Formation and biological activities of foreign body giant cells in response to biomaterials

The integration of biomaterials in medical applications triggers the foreign body response (FBR), a multi-stage immune reaction characterized by the formation of foreign body giant cells (FBGCs). Originating from the fusion of monocyte/macrophage lineage cells, FBGCs are pivotal participants during tissue-material interactions. This review provides an in-depth examination of the molecular processes during FBGC formation, highlighting signaling pathways and fusion mediators in response to both exogenous and endogenous stimuli. Moreover, a wide range of material-specific characteristics, such as surface chemical and physical properties, has been proven to influence the fusion of macrophages into FBGCs. Multifaceted biological activities of FBGCs are also explored, with emphasis on their phagocytic capabilities and extracellular secretory functions, which profoundly affect the vascularization, degradation, and encapsulation of the biomaterials. This review further elucidates the heterogeneity of FBGCs and their diverse roles during FBR, as demonstrated by their distinct behaviors in response to different materials. By presenting a comprehensive understanding of FBGCs, this review intends to provide strategies and insights into optimizing biocompatibility and the therapeutic potential of biomaterials for enhanced stability and efficacy in clinical applications. STATEMENT OF SIGNIFICANCE: As a hallmark of the foreign body response (FBR), foreign body giant cells (FBGCs) significantly impact the success of implantable biomaterials, potentially leading to complications such as chronic inflammation, fibrosis, and device failure. Understanding the role of FBGCs and modulating their responses are vital for successful material applications. This review provides a comprehensive overview of the molecules and signaling pathways guiding macrophage fusion into FBGCs. By elucidating the physical and chemical properties of materials inducing distinct levels of FBGCs, potential strategies of materials in modulating FBGC formation are investigated. Additionally, the biological activities of FBGCs and their heterogeneity in responses to different material categories in vivo are highlighted in this review, offering crucial insights for improving the biocompatibility and efficacy of biomaterials.

Beyond resorption: osteoclasts as drivers of bone formation

Emerging evidence illustrates that osteoclasts (OCs) play diverse roles beyond bone resorption, contributing significantly to bone formation and regeneration. Despite this, OCs remain mysterious cells, with aspects of their lifespan-from origin, fusion, alterations in cellular characteristics, to functions-remaining incompletely understood. Recent studies have identified that embryonic osteoclastogenesis is primarily driven by osteoclast precursors (OCPs) derived from erythromyeloid progenitors (EMPs). These precursor cells subsequently fuse into OCs essential for normal bone development and repair. Postnatally, hematopoietic stem cells (HSCs) become the primary source of OCs, gradually replacing EMP-derived OCs and assuming functional roles in adulthood. The absence of OCs during bone development results in bone structure malformation, including abnormal bone marrow cavity formation and shorter long bones. Additionally, OCs are reported to have intimate interactions with blood vessels, influencing bone formation and repair through angiogenesis regulation. Upon biomaterial implantation, activation of the innate immune system ensues immediately. OCs, originating from macrophages, closely interact with the immune system. Furthermore, evidence from material-induced bone formation events suggests that OCs are pivotal in these de novo bone formation processes. Nevertheless, achieving a pure OC culture remains challenging, and interpreting OC functions in vivo faces difficulties due to the presence of other multinucleated cells around bone-forming biomaterials. We here describe the fusion characteristics of OCPs and summarize reliable markers and morphological changes in OCs during their fusion process, providing guidance for researchers in identifying OCs both in vitro and in vivo. This review focuses on OC formation, characterization, and the roles of OCs beyond resorption in various bone pathophysiological processes. Finally, therapeutic strategies targeting OCs are discussed.

Transformable Carbonate Apatite Chains as a Novel Type of Bone Graft

The aging global population is generating an ever-increasing demand for bone regeneration. Various materials, including blocks, granules, and sponges, are developed for bone regeneration. However, blocks require troublesome shaping and exhibit poor bone-defect conformities; granules migrate into the surrounding tissues during and after filling of the defect, causing handling difficulties and complications; and sponges contain polymers that are subject to religious restrictions, lack osteoconductivity, and may cause inflammation and allergies. Herein, carbonate apatite chains that overcome the limitations of conventional materials are presented. Although carbonate apatite granules migrate, causing inflammation and ectopic calcification, the chains remain in the defects without causing any complications. The chains conform to the defect shape and transform into 3D porous structures, resulting in faster bone regeneration than that observed using granules. Thus, these findings indicate that even traditional calcium phosphates materials can be converted to state-of-the-art materials via shape control.

Transcriptomic Signature and Pro-Osteoclastic Secreted Factors of Abnormal Bone-Marrow Stromal Cells in Fibrous Dysplasia

Fibrous dysplasia (FD) is a mosaic skeletal disorder caused by somatic activating variants of GNAS encoding for Gαs and leading to excessive cyclic adenosine monophosphate signaling in bone-marrow stromal cells (BMSCs). The effect of Gαs activation in the BMSC transcriptome and how it influences FD lesion microenvironment are unclear. We analyzed changes induced by Gαs activation in the BMSC transcriptome and secretome. RNAseq analysis of differential gene expression of cultured BMSCs from patients with FD and healthy volunteers, and from an inducible mouse model of FD, was performed, and the transcriptomic profiles of both models were combined to build a robust FD BMSC genetic signature. Pathways related to Gαs activation, cytokine signaling, and extracellular matrix deposition were identified. To assess the modulation of several key secreted factors in FD pathogenesis, cytokines and other factors were measured in culture media. Cytokines were also screened in a collection of plasma samples from patients with FD, and positive correlations of several cytokines to their disease burden score, as well as to one another and bone turnover markers, were found. These data support the pro-inflammatory, pro-osteoclastic behavior of FD BMSCs and point to several cytokines and other secreted factors as possible therapeutic targets and/or circulating biomarkers for FD.

Tm4sf19 deficiency inhibits osteoclast multinucleation and prevents bone loss

BACKGROUND

Multinucleation is a hallmark of osteoclast formation and has a unique ability to resorb bone matrix. During osteoclast differentiation, the cytoskeleton reorganization results in the generation of actin belts and eventual bone resorption. Tetraspanins are involved in adhesion, migration and fusion in various cells. However, its function in osteoclast is still unclear. In this study, we identified Tm4sf19, a member of the tetraspanin family, as a regulator of osteoclast function.

MATERIALS AND METHODS

We investigate the effect of Tm4sf19 deficiency on osteoclast differentiation using bone marrow-derived macrophages obtained from wild type (WT), Tm4sf19 knockout (KO) and Tm4sf19 LELΔ mice lacking the large extracellular loop (LEL). We analyzed bone mass of young and aged WT, KO and LELΔ mice by μCT analysis. The effects of Tm4sf19 LEL-Fc fusion protein were accessed in osteoclast differentiation and osteoporosis animal model.

