The osteoclast: friend or foe?

Cutaneous granulomas: mechanisms, cellular interactions and therapeutic insights

Granulomas are specialized biologic defence mechanisms that form in response to infections by pathogens, foreign bodies or specific stimuli such as antimicrobials or fungi. These structures function to isolate foreign materials and pathogens that cannot be eliminated by immune cells, primarily through macrophage activity. In the skin, granulomas are a hallmark of several conditions, including sarcoidosis, granuloma annulare, tuberculosis and leprosy, each exhibiting distinct pathological and immunological features. Granulomas can also arise from lipid accumulation, as observed in xanthogranuloma, or be triggered by inflammatory processes associated with unidentified antigens. Among their cellular components, Langhans-type multinucleated giant cells play a pivotal role in granuloma structure and function, contributing to pathogen containment and tissue remodelling, although their precise mechanisms of action remain an area of active investigation. In addition to these giant cells, recent studies have identified triggering receptors expressed on myeloid cells 2 (TREM2)+ macrophages as key contributors to granuloma formation and maintenance. These macrophages are involved in extracellular degradation of foreign substances and play a role in adapting to the hypoxic and nutrient-poor microenvironment of granulomas through metabolic reprogramming, including the pentose phosphate pathway. Recent advances in molecular biology, such as single-cell RNA sequencing, have provided unprecedented insights into the cellular heterogeneity and molecular pathways involved in granuloma formation. These techniques have revealed disease-specific differences in immune cell profiles and activation states, offering new perspectives on the underlying mechanisms of granulomatous diseases. Despite these advances, the precise processes driving granuloma formation and their functional significance remain largely unclear. This review addresses the central question, 'What is a granuloma?', by synthesizing recent findings, with a particular focus on cutaneous granulomas, and presenting interpretations grounded in the current body of literature. We also discuss the implications of these findings for the development of novel therapeutic strategies, including targeted immunomodulation and cytokine blockade, which hold promise for treating granulomatous diseases while preserving host defence.

Self-assembled water soluble and bone-targeting phosphorylated quercetin ameliorates postmenopausal osteoporosis in ovariectomy mice

Natural compounds have shown promising application prospects in preventing or treating various diseases, including osteoporosis on account of their abundant sources, low price, multi-targeting and multiple biological effects. As a bioactive natural product, quercetin (Que) has previously demonstrated to ameliorate osteoporosis (OP), however, its poor bioavailability resulting from low water solubility, poor stability and lack of bone-targeting largely restricted its efficacy and clinical applications. Inspired by the bone-targeting capability of phosphate compounds, we reported a one-step procedure for synthesis of phosphorylated Que (p-Que) by direct phosphorylating phenol groups of Que for the first time. The phosphate groups on p-Que could not only improve the water dispersibility of Que, but also endow p-Que desirable bioavailability and bone-targeting feature. The results from biological assays suggested that p-Que could inhibit osteoclastogenesis and bone resorption and alleviate trabeculae loss in osteoporotic mice. In conclusion, this work demonstrated that phosphorylation strategy can effectively solve low water solubility, lack of bone-targeting capability and poor bioavailability of natural compounds, providing a novel and efficient approach for development of OP nanomedicines.

A comprehensive review of pathology and treatment of staphylococcus aureus osteomyelitis

Osteomyelitis (OM) is an inflammation of the bone and bone marrow triggered by infectious pathogens which may induce progressive bone destruction. The majority of OM cases, especially the chronic OM cases, are induced by the most prevalent and devastating pathogen Staphylococcus aureus (S. aureus), partially due to its resistance mechanisms against the immune system and antibiotic therapies. Regarding the high rate of morbidity and recurrence in patients, it is pivotal to elucidate underlying mechanisms that how S. aureus enter and survive in hosts. The accumulated discoveries have identified multiple distinct strategies associated with chronicity and recurrence include biofilm development, small colony variants (SCVs), staphylococcus abscess communities (SACs), the osteocyte lacuno-canalicular network invasion (OLCN) of cortical bones, and S. aureus protein A (SpA). Unfortunately, little clinical progress has been achieved for the diagnosis and therapeutic treatment for OM patients, indicating that numerous questions remain to be solved. Therefore, we still have a long way to obtain the clear elucidation of the host-pathogen interactions which could be applied for clinical treatment of OM. In this review, we provide insights of current knowledge about how S. aureus evades immune eradication and remains persistent in hosts with recent discoveries. The common and novel treatment strategies for OM are also described. The purpose of this review is to have in-dept understanding of S. aureus OM and bring new perspectives to therapeutic fields which may be translated to the clinic.

Atractylenolide I Attenuates Glucocorticoid-Induced Osteoporosis via Inhibiting NF-κB Signaling Pathway

Long-term treatment with glucocorticoids significantly impacts bone health, with glucocorticoid-induced osteoporosis (GIOP) being the most prevalent consequence. Previous studies have established that Atractylenolide I (Atr I) possesses anti-inflammatory, antioxidant and anti-tumor properties, however, its specific effects on osteoclastogenesis and GIOP are still unclear. In this study, our in vitro results revealed that Atr I inhibited RANKL-stimulated osteoclast differentiation in a dose-dependent manner, disrupted the structure of the F-actin belt in mature osteoclasts, blocked RANKL-induced ROS production, and suppressed the expression of osteoclast-associated genes. Mechanistically, the findings indicated that Atr I inhibited the RANKL-induced activation of the NF-κB signaling pathway. In vivo, the micro-CT, bone histomorphometric analysis and histological data demonstrated that Dex administration led to significant bone loss, accompanied by a considerable increase in the number of osteoclasts on the bone surface. Conversely, treatment with Atr I effectively prevented these Dex-induced alterations. Taken together, this study suggests that Atr I may hold potential as a therapeutic agent for the treatment of GIOP.

Dectin-2 depletion alleviates osteoclast-induced bone loss in periodontitis via Syk/NOX2/ROS signaling

Periodontitis is the sixth most common disease worldwide and is closely associated with various systemic diseases, impacting overall health. It is characterized by the over-differentiation and activity of osteoclasts, leading to increased bone resorption and subsequent bone loss. Current treatments for bone loss are not ideal, highlighting the need for new targeted therapeutic strategies. Dectin-2, a member of the C-type lectin receptor (CLR) family, has recently been reported to play an important role in immune regulation, but its role in osteoclastogenesis has not been documented. This study identified a significant upregulation of Dectin-2 expression during osteoclast differentiation through single-cell sequencing and transcriptomic analysis. Knocking down Dectin-2 significantly inhibits the differentiation of RAW264.7 cells and bone marrow-derived macrophages (BMDMs) into osteoclasts, while overexpressing Dectin-2 enhances osteoclast differentiation and function. Mechanistically, transcriptomic analysis indicates that Dectin-2 deficiency disrupts redox homeostasis and affects the MAPK signaling pathway. Furthermore, the study demonstrates that Dectin-2 promotes osteoclastogenesis via the Syk/NOX2/ROS/MAPK signaling axis. In vivo, Dectin-2 knockout mice show reduced osteoclast numbers and decreased alveolar bone resorption in a periodontitis model. In conclusion, these findings suggest that Dectin-2 is a key regulatory factor in osteoclast-mediated bone resorption and may serve as a promising therapeutic target for bone diseases characterized by osteoclast overactivity, such as periodontitis.

The physiological and pathogenic roles of yes-associated protein/transcriptional co-activator with PDZ-binding motif in bone or skeletal motor system-related cells

Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are the primary downstream effectors of the Hippo signaling pathway. This pathway plays a crucial role in regulating organ size, maintaining tissue homeostasis, and controlling cellular processes such as fate determination and tissue development. This review provides an overview of the current understanding of how the transcriptional regulators YAP and TAZ contribute to the physiological and pathological processes in tissues and cells associated with the skeletal motor system. The underlying molecular mechanisms and mechanical transduction were reviewed.

