Regulation of osteoclast formation and function

Effects of Amnion Lyophilization Sterile Radiation against the Number of Osteoblasts and Osteocytes in Nonunion Fractures: An Experimental Research Study

Background: This study used an experimental posttest-only control group design that involved white rats of the Sprague Dawley strain. Methods: The samples used were 8-week-old rats, weighing 250–350 grams of the male sex. The study was conducted on five groups of rats with a total of seven rats per group. Results: The mean value of osteocytes in the control group was 00.00 ± 00.00 and in the ALSR group it was 87.14 ± 44.85. The mean value of osteoblasts in the control group was 50.06 ± 5.76 and in the ALSR group it was 283.63 ± 22.86. This study showed that there were differences in the number of osteocytes and osteoblasts in the two groups. Conclusion: The study reported that the ALSR group had significantly different numbers of osteoblasts and osteocytes than the control group.

Inhibition of Neddylation Suppresses Osteoclast Differentiation and Function In Vitro and Alleviates Osteoporosis In Vivo

Neddylation, or the covalent addition of NEDD8 to specific lysine residue of proteins, is a reversible posttranslational modification, which regulates numerous biological functions; however, its involvement and therapeutic significance in osteoporosis remains unknown. Our results revealed that during the soluble receptor activator of nuclear factor-κB ligand (sRANKL)-stimulated osteoclast differentiation, the neddylation and expression of UBA3, the NEDD8-activating enzyme (NAE) catalytic subunit, were dose- and time-dependently upregulated in RAW 264.7 macrophages. UBA3 knockdown for diminishing NAE activity or administering low doses of the NAE inhibitor MLN4924 significantly suppressed sRANKL-stimulated osteoclast differentiation and bone-resorbing activity in the macrophages by inhibiting sRANKL-stimulated neddylation and tumor necrosis factor receptor-associated factor 6 (TRAF6)-activated transforming growth factor-β-activated kinase 1 (TAK1) downstream signaling for diminishing nuclear factor-activated T cells c1 (NFATc1) expression. sRANKL enhanced the interaction of TRAF6 with the neddylated proteins and the polyubiquitination of TRAF6's lysine 63, which activated TAK1 downstream signaling; however, this process was inhibited by MLN4924. MLN4924 significantly reduced osteoporosis in an ovariectomy- and sRANKL-induced osteoporosis mouse model in vivo. Our novel finding was that NAE-mediated neddylation participates in RANKL-activated TRAF6-TAK1-NFATc1 signaling during osteoclast differentiation and osteoporosis, suggesting that neddylation may be a new target for treating osteoporosis.

Comparison of Synthetic vs. Biogenic Polymeric Process-Directing Agents for Intrafibrillar Mineralization of Collagen

With the aging population, there is a growing need for mineralized tissue restoration and synthetic bone substitutes. Previous studies have shown that a polymer-induced liquid-precursor (PILP) process can successfully mineralize collagen substrates to achieve compositions found in native bone and dentin. This process also leads to intrafibrillar apatitic crystals with their [001] axes aligned roughly parallel to the long axis of the collagen fibril, emulating the nanostructural organization found in native bone and dentin. When demineralized bovine bone was remineralized via the PILP process using osteopontin (OPN), the samples were able to activate mouse marrow-derived osteoclasts to similar levels to those of native bone, suggesting a means for fabricating bioactive bone substitutes that could trigger remodeling through the native bone multicellular unit (BMU). In order to determine if OPN derived from bovine milk could be a cost-effective process-directing agent, the mineralization of type I collagen scaffolds using this protein was compared to the benchmark polypeptide of polyaspartic acid (sodium salt; pAsp). In this set of experiments, we found that OPN led to much faster and more uniform mineralization when compared with pAsp, making it a cheaper and commercially attractive alternative for mineralized tissue restorations.

Age-related changes and reference intervals of RANKL, OPG, and bone turnover markers in Indian women

RANKL and OPG are cytokines involved in bone remodeling that makes them potential bone biomarkers. The reference interval for these cytokines, their ratio, and bone turnover markers CTX and PINP were established in Indian women, which may serve in diagnosis and management of osteoporosis.

PURPOSE

The aim of the study was to establish reference interval for RANKL, OPG, RANKL/OPG, and bone turnover markers CTX and PINP in healthy Indian women.

METHODS

This was a cross-sectional study on 374 healthy Indian women in the age group of 20-65 years. Serum levels of total RANKL, OPG, CTX, PINP, and estradiol were determined by commercial ELISA kits. The reference intervals for these cytokines and bone turnover markers were based on the 95% centrally distributed data.

RESULTS

Median RANKL (245.6 pmol/L vs. 149 pmol/L) and RANKL/OPG (38.7 vs. 20.4) were higher, while sCTX (380 ng/L vs. 551 ng/L) and OPG levels (6.1 pmol/L vs. 7.4 pmol/L) were lower in premenopausal women than those in postmenopausal women. PINP levels were comparable in both groups. Women were classified into 5 groups according to decades of age and the reference intervals for RANKL, OPG, RANKL/OPG ratio, and CTX and PINP in each group were reported.

CONCLUSION

We reported menopausal status-based and age-related reference intervals for serum RANKL, OPG, RANKL/OPG ratio, and CTX and PINP in healthy Indian women.

Genetically-programmed, mesenchymal stromal cell-laden & mechanically strong 3D bioprinted scaffolds for bone repair

Additive manufacturing processes used to create regenerative bone tissue engineered implants are not biocompatible, thereby restricting direct use with stem cells and usually require cell seeding post-fabrication. Combined delivery of stem cells with the controlled release of osteogenic factors, within a mechanically-strong biomaterial combined during manufacturing would replace injectable defect fillers (cements) and allow personalized implants to be rapidly prototyped by 3D bioprinting. Through the use of direct genetic programming via the sustained release of an exogenously delivered transcription factor RUNX2 (delivered as recombinant GET-RUNX2 protein) encapsulated in PLGA microparticles (MPs), we demonstrate that human mesenchymal stromal (stem) cells (hMSCs) can be directly fabricated into a thermo-sintered 3D bioprintable material and achieve effective osteogenic differentiation. Importantly we observed osteogenic programming of gene expression by released GET-RUNX2 (8.2-, 3.3- and 3.9-fold increases in OSX, RUNX2 and OPN expression, respectively) and calcification (von Kossa staining) in our scaffolds. The developed biodegradable PLGA/PEG paste formulation augments high-density bone development in a defect model (~2.4-fold increase in high density bone volume) and can be used to rapidly prototype clinically-sized hMSC-laden implants within minutes using mild, cytocompatible extrusion bioprinting. The ability to create mechanically strong 'cancellous bone-like' printable implants for tissue repair that contain stem cells and controlled-release of programming factors is innovative, and will facilitate the development of novel localized delivery approaches to direct cellular behaviour for many regenerative medicine applications including those for personalized bone repair.

