The cell biology of osteoclast function

The Mechanism Switching the Osteoclast From Short to Long Duration Bone Resorption

The current models of osteoclastic bone resorption focus on immobile osteoclasts sitting on the bone surface and drilling a pit into the bone matrix. It recently appeared that many osteoclasts also enlarge their pit by moving across the bone surface while resorbing. Drilling a pit thus represents only the start of a resorption event of much larger amplitude. This prolonged resorption activity significantly contributes to pathological bone destruction, but the mechanism whereby the osteoclast engages in this process does not have an answer within the standard bone resorption models. Herein, we review observations that lead to envision how prolonged resorption is possible through simultaneous resorption and migration. According to the standard pit model, the “sealing zone” which surrounds the ruffled border (i.e., the actual resorption apparatus), “anchors” the ruffled border against the bone surface to be resorbed. Herein, we highlight that continuation of resorption demands that the sealing zone “glides” inside the cavity. Thereby, the sealing zone emerges as the structure responsible for orienting and displacing the ruffled border, e.g., directing resorption against the cavity wall. Importantly, sealing zone displacement stringently requires thorough collagen removal from the cavity wall - which renders strong cathepsin K collagenolysis indispensable for engagement of osteoclasts in cavity-enlargement. Furthermore, the sealing zone is associated with generation of new ruffled border at the leading edge, thereby allowing the ruffled border to move ahead. The sealing zone and ruffled border displacements are coordinated with the migration of the cell body, shown to be under control of lamellipodia at the leading edge and of the release of resorption products at the rear. We propose that bone resorption demands more attention to osteoclastic models integrating resorption and migration activities into just one cell phenotype.

Biological Evaluation and Transcriptomic Analysis of Corylin as an Inhibitor of Osteoclast Differentiation

Corylin, a flavonoid isolated from the fruit of Psoralea corylifolia, has an osteogenic effect on osteoblasts in vitro and bone micromass ex vivo. However, the effect and mechanism of corylin in regulating osteoclastogenesis remain unknown. By using murine bone marrow macrophages as the osteoclast precursor, corylin was found to inhibit the receptor activator of nuclear factor (NF) κB ligand (RANKL)-induced osteoclast differentiation via down-regulating osteoclastic marker genes. In parallel, F-actin formation and osteoclast migration were diminished in corylin-treated cultured osteoclasts, and subsequently the expressions of osteoclastic proteins were suppressed: the suppression of protein expression was further illustrated by transcriptomic analysis. Furthermore, corylin inhibited the nuclear translocation of p65, giving rise to a restraint in osteoclastic differentiation through the attenuation of transcription factors nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and nuclear factor of activated T cells c1 (NFATc1). There was no obvious change in apoptosis when the RANKL-induce osteoclasts were cultured in the presence of corylin. The finding supports the potential development of corylin as an osteoclast inhibitor against osteoporosis.

Pathophysiological mechanisms of root resorption after dental trauma: a systematic scoping review

Background

The objective of this scoping review was to systematically explore the current knowledge of cellular and molecular processes that drive and control trauma-associated root resorption, to identify research gaps and to provide a basis for improved prevention and therapy.

Methods

Four major bibliographic databases were searched according to the research question up to February 2021 and supplemented manually. Reports on physiologic, histologic, anatomic and clinical aspects of root resorption following dental trauma were included. Duplicates were removed, the collected material was screened by title/abstract and assessed for eligibility based on the full text. Relevant aspects were extracted, organized and summarized.

Results

846 papers were identified as relevant for a qualitative summary. Consideration of pathophysiological mechanisms concerning trauma-related root resorption in the literature is sparse. Whereas some forms of resorption have been explored thoroughly, the etiology of others, particularly invasive cervical resorption, is still under debate, resulting in inadequate diagnostics and heterogeneous clinical recommendations. Effective therapies for progressive replacement resorptions have not been established. Whereas the discovery of the RANKL/RANK/OPG system is essential to our understanding of resorptive processes, many questions regarding the functional regulation of osteo-/odontoclasts remain unanswered.

Conclusions

This scoping review provides an overview of existing evidence, but also identifies knowledge gaps that need to be addressed by continued laboratory and clinical research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-021-01510-6.

Isovaleric acid ameliorates ovariectomy-induced osteoporosis by inhibiting osteoclast differentiation

Osteoclasts (OCs) play important roles in bone remodelling and contribute to bone loss by increasing bone resorption activity. Excessively activated OCs cause diverse bone disorders including osteoporosis. Isovaleric acid (IVA), also known as 3-methylbutanoic acid is a 5-carbon branched-chain fatty acid (BCFA), which can be generated by bacterial fermentation of a leucine-rich diet. Here, we find that IVA suppresses differentiation of bone marrow-derived macrophages into OCs by RANKL. IVA inhibited the expression of OC-related genes. IVA-induced inhibitory effects on OC generation were attenuated by pertussis toxin but not by H89, suggesting a G_i -coupled receptor-dependent but protein kinase A-independent response. Moreover, IVA stimulates AMPK phosphorylation, and treatment with an AMPK inhibitor blocks IVA-induced inhibition of OC generation. In an ovariectomized mouse model, addition of IVA to the drinking water resulted in significant decrease of body weight gain and inhibited the expression of not only OC-related genes but also fusogenic genes in the bone tissue. IVA exposure also blocked bone destruction and OC generation in the bone tissue of ovariectomized mice. Collectively, the results demonstrate that IVA is a novel bioactive BCFA that inhibits OC differentiation, suggesting that IVA can be considered a useful material to control osteoclast-associated bone disorders, including osteoporosis.

Expression of tartrate-resistant acid phosphatase and cathepsin K during osteoclast differentiation in developing mouse mandibles

The present study was designed to test the hypothesis that osteoclasts appear after or at the same time as the initiation of bone mineralization in developing intramembranous bones. We examined mineral deposition via Von Kossa staining to determine when bone mineralization begins, tartrate-resistant acid phosphatase (TRAP) activity and cathepsin K immunoreactivity to identify the presence of osteoclasts, and their mRNA expression levels to assess osteoclastic differentiation in the embryonic mouse mandible. Cathepsin K-immunopositive cells were detected around the same time as the onset of bone mineralization, whereas TRAP-positive cells appeared prior to bone mineralization. Cathepsin K protein was expressed only in multinucleated osteoclasts, whereas TRAP activity was identified in both mono- and multinucleated cells. During bone development, TRAP-positive cells altered their morphology, which was related to the number of their nuclei. The elevated mRNA levels of TRAP and cathepsin K were consistent with the increased percentage of multinucleated osteoclasts and the progression of bone development. Our study revealed that TRAP-positive cells appear prior to bone mineralization, and TRAP- and cathepsin K-positive multinucleated osteoclasts appear at the same time as the initiation of bone mineralization in embryonic mouse mandibles, suggesting that osteoclasts contribute to bone matrix maturation during intramembranous ossification.

Two macrophages, osteoclasts and microglia: from development to pleiotropy

Tissue-resident macrophages are highly specialized to their tissue-specific microenvironments, activated by various inflammatory signals and modulated by genetic and environmental factors. Osteoclasts and microglia are distinct tissue-resident cells of the macrophage lineage in bone and brain that are responsible for pathological changes in osteoporosis and Alzheimer’s disease (AD), respectively. Osteoporosis is more frequently observed in individuals with AD compared to the prevalence in general population. Diagnosis of AD is often delayed until underlying pathophysiological changes progress and cause irreversible damages in structure and function of brain. As such earlier diagnosis and intervention of individuals at higher risk would be indispensable to modify clinical courses. Pleiotropy is the phenomenon that a genetic variant affects multiple traits and the genetic correlation between two traits could suggest a shared molecular mechanism. In this review, we discuss that the Pyk2-mediated actin polymerization pathway in osteoclasts and microglia in bone and brain, respectively, is the horizontal pleiotropic mediator of shared risk factors for osteoporosis and AD.

