Coupling the activities of bone formation and resorption: a multitude of signals within the basic multicellular unit

Chemokines network in bone metastasis: Vital regulators of seeding and soiling

Chemokines are well equipped with chemo-attractive signals that can regulate cancer cell trafficking to specific organ sites. Currently, updated concepts have revealed the diverse role of chemokines in the biology of cancer initiation and progression. Genomic instabilities and alterations drive tumor heterogeneity, providing more options for the selection and metastatic progression to cancer cells. Tumor heterogeneity and acquired drug resistance are the main obstacles in managing cancer therapy and the primary root cause of metastasis. Studies emphasize that multiple chemokine/receptor axis are involved in cancer cell-mediated organ-specific distant metastasis. One of the persuasive mechanisms for heterogeneity and subsequent events is sturdily interlinked with the crosstalk between chemokines and their receptors on cancer cells and tissue-specific microenvironment. Among different metastatic niches, skeletal metastasis is frequently observed in the late stages of prostate, breast, and lung cancer and significantly reduces the survival of cancer patients. Therefore, it is crucial to elucidate the role of chemokines and their receptors in metastasis and bone remodeling. Here, we review the potential chemokine/receptor axis in tumorigenesis, tumor heterogeneity, metastasis, and vicious cycle in bone microenvironment.

Analysis of Bone Histomorphometry in Rat and Guinea Pig Animal Models Subject to Hypoxia

Hypoxia may be associated with alterations in bone remodeling, but the published results are contradictory. The aim of this study was to characterize the bone morphometry changes subject to hypoxia for a better understanding of the bone response to hypoxia and its possible clinical consequences on the bone metabolism. This study analyzed the bone morphometry parameters by micro-computed tomography (μCT) in rat and guinea pig normobaric hypoxia models. Adult male and female Wistar rats were exposed to chronic hypoxia for 7 and 15 days. Additionally, adult male guinea pigs were exposed to chronic hypoxia for 15 days. The results showed that rats exposed to chronic constant and intermittent hypoxic conditions had a worse trabecular and cortical bone health than control rats (under a normoxic condition). Rats under chronic constant hypoxia were associated with a more deteriorated cortical tibia thickness, trabecular femur and tibia bone volume over the total volume (BV/TV), tibia trabecular number (Tb.N), and trabecular femur and tibia bone mineral density (BMD). In the case of chronic intermittent hypoxia, rats subjected to intermittent hypoxia had a lower cortical femur tissue mineral density (TMD), lower trabecular tibia BV/TV, and lower trabecular thickness (Tb.Th) of the tibia and lower tibia Tb.N. The results also showed that obese rats under a hypoxic condition had worse values for the femur and tibia BV/TV, tibia trabecular separation (Tb.Sp), femur and tibia Tb.N, and BMD for the femur and tibia than normoweight rats under a hypoxic condition. In conclusion, hypoxia and obesity may modify bone remodeling, and thus bone microarchitecture, and they might lead to reductions in the bone strength and therefore increase the risk of fragility fracture.

Impact of the host response and osteoblast lineage cells on periodontal disease

Periodontitis involves the loss of connective tissue attachment and alveolar bone. Single cell RNA-seq experiments have provided new insight into how resident cells and infiltrating immune cells function in response to bacterial challenge in periodontal tissues. Periodontal disease is induced by a combined innate and adaptive immune response to bacterial dysbiosis that is initiated by resident cells including epithelial cells and fibroblasts, which recruit immune cells. Chemokines and cytokines stimulate recruitment of osteoclast precursors and osteoclastogenesis in response to TNF, IL-1β, IL-6, IL-17, RANKL and other factors. Inflammation also suppresses coupled bone formation to limit repair of osteolytic lesions. Bone lining cells, osteocytes and periodontal ligament cells play a key role in both processes. The periodontal ligament contains cells that exhibit similarities to tendon cells, osteoblast-lineage cells and mesenchymal stem cells. Bone lining cells consisting of mesenchymal stem cells, osteoprogenitors and osteoblasts are influenced by osteocytes and stimulate formation of osteoclast precursors through MCSF and RANKL, which directly induce osteoclastogenesis. Following bone resorption, factors are released from resorbed bone matrix and by osteoclasts and osteal macrophages that recruit osteoblast precursors to the resorbed bone surface. Osteoblast differentiation and coupled bone formation are regulated by multiple signaling pathways including Wnt, Notch, FGF, IGF-1, BMP, and Hedgehog pathways. Diabetes, cigarette smoking and aging enhance the pathologic processes to increase bone resorption and inhibit coupled bone formation to accelerate bone loss. Other bone pathologies such as rheumatoid arthritis, post-menopausal osteoporosis and bone unloading/disuse also affect osteoblast lineage cells and participate in formation of osteolytic lesions by promoting bone resorption and inhibiting coupled bone formation. Thus, periodontitis involves the activation of an inflammatory response that involves a large number of cells to stimulate bone resorption and limit osseous repair processes.

Postmenopausal Osteoporosis: A Literature Review

A substantial proportion of the population of females in India falls in the perimenopausal and postmenopausal age groups. One of the complications associated with older age in women is the weakening of bones and the fall in bone mineral density (BMD). This has a severe debilitating consequence in a woman’s life and leads to reduced quality of life along with a greater incidence of fractures. If the fracture involves the hip or the vertebrae, it can cause immobility and be devastating. Postmenopausal osteoporosis is linked with the deficiency of estrogen that occurs with the cessation of the function of the ovaries as age progresses. The function of estrogen in the bone remodeling process is very well understood after years of research; estrogen plays a part in both the formation of bone as well as the prevention of the resorption of bone. A diagnosis can be made by dual-energy X-ray absorptiometry (DEXA). It is the gold standard and can spot low bone density at particular sites. The treatment options are selected according to the severity and rate of progression and factors pertaining to each patient. All postmenopausal women should be made aware of this disorder, and they should be encouraged to cultivate a healthy lifestyle through the implementation of a proper diet and inculcation of a regular exercise routine. Smoking and drinking alcohol should be limited, and calcium and vitamin D supplementation should be started in all women of the postmenopausal age group with or without osteoporosis. In patients who have been diagnosed with the disorder, pharmacological intervention is done. Drugs should be selected based on their side effects and contradictions. Follow-up is essential, and patient compliance should be carefully monitored. This article attempts to review the existing literature on this very prevalent disorder to spread awareness about it so that all postmenopausal women can take the necessary steps to prevent the weakening of their bones, and deal with its progression.

Traditional Chinese decoction Si Zhi Wan attenuates ovariectomy (OVX)-induced bone loss by inhibiting osteoclastogenesis and promoting apoptosis of mature osteoclasts

Si Zhi Wan (SZW) is a traditional Chinese decoction used for osteoporosis treatment. Currently, the effect of SZW on ovariectomy (OVX)-induced bone loss and the underlying mechanisms remain unknown. Herein, we investigated the therapeutic effect of SZW on osteoporosis and explored the underlying mechanisms in vitro and in vivo. An OVX-induced bone loss model was established in vivo. After administration of SZW for 8 weeks, rats were sacrificed, and the uterus was weighted to calculate its index. The femur change was pathologically evaluated using hematoxylin and eosin (H&E) staining. The mineral density of the femur was observed by micro-CT. RAW264.7 cells were activated by receptor activator of nuclear factor-κB ligand (RANKL) in vitro. The effect of SZW on osteoclastogenesis was evaluated using tartrate-resistant acid phosphatase (TRAP) staining, Western blotting, and RT-PCR. The pro-apoptosis effect of SZW on mature osteoclasts was examined after induction of osteoclast maturation. Finally, the effect of SZW on the NF-κB pathway was evaluated. Our results demonstrated that SZW ameliorated OVX-induced bone loss in rats. In addition, SZW inhibited osteoclastogenesis and attenuated osteoclast-mediated bone resorption in vitro and in vivo. SZW also promoted apoptosis of mature osteoclasts. Mechanically, SZW exerts its effects by suppressing the NF-κB pathway. Overall, these findings demonstrated that SZW may be a potentially effective alternative treatment for osteoporosis.

