DC-STAMP is essential for cell-cell fusion in osteoclasts and foreign body giant cells

Flrt2 is involved in fine-tuning of osteoclast multinucleation

Osteoclasts are multinucleated giant cells derived from myeloid progenitors. Excessive bone resorption by osteoclasts can result in serious clinical outcomes for which better treatment options are needed. Here, we identified fibronectin leucine-rich transmembrane protein 2 (Flrt2), a ligand of the Unc5 receptor family for neurons, as a novel target associated with the late/maturation stage of osteoclast differentiation. Flrt2 expression is induced by stimulation with receptor activator of nuclear factor-kB ligand (RANKL). Flrt2 deficiency in osteoclasts results in reduced hyper-multinucleation, which could be restored by RNAi-mediated knockdown of Unc5b. Treatment with Netrin1, another ligand of Unc5b which negatively controls osteoclast multinucleation through down regulation of RANKL-induced Rac1 activation, showed no inhibitory effects on Flrt2-deficient cells. In addition, RANKL-induced Rac1 activation was attenuated in Flrt2-deficient cells. Taken together, these results suggest that Flrt2 regulates osteoclast multinucleation by interfering with Netrin 1-Unc5b interaction and may be a suitable therapeutic target for diseases associated with bone remodeling.

Microtubule actin crosslinking factor 1 (MACF1) knockdown inhibits RANKL-induced osteoclastogenesis via Akt/GSK3β/NFATc1 signalling pathway

Osteoclasts are responsible for bone resorption and play essential roles in causing bone diseases such as osteoporosis. Microtubule actin crosslinking factor 1 (MACF1) is a large spectraplakin protein that has been implicated in regulating cytoskeletal distribution, cell migration, cell survival and cell differentiation. However, whether MACF1 regulates the differentiation of osteoclasts has not been elucidated. In this study, we found that the expression of MACF1 was increased in primary bone marrow-derived monocytes (BMMs) of osteoporotic mice and was downregulated during receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis of pre-osteoclast cell lines RAW264.7 cells. RAW264.7 cells were transfected with shMACF1 using a lentiviral vector to study the role of MACF1 in osteoclastogenic differentiation. Knockdown of MACF1 in RAW264.7 cells inhibited the formation of multinucleated osteoclasts and decreased the expression of osteoclast-marker genes (Ctsk, Acp5, Mmp9 and Oscar) during RANKL-induced osteoclastogenesis. Additionally, knockdown of MACF1 disrupted actin ring formation in osteoclasts and further blocked the bone resorption activity of osteoclasts by reducing the area and depth of pits. Knockdown of MACF1 had no effect on the survival of pre-osteoclasts and mature osteoclasts. We further established that knockdown of MACF1 attenuated the phosphorylation of Akt and GSK3β and inhibited the expression of its downstream target NFATc1. Akt activator rescued the inhibition of osteoclast differentiation by MACF1 knockdown. These data demonstrate that MACF1 positively regulates osteoclast differentiation via the Akt/GSK3β/NFATc1 signalling pathway, suggesting that targeting MACF1 may be a novel therapeutic approach against osteoporosis.

Role of the captured retroviral envelope syncytin-B gene in the fusion of osteoclast and giant cell precursors and in bone resorption, analyzed ex vivo and in vivo in syncytin-B knockout mice

Syncytin-A and -B are envelope genes of retroviral origin that have been captured in evolution for a role in placentation. They trigger cell-cell fusion and were shown to be essential for the formation of the syncytiotrophoblast layer during mouse placenta formation. Syncytin-A and -B expression has been described in other tissues and their highly fusogenic properties suggested that they might be involved in the fusion of other cell types. Here, taking advantage of mice knocked out for syncytin-B, SynB^-/- mice, we investigated the potential role of syncytin-B in the fusion of cells from the monocyte/macrophage lineage into multinucleated osteoclasts (OCs) -in bone- or multinucleated giant cells -in soft tissues. In ex vivo experiments, a significant reduction in fusion index and in the number of multinucleated OCs and giant cells was observed as soon as Day3 in SynB^-/- as compared to wild-type cell cultures. Interestingly, the number of nuclei per multinucleated OC or giant cell remained unchanged. These results, together with the demonstration that syncytin-B expression is maximal in the first 2 days of OC differentiation, argue for syncytin-B playing a role in the fusion of OC and giant cell mononucleated precursors, at initial stages. Finally, ex vivo, the observed reduction in multinucleated OC number had no impact on the expression of OC differentiation markers, and a dentin resorption assay did not evidence any difference in the osteoclastic resorption activity, suggesting that syncytin-B is not required for OC activity. In vivo, syncytin-B was found to be expressed in the periosteum of embryos at embryonic day 16.5, where TRAP-positive cells were observed. Yet, in adults, no significant reduction in OC number or alteration in bone phenotype was observed in SynB^-/- mice. In addition, SynB^-/- mice did not show any change in the number of foreign body giant cells (FBGCs) that formed in response to implantation of foreign material, as compared to wild-type mice. Altogether the results suggest that in addition to its essential role in placenta formation, syncytin-B plays a role in OCs and macrophage fusion; yet it is not essential in vivo for OC and FBGC formation, or maintenance of bone homeostasis, at least under the conditions tested.

Mesenchymal stem cell-based bioengineered constructs: foreign body response, cross-talk with macrophages and impact of biomaterial design strategies for pelvic floor disorders

An excessive foreign body response (FBR) has contributed to the adverse events associated with polypropylene mesh usage for augmenting pelvic organ prolapse surgery. Consequently, current biomaterial research considers the critical role of the FBR and now focuses on developing better biocompatible biomaterials rather than using inert implants to improve the clinical outcomes of their use. Tissue engineering approaches using mesenchymal stem cells (MSCs) have improved outcomes over traditional implants in other biological systems through their interaction with macrophages, the main cellular player in the FBR. The unique angiogenic, immunomodulatory and regenerative properties of MSCs have a direct impact on the FBR following biomaterial implantation. In this review, we focus on key aspects of the FBR to tissue-engineered MSC-based implants for supporting pelvic organs and beyond. We also discuss the immunomodulatory effects of the recently discovered endometrial MSCs on the macrophage response to new biomaterials designed for use in pelvic floor reconstructive surgery. We conclude with a focus on considerations in biomaterial design that take into account the FBR and will likely influence the development of the next generation of biomaterials for gynaecological applications.

Bone morphogenetic proteins: Their role in regulating osteoclast differentiation

The ability to create recombinant bone morphogenetic proteins (BMPs) in recent years has led to their rise as a common clinical adjuvant. Their application varies, from spinal fixation to repairing palatal clefts, to coating implants for osseointegration. In recent years questions have been raised as to the efficacy of BMPs in several of these procedures. These questions are due to the unwanted side effect of BMPs on other cell types, such as osteoclasts which can resorb bone at the graft/implant site. However, most BMP research focuses on the anabolic osteoinductive effects of BMPs on osteoblasts rather than its counterpart- stimulation of the osteoclasts, which are cells responsible for resorbing bone. In this review, we discuss the data available from multiple in-vitro and in-vivo BMP-related knockout models to elucidate the different functions BMPs have on osteoclast differentiation and activity.

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BMPs can act directly on osteoclasts to regulate differentiation and activity.

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Osteoclasts express multiple BMP signaling components.

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BMPs signal through both SMAD independent and dependent mechanisms in osteoclasts.

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SMAD dependent BMP signaling regulates osteoclast-osteoblast coupling factors.

