IL-6 Reduces Spheroid Sizes of Osteophytic Cells Derived from Osteoarthritis Knee Joint via Induction of Apoptosis

Osteophytes in osteoarthritis (OA) joints contribute to restriction of joint movement, joint pain, and OA progression, but little is known about osteophyte regulators. Examination of gene expression related to cartilage extracellular matrix, endochondral ossification, and growth factor signaling in articular cartilage and osteophytes obtained from OA knee joints showed that several genes such as COL1A1, VCAN, BGLAP, BMP8B, RUNX2, and SOST were overexpressed in osteophytes compared with articular cartilage. Ratios of mesenchymal stem/progenitor cells, which were characterized by co-expression of CD105 and CD166, were significantly higher in osteophytic cells than articular cells. A three-dimensional culture method for cartilage and osteophyte cells was developed by modification of cultures of self-assembled spheroid cell organoids (spheroids). These spheroids cultured in the media for mesenchymal stem cells containing transforming growth factor-β3 showed characteristic morphologies and gene expression profiles of articular cartilage and osteophytes, respectively. The effects of IL-1β, tumor necrosis factor-α, and IL-6 on the spheroids of articular and osteophytic cells were studied. To the best of our knowledge, they provide the first evidence that IL-6 suppresses the spheroid size of osteophytic cells by inducing apoptosis and reducing extracellular matrix molecules. These data show that IL-6 is the suppressor of osteophyte growth and suggest that IL-6 expression and/or activity are implicated in the regulation of osteophyte formation in pathologic joints.

Medial meniscus extrusion is invariably observed and consistent with tibial osteophyte width in elderly populations: The Bunkyo Health Study

We reported that the full-length width of medial tibial osteophytes comprising cartilage and bone parts correlates with medial meniscus extrusion (MME) in early-stage knee osteoarthritis (OA). However, no data exist on the prevalence of MME and its relationship with osteophytes in the elderly population. 1191 elderly individuals (females 57%; 72.9 years old on average) in the Bunkyo Health Study underwent standing plain radiograph and proton density-weighted MRI on knee joints. MRI-detected OA changes were evaluated according to the Whole-Organ Magnetic Resonance Imaging Score. A new method of assessing the cartilage and bone parts of osteophytes was developed using pseudo-coloring images of proton density-weighted fat-suppressed MRI. Most subjects showed Kellgren-Lawrence grade 1 or 2 radiographic medial knee OA (88.1%), MME (98.7%, 3.90 ± 2.01 mm), and medial tibial osteophytes (99.3%, 3.27 ± 1.50 mm). Regarding OA changes, MME was closely associated with the full-length width of medial tibial osteophytes (β = 1.114; 95% CI 1.069-1.159; p < 0.001) in line with osteophyte width (intraclass correlation coefficient, 0.804; 95% CI 0.783-0.823). Our data revealed that MME and medial tibial osteophytes are observed in the elderly and demonstrate that the degree of MME is consistent with the full-length width of medial tibial osteophytes, suggesting that osteophytes might be implicated in MME.

Anterior meniscus extrusion is associated with anterior tibial osteophyte width in knee osteoarthritis - The Bunkyo Health Study

Background

In knee osteoarthritis (OA), medial meniscus extrudes both medially and anteriorly. We reported that full-length width of medial tibial osteophyte, which comprises cartilage and bone parts, is directly associated with medial meniscus extrusion in early-stage knee OA and hypothesized that anterior tibial osteophyte (ATO) is also associated with anterior meniscus extrusion (AME). Thus, we aimed to examine their prevalence and relationship.

Methods

Elderly subjects (638 females and 507 males; average 72.9 years old) in the Bunkyo Health Study cohort were enrolled. MRI-detected OA changes were evaluated according to the Whole Organ Magnetic Resonance Imaging Score. ATO was evaluated using the method which can assess both cartilage and bone parts of osteophyte by pseudo-coloring images of proton density-weighted fat-suppressed MRI.

Results

Most subjects showed the Kellgren-Lawrence grade 1/2 of the medial knee OA (88.1%), AME (94.3%, 3.7 ​± ​2.2 ​mm), and ATO (99.6%, 4.2 ​± ​1.5 ​mm). Among the OA changes, AME was most closely associated with full-length width of ATO (multivariable β ​= ​0.877, p ​< ​0.001). The area under the receiver operating characteristic curve for determining the presence of AME as evaluated by ATO width was 0.75 (95% confidence interval 0.60-0.84, p ​< ​0.001). The odds ratio for the presence of AME as evaluated by ATO width at 2.9 ​mm was 7.16 (4.23-12.15, p ​< ​0.001, age, gender, BMI, and K-L adjusted).

