Osteoprotegerin-deficient male mice as a model for severe alveolar bone loss: comparison with RANKL-overexpressing transgenic male mice

Proton FLASH Radiotherapy Ameliorates Radiation-induced Salivary Gland Dysfunction and Oral Mucositis and Increases Survival in a Mouse Model of Head and Neck Cancer

Head and neck cancer radiotherapy often damages salivary glands and oral mucosa, severely negatively impacting patients' quality of life. The ability of FLASH proton radiotherapy (F-PRT) to decrease normal tissue toxicity while maintaining tumor control compared with standard proton radiotherapy (S-PRT) has been previously demonstrated for several tissues. However, its potential in ameliorating radiation-induced salivary gland dysfunction and oral mucositis and controlling orthotopic head and neck tumor growth has not been reported. The head and neck area of C57BL/6 mice was irradiated with a single dose of radiotherapy (ranging from 14-18 Gy) or a fractionated dose of 8 Gy × 3 of F-PRT (128 Gy/second) or S-PRT (0.95 Gy/second). Following irradiation, the mice were studied for radiation-induced xerostomia by measuring their salivary flow. Oral mucositis was analyzed by histopathologic examination. To determine the ability of F-PRT to control orthotopic head and neck tumors, tongue tumors were generated in the mice and then irradiated with either F-PRT or S-PRT. Mice treated with either a single dose or fractionated dose of F-PRT showed significantly improved survival than those irradiated with S-PRT. F-PRT-treated mice showed improvement in their salivary flow. S-PRT-irradiated mice demonstrated increased fibrosis in their tongue epithelium. F-PRT significantly increased the overall survival of the mice with orthotopic tumors compared with the S-PRT-treated mice. The demonstration that F-PRT decreases radiation-induced normal tissue toxicity without compromising tumor control, suggests that this modality could be useful for the clinical management of patients with head and neck cancer.

Omics-Based Mathematical Modeling Unveils Pathogenesis of Periodontitis in an Experimental Murine Model

Periodontitis is a multifactorial disease that progresses via dynamic interaction between bacterial and host-derived genetic factors. The recent trend of omics analyses has discovered many periodontitis-related risk factors. However, how much the individual factor affects the pathogenesis of periodontitis is still unknown. This article aims to identify multiple key factors related to the pathogenesis of periodontitis and quantitatively predict the influence of each factor on alveolar bone resorption by omics analysis and mathematical modeling. First, we induced periodontitis in mice (n = 3 or 4 at each time point) by tooth ligation. Next, we assessed alveolar bone resorption by micro-computed tomography, alterations in the gene expression by RNA sequencing, and the microbiome of the gingivae by 16S ribosomal RNA sequencing during disease pathogenesis. Omics data analysis identified key players (bacteria and molecules) involved in the pathogenesis of periodontitis. We then constructed a mathematical model of the pathogenesis of periodontitis by employing ordinary differential equations that described the dynamic regulatory interplay between the key players and predicted the alveolar bone integrity as output. Finally, we estimated the model parameters using our dynamic experimental data and validated the model prediction of influence on alveolar bone resorption by in vivo experiments. The model predictions and experimental results revealed that monocyte recruitment induced by bacteria-mediated Toll-like receptor activation was the principal reaction regulating alveolar bone resorption in a periodontitis condition. On the other hand, osteoblast-mediated osteoclast differentiation had less impact on bone integrity in a periodontitis condition.