RESULTS

We found that deficiency of Tm4sf19 inhibited osteoclast function and LEL of Tm4sf19 was responsible for its function in osteoclasts in vitro. KO and LELΔ mice exhibited higher trabecular bone mass compared to WT mice. We found that Tm4sf19 interacts with integrin αvβ3 through LEL, and that this binding is important for cytoskeletal rearrangements in osteoclast by regulating signaling downstream of integrin αvβ3. Treatment with LEL-Fc fusion protein inhibited osteoclast function in vitro and administration of LEL-Fc prevented bone loss in an osteoporosis mouse model in vivo.

CONCLUSION

We suggest that Tm4sf19 regulates osteoclast function and that LEL-Fc may be a promising drug to target bone destructive diseases caused by osteoclast hyper-differentiation.

Monocyte Differentiation on Atomic Layer-Deposited (ALD) Hydroxyapatite Coating on Titanium Substrate

Hydroxyapatite (HA; Ca₁₀(PO₄)₆(OH)₂) coating of bone implants has many beneficial properties as it improves osseointegration and eventually becomes degraded and replaced with new bone. We prepared HA coating on a titanium substrate with atomic layer deposition (ALD) and compared monocyte differentiation and material resorption between ALD-HA and bone. After stimulation with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa-B ligand (RANKL), human peripheral blood monocytes differentiated into resorbing osteoclasts on bovine bone, but non-resorbing foreign body cells were observed on ALD-HA. The analysis of the topography of ALD-HA and bone showed no differences in wettability (water contact angle on ALD-HA 86.2° vs. 86.7° on the bone), but the surface roughness of ALD-HA (Ra 0.713 µm) was significantly lower compared to bone (Ra 2.30 µm). The cellular reaction observed on ALD-HA might be a consequence of the topographical properties of the coating. The absence of resorptive osteoclasts on ALD-HA might indicate inhibition of their differentiation or the need to modify the coating to induce osteoclast differentiation.

Osteoclastogenesis of human peripheral blood, bone marrow, and cord blood monocytes

Osteoclasts are multinucleated bone resorbing cells that can be differentiated from human monocytes in vitro. There are few studies comparing osteoclastogenesis of different monocyte sources. We compared monocytes from human bone marrow (BM), peripheral blood (PB), and umbilical cord blood (CB) and their osteoclastogenic potential by culturing them with RANKL (20 and 80 ng/ml) and M-CSF (10 ng/ml) for 14 days. We also cultured cells without growth factors, as umbilical cord blood monocytes have been reported to be able to fuse spontaneously into osteoclasts. The data was analysed on d4, d8, d11, and d14. After culture with RANKL and M-CSF, all types of cell cultures developed TRACP -positive multinuclear cells that were able to form resorption pits on human bone slices. Only occasional multinuclear cells and small infrequent resorbed areas could be found in PB and CB-derived cultures without growth factors. BM-derived cells formed greater resorption areas than PB- and CB-derived monocytes. The greatest monocyte population in BM samples were intermediate (CD14⁺⁺CD16⁺) and in PB and CB classical monocytes (76.3% and 54.4%, respectively). In conclusion, our data demonstrates that bone resorbing osteoclasts can be differentiated from BM, PB and CB. However, the osteoclast precursor origin can affect the osteoclast properties and function.

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.

Apoptotic mesenchymal stromal cells support osteoclastogenesis while inhibiting multinucleated giant cells formation in vitro

In bone regeneration induced by the combination of mesenchymal stromal cells (MSCs) and calcium-phosphate (CaP) materials, osteoclasts emerge as a pivotal cell linking inflammation and bone formation. Favorable outcomes are observed despite short-term engraftments of implanted MSCs, highlighting their major paracrine function and the possible implication of cell death in modulating their secretions. In this work, we focused on the communication from MSCs towards osteoclasts-like cells in vitro. MSCs seeded on a CaP biomaterial or undergoing induced apoptosis produced a conditioned media favoring the development of osteoclasts from human CD14+ monocytes. On the contrary, MSCs’ apoptotic secretion inhibited the development of inflammatory multinucleated giant cells formed after IL-4 stimulation. Components of MSCs’ secretome before and after apoptotic stress were compared using mass spectrometry-based quantitative proteomics and a complementary immunoassay for major cytokines. CXCR-1 and CXCR-2 ligands, primarily IL-8/CXCL-8 but also the growth-regulated proteins CXCL-1, -2 or -3, were suggested as the major players of MSCs’ pro-osteoclastic effect. These findings support the hypothesis that osteoclasts are key players in bone regeneration and suggest that apoptosis plays an important role in MSCs’ effectiveness.

CD13 is a critical regulator of cell-cell fusion in osteoclastogenesis

The transmembrane aminopeptidase CD13 is highly expressed in cells of the myeloid lineage, regulates dynamin-dependent receptor endocytosis and recycling and is a necessary component of actin cytoskeletal organization. Here, we show that CD13-deficient mice present a low bone density phenotype with increased numbers of osteoclasts per bone surface, but display a normal distribution of osteoclast progenitor populations in the bone marrow and periphery. In addition, the bone formation and mineral apposition rates are similar between genotypes, indicating a defect in osteoclast-specific function in vivo. Lack of CD13 led to exaggerated in vitro osteoclastogenesis as indicated by significantly enhanced fusion of bone marrow-derived multinucleated osteoclasts in the presence of M-CSF and RANKL, resulting in abnormally large cells containing remarkably high numbers of nuclei. Mechanistically, while expression levels of the fusion-regulatory proteins dynamin and DC-STAMP1 must be downregulated for fusion to proceed, these are aberrantly sustained at high levels even in CD13-deficient mature multi-nucleated osteoclasts. Further, the stability of fusion-promoting proteins is maintained in the absence of CD13, implicating CD13 in protein turnover mechanisms. Together, we conclude that CD13 may regulate cell–cell fusion by controlling the expression and localization of key fusion regulatory proteins that are critical for osteoclast fusion.

The Macrophage-Osteoclast Axis in Osteoimmunity and Osteo-Related Diseases

Osteoimmunity is involved in regulating the balance of bone remodeling and resorption, and is essential for maintaining normal bone morphology. The interaction between immune cells and osteoclasts in the bone marrow or joint cavity is the basis of osteoimmunity, in which the macrophage-osteoclast axis plays a vital role. Monocytes or tissue-specific macrophages (macrophages resident in tissues) are an important origin of osteoclasts in inflammatory and immune environment. Although there are many reports on macrophages and osteoclasts, there is still a lack of systematic reviews on the macrophage-osteoclast axis in osteoimmunity. Elucidating the role of the macrophage-osteoclast axis in osteoimmunity is of great significance for the research or treatment of bone damage caused by inflammation and immune diseases. In this article, we introduced in detail the concept of osteoimmunity and the mechanism and regulators of the differentiation of macrophages into osteoclasts. Furthermore, we described the role of the macrophage-osteoclast axis in typical bone damage caused by inflammation and immune diseases. These provide a clear knowledge framework for studying macrophages and osteoclasts in inflammatory and immune environments. And targeting the macrophage-osteoclast axis may be an effective strategy to treat bone damage caused by inflammation and immune diseases.

