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

A benzamide-linked small molecule NDMC101 inhibits NFATc1 and NF-κB activity: a potential osteoclastogenesis inhibitor for experimental arthritis

PURPOSE

Using receptor activator of NF-κB ligand (RANKL) induced osteoclast differentiation on RAW264.7 as a screening tool; we synthesize and identify small-molecule inhibitors preserving immunomodulatory effects as therapeutics for rheumatoid arthritis.

METHODS

Differentiation into osteoclast-like cells was examined by tartrate-resistant acid phosphatase (TRAP) staining and expression of osteoclast differentiation markers. Collagen-induced arthritis (CIA) mice were administered test articles by gavages to assess its efficacy. Then clinical, histological, and biochemical parameters were assessed to determine the effects of N-(4-chloro-2-fluorophenyl)-2-hydroxybenzamide (NDMC101) on synovial inflammation and bone erosion by hematoxlin and eosin staining and Enzyme-linked immunosorbent assay (ELISA).

RESULTS

NDMC101 markedly inhibited RANKL-induced formation of TRAP+ multinucleated cells in RAW264.7 and bone marrow macrophage cells (BMMs). Moreover, pit formation assay showed that NDMC101 significantly reduced the bone-resorbing activity of mature osteoclasts. In CIA mice, oral administration of NDMC101 reduced arthritic index and mitigated bone erosion. Serum TNF-α and IL-1β concentrations in these mice were decreased significantly at the higher dose of 62.5 mg/kg.

CONCLUSIONS

Screening of our chemical library, our findings suggest that NDMC101 inhibits osteoclastogenesis which also ameliorates paw swelling and inflammatory bone destruction. Its efficacy is associated with the inhibition of such transcription factors as NF-κB and NFATc1 as well as multiple protein kinases, including p38, ERK, and JNK. There results guarantee further clinical tests of NDMC101 for its therapeutic potential in the treatment of inflammation-induced bone diseases.

The transient appearance of zipper-like actin superstructures during the fusion of osteoclasts

Multinucleated osteoclasts are responsible for bone resorption. Hypermultinucleated osteoclasts are often observed in some bone-related diseases such as Paget's disease and cherubism. The cellular mechanics controlling the size of osteoclasts is poorly understood. We introduced EGFP–actin into RAW 264.7 cells to monitor actin dynamics during osteoclast differentiation. Before their terminal differentiation into osteoclasts, syncytia displayed two main types of actin assembly, podosome clusters and clusters of zipper-like structures. The zipper-like structures morphologically resembled the adhesion zippers found at the initial stage of cell–cell adhesion in keratinocytes. In the zipper-like structure, Arp3 and cortactin overlapped with the distribution of dense F-actin, whereas integrin β3, paxillin and vinculin were localized to the periphery of the structure. The structure was negative for WGA–lectin staining and biotin labeling. The zipper-like structure broke down and transformed into a large actin ring, called a podosome belt. Syncytia containing clusters of zipper-like structures had more nuclei than those with podosome clusters. Differentiated osteoclasts with a podosome belt also formed the zipper-like structure at the cell contact site during cell fusion. The breakdown of the cell contact site resulted in the fusion of the podosome belts following plasma membrane fusion. Additionally, osteoclasts in mouse calvariae formed the zipper-like structure in the sealing zone. Therefore, we propose that the zipper-like actin superstructures might be involved in cell–cell interaction to achieve efficient multinucleation of osteoclasts. Understanding of the zipper-like structure might lead to selective therapeutics for bone diseases caused by hypermultinucleated osteoclasts.

Phospholipase Cγ2 is required for basal but not oestrogen deficiency-induced bone resorption

BACKGROUND

Osteoclasts play a critical role in bone resorption under basal conditions, but they also contribute to pathological bone loss during diseases including postmenopausal osteoporosis. Phospholipase Cγ2 (PLCγ2) is an important signalling molecule in diverse haematopoietic lineages. Here, we tested the role of PLCγ2 in basal and ovariectomy-induced bone resorption, as well as in in vitro osteoclast cultures using PLCγ2-deficient (PLCγ2(-/-) ) mice.

MATERIALS AND METHODS

The trabecular architecture of long bone metaphyses was tested by micro-CT and histomorphometric analyses. Postmenopausal osteoporosis was modelled by surgical ovariectomy. Osteoclast development and function, gene expression and PLCγ2 phosphorylation were tested on in vitro osteoclast and macrophage cultures.

RESULTS

  PLCγ2(-/-) mice had significantly higher trabecular bone mass under basal conditions than wild-type mice. PLCγ2 was required for in vitro development and resorptive function of osteoclasts, but not for upregulation of osteoclast-specific gene expression. PLCγ2 was phosphorylated in a Src-family-dependent manner upon macrophage adhesion but not upon stimulation by M-CSF or RANKL. Surprisingly, ovariectomy-induced bone resorption in PLCγ2(-/-) mice was similar to, or even more robust than, that in wild-type animals.

CONCLUSIONS

Our results indicate that PLCγ2 participates in bone resorption under basal conditions, likely because of its role in adhesion receptor signalling during osteoclast development. In contrast, PLCγ2 does not appear to play a major role in ovariectomy-induced bone loss. These results suggest that basal and oestrogen deficiency-induced bone resorption utilizes different signalling pathways and that PLCγ2 may not be a suitable therapeutic target in postmenopausal osteoporosis.

Biomarkers to Diagnose Early Arthritis in Patients With Psoriasis

BACKGROUND

Psoriatic arthritis is a potentially destructive, inflammatory joint disease that affects 20% to 30% of patients with psoriasis. Psoriasis precedes the onset of joint inflammation by approximately 10 years, providing a unique opportunity to intervene and prevent or delay onset of musculoskeletal manifestations. The emergence of sensitive imaging modalities and cellular biomarkers may facilitate early identification of patients with psoriasis who have subclinical joint disease and might help stratify patients with an early onset of arthritis.

METHODS

The translational studies described herein are focused on the development of cellular biomarkers identified with flow cytometry and cell culture techniques in patients with psoriasis and psoriatic arthritis.

RESULTS AND CONCLUSION

The combination of power Doppler ultrasound imaging and cellular biomarkers (ie, CD16 and dendritic cell specific transmembrane protein) to diagnose early psoriatic arthritis and to stratify patients with established psoriatic arthritis provides a new opportunity to optimize treatment outcomes in this potentially disabling disease.

Regulation of human osteoclast development by dendritic cell-specific transmembrane protein (DC-STAMP)

Osteoclasts (OC) are bone-resorbing, multinucleated cells that are generated via fusion of OC precursors (OCP). The frequency of OCP is elevated in patients with erosive inflammatory arthritis and metabolic bone diseases. Although many cytokines and cell surface receptors are known to participate in osteoclastogenesis, the molecular mechanisms underlying the regulation of this cellular transformation are poorly understood. Herein, we focused our studies on the dendritic cell-specific transmembrane protein (DC-STAMP), a seven-pass transmembrane receptor-like protein known to be essential for cell-to-cell fusion during osteoclastogenesis. We identified an immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic tail of DC-STAMP, and developed an anti-DC-STAMP monoclonal antibody 1A2 that detected DC-STAMP expression on human tumor giant cells, blocked OC formation in vitro, and distinguished four patterns of human PBMC with a positive correlation to OC potential. In freshly isolated monocytes, DC-STAMP(high) cells produced a higher number of OC in culture than DC-STAMP(low) cells and the surface expression of DC-STAMP gradually declined during osteoclastogenesis. Importantly, we showed that DC-STAMP is phosphorylated on its tyrosine residues and physically interacts with SHP-1 and CD16, an SH2-domain-containing tyrosine phosphatase and an ITAM-associated protein, respectively. Taken together, these data show that DC-STAMP is a potential OCP biomarker in inflammatory arthritis. Moreover, in addition to its effect on cell fusion, DC-STAMP dynamically regulates cell signaling during osteoclastogenesis.