Lipopolysaccharide Impedes Bone Repair in FcγRIIB-Deficient Mice

Chronic inflammation contributes to the development of skeletal disorders in patients with systemic lupus erythematosus (SLE). Activation of the host immune response stimulates osteoclast activity, which in turn leads to bone loss. Regenerating bone in the inflammatory microenvironments of SLE patients with critical bone defects remains a great challenge. In this study, we utilized lipopolysaccharide (LPS) to imitate locally and systemically pathogenic bacterial infection and examined the bone regeneration performance of LPS-associated mandibular and tibial bone regeneration impairment in FcγRIIB-/- mice. Our results indicated that a loss of FcγRIIB alleviates bone regeneration in both mandibles and tibiae. After LPS induction, FcγRIIB-/- mice were susceptible to impaired fracture healing in tibial and mandibular bones. LPS decreased the mineralization to collagen ratio in FcγRIIB-/- mice, indicating a mineralization defect during bone repair. An osteoblast-associated gene (Col1a1) was attenuated in FcγRIIB-deficient mice, whereas Bglap, Hhip, and Creb5 were further downregulated with LPS treatment in FcγRIIB-/- mice compared to FcγRIIB-/- mice. Alpl and Bglap expression was dcreased in osteoblasts derived from bone chips. An osteoclast-associated gene, Tnfsf11/Tnfrsf11 ratio, ewas increased in LPS-induced FcγRIIB-/- mice and in vitro. Furthermore, systemic LPS was relatively potent in stimulating production of pro-inflammatory cytokines including TNF-α, IL-6, and MCP-1 in FcγRIIB-/- mice compared to FcγRIIB-/- mice. The levels of TNF-α, IFN-β, IL-1α, and IL-17A were increased, whereas IL-10 and IL-23 were decreased in FcγRIIB-/- mice treated locally with LPS. These findings suggest that both local and systemic LPS burden can exacerbate bone regeneration impairment, delay mineralization and skeletal repair, and induce inflammation in SLE patients.

Fluoxetine treatment during the postpartal period may have short-term impacts on murine maternal skeletal physiology

Postpartum depression affects many individuals after parturition, and selective serotonin reuptake inhibitors (SSRIs) are often used as the first-line treatment; however, both SSRIs and lactation are independently associated with bone loss due to the role of serotonin in bone remodeling. Previously, we have established that administration of the SSRI fluoxetine during the peripartal period results in alterations in long-term skeletal characteristics. In the present study, we treated mice with either a low or high dose of fluoxetine during lactation to determine the consequences of the perturbation of serotonin signaling during this time period on the dam skeleton. We found that lactational fluoxetine exposure affected both cortical and trabecular parameters, altered gene expression and circulating markers of bone turnover, and affected mammary gland characteristics, and that these effects were more pronounced in the dams that were exposed to the low dose of fluoxetine in comparison to the high dose. Fluoxetine treatment during the postpartum period in rodents had short term effects on bone that were largely resolved 3 months post-weaning. Despite the overall lack of long-term insult to bone, the alterations in serotonin-driven lactational bone remodeling raises the question of whether fluoxetine is a safe option for the treatment of postpartum depression.

Periostin plays a key role in maintaining the osteogenic abilities of dental follicle stem cells in the inflammatory microenvironment

OBJECTIVE

This study aimed to explore the effect of periostin in the osteogenic abilities of dental follicle stem cells (DFSCs) and DFSC sheets in the inflammatory microenvironment.

DESIGN

DFSCs were isolated from dental follicles and identified. A lentiviral vector was used to knock down periostin in DFSCs. 250 ng/ml lipopolysaccharide from Porphyromonas gingivalis (P.g-LPS) was used to construct the inflammatory microenvironment. Osteogenic differentiation was evaluated by alizarin red staining, quantitative real-time polymerase chain reaction (qRT-PCR), and western blot. The formation of extracellular matrix was assessed by qRT-PCR and immunofluorescence. The expressions of receptor activator of nuclear factor kappa-B ligand (RANKL) and osteoprotegerin (OPG) were measured by western blot.

RESULTS

Knockdown of periostin inhibited osteogenic differentiation and promoted adipogenic differentiation of DFSCs. In an inflammatory microenvironment, knockdown of periostin attenuated the proliferation and osteogenic differentiation of DFSCs. Knockdown of periostin inhibited the formation of extracellular matrix collagen I (COL-I), fibronectin, and laminin in DFSC sheets, but did not affect the expression of osteogenesis-related markers alkaline phosphatase (ALP) and osteocalcin (OCN). In the inflammatory microenvironment, knocking down periostin inhibited the expression of OCN and OPG in DFSC sheets, and promoted the expression of RANKL.