Rethinking Biomedical Titanium Alloy Design: A Review of Challenges from Biological and Manufacturing Perspectives

Current biomedical titanium alloys have been repurposed from other industries, which has contributed to several biologically driven implant failure mechanisms. This review highlights the added value that may be gained by building an appreciation of implant biological responses at the onset of alloy design. Specifically, the fundamental mechanisms associated with immune response, angiogenesis, osseointegration and the potential threat of infection are discussed, including how elemental selection can modulate these pivotal systems. With a view to expedite inclusion of these interactions in alloy design criteria, methods to analyze these performance characteristics are also summarized. While machine learning techniques are being increasingly used to unearth complex relationships between alloying elements and material properties, much is still unknown about the correlation between composition and some bio-related properties. To bridge this gap, high-throughput methods are also reviewed to validate biological response along with cutting edge manufacturing approaches that may support rapid discovery. Taken together, this review encourages the alloy development community to rethink their approach to enable a new generation of biomedical implants intrinsically designed for a life in the body, including functionality to tackle biological challenges thereby offering improved patient outcomes.

Albiflorin inhibits osteoclastogenesis and titanium particles-induced osteolysis via inhibition of ROS accumulation and the PI3K/AKT signaling pathway

Periprosthetic osteolysis (PPO), caused by wear particles, is a significant complication of total joint replacement, leading to prosthesis failure. Previous research has highlighted the crucial role of osteoclast-induced bone destruction in PPO progression. Albiflorin (AF), a monoterpene glycoside from Paeonia lactiflora, is a key active ingredient known for its antioxidant and anti-inflammatory properties. Although AF has shown promise in treating various conditions, its impact on osteoclasts and PPO remains unexplored. Our study revealed that AF could effectively inhibit osteoclast differentiation to reduce overactivated bone resorption and effectively inhibit the accumulation of reactive oxygen species (ROS) induced by wear particles. In vitro experiments also confirmed that AF could effectively inhibit the PI3K/AKT signaling pathway and inhibit inflammation to regulate osteoclast generation. Studies in animal models have also verified the antioxidant and anti-inflammatory properties of AF. In summary, the above studies indicate that AF inhibits osteoclastogenesis via inhibiting ROS accumulation and the PI3K/AKT signaling pathway, which may be a potential therapeutic method for PPO.

Application of loaded graphene oxide biomaterials in the repair and treatment of bone defects

Addressing bone defects is a complex medical challenge that involves dealing with various skeletal conditions, including fractures, osteoporosis (OP), bone tumours, and bone infection defects. Despite the availability of multiple conventional treatments for these skeletal conditions, numerous limitations and unresolved issues persist. As a solution, advancements in biomedical materials have recently resulted in novel therapeutic concepts. As an emerging biomaterial for bone defect treatment, graphene oxide (GO) in particular has gained substantial attention from researchers due to its potential applications and prospects. In other words, GO scaffolds have demonstrated remarkable potential for bone defect treatment. Furthermore, GO-loaded biomaterials can promote osteoblast adhesion, proliferation, and differentiation while stimulating bone matrix deposition and formation. Given their favourable biocompatibility and osteoinductive capabilities, these materials offer a novel therapeutic avenue for bone tissue regeneration and repair. This comprehensive review systematically outlines GO scaffolds' diverse roles and potential applications in bone defect treatment.

Cellular signatures in human blood track bone mineral density in postmenopausal women

Osteoclasts are the sole bone-resorbing cells and are formed by the fusion of osteoclast precursor cells (OCPs) derived from myeloid lineage cells. Animal studies reveal that circulating OCPs (cOCPs) in blood travel to bone and fuse with bone-resident osteoclasts. However, the characteristics of human cOCPs and their association with bone diseases remain elusive. We have identified and characterized human cOCPs and found a positive association between cOCPs and osteoclast activity. Sorted cOCPs have a higher osteoclastogenic potential than other myeloid cells and effectively differentiate into osteoclasts. cOCPs exhibit distinct morphology and transcriptomic signatures. The frequency of cOCPs in the blood varies among treatment-naive postmenopausal women and has an inverse correlation with lumbar spine bone density and a positive correlation with serum CTX, a bone resorption marker. The increased cOCPs in treatment-naive patients with osteoporosis were significantly diminished by denosumab, a widely used antiresorptive therapy. Our study reveals the distinctive identity of human cOCPs and the potential link between the dynamic regulation of cOCPs and osteoporosis and its treatment. Taken together, our study enhances our understanding of human cOCPs and highlights a potential opportunity to measure cOCPs through a simple blood test, which could potentially identify high-risk individuals.

Preventing osteoporotic bone loss in mice by promoting balanced bone remodeling through M-CSFRGD, a dual antagonist to c-FMS and αvβ3 receptors

Osteoporosis is a common, age-related disease caused by imbalanced bone remodeling. Current treatments either shut down bone resorption or robustly stimulate bone formation. Here, we describe a novel compound that inhibits osteoclast activity without causing apparent disruptions to bone formation by targeting both c-FMS (i.e., osteoclast differentiation) and αvβ3 integrin (i.e., osteoclastic bone resorption) receptors. We show that human serum albumin (HSA)-conjugated M-CSFRGD protein (M-CSFRGD-HSA) effectively inhibits the activity of both receptors, with a three-fold higher serum half-life compared to the unconjugated M-CSFRGD. We then treated ovariectomized mice with different doses of M-CSFRGD-HSA, alendronate, or a monospecific control protein. The bispecific M-CSFRGD-HSA was superior to a monospecific control in alleviating bone loss and reducing osteoclast distribution and function. M-CSFRGD-HSA and alendronate effectively prevented ovariectomy-induced bone loss, but M-CSFRGD-HSA had a milder inhibitory effect on osteoclast distribution and activity. Moreover, alendronate halted bone formation, while M-CSFRGD-HSA-treated mice showed an increased level of serum amino-terminal propeptide of type I collagen, a bone formation marker. Our data indicate that the mild reduction in osteoclast activity facilitated by the bispecific M-CSFRGD-HSA allows the maintenance of certain levels of bone formation and may be superior to treatments that induce osteoclast depletion.

PRMT6 Epigenetically Drives Metabolic Switch from Fatty Acid Oxidation toward Glycolysis and Promotes Osteoclast Differentiation During Osteoporosis

Epigenetic regulation of metabolism profoundly influences cell fate commitment. During osteoclast differentiation, the activation of RANK signaling is accompanied by metabolic reprogramming, but the epigenetic mechanisms by which RANK signaling induces this reprogramming remain elusive. By transcriptional sequence and ATAC analysis, this study identifies that activation of RANK signaling upregulates PRMT6 by epigenetic modification, triggering a metabolic switching from fatty acids oxidation toward glycolysis. Conversely, Prmt6 deficiency reverses this shift, markedly reducing HIF-1α-mediated glycolysis and enhancing fatty acid oxidation. Consequently, PRMT6 deficiency or inhibitor impedes osteoclast differentiation and alleviates bone loss in ovariectomized (OVX) mice. At the molecular level, Prmt6 deficiency reduces asymmetric dimethylation of H3R2 at the promoters of genes including Ppard, Acox3, and Cpt1a, enhancing genomic accessibility for fatty acid oxidation. PRMT6 thus emerges as a metabolic checkpoint, mediating metabolic switch from fatty acid oxidation to glycolysis, thereby supporting osteoclastogenesis. Unveiling PRMT6's critical role in epigenetically orchestrating metabolic shifts in osteoclastogenesis offers a promising target for anti-resorptive therapy.