Current use of bone turnover markers in the management of osteoporosis

The adult bone is continuously being remodelled to repair microdamage, preserve bone strength and mechanical competence as well as maintain calcium homeostasis. Bone turnover markers are products of osteoblasts (bone formation markers) and osteoclasts (bone resorption markers) providing a dynamic assessment of remodelling (turnover). Resorption-specific bone turnover markers are typically degradation products of bone collagen molecules (N- [NTX] and C-telopeptide cross-linked type 1 collagen [CTX]), which are released into the circulation and excreted in urine; or enzymatic activities reflecting osteoclastic resorption, tartrate-resistant acid phosphatase [TRACP]. Formation-specific bone turnover markers embrace different osteoblastic activities: type 1 collagen synthesis (Procollagen type I N- propeptide [PINP]), osteoblast enzymes (bone-specific alkaline phosphatase [BALP]), or bone matrix proteins [osteocalcin]. Among individuals not receiving osteoporosis treatment, resorption and formation markers are tightly linked and highly correlated (r= 0.6-0.8). Significant biological variability was reported in the past, but these issues have been greatly improved with automated assays and attention to pre-analytical and analytical factors that are known to influence bone turnover marker levels. Bone turnover markers are not useful in the diagnosis of osteoporosis, the individual prediction of bone loss, fracture, or rare complications, or in the selection of pharmacological treatment. Despite remaining issues with reference intervals and assays harmonization, bone turnover markers have proven to be useful in elucidating the pharmacodynamics and effectiveness of osteoporosis medications in clinical trials. As an alternative to BMD testing, BTMs may be useful to monitor osteoporosis therapies.

Neuronal Guidance Molecules in Bone Remodeling and Orthodontic Tooth Movement

During orthodontic tooth movement, mechanically induced remodeling occurs in the alveolar bone due to the action of orthodontic forces. The number of factors identified to be involved in mechanically induced bone remodeling is growing steadily. With the uncovering of the functions of neuronal guidance molecules (NGMs) for skeletal development as well as for bone homeostasis, NGMs are now also among the potentially significant factors for the regulation of bone remodeling during orthodontic tooth movement. This narrative review attempts to summarize the functions of NGMs in bone homeostasis and provides insight into the currently sparse literature on the functions of these molecules during orthodontic tooth movement. Presently, four families of NGMs are known: Netrins, Slits, Semaphorins, ephrins and Eph receptors. A search of electronic databases revealed roles in bone homeostasis for representatives from all four NGM families. Functions during orthodontic tooth movement, however, were only identified for Semaphorins, ephrins and Eph receptors. For these, crucial prerequisites for participation in the regulation of orthodontically induced bone remodeling, such as expression in cells of the periodontal ligament and in the alveolar bone, as well as mechanical inducibility, were shown, which suggests that the importance of NGMs in orthodontic tooth movement may be underappreciated to date and further research might be warranted.

Altered type I collagen networking in osteoporotic human femoral head revealed by histomorphometric and Fourier transform infrared imaging correlated analyses

Bone homeostasis is the equilibrium between organic and inorganic components of the extracellular matrix (ECM) and cells. Alteration of this balance has consequences on bone mass and architecture, resulting in conditions such as osteoporosis (OP). Given ECM protein mutual regulation and their effects on bone structure and mineralization, further insight into their expression is crucial to understanding bone biology under normal and pathological conditions. This study focused on Type I Collagen, which is mainly responsible for structural properties and mineralization of bone, and selected proteins implicated in matrix composition, mineral deposition, and cell-matrix interaction such as Decorin, Osteocalcin, Osteopontin, Bone Sialoprotein 2, Osteonectin and Transforming Growth Factor beta. We developed a novel multidisciplinary approach in order to assess bone matrix in healthy and OP conditions more comprehensively by exploiting the Fourier Transform Infrared Imaging (FTIRI) technique combined with histomorphometry, Sirius Red staining, immunohistochemistry, and Western Blotting. This innovatory procedure allowed for the analysis of superimposed tissue sections and revealed that the alterations in OP bone tissue architecture were associated with warped Type I Collagen structure and deposition but not with changes in the total protein amount. The detected changes in the expression and/or cooperative or antagonist role of Decorin, Osteocalcin, Osteopontin, and Bone Sialoprotein-2 indicate the deep impact of these NCPs on collagen features of OP bone. Overall, our strategy may represent a starting point for designing targeted clinical strategies aimed at bone mass preservation and sustain the FTIRI translational capability as upcoming support for traditional diagnostic methods.

Anti-Osteoporosis Effects of the Fruit of Sea Buckthorn (Hippophae rhamnoides) through Promotion of Osteogenic Differentiation in Ovariectomized Mice

The fruit of Hippophae rhamnoides has been widely used for medicinal purposes because of its anti-inflammatory, antioxidant, antiplatelet, and antimicrobial effects. Since there are no clear reports on the therapeutic efficacy of H. rhamnoides in osteoporosis, this study aimed to confirm the potential use of H. rhamnoides for the treatment of osteoporosis through its osteogenic differentiation-promoting effect in ovariectomized mice. Through an in vitro study, we compared the effects of the EtOH extract of H. rhamnoides fruits (EHRF) on the differentiation of C3H10T1/2, a mouse mesenchymal stem cell line, into osteoblasts based on alkaline phosphatase (ALP) staining and the relative expression of osteogenesis-related mRNAs. The EHRF significantly stimulated the differentiation of mesenchymal stem cells into osteoblasts and showed 7.5 times (* p < 0.05) higher osteogenesis than in the untreated control. A solvent fractionation process of EHRF showed that the hexane-soluble fraction (HRH) showed 10.4 times (** p < 0.01) higher osteogenesis than in the untreated control. Among the subfractions derived from the active HRH by preparative HPLC fractionation, HRHF4 showed 7.5 times (* p < 0.05) higher osteogenesis than in the untreated naïve cells, and HRH and HRHF4 fractions showed 22.6 times (*** p < 0.001) stronger osteogenesis activity than in the negative control. Osteoporosis was induced by excision of both ovaries in 9-week-old female ICR mice for in vivo analysis, and two active fractions, HRH and HRHF4, were administered orally for three months. During the oral administration period, body weight was measured weekly, and bone mineral density (BMD) and body fat density were measured simultaneously using a DEXA machine once a month. In particular, during the in vivo study, the average BMD of the ovariectomized group decreased by 0.0009 g/cm2, whereas the average BMD of the HRH intake group increased by 0.0033 g/cm2 (* p < 0.05) and that of the HRHF4 intake group increased by 0.0059 g/cm2 (** p < 0.01). The HRH and HRHF4 intake groups significantly recovered the mRNA and protein expression of osteogenic genes, including ALP, Osteopontin, Runx2, and Osterix, in the osteoporosis mouse tibia. These findings suggest that the active fractions of H. rhamnoides fruit significantly promoted osteoblast differentiation in mesenchymal stem cells and increased osteogenic gene expression, resulting in an improvement in bone mineral density in the osteoporosis mouse model. Taken together, H. rhamnoides fruits are promising candidates for the prevention and treatment of osteoporosis.

