The role of cadherin in the generation of multinucleated osteoclasts from mononuclear precursors in murine marrow

Stage-specific modulation of multinucleation, fusion, and resorption by the long non-coding RNA DLEU1 and miR-16 in human primary osteoclasts

Osteoclasts are the only cells able to resorb all the constituents of the bone matrix. While the modulation of osteoclast activity is well established for preventing bone-related diseases, there is an increasing demand for novel classes of anti-resorption agents. Herein, we investigated non-coding RNA molecules and proposed DLEU1 and miR-16 as potential candidates for modulating osteoclast functions. DLEU1 and miR-16 target cell fusion at both the early and late stages of osteoclastogenesis but operate through independent pathways. DLEU1 silencing hinders the fusion process, leading to abrogation of the phagocytic cup fusion modality and a reduction in the fusion events between mononucleated precursors and multinucleated osteoclasts, while miR-16 influences monocyte-to-osteoclast differentiation, impairing osteoclasts formation but not the number of nuclei at early stages. On the other hand, using these non-coding RNAs to engineer mature osteoclasts has implications for bone resorption. Both DLEU1 and miR-16 influence the speed of resorption in pit-forming osteoclasts, without affecting the resorbed area. However, the impact of increasing miR-16 levels extends more broadly, affecting trench-forming osteoclasts as well, leading to a reduction in their percentage, speed, and resorbed area. These findings offer potential new therapeutic targets to ameliorate bone destruction in skeletal diseases.

UBAP2 plays a role in bone homeostasis through the regulation of osteoblastogenesis and osteoclastogenesis

Osteoporosis is a condition characterized by decreased bone mineral density (BMD) and reduced bone strength, leading to an increased risk of fractures. Here, to identify novel risk variants for susceptibility to osteoporosis-related traits, an exome-wide association study is performed with 6,485 exonic single nucleotide polymorphisms (SNPs) in 2,666 women of two Korean study cohorts. The rs2781 SNP in UBAP2 gene is suggestively associated with osteoporosis and BMD with p-values of 6.1 × 10-7 (odds ratio = 1.72) and 1.1 × 10-7 in the case-control and quantitative analyzes, respectively. Knockdown of Ubap2 in mouse cells decreases osteoblastogenesis and increases osteoclastogenesis, and knockdown of ubap2 in zebrafish reveals abnormal bone formation. Ubap2 expression is associated with E-cadherin (Cdh1) and Fra1 (Fosl1) expression in the osteclastogenesis-induced monocytes. UBAP2 mRNA levels are significantly reduced in bone marrow, but increased in peripheral blood, from women with osteoporosis compared to controls. UBAP2 protein level is correlated with the blood plasma level of the representative osteoporosis biomarker osteocalcin. These results suggest that UBAP2 has a critical role in bone homeostasis through the regulation of bone remodeling.

Physiological functions of podosomes: From structure and function to therapy implications in osteoclast biology of bone resorption

With increasing age, bone tissue undergoes significant alterations in composition, architecture, and metabolic functions, probably causing senile osteoporosis. Osteoporosis possess the vast majority of bone disease and associates with a reduction in bone mass and increased fracture risk. Bone loss is on account of the disorder in osteoblast-induced bone formation and osteoclast-induced bone resorption. As a unique bone resorptive cell type, mature bone-resorbing osteoclasts exhibit dynamic actin-based cytoskeletal structures called podosomes that participate in cell-matrix adhesions specialized in the degradation of mineralized bone matrix. Podosomes share many of the same molecular constitutions as focal adhesions, but they have a unique structural organization, with a central core abundant in F-actin and encircled by scaffolding proteins, kinases and integrins. Here, we conclude recent advancements in our knowledge of the architecture and the functions of podosomes. We also discuss the regulatory pathways in osteoclast podosomes, providing a reference for future research on the podosomes of osteoclasts and considering podosomes as a therapeutic target for inhibiting bone resorption.

Multinucleated Giant Cells: Current Insights in Phenotype, Biological Activities, and Mechanism of Formation

Monocytes and macrophages are innate immune cells with diverse functions ranging from phagocytosis of microorganisms to forming a bridge with the adaptive immune system. A lesser-known attribute of macrophages is their ability to fuse with each other to form multinucleated giant cells. Based on their morphology and functional characteristics, there are in general three types of multinucleated giant cells including osteoclasts, foreign body giant cells and Langhans giant cells. Osteoclasts are bone resorbing cells and under physiological conditions they participate in bone remodeling. However, under pathological conditions such as rheumatoid arthritis and osteoporosis, osteoclasts are responsible for bone destruction and bone loss. Foreign body giant cells and Langhans giant cells appear only under pathological conditions. While foreign body giant cells are found in immune reactions against foreign material, including implants, Langhans giant cells are associated with granulomas in infectious and non-infectious diseases. The functionality and fusion mechanism of osteoclasts are being elucidated, however, our knowledge on the functions of foreign body giant cells and Langhans giant cells is limited. In this review, we describe and compare the phenotypic aspects, biological and functional activities of the three types of multinucleated giant cells. Furthermore, we provide an overview of the multinucleation process and highlight key molecules in the different phases of macrophage fusion.

Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune

Different types of multinucleated giant cells (MGCs) of myeloid origin have been described; osteoclasts are the most extensively studied because of their importance in bone homeostasis. MGCs are formed by cell-to-cell fusion, and most types have been observed in pathological conditions, especially in infectious and non-infectious chronic inflammatory contexts. The precise role of the different MGCs and the mechanisms that govern their formation remain poorly understood, likely due to their heterogeneity. First, we will introduce the main populations of MGCs derived from the monocyte/macrophage lineage. We will then discuss the known molecular actors mediating the early stages of fusion, focusing on cell-surface receptors involved in the cell-to-cell adhesion steps that ultimately lead to multinucleation. Given that cell-to-cell fusion is a complex and well-coordinated process, we will also describe what is currently known about the evolution of F-actin-based structures involved in macrophage fusion, i.e., podosomes, zipper-like structures, and tunneling nanotubes (TNT). Finally, the localization and potential role of the key fusion mediators related to the formation of these F-actin structures will be discussed. This review intends to present the current status of knowledge of the molecular and cellular mechanisms supporting multinucleation of myeloid cells, highlighting the gaps still existing, and contributing to the proposition of potential disease-specific MGC markers and/or therapeutic targets.

Flagging fusion: Phosphatidylserine signaling in cell-cell fusion

Formations of myofibers, osteoclasts, syncytiotrophoblasts, and fertilized zygotes share a common step, cell–cell fusion. Recent years have brought about considerable progress in identifying some of the proteins involved in these and other cell-fusion processes. However, even for the best-characterized cell fusions, we still do not know the mechanisms that regulate the timing of cell-fusion events. Are they fully controlled by the expression of fusogenic proteins or do they also depend on some triggering signal that activates these proteins? The latter scenario would be analogous to the mechanisms that control the timing of exocytosis initiated by Ca²⁺ influx and virus-cell fusion initiated by low pH- or receptor interaction. Diverse cell fusions are accompanied by the nonapoptotic exposure of phosphatidylserine at the surface of fusing cells. Here we review data on the dependence of membrane remodeling in cell fusion on phosphatidylserine and phosphatidylserine-recognizing proteins and discuss the hypothesis that cell surface phosphatidylserine serves as a conserved “fuse me” signal regulating the time and place of cell-fusion processes.