Molecular Signaling Pathways and Essential Metabolic Elements in Bone Remodeling: An Implication of Therapeutic Targets for Bone Diseases

Bone is one of the dynamic tissues in the human body that undergoes continuous remodelling through subsequent actions of bone cells, osteoclasts, and osteoblasts. Several signal transduction pathways are involved in the transition of mesenchymal stem cells into osteoblasts. These primarily include Runx2, ATF4, Wnt signaling and sympathetic signalling. The differentiation of osteoclasts is controlled by M-CSF, RANKL, and costimulatory signalling. It is well known that bone remodelling is regulated through receptor activator of nuclear factor-kappa B ligand followed by binding to RANK, which eventually induces the differentiation of osteoclasts. The resorbing osteoclasts secrete TRAP, cathepsin K, MMP-9 and gelatinase to digest the proteinaceous matrix of type I collagen and form a saucer-shaped lacuna along with resorption tunnels in the trabecular bone. Osteoblasts secrete a soluble decoy receptor, osteoprotegerin that prevents the binding of RANK/RANKL and thus moderating osteoclastogenesis. Moreover, bone homeostasis is also regulated by several growth factors like, cytokines, calciotropic hormones, parathyroid hormone and sex steroids. The current review presents a correlation of the probable molecular targets underlying the regulation of bone mass and the role of essential metabolic elements in bone remodelling. Targeting these signaling pathways may help to design newer therapies for treating bone diseases.

Poncirin Inhibits Osteoclast Differentiation and Bone Loss through Down-Regulation of NFATc1 In Vitro and In Vivo

Activation of osteoclast and inactivation of osteoblast result in loss of bone mass with bone resorption, leading to the pathological progression of osteoporosis. The receptor activator of NF-κB ligand (RANKL) is a member of the TNF superfamily, and is a key mediator of osteoclast differentiation. A flavanone glycoside isolated from the fruit of Poncirus trifoliata, poncirin has anti-allergic, hypocholesterolemic, anti-inflammatory and anti-platelet activities. The present study investigates the effect of poncirin on osteoclast differentiation of RANKL-stimulated RAW264.7 cells. We observed reduced formation of RANKL-stimulated TRAP-positive multinucleated cells (a morphological feature of osteoclasts) after poncirin exposure. Real-time qPCR analysis showed suppression of the RANKL-mediated induction of key osteoclastogenic molecules such as NFATc1, TRAP, c-Fos, MMP9 and cathepsin K after poncirin treatment. Poncirin also inhibited the RANKL-mediated activation of NF-κB and, notably, JNK, without changes in ERK and p38 expression in RAW264.7 cells. Furthermore, we assessed the in vivo efficacy of poncirin in the lipopolysaccharide (LPS)-induced bone erosion model. Evaluating the micro-CT of femurs revealed that bone erosion in poncirin treated mice was markedly attenuated. Our results indicate that poncirin exerts anti-osteoclastic effects in vitro and in vivo by suppressing osteoclast differentiation. We believe that poncirin is a promising candidate for inflammatory bone loss therapeutics.

Modulated podosome patterning in osteoclasts by fullerenol nanoparticles disturbs the bone resorption for osteoporosis treatment

Overactivation and excessive differentiation of osteoclasts (OCs) has been implicated in the course of bone metabolism-related diseases. Although fullerenol nanoparticles (fNPs) have been suggested to inhibit OC differentiation and OC function in our previous work, systemic studies on the effect of fNPs on bone diseases, e.g., osteoporosis (OP), in vivo remain elusive. Herein, it is demonstrated that fNPs significantly suppress the differentiation of OCs that derived from the murine bone marrow monocytes and inhibit the formation of the sealing zone by blocking the formation and patterning of podosomes in OCs spatiotemporally. In vivo, fNPs are supposed to be an efficient inhibitor of the overactivation of OCs in a LPS-induced bone erosion mouse model. The therapeutic effect of fNPs on osteoporosis is also investigated in an ovariectomy-induced osteoporosis rat model. The well-organized trabecular bone, the reduction in the number of TRAP positive cells, the improvement of bone-associated parameters, and the mechanical properties all demonstrate that fNPs, similar to diphosphonates, can be a promising candidate for the effective treatment of osteoporosis.

Zeolitic Imidazolate Framework-8 Encapsulating Risedronate Synergistically Enhances Osteogenic and Antiresorptive Properties for Bone Regeneration

Bisphosphonates (BPs) are routinely administered for the treatment of turnover bone diseases. To avoid the undesirable adverse effects of long-term usage of bisphosphonates and improve their bioavailability in the bone microenvironment, we initially encapsulated risedronate (RIS) molecules inside nanoscale zeolitic imidazolate framework-8 particles (nZIF-8) by a one-step synthesis method to generate RIS@ZIF-8 nanoparticles. RIS@ZIF-8 nanoparticles displayed high loading encapsulation efficiency (64.21 ± 2.48%), good biocompatibility, controlled drug release capacity, and dual effects for bone regeneration. This work explored the potential of RIS@ZIF-8 nanoparticles, which could not only enhance ATP production, induce extracellular matrix (ECM) mineralization, and upregulate the expression levels of osteogenic genes but also effectively inhibit the formation of multinucleated giant osteocasts and decrease the Rankl/Opg ratio. Overall, RIS@ZIF-8 nanoparticles could be a very promising approach to synergistically enhance osteogenic and antiresorptive properties for bone regeneration, which could be utilized for the local treatment of bone defects.

Carboxylated gold nanoparticles inhibit bone erosion by disturbing the acidification of an osteoclast absorption microenvironment

Hyperactive osteoclasts (OCs) are a fundamental reason for excessive bone resorption and consequent osteoporosis that lead to one-third of the patients sustaining a fracture. OCs, with the help of acidifying vesicles containing vacuolar-type H⁺-ATPase (V-ATPase), transport cytoplasmic protons into a resorptive pit and create an acidic microenvironment where proteolytic enzymes degrade the bone matrix. Here, we report a previously undescribed application of gold nanoparticles (AuNPs) to inhibit excessive bone resorption by regulating the acidic microenvironment in which OCs resorb bone. Internalized AuNPs, with relatively abundant carboxyl groups, eventually accumulate in the membrane of the intracellular vesicles and interact with the V0 domain of V-ATPase, which prevents it from recruiting the V1 domain. This destroys the acid-secretion function of OCs. The therapeutic effect of AuNPs on bone resorption was assessed in an established lipopolysaccharide-induced bone erosion mouse model. Micro-computed tomography, histology, and tartrate-resistant acid phosphatase staining showed that AuNPs significantly reduced bone erosion. In summary, AuNPs are promising nano-functional materials for repairing bone defects by regulating OC acid secretion.

Role of Nrf2 in Fracture Healing: Clinical Aspects of Oxidative Stress

Fracture healing is a natural process that recapitulates embryonic skeletal development. In the early phase after fracture, reactive oxygen species (ROS) are produced under inflammatory and ischemic conditions due to vessel injury and soft tissue damage, leading to cell death. Usually, such damage during the course of fracture healing can be largely prevented by protective mechanisms and functions of antioxidant enzymes. However, intrinsic oxidative stress can cause excessive toxic radicals, resulting in irreversible damage to cells associated with bone repair during the fracture healing process. Clinically, patients with type-2 diabetes mellitus, osteoporosis, habitual drinkers, or heavy smokers are at risk of impaired fracture healing due to elevated oxidative stress. Although increased levels of oxidative stress markers upon fracture and effects of antioxidants on fracture healing have been reported, a detailed understanding of what causes impaired fracture healing under intrinsic conditions of oxidative stress is lacking. Nuclear factor erythroid 2-related factor 2 (Nrf2) has been identified as a key transcriptional regulator of the expression of antioxidants and detoxifying enzymes. It further not only plays a crucial role in preventing degenerative diseases in multiple organs, but also during fracture healing. This narrative review evaluates the influence of intrinsic oxidative stress on fracture healing and sheds new light on the intriguing role of Nrf2 during bone regeneration in pathological fractures.

