Podosome and sealing zone: specificity of the osteoclast model

Repurposing the multiple sclerosis drug Siponimod for osteoporosis treatment

Osteoporosis is the most common bone disorder, in which an imbalance between osteoclastic bone resorption and osteoblastic bone formation disrupts bone homeostasis. Osteoporosis management using anti-osteoclastic agents is a promising strategy; however, this remains an unmet need. Sphingosine-1-phosphate (S1P) and its receptors (S1PRs) are essential for maintaining bone homeostasis. Here, we identified that Siponimod, a Food and Drug Administration-approved S1PR antagonist for the treatment of multiple sclerosis, shows promising therapeutic effects against osteoporosis by inhibiting osteoclast formation and function. We found that Siponimod inhibited osteoclast formation in a dose-dependent manner without causing cytotoxicity. Podosome belt staining and bone resorption assays indicated that Siponimod treatment impaired osteoclast function. Western blot and qPCR assays demonstrated that Siponimod suppressed the expression of osteoclast-specific markers, including C-Fos, Nftac1, and Ctsk. Mechanistically, we validated that Siponimod downregulated receptor activator of nuclear factor kappa B ligand (RANKL)-induced Mitogen-activated protein kinases (MAPKs) and nuclear factor kappa B (NF-κB) signaling pathways during osteoclastogenesis. Moreover, in a preclinical mouse model, Siponimod prevented ovariectomy-induced bone loss by suppressing osteoclast activity in vivo. Collectively, these results suggest that Siponimod could serve as an alternative therapeutic agent for the treatment of osteoporosis.

In Vitro Cell Culture Model for Osteoclast Activation during Estrogen Withdrawal

Estrogen (17β-estradiol) deficiency post-menopause alters bone homeostasis whereby bone resorption by osteoclasts exceeds bone formation by osteoblasts, leading to osteoporosis in females. We established an in vitro model to examine the consequences of estrogen withdrawal (E2-WD) on osteoclasts derived from the mouse macrophage RAW 264.7 cell line and utilized it to investigate the mechanism behind the enhanced osteoclast activity post-menopause. We found that a greater population of osteoclasts that underwent E2-WD contained a podosome belt necessary for osteoclasts to adhere and resorb bone and possessed elevated resorptive activity compared to osteoclasts exposed to estrogen (E2) continuously. Our results show that compared to osteoclasts that received E2 continuously, those that underwent E2-WD had a faster rate of microtubule (MT) growth, reduced RhoA activation, and shorter podosome lifespan. Thus, altered podosome and MT dynamics induced by the withdrawal of estrogen supports podosome belt assembly/stability in osteoclasts, which may explain their enhanced bone resorption activity.

Moesin activation controls bone resorption and tunneling nanotube-dependent osteoclast fusion

Osteoclasts are multinucleated cells unique in their ability to resorb bone. Osteoclastogenesis involves several steps of actin-driven rearrangements that participate not only in the cell-cell fusion process, but also in the formation of the sealing zone, the adhesive structure determining the resorption area. Despite the importance of these actin cytoskeleton-based processes, their precise mechanisms of regulation are still poorly characterized. Here, we found that moesin, a member of the Ezrin/Radixin/Moesin (ERM) protein family, is activated during osteoclast maturation and plays an instrumental role for both osteoclast fusion and function. In mouse and human osteoclast precursors, moesin is negatively regulated to potentiate their ability to fuse and degrade bone. Accordingly, we demonstrated that moesin depletion decreases membrane-to-cortex attachment and enhances formation of tunneling nanotubes (TNTs), F-actin-containing intercellular bridges that we revealed to trigger osteoclast fusion. In addition, via a β3-integrin/RhoA/SLK pathway and independently of its role in fusion, moesin regulates the number and organization of sealing zones in mature osteoclast, and thus participates in the control of bone resorption. Supporting these findings, we found that moesin-deficient mice are osteopenic with a reduced density of trabecular bones and increased osteoclast abundance and activity. These findings provide a better understanding of the regulation of osteoclast biology, and open new opportunities to specifically target osteoclast activity in bone disease therapy.

Synthetic composites versus calcium phosphate cements in bone regeneration: A narrative review

BACKGROUND

When the natural process of bone remodeling is disturbed, the need arises for a stimulant material in order to enhance the formation of a new healthy and strong osseous tissue to replace the damaged one. Recent studies have reported synthetic biomaterials to be a very good option for supporting bone regeneration.

STUDY DESIGN

Narrative review.

OBJECTIVE

This review aims to provide a brief presentation of two of the most recently developed synthetic biomaterials, i.e. calcium phosphate cements and synthetic composites, that are currently being used in bone regeneration with promising results.

METHODS

Literature searches using broad terms such as "bone regeneration," "biomaterials," "synthetic composites" and "calcium phosphate cements" were performed using PubMed. The osteal cells state of the art was explored by searching topic-specific full text keywords using Google Scholar.

CONCLUSIONS

Synthetic polymers such as PCL (poly-ε-caprolactone) and PLGA (poly lactic-co-glycolic acid) can improve the effectiveness of biomaterials like HA (hydroxyapatite) and BG (bioglass). Calcium phosphate, although being a suitable material for stimulating bone regeneration, needs an adjuvant in order to be effective in larger bone defects.

Myeloid zinc finger 1 knockdown promotes osteoclastogenesis and bone loss in part by regulating RANKL-induced ferroptosis of osteoclasts through Nrf2/GPX4 signaling pathway

The overactivation of the osteoclasts is a crucial pathological factor in the development of osteoporosis. MZF1, belonging to the scan-zinc finger family, plays a significant role in various processes associated with tumor malignant progression and acts as an essential transcription factor regulating osteoblast expression. However, the exact role of MZF1 in osteoclasts has not been determined. In this study, the purpose of our study was to elucidate the role of MZF1 in osteoclastogenesis. First, we established MZF1-deficient female mice and evaluated the femur bone phenotype by micro-computed tomography and histological staining. Our findings indicate that MZF1-/- mice exhibited a low bone mass osteoporosis phenotype. RANKL could independently induce the differentiation of RAW264.7 cells into osteoclasts, and we found that the expression level of MZF1 protein decreased gradually. Then, the CRISPR/Cas 9 gene-editing technique was used to build a RAW264.7 cell model with MZF1 knockout, and RANKL was used to independently induce MZF1-/- and wild-type cells to differentiate into mature osteoclasts. Tartrate-resistant acid phosphatase staining and F-actin fluorescence results showed that the MZF1-/- group produced more tartrate-resistant acid phosphatase-positive mature osteoclasts and larger actin rings. The expression of osteoclast-associated genes (including tartrate-resistant acid phosphatase, CTSK, c-Fos, and NFATc1) was evaluated by reverse transcription quantitative polymerase chain reaction and Western blot. The expression of key genes of osteoclast differentiation in the MZF1-/- group was significantly increased. Furthermore, we found that cell viability was increased in the early stages of RANKL-induced cell differentiation in the MZF1-/- group cells. We examined some prevalent ferroptosis markers, including malondialdehyde, glutathione, and intracellular Fe, the active form of iron in the cytoplasm during the early stages of osteoclastogenesis. The results suggest that MZF1 may be involved in osteoclast differentiation by regulating RANKL-induced ferroptosis of osteoclasts. Collectively, our findings shed light on the essential involvement of MZF1 in the regulation of osteoclastogenesis in osteoporosis and provide insights into its potential underlying mechanism.

Hem1 is essential for ruffled border formation in osteoclasts and efficient bone resorption

Bone resorption is highly dependent on the dynamic rearrangement of the osteoclast actin cytoskeleton to allow formation of actin rings and a functional ruffled border. Hem1 is a hematopoietic-specific subunit of the WAVE-complex which regulates actin polymerization and is crucial for lamellipodia formation in hematopoietic cell types. However, its role in osteoclast differentiation and function is still unknown. Here, we show that although the absence of Hem1 promotes osteoclastogenesis, the ability of Hem1-/- osteoclasts to degrade bone was severely impaired. Global as well as osteoclast-specific deletion of Hem1 in vivo revealed increased femoral trabecular bone mass despite elevated numbers of osteoclasts in vivo. We found that the resorption defect derived from the morphological distortion of the actin-rich sealing zone and ruffled border deformation in Hem1-deficient osteoclasts leading to impaired vesicle transport and increased intracellular acidification. Collectively, our data identify Hem1 as a yet unknown key player in bone remodeling by regulating ruffled border formation and consequently the resorptive capacity of osteoclasts.