Anacardic acid inhibits RANKL-induced osteoclastogenesis in vitro and prevents ovariectomy-induced bone loss in vivo

Postmenopausal osteoporosis is the most common form of primary osteoporosis, and the incidence of the condition is rapidly increasing. In consideration of the limitations of current therapeutic options for the treatment of postmenopausal osteoporosis, there is an urgent need to develop safer alternatives. Anacardic acid, a natural phenolic acid compound extracted from cashew nut shell, possesses potent antitumor and anti-inflammatory effects and inhibits NF-κB signaling. However, its effect on osteoclasts remains unknown. This study reports the first evidence for the antiosteoclastogenic and antiresorptive effects of anacardic acid on bone marrow-derived macrophage-derived osteoclasts. Mechanistically, anacardic acid disrupts the phosphorylation of TGF-β activated kinase 1 and subsequently suppresses multiple receptor activator of NF-κB ligand-induced signaling cascades, ultimately inhibiting the induction and activation of the crucial osteoclast transcriptional factor nuclear factor of activated T-cell cytoplasmic 1. Consistent with cellular results in vitro, anacardic acid treatment improves bone density in the murine model of ovariectomy-induced bone loss. Taken together, our study provides promising evidence for the therapeutic application of anacardic acid as a new potential pharmacological treatment for osteoporosis.-Zhao, K., Jia, Y., Peng, J., Pang, C., Zhang, T., Han, W., Jiang, J., Lu, X., Zhu, J., Qian, Y. Anacardic acid inhibits RANKL-induced osteoclastogenesis in vitro and prevents ovariectomy-induced bone loss in vivo.

Bee venom attenuates Porphyromonas gingivalis and RANKL-induced bone resorption with osteoclastogenic differentiation

Porphyromonas gingivalis (P. gingivalis) is one of the major periodontal pathogens leading to inflammation and alveolar bone resorption. Bone resorption is induced by osteoclasts, which are multinucleated giant cells. Osteoclastic bone resorption is mediated by enhanced receptor activator of nuclear factor-kappa B ligand (RANKL) signaling. Therefore, the down-regulation of RANKL downstream signals is regarded as an effective therapeutic target in the treatment of bone loss-associated disorders. The aim of this study was to evaluate whether purified bee venom (BV) could attenuate P. gingivalis-induced inflammatory periodontitis and RANKL-induced osteoclast differentiation. Inflammatory periodontitis induced by P. gingivalis increased alveolar bone resorption and increased expression of TNF-α and IL-1β, while BV treatment resulted in decreased bone loss and pro-inflammatory cytokines. Similarly, RANKL-induced multinucleated osteoclast differentiation and osteoclast-specific gene expression, such as nuclear factor of activated T cells 1 (NFATc1), cathepsin K, tartrate-resistant acid phosphatase (TRAP), and integrin αvβ3 were significantly suppressed by treatment with BV. We show that BV reduces P. gingivalis-induced inflammatory bone loss-related periodontitis in vivo and RANKL-induced osteoclast differentiation, activation, and function in vitro. These results suggest that BV exerts positive effects on inflammatory periodontitis associated osteoclastogenesis.

Paget's Disease of Bone

Paget's disease of bone (PDB) is a chronic and focal bone disorder, characterized by increased osteoclast-mediated bone resorption and a subsequent compensatory increase in bone formation, resulting in a disorganized mosaic of woven and lamellar bone at one or more affected skeletal sites. As a result, bone pain, noticeable deformities, arthritis at adjacent joints, and fractures can occur. In a small proportion of cases neoplastic degeneration in osteosarcoma, or, less frequently, giant cell tumor has been also described at PDB sites. While recent epidemiological evidences clearly indicate a decrease in the prevalence and the severity of PDB, over the past 2 decades there have been consistent advances on the genetic mechanisms of disease. It is now clear that PDB is a genetically heterogeneous disorder, with mutations in at least two different genes (SQSTM1, ZNF687) and more common predisposing variants. As a counterpart to the genetic hypothesis, the focal nature of lesions, the decline in prevalence rates, and the incomplete penetrance of the disease among family members suggest that one or more environmental triggers may play a role in the pathophysiology of PDB. The exact nature of these triggers and how they might interact with the genetic factors are less understood, but recent experimental data from mice models suggest the implication of paramixoviral infections. The clinical management of PDB has also evolved considerably, with the development of potent aminobisphosphonates such as zoledronic acid which, given as a single intravenous infusion, now allows a long-term disease remission in the majority of patients.

Inhibitory Effect of KP-A038 on Osteoclastogenesis and Inflammatory Bone Loss Is Associated With Downregulation of Blimp1

Excessive osteoclastic activity results in pathological bone resorptive diseases, such as osteoporosis, periodontitis, and rheumatoid arthritis. As imidazole-containing compounds possess extensive therapeutic potential for the management of diverse diseases, we synthesized a series of imidazole derivatives and investigated their effects on osteoclast differentiation and function. In the present study, we found that a novel imidazole derivative, KP-A038, suppressed receptor activator of nuclear factor-κB ligand (RANKL)-mediated osteoclastogenesis and bone-resorbing activity in vitro and attenuated lipopolysaccharide (LPS)-induced bone destruction in vivo. KP-A038 significantly inhibited the induction of nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) and the expression of its target genes, including tartrate-resistant acid phosphatase (Acp5), cathepsin K (Ctsk), dendritic cell-specific transmembrane protein (Dcstamp), and matrix metallopeptidase 9 (Mmp9). KP-A038 upregulated the expression of negative regulators of osteoclast differentiation, such as interferon regulatory factor-8 (Irf8) and B-cell lymphoma 6 (Bcl6). Consistently, KP-A038 downregulated the expression of B lymphocyte-induced maturation protein-1 (Blimp1 encoded by Prdm1), a repressor for Irf8 and Bcl6. Moreover, administration of KP-A038 reduced LPS-induced bone erosion by suppressing osteoclast formation in vivo. Thus, our findings suggest that KP-A038 may serve as an effective therapeutic agent for the treatment and/or prevention of bone loss in pathological bone diseases, including osteoporosis and periodontitis.

Pentamidine Inhibits Titanium Particle-Induced Osteolysis In Vivo and Receptor Activator of Nuclear Factor-κB Ligand-Mediated Osteoclast Differentiation In Vitro

Background

Wear debris-induced osteolysis leads to periprosthetic loosening and subsequent prosthetic failure. Since excessive osteoclast formation is closely implicated in periprosthetic osteolysis, identification of agents to suppress osteoclast formation and/or function is crucial for the treatment and prevention of wear particle-induced bone destruction. In this study, we examined the potential effect of pentamidine treatment on titanium (Ti) particle-induced osteolysis, and receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis.

Methods

The effect of pentamidine treatment on bone destruction was examined in Ti particle-induced osteolysis mouse model. Ti particles were implanted onto mouse calvaria, and vehicle or pentamidine was administered for 10 days. Then, calvarial bone tissue was analyzed using micro-computed tomography and histology. We performed in vitro osteoclastogenesis assay using bone marrow-derived macrophages (BMMs) to determine the effect of pentamidine on osteoclast formation. BMMs were treated with 20 ng/mL RANKL and 10 ng/mL macrophage colony-stimulating factor in the presence or absence of pentamidine. Osteoclast differentiation was determined by tartrate-resistant acid phosphatase staining, real-time polymerase chain reaction, and immunofluorescence staining.

Results

Pentamidine administration decreased Ti particle-induced osteoclast formation significantly and prevented bone destruction compared to the Ti particle group in vivo. Pentamidine also suppressed RANKL-induced osteoclast differentiation and actin ring formation markedly, and inhibited the expression of nuclear factor of activated T cell c1 and osteoclast-specific genes in vitro. Additionally, pentamidine also attenuated RANKL-mediated phosphorylation of IκBα in BMMs.

Conclusion

These results indicate that pentamidine is effective in inhibiting osteoclast formation and significantly attenuates wear debris-induced bone loss in mice.

How cells fuse

Brukman et al. review cell–cell fusion mechanisms, focusing on the identity of the fusogens that mediate these processes and the regulation of their activities.

