Energy metabolism in osteoclast formation and activity

Bioenergetic programs of cancellous and cortical bone are distinct and differ with age and mechanical loading

Mechanical loading induces bone formation in young rodents, but mechanoresponsiveness is reduced with age. Glycolytic activity and mitochondrial dysfunction increase with age and may change bone mechanotransduction. To evaluate load-induced changes to bioenergetic activity in young and adult animals, we loaded the tibia of 10-wk and 26-wk female C57BL/6J mice and examined transcriptomic responses at the mid-diaphysis, and metaphyseal cortical shell and cancellous core. Across all biological processes, oxidative phosphorylation and mitochondrial pathways were most often enriched with loading and had contrasting enrichment in young and adult animals. Following loading, young animals had temporally-coordinated differential expression of mitochondrial-associated genes, with greatest expression at the mid-diaphysis. In adults, bioenergetic gene expression was lower compared to young animals. To assess individual contributions of glycolysis and pyruvate-mediated oxidative phosphorylation to load-induced bone formation in vivo, we inhibited each pathway therapeutically and loaded the tibia of young and adult female mice for 2 weeks. In both young and adult mice, loading increased cortical bone mass, but inhibition of oxidative phosphorylation reduced cortical area and moment of inertia in both loaded and control limbs. Conversely, load-induced improvements of adult cancellous bone depended on glycolysis. In summary, mechanical loading transcriptionally activated mitochondrial pathways in an age-specific manner and bioenergetic inhibition revealed unique metabolic programs for cortical and cancellous bone.

Heme metabolism mediates RANKL-induced osteoclastogenesis via mitochondrial oxidative phosphorylation

Bone undergoes life-long remodeling, in which disorders of bone remodeling could occur in many pathological conditions including osteoporosis. Understanding the cellular metabolism of osteoclasts (OCs) is key to developing new treatments for osteoporosis, a disease that affects over 200 million women worldwide per annum. We found that human OC differentiation from peripheral blood mononuclear cells derived from 8 female patients is featured with a distinct gene expression profile of mitochondrial biogenesis. Elevated mitochondrial membrane potential (MMP, Δψm) was also observed in receptor activator of NF-κB ligand (RANKL)-induced OCs. Interestingly, the gene pathways of heme synthesis and metabolism were activated upon RANKL stimulation, featured by transcriptomic profiling in murine cells at a single-cell resolution, which revealed a stepwise expression pattern of heme-related genes. The real-world human data also divulges potential links between heme-related genes and bone mineral density. Heme is known to have a role in the formation of functional mitochondrial complexes that regulate MMP. Disruption of heme biosynthesis via genetically silencing Ferrochelatase or a selective inhibitor, N-methyl Protoporphyrin IX (NMPP), demonstrated potent inhibition of OC differentiation, with a dose-dependent effect observed in NMPP treatment and a substantial efficacy even at a single dose. In vivo study further showed the protective effect of NMPP on ovariectomy-induced bone loss in female mice. Collectively, we found that RANKL-mediated signaling regulated mitochondrial formation and heme metabolism to synergistically support osteoclastogenesis. Inhibition of heme synthesis impaired OC formation and reversed excessive bone loss, representing a new therapeutic target for metabolic skeletal disorders.

Role of TRPM2 in Oxidative Stress-Mediated Bone Loss in Periodontitis

Oxidative stress has emerged as a critical player in the development and progression of periodontitis. Transient receptor potential melastatin 2 (TRPM2) is a crucial oxidative stress sensor, while its role in periodontitis and its relationship with the oxidative stress microenvironment remains poorly understood. The objective of this research is to unravel the mechanism by which reactive oxygen species (ROS) activate the TRPM2 channel, driving osteoclast differentiation and eventually leading to bone degradation in periodontitis. By doing so, we aim to provide novel insights into the initiation, progress, and potential treatment methodologies for bone loss instigated by periodontitis. In this study, our results revealed significant upregulation of TRPM2 expression in inflamed periodontal tissues and a close alliance with osteoclast differentiation. First, significant upregulation of TRPM2 in periodontitis, with a clear association with osteoclast differentiation, was observed based on the GEO database. In addition, enhanced levels of TRPM2 and oxidative stress markers were evident in samples from both periodontitis patients and the mouse model of periodontitis. Importantly, the ablation of TRPM2 distinctly alleviated alveolar bone resorption in periodontitis-affected mice. In vitro assays concluded that ROS-induced TRPM2 activation fostered osteoclast differentiation and amplification of osteoclast-related genes. Moreover, RNA-seq results illuminated a close alliance of TRPM2 with osteoclast differentiation, oxidative phosphorylation, mitochondrial inner membrane, and mitochondrial protein complexes. Further validation indicated that damaged mitophagy could overproduce ROS to activate TRPM2 as a positive regulator of osteoclast differentiation via the Ca2+/NFATc1 signaling pathway. Finally, we conducted in vivo and in vitro interventions using a TRPM2 inhibitor and found that the inhibition of TRPM2 significantly alleviated bone loss induced by periodontitis. Consequently, our results suggest that TRPM2 plays a crucial role in triggering osteoclast differentiation in periodontitis's oxidative stress microenvironment, signifying a potential therapeutic target for prevention and treatment of bone erosion induced by periodontitis.

Curcumin Ameliorated Glucocorticoid-Induced Osteoporosis While Modulating the Gut Microbiota and Serum Metabolome

Glucocorticoid-induced osteoporosis (GIOP) is the leading cause of secondary osteoporosis. Recently, the "bone-gut axis" theory has linked bone development with gut microbial diversity, community composition, and metabolites. Curcumin, a well-studied polyphenol, shows potential in mitigating bone loss and osteoporosis. Alendronate, a standard therapeutic agent for osteoporosis, serves as a positive control in this investigation. The study demonstrates the potency of curcumin in reducing bone loss and restoring bone mineral density, enhancing trabecular parameters notably through increased trabecular number, volume, and thickness and reduced bone marrow cavity size. Gut microbiome sequencing revealed that both curcumin and alendronate treatments similarly enhanced gut microbial diversity and altered microbiota composition, increasing beneficial bacteria (Akkermansia_muciniphila, Dubosiella_sp910585105, and Ruminococcus_sp910584195) while reducing harmful bacteria (Treponema_D_sp910584475 and Duncaniella_sp910584825). Furthermore, significant changes in serum levels of metabolites including raffinose, ursolic acid, spermidine, inosine, hypoxanthine, thiamine, and pantothenic acid were observed post-treatment with curcumin or alendronate. Importantly, these beneficial metabolites and microorganisms were negatively correlated with inflammatory cytokines. In conclusion, curcumin holds promise for use against GIOP by modulating the gut microbiome and serum metabolome as well as reducing systemic inflammation.

FSH exacerbates bone loss by promoting osteoclast energy metabolism through the CREB-MDH2-NAD+ axis

AIMS

Osteoclast energy metabolism is a promising target for treating diseases characterized by high osteoclast activity, such as osteoporosis. However, the regulatory factors involved in osteoclast bioenergetic processes are still in the early stages of being fully understood. This study reveals the effects of follicle-stimulating hormone (FSH) on osteoclast energy metabolism.

METHODS

The Lyz2-Cre-Flox model selectively deletes FSH receptor (FSHR) from osteoclast precursor cells to generate Fshrf/f; Lyz2-Cre (Fshrf/f; Cre) mice. Bone quality was assessed using micro-computed tomography, histomorphometric analysis, and dual-fluorescence labeling. The in vitro assays measured oxygen consumption rate, extracellular acidification rate, pyruvate content, and mitochondrial membrane potential to determine metabolic flux. RNA-seq, LC-MS, dual-luciferase reporter assays, and chromatin immunoprecipitation (ChIP) assays were used to elucidate the underlying mechanisms.

RESULTS

FSHR deficiency in osteoclasts protected bone from resorption under normal and ovariectomized conditions. FSHR-deficient osteoclasts have reduced nicotinamide adenine dinucleotide (NAD+) levels, impairing osteoclast activity and energy metabolism. Mechanistically, FSH influenced NAD+ levels via the CREB/MDH2 axis. Treatment with FSH monoclonal antibodies rescued bone loss in OVX mice and reduced bone marrow NAD+ levels.

CONCLUSIONS

Targeting FSH may be a promising metabolic modulation strategy for treating osteoporosis and other diseases associated with high osteoclast activity.

