RANK-Independent Osteoclast Formation and Bone Erosion in Inflammatory Arthritis

The miR-29-3p family suppresses inflammatory osteolysis

Osteoclasts are the cells primarily responsible for inflammation-induced bone loss, as is particularly seen in rheumatoid arthritis. Increasing evidence suggests that osteoclasts formed under homeostatic versus inflammatory conditions may differ in phenotype. While microRNA-29-3p family members (miR-29a-3p, miR-29b-3p, miR-29c-3p) promote the function of RANKL-induced osteoclasts, the role of miR-29-3p during inflammatory TNF-α-induced osteoclastogenesis is unknown. We used bulk RNA-seq, histology, qRT-PCR, reporter assays, and western blot analysis to examine bone marrow monocytic cell cultures and tissue from male mice in which the function of miR-29-3p family members was decreased by expression of a miR-29-3p tough decoy (TuD) competitive inhibitor in the myeloid lineage (LysM-cre). We found that RANKL-treated monocytic cells expressing the miR-29-3p TuD developed a hypercytokinemia/proinflammatory gene expression profile in vitro, which is associated with macrophages. These data support the concept that miR-29-3p suppresses macrophage lineage commitment and may have anti-inflammatory effects. In correlation, when miR-29-3p activity was decreased, TNF-α-induced osteoclast formation was accentuated in an in vivo model of localized osteolysis and in a cell-autonomous manner in vitro. Further, miR-29-3p targets mouse TNF receptor 1 (TNFR1/Tnfrsf1a), an evolutionarily conserved regulatory mechanism, which likely contributes to the increased TNF-α signaling sensitivity observed in the miR-29-3p decoy cells. Whereas our previous studies demonstrated that the miR-29-3p family promotes RANKL-induced bone resorption, the present work shows that miR-29-3p dampens TNF-α-induced osteoclastogenesis, indicating that miR-29-3p has pleiotropic effects in bone homeostasis and inflammatory osteolysis. Our data supports the concept that the knockdown of miR-29-3p activity could prime myeloid cells to respond to an inflammatory challenge and potentially shift lineage commitment toward macrophage, making the miR-29-3p family a potential therapeutic target for modulating inflammatory response.

Musculoskeletal crosstalk in chronic obstructive pulmonary disease and comorbidities: Emerging roles and therapeutic potentials

Chronic Obstructive Pulmonary Disease (COPD) is a multifaceted respiratory disorder characterized by progressive airflow limitation and systemic implications. It has become increasingly apparent that COPD exerts its influence far beyond the respiratory system, extending its impact to various organ systems. Among these, the musculoskeletal system emerges as a central player in both the pathogenesis and management of COPD and its associated comorbidities. Muscle dysfunction and osteoporosis are prevalent musculoskeletal disorders in COPD patients, leading to a substantial decline in exercise capacity and overall health. These manifestations are influenced by systemic inflammation, oxidative stress, and hormonal imbalances, all hallmarks of COPD. Recent research has uncovered an intricate interplay between COPD and musculoskeletal comorbidities, suggesting that muscle and bone tissues may cross-communicate through the release of signalling molecules, known as "myokines" and "osteokines". We explored this dynamic relationship, with a particular focus on the role of the immune system in mediating the cross-communication between muscle and bone in COPD. Moreover, we delved into existing and emerging therapeutic strategies for managing musculoskeletal disorders in COPD. It underscores the development of personalized treatment approaches that target both the respiratory and musculoskeletal aspects of COPD, offering the promise of improved well-being and quality of life for individuals grappling with this complex condition. This comprehensive review underscores the significance of recognizing the profound impact of COPD on the musculoskeletal system and its comorbidities. By unravelling the intricate connections between these systems and exploring innovative treatment avenues, we can aspire to enhance the overall care and outcomes for COPD patients, ultimately offering hope for improved health and well-being.

New Horizons: Translational Aspects of Osteomorphs

Osteomorphs are a newly described osteoclast lineage cell in mice, which are suggested to play a significant role in the maintenance of bone resorption. Preclinical investigations revealed that osteomorphs are generated through the fission of multinucleated bone-resorbing osteoclasts and can also re-fuse with existing osteoclasts. Modifications to RANKL signaling have been shown to alter cycles of fission and re-fusion of osteomorphs in mice. These novel findings were also shown to contribute to the rebound phenomenon after cessation of anti-RANKL therapy in mice. Moreover, the absence of osteomorph-specific genes in mice exhibits bone structural and quality phenotypes. Given these insights, it could be speculated that osteomorphs play a significant role in bone homeostasis, bone metabolic diseases, and response to therapeutics. In this review, we discuss these potential translational roles for osteomorphs. Importantly, we highlight the need for future preclinical and clinical studies to verify the presence of osteomorphs in humans and explore further the translational implications of this discovery.

Bub1 suppresses inflammatory arthritis-associated bone loss in mice through inhibition of TNFα-mediated osteoclastogenesis

Rheumatoid arthritis (RA) is an inflammatory autoimmune disease characterized by synovitis, bone and cartilage destruction, and increased fracture risk with bone loss. Although disease-modifying antirheumatic drugs have dramatically improved clinical outcomes, these therapies are not universally effective in all patients because of the heterogeneity of RA pathogenesis. Therefore, it is necessary to elucidate the molecular mechanisms underlying RA pathogenesis, including associated bone loss, in order to identify novel therapeutic targets. In this study, we found that Budding uninhibited by benzimidazoles 1 (BUB1) was highly expressed in RA patients' synovium and murine ankle tissue with arthritis. As CD45+CD11b+ myeloid cells are a Bub1 highly expressing population among synovial cells in mice, myeloid cell-specific Bub1 conditional knockout (Bub1ΔLysM) mice were generated. Bub1ΔLysM mice exhibited reduced femoral bone mineral density when compared with control (Ctrl) mice under K/BxN serum-transfer arthritis, with no significant differences in joint inflammation or bone erosion based on a semi-quantitative erosion score and histological analysis. Bone histomorphometry revealed that femoral bone mass of Bub1ΔLysM under arthritis was reduced by increased osteoclastic bone resorption. RNA-seq and subsequent Gene Set Enrichment Analysis demonstrated a significantly enriched nuclear factor-kappa B pathway among upregulated genes in receptor activator of nuclear factor kappa B ligand (RANKL)-stimulated bone marrow-derived macrophages (BMMs) obtained from Bub1ΔLysM mice. Indeed, osteoclastogenesis using BMMs derived from Bub1ΔLysM was enhanced by RANKL and tumor necrosis factor-α or RANKL and IL-1β treatment compared with Ctrl. Finally, osteoclastogenesis was increased by Bub1 inhibitor BAY1816032 treatment in BMMs derived from wildtype mice. These data suggest that Bub1 expressed in macrophages plays a protective role against inflammatory arthritis-associated bone loss through inhibition of inflammation-mediated osteoclastogenesis.

Osteoclast differentiation in rheumatoid arthritis

Osteoclasts, derived from the monocyte/macrophage line of bone marrow hematopoietic stem cell progenitors, are the sole bone-resorbing cells of the body. Conventional osteoclast differentiation requires macrophage colony-stimulating factor and receptor activator of nuclear factor kappa-B ligand (RANKL) signaling. Rheumatoid arthritis (RA) is the most prevalent systemic autoimmune disease and inflammatory arthritis characterized by bone destruction. Increased levels of proinflammatory cytokines, such as tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6), in the serum and joints, cause excessive bone destruction. We have recently reported that stimulation of human peripheral blood monocytes with TNF-α and IL-6 induces the differentiation of osteoclasts with bone resorption activity. This review presents the functional differences between representative osteoclasts, conventional RANKL-induced osteoclasts, and recently identified proinflammatory cytokine (TNF-α and IL-6)-induced osteoclasts in RA patients. We believe novel pathological osteoclasts associated with RA will be identified, and new therapeutic strategies will be developed to target these osteoclasts and prevent the progression of bone destruction.

Osteoimmunology: The Crosstalk between T Cells, B Cells, and Osteoclasts in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an ongoing inflammatory condition that affects the joints and can lead to severe damage to cartilage and bones, resulting in significant disability. This condition occurs when the immune system becomes overactive, causing osteoclasts, cells responsible for breaking down bone, to become more active than necessary, leading to bone breakdown. RA disrupts the equilibrium between osteoclasts and osteoblasts, resulting in serious complications such as localized bone erosion, weakened bones surrounding the joints, and even widespread osteoporosis. Antibodies against the receptor activator of nuclear factor-κB ligand (RANKL), a crucial stimulator of osteoclast differentiation, have shown great effectiveness both in laboratory settings and actual patient cases. Researchers are increasingly focusing on osteoclasts as significant contributors to bone erosion in RA. Given that RA involves an overactive immune system, T cells and B cells play a pivotal role by intensifying the immune response. The imbalance between Th17 cells and Treg cells, premature aging of T cells, and excessive production of antibodies by B cells not only exacerbate inflammation but also accelerate bone destruction. Understanding the connection between the immune system and osteoclasts is crucial for comprehending the impact of RA on bone health. By delving into the immune mechanisms that lead to joint damage, exploring the interactions between the immune system and osteoclasts, and investigating new biomarkers for RA, we can significantly improve early diagnosis, treatment, and prognosis of this condition.