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

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

Critical Roles of NF-κB Signaling Molecules in Bone Metabolism Revealed by Genetic Mutations in Osteopetrosis

The nuclear factor-κB (NF-κB) transcription factor family consists of five related proteins, RelA (p65), c-Rel, RelB, p50/p105 (NF-κB1), and p52/p100 (NF-κB2). These proteins are important not only for inflammation and the immune response but also for bone metabolism. Activation of NF-κB occurs via the classic and alternative pathways. Inflammatory cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL)-1β, activate the former, and cytokines involved in lymph node formation, such as receptor activator of NF-κB ligand (RANKL) and CD40L, activate the latter. p50 and p52 double-knockout mice revealed severe osteopetrosis due to the total lack of osteoclasts, which are specialized cells for bone resorption. This finding suggests that the activation of NF-κB is required for osteoclast differentiation. The NF-κB signaling pathway is controlled by various regulators, including NF-κB essential modulator (NEMO), which is encoded by the IKBKG gene. In recent years, mutant forms of the IKBKG gene have been reported as causative genes of osteopetrosis, lymphedema, hypohidrotic ectodermal dysplasia, and immunodeficiency (OL-EDA-ID). In addition, a mutation in the RELA gene, encoding RelA, has been reported for the first time in newborns with high neonatal bone mass. Osteopetrosis is characterized by a diffuse increase in bone mass, ranging from a lethal form observed in newborns to an asymptomatic form that appears in adulthood. This review describes the genetic mutations in NF-κB signaling molecules that have been identified in patients with osteopetrosis.

Intercellular signaling between ameloblastoma and osteoblasts

Ameloblastoma is an odontogenic tumor located in the bone jaw with clinical characteristics of extensive bone resorption. It is a locally invasive tumor with a high recurrence rate despite adequate surgical removal. In bone disease, tumors and other cells including osteoblasts, osteoclasts, and osteocytes in the bone microenvironment contribute to the pathogenesis of tumor growth. However, the effect of osteoblasts on ameloblastoma cells is not well-understood, and there has been limited research on interactions between them.

This study investigated interactions between ameloblastoma cells and osteoblasts using a human ameloblastoma cell line (AM-3 ameloblastoma cells) and a murine pre-osteoblast cell line (MC3T3-E1 cells). We treated each cell type with the conditioned medium by the other cell type. We analyzed the effect on cytokine production by MC3T3-E1 cells and the production of MMPs by AM-3 cells. Treatment with AM-3-conditioned medium induced inflammatory cytokine production of IL-6, MCP-1, and RANTES from MC3T3-E1 cells. The use of an IL-1 receptor antagonist suppressed the production of these inflammatory cytokines by MC3T3-E1 cells stimulated with AM-3-conditioned medium. The MC3T3-E1-conditioned medium triggered the expression of MMP-2 from AM-3 cells. Furthermore, we have shown that the proliferation and migration activity of AM-3 cells were accelerated by MC3T3-E1 conditioned media.

In conclusion, these intercellular signalings between ameloblastoma cells and osteoblasts may play multiple roles in the pathogenesis of ameloblastoma.

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Ameloblastoma conditioned medium induced IL-6, MCP-1 and RANTES production from osteoblast cell in IL-1 dependent manner.

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Unidentified factors from osteoblast conditioned medium induced MMP-2 production and stimulate proliferation and cellular motility of ameloblastoma cells.

Tetracyclines and bone: Unclear actions with potentially lasting effects

Tetracyclines are a broad-spectrum class of antibiotics that have unclear actions with potentially lasting effects on bone metabolism. Initially isolated from Streptomyces, tetracycline proved to be an effective treatment for Gram +/- infections. The emergence of resistant bacterial strains commanded the development of later generation agents, including minocycline, doxycycline, tigecycline, sarecycline, omadacycline, and eravacycline. In 1957, it was realized that tetracyclines act as bone fluorochrome labels due to their high affinity for the bone mineral matrix. Over the course of the next decade, researchers discerned that these compounds are retained in the bone matrix at high levels after the termination of antibiotic therapy. Studies during this period provided evidence that tetracyclines could disrupt prenatal and early postnatal skeletal development. Currently, tetracyclines are most commonly prescribed as a long-term systemic therapy for the treatment of acne in healthy adolescents and young adults. Surprisingly, the impact of tetracyclines on physiologic bone modeling/remodeling is largely unknown. This article provides an overview of the pharmacology of tetracycline drugs, summarizes current knowledge about the impact of these agents on skeletal development and homeostasis, and reviews prior work targeting tetracyclines' effects on bone cell physiology. The need for future research to elucidate unclear effects of tetracyclines on the skeleton is addressed, including drug retention/release mechanisms from the bone matrix, signaling mechanisms at bone cells, the impact of newer third generation tetracycline antibiotics, and the role of the gut-bone axis.

Bone Metastases: From Mechanisms to Treatment

OBJECTIVES

Bone metastases are of high clinical relevance because they are a frequent complication of most types of common cancers, such as breast and prostate. The metastatic process is complex, requiring the completion of several different steps to allow successful dissemination and homing. In addition, preparation of the metastatic niche changes the constant cycle of bone matrix formation and degradation, leading to the clinical phenotypes of lytic and sclerotic lesions. We review our current knowledge on this topic and briefly explain the current treatment landscape of bone metastasis.

DATA SOURCES

These include PubMed, international guidelines, and clinician experience.

CONCLUSION

Bone metastases remain a clinical challenge that negatively impacts patients prognosis and quality of life. A comprehensive understanding of the complex molecular mechanisms that results in bone metastasis is the basis for successful treatment of affected patients. The disruption of bone matrix metabolism is already recognized as the prerequisite for metastasis formation, but many open questions remain that need to be addressed in future research to establish individually tailored treatment approaches.

IMPLICATIONS FOR NURSING PRACTICE

Patient-centered therapy of bone metastases requires suitable pharmacological options, and importantly a holistic approach in care delivery across the multidisciplinary team. Nurses provide the cornerstone of the multidisciplinary team and provide the closest and the most frequent contact to the patient and their families to provide timely intervention. Nurses require a basic understanding of the complex physiology of metastasis to inform practice.

Calcium released by osteoclastic resorption stimulates autocrine/paracrine activities in local osteogenic cells to promote coupled bone formation

A major cause of osteoporosis is impaired coupled bone formation. Mechanistically, both osteoclast-derived and bone-derived growth factors have been previously implicated. We hypothesize that the release of bone calcium during osteoclastic bone resorption is essential for coupled bone formation. Osteoclastic resorption increases interstitial fluid calcium locally from the normal 1.8 mM up to 5 mM. MC3T3-E1 osteoprogenitors, cultured in a 3.6 mM calcium medium, demonstrated that calcium signaling stimulated osteogenic cell proliferation, differentiation, and migration. Calcium channel knockdown studies implicated calcium channels, Cav1.2, store-operated calcium entry (SOCE), and calcium-sensing receptor (CaSR) in regulating bone cell anabolic activities. MC3T3-E1 cultured in a 3.6 mM calcium medium expressed increased gene expression of Wnt signaling and growth factors platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and bone morphogenic protein-2 (BMP 2). Our coupling model of bone formation, the Receptor activator of nuclear factor-kappa-Β ligand (RANKL) treated mouse calvaria, confirmed the role of calcium signaling in coupled bone formation by exhibiting increased gene expression for osterix and osteocalcin. Critically, dual immunocytochemistry showed that RANKL treatment increased osterix positive cells and increased fluorescence intensity of Cav1.2 and CaSR protein expression per osterix positive cell. The data established that calcium released by osteoclasts contributed to the regulation of coupled bone formation. CRISPR/Cas-9 knockout of Cav1.2 in osteoprogenitors cultured in basal calcium medium caused a >80% decrease in the expression of downstream osteogenic genes, emphasizing the large magnitude of the effect of calcium signaling. Thus, calcium signaling is a major regulator of coupled bone formation.

Molecular Mechanisms Involved in Hypoxia-Induced Alterations in Bone Remodeling

Bone is crucial for the support of muscles and the protection of vital organs, and as a reservoir of calcium and phosphorus. Bone is one of the most metabolically active tissues and is continuously renewed to adapt to the changes required for healthy functioning. To maintain normal cellular and physiological bone functions sufficient oxygen is required, as evidence has shown that hypoxia may influence bone health. In this scenario, this review aimed to analyze the molecular mechanisms involved in hypoxia-induced bone remodeling alterations and their possible clinical consequences. Hypoxia has been associated with reduced bone formation and reduced osteoblast matrix mineralization due to the hypoxia environment inhibiting osteoblast differentiation. A hypoxic environment is involved with increased osteoclastogenesis and increased bone resorptive capacity of the osteoclasts. Clinical studies, although with contradictory results, have shown that hypoxia can modify bone remodeling.

Exosome as a target for cancer treatment

Exosomes are small vesicles covered by a lipid bilayer, ranging in size from 50 nm to 90 nm, secreted by different cell types in the body under normal and pathological conditions. They are surrounded by cell-segregated membrane complexes and play a role in the pathological and physiological environments of target cells by transfer of different molecules such as microRNA (miRNA). Exosomes have been detected in many body fluids, such as in the amniotic fluid, urine, breast milk, blood, saliva, ascites, semen, and bile. They include proteins, lipids, and nucleic acids such as DNA, RNA, and miRNA, which have many functions in target cells under pathological and physiological conditions. They participate in pathological processes such as tumor growth and survival, autoimmunity, neurodegenerative disorders, infectious diseases, inflammation conditions, and others. Biomarkers in exosomes isolated from body fluids have allowed for a more precise and consistent diagnostic method than previous approaches. Exosomes can be used in a variety of intracellular functions, and with advances in molecular techniques they can be used in the treatment and diagnosis of many diseases, including cancer. These vesicles play a significant role in various stages of cancer. Tumor-derived exosomes have an important role in tumor growth, survival, and metastasis. In contrast, the use of stem cells in cancer treatment is a relatively new scientific area. We hope to address targeted use of miRNA-carrying exosomes in cancer therapy in this review paper.

Comparison of Synthetic vs. Biogenic Polymeric Process-Directing Agents for Intrafibrillar Mineralization of Collagen

With the aging population, there is a growing need for mineralized tissue restoration and synthetic bone substitutes. Previous studies have shown that a polymer-induced liquid-precursor (PILP) process can successfully mineralize collagen substrates to achieve compositions found in native bone and dentin. This process also leads to intrafibrillar apatitic crystals with their [001] axes aligned roughly parallel to the long axis of the collagen fibril, emulating the nanostructural organization found in native bone and dentin. When demineralized bovine bone was remineralized via the PILP process using osteopontin (OPN), the samples were able to activate mouse marrow-derived osteoclasts to similar levels to those of native bone, suggesting a means for fabricating bioactive bone substitutes that could trigger remodeling through the native bone multicellular unit (BMU). In order to determine if OPN derived from bovine milk could be a cost-effective process-directing agent, the mineralization of type I collagen scaffolds using this protein was compared to the benchmark polypeptide of polyaspartic acid (sodium salt; pAsp). In this set of experiments, we found that OPN led to much faster and more uniform mineralization when compared with pAsp, making it a cheaper and commercially attractive alternative for mineralized tissue restorations.

Effects of Exercise Training on Bone Health Parameters in Individuals With Obesity: A Systematic Review and Meta-Analysis

Background

Osteoporosis causes bone fragility, increasing the risk of fractures. Evidence suggests a strong correlation between obesity and fracture risk. Physical training is known to enhance bone resistance and protect from fracture; however, its osteogenic effect in the presence of obesity remains unknown.

Objective

We sought to evaluate the influence of exercise training on bone health indices in individuals with obesity.

Methods

This systematic literature search was conducted using common electronic databases from inception - December 2019. The following key terms (and synonyms searched for by the MeSH database) were included and combined using the operators “AND,” “OR,” “NOT”: [(“body mass index” OR obesity OR obese OR overweight OR fat mass) AND (“bone mineral density” OR “bone mineral content” OR “peak bone mass” OR “mechanical loading” OR “Osteoporosis” OR “bone geometry” OR “bone resistance”) AND (“exercise training” OR “physical training” OR “strength training,” OR “resistance training” OR “aerobic training” OR “combined training”)].

Results

After screening, 10 studies (889 initial records) were included in the final analysis (8 different countries, 263 participants). Two studies investigated males, six females, and two, both sexes. The training duration was at least eight weeks with 2–3 sessions/week. Physical training displayed a significant trivial impact on the whole body (WB) BMD (0.13 SMD; 95% CI [0.00, 0.26], p = 0.046). Subgroup analyses indicated a significant small increase in the WB BMD (0.27 SMD; 95% CI [0.00, 0.53], p = 0.048) in the endurance training group, a non-significant trivial increase in the WB BMD (0.11 SMD; 95% CI [−0.06, 0.29], p = 0.203) in the resistance group, and a non-significant trivial increase in the WB BMD (0.03 SMD; 95% CI [−0.26, 0.32], p = 0.86) in the combined training group. In addition, a significant small decrease was found in the weight of trained subjects (−0.24 SMD; 95% CI [−0.42, −0.05], p = 0.011).

Conclusion

Physical training has little to no effect on the WB BMD in subjects with overweight/obesity. Currently, insufficient evidence to advocate for any specific type of exercise for enhancing bone health exists for overweight/obese individuals. Investigations examining the impact of varying types of physical exercise on WB BMD of obese individuals are needed.

A Tailored Approach for Appendicular Impending and Pathologic Fractures in Solid Cancer Metastases

Simple Summary

Patients with bone metastases often suffer with complications, such as bone fractures, which have a substantial negative impact on clinical outcomes. To optimize clinical results, a tailored approach should be defined for managing impending or pathologic fractures in each individual case. The ability to control systemic disease, the extent, location and nature of bone metastases, and the biology of the underlying tumor, are the main factors that will define the strategy to follow.

Abstract

Advances in medical and surgical treatment have played a major role in increasing the survival rates of cancer patients with metastatic bone disease. The clinical course of patients with bone metastases is often impaired by bone complications, such as bone fractures, which have a substantial negative impact on clinical outcomes. To optimize clinical results and prevent a detrimental impact on patients’ health, a tailored approach should be defined for any given patient. The optimal management of impending or pathologic fractures is unknown and relies on a multidisciplinary approach to tailor clinical decisions to each individual patient. The ability to control systemic disease, the extent, location and nature of bone metastases, and the biology of the underlying tumor, are the main factors that will define the strategy to follow. The present review covers the most recent data regarding impending and pathologic fractures in patients with bone metastases, and discusses the medical and surgical management of patients presenting with metastatic bone disease in different clinical settings.

Dissociation of Bone Resorption and Formation in Spaceflight and Simulated Microgravity: Potential Role of Myokines and Osteokines?

The dissociation of bone formation and resorption is an important physiological process during spaceflight. It also occurs during local skeletal unloading or immobilization, such as in people with neuromuscular disorders or those who are on bed rest. Under these conditions, the physiological systems of the human body are perturbed down to the cellular level. Through the absence of mechanical stimuli, the musculoskeletal system and, predominantly, the postural skeletal muscles are largely affected. Despite in-flight exercise countermeasures, muscle wasting and bone loss occur, which are associated with spaceflight duration. Nevertheless, countermeasures can be effective, especially by preventing muscle wasting to rescue both postural and dynamic as well as muscle performance. Thus far, it is largely unknown how changes in bone microarchitecture evolve over the long term in the absence of a gravity vector and whether bone loss incurred in space or following the return to the Earth fully recovers or partly persists. In this review, we highlight the different mechanisms and factors that regulate the humoral crosstalk between the muscle and the bone. Further we focus on the interplay between currently known myokines and osteokines and their mutual regulation.

The osteoprotective role of USP26 in coordinating bone formation and resorption

Bone homeostasis is maintained through a balance of bone formation by osteoblasts and bone resorption by osteoclasts. Ubiquitin-specific proteases (USPs) are involved in regulating bone metabolism by preserving bone formation or antagonizing bone resorption. However, the specific USPs that maintain bone homeostasis by orchestrating bone formation and bone resorption simultaneously are poorly understood. Here, we identified USP26 as a previously unknown regulator of bone homeostasis that coordinates bone formation and resorption. Mechanistically, USP26 stabilizes β-catenin to promote the osteogenic activity of mesenchymal cells (MSCs) and impairs the osteoclastic differentiation of bone myelomonocytes (BMMs) by stabilizing inhibitors of NF-κBα (IκBα). Gain-of-function experiments revealed that Usp26 supplementation significantly increased bone regeneration in bone defects in aged mice and decreased bone loss resulting from ovariectomy. Taken together, these data show the osteoprotective effect of USP26 via the coordination of bone formation and resorption, suggesting that USP26 represents a potential therapeutic target for osteoporosis.

Hydroxyapatite-decorated Fmoc-hydrogel as a bone-mimicking substrate for osteoclast differentiation and culture

Hydrogels are water-swollen networks with great potential for tissue engineering applications. However, their use in bone regeneration is often hampered due to a lack of materials' mineralization and poor mechanical properties. Moreover, most studies are focused on osteoblasts (OBs) for bone formation, while osteoclasts (OCs), cells involved in bone resorption, are often overlooked. Yet, the role of OCs is pivotal for bone homeostasis and aberrant OC activity has been reported in several pathological diseases, such as osteoporosis and bone cancer. For these reasons, the aim of this work is to develop customised, reinforced hydrogels to be used as material platform to study cell function, cell-material interactions and ultimately to provide a substrate for OC differentiation and culture. Here, Fmoc-based RGD-functionalised peptide hydrogels have been modified with hydroxyapatite nanopowder (Hap) as nanofiller, to create nanocomposite hydrogels. Atomic force microscopy showed that Hap nanoparticles decorate the peptide nanofibres with a repeating pattern, resulting in stiffer hydrogels with improved mechanical properties compared to Hap- and RGD-free controls. Furthermore, these nanocomposites supported adhesion of Raw 264.7 macrophages and their differentiation in 2D to mature OCs, as defined by the adoption of a typical OC morphology (presence of an actin ring, multinucleation, and ruffled plasma membrane). Finally, after 7 days of culture OCs showed an increased expression of TRAP, a typical OC differentiation marker. Collectively, the results suggest that the Hap/Fmoc-RGD hydrogel has a potential for bone tissue engineering, as a 2D model to study impairment or upregulation of OC differentiation. STATEMENT OF SIGNIFICANCE: Altered osteoclasts (OC) function is one of the major cause of bone fracture in the most commonly skeletal disorders (e.g. osteoporosis). Peptide hydrogels can be used as a platform to mimic the bone microenvironment and provide a tool to assess OC differentiation and function. Moreover, hydrogels can incorporate different nanofillers to yield hybrid biomaterials with enhanced mechanical properties and improved cytocompatibility. Herein, Fmoc-based RGD-functionalised peptide hydrogels were decorated with hydroxyapatite (Hap) nanoparticles to generate a hydrogel with improved rheological properties. Furthermore, they are able to support osteoclastogenesis of Raw264.7 cells in vitro as confirmed by morphology changes and expression of OC-markers. Therefore, this Hap-decorated hydrogel can be used as a template to successfully differentiate OC and potentially study OC dysfunction.

Protocol of Co-Culture of Human Osteoblasts and Osteoclasts to Test Biomaterials for Bone Tissue Engineering

New biomaterials and scaffolds for bone tissue engineering (BTE) applications require to be tested in a bone microenvironment reliable model. On this assumption, the in vitro laboratory protocols with bone cells represent worthy experimental systems improving our knowledge about bone homeostasis, reducing the costs of experimentation. To this day, several models of the bone microenvironment are reported in the literature, but few delineate a protocol for testing new biomaterials using bone cells. Herein we propose a clear protocol to set up an indirect co-culture system of human-derived osteoblasts and osteoclast precursors, providing well-defined criteria such as the cell seeding density, cell:cell ratio, the culture medium, and the proofs of differentiation. The material to be tested may be easily introduced in the system and the cell response analyzed. The physical separation of osteoblasts and osteoclasts allows distinguishing the effects of the material onto the two cell types and to evaluate the correlation between material and cell behavior, cell morphology, and adhesion. The whole protocol requires about 4 to 6 weeks with an intermediate level of expertise. The system is an in vitro model of the bone remodeling system useful in testing innovative materials for bone regeneration, and potentially exploitable in different application fields. The use of human primary cells represents a close replica of the bone cell cooperation in vivo and may be employed as a feasible system to test materials and scaffolds for bone substitution and regeneration.

The Role of SIRT3 in the Osteoporosis

SIRT3 is an NAD⁺-dependent deacetylase in the mitochondria with an extensive ability to regulate mitochondrial morphology and function. It has been reported that SIRT3 participates in the occurrence and development of many aging-related diseases. Osteoporosis is a common aging-related disease characterized by decreased bone mass and fragility fractures, which has caused a huge burden on society. Current research shows that SIRT3 is involved in the physiological processes of senescence of bone marrow mesenchymal stem cells (BMSCs), differentiation of BMSCs and osteoclasts. However, the specific effects and mechanisms of SIRT3 in osteoporosis are not clear. In the current review, we elaborated on the physiological functions of SIRT3, the cell types involved in bone remodeling, and the role of SIRT3 in osteoporosis. Furthermore, it also provided a theoretical basis for SIRT3 as a therapeutic target for osteoporosis.

Knowledge Domains and Emerging Trends of Osteoblasts-Osteoclasts in Bone Disease From 2002 to 2021: A Bibliometrics Analysis and Visualization Study

BACKGROUND

Osteoblasts-Osteoclasts has been a major area in bone disease research for a long time. However, there are few systematic studies in this field using bibliometric analysis. We aimed to perform a bibliometric analysis and visualization study to determine hotspots and trends of osteoblasts-osteoclasts in bone diseases, identify collaboration and influence among authors, countries, institutions, and journals, and assess the knowledge base to develop basic and clinical research in the future.

METHODS

We collected articles and reviews for osteoblasts-osteoclasts in bone diseases from the Web of Science Core Collection. In addition, we utilized scientometrics software (CiteSpace5.8 and VOSviewer1.6.18) for visual analysis of countries/regions, institutions, authors, references, and keywords in the field.

RESULTS

In total, 16,832 authors from 579 institutions in 73 countries/regions have published 3,490 papers in 928 academic journals. The literature in this field is rapidly increasing, with Bone publishing the most articles, whereas Journal of Bone and Mineral Research had the most co-cited journals. These two journals mainly focused on molecular biology and the clinical medicine domain. The countries with the highest number of publications were the US and China, and the University of Arkansas for Medical Sciences was the most active institution. Regarding authors, Stavros C. Manolagas published the most articles, and Hiroshi Takayanagi had the most co-cited papers. Research in this field mainly includes molecular expression and regulatory mechanisms, differentiation, osteoprotection, inflammation, and tumors. The latest research hotspots are oxidative stress, mutation, osteocyte formation and absorption, bone metabolism, tumor therapy, and in-depth mechanisms.

CONCLUSION

We identified the research hotspots and development process of osteoblasts-osteoclasts in bone disease using bibliometric and visual methods. Osteoblasts-osteoclasts have attracted increasing attention in bone disease. This study will provide a valuable reference for researchers concerned with osteoblasts-osteoclasts in bone diseases.