Efficient evaluation of osteotoxicity and mechanisms of endocrine disrupting chemicals using network toxicology and molecular docking approaches: triclosan as a model compound

This study aimed to demonstrate the utility of a network toxicology strategy in elucidating osteotoxicity and the molecular mechanisms of endocrine-disrupting chemicals (EDCs) using triclosan exposure in postmenopausal osteoporosis (PMOP) as a case study. The potential targets of triclosan were identified using the Comparative Toxicogenomics Database, SwissTargetPrediction, and TargetNet. PMOP-related targets were obtained from GeneCards, DisGeNET, and DrugBank. A total of 478 overlapping genes between disease targets and triclosan effectors were identified. Subsequent analysis using STRING and Cytoscape, applying the Matthews correlation coefficient algorithm, identified five core genes: STAT3, TP53, EGFR, MYC, and JUN. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses performed using R revealed that triclosan-induced PMOP is primarily associated with disrupted endocrine signaling and activation of the Phosphoinositide 3-kinase (PI3K)-Protein kinase B (Akt) signaling pathway. Molecular docking using CB-Dock2 confirmed strong binding affinities between triclosan and the core targets. Collectively, these results indicate that triclosan adversely affects bone health by disrupting endocrine regulation and energy metabolism through the PI3K-Akt pathway. This study establishes a theoretical framework for understanding how long-term triclosan exposure induces or exacerbates PMOP by investigating the underlying molecular mechanisms. These findings present a novel paradigm for evaluating the health risks posed by environmental pollutants.

Metabolomics Approach Revealed Polyunsaturated Fatty Acid Disorders as Pathogenesis for Chronic Pancreatitis-Induced Osteoporosis in Mice

Background: Osteoporosis is frequently observed in patients with chronic pancreatitis, and both conditions are closely associated with systemic metabolic disorders. However, the underlying mechanisms linking chronic pancreatitis and osteoporosis remain unclear. Methods: In this study, we utilized high-performance liquid chromatography-mass spectrometry (HPLC-MS) to conduct metabolomics and lipidomics analyses on pancreatic, serum, and other tissues from a mouse model of chronic pancreatitis-induced osteoporosis (CP-OP), with the aim to elucidate the metabolism-related pathogenic mechanisms of CP-OP. Results: We identified over 405 metabolites and 445 lipids, and our findings revealed that several metabolites involving the tricarboxylic acid (TCA) cycle, as well as triacylglycerols and diacylglycerols with higher saturation, were significantly increased in the CP-OP model. In contrast, triglycerides with higher unsaturation were decreased. Differential pathways were enriched in n-3 long-chain polyunsaturated fatty acid metabolism in both pancreatic and bone tissues, and these pathways exhibited positive correlations with bone-related parameters. Furthermore, the modulation of these polyunsaturated fatty acids by Qingyi granules demonstrated significant therapeutic effects on CP-OP, as validated in mouse models. Conclusions: Through the metabolomics approach, we uncovered that disorders in polyunsaturated fatty acids play a critical role in the pathogenesis of CP-OP. This study not only enhances our understanding of the pathogenesis of CP-OP but also highlights the therapeutic potential of targeting polyunsaturated fatty acids as a future intervention strategy for osteoporosis treatment.

A call for standardization: Evaluating different methodologies to induce in vitro foreign body giant cell formation for biomaterials research and design

Foreign body giant cells (FBGCs) are crucial in the foreign body reaction at the biomaterial-tissue interface, forming through the fusion of cells from the monocyte/macrophage lineage and performing functions such as material degradation and fibrous encapsulation. Yet, their presence and role in biomaterials research is only slowly unveiled. This review analyzed existing FBGC literature identified through a search string and sources from FBGC articles to evaluate the most commonly used methods and highlight the challenges in establishing a standardized protocol. Our findings revealed a fragmented research landscape marked by significant variability in in vitro culture conditions, i.e., cell origin and type, culture media and sera, fusion-inducing factors, seeding density, culture surface, and inconsistencies in the read-outs. This complicates efforts toward standardization and hampers cross-study comparisons. Based on these results, we highlight the need and propose guidelines for standardized culture protocols for FBGC research. Overall, this review aims to underscore the relevance of improving reproducibility and reliability in FBGC research, facilitating effective cross-study comparisons and advancing understanding of FBGC formation and function, ultimately contributing to designing more effective biomaterial-based therapies. STATEMENT OF SIGNIFICANCE: Foreign body giant cells (FBGCs) are crucial in the body's response to implanted biomaterials. Yet, current research addressing their role and impact is highly fragmented. This review comprehensively and systematically examines the diverse methodologies and definitions used in FBGC research and identifies critical gaps and inconsistencies hindering the reproducibility and comparison of findings. By advocating for standardized protocols, we aim to enhance the reliability and equivalence of research, thus providing a stronger foundation for understanding biomaterial-driven FBGC formation and function. Establishing such a framework will impact biomaterial-based therapies, supporting their effectiveness and safety in medical applications, and is thus of relevance for scientists, companies, and clinicians in the biomaterial and medical device communities.

Molecular mechanism of mitochondrial autophagy mediating impaired energy metabolism leading to osteoporosis

Osteoporosis (OP) is a bone metabolic disease caused by decreased bone mass leading to destruction of bone microstructure. Treatment of OP is characterized by a lifelong nature, causing extreme financial and psychological burdens to patients. Hormonal abnormalities, cellular autophagy, Ferroptosis, and oxidative stress are all part of the intricate and varied pathophysiology of OP. Recent research has revealed that mitochondrial dysfunction is a significant factor in the onset and progression of OP. By regulating bone marrow mesenchymal stem cell differentiation through various signaling pathways and cytokines, abnormal mitochondrial energy metabolism brought on by oxidative stress processes impacts osteoblast and osteoclast proliferation and differentiation, causing an imbalance in bone metabolism that ultimately results in OP. Therefore, one possible method to prevent and manage OP may be to use mitochondria as a carrier to trigger osteogenic differentiation of bone marrow mesenchymal stem cells from mitochondrial energy consumption, oxidative stress, autophagy, and osteoclast death. In order to offer some theoretical references and therapeutic approaches for the clinical prevention and treatment of OP, we will examine the pathophysiology of OP from mitochondrial dysfunction in this work.

Mechanisms of Ferroptosis in bone disease: A new target for osteoporosis treatment

Osteoporosis (OP) is a common disease in the elderly, characterized by decreased bone strength, reduced bone density, and increased fracture risk. There are two clinical types of osteoporosis: primary osteoporosis and secondary osteoporosis. The most common form is postmenopausal osteoporosis, which is caused by decreased estrogen production after menopause. Secondary osteoporosis, on the other hand, occurs when certain medications, diabetes, or nutritional deficiencies lead to a decrease in bone density. Ferroptosis, a new iron-dependent programmed cell death process, is critical in regulating the development of osteoporosis, but the underlying molecular mechanisms are complex. In the pathologic process of osteoporosis, several studies have found that ferroptosis may occur in osteocytes, osteoblasts, and osteoclasts, cell types closely related to bone metabolism. The imbalance of iron homeostasis in osteoblasts and excessive iron accumulation can promote lipid peroxidation through the Fenton reaction, which induces ferroptosis in osteoblasts and affects their role in regulating bone metabolism. Ferroptosis in osteoblasts inhibits bone formation and reduces the amount of new bone production. Osteoclast-associated ferroptosis abnormalities, on the other hand, may alter the homeostasis of bone resorption. In this paper, we start from the molecular mechanism of ferroptosis, and introduce the ways in which ferroptosis affects the physiological and pathological processes of the body. After that, the effects of ferroptosis on osteoblasts and osteoclasts will be discussed separately to elucidate the molecular mechanism between ferroptosis and osteoporosis, which will provide a new breakthrough for the prevention and treatment of osteoporosis and a more effective and better idea for the treatment strategy of osteoporosis.

FTO-associated osteoclastogenesis promotes alveolar bone resorption in apical periodontitis male rat via the HK1/USP14/RANK pathway

Alveolar bone resorption (ABR) is a key pathological manifestation in the development of apical periodontitis (AP) and contributes to the AP-associated tooth loss among AP patients in the clinic. However, the underlying mechanism of ABR development is largely unknown. Here we show, the total levels of N6-methyladenosine (m6A) were reduced in AP male rat alveolar bone tissues and BMDM-derived osteoclasts (OC), which was associated with the up-regulation of obesity-associated protein (FTO). Subsequently FTO-mediated hexokinase (HK1) demethylation modification enhancing glycolytic pathway that stabilizes receptor activator of NF-κB (RANK) protein via the deubiquitination activity of ubiquitin-specific protease 14 (USP14), which further promotes osteoclastogenesis to participate in the AP-related ABR development. Finally, Dac51 (an FTO inhibitor) and 2-DG (an HK1 inhibitor) both exhibit the inhibitory activity of osteoclastogenesis. Our current study reveals a molecular mechanism on osteoclastogenesis-related ABR and provides a therapeutic target of AP via modulating the FTO/HK1/USP14/RANK axis.