The Role of Substance P in the Regulation of Bone and Cartilage Metabolic Activity

Substance P (SP) is a neuropeptide that is released from sensory nerve endings and is widely present in nerve fibers. It acts on bones and related tissues by binding to receptors, thereby regulating bone metabolism, cartilage metabolism, and fracture healing. SP has attracted widespread attention as a signaling substance that can be recognized by both the immune system and the nervous system. Previous studies have shown that bone and chondrocytes can synthesize and secrete sensory neuropeptides and express their receptors, and can promote proliferation, differentiation, apoptosis, matrix synthesis, and the degradation of target cells through autocrine/paracrine modes. In this paper, we review the research progress made in this field in recent years in order to provide a reference for further understanding the regulatory mechanism of bone and cartilage physiology and pathological metabolism.

Current and emerging osteoporosis pharmacotherapy for women: state of the art therapies for preventing bone loss

INTRODUCTION

Pharmacological options to address the imbalance between bone resorption and accrual in osteoporosis include anti-resorptive and osteoanabolic agents. Unique biologic pathways such as the Wnt/β-catenin pathway have been targeted in the quest for new emerging therapeutic strategies.

AREAS COVERED

This review provides an overview of existing pharmacotherapy for osteoporosis in women and explore state-of-the-art and emerging therapies to prevent bone loss, with an emphasis on the mechanism of action, indications and side effects.

EXPERT OPINION

Bisphosphonates appear to be a reliable and cost-effective option, whereas denosumab has introduced a simpler dosing regimen and may achieve a linear increase in bone mineral density (BMD) with no plateau being observed, along with continuous anti-fracture efficacy. Abaloparatide, a parathyroid-hormone-related peptide (PTHrP)-analogue, approved by the FDA in April 2017, constitutes the first new anabolic osteoporosis drug in the US for nearly 15 years and has also proven its anti-fracture efficacy. Romosozumab, a sclerostin inhibitor, which induces bone formation and suppresses bone resorption, has also been developed and shown a significant reduction in fracture incidence; however, concerns have arisen with regard to increased cardiovascular risk.

Alveolar bone healing in mice genetically selected in the maximum (AIRmax) or minimum (AIRmin) inflammatory reaction

The exact role of inflammatory immune response in bone healing process is still unclear, but the success of the alveolar bone healing process seems to be associated with a moderate and transitory inflammatory response, while insufficient or exacerbated responses seems to have a detrimental influence in the healing outcome. In this context, we performed a comparative analysis of mice strains genetically selected for maximum (AIRmax) or minimum (AIRmin) acute inflammatory response to address the influence of inflammation genes in alveolar bone healing outcome. Experimental groups comprised 8-week-old male or female AIRmax and AIRmin submitted to extraction of upper right incisor, and evaluated at 0, 3, 7, 14 and 21 days after upper incision extraction by micro-computed tomography (μCT), histomorphometry, birefringence, immunohistochemistry and molecular (PCRArray) analysis. Overall, the results demonstrate a similar successful bone healing outcome at the endpoint was evidenced in both AIRmin and AIRmax strains. The histormophometric analysis reveal a slight but significant decrease in blood clot and inflammatory cells density, as well a delay in the bone formation in AIRmax strain in the early times, associated with a decreased expression of BMP2, BMP4, BMP7, TGFb1, RUNX2, and ALP. The evaluation of inflammatory cells nature reveals increased GR1+ cells counts in AIRmax strain at 3d, associated with increased levels of neutrophil chemoattractants such as CXCL1 and CXCL2, and its receptor CXCR1, while F4/80+ cell prevails in AIRmin strain at 7d. Also, our results demonstrate a relative predominance of M2 macrophages in AIRmin strain, associated with an increased expression of ARG1, IL10, TGFb, while M1 macrophages prevail in AIRmax, which parallel with increased IL-1B, IL-6 and TNF expression. At late repair stage, AIRmax presents evidences of increased bone remodeling, characterized by increased density of blood vessels and osteoclasts in parallel with decreased bone matrix density, as well increased levels of MMPs, osteoclastogenic and osteocyte markers. In the view of contrasting inflammatory and healing phenotypes of AIRmin and AIRmax strains in other models, the unpredicted phenotype observed suggests the existence of specific QTLs (Quantitative trait loci) responsible for the regulation 'sterile' inflammation and bone healing events. Despite the similar endpoint healing, AIRmax strain delayed repair was associated with increased presence of neutrophils and M1 macrophages, supporting the association of M2 cells with faster bone healing. Further studies are required to clarify the elements responsible for the regulation of inflammatory events at bone healing sites, as well the determinants of bone healing outcome.

Safety and efficacy of denosumab in osteoporotic patients previously treated with other medications: a systematic review and meta-analysis

INTRODUCTION

Denosumab is a monoclonal antibody that received approval by the FDA for the treatment of osteoporosis in 2010. Available higher level research evidence concerns the treatment of patients that have not received any anti-osteoporotic medication in the past. Further investigation is warranted, since clinicians often face the challenge of administering the most efficacious drug in patients, pretreated with other medications.

AREAS COVERED

We conducted a systematic review and meta-analysis to evaluate the efficacy and safety of denosumab compared to other active anti-osteoporotic agents in patients formerly receiving other treatments. We searched MEDLINE, EMBASE, CENTRAL, the metaRegister of Controlled Trials (mRCT) and clinicaltrials.gov up to April 2017 to identify eligible trials in patients with primary osteoporosis.

EXPERT OPINION

Our meta-analysis included 6 Randomised Controlled Trials encompassing 2968 patients formerly treated with anti-osteoporotic medications. Quantitative data synthesis demonstrated superiority of denosumab in augmenting Bone Mineral Density in all skeletal sites studied compared to controls [treatment difference in total hip: 1.59% (95% CI 1.01, 2.17)], whereas the overall incidence of serious adverse events was not increased (OR 1.12, 95% CI 0.85 to 1.47, p = 0.42). Future research geared towards the fracture incidence, quality of life and patient reported outcomes is warranted.

Biologically active γ-lactams: synthesis and natural sources

The γ-lactam moiety is present in a large number of natural and non-natural biologically active compounds. The range of biological activities covered by these compounds is very broad. Functionalized γ-lactams are thus of high interest and have great potential in medicinal chemistry. This review provides a description of the title compounds by focusing on their synthesis, natural sources and biological activities.

Pathophysiology of Osteonecrosis of the Jaws

Osteonecrosis of the jaw (ONJ) is a multifactorial disease in patients with primary or metastatic bone malignancy or osteoporosis undergoing systemic antiresorptive therapy, where pathophysiology has not yet been fully determined. The staging of ONJ is based on severity of symptoms and extent of clinical and radiographic findings. Treatment strategies range from conservative local wound care to aggressive resective surgery of all necrotic bone. The first ONJ cases were reported in 2003 and 2004, and although significant progress has been made in our understanding of the disease, much more work needs to be done to completely explain its pathophysiology.