Pyruvate dehydrogenase kinase 4 induces bone loss at unloading by promoting osteoclastogenesis

Disuse osteoporosis, which occurs commonly in prolonged bed rest and immobilization, is becoming a major problem in modern societies; however, the molecular mechanisms underlying unloading-driven bone loss have not been fully elucidated. The osteocyte network is considered to be an ideal mechanosensor and mechanotransduction system. We searched for the molecules responsible for disuse osteoporosis using BCL2 transgenic mice, in which the osteocyte network was disrupted. Pyruvate dehydrogenase kinase 4 (Pdk4), which inactivates pyruvate dehydrogenase complex (PDC), was upregulated in femurs and tibiae of wild-type mice but not of BCL2 transgenic mice after tail suspension. Bone in Pdk4(-/-) mice developed normally and was maintained. At unloading, however, bone mass was reduced due to enhanced osteoclastogenesis and Rankl expression in wild-type mice but not in Pdk4(-/-) mice. Osteoclast differentiation of Pdk4(-/-) bone marrow-derived monocyte/macrophage lineage cells (BMMs) in the presence of M-CSF and RANKL was suppressed, and osteoclastogenesis was impaired in the coculture of wild-type BMMs and Pdk4(-/-) osteoblasts, in which Rankl expression and promoter activity were reduced. Further, introduction of Pdk4 into Pdk4(-/-) BMMs and osteoblasts enhanced osteoclastogenesis and Rankl expression and activated Rankl promoter. These findings indicate that Pdk4 plays an important role in bone loss at unloading by promoting osteoclastogenesis.

Orai1-mediated calcium entry plays a critical role in osteoclast differentiation and function by regulating activation of the transcription factor NFATc1

Bone diseases such as postmenopausal osteoporosis are primarily caused by excessive formation and activity of osteoclasts (OCLs). Receptor activator of nuclear factor-κB ligand (RANKL) is a key initiating cytokine for OCL differentiation and function. RANKL induces calcium (Ca(2+)) oscillations, resulting in selective and robust induction of nuclear factor of activated T cells c1 (NFATc1), a Ca(2+)-responsive transcription factor that drives osteoclastogenesis. Store-operated Ca(2+) entry (SOCE) is a major Ca(2+) influx pathway in most nonexcitable cell types and is activated by any stimulus that depletes Ca(2+) stores in the endoplasmic reticulum. Although the role of Orai1, a SOCE channel in the plasma membrane, in maintaining Ca(2+) oscillations and transactivation of NFAT in other cell types is well known, its contribution to osteoclastogenesis remains unclear. We show here that silencing of the Orai1 gene with viral delivery of shRNA reduces SOCE and inhibits RANKL-induced osteoclastogenesis of RAW264.7 cells, a murine monocyte/macrophage cell line, by suppressing the induction of NFATc1. This was accompanied by defective induction of OCL-specific genes, such as tartrate-resistant acid phosphatase and immunoreceptor OCL-associated receptor, which are known to be direct transcriptional targets of NFATc1 during osteoclastogenesis. In addition, maturation of OCLs was abrogated by defective cell fusion of pre-OCLs depleted of Orai1, consistent with defective RANKL-mediated induction of d2 isoform of vacuolar ATPase V(o) domain that is involved in cell fusion of pre-OCLs. We found that the functional bone resorbing capacity was severely impaired in OCLs depleted of Orai1, potentially related to the observed decrease in the induction of cathepsin K, a major bone matrix degrading protease. Our results indicate that Orai1 plays a critical role in the differentiation and function of OCLs, suggesting that Orai1 might be a potential therapeutic target for the treatment or prevention of bone loss caused by OCLs.

Regulation and function of the E-cadherin/catenin complex in cells of the monocyte-macrophage lineage and DCs

E-cadherin is best characterized as adherens junction protein, which through homotypic interactions contributes to the maintenance of the epithelial barrier function. In epithelial cells, the cytoplasmic tail of E-cadherin forms a dynamic complex with catenins and regulates several intracellular signal transduction pathways, including Wnt/β-catenin, PI3K/Akt, Rho GTPase, and NF-κB signaling. Recent progress uncovered a novel and critical role for this adhesion molecule in mononuclear phagocyte functions. E-cadherin regulates the maturation and migration of Langerhans cells, and its ligation prevents the induction of a tolerogenic state in bone marrow-derived dendritic cells (DCs). In this respect, the functionality of β-catenin could be instrumental in determining the balance between immunogenicity and tolerogenicity of DCs in vitro and in vivo. Fusion of alternatively activated macrophages and osteoclasts is also E-cadherin-dependent. In addition, the E-cadherin ligands CD103 and KLRG1 are expressed on DC-, T-, and NK-cell subsets and contribute to their interaction with E-cadherin-expressing DCs and macrophages. Here we discuss the regulation, function, and implications of E-cadherin expression in these central orchestrators of the immune system.

Platinum nanoparticles suppress osteoclastogenesis through scavenging of reactive oxygen species produced in RAW264.7 cells

Recent research has shown that platinum nanoparticles (nano-Pt) efficiently quench reactive oxygen species (ROS) as a reducing catalyst. ROS have been suggested to regulate receptor activator of NF-κB ligand (RANKL)-stimulated osteoclast differentiation. In the present study, we examined the direct effects of platinum nano-Pt on RANKL-induced osteoclast differentiation of murine pre-osteoclastic RAW 264.7 cells. The effect of the nano-Pt on the number of osteoclasts was measured and their effect on the mRNA expression for osteoclast differentiation was assayed using real-time PCR. Nano-Pt appeared to have a ROS-scavenging activity. Nano-Pt decreased the number of osteoclasts (2+ nuclei) and large osteoclasts (8+ nuclei) in a dose-dependent manner without affecting cell viability. In addition, this agent significantly blocked RANKL-induced mRNA expression of osteoclastic differentiation genes such as c-fms, NFATc1, NFATc2, and DC-STAMP as well as that of osteoclast-specific marker genes including MMP-9, Cath-K, CLC7, ATP6i, CTR, and TRAP. Although nano-Pt attenuated expression of the ROS-producing NOX-family oxidases, Nox1 and Nox4, they up-regulated expression of Nox2, the major Nox enzyme in macrophages. These findings suggest that the nano-Pt inhibit RANKL-stimulated osteoclast differentiation via their ROS scavenging property. The use of nano-Pt as scavengers of ROS that is generated by RANKL may be a novel and innovative therapy for bone diseases.

The influence of interleukin-32γ on osteoclastogenesis with a focus on fusion-related genes

We previously reported that interleukin-32 gamma (IL-32γ) has a direct effect on osteoclast differentiation and activation in vitro in the context of receptor activator of NF-κB ligand (RANKL) co-stimulation. However, the stage of osteoclast differentiation at which IL-32γ exerts its effect was not determined. Here, we demonstrated that IL-32γ plays an important role in the fusion of preosteoclasts to yield multinuclear osteoclasts, particularly large osteoclasts. The synergistic effect of IL-32γ on RANKL-induced formation of multinuclear osteoclasts was readily apparent when cells were treated with IL-32γ at the fusion stage. In addition, we demonstrated that IL-32γ induced the expression of dendritic cell-specific transmembrane protein (DC-STAMP) and nuclear factor of activated T cells cytoplasmic 1 (NFATc1), and NFATc1 inactivation by cyclosporine treatment attenuated the effect of IL-32γ. These results indicate that IL-32γ is a potential mediator of osteoclast fusion, likely through up-regulation of NFATc1 and DC-STAMP.