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.

Yoga maintains Th17/Treg cell homeostasis and reduces the rate of T cell aging in rheumatoid arthritis: a randomized controlled trial

The pathogenesis of rheumatoid arthritis (RA) is characterized by a Th17/Treg cell imbalance. A pro-inflammatory cytokine milieu that promotes the continued proliferation of Th17 cells is related to the development of autoinflammation. In RA, T cells have several hallmarks of cellular aging, and they accumulate DNA damage, predisposing to the occurrence of mutations and epigenetic alterations. Since the onset, progression, and treatment response are influenced by a variety of external stressors and environmental factors, this study aimed to evaluate the impact of 8-week yoga practice on disease severity, T cell subsets, markers of T cell ageing and inflammation, epigenetic alterations and gene expression patterns in active RA patients on standard disease-modifying anti-rheumatic drugs (DMARDs). A total of 64 participants with active RA were randomized into 2 groups, yoga group (n = 32) or non-yoga group (n = 32); that were assessed for disease severity, at baseline and after 8 week duration, for Disease Activity Score (DAS28-ESR), T cell subsets [Th17 (CD3+ CD4+ IL17+ RORγt+) cells and Treg (CD3+ CD4+ CD25+ CD127-Foxp3+) cells], markers of T cell aging [aged Th17 cells (CD3+ CD4+ IL17+ RORγt+ CD28-) and aged Treg cells (CD3+ CD4+ CD25+ CD127-Foxp3+ CD28-)], pro-inflammatory markers [IL-6, and IL-17], anti-inflammatory markers [TGF-β, and IL-10], epigenetic alterations [5-methyl cytosine, 5-hydroxymethyl cytosine, and HDAC1] and gene expression patterns [RORγt, FoxP3, IL-17, IL-6, TGF-β, CXCL2, CXCR2, and JUN]. In yoga group, there was a significant improvement in DAS28-ESR scores at the end of 8-weeks of yoga program. The Th17 cells and aged T cell subsets showed a significant decline whereas Treg cell population showed a significant elevation in yoga group. There were significant improvements observed in epigenetic markers as well as inflammatory markers post 8-weeks of yoga practice. The yoga group showed downregulation of RORγt, IL-17, IL-6, CXCL2, CXCR2, and upregulation of FoxP3 and TGF-β transcripts. Yoga enables the maintenance of immune-homeostasis as evident by increased Treg cell population and reduced Th17 cell population. Yoga reduces the rate of immunological aging in T cells, as seen by the reduction in population of aged Th17 cells and aged Treg cells. Yoga positively modifies transcriptome and epigenome by normalization of various inflammatory markers, gene expression patterns and epigenetic alterations. Taken together, yoga reduces RA severity, and aids in immune-modulation and hence can be beneficial as an adjunct therapy.

Impacts and mechanisms of PM2.5 on bone

Osteoporosis is a metabolic bone disease, which is characterized by a decreased bone mass and deterioration of bone microstructure, resulting in increased bone fragility and a higher risk of fracture. The main pathological process of osteoporosis is the dynamic imbalance between bone absorption and bone formation, which can be caused by various factors such as air pollution. Particulate matter (PM)_2.5 refers to the fine particles in the atmosphere, which are small in volume and large in specific surface area. These particles are prone to carrying toxic substances and have negative effects on several extrapulmonary organs, including bones. In this review, we present relevant data from studies, which show that PM_2.5 is associated with abnormal bone turnover and osteoporosis. PM_2.5 may cause or aggravate bone loss by stimulating an inflammatory response, inducing oxidative damage, reducing estrogen efficiency by competitive binding to estrogen receptors, or endocrine disorder mediated by binding with aromatic hydrocarbon receptors, and affecting the synthesis of vitamin D to reduce calcium absorption. The cellular and molecular mechanisms involved in these processes are also summarized in this review.

Interleukin-23 Regulates Inflammatory Osteoclastogenesis via Activation of CLEC5A(+) Osteoclast Precursors

OBJECTIVE

To investigate the role of interleukin-23 (IL-23) in pathologic bone remodeling in inflammatory arthritis.

METHODS

In this study we investigated the role of IL-23 in osteoclast differentiation and activation using in vivo gene transfer techniques in wild-type and myeloid DNAX-activation protein 12-associating lectin-1 (MDL-1)-deficient mice, and by performing in vitro and in vivo osteoclastogenesis assays using spectral flow cytometry, micro-computed tomography analysis, Western blotting, and immunoprecipitation.

RESULTS

Herein, we show that IL-23 induces the expansion of a myeloid osteoclast precursor population and supports osteoclastogenesis and bone resorption in inflammatory arthritis. Genetic ablation of C-type lectin domain family member 5A, also known as MDL-1, prevents the induction of osteoclast precursors by IL-23 that is associated with bone destruction, as commonly observed in inflammatory arthritis. Moreover, osteoclasts derived from the bone marrow of MDL-1-deficient mice showed impaired osteoclastogenesis, and MDL-1-/- mice had increased bone mineral density.

CONCLUSION

Our data show that IL-23 signaling regulates the availability of osteoclast precursors in inflammatory arthritis that could be effectively targeted for the treatment of inflammatory bone loss in inflammatory arthritis.

Schnurri-3 inhibition suppresses bone and joint damage in models of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease that leads to systemic and articular bone loss by activating bone resorption and suppressing bone formation. Despite current therapeutic agents, inflammation-induced bone loss in RA continues to be a significant clinical problem due to joint deformity and lack of articular and systemic bone repair. Here, we identify the suppressor of bone formation, Schnurri-3 (SHN3), as a potential target to prevent bone loss in RA. SHN3 expression in osteoblast-lineage cells is induced by proinflammatory cytokines. Germline deletion or conditional deletion of Shn3 in osteoblasts limits articular bone erosion and systemic bone loss in mouse models of RA. Similarly, silencing of SHN3 expression in these RA models using systemic delivery of a bone-targeting recombinant adenoassociated virus protects against inflammation-induced bone loss. In osteoblasts, TNF activates SHN3 via ERK MAPK-mediated phosphorylation and, in turn, phosphorylated SHN3 inhibits WNT/β-catenin signaling and up-regulates RANKL expression. Accordingly, knock-in of a mutation in Shn3 that fails to bind ERK MAPK promotes bone formation in mice overexpressing human TNF due to augmented WNT/β-catenin signaling. Remarkably, Shn3-deficient osteoblasts are not only resistant to TNF-induced suppression of osteogenesis, but also down-regulate osteoclast development. Collectively, these findings demonstrate SHN3 inhibition as a promising approach to limit bone loss and promote bone repair in RA.

Role of the Osteocyte in Musculoskeletal Disease

PURPOSE OF THE REVIEW

The purpose of this review is to summarize the role of the osteocyte in muscle atrophy in cancer patients, sarcopenia, spinal cord injury, Duchenne's muscular dystrophy, and other conditions associated with muscle deterioration.

RECENT FINDINGS

One type of bone cell, the osteocyte, appears to play a major role in muscle and bone crosstalk, whether physiological or pathological. Osteocytes are cells living within the bone-mineralized matrix. These cells are connected to each other by means of dendrites to create an intricately connected network. The osteocyte network has been shown to respond to different types of stimuli such as mechanical unloading, immobilization, aging, and cancer by producing osteocytes-derived factors. It is now becoming clear that some of these factors including sclerostin, RANKL, TGF-β, and TNF-α have detrimental effects on skeletal muscle. Bone and muscle not only communicate mechanically but also biochemically. Osteocyte-derived factors appear to contribute to the pathogenesis of muscle disease and could be used as a cellular target for new therapeutic approaches.