Cell–cell fusion remains the least understood type of membrane fusion process. However, the last few years have brought about major advances in understanding fusion between gametes, myoblasts, macrophages, trophoblasts, epithelial, cancer, and other cells in normal development and in diseases. While different cell fusion processes appear to proceed via similar membrane rearrangements, proteins that have been identified as necessary and sufficient for cell fusion (fusogens) use diverse mechanisms. Some fusions are controlled by a single fusogen; other fusions depend on several proteins that either work together throughout the fusion pathway or drive distinct stages. Furthermore, some fusions require fusogens to be present on both fusing membranes, and in other fusions, fusogens have to be on only one of the membranes. Remarkably, some of the proteins that fuse cells also sculpt single cells, repair neurons, promote scission of endocytic vesicles, and seal phagosomes. In this review, we discuss the properties and diversity of the known proteins mediating cell–cell fusion and highlight their different working mechanisms in various contexts.

An Overview of the Derivation and Function of Multinucleated Giant Cells and Their Role in Pathologic Processes

Monocyte lineage cells play important roles in health and disease. Their differentiation into macrophages is crucial for a broad array of immunologic processes that regulate inflammation, neoplasia, and infection. In certain pathologic conditions, such as foreign body reactions and peripheral inflammatory lesions, monocytes fuse to form large, multinucleated giant cells (MGCs). Currently, our knowledge of the fusion mechanisms of monocytes and the regulation of MGC formation and function in discrete pathologies is limited. Herein, we consider the types and function of MGCs in disease and assess the mechanisms by which monocyte fusion contributes to the formation of MGCs. An improved understanding of the cellular origins and metabolic functions of MGCs will facilitate their identification and ultimately the treatment of diseases and disorders that involve MGCs.

TNX deficiency results in bone loss due to an increase in multinucleated osteoclasts

Tenascin-X (TNX), a glycoprotein of the extracellular matrix (ECM), is expressed in various tissues and plays an important role in ECM architecture. The TNXB gene encoding TNX is known as the gene responsible for classic-like Ehlers-Danlos syndrome (clEDS). To date, the role of TNX in dermal, muscular and obstetric features has been reported, but its role in bone homeostasis remains to be clarified. In this study, we found significant bone loss and upregulation of osteoclast marker gene expression in TNX-deficient mice. Further, TNX deficiency in the bone marrow promoted multinucleation of osteoclasts and resulted in increased bone resorption activity. These results indicate that multinucleated osteoclasts are the cause of bone loss in a TNX-deficient environment. Our findings provide new insight into the mechanism of osteoclast differentiation mediated by TNX and the pathology of clEDS.

The Molecular Mechanism of Vitamin E as a Bone-Protecting Agent: A Review on Current Evidence

Bone remodelling is a tightly-coordinated and lifelong process of replacing old damaged bone with newly-synthesized healthy bone. In the bone remodelling cycle, bone resorption is coupled with bone formation to maintain the bone volume and microarchitecture. This process is a result of communication between bone cells (osteoclasts, osteoblasts, and osteocytes) with paracrine and endocrine regulators, such as cytokines, reactive oxygen species, growth factors, and hormones. The essential signalling pathways responsible for osteoclastic bone resorption and osteoblastic bone formation include the receptor activator of nuclear factor kappa-B (RANK)/receptor activator of nuclear factor kappa-B ligand (RANKL)/osteoprotegerin (OPG), Wnt/β-catenin, and oxidative stress signalling. The imbalance between bone formation and degradation, in favour of resorption, leads to the occurrence of osteoporosis. Intriguingly, vitamin E has been extensively reported for its anti-osteoporotic properties using various male and female animal models. Thus, understanding the underlying cellular and molecular mechanisms contributing to the skeletal action of vitamin E is vital to promote its use as a potential bone-protecting agent. This review aims to summarize the current evidence elucidating the molecular actions of vitamin E in regulating the bone remodelling cycle.

Targeted sequencing of DCSTAMP in familial Paget's disease of bone

Paget's disease of bone (PDB) has a strong genetic component. Variants in SQSTM1 are found in up to 40% of patients with a family history of the disease, where a pattern of autosomal dominance with incomplete penetrance is apparent. By contrast, SQSTM1 variants are only found in up to 10% of patients with sporadic disease. It has been hypothesised that the remaining genetic susceptibility to PDB, particularly in familial cases, could be explained by rare genetic variants in loci previously identified by Genome Wide Association Studies. It is likely that polygenic factors are involved in many individuals. In this study we utilised whole exome sequencing to investigate predisposing genetic factors in an unsolved PDB kindred and identified a c.1189C > T p.L397F variant in DC-STAMP, also known as TM7SF4, that co-segregated with disease. DCSTAMP was identified as a gene of interest in PDB following Genome Wide Association Studies and has been previously shown to play critical roles in osteoclast fusion. The variant we identified has also been reported in association with PDB in a French-Canadian cohort however the significance of this variant was inconclusive. Targeted screening of DCSTAMP in our familial cohort of PDB patients revealed an additional 8 variants; however we did not find a significant association between any of these, including p.L397F, with PDB. Osteoclastogenesis assays from the affected proband and his unaffected brother demonstrated an increase in osteoclast number and nucleation, consistent with the pagetic phenotype. In converse to other established Paget's associated genetic variations such as SQSTM1, TNFRSF11A and OPTN, expression of the mutant DC-STAMP protein attenuated the activation of transcription factors NFκB and AP-1 when exogenously expressed. We found that the p.L397F variant did not influence the subcellular localization of the protein. Based on these findings we conclude that genetic variation in DCSTAMP is not a significant predisposing factor in our specific cohort of PDB patients and the p.L397F variant is unlikely to be a contributing factor in PDB pathogenesis.

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Variants in DC-STAMP do not appear to be significantly associated with Paget’s disease of bone in our cohort.

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The DC-STAMP p.L397F variant attenuates both NFkB and AP-1 signalling compared to the wild type protein.

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No significant differences in cellular colocalisation were found between DC-STAMP wild type and p.L397F.

G-protein Gα13 functions as a cytoskeletal and mitochondrial regulator to restrain osteoclast function

Excessive osteoclastic bone erosion disrupts normal bone remodeling and leads to bone loss in many skeletal diseases, including inflammatory arthritis, such as rheumatoid arthritis (RA) and psoriatic arthritis, periodontitis and peri-prosthetic loosening. Functional control of osteoclasts is critical for the maintenance of bone homeostasis. However, the mechanisms that restrain osteoclast resorptive function are not fully understood. In this study, we identify a previously unrecognized role for G-protein Gα13 in inhibition of osteoclast adhesion, fusion and bone resorptive function. Gα13 is highly expressed in mature multinucleated osteoclasts, but not during early differentiation. Deficiency of Gα13 in myeloid osteoclast lineage (Gα13ΔM/ΔM mice) leads to super spread morphology of multinucleated giant osteoclasts with elevated bone resorptive capacity, corroborated with an osteoporotic bone phenotype in the Gα13ΔM/ΔM mice. Mechanistically, Gα13 functions as a brake that restrains the c-Src, Pyk2, RhoA-Rock2 mediated signaling pathways and related gene expressions to control the ability of osteoclasts in fusion, adhesion, actin cytoskeletal remodeling and resorption. Genome wide analysis reveals cytoskeleton related genes that are suppressed by Gα13, identifying Gα13 as a critical cytoskeletal regulator in osteoclasts. We also identify a genome wide regulation of genes responsible for mitochondrial biogenesis and function by Gα13 in osteoclasts. Furthermore, the significant correlation between Gα13 expression levels, TNF activity and RA disease activity in RA patients suggests that the Gα13 mediated mechanisms represent attractive therapeutic targets for diseases associated with excessive bone resorption.