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.

Osteoclast Multinucleation: Review of Current Literature

Multinucleation is a hallmark of osteoclast maturation. The unique and dynamic multinucleation process not only increases cell size but causes functional alterations through reconstruction of the cytoskeleton, creating the actin ring and ruffled border that enable bone resorption. Our understanding of the molecular mechanisms underlying osteoclast multinucleation has advanced considerably in this century, especially since the identification of DC-STAMP and OC-STAMP as “master fusogens”. Regarding the molecules and pathways surrounding these STAMPs, however, only limited progress has been made due to the absence of their ligands. Various molecules and mechanisms other than the STAMPs are involved in osteoclast multinucleation. In addition, several preclinical studies have explored chemicals that may be able to target osteoclast multinucleation, which could enable us to control pathogenic bone metabolism more precisely. In this review, we will focus on recent discoveries regarding the STAMPs and other molecules involved in osteoclast multinucleation.

Multinucleated giant cell phenotype in response to stimulation

Macrophages fuse into multinucleated giant cells (MGC) in many pathological conditions. Despite MGC correlations with granulomas, their functional contribution to inflammation is relatively unknown. An in vitro mouse model of IL-4-induced bone marrow-derived macrophage fusion and microfiltration were used to generate enriched MGC and macrophage populations. Phenotypes were compared in response to well-known inflammatory stimuli, including lipopolysaccharide and crocidolite asbestos. Surface markers were assessed by flow cytometry: CD11b, CD11c, F4/80, and MHC II. Secreted cytokines were assessed by multiplex immunoassay: IFN-γ, IL-1β, IL-6, TNF-α, IL-10, IL-13, and IL-33. Results show that MGC maintained macrophage surface protein expression but lost the ability to produce a cytokine response. This suggests a potentially beneficial role of MGC in isolating the host from a foreign body without contributing to excessive inflammation. This study and future research using other stimulants and environments are important to gaining a fundamental MGC cell biology understanding. This will inform approaches to controlling the foreign body response to particle exposure, medical implants, and many diseases associated with granulomas.

Osteoclast stimulatory transmembrane protein (OC-STAMP) is a promising molecular prognostic indicator for multiple myeloma

BACKGROUND

Currently, the prognostic stratification and therapeutic evaluation systems for multiple myeloma (MM) lack specific molecular indicators. OC-STAMP is a new gene and is also highly expressed in MM.

METHODS

A total of 160 MM patients have been investigated with both quantitative reverse transcription PCR (RT-qPCR), flow cytometry (FCM) and cytogenetic FISH on the same mononuclear cells isolated from bone marrow specimens.

RESULTS

We found that OC-STAMP mRNA levels were significantly higher in newly diagnosed cases of MM than in healthy donors (median, 0.52% vs. 0.02%, P < .001). Moreover, the changes in the OC-STAMP mRNA levels paralleled the disease stages and minimal residual disease, as detected by FCM. Furthermore, we found that patients with high OC-STAMP mRNA levels were more likely to develop ≥3 bone lesions, be diagnosed with Durie-Salmon stages III, and have the P53 (17p13) deletion. In addition, advanced stage patients with high OC-STAMP mRNA levels had a lower 4-year progression-free survival (5.6% vs. 22.9%, P = .0055) and a worse 4-year overall survival (25.8% vs. 48.8%, P = .0137) compared to patients with low mRNA levels of this indicator.

CONCLUSIONS

OC-STAMP may be a promising molecular indicator to monitor treatment effects and participate in the prognostic stratification of MM.

Factors influencing multinucleated giant cell formation in vitro

Macrophages fuse together to form multinucleated giant cells (MGC) in granulomas associated with various pathological conditions. Improved in vitro methods are required to better enable investigations of MGC biology and potential contribution to disease. There is a need for standardization of MGC quantification, purification of MGC populations, and characterization of how cell culture variables influence MGC formation. This study examined solutions to address these needs while providing context with other current and alternative methods. Primary mouse bone marrow-derived macrophages were treated with interleukin-4, a cytokine known to induce fusion into MGC. This model was used to systematically assess the influence of cell stimulant timing, cell seeding density, colony stimulating factors, and culture vessel type. Results indicated that MGC formation is greatly impacted by alterations in certain culture variables. An assessment of previously published research showed that these culture conditions varied widely between different laboratories, which may explain inconsistencies in the literature. A particularly novel and unexpected observation was that MGC formation appears to be greatly increased by silicone, which is a component of a chamber slide system commonly used for MGC studies. The most successful quantification method was fluorescent staining with semi-automated morphological evaluation. The most successful enrichment method was microfiltration. Overall, this study takes steps toward standardizing in vitro methods, enhancing replicability, and guiding investigators attempting to culture, quantify, and enrich MGC.

Resorption of the calcium phosphate layer on S53P4 bioactive glass by osteoclasts

Clinically, S53P4 bioactive glass (BAG) has shown very promising results in bone infection treatment, but it is also known to degrade very slowly in vivo. To evaluate which mechanisms (cellular or dissolution) can play a role in the degradation of S53P4 BAG and S53P4 BAG putty, in vitro degradation experiments at different pH (7.4 and 4.6) were performed. Micro computed tomography showed a rapid dissolution of the synthetic binder in the putty formulation, within 12 h is simulated body fluid (pH = 7.4), leaving behind only loose granules. Therefore the degradation of the loose granules was investigated further. Significant weight loss was observed and ion chromatography showed that Ca2+, Na+ and PO43- ions were released from S54P4 BAG granules in the two fluids. It was observed that the weight loss and ion release were increased when the pH of the fluid was decreased to 4.6. Osteoclasts are known to create such a low pH when resorbing bone and therefore their capacity to degrade S53P4 surfaces were studied as well. Scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed that osteoclasts were able to create resorption pits in the calcium phosphate layer on S53P4 BAG surfaces. The silica of the BAG, located underneath the calcium phosphate, seemed to hinder further osteclastic resorption of the material. To our knowledge we were the first to observe actively resorbing osteoclasts on S53P4 bioactive glass surfaces, in vitro. Future research is needed to define the specific role osteoclasts play in the degradation of BAG in vivo.

Anti-osteoclastogenic effect of epigallocatechin gallate-functionalized gold nanoparticles in vitro and in vivo

Background: Epigallocatechin gallate (EGCG), the major anti-inflammatory compound in green tea, has been shown to suppress osteoclast (OC) differentiation. However, the low aqueous solubility of EGCG always leads to poor bioavailability, adverse effects, and several drawbacks for clinical applications.

Purpose: In this study, we synthesized EGCG-capped gold nanoparticles (EGCG-GNPs) to solve the drawbacks for clinical uses of EGCG in bone destruction disorders by direct reduction of HAuCl₄ in EGCG aqueous solution.

Methods and Results: The obtained EGCG-GNPs were negatively charged and spherical. Theoretical calculation results suggested that EGCG was released from GNPs in an acidic environment. Cellular uptake study showed an obviously large amount of intracellular EGCG-GNPs without cytotoxicity. EGCG-GNPs exhibited better effects in reducing intracellular reactive oxygen species levels than free EGCG. A more dramatic anti-osteoclastogenic effect induced by EGCG-GNPs than free EGCG was observed in lipopolysaccharide (LPS)-stimulated bone marrow macrophages, including decreased formation of TRAP-positive multinuclear cells and actin rings. Meanwhile, EGCG-GNPs not only suppressed the mRNA expression of genetic markers of OC differentiation but also inhibited MAPK signaling pathways. Furthermore, we confirmed that EGCG-GNPs greatly reversed bone resorption in the LPS-induced calvarial bone erosion model in vivo, which was more effective than applying free EGCG, specifically in inhibiting the number of OCs, improving bone density, and preventing bone loss.

Conclusion: EGCG-GNPs showed better anti-osteoclastogenic effect than free EGCG in vitro and in vivo, indicating their potential in anti-bone resorption treatment strategy.

Immune Modulation by Transplanted Calcium Phosphate Biomaterials and Human Mesenchymal Stromal Cells in Bone Regeneration

A wide variety of biomaterials have been developed as both stabilizing structures for the injured bone and inducers of bone neoformation. They differ in chemical composition, shape, porosity, and mechanical properties. The most extensively employed and studied subset of bioceramics are calcium phosphate materials (CaPs). These materials, when transplanted alongside mesenchymal stem cells (MSCs), lead to ectopic (intramuscular and subcutaneous) and orthotopic bone formation in preclinical studies, and effective fracture healing in clinical trials. Human MSC transplantation in pre-clinical and clinical trials reveals very low engraftment in spite of successful clinical outcomes and their therapeutic actions are thought to be primarily through paracrine mechanisms. The beneficial role of transplanted MSC could rely on their strong immunomodulatory effect since, even without long-term engraftment, they have the ability to alter both the innate and adaptive immune response which is critical to facilitate new bone formation. This study presents the current knowledge of the immune response to the implantation of CaP biomaterials alone or in combination with MSC. In particular the central role of monocyte-derived cells, both macrophages and osteoclasts, in MSC-CaP mediated bone formation is emphasized. Biomaterial properties, such as macroporosity and surface microstructure, dictate the host response, and the ultimate bone healing cascade. Understanding intercellular communications throughout the inflammation, its resolution and the bone regeneration phase, is crucial to improve the current therapeutic strategies or develop new approaches.

Tatarinan N inhibits osteoclast differentiation through attenuating NF-κB, MAPKs and Ca2+-dependent signaling

Osteoclasts are multinucleated cells that originate from hemopoietic stem cells. Targeting over activated osteoclasts is thought to be an effective therapeutic approach to osteoporosis. In a previous study, we reported that Tatarinan O, a lignin-like compound, suppressed RANKL-induced osteoclastogenesis. In this study, we further examined the effects on osteoclast formation of three lignin-like compounds including Tatarinan N (TN), Tatarinan U (TU) and Tatarinan V (TV), all containing a common structure of asarone. We found that only TN suppressed RANKL-induced osteoclast differentiation, bone resorption pit formation and F-acting ring formation. TU and TV did not influence RANKL-induced osteoclastogenesis. We also found that TN dose-dependently inhibited the expression of osteoclastogenesis-associated genes, including TRAP, cathepsin K and MMP-9. Furthermore, we found that TN down-regulated the key transcription factor NFATc1 and c-Fos by preventing the activation of NF-κB and phosphorylation of MAPKs including ERK1/2 and p38 but not JNK. TN attenuated calcineurin expression via suppression of the Btk-PLCγ2 cascade and reduction of intracellular Ca²⁺, modulating NFATc1 activation. Taking together, our results indicated that TN might have therapeutic potential for osteoporosis.

Visual Osteoclast Fusion via A Fluorescence Method

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

Common signalling pathways in macrophage and osteoclast multinucleation

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

Characterization of different osteoclast phenotypes in the progression of bone invasion by oral squamous cell carcinoma

The present study aimed to characterize different phenotypes of osteoclasts in the progression of bone invasion by oral squamous cell carcinoma (OSCC). A local bone invasion model of OSCC was established by injecting SCC25 human OSCC cells into the center of calvariae in nude mice, and all mice were found to have a typical bone resorption area. Staining for tartrate-resistant acid phosphatase (TRAP) revealed various types of giant osteoclasts in the tumour-bone interface. Bone marrow cells (BMCs) were isolated from the nude mice for primary osteoclast culture, but only a few giant osteoclasts were generated. Additionally, special blood centrifuge tubes were utilized to obtain large numbers of peripheral blood mononuclear cells (PBMCs). Using magnetic activated cell sorting (MACS) and the cytokines colony-stimulating factor (CSF) and receptor activator of nuclear factor-κb ligand (RANKL), we differentiated human osteoclasts from CD14+ monocytes of PBMCs. Bone resorption was further confirmed by a bone resorption assay. Finally, Transwell inserts were used for indirect cell co-culture of SCC25 cells and CD14+ monocytes. Expression of specific osteoclast markers was detected by real-time PCR and western blotting. After co-culture for 3 and 6 days, conditioned medium (CM) of SCC25 cells stimulated the expression of osteoclast markers, and additional osteoclasts were detected through staining of TRAP and F-actin. In the present study distinct osteoclast phenotypes were observed in the established bone invasion animal model, and were confirmed using various primary osteoclast cultures. CM of OSCC cells may promote the expression of osteoclast markers and induce the differentiation of monocytes to mature osteoclasts, which can resorb adjacent bone tissue.

Osteoclast stimulatory transmembrane protein induces a phenotypic switch in macrophage polarization suppressing an M1 pro-inflammatory state

Macrophages are the key cells in metabolic syndrome and are also a risk factor for metabolic disease. Macrophages have different functions and transcriptional profiles, but all are required for maintaining homeostasis. It is well known that macrophages play a key role in inflammation and early atherogenesis, and are present in two phenotypes: pro-inflammatory (M1) and anti-inflammatory (M2). Osteoclast stimulatory transmembrane protein (oc-stamp) is a multiple-pass transmembrane protein; however, its function remains unclear. In this study, we explored the role of oc-stamp in macrophages physiology. The results showed that oc-stamp was notably decreased under LPS and IFN-γ stimulation, while it was increased with IL-4 treatment. Furthermore, oc-stamp induced a phenotypic switch in macrophage polarization, suppressing the M1 pro-inflammatory state in the overexpression group, and promoting the M1 pro-inflammatory state in the knockdown group. Further study revealed that oc-stamp regulated macrophage polarization possibly via STAT6. Taken together, our results are the first to demonstrate that oc-stamp may play an important role in macrophage polarization and inhibit the M1 pro-inflammatory state.

