Intracellular and extracellular ATP coordinately regulate the inverse correlation between osteoclast survival and bone resorption

Obesity induced caveolin-1 impairs osteogenesis via activating mitophagy and inhibiting Sirt1 signaling

Obesity has become a major global health problem and the effect on bone formation has received increasing attention. However, the interaction between obesity and bone metabolism is complex and still not fully understood. Here, we show that caveolin-1 (Cav1), a membrane scaffold protein involved in regulating a variety of cellular processes, plays a key regulatory role as a bridge connecting obesity and bone metabolism. High-fat diet (HFD)-induced obese C57BL/6J mouse displayed a significant increase in Cav1 expression and lower osteogenic activity; In vitro treatment of osteoblastic MC3T3-E1 cells with 1 mM free fatty acids (FFA) significantly promoted Cav1 expression and PINK1/Parkin regulated mitophagy, but inhibited the expression of osteogenic marker genes. Conversely, reduced expression of the Cav1 gene prevented these effects. Both endogenous oxidative stress and Sirt1 pathway were also significantly reduced after Cav1 knockdown in FFA-treated cells. Finally, Cav1-Sirt1 docking and co-immunoprecipitation results showed that Cav1 interacted with Sirt1 and FFA enhanced the interaction. Taken together, these results suggest that obesity impairs bone development and formation through up-regulation of the Cav1 gene, which lead to inhibition of Sirt1/FOXO1 and Sirt1/PGC-1α signaling pathways through interacting with Sirt1 molecule, and an increase of mitophagy level.

Impdh2 deficiency suppresses osteoclastogenesis through mitochondrial oxidative phosphorylation and alleviates ovariectomy-induced osteoporosis

Abnormalities in osteoclastic generation or activity disrupt bone homeostasis and are highly involved in many pathologic bone-related diseases, including rheumatoid arthritis, osteopetrosis, and osteoporosis. Control of osteoclast-mediated bone resorption is crucial for treating these bone diseases. However, the mechanisms of control of osteoclastogenesis are incompletely understood. In this study, we identified that inosine 5'-monophosphate dehydrogenase type II (Impdh2) positively regulates bone resorption. By histomorphometric analysis, Impdh2 deletion in mouse myeloid lineage cells (Impdh2LysM-/- mice) showed a high bone mass due to the reduced osteoclast number. qPCR and western blotting results demonstrated that the expression of osteoclast marker genes, including Nfatc1, Ctsk, Calcr, Acp5, Dcstamp, and Atp6v0d2, was significantly decreased in the Impdh2LysM-/- mice. Furthermore, the Impdh inhibitor MPA treatment inhibited osteoclast differentiation and induced Impdh2-cytoophidia formation. The ability of osteoclast differentiation was recovered after MPA deprivation. Interestingly, genome-wide analysis revealed that the osteoclastic mitochondrial biogenesis and functions, such as oxidative phosphorylation, were impaired in the Impdh2LysM-/- mice. Moreover, the deletion of Impdh2 alleviated ovariectomy-induced bone loss. In conclusion, our findings revealed a previously unrecognized function of Impdh2, suggesting that Impdh2-mediated mechanisms represent therapeutic targets for osteolytic diseases.

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.

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.

Mitochondria: An Emerging Unavoidable Link in the Pathogenesis of Periodontitis Caused by Porphyromonas gingivalis

Porphyromonas gingivalis (P. gingivalis) is a key pathogen of periodontitis. Increasing evidence shows that P. gingivalis signals to mitochondria in periodontal cells, including gingival epithelial cells, gingival fibroblast cells, immune cells, etc. Mitochondrial dysfunction affects the cellular state and participates in periodontal inflammatory response through the aberrant release of mitochondrial contents. In the current review, it was summarized that P. gingivalis induced mitochondrial dysfunction by altering the mitochondrial metabolic state, unbalancing mitochondrial quality control, prompting mitochondrial reactive oxygen species (ROS) production, and regulating mitochondria-mediated apoptosis. This review outlines the impacts of P. gingivalis and its virulence factors on the mitochondrial function of periodontal cells and their role in periodontitis.

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.

An NIR-II Probe with High PSMA Affinity Demonstrates an Unexpected Excellent Bone Imaging Ability

(S)-3-(Carboxyformamido)-2-(3-(carboxymethyl)ureido)propanoic acid (EuK) is a known binder toward the prostate-specific membrane agent (PSMA) with strong affinity, making it a popular choice for prostate cancer medicine development. However, during the probe modification, a new EuK-based PSMA tetramer, Bone-1064, was discovered to have an unexpected and intense uptake in bone, which has not yet been reported in any previous studies yet. After administration, Bone-1064 allowed for high contrast visualization of the bone from surrounding tissues with a signal-to-background ratio of 10.22 at 24 h postinjection. In contrast, the tumor had a blurry contour, and the maximum tumor-to-normal-tissue ratio was only 2.22. Further imaging studies revealed that Bone-1064 binds specifically to hydroxyapatite in bone tissues, instead of PSMA. Overall, Bone-1064 is an excellent bone probe with a unique structure that can be used for NIR-II fluorescence imaging in animal models. Meanwhile, this modification study might also inspire further PSMA probe designations.

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.

The role of reactive oxygen species in bone cell physiology and pathophysiology

Hydrogen peroxide (H2O2), superoxide anion radical (O2-•), and other forms of reactive oxygen species (ROS) are produced by the vast majority of mammalian cells and can contribute both to cellular homeostasis and dysfunction. The NADPH oxidases (NOX) enzymes and the mitochondria electron transport chain (ETC) produce most of the cellular ROS. Multiple antioxidant systems prevent the accumulation of excessive amounts of ROS which cause damage to all cellular macromolecules. Many studies have examined the contribution of ROS to different bone cell types and to skeletal physiology and pathophysiology. Here, we discuss the role of H2O2 and O2-• and their major enzymatic sources in osteoclasts and osteoblasts, the fundamentally different ways via which these cell types utilize mitochondrial derived H2O2 for differentiation and function, and the molecular mechanisms that impact and are altered by ROS in these cells. Particular emphasis is placed on evidence obtained from mouse models describing the contribution of different sources of ROS or antioxidant enzymes to bone resorption and formation. Findings from studies using pharmacological or genetically modified mouse models indicate that an increase in H2O2 and perhaps other ROS contribute to the loss of bone mass with aging and estrogen deficiency, the two most important causes of osteoporosis and increased fracture risk in humans.

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.

BAP1 promotes osteoclast function by metabolic reprogramming

Treatment of osteoporosis commonly diminishes osteoclast number which suppresses bone formation thus compromising fracture prevention. Bone formation is not suppressed, however, when bone degradation is reduced by retarding osteoclast functional resorptive capacity, rather than differentiation. We find deletion of deubiquitinase, BRCA1-associated protein 1 (Bap1), in myeloid cells (Bap1∆LysM), arrests osteoclast function but not formation. Bap1∆LysM osteoclasts fail to organize their cytoskeleton which is essential for bone degradation consequently increasing bone mass in both male and female mice. The deubiquitinase activity of BAP1 modifies osteoclast function by metabolic reprogramming. Bap1 deficient osteoclast upregulate the cystine transporter, Slc7a11, by enhanced H2Aub occupancy of its promoter. SLC7A11 controls cellular reactive oxygen species levels and redirects the mitochondrial metabolites away from the tricarboxylic acid cycle, both being necessary for osteoclast function. Thus, in osteoclasts BAP1 appears to regulate the epigenetic-metabolic axis and is a potential target to reduce bone degradation while maintaining osteogenesis in osteoporotic patients.