New Insights into the Roles of Monocytes/Macrophages in Cardiovascular Calcification Associated with Chronic Kidney Disease

Cardiovascular disease (CVD) is an important cause of death in patients with chronic kidney disease (CKD), and cardiovascular calcification (CVC) is one of the strongest predictors of CVD in this population. Cardiovascular calcification results from complex cellular interactions involving the endothelium, vascular/valvular cells (i.e., vascular smooth muscle cells, valvular interstitial cells and resident fibroblasts), and monocyte-derived macrophages. Indeed, the production of pro-inflammatory cytokines and oxidative stress by monocyte-derived macrophages is responsible for the osteogenic transformation and mineralization of vascular/valvular cells. However, monocytes/macrophages show the ability to modify their phenotype, and consequently their functions, when facing environmental modifications. This plasticity complicates efforts to understand the pathogenesis of CVC-particularly in a CKD setting, where both uraemic toxins and CKD treatment may affect monocyte/macrophage functions and thereby influence CVC. Here, we review (i) the mechanisms by which each monocyte/macrophage subset either promotes or prevents CVC, and (ii) how both uraemic toxins and CKD therapies might affect these monocyte/macrophage functions.

Assessing the Effectiveness of Panchatikta Ghrita, a Classical Ayurvedic Formulation as Add-on Therapy to Vitamin D3 and Calcium Supplements in Patients with Osteopenia: A Randomized, Open-Labeled, Comparative, Controlled Clinical Study

Objectives: To assess the change in the bone mineral density (BMD) score, bone-specific biomarkers (serum vitamin D₃, tartrate-resistant acid phosphatase 5b [TRAP-5b], and osteocalcin), quality of life, Ayurvedic symptoms (Asthikshaya Lakshanas), and fracture risk assessment tool (FRAX) scores following treatment with Panchatikta Ghrita (PG), a classical herbal formulation as add-on therapy to calcium and vitamin D₃ supplements. Study design: Randomized, open-labeled, comparative, controlled clinical study. Location: TN Medical College and BYL Nair Hospital, Mumbai, India. Study participants: Eighty adult patients, aged between 40 and 75 years, diagnosed to have osteopenia (BMD T-score between -1 and -2.5 in at least two of the three joints tested-lumbar spine L1-L4, left femur-neck, left forearm-radius total). Study intervention: Treatment group received two tablespoons of PG (10 mL in lukewarm milk) along with calcium and vitamin D₃ supplements twice a day, whereas control group received only calcium and vitamin D₃ supplements twice a day for a period of 12 months. Outcome measures: BMD, bone-specific biomarkers (vitamin D₃, TRAP-5b, and osteocalcin), quality of life, Ayurvedic symptoms, and FRAX scores were evaluated before and at 6 and 12 months. Results: Eighty patients were enrolled; of which, 65 patients completed the study while 15 patients dropped out. Improvement in the BMD scores was observed at 6 and 12 months with the maximum benefit in the lumbar spine region. Significant improvement in the bone-specific biomarkers, namely serum vitamin D₃ (p < 0.001), osteocalcin (p < 0.001), and TRAP-5b (p < 0.05), was observed in the PG-treated group compared with the standard treatment group. Improvement in the quality of life, Ayurvedic symptoms scores, and risk reduction in FRAX scores of major osteoporotic fracture risk and hip fracture risk was greater with PG, although not statistically significant. Conclusions: The study findings demonstrate that PG slows down the bone degeneration processes by its stabilizing effect on the bone-specific biomarkers, indicating its potential usefulness as preventive therapy in osteopenia. The positive improvement noted in this study needs to be confirmed in studies with a larger sample size and longer duration.

Activation of PPARs Modulates Signalling Pathways and Expression of Regulatory Genes in Osteoclasts Derived from Human CD14+ Monocytes

Osteoclasts are the sole bone resorbing cell in the body and their over activity is key in the development of osteoporosis. Osteoclastogenesis is mediated by receptor activator of nuclear factor κB ligand (RANKL) signalling pathways. Unsaturated fatty acids (UFA) are known to inhibit osteoclastogenesis by targeting RANKL signalling. However, the mechanisms of action remain unclear. Peroxisome proliferator activated receptors (PPARs) are a family of nuclear receptors, with three known isoforms (PPAR-α, PPAR-β/δ and PPAR-γ), that are known to bind UFAs and are expressed in osteoclasts. In this study, we aimed to determine how different families of UFAs activate PPARs and how PPAR activation influences osteoclast signalling. Human CD14+ monocytes were seeded into cluster plates with RANKL and macrophage colony stimulating factor (M-CSF) in the presence of PPAR agonists or different types of UFAs. All the PPAR agonists were shown to upregulate the activity of their respective receptors. Polyunsaturated fatty acids increased PPAR-α to a greater extent than monounsaturated fatty acids (MUFAs), which favoured PPAR-β/δ activation. All PPAR agonists inhibited osteoclastogenesis. The activation of RANKL signalling pathways and expression of key osteoclast genes were downregulated by PPAR agonists. This study reveals that PPAR activation can inhibit osteoclastogenesis through modulation of RANKL signalling.

G-protein Gα13 functions as a cytoskeletal and mitochondrial regulator to restrain osteoclast function

Excessive osteoclastic bone erosion disrupts normal bone remodeling and leads to bone loss in many skeletal diseases, including inflammatory arthritis, such as rheumatoid arthritis (RA) and psoriatic arthritis, periodontitis and peri-prosthetic loosening. Functional control of osteoclasts is critical for the maintenance of bone homeostasis. However, the mechanisms that restrain osteoclast resorptive function are not fully understood. In this study, we identify a previously unrecognized role for G-protein Gα13 in inhibition of osteoclast adhesion, fusion and bone resorptive function. Gα13 is highly expressed in mature multinucleated osteoclasts, but not during early differentiation. Deficiency of Gα13 in myeloid osteoclast lineage (Gα13ΔM/ΔM mice) leads to super spread morphology of multinucleated giant osteoclasts with elevated bone resorptive capacity, corroborated with an osteoporotic bone phenotype in the Gα13ΔM/ΔM mice. Mechanistically, Gα13 functions as a brake that restrains the c-Src, Pyk2, RhoA-Rock2 mediated signaling pathways and related gene expressions to control the ability of osteoclasts in fusion, adhesion, actin cytoskeletal remodeling and resorption. Genome wide analysis reveals cytoskeleton related genes that are suppressed by Gα13, identifying Gα13 as a critical cytoskeletal regulator in osteoclasts. We also identify a genome wide regulation of genes responsible for mitochondrial biogenesis and function by Gα13 in osteoclasts. Furthermore, the significant correlation between Gα13 expression levels, TNF activity and RA disease activity in RA patients suggests that the Gα13 mediated mechanisms represent attractive therapeutic targets for diseases associated with excessive bone resorption.

Adenosine A2A Receptor Mediates Inhibition of Synovitis and Osteoclastogenesis after Electroacupuncture in Rats with Collagen-Induced Arthritis

Background

This study was to investigate the role of adenosine A2A receptors (A2AR) in inhibiting the effect of electroacupuncture (EA) on osteoclastogenesis in collagen-induced arthritis (CIA).

Methods

Wistar rats were divided into four groups: sham-control group, CIA-control group, CIA-EA group, and CIA-EA-SCH58261 (A2AR antagonist) group. We detected tumor necrosis factor-α (TNF-α), nuclear transcription factor-κB (NF-κB), receptor activator of NF-κB ligand (RANKL), protein kinase A (PKA), and extracellular regulatory protein kinase 1/2 (ERK1/2) in peripheral blood by ELISA. PKA, ERK1/2, and NF-κB in ankle joints were determined by western blotting. We evaluated the arthritis damage by histological examination and determined the number of osteoclasts by tartrate-resistant acid phosphatase (TRAP) staining.

Results

EA treatment downregulated the expression of TNF-α, RANKL, PKA, ERK1/2, and NF-κB in peripheral blood but increased the levels of PKA and ERK1/2 in ankle joints. Importantly, EA treatment reduced bone erosion as evidenced by the histological findings and inhibited osteoclastogenesis as revealed by TRAP staining. All these effects of the EA treatment were reversed by combining EA treatment with the A2AR antagonist SCH58261.

Conclusion

Our data suggest that EA treatment activated A2AR. The effects of the A2AR antagonist SCH58261 suggest that the inhibition of osteoclast formation, the inhibition of TNF-α, RANKL, and NF-κB expression, and the increase of ERK1/2 are all dependent on this EA-induced A2AR activation. It is therefore likely that these pathways with clearly defined roles in inflammation and bone erosion are at least partially involved in the mediation of the inhibition of synovitis and osteoclast formation induced by EA.

Membrane trafficking in osteoclasts and implications for osteoporosis

Osteoclasts are large multinucleated cells exquisitely adapted to resorb bone matrix. Like other eukaryotes, osteoclasts possess an elaborate ensemble of intracellular organelles through which solutes, proteins and other macromolecules are trafficked to their target destinations via membrane-bound intermediaries. During bone resorption, membrane trafficking must be tightly regulated to sustain the structural and functional polarity of the osteoclasts’ membrane domains. Of these, the ruffled border (RB) is most characteristic, functioning as the osteoclasts' secretory apparatus. This highly convoluted organelle is classically considered to be formed by the targeted fusion of acidic vesicles with the bone-facing plasma membrane. Emerging findings disclose new evidence that the RB is far more complex than previously envisaged, possessing discrete subdomains that are serviced by several intersecting endocytic, secretory, transcytotic and autophagic pathways. Bone-resorbing osteoclasts therefore serve as a unique model system for studying polarized membrane trafficking. Recent advances in high-resolution microscopy together with the convergence of genetic and cell biological studies in humans and in mice have helped illuminate the major membrane trafficking pathways in osteoclasts and unmask the core molecular machinery that governs these distinct vesicle transport routes. Among these, small Rab GTPases, their binding partners and members of the endocytic sorting nexin family have emerged as critical regulators. This mini review summarizes our current understanding of membrane trafficking in osteoclasts, the key molecular participants, and discusses how these transport machinery may be exploited for the development of new therapies for metabolic disorders of bone-like osteoporosis.

Molecular signaling in bone cells: Regulation of cell differentiation and survival

The achievement of proper bone mass and architecture, and their maintenance throughout life requires the concerted actions of osteoblasts, the bone forming cells, and osteoclasts, the bone resorbing cells. The differentiation and activity of osteoblasts and osteoclasts are regulated by molecules produced by matrix-embedded osteocytes, as well as by cross talk between osteoblasts and osteoclasts through secreted factors. In addition, it is likely that direct contact between osteoblast and osteoclast precursors, and the contact of these cells with osteocytes and cells in the bone marrow, also modulates bone cell differentiation and function. With the advancement of molecular and genetic tools, our comprehension of the intracellular signals activated in bone cells has evolved significantly, from early suggestions that osteoblasts and osteoclasts have common precursors and that osteocytes are inert cells in the bone matrix, to the very sophisticated understanding of a network of receptors, ligands, intracellular kinases/phosphatases, transcription factors, and cell-specific genes that are known today. These advances have allowed the design and FDA-approval of new therapies to preserve and increase bone mass and strength in a wide variety of pathological conditions, improving bone health from early childhood to the elderly. We have summarized here the current knowledge on selected intracellular signal pathways activated in osteoblasts, osteocytes, and osteoclasts.

Phenobarbital inhibits osteoclast differentiation and function through NF-κB and MAPKs signaling pathway

The purpose of this study was to determine the direct effects of phenobarbital (PB) on receptor activator of nuclear factor kappa-B ligand (RANKL) induced osteoclast differentiation and function in vitro and in vivo. Here, PB significantly inhibited osteoclast formation and bone resorption ability induced by RANKL in vitro. Meanwhile, intracellular signaling transduction analysis revealed PB specifically decreasing the phosphorylation level of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK), respectively. Besides, oral administration of PB at the dose of 60 mg/kg/day for 6 weeks led to improve the bone loss and to decrease the activity on both osteoblast and osteoclast. This suppression effect is more obvious in osteoblast-induced bone formation than that on osteoclast-induced bone resorption. Taken together, our findings demonstrated that PB down-regulate osteoclast differentiation and activity through modulation of NF-κB and MAPKs signaling pathway. The direct suppression effect on osteoclast can induce bone loss after long term oral administration. This bone loss is due to reducing bone turnover rate on both sides of bone formation and bone resorption.

Bone Tissue Engineering Using Human Cells: A Comprehensive Review on Recent Trends, Current Prospects, and Recommendations

The use of proper cells for bone tissue engineering remains a major challenge worldwide. Cells play a pivotal role in the repair and regeneration of the bone tissue in vitro and in vivo. Currently, a large number of differentiated (somatic) and undifferentiated (stem) cells have been used for bone reconstruction alone or in combination with different biomaterials and constructs (e.g., scaffolds). Although the results of the cell transplantation without any supporting or adjuvant material have been very effective with regard to bone healing. Recent advances in bone scaffolding are now becoming new players affecting the osteogenic potential of cells. In the present study, we have critically reviewed all the currently used cell sources for bone reconstruction and discussed the new horizons that are opening up in the context of cell-based bone tissue engineering strategies.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Highly Dispersed Fullerenols Hamper Osteoclast Ruffled Border Formation by Perturbing Ca2+ Bundles

Osteoporosis, a common and serious bone disorder affecting aged people and postmenopausal women, is characterized by osteoclast overactivity. One therapeutic strategy is suppressing the bone resorption function of hyperactive osteoclasts, but there is no effective drug in clinical practice so far. Herein, it is demonstrated that fullerenols suppress the bone resorption of osteoclasts by inhibiting ruffled borders (RBs) formation. The RBs formation, which is supported by well-aligned actin bundles (B-actins), is a critical event for osteoclast bone resorption. To facilitate this function, osteoclast RBs dynamics is regulated by variable microenvironments to bundle F-actins, protrude cell membrane, and so on. B-actin perturbation by fullerenols is determined here, offering an opportunity to regulate osteoclast function by destroying RBs. In vivo, the therapeutic effect of fullerenols on overactive osteoclasts is confirmed in a mouse model of lipopolysaccharide-induced bone erosion. Collectively, the findings suggest that fullerenols adhere to F-actin surfaces and inhibit RBs formation in osteoclasts, mainly through hampering Ca²⁺ from bundling F-actins, and this is likely due to the stereo-hindrance effect caused by adherent fullerenols.

Low-Dose Radiotherapy Has No Harmful Effects on Key Cells of Healthy Non-Inflamed Joints

Low-dose radiotherapy (LD-RT) for benign inflammatory and/or bone destructive diseases has been used long. Therefore, mechanistic investigations on cells being present in joints are mostly made in an inflammatory setting. This raises the question whether similar effects of LD-RT are also seen in healthy tissue and thus might cause possible harmful effects. We performed examinations on the functionality and phenotype of key cells within the joint, namely on fibroblast-like synoviocytes (FLS), osteoclasts and osteoblasts, as well as on immune cells. Low doses of ionizing radiation showed only a minor impact on cytokine release by healthy FLS as well as on molecules involved in cartilage and bone destruction and had no significant impact on cell death and migration properties. The bone resorbing abilities of healthy osteoclasts was slightly reduced following LD-RT and a positive impact on bone formation of healthy osteoblasts was observed after in particular exposure to 0.5 Gray (Gy). Cell death rates of bone-marrow cells were only marginally increased and immune cell composition of the bone marrow showed a slight shift from CD8⁺ to CD4⁺ T cell subsets. Taken together, our results indicate that LD-RT with particularly a single dose of 0.5 Gy has no harmful effects on cells of healthy joints.