Unraveling the Mechanisms That Regulate Osteoclast Differentiation: A Review of Current Advances

Osteoporosis is a metabolic bone disease primarily caused by a decreased bone formation and increased bone resorption. Osteoclasts are a special class of terminally differentiated cells that play an important role in normal bone remodeling and bone loss in osteoporosis as well as in a variety of osteolytic diseases. Osteoclasts can be differentiated from monocyte-macrophage cells of the hematopoietic system; they are the key cells in bone resorption. Osteoclast formation and differentiation are regulated by various cytokines and transcription factors. In this review, we summarize recent advances in research on the regulation of osteoclast differentiation and function by factors such as M-CSF, RANKL, AP-1, NFATC1, MITF, and PU.1. Understanding these cytokines and transcription factors can not only help identify targets for osteoclast differentiation but also aid in intervening in the treatment of osteoclast-related diseases.

The SIRT5-JIP4 interaction promotes osteoclastogenesis by modulating RANKL-induced signaling transduction

Receptor activator of nuclear factor kappa-B ligand (RANKL) initiates a complex signaling cascade that is crucial for inducing osteoclast differentiation and activation. RANKL-induced signaling has been analyzed in detail, and the involvement of TNF receptor-associated factor 6 (TRAF6), calmodulin-dependent protein kinase (CaMK), NF-κB, mitogen-activated protein kinase (MAPK), activator protein-1 (AP-1), and molecules that contain an immunoreceptor tyrosine-based activation motif (ITAM) has been reported. However, the precise molecular steps that regulate RANKL signaling remain largely unknown. Here, we revealed the indispensable role of a class III histone deacetylase (SIRT5) in the processes of RANKL-induced osteoclast differentiation and activation. SIRT5 expression in osteoclasts was increased during osteoclastogenesis upon stimulation with RANKL. The RANKL-induced signaling activation was suppressed in SIRT5-deficient osteoclasts but enhanced by SIRT5 overexpression. Mice with global or conditional monocytic lineage knockout of SIRT5 had increased bone mass and reduced osteoclast numbers. In the cytoplasm, SIRT5 interacted with the scaffold protein JNK-interacting protein 4 (JIP4) to finely regulate MAPK signaling, which was critical for osteoclast differentiation and activation. Pharmacological inhibition of the catalytic activity of SIRT5 effectively reversed bone loss in ovariectomized mice. Taken together, the results of this study reveal that the SIRT5-JIP4 axis is a novel positive regulator that finely regulates RANKL-induced osteoclast differentiation and suggest that targeting this axis is a therapeutic strategy for preventing osteoporotic bone loss.

Important Role of Mitochondrial Dysfunction in Immune Triggering and Inflammatory Response in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an inflammatory autoimmune disease, primarily characterized by chronic symmetric synovial inflammation and erosive bone destruction.Mitochondria, the primary site of cellular energy production, play a crucial role in energy metabolism and possess homeostatic regulation capabilities. Mitochondrial function influences the differentiation, activation, and survival of both immune and non-immune cells involved in RA pathogenesis. If the organism experiences hypoxia, genetic predisposition, and oxidative stress, it leads to mitochondrial dysfunction, which further affects immune cell energy metabolism, synovial cell proliferation, apoptosis, and inflammatory signaling, causing the onset and progression of RA; and, mitochondrial regulation is becoming increasingly important in the treatment of RA.In this review, we examine the structure and function of mitochondria, analyze the potential causes of mitochondrial dysfunction in RA, and focus on the mechanisms by which mitochondrial dysfunction triggers chronic inflammation and immune disorders in RA. We also explore the effects of mitochondrial dysfunction on RA immune cells and osteoblasts, emphasizing its key role in the immune response and inflammatory processes in RA. Furthermore, we discuss potential biological processes that regulate mitochondrial homeostasis, which are of great importance for the prevention and treatment of RA.

Niacin regulates glucose metabolism and osteogenic differentiation via the SIRT2-C/EBPβ-AREG signaling axis

The pathogenesis of osteoporosis is driven by several mechanisms including the imbalance between osteoblastic bone formation and osteoclastic bone resorption. Currently, the role of Niacin (NA), also known as vitamin B3, in the regulation of osteoblastic differentiation is not fully understood. Data from the NHANES database were employed to investigate the association of NA intake with the prevalence of osteoporosis. Alterations in mRNA and protein levels of genes and proteins involved in osteogenic differentiation were evaluated via techniques including qRT-PCR, protein immunoblotting, Alkaline Phosphatase (ALP) activity analysis, ALP staining, and Alizarin Red staining. Changes in the mouse skeletal system were investigated by organizational analysis and Micro-CT. The results indicated that NA promoted osteogenic differentiation. Co-immunoprecipitation and chromatin immunoprecipitation were performed to explore the underlying mechanisms. It was observed that NA promoted AREG expression by deacetylating C/EBPβ via SIRT2, thereby activating the PI3K-AKT signaling pathway. It also enhanced the activity of the pivotal glycolytic enzyme, PFKFB3. This cascade amplified osteoblast glycolysis, facilitating osteoblast differentiation. These findings demonstrate that NA modulates glucose metabolism and influences osteogenic differentiation via the SIRT2-C/EBPβ-AREG pathway, suggesting that NA may be a potential therapeutic agent for the management of osteoporosis, and AREG could be a plausible target.

Metabolic reprogramming in skeletal cell differentiation

The human skeleton is a multifunctional organ made up of multiple cell types working in concert to maintain bone and mineral homeostasis and to perform critical mechanical and endocrine functions. From the beginning steps of chondrogenesis that prefigures most of the skeleton, to the rapid bone accrual during skeletal growth, followed by bone remodeling of the mature skeleton, cell differentiation is integral to skeletal health. While growth factors and nuclear proteins that influence skeletal cell differentiation have been extensively studied, the role of cellular metabolism is just beginning to be uncovered. Besides energy production, metabolic pathways have been shown to exert epigenetic regulation via key metabolites to influence cell fate in both cancerous and normal tissues. In this review, we will assess the role of growth factors and transcription factors in reprogramming cellular metabolism to meet the energetic and biosynthetic needs of chondrocytes, osteoblasts, or osteoclasts. We will also summarize the emerging evidence linking metabolic changes to epigenetic modifications during skeletal cell differentiation.

Glutaminolysis provides nucleotides and amino acids to regulate osteoclast differentiation in mice

Osteoclasts are bone resorbing cells that are essential to maintain skeletal integrity and function. While many of the growth factors and molecular signals that govern osteoclastogenesis are well studied, how the metabolome changes during osteoclastogenesis is unknown. Using a multifaceted approach, we identified a metabolomic signature of osteoclast differentiation consisting of increased amino acid and nucleotide metabolism. Maintenance of the osteoclast metabolic signature is governed by elevated glutaminolysis. Mechanistically, glutaminolysis provides amino acids and nucleotides which are essential for osteoclast differentiation and bone resorption in vitro. Genetic experiments in mice found that glutaminolysis is essential for osteoclastogenesis and bone resorption in vivo. Highlighting the therapeutic implications of these findings, inhibiting glutaminolysis using CB-839 prevented ovariectomy induced bone loss in mice. Collectively, our data provide strong genetic and pharmacological evidence that glutaminolysis is essential to regulate osteoclast metabolism, promote osteoclastogenesis and modulate bone resorption in mice.

Substrate-Mediated Regulation of Src Expression Drives Osteoclastogenesis Divergence

BACKGROUND/OBJECTIVES

Glass, bone, and dentin are commonly applied substrates for osteoclast cultures; however, the impact of these substrates on osteoclastogenesis remains underexplored. This study aimed to address a significant gap in understanding how different substrates influence the process of osteoclastogenesis.

METHODS

RAW 264.7 cells were cultured and induced with RANKL on glass, bone, and dentin slides. Histological and molecular techniques were used to identify patterns and differences in osteoclast behavior on each substrate.

RESULTS

Osteoclasts cultured on glass slides possessed the greatest number of nuclei and the highest expression levels of ACP5 (TRAP) and CTSK, with osteoclasts on bone and dentin slides displaying progressively lower levels. Src expression was also most pronounced in osteoclasts on glass slides, with decreased levels observed on bone and dentin. This variation in Src expression likely contributed to differences in cytoskeletal remodeling and oxidative phosphorylation (OXPHOS), resulting in substrate-dependent divergences in osteoclastogenesis.

CONCLUSIONS

Glass slides were the most favorable substrate for inducing osteoclastogenesis, while bone and dentin slides were less effective. The substrate-induced expression of Src played a fundamental role in shaping the phenotypic divergence of osteoclasts. These insights fill important knowledge gaps and have significant implications for the development and selection of in vitro models for bone-related diseases and drug screening platforms.