Conclusions

AME and ATO were inevitably observed in the elderly subjects and AME was closely associated with full-length width of ATO. Our study provides the first evidence on the close relationship between AME and ATO in knee OA.

Periosteal stem cells control growth plate stem cells during postnatal skeletal growth

The ontogeny and fate of stem cells have been extensively investigated by lineage-tracing approaches. At distinct anatomical sites, bone tissue harbors multiple types of skeletal stem cells, which may independently supply osteogenic cells in a site-specific manner. Periosteal stem cells (PSCs) and growth plate resting zone stem cells (RZSCs) critically contribute to intramembranous and endochondral bone formation, respectively. However, it remains unclear whether there is functional crosstalk between these two types of skeletal stem cells. Here we show PSCs are not only required for intramembranous bone formation, but also for the growth plate maintenance and prolonged longitudinal bone growth. Mice deficient in PSCs display progressive defects in intramembranous and endochondral bone formation, the latter of which is caused by a deficiency in PSC-derived Indian hedgehog (Ihh). PSC-specific deletion of Ihh impairs the maintenance of the RZSCs, leading to a severe defect in endochondral bone formation in postnatal life. Thus, crosstalk between periosteal and growth plate stem cells is essential for post-developmental skeletal growth.

Intramembranous and endochondral bone formation have been considered to be independent processes mediated by independent stem cells. Here the authors show that periosteal stem cells participate in both types of bone formation, supporting endochondral formation by producing Ihh.

Inhibition of hepatocyte growth factor/c-Met signalling abrogates joint destruction by suppressing monocyte migration in rheumatoid arthritis

OBJECTIVES

To determine the expression of hepatocyte growth factor (HGF) in RA biological fluids, the role of HGF in monocyte migration and the therapeutic effect of the c-Met inhibitor savolitinib in an arthritis model mice.

METHODS

HGF/c-Met expression in serum, SF and synovial tissues (STs) obtained from RA patients and controls, as well as RA fibroblast-like synoviocytes (FLSs), was evaluated by ELISA and immunostaining. To determine the function of HGF in RA SF, we preincubated RA SF with a neutralizing anti-HGF antibody and measured the chemotactic ability of a human acute monocytic leukaemia cell line (THP-1). Additionally, examinations were conducted of SKG mice treated with savolitinib for 4 weeks.

RESULTS

HGF levels in serum from RA patients were significantly higher than those in the controls and were decreased by drug treatment for 24 weeks. Additionally, the HGF level in SF from RA patients was higher than that in SF from OA patients. HGF and c-Met expression was also noted in RA STs. Stimulation of RA FLSs with TNF-α increased HGF/c-Met expression in a concentration-dependent manner, and c-Met signal inhibition suppressed production of fractalkine/CX3CL1 and macrophage inflammatory protein-1α/CCL3. When HGF was removed by immunoprecipitation, migration of THP-1 in RA SF was suppressed. In SKG mice, savolitinib significantly suppressed ankle bone destruction on µCT, with an associated reduction in the number of tartrate-resistant acid phosphatase-positive osteoclasts.

CONCLUSION

HGF produced by inflammation in synovium of RA patients activates monocyte migration to synovium and promotes bone destruction via a chemotactic effect and enhanced chemokine production.

Novel gene Merlot inhibits differentiation and promotes apoptosis of osteoclasts

Extended osteoclast longevity is deeply involved in the pathogenesis of bone diseases such as osteoporosis and rheumatoid arthritis, though the mechanisms that determine osteoclast lifespan are not fully understood. Here we present findings indicating that the newly characterized gene Merlot, which encodes a highly conserved yet uncharacterized protein in vertebrates, is an important regulator for termination of osteoclastogenesis via induction of apoptosis. Mice lacking Merlot exhibited low bone mass due to increased osteoclast and bone resorption. Furthermore, osteoclast precursors overexpressing Merlot failed to differentiate into mature osteoclasts, while Merlot deficiency led to hyper-nucleation and prolonged survival of osteoclasts, accompanied by sustained nuclear localization of nuclear factor of activated T cell c1 (NFATc1) and derepression of glycogen synthase kinase-3β (GSK3β) activity, known to regulate NFATc1 activity and induce apoptosis. Merlot-deficient osteoclasts were found to represent suppression of caspase-3-mediated apoptosis and Merlot deficiency caused transcriptional downregulation of a proapoptotic cascade, including Bax, Bak, Noxa, and Bim, as well as the executor caspase members Casp-3, -6, and -7, and upregulation of anti-apoptotic Bcl2, resulting in a low apoptotic threshold. Thus, Merlot regulates osteoclast lifespan by inhibition of differentiation and simultaneous induction of apoptosis via regulation of the NFATc1-GSK3β axis.

