Podosome and sealing zone: specificity of the osteoclast model

Linarin and its aglycone acacetin abrogate actin ring formation and focal contact to bone matrix of bone-resorbing osteoclasts through inhibition of αvβ3 integrin and core-linked CD44

BACKGROUND

Since enhanced bone resorption due to osteoclast differentiation and activation cause skeletal diseases, there is a growing need in therapeutics for combating bone-resorbing osteoclasts. Botanical antioxidants are being increasingly investigated for their health-promoting effects on bone. Edible Cirsium setidens contains various polyphenols of linarin, pectolinarin, and apigenin with antioxidant and hepatoprotective effects.

PURPOSE

This study aimed to determine whether linarin present in Cirsium setidens water extracts (CSE) and its aglycone acacetin inhibited osteoclastogenesis of RANKL-exposed RAW 264.7 murine macrophages for 5 days.

METHODS

This study assessed the osteoprotective effects of CSE, linarin and acacetin on RANKL-induced differentiation and activation of osteoclasts by using MTT assay, TRAP staining, Western blot analysis, bone resorption assay actin ring staining, adhesion assay and immunocytochemical assay. This study explored the underlying mechanisms of their osteoprotection, and identified major components present in CSE by HPLC analysis.

RESULTS

Linarin and pectolinarin were identified as major components of CSE. Nontoxic linarin and acacetin as well as CSE, but not pectolinarin attenuated the RANKL-induced macrophage differentiation into multinucleated osteoclasts, and curtailed osteoclastic bone resorption through reducing lacunar acidification and bone matrix degradation in the osteoclast-bone interface. Linarin and acacetin in CSE reduced the transmigration and focal contact of osteoclasts to bone matrix-mimicking RGD peptide. Such reduction was accomplished by inhibiting the induction of integrins, integrin-associated proteins of paxillin and gelsolin, cdc42 and CD44 involved in the formation of actin rings. The inhibition of integrin-mediated actin ring formation by linarin and acacetin entailed the disruption of TRAF6-c-Src-PI3K signaling of bone-resorbing osteoclasts. The functional inhibition of c-Src was involved in the loss of F-actin-enriched podosome core protein cortactin-mediated actin assembly due to linarin and acacetin.

CONCLUSION

These observations demonstrate that CSE, linarin and acacetin were effective in retarding osteoclast function of focal adhesion to bone matrix and active bone resorption via inhibition of diffuse cloud-associated αvβ3 integrin and core-linked CD44.

Aesculetin Inhibits Osteoclastic Bone Resorption through Blocking Ruffled Border Formation and Lysosomal Trafficking

For the optimal resorption of mineralized bone matrix, osteoclasts require the generation of the ruffled border and acidic resorption lacuna through lysosomal trafficking and exocytosis. Coumarin-type aesculetin is a naturally occurring compound with anti-inflammatory and antibacterial effects. However, the direct effects of aesculetin on osteoclastogenesis remain to be elucidated. This study found that aesculetin inhibited osteoclast activation and bone resorption through blocking formation and exocytosis of lysosomes. Raw 264.7 cells were differentiated in the presence of 50 ng/mL receptor activator of nuclear factor-κB ligand (RANKL) and treated with 1–10 μM aesculetin. Differentiation, bone resorption, and lysosome biogenesis of osteoclasts were determined by tartrate-resistance acid phosphatase (TRAP) staining, bone resorption assay, Western blotting, immunocytochemical analysis, and LysoTracker staining. Aesculetin inhibited RANKL-induced formation of multinucleated osteoclasts with a reduction of TRAP activity. Micromolar aesculetin deterred the actin ring formation through inhibition of induction of αvβ3 integrin and Cdc42 but not cluster of differentiation 44 (CD44) in RANKL-exposed osteoclasts. Administering aesculetin to RANKL-exposed osteoclasts attenuated the induction of autophagy-related proteins, microtubule-associated protein light chain 3, and small GTPase Rab7, hampering the lysosomal trafficking onto ruffled border crucial for bone resorption. In addition, aesculetin curtailed cellular induction of Pleckstrin homology domain-containing protein family member 1 and lissencephaly-1 involved in lysosome positioning to microtubules involved in the lysosomal transport within mature osteoclasts. These results demonstrate that aesculetin retarded osteoclast differentiation and impaired lysosomal trafficking and exocytosis for the formation of the putative ruffled border. Therefore, aesculetin may be a potential osteoprotective agent targeting RANKL-induced osteoclastic born resorption for medicinal use.

Biomechanical regulation of focal adhesion and invadopodia formation

Integrin adhesions are a structurally and functionally diverse family of transmembrane, multi-protein complexes that link the intracellular cytoskeleton to the extracellular matrix (ECM). The different members of this family, including focal adhesions (FAs), focal complexes, fibrillar adhesions, podosomes and invadopodia, contain many shared scaffolding and signaling ‘adhesome’ components, as well as distinct molecules that perform specific functions, unique to each adhesion form. In this Hypothesis, we address the pivotal roles of mechanical forces, generated by local actin polymerization or actomyosin-based contractility, in the formation, maturation and functionality of two members of the integrin adhesions family, namely FAs and invadopodia, which display distinct structures and functional properties. FAs are robust and stable ECM contacts, associated with contractile stress fibers, while invadopodia are invasive adhesions that degrade the underlying matrix and penetrate into it. We discuss here the mechanisms, whereby these two types of adhesion utilize a similar molecular machinery to drive very different – often opposing cellular activities, and hypothesize that early stages of FAs and invadopodia assembly use similar biomechanical principles, whereas maturation of the two structures, and their ‘adhesive’ and ‘invasive’ functionalities require distinct sources of biomechanical reinforcement.

Influence of the TGF-β Superfamily on Osteoclasts/Osteoblasts Balance in Physiological and Pathological Bone Conditions

The balance between bone forming cells (osteoblasts/osteocytes) and bone resorbing cells (osteoclasts) plays a crucial role in tissue homeostasis and bone repair. Several hormones, cytokines, and growth factors-in particular the members of the TGF-β superfamily such as the bone morphogenetic proteins-not only regulate the proliferation, differentiation, and functioning of these cells, but also coordinate the communication between them to ensure an appropriate response. Therefore, this review focuses on TGF-β superfamily and its influence on bone formation and repair, through the regulation of osteoclastogenesis, osteogenic differentiation of stem cells, and osteoblasts/osteoclasts balance. After introducing the main types of bone cells, their differentiation and cooperation during bone remodeling and fracture healing processes are discussed. Then, the TGF-β superfamily, its signaling via canonical and non-canonical pathways, as well as its regulation by Wnt/Notch or microRNAs are described and discussed. Its important role in bone homeostasis, repair, or disease is also highlighted. Finally, the clinical therapeutic uses of members of the TGF-β superfamily and their associated complications are debated.

TANK-binding kinase 1 mediates osteoclast differentiation by regulating NF-κB, MAPK and Akt signaling pathways

TANK-binding kinase 1 (TBK1) belongs to the noncanonical IκB kinase (IKK) family. The ubiquitously expressed protein is well known to play a pivotal role in innate immune response and inflammation. Although excessive inflammatory activities have been shown to affect osteoclast (OC) differentiation and function, direct relevance of TBK1 in bone turnover is not known. In this work, we specifically altered the TBK1 protein level by knocking down or overexpressing it without affecting its homologous protein IKKε expression, and demonstrated the effect of TBK1 on OC differentiation in bone marrow macrophages (BMMs) and RAW264.7 cells upon induction by receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL). TBK1 knockdown was found to markedly inhibit the OC differentiation and function, while TBK1 overexpression enhanced OC formation. Downregulation of TBK1 greatly suppressed RANKL-induced gene expression of Mmp9, Atp6v0d2, Acp5, Ctsk andNfatc1 involved in the regulation of OC formation and function in both BMM and RAW264.7 cells. Mechanistic studies indicated that TBK1 affected the NF-κB signaling pathway as well as mitogen-activated protein kinases (MAPKs) and protein kinase B (Akt) activation during OC differentiation. Moreover, the protein level of TNF receptor-associated factor 6 (TRAF6) was increased, and the interaction of TRAF6 with TBK1 was potentiated, by RANKL. Collectively, we provide direct evidence showing that TBK1 effectively mediates OC differentiation and function by regulating NF-κB, MAPKs and Akt signals. A TBK1-targeted therapeutic strategy may be useful for the treatment of bone-related disorders.

