Osteoclast‐specific Dicer gene deficiency suppresses osteoclastic bone resorption

Osteoclasts are unique cells that resorb bone, and are involved in not only bone remodeling but also pathological bone loss such as osteoporosis and rheumatoid arthritis. The regulation of osteoclasts is based on a number of molecules but full details of these molecules have not yet been understood. MicroRNAs are produced by Dicer cleavage an emerging regulatory system for cell and tissue function. Here, we examine the effects of Dicer deficiency in osteoclasts on osteoclastic activity and bone mass in vivo. We specifically knocked out Dicer in osteoclasts by crossing Dicer flox mice with cathepsin K‐Cre knock‐in mice. Dicer deficiency in osteoclasts decreased the number of osteoclasts (N.Oc/BS) and osteoclast surface (Oc.S/BS) in vivo. Intrinsically, Dicer deficiency in osteoclasts suppressed the levels of TRAP positive multinucleated cell development in culture and also reduced NFATc1 and TRAP gene expression. MicroRNA analysis indicated that expression of miR‐155 was suppressed by RANKL treatment in Dicer deficient cells. Dicer deficiency in osteoclasts suppressed osteoblastic activity in vivo including mineral apposition rate (MAR) and bone formation rate (BFR) and also suppressed expression of genes encoding type I collagen, osteocalcin, Runx2, and Efnb2 in vivo. Dicer deficiency in osteoclasts increased the levels of bone mass indicating that the Dicer deficiency‐induced osteoclastic suppression was dominant over Dicer deficiency‐induced osteoblastic suppression. On the other hand, conditional Dicer deletion in osteoblasts by using 2.3 kb type I collagen‐Cre did not affect bone mass. These results indicate that Dicer in osteoclasts controls activity of bone resorption in vivo. J. Cell. Biochem. 109: 866–875, 2010. © 2009 Wiley‐Liss, Inc.     

Attenuated Monocyte Apoptosis,a New Mechanism for Osteoporosis Suggested by a Transcriptome-Wide Expression Study of Monocytes

BackgroundOsteoporosis is caused by excessive bone resorption (by osteoclasts) over bone formation (by osteoblasts). Monocytes are important to osteoporosis by serving as progenitors of osteoclasts and produce cytokines for osteoclastogenesis.AimTo identify osteoporosis-related genes, we performed microarray analyses of monocytes using Affymetrix 1.0 ST arrays in 42 (including 16 pre- and 26 postmenopausal) high hip BMD (bone mineral density) vs. 31 (including 15 pre- and 16 postmenopausal) low hip BMD Caucasian female subjects. Here, high vs. low BMD is defined as belonging to top vs. bottom 30% of BMD values in population.MethodDifferential gene expression analysis in high vs. low BMD subjects was conducted in the total cohort as well as pre- and post-menopausal subjects. Focusing on the top differentially expressed genes identified in the total, the pre- and the postmenopausal subjects (with a p <5E-03), we performed replication of the findings in 3 independent datasets of microarray analyses of monocytes (total N = 125).ResultsWe identified (in the 73 subjects) and successfully replicated in all the 3 independent datasets 2 genes, DAXX and PLK3. Interestingly, both genes are apoptosis induction genes and both down-regulated in the low BMD subjects. Moreover, using the top 200 genes identified in the meta-analysis across all of the 4 microarray datasets, GO term enrichment analysis identified a number of terms related to induction of apoptosis, for which the majority of component genes are also down-regulated in the low BMD subjects. Overall, our result may suggest that there might be a decreased apoptosis activity of monocytes in the low BMD subjects.ConclusionOur study for the first time suggested a decreased apoptosis rate (hence an increased survival) of monocytes, an important osteoclastogenic cell, as a novel mechanism for osteoporosis.     

Osteoclast Derivation from Mouse Bone Marrow

Osteoclasts are highly specialized cells that are derived from the monocyte/macrophage lineage of the bone marrow. Their unique ability to resorb both the organic and inorganic matrices of bone means that they play a key role in regulating skeletal remodeling. Together, osteoblasts and osteoclasts are responsible for the dynamic coupling process that involves both bone resorption and bone formation acting together to maintain the normal skeleton during health and disease.As the principal bone-resorbing cell in the body, changes in osteoclast differentiation or function can result in profound effects in the body. Diseases associated with altered osteoclast function can range in severity from lethal neonatal disease due to failure to form a marrow space for hematopoiesis, to more commonly observed pathologies such as osteoporosis, in which excessive osteoclastic bone resorption predisposes to fracture formation.An ability to isolate osteoclasts in high numbers in vitro has allowed for significant advances in the understanding of the bone remodeling cycle and has paved the way for the discovery of novel therapeutic strategies that combat these diseases. Here, we describe a protocol to isolate and cultivate osteoclasts from mouse bone marrow that will yield large numbers of osteoclasts.     

N-[2-(4-benzoyl-1-piperazinyl)phenyl]-2-(4-chlorophenoxy) acetamide is a novel inhibitor of resorptive bone loss in mice

The dynamic balance between bone formation and bone resorption is vital for the retention of bone mass. The abnormal activation of osteoclasts, unique cells that degrade the bone matrix, may result in many bone diseases such as osteoporosis. Osteoporosis, a bone metabolism disease, occurs when extreme osteoclast-mediated bone resorption outstrips osteoblast-related bone synthesis. Therefore, it is of great interest to identify agents that can regulate the activity of osteoclasts and prevent bone loss-induced bone diseases. In this study, we found that N-[2-(4-benzoyl-1-piperazinyl)phenyl]-2-(4-chlorophenoxy) acetamide (PPOAC-Bz) exerted a strong inhibitory effect on osteoclastogenesis. PPOAC-Bz altered the mRNA expressions of several osteoclast-specific marker genes and blocked the formation of mature osteoclasts, suppressing F-actin belt formation and bone resorption activity in vitro. In addition, PPOAC-Bz prevented OVX-induced bone loss in vivo. These findings highlighted the potential of PPOAC-Bz as a prospective drug for the treatment of osteolytic disorders.     

Correlation Between Bone Markers and Bone Mineral Density in Postmenopausal Women with Osteoporosis

ObjectiveTo study the relationship between bone markers and bone mineral density (BMD) in an effort to identify their utility in postmenopausal women with osteoporosis.MethodsEighty-two consecutive postmenopausal women with untreated osteoporosis were included in the study. Forearm, spinal, and femoral BMD by dual-energy x-ray absorptiometry and markers of bone formation (serum osteocalcin and bone-specific alkaline phosphatase) and bone resorption (urinary free deoxypyridinoline) were measured in all patients. Patients with low serum vitamin D levels, secondary osteoporosis, or clinically significant systemic disease were excluded from the study. The patients were classified on the basis of BMD of the lumbar spine into the following 3 groups: mild (n = 23) (T score -2.5 through -3), moderate (n = 42) (T score -3.1 through -4), or severe (n = 17) (T score ≤-4.1) osteoporosis. One-way analysis of variance and Pearson correlation were used for statistical analysis, with a P value < .05 being considered significant.ResultsSerum osteocalcin was significantly different among the 3 study groups (4.1 ± 2.7, 4.5 ± 3.1, and 6.7 ± 5.6 ng/mL, respectively; P = .0349) and had a significant negative correlation with BMD (r² = -0.0779; P = .0168). Other bone markers such as bone-specific alkaline phosphatase and urinary free deoxypyridinoline did not correlate with the underlying BMD.ConclusionIn our study, osteocalcin was significantly correlated with BMD in postmenopausal women with osteoporosis. Other bone markers did not correlate with BMD. Further large-scale population data and analyses are needed to confirm these findings. (Endocr Pract. 2008;14:1102-1107)     

Osteocalcin: genetic and physical mapping of the human gene BGLAP and its potential role in postmenopausal osteoporosis.