Monocyte/Macrophage Lineage Cells From Fetal Erythromyeloid Progenitors Orchestrate Bone Remodeling and Repair

A third of the population sustains a bone fracture, and the pace of fracture healing slows with age. The slower pace of repair is responsible for the increased morbidity in older individuals who sustain a fracture. Bone healing progresses through overlapping phases, initiated by cells of the monocyte/macrophage lineage. The repair process ends with remodeling. This last phase is controlled by osteoclasts, which are bone-specific multinucleated cells also of the monocyte/macrophage lineage. The slower rate of healing in aging can be rejuvenated by macrophages from young animals, and secreted proteins from macrophage regulate undifferentiated mesenchymal cells to become bone-forming osteoblasts. Macrophages can derive from fetal erythromyeloid progenitors or from adult hematopoietic progenitors. Recent studies show that fetal erythromyeloid progenitors are responsible for the osteoclasts that form the space in bone for hematopoiesis and the fetal osteoclast precursors reside in the spleen postnatally, traveling through the blood to participate in fracture repair. Differences in secreted proteins between macrophages from old and young animals regulate the efficiency of osteoblast differentiation from undifferentiated mesenchymal precursor cells. Interestingly, during the remodeling phase osteoclasts can form from the fusion between monocyte/macrophage lineage cells from the fetal and postnatal precursor populations. Data from single cell RNA sequencing identifies specific markers for populations derived from the different precursor populations, a finding that can be used in future studies. Here, we review the diversity of macrophages and osteoclasts, and discuss recent finding about their developmental origin and functions, which provides novel insights into their roles in bone homeostasis and repair.

Switch of macrophage fusion competency by 3D matrices

Foreign body reaction reflects the integration between biomaterials and host cells. At the implantation microenvironment, macrophages usually fuse into multinuclear cells, also known as foreign body giant cells, to respond to the biomaterial implants. To understand the biomaterial-induced macrophage fusion, we examined whether biomaterial alone can initiate and control the fusion rate without exogenous cytokines and chemicals. We introduced a collagen-based 3D matrix to embed Raw264.7 cell line and primary rat bone marrow-derived macrophages. We found the biomaterial-stimuli interacted regional macrophages and altered the overall fusogenic protein expressions to regulate the macrophage fusion rate. The fusion rate could be altered by modulating the cell-matrix and cell-cell adhesions. The fused macrophage morphologies, the nuclei number in the fused macrophage, and the fusion rates were matrix dependent. The phenomena were also observed in the in vivo models. These results suggest that the biomaterial-derived stimuli exert similar functions as cytokines to alter the competency of macrophage fusion as well as their drug sensitivity in the biomaterial implanted tissue environment. Furthermore, this in vitro 3D-matrix model has the potential to serve as a toolbox to predict the host tissue response on implanted biomaterials.

Tuning Ligand Density To Optimize Pharmacokinetics of Targeted Nanoparticles for Dual Protection against Tumor-Induced Bone Destruction

Breast cancer patients are at high risk for bone metastasis. Metastatic bone disease is a major clinical problem that leads to a reduction in mobility, increased risk of pathologic fracture, severe bone pain, and other skeletal-related events. The transcription factor Gli2 drives expression of parathyroid hormone-related protein (PTHrP), which activates osteoclast-mediated bone destruction, and previous studies showed that Gli2 genetic repression in bone-metastatic tumor cells significantly reduces tumor-induced bone destruction. Small molecule inhibitors of Gli2 have been identified; however, the lipophilicity and poor pharmacokinetic profile of these compounds have precluded their success in vivo. In this study, we designed a bone-targeted nanoparticle (BTNP) comprising an amphiphilic diblock copolymer of poly[(propylene sulfide)-block-(alendronate acrylamide-co-N,N-dimethylacrylamide)] [PPS-b-P(Aln-co-DMA)] to encapsulate and preferentially deliver a small molecule Gli2 inhibitor, GANT58, to bone-associated tumors. The mol % of the bisphosphonate Aln in the hydrophilic polymer block was varied in order to optimize BTNP targeting to tumor-associated bone by a combination of nonspecific tumor accumulation (presumably through the enhanced permeation and retention effect) and active bone binding. Although 100% functionalization with Aln created BTNPs with strong bone binding, these BTNPs had highly negative zeta-potential, resulting in shorter circulation time, greater liver uptake, and less distribution to metastatic tumors in bone. However, 10 mol % of Aln in the hydrophilic block generated a formulation with a favorable balance of systemic pharmacokinetics and bone binding, providing the highest bone/liver biodistribution ratio among formulations tested. In an intracardiac tumor cell injection model of breast cancer bone metastasis, treatment with the lead candidate GANT58-BTNP formulation decreased tumor-associated bone lesion area 3-fold and increased bone volume fraction in the tibiae of the mice 2.5-fold. Aln conferred bone targeting to the GANT58-BTNPs, which increased GANT58 concentration in the tumor-associated bone relative to untargeted NPs, and also provided benefit through the direct antiresorptive therapeutic function of Aln. The dual benefit of the Aln in the BTNPs was supported by the observations that drug-free Aln-containing BTNPs improved bone volume fraction in bone-tumor-bearing mice, while GANT58-BTNPs created better therapeutic outcomes than both unloaded BTNPs and GANT58-loaded untargeted NPs. These findings suggest GANT58-BTNPs have potential to potently inhibit tumor-driven osteoclast activation and resultant bone destruction in patients with bone-associated tumor metastases.

Osteoclastogenesis inhibition by mutated IGSF23 results in human osteopetrosis

OBJECTIVES

Osteopetrosis is a rare inherited skeletal disease characterized by increased bone mineral density due to the loss of osteoclast function or differentiation potential.

MATERIALS AND METHODS

The study involved a Chinese patient with osteopetrosis (the proband) and her immediate family members and 180 controls without osteopetrosis. Bone density of the femoral neck, lumbar spine and total body was measured using dual-energy x-ray absorptiometry. Osteoclast differentiation by the participants' peripheral blood mononuclear cells (PBMCs) was investigated using tartrate-resistant acid phosphatase (TRAP) staining. Osteoblast differentiation was examined with Alizarin Red S staining. Reverse transcription-quantitative PCR was used to amplify immunoglobulin superfamily member 23 (IGSF23), c-FOS and nuclear factor of activated T cells 1 (NFATC1).

RESULTS

We found a homozygous mutation (c.295C>T) in the IGSF23 gene in two osteopetrosis samples. The mutation led to the formation of a stop codon, causing loss of the immunoglobulin-like domain and the whole transmembrane domain. PBMCs from the proband (IGSF23^-/- ) exhibited poor ability for differentiating into mature osteoclasts in vitro. Overexpression of IGSF23 rescued the ability of IGSF23^-/- PBMCs to differentiate into osteoclasts. Moreover, knockdown of IGSF23 reversed the bone loss in OVX mice by injecting AAV-shIGSF23 into mice femoral bone marrow cavity. Furthermore, we also found that the IGSF23 mutation led to decreased c-Fos and NFATC1 expression levels by inhibiting the mitogen-activated protein kinase signalling pathways.

