Non-Canonical (RANKL-Independent) Pathways of Osteoclast Differentiation and Their Role in Musculoskeletal Diseases

Contribution of macrophage polarization in bone metabolism: A literature review

Macrophage polarization divides macrophages into two main cell subpopulations, classically and alternatively activated macrophages (M1 and M2, respectively). M1 polarization promotes osteoclastogenesis, while M2 polarization promotes osteogenesis. The physiological homeostasis of bone metabolism involves a high dynamic balance between osteoclastic-mediated bone resorption and formation. Reportedly, M1/M2 imbalance causes the onset and persistence of inflammation-related bone diseases. Therefore, understanding the research advances in functions and roles of macrophages in such diseases will provide substantial guidance for improved treatment of bone diseases. In this review, we underscore and summarize the research advances in macrophage polarization, and bone-related diseases, such as rheumatoid arthritis, osteoarthritis, and osteoporosis, over the last 5 years. Our findings showed that targeting macrophages and balancing macrophage polarization can effectively reduce inflammation and decrease bone destruction while promoting bone formation and vascular repair. These results indicate that regulating macrophage and macrophage polarization to restore homeostasis is a prospective approach for curing bone-related diseases.

The role of infection in signalling root resorption: A narrative review

BACKGROUND

Root resorption consists of complex, multistep processes that involve cell signalling caused by inflammation and stromal cells, which promotes the secretion of receptor activator of nuclear factor κB ligand/ macrophage-colony stimulating factor (RANKL/M-CSF) resulting in a resorptive process.

OBJECTIVE

The aim of this narrative review was to analyse the literature related to root resorption resulting from microbial infection and to comparing it with non-microbial infection.

METHODS

An electronic literature search was performed using the PubMed database and applying keywords of articles published in English. Eligible papers were reviewed to reveal the descriptions of bone and root resorption processes. The abstracts were searched manually to identify articles about infection-stimulating bone and root resorption.

RESULTS

Three main types of root resorption were identified, two associated with primary bacterial infection and one secondary to bacterial infection. These include external inflammatory resorption, internal inflammatory resorption and external cervical (invasive) resorption.

DISCUSSION

The magnitude of cytokine involvement that promotes resorption and M-CSF/RANKL production depends on multiple factors, including pathogen virulence, site of infection and host genetic factors that activate the inflammation at the infection site. Two mechanisms activate the resorption mechanisms-the canonical and non-canonical pathways that can activate clastic cells independently of the RANKL/RANK canonical pathways.

CONCLUSIONS

Two pathways of root resorption co-exist in the body. When resorption is caused by infection, chronic inflammation due to bacterial infection prolongs the secretions of pro-inflammatory cytokines that intensify root and bone resorption. The second pathway is bacterial independent of the non-infection root resorption that is part of the wound healing process, which is limited in time due to its innate ability.

The effect of low-level laser therapy on osteoclast differentiation: Clinical implications for tooth movement and bone density

Background/purpose

Osteoclast differentiation is crucial for orchestrating both tooth movement and the maintenance of bone density. Therefore, the current study sought to explore the impact of low-level laser therapy (LLLT) on osteoclast differentiation, functional gene expression, molecular signaling pathways, and orthodontic tooth movement in clinical settings.

Materials and methods

The RAW 264.7 cell line served as the precursor for osteoclasts, and these cells underwent irradiation using a 808-nm LLLT. Osteoclast differentiation was assessed through tartrate-resistant acid phosphatase (TRAP) staining. Functional gene expression levels were evaluated using real-time quantitative polymerase chain reaction (RT-qPCR) while signaling molecules were examined through Western blot analysis. In the clinical study, 12 participants were enrolled. Their tooth movement was monitored using a TRIOS desktop scanner. Bone density measurements were conducted using Mimics software, which processed cone-beam computed tomography (CBCT) images exported in Digital Imaging and Communications in Medicine (DICOM) format.

Results

We found that LLLT effectively promoted receptor activator of nuclear factor-κB ligand (RANKL)-dependent osteoclast differentiation and the expression of osteoclast functional genes, including matrix metallopeptidase 9 (MMP9), nuclear factor of activated T-cells cytoplasmic 1(NFATc1), tartrate-resistant acid phosphatase (TRAP) and cathepsin K (CTSK) in RAW264.7 cells. Clinically, the cumulative tooth movement over 90 days was significantly higher in the laser group than in the control group.

Conclusion

Our research demonstrates that LLLT not only significantly promotes osteoclast differentiation but is also a valuable adjunct in orthodontic therapy.

Aseptic loosening around total joint replacement in humans is regulated by miR-1246 and miR-6089 via the Wnt signalling pathway

BACKGROUND

Total joint replacement for osteoarthritis is one of the most successful surgical procedures in modern medicine. However, aseptic loosening continues to be a leading cause of revision arthroplasty. The diagnosis of aseptic loosening remains a challenge as patients are often asymptomatic until the late stages. MicroRNA (miRNA) has been demonstrated to be a useful diagnostic tool and has been successfully used in the diagnosis of other diseases. We aimed to identify differentially expressed miRNA in the plasma of patients with aseptic loosening.

METHODS

Adult patients undergoing revision arthroplasty for aseptic loosening and age- and gender-matched controls were recruited. Samples of bone, tissue and blood were collected, and RNA sequencing was performed in 24 patients with aseptic loosening and 26 controls. Differentially expressed miRNA in plasma was matched to differentially expressed mRNA in periprosthetic bone and tissue. Western blot was used to validate protein expression.

RESULTS

Seven miRNA was differentially expressed in the plasma of patients with osteolysis (logFC >|2|, adj-P < 0.05). Three thousand six hundred and eighty mRNA genes in bone and 427 mRNA genes in tissue samples of osteolysis patients were differentially expressed (logFC >|2|, adj-P < 0.05). Gene enrichment analysis and pathway analysis revealed two miRNA (miR-1246 and miR-6089) had multiple gene targets in the Wnt signalling pathway in the local bone and tissues which regulate bone metabolism.

CONCLUSION

These results suggest that aseptic loosening may be regulated by miR-1246 and miR-6089 via the Wnt signalling pathway.

Nano wear particles and the periprosthetic microenvironment in aseptic loosening induced osteolysis following joint arthroplasty

Joint arthroplasty is an option for end-stage septic arthritis due to joint infection after effective control of infection. However, complications such as osteolysis and aseptic loosening can arise afterwards due to wear and tear caused by high joint activity after surgery, necessitating joint revision. Some studies on tissue pathology after prosthesis implantation have identified various cell populations involved in the process. However, these studies have often overlooked the complexity of the altered periprosthetic microenvironment, especially the role of nano wear particles in the etiology of osteolysis and aseptic loosening. To address this gap, we propose the concept of the "prosthetic microenvironment". In this perspective, we first summarize the histological changes in the periprosthetic tissue from prosthetic implantation to aseptic loosening, then analyze the cellular components in the periprosthetic microenvironment post prosthetic implantation. We further elucidate the interactions among cells within periprosthetic tissues, and display the impact of wear particles on the disturbed periprosthetic microenvironments. Moreover, we explore the origins of disease states arising from imbalances in the homeostasis of the periprosthetic microenvironment. The aim of this review is to summarize the role of relevant factors in the microenvironment of the periprosthetic tissues, in an attempt to contribute to the development of innovative treatments to manage this common complication of joint replacement surgery.

