Prostaglandin E2 induces expression of receptor activator of nuclear factor-kappa B ligand/osteoprotegrin ligand on pre-B cells: implications for accelerated osteoclastogenesis in estrogen deficiency

Exercise and osteoimmunology in bone remodeling

Bones can form the scaffolding of the body, support the organism, coordinate somatic movements, and control mineral homeostasis and hematopoiesis. The immune system plays immune supervisory, defensive, and regulatory roles in the organism, which mainly consists of immune organs (spleen, bone marrow, tonsils, lymph nodes, etc.), immune cells (granulocytes, platelets, lymphocytes, etc.), and immune molecules (immune factors, interferons, interleukins, tumor necrosis factors, etc.). Bone and the immune system have long been considered two distinct fields of study, and the bone marrow, as a shared microenvironment between the bone and the immune system, closely links the two. Osteoimmunology organically combines bone and the immune system, elucidates the role of the immune system in bone, and creatively emphasizes its interdisciplinary characteristics and the function of immune cells and factors in maintaining bone homeostasis, providing new perspectives for skeletal-related field research. In recent years, bone immunology has gradually become a hot spot in the study of bone-related diseases. As a new branch of immunology, bone immunology emphasizes that the immune system can directly or indirectly affect bones through the RANKL/RANK/OPG signaling pathway, IL family, TNF-α, TGF-β, and IFN-γ. These effects are of great significance for understanding inflammatory bone loss caused by various autoimmune or infectious diseases. In addition, as an external environment that plays an important role in immunity and bone, this study pays attention to the role of exercise-mediated bone immunity in bone reconstruction.

Osteoinduction and osteoimmunology: Emerging concepts

The recognition and importance of immune cells during bone regeneration, including around bone biomaterials, has led to the development of an entire field termed "osteoimmunology," which focuses on the connection and interplay between the skeletal system and immune cells. Most studies have focused on the "osteogenic" capacity of various types of bone biomaterials, and much less focus has been placed on immune cells despite being the first cell type in contact with implantable devices. Thus, the amount of literature generated to date on this topic makes it challenging to extract needed information. This review article serves as a guide highlighting advancements made in the field of osteoimmunology emphasizing the role of the osteoimmunomodulatory properties of biomaterials and their impact on osteoinduction. First, the various immune cell types involved in bone biomaterial integration are discussed, including the prominent role of osteal macrophages (OsteoMacs) during bone regeneration. Thereafter, key biomaterial properties, including topography, wettability, surface charge, and adsorption of cytokines, growth factors, ions, and other bioactive molecules, are discussed in terms of their impact on immune responses. These findings highlight and recognize the importance of the immune system and osteoimmunology, leading to a shift in the traditional models used to understand and evaluate biomaterials for bone regeneration.

Anti-NF-κB peptide derived from nuclear acidic protein attenuates ovariectomy-induced osteoporosis in mice

NF-κB is a transcription factor that is activated with aging. It plays a key role in the development of osteoporosis by promoting osteoclast differentiation and inhibiting osteoblast differentiation. In this study, we developed a small anti-NF-κB peptide called 6A-8R from a nuclear acidic protein (also known as macromolecular translocation inhibitor II, Zn2+-binding protein, or parathymosin) that inhibits transcriptional activity of NF-κB without altering its nuclear translocation and binding to DNA. Intraperitoneal injection of 6A-8R attenuated ovariectomy-induced osteoporosis in mice by inhibiting osteoclast differentiation, promoting osteoblast differentiation, and inhibiting sclerostin production by osteocytes in vivo with no apparent side effects. Conversely, in vitro, 6A-8R inhibited osteoclast differentiation by inhibiting NF-κB transcriptional activity, promoted osteoblast differentiation by promoting Smad1 phosphorylation, and inhibited sclerostin expression in osteocytes by inhibiting myocyte enhancer factors 2C and 2D. These findings suggest that 6A-8R has the potential to be an antiosteoporotic therapeutic agent with uncoupling properties.

Guided Bone Regeneration Using Barrier Membrane in Dental Applications

Guided bone regeneration (GBR) is a widely used technique in preclinical and clinical studies due to its predictability. Its main purpose is to prevent the migration of soft tissue into the osseous wound space, while allowing osseous cells to migrate to the site. GBR is classified into two main categories: resorbable and non-resorbable membranes. Resorbable membranes do not require a second surgery but tend to have a short resorption period. Conversely, non-resorbable membranes maintain their mechanical strength and prevent collapse. However, they require removal and are susceptible to membrane exposure. GBR is often used with bone substitute graft materials to fill the defect space and protect the bone graft. The membrane can also undergo various modifications, such as surface modification and biological factor loading, to improve barrier functions and bone regeneration. In addition, bone regeneration is largely related to osteoimmunology, a new field that focuses on the interactions between bone and the immune system. Understanding these interactions can help in developing new treatments for bone diseases and injuries. Overall, GBR has the potential to be a powerful tool in promoting bone regeneration. Further research in this area could lead to advancements in the field of bone healing. This review will highlight resorbable and non-resorbable membranes with cellular responses during bone regeneration, provide insights into immunological response during bone remodeling, and discuss antibacterial features.

Immune-related gene IL17RA as a diagnostic marker in osteoporosis

Objectives: Bone immune disorders are major contributors to osteoporosis development. This study aims to identify potential diagnostic markers and molecular targets for osteoporosis treatment from an immunological perspective. Method: We downloaded dataset GSE56116 from the Gene Expression Omnibus database, and identified differentially expressed genes (DEGs) between normal and osteoporosis groups. Subsequently, differentially expressed immune-related genes (DEIRGs) were identified, and a functional enrichment analysis was performed. A protein-protein interaction network was also constructed based on data from STRING database to identify hub genes. Following external validation using an additional dataset (GSE35959), effective biomarkers were confirmed using RT-qPCR and immunohistochemical (IHC) staining. ROC curves were constructed to validate the diagnostic values of the identified biomarkers. Finally, a ceRNA and a transcription factor network was constructed, and a Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis was performed to explore the biological functions of these diagnostic markers. Results: In total, 307 and 31 DEGs and DEIRGs were identified, respectively. The enrichment analysis revealed that the DEIRGs are mainly associated with Gene Ontology terms of positive regulation of MAPK cascade, granulocyte chemotaxis, and cytokine receptor. protein-protein interaction network analysis revealed 10 hub genes: FGF8, KL, CCL3, FGF4, IL9, FGF9, BMP7, IL17RA, IL12RB2, CD40LG. The expression level of IL17RA was also found to be significantly high. RT-qPCR and immunohistochemical results showed that the expression of IL17RA was significantly higher in osteoporosis patients compared to the normal group, as evidenced by the area under the curve Area Under Curve of 0.802. Then, we constructed NEAT1-hsa-miR-128-3p-IL17RA, and SNHG1-hsa-miR-128-3p-IL17RA ceRNA networks in addition to ERF-IL17RA, IRF8-IL17RA, POLR2A-IL17RA and ERG-IL17RA transcriptional networks. Finally, functional enrichment analysis revealed that IL17RA was involved in the development and progression of osteoporosis by regulating local immune and inflammatory processes in bone tissue. Conclusion: This study identifies the immune-related gene IL17RA as a diagnostic marker of osteoporosis from an immunological perspective, and provides insight into its biological function.

Role of Mast-Cell-Derived RANKL in Ovariectomy-Induced Bone Loss in Mice

Mast cells may contribute to osteoporosis development, because patients with age-related or post-menopausal osteoporosis exhibit more mast cells in the bone marrow, and mastocytosis patients frequently suffer from osteopenia. We previously showed that mast cells crucially regulated osteoclastogenesis and bone loss in ovariectomized, estrogen-depleted mice in a preclinical model for post-menopausal osteoporosis and found that granular mast cell mediators were responsible for these estrogen-dependent effects. However, the role of the key regulator of osteoclastogenesis, namely, receptor activator of NFκB ligand (RANKL), which is secreted by mast cells, in osteoporosis development has, to date, not been defined. Here, we investigated whether mast-cell-derived RANKL participates in ovariectomy (OVX)-induced bone loss by using female mice with a conditional Rankl deletion. We found that this deletion in mast cells did not influence physiological bone turnover and failed to protect against OVX-induced bone resorption in vivo, although we demonstrated that RANKL secretion was significantly reduced in estrogen-treated mast cell cultures. Furthermore, Rankl deletion in mast cells did not influence the immune phenotype in non-ovariectomized or ovariectomized mice. Therefore, other osteoclastogenic factors released by mast cells might be responsible for the onset of OVX-induced bone loss.