SNX10 regulates osteoclastogenic cell fusion and osteoclast size in mice

Bone-resorbing osteoclasts (OCLs) are formed by differentiation and fusion of monocyte precursor cells, generating large multinucleated cells. Tightly regulated cell fusion during osteoclastogenesis leads to formation of resorption-competent OCLs, whose sizes fall within a predictable physiological range. The molecular mechanisms that regulate the onset of OCL fusion and its subsequent arrest are, however, largely unknown. We have previously shown that OCLs cultured from mice homozygous for the R51Q mutation in the vesicle trafficking-associated protein sorting nexin 10, a mutation that induces autosomal recessive osteopetrosis in humans and in mice, display deregulated and continuous fusion that generates gigantic, inactive OCLs. Fusion of mature OCLs is therefore arrested by an active, genetically encoded, cell-autonomous, and SNX10-dependent mechanism. To directly examine whether SNX10 performs a similar role in vivo, we generated SNX10-deficient (SKO) mice and demonstrated that they display massive osteopetrosis and that their OCLs fuse uncontrollably in culture, as do homozygous R51Q SNX10 (RQ/RQ) mice. OCLs that lack SNX10 exhibit persistent presence of DC-STAMP protein at their periphery, which may contribute to their uncontrolled fusion. To visualize endogenous SNX10-mutant OCLs in their native bone environment, we genetically labeled the OCLs of WT, SKO, and RQ/RQ mice with enhanced Green Fluorescent Protein (EGFP), and then visualized the 3D organization of resident OCLs and the pericellular bone matrix by 2-photon, confocal, and second harmonics generation microscopy. We show that the volumes, surface areas and, in particular, the numbers of nuclei in the OCLs of both mutant strains were on average 2-6-fold larger than those of OCLs from WT mice, indicating that deregulated, excessive fusion occurs in the mutant mice. We conclude that the fusion of OCLs, and consequently their size, is regulated in vivo by SNX10-dependent arrest of fusion of mature OCLs.

Moesin controls cell-cell fusion and osteoclast function

Cell-cell fusion is an evolutionarily conserved process that is essential for many functions, including fertilisation and the formation of placenta, muscle and osteoclasts, multinucleated cells that are unique in their ability to resorb bone. The mechanisms of osteoclast multinucleation involve dynamic interactions between the actin cytoskeleton and the plasma membrane that are still poorly characterized. Here, we found that moesin, a cytoskeletal linker protein member of the Ezrin/Radixin/Moesin (ERM) protein family, is activated during osteoclast maturation and plays an instrumental role in both osteoclast fusion and function. In mouse and human osteoclast precursors, moesin inhibition favors their ability to fuse into multinucleated osteoclasts. Accordingly, we demonstrated that moesin depletion decreases membrane-to-cortex attachment and enhances the formation of tunneling nanotubes (TNTs), F-actin-based intercellular bridges that we reveal here to trigger cell-cell fusion. Moesin also controls HIV-1- and inflammation-induced cell fusion. In addition, moesin regulates the formation of the sealing zone, the adhesive structure determining osteoclast bone resorption area, and thus controls bone degradation, via a β3-integrin/RhoA/SLK pathway. Supporting our results, moesin - deficient mice present a reduced density of trabecular bones and increased osteoclast abundance and activity. These findings provide a better understanding of the regulation of cell-cell fusion and osteoclast biology, opening new opportunities to specifically target osteoclast activity in bone disease therapy.

Osteoporosis and Alzheimer´s disease (or Alzheimer´s disease and Osteoporosis)

Alzheimer's disease (AD) and osteoporosis are two diseases that mainly affect elderly people, with increases in the occurrence of cases due to a longer life expectancy. Several epidemiological studies have shown a reciprocal association between both diseases, finding an increase in incidence of osteoporosis in patients with AD, and a higher burden of AD in osteoporotic patients. This epidemiological relationship has motivated the search for molecules, genes, signaling pathways and mechanisms that are related to both pathologies. The mechanisms found in these studies can serve to improve treatments and establish better patient care protocols.

A p38 MAP kinase inhibitor suppresses osteoclastogenesis and alleviates ovariectomy-induced bone loss through the inhibition of bone turnover

Inhibition of excessive osteoclastic activity is an efficient therapeutic strategy for many bone diseases induced by increased bone resorption, such as osteoporosis. BMS-582949, a clinical p38α inhibitor, is a promising drug in Phase II studies for treating rheumatoid arthritis. However, its function on bone resorption is largely unknown. In this study, we find that BMS-582949 represses RANKL-induced osteoclast differentiation in a dose-dependent manner. Moreover, BMS-582949 inhibits osteoclastic F-actin ring formation and osteoclast-specific gene expression. Mechanically, BMS-582949 treatment attenuates RANKL-mediated osteoclastogenesis through mitogen-activated protein kinases (MAPKs) and protein kinase B (AKT) signaling pathways without disturbing nuclear factor-κB (NF-κB) signaling. Interestingly, BMS-582949 impairs osteoclastic mitochondrial biogenesis and functions, such as oxidative phosphorylation (OXPHOS). Furthermore, BMS-582949 administration prevents bone loss in ovariectomized mouse mode by inhibiting both bone resorption and bone formation in vivo. Taken together, these findings indicate that BMS-582949 may be a potential and effective drug for the therapy of osteolytic diseases.

Ginkgetin attenuates bone loss in OVX mice by inhibiting the NF-κB/IκBα signaling pathway

Background

Osteoporosis is a disease associated with bone resorption, characterized primarily by the excessive activation of osteoclasts. Ginkgetin is a compound purified from natural ginkgo leaves which has various biological properties, including anti-inflammation, antioxidant, and anti-tumor effects. This study investigated the bone-protective effects of ginkgetin in ovariectomized (OVX) mice and explored their potential signaling pathway in inhibiting osteoclastogenesis in a mouse model of osteoporosis.

Methods

Biochemical assays were performed to assess the levels of Ca, ALP, and P in the blood. Micro CT scanning was used to evaluate the impact of ginkgetin on bone loss in mice. RT-PCR was employed to detect the expression of osteoclast-related genes (ctsk, c-fos, trap) in their femoral tissue. Hematoxylin and eosin (H&E) staining was utilized to assess the histopathological changes in femoral tissue due to ginkgetin. The TRAP staining was used to evaluate the impact of ginkgetin osteoclast generation in vivo. Western blot analysis was conducted to investigate the effect of ginkgetin on the expression of p-NF-κB p65 and IκBα proteins in mice.

Results

Our findings indicate that ginkgetin may increase the serum levels of ALP and P, while decreasing the serum level of Ca in OVX mice. H&E staining and micro CT scanning results suggest that ginkgetin can inhibit bone loss in OVX mice. The TRAP staining results showed ginkgetin suppresses the generation of osteoclasts in OVX mice. RT-PCR results demonstrate that ginkgetin downregulate the expression of osteoclast-related genes (ctsk, c-fos, trap) in the femoral tissue of mice, and this effect is dose-dependent. Western blot analysis results reveal that ginkgetin can inhibit the expression of p-NF-κB p65 and IκBα proteins in mice.

Conclusion

Ginkgetin can impact osteoclast formation and activation in OVX mice by inhibiting the NF-κB/IκBα signaling pathway, thereby attenuating bone loss in mice.

Advancing skeletal health and disease research with single-cell RNA sequencing

Orthopedic conditions have emerged as global health concerns, impacting approximately 1.7 billion individuals worldwide. However, the limited understanding of the underlying pathological processes at the cellular and molecular level has hindered the development of comprehensive treatment options for these disorders. The advent of single-cell RNA sequencing (scRNA-seq) technology has revolutionized biomedical research by enabling detailed examination of cellular and molecular diversity. Nevertheless, investigating mechanisms at the single-cell level in highly mineralized skeletal tissue poses technical challenges. In this comprehensive review, we present a streamlined approach to obtaining high-quality single cells from skeletal tissue and provide an overview of existing scRNA-seq technologies employed in skeletal studies along with practical bioinformatic analysis pipelines. By utilizing these methodologies, crucial insights into the developmental dynamics, maintenance of homeostasis, and pathological processes involved in spine, joint, bone, muscle, and tendon disorders have been uncovered. Specifically focusing on the joint diseases of degenerative disc disease, osteoarthritis, and rheumatoid arthritis using scRNA-seq has provided novel insights and a more nuanced comprehension. These findings have paved the way for discovering novel therapeutic targets that offer potential benefits to patients suffering from diverse skeletal disorders.

mmu-miR-185 regulates osteoclasts differentiation and migration by targeting Btk

BACKGROUND

Bones undergo a constant remodeling, a process involving osteoclast-mediated bone resorption and osteoblast-mediated bone formation, crucial for maintaining healthy bone mass. We previously observed that miR-185 depletion may promote bone formation by regulating Bgn expression and the BMP/Smad signaling pathway. However, the effects of miR-185-5p on the osteoclasts and bone remodeling have not been elucidated, warranting further exploration.