Spontaneous Osteoclastogenesis, a risk factor for bone metastasis in advanced luminal A-type breast cancer patients

Introduction

Osteolytic bone metastasis in advanced breast cancer stages are a major complication for patient´s quality life and a sign of low survival prognosis. Permissive microenvironments which allow cancer cell secondary homing and later proliferation are fundamental for metastatic processes. The causes and mechanisms behind bone metastasis in breast cancer patients are still an unsolved puzzle. Therefore, in this work we contribute to describe bone marrow pre-metastatic niche in advanced breast cancer patients.

Results

We show an increase in osteoclasts precursors with a concomitant imbalance towards spontaneous osteoclastogenesis which can be evidenced at bone marrow and peripheral levels. Pro-osteoclastogenic factors RANKL and CCL-2 may contribute to bone resorption signature observed in bone marrow. Meanwhile, expression levels of specific microRNAs in primary breast tumors may already indicate a pro-osteoclastogenic scenario prior to bone metastasis.

Discussion

The discovery of prognostic biomarkers and novel therapeutic targets linked to bone metastasis initiation and development are a promising perspective for preventive treatments and metastasis management in advanced breast cancer patients.

Emerging roles of growth factors in osteonecrosis of the femoral head

Osteonecrosis of the femoral head (ONFH) is a potentially disabling orthopedic condition that requires total hip arthroplasty in most late-stage cases. However, mechanisms underlying the development of ONFH remain unknown, and the therapeutic strategies remain limited. Growth factors play a crucial role in different physiological processes, including cell proliferation, invasion, metabolism, apoptosis, and stem cell differentiation. Recent studies have reported that polymorphisms of growth factor-related genes are involved in the pathogenesis of ONFH. Tissue and genetic engineering are attractive strategies for treating early-stage ONFH. In this review, we summarized dysregulated growth factor-related genes and their role in the occurrence and development of ONFH. In addition, we discussed their potential clinical applications in tissue and genetic engineering for the treatment of ONFH.

Interplay between Inflammation and Pathological Bone Resorption: Insights into Recent Mechanisms and Pathways in Related Diseases for Future Perspectives

Bone is a mineralized and elastic connective tissue that provides fundamental functions in the human body, including mechanical support to the muscles and joints, protection of vital organs and storage of minerals. Bone is a metabolically active organ that undergoes continuous remodeling processes to maintain its architecture, shape, and function throughout life. One of the most important medical discoveries of recent decades has been that the immune system is involved in bone remodeling. Indeed, chronic inflammation has been recognized as the most significant factor influencing bone homeostasis, causing a shift in the bone remodeling process toward pathological bone resorption. Bone osteolytic diseases typified by excessive bone resorption account for one of the greatest causes of disability worldwide, with significant economic and public health burdens. From this perspective, we discuss the recent findings and discoveries highlighting the cellular and molecular mechanisms that regulate this process in the bone microenvironment, in addition to the current therapeutic strategies for the treatment of osteolytic bone diseases.

Inhibitory Effects of Combined Bone Morphogenetic Protein 2, Vascular Endothelial Growth Factor, and Basic Fibroblast Growth Factor on Osteoclast Differentiation and Activity

Bone morphogenetic protein 2 (BMP-2), vascular endothelial growth factor (VEGF), and basic fibroblast growth factors (bFGF) are important regulators of bone development and bone remodeling involving the coordination of osteoblast-mediated bone formation and osteoclast-mediated bone resorption. The synergistic promotions of these growth factors on osteogenesis in the appropriate combination have been confirmed by a lot of studies, but the effect of this combined application on osteoclastogenesis still remains ambiguous. On the basis of comparing the osteoclastic potentials under stimulation of BMP-2, VEGF, or bFGF alone, this study focused on their combined effects on the differentiation and activity of osteoclasts. Our results showed that osteoclastogenesis was enhanced to some extent under the stimulation of BMP-2, VEGF, or bFGF alone, and the potential of these three growth factors to stimulate osteoclastogenesis was VEGF > BMP-2 > bFGF. However, the treatment with the combination of BMP-2 (50 ng/mL), VEGF (1 ng/mL), and bFGF (10 ng/mL), the most suitable dose combination for osteogenesis optimized in our previous study, weakened osteoclast differentiation confirmed by smaller tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells, lower TRAP activity, and lower expression of dendritic cell-specific transmembrane protein, an important molecule regulating osteoclast fusion. Moreover, BMP-2, VEGF, and bFGF in combination also moderately inhibited the bone-resorbing activity of mature osteoclasts by suppressing the expression of osteoclast-specific genes cathepsin K, and matrix metalloproteinase-9. The underlying molecular mechanisms involved the suppression of the receptor activator of nuclear factor-κB ligand-induced c-Fos levels and the activation of nuclear factor of activated T cells c1, two major transcription factors in osteoclast differentiation. Taken together, our study showed that the combination of BMP-2 (50 ng/mL), VEGF (1 ng/mL), and bFGF (10 ng/mL) promoted osteoblastogenesis but inhibited osteoclastogenesis. Thus, the simultaneous use of BMP-2 (50 ng/mL), VEGF (1 ng/mL), and bFGF (10 ng/mL) in an appropriate combination might improve efficacious bone regeneration in a clinical setting. Impact statement Few studies have addressed the combined effects of multiple growth factors on osteoclasts. This study demonstrated that the simultaneous use of bone morphogenetic protein 2 (BMP-2; 50 ng/mL), vascular endothelial growth factor (VEGF; 1 ng/mL), and basic fibroblast growth factors (bFGF; 10 ng/mL), the most suitable dose combination for osteogenesis optimized in our previous study, showed inhibitory effects on the differentiation and activity of osteoclasts. Our results suggest that the growth factor signaling pathways in osteoclasts may interact with each other. Furthermore, this study could provide new insights into the optimal application of BMP-2, VEGF, and bFGF for bone repair and regeneration.

Targeting thymidine phosphorylase as a potential therapy for bone loss associated with periprosthetic osteolysis

Macrophages are generally thought to play a key role in the pathogenesis of aseptic loosening through initiating periprosthetic inflammation and pathological bone resorption. The aim of this study was to identify macrophage-derived factors that promote osteoclast differentiation and periprosthetic bone destruction. To achieve this, we examined the effects of 12 macrophage-derived factors that were identified by RNA-seq analysis of stimulated macrophages on osteoclast differentiation. Surprisingly, thymidine phosphorylase (TYMP) was found to trigger significant number of osteoclasts that exhibited resorbing activities on dentine slices. Functionally, TYMP knockdown reduced the number of osteoclasts in macrophages that had been stimulated with polyethylene debris. TYMP were detected in serum and synovial tissues of patients that had been diagnosed with aseptic loosening. Moreover, the administration of TYMP onto calvariae of mice induced pathological bone resorption that was accompanied by an excessive infiltration of inflammatory cells and osteoclasts. The RNA-seq for TYMP-induced-osteoclasts was then performed in an effort to understand action mode of TYMP. TYMP stimulation appeared to activate the tyrosine kinase FYN signaling associated with osteoclast formation. Oral administration of saracatinib, a FYN kinase inhibitor, significantly suppressed formation of bone osteolytic lesions in a polyethylene debris-induced osteolysis model. Our findings highlight a novel molecular target for therapeutic intervention in periprosthetic osteolysis.

Osteomorphs as a tool for personalized medicine

McDonald and colleagues reported osteoclast-related dynamic mechanisms that lead, by fission, to osteomorphs; motile, fusion-competent cells capable of forming bone-resorbing osteoclasts. scRNA-seq analyses revealed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and might be implicated in rare and common bone diseases in humans.

[Research progress on mechanism of myokines regulating bone tissue cells]

Objective

To review the effects and mechanisms of various myokines secreted by skeletal muscle on various bone tissue cells.

Methods

Literature related to myokines and their regulation of bone tissue cells was reviewed and analyzed comprehensively in recent years.

Results

Bone and skeletal muscle are important members of the motor system, and they are closely related in anatomy, genetics, and physiopathology. In recent years, it has been found that skeletal muscle can secrete a variety of myokines to regulate bone marrow mesenchymal stem cells, osteoblasts, osteoclasts, and bone cells; these factors mutual crosstalk between myoskeletal unit, contact each other and influence each other, forming a complex myoskeletal micro-environment, and to some extent, it has a positive impact on bone repair and reconstruction.

Conclusion

Myokines are potential targets for the dynamic balance of bone tissue cells. In-depth study of its mechanism is helpful to the prevention and treatment of myoskeletal diseases.

Autophagy inhibitors 3-MA and LY294002 repress osteoclastogenesis and titanium particle-stimulated osteolysis

Aseptic loosening caused by peri-implant osteolysis (PIO) is a common complication after joint replacement, and there is still no better treatment than revision surgery. The wear particle-induced inflammation response, especially subsequent osteoclastic bone resorption, is responsible for PIO. As the importance of wear particles in inducing autophagy in cells around the prosthesis in PIO has been discovered, this might be a central process underlying aseptic loosening. However, the role of autophagy induced by wear particles in osteoclastogenesis during PIO remains unclear. In this study, we investigated the role of autophagy in osteoclastogenesis and verified it in a mouse calvarial osteolysis model. We found that osteoclasts were increased in the interface membranes of patients with aseptic loosening. In vitro, knocking down the Atg5 gene or using autophagy inhibitors (3-MA, LY294002) to inhibit autophagy was found to repress osteoclastogenesis and decrease expression of the osteoclast-related genes TRAP, cathepsin K, and matrix metalloprotein 9 (MMP-9) with or without titanium (Ti) particles. In vivo, 3-MA and LY294002 repressed Ti particle-stimulated osteolysis and osteoclastogenesis and reduced expression of the pro-inflammatory factors TNF-α, IL-1β, and IL-6. Our results suggest that 3-MA and LY294002 might be the potential medicines to prevent and treat PIO and aseptic loosening.

The cytokine interleukin-11 crucially links bone formation, remodeling and resorption

Bone development is a complex process that requires the activity of several different signaling pathways and cell types. It involves the coordinated action of osteoclasts (cells that are capable of resorbing bone), osteoblasts (cells that are able to form bone), osteocytes (cells that form a syncytial network within the bone), skeletal muscle cells and the bone marrow. In recent years, the cytokine interleukin-11 (IL-11), a member of the IL-6 family of cytokines, has emerged as an important regulatory protein for bone formation, remodeling and resorption. Furthermore, coding missense mutations in the IL11RA gene, which encodes the IL-11 receptor (IL-11R), have recently been linked to craniosynostosis, a human disease in which the sutures that line the head bones close prematurely. This review summarizes current knowledge about IL-11 and highlights its role in bone development and homeostasis. It further discusses the specificity and redundancy provided by the other members of the IL-6 cytokine family and how they facilitate signaling and cross-talk between skeletal muscle cells, bone cells and the bone marrow. We describe their actions in physiological and in pathological states and discuss how this knowledge could be translated into therapy.

Small extracellular vesicles deliver osteolytic effectors and mediate cancer-induced osteolysis in bone metastatic niche

Extracellular vesicles (EVs) play critical roles in regulating bone metastatic microenvironment through mediating intercellular crosstalks. However, little is known about the contribution of EVs derived from cancer cells to the vicious cycle of bone metastasis. Here, we report a direct regulatory mode between tumour cells and osteoclasts in metastatic niche of prostate cancer via vesicular miRNAs transfer. Combined analysis of miRNAs profiles both in tumour‐derived small EVs (sEVs) and osteoclasts identified miR‐152‐3p as a potential osteolytic molecule. sEVs were enriched in miR‐152‐3p, which targets osteoclastogenic regulator MAFB. Blocking miR‐152‐3p in sEVs upregulated the expression of MAFB and impaired osteoclastogenesis in vitro. In vivo experiments of xenograft mouse model found that blocking of miR‐152‐3p in sEVs significantly slowed down the loss of trabecular architecture, while systemic inhibition of miR‐152‐3p using antagomir‐152‐3p reduced the osteolytic lesions of cortical bone while preserving basic trabecular architecture. Our findings suggest that miR‐152‐3p carried by prostate cancer‐derived sEVs deliver osteolytic signals from tumour cells to osteoclasts, facilitating osteolytic progression in bone metastasis.

LncRNA-mRNA expression profiles and functional networks in osteoclast differentiation

Human osteoclasts are differentiated from CD14⁺ monocytes and are responsible for bone resorption. Long non‐coding RNAs (lncRNAs) have been proved to be significantly involved in multiple biologic processes, especially in cell differentiation. However, the effect of lncRNAs in osteoclast differentiation is less appreciated. In our study, RNA sequencing (RNA‐seq) was used to identify the expression profiles of lncRNAs and mRNAs in osteoclast differentiation. The results demonstrated that expressions of 1117 lncRNAs and 296 mRNAs were significantly altered after osteoclast differentiation. qRT‐PCR assays were performed to confirm the expression profiles, and the results were almost consistent with the RNA‐seq data. GO and KEGG analyses were used to predict the functions of these differentially expressed mRNA and lncRNAs. The Path‐net analysis demonstrated that MAPK pathway, PI3K‐AKT pathway and NF‐kappa B pathway played important roles in osteoclast differentiation. Co‐expression networks and competing endogenous RNA networks indicated that ENSG00000257764.2‐miR‐106a‐5p‐TIMP2 may play a central role in osteoclast differentiation. Our study provides a foundation to further understand the role and underlying mechanism of lncRNAs in osteoclast differentiation, in which many of them could be potential targets for bone metabolic disease.

QKI deficiency leads to osteoporosis by promoting RANKL-induced osteoclastogenesis and disrupting bone metabolism

Quaking (QKI), an RNA-binding protein, has been reported to exhibit numerous biological functions, such as mRNA regulation, cancer suppression, and anti-inflammation. However, little known about the effects of QKI on bone metabolism. In this study, we used a monocyte/macrophage-specific QKI knockout transgenic mouse model to investigate the effects of QKI deficiency on receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis. The loss of QKI promoted the formation of multinucleated tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts (OCs) from bone marrow macrophages, and upregulated the expression of OC-specific markers, including TRAP (Acp5) and cathepsin K (Ctsk). The pro-osteoclastogenesis effect of QKI deficiency was achieved by amplifying the signaling cascades of the NF-κB and mitogen-activated protein kinase (MAPK) pathways; then, signaling upregulated the activation of nuclear factor of activated T cells c1 (NFATc1), which is considered to be the core transcription factor that regulates OC differentiation. In addition, QKI deficiency could inhibit osteoblast (OB) formation through the inflammatory microenvironment. Taken together, our data suggest that QKI deficiency promoted OC differentiation and disrupted bone metabolic balance, and eventually led to osteopenia under physiological conditions and aggravated the degree of osteoporosis under pathological conditions.