CONCLUSION

Periostin played a key role in maintaining the osteogenic abilities of DFSCs and DFSC sheets in the inflammatory microenvironment and might be an important molecule in the process of DFSCs coping with inflammatory microenvironment and promoting periodontal tissues regeneration.

Diagnostic potential of salivary interleukin-17, RANKL, and OPG to differentiate between periodontal health and disease and discriminate stable and unstable periodontitis: A case-control study

Background and Aims

Limitations of the conventional diagnostic techniques urged researchers to seek novel methods to predict, diagnose, and monitor periodontal disease. Use of the biomarkers available in oral fluids could be a revolutionary surrogate for the manual probing/diagnostic radiograph. Several salivary biomarkers have the potential to accurately discriminate periodontal health and disease. This study aimed to determine the diagnostic sensitivity and specificity of salivary interleukin (IL)-17, receptor activator of nuclear factor-κB ligand (RANKL), osteoprotegerin (OPG), RANKL/OPG for differentiating (1) periodontal health from disease and (2) stable and unstable periodontitis.

Methods

Participants with periodontitis (n = 50) and gingivitis (n = 25), both diseases represented the cases, and subjects with healthy periodontium (n = 15) as a control were recruited for this study. Periodontitis cases were further equally subdivided into stable and unstable. Whole unstimulated salivary sample were collected from all participants. Periodontal parameters including bleeding on probing, probing pocket depth, clinical attachment loss, and number of missing teeth were recorded. The protein levels of salivary IL-17, RANKL, and OPG were determined by using enzyme-linked immunosorbent assays technique.

Results

Salivary IL-17, OPG, RANKL, and RANKL/OPG showed high sensitivity and specificity to differentiate periodontal health from gingivitis and periodontitis. Similar pattern was observed in discriminating stable and unstable periodontitis. Salivary IL-17 and RANKL showed a good accuracy to differentiate gingivitis from periodontitis. However, OPG and RANKL/OPG did not exhibit enough sensitivity and specificity to differentiate the latter conditions.

Conclusion

Salivary IL-17, RANKL, OPG, and RANKL/OPG system are potential candidates for differentiating periodontal health and disease and discriminate stable and unstable periodontitis.

[RANKL and periodontitis]

Periodontal disease is characterized by inflammation of the periodontal tissue and subsequent destruction of the alveolar bone. It is one of the most common infectious diseases in humans, being the leading cause of tooth loss in adults. Recently, it has been shown that the receptor activator of NF-κB ligand (RANKL) produced by osteoblasts and periodontal ligament fibroblasts critically contributes to the bone destruction caused by periodontal disease. Activation of the immune system plays an important role in the induction of RANKL during periodontal inflammation. Here we discuss the molecular mechanisms of periodontal bone destruction by focusing on the osteoimmune molecule RANKL.

Osteoimmunology in periodontitis; a paradigm for Th17/IL-17 inflammatory bone loss

Periodontitis is a prevalent human disease of inflammation-induced bone destruction. Through studies in patient lesions of rare and common forms of periodontitis and animal model experimentation, Th17/IL-17 related immune pathways have emerged as mediators of disease pathology. In this focused review, we examine mechanisms of induction, amplification and pathogenicity of Th17 cells in periodontitis.