Smart Nanosacrificial Layer on the Bone Surface Prevents Osteoporosis through Acid-Base Neutralization Regulated Biocascade Effects

Osteoporosis is a global chronic disease characterized by severe bone loss and high susceptibility to fragile fracture. It is widely accepted that the origin acidified microenvironment created by excessive osteoclasts causes irreversible bone mineral dissolution and organic degradation during osteoclastic resorption. However, current clinically available approaches are mainly developed from the perspective of osteoclast biology rather than the critical acidified niche. Here, we developed a smart "nanosacrificial layer" consisting of sodium bicarbonate (NaHCO₃)-containing and tetracycline-functionalized nanoliposomes (NaHCO₃-TNLs) that can target bone surfaces and respond to external secreted acidification from osteoclasts, preventing osteoporosis. In vitro and in vivo results prove that this nanosacrificial layer precisely inhibits the initial acidification of osteoclasts and initiates a chemically regulated biocascade to remodel the bone microenvironment and realize bone protection: extracellular acid-base neutralization first inhibits osteoclast function and also promotes its apoptosis, in which the apoptosis-derived extracellular vesicles containing RANK (receptor activator of nuclear factor-κ B) further consume RANKL (RANK ligand) in serum, achieving comprehensive osteoclast inhibition. Our therapeutic strategy for osteoporosis is based on original and precise acid-base neutralization, aiming to reestablish bone homeostasis by using a smart nanosacrificial layer that is able to induce chemically regulated biocascade effects. This study also provides a novel understanding of osteoporosis therapy in biomedicine and clinical treatments.

Mechanical Cues Affect Migration and Invasion of Cells From Three Different Directions

Cell migration and invasion is a key driving factor for providing essential cellular functions under physiological conditions or the malignant progression of tumors following downward the metastatic cascade. Although there has been plentiful of molecules identified to support the migration and invasion of cells, the mechanical aspects have not yet been explored in a combined and systematic manner. In addition, the cellular environment has been classically and frequently assumed to be homogeneous for reasons of simplicity. However, motility assays have led to various models for migration covering only some aspects and supporting factors that in some cases also include mechanical factors. Instead of specific models, in this review, a more or less holistic model for cell motility in 3D is envisioned covering all these different aspects with a special emphasis on the mechanical cues from a biophysical perspective. After introducing the mechanical aspects of cell migration and invasion and presenting the heterogeneity of extracellular matrices, the three distinct directions of cell motility focusing on the mechanical aspects are presented. These three different directions are as follows: firstly, the commonly used invasion tests using structural and structure-based mechanical environmental signals; secondly, the mechano-invasion assay, in which cells are studied by mechanical forces to migrate and invade; and thirdly, cell mechanics, including cytoskeletal and nuclear mechanics, to influence cell migration and invasion. Since the interaction between the cell and the microenvironment is bi-directional in these assays, these should be accounted in migration and invasion approaches focusing on the mechanical aspects. Beyond this, there is also the interaction between the cytoskeleton of the cell and its other compartments, such as the cell nucleus. In specific, a three-element approach is presented for addressing the effect of mechanics on cell migration and invasion by including the effect of the mechano-phenotype of the cytoskeleton, nucleus and the cell's microenvironment into the analysis. In precise terms, the combination of these three research approaches including experimental techniques seems to be promising for revealing bi-directional impacts of mechanical alterations of the cellular microenvironment on cells and internal mechanical fluctuations or changes of cells on the surroundings. Finally, different approaches are discussed and thereby a model for the broad impact of mechanics on cell migration and invasion is evolved.

Maackiain dampens osteoclastogenesis via attenuating RANKL-stimulated NF-κB signalling pathway and NFATc1 activity

Osteolytic diseases are typified by over‐enhanced formation and resorbing function of osteoclasts and have a major impact on human health. Inhibition of osteoclastic differentiation and function is a key strategy for clinical therapy of osteolytic conditions. Maackiain is a natural compound extracted from Sophora flavescens, which has been applied to anti‐allergic and anti‐tumour treatments. The present results showed that Maackiain could restrain receptor activator of nuclear factor‐κB ligand (RANKL)‐stimulated osteoclast formation and hydroxyapatite resorption dose‐dependently, and interrupt the structures of F‐actin belts in the mature osteoclasts. It also repressed the expressions of osteoclast‐specific genes and proteins. Furthermore, Maackiain could inhibit RANKL‐stimulated NF‐κB and calcium signalling pathways, and dampen Nuclear factor of activated T cell cytoplasmic 1 activity, protein expression and translocation into the nucleus. These results revealed that Maackiain may have a potential therapeutic effect on osteoclast‐related disorders.

Heterogeneity and Actin Cytoskeleton in Osteoclast and Macrophage Multinucleation

Osteoclast signatures are determined by two transcriptional programs, the lineage-determining transcription pathway and the receptor activator of nuclear factor kappa-B ligand (RANKL)-dependent differentiation pathways. During differentiation, mononuclear precursors become multinucleated by cell fusion. Recently, live-cell imaging has revealed a high level of heterogeneity in osteoclast multinucleation. This heterogeneity includes the difference in the differentiation states and the mobility of the fusion precursors, as well as the mode of fusion among the fusion precursors with different numbers of nuclei. In particular, fusion partners often form morphologically distinct actin-based linkages that allow two cells to exchange lipids and proteins before membrane fusion. However, the origin of this heterogeneity remains elusive. On the other hand, osteoclast multinucleation is sensitive to the environmental cues. Such cues promote the reorganization of the actin cytoskeleton, especially the formation and transformation of the podosome, an actin-rich punctate adhesion. This review covers the heterogeneity of osteoclast multinucleation at the pre-fusion stage with reference to the environment-dependent signaling pathway responsible for reorganizing the actin cytoskeleton. Furthermore, we compare osteoclast multinucleation with macrophage fusion, which results in multinucleated giant macrophages.

Rho-GEF trio regulates osteoclast differentiation and function by Rac1/Cdc42

Many bone diseases result from abnormal bone resorption by osteoclasts (OCs). Studying OC related regulatory genes is necessary for the development of new therapeutic strategies. Rho GTPases have been proven to regulate OC differentiation and function and only mature OCs can carry out bone resorption. Here we demonstrate that Rac1 and Cdc42 exchange factor Triple functional domain (Trio) is critical for bone resorption caused by OCs. In this study, we created LysM-Cre;Trio^fl/fl conditional knockout mice in which Trio was conditionally ablated in monocytes. LysM-Cre;Trio^fl/fl mice showed increased bone mass due to impaired bone resorption caused by OCs. Furthermore, our in vitro analysis indicated that Trio conditional deficiency significantly suppressed OC differentiation and function. At the molecular level, Trio deficiency significantly inhibited the expression of genes critical for osteoclastogenesis and OC function. Mechanistically, our researches suggested that perturbed Rac1/Cdc42-PAK1-ERK/p38 signaling could be used to explain the lower ability of bone resorption in CKO mice. Taken together, this study indicates that Trio is a regulator of OCs. Studying the role of Trio in OCs provides a potential new insight for the treatment of OC related bone diseases.

