonstrate a unique role of Mmp13 in skeletal development.collagen ͉ extracellular matrix ͉ vascularization C ollagenases, a group of matrix metalloproteinases (MMPs) that act at neutral pH (1-4), have been postulated to have a role in skeletal development and bone remodeling (5-8). The MMPs are members of a large family of proteinases that have several structural features in common including the presence of a conserved zinc-binding catalytic domain (1-4). Only the products of specific MMP genes, MMP1, -2, -8, -13, and -14, however, have the capacity to cleave native, undenatured, interstitial collagens at a specific helical locus (9-13). Of the collagenases, MMP13 (collagenase-3) has been considered to have an important role in skeletal biology in view of its exclusive presence in the skeleton during embryonic development in cartilaginous growth plates and primary centers of ossification (5-8). MMP13 is also a downstream target of parathyroid hormone (PTH)-related protein (PTHrP) (14) and the transcription factor Osf2͞Cbfa1͞Runx2 in growth plate chondrocytes (15,16). In contrast to humans, where MMP1 may be strongly expressed, e.g., in inflammation, the orthologue of MMP1, McolA (12), is expressed in mice only at low levels.To examine possible functional roles of collagenases during skeletal development in vivo, we targeted a null mutation to the Mmp13 gene in mice. Our targeting strategy resulted in splicing out exon 5 that encodes the zinc-binding residues in the catalytic domain. As described here, deletion of functional Mmp13 had profound effects on skeletal development. In Mmp13 Ϫ/Ϫ embryos compared with WT embryos, the growth plates were strikingly lengthened, a defect ascribable predominantly to a delay in terminal events in the growth plates, with failure to resorb collagens, as well as a delay in ossification at the primary centers. Materials and MethodsGeneration of Mmp13 ؊/؊ Mice. We isolated two Mmp13 genomic clones from a 129͞J1 library to construct the knockout vector.The first was a BamHI͞SalI fragment that spanned from Ϸ3.4 kb of promoter sequence through the first...