Denosumab,a Fully Human Monoclonal Antibody to RANKL,Inhibits Bone Resorption and Increases BMD in Knock‐In Mice That Express Chimeric (Murine/Human) RANKL

RANKL is a TNF family member that mediates osteoclast formation, activation, and survival by activating RANK. The proresorptive effects of RANKL are prevented by binding to its soluble inhibitor osteoprotegerin (OPG). Recombinant human OPG‐Fc recognizes RANKL from multiple species and reduced bone resorption and increased bone volume, density, and strength in a number of rodent models of bone disease. The clinical development of OPG‐Fc was discontinued in favor of denosumab, a fully human monoclonal antibody that specifically inhibits primate RANKL. Direct binding assays showed that denosumab bound to human RANKL but not to murine RANKL, human TRAIL, or other human TNF family members. Denosumab did not suppress bone resorption in normal mice or rats but did prevent the resorptive response in mice challenged with a human RANKL fragment encoded primarily by the fifth exon of the RANKL gene. To create mice that were responsive to denosumab, knock‐in technology was used to replace exon 5 from murine RANKL with its human ortholog. The resulting “huRANKL” mice exclusively express chimeric (human/murine) RANKL that was measurable with a human RANKL assay and that maintained bone resorption at slightly reduced levels versus wildtype controls. In young huRANKL mice, denosumab and OPG‐Fc each reduced trabecular osteoclast surfaces by 95% and increased bone density and volume. In adult huRANKL mice, denosumab reduced bone resorption, increased cortical and cancellous bone mass, and improved trabecular microarchitecture. These huRANKL mice have potential utility for characterizing the activity of denosumab in a variety of murine bone disease models.     

Regulating DeltaFosB expression in adult Tet-Off-DeltaFosB transgenic mice alters bone formation and bone mass.

The DeltaFosB isoforms are naturally occurring AP-1 family members that increase bone volume via a cell-autonomous effect on osteoblastic bone formation. Mice overexpressing DeltaFosB demonstrate a very high level of bone formation, resulting in a progressive osteosclerosis. Despite the linkage of bone formation and resorption in physiological systems, no alteration in bone resorption was detected in mice overexpressing DeltaFosB. To determine whether altering DeltaFosB expression can regulate bone formation independently of bone resorption in adult mice, we used the Tet-Off-inducible transgene system to induce or block transgenic DeltaFosB overexpression and thereby regulate bone formation in vivo. Overexpression of DeltaFosB after skeletal maturity increased trabecular bone volume by increasing bone formation, again without altering bone resorption, indicating that developmental DeltaFosB overexpression is not required for the osteosclerotic phenotype. Similarly, switching off DeltaFosB overexpression after osteosclerosis had developed led to a marked decrease in bone formation and loss of bone mass such that trabecular bone volume approached normal levels. Despite this dramatic reduction, no alteration in bone resorption was detected. These results clearly demonstrate that DeltaFosB regulates bone formation and bone mass in adult mice with no effect on bone resorption.     

Transgenic mice overexpressing macrophage migration inhibitory factor (MIF) exhibit high-turnover osteoporosis.

The bone phenotype of mice overexpressing MIF was studied. These mice showed decreased trabecular bone, increased bone formation rate, and increased MMP-3, -9, and -13 mRNA expression in the femora and tibias. This model provides evidence of the role played by MIF in bone remodeling and balance in vivo. INTRODUCTION: The role of macrophage migration inhibitory factor (MIF) in in vivo bone remodeling remains unelucidated. We describe disordered bone metabolism in transgenic mice overexpressing MIF. MATERIALS AND METHODS: For in vivo study, muCT, bone histomorphometry, blood and urine biochemical data, and gene expression of MIF transgenic (MIF Tg) mice and littermate wildtype (WT) mice were examined. For in vitro study, osteoclastogenesis in the co-culture of bone marrow cells and osteoblasts from MIF Tg and WT were assessed. RESULTS: muCT analyses revealed a significant reduction in the trabecular bone of distal femur in MIF Tg at 8-12 weeks of age. Histomorphometric analysis revealed increase in several measures of bone formation. Osteoclastogenesis was not influenced by the origin of bone marrow cells or osteoblasts. Urine level of deoxypyridinoline/creatinine and the mRNA levels of matrix metalloproteinase (MMP) -3, -9, and -13 in femurs were elevated in MIF Tg. CONCLUSIONS: Overexpression of MIF causes high-turnover osteoporosis in mice. The increased expression of MMPs in bone was suggested, at least in part, as one cause of this phenotype, because MMPs plays important roles for bone resorption without affecting the formation of osteoclasts. This model provides evidence of the role played by MIF in bone remodeling and balance.     