Targeting autophagy in osteoporosis: From pathophysiology to potential therapy

Osteoporosis is a highly prevalent disorder characterized by the loss of bone mass and microarchitecture deterioration of bone tissue, attributed to various factors, including menopause (primary), aging (primary) and adverse effects of relevant medications (secondary). In recent decades, knowledge regarding the etiological mechanisms underpinning osteoporosis emphasizes that bone cellular homeostasis, including the maintenance of cell functions, differentiation, and the response to stress, is tightly regulated by autophagy, which is a cell survival mechanism for eliminating and recycling damaged proteins and organelles. With the important roles in the maintenance of cellular homeostasis and organ function, autophagy has emerged as a potential target for the prevention and treatment of osteoporosis. In this review, we update and discuss the pathophysiology of autophagy in normal bone cell life cycle and metabolism. Then, the alternations of autophagy in primary and secondary osteoporosis, and the accompanied pathological process are discussed. Finally, we discuss current strategies, limitations, and challenges involved in targeting relevant pathways and propose strategies by which such hurdles may be circumvented in the future for their translation into clinical validations and applications for the prevention and treatment of osteoporosis.

Pharmacological evaluation of imidazole-derived bisphosphonates on receptor activator of nuclear factor-κB ligand-induced osteoclast differentiation and function

Bisphosphonates (BPs) have been commonly used in the treatment of osteolytic bone lesions, such as osteoporosis and osteogenesis imperfecta. However, serious side-effects can occur during the therapy. To search for novel potent BPs with lower side-effects, a series of imidazole-containing BPs (zoledronic acid [ZOL]; ZOL derivatives by substitution of the hydrogen at the 2-position on the imidazole ring with a methyl [MIDP], ethyl [EIDP], n-propyl [PIDP], or n-butyl group [BIDP]) were developed and the effects on receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation were investigated using the murine macrophage RAW 264.7 cells at the protein, gene, and morphological and functional levels. Influences of these BPs on the cell growth and proliferation of RAW 264.7 were also studied in order to determine cytotoxicity. The results showed that PIDP significantly inhibited the RANKL-induced osteoclast formation in a dose-dependent fashion without inducing cytotoxicity under the concentration of 12.5 μM. It exerted remarkable suppressive effects on the development of actin rings, the bone resorption, and the expressions of osteoclastogenesis-related gene and protein markers. The down-regulation of c-Jun N-terminal kinase (JNK), protein kinase B (Akt), and inhibitor of nuclear factor kappa-B (IκB) phosphorylation in the early signaling event and subsequent inhibition of the expression of c-Fos and nuclear factor of activated T cells (NFATc1) might be involved in these effects. All these results indicated that PIDP might be a promising drug to treat bone-related disorders.

Ostm1 from Mouse to Human: Insights into Osteoclast Maturation

The maintenance of bone mass is a dynamic process that requires a strict balance between bone formation and resorption. Bone formation is controlled by osteoblasts, while osteoclasts are responsible for resorption of the bone matrix. The opposite functions of these cell types have to be tightly regulated not only during normal bone development, but also during adult life, to maintain serum calcium homeostasis and sustain bone integrity to prevent bone fractures. Disruption of the control of bone synthesis or resorption can lead to an over accumulation of bone tissue in osteopetrosis or conversely to a net depletion of the bone mass in osteoporosis. Moreover, high levels of bone resorption with focal bone formation can cause Paget’s disease. Here, we summarize the steps toward isolation and characterization of the osteopetrosis associated trans-membrane protein 1 (Ostm1) gene and protein, essential for proper osteoclast maturation, and responsible when mutated for the most severe form of osteopetrosis in mice and humans.

Nuclear receptors in osteoclasts

Osteoclasts are bone-resorbing cells that play an essential role in the remodeling of bone under physiological conditions and numerous pathological conditions, such as osteoporosis, bone metastasis, and inflammatory bone erosion. Nuclear receptors are crucial to various physiological processes, including metabolism, development and inflammation, and function as transcription factors to activate target genes. Synthetic ligands of nuclear receptors are also available for the treatment of metabolic and inflammatory diseases. However, dysregulated bone phenotypes have been documented in patients who take synthetic nuclear receptor ligands as a therapy. Therefore, the effect of nuclear receptors on bone cells has become an important area of exploration; additionally, the molecular mechanisms underlying the action of nuclear receptors in osteoclasts have not been completely understood. Here, we cover the recent progress in our understanding of the roles of nuclear receptors in osteoclasts.

Tetraspanin 7 and its closest paralog tetraspanin 6: membrane organizers with key functions in brain development, viral infection, innate immunity, diabetes and cancer

Tetraspanin (TSPAN) protein family forms a family of transmembrane proteins that act as organizers/scaffold for other proteins. TSPANs are primarily present on plasma membranes although they are also found in other biological membranes. They are organized in tetraspanin-enriched microdomains (TEMs), which allow spatiotemporal tuning of protein functions through the control of their membrane localization. TSPAN6 and TSPAN7 are close paralogs expressed in different tissues, TSPAN7 being highly expressed in the brain. Their functions only started to be unveiled in the late 2000's and are still poorly understood. Here, we introduce how TSPAN7 was first highlighted has a protein mutated in some forms of X-linked mental retardation, which was later proposed to be caused by defects in neuronal morphogenesis and synaptic transmission. We then discuss the impacts TSPAN7 has on cell morphology of dendritic cells and osteoclasts, through rearrangement of actin cytoskeleton and how TSPAN7 was shown to be a target of autoantibody in patients suffering from type 1 diabetes. Finally, we are addressing the double edge sword that is TSPAN7 in cancer. In the second part of this review, we address the known roles of TSPAN6 and how this protein was shown to participate in synaptic transmission and in amyloid precursor protein secretion, which may contribute to Alzheimer's disease pathology. We conclude this review by discussing the anti-inflammatory effect of TSPAN6.

The role of Rho GTPases' substrates Rac and Cdc42 in osteoclastogenesis and relevant natural medicinal products study

Recently, Rho GTPases substrates include Rac (Rac1 and Rac2) and Cdc42 that have been reported to exert multiple cellular functions in osteoclasts, the most prominent of which includes regulating the dynamic actin cytoskeleton rearrangements. In addition, natural products and their molecular frameworks have a long tradition as valuable starting points for medicinal chemistry and drug discovery. Although currently, there are reports about the natural product, which could play a therapeutic role in bone loss diseases (osteoporosis and osteolysis) through the regulation of Rac1/2 and Cdc42 during osteoclasts cytoskeletal structuring. There have been several excellent studies for exploring the therapeutic potentials of various natural products for their role in inhibiting cancer cells migration and function via regulating the Rac1/2 and Cdc42. Herein in this review, we try to focus on recent advancement studies for extensively understanding the role of Rho GTPases substrates Rac1, Rac2 and Cdc42 in osteoclastogenesis, as well as therapeutic potentials of natural medicinal products for their properties on the regulation of Rac1, and/or Rac2 and Cdc42, which is in order to inspire drug discovery in regulating osteoclastogenesis.