Impact of the SIK3 pathway inhibition on osteoclast differentiation via oxidative phosphorylation

Maintenance of bone homeostasis and the balance between bone resorption and formation are crucial for maintaining skeletal integrity. This study sought to investigate the role of salt-inducible kinase 3 (SIK3), a key regulator in cellular energy metabolism, during the differentiation of osteoclasts. Despite osteoclasts being high energy-consuming cells essential for breaking down mineralized bone tissue, the specific function of SIK3 in this process remains unclear. To address this issue, we generated osteoclast-specific SIK3 conditional knockout mice and assessed the impact of SIK3 deletion on bone homeostasis. Our findings revealed that SIK3 conditional knockout mice exhibited increased bone mass and an osteopetrosis phenotype, suggesting a pivotal role for SIK3 in bone resorption. Moreover, we assessed the impact of pterosin B, a SIK3 inhibitor, on osteoclast differentiation. The treatment with pterosin B inhibited osteoclast differentiation, reduced the numbers of multinucleated osteoclasts, and suppressed resorption activity in vitro. Gene expression analysis demonstrated that SIK3 deletion and pterosin B treatment influence a common set of genes involved in osteoclast differentiation and bone resorption. Furthermore, pterosin B treatment altered intracellular metabolism, particularly affecting key metabolic pathways, such as the tricarboxylic acid cycle and oxidative phosphorylation. These results provide valuable insights into the involvement of SIK3 in osteoclast differentiation and the molecular mechanisms underlying osteoclast function and bone diseases.

Osteoclasts control endochondral ossification via regulating acetyl-CoA availability

Osteoclast is critical in skeletal development and fracture healing, yet the impact and underlying mechanisms of their metabolic state on these processes remain unclear. Here, by using osteoclast-specific small GTPase Rheb1-knockout mice, we reveal that mitochondrial respiration, rather than glycolysis, is essential for cathepsin K (CTSK) production in osteoclasts and is regulated by Rheb1 in a mechanistic target of rapamycin complex 1 (mTORC1)-independent manner. Mechanistically, we find that Rheb1 coordinates with mitochondrial acetyl-CoA generation to fuel CTSK, and acetyl-CoA availability in osteoclasts is the central to elevating CTSK. Importantly, our findings demonstrate that the regulation of CTSK by acetyl-CoA availability is critical and may confer a risk for abnormal endochondral ossification, which may be the main cause of poor fracture healing on alcohol consumption, targeting Rheb1 could successfully against the process. These findings uncover a pivotal role of mitochondria in osteoclasts and provide a potent therapeutic opportunity in bone disorders.

The Emerging Role of the Mitochondrial Respiratory Chain in Skeletal Aging

Maintenance of mitochondrial homeostasis is crucial for ensuring healthy mitochondria and normal cellular function. This process is primarily responsible for regulating processes that include mitochondrial OXPHOS, which generates ATP, as well as mitochondrial oxidative stress, apoptosis, calcium homeostasis, and mitophagy. Bone mesenchymal stem cells express factors that aid in bone formation and vascular growth. Positive regulation of hematopoietic stem cells in the bone marrow affects the differentiation of osteoclasts. Furthermore, the metabolic regulation of cells that play fundamental roles in various regions of the bone, as well as interactions within the bone microenvironment, actively participates in regulating bone integrity and aging. The maintenance of cellular homeostasis is dependent on the regulation of intracellular organelles, thus understanding the impact of mitochondrial functional changes on overall bone metabolism is crucially important. Recent studies have revealed that mitochondrial homeostasis can lead to morphological and functional abnormalities in senescent cells, particularly in the context of bone diseases. Mitochondrial dysfunction in skeletal diseases results in abnormal metabolism of bone-associated cells and a secondary dysregulated microenvironment within bone tissue. This imbalance in the oxidative system and immune disruption in the bone microenvironment ultimately leads to bone dysplasia. In this review, we examine the latest developments in mitochondrial respiratory chain regulation and its impacts on maintenance of bone health. Specifically, we explored whether enhancing mitochondrial function can reduce the occurrence of bone cell deterioration and improve bone metabolism. These findings offer prospects for developing bone remodeling biology strategies to treat age-related degenerative diseases.

Citric Acid: A Nexus Between Cellular Mechanisms and Biomaterial Innovations

Citrate-based biodegradable polymers have emerged as a distinctive biomaterial platform with tremendous potential for diverse medical applications. By harnessing their versatile chemistry, these polymers exhibit a wide range of material and bioactive properties, enabling them to regulate cell metabolism and stem cell differentiation through energy metabolism, metabonegenesis, angiogenesis, and immunomodulation. Moreover, the recent US Food and Drug Administration (FDA) clearance of the biodegradable poly(octamethylene citrate) (POC)/hydroxyapatite-based orthopedic fixation devices represents a translational research milestone for biomaterial science. POC joins a short list of biodegradable synthetic polymers that have ever been authorized by the FDA for use in humans. The clinical success of POC has sparked enthusiasm and accelerated the development of next-generation citrate-based biomaterials. This review presents a comprehensive, forward-thinking discussion on the pivotal role of citrate chemistry and metabolism in various tissue regeneration and on the development of functional citrate-based metabotissugenic biomaterials for regenerative engineering applications.

Attenuating bone loss in osteoporosis: the potential of corylin (CL) as a therapeutic agent

The global prevalence of osteoporosis is being exacerbated by the increasing number of aging societies and longer life expectancies. In response, numerous drugs have been developed in recent years to mitigate bone resorption and enhance bone density. Nonetheless, the efficacy and safety of these pharmaceutical interventions remain constrained. Corylin (CL), a naturally occurring compound derived from the anti-osteoporosis plant Psoralea corylifolia L., has exhibited promising potential in impeding osteoclast differentiation. This study aims to evaluate the effect and molecular mechanisms of CL regulating osteoclast differentiation in vitro and its potential as a therapeutic agent for osteoporosis treatment in vivo. Our investigation revealed that CL effectively inhibits osteoclast formation and their bone resorption capacity by downregulating the transcription factors NFATc1 and c-fos, consequently resulting in the downregulation of genes associated with bone resorption. Furthermore, it has been observed that CL can effectively mitigate the migration and fusion of pre-osteoclast, while also attenuating the activation of mitochondrial mass and function. The results obtained from an in vivo study have demonstrated that CL is capable of attenuating the bone loss induced by ovariectomy (OVX). Based on these significant findings, it is proposed that CL exhibits considerable potential as a novel drug strategy for inhibiting osteoclast differentiation, thereby offering a promising approach for the treatment of osteoporosis.

Oestradiol and osteoclast differentiation: Effects on p53 and mitochondrial metabolism

BACKGROUND

Oestrogen deficiency increases bone resorption, contributing to osteoporosis development. Yet, the mechanisms mediating the effects of oestrogen on osteoclasts remain unclear. This study aimed to elucidate the early metabolic alteration induced by RANKL, the essential cytokine in osteoclastogenesis and 17-beta-oestradiol (E2) on osteoclast progenitor cells, using RAW 264.7 macrophage cell line and primary bone marrow-derived macrophages as biological models.

RESULTS

This research demonstrated that, in osteoclast precursors, RANKL stimulates complex I activity, oxidative phosphorylation (OXPHOS) and mitochondria-derived ATP production as early as 3 h of exposure. This effect on mitochondrial bioenergetics is associated with an increased capacity to oxidize TCA cycle substrates, fatty acids and amino acids. E2 inhibited all effects of RANKL on mitochondria metabolism. In the presence of RANKL, E2 also decreased cell number and stimulated the mitochondrial-mediated apoptotic pathway, detected as early as 3 h. Further, the pro-apoptotic effects of E2 during osteoclast differentiation were associated with an accumulation of p392S-p53 in mitochondria.

CONCLUSIONS

These findings elucidate the early effects of RANKL on osteoclast progenitor metabolism and suggest novel p53-mediated mechanisms that contribute to postmenopausal osteoporosis.

Isobavachin attenuates osteoclastogenesis and periodontitis-induced bone loss by inhibiting cellular iron accumulation and mitochondrial biogenesis

As bone-resorbing cells rich in mitochondria, osteoclasts require high iron uptake to promote mitochondrial biogenesis and maintain a high-energy metabolic state for active bone resorption. Given that abnormal osteoclast formation and activation leads to imbalanced bone remodeling and osteolytic bone loss, osteoclasts may be crucial targets for treating osteolytic diseases such as periodontitis. Isobavachin (IBA), a natural flavonoid compound, has been confirmed to be an inhibitor of receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation from bone marrow-derived macrophages (BMMs). However, its effects on periodontitis-induced bone loss and the potential mechanism of its anti-osteoclastogenesis effect remain unclear. Our study demonstrated that IBA suppressed RANKL-induced osteoclastogenesis in BMMs and RAW264.7 cells and inhibited osteoclast-mediated bone resorption in vitro. Transcriptomic analysis indicated that iron homeostasis and reactive oxygen species (ROS) metabolic process were enriched among the differentially expressed genes following IBA treatment. IBA exerted its anti-osteoclastogenesis effect by inhibiting iron accumulation in osteoclasts. Mechanistically, IBA attenuated iron accumulation in RANKL-induced osteoclasts by inhibiting the mitogen-activated protein kinase (MAPK) pathway to upregulate ferroportin1 (Fpn1) expression and promote Fpn1-mediated intracellular iron efflux. We also found that IBA inhibited mitochondrial biogenesis and function, and reduced RANKL-induced ROS generation in osteoclasts. Furthermore, IBA attenuated periodontitis-induced bone loss by reducing osteoclastogenesis in vivo. Overall, these results suggest that IBA may serve as a promising therapeutic strategy for bone diseases characterized by osteoclastic bone resorption.