Perturbations in bone formation and resorption in insulin-like growth factor binding protein-3 transgenic mice.

IGF-I and their binding proteins are important in bone health. Examination of BMD, osteoblast proliferation, and markers of bone resorption in transgenic mice that constitutively overexpress IGFBP-3 indicates that overexpression of IGFBP-3 increases osteoclast number and bone resorption, impairs osteoblast proliferation, and has a significant negative effect on bone formation. INTRODUCTION: Low serum insulin-like growth factor I (IGF-I) levels correlate with an increased risk of osteoporotic fractures. Serum IGF-I is largely bound to IGF-binding protein-3 (IGFBP-3), which can inhibit IGF-I action and enhance delivery of IGF-I to tissues. Its role in bone biology is unclear. METHODS: Bone mineral density (BMD), osteoblast proliferation, and markers of bone resorption were examined in transgenic (Tg) mice that constitutively overexpressed human IGFBP-3 cDNA driven by either the cytomegalovirus (CMV) or phosphoglycerate kinase (PGK) promoter. RESULTS: Cultured calvarial osteoblasts from Tg mice expressed the transgene and grew more slowly than cells from wild-type (Wt) mice, and the mitogenic response to IGF-I was attenuated in osteoblasts from Tg mice. Total volumetric BMD and cortical BMD, measured in the femur using peripheral quantitative computed tomography (pQCT) were significantly reduced in both Tg mouse strains compared with Wt mice. PGKBP-3 Tg mice showed the most marked reduction in bone density. Osteocalcin levels were similar in Wt and CMVBP-3 Tg mice but were significantly reduced in PGKBP-3 Tg mice. Urinary deoxypyridinoline and osteoclast perimeter, markers of bone resorption, were significantly increased in both Tg mouse strains compared with Wt mice. Using double labeling with tetracycline, we demonstrated that pericortical and endocortical mineral apposition rate was significantly reduced in PGKBP-3 Tg mice compared with Wt mice. CONCLUSIONS: These data show that overexpression of IGFBP-3 increases osteoclast number and bone resorption, impairs osteoblast proliferation, and has a significant negative effect on bone formation.     

Evaluation of Pharmaceuticals With a Novel 50‐Hour Animal Model of Bone Loss

Osteoporosis remains a major public health problem through its associated fragility fractures. Several animal models for the study of osteoporotic bone loss, such as ovariectomy (OVX) and denervation, require surgical skills and several weeks to establish. Osteoclast differentiation and activation is mediated by RANKL. Here we report the establishment of a novel and rapid bone loss model by the administration of soluble RANKL (sRANKL) to mice. Mice were injected intraperitoneally with sRANKL and used to evaluate existing anti‐osteoporosis drugs. sRANKL decreased BMD within 50 h in a dose‐dependent manner. The marked decrease in femoral trabecular BMD shown by pQCT and the 3D images obtained by μCT were indistinguishable from those observed in the OVX model. Histomorphometry showed that osteoclastic activity was significantly increased in the sRANKL‐injected mice. In addition, serum biochemical markers of bone turnover such as Ca, C‐telopeptide of type 1 collagen (CTX), and TRACP5b were also significantly increased in the sRANKL‐injected mice in a dose‐dependent manner. Bisphosphonates (BPs), selective estrogen receptor modulators (SERMs), and PTH are commonly used for the treatment of osteoporosis. We successfully evaluated the effects of anti–bone‐resorbing agents such as BPs, a SERM, and anti–RANKL‐neutralizing antibody on bone resorption in a couple of weeks. We also evaluated the effects of PTH on bone formation in 2 wk. A combination of sRANKL injections and OVX made it possible to evaluate a SERM. The sRANKL model is the simplest, fastest, and easiest of all osteoporosis models and could be useful in the evaluation of drug candidates for osteoporosis.     

