Osteoblasts and endothelium constitute functional niches that support hematopoietic stem cells (HSC) in mammalian bone marrow (BM) 1,2,3 . Adult BM also contains adipocytes, whose numbers correlate inversely with the hematopoietic activity of the marrow. Fatty infiltration of hematopoietic red marrow follows irradiation or chemotherapy and is a diagnostic feature in biopsies from patients with marrow aplasia 4. To explore whether adipocytes influence hematopoiesis or simply fill marrow space, we compared the hematopoietic activity of distinct regions of the mouse skeleton that differ in adiposity. By flow cytometry, colony forming activity, and competitive repopulation assay, HSCs and short-term progenitors are reduced in frequency in the adipocyte-rich vertebrae of the mouse tail relative to the adipocyte-free vertebrae of the thorax. In lipoatrophic A-ZIP/F1 “fatless” mice, which are genetically incapable of forming adipocytes8, and in mice treated with the PPARγ inhibitor Bisphenol-A-DiGlycidyl-Ether (BADGE), which inhibits adipogenesis9, post-irradiation marrow engraftment is accelerated relative to wild type or untreated mice. These data implicate adipocytes as predominantly negative regulators of the bone marrow microenvironment, and suggest that antagonizingmarrow adipogenesis may enhance hematopoietic recovery in clinical bone marrow transplantation.

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Many proteins found in mineralized tissues have been proposed to function as regulators of the mineralization process, either as nucleators or inhibitors of hydroxyapatite (HA) formation. We have studied the HA-nucleating and HA-inhibiting properties of proteins from bone [osteocalcin (OC), osteopontin (OPN), osteonectin (ON) and bone sialoprotein (BSP)], dentine [phosphophoryn (DPP)] and calcified cartilage [chondrocalcin (CC)] over a wide range of concentrations. Nucleation of HA was studied with a steady-state agarose gel system at sub-threshold [Ca] x [PO4] product. BSP and DPP exhibited nucleation activity at minimum concentrations of 0.3 microgram/ml (9 nM) and 10 micrograms/ml (67 nM) respectively. OC, OPN, ON and CC all lacked nucleation activity at concentrations up to 100 micrograms/ml. Inhibition of HA formation de novo was studied with calcium phosphate solutions buffered by autotitration. OPN was found to be a potent inhibitor of HA formation [IC50 = 0.32 microgram/ml (0.01 microM)] whereas OC was of lower potency [IC50 = 6.1 micrograms/ml (1.1 microM)]; BSP, ON and CC all lacked inhibitory activity at concentrations up to 10 micrograms/ml. The effect of OPN on HA formation de novo is mainly to inhibit crystal growth, whereas OC delays nucleation. These findings are consistent with the view that BSP and DPP may play roles in the initiation of mineralization in bone and dentine respectively. OPN seems to be the mineralized tissue protein most likely to function in the inhibition of HA formation, possibly by preventing phase separation in tissue fluids of high supersaturation.

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Estrogen deficiency in menopause is a major cause of osteoporosis in women. Estrogen acts to maintain the appropriate ratio between bone-forming osteoblasts and bone-resorbing osteoclasts in part through the induction of osteoclast apoptosis. Recent studies have suggested a role for Fas ligand (FasL) in estrogen-induced osteoclast apoptosis by an autocrine mechanism involving osteoclasts alone. In contrast, we describe a paracrine mechanism in which estrogen affects osteoclast survival through the upregulation of FasL in osteoblasts (and not osteoclasts) leading to the apoptosis of pre-osteoclasts. We have characterized a cell-type-specific hormone-inducible enhancer located 86 kb downstream of the FasL gene as the target of estrogen receptor-alpha induction of FasL expression in osteoblasts. In addition, tamoxifen and raloxifene, two selective estrogen receptor modulators that have protective effects in bone, induce apoptosis in pre-osteoclasts by the same osteoblast-dependent mechanism. These results demonstrate that estrogen protects bone by inducing a paracrine signal originating in osteoblasts leading to the death of pre-osteoclasts and offer an important new target for the prevention and treatment of osteoporosis.

Osteocalcin is an abundant Ca2+-binding protein of bone containing three residues of vitamin K dependent gamma-carboxyglutamic acid (Gla) among its 49 (human, monkey, cow) or 50 (chicken) amino acids. Gla side chains participate directly in the binding of Ca2+ ions and the adsorption of osteocalcin to hydroxylapatite (HA) surfaces in vivo and in vitro. Osteocalcin exhibits a major conformational change when Ca2+ is bound. Metal-free chicken osteocalcin is a random coil with only 8% of its residues in the alpha helix as revealed by circular dichroism. In the presence of physiological levels of Ca2+, 38% of the protein adopts the alpha-helical conformation with a transition midpoint at 0.75 mM Ca2+ in a rapid, reversible fashion which (1) requires an intact disulfide bridge, (2) is proportionally diminished when Gla residues are decarboxylated to Glu, (3) is insensitive to 1.5 m NaCl, and (4) can be mimicked by other cations. Tyr fluorescence, UV difference spectra, and Tyr reactivity to tetranitromethane corroborate the conformational change. Homologous monkey osteocalcin also exhibits Ca2+-dependent structure. Integration of predictive calculations from osteocalcin sequence has yielded a structural model for the protein, the dominant features of which include two opposing alpha-helical domains of 9-12 residues each, connected by a bea turn and stabilized by the Cys23-Cys29 disulfide bond. Cation binding permits realization of the full alph a-helical potential by partial neutralization of high anionic charge in the helical domains. Periodic Gla occurrence at positions 17, 21, and 24 has been strongly conserved throughout evolution and places all Gla side chains on the same face of one alpha helix spaced at intervals of approximately 5.4 A, closely paralleling the interatomic separation of Ca2+ in the HA lattice. Helical osteocalcin has greatly increased affinity for HA; thus, the Ca2+-induced structural transition may perform an informational role related to bone metabolism.