Oxidative phosphorylation in bone cells

The role of energy metabolism in bone cells is an active field of investigation. Bone cells are metabolically very active and require high levels of energy in the form of adenosine triphosphate (ATP) to support their function. ATP is generated in the cytosol via glycolysis coupled with lactic acid fermentation and in the mitochondria via oxidative phosphorylation (OXPHOS). OXPHOS is the final convergent metabolic pathway for all oxidative steps of dietary nutrients catabolism. The formation of ATP is driven by an electrochemical gradient that forms across the mitochondrial inner membrane through to the activity of the electron transport chain (ETC) complexes and requires the presence of oxygen as the final electron acceptor. The current literature supports a model in which glycolysis is the main source of energy in undifferentiated mesenchymal progenitors and terminally differentiated osteoblasts, whereas OXPHOS appears relevant in an intermediate stage of differentiation of those cells. Conversely, osteoclasts progressively increase OXPHOS during differentiation until they become multinucleated and mitochondrial-rich terminal differentiated cells. Despite the abundance of mitochondria, mature osteoclasts are considered ATP-depleted, and the availability of ATP is a critical factor that regulates the low survival capacity of these cells, which rapidly undergo death by apoptosis. In addition to ATP, bioenergetic metabolism generates reactive oxygen species (ROS) and intermediate metabolites that regulate a variety of cellular functions, including epigenetics changes of genomic DNA and histones. This review will briefly discuss the role of OXPHOS and the cross-talks OXPHOS-glycolysis in the differentiation process of bone cells.

SIRT6-PAI-1 axis is a promising therapeutic target in aging-related bone metabolic disruption

The mechanistic regulation of bone mass in aged animals is poorly understood. In this study, we examined the role of SIRT6, a longevity-associated factor, in osteocytes, using mice lacking Sirt6 in Dmp-1-expressing cells (cKO mice) and the MLO-Y4 osteocyte-like cell line. cKO mice exhibited increased osteocytic expression of Sost, Fgf23 and senescence inducing gene Pai-1 and the senescence markers p16 and Il-6, decreased serum phosphate levels, and low-turnover osteopenia. The cKO phenotype was reversed in mice that were a cross of PAI-1-null mice with cKO mice. Furthermore, senescence induction in MLO-Y4 cells increased the Fgf23 and Sost mRNA expression. Sirt6 knockout and senescence induction increased HIF-1α binding to the Fgf23 enhancer sequence. Bone mass and serum phosphate levels were higher in PAI-1-null aged mice than in wild-type mice. Therefore, SIRT6 agonists or PAI-1 inhibitors may be promising therapeutic options for aging-related bone metabolism disruptions.

ECSIT is essential for RANKL-induced stimulation of mitochondria in osteoclasts and a target for the anti-osteoclastogenic effects of estrogens

Introduction

Estrogens inhibit bone resorption and preserve bone mass, at least in part, via direct effects on osteoclasts. The binding of RANKL, the critical cytokine for osteoclast differentiation, to its receptor in osteoclast precursor cells of the monocyte lineage recruits the adaptor protein TRAF6 and activates multiple signaling pathways. Early effects of RANKL include stimulation of mitochondria. 17β-estradiol (E2) prevents the effects of RANKL on mitochondria and promotes mitochondria mediated apoptotic cell death. However, the molecular mechanisms responsible for the actions of RANKL and estrogens on mitochondria remain unknown. Evolutionarily Conserved Signaling Intermediate in Toll Pathway (ECSIT) is a complex I-associated protein that regulates immune responses in macrophages following the engagement of Toll-like receptors, which also recruit TRAF6. Here, we examined whether ECSIT could be implicated in the rapid effects of RANKL and E2 on osteoclast progenitors.

Methods

Bone marrow-derived macrophages (BMMs) from C57BL/6 mice were cultured with RANKL (30 ng/ml) with or without E2 (10-8 M). ECSIT-TRAF6 interaction was evaluated by co-immunoprecipitation and ECSIT levels in mitochondria and cytosolic fractions by Western blot. ShRNA lentivirus particles were used to knockdown ECSIT. Osteoclasts were enumerated after tartrate-resistant acid phosphatase staining. Oxygen consumption and extracellular acidification rates were measured with Seahorse XFe96 Analyzer. ATP, lactate, and NAD/NADH were measured with commercial assay kits. NADH oxidation to NAD was used to evaluate Complex I activity. Total and mitochondrial ROS, and mitochondrial membrane potential were measured with H2DCFDA, MitoSOX, and TMRM probes, respectively. Degradation of DEVD-AFC was used to measure Caspase-3 activity.

Results

We found that RANKL promoted ECSIT-TRAF6 interaction and increased the levels of ECSIT in mitochondria. E2 abrogated these effects of RANKL. Silencing of ECSIT decreased osteoclast differentiation and abrogated the inhibitory effects of E2 on osteoclastogenesis. Loss of ECSIT decreased complex I activity, oxygen consumption, NAD+/NADH redox ratio, and ATP production and increased mitochondrial ROS. In the absence of ECSIT, the stimulatory actions of RANKL on complex I activity and all other markers of oxidative phosphorylation, as well as their inhibition by E2, were prevented. Instead, RANKL stimulated apoptosis of osteoclast progenitors.

Discussion

These findings suggest that dysregulated mitochondria cause a switch in RANKL signaling from pro-survival to pro-apoptotic. In addition, our results indicate that ECSIT represents a central node for the early effects of RANKL on mitochondria and that inhibition of ECSIT-mediated mitochondria stimulation might contribute to the bone protective actions of estrogens.

Multiple Genetic Loci Associated with Pug Dog Thoracolumbar Myelopathy

Pug dogs with thoracolumbar myelopathy (PDM) present with a specific clinical phenotype that includes progressive pelvic limb ataxia and paresis, commonly accompanied by incontinence. Vertebral column malformations and lesions, excessive scar tissue of the meninges, and central nervous system inflammation have been described. PDM has a late onset and affects more male than female dogs. The breed-specific presentation of the disorder suggests that genetic risk factors are involved in the disease development. To perform a genome-wide search for PDM-associated loci, we applied a Bayesian model adapted for mapping complex traits (BayesR) and a cross-population extended haplotype homozygosity test (XP-EHH) in 51 affected and 38 control pugs. Nineteen associated loci (harboring 67 genes in total, including 34 potential candidate genes) and three candidate regions under selection (with four genes within or next to the signal) were identified. The multiple candidate genes identified have implicated functions in bone homeostasis, fibrotic scar tissue, inflammatory responses, or the formation, regulation, and differentiation of cartilage, suggesting the potential relevance of these processes to the pathogenesis of PDM.

GIP reduces osteoclast activity and improves osteoblast survival in primary human bone cells

OBJECTIVE

Drugs targeting the glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) are emerging as treatments for type-2 diabetes and obesity. GIP acutely decreases serum markers of bone resorption and transiently increases bone formation markers in short-term clinical investigations. However, it is unknown whether GIP acts directly on bone cells to mediate these effects. Using a GIPR-specific antagonist, we aimed to assess whether GIP acts directly on primary human osteoclasts and osteoblasts.

METHODS

Osteoclasts were differentiated from human CD14+ monocytes and osteoblasts from human bone. GIPR expression was determined using RNA-seq in primary human osteoclasts and in situ hybridization in human femoral bone. Osteoclastic resorptive activity was assessed using microscopy. GIPR signaling pathways in osteoclasts and osteoblasts were assessed using LANCE cAMP and AlphaLISA phosphorylation assays, intracellular calcium imaging and confocal microscopy. The bioenergetic profile of osteoclasts was evaluated using Seahorse XF-96.