Energy homeostasis in the bone

The bone serves as an energy reservoir and actively engages in whole-body energy metabolism. Numerous studies have determined fuel requirements and bioenergetic properties of bone under physiological conditions as well as the dysregulation of energy metabolism associated with bone metabolic diseases. Here, we review the main sources of energy in bone cells and their regulation, as well as the endocrine role of the bone in systemic energy homeostasis. Moreover, we discuss metabolic changes that occur as a result of osteoporosis. Exploration in this area will contribute to an enhanced comprehension of bone energy metabolism, presenting novel possibilities to address metabolic diseases.

M2 macrophages secrete glutamate-containing extracellular vesicles to alleviate osteoporosis by reshaping osteoclast precursor fate

Osteoclast precursors (OCPs) are thought to commit to osteoclast differentiation, which is accelerated by aging-related chronic inflammation, thereby leading to osteoporosis. However, whether the fate of OCPs can be reshaped to transition into other cell lineages is unknown. Here, we showed that M2 macrophage-derived extracellular vesicles (M2-EVs) could reprogram OCPs to downregulate osteoclast-specific gene expression and convert OCPs to M2 macrophage-like lineage cells, which reshaped the fate of OCPs by delivering the molecular metabolite glutamate. Upon delivery of glutamate, glutamine metabolism in OCPs was markedly enhanced, resulting in the increased production of α-ketoglutarate (αKG), which participates in Jmjd3-dependent epigenetic reprogramming, causing M2-like macrophage differentiation. Thus, we revealed a novel transformation of OCPs into M2-like macrophages via M2-EVs-initiated metabolic reprogramming and epigenetic modification. Our findings suggest that M2-EVs can reestablish the balance between osteoclasts and M2 macrophages, alleviate the symptoms of bone loss, and constitute a new approach for bone-targeted therapy to treat osteoporosis.

A novel proteomic signature of osteoclast differentiation unveils the deubiquitinase UCHL1 as a necessary osteoclastogenic driver

Bone destruction, a major source of morbidity, is mediated by heightened differentiation and activity of osteoclasts (OC), highly specialized multinucleated myeloid cells endowed with unique bone-resorptive capacity. The molecular mechanisms regulating OC differentiation in the bone marrow are still partly elusive. Here, we aimed to identify new regulatory circuits and actionable targets by comprehensive proteomic characterization of OCgenesis from mouse bone marrow monocytes, adopting two parallel unbiased comparative proteomic approaches. This work disclosed an unanticipated protein signature of OCgenesis, with most gene products currently unannotated in bone-related functions, revealing broad structural and functional cellular reorganization and divergence from macrophagic immune activity. Moreover, we identified the deubiquitinase UCHL1 as the most upregulated cytosolic protein in differentiating OCs. Functional studies proved it essential, as UCHL1 genetic and pharmacologic inhibition potently suppressed OCgenesis. Furthermore, proteomics and mechanistic dissection showed that UCHL1 supports OC differentiation by restricting the anti-OCgenic activity of NRF2, the transcriptional activator of the canonical antioxidant response, through redox-independent stabilization of the NRF2 inhibitor, KEAP1. Besides offering a valuable experimental framework to dissect OC differentiation, our study discloses the essential role of UCHL1, exerted through KEAP1-dependent containment of NRF2 anti-OCgenic activity, yielding a novel potential actionable pathway against bone loss.

Mesoporous bioactive glass scaffolds for the delivery of bone marrow stem cell-derived osteoinductive extracellular vesicles lncRNA promote senescent bone defect repair by targeting the miR-1843a-5p/Mob3a/YAP axis

Bone repair in elderly patients poses a huge challenge due to the age-related progressive decline in regenerative abilities attributed to the senescence of bone marrow stem cells (BMSCs). Bioactive scaffolds have been applied in bone regeneration due to their various biological functions. In this study, we aimed to fabricate functionalized bioactive scaffolds through loading osteoinductive extracellular vesicles (OI-EVs) based on mesoporous bioactive glass (MBG) scaffolds (1010 particles/scaffold) and to investigate its effects on osteogenesis and senescence of BMSCs. The results suggested that OI-EVs upregulate the proliferative and osteogenic capacities of senescent BMSCs. More importantly, The results showed that loading OI-EVs into MBG scaffolds achieved better bone regeneration. Furthermore, OI-EVs and BMSCs RNAs bioinformatics analysis indicated that OI-EVs play roles through transporting pivotal lncRNA acting as a "sponge" to compete with Mob3a for miR-1843a-5p to promote YAP dephosphorylation and nuclear translocation, ultimately resulting in elevated proliferation and osteogenic differentiation and reduced senescence-related phenotypes. Collectively, these results suggested that the OI-EVs lncRNA ceRNA regulatory networks might be the key point for senescent osteogenesis. More importantly, the study indicated the feasibility of loading OI-EVs into scaffolds and provided novel insights into biomaterial design for facilitating bone regeneration in the treatment of senescent bone defects. STATEMENT OF SIGNIFICANCE: Constructing OI-EVs/MBG delivering system and verification of its bone regeneration enhancement in senescent defect repair. Aging bone repair poses a huge challenge due to the age-related progressive degenerative decline in regenerative abilities attributed to the senescence of BMSCs. OI-EVs/MBG delivering system were expected as promising treatment for senescent bone repair, which could provide an effective strategy for bone regeneration in elderly patients. Clarification of potential OI-EVs lncRNA ceRNA regulatory mechanism in senescent bone regeneration OI-EVs play important roles through transferring lncRNA-ENSRNOG00000056625 sponging miR-1843a-5p that targeted Mob3a to activate YAP translocation into nucleus, ultimately alleviate senescence, promote proliferation and osteogenic differentiation in O-BMSCs, which provides theoretical basis for EVs-mediated therapy in future clinical works.

PINK1 restrains periodontitis-induced bone loss by preventing osteoclast mitophagy impairment

The oral colonization of periodontal pathogens onto gingival tissues establishes hypoxic microenvironment, often disrupting periodontal homeostasis in conjunction with oxidative stress. The association between reactive oxygen species (ROS) and osteolytic periodontitis have been suggested by recent studies. PTEN-induced kinase 1 (PINK1), a mitochondrial serine/threonine kinase, is an essential protein for mitochondrial quality control as it protects cells from oxidative stress by promoting degradation of damaged mitochondria through mitophagy. However, the pathophysiological roles of PINK1 in osteoclast-mediated bone loss have not been explored. Here we aimed to determine whether PINK1 plays a role in the regulation of osteoclastogenesis and alveolar bone resorption associated with periodontitis. C57BL/6 wild type (WT) and Pink1 knockout (KO) mice were subjected to ligature-induced periodontitis (LIP), and alveolar bones were evaluated by μCT-analysis and tartrate-resistant acid phosphatase (TRAP) staining. The μCT-analysis showed that bone volume fraction and travecular thickness were lower in Pink1 KO compared to WT mice. The number of TRAP-positive osteoclasts was markedly increased in the periodontal tissues of Pink1 KO mice with LIP. The genetic silencing or deletion of Pink1 promoted excessive osteoclast differentiation and bone resorption in vitro, as respectively indicated by TRAP staining and resorption pits on dentin slices. PINK1 deficiency led to mitochondrial instabilities as indicated by confocal microscopy of mitochondrial ROS, mitochondrial oxygen consumption rate (OCR) analysis, and transmission electron microscopy (TEM). Consequently, a significant increase in Ca2+-nuclear factor of activated T cells 1 (NFATc1) signaling was also found. On the other hand, restoration of mitophagy and autophagy by spermidine (SPD) treatment and the resolution of oxidative stress by N-acetyl-l-cysteine (NAC) treatment protected PINK1 deficiency-induced excessive generation of osteoclasts. Taken together, our findings demonstrate that PINK1 is essential for maintaining mitochondrial homeostasis during osteoclast differentiation. Therefore, targeting PINK1 may provide a novel therapeutic strategy for severe periodontitis with fulminant osteolysis.

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.