Daily administration of eldecalcitol (ED-71), an active vitamin D analog, increases bone mineral density by suppressing RANKL expression in mouse trabecular bone

Eldecalcitol (ED‐71) is a new vitamin D₃ derivative recently approved for the treatment of osteoporosis in Japan. Previous studies have shown that the daily administration of ED‐71 increases bone mineral density (BMD) by suppressing bone resorption in various animal models. In this study, we examined how ED‐71 suppresses bone resorption in vivo, by analyzing bone histomorphometry and ex vivo osteoclastogenesis assays. Daily administration of ED‐71 (50 ng/kg body weight) to 8‐week‐old male mice for 2 and 4 weeks increased BMD in the femoral metaphysis without causing hypercalcemia. Bone and serum analyses revealed that ED‐71 inhibited bone resorption and formation, indicating that the increase in BMD is the result of the suppression of bone resorption. This suppression was associated with a decrease in the number of osteoclasts in trabecular bone. We previously identified cell cycle‐arrested receptor activator of NF‐κB (RANK)‐positive bone marrow cells as quiescent osteoclast precursors (QOPs) in vivo. Daily administration of ED‐71 affected neither the number of RANK‐positive cells in vivo nor the number of osteoclasts formed from QOPs in ex vivo cultures. In contrast, ED‐71 suppressed the expression of RANK ligand (RANKL) mRNA in femurs. Immunohistochemical experiments also showed that the perimeter of the RANKL‐positive cell surface around the trabecular bone was significantly reduced in ED‐71‐treated mice than in the control mice. ED‐71 administration also increased BMD in 12‐week‐old ovariectomized mice, through the suppression of RANKL expression in the trabecular bone. These results suggest that the daily administration of ED‐71 increases BMD by suppressing RANKL expression in trabecular bone in vivo. © 2012 American Society for Bone and Mineral Research     

Swedish mutant APP suppresses osteoblast differentiation and causes osteoporotic deficit,which are ameliorated by N‐acetyl‐L‐cysteine

Reduced bone mineral density and hip fracture are frequently observed in patients with Alzheimer's disease (AD). However, mechanisms underlying their association remain poorly understood. Amyloid precursor protein (APP) is a transmembrane protein that is ubiquitously expressed in bone marrow stromal cells (BMSCs), osteoblasts (OBs), macrophages (BMMs), and osteoclasts (OCs). Mutations in the APP gene identified in early‐onset AD patients are believed to cause AD. But little is known about APP's role in bone remodeling. Here, we present evidence for Swedish mutant APP (APPswe) in suppression of OB differentiation and function in culture and in mouse. APP expression in BMSCs increases during aging. Ubiquitous expression of APPswe in young adult Tg2576 transgenic mice (under the control of a prion promoter) recaptured skeletal “aging‐like” deficits, including decreased OB genesis and bone formation, increased adipogenesis and bone marrow fat, and enhanced OC genesis and bone resorption. Remarkably, selective expression of APPswe in mature OB‐lineage cells in TgAPPswe‐Ocn mice (under the control of osteocalcin [Ocn] promoter‐driven Cre) also decreased OB genesis and increased OC formation, resulting in a trabecular bone loss. These results thus suggest a cell‐autonomous role for APPswe in suppressing OB formation and function, but a nonautonomous effect on OC genesis. Notably, increased adipogenesis and elevated bone marrow fat were detected in young adult Tg2576 mice, but not in TgAPPswe‐Ocn mice, implying that APPswe in BMSCs and/or multicell types in bone marrow promotes bone marrow adipogenesis. Intriguingly, the skeletal aging‐like deficits in young adult Tg2576 mice were prevented by treatment with N‐acetyl‐L‐cysteine (NAC), an antioxidant, suggesting that reactive oxygen species (ROS) may underlie APPswe‐induced osteoporotic deficits. Taken together, these results demonstrate a role for APPswe in suppressing OB differentiation and bone formation, implicate APPswe as a detrimental factor for AD‐associated osteoporotic deficit, and reveal a potential clinical value of NAC in the treatment of osteoporotic deficits. © 2013 American Society for Bone and Mineral Research.     

Transgenic mice overexpressing tartrate-resistant acid phosphatase exhibit an increased rate of bone turnover.