RESULTS

GIPR is robustly expressed in mature human osteoclasts. GIP inhibits osteoclastogenesis, delays bone resorption, and increases osteoclast apoptosis by acting upon multiple signaling pathways (Src, cAMP, Akt, p38, Akt, NFκB) to impair nuclear translocation of nuclear factor of activated T cells-1 (NFATc1) and nuclear factor-κB (NFκB). Osteoblasts also expressed GIPR, and GIP improved osteoblast survival. Decreased bone resorption and improved osteoblast survival were also observed after GIP treatment of osteoclast-osteoblast co-cultures. Antagonizing GIPR with GIP(3-30)NH2 abolished the effects of GIP on osteoclasts and osteoblasts.

CONCLUSIONS

GIP inhibits bone resorption and improves survival of human osteoblasts, indicating that drugs targeting GIPR may impair bone resorption, whilst preserving bone formation.

A novel MTX2 gene splice site variant resulting in exon skipping, causing the recently described mandibuloacral dysplasia progeroid syndrome

Until recently, mandibuloacral dysplasia (MAD) with type A and type B lipodystrophy was the first to come to mind in the association of mandibular hypoplasia, lipodystrophy, and acro-osteolysis. However, it has recently been added to the differential diagnosis of MAD, a newly defined syndrome, called MDPS. MDPS is a skeletal dysplasia characterized by postnatal growth retardation, hypotonia, generalized lipodystrophy, skin changes, progeroid traits, and dysmorphic facial features, including prominent eyes, long pinched nose, mandibular hypoplasia, and a small mouth. Biallelic null variants of the MTX2 gene are responsible for this syndrome. We performed whole-exome sequencing (WES) in a 6-year-old patient with skeletal dysplasia. WES revealed a novel homozygous c.543+1G>T splice site variant in the MTX2 gene. We also extracted total RNA from peripheral blood and used reverse transcription-polymerase chain reaction to generate cDNA. Sanger sequencing from cDNA showed that exon 8 of MTX2 was skipped. This study adds to the genetics and phenotype of MDPS and underlines the importance of comprehensive clinical and molecular research.

Osteometabolism: Metabolic Alterations in Bone Pathologies

Renewing interest in the study of intermediate metabolism and cellular bioenergetics is brought on by the global increase in the prevalence of metabolic illnesses. Understanding of the mechanisms that integrate energy metabolism in the entire organism has significantly improved with the application of contemporary biochemical tools for quantifying the fuel substrate metabolism with cutting-edge mouse genetic procedures. Several unexpected findings in genetically altered mice have prompted research into the direction of intermediate metabolism of skeletal cells. These findings point to the possibility of novel endocrine connections through which bone cells can convey their energy status to other metabolic control centers. Understanding the expanded function of skeleton system has in turn inspired new lines of research aimed at characterizing the energy needs and bioenergetic characteristics of these bone cells. Bone-forming osteoblast and bone-resorbing osteoclast cells require a constant and large supply of energy substrates such as glucose, fatty acids, glutamine, etc., for their differentiation and functional activity. According to latest research, important developmental signaling pathways in bone cells are connected to bioenergetic programs, which may accommodate variations in energy requirements during their life cycle. The present review article provides a unique perspective of the past and present research in the metabolic characteristics of bone cells along with mechanisms governing energy substrate utilization and bioenergetics. In addition, we discussed the therapeutic inventions which are currently being utilized for the treatment and management of bone-related diseases such as osteoporosis, rheumatoid arthritis (RA), osteogenesis imperfecta (OIM), etc., by modulating the energetics of bone cells. We further emphasized on the role of GUT-associated metabolites (GAMs) such as short-chain fatty acids (SCFAs), medium-chain fatty acids (MCFAs), indole derivates, bile acids, etc., in regulating the energetics of bone cells and their plausible role in maintaining bone health. Emphasis is importantly placed on highlighting knowledge gaps in this novel field of skeletal biology, i.e., "Osteometabolism" (proposed by our group) that need to be further explored to characterize the physiological importance of skeletal cell bioenergetics in the context of human health and bone related metabolic diseases.

Glucose metabolism in skeletal cells

The mammalian skeleton is integral to whole body physiology with a multitude of functions beyond mechanical support and locomotion, including support of hematopoiesis, mineral homeostasis and potentially other endocrine roles. Formation of the skeleton begins in the embryo and mostly from a cartilage template that is ultimately replaced by bone through endochondrial ossification. Skeletal development and maturation continue after birth in most species and last into the second decade of postnatal life in humans. In the mature skeleton, articular cartilage lining the synovial joint surfaces is vital for bodily movement and damages to the cartilage are a hallmark of osteoarthritis. The mature bone tissue undergoes continuous remodeling initiated with bone resorption by osteoclasts and completed with bone formation from osteoblasts. In a healthy state, the exquisite balance between bone resorption and formation is responsible for maintaining a stable bone mass and structural integrity, while meeting the physiological needs for minerals via controlled release from bone. Disruption of the balance in favor of bone resorption is the root cause for osteoporosis. Whereas osteoclasts pump molar quantities of hydrochloric acid to dissolve the bone minerals in a process requiring ATP hydrolysis, osteoblasts build bone mass by synthesizing and secreting copious amounts of bone matrix proteins. Thus, both osteoclasts and osteoblasts engage in energy-intensive activities to fulfill their physiological functions, but the bioenergetics of those and other skeletal cell types are not well understood. Nonetheless, the past ten years have witnessed a resurgence of interest in studies of skeletal cell metabolism, resulting in an unprecedented understanding of energy substrate utilization and its role in cell fate and activity regulation. The present review attempts to synthesize the current findings of glucose metabolism in chondrocytes, osteoblasts and osteoclasts. Advances with the other relevant cell types including skeletal stem cells and marrow adipocytes will not be discussed here as they have been extensively reviewed recently by others (van Gastel and Carmeliet, 2021). Elucidation of the bioenergetic mechanisms in the skeletal cells is likely to open new avenues for developing additional safe and effective bone therapies.

Glycemic Control and Bone in Diabetes

PURPOSE OF REVIEW

This review summarizes recent developments on the effects of glycemic control and diabetes on bone health. We discuss the foundational cellular mechanisms through which diabetes and impaired glucose control impact bone biology, and how these processes contribute to bone fragility in diabetes.

RECENT FINDINGS

Glucose is important for osteoblast differentiation and energy consumption of mature osteoblasts. The role of insulin is less clear, but insulin receptor deletion in mouse osteoblasts reduces bone formation. Epidemiologically, type 1 (T1D) and type 2 diabetes (T2D) associate with increased fracture risk, which is greater among people with T1D. Accumulation of cortical bone micro-pores, micro-vascular complications, and AGEs likely contribute to diabetes-related bone fragility. The effects of youth-onset T2D on peak bone mass attainment and subsequent skeletal fragility are of particular concern. Further research is needed to understand the effects of hyperglycemia on skeletal health through the lifecycle, including the related factors of inflammation and microvascular damage.

In vivo Imaging Enables Understanding of Seamless Plant Defense Responses to Wounding and Pathogen Attack

Plants are exposed to varied biotic stresses, including sequential or simultaneous attack by insects and pathogens. To overcome these complex stresses, plants must perceive each of the stresses, then integrate and relay the information throughout the plant body and eventually activate local and systemic resistance responses. Previous molecular genetic studies identified jasmonic acid and salicylic acid as key plant hormones of wound and immune responses. These hormones, combined with their antagonistic interaction, play critical roles in the initiation and regulation of defense responses against insects and pathogens. Aside from molecular and genetic information, the latest in vivo imaging technology has revealed that plant defense responses are regulated spatially and temporally. In this review, we summarize the current knowledge of local and systemic defense responses against wounding and diseases with a focus on past and recent advances in imaging technologies. We discuss how imaging-based multiparametric analysis has improved our understanding of the spatiotemporal regulation of dynamic plant stress responses. We also emphasize the importance of compiling the knowledge generated from individual studies on plant wounding and immune responses for a more seamless understanding of plant defense responses in the natural environment.