Effects on the hepatic transcriptome of chicken embryos in ovo exposed to phenobarbital

This work aimed at evaluating the toxic effects of in ovo exposure to phenobarbital (PB) and unveiling the mode of action by transcriptome analysis in the embryonic liver of a model avian species, chicken (Gallus gallus). Embryos were initially treated with saline or 1 μg PB /g egg at Hamburger Hamilton Stage (HHS) 1 (1st day), followed by 20 days of incubation to HHS 46. At 21st day, chicks that pipped successfully were euthanized and dissected for assessing the PB caused effects on phenotypes and the liver transcriptome in both genders. In the PB treatment group, a 7% attenuation in tarsus length was found in females. While no adverse phenotypic effect on the liver somatic index (LSI) was observed, PB caused significant changes in the expressions of 52 genes in males and 516 genes in females (False Discovery Rate < 0.2, p value < 0.05, and absolute fold change > 2). PB exposure modulated the genes primarily enriched in the biological pathways of the cancer, cardiac development, immune response, lipid metabolism, and skeletal development in both genders, and altered expressions of genes related to the cellular process and neural development in females. However, mRNA expressions of chicken xenobiotic receptor (CXR)-mediated CYP genes were not induced in the PB treatment groups, regardless of males and females. On the contrary, PB exposure repressed the mRNA expressions of CYP2AC2 in males and CYP2R1, CYP3A37, and CYP8B1 in females. Although transcription factors (TFs) including SREBF1 and COUP-TFII were predicted to be commonly activated in both genders, some TFs were activated in a gender-dependent manner, such as PPARa in males and BRCA1 and IRF9 in females. Taken together, our results provided an insight into the mode of action of PB on the chicken embryos.

Identification of N-(5-(phenoxymethyl)-1,3,4-thiadiazol-2-yl)acetamide derivatives as novel protein tyrosine phosphatase epsilon inhibitors exhibiting anti-osteoclastic activity

Cytosolic protein tyrosine phosphatase epsilon (cyt-PTPε) plays a central role in controlling differentiation and function of osteoclasts, whose overactivation causes osteoporosis. Based on our previous study reporting a number of cyt-PTPε inhibitory chemical compounds, we carried out a further and extended analysis of our compounds to examine their effects on cyt-PTPε-mediated dephosphorylation and on osteoclast organization and differentiation. Among five compounds showing target selectivity to cyt-PTPε over three other phosphatases in vitro, two compounds exhibited an inhibitory effect against the dephosphorylation of cellular Src protein, the cyt-PTPε substrate. Moreover, these two compounds caused destabilization of the podosome structure that is necessary for the bone-resorbing activity of osteoclasts, and also attenuated cellular differentiation of monocytes into osteoclasts, without affecting cell viability. Therefore, these findings not only verified anti-osteoclastic effects of our cyt-PTPε inhibitory compounds, but also showed that cyt-PTPε expressed in osteoclasts could be a putative therapeutic target worth considering.

Genetic Analysis of CLCN7 in an Old Female Patient with Type II Autosomal Dominant Osteopetrosis

BACKGROUND

Type II autosomal dominant osteopetrosis (ADO II) is a rare genetically heterogeneous disorder characterized by osteosclerosis and increased bone mass, predominantly involving spine, pelvis, and skull. It is closely related to functional defect of osteoclasts caused by chloride voltage-gated channel 7 (CLCN7) gene mutations. In this study, we aimed to identify the pathogenic mutation in a Korean patient with ADO II using whole exome sequencing.

METHODS

We evaluated the clinical, biochemical, and radiographic analysis of a 68-year-old woman with ADO II. We also performed whole exome sequencing to identify pathogenic mutation of a rare genetic disorder of the skeleton. Moreover, a polymorphism phenotyping program, Polymorphism Phenotyping v2 (PolyPhen-2), was used to assess the effect of the identified mutation on protein function.

RESULTS

Whole exome sequencing using peripheral leukocytes revealed a heterozygous c.296A>G missense mutation in the CLCN7 gene. The mutation was also confirmed using Sanger sequencing. The mutation c.296A>G was regarded to have a pathogenic effect by PolyPhen-2 software.

CONCLUSION

We detect a heterozygous mutation in CLCN7 gene of a patient with ADO II, which is the first report in Korea. Our present findings suggest that symptoms and signs of ADO II patient having a c.296A>G mutation in CLCN7 may appear at a very late age. The present study would also enrich the database of CLCN7 mutations and improve our understanding of ADO II.

Effects of Ca2+/calmodulin‑dependent protein kinase pathway inhibitor KN93 on osteoclastogenesis

The aim of the present study was to determine the effects of the Ca2+/calmodulin‑dependent protein kinase pathway inhibitor KN93 on osteoclastogenesis. RAW264.7 cells were incubated with macrophage colony‑stimulating factor (M‑CSF) + receptor activator of nuclear factor kappa‑light‑chain‑enhancer of activated B cells ligand (RANKL) to stimulate osteoclastogenesis and then treated with 10 µM KN93. The methods included tartrate‑resistant acid phosphatase (TRAP) staining, bone resorption activity assays, filamentous (F)‑actin staining, determination of intracellular calcium ([Ca2+]i) levels, monitoring of osteoclast‑specific gene expression levels and measurement of key transcription factors protein levels. The results suggested that KN93 inhibited the formation of TRAP‑positive multinucleated cells, shaping of F‑actin rings and resorption activity of the cells. In addition, KN93 decreased the concentration of [Ca2+]i, expression levels of osteoclast specific genes and protein levels of critical transcription factors in the M‑CSF + RANKL‑induced osteoclast model. In summary, KN93 may directly affect the differentiation and activation of osteoclasts, potentially through the Ca2+/calmodulin‑dependent protein kinase signaling pathway.

WITHDRAWN: Catalytically inactive phosphatase MTMR12 is a novel regulator of osteoclast function through F-actin ring formation

This article has been withdrawn as it was submitted without the knowledge of several co-authors. Under journal policies, all listed authors must have provided final approval of the submitted manuscript and the Corresponding Author is asked to confirm this approval during the submission process. Several of the listed co-authors have stated that they were not involved in the drafting of the manuscript and were not made aware of their inclusion as authors. Therefore they have been removed from this record. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

The Induction of Bone Formation: The Translation Enigma

A paradigmatic shift in the way of thinking is what bone tissue engineering science requires to decrypt the translation conundrum from animal models into human. The deductive work of Urist (1965), who discerned the principle of bone induction from the pioneering works of Senn, Huggins, Lacroix, Levander, and other bone regenerative scientists, provided the basis that has assisted future bone tissue regenerative scientists to extend the bone tissue engineering field and its potential uses for bone regenerative medicine in humans. However, major challenges remain that are preventing the formation of bone by induction clinically. Growing experimental evidence is indicating that bone inductive studies are non-translatable from animal models into a clinical environment. This is preventing bone tissue engineering from reaching the next phase in development. Countless studies are trying to discern how the formation of bone by induction functions mechanistically, so as to try and solve this enigmatic problem. However, are the correct questions being asked? Why do bone inductive animal studies not translate into humans? Why do bone induction principles not yield the same extent of bone formation as an autogenous bone graft? What are bone tissue engineering scientists missing? By critically re-assessing the past and present discoveries of the bone induction field, this review article attempts to re-discover the field of bone formation by induction, identifying some key features that may have been missed. These include a detailed library of all proteins in bones and their arrangement in the 3D superstructure of the bone together with some other important criteria not considered by tissue engineering scientists. The review therefore not only re-iterates possible avenues of research that need to be re-explored but also seeks to guide present and future scientists in how they assess their own research in light of experimental design and results. By addressing these issues bone formation by induction without autografts might finally become clinically viable.