Interactions of B-lymphocytes and bone cells in health and disease

Bone remodeling occurs through the interactions of three major cell lineages, osteoblasts, which mediate bone formation, osteocytes, which derive from osteoblasts, sense mechanical force and direct bone turnover, and osteoclasts, which mediate bone resorption. However, multiple additional cell types within the bone marrow, including macrophages, T lymphocytes and B lymphocytes influence the process. The bone marrow microenvironment, which is supported, in part, by bone cells, forms a nurturing network for B lymphopoiesis. In turn, developing B lymphocytes influence bone cells. Bone health during homeostasis depends on the normal interactions of bone cells with other lineages in the bone marrow. In disease state these interactions become pathologic and can cause abnormal function of bone cells and inadequate repair of bone after a fracture. This review summarizes what is known about the development of B lymphocytes and the interactions of B lymphocytes with bone cells in both health and disease.

Hematopoietic cytoplasmic adaptor protein Hem1 promotes osteoclast fusion and bone resorption in mice

Hem1 (hematopoietic protein 1), a hematopoietic cell-specific member of the Hem family of cytoplasmic adaptor proteins, is essential for lymphopoiesis and innate immunity as well as for the transition of hematopoiesis from the fetal liver to the bone marrow. However, the role of Hem1 in bone cell differentiation and bone remodeling is unknown. Here, we show that deletion of Hem1 resulted in a markedly increase in bone mass because of defective bone resorption in mice of both sexes. Hem1-deficient osteoclast progenitors were able to differentiate into osteoclasts, but the osteoclasts exhibited impaired osteoclast fusion and decreased bone-resorption activity, potentially because of decreased mitogen-activated protein kinase and tyrosine kinase c-Abl activity. Transplantation of bone marrow hematopoietic stem and progenitor cells from wildtype into Hem1 knockout mice increased bone resorption and normalized bone mass. These findings indicate that Hem1 plays a pivotal role in the maintenance of normal bone mass.

Reduction of mature B cells and immunoglobulins results in increased trabecular bone

Inflammation has a significant effect on bone remodeling and can result in bone loss via increased stimulation of osteoclasts. Activated immunoglobulins, especially autoantibodies, can increase osteoclastogenesis and are associated with pathological bone loss. Whether immunoglobulins and mature B lymphocytes are important for general bone architecture has not been completely determined. Here we demonstrate, using a transgenic mouse model, that reduction of mature B cells and immunoglobulins leads to increased trabecular bone mass compared to wild‐type (WT) littermate controls. This bone effect is associated with a decrease in the number of osteoclasts and reduced bone resorption, despite decreased expression of osteoprotegerin. We also demonstrate that the reduction of mature B cells and immunoglobulins do not prevent bone loss caused by estrogen deficiency or arthritis compared to WT littermate controls. In conclusion, the reduction of mature B cells and immunoglobulins results in disturbed regulation of trabecular bone turnover in healthy conditions but is dispensable for pathological bone loss. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Immunomodulation Effect of Biomaterials on Bone Formation

Traditional bone replacement materials have been developed with the goal of directing the osteogenesis of osteoblastic cell lines toward differentiation and therefore achieving biomaterial-mediated osteogenesis, but the osteogenic effect has been disappointing. With advances in bone biology, it has been revealed that the local immune microenvironment has an important role in regulating the bone formation process. According to the bone immunology hypothesis, the immune system and the skeletal system are inextricably linked, with many cytokines and regulatory factors in common, and immune cells play an essential role in bone-related physiopathological processes. This review combines advances in bone immunology with biomaterial immunomodulatory properties to provide an overview of biomaterials-mediated immune responses to regulate bone regeneration, as well as methods to assess the bone immunomodulatory properties of bone biomaterials and how these strategies can be used for future bone tissue engineering applications.

RANKL Blockade Reduces Cachexia and Bone Loss Induced by Non-Metastatic Ovarian Cancer in Mice

Tumor- and bone-derived soluble factors have been proposed to participate in the alterations of skeletal muscle size and function in cachexia. We previously showed that mice bearing ovarian cancer (OvCa) exhibit cachexia associated with marked bone loss, whereas bone-targeting agents, such as bisphosphonates, are able to preserve muscle mass in animals exposed to anticancer drugs. De-identified CT images and plasma samples from female patients affected with OvCa were used for body composition assessment and quantification of circulating cross-linked C-telopeptide type I (CTX-I) and receptor activator of NF-kB ligand (RANKL), respectively. Female mice bearing ES-2 tumors were used to characterize cancer- and RANKL-associated effects on muscle and bone. Murine C2C12 and human HSMM myotube cultures were used to determine the OvCa- and RANKL-dependent effects on myofiber size. To the extent of isolating new regulators of bone and muscle in cachexia, here we demonstrate that subjects affected with OvCa display evidence of cachexia and increased bone turnover. Similarly, mice carrying OvCa present high RANKL levels. By using in vitro and in vivo experimental models, we found that elevated circulating RANKL is sufficient to cause skeletal muscle atrophy and bone resorption, whereas bone preservation by means of antiresorptive and anti-RANKL treatments concurrently benefit muscle mass and function in cancer cachexia. Altogether, our data contribute to identifying RANKL as a novel therapeutic target for the treatment of musculoskeletal complications associated with RANKL-expressing non-metastatic cancers. © 2021 American Society for Bone and Mineral Research (ASBMR).

The Impact of High Dose Glucocorticoids on Bone Health and Fracture Risk in Systemic Vasculitides

Systemic vasculitides are a range of conditions characterized by inflammation of blood vessels which may manifest as single organ or life-threatening multisystem disease. The treatment of systemic vasculitis varies depending on the specific disease but historically has involved initial treatment with high dose glucocorticoids alone or in conjunction with other immunosuppressive agents. Prolonged glucocorticoid treatment is frequently required as maintenance treatment. Patients with small and large vessel vasculitis are at increased risk of fracture. Osteoporosis may occur due to intrinsic factors such as chronic inflammation, impaired renal function and to a large extent due to pharmacological therapy with high dose glucocorticoid or combination treatments. This review will outline the known mechanism of bone loss in vasculitis and will summarize factors attributing to fracture risk in different types of vasculitis. Osteoporosis treatment with specific consideration for patients with vasculitis will be discussed. The use of glucocorticoid sparing immunosuppressive agents in the treatment of systemic vasculitis is a significant area of ongoing research. Adjunctive treatments are used to reduce cumulative doses of glucocorticoids and therefore may significantly decrease the associated fracture risk in patients with vasculitis. Lastly, we will highlight the many unknowns in the relation between systemic vasculitis, its treatment and bone health and will outline key research priorities for this field.

