Effects of Advanced Glycation End Products on Differentiation and Function of Osteoblasts and Osteoclasts

BACKGROUND

Risk of fragility fractures increases in patients with diabetes mellitus, independent of bone mineral density. In the present study, the effects of advanced glycation end products (AGEs) on differentiation and function of osteoblasts and osteoclasts were investigated.

METHODS

AGEs and 25 mM glucose were administered to marrow-derived macrophages and MCT3T3-E1 cells. The effects of AGEs on osteoclast differentiation was investigated using tartrate-resistant acid phosphatase (TRAP) assay. The effects of AGEs on osteoblast differentiation was investigated using alkaline phosphatase (ALP) activity and bone nodule formation assays. Expression of osteoclast-specific and osteoblast-specific genes and effects on cell signaling pathways associated with cell differentiation were analyzed using reverse transcription polymerase chain reaction and western blotting.

RESULTS

AGEs significantly decreased TRAP-positive multinucleated cell formation in receptor activator of nuclear factor-κB ligand-induced marrow-derived macrophages in a dose-dependent manner. AGEs suppressed the expression of osteoclast-specific genes, JNK, p38, AKT, intercellular adhesion molecule 1, and lymphocyte function-associated antigen 1 in marrow-derived macrophages. AGEs decreased ALP activity and showed a tendency to decrease bone nodule formation in MC3T3-E1 cells. AGEs suppressed the expression of osteoblast-specific genes, lysyl hydroxylase and lysyl oxidase in MC3T3-E1 cells.

CONCLUSION

AGEs suppressed differentiation and function of osteoclasts and osteoblasts, and collagen cross-linking activity. It suggests that AGE may induce bone fragility through low bone turnover and deterioration of bone quality.

L-Plastin Phosphorylation: Possible Regulation by a TNFR1 Signaling Cascade in Osteoclasts

Tumor necrosis factor-alpha (TNF-α) signaling regulates phosphorylation of L-plastin, which is involved in forming the nascent sealing zone, a precursor zone for the matured sealing ring. This study aimed to illustrate the molecular mechanisms of L-plastin phosphorylation and the subsequent formation of the nascent sealing zone in osteoclasts treated with TNF-α. Here, we report that anti-TNF-receptor 1, inhibitors of signaling proteins (Src, PI3-K, Rho, and Rho-kinase), and siRNA of TRAF-6 attenuated the phosphorylation of LPL and filamentous actin content significantly in the presence of TNF-α. An inhibitor of integrin αvβ3, PKC, or PKA did not inhibit TNF-α-induced L-plastin phosphorylation. Inhibitors of Src and PI3-K and not Rho or Rho-kinase reduced tyrosine phosphorylation of TRAF-6, suggesting that Src and PI3-K regulate TRAF-6 phosphorylation, and Rho and Rho-kinase are downstream of TRAF-6 regulation. Osteoclasts expressing constitutively active or kinase-defective Src proteins were used to determine the role of Src on L-plastin phosphorylation; similarly, the effect of Rho was confirmed by transducing TAT-fused constitutively active (V14) or dominant-negative (N19) Rho proteins into osteoclasts. Pull-down analysis with glutathione S-transferase-fused SH2 and SH3 domains of Src and PI3-K demonstrated coprecipitation of L-plastin and TRAF-6 with the SH3 and SH2 domains of the PI3-K and Src proteins. However, the actual order of the interaction of proteins requires further elucidation; a comprehensive screening should corroborate the initial findings of protein interactions via the SH2/SH3 domains. Ultimately, inhibition of the interaction of proteins with SH2/SH3 could reduce L-plastin phosphorylation and affect NSZ formation and bone resorption in conditions that display osteoclast activation and bone loss.

The roles of RUNX2 and osteoclasts in regulating expression of steroidogenic enzymes in castration-resistant prostate cancer cells

Intratumoral steroidogenesis is involved in development of castration-resistant prostate cancer (CRPC) as bone metastases. The osteoblast transcription factor RUNX2 influences steroidogenesis and is induced in CRPC cells by osteoblasts. This study investigates osteoclastic influence on RUNX2 in intratumoral steroidogenesis. Steroidogenic enzymes and steroid receptors were detected with immunohistochemistry in xenograft intratibial tumors from CRPC cells. In vitro, expression of RUNX2 was increased by osteoclasts in osteoblastic LNCaP-19 cells, but not in osteolytic PC-3. Silencing of RUNX2 downregulates expression of CYP11A1, CYP17A1 and HSD3B1 in LNCaP-19 cells co-cultured with osteoclasts, leading to inhibition of KLK3 expression. Osteoclasts promoted CYP11A1 and RUNX2 promoted AKR1C3, HSD17B3 and CYP19A1, but suppressed ESR2 in PC-3 cells. This study shows that osteoclasts promote RUNX2 regulated induction of key steroidogenic enzymes, influencing activation of androgen receptor in CRPC cells. The potential of RUNX2 as a target to inhibit progression of skeletal metastases of CRPC needs further investigation.

