NFATc1 in mice represses osteoprotegerin during osteoclastogenesis and dissociates systemic osteopenia from inflammation in cherubism

Loss-of-function OGFRL1 variants identified in autosomal recessive cherubism families

Cherubism (OMIM 118400) is a rare craniofacial disorder in children characterized by destructive jawbone expansion due to the growth of inflammatory fibrous lesions. Our previous studies have shown that gain-of-function mutations in SH3 domain-binding protein 2 (SH3BP2) are responsible for cherubism and that a knock-in mouse model for cherubism recapitulates the features of cherubism, such as increased osteoclast formation and jawbone destruction. To date, SH3BP2 is the only gene identified to be responsible for cherubism. Since not all patients clinically diagnosed with cherubism had mutations in SH3BP2, we hypothesized that there may be novel cherubism genes and that these genes may play a role in jawbone homeostasis. Here, using whole exome sequencing, we identified homozygous loss-of-function variants in the opioid growth factor receptor like 1 (OGFRL1) gene in 2 independent autosomal recessive cherubism families from Syria and India. The newly identified pathogenic homozygous variants were not reported in any variant databases, suggesting that OGFRL1 is a novel gene responsible for cherubism. Single cell analysis of mouse jawbone tissue revealed that Ogfrl1 is highly expressed in myeloid lineage cells. We generated OGFRL1 knockout mice and mice carrying the Syrian frameshift mutation to understand the in vivo role of OGFRL1. However, neither mouse model recapitulated human cherubism or the phenotypes exhibited by SH3BP2 cherubism mice under physiological and periodontitis conditions. Unlike bone marrow-derived M-CSF-dependent macrophages (BMMs) carrying the SH3BP2 cherubism mutation, BMMs lacking OGFRL1 or carrying the Syrian mutation showed no difference in TNF-ɑ mRNA induction by LPS or TNF-ɑ compared to WT BMMs. Osteoclast formation induced by RANKL was also comparable. These results suggest that the loss-of-function effects of OGFRL1 in humans differ from those in mice and highlight the fact that mice are not always an ideal model for studying rare craniofacial bone disorders.

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.

StemRegenin 1 attenuates the RANKL-induced osteoclastogenesis via inhibiting AhR-c-src-NF-κB/p-ERK MAPK-NFATc1 signaling pathway

The aryl hydrocarbon receptor (AhR) pathway may play an important role in the regulation of osteoclasts, but there are still conflicting studies on this aspect, and the specific mechanism of action has not been fully elucidated. Therefore, we conducted this study to find a drug to treat osteoporosis that targets AhR. We found that StemRegenin 1 inhibited RANKL-induced osteoclastogenesis in a concentration-dependent and time-dependent manner. Through further experiments, we found that SR1 can inhibit nuclear transcription of AhR and inhibit c-src phosphorylation, and ultimately regulates the activation of the NF-κB and p-ERK/mitogen-activated protein kinase pathways. Therefore, for the first time, we discovered the way in which the AhR-c-src-NF-κB/p-ERK MAPK-NFATc1 signaling pathway regulates the expression of osteoclast differentiation-associated proteins. Finally, SR1 was shown to successfully reverse bone loss in OVX mice. These studies provide us with ideas for finding new way to treat osteoporosis.

Chordoma cells possess bone-dissolving activity at the bone invasion front

PURPOSE

Chordomas are malignant tumors that destroy bones, compress surrounding nerve tissues and exhibit phenotypes that recapitulate notochordal differentiation in the axial skeleton. Chordomas recur frequently, as they resist radio-chemotherapy and are difficult to completely resect, leading to repeated bone destruction and local expansion via unknown mechanisms. Here, using chordoma specimens and JHC7 chordoma cells, we asked whether chordoma cells possess bone-dissolving activity.

METHODS

CT imaging and histological analysis were performed to evaluate the structure and mineral density of chordoma-invaded bone and osteolytic marker expression. JHC7 cells were subjected to immunocytochemistry, imaging of cell fusion, calcium dynamics and acidic vacuoles, and bone lysis assays.

RESULTS

In patients, we found that the skull base invaded by chordoma was highly porous, showed low mineral density and contained brachyury-positive chordoma cells and conventional osteoclasts both expressing the osteolytic markers tartrate-resistant acid phosphatase (TRAP) and collagenases. JHC7 cells expressed TRAP and cathepsin K, became multinucleated via cell-cell fusion, showed spontaneous calcium oscillation, and were partly responsive to the osteoclastogenic cytokine RANKL. JHC7 cells exhibited large acidic vacuoles, and nonregulatory bone degradation without forming actin rings. Finally, bone-derived factors, calcium ions, TGF-β1, and IGF-1 enhanced JHC7 cell proliferation.

CONCLUSION

In chordoma, we propose that in addition to conventional bone resorption by osteoclasts, chordoma cells possess bone-dissolving activity at the tumor-bone boundary. Furthermore, bone destruction and tumor expansion may occur in a positive feedback loop.

Napyradiomycin B4 Suppresses RANKL-Induced Osteoclastogenesis and Prevents Alveolar Bone Destruction in Experimental Periodontitis

The unique structure and beneficial biological properties of marine natural products have drawn interest in drug development. Here, we examined the therapeutic potential of napyradiomycin B4 isolated from marine-derived Streptomyces species for osteoclast-related skeletal diseases. Bone marrow-derived macrophages were treated with napyradiomycin B4 in an osteoclast-inducing medium, and osteoclast formation, osteoclast-specific gene expression, and nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) localization were evaluated using tartrate-resistant acid phosphatase staining, real-time PCR, and immunostaining, respectively. Phosphorylation levels of signaling proteins were assessed by immunoblot analysis to understand the molecular action of napyradiomycin B4. The in vivo efficacy of napyradiomycin B4 was examined under experimental periodontitis, and alveolar bone destruction was evaluated by microcomputed tomography (micro-CT) and histological analyses. Among the eight napyradiomycin derivatives screened, napyradiomycin B4 considerably inhibited osteoclastogenesis. Napyradiomycin B4 significantly suppressed the receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation and disrupted the expression of NFATc1 and its target genes. Mitogen-activated extracellular signal-regulated kinase (MEK) and extracellular signal-regulated kinase (ERK) phosphorylation levels were reduced by napyradiomycin B4 in response to RANKL. Under in vivo experimental periodontitis, napyradiomycin B4 significantly attenuated osteoclast formation and decreased the distance between the cementoenamel junction and alveolar bone crest. Our findings demonstrate the antiosteoclastogenic activity of napyradiomycin B4 by inhibiting the RANKL-induced MEK-ERK signaling pathway and its protective effect on alveolar bone destruction.

Microdroplets Encapsulated with NFATc1-siRNA and Exosomes-Derived from MSCs Onto 3D Porous PLA Scaffold for Regulating Osteoclastogenesis and Promoting Osteogenesis

Introduction

Osteoporotic-related fractures remains a significant public health concern, thus imposing substantial burdens on our society. Excessive activation of osteoclastic activity is one of the main contributing factors for osteoporosis-related fractures. While polylactic acid (PLA) is frequently employed as a biodegradable scaffold in tissue engineering, it lacks sufficient biological activity. Microdroplets (MDs) have been explored as an ultrasound-responsive drug delivery method, and mesenchymal stem cell (MSC)-derived exosomes have shown therapeutic effects in diverse preclinical investigations. Thus, this study aimed to develop a novel bioactive hybrid PLA scaffold by integrating MDs-NFATc1-silencing siRNA to target osteoclast formation and MSCs-exosomes (MSC-Exo) to influence osteogenic differentiation (MDs-NFATc1/PLA-Exo).

Methods

Human bone marrow-derived mesenchymal stromal cells (hBMSCs) were used for exosome isolation. Transmission electron microscopy (TEM) and confocal laser scanning microscopy were used for exosome and MDs morphological characterization, respectively. The MDs-NFATc1/PLA-Exo scaffold was fabricated through poly(dopamine) and fibrin gel coating. Biocompatibility was assessed using RAW 264.7 macrophages and hBMSCs. Osteoclast formations were examined via TRAP staining. Osteogenic differentiation of hBMSCs and cytokine expression modulation were also investigated.