METHODS

Tartrate-resistant acid phosphatase staining was utilized to assess the differentiation ability of bone marrow mononuclear macrophages (BMMs) from mmu-miR-185 gene knockout (KO) mice and wild-type (WT) mice. A reverse transcriptase-quantitative PCR was conducted to compare differences in miR-185-5p and osteoclast marker molecules, including Trap, Dcstamp, Ctsk and Nfatc1, between the KO group and WT group BMMs. Western blot analysis was employed to observe the expression of osteoclast marker molecules. A cell-counting kit-8 was used to analyze cell proliferation ability. Transwell experiments were conducted to detect cell migration. Dual-luciferase reporter assays were employed to confirm whether Btk is a downstream target gene of miR-185-5p.

RESULTS

miR-185 depletion promoted osteoclast differentiation in bone marrow-derived monocytes/macrophages. Overexpression of miR-185-5p in RAW264.7 cells inhibited differentiation and migration of osteoclasts. Furthermore, Btk was identified as a downstream target gene of miR-185-5p, suggesting that miR-185-5p may inhibit osteoclast differentiation and migration by targeting Btk.

CONCLUSIONS

miR-185 regulates osteoclasts differentiation, with overexpression of miR-185-5p inhibiting osteoclast differentiation and migration in vitro. Additionally, miR-185-5p may modulate osteoclastic differentiation and migration by regulating Btk expression.

Mouse mandibular-derived osteoclast progenitors have differences in intrinsic properties compared with femoral-derived progenitors

Craniofacial osteoclasts are essential for site-specific processes such as alveolar bone resorption, tooth eruption, and orthodontic tooth movement. Much of the current understanding of osteoclast development and function comes from studies using long bone-derived cells. Minimal investigation has been done to explore skeletal site differences. The overall goal of this study was to determine if mandibular- and femoral-derived osteoclasts represent distinct populations. To test this hypothesis, bone marrow cells were initially analyzed from the mandible and femur of 2-month-old mice. It was shown that mandibular-derived osteoclasts have enhanced size (mm2) compared with femoral-derived osteoclasts. Since bone marrow macrophages are a heterogenous population, we additionally selected for monocytes and demonstrated that mandibular-derived monocytes also form osteoclasts with increased size compared with femoral-derived monocytes. Osteoclast precursor populations from both skeletal sites were analyzed by flow cytometry. A newly described Ly6CHigh+ population as well as the Ly6Cint population was increased in the mandibular-derived cells. The difference in differentiation potential between monocyte cultures suggests that the increase in the Ly6CHigh+ population may explain the enhanced differentiation potential in mandibular-derived cells. Monocyte genes such as Pu.1, C/ebp-a, and Prdm1 are increased in expression in mandibular-derived monocytes compared with femoral-derived monocytes. As expected with enhanced differentiation, osteoclast genes including Nfatc1, Dc-stamp, Ctsk, and Rank are upregulated in mandibular-derived osteoclast precursors. Future studies will determine how changes in the environment of the mandible lead to changes in percentages of osteoclast progenitors and their differentiation potential.

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.

Osteoclasts and osteoarthritis: Novel intervention targets and therapeutic potentials during aging

Osteoarthritis (OA), a chronic degenerative joint disease, is highly prevalent among the aging population, and often leads to joint pain, disability, and a diminished quality of life. Although considerable research has been conducted, the precise molecular mechanisms propelling OA pathogenesis continue to be elusive, thereby impeding the development of effective therapeutics. Notably, recent studies have revealed subchondral bone lesions precede cartilage degeneration in the early stage of OA. This development is marked by escalated osteoclast-mediated bone resorption, subsequent imbalances in bone metabolism, accelerated bone turnover, and a decrease in bone volume, thereby contributing significantly to the pathological changes. While the role of aging hallmarks in OA has been extensively elucidated from the perspective of chondrocytes, their connection with osteoclasts is not yet fully understood. There is compelling evidence to suggest that age-related abnormalities such as epigenetic alterations, proteostasis network disruption, cellular senescence, and mitochondrial dysfunction, can stimulate osteoclast activity. This review intends to systematically discuss how aging hallmarks contribute to OA pathogenesis, placing particular emphasis on the age-induced shifts in osteoclast activity. It also aims to stimulate future studies probing into the pathological mechanisms and therapeutic approaches targeting osteoclasts in OA during aging.

Unraveling the intricacies of osteoclast differentiation and maturation: insight into novel therapeutic strategies for bone-destructive diseases

Osteoclasts are the principal cells that efficiently resorb bone. Numerous studies have attempted to reveal the molecular pathways leading to the differentiation and activation of osteoclasts to improve the treatment and prevention of osteoporosis and other bone-destructive diseases. While the cumulative knowledge of osteoclast regulatory molecules, such as receptor activator of nuclear factor-kB ligand (RANKL) and nuclear factor of activated T cells 1 (NFATc1), contributes to the understanding of the developmental progression of osteoclasts, little is known about how the discrete steps of osteoclastogenesis modify osteoclast status but not the absolute number of osteoclasts. The regulatory mechanisms involved in osteoclast maturation but not those involved in differentiation deserve special attention due to their potential use in establishing a more effective treatment strategy: targeting late-phase differentiation while preserving coupled bone formation. Recent studies have shed light on the molecules that govern late-phase osteoclast differentiation and maturation, as well as the metabolic changes needed to adapt to shifting metabolic demands. This review outlines the current understanding of the regulation of osteoclast differentiation, as well as osteoclast metabolic adaptation as a differentiation control mechanism. Additionally, this review introduces molecules that regulate the late-phase osteoclast differentiation and thus minimally impact coupled bone formation.

Plant molecules reinforce bone repair: Novel insights into phenol-modified bone tissue engineering scaffolds for the treatment of bone defects

Bone defects have become a major cause of disability and death. To overcome the limitations of natural bone implants, including donor shortages and immune rejection risks, bone tissue engineering (BTE) scaffolds have emerged as a promising therapy for bone defects. Despite possessing good biocompatibility, these metal, ceramic and polymer-based scaffolds are still challenged by the harsh conditions in bone defect sites. ROS accumulation, bacterial infection, excessive inflammation, compromised blood supply deficiency and tumor recurrence negatively impact bone tissue cells (BTCs) and hinder the osteointegration of BTE scaffolds. Phenolic compounds, derived from plants and fruits, have gained growing application in treating inflammatory, infectious and aging-related diseases due to their antioxidant ability conferred by phenolic hydroxyl groups. The prevalent interactions between phenols and functional groups also facilitate their utilization in fabricating scaffolds. Consequently, phenols are increasingly incorporated into BTE scaffolds to boost therapeutic efficacy in bone defect. This review demonstrated the effects of phenols on BTCs and bone defect microenvironment, summarized the intrinsic mechanisms, presented the advances in phenol-modified BTE scaffolds and analyzed their potential risks in practical applications. Overall, phenol-modified BTE scaffolds hold great potential for repairing bone defects, offering novel patterns for BTE scaffold construction and advancing traumatological medicine.

Osteoclast Methods in Protein Phosphatase Research

Osteoclasts are specialized cells that degrade bone and are essential for bone formation and maintaining bone homeostasis. Excess or deficient activity of these cells can significantly alter bone mass, structure, and physical strength, leading to significant morbidity, as in osteoporosis or osteopetrosis, among many other diseases. Protein phosphorylation in osteoclasts plays critical roles in the signaling pathways that govern the production of osteoclasts and regulate their bone-resorbing activity. In this chapter, we describe the isolation of mouse splenocytes and their differentiation into mature osteoclasts on resorptive (e.g., bone) and non-resorptive (e.g., plastic or glass) surfaces, examining matrix resorption by osteoclasts, immunofluorescence staining of these cells, and knocking out genes by CRISPR in the mouse osteoclastogenic cell line RAW264.7.