Comparison of osteoclastogenesis and resorption activity of human osteoclasts on tissue culture polystyrene and on natural extracellular bone matrix in 2D and 3D

Bone homeostasis is maintained by osteoblasts (bone formation) and osteoclasts (bone resorption). While there have been numerous studies investigating mesenchymal stem cells and their potential to differentiate into osteoblasts as well as their interaction with different bone substitute materials, there is only limited knowledge concerning in vitro generated osteoclasts. Due to the increasing development of degradable bone-grafting materials and the need of sophisticated in vitro test methods, it is essential to gain deeper insight into the process of osteoclastogenesis and the resorption functionality of human osteoclasts. Therefore, we focused on the comparison of osteoclastogenesis and resorption activity on tissue culture polystyrene (TCPS) and bovine extracellular bone matrices (BMs). Cortical bone slices were used as two-dimensional (2D) substrates, whereas a thermally treated cancellous bone matrix was used for three-dimensional (3D) experiments. We isolated primary human monocytes and induced osteoclastogenesis by medium supplementation. Subsequently, the expression of the vitronectin receptor (αVβ3) and cathepsin K as well as the characteristic actin formation on TCPS and the two BMs were examined. The cell area of human osteoclasts was analyzed on TCPS and on BMs, whereas significantly larger osteoclasts could be detected on BMs. Additionally, we compared the diameter of the sealing zones with the measured diameter of the resorption pits on the BMs and revealed similar diameters of the sealing zones and the resorption pits. We conclude that using TCPS as culture substrate does not affect the expression of osteoclast-specific markers. The analysis of resorption activity can successfully be conducted on cortical as well as on cancellous bone matrices. For new in vitro test systems concerning bone resorption, we suggest the establishment of a 2D assay for high throughput screening of new degradable bone substitute materials with osteoclasts.

Inhibition of bone resorption by the cathepsin K inhibitor odanacatib is fully reversible

The cathepsin K (CatK) inhibitor odanacatib (ODN) is currently being developed for the treatment of osteoporosis. In clinical trials, efficacy and resolution of effect of ODN treatment on bone turnover biomarkers and accrued bone mass have been demonstrated. Here, we examine the effects of continuing treatment and discontinuation of ODN versus alendronate (ALN) on osteoclast (OC) function. First, accessibility and reversible engagement of active CatK in intracellular vesicles and resorption lacunae of actively resorbing OCs were demonstrated by the selective and reversible CatK inhibitors, BODIPY-L-226 (IC50=39nM) and L-873,724 (IC50=0.5nM). Next, mature human OCs on bone slices were treated with vehicle, ODN, or ALN for 2days, followed by either continuing with the same treatment, or replacement of the inhibitors by vehicle for additional times as specified per experimental conditions. Maintaining OCs on ODN or ALN significantly reduced CTx-I release compared to vehicle controls. However, only the treatment of OCs with ODN resulted in the formation of small shallow discrete resorption pits, retention of intracellular vesicles enriched with CatK and other lysosomal enzymes, increase in 1-CTP release and number of TRAP(+) OCs. Upon discontinuation of ODN treatment, OCs rapidly resumed bone resorption activity, as demonstrated by a return of OC functional markers (CTx-I, 1-CTP), cell number and size, morphology and number of resorption pits, and vesicular secretion of CatK toward the respective vehicle levels. As expected, discontinuation of ALN did not reverse the treatment-related inhibition of OC activity in the time frame of the experiment. In summary, this study demonstrated rapid kinetics of inhibition and reversibility of the effects of ODN on OC bone resorption, that differentiated the cellular mechanism of CatK inhibition from that of the bisphosphate antiresorptive ALN.

Cyclic nucleotide phosphodiesterases: important signaling modulators and therapeutic targets

By catalyzing hydrolysis of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), cyclic nucleotide phosphodiesterases are critical regulators of their intracellular concentrations and their biological effects. As these intracellular second messengers control many cellular homeostatic processes, dysregulation of their signals and signaling pathways initiate or modulate pathophysiological pathways related to various disease states, including erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, intermittent claudication, chronic obstructive pulmonary disease, and psoriasis. Alterations in expression of PDEs and PDE-gene mutations (especially mutations in PDE6, PDE8B, PDE11A, and PDE4) have been implicated in various diseases and cancer pathologies. PDEs also play important role in formation and function of multimolecular signaling/regulatory complexes, called signalosomes. At specific intracellular locations, individual PDEs, together with pathway-specific signaling molecules, regulators, and effectors, are incorporated into specific signalosomes, where they facilitate and regulate compartmentalization of cyclic nucleotide signaling pathways and specific cellular functions. Currently, only a limited number of PDE inhibitors (PDE3, PDE4, PDE5 inhibitors) are used in clinical practice. Future paths to novel drug discovery include the crystal structure-based design approach, which has resulted in generation of more effective family-selective inhibitors, as well as burgeoning development of strategies to alter compartmentalized cyclic nucleotide signaling pathways by selectively targeting individual PDEs and their signalosome partners.

MicroRNAs and post-transcriptional regulation of skeletal development

MicroRNAs (miRNAs) have become integral nodes of post-transcriptional control of genes that confer cellular identity and regulate differentiation. Cell-specific signaling and transcriptional regulation in skeletal biology are extremely dynamic processes that are highly reliant on dose-dependent responses. As such, skeletal cell-determining genes are ideal targets for quantitative regulation by miRNAs. So far, large amounts of evidence have revealed a characteristic temporal miRNA signature in skeletal cell differentiation and confirmed the essential roles that numerous miRNAs play in bone development and homeostasis. In addition, microarray expression data have provided evidence for their role in several skeletal pathologies. Mouse models in which their expression is altered have provided evidence of causal links between miRNAs and bone abnormalities. Thus, a detailed understanding of the function of miRNAs and their tight relationship with bone diseases would constitute a powerful tool for early diagnosis and future therapeutic approaches.

Multifunctional role of osteopontin in directing intrafibrillar mineralization of collagen and activation of osteoclasts

Mineralized collagen composites are of interest because they have the potential to provide a bone-like scaffold that stimulates the natural processes of resorption and remodeling. Working towards this goal, our group has previously shown that the nanostructure of bone can be reproduced using a polymer-induced liquid-precursor (PILP) process, which enables intrafibrillar mineralization of collagen with hydroxyapatite to be achieved. This prior work used polyaspartic acid (pASP), a simple mimic for acidic non-collagenous proteins, to generate nanodroplets/nanoparticles of an amorphous mineral precursor which can infiltrate the interstices of type-I collagen fibrils. In this study we show that osteopontin (OPN) can similarly serve as a process-directing agent for the intrafibrillar mineralization of collagen, even though OPN is generally considered a mineralization inhibitor. We also found that inclusion of OPN in the mineralization process promotes the interaction of mouse marrow-derived osteoclasts with PILP-remineralized bone that was previously demineralized, as measured by actin ring formation. While osteoclast activation occurred when pASP was used as the process-directing agent, using OPN resulted in a dramatic effect on osteoclast activation, presumably because of the inherent arginine-glycine-aspartate acid ligands of OPN. By capitalizing on the multifunctionality of OPN, these studies may lead the way to producing biomimetic bone substitutes with the capability of tailorable bioresorption rates.