CONCLUSIONS

IGSF23-mediated osteoclast differentiation of PBMCs may serve as a potential target in osteoporosis therapy.

Cell-substrate traction force regulates the fusion of osteoclast precursors through cell-cell interaction

The adhesion morphology of a cell monolayer results in a mechanical force inside cells, between cells, or between cells and substrates. The mechanical force regulates the differentiation of stem cells, but its influence on cell fusion is seldom studied. The present study is focused on osteoclast precursors, RAW264.7 monocytes, which can fuse into multinucleated cells (MNCs) responsible for bone resorption. Cells were cultured on circular and ring-like patterned substrates. Then, cell fusion, cell-substrate traction force, and force-sensitive molecules in different regions were measured and analyzed. Results showed that MNCs mainly appeared in the interior of the ring-like pattern and the central zone of the circular pattern, where both cell-substrate traction force and in-plane maximal shear stress were smaller than that at the patterns' edge. The immunostaining results revealed that F-actin, vinculin, β-catenin, and E-cadherin were highly distributed at the edge of patterns. High seeding density of cells promoted mechanical force-dependent fusion. When calcium-dependent cell-cell connections were inhibited by E-cadherin antibody or low-calcium medium, the fusion into MNCs was greatly reduced. Thus, the morphology of cell monolayer decides the mechanical state of cell-substrate interaction and cell-cell connection, ultimately regulating the fusion of osteoclast precursors.

Systemic delivery of a Gli inhibitor via polymeric nanocarriers inhibits tumor-induced bone disease

Solid tumors frequently metastasize to bone and induce bone destruction leading to severe pain, fractures, and other skeletal-related events (SREs). Osteoclast inhibitors such as bisphosphonates delay SREs but do not prevent skeletal complications or improve overall survival. Because bisphosphonates can cause adverse side effects and are contraindicated for some patients, we sought an alternative therapy to reduce tumor-associated bone destruction. Our previous studies identified the transcription factor Gli2 as a key regulator of parathyroid hormone-related protein (PTHrP), which is produced by bone metastatic tumor cells to promote osteoclast-mediated bone destruction. In this study, we tested the treatment effect of a Gli antagonist GANT58, which inhibits Gli2 nuclear translocation and PTHrP expression in tumor cells. In initial testing, GANT58 did not have efficacy in vivo due to its low water solubility and poor bioavailability. We therefore developed a micellar nanoparticle (NP) to encapsulate and colloidally stabilize GANT58, providing a fully aqueous, intravenously injectable formulation based on the polymer poly(propylene sulfide)₁₃₅-b-poly[(oligoethylene glycol)₉ methyl ether acrylate]₁₇ (PPS₁₃₅-b-POEGA₁₇). POEGA forms the hydrophilic NP surface while PPS forms the hydrophobic NP core that sequesters GANT58. In response to reactive oxygen species (ROS), PPS becomes hydrophilic and degrades to enable drug release. In an intratibial model of breast cancer bone metastasis, treatment with GANT58-NPs decreased bone lesion area by 49% (p<.01) and lesion number by 38% (p<.05) and resulted in a 2.5-fold increase in trabecular bone volume (p<.001). Similar results were observed in intracardiac and intratibial models of breast and lung cancer bone metastasis, respectively. Importantly, GANT58-NPs reduced tumor cell proliferation but did not alter mesenchymal stem cell proliferation or osteoblast mineralization in vitro, nor was there evidence of cytotoxicity after repeated in vivo treatment. Thus, inhibition of Gli2 using GANT58-NPs is a potential therapy to reduce bone destruction that should be considered for further testing and development toward clinical translation.

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.

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.

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.

Atorvastatin inhibits osteoclast differentiation by suppressing NF-κB and MAPK signaling during IL-1β-induced osteoclastogenesis

BACKGROUND/AIMS

To define the effect of statins on interleukin 1β (IL-1β)-induced osteoclastogenesis and elucidate the underlying mechanisms.

METHODS

Bone marrow cells were obtained from 5-week-old male ICR (Institute for Cancer Research) mice, and they were cultured to differentiate them into osteoclasts with macrophage colony-stimulating factor and the receptor activator of nuclear factor (NF)-κB ligand in the presence or absence of IL-1β or atorvastatin. The formation of osteoclasts was evaluated by tartrate-resistant acid phosphatase (TRAP) staining and resorption pit assay with dentine slice. The molecular mechanisms of the effects of atorvastatin on osteoclastogenesis were investigated using reverse transcription polymerase chain reaction and immunoblotting for osteoclast specific molecules.

RESULTS

Atorvastatin significantly reduced the number of TRAP-positive multinucleated cells as well as the bone resorption area. Atorvastatin also downregulated the expression of the NF of activated T-cell c1 messenger RNA and inhibited the expression of osteoclast-specific genes. A possible underlying mechanism may be that atorvastatin suppresses the degradation of the inhibitors of NF-κB and blocks the activation of the c-Jun N-terminal kinase, extracellular signal-regulated kinase, and p38; thus, implicating the NF-κB and mitogen-activated protein kinases pathway in this process.

CONCLUSIONS

Atorvastatin is a strong inhibitor of inflammation-induced osteoclastogenesis in inflammatory joint diseases.

Symmetrical retrograde actin flow in the actin fusion structure is involved in osteoclast fusion

The aim of this study was to elucidate the role of the zipper-like structure (ZLS), a podosome-related structure that transiently appears at the cell contact zone, in osteoclast fusion. Live-cell imaging of osteoclasts derived from RAW264.7 cells transfected with EGFP-actin revealed consistent symmetrical retrograde actin flow in the ZLS, but not in the podosome cluster, the podosome ring or the podosome belt. Confocal imaging showed that the distributions of F-actin, vinculin, paxillin and zyxin in the ZLS were different from those in the podosome belt. Thick actin filament bundles running outside the ZLS appeared to recruit non-muscle myosin IIA. The F-actin-rich domain of the ZLS contained actin-related protein 2/3 complex (Arp2/3). Inhibition of Arp2/3 activity disorganized the ZLS, disrupted actin flow, deteriorated cell-cell adhesion and inhibited osteoclast hypermultinucleation. In contrast, ML-7, an inhibitor of myosin light chain kinase, had little effect on the structure of ZLS and promoted osteoclast hypermultinucleation. These results reveal a link between actin flow in the ZLS and osteoclast fusion. Osteoclast fusion was promoted by branched actin elongation and negatively regulated by actomyosin contraction.

Summary: Multinucleated osteoclasts form a podosome-derived fusion structure during cell fusion. Juxtaposition of fusion partner cells is probably maintained via force generated by symmetrical retrograde actin flow in the fusion structure.