Bioengineered cartilaginous grafts for repairing segmental mandibular defects

Reconstructing critical-sized craniofacial bone defects is a global healthcare challenge. Current methods, like autologous bone transplantation, face limitations. Bone tissue engineering offers an alternative to autologous bone, with traditional approaches focusing on stimulating osteogenesis via the intramembranous ossification (IMO) pathway. However, IMO falls short in addressing larger defects, particularly in clinical scenarios where there is insufficient vascularisation. This review explores redirecting bone regeneration through endochondral ossification (ECO), a process observed in long bone healing stimulated by hypoxic conditions. Despite its promise, gaps exist in applying ECO to bone tissue engineering experiments, requiring the elucidation of key aspects such as cell sources, biomaterials and priming protocols. This review discusses various scaffold biomaterials and cellular sources for chondrogenesis and hypertrophic chondrocyte priming, mirroring the ECO pathway. The review highlights challenges in current endochondral priming and proposes alternative approaches. Emphasis is on segmental mandibular defect repair, offering insights for future research and clinical application. This concise review aims to advance bone tissue engineering by addressing critical gaps in ECO strategies.

Metallothionein 3 promotes osteoclast differentiation and survival by regulating the intracellular Zn2+ concentration and NRF2 pathway

In osteoclastogenesis, the metabolism of metal ions plays an essential role in controlling reactive oxygen species (ROS) production, mitochondrial biogenesis, and survival, and differentiation. However, the mechanism regulating metal ions during osteoclast differentiation remains unclear. The metal-binding protein metallothionein (MT) detoxifies heavy metals, maintains metal ion homeostasis, especially zinc, and manages cellular redox levels. We carried out tests using murine osteoclast precursors to examine the function of MT in osteoclastogenesis and evaluated their potential as targets for future osteoporosis treatments. MT genes were significantly upregulated upon differentiation from osteoclast precursors to mature osteoclasts in response to receptor activators of nuclear factor-κB (NF-κB) ligand (RANKL) stimulation, and MT3 expression was particularly pronounced in mature osteoclasts among MT genes. The knockdown of MT3 in osteoclast precursors demonstrated a remarkable inhibition of differentiation into mature osteoclasts. In preosteoclasts, MT3 knockdown suppressed the activity of mitogen-activated protein kinase (MAPK) and NF-κB signaling pathways upon RANKL stimulation, leading to affect cell survival through elevated cleaved Caspase 3 and poly (ADP-ribose) polymerase (PARP) levels. Additionally, ROS levels were decreased, and nuclear factor erythroid 2-related factor 2 (NRF2) (a suppressor of ROS) and the downstream antioxidant proteins, such as catalase (CAT) and heme oxygenase 1 (HO-1), were more highly expressed in the MT3 preosteoclast knockdowns. mitochondrial ROS, which is involved in mitochondrial biogenesis and the production of reactive oxygen species, were similarly decreased because cAMP response element-binding (CREB) and peroxisome proliferator-activated receptor γ coactivator 1β (PGC-1β) were less activated due to MT3 depletion. Thus, by modulating ROS through the NRF2 pathway, MT3 plays a crucial role in regulating osteoclast differentiation and survival, acting as a metabolic modulator of intracellular zinc ions.

Jagged1 Acts as an RBP-J Target and Feedback Suppresses TNF-Mediated Inflammatory Osteoclastogenesis

TNF plays a crucial role in inflammation and bone resorption in various inflammatory diseases, including rheumatoid arthritis (RA). However, its direct ability to drive macrophages to differentiate into osteoclasts is limited. Although RBP-J is recognized as a key inhibitor of TNF-mediated osteoclastogenesis, the precise mechanisms that restrain TNF-induced differentiation of macrophages into osteoclasts are not fully elucidated. In this study, we identified that the Notch ligand Jagged1 is a previously unrecognized RBP-J target. The expression of Jagged1 is significantly induced by TNF mainly through RBP-J. The TNF-induced Jagged1 in turn functions as a feedback inhibitory regulator of TNF-mediated osteoclastogenesis. This feedback inhibition of osteoclastogenesis by Jagged1 does not exist in RANKL-induced mouse osteoclast differentiation, as RANKL does not induce Jagged1 expression. The Jagged1 level in peripheral blood monocytes/osteoclast precursors is decreased in RA compared with the nonerosive inflammatory disease systemic lupus erythematosus, suggesting a mechanism that contributes to increased osteoclast formation in RA. Moreover, recombinant Jagged1 suppresses human inflammatory osteoclastogenesis. Our findings identify Jagged1 as an RBP-J direct target that links TNF and Notch signaling pathways and restrains TNF-mediated osteoclastogenesis. Given that Jagged1 has no effect on TNF-induced expression of inflammatory genes, its use may present a new complementary therapeutic approach to mitigate inflammatory bone loss with little impact on the immune response in disease conditions.

Ginsenoside Rg2 inhibits osteoclastogenesis by downregulating the NFATc1, c-Fos, and MAPK pathways

Ginsenosides, among the most active components of ginseng, exhibit several therapeutic effects against cancer, diabetes, and other metabolic diseases. However, the molecular mechanism underlying the anti-osteoporotic activity of ginsenoside Rg2, a major ginsenoside, has not been clearly elucidated. This study aimed to determine the effects of ginsenoside Rg2 on receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation. Results indicate that ginsenoside Rg2 inhibits RANKLinduced osteoclast differentiation of bone marrow macrophages (BMMs) without cytotoxicity. Pretreatment with ginsenoside Rg2 significantly reduced the RANKL-induced gene expression of c-fos and nuclear factor of activated T-cells (Nfatc1), as well as osteoclast-specific markers tartrate-resistant acid phosphatase (TRAP, Acp5) and osteoclast-associated receptor (Oscar). Moreover, RANKL-induced phosphorylation of mitogen-activated protein kinases (MAPKs) was decreased by ginsenoside Rg2 in BMM. Therefore, we suggest that ginsenoside Rg2 suppresses RANKLinduced osteoclast differentiation through the regulation of MAPK signaling-mediated osteoclast markers and could be developed as a therapeutic drug for the prevention and treatment of osteoporosis. [BMB Reports 2023; 56(10): 551-556].

Ginsenoside Rg2 inhibits osteoclastogenesis by downregulating the NFATc1, c-Fos, and MAPK pathways

Ginsenosides, among the most active components of ginseng, exhibit several therapeutic effects against cancer, diabetes, and other metabolic diseases. However, the molecular mechanism underlying the anti-osteoporotic activity of ginsenoside Rg2, a major ginsenoside, has not been clearly elucidated. This study aimed to determine the effects of ginsenoside Rg2 on receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation. Results indicate that ginsenoside Rg2 inhibits RANKLinduced osteoclast differentiation of bone marrow macrophages (BMMs) without cytotoxicity. Pretreatment with ginsenoside Rg2 significantly reduced the RANKL-induced gene expression of c-fos and nuclear factor of activated T-cells (Nfatc1), as well as osteoclast-specific markers tartrate-resistant acid phosphatase (TRAP, Acp5) and osteoclast-associated receptor (Oscar). Moreover, RANKL-induced phosphorylation of mitogen-activated protein kinases (MAPKs) was decreased by ginsenoside Rg2 in BMM. Therefore, we suggest that ginsenoside Rg2 suppresses RANKLinduced osteoclast differentiation through the regulation of MAPK signaling-mediated osteoclast markers and could be developed as a therapeutic drug for the prevention and treatment of osteoporosis. [BMB Reports 2023; 56(10): 551-556].

Sulforaphene suppresses RANKL-induced osteoclastogenesis and LPS-induced bone erosion by activating Nrf2 signaling pathway

BACKGROUND AND PURPOSE

Inflammatory disorders have been found to induce bone loss through sustained and persistent activation of osteoclast differentiation, leading to heightened bone resorption. The current pharmacological interventions for combating bone loss to harbor adverse effects or contraindications. There is a pressing need to identify drugs with fewer side effects.

EXPERIMENTAL APPROACH

The effect and underlying mechanism of sulforaphene (LFS) on osteoclast differentiation were illustrated in vitro and in vivo with RANKL-induced Raw264.7 cell line osteoclastogenesis and lipopolysaccharide (LPS)-induced bone erosion model.

