Advances in osteoclast biology: old findings and new insights from mouse models

Sirt1: An Increasingly Interesting Molecule with a Potential Role in Bone Metabolism and Osteoporosis

Osteoporosis (OP) is a common metabolic bone disease characterized by low bone mass, decreased bone mineral density, and degradation of bone tissue microarchitecture. However, our understanding of the mechanisms of bone remodeling and factors affecting bone mass remains incomplete. Sirtuin1 (SIRT1) is a nicotinamide adenine dinucleotide-dependent deacetylase that regulates a variety of cellular metabolisms, including inflammation, tumorigenesis, and bone metabolism. Recent studies have emphasized the important role of SIRT1 in bone homeostasis. This article reviews the role of SIRT1 in bone metabolism and OP and also discusses therapeutic strategies and future research directions for targeting SIRT1.

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

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

Critical signaling molecules in the temporomandibular joint osteoarthritis under different magnitudes of mechanical stimulation

The mechanical stress environment in the temporomandibular joint (TMJ) is constantly changing due to daily mandibular movements. Therefore, TMJ tissues, such as condylar cartilage, the synovial membrane and discs, are influenced by different magnitudes of mechanical stimulation. Moderate mechanical stimulation is beneficial for maintaining homeostasis, whereas abnormal mechanical stimulation leads to degeneration and ultimately contributes to the development of temporomandibular joint osteoarthritis (TMJOA), which involves changes in critical signaling molecules. Under abnormal mechanical stimulation, compensatory molecules may prevent degenerative changes while decompensatory molecules aggravate. In this review, we summarize the critical signaling molecules that are stimulated by moderate or abnormal mechanical loading in TMJ tissues, mainly in condylar cartilage. Furthermore, we classify abnormal mechanical stimulation-induced molecules into compensatory or decompensatory molecules. Our aim is to understand the pathophysiological mechanism of TMJ dysfunction more deeply in the ever-changing mechanical environment, and then provide new ideas for discovering effective diagnostic and therapeutic targets in TMJOA.

Bone Marrow Adipose Tissue as a Critical Regulator of Postmenopausal Osteoporosis - A Concise Review

Postmenopausal osteoporosis (PMOP) is a major health problem affecting millions of women worldwide. PMOP patients are often accompanied by abnormal accumulation of bone marrow adipose tissue (BMAT). BMAT is a critical regulator of bone homeostasis, and an increasing BMAT volume is negatively associated with bone mass reduction or fracture. BMAT regulates bone metabolism via adipokines, cytokines and the immune system, but the specific mechanisms are largely unknown. This review emphasizes the impact of estrogen deficiency on bone homeostasis and BMAT expansion, and the mechanism by which BMAT regulates PMOP, providing a promising strategy for targeting BMAT in preventing and treating PMOP.

The role of N-acetylcysteine in osteogenic microenvironment for bone tissue engineering

Bone defect is a common clinical symptom which can arise from various causes. Currently, bone tissue engineering has demonstrated positive therapeutic effects for bone defect repair by using seeding cells such as mesenchymal stem cells and precursor cells. N-acetylcysteine (NAC) is a stable, safe and highly bioavailable antioxidant that shows promising prospects in bone tissue engineering due to the ability to attenuate oxidative stress and enhance the osteogenic potential and immune regulatory function of cells. This review systematically introduces the antioxidant mechanism of NAC, analyzes the advancements in NAC-related research involving mesenchymal stem cells, precursor cells, innate immune cells and animal models, discusses its function using the classic oral microenvironment as an example, and places particular emphasis on the innovative applications of NAC-modified tissue engineering biomaterials. Finally, current limitations and future prospects are proposed, with the aim of providing inspiration for targeted readers in the field.

Cortactin controls bone homeostasis through regulating the differentiation of osteoblasts and osteoclasts

Cortactin (CTTN), a cytoskeletal protein and substrate of Src kinase, is implicated in tumor aggressiveness. However, its role in bone cell differentiation remains unknown. The current study revealed that CTTN was upregulated during osteoblast and adipocyte differentiation. Functional experiments demonstrated that CTTN promoted the in vitro differentiation of mesenchymal stem/progenitor cells into osteogenic and adipogenic lineages. Mechanistically, CTTN was able to stabilize the protein level of mechanistic target of rapamycin kinase (mTOR), leading to the activation of mTOR signaling. In-depth investigation revealed that CTTN could bind with casitas B lineage lymphoma-c (c-CBL) and counteract the function of c-CBL, a known E3 ubiquitin ligase responsible for the proteasomal degradation of mTOR. Silencing c-Cbl alleviated the impaired differentiation of osteoblasts and adipocytes caused by CTTN siRNA, while silencing mTOR mitigated the stimulation of osteoblast and adipocyte differentiation induced by CTTN overexpression. Notably, transplantation of CTTN-silenced bone marrow stromal cells (BMSCs) into the marrow of mice led to a reduction in trabecular bone mass, accompanied by a decrease in osteoblasts and an increase in osteoclasts. Furthermore, CTTN-silenced BMSCs expressed higher levels of receptor activator of nuclear factor κB ligand (RANKL) than control BMSCs did and promoted osteoclast differentiation when cocultured with bone marrow-derived osteoclast precursor cells. This study provides evidence that CTTN favors osteoblast differentiation by counteracting the c-CBL-induced degradation of mTOR and inhibits osteoclast differentiation by downregulating the expression of RANKL. It also suggests that maintaining an appropriate level of CTTN expression may be advantageous for maintaining bone homeostasis.

Nanoparticulate bioceramic putty suppresses osteoclastogenesis and inflammatory bone loss in mice via inhibition of TRAF6-mediated signalling pathways: A laboratory investigation

AIM

This study aimed to determine the effects of iRoot BP Plus on receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis in vitro and inflammation-mediated bone resorption in vivo and investigated the underlying molecular mechanisms.

METHODOLOGY

CCK-8 was performed to test cell viability in RANKL-induced RAW 264.7 cells and BMDMs in response to iRoot BP Plus. The effect of iRoot BP Plus on osteoclastogenesis was determined using TRAP staining and phalloidin staining, respectively. Pit formation assay was conducted to measure osteoclast resorptive capacity. Western blot and qPCR were performed to examine osteoclast-related proteins and gene expression, respectively. Western blot was also used to investigate the signalling pathways involved. For in vivo experiments, an LPS-induced mouse calvarial bone resorption model was established to analyse the effect of iRoot BP Plus on bone resorption (n = 6 per group). At 7 days, mouse calvaria were collected and prepared for histological analysis.

RESULTS

We identified that iRoot BP Plus extracts significantly attenuated RANKL-induced osteoclastogenesis, reduced sealing zone formation, restrained osteolytic capacity and decreased osteoclast-specific gene expression (p < .01). Mechanistically, iRoot BP Plus extracts reduced TRAF6 via proteasomal degradation, then suppressed the phosphorylation of mitogen-activated protein kinases (MAPKs), blocked the nuclear translocation of c-Fos and diminished nuclear factor-κB (NF-κB) p65 and NFATc1 accumulation. Consistent with the in vitro results, iRoot BP Plus extracts attenuated osteoclast activity thus protecting against inflammatory bone resorption in vivo (p < .05), which was accompanied by a suppression of TRAF6, c-Fos, NFATc1 and cathepsin K expression.

CONCLUSION

These findings provide valuable insights into the signalling mechanisms underlying nanoparticulate bioceramic putty-mediated bone homeostasis.

The role and mechanism of RNA-binding proteins in bone metabolism and osteoporosis

Osteoporosis is a prevalent chronic metabolic bone disease that poses a significant risk of fractures or mortality in elderly individuals. Its pathophysiological basis is often attributed to postmenopausal estrogen deficiency and natural aging, making the progression of primary osteoporosis among elderly people, especially older women, seemingly inevitable. The treatment and prevention of osteoporosis progression have been extensively discussed. Recently, as researchers delve deeper into the molecular biological mechanisms of bone remodeling, they have come to realize the crucial role of posttranscriptional gene control in bone metabolism homeostasis. RNA-binding proteins, as essential actors in posttranscriptional activities, may exert influence on osteoporosis progression by regulating the RNA life cycle. This review compiles recent findings on the involvement of RNA-binding proteins in abnormal bone metabolism in osteoporosis and describes the impact of some key RNA-binding proteins on bone metabolism regulation. Additionally, we explore the potential and rationale for modulating RNA-binding proteins as a means of treating osteoporosis, with an overview of drugs that target these proteins.