Versatile roles of V-ATPases accessory subunit Ac45 in osteoclast formation and function

Vacuolar-type H(+)-ATPases (V-ATPases) are macromolecular proton pumps that acidify intracellular cargos and deliver protons across the plasma membrane of a variety of specialized cells, including bone-resorbing osteoclasts. Extracellular acidification is crucial for osteoclastic bone resorption, a process that initiates the dissolution of mineralized bone matrix. While the importance of V-ATPases in osteoclastic resorptive function is well-defined, whether V-ATPases facilitate additional aspects of osteoclast function and/or formation remains largely obscure. Here we report that the V-ATPase accessory subunit Ac45 participates in both osteoclast formation and function. Using a siRNA-based approach, we show that targeted suppression of Ac45 impairs intracellular acidification and endocytosis, both are prerequisite for osteoclastic bone resorptive function in vitro. Interestingly, we find that knockdown of Ac45 also attenuates osteoclastogenesis owing to a reduced fusion capacity of osteoclastic precursor cells. Finally, in an effort to gain more detailed insights into the functional role of Ac45 in osteoclasts, we attempted to generate osteoclast-specific Ac45 conditional knockout mice using a Cathepsin K-Cre-LoxP system. Surprisingly, however, insertion of the neomycin cassette in the Ac45-Flox(Neo) mice resulted in marked disturbances in CNS development and ensuing embryonic lethality thus precluding functional assessment of Ac45 in osteoclasts and peripheral bone tissues. Based on these unexpected findings we propose that, in addition to its canonical function in V-ATPase-mediated acidification, Ac45 plays versatile roles during osteoclast formation and function.

Sperm-egg interaction

A crucial step of fertilization is the sperm-egg interaction that allows the two gametes to fuse and create the zygote. In the mouse, CD9 on the egg and IZUMO1 on the sperm stand out as critical players, as Cd9(-/-) and Izumo1(-/-) mice are healthy but infertile or severely subfertile due to defective sperm-egg interaction. Moreover, work on several nonmammalian organisms has identified some of the most intriguing candidates implicated in sperm-egg interaction. Understanding of gamete membrane interactions is advancing through characterization of in vivo and in vitro fertilization phenotypes, including insights from less robust phenotypes that highlight potential supporting (albeit not absolutely essential) players. An emerging theme is that there are varied roles for gamete molecules that participate in sperm-egg interactions. Such roles include not only functioning as fusogens, or as adhesion molecules for the opposite gamete, but also functioning through interactions in cis with other proteins to regulate membrane order and functionality.

The CD40-CD40L axis and IFN-γ play critical roles in Langhans giant cell formation

The presence of Langhans giant cells (LGCs) is one of the signatures of systemic granulomatous disorders such as tuberculosis and sarcoidosis. However, the pathophysiological mechanism leading to LGC formation, especially the contribution of the T cells abundantly found in granulomas, has not been fully elucidated. To examine the role of T cells in LGC formation, a new in vitro method for the induction of LGCs was developed by co-culturing human monocytes with autologous T cells in the presence of concanavalin A (ConA). This system required close contact between monocytes and T cells, and CD4+ T cells were more potent than CD8+ T cells in inducing LGC formation. Antibody inhibition revealed that a CD40-CD40 ligand (CD40L) interaction and IFN-γ were essential for LGC formation, and the combination of exogenous soluble CD40L (sCD40L) and IFN-γ efficiently replaced the role of T cells. Dendritic cell-specific transmembrane protein (DC-STAMP), a known fusion-related molecule in monocytes, was up-regulated during LGC formation. Moreover, knock-down of DC-STAMP by siRNA inhibited LGC formation, revealing that DC-STAMP was directly involved in LGC formation. Taken together, these results demonstrate that T cells played a pivotal role in a new in vitro LGC formation system, in which DC-STAMP was involved, and occurred via a molecular mechanism that involved CD40-CD40L interaction and IFN-γ secretion.

Tal1 regulates osteoclast differentiation through suppression of the master regulator of cell fusion DC-STAMP

The balance between bone-forming osteoblasts and bone-resorbing osteoclasts is crucial to bone homeostasis, an equilibrium that is disturbed in many bone diseases. The transcription factor Tal1 is involved in the establishment of hematopoietic stem cells in the embryo and is a master regulator of hematopoietic gene expression in the adult. Here, we show that Tal1 is expressed in osteoclasts and that loss of Tal1 in osteoclast progenitors leads to altered expression of >1200 genes. We found that DC-STAMP, a key regulator of osteoclast cell fusion, is a direct target gene of Tal1 and show that Tal1 represses DC-STAMP expression by counteracting the activating function of the transcription factors PU.1 and MITF. The identification of Tal1 as a factor involved in cell fusion contributes to the understanding of osteoclast-associated diseases, including osteoporosis.

DC-STAMP knock-down deregulates cytokine production and T-cell stimulatory capacity of LPS-matured dendritic cells

BACKGROUND

Dendritic cells (DCs) are the highly specialized antigen presenting cells of the immune system that play a key role in regulating immune responses. DCs can efficiently initiate immune responses or induce tolerance. Due to this dual function, DCs are studied in the context of immunotherapy for both cancer and autoimmune diseases. Characterization of DC-specific genes, leading to better understanding of DC immunobiology, will help to guide their use in clinical settings. We previously identified DC-STAMP, a multi-membrane spanning protein preferentially expressed by DCs. DC-STAMP resides in the endoplasmic reticulum (ER) of immature DCs and translocates towards the Golgi compartment upon maturation. In this study we knocked down DC-STAMP in mouse bone marrow-derived DCs (mBMDCs) to determine its function.

RESULTS

We demonstrate that DC-STAMP knock-down mBMDCs secrete less IL-6, IL-12, TNF-α and IL-10 while IL-1 production is enhanced. Moreover, LPS-matured DC-STAMP knock-down mBMDCs show impaired T cell activation potential and induction of Th1 responses in an alloreaction.

CONCLUSIONS

We show that DC-STAMP plays an important role in cytokine production by mBMDCs following LPS exposure. Our results reveal a novel function of DC-STAMP in regulating DC-initiated immune responses.

IL-1β and TNFα-initiated IL-6-STAT3 pathway is critical in mediating inflammatory cytokines and RANKL expression in inflammatory arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease that causes irreversible joint damage and significant disability. However, the fundamental mechanisms underlying how inflammation and joint destruction in RA develop and are sustained chronically remain largely unknown. Here, we show that signal transducer and activator of transcription 3 (STAT3) is the key mediator of both chronic inflammation and joint destruction in RA. We found that inflammatory cytokines highly expressed in RA patients, such as IL-1β, tumor necrosis factor alpha and IL-6, activated STAT3 either directly or indirectly and in turn induced expression of IL-6 family cytokines, further activating STAT3 in murine osteoblastic and fibroblastic cells. STAT3 activation also induced expression of receptor activator of nuclear factor kappa B ligand (RANKL), a cytokine essential for osteoclastogenesis, and STAT3 deficiency or pharmacological inhibition promoted significant reduction in expression of both IL-6 family cytokines and RANKL in vitro. STAT3 inhibition was also effective in treating an RA model, collagen-induced arthritis, in vivo through significant reduction in expression of IL-6 family cytokines and RANKL, inhibiting both inflammation and joint destruction. Leukemia inhibitory factor expression and STAT3 activation by IL-1β were mainly promoted by IL-6 but still induced in IL-6-deficient cells. Thus, our data provide new insight into RA pathogenesis and provide evidence that inflammatory cytokines trigger a cytokine amplification loop via IL-6-STAT3 that promotes sustained inflammation and joint destruction.

Comparative promoter analysis in vivo: identification of a dendritic cell-specific promoter module

Dendritic cells (DCs) are important immune cells. This study focused on transcriptional networks active in murine DCs, but DCs are difficult to study using conventional molecular techniques. Therefore, comparative promoter analysis was used to identify evolutionarily conserved features between the murine CD11c and DC-STAMP promoters. A promoter framework consisting of 4 transcription factor binding sites was identified that included signal transducer and activator of transcription, homeodomain transcription factors, and 2 members of the Brn POU domain factors family. This promoter module was functionally verified by in vivo promoter analysis and site-directed mutagenesis. Hematopoietic stem cells were engineered by lentiviral vectors and expression of green fluorescent protein reporter was monitored in primary hematopoietic cell types that develop without further manipulation in irradiated recipient mice. The verified promoter module was then modeled and used in a bioinformatics-based search for other potential coregulated genes in murine DCs. A promoter database search identified 2 additional genes, Ppef2 and Pftk1, which have a similar promoter organization and are preferentially expressed in murine DCs. The results define a regulatory network linked to development of murine DCs.

Plectranthus amboinicus attenuates inflammatory bone erosion in mice with collagen-induced arthritis by downregulation of RANKL-induced NFATc1 expression

OBJECTIVE

Plectranthus amboinicus has been known to treat inflammatory diseases or swelling symptoms. We investigated whether P. amboinicus exhibited an inhibitory effect on osteoclastogenesis in vitro and inflammatory bone erosion in collagen-induced arthritis (CIA) mice, an animal model of rheumatoid arthritis. We attempted to identify the active component of P. amboinicus involved in regulation of osteoclastogenesis.

METHODS

We treated M-CSF- and RANKL-stimulated murine bone marrow-derived macrophages (BMM) and RANKL-induced RAW264.7 cells with different concentrations of P. amboinicus or rosmarinic acid, a phytopolyphenol purified from P. amboinicus, to monitor osteoclast formation by TRAP staining. The mechanism of the inhibition was studied by biochemical analysis such as RT-PCR and immunoblotting. CIA mice were administered gavages of P. amboinicus (375 mg/kg) or placebo. Then clinical, histological, and biochemical measures were assessed to determine the effects of P. amboinicus on synovial inflammation and bone erosion by H&E staining of the inflamed joints and ELISA.

RESULTS

Rosmarinic acid strongly inhibited RANKL-induced NF-κB activation and nuclear factor of activated T cells c1 (NFATc1) nuclear translocation in BMM, and also inhibited RANKL-induced formation of TRAP-positive multinucleated cells. A pit formation assay and the CIA animal model showed that P. amboinicus significantly inhibited the bone-resorbing activity of mature osteoclasts.

CONCLUSION

We postulated that rosmarinic acid conferred the inhibitory activity on P. amboinicus for inhibition of osteoclastogenesis via downregulation of RANKL-induced NFATc1 expression. Our results indicated the possibility of P. amboinicus as a new remedy against inflammatory bone destruction.

Inhibitory effect of chlorpromazine on RANKL-induced osteoclastogenesis in mouse bone marrow cells

Chlorpromazine (CPZ), the first widely used phenothiazine tranquilizer, is shown to inhibit the action of intracellular calmodulin (CaM) and bone resorption in vivo and in vitro. In this study, CPZ (0.63 - 10 µM) dose-dependently inhibited the formation of tartrate-resistant acid phosphatase (TRAP) staining-positive osteoclast-like cells in mouse bone marrow cells (BMCs) treated with 1α,25(OH)(2)D(3) (10 nM) or soluble receptor activator of nuclear factor-κB ligand (s-RANKL) (20 ng/ml). Expressions of mRNA for the nuclear factor of activated T-cells c1 (NFATc1), a key regulator of osteoclast differentiation; dendritic cell-specific transmembrane protein (DC-STAMP), an essential protein for cell-cell fusion; and characteristic markers of osteoclasts such as TRAP, cathepsin K, carbonic anhydrase II, and calcitonin receptor in BMCs were up-regulated by s-RANKL and decreased by the addition of CPZ (5 µM) or the selective CaM antagonist W7, but not the inactive analog W5. The general CaM kinase (CaMK) inhibitor KN-93 and CaM-dependent phosphatase calcineurin inhibitor FK-506 also inhibited s-RANKL-induced osteoclastogenesis. Phenothiazines such as CPZ, trifluoperazine (TFPZ), and promethazine (PMZ) inhibited s-RANKL-induced osteoclast-like cell formation in mouse BMCs. Osteoclastogenesis inhibitory effects decreased in the order of TFPZ, CPZ, PMZ, depending on their anti-CaM potency. These findings suggest that CPZ inhibits RANKL-induced osteoclastogenesis by its anti-CaM action.

Lack of TNF-α-induced MMP-9 production and abnormal E-cadherin redistribution associated with compromised fusion in MCP-1-null macrophages

Homotypic cell fusion occurs in several cell types including macrophages in the formation of foreign body giant cells. Previously, monocyte chemoattractant protein-1 (MCP-1) was demonstrated to be required for foreign body giant cell formation in the foreign body response. The present study investigated the fusion defect in MCP-1-null macrophages by implanting biomaterials intraperitoneally in wild-type and MCP-1-null mice and monitoring the macrophage response at 12 hours to 4 weeks. MCP-1-null mice exhibited reduced accumulation and fusion of macrophages on implants, which was associated with attenuation of the foreign body response. Consistent with previous in vitro findings, the level of matrix metalloproteinase-9 (MMP-9) was reduced in MCP-1-null macrophages adherent to implants. In contrast, CCR2 expression was unaffected. In vitro studies revealed reduced tumor necrosis factor-α (TNF-α) production and abnormal subcellular redistribution of E-cadherin and β-catenin during fusion in MCP-1-null macrophages. Exogenous TNF-α caused an increase in the production of MMP-9 and rescued the fusion defect. Addition of GM6001 (MMP inhibitor) or NSC23766 (Rac1 inhibitor) indicated two distinct induction pathways, one for E-cadherin/β-catenin and one for MCP-1, TNF-α, and MMP-9. Considered together, these observations demonstrate that induction of E-cadherin/β-catenin is not sufficient for fusion in the absence of MCP-1 or the downstream mediators TNF-α and MMP-9. Moreover, attenuation of the foreign body response in intraperitoneal implants in MCP-1-null mice demonstrates that the process depends on tissue-specific factors.

The P2X7 receptor and Pannexin-1 are both required for the promotion of multinucleated macrophages by the inflammatory cytokine GM-CSF

The P2X(7) receptor (P2X(7)R), an ATP-gated ion channel, has been implicated in the process of cell-to-cell fusion into multinucleated macrophages (MA), but its contribution to MA fusion driven by physiological/pathological stimuli is not clearly established. Based on several lines of evidence, we demonstrate that P2X(7)R is critical for the induction of multinucleated MA by the inflammatory cytokine GM-CSF: 1) pharmacological inhibition of P2X(7)R with oxidized ATP (oATP), KN-62, and the selective antagonist A740003 abrogated GM-CSF action on rat alveolar MA and murine peritoneal MA; 2) a murine J774 P2X(7) low MA clone, selected for defective P2X(7)R function, was unresponsive; 3) MA from mice lacking P2X(7)R failed to respond to GM-CSF, in contrast to wild-type. GM-CSF also stimulated ATP-induced membrane permeabilization in J774 P2X(7) high MA and rat alveolar MA, an effect absent in the P2X(7) low MA clone and inhibited by the P2X(7) blockers oATP and KN-62. Notably, the stimulatory effects of GM-CSF on pore formation and MA fusion were both inhibited by blocking functional Pannexin-1 (Panx-1), and GM-CSF failed to stimulate MA fusion in cells from Panx-1 knockout mice. We provide further evidence that extracellular ATP release from peritoneal MA is dependent on P2X(7) but not on Panx-1 expression and that its metabolism to adenosine mediates P2X(7)-dependent MA fusion. These data demonstrate that both P2X(7) and Panx-1 are required for GM-CSF promotion of MA fusion but likely act independently through different signaling pathway(s).