Multifunctional Janus Nanoplatform for Efficiently Synergistic Theranostics of Rheumatoid Arthritis

Progress has been made in the application of nanomedicine in rheumatoid arthritis (RA) treatment. However, the whole process of monitoring and treatment of RA remains a formidable challenge due to the complexity of the chronic autoimmune disease. In this study, we develop a Janus nanoplatform (denoted as Janus-CPS) composed of CeO₂-Pt nanozyme subunit on one side and periodic mesoporous organosilica (PMO) subunit on another side for simultaneous early diagnosis and synergistic therapy of RA. The Janus nanostructure, which enables more active sites to be exposed, enhances the reactive oxygen species scavenging capability of CeO₂-Pt nanozyme subunit as compared to their core-shell counterpart. Furthermore, micheliolide (MCL), an extracted compound from natural plants with anti-osteoclastogenesis effects, is loaded into the mesopores of PMO subunit to synergize with the anti-inflammation effect of nanozymes for efficient RA treatment, which has been demonstrated by in vitro cellular experiments and in vivo collagen-induced arthritis (CIA) model. In addition, by taking advantage of the second near-infrared window (NIR-II) fluorescent imaging, indocyanine green (ICG)-loaded Janus-CPS exhibits desirable effectiveness in detecting RA lesions at a very early stage. It is anticipated that such a Janus nanoplatform may offer an alternative strategy of functional integration for versatile theranostics.

A Low Concentration of Citreoviridin Prevents Both Intracellular Calcium Deposition in Vascular Smooth Muscle Cell and Osteoclast Activation In Vitro

Vascular calcification (VC) and osteoporosis are age-related diseases and significant risk factors for the mortality of elderly. VC and osteoporosis may share common risk factors such as renin-angiotensin system (RAS)-related hypertension. In fact, inhibitors of RAS pathway, such as angiotensin type 1 receptor blockers (ARBs), improved both vascular calcification and hip fracture in elderly. However, a sex-dependent discrepancy in the responsiveness to ARB treatment in hip fracture was observed, possibly due to the estrogen deficiency in older women, suggesting that blocking the angiotensin signaling pathway may not be effective to suppress bone resorption, especially if an individual has underlying osteoclast activating conditions such as estrogen deficiency. Therefore, it has its own significance to find alternative modality for inhibiting both vascular calcification and osteoporosis by directly targeting osteoclast activation to circumvent the shortcoming of ARBs in preventing bone resorption in estrogen deficient individuals. In the present study, a natural compound library was screened to find chemical agents that are effective in preventing both calcium deposition in vascular smooth muscle cells (vSMCs) and activation of osteoclast using experimental methods such as Alizarin red staining and Tartrate-resistant acid phosphatase staining. According to our data, citreoviridin (CIT) has both an anti-VC effect and anti-osteoclastic effect in vSMCs and in Raw 264.7 cells, respectively, suggesting its potential as an effective therapeutic agent for both VC and osteoporosis.

Osteoclasts, Master Sculptors of Bone

Osteoclasts are multinucleated cells with the unique ability to resorb bone matrix. Excessive production or activation of osteoclasts leads to skeletal pathologies that affect a significant portion of the population. Although therapies that effectively target osteoclasts have been developed, they are associated with sometimes severe side effects, and a fuller understanding of osteoclast biology may lead to more specific treatments. Along those lines, a rich body of work has defined essential signaling pathways required for osteoclast formation, function, and survival. Nonetheless, recent studies have cast new light on long-held views regarding the origin of these cells during development and homeostasis, their life span, and the cellular sources of factors that drive their production and activity during homeostasis and disease. In this review, we discuss these new findings in the context of existing work and highlight areas of ongoing and future investigation.

Mechanisms of cancer pain

Personalised and targeted interventions have revolutionised cancer treatment and dramatically improved survival rates in recent decades. Nonetheless, effective pain management remains a problem for patients diagnosed with cancer, who continue to suffer from the painful side effects of cancer itself, as well as treatments for the disease. This problem of cancer pain will continue to grow with an ageing population and the rapid advent of more effective therapeutics to treat the disease. Current pain management guidelines from the World Health Organisation are generalised for different pain severities, but fail to address the heterogeneity of mechanisms in patients with varying cancer types, stages of disease and treatment plans. Pain is the most common complaint leading to emergency unit visits by patients with cancer and over one-third of patients that have been diagnosed with cancer will experience under-treated pain. This review summarises preclinical models of cancer pain states, with a particular focus on cancer-induced bone pain and chemotherapy-associated pain. We provide an overview of how preclinical models can recapitulate aspects of pain and sensory dysfunction that is observed in patients with persistent cancer-induced bone pain or neuropathic pain following chemotherapy. Peripheral and central nervous system mechanisms of cancer pain are discussed, along with key cellular and molecular mediators that have been highlighted in animal models of cancer pain. These include interactions between neuronal cells, cancer cells and non-neuronal cells in the tumour microenvironment. Therapeutic targets beyond opioid-based management are reviewed for the treatment of cancer pain.

Ultrasmall PtAu2 nanoclusters activate endogenous anti-inflammatory and anti-oxidative systems to prevent inflammatory osteolysis

Rationale: Inflammatory osteolysis, characterized by abundant immune cell infiltration and osteoclast (OC) formation, is a common complication induced by bacterial products and/or wear particles at the bone-prosthesis interface that severely reduces long-term stability after implantation. Molecular nanoclusters are ultrasmall particles with unique physicochemical and biological properties that have great potential as theranostic agents for treating inflammatory diseases. Methods: In this study, heterometallic PtAu₂ nanoclusters with sensitive nitric oxide-responsive phosphorescence turn-on characteristics and strong binding interactions with cysteine were designed, making them desirable candidates for the treatment of inflammatory osteolysis. Results: PtAu₂ clusters exhibited satisfactory biocompatibility and cellular uptake behavior, with potent anti-inflammatory and anti-OC activities in vitro. In addition, PtAu₂ clusters alleviated lipopolysaccharide-induced calvarial osteolysis in vivo and activated nuclear factor erythroid 2-related factor 2 (Nrf2) expression by disrupting its association with Kelch-like ECH-associated protein 1 (Keap1), thereby upregulating the expression of endogenous anti-inflammatory and anti-oxidative products. Conclusion: Through the rational design of novel heterometallic nanoclusters that activate the endogenous anti-inflammatory system, this study provides new insights into the development of multifunctional molecular therapeutic agents for inflammatory osteolysis and other inflammatory diseases.

Anti-Inflammatory Effects of Geniposidic Acid on Porphyromonas gingivalis-Induced Periodontitis in Mice

Periodontal disease is predominantly caused by the pathogenic bacterium Porphyromonas gingivalis that produces inflammation-inducing factors in the host. Eucommia ulmoides is a plant native to China that has been reported to reduce blood pressure, promote weight loss, and exhibit anti-inflammatory effects. Geniposidic acid (GPA) is the major component of E. ulmoides. Herein, we investigated the effects of GPA on P. gingivalis-induced periodontitis by measuring the inflammatory responses in human gingival epithelial cells (HGECs) after P. gingivalis stimulation and GPA addition in a P. gingivalis-induced periodontitis mouse model. We found that GPA addition suppressed interleukin (IL)-6 mRNA induction (33.8% suppression), IL-6 production (69.2% suppression), toll-like receptor (TLR) 2 induction, and mitogen-activated protein kinase (MAPK) phosphorylation in HGECs stimulated by P. gingivalis. Inoculation of mice with GPA inhibited P. gingivalis-induced alveolar bone resorption (25.6% suppression) by suppressing IL-6 and TLR2 production in the serum and gingiva. GPA suppressed osteoclast differentiation of bone marrow cells induced by M-CSF and sRANKL in mice (56.7% suppression). GPA also suppressed the mRNA expression of OSCAR, NFATc1, c-Fos, cathepsin K, and DC-STAMP. In summary, GPA exerts an anti-inflammatory effect on periodontal tissue and may be effective in preventing periodontal disease.