The roles and regulatory mechanisms of TGF-β and BMP signaling in bone and cartilage development, homeostasis and disease

Transforming growth factor-βs (TGF-βs) and bone morphometric proteins (BMPs) belong to the TGF-β superfamily and perform essential functions during osteoblast and chondrocyte lineage commitment and differentiation, skeletal development, and homeostasis. TGF-βs and BMPs transduce signals through SMAD-dependent and -independent pathways; specifically, they recruit different receptor heterotetramers and R-Smad complexes, resulting in unique biological readouts. BMPs promote osteogenesis, osteoclastogenesis, and chondrogenesis at all differentiation stages, while TGF-βs play different roles in a stage-dependent manner. BMPs and TGF-β have opposite functions in articular cartilage homeostasis. Moreover, TGF-β has a specific role in maintaining the osteocyte network. The precise activation of BMP and TGF-β signaling requires regulatory machinery at multiple levels, including latency control in the matrix, extracellular antagonists, ubiquitination and phosphorylation in the cytoplasm, nucleus-cytoplasm transportation, and transcriptional co-regulation in the nuclei. This review weaves the background information with the latest advances in the signaling facilitated by TGF-βs and BMPs, and the advanced understanding of their diverse physiological functions and regulations. This review also summarizes the human diseases and mouse models associated with disordered TGF-β and BMP signaling. A more precise understanding of the BMP and TGF-β signaling could facilitate the development of bona fide clinical applications in treating bone and cartilage disorders.

Dancr-BRG1 regulates Nfatc1 transcription and Pgc1β-dependent metabolic shifts in osteoclastogenesis

Long non-coding RNA (lncRNA) serves as a vital regulator of bone metabolism, but its role in pathologically overactive osteoclast differentiation remains elusive. Here, we identify lncRNA Dancr (Differentiation Antagonizing Non-protein Coding RNA) as a critical suppressor of osteoclastogenesis and bone resorption, which is down-regulated in response to estrogen deficiency. Global or osteoclast-specific Dancr Knockout mice display significant trabecular bone deterioration and enhanced osteoclast activity, but minimal alteration of bone formation. Moreover, the bone-targeted delivery of Dancr by Adeno-associated viral remarkably attenuates ovariectomy-induced osteopenia in mice. Mechanistically, Dancr establishes a direct interaction with Brahma-related gene 1 to prevent its binding and preserve H3K27me3 enrichment at the nuclear factor of activated T cells 1 and proliferator-activated receptor gamma coactivator 1-beta promoters, thereby maintaining appropriate expression of osteoclastic genes and metabolic programs during osteoclastogenesis. These results demonstrate that Dancr is a key molecule maintaining proper osteoclast differentiation and bone homeostasis under physiological conditions, and Dancr overexpression constitutes a potential strategy for treating osteoporosis.

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.

Prostate transmembrane androgen inducible protein 1 (PMEPA1): regulation and clinical implications

Prostate transmembrane androgen inducible protein 1 (PMEPA1) can promote or inhibit prostate cancer cell growth based on the cancer cell response to the androgen receptor (AR). Further, it can be upregulated by transforming growth factor (TGF), which downregulates transforming growth factor-β (TGF-β) signaling by interfering with R-Smad phosphorylation to facilitate TGF-β receptor degradation. Studies have indicated the increased expression of PMEPA1 in some solid tumors and its functioning as a regulator of multiple signaling pathways. This review highlights the multiple potential signaling pathways associated with PMEPA1 and the role of the PMEPA1 gene in regulating prognosis, including transcriptional regulation and epithelial mesenchymal transition (EMT). Moreover, the relevant implications in and outside tumors, for example, as a biomarker and its potential functions in lysosomes have also been discussed.

Effect of phosphodiester composition in polyphosphoesters on the inhibition of osteoclastic differentiation of murine bone marrow mononuclear cells

Osteoporosis is a common bone disorder characterized by reduced bone density and increased risk of fractures. The modulation of bone cell functions, particularly the inhibition of osteoclastic differentiation, plays a crucial role in osteoporosis treatment. Polyphosphoesters (PPEs) have shown the potential in reducing the function of osteoclast cells, but the effect of their chemical structure on osteoclastic differentiation remains largely unexplored. In this study, we evaluated the effect of PPE's chemical structure on the inhibition of osteoclastic differentiation of murine bone marrow mononuclear cells (BMNCs). PPEs containing phosphotriester and phosphodiester units at varying compositions were synthesized. Cytotoxicity testing confirmed the biocompatibility of the copolymers at concentrations below 0.5 mg/mL. Isolated from long bones, BMNCs were cultured in a differentiation medium supplemented with different PPE concentrations. Osteoclast formation was assessed through tartrate-resistant acid phosphatase and phalloidin staining. A significant decrease in the size of osteoclast cells formed upon BMNC contact with PPEs was observed, with a more pronounced effect observed at higher PPE concentrations. In addition, an increased composition of phosphodiester units in the PPEs yielded a decreased density of differentiated osteoclasts. Furthermore, real-time PCR analysis of major osteoclastic markers provided gene expression data that correlated with microscopic observations, confirming the effect of phosphodiester units in suppressing osteoclast differentiation of BMNCs from the early stages. These findings highlight the potential of PPEs as polymers are capable of modulating bone cell functions through their chemical structures.

Role and Regulation of Transcription Factors in Osteoclastogenesis

Bones serve mechanical and defensive functions, as well as regulating the balance of calcium ions and housing bone marrow.. The qualities of bones do not remain constant. Instead, they fluctuate throughout life, with functions increasing in some situations while deteriorating in others. The synchronization of osteoblast-mediated bone formation and osteoclast-mediated bone resorption is critical for maintaining bone mass and microstructure integrity in a steady state. This equilibrium, however, can be disrupted by a variety of bone pathologies. Excessive osteoclast differentiation can result in osteoporosis, Paget's disease, osteolytic bone metastases, and rheumatoid arthritis, all of which can adversely affect people's health. Osteoclast differentiation is regulated by transcription factors NFATc1, MITF, C/EBPα, PU.1, NF-κB, and c-Fos. The transcriptional activity of osteoclasts is largely influenced by developmental and environmental signals with the involvement of co-factors, RNAs, epigenetics, systemic factors, and the microenvironment. In this paper, we review these themes in regard to transcriptional regulation in osteoclastogenesis.

Disease-related PSS1 mutant impedes the formation and function of osteoclasts

Phosphatidylserine (PS) is an acidic phospholipid that is involved in various cellular events. Heterologous dominant mutations have been identified in the gene encoding PS synthase 1 (PSS1) in patients with a congenital disease called Lenz-Majewski syndrome (LMS). Patients with LMS show various symptoms, including craniofacial/distal-limb bone dysplasia and progressive hyperostosis. The LMS-causing gain-of-function mutants of PSS1 (PSS1LMS) have been shown to synthesize PS without control, but why the uncontrolled synthesis would lead to LMS is unknown. Here we investigated the effect of PSS1LMS on osteoclasts (OCs) to elucidate the causative mechanism of LMS. PSS1LMS did not affect the expression of OC-related genes but inhibited the formation, multinucleation, and activity of OCs. Especially, OCs expressing PSS1LMS showed abnormal patterns and dynamics of actin podosome clusters, which have roles in OC migration and fusion. PSS1LMS did not affect the level of PS but changed the acyl chain compositions of PS and phosphatidylethanolamine, and decreased the level of phosphatidylinositol. The introduction of a catalytically inactive mutation into PSSLMS canceled the changes in phospholipids and the phenotypes observed in OCs expressing PSS1LMS. A gain-of-function mutant of PSS2 (PSS2 R97K) also impaired OC formation and caused changes in phospholipid composition similar to the changes caused by PSS1LMS. Our results suggest that uncontrolled PS synthesis by PSS1LMS causes changes in the quantity or fatty acid composition of certain phospholipid classes, impairing OC formation and function, which might be a cause of osteosclerosis in patients with LMS.

