Transcriptional activity of nuclei in multinucleated osteoclasts and its modulation by calcitonin

SNX10 regulates osteoclastogenic cell fusion and osteoclast size in mice

Bone-resorbing osteoclasts (OCLs) are formed by differentiation and fusion of monocyte precursor cells, generating large multinucleated cells. Tightly regulated cell fusion during osteoclastogenesis leads to formation of resorption-competent OCLs, whose sizes fall within a predictable physiological range. The molecular mechanisms that regulate the onset of OCL fusion and its subsequent arrest are, however, largely unknown. We have previously shown that OCLs cultured from mice homozygous for the R51Q mutation in the vesicle trafficking-associated protein sorting nexin 10, a mutation that induces autosomal recessive osteopetrosis in humans and in mice, display deregulated and continuous fusion that generates gigantic, inactive OCLs. Fusion of mature OCLs is therefore arrested by an active, genetically encoded, cell-autonomous, and SNX10-dependent mechanism. To directly examine whether SNX10 performs a similar role in vivo, we generated SNX10-deficient (SKO) mice and demonstrated that they display massive osteopetrosis and that their OCLs fuse uncontrollably in culture, as do homozygous R51Q SNX10 (RQ/RQ) mice. OCLs that lack SNX10 exhibit persistent presence of DC-STAMP protein at their periphery, which may contribute to their uncontrolled fusion. To visualize endogenous SNX10-mutant OCLs in their native bone environment, we genetically labeled the OCLs of WT, SKO, and RQ/RQ mice with enhanced Green Fluorescent Protein (EGFP), and then visualized the 3D organization of resident OCLs and the pericellular bone matrix by 2-photon, confocal, and second harmonics generation microscopy. We show that the volumes, surface areas and, in particular, the numbers of nuclei in the OCLs of both mutant strains were on average 2-6-fold larger than those of OCLs from WT mice, indicating that deregulated, excessive fusion occurs in the mutant mice. We conclude that the fusion of OCLs, and consequently their size, is regulated in vivo by SNX10-dependent arrest of fusion of mature OCLs.

NVP-BHG712 alleviates ovariectomy-induced osteoporosis by modulating osteoclastogenesis

Postmenopausal osteoporosis (PMOP) is closely related to the pathogenesis of osteoclasts, with the Cathepsin K (CTSK) protein playing a crucial role. Our study aimed to screen small molecule compounds targeting CTSK and evaluate their impact on PMOP. Through molecular docking, we identified NVP-BHG712 as significantly inhibiting osteoclast differentiation and bone resorption. NVP-BHG712 also effectively suppressed CTSK activity and exhibited strong binding affinity to CTSK protein. Furthermore, NVP-BHG712 regulated the expression of inflammatory factors and modulated the balance between M1 and M2 macrophage polarization. In the mouse model of ovariectomy-induced osteoporosis, NVP-BHG712 rescued bone loss by inhibiting excessive osteoclast activation. These findings suggest that NVP-BHG712 may be a promising treatment for pathological osteoporosis by alleviating osteoclast function.

Increased nerve growth factor expression and osteoclast density are associated with subchondral bone marrow lesions in osteoarthritic knees

Objectives

Subchondral bone marrow lesions (BMLs) detected on magnetic resonance imaging in knee osteoarthritis (OA) are associated with knee pain, though the mechanisms remain unknown. Increased nerve growth factor (NGF) expression and osteoclast density in subchondral bone appear to be the key features associated with bone pain in knee OA. Therefore, we aimed to identify associations among NGF, osteoclasts, and BMLs in knee OA.

Methods

Twenty tibial plateaus were obtained from patients undergoing total knee arthroplasty for medial knee OA with BMLs at the medial tibial plateau (MTP). Osteochondral tissue samples from the weight-bearing part of the MTP, with and without BML, and from the weight-bearing part of the lateral tibial plateau (LTP), without BML, were collected. NGF expression and density of osteoclasts were compared among the three osteochondral tissue types.

Results

MTP bone with BMLs exhibited significantly higher NGF expression in bone marrow space and osteochondral channel, and higher osteoclast density than MTP bone without BML and LTP bone. The mean differences in NGF-positive area in the bone marrow space and the percentage of NGF-positive channels between MTP bones with and without BML were 9.0% (95% confidence interval [CI]: 5.9-12.1%) and 23.1% (95% CI: 11.3-35.0%), respectively. The difference in osteoclast density between MTP bones with and without BML was 0.6 osteoclasts per mm (95% CI: 0.3-0.9 osteoclasts per mm).

Conclusions

Increased NGF expression and osteoclast density are associated with subchondral BMLs in knee OA, contribute to understanding the mechanisms underlying BML-related bone pain in knee OA.

Real-time analysis of osteoclast resorption and fusion dynamics in response to bone resorption inhibitors

Cathepsin K (CatK), an essential collagenase in osteoclasts (OCs), is a potential therapeutic target for the treatment of osteoporosis. Using live-cell imaging, we monitored the bone resorptive behaviour of OCs during dose-dependent inhibition of CatK by an ectosteric (Tanshinone IIA sulfonate) and an active site inhibitor (odanacatib). CatK inhibition caused drastic reductions in the overall resorption speed of OCs. At IC50 CatK-inhibitor concentration, OCs reduced about 40% of their trench-forming capacity and at fourfold IC50 concentrations, a > 95% reduction was observed. The majority of CatK-inhibited OCs (~ 75%) were involved in resorption-migration-resorption episodes forming adjacent pits, while ~ 25% were stagnating OCs which remained associated with the same excavation. We also observed fusions of OCs during the resorption process both in control and inhibitor-treated conditions, which increased their resorption speeds by 30-50%. Inhibitor IC50-concentrations increased OC-fusion by twofold. Nevertheless, more fusion could not counterweigh the overall loss of resorption activity by inhibitors. Using an activity-based probe, we demonstrated the presence of active CatK at the resorbing front in pits and trenches. In conclusion, our data document how OCs respond to CatK-inhibition with respect to movement, bone resorption activity, and their attempt to compensate for inhibition by activating fusion.

Effect of phosphodiester composition in polyphosphoesters on the inhibition of osteoclastic differentiation of murine bone marrow mononuclear cells

Osteoporosis is a common bone disorder characterized by reduced bone density and increased risk of fractures. The modulation of bone cell functions, particularly the inhibition of osteoclastic differentiation, plays a crucial role in osteoporosis treatment. Polyphosphoesters (PPEs) have shown the potential in reducing the function of osteoclast cells, but the effect of their chemical structure on osteoclastic differentiation remains largely unexplored. In this study, we evaluated the effect of PPE's chemical structure on the inhibition of osteoclastic differentiation of murine bone marrow mononuclear cells (BMNCs). PPEs containing phosphotriester and phosphodiester units at varying compositions were synthesized. Cytotoxicity testing confirmed the biocompatibility of the copolymers at concentrations below 0.5 mg/mL. Isolated from long bones, BMNCs were cultured in a differentiation medium supplemented with different PPE concentrations. Osteoclast formation was assessed through tartrate-resistant acid phosphatase and phalloidin staining. A significant decrease in the size of osteoclast cells formed upon BMNC contact with PPEs was observed, with a more pronounced effect observed at higher PPE concentrations. In addition, an increased composition of phosphodiester units in the PPEs yielded a decreased density of differentiated osteoclasts. Furthermore, real-time PCR analysis of major osteoclastic markers provided gene expression data that correlated with microscopic observations, confirming the effect of phosphodiester units in suppressing osteoclast differentiation of BMNCs from the early stages. These findings highlight the potential of PPEs as polymers are capable of modulating bone cell functions through their chemical structures.

In vitro osteoclastogenesis in autoimmune diseases - Strengths and pitfalls of a tool for studying pathological bone resorption and other disease characteristics

Osteoclasts play a critical role in bone pathology frequently associated with autoimmune diseases. Studying the etiopathogenesis of these diseases and their clinical manifestations can involve in vitro osteoclastogenesis, an experimental technique that utilizes osteoclast precursors that are relatively easily accessible from peripheral blood or synovial fluid. However, the increasing number of methodical options to study osteoclastogenesis in vitro poses challenges in translating findings to clinical research and practice. This review compares and critically evaluates previous research work based on in vitro differentiation of human osteoclast precursors originating from patients, which aimed to explain autoimmune pathology in rheumatic and enteropathic diseases. The discussion focuses primarily on methodical differences between the studies, including the origin of osteoclast precursors, culture conditions, and methods for identifying osteoclasts and assessing their activity. Additionally, the review examines the clinical significance of the three most commonly used in vitro approaches: induced osteoclastogenesis, spontaneous osteoclastogenesis, and cell co-culture. By analyzing and integrating the gathered information, this review proposes general connections between different studies, even in cases where their results are seemingly contradictory. The derived conclusions and future directions aim to enhance our understanding of a potential and limitations of in vitro osteoclastogenesis and provide a foundation for discussing novel methods (such as osteoclastogenesis dynamic) and standardized approaches (such as spontaneous osteoclastogenesis) for future use in autoimmune disease research.