Utilization of E-cadherin by monocytes from tumour cells plays key roles in the progression of bone invasion by oral squamous cell carcinoma

E-cadherin (E-cad) is recently reported to be expressed in early stages of osteoclastogenesis, and blocking E-cad with neutralizing antibodies decreases osteoclast differentiation. Since our previous research demonstrates the loss of E-cad protein in the bone invasion by oral squamous cell carcinoma (OSCC), we hypothesize that E-cad may be utilized by monocytes to fuse and differentiate into osteoclasts. Two research models are used in the present study to explore our hypothesis. On one hand, we use OSCC cells of SCC25 to establish an animal model of bone invasion by OSCC, and investigate whether E-cad protein disappears in vivo; on the other hand, we use the indirect co-culture model of SCC25 and RAW 264.7 cells, with the treatment of transforming growth factor-β1 (TGF-β1), and observe whether the decreased E-cad protein is ‘hijacked’ in vitro. Results showed the animal model of OSCC with bone invasion was successfully established. Immunohistochemistry (IHC) found similar changes of E-cad protein, which was weakly stained by tumour cells. By using 5 ng/ml of TGF-β1, we confirmed the artificial epithelial-mesenchymal transition (EMT) of SCC25 cells, with changes of EMT marker expression and cell morphology. Real-time PCR showed E-cad mRNA decreased in SCC25 while increased in RAW 264.7 of the indirect cell co-culture model, and immunofluoresence (IF) observed the evident switch of E-cad staining from SCC25 to RAW 264.7. With the supplement of receptor activator of NF-κB ligand (RANKL), tartrate-resistant acid phosphatase (TRAP) and F-actin staining confirmed the increased number of osteoclasts. Taken together, our study found the switch of E-cad protein in the progression of bone invasion by OSCC. The loss of E-cad in tumour cells may be utilized by monocytes to differentiate into osteoclasts, thus further explaining the underlying mechanisms of bone invasion by OSCC, which may supply clues for future molecular biotherapies.

Osteoclast Fusion: Time-Lapse Reveals Involvement of CD47 and Syncytin-1 at Different Stages of Nuclearity

Investigations addressing the molecular keys of osteoclast fusion are primarily based on end-point analyses. No matter if investigations are performed in vivo or in vitro the impact of a given factor is predominantly analyzed by counting the number of multi-nucleated cells, the number of nuclei per multinucleated cell or TRAcP activity. But end-point analyses do not show how the fusion came about. This would not be a problem if fusion of osteoclasts was a random process and occurred by the same molecular mechanism from beginning to end. However, we and others have in the recent period published data suggesting that fusion partners may specifically select each other and that heterogeneity between the partners seems to play a role. Therefore, we set out to directly test the hypothesis that fusion factors have a heterogenic involvement at different stages of nuclearity. Therefore, we have analyzed individual fusion events using time-lapse and antagonists of CD47 and syncytin-1. All time-lapse recordings have been studied by two independent observers. A total of 1808 fusion events were analyzed. The present study shows that CD47 and syncytin-1 have different roles in osteoclast fusion depending on the nuclearity of fusion partners. While CD47 promotes cell fusions involving mono-nucleated pre-osteoclasts, syncytin-1 promotes fusion of two multi-nucleated osteoclasts, but also reduces the number of fusions between mono-nucleated pre-osteoclasts. Furthermore, CD47 seems to mediate fusion mostly through broad contact surfaces between the partners' cell membrane while syncytin-1 mediate fusion through phagocytic-cup like structure. J. Cell. Physiol. 232: 1396-1403, 2017. © 2016 Wiley Periodicals, Inc.

Cysteine cathepsin activity suppresses osteoclastogenesis of myeloid-derived suppressor cells in breast cancer

Cysteine cathepsin proteases contribute to many normal cellular functions, and their aberrant activity within various cell types can contribute to many diseases, including breast cancer. It is now well accepted that cathepsin proteases have numerous cell-specific functions within the tumor microenvironment that function to promote tumor growth and invasion, such that they may be valid targets for anti-metastatic therapeutic approaches. Using activity-based probes, we have examined the activity and expression of cysteine cathepsins in a mouse model of breast cancer metastasis to bone. In mice bearing highly metastatic tumors, we detected abundant cysteine cathepsin expression and activity in myeloid-derived suppressor cells (MDSCs). These immature immune cells have known metastasis-promoting roles, including immunosuppression and osteoclastogenesis, and we assessed the contribution of cysteine cathepsins to these functions. Blocking cysteine cathepsin activity with multiple small-molecule inhibitors resulted in enhanced differentiation of multinucleated osteoclasts. This highlights a potential role for cysteine cathepsin activity in suppressing the fusion of osteoclast precursor cells. In support of this hypothesis, we found that expression and activity of key cysteine cathepsins were downregulated during MDSC-osteoclast differentiation. Another cysteine protease, legumain, also inhibits osteoclastogenesis, in part through modulation of cathepsin L activity. Together, these data suggest that cysteine protease inhibition is associated with enhanced osteoclastogenesis, a process that has been implicated in bone metastasis.

Proteomic approaches to study osteoclast biology

Bone is a dynamic tissue whose remodeling throughout life is orchestrated by repeated cycles of destruction mediated by osteoclasts and rebuilding by osteoblasts. Current understanding of osteoclast biology has largely relied on the generation of knockout mice exhibiting an abnormal bone phenotype. This has provided a better understanding of osteoclast biology and the key proteins that support osteoclast function. However, mouse models alone do not provide an integrated view on protein networks and post-translational modifications that might be important for osteoclast function. During the past years, a number of MS-based quantitative methods have been developed to investigate the complexity of biological systems. This review will summarize how such approaches have contributed to the understanding of osteoclast differentiation and function.

Osteoclasts-Key Players in Skeletal Health and Disease

The differentiation of osteoclasts (OCs) from early myeloid progenitors is a tightly regulated process that is modulated by a variety of mediators present in the bone microenvironment. Once generated, the function of mature OCs depends on cytoskeletal features controlled by an αvβ3-containing complex at the bone-apposed membrane and the secretion of protons and acid-protease cathepsin K. OCs also have important interactions with other cells in the bone microenvironment, including osteoblasts and immune cells. Dysregulation of OC differentiation and/or function can cause bone pathology. In fact, many components of OC differentiation and activation have been targeted therapeutically with great success. However, questions remain about the identity and plasticity of OC precursors and the interplay between essential networks that control OC fate. In this review, we summarize the key principles of OC biology and highlight recently uncovered mechanisms regulating OC development and function in homeostatic and disease states.