Expression of PTHrP and RANKL in acquired middle ear cholesteatoma epithelium

Background: Regarded as the most important clinical characteristic of middle ear cholesteatoma, the exact mechanism of bone resorption in cholesteatoma still remains unknown.Objectives: To investigate protein expression of PTHrP and RANKL in acquired middle ear cholesteatoma epithelium and analyze their functional roles in the etiopathogenesis of bone resorption in middle ear cholesteatoma.Material and methods: A total of 22 patients who underwent surgical treatment for middle ear cholesteatoma were recruited in the study. Protein expression of PTHrP and RANKL in middle ear cholesteatoma and normal postauricular skin was investigated by immunohistochemical staining. Correlations between bone resorption degree and expression of PTHrP and RANKL were also analyzed.Results: Protein expression of PTHrP and RANKL in cholesteatoma epithelium significantly increased when compared with normal postauricular skin epithelium. In cholesteatoma epithelium, a significantly positive association was observed between PTHrP and RANKL expression. Meanwhile, obviously positive correlations between protein expression of PTHrP and RANKL and bone resorption degree were discovered.Conclusions and significance: The increased protein expression of PTHrP and RANKL in cholesteatoma epithelium, and their associations with the degree of bone resorption, revealing that PTHrP might promote bone resorption process in middle ear cholesteatoma through RANKL signaling pathway.

Immune checkpoint inhibitor-induced inflammatory arthritis as a model of autoimmune arthritis

The development of inflammatory arthritis in patients receiving immune checkpoint inhibitor therapy is increasingly recognized due to the growing use of these drugs for the treatment of cancer. This represents an important opportunity not only to define the mechanisms responsible for the development of this immune-related adverse event and to ultimately predict or prevent its development, but also to provide a unique window into early events in the development of inflammatory arthritis. Knowledge gained through the study of this patient population, for which the inciting event is known, could shed light into the pathogenesis of autoimmune arthritis. This review will highlight the clinical and immunologic features of these entities to define common elements for future study.

Interaction of Cutibacterium acnes with human bone marrow derived mesenchymal stem cells: a step toward understanding bone implant- associated infection development

Crosstalk between mesenchymal stem cells (MSCs) and bacteria plays an important role in regulating the regenerative capacities of MSCs, fighting infections, modulating immune responses and maintaining tissue homeostasis. Commensal Cutibacterium acnes (C. acnes) bacterium becomes an opportunistic pathogen causing implant-associated infections. Herein, we examined MSCs/C. acnes interaction and analysed the subsequent bacteria and MSCs behaviours following infection. Human bone marrow derived MSCs were infected by two clinical and one laboratory C. acnes strains. Following 3h of interaction, all bacterial strains were able to invade MSCs. Viable intracellular bacteria acquired virulence factors by increasing biofilm formation and/or by affecting macrophage phagocytosis. Although the direct and indirect (through neutrophil stimulation) antibacterial effects of the MSCs secretome were not enhanced following C. acnes infection, ELISA analysis revealed that C. acnes clinical strains are able to license MSCs to become immunosuppressive cell-like by increasing the secretion of IL-6, IL-8, PGE-2, VEGF, TGF-β and HGF. Overall, these results showed a direct impact of C. acnes on bone marrow derived MSCs, providing new insights into the development of C. acnes during implant-associated infections. STATEMENT OF SIGNIFICANCE: The originality of this work relies on the study of relationship between human bone marrow derived mesenchymal stem cells (MSCs) phenotype and C. acnes clinical strains virulence following cell infection. Our major results showed that C. acnes are able to invade MSCs, inducing a transition of commensal to an opportunistic pathogen behaviour. Although the direct and indirect antibacterial effects were not enhanced following C. acnes infection, secretome analysis revealed that C. acnes clinical strains were able to license MSCs to become immunosuppressive and anti-fibrotic cell-like. These results showed a direct impact of C. acnes on bone marrow derived MSCs, providing new insights into the development of C. acnes during associated implant infections.

Extracellular Vesicles From Osteotropic Breast Cancer Cells Affect Bone Resident Cells

Extracellular vesicles (EVs) are emerging as mediators of a range of pathological processes, including cancer. However, their role in bone metastases has been poorly explored. We investigated EV-mediated effects of osteotropic breast cancer cells (MDA-MB-231) on bone resident cells and endothelial cells. Pretreatment of osteoblasts with conditioned medium (CM) of MDA-MB-231 (MDA) cells promoted pro-osteoclastogenic and pro-angiogenic effects by osteoblast EVs (OB-EVs), as well as an increase of RANKL-positive OB-EVs. Moreover, when treating osteoblasts with MDA-EVs, we observed a reduction of their number, metabolic activity, and alkaline phosphatase (Alp) activity. MDA-EVs also reduced transcription of Cyclin D1 and of the osteoblast-differentiating genes, while enhancing the expression of the pro-osteoclastogenic factors Rankl, Lcn2, Il1b, and Il6. Interestingly, a cytokine array on CM from osteoblasts treated with MDA-EVs showed an increase of the cytokines CCL3, CXCL2, Reg3G, and VEGF, while OPG and WISP1 were downregulated. MDA-EVs contained mRNAs of genes involved in bone metabolism, as well as cytokines, including PDGF-BB, CCL3, CCL27, VEGF, and Angiopoietin 2. In line with this profile, MDA-EVs increased osteoclastogenesis and in vivo angiogenesis. Finally, intraperitoneal injection of MDA-EVs in mice revealed their ability to reach the bone microenvironment and be integrated by osteoblasts and osteoclasts. In conclusion, we showed a role for osteoblast-derived EVs and tumor cell-derived EVs in the deregulation of bone and endothelial cell physiology, thus fueling the vicious cycle induced by bone tumors. © 2019 American Society for Bone and Mineral Research.

Characterization and immunogenicity of bone marrow-derived mesenchymal stem cells under osteoporotic conditions

Mesenchymal stem cells (MSCs) are characterized by their multilineage potential and low immunogenicity. However, the properties of MSCs under pathological conditions are unclear. The current study investigated the differentiation potential and immunological characteristics of bone marrow-derived MSCs from ovariectomized-osteoporotic rats (OP-BMSCs). Although the expression of cell morphology- and stemness-related surface markers was similar between OP-BMSCs and BMSCs from healthy rats (H-BMSCs), the proliferation rate was significantly decreased compared with that of H-BMSCs. Regarding multilineage potential, osteogenesis and chondrogenesis abilities of OP-BMSCs decreased, but the adipogenesis ability was significantly enhanced compared with that of H-BMSCs. As expected, decreased osteogenesis following osteogenic induction resulted in reduced expression of β-catenin, osteocalcin, and runt-related transcription factor 2 in OP-BMSCs. Remarkably, the expression of the co-stimulatory proteins CD40 and CD80 was significantly higher, whereas the expression of the negative co-stimulatory molecule programmed cell death ligand 1 was significantly lower in the OP-BMSCs than that in H-BMSCs. Consequently, H-BMSCs inhibited the proliferation and secretion of inflammatory cytokines from anti-CD3 antibody-activated T cells, whereas OP-BMSCs did not. These results indicate that decreased osteogenesis and increased immunogenicity of OP-BMSCs contribute to bone loss in osteoporosis.