Insights and perspectives on calcium channel functions in the cockpit of cancerous space invaders

Development of metastasis causes the most serious clinical consequences of cancer and is responsible for over 90 % of cancer-related deaths. Hence, a better understanding of the mechanisms that drive metastasis formation appears critical for drug development designed to prevent the spread of cancer and related mortality. Metastasis dissemination is a multistep process supported by the increased motility and invasiveness capacities of tumor cells. To succeed in overcoming the mechanical constraints imposed by the basement membrane and surrounding tissues, cancer cells reorganize their focal adhesions or extend acto-adhesive cellular protrusions, called invadosomes, that can both contact the extracellular matrix and tune its degradation through metalloprotease activity. Over the last decade, accumulating evidence has demonstrated that altered Ca²⁺ channel activities and/or expression promote tumor cell-specific phenotypic changes, such as exacerbated migration and invasion capacities, leading to metastasis formation. While several studies have addressed the molecular basis of Ca²⁺ channel-dependent cancer cell migration, we are still far from having a comprehensive vision of the Ca²⁺ channel-regulated mechanisms of migration/invasion. This is especially true regarding the specific context of invadosome-driven invasion. This review aims to provide an overview of the current evidence supporting a central role for Ca²⁺ channel-dependent signaling in the regulation of these dynamic degradative structures. It will present available data on the few Ca²⁺ channels that have been studied in that specific context and discuss some potential interesting actors that have not been fully explored yet.

Cytoplasmic PCNA is located in the actin belt and involved in osteoclast differentiation

Osteoporosis (OP) is an age-related osteolytic disease and characterized by low bone mass and more prone to fracture due to active osteoclasts. Proliferating cell nuclear antigen (PCNA) has been long identified as a nuclear protein playing critical roles in the regulation of DNA replication and repair. Recently, a few studies have demonstrated the cytoplasmic localization of PCNA and its function associated with apoptosis in neutrophil and neuroblastoma cells. However, the involvement of PCNA, including the cytoplasmic PCNA, in the osteoclast differentiation remains unclear. In the present study, we show that PCNA is translocated from nucleus to cytoplasm during the RANKL-induced osteoclast differentiation, and localized in the actin belt of mature osteoclast. Knockdown of PCNA significantly affected the integrity of actin belt, the formation of multinucleated osteoclasts, the expression of osteoclast-specific genes, and the in vitro bone resorption. Interactomic study has revealed β-actin as the major interacting partner of the cytoplasmic PCNA, suggesting that cytoplasmic PCNA might play a critical role in the differentiation of osteoclast through regulation of actin-cytoskeleton remodeling. Taken together, our results demonstrate the critical role of cytoplasmic PCNA during the process of osteoclast differentiation, and provided a potential therapeutic target for treatment of osteoclast-related bone diseases.

Osteoclast formation at the bone marrow/bone surface interface: Importance of structural elements, matrix, and intercellular communication

Osteoclasts, the multinucleated cells responsible for bone resorption, have an enormous destructive power which demands to be kept under tight control. Accordingly, the identification of molecular signals directing osteoclastogenesis and switching on their resorptive activity have received much attention. Mandatory factors were identified, but a very essential aspect of the control mechanism of osteoclastic resorption, i.e. its spatial control, remains poorly understood. Under physiological conditions, multinucleated osteoclasts are only detected on the bone surface, while their mono-nucleated precursors are only in the bone marrow. How are pre-osteoclasts targeted to the bone surface? How is their progressive differentiation coordinated with their approach to the bone surface sites to be resorbed, which is where they finally fuse? Here we review the information on the bone marrow distribution of differentiating pre-osteoclasts relative to the position of the mandatory factors for their differentiation as well as relative to physical entities that may affect their access to the remodelling sites. This info allows recognizing an "osteoclastogenesis route" through the bone marrow and leading to the coincident fusion/resorption site - but also points to what still remains to be clarified regarding this route and regarding the restriction of fusion at the resorption site. Finally, we discuss the mechanism responsible for the start of resorption and its spatial extension. This review underscores that fully understanding the control of bone resorption requires to consider it in both space and time - which demands taking into account the context of bone tissue.

Modulated podosome patterning in osteoclasts by fullerenol nanoparticles disturbs the bone resorption for osteoporosis treatment

Overactivation and excessive differentiation of osteoclasts (OCs) has been implicated in the course of bone metabolism-related diseases. Although fullerenol nanoparticles (fNPs) have been suggested to inhibit OC differentiation and OC function in our previous work, systemic studies on the effect of fNPs on bone diseases, e.g., osteoporosis (OP), in vivo remain elusive. Herein, it is demonstrated that fNPs significantly suppress the differentiation of OCs that derived from the murine bone marrow monocytes and inhibit the formation of the sealing zone by blocking the formation and patterning of podosomes in OCs spatiotemporally. In vivo, fNPs are supposed to be an efficient inhibitor of the overactivation of OCs in a LPS-induced bone erosion mouse model. The therapeutic effect of fNPs on osteoporosis is also investigated in an ovariectomy-induced osteoporosis rat model. The well-organized trabecular bone, the reduction in the number of TRAP positive cells, the improvement of bone-associated parameters, and the mechanical properties all demonstrate that fNPs, similar to diphosphonates, can be a promising candidate for the effective treatment of osteoporosis.

HIV-1-Infected Human Macrophages, by Secreting RANK-L, Contribute to Enhanced Osteoclast Recruitment

HIV-1 infection is frequently associated with low bone density, which can progress to osteoporosis leading to a high risk of fractures. Only a few mechanisms have been proposed to explain the enhanced osteolysis in the context of HIV-1 infection. As macrophages are involved in bone homeostasis and are critical host cells for HIV-1, we asked whether HIV-1-infected macrophages could participate in bone degradation. Upon infection, human macrophages acquired some osteoclast features: they became multinucleated, upregulated the osteoclast markers RhoE and β3 integrin, and organized their podosomes as ring superstructures resembling osteoclast sealing zones. However, HIV-1-infected macrophages were not fully differentiated in osteoclasts as they did not upregulate NFATc-1 transcription factor and were unable to degrade bone. Investigating whether infected macrophages participate indirectly to virus-induced osteolysis, we showed that they produce RANK-L, the key osteoclastogenic cytokine. RANK-L secreted by HIV-1-infected macrophages was not sufficient to stimulate multinucleation, but promoted the protease-dependent migration of osteoclast precursors. In conclusion, we propose that, by stimulating RANK-L secretion, HIV-1-infected macrophages contribute to create a microenvironment that favors the recruitment of osteoclasts, participating in bone disorders observed in HIV-1 infected patients.

Cell-Substrate Patterns Driven by Curvature-Sensitive Actin Polymerization: Waves and Podosomes

Cells adhered to an external solid substrate are observed to exhibit rich dynamics of actin structures on the basal membrane, which are distinct from those observed on the dorsal (free) membrane. Here we explore the dynamics of curved membrane proteins, or protein complexes, that recruit actin polymerization when the membrane is confined by the solid substrate. Such curved proteins can induce the spontaneous formation of membrane protrusions on the dorsal side of cells. However, on the basal side of the cells, such protrusions can only extend as far as the solid substrate and this constraint can convert such protrusions into propagating wave-like structures. We also demonstrate that adhesion molecules can stabilize localized protrusions that resemble some features of podosomes. This coupling of curvature and actin forces may underlie the differences in the observed actin-membrane dynamics between the basal and dorsal sides of adhered cells.

Plectin stabilizes microtubules during osteoclastic bone resorption by acting as a scaffold for Src and Pyk2

Osteoclasts are multinuclear cells which maintain bone homeostasis by resorbing bone. During bone resorption, osteoclasts attach to the bone matrix via a sealing zone formed by an actin ring. Rous sarcoma oncogene (Src) is essential for actin ring formation and bone resorption. Recently, we demonstrated that plectin, a cytolinker protein, is a Src-binding protein in osteoclasts. However, the function of plectin in osteoclasts remains unknown. In this study, we demonstrated that shRNA knockdown of plectin in RAW 264.7 cells resulted in tartrate resistant acid phosphatase positive multinuclear cells (TRAP (+) MNCs) with impaired actin ring formation and bone resorption activity. Moreover, we found that in plectin-silenced TRAP (+) MNCs, Src and protein tyrosine kinase 2 beta (Pyk2), two critical kinases in osteoclastic bone resorption, were inactivated and microtubule polarity was disturbed. These results suggest that plectin plays a critical role in osteoclast biology by acting as a scaffold to facilitate Src and Pyk2 activation during microtubule organization.