Osteocalcin is an abundant, highly conserved bone-specific protein that is synthesized by osteoblasts. Temporally, osteocalcin appears in embryonic bone at the time of mineral deposition, where it binds to hydroxyapatite in a calcium-dependent manner. A role for osteocalcin in bone resorption has been suggested because of its ability to influence recruitment and differentiation of osteoclasts at the bone surface. The human osteocalcin gene has been mapped to 1q25-1q31 by somatic cell hybridization. In this paper, we refine both the genetic map and the physical map of osteocalcin and describe a new microsatellite (CA) marker, D1S3737, which is tightly linked to the gene. This marker and two other closely linked markers were used to identify alleles of the osteocalcin gene in case and control samples of postmenopausal white Iowans with low and high bone mineral density (BMD), respectively. A significant difference (P = 0.007) was observed between allele frequency distributions of case and control women with one of the markers, D1S3737. Further, logistic regression analysis determined one allele of D1S3737 as associated with BMD status in this population (P = 0.03). Our data suggest that genetic variation at the osteocalcin locus impacts BMD levels in the postmenopausal period and may predispose some women to osteoporosis.     

VKORC1 common variation and bone mineral density in the Third National Health and Nutrition Examination Survey

Osteoporosis, defined by low bone mineral density (BMD), is common among postmenopausal women. The distribution of BMD varies across populations and is shaped by both environmental and genetic factors. Because the candidate gene vitamin K epoxide reductase complex subunit 1 (VKORC1) generates vitamin K quinone, a cofactor for the gamma-carboxylation of bone-related proteins such as osteocalcin, we hypothesized that VKORC1 genetic variants may be associated with BMD and osteoporosis in the general population. To test this hypothesis, we genotyped six VKORC1 SNPs in 7,159 individuals from the Third National Health and Nutrition Examination Survey (NHANES III). NHANES III is a nationally representative sample linked to health and lifestyle variables including BMD, which was measured using dual energy x-ray absorptiometry (DEXA) on four regions of the proximal femur. In adjusted models stratified by race/ethnicity and sex, SNPs rs9923231 and rs9934438 were associated with increased BMD (p=0.039 and 0.024, respectively) while rs8050894 was associated with decreased BMD (p=0.016) among non-Hispanic black males (n=619). VKORC1 rs2884737 was associated with decreased BMD among Mexican-American males (n=795; p=0.004). We then tested for associations between VKORC1 SNPs and osteoporosis, but the results did not mirror the associations observed between VKORC1 and BMD, possibly due to small numbers of cases. This is the first report of VKORC1 common genetic variation associated with BMD, and one of the few reports available that investigate the genetics of BMD and osteoporosis in diverse populations.     

1,25(OH)2D3 acts as a bone-forming agent in the hormone-independent senescence-accelerated mouse (SAM-P/6)

Recent studies suggest that vitamin D signaling regulates bone formation. However, the overall effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on bone turnover in vivo is still unclear. In this study, our aim was to examine the effect of 1,25(OH)2D3 on bone turnover in SAM-P/6, a hormone-independent mouse model of senile osteoporosis characterized by a decrease in bone formation. Male and female 4-mo-old SAM-P/6 mice were treated with 1,25(OH)2D3 (18 pmol/24 h) or vehicle for a period of 6 wk, and a group of age- and sex-matched nonosteoporotic animals was used as control. Bone mineral density (BMD) at the lumbar spine increased rapidly by >30 +/- 5% (P < 0.001) in 1,25(OH)2D3-treated SAM-P/6 animals, whereas BMD decreased significantly by 18 +/- 2% (P < 0.01) in vehicle-treated SAM-P/6 animals and remained stable in control animals during the same period. Static and dynamic bone histomorphometry indicated that 1,25(OH)2D3 significantly increased bone volume and other parameters of bone quality as well as subperiosteal bone formation rate compared with vehicle-treated SAM-P/6 mice. However, no effect on trabecular bone formation was observed. This was accompanied by a marked decrease in the number of osteoclasts and eroded surfaces. A significant increase in circulating bone formation markers and a decrease in bone resorption markers was also observed. Finally, bone marrow cells, obtained from 1,25(OH)2D3-treated animals and cultured in the absence of 1,25(OH)2D3, differentiated more intensely into osteoblasts compared with those derived from vehicle-treated mice cultured in the same conditions. Taken together, these findings demonstrate that 1,25(OH)2D3 acts simultaneously on bone formation and resorption to prevent the development of senile osteoporosis.     