Factors that Affect the Osteoclastogenesis of RAW264.7 Cells

Osteoclasts and their activity are key regulators of bone formation. However, studying osteoclasts is difficult. Primary osteoclast cultures are difficult to maintain and isolate. Also, the amount of cells that are isolated and their properties depend on the origin and differentiation protocols. These protocols are usually developed in a distinct lab and multiple protocols exist. A cell line to study osteoclasts and a thorough study of osteoclast differentiation and culturing is currently lacking. The RAW264.7 cell line is most commonly used to study osteoclast differentiation and its signaling pathways. RAW264.7 cells are not a homogenous cell line. They don’t often exclusively differentiate into osteoclast but also into other multinucleated cells as well including macrophage polykaryons. A challenge of culturing RAW264.7 cells are culture conditions. Different conditions can affect survival, proliferation, and differentiation of RAW264.7 cells. Currently published protocols of culturing RAW264.7 cells often assume multinucleated cells that have three or more nuclei with distinguished osteoclast characteristics (such as TRAP+) as osteoclasts. However, osteoclasts and macrophage polykaryons are almost indistinguishable under a light microscope (TRAP+ with three or more nuclei). The goal of this paper is to examine the effect of culture conditions on the osteoclastogenesis ability of RAW264.7 cells. The focus will be on establishing the crucial parameters for culture density, time of stimulation, RANKL, and L-Gln concentrations. Although we are unable to establish the condition that offers a homogenous population of osteoclasts; nevertheless, we are able to identify the optimal conditions at which osteoclasts are found to be more than macrophage polykaryons. Finally, this article also demonstrates that osteoclasts and macrophage polykaryons can be distinguished by immunofluorescence staining for cathepsin K.

Multinucleated giant cells in the implant bed of bone substitutes are foreign body giant cells-New insights into the material-mediated healing process

In addition to macrophages, multinucleated giant cells (MNGCs) are involved in the tissue reaction to a variety of biomaterials. Especially in the case of bone substitute materials it has been assumed that the MNGCs are osteoclasts, based on the chemical and physical similarity of many materials to the calcified matrix and the bony environment in which they are used. However, many studies indicate that these cells belong to the cell line of the foreign body giant cells (FBGCs), which are of "inflammatory origin", although they have been shown to possess both a pro- and also anti-inflammatory phenotype. Moreover, no information is available about their role in the tissue reaction to bone substitute materials. The present study was conducted to analyze the origin of MNGCs in the implant beds of a synthetic and a xenogeneic bone substitute and focused on the application of immunohistochemical methods. Two antibodies against integrin molecules specific for osteoclasts (β-3 integrin) or FBGCs (β-2 integrin) were used to distinguish both giant cell types. The results of the present study indicate that the MNGCs induced by both kinds of bone substitutes are FBGCs, as they express only β-2 integrin in contrast to the osteoclasts outside of the immediate implantation areas, which only demonstrate β-3 integrin expression. These data give new insight into the tissue reaction to both xenogeneic and synthetic bone substitutes. Based on this new knowledge further research concerning the proteomic profile of the FBGCs especially based on the different physicochemical properties of bone substitutes is necessary. This may show that specific characteristics of bone substitutes may exhibit a substantial influence on the regeneration process via the expression of anti-inflammatory molecules by FBGCs. Based on this information it may be possible to formulate and choose bone substitutes that can guide the process of bone tissue regeneration on the molecular level. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1105-1111, 2017.

Sulforaphane inhibits osteoclast differentiation by suppressing the cell-cell fusion molecules DC-STAMP and OC-STAMP

Sulforaphane (SFN), a kind of isothiocyanate, is derived from broccoli sprouts. It has anti-tumor, anti-inflammatory, and anti-oxidation activity. The molecular function of SFN in the inhibition of osteoclast differentiation is not well-documented. In this study, we assessed the effect of SFN on osteoclast differentiation in vitro. SFN inhibited osteoclast differentiation in both bone marrow cells and RAW264.7 cells. Key molecules involved in the inhibitory effects of SFN on osteoclast differentiation were determined using a microarray analysis, which showed that SFN inhibits osteoclast-associated genes, such as osteoclast-associated receptor (OSCAR), nuclear factor of activated T cells cytoplasmic-1, tartrate-resistant acid phosphatase, and cathepsin K. Moreover, the mRNA expression levels of the cell-cell fusion molecules dendritic cell specific transmembrane protein (DC-STAMP) and osteoclast stimulatory transmembrane protein (OC-STAMP) were strongly suppressed in cells treated with SFN. Furthermore, SFN increased the phosphorylation of signal transducer and activator of transcription 1 (STAT1), a regulator of macrophage and osteoclast cell fusion. Thus, our data suggested that SFN significantly inhibits the cell-cell fusion molecules DC-STAMP and OC-STAMP by inducing the phosphorylation of STAT1 (Tyr701), which might be regulated by interactions with OSCAR.

Strontium-Substituted Bioceramics Particles: A New Way to Modulate MCP-1 and Gro-α Production by Human Primary Osteoblastic Cells

Background: To avoid morbidity and limited availability associated with autografts, synthetic calcium phosphate (CaP) ceramics were extensively developed and used as bone filling materials. Controlling their induced-inflammatory response nevertheless remained a major concern. Strontium-containing CaP ceramics were recently demonstrated for impacting cytokines’ secretion pattern of human primary monocytes. The present study focuses on the ability of strontium-containing CaP to control the human primary bone cell production of two major inflammatory and pro-osteoclastogenic mediators, namely MCP-1 and Gro-α, in response to ceramics particles. Methods: This in vitro study was performed using human primary osteoblasts in which their response to ceramics was evaluated by PCR arrays, antibody arrays were used for screening and real-time PCR and ELISA for more focused analyses. Results: Study of mRNA and protein expression highlights that human primary bone cells are able to produce these inflammatory mediators and reveal that the adjunction of CaP in the culture medium leads to their enhanced production. Importantly, the current work determines the down-regulating effect of strontium-substituted CaP on MCP-1 and Gro-α production. Conclusion: Our findings point out a new capability of strontium to modulate human primary bone cells’ communication with the immune system.

Giant cells around bone biomaterials: Osteoclasts or multi-nucleated giant cells?