Potential of Hibiscus sabdariffa L. and Hibiscus Acid to Reverse Skin Aging

Hibiscus sabdariffa L. (HS) has a long history of edible and medicinal uses. In this study, the biological activities of the extracts, chromatographic fractions, and hibiscus acid obtained from HS were evaluated for their potential bioactivities. Their ability to promote extracellular matrix synthesis in skin fibroblasts was evaluated by enzyme-linked immunosorbent assays. Their anti-inflammatory activity was evaluated in a nitric oxide (NO)–Griess inflammatory experiment. Furthermore, hibiscus acid was found to have a strong anti-oxidative stress effect through the establishment of an oxidative stress model induced by hydrogen peroxide. Several assays indicated that hibiscus acid treatment can effectively reduce extracellular adenosine triphosphate (ATP) secretion and carbonyl protein production, as well as maintain a high level of reduced/oxidized glutathione (GSH/GSSG) in skin cells, thus providing a possible mechanism by which hibiscus acid can counter antioxidative stress. The present study is the first to explore the reversing skin aging potential and the contributory component of HS.

Mitochondrial fatty acid β-oxidation is important for normal osteoclast formation in growing female mice

Skeletal remodeling is an energy demanding process that is linked to nutrient availability and the levels of metabolic hormones. While recent studies have examined the metabolic requirements of bone formation by osteoblasts, much less is known about the energetic requirements of bone resorption by osteoclasts. The abundance of mitochondria in mature osteoclasts suggests that the production of an acidified micro-environment conducive to the ionization of hydroxyapatite, secretion of matrix-degrading enzymes, and motility during resorption requires significant energetic capacity. To investigate the contribution of mitochondrial long chain fatty acid β-oxidation to osteoclast development, we disrupted the expression of carnitine palmitoyltransferase-2 (Cpt2) in myeloid-lineage cells. Fatty acid oxidation increases dramatically in bone marrow cultures stimulated with RANKL and M-CSF and microCT analysis revealed that the genetic inhibition of long chain fatty acid oxidation in osteoclasts significantly increases trabecular bone volume in female mice secondary to reduced osteoclast numbers. In line with these data, osteoclast precursors isolated from Cpt2 mutants exhibit reduced capacity to form large-multinucleated osteoclasts, which was not rescued by exogenous glucose or pyruvate, and signs of an energetic stress response. Together, our data demonstrate that mitochondrial long chain fatty acid oxidation by the osteoclast is required for normal bone resorption as its inhibition produces an intrinsic defect in osteoclast formation.

Mechanical Disturbance of Osteoclasts Induces ATP Release That Leads to Protein Synthesis in Skeletal Muscle through an Akt-mTOR Signaling Pathway

Muscle and bone are tightly integrated through mechanical and biochemical signals. Osteoclasts are cells mostly related to pathological bone loss; however, they also start physiological bone remodeling. Therefore, osteoclast signals released during bone remodeling could improve both bone and skeletal muscle mass. Extracellular ATP is an autocrine/paracrine signaling molecule released by bone and muscle cells. Then, in the present work, it was hypothesized that ATP is a paracrine mediator released by osteoclasts and leads to skeletal muscle protein synthesis. RAW264.7-derived osteoclasts were co-cultured in Transwell^® chambers with flexor digitorum brevis (FDB) muscle isolated from adult BalbC mice. The osteoclasts at the upper chamber were mechanically stimulated by controlled culture medium perturbation, resulting in a two-fold increase in protein synthesis in FDB muscle at the lower chamber. Osteoclasts released ATP to the extracellular medium in response to mechanical stimulation, proportional to the magnitude of the stimulus and partly dependent on the P2X₇ receptor. On the other hand, exogenous ATP promoted Akt phosphorylation (S473) in isolated FDB muscle in a time- and concentration-dependent manner. ATP also induced phosphorylation of proteins downstream Akt: mTOR (S2448), p70S6K (T389) and 4E-BP1 (T37/46). Exogenous ATP increased the protein synthesis rate in FDB muscle 2.2-fold; this effect was blocked by Suramin (general P2X/P2Y antagonist), LY294002 (phosphatidylinositol 3 kinase inhibitor) and Rapamycin (mTOR inhibitor). These blockers, as well as apyrase (ATP metabolizing enzyme), also abolished the induction of FDB protein synthesis evoked by mechanical stimulation of osteoclasts in the co-culture model. Therefore, the present findings suggest that mechanically stimulated osteoclasts release ATP, leading to protein synthesis in isolated FDB muscle, by activating the P2-PI3K-Akt-mTOR pathway. These results open a new area for research and clinical interest in bone-to-muscle crosstalk in adaptive processes related to muscle use/disuse or in musculoskeletal pathologies.

Extracellular ATP and its derivatives provide spatiotemporal guidance for bone adaptation to wide spectrum of physical forces

ATP is a ubiquitous intracellular molecule critical for cellular bioenergetics. ATP is released in response to mechanical stimulation through vesicular release, small tears in cellular plasma membranes, or when cells are destroyed by traumatic forces. Extracellular ATP is degraded by ecto-ATPases to form ADP and eventually adenosine. ATP, ADP, and adenosine signal through purinergic receptors, including seven P2X ATP-gated cation channels, seven G-protein coupled P2Y receptors responsive to ATP and ADP, and four P1 receptors stimulated by adenosine. The goal of this review is to build a conceptual model of the role of different components of this complex system in coordinating cellular responses that are appropriate to the degree of mechanical stimulation, cell proximity to the location of mechanical injury, and time from the event. We propose that route and amount of ATP release depend on the scale of mechanical forces, ranging from vesicular release of small ATP boluses upon membrane deformation, to leakage of ATP through resealable plasma membrane tears, to spillage of cellular content due to destructive forces. Correspondingly, different P2 receptors responsive to ATP will be activated according to their affinity at the site of mechanical stimulation. ATP is a small molecule that readily diffuses through the environment, bringing the signal to the surrounding cells. ATP is also degraded to ADP which can stimulate a distinct set of P2 receptors. We propose that depending on the magnitude of mechanical forces and distance from the site of their application, ATP/ADP profiles will be different, allowing the relay of information about tissue level injury and proximity. Lastly, ADP is degraded to adenosine acting via its P1 receptors. The presence of large amounts of adenosine without ATP, indicates that an active source of ATP release is no longer present, initiating the transition to the recovery phase. This model consolidates the knowledge regarding the individual components of the purinergic system into a conceptual framework of choreographed responses to physical forces.

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Cellular bioenergetic molecule ATP is released when cell is mechanically stimulated.

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ATP release is proportional to the amount of cellular damage.

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ATP diffusion and transformation to ADP indicates the proximity to the damage.

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Purinergic receptors form a network choreographing cell response to physical forces.

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Complete transformation of ATP to adenosine initiates the recovery phase.