Pharmacologic stem cell based intervention as a new approach to osteoporosis treatment in rodents

Background

Osteoporosis is the most prevalent skeletal disorder, characterized by a low bone mineral density (BMD) and bone structural deterioration, leading to bone fragility fractures. Accelerated bone resorption by osteoclasts has been established as a principal mechanism in osteoporosis. However, recent experimental evidences suggest that inappropriate apoptosis of osteoblasts/osteocytes accounts for, at least in part, the imbalance in bone remodeling as occurs in osteoporosis. The aim of this study is to examine whether aspirin, which has been reported as an effective drug improving bone mineral density in human epidemiology studies, regulates the balance between bone resorption and bone formation at stem cell levels.

Methods and Findings

We found that T cell-mediated bone marrow mesenchymal stem cell (BMMSC) impairment plays a crucial role in ovariectomized-induced osteoporosis. Ex vivo mechanistic studies revealed that T cell-mediated BMMSC impairment was mainly attributed to the apoptosis of BMMSCs via the Fas/Fas ligand pathway. To explore potential of using pharmacologic stem cell based intervention as an approach for osteoporosis treatment, we selected ovariectomy (OVX)-induced ostoeporosis mouse model to examine feasibility and mechanism of aspirin-mediated therapy for osteoporosis. We found that aspirin can inhibit T cell activation and Fas ligand induced BMMSC apoptosis in vitro. Further, we revealed that aspirin increases osteogenesis of BMMSCs by aiming at telomerase activity and inhibits osteoclast activity in OVX mice, leading to ameliorating bone density.

Conclusion

Our findings have revealed a novel osteoporosis mechanism in which activated T cells induce BMMSC apoptosis via Fas/Fas ligand pathway and suggested that pharmacologic stem cell based intervention by aspirin may be a new alternative in osteoporosis treatment including activated osteoblasts and inhibited osteoclasts.     

Inhibition of osteoblastic metalloproteinases in mice prevents bone loss induced by oestrogen deficiency

Matrix metalloproteinases (MMPs) are key mediators in extra-cellular matrix remodelling and implicated primarily in bone growth, and particularly in osteoclastic bone resorption. We hypothesise that MMPs have a role in the increased bone remodelling resulting from oestrogen deficiency. Transgenic (TG) mice overexpressing TIMP-1 in their osteoblastic cells and their wild-type (WT) littermates were ovariectomised. One month after surgery, bone mineral density (BMD) and bone microarchitecture were assessed. Primary cells from WT and TG mice were used to determine how TIMP-1 affects osteoclast and osteoblastic cells. The reduction of BMD induced by ovariectomy in WT mice was not observed in the transgenic mice. The transgene overexpression also dampened the post-ovariectomy increase in bone resorption in contrast to the WT mice. In vivo, osteoclastic surfaces and D-pyridinoline were not increased in TG mice, and ex vivo, the differentiation of osteoclasts from TG bone marrow precursor cells were unaffected by in vivo oestrogen deficiency or treatment. We showed also that TIMP-1 overexpression reduces and delays the osteoblastic proliferation and differentiation respectively, and reduced the generation of the active form of TGFbeta1 in the supernatant of TG osteoblasts. Our findings support the hypothesis that in vivo inhibition of osteoblastic MMPs prevented the bone loss induced by oestrogen deficiency, with a significant decrease in bone resorption. This effect was presumably resulting from (1) a direct inhibition of osteoclastic resorption activity by the TIMP-1 and (2) the modification in the local activation of extra-cellular signalling factors such as TGFbeta1 and the OPG/RANKL ratio.     