Persistent Abnormal Immunocytes Induced Systemic Bone Loss in Locally Irradiated Rats

Chronic and systemic bone complications frequently occur in patients who undergo radiotherapy; however, the pathological mechanisms underlying these complications remain unclear. This study aimed to observe persistent and systemic changes in locally irradiated rats and to determine the systemic pathological changes that persistently affect bone metabolism. We examined the inflammatory and oxidative stress responses that occurred after local irradiation using enzyme immunoassays and biochemical analyses. Lymphocytes obtained from the blood, spleen, thymus, and bone marrow were evaluated using flow cytometry. The proliferation and apoptosis characteristics of co-cultured bone marrow-derived mesenchymal stem cells (BMSCs) were detected by MTT assay and PI/Annexin V-FITC staining, respectively, and the differentiation of BMSCs was measured according to alkaline phosphatase (ALP) staining, alizarin red staining, and Oil Red O staining and by evaluating the mRNA expression of ALP, osteocalcin (OCN), osteopontin (OPN), collagen I, Runx2, and PPARγ. Our results revealed that no significant or continuous differences were present in the inflammatory response or the oxidative stress response throughout the body after local irradiation. B lymphocyte levels increased continuously in the blood, spleen, and bone marrow after local irradiation. T lymphocyte levels were decreased at 2 weeks after local irradiation, and CD8⁺T lymphocyte levels were increased in the blood, thymus, and bone marrow at 12 weeks after local irradiation. The ratio of CD4⁺/CD8⁺T lymphocytes began to decrease during the early phase after local irradiation and became significantly decreased at 12 weeks after local irradiation. Normal BMSCs co-cultured with lymphocytes derived from irradiated rats exhibited decreased proliferation and increased apoptosis, and the ALP staining intensity, alizarin red staining intensity, and mRNA expression of related genes were all also decreased. Oil Red O staining intensity and mRNA expression of PPARγ were both increased. Lymphocyte levels contribute to chronic and systemic bone complications after radiotherapy by inhibiting the proliferation and osteoblastogenesis of BMSCs.

Role of irisin in effects of chronic exercise on muscle and bone in ovariectomized mice

INTRODUCTION

Exercise is beneficial for the prevention and treatment of osteoporosis. Skeletal muscle affects other tissues via myokines, the release of which is regulated by acute exercise. However, the effects of chronic exercise on myokines linking muscle to bone have not been fully elucidated. Therefore, we investigated the effects of chronic exercise on bone and myokines using ovariectomized (OVX) mice.

MATERIALS AND METHODS

Treadmill exercise with moderate intensity was performed for 8 weeks after OVX or sham surgery. We measured bone mineral density (BMD) at the femurs and tibias of mice by quantitative computed tomography and myokine mRNA levels in the gastrocnemius and soleus muscles.

RESULTS

Treadmill exercise ameliorated decreases in trabecular and cortical BMD in the femurs of OVX mice. Irisin is a proteolytic product of fibronectin type III domain-containing 5 (Fndc5). Among the myokines examined, treadmill exercise increased irisin protein and Fndc5 mRNA levels in the gastrocnemius and soleus muscles of sham and OVX mice. Treadmill exercise increased peroxisome proliferator-activated receptor γ coactivator-1α mRNA levels in the gastrocnemius muscles of mice. Fndc5 mRNA levels in the gastrocnemius muscles positively correlated with trabecular BMD, but not with cortical BMD, at the femurs and tibias of mice in simple regression analyses.

CONCLUSIONS

We demonstrated that chronic exercise elevated irisin expression in the gastrocnemius and soleus muscles of estrogen-deficient mice. Irisin might be related to increases in trabecular BMD in mice; however, further studies are needed to clarify the involvement of irisin in the effects of chronic exercise on muscle/bone interactions.

The hidden secrets of soluble RANKL in bone biology

The discovery of cytokine tumor necrosis factor (TNF) in the 20th century revealed numerous secrets about organ development. In particular, the functions identified for the receptor activator of nuclear factor kappa-β (NF-κβ) ligand (also known as the RANKL/osteoprotegerin ligand (OPGL) or RANK ligand/TNFSF11) in the homeostasis of skeletal structure, function and regulation were not anticipated. Empirical evidence established the receptor-ligand interaction of RANKL with RANK in osteoclast formation. Reverse signaling of RANKL triggers NF-κβ for the degradation of β-catenin to inhibit bone formation. There is also evidence that RANKL modifies the behavior of other cells in the bone microenvironment, including osteoblasts, chondrocytes, endothelial cells and lymphocytes during normal (homeostatic) and diseased (osteoimmune) states. Two forms of RANKL, i.e., soluble and membrane-bound RANKL, are produced by bone cells. Even though soluble RANKL (sRANKL) and membrane-bound RANKL (mRANKL) both stimulate osteoclast formation in vitro, their biological roles are different. mRANKL triggers osteoclastogenesis by binding to RANK through cell-cell interaction; however, sRANKL released from osteogenic cells binds to RANK without cell-cell interaction. This review attempts to hypothesize how sRANKL functions biologically in bone and explore how this hypothesis might influence future research.

Plasma Fatty Acids and Quantitative Ultrasound, DXA and pQCT Derived Parameters in Postmenopausal Spanish Women

Different factors may contribute to the development of osteopenia or osteoporosis. Fatty acids are key nutrients for health, and a number of studies have reported an association between bone mineral density (BMD) and fatty acid intake. We aimed to investigate the relationships between serum levels of different fatty acids and bone parameters determined by quantitative bone ultrasound (QUS), peripheral quantitative computed tomography (pQCT), and dual-energy X-ray absorptiometry (DXA) in a sample of Spanish postmenopausal women. We enrolled a total of 301 postmenopausal women (median age 59 years; interquartile range (IQR) 7) in this study. All participants underwent full densitometric screening, including calcaneal quantitative ultrasound (QUS), peripheral quantitative computed tomography (pQCT), and dual-energy X-ray absorptiometry (DXA), as well as plasma fatty acid measurement. After adjustment for potential confounders, plasma n-3 polyunsaturated fatty acid (PUFA) levels correlated with BMD in the spine (r = 0.150; p = 0.014) and femoral neck (r = 0.143; p = 0.019). By multiple linear regression, an independent statistically significant positive relationship was observed between BMD in the spine and BMI (β = 0.288; p = 0.001) as well as total plasma n-3 PUFAs (β = 0.155; p = 0.009). The plasma n-3 PUFA level was also a significant and positive predictor of BMD at the femoral neck (β = 0.146; p = 0.009). Independent risk factors for low BMD (T-score ≤ 1) were determined by logistic regression analysis, and a relatively high level of plasma n-3 PUFAs (OR = 0.751; 95% CI 0.587-0.960, p = 0.022) was identified as a protective factor against low bone mass. In this single-center sample of Spanish postmenopausal women, we reported a significant positive and statistically independent association between BMD and plasma levels of n-3 PUFAs.

The Role of the Immune Microenvironment in Bone Regeneration

Bone is an active tissue, being constantly renewed in healthy individuals with participation of the immune system to a large extent. Any imbalance between the processes of bone formation and bone resorption is linked to various inflammatory bone diseases. The immune system plays an important role in tissue formation and bone resorption. Recently, many studies have demonstrated complex interactions between the immune and skeletal systems. Both of immune cells and cytokines contribute to the regulation of bone homeostasis, and bone cells, including osteoblasts, osteoclasts, osteocytes, also influence the cellular functions of immune cells. These crosstalk mechanisms between the bone and immune system finally emerged, forming a new field of research called osteoimmunology. Therefore, the immune microenvironment is crucial in determining the speed and outcome of bone healing, repair, and regeneration. In this review, we summarise the role of the immune microenvironment in bone regeneration from the aspects of immune cells and immune cytokines. The elucidation of immune mechanisms involved in the process of bone regeneration would provide new therapeutic targets for improving the curative effects of bone injury treatment.

ROCK-II inhibition suppresses impaired mechanobiological responses in early estrogen deficient osteoblasts

Mechanobiological responses by osteoblasts are governed by downstream Rho-ROCK signalling through actin cytoskeleton re-arrangements but whether these responses are influenced by estrogen deficiency during osteoporosis remains unknown. The objective of this study was to determine alterations in the mechanobiological responses of estrogen-deficient osteoblasts and investigate whether an inhibitor of the Rho-ROCK signalling can revert these changes. MC3T3-E1 cells were pre-treated with 10 nM 17-β estradiol for 7 days and further cultured with or without estradiol for next 2 days. These cells were treated with or without ROCK-II inhibitor, Y-27632, and oscillatory fluid flow (OFF, 1Pa, 0.5 Hz, 1 h) was applied. Here, we report that Prostaglandin E₂ release, Runt-related transcription factor 2 and Osteopontin gene expression were significantly enhanced in response to OFF in estrogen-deficient cells than in cells with estrogen (3.73 vs 1.63 pg/ng DNA; 13.5 vs 2.6 fold, 2.1 vs 0.4 fold respectively). Upon ROCK-II inhibition, these enhanced effects of estrogen deficiency were downregulated. OFF increased the fibril anisotropy in cells pre-treated with estrogen and this increase was suppressed upon ROCK-II inhibition. This study is the first to demonstrate altered mechanobiological responses by osteoblasts during early estrogen deficiency and that these responses to OFF can be suppressed upon ROCK inhibition.