Diallyl disulfide alleviates inflammatory osteolysis by suppressing osteoclastogenesis via NF-κB-NFATc1 signal pathway

Skeletal homeostasis is closely effectuated by the regulation of bone formation and bone resorption. Osteoclasts are multinuclear giant cells responsible for bone resorption. Overactivated osteoclasts and excessive bone resorption result in various lytic bone diseases, such as osteoporosis, osteoarthritis, periprosthetic infection, and inflammatory aseptic loosening of orthopedic implants. In consideration of the severe side effects caused by the currently available drugs, exploitation of novel drugs has gradually attracted attention. Because of its anti-inflammatory, antioxidant, and antitumor capacities, diallyl disulfide (DADS), a major oil-soluble organosulfur ingredient compound derived from garlic, has been widely researched. However, the effects of DADS on osteoclasts and lytic bone diseases are still unknown. In this study, we investigated the effects of DADS on receptor activator of NF-κB ligand (RANKL)- and LPS-mediated osteoclastogenesis, LPS-stimulated proinflammatory cytokines related to osteoclasts, and LPS-induced inflammatory osteolysis. The results showed that DADS significantly inhibited RANKL-mediated osteoclast formation, fusion, and bone resorption in a dose-dependent manner via inhibiting the NF-κB and signal transducer and activator of transcription 3 signaling and restraining the interaction of NF-κB p65 with nuclear factor of activated T cells cytoplasmic 1. Furthermore, DADS also markedly suppressed LPS-induced osteoclastogenesis and reduced the production of proinflammatory cytokines with LPS stimulation to indirectly mediate osteoclast formation. Consistent with the in vitro results, DADS prevented the LPS-induced severe bone loss by blocking the osteoclastogenesis. All of the results indicate that DADS may be a potential and exploitable drug used for preventing and impeding osteolytic lesions.-Yang, J., Tang, R., Yi, J., Chen, Y., Li, X., Yu, T., Fei, J. Diallyl disulfide alleviates inflammatory osteolysis by suppressing osteoclastogenesis via NF-κB-NFATc1 signal pathway.

Vitexin suppresses RANKL-induced osteoclastogenesis and prevents lipopolysaccharide (LPS)-induced osteolysis

Osteolytic diseases are characterized by an increase in the number and/or activity of bone-resorbing osteoclasts. Identification of natural compounds that can suppress osteoclast formation and function is crucial for the prevention and treatment of osteolytic diseases. Vitexin, a naturally-derived flavonoid extracted from various medicinal plant species, demonstrates a broad range of pharmacological properties including anticancer and anti-inflammatory effects. Here in this study, we showed that vitexin exerts antiosteoclastogenic effects by directly inhibiting receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation and bone resorption in vitro and protected against lipopolysaccharide (LPS)-induced inflammatory osteolysis in vivo. Vitexin suppressed the early activation of ERK and p38 MAPK pathways in response to RANKL thereby attenuating the downstream induction of c-Fos and NFATc1, and abrogating the expression of osteoclast marker genes. Collectively, these results provide evidence for the therapeutic application of vitexin in the treatment of osteoclast-mediated bone lytic diseases.

Molecular signaling in bone cells: Regulation of cell differentiation and survival

The achievement of proper bone mass and architecture, and their maintenance throughout life requires the concerted actions of osteoblasts, the bone forming cells, and osteoclasts, the bone resorbing cells. The differentiation and activity of osteoblasts and osteoclasts are regulated by molecules produced by matrix-embedded osteocytes, as well as by cross talk between osteoblasts and osteoclasts through secreted factors. In addition, it is likely that direct contact between osteoblast and osteoclast precursors, and the contact of these cells with osteocytes and cells in the bone marrow, also modulates bone cell differentiation and function. With the advancement of molecular and genetic tools, our comprehension of the intracellular signals activated in bone cells has evolved significantly, from early suggestions that osteoblasts and osteoclasts have common precursors and that osteocytes are inert cells in the bone matrix, to the very sophisticated understanding of a network of receptors, ligands, intracellular kinases/phosphatases, transcription factors, and cell-specific genes that are known today. These advances have allowed the design and FDA-approval of new therapies to preserve and increase bone mass and strength in a wide variety of pathological conditions, improving bone health from early childhood to the elderly. We have summarized here the current knowledge on selected intracellular signal pathways activated in osteoblasts, osteocytes, and osteoclasts.

Genetic regulatory mechanisms in human osteoclasts suggest a role for the STMP1 and DCSTAMP genes in Paget's disease of bone

Paget’s disease of bone (PDB) is characterised by focal abnormalities of bone remodelling, with increased osteoclastic resorption the primary feature of the disease. Genetic factors have been shown to play an important role in PDB, and genome-wide association studies (GWAS) have identified 7 genetic loci as associated with PDB at the genome-wide level. Expression quantitative trait locus (eQTL) studies using cell types that are directly relevant to the disease of interest are increasingly being used to identify putative effector genes for GWAS loci. We have recently constructed a unique osteoclast-specific eQTL resource using cells differentiated in vitro from 158 subjects for study of the genetics of bone disease. Considering the major role osteoclasts have in PDB, we used this resource to investigate potential genetic regulatory effects for the 7 PDB genome-wide significant loci on genes located within 500 kb of each locus. After correction for multiple testing, we observed statistically significant associations for rs4294134 with expression of the gene STMP1, and rs2458413 with expression of the genes DPYS and DCSTAMP. The eQTL associations observed for rs4294134 with STMP1, and rs2458413 with DCSTAMP were further supported by eQTL data from other tissue types. The product of the STMP1 gene has not been extensively studied, however the DCSTAMP gene has an established role in osteoclast differentiation and the associations seen between rs2458413 and PDB are likely mediated through regulatory effects on this gene. This study highlights the value of eQTL data in determining which genes are relevant to GWAS loci.

Ninjurin1 positively regulates osteoclast development by enhancing the survival of prefusion osteoclasts

Osteoclasts (OCs) are bone-resorbing cells that originate from hematopoietic stem cells and develop through the fusion of mononuclear myeloid precursors. Dysregulation of OC development causes bone disorders such as osteopetrosis, osteoporosis, and rheumatoid arthritis. Although the molecular mechanisms underlying osteoclastogenesis have been well established, the means by which OCs maintain their survival during OC development remain unknown. We found that Ninjurin1 (Ninj1) expression is dynamically regulated during osteoclastogenesis and that Ninj1^-/- mice exhibit increased trabecular bone volume owing to impaired OC development. Ninj1 deficiency did not alter OC differentiation, transmigration, fusion, or actin ring formation but increased Caspase-9-dependent intrinsic apoptosis in prefusion OCs (preOCs). Overexpression of Ninj1 enhanced the survival of mouse macrophage/preOC RAW264.7 cells in osteoclastogenic culture, suggesting that Ninj1 is important for the survival of preOCs. Finally, analysis of publicly available microarray data sets revealed a potent correlation between high NINJ1 expression and destructive bone disorders in humans. Our data indicate that Ninj1 plays an important role in bone homeostasis by enhancing the survival of preOCs.

Propofol promotes osteoclastic bone resorption by increasing DC-STAMP expression

Background

Propofol is an intravenous anesthetic which has antioxidant effects due to its similarity in molecular structure to α-tocopherol. It has been reported that α-tocopherol increases osteoclast fusion and bone resorption. Here, we investigated the effects of propofol on signaling pathways of osteoclastogenic gene expression, as well as osteoclastogenesis and bone resorption using bone marrow-derived macrophages (BMMs).

Methods

BMMs were cultured with macrophage colony-stimulating factor (M-CSF) alone or M-CSF plus receptor activator of nuclear factor kappa B ligand (RANKL) in the presence of propofol (0-50 µM) for 4 days. Mature osteoclasts were stained for tartrate-resistant acid phosphatase (TRAP) and the numbers of TRAP-positive multinucleated osteoclasts were counted. To examine the resorption activities of osteoclasts, a bone resorption assay was performed. To identify the mechanism of action of propofol on the formation of multinucleated osteoclasts, we focused on dendritic cell-specific transmembrane protein (DC-STAMP), a protein essential for pre-osteoclastic cell fusion.