Aberrant Activation of Immune and Non-Immune Cells Contributes to Joint Inflammation and Bone Degradation in Rheumatoid Arthritis

Abnormal activation of multiple immune and non-immune cells and proinflammatory factors mediate the development of joint inflammation in genetically susceptible individuals. Although specific environmental factors like smoking and infections are associated with disease pathogenesis, until now, we did not know the autoantigens and arthritogenic factors that trigger the initiation of the clinical disease. Autoantibodies recognizing specific post-translationally modified and unmodified antigens are generated and in circulation before the onset of the joint disease, and could serve as diagnostic and prognostic markers. The characteristic features of autoantibodies change regarding sub-class, affinity, glycosylation pattern, and epitope spreading before the disease onset. Some of these antibodies were proven to be pathogenic using animal and cell-culture models. However, not all of them can induce disease in animals. This review discusses the aberrant activation of major immune and non-immune cells contributing to joint inflammation. Recent studies explored the protective effects of extracellular vesicles from mesenchymal stem cells and bacteria on joints by targeting specific cells and pathways. Current therapeutics in clinics target cells and inflammatory pathways to attenuate joint inflammation and protect the cartilage and bones from degradation, but none cure the disease. Hence, more basic research is needed to investigate the triggers and mechanisms involved in initiating the disease and relapses to prevent chronic inflammation from damaging joint architecture.

Obacunone targets macrophage migration inhibitory factor (MIF) to impede osteoclastogenesis and alleviate ovariectomy-induced bone loss

INTRODUCTION

Osteoporosis is the most common bone disorder where the hyperactive osteoclasts represent the leading role during the pathogenesis. Targeting hyperactive osteoclasts is currently the primary therapeutic strategy. However, concerns about the long-term efficacy and side effects of current frontline treatments persist. Alternative therapeutic agents are still needed.

OBJECTIVES

Obacunone (OB) is a small molecule with a broad spectrum of biological activities, particularly antioxidant and anti-inflammatory effects. This study aims to examine OB's therapeutic potential on osteoporosis and explore the rudimentary mechanisms.

METHODS

Osteoclast formation and osteoclastic resorption assays were carried out to examine OB's inhibitory effects in vitro, followed by the in-vivo studies of OB's therapeutic effects on ovariectomy-induced osteoporotic preclinical model. To further study the underlying mechanisms, mRNA sequencing and analysis were used to investigate the changes of downstream pathways. The molecular targets of OB were predicted, and in-silico docking analysis was performed. Ligand-target binding was verified by surface plasmon resonance (SPR) assay and Western Blotting assay.

RESULTS

The results indicated that OB suppressed the formation of osteoclast and its resorptive function in vitro. Mechanistically, OB interacts with macrophage migration inhibitory factor (MIF) which attenuates receptor activator of nuclear factor kappa B (NF-κB) ligand (RANKL)-induced signaling pathways, including reactive oxygen species (ROS), NF-κB pathway, and mitogen-activated protein kinases (MAPKs). These effects eventually caused the diminished expression level of the master transcriptional factor of osteoclastogenesis, nuclear factor of activated T cells 1 (NFATc1), and its downstream osteoclast-specific proteins. Furthermore, our data revealed that OB alleviated estrogen deficiency-induced osteoporosis by targeting MIF and thus inhibiting hyperactive osteoclasts in vivo.

CONCLUSION

These results together implicated that OB may represent as a therapeutic candidate for bone disorders caused by osteoclasts, such as osteoporosis.

Integrative tissue, cellular and molecular responsiveness of an innovative ex vivo model of the Staphylococcus aureus-mediated bone infection

Osteomyelitis is a pathological condition of the bone, frequently associated with the presence of infectious agents - namely Staphylococcus aureus - that induce inflammation and tissue destruction. Recent advances in the understanding of its pathophysiology and the identification of innovative therapeutic approaches were gathered from experimental in vitro and in vivo systems. However, cell culture models offer limited representativeness of the cellular functionality and the cell-cell and cell-matrix interactions, further failing to mimic the three-dimensional tissue organization; and animal models allow for limited mechanistic assessment given the complex nature of systemic and paracrine regulatory systems and are endorsed with ethical constraints. Accordingly, this study aims at the establishment and assessment of a new ex vivo bone infection model, upon the organotypic culture of embryonic chicken femurs colonized with S. aureus, highlighting the model responsiveness at the molecular, cellular, and tissue levels. Upon infection with distinct bacterial inoculums, data reported an initial exponential bacterial growth, followed by diminished metabolic activity. At the tissue level, evidence of S. aureus-mediated tissue destruction was attained and demonstrated through distinct methodologies, conjoined with decreased osteoblastic/osteogenic and increased osteoclastic/osteoclastogenic functionalities-representative of the osteomyelitis clinical course. Overall, the establishment and characterization of an innovative bone tissue infection model that is simple, reproducible, easily manipulated, cost-effective, and simulates many features of human osteomyelitis, further allowing the maintenance of the bone tissue's three-dimensional morphology and cellular arrangement, was achieved. Model responsiveness was further demonstrated, showcasing the capability to improve the research pipeline in bone tissue infection-related research.

Phylobone: a comprehensive database of bone extracellular matrix proteins in human and model organisms

The bone extracellular matrix (ECM) contains minerals deposited on highly crosslinked collagen fibrils and hundreds of non-collagenous proteins. Some of these proteins are key to the regulation of bone formation and regeneration via signaling pathways, and play important regulatory and structural roles. However, the complete list of bone extracellular matrix proteins, their roles, and the extent of individual and cross-species variations have not been fully captured in both humans and model organisms. Here, we introduce the most comprehensive resource of bone extracellular matrix (ECM) proteins that can be used in research fields such as bone regeneration, osteoporosis, and mechanobiology. The Phylobone database (available at https://phylobone.com ) includes 255 proteins potentially expressed in the bone extracellular matrix (ECM) of humans and 30 species of vertebrates. A bioinformatics pipeline was used to identify the evolutionary relationships of bone ECM proteins. The analysis facilitated the identification of potential model organisms to study the molecular mechanisms of bone regeneration. A network analysis showed high connectivity of bone ECM proteins. A total of 214 functional protein domains were identified, including collagen and the domains involved in bone formation and resorption. Information from public drug repositories was used to identify potential repurposing of existing drugs. The Phylobone database provides a platform to study bone regeneration and osteoporosis in light of (biological) evolution, and will substantially contribute to the identification of molecular mechanisms and drug targets.

Antidementia medication acetylcholinesterase inhibitors have therapeutic benefits on osteoporotic bone by attenuating osteoclastogenesis and bone resorption

This study was designed to determine whether the use of acetylcholinesterase inhibitors (AChEIs), a group of drugs that stimulate acetylcholine receptors and are used to treat Alzheimer's disease (AD), is associated with osteoporosis protection and inhibition of osteoclast differentiation and function. Firstly, we examined the effects of AChEIs on RANKL-induced osteoclast differentiation and function with osteoclastogenesis and bone resorption assays. Next, we investigated the impacts of AChEIs on RANKL-induced nuclear factor κB and NFATc1 activation and expression of osteoclast marker proteins CA-2, CTSK and NFATc1, and dissected the MAPK signaling in osteoclasts in vitro by using luciferase assay and Western blot. Finally, we assessed the in vivo efficacy of AChEIs using an ovariectomy-induced osteoporosis mouse model, which was analyzed using microcomputed tomography, in vivo osteoclast and osteoblast parameters were assessed using histomorphometry. We found that Donepezil and Rivastigmine inhibited RANKL-induced osteoclastogenesis and impaired osteoclastic bone resorption. Moreover, AChEIs reduced the RANKL-induced transcription of Nfatc1, and expression of osteoclast marker genes to varying degrees (mainly Donepezil and Rivastigmine but not Galantamine). Furthermore, AChEIs variably inhibited RANKL-induced MAPK signaling accompanied by downregulation of AChE transcription. Finally, AChEIs protected against OVX-induced bone loss mainly by inhibiting osteoclast activity. Taken together, AChEIs (mainly Donepezil and Rivastigmine) exerted a positive effect on bone protection by inhibiting osteoclast function through MAPK and NFATc1 signaling pathways through downregulating AChE. Our findings have important clinical implications that elderly patients with dementia who are at risk of developing osteoporosis may potentially benefit from therapy with the AChEI drugs. Our study may influence drug choice in those patients with both AD and osteoporosis.