Aged skeletal stem cells generate an inflammatory degenerative niche

Loss of skeletal integrity during ageing and disease is associated with an imbalance in the opposing actions of osteoblasts and osteoclasts1. Here we show that intrinsic ageing of skeletal stem cells (SSCs)2 in mice alters signalling in the bone marrow niche and skews the differentiation of bone and blood lineages, leading to fragile bones that regenerate poorly. Functionally, aged SSCs have a decreased bone- and cartilage-forming potential but produce more stromal lineages that express high levels of pro-inflammatory and pro-resorptive cytokines. Single-cell RNA-sequencing studies link the functional loss to a diminished transcriptomic diversity of SSCs in aged mice, which thereby contributes to the transformation of the bone marrow niche. Exposure to a youthful circulation through heterochronic parabiosis or systemic reconstitution with young haematopoietic stem cells did not reverse the diminished osteochondrogenic activity of aged SSCs, or improve bone mass or skeletal healing parameters in aged mice. Conversely, the aged SSC lineage promoted osteoclastic activity and myeloid skewing by haematopoietic stem and progenitor cells, suggesting that the ageing of SSCs is a driver of haematopoietic ageing. Deficient bone regeneration in aged mice could only be returned to youthful levels by applying a combinatorial treatment of BMP2 and a CSF1 antagonist locally to fractures, which reactivated aged SSCs and simultaneously ablated the inflammatory, pro-osteoclastic milieu. Our findings provide mechanistic insights into the complex, multifactorial mechanisms that underlie skeletal ageing and offer prospects for rejuvenating the aged skeletal system.

Osteomorphs as a tool for personalized medicine

McDonald and colleagues reported osteoclast-related dynamic mechanisms that lead, by fission, to osteomorphs; motile, fusion-competent cells capable of forming bone-resorbing osteoclasts. scRNA-seq analyses revealed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and might be implicated in rare and common bone diseases in humans.

Melatonin Inhibits Osteoclastogenesis and Osteolytic Bone Metastasis: Implications for Osteoporosis

Osteoblasts and osteoclasts are major cellular components in the bone microenvironment and they play a key role in the bone turnover cycle. Many risk factors interfere with this cycle and contribute to bone-wasting diseases that progressively destroy bone and markedly reduce quality of life. Melatonin (N-acetyl-5-methoxy-tryptamine) has demonstrated intriguing therapeutic potential in the bone microenvironment, with reported effects that include the regulation of bone metabolism, acceleration of osteoblastogenesis, inhibition of osteoclastogenesis and the induction of apoptosis in mature osteoclasts, as well as the suppression of osteolytic bone metastasis. This review aims to shed light on molecular and clinical evidence that points to possibilities of melatonin for the treatment of both osteoporosis and osteolytic bone metastasis. It appears that the therapeutic qualities of melatonin supplementation may enable existing antiresorptive osteoporotic drugs to treat osteolytic metastasis.

Hexosamine Biosynthetic Pathway-Derived O-GlcNAcylation Is Critical for RANKL-Mediated Osteoclast Differentiation

O-linked-N-acetylglucosaminylation (O-GlcNAcylation) performed by O-GlcNAc transferase (OGT) is a nutrient-responsive post-translational modification (PTM) via the hexosamine biosynthetic pathway (HBP). Various transcription factors (TFs) are O-GlcNAcylated, affecting their activities and significantly contributing to cellular processes ranging from survival to cellular differentiation. Given the pleiotropic functions of O-GlcNAc modification, it has been studied in various fields; however, the role of O-GlcNAcylation during osteoclast differentiation remains to be explored. Kinetic transcriptome analysis during receptor activator of nuclear factor-kappaB (NF-κB) ligand (RANKL)-mediated osteoclast differentiation revealed that the nexus of major nutrient metabolism, HBP was critical for this process. We observed that the critical genes related to HBP activation, including Nagk, Gfpt1, and Ogt, were upregulated, while the global O-GlcNAcylation was increased concomitantly during osteoclast differentiation. The O-GlcNAcylation inhibition by the small-molecule inhibitor OSMI-1 reduced osteoclast differentiation in vitro and in vivo by disrupting the translocation of NF-κB p65 and nuclear factor of activated T cells c1 (NFATc1) into the nucleus by controlling their PTM O-GlcNAcylation. Furthermore, OSMI-1 had a synergistic effect with bone target therapy on osteoclastogenesis. Lastly, knocking down Ogt with shRNA (shOgt) mimicked OSMI-1’s effect on osteoclastogenesis. Targeting O-GlcNAcylation during osteoclast differentiation may be a valuable therapeutic approach for osteoclast-activated bone diseases.