KEY RESULTS

In this study, LFS has been shown to effectively impede the formation of mature osteoclasts induced from both Raw264.7 cell line and bone marrow macrophages (BMMs), mainly at the early stage. Further mechanistic investigations uncovered that LFS suppressed AKT phosphorylation. SC-79, a potent AKT activator, was found to reverse the inhibitory impact of LFS on osteoclast differentiation. Moreover, transcriptome sequencing analysis revealed that treatment with LFS led to a significant upregulation in the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and antioxidant-related genes. Then it's validated that LFS could promote NRF2 expression and nuclear translocation, as well as effectively resist oxidative stress. NRF2 knockdown reversed the suppression effect of LFS on osteoclast differentiation. In vivo experiments provide convincing evidence that LFS is protective against LPS-induced inflammatory osteolysis.

CONCLUSION AND IMPLICATIONS

These well-grounded and promising findings suggest LFS as a promising agent to addressing oxidative-stress related diseases and bone loss disorders.

Mechanistic insight of interleukin-9 induced osteoclastogenesis

Interleukin (IL)-9 is an emerging player in the pathogenesis of various chronic inflammatory diseases including bone disorders like rheumatoid arthritis (RA) and psoriatic arthritis. Recently, IL-9 was shown to enhance the osteoclast formation and their function in RA. However, the mechanisms by which IL-9 influences osteoclastogenesis are not known. Therefore, in this study we aimed to unravel the direct and indirect ways by which IL-9 can influence osteoclast formation. We used mouse bone marrow precursor cells for checking the effect of IL-9 on osteoclast differentiation and its function. Next, IL-9 induced signalling pathway were checked in the process of osteoclastogenesis. T cells play an important role in enhancing osteoclastogenesis in inflammatory conditions. We used splenic T cells to understand the impact of IL-9 on the functions of T effector (Teff) and regulatory T (Treg) cells. Furthermore, the effect of IL-9 mediated modulation of the T cell response on osteoclasts was checked using a coculture model of T cells with osteoclast precursors. We showed that IL-9 enhanced osteoclast formation and its function. We found that IL-9 activates STAT3, P38 MAPK, ERK1/2, NFκB and we hypothesize that it mediates the effect on osteoclastogenesis by accelerating mitochondrial biogenesis. Additionally, IL-9 was observed to facilitate the functions of pro-osteoclastogenic IL-17 producing T cells, but inhibits the function of anti-osteoclastogenic Treg cells. Our observations suggest that IL-9 can influence osteoclastogenesis directly by modulating the signalling cascade in the precursor cells; indirectly by enhancing IL-17 producing T cells and by reducing the functions of Treg cells.

The rules and regulatory mechanisms of FOXO3 on inflammation, metabolism, cell death and aging in hosts

The FOX family of transcription factors was originally identified in 1989, comprising the FOXA to FOXS subfamilies. FOXO3, a well-known member of the FOXO subfamily, is widely expressed in various human organs and tissues, with higher expression levels in the ovary, skeletal muscle, heart, and spleen. The biological effects of FOXO3 are mostly determined by its phosphorylation, which occurs in the nucleus or cytoplasm. Phosphorylation of FOXO3 in the nucleus can promote its translocation into the cytoplasm and inhibit its transcriptional activity. In contrast, phosphorylation of FOXO3 in the cytoplasm leads to its translocation into the nucleus and exerts regulatory effects on biological processes, such as inflammation, aerobic glycolysis, autophagy, apoptosis, oxidative stress, cell cycle arrest and DNA damage repair. Additionally, FOXO3 isoform 2 acts as an important suppressor of osteoclast differentiation. FOXO3 can also interfere with the development of various diseases, including inhibiting the proliferation and invasion of tumor cells, blocking the production of inflammatory factors in autoimmune diseases, and inhibiting β-amyloid deposition in Alzheimer's disease. Furthermore, FOXO3 slows down the aging process and exerts anti-aging effects by delaying telomere attrition, promoting cell self-renewal, and maintaining genomic stability. This review suggests that changes in the levels and post-translational modifications of FOXO3 protein can maintain organismal homeostasis and improve age-related diseases, thus counteracting aging. Moreover, this may indicate that alterations in FOXO3 protein levels are also crucial for longevity, offering new perspectives for therapeutic strategies targeting FOXO3.

Origin of Osteoclasts: Osteoclast Precursor Cells

Osteoclasts are multinucleated bone-resorbing cells and a key player in bone remodeling for health and disease. Since the discovery of osteoclasts in 1873, the structure and function of osteoclasts and the molecular and cellular mechanisms of osteoclastogenesis have been extensively studied. Moreover, it has been well established that osteoclasts are differentiated in vitro from myeloid cells such as bone marrow macrophages or monocytes. The concept showing that osteoclasts are derived from a specific population (named osteoclast precursor cells [OCPs]) among myeloid cells has been long hypothesized. However, the specific precursor population of osteoclasts is not clearly defined yet. A growing body of work provides evidence of the developmental origin and lifespan of murine osteoclasts, particularly in vivo. Here, we review the emerging evidence that supports the existence of OCPs and discuss current insights into their identity.

Bioengineering extracellular vesicles: smart nanomaterials for bone regeneration

In the past decade, extracellular vesicles (EVs) have emerged as key regulators of bone development, homeostasis and repair. EV-based therapies have the potential to circumnavigate key issues hindering the translation of cell-based therapies including functional tissue engraftment, uncontrolled differentiation and immunogenicity issues. Due to EVs' innate biocompatibility, low immunogenicity, and high physiochemical stability, these naturally-derived nanoparticles have garnered growing interest as potential acellular nanoscale therapeutics for a variety of diseases. Our increasing knowledge of the roles these cell-derived nanoparticles play, has made them an exciting focus in the development of novel pro-regenerative therapies for bone repair. Although these nano-sized vesicles have shown promise, their clinical translation is hindered due to several challenges in the EV supply chain, ultimately impacting therapeutic efficacy and yield. From the biochemical and biophysical stimulation of parental cells to the transition to scalable manufacture or maximising vesicles therapeutic response in vivo, a multitude of techniques have been employed to improve the clinical efficacy of EVs. This review explores state of the art bioengineering strategies to promote the therapeutic utility of vesicles beyond their native capacity, thus maximising the clinical potential of these pro-regenerative nanoscale therapeutics for bone repair.

Identification of Differentially Expressed Genes and Molecular Pathways Involved in Osteoclastogenesis Using RNA-seq

Osteoporosis is a disease that is characterised by reduced bone mineral density (BMD) and can be exacerbated by the excessive bone resorption of osteoclasts (OCs). Bioinformatic methods, including functional enrichment and network analysis, can provide information about the underlying molecular mechanisms that participate in the progression of osteoporosis. In this study, we harvested human OC-like cells differentiated in culture and their precursor peripheral blood mononuclear cells (PBMCs) and characterised the transcriptome of the two cell types using RNA-sequencing in order to identify differentially expressed genes. Differential gene expression analysis was performed in RStudio using the edgeR package. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed to identify enriched GO terms and signalling pathways, with inter-connected regions characterised using protein-protein interaction analysis. In this study, we identified 3201 differentially expressed genes using a 5% false discovery rate; 1834 genes were upregulated, whereas 1367 genes were downregulated. We confirmed a significant upregulation of several well-established OC genes including CTSK, DCSTAMP, ACP5, MMP9, ITGB3, and ATP6V0D2. The GO analysis suggested that upregulated genes are involved in cell division, cell migration, and cell adhesion, while the KEGG pathway analysis highlighted oxidative phosphorylation, glycolysis and gluconeogenesis, lysosome, and focal adhesion pathways. This study provides new information about changes in gene expression and highlights key biological pathways involved in osteoclastogenesis.