Effects of N-methyl-d-aspartate receptor antagonist MK-801 (dizocilpine) on bone homeostasis in mice

OBJECTIVES

To gain insight into the role of the N-methyl-d-aspartate (NMDA) receptor in bone metabolism by examining the effects of its noncompetitive antagonist, MK-801 (dizocilpine), on bone homeostasis and bone healing in mice.

METHODS

MK-801 (2.5 mg/kg) or saline (in control groups) was intravenously administered to healthy mice and mice with bone-defects daily for seven to 14 days. Bone defects were artificially created in femurs using a drill and reamer. Following euthanasia, bones were extracted and processed for microcomputed tomography (μCT) and histological analyses. The effects of MK-801 on osteoclast differentiation by bone marrow macrophages (BMMs) were examined in vitro. mRNA expressionlevels of Grin3b levels were also examined using reverse-transcription polymerase chain reaction (RT-PCR).

RESULTS

Bone volume was significantly decreased in mice administered MK-801 for 14 days. Additionally, the number of osteoclasts was reduced, while number of osteoblasts and rate of bone formation were increased in these mice. MK-801 inhibited osteoclast differentiation dose-dependently in vitro. RT-PCR findings suggested expression of Grin3b, a subunit of the NMDA receptor, in BMMs. During the healing process of artificially created defects in femurs, no significant differences were found between the control and MK-801-treated groups, indicating no stimulatory or inhibitory effects by MK-801 administration.

CONCLUSIONS

These results indicate that blockade of the NMDA receptor by MK-801 administration affects bone metabolism but not the healing process of artificial bone defects.

Myelination during fracture healing in vivo in myelin protein zero (p0) transgenic medaka line

During the fracture healing process, osteoblasts and osteoclasts, as well as the nervous system are known to play important roles for signaling in the body. Glia cells contribute to the healing process by myelination, which can increase the speed of signals transmitted between neurons. However, the behavior of myelinating cells at a fracture site remains unclear. We developed a myelin protein zero (mpz)-EGFP transgenic medaka line for tracing myelinating cells. Mpz-enhanced green fluorescence protein (EGFP)-positive (mpz⁺) cells are driven by the 2.9-kb promoter of the medaka mpz gene, which is distributed throughout the nervous system, such as the brain, spinal cord, lateral line, and peripheral nerves. In the caudal fin region, mpz⁺ cells were found localized parallel with the fin ray (bone) in the adult stage. mpz⁺ cells were not distributed with fli-DsRed positive (fli⁺) blood vessels, but with some nerve fibers, and were dyed with the anti-acetylated tubulin antibody. We then fractured one side of the caudal lepidotrichia in a caudal fin of mpz-EGFP medaka and found a unique phenomenon, in that mpz⁺ cells were accumulated at 1 bone away from the fracture site. This mpz⁺ cell accumulation phenomenon started from 4 days after fracture of the proximal bone. Thereafter, mpz⁺ cells became elongated from the proximal bone to the distal bone and finally showed a crosslink connection crossing the fracture site to the distal bone at 28 days after fracture. Finally, the effects of rapamycin, known as a mTOR inhibitor, on myelination was examined. Rapamycin treatment of mpz-EGFP/osterix-DsRed double transgenic medaka inhibited not only the crosslink connection of mpz⁺ cells but also osterix⁺ osteoblast accumulation at the fracture site, accompanied with a fracture healing defect. These findings indicated that mTOR signaling plays important roles in bone formation and neural networking during fracture healing. Taken together, the present results are the first to show the dynamics of myelinating cells in vivo.

Effects of lipid metabolism on mouse incisor dentinogenesis

Tooth formation can be affected by various factors, such as oral disease, drug administration, and systemic illness, as well as internal conditions including dentin formation. Dyslipidemia is an important lifestyle disease, though the relationship of aberrant lipid metabolism with tooth formation has not been clarified. This study was performed to examine the effects of dyslipidemia on tooth formation and tooth development. Dyslipidemia was induced in mice by giving a high-fat diet (HFD) for 12 weeks. Additionally, LDL receptor-deficient (Ldlr^−/−) strain mice were used to analyze the effects of dyslipidemia and lipid metabolism in greater detail. In the HFD-fed mice, incisor elongation was decreased and pulp was significantly narrowed, while histological findings revealed disappearance of predentin. In Ldlr^−/− mice fed regular chow, incisor elongation showed a decreasing trend and pulp a narrowing trend, while predentin changes were unclear. Serum lipid levels were increased in the HFD-fed wild-type (WT) mice, while Ldlr^−/− mice given the HFD showed the greatest increase. These results show important effects of lipid metabolism, especially via the LDL receptor, on tooth homeostasis maintenance. In addition, they suggest a different mechanism for WT and Ldlr^−/− mice, though the LDL receptor pathway may not be the only factor involved.

