Fibroblast growth factor (FGF) signaling is involved in skeletal development of the vertebrate. Gain-of-function mutations of FGF receptors (FGFR) cause craniosynostosis, premature fusion of the skull, and dwarfism syndromes. Disruption of Fgfr3 results in prolonged growth of long bones and vertebrae. However, the role that FGFs actually play in skeletal development in the embryo remains unclear. Here we show that Fgf18 is expressed in and required for osteogenesis and chondrogenesis in the mouse embryo. Fgf18 is expressed in both osteogenic mesenchymal cells and differentiating osteoblasts during calvarial bone development. In addition, Fgf18 is expressed in the perichondrium and joints of developing long bones. In calvarial bone development of Fgf18-deficient mice generated by gene targeting, the progress of suture closure is delayed. Furthermore, proliferation of calvarial osteogenic mesenchymal cells is decreased, and terminal differentiation to calvarial osteoblasts is specifically delayed. Delay of osteogenic differentiation is also observed in the developing long bones of this mutant. Conversely, chondrocyte proliferation and the number of differentiated chondrocytes are increased. Therefore, FGF18 appears to regulate cell proliferation and differentiation positively in osteogenesis and negatively in chondrogenesis.

Sclerosteosis is a progressive sclerosing bone dysplasia. Sclerostin (the SOST gene) was originally identified as the sclerosteosis-causing gene. However, the physiological role of sclerostin remains to be elucidated. Sclerostin was intensely expressed in developing bones of mouse embryos. Punctuated expression of sclerostin was localized on the surfaces of both intramembranously forming skull bones and endochondrally forming long bones. Sclerostin-positive cells were identified as osteoclasts. Recombinant sclerostin protein produced in cultured cells was efficiently secreted as a monomer. We examined effects of sclerostin on the activity of BMP2, BMP4, BMP6, and BMP7 for mouse preosteoblastic MC3T3-E1 cells. Sclerostin inhibited the BMP6 and BMP7 activity but not the BMP2 and BMP4 activity. Sclerostin bound to BMP6 and BMP7 with high affinity but bound to BMP2 and BMP4 with lower affinity. In conclusion, sclerostin is a novel secreted osteoclast-derived BMP antagonist with unique ligand specificity. We suggest that sclerostin negatively regulates the formation of bone by repressing the differentiation and/or function of osteoblasts induced by BMPs. Since sclerostin expression is confined to the bone-resorbing osteoclast, it provides a mechanism whereby bone apposition is inhibited in the vicinity of resorption. Our findings indicate that sclerostin plays an important role in bone remodeling and links bone resorption and bone apposition.Sclerosteosis is a progressive sclerosing bone dysplasia with an autosomal recessive mode of inheritance. Scleroteosis is clinically and radiologically very similar to van Buchem disease (1, 2). By linkage analysis of families with these diseases, the disease-causing genes were mapped to the same chromosomal 17q12-q21 region, supporting the hypothesis that both diseases are caused by mutations in the same gene. By the positional cloning strategy, sclerostin (the SOST gene), which was mutated in sclerosteosis patients, was identified (1, 2). Sclerostin was found to be expressed in human long bones and cartilage using the polymerase chain reaction. However, the expression of sclerostin in the bones and cartilage was not examined in detail. The pathogenesis and genetics of sclerosteosis suggest that inhibition of sclerostin could lead to increased bone density. This definitely makes sclerostin and its pathway interesting targets for the development of anabolic agents against osteoporosis (1, 2). Sclerostin encodes a protein of 213 amino acids with a putative signal peptide for secretion, and sclerostin has six conserved cysteine residues and one conserved glycine residue that are essential to form a cystine knot. The spacing of cysteine residues is highly homologous to that of bone morphogenetic protein (BMP) 1 antagonists of the DAN/ cerberus family, indicating that sclerostin might be a BMP antagonist (1, 2). However, the biological activity of sclerostin is not known. Therefore, the physiological role of sclerostin and its mechanism of action remain to be elucidated.We examined t...