Structure-Based Discovery of Receptor Activator of Nuclear Factor-κB Ligand (RANKL)-Induced Osteoclastogenesis Inhibitors

Receptor activator of nuclear factor-κB ligand (RANKL) has been actively pursued as a therapeutic target for osteoporosis, given that RANKL is the master mediator of bone resorption as it promotes osteoclast differentiation, activity and survival. We employed a structure-based virtual screening approach comprising two stages of experimental evaluation and identified 11 commercially available compounds that displayed dose-dependent inhibition of osteoclastogenesis. Their inhibitory effects were quantified through TRAP activity at the low micromolar range (IC₅₀ < 5 μΜ), but more importantly, 3 compounds displayed very low toxicity (LC₅₀ > 100 μΜ). We also assessed the potential of an N-(1-aryl-1H-indol-5-yl)aryl-sulfonamide scaffold that was based on the structure of a hit compound, through synthesis of 30 derivatives. Their evaluation revealed 4 additional hits that inhibited osteoclastogenesis at low micromolar concentrations; however, cellular toxicity concerns preclude their further development. Taken together with the structure-activity relationships provided by the hit compounds, our study revealed potent inhibitors of RANKL-induced osteoclastogenesis of high therapeutic index, which bear diverse scaffolds that can be employed in hit-to-lead optimization for the development of therapeutics against osteolytic diseases.

Discovery and optimized extraction of the anti-osteoclastic agent epicatechin-7-O-β-D-apiofuranoside from Ulmus macrocarpa Hance bark

Ulmus macrocarpa Hance bark (UmHb) has been used as a traditional herbal medicine in East Asia for bone concern diseases for a long time. To find a suitable solvent, we, in this study, compared the efficacy of UmHb water extract and ethanol extract which can inhibit osteoclast differentiation. Compared with two ethanol extracts (70% and 100% respectively), hydrothermal extracts of UmHb more effectively inhibited receptor activators of nuclear factor κB ligand-induced osteoclast differentiation in murine bone marrow-derived macrophages. We identified for the first time that (2R,3R)-epicatechin-7-O-β-D-apiofuranoside (E7A) is a specific active compound in UmHb hydrothermal extracts through using LC/MS, HPLC, and NMR techniques. In addition, we confirmed through TRAP assay, pit assay, and PCR assay that E7A is a key compound in inhibiting osteoclast differentiation. The optimized condition to obtain E7A-rich UmHb extract was 100 mL/g, 90 °C, pH 5, and 97 min. At this condition, the content of E7A was 26.05 ± 0.96 mg/g extract. Based on TRAP assay, pit assay, PCR, and western blot, the optimized extract of E7A-rich UmHb demonstrated a greater inhibition of osteoclast differentiation compared to unoptimized. These results suggest that E7A would be a good candidate for the prevention and treatment of osteoporosis-related diseases.

Messages from the Mineral: How Bone Cells Communicate with Other Tissues

Bone is a highly dynamic tissue, and the constant actions of bone-forming and bone-resorbing cells are responsible for attaining peak bone mass, maintaining bone mass in the adults, and the subsequent bone loss with aging and menopause, as well as skeletal complications of diseases and drug side-effects. It is now accepted that the generation and activity of bone-forming osteoblasts and bone-resorbing osteoclasts is modulated by osteocytes, osteoblast-derived cells embedded in the bone matrix. The interaction among bone cells occurs through direct contact and via secreted molecules. In addition to the regulation of bone cell function, molecules released by these cells are also able to reach the circulation and have effects in other tissues and organs in healthy individuals. Moreover, bone cell products have also been associated with the establishment or progression of diseases, including cancer and muscle weakness. In this review, we will discuss the role of bone as an endocrine organ, and the effect of selected, osteoblast-, osteocyte-, and osteoclast-secreted molecules on other tissues.

Rational polyelectrolyte nanoparticles endow preosteoclast-targeted siRNA transfection for anabolic therapy of osteoporosis

Targeted transfection of siRNA to preosteoclasts features the potential of anti-osteoporosis, yet challenge arises from the development of satisfied delivery vehicles. Here, we design a rational core-shell nanoparticle (NP) composed of cationic and responsive core for controlled load and release of small interfering RNA (siRNA) and compatible polyethylene glycol shell modified with alendronate for enhanced circulation and bone-targeted delivery of siRNA. The designed NPs perform well on transfection of an active siRNA (siDcstamp) that interferes Dcstamp mRNA expression, leading to impeded preosteoclast fusion and bone resorption, as well as promoted osteogenesis. In vivo results corroborate the abundant siDcstamp accumulation on bone surfaces and the enhanced trabecular bone mass volume and microstructure in treating osteoporotic OVX mice by rebalancing bone resorption, formation, and vascularization. Our study validates the hypothesis that satisfied transfection of siRNA enables preserved preosteoclasts that regulate bone resorption and formation simultaneously as potential anabolic treatment for osteoporosis.

Klf4 haploinsufficiency in Sp7+ lineage leads to underdeveloped mandibles and insufficient elongation of mandibular incisor

The mandible is an important component of the craniofacial bones, whose development is regulated by complex molecular networks and involves the well-coordinated development of the bone, cartilage, and teeth. Previously, we demonstrated that Krüppel-like factor 4 (KLF4) promoted dentinogenesis and osteogenesis, but it was enigmatic whether Klf4 participated in the development of the mandible. In this study, the Sp7-Cre; Klf4^f/+ mice exhibited underdeveloped mandibles and insufficient elongation of the mandibular incisor when compared with Klf4^f/+ and Sp7-Cre mice. Moreover, morphological and molecular analysis showed that the alveolar bone mass was significantly decreased in KLF4 deficient mice, accompanied by reduced expression of osteoblast-related genes. Meanwhile, the KLF4 deficient mice had decreased expression of receptor activator of nuclear factor kappa-Β ligand (RANKL) and no significant change of osteoprotegerin (OPG) in the alveolar bone near the mandibular incisor. Simultaneously, the osteoclastogenesis in the alveolar bone of KLF4 deficient mice was attenuated, which was demonstrated by a diminished number of tartrate-resistant acid phosphatase positive (TRAP+), matrix metallopeptidase 9 positive (MMP9+), and cathepsin K positive (CTSK+) multinucleated osteoclasts, respectively. Collectively, our study suggested that Klf4 participated in mandibular development, and Klf4 in Sp7+ lineage affected osteogenesis directly and osteoclastogenesis indirectly.

MRL/MpJ Mice Resist to Age-Related and Long-Term Ovariectomy-Induced Bone Loss: Implications for Bone Regeneration and Repair

Osteoporosis and age-related bone loss increase bone fracture risk and impair bone healing. The need for identifying new factors to prevent or treat bone loss is critical. Previously, we reported that young MRL/MpJ mice have superior bone microarchitecture and biomechanical properties as compared to wild-type (WT) mice. In this study, MRL/MpJ mice were tested for resistance to age-related and long-term ovariectomy-induced bone loss to uncover potential beneficial factors for bone regeneration and repair. Bone tissues collected from 14-month-old MRL/MpJ and C57BL/6J (WT) mice were analyzed using micro-CT, histology, and immunohistochemistry, and serum protein markers were characterized using ELISAs or multiplex assays. Furthermore, 4-month-old MRL/MpJ and WT mice were subjected to ovariectomy (OV) or sham surgery and bone loss was monitored continuously using micro-CT at 1, 2, 4, and 6 months (M) after surgery with histology and immunohistochemistry performed at 6 M post-surgery. Sera were collected for biomarker detection using ELISA and multiplex assays at 6 M after surgery. Our results indicated that MRL/MpJ mice maintained better bone microarchitecture and higher bone mass than WT mice during aging and long-term ovariectomy. This resistance of bone loss observed in MRL/MpJ mice correlated with the maintenance of higher OSX⁺ osteoprogenitor cell pools, higher activation of the pSMAD5 signaling pathway, more PCNA⁺ cells, and a lower number of osteoclasts. Systemically, lower serum RANKL and DKK1 with higher serum IGF1 and OPG in MRL/MpJ mice relative to WT mice may also contribute to the maintenance of higher bone microarchitecture during aging and less severe bone loss after long-term ovariectomy. These findings may be used to develop therapeutic approaches to maintain bone mass and improve bone regeneration and repair due to injury, disease, and aging.