Multinucleation resets human macrophages for specialized functions at the expense of their identity

Macrophages undergo plasma membrane fusion and cell multinucleation to form multinucleated giant cells (MGCs) such as osteoclasts in bone, Langhans giant cells (LGCs) as part of granulomas or foreign-body giant cells (FBGCs) in reaction to exogenous material. How multinucleation per se contributes to functional specialization of mature mononuclear macrophages remains poorly understood in humans. Here, we integrate comparative transcriptomics with functional assays in purified mature mononuclear and multinucleated human osteoclasts, LGCs and FBGCs. Strikingly, in all three types of MGCs, multinucleation causes a pronounced downregulation of macrophage identity. We show enhanced lysosome-mediated intracellular iron homeostasis promoting MGC formation. The transition from mononuclear to multinuclear state is accompanied by cell specialization specific to each polykaryon. Enhanced phagocytic and mitochondrial function associate with FBGCs and osteoclasts, respectively. Moreover, human LGCs preferentially express B7-H3 (CD276) and can form granuloma-like clusters in vitro, suggesting that their multinucleation potentiates T cell activation. These findings demonstrate how cell-cell fusion and multinucleation reset human macrophage identity as part of an advanced maturation step that confers MGC-specific functionality.

The effects of epigenetic modifications on bone remodeling in age-related osteoporosis

PURPOSE

As the population ages, there is an increased risk of fracture and morbidity diseases associated with aging, such as age-related osteoporosis and other bone diseases linked to aging skeletons.

RESULTS

Several bone-related cells, including multipotent bone mesenchymal stem cells, osteoblasts that form bone tissue, and osteoclasts that break it down, are in symbiotic relationships throughout life. Growing evidence indicates that epigenetic modifications of cells caused by aging contribute to compromised bone remodeling and lead to osteoporosis. A number of epigenetic mechanisms are at play, including DNA/RNA modifications, histone modifications, microRNAs (miRNAs), and long noncoding RNAs (lncRNAs), as well as chromatin remodeling.

CONCLUSION

In this review, we summarized the epigenetic modifications of different bone-related cells during the development and progression of osteoporosis associated with aging. Additionally, we described a compensatory recovery mechanism under epigenetic regulation that may lead to new strategies for regulating bone remodeling in age-related osteoporosis.

Suppressive effects of (-)-tubaic acid on RANKL-induced osteoclast differentiation and bone resorption

Regulation of osteoclastogenesis and bone-resorbing activity can be an efficacious strategy for treating bone loss diseases because excessive osteoclastic bone resorption leads to the development of such diseases. Here, we investigated the role of (-)-tubaic acid, a thermal degradation product of rotenone, in osteoclast formation and function in an attempt to identify alternative natural compounds. (-)-Tubaic acid significantly inhibited receptor activator of nuclear factor-κB ligand (RANKL)-mediated osteoclast differentiation at both the early and late stages, suggesting that (-)-tubaic acid affects the commitment and differentiation of osteoclast progenitors as well as the cell-cell fusion of mononuclear osteoclasts. (-)-Tubaic acid attenuated the activation of extracellular signal-regulated kinase (ERK) and expression of nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) and its target genes in response to RANKL. Furthermore, a pit-formation assay revealed that (-)-tubaic acid significantly impaired the bone-resorbing activity of osteoclasts. Our results demonstrated that (-)-tubaic acid exhibits anti-osteoclastogenic and anti-resorptive effects, indicating its therapeutic potential in the management of osteoclast-related bone diseases.

RANKL-responsive epigenetic mechanism reprograms macrophages into bone-resorbing osteoclasts

Monocyte/macrophage lineage cells are highly plastic and can differentiate into various cells under different environmental stimuli. Bone-resorbing osteoclasts are derived from the monocyte/macrophage lineage in response to receptor activator of NF-κB ligand (RANKL). However, the epigenetic signature contributing to the fate commitment of monocyte/macrophage lineage differentiation into human osteoclasts is largely unknown. In this study, we identified RANKL-responsive human osteoclast-specific superenhancers (SEs) and SE-associated enhancer RNAs (SE-eRNAs) by integrating data obtained from ChIP-seq, ATAC-seq, nuclear RNA-seq and PRO-seq analyses. RANKL induced the formation of 200 SEs, which are large clusters of enhancers, while suppressing 148 SEs in macrophages. RANKL-responsive SEs were strongly correlated with genes in the osteoclastogenic program and were selectively increased in human osteoclasts but marginally presented in osteoblasts, CD4+ T cells, and CD34+ cells. In addition to the major transcription factors identified in osteoclasts, we found that BATF binding motifs were highly enriched in RANKL-responsive SEs. The depletion of BATF1/3 inhibited RANKL-induced osteoclast differentiation. Furthermore, we found increased chromatin accessibility in SE regions, where RNA polymerase II was significantly recruited to induce the extragenic transcription of SE-eRNAs, in human osteoclasts. Knocking down SE-eRNAs in the vicinity of the NFATc1 gene diminished the expression of NFATc1, a major regulator of osteoclasts, and osteoclast differentiation. Inhibiting BET proteins suppressed the formation of some RANKL-responsive SEs and NFATc1-associated SEs, and the expression of SE-eRNA:NFATc1. Moreover, SE-eRNA:NFATc1 was highly expressed in the synovial macrophages of rheumatoid arthritis patients exhibiting high-osteoclastogenic potential. Our genome-wide analysis revealed RANKL-inducible SEs and SE-eRNAs as osteoclast-specific signatures, which may contribute to the development of osteoclast-specific therapeutic interventions.

TET2 regulates osteoclastogenesis by modulating autophagy in OVX-induced bone loss

Increased bone resorption by osteoclasts after estrogen deficiency is the main cause of postmenopausal osteoporosis. TET2 (tet methylcytosine dioxygenase 2) is a DNA demethylase that regulates cellular function and differentiation potential. Macroautophagy/autophagy maintains cellular homeostasis by recycling unnecessary and damaged organelles. This study revealed that TET2 promoted bone loss in oophorectomized (OVX) mice and that TET2 promoted osteoclast differentiation by regulating autophagy. Tet2 knockdown inhibited autophagy and osteoclast differentiation in vitro. Mechanistically, Tet2 knockdown increased BCL2 (B cell leukemia/lymphoma 2) expression and BCL2 exhibited increased binding to BECN1 and negatively regulated autophagy. Small interfering RNA specific to Bcl2 interfered with BCL2 expression in Tet2-knockdown bone marrow cells/precursors, partially reversing autophagy dysregulation and promoting osteoclast differentiation. Moreover, the LV-shTet2 lentivirus prevented bone loss in OVX mice. In summary, our findings provide evidence that TET2 promotes osteoclast differentiation by inhibiting BCL2 expression and positively regulating BECN1-dependent autophagy.Abbreviations: ACP5/TRAP: acid phosphatase 5, tartrate resistant; ATP6V0D2: ATPase, H+ transporting, lysosomal V0 subunit D2; BCL2: B cell leukemia/lymphoma 2; BECN1: beclin 1, autophagy related; BMs: bone marrow cells; CTSK: cathepsin K; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MMP9: matrix metallopeptidase 9; OVX: oophorectomy; RUNX1: runt related transcription factor 1; SOCS3: suppressor of cytokine signaling 3; SPI1/PU.1: Spi-1 proto-oncogene; TNFSF11/RANKL: tumor necrosis factor (ligand) superfamily, member 11; TET2: tet methylcytosine dioxygenase 2.

Pathophysiology and therapeutic advances in myeloma bone disease

Bone disease is the most common complication in patients with multiple myeloma (MM), and it may lead to skeletal-related events (SREs) such as bone pain, pathological fractures, and spinal cord compression, which impair a patients' quality of life and survival. The pathogenesis of myeloma bone disease (MBD) involves disruption of bone reconstitution balance including excessive activation of osteoclasts, inhibition of osteoblasts, and participation of osteocytes and bone marrow stromal cells. Various factors, such as the receptor activator of nuclear factor-κB ligand (RANKL)/osteoprotegerin (OPG), dickkopf-1 (DKK-1), sclerostin, and activin-A, are involved in the development of MBD. Bisphosphonates and the anti-RANKL antibody denosumab are currently the main treatment options for MBD, delaying the onset of SREs. Denosumab is preferred in patients with MM and renal dysfunction. Although effective drugs have been approved, antimyeloma therapy is the most important method for controlling bone disease.