Results

Propofol increased the formation of TRAP-positive multinucleated osteoclasts. In addition, the bone resorption assay revealed that propofol increased the bone resorption area on dentin discs. The mRNA expression of DC-STAMP was upregulated most strongly in the presence of both RANKL and propofol. However, SB203580, a p38 inhibitor, significantly suppressed the propofol/RANKL-induced increase in mRNA expression of DC-STAMP.

Conclusion

We have demonstrated that propofol enhances osteoclast differentiation and maturation, and subsequently increases bone resorption. Additionally, we identified the regulatory pathway underlying osteoclast cell-cell fusion, which was enhanced by propofol through p38-mediated DC-STAMP expression.

Osteoimmunology

Bone is a crucial element of the skeletal-locomotor system, but also functions as an immunological organ that harbors hematopoietic stem cells (HSCs) and immune progenitor cells. Additionally, the skeletal and immune systems share a number of regulatory molecules, including cytokines and signaling molecules. Osteoimmunology was created as an interdisciplinary field to explore the shared molecules and interactions between the skeletal and immune systems. In particular, the importance of an inseparable link between the two systems has been highlighted by studies on the pathogenesis of rheumatoid arthritis (RA), in which pathogenic helper T cells induce the progressive destruction of multiple joints through aberrant expression of receptor activator of nuclear factor (NF)-κB ligand (RANKL). The conceptual bridge of osteoimmunology provides not only a novel framework for understanding these biological systems but also a molecular basis for the development of therapeutic approaches for diseases of bone and/or the immune system.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Myosins in Osteoclast Formation and Function

Skeletal quantity and quality are determined by processes of bone modeling and remodeling, which are undertaken by cells that build and resorb bone as they respond to mechanical, hormonal, and other external and internal signals. As the sole bone resorptive cell type, osteoclasts possess a remarkably dynamic actin cytoskeleton that drives their function in this enterprise. Actin rearrangements guide osteoclasts’ capacity for precursor fusion during differentiation, for migration across bone surfaces and sensing of their composition, and for generation of unique actin superstructures required for the resorptive process. In this regard, it is not surprising that myosins, the superfamily of actin-based motor proteins, play key roles in osteoclast physiology. This review briefly summarizes current knowledge of the osteoclast actin cytoskeleton and describes myosins’ roles in osteoclast differentiation, migration, and actin superstructure patterning.

Neotuberostemonine inhibits osteoclastogenesis via blockade of NF-κB pathway

Osteoporosis has been attributed to low bone mass arising from cellular communications between bone formation and bone resorption. Osteoclastogenesis is induced by M-CSF and RANKL in hematopoietic lineage cells. Once RANK/RANKL complex is formed, TRAF6 is recruited and triggers the activation of NF-κB pathway and the expression of osteoclast-related genes including NFATc1. Neotuberostemonine (NTS) is an active compound isolated from Stemona tuberosa Lour. Pharmacologically, NTS has been known to possess antitussive, anti-fibrotic and anti-inflammatory activities through regulation of macrophage. However, the influence of NTS to osteoclastogenesis has not been reported. The purpose of this study is to investigate whether NTS can modulate the osteoclastogenesis induced by RANKL or cancer cells. We found that NTS inhibits RANKL- or cancer cell-mediated osteoclastogenesis via blockade of TRAF6 and NF-κB activation. NTS also impairs the formation of F-actin ring structure, an important feature of osteoclast differentiation and function. These results indicate that NTS can be a preventive and therapeutic candidate for bone-related disease and that NTS provides insights underlying molecular mechanisms that influence osteoclastogenesis.

Inhibition of STAT5A promotes osteogenesis by DLX5 regulation

The regulation of osteogenesis is important for bone formation and fracture healing. Despite advances in understanding the molecular mechanisms of osteogenesis, crucial modulators in this process are not well-characterized. Here we demonstrate that suppression of signal transducer and activator of transcription 5A (STAT5A) activates distal-less homeobox 5 (DLX5) in human bone marrow-derived stromal cells (hBMSCs) and enhances osteogenesis in vitro and in vivo. We show that STAT5A negatively regulates expression of Dlx5 in vitro and that STAT5A deletion results in increased trabecular and cortical bone mass and bone mineral density in mice. Additionally, STAT5A deletion prevents age-related bone loss. In a murine fracture model, STAT5A deletion was found to significantly enhance bone remodeling by stimulating the formation of a fracture callus. Our findings indicate that STAT5A inhibition enhances bone formation by promoting osteogenesis of BMSCs.

Tumor necrosis factor receptor-associated factor 6 is required to inhibit foreign body giant cell formation and activate osteoclasts under inflammatory and infectious conditions

Osteoclasts and foreign body giant cells (FBGCs) are derived from common progenitors and share properties such as multi-nucleation capacity induced by cell-cell fusion; however, mechanisms underlying lineage determination between these cells remain unclear. Here we show that, under inflammatory conditions, osteoclasts are stimulated in a manner similar to M1 macrophages, while formation of FBGCs, which exhibit M2-like phenotypes, is inhibited in a manner similar to that seen in M1/M2 macrophage polarization. FBGC/osteoclast polarization was inhibited by conditional knockout of tumor necrosis factor receptor associated factor 6 (Traf6) in adults in vivo and in vitro. Traf6-null mice were previously reported to die soon after birth, but we found that Traf6 deletion in adults did not cause lethality but rather inhibited osteoclast activation and prevented FBGC inhibition under inflammatory conditions. Accordingly, basal osteoclastogenesis was significantly inhibited by Traf6 deletion in vivo and in vitro and accompanied by increased bone mass. Lipopolysaccharide-induced osteoclast formation and osteolysis were significantly inhibited in Traf6 conditional knockout mice. Our results suggest that Traf6 plays a crucial role in regulating M1 osteoclast and M2 FBGC polarization and is a potential therapeutic target in blocking FBGC inhibition, antagonizing osteolysis in inflammatory conditions, and increasing bone mass without adverse effects in adults.

BCPA {N,N'-1,4-Butanediylbis[3-(2-chlorophenyl)acrylamide]} Inhibits Osteoclast Differentiation through Increased Retention of Peptidyl-Prolyl cis-trans Isomerase Never in Mitosis A-Interacting 1

Osteoporosis is caused by an imbalance of osteoclast and osteoblast activities and it is characterized by enhanced osteoclast formation and function. Peptidyl-prolyl cis-trans isomerase never in mitosis A (NIMA)-interacting 1 (Pin1) is a key mediator of osteoclast cell-cell fusion via suppression of the dendritic cell-specific transmembrane protein (DC-STAMP). We found that N,N′-1,4-butanediylbis[3-(2-chlorophenyl)acrylamide] (BCPA) inhibited receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis in a dose-dependent manner without cytotoxicity. In addition, BCPA attenuated the reduction of Pin1 protein during osteoclast differentiation without changing Pin1 mRNA levels. BCPA repressed the expression of osteoclast-related genes, such as DC-STAMP and osteoclast-associated receptor (OSCAR), without altering the mRNA expression of nuclear factor of activated T cells (NFATc1) and cellular oncogene fos (c-Fos). Furthermore, Tartrate-resistant acid phosphatase (TRAP)-positive mononuclear cells were significantly decreased by BCPA treatment compared to treatment with the Pin1 inhibitor juglone. These data suggest that BCPA can inhibit osteoclastogenesis by regulating the expression of the DC-STAMP osteoclast fusion protein by attenuating Pin1 reduction. Therefore, BCPA may be used to treat osteoporosis.