Nonclinical and clinical pharmacological characterization of the potent and selective cathepsin K inhibitor MIV-711

BACKGROUND

Cathepsin K is an attractive therapeutic target for diseases in which bone resorption is excessive such as osteoporosis and osteoarthritis (OA). The current paper characterized the pharmacological profile of the potent and selective cathepsin K inhibitor, MIV-711, in vitro and in cynomolgus monkeys, and assessed translation to human based on a single dose clinical study in man.

METHODS

The potency and selectivity of MIV-711 were assessed in vitro using recombinant enzyme assays and differentiated human osteoclasts. MIV-711 was administered to healthy cynomolgus monkeys (3-30 µmol/kg, p.o.). Plasma levels of MIV-711 and the bone resorption biomarker CTX-I were measured after single dose experiments, and urine levels of CTX-I, NTX-I and CTX-II biomarkers were measured after repeat dose experiments. The safety, pharmacokinetics and pharmacodynamics (serum CTX-I) of MIV-711 were assessed in human healthy subjects after single ascending doses from 20 to 600 mg.

RESULTS

MIV-711 was a potent inhibitor of human cathepsin K (K_i: 0.98 nmol/L) with > 1300-fold selectivity towards other human cathepsins. MIV-711 inhibited human osteoclast-mediated bone resorption with an IC₅₀ value of 43 nmol/L. Single oral doses of MIV-711 to monkeys reduced plasma levels of CTX-I in a dose-dependent fashion by up to 57% at trough. The effect on CTX-I was linearly correlated to the plasma exposure of MIV-711, while the efficacy duration outlasted plasma exposure. Repeat oral dosing with MIV-711 also reduced urinary levels of the bone resorption biomarkers CTX-I (by 93%) and NTX-I (by 71%) and the cartilage degradation biomarker CTX-II (by 71%). MIV-711 was safe and well-tolerated when given as single ascending doses to healthy subjects. MIV-711 reduced serum CTX-I levels in a dose-dependent manner by up to 79% at trough. The relationship between MIV-711 exposure and effects on these biomarkers in humans was virtually identical when compared to the corresponding monkey data.

CONCLUSIONS

MIV-711 is a potent and selective cathepsin K inhibitor with dose-dependent effects on biomarkers of bone and cartilage degradation in monkey and human. Taken together, MIV-711 shows promise for the treatment of bone and cartilage related disorders in humans, such as OA. Trial Registration EudraCT number 2011-003024-12, registered on June 22nd 2011.

Bone remodelling in vitro: Where are we headed?: -A review on the current understanding of physiological bone remodelling and inflammation and the strategies for testing biomaterials in vitro

Bone remodelling is a dynamic process required for the maintenance of bone architecture in response to the changing mechanical needs. It is also a vital process during the repair of bone tissue following injury. Clinical intervention in terms of autografting or allografting is often required to heal bone injuries where physiological healing fails. The use of biomaterials as alternatives to autografts and allografts has spurred a significant research interest into further development of biomaterials for better clinical outcomes. Unfortunately, many biomaterials fail to make it to the clinic or fail after implantation due to the inconsistencies observed between in vitro and in vivo studies. It is therefore important to mimic the in vivo situation as closely as possible in an in vitro setting for testing biomaterials. The current in vitro models focus mostly on investigating the behaviour of osteoblast progenitors with the biomaterial under development as well as assessing the behaviour of osteoclasts, endothelial cells etc. However, the sequence of events that take place during bone healing or remodelling are not incorporated into the current in vitro models. This review highlights our current understanding of the physiological bone remodelling and the bone healing process followed by strategies to incorporate both the physiological and pathophysiological events into an in vitro environment. Here, we propose three strategies for the assessment of biomaterials for bone, which includes; (1) testing biomaterials in the presence of immune cells, (2) testing biomaterials for osteogenesis, and (3) testing biomaterials in the presence of osteoclasts followed by osteoblasts to recapitulate the physiological events of bone resorption prior to bone formation. The focus of this review is to discuss the third strategy in details as the first two strategies are currently incorporated into a majority of in vitro experiments.

Essential Role of the a3 Isoform of V-ATPase in Secretory Lysosome Trafficking via Rab7 Recruitment

Secretory lysosomes are required for the specialised functions of various types of differentiated cells. In osteoclasts, the lysosomal proton pump V-ATPase (vacuolar-type ATPase) is targeted to the plasma membrane via secretory lysosomes and subsequently acidifies the extracellular compartment, providing optimal conditions for bone resorption. However, little is known about the mechanism underlying this trafficking of secretory lysosomes. Here, we demonstrate that the lysosome-specific a3 isoform of the V-ATPase a subunit plays an indispensable role in secretory lysosome trafficking, together with Rab7, a small GTPase involved in organelle trafficking. In osteoclasts lacking a3, lysosomes were not transported to the cell periphery, and Rab7 was not localised to lysosomes but diffused throughout the cytoplasm. Expression of dominant-negative (GDP-bound form) Rab7 inhibited lysosome trafficking in wild-type cells. Furthermore, a3 directly interacted with the GDP-bound forms of Rab7 and Rab27A. These findings reveal a novel role for the proton pump V-ATPase in secretory lysosome trafficking and an unexpected mechanistic link with Rab GTPases.

Osteoclasts degrade bone and cartilage knee joint compartments through different resorption processes

Background

Osteoclasts have been strongly implicated in osteoarthritic cartilage degradation, at least indirectly via bone resorption, and have been shown to degrade cartilage in vitro. The osteoclast resorption processes required to degrade subchondral bone and cartilage—the remodeling of which is important in the osteoarthritic disease process—have not been previously described, although cathepsin K has been indicated to participate. In this study we profile osteoclast-mediated degradation of bovine knee joint compartments in a novel in vitro model using biomarkers of extracellular matrix (ECM) degradation to assess the potential of osteoclast-derived resorption processes to degrade different knee joint compartments.

Methods

Mature human osteoclasts were cultured on ECMs isolated from bovine knees—articular cartilage, cortical bone, and osteochondral junction ECM (a subchondral bone-calcified cartilage mixture)—in the presence of inhibitors: the cystein protease inhibitor E-64, the matrix metalloproteinase (MMP) inhibitor GM6001, or the vacuolar-type H⁺-ATPase (V-ATPase) inhibitor diphyllin. Biomarkers of bone (calcium and C-terminal type I collagen (CTX-I)) and cartilage (C2M) degradation were measured in the culture supernatants. Cultures without osteoclasts were used as background samples. Background-subtracted biomarker levels were normalized to the vehicle condition and were analyzed using analysis of variance with Tukey or Dunnett’s T3 post hoc test, as applicable.

Results

Osteochondral CTX-I release was inhibited by E-64 (19% of vehicle, p = 0.0008), GM6001 (51% of vehicle, p = 0.013), and E-64/GM6001 combined (4% of vehicle, p = 0.0007)—similarly to bone CTX-I release. Diphyllin also inhibited osteochondral CTX-I release (48% of vehicle, p = 0.014), albeit less than on bone (4% of vehicle, p < 0.0001). Osteochondral C2M release was only inhibited by E-64 (49% of vehicle, p = 0.07) and GM6001 (14% of vehicle, p = 0.006), with complete abrogation when combined (0% of vehicle, p = 0.004). Cartilage C2M release was non-significantly inhibited by E-64 (69% of vehicle, p = 0.98) and was completely abrogated by GM6001 (0% of vehicle, p = 0.16).