Changes in Metabolism and Mitochondrial Bioenergetics during Polyethylene-Induced Osteoclastogenesis

Changes in mitochondrial bioenergetics are believed to take place during osteoclastogenesis. This study aims to assess changes in mitochondrial bioenergetics and reactive oxygen species (ROS) levels during polyethylene (PE)-induced osteoclastogenesis in vitro. For this purpose, RAW264.7 cells were cultured for nine days and allowed to differentiate into osteoclasts in the presence of PE and RANKL. The total TRAP-positive cells, resorption activity, expression of osteoclast marker genes, ROS level, mitochondrial bioenergetics, glycolysis, and substrate utilization were measured. The effect of tocotrienols-rich fraction (TRF) treatment (50 ng/mL) on those parameters during PE-induced osteoclastogenesis was also studied. During PE-induced osteoclastogenesis, as depicted by an increase in TRAP-positive cells and gene expression of osteoclast-related markers, higher proton leak, higher extracellular acidification rate (ECAR), as well as higher levels of ROS and NADPH oxidases (NOXs) were observed in the differentiated cells. The oxidation level of some substrates in the differentiated group was higher than in other groups. TRF treatment significantly reduced the number of TRAP-positive osteoclasts, bone resorption activity, and ROS levels, as well as modulating the gene expression of antioxidant-related genes and mitochondrial function. In conclusion, changes in mitochondrial bioenergetics and substrate utilization were observed during PE-induced osteoclastogenesis, while TRF treatment modulated these changes.

Detecting respiratory chain defects in osteoblasts from osteoarthritic patients using imaging mass cytometry

Osteoporosis is a skeletal disease which is characterised by reduced bone mass and microarchitecture, with a subsequent loss of strength that predisposes to fragility and risk of fractures. The pathogenesis of falling bone mineral density, ultimately leading to a diagnosis of osteoporosis is incompletely understood but the disease is currently thought to be multifactorial. Humans are known to accumulate mitochondrial mutations and respiratory chain deficiency with age and mounting evidence suggests that this may indeed be the overarching cause intrinsic to the changing phenotype in advancing age and age-related disease. Mitochondrial mutations are detectable from the age of about 30 years onwards. Mitochondria contain their own genome which encodes 13 essential mitochondrial proteins and accumulates somatic variants at up to 10 times the rate of the nuclear genome. Once the concentration of any pathogenic mitochondrial genome variant exceeds a threshold, respiratory chain deficiency and cellular dysfunction occur. The Polg^D257A/D257A mouse model is a knock-in mutant that expresses a proof-reading-deficient version of PolgA, a nuclear encoded subunit of mtDNA polymerase. These mice are a useful model of age-related accumulation of mtDNA mutations in humans since their defective proof-reading mechanism leads to a mitochondrial DNA mutation rate 3-5 times higher than in wild-type mice. These mice showed enhanced levels of age-related osteoporosis along with respiratory chain deficiency in osteoblasts. To explore whether respiratory chain deficiency is also seen in human osteoblasts, we developed a protocol and analysis framework for imaging mass cytometry in bone tissue sections to analyse osteoblasts in situ. By comparing bone tissue sampled at one timepoint from femoral neck of 10 older healthy volunteers aged 40-85 with samples from young patients aged 1-19, we have identified complex I defect in osteoblasts from 6 out of 10 older volunteers, complex II defect in 2 out of 10 older volunteers, complex IV defect in 1 out of 10 older volunteers and complex V defect in 4 out of 10 older volunteers. These observations are consistent with findings from the Polg^D257A/D257A mouse model and suggest that respiratory chain deficiency, as a consequence of the accumulation of age-related pathogenic mitochondrial DNA mutations, may play a significant role in the pathogenesis of human age-related osteoporosis.

The effect of P2X7R- mediated Ca2+ and MAPK signaling in OPG-induced duck embryo osteoclasts differentiation and adhesive structure damage

Various factors cause animal bone malnutrition disease during intensive culture. Osteoclasts play an important role in regulating bone metabolism disease. Osteoprotegerin (OPG) modulates osteoclast function; however, the mechanism underlying this effect is unknown. Therefore, the present study aimed to explore whether OPG affects duck embryo osteoclast function via purinergic receptor P2X7. OPG significantly inhibited duck embryo osteoclast differentiation and bone resorption, and suppressed F-actin formation. In addition, OPG remarkably impaired duck embryo osteoclasts' adhesive structure. After OPG treatment, the expression of P2X7R significantly reduced, the ATP level and Ca²⁺-ATPase activity decreased rapidly, and concomitantly suppressed calcium and MAPK signaling. A438079 (a selective P2X7R inhibitor) significantly inhibited duck embryo osteoclast differentiation and bone resorption, and the phosphorylation of Ca²⁺ regulated proteins (CAM, CAMKII, CAMKIV) and MAPKs (ERK, JNK, and P38) were markedly suppressed. Pretreatment of duck embryo osteoclasts with BzATP, a P2X7R agonist, activated Ca²⁺ and MAPK signaling. BzATP alleviated OPG-induced duck embryo osteoclast differentiation and adhesive structure damage, and recovered the distribution of adhesion-related proteins in mature duck embryo osteoclasts. Thus, P2RX7-mediated Ca²⁺ and MAPK signaling has a key function in OPG-induced duck embryo osteoclast differentiation and adhesive structure damage. P2X7R might be an ideal target to treat bone diseases through regulating bone cell activation.

Andrographolide prevents bone loss via targeting estrogen-related receptor-α-regulated metabolic adaption of osteoclastogenesis

BACKGROUND AND PURPOSE

Metabolic adaptation driven by oestrogen-related receptor-α (ERRα/NR3B1) is required to meet the increased energy demand during osteoclast differentiation. Here, we hypothesize that natural product, andrographolide, acts as an ERRα inverse agonist to inhibit osteoclastogenesis.

EXPERIMENTAL APPROACH

Virtual docking and site-directed mutagenesis analysis were employed to study the binding mode of andrographolide to ERRα. Co-immunoprecipitation, luciferase reporter assay, real-time polymerase chain reaction (PCR) and immunoblot analyses were performed to identify andrographolide as an ERRα inverse agonist. The pharmacological effects of andrographolide in vivo were assessed in mice models of osteopenia induced by either a high-fat diet in male or ovariectomy in female mice.

KEY RESULTS

ERRα-dependent expression of glutaminase, a rate-limiting enzyme of mitochondrial glutamine anaplerosis, is required for ex vivo bone marrow osteoclast differentiation. Andrographolide inhibited glutaminase expression induced by ERRα and co-activator peroxisome proliferator-activated receptor γ co-activator-1β (PGC-1β), leading to reduction in osteoclastogenesis. Andrographolide acted as an inverse agonist of ERRα by disrupting its interaction with co-activator PGC-1β. Phenylalanine 232, valine 395 and phenylalanine 399 of ERRα ligand-binding domain were confirmed to be essential for this effect. In contrast, glutaminase overexpression restored the impairment triggered by andrographolide. Accordingly, andrographolide suppressed osteoclastic bone resorption and attenuated bone loss in vivo.

CONCLUSIONS AND IMPLICATIONS

These findings demonstrate that andrographolide acts as an ERRα inverse agonist for perturbation of ERRα/PGC-1β/glutaminase axis-driven metabolic adaption during osteoclast differentiation, implying that andrographolide may be a promising natural compound for preventing physiological and pathological bone loss.