Tomatidine suppresses osteoclastogenesis and mitigates estrogen deficiency-induced bone mass loss by modulating TRAF6-mediated signaling

Postmenopausal osteoporosis is initiated by estrogen withdrawal and is characterized mainly by overactivated osteoclastic bone resorption. Targeting TNF receptor-associated factor 6 (TRAF6) or its downstream signaling pathways to modulate osteoclast formation and function is an appealing strategy for osteoclast-related disorders. In the present study, we determined the effect of tomatidine, a steroidal alkaloid derived from Solanaceae, on the formation and function of receptor activator of NF-κB (RANK) ligand-induced osteoclasts and the underlying mechanism. Tomatidine inhibited osteoclast formation in a dose-dependent manner and decreased the expression of osteoclast marker genes. Actin ring formation and osteoclastic bone resorption were attenuated in the presence of tomatidine in vitro. Eight weeks after ovariectomy, tomatidine prevented estrogen deficiency-induced bone loss and restored the mechanical properties of the femur. At the molecular level, tomatidine abrogated phosphorylation of c-Jun N-terminal kinase (JNK)/p38, NF-κB, and protein kinase B (Akt) pathway proteins by suppressing RANK expression, inhibiting the binding of TRAF6 to RANK, and downregulating the osteoclastogenesis marker-related protein expression. In summary, these data demonstrated that tomatidine attenuated osteoclast formation and function by modulating multiple TRAF6-mediated pathways. Therefore, tomatidine could be a novel candidate for the treatment of osteoclast-related disorders, including osteoporosis.-Hu, B., Sun, X., Yang, Y., Ying, Z., Meng, J., Zhou, C., Jiang, G., Li, S., Wu, F., Zhao, X., Zhu, H., Wu, H., Cai, X., Shi, Z., Yan, S. Tomatidine suppresses osteoclastogenesis and mitigates estrogen deficiency-induced bone mass loss by modulating TRAF6-mediated signaling.

Skullcapflavone II inhibits osteoclastogenesis by regulating reactive oxygen species and attenuates the survival and resorption function of osteoclasts by modulating integrin signaling

Many bone diseases, such as osteoporosis and rheumatoid arthritis, are attributed to an increase in osteoclast number or activity; therefore, control of osteoclasts has significant clinical implications. This study shows how skullcapflavone II (SFII), a flavonoid with anti-inflammatory activity, regulates osteoclast differentiation, survival, and function. SFII inhibited osteoclastogenesis with decreased activation of MAPKs, Src, and cAMP response element-binding protein (CREB), which have been known to be redox sensitive. SFII decreased reactive oxygen species by scavenging them or activating nuclear factor-erythroid 2-related factor 2 (Nrf2), and its effects were partially reversed by hydrogen peroxide cotreatment or Nrf2 deficiency. In addition, SFII attenuated survival, migration, and bone resorption, with a decrease in the expression of integrin β₃, Src, and p130 Crk-associated substrate, and the activation of RhoA and Rac1 in differentiated osteoclasts. Furthermore, SFII inhibited osteoclast formation and bone loss in an inflammation- or ovariectomy-induced osteolytic mouse model. These findings suggest that SFII inhibits osteoclastogenesis through redox regulation of MAPKs, Src, and CREB and attenuates the survival and resorption function by modulating the integrin pathway in osteoclasts. SFII has therapeutic potential in the treatment and prevention of bone diseases caused by excessive osteoclast activity.-Lee, J., Son, H. S., Lee, H. I., Lee, G.-R., Jo, Y.-J., Hong, S.-E., Kim, N., Kwon, M., Kim, N. Y., Kim, H. J., Lee, Y. J., Seo, E. K., Jeong, W. Skullcapflavone II inhibits osteoclastogenesis by regulating reactive oxygen species and attenuates the survival and resorption function of osteoclasts by modulating integrin signaling.

HDL impairs osteoclastogenesis and induces osteoclast apoptosis via upregulation of ABCG1 expression

Cholesterol is one of the major components of biological membranes and has an important function in osteoclast formation and survival. It has been reported that high-density lipoprotein (HDL) promotes cholesterol efflux from osteoclasts and induces their apoptosis, but the underlying mechanisms are unclear. In this study, we investigated how HDL promotes osteoclast cholesterol efflux and explored its effect on osteoclast formation and survival. Our results showed that the maximum diameter and fusion index of osteoclasts were decreased, while the ratios of osteoclasts with pyknotic nuclei were increased when cells were treated with HDL (600 ng/ml), as revealed by tartrate-resistant acid phosphatase-positive staining and microscopy assay. HDL enhanced cellular cholesterol efflux from osteoclasts in both concentration- and time-dependent manners. The ability of HDL3 to stimulate cholesterol efflux was stronger than preβ-HDL, HDL2, and ApoAI. Knockdown of ABCG1 expression reduced HDL-mediated cholesterol efflux and restored the HDL-induced reduction in osteoclast formation. Finally, HDL3 promoted sphingomyelin efflux from osteoclasts and reduced the expression of caveolin-1. Together, the findings demonstrate that HDL3 upregulates ABCG1 expression and promotes cholesterol efflux from osteoclast, impairs cholesterol homeostasis in osteoclasts, and consequently enhances osteoclast apoptosis.

Visual Osteoclast Fusion via A Fluorescence Method

Osteoclasts are multinucleated giant cells. Fusion is an essential element in the formation of osteoclasts. However, the exact cellular events and mechanisms remain largely unknown because of limited and insufficient methods for observing fusion process. In this work, a fluorescence reporter strategy was established to monitor osteoclast fusion. After fusing with cells expressing Cre recombinase, those cells with double fluorescence switch its expression from red to green fluorescent protein. The effect of RANKL and PTH on osteoclast fusion were both quantitatively and visually detected utilizing this strategy. Furthermore, a combination of this strategy with a technique of fluorescence-activated cell sorting revealed two different populations of fused osteoclasts, tdTomato+ GFP+ cells (TG cells) and GFP+ cells (G cells). The results argue for the potential of combining this technique with other bio-technologies to gain more information about osteoclast fusion. Overall, these data demonstrated that this visual fluorescence switch strategy is useful for further analysis of osteoclast fusion mechanisms.

Non-classical monocytes as mediators of tissue destruction in arthritis

Objectives

Bone destruction in rheumatoid arthritis is mediated by osteoclasts (OC), which are derived from precursor cells of the myeloid lineage. The role of the two monocyte subsets, classical monocytes (expressing CD115, Ly6C and CCR2) and non-classical monocytes (which are CD115 positive, but low in Ly6C and CCR2), in serving as precursors for OC in arthritis is still elusive.

Methods

We investigated CCR2^−/− mice, which lack circulating classical monocytes, crossed into hTNFtg mice for the extent of joint damage. We analysed monocyte subsets in hTNFtg and K/BxN serum transfer arthritis by flow cytometry. We sorted monocyte subsets and analysed their potential to differentiate into OC and their transcriptional response in response to RANKL by RNA sequencing. With these data, we performed a gene ontology enrichment analysis and gene set enrichment analysis.

Results

We show that in hTNFtg arthritis local bone erosion and OC generation are even enhanced in the absence of CCR2. We further show the numbers of non-classical monocytes in blood are elevated and are significantly correlated with histological signs of joint destruction. Sorted non-classical monocytes display an increased capacity to differentiate into OCs. This is associated with an increased expression of signal transduction components of RANK, most importantly TRAF6, leading to an increased responsiveness to RANKL.

Conclusion

Therefore, non-classical monocytes are pivotal cells in arthritis tissue damage and a possible target for therapeutically intervention for the prevention of inflammatory joint damage.