Adipose Tissue Dysfunction: Impact on Bone and Osseointegration

Bone metabolism may be adversely affected in metabolic diseases such as obesity and metabolic syndrome, which are characterised by weight gain, due to the expansion of adipose tissue deposits. As an important regulator of energy metabolism, adipose tissues synthesise and secrete several key regulatory adipokines that influence a range of metabolic functions. This narrative review outlines the evidence for the mechanisms by which adipose tissue dysfunction may alter bone metabolism prior to the development of frank hyperglycaemia and presents the emerging evidence for the impact of diet-induced expansion of adipose tissue on implant osseointegration. Successful osseointegration requires normal bone cell function, and the expansion of adipose tissue deposits results in dysregulated adipokine production favouring an increase in pro-inflammatory adipokines, contributing to the development of a chronic inflammatory state and insulin resistance. The increase in inflammatory cytokines promotes the growth and differentiation of osteoclasts indirectly through the modulation of osteoblastic RANKL production and directly by reducing osteoclast apoptosis and increased osteoclastic expression of RANK. Conversely, the suppression of osteoblastic regulatory genes results in reduced osteoblast numbers and function contributing to compromised bone turnover. Compromised osseointegration has been established in hyperglycaemia; however, as discussed in this review, it may not be the only driver of altered bone metabolism. The incidence of metabolic disease in the community is rising, patients may present for implant treatment with undiagnosed, underlying changes to bone cell metabolism due to adipose tissue dysmetabolism.

Multifunctional Coatings of Titanium Implants Toward Promoting Osseointegration and Preventing Infection: Recent Developments

Titanium and its alloys are dominant material for orthopedic/dental implants due to their stable chemical properties and good biocompatibility. However, aseptic loosening and peri-implant infection remain problems that may lead to implant removal eventually. The ideal orthopedic implant should possess both osteogenic and antibacterial properties and do proper assistance to in situ inflammatory cells for anti-microbe and tissue repair. Recent advances in surface modification have provided various strategies to procure the harmonious relationship between implant and its microenvironment. In this review, we provide an overview of the latest strategies to endow titanium implants with bio-function and anti-infection properties. We state the methods they use to preparing these efficient surfaces and offer further insight into the interaction between these devices and the local biological environment. Finally, we discuss the unmet needs and current challenges in the development of ideal materials for bone implantation.

Transcriptional responses of skeletal stem/progenitor cells to hindlimb unloading and recovery correlate with localized but not systemic multi-systems impacts

Disuse osteoporosis (DO) results from mechanical unloading of weight-bearing bones and causes structural changes that compromise skeletal integrity, leading to increased fracture risk. Although bone loss in DO results from imbalances in osteoblast vs. osteoclast activity, its effects on skeletal stem/progenitor cells (SSCs) is indeterminate. We modeled DO in mice by 8 and 14 weeks of hindlimb unloading (HU) or 8 weeks of unloading followed by 8 weeks of recovery (HUR) and monitored impacts on animal physiology and behavior, metabolism, marrow adipose tissue (MAT) volume, bone density and micro-architecture, and bone marrow (BM) leptin and tyrosine hydroxylase (TH) protein expression, and correlated multi-systems impacts of HU and HUR with the transcript profiles of Lin^-LEPR⁺ SSCs and mesenchymal stem cells (MSCs) purified from BM. Using this integrative approach, we demonstrate that prolonged HU induces muscle atrophy, progressive bone loss, and MAT accumulation that paralleled increases in BM but not systemic leptin levels, which remained low in lipodystrophic HU mice. HU also induced SSC quiescence and downregulated bone anabolic and neurogenic pathways, which paralleled increases in BM TH expression, but had minimal impacts on MSCs, indicating a lack of HU memory in culture-expanded populations. Although most impacts of HU were reversed by HUR, trabecular micro-architecture remained compromised and time-resolved changes in the SSC transcriptome identified various signaling pathways implicated in bone formation that were unresponsive to HUR. These findings indicate that HU-induced alterations to the SSC transcriptome that persist after reloading may contribute to poor bone recovery.

6-Shogaol promotes bone resorption and accelerates orthodontic tooth movement through the JNK-NFATc1 signaling axis

INTRODUCTION

Corticotomy is widely used in clinical practice to accelerate tooth movement and shorten the duration of orthodontic treatment. It is effective, but an invasive surgery is needed to induce alveolar bone osteopenia that enable rapid tooth movement. In this study, we discovered the potential of 6-shogaol as a more patient-friendly non-invasive alternative to induce transient osteopenia and accelerate tooth movement.

MATERIALS AND METHODS

The effects of 6-shogaol on the bone marrow macrophages (BMM) proliferation and osteoclast differentiation, and bone resorption were determined in vitro. Sprague-Dawley rats were distributed into three groups: CON, IPinj or Localinj and euthanized at day 28. Micro-CT, histology, immunohistological, and TUNEL analysis were performed to evaluate the tooth movement acceleration effect of 6-shogaol.

RESULTS

In vitro, 6-shogaol promotes osteoclast differentiation and functional demineralization of alveolar bone. RANKL-induced mRNA expression of osteoclastic-specific genes was significantly higher in the presence of 6-shogaol. A dose-dependent increase in the area of TRAP-positive cells was observed with 6-shogaol treatment. F-actin ring formation and increased bone resorption confirmed that osteoclasts treated with 6-shogaol were mature and functional. 6-shogaol stimulated JNK activation and NFATc1 expression during osteoclast differentiation. In vivo, 6-shogaol promotes alveolar bone transient osteopenia and accelerates orthodontic tooth movement. Alveolar bone mass was reduced, more osteoclasts were observed in bone resorption lacunae on the compression side, and the expression of RANKL and sclerostin were higher than the control group. In conclusion, our results suggest that 6-shogoal accelerates tooth movement by inducing osteopenia by a mechanism similar to surgically induced bone injury.

Cell Biology of Giant Cell Tumour of Bone: Crosstalk between m/wt Nucleosome H3.3, Telomeres and Osteoclastogenesis

Giant cell tumour of bone (GCTB) is a rare and intriguing primary bone neoplasm. Worrisome clinical features are its local destructive behaviour, its high tendency to recur after surgical therapy and its ability to create so-called benign lung metastases (lung 'plugs'). GCTB displays a complex and difficult-to-understand cell biological behaviour because of its heterogenous morphology. Recently, a driver mutation in histone H3.3 was found. This mutation is highly conserved in GCTB but can also be detected in glioblastoma. Denosumab was recently introduced as an extra option of medical treatment next to traditional surgical and in rare cases, radiotherapy. Despite these new insights, many 'old' questions about the key features of GCTB remain unanswered, such as the presence of telomeric associations (TAs), the reactivation of hTERT, and its slight genomic instability. This review summarises the recent relevant literature of histone H3.3 in relation to the GCTB-specific G34W mutation and pays specific attention to the G34W mutation in relation to the development of TAs, genomic instability, and the characteristic morphology of GCTB. As pieces of an etiogenetic puzzle, this review tries fitting all these molecular features and the unique H3.3 G34W mutation together in GCTB.

Extracellular Vesicles in Bone Tumors: How to Seed in the Surroundings Molecular Information for Malignant Transformation and Progression

Bone is a very dynamic tissue hosting different cell types whose functions are regulated by a plethora of membrane-bound and soluble molecules. Intercellular communication was recently demonstrated to be also sustained by the exchange of extracellular vesicles (EVs). These are cell-derived nanosized structures shuttling biologically active molecules, such as nucleic acids and proteins. The bone microenvironment is a preferential site of primary and metastatic tumors, in which cancer cells find a fertile soil to “seed and blossom”. Nowadays, many oncogenic processes are recognized to be sustained by EVs. For example, EVs can directly fuel the vicious cycle in the bone/bone marrow microenvironment. EVs create a favourable environment for tumor growth by affecting osteoblasts, osteoclasts, osteocytes, adipocytes, leukocytes, and endothelial cells. At the same time other crucial tumor-mediated events, such as the premetastatic niche formation, tumor cell dormancy, as well as drug resistance, have been described to be fostered by tumor-derived EVs. In this review, we will discuss the main body of literature describing how the cancer cells use the EVs for their growth into the bone and for educating the bone microenvironment to host metastases.