Hyperoside suppresses osteoclasts differentiation and function through downregulating TRAF6/p38 MAPK signaling pathway

Hyperoside (HP), as a natural product, can promote proliferation and differentiation of osteoblasts and presents a protective effect on ovariectomized (OVX) mice. However, the inhibitory effect of HP on osteoclasts (OCs) and the potential mechanism remain to be elucidated. In this study, it was found that HP could effectively inhibit the differentiation and bone resorption of OCs, and its intrinsic molecular mechanism was related to the inhibition of TRAF6/p38 MAPK signaling pathway. Therefore, HP could be a promising natural compound for lytic bone diseases.

MiR-148a deletion protects from bone loss in physiological and estrogen-deficient mice by targeting NRP1

Bone metabolic homeostasis is largely dependent on the dynamic balance between osteoblasts and osteoclasts. MicroRNAs (miRNAs) play critical roles in regulating bone metabolism. In this study, we explored the role of a new miRNA (miR-148a) in osteoporosis. We compared the bone phenotype between miR-148a knockout (KO) mice and the wild-type (WT) littermates. We found miR-148a KO mice exhibited an increased bone mass phenotype and decreased osteoclastogenesis compared to the WT group. In vitro, miR-148a overexpression promoted osteoclastogenesis and bone resorption function. Mechanistically, NRP1 was identified as a novel direct target of miR-148a, and NRP1 silencing reversed the effect of miR-148a knockout. In OVX and calvarial osteolysis models, miR-148a KO protects mice against excessive bone resorption, while miR-148a agomiR/AAV-shNRP1 accelerates pathologic bone loss. Finally, the miR-148a level was found to be positively correlated with β-CTX in postmenopausal osteoporosis (PMOP) serum specimens. In summary, our findings revealed that miR-148a genetic deletion ameliorates bone loss under physiological and pathological conditions by targeting NRP1. In osteoclast-related bone metabolic diseases such as PMOP, miR-148a may be an attractive therapeutic target in the future.

Osteoclast activity sculpts craniofacial form to permit sensorineural patterning in the zebrafish skull

Efforts to understand the morphogenesis of complex craniofacial structures have largely focused on the role of chondrocytes and osteoblasts. Along with these bone-creating cells, bone-resorbing osteoclasts are critical in homeostasis of adult skeletal structures, but there is currently limited information on their role in the complex morphogenetic events of craniofacial development. Fundamental aspects of skull formation and general skeletal development are conserved from zebrafish to mammals. Using a cathepsinK reporter, we documented osteoclast location in the developing zebrafish skull over several weeks, from 5.18 mm to 9.6 mm standard length (approximately 15 to 34 days post fertilization). While broad distribution of osteoclasts is consistent across individuals, they are sparse and the exact locations vary among fish and across developmental time points. Interestingly, we observed osteoclasts concentrating at areas associated with neuromasts and their associated nerves, in particular the hyomandibular foramina and around the supraorbital lateral line. These are areas of active remodeling. In contrast, other areas of rapid bone growth, such as the osteogenic fronts of the frontal and parietal bones, show no particular concentration of osteoclasts, suggesting that they play a special role in shaping bone near neuromasts and nerves. In csf1ra mutants lacking functional osteoclasts, the morphology of the cranial bone was disrupted in both areas. The hyomandibular foramen is present in the initial cartilage template, but after the initiation of ossification, the diameter of the canal is significantly smaller in the absence of osteoclasts. The diameter of the supraorbital lateral line canals was also reduced in the mutants, as was the number of pores associated with neuromasts, which allow for the passage of associated nerves through the bone. Our findings define important and previously unappreciated roles for osteoclast activity in shaping craniofacial skeletal structures with a particular role in bone modeling around peripheral cranial nerves, providing a scaffold for wiring the sensioneural system during craniofacial development. This has important implications for the formation of the evolutionarily diverse lateral line system, as well understanding the mechanism of neurologic sequelae of congenital osteoclast dysfunction in human craniofacial development.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

Inhibition of myeloid PD-L1 suppresses osteoclastogenesis and cancer bone metastasis

Programmed death-ligand 1 (PD-L1) is predominantly expressed in the antigen-presenting cells (APCs) that are originated and are abundant in the bone marrow. The roles of PD-L1 in bone cell differentiation and cancer bone metastasis remain unclear. Here we show that PD-L1 antibody or PD-L1 conditional knockout in the hematopoietic or myeloid lineage suppresses osteoclast differentiation in vitro and in vivo. Bone metastases of breast cancer and melanoma are diminished by PD-L1 antibody or PD-L1 deletion in the myeloid lineage. Transcriptional profiling of bone marrow cells reveals that PD-L1 deletion in the myeloid cells upregulates immune-stimulatory genes, leading to increased macrophage M1 polarization, decreased M2 polarization, enhanced IFNγ signaling, and elevated T cell recruitment and activation. All these alterations result in heightened anti-tumor immunity in the cancer microenvironment. Our findings support PD-L1 antibody as a potent therapy for bone metastasis of breast cancer and melanoma by simultaneously suppressing osteoclast and enhancing immunity.

CCN2 Facilitates IL-17 Production and Osteoclastogenesis in Human Osteoarthritis Synovial Fibroblasts by Inhibiting miR-655 Expression

Osteoarthritis (OA) is associated with extensive upregulation of osteoclastogenesis and subsequent bone breakdown. The CCN family protein connective tissue growth factor (CCN2, also called CCN2) enhances inflammatory cytokine production in OA disease. The cytokine interleukin (IL)-17 is known to induce osteoclastogenesis and bone erosion in arthritic disease. Our retrieval of data from the Gene Expression Omnibus (GEO) data set and clinical tissues exhibited higher CCN2 and IL-17 expression in OA synovial sample than in normal healthy samples. We observed the same phenomenon in synovial tissue from rats with anterior cruciate ligament transaction (ACLT)-elicited OA compared with synovial tissue from control healthy rats. We also found that CCN2 facilitated increases in IL-17 synthesis in human OA synovial fibroblasts (OASFs) and promoted osteoclast formation. CCN2 affected IL-17 production by reducing miR-655 expression through the ILK and Syk signaling cascades. Our findings improve our understanding about the effect of CCN2 in OA pathogenesis and, in particular, IL-17 production and osteoclastogenesis, which may help with the design of more effective OA treatments. © 2022 American Society for Bone and Mineral Research (ASBMR).

Vibration accelerates orthodontic tooth movement by inducing osteoclastogenesis via transforming growth factor-β signalling in osteocytes

Background

We previously found the conditions of supplementary vibration that accelerated tooth movement and induced bone resorption in an experimental rat tooth movement model. However, the molecular biological mechanisms underlying supplementary vibration-induced orthodontic tooth movement are not fully understood. Transforming growth factor (TGF)-β upregulates osteoclastogenesis via induction of the receptor activator of nuclear factor kappa B ligand expression, thus TGF-β is considered an essential cytokine to induce bone resorption.

Objectives

The aim of this study is to examine the role of TGF-β during the acceleration of orthodontic tooth movement by supplementary vibration.

Materials and methods

In experimental tooth movement, 15 g of orthodontic force was loaded onto the maxillary right first molar for 28 days. Supplementary vibration (3 g, 70 Hz) was applied to the maxillary first molar for 3 min on days 0, 7, 14, and 21. TGF-β receptor inhibitor SB431542 was injected into the submucosal palatal and buccal areas of the maxillary first molars once every other day. The co-culture of RAW264.7 cells and MLO-Y4 cells was used as an in vitro osteoclastogenesis model.

Results

SB431542 suppressed the acceleration of tooth movement and the increase in the number of osteoclasts by supplementary vibration in our experimental rat tooth movement model. Immunohistochemical analysis showed supplementary vibration increased the number of TGF-β1-positive osteocytes in the alveolar bone on the compression side during the experimental tooth movement. Moreover, vibration-upregulated TGF-β1 in MLO-Y4 cells induced osteoclastogenesis.

Conclusions

Orthodontic tooth movement was accelerated by supplementary vibration through the promotion of the production of TGF-β1 in osteocytes and subsequent osteoclastogenesis.