Kisspeptin-10 binding to Gpr54 in osteoclasts prevents bone loss by activating Dusp18-mediated dephosphorylation of Src

Osteoclasts are over-activated as we age, which results in bone loss. Src deficiency in mice leads to severe osteopetrosis due to a functional defect in osteoclasts, indicating that Src function is essential in osteoclasts. G-protein-coupled receptors (GPCRs) are the targets for ∼35% of approved drugs but it is still unclear how GPCRs regulate Src kinase activity. Here, we reveal that GPR54 activation by its natural ligand Kisspeptin-10 (Kp-10) causes Dusp18 to dephosphorylate Src at Tyr 416. Mechanistically, Gpr54 recruits both active Src and the Dusp18 phosphatase at its proline/arginine-rich motif in its C terminus. We show that Kp-10 binding to Gpr54 leads to the up-regulation of Dusp18. Kiss1, Gpr54 and Dusp18 knockout mice all exhibit osteoclast hyperactivation and bone loss, and Kp-10 abrogated bone loss by suppressing osteoclast activity in vivo. Therefore, Kp-10/Gpr54 is a promising therapeutic target to abrogate bone resorption by Dusp18-mediated Src dephosphorylation.

Hypoxia inducible factor-1α related mechanism and TCM intervention in process of early fracture healing

As a common clinical disease, fracture is often accompanied by pain, swelling, bleeding as well as other symptoms and has a high disability rate, even threatening life, seriously endangering patients' physical and psychological health and quality of life. Medical practitioners take many strategies for the treatment of fracture healing, including Traditional Chinese Medicine (TCM). In the early stage of fracture healing, the local fracture is often in a state of hypoxia, accompanied by the expression of hypoxia inducible factor-1α (HIF-1α), which is beneficial to wound healing. Through literature mining, we thought that hypoxia, HIF-1α and downstream factors affected the mechanism of fracture healing, as well as dominated this process. Therefore, we reviewed the local characteristics and related signaling pathways involved in the fracture healing process and summarized the intervention of TCM on these mechanisms, in order to inspirit the new strategy for fracture healing, as well as elaborate on the possible principles of TCM in treating fractures based on the HIF molecular mechanism.

Functional and analytical recapitulation of osteoclast biology on demineralized bone paper

Osteoclasts are the primary target for osteoporosis drug development. Recent animal studies revealed the crucial roles of osteoblasts in regulating osteoclastogenesis and the longer lifespans of osteoclasts than previously thought with fission and recycling. However, existing culture platforms are limited to replicating these newly identified cellular processes. We report a demineralized bone paper (DBP)-based osteoblast culture and osteoclast assay platform that replicates osteoclast fusion, fission, resorption, and apoptosis with high fidelity and analytical power. An osteoid-inspired DBP supports rapid and structural mineral deposition by osteoblasts. Coculture osteoblasts and bone marrow monocytes under biochemical stimulation recapitulate osteoclast differentiation and function. The DBP-based bone model allows longitudinal quantitative fluorescent monitoring of osteoclast responses to bisphosphonate drug, substantiating significantly reducing their number and lifespan. Finally, we demonstrate the feasibility of humanizing the bone model. The DBP-based osteo assay platforms are expected to advance bone remodeling-targeting drug development with improved prediction of clinical outcomes.

Characterization of Rab32- and Rab38-positive lysosome-related organelles in osteoclasts and macrophages

Both the biogenesis and functions of osteoclasts and macrophages involves dynamic membrane traffic. We screened transcript levels for Rab family small GTPases related to osteoclasts and identified Rab38. Rab38 expression is upregulated during osteoclast differentiation and maturation. In osteoclasts, both Rab38 and its paralog, Rab32, colocalize to lysosome-related organelles (LROs). In macrophages, Rab32 is also found in LROs. LROs are part of the endocytic pathway but are distinct from lysosomes. After receptor activator of NF-κB ligand stimulation, LROs contain cathepsin K and tartrate-resistant acid phosphatase inside and help both proteins to accumulate around bone resorption pits. After osteoclast maturation, these enzymes are hardly found within LROs. In macrophages derived from Rab32 and Rab38 double knockout mice, both acidification and V-ATPase a3 localization were severely compromised. Both the double knockout macrophage and bafilomycin-treated wildtype macrophage show an increase in Lamp1-positive organelles, implying that biogenesis of lysosomes and LROs are related. These results indicate that Rab32 and Rab38 both play a crucial role in LRO biogenesis in macrophages and in osteoclasts.

BASP1 down-regulates RANKL-induced osteoclastogenesis

The cytokine RANKL (Receptor Activator of NFκB Ligand) is the key driver of differentiation of monocytes/macrophages to form multi-nucleated, bone-resorbing osteoclasts, a process that is accompanied by significant changes in gene expression. We show that exposure to RANKL rapidly down-regulates expression of Brain Acid Soluble Protein 1 (BASP1) in cultured primary mouse bone marrow macrophages (BMMs), and that this reduced expression is causally linked to the osteoclastogenic process in vitro. Knocking down BASP1 expression in BMMs or eliminating its expression in these cells or in RAW 264.7 cells enhanced RANKL-induced osteoclastogenesis, promoted cell-cell fusion, and generated cultures containing larger osteoclasts with increased mineral degrading abilities relative to controls. Expression of exogenous BASP1 in BMMs undergoing osteoclastogenic differentiation produced the opposite effects. Upon exposure to RANKL, primary mouse BMMs in which BASP1 had been knocked down exhibited increased expression of the key osteoclastogenic transcription factor Nfatc1and of its downstream target genes Dc-stamp, Ctsk, Itgb3, and Mmp9 relative to controls. The knock-down cells also exhibited increased sensitivity to the pro-osteoclastogenic effects of RANKL. We conclude that BASP1 is a negative regulator of RANKL-induced osteoclastogenesis, which down-regulates the pro-osteoclastogenic gene expression pattern induced by this cytokine. Decreased expression of BASP1 upon exposure of BMMs to RANKL removes a negative regulator of osteoclastogenesis and promotes this process.

Differentiation-inducing effect of osteoclast microgrooves for the purpose of three-dimensional design of regenerated bone

In vivo bone remodeling is promoted by the balance between osteoclast and osteoblast activity. Conventional research on bone regeneration has mainly focused on increasing osteoblast activity, with limited studies on the effects of scaffold topography on cell differentiation. Here, we examined the effect of microgroove-patterned substrate with spacings ranging from 1 to 10 μm on the differentiation of rat bone marrow-derived osteoclast precursors. Tartrate-resistant acid phosphatase (TRAP) staining and relative gene expression quantification showed that osteoclast differentiation was enhanced in substrate with 1 µm microgroove spacing compared with that in the other groups. Additionally, the ratio of podosome maturation stages in substrate with 1 μm microgroove spacing exhibited a distinct pattern, which was characterized by an increase in the ratio of belts and rings and a decrease in that of clusters. However, myosin II abolished the effects of topography on osteoclast differentiation. Overall, these showed that the reduction of myosin II tension in the podosome core by an integrin vertical vector increased podosome stability and promoted osteoclast differentiation in substrates with 1 μm microgroove spacing, including that microgroove design plays an important role in scaffolds for bone regeneration. STATEMENT OF SIGNIFICANCE: Reduction of myosin II tension in the podosome core, facilitated by an integrin vertical vector, resulted in an enhanced osteoclast differentiation, concomitant with an increase in podosome stability within 1-μm-spaced microgrooves. These findings are anticipated to serve as valuable indicators for the regulation of osteoclast differentiation through the manipulation of biomaterial surface topography in tissue engineering. Furthermore, this study contributes to the lucidation of the underlying mechanisms governing cellular differentiation by providing insights into the impact of the microtopographical environment.