The hypertrophic chondrocyte: To be or not to be

Hypertrophic chondrocytes are the master regulators of endochondral ossification; however, their ultimate cell fates cells remain largely elusive due to their transient nature. Historically, hypertrophic chondrocytes have been considered as the terminal state of growth plate chondrocytes, which are destined to meet their inevitable demise at the primary spongiosa. Chondrocyte hypertrophy is accompanied by increased organelle synthesis and rapid intracellular water uptake, which serve as the major drivers of longitudinal bone growth. This process is delicately regulated by major signaling pathways and their target genes, including growth hormone (GH), insulin growth factor-1 (IGF-1), indian hedgehog (Ihh), parathyroid hormone-related protein (PTHrP), bone morphogenetic proteins (BMPs), sex determining region Y-box 9 (Sox9), runt-related transcription factors (Runx) and fibroblast growth factor receptors (FGFRs). Hypertrophic chondrocytes orchestrate endochondral ossification by regulating osteogenic-angiogenic and osteogenic-osteoclastic coupling through the production of vascular endothelial growth factor (VEGF), receptor activator of nuclear factor kappa-B ligand (RANKL) and matrix metallopeptidases-9/13 (MMP-9/13). Hypertrophic chondrocytes also indirectly regulate resorption of the cartilaginous extracellular matrix, by controlling formation of a special subtype of osteoclasts termed "chondroclasts". Notably, hypertrophic chondrocytes may possess innate potential for plasticity, reentering the cell cycle and differentiating into osteoblasts and other types of mesenchymal cells in the marrow space. We may be able to harness this unique plasticity for therapeutic purposes, for a variety of skeletal abnormalities and injuries. In this review, we discuss the morphological and molecular properties of hypertrophic chondrocytes, which carry out important functions during skeletal growth and regeneration.

The Inducible Nitric Oxide Synthase Pathway Promotes Osteoclastogenesis under Hypoxic Culture Conditions

Bone homeostasis depends on the balance between bone resorption by osteoclasts (OCs) and bone formation by osteoblasts. Bone resorption can become excessive under various pathologic conditions, including rheumatoid arthritis. Previous studies have shown that OC formation is promoted under hypoxia. However, the precise mechanisms behind OC formation under hypoxia have not been elucidated. The present study investigated the role of inducible nitric oxide synthase (iNOS) in OC differentiation under hypoxia. Primary bone marrow cells obtained from mice were stimulated with receptor activator of NF-κB ligand and macrophage colony-stimulating factor to induce OC differentiation. The number of OCs increased in culture under hypoxia (oxygen concentration, 5%) compared with that under normoxia (oxygen concentration, 20%). iNOS gene and protein expression increased in culture under hypoxia. Addition of an iNOS inhibitor under hypoxic conditions suppressed osteoclastogenesis. Addition of a nitric oxide donor to the normoxic culture promoted osteoclastogenesis. Furthermore, insulin-like growth factor 2 expression was significantly altered in both iNOS inhibition experiments and nitric oxide donor experiments. These data might provide clues to therapies for excessive osteoclastogenesis under several hypoxic pathologic conditions, including rheumatoid arthritis.

New Insights Into Osteoclast Biology

Osteoclasts are multinucleated cells that are characterized by their unique ability to resorb large quantities of bone. Therefore, they are frequently the target of therapeutic interventions to ameliorate bone loss. In an adult organism, osteoclasts derive from hematopoietic stem cells and differentiate into osteoclasts within a multistep process under the influence of macrophage colony‐stimulating factor (M‐CSF) and receptor activator of NF‐κB ligand (RANKL). Historically, the osteoclast life cycle has been defined as linear, whereby lineage‐committed mononuclear precursors fuse to generate multinucleated highly specialized and localized bone phagocytic cells, which then undergo apoptosis within weeks. Recent advances through lineage tracing, single cell RNA sequencing, parabiosis, and intravital imaging approaches have challenged this dogma, revealing they have greater longevity and the capacity to circulate and undergo cell recycling. Indeed, these new insights highlight that under homeostatic conditions very few incidences of osteoclast apoptosis occur. More importantly, as we revisit the formation and fate of the osteoclast, novel methods to target osteoclast biology in bone pathology and regeneration are emerging. This review briefly summarizes the historical life cycle of osteoclasts and highlights recent discoveries made through advanced methodologies, which have led to a paradigm shift in osteoclast biology. These findings are discussed in light of both existing and emerging bone targeted therapeutics, bone pathologies, and communication between osteoclasts and cells resident in bone or at distant sites. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Anti-Müllerian Hormone Negatively Regulates Osteoclast Differentiation by Suppressing the Receptor Activator of Nuclear Factor-κB Ligand Pathway

Background

Multiple members of the transforming growth factor-β (TGF-β) superfamily have well-established roles in bone homeostasis. Anti-Müllerian hormone (AMH) is a member of TGF-β superfamily of glycoproteins that is responsible for the regression of fetal Müllerian ducts and the transcription inhibition of gonadal steroidogenic enzymes. However, the involvement of AMH in bone remodeling is unknown. Therefore, we investigated whether AMH has an effect on bone cells as other TGF-β superfamily members do.

Methods

To identify the roles of AMH in bone cells, we administered AMH during osteoblast and osteoclast differentiation, cultured the cells, and then stained the cultured cells with Alizarin red and tartrate-resistant acid phosphatase, respectively. We analyzed the expression of osteoblast- or osteoclast-related genes using real-time polymerase chain reaction and western blot.

Results

AMH does not affect bone morphogenetic protein 2-mediated osteoblast differentiation but inhibits receptor activator of nuclear factor-κB (NF-κB) ligand-induced osteoclast differentiation. The inhibitory effect of AMH on osteoclast differentiation is mediated by IκB-NF-κB signaling.

Conclusions

AMH negatively regulates osteoclast differentiation without affecting osteoblast differentiation.

CKIP-1 contributes to osteogenic differentiation of mouse bone marrow mesenchymal stem cells

Background: Osteogenesis greatly depends on the differentiation of bone marrow mesenchymal stem cells (BMSCs). CKIP-1 is considered to be a negative regulator of BMSCs. Methods: We established a CKIP-1 knockout mouse model, then isolated and cultured BMSCs from wild-type and knockout groups. Results: Our data demonstrated that CKIP-1 knockout significantly increased bone structure in the experimental mouse model and enhanced BMSC proliferation. CKIP-1 knockout contributed to osteoblastic and adipogenic differentiation. Furthermore, CKIP-1 regulated osteogenesis in BMSCs via the MAPK signaling pathway, and BMSCs from the CKIP-1 knockout mice were effective in repairing the skull defect null mice. Conclusion: Our results concluded that silencing of CKIP-1 promoted osteogenesis in experimental mice and increased BMSCs differentiation via upregulation of the MAPK signaling pathway.

Systemic osteoprotegerin does not improve peri-implant bone volume or osseointegration in rabbits

Anti-RANKL (receptor activator of nuclear factor kappa-B ligand) agents function by blocking the differentiation of osteoclasts, thereby proving useful in the clinical management of postmenopausal osteoporosis. The effects of such agents on osseointegration is less well understood. The purpose of the current study was to investigate whether osteoprotegerin (OPG), an osteoclast inhibitor, enhances the known anabolic effects of mechanical loading (VEH) and intermittent PTH (iPTH) using a well-established rabbit model of osseointegration. In the first set of experiments, OPG was administered either alone or combined with iPTH to study its effects on measured bone mass. The second set of experiments was conducted using a higher dosage of OPG (10 mg/kg) to explore its early impact at the cellular and molecular levels. All subjects had mechanical load applied to the implant on one extremity, and no load applied on the contralateral side. In the first set of experiments, OPG alone decreased peri-implant bone mass compared to the mechanical loading group, whereas OPG + iPTH increased peri-implant bone mass compared to the OPG group. In the second set of experiments, high-dose OPG significantly decreased osteoclast number (-74.3%) at 1 week. However, this effect was not sustained as osteoclast number returned to baseline by 2 weeks. These results suggest that systemic administration of OPG does not enhance osseointegration, but rather has a detrimental effect.