Recently accumulating evidence has put into question the role of large multinucleated giant cells (MNGCs) around bone biomaterials. While cells derived from the monocyte/macrophage lineage are one of the first cell types in contact with implanted biomaterials, it was originally thought that specifically in bone tissues, all giant cells were bone-resorbing osteoclasts whereas foreign body giant cells (FBGCs) were found associated with a connective tissue foreign body reaction resulting in fibrous encapsulation and/or material rejection. Despite the great majority of bone grafting materials routinely found with large osteoclasts, a special subclass of bone biomaterials has more recently been found surrounded by large giant cells virtually incapable of resorbing bone grafts even years after their implantation. While original hypotheses believed that a 'foreign body reaction' may be taking place, histological data retrieved from human samples years after their implantation have put these original hypotheses into question by demonstrating better and more stable long-term bone volume around certain bone grafts. Exactly how or why this 'special' subclass of giant cells is capable of maintaining long-term bone volume, or methods to scientifically distinguish them from osteoclasts remains extremely poorly studied. The aim of this review article was to gather the current available literature on giant cell markers and differences in expression patterns between osteoclasts and MNGCs utilizing 19 specific markers including an array of CD-cell surface markers. Furthermore, the concept of now distinguishing between pro-inflammatory M1-MNGCs (previously referred to as FBGCs) as well as wound-healing M2-MNGCs is introduced and discussed.

STATEMENT OF SIGNIFICANCE

This review article presents 19 specific cell-surface markers to distinguish between osteoclasts and MNGCs including an array of CD-cell surface markers. Furthermore, the concept of now distinguishing between pro-inflammatory M1-MNGCs (often previously referred to as FBGCs) as well as wound-healing M2-MNGCs is introduced and discussed. The proposed concepts and guidelines aims to guide the next wave of research facilitating the differentiation between osteoclast/MNGCs formation, as well as provides the basis for increasing our understanding of the exact function of MNGCs in bone tissue/biomaterial homeostasis.

RAW 264.7 co-cultured with ultra-high molecular weight polyethylene particles spontaneously differentiate into osteoclasts: an in vitro model of periprosthetic osteolysis

Wear-particle osteolysis affects prosthesis survival leading to implant loosening up to 70% of revisions. Therapeutic strategies are increasing, however alternative testing methods to experimentally evaluate such treatments are lacking. The aim of this study was to reproduce an in vitro osteolysis model recapitulating the events that, starting from the exposure of macrophages to polyethylene, lead to the establishment of osteoclastogenesis and inflammation. Responses to polyethylene, at 3 and 7 days, in a macrophage cell line, RAW 264.7, were determined by DNA quantification, immunofluorescence, pit assay, gene expression, cytokine production and NF-kB activation. Results showed that 3 days exposure to particles could induce a significant production of Tumor Necrosis Factor alpha (p < 0.0005) and Prostaglandin E₂ (p < 0.005) compared to controls. Particles also induced macrophages to spontaneously differentiate into mature and active osteoclasts, in terms of identification of multinucleated cells by Phalloidin staining and by the analysis of osteoclast-specific gene markers. In particular, at 3 days polyethylene induced a significant up-regulation of Nuclear Factor of Activated T-cells, cytoplasmic 1, Receptor Activator of Nuclear factor Kappa-B and Receptor Activator of Nuclear Factor Kappa-B Ligand genes (p < 0.0005) compared to controls. At protein level, the particles induced a significant increase of Receptor Activator of Nuclear Factor Kappa-B Ligand at day 7 over controls (p < 0.0005). Osteoclasts were capable to resorb bone even in absence of differentiating factors. The possible mechanism, beside spontaneous osteoclastogenesis mediated by wear debris, was identified in an autocrine up-regulation of Receptor activator of nuclear factor kappa-B ligand gene expression and protein synthesis. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 510-520, 2017.

CCL2 and CCR2 are Essential for the Formation of Osteoclasts and Foreign Body Giant Cells

Osteoclasts are multinucleated cells responsible for bone resorption. They are derived from the fusion of cells in the monocyte/macrophage lineage. Monocytes and macrophages can also fuse to form foreign body giant cells (FBGC). Foreign body giant cells are observed at the interface between a host and a foreign body such as implants during a foreign body reaction. Macrophages are attracted to the site of bone resorption and foreign body reactions by different cytokines. Chemokine (C-C) ligand-2 (CCL2) is an important chemotactic factor and binds to a receptor CCR2. In this study we investigated the importance of CCL2 and the receptor CCR2 in the formation of osteoclasts and FBGC. CCL2 mRNA was more highly expressed in giant cell culture than macrophages, being 9-fold and 16-fold more abundant in osteoclasts and FBGC respectively. Significantly fewer osteoclasts and FBGC were cultured from the bone marrow of CCL2 and CCR2 knockout mice, when compared to wild type. Not only were the number of giant cells reduced but there was a significant reduction in the number of nuclei and the size of these cells in the cultures of CCL2 and CCR2 knockout mice. Formation of osteoclasts and FBGC were recovered in cultures by addition of exogenous CCL2 to the media containing marrow cells from CCL2-/- mice. We conclude that CCL2 and its receptor CCR2 are important for the formation of osteoclasts and FBGC and absence of these genes causes inhibition of osteoclast and FBGC formation.

Diversity of multinucleated giant cells by microstructures of hydroxyapatite and plasma components in extraskeletal implantation model

Foreign body giant cells (FBGCs) and osteoclasts are multinucleated giant cells (MNGCs), both of which are formed by the fusion of macrophage-derived mononuclear cells. Osteoclasts are distinct from FBGCs due to their bone resorption ability; however, not only morphological, but also functional similarities may exist between these cells. The characterization and diversity of FBGCs that appear in an in vivo foreign body reaction currently remain incomplete. In the present study, we investigated an in vivo foreign body reaction using an extraskeletal implantation model of hydroxyapatite (HA) with different microstructures. The implantation of HA granules in rat subcutaneous tissue induced a foreign body reaction that was accompanied by various MNGCs. HA granules composed of rod-shaped particles predominantly induced cathepsin K (CTSK)-positive FBGCs, whereas HA granules composed of globular-shaped particles predominantly induced CTSK-negative FBGCs. Plasma, which was used as the binder of ceramic granules, stimulated the induction of CTSK-positive FBGCs more strongly than purified fibrin. Furthermore, the implantation of HA composed of rod-shaped particles with plasma induced tartrate-resistant acid phosphatase (TRAP)-positive MNGCs in contrast to HA composed of globular-shaped particles with purified fibrin, which predominantly induced CTSK-negative and TRAP-negative typical FBGCs. These results suggest that CTSK-positive, TRAP-positive, and CTSK- and TRAP-negative MNGCs are induced in this subcutaneous implantation model in a manner that is dependent on the microstructure of HA and presence or absence of plasma.