Epiphyseal abnormalities, trabecular bone loss and articular chondrocyte hypertrophy develop in the long bones of postnatal Ext1-deficient mice

Long bones are integral components of the limb skeleton. Recent studies have indicated that embryonic long bone development is altered by mutations in Ext genes and consequent heparan sulfate (HS) deficiency, possibly due to changes in activity and distribution of HS-binding/growth plate-associated signaling proteins. Here we asked whether Ext function is continuously required after birth to sustain growth plate function and long bone growth and organization. Compound transgenic Ext1(f/f);Col2CreERT mice were injected with tamoxifen at postnatal day 5 (P5) to ablate Ext1 in cartilage and monitored over time. The Ext1-deficient mice exhibited growth retardation already by 2weeks post-injection, as did their long bones. Mutant growth plates displayed a severe disorganization of chondrocyte columnar organization, a shortened hypertrophic zone with low expression of collagen X and MMP-13, and reduced primary spongiosa accompanied, however, by increased numbers of TRAP-positive osteoclasts at the chondro-osseous border. The mutant epiphyses were abnormal as well. Formation of a secondary ossification center was significantly delayed but interestingly, hypertrophic-like chondrocytes emerged within articular cartilage, similar to those often seen in osteoarthritic joints. Indeed, the cells displayed a large size and round shape, expressed collagen X and MMP-13 and were surrounded by an abundant Perlecan-rich pericellular matrix not seen in control articular chondrocytes. In addition, ectopic cartilaginous outgrowths developed on the lateral side of mutant growth plates over time that resembled exostotic characteristic of children with Hereditary Multiple Exostoses, a syndrome caused by Ext mutations and HS deficiency. In sum, the data do show that Ext1 is continuously required for postnatal growth and organization of long bones as well as their adjacent joints. Ext1 deficiency elicits defects that can occur in human skeletal conditions including trabecular bone loss, osteoarthritis and HME.

Effects of electroacupuncture on bone mineral density, oestradiol level and osteoprotegerin ligand expression in ovariectomised rabbits

OBJECTIVES

To investigate the effects of electroacupuncture (EA) on the oestradiol level, bone mineral density and osteoprotegerin ligand (OPGL) expression, and to explore whether EA might be a complementary method to prevent and treat osteoporosis.

METHODS

A total of 21 New Zealand rabbits were randomly divided into three groups: a normal control (NC) group undergoing no surgery or EA; an ovariectomised (OVX) group, in which rabbits were ovariectomised but did not receive EA; an EA group, in which rabbits were ovariectomised and treated with EA. Acupuncture was applied at ST35, BL20 and BL23 points bilaterally. EA (10 Hz, 2 mA) was applied bilaterally at BL20 and BL23 for 30 min a day for 14 days. After 14 days, all animals were killed. OPGL expression level was determined by immunohistochemistry. Blood serum levels of oestradiol were measured by ELISA and bone mineral density was detected by dual-energy x-ray absorptiometry.

RESULTS

After ovariectomy, the bone mineral density and oestradiol level decreased significantly in the OVX group compared with the NC group (p=0.001), whereas the OPGL expression level increased. After EA, the bone mineral density and oestradiol level increased compared with the OVX group (p=0.049 and p=0.012, respectively). The OPGL level OPGL level in the EA group was lower than that in the OVX group (p=0.022).

CONCLUSIONS

EA restored bone mineral density towards normal and was associated with increased plasma oestradiol level and reduced OPGL expression in an ovariectomised rabbit model of osteoporosis.

Effects of Osteochondrin S and select connective tissue ribonucleinate components on human osteoclasts in vitro

Objectives

Osteochondrin S, a natural product derived from connective tissues and yeast, is used to treat osteoarthritis. The aim of this study was to determine the effect of Osteochondrin S on human osteoclast activity in vitro.

Methods

Osteoclasts were derived from human peripheral blood mononuclear cells stimulated with macrophage colony-stimulating factor and receptor activator of nuclear factor kappa B (RANK) ligand. Cells were treated with 23.5–587.2 ng/ml Osteochondrin S or 0.2–5 mg/ml of RNA components (synovia, placenta, intervertebral disc or cartilage). The effects on osteoclast formation and resorptive activity were assessed. Real-time polymerase chain reaction was conducted to assess the expression of key osteoclast genes.

Key findings

Osteochondrin S and the individual RNA extracts resulted in a concentration-dependent inhibition of human osteoclast activity. Osteochondrin S did not affect RANK, nuclear factor of activated T cells (NFATc1), osteoclast-associated receptor or cathepsin K expression. However, there was a significant (P < 0.05) reduction in mRNA expression of calcitonin receptor. Osteochondrin S treatment also significantly increased the expression of osteoclast inhibitory factor interferon-β and, interestingly, increased the expression of tumour necrosis-α-like weak inducer of apoptosis (TWEAK).

Conclusions

Osteochondrin S inhibited the resorptive ability of osteoclasts. These actions are likely to occur at a late stage during osteoclast formation, downstream of NFATc1. Overall, the findings show that Osteochondrin S inhibition of osteoclast activity may be responsible for its beneficial effects on diseases such as osteoarthritis.

Therapeutic inhibition of cathepsin K-reducing bone resorption while maintaining bone formation

Osteoporosis is a disease of high bone remodeling with an imbalance of bone resorption over bone formation, resulting in decreased bone mineral density and deterioration of bone microarchitecture. From the emerging understandings of the molecular and cellular regulators of bone remodeling, potential new targets for therapeutic intervention for this disease have been identified. Cathepsin K (CatK), a cysteine protease produced by osteoclasts, is the primary enzyme mediating the degradation of the demineralized bone matrix. Current genetic and pharmacological evidence from studies in multiple preclinical species have consistently demonstrated that inhibition of CatK results in the reduction of bone resorption while allowing bone formation to continue. Early results from clinical studies with several investigational CatK inhibitors indicate that the impact of CatK inhibition on bone formation is distinct from that of either the bisphosphonates or the anti-receptor activator of nuclear factor-κB ligand antibody, denosumab. Odanacatib, a highly selective, reversible and potent inhibitor of CatK, is currently in phase III clinical trials for the treatment of postmenopausal osteoporosis.

The effects of the cathepsin K inhibitor odanacatib on osteoclastic bone resorption and vesicular trafficking

Odanacatib (ODN) is a selective, potent and reversible inhibitor of cathepsin K (CatK) that inhibits bone loss in postmenopausal osteoporosis. Evidence from osteoclast (OC) formation from bone marrow of CatK(-/-) mice or human OC progenitors treated with ODN, demonstrated that CatK inhibition has no effect on osteoclastogenesis or survival of OCs. Although having no impact on OC activation, ODN reduces resorption activity as measured by CTx release (IC(50)=9.4 nM) or resorption area (IC(50)=6.5 nM). While untreated cells generate deep trail-like resorption lacunae, treated OCs form small discrete shallow pits. ODN leads to significant accumulation of intracellular vesicles intensely stained for CatK and TRAP. CatK (+) vesicles localize toward the basolateral and functional secretory membranes of the polarized OC and TRAP(+) vesicles evenly distribute in the cytoplasm, suggesting that ODN disrupts multiple vesicular trafficking pathways. Intracellular levels of both precursor and mature TRAP were increased by 2-fold and the pre-pro and mature CatK by 6- and 2-fold in ODN-treated OCs compared to untreated controls. ODN treated OC accumulates labeled degraded bone matrix proteins in CatK containing vesicles. In summary, ODN treatment inhibits bone resorption by blocking degradation of demineralized collagen in the resorption lacunae, and retarding transcytosis for further processing of degraded proteins.

NPY regulation of bone remodelling

Neuropeptide Y (NPY), a classic neuronal regulator of energy homeostasis, is now also known to be involved in the control of bone homeostasis. Of the five known Y receptors through which the NPY family of ligands signals, the Y1 and Y2 receptors have so far been implicated in the control of osteoblast activity and thus bone formation. Analysis of brain specific NPY overexpressing and Y receptor knockout models has revealed a powerful anabolic pathway likely involving hypothalamic Y2 receptors and osteoblastic Y1 receptors. Furthering our understanding of the mechanisms underlying the involvement of the NPY system in the control of bone could lead to the development of therapies to improve bone mass in patients with diseases such as osteoporosis.

In vivo genome-wide expression study on human circulating B cells suggests a novel ESR1 and MAPK3 network for postmenopausal osteoporosis

INTRODUCTION

Osteoporosis is characterized by low BMD. Studies have shown that B cells may participate in osteoclastogenesis through expression of osteoclast-related factors, such as RANKL, transforming growth factor beta (TGFB), and osteoprotegerin (OPG). However, the in vivo significance of B cells in human bone metabolism and osteoporosis is still largely unknown, particularly at the systematic gene expression level.

MATERIALS AND METHODS

In this study, Affymetrix HG-U133A GeneChip arrays were used to identify genes differentially expressed in B cells between 10 low and 10 high BMD postmenopausal women. Significance of differential expression was tested by t-test and adjusted for multiple testing with the Benjamini and Hochberg (BH) procedure (adjusted p </= 0.05).

RESULTS

Twenty-nine genes were downregulated in the low versus high BMD group. These genes were further analyzed using Ingenuity Pathways Analysis (Ingenuity Systems). A network involving estrogen receptor 1 (ESR1) and mitogen activated protein kinase 3 (MAPK3) was identified. Real-time RT-PCR confirmed differential expression of eight genes, including ESR1, MAPK3, methyl CpG binding protein 2 (MECP2), proline-serine-threonine phosphatase interacting protein 1 (PSTPIP1), Scr-like-adaptor (SLA), serine/threonine kinase 11 (STK11), WNK lysine-deficient protein kinase 1 (WNK1), and zinc finger protein 446 (ZNF446).

CONCLUSIONS

This is the first in vivo genome-wide expression study on human B cells in relation to osteoporosis. Our results highlight the significance of B cells in the etiology of osteoporosis and suggest a novel mechanism for postmenopausal osteoporosis (i.e., that downregulation of ESR1 and MAPK3 in B cells regulates secretion of factors, leading to increased osteoclastogenesis or decreased osteoblastogenesis).

Small GTPase Rho signaling is involved in beta1 integrin-mediated up-regulation of intercellular adhesion molecule 1 and receptor activator of nuclear factor kappaB ligand on osteoblasts and osteoclast maturation

We assessed the characteristics of human osteoblasts, focusing on small GTPase Rho signaling. Beta1 Integrin were highly expressed on osteoblasts. Engagement of beta1 integrins by type I collagen augmented expression of intercellular adhesion molecule 1 (ICAM-1) and receptor activator of nuclear factor kappaB ligand (RANKL) on osteoblasts. Rho was activated by beta1 stimulation in osteoblasts. Beta1 Integrin-induced up-regulation of ICAM-1 and RANKL was inhibited by transfection with adenoviruses encoding C3 transferase or pretreated with Y-27632, specific Rho and Rho-kinase inhibitors. Engagement of beta1 integrin on osteoblasts induced formation of tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells (MNC) in a coculture system of osteoblasts and peripheral monocytes, but this action was completely abrogated by transfection of C3 transferase. Our results indicate the direct involvement of Rho-mediated signaling in beta1 integrin-induced up-regulation of ICAM-1 and RANKL and RANKL-dependent osteoclast maturation. Thus, Rho-mediated signaling in osteoblasts seems to introduce major biases to bone resorption.

Effect of Modified Pectin Molecules on the Growth of Bone Cells

The aim of this study was to investigate molecular candidates for bone implant nanocoatings, which could improve biocompatibility of implant materials. Primary rat bone cells and murine preosteoblastic MC3T3-E1 cells were cultured on enzymatically modified hairy regions (MHR-A and MHR-B) of apple pectins. MHRs were covalently attached to tissue culture polystyrene (TCPS) or glass. Uncoated substrata or bone slices were used as controls. Cell attachment, proliferation, and differentiation were investigated with fluorescence and confocal microscopy. Bone cells seem to prefer MHR-B coating to MHR-A coating. On MHR-A samples, the overall numbers as well as proportions of active osteoclasts were diminished compared to those on MHR-B, TCPS, or bone. Focal adhesions indicating attachment of the osteoblastic cells were detected on MHR-B and uncoated controls but not on MHR-A. These results demonstrate the possibility to modify surfaces with pectin nanocoatings.

Geranylgeranyl-pyrophosphate (GGPP) synthase is down-regulated during differentiation of osteoblastic cell line MC3T3-E1

Isoprenylation of geranylgeranyl-pyrophosphate (GGPP) is critical for activation of small GTPases. We examined the roles of GGPP synthase (GGPPS) during the differentiation induced by the cell-to-cell contact in osteoblastic cell line MC3T3-E1 cells. We found that (1) both mRNA and protein expression of GGPPS was reduced with decrement of its activity during the differentiation, (2) GGOH, which is converted to GGPP in the cells, inhibited differentiation. These results suggest that the decrement of GGPP is critical for the cell-to-cell contact-induced differentiation, in which the down-regulation of GGPPS might be involved.

Dexamethasone inhibits the formation of multinucleated osteoclastsvia down-regulation of β3 integrin expression

Although glucocorticoids are known to affect osteoclast differentiation and function, there have been conflicting reports about the effect of glucocorticoids on osteoclast formation, leading to the assumption that microenvironment and cell type influence their action. We explored the effect of the synthetic glucocorticoid analog dexamethasone on the formation of osteoclasts. Dexamethasone inhibited the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclasts without affecting the formation of TRAP-positive mononuclear cells in a coculture of mouse osteoblasts and bone marrow cells. Dexamethasone did not inhibit mRNA expression levels of the receptor activator of nuclear factor-kappaB ligand and osteoprotegerin, the essential regulators of osteoclastogenesis. Dexamethasone down-regulated the expression of beta3 integrin mRNA and protein but did not alter expression of other osteoclast differentiation marker genes. Both dexamethasone and echistatin, a beta3 integrin function blocker, inhibited TRAP-positive multinucleated osteoclast formation but not TRAP-positive mononuclear cell formation. These results suggest that dexamethasone inhibits the formation of multinucleated osteoclasts, at least in part, through the down-regulation of beta3 integrin, which plays an important role in the formation of multinucleated osteoclasts.