Results

MSC-Exo exhibited a cup-shaped structure and effective internalization into cells, while MDs displayed a spherical morphology with a well-defined core-shell structure. Following ultrasound stimulation, the internalization study demonstrated efficient delivery of bioactive MDs into recipient cells. Biocompatibility studies indicated no cytotoxicity of MDs-NFATc1/PLA-Exo scaffolds in RAW 264.7 macrophages and hBMSCs. Both MDs-NFATc1/PLA and MDs-NFATc1/PLA-Exo treatments significantly reduced osteoclast differentiation and formation. In addition, our results further indicated MDs-NFATc1/PLA-Exo scaffold significantly enhanced osteogenic differentiation of hBMSCs and modulated cytokine expression.

Discussion

These findings suggest that the bioactive MDs-NFATc1/PLA-Exo scaffold holds promise as an innovative structure for bone tissue regeneration. By specifically targeting osteoclast formation and promoting osteogenic differentiation, this hybrid scaffold may address key challenges in osteoporosis-related fractures.

Hydroxychavicol Inhibits In Vitro Osteoclastogenesis via the Suppression of NF-κB Signaling Pathway

Hydroxychavicol, a primary active phenolic compound of betel leaves, previously inhibited bone loss in vivo by stimulating osteogenesis. However, the effect of hydroxychavicol on bone remodeling induced by osteoclasts is unknown. In this study, the anti-osteoclastogenic effects of hydroxychavicol and its mechanism were investigated in receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclasts. Hydroxychavicol reduced the number of tartrate resistance acid phosphatase (TRAP)-positive multinucleated, F-actin ring formation and bone-resorbing activity of osteoclasts differentiated from RAW264.7 cells in a concentration-dependent manner. Furthermore, hydroxychavicol decreased the expression of osteoclast-specific genes, including cathepsin K, MMP-9, and dendritic cell-specific transmembrane protein (DC-STAMP). For mechanistic studies, hydroxychavicol suppressed RANKL-induced expression of major transcription factors, including the nuclear factor of activated T-cells 1 (NFATc1), c-Fos, and c-Jun. At the early stage of osteoclast differentiation, hydroxychavicol blocked the phosphorylation of NF-κB subunits (p65 and Iκβα). This blockade led to the decrease of nuclear translocation of p65 induced by RANKL. In addition, the anti-osteoclastogenic effect of hydroxychavicol was confirmed by the inhibition of TRAP-positive multinucleated differentiation from human peripheral mononuclear cells (PBMCs). In conclusion, hydroxychavicol inhibits osteoclastogenesis by abrogating RANKL-induced NFATc1 expression by suppressing the NF-κB signaling pathway in vitro.

Rare coding variants in NOX4 link high ROS levels to psoriatic arthritis mutilans

Psoriatic arthritis mutilans (PAM) is the rarest and most severe form of psoriatic arthritis, characterized by erosions of the small joints and osteolysis leading to joint disruption. Despite its severity, the underlying mechanisms are unknown, and no susceptibility genes have hitherto been identified. We aimed to investigate the genetic basis of PAM by performing massive parallel sequencing in sixty-one patients from the PAM Nordic cohort. We found rare variants in the NADPH oxidase 4 (NOX4) in four patients. In silico predictions show that the identified variants are potentially damaging. NOXs are the only enzymes producing reactive oxygen species (ROS). NOX4 is specifically involved in the differentiation of osteoclasts, the cells implicated in bone resorption. Functional follow-up studies using cell culture, zebrafish models, and measurement of ROS in patients uncovered that these NOX4 variants increase ROS levels both in vitro and in vivo. We propose NOX4 as the first candidate susceptibility gene for PAM. Our study links high levels of ROS caused by NOX4 variants to the development of PAM, offering a potential therapeutic target.

Tm4sf19 deficiency inhibits osteoclast multinucleation and prevents bone loss

BACKGROUND

Multinucleation is a hallmark of osteoclast formation and has a unique ability to resorb bone matrix. During osteoclast differentiation, the cytoskeleton reorganization results in the generation of actin belts and eventual bone resorption. Tetraspanins are involved in adhesion, migration and fusion in various cells. However, its function in osteoclast is still unclear. In this study, we identified Tm4sf19, a member of the tetraspanin family, as a regulator of osteoclast function.

MATERIALS AND METHODS

We investigate the effect of Tm4sf19 deficiency on osteoclast differentiation using bone marrow-derived macrophages obtained from wild type (WT), Tm4sf19 knockout (KO) and Tm4sf19 LELΔ mice lacking the large extracellular loop (LEL). We analyzed bone mass of young and aged WT, KO and LELΔ mice by μCT analysis. The effects of Tm4sf19 LEL-Fc fusion protein were accessed in osteoclast differentiation and osteoporosis animal model.

RESULTS

We found that deficiency of Tm4sf19 inhibited osteoclast function and LEL of Tm4sf19 was responsible for its function in osteoclasts in vitro. KO and LELΔ mice exhibited higher trabecular bone mass compared to WT mice. We found that Tm4sf19 interacts with integrin αvβ3 through LEL, and that this binding is important for cytoskeletal rearrangements in osteoclast by regulating signaling downstream of integrin αvβ3. Treatment with LEL-Fc fusion protein inhibited osteoclast function in vitro and administration of LEL-Fc prevented bone loss in an osteoporosis mouse model in vivo.

CONCLUSION

We suggest that Tm4sf19 regulates osteoclast function and that LEL-Fc may be a promising drug to target bone destructive diseases caused by osteoclast hyper-differentiation.

Corylifol A suppresses osteoclastogenesis and alleviates ovariectomy-induced bone loss via attenuating ROS production and impairing mitochondrial function

Osteoporosis is a systemic disease characterized by an imbalance in bone homeostasis, where osteoblasts fail to fully compensate for the bone resorption induced by osteoclasts. Corylifol A, a flavonoid extracted from Fructus psoraleae, has been identified as a potential treatment for this condition. Predictions from network pharmacology and molecular docking studies suggest that Corylifol A exhibits strong binding affinity with NFATc1, Nrf2, PI3K, and AKT1. Empirical evidence from in vivo experiments indicates that Corylifol A significantly mitigates systemic bone loss induced by ovariectomy by suppressing both the generation and activation of osteoclasts. In vitro studies further showed that Corylifol A inhibited the activation of PI3K-AKT and MAPK pathways and calcium channels induced by RANKL in a time gradient manner, and specifically inhibited the phosphorylation of PI3K, AKT, GSK3 β, ERK, CaMKII, CaMKIV, and Calmodulin. It also diminishes ROS production through Nrf2 activation, leading to a decrease in the expression of key regulators such as NFATcl, C-Fos, Acp5, Mmp9, and CTSK that are involved in osteoclastogenesis. Notably, our RNA-seq analysis suggests that Corylifol A primarily impacts mitochondrial energy metabolism by suppressing oxidative phosphorylation. Collectively, these findings demonstrate that Corylifol A is a novel inhibitor of osteoclastogenesis, offering potential therapeutic applications for diseases associated with excessive bone resorption.

Activating transcriptional coactivator with PDZ-binding motif by (R)-PFI-2 attenuates osteoclastogenesis and prevents ovariectomized-induced osteoporosis

Excessive osteoclast activation is a leading cause of osteoporosis. Therefore, identifying molecular targets and relevant pharmaceuticals that inhibit osteoclastogenesis is of substantial clinical importance. Prior research has indicated that transcriptional coactivator with PDZ-binding motif (TAZ) impedes the process of osteoclastogenesis by engaging the nuclear factor (NF)-κB signaling pathway, thereby suggesting TAZ activation as a potential therapeutic approach to treat osteoporosis. (R)-PFI-2 is a novel selective inhibitor of SETD7 methyltransferase activity, which prevents the nuclear translocation of YAP, a homolog of TAZ. Therefore, we hypothesized that (R)-PFI-2 could be an effective therapeutic agent in the treatment of osteoporosis. To test this hypothesis and explore the underlying mechanism, we first examined the impact of (R)-PFI-2 on osteoclastogenesis in bone marrow macrophages (BMMs) in vitro. (R)-PFI-2 treatment inhibited TAZ phosphorylation induced by NF-κB, thereby enhancing its nuclear localization, protein expression, and activation in BMMs. Moreover, (R)-PFI-2-induced TAZ activation inhibited osteoclast formation in a dose-dependent manner, which involved inhibition of osteoclastogenesis through the TAZ and downstream NF-κB pathways. Furthermore, (R)-PFI-2 inhibited osteoclastogenesis and prevented ovariectomy-induced bone loss in vivo in a mouse model. Overall, our findings suggest that TAZ activation by (R)-PFI-2 inhibits osteoclastogenesis and prevents osteoporosis, indicating an effective strategy for treating osteoclast-induced osteoporosis.

Development of bioactive and ultrasound-responsive microdroplets for preventing ovariectomy (OVX)-induced osteoporosis

As a common bone disease in the elderly population, osteoporosis-related bone loss and bone structure deterioration represent a major public health problem. Therapeutic strategies targeting excessive osteoclast formation are frequently used for osteoporosis treatment; however, potential side effects have been recorded. Here, we have developed a novel therapeutic strategy using microdroplets (MDs) encapsulated with NFATc1-siRNA and investigated the role of bioactive MDs-NFATc1 biocompatibility in RAW 264.7 macrophages and human mesenchymal stem cells (hBMSCs), respectively. Its role in regulating osteoclast differentiation and formation was also investigated in vitro. We first fabricated MDs with spherical morphology along with a well-defined core-shell structure. The ultrasound-responsive study demonstrated time-dependent responsive structural changes following ultrasound stimulation. The internalization study into unstimulated macrophages, inflammatory macrophages, and hBMSCs indicated good delivery efficiency. Furthermore, the results from the MTT assay, the live/dead assay, and the cellular morphological analysis further indicated good biocompatibility of our bioactive MDs-NFATc1. Following MDs-NFATc1 treatment, the number of osteoclasts was greatly reduced, indicating their inhibitory effect on osteoclastogenesis and osteoclast formation. Subsequently, osteoporotic rats that underwent ovariectomy (OVX) were used for the in vivo studies. The rats treated with MDs-NFATc1 exhibited significant resistance to bone loss induced by OVX. In conclusion, our results demonstrate that MDs-NFATc1 could become an important regulator in osteoclast differentiation and functions, thus having potential applications in osteoclast-related bone diseases.

RNF125, transcriptionally regulated by NFATC2, alleviates osteoarthritis via inhibiting the Wnt/β-catenin signaling pathway through degrading TRIM14

Osteoarthritis (OA) is a chronic joint disease characterized by the progressive degradation of articular cartilage. In this study, as determined by histological staining, the cartilage surface of the OA rats was damaged, defective and broken, and chondrocytes and proteoglycan were reduced. While moderate physical exercise showed protective effects on the cartilage. Besides, RNA-seq was performed to select a target protein and RNF125 (an E3 ubiquitin ligase) was decreased in the cartilage tissues of OA rats and increased after physiological exercise. However, the precise role of RNF125 in OA is still unknown. This work aimed to investigate the involvement and underlying mechanism of RNF125 in OA pathogenesis. Our results defined that adenovirus-mediated overexpression of RNF125 inhibited the degradation of extracellular matrix of chondrocytes induced by IL-1β, as revealed by increased chondrocyte viability, upregulated COL2A1 and ACAN levels, and downregulated MMP1, MMP13 and ADAMTS5 levels, which was abrogated by NR4A2 knockdown. In vivo, RNF125 relieved OA, manifested as reduced cartilage injury and increased chondrocytes. Mechanically, NFATC2 bound to the RNF125 promoter and directly regulated RNF125 transcription, as illustrated by luciferase reporter, Ch-IP and DNA pull-down assays. Furthermore, RNF125 overexpression inhibited the nuclear translocation of β-catenin, thus suppressing activation of the Wnt/β-catenin signaling pathway. Also, RNF125 as E3 ubiquitin ligase led to the ubiquitination and degradation of TRIM14 protein, and TRIM14 overexpression efficiently reversed the effects of RNF125 overexpression on OA progression. Totally, this study provides new insights into OA pathogenesis regulated by RNF125. RNF125 may be a novel biomarker for OA therapy.

Efficacy and Toxicity of Calcitonin Treatment in Children with Cherubism: A Single-Center Cohort Study

Cherubism is a rare autosomal dominant disease characterized by expansile osteolytic jawbone lesions. The effect and safety of off-label calcitonin treatment during the progressive phase of the disease are not well described. In this retrospective study, we present data on the radiological response and adverse effects of subcutaneously administered calcitonin in a cohort of nine cherubism children (three female, six male). Two of the nine patients underwent two separate treatment courses with a significant off-treatment interval in between; therefore, a total of 11 treatment courses with a mean duration of 17.9 months (range <1 to 35, SD 10.8) were studied. To measure the response, the cumulative volume of cherubism lesions was calculated from available three-dimensional imaging. The primary outcome was the change in the volume of lesions during calcitonin treatment and only assessed for the eight treatment courses with a minimal duration of 6 months. A statistically significant reduction in the mean cumulative volume of lesions was seen regardless of treatment duration. Average volume reduction was highest in the first half year of treatment, with a gradual, ongoing reduction thereafter. For the secondary outcome, the change in the cumulative volume of lesions after treatment cessation was assessed for the seven treatment courses with follow-up imaging available. After six of these seven treatment courses, the cumulative volume increased again but remained undoubtedly smaller than the initial volume at the start of therapy. Adverse effects were assessed for all 11 treatment courses and occurred in 73% of them. Most adverse effects were mild and low grade, with the most severe being one grade 3 symptomatic hypocalcemia requiring hospitalization and early treatment termination. Calcitonin treatment seems effective and tolerable in treating actively progressing cherubism in children. However, further research is required to better understand the pharmacological treatment of cherubism, including also other drugs, dosing, and protocols. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Synthesis and evaluation of 2-NMPA derivatives as potential agents for prevention of osteoporosis in vitro and in vivo

Abnormal osteoclast differentiation causes various bone disorders such as osteoporosis. Targeting the formation and activation of osteoclasts has been recognized as an effective approach for preventing osteoporosis. Herein, we synthesized eleven 2-NMPA derivatives which are (2-(2-chlorophenoxy)-N-(4-alkoxy-2-morpholinophenyl) acetamides, and evaluated their suppression effects on osteoclastogenesis in vitro by using TRAP-staining assay. Among the synthesized eleven novel 2-NMPAs, 4-(2-(2-chlorophenoxy)acetamido)-3-morpholinophenyl trifluoromethanesulfonate (11b), 4-(2-(2-chlorophenoxy) acetamido)-3-morpholinophenyl-3-(N-(2-oxo-2-((2-(phenylthio) phenyl) amino) ethyl)methylsulfonamido)benzoate (11d), and 4-(2-(2-chlorophenoxy) acetamido)-3-morpholinophenyl 4-acetamidobenzenesulfonate (11h) displayed highly inhibitory bioactivity on the differentiation of primary osteoclasts. 11h was selected for further investigation of the inhibitory effects and potential mechanism involved in the suppression of osteoclastogenesis. In vitro analysis suggested that 11h inhibited osteoclastogenesis with an IC50 of 358.29 nM, decreased the formation of F-action belts and bone resorption, without interfering cell viability and osteoblast differentiation. Furthermore, the mRNA expressions of osteoclast-specific genes such as Acp5, Nfatc1, Dc-stamp, Atp6v0d2, Mmp9, and Ctsk significantly decreased following 11h treatment. RANKL-induced osteoclast-specific proteins analysis demonstrated that 11h suppressed osteoclast differentiation by downregulating of RANKL-mediated TRAF6 expression, followed by inactivation of PI3K/AKT and IκBα/NF-κB signaling pathways. Finally, 11h inhibited ovariectomy-induced bone loss in vivo. Therefore, the current work highlighted the therapeutic potential of 11h as an anti-osteoporosis lead compound.

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.

A novel Glycyrrhiza glabra extract liquiritin targeting NFATc1 activity and ROS levels to counteract ovariectomy-induced osteoporosis and bone loss in murine model

Osteoporosis, a prevalent osteolytic condition worldwide, necessitates effective strategies to inhibit excessive bone resorption by curbing osteoclast hyperactivation. Liquiritin (LIQ), an flavanone derivative employed in acute lung injury and rheumatoid arthritis treatment, possesses an unclear role in addressing excessive bone resorption. In this investigation, we found that LIQ demonstrates the ability to inhibit osteoclast formation and the bone-resorbing activity induced by RANKL. At a specific concentration, LIQ significantly attenuated NF-κB-Luc activity induced by RANKL and curtailed NF-κB activation in RANKL-stimulated RAW264.7 cells, resulting in reduced IκB-α breakdown and diminished nuclear NF-κB levels. Furthermore, LIQ markedly inhibited RANKL-induced NFATc1 activation, as evidenced by diminished NFATc1 luciferase activity, reduced NFATc1 mRNA levels, and decreased nuclear NFATc1 protein levels. Subsequent experiments demonstrated that LIQ effectively restrained the RANKL-induced elevation of intracellular calcium as well as reactive oxygen species. Additionally, LIQ exhibited a downregulating effect on the expression of osteoclast-specific genes, which include Acp5, Cathepsin K, Atp6v0d2, Nfatc1, c-Fos, and Mmp9. Notably, our findings revealed the potential of LIQ to counteract decreased bone density in mice that underwent ovariectomy. Collectively, the data indicate that LIQ impedes osteoclast formation triggered by RANKL and the subsequent reduction in bone mass by mitigating ROS levels and suppressing the Ca2+/MAPK-NFATc1 signaling pathway, suggesting its promising candidacy as a therapeutic agent for RANKL-mediated osteoporosis.