A simple and robust reporter gene assay for measuring the bioactivity of anti-RANKL therapeutic antibodies

RANKL (receptor activator of nuclear factor κB ligand) plays a key role in the differentiation, activation and survival of osteoclasts. Denosumab, which targets RANKL, is approved for osteoporosis or bone loss that has a high risk for fracture and bone metastases from solid tumors. Bioactivity determination is essential for the safety and efficacy of therapeutic antibodies. At present, the mechanism of action (MOA) based bioassay for anti-RANKL monoclonal antibodies (mAbs) is the measurement of tartrate resistant acid phosphatase (TRAP) activity, which takes about five days and has complex operation and relatively high variation. In this study, we developed a reporter gene assay (RGA) based on a RAW264.7 cell line stably expressing luciferase reporter under the control of nuclear factor-κB (NF-κB) response elements. After optimizing the key parameters, the validation results based on ICH-Q2 not only show superior specificity, precision, linearity, accuracy and passage stability, but also a short duration and simple operation. These results demonstrate the RGA based on the RANKL-RANK-NF-κB pathway can be an excellent alternative for measuring the bioactivity of anti-RANKL mAbs.

Periprosthetic Osteolysis: Mechanisms, Prevention and Treatment

Clinical studies, as well as in vitro and in vivo experiments have demonstrated that byproducts from joint replacements induce an inflammatory reaction that can result in periprosthetic osteolysis (PPOL) and aseptic loosening (AL). Particle-stimulated macrophages and other cells release cytokines, chemokines, and other pro-inflammatory substances that perpetuate chronic inflammation, induce osteoclastic bone resorption and suppress bone formation. Differentiation, maturation, activation, and survival of osteoclasts at the bone-implant interface are under the control of the receptor activator of nuclear factor kappa-Β ligand (RANKL)-dependent pathways, and the transcription factors like nuclear factor κB (NF-κB) and activator protein-1 (AP-1). Mechanical factors such as prosthetic micromotion and oscillations in fluid pressures also contribute to PPOL. The treatment for progressive PPOL is only surgical. In order to mitigate ongoing loss of host bone, a number of non-operative approaches have been proposed. However, except for the use of bisphosphonates in selected cases, none are evidence based. To date, the most successful and effective approach to preventing PPOL is usage of wear-resistant bearing couples in combination with advanced implant designs, reducing the load of metallic and polymer particles. These innovations have significantly decreased the revision rate due to AL and PPOL in the last decade.

MicroRNA-29a Counteracts Glucocorticoid Induction of Bone Loss through Repressing TNFSF13b Modulation of Osteoclastogenesis

Glucocorticoid excess escalates osteoclastic resorption, accelerating bone mass loss and microarchitecture damage, which ramps up osteoporosis development. MicroRNA-29a (miR-29a) regulates osteoblast and chondrocyte function; however, the action of miR-29a to osteoclastic activity in the glucocorticoid-induced osteoporotic bone remains elusive. In this study, we showed that transgenic mice overexpressing an miR-29a precursor driven by phosphoglycerate kinase exhibited a minor response to glucocorticoid-mediated bone mineral density loss, cortical bone porosity and overproduction of serum resorption markers C-teleopeptide of type I collagen and tartrate-resistant acid phosphatase 5b levels. miR-29a overexpression compromised trabecular bone erosion and excessive osteoclast number histopathology in glucocorticoid-treated skeletal tissue. Ex vivo, the glucocorticoid-provoked osteoblast formation and osteoclastogenic markers (NFATc1, MMP9, V-ATPase, carbonic anhydrase II and cathepsin K) along with F-actin ring development and pit formation of primary bone-marrow macrophages were downregulated in miR-29a transgenic mice. Mechanistically, tumor necrosis factor superfamily member 13b (TNFSF13b) participated in the glucocorticoid-induced osteoclast formation. miR-29a decreased the suppressor of cytokine signaling 2 (SOCS2) enrichment in the TNFSF13b promoter and downregulated the cytokine production. In vitro, forced miR-29a expression and SOCS2 knockdown attenuated the glucocorticoid-induced TNFSF13b expression in osteoblasts. miR-29a wards off glucocorticoid-mediated excessive bone resorption by repressing the TNFSF13b modulation of osteoclastic activity. This study sheds new light onto the immune-regulatory actions of miR-29a protection against glucocorticoid-mediated osteoporosis.

IGF-1 polymorphisms modulate the susceptibility to osteonecrosis of the femoral head among Chinese Han population

The study was performed to investigate the genetic associations of IGF-1 polymorphisms rs35767, rs5742714, and rs972936 with susceptibility to osteonecrosis of the femoral head (ONFH) among Chinese Han population.Totally, 101 ONFH patients and 128 healthy controls were enrolled. Hardy-Weinberg equilibrium (HWE) was detected with chi-square test in control group. Odds ratios (ORs) with 95% confidence intervals (95% CIs) were calculated to estimate the relationship between IGF-1 polymorphisms and ONFH risk. Besides, hyplotype analysis was performed to examine linkage disequilibrium between the studied polymorphisms.Genotype AA and allele A of polymorphism rs35767 were more frequent in control group, and offered protection for ONFH onset (AA: OR = 0.382, 95% CI = 0.158-0.923; A: OR = 0.650, 95% CI = 0.442-0.956). Furthermore, the negative relationship was also observed between ONFH risk and polymorphism rs5742714 under the comparisons CG vs CC, and G vs C (OR = 0.395, 95%CI = 0.199-0.787; OR = 0.346, 95%CI = 0.191-0.627). While the polymorphism rs972936 significantly enhanced the disease risk (CT vs CC: OR = 2.434, 95% CI = 1.184-5.003; TT vs CC: OR = 2.497, 95% CI = 1.040-5.990). Furthermore, haplotype analysis demonstrated that C-T (rs5742714-rs972936) could increase ONFH risk (OR = 2.177, 95% CI = 1.444-3.283), while G-T might be a protective factor for ONFH (OR = 0.472, 95% CI = 0.254-0.878).IGF-1 polymorphisms rs35767, rs5742714, and rs972936 show significant association with ONFH risk.

IL-11 is essential in promoting osteolysis in breast cancer bone metastasis via RANKL-independent activation of osteoclastogenesis

A variety of osteolytic factors have been identified from breast cancer cells leading to osteolysis, but less is known about which factor plays an essential role in the initiation process prior to the overt vicious osteolytic cycle. Here, we present in vitro and in vivo evidences to clarify the role of interleukin-11 (IL-11) as an essential contributor to breast cancer bone metastasis mediated osteolysis. Animal studies showed that bone specific metastatic BoM-1833 cells induce earlier onset of osteolysis and faster tumor growth compared with MCF7 and parental MDA-MB-231 cells in BALB/c-nu/nu nude mice. IL-11 was further screened and identified as the indispensable factor secreted by BoM-1833 cells inducing osteoclastogenesis independently of receptor activator of nuclear factor κB ligand (RANKL). Mechanistic investigation revealed that the JAK1/STAT3 signaling pathway as a downstream effector of IL-11, STAT3 activation further induces the expression of c-Myc, a necessary factor required for osteoclastogenesis. By inhibiting STAT3 phosphorylation, AG-490 was shown effective in reducing osteolysis and tumor growth in the metastatic niche. Overall, our results revealed the essential role and the underlying molecular mechanism of IL-11 in breast cancer bone metastasis mediated osteolysis. STAT3 targeting through AG-490 is a potential therapeutic strategy for mitigating osteolysis and tumor growth of bone metastatic breast cancer.

TG-interacting factor 1 (Tgif1)-deficiency attenuates bone remodeling and blunts the anabolic response to parathyroid hormone

Osteoporosis is caused by increased bone resorption and decreased bone formation. Intermittent administration of a fragment of Parathyroid hormone (PTH) activates osteoblast-mediated bone formation and is used in patients with severe osteoporosis. However, the mechanisms by which PTH elicits its anabolic effect are not fully elucidated. Here we show that the absence of the homeodomain protein TG-interacting factor 1 (Tgif1) impairs osteoblast differentiation and activity, leading to a reduced bone formation. Deletion of Tgif1 in osteoblasts and osteocytes decreases bone resorption due to an increased secretion of Semaphorin 3E (Sema3E), an osteoclast-inhibiting factor. Tgif1 is a PTH target gene and PTH treatment failed to increase bone formation and bone mass in Tgif1-deficient mice. Thus, our study identifies Tgif1 as a novel regulator of bone remodeling and an essential component of the PTH anabolic action. These insights contribute to a better understanding of bone metabolism and the anabolic function of PTH.

Myelofibrosis osteoclasts are clonal and functionally impaired

Bone marrow (BM) sclerosis is commonly found in patients with late-stage myelofibrosis (MF). Because osteoclasts (OCs) and osteoblasts play a key role in bone remodeling, and MF monocytes, the OC precursors, are derived from the neoplastic clone, we wondered whether decreased OC numbers or impairment in their osteolytic function affects the development of osteosclerosis. Analysis of BM biopsies from 50 MF patients showed increased numbers of multinucleated tartrate-resistant acid phosphatase (TRAP)/cathepsin K⁺ OCs expressing phosphorylated Janus kinase 2 (JAK2). Randomly microdissected TRAP⁺ OCs from 16 MF patients harbored JAK2 or calreticulin (CALR) mutations, confirming MF OCs are clonal. To study OC function, CD14⁺ monocytes from MF patients and healthy individuals were cultured and differentiated into OCs. Unlike normal OCs, MF OCs appeared small and round, with few protrusions, and carried the mutations and chromosomal abnormalities of neoplastic clones. In addition, MF OCs lacked F-actin-rich ring-like structures and had fewer nuclei and reduced colocalization signals, compatible with decreased fusion events, and their mineral resorption capacity was significantly reduced, indicating impaired osteolytic function. Taken together, our data suggest that, although the numbers of MF OCs are increased, their impaired osteolytic activity distorts bone remodeling and contributes to the induction of osteosclerosis.

Sera of patients with axial spondyloarthritis (axSpA) enhance osteoclastogenic potential of monocytes isolated from healthy individuals

Background

Axial spondyloarthritis (axSpA) is characterized by significant bone loss caused by dysregulation of physiological bone turnover, possibly resulting from intensified differentiation of osteoclasts. The aim of this study was to reevaluate the levels of osteoclastogenesis-mediating factors: soluble RANKL, M-CSF, OPG and other cytokines in sera of untreated, with sDMARDs and/or bDMARDs, axSpA patients and to test whether these sera influence differentiation of healthy monocytes towards osteoclast lineage.

Methods

Bone remodeling molecules (RANKL, M-CSF, OPG, IL-6, OSM, IL-17A, TGFβ, and TNFα) were evaluated in 27 patients with axSpA and 23 age and sex-matched controls. Disease activity (BASDAI, ASDAS) and inflammatory markers (ESR, CRP) were assessed. Monocytes obtained from healthy individuals were cultured in vitro in presence of sera from 11 randomly chosen axSpA patients and 10 controls, with addition of exogenous M-CSF and/or RANKL or without. Osteoclastic differentiation was assessed analyzing osteoclast markers (cathepsin K and RANK at mRNA level) and with osteoclast-specific staining.

Results

axSpA patients’ sera levels of soluble RANKL were significantly lower and M-CSF, IL-6, OSM, IL-17A and TNFα significantly higher in comparison to controls, whereas of OPG and TGFβ were comparable in both groups. Numbers of generated in vitro osteoclasts and cathepsin K mRNA levels did not differ between cultures supplemented with sera of healthy and axSpA patients, both in the absence and presence of M-CSF. Instead, addition of exogenous RANKL boosted osteoclastogenesis, which was significantly higher in cultures with axSpA sera. Furthermore, sera from axSpA patients induced substantially higher levels of RANK mRNA, independently of M-CSF and RANKL stimulation.

Conclusion

We show that, paradoxically, serum levels of soluble RANKL observed in axSpA are in fact significantly lower in comparison to healthy blood donors. Our results indicate that sera of axSpA patients - in contrary to healthy subjects - contain circulating, soluble factors (presumably IL-6, OSM, IL-17A, TNFα and others) able to stimulate healthy monocytes responsiveness to even relative low RANKL serum levels, by inducing high RANK mRNA expression and - as a net effect - boosting their osteoclastogenic potential. We suggest also that locally produced RANKL in axSpA may induce overactive osteoclasts from their precursors.

Differential Effect of Cobalt and Chromium Ions as Well as CoCr Particles on the Expression of Osteogenic Markers and Osteoblast Function

The balance of bone formation and resorption is the result of a regulated crosstalk between osteoblasts, osteoclasts, and osteocytes. Inflammation, mechanical load, and external stimuli modulate this system. Exposure of bone cells to metal ions or wear particles are thought to cause osteolysis via activation of osteoclasts and inhibition of osteoblast activity. Co²⁺ ions have been shown to impair osteoblast function and the expression of the three transforming growth factor (TGF)-β isoforms. The current study was performed to analyze how Co²⁺ and Cr³⁺ influence the expression, proliferation, and migration profile of osteoblast-like cells. The influence of Co²⁺, Cr³⁺, and CoCr particles on gene expression was analyzed using an osteogenesis PCR Array. The expression of different members of the TGF-β signaling cascade were down-regulated by Co²⁺, as well as several TGF-β regulated collagens, however, Cr³⁺ had no effect. CoCr particles partially affected similar genes as the Co²⁺treatment. Total collagen production of Co²⁺ treated osteoblasts was reduced, which can be explained by the reduced expression levels of various collagens. While proliferation of MG63 cells appears unaffected by Co²⁺, the migration capacity was impaired. Our data may improve the knowledge of changes in gene expression patterns, and the proliferation and migration effects caused by artificial materials.

Involvement of Osteocytes in the Action of Pasteurella multocida Toxin

Pasteurella multocida toxin (PMT) causes progressive atrophic rhinitis with severe turbinate bone degradation in pigs. It has been reported that the toxin deamidates and activates heterotrimeric G proteins, resulting in increased differentiation of osteoclasts and blockade of osteoblast differentiation. So far, the action of PMT on osteocytes, which is the most abundant cell type in bone tissue, is not known. In MLO-Y4 osteocytes, PMT deamidated heterotrimeric G proteins, resulting in loss of osteocyte dendritic processes, stress fiber formation, cell spreading and activation of RhoC but not of RhoA. Moreover, the toxin caused processing of membrane-bound receptor activator of NF-κB ligand (RANKL) to release soluble RANKL and enhanced the secretion of osteoclastogenic TNF-α. In a co-culture model of osteocytes and bone marrow cells, PMT-induced osteoclastogenesis was largely increased as compared to the mono-culture model. The enhancement of osteoclastogenesis observed in the co-culture was blocked by sequestering RANKL with osteoprotegerin and by an antibody against TNF-α indicating involvement of release of the osteoclastogenic factors from osteocytes. Data support the crucial role of osteocytes in bone metabolism and osteoclastogenesis and identify osteocytes as important target cells of PMT in progressive atrophic rhinitis.