The checkpoint inhibitor PD-1H/VISTA controls osteoclast-mediated multiple myeloma bone disease

Multiple myeloma bone disease is characterized by the development of osteolytic bone lesions. Recent work identified matrix metalloproteinase 13 as a myeloma-derived fusogen that induces osteoclast activation independent of its proteolytic activity. We now identify programmed death-1 homolog, PD-1H, as the bona fide MMP-13 receptor on osteoclasts. Silencing PD-1H or using Pd-1h^-/- bone marrow cells abrogates the MMP-13-enhanced osteoclast fusion and bone-resorptive activity. Further, PD-1H interacts with the actin cytoskeleton and plays a necessary role in supporting c-Src activation and sealing zone formation. The critical role of PD-1H in myeloma lytic bone lesions was confirmed using a Pd-1h^-/- myeloma bone disease mouse model wherein myeloma cells injected into Pd-1h^-/-Rag2^-/- results in attenuated bone destruction. Our findings identify a role of PD-1H in bone biology independent of its known immunoregulatory functions and suggest that targeting the MMP-13/PD-1H axis may represent a potential approach for the treatment of myeloma associated osteolysis.

Microglia degrade Tau oligomers deposit via purinergic P2Y12-associated podosome and filopodia formation and induce chemotaxis

BACKGROUND

Tau protein forms neurofibrillary tangles and becomes deposited in the brain during Alzheimer's disease (AD). Tau oligomers are the most reactive species, mediating neurotoxic and inflammatory activity. Microglia are the immune cells in the central nervous system, sense the extracellular Tau via various cell surface receptors. Purinergic P2Y12 receptor can directly interact with Tau oligomers and mediates microglial chemotaxis via actin remodeling. The disease-associated microglia are associated with impaired migration and express a reduced level of P2Y12, but elevate the level of reactive oxygen species and pro-inflammatory cytokines.

RESULTS

Here, we studied the formation and organization of various actin microstructures such as-podosome, filopodia and uropod in colocalization with actin nucleator protein Arp2 and scaffold protein TKS5 in Tau-induced microglia by fluorescence microscopy. Further, the relevance of P2Y12 signaling either by activation or blockage was studied in terms of actin structure formations and Tau deposits degradation by N9 microglia. Extracellular Tau oligomers facilitate the microglial migration via Arp2-associated podosome and filopodia formation through the involvement of P2Y12 signaling. Similarly, Tau oligomers induce the TKS5-associated podosome clustering in microglial lamella in a time-dependent manner. Moreover, the P2Y12 was evidenced to localize with F-actin-rich podosome and filopodia during Tau-deposit degradation. The blockage of P2Y12 signaling resulted in decreased microglial migration and Tau-deposit degradation.

CONCLUSIONS

The P2Y12 signaling mediate the formation of migratory actin structures like- podosome and filopodia to exhibit chemotaxis and degrade Tau deposit. These beneficial roles of P2Y12 in microglial chemotaxis, actin network remodeling and Tau clearance can be intervened as a therapeutic target in AD.

Exploring the Role of Urocortin in Osteoporosis

Osteoporosis is a debilitating disease that affects over 200 million people worldwide. Overactive osteoclast activity leads to micro-architectural defects and low bone mass. This culminates in fragility fractures, such as femoral neck fractures. Treatments currently available either are not completely effective or have considerable side effects; thus, there is a need for more effective treatments. The urocortin (Ucn) family, composed of urocortin 1 (Ucn1), urocortin 2 (Ucn2), urocortin 3 (Ucn3), corticotropin-releasing factor (CRF) and corticotropin-releasing factor-binding protein (CRF-BP), exerts a wide range of effects throughout the body. Ucn1 has been shown to inhibit murine osteoclast activity. This review article will aim to bridge the gap between existing knowledge of Ucn and whether it can affect human osteoclasts.

Functional Role of Phospholipase D in Bone Metabolism

Phospholipase D (PLD) proteins are major enzymes that regulate various cellular functions, such as cell growth, cell migration, membrane trafficking, and cytoskeletal dynamics. As they are responsible for such important biological functions, PLD proteins have been considered promising therapeutic targets for various diseases, including cancer and vascular and neurological diseases. Intriguingly, emerging evidence indicates that PLD1 and PLD2, 2 major mammalian PLD isoenzymes, are the key regulators of bone remodeling; this suggests that these isozymes could be used as potential therapeutic targets for bone diseases, such as osteoporosis and rheumatoid arthritis. PLD1 or PLD2 deficiency in mice can lead to decreased bone mass and dysregulated bone homeostasis. Although both mutant mice exhibit similar skeletal phenotypes, PLD1 and PLD2 play distinct and nonredundant roles in bone cell function. This review summarizes the physiological roles of PLD1 and PLD2 in bone metabolism, focusing on recent findings of the biological functions and action mechanisms of PLD1 and PLD2 in bone cells.

Tau aggregates improve the purinergic receptor P2Y12-associated podosome rearrangements in microglial cells

Alzheimer's disease (AD) is a progressive neurodegenerative disease that is associated with protein misfolding, plaque accumulation, neuronal dysfunction, synaptic loss, and cognitive decline. The pathological cascade of AD includes the intracellular Tau hyperphosphorylation and its subsequent aggregation, extracellular Amyloid-β plaque formation and microglia-mediated neuroinflammation. The extracellular release of aggregated Tau is sensed by surveilling microglia through the involvement of various cell surface receptors. Among all, purinergic P2Y12R signaling is involved in microglial chemotaxis towards the damaged neurons. Microglial migration is highly linked with membrane-associated actin remodeling leading to the phagocytosis of extracellular Tau species. Here, we studied the formation of various actin structures such as podosome, lamellipodia and filopodia, in response to extracellular Tau monomers and aggregates. Microglial podosomes are colocalized with actin nucleator protein WASP, Arp2 and TKS5 adaptor protein during Tau-mediated migration. Moreover, the P2Y12 receptors were associated with F-actin-rich podosome structures, which signify the potential of Tau aggregates in microglial chemotaxis through the involvement of actin remodeling.

Dysfunction in parkin aggravates inflammatory bone erosion by reinforcing osteoclast activity

BACKGROUND

Parkin dysfunction associated with the progression of parkinsonism contributes to a progressive systemic skeletal disease characterized by low bone mineral density. However, the role of parkin in bone remodeling has not yet been elucidated in detail.

RESULT

We observed that decreased parkin in monocytes is linked to osteoclastic bone-resorbing activity. siRNA-mediated knockdown of parkin significantly enhanced the bone-resorbing activity of osteoclasts (OCs) on dentin without any changes in osteoblast differentiation. Moreover, Parkin-deficient mice exhibited an osteoporotic phenotype with a lower bone volume accompanied by increased OC-mediated bone-resorbing capacity displaying increased acetylation of α-tubulin compared to wild-type (WT) mice. Notably, compared to WT mice, the Parkin-deficient mice displayed increased susceptibility to inflammatory arthritis, reflected by a higher arthritis score and a marked bone loss after arthritis induction using K/BxN serum transfer, but not ovariectomy-induced bone loss. Intriguingly, parkin colocalized with microtubules and parkin-depleted-osteoclast precursor cells (Parkin^-/- OCPs) displayed augmented ERK-dependent acetylation of α-tubulin due to failure of interaction with histone deacetylase 6 (HDAC6), which was promoted by IL-1β signaling. The ectopic expression of parkin in Parkin^-/- OCPs limited the increase in dentin resorption induced by IL-1β, accompanied by the reduced acetylation of α-tubulin and diminished cathepsin K activity.

CONCLUSION

These results indicate that a deficiency in the function of parkin caused by a decrease in parkin expression in OCPs under the inflammatory condition may enhance inflammatory bone erosion by altering microtubule dynamics to maintain OC activity.