An inducible explant model of osteoclast-osteoprogenitor coordination in exacerbated osteoclastogenesis

Elucidating a basic blueprint of osteoclast-osteoblast coordination in skeletal remodeling and understanding how this coordination breaks down with age and disease is essential for addressing the growing skeletal health problem in our aging population. The paucity of simple, activatable, biologically relevant models of osteoclast-osteoblast coordination has hindered our understanding of how skeletal remolding is regulated. Here, we describe an inducible ex vivo model of osteoclast-osteoblast progenitor coordination. Induction activates the release of osteoclastogenic factors from osteoprogenitors, which elicits the differentiation and fusion of neighboring preosteoclasts. In turn, multinucleated osteoclasts release soluble coupling factors, RANK⁺ extracellular vesicles and promote osteoprogenitor proliferation, recapitulating aspects of perturbed coordination in diseases underpinned by excessive osteoclast formation. We expect this model to expedite the investigation of cell-cell fusion, osteoclast-osteoblast progenitor coordination, and extracellular vesicle signaling during bone remodeling and offer a powerful tool for evaluating signaling cascades and novel therapeutic interventions in osteoclast-linked skeletal disease.

The role of insulin-like growth factor-1 in bone remodeling: A review

Insulin-like growth factor (IGF)-1 is a polypeptide hormone with vital biological functions in bone cells. The abnormal expression of IGF-1 has a serious effect on bone growth, particularly bone remodeling. Evidence from animal models and human disease suggested that both IGF-1 deficiency and excess cause changes in bone remodeling equilibrium, resulting in profound alterations in bone mass and development. Here, we first introduced the functions and mechanisms of the members of IGFs in bone. Subsequently, the critical role of IGF-1 in the process of bone remodeling were emphasized from the aspects of bone resorption and bone formation respectively. This review explains the mechanism of IGF-1 in maintaining bone mass and bone homeostasis to a certain extent and provides a theoretical basis for further research.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

Functional Heterogeneity Within Osteoclast Populations-a Critical Review of Four Key Publications that May Change the Paradigm of Osteoclasts

PURPOSE OF REVIEW

In this review, we critically evaluate the literature for osteoclast heterogeneity, including heterogeneity in osteoclast behavior, which has hitherto been unstudied and has only recently come to attention. We give a critical review centered on four recent high-impact papers on this topic and aim to shed light on the elusive biology of osteoclasts and focus on the variant features of osteoclasts that diverge from the classical viewpoint.

RECENT FINDINGS

Osteoclasts originate from the myeloid lineage and are best known for their unique ability to resorb bone. For decades, osteoclasts have been defined simply as multinucleated cells positive for tartrate-resistant acid phosphatase activity and quantified relative to the bone perimeter or surface in histomorphometric analyses. However, several recent, high-profile studies have demonstrated the existence of heterogeneous osteoclast populations, with variable origins and functions depending on the microenvironment. This includes long-term persisting osteoclasts, inflammatory osteoclasts, recycling osteoclasts (osteomorphs), and bone resorption modes. Most of these findings have been revealed through murine studies and have helped identify new targets for human studies. These studies have also uncovered distinct sets of behavioral patterns in heterogeneous osteoclast cultures. The underlying osteoclast heterogeneity likely drives differences in bone remodeling, altering patient risk for osteoporosis and fracture. Thus, identifying the underlying key features of osteoclast heterogeneity may help in better targeting bone diseases.

Suppression of osteoclast multinucleation via a posttranscriptional regulation-based spatiotemporally selective delivery system

Redundancy of multinucleated mature osteoclasts, which results from the excessive fusion of mononucleated preosteoclasts (pOCs), leads to osteolytic diseases such as osteoporosis. Unfortunately, the currently available clinical drugs completely inhibit osteoclasts, thus interfering with normal physiological bone turnover. pOC-specific regulation may be more suitable for maintaining bone homeostasis. Here, circBBS9, a previously unidentified circular RNA, was found to exert regulatory effects via the circBBS9/miR-423-3p/Traf6 axis in pOCs. To overcome the long-standing challenge of spatiotemporal RNA delivery to cells, we constructed biomimetic nanoparticles to achieve the pOC-specific targeted delivery of circBBS9. pOC membranes (POCMs) were extracted to camouflage cationic polymer for RNA interference with circBBS9 (POCM-NPs@siRNA/shRNA^circBBS9). POCM-NPs endowed the nanocarriers with improved stability, accurate pOC targeting, fusogenic uptake, and reactive oxygen species-responsive release. In summary, our findings may provide an alternative strategy for multinucleated cell-related diseases that involves restriction of mononucleated cell multinucleation through a spatiotemporally selective delivery system.

Monocyte-Macrophage Lineage Cell Fusion

Cell fusion (fusogenesis) occurs in natural and pathological conditions in prokaryotes and eukaryotes. Cells of monocyte–macrophage lineage are highly fusogenic. They create syncytial multinucleated giant cells (MGCs) such as osteoclasts (OCs), MGCs associated with the areas of infection/inflammation, and foreign body-induced giant cells (FBGCs). The fusion of monocytes/macrophages with tumor cells may promote cancer metastasis. We describe types and examples of monocyte–macrophage lineage cell fusion and the role of actin-based structures in cell fusion.

An antibody against Siglec-15 promotes bone formation and fracture healing by increasing TRAP+ mononuclear cells and PDGF-BB secretion

Osteoporosis (OP) is a common age-related disease characterized by a deterioration of bone mass and structure that predisposes patients to fragility fractures. Pharmaceutical therapies that promote anabolic bone formation in OP patients and OP-induced fracture are needed. We investigated whether a neutralizing antibody against Siglec-15 can simultaneously inhibit bone resorption and stimulate bone formation. We found that the multinucleation of osteoclasts was inhibited in SIGLEC-15 conditional knockout mice and mice undergoing Siglec-15 neutralizing antibody treatment. The secretion of platelet-derived growth factor-BB (PDGF-BB), the number of tartrate-resistant acid phosphatase-positive (TRAP⁺) mononuclear cells, and bone formation were significantly increased in the SIGLEC-15 conditional knockout mice and antibody-treated mice. The anabolic effect of the Siglec-15 neutralizing antibody on bone formation was blunted in mice with Pdgfb deleted in TRAP⁺ cells. These findings showed that the anabolic effect of the Siglec-15 neutralizing antibody was mediated by elevating PDGF-BB production of TRAP⁺ mononuclear cells. To test the therapeutic potential of the Siglec-15 neutralizing antibody, we injected the antibody in an ovariectomy-induced osteoporotic mouse model, which mimics postmenopausal osteoporosis in women, and in two fracture healing models because fracture is the most serious health consequence of osteoporosis. The Siglec-15 neutralizing antibody effectively reduced bone resorption and stimulated bone formation in estrogen deficiency-induced osteoporosis. Of note, the Siglec-15 neutralizing antibody promoted intramembranous and endochondral ossification at the damaged area of cortical bone in fracture healing mouse models. Thus, the Siglec-15 neutralizing antibody shows significant translational potential as a novel therapy for OP and bone fracture.