Sclareol prevents ovariectomy-induced bone loss in vivo and inhibits osteoclastogenesis in vitro via suppressing NF-κB and MAPK/ERK signaling pathways

Postmenopausal osteoporosis (PMO) is a progressive disease occurring in elderly postmenopausal women that is characterized by low bone mass and impaired bone quality. Sclareol is a natural product (initially isolated from the leaves and flowers of Salvia Sclarea) that possesses immune-regulation and anti-inflammatory effects, but its role in osteoclastic formation and function as well as the PMO remains unknown. In the current study, we investigated the inhibitory effect of sclareol on osteoclastogenesis and progression of PMO. In vitro, sclareol not only inhibited osteoclast formation but also suppressed osteoclast function. The expression of the receptor activator of NF-κB ligand (RANKL)-induced osteoclast marker gene and protein was also reduced by sclareol treatment. Mechanistically, we found that sclareol inhibits RANKL-induced NF-κB and MAPK/ERK pathway activation. Furthermore, sclareol exerted a protective effect against bone loss in an ovariectomy-induced mouse model. Taken together, our findings suggest that sclareol has potential value as a therapeutic agent for PMO.

Carboxylated gold nanoparticles inhibit bone erosion by disturbing the acidification of an osteoclast absorption microenvironment

Hyperactive osteoclasts (OCs) are a fundamental reason for excessive bone resorption and consequent osteoporosis that lead to one-third of the patients sustaining a fracture. OCs, with the help of acidifying vesicles containing vacuolar-type H⁺-ATPase (V-ATPase), transport cytoplasmic protons into a resorptive pit and create an acidic microenvironment where proteolytic enzymes degrade the bone matrix. Here, we report a previously undescribed application of gold nanoparticles (AuNPs) to inhibit excessive bone resorption by regulating the acidic microenvironment in which OCs resorb bone. Internalized AuNPs, with relatively abundant carboxyl groups, eventually accumulate in the membrane of the intracellular vesicles and interact with the V0 domain of V-ATPase, which prevents it from recruiting the V1 domain. This destroys the acid-secretion function of OCs. The therapeutic effect of AuNPs on bone resorption was assessed in an established lipopolysaccharide-induced bone erosion mouse model. Micro-computed tomography, histology, and tartrate-resistant acid phosphatase staining showed that AuNPs significantly reduced bone erosion. In summary, AuNPs are promising nano-functional materials for repairing bone defects by regulating OC acid secretion.

L-Plastin deficiency produces increased trabecular bone due to attenuation of sealing ring formation and osteoclast dysfunction

Bone resorption requires the formation of complex, actin-rich cytoskeletal structures. During the early phase of sealing ring formation by osteoclasts, L-plastin regulates actin-bundling to form the nascent sealing zones (NSZ). Here, we show that L-plastin knockout mice produce osteoclasts that are deficient in the formation of NSZs, are hyporesorptive, and make superficial resorption pits in vitro. Transduction of TAT-fused full-length L-plastin peptide into osteoclasts from L-plastin knockout mice rescued the formation of nascent sealing zones and sealing rings in a time-dependent manner. This response was not observed with mutated full-length L-plastin (Ser-5 and -7 to Ala-5 and -7) peptide. In contrast to the observed defect in the NSZ, L-plastin deficiency did not affect podosome formation or adhesion of osteoclasts in vitro or in vivo. Histomorphometry analyses in 8- and 12-week-old female L-plastin knockout mice demonstrated a decrease in eroded perimeters and an increase in trabecular bone density, without a change in bone formation by osteoblasts. This decrease in eroded perimeters supports that osteoclast function is attenuated in L-plastin knockouts. Micro-CT analyses confirmed a marked increase in trabecular bone mass. In conclusion, female L-plastin knockout mice had increased trabecular bone density due to impaired bone resorption by osteoclasts. L-plastin could be a potential target for therapeutic interventions to treat trabecular bone loss.

Probing the mechanical landscape – new insights into podosome architecture and mechanics

Podosomes are dynamic adhesion structures formed constitutively by macrophages, dendritic cells and osteoclasts and transiently in a wide variety of cells, such as endothelial cells and megakaryocytes. They mediate numerous functions, including cell–matrix adhesion, extracellular matrix degradation, mechanosensing and cell migration. Podosomes present as micron-sized F-actin cores surrounded by an adhesive ring of integrins and integrin–actin linkers, such as talin and vinculin. In this Review, we highlight recent research that has considerably advanced our understanding of the complex architecture–function relationship of podosomes by demonstrating that the podosome ring actually consists of discontinuous nano-clusters and that the actin network in between podosomes comprises two subsets of unbranched actin filaments, lateral and dorsal podosome-connecting filaments. These lateral and dorsal podosome-connecting filaments connect the core and ring of individual podosomes and adjacent podosomes, respectively. We also highlight recent insights into the podosome cap as a novel regulatory module of actomyosin-based contractility. We propose that these newly identified features are instrumental for the ability of podosomes to generate protrusion forces and to mechanically probe their environment. Furthermore, these new results point to an increasing complexity of podosome architecture and have led to our current view of podosomes as autonomous force generators that drive cell migration.

Astilbin prevents bone loss in ovariectomized mice through the inhibition of RANKL-induced osteoclastogenesis

Osteoporosis is the most common osteolytic disease characterized by excessive osteoclast formation and resultant bone loss, which afflicts millions of patients around the world. Astilbin, a traditional herb, is known to have anti-inflammatory, antioxidant and antihepatic properties, but its role in osteoporosis treatment has not yet been confirmed. In our study, astilbin was found to have an inhibitory effect on the RANKL-induced formation and function of OCs in a dose-dependent manner without cytotoxicity. These effects were attributed to its ability to suppress the activity of two transcription factors (NFATc1 and c-Fos) indispensable for osteoclast formation, followed by inhibition of the expression of bone resorption-related genes and proteins (Acp5/TRAcP, CTSK, V-ATPase-d2 and integrin β3). Furthermore, we examined the underlying mechanisms and found that astilbin repressed osteoclastogenesis by blocking Ca2+ oscillations and the NF-κB and MAPK pathways. In addition, the therapeutic effect of MA on preventing bone loss in vivo was further confirmed in an ovariectomized mouse model. Therefore, considering its ability to inhibit RANKL-mediated osteoclastogenesis and the underlying mechanisms, astilbin might be a potential candidate for treating osteolytic bone diseases.

Osteoclasts may contribute bone substitute materials remodeling and bone formation in bone augmentation

Bone augmentation is increasingly important in implantology. Bone substitute materials exert essential roles during bone augmentation process. However, accelerating bone substitute materials remodeling and acquiring high bone architecture quality was still the challenges of bone augmentation. Accumulated studies had suggested osteoclasts is the key cell type to resorb bone or bone substitute materials. Our previous study and other studies suggested osteoclasts contributed to bone formation by promoting osteoblast function and facilitate angiogenesis. We hypothesized that bone substitute materials loaded osteoclastogenic cytokines or osteoclast progenitors will help to bone substitute materials rapid remodeling and subsequent bone formation. Our hypothesis could help to lessen long-term post-bone augmentation period and acquire better bone quality for osseointegration.

The osteoclast, a target cell for microorganisms

Bone is a highly adaptive tissue with regenerative properties that is subject to numerous diseases. Infection is one of the causes of altered bone homeostasis. Bone infection happens subsequently to bone surgery or to systemic spreading of microorganisms. In addition to osteoblasts, osteoclasts (OCs) also constitute cell targets for pathogens. OCs are multinucleated cells that have the exclusive ability to resorb bone mineral tissue. However, the OC is much more than a bone eater. Beyond its role in the control of bone turnover, the OC is an immune cell that produces and senses inflammatory cytokines, ingests microorganisms and presents antigens. Today, increasing evidence shows that several pathogens use OC as a host cell to grow, generating debilitating bone defects. In this review, we exhaustively inventory the bacteria and viruses that infect OC and report the present knowledge in this topic. We point out that most of the microorganisms enhance the bone resorption activity of OC. We notice that pathogen interactions with the OC require further investigation, in particular to validate the OC as a host cell in vivo and to identify the cellular mechanisms involved in altered bone resorption. Thus, we conclude that the OC is a new cell target for pathogens; this new research area paves the way for new therapeutic strategies in the infections causing bone defects.

Stachydrine prevents LPS-induced bone loss by inhibiting osteoclastogenesis via NF-κB and Akt signalling

Osteoclast overactivation‐induced imbalance in bone remodelling leads to pathological bone destruction, which is a characteristic of many osteolytic diseases such as rheumatoid arthritis, osteoporosis, periprosthetic osteolysis and periodontitis. Natural compounds that suppress osteoclast formation and function have therapeutic potential for treating these diseases. Stachydrine (STA) is a bioactive alkaloid isolated from Leonurus heterophyllus Sweet and possesses antioxidant, anti‐inflammatory, anticancer and cardioprotective properties. However, its effects on osteoclast formation and function have been rarely described. In the present study, we found that STA suppressed receptor activator of nuclear factor‐κB (NF‐κB) ligand (RANKL)‐induced osteoclast formation and bone resorption, and reduced osteoclast‐related gene expression in vitro. Mechanistically, STA inhibited RANKL‐induced activation of NF‐κB and Akt signalling, thus suppressing nuclear factor of activated T cells c1 induction and nuclear translocation. In addition, STA alleviated bone loss and reduced osteoclast number in a murine model of LPS‐induced inflammatory bone loss. STA also inhibited the activities of NF‐κB and NFATc1 in vivo. Together, these results suggest that STA effectively inhibits osteoclastogenesis both in vitro and in vivo and therefore is a potential option for treating osteoclast‐related diseases.

Clathrin-containing adhesion complexes

An understanding of the mechanisms whereby cell adhesion complexes (ACs) relay signals bidirectionally across the plasma membrane is necessary to interpret the role of adhesion in regulating migration, differentiation, and growth. A range of AC types has been defined, but to date all have similar compositions and are dependent on a connection to the actin cytoskeleton. Recently, a new class of AC has been reported that normally lacks association with both the cytoskeleton and integrin-associated adhesome components, but is rich in components of the clathrin-mediated endocytosis machinery. The characterization of this new type of adhesion structure, which is emphasized by mitotic cells and cells in long-term culture, identifies a hitherto underappreciated link between the adhesion machinery and clathrin structures at the plasma membrane. While this discovery has implications for how ACs are assembled and disassembled, it raises many other issues. Consequently, to increase awareness within the field, and stimulate research, we explore a number of the most significant questions below.

Scattered podosomes and podosomes associated with the sealing zone architecture in cultured osteoclasts revealed by cell shearing, quick freezing, and platinum-replica electron microscopy

Osteoclasts (OCs) can adhere to a variety of substrate surfaces by highly dynamic actin-based cytoskeletal structures termed podosomes. This tight attachment is established by a sealing zone (SZ), which is made of interconnected individual podosomes. Compared with scattered podosomes in various cell types, the architecture of the SZ is still unclear. Especially, ultrastructural studies on the details of the cytoskeletal structure of an OC have been challenging, because the high density of filaments in their podosomes obscure visualization of individual filaments. Therefore, to study this organization in more exact detail, we employed shearing open combined with replica electron microscopy. The present study provides several new details of the podosome and SZ structure, which were previously unrecognized: (a) the SZ consists of recognizable podosomes with a dense actin network of interpodosomal regions characterized by multiple layers of crossing, branching and anastomosing actin filament networks; (b) the Arp2/3 complex is distributed throughout the actin network of podosomes and SZ, indicating that actin polymerization is concentrated at these regions; (c) a close spatial relationship between the podosome and the dorsal membrane; and (d) a network of membranous organelles in close proximity to the podosomes in the SZ. Taken together, the present study reveals that a more complicated interpodosomal actin network among neighboring individual podosomes, which is more complicated than previously thought, appears to form the SZ. Indeed, individual podosomes are not an isolated structural unit from other organelles; and, in turn, their dynamism might affect the surrounding interpodosomal cytoskeletons, membranous organelles, and plasma membrane.

Deficiency in the phosphatase PHLPP1 suppresses osteoclast-mediated bone resorption and enhances bone formation in mice

Enhanced osteoclast-mediated bone resorption and diminished formation may promote bone loss. Pleckstrin homology (PH) domain and leucine-rich repeat protein phosphatase 1 (Phlpp1) regulates protein kinase C (PKC) and other proteins in the control of bone mass. Germline Phlpp1 deficiency reduces bone volume, but the mechanisms remain unknown. Here, we found that conditional Phlpp1 deletion in murine osteoclasts increases their numbers, but also enhances bone mass. Despite elevating osteoclasts, Phlpp1 deficiency did not increase serum markers of bone resorption, but elevated serum markers of bone formation. These results suggest that Phlpp1 suppresses osteoclast formation and production of paracrine factors controlling osteoblast activity. Phlpp1 deficiency elevated osteoclast numbers and size in ex vivo osteoclastogenesis assays, accompanied by enhanced expression of proto-oncogene C-Fms (C-Fms) and hyper-responsiveness to macrophage colony-stimulating factor (M-CSF) in bone marrow macrophages. Although Phlpp1 deficiency increased TRAP⁺ cell numbers, it suppressed actin-ring formation and bone resorption in these assays. We observed that Phlpp1 deficiency increases activity of PKCζ, a PKC isoform controlling cell polarity, and that addition of a PKCζ pseudosubstrate restores osteoclastogenesis and bone resorption of Phlpp1-deficient osteoclasts. Moreover, Phlpp1 deficiency increased expression of the bone-coupling factor collagen triple helix repeat-containing 1 (Cthrc1). Conditioned growth medium derived from Phlpp1-deficient osteoclasts enhanced mineralization of ex vivo osteoblast cultures, an effect that was abrogated by Cthrc1 knockdown. In summary, Phlpp1 critically regulates osteoclast numbers, and Phlpp1 deficiency enhances bone mass despite higher osteoclast numbers because it apparently disrupts PKCζ activity, cell polarity, and bone resorption and increases secretion of bone-forming Cthrc1.

Zoledronic acid inhibits osteoclast differentiation and function through the regulation of NF-κB and JNK signalling pathways

It is well known that extensive osteoclast formation plays a key role in osteoporosis in post‑menopausal women and the elderly. The suppression of extensive osteoclastogenesis and bone resorption may be an effective preventive strategy for osteoporosis. Zoledronic acid (ZOL) has been indicated to play an essential role in regulating bone mineral density and has already been used in large clinical trials. However, the effects of ZOL on osteoclastogenesis remain to be fully elucidated. Therefore, the present study aimed to determine the effects of ZOL on osteoclastogenesis, and to explore the corresponding signalling pathways. By using a cell viability assay, as well as in vitro osteoclastogenesis, immunofluorescence and resorption pit assays, we demonstrated that ZOL (0.1‑5 µM) suppressed receptor activator of nuclear factor‑κB ligand (RANKL)‑induced osteoclast differentiation and bone resorptive activity. Furthermore, western blot analysis and reverse transcription‑quantitative PCR indicated that ZOL inhibited the RANKL‑induced activation of NF‑κB and the phosphorylation of JNK in RAW264.7 cells, and subsequently decreased the expression of osteoclastogenesis‑associated genes, including calcitonin receptor, tartrate‑resistant acid phosphatase and dendritic cell‑specific transmembrane protein. ZOL inhibited osteoclast formation and resorption in vitro by specifically suppressing NF‑κB and JNK signalling. On the whole, the findings of this study indicate that ZOL may serve as a potential agent for the treatment of osteoclast‑associated diseases, including osteoporosis.

Bone morphogenetic proteins: Their role in regulating osteoclast differentiation

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

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

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Osteoclasts express multiple BMP signaling components.

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

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SMAD dependent BMP signaling regulates osteoclast-osteoblast coupling factors.

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

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

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

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

L-caldesmon alters cell spreading and adhesion force in RANKL-induced osteoclasts

Background

Osteoclasts (OCs) are motile multinucleated cells derived from differentiation and fusion of hematopoietic progenitors of the monocyte-macrophage lineage that undergo a multistep process called osteoclastogenesis. The biological function of OCs is to resorb bone matrix for controlling bone strength and integrity, which is essential for bone development. The bone resorption function is based on the remodelling of the actin cytoskeleton into an F-actin-rich structure known as the sealing zone for bone anchoring and matrix degradation. Non-muscle caldesmon (l-CaD) is known to participate in the regulation of actin cytoskeletal remodeling, but its function in osteoclastogenesis remains unclear.

Methods/results

In this study, gain and loss of the l-CaD level in RAW264.7 murine macrophages followed by RANKL induction was used as an experimental approach to examine the involvement of l-CaD in the control of cell fusion into multinucleated OCs in osteoclastogenesis. In comparison with controls, l-CaD overexpression significantly increased TRAP activity, actin ring structure and mineral substrate resorption in RANKL-induced cells. In contrast, gene silencing against l-CaD decreased the potential for RANKL-induced osteoclastogenesis and mineral substrate resorption. In addition, OC precursor cells with l-CaD overexpression and gene silencing followed by RANKL induction caused 13% increase and 24% decrease, respectively, in cell fusion index. To further understand the mechanistic action of l-CaD in the modulation of OC fusion, atomic force microscopy was used to resolve the mechanical changes of cell spreading and adhesion force in RANKL-induced cells with and without l-CaD overexpression or gene silencing.

Conclusions

l-CaD plays a key role in the regulation of actin cytoskeletal remodeling for the formation of actin ring structure at the cell periphery, which may in turn alter the mechanical property of cell-spreading and cell surface adhesion force, thereby facilitating cell-cell fusion into multinucleated OCs during osteoclastogenesis.

Electronic supplementary material

The online version of this article (10.1186/s12929-019-0505-1) contains supplementary material, which is available to authorized users.

Madecassoside inhibits estrogen deficiency-induced osteoporosis by suppressing RANKL-induced osteoclastogenesis

Osteoporosis is the most common osteolytic disease characterized by excessive osteoclast formation and resultant bone loss, which afflicts millions of patients around the world. Madecassoside (MA), isolated from Centella asiatica, was reported to have anti-inflammatory and antioxidant activities, but its role in osteoporosis treatment has not yet been confirmed. In our study, MA was found to have an inhibitory effect on the RANKL-induced formation and function of OCs in a dose-dependent manner without cytotoxicity. These effects were attributed to its ability to suppress the activity of two transcription factors (NFATc1 and c-Fos) indispensable for osteoclast formation, followed by inhibition of the expression of bone resorption-related genes and proteins (Acp5/TRAcP, CTSK, ATP6V0D2/V-ATPase-d2, and integrin β3). Furthermore, we examined the underlying mechanisms and found that MA represses osteoclastogenesis by blocking Ca2+ oscillations and the NF-κB and MAPK pathways. In addition, the therapeutic effect of MA on preventing bone loss in vivo was further confirmed in an ovariectomized mouse model. Therefore, considering its ability to inhibit RANKL-mediated osteoclastogenesis and the underlying mechanisms, MA might be a potential candidate for treating osteolytic bone diseases.

Myosins in Osteoclast Formation and Function

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

L-plastin phosphorylation regulates the early phase of sealing ring formation by actin bundling process in mouse osteoclasts

The process of sealing ring formation requires major actin filament reorganization. We previously demonstrated that an actin-bundling protein L-plastin has a role in the cross-linking of actin filaments into tight bundles and forms actin aggregates (denoted as nascent sealing zones). These nascent sealing zones mature into fully functional sealing rings. We have shown here that TNF-alpha signaling regulates the phosphorylation of serine-5 and -7 in L-plastin which increases the actin bundling capacity of L-plastin and hence the formation of nascent sealing zones in mouse osteoclasts. Using the TAT-mediated transduction method, we confirmed the role of L-plastin in nascent sealing zones formation at the early phase of the sealing ring assembly. Transduction of TAT-fused full-length L-plastin peptide significantly increases the number of nascent sealing zones and therefore sealing rings. But, transduction of amino-terminal L-plastin peptides consisting of the serine-5 and -7 reduces the formation of both nascent sealing zones and sealing rings. Therefore, bone resorption in vitro was reduced considerably. The decrease was associated with the selective inhibition of cellular L-plastin phosphorylation by the transduced peptides. Neither the formation of podosomes nor the migration was affected in these osteoclasts. Phosphorylation of L- plastin on serine 5 and -7 residues increases the F-actin bundling capacity. The significance of our studies stands on laying the groundwork for a better understanding of L-plastin as a potential regulator at the early phase of sealing ring formation and could be a new therapeutic target to treat bone loss.

Progranulin in the hematopoietic compartment protects mice from atherosclerosis

BACKGROUND AND AIMS

Progranulin is a circulating protein that modulates inflammation and is found in atherosclerotic lesions. Here we determined whether inflammatory cell-derived progranulin impacts atherosclerosis development.

METHODS

Ldlr^-/- mice were transplanted with bone marrow from wild-type (WT) or Grn^-/- (progranulin KO) mice (referred to as Tx-WT and Tx-KO, respectively).

RESULTS

After 10 weeks of high-fat diet feeding, both groups displayed similarly elevated plasma levels of cholesterol and triglycerides. Despite abundant circulating levels of progranulin, the size of atherosclerotic lesions in Tx-KO mice was increased by 47% in aortic roots and by 62% in whole aortas. Aortic root lesions in Tx-KO mice had increased macrophage content and larger necrotic cores, consistent with more advanced lesions. Progranulin staining was markedly reduced in the lesions of Tx-KO mice, indicating little or no uptake of circulating progranulin. Mechanistically, cultured progranulin-deficient macrophages exhibited increased lysosome-mediated exophagy of aggregated low-density lipoproteins resulting in increased cholesterol uptake and foam cell formation.

CONCLUSIONS

We conclude that hematopoietic progranulin deficiency promotes diet-induced atherosclerosis in Ldlr^-/- mice, possibly due to increased exophagy-mediated cholesterol uptake. Circulating progranulin was unable to prevent the increased lesion development, consistent with the importance of progranulin acting via cell-autonomous or local effects.