Tartrate-resistant acid phosphatase (TRAP) is a secreted product of osteoclasts and a lysosomal hydrolase of some tissue macrophages. To determine whether TRAP expression is rate-limiting in bone resorption, we overexpressed TRAP in transgenic mice by introducing additional copies of the TRAP gene that contained the SV40 enhancer. In multiple independent mouse lines, the transgene gave a copy number-dependent increase in TRAP mRNA levels and TRAP activity in osteoclasts, macrophages, serum, and other sites of normal low-level expression (notably, liver parenchymal cells, kidney mesangial cells, and pancreatic secretory acinar cells). Transgenic mice had decreased trabecular bone consistent with mild osteoporosis. Measurements of the bone formation rate suggest that the animals compensate for the increased resorption by increasing bone synthesis, which partly ameliorates the phenotype. These mice provide evidence that inclusion of an irrelevant enhancer does not necessarily override a tissue-specific promoter.     

MicroRNA-29a mitigates glucocorticoid induction of bone loss and fatty marrow by rescuing Runx2 acetylation

Glucocorticoid treatment reportedly increases the morbidity of osteoporotic or osteonecrotic disorders. Exacerbated bone acquisition and escalated marrow adipogenesis are prominent pathological features of glucocorticoid-mediated skeletal disorders. MicroRNAs reportedly modulate tissue metabolism and remodeling. This study was undertaken to investigate the biological roles of microRNA-29a (miR-29a) in skeletal and fat metabolism in the pathogenesis of glucocorticoid-induced osteoporosis. Transgenic mice overexpressing miR-29a precursor or wild-type mice were given methylprednisolone. Bone mass, microarchitecture and histology were assessed by dual energy X-ray absorptiometry, μCT and histomorphometry. Differential gene expression and signaling components were delineated by quantitative RT-PCR and immunoblotting. Glucocorticoid treatment accelerated bone loss and marrow fat accumulation in association with decreased miR-29a expression. The miR-29a transgenic mice had high bone mineral density, trabecular microarchitecture and cortical thickness. miR-29a overexpression mitigated the glucocorticoid-induced impediment of bone mass, skeletal microstructure integrity and mineralization reaction and attenuated fatty marrow histopathology. Ex vivo, miR-29a increased osteogenic differentiation capacity and alleviated the glucocorticoid-induced promotion of adipocyte formation in primary bone-marrow mesenchymal progenitor cell cultures. Through inhibition of histone deacetylase 4 (HDAC4) expression, miR-29a restored acetylated Runx2 and β-catenin abundances and reduced RANKL, leptin and glucocorticoid receptor expression in glucocorticoid-mediated osteoporosis bone tissues. Taken together, glucocorticoid suppression of miR-29a signaling disturbed the balances between osteogenic and adipogenic activities, and thereby interrupted bone formation and skeletal homeostasis. miR-29a inhibition of HDAC4 stabilized the acetylation state of Runx2 and β-catenin that ameliorated the detrimental effects of glucocorticoid on mineralization and lipogenesis reactions in bone tissue microenvironments. This study highlighted emerging skeletal-anabolic actions of miR-29a signaling in the progression of glucocorticoid-induced bone tissue destruction. Sustaining miR-29a actions is beneficial in protecting against glucocorticoid-mediated osteoporosis.     

Transgenic mice overexpressing soluble osteoclast differentiation factor (sODF) exhibit severe osteoporosis

Osteoclast differentiation factor, ODF, also called RANKL, TRANCE, or OPGL, is a key molecule for osteoclast differentiation and activation, and is thought to act as a membrane-associated molecule in bone remodeling. Recent study suggested that soluble ODF (sODF) released from T cells also has some roles in bone resorption. To investigate the physiological and pathological function of sODF, we generated two types of transgenic mice overexpressing sODF. Mice overexpressing sODF ubiquitously from the early developmental stage died at the late fetal stage. The other type of mice, expressing sODF only in the liver after birth, grew to maturity with normal body size and weight. However, they exhibited a marked decrease in bone mineral density with aging compared with their non-transgenic littermates, and in addition, the strength of their femurs was extremely reduced. Histological analysis showed that the trabecular bone mass was decreased at 6 weeks of age and was sparse at age 3-4 months. The number of osteoclasts was significantly increased, while the number of osteoblasts was not altered on the surface of young trabecular bone. These results indicate that excessive production of sODF causes osteoporosis by accelerated osteoclastogenesis. The transgenic mouse overexpressing sODF in the liver could serve as a useful animal model for studying bone remodeling and evaluating therapeutic agents for osteoporosis.     