Conclusions

Our study supports that osteoclasts can resorb non-calcified and calcified cartilage independently of acidification. We demonstrated both MMP-mediated and cysteine protease-mediated resorption of calcified cartilage. Osteoclast functionality was highly dependent on the resorbed substrate, as different ECMs required different osteoclast processes for degradation. Our novel culture system has potential to facilitate drug and biomarker development aimed at rheumatic diseases, e.g. osteoarthritis, where pathological osteoclast processes in specific joint compartments may contribute to the disease process.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1564-5) contains supplementary material, which is available to authorized users.

The Sealing Zone in Osteoclasts: A Self-Organized Structure on the Bone

Osteoclasts form a specialized cell-matrix adhesion structure, known as the "sealing zone", during bone resorption. The sealing zone is a dynamic actin-rich structure that defines the resorption area of the bone. The detailed dynamics and fine structure of the sealing zone have been elusive. Osteoclasts plated on glass do not form a sealing zone, but generate a separate supra-molecular structure called the "podosome belt". Podosomes are integrin-based adhesion complexes involved in matrix adhesion, cell migration, matrix degradation, and mechanosensing. Invadopodia, podosome-like protrusions in cancer cells, are involved in cell invasion into other tissues by promoting matrix degradation. Both podosomes and invadopodia exhibit actin pattern transitions during maturation. We previously found that Arp2/3-dependent actin flow occurs in all observed assembly patterns of podosomes in osteoclasts on glass. It is known that the actin wave in Dictyostelium cells exhibits a similar pattern transition in its evolution. Because of significant advances in our understanding regarding the mechanism of podosomes/invadopodia formation over the last decade, we revisited the structure and function of the sealing zone in this review, highlighting the possible involvement of self-organized actin waves in the organogenesis of the sealing zone.

Bone degradation machinery of osteoclasts: An HIV-1 target that contributes to bone loss

Bone deficits are frequent in HIV-1-infected patients. We report here that osteoclasts, the cells specialized in bone resorption, are infected by HIV-1 in vivo in humanized mice and ex vivo in human joint biopsies. In vitro, infection of human osteoclasts occurs at different stages of osteoclastogenesis via cell-free viruses and, more efficiently, by transfer from infected T cells. HIV-1 infection markedly enhances adhesion and osteolytic activity of human osteoclasts by modifying the structure and function of the sealing zone, the osteoclast-specific bone degradation machinery. Indeed, the sealing zone is broader due to F-actin enrichment of its basal units (i.e., the podosomes). The viral protein Nef is involved in all HIV-1-induced effects partly through the activation of Src, a regulator of podosomes and of their assembly as a sealing zone. Supporting these results, Nef-transgenic mice exhibit an increased osteoclast density and bone defects, and osteoclasts derived from these animals display high osteolytic activity. Altogether, our study evidences osteoclasts as host cells for HIV-1 and their pathological contribution to bone disorders induced by this virus, in part via Nef.

Glucagon-like peptide 2 decreases osteoclasts by stimulating apoptosis dependent on nitric oxide synthase

OBJECTIVES

Glucagon-like peptide 2 (GLP2) is involved in the regulation of energy absorption and metabolism. Despite the importance of the GLP2 signalling mechanisms on osteoclast, little has been studied on how GLP2 works during osteoclastogenesis.

MATERIALS AND METHODS

RAW264.7 cells were infected with rLV-Green-GLP2. The induction of osteoclasts was performed by RANKL. TRAP were detected by RT-PCR, Western blotting and staining. Total nitric oxide and total NOS activity were measured. Cells apoptosis was detected by Hoest33258 and Annix V staining. Animal test, chromatin immunoprecipitation (CHIP), co-immunoprecipitation(IP) and luciferase reporter assay were also performed.

RESULTS

We indicate that GLP2 is associated with osteoporosis-related factors in aged rats, including BALP, TRAP, IL6, TNFα, Nitric Oxide (NO), iNOS, calcitonin and occludin. Moreover, GLP2 is demonstrated to result in negative action during proliferation of tartrate-resistant acid phosphatase-positive (TRAP+) osteoclasts. Furthermore, GLP2 decreases osteoclasts induced from monocyte/macrophage cells RAW264.7 as well as the serum TRAP activity in aged rats. Mechanistic investigations reveal GLP2 enhances the expression of iNOS through stimulating the activity of TGFβ-Smad2/3 signalling in osteoclasts. In particular, inhibition of TGFβ fully abrogates this function of GLP2 in osteoclasts. Strikingly, overexpression of GLP2 significantly increases the product of nitric oxide via iNOS which promotes apoptosis of osteoclasts by decreasing bcl2 or increasing caspase3. Thereby, the ability of GLP2 to regulate apoptosis depends on TGFβ-Smad2/3-iNOS-NO signalling pathway since total NOS inhibitor L-NMMA specifically inhibits the actions by GLP2.

CONCLUSIONS

GLP2 induces apoptosis via TGFβ-Smad2/3 signalling, which contributes to the inhibition of the proliferation of osteoclasts and which may provide potential therapeutic targets for the treatment of osteoporosis.

New bioactive glass scaffolds with exceptional qualities for bone tissue regeneration: response of osteoblasts and osteoclasts

Tissue regeneration is a significantly improved alternative to tissue replacement by implants. It requires porous bioscaffolds for the restoration of natural tissue rather than relying on bio-inactive, often metallic implants. Recently, we developed technology for fabricating novel, nano-macroporous bioactive 'tailored amorphous multi-porous (TAMP)' hard tissue scaffolds using a 70 mol% SiO₂-30 mol% CaO model composition. The TAMP silicate scaffolds, fabricated by a modified sol-gel process, have shown excellent biocompatibility via the rapid formation of hydroxyapatite in biological fluids as well as in early tests with bone forming cells. Here we report an in depth investigation of the response of MC3T3-E1 pre-osteoblast cells and bone marrow derived (BMD) osteoclasts to these TAMP scaffolds. Light and electron microscopic imaging, gene and protein expression, and enzyme activity analyses demonstrate that MC3T3-E1 pre-osteoblasts adhere, proliferate, colonize, and differentiate on and inside the bioactive TAMP scaffolds. Additionally, BMD precursor cells mature into active osteoclasts and remodel the scaffold, highlighting the exceptional qualities of this novel scaffold material for bone tissue regeneration.

The Oxygen Paradox, the French Paradox, and age-related diseases

A paradox is a seemingly absurd or impossible concept, proposition, or theory that is often difficult to understand or explain, sometimes apparently self-contradictory, and yet ultimately correct or true. How is it possible, for example, that oxygen "a toxic environmental poison" could be also indispensable for life (Beckman and Ames Physiol Rev 78(2):547-81, 1998; Stadtman and Berlett Chem Res Toxicol 10(5):485-94, 1997)?: the so-called Oxygen Paradox (Davies and Ursini 1995; Davies Biochem Soc Symp 61:1-31, 1995). How can French people apparently disregard the rule that high dietary intakes of cholesterol and saturated fats (e.g., cheese and paté) will result in an early death from cardiovascular diseases (Renaud and de Lorgeril Lancet 339(8808):1523-6, 1992; Catalgol et al. Front Pharmacol 3:141, 2012; Eisenberg et al. Nat Med 22(12):1428-1438, 2016)?: the so-called, French Paradox. Doubtless, the truth is not a duality and epistemological bias probably generates apparently self-contradictory conclusions. Perhaps nowhere in biology are there so many apparently contradictory views, and even experimental results, affecting human physiology and pathology as in the fields of free radicals and oxidative stress, antioxidants, foods and drinks, and dietary recommendations; this is particularly true when issues such as disease-susceptibility or avoidance, "healthspan," "lifespan," and ageing are involved. Consider, for example, the apparently paradoxical observation that treatment with low doses of a substance that is toxic at high concentrations may actually induce transient adaptations that protect against a subsequent exposure to the same (or similar) toxin. This particular paradox is now mechanistically explained as "Adaptive Homeostasis" (Davies Mol Asp Med 49:1-7, 2016; Pomatto et al. 2017a; Lomeli et al. Clin Sci (Lond) 131(21):2573-2599, 2017; Pomatto and Davies 2017); the non-damaging process by which an apparent toxicant can activate biological signal transduction pathways to increase expression of protective genes, by mechanisms that are completely different from those by which the same agent induces toxicity at high concentrations. In this review, we explore the influences and effects of paradoxes such as the Oxygen Paradox and the French Paradox on the etiology, progression, and outcomes of many of the major human age-related diseases, as well as the basic biological phenomenon of ageing itself.