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

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

CR6-Interacting Factor-1 Promotes Osteoclastogenesis Through the NF-κB Signaling Pathway after Irradiation

Radiation exposure arising from radiotherapy may induce rapid bone loss and an increase in the extent of bone resorption. Reactive oxygen species (ROS) caused by radiation exposure play a crucial role during the process of osteoclastogenesis. However, the pathological mechanisms underlying radiation-induced osteoclastogenesis have yet to be fully elucidated. CR6-interacting factor-1 (Crif1) as a multifunctional protein is involved in regulating multiple biological functions in cells. Here, we investigated the role of Crif1 in radiation-induced osteoclastogenesis and found that radiation exposure induced an increase in the expression level of Crif1 and enhanced osteoclastogenesis in osteoclast progenitors. Crif1 and NF-κB p65 co-localized in the cytoplasm after radiation exposure. Crif1 knockdown did not affect the phosphorylation and total protein levels of extracellular signal-regulated kinases (ERK), c-Jun amino (N)-terminal kinases (JNK), p38, and IκB-α before and after irradiation. However, Crif1 knockdown did lead to the reduced phosphorylation and nuclear translocation of NF-κB p65 after irradiation and resulted in a reduced level of osteoclastogenesis in RAW264.7 cells after irradiation. In vivo studies involving Lyz2Cre;Crif1fl/fl mice possessing the myeloid-specific deletion of Crif1 demonstrated the alleviation of bone loss after irradiation when compared with Crif1fl/fl mice. Our findings demonstrate that Crif1 mediated the phosphorylation and nuclear translocation of NF-κB p65 and promoted osteoclastogenesis via the NF-κB signaling pathway after radiation exposure. Thus, our analysis revealed a specific role for Crif1 in the mediation of radiation-induced bone loss and may provide new insight into potential therapeutic strategies for radiation-induced bone loss.

5'-Methylthioadenosine strongly suppresses RANKL-induced osteoclast differentiation and function via inhibition of RANK-NFATc1 signalling pathways

Excessive osteoclast-mediated bone resorption is a critical cause of osteoporosis affecting many aging people worldwide. 5'-Methylthioadenosine (MTA) is a natural sulfur-containing nucleoside normally produced in prokaryotes, plants, yeast, and higher eukaryotes via polyamine metabolism. MTA affects various physiological responses particularly the inflammatory pathway in both normal and cancerous cells and modulates the activation of nuclear factor-κB involved in the osteoclastogenesis signalling process. While several studies have reported that natural products possess anti-osteoclastogenesis phenolics and flavonoids, the effect of nucleoside derivatives on osteoclastogenesis remains limited. Therefore, this study aimed to explore the molecular mechanisms by which MTA affects pre-osteoclastic RAW 264.7 cells as a potential alleviation compound for inflammation-mediated bone loss. Osteoclasts were established by incubating RAW264.7 macrophage cells with receptor activator of nuclear factor kappa B ligand (RANKL) and macrophage colony-stimulating factor, the vital cytokines for activation of osteoclast differentiation. Cell viability was measured using MTT assays at 24, 48, and 72 h. The suppressive effect of MTA on RANKL-induced osteoclast differentiation and function was assessed using tartrate-resistant acid phosphatase (TRAP) analysis, qRT-PCR, and pit formation, Western blot, and immunofluorescence assays. MTA showed dose-dependent anti-osteoclastogenic activity by inhibiting TRAP-positive cell and pit formation and reducing essential digestive enzymes, including TRAP, cathepsin K, and matrix metallopeptidase 9. MTA was observed to suppress the osteoclast transduction pathway through (RANKL)-induced nuclear factor kappa-light-chain-enhancer of activated B cells (NFƘB); it attenuated NFƘB-P65 expression and down-regulated cFos proto-oncogene and nuclear factor of activated T cell c1 (NFATc1), the main regulators of osteoclasts. Moreover, the suppression of RANK (the initial receptor triggering several osteoclastogenic transduction pathways) was observed. Thus, this study highlights the potential of MTA as an effective therapeutic compound for restoring bone metabolic disease by inhibiting the RANK-NFATc1 signal pathway.

Essentiality of Nfatc1 short isoform in osteoclast differentiation and its self-regulation

During osteoclast differentiation, the expression of the transcription factor nuclear factor of activated T cell 1 (Nfatc1) increases in an autoproliferative manner. Nfatc1 isoforms are of three sizes, and only the short isoform increases during osteoclast differentiation. Genetic ablation of the whole Nfatc1 gene demonstrated that it is essential for osteoclastogenesis; however, the specific role of the Nfatc1 short form (Nfatc1/αA) remains unknown. In this study, we engineered Nfatc1 short form-specific knockout mice and found that these mice died in utero by day 13.5. We developed a novel osteoclast culture system in which hematopoietic stem cells were cultured, proliferated, and then differentiated into osteoclasts in vitro. Using this system, we show that the Nfatc1/αA isoform is essential for osteoclastogenesis and is responsible for the expression of various osteoclast markers, the Nfatc1 short form itself, and Nfatc1 regulators.

The effect of the Litcubanine A on the treatment of murine experimental periodontitis by inhibiting monocyte-macrophage chemotaxis and osteoclast differentiation

BACKGROUND

Periodontal disease is an inflammatory disease of periodontal tissues that is closely connected with systemic diseases. During periodontitis, the inappropriate recruitment and activation of monocytes-macrophages causes an increase in osteoclast activity and disrupts bone homeostasis. Therefore, it is a promising therapeutic strategy to treat periodontitis by regulating the functions of monocytes-macrophages. Litcubanine A (LA) is an isoquinoline alkaloid extracted from the traditional Chinese medicine Litsea cubeba, which was proven to have reproducible anti-inflammatory effects, but its regulatory role on bone homeostasis in periodontitis is still not clear.

METHODS

In this study, zebrafish experiments and a mouse ligature-induced periodontitis model were performed, and histological analysis was used to investigate the effect of LA on macrophage chemotaxis under the inflammatory environment. Real-time PCR was used to detect the regulatory effect of LA (100 nM ~ 100 μM) on the chemotaxis function of macrophages induced by LPS. Apoptosis assay and flow cytometry were used to elucidate the influence of LA on macrophage apoptosis and proliferation. To further clarify the regulatory role of LA on macrophage osteoclast differentiation, real-time PCR, histological analysis, western blot, and micro-computed tomography (micro-CT) were performed in vivo and in vitro to verify the impact of LA on bone homeostasis.

RESULTS

Compared with the control group, the chemotaxis function of macrophage was significantly attenuated by LA in vivo. LA could significantly inhibit the expression of genes encoding the chemokine receptors Ccr1 and Cxcr4, and its ligand chemokine Cxcl12 in macrophages, and suppresses the differentiation of osteoclastic precursors to osteoclasts through the MAPK signaling pathway. There were significantly lower osteoclast differentiation and bone loss in the LA group compared with the control in the ligature-induced periodontitis model.

CONCLUSION

LA is a promising candidate for the treatment of periodontitis through its reproducible functions of inhibiting monocyte-macrophage chemotaxis and osteoclast differentiation.

Pharmacological inhibition of protein S-palmitoylation suppresses osteoclastogenesis and ameliorates ovariectomy-induced bone loss

Background

Excessive osteoclast formation disrupts bone homeostasis, thereby significantly contributing to pathological bone loss associated with a variety of diseases. Protein S-palmitoylation is a reversible post-translational lipid modification catalyzed by ZDHHC family of palmitoyl acyltransferases, which plays an important role in various physiological and pathological processes. However, the role of palmitoylation in osteoclastogenesis has never been explored. Consequently, it is unclear whether this process can be targeted to treat osteolytic bone diseases that are mainly caused by excessive osteoclast formation.