Gulp1 deficiency augments bone mass in male mice by affecting osteoclasts due to elevated 17β-estradiol levels

The engulfment adaptor phosphotyrosine-binding domain containing 1 (GULP1) is an adaptor protein involved in the engulfment of apoptotic cells via phagocytosis. Gulp1 was first found to promote the phagocytosis of apoptotic cells by macrophages, and its role in various tissues, including neurons and ovaries, has been well studied. However, the expression and function of GULP1 in bone tissue are poorly understood. Consequently, to determine whether GULP1 plays a role in the regulation of bone remodeling in vitro and in vivo, we generated Gulp1 knockout (KO) mice. Gulp1 was expressed in bone tissue, mainly in osteoblasts, while its expression is very low in osteoclasts. Microcomputed tomography and histomorphometry analysis in 8-week-old male Gulp1 KO mice revealed a high bone mass in comparison with male wild-type (WT) mice. This was a result of decreased osteoclast differentiation and function in vivo and in vitro as confirmed by a reduced actin ring and microtubule formation in osteoclasts. Gas chromatography-mass spectrometry analysis further showed that both 17β-estradiol (E2) and 2-hydroxyestradiol levels, and the E2/testosterone metabolic ratio, reflecting aromatase activity, were also higher in the bone marrow of male Gulp1 KO mice than in male WT mice. Consistent with mass spectrometry analysis, aromatase enzymatic activity was significantly higher in the bone marrow of male Gulp1 KO mice. Altogether, our results suggest that GULP1 deficiency decreases the differentiation and function of osteoclasts themselves and increases sex steroid hormone-mediated inhibition of osteoclast differentiation and function, rather than affecting osteoblasts, resulting in a high bone mass in male mice. To the best of our knowledge, this is the first study to explore the direct and indirect roles of GULP1 in bone remodeling, providing new insights into its regulation.

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

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

The nanoformula of zoledronic acid and calcium carbonate targets osteoclasts and reverses osteoporosis

Osteoporosis is known as an imbalance in bone catabolism and anabolism. Overactive bone resorption causes bone mass loss and increased incidence of fragility fractures. Antiresorptive drugs are widely used for osteoporosis treatment, and their inhibitory effects on osteoclasts (OCs) have been well established. However, due to the lack of selectivity, their off-target and side effects often bring suffering to patients. Herein, an OCs' microenvironment-responsive nanoplatform HA-MC/CaCO₃/ZOL@PBAE-SA (HMCZP) is developed, consisting of succinic anhydride (SA)-modified poly(β-amino ester) (PBAE) micelle, calcium carbonate shell, minocycline-modified hyaluronic acid (HA-MC) and zoledronic acid (ZOL). Results indicate that HMCZP, as compared with the first-line therapy, could more effectively inhibit the activity of mature OCs and significantly reverse the systemic bone mass loss in ovariectomized mice. In addition, the OCs-targeted capacity of HMCZP makes it therapeutically efficient at sites of severe bone mass loss and allows it to reduce the adverse effects of ZOL, such as acute phase reaction. High-throughput RNA sequencing (RNA-seq) reveals that HMCZP could down-regulate a critical osteoporotic target, tartrate-resistant acid phosphatase (TRAP), as well as other potential therapeutical targets for osteoporosis. These results suggest that an intelligent nanoplatform targeting OCs is a promising strategy for osteoporosis therapy.

Mechanisms and roles of podosomes and invadopodia

Cell invasion into the surrounding extracellular matrix or across tissue boundaries and endothelial barriers occurs in both physiological and pathological scenarios such as immune surveillance or cancer metastasis. Podosomes and invadopodia, collectively called 'invadosomes', are actin-based structures that drive the proteolytic invasion of cells, by forming highly regulated platforms for the localized release of lytic enzymes that degrade the matrix. Recent advances in high-resolution microscopy techniques, in vivo imaging and high-throughput analyses have led to considerable progress in understanding mechanisms of invadosomes, revealing the intricate inner architecture of these structures, as well as their growing repertoire of functions that extends well beyond matrix degradation. In this Review, we discuss the known functions, architecture and regulatory mechanisms of podosomes and invadopodia. In particular, we describe the molecular mechanisms of localized actin turnover and microtubule-based cargo delivery, with a special focus on matrix-lytic enzymes that enable proteolytic invasion. Finally, we point out topics that should become important in the invadosome field in the future.

The Ethanol Extracts of Osmanthus fragrans Leaves Ameliorate the Bone Loss via the Inhibition of Osteoclastogenesis in Osteoporosis

The aim of this study was to evaluate the anti-osteoporosis effects of Osmanthus fragrans leaf ethanol extract (OFLEE) in bone marrow-derived macrophages (BMM) and animals with osteoporosis. OFLEE not only suppressed tartrate-resistant acid phosphatase (TRAP)-positive cells with multiple nuclei but also decreased TRAP activity in BMM treated with macrophage colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL). The formation of F-actin rings and the expression and activation of matrix metalloproteinases were decreased by OFLEE in BMM treated with M-CSF and RANKL. OFLEE suppressed M-CSF- and RANKL-induced osteoclastogenesis by inhibiting NF-κB phosphorylation, tumor necrosis factor receptor-associated factor 6, c-fos, the nuclear factor of activated T-cells, cytoplasmic 1, and cathepsin K in BMM. OFLEE downregulated reactive oxygen species, cyclooxygenase-2, inducible nitric oxide synthase, prostaglandin E₂, tumor necrosis factor α, interleukin (IL)-1β, IL-6, IL-17, and RANKL in BMM treated with M-CSF and RANKL. Oral administration of OFLEE suppressed osteoporotic bone loss without hepatotoxicity in ovariectomy-induced osteoporosis animals. Our findings suggest that OFLEE, with anti-inflammatory effects, prevents osteoporotic bone loss through the suppression of osteoclastic differentiation in BMM and animals with osteoporosis.

A Novel RANKL-Targeted Furoquinoline Alkaloid Ameliorates Bone Loss in Ovariectomized Osteoporosis through Inhibiting the NF-κB Signal Pathway and Reducing Reactive Oxygen Species

Dysregulation of osteoclast-osteoblast balance, resulting in abnormal bone remodeling, is responsible for postmenopausal osteoporosis (PMOP) or other secondary forms of osteoporosis. We demonstrated that dictamnine (DIC), a novel RANKL-targeted furoquinoline alkaloid, inhibits osteoclastogenesis by facilitating the activities of reactive oxygen species (ROS), NF-κB, and NFATc1 in vitro and prevents the development of OVX-induced osteoporosis mouse models in vivo. Methods. The docking mechanism of DIC and RANKL was initially identified by protein–ligand molecular docking. RNA sequencing was performed and analyzed to reveal the potential mechanism and signaling pathway of the antiosteoporosis effects of DIC. To verify the sequencing results, we examined the impact of DIC on RANKL-induced osteoclast differentiation, bone resorption, F-actin ring production, ROS generation, and NF-κB activation in osteoclasts in vitro. Moreover, a luciferase assay was performed to determine the binding and transcriptional activity of Nrf2 and NF-κB. The in vivo efficacy of DIC was assessed with an ovariectomy- (OVX-) induced osteoporosis model, which was analyzed using micro-CT and bone histomorphometry. Results. The molecular docking results indicated that DIC could bind particularly to RANKL. RNA-seq confirmed that DIC could regulate the osteoclast-related pathway. DIC suppressed osteoclastogenesis, bone resorption, F-actin belt formation, osteoclast-specific gene expression, and ROS activity by preventing NFATc1 expression and affecting NF-κB signaling pathways in vitro. The luciferase assay showed that DIC not only suppressed the activity of Nrf2 but also contributed to the combination of Nrf2 and NF-κB. Our in vivo study indicated that DIC protects against OVX-induced osteoporosis and preserves bone volume by inhibiting osteoclast activity and function. Conclusions. DIC can ameliorate osteoclast formation and OVX-induced osteoporosis and therefore is a potential therapeutic treatment for osteoporosis.