Skeletal deterioration induced by RANKL infusion: a model for high-turnover bone disease

Summary RANKL was administered continuously to rats for 28 days to investigate its potential as a disease model for the skeletal system. Bone turnover rates, bone material, structural and mechanical properties were evaluated. RANKL infusion caused overall skeletal complications comparable to those in high bone-turnover conditions, such as postmenopausal osteoporosis. Introduction RANKL is an essential mediator for osteoclast development. No study has examined in detail the direct skeletal consequences of excess RANKL on bone turnover, mineralization, architecture, and vascular calcification. We, therefore, administrated soluble RANKL continuously into mature rats and created a bone-loss model. Methods Six-month-old Sprague-Dawley (SD) rats were assigned to three groups (n = 12) receiving continuous administration of saline (VEH) or human RANKL (35 μg/kg/day, LOW or 175 μg/kg/day, HI) for 28 days. Blood was collected routinely during the study. At sacrifice, hind limbs and aorta were removed and samples were analyzed. Results High dose RANKL markedly stimulated serum osteocalcin and TRAP-5b levels and reduced femur cortical bone volume (−7.6%) and trabecular volume fraction (BV/TV) at the proximal tibia (−64% vs. VEH). Bone quality was significantly degraded in HI, as evidenced by decreased femoral percent mineralization, trabecular connectivity, and increased endocortical bone resorption perimeters. Both cortical and trabecular bone mechanical properties were reduced by high dose RANKL. No differences were observed in the mineral content of the abdominal aorta. Conclusions Continuous RANKL infusion caused general detrimental effects on rat skeleton. These changes are comparable to those commonly observed in high-turnover bone diseases such as postmenopausal osteoporosis.     

Targeted disruption of leucine‐rich repeat kinase 1 but not leucine‐rich repeat kinase 2 in mice causes severe osteopetrosis

To assess the roles of Lrrk1 and Lrrk2, we examined skeletal phenotypes in Lrrk1 and Lrrk2 knockout (KO) mice. Lrrk1 KO mice exhibit severe osteopetrosis caused by dysfunction of multinucleated osteoclasts, reduced bone resorption in endocortical and trabecular regions, and increased bone mineralization. Lrrk1 KO mice have lifelong accumulation of bone and respond normally to the anabolic actions of teriparatide treatment, but are resistant to ovariectomy‐induced bone boss. Precursors derived from Lrrk1 KO mice differentiate into multinucleated cells in response to macrophage colony‐stimulating factor (M‐CSF)/receptor activator of NF‐κB ligand (RANKL) treatment, but these cells fail to form peripheral sealing zones and ruffled borders, and fail to resorb bone. The phosphorylation of cellular Rous sarcoma oncogene (c‐Src) at Tyr‐527 is significantly elevated whereas at Tyr‐416 is decreased in Lrrk1‐deficient osteoclasts. The defective osteoclast function is partially rescued by overexpression of the constitutively active form of Y527F c‐Src. Immunoprecipitation assays in osteoclasts detected a physical interaction of Lrrk1 with C‐terminal Src kinase (Csk). Lrrk2 KO mice do not show obvious bone phenotypes. Precursors derived from Lrrk2 KO mice differentiate into functional multinucleated osteoclasts. Our finding of osteopetrosis in Lrrk1 KO mice provides convincing evidence that Lrrk1 plays a critical role in negative regulation of bone mass in part through modulating the c‐Src signaling pathway in mice.     

MiR‐503 Regulates Osteoclastogenesis via Targeting RANK

MicroRNAs (miRNAs) play important roles in osteoclastogenesis and bone resorption. However, no study has investigated the role of miRNA in postmenopausal osteoporosis. Here, we report that miR‐503 was markedly reduced in circulating progenitors of osteoclasts–CD14⁺ peripheral blood mononuclear cells (PBMCs) from postmenopausal osteoporosis patients compared with those from postmenopausal healthy women. Overexpression of miR‐503 in CD14⁺ PBMCs inhibited receptor activator of nuclear factor‐κB ligand (RANKL)‐induced osteoclastogenesis. Conversely, silencing of miR‐503 in CD14⁺ PBMCs promoted osteoclastogenesis. RANK, which is activated by the binding of RANKL and inducing osteoclast differentiation, was confirmed to be a target of miR‐503. In vivo, silencing of miR‐503 using a specific antagomir in ovariectomy (OVX) mice increased RANK protein expression, promoted bone resorption, and decreased bone mass, whereas overexpression of miR‐503 with agomir inhibited bone resorption and prevented bone loss in OVX mice. Thus, our study revealed that miR‐503 plays an important role in the pathogenesis of postmenopausal osteoporosis and contributes to a new therapeutic way for osteoporosis. © 2014 American Society for Bone and Mineral Research.     