Grafting aminocyclopentane carboxylic acids onto the RGD tripeptide sequence generates low nanomolar alphaVbeta3/alphaVbeta5 integrin dual binders

Eleven gamma-aminocyclopentane carboxylic acid (Acpca) platforms, including four dihydroxy representatives (19-22), three hydroxy analogues (34-36), and four deoxy derivatives (30-33), were prepared in a chiral nonracemic format. These simple units were then grafted onto an Arg-Gly-Asp (RGD) tripeptide framework by a mixed solid phase/solution protocol delivering an ensemble of 11 macrocyclic analogues of type cyclo-[-Arg-Gly-Asp-Acpca-], 1-11. The individual compounds were evaluated for their binding affinity toward the alphaVbeta3 and alphaVbeta5 integrin receptors. The analogue 10 exhibited a very interesting activity profile (IC50/alphaVbeta3= 1.5 nM; IC50/alphaVbeta5= 0.59 nM), comparable to that of reference compounds EMD121974 and ST1646. Closely related congeners 6, 8, and 9 also proved to be excellent dual binders with activity levels in the low nanomolar range. The three-dimensional (3D) NMR solution structures were determined, and docking studies to X-ray crystal structure of the extracellular segment of integrin alphaVbeta3 in complex with the reference compound EMD121974 were performed on selected analogues to elucidate the interplay between structure and function in these systems and to evidence the subtle bases for receptorial recognition. The results prove that the principle of isosteric dipeptide replacement for peptidomimetics design and synthesis can be violated, without detriment to the development of highly effective integrin binders.

Osteoprotegerin as a potential therapy for osteoporosis

The discovery and characterization of the RANKL/RANK/OPG signaling pathway and the identification of its role in the pathogenesis of bone loss have provided the rationale for the development of drugs with the ability to modulate RANK-induced osteoclastogenesis. In vivo studies have identified interfering with the RANKL/RANK interaction as a potential therapeutic target in the management of osteoporosis. Two agents capable of blocking the binding of RANKL to RANK have been so far tested in clinical studies--osteoprotegerin (Fc-OPG fusion molecule) and the RANKL-antibody (AMG 162). Both have been found to have profound inhibitory effects on bone resorption, with AMG 162 appearing to be overall superior to OPG. Data are still very scarce, however, and much remains to be uncovered before novel strategies capable of modulating the RANKL/OPG signaling pathway could be safely and effectively used in the management of osteoporosis.

Osteoprotegerin as a potential therapy for osteoporosis

The discovery and characterization of the RANKL/RANK/OPG signaling pathway and the identification of its role in the pathogenesis of bone loss have provided the rationale for the development of drugs with the ability to modulate RANK-induced osteoclastogenesis. In vivo studies have identified interfering with the RANKL/RANK interaction as a potential therapeutic target in the management of osteoporosis. Two agents capable of blocking the binding of RANKL to RANK have been so far tested in clinical studies--osteoprotegerin (Fc-OPG fusion molecule) and the RANKL-antibody (AMG 162). Both have been found to have profound inhibitory effects on bone resorption, with AMG 162 appearing to be overall superior to OPG. Data are still very scarce, however, and much remains to be uncovered before novel strategies capable of modulating the RANKL/OPG signaling pathway could be safely and effectively used in the management of osteoporosis.

[Osteoprotegerin--a neutralizing receptor, protector of bones and a potential antiresorptive agent]

INTRODUCTION

The receptor activator of NF-B ligand (RANKL), expressed on the surface of osteoblasts and stromal cells in the bone marrow, plays an essential role in the formation and differentiation of osteoclasts and bone resorption. RANKL binds to its functional receptor, receptor activator of NF-B (RANK), expressed as a transmembrane heterotrimer on the surface of hematopoietic osteoclasts progenitors and mature osteoclasts. RANKL-RANK interaction is inhibited by soluble receptor-osteoprotegerin.

OSTEOPROTEGERIN

Osteoprotegerin (OPG), a member of the tumor necrosis factor receptor superfamily, acts as a natural decoy receptor that blocks the interaction between RANKL and RANK. The balance between RANKL and osteoprotegerin is of major importance in bone homeostasis. Osteoprotegerin inhibits differentiation and formation of osteoclasts and induces apoptosis of osteoclasts. OPG knock-out mice develop severe osteoporosis. In contrast, overexpression of OPG in transgenic mice causes osteopetrosis.

OSTEOPROTEGERIN AS A THERAPEUTIC AGENT

Antiresorptive activity of osteoprotegerin is proved in numerous experimental models. Osteoprotective effect of osteoprotegerin has recently been proved in postmenopausal women with osteoporosis as well as in patients with multiple myeloma and osteolytic metastases of breast cancer.

CONCLUSION

Osteoprotegerin is a potent antiosteoclast agent that may prove useful in the treatment of bone disorders with net bone loss, such as postmenopausal osteoporosis and cancer metastases.

Aging increases stromal/osteoblastic cell-induced osteoclastogenesis and alters the osteoclast precursor pool in the mouse

Stromal/osteoblastic cell expression of RANKL and M-CSF regulates osteoclastogenesis. We show that aging is accompanied by increased RANKL and M-CSF expression, increased stromal/osteoblastic cell-induced osteoclastogenesis, and expansion of the osteoclast precursor pool. These changes correlate with age-related alterations in the relationship between osteoblasts and osteoclasts in cancellous bone.

INTRODUCTION

Bone mass is maintained through a balance between osteoblast and osteoclast activity. Osteoblasts regulate the number and activity of osteoclasts through expression of RANKL, osteoprotegerin (OPG), and macrophage-colony stimulation factor (M-CSF). To determine whether age-related changes in stromal/osteoblastic cell expression of RANKL, OPG, and M-CSF are associated with stimulation of osteoclastogenesis and whether the osteoclast precursor pool changes with age, we studied cultures of stromal/osteoblastic cells and osteoclast precursor cells from animals of different ages and examined how aging influences bone cell populations in vivo.

MATERIALS AND METHODS

Osteoclast precursors from male C57BL/6 mice of 6 weeks (young), 6 months (adult), and 24 months (old) of age were either co-cultured with stromal/osteoblastic cells from young, adult, or old mice or treated with M-CSF, RANKL, and/or OPG. Osteoclast precursor pool size was determined by fluorescence-activated cell sorting (FACS), and osteoclast formation was assessed by measuring the number of multinucleated TRACP(+) cells and pit formation. The levels of mRNA for RANKL, M-CSF, and OPG were determined by quantitative RT-PCR, and transcription was measured by PCR-based run-on assays. Osteoblast and osteoclast numbers in bone were measured by histomorphometry.

RESULTS

Osteoclast formation increased dramatically when stromal/osteoblastic cells from old compared with young donors were used to induce osteoclastogenesis. Regardless of the origin of the stromal/osteoblastic cells, the number of osteoclasts formed from the nonadherent population of cells increased with increasing age. Stromal/osteoblastic cell expression of RANKL and M-CSF increased, whereas OPG decreased with aging. Exogenously administered RANKL and M-CSF increased, dose-dependently, osteoclast formation from all donors, but the response was greater in cells from old donors. Osteoclast formation in vitro positively, and the ratio of osteoblasts to osteoclasts in vivo negatively, correlated with the ratio of RANKL to OPG expression in stromal/osteoblastic cells for all ages. The effects of RANKL-induced osteoclastogenesis in vitro were blocked by OPG, suggesting a causal relationship between RANKL expression and osteoclast-inducing potential. The osteoclast precursor pool and expression of RANK and c-fms increased with age.