E-cadherin is important for cell differentiation during osteoclastogenesis

E-cadherin, a protein responsible for intercellular adhesion between epithelial cells, is also expressed in the monocyte/macrophage lineage. In this study we have explored the involvement of E-cadherin during receptor activator of nuclear factor-κB ligand (RANKL)-stimulated osteoclast differentiation. Osteoclastogenesis involves a period of precursor expansion followed by multiple fusion events to generate a multinuclear osteoclast that is capable of bone resorption. We asked whether E-cadherin participated in early precursor interactions and recognition or was a component of the osteoclast fusion machinery. Here, we show that endogenous E-cadherin expression is the highest during early stages of osteoclast differentiation, with surface expression visible on small precursor cells (fewer than four nuclei per cell) in both RAW 264.7 cells and primary macrophages. Blocking E-cadherin function with neutralizing antibodies prior to the onset of fusion delayed the expression of TRAP, Cathepsin K, DC-STAMP and NFATc1 and significantly diminished multinucleated osteoclast formation. Conversely, E-cadherin-GFP overexpressing macrophages displayed earlier NFATc1 nuclear translocation along with faster formation of multinucleated osteoclasts compared to control macrophages. Through live imaging we identified that disrupting E-cadherin function prolonged the proliferative phase of the precursor population while concomitantly decreasing the proportion of migrating precursors. The lamellipodium and polarized membrane extensions appeared to be the principal sites of fusion, indicating precursor migration was a critical factor contributing to osteoclast fusion. These findings demonstrate that E-cadherin-mediated cell-cell contacts can modulate osteoclast-specific gene expression and prompt differentiating osteoclast precursors toward migratory and fusion activities.

Galectin-3 Cleavage Alters Bone Remodeling: Different Outcomes in Breast and Prostate Cancer Skeletal Metastasis

Management of bone metastasis remains clinically challenging and requires the identification of new molecular target(s) that can be therapeutically exploited to improve patient outcome. Galectin-3 (Gal-3) has been implicated as a secreted factor that alters the bone microenvironment. Proteolytic cleavage of Gal-3 may also contribute to malignant cellular behaviors, but has not been addressed in cancer metastasis. Here, we report that Gal-3 modulates the osteolytic bone tumor microenvironment in the presence of RANKL. Gal-3 was localized on the osteoclast cell surface, and its suppression by RNAi or a specific antagonist markedly inhibited osteoclast differentiation markers, including tartrate-resistant acid phosphatase, and reduced the number of mature osteoclasts. Structurally, the 158-175 amino acid sequence in the carbohydrate recognition domain (CRD) of Gal-3 was responsible for augmented osteoclastogenesis. During osteoclast maturation, Gal-3 interacted and colocalized with myosin-2A along the surface of cell-cell fusion. Pathologically, bone metastatic cancers expressed and released an intact form of Gal-3, mainly detected in breast cancer bone metastases, as well as a cleaved form, more abundant in prostate cancer bone metastases. Secreted intact Gal-3 interacted with myosin-2A, leading to osteoclastogenesis, whereas a shift to cleaved Gal-3 attenuated the enhancement in osteoclast differentiation. Thus, our studies demonstrate that Gal-3 shapes the bone tumor microenvironment through distinct roles contingent on its cleavage status, and highlight Gal-3 targeting through the CRD as a potential therapeutic strategy for mitigating osteolytic bone remodeling in the metastatic niche.

ADP-Ribosylation Factor 1 Regulates Proliferation, Migration, and Fusion in Early Stage of Osteoclast Differentiation

Small G-protein adenosine diphosphate (ADP)-ribosylation factors (ARFs) regulate a variety of cellular functions, including actin cytoskeleton remodeling, plasma membrane reorganization, and vesicular transport. Here, we propose the functional roles of ARF1 in multiple stages of osteoclast differentiation. ARF1 was upregulated during receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast differentiation and transiently activated in an initial stage of their differentiation. Differentiation of ARF1-deficient osteoclast precursors into mature osteoclasts temporarily increased in pre-maturation stage of osteoclasts followed by reduced formation of mature osteoclasts, indicating that ARF1 regulates the osteoclastogenic process. ARF1 deficiency resulted in reduced osteoclast precursor proliferation and migration as well as increasing cell-cell fusion. In addition, ARF1 silencing downregulated c-Jun N-terminal kinase (JNK), Akt, osteopontin, and macrophage colony-stimulating factor (M-CSF)-receptor c-Fms as well as upregulating several fusion-related genes including CD44, CD47, E-cadherin, and meltrin-α. Collectively, we showed that ARF1 stimulated proliferation and migration of osteoclast precursors while suppressing their fusion, suggesting that ARF1 may be a plausible inter-player that mediates the transition to osteoclast fusion at multiple steps during osteoclast differentiation.

Spatiotemporal regulation of cAMP signaling controls the human trophoblast fusion

During human placentation, mononuclear cytotrophoblasts fuse to form multinucleated syncytia ensuring hormonal production and nutrient exchanges between the maternal and fetal circulation. Syncytial formation is essential for the maintenance of pregnancy and for fetal growth. The cAMP signaling pathway is the major route to trigger trophoblast fusion and its activation results in phosphorylation of specific intracellular target proteins, in transcription of fusogenic genes and assembly of macromolecular protein complexes constituting the fusogenic machinery at the plasma membrane. Specificity in cAMP signaling is ensured by generation of localized pools of cAMP controlled by cAMP phosphodiesterases (PDEs) and by discrete spatial and temporal activation of protein kinase A (PKA) in supramolecular signaling clusters inside the cell organized by A-kinase-anchoring proteins (AKAPs) and by organization of signal termination by protein phosphatases (PPs). Here we present original observations on the available components of the cAMP signaling pathway in the human placenta including PKA, PDE, and PP isoforms as well as AKAPs. We continue to discuss the current knowledge of the spatiotemporal regulation of cAMP signaling triggering trophoblast fusion.

Global epigenomic analysis indicates protocadherin-7 activates osteoclastogenesis by promoting cell-cell fusion

Gene expression is dependent not only on genomic sequences, but also epigenetic control, in which the regulation of chromatin by histone modification plays a crucial role. Histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 27 trimethylation (H3K27me3) are related to transcriptionally activated and silenced sequences, respectively. Osteoclasts, the multinucleated cells that resorb bone, are generated by the fusion of precursor cells of monocyte/macrophage lineage. To elucidate the molecular and epigenetic regulation of osteoclast differentiation, we performed a chromatin immunoprecipitation sequencing (ChIP-seq) analysis for H3K4me3 and H3K27me3 in combination with RNA sequencing. We focused on the histone modification change from H3K4me3(+)H3K27me3(+) to H3K4me3(+)H3K27me3(-) and identified the protocadherin-7 gene (Pcdh7) to be among the genes epigenetically regulated during osteoclastogenesis. Pcdh7 was induced by RANKL stimulation in an NFAT-dependent manner. The knockdown of Pcdh7 inhibited RANKL-induced osteoclast differentiation due to the impairment of cell-cell fusion, accompanied by a decreased expression of the fusion-related genes Dcstamp, Ocstamp and Atp6v0d2. This study demonstrates that Pcdh7 plays a key role in osteoclastogenesis by promoting cell-cell fusion.

Pin1 regulates osteoclast fusion through suppression of the master regulator of cell fusion DC-STAMP

Cell fusion is a fundamental biological event that is essential for the development of multinucleated cells such as osteoclasts. Fusion failure leads to the accumulation of dense bone such as in osteopetrosis, demonstrating the importance of fusion in osteoclast maturity and bone remodeling. In a recent study, we reported that Pin1 plays a role in the regulation of bone formation and Runx2 regulation. In this study, we explored the role of Pin1 in osteoclast formation and bone resorption. Pin1 null mice have low bone mass and increased TRAP staining in histological sections of long bones, compared to Pin1 wild-type mice. In vitro osteoclast forming assays with bone marrow-derived monocyte/macrophage revealed that Pin1-deficient osteoclasts are larger than wild-type osteoclasts and have higher nuclei numbers, indicating greater extent of fusion. Pin1 deficiency also highly enhanced foreign body giant cell formation both in vitro and in vivo. Among the known fusion proteins, only DC-STAMP was significantly increased in Pin1(-/-) osteoclasts. Immunohistochemistry showed that DC-STAMP expression was also significantly increased in tibial metaphysis of Pin1 KO mice. We found that Pin1 binds and isomerizes DC-STAMP and affects its expression levels and localization at the plasma membrane. Taken together, our data indicate that Pin1 is a determinant of bone mass through the regulation of the osteoclast fusion protein DC-STAMP. The identification of Pin1 as a factor involved in cell fusion contributes to the understanding of osteoclast-associated diseases, including osteoporosis, and opens new avenues for therapeutic targets.

Essential role for CD103+ cells in the pathogenesis of spondyloarthritides

The clinical features of spondyloarthritides include extraarticular manifestations involving the skin, eyes, and gastrointestinal tract. At these sites, a membrane integrin can be acquired by virtue of the presence of CD4+ T cells and specific dendritic cells and correlates with a regulatory behavior of these cells. This membrane integrin conjugates the beta7 subunit and the alphaE subunit, also known as CD103. CD103 expression requires high levels of TGF-beta and retinoic acid; in addition, expression of CD103 by T cells requires antigen recognition. Whether CD103 is found in the entheses has not yet been investigated. CD103 is expressed at high levels in the skin, eyes, and bowel but it is found in only very low levels in the bloodstream. CD8+ CD103+ T cells differ markedly from other CD103+ cells in that they are resident cells with no tendency to migrate and usually exert predominantly cytotoxic functions as opposed to regulatory functions. Several bacteria, such as Salmonella, can become dormant within the mucous membranes and/or their lymph nodes, where they use CD103+ dendritic cells and CD4+ CD103+ regulatory T cells (Tregs) to evade the immune response. This phenomenon could be studied in other tissues targeted by spondyloarthritides, where dormant microorganisms can migrate by using M2 macrophages as Trojan horses, since M2 macrophages express the CD103 ligand E-cadherin. Microorganism peptide recognition by CD8+ CD103+ T cells (which are overrepresented in psoriasis and joint fluid in some forms of spondyloarthritis) induces an inflammatory response that may be sufficient to transiently reverse the regulatory function of the CD103+ dendritic cells and CD4+ CD103+ T cells during disease flares. The sensitivity of these diseases to retinoids further supports a pathogenic role for transient CD103+ cell failure.

MiR-7b directly targets DC-STAMP causing suppression of NFATc1 and c-Fos signaling during osteoclast fusion and differentiation

DC-STAMP is a key regulating molecule of osteoclastogenesis and osteoclast precursor (OCP) fusion. Emerging lines of evidence showed that microRNAs play crucial roles in bone metabolism and osteoclast differentiation, but no microRNA has yet been reported to be directly related to OCPs fusion. Through a microarray, we found that the expression of miR-7b in RAW264.7 cells was significantly decreased after induction with M-CSF and RANKL. The overexpression of miR-7b in RAW264.7 cells attenuated the number of TRAP-positive cells number and the formation of multinucleated cells, whereas the inhibition of miR-7b enhanced osteoclastogenesis. Through a dual luciferase reporter assay, we confirmed that miR-7b directly targets DC-STAMP. Other fusogenic molecules, such as CD47, ATP6v0d2, and OC-STAMP, were detected to be down-regulated in accordance with the inhibition of DC-STAMP. Because DC-STAMP also participates in osteoclast differentiation through the ITAM-ITIM network, multiple osteoclast-specific genes in the ITAM-ITIM network were detected to identify how DC-STAMP is involved in this process. The results showed that molecules associated with the ITAM-ITIM network, such as NFATc1 and OSCAR, which are crucial in osteoclastogenesis, were consistently altered due to DC-STAMP inhibition. These findings suggest that miR-7b inhibits osteoclastogenesis and cell-cell fusion by directly targeting DC-STAMP. In addition, the inhibition of DC-STAMP and its downstream signals changed the expression of other fusogenic genes and key regulating genes, such as Nfatc1, c-fos, Akt, Irf8, Mapk1, and Traf6. In conclusion, our findings indicate that miR-7b may be a potential therapeutic target for the treatment of osteoclast-related bone disorders.

Gene expression profiling of peripheral blood cells: new insights into Ewing sarcoma biology and clinical applications

Ewing sarcoma (ES) is a group of highly aggressive small round cell tumors of bone or soft tissue with high metastatic potential and low cure rate. ES tumors are associated with a rapid osteolysis and necrosis. The currently accepted clinical prognostic parameters do not accurately predict survival of high-risk patients. Moreover, neither the subtype of EWS-FLI1/ERG in the tumor, nor the detection of fusion transcripts in the peripheral blood (PB) samples, has prognostic value in ES patients. We evaluated the prevalence of circulating tumor cells (CTCs) in 34 adult ES patients. Since CTCs were confirmed in only small subset of patients, we further explored the expression profiles of PB leukocytes using a panel of genes associated with immune system status and increased tumor invasiveness. Moreover, we analyzed the alterations of the routine blood tests in the examined cohort of patients and correlated our findings with the clinical outcome. A uniform decrease in ZAP70 expression in PB cells among all ES patients, as compared to healthy individuals, was observed. Monocytosis and the abnormal expression of CDH2 and CDT2 genes in the PB cells significantly correlated with poor prognosis in ES patients. Our study supports the previously proposed hypothesis of systemic nature of ES. Based on the PB cell expression profiles, we propose a mechanism by which immune system may be involved in intensification of osteoclastogenesis and disease progression in ES patients. Moreover, we demonstrate the prognostic value of molecular PB testing at the time of routine histopathological diagnosis.

A PKA-ezrin-Cx43 signaling complex controls gap junction communication and thereby trophoblast cell fusion

Cell fusion occurs as part of the differentiation of some cell types, including myotubes in muscle and osteoclasts in remodeling bone. In the human placenta, mononuclear cytotrophoblasts in a human chorionic gonadotropin (hCG)-driven process fuse to form multinucleated syncytia that allow the exchange of nutrients and gases between the maternal and fetal circulation. Experiments in which protein kinase A (PKA) is displaced from A-kinase anchoring proteins (AKAPs), or in which specific AKAPs are depleted by siRNA-mediated knockdown, point to ezrin as a scaffold required for hCG-, cAMP- and PKA-mediated regulation of the fusion process. By a variety of immunoprecipitation and immunolocalization experiments, we show that ezrin directs PKA to a molecular complex of connexin 43 (Cx43, also known as GJA1) and zona occludens-1 (ZO-1, also known as TJP1). A combination of knockdown experiments and reconstitution with ezrin or Cx43 with or without the ability to bind to its interaction partner or to PKA demonstrate that ezrin-mediated coordination of the localization of PKA and Cx43 is necessary for discrete control of Cx43 phosphorylation and hCG-stimulated gap junction communication that triggers cell fusion in cytotrophoblasts.