Biological Effects of Anti-RANKL Antibody and Zoledronic Acid on Growth and Tooth Eruption in Growing Mice

The anti-bone resorptive drugs denosumab, an anti-human-RANKL antibody, and zoledronic acid (ZOL), a nitrogen-containing bisphosphonate, have recently been applied for treatment of pediatric patients with bone diseases, though details regarding their effects in growing children have yet to be fully elucidated. In the present study, we administered these anti-resorptive drugs to mice from the age of 1 week and continued once-weekly injections for a total of 7 times. Mice that received the anti-RANKL antibody displayed normal growth and tooth eruption, though osteopetrotic bone volume gain in long and alveolar bones was noted, while there were nearly no osteoclasts and a normal of number osteoblasts observed. In contrast, ZOL significantly delayed body growth, tooth root formation, and tooth eruption, with increased osteoclast and decreased osteoblast numbers. These findings suggest regulation of tooth eruption via osteoblast differentiation by some types of anti-resorptive drugs.

A Delphinidin-Enriched Maqui Berry Extract Improves Bone Metabolism and Protects against Bone Loss in Osteopenic Mouse Models

In our previous investigation, delphinidin, one of the most abundant anthocyanins found in vegetables and berry fruits, had been shown to inhibit osteoclasts and prevent bone loss in mouse models of osteoporosis. In the present study, we investigated whether a delphinidin glycoside-enriched maqui berry extract (MBE, Delphinol^®) exhibits beneficial effects on bone metabolism both in vitro and in vivo. MBE stimulated the osteoblastic differentiation of MC3T3-E1 cells, as indicated by enhanced mineralized nodule formation, and increased alkaline phosphatase activity, through the upregulation of bone morphogenetic protein 2 (Bmp2), runt-related transcription factor 2 (Runx2), osterix (Osx), osteocalcin (Ocn), and matrix extracellular phosphoglycoprotein (Mepe) mRNA expression. Immunostaining and immunoprecipitation assays demonstrated that MBE suppressed NF-κB transnucleation through acting as a superoxide anion/peroxynitrite scavenger in MC3T3-E1 cells. Simultaneously, MBE inhibited both osteoclastogenesis in primary bone marrow macrophages and pit formation by maturated osteoclasts on dentine slices. Microcomputed tomography (micro-CT) and bone histomorphometry analyses of femurs demonstrated that the daily ingestion of MBE significantly increased BV/TV (ratio of bone volume to tissue volume), Tb.Th (trabecular thickness), Tb.N (trabecular number), N.Nd/N.Tm (node to terminus ratio), OV/TV (ratio of osteoid volume to tissue volume), BFR/TV (bone formation rate per tissue volume), and significantly decreased Tb.Sp (trabecular separation), ES/BS (ratio of eroded surface to bone surface) and N.Oc/BS (number of osteoclast per unit of bone surface), compared to vehicle controls in osteopenic mouse models. These findings suggest that MBE can be a promising natural agent for the prevention of bone loss in osteopenic conditions by not only inhibiting bone resorption, but also stimulating bone formation.

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.

Biological effects of anti-RANKL antibody administration in pregnant mice and their newborns

Denosumab, a fully human monoclonal antibody that neutralizes receptor activator of nuclear factor-κB ligand (RANKL) and blocks osteoclast differentiation, has received approval in Japan for use as an anti-resorptive drug for osteoporosis and skeletal-related events (SREs) in patients with solid cancer. Denosumab is contraindicated during pregnancy, though the effects of blocking RANKL activity on pregnant mothers and their newborns are unclear. We used mice to investigate the effects of an anti-RANKL antibody on maternal and newborn health. Mothers injected with the anti-RANKL antibody had increased bone mass as compared with the controls, while osteoclast number and the level of tartrate-resistant acid phosphatase (TRAP) in serum were increased at the end of pregnancy. Newborn mice exposed to the antibody in utero were normally born, but showed increased bone mass and died within 48 h after birth. None of the newborns were found to have milk in their stomachs, suggesting that they died due to a maternal defect in lactation. Consistent with this, anti-RANKL antibody-injected mothers displayed impaired mammary gland development. However, fostering by healthy surrogate mothers rescued only 33% of the antibody-exposed newborns, suggesting that neonatal mortality was due, at least in part, to an intrinsic defect in the newborns. Our findings show that anti-RANKL antibody administration during pregnancy results in not only an undesirable increase in bone mass, but also has harmful effects on newborn survival.