WNT-5a and SOST Levels in Gingival Crevicular Fluid Depend on the Inflammatory and Osteoclastogenic Activities of Periodontal Tissues

Background and Objectives: Wnt signaling leads to stimulation of osteoblasts and it reduces osteoclastogenesis and bone resorption via the regulation of the osteprotegrin and receptor activator of nuclear factor kappa-Β ligan (RANKL). Wnt signaling pathways are regulated by their physiological antagonists such as sclerostin (SOST) as well as WNT-5a. The aim of this study was to determine the total amount of Sclerostin and WNT-5a in the gingival crevicular fluid (GCF) in sites with a continuum from a healthy to diseased periodontium. Materials and Methods: In this cross-sectional study, a total of 20 patients with generalized periodontitis, 10 subjects with gingivitis as well as 14 individuals with a healthy periodontium were recruited upon clinical and radiographic periodontal examination. In patients diagnosed with periodontitis, GCF samples were collected from periodontitis, gingivitis and healthy sites, while gingivitis patients provided samples from gingivitis and healthy sites. In healthy patients, only healthy sites were sampled. Protein total amount of SOST and WNT-5a were quantified by sandwich enzyme-linked immunosorbent assay (ELISA). Results: A total of 108 GCF samples were collected from a total of 44 individuals. When all periodontitis (n = 51), gingivitis (n = 12) and healthy (n = 45) sites were analyzed regardless of the patient diagnosis, periodontitis sites demonstrated significantly elevated WNT-5a total amounts (p = 0.03) when compared to gingivitis sites. Gingivitis sites demonstrated a trend of more total SOST (p = 0.09) when compared to periodontitis and healthy sites. Within each patient diagnostic category, sites showed similar SOST and WNT-5a total amounts (p > 0.05). Conclusions: WNT-5a levels in GCF depend on the stage of periodontitis sites. SOST trended higher in the GCF of gingivitis sites but similar in chronic periodontitis and healthy sites. WNT-5a and SOST play a crucial role in periodontal tissue remodeling and depend on the inflammatory and osteoclastogenic activities.

Serum exosomes from young rats improve the reduced osteogenic differentiation of BMSCs in aged rats with osteoporosis after fatigue loading in vivo

BACKGROUND

Osteoporosis is a major public health concern for the elderly population and is characterized by fatigue load resulting in bone microdamage. The ability of bone mesenchymal stem cells (BMSCs) to repair bone microdamage diminishes with age, and the accumulation of bone microdamage increases the risk of osteoporotic fracture. There is a lack of effective means to promote the repair of bone microdamage in aged patients with osteoporosis. Exosomes have been shown to be related to the osteogenic differentiation of BMSCs. Here, we aimed to evaluate the changes in the osteogenic differentiation capacity of BMSCs in aged osteoporotic rats after fatigue loading and the treatment potential of serum exosomes from young rats.

METHODS

The tibias of six aged osteoporotic rats were subjected to fatigue loading in vivo for 2 weeks, and the bone microdamage, microstructures, and mechanical properties were assessed. Subsequently, BMSCs were extracted to evaluate their proliferation and osteogenic differentiation abilities. In addition, the BMSCs of aged osteoporotic rats after fatigue loading were treated with serum exosomes from young rats under osteogenic induction conditions, and the expression of osteogenic-related miRNAs was quantified. The osteogenetic effects of miRNA-19b-3p in exosomes and the possible target protein PTEN was detected.

RESULTS

Obvious bone microdamage at the fatigue load stress point, the bone microstructure and biomechanical properties were not obviously changed. A decreased osteogenic differentiation ability of BMSCs was observed after fatigue loading, while serum exosomes from young rats highly expressing miRNA-19b-3p improved the decreased osteogenic differentiation ability of BMSCs. Transfection with miRNA-19b-3p mimic could promote osteoblastic differentiation of BMSCs and decreased the expression of PTEN. After transfection of miRNA-19b-3p inhibitor, the promotional effect of exosomes on bone differentiation was weakened. Treatment with transfected exosomes increased the expression of PTEN.

CONCLUSION

Serum exosomes derived from young rats can improve the decreased osteogenic differentiation ability of BMSCs in aged rats with osteoporosis after fatigue loading and can provide a new treatment strategy for the repair of bone microdamage and prevention of fractures.

Depression of Bone Density at the Weight-Bearing Joints in Wistar Hannover Rats by a Simulated Mechanical Stress Associated With Partial Gravity Environment

The partial gravity environment in space can negatively affect bone health. This survey aimed to study the reaction of different parts of the lower limb bones of rats to partial gravity and the effects of different degrees of gravity on these bony parts. We used 15 8-week-old male Wistar Hannover rats were used at the beginning of the experiment. The degree of mechanical stress was modified, but the ankle joint was maintained at ∼30°, ∼120°, or ∼160° with or without plaster fixation during 10-day hindlimb suspension. Computed tomography was performed to measure the bone parameters [bone mineral density (BMD), trabecular BMD, cortical BMD, and cortical thickness] of each studied group of the whole, proximal, middle, and distal femur and distal tibia. BMD, trabecular BMD, and cortical thickness of the distal femur and proximal tibia of the simulated mechanical stress associated with partial gravity groups were significantly lower than those of the control group; the effect of different degrees of gravity on the same area of hindlimb bone had no significant difference. The simulated mechanical stress associated with partial gravity had the most significant effect on the bone close to the knee joint, with the largest weight-bearing response.

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

Mechanism of Exosomes Involved in Osteoimmunity Promoting Osseointegration Around Titanium Implants With Small-Scale Topography

Exosomes are nanoscale extracellular vesicles. Several studies have shown that exosomes participate in intercellular communication and play a key role in osseointegration. However, it is unclear whether exosomes and their contents participate in the communication between the immune and skeletal systems in the process of osseointegration. In this study, we obtained smooth titanium disks by polishing and small-scale topography titanium disks by sandblasted large-grit acid-etched (SLA) technology combined with alkali thermal reaction. After stimulating mouse RAW264.7 cells with these two kinds of titanium disks, we co-cultured the MC3T3-E1 cells and the RAW264.7 cells, obtained and identified the exosomes derived from RAW264.7 cells, and studied the effect of the osteoimmune microenvironment and the exosomes on the osseointegration of mouse MC3T3-E1 cells. Cell counting kit-8 (CCK-8), real time quantitative PCR, western blotting, alizarin red staining, and quantitative and confocal fluorescence microscopy were used to study the effects of exosomes on MC3T3-E1 cells; RNA sequencing and correlation analysis were performed. We found that the osteoimmune microenvironment could promote the osseointegration of MC3T3-E1 cells. We successfully isolated exosomes and found that RAW264.7 cell-derived exosomes can promote osteogenic differentiation and mineralization of MC3T3-E1 cells. Through RNA sequencing and gene analysis, we found differentially expressed microRNAs that targeted the signal pathways that may be related, such as mTOR, AMPK, Wnt, etc., and thus provide a reference for the mechanism of osteoimmunue regulation of implant osseointegration. The study further elucidated the mechanism of implant osseointegration and provided new insights into the effect of exosomes on implant osseointegration, and provided reference for clinical improvement of implant osseointegration and implant success rate.