NF-κB signaling participates in both RANKL- and IL-4-induced macrophage fusion: receptor cross-talk leads to alterations in NF-κB pathways

NF-κB activation is essential for receptor activator for NF-κB ligand (RANKL)-induced osteoclast formation. IL-4 is known to inhibit the RANKL-induced osteoclast differentiation while at the same time promoting macrophage fusion to form multinucleated giant cells (MNG). Several groups have proposed that IL-4 inhibition of osteoclastogenesis is mediated by suppressing the RANKL-induced activation of NF-κB. However, we found that IL-4 did not block proximal, canonical NF-κB signaling. Instead, we found that IL-4 inhibited alternative NF-κB signaling and induced p105/50 expression. Interestingly, in nfκb1(-/-) bone marrow-derived macrophages (BMM), the formation of both multinucleated osteoclast and MNG induced by RANKL or IL-4, respectively, was impaired. This suggests that NF-κB signaling also plays an important role in IL-4-induced macrophage fusion. Indeed, we found that the RANKL-induced and IL-4-induced macrophage fusion were both inhibited by the NF-κB inhibitors IκB kinase 2 inhibitor and NF-κB essential modulator inhibitory peptide. Furthermore, overexpression of p50, p65, p52, and RelB individually in nfκb1(-/-) or nfκb1(+/+) BMM enhanced both giant osteoclast and MNG formation. Interestingly, knockdown of nfκb2 in wild-type BMM dramatically enhanced both osteoclast and MNG formation. In addition, both RANKL- and IL-4-induced macrophage fusion were impaired in NF-κB-inducing kinase(-/-) BMM. These results suggest IL-4 influences NF-κB pathways by increasing p105/p50 and suppressing RANKL-induced p52 translocation and that NF-κB pathways participate in both RANKL- and IL-4-induced giant cell formation.

The small molecule harmine regulates NFATc1 and Id2 expression in osteoclast progenitor cells

Small molecule compounds that potently affect osteoclastogenesis could be useful as chemical probes for elucidating the mechanisms of various biological phenomena and as effective therapeutic strategies against bone resorption. An osteoclast progenitor cell-based high-throughput screening system was designed to target activation of NFAT, which is a key event for osteoclastogenesis. Orphan ligand library screening using this system identified the β-carboline derivative harmine, which is a highly potent inhibitor of dual-specificity tyrosine-phosphorylation regulated kinase 1A (DYRK1A), to be an NFAT regulator in osteoclasts. RAW264.7 cells highly expressed DYRK1A protein, and in vitro phosphorylation assay demonstrated that harmine directly inhibited the DYRK1A-mediated phosphorylation (in-activation) of NFATc1. Harmine promoted the dephosphorylation (activation) of NFATc1 in RAW264.7 cells within 24h, and it significantly increased the expression of NFATc1 in RAW264.7 cells and mouse primary bone marrow macrophages (BMMs) both in the presence and absence of RANKL stimulation. Although harmine promoted NFATc1 expression and stimulated target genes for osteoclastogenesis, cell-cell fusion and the formation of TRAP-positive multinucleated osteoclasts from RAW264.7 cells and BMMs was significantly inhibited by harmine treatment. Meanwhile, harmine remarkably promoted the expression of inhibitor of DNA binding/differentiation-2 (Id2), which is a negative regulator for osteoclastogenesis, in RAW264.7 cells and BMMs. An Id2-null-mutant showed slightly increased osteoclast formation from BMMs, and the harmine-mediated inhibition of osteoclast formation was abolished in the BMMs of Id2-null-mutant mice. These results suggest that harmine is a potent activator of NFATc1 that interferes with the function of DYRK1A in osteoclast precursors and also up-regulates Id2 protein, which may dominantly inhibit expression pathways associated with cell-cell fusion, thereby leading to the disruption of the fusion events mediating osteoclastogenesis. The small molecule harmine is therefore expected to provide an experimental tool for investigating signaling cascades in osteoclastogenesis, especially those centered on DYRK1A-mediated NFATc1 and Id2 regulation.

Molecular and cellular mechanisms of mammalian cell fusion

The fusion of one cell with another occurs in development, injury and disease. Despite the diversity of fusion events, five steps in sequence appear common. These steps include programming fusion-competent status, chemotaxis, membrane adhesion, membrane fusion, and post-fusion resetting. Recent advances in the field start to reveal the molecules involved in each step. This review focuses on some key molecules and cellular events of cell fusion in mammals. Increasing evidence demonstrates that membrane lipid rafts, adhesion proteins and actin rearrangement are critical in the final step of membrane fusion. Here we propose a new model for the formation and expansion of membrane fusion pores based on recent observations on myotube formation. In this model, membrane lipid rafts first recruit adhesion molecules and align with opposing membranes, with the help of a cortical actin "wall" as a rigid supportive platform. Second, the membrane adhesion proteins interact with each other and trigger actin rearrangement, which leads to rapid dispersion of lipid rafts and flow of a highly fluidic phospholipid bilayer into the site. Finally, the opposing phospholipid bilayers are then pushed into direct contact leading to the formation of fusion pores by the force generated through actin polymerization. The actin polymerization generated force also drives the expansion of the fusion pores. However, several key questions about the process of cell fusion still remain to be explored. The understanding of the mechanisms of cell fusion may provide new opportunities in correcting development disorders or regenerating damaged tissues by inhibiting or promoting molecular events associated with fusion.

Activation of liver X receptor (LXR) inhibits receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation in an LXRβ-dependent mechanism

Bone destruction is the major pathological process in many bone metabolic diseases and is a result of increased osteoclast formation and bone resorption. The liver X receptors (α,β), important regulators of cholesterol metabolism and inflammatory signaling, have recently been observed to play a role in both physiological and pathological bone turnover. However, the relationship between liver X receptors (LXR) and osteoclast differentiation/formation remains unknown. Here, we report that the LXR ligand GW3965 is able to clearly and potently inhibit the formation of mature osteoclasts from receptor activator of nuclear factor κB ligand (RANKL)-stimulated human and murine osteoclast precursors. This results in a significant inhibition of bone resorption. We observed that GW3965 significantly inhibited expression of the osteoclast markers tartrate-resistant acid phosphatase, cathepsin K, osteoclast-associated receptor (OSCAR), and calcitonin receptor, appearing to act in an NFATc1/p38/microphthalmia-associated transcription factor (MITF)-dependent mechanism, independently of receptor activator of nuclear factor κB or c-Fos and not directly involving the NFκB pathways. GW3965 was less effective in RAW264.7 monocyte/macrophage cells, which are more committed into the osteoclast lineage. Also, GW3965 seemed to act differently depending on the source of the progenitor cells as it had no effect on calvarial osteoclasts, compared with marrow or blood-derived monocytes. As these effects were abolished in osteoclast precursors derived from LXRβ(-/-) mice, we suggest that GW3965 acts via an LXRβ-dependent mechanism. Taken together, our results suggest that the LXR can act as an important inhibitor of RANKL-mediated osteoclast differentiation.

Frequent upregulation of cyclin D1 and p16 expression with low Ki-67 scores in multinucleated giant cells

BACKGROUND/AIMS

Multinucleated giant cells are formed from the fusion of macrophages and are classified into foreign body-type giant cells (FBGCs), osteoclast-type giant cells (OCGCs) and Langhans-type giant cells (LHGCs). OCGCs display upregulated cyclin D1 expression with low Ki-67 activity. However, little is known about the expression of cell cycle regulators in the other types of multinucleated giant cells. We aimed to investigate the cell cycle status of multinucleated giant cells.

METHODS

The immunohistochemical expressions of cyclin D1, p16(INK4a) and Ki-67 were analyzed in a total of 127 cases showing multinucleated giant cells.