Osteoclasts in teleost fish: Light-and electron-microscopical observations

Summary This paper reports the common occurrence of osteoclasts during normal and experimental bone resorption in a number of teleost fishes. Light-microscopical observations on osteoclasts are presented in resorption areas on perichondral bone (mandibula and pharyngeal jaws of cichlids and vertebrae of gymnotids), on dermal bone (mandibula of salmonids and characoids and frontal bone of cichlids), on chondroid bone (pharyngeal jaws of cichlids), and on elasmoid body scales (eichlids and gymnotids). Osteoclasts acting along the bone surface usually lie in a Howship's lacuna whereas others are wrapped around bone extremities. Electronmicroscopical observations reveal that teleost osteoclasts show features similar to those of higher vertebrate osteoclasts, c.g., the presence of a ruffled border and the occurrence of numerous vacuoles, lysosomes and mitochondria. The multinucleated aspect that characterizes osteoclasts in other vertebrate groups is not a distinct feature of teleost osteoclasts since some are possibly mononucleated. Teleost osteoclasts are also able to resorb uncalcified tissues adjoining bone resorption areas, either as a primary process directed toward the tissue (basal plate of elasmoid scale) or as a secondary phenomenon (cartilage).     

The Foreign Body Giant Cell Cannot Resorb Bone,But Dissolves Hydroxyapatite Like Osteoclasts

Foreign body multinucleated giant cells (FBGCs) and osteoclasts share several characteristics, like a common myeloid precursor cell, multinuclearity, expression of tartrate-resistant acid phosphatase (TRAcP) and dendritic cell-specific transmembrane protein (DC-STAMP). However, there is an important difference: osteoclasts form and reside in the vicinity of bone, while FBGCs form only under pathological conditions or at the surface of foreign materials, like medical implants. Despite similarities, an important distinction between these cell types is that osteoclasts can resorb bone, but it is unknown whether FBGCs are capable of such an activity. To investigate this, we differentiated FBGCs and osteoclasts in vitro from their common CD14⁺ monocyte precursor cells, using different sets of cytokines. Both cell types were cultured on bovine bone slices and analyzed for typical osteoclast features, such as bone resorption, presence of actin rings, formation of a ruffled border, and characteristic gene expression over time. Additionally, both cell types were cultured on a biomimetic hydroxyapatite coating to discriminate between bone resorption and mineral dissolution independent of organic matrix proteolysis. Both cell types differentiated into multinucleated cells on bone, but FBGCs were larger and had a higher number of nuclei compared to osteoclasts. FBGCs were not able to resorb bone, yet they were able to dissolve the mineral fraction of bone at the surface. Remarkably, FBGCs also expressed actin rings, podosome belts and sealing zones—cytoskeletal organization that is considered to be osteoclast-specific. However, they did not form a ruffled border. At the gene expression level, FBGCs and osteoclasts expressed similar levels of mRNAs that are associated with the dissolution of mineral (e.g., anion exchange protein 2 (AE2), carbonic anhydrase 2 (CAII), chloride channel 7 (CIC7), and vacuolar-type H⁺-ATPase (v-ATPase)), in contrast the matrix degrading enzyme cathepsin K, which was hardly expressed by FBGCs. Functionally, the latter cells were able to dissolve a biomimetic hydroxyapatite coating in vitro, which was blocked by inhibiting v-ATPase enzyme activity. These results show that FBGCs have the capacity to dissolve the mineral phase of bone, similar to osteoclasts. However, they are not able to digest the matrix fraction of bone, likely due to the lack of a ruffled border and cathepsin K.     

Microscopic study on resorption of β-tricalcium phosphate materials by osteoclasts

Sintered compounds prepared with β-tricalcium phosphate (β-TCP) are commonly used as biocompatible materials for bone regenerative medicine. Although implanted β-TCP is gradually replaced with new bone after resorption by osteoclasts, exactly how osteoclasts resorb β-TCP is not well understood. To elucidate this mechanism, we analyzed the structure of β-TCP discs on which mouse mature osteoclasts were cultured using scanning electron microscopy. We found that β-TCP was resorbed by mature osteoclasts on one side of each disc, as evidenced by the formation of multiple spine-like crystals at the exposed areas. Because osteoclasts secrete acid to resorb bone minerals, we mimicked this acidification by dipping β-TCP slices into HCl solution (pH 2.0). However, no spine-like crystals appeared even though the size of each β-TCP particle was reduced. On dentin slices, osteoclasts formed clear actin rings, which are cytoskeletal structures characteristic of bone-resorbing osteoclasts. No clear actin rings were observed in osteoclasts cultured on β-TCP slices, although small actin dots were observed. Analysis by transmission electron microscopy showed that osteoclasts attached to β-TCP particles. These results suggest that osteoclasts resorb β-TCP particles independently of clear actin ring formation.     