The role played by ailanthone in inhibiting bone metastasis of breast cancer by regulating tumor-bone microenvironment through the RANKL-dependent pathway

Introduction: Bone metastasis of breast cancer (BC) is a process in which the disruption of the bone homeostatic microenvironment leads to an increase in osteoclast differentiation. Ailanthus altissima shows an inhibitory effect on osteoclast differentiation. Ailanthone (AIL) refers to a natural compound isolated from Ailanthus altissima, a Chinese herbal medicine, and has effective anti-tumor activity in numerous cell lines. Its impact on bone metastases for BC is yet unclear. Methods: We measured the effect of AIL on MDA-MB-231 cells by wound healing experiments, Transwell and colony formation experiment. Using the Tartrate-resistant Acid Phosphatase (TRAP) staining tests, filamentous (F-actin) staining and bone resorption test to detect the effect of AIL on the osteoclast cell differentiation of the Bone Marrow-derived Macrophages (BMMs), activated by the MDA-MB-231 cell Conditioned Medium (MDA-MB-231 CM) and the Receptor Activator of Nuclear factor-κB Ligand (RANKL),and to explore its possibility Mechanisms. In vivo experiments verified the effect of AIL on bone destruction in breast cancer bone metastasis model mice. Results: In vitro, AIL significantly decrease the proliferation, migration and infiltration abilities of MDA-MB-231 cells at a safe concentration, and also reduced the expression of genes and proteins involved in osteoclast formation in MDA-MB-231 cells. Osteoclast cell differentiation of the BMMs, activated by MDA-MB-231 CM and RANKL, were suppressed by AIL in the concentration-dependent manner. Additionally, it inhibits osteoclast-specific gene and protein expression. It was noted that AIL inhibited the expression of the osteoclast differentiation-related cytokines RANKL and interleukin-1β (IL-1β) that were secreted by the MDA-MB-231 cells after upregulating the Forkhead box protein 3 (FOXP3) expression. Furthermore, AIL also inhibits the expression of the Mitogen-Activated Protein Kinase (MAPK), Phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT), and Nuclear factor-κB Ligand (NF-κB) signaling pathways, which then suppresses the MDA-MB-231CM-induced development of Osteoclasts. Conclusion: Our study shows that AIL blocks osteoclast differentiation in the bone metastasis microenvironment by inhibiting cytokines secreted by BC cells, which may be a potential agent for the treatment of BC and its secondary bone metastasis.

Role of MOTS-c in the regulation of bone metabolism

MOTS-c, a mitochondrial-derived peptide (MDP), is an essential regulatory mediator of cell protection and energy metabolism and is involved in the development of specific diseases. Recent studies have revealed that MOTS-c promotes osteoblast proliferation, differentiation, and mineralization. Furthermore, it inhibits osteoclast production and mediates the regulation of bone metabolism and bone remodeling. Exercise effectively upregulates the expression of MOTS-c, but the specific mechanism of MOTS-c regulation in bone by exercise remains unclear. Therefore, this article reviewed the distribution and function of MOTS-c in the tissue, discussed the latest research developments in the regulation of osteoblasts and osteoclasts, and proposed potential molecular mechanisms for the effect of exercise on the regulation of bone metabolism. This review provides a theoretical reference for establishing methods to prevent and treat skeletal metabolic diseases.

E3 Ubiquitin Ligases: Potential Therapeutic Targets for Skeletal Pathology and Degeneration

The ubiquitination-proteasome system (UPS) is crucial in regulating a variety of cellular processes including proliferation, differentiation, and survival. Ubiquitin protein ligase E3 is the most critical molecule in the UPS system. Dysregulation of the UPS system is associated with many conditions. Over the past few decades, there have been an increasing number of studies focusing on the UPS system and how it affects bone metabolism. Multiple E3 ubiquitin ligases have been found to mediate osteogenesis or osteolysis through a variety of pathways. In this review, we describe the mechanisms of UPS, especially E3 ubiquitin ligases on bone metabolism. To date, many E3 ubiquitin ligases have been found to regulate osteogenesis or osteoclast differentiation. We review the classification of these E3 enzymes and the mechanisms that influence upstream and downstream molecules and transduction pathways. Finally, this paper reviews the discovery of the relevant UPS inhibitors, drug molecules, and noncoding RNAs so far and prospects the future research and treatment.

Biological mechanism of surgery-mediated acceleration of orthodontic tooth movement: A narrative review

Surgery-mediated acceleration of orthodontic tooth movement (SAOTM) has been proven effective for decades. Research has confirmed that surgical approaches play an important role in adult patients with a short orthodontic treatment time. The mechanism of SAOTM involves short-term acceleration of localized hard and soft tissue remodeling, known as the regional acceleratory phenomenon. However, no relevant review on the biological mechanism of SAOTM has been performed to date. The proposed biological mechanism of acceleration of OTM involves the participation of various cells, cytokines, and signaling pathways. We herein review the relevant literature and summarize the biological mechanism of SAOTM to provide new insights for further research on acceleration of OTM.

Isoliensinine Suppresses Osteoclast Formation Through NF-κB Signaling Pathways and Relieves Ovariectomy-Induced Bone Loss

Osteoporosis is among the major contributors of pathologic fracture in postmenopausal women, which is caused by the bone metabolic disorder owing to the over-activation of osteoclasts. Inhibition of osteoclast differentiation and maturation has become a mainstream research interest in the prevention of osteoporosis. Isoliensinine (Iso) is a dibenzyl isoquinoline alkaloid with antioxidant, anti-inflammatory, and anti-cancer activities. However, whether it can be used as a potential treatment for osteoporosis remains undiscovered. Here, we investigated whether Iso might suppress the differentiation of osteoclasts in vitro and in vivo to play an anti-osteoporosis role. Our results showed that Iso inhibits the formation of mature multinuclear osteoclasts induced by RANKL, the bone resorption, and the osteoclast-specific genes expression by blocking the nuclear translocation of NF-κB p65, and the effect was in a dosage-dependent way. Furthermore, we investigated the therapeutic effect of Iso on osteoporosis in ovariectomized (OVX) mice. We found that Iso attenuated bone loss in the OVX mice and significantly promoted BS, Conn. DN, Tb.Th, TB.N, and BV/TV Index. All in all, Iso showed a prominent effect of osteoclast inhibition, with great promise for treating osteoporosis.

Proteus mirabilis Vesicles Induce Mitochondrial Apoptosis by Regulating miR96-5p/Abca1 to Inhibit Osteoclastogenesis and Bone Loss

Bone loss due to an increased osteoclast activity is common in osteoporosis and rheumatoid arthritis. For the first time, we observed an inhibition of osteoclast formation and bone resorption by outer-membrane vesicles (OMVs) from a Gram-negative, pathogenic bacterium, Proteus mirabilis (P.M). Gene ontogeny and KEGG enrichment analyses of miRNA and mRNA sequencing data demonstrated a significant effect of P.M OMVs on mitochondrial functions and apoptotic pathways. OMVs induced mitochondrial dysfunction through an increased level of intracellular ROS, collapse of mitochondrial membrane potential (ΔΨm), and modulation of Bax, Bcl-2, caspase-3, and cytochrome c expression. In addition, P.M OMVs strongly inhibited miR-96-5p expression, which caused an upregulation of ATP binding cassette subfamily A member 1 (Abca1) in osteoclasts leading to an increased level of mitochondria-dependent apoptosis. Moreover, treatment with P.M but not Escherichia coli OMVs attenuated bone loss in experimental osteoporosis and collagen-induced arthritis. Collectively, we demonstrated osteoprotective functions of OMVs from Proteus mirabilis, which downregulated miR-96-5p causing an increased Abca1 expression and mitochondria-dependent apoptosis.

Neuropilin-1-Mediated SARS-CoV-2 Infection in Bone Marrow-Derived Macrophages Inhibits Osteoclast Differentiation

In humans, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection can cause medical complications across various tissues and organs. Despite the advances to understanding the pathogenesis of SARS-CoV-2, its tissue tropism and interactions with host cells have not been fully understood. Existing clinical data have revealed disordered calcium and phosphorus metabolism in Coronavirus Disease 2019 (COVID-19) patients, suggesting possible infection or damage in the human skeleton system by SARS-CoV-2. Herein, SARS-CoV-2 infection in mouse models with wild-type and beta strain (B.1.351) viruses is investigated, and it is found that bone marrow-derived macrophages (BMMs) can be efficiently infected in vivo. Single-cell RNA sequencing (scRNA-Seq) analyses of infected BMMs identify distinct clusters of susceptible macrophages, including those related to osteoblast differentiation. Interestingly, SARS-CoV-2 entry on BMMs is dependent on the expression of neuropilin-1 (NRP1) rather than the widely recognized receptor angiotensin-converting enzyme 2 (ACE2). The loss of NRP1 expression during BMM-to-osteoclast differentiation or NRP1 neutralization and knockdown can significantly inhibit SARS-CoV-2 infection in BMMs. Importantly, it is found that authentic SARS-CoV-2 infection impedes BMM-to-osteoclast differentiation. Collectively, this study provides evidence for NRP1-mediated SARS-CoV-2 infection in BMMs and establishes a potential link between disturbed osteoclast differentiation and disordered skeleton metabolism in COVID-19 patients.

Targeting adipocytic discoidin domain receptor 2 impedes fat gain while increasing bone mass

Obesity is closely associated with low-bone-mass disorder. Discoidin domain receptor 2 (DDR2) plays essential roles in skeletal metabolism, and is probably involved in fat metabolism. To test the potential role of DDR2 in fat and fat-bone crosstalk, Ddr2 conditional knockout mice (Ddr2^Adipo) were generated in which Ddr2 gene is exclusively deleted in adipocytes by Adipoq Cre. We found that Ddr2^Adipo mice are protected from fat gain on high-fat diet, with significantly decreased adipocyte size. Ddr2^Adipo mice exhibit significantly increased bone mass and mechanical properties, with enhanced osteoblastogenesis and osteoclastogenesis. Marrow adipocyte is diminished in the bone marrow of Ddr2^Adipo mice, due to activation of lipolysis. Fatty acid in the bone marrow was reduced in Ddr2^Adipo mice. RNA-Seq analysis identified adenylate cyclase 5 (Adcy5) as downstream molecule of Ddr2. Mechanically, adipocytic Ddr2 modulates Adcy5-cAMP-PKA signaling, and Ddr2 deficiency stimulates lipolysis and supplies fatty acid for oxidation in osteoblasts, leading to the enhanced osteoblast differentiation and bone mass. Treatment of Adcy5 specific inhibitor abolishes the increased bone mass gain in Ddr2^Adipo mice. These observations establish, for the first time, that Ddr2 plays an essential role in the crosstalk between fat and bone. Targeting adipocytic Ddr2 may be a potential strategy for treating obesity and pathological bone loss simultaneously.

Roles of the RANKL-RANK Axis in Immunity-Implications for Pathogenesis and Treatment of Bone Metastasis

A substantial amount patients with cancer will develop bone metastases, with 70% of metastatic prostate and breast cancer patients harboring bone metastasis. Despite advancements in systemic therapies for advanced cancer, survival remains poor for those with bone metastases. The interaction between bone cells and the immune system contributes to a better understanding of the role that the immune system plays in the bone metastasis of cancer. The immune and bone systems share various molecules, including transcription factors, signaling molecules, and membrane receptors, which can stimulate the differentiation and activation of bone-resorbing osteoclasts. The process of cancer metastasis to bone, which deregulates bone turnover and results in bone loss and skeletal-related events (SREs), is also controlled by primary cancer-related factors that modulate the intratumoral microenvironment as well as cellular immune process. The nuclear factor kappa B ligand (RANKL) and the receptor activator of nuclear factor kappa B (RANK) are key regulators of osteoclast development, bone metabolism, lymph node development, and T-cell/dendritic cell communication. RANKL is an osteoclastogenic cytokine that links the bone and the immune system. In this review, we highlight the role of RANKL and RANK in the immune microenvironment and bone metastases and review data on the role of the regulatory mechanism of immunity in bone metastases, which could be verified through clinical efficacy of RANKL inhibitors for cancer patients with bone metastases. With the discovery of the specific role of RANK signaling in osteoclastogenesis, the humanized monoclonal antibody against RANKL, such as denosumab, was available to prevent bone loss, SREs, and bone metastases, providing a unique opportunity to target RANKL/RANK as a future strategy to prevent bone metastases.

Effects of Vitamin A (Retinol) Release from Calcium Phosphate Matrices and Porous 3D Printed Scaffolds on Bone Cell Proliferation and Maturation

Vitamin A is a fat-soluble compound widely known for vision health. Highly variable reports on its effects on bone health have necessitated further research to truly understand its role on bone cell proliferation. Retinol, one bioactive form of vitamin A, is incorporated into synthetic bone graft scaffolds for low load-bearing clinical bone treatment. The objective of this work is to understand the effects of retinol on osteoblast and osteoclast cells when embedded within calcium phosphate matrices, including interconnected porous 3D printed tricalcium phosphate scaffolds. Results show that hydrophobic retinol can be released from bone scaffolds when a combination of biodegradable polymers, polycaprolactone and polyethylene glycol, are employed as drug carriers. The release of retinol in vitro can support a 20 ± 1% increase in osteoblast (bone-forming) cell proliferation with proper cell adhesion and filopodial extensions. Osteoclast cell morphology is necrosed and torn with a reduction in proliferation at approximately 6 ± 1% when retinol is present. In addition, inhibition of osteoclastic resorption pit bays is noted using scanning electron microscopy. With the scaffolds' round pore interconnectivity facilitating retinol release, this system can provide an alternative to traditional bone grafts while additionally supporting bone healing through enhanced osteoblast cell proliferation and inhibition of osteoclast resorption activity.

A Nitrobenzoyl Sesquiterpenoid Insulicolide A Prevents Osteoclast Formation via Suppressing c-Fos-NFATc1 Signaling Pathway

It is a viable strategy to inhibit osteoclast differentiation for the treatment of osteolytic diseases such as osteoporosis, rheumatoid arthritis and tumor bone metastases. Here we assessed the effects of insulicolide A, a natural nitrobenzoyl sesquiterpenoid derived from marine fungus, on receptor activator of nuclear factor-κB ligand (RANKL)-stimulated osteoclastogenesis in vitro and its protective effects on LPS-induced osteolysis mice model in vivo. The results demonstrated that insulicolide A inhibited osteoclastogenesis from 1 μM in vitro. Insulicolide A could prevent c-Fos and nuclear factor of activated T-cell cytoplasmic 1 (NFATc1) nuclear translocation and attenuate the expression levels of osteoclast-related genes and DC-STAMP during RANKL-stimulated osteoclastogenesis but have no effects on NF-κB and MAPKs. Insulicolide A can also protect the mice from LPS-induced osteolysis. Our research provides the first evidence that insulicolide A may inhibit osteoclastogenesis both in vitro and in vivo, and indicates that it may have potential for the treatment of osteoclast-related diseases.

Sirtuin function and metabolism: Role in pancreas, liver, and adipose tissue and their crosstalk impacting bone homeostasis

Mammalian sirtuins (SIRT1-7) are members of the nicotine adenine dinucleotide (NAD+)-dependent family of enzymes critical for histone deacetylation and posttranslational modification of proteins. Sirtuin family members regulate a wide spectrum of biological processes and are best known for maintaining longevity. Sirtuins are well characterized in metabolic tissues such as the pancreas, liver and adipose tissue (AT). They are regulated by a diverse range of stimuli, including nutrients and metabolic changes within the organism. Indeed, nutrient-associated conditions, such as obesity and anorexia nervosa (AN), were found to be associated with bone fragility development in osteoporosis. Interestingly, it has also been demonstrated that sirtuins, more specifically SIRT1, can regulate bone activity. Various studies have demonstrated the importance of sirtuins in bone in the regulation of bone homeostasis and maintenance of the balance between bone resorption and bone formation. However, to understand the molecular mechanisms involved in the negative regulation of bone homeostasis during overnutrition (obesity) or undernutrition, it is crucial to examine a wider picture and to determine the pancreatic, liver and adipose tissue pathway crosstalk responsible for bone loss. Particularly, under AN conditions, sirtuin family members are highly expressed in metabolic tissue, but this phenomenon is reversed in bone, and severe bone loss has been observed in human subjects. AN-associated bone loss may be connected to SIRT1 deficiency; however, additional factors may interfere with bone homeostasis. Thus, in this review, we focus on sirtuin activity in the pancreas, liver and AT in cases of over- and undernutrition, especially the regulation of their secretome by sirtuins. Furthermore, we examine how the secretome of the pancreas, liver and AT affects bone homeostasis, focusing on undernutrition. This review aims to lead to a better understanding of the crosstalk between sirtuins, metabolic organs and bone. In long term prospective it should contribute to promote improvement of therapeutic strategies for the prevention of metabolic diseases and the development of osteoporosis.

Mesenchymal Stem Cell Senescence and Osteogenesis

Mesenchymal stem cells (MSCs) are stem cells with the potential ability to differentiate into various cells and the ability to self-renew and resemble fibroblasts. These cells can adhere to plastic to facilitate the culture process. MSCs can be used in research into tissue biotechnology and rejuvenation medicine. MSCs are also beneficial in recipient tissue and differentiate as a breakthrough strategy through paracrine activity. Many databases have shown MSC-based treatment can be beneficial in the reduction of osteogenesis induced by senescence. In this article, we will discuss the potential effect of MSCs in senescence cells related to osteogenesis.