Inhibition of miR-29 Activity in the Myeloid Lineage Increases Response to Calcitonin and Trabecular Bone Volume in Mice

The miR-29-3p family (miR-29a, miR-29b, miR-29c) of microRNAs is increased during receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis. In vivo, activation of a miR-29-3p tough decoy inhibitor in Cre recombinase under the control of the lysozyme 2 promoter-expressing cells (myeloid lineage) resulted in mice displaying enhanced trabecular and cortical bone volume because of decreased bone resorption. Calcitonin receptor (Calcr) is a miR-29 target that negatively regulates bone resorption. CALCR was significantly increased in RANKL-treated miR-29-decoy osteoclasts, and these cells were more responsive to the inhibitory effect of calcitonin on osteoclast formation. Further, cathepsin K (Ctsk), which is critical for resorption, was decreased in miR-29-decoy cells. CALCR is a Gs-coupled receptor and its activation raises cAMP levels. In turn, cAMP suppresses cathepsin K, and cAMP levels were increased in miR-29-decoy cells. siRNA-mediated knock-down of Calcr in miR-29 decoy osteoclasts allowed recovery of cathepsin K levels in these cells. Overall, using a novel knockin tough decoy mouse model, we identified a new role for miR-29-3p in bone homeostasis. In RANKL-driven osteoclastogenesis, as seen in normal bone remodeling, miR-29-3p promotes resorption. Consequently, inhibition of miR-29-3p activity in the myeloid lineage leads to increased trabecular and cortical bone. Further, this study documents an interrelationship between CALCR and CTSK in osteoclastic bone resorption, which is modulated by miR-29-3p.

An SNX10-dependent mechanism downregulates fusion between mature osteoclasts

Homozygosity for the R51Q mutation in sorting nexin 10 (SNX10) inactivates osteoclasts (OCLs) and induces autosomal recessive osteopetrosis in humans and in mice. We show here that the fusion of wild-type murine monocytes to form OCLs is highly regulated, and that its extent is limited by blocking fusion between mature OCLs. In contrast, monocytes from homozygous R51Q SNX10 mice fuse uncontrollably, forming giant dysfunctional OCLs that can become 10- to 100-fold larger than their wild-type counterparts. Furthermore, mutant OCLs display reduced endocytotic activity, suggesting that their deregulated fusion is due to alterations in membrane homeostasis caused by loss of SNX10 function. This is supported by the finding that the R51Q SNX10 protein is unstable and exhibits altered lipid-binding properties, and is consistent with a key role for SNX10 in vesicular trafficking. We propose that OCL size and functionality are regulated by a cell-autonomous SNX10-dependent mechanism that downregulates fusion between mature OCLs. The R51Q mutation abolishes this regulatory activity, leading to excessive fusion, loss of bone resorption capacity and, consequently, to an osteopetrotic phenotype in vivo.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: Fusion of monocytes to become bone-resorbing osteoclasts is limited by an SNX10-dependent cell-autonomous mechanism. Loss of SNX10 function deregulates fusion and generates giant inactive osteoclasts.

Origins, Biology, and Diseases of Tissue Macrophages

Tissue-resident macrophages are present in most tissues with developmental, self-renewal, or functional attributes that do not easily fit into a textbook picture of a plastic and multifunctional macrophage originating from hematopoietic stem cells; nor does it fit a pro- versus anti-inflammatory paradigm. This review presents and discusses current knowledge on the developmental biology of macrophages from an evolutionary perspective focused on the function of macrophages, which may aid in study of developmental, inflammatory, tumoral, and degenerative diseases. We also propose a framework to investigate the functions of macrophages in vivo and discuss how inherited germline and somatic mutations may contribute to the roles of macrophages in diseases.

The Macrophage-Osteoclast Axis in Osteoimmunity and Osteo-Related Diseases

Osteoimmunity is involved in regulating the balance of bone remodeling and resorption, and is essential for maintaining normal bone morphology. The interaction between immune cells and osteoclasts in the bone marrow or joint cavity is the basis of osteoimmunity, in which the macrophage-osteoclast axis plays a vital role. Monocytes or tissue-specific macrophages (macrophages resident in tissues) are an important origin of osteoclasts in inflammatory and immune environment. Although there are many reports on macrophages and osteoclasts, there is still a lack of systematic reviews on the macrophage-osteoclast axis in osteoimmunity. Elucidating the role of the macrophage-osteoclast axis in osteoimmunity is of great significance for the research or treatment of bone damage caused by inflammation and immune diseases. In this article, we introduced in detail the concept of osteoimmunity and the mechanism and regulators of the differentiation of macrophages into osteoclasts. Furthermore, we described the role of the macrophage-osteoclast axis in typical bone damage caused by inflammation and immune diseases. These provide a clear knowledge framework for studying macrophages and osteoclasts in inflammatory and immune environments. And targeting the macrophage-osteoclast axis may be an effective strategy to treat bone damage caused by inflammation and immune diseases.

Osteoclasts recycle via osteomorphs during RANKL-stimulated bone resorption

Osteoclasts are large multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage-derived precursors that are thought to undergo apoptosis once resorption is complete. Here, by intravital imaging, we reveal that RANKL-stimulated osteoclasts have an alternative cell fate in which they fission into daughter cells called osteomorphs. Inhibiting RANKL blocked this cellular recycling and resulted in osteomorph accumulation. Single-cell RNA sequencing showed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and express a number of non-canonical osteoclast genes that are associated with structural and functional bone phenotypes when deleted in mice. Furthermore, genetic variation in human orthologs of osteomorph genes causes monogenic skeletal disorders and associates with bone mineral density, a polygenetic skeletal trait. Thus, osteoclasts recycle via osteomorphs, a cell type involved in the regulation of bone resorption that may be targeted for the treatment of skeletal diseases.

Intracellular Staphylococcus aureus in bone and joint infections: A mechanism of disease recurrence, inflammation, and bone and cartilage destruction

Bone and joint infections are devastating afflictions. Although medical interventions and advents have improved their care, bone and joint infections still portend dismal outcomes. Indeed, bone and joint infections are associated with extremely high mortality and morbidity rates and, generally, occur secondary to the aggressive pathogen Staphylococcus aureus. The consequences of bone and joint infections are further compounded by the fact that although they are aggressively treated, they frequently recur and result in massive bone and articular cartilage loss. Here, we review the literature and chronicle the fact that the fundamental cellular components of the musculoskeletal system can be internally infected with Staphylococcus aureus, which explains the ready recurrence of bone and joint infections even after extensive administration of antibiotic therapy and debridement and offer potential treatment solutions for further study. Moreover, we review the ramifications of intracellular infection and expound that the massive bone and articular cartilage loss is caused by the sustained proinflammatory state induced by infection and offer potential combination therapies for further study to protect bone and cartilage.

Osteoclast Fusion: Physiological Regulation of Multinucleation through Heterogeneity-Potential Implications for Drug Sensitivity

Classically, osteoclast fusion consists of four basic steps: (1) attraction/migration, (2) recognition, (3) cell-cell adhesion, and (4) membrane fusion. In theory, this sounds like a straightforward simple linear process. However, it is not. Osteoclast fusion has to take place in a well-coordinated manner-something that is not simple. In vivo, the complex regulation of osteoclast formation takes place within the bone marrow-in time and space. The present review will focus on considering osteoclast fusion in the context of physiology and pathology. Special attention is given to: (1) regulation of osteoclast fusion in vivo, (2) heterogeneity of osteoclast fusion partners, (3) regulation of multi-nucleation, (4) implications for physiology and pathology, and (5) implications for drug sensitivity and side effects. The review will emphasize that more attention should be given to the human in vivo reality when interpreting the impact of in vitro and animal studies. This should be done in order to improve our understanding of human physiology and pathology, as well as to improve anti-resorptive treatment and reduce side effects.

Heterogeneity and Actin Cytoskeleton in Osteoclast and Macrophage Multinucleation

Osteoclast signatures are determined by two transcriptional programs, the lineage-determining transcription pathway and the receptor activator of nuclear factor kappa-B ligand (RANKL)-dependent differentiation pathways. During differentiation, mononuclear precursors become multinucleated by cell fusion. Recently, live-cell imaging has revealed a high level of heterogeneity in osteoclast multinucleation. This heterogeneity includes the difference in the differentiation states and the mobility of the fusion precursors, as well as the mode of fusion among the fusion precursors with different numbers of nuclei. In particular, fusion partners often form morphologically distinct actin-based linkages that allow two cells to exchange lipids and proteins before membrane fusion. However, the origin of this heterogeneity remains elusive. On the other hand, osteoclast multinucleation is sensitive to the environmental cues. Such cues promote the reorganization of the actin cytoskeleton, especially the formation and transformation of the podosome, an actin-rich punctate adhesion. This review covers the heterogeneity of osteoclast multinucleation at the pre-fusion stage with reference to the environment-dependent signaling pathway responsible for reorganizing the actin cytoskeleton. Furthermore, we compare osteoclast multinucleation with macrophage fusion, which results in multinucleated giant macrophages.

Fusion Potential of Human Osteoclasts In Vitro Reflects Age, Menopause, and In Vivo Bone Resorption Levels of Their Donors-A Possible Involvement of DC-STAMP

It is well established that multinucleation is central for osteoclastic bone resorption. However, our knowledge on the mechanisms regulating how many nuclei an osteoclast will have is limited. The objective of this study was to investigate donor-related variations in the fusion potential of in vitro-generated osteoclasts. Therefore, CD14⁺ monocytes were isolated from 49 healthy female donors. Donor demographics were compared to the in vivo bone biomarker levels and their monocytes’ ability to differentiate into osteoclasts, showing that: (1) C-terminal telopeptide of type I collagen (CTX) and procollagen type I N-terminal propeptide (PINP) levels increase with age, (2) the number of nuclei per osteoclast in vitro increases with age, and (3) there is a positive correlation between the number of nuclei per osteoclast in vitro and CTX levels in vivo. Furthermore, the expression levels of the gene encoding dendritic cell-specific transmembrane protein (DCSTAMP) of osteoclasts in vitro correlated positively with the number of nuclei per osteoclast, CTX levels in vivo, and donor age. Our results furthermore suggest that these changes in gene expression may be mediated through age-related changes in DNA methylation levels. We conclude that both intrinsic factors and age-induced increase in fusion potential of osteoclasts could be contributing factors for the enhanced bone resorption in vivo, possibly caused by increased expression levels of DCSTAMP.

Multinucleated giant cell phenotype in response to stimulation

Macrophages fuse into multinucleated giant cells (MGC) in many pathological conditions. Despite MGC correlations with granulomas, their functional contribution to inflammation is relatively unknown. An in vitro mouse model of IL-4-induced bone marrow-derived macrophage fusion and microfiltration were used to generate enriched MGC and macrophage populations. Phenotypes were compared in response to well-known inflammatory stimuli, including lipopolysaccharide and crocidolite asbestos. Surface markers were assessed by flow cytometry: CD11b, CD11c, F4/80, and MHC II. Secreted cytokines were assessed by multiplex immunoassay: IFN-γ, IL-1β, IL-6, TNF-α, IL-10, IL-13, and IL-33. Results show that MGC maintained macrophage surface protein expression but lost the ability to produce a cytokine response. This suggests a potentially beneficial role of MGC in isolating the host from a foreign body without contributing to excessive inflammation. This study and future research using other stimulants and environments are important to gaining a fundamental MGC cell biology understanding. This will inform approaches to controlling the foreign body response to particle exposure, medical implants, and many diseases associated with granulomas.

Endogenous Collagenases Regulate Osteoclast Fusion

The precise regulation of osteoclast differentiation and function is crucial for the maintenance of healthy bone. Despite several reports of collagenase expression in bone tissues, the precise isoform expression as well as the role in osteoclasts are still unclear. In the present report, the expression of matrix metalloprotease (MMP)8 and MMP13 was confirmed in mouse bone marrow macrophage osteoclast precursors. The mRNA and protein expressions of both collagenases were significantly reduced by receptor activator of nuclear factor κB ligand (RANKL) stimulation. Notably, either inhibition of MMP expression by siRNA or treatment of cells with collagenase inhibitor Ro 32-3555 significantly augmented osteoclast fusion and resorption activity without affecting the osteoclast number. The inhibition of collagenase by Ro 32-3555 increased the expression of osteoclast fusion genes, Atp6v0d2 and Dcstamp, without affecting nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) protein expression. The enhanced osteoclast fusion by collagenase inhibition appears to be mediated through an extracellular signal regulated kinase (ERK)-dependent pathway. Collectively, these data provide novel information on the regulation of osteoclast fusion process.

Interaction of Brucella abortus with Osteoclasts: a Step toward Understanding Osteoarticular Brucellosis and Vaccine Safety

Osteoarticular disease is a frequent complication of human brucellosis. Vaccination remains a critical component of brucellosis control, but there are currently no vaccines for use in humans, and no in vitro models for assessing the safety of candidate vaccines in reference to the development of bone lesions currently exist. While the effect of Brucella infection on osteoblasts has been extensively evaluated, little is known about the consequences of osteoclast infection. Murine bone marrow-derived macrophages were derived into mature osteoclasts and infected with B. abortus 2308, the vaccine strain S19, and attenuated mutants S19vjbR and B. abortus ΔvirB2 While B. abortus 2308 and S19 replicated inside mature osteoclasts, the attenuated mutants were progressively killed, behavior that mimics infection kinetics in macrophages. Interestingly, B. abortus 2308 impaired the growth of osteoclasts without reducing resorptive activity, while osteoclasts infected with B. abortus S19 and S19vjbR were significantly larger and exhibited enhanced resorption. None of the Brucella strains induced apoptosis or stimulated nitric oxide or lactose dehydrogenase production in mature osteoclasts. Finally, infection of macrophages or osteoclast precursors with B. abortus 2308 resulted in generation of smaller osteoclasts with decreased resorptive activity. Overall, Brucella exhibits similar growth characteristics in mature osteoclasts compared to the primary target cell, the macrophage, but is able to impair the maturation and alter the resorptive capacity of these cells. These results suggest that osteoclasts play an important role in osteoarticular brucellosis and could serve as a useful in vitro model for both analyzing host-pathogen interactions and assessing vaccine safety.

Common signalling pathways in macrophage and osteoclast multinucleation

Macrophage cell fusion and multinucleation are fundamental processes in the formation of multinucleated giant cells (MGCs) in chronic inflammatory disease and osteoclasts in the regulation of bone mass. However, this basic cell phenomenon is poorly understood despite its pathophysiological relevance. Granulomas containing multinucleated giant cells are seen in a wide variety of complex inflammatory disorders, as well as in infectious diseases. Dysregulation of osteoclastic bone resorption underlies the pathogenesis of osteoporosis and malignant osteolytic bone disease. Recent reports have shown that the formation of multinucleated giant cells and osteoclast fusion display a common molecular signature, suggesting shared genetic determinants. In this Review, we describe the background of cell-cell fusion and the similar origin of macrophages and osteoclasts. We specifically focus on the common pathways involved in osteoclast and MGC fusion. We also highlight potential approaches that could help to unravel the core mechanisms underlying bone and granulomatous disorders in humans.

Divergent effects of strontium and calcium-sensing receptor positive allosteric modulators (calcimimetics) on human osteoclast activity

BACKGROUND AND PURPOSE

Strontium ranelate, a drug approved and until recently used for the treatment of osteoporosis, mediates its effects on bone at least in part via the calcium-sensing (CaS) receptor. However, it is not known whether bone-targeted CaS receptor positive allosteric modulators (PAMs; calcimimetics) represent an alternative (or adjunctive) therapy to strontium (Sr²⁺ _o ).

EXPERIMENTAL APPROACH

We assessed three structurally distinct calcimimetics [cinacalcet, AC-265347 and a benzothiazole tri-substituted urea (BTU-compound 13)], alone and in combination with extracellular calcium (Ca²⁺ _o ) or Sr²⁺ _o , in G protein-dependent signalling assays and trafficking experiments in HEK293 cells and their effects on cell differentiation, tartrate-resistant acid phosphatase (TRAP) activity and hydroxyapatite resorption assays in human blood-derived osteoclasts.

KEY RESULTS

Sr²⁺ _o activated CaS receptor-dependent signalling in HEK293 cells in a similar manner to Ca²⁺ _o , and inhibited the maturation, TRAP expression and hydroxyapatite resorption capacity of human osteoclasts. Calcimimetics potentiated Ca²⁺ _o - and Sr²⁺ _o -mediated CaS receptor signalling in HEK293 cells with distinct biased profiles, and only cinacalcet chaperoned an endoplasmic reticulum-retained CaS mutant receptor to the cell surface in HEK293 cells, indicative of a conformational state different from that engendered by AC-265347 and BTU-compound 13. Intriguingly, only cinacalcet modulated human osteoclast function, reducing TRAP activity and profoundly inhibiting resorption.

CONCLUSION AND IMPLICATIONS

Although AC-265347 and BTU-compound 13 potentiated Ca²⁺ _o - and Sr²⁺ _o -induced CaS receptor activation, they neither replicated nor potentiated the ability of Sr²⁺ _o to inhibit human osteoclast function. In contrast, the FDA-approved calcimimetic, cinacalcet, inhibited osteoclast TRAP activity and hydroxyapatite resorption, which may contribute to its clinical effects on bone mineral density LINKED ARTICLES: This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Regulation of Osteoclast Growth and Fusion by mTOR/raptor and mTOR/rictor/Akt

Osteoclasts are giant bone cells formed by fusion from monocytes and uniquely capable of a complete destruction of mineralized tissues. Previously, we have demonstrated that in energy-rich environment not only osteoclast fusion index (the number of nuclei each osteoclast contains), but also cytoplasm volume per single nucleus was increased. The goal of this study was to investigate the regulation of metabolic sensor mTOR during osteoclast differentiation in energy-rich environment simulated by addition of pyruvate. We have found that in the presence of pyruvate, the proportion of mTOR associated with raptor increased, while mTOR-rictor-mediated Akt phosphorylation decreased. Inhibition of mTOR with rapamycin (10 nM) significantly interfered with all aspects of osteoclastogenesis. However, rapamycin at 1 nM, which preferentially targets mTOR-raptor complex, was only effective in control cultures, while in the presence of pyruvate osteoclast fusion index was successfully increased. Inhibition of Akt drastically reduced osteoclast fusion, however in energy-rich environment, osteoclasts of comparable size were formed through increased cytoplasm growth. These data suggest that mTOR-rictor mediated Akt signaling regulates osteoclast fusion, while mTOR-raptor regulation of protein translation contributes to fusion-independent cytoplasm growth. We demonstrate that depending on the bioenergetics microenvironment osteoclastogenesis can adjust to occur through preferential multinucleation or through cell growth, implying that attaining large cell size is part of the osteoclast differentiation program.

High throughput, quantitative analysis of human osteoclast differentiation and activity

Osteoclasts are multinuclear cells that degrade bone under both physiological and pathophysiological conditions. Osteoclasts are therefore a major target of osteoporosis therapeutics aimed at preserving bone. Consequently, analytical methods for osteoclast activity are useful for the development of novel biomarkers and/or pharmacological agents for the treatment of osteoporosis. The nucleation state of an osteoclast is indicative of its maturation and activity. To date, activity is routinely measured at the population level with only approximate consideration of the nucleation state (an 'osteoclast population' is typically defined as cells with ≥3 nuclei). Using a fluorescent substrate for tartrate-resistant acid phosphatase (TRAP), a routinely used marker of osteoclast activity, we developed a multi-labelled imaging method for quantitative measurement of osteoclast TRAP activity at the single cell level. Automated image analysis enables interrogation of large osteoclast populations in a high throughput manner using open source software. Using this methodology, we investigated the effects of receptor activator of nuclear factor kappa-B ligand (RANK-L) on osteoclast maturation and activity and demonstrated that TRAP activity directly correlates with osteoclast maturity (i.e. nuclei number). This method can be applied to high throughput screening of osteoclast-targeting compounds to determine changes in maturation and activity.

Ablation of Y1 receptor impairs osteoclast bone-resorbing activity

Y₁ receptor (Y₁R)-signalling pathway plays a pivotal role in the regulation of bone metabolism. The lack of Y₁R-signalling stimulates bone mass accretion that has been mainly attributed to Y₁R disruption from bone-forming cells. Still, the involvement of Y₁R-signalling in the control of bone-resorbing cells remained to be explored. Therefore, in this study we assessed the role of Y₁R deficiency in osteoclast formation and resorption activity. Here we demonstrate that Y₁R germline deletion (Y₁R^−/−) led to increased formation of highly multinucleated (n > 8) osteoclasts and enhanced surface area, possibly due to monocyte chemoattractant protein-1 (MCP-1) overexpression regulated by RANKL-signalling. Interestingly, functional studies revealed that these giant Y₁R^−/− multinucleated cells produce poorly demineralized eroded pits, which were associated to reduce expression of osteoclast matrix degradation markers, such as tartrate-resistant acid phosphatase-5b (TRAcP5b), matrix metalloproteinase-9 (MMP-9) and cathepsin-K (CTSK). Tridimensional (3D) morphologic analyses of resorption pits, using an in-house developed quantitative computational tool (BonePit), showed that Y₁R^−/− resorption pits displayed a marked reduction in surface area, volume and depth. Together, these data demonstrates that the lack of Y₁Rs stimulates the formation of larger multinucleated osteoclasts in vitro with reduced bone-resorbing activity, unveiling a novel therapeutic option for osteoclastic bone diseases based on Y₁R-signalling ablation.

Advanced glycation end products biphasically modulate bone resorption in osteoclast-like cells

Advanced glycation end products (AGEs) disturb bone remodeling during aging, and this process is accelerated in diabetes. However, their role in modulation of osteoclast-induced bone resorption is controversial, with some studies indicating that AGEs enhance bone resorption and others showing the opposite effect. We determined whether AGEs present at different stages of osteoclast differentiation affect bone resorption differently. Based on increased levels of tartrate-resistant acid phosphatase (TRAP) and cathepsin K (CTSK), we identified day 4 of induction as the dividing time of cell fusion stage and mature stage in RAW264.7 cell-derived osteoclast-like cells (OCLs). AGE-modified BSA (50-400 μg/ml) or control BSA (100 μg/ml) was then added at the beginning of each stage. Results showed that the presence of AGEs at the cell fusion stage reduced pit numbers, resorption area, and CTSK expression. Moreover, expression of receptor activator of nuclear factor-κB (RANK) as well as the number of TRAP-positive cells, nuclei per OCL, actin rings, and podosomes also decreased. However, the presence of AGEs at the mature stage enlarged the resorption area markedly and increased pit numbers slightly. Intriguingly, only the number of nuclei per OCL and podosomes increased. These data indicate that AGEs biphasically modulate bone resorption activity of OCLs in a differentiation stage-dependent manner. AGEs at the cell fusion stage reduce bone resorption dramatically, mainly via suppression of RANK expression in osteoclast precursors, whereas AGEs at the mature stage enhance bone resorption slightly, most likely by increasing the number of podosomes in mature OCLs.

Effect of Tumor Necrosis Factor Inhibitor Therapy on Osteoclasts Precursors in Ankylosing Spondylitis

Introduction

Ankylosing Spondylitis (AS) is characterized by excessive local bone formation and concomitant systemic bone loss. Tumor necrosis factor (TNF) plays a central role in the inflammation of axial skeleton and enthesis of AS patients. Despite reduction of inflammation and systemic bone loss, AS patients treated with TNF inhibitors (TNFi) have ongoing local bone formation. The aim of this study was to assess the effect of TNFi in the differentiation and activity of osteoclasts (OC) in AS patients.

Methods

13 AS patients treated with TNFi were analyzed at baseline and after a minimum follow-up period of 6 months. 25 healthy donors were recruited as controls. Blood samples were collected to assess receptor activator of nuclear factor kappa-B ligand (RANKL) surface expression on circulating leukocytes and frequency and phenotype of monocyte subpopulations. Quantification of serum levels of bone turnover markers and cytokines, in vitro OC differentiation assay and qRT-PCR for OC specific genes were performed.

Results

RANKL⁺ circulating lymphocytes (B and T cells) and IL-17A, IL-23 and TGF-β levels were decreased after TNFi treatment. We found no differences in the frequency of the different monocyte subpopulations, however, we found decreased expression of CCR2 and increased expression of CD62L after TNFi treatment. OC number was reduced in patients at baseline when compared to controls. OC specific gene expression was reduced in circulating OC precursors after TNFi treatment. However, when cultured in OC differentiating conditions, OC precursors from AS TNFi-treated patients showed increased activity as compared to baseline.

Conclusion

In AS patients, TNFi treatment reduces systemic pro osteoclastogenic stimuli. However, OC precursors from AS patients exposed to TNFi therapy have increased in vitro activity in response to osteoclastogenic stimuli.