橙皮苷对去卵巢骨质疏松症大鼠骨质流失和机制的影响

目的研究橙皮苷对去卵巢骨质疏松大鼠骨质流失和骨保护素(osteoprotegerin,OPG)/核因子kappa B配体的受体激活物(receptor activator of nuclear factor kappa B ligand,RANKL)/RANK通路的影响。方法将48只雌性Sprague-Dawley(SD)大鼠随机分为假手术组、去卵巢骨质疏松组、橙皮苷低剂量组、橙皮苷中剂量、橙皮苷高剂量组和雌激素组。检测骨组织骨体积分数(bone volume fraction,BV/TV)、骨小梁厚度(trabecular thickness,Tb.Th)、骨密度(bone mineral density,BMD)、骨小梁数量(trabecular number,Tb.N)、骨小梁分离度(trabecular separation,Tb.Sp),通过HE染色观察骨组织病理损伤,采用酶联免疫吸附法(enzyme linked immunosorbent assay,ELISA)检测人Ⅰ型胶原C端肽(C-terminal peptide collagen typeⅠ,CTX-Ⅰ)和血清骨钙素(bone glaprotein,BGP)的表达量。运用qRT-PCR和Western bolt分别检测骨组织OPG、RANKL、RANK的mRNA和蛋白表达。结果在橙皮苷治疗后,与去卵巢骨质疏松组相比,骨质疏松标志物BV/TV、BMD、Tb.Th和Tb.N水平升高,Tb.Sp水平降低;大鼠骨小梁数量明显增加,且骨小梁连接更为紧密;骨吸收标志物CTX-Ⅰ水平降低;骨形成标志物BGP水平升高。OPG mRNA相对表达量和蛋白的表达量升高,RANKL和RANK mRNA相对表达量和蛋白的表达量降低。结论橙皮苷能缓解去卵巢骨质疏松大鼠骨质流失并调节OPG/RANKL/RANK信号通路。     

Interferon‐γ plays a role in bone formation in vivo and rescues osteoporosis in ovariectomized mice

Interferon γ (IFN‐γ) is a cytokine produced locally in the bone microenvironment by cells of immune origin as well as mesenchymal stem cells. However, its role in normal bone remodeling is still poorly understood. In this study we first examined the consequences of IFN‐γ ablation in vivo in C57BL/6 mice expressing the IFN‐γ receptor knockout phenotype (IFNγR1^?/?). Compared with their wild‐type littermates (IFNγR1^+/+), IFNγR1^?/? mice exhibit a reduction in bone volume associated with significant changes in cortical and trabecular structural parameters characteristic of an osteoporotic phenotype. Bone histomorphometry of IFNγR1^?/? mice showed a low‐bone‐turnover pattern with a decrease in bone formation, a significant reduction in osteoblast and osteoclast numbers, and a reduction in circulating levels of bone‐formation and bone‐resorption markers. Furthermore, administration of IFN‐γ (2000 and 10,000 units) to wild‐type C57BL/6 sham‐operated (SHAM) and ovariectomized (OVX) female mice significantly improved bone mass and microarchitecture, mechanical properties of bone, and the ratio between bone formation and bone resorption in SHAM mice and rescued osteoporosis in OVX mice. These data therefore support an important physiologic role for IFN‐γ signaling as a potential new anabolic therapeutic target for osteoporosis. © 2011 American Society for Bone and Mineral Research.     