The presence of neutrophils causes RANKL expression in periodontal tissue, giving rise to osteoclast formation

BACKGROUNDS AND OBJECTIVE

Increased neutrophil infiltration and osteoclast formation are key characteristics of periodontitis. The effect of these neutrophils on osteoclast formation in periodontitis remains unclear. Therefore, we investigated the effects of neutrophils on osteoclast formation in a neutrophil-deficient mouse model of periodontitis.

METHODS

Anti-Ly6G antibody (Ab) was used for neutrophil depletion in two mouse models: periodontitis and air pouch. In the periodontitis experiments, mice were divided into PBS-administered control (C), control Ab-administered periodontitis (P), and anti-Ly6G Ab-administered periodontitis (P + Ly6G) groups. Periodontitis was induced by ligature of mandibular first molars. In the air pouch experiments, mice were divided into PBS-administered (C), LPS and control Ab-administered (LPS), and LPS and anti-Ly6G Ab-administered (LPS + Ly6G) groups. Neutrophil migration into air pouches was induced by LPS injection. Flow cytometry was used to examine CD11b⁺ Ly6G⁺ neutrophils in the blood of periodontitis mice and CD11b⁺ Ly6G⁺ RANKL⁺ neutrophils in exudates of air pouch mice. In periodontal tissue, Ly6G⁺ neutrophil and RANKL⁺ cell numbers in periodontal ligament and alveolar bone areas were estimated using immunohistochemistry, osteoclast numbers were measured using TRAP assay, and alveolar bone loss was determined by H&E staining.

RESULTS

In blood, CD11b⁺ Ly6G⁺ neutrophils were found in greater percentage in the P group than in the C group on days 3 and 7. However, the percentage of neutrophils was lower in the P + Ly6G group than in the C and P groups. In periodontal tissue, the numbers of Ly6G⁺ neutrophils and RANKL⁺ cells were lower in the P + Ly6G group than in the P group on day 3. Ly6G⁺ neutrophil numbers decreased more in the P + Ly6G group than in the P group on day 7, but RANKL⁺ cell numbers did not decrease in the P + Ly6G group. In exudates, the number of CD11b⁺ Ly6G⁺ RANKL⁺ neutrophils was greater in the LPS group than in the C and LPS + Ly6G groups. On days 3 and 7, the numbers of osteoclasts and alveolar bone loss were greater in periodontal tissue in the P and P + Ly6G groups than in the C group. Interestingly, there were fewer osteoclasts in the P + Ly6G group than in the P group on day 3.

CONCLUSION

Neutrophil deficiency caused a reduction in numbers of both RANKL⁺ cells and osteoclasts in periodontitis-induced tissues only on day 3. Furthermore, in the LPS-injected air pouch model, neutrophil deficiency reduced the influx of RANKL⁺ neutrophils. These findings suggest that the presence of neutrophils induces RANKL expression and could induce osteoclast formation in the early stages of periodontitis.

A Novel 3D Osteoblast and Osteocyte Model Revealing Changes in Mineralization and Pro-osteoclastogenic Paracrine Signaling During Estrogen Deficiency

Recent in vitro studies have revealed that the mechanobiological responses of osteoblasts and osteocytes are fundamentally impaired during estrogen deficiency. However, these two-dimensional (2D) cell culture studies do not account for in vivo biophysical cues. Thus, the objectives of this study are to (1) develop a three-dimensional (3D) osteoblast and osteocyte model integrated into a bioreactor and (2) apply this model to investigate whether estrogen deficiency leads to changes in osteoblast to osteocyte transition, mechanosensation, mineralization, and paracrine signaling associated with bone resorption by osteoclasts. MC3T3-E1s were expanded in media supplemented with estrogen (17β-estradiol). These cells were encapsulated in gelatin-mtgase before culture in (1) continued estrogen (E) or (2) no further estrogen supplementation. Constructs were placed in gas permeable and water impermeable cell culture bags and maintained at 5% CO2 and 37°C. These bags were either mechanically stimulated in a custom hydrostatic pressure (HP) bioreactor or maintained under static conditions (control). We report that osteocyte differentiation, characterized by the presence of dendrites and staining for osteocyte marker dentin matrix acidic phosphoprotein 1 (DMP1), was significantly greater under estrogen withdrawal (EW) compared to under continuous estrogen treatment (day 21). Mineralization [bone sialoprotein (BSP), osteopontin (OPN), alkaline phosphatase (ALP), calcium] and gene expression associated with paracrine signaling for osteoclastogenesis [receptor activator of nuclear factor kappa-β ligand (RANKL)/osteoprotegerin OPG ratio] were significantly increased in estrogen deficient and mechanically stimulated cells. Interestingly, BSP and DMP-1 were also increased at day 1 and day 21, respectively, which play a role in regulation of biomineralization. Furthermore, the increase in pro-osteoclastogenic signaling may be explained by altered mechanoresponsiveness of osteoblasts or osteocytes during EW. These findings highlight the impact of estrogen deficiency on bone cell function and provide a novel in vitro model to investigate the mechanisms underpinning changes in bone cells after estrogen deficiency.

Age and leptinemia association with anxiety and depression symptoms in overweight middle-aged women

OBJECTIVE

The aim of the study was to investigate the effect of menopause and of postmenopausal stages on depression and anxiety symptoms, and whether these symptoms associate with anthropometric, metabolic, and hormonal parameters in midlife women.

METHODS

Postmenopausal women (age 50-65), either at early (EPM, n=33) or late (LPM, n = 23) postmenopause, and 23 premenopausal controls (PreM, age 40-50), matched for BMI with the PM groups, were studied. Blood biochemical and hormonal determinations, bioimpedance anthropometry, and depression and anxiety symptoms (Beck's depression [BDI] and anxiety [BAI] inventories) were conducted.

RESULTS

The BAI score was higher in both PM groups than in the PreM group. In contrast, only the LPM group showed a significantly elevated BDI score. All groups presented overweight and abdominal obesity, having similar BMI and waist/hip ratio values. Both PM groups showed insulin resistance, whereas only the LPM group presented decreased skeletal muscle mass and basal metabolic rate. Correlation analysis, including all 79 middle-aged women, showed age, percentage body fat, waist/hip ratio, and leptinemia to correlate positively with the anxiety and depression scores. Multivariate regression showed leptin and age to associate positively with depressive- and anxious-like symptoms.

CONCLUSIONS

Postmenopausal women presented impaired body composition, energy expenditure, insulin sensitivity, and mental symptoms, in comparison to similarly overweight premenopausal women. Among all the overweight midlife women, these symptoms were more strongly associated with age and leptin levels than with reproductive aging itself. The data indicate that, among overweight middle-aged women with abdominal obesity, the aging process and the development of leptin resistance are associated with impairment of mental health.