Plastrum testudinis extract suppresses osteoclast differentiation via the NF-κB signaling pathway and ameliorates senile osteoporosis

ETHNOPHARMACOLOGICAL RELEVANCE

Plastrum testudinis (PT) is a kind of single traditional Chinese medicine that can tonify kidney and strengthen bone. Plastrum testudinis extract (PTE) has been approved to promote the osteogenic differentiation of bone marrow-derived mesenchymal stem cells in vitro. However, the mechanism by which PTE reduces osteoclast differentiation has not yet been reported.

AIM OF THE STUDY

To explore the potential of PTE as a therapeutic treatment for bone loss caused by senile osteoporosis (SOP).

MATERIALS AND METHODS

We evaluated whether PTE could inhibit RANKL-induced osteoclast differentiation both in vitro and in vivo, and investigated PTE-induced phenotypes of human peripheral blood monocytes.

RESULTS

We found that PTE inhibited osteoclast differentiation and bone resorption in vitro in a concentration-dependent manner and that PTE treatment is most effective during the early stages of osteoclastogenesis. Moreover, we found that PTE could block the NF-κB signaling pathway in vitro, leading to the down-regulation of osteoclast-specific genes including C-FOS and NFATC1. The results from our in vivo mouse study suggest that PTE treatment suppresses osteoclast formation and mitigates bone loss caused by SOP. Notably, we also found that PTE inhibited RANKL-induced osteoclast differentiation in human peripheral blood monocytes.

CONCLUSION

Our results suggest that PTE treatment suppresses osteoclastogenesis and ameliorates bone loss caused by SOP by selectively blocking the nuclear translocation of NF-κB/p50.

RANKL-Induced Btn2a2 - A T Cell Immunomodulatory Molecule - During Osteoclast Differentiation Fine-Tunes Bone Resorption

Butyrophilins, which are members of the extended B7 family of immunoregulators structurally related to the B7 family, have diverse functions on immune cells as co-stimulatory and co-inhibitory molecules. Despite recent advances in the understanding on butyrophilins' role on adaptive immune cells during infectious or autoimmune diseases, nothing is known about their role in bone homeostasis. Here, we analyzed the role of one specific butyrophilin, namely Btn2a2, as we have recently shown that Btn2a2 is expressed on the monocyte/macrophage lineage that also gives rise to bone degrading osteoclasts. We found that expression of Btn2a2 on monocytes and pre-osteoclasts is upregulated by the receptor activator of nuclear factor κ-B ligand (RANKL), an essential protein required for osteoclast formation. Interestingly, in Btn2a2-deficient osteoclasts, typical osteoclast marker genes (Nfatc1, cathepsin K, TRAP, and RANK) were downregulated following RANKL stimulation. In vitro osteoclast assays resulted in decreased TRAP positive osteoclast numbers in Btn2a2-deficient cells. However, Btn2a2-deficient osteoclasts revealed abnormal fusion processes shown by their increased size. In vivo steady state µCT and histological analysis of bone architecture in complete Btn2a2-deficient mice showed differences in bone parameters further highlighting the fine-tuning effect of BTN2a2. Moreover, in rheumatoid arthritis patients and experimental arthritis, we detected significantly decreased serum levels of the secreted soluble Btn2a2 protein. Taken together, we identified the involvement of the immunomodulatory molecule Btn2a2 in osteoclast differentiation with potential future implications in basic and translational osteoimmunology.