STATEMENT OF SIGNIFICANCE

We attempted to elucidate the mechanisms responsible for the foreign body reaction induced by the implantation of hydroxyapatite granules with different microstructures in rat subcutaneous tissue with or without plasma components as the binder of ceramic granules. By analyzing the expression of two reliable osteoclast markers, we detected tartrate-resistant acid phosphatase-positive multinucleated giant cells, cathepsin K-positive multinucleated giant cells, and tartrate-resistant acid phosphatase- and cathepsin K-negative multinucleated giant cells. The induction of tartrate-resistant acid phosphatase-positive multinucleated giant cells was plasma component-dependent while the induction of cathepsin K-positive multinucleated giant cells was influenced by the microstructure of hydroxyapatite. This is the first study to show the conditions dividing the three kinds of multinucleated giant cells in the foreign body reaction.

Regulation and Biological Significance of Formation of Osteoclasts and Foreign Body Giant Cells in an Extraskeletal Implantation Model

The implantation of biomaterials induces a granulomatous reaction accompanied by foreign body giant cells (FBGCs). The characterization of multinucleated giant cells (MNGCs) around bone substitutes implanted in bone defects is more complicated because of healing with bone admixed with residual bone substitutes and their hybrid, and the appearance of two kinds of MNGCs, osteoclasts and FBGCs. Furthermore, the clinical significance of osteoclasts and FBGCs in the healing of implanted regions remains unclear. The aim of the present study was to characterize MNGCs around bone substitutes using an extraskeletal implantation model and evaluate the clinical significance of osteoclasts and FBGCs. Beta-tricalcium phosphate (β-TCP) granules were implanted into rat subcutaneous tissue with or without bone marrow mesenchymal cells (BMMCs), which include osteogenic progenitor cells. We also compared the biological significance of plasma and purified fibrin, which were used as binders for implants. Twelve weeks after implantation, osteogenesis was only detected in specimens implanted with BMMCs. The expression of two typical osteoclast markers, tartrate-resistant acid phosphatase (TRAP) and cathepsin-K (CTSK), was analyzed, and TRAP-positive and CTSK-positive osteoclasts were only detected beside bone. In contrast, most of the MNGCs in specimens without the implantation of BMMCs were FBGCs that were negative for TRAP, whereas the degradation of β-TCP was detected. In the region implanted with β-TCP granules with plasma, FBGCs tested positive for CTSK, and when β-TCP granules were implanted with purified fibrin, FBGCs tested negative for CTSK. These results showed that osteogenesis was essential to osteoclastogenesis, two kinds of FBGCs, CTSK-positive and CTSK-negative, were induced, and the expression of CTSK was plasma-dependent. In addition, the implantation of BMMCs was suggested to contribute to osteogenesis and the replacement of implanted β-TCP granules to bone.

Double Stranded RNA-Dependent Protein Kinase is Necessary for TNF-α-Induced Osteoclast Formation In Vitro and In Vivo

Double-stranded RNA-dependent protein kinase (PKR) is involved in cell cycle progression, cell proliferation, cell differentiation, tumorgenesis, and apoptosis. We previously reported that PKR is required for differentiation and calcification in osteoblasts. TNF-α plays a key role in osteoclast differentiation. However, it is unknown about the roles of PKR in the TNF-α-induced osteoclast differentiation. The expression of PKR in osteoclast precursor RAW264.7 cells increased during TNF-α-induced osteoclastogenesis. The TNF-α-induced osteoclast differentiation in bone marrow-derived macrophages and RAW264.7 cells was markedly suppressed by the pretreatment of PKR inhibitor, 2-aminopurine (2AP), as well as gene silencing of PKR. The expression of gene markers in the differentiated osteoclasts including TRAP, Calcitonin receptor, cathepsin K, and ATP6V0d2 was also suppressed by the 2AP treatment. Bone resorption activity of TNF-α-induced osteoclasts was also supressed by 2AP treatment. Inhibition of PKR supressed the TNF-α-induced activation of NF-κB and MAPK in RAW264.7 cells. 2AP inhibited both the nuclear translocation of NF-κB and its transcriptional activity in RAW264.7 cells. 2AP inhibited the TNF-α-induced expression of NFATc1 and c-fos, master transcription factors in osteoclastogenesis. TNF-α-induced nuclear translocation of NFATc1 in mature osteoclasts was clearly inhibited by the 2AP treatment. The PKR inhibitor C16 decreased the TNF-α-induced osteoclast formation and bone resorption in mouse calvaria. The present study indicates that PKR is necessary for the TNF-α-induced osteoclast differentiation in vitro and in vivo.

Production and Functional Characterization of Murine Osteoclasts Differentiated from ER-Hoxb8-Immortalized Myeloid Progenitor Cells

In vitro differentiation into functional osteoclasts is routinely achieved by incubation of embryonic stem cells, induced pluripotent stem cells, or primary as well as cryopreserved spleen and bone marrow-derived cells with soluble receptor activator of nuclear factor kappa-B ligand and macrophage colony-stimulating factor. Additionally, osteoclasts can be derived from co-cultures with osteoblasts or by direct administration of soluble receptor activator of nuclear factor kappa-B ligand to RAW 264.7 macrophage lineage cells. However, despite their benefits for osteoclast-associated research, these different methods have several drawbacks with respect to differentiation yields, time and animal consumption, storage life of progenitor cells or the limited potential for genetic manipulation of osteoclast precursors. In the present study, we therefore established a novel protocol for the differentiation of osteoclasts from murine ER-Hoxb8-immortalized myeloid stem cells. We isolated and immortalized bone marrow cells from wild type and genetically manipulated mouse lines, optimized protocols for osteoclast differentiation and compared these cells to osteoclasts derived from conventional sources. In vitro generated ER-Hoxb8 osteoclasts displayed typical osteoclast characteristics such as multi-nucleation, tartrate-resistant acid phosphatase staining of supernatants and cells, F-actin ring formation and bone resorption activity. Furthermore, the osteoclast differentiation time course was traced on a gene expression level. Increased expression of osteoclast-specific genes and decreased expression of stem cell marker genes during differentiation of osteoclasts from ER-Hoxb8-immortalized myeloid progenitor cells were detected by gene array and confirmed by semi-quantitative and quantitative RT-PCR approaches. In summary, we established a novel method for the quantitative production of murine bona fide osteoclasts from ER-Hoxb8 stem cells generated from wild type or genetically manipulated mouse lines. These cells represent a standardized and theoretically unlimited source for osteoclast-associated research projects.