LIGHT/TNFSF14 regulates estrogen deficiency-induced bone loss

Bone loss induced by ovariectomy is due to the direct activity on bone cells and mesenchymal cells and to the dysregulated activity of bone marrow cells, including immune cells and stromal cells, but the underlying mechanisms are not completely known. Here, we demonstrate that ovariectomy induces the T-cell co-stimulatory cytokine LIGHT, which stimulates both osteoblastogenesis and osteoclastogenesis by modulating osteoclastogenic cytokine expression, including TNF, osteoprotegerin, and the receptor activator of nuclear factor-κB ligand (RANKL). Predictably, LIGHT-deficient (Tnfsf14^-/- ) mice are protected from ovariectomy-dependent bone loss, whereas trabecular bone mass increases in mice deficient in both LIGHT and T and B lymphocytes (Rag ^-/- Tnfsf14 ^-/- ) and is associated with an inversion of the TNF and RANKL/OPG ratio. Furthermore, women with postmenopausal osteoporosis display high levels of LIGHT in circulating T cells and monocytes. Taken together, these results indicate that LIGHT mediates bone loss induced by ovariectomy, suggesting that patients with postmenopausal osteoporosis may benefit from LIGHT antagonism. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Ovariectomy increases RANKL protein expression in bone marrow adipocytes of C3H/HeJ mice

Estrogen deficiency induces bone loss by increasing bone resorption, in part through upregulation of receptor activator of nuclear factor-κB ligand (RANKL). RANKL is secreted by osteoblasts and osteocytes, but more recently bone marrow (pre)adipocytes have also been shown to express RANKL. Estrogen deficiency increases bone marrow adipose tissue (BMAT). The aim of this study was to determine the effect of ovariectomy (OVX) on RANKL protein expression by bone marrow adipocytes in C3H/HeJ mice. Fourteen-week-old female C3H/HeJ mice (n = 20) were randomized to sham surgery (Sham) or OVX. After 4 wk animals were euthanized. BMAT volume fraction (BMAT volume/marrow volume) was quantified by polyoxometalate-based contrast-enhanced nano-computed tomography. The percentage of RANKL-positive bone marrow adipocytes (RANKL-positive bone marrow adipocytes/total adipocytes) and the percentage of RANKL-positive osteoblasts covering the bone surface (bone surface covered in RANKL-positive osteoblasts/total bone surface) were quantified in the distal metaphysis of immunohistochemically stained sections of the left femur. The effects of OVX were analyzed by Student's t test or Mann-Whitney U test. RANKL was detected in osteoblasts, osteocytes, and bone marrow adipocytes. OVX significantly increased mean percentage of RANKL-positive bone marrow adipocytes [mean (SD): Sham 42 (18)%; OVX 64 (12)%; P = 0.029] as well as BMAT volume/marrow volume [median (interquartile range): Sham 1.4 (4.9)%; OVX 7.2 (7.3)%; P = 0.008] compared with Sham. We show that OVX increased both the percentage of RANKL-positive bone marrow adipocytes and the total BMAT volume fraction in C3H/HeJ mice. Therefore, RANKL produced by bone marrow adipocytes could be an important contributor to OVX-induced bone loss in C3H/HeJ mice.

Omnipresence of inflammasome activities in inflammatory bone diseases

The inflammasomes are intracellular protein complexes that are assembled in response to a variety of perturbations including infections and injuries. Failure of the inflammasomes to rapidly clear the insults or restore tissue homeostasis can result in chronic inflammation. Recurring inflammation is also provoked by mutations that cause the constitutive assembly of the components of these protein platforms. Evidence suggests that chronic inflammation is a shared mechanism in bone loss associated with aging, dysregulated metabolism, autoinflammatory, and autoimmune diseases. Mechanistically, inflammatory mediators promote bone resorption while suppressing bone formation, an imbalance which over time leads to bone loss and increased fracture risk. Thus, while acute inflammation is important for the maintenance of bone integrity, its chronic state damages this tissue. In this review, we discuss the role of the inflammasomes in inflammation-induced osteolysis.

Activated B Lymphocyte Inhibited the Osteoblastogenesis of Bone Mesenchymal Stem Cells by Notch Signaling

Estrogen is very important to the differentiation of B lymphocytes; B lymphopoiesis induced by OVX was supposedly involved in osteoporosis. But the effects of B lymphocytes on the osteogenic differentiation of bone mesenchymal stem cells (BMSCs) are not clear. In this study, we detected bone quality and bone loss in a trabecular bone by electronic universal material testing machine and microcomputed tomography (micro-CT) in OVX and splenectomized-ovariectomy (SPX-OVX) rats. Additionally, changes in lymphocytes (B lymphocyte, CD4⁺ and CD8⁺ T lymphocytes, and macrophages) in the bone marrow were analyzed by flow cytometry. The osteogenesis of BMSCs cocultured with normal and LPS-pretreated B lymphocytes was detected by BCIP/NBT and Alizarin red S staining. Measurement of the Notch2, Notch4, Hey1, Hey2, Hes1, and runt-related transcription factor 2 (Runx2) expression in BMSCs cocultured with B lymphocytes was done using real-time PCR. The effects of dexamethasone and DAPT (inhibitor of Notch signaling) on osteogenesis of BMSCs were detected by BCIP/NBT, Alizarin red S staining, and real-time PCR. Osteoporosis happened in OVX rats, more serious in SPX-OVX rats, B lymphocytes increased in OVX rats, and sharply higher in SPX-OVX rats. Osteoporosis did not happen in SPX rats which is still companied with a high increase of B lymphocytes. LPS-pretreated B lymphocytes suppressed the osteogenesis of BMSCs, but the normal B lymphocytes could not. The LPS-pretreated B lymphocytes upregulated the expression of Notch4, Hes1, and Hey2 and downregulated the expression of Runx2 in BMSCs. Dexamethasone and DAPT could downregulate the high expression of Notch4, Hes1, Hey2 and upregulate the low expression of Runx2 in BMSCs which cocultured with LPS treated B lymphocytes, the inhibited ALP and Alizarin red staining in BMSCs which cocultured with LPS treated B lymphocytes also partly restored.

Vibration enhances osteoclastogenesis by inducing RANKL expression via NF-κB signaling in osteocytes

To shorten the duration of orthodontic treatment it is important not only to reduce risks such as dental caries, periodontal disease, and root resorption, but also to decrease pain and discomfort caused by a fixed appliance. Several studies have investigated the effect of vibration applied to fixed appliances to accelerate tooth movement. Although it was reported that vibration accelerates orthodontic tooth movement by enhancing alveolar bone resorption, the underlying cellular and molecular mechanisms remain unclear. In this study, we investigated the effects of vibration on osteoclastogenesis in vitro and in vivo. Vibration applied to pre-osteoclast cell line RAW264.7 cells enhanced cell proliferation but did not affect their differentiation into osteoclasts. Osteocytes in bone are known to be mechanosensitive and to act as receptor activator of nuclear factor kappa B (NF-κB) ligand (RANKL). Therefore, in the present study, vibration was applied to cells from the osteocyte-like cell line MLO-Y4. In MLO-Y4 cells, vibration induced phosphorylation of the inhibitor of NF-κB (IκB) and caused nuclear localization of NF-κB p65. Additionally, vibration increased RANKL mRNA expression, but did not affect osteoprotegerin (OPG) mRNA expression in MLO-Y4 cells, thus resulting in an increased RANKL/OPG ratio. Consistent with these findings, vibration applied during experimental tooth movement increased NF-κB activation and RANKL expression in osteocytes on the compression side of alveolar bone in vivo, whereas vibration had no such effects on the tension side. Furthermore, in a co-culture of MLO-Y4 cells and RAW264.7 cells, vibration applied to MLO-Y4 cells enhanced osteoclastogenesis. These findings suggest that vibration could accelerate orthodontic tooth movement by enhancing osteoclastogenesis through increasing the number of pre-osteoclasts and up-regulating RANKL expression in osteocytes on the compression side of alveolar bone via NF-κB activation.

Effects of 12-week combined exercise on RANKL/RANK/OPG signaling and bone-resorption cytokines in healthy college females

[Purpose]

The OPG/RANK/RANKL signaling is a new family of bone metabolism biomarkers belonging to the immune system. However, the bone metabolism response to long-term exercise in the RANKL/RANK/OPG signaling is less evident. The purpose of this study was to examine these biomarkers in healthy college females after 12-weeks combined exercise intervention.

[Methods]

Participants (N=22, 22.4±1.3yrs) were randomly divided in two different group: 12 in the control group and 10 in the exercise group performing combined exercise program that interventions was conducted 3 times per week for 12 weeks. The outcome measures included serum concentrations of RANKL, OPG and bone metabolic cytokines such as TNF-α and IL-6, and mRNA expressions of same variables from PBMC. VO_2max and bone mineral density (BMD) were measured at before and after exercise intervention.