Pin1 regulates osteoclast fusion through suppression of the master regulator of cell fusion DC-STAMP

Cell fusion is a fundamental biological event that is essential for the development of multinucleated cells such as osteoclasts. Fusion failure leads to the accumulation of dense bone such as in osteopetrosis, demonstrating the importance of fusion in osteoclast maturity and bone remodeling. In a recent study, we reported that Pin1 plays a role in the regulation of bone formation and Runx2 regulation. In this study, we explored the role of Pin1 in osteoclast formation and bone resorption. Pin1 null mice have low bone mass and increased TRAP staining in histological sections of long bones, compared to Pin1 wild-type mice. In vitro osteoclast forming assays with bone marrow-derived monocyte/macrophage revealed that Pin1-deficient osteoclasts are larger than wild-type osteoclasts and have higher nuclei numbers, indicating greater extent of fusion. Pin1 deficiency also highly enhanced foreign body giant cell formation both in vitro and in vivo. Among the known fusion proteins, only DC-STAMP was significantly increased in Pin1(-/-) osteoclasts. Immunohistochemistry showed that DC-STAMP expression was also significantly increased in tibial metaphysis of Pin1 KO mice. We found that Pin1 binds and isomerizes DC-STAMP and affects its expression levels and localization at the plasma membrane. Taken together, our data indicate that Pin1 is a determinant of bone mass through the regulation of the osteoclast fusion protein DC-STAMP. The identification of Pin1 as a factor involved in cell fusion contributes to the understanding of osteoclast-associated diseases, including osteoporosis, and opens new avenues for therapeutic targets.

Dual impact of live Staphylococcus aureus on the osteoclast lineage, leading to increased bone resorption

BACKGROUND

Bone and joint infection, mainly caused by Staphylococcus aureus, is associated with significant morbidity and mortality, characterized by severe inflammation and progressive bone destruction. Studies mostly focused on the interaction between S. aureus and osteoblasts, the bone matrix-forming cells, while interactions between S. aureus and osteoclasts, the only cells known to be able to degrade bone, have been poorly explored.

METHODS

We developed an in vitro infection model of primary murine osteoclasts to study the direct impact of live S. aureus on osteoclastogenesis and osteoclast resorption activity.

RESULTS

Staphylococcal infection of bone marrow-derived osteoclast precursors induced their differentiation into activated macrophages that actively secreted proinflammatory cytokines. These cytokines enhanced the bone resorption capacity of uninfected mature osteoclasts and promoted osteoclastogenesis of the uninfected precursors at the site of infection. Moreover, infection of mature osteoclasts by live S. aureus directly enhanced their ability to resorb bone by promoting cellular fusion.

CONCLUSIONS

Our results highlighted two complementary mechanisms involved in bone loss during bone and joint infection, suggesting that osteoclasts could be a pivotal target for limiting bone destruction.

Expression of metastasin S100A4 is essential for bone resorption and regulates osteoclast function

OBJECTIVE

S100A4 is a Ca-binding protein that regulates cell growth, survival, and motility. The abundant expression of S100A4 in rheumatiod arthritis contributes to the invasive growth of joint tissue and to bone damage. In the present study, we analysed the role of S100A4 in bone homeostasis.

METHODS

Peripheral quantitative computed tomography and histomorphometric analysis were performed in mice lacking the entire S100A4 protein (S100A4KO) and in wild-type (WT) counterparts treated with shRNA-lentiviral constructs targeting S100A4 (S100A4-shRNA). Control groups consisted of sex-matched WT counterparts and WT mice treated with a non-targeting RNA construct.

RESULTS

S100A4 deficiency was associated with higher trabecular and cortical bone mass, increased number and thickness of trabeculi combined with larger periosteal circumference and higher predicted bone strength. S100A4 inhibition by shRNA led to an increase in cortical bone in WT mice. S100A4-deficieny was associated with a reduced number of functional osteoclasts. S100A4KO and S100A4-shRNA-treated bone marrow progenitors gave rise to a large number of small TRAP+ cells with few nuclei and few pseudopodial processes. Poor osteoclastogenesis and the low resorptive capacity in S100A4Ko mice may be linked to low levels of surface integrins, impaired adhesion capacity, and poor multinucleation in S100A4-deficient osteoclasts, as well as a low content of proteolytic enzymes cathepsin K and MMP3 and MMP9 to break down the organic matrix.

CONCLUSION

S100A4 emerges as a negative regulator of bone metabolism potentially responsible for the excessive bone turnover in conditions marked by high levels of S100A4 protein, such as inflammation and rheumatoid arthritis.

E proteins regulate osteoclast maturation and survival

Osteoclasts are bone-specific polykaryons derived from myeloid precursors under the stimulation of macrophage colony stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL). E proteins are basic helix-loop-helix (bHLH) transcription factors that modulate lymphoid versus myeloid cell fate decisions. To study the role of E proteins in osteoclasts, myeloid-specific E protein gain-of-function transgenic mice were generated. These mice have high bone mass due to decreased osteoclast numbers and increased osteoclast apoptosis leading to overall reductions in resorptive capacity. The molecular mechanism of decreased osteoclast numbers and resorption is in part a result of elevated expression of CD38, a regulator of intracellular calcium pools with known antiosteoclastogenic properties, which increases sensitivity to apoptosis. In vivo, exogenous RANKL stimulation can overcome this inhibition to drive osteoclastogenesis and bone loss. In vitro-derived ET2 osteoclasts are more spread and more numerous with increases in RANK, triggering receptor expressed on myeloid cells 2 (TREM2), and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) compared to wild type. However, their resorptive capacity does not increase accordingly. Thus, E proteins participate in osteoclast maturation and survival in homeostatic bone remodeling.

Purinergic signalling in bone

Purinergic signaling in bone was first proposed in the early 1990s with the observation that extracellular ATP could modulate events crucial to the normal functioning of bone cells. Since then the expression of nearly all the P2Y and P2X receptors by osteoblasts and osteoclasts has been reported, mediating multiple processes including cell proliferation, differentiation, function, and death. This review will highlight the most recent developments in the field of purinergic signaling in bone, with a special emphasis on recent work resulting from the European Framework 7 funded collaboration ATPBone, as well as Arthritis Research UK and Bone Research Society supported projects.

A 3D scanning confocal imaging method measures pit volume and captures the role of Rac in osteoclast function

Modulation of Rho GTPases Rac1 and Rac2 impacts bone development, remodeling, and disease. In addition, GTPases are considered treatment targets for dysplastic and erosive bone diseases including Neurofibromatosis type 1. While it is important to understand the effects of Rac modulation on osteoclast function, two-dimensional resorption pit area measurements fall short in elucidating the volume aspect of bone resorption activity. Bone marrow from wild-type, Rac1 and Rac2 null mice was isolated from femora. Osteoclastogenesis was induced by adding M-CSF and RANKL in culture plates containing dentin slices and later stained with Picro Sirius Red to image resorption lacunae. Osteoclasts were also plated on glass cover slips and stained with phalloidin and DAPI to measure their surface area and the number of nuclei. Volumetric images were collected on a laser-scanning confocal system. Sirius Red confocal imaging provided an unambiguous, continuous definition of the pit boundary compared to reflected and transmitted light imaging. Rac1- and Rac2-deficient osteoclasts had fewer nuclei in comparison to wild-type counterparts. Rac1-deficient osteoclasts showed reduced resorption pit volume and surface area. Lacunae made by single Rac2 null osteoclasts had reduced volume but surprisingly surface area was unaffected. Surface area measures are deceiving since volume changed independently in resorption pits made by individual Rac2 null osteoclasts. Our innovative confocal imaging technique allows us to derive novel conclusions about Rac1 and Rac2 in osteoclast function. The data and method can be applied to study effects of genes and drugs including Rho GTPase modulators on osteoclast function and to develop pharmacotherapeutics to treat bone lytic disorders.

Hyperactivation of mTOR critically regulates abnormal osteoclastogenesis in neurofibromatosis Type 1

Individuals with nerofibromatosis Type 1 (NF1) frequently suffer a spectrum of bone pathologies, such as abnormal skeletal development (scoliosis, congenital bowing, and congenital pseudoarthroses, etc), lower bone mineral density with increased fracture risk. These skeletal problems may result, in part, from abnormal osteoclastogenesis. Enhanced RAS/PI3K activity has been reported to contribute to abnormal osteoclastogenesis in Nf1 heterozygous (Nf1+/-) mice. However, the specific downstream pathways linked to NF1 abnormal osteoclastogenesis have not been defined. Our aim was to determine whether mammalian target of rapamycin (mTOR) was a key effector responsible for abnormal osteoclastogenesis in NF1. Primary osteoclast-like cells (OCLs) were cultured from Nf1 wild-type (Nf1+/+) and Nf1+/- mice. Compared to Nf1+/+ controls, there were 20% more OCLs induced from Nf1+/- mice. Nf1+/- OCLs were larger and contained more nuclei. Hyperactive mTOR signaling was detected in Nf1+/- OCLs. Inhibition of mTOR signaling by rapamycin in Nf1+/- OCLs abrogated abnormalities in cellular size and number. Moreover, we found that hyperactive mTOR signaling induced abnormal osteoclastogenesis major through hyper-proliferation. Our research suggests that neurofibromin directly regulates osteoclastogenesis through mTOR signaling pathway. Inhibiting mTOR may represent a viable strategy to treat NF1 bone diseases.