Materials and methods

In this study, we employed acyl-biotin exchange (ABE) assay to reveal protein S-palmitoylation in differentiating osteoclasts (OCs). We utilized 2-bromopalmitic acid (2-BP), a pharmacological inhibitor of protein S-palmitoylation, to inhibit protein palmitoylation in mouse bone marrow-derived macrophages (BMMs), and tested its effect on receptor activator of nuclear factor κβ ligand (RANKL)-induced osteoclast differentiation and activity by TRAP staining, phalloidin staining, qPCR analyses, and pit formation assays. We also evaluated the protective effect of 2-BP against estrogen deficiency-induced bone loss and bone resorption in ovariectomized (OVX) mice using μCT, H&E staining, TRAP staining, and ELISA assay. Furthermore, we performed western blot analyses to explore the molecular mechanism underlying the inhibitory effect of 2-BP on osteoclastogenesis.

Results

We found that many proteins were palmitoylated in differentiating OCs and that pharmacological inhibition of palmitoylation impeded RANKL-induced osteoclastogenesis, osteoclast-specific gene expression, F-actin ring formation and osteoclastic bone resorption in vitro, and to a lesser extent, osteoblast formation from MC3T3-E1 cells. Furthermore, we demonstrated that administration of 2-BP protected mice from ovariectomy-induced osteoporosis and bone resorption in vivo. Mechanistically, we showed that 2-BP treatment inhibited osteoclastogenesis partly by downregulating the expression of c-Fos and NFATc1 without overtly affecting RANKL-induced activation of osteoclastogenic AKT, MAPK, and NF-κB pathways.

Conclusion

Pharmacological inhibition of palmitoylation potently suppresses RANKL-mediated osteoclast differentiation in vitro and protects mice against OVX-induced osteoporosis in vivo. Mechanistically, palmitoylation regulates osteoclast differentiation partly by promoting the expression of c-Fos and NFATc1. Thus, palmitoylation plays a key role in promoting osteoclast differentiation and activity, and could serve as a potential therapeutic target for the treatment of osteoporosis and other osteoclast-related diseases.

The translational potential of this article

The translation potential of this article is that we first revealed palmitoylation as a key mechanism regulating osteoclast differentiation, and therefore provided a potential therapeutic target for treating osteolytic bone diseases.

Accelerated Bone Loss in Transgenic Mice Expressing Constitutively Active TGF-β Receptor Type I

Transforming growth factor beta (TGF-β) is a key factor mediating the intercellular crosstalk between the hematopoietic stem cells and their microenvironment. Here, we investigated the skeletal phenotype of transgenic mice expressing constitutively active TGF-β receptor type I under the control of Mx1-Cre (Mx1;TβRI^CA mice). μCT analysis showed decreased cortical thickness, and cancellous bone volume in both femurs and mandibles. Histomorphometric analysis confirmed a decrease in cancellous bone volume due to increased osteoclast number and decreased osteoblast number. Primary osteoblasts showed decreased ALP and mineralization. Constitutive TβRI activation increased osteoclast differentiation. qPCR analysis showed that Tnfsf11/Tnfrsf11b ratio, Ctsk, Sufu, and Csf1 were increased whereas Runx2, Ptch1, and Ptch2 were decreased in Mx1;TβRI^CA femurs. Interestingly, Gli1, Wnt3a, Sp7, Alpl, Ptch1, Ptch2, and Shh mRNA expression were reduced whereas Tnfsf11/Tnfrsf11b ratio was increased in Mx1;TβRI^CA mandibles. Similarly, osteoclast-related genes were increased in Mx1;TβRI^CA osteoclasts whereas osteoblast-related genes were reduced in Mx1;TβRI^CA osteoblasts. Western blot analysis indicated that SMAD2 and SMAD3 phosphorylation was increased in Mx1;TβRI^CA osteoblasts, and SMAD3 phosphorylation was increased in Mx1;TβRI^CA osteoclasts. CTSK was increased while RUNX2 and PTCH1 was decreased in Mx1;TβRI^CA mice. Microindentation analysis indicated decreased hardness in Mx1;TβRI^CA mice. Our study indicated that Mx1;TβRI^CA mice were osteopenic by increasing osteoclast number and decreasing osteoblast number, possibly by suppressing Hedgehog signaling pathways.

Craniofacial dysmorphology in Down syndrome is caused by increased dosage of Dyrk1a and at least three other genes

Down syndrome (DS), trisomy of human chromosome 21 (Hsa21), occurs in 1 in 800 live births and is the most common human aneuploidy. DS results in multiple phenotypes, including craniofacial dysmorphology, which is characterised by midfacial hypoplasia, brachycephaly and micrognathia. The genetic and developmental causes of this are poorly understood. Using morphometric analysis of the Dp1Tyb mouse model of DS and an associated mouse genetic mapping panel, we demonstrate that four Hsa21-orthologous regions of mouse chromosome 16 contain dosage-sensitive genes that cause the DS craniofacial phenotype, and identify one of these causative genes as Dyrk1a. We show that the earliest and most severe defects in Dp1Tyb skulls are in bones of neural crest (NC) origin, and that mineralisation of the Dp1Tyb skull base synchondroses is aberrant. Furthermore, we show that increased dosage of Dyrk1a results in decreased NC cell proliferation and a decrease in size and cellularity of the NC-derived frontal bone primordia. Thus, DS craniofacial dysmorphology is caused by an increased dosage of Dyrk1a and at least three other genes.

Protein kinase D3 conditional knockout impairs osteoclast formation and increases trabecular bone volume in male mice

Studies using kinase inhibitors have shown that the protein kinase D (PRKD) family of serine/threonine kinases are required for formation and function of osteoclasts in culture. However, the involvement of individual protein kinase D genes and their in vivo significance to skeletal dynamics remains unclear. In the current study we present data indicating that protein kinase D3 is the primary form of PRKD expressed in osteoclasts. We hypothesized that loss of PRKD3 would impair osteoclast formation, thereby decreasing bone resorption and increasing bone mass. Conditional knockout (cKO) of Prkd3 using a murine Cre/Lox system driven by cFms-Cre revealed that its loss in osteoclast-lineage cells reduced osteoclast differentiation and resorptive function in culture. Examination of the Prkd3 cKO mice showed that bone parameters were unaffected in the femur at 4 weeks of age, but consistent with our hypothesis, Prkd3 conditional knockout resulted in 18 % increased trabecular bone mass in male mice at 12 weeks and a similar increase at 6 months. These effects were not observed in female mice. As a further test of our hypothesis, we asked if Prkd3 cKO could protect against bone loss in a ligature-induced periodontal disease model but did not see any reduction in bone destruction in this system. Together, our data indicate that PRKD3 promotes osteoclastogenesis both in vitro and in vivo.

RhoA promotes osteoclastogenesis and regulates bone remodeling through mTOR-NFATc1 signaling

BACKGROUND

The cytoskeletal architecture of osteoclasts (OCs) and bone resorption activity must be appropriately controlled for proper bone remodeling, which is associated with osteoporosis. The RhoA protein of GTPase plays a regulatory role in cytoskeletal components and contributes to osteoclast adhesion, podosome positioning, and differentiation. Although osteoclast investigations have traditionally been performed by in vitro analysis, however, the results have been inconsistent, and the significance of RhoA in bone physiology and pathology is still unknown.

METHODS

We generated RhoA knockout mice by specifically deleting RhoA in the osteoclast lineage to understand more about RhoA's involvement in bone remodeling. The function of RhoA in osteoclast differentiation and bone resorption and the mechanisms were assessed using bone marrow macrophages (BMMs) in vitro. The ovariectomized (OVX) mouse model was adopted to examine the pathological effect of RhoA in bone loss.

RESULTS

Conditional deletion of RhoA in the osteoclast lineage causes a severe osteopetrosis phenotype, which is attributable to a bone resorption suppression. Further mechanistic studies suggest that RhoA deficiency suppresses Akt-mTOR-NFATc1 signaling during osteoclast differentiation. Additionally, RhoA activation is consistently related to the significant enhancement the osteoclast activity, which culminates in the development of an osteoporotic bone phenotype. Furthermore, in mice, the absence of RhoA in osteoclast precursors prevented occurring OVX-induced bone loss.

CONCLUSION

RhoA promoted osteoclast development via the Akt-mTOR-NFATc1 signaling pathway, resulting a osteoporosis phenotype, and that manipulating RhoA activity might be a therapeutic strategy for osteoporotic bone loss.

Cedrol, a Ginger-derived sesquiterpineol, suppresses estrogen-deficient osteoporosis by intervening NFATc1 and reactive oxygen species

Osteoporosis is a prevalent bone metabolic disease in menopause, and long-term medication is accompanied by serious side effects. Ginger, a food spice and traditional medicine with ancient history, exhibits the potential to alleviate osteoporosis in preclinical experiments, whereas its complex composition leads to ambiguous pharmacological mechanisms. The purpose of this study was to investigate the effect and mechanism of Ced in estrogen-deficient osteoporosis, a sesquiterpene alcohol recently discovered from Ginger with multiple pharmacological properties. RANKL was stimulated BMM (bone marrow macrophages) differentiation into osteoclasts in vitro. And the osteoclast activity and number were assessed by TRAcP and SEM. We found that Ced mitigated RANKL-induced osteoclastogenesis by descending the ROS content and obstructing NFATc1, NF-κB, and MAPK signaling. Also, Ced-mediated anti-osteolytic property was found in ovariectomized mice by Micro-CT scanning and histological staining. Summarily, our works demonstrated the anti-osteoporotic potential of Cedrol in Ginger for the first time, which also offered more pharmacological evidence for Ginger as food or medicine used for bone metabolic disease.

Isosinensetin alleviates estrogen deficiency-induced osteoporosis via suppressing ROS-mediated NF-κB/MAPK signaling pathways

The formation of osteoclasts and their hyperactive bone resorption are related to the aggregation of intracellular reactive oxygen species (ROS). Flavonoids, derived from plant active ingredients, can alleviate the symptoms of osteoporosis (OP). Isosinensetin (Iss) is a flavonoid with antioxidant effects obtained mainly from citrus fruits, and its effect on osteoclastogenesis has not been reported. In this study, we investigated the antioxidant activity of Iss on osteoclast differentiation and function, as well as the therapeutic impact of Iss on OP. We found that Iss inhibited osteoclastogenesis and suppressed the bone resorption function of osteoclasts. Additionally, Iss reduced receptor activator of nuclear factor-κB ligand (RANKL)-induced intracellular ROS. Using quantitative real-time polymerase chain reaction and western blot, we further found that Iss inhibited osteoclast-specific genes and related proteins, while promoting the expression of antioxidant enzyme-related genes and proteins. Mechanistically, Iss reduces intracellular ROS by activating nuclear factor-erythroid 2-related factor 2 (Nrf2) and its related antioxidant enzymes and inhibits the downstream nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways of ROS, which in turn inhibits nuclear factor of activated T cells 1 (NFATc1), and ultimately inhibits osteoclastogenesis. In vivo, by micro-computed tomography (Micro-CT) assay and histological analyses, we found that Iss could reduce bone loss in ovariectomized (OVX) mice. Therefore, Iss has the potential as an OP preventative and therapeutic drug option.

NFATc1 induction by an intronic enhancer restricts NKT γδ cell formation

In thymus, the ablation of T cell receptor (TCR)-activated transcription factor NFATc1 or its inducible isoforms during the double-negative (DN) stages of thymocyte development leads to a marked increase in γδ thymocytes whereas the development of αβ thymocytes remains mostly unaffected. These γδ thymocytes are characterized by the upregulation of the promyelocytic leukemia zinc-finger factor (PLZF), the "master regulator" of natural killer T (NKT) cell development, and the acquisition of an NKT γδ cell phenotype with higher cell survival rates. The suppressive function of NFATc1 in NKT γδ cell formation critically depends on the remote enhancer E2, which is essential for the inducible expression of NFATc1 directed by its distal promoter P1. Thus, the enhancer deciphers a strong γδ TCR signal into the expression of inducible NFATc1 isoforms resulting in high levels of NFATc1 protein that are essential to control the numbers of NKT γδ cells.

High Bone Mass Disorders: New Insights From Connecting the Clinic and the Bench

Monogenic high bone mass (HBM) disorders are characterized by an increased amount of bone in general, or at specific sites in the skeleton. Here, we describe 59 HBM disorders with 50 known disease-causing genes from the literature, and we provide an overview of the signaling pathways and mechanisms involved in the pathogenesis of these disorders. Based on this, we classify the known HBM genes into HBM (sub)groups according to uniform Gene Ontology (GO) terminology. This classification system may aid in hypothesis generation, for both wet lab experimental design and clinical genetic screening strategies. We discuss how functional genomics can shape discovery of novel HBM genes and/or mechanisms in the future, through implementation of omics assessments in existing and future model systems. Finally, we address strategies to improve gene identification in unsolved HBM cases and highlight the importance for cross-laboratory collaborations encompassing multidisciplinary efforts to transfer knowledge generated at the bench to the clinic. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

NFATC1 and NFATC2 expression patterns in human osteochondromas

Background

Our previous study in genetic mouse models found that NFATc1 and NFATc2 suppress osteochondroma formation from entheseal progenitors. However, it remains unclear whether NFAT signaling is also involved in human osteochondromagenesis. As the first step in addressing this question, the current study aimed to determine the expression patterns of NFATC1 and NFATC2 in human osteochondroma samples.

Methods

Immunohistochemistry (IHC) was used to examine and analyze NFATC1 and NFATC2 expression in human osteochondroma samples. The human periosteum was used to map the expression of NFATC1 under physiological conditions by IHC. Furthermore, human periosteal progenitors were isolated and identified from the periosteal tissues of bone fracture healing patients. The expression of NFATC1 in human periosteal progenitors was characterized by Western blotting compared to human bone marrow stromal cells (BMSC).

Results

The IHC results showed that the expression of NFATC1 was undetectable in most human osteochondromas cells, and only a small proportion of osteochondroma cells, especially clonally grown chondrocytes, showed positive staining of NFATC1. NFATC2 expression was also undetectable in most chondrocytes in human osteochondromas. The mouse and human periosteum showed a comparable ratio of NFATC1 positive cells (9.56 ± 0.80% vs 11.04 ± 2.05%, P = 0.3101). Furthermore, Western blotting analysis revealed that NFATC1 expression was highly enriched in human periosteal progenitors compared to BMSC.

Conclusions

NFATC1 and NFATC2 are undetectable in most human osteochondroma chondrocytes. The expression pattern of NFATC1 in human osteochondromas and the normal periosteum suggests that NFAT signaling could be suppressed during human osteochondromagenesis.

FTY720 Attenuates LPS-Induced Inflammatory Bone Loss by Inhibiting Osteoclastogenesis via the NF-κB and HDAC4/ATF Pathways

Osteoclast (OC) abnormalities lead to many osteolytic diseases, such as osteoporosis, inflammatory bone erosion, and tumor-induced osteolysis. Exploring effective strategies to remediate OCs dysregulation is essential. FTY720, also known as fingolimod, has been approved for the treatment of multiple sclerosis and has anti-inflammatory and immunosuppressive effects. Here, we found that FTY720 inhibited osteoclastogenesis and OC function by inhibiting nuclear factor kappa-B (NF-κB) signaling. Interestingly, we also found that FTY720 inhibited osteoclastogenesis by upregulating histone deacetylase 4 (HDAC4) expression levels and downregulating activating transcription factor 4 (ATF4) expression levels. In vivo, FTY720 treatment prevented lipopolysaccharide- (LPS-) induced calvarial osteolysis and significantly reduced the number of tartrate-resistant acid phosphatase- (TRAP-) positive OCs. Taken together, these results demonstrate that FTY720 can inhibit osteoclastogenesis and ameliorate inflammation-induced bone loss. Which may provide evidence of a new therapeutic target for skeletal diseases caused by OC abnormalities.

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

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

Effects of chondroitin sulfate oligosaccharides on osteoclast differentiation of RAW264 cells, and myotube differentiation of C2C12 cells

Chondroitin sulfate (CS) is a glycosaminoglycan, and CS derived from various animal species is used in drugs and food supplements to alleviate arthralgia. The CS is a high molecular weight compound, and hydrolysis of CS by intestinal microbiota is thought to be required for absorption in mammalians. Chondroitin sulfate oligosaccharides (Oligo-CS) are produced by hydrolysis with subcritical water from CS isolated from a species of skate, Raja pulchra for the improvement of bioavailability. The present study conducted in vitro experiments using murine cell lines, to compare the biological activities of Oligo-CS and high molecular weight CS composed with the similar disaccharide isomer units of D-glucuronic acid and N-acetyl-D-glucosamine (CS-C). The results show that Oligo-CS inhibits osteoclast differentiation of RAW264 cells significantly at lower concentrations than in CS. The cell viability of a myoblast cell line, C2C12 cells, was increased when the cells were grown in a differentiated medium for myotubes with Oligo-CS, where there were no effects on the cell viability in CS. These results suggest that in vitro Oligo-CS exhibits stronger bioactivity than high-molecular weight CS.

Macrophage P2Y6 Receptor Signaling Selectively Activates NFATC2 and Suppresses Allergic Lung Inflammation

Innate immune responses to innocuous Ags can either prevent or facilitate adaptive type 2 allergic inflammation, but the mechanisms are incompletely understood. We now demonstrate that macrophage UDP-specific type 6 purinergic (P2Y6) receptors selectively activate NFATC2, a member of the NFAT family, to drive an innate IL-12/IFN-γ axis that prevents type 2 allergic inflammation. UDP priming potentiated IL-12p40 production in bone marrow-derived macrophages (BMMs) stimulated by the house dust mite Dermatophagoides farinae (Df) in a P2Y6-dependent manner. Inhibitions of phospholipase C, calcium increase, and calcineurin eliminated UDP-potentiated Df-induced IL-12p40 production. UDP specifically induced nuclear translocation of NFATC2, but not NFATC1 and NFATC3, in BMMs in a P2Y6-dependent manner. UDP-potentiated IL-12p40 production by BMMs and Df-induced IL-12p40 gene expression by alveolar macrophages were abrogated in cells from Nfatc2 knockout mice. Pulmonary transplantation of wild-type but not Nfatc2 knockout macrophages increased Df-induced IL-12 production and IFN-γ expression in P2ry6 fl/fl/Cre/+ recipient mice. Finally, Nfatc2 knockout mice showed significantly increased indices of type 2 immunopathology in response to Df challenge, similar to P2ry6 fl/fl/Cre/+ mice. Thus, macrophage P2Y6 receptor signaling selectively utilizes NFATC2 to potentiate an innate IL-12/IFN-γ axis, a potential mechanism that protects against inappropriate type 2 immune responses.

NFATc1 signaling drives chronic ER stress responses to promote NAFLD progression

OBJECTIVES

Non-alcoholic fatty liver disease (NAFLD) can persist in the stage of simple hepatic steatosis or progress to steatohepatitis (NASH) with an increased risk for cirrhosis and cancer. We examined the mechanisms controlling the progression to severe NASH in order to develop future treatment strategies for this disease.

DESIGN

NFATc1 activation and regulation was examined in livers from patients with NAFLD, cultured and primary hepatocytes and in transgenic mice with differential hepatocyte-specific expression of the transcription factor (Alb-cre, NFATc1^c.a . and NFATc1^Δ/Δ ). Animals were fed with high-fat western diet (WD) alone or in combination with tauroursodeoxycholic acid (TUDCA), a candidate drug for NAFLD treatment. NFATc1-dependent ER stress-responses, NLRP3 inflammasome activation and disease progression were assessed both in vitro and in vivo.

RESULTS

NFATc1 expression was weak in healthy livers but strongly induced in advanced NAFLD stages, where it correlates with liver enzyme values as well as hepatic inflammation and fibrosis. Moreover, high-fat WD increased NFATc1 expression, nuclear localisation and activation to promote NAFLD progression, whereas hepatocyte-specific depletion of the transcription factor can prevent mice from disease acceleration. Mechanistically, NFATc1 drives liver cell damage and inflammation through ER stress sensing and activation of the PERK-CHOP unfolded protein response (UPR). Finally, NFATc1-induced disease progression towards NASH can be blocked by TUDCA administration.

CONCLUSION

NFATc1 stimulates NAFLD progression through chronic ER stress sensing and subsequent activation of terminal UPR signalling in hepatocytes. Interfering with ER stress-responses, for example, by TUDCA, protects fatty livers from progression towards manifest NASH.

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

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

Genistein Modified with 8-Prenyl Group Suppresses Osteoclast Activity Directly via Its Prototype but Not Metabolite by Gut Microbiota

Postmenopausal osteoporosis is a significant threat to human health globally. Genistein, a soy-derived isoflavone, is regarded as a promising anti-osteoporosis drug with the effects of promoting osteoblastogenesis and suppressing osteoclastogenesis. However, its oral bioavailability (6.8%) is limited by water solubility, intestinal permeability, and biotransformation. Fortunately, 8-prenelylated genistein (8PG), a derivative of genistein found in Erythrina Variegate, presented excellent predicted oral bioavailability (51.64%) with an improved osteoblastogenesis effect, although its effects on osteoclastogenesis and intestinal biotransformation were still unclear. In this study, an in vitro microbial transformation platform and UPLC-QTOF/MS analysis method were developed to explore the functional metabolites of 8PG. RANKL-induced RAW264.7 cells were utilized to evaluate the effects of 8PG on osteoclastogenesis. Our results showed that genistein was transformed into dihydrogenistein and 5-hydroxy equol, while 8PG metabolites were undetectable under the same conditions. The 8PG (10^-6 M) was more potent in inhibiting osteoclastogenesis than genistein (10^-5 M) and it down-regulated NFATC1, cSRC, MMP-9 and Cathepsin K. It was concluded that 8-prenyl plays an important role in influencing the osteoclast activity and intestinal biotransformation of 8PG, which provides evidence supporting the further development of 8PG as a good anti-osteoporosis agent.

Lophatherum gracile Bronghiart Suppresses Receptor Activator of Nuclear Factor Kappa-B Ligand-Stimulated Osteoclastogenesis and Prevents Ovariectomy-Induced Osteoporosis

Lophatherum gracile Bronghiart, used in traditional herbal medicine, has many biological properties including antiviral, antipyretic, antitumor, vasorelaxation, and neutrophilic inflammatory effects. However, its modulatory effects on bone metabolism have not been investigated previously. In this study, we examined the effects of a water extract of the leaves of L. gracile (WELG) on osteoclast differentiation and bone loss, and explored its underlying mechanisms. We found that WELG inhibits osteoclastogenesis by suppressing both receptor activator of nuclear factor-κB ligand (RANKL)-induced early activation of mitogen-activated protein kinases (MAPKs) and nuclear factor-κB (NF-κB)- and RANKL-induced modulation of the positive and negative regulators of osteoclastogenesis in osteoclast precursors. In vivo study demonstrated that WELG protects against bone loss, weight gain, and fat accumulation without affecting uterine atrophy in an ovariectomy-induced postmenopausal osteoporosis mice model. In addition, photochemical analysis of WELG identified active constituents known to have bone-protective effects. Overall, the results of this study suggest that WELG can be a potential candidate for therapy and prevention of postmenopausal osteoporosis.

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

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

The origins and formation of bone-resorbing osteoclasts

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

MCU Inhibitor Ruthenium Red Alleviates the Osteoclastogenesis and Ovariectomized Osteoporosis via Suppressing RANKL-Induced ROS Production and NFATc1 Activation through P38 MAPK Signaling Pathway

Osteoporosis is a disorder of bone metabolism that is extremely common in elderly patients as well as in postmenopausal women. The main manifestation is that the bone resorption capacity is greater than the bone formation capacity, which eventually leads to a decrease in bone mass, increasing the risk of fracture. There is growing evidence that inhibiting osteoclast formation and resorption ability can be effective in treating and preventing the occurrence of osteoporosis. Our study is the first time to explore the role of the mitochondrial calcium uniporter (MCU) and its inhibitor ruthenium red (RR) in bone metabolism, clarifying the specific mechanism by which it inhibits osteoclast formation in vitro and plays a therapeutic role in osteoporosis in vivo. We verified the suppressive effects of RR on the receptor activator of nuclear factor-κB ligand (RANKL-)-induced differentiation and bone resorption function of osteoclasts in vitro. The reactive oxygen species (ROS) production stimulated by RANKL and the expression level of P38 MAPK/NFATc1 were also found to be inhibited by RR. Moreover, the promotion of RR on osteogenesis differentiation was investigated by alkaline phosphatase (ALP) and alizarin red S (ARS) staining and the detection of osteogenesis-specific gene expression levels by quantitative polymerase chain reaction (qPCR) and western blotting. Moreover, in ovariectomy (OVX-)-induced osteoporosis models, RR can downregulate the expression and function of the MCU, relieving bone loss and promoting osteogenesis to present a therapeutic effect on osteoporosis. This new finding will provide an important direction for the study of RR and MCU in the study of bone metabolism therapy targets.