Phospholipase D2 controls bone homeostasis by modulating M-CSF-dependent osteoclastic cell migration and microtubule stability

Phospholipase D2 (PLD2), a signaling protein, plays a central role in cellular communication and various biological processes. Here, we show that PLD2 contributes to bone homeostasis by regulating bone resorption through osteoclastic cell migration and microtubule-dependent cytoskeletal organization. Pld2-deficient mice exhibited a low bone mass attributed to increased osteoclast function without altered osteoblast activity. While Pld2 deficiency did not affect osteoclast differentiation, its absence promoted the migration of osteoclast lineage cells through a mechanism involving M-CSF-induced activation of the PI3K–Akt–GSK3β signaling pathway. The absence of Pld2 also boosted osteoclast spreading and actin ring formation, resulting in elevated bone resorption. Furthermore, Pld2 deletion increased microtubule acetylation and stability, which were later restored by treatment with a specific inhibitor of Akt, an essential molecule for microtubule stabilization and osteoclast bone resorption activity. Interestingly, PLD2 interacted with the M-CSF receptor (c-Fms) and PI3K, and the association between PLD2 and c-Fms was reduced in response to M-CSF. Altogether, our findings indicate that PLD2 regulates bone homeostasis by modulating osteoclastic cell migration and microtubule stability via the M-CSF-dependent PI3K–Akt–GSK3β axis.

A signaling protein that regulates bone resorption may prove a useful target in treating skeletal conditions such as osteoporosis and rheumatoid arthritis. Bone is synthesized by cells called osteoblasts, while osteoclasts trigger bone resorption, keeping the skeleton healthy. Imbalances in this recycling process are common in bone disorders. Je-Young Choi and Hyun-Ju Kim at Kyungpook National University in Daegu, South Korea, and co-workers demonstrated that phospholipase D2 (PLD2), a membrane protein, directly regulates bone resorption in mice. Mice without the Pld2 gene had increased osteoclast activity, resulting in low bone mass. The absence of PLD2 promotes the migration of osteoclasts via a particular signaling pathway. This increased the organization of microtubules, polymers that help form the cytoskeleton. The results suggest that regulating PLD2 activity could form the basis of a future treatment method.

Effect of cadmium on Rho GTPases signal transduction during osteoclast differentiation

Osteoclasts are the key target cells for cadmium (Cd)-induced bone metabolism diseases, while Rho GTPases play an important role in osteoclast differentiation and bone resorption. To identify new therapeutic targets of Cd-induced bone diseases; we evaluated signal transduction through Rho GTPases during osteoclast differentiation under the influence of Cd. In osteoclastic precursor cells, 10 nM Cd induced pseudopodia stretching, promoted cell migration, upregulated the levels of Cdc42, and RhoQ mRNAs and downstream Rho-associated coiled-coil kinase 1 (ROCK1) and ROCK2 proteins, and downregulated the actin-related protein 2/3 (ARP2/3) levels. Cd at 2 and 5 μM shortened the pseudopodia, inhibited cell migration, and decreased ROCK1, ROCK2, and ARP2/3 protein levels; Cd at 5 μM also reduced the mRNA expression levels of Rac1, Rac2, and RhoU mRNAs and decreased the level of phosphorylated (p)-cofilin. In osteoclasts, 10 nM Cd induced the formation of sealing zones, slightly upregulated Cdc42 mRNA levels and ROCK2 and ARP2/3 protein levels and significantly reduced p-cofilin levels. Cd at 2 μM and 5 μM Cd blocked the fusion of precursor cells; and 5 μM Cd downregulated the expression levels of RhoB, Rac1, Rac3, and RhoU mRNAs, and ROCK1, p-cofilin and ARP2/3 protein levels, significantly. In vivo, Cd (at 5 or 25 mg/L) increased the levels of key proteins RhoA, Rac1/2/3, Cdc42, and RhoU and their mRNAs in bone marrow cells. In summary, the results suggested that Cd affected the differentiation process of osteoclast and altered the expression of several Rho GTPases, which might be crucial targets of Cd during the differentiation of osteoclasts.

Nanoscale architecture and coordination of actin cores within the sealing zone of human osteoclasts

Osteoclasts are unique in their capacity to degrade bone tissue. To achieve this process, osteoclasts form a specific structure called the sealing zone, which creates a close contact with bone and confines the release of protons and hydrolases for bone degradation. The sealing zone is composed of actin structures called podosomes nested in a dense actin network. The organization of these actin structures inside the sealing zone at the nano scale is still unknown. Here, we combine cutting-edge microscopy methods to reveal the nanoscale architecture and dynamics of the sealing zone formed by human osteoclasts on bone surface. Random illumination microscopy allowed the identification and live imaging of densely packed actin cores within the sealing zone. A cross-correlation analysis of the fluctuations of actin content at these cores indicates that they are locally synchronized. Further examination shows that the sealing zone is composed of groups of synchronized cores linked by a-actinin1 positive filaments, and encircled by adhesion complexes. Thus, we propose that the confinement of bone degradation mediators is achieved through the coordination of islets of actin cores and not by the global coordination of all podosomal subunits forming the sealing zone.

Role of chromatin modulator Dpy30 in osteoclast differentiation and function

Osteoclasts are the principal bone resorption cells crucial for homeostatic bone remodeling and pathological bone destruction. Increasing data demonstrate a vital role of histone methylation in osteoclastogenesis. As an integral core subunit of H3K4 methyltransferases, Dpy30 is notal as a key chromatin regulator for cell growth and differentiation and stem cell fate determination, particularly in the hematopoietic system. However, its role in osteoclastogenesis is currently unknown. Herein, we generated Dpy30^F/F; LysM-Cre^+/+ mice, which deletes Dpy30 in myeloid cells, to characterize its involvement in osteoclast differentiation and function. Dpy30^F/F; LysM-Cre^+/+ mice showed increased bone mass, evident by impaired osteoclastogenesis and defective osteoclast activity, but no alteration of osteoblast numbers and bone formation. Additionally, our ex vivo analysis showed that the loss of Dpy30 significantly impedes osteoclast differentiation and suppresses osteoclast-related gene expression. Moreover, Dpy30 deficiency significantly decreased the enrichment of H3K4me3 on the promoter region of NFATc1. Thus, we revealed a novel role for Dpy30 in osteoclastogenesis through epigenetic mechanisms, and that it could potentially be a therapeutic target for bone destruction diseases.

Tyrosine Kinase Src Is a Regulatory Factor of Bone Homeostasis

Osteoclasts, which resorb the bone, and osteoblasts, which form the bone, are the key cells regulating bone homeostasis. Osteoporosis and other metabolic bone diseases occur when osteoclast-mediated bone resorption is increased and bone formation by osteoblasts is decreased. Analyses of tyrosine kinase Src-knockout mice revealed that Src is essential for bone resorption by osteoclasts and suppresses bone formation by osteoblasts. Src-knockout mice exhibit osteopetrosis. Therefore, Src is a potential target for osteoporosis therapy. However, Src is ubiquitously expressed in many tissues and is involved in various biological processes, such as cell proliferation, growth, and migration. Thus, it is challenging to develop effective osteoporosis therapies targeting Src. To solve this problem, it is necessary to understand the molecular mechanism of Src function in the bone. Src expression and catalytic activity are maintained at high levels in osteoclasts. The high activity of Src is essential for the attachment of osteoclasts to the bone matrix and to resorb the bone by regulating actin-related molecules. Src also inhibits the activity of Runx2, a master regulator of osteoblast differentiation, suppressing bone formation in osteoblasts. In this paper, we introduce the molecular mechanisms of Src in osteoclasts and osteoblasts to explore its potential for bone metabolic disease therapy.

Acid Neutralization and Immune Regulation by Calcium-Aluminum-Layered Double Hydroxide for Osteoporosis Reversion

Osteoporosis is a kind of global chronic bone disease characterized by progressive loss of bone mass and bone quality reduction, leading to a largely increased risk of bone fragility. In clinics, the current treatment of osteoporosis relies on the inhibition of bone damage by osteoclasts but ignores the function of immune cells in the progress of osteoporosis, leading to much compromised therapeutic efficacy. In this work, a highly effective osteoporosis-immunotherapeutic modality is established for the treatment of osteoporosis based on acid neutralization in synergy with immune microenvironment regulation by a specially designed nanocatalytic medicine, calcein functionalized calcium-aluminum-layered double hydroxide (CALC) nanosheets. Briefly, the mildly alkaline CALC nanosheets could neutralize the acidic microenvironment of osteoporosis accompanying the acidity-responsive LDH degradation. Subsequently, calcium phosphate nanoparticles (CAPs) are generated by the reaction between the released Ca²⁺ from LDH degradation and endogenous phosphates, resulting in M2 phenotype anti-inflammatory differentiation of bone macrophages through a c-Maf transcriptional factor pathway and the following activity enhancements of regulatory T cells (T_reg) and the deactivation of T helper 17 cells (T_H17). Both in vitro and in vivo results show an excellent therapeutic efficacy on osteoporosis featuring a significant BV/TV (%) enhancement of femurs from 6.2 to 10.7, demonstrating high feasibility of this therapeutic concept through the combined acid neutralization and immune regulation. Such an inorganic nanomaterial-based strategy provides a novel, efficient, and biosafe therapeutic modality for intractable osteoporosis treatment, which will benefit patients suffering from osteoporosis.

Terminally differentiated osteoclasts organize centrosomes into large clusters for microtubule nucleation and bone resorption

Osteoclasts are highly specialized, multinucleated cells responsible for the selective resorption of the dense, calcified bone matrix. Microtubules (MTs) contribute to the polarization and trafficking events involved in bone resorption by osteoclasts, however the origin of these elaborate arrays is less clear. Osteoclasts arise through cell fusion of precursor cells. Previous studies have suggested that centrosome MT nucleation is lost during this process, with the nuclear membrane and its surrounding Golgi serving as the major microtubule organizing centres (MTOCs) in these cells. Here we reveal that precursor cell centrosomes are maintained and functional in the multinucleated osteoclast and interestingly form large MTOC clusters, with the clusters organizing significantly more MTs, compared to individual centrosomes. MTOC cluster formation requires dynamic microtubules and minus-end directed MT motor activity. Inhibition of these centrosome clustering elements had a marked impact on both F-actin ring formation and bone resorption. Together these findings show that multinucleated osteoclasts employ unique centrosomal clusters to organize the extensive microtubules during bone attachment and resorption. [Media: see text].

CPT1A-Mediated Fatty Acid Oxidation Promotes Precursor Osteoclast Fusion in Rheumatoid Arthritis

The overproduction of osteoclasts, leading to bone destruction in patients with rheumatoid arthritis (RA), is well established. However, little is known about the metabolic dysfunction of osteoclast precursors (OCPs) in RA. Herein, we show that increasing fatty acid oxidation (FAO) induces OCP fusion. Carnitine palmitoyltransferase IA (CPT1A), which is important for carnitine transportation and is involved in FAO in the mitochondria, is upregulated in RA patients. This metabolic change further increases the expression of clathrin heavy chain (CLTC) and clathrin light chain A (CLTA) by enhancing the binding of the transcription factor CCAAT/enhancer binding protein β (C/EBPβ) to the promoters of CLTA and CLTC. This drives clathrin-dependent endocytosis pathway, which attenuates fusion receptors in the cellular membrane and contributes to increased podosome structure formation. This study reveals a new mechanism through which FAO metabolism participates in joint destruction in RA and provides a novel therapeutic direction for the development of drugs against bone destruction in patients with RA.

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

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

Substrate stiffness regulates the differentiation profile and functions of osteoclasts via cytoskeletal arrangement

Objectives

Aging and common diseases alter the stiffness of bone tissue, causing changes to the microenvironment of the mechanosensitive bone cells. Osteoclasts, the sole bone‐resorbing cells, play a vital role in bone remodeling. This study was performed to elucidate the mechanism through which osteoclasts sense and react to substrate stiffness signals.

Materials and methods

We fabricated polydimethylsiloxane (PDMS) substrates of different stiffness degrees for osteoclast formation progressed from osteoclast precursors including bone marrow‐derived macrophages (BMMs) and RAW264.7 monocytes. Osteoclast differentiation in response to the stiffness signals was determined by examining the cell morphology, fusion/fission activities, transcriptional profile, and resorption function. Cytoskeletal changes and mechanosensitive adhesion molecules were also assessed.

Results

Stiffer PDMS substrates accelerated osteoclast differentiation, firstly observed by variations in their morphology and fusion/fission activities. Upregulation of canonical osteoclast markers (Nfatc1, Acp5, Ctsk, Camk2a, Mmp9, Rela, and Traf6) and the fusion master regulator DC‐stamp were detected on stiffer substrates, with similar increases in their bone resorption functions. Additionally, the activation of cytoskeleton‐associated adhesion molecules, including fibronectin and integrin αvβ3, followed by biochemical signaling cascades of paxillin, FAK, PKC, and RhoA, was detected on the stiffer substrates.

Conclusions

This is the first study to provide evidence proving that extracellular substrate stiffness is a strong determinant of osteoclast differentiation and functions. Higher stiffness upregulated the differentiation profile and activity of osteoclasts, revealing the mechanical regulation of osteoclast activity in bone homeostasis and diseases.

The Neuropeptide VIP Limits Human Osteoclastogenesis: Clinical Associations with Bone Metabolism Markers in Patients with Early Arthritis

We aimed to evaluate the direct action of VIP on crucial molecules involved in human osteoclast differentiation and function. We also investigated the relationship between VIP serum levels and bone remodeling mediators in early arthritis patients. The expression of VIP receptors and osteoclast gene markers in monocytes and in vitro differentiated osteoclasts was studied by real-time PCR. NFATc1 activity was measured using a TransAM^® kit. Osteoclastogenesis was confirmed by quantification of tartrate-resistant acid phosphatase positive multinucleated cells. OsteoAssay^® Surface Multiple Well Plate was used to evaluate bone-resorbing activity. The ring-shaped actin cytoskeleton and the VPAC1 and VPAC2 expression were analyzed by immunofluorescence. We described the presence of VIP receptors in monocytes and mature osteoclasts. Osteoclasts that formed in the presence of VIP showed a decreased expression of osteoclast differentiation gene markers and proteolytic enzymes involved in bone resorption. VIP reduced the resorption activity and decreased both β3 integrin expression and actin ring formation. Elevated serum VIP levels in early arthritis patients were associated with lower BMD loss and higher serum OPG concentration. These results demonstrate that VIP exerts an anti-osteoclastogenic action impairing both differentiation and resorption activity mainly through the negative regulation of NFATc1, evidencing its bone-protective effects in humans.

Low kindlin-3 levels in osteoclasts of kindlin-3 hypomorphic mice result in osteopetrosis due to leaky sealing zones

Osteoclasts form special integrin-mediated adhesion structures called sealing zones that enable them to adhere to and resorb bone. Sealing zones consist of densely packed podosomes tightly interconnected by actin fibers. Their formation requires the presence of the hematopoietic integrin regulator kindlin-3 (also known as Fermt3). In this study, we investigated osteoclasts and their adhesion structures in kindlin-3 hypomorphic mice expressing only 5-10% of the kindlin-3 level of wild-type mice. Low kindlin-3 expression reduces integrin activity, results in impaired osteoclast adhesion and signaling, and delays cell spreading. Despite these defects, in vitro-generated kindlin-3-hypomorphic osteoclast-like cells arrange their podosomes into adhesion patches and belts, but their podosome and actin organization is abnormal. Remarkably, kindlin-3-hypomorphic osteoclasts form sealing zones when cultured on calcified matrix in vitro and on bone surface in vivo. However, functional assays, immunohistochemical staining and electron micrographs of bone sections showed that they fail to seal the resorption lacunae properly, which is required for secreted proteinases to digest bone matrix. This results in mild osteopetrosis. Our study reveals a new, hitherto understudied function of kindlin-3 as an essential organizer of integrin-mediated adhesion structures, such as sealing zones.

Are Osteoclasts Mechanosensitive Cells?