Oroxin B Attenuates Ovariectomy-Induced Bone Loss by Suppressing Osteoclast Formation and Activity

Background

Osteoclasts are the major players in bone resorption and have always been studied in the prevention and treatment of osteoporosis. Previous studies have confirmed that a variety of flavonoids inhibit osteoporosis and improve bone health mainly through inhibiting osteoclastogenesis. Oroxin B (OB) is a flavonoid compound extracted from traditional Chinese herbal medicine Oroxylum indicum (L.) Vent, exerts potent antitumor and anti-inflammation effect, but its effect on osteoclastogensis remains unknown.

Methods

We comprehensively evaluated the effect of OB on the formation and function of osteoclasts and the underling mechanism by bone marrow-derived macrophage in vitro. In vivo, we used mice ovariectomized model to verify the protective effect of OB.

Results

OB was found to inhibit osteoclast formation and bone resorption function in vitro, in a dose-dependent manner and the increased osteoclastic-related genes induced by RANKL (NFATc1, c-fos, cathepsin K, RANK, MMP9 and TRAP) were also attenuated following OB treatment. Mechanistical investigation showed OB abrogated the increased phosphorylation level of MAPK and NF-κB pathway, and diminished the expression of the vital transcription factors for osteoclastogenesis. OB also prevented ovariectomy (OVX)-induced bone loss by inhibiting osteoclast formation and activity in mice.

Conclusion

Our study demonstrated that OB may act as an anti-osteoporosis agent by inhibiting osteoclast maturation and attenuating bone resorption.

Roburic acid attenuates osteoclastogenesis and bone resorption by targeting RANKL-induced intracellular signaling pathways

Excessive activity of osteoclasts contributes to skeletal diseases such as osteoporosis and osteolysis. However, current drugs targeting osteoclast have various deficiencies, placing natural compounds as substitutions of great potential. Roburic acid (RA) is a triterpenoid exacted from Radix Gentianae Macrophyllae, which exhibits inhibitory effects on inflammation and oxidation. By employing an in vitro osteoclastogenesis model, this study investigates the effects and mechanisms of RA on intracellular signaling induced by receptor activator of nuclear factor-κB ligand (RANKL). As expected, RA at a concentration scope from 1 to 10 μM dampened the osteoclast differentiation of bone marrow macrophages (BMMs) but without cell toxicity. Interestingly, RA showed no effect on osteoblastogenesis in vitro. Furthermore, RA mitigated F-actin ring formation, hydroxyapatite resorption, and gene expression in osteoclasts. Mechanistically, RA suppressed TNF receptor-associated factor 6 (TRAF6), the crucial adaptor protein following RANKL-RANK binding. On the one hand, RA downregulated the nuclear factor-κB (NF-κB) activity, extracellular regulated protein kinases (ERK) phosphorylation, and calcium oscillations. On the other hand, RA upregulated the antioxidative response element (ARE) response and the protein expression of heme oxygenase (HO)-1. These upstream alterations eventually led to the suppression of the nuclear factor of activated T cells 1 (NFATc1) activity and the expression of proteins involved in osteoclastogenesis and bone resorption. Furthermore, by using an ovariectomized (OVX) mice model, RA was found to have therapeutic effects against bone loss. On account of these findings, RA could be used to restrain osteoclasts for treating osteoporosis and other osteolytic diseases.

Skeleton-secreted PDGF-BB mediates arterial stiffening

Evidence links osteoporosis and cardiovascular disease but the cellular and molecular mechanisms are unclear. Here we identify skeleton-secreted platelet-derived growth factor-BB (PDGF-BB) as a key mediator of arterial stiffening in response to aging and metabolic stress. Aged mice and those fed high-fat diet (HFD), relative to young mice and those fed normal chow food diet, respectively, had higher serum PDGF-BB and developed bone loss and arterial stiffening. Bone/bone marrow preosteoclasts in aged mice and HFD mice secrete an excessive amount of PDGF-BB, contributing to the elevated PDGF-BB in blood circulation. Conditioned medium prepared from preosteoclasts stimulated proliferation and migration of the vascular smooth muscle cells. Conditional transgenic mice, in which PDGF-BB is overexpressed in preosteoclasts, had 3-fold higher serum PDGF-BB concentration and developed simultaneous bone loss and arterial stiffening spontaneously at a young age. Conversely, in conditional knockout mice, in which PDGF-BB is deleted selectively in preosteoclasts, HFD did not affect serum PDGF-BB concentration; as a result, HFD-induced bone loss and arterial stiffening were attenuated. These studies confirm that preosteoclasts are a main source of excessive PDGF-BB in blood circulation during aging and metabolic stress and establish the role of skeleton-derived PDGF-BB as an important mediator of vascular stiffening.

Molecular profiling of bone remodeling occurring in musculoskeletal tumors

Musculoskeletal malignancy is often accompanied by aberrant bone remodeling, leading to tumor cell invasion into skeletal tissues and causing severe pain. BMPs, FGF-2, and RANKL have been identified as promising regulators in physiological bone remodeling. In this study, we explored the expressional profile of BMPs, FGF-2, and RANKL in 1361 patients with 22 varieties of musculoskeletal tumors. Notably, the expression of FGF-2 and RANKL was under detected in all patients. Among BMP1 to BMP15, we found that BMP1, BMP2, BMP4, BMP5, BMP6, and BMP7 were prevalent. In comparison with normal bones, osteosarcoma highly expressed BMP1, BMP2, BMP4, and BMP7 with statistical significance. Synovial sarcoma upregulated BMP4, BMP5, and BMP7; rhabdomyosarcoma increased BMP1 and BMP4; and alveolar soft part sarcoma upregulated BMP1, BMP4, and BMP7. To visualize the BMP-oriented interactions in a bone tumor microenvironment, we have developed novel software that analyzes numerous cell-to-cell and ligand-to-receptor interactions, that is, Environmentome, delineating that osteosarcoma-secreted BMP-4 and synovial sarcoma-secreted BMP7 potently interact with osteoblasts, osteocytes, osteoclast precursors, and mature osteoclasts. Specifically, quantification analysis revealed that the relationship between osteosarcoma and mature osteoclast/precursor, BMP4-BMPR2 and BMP4-ACVR2A interactions were most potent. Regarding the association between osteosarcoma and osteocyte/osteoblast, BMP4-ACVR1 and BMP4-BMPR2 were the key interactions. In the connection between synovial sarcoma and mature osteoclast/precursor, BMP7-ACVR2A and BMP7-BMPR2 interactions were most remarkable. With regard to the cellular link between synovial sarcoma and osteocyte/osteoblast, BMP7-BMPR2 was identified as a potent interaction. In conclusion, our new outlook suggests delivering the pathological events that clinically underlie behind severe skeletal pain or fracture in musculoskeletal tumors.

Biomechanical and biological responses of periodontium in orthodontic tooth movement: up-date in a new decade

Nowadays, orthodontic treatment has become increasingly popular. However, the biological mechanisms of orthodontic tooth movement (OTM) have not been fully elucidated. We were aiming to summarize the evidences regarding the mechanisms of OTM. Firstly, we introduced the research models as a basis for further discussion of mechanisms. Secondly, we proposed a new hypothesis regarding the primary roles of periodontal ligament cells (PDLCs) and osteocytes involved in OTM mechanisms and summarized the biomechanical and biological responses of the periodontium in OTM through four steps, basically in OTM temporal sequences, as follows: (1) Extracellular mechanobiology of periodontium: biological, mechanical, and material changes of acellular components in periodontium under orthodontic forces were introduced. (2) Cell strain: the sensing, transduction, and regulation of mechanical stimuli in PDLCs and osteocytes. (3) Cell activation and differentiation: the activation and differentiation mechanisms of osteoblast and osteoclast, the force-induced sterile inflammation, and the communication networks consisting of sensors and effectors. (4) Tissue remodeling: the remodeling of bone and periodontal ligament (PDL) in the compression side and tension side responding to mechanical stimuli and root resorption. Lastly, we talked about the clinical implications of the updated OTM mechanisms, regarding optimal orthodontic force (OOF), acceleration of OTM, and prevention of root resorption.