Transcriptionally active nuclei are selective in mature multinucleated osteoclasts

Multinucleation is indispensable for the bone-resorbing activity of mature osteoclasts. Although multinucleation is evident in mature osteoclasts and certain other cell types, putative regulatory networks among nuclei remain poorly characterized. To address this issue, transcriptional activity of each nucleus in a multinucleated osteoclast was assessed by detecting the distributions of nuclear proteins by immunocytochemistry and primary transcripts by RNA FISH. Patterns of epigenetic histone markers governing transcription as well as localization of tested nuclear receptor proteins appeared indistinguishable among nuclei in differentiated Raw264 cells and mouse mature osteoclasts. However, RNAPII-Ser5P/2P and NFATc1 proteins were selectively distributed in certain nuclei in the same cell. Similarly, the distributions of primary transcripts for osteoclast-specific genes (Nfatc1, Ctsk and Acp5) as well as a housekeeping gene (beta-tubulin) were limited in certain nuclei within individual cells. By fusing two Raw264 cell lines that stably expressed ZsGreen-NLS and DsRed-NLS proteins, transmission of nuclear proteins across all of the nuclei in a cell could be observed, presumably through the shared cytoplasm. Taken together, we conclude that although nuclear proteins are diffusible among nuclei, only certain nuclei within a multinucleated osteoclast are transcriptionally active.

Regulation of sealing ring formation by L-plastin and cortactin in osteoclasts

The aim of this study is to identify the exact mechanism(s) by which cytoskeletal structures are modulated during bone resorption. In this study, we have shown the possible role of different actin-binding and signaling proteins in the regulation of sealing ring formation. Our analyses have demonstrated a significant increase in cortactin and a corresponding decrease in L-plastin protein levels in osteoclasts subjected to bone resorption for 18 h in the presence of RANKL, M-CSF, and native bone particles. Time-dependent changes in the localization of L-plastin (in actin aggregates) and cortactin (in the sealing ring) suggest that these proteins may be involved in the initial and maturation phases of sealing ring formation, respectively. siRNA to cortactin inhibits this maturation process but not the formation of actin aggregates. Osteoclasts treated as above but with TNF-α demonstrated very similar effects as observed with RANKL. Osteoclasts treated with a neutralizing antibody to TNF-α displayed podosome-like structures in the entire subsurface and at the periphery of osteoclast. It is possible that TNF-α and RANKL-mediated signaling may play a role in the early phase of sealing ring configuration (i.e. either in the disassembly of podosomes or formation of actin aggregates). Furthermore, osteoclasts treated with alendronate or αv reduced the formation of the sealing ring but not actin aggregates. The present study demonstrates a novel mechanistic link between L-plastin and cortactin in sealing ring formation. These results suggest that actin aggregates formed by L-plastin independent of integrin signaling function as a core in assembling signaling molecules (integrin αvβ3, Src, cortactin, etc.) involved in the maturation process.

A comparative study on BMP-induced osteoclastogenesis and osteoblastogenesis in primary cultures of adult rat bone marrow cells

Bone remodeling is a process consisting of bone formation and resorption. The present study compared the relative osteoclastic and osteoblastic potency of bone morphogenetic proteins (BMP)-2, -4, -5, -6, and -7 in primary murine bone marrow cultures. All five BMPs stimulated, to varying degree, formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells in a time- and protein concentration-dependent manner. The TRAP staining intensity correlated positively with the number of nuclei per TRAP-positive cell and the mRNA levels of colony-stimulating factor (CSF-1), receptor activator of nuclear factor kappaB ligand (RANKL), TRAP, and cathepsin K. Under osteogenic conditions, all five BMPs stimulated AP activity and mineralized bone nodule formation in a protein concentration-dependent manner in the same primary murine bone marrow cell culture system. These findings should be useful in designing treatment strategies for bone regeneration.

Giant osteoclast formation and long-term oral bisphosphonate therapy

BACKGROUND

Bisphosphonates decrease bone resorption and are commonly used to treat or prevent osteoporosis. However, the effect of bisphosphonates on their target cells remains enigmatic, since in patients benefiting from therapy, little change, if any, has been observed in the number of osteoclasts, which are the cells responsible for bone resorption.

METHODS

We examined 51 bone-biopsy specimens obtained after a 3-year, double-blind, randomized, placebo-controlled, dose-ranging trial of oral alendronate to prevent bone resorption among healthy postmenopausal women 40 through 59 years of age. The patients were assigned to one of five groups: those receiving placebo for 3 years; alendronate at a dose of 1, 5, or 10 mg per day for 3 years; or alendronate at a dose of 20 mg per day for 2 years, followed by placebo for 1 year. Formalin-fixed, undecalcified planar sections were assessed by bone histomorphometric methods.

RESULTS

The number of osteoclasts was increased by a factor of 2.6 in patients receiving 10 mg of alendronate per day for 3 years as compared with the placebo group (P<0.01). Moreover, the number of osteoclasts increased as the cumulative dose of the drug increased (r=0.50, P<0.001). Twenty-seven percent of these osteoclasts were giant cells with pyknotic nuclei that were adjacent to superficial resorption cavities. Furthermore, giant, hypernucleated, detached osteoclasts with 20 to 40 nuclei were found after alendronate treatment had been discontinued for 1 year. Of these large cells, 20 to 37% were apoptotic, according to both their morphologic features and positive findings from in situ end labeling.

CONCLUSIONS

Long-term alendronate treatment is associated with an increase in the number of osteoclasts, which include distinctive giant, hypernucleated, detached osteoclasts that are undergoing protracted apoptosis.

The osteoclast: a multinucleated, hematopoietic-origin, bone-resorbing osteoimmune cell

Osteoclasts are multinucleated cells that derive from hematopoietic progenitors in the bone marrow which also give rise to monocytes in peripheral blood, and to the various types of tissue macrophages. Osteoclasts are formed by the fusion of precursor cells. They function in bone resorption and are therefore critical for normal skeletal development (growth and modeling), for the maintenance of its integrity throughout life, and for calcium metabolism (remodeling). To resorb bone, the osteoclasts attach to the bone matrix, their cytoskeleton reorganizes, and they assume polarized morphology and form ruffled borders to secrete acid and collagenolytic enzymes and a sealing zone to isolate the resorption site. Identification of the osteoclastogenesis inducer, the receptor activator of nuclear factor-kappaB ligand (RANKL), its cognate receptor RANK, and its decoy receptor osteoprotegerin (OPG), has contributed enormously to the dramatic advance in our understanding of the molecular mechanisms involved in osteoclast differentiation and activity. This explosion in osteoclast biology is reflected by the large number of reviews which appeared during the last decade. Here I will summarize the "classical" issues (origin, differentiation, and activity) in a general manner, and will discuss an untouched issue (multinucleation) and a relatively novel aspect of osteoclast biology (osteoimmunology).

Osteoclast nuclei of myeloma patients show chromosome translocations specific for the myeloma cell clone: a new type of cancer-host partnership?

A major clinical manifestation of bone cancers is bone destruction. It is widely accepted that this destruction is not caused by the malignant cells themselves, but by osteoclasts, multinucleated cells of monocytic origin that are considered to be the only cells able to degrade bone. The present study demonstrates that bone-resorbing osteoclasts from myeloma patients contain nuclei with translocated chromosomes of myeloma B-cell clone origin, in addition to nuclei without these translocations, by using combined FISH and immunohistochemistry on bone sections. These nuclei of malignant origin are transcriptionally active and appear fully integrated amongst the other nuclei. The contribution of malignant nuclei to the osteoclast population analysed in this study was greater than 30%. Osteoclast-myeloma clone hybrids contained more nuclei than normal osteoclasts and their occurrence correlated with the proximity of myeloma cells. Similar hybrid cells were generated in myeloma cell-osteoclast co-cultures, as revealed by tracing myeloma nuclei using translocations, bromo-deoxyuridine, or the Y chromosome of male myeloma cells in female osteoclasts. These observations indicate that hybrid cells can originate through fusion between myeloma cells and osteoclasts. In conclusion, malignant cells contribute significantly to the formation of bone-resorbing osteoclasts in multiple myeloma. Osteoclast-myeloma clone hybrids reflect a previously unrecognized mechanism of bone destruction in which malignant cells participate directly. The possibility that malignant cells corrupt host cells by the transfer of malignant DNA may have been underestimated to date in cancer research.