Metagenomic Analysis Reveals a Mitigating Role for Lactobacillus paracasei and Bifidobacterium animalis in Experimental Periodontitis

Probiotics have aroused increasing concern as an intervention strategy for periodontitis (PD), but their underlying mechanism of action remains poorly characterized. Regarding the significance of oral microbiota dysbiosis related to PD, we predicted that the preventive activity of probiotics may be influenced by suppressing the bacterial pathogenicity. Herein, we investigated the effects of Lactobacillus paracasei L9 (L9) and Bifidobacterium animalis A6 (A6) on PD using a rat model, and demonstrated a regulatory effect of probiotics on oral flora from a metagenomics perspective. Oral administration of A6 or L9 effectively relieved gingival bleeding, periodontal inflammatory infiltration, and alveolar bone resorption. In addition, A6 or L9 treatment reduced the inflammatory response and increased the expression of anti-inflammatory cytokines, which we expected to ameliorate alveolar bone resorption as mediated by the receptor activator of the nuclear factor-κB ligand/OPG signaling pathway. More importantly, using metagenomic sequencing, we showed that probiotics significantly altered the taxonomic composition of the subgingival microbiome, and reduced the relative proportions of pathogenic bacterial genera such as Streptococcus, Fusobacterium, Veillonella, and Escherichia. Both probiotics significantly inhibited levels of bacterial virulence factors related to adherence, invasion, exoenzyme, and complement protease functions that are strongly correlated with the pathogenesis of PD. Our overall results suggest that A6 and L9 may constitute promising prophylactic agents for PD, and should thus be further explored in the future.

Metagenomic Analysis Reveals a Mitigating Role for Lactobacillus paracasei and Bifidobacterium animalis in Experimental Periodontitis

Probiotics have aroused increasing concern as an intervention strategy for periodontitis (PD), but their underlying mechanism of action remains poorly characterized. Regarding the significance of oral microbiota dysbiosis related to PD, we predicted that the preventive activity of probiotics may be influenced by suppressing the bacterial pathogenicity. Herein, we investigated the effects of Lactobacillus paracasei L9 (L9) and Bifidobacterium animalis A6 (A6) on PD using a rat model, and demonstrated a regulatory effect of probiotics on oral flora from a metagenomics perspective. Oral administration of A6 or L9 effectively relieved gingival bleeding, periodontal inflammatory infiltration, and alveolar bone resorption. In addition, A6 or L9 treatment reduced the inflammatory response and increased the expression of anti-inflammatory cytokines, which we expected to ameliorate alveolar bone resorption as mediated by the receptor activator of the nuclear factor-κB ligand/OPG signaling pathway. More importantly, using metagenomic sequencing, we showed that probiotics significantly altered the taxonomic composition of the subgingival microbiome, and reduced the relative proportions of pathogenic bacterial genera such as Streptococcus, Fusobacterium, Veillonella, and Escherichia. Both probiotics significantly inhibited levels of bacterial virulence factors related to adherence, invasion, exoenzyme, and complement protease functions that are strongly correlated with the pathogenesis of PD. Our overall results suggest that A6 and L9 may constitute promising prophylactic agents for PD, and should thus be further explored in the future.

Metagenomic Analysis Reveals a Mitigating Role for Lactobacillus paracasei and Bifidobacterium animalis in Experimental Periodontitis

Probiotics have aroused increasing concern as an intervention strategy for periodontitis (PD), but their underlying mechanism of action remains poorly characterized. Regarding the significance of oral microbiota dysbiosis related to PD, we predicted that the preventive activity of probiotics may be influenced by suppressing the bacterial pathogenicity. Herein, we investigated the effects of Lactobacillus paracasei L9 (L9) and Bifidobacterium animalis A6 (A6) on PD using a rat model, and demonstrated a regulatory effect of probiotics on oral flora from a metagenomics perspective. Oral administration of A6 or L9 effectively relieved gingival bleeding, periodontal inflammatory infiltration, and alveolar bone resorption. In addition, A6 or L9 treatment reduced the inflammatory response and increased the expression of anti-inflammatory cytokines, which we expected to ameliorate alveolar bone resorption as mediated by the receptor activator of the nuclear factor-κB ligand/OPG signaling pathway. More importantly, using metagenomic sequencing, we showed that probiotics significantly altered the taxonomic composition of the subgingival microbiome, and reduced the relative proportions of pathogenic bacterial genera such as Streptococcus, Fusobacterium, Veillonella, and Escherichia. Both probiotics significantly inhibited levels of bacterial virulence factors related to adherence, invasion, exoenzyme, and complement protease functions that are strongly correlated with the pathogenesis of PD. Our overall results suggest that A6 and L9 may constitute promising prophylactic agents for PD, and should thus be further explored in the future.