PMEPA1 and NEDD4 control the proton production of osteoclasts by regulating vesicular trafficking

Osteoclast bone resorption activity is critically regulated to maintain bone homeostasis. Osteoclasts resorb bone by producing protons and acid hydrolase via lysosomal secretion, however, a detailed mechanism remains elusive. PMEPA1 is a vesicular membrane protein, which binds to the NEDD4 family member of ubiquitin ligases. We have previously reported that Pmepa1 is highly expressed in bone resorbing osteoclasts, and regulates bone resorption. Here, we investigated the mechanism of bone resorption regulated by PMEPA1. Mutant mice lacking NEDD4-binding domains of PMEPA1 displayed enhanced bone volume, and reduced bone resorption activity in comparison with those of WT mice. Analysis with pH-sensitive fluorescence probe revealed that proton secretion from osteoclasts significantly decreased in Pmepa1 mutant osteoclasts. Immunofluorescence analysis revealed that PMEPA1 was colocalized with NEDD4, V0A3, and V0D2 subunits of vacuolar ATPase, which regulate the proton production of osteoclasts. In addition, Nedd4 knockdown reduced bone resorption and proton secretion of osteoclasts. Furthermore, Pmepa1 mutation and Nedd4 knockdown altered the cytoplasmic distribution of components of V-ATPase and expression of autophagy-related proteins, suggesting that lysosomal secretion is affected. Collectively, these findings indicate that PMEPA1 controls proton secretion from osteoclasts via NEDD4 by regulating vesicular trafficking, and NEDD4 is an important regulator of bone resorption.

YTHDF2 mediates LPS-induced osteoclastogenesis and inflammatory response via the NF-κB and MAPK signaling pathways

Aberrant elevation of osteoclast differentiation and function is responsible for disrupting bone homeostasis in various inflammatory bone diseases. YTH domain family 2 (YTHDF2) is a well-known m⁶A-binding protein that plays an essential role in regulating cell differentiation and inflammatory processes by mediating mRNA degradation. However, the regulatory role of YTHDF2 in inflammatory osteoclast differentiation remains unelucidated. Here, we detected the expression of m⁶A-related genes and found that YTHDF2 was upregulated in RANKL-primed osteoclast precursors stimulated with lipopolysaccharide (LPS). Ythdf2 knockdown in RAW264.7 cells and primary bone marrow-derived macrophages (BMMs) enhanced osteoclast formation and bone resorption, which was assessed by TRAP staining assay and pit formation assay. Ythdf2 depletion upregulated osteoclast-related gene expression and proinflammatory cytokine secretion. In contrast, overexpression of Ythdf2 produced the reverse effect. Furthermore, Ythdf2 knockdown enhanced the phosphorylation of IKKα/β, IκBα, ERK, P38 and JNK. NF-κB and MAPK signaling pathway inhibitors effectively abrogated the enhanced expression of Nfact1, c-Fos, IL-1β and TNF-α caused by Ythdf2 knockdown. Mechanistically, the mRNA stability assay revealed that Ythdf2 depletion led to stabilization of Tnfrsf11a, Traf6, Map4k4, Map2k3, Map2k4 and Nfatc1 mRNA. In summary, our findings demonstrated that YTHDF2 has a negative regulatory role in LPS-induced osteoclast differentiation and the inflammatory response via the NF-κB and MAPK signaling pathways.

Austalide K from the Fungus Penicillium rudallense Prevents LPS-Induced Bone Loss in Mice by Inhibiting Osteoclast Differentiation and Promoting Osteoblast Differentiation

Osteoporosis is a chronic disease that has become a serious public health problem due to the associated reduction in quality of life and its increasing financial burden. It is known that inhibiting osteoclast differentiation and promoting osteoblast formation prevents osteoporosis. As there is no drug with this dual activity without clinical side effects, new alternatives are needed. Here, we demonstrate that austalide K, isolated from the marine fungus Penicillium rudallenes, has dual activities in bone remodeling. Austalide K inhibits the receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation and improves bone morphogenetic protein (BMP)-2-mediated osteoblast differentiation in vitro without cytotoxicity. The nuclear factor of activated T cells c1 (NFATc1), tartrate-resistant acid phosphatase (TRAP), dendritic cell-specific transmembrane protein (DC-STAMP), and cathepsin K (CTSK) osteoclast-formation-related genes were reduced and alkaline phosphatase (ALP), runt-related transcription factor 2 (Runx2), osteocalcin (OCN), and osteopontin (OPN) (osteoblast activation-related genes) were simultaneously upregulated by treatment with austalide K. Furthermore, austalide K showed good efficacy in an LPS-induced bone loss in vivo model. Bone volume, trabecular separation, trabecular thickness, and bone mineral density were recovered by austalide K. On the basis of these results, austalide K may lead to new drug treatments for bone diseases such as osteoporosis.

Malfunctioning CD106-positive, short-term hematopoietic stem cells trigger diabetic neuropathy in mice by cell fusion

Diabetic neuropathy is an incurable disease. We previously identified a mechanism by which aberrant bone marrow-derived cells (BMDCs) pathologically expressing proinsulin/TNF-α fuse with residential neurons to impair neuronal function. Here, we show that CD106-positive cells represent a significant fraction of short-term hematopoietic stem cells (ST-HSCs) that contribute to the development of diabetic neuropathy in mice. The important role for these cells is supported by the fact that transplantation of either whole HSCs or CD106-positive ST-HSCs from diabetic mice to non-diabetic mice produces diabetic neuronal dysfunction in the recipient mice via cell fusion. Furthermore, we show that transient episodic hyperglycemia produced by glucose injections leads to abnormal fusion of pathological ST-HSCs with residential neurons, reproducing neuropathy in nondiabetic mice. In conclusion, we have identified hyperglycemia-induced aberrant CD106-positive ST-HSCs underlie the development of diabetic neuropathy. Aberrant CD106-positive ST-HSCs constitute a novel therapeutic target for the treatment of diabetic neuropathy.

Katagi et al. show that abnormal bone marrow-derived cells originated from hematopoietic stem cells (CD106-positive short-term HSCs) aberrantly fuse with neurons to develop diabetic neuropathy. This study suggests that the pathological abnormality is memorized in the bone marrow and that it cannot be erased by conventional therapy.

Inhibitory Effects of Gold and Silver Nanoparticles on the Differentiation into Osteoclasts In Vitro

Gold nanoparticles (GNPs) have been widely studied to inhibit differentiation into osteoclasts. However, reports of the inhibitory effects of silver nanoparticles (SNPs) during the process of differentiation into osteoclasts are rare. We compared the inhibitory effect of GNPs and SNPs during the process of differentiation into osteoclasts. Bone marrow-derived cells were differentiated into osteoclasts by the receptor activator of the nuclear factor-kappa-Β ligand (RANKL). The inhibitory effect of GNPs or SNPs during the process of differentiation into osteoclasts was investigated using tartrate-resistant acid phosphatase (TRAP) and actin ring staining. The formation of TRAP positive ⁽⁺⁾ multinuclear cells (MNCs) with the actin ring structure was most inhibited in the SNP group. In addition, the expression of specific genes related to the differentiation into osteoclasts, such as c-Fos, the nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), TRAP, and Cathepsin K (CTSK) were also inhibited in the SNP groups. As a result, the levels related to differentiation into osteoclasts were consistently lower in the SNP groups than in the GNP groups. Our study suggests that SNPs can be a useful material for inhibiting differentiation into osteoclasts and they can be applied to treatments for osteoporosis patients.

Genome-Wide Association Identifies Risk Pathways for SAPHO Syndrome

SAPHO syndrome is a rare chronic inflammatory disease which is characterized by the comprehensive manifestations of bone, joint, and skin. However, little is known about the pathogenesis of SAPHO syndrome. A genome-wide association study (GWAS) of 49 patients and 121 control subjects have primarily focused on identification of common genetic variants associated with SAPHO, the data were analyzed by classical multiple logistic regression. Later, GWAS findings were further validated using whole exome sequencing (WES) in 16 patients and 15 controls to identify potentially functional pathways involved in SAPHO pathogenesis. In general, 40588 SNPs in genomic regions were associated with P < 0.05 after filter process, only 9 SNPs meet the expected cut-off P-value, however, none of them had association with SAPHO syndrome based on published literatures. And then, 15 pathways were found involved in SAPHO pathogenesis, of them, 6 pathways including osteoclast differentiation, bacterial invasion of epithelial cells, et al., had strong association with skin, osteoarticular manifestations of SAPHO or inflammatory reaction based published research. This study identified aberrant osteoclast differentiation and other pathways were involved in SAPHO syndrome. This finding may give insight into the understanding of pathogenic genes of SAPHO and provide the basis for SAPHO research and treatment.