NOTCH2 promotes osteoclast maturation and metabolism and modulates the transcriptome profile during osteoclastogenesis

Notch signaling plays a key regulatory role in bone remodeling and NOTCH2 enhances osteoclastogenesis, an effect that is mostly mediated by its target gene Hes1. In the present study, we explored mechanisms responsible for the enhanced osteoclastogenesis in bone marrow-derived macrophages (BMM) from Notch2tm1.1Ecan, harboring a NOTCH2 gain-of-function mutation, and control mice. Notch2tm1.1Ecan mice are osteopenic and have enhanced osteoclastogenesis. Bulk RNA-Seq and gene set enrichment analysis of Notch2tm1.1Ecan BMMs cultured in the presence of macrophage colony stimulating factor (M-CSF) and receptor activator of NF-κB ligand revealed enrichment of genes associated with enhanced cell metabolism, aerobic respiration, and mitochondrial function, all associated with osteoclastogenesis. These pathways were not enhanced in the context of a Hes1 inactivation. Analysis of single cell RNA-Seq data of pooled control and Notch2tm1.1Ecan BMMs treated with M-CSF or M-CSF and receptor activator of NF-κB ligand for 3 days identified 11 well-defined cellular clusters. Pseudotime trajectory analysis indicated a trajectory of clusters expressing genes associated with osteoclast progenitors, osteoclast precursors, and mature cells. There were an increased number of cells expressing gene markers associated with the osteoclast and with an unknown, albeit related, cluster in Notch2tm1.1Ecan than in control BMMs as well as enhanced expression of genes associated with osteoclast progenitors and precursors in Notch2tm1.1Ecan cells. In conclusion, BMM cultures display cellular heterogeneity, and NOTCH2 enhances osteoclastogenesis, increases mitochondrial and metabolic activity of osteoclasts, and affects cell cluster allocation in BMMs.

L-arginine metabolism inhibits arthritis and inflammatory bone loss

OBJECTIVES

To investigate the effect of the L-arginine metabolism on arthritis and inflammation-mediated bone loss.

METHODS

L-arginine was applied to three arthritis models (collagen-induced arthritis, serum-induced arthritis and human TNF transgenic mice). Inflammation was assessed clinically and histologically, while bone changes were quantified by μCT and histomorphometry. In vitro, effects of L-arginine on osteoclast differentiation were analysed by RNA-seq and mass spectrometry (MS). Seahorse, Single Cell ENergetIc metabolism by profilIng Translation inHibition and transmission electron microscopy were used for detecting metabolic changes in osteoclasts. Moreover, arginine-associated metabolites were measured in the serum of rheumatoid arthritis (RA) and pre-RA patients.

RESULTS

L-arginine inhibited arthritis and bone loss in all three models and directly blocked TNFα-induced murine and human osteoclastogenesis. RNA-seq and MS analyses indicated that L-arginine switched glycolysis to oxidative phosphorylation in inflammatory osteoclasts leading to increased ATP production, purine metabolism and elevated inosine and hypoxanthine levels. Adenosine deaminase inhibitors blocking inosine and hypoxanthine production abolished the inhibition of L-arginine on osteoclastogenesis in vitro and in vivo. Altered arginine levels were also found in RA and pre-RA patients.

CONCLUSION

Our study demonstrated that L-arginine ameliorates arthritis and bone erosion through metabolic reprogramming and perturbation of purine metabolism in osteoclasts.

The serine synthesis pathway drives osteoclast differentiation through epigenetic regulation of NFATc1 expression

Bone-resorbing osteoclasts are vital for postnatal bone health, as increased differentiation or activity results in skeletal pathologies such as osteoporosis. The metabolism of mature osteoclasts differs from their progenitor cells, but whether the observed metabolic changes are secondary to the altered cell state or actively drive the process of cell differentiation is unknown. Here, we show that transient activation of the serine synthesis pathway (SSP) is essential for osteoclastogenesis, as deletion of the rate-limiting enzyme phosphoglycerate dehydrogenase in osteoclast progenitors impairs their differentiation and results in increased bone mass. In addition, pharmacological phosphoglycerate dehydrogenase inhibition abrogated bone loss in a mouse model of postmenopausal osteoporosis by blocking bone resorption. Mechanistically, SSP-derived α-ketoglutarate is necessary for histone demethylases that remove repressive histone methylation marks at the nuclear factor of activated T cells, cytoplasmic 1 (Nfatc1) gene locus, thereby inducing NFATc1 expression and consequent osteoclast maturation. Taken together, this study reveals a metabolic-epigenetic coupling mechanism that directs osteoclast differentiation and suggests that the SSP can be therapeutically targeted to prevent osteoporotic bone loss.

Sexual dimorphism of osteoclast reliance on mitochondrial oxidation of energy substrates in the mouse

Osteoclasts specialize in bone resorption and are critical for bone remodeling. Previous studies have shown that osteoclasts possess abundant mitochondria and derive most energy through oxidative phosphorylation (OXPHOS). However, the energy substrates fueling OXPHOS in osteoclasts remain to be fully defined. Here, we showed that osteoclast differentiation was coupled with increased oxidation of glucose, glutamine, and oleate. Transcriptomic analyses with RNA sequencing revealed marked upregulation of genes participating in OXPHOS and mitochondrial fatty acid oxidation, during osteoclast differentiation. Increased mitochondrial oxidation of long-chain fatty acids was required for osteoclast differentiation in vitro. However, blocking fatty acid oxidation in vivo, by deletion of carnitine palmitoyltransferase 1a (Cpt1a) in osteoclast progenitors, impaired osteoclast formation only in the female mice. The Cpt1a-deficient females were further protected from osteoclast activation by a high-fat diet. The males, on the contrary, exhibited normal bone resorption despite Cpt1a deletion, regardless of the dietary fat content. Moreover, concurrent deletion of mitochondrial pyruvate carrier 1 and Cpt1a, blocking mitochondrial oxidation of both glucose and fatty acids in the osteoclast lineage, failed to impede bone resorption in the males. The study therefore uncovers a female-specific dependence on mitochondrial oxidation of fatty acids and glucose in osteoclasts in vivo.

Irisin Enhances Mitochondrial Function in Osteoclast Progenitors during Differentiation

Irisin is a myokine released from muscle during exercise with distinct signaling effects on tissues throughout the body, including an influence on skeletal remodeling. Our previous work has shown that irisin stimulates resorption, a key first step in bone remodeling, by enhancing osteoclastogenesis. The present study further investigates the action of irisin on the metabolic function of osteoclast progenitors during differentiation. Fluorescent imaging showed increased mitochondrial content and reactive oxygen species production with irisin treatment in osteoclast progenitors after 48 h of osteoclastogenic culture. Mitochondrial stress testing demonstrated a significant increase in maximal oxygen consumption rate and spare capacity after 48 h of preconditioning with irisin treatment. Together, these findings further elucidate the stimulatory action of irisin on osteoclastogenesis, demonstrating an enhancement of metabolism through mitochondrial respiration in the progenitor to support the energy demands of their differentiation into mature osteoclasts.

The protective role of CD73 in periodontitis: preventing hyper-inflammatory fibroblasts and driving osteoclast energy metabolism

Introduction

Periodontitis is an immune-mediated inflammatory disease affecting almost half of the adult population and is the leading cause of tooth loss in the United States. The role of extracellular nucleotide signaling including nucleotide metabolizing enzyme CD73 adds an important layer of interaction of purine mediators capable of orchestrating inflammatory outcomes. CD73 is able to catabolize 5'-adenosine monophosphate into adenosine at the extracellular level, playing a critical role in regulating many processes under physiological and pathological conditions. Here, we explored the role of CD73 in ligature-induced periodontitis in vivo comparing wild-type C57Bl/6J and CD73-deficient mice.

Methods

We assessed gingival levels of inflammatory cytokines in vivo and in murine gingival fibroblasts in vitro, as well as bone loss, and RANKL-induced osteoclastogenesis. We have also analyzed CD73 mRNA in samples derived from patients diagnosed with severe periodontitis.

Results

Our results in mice show that lack of CD73 resulted in increased inflammatory cytokines and chemokines such as IL-1β, IL-17, Cxcl1 and Cxcl2 in diseased gingiva relative to the healthy-controls and in comparison with the wild type. CD73-deficient gingival fibroblasts also manifested a defective healing response with higher MMP-13 levels. CD73-deficient animals also showed increased osteoclastogenesis in vitro with increased mitochondrial metabolism typified by excessive activation of oxidative phosphorylation, increased mitochondrial membrane potential and accumulation of hydrogen peroxide. Micro-CT analysis revealed that lack of CD73 resulted in decreased bone mineral density, decreased trabecular bone volume and thickness as well as decreased bone volume in long bones. CD73 deficiency also resulted in increased alveolar bone loss in experimental periodontitis. Correlative studies of gingival samples from severe (Grade C) periodontitis showed decreased levels of CD73 compared to healthy controls, further supporting the relevance of our murine results.

Conclusion

In conclusion, CD73 appears to play a protective role in the gingival periodontal tissue and bone homeostasis, regulating hyper-inflammatory state of stromal fibroblasts and osteoclast energy metabolism and being an important candidate for future target therapies to prevent or control immune-mediated inflammatory and osteolytic diseases.

Lactate dehydrogenase A inhibition prevents RANKL-induced osteoclastogenesis by reducing enhanced glycolysis

Osteoclasts are multinucleated, specializes bone-resorbing cells that are derived from the monocyte/macrophage lineage. Excessive resorbing activities of osteoclasts are involved in destructive bone diseases. The detailed mechanism of acidification at the bone adhesion surface during the bone resorption process of osteoclasts remains to be defined. During glycolysis, pyruvate proceeds to the tricarboxylic cycle under aerobic conditions and pyruvate is converted to lactate via lactate dehydrogenase A (LDHA) under anaerobic conditions. However, tumor cells produce ATP during aerobic glycolysis and large amounts of pyruvate are converted to lactate and H+ by LDHA. Lactate and H+ are excreted outside the cell, whereby they are involved in invasion of tumor cells due to the pH drop around the cell. In this study, we focused on aerobic glycolysis and investigated the production of lactate by LDHA in osteoclasts. Expression of LDHA and monocarboxylate transporter 4 (MCT4) was upregulated during osteoclast differentiation. Intracellular and extracellular lactate levels increased with upregulation of LDHA and MCT4, respectively. FX11 (an LDHA inhibitor) inhibited osteoclast differentiation and suppressed the bone-resorbing activity of osteoclasts. We propose that inhibition of LDHA may represent a novel therapeutic strategy for controlling excessive bone resorption in osteoporosis and rheumatoid arthritis.

AMPKα1 negatively regulates osteoclastogenesis and mitigates pathological bone loss

Osteoclasts are specialized cells responsible for bone resorption, a highly energy-demanding process. Focus on osteoclast metabolism could be a key for the treatment of osteolytic diseases including osteoporosis. In this context, AMP-activated protein kinase α1 (AMPKα1), an energy sensor highly expressed in osteoclasts, participates in the metabolic reconfiguration during osteoclast differentiation and activation. This study aimed to elucidate the role of AMPKα1 during osteoclastogenesis in vitro and its impact in bone loss in vivo. Using LysMcre/0AMPK⍺1f/f animals and LysMcre/0 as control, we evaluated how AMPKα1 interferes with osteoclastogenesis and bone resorption activity in vitro. We found that AMPKα1 is highly expressed in the early stages of osteoclastogenesis. Genetic deletion of AMPKα1 leads to increased gene expression of osteoclast differentiation and fusion markers. In addition, LysMcre/0AMPK⍺1f/f mice had an increased number and size of differentiated osteoclast. Accordingly, AMPKα1 negatively regulates bone resorption in vitro, as evidenced by the area of bone resorption in LysMcre/0AMPK⍺1f/f osteoclasts. Our data further demonstrated that AMPKα1 regulates mitochondrial fusion and fission markers upregulating Mfn2 and downregulating DRP1 (dynamics-related protein 1) and that Ctskcre/0AMPK⍺1f/f osteoclasts lead to an increase in the number of mitochondria in AMPK⍺1-deficient osteoclast. In our in vivo study, femurs from Ctskcre/0AMPK⍺1f/f animals exhibited bone loss associated with the increased number of osteoclasts, and there was no difference between Sham and ovariectomized group. Our data suggest that AMPKα1 acts as a negative regulator of osteoclastogenesis, and the depletion of AMPKα1 in osteoclast leads to a bone loss state similar to that observed after ovariectomy.

Novel Insights into Osteoclast Energy Metabolism

PURPOSE OF REVIEW

Osteoclasts are crucial for the dynamic remodeling of bone as they resorb old and damaged bone, making space for new bone. Metabolic reprogramming in these cells not only supports phenotypic changes, but also provides the necessary energy for their highly energy-consuming activity, bone resorption. In this review, we highlight recent developments in our understanding of the metabolic adaptations that influence osteoclast behavior and the overall remodeling of bone tissue.

RECENT FINDINGS

Osteoclasts undergo metabolic reprogramming to meet the energy demands during their transition from precursor cells to fully mature bone-resorbing osteoclasts. Recent research has made considerable progress in pinpointing crucial metabolic adaptations and checkpoint proteins in this process. Notably, glucose metabolism, mitochondrial biogenesis, and oxidative respiration were identified as essential pathways involved in osteoclast differentiation, cytoskeletal organization, and resorptive activity. Furthermore, the interaction between these pathways and amino acid and lipid metabolism adds to the complexity of the process. These interconnected processes can function as diverse fuel sources or have independent regulatory effects, significantly influencing osteoclast function. Energy metabolism in osteoclasts involves various substrates and pathways to meet the energetic requirements of osteoclasts throughout their maturation stages. This understanding of osteoclast biology may provide valuable insights for modulating osteoclast activity during the pathogenesis of bone-related disorders and may pave the way for the development of innovative therapeutic strategies.

Tussilagone inhibits osteoclastogenesis by modulating mitochondrial function and ROS production involved Nrf2 activation

Reactive Oxygen Species (ROS) play an essential role in the pathogenesis of osteoporosis mainly characterized by excessive osteoclasts (OCs) activity. OCs are rich in mitochondria for energy support, which is a major source of total ROS. Tussilagone (TSG), a natural Sesquiterpenes from the flower of Tussilago farfara, has plentiful beneficial pharmacological characteristics with anti-inflammatory and anti-oxidative activity, but its effects and mechanism in osteopathology are still unclear. In our study, we investigated the regulation of ROS generated from the mitochondria in OCs. We found that TSG inhibited OCs differentiation and bone resorption without any cytotoxicity. Mechanistically, TSG reduced RANKL-mediated total ROS level by down-regulating intracellular ROS production and mitochondrial function, leading to the suppression of NFATc1 transcription. We also found that nuclear factor erythroid 2-related factor 2 (Nrf2) could enhance ROS scavenging enzymes in response to RANKL-induced oxidative stress. Furthermore, TSG up-regulated the expression of Nrf2 by inhibiting its proteosomal degradation. Interestingly, Nrf2 deficiency reversed the suppressive effect of TSG on mitochondrial activity and ROS signaling in OCs. Consistent with this finding, TSG attenuated post-ovariectomy (OVX)- and lipopolysaccharide (LPS) induced bone loss by ameliorating osteoclastogenesis. Taken together, TSG has an anti-bone resorptive effect by modulating mitochondrial function and ROS production involved Nrf2 activation.

Metabolomics insights into the effects of pre-pregnancy lead exposure on bone metabolism in pregnant rats

Today's women of childbearing age with a history of high lead (Pb) exposure in childhood have large Pb body burdens, which increases Pb release during pregnancy by promoting bone Pb mobilisation. The purpose of this study was to investigate the metabolic mechanisms underlying bone Pb mobilisation and explore the bone metabolism-related pathways during pregnancy. Drinking water containing 0.05% sodium acetate or Pb acetate was provided to weaned female rats for 4 weeks followed by a 4-week washout period, and then rats were co-caged with healthy males of the same age until pregnancy. Blood and bone tissues of the female rats were collected at gestational day (GD) 3 (early pregnancy), GD 10 (middle pregnancy), and GD 17 (late pregnancy), respectively. Pb and calcium concentrations, biomarkers for bone turnover, bone microstructure, serum metabolomics, and metabolic indicators were intensively analyzed. The results demonstrated that pre-pregnancy Pb exposure elevated blood lead levels (BLLs) at GD17, accompanied by a negative correlation between BLLs and trabecular bone Pb levels. Meanwhile, Pb-exposed rats had low bone mass and aberrant bone architecture with a larger number of mature osteoclasts (OCs) compared to the control group. Moreover, the metabolomics uncovered that Pb exposure caused mitochondrial dysfunction, such as enhanced oxidative stress and inflammatory response, and suppressed energy metabolism. Additionally, the levels of ROS, MDA, IL-1β, and IL-18 involved in redox and inflammatory pathways of bone tissues were significantly increased in the Pb-exposed group, while antioxidant SOD and energy metabolism-related indicators including ATP levels, Na+-K+-ATPase, and Ca2+-Mg2+-ATPase activities were significantly decreased. In conclusion, pre-pregnancy Pb exposure promotes bone Pb mobilisation and affects bone microstructure in the third trimester of pregnancy, which may be attributed to OC activation and mitochondrial dysfunction.