Use of Network Pharmacology and Molecular Docking Technology to Analyze the Mechanism of Action of Velvet Antler in the Treatment of Postmenopausal Osteoporosis

Deer velvet antlers are the young horns of male deer that are not ossified and densely overgrown. Velvet antler and its preparations have been widely used in the treatment of postmenopausal osteoporosis (PMOP) in recent years, although its mechanism of action in the human body remains unclear. To screen the effective ingredients and targets of velvet antler in the treatment of PMOP using network pharmacology and to explore the potential mechanisms of velvet antler action in such treatments, we screened the active ingredients and targets of velvet antler in the BATMAN-TCM database. We also screened the relevant targets of PMOP in the GeneCards and OMIM databases and then compared the targets at the intersection of both velvet antler and PMOP. We used Cytoscape 3.7.2 software to construct a network diagram of "disease-drug-components-targets" and a protein-protein interaction (PPI) network through the STRING database and screened out the core targets; the R language was then used to analyze the shared targets between antler and PMOP for GO-enrichment analysis and KEGG pathway-annotation analysis. Furthermore, we used the professional software Maestro 11.1 to verify the predictive analysis based on network pharmacology. Hematoxylin-eosin (H&E) staining and micro-CT were used to observe the changes in trabecular bone tissue, further confirming the results of network pharmacological analysis. The potentially effective components of velvet antler principally include 17β-E2, adenosine triphosphate, and oestrone. These components act on key target genes such as AKT1, IL6, MAPK3, TP53, EGFR, SRC, and TNF and regulate the PI3K/Akt-signaling and MAPK-signaling pathways. These molecules participate in a series of processes such as cellular differentiation, apoptosis, metabolism, and inflammation and can ultimately be used to treat PMOP; they reflect the overall regulation, network regulation, and protein interactions.

The investigation of energy metabolism in osteoblasts and osteoclasts

The maintenance of bone homeostasis is critical for bone health. It is vulnerable to cause bone loss, even severely osteoporosis when the balance between bone formation and absorption is interrupted. Growing evidence has shown that energy metabolism disorders, such as abnormal glucose metabolism, irregular amino acid metabolism, and aberrant lipid metabolism, can damage bone homeostasis, causing or exacerbating bone mass loss and osteoporosis-related fractures. Here, we summarize the studies of energy metabolism in osteoblasts and osteoclasts and provide a better appreciation of how energy metabolism, especially glucose metabolism maintains bone homeostasis. With this knowledge, new avenues will be unraveled to understand and cue bone-related diseases such as osteoporosis.

Mitochondrial dysfunction-induced H3K27 hyperacetylation perturbs enhancers in Parkinson's disease

Mitochondrial dysfunction is a major pathophysiological contributor to the progression of Parkinson’s disease (PD); however, whether it contributes to epigenetic dysregulation remains unknown. Here, we show that both chemically and genetically driven mitochondrial dysfunctions share a common mechanism of epigenetic dysregulation. Under both scenarios, lysine 27 acetylation of likely variant H3.3 (H3.3K27ac) increased in dopaminergic neuronal models of PD, thereby opening that region to active enhancer activity via H3K27ac. These vulnerable epigenomic loci represent potential transcription factor motifs for PD pathogenesis. We further confirmed that mitochondrial dysfunction induces H3K27ac in ex vivo and in vivo (MitoPark) neurodegenerative models of PD. Notably, the significantly increased H3K27ac in postmortem PD brains highlights the clinical relevance to the human PD population. Our results reveal an exciting mitochondrial dysfunction-metabolism-H3K27ac-transcriptome axis for PD pathogenesis. Collectively, the mechanistic insights link mitochondrial dysfunction to epigenetic dysregulation in dopaminergic degeneration and offer potential new epigenetic intervention strategies for PD.

The Role of Osteoclast Energy Metabolism in the Occurrence and Development of Osteoporosis

In recent decades, the mechanism underlying bone metabolic disorders based on energy metabolism has been heavily researched. Bone resorption by osteoclasts plays an important role in the occurrence and development of osteoporosis. However, the mechanism underlying the osteoclast energy metabolism disorder that interferes with bone homeostasis has not been determined. Bone resorption by osteoclasts is a process that consumes large amounts of adenosine triphosphate (ATP) produced by glycolysis and oxidative phosphorylation. In addition to glucose, fatty acids and amino acids can also be used as substrates to produce energy through oxidative phosphorylation. In this review, we summarize and analyze the energy-based phenotypic changes, epigenetic regulation, and coupling with systemic energy metabolism of osteoclasts during the development and progression of osteoporosis. At the same time, we propose a hypothesis, the compensatory recovery mechanism (involving the balance between osteoclast survival and functional activation), which may provide a new approach for the treatment of osteoporosis.

The Novel Role of PGC1α in Bone Metabolism

Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) is a protein that promotes transcription of numerous genes, particularly those responsible for the regulation of mitochondrial biogenesis. Evidence for a key role of PGC1α in bone metabolism is very recent. In vivo studies showed that PGC1α deletion negatively affects cortical thickness, trabecular organization and resistance to flexion, resulting in increased risk of fracture. Furthermore, in a mouse model of bone disease, PGC1α activation stimulates osteoblastic gene expression and inhibits atrogene transcription. PGC1α overexpression positively affects the activity of Sirtuin 3, a mitochondrial nicotinammide adenina dinucleotide (NAD)-dependent deacetylase, on osteoblastic differentiation. In vitro, PGC1α overexpression prevents the reduction of mitochondrial density, membrane potential and alkaline phosphatase activity caused by Sirtuin 3 knockdown in osteoblasts. Moreover, PGC1α influences the commitment of skeletal stem cells towards an osteogenic lineage, while negatively affects marrow adipose tissue accumulation. In this review, we will focus on recent findings about PGC1α action on bone metabolism, in vivo and in vitro, and in pathologies that cause bone loss, such as osteoporosis and type 2 diabetes.

Deletion of SIRT3 inhibits osteoclastogenesis and alleviates aging or estrogen deficiency-induced bone loss in female mice

SIRT3 has been speculated to affect osteoclast activity through its important roles in regulating mitochondrial function. It remains unclear whether SIRT3 affects osteoclast activity in female mice which is relevant to postmenopausal osteoporosis. We hypothesized that deletion of SIRT3 could modulate bone remodeling in female mice under physiological aging process or ovariectomy (OVX)-induced bone loss. We found that SIRT3 level was markedly increased in primary bone marrow-derived macrophages (BMMs) from both 26-month-old aged mice and OVX mice. Knockdown of SIRT3 in vitro inhibited osteoclast differentiation and mitochondrial biogenesis, and deletion of SIRT3 increased trabecular bone mass in female mice due to impaired osteoclastogenesis. The effect of SIRT3 on bone remodeling appears to be age-dependent as revealed by comparing the effect of SIRT3 deletion on 5-week-old, 3-month-old and 6-month-old female mice. Interestingly, Sirt3-/- mice were more resistant to bone loss following estrogen deficiency resulting from OVX. Our findings demonstrated that SIRT3 could play critical roles in bone remodeling and estrogen deficiency-induced bone loss in female mice, suggesting that SIRT3 and its downstream effectors might be potential novel therapeutic targets for the management of postmenopausal osteoporosis.

Role of sirtuins in bone biology: Potential implications for novel therapeutic strategies for osteoporosis

The decline in bone mass and bone strength and musculoskeletal problems associated with aging constitute a major challenge for affected individuals and the healthcare system globally. Sirtuins 1-7 (SIRT1-SIRT7) are a family of nicotinamide adenine dinucleotide-dependent deacetylases with remarkable abilities to promote longevity and counteract age-related diseases. Sirtuin knockout and transgenic models have provided novel insights into the function and signaling of these proteins in bone homeostasis. Studies have revealed that sirtuins play a critical role in normal skeletal development and homeostasis through their direct action on bone cells and that their dysregulation might contribute to different bone diseases. Preclinical studies have demonstrated that mice treated with sirtuin agonists show protection against age-related, postmenopausal, and immobilization-induced osteoporosis. These findings suggest that sirtuins could be potential targets for the modulation of the imbalance in bone remodeling and treatment of osteoporosis and other bone disorders. The aim of this review was to provide a comprehensive updated review of the current knowledge on sirtuin biology, focusing specifically on their roles in bone homeostasis and osteoporosis, and potential pharmacological interventions targeting sirtuins for the treatment of osteoporosis.

Loss of MTX2 causes mandibuloacral dysplasia and links mitochondrial dysfunction to altered nuclear morphology

Mandibuloacral dysplasia syndromes are mainly due to recessive LMNA or ZMPSTE24 mutations, with cardinal nuclear morphological abnormalities and dysfunction. We report five homozygous null mutations in MTX2, encoding Metaxin-2 (MTX2), an outer mitochondrial membrane protein, in patients presenting with a severe laminopathy-like mandibuloacral dysplasia characterized by growth retardation, bone resorption, arterial calcification, renal glomerulosclerosis and severe hypertension. Loss of MTX2 in patients' primary fibroblasts leads to loss of Metaxin-1 (MTX1) and mitochondrial dysfunction, including network fragmentation and oxidative phosphorylation impairment. Furthermore, patients' fibroblasts are resistant to induced apoptosis, leading to increased cell senescence and mitophagy and reduced proliferation. Interestingly, secondary nuclear morphological defects are observed in both MTX2-mutant fibroblasts and mtx-2-depleted C. elegans. We thus report the identification of a severe premature aging syndrome revealing an unsuspected link between mitochondrial composition and function and nuclear morphology, establishing a pathophysiological link with premature aging laminopathies and likely explaining common clinical features.

Energy metabolism: A newly emerging target of BMP signaling in bone homeostasis

Energy metabolism is the process of generating energy (i.e. ATP) from nutrients. This process is indispensable for cell homeostasis maintenance and responses to varying conditions. Cells require energy for growth and maintenance and have evolved to have multiple pathways to produce energy. Both genetic and functional studies have demonstrated that energy metabolism, such as glucose, fatty acid, and amino acid metabolism, plays important roles in the formation and function of bone cells including osteoblasts, osteocytes, and osteoclasts. Dysregulation of energy metabolism in bone cells consequently disturbs the balance between bone formation and bone resorption. Metabolic diseases have also been reported to affect bone homeostasis. Bone morphogenic protein (BMP) signaling plays critical roles in regulating the formation and function of bone cells, thus affecting bone development and homeostasis. Mutations of BMP signaling-related genes in mice have been reported to show abnormalities in energy metabolism in many tissues, including bone. In addition, BMP signaling correlates with critical signaling pathways such as mTOR, HIF, Wnt, and self-degradative process autophagy to coordinate energy metabolism and bone homeostasis. These findings will provide a newly emerging target of BMP signaling and potential therapeutic strategies and the improved management of bone diseases. This review summarizes the recent advances in our understanding of (1) energy metabolism in regulating the formation and function of bone cells, (2) function of BMP signaling in whole body energy metabolism, and (3) mechanistic interaction of BMP signaling with other signaling pathways and biological processes critical for energy metabolism and bone homeostasis.

Both aerobic glycolysis and mitochondrial respiration are required for osteoclast differentiation

Excessive bone resorption over bone formation is the root cause for bone loss leading to osteoporotic fractures. Development of new antiresorptive therapies calls for a holistic understanding of osteoclast differentiation and function. Although much has been learned about the molecular regulation of osteoclast biology, little is known about the metabolic requirement and bioenergetics during osteoclastogenesis. Here, we report that glucose metabolism through oxidative phosphorylation (OXPHOS) is the predominant bioenergetic pathway to support osteoclast differentiation. Meanwhile, increased lactate production from glucose, known as aerobic glycolysis when oxygen is abundant, is also critical for osteoclastogenesis. Genetic deletion of Glut1 in osteoclast progenitors reduces aerobic glycolysis without compromising OXPHOS, but nonetheless diminishes osteoclast differentiation in vitro. Glut1 deficiency in the progenitors leads to osteopetrosis due to fewer osteoclasts specifically in the female mice. Thus, Glut1-mediated glucose metabolism through both lactate production and OXPHOS is necessary for normal osteoclastogenesis.

The Role of Autophagy and Mitophagy in Bone Metabolic Disorders

Bone metabolic disorders include osteolysis, osteoporosis, osteoarthritis and rheumatoid arthritis. Osteoblasts and osteoclasts are two major types of cells in bone constituting homeostasis. The imbalance between bone formation by osteoblasts and bone resorption by osteoclasts has been shown to have a direct contribution to the onset of these diseases. Recent evidence indicates that autophagy and mitophagy, the selective autophagy of mitochondria, may play a vital role in regulating the proliferation, differentiation and function of osteoblasts and osteoclasts. Several signaling pathways, including PINK1/Parkin, SIRT1, MAPK8/FOXO3, Beclin-1/BECN1, p62/SQSTM1, and mTOR pathways, have been implied in the regulation of autophagy and mitophagy in these cells. Here we review the current progress about the regulation of autophagy and mitophagy in osteoblasts and osteoclasts in these bone metabolic disorders, as well as the molecular signaling activated or deactivated during this process. Together, we hope to draw attention to the role of autophagy and mitophagy in bone metabolic disorders, and their potential as a new target for the treatment of bone metabolic diseases and the requirements of further mechanism studies.

Involvement of P2X7 Receptors in the Osteogenic Differentiation of Mesenchymal Stromal/Stem Cells Derived from Human Subcutaneous Adipose Tissue

The ionotropic P2X7 receptor (P2X7R) is involved in bone homeostasis but its role in osteogenesis is controversial. Thus, we investigated the expression of P2X7R and the effects exerted by its modulation in mesenchymal stromal cells from human subcutaneous adipose tissue (S-ASCs), which have potential therapeutic application in bone regenerative medicine. We found that undifferentiated S-ASCs expressed P2X7R and its functional splice variants P2X7AR and P2X7BR. Cell stimulation by P2X7R agonist BzATP (100 μM) neither modified proliferation nor caused membrane pore opening while increasing intracellular Ca²⁺ levels and migration. The P2X7R antagonist A438079 reversed these effects. However, 25-100 μM BzATP, administered to S-ASCs undergoing osteogenic differentiation, dose-dependently decreased extracellular matrix mineralization and expression of osteogenic transcription factors Runx2, alkaline phosphatase and osteopontin. These effects were not coupled to cell proliferation reduction or to cell death increase, but were associated to decrease in P2X7AR and P2X7BR expression. In contrast, expression of P2X7R, especially P2X7BR isoform, significantly increased during the osteogenic process. Noteworthy, the antagonist A438079, administered alone, at first restrained cell differentiation, enhancing it later. Accordingly, A438079 reversed BzATP effects only in the second phase of S-ASCs osteogenic differentiation. Apyrase, a diphosphohydrolase converting ATP/ADP into AMP, showed a similar behavior. Altogether, findings related to A438079 or apyrase effects suggest an earlier and prevailing pro-osteogenic activity by endogenous ATP and a later one by adenosine derived from endogenous ATP metabolism. Conversely, P2X7R pharmacological stimulation by BzATP, mimicking the effects of high ATP levels occurring during tissue injuries, depressed receptor expression/activity impairing MSC osteogenic differentiation.

Estrogens decrease osteoclast number by attenuating mitochondria oxidative phosphorylation and ATP production in early osteoclast precursors

Loss of estrogens at menopause is a major cause of osteoporosis and increased fracture risk. Estrogens protect against bone loss by decreasing osteoclast number through direct actions on cells of the myeloid lineage. Here, we investigated the molecular mechanism of this effect. We report that 17β-estradiol (E₂) decreased osteoclast number by promoting the apoptosis of early osteoclast progenitors, but not mature osteoclasts. This effect was abrogated in cells lacking Bak/Bax-two pro-apoptotic members of the Bcl-2 family of proteins required for mitochondrial apoptotic death. FasL has been previously implicated in the pro-apoptotic actions of E₂. However, we show herein that FasL-deficient mice lose bone mass following ovariectomy indistinguishably from FasL-intact controls, indicating that FasL is not a major contributor to the anti-osteoclastogenic actions of estrogens. Instead, using microarray analysis we have elucidated that ERα-mediated estrogen signaling in osteoclast progenitors decreases "oxidative phosphorylation" and the expression of mitochondria complex I genes. Additionally, E₂ decreased the activity of complex I and oxygen consumption rate. Similar to E₂, the complex I inhibitor Rotenone decreased osteoclastogenesis by promoting osteoclast progenitor apoptosis via Bak/Bax. These findings demonstrate that estrogens decrease osteoclast number by attenuating respiration, and thereby, promoting mitochondrial apoptotic death of early osteoclast progenitors.

