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...

The homeostasis of the plasma phosphate level is essential for many biological processes including skeletal mineralization. The reabsorption of phosphate in the kidney is a major determinant of the plasma levels of phosphate. Phosphatonin is a hormone-like factor that specifically inhibits phosphate uptake in renal proximal epithelial cells. Recent studies on tumor-induced osteomalacia suggested that phosphatonin was potentially identical to fibroblast growth factor (FGF)-23. However, as purified recombinant FGF-23 could not inhibit phosphate uptake in renal proximal epithelial cells, the mechanism of action of FGF-23 remains to be elucidated. Therefore, we examined the mechanism of action of FGF-23 in cultured renal proximal epithelial cells, opossum kidney cells. FGF-23 was found to require heparinlike molecules for its inhibitory activity on phosphate uptake. FGF-23 binds to the FGF receptor 3c, which is mainly expressed in opossum kidney cells, with high affinity. An inhibitor for tyrosine kinases of the FGF receptor, SU 5402, blocked the activity of FGF-23. FGF-23 activated the mitogen-activated protein kinase (MAPK) pathway, which is the major intracellular signaling pathway of FGF. Inhibitors of the MAPK pathway, PD98059 and SB203580, also blocked the activity of FGF-23.ThepresentfindingshaverevealedanovelMAPKdependent mechanism of the regulation of phosphate uptake by FGF signaling.Phosphate is a nutrient essential for many biological processes including skeletal mineralization and energy metabolism (1). The homeostasis of the plasma phosphate level is essential for these processes. The reabsorption of phosphate in the kidney is a major determinant of the plasma phosphate level. Reabsorption is largely regulated by the type II sodium-dependent phosphate (Na/P i ) cotransporter that is expressed in renal proximal epithelial cells (1). The activity of the type-II Na/P i cotransporter is regulated by hormones, such as parathyroid hormone (PTH) 1 and 1,25-dihydroxyvitamin D (1,25(OH) 2 D), which have opposite effects. PTH and 1,25(OH) 2 D decrease and increase the reabsorption of phosphate in renal proximal tubules, respectively (1). Tumor-induced osteomalacia is a renal phosphate-wasting disorder resulting in low serum phosphorus concentration and osteomalacia. Removal of the tumors responsible for tumorinduced osteomalacia normalizes phosphate metabolism. The responsible tumors secrete a heat-sensitive molecule of ϳ25 kDa designated as "phosphatonin" that specifically inhibits sodium-dependent phosphate transport in cultured renal proximal epithelial cells. Recent studies on tumor-induced osteomalacia revealed that phosphatonin was potentially identical to fibroblast growth factor (FGF)-23, which is a new member of the FGF family (2, 3, 5). Autosomal dominant hypophosphataemic rickets is also a renal phosphate-wasting disorder resulting in low serum phosphorus concentration, rickets, and osteomalacia. The ADHR gene was also potentially identified to be FGF-23 with missense mutations (4). However, as puri...

We isolated the cDNA encoding a novel member of the fibroblast growth factor (FGF) family from rat embryos by homology-based polymerase chain reaction. The FGF-related cDNA encodes a protein of 215 amino acids (ϳ24 kDa), which has a conserved ϳ120-amino acid core with ϳ30 -60% amino acid sequence identity with the FGF family. This protein with a hydrophobic amino terminus appears to be a secreted protein. The cDNA was translated in a coupled in vitro transcription-translation system. The molecular mass of the translation product was observed to be ϳ26 kDa. The expression of the FGF-related mRNA in the rat embryo and adult tissues was determined by Northern analysis and in situ hybridization. The mRNA was expressed in several discrete regions of the embryo. In adult tissues, the mRNA was preferentially expressed in the lung. The expression profile of the FGF-related mRNA was different from those of other FGF family mRNAs. As this protein is the 10th documented protein related to FGFs, we tentatively term this protein FGF-10.

We identified a novel secreted protein and named it neudesin. Mouse neudesin of 171 amino acids is unique with no primary structural similarity to any known proteins. The neudesin protein produced in cultured cells was secreted efficiently into the culture medium. Mouse neudesin mRNA was expressed abundantly in the developing brain and spinal cord in embryos, but was expressed widely in postnatal tissues including brain, heart, lung, and kidney. Mouse neudesin mRNA was expressed in neurons but not glial cells of the brain. The protein exhibited significant neurotrophic activity in primary cultured mouse neurons but not mitogenic activity in primary cultured mouse astrocytes. Neudesin activated the mitogen-activated protein (MAP) and phosphatidylinositol-3 (PI-3) kinase pathways. The activity of neudesin was inhibited by the inhibitor pertussis toxin for Gi/Go-protein but not by inhibitors for receptor tyrosine kinases. These results indicated that the activity was mediated via the activation of the MAP and PI-3 kinase pathways, potentially by the activation of a Gi/Go-protein-coupled receptor. Human neudesin of 172 amino acids with high similarity ( approximately 91% identity) to mouse neudesin was also identified. The human neudesin gene was mapped to chromosome 1p33. The identification of neudesin, a novel secreted protein with a unique primary structure and neurotrophic activity, will provide new insights into the development and maintenance of neuron