PTH1R translocation to primary cilia in mechanically-stimulated ostecytes prevents osteoclast formation via regulation of CXCL5 and IL-6 secretion

Osteocytes respond to mechanical forces controlling osteoblast and osteoclast function. Mechanical stimulation decreases osteocyte apoptosis and promotes bone formation. Primary cilia have been described as potential mechanosensors in bone cells. Certain osteogenic responses induced by fluid flow (FF) in vitro are decreased by primary cilia inhibition in MLO-Y4 osteocytes. The parathyroid hormone (PTH) receptor type 1 (PTH1R) modulates osteoblast, osteoclast, and osteocyte effects upon activation by PTH or PTH-related protein (PTHrP) in osteoblastic cells. Moreover, some actions of PTH1R seem to be triggered directly by mechanical stimulation. We hypothesize that PTH1R forms a signaling complex in the primary cilium that is essential for mechanotransduction in osteocytes and affects osteocyte-osteoclast communication. MLO-Y4 osteocytes were stimulated by FF or PTHrP (1-37). PTH1R and primary cilia signaling were abrogated using PTH1R or primary cilia specific siRNAs or inhibitors, respectively. Conditioned media obtained from mechanically- or PTHrP-stimulated MLO-Y4 cells inhibited the migration of preosteoclastic cells and osteoclast differentiation. Redistribution of PTH1R along the entire cilium was observed in mechanically stimulated MLO-Y4 osteocytic cells. Preincubation of MLO-Y4 cells with the Gli-1 antagonist, the adenylate cyclase inhibitor (SQ22536), or with the phospholipase C inhibitor (U73122), affected the migration of osteoclast precursors and osteoclastogenesis. Proteomic analysis and neutralizing experiments showed that FF and PTH1R activation control osteoclast function through the modulation of C-X-C Motif Chemokine Ligand 5 (CXCL5) and interleukin-6 (IL-6) secretion in osteocytes. These novel findings indicate that both primary cilium and PTH1R are necessary in osteocytes for proper communication with osteoclasts and show that mechanical stimulation inhibits osteoclast recruitment and differentiation through CXCL5, while PTH1R activation regulate these processes via IL-6.

Multimodal regulation of the osteoclastogenesis process by secreted group IIA phospholipase A2

Increasing evidence points to the involvement of group IIA secreted phospholipase A₂ (sPLA₂-IIA) in pathologies characterized by abnormal osteoclast bone-resorption activity. Here, the role of this moonlighting protein has been deepened in the osteoclastogenesis process driven by the RANKL cytokine in RAW264.7 macrophages and bone-marrow derived precursor cells from BALB/cJ mice. Inhibitors with distinct selectivity toward sPLA₂-IIA activities and recombinant sPLA₂-IIA (wild-type or catalytically inactive forms, full-length or partial protein sequences) were instrumental to dissect out sPLA₂-IIA function, in conjunction with reduction of sPLA₂-IIA expression using small-interfering-RNAs and precursor cells from Pla2g2a knock-out mice. The reported data indicate sPLA₂-IIA participation in murine osteoclast maturation, control of syncytium formation and resorbing activity, by mechanisms that may be both catalytically dependent and independent. Of note, these studies provide a more complete understanding of the still enigmatic osteoclast multinucleation process, a crucial step for bone-resorbing activity, uncovering the role of sPLA₂-IIA interaction with a still unidentified receptor to regulate osteoclast fusion through p38 SAPK activation. This could pave the way for the design of specific inhibitors of sPLA₂-IIA binding to interacting partners implicated in osteoclast syncytium formation.

Pathomechanisms of bone loss in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease, in which the inflammatory processes involve the skeletal system and there is marked destruction of the bones and the surrounding structures. In this review, we discuss the current concepts of osteoimmunology in RA, which represent the molecular crosstalk between the immune and skeletal systems, resulting in the disruption of bone remodeling. Bone loss in RA can be focal or generalized, leading to secondary osteoporosis. We have summarized the recent studies of bone loss in RA, which focused on the molecular aspects, such as cytokines, autoantibodies, receptor activator of nuclear kappa-β ligand (RANKL) and osteoprotegerin (OPG). Apart from the above molecules, the role of aryl hydrocarbon receptor (Ahr), which is a potential key mediator in this process through the generation of the Th17 cells, is discussed. Hence, this review highlights the key insights into molecular mechanisms of bone loss in RA.

Advance in bone destruction participated by JAK/STAT in rheumatoid arthritis and therapeutic effect of JAK/STAT inhibitors

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic joint inflammation and bone erosion. The bones in the human body are constantly undergoing bone remodeling throughout their lives, which is the process of bone resorption by osteoclasts to damaged bone tissue and new bone formation by osteoblasts. Osteoblasts (OBs) are the main functional cells in bone formation, responsible for the synthesis, secretion and mineralization of the bone matrix. On the contrary, osteoclasts (OCs) mediate bone breakdown during natural bone turnover, but excessive breakdown occurs in RA. Under the condition of RA inflammation, many molecules, such as IL-1β, IL-6, TNF-α, IL-17 and hypoxia-inducible factor-1α (HIF-1α) are produced that could mediate bone loss. Studies have shown that cytokines mainly promote the formation of OCs and play a role in bone resorption by stimulating OBs to express receptor activator of NF-κB ligand (RANKL). JAK/STAT plays a crucial role in the process of bone destruction. And JAK/STAT pathway mediates the RANKL/receptor activator of NF-κB (RANK)/osteoprotegerin (OPG) axis. Tofacitinib, Baricitinib, Peficitinib and Filgotinib are now being used in patients with moderate to severe RA, as well as in patients with RA who have an inadequate response to methotrexate therapy and bone destruction. Currently, Tofacitiniband Baritinib areapprovedfor thetreatmentof moderate-to-severely active RA. JAK inhibitors have been reported to have better efficacy and lower adverse effects compared with methotrexate and adalimumab. In addition, two JAK inhibitors are currently in development: the JAK1 selective Upadacitinib, and the JAK3 selective inhibitor Decernotinib. In addition to the above JAK inhibitors, some small molecular compounds inhibit bone destruction by inhibiting the Phosphorylation of STAT3. In this paper, the research progress of bone destruction participated by JAK/ STAT in rheumatoid arthritis and therapeutic effect of JAK/STAT inhibitors were reviewed.

IL-34 and protein-tyrosine phosphatase receptor type-zeta-dependent mechanisms limit arthritis in mice

Myeloid cell mediated mechanisms regulate synovial joint inflammation. IL-34, a macrophage (Mø) growth and differentiation molecule, is markedly expressed in neutrophil and Mø-rich arthritic synovium. IL-34 engages a newly identified independent receptor, protein-tyrosine phosphatase, receptor-type, zeta (PTPRZ), that we find is expressed by Mø. As IL-34 is prominent in rheumatoid arthritis, we probed for the IL-34 and PTPRZ-dependent myeloid cell mediated mechanisms central to arthritis using genetic deficient mice in K/BxN serum-transfer arthritis. Unanticipatedly, we now report that IL-34 and PTPRZ limited arthritis as intra-synovial pathology and bone erosion were more severe in IL-34 and PTPRZ KO mice during induced arthritis. We found that IL-34 and PTPRZ: (i) were elevated, bind, and induce downstream signaling within the synovium in arthritic mice and (ii) were upregulated in the serum and track with disease activity in rheumatoid arthritis patients. Mechanistically, IL-34 and PTPRZ skewed Mø toward a reparative phenotype, and enhanced Mø clearance of apoptotic neutrophils, thereby decreasing neutrophil recruitment and intra-synovial neutrophil extracellular traps. With fewer neutrophils and neutrophil extracellular traps in the synovium, destructive inflammation was restricted, and joint pathology and bone erosion diminished. These novel findings suggest that IL-34 and PTPRZ-dependent mechanisms in the inflamed synovium limit, rather than promote, inflammatory arthritis.

Antibody-induced pain-like behavior and bone erosion: links to subclinical inflammation, osteoclast activity, and acid-sensing ion channel 3-dependent sensitization

ABSTRACT

Several bone conditions, eg, bone cancer, osteoporosis, and rheumatoid arthritis (RA), are associated with a risk of developing persistent pain. Increased osteoclast activity is often the hallmark of these bony pathologies and not only leads to bone remodeling but is also a source of pronociceptive factors that sensitize the bone-innervating nociceptors. Although historically bone loss in RA has been believed to be a consequence of inflammation, both bone erosion and pain can occur years before the symptom onset. Here, we have addressed the disconnection between inflammation, pain, and bone erosion by using a combination of 2 monoclonal antibodies isolated from B cells of patients with RA. We have found that mice injected with B02/B09 monoclonal antibodies (mAbs) developed a long-lasting mechanical hypersensitivity that was accompanied by bone erosion in the absence of joint edema or synovitis. Intriguingly, we have noted a lack of analgesic effect of naproxen and a moderate elevation of few inflammatory factors in the ankle joints suggesting that B02/B09-induced pain-like behavior does not depend on inflammatory processes. By contrast, we found that inhibiting osteoclast activity and acid-sensing ion channel 3 signaling prevented the development of B02/B09-mediated mechanical hypersensitivity. Moreover, we have identified secretory phospholipase A2 and lysophosphatidylcholine 16:0 as critical components of B02/B09-induced pain-like behavior and shown that treatment with a secretory phospholipase A2 inhibitor reversed B02/B09-induced mechanical hypersensitivity and bone erosion. Taken together, our study suggests a potential link between bone erosion and pain in a state of subclinical inflammation and offers a step forward in understanding the mechanisms of bone pain in diseases such as RA.

Chronic nonbacterial osteomyelitis in children: a multicentre Belgian cohort of 30 children

BACKGROUND

To evaluate clinical characteristics, imaging findings, therapeutic approach and outcome of paediatric patients with Chronic Non-Bacterial Osteomyelitis (CNO).

METHODS

Retrospective review of 30 children diagnosed with CNO at two tertiary care centres in Belgium. Imaging data were evaluated by blinded paediatric radiologists.

RESULTS

Mean age at onset was 10.3 years and mean age at diagnosis was 11.7 years. Bone pain was the leading symptom (29/30 patients). Out of 180 symptomatic lesions, 131 were confirmed on MRI as hyperintense geographic lesions on STIR images at the metaphysis and epiphysis adjacent to growth plates of tubular bones. The most common sites of involvement were the lower limbs, spine, sternoclavicular joint and humerus. For nearly half of the patients (14/30) monotherapy with NSAIDs was sufficient to obtain remission. The remaining 16 patients received second-line therapy: bisphosphonates (n = 15/30), disease-modifying antirheumatic drugs (n = 7/30), etanercept (n = 4/30) and tocilizumab (n = 1/30). Remission was reached after a mean time of 37.6 months in 26/30 patients. The prognosis was worse for patients with spinal involvement, resulting in more long-term sequelae.

CONCLUSIONS

We present a multicentre paediatric cohort of 30 CNO patients. A typical pattern of bone involvement could be found on MRI. NSAIDs were administered as first-line treatment. Second-line strategies included bisphosphonates, corticosteroids, methotrexate, etanercept and tocilizumab.

TRIAL REGISTRATION

Retrospectively registered. Registratienummer EC KUL: MP018023.

Soluble CD13 induces inflammatory arthritis by activating the bradykinin receptor B1

CD13, an ectoenzyme on myeloid and stromal cells, also circulates as a shed, soluble protein (sCD13) with powerful chemoattractant, angiogenic, and arthritogenic properties, which require engagement of a G protein-coupled receptor (GPCR). Here we identify the GPCR that mediates sCD13 arthritogenic actions as the bradykinin receptor B1 (B1R). Immunofluorescence and immunoblotting verified high expression of B1R in rheumatoid arthritis (RA) synovial tissue and fibroblast-like synoviocytes (FLSs), and demonstrated binding of sCD13 to B1R. Chemotaxis, and phosphorylation of Erk1/2, induced by sCD13, were inhibited by B1R antagonists. In ex vivo RA synovial tissue organ cultures, a B1R antagonist reduced secretion of inflammatory cytokines. Several mouse arthritis models, including serum transfer, antigen-induced, and local innate immune stimulation arthritis models, were attenuated in Cd13-/- and B1R-/- mice and were alleviated by B1R antagonism. These results establish a CD13/B1R axis in the pathogenesis of inflammatory arthritis and identify B1R as a compelling therapeutic target in RA and potentially other inflammatory diseases.

Bone Health in Children with Rheumatic Disorders: Focus on Molecular Mechanisms, Diagnosis, and Management

Bone is an extremely dynamic and adaptive tissue, whose metabolism and homeostasis is influenced by many different hormonal, mechanical, nutritional, immunological and pharmacological stimuli. Genetic factors significantly affect bone health, through their influence on bone cells function, cartilage quality, calcium and vitamin D homeostasis, sex hormone metabolism and pubertal timing. In addition, optimal nutrition and physical activity contribute to bone mass acquisition in the growing age. All these factors influence the attainment of peak bone mass, a critical determinant of bone health and fracture risk in adulthood. Secondary osteoporosis is an important issue of clinical care in children with acute and chronic diseases. Systemic autoimmune disorders, like juvenile idiopathic arthritis, can affect the skeletal system, causing reduced bone mineral density and high risk of fragility fractures during childhood. In these patients, multiple factors contribute to reduce bone strength, including systemic inflammation with elevated cytokines, reduced physical activity, malabsorption and nutritional deficiency, inadequate daily calcium and vitamin D intake, use of glucocorticoids, poor growth and pubertal delay. In juvenile arthritis, osteoporosis is more prominent at the femoral neck and radius compared to the lumbar spine. Nevertheless, vertebral fractures are an important, often asymptomatic manifestation, especially in glucocorticoid-treated patients. A standardized diagnostic approach to the musculoskeletal system, including prophylaxis, therapy and follow up, is therefore mandatory in at risk children. Here we discuss the molecular mechanisms involved in skeletal homeostasis and the influence of inflammation and chronic disease on bone metabolism.

Xanthine Derivative KMUP-1 Attenuates Experimental Periodontitis by Reducing Osteoclast Differentiation and Inflammation

Periodontitis is an inflammatory disease of gum that may predispose to serious systemic complications such as diabetes and cardiovascular diseases. Activation of macrophages and osteoclasts around periodontal tissue can accelerate gum inflammation. In addition, alteration of cyclic nucleotide levels is associated with the severity of periodontitis. Our previous study has shown that KMUP-1, a xanthine derivative exhibiting phosphodiesterase inhibition and soluble guanylyl cyclase activation, can inhibit lipopolysaccharide (LPS)-induced inflammation and receptor activator of nuclear factor kappa-Β ligand (RANKL)-induced osteoclastogenesis. This study was aimed to investigate whether KMUP-1 could attenuate periodontitis both in vitro and in vivo. In vitro, the protective effect of KMUP-1 on inflammation and osteoclastogenesis was investigated in RANKL-primed RAW264.7 cells treated by Porphyromonas gingivalis LPS (PgLPS). The results showed that KMUP-1 attenuated PgLPS-induced osteoclast differentiation as demonstrated by decreased TRAP-positive multinuclear cells and TRAP activity. This reduction of osteoclast differentiation by KMUP-1 was reversed by KT5823, a protein kinase G inhibitor. Similarly, pro-inflammatory cytokine levels induced by PgLPS were inhibited by KMUP-1 in a dose-dependent manner whereas reversed by KT5823. Mechanistically, suppression of MAPKs, PI3K/Akt, and NF-κB signaling pathways and decrease of c-Fos and NFATc1 expression in osteoclast precursors by KMUP-1 may mediate its protective effect. In vivo, two models of periodontitis in rats were induced by gingival injections of PgLPS and ligature placement around molar teeth, respectively. Our results showed that KMUP-1 inhibited alveolar bone loss in both rat models, and this effect mediated at least partly by reduced osteoclastogenesis. In conclusion, our study demonstrated the therapeutic potential of KMUP-1 on periodontitis through suppression of inflammation and osteoclast differentiation.

Connection between Mesenchymal Stem Cells Therapy and Osteoclasts in Osteoarthritis

The use of mesenchymal stem cells constitutes a promising therapeutic approach, as it has shown beneficial effects in different pathologies. Numerous in vitro, pre-clinical, and, to a lesser extent, clinical trials have been published for osteoarthritis. Osteoarthritis is a type of arthritis that affects diarthritic joints in which the most common and studied effect is cartilage degradation. Nowadays, it is known that osteoarthritis is a disease with a very powerful inflammatory component that affects the subchondral bone and the rest of the tissues that make up the joint. This inflammatory component may induce the differentiation of osteoclasts, the bone-resorbing cells. Subchondral bone degradation has been suggested as a key process in the pathogenesis of osteoarthritis. However, very few published studies directly focus on the activity of mesenchymal stem cells on osteoclasts, contrary to what happens with other cell types of the joint, such as chondrocytes, synoviocytes, and osteoblasts. In this review, we try to gather the published bibliography in relation to the effects of mesenchymal stem cells on osteoclastogenesis. Although we find promising results, we point out the need for further studies that can support mesenchymal stem cells as a therapeutic tool for osteoclasts and their consequences on the osteoarthritic joint.

Multiple idiopathic external cervical root resorption in patient treated continuously with denosumab: a case report

Background

External root resorption is an irreversible loss of dental hard tissue as a result of odontoclastic action. Multiple external cervical root resorptions in permanent teeth are rare. The exact cause of external cervical root resorption is unclear. It is currently well established that RANK/RANKL signaling is essential for osteoclastogenesis and osteoclast-mediated bone resorption. Denosumab is an anti-RANKL antibody used for the treatment of postmenopausal osteoporosis. RANK/RANKL pathway suppression by denosumab is expected to suppress the activity of clastic cells responsible for hard tissue resorption involving both osteoclasts and odontoclasts.

Case presentation

This case report demonstrates aggressive and generalized idiopathic external cervical root resorption that started and advanced during ongoing antiresorptive therapy with the human monoclonal RANKL-blocking antibody denosumab without discontinuation of therapy in a 74-year-old female patient treated for postmenopausal osteoporosis. The extent of resorptive defects was too large and progressively led to fractures of the teeth. The number of teeth involved and the extend of destruction excluded conservative treatment. The affected teeth had to be extracted for functional prosthetic reconstruction.

Conclusions

This finding suggests that treatment with denosumab may be associated with severe and aggressive odontoclastic resorption of multiple dental roots despite an adequate inhibitory effect on osteoclasts in the treatment of osteoporosis. The RANKL-independent pathways of clastic cell formation are likely to be involved in this pathological process.

Differential effects of anti-RANKL monoclonal antibody and zoledronic acid on necrotic bone in a murine model of Staphylococcus aureus-induced osteomyelitis

Osteomyelitis is characterized by progressive inflammatory bone destruction accompanied by severe pain and disability. However, with the exception of antibiotic therapies, there is no established therapy to protect the bone from infectious osteolysis. The anti-receptor activator of nuclear factor-kB ligand (RANKL) monoclonal antibody (anti-RANKL Ab) is a potential drug based on its proven effectiveness in preventing joint bone erosion in rheumatoid arthritis; however, the efficacy and adverse effects of anti-RANKL Ab in osteomyelitis remain to be investigated. In this study, we investigated the effects of anti-mouse RANKL Ab on acute osteomyelitis and compared them with those of zoledronic acid (ZA) using a murine model. Mice were inoculated with bioluminescent Staphylococcus aureus (Xen 29) on their left femur and then treated with ZA, anti-RANKL Ab, or phosphate-buffered saline as control. A 21-day longitudinal observational study using microcomputed tomography showed that both anti-RANKL Ab and ZA had an osteoprotective effect against infectious osteolysis. However, it was also demonstrated through bioluminescence imaging that ZA delayed the spontaneous reduction of bacterial load and through histology that it increased the amount of necrotic bone, while anti-RANKL Ab did not. Findings from histopathological and in vitro studies suggest that an intense inflammatory response around the necrotic bone could induce osteoclasts in a RANKL-independent manner, leading to the removal of necrotic bone, even after administration of the anti-RANKL Ab therapy. Collectively, anti-RANKL Ab may exert an osteoprotective effect without hampering the removal of the necrotic bone, which serves as a nidus for infection in osteomyelitis.

A meta-analysis of dietary inflammatory index and bone health status

Background

The inflammatory potential of diets is associated with several diseases and can affect bone health. We aimed to systematically review and pool the current evidence on the association of DII with bone health in observational studies.

Methods

We searched PubMed and NLM Gateway (for Medline), Web of Science, Scopus and EMBASE up to December 16, 2020 for studies that examined the relationship between DII score and bone mineral density (BMD) or fracture. All observational studies were included in this meta-analysis. Heterogeneity between studies was evaluated using Cochran Q-statistic and I2 statistics. Random effect meta-analysis method was used to pool the effect size. Stratified meta-analysis according to the type of study (cohort/ non-cohort) was performed to assess the relationship of DII with BMD and fracture.

Results

In total, 13 articles were included in the present systematic review, including five cohorts, five cross-sectional, and three case-control studies. The total sample size of these studies was 211,938 individuals aged 5 to 85 years. According to random-effect meta-analysis, DII was associated with increased odds of fracture in non-cohort studies (pooled OR=1.42, 95%CI: 1.17, 1.67), but this association was not statistically significant in cohort studies (pooled OR=1.03, 95%CI: 0.97, 1.09). Moreover, only in non-cohort studies, the mean of BMD in subjects in the highest DII category was significantly lower than those in the lowest DII category (SMD: -9.59, 95%CI: -10.84,-8.33).

Conclusions

Our findings showed that high score of DII can have devastating effects on bone health. Further longitudinal studies are necessary to confirm these findings among more diverse populations.

TBK1 recruitment to STING mediates autoinflammatory arthritis caused by defective DNA clearance

Defective DNA clearance in DNase II-/- mice leads to lethal inflammatory diseases that can be rescued by deleting cGAS or STING, but the role of distinct signaling pathways downstream of STING in the disease manifestation is not known. We found that the STING S365A mutation, which abrogates IRF3 binding and type I interferon induction, rescued the embryonic lethality of DNase II-/- mice. However, the STING S365A mutant retains the ability to recruit TBK1 and activate NF-κB, and DNase II-/-STING-S365A mice exhibited severe polyarthritis, which was alleviated by neutralizing antibodies against TNF-α or IL-6 receptor. In contrast, the STING L373A mutation or C-terminal tail truncation, which disrupts TBK1 binding and therefore prevents activation of both IRF3 and NF-κB, completely rescued the phenotypes of DNase II-/- mice. These results demonstrate that TBK1 recruitment to STING mediates autoinflammatory arthritis independently of type I interferons. Inhibiting TBK1 binding to STING may be a therapeutic strategy for certain autoinflammatory diseases instigated by self-DNA.

The Imbalance of Cytokines and Lower Levels of Tregs in Elderly Male Primary Osteoporosis

INTRODUCTION

Osteoporosis (OP) is a debilitating disease that brings a heavy burden to individuals and society with reduced quality of life and lifespan. However, it's frequently overlooked and poorly studied in elderly male patients. Worse still, few anti-osteoporosis drugs are effective at the prevention and treatment of osteoporosis in men. It has been reported that the cells of bone and the immune system share common progenitors, cytokines and growth factors, and that reciprocal interactions occur during health and disease. Nevertheless, the role of immune system in OP is not fully understood, especially in male patients. Therefore, this study aimed to investigate molecular alterations in immune cells in men with OP and to identify immunomodulatory strategies with potential therapeutic value.

MATERIALS AND METHODS

A population of 121 men aged between 51 and 80 years old was recruited. Bone mineral density (BMD) was measured at the lumbar spine L1-4 and femoral neck using dual-energy X-ray absorptiometry (DXA). Twenty people were healthy, 66 people had osteopenia and 35 people had OP. Bone metabolic markers, Th1, Th2, Tregs and immune molecules were evaluated at the time of enrollment.

RESULTS

Smoking was a risk factor for OP. C-terminal crosslinking of type I collagen (β-CTX) and the ratio of receptor activator of nuclear factor-κB ligand (RANKL) to osteoprotegerin (OPG) were higher in OP group, which had lower 25-hydroxyvitamin D [25(OH)D] levels. OP had the higher levels of IL-6 and TNF-α and lower levels of IFN-γ and IL-10. CD4⁺CD25⁺CD127^-/low Tregs were significantly lower in the OP group. The imbalance of Th1/Th2 cells may play an important role in the development of OP. 25(OH)D may play essential roles in maintaining bone health. The low level of Tregs is also one of the underlying immune mechanism that leads to male primary OP.

CONCLUSION

The active function of osteoclasts and the decline in osteoblasts were characteristics of OP, and the imbalance in cytokines and lower levels of Tregs were observed in elderly male patients with primary OP.

Combined injury: irradiation with skin or bone wounds in rodent models

A radiation combined injury is defined as an injury that occurs in the setting of irradiation, such as those expected after a nuclear accident, radiation dispersal device release (a 'dirty bomb'), or a nuclear weapon detonation. There is much research on irradiation-associated burns and their healing, but there is less known about other injuries sustained in the context of irradiation. Animal models are limited in their correlations to clinical situations but can support research on specific questions about injuries and their healing. Mouse models of irradiation with skin or bone wounds are validated as highly reproducible and quantitative. They show dose-dependent impairment of wound healing, with later recovery. Irradiation-induced delay of bone wound healing was mitigated to different extents by single doses of gramicidin S-nitroxide JP4-039, a plasmid expressing manganese superoxide dismutase, amifostine/WR2721, or the bifunctional sulfoxide MMS-350. These models should be useful for research on mechanisms of radiation dermal and osseous damage and for further development of new radioprotectors. They also provide information of potential relevance to the effects of clinical radiation therapies.

Systemic oxidative stress in old rats is associated with both osteoporosis and cognitive impairment

Aging is associated both with an increase in memory loss and with comorbidities such as Osteoporosis, which could be causatively linked. In the present study, a deleterious effect on bone is demonstrated for the first time in a model of aged rats with impaired memory. We show that bone marrow progenitor cells obtained from rats with memory deficit have a decrease in their osteogenic capacity, and an increase both in their osteoclastogenic profile and adipogenic capacity, when compared to aged rats with preserved memory. Rats with impaired (versus preserved) memory also show alterations in long-bone micro-architecture (decreased trabecular bone and osteocyte density, increased TRAP-positive osteoclasts), lower bone quality (decreased trabecular bone mineral content and density) and an increase in bone marrow adiposity. Interestingly, the development of bone alterations and memory deficit in old rats is associated with significantly higher levels of serum oxidative stress (versus unaffected aged rats). In conclusion, we have found for the first time in an aged rat model, a relationship between alterations in bone quality and memory impairment, with increased systemic oxidative stress as a possible unifying mechanism.