Immune complexes regulate bone metabolism through FcRγ signalling

Autoantibody production and immune complex (IC) formation are frequently observed in autoimmune diseases associated with bone loss. However, it has been poorly understood whether ICs regulate bone metabolism directly. Here we show that the level of osteoclastogenesis is determined by the strength of FcRγ signalling, which is dependent on the relative expression of positive and negative FcγRs (FcγRI/III/IV and IIB, respectively) as well as the availability of their ligands, ICs. Under physiological conditions, unexpectedly, FcγRIII inhibits osteoclastogenesis by depriving other osteoclastogenic Ig-like receptors of FcRγ. Fcgr2b(-/-) mice lose bone upon the onset of a hypergammaglobulinemia or the administration of IgG1 ICs, which act mainly through FcγRIII. The IgG2 IC activates osteoclastogenesis by binding to FcγRI and FcγRIV, which is induced under inflammatory conditions. These results demonstrate a link between the adaptive immunity and bone, suggesting a regulatory role for ICs in bone resorption in general, and not only in inflammatory diseases.

OSCAR is a collagen receptor that costimulates osteoclastogenesis in DAP12-deficient humans and mice

Osteoclasts are terminally differentiated leukocytes that erode the mineralized bone matrix. Osteoclastogenesis requires costimulatory receptor signaling through adaptors containing immunoreceptor tyrosine-based activation motifs (ITAMs), such as Fc receptor common γ (FcRγ) and DNAX-activating protein of 12 kDa. Identification of these ITAM-containing receptors and their ligands remains a high research priority, since the stimuli for osteoclastogenesis are only partly defined. Osteoclast-associated receptor (OSCAR) was proposed to be a potent FcRγ-associated costimulatory receptor expressed by preosteoclasts in vitro, but OSCAR lacks a cognate ligand and its role in vivo has been unclear. Using samples from mice and patients deficient in various ITAM signaling pathways, we show here that OSCAR costimulates one of the major FcRγ-associated pathways required for osteoclastogenesis in vivo. Furthermore, we found that OSCAR binds to specific motifs within fibrillar collagens in the ECM that become revealed on nonquiescent bone surfaces in which osteoclasts undergo maturation and terminal differentiation in vivo. OSCAR promoted osteoclastogenesis in vivo, and OSCAR binding to its collagen motif led to signaling that increased numbers of osteoclasts in culture. Thus, our results suggest that ITAM-containing receptors can respond to exposed ligands in collagen, leading to the functional differentiation of leukocytes, which provides what we believe to be a new concept for ITAM regulation of cytokine receptors in different tissue microenvironments.

Ca2+-NFATc1 signaling is an essential axis of osteoclast differentiation

Osteoclasts are unique, multinucleated giant cells that decalcify and degrade the bone matrix. They originate from hematopoietic cells and their differentiation is dependent on a tumor necrosis factor (TNF) family cytokine, receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL), as well as macrophage-colony stimulating factor (M-CSF). Recent studies have unveiled the precise molecular mechanism underlying osteoclastogenesis. In particular, the discovery of nuclear factor of activated T cells c1 (NFATc1), the master regulator of osteoclastogenesis, has proven to be a breakthrough in this field. NFATc1 is activated by Ca2+ signaling induced by the activation of the immunoglobulin-like receptor signaling associated with immunoreceptor tyrosine-based activation motif (ITAM)-harboring adapters. The long-lasting Ca2+ oscillation, which is evident during osteoclastogenesis, may ensure the robust induction of NFATc1 through an autoamplification mechanism. Thus, intracellular Ca2+ is a critical attribute of osteoclastogenic signaling. In addition, osteoclasts are exposed to a very high extracellular Ca2+ concentration ([Ca2+]o) in the bone microenvironment and respond to the change in [Ca2+]o by increasing the intracellular Ca2+, which regulates diverse cellular functions. Investigation of the molecular mechanisms underlying the regulation of intracellular Ca2+ dynamics may open up new directions for therapeutic strategies in bone disease.