Angiotensin II Induces Aortic Rupture and Dissection in Osteoprotegerin-Deficient Mice

Background

The biological mechanism of action for osteoprotegerin, a soluble decoy receptor for the receptor activator of nuclear factor‐kappa B ligand in the vascular structure, has not been elucidated. The study aim was to determine if osteoprotegerin affects aortic structural integrity in angiotensin II (Ang II)‐induced hypertension.

Methods and Results

Mortality was higher (P<0.0001 by log‐rank test) in 8‐week‐old male homozygotes of osteoprotegerin gene‐knockout mice given subcutaneous administration of Ang II for 28 days, with an incidence of 21% fatal aortic rupture and 23% aortic dissection, than in age‐matched wild‐type mice. Ang II‐infused aorta of wild‐type mice showed that osteoprotegerin immunoreactivity was present with proteoglycan. The absence of osteoprotegerin was associated with decreased medial and adventitial thickness and increased numbers of elastin breaks as well as with increased periostin expression and soluble receptor activator of nuclear factor‐kappa B ligand concentrations. PEGylated human recombinant osteoprotegerin administration decreased all‐cause mortality (P<0.001 by log‐rank test), the incidence of fatal aortic rupture (P=0.08), and aortic dissection (P<0.001) with decreasing numbers of elastin breaks, periostin expressions, and soluble receptor activator of nuclear factor‐kappa B ligand concentrations in Ang II‐infused osteoprotegerin gene‐knockout mice.

Conclusions

These data suggest that osteoprotegerin protects against aortic rupture and dissection in Ang II‐induced hypertension by inhibiting receptor activator of nuclear factor‐kappa B ligand activity and periostin expression.

Oral Osteomicrobiology: The Role of Oral Microbiota in Alveolar Bone Homeostasis

Osteomicrobiology is a new research field in which the aim is to explore the role of microbiota in bone homeostasis. The alveolar bone is that part of the maxilla and mandible that supports the teeth. It is now evident that naturally occurring alveolar bone loss is considerably stunted in germ-free mice compared with specific-pathogen-free mice. Recently, the roles of oral microbiota in modulating host defense systems and alveolar bone homeostasis have attracted increasing attention. Moreover, the mechanistic understanding of oral microbiota in mediating alveolar bone remodeling processes is undergoing rapid progress due to the advancement in technology. In this review, to provide insight into the role of oral microbiota in alveolar bone homeostasis, we introduced the term “oral osteomicrobiology.” We discussed regulation of alveolar bone development and bone loss by oral microbiota under physiological and pathological conditions. We also focused on the signaling pathways involved in oral osteomicrobiology and discussed the bridging role of osteoimmunity and influencing factors in this process. Finally, the critical techniques for osteomicrobiological investigations were introduced.

The Development of Molecular Biology of Osteoporosis

Osteoporosis is one of the major bone disorders that affects both women and men, and causes bone deterioration and bone strength. Bone remodeling maintains bone mass and mineral homeostasis through the balanced action of osteoblasts and osteoclasts, which are responsible for bone formation and bone resorption, respectively. The imbalance in bone remodeling is known to be the main cause of osteoporosis. The imbalance can be the result of the action of various molecules produced by one bone cell that acts on other bone cells and influence cell activity. The understanding of the effect of these molecules on bone can help identify new targets and therapeutics to prevent and treat bone disorders. In this article, we have focused on molecules that are produced by osteoblasts, osteocytes, and osteoclasts and their mechanism of action on these cells. We have also summarized the different pharmacological osteoporosis treatments that target different molecular aspects of these bone cells to minimize osteoporosis.