[Results]

There were no significant differences in the serum RANKL, OPG concentrations and all RANKL/RANK/OPG signaling mRNA expression on interaction effect between group and time (NS). Also no significant differences were found in the serum TNF-α and IL-6 concentrations and mRNA expression (NS). The IL-6 mRNA expression only showed significant difference in the main effect of groups (p<.05). There were also no significant differences in the VO_2max and BMD on interaction effect between group and time (NS).

[Conclusion]

These results suggested that there were no effects on bone mineral density and RANKL/RANK/OPG signaling without the effect of 8-weeks combined exercise on cardiovascular endurance fitness.

Mesenchymal lineage cells and their importance in B lymphocyte niches

Early B lymphopoiesis occurs in the bone marrow and is reliant on interactions with numerous cell types in the bone marrow microenvironment, particularly those of the mesenchymal lineage. Each cellular niche that supports the distinct stages of B lymphopoiesis is unique. Different cell types and signaling molecules are important for the progressive stages of B lymphocyte differentiation. Cells expressing CXCL12 and IL-7 have long been recognized as having essential roles in facilitating progression through stages of B lymphopoiesis. Recently, a number of other factors that extrinsically mediate B lymphopoiesis (positively or negatively) have been identified. In addition, the use of transgenic mouse models to delete specific genes in mesenchymal lineage cells has further contributed to our understanding of how B lymphopoiesis is regulated in the bone marrow. This review will cover the current understanding of B lymphocyte niches in the bone marrow and key extrinsic molecules and signaling pathways involved in these niches, with a focus on how mesenchymal lineage cells regulate B lymphopoiesis.

Effects of mechanical forces on osteogenesis and osteoclastogenesis in human periodontal ligament fibroblasts: A systematic review of in vitro studies

OBJECTIVES

Many in vitro studies have investigated the mechanism by which mechanical signals are transduced into biological signals that regulate bone homeostasis via periodontal ligament fibroblasts during orthodontic treatment, but the results have not been systematically reviewed. This review aims to do this, considering the parameters of various in vitro mechanical loading approaches and their effects on osteogenic and osteoclastogenic properties of periodontal ligament fibroblasts.

METHODS

Specific keywords were used to search electronic databases (EMBASE, PubMed, and Web of Science) for English-language literature published between 1995 and 2017.

RESULTS

A total of 26 studies from the 555 articles obtained via the database search were ultimately included, and four main types of biomechanical approach were identified. Compressive force is characterized by static and continuous application, whereas tensile force is mainly cyclic. Only nine studies investigated the mechanisms by which periodontal ligament fibroblasts transduce mechanical stimulus. The studies provided evidence from in vitro mechanical loading regimens that periodontal ligament fibroblasts play a unique and dominant role in the regulation of bone remodelling during orthodontic tooth movement.

CONCLUSION

Evidence from the reviewed studies described the characteristics of periodontal ligament fibroblasts exposed to mechanical force. This is expected to benefit subsequent research into periodontal ligament fibroblasts and to provide indirectly evidence-based insights regarding orthodontic treatment. Further studies should be performed to explore the effects of static tension on cytomechanical properties, better techniques for static compressive force loading, and deeper analysis of underlying regulatory systems.Cite this article: M. Li, C. Zhang, Y. Yang. Effects of mechanical forces on osteogenesis and osteoclastogenesis in human periodontal ligament fibroblasts: A systematic review of in vitro studies. Bone Joint Res 2019;8:19-31. DOI: 10.1302/2046-3758.81.BJR-2018-0060.R1.

Schlafen2 mutation in mice causes an osteopetrotic phenotype due to a decrease in the number of osteoclast progenitors

Osteoclasts are the bone resorbing cells that derive from myeloid progenitor cells. Although there have been recent advancements in the ability to identify osteoclast progenitors, very little is known about the molecular mechanisms governing their homeostasis. Here, by analyzing the normalized phylogenetic profiles of the Schlafen (Slfn) gene family, we found that it co-evolved with osteoclast-related genes. Following these findings, we used a Slfn2 loss-of-function mutant mouse, elektra, to study the direct role of Slfn2 in osteoclast development and function. Slfn2^eka/eka mice exhibited a profound increase in their cancellous bone mass and a significant reduction in osteoclast numbers. In addition, monocyte cultures from the bone marrow of Slfn2^eka/eka mice showed a reduction in osteoclast number and total resorption area. Finally, we show that the bone marrow of Slfn2^eka/eka mice have significantly less CD11b^-Ly6C^hi osteoclast precursors. Overall, our data suggest that Slfn2 is required for normal osteoclast differentiation and that loss of its function in mice results in an osteopetrotic phenotype.

Updating osteoimmunology: regulation of bone cells by innate and adaptive immunity

Osteoimmunology encompasses all aspects of the cross-regulation of bone and the immune system, including various cell types, signalling pathways, cytokines and chemokines, under both homeostatic and pathogenic conditions. A number of key areas are of increasing interest and relevance to osteoimmunology researchers. Although rheumatoid arthritis has long been recognized as one of the most common autoimmune diseases to affect bone integrity, researchers have focused increased attention on understanding how molecular triggers and innate signalling pathways (such as Toll-like receptors and purinergic signalling pathways) related to pathogenic and/or commensal microbiota are relevant to bone biology and rheumatic diseases. Additionally, although most discussions relating to osteoimmune regulation of homeostasis and disease have focused on the effects of adaptive immune responses on bone, evidence exists of the regulation of immune cells by bone cells, a concept that is consistent with the established role of the bone marrow in the development and homeostasis of the immune system. The active regulation of immune cells by bone cells is an interesting emerging component of investigations that seek to understand how to control immune-associated diseases of the bone and joints.

Dihydromyricetin Protects against Bone Loss in Ovariectomized Mice by Suppressing Osteoclast Activity

Dihydromyricetin (DMY), the main flavonoid component of Ampelopsis grossedentata, possesses pharmacological activities useful for treatment of diseases associated with inflammation and oxidative damage. Because osteoclasts are often involved in chronic low-grade systemic inflammation and oxidative damage, we hypothesized that DMY may be an effective treatment for osteoclast-related diseases. The effects of DMY on osteoclast formation and activity were examined in vitro. Female C57BL/6 mice were ovariectomized to mimic menopause-induced bone loss and treated with DMY, and femur samples were subjected to bone structure and histological analysis, serum biochemical indicators were also measured. DMY suppressed the activation of nuclear factor-κB, c-Fos and mitogen-activated protein kinase, and prevented production of reactive oxygen species. DMY decreased expression of osteoclast-specific genes, including Trap, Mmp-9, Cathepsin K, C-Fos, Nfatc1, and Rank. In addition, DMY prevented bone loss and decreased serum levels of tumor necrosis factor-α, interleukin-1β, and interleukin-6, and with a decrease in the ratio between receptor activator of nuclear factor-κB (RANK) ligand (RANKL) and osteoprotegerin (OPG) in vivo. These findings demonstrate that DMY attenuates bone loss and inhibits osteoclast formation and activity through modulation of multiple pathways both upstream and downstream of RANKL signaling. DMY may thus be a useful option for treatment of osteoclast-related diseases such as rheumatoid arthritis and osteoporosis.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.

The role of stromal cells in inflammatory bone loss

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation, local and systemic bone loss and a lack of compensatory bone repair. Fibroblast-like synoviocytes (FLS) are the most abundant cells of the stroma and a key population in autoimmune diseases such as RA. An increasing body of evidence suggests that these cells play not only an important role in chronic inflammation and synovial hyperplasia, but also impact bone remodelling. Under inflammatory conditions FLS release inflammatory cytokines, regulate bone destruction and formation and communicate with immune cells to control bone homeostasis. Other stromal cells, such as osteoblasts and terminally differentiated osteoblasts, termed osteocytes, are also involved in the regulation of bone homeostasis and are dysregulated during inflammation. This review highlights our current understanding of how stromal cells influence the balance between bone formation and bone destruction. Increasing our understanding of these processes is critical to enable the development of novel therapeutic strategies with which to treat bone loss in RA.

Deletion of a Distal RANKL Gene Enhancer Delays Progression of Atherosclerotic Plaque Calcification in Hypercholesterolemic Mice

Receptor activator of NF-κB ligand (RANKL) is a TNF-like cytokine which mediates diverse physiological functions including bone remodeling and immune regulation. RANKL has been identified in atherosclerotic lesions; however, its role in atherosclerotic plaque development remains elusive. An enhancer located 75 kb upstream of the murine Rankl gene's transcription start site designated D5 is important for its calciotropic hormone- and cytokine-mediated expression. Here, we determined the impact of RANKL levels in atherosclerotic plaque development in the D5 enhancer-null (D5^-/- ) mice in an atherogenic Apoe^-/- background fed a high-fat diet (HFD). Rankl mRNA transcripts were increased in aortic arches and thoracic aortae of Apoe^-/- mice; however, this increase was blunted in Apoe^-/- ;D5^-/- mice. Similarly, higher Rankl transcripts were identified in splenic T lymphocytes in Apoe^-/- mice, and their levels were reduced in Apoe^-/- ;D5^-/- mice. When analyzed by micro-computed tomography (µCT), atherosclerotic plaque calcification was identified in six out of eight Apoe^-/- mice, whereas only one out of eight Apoe^-/- ;D5^-/- mice developed plaque calcification after 12 weeks of HFD. However, following 18 weeks of HFD challenge, all of Apoe^-/- and Apoe^-/- ;D5^-/- animals developed atherosclerotic plaque calcification. Likewise, atherosclerotic lesion sizes were site-specifically reduced in the aortic arch of Apoe^-/- ;D5^-/- mice at initial stage of atherosclerosis and this effect was diminished as atherosclerosis proceeded to a more advanced stage. Our data suggest that deletion of the RANKL D5 enhancer delays the progression of atherosclerotic plaque development and plaque calcification in hypercholesterolemic mice. This work provides important insight into RANKL's regulatory role in atherosclerosis. J. Cell. Biochem. 118: 4240-4253, 2017. © 2017 Wiley Periodicals, Inc.

Dysregulated systemic lymphocytes affect the balance of osteogenic/adipogenic differentiation of bone mesenchymal stem cells after local irradiation

Background

While it is known that irradiation can induce local and systemic bone loss over time, how focal irradiation induces systemic bone complications remains unclear. Immune cells are thought to be crucial to bone homeostasis, and abnormal immune cells lead to serious disruption of bone homeostasis, such as in acute lymphoblastic leukaemia. This disruption primarily occurs due to inhibition of the osteogenic differentiation of bone mesenchymal stem cells (BMSCs).

Methods

In this study, we detected local and systemic bone loss in trabecular bone by micro-computed tomography (micro-CT) and measurement of peroxisome proliferator-activated receptor gamma (PPARγ) and runt-related transcription factor 2 (RUNX2) expression in BMSCs using real-time polymerase chain reaction and western blotting. Additionally, changes in lymphocytes (B cells and CD4⁺ and CD8⁺ T cells) in the peripheral blood and bone marrow were analysed by flow cytometry. BMSC-derived osteoblasts and adipocytes, cultured in osteogenic or adipogenic media or co-cultured with lymphocytes, were detected by BCIP/NBT, Alizarin Red S and Oil Red O staining.

Results

Focal irradiation induced local and systemic bone loss in trabecular bone. Increased PPARγ expression and decreased RUNX2 expression were observed, accompanied by upregulated adipogenesis and downregulated osteogenesis of BMSCs. B cells and CD8⁺ T lymphocytes were increased in the blood and bone marrow after irradiation, while CD4⁺ T lymphocytes were decreased in the blood. Inhibition of RUNX2 expression and reduction of alkaline phosphatase activity and mineralization deposits were observed in lymphocyte-co-cultured BMSCs, accompanied by an increase in PPARγ expression and in the number of lipid droplets.

Conclusions

Focal irradiation induced local and systemic bone loss in trabecular bone. Increased B cells and CD8⁺ T lymphocytes led to systemic bone loss by decreasing BMSC osteogenesis.

Interleukin-32 Gamma Stimulates Bone Formation by Increasing miR-29a in Osteoblastic Cells and Prevents the Development of Osteoporosis

Interleukin-32 gamma (IL-32γ) is a recently discovered cytokine that is elevated in inflamed tissues and contributes to pathogenic features of bone in human inflammatory rheumatic diseases. Nevertheless, the role of IL-32γ and its direct involvement in bone metabolism is unclear. We investigated the molecular mechanism of IL-32γ in bone remodeling and the hypothetical correlation between IL-32γ and disease activity in osteoporosis patients. Transgenic (TG) mice overexpressing human IL-32γ showed reduced bone loss with advancing age, increased bone formation, and high osteogenic capacity of osteoblast compared to wild-type (WT) mice through the upregulation of miR-29a, which caused a reduction of Dickkopf-1 (DKK1) expression. IL-32γ TG mice were protected against ovariectomy (OVX)induced osteoporosis compared with WT mice. Decreased plasma IL-32γ levels were associated with bone mineral density (BMD) in human patients linked to increased DKK1 levels. These results indicate that IL-32γ plays a protective role for bone loss, providing clinical evidence of a negative correlation between IL-32γ and DKK1 as bone metabolic markers.

Absence of the Vitamin D Receptor Inhibits Atherosclerotic Plaque Calcification in Female Hypercholesterolemic Mice

Epidemiological and clinical data suggest adverse cardiovascular outcomes with respect to vitamin D deficiency. Here, we explored the effects of vitamin D in atherosclerotic plaque calcification in vivo by utilizing vitamin D receptor (Vdr)-deficient mice in an Apoe^-/- background. Animals were fed a high-fat diet (HFD) for either 12 or 18 weeks and then examined for atherosclerotic plaque development. In order to prevent calcium deficiency, Vdr^-/- and Apoe^-/- ;Vdr^-/- animals were fed a high-calcium rescue diet prior to initiation of the HFD feeding and supplemented with high-calcium water during HFD feeding. Although calcium supplementation improved bone mass in Vdr^-/- and Apoe^-/- ;Vdr^-/- mice, neither strain was fully rescued. Systemic inflammatory responses observed in the absence of VDR were exaggerated in Apoe^-/- mice. Whereas, hyperlipidemic profiles seen in Apoe^-/- mice were ameliorated in the absence of VDR. Micro-computed tomography (µCT) analysis revealed that six out of eight Apoe^-/- animals developed atherosclerotic plaque calcification following 12 weeks of HFD feeding and 100% of the mice developed plaque calcification after 18 weeks. In contrast, although atherosclerotic lesions were evident in Apoe^-/- ;Vdr^-/- mice at 12 and 18 weeks of HFD challenge, none of these animals developed plaque calcification at either time point. The active vitamin D hormone, 1,25(OH)₂ D₃ likely increased calcification in aortic smooth muscle cells perhaps by directly modulating expression of Alpl, Rankl, and Opg. Our data suggest that the absence of VDR inhibits atherosclerotic plaque calcification in hypercholesterolemic Apoe^-/- mice, providing additional insight into the role of vitamin D in atherosclerotic plaque calcification. J. Cell. Biochem. 118: 1050-1064, 2017. © 2016 Wiley Periodicals, Inc.

Animal Models of Bone Loss in Inflammatory Arthritis: from Cytokines in the Bench to Novel Treatments for Bone Loss in the Bedside-a Comprehensive Review

Throughout life, bone is continuously remodelled. Bone is formed by osteoblasts, from mesenchymal origin, while osteoclasts induce bone resorption. This process is tightly regulated. During inflammation, several growth factors and cytokines are increased inducing osteoclast differentiation and activation, and chronic inflammation is a condition that initiates systemic bone loss. Rheumatoid arthritis (RA) is a chronic inflammatory auto-immune disease that is characterised by active synovitis and is associated with early peri-articular bone loss. Peri-articular bone loss precedes focal bone erosions, which may progress to bone destruction and disability. The incidence of generalised osteoporosis is associated with the severity of arthritis in RA and increased osteoporotic vertebral and hip fracture risk. In this review, we will give an overview of different animal models of inflammatory arthritis related to RA with focus on bone erosion and involvement of pro-inflammatory cytokines. In addition, a humanised endochondral ossification model will be discussed, which can be used in a translational approach to answer osteoimmunological questions.

Activation of mTORC1 in B Lymphocytes Promotes Osteoclast Formation via Regulation of β-Catenin and RANKL/OPG

The cytokine receptor activator of nuclear factor-κB ligand (RANKL) induces osteoclast formation from monocyte/macrophage lineage cells. However, the mechanisms by which RANKL expression is controlled in cells that support osteoclast differentiation are still unclear. We show that deletion of TSC1 (tuberous sclerosis complex 1) in murine B cells causes constitutive activation of mechanistic target of rapamycin complex 1 (mTORC1) and stimulates RANKL but represses osteoprotegerin (OPG) expression and subsequently promotes osteoclast formation and causes osteoporosis in mice. Furthermore, the regulation of RANKL/OPG and stimulation of osteoclastogenesis by mTORC1 was confirmed in a variety of RANKL-expressing cells and in vivo. Mechanistically, mTORC1 controls RANKL/OPG expression through negative feedback inactivation of Akt, destabilization of β-catenin mRNA, and downregulation of β-catenin. Our findings demonstrate that mTORC1 activation-stimulated RANKL expression in B cells is sufficient to induce bone loss and osteoporosis. The study also established a link between mTORC1 and the RANKL/OPG axis via negative regulation of β-catenin. © 2016 American Society for Bone and Mineral Research.

Osteoimmunology: from mice to humans

The immune system has been recognized as one of the most important regulators of bone turnover and its deregulation is implicated in several bone diseases such as postmenopausal osteoporosis and inflammatory bone loss; recently it has been suggested that the gut microbiota may influence bone turnover by modulation of the immune system. The study of the relationship between the immune system and bone metabolism is generally indicated under the term 'osteoimmunology'. The vast majority of these studies have been performed in animal models; however, several data have been confirmed in humans as well: this review summarizes recent data on the relationship between the immune system and bone with particular regard to the data confirmed in humans.

Brucella abortus-infected B cells induce osteoclastogenesis

Brucella abortus is an intracellular bacterium that establishes lifelong infections in livestock and humans although the mechanisms of its chronicity are poorly understood. Activated B cells have long lifespan and B. abortus infection activates B cells. Our results indicate that the direct infection of B cells with B. abortus induced matrix metalloproteinase-9 (MMP-9), receptor activator for NF κB ligand (RANKL), tumor necrosis factor (TNF)-α and interleukin (IL)-6 secretion. In addition, supernatants from B. abortus-infected B cells induced bone marrow-derived monocytes to undergo osteoclastogenesis. Using osteoprotegerin, RANKL's decoy receptor, we determined that RANKL is involved in osteoclastogenesis induced by supernatants from B. abortus-infected B cells. The results presented here shed light on how the interactions of B. abortus with B cells may have a role in the pathogenesis of brucellar osteoarticular disease.

Unique Distal Enhancers Linked to the Mouse Tnfsf11 Gene Direct Tissue-Specific and Inflammation-Induced Expression of RANKL

Receptor activator of nuclear factor κB ligand (RANKL) is expressed by a number of cell types to participate in diverse physiological functions. We have previously identified 10 distal RANKL enhancers. Earlier studies have shown that RL-D5 is a multifunctional RANKL enhancer. Deletion of RL-D5 from the mouse genome leads to lower skeletal and lymphoid tissue RANKL, causing a high bone mass phenotype. Herein, we determine the physiological role and lineage specificity of 2 additional RANKL enhancers, RL-D6 and RL-T1, which are located 83 and 123 kb upstream of the gene's transcriptional start site, respectively. Lack of RL-D6 or RL-T1 did not alter skeletal RANKL or bone mineral density up to 48 weeks of age. Although both RL-D5 and RL-T1 contributed to activation induction of T-cell RANKL, RL-T1 knockout mice had drastically low lymphocyte and lymphoid tissue RANKL levels, indicating that RL-T1 is the major regulator of lymphocyte RANKL. Moreover, RL-T1 knockout mice had lower circulating soluble RANKL, suggesting that lymphocytes are important sources of circulating soluble RANKL. Under physiological conditions, lack of RL-D6 did not alter RANKL expression. However, lack of RL-D5 or RL-D6, but not of RL-T1, blunted the oncostatin M and lipopolysaccharide induction of RANKL ex vivo and in vivo, suggesting that RL-D5 and RL-D6 coregulate the inflammation-mediated induction of RANKL in osteocytes and osteoblasts while lack of RL-D6 did not alter secondary hyperparathyroidism or lactation induction of RANKL or bone loss. These results suggest that although RL-D5 mediates RANKL expression in multiple lineages, other cell type- or factor-specific enhancers are required for its appropriate control, demonstrating the cell type-specific and complex regulation of RANKL expression.

Osteoclast fusion is initiated by a small subset of RANKL-stimulated monocyte progenitors, which can fuse to RANKL-unstimulated progenitors

Osteoclasts are multinucleated, bone-resorbing cells formed via fusion of monocyte progenitors, a process triggered by prolonged stimulation with RANKL, the osteoclast master regulator cytokine. Monocyte fusion into osteoclasts has been shown to play a key role in bone remodeling and homeostasis; therefore, aberrant fusion may be involved in a variety of bone diseases. Indeed, research in the last decade has led to the discovery of genes regulating osteoclast fusion; yet the basic cellular regulatory mechanism underlying the fusion process is poorly understood. Here, we applied a novel approach for tracking the fusion processes, using live-cell imaging of RANKL-stimulated and non-stimulated progenitor monocytes differentially expressing dsRED or GFP, respectively. We show that osteoclast fusion is initiated by a small (~2.4%) subset of precursors, termed "fusion founders", capable of fusing either with other founders or with non-stimulated progenitors (fusion followers), which alone, are unable to initiate fusion. Careful examination indicates that the fusion between a founder and a follower cell consists of two distinct phases: an initial pairing of the two cells, typically lasting 5-35 min, during which the cells nevertheless maintain their initial morphology; and the fusion event itself. Interestingly, during the initial pre-fusion phase, a transfer of the fluorescent reporter proteins from nucleus to nucleus was noticed, suggesting crosstalk between the founder and follower progenitors via the cytoplasm that might directly affect the fusion process, as well as overall transcriptional regulation in the developing heterokaryon.

Interplay between low plasma RANKL and VDR-FokI polymorphism in lumbar disc herniation independently from age, body mass, and environmental factors: a case-control study in the Italian population

PURPOSE

Aim of this study was to investigate RANKL and osteoprotegerin plasma concentrations in patients affected by disc herniation, the most common epiphenomenon of disc degenerative diseases, and in a matched cohort of healthy subjects and whether the expression of these markers was associated to a polymorphism of the vitamin D receptor gene.

METHODS

For this case-control study, 110 consecutive cases affected by lumbar disc herniation (confirmed by MRI) and 110 healthy age- and sex-matched controls were enrolled. Subjects affected by any other pathology were excluded. RANKL and osteoprotegerin were measured in plasma by immunoassays. The difference in these markers between cases and controls was assessed by t test. The correlation between osteoimmunological markers concentrations, anthropometrical variables, and the expression of the pathology was statistically assessed (Pearson's test) along with the association (Fisher's exact test) with the vitamin D receptor gene genotype, determined elsewhere.

RESULTS

Despite comparable osteoprotegerin concentrations, cases, altogether or grouped for gender, express lower RANKL and, consequently, RANKL-to-osteoprotegerin ratio. While in cases RANKL and osteoprotegerin concentrations were independent from age and BMI, in controls they increased with age. Disc herniation was strongly associated with RANKL and the presence of the F allele of the VDR gene.

CONCLUSIONS

Whether vertebral bone changes precede or follow cartilage deterioration in intervertebral disc degeneration is not known. Our results suggest a reduced bone turnover rate, associated to a specific genetic background, in patients affected by lumbar disc herniation which could be one of the favoring factors for disc degeneration.