RESULTS

Cyclin D1 was overexpressed in 45 (88%) of 51 FBGC cases, 25 (86%) of 29 OCGC cases and 22 (47%) of 47 LHGC cases. p16(INK4a) showed diffuse nuclear and/or cytoplasmic overexpression in 45 (88%) of 51 FBGC cases, 27 (93%) of 29 OCGC cases and 24 (51%) of 47 LHGC cases. Ki-67 immunostaining was negative in almost all FBGC, OCGC and LHGC cases.

CONCLUSION

This study demonstrates that FBGCs and OCGCs frequently show upregulation of cyclin D1 and p16(INK4a) expression with low Ki-67 scores. This suggests that multinucleated giant cells are arrested in the G1/S cell cycle transition.

3BP2-deficient mice are osteoporotic with impaired osteoblast and osteoclast functions

A fine balance between bone resorption by osteoclasts and bone formation by osteoblasts maintains bone homeostasis. In patients with cherubism, gain-of-function mutations in 3BP2, which is encoded by SH3-domain binding protein 2 (SH3BP2), cause cystic lesions with activated osteoclasts that lead to craniofacial abnormalities. However, little is known about the function of wild-type 3BP2 in regulating bone homeostasis. Here we have shown that 3BP2 is required for the normal function of both osteoblasts and osteoclasts. Initial analysis showed that Sh3bp2-/-mice developed osteoporosis as a result of reduced bone formation despite the fact that bone resorption was impaired. We demonstrated using reciprocal bone marrow chimeras, a cell-intrinsic defect of the osteoblast and osteoclast compartments in vivo. Further, Sh3bp2-/- osteoblasts failed to mature and form mineralized nodules in vitro, while Sh3bp2-/- osteoclasts spread poorly and were unable to effectively degrade dentine matrix in vitro. Finally, we showed that 3BP2 was required for Abl activation in osteoblasts and Src activation in osteoclasts, and demonstrated that the in vitro defect of each cell type was restored by the respective expression of activated forms of these kinases. These findings reveal an unanticipated role for the 3BP2 adapter protein in osteoblast function and in coordinating bone homeostatic signals in both osteoclast and osteoblast lineages.

Role of RANKL-RANK/osteoprotegerin molecular complex in bone remodeling and its immunopathologic implications

Bone remodeling is a cyclic and continuous physiological process, which ensures the conservation and renewal of the bone matrix. Osteosynthesis of the bone matrix is achieved by osteoblasts and coordinated within this complex machinery of bone remodeling with resorption of extracellular bone matrix performed by osteoclasts. The mismatch between the activities of osteoblasts and osteoclasts has immunopathologic implications associated with either a decrease or increase of bone mass mineral density. The balance of the trimolecular control factor complex composed of osteoprotegerin (OPG), RANKL (osteoprotegerin ligand) and RANK maintains physiologic bone remodeling. This trimolecular complex functions as receptors and ligands and belongs to the superfamily of tumor necrosis factor (TNF). This mini review highlights the complex interplay of the RANKL-RANK/OPG axis and their immunopathologic implications in clinical medicine.

Immune responses to implants - a review of the implications for the design of immunomodulatory biomaterials

A key for long-term survival and function of biomaterials is that they do not elicit a detrimental immune response. As biomaterials can have profound impacts on the host immune response the concept emerged to design biomaterials that are able to trigger desired immunological outcomes and thus support the healing process. However, engineering such biomaterials requires an in-depth understanding of the host inflammatory and wound healing response to implanted materials. One focus of this review is to outline the up-to-date knowledge on immune responses to biomaterials. Understanding the complex interactions of host response and material implants reveals the need for and also the potential of "immunomodulating" biomaterials. Based on this knowledge, we discuss strategies of triggering appropriate immune responses by functional biomaterials and highlight recent approaches of biomaterials that mimic the physiological extracellular matrix and modify cellular immune responses.

L-type Ca(2+) channel agonist inhibits RANKL-induced osteoclast formation via NFATc1 down-regulation

AIMS

BayK 8644 is an L-type Ca(2+) channel agonist that enhances Ca(2+) influx and elevates cytosolic Ca(2+). As intracellular calcium plays a key role in osteoclast formation, we investigated the effects of BayK 8644 in cultures of bone marrow-derived precursor cells with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappaB ligand (RANKL).

MAIN METHODS

We performed an osteoclast formation assay, a pit formation assay, real-time PCR, and Western blot analysis.

KEY FINDINGS

BayK 8644 concentration-dependently suppressed osteoclastogenesis, as well as the expression of osteoclastic marker genes. It also decreased osteoclastic bone resorption on a dentine slice. While the RANKL-stimulated induction of IL-1β and IL-6 was not affected, TNF-α induction was reduced by BayK 8644 treatment. In addition, BayK 8644 blocked IκB degradation and the induction of nuclear factor of activated T cells c1 (NFATc1), the master regulator of osteoclast differentiation, following RANKL stimulation. Finally, forced expression of NFATc1 reversed the inhibitory effect of BayK 8644 on osteoclastogenesis, suggesting that NFATc1 is a downstream target for the anti-osteoclastogenic action of BayK 8644. Taken together, our data suggest that BayK 8644 has an anti-osteoclastogenic effect by inhibiting RANKL-induced activation of NF-κB pathways, thereby suppressing the gene expression of NFATc1 in osteoclast precursors.

SIGNIFICANCE

Our results provide a molecular understanding of the inhibitory effect of the L-type Ca(2+) channel agonist, BayK 8644, on osteoclastogenesis.

Genome-wide association identifies three new susceptibility loci for Paget's disease of bone

Paget's disease of bone (PDB) is a common disorder characterized by focal abnormalities of bone remodeling. We previously identified variants at the CSF1, OPTN and TNFRSF11A loci as risk factors for PDB by genome-wide association study. Here we extended this study, identified three new loci and confirmed their association with PDB in 2,215 affected individuals (cases) and 4,370 controls from seven independent populations. The new associations were with rs5742915 within PML on 15q24 (odds ratio (OR) = 1.34, P = 1.6 × 10(-14)), rs10498635 within RIN3 on 14q32 (OR = 1.44, P = 2.55 × 10(-11)) and rs4294134 within NUP205 on 7q33 (OR = 1.45, P = 8.45 × 10(-10)). Our data also confirmed the association of TM7SF4 (rs2458413, OR = 1.40, P = 7.38 × 10(-17)) with PDB. These seven loci explained ∼13% of the familial risk of PDB. These studies provide new insights into the genetic architecture and pathophysiology of PDB.

Acidosis environment promotes osteoclast formation by acting on the last phase of preosteoclast differentiation: a study to elucidate the action points of acidosis and search for putative target molecules

Acidosis promoted tartaric acid-resistant acid phosphatase-positive multinuclear cell (TRAP+MNC) or osteoclast formation. Large osteoclast or TRAP+LMNC formation was observed far more in an acidosis environment than in a physiologically neutral environment. One of the major action points of acidosis was determined to be located in the last phase of preosteoclast differentiation using a co-culture system and a soluble RANKL-dependent bone marrow cell culture system. On-going osteoclast formation in an acidosis environment markedly deteriorated when the medium was replaced with physiologically neutral medium within the first 6h; however, bone marrow cells previously stimulated in an acidosis environment for 9h differentiated into TRAP+LMNC in pH 7.4 medium. Messenger RNA (mRNA) expression levels of DC-STAMP, a key molecule in cell fusion, and NFATc1 did not increase in the acidosis environment compared with those under physiologically neutral conditions. Ruthenium red, a general TRP antagonist, deteriorated acidosis-promoted TRAP+LMNC formation. 4-Alpha-PDD, a TRPV4-specific agonist, added in the last 21 h of preosteoclast differentiation, potentiated TRAP+LMNC formation in a mild acidosis environment, showing synergism between TRPV4 activation and acidosis. RN1734, a TRPV4-specific antagonist, partly inhibited acidosis-promoted TRAP+LMNC formation. We thus narrowed down the major action points of acidosis in osteoclast formation and elucidated the characteristics of this system in detail. Our results show that acidosis effectively uses TRPV4 to drive large-scale cell fusion and also utilizes systems independently of TRPV4.