Porphyromonas gingivalis induces periodontitis, causes immune imbalance, and promotes rheumatoid arthritis

Periodontitis induced by bacteria especially Porphyromonas gingivalis (P. gingivalis) is the most prevalent microbial disease worldwide and is a significant risk factor for systemic diseases such as rheumatoid arthritis (RA). RA and periodontitis share similar clinical and pathologic features. Moreover, the prevalence of RA is much higher in patients with periodontitis than in those without periodontitis. To explore the immunologic mechanism of periodontitis involved in RA, we established a mouse model of periodontitis and then induced RA. According to the results of paw thickness, arthritis clinical score, arthritis incidence, microscopic lesion using H&E staining, and micro-CT analysis, periodontitis induced by P. gingivalis promoted the occurrence and development of collagen-induced arthritis (CIA) in mice. Furthermore, periodontitis enhanced the frequency of CD19⁺ B cells, Th17, Treg, gMDSCs, and mMDSCs, whereas down-regulated IL-10 producing regulatory B cells (B10) in CIA mice preinduced for periodontitis with P. gingivalis. In vitro stimulation with splenic cells revealed that P. gingivalis directly enhanced differentiation of Th17, Treg, and mMDSCs but inhibited the process of B cell differentiation into B10 cells. Considering that adoptive transfer of B10 cells prevent RA development, our study, although preliminary, suggests that down-regulation of B10 cells may be the key mechanism that periodontitis promotes RA as the other main immune suppressive cells such as Treg and MDSCs are up-regulated other than down-regulated in group of P. gingivalis plus CIA.

Osteoclast differentiation by RANKL and OPG signaling pathways

INTRODUCTION

In bone tissue, bone resorption by osteoclasts and bone formation by osteoblasts are repeated continuously. Osteoclasts are multinucleated cells that derive from monocyte-/macrophage-lineage cells and resorb bone. In contrast, osteoblasts mediate osteoclastogenesis by expressing receptor activator of nuclear factor-kappa B ligand (RANKL), which is expressed as a membrane-associated cytokine. Osteoprotegerin (OPG) is a soluble RANKL decoy receptor that is predominantly produced by osteoblasts and which prevents osteoclast formation and osteoclastic bone resorption by inhibiting the RANKL-RANKL receptor interaction.

MATERIALS AND METHODS

In this review, we would like to summarize our experimental results on signal transduction that regulates the expression of RANKL and OPG.

RESULTS

Using OPG gene-deficient mice, we have demonstrated that OPG and sclerostin produced by osteocytes play an important role in the maintenance of cortical and alveolar bone. In addition, it was shown that osteoclast-derived leukemia inhibitory factor (LIF) reduces the expression of sclerostin in osteocytes and promotes bone formation. WP9QY (W9) is a peptide that was designed to be structurally similar to one of the cysteine-rich TNF-receptortype-I domains. Addition of the W9 peptide to bone marrow culture simultaneously inhibited osteoclast differentiation and stimulated osteoblastic cell proliferation. An anti-sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) antibody inhibited multinucleated osteoclast formation induced by RANKL and macrophage colony-stimulating factor (M-CSF). Pit-forming activity of osteoclasts was also inhibited by the anti-Siglec-15 antibody. In addition, anti-Siglec-15 antibody treatment stimulated the appearance of osteoblasts in cultures of mouse bone marrow cells in the presence of RANKL and M-CSF.

CONCLUSIONS

Bone mass loss depends on the RANK-RANKL-OPG system, which is a major regulatory system of osteoclast differentiation induction, activation, and survival.

RANKL and osteoimmunology in periodontitis

Periodontitis, one of the most common infectious diseases in humans, is characterized by inflammation of the periodontal tissue and subsequent destruction of the alveolar bone, which ultimately leads to tooth loss. Recently, it was revealed that the osteoclastic bone damage that occurs during periodontitis is dependent on the receptor activator of NF-kB ligand (RANKL) produced by osteoblastic cells and periodontal ligament cells. Immune cells provide essential cues for the RANKL induction that takes place during periodontal inflammation. The knowledge accumulated and experimental tools established in the field of "osteoimmunology" have made crucