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SOST encodes sclerostin, an inhibitor of bone formation that antagonizes canonical Wnt signaling. Variations of SOST expression have an impact on bone mineral density (BMD) and bone strength. We hypothesized that genetic and epigenetic DNA modifications have an impact on SOST gene expression. By analyzing 43 bone samples from women, we found that rs851054 (G/A) is associated with SOST mRNA expression, donors with one or two G allele(s) displaying higher SOST expression (p<0.05). Beside this polymorphism, we also investigated the role of CpG methylation in SOST mRNA expression, and analyzed methylation variation at 13 CpG sites on the 1st exon of SOST in 14 human bone samples. Principal component analysis identified three groups of CpG sites that explained most of the methylation variation. We calculated the percentage of methylation in the main group of CpGs, and showed that higher rates of methylated CpGs are associated with higher SOST mRNA expression. To demonstrate that change in SOST expression might be related to human bone disease, we analyzed 131 patients with osteogenesis imperfecta (OI), a rare disease characterized by low BMD, bone fragility, and marked intra-familial variability of bone phenotypes. We found an association between rs851054 of the SOST promoter and the fracture rate only during childhood (p<0.01). In conclusion, genetic and epigenetic changes contribute to variation in SOST expression in human bone. Our data also indicate that these variations may be related to the severity of OI., (Copyright © 2016 Elsevier Inc. All rights reserved.)

The monoamine serotonin (5-HT), a well-known neurotransmitter, is also important in peripheral tissues. Several studies have suggested that 5-HT is involved in bone metabolism. Starting from our original observation of increased 5-HT(2B) receptor (5-HT(2B)R) expression during in vitro osteoblast differentiation, we investigated a putative bone phenotype in vivo in 5-HT(2B)R knockout mice. Of interest, 5-HT(2B)R mutant female mice displayed reduced bone density that was significant from age 4 months and had intensified by 12 and 18 months. This histomorphometrically confirmed osteopenia seems to be due to reduced bone formation because 1) the alkaline phosphatase-positive colony-forming unit capacity of bone marrow precursors was markedly reduced in the 5-HT(2B)R mutant mice from 4 to 12 months of age, 2) ex vivo primary osteoblasts from mutant mice exhibited reduced proliferation and delayed differentiation, and 3) calcium incorporation was markedly reduced in osteoblasts after 5-HT(2B)R depletion (produced genetically or by pharmacological inactivation). These findings support the hypothesis that the 5-HT(2B)R receptor facilitates osteoblast recruitment and proliferation and that its absence leads to osteopenia that worsens with age. We show here, for the first time, that the 5-HT(2B)R receptor is a physiological mediator of 5-HT in bone formation and, potentially, in the onset of osteoporosis in aging women.

Intermittent PTH treatment induces structural changes that affect cancellous bone mass and have led to its indication for the treatment of osteoporosis. PTH is also known to upregulate the expression of matrix metalloproteinases (MMP) in osteoblasts. We wanted to find out whether inhibiting osteoblastic MMPs can affect the anabolic action of PTH in vivo. We had shown previously that mice over-expressing TIMP-1 (tissue inhibitor of MMPs) specifically in osteoblasts display an increase in bone mineral density and bone mass combined with an overall decrease in bone turnover. In the present study, 10-week-old wild-type (WT) and transgenic (TG) mice were treated with PTH at 40 microg/kg/day for 1.5 months. DEXA analysis was performed before and after treatment, and histomorphometric and molecular analysis were carried out at the end of the experiment. Our findings indicate that the transgene boosted the anabolic action of PTH. The femurs of PTH-treated TG mice displayed a greater increase in bone mineral density and trabecular bone volume than treated WT mice. Interestingly, the positive effect of the transgene on the action of PTH resulted from both reduced bone resorption activity and an increase in the bone formation rate. Osteoclastic surfaces that were increased in PTH-treated WT mice remained unchanged in TG mice, suggesting a decrease in osteoclastic differentiation. Histomorphometric data also indicate that PTH administration increased osteoblast activity in TG mice and affected the number of osteoblasts in WT mice. In conclusion, we demonstrate that inhibiting osteoblastic MMPs can potentiate the anabolic effect of PTH by decreasing osteoclast activity and increasing osteoblast activity. Our data also suggest that osteoblastic MMPs have some role in mediating the anabolic effects of PTH in vivo and indicate that inhibitors of MMPs could constitute a new therapy for degenerative diseases.

Unlabelled: Chronic thiazide treatment is associated with high BMD. We report that patients and mice with null mutations in the thiazide-sensitive NaCl cotransporter (NCC) have higher renal tubular Ca reabsorption, higher BMD, and lower bone remodeling than controls, as well as abnormalities in Ca metabolism, mainly caused by Mg depletion., Introduction: Chronic thiazide treatment decreases urinary Ca excretion (UVCa) and increases BMD. To understand the underlying mechanisms, Ca and bone metabolism were studied in two models of genetic inactivation of the thiazide-sensitive NaCl cotransporter (NCC): patients with Gitelman syndrome (GS) and Ncc knockout (Ncc(-/-)) mice., Materials and Methods: Ca metabolism was analyzed in GS patients and Ncc(-/-) mice under conditions of low dietary Ca. BMD was measured by DXA in patients and mice, and bone histomorphometry was analyzed in mice., Results: GS patients had low plasma Mg. They exhibited reduced UVCa, but similar serum Ca and GFR as control subjects, suggesting increased renal Ca reabsorption. Blood PTH was lower despite lower serum ionized Ca, and Mg repletion almost corrected both relative hypoparathyroidism and low UVCa. BMD was significantly increased in GS patients at both lumbar (+7%) and femoral (+16%) sites, and osteocalcin was reduced. In Ncc(-/-) mice, serum Ca and GFR were unchanged, but UVCa was reduced and PTH was elevated; Mg repletion largely corrected both abnormalities. Trabecular and cortical BMD were higher than in Ncc(+/+) mice (+4% and +5%, respectively), and despite elevated PTH, were associated with higher cortical thickness and lower endosteal osteoclastic surface., Conclusions: Higher BMD is observed in GS patients and Ncc(-/-) mice. Relative hypoparathyroidism (human) and bone resistance to PTH (mice), mainly caused by Mg depletion, can explain the low bone remodeling and normal/low serum Ca despite increased renal Ca reabsorption.

Summary: Young mice over-expressing Runx2 fail to gain bone relative to wild type mice with growth and present spontaneous fractures. It allows, for the first time in rodents, direct assessment of anti-fracture efficacy of strontium ranelate which was able to decrease caudal vertebrae fracture incidence through an improvement of trabecular and cortical architecture. Introduction: The aim was to investigate whether strontium ranelate was able to decrease fracture incidence in mice over-expressing Runx2, model of severe developmental osteopenia associated with spontaneous vertebral fractures. Methods: Transgenic mice and their wild type littermates were treated by oral route with strontium ranelate or vehicle for 9 weeks. Caudal fracture incidence was assessed by repeated X-rays, resistance to compressive loading by biochemical tests, and bone microarchitecture by histomorphometry. Results: Transgenic mice receiving strontium ranelate had significantly fewer new fractures occurring during the 9 weeks of the study (−60%, p < 0.05). In lumbar vertebrae, strontium ranelate improves resistance to compressive loading (higher ultimate force to failure, +120%, p < 0.05) and trabecular microarchitecture (higher bone volume and trabecular number, lower trabecular separation, +60%, +50%, −39%, p < 0.05) as well as cortical thickness (+17%, p < 0.05). In tibiae, marrow cavity cross-section area and equivalent diameter were lower (−39%, −21%, p < 0.05). The strontium level in plasma and bone was in the same range as the values measured in treated postmenopausal women. Conclusions: This model allows, for the first time, direct assessment of anti-fracture efficacy of strontium ranelate treatment in rodents. In these transgenic mice, strontium ranelate was able to decrease caudal vertebral fracture incidence through an improvement of trabecular and cortical architecture. [ABSTRACT FROM AUTHOR]

The monoamine serotonin (5-HT), a well-known neurotransmitter, is also important in peripheral tissues. Several studies have suggested that 5-HT is involved in bone metabolism. Starting from our original observation of increased 5-HT2B receptor (5- HT2BR) expression during in vitro osteoblast differentiation, we investigated a putative bone phenotype in vivo in 5-HT2B knockout mice. Of interest, 5-HT2BR mutant female mice displayed reduced bone density that was significant from age 4 months and had intensified by 12 and 18 months. This histomorphometrically confirmed osteopenia seems to be due to reduced bone formation because I) the alkaline phosphatase-positive colony-forming unit capacity of bone marrow precursors was markedly reduced in the 5-HT2BR mutant mice from 4 to 12 months of age, 2) ex vivo primary osteoblasts from mutant mice exhibited reduced proliferation and delayed differentiation, and 3) calcium incorporation was markedly reduced in osteoblasts after 5-HT2BR depletion (produced genetically or by pharmacological inactivation). These findings support the hypothesis that the 5-HT2BR receptor facilitates osteoblast recruitment and proliferation and that its absence leads to osteopenia that worsens with age. We show here, for the first time, that the 5-HT2BR receptor is a physiological mediator of 5-HT in bone formation and, potentially, in the onset of osteoporosis in aging women. [ABSTRACT FROM AUTHOR]