The Osteoclast in Bone Metastasis: Player and Target

Bone metastases are frequently the final fate of breast and prostate cancer patients. According to the definition of metastasis as an incurable disease, to date there are no effective treatments for tumor-associated bone metastases and this represents a real challenge for the researchers in the field. The bone is a heterogeneous environment that represents a fertile soil for tumor cells, supporting their growth. Among the different cell types present in the bone, in this review we will focus our attention on the osteoclasts, which are crucial players in the so called “vicious cycle”, a phenomenon triggered by tumor cells eventually leading to both tumor proliferation as well as bone deregulation, thus fueling the development of bone metastasis. The complex network, linking tumor cells to the bone by activating osteoclasts, represents a fruitful target for the treatment of bone metastases. In this review we will describe how tumor cells perturb the bone microenvironment by actively influencing osteoclast formation and activity. Moreover, we will describe the current antiresorptive drugs employed in the treatment of bone metastases as well as new, targeted therapies able to affect both cancer cells and osteoclasts.

Dehydrodiconiferyl Alcohol Inhibits Osteoclast Differentiation and Ovariectomy-Induced Bone Loss through Acting as an Estrogen Receptor Agonist

Estrogen deficiency after menopause increases bone loss by activating RANKL-induced osteoclast differentiation. Dehydrodiconiferyl alcohol (DHCA), a lignan originally isolated from Cucurbita moschata, has been thought to be a phytoestrogen based on its structure. In this study, we tested whether DHCA could affect RANKL-induced osteoclastogenesis in vitro and ovariectomy-induced bone loss in vivo. In RAW264.7 cells, DHCA inhibited RANKL-induced differentiation of osteoclasts. Consistently, expression of the six osteoclastogenic genes induced by RANKL was down-regulated. DHCA was also shown to suppress the NF-κB and p38 MAPK signaling pathways by activating AMPK. Data from transient transfection assays suggested that DHCA might activate the estrogen receptor signaling pathway. Effects of DHCA on RANKL-induced osteoclastogenesis were reduced when cells were treated with specific siRNA to ERα, but not to ERβ. Interestingly, DHCA was predicted from molecular docking simulation to bind to both ERα and ERβ. Indeed, data from an estrogen receptor competition assay revealed that DHCA acted as an agonist on both estrogen receptors. In the ovariectomized (Ovx) mouse model, DHCA prevented Ovx-induced bone loss by inhibiting osteoclastogenesis. Taken together, our results suggest that DHCA may be developed as an efficient therapeutic for osteoporosis by regulating osteoclastogenesis through its estrogenic effects.

Actin binding LIM 1 (abLIM1) negatively controls osteoclastogenesis by regulating cell migration and fusion

Actin binding LIM 1 (abLIM1) is a cytoskeletal actin-binding protein that has been implicated in interactions between actin filaments and cytoplasmic targets. Previous biochemical and cytochemical studies have shown that abLIM1 interacts and co-localizes with F-actin in the retina and muscle. However, whether abLIM1 regulates osteoclast differentiation has not yet been elucidated. In this study, we examined the role of abLIM1 in osteoclast differentiation and function. We found that abLIM1 expression was upregulated during receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast differentiation, and that a novel transcript of abLIM1 was exclusively expressed in osteoclasts. Overexpression of abLIM1 in the murine monocytic cell line, RAW-D suppressed osteoclast differentiation and decreased expression of several osteoclast-marker genes. By contrast, small interfering RNA-induced knockdown of abLIM1 enhanced the formation of multinucleated osteoclasts and markedly increased the expression of the osteoclast-marker genes. Mechanistically, abLIM1 regulated the localization of tubulin, migration, and fusion in osteoclasts. Thus, these results indicate that abLIM1 negatively controls osteoclast differentiation by regulating cell migration and fusion mediated via actin formation.

Monocyte Subsets Have Distinct Patterns of Tetraspanin Expression and Different Capacities to Form Multinucleate Giant Cells

Monocytes are able to undergo homotypic fusion to produce different types of multinucleated giant cells, such as Langhans giant cells in response to M. tuberculosis infection or foreign body giant cells in response to implanted biomaterials. Monocyte fusion is highly coordinated and complex, with various soluble, intracellular, and cell-surface components mediating different stages of the process. Tetraspanins, such as CD9, CD63, and CD81, are known to be involved in cell:cell fusion and have been suggested to play a role in regulating homotypic monocyte fusion. However, peripheral human monocytes are not homogenous: they exist as a heterogeneous population consisting of three subsets, classical (CD14⁺⁺CD16^-), intermediate (CD14⁺⁺CD16⁺), and non-classical (CD14⁺CD16⁺), at steady state. During infection with mycobacteria, the circulating populations of intermediate and non-classical monocytes increase, suggesting they may play a role in the disease outcome. Human monocytes were separated into subsets and then induced to fuse using concanavalin A. The intermediate monocytes were able to fuse faster and form significantly larger giant cells than the other subsets. When antibodies targeting tetraspanins were added, the intermediate monocytes responded to anti-CD63 by forming smaller giant cells, suggesting an involvement of tetraspanins in fusion for at least this subset. However, the expression of fusion-associated tetraspanins on monocyte subsets did not correlate with the extent of fusion or with the inhibition by tetraspanin antibody. We also identified a CD9^High and a CD9^Low monocyte population within the classical subset. The CD9^High classical monocytes expressed higher levels of tetraspanin CD151 compared to CD9^Low classical monocytes but the CD9^High classical subset did not exhibit greater potential to fuse and the role of these cells in immunity remains unknown. With the exception of dendrocyte-expressed seven transmembrane protein, which was expressed at higher levels on the intermediate monocyte subset, the expression of fusion-related proteins between the subsets did not clearly correlate with their ability to fuse. We also did not observe any clear correlation between giant cell formation and the expression of pro-inflammatory or fusogenic cytokines. Although tetraspanin expression appears to be important for the fusion of intermediate monocytes, the control of multinucleate giant cell formation remains obscure.

Common signalling pathways in macrophage and osteoclast multinucleation

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

The Caenorhabditis elegans spe-49 gene is required for fertilization and encodes a sperm-specific transmembrane protein homologous to SPE-42

Fertilization, the fusion of sperm and oocyte to form a zygote, is the first and arguably the most important cell-cell interaction event in an organism's life. Forward and reverse genetic approaches in the nematode Caenorhabditis elegans have identified many genes that are required for gametogenesis and fertilization and thus are beginning to elucidate the molecular pathways that underlie these processes. We identified an allele of the spe-49 gene in a second filial generation (F₂ ) mutagenesis screen for spermatogenesis-defective (spe) mutants. Mutant worms for spe-49 produce sperm that have normal morphology, activate to form ameboid spermatozoa, and can migrate to and maintain their position in the hermaphrodite reproductive tract but fail to fertilize oocytes. This phenotype puts spe-49 in the spe-9 class of late-acting genes that function in sperm at the time of fertilization. We cloned the spe-49 gene through a combination of deficiency mapping, transgenic rescue, and genomic sequencing. spe-49 messenger RNA (mRNA) is enriched in male germ cells, and the complementary DNA (cDNA) encodes a predicted 772-amino-acid six-pass transmembrane protein that is homologous to SPE-42. Indeed, SPE-49 and SPE-42 have identical predicted membrane topology and domain structure, including a large extracellular domain with six conserved cysteine residues, a DC-STAMP domain, and a C-terminal cytoplasmic domain containing a C4-C4 RING finger motif. The presence of two SPE-42 homologs in animal genomes from worms to humans suggests that these proteins are highly conserved components of the molecular apparatus required for the sperm-oocyte recognition, binding, and fusion.