HIF-1α and periodontitis: Novel insights linking host-environment interplay to periodontal phenotypes

Periodontitis, the sixth most prevalent epidemic disease globally, profoundly impacts oral aesthetics and masticatory functionality. Hypoxia-inducible factor-1α (HIF-1α), an oxygen-dependent transcriptional activator, has emerged as a pivotal regulator in periodontal tissue and alveolar bone metabolism, exerts critical functions in angiogenesis, erythropoiesis, energy metabolism, and cell fate determination. Numerous essential phenotypes regulated by HIF are intricately associated with bone metabolism in periodontal tissues. Extensive investigations have highlighted the central role of HIF and its downstream target genes and pathways in the coupling of angiogenesis and osteogenesis. Within this concise perspective, we comprehensively review the cellular phenotypic alterations and microenvironmental dynamics linking HIF to periodontitis. We analyze current research on the HIF pathway, elucidating its impact on bone repair and regeneration, while unraveling the involved cellular and molecular mechanisms. Furthermore, we briefly discuss the potential application of targeted interventions aimed at HIF in the field of bone tissue regeneration engineering. This review expands our biological understanding of the intricate relationship between the HIF gene and bone angiogenesis in periodontitis and offers valuable insights for the development of innovative therapies to expedite bone repair and regeneration.

The Physiological and Pathological Role of Acyl-CoA Oxidation

Fatty acid metabolism, including β-oxidation (βOX), plays an important role in human physiology and pathology. βOX is an essential process in the energy metabolism of most human cells. Moreover, βOX is also the source of acetyl-CoA, the substrate for (a) ketone bodies synthesis, (b) cholesterol synthesis, (c) phase II detoxication, (d) protein acetylation, and (d) the synthesis of many other compounds, including N-acetylglutamate-an important regulator of urea synthesis. This review describes the current knowledge on the importance of the mitochondrial and peroxisomal βOX in various organs, including the liver, heart, kidney, lung, gastrointestinal tract, peripheral white blood cells, and other cells. In addition, the diseases associated with a disturbance of fatty acid oxidation (FAO) in the liver, heart, kidney, lung, alimentary tract, and other organs or cells are presented. Special attention was paid to abnormalities of FAO in cancer cells and the diseases caused by mutations in gene-encoding enzymes involved in FAO. Finally, issues related to α- and ω- fatty acid oxidation are discussed.

Icaritin ameliorates RANKL-mediated osteoclastogenesis and ovariectomy-induced osteoporosis

A rapidly aging society and longer life expectancy are causing osteoporosis to become a global epidemic. Over the last five decades, a number of drugs aimed at reducing bone resorption or restoring bone mass have been developed, but their efficacy and safety are limited. Icaritin (ICT) is a natural compound extracted from anti-osteoporosis herb Epimedium spp. and has been shown to inhibit osteoclast differentiation. However, the molecular mechanism by which ICT weaken RANKL-induced osteoclast differentiation has not been completely investigated. Here, we evaluated the anti-osteoclastogenic effect of ICT in vitro and the potential drug candidate for treating osteoporosis in vivo. In vitro study, ICT was found to inhibit osteoclast formation and bone resorption function via downregulating transcription factors activated T cell cytoplasm 1 (NFATc1) and c-fos, which further downregulate osteoclastogenesis-specific gene. In addition, the enhanced mitochondrial mass and function required for osteoclast differentiation was mitigated by ICT. The histomorphological results from an in vivo study showed that ICT attenuated the bone loss associated with ovariectomy (OVX). Based on these results, we propose ICT as a promising new drug strategy for osteoporosis that inhibits osteoclast differentiation.

Myeloid Cells As a Promising Target for Brain-Bone Degenerative Diseases from a Metabolic Point of View

Brain and bone degenerative diseases such as Alzheimer's disease and osteoporosis are common in the aging population and lack efficient pharmacotherapies. Myeloid cells are a diverse group of mononuclear cells that plays important roles in development, immune defense, and tissue homeostasis. Aging drastically alters the expansion and function of myeloid cells, which might be a common pathogenesis of the brain-bone degenerative diseases. From this perspective, the role of myeloid cells in brain-bone degenerative diseases is discussed, with a particular focus on metabolic alterations in myeloid cells. Furthermore, targeting myeloid cells through metabolic regulation via drugs such as metformin and melatonin is proposed as a potential therapy for the clinical treatment of brain-bone diseases.

Systems level identification of a matrisome-associated macrophage polarisation state in multi-organ fibrosis

Tissue fibrosis affects multiple organs and involves a master-regulatory role of macrophages which respond to an initial inflammatory insult common in all forms of fibrosis. The recently unravelled multi-organ heterogeneity of macrophages in healthy and fibrotic human disease suggests that macrophages expressing osteopontin (SPP1) associate with lung and liver fibrosis. However, the conservation of this SPP1+ macrophage population across different tissues and its specificity to fibrotic diseases with different etiologies remain unclear. Integrating 15 single-cell RNA-sequencing datasets to profile 235,930 tissue macrophages from healthy and fibrotic heart, lung, liver, kidney, skin, and endometrium, we extended the association of SPP1+ macrophages with fibrosis to all these tissues. We also identified a subpopulation expressing matrisome-associated genes (e.g., matrix metalloproteinases and their tissue inhibitors), functionally enriched for ECM remodelling and cell metabolism, representative of a matrisome-associated macrophage (MAM) polarisation state within SPP1+ macrophages. Importantly, the MAM polarisation state follows a differentiation trajectory from SPP1+ macrophages and is associated with a core set of regulon activity. SPP1+ macrophages without the MAM polarisation state (SPP1+MAM-) show a positive association with ageing lung in mice and humans. These results suggest an advanced and conserved polarisation state of SPP1+ macrophages in fibrotic tissues resulting from prolonged inflammatory cues within each tissue microenvironment.

Arginine methylation of PPP1CA by CARM1 regulates glucose metabolism and affects osteogenic differentiation and osteoclastic differentiation

BACKGROUND

The imbalance between osteoblasts and osteoclasts may lead to osteoporosis. Osteoblasts and osteoclasts have different energy requirements, with aerobic glycolysis being the prominent metabolic feature of osteoblasts, while osteoclast differentiation and fusion are driven by oxidative phosphorylation.

METHODS

By polymerase chain reaction as well as Western blotting, we assayed coactivator-associated arginine methyltransferase 1 (CARM1) expression in bone tissue, the mouse precranial osteoblast cell line MC3T3-E1 and the mouse monocyte macrophage leukaemia cell line RAW264.7, and expression of related genes during osteogenic differentiation and osteoclast differentiation. Using gene overexpression (lentivirus) and loss-of-function approach (CRISPR/Cas9-mediated knockout) in vitro, we examined whether CARM1 regulates osteogenic differentiation and osteoblast differentiation by metabolic regulation. Transcriptomic assays and metabolomic assays were used to find the mechanism of action of CARM1. Furthermore, in vitro methylation assays were applied to clarify the arginine methylation site of PPP1CA by CARM1.

RESULTS

We discovered that CARM1 reprogrammed glucose metabolism in osteoblasts and osteoclasts from oxidative phosphorylation to aerobic glycolysis, thereby promoting osteogenic differentiation and inhibiting osteoclastic differentiation. In vivo experiments revealed that CARM1 significantly decreased bone loss in osteoporosis model mice. Mechanistically, CARM1 methylated R23 of PPP1CA, affected the dephosphorylation of AKT-T450 and AMPK-T172, and increased the activities of phosphofructokinase-1 and pructose-2,6-biphosphatase3, causing an up-regulation of glycolytic flux. At the same time, as a transcriptional coactivator, CARM1 regulated the expression of pyruvate dehydrogenase kinase 3, which resulted in the inhibition of pyruvate dehydrogenase activity and inhibition of the tricarboxylic acid cycle, leading to a subsequent decrease in the flux of oxidative phosphorylation.

CONCLUSIONS

These findings reveal for the first time the mechanism by which CARM1 affects both osteogenesis and osteoclast differentiation through metabolic regulation, which may represent a new feasible treatment strategy for osteoporosis.

Gut microbiota as a target in the bone health of livestock and poultry: roles of short-chain fatty acids

The regulation and maintenance of bone metabolic homeostasis are crucial for animal skeletal health. It has been established that structural alterations in the gut microbiota and ecological dysbiosis are closely associated with bone metabolic homeostasis. The gut microbiota and its metabolites, especially short-chain fatty acids (SCFAs), affect almost all organs, including the bone. In this process, SCFAs positively affect bone healing by acting directly on cells involved in bone repair after or by shaping appropriate anti-inflammatory and immunomodulatory responses. Additionally, SCFAs have the potential to maintain bone health in livestock and poultry because of their various biological functions in regulating bone metabolism, including immune function, calcium absorption, osteogenesis and osteolysis. This review primarily focuses on the role of SCFAs in the regulation of bone metabolism by gut microbiota and provides insight into studies related to bone health in livestock and poultry.

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.