IntroductionThere are 2 major modes of bone development, intramembranous ossification and endochondral ossification. The former occurs when mesenchymal precursor cells directly differentiate into bone-forming osteoblasts, a process by which all flat bones are formed. The latter entails the conversion of an initial cartilage template into bone and is responsible for generating the long bones of the skeleton (1). Fibroblast growth factor receptors (FGFRs) have been implicated in both processes of bone formation. Mutations in 3 FGF receptors, FGFR1-3, result in craniosynostoses, including Pfeiffer, Crouzon, Apert, JacksonWeiss, and Beare-Stevenson cutis gyrata syndromes (2-4). Mutations in FGFR3 were also found to cause several skeletal dysplasias that involve primarily long bone growth, including achondroplasia (ACH), hypochondroplasia (HCH), and thanatophoric dysplasia (TD) (5-9). ACH is the most common form of dwarfism, with a frequency of approximately 1 in 20,000 live births. Patients with ACH exhibit a characteristic phenotype of rhizomelic dwarfism, relative macrocephaly, exaggerated lumbar lordosis, and minimal chondrocyte proliferation in the growth plate cartilage of long bones. ACH is most frequently caused by a glycine-to-arginine substitution at position 380 (Gly380Arg), and can also be caused by a change from glycine to cysteine at position 375 (Gly375Cys) (5, 6, 10).The effect of the Gly380Arg mutation on functions of FGFR3 has been investigated extensively. The Gly380Arg mutation causes ligand-independent activation of FGFR3 in vitro and results in dwarfism in vivo (11)(12)(13)(14). Little is known, however, about the possible effect of the Gly375Cys mutation on the functions of FGFR3. In this study, we examined the effect of the Gly375Cys mutation using both in vitro and in vivo approaches. Our data indicated that the Gly375Cys mutation caused activation of FGFR3 by inducing ligand-independent dimerization of the receptor in cultured cells. To study further the function of FGFR3 in bone growth, and to create a mouse model for the FGFR3-related inherited skeletal disorders, we introduced a Gly369Cys mutation, which corresponds to the Gly375Cys mutation in human, into the mouse genome using gene targeting. Mice carrying Missense mutations in fibroblast growth factor receptor 3 (FGFR3) result in several human skeletal dysplasias, including the most common form of dwarfism, achondroplasia. Here we show that a glycine-to-cysteine substitution at position 375 (Gly375Cys) in human FGFR3 causes ligand-independent dimerization and phosphorylation of FGFR3 and that the equivalent substitution at position 369 (Gly369Cys) in mouse FGFR3 causes dwarfism with features mimicking human achondroplasia. Accordingly, homozygous mice were more severely affected than heterozygotes. The resulting mutant mice exhibited macrocephaly and shortened limbs due to retarded endochondral bone growth and premature closure of cranial base synchondroses. Compared with their wild-type littermates, mutant mice growth plates shared an...

We find anomalously high gadolinium (Gd) concentrations in the femoral head bones of patients exposed to chelated Gd, commonly used as a contrast agent for medical imaging. Gd is introduced in chelated form to protect patients from exposure to toxic free Gd(3+), a calcium antagonist which disrupts cellular processes. Recent studies suggest Gd chelates break down in vivo, and Gd accumulation in tissue is linked to medical conditions such as nephrogenic systemic fibrosis (NSF), acute kidney failure, and in some cases death. We measure Gd and other rare earth element (REE) concentrations in 35 femoral heads by solution based ICP-MS. Gd concentrations in patients with documented exposure to Gd-based contrast agents (n = 13: Gd DTPA-BMA (Omniscan) n = 6; Gd HP-DO3A (Prohance) n = 5; unknown type n = 4) are significantly higher (p < 0.001) than the control group (n = 17). We use our control group to establish the 'natural' background level of Gd in human bone (cortical 95% CI: 0.023, 0.041 nmol/g; trabecular 95% CI: 0.054, 0.107 nmol/g). A control group outlier reveals the occurrence of individuals with high concentrations of all REEs, including Gd. Because of this, we calculate Gd anomalies from the concentrations of adjacent REEs and normalize to the control group mean to isolate Gd input from contrast agents. Normalized Gd anomalies, (Gd/Gd*)(N), for exposed patients range up to >800 times the 'natural' level (95% CI: 124, 460). Our data confirm that Gd, introduced in chelated form, incorporates into bone and is retained for more than 8 years. No difference was observed in bone Gd concentrations and anomalies between patients dosed with Gd DTPA-BMA (Omniscan; n = 6) and Gd HP-DO3A (Prohance; n = 5). Osteoporotic fracture patients exposed to Gd have significantly lower Gd concentrations than osteoarthritis patients (p < 0.001). This indicates different mechanisms of metal incorporation and/or retention in osteoporotic bone tissues, and may signal an increased risk of endogenous Gd release for patients with increased rates of bone resorption (e.g. osteoporosis patients and menopausal, pregnant, and lactating women) who are exposed to Gd-based contrast agents.