CONCLUSIONS

Our results show that aging significantly increases stromal/osteoblastic cell-induced osteoclastogenesis, promotes expansion of the osteoclast precursor pool and alters the relationship between osteoblasts and osteoclasts in cancellous bone.

Osteoprotegerin and RANKL in alcoholic liver cirrhosis

BACKGROUND/AIMS

The mechanisms leading to osteoporosis in alcoholic liver disease remain poorly understood. Recently identified soluble circulating osteoprotegerin (OPG), is the osteoclastogenesis inhibitory factor. It acts as a decoy receptor for osteoclast activating factor, receptor activator of nuclear factor-kappaB ligand (RANKL), and impairs osteoclast function. The aim of our study was to investigate the OPG/RANKL system in alcoholic cirrhotic patients and their correlation with biochemical marker of bone turnover.

PATIENTS AND METHODS

Serum OPG, RANKL, osteocalcin (OC), C-terminal cross-linking telopeptide of type I collagen (CTX-I), bone alkaline phosphatase activity (bALP), and urinary hydroxyproline were measured in 30 patients with alcoholic cirrhosis, and in 20 age- and sex-matched healthy controls.

RESULTS

OPG levels were significantly increased in patients with alcoholic cirrhosis compared with healthy subjects (5.9 pmol/l, range 2.7-9.0 vs 4.1 pmol/l, range 1.2-6.6; P < 0.001). RANKL levels were significantly higher in patients with cirrhosis (0.48 pmol/l, range 0.01-1.34) than in healthy subjects (0.11 pmol/l, range 0.01-0.90). There was a positive correlation between serum OPG and RANKL (r = 0.37; P < 0.001), bALP (r = 0.66; P < 0.001) and urinary hydroxyproline (r = 0.51; P < 0.05) but not with OC and CTX-I.

CONCLUSIONS

OPG might partly represent a compensating mechanism to the negative balance of bone remodelling in patients with alcoholic cirrhosis.

Modulation of osteoclastogenesis in porcine bone marrow cultures by quercetin and rutin

Flavonols, in contrast to soybean isoflavones, are the most abundant phytoestrogens in western diets, being present in onions, beans, fruits, red wine, and tea. They may protect against atherosclerosis, inhibit certain cancer cell types, and reduce bone resorption. The most widely distributed flavonol is quercetin, which occurs mainly as its glycoside, rutin, but data are very scarce regarding the precise mechanism of action of these compounds on bone-resorbing cells at concentrations similar to those detected in human plasma. We have therefore investigated the effects of nanomolar concentrations of quercetin and rutin on the development and activity of osteoclasts in vitro compared with the effects of 17beta-estradiol. Nonadherent porcine bone marrow cells were cultured on dentine slices in the presence of 10 nM 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), with or without 10 nM quercetin, 10 nM rutin or 10 nM 17beta-estradiol for 11 days. Multinuclear TRAP+ cells that resorbed dentine (osteoclasts) developed in the presence of 1,25(OH)2D3, but their number was significantly reduced by quercetin, rutin, and 17beta-estradiol (P < 0.05). Like 17beta-estradiol, both flavonols also significantly reduced resorption (P<0.05) as assessed by the size of pits resorbed on dentine slices. Osteoclasts and osteoclast progenitors contained estrogen receptor alpha (ERalpha), ERbeta, and RANK proteins. Both flavonols increased nuclear ERbeta protein and decreased ERalpha protein of osteoclast progenitors. Moreover, rutin reduced RANK protein, whereas 17beta-oestradiol and quercetin promoted apoptosis by cleavage of caspase-8 and caspase-3. All the effects of flavonols were reversed by 1 microM ICI 182,780, an estrogen antagonist. Thus, the anti-resorbing properties of flavonols are mainly mediated by ER proteins through the inhibition of RANK protein or the activation of caspases.

Nasu-Hakola disease (polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy--PLOSL): a dementia associated with bone cystic lesions. From clinical to genetic and molecular aspects

The authors review the clinical, radiological, electrophysiological, pathological, and molecular aspects of Nasu-Hakola disease (polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy or PLOSL). Nasu-Hakola disease is a unique disease characterized by multiple bone cysts associated with a peculiar form of neurodegeneration that leads to dementia and precocious death usually during the fifth decade of life. The diagnosis can be established on the basis of clinical and radiological findings. Recently, molecular analysis of affected families revealed mutations in the DAP12 (TYROBP) or TREM2 genes, providing an interesting example how mutations in two different subunits of a multi-subunit receptor complex result in an identical human disease phenotype. The association of PLOSL with mutations in the DAP12 or TREM2 genes has led to improved diagnosis of affected individuals. Also, the possible roles of the DAP12/TREM2 signaling pathway in microglia and osteoclasts in humans are just beginning to be elucidated. Some aspects of this peculiar signaling pathway are discussed here.

M-CSF, TNFalpha and RANK ligand promote osteoclast survival by signaling through mTOR/S6 kinase

Multinucleated bone-resorbing osteoclasts (Ocl) are cells of hematopoietic origin that play a major role in osteoporosis pathophysiology. Ocl survival and activity require M-CSF and RANK ligand (RANKL). M-CSF signals to Akt, while RANKL, like TNFalpha, activates NF-kappaB. We show here that although these are separate pathways in the Ocl, signaling of all three cytokines converges on mammalian target of rapamycin (mTOR) as part of their antiapoptotic action. Accordingly, rapamycin blocks M-CSF- and RANKL-dependent Ocl survival inducing apoptosis, and suppresses in vitro bone resorption proportional to the reduction in Ocl number. The cytokine signaling intermediates for mTOR/ribosomal protein S6 kinase (S6K) activation include phosphatidylinositol-3 kinase, Akt, Erks and geranylgeranylated proteins. Inhibitors of these intermediates suppress cytokine activation of S6K and induce Ocl apoptosis. mTOR regulates protein translation acting via S6K, 4E-BP1 and S6. We find that inhibition of translation by other mechanisms also induces Ocl apoptosis, demonstrating that Ocl survival is highly sensitive to continuous de novo protein synthesis. This study thus identifies mTOR/S6K as an essential signaling pathway engaged in the stimulation of cell survival in osteoclasts.

N-Aryl-γ-lactams as integrin αvβ3 antagonists

Novel alphavbeta3 antagonists based on the N-aryl-gamma-lactam scaffold were prepared. SAR studies led to the identification of potent antagonists for alphavbeta3 receptor with excellent selectivity against the structurally related alpha(IIb)beta3 receptor. Additional interactions of N-aryl-gamma-lactam derivatives with alphavbeta3 were found when compared to c(-RGDf[NMe]V-) peptide antagonist. The effects of the conformation and configuration of the gamma-lactam core on the binding were also assessed.