Roles of osteoclasts in alveolar bone remodeling

Osteoclasts are large multinucleated cells from hematopoietic origin and are responsible for bone resorption. A balance between osteoclastic bone resorption and osteoblastic bone formation is critical to maintain bone homeostasis. The alveolar bone, also called the alveolar process, is the part of the jawbone that holds the teeth and supports oral functions. It differs from other skeletal bones in several aspects: its embryonic cellular origin, the form of ossification, and the presence of teeth and periodontal tissues; hence, understanding the unique characteristic of the alveolar bone remodeling is important to maintain oral homeostasis. Excessive osteoclastic bone resorption is one of the prominent features of bone diseases in the jaw such as periodontitis. Therefore, inhibiting osteoclast formation and bone resorptive process has been the target of therapeutic intervention. Understanding the mechanisms of osteoclastic bone resorption is critical for the effective treatment of bone diseases in the jaw. In this review, we discuss basic principles of alveolar bone remodeling with a specific focus on the osteoclastic bone resorptive process and its unique functions in the alveolar bone. Lastly, we provide perspectives on osteoclast-targeted therapies and regenerative approaches associated with bone diseases in the jaw.

Role of nuclear factor of activated T cells in chondrogenesis osteogenesis and osteochondroma formation

PURPOSE

Nuclear factor of activated T cells (NFATc) are transcription factors that play a function in the immune response and in osteoclast differentiation. In the present work, we define the function of NFATc2 in chondrogenic and osteogenic cells.

METHODS

Nfatc2^loxP/loxP and Nfatc1^loxP/loxP;Nfatc2^loxP/loxP conditional mice were crossed with Prx1-Cre transgenics to inactivate Nfatc2 singly and with Nfatc1. Femurs and vertebrae were examined by microcomputed tomography (µCT) X-Ray images and histology and analyzed for the presence of osteochondromas.

RESULTS

µCT demonstrated that Prx1-Cre;Nfatc2^∆/∆ female mice had transient osteopenia and male mice did not have a cancellous or a cortical bone phenotype when compared to control mice. In contrast, the dual inactivation of Nfatc1 and Nfatc2 in Prx1-expressing cells resulted in cancellous osteopenia and small bones at 1 month of age in both sexes. Nfatc1;Nfatc2 deleted mice exhibited a ~ 50% decrease in bone volume and connectivity. Total bone area, periosteal and endocortical bone perimeters and femoral length were reduced indicating smaller bones. As the mice matured, the shortening of the femoral length persisted, but the osteopenic phenotype resolved and cancellous femoral bone of 4-month-old Nfatc1;Nfatc2 deleted mice was not different from controls although male mice had vertebral osteopenia. In addition, Nfatc1;Nfatc2 deleted mice displayed distortion of the distal metaphysis and, as they matured, the articular presence of mineralized tumors with the appearance of osteochondromas.

CONCLUSION

Our studies reveal that NFATc1 and NFATc2 are necessary for optimal bone homeostasis and the suppression of osteochondroma formation.

LY450139 Inhibited Ti-Particle-Induced Bone Dissolution via Suppressing Notch and NF-κB Signaling Pathways

Aseptic loosening of the prosthesis caused by wear-particle-induced osteolysis is a long-term complication and one of the most common reasons for the failure of joint implants. The primary cause of aseptic loosening of the prosthesis is overactive bone resorption caused by wear-particle-activated osteoclasts in both direct and indirect ways. Therefore, drugs that can inhibit differentiation and bone resorption of osteoclasts need investigation as a potential therapeutic strategy to prevent and treat peri-prosthetic osteolysis and thereby prolong the service life of the prosthesis. This study has verified the potential inhibitory effect of LY450139 on inflammatory osteolysis induced by titanium particles in a mice skull model. In addition, we found that LY450139 inhibited receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis, bone resorption, and podosomal actin belt formation in a dose-dependent manner without evidence of cytotoxicity in vitro. In addition, LY450139 significantly decreased the expression of osteoclast-specific markers, including TRAP, CTSK, V-ATPase d2, CTR, DC-STAMP, NFATc1, and the downstream target gene Hes1 in Notch signaling pathway. Further investigation of the molecular mechanism demonstrated that LY450139 inhibited the formation of osteoclasts via inhibition of the NF-κB and Notch signaling pathways. In summary, LY450139 inhibited the formation of RANKL-mediated osteoclasts via NF-κB and Notch signaling and inhibited Ti particle-induced inflammatory osteolysis in vivo. LY450139 is a potential targeted drug for the treatment of peri-prosthetic osteolysis and other osteolytic disease associated with overactive osteoclasts.

Babam2 negatively regulates osteoclastogenesis by interacting with Hey1 to inhibit Nfatc1 transcription

Osteoclast-mediated excessive bone resorption was highly related to diverse bone diseases including osteoporosis. BRISC and BRCA1-A complex member 2 (Babam2) was an evolutionarily conserved protein that is highly expressed in bone tissues. However, whether Babam2 is involved in osteoclast formation is still unclear. In this study, we identify Babam2 as an essential negative regulator of osteoclast formation. We demonstrate that Babam2 knockdown significantly accelerated osteoclast formation and activity, while Babam2 overexpression blocked osteoclast formation and activity. Moreover, we demonstrate that the bone resorption activity was significantly downregulated in Babam2-transgenic mice as compared with wild-type littermates. Consistently, the bone mass of the Babam2-transgenic mice was increased. Furthermore, we found that Babam2-transgenic mice were protected from LPS-induced bone resorption activation and thus reduced the calvarial bone lesions. Mechanistically, we demonstrate that the inhibitory effects of Babam2 on osteoclast differentiation were dependent on Hey1. As silencing Hey1 largely diminished the effects of Babam2 on osteoclastogenesis. Finally, we show that Babam2 interacts with Hey1 to inhibit Nfatc1 transcription. In sum, our results suggested that Babam2 negatively regulates osteoclastogenesis and bone resorption by interacting with Hey1 to inhibit Nfatc1 transcription. Therefore, targeting Babam2 may be a novel therapeutic approach for osteoclast-related bone diseases.

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.

Transferrin receptor 1-mediated iron uptake regulates bone mass in mice via osteoclast mitochondria and cytoskeleton

Increased intracellular iron spurs mitochondrial biogenesis and respiration to satisfy high-energy demand during osteoclast differentiation and bone-resorbing activities. Transferrin receptor 1 (Tfr1) mediates cellular iron uptake through endocytosis of iron-loaded transferrin, and its expression increases during osteoclast differentiation. Nonetheless, the precise functions of Tfr1 and Tfr1-mediated iron uptake in osteoclast biology and skeletal homeostasis remain incompletely understood. To investigate the role of Tfr1 in osteoclast lineage cells in vivo and in vitro, we crossed Tfrc (encoding Tfr1)-floxed mice with Lyz2 (LysM)-Cre and Cathepsin K (Ctsk)-Cre mice to generate Tfrc conditional knockout mice in myeloid osteoclast precursors (Tfr1^ΔLysM) or differentiated osteoclasts (Tfr1^ΔCtsk), respectively. Skeletal phenotyping by µCT and histology unveiled a significant increase in trabecular bone mass with normal osteoclast number in long bones of 10-week-old young and 6-month-old adult female but not male Tfr1^ΔLysM mice. Although high trabecular bone volume in long bones was observed in both male and female Tfr1^ΔCtsk mice, this phenotype was more pronounced in female knockout mice. Consistent with this gender-dependent phenomena, estrogen deficiency induced by ovariectomy decreased trabecular bone mass in Tfr1^ΔLysM mice. Mechanistically, disruption of Tfr1 expression attenuated mitochondrial metabolism and cytoskeletal organization in mature osteoclasts in vitro by attenuating mitochondrial respiration and activation of the Src-Rac1-WAVE regulatory complex axis, respectively, leading to decreased bone resorption with little impact on osteoclast differentiation. These results indicate that Tfr1-mediated iron uptake is specifically required for osteoclast function and is indispensable for bone remodeling in a gender-dependent manner.