Hematopoietic stem cell-targeted neonatal gene therapy reverses lethally progressive osteopetrosis in oc/oc mice

Infantile malignant osteopetrosis (IMO) is a fatal disease caused by lack of functional osteoclasts, and the only available treatment is hematopoietic stem cell (HSC) transplantation. In the majority of patients, the TCIRG1 gene, coding for a subunit of a proton pump essential for bone resorption, is mutated. Oc/oc mice have a deletion in the homologue gene (tcirg1) and die at 3 to 4 weeks, but can be rescued by neonatal transplantation of HSCs. Here, HSC-targeted gene therapy of osteopetrosis in the oc/oc mouse model was developed. Oc/oc fetal liver cells depleted of Ter119-expressing erythroid cells were transduced with a retroviral vector expressing tcirg1 and GFP, and subsequently transplanted intraperitoneally to irradiated neonatal oc/oc mice. Eight of 15 mice survived past the normal life span of oc/oc mice. In vitro osteoclastogenesis revealed formation of GFP-positive osteoclasts and bone resorption, albeit at a lower level than from wild-type cells. The skeletal phenotype was analyzed by X-ray and histopathology and showed partial correction at 8 weeks and almost normalization after 18 weeks. In summary, osteopetrosis in oc/oc mice can be reversed by neonatal transplantation of gene-modified HSCs leading to long-term survival. This represents a significant step toward the development of gene therapy for osteopetrosis.

Association of the CT gene (CA) polymorphism with BMD in osteoporotic mexican women

Calcitonin (CT) plays a role in the pathogenesis of osteoporosis and genetic variations in or adjacent to the CT gene may be associated with loss of bone mineral density (BMD). The correlation between a dinucleotide (cytosine-adenine) repeat polymorphism at the CT locus and BMD was examined in 70 osteoporotic women, 70 non-osteoporotic women and 500 subjects from the Mexican population. The allele A and genotype AA frequencies were significantly higher in osteoporotic women than in non-osteoporotic women (60% vs 32%; p < 0.0001 and 41% vs 14%; p = 0.0007, respectively). Genotype AA was associated with the presence of osteoporosis [odds ratio 2.58; 95% confidence interval (CI); 1.62-4.12]. Likewise, the loss of lumbar BMD and T scores were related to the presence of allele A: subjects with a single A allele displayed lower values for lumbar BMD and T score (84.02% and -1.51, respectively) than those who do not present any A allele (89.61% and -0.88, respectively). Individuals with two alleles A showed the lowest lumbar BMD and T-score values (73.77% and -2.51, respectively). Analysis of potential confounder demonstrated that aging has a significant effect on osteoporosis development (odds ratio 1.1; 95% CI; 1.1052-1.152).

Role of IGF-I signaling in regulating osteoclastogenesis

We showed that IGF-I deficiency impaired osteoclastogenesis directly and/or indirectly by altering the interaction between stromal/osteoblastic cells and osteoclast precursors, reducing RANKL and M-CSF production. These changes lead to impaired bone resorption, resulting in high BV/TV in IGF-I null mice.

INTRODUCTION

Although IGF-I has been clearly identified as an important growth factor in regulating osteoblast function, information regarding its role in osteoclastogenesis is limited. Our study was designed to analyze the role of IGF-I in modulating osteoclastogenesis using IGF-I knockout mice (IGF-I(-/-)).

MATERIALS AND METHODS

Trabecular bone volume (BV/TV), osteoclast number, and morphology of IGF-I(-/-) or wildtype mice (IGF-I(+/+)) were evaluated in vivo by histological analysis. Osteoclast precursors from these mice were cultured in the presence of RANKL and macrophage-colony stimulating factor (M-CSF) or co-cultured with stromal/osteoblastic cells from either genotype. Osteoclast formation was assessed by measuring the number of multinucleated TRACP+ cells and pit formation. The mRNA levels of osteoclast regulation markers were determined by quantitative RT-PCR.

RESULTS

In vivo, IGF-I(-/-) mice have higher BV/TV and fewer (76% of IGF-I(+/+)) and smaller osteoclasts with fewer nuclei. In vitro, in the presence of RANKL and M-CSF, osteoclast number (55% of IGF-I(+/+)) and resorptive area (30% of IGF-I(+/+)) in osteoclast precursor cultures from IGF-I(-/-) mice were significantly fewer and smaller than that from the IGF-I(+/+) mice. IGF-I (10 ng/ml) increased the size, number (2.6-fold), and function (resorptive area, 2.7-fold) of osteoclasts in cultures from IGF-I(+/+) mice, with weaker stimulation in cultures from IGF-I(-/-) mice. In co-cultures of IGF-I(-/-) osteoblasts with IGF-I(+/+) osteoclast precursors, or IGF-I(+/+) osteoblasts with IGF-I(-/-) osteoclast precursors, the number of osteoclasts formed was only 11% and 48%, respectively, of that from co-cultures of IGF-I(+/+) osteoblasts and IGF-I(+/+) osteoclast precursors. In the long bones from IGF-I(-/-) mice, mRNA levels of RANKL, RANK, M-CSF, and c-fms were 55%, 33%, 60%, and 35% of that from IGF-I(+/+) mice, respectively.

CONCLUSIONS

Our results indicate that IGF-I regulates osteoclastogenesis by promoting their differentiation. IGF-I is required for maintaining the normal interaction between the osteoblast and osteoclast to support osteoclastogenesis through its regulation of RANKL and RANK expression.

TREM2, a DAP12-associated receptor, regulates osteoclast differentiation and function

Deficiency of the signaling adapter protein DAP12 or its associated receptor TREM2 is associated with abnormal OC development in humans. Here we examine the role of TREM2 in mouse OC development and function, including migration and resorption in vitro. These results provide new evidence that TREM2 regulates OC function independent of its effects on multinucleated OC differentiation.

INTRODUCTION

TREM2 (triggering receptor expressed in myeloid cells-2) associates with the signaling adapter DAP12 in osteoclasts (OCs). Genetic mutation or deletion of either the TYROBP (DAP12) or TREM2 gene is associated with the human disorder of brain and bone, Nasu-Hakola disease. We and others recently showed the critical requirement for immunoreceptor tyrosine-based activation motif (ITAM) signals through DAP12 and the Fc Receptor gamma chain (FcRgamma) during OC development. Here, we further define the role of TREM2 in OC differentiation and describe a role for TREM2 in OC migration and bone resorption.

MATERIALS AND METHODS

We generated monoclonal anti-mouse TREM2 antibodies (mAb), analyzed pre-osteoclasts and mature OCs for TREM2 surface expression, and determined the effect of antibody ligation on in vitro OC differentiation, resorption, and migration. TREM2 RNA interference (RNAi) was used to disrupt expression of TREM2 in pre-osteoclasts.

RESULTS

Using flow cytometry, our studies reveal that TREM2 is weakly expressed on C57BL/6 bone marrow macrophages (BMMs) and is upregulated during culture with RANKL and macrophage-colony stimulating factor (M-CSF). The expression of TREM2 is unaltered in DAP12-deficient OCs. Using C57BL/6 BMMs or RAW264.7 precursors, anti-TREM2 mAb treatment with RANKL and M-CSF enhances the formation of multinuclear TRACP+ OCs compared with control mAb treatment. In contrast, these agents have no effect on DAP12-deficient precursors. Monoclonal Ab blockade of TREM2 on OCs generated from C57BL/6 BMMs results in decreased resorption of artificial calcium-phosphate substrate and dentine. Reduction of TREM2 expression in RAW264.7 cells by RNAi results in loss of OC formation in response to RANKL and M-CSF. Anti-TREM2 cross-linking enhances migration of C57BL/6 OCs and RAW246.7 OCs in response to M-CSF.

CONCLUSIONS

Our studies indicate that the TREM2 receptor regulates OC multinucleation as well as resorption and migration of mature OCs. Thus, TREM2-DAP12 signals regulate both OC formation and function.