Régulation de la masse osseuse par la leptine : un contrôle hypothalamique de la formation osseuse

Bone mass is maintained constant between puberty and menopause by the balance between osteoblasts and osteoclasts activity. The existence of a hormonal control of osteoblast activity has been speculated for years by analogy to osteoclast biology. Through the search for such humoral signal(s) regulating bone formation, leptin has been identified as a powerful inhibitor of bone formation. Furthermore, by means of intracerebroventricular infusion of leptin, it has been shown that the effect of this adipocyte-derived hormone on bone is mediated via a brain relay, like all its other functions. Subsequent studies have led to the identification of hypothalamic neurons involved in leptin's antiosteogenic function. In addition, it has been shown that those neurons or neuronal pathways are distinct from neurons responsible for the regulation of energy metabolism. Finally, the peripheral mediator of leptin's antiosteogenic function has been identified as being the sympathetic nervous system. Catecholamine-deficient mice have a high bone mass and sympathomimetics administered to mice decreased bone formation and bone mass. Conversely, beta-blockers increased bone formation and bone mass and blunt the bone loss induced by ovariectomy.

17beta-estradiol downregulates beta3-integrin expression in differentiating and mature human osteoclasts

The increased bone resorption observed after estrogen withdrawal is responsible for bone loss and may lead to osteoporosis. The mechanism by which estradiol inhibits bone resorption is known to involve decreased osteoclastogenesis, however, the effect on osteoclast adhesion remains unclear. We examined the in vitro effect of estradiol and raloxifene on human osteoclast differentiation and function. Human peripheral blood mononuclear cells were cultured with M-CSF/RANK-L for 18 days, and we evaluated bone resorption, the expression of the protein and mRNA of the integrins, c-jun and c-fos in the presence or absence of estradiol. In this human model, beta3-integrin expression increased at the mRNA and protein levels during osteoclast differentiation, whereas that of beta5-integrin did not. We found that estradiol and raloxifene directly inhibited bone resorption on bone slices by 50%, and decreased the expression of beta3-integrin mRNA (60%) and protein (20%) in a time-dependent manner. Moreover, the mRNAs of c-fos and c-jun were both diminished by estradiol and raloxifene, particularly in early osteoclasts, but also to a lesser extent in mature cells. These findings suggest that the direct inhibitory action of estradiol on bone resorption may affect human osteoclast differentiation through downregulation of c-fos and c-jun and adhesion through modulation of beta3-integrin.

RANKL/RANK/osteoprotegerin system as novel therapeutic target in the treatment of primary bone tumors and osteolytic metastases

Primary bone tumors and cancers that metastasize to bone require osteoclastic activity to release tumor-supportive growth factors from bone tissue. A number of systemic and locally acting factors are known to influence osteoclast formation, fusion, activation, and survival. Recently, two critical extracellular regulators of osteoclast differentiation and activation have been identified: receptor activator of nuclear factor (NF-kappaB) ligand (RANKL) and osteoprotegerin (OPG). RANKL is a tumor necrosis factor (TNF)-related cytokine that stimulates osteoclast differentiation from hematopoietic precursor cells and activation of mature osteoclasts. RANKL activates its specific receptor, receptor activator of NF- kappaB (RANK), located on osteoclasts, chondrocytes and dendritic cells. Binding of the RANK ligand to its receptor and osteoclastogenesis are prevented by osteoprotegerin, a decoy receptor produced by osteoblasts and marrow stromal cells. The balance between RANKL and OPG is of major importance in bone homeostasis. Disorders of the RANKL/RANK/OPG system have been linked to several human diseases, including primary bone tumors skeletal metastases, and hypercalcemia of malignancy. The discovery and characterization of RANKL, RANK and OPG and subsequent studies have changed the concepts of bone metabolism and may form the basis of innovative therapeutic strategies. Novel treatment strategies for bone tumors are emerging based on blockade of the RANKL/RANK interaction. The advantage of these strategies is their potential to selectively target tumor cells. Combining these new strategies with currently available treatments such as chemotherapy and radiation therapy is under investigation, with promising results.

Primitive adult hematopoietic stem cells can function as osteoblast precursors

Osteoblasts are continually recruited from stem cell pools to maintain bone. Although their immediate precursor is a plastic-adherent mesenchymal stem cell able to generate tissues other than bone, increasing evidence suggests the existence of a more primitive cell that can differentiate to both hematopoietic and mesenchymal cells. We show here that the "side population" (SP) of marrow stem cells, defined by their ability to rapidly expel a DNA-binding dye and to regenerate the hematopoietic compartment, can differentiate to osteoblasts through a mesenchymal intermediate. When transplanted into lethally irradiated mice, single gene-marked murine SP cells reconstituted depleted osteoprogenitor pools, such that a large proportion of the osteogenic cells in the epiphysis of long bone carried the donor SP cell marker. These findings suggest that the developmental capacity of SP cells is not restricted to the hematopoietic lineages but extends to osteogenic differentiation. This property not only elucidates a previously unrecognized step in osteoblast development, but also has intriguing implications for the use of SP cells in clinical orthopedics and stem cell-based disorders of bone.

Beta1 integrin/focal adhesion kinase-mediated signaling induces intercellular adhesion molecule 1 and receptor activator of nuclear factor kappaB ligand on osteoblasts and osteoclast maturation

We have assessed characteristics of primary human osteoblasts, shedding light on signaling mediated by beta1 integrin. beta1 integrins are major receptors for these matrix glycoproteins. 1) Integrins beta1, alpha2, alpha3, alpha4, alpha5, alpha6, and alphav were highly expressed on primary osteoblasts. 2) Engagement of beta1 integrins on osteoblasts by cross-linking with specific antibody or ligand matrices, such as fibronectin or collagen, augmented expression of intercellular adhesion molecule 1 (ICAM-1) and receptor activator of nuclear factor kappaB ligand (RANKL) on the surface. 3) Up-regulation of ICAM-1 and RANKL on osteoblasts by beta1 stimulation was completely abrogated by pretreatment with herbimycin A and genistein, tyrosine kinase inhibitors, or transfection of dominant negative truncations of focal adhesion kinase (FAK). 4) Engagement of beta1 integrins on osteoblasts induced tartrate-resistant acid phosphatase-positive multinuclear cell formation in the coculture system of osteoblasts and peripheral monocytes. 5) Up-regulation of tartrate-resistant acid phosphatase-positive multinuclear cell formation by beta1 stimulation was completely abrogated by transfection of dominant negative truncations of FAK. Our results indicate that beta1 integrin-dependent adhesion of osteoblasts to bone matrices induces ICAM-1 and RANKL expression and osteoclast formation via tyrosine kinase, especially FAK. We here propose that beta1 integrin/FAK-mediated signaling on osteoblasts could be involved in ICAM-1- and RANKL-dependent osteoclast maturation.

Reaching a genetic and molecular understanding of skeletal development

In the last ten years, we have made considerable progress in our genetic and molecular understanding of all aspects of skeletal development, chondrogenesis, joint formation, and osteogenesis. This review addresses the role of the principal growth factors and transcription factors affecting these different processes and presents, in several cases, the genetic cascade leading to cell differentiation.