A dual-specific macrophage colony-stimulating factor antagonist of c-FMS and αvβ3 integrin for osteoporosis therapy

There is currently a demand for new highly efficient and specific drugs to treat osteoporosis, a chronic bone disease affecting millions of people worldwide. We have developed a combinatorial strategy for engineering bispecific inhibitors that simultaneously target the unique combination of c-FMS and α_vβ₃ integrin, which act in concert to facilitate bone resorption by osteoclasts. Using functional fluorescence-activated cell sorting (FACS)-based screening assays of random mutagenesis macrophage colony-stimulating factor (M-CSF) libraries against c-FMS and α_vβ₃ integrin, we engineered dual-specific M-CSF mutants with high affinity to both receptors. These bispecific mutants act as functional antagonists of c-FMS and α_vβ₃ integrin activation and hence of osteoclast differentiation in vitro and osteoclast activity in vivo. This study thus introduces a versatile platform for the creation of new-generation therapeutics with high efficacy and specificity for osteoporosis and other bone diseases. It also provides new tools for studying molecular mechanisms and the cell signaling pathways that mediate osteoclast differentiation and function.

Many bone diseases—including osteoporosis, in which the bones become brittle and fragile from loss of tissue—are characterized by excessive and uncontrolled bone resorption by bone-destroying cells known as osteoclasts. Therefore, controlled and specific inhibition of osteoclast activity is a desired outcome in treatments for bone diseases. Osteoclast differentiation and function are coordinated by cell surface receptors, including c-FMS and α_vβ₃ integrin, which cooperate with one another to drive signals that are essential for osteoclast functions. Here, we describe the engineering, characterization, and testing of novel proteins that can target and inhibit both c-FMS and α_vβ₃ integrin at the same time, thereby providing a way of controlling osteoclast function. The study represents the first example of engineering a natural ligand, which acts as a signaling molecule, as a scaffold for binding not only its target protein but also a second target. We show that these engineered proteins inhibit osteoclast activity in a mouse model of osteoporosis. Our study describes potential inhibitors that target all the known functions resulting from c-FMS/integrin α_vβ₃ crosstalk and paves the way to create novel targeting proteins that could be used to treat osteoporosis. It also expands our understanding of the role of the c-FMS/α_vβ₃ integrin pathway in the regulation of osteoclast differentiation and function.

Manipulation of osteoclastogenesis: Bioactive multiphasic silica/collagen composites and their effects of surface and degradation products

The intent of the present study was to demonstrate that multiphasic silica/collagen xerogels are able to manipulate cellular processes. These xerogels were prepared by a sol-gel approach allowing the incorporation of mineral phases. The resulting nanocomposites are designed as biomaterial for bone regeneration. Human osteoclasts derived from peripheral blood mononuclear cells were cultured both indirectly and directly, either in presence of different xerogel types or on their surface, to investigate the factor with the main influence on osteoclastogenesis. To this end, the incorporation of a third phase to silica/collagen xerogels was used to affect osteoclastogenesis. In cell culture, ambient ion conditions controlled by both the degradation products of the xerogel and the bioactivity-dependent ion release and reprecipitation were shown to have the main effect on osteoclast specific enzyme tartrate-resistant acid phosphatase (TRAP) 5b. Late stage of osteoclastogenesis characterized by resorption was strongly dependent on the xerogels composition. Surface chemistry of the xerogels was displayed to play an important role in osteoclast resorption. Biphasic silica/collagen xerogels and triphasic xerogels with calcium carbonate offered widespread resorbed areas, whereas hydroxyapatite containing xerogels showed distinctly reduced resorption. The incorporation of strontium carbonate and phosphate, respectively, as third phase changed TRAP 5b activity dose-dependently and inhibited resorption within 21 days. Quantitative evaluation on osteoclast differentiation was carried out using biochemical methods (TRAP 5b, cathepsin K) and was supported by confocal laser scanning microscopy and scanning electron microscopy (SEM). Qualitative estimation of resorption was carried out by SEM.

The effect of surface immobilized NBD peptide on osteoclastogenesis of rough titanium plates in vitro and osseointegration of rough titanium implants in ovariectomized rats in vivo

Successful osseointegration in dental implants depends on balanced activation of osteoclasts and osteoblasts. Osteoporosis up-regulates osteoclast activity, so it is desirable to find effective interventions to inhibit osteoclastogenesis and enhance the osseointegration of implants under these conditions. It has been reported that the NF-κB essential modulator (NEMO)-binding domain (NBD) peptide can prevent osteoclast formation and bone resorption. In this study, we conjugated NBD peptide onto the surface of rough pure titanium (Ti) using the layer by layer technique. We analyzed the surface characteristics and determined the successful NBD integration by the presence of trivial granular structures, increased S elements and hydrophilia. Importantly, we first reported that Ti surface-conjugated NBD peptide retained its inhibitory effects on osteoclastogenesis by reducing osteoclast sealing zone formation and function. These effects were mediated by a reduction in NFATc1 expression, which in turn regulated integrin ανβ₃ and MMP9 by targeting the P65 signaling pathway. In vivo TRAP staining suggested NBD-coating decreased osteoclast formation with less pseudopodia. Micro-CT and histomorphometric analysis demonstrated that NBD-coating enhanced pronounced osseointegration in vivo in ovariectomized rats. This study holds great promise for in vivo use of immobilized NBD peptide and offers an effective therapeutic approach to select more suitable Ti-implant surface modifications for improving implant osseointegration in osteoporotic patients.

Successful osseointegration in dental implants depends on balanced activation of osteoclasts and osteoblasts.

Poria cocos polysaccharide attenuates RANKL-induced osteoclastogenesis by suppressing NFATc1 activity and phosphorylation of ERK and STAT3

Pathological fractures caused by osteolytic lesions seriously threaten the health of patients. Osteoclasts play important roles in bone resorption whose hyperfunction are closely related to osteolytic lesions. Studies on osteoclast differentiation and function assist in the prevention of excessive bone loss associated diseases. We screened a variety of natural compounds with anti-inflammatory effect and found that poria cocos polysaccharide (PCP) inhibited RANKL-induced osteoclast formation and bone resorption via TRAcP staining, immunofluorescence, RT-PCR and western blot. PCP down-regulated phosphorylation of STAT3, P38, ERK and JNK, and thus repressed the expression of NFAcT1 and c-Fos during RANKL-induced osteoclastogenesis. Besides, the expression of bone resorption related genes such as TRAcP and CTSK was suppressed by PCP. The results suggest that PCP can be invoked as a candidate for the treatment of osteolytic diseases by inhibiting osteoclastogenesis.

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

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

The Sealing Zone in Osteoclasts: A Self-Organized Structure on the Bone

Osteoclasts form a specialized cell-matrix adhesion structure, known as the "sealing zone", during bone resorption. The sealing zone is a dynamic actin-rich structure that defines the resorption area of the bone. The detailed dynamics and fine structure of the sealing zone have been elusive. Osteoclasts plated on glass do not form a sealing zone, but generate a separate supra-molecular structure called the "podosome belt". Podosomes are integrin-based adhesion complexes involved in matrix adhesion, cell migration, matrix degradation, and mechanosensing. Invadopodia, podosome-like protrusions in cancer cells, are involved in cell invasion into other tissues by promoting matrix degradation. Both podosomes and invadopodia exhibit actin pattern transitions during maturation. We previously found that Arp2/3-dependent actin flow occurs in all observed assembly patterns of podosomes in osteoclasts on glass. It is known that the actin wave in Dictyostelium cells exhibits a similar pattern transition in its evolution. Because of significant advances in our understanding regarding the mechanism of podosomes/invadopodia formation over the last decade, we revisited the structure and function of the sealing zone in this review, highlighting the possible involvement of self-organized actin waves in the organogenesis of the sealing zone.