Cytokine Networks in the Pathogenesis of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic systemic inflammation causing progressive joint damage that can lead to lifelong disability. The pathogenesis of RA involves a complex network of various cytokines and cells that trigger synovial cell proliferation and cause damage to both cartilage and bone. Involvement of the cytokines tumor necrosis factor (TNF)-α and interleukin (IL)-6 is central to the pathogenesis of RA, but recent research has revealed that other cytokines such as IL-7, IL-17, IL-21, IL-23, granulocyte macrophage colony-stimulating factor (GM-CSF), IL-1β, IL-18, IL-33, and IL-2 also play a role. Clarification of RA pathology has led to the development of therapeutic agents such as biological disease-modifying anti-rheumatic drugs (DMARDs) and Janus kinase (JAK) inhibitors, and further details of the immunological background to RA are emerging. This review covers existing knowledge regarding the roles of cytokines, related immune cells and the immune system in RA, manipulation of which may offer the potential for even safer and more effective treatments in the future.

Immunoporosis: Role of Innate Immune Cells in Osteoporosis

Osteoporosis or porous bone disorder is the result of an imbalance in an otherwise highly balanced physiological process known as 'bone remodeling'. The immune system is intricately involved in bone physiology as well as pathologies. Inflammatory diseases are often correlated with osteoporosis. Inflammatory mediators such as reactive oxygen species (ROS), and pro-inflammatory cytokines and chemokines directly or indirectly act on the bone cells and play a role in the pathogenesis of osteoporosis. Recently, Srivastava et al. (Srivastava RK, Dar HY, Mishra PK. Immunoporosis: Immunology of Osteoporosis-Role of T Cells. Frontiers in immunology. 2018;9:657) have coined the term "immunoporosis" to emphasize the role of immune cells in the pathology of osteoporosis. Accumulated pieces of evidence suggest both innate and adaptive immune cells contribute to osteoporosis. However, innate cells are the major effectors of inflammation. They sense various triggers to inflammation such as pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), cellular stress, etc., thus producing pro-inflammatory mediators that play a critical role in the pathogenesis of osteoporosis. In this review, we have discussed the role of the innate immune cells in great detail and divided these cells into different sections in a systemic manner. In the beginning, we talked about cells of the myeloid lineage, including macrophages, monocytes, and dendritic cells. This group of cells explicitly influences the skeletal system by the action of production of pro-inflammatory cytokines and can transdifferentiate into osteoclast. Other cells of the myeloid lineage, such as neutrophils, eosinophils, and mast cells, largely impact osteoporosis via the production of pro-inflammatory cytokines. Further, we talked about the cells of the lymphoid lineage, including natural killer cells and innate lymphoid cells, which share innate-like properties and play a role in osteoporosis. In addition to various innate immune cells, we also discussed the impact of classical pro-inflammatory cytokines on osteoporosis. We also highlighted the studies regarding the impact of physiological and metabolic changes in the body, which results in chronic inflammatory conditions such as ageing, ultimately triggering osteoporosis.

JTE-952 Suppresses Bone Destruction in Collagen-Induced Arthritis in Mice by Inhibiting Colony Stimulating Factor 1 Receptor

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation and structural destruction of the joints. Bone damage occurs in an early stage after onset and osteoclast activation plays a substantial role in its progression. Colony stimulating factor 1 receptor (CSF1R) is a receptor protein tyrosine kinase specifically expressed in monocytic-lineage cells such as macrophages and osteoclasts. Here, we investigated the effect of JTE-952, a novel CSF1R tyrosine kinase inhibitor, on osteoclast formation in vitro and on bone destruction in a mouse model of collagen-induced arthritis. JTE-952 completely inhibited osteoclast differentiation from human monocytes, with an IC₅₀ of 2.8 nmol/L, and reduced osteoclast formation from the synovial cells of RA patients. Detectable levels of colony stimulating factor 1 (CSF1), a ligand of CSF1R, were observed in the synovial tissues of the arthritis model, similar to those observed in the pathology of human RA. JTE-952 significantly suppressed increases in the bone destruction score, the number of tartrate-resistant-acid-phosphatase-positive cells, and the severity of arthritis in the model mice. We also examined the efficacy of JTE-952 combined with methotrexate. This combination therapy more effectively reduced the severity of bone destruction and arthritis than monotherapy with either agent alone. In summary, JTE-952 potently inhibited human osteoclast formation in vitro and suppressed bone destruction in an experimental arthritis model, especially when combined with methotrexate. These results indicate that JTE-952 should strongly inhibit bone destruction and joint inflammation in RA patients and effectively prevent the progression of the structural destruction of joints.

IL-1β-driven osteoclastogenic T regulatory cells accelerate bone erosion in arthritis

IL-1β is a pro-inflammatory mediator with roles in innate and adaptive immunity. Here we show that IL-1β contributes to autoimmune arthritis by inducing osteoclastogenic capacity in T regulatory cells (Tregs). Using mice with joint inflammation arising through deficiency of the IL-1 receptor antagonist (Il1rn-/-), we observed that IL-1β blockade attenuated disease more effectively in early arthritis than in established arthritis, especially with respect to bone erosion. Protection was accompanied by a reduction in synovial CD4+Foxp3+ Tregs that displayed preserved suppressive capacity and aerobic metabolism but aberrant expression of RANKL and a striking capacity to drive RANKL-dependent osteoclast differentiation. Both Il1rn-/- Tregs and wild-type Tregs differentiated with IL-1β accelerated bone erosion upon adoptive transfer. Human Tregs exhibited analogous differentiation, and corresponding RANKLhiFoxp3+ T cells could be identified in rheumatoid arthritis synovial tissue. Together, these findings identify IL-1β-induced osteoclastogenic Tregs (O-Tregs) as a contributor to bone erosion in arthritis.

Role of interleukin-6 in bone destruction and bone repair in rheumatoid arthritis

Rheumatoid arthritis (RA) is a common inflammatory form of arthritis leading to the progressive bone and joint destruction. Many factors are closely involved in the pathology of RA, in particular bone-related cells and inflammatory cytokines such as TNF-α and interleukin-6 (IL-6). Because RA patients with progressive bone destruction experience accelerated deterioration of their quality of life, inhibition of disease progression and joint destruction has become an important clinical goal. Recent studies have also found that drug intervention targeting proinflammatory cytokines such as IL-6 results in bone repair in addition to suppression of bone and joint destruction, and these results suggest the potential for new therapeutic goals. Regarding the relationship between IL-6 and bone destruction, essential roles of osteoclasts have been reported over many years; however, more recent studies have explored the relationship of IL-6 with osteoblasts and osteocytes. In this review, we highlight the perspectives of basic and clinical research, adding new findings on the relationships between IL-6 and bone-related cells associated with inflammation, and the possibility of bone repair by blocking IL-6.

Characterization and Function of Tumor Necrosis Factor and Interleukin-6-Induced Osteoclasts in Rheumatoid Arthritis

Objective

We have previously reported that stimulation of mouse bone marrow–derived macrophages with tumor necrosis factor (TNF) and interleukin‐6 (IL‐6) induces differentiation of osteoclast‐like cells. We undertook this study to clarify the characterization and function of human TNF and IL‐6–induced osteoclasts using peripheral blood collected from patients with rheumatoid arthritis (RA) and healthy donors.

Methods

Peripheral blood monocytes were cultured with a combination of TNF and IL‐6, TNF alone, IL‐6 alone, or with RANKL, and their bone resorption ability was evaluated. Expression levels of NFATc1, proinflammatory cytokines, and matrix metalloproteinase 3 were analyzed. The effects of NFAT inhibitor and JAK inhibitor were examined. Furthermore, the relationship between the number of TNF and IL‐6–induced osteoclasts or RANKL‐induced osteoclasts differentiated from peripheral blood mononuclear cells (PBMCs) in patients with RA and the modified total Sharp score (mTSS) or whole‐body bone mineral density (BMD) was examined.