Protective Effect of Ciclopirox against Ovariectomy-Induced Bone Loss in Mice by Suppressing Osteoclast Formation and Function

Postmenopausal osteoporosis is closely associated with excessive osteoclast formation and function, resulting in the loss of bone mass. Osteoclast-targeting agents have been developed to manage this disease. We examined the effects of ciclopirox on osteoclast differentiation and bone resorption in vitro and in vivo. Ciclopirox significantly inhibited osteoclast formation from primary murine bone marrow macrophages (BMMs) in response to receptor activator of nuclear factor kappa B ligand (RANKL), and the expression of genes associated with osteoclastogenesis and function was decreased. The formation of actin rings and resorption pits was suppressed by ciclopirox. Analysis of RANKL-mediated early signaling events in BMMs revealed that ciclopirox attenuates IκBα phosphorylation without affecting mitogen-activated protein kinase activation. Furthermore, the administration of ciclopirox suppressed osteoclast formation and bone loss in ovariectomy-induced osteoporosis in mice and reduced serum levels of osteocalcin and C-terminal telopeptide fragment of type I collagen C-terminus. These results indicate that ciclopirox exhibits antiosteoclastogenic activity both in vitro and in vivo and represents a new candidate compound for protection against osteoporosis and other osteoclast-related bone diseases.

Solanum nigrum Line inhibits osteoclast differentiation and suppresses bone mineral density reduction in the ovariectomy‑induced osteoporosis model

Bone homeostasis is maintained by osteoclasts that absorb bone and osteoblasts that form bone tissue. Menopausal osteoporosis is a disease associated with aging and hormonal changes due to menopause causing abnormal activation of osteoclasts, resulting in a decrease in bone density. Existing treatments for osteoporosis have been reported to have serious side effects, such as jawbone necrosis and breast and uterine cancer; therefore, their use by patients is decreasing, whilst studies focusing on alternative treatments are increasingly popular. Solanum nigrum Line (SL) has been used as a medicinal plant that possesses several pharmacological effects, such as anti‑inflammatory and hepatotoxic protective effects. To the best of our knowledge, however, its effects on osteoporosis and osteoclasts have not been demonstrated previously. In the present study, the anti‑osteoporotic effect of SL was investigated using a postmenopausal model of osteoporosis in which Sprague‑Dawley rat ovaries were extracted. In addition, the inhibitory effects on osteoclast differentiation and function of SL was confirmed using an osteoclast model treated with receptor activator of NF‑κB ligand (RANKL) on murine RAW 264.7 macrophages. In vivo experiments showed that SL reduced the decrease in bone mineral density and improved changes in the morphological index of bone microstructure, such as trabecular number and separation. In addition, the number of tartrate resistant acid phosphatase‑positive cells in the femur and the expression levels of nuclear factor of activated T‑cells cytoplasmic 1 (NFATc1) and cathepsin K protein were inhibited. In vitro, SL suppressed RANKL‑induced osteoclast differentiation and bone resorption ability; this was mediated by NFATc1/c‑Fos, a key transcription factor involved in osteoclast differentiation, ultimately inhibiting expression of various osteoclast‑associated genes. These experimental results show that SL may be an alternative treatment for osteoporosis caused by abnormal activation of osteoclasts in the future.

Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune

Different types of multinucleated giant cells (MGCs) of myeloid origin have been described; osteoclasts are the most extensively studied because of their importance in bone homeostasis. MGCs are formed by cell-to-cell fusion, and most types have been observed in pathological conditions, especially in infectious and non-infectious chronic inflammatory contexts. The precise role of the different MGCs and the mechanisms that govern their formation remain poorly understood, likely due to their heterogeneity. First, we will introduce the main populations of MGCs derived from the monocyte/macrophage lineage. We will then discuss the known molecular actors mediating the early stages of fusion, focusing on cell-surface receptors involved in the cell-to-cell adhesion steps that ultimately lead to multinucleation. Given that cell-to-cell fusion is a complex and well-coordinated process, we will also describe what is currently known about the evolution of F-actin-based structures involved in macrophage fusion, i.e., podosomes, zipper-like structures, and tunneling nanotubes (TNT). Finally, the localization and potential role of the key fusion mediators related to the formation of these F-actin structures will be discussed. This review intends to present the current status of knowledge of the molecular and cellular mechanisms supporting multinucleation of myeloid cells, highlighting the gaps still existing, and contributing to the proposition of potential disease-specific MGC markers and/or therapeutic targets.

Protocatechualdehyde Inhibits the Osteoclast Differentiation of RAW264.7 and BMM Cells by Regulating NF-κB and MAPK Activity

Protocatechualdehyde (PCA), an important component of Salvia miltiorrhiza, has many activities, such as anti-inflammatory and antisepsis activities. However, the role of PCA in osteoclasts is not clear. We used RAW264.7 cells (a mouse leukemic monocyte/macrophage cell line) and bone marrow macrophages (BMMs) to probe the role of PCA in osteoclasts and the underlying mechanism. The effects of PCA on cell activity were evaluated with CCK-8 assays. TRAP staining detected mature osteoclasts. Corning Osteo Assay Surface plates were used to examine absorption. The levels of RNA and protein were analyzed, respectively, using RT-PCR and Western blotting. PCA (5 μg/ml) was not toxic to the two cell types but reduced the formation of osteoclasts and bone absorption. Furthermore, PCA restrained the expression of mRNAs encoding proteins associated with osteoclasts and reduced the phosphorylation of proteins in important signaling pathways. The results indicate that PCA inhibits osteoclast differentiation by suppressing NF-κB and MAPK activity.

The V-ATPase a3 Subunit: Structure, Function and Therapeutic Potential of an Essential Biomolecule in Osteoclastic Bone Resorption

This review focuses on one of the 16 proteins composing the V-ATPase complex responsible for resorbing bone: the a3 subunit. The rationale for focusing on this biomolecule is that mutations in this one protein account for over 50% of osteopetrosis cases, highlighting its critical role in bone physiology. Despite its essential role in bone remodeling and its involvement in bone diseases, little is known about the way in which this subunit is targeted and regulated within osteoclasts. To this end, this review is broadened to include the three other mammalian paralogues (a1, a2 and a4) and the two yeast orthologs (Vph1p and Stv1p). By examining the literature on all of the paralogues/orthologs of the V-ATPase a subunit, we hope to provide insight into the molecular mechanisms and future research directions specific to a3. This review starts with an overview on bone, highlighting the role of V-ATPases in osteoclastic bone resorption. We then cover V-ATPases in other location/functions, highlighting the roles which the four mammalian a subunit paralogues might play in differential targeting and/or regulation. We review the ways in which the energy of ATP hydrolysis is converted into proton translocation, and go in depth into the diverse role of the a subunit, not only in proton translocation but also in lipid binding, cell signaling and human diseases. Finally, the therapeutic implication of targeting a3 specifically for bone diseases and cancer is discussed, with concluding remarks on future directions.

S100 Calcium-Binding Protein P Secreted from Megakaryocytes Promotes Osteoclast Maturation

Megakaryocytes (MKs) differentiate from hematopoietic stem cells and produce platelets at the final stage of differentiation. MKs directly interact with bone cells during bone remodeling. However, whether MKs are involved in regulating bone metabolism through indirect regulatory effects on bone cells is unclear. Here, we observed increased osteoclast differentiation of bone marrow-derived macrophages (BMMs) cultured in MK-cultured conditioned medium (MK CM), suggesting that this medium contains factors secreted from MKs that affect osteoclastogenesis. To identify the MK-secreted factor, DNA microarray analysis of the human leukemia cell line K562 and MKs was performed, and S100 calcium-binding protein P (S100P) was selected as a candidate gene affecting osteoclast differentiation. S100P was more highly expressed in MKs than in K562 cells, and showed higher levels in MK CM than in K562-cultured conditioned medium. In BMMs cultured in the presence of recombinant human S100P protein, osteoclast differentiation was promoted and marker gene expression was increased. The resorption area was significantly larger in S100P protein-treated osteoclasts, demonstrating enhanced resorption activity. Overall, S100P secreted from MKs promotes osteoclast differentiation and resorption activity, suggesting that MKs indirectly regulate osteoclast differentiation and activity through the paracrine action of S100P.

Nitrogen-Doped Multiwalled Carbon Nanotubes Enhance Bone Remodeling through Immunomodulatory Functions

It has been reported that multiwalled carbon nanotubes (MWCNTs) can reportedly positively affect growth and differentiation of bone-related cells and therefore offer great potential in biomedical applications. To overcome negative immune responses that limit their application, specific doping and functionalization can improve their biocompatibility. Here, we demonstrated that nitrogen-doped carboxylate-functionalized MWCNTs (N-MWCNTs) enhance bone remodeling both in vitro and in vivo with excellent biocompatibility, via stimulation of both bone resorption and formation. We revealed that 0.2 μg/mL N-MWCNTs not only increase the transcription of osteoblastogenic and osteoclastogenic genes but also up-regulate the activities of both TRAP and AKP in the differentiation of bone marrow stromal cells (BMSCs). Additionally, intramuscular administration of N-MWCNTs at a dosage of 1.0 mg/kg body weight enhances bone mineral density and bone mass content in mice, as well as induces potentiated degree of TRAP- and ARS-positive staining in the femur. The positive regulation of N-MWCNTs on bone remodeling is initiated by macrophage phagocytosis, which induces altered production of inflammatory cytokines by immune response pathways, and consequently up-regulates IL1α, IL10, and IL16. These cytokines collectively regulate the central osteoclastogenic transcription factor NFATc1 and osteoblastogenic BMP signaling, the suppression of which confirmed that these factors respectively participate in N-MWCNT-mediated regulation of osteoclastic and osteoblastic bone marrow stem cell activities. These results suggest that N-MWCNTs can be readily generalized for use as biomaterials in bone tissue engineering for metabolic bone disorders.