RANKL-independent human osteoclast formation with APRIL, BAFF, NGF, IGF I and IGF II

Non-canonical pathways of osteoclastogenesis have been described in which several cytokines are able to substitute for RANKL. These cytokines are few in number and their role(s) in pathological bone resorption has not been ascertained. We have identified five additional cytokines, APRIL, BAFF, NGF, IGF I and IGF II, that can induce RANKL-independent osteoclastogenesis. All five cytokines induced both osteoclast differentiation and activation with respect to the formation of significant numbers of TRAP(+) and VNR(+) multinucleated cells that were capable of resorbing bone. The number of TRAP(+) multinucleated cells that formed was in the range of 40-75% of that supported by MCSF plus RANKL. Resorption was at a similar level to that induced by the other known RANKL substitutes TNFα, IL-6 and TGF-β. The addition of osteoprotegrin, the endogenous decoy receptor of RANKL, revealed that this resorption was independent of RANKL. APRIL, BAFF, IGF I and IGF II were found to be expressed in giant cell tumour of bone. IGF I and IGF II demonstrated very strong expression in the stromal cell population of all tumour samples. This data suggests that non-canonical osteoclastogenesis plays a role in both normal and pathological bone resorption.

Interleukin-33 is expressed in differentiated osteoblasts and blocks osteoclast formation from bone marrow precursor cells

Since the hematopoetic system is located within the bone marrow, it is not surprising that recent evidence has demonstrated the existence of molecular interactions between bone and immune cells. While interleukin 1 (IL-1) and IL-18, two cytokines of the IL-1 family, have been shown to regulate differentiation and activity of bone cells, the role of IL-33, another IL-1 family member, has not been addressed yet. Since we observed that the expression of IL-33 increases during osteoblast differentiation, we analyzed its possible influence on bone formation and observed that IL-33 did not affect matrix mineralization but enhanced the expression of Tnfsf11, the gene encoding RANKL. This finding led us to analyze the skeletal phenotype of Il1rl1-deficient mice, which lack the IL-33 receptor ST2. Unexpectedly, these mice displayed normal bone formation but increased bone resorption, thereby resulting in low trabecular bone mass. Since this finding suggested a negative influence of IL-33 on osteoclastogenesis, we next analyzed osteoclast differentiation from bone marrow precursor cells and observed that IL-33 completely abolished the generation of TRACP(+) multinucleated osteoclasts, even in the presence of RANKL and macrophage colony-stimulating factor (M-CSF). Although our molecular studies revealed that IL-33 treatment of bone marrow cells caused a shift toward other hematopoetic lineages, we further observed a direct negative influence of IL-33 on the osteoclastogenic differentiation of RAW264.7 macrophages, where IL-33 repressed the expression of Nfatc1, which encodes one of the key transciption factors of osteoclast differentiation. Taken together, these findings have uncovered a previously unknown function of IL-33 as an inhibitor of bone resorption.

TGF-β inducible early gene 1 regulates osteoclast differentiation and survival by mediating the NFATc1, AKT, and MEK/ERK signaling pathways

TGF-β Inducible Early Gene-1 (TIEG1) is a Krüppel-like transcription factor (KLF10) that was originally cloned from human osteoblasts as an early response gene to TGF-β treatment. As reported previously, TIEG1(-/-) mice have decreased cortical bone thickness and vertebral bone volume and have increased spacing between the trabeculae in the femoral head relative to wildtype controls. Here, we have investigated the role of TIEG1 in osteoclasts to further determine their potential role in mediating this phenotype. We have found that TIEG1(-/-) osteoclast precursors differentiated more slowly compared to wildtype precursors in vitro and high RANKL doses are able to overcome this defect. We also discovered that TIEG1(-/-) precursors exhibit defective RANKL-induced phosphorylation and accumulation of NFATc1 and the NFATc1 target gene DC-STAMP. Higher RANKL concentrations reversed defective NFATc1 signaling and restored differentiation. After differentiation, wildtype osteoclasts underwent apoptosis more quickly than TIEG1(-/-) osteoclasts. We observed increased AKT and MEK/ERK signaling pathway activation in TIEG1(-/-) osteoclasts, consistent with the roles of these kinases in promoting osteoclast survival. Adenoviral delivery of TIEG1 (AdTIEG1) to TIEG1(-/-) cells reversed the RANKL-induced NFATc1 signaling defect in TIEG1(-/-) precursors and eliminated the differentiation and apoptosis defects. Suppression of TIEG1 with siRNA in wildtype cells reduced differentiation and NFATc1 activation. Together, these data provide evidence that TIEG1 controls osteoclast differentiation by reducing NFATc1 pathway activation and reduces osteoclast survival by suppressing AKT and MEK/ERK signaling.

Fertilization in C. elegans requires an intact C-terminal RING finger in sperm protein SPE-42

Background

The C. elegans sperm protein SPE-42, a membrane protein of unknown structure and molecular function, is required for fertilization. Sperm from worms with spe-42 mutations appear normal but are unable to fertilize eggs. Sequence analysis revealed the presence of 8 conserved cysteine residues in the C-terminal cytoplasmic domain of this protein suggesting these residues form a zinc-coordinating RING finger structure.

Results

We made an in silico structural model of the SPE-42 RING finger domain based on primary sequence analysis and previously reported RING structures. To test the model, we created spe-42 transgenes coding for mutations in each of the 8 cysteine residues predicted to coordinate Zn⁺⁺ ions in the RING finger motif. Transgenes were crossed into a spe-42 null background and protein function was measured by counting progeny. We found that all 8 cysteines are required for protein function. We also showed that sequence differences between the C-terminal 29 and 30 amino acids in C. elegans and C. briggsae SPE-42 following the RING finger domain are not responsible for the failure of the C. briggsae SPE-42 homolog to rescue C. elegans spe-42 mutants.

Conclusions

The results suggest that a bona fide RING domain is present at the C-terminus of the SPE-42 protein and that this motif is required for sperm-egg interactions during C. elegans fertilization. Our structural model of the RING domain provides a starting point for further structure-function analysis of this critical region of the protein. The C-terminal domain swap experiment suggests that the incompatibility between the C. elegans and C. briggsae SPE-42 proteins is caused by small amino acid differences outside the C-terminal domain.

Sbds is required for Rac2-mediated monocyte migration and signaling downstream of RANK during osteoclastogenesis

Shwachman-Diamond syndrome (SDS) results from mutations in the SBDS gene, characterized by exocrine pancreatic insufficiency and hematologic and skeletal abnormalities. Neutropenia and neutrophil dysfunction are hallmark features of SDS; however, causes for the bone defects are unknown. Dysfunction of bone-resorbing osteoclasts, formed by the fusion of monocytic progenitors derived from the same granulocytic precursors as neutrophils, could be responsible. We report that Sbds is required for in vitro and in vivo osteoclastogenesis (OCG). Sbds-null murine monocytes formed osteoclasts of reduced number and size because of impaired migration and fusion required for OCG. Phenotypically, Sbds-null mice exhibited low-turnover osteoporosis consistent with findings in SDS patients. Western blotting of Rho GTPases that control actin dynamics and migration showed a 5-fold decrease in Rac2, whereas Rac1, Cdc42, and RhoA were unchanged or only mildly reduced. Although migration was rescued on Rac2 supplementation, OCG was not. This was attributed to impaired signaling downstream of receptor activator of nuclear factor-κB (RANK) and reduced expression of the RANK-ligand-dependent fusion receptor DC-STAMP. We conclude that Sbds is required for OCG by regulating monocyte migration via Rac2 and osteoclast differentiation signaling downstream of RANK. Impaired osteoclast formation could disrupt bone homeostasis, resulting in skeletal abnormalities seen in SDS patients.

CCN family 2/connective tissue growth factor (CCN2/CTGF) promotes osteoclastogenesis via induction of and interaction with dendritic cell-specific transmembrane protein (DC-STAMP)

CCN family 2/connective tissue growth factor (CCN2/CTGF) promotes endochondral ossification. However, the role of CCN2 in the replacement of hypertrophic cartilage with bone is still unclear. The phenotype of Ccn2 null mice, having an expanded hypertrophic zone, indicates that the resorption of the cartilage extracellular matrix is impaired therein. Therefore, we analyzed the role of CCN2 in osteoclastogenesis because cartilage extracellular matrix is resorbed mainly by osteoclasts during endochondral ossification. Expression of the Ccn2 gene was upregulated in mouse macrophage cell line RAW264.7 on day 6 after treatment of glutathione S transferase (GST) fusion mouse receptor activator of NF-κB ligand (GST-RANKL), and a combination of recombinant CCN2 (rCCN2) and GST-RANKL significantly enhanced tartrate-resistant acid phosphatase (TRACP)-positive multinucleated cell formation compared with GST-RANKL alone. Therefore, we suspected the involvement of CCN2 in cell-cell fusion during osteoclastogenesis. To clarify the mechanism, we performed real-time PCR analysis of gene expression, coimmunoprecipitation analysis, and solid-phase binding assay of CCN2 and dendritic cell-specific transmembrane protein (DC-STAMP), which is involved in cell-cell fusion. The results showed that CCN2 induced and interacted with DC-STAMP. Furthermore, GST-RANKL-induced osteoclastogenesis was impaired in fetal liver cells from Ccn2 null mice, and the impaired osteoclast formation was rescued by the addition of exogenous rCCN2 or the forced expression of DC-STAMP by a retroviral vector. These results suggest that CCN2 expressed during osteoclastogenesis promotes osteoclast formation via induction of and interaction with DC-STAMP.

P2X7 receptor drives osteoclast fusion by increasing the extracellular adenosine concentration

Defects in bone homeostasis are a major health problem. Osteoclast differentiation and activation have a crucial role in bone remodeling in health and disease. Osteoclasts are bone-resorbing cells derived from mononuclear phagocyte progenitors. The key event in osteoclast formation is fusion of mononucleate precursors to form mature multinucleated osteclasts. Here we provide evidence of an absolute requirement for the P2X7 receptor, ATP release, and adenosine signaling in human osteoclast formation, as shown by the following findings: macrophage-colony stimulating factor/receptor activator for nuclear factor-κB ligand (M-CSF/RANKL)-stimulated fusion of human monocytes is fully prevented by an anti-P2X7 mAb, by specific P2X7 pharmacological antagonists, or by inhibition of CD39/NTPDase; fusion-competent monocytes release ATP via the P2X7 receptor; accelerated degradation of released ATP by addition of either apyrase or hexokinase strongly increases fusion; removal of extracellular adenosine by adenosine deaminase blocks, while addition of exogenous adenosine strongly potentiates, fusion; and pharmacologic stimulation of the adenosine A2A receptor increases, while selective A2A blockade inhibits, fusion. These results show that the purinergic axis plays a crucial and as yet undescribed role in osteoclast formation and reconcile previous evidence advocating a key role for either ATP or adenosine receptors in multinucleated giant cell formation.

Contributions of the measles virus nucleocapsid gene and the SQSTM1/p62(P392L) mutation to Paget's disease

Paget's disease (PD) is characterized by abnormal osteoclasts (OCL) that secrete high IL-6 levels and induce exuberant bone formation. Because measles virus nucleocapsid gene (MVNP) and the p62(P392L) mutation are implicated in PD, marrows from 12 PD patients harboring p62(P392L) and eight normals were tested for MVNP expression and pagetic OCL formation. Eight out of twelve patients expressed MVNP and formed pagetic OCL in vitro, which were inhibited by antisense-MVNP. Four out of twelve patients lacked MVNP and formed normal OCL that were hyperresponsive to RANKL but unaffected by antisense-MVNP. Similarly, mice expressing only p62(P394L) formed normal OCL, while mice expressing MVNP in OCL, with or without p62(P394L), developed pagetic OCL and expressed high IL-6 levels dependent on p38MAPK activation. IL-6 deficiency in MVNP mice abrogated pagetic OCL development in vitro. Mice coexpressing MVNP and p62(P394L) developed dramatic Paget's-like bone lesions. These results suggest that p62(P394L) and IL-6 induction by MVNP play key roles in PD.

2-Methoxystypandrone represses RANKL-mediated osteoclastogenesis by down-regulating formation of TRAF6-TAK1 signalling complexes

BACKGROUND AND PURPOSE

2-Methoxystypandrone (2-MS) is a naphthoquinone isolated from Polygonum cuspidatum, a Chinese herb used to treat bone diseases. Here we have determined whether 2-MS antagonised osteoclast development and bone resorption.

EXPERIMENTAL APPROACH

RAW264.7 cells were treated with receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL) to induce differentiation into osteoclasts. RT-PCR and Western blot were used to analyse osteoclast-associated gene expression and signalling pathways.

KEY RESULTS

The number of multinuclear osteoclasts, actin rings and resorption pit formation were markedly inhibited by 2-MS, targeting osteoclast differentiation at an early stage and without significant cytotoxicity. The anti-resorption effect of 2-MS was accompanied by decreasing dendritic cell-specific transmembrane protein and matrix metalloproteinase-9 (MMP-9) mRNA expression. RANKL-increased MMP-9 gelatinolytic activity was also attenuated by concurrent, but not by subsequent addition of 2-MS. 2-MS markedly inhibited not only the RANKL-triggered nuclear translocations of NF-kappaB, c-Fos and nuclear factor of activated T cells c1 (NFATc1), but also the subsequent NFATc1 induction. Degradation of IkappaB and phosphorylation of mitogen-activated protein kinases were also suppressed. RANKL facilitated the formation of signaling complexes of tumour necrosis factor receptor-associated factor 6 and transforming growth factor beta-activated kinase 1 (TRAF6-TAK1), important for osteoclastogenesis and formation of such signalling complexes was prevented by 2-MS.

CONCLUSIONS AND IMPLICATIONS

The anti-osteoclastogenic effects of 2-MS could reflect the block of RANKL-induced association of TRAF6-TAK1 complexes with consequent decrease of IkappaB-mediated NF-kappaB and mitogen-activated protein kinases-mediated c-Fos activation pathways and suppression of NFATc1 and other gene expression, essential for bone resorption.

Osteoclast motility: putting the brakes on bone resorption

As the skeleton ages, the balanced formation and resorption of normal bone remodeling is lost, and bone loss predominates. The osteoclast is the specialized cell that is responsible for bone resorption. It is a highly polarized cell that must adhere to the bone surface and migrate along it while resorbing, and cytoskeletal reorganization is critical. Podosomes, highly dynamic actin structures, mediate osteoclast motility. Resorbing osteoclasts form a related actin complex, the sealing zone, which provides the boundary for the resorptive microenvironment. Similar to podosomes, the sealing zone rearranges itself to allow continuous resorption while the cell is moving. The major adhesive protein controlling the cytoskeleton is αvβ3 integrin, which collaborates with the growth factor M-CSF and the ITAM receptor DAP12. In this review, we discuss the signaling complexes assembled by these molecules at the membrane, and their downstream mediators that control OC motility and function via the cytoskeleton.