Skeleton metabolism is a process in which osteoclasts constantly remove old bone and osteoblasts form new osteoid and induce mineralization; disruption of this balance may cause diseases. Osteoclasts play a key role in bone metabolism, as osteoclastogenesis marks the beginning of each bone remodeling cycle. As the only cell capable of bone resorption, osteoclasts are derived from the monocyte/macrophage hematopoietic precursors that terminally adhere to mineralized extracellular matrix, and they subsequently break down the extracellular compartment. Bone is generally considered the load-burdening tissue, bone homeostasis is critically affected by mechanical conductions, and the bone cells are mechanosensitive. The functions of various bone cells under mechanical forces such as chondrocytes and osteoblasts have been reported; however, the unique bone-resorbing osteoclasts are less studied. The oversuppression of osteoclasts in mechanical studies may be because of its complicated differentiation progress and flexible structure, which increases difficulty in targeting mechanical structures. This paper will focus on recent findings regarding osteoclasts and attempt to uncover proposed candidate mechanosensing structures in osteoclasts including podosome-associated complexes, gap junctions and transient receptor potential family (ion channels). We will additionally describe possible mechanotransduction signaling pathways including GTPase ras homologue family member A (RhoA), Yes-associated protein/transcriptional co-activator with PDZ-binding motif (TAZ), Ca²⁺ signaling and non-canonical Wnt signaling. According to numerous studies, evaluating the possible influence of various physical environments on osteoclastogenesis is conducive to the study of bone homeostasis.

Evaluate the effects of serum urate level on bone mineral density: a genome-wide gene-environment interaction analysis in UK Biobank cohort

INTRODUCTION

Serum urate is associated with BMD and may be a protective factor. However, the exact association and mechanism are still unclear. We performed a genome-wide gene-environmental interaction study (GWGEIS) to explore the interaction effects between gene and urate on BMD, using data from the UK Biobank cohort.

METHODS

A total of 4575 participants for femur total BMD, 4561 participants for L1-L4 BMD, and 237799 participants for heel BMD were included in the present study. Linear regression models were used to test for associations between urate and BMD (femur total BMD, L1-L4 BMD, heel BMD) by R software. GWGEIS was conducted by PLINK 2.0 using a generalize linear model, adjusted for age, sex, weight, smoking behavior, drinking behavior, physical activity and 10 principle components for population structure.

RESULTS

Results showed that urate was positively associated with femur total BMD, L1-L4 BMD and heel BMD and similar findings were observed in both the male and female subgroups. GWGEIS identified 261 genome-wide significant (P < 5.00 × 10^-8) SNP × urate interaction effects for femur total BMD (rs8192585 in NOTCH4, rs116080577 in PBX1, rs9409991 in COL5A1), 17 genome-wide significant SNP × urate interaction effects for heel BMD (rs145344540 in PDE11A and rs78485379 in DKK2), 17 suggestive genome-wide SNP × urate interaction effects (P < 1.00 × 10^-5) for L1-L4 BMD (rs10977015 in PTPRD). We also detected genome-wide significant and suggestive SNP × urate interaction effects for BMD in both the male and female subgroups.

CONCLUSIONS

This study reported several novel candidate genes, and strengthen the evidence of the interactive effects between gene and urate on the variations of BMD.

Small GTPases all over invadosomes

Cell invasion is associated with numerous patho-physiologic states including cell development and metastatic dissemination. This process couples the activation of cell motility with the capacity to degrade the extracellular matrix, thereby permitting cells to pass through basal membranes. Invasion is sustained by the actions of invadosomes, an ensemble of subcellular structures with high functional homology. Invadosomes are 3D acto-adhesive structures that can also mediate local extracellular matrix degradation through the controlled delivery of proteases. Intracellular RHO GTPases play a central role in the regulation of invadosomes where their complex interplay regulates multiple invadosome functions. This review aims to provide an overview of the synergistic activities of the small GTPases in invadosome biology. This broad-based review also reinforces the importance of the spatiotemporal regulation of small GTPases and the impact of this process on invadosome dynamics.

Effects of Melandrium firmum Rohrbach on RANKL‑induced osteoclast differentiation and OVX rats

Osteoporosis is a systemic skeletal disease characterized by reduced bone mineral density (BMD), which results in an increased risk of fracture. Melandrium firmum (Siebold & Zucc.) Rohrbach (MFR), 'Wangbulryuhaeng' in Korean, is the dried aerial portion of Melandrii Herba Rohrbach, which is a member of the Caryophyllaceae family and has been used to treat several gynecological conditions as a traditional medicine. However, to the best of our knowledge, the effect of MFR on osteoclast differentiation and osteoporosis has not been assessed. To evaluate the effects of MFR on osteoclast differentiation, tartrate‑resistant acid phosphatase staining, actin ring formation and bone resorption assays were used. Additionally, receptor activator of nuclear factor‑κB ligand‑induced expression of nuclear factor of activated T cell, cytoplasmic 1 (NFATc1) and c‑Fos were measured using western blotting and reverse transcription‑PCR. The expression levels of osteoclast‑related genes were also examined. To further investigate the anti‑osteoporotic effects of MFR in vivo, an ovariectomized (OVX) rat model of menopausal osteoporosis was established. Subsequently, the femoral head was scanned using micro‑computed tomography. The results revealed that MFR suppressed osteoclast differentiation, formation and function. Specifically, MFR reduced the expression levels of osteoclast‑related genes by downregulating transcription factors, such as NFATc1 and c‑Fos. Consistent with the in vitro results, administration of MFR water extract to OVX rats reduced BMD loss, and reduced the expression levels of NFATc1 and cathepsin K in the femoral head. In conclusion, MFR may contribute to alleviate osteoporosis‑like symptoms. These results suggested that MFR may exhibit potential for the prevention and treatment of postmenopausal osteoporosis.

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.

IgG Immunocomplexes Drive the Differentiation of a Novel Subset of Osteoclasts Independent of RANKL and Inflammatory Cytokines

Potentiation of receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis by IgG immunocomplexes (ICs) is generally considered an important pathway leading to cartilage and bone destruction in rheumatoid arthritis (RA). However, whether IgG ICs possess pro-osteoclastogenic potential independent of RANKL and inflammatory cytokines is unclear. Here we demonstrate that by fully cross-linking human FcγRIIa (hFcγRIIa) or co-ligating hFcγRIIa and TLR4, IgG ICs alone could drive the differentiation of human blood monocytes into nuclear factor of activated T cells cytoplasmic 1 (NFATc1-negative nonclassical osteoclasts (NOCs). Surprisingly, IgG ICs could also overrule RANKL-induced classical osteoclast (COC) differentiation in vitro. In mouse model of collagen-induced arthritis, hFcγRIIa-transgenic, but not nontransgenic control, mice suffered from cartilage/bone destruction accompanied by the presence of NFATc1^- NOCs lining the eroded cartilage surface in affected joints. Our results not only identify a novel subset of IC-induced NOCs but also provide a possible explanation for the uncoupling of FcγR-mediated cartilage destruction from RANKL-related bone erosion in autoinflammatory arthritis. © 2021 American Society for Bone and Mineral Research (ASBMR)..

Characterization of unique functionalities in c-Src domains required for osteoclast podosome belt formation

Deletion of c-Src, a ubiquitously expressed tyrosine kinase, results in osteoclast dysfunction and osteopetrosis, in which bones harden into "stone." In contrast, deletion of the genes encoding other members of the Src family kinase (SFK) fails to produce an osteopetrotic phenotype. This suggests that c-Src performs a unique function in the osteoclast that cannot be compensated for by other SFKs. We aimed to identify the molecular basis of this unique role in osteoclasts and bone resorption. We found that c-Src, Lyn, and Fyn were the most highly expressed SFKs in WT osteoclasts, whereas Hck, Lck, Blk, and Fgr displayed low levels of expression. Formation of the podosome belt, clusters of unique actin assemblies, was disrupted in src^-/- osteoclasts; introduction of constitutively activated SFKs revealed that only c-Src and Fyn could restore this process. To identify the key structural domains responsible, we constructed chimeric Src-Hck and Src-Lyn constructs in which the unique, SH3, SH2, or catalytic domains had been swapped. We found that the Src unique, SH3, and kinase domains were each crucial to establish Src functionality. The SH2 domain could however be substituted with Lyn or Hck SH2 domains. Furthermore, we demonstrate that c-Src's functionality is, in part, derived from an SH3-proximal proline-rich domain interaction with c-Cbl, leading to phosphorylation of c-Cbl Tyr700. These data help clarify Src's unique functionality in the organization of the cytoskeleton in osteoclasts, required for efficient bone resorption and explain why c-Src cannot be replaced, in osteoclasts, by other SFKs.