Cadherin-mediated cell-cell adhesion and signaling in the skeleton

Direct cell-to-cell interactions via cell adhesion molecules, in particular cadherins, are critical for morphogenesis, tissue architecture, and cell sorting and differentiation. Partially overlapping, yet distinct roles of N-cadherin (cadherin-2) and cadherin-11 in the skeletal system have emerged from mouse genetics and in vitro studies. Both cadherins are important for precursor commitment to the osteogenic lineage, and genetic ablation of Cdh2 and Cdh11 results in skeletal growth defects and impaired bone formation. While Cdh11 defines the osteogenic lineage, persistence of Cdh2 in osteoblasts in vivo actually inhibits their terminal differentiation and impairs bone formation. The action of cadherins involves both cell-cell adhesion and interference with intracellular signaling, and in particular the Wnt/β-catenin pathway. Both cadherin-2 and cadherin-11 bind to β-catenin, thus modulating its cytoplasmic pools and transcriptional activity. Recent data demonstrate that cadherin-2 also interferes with Lrp5/6 signaling by sequestering these receptors in inactive pools via axin binding. These data extend the biologic action of cadherins in bone forming cells, and provide novel mechanisms for development of therapeutic strategies aimed at enhancing bone formation.

Self-contact elimination by membrane fusion

Mutual, homophilic cell-cell adhesion between epithelial cells is required for proper maintenance of epithelial barrier function. Whereas opposing membranes from neighboring cells rapidly assemble junctional complexes, self-contacting membranes curiously do not, suggesting that cells have the ability to prevent the maturation of self-junctions. Using a self-contact-inducing microfabricated substrate, we show that self-contacts of normal epithelial cells are rapidly eliminated by membrane fusion between two opposing plasma membranes of a single cell. This membrane fusion is most frequently observed in E-cadherin-expressing epithelial cells, but not in fibroblasts. The efficiency of self-contact elimination depends on extracellular calcium concentration and the level of E-cadherin, suggesting that E-cadherin, although not required, enhances membrane fusion efficiency by bringing opposing membranes into close apposition to one another. Additionally, Rho-associated protein kinase inhibition decreases self-contact-induced membrane fusion of epithelial cells, suggesting that this fusion may be mechanically regulated through the actin-myosin network. This self-contact-induced membrane fusion is a key elimination mechanism for unwanted self-junctions and may be a feature of cell self-recognition.

Possible role of IRTKS in Tks5-driven osteoclast fusion

Podosomes and invadopodia seen in osteoclasts and cancer cells, respectively, are actin-rich membrane protrusions. We recently demonstrated that an adaptor protein, Tks5, which is an established regulator of invadopodia in cancer cells, drives osteoclast-osteoclast fusion as well as osteoclast-cancer cell fusion by generating circumferential podosomes/invadopodia. This finding revealed an unexpected potential of podosomes/invadopodia to act as fusion-competent protrusions. Fusion of biological membranes involves the intricate orchestration of various proteins and lipids. Recent literature suggests the importance of membrane curvature formation in lipid bilayer fusion. In this study, we investigated the expression of Bin-Amphiphysin-Rvs161/167 (BAR) domain superfamily proteins, which have membrane deforming activity, during osteoclastogenesis. We found that IRTKS was specifically induced during osteoclast fusion and interacted with Tks5, suggesting the role of IRTKS in the formation of fusion-competent protrusions via its BAR domain.

STATs and macrophage fusion

Macrophages play a pivotal role in host defense against multiple foreign materials such as bacteria, parasites and artificial devices. Some macrophage lineage cells, namely osteoclasts and foreign body giant cells (FBGCs), form multi-nuclear giant cells by the cell-cell fusion of mono-nuclear cells. Osteoclasts are bone-resorbing cells, and are formed in the presence of RANKL on the surface of bones, while FBGCs are formed in the presence of IL-4 or IL-13 on foreign materials such as artificial joints, catheters and parasites. Recently, fusiogenic mechanisms and the molecules required for the cell-cell fusion of these macrophage lineage cells were, at least in part, clarified. Dendritic cell specific transmembrane protein (DC-STAMP) and osteoclast stimulatory transmembrane protein (OC-STAMP), both of which comprise seven transmembrane domains, are required for both osteoclast and FBGC cell-cell fusion. STAT6 was demonstrated to be required for the cell-cell fusion of FBGCs but not osteoclasts. In this review, advances in macrophage cell-cell fusion are discussed.

Assessing cell fusion and cytokinesis failure as mechanisms of clone 9 hepatocyte multinucleation in vitro

In this in vitro model of hepatocyte multinucleation, separate cultures of rat Clone 9 cells are labeled with either red or green cell tracker dyes (Red Cell Tracker CMPTX or Vybrant CFDA SE Cell Tracer), plated together in mixed-color colonies, and treated with positive or negative control agents for 4 days. The fluorescent dyes become cell-impermeant after entering cells and are not transferred to adjacent cells in a population, but are inherited by daughter cells after fusion. The mixed-color cultures are then evaluated microscopically for multinucleation and analysis of the underlying mechanism (cell fusion/cytokinesis). Multinucleated cells containing only one dye have undergone cytokinesis failure, whereas dual-labeled multinucleated cells have resulted from fusion.

Short-term mechanical stress inhibits osteoclastogenesis via suppression of DC-STAMP in RAW264.7 cells

Mechanical stress is an important factor in bone homeostasis, which is maintained by a balance between bone resorption by osteoclasts and bone formation by osteoblasts. However, little is known about the effects of mechanical stress on osteoclast differentiation. In this study, we examined the effects of short-term mechanical stress on osteoclastogenesis by applying tensile force to RAW264.7 cells stimulated with receptor activator of nuclear factor-κB ligand (RANKL) using a Flexercell tension system. We counted the number of osteoclasts that were tartrate-resistant acid phosphatase (TRAP)-positive and multinucleated (two or more nuclei) with or without application of mechanical stress for 24 h. Osteoclast number was lower after mechanical stress compared with no mechanical stress. Furthermore, mechanical stress for up to 24 h caused downregulation of osteoclast-specific gene expression and fusion-related molecule [dendritic cell specific transmembrane protein (DC-STAMP), osteoclast stimulatory transmembrane protein (OC-STAMP), E-cadherin, Integrin αV and Integrin β3] mRNA levels. Protein expression of DC-STAMP decreased with mechanical stress for 24 h compared to the control without mechanical stress, whereas the expression of E-cadherin, Integrin αV and Integrin β3 was slightly decreased. Nuclear factor of activated T cells c1 (NFATc1) mRNA levels were decreased at 6 h and increased at 12 and 24 h compared with the control. The levels of NFATc2, NFATc3 mRNA did not change compared with the control group. By contrast, mechanical stress for 24 h significantly enhanced NFAT transcriptional activity compared with the control, despite a decrease in DC-STAMP mRNA and protein levels. These results suggest that short-term mechanical stress strongly inhibits osteoclastogenesis through the downregulation of DC-STAMP and other fusion-related molecules and that short-term mechanical stress induces a negative regulatory mechanism that cancels the enhancement of NFAT transcriptional activity.