Autoimmunity in 2016

The number of peer-reviewed articles published during the 2016 solar year and retrieved using the "autoimmunity" key word remained stable while gaining a minimal edge among the immunology articles. Nonetheless, the quality of the publications has been rising significantly and, importantly, acquisitions have become available through scientific journals dedicated to immunology or autoimmunity. Major discoveries have been made in the fields of systemic lupus erythematosus, rheumatoid arthritis, autoimmunity of the central nervous system, vasculitis, and seronegative spondyloarthrithritides. Selected examples include the role of IL17-related genes and long noncoding RNAs in systemic lupus erythematosus or the effects of anti-pentraxin 3 (PTX3) in the treatment of this paradigmatic autoimmune condition. In the case of rheumatoid arthritis, there have been reports of the role of induced regulatory T cells (iTregs) or fibrocytes and T cell interactions with exciting implications. The large number of studies dealing with neuroimmunology pointed to Th17 cells, CD56(bright) NK cells, and low-level TLR2 ligands as involved in multiple sclerosis, along with a high salt intake or the micriobiome-derived Lipid 654. Lastly, we focused on the rare vasculitides to which numerous studies were devoted and suggested that unsuspected cell populations, including monocytes, mucosal-associated invariant T cells, and innate lymphoid cells, may be crucial to ANCA-associated manifestations. This brief and arbitrary discussion of the findings published in 2016 is representative of a promising background for developments that will enormously impact the work of laboratory scientists and physicians at an exponential rate.

Biological effects of metal degradation in hip arthroplasties

Metals and metal alloys are the most used materials in orthopedic implants. The focus is on total hip arthroplasty (THA) that, though well tolerated, may be associated with local and remote adverse effects in the medium-long term. This review aims to summarize data on the biological consequences of the metal implant degradation that have been attributed predominantly to metal-on-metal (MoM) THA. Local responses to metals consist of a broad clinical spectrum ranging from small asymptomatic tissue lesions to severe destruction of bone and soft tissues, which are designated as metallosis, adverse reactions to metal debris (ARMD), aseptic lymphocytic vasculitis associated lesion (ALVAL), and pseudotumors. In addition, the dissemination of metal particles and ions throughout the body has been associated with systemic adverse effects, including organ toxicity, cancerogenesis, teratogenicity, and immunotoxicity. As proved by the multitude of studies in this field, metal degradation may increase safety issues associated with THA, especially with MoM hip systems. Data collection regarding local, systemic and long-term effects plays an essential role to better define any safety risks and to generate scientifically based recommendations.

Inflammation and bone mineral density: A Mendelian randomization study

Osteoporosis is a common age-related disorder leading to an increase in osteoporotic fractures and resulting in significant suffering and disability. Inflammation may contribute to osteoporosis, as it does to many other chronic diseases. We examined whether inflammation is etiologically relevant to osteoporosis, assessed from bone mineral density (BMD), as a new potential target of intervention, or whether it is a symptom/biomarker of osteoporosis. We obtained genetic predictors of inflammatory markers from genome-wide association studies and applied them to a large genome wide association study of BMD. Using two-sample Mendelian randomization, we obtained unconfounded estimates of the effect of high-sensitivity C-reactive protein (hsCRP) on BMD at the forearm, femoral neck, and lumbar spine. After removing potentially pleiotropic single nucleotide polymorphisms (SNPs) possibly acting via obesity-related traits, hsCRP, based on 16 SNPs from genes including CRP, was not associated with BMD. A causal relation of hsCRP with lower BMD was not evident in this study.

Kinase inhibitors of the IGF-1R as a potential therapeutic agent for rheumatoid arthritis

We have previously shown that the inhibition of connective tissue growth factor (CTGF) is a potential therapeutic strategy against rheumatoid arthritis (RA). CTGF consists of four distinct modules, including the insulin-like growth factor binding protein (IGFBP). In serum, insulin-like growth factors (IGFs) bind IGFBPs, interact with the IGF-1 receptor (IGF-1 R), and regulate anabolic effects and bone metabolism. We investigated the correlation between IGF-1 and the pathogenesis of RA, and the inhibitory effect on osteoclastogenesis and angiogenesis of the small molecular weight kinase inhibitor of the IGF-1 R, NVP-AEW541, against pathogenesis of RA in vitro. Cell proliferation was evaluated by cell count and immunoblotting. The expression of IGF-1 and IGF-1 R was evaluated by RT-PCR. Osteoclastogenesis was evaluated using tartrate-resistant acid phosphatase staining, a bone resorption assay, and osteoclast-specific enzyme production. Angiogenesis was evaluated by a tube formation assay using human umbilical vein endothelial cells (HUVECs). The proliferation of MH7A cells was found to be inhibited in the presence of NVP-AEW541, and the phosphorylation of extracellular signal-regulated kinase (ERK) and Akt was downregulated in MH7A cells. IGF-1 and IGF-1 R mRNA expression levels were upregulated during formation of M-colony stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL)-mediated osteoclast formation. Moreover, osteoclastogenesis was suppressed in the presence of NVP-AEW541. The formation of the tubular network was enhanced by IGF-1, and this effect was neutralized by NVP-ARE541. Our findings suggest that NVP-AEW541 may be utilized as a potential therapeutic agent in the treatment of RA.

Role of LIGHT in the pathogenesis of joint destruction in rheumatoid arthritis

AIM

To characterise the role of substitutes for receptor-activator nuclear factor kappa-B ligand (RANKL) in rheumatoid arthritis (RA) joint destruction.

METHODS

Synovial fluid (SF) macrophages isolated from the knee joint of RA patients were incubated with 25 ng/mL macrophage-colony stimulating factor (M-CSF) and 50 ng/mL LIGHT (lymphotoxin-like, exhibits inducible expression and competes with herpes simplex virus glycoprotein D for herpes virus entry mediator, a receptor expressed by T lymphocytes) in the presence and absence of 25 ng/mL RANKL and 100 ng/mL osteoprotegerin (OPG) on glass coverslips and dentine slices. Osteoclastogenesis was assessed by the formation of multinucleated cells (MNCs) expressing tartrate-resistant acid phosphatase (TRAP) on coverslips and the extent of lacunar resorption pit formation on dentine slices. The concentration of LIGHT in RA and osteoarthritis (OA) synovial fluid was measured by an enzyme-linked immunosorbent assay (ELISA) and the expression of LIGHT in RA and OA synovium was determined by immunohistochemistry using an indirect immunoperoxidase technique.

RESULTS

In cultures of RA SF macrophages treated with LIGHT and M-CSF, there was significant formation of TRAP + MNCs on coverslips and extensive lacunar resorption pit formation on dentine slices. SF-macrophage-osteoclast differentiation was not inhibited by the addition of OPG, a decoy receptor for RANKL. Resorption pits were smaller and less confluent than in RANKL-treated cultures but the overall percentage area of the dentine slice resorbed was comparable in LIGHT- and RANKL-treated cultures. LIGHT significantly stimulated RANKL-induced lacunar resorption compared with RA SF macrophages treated with either RANKL or LIGHT alone. LIGHT was strongly expressed by synovial lining cells, subintimal macrophages and endothelial cells in RA synovium and the concentration of LIGHT was much higher in RA compared with OA SF.