Increased trabecular bone formation in mice lacking the growth factor midkine

Midkine (Mdk) and pleiotrophin (Ptn) comprise a family of heparin‐binding growth factors known primarily for their effects on neuronal cells. Since transgenic mice overexpressing Ptn have been reported to display increased bone density, we have previously analyzed Ptn‐deficient mice but failed to detect any abnormality of skeletal development and remodeling. Together with the finding that Mdk expression increases in the course of primary osteoblast differentiation, we reasoned that Mdk, rather than Ptn, could play a physiologic role in bone formation. Here, we show that Mdk‐deficient mice display an increased trabecular bone volume at 12 and 18 months of age, accompanied by cortical porosity. Histomorphometric quantification demonstrated an increased bone‐formation rate compared with wild‐type littermates, whereas bone resorption was differentially affected in trabecular and cortical bone of Mdk‐deficient mice. To understand the effect of Mdk on bone formation at the molecular level, we performed a genome‐wide expression analysis of primary osteoblasts and identified Ank and Enpp1 as Mdk‐induced genes whose decreased expression in Mdk‐deficient osteoblasts may explain, at least in part, the observed skeletal phenotype. Finally, we performed ovariectomy and observed bone loss only in wild‐type but not in Mdk‐deficient animals. Taken together, our data demonstrate that Mdk deficiency, at least in mice, results in an increased trabecular bone formation, thereby raising the possibility that Mdk‐specific antagonists might prove beneficial in osteoporosis therapy. © 2010 American Society for Bone and Mineral Research     

Activation of Renin‐Angiotensin System Induces Osteoporosis Independently of Hypertension

Hypertension and osteoporosis are two major age‐related disorders; however, the underlying molecular mechanism for this comorbidity is not known. The renin‐angiotensin system (RAS) plays a central role in the control of blood pressure and has been an important target of antihypertensive drugs. Using a chimeric RAS model of transgenic THM (Tsukuba hypertensive mouse) expressing both the human renin and human angiotensinogen genes, we showed in this study that activation of RAS induces high turnover osteoporosis with accelerated bone resorption. Transgenic mice that express only the human renin gene were normotensive and yet exhibited a low bone mass, suggesting that osteoporosis occurs independently of the development of hypertension per se. Ex vivo cultures showed that angiotensin II (AngII) acted on osteoblasts and not directly on osteoclast precursor cells and increased osteoclastogenesis‐supporting cytokines, RANKL and vascular endothelial growth factor (VEGF), thereby stimulating the formation of osteoclasts. Knockdown of AT2 receptor inhibited the AngII activity, whereas silencing of the AT1 receptor paradoxically enhanced it, suggesting a functional interaction between the two AngII receptors on the osteoblastic cell surface. Finally, treatment of THM mice with an ACE inhibitor, enalapril, improved osteoporosis and hypertension, whereas treatment with losartan, an angiotensin receptor blockers specific for AT1, resulted in exacerbation of the low bone mass phenotype. Thus, blocking the synthesis of AngII may be an effective treatment of osteoporosis and hypertension, especially for those afflicted with both conditions.     

Overexpression of GαS in Murine Osteoblasts In Vivo Leads to Increased Bone Mass and Decreased Bone Quality

Gα_S is a heterotrimeric G protein that transduces signals from activated G protein‐coupled receptors on the cell surface to stimulate adenylyl cyclase/cyclic adenosine monophosphate (AMP) signaling. Gα_S plays a central role in mediating numerous growth and maintenance processes including osteogenesis and bone turnover. Decreased Gα_S expression or activating mutations in Gα_S both affect bone, suggesting that modulating Gα_S protein levels may be important for bone health and development. To examine the effects of increased osteoblastic Gα_S expression on bone development in vivo, we generated transgenic mice with Gα_S overexpression in osteoblasts (HOM‐Gs mice) driven by the 3.6‐kilobase (kb) Col1A1 promoter. Both male and female HOM‐Gs mice exhibit increased bone turnover with overactive osteoblasts and osteoclasts, resulting in a high bone mass phenotype with significantly reduced bone quality. At 9 weeks of age, HOM‐Gs mice have increased trabecular number, volumetric BMD (vBMD), and bone volume; however, the bone was woven and disorganized. There was also increased cortical bone volume despite an overall reduction in size in HOM‐Gs mice along with increased cortical porosity and brittleness. The skeletal phenotype of HOM‐Gs mice progressed into maturity at 26 weeks of age with further accrual of trabecular bone, whereas WT mice lost trabecular bone at this age. Although cortical bone volume and geometry were similar between mature HOM‐Gs and WT mice, increased porosity persisted and the bone was weaker. At the cellular level, these alterations were mediated by an increase in bone resorption by osteoclasts and an overwhelmingly higher increase in bone formation by osteoblasts. In summary, our findings demonstrate that high osteoblastic Gα_S expression results in aberrant skeletal development in which bone production is favored at the cost of bone quality. © 2017 American Society for Bone and Mineral Research.