Effects of varying gelatin coating concentrations on RANKL induced osteoclastogenesis

Here, we assessed the effects of varying concentrations of gelatin coating on Receptor Activator of Nuclear Factor κ-B Ligand (RANKL)-induced RAW264.7 murine macrophage differentiation into osteoclast (OC) via osteoclastogenesis. The microstructures of coating surfaces with different concentrations of gelatin were examined by scanning electron microscopy and atomic force microscopy. Increased gelatin coating concentrations led to decreased gel rigidity but increased surface adhesion force attenuated OC differentiation and the decreased actin ring formation in RANKL-induced osteoclastogenesis. The decreased actin ring formation is associated with decreased lysosomal-associated membrane protein 1 (LAMP1) activity and bone resorption in the differentiated OCs with different gelatin coating concentrations as compared to the cells differentiated without gelatin coatings. In addition, increasing concentrations of gelatin coating attenuated the medium TGF-β1 protein levels and the expression levels of TGF-β and type-I (R1) and type-II (R2) TGF-β receptors in OCs, suggesting the gelatin-induced suppression of TGF-β signaling for the regulation of RNAKL-induced OC differentiation. Taken together, these findings showed that changes in gelatin coating concentrations, which were associated with altered gel thickness and substrate rigidity, might attenuate TGF-β signaling events to modulate OC differentiation and concomitant actin ring formation and bone matrix resorption in RANKL-induced osteoclastogenesis.

Targeted inhibition of ATP5B gene prevents bone erosion in collagen-induced arthritis by inhibiting osteoclastogenesis

Bone resorption by osteoclasts is an energy consuming activity, which depends on mitochondrial ATP. ATP5B, a mitochondrial ATP synthase beta subunit, is a catalytic core involved in producing ATP. Here, we investigated the contribution of ATP5B in osteoclast differentiation and joint destruction. ATP5B (LV-ATP5B) targeting or non-targeting (LV-NC) siRNA containing lentivirus particles were transduced into bone marrow macrophage derived osteoclasts or locally administered to arthritic mouse joints. Inhibition of ATP5B reduced the expression of osteoclast related genes and proteins, suppressed bone resorption by significantly impairing F-actin formation and decreased the levels of adhesion-associated proteins. In addition, ATP5B deficiency caused osteoclast mitochondrial dysfunction and, impaired the secretion of vacuole protons and MMP9. Importantly, inhibition of ATP5B expression, protected arthritis mice from joint destructions although serum levels of inflammatory mediators (TNF-α, IL-1β) and IgG2α antibodies were unaffected. These results demonstrate an essential function of ATP5B in osteoclast differentiation and bone resorption, and suggest it as a potential therapeutic target for protecting bones in RA.

Pristimerin Protects Against OVX-Mediated Bone Loss by Attenuating Osteoclast Formation and Activity via Inhibition of RANKL-Mediated Activation of NF-κB and ERK Signaling Pathways

Introduction

Osteoporosis is an osteolytic bone condition characterized by decreased bone strength and increased bone fragility. It is the result of elevated formation or activity of bone-resorbing osteoclasts. Although current therapeutic agents are efficacious against osteoclast-mediated bone loss, detrimental side effects preclude the long-term use of these agents. Pristimerin (PRI) is a naturally occurring quinone-methide triterpenoid that has been revealed to exert anti-inflammatory and anti-tumor effects via regulating various signaling cascades including NF-κB and MAPK activation.

Methods

The bone marrow macrophages were used to confirm the anti-osteoclastic and anti-resorptive functions of PRI in vitro. An in vivo ovariectomy (OVX) model was applied to verify the function of PRI protecting bone loss.

Results

PRI abolished the early activation of NF-κB and ERK MAPK signal cascades thereby thwarting the downstream expression of c-Fos and NFATc1, which prevented the production of mature osteoclasts. In vivo, PRI protects mice against ovariectomy (OVX)-mediated bone loss by diminishing osteoclast formation and bone resorptive activity.

Conclusion

Our study shows that PRI demonstrates therapeutic potential in the effective treatment against osteoclast-induced osteolytic diseases like osteoporosis.

The Regulatory Role of Rho GTPases and their Substrates in Osteoclastogenesis

Pathological bone loss diseases (osteolysis, Paget's diseases) are commonly caused by the excessive differentiation and activity of osteoclasts. The Rho GTPases family members Rac1/2 (Rac1 and Rac2) have been reported for their special role in exerting multiple cellular functions during osteoclastic differentiation, which includes the most prominent function on dynamic actin cytoskeleton rearranging. Besides that, the increasing studies demonstrated that the regulating effects of Rac1/2 on the osteoclastic cytoskeletal organization are through the GEFs member Dock5. Although the amount of relevant studies on this topic is still limited, several excellent studies have been reported that extensively explored the molecular mechanisms involved in Rac1/2 and Dock5 during the osteoclastogenesis regulation, as well as their role as the therapeutic target in bone loss diseases. Herein, in this review, we aim to focus on recent advances studies for extensively understanding the role of Rho GTPases Rac1/2 and Dock5 in osteoclastogenesis, as well as their role as a potential therapeutic target in regulating osteoclastogenesis.

Pantoprazole (PPZ) Inhibits RANKL-Induced Osteoclast Formation and Function In Vitro and Prevents Lipopolysaccharide- (LPS-) Induced Inflammatory Calvarial Bone Loss In Vivo

Bone remodeling is a process delicately balanced between osteoclastic bone resorption and osteoblastic bone formation. Osteoclasts (OCs) are multinucleated giant cells formed through the fusion of monocytic precursors of the hematopoietic stem cells lineage. OCs are the exclusive cells responsible for the resorption and degradation of the mineralized bone matrix. Pantoprazole (PPZ), a proton pump inhibitor (PPI), is commonly prescribed to reduce excess gastric acid production for conditions such as gastroesophageal reflux disease and peptic ulcer disease. Studies have found contradictory effects of PPI therapy on bone metabolism due to the lack of understanding of the exact underlying mechanism. In this study, we found that PPZ inhibits receptor activator of nuclear factor-κB (NF-κB) ligand- (RANKL-) induced osteoclastogenesis from bone marrow monocytic/macrophage (BMMs) precursors and the bone-resorbing activity of mature OCs. Correspondingly, the expression of OC marker genes was also attenuated. At the molecular level, PPZ treatment was associated with reduced activation of the ERK MAPK signaling pathways crucial to OC differentiation. Additionally, the in vivo administration of PPZ protected mice against lipopolysaccharide- (LPS-) induced inflammatory calvarial bone erosion, as a result of the reduced number and activity of OCs on the calvarial bone surface. Although PPI use is associated with increased risk of osteoporosis and bone fractures, our study provides evidence for the direct inhibitory effect of PPZ on OC formation and bone resorption in vitro and in vivo, suggesting a potential therapeutic use of PPZ in the treatment of osteolytic disease with localized bone destruction.

Surface distribution of heterogenous clathrin assemblies in resorbing osteoclasts

Osteoclasts seeded on either glass coverslips or apatite pellets have at least two morphologically distinct substrate adhesion sites: actin-based adhesion structures including podosome belts and sealing zones, and adjacent clathrin sheets. Clathrin-coated structures are exclusively localized at the podosome belts and sealing zone, in both of which the plasma membrane forms a tight attachment to the substrate surface. When cultured on apatite osteoclasts can degrade the apatite leading to the formation of resorption lacunae. The sealing zone divides the ventral membrane into different domains, outside and inside of the sealing zones. The former facing the smooth-surfaced intact apatite contains relatively solitary or networks of larger flat clathrin structures; and the latter, facing the rough-surfaced degraded apatite in the resorption lacunae contain clathrin in various shapes and sizes. Clathrin assemblies on the membrane domain facing not only a resorption lacuna, or trails but also intact apatite indeed were observed to be heterogeneous in size and intensity, suggesting that they appeared to follow variations in the surface topography of the apatite surface. These results provide a detailed insight into the flat clathrin sheets that have been suggested to be the sites of adhesion and mechanosensing in co-operation with podosomes.

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.