CONCLUSION

LIGHT is highly expressed in RA synovium and SF, stimulates RANKL-independent/dependent osteoclastogenesis from SF macrophages and may contribute to marginal erosion formation.

Pasteurella multocida Toxin Triggers RANKL-Independent Osteoclastogenesis

Bone remodeling is a continuous process to retain the structural integrity and function of the skeleton. A tight coupling is maintained between osteoclast-mediated resorption of old or damaged bones and osteoblast-mediated formation of new bones for bone homeostasis. While osteoblasts differentiate from mesenchymal stem cells, osteoclasts are hematopoietic in origin and derived from myeloid precursor cells. Osteoclast differentiation is driven by two cytokines, cytokine receptor activator of NF-κB ligand (RANKL), and macrophage colony-stimulating factor. Imbalances in the activity of osteoblasts and osteoclasts result in the development of bone disorders. Bacterially caused porcine atrophic rhinitis is characterized by a loss of nasal ventral conche bones and a distortion of the snout. While Bordetella bronchiseptica strains cause mild and reversible symptoms, infection of pigs with toxigenic Pasteurella multocida strains causes a severe and irreversible decay. The responsible virulence factor Pasteurella multocida toxin (PMT) contains a deamidase activity in its catalytical domain that constitutively activates specific heterotrimeric G proteins to induce downstream signaling cascades. While osteoblasts are inhibited by the toxin, osteoclasts are activated, thus skewing bone remodeling toward excessive bone degradation. Still, the mechanism by which PMT interferes with bone homeostasis, and the reason for this unusual target tissue is not yet well understood. Here, we show that PMT has the potential to differentiate bone marrow-derived macrophages into functional osteoclasts. This toxin-mediated differentiation process is independent of RANKL, a cytokine believed to be indispensable for triggering osteoclastogenesis, as addition of osteoprotegerin to PMT-treated macrophages does not show any effect on PMT-induced osteoclast formation. Although RANKL is not a prerequisite, toxin-primed macrophages show enhanced responsiveness to low concentrations of RANKL, suggesting that the PMT-generated microenvironment offers conditions where low concentrations of RANKL lead to an increase in the number of osteoclasts resulting in increased resorption. PMT-mediated release of the osteoclastogenic cytokines such as IL-6 and TNF-α, but not IL-1, supports the differentiation process. Although the production of cytokines and the subsequent activation of signaling cascades are necessary for PMT-mediated differentiation into osteoclasts, they are not sufficient and PMT-induced activation of G protein signaling is essential for efficient osteoclastogenesis.

NADPH oxidase gp91phox contributes to RANKL-induced osteoclast differentiation by upregulating NFATc1

Bone-marrow derived monocyte-macrophages (BMMs) differentiate into osteoclasts by M-CSF along subsequent RANKL stimulation possibly in collaboration with many other unknown cytokines released by pre- or mature osteoblasts. The differentiation process requires receptor activator of nuclear factor kappa-B ligand (RANKL)/RANK signaling and reactive oxygen species (ROS) such as superoxide anion (O₂^•−). Gp91^phox, a plasma membrane subunit of NADPH oxidase (Nox), is constitutively expressed in BMMs and plays a major role in superoxide anion production. In this study, we found that mice deficient in gp91^phox (gp91^phox−/−) showed defects in osteoclast differentiation. Femurs of these mice produced osteoclasts at about 70% of the levels seen in femurs from wild-type mice, and accordingly exhibited excessive bone density. This abnormal bone growth in the femurs of gp91^phox−/− mice resulted from impaired osteoclast differentiation. In addition, gp91^phox−/− mice were defective for RANKL-induced expression of nuclear factor of activated T cells c1 (NFATc1). However, H₂O₂ treatment compensated for gp91^phox deficiency in BMMs, almost completely rescuing osteoclast differentiation. Treating wild-type BMMs with antioxidants and superoxide inhibitors resulted in a differentiation defect resembling the phenotype of gp91^phox−/− BMMs. Therefore, our results demonstrate that gp91^phox-derived superoxide is important for promoting efficient osteoclast differentiation by inducing NFATc1 as a downstream signaling mediator of RANK.

Recombinant Human Endostatin Suppresses Mouse Osteoclast Formation by Inhibiting the NF-κB and MAPKs Signaling Pathways

Rheumatoid arthritis is an autoimmune disease characterized by synovial hyperplasia and progressive joint destruction. As reported previously, recombinant human endostatin (rhEndostatin) is associated with inhibition of joint bone destruction present in rat adjuvant-induced arthritis; however, the effect of rhEndostatin on bone destruction is not known. This study was designed to assess the inhibitory effect and mechanisms of rhEndostatin on formation and function of osteoclasts in vitro, and to gain insight into the mechanism underlying the inhibitory effect of bone destruction. Bone marrow-derived macrophages isolated from BALB/c mice were stimulated with receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor to establish osteoclast formation. Osteoclast formation was determined by TRAP staining. Cell viability of BMMs affected by rhEndostatin was determined using a MTT assay. Bone resorption was examined with a bone resorption pits assay. The expression of osteoclast-specific markers was analyzed using quantitative real-time PCR. The related signaling pathways were examined using a Luciferase reporter assay and western blot analysis. Indeed, rhEndostatin showed a significant reduction in the number of osteoclast-like cells and early-stage bone resorption. Moreover, molecular analysis demonstrated that rhEndostatin attenuated RANKL-induced NF-κB signaling by inhibiting the phosphorylation of IκBα and NF-κB p65 nuclear translocation. Furthermore, rhEndostatin significantly inhibited the activation of RANKL-dependent mitogen-activated protein kinases, such as ERK1/2, JNK, and p38. Hence, we demonstrated for the first time that preventing the formation and function of osteoclasts is an important anti-bone destruction mechanism of rhEndostatin, which might be useful in the prevention and treatment of bone destruction in RA.

Clinical joint inactivity predicts structural stability in patients with established rheumatoid arthritis

OBJECTIVES

Clinical joint activity is a strong predictor of joint damage in rheumatoid arthritis (RA), but progression of damage might increase despite clinical inactivity of the respective joint (silent progression). The aim of this study was to evaluate the prevalence of silent joint progression, but particularly on the patient level and to investigate the duration of clinical inactivity as a marker for non-progression on the joint level.

METHODS

279 patients with RA with any radiographic progression over an observational period of 3-5 years were included. We obtained radiographic and clinical data of 22 hand/finger joints over a period of at least 3 years. Prevalence of silent progression and associations of clinical joint activity and radiographic progression were evaluated.

RESULTS

120 (43.0%) of the patients showed radiographic progression in at least one of their joints without any signs of clinical activity in that respective joint. In only 7 (5.8%) patients, such silent joint progression would go undetected, as the remainder had other joints with clinical activity, either with (n=84; 70.0%) or without (n=29; 24.2%) accompanying radiographic progression. Also, the risk of silent progression decreases with duration of clinical activity.

CONCLUSIONS

Silent progression of a joint without accompanying apparent clinical activity in any other joint of a patient was very rare, and would therefore be most likely detected by the assessment of the patient. Thus, full clinical remission is an excellent marker of structural stability in patients with RA, and the maintenance of this state reduces the risk of progression even further.