Tks5-dependent formation of circumferential podosomes/invadopodia mediates cell-cell fusion

Tks5, a master regulator of invadopodia in cancer cells, is also crucial for osteoclast cell–cell fusion.

Osteoclasts fuse to form multinucleated cells during osteoclastogenesis. This process is mediated by dynamic rearrangement of the plasma membrane and cytoskeleton, and it requires numerous factors, many of which have been identified. The underlying mechanism remains obscure, however. In this paper, we show that Tks5, a master regulator of invadopodia in cancer cells, is crucial for osteoclast fusion downstream of phosphoinositide 3-kinase and Src. Expression of Tks5 was induced during osteoclastogenesis, and prevention of this induction impaired both the formation of circumferential podosomes and osteoclast fusion without affecting cell differentiation. Tyrosine phosphorylation of Tks5 was attenuated in Src^−/− osteoclasts, likely accounting for defects in podosome organization and multinucleation in these cells. Circumferential invadopodia formation in B16F0 melanoma cells was also accompanied by Tks5 phosphorylation. Co-culture of B16F0 cells with osteoclasts in an inflammatory milieu promoted the formation of melanoma–osteoclast hybrid cells. Our results thus reveal an unexpected link between circumferential podosome/invadopodium formation and cell–cell fusion in and beyond osteoclasts.

An essential role for STAT6-STAT1 protein signaling in promoting macrophage cell-cell fusion

Macrophage lineage cells such as osteoclasts and foreign body giant cells (FBGCs) form multinuclear cells by cell-cell fusion of mononuclear cells. Recently, we reported that two seven-transmembrane molecules, osteoclast stimulatory transmembrane protein (OC-STAMP) and dendritic cell-specific transmembrane protein (DC-STAMP), were essential for osteoclast and FBGC cell-cell fusion in vivo and in vitro. However, signaling required to regulate FBGC fusion remained largely unknown. Here, we show that signal transducer and activator of transcription 1 (STAT1) deficiency in macrophages enhanced cell-cell fusion and elevated DC-STAMP expression in FBGCs. By contrast, lack of STAT6 increased STAT1 activation, significantly inhibiting cell-cell fusion and decreasing OC-STAMP and DC-STAMP expression in IL-4-induced FBGCs. Furthermore, either STAT1 loss or co-expression of OC-STAMP/DC-STAMP was sufficient to induce cell-cell fusion of FBGCs without IL-4. We conclude that the STAT6-STAT1 axis regulates OC-STAMP and DC-STAMP expression and governs fusogenic mechanisms in FBGCs.

Osteoblastogenesis and osteoprotection enhanced by flavonolignan silibinin in osteoblasts and osteoclasts

Bone-remodeling imbalance induced by decreased osteoblastogenesis and increased bone resorption is known to cause skeletal diseases such as osteoporosis. Silibinin is the major active constituent of silymarin, the mixture of flavonolignans extracted from blessed milk thistle (Silybum marianum). Numerous studies suggest that silibinin is a powerful antioxidant and has anti-hepatotoxic properties and anti-cancer effects against carcinoma cells. This study investigated that silibinin had bone-forming and osteoprotective effects in in vitro cell systems of murine osteoblastic MC3T3-E1 cells and RAW 264.7 murine macrophages. MC3T3-E1 cells were incubated in osteogenic media in the presence of 1-20 µM silibinin up to 15 days. Silibinin accelerated cell proliferation and promoted matrix mineralization by enhancing bone nodule formation by calcium deposits. In addition, silibinin furthered the induction of osteoblastogenic biomarkers of alkaline phosphatase, collagen type 1, connective tissue growth factor, and bone morphogenetic protein-2. Differentiated MC3T3-E1 cells enhanced secretion of receptor activator of nuclear factor-κB ligand (RANKL) essential for osteoclastogenesis, which was reversed by silibinin. On the other hand, RAW 264.7 cells were pre-incubated with 1-20 µM silibinin for 5 days in the presence of RANKL. Non-toxic silibinin markedly attenuated RANK transcription and intracellular adhesion molecule-1 expression elevated by RANKL, thereby suppressing the differentiation of macrophages to multi-nucleated osteoclasts. It was also found that silibinin retarded tartrate-resistant acid phosphatase and cathepsin K induction and matrix metalloproteinase-9 activity elevated by RANKL through disturbing TRAF6-c-Src signaling pathways. These results demonstrate that silibinin was a potential therapeutic agent promoting bone-forming osteoblastogenesis and encumbering osteoclastic bone resorption.

Osteoclast stimulatory transmembrane protein and dendritic cell–specific transmembrane protein cooperatively modulate cell–cell fusion to form osteoclasts and foreign body giant cells

Cell–cell fusion is a dynamic phenomenon promoting cytoskeletal reorganization and phenotypic changes. To characterize factors essential for fusion of macrophage lineage cells, we identified the multitransmembrane protein, osteoclast stimulatory transmembrane protein (OC-STAMP), and analyzed its function. OC-STAMP–deficient mice exhibited a complete lack of cell–cell fusion of osteoclasts and foreign body giant cells (FBGCs), both of which are macrophage-lineage multinuclear cells, although expression of dendritic cell specific transmembrane protein (DC-STAMP), which is also essential for osteoclast/FBGC fusion, was normal. Crossing OC-STAMP–overexpressing transgenic mice with OC-STAMP–deficient mice restored inhibited osteoclast and FBGC cell–cell fusion seen in OC-STAMP–deficient mice. Thus, fusogenic mechanisms in macrophage-lineage cells are regulated via OC-STAMP and DC-STAMP.

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.

The role of T-cell leukemia translocation-associated gene protein in human tumorigenesis and osteoclastogenesis

Synovial tissues of patients with rheumatoid arthritis (RA) include factors regulating bone resorption, such as receptor activator NF-κB ligand (RANKL), TNF-α, IL-6, IL-17, and IFN-γ. However, in addition to these cytokines, other factors expressed in synovial tissues may play a role in regulating bone resorption. In 2009, we demonstrated that novel peptides from T-cell leukemia translocation-associated gene (TCTA) protein expressed in synovial tissues from patients with RA inhibit human osteoclastogenesis, preventing cellular fusion via the interaction between TCTA protein and a putative counterpart molecule. Only a few studies on the role of TCTA protein have been reported. Genomic Southern blots demonstrated a reduced TCTA signal in three of four small cell lung cancer cell lines, suggesting the loss of one of the two copies of the gene. In the current paper, we reviewed the roles of TCTA protein in lung cancer cell lines and human osteoclastogenesis.

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.