The endocannabinoid system and retinoic acid signaling combine to influence bone growth

Osteoporosis is an increasing burden on public health as the world-wide population ages and effective therapeutics are severely needed. Two pathways with high potential for osteoporosis treatment are the retinoic acid (RA) and endocannabinoid system (ECS) signaling pathways. We sought to elucidate the roles that these pathways play in bone development and maturation. Here, we use chemical treatments to modulate the RA and ECS pathways at distinct early, intermediate, and late times bone development in zebrafish. We further assessed osteoclast activity later in zebrafish and medaka. Finally, by combining sub-optimal doses of AR and ECS modulators, we show that enhancing RA signaling or reducing the ECS promote bone formation and decrease osteoclast abundance and activity. These data demonstrate that RA signaling and the ECS can be combined as sub-optimal doses to influence bone growth and may be key targets for potential therapeutics.

Vitamin D receptor expression in mature osteoclasts reduces bone loss due to low dietary calcium intake in male mice

Mature osteoclasts express the vitamin D receptor (VDR) and are able to respond to active vitamin D (1α, 25-dihydroxyvitamin D3; 1,25(OH)2D3) by regulating cell maturation and activity. However, the in vivo consequences of vitamin D signalling directly within functionally mature osteoclasts is only partially understood. To investigate the in vivo role of VDR in mature osteoclasts, conditional deletion of the VDR under control of the cathepsin K promoter (CtskCre/Vdr-/-), was assessed in 6 and 12-week-old mice, either under normal dietary conditions (NormCaP) or when fed a low calcium (0.03 %), low phosphorous (0.08 %) diet (LowCaP). Splenocytes from CtskCre/Vdr-/- mice were co-cultured with MLO-Y4 osteocyte-like cells to assess the effect on osteoclastogenesis. Six-week-old CtskCre/Vdr-/- mice demonstrated a 10 % decrease in vertebral bone volume (p < 0.05), which was associated with increased osteoclast size (p < 0.05) when compared to Vdrfl/fl control mice. Control mice fed a LowCaP diet exhibited extensive trabecular bone loss associated with increased osteoclast surface, number and size (p < 0.0001). Interestingly, CtskCre/Vdr-/- mice fed a LowCaP diet showed exacerbated loss of bone volume fraction (BV/TV%) and trabecular number (Tb.N), by a further 22 % and 21 %, respectively (p < 0.05), suggesting increased osteoclastic bone resorption activity with the loss of VDR in mature osteoclasts under these conditions. Co-culture of CtskCre/Vdr-/- splenocytes with MLO-Y4 cells increased resulting osteoclast numbers 2.5-fold, which were greater in nuclei density and exhibited increased resorption of dentine compared to osteoclasts derived from Vdrfl/fl splenocyte cultures. These data suggest that in addition to RANKL-mediated osteoclastogenesis, intact VDR signalling is required for the direct regulation of the differentiation and activity of osteoclasts in both in vivo and ex vivo settings.

Regulation of Bone Cell Differentiation and Activation by Microbe-Associated Molecular Patterns

Gut microbiota has emerged as an important regulator of bone homeostasis. In particular, the modulation of innate immunity and bone homeostasis is mediated through the interaction between microbe-associated molecular patterns (MAMPs) and the host pattern recognition receptors including Toll-like receptors and nucleotide-binding oligomerization domains. Pathogenic bacteria such as Porphyromonas gingivalis and Staphylococcus aureus tend to induce bone destruction and cause various inflammatory bone diseases including periodontal diseases, osteomyelitis, and septic arthritis. On the other hand, probiotic bacteria such as Lactobacillus and Bifidobacterium species can prevent bone loss. In addition, bacterial metabolites and various secretory molecules such as short chain fatty acids and cyclic nucleotides can also affect bone homeostasis. This review focuses on the regulation of osteoclast and osteoblast by MAMPs including cell wall components and secretory microbial molecules under in vitro and in vivo conditions. MAMPs could be used as potential molecular targets for treating bone-related diseases such as osteoporosis and periodontal diseases.

Role of Biomolecules in Osteoclasts and Their Therapeutic Potential for Osteoporosis

Osteoclasts (OCs) are important cells that are involved in the regulation of bone metabolism and are mainly responsible for coordinating bone resorption with bone formation to regulate bone remodeling. The imbalance between bone resorption and formation significantly affects bone metabolism. When the activity of osteoclasts exceeds the osteoblasts, it results in a condition called osteoporosis, which is characterized by reduced bone microarchitecture, decreased bone mass, and increased occurrences of fracture. Molecules, including transcription factors, proteins, hormones, nucleic acids, such as non-coding RNAs, play an important role in osteoclast proliferation, differentiation, and function. In this review, we have highlighted the role of these molecules in osteoclasts regulation and osteoporosis. The developed therapeutics targeting these molecules for the treatment of osteoporosis in recent years have also been discussed with challenges faced in clinical application.

TRPM7 kinase-mediated immunomodulation in macrophage plays a central role in magnesium ion-induced bone regeneration

Despite the widespread observations on the osteogenic effects of magnesium ion (Mg2+), the diverse roles of Mg2+ during bone healing have not been systematically dissected. Here, we reveal a previously unknown, biphasic mode of action of Mg2+ in bone repair. During the early inflammation phase, Mg2+ contributes to an upregulated expression of transient receptor potential cation channel member 7 (TRPM7), and a TRPM7-dependent influx of Mg2+ in the monocyte-macrophage lineage, resulting in the cleavage and nuclear accumulation of TRPM7-cleaved kinase fragments (M7CKs). This then triggers the phosphorylation of Histone H3 at serine 10, in a TRPM7-dependent manner at the promoters of inflammatory cytokines, leading to the formation of a pro-osteogenic immune microenvironment. In the later remodeling phase, however, the continued exposure of Mg2+ not only lead to the over-activation of NF-κB signaling in macrophages and increased number of osteoclastic-like cells but also decelerates bone maturation through the suppression of hydroxyapatite precipitation. Thus, the negative effects of Mg2+ on osteogenesis can override the initial pro-osteogenic benefits of Mg2+. Taken together, this study establishes a paradigm shift in the understanding of the diverse and multifaceted roles of Mg2+ in bone healing.

Deletion of a putative promoter-proximal Tnfsf11 regulatory region in mice does not alter bone mass or Tnfsf11 expression in vivo

The cytokine RANKL is essential for osteoclast formation during physiological and pathological bone resorption. RANKL also contributes to lymphocyte production, development of lymph nodes and mammary glands, as well as other biological activities. Transcriptional control of the Tnfsf11 gene, which encodes RANKL, is complex and involves distant regulatory regions. Nevertheless, cell culture studies suggest that an enhancer region near the transcription start site is involved in the control of Tnfsf11 expression by hormones such as 1,25-(OH)₂ vitamin D₃ and parathyroid hormone, as well as the sympathetic nervous system. To address the significance of this region in vivo, we deleted the sequence between -510 to -1413 bp, relative to Tnfsf11 exon 1, from mice using CRISPR-based gene editing. MicroCT analysis of the femur and fourth lumbar vertebra of enhancer knockout mice showed no differences in bone mass compared to wild type littermates at 5 weeks and 6 months of age, suggesting no changes in osteoclast formation. RNA extracted from the tibia, fifth lumbar vertebra, thymus, and spleen at 6 months of age also showed no reduction in Tnfsf11 mRNA abundance between these groups. However, maximal stimulation of Tnfsf11 mRNA abundance in cultured stromal cells by PTH was reduced approximately 40% by enhancer deletion, while stimulation by 1,25-(OH)₂ vitamin D₃ was unaffected. The abundance of B and T lymphocytes in the bone marrow did not differ between genotypes. These results demonstrate that the region between -510 and -1413 does not contribute to Tnfsf11 expression, osteoclast support, or lymphocyte production in mice under normal physiological conditions but may be involved in situations of elevated parathyroid hormone.

Effects of fluid flow shear stress to mouse muscle cells on the bone actions of muscle cell-derived extracellular vesicless

The interactions between skeletal muscle and bone have been recently noted, and muscle-derived humoral factors related to bone metabolism play crucial roles in the muscle/bone relationships. We previously reported that extracellular vesicles from mouse muscle C2C12 cells (Myo-EVs) suppress osteoclast formation in mice. Although mechanical stress is included in extrinsic factors which are important for both muscle and bone, the detailed roles of mechanical stress in the muscle/bone interactions have still remained unknown. In present study, we examined the effects of fluid flow shear stress (FFSS) to C2C12 cells on the physiological actions of muscle cell-derived EV. Applying FFSS to C2C12 cells significantly enhanced muscle cell-derived EV-suppressed osteoclast formation and several osteoclast-related gene levels in mouse bone marrow cells in the presence of receptor activator nuclear factor κB ligand (RANKL). Moreover, FFSS to C2C12 cells significantly enhanced muscle cell-derived EV-suppressed mitochondria biogenesis genes during osteoclast formation with RANKL treatment. In addition, FFSS to C2C12 cells significantly enhanced muscle cell-derived EV-suppressed osteoclast formation and several osteoclast-related gene levels in Raw264.7 cells in the presence of RANKL. Small RNA-seq-analysis showed that FFSS elevated the expression of miR196a-5p and miR155-5p with the suppressive actions of osteoclast formation and low expression in mouse bone cells. On the other hand, muscle cell-derived EVs with or without FFSS to C2C12 cells did not affect the expression of osteogenic genes, alkaline phosphatase activity and mineralization in mouse osteoblasts. In conclusion, we first showed that FFSS to C2C12 cells enhances the suppressive effects of muscle cell-derived EVs on osteoclast formation in mouse cells. Muscle cell-derived EVs might be partly involved in the effects of mechanical stress on the muscle/bone relationships.

Cell cytoskeleton and proliferation study for the RANKL-induced RAW264.7 differentiation

Although document studies (including ours) have been reported the achieved in vitro osteoclastic cellular model establishment from the RAW264.7 cell lineage, there was no study directly reported that American Type Culture Collection (ATCC) cell bank has various RAW264.7 cell lineages. Besides that, for our knowledge there was only one study compared the two different RAW264.7TIB-71 and RAW264.7CRL-2278 cell lineages for their osteoclastic differentiation, and they concluded that the RAW264.7CRL-2278 demonstrated to generate much osteoclast than RAW264.7TIB-71 . However, on the contrary to their results we noticed the fusion of RAW264.7TIB-71 in our previous studies was much compromising. Therefore, we try to explore the two cell lineages for their properties in osteoclastic differentiation with an in-depth cellular cytoskeletal study. Our current study has showed that comparing to the RAW264.7CRL-2278 , RAW264.7TIB-71 demonstrated a much higher efficacies for RANKL-stimulated osteoclastic differentiation. Besides that, in our depth cytoskeletal studies, we found that the RANKL-induced RAW264.7TIB-71 cells could finally differentiate into mature osteoclasts. However, regardless the various pre-treatment conditions, there was no mature osteoclast formed in RANKL-induced RAW264.7CRL-2278 cell lineage.

Activation of (pro)renin by (pro)renin receptor in extracellular vesicles from osteoclasts

The (pro)renin receptor (PRR) is a multifunctional integral membrane protein that serves as a component of the vacuolar H+-ATPase (V-ATPase) and also activates (pro)renin. We recently showed that full-length PRR, found as part of a V-ATPase sub-complex, is abundant in extracellular vesicles shed by osteoclasts. Here, we tested whether these extracellular vesicles stimulate (pro)renin. Extracellular vesicles isolated from the conditioned media of RAW 264.7 osteoclast-like cells or primary osteoclasts were characterized and counted by nanoparticle tracking. Immunoblotting confirmed that full-length PRR was present. Extracellular vesicles from osteoclasts dose-dependently stimulated (pro)renin activity, while extracellular vesicles from 4T1 cancer cells, in which we did not detect PRR, did not activate (pro)renin. To confirm that the ability of extracellular vesicles from osteoclasts to stimulate (pro)renin activity was due to the PRR, the "handle region peptide" from the PRR, a competitive inhibitor of PRR activity, was tested. It dose-dependently blocked the ability of extracellular vesicles to stimulate the enzymatic activity of (pro)renin. In summary, the PRR, an abundant component of extracellular vesicles shed by osteoclasts, stimulates (pro)renin activity. This represents a novel mechanism by which extracellular vesicles can function in intercellular regulation, with direct implications for bone biology.