Calcitonin inhibits SDCP-induced osteoclast apoptosis and increases its efficacy in a rat model of osteoporosis

Introduction

Treatment for osteoporosis commonly includes the use of bisphosphonates. Serious side effects of these drugs are caused by the inhibition of bone resorption as a result of osteoclast apoptosis. Treatment using calcitonin along with bisphosphonates overcomes these side-effects in some patients. Calcitonin is known to inhibit bone resorption without reducing the number of osteoclasts and is thought to prolong osteoclast survival through the inhibition of apoptosis. Further understanding of how calcitonin inhibits apoptosis could prove useful to the development of alternative treatment regimens for osteoporosis. This study aimed to analyze the mechanism by which calcitonin influences osteoclast apoptosis induced by a bisphosphate analog, sintered dicalcium pyrophosphate (SDCP), and to determine the effects of co-treatment with calcitonin and SDCP on apoptotic signaling in osteoclasts.

Methods

Isolated osteoclasts were treated with CT, SDCP or both for 48 h. Osteoclast apoptosis assays, pit formation assays, and tartrate-resistant acid phosphatase (TRAP) staining were performed. Using an osteoporosis rat model, ovariectomized (OVX) rats received calcitonin, SDCP, or calcitonin + SDCP. The microarchitecture of the fifth lumbar trabecular bone was investigated, and histomorphometric and biochemical analyses were performed.

Results

Calcitonin inhibited SDCP-induced apoptosis in primary osteoclast cultures, increased Bcl-2 and Erk activity, and decreased Mcl-1 activity. Calcitonin prevented decreased osteoclast survival but not resorption induced by SDCP. Histomorphometric analysis of the tibia revealed increased bone formation, and microcomputed tomography of the fifth lumbar vertebrate showed an additive effect of calcitonin and SDCP on bone volume. Finally, analysis of the serum bone markers CTX-I and P1NP suggests that the increased bone volume induced by co-treatment with calcitonin and SDCP may be due to decreased bone resorption and increased bone formation.

Conclusions

Calcitonin reduces SDCP-induced osteoclast apoptosis and increases its efficacy in an in vivo model of osteoporosis.     

Up‐regulated CST5 inhibits bone resorption and activation of osteoclasts in rat models of osteoporosis via suppression of the NF‐κB pathway

Here, we aim at exploring the effect of CST5 on bone resorption and activation of osteoclasts in osteoporosis (OP) rats through the NF‐κB pathway. Microarray analysis was used to screen the OP‐related differentially expressed genes. Osteoporosis was induced in rats by intragastric retinoic acid administration. The serum levels of tartrate‐resistant acid phosphatase (TRAP), bone alkaline phosphatase (BALP) and osteocalcin (OC) and the expression of CD61 on the surface of osteoclasts were examined. The number of osteoclasts and the number and area of resorption pits were detected. Besides, the pathological changes and bone mineral density in bone tissues of rats were assessed. Also, the relationship between CST5 and the NF‐κB pathway was identified through determining the expression of CST5, RANKL, RANK, OPG, p65 and IKB. Poorly expressed CST5 was indicated to affect the OP. CST5 elevation and inhibition of the NF‐κB pathway decreased serum levels of TRAP, BALP and OC and expression of CD61 in vivo and in vitro. In OP rats, CST5 overexpression increased trabecular bones and bone mineral density of bone tissues, but decreased trabecular separation, fat within the bone marrow cavities and the number of osteoclasts through inhibiting the NF‐κB pathway. In vivo experiments showed that CST5 elevation inhibited growth in number and area of osteoclastic resorption pits and restrained osteoclastic bone absorption by inhibiting the NF‐κB pathway. In summary, overexpression of CST5 suppresses the activation and bone resorption of osteoclasts by inhibiting the activation of the NF‐κB pathway.