Bone morphogenetic protein signaling in bone homeostasis

Bone morphogenetic proteins (BMPs) are cytokines belonging to the transforming growth factor-β (TGF-β) superfamily. They play multiple functions during development and tissue homeostasis, including regulation of the bone homeostasis. The BMP signaling pathway consists in a well-orchestrated manner of ligands, membrane receptors, co-receptors and intracellular mediators, that regulate the expression of genes controlling the normal functioning of the bone tissues. Interestingly, BMP signaling perturbation is associated to a variety of low and high bone mass diseases, including osteoporosis, bone fracture disorders and heterotopic ossification. Consistent with these findings, in vitro and in vivo studies have shown that BMPs have potent effects on the activity of cells regulating bone function, suggesting that manipulation of the BMP signaling pathway may be employed as a therapeutic approach to treat bone diseases. Here we review the recent advances on BMP signaling and bone homeostasis, and how this knowledge may be used towards improved diagnosis and development of novel treatment modalities. This article is part of a Special Issue entitled "Muscle Bone Interactions".

The Foreign Body Giant Cell Cannot Resorb Bone, But Dissolves Hydroxyapatite Like Osteoclasts

Foreign body multinucleated giant cells (FBGCs) and osteoclasts share several characteristics, like a common myeloid precursor cell, multinuclearity, expression of tartrate-resistant acid phosphatase (TRAcP) and dendritic cell-specific transmembrane protein (DC-STAMP). However, there is an important difference: osteoclasts form and reside in the vicinity of bone, while FBGCs form only under pathological conditions or at the surface of foreign materials, like medical implants. Despite similarities, an important distinction between these cell types is that osteoclasts can resorb bone, but it is unknown whether FBGCs are capable of such an activity. To investigate this, we differentiated FBGCs and osteoclasts in vitro from their common CD14+ monocyte precursor cells, using different sets of cytokines. Both cell types were cultured on bovine bone slices and analyzed for typical osteoclast features, such as bone resorption, presence of actin rings, formation of a ruffled border, and characteristic gene expression over time. Additionally, both cell types were cultured on a biomimetic hydroxyapatite coating to discriminate between bone resorption and mineral dissolution independent of organic matrix proteolysis. Both cell types differentiated into multinucleated cells on bone, but FBGCs were larger and had a higher number of nuclei compared to osteoclasts. FBGCs were not able to resorb bone, yet they were able to dissolve the mineral fraction of bone at the surface. Remarkably, FBGCs also expressed actin rings, podosome belts and sealing zones--cytoskeletal organization that is considered to be osteoclast-specific. However, they did not form a ruffled border. At the gene expression level, FBGCs and osteoclasts expressed similar levels of mRNAs that are associated with the dissolution of mineral (e.g., anion exchange protein 2 (AE2), carbonic anhydrase 2 (CAII), chloride channel 7 (CIC7), and vacuolar-type H+-ATPase (v-ATPase)), in contrast the matrix degrading enzyme cathepsin K, which was hardly expressed by FBGCs. Functionally, the latter cells were able to dissolve a biomimetic hydroxyapatite coating in vitro, which was blocked by inhibiting v-ATPase enzyme activity. These results show that FBGCs have the capacity to dissolve the mineral phase of bone, similar to osteoclasts. However, they are not able to digest the matrix fraction of bone, likely due to the lack of a ruffled border and cathepsin K.

Studies of OC-STAMP in Osteoclast Fusion: A New Knockout Mouse Model, Rescue of Cell Fusion, and Transmembrane Topology

The fusion of monocyte/macrophage lineage cells into fully active, multinucleated, bone resorbing osteoclasts is a complex cell biological phenomenon that utilizes specialized proteins. OC-STAMP, a multi-pass transmembrane protein, has been shown to be required for pre-osteoclast fusion and for optimal bone resorption activity. A previously reported knockout mouse model had only mononuclear osteoclasts with markedly reduced resorption activity in vitro, but with paradoxically normal skeletal micro-CT parameters. To further explore this and related questions, we used mouse ES cells carrying a gene trap allele to generate a second OC-STAMP null mouse strain. Bone histology showed overall normal bone form with large numbers of TRAP-positive, mononuclear osteoclasts. Micro-CT parameters were not significantly different between knockout and wild type mice at 2 or 6 weeks old. At 6 weeks, metaphyseal TRAP-positive areas were lower and mean size of the areas were smaller in knockout femora, but bone turnover markers in serum were normal. Bone marrow mononuclear cells became TRAP-positive when cultured with CSF-1 and RANKL, but they did not fuse. Expression levels of other osteoclast markers, such as cathepsin K, carbonic anhydrase II, and NFATc1, were not significantly different compared to wild type. Actin rings were present, but small, and pit assays showed a 3.5-fold decrease in area resorbed. Restoring OC-STAMP in knockout cells by lentiviral transduction rescued fusion and resorption. N- and C-termini of OC-STAMP were intracellular, and a predicted glycosylation site was shown to be utilized and to lie on an extracellular loop. The site is conserved in all terrestrial vertebrates and appears to be required for protein stability, but not for fusion. Based on this and other results, we present a topological model of OC-STAMP as a 6-transmembrane domain protein. We also contrast the osteoclast-specific roles of OC- and DC-STAMP with more generalized cell fusion mechanisms.

EphA4 receptor is a novel negative regulator of osteoclast activity

Of the ephrin (Eph) receptors, mature osteoclasts express predominantly EphA4. This study sought to determine if EphA4 has a regulatory role in osteoclasts. Treatment of RAW/C4 cells with Epha4 small interfering RNAs (siRNAs) increased average size, Ctsk mRNA expression level, and bone resorption activity of the derived osteoclast-like cells. Activation of the EphA4 signaling in osteoclast precursors with EfnA4-fc chimeric protein reduced cell size and resorption activity of the derived osteoclasts. Homozygous Epha4 null mice had substantially less trabecular bone in femur and vertebra compared to wild-type controls. The bone loss was due to a decrease in trabecular number and an increase in trabecular spacing, but not to an increase in osteoclast-lined bone surface or an increase in the number of osteoclasts on bone surface. Dynamic histomorphometry and serum biomarker analyses indicate that bone formation in Epha4 null mice was reduced slightly but not significantly. Osteoclasts of Epha4 null mice were also larger, expressed higher levels of Mmp3 and Mmp9 mRNAs, and exhibited greater bone resorption activity than wild-type osteoclasts in vitro. Deficient Epha4 expression had no effects on the total number of osteoclast formed in response to receptor activator of NF-κB ligand nor on apoptosis of osteoclasts in vitro. It also did not affect the protein-tyrosine phosphorylation status of its ligands, EfnB2, EfnA2, and EfnA4, in osteoclasts. Deficient Epha4 expression in Epha4 null osteoclasts activated the β3 -integrin signaling through reduced phosphorylation of the tyr-747 residue, which led to increased binding of the stimulatory talin and reduced binding of the inhibitory Dok1 to β3 -integrin. This in turn activated Vav3 and the bone resorption activity of osteoclasts. In conclusion, we demonstrate for the first time that EphA4 is a potent negative regulator of osteoclastic activity, mediated in part through increased Dok1 binding to β3 -integrin via an increase in EphA4-dependent tyr-747 phosphorylation.

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.