Novel mutations of TCIRG1 cause a malignant and mild phenotype of autosomal recessive osteopetrosis (ARO) in four Chinese families

Human autosomal recessive osteopetrosis (ARO), also known as infantile malignant osteopetrosis, is a rare genetic bone disorder that often causes death. Mutations in T-cell immune regulator 1 (TCIRG1) are a frequent cause of human ARO. Six additional genes (TNFSF11, TNFRSF11A, CLCN7, OSTM1, SNX10, PLEKHM1) were also found to be associated with human ARO. In order to expand the mutation spectrum and clinical diversity for a better understanding of the ARO phenotype and to further investigate the clinical characteristics of benign subjects with ARO, we here report five individuals with ARO from four unrelated Chinese families. X-ray examination was conducted and bone turnover markers were assayed. The gene of T-cell immune regulator 1 (TCIRG1) was screened and analyzed. Monocyte-induced osteoclasts were prepared and their resorption ability was studied in vitro. We identified five novel mutations (c.66delC, c.1020+1_1020+5dup, c.2181C>A, c.2236+6T>G, c.692delA) in these patients. Four patients displayed a malignant phenotype, three of them died, and one who received bone marrow transplantation survived. The remaining one, a 24-year-old male from a consanguineous family, was diagnosed based on radiological findings but presented no neurological or hematological defects. He was homozygous for c.2236+6T>G in intron 18; this mutation influenced the splicing process. An in vitro functional study of this novel splicing defect showed no resorption pits on dentine slices. TCIRG1-dependent osteopetrosis with a mild clinical course was observed for the first time in Chinese population. The present findings add to the wide range of phenotypes of Chinese patients with TCIRG1-dependent ARO and enrich the database of TCIRG1 mutations.

CD169+ macrophages are critical for osteoblast maintenance and promote intramembranous and endochondral ossification during bone repair

Osteal macrophages (osteomacs) contribute to bone homeostasis and regeneration. To further distinguish their functions from osteoclasts, which share many markers and growth factor requirements, we developed a rapid, enzyme-free osteomac enrichment protocol that permitted characterization of minimally manipulated osteomacs by flow cytometry. Osteomacs differ from osteoclasts in expression of Siglec1 (CD169). This distinction was confirmed using the CD169-diphtheria toxin (DT) receptor (DTR) knock-in model. DT treatment of naïve CD169-DTR mice resulted in selective and striking loss of osteomacs, whilst osteoclasts and trabecular bone area were unaffected. Consistent with a previously-reported trophic interaction, osteomac loss was accompanied by a concomitant and proportionately striking reduction in osteoblasts. The impact of CD169⁺ macrophage depletion was assessed in two models of bone injury that heal via either intramembranous (tibial injury) or endochondral (internally-plated femoral fracture model) ossification. In both models, CD169⁺ macrophage, including osteomac depletion compromised bone repair. Importantly, DT treatment in CD169-DTR mice did not affect osteoclast frequency in either model. In the femoral fracture model, the magnitude of callus formation correlated with the number of F4/80⁺ macrophages that persisted within the callus. Overall these observations provide compelling support that CD169⁺ osteomacs, independent of osteoclasts, provide vital pro-anabolic support to osteoblasts during both bone homeostasis and repair.

Molecular mechanisms of the inhibitory effects of jiangu granule‑containing serum on RANKL‑induced osteoclastogenesis

Postmenopausal osteoporosis (PMOP) is characterized by increased bone loss due to enhanced osteoclastogenesis and bone resorption. A Chinese herbal formula, jiangugranule (JG), exhibited great efficacy in the clinical treatment of PMOP. However, the molecular mechanisms underlying the therapeutic effects remain unclear. The present study aimed to examine the effects of JG‑containing serum on receptor activator of nuclear factor‑κB (NF‑κB) ligand (RANKL)‑induced osteoclastogenesis. Osteoclast precursor RAW264.7 cells were cultured and treated with JG‑containing serum in the presence of RANKL. Following 6 days of culture, the cells were stained with tartrate‑resistant acid phosphatase and the rate of differentiation was calculated. In addition, cells were treated with JG‑containing serum for 24, 48 and 96 h and total RNA and proteins were extracted for reverse transcription‑quantitative polymerase chain reaction and western blot analysis to detect mRNA and protein expression, respectively, of key molecules in the RANK/RANKL signaling pathway, including RANK, tumor necrosis factor receptor‑associated factor 6, NF‑κB (p50 and p52 subunits), c‑Fos and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1). The results revealed that JG‑containing serum inhibited RANKL‑induced osteoclastogenesis and reduced mRNA and protein expression of RANK, c‑Fos and NFATc1. The results suggested that JG may regulate osteoclast differentiation through the RANK/RANKL signaling pathway, which may be a possible mechanism for the therapeutic effects of JG on PMOP.

The biology of normal bone remodelling

During life, bone undergoes modelling and remodelling in order to grow or change shape. Bone modelling is the process by which bones change shape or size in response to physiologic influences or mechanical forces that are encountered by the skeleton, while bone remodelling takes place so that bone may maintain its strength and mineral homeostasis. During early childhood, both bone modelling (the formation and shaping of bone) and bone remodelling (the replacement or renewal of old bone) occur. The predominant process in childhood is bone modelling, while in adulthood bone remodelling predominates. The exception to this is after a fracture when we see massive increases in bone formation. During childhood and adolescence growth occurs in the bones longitudinally and radially, while in the growth plates it occurs longitudinally, thus promoting growth in size. Cartilage first proliferates in the epiphyseal and metaphyseal areas of long bones before undergoing mineralisation to form new bone.

Skeletal energy homeostasis: a paradigm of endocrine discovery

Throughout the last decade, significant developments in cellular, molecular and mouse models have revealed major endocrine functions of the skeleton. More recent studies have evolved the interplay between bone-specific hormones, the skeleton, marrow adipose tissue, muscle and the brain. This review focuses on literature from the last decade, addressing the endocrine regulation of global energy metabolism via the skeleton. In addition, we will highlight several recent studies that further our knowledge of new endocrine functions of some organs; explore remaining unanswered questions; and, finally, we will discuss future directions for this more complex era of bone biology research.

Invadosomes are coming: new insights into function and disease relevance

Invadopodia and podosomes are discrete, actin-based molecular protrusions that form in cancer cells and normal cells, respectively, in response to diverse signaling pathways and extracellular matrix cues. Although they participate in a host of different cellular processes, they share a common functional theme of controlling pericellular proteolytic activity, which sets them apart from other structures that function in migration and adhesion, including focal adhesions, lamellipodia, and filopodia. In this review, we highlight research that explores the function of these complex structures, including roles for podosomes in embryonic and postnatal development, in angiogenesis and remodeling of the vasculature, in maturation of the postsynaptic membrane, in antigen sampling and recognition, and in cell-cell fusion mechanisms, as well as the involvement of invadopodia at multiple steps of the metastatic cascade, and how all of this may apply in the treatment of human disease states. Finally, we explore recent research that implicates a novel role for exosomes and microvesicles in invadopodia-dependent and invadopodia-independent mechanisms of invasion, respectively.