Mitochondrial dysfunction impairs osteogenesis, increases osteoclast activity, and accelerates age related bone loss

The pathogenesis of declining bone mineral density, a universal feature of ageing, is not fully understood. Somatic mitochondrial DNA (mtDNA) mutations accumulate with age in human tissues and mounting evidence suggests that they may be integral to the ageing process. To explore the potential effects of mtDNA mutations on bone biology, we compared bone microarchitecture and turnover in an ageing series of wild type mice with that of the PolgA^mut/mut mitochondrial DNA ‘mutator’ mouse. In vivo analyses showed an age-related loss of bone in both groups of mice; however, it was significantly accelerated in the PolgA^mut/mut mice. This accelerated rate of bone loss is associated with significantly reduced bone formation rate, reduced osteoblast population densities, increased osteoclast population densities, and mitochondrial respiratory chain deficiency in osteoblasts and osteoclasts in PolgA^mut/mut mice compared with wild-type mice. In vitro assays demonstrated severely impaired mineralised matrix formation and increased osteoclast resorption by PolgA^mut/mut cells. Finally, application of an exercise intervention to a subset of PolgA^mut/mut mice showed no effect on bone mass or mineralised matrix formation in vitro. Our data demonstrate that mitochondrial dysfunction, a universal feature of human ageing, impairs osteogenesis and is associated with accelerated bone loss.

p53 positively regulates osteoprotegerin-mediated inhibition of osteoclastogenesis by downregulating TSC2-induced autophagy in vitro

Osteoclasts are terminally multinucleated cells that are regulated by nuclear factor-activated T cells c1 (NFATc1), and are responsible for bone resorption while the tartrate resistant acid phosphatase (TRAP) enzymes releases into bone resorption lacunae. Furthermore, tumor suppressor p53 is a negative regulator during osteoclastogenesis. Osteoprotegerin (OPG) inhibits osteoclastogenesis and bone resorption by activating autophagy, however, whether p53 is involved in OPG-mediated inhibition of osteoclastogenesis remains unclear. In the current study, OPG could enhance the expression of p53 and tuberin sclerosis complex 2 (TSC2). Moreover, the expression of p53 is regulated by autophagy during OPG-mediated inhibition of osteoclastogenesis. Inhibition of p53 by treated with pifithrin-α (PFTα) causing augments of osteoclastogenesis and bone resorption, also reversed OPG-mediated inhibition of osteoclastogenesis by reducing the expression of TSC2. In addition, knockdown of TSC2 using siRNA could rescue OPG-mediated inhibition of osteoclastogenesis by reducing autophagy, which is manifested by the decrease of the expression of Beclin1 and the phosphorylation of mammalian target of rapamycin (mTOR) and ribosomal protein S6 kinase beta 1 (S6K1, also known as p70S6K). Collectively, p53 plays a critical role during OPG-mediated inhibition of osteoclastogenesis via regulating the TSC2-induced autophagy in vitro.

To Inhibit or Enhance? Is There a Benefit to Positive Allosteric Modulation of P2X Receptors?

The family of ligand-gated ion channels known as P2X receptors were discovered several decades ago. Since the cloning of the seven P2X receptors (P2X1-P2X7), a huge research effort has elucidated their roles in regulating a range of physiological and pathophysiological processes. Transgenic animals have been influential in understanding which P2X receptors could be new therapeutic targets for disease. Furthermore, understanding how inherited mutations can increase susceptibility to disorders and diseases has advanced this knowledge base. There has been an emphasis on the discovery and development of pharmacological tools to help dissect the individual roles of P2X receptors and the pharmaceutical industry has been involved in pushing forward clinical development of several lead compounds. During the discovery phase, a number of positive allosteric modulators have been described for P2X receptors and these have been useful in assigning physiological roles to receptors. This review will consider the major physiological roles of P2X1-P2X7 and discuss whether enhancement of P2X receptor activity would offer any therapeutic benefit. We will review what is known about identified compounds acting as positive allosteric modulators and the recent identification of drug binding pockets for such modulators.

P2X7 receptor acts as an efficient drug target in regulating bone metabolism system

Skeletal system is a highly dynamic system going through continuous resorption and reconstruction to maintain homeostasis, which is influenced by numerous factors. Once the balance is disrupted, various kinds of bone diseases may occur such as osteoporosis. It has been well known that ATP (adenosine triphosphate), an important signaling molecule, is important in maintaining the dynamic balance of bone matrix. ATP mainly functions through P2X receptors, a kind of ATP receptors expressed by various kinds of bone cells to regulate the whole network of skeleton system. Among P2X receptors, P2X7 plays a crucial role in bone since P2X7 is widely expressed by bone cells and the mutation of P2X7 receptor is associated with kinds of bone diseases. It's acknowledged that P2X7 acts as a potential therapeutic target for clinical treatment of bone-related diseases but further investigations are needed for the practical application. However, since P2X7 has a complicated effect in many aspects, the exact role of P2X7 in skeleton system is ambiguous. This review discusses the function of P2X7 in bone and other cells and their general effect on skeleton system, especially focusing on the possible clinical application for bone diseases.

Corylin, a flavonoid derived from Psoralea Fructus, induces osteoblastic differentiation via estrogen and Wnt/β-catenin signaling pathways

Corylin is a naturally occurring flavonoid isolated from the fruit of Psoralea corylifolia L. (Fabaceae), which is a Chinese medicinal herb in treating osteoporosis. Although a variety of pharmacological activities of corylin have been reported, its osteogenic action and the underlying mechanism in bone development remain unclear. In the present study, the involvement of bone-specific genes in corylininduced differentiated osteoblasts was analyzed by RT-PCR, promoter-reporter assay, and Western blotting. In cultured osteoblasts, corylin-induced cell differentiation and mineralization, as well as increased the expressions of vital biological markers for osteogenesis, such as Runx2, Osterix, Col1, and ALP. Corylin was proposed to have dual pathways in triggering the osteoblastic differentiation. First, the osteogenic function of corylin acted through the activation of Wnt/β-catenin signaling. The nuclear translocation of β-catenin of cultured osteoblasts, as determined by flow cytometry and confocal microscopy, was triggered by applied corylin, and which was blocked by DKK-1, an inhibitor of Wnt/β-catenin signaling. Second, the application of corylin-induced estrogenic response in a dose-dependent manner, and which was blocked by ICI 182 780, an antagonist of estrogen receptor. Furthermore, the activation of Runx2 promoter by corylin was abolished by both DKK-1 and ICI 182,780, indicating that the corylin exhibited its osteogenic effect via estrogen and Wnt/β-catenin signaling pathways. In addition, corylin regulated the metabolic profiles, as well as the membrane potential of mitochondria, in cultured osteoblasts. Corylin also stimulated the osteogenesis in bone micromass derived from mesenchymal progenitor cells. This study demonstrated the osteogenic activities of corylin in osteoblasts and micromass, suggesting that corylin has the potential to be developed as a novel pro-osteogenic agent in targeting for the treatment of osteoblast-mediated osteoporosis.