Roles of Enhancer RNAs in RANKL-induced Osteoclast Differentiation Identified by Genome-wide Cap-analysis of Gene Expression using CRISPR/Cas9

Bidirectional transcription has been proposed to play a role associated with enhancer activity. Transcripts called enhancer RNAs (eRNAs) play important roles in gene regulation; however, their roles in osteoclasts are unknown. To analyse eRNAs in osteoclasts comprehensively, we used cap-analysis of gene expression (CAGE) to detect adjacent transcription start sites (TSSs) that were distant from promoters for protein-coding gene expression. When comparing bidirectional TSSs between osteoclast precursors and osteoclasts, we found that bidirectional TSSs were located in the 5′-flanking regions of the Nrp2 and Dcstamp genes. We also detected bidirectional TSSs in the intron region of the Nfatc1 gene. To investigate the role of bidirectional transcription in osteoclasts, we performed loss of function analyses using the CRISPR/Cas9 system. Targeted deletion of the DNA regions between the bidirectional TSSs led to decreased expression of the bidirectional transcripts, as well as the protein-coding RNAs of Nrp2, Dcstamp, and Nfatc1, suggesting that these transcripts act as eRNAs. Furthermore, osteoclast differentiation was impaired by targeted deletion of bidirectional eRNA regions. The combined results show that eRNAs play important roles in osteoclastogenic gene regulation, and may therefore provide novel insights to elucidate the transcriptional mechanisms that control osteoclast differentiation.

Inhibitory effects of sodium pentosan polysulfate on formation and function of osteoclasts derived from canine bone marrow

BACKGROUND

Sodium pentosan polysulfate (NaPPS) was testified as a chondroprotective drug in with a detailed rationale of the disease-modifying activity. This study was undertaken to determine whether anti-osteoarthritis drug, NaPPS inhibited osteoclasts (OC) differentiation and function. Canine bone marrow mononuclear cells (n = 6) were differentiated to OC by maintaining with receptor activator of nuclear factor kappa B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) for up to 7 days with the treatment of NaPPS at concentration of 0, 0.2, 1 and 5 μg/mL. Differentiation and function of OC were accessed using tartrate-resistant acid phosphate (TRAP) staining and bone resorption assay, while monitoring actin ring formation. Invasion and colocalization patterns of fluorescence-labeled NaPPS with transcribed gene in OC were monitored. Gene expression of OC for cathepsin K (CTK), matrix metallopeptidase-9 (MMP-9), nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), c-Fos, activator protein-1(AP-1) and carbonic anhydrase II was examined using real-time PCR.

RESULTS

Significant inhibition of OC differentiation was evident at NaPPS concentration of 1 and 5 μg/mL (p < 0.05). In the presence of 0.2 to 5 μg/mL NaPPS, bone resorption was attenuated (p < 0.05), while 1 and 5 μg/mL NaPPS achieved significant reduction of actin ring formation. Intriguingly, fluorescence-labeled NaPPS invaded in to cytoplasm and nucleus while colocalizing with actively transcribed gene. Gene expression of CTK, MMP-9 and NFATc1 were significantly inhibited at 1 and 5 μg/mL (p < 0.05) of NaPPS whereas inhibition of c-Fos and AP-1 was identified only at concentration of 5 μg/mL (p < 0.05).

CONCLUSIONS

Taken together, all the results suggest that NaPPS is a novel inhibitor of RANKL and M-CSF-induced CTK, MMP-9, NFATc1, c-Fos, AP-1 upregulation, OC differentiation and bone resorption which might be a beneficial for treatment of inflammatory joint diseases and other bone diseases associated with excessive bone resorption.

Ostm1 Bifunctional Roles in Osteoclast Maturation: Insights From a Mouse Model Mimicking a Human OSTM1 Mutation

Ostm1 mutations are responsible for the most severe form of osteopetrosis in human and mice. To gain insight into Ostm1 cellular functions, we engineered a conditional in-frame deletion of the Ostm1 transmembrane domain and generated the first Ostm1 mouse model with a human mutation. Systemic targeting of Ostm1 loss of transmembrane domain produced osteopetrosis, as in the null Ostm1 gl/gl mouse. Significantly, conditional osteoclast targeting of Ostm1 resulted in similar osteopetrosis, thereby demonstrating that the intrinsic Ostm1 osteoclast deficiency is solely responsible for the mouse phenotype. Our analysis showed oversized osteoclasts with enhanced multinucleation associated with stimulation of intracellular calcium levels, of Nfatc1 nuclear re-localization, and of specific downstream Nfatc1 target genes, providing compelling evidence that Ostm1 is a negative regulator of preosteoclast fusion. Moreover, mature OCs with Ostm1 loss of transmembrane domain show appropriate levels of intracellular acidification but an altered distribution pattern, highlighting misregulation of endolysosome localization and dispersion. Consistently, the hydrolases tartrate-resistant acid phosphatase (TRAP) and cathepsin K (Ctsk) normally produced are sequestered within the osteoclasts and are not extracellularly secreted. These studies defined bifunctional roles for Ostm1 as a major regulator of preosteoclast cytoskeletal rearrangements toward cell multinucleation and of mature osteoclast intracellular lysosomal trafficking and exocytosis mechanism, both of which are essential for bone resorption. Importantly, these Ostm1 molecular and regulatory functions could serve as preclinical targets in this mouse model toward osteoclastogenic pathologies as osteoporosis and inflammation-induced bone loss. © 2018 American Society for Bone and Mineral Research.

Transglutaminase activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics

Osteoclasts, bone resorbing cells, derive from monocyte/macrophage cell lineage. Increased osteoclast activity is responsible for bone destruction in diseases such as osteoporosis, periodontitis and rheumatoid arthritis. Transglutaminases (TGs), protein crosslinking enzymes, were recently found involved in osteoclastogenesis in vivo, however their mechanisms of action have remained unknown. In this study, we have investigated the role of TG activity in osteoclastogenesis in vitro using four TG inhibitors, NC9, Z006, T101, and monodansyl cadaverine. Our results showed that all TG inhibitors were capable of blocking the entire osteoclastogenesis process. The most potent of the inhibitors, NC9 when added to cultures at different phases of osteoclastogenesis, inhibited differentiation, migration, and fusion of pre-osteoclasts as well as resorption activity of mature osteoclasts. Further investigation into the mechanisms revealed that NC9 increased RhoA levels and blocked podosome belt formation suggesting that TG activity regulates actin dynamics in pre-osteoclasts. The inhibitory effect of NC9 on osteoclastogenesis as well as podosome belt formation was completely reversed with a Rho-family inhibitor Exoenzyme C3. Microtubule architecture, acetylation, and detyrosination of α-tubulin were not affected. Finally, we demonstrated that macrophages and osteoclasts expressed mRNA of three TGs:TG1, TG2, and Factor XIII-A which were all differentially regulated in these cells during differentiation. Immunofluoresence microscopic analysis showed that all three enzymes co-localized to podosomes in osteoclasts. Taken together, our data suggests that TG activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics and that this may involve contribution from all three TG enzymes.

Osteoclast-secreted SLIT3 coordinates bone resorption and formation

Coupling is the process that links bone resorption to bone formation in a temporally and spatially coordinated manner within the remodeling cycle. Several lines of evidence point to the critical roles of osteoclast-derived coupling factors in the regulation of osteoblast performance. Here, we used a fractionated secretomic approach and identified the axon-guidance molecule SLIT3 as a clastokine that stimulated osteoblast migration and proliferation by activating β-catenin. SLIT3 also inhibited bone resorption by suppressing osteoclast differentiation in an autocrine manner. Mice deficient in Slit3 or its receptor, Robo1, exhibited osteopenic phenotypes due to a decrease in bone formation and increase in bone resorption. Mice lacking Slit3 specifically in osteoclasts had low bone mass, whereas mice with either neuron-specific Slit3 deletion or osteoblast-specific Slit3 deletion had normal bone mass, thereby indicating the importance of SLIT3 as a local determinant of bone metabolism. In postmenopausal women, higher circulating SLIT3 levels were associated with increased bone mass. Notably, injection of a truncated recombinant SLIT3 markedly rescued bone loss after an ovariectomy. Thus, these results indicate that SLIT3 plays an osteoprotective role by synchronously stimulating bone formation and inhibiting bone resorption, making it a potential therapeutic target for metabolic bone diseases.