Results

Peripheral blood monocytes stimulated with a TNF and IL‐6–induced osteoclasts were shown to demonstrate the ability to absorb bone matrix. Cell differentiation was not inhibited by the addition of osteoprotegerin. Stimulation with a combination of TNF and IL‐6 promoted NFATc1 expression, whereas the NFAT and JAK inhibitors prevented TNF and IL‐6–induced osteoclast formation. Expression levels of IL1β, TNF, IL12p40, and MMP3 were significantly increased in TNF and IL‐6–induced osteoclasts, but not in RANKL‐induced osteoclasts. The number of TNF and IL‐6–induced osteoclasts differentiated from PBMCs in patients with RA positively correlated with the mTSS, whereas RANKL‐induced osteoclast numbers negatively correlated with the whole‐body BMD of the same patients.

Conclusion

Our results demonstrate that TNF and IL‐6–induced osteoclasts may contribute to the pathology of inflammatory arthritis associated with joint destruction, such as RA.

Targeted apoptosis of macrophages and osteoclasts in arthritic joints is effective against advanced inflammatory arthritis

Insufficient apoptosis of inflammatory macrophages and osteoclasts (OCs) in rheumatoid arthritis (RA) joints contributes toward the persistent progression of joint inflammation and destruction. Here, we deliver celastrol (CEL) to selectively induce apoptosis of OCs and macrophages in arthritic joints, with enzyme-responsive nanoparticles (termed PRNPs) composed of RGD modified nanoparticles (termed RNPs) covered with cleavable PEG chains. CEL-loaded PRNPs (CEL-PRNPs) dually target OCs and inflammatory macrophages derived from patients with RA via an RGD-αvβ3 integrin interaction after PEG cleavage by matrix metalloprotease 9, leading to increased apoptosis of these cells. In an adjuvant-induced arthritis rat model, PRNPs have an arthritic joint-specific distribution and CEL-PRNPs efficiently reduce the number of OCs and inflammatory macrophages within these joints. Additionally, rats with advanced arthritis go into inflammatory remission with bone erosion repair and negligible side effects after CEL-PRNPs treatment. These findings indicate potential for targeting chemotherapy-induced apoptosis in the treatment of advanced inflammatory arthritis.

Low magnetic field promotes recombinant human BMP-2-induced bone formation and influences orientation of trabeculae and bone marrow-derived stromal cells

Effects of high magnetic fields [MFs, ≥ 1 T (T)] on osteoblastic differentiation and the orientation of cells or matrix proteins have been reported. However, the effect of low MFs (< 1 T) on the orientation of bone formation is not well known. This study was performed to verify the effects of low MFs on osteoblastic differentiation, bone formation, and orientation of both cells and newly formed bone. An apparatus was prepared with two magnets (190 mT) aligned in parallel to generate a parallel MF. In vitro, bone marrow-derived stromal cells of rats were used to assess the effects of low MFs on cell orientation, osteoblastic differentiation, and mineralization. A bone morphogenetic protein (BMP)-2-induced ectopic bone model was used to elucidate the effect of low MFs on microstructural indices, trabecula orientation, and the apatite c-axis orientation of newly formed bone. Low MFs resulted in an increased ratio of cells oriented perpendicular to the direction of the MF and promoted osteoblastic differentiation in vitro. Moreover, in vivo analysis demonstrated that low MFs promoted bone formation and changed the orientation of trabeculae and apatite crystal in a direction perpendicular to the MF. These changes led to an increase in the mechanical strength of rhBMP-2-induced bone. These results suggest that the application of low MFs has potential to facilitate the regeneration of bone with sufficient mechanical strength by controlling the orientation of newly formed bone.

Iguratimod inhibits osteoclastogenesis by modulating the RANKL and TNF-α signaling pathways

BACKGROUND

Iguratimod, a small molecular drug, has been proven to have effective bone protection for treatment of patients with bone loss-related diseases, such as rheumatoid arthritis (RA). However, the exact bone protective mechanism of iguratimod remains to be determined. The purpose of this study was to better explore the underlying mechanism of bone protection of iguratimod.

METHODS

Bone marrow monocytes from C57/BL6 mice were stimulated with either RANKL or TNF-α plus M-CSF. The effects of iguratimod on morphology and function of osteoclasts were confirmed by TRAP staining and bone resorption assay, respectively. The expression of osteoclast related genes was detected by RT-PCR and the activation of signal pathway was detected by Western blotting. We used rodent models of osteoporosis (ovariectomy) and of arthritis (modified TNF-α-induced osteoclastogenesis) to evaluate the osteoprotective effect of iguratimod in vivo.

RESULTS

Iguratimod potently inhibited osteoclast formation in a dose-dependent manner at the early stage of RANKL-induced osteoclastogenesis, whereas iguratimod had no effect on M-CSF-induced proliferation and RANK expression in bone marrow monocytes. Bone resorption was significantly reduced by both early and late addition of iguratimod. Administration of iguratimod prevented bone loss in ovariectomized mice. The blockage of osteoclastogenesis elicited by iguratimod results from abrogation of the p38、ERK and NF-κB pathways induced by RANKL. Importantly, Iguratimod also dampened TNF-α-induced osteoclastogenesis in vitro and attenuated osteoclasts generation in vivo through disrupting NF-κB late nuclear translocation without interfering with IκBα degradation.

CONCLUSIONS

Iguratimod not only suppresses osteoclastogenesis by interfering with RANKL and TNF-α signals, but also inhibits the bone resorption of mature osteoclasts. These results provided promising evidence for the therapeutic application of iguratimod as a unique treatment option against RA and especially in prevention of bone loss.

RANKL as the master regulator of osteoclast differentiation

RANKL, the essential cue for osteoclast differentiation, is the membrane-bound factor expressed by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes. In vivo evidence indicates that RANKL functions as the indispensable and irreplaceable in the program of osteoclast differentiation. The reason why RANKL plays a critical role in osteoclastogenesis is discussed from the viewpoint of the distinct signaling pathways mediated by co-stimulatory receptors and the key transcription factor NFATc1.

The Osteocyte: New Insights

Osteocytes are an ancient cell, appearing in fossilized skeletal remains of early fish and dinosaurs. Despite its relative high abundance, even in the context of nonskeletal cells, the osteocyte is perhaps among the least studied cells in all of vertebrate biology. Osteocytes are cells embedded in bone, able to modify their surrounding extracellular matrix via specialized molecular remodeling mechanisms that are independent of the bone forming osteoblasts and bone-resorbing osteoclasts. Osteocytes communicate with osteoclasts and osteoblasts via distinct signaling molecules that include the RankL/OPG axis and the Sost/Dkk1/Wnt axis, among others. Osteocytes also extend their influence beyond the local bone environment by functioning as an endocrine cell that controls phosphate reabsorption in the kidney, insulin secretion in the pancreas, and skeletal muscle function. These cells are also finely tuned sensors of mechanical stimulation to coordinate with effector cells to adjust bone mass, size, and shape to conform to mechanical demands.

Is There a Governing Role of Osteocytes in Bone Tissue Regeneration?

PURPOSE OF REVIEW

Bone regeneration plays an important role in contemporary clinical treatment. Bone tissue engineering should result in successful bone regeneration to restore congenital or acquired bone defects in the human skeleton. Osteocytes are thought to have a governing role in bone remodeling by regulating osteoclast and osteoblast activity, and thus bone loss and formation. In this review, we address the so far largely unknown role osteocytes may play in bone tissue regeneration.

RECENT FINDINGS

Osteocytes release biochemical signaling molecules involved in bone remodeling such as prostaglandins, nitric oxide, Wnts, and insulin-like growth factor-1 (IGF-1). Treatment of mesenchymal stem cells in bone tissue engineering with prostaglandins (e.g., PGE₂, PGI₂, PGF_2α), nitric oxide, IGF-1, or Wnts (e.g., Wnt3a) improves osteogenesis. This review provides an overview of the functions of osteocytes in bone tissue, their interaction with other bone cells, and their role in bone remodeling. We postulate that osteocytes may have a pivotal role in bone regeneration as well, and consequently that the bone regeneration process may be improved effectively and rapidly if osteocytes are optimally used and stimulated.

Multitasking by the OC Lineage during Bone Infection: Bone Resorption, Immune Modulation, and Microbial Niche

Bone infections, also known as infectious osteomyelitis, are accompanied by significant inflammation, osteolysis, and necrosis. Osteoclasts (OCs) are the bone-resorbing cells that work in concert with osteoblasts and osteocytes to properly maintain skeletal health and are well known to respond to inflammation by increasing their resorptive activity. OCs have typically been viewed merely as effectors of pathologic bone resorption, but recent evidence suggests they may play an active role in the progression of infections through direct effects on pathogens and via the immune system. This review discusses the host- and pathogen-derived factors involved in the in generation of OCs during infection, the crosstalk between OCs and immune cells, and the role of OC lineage cells in the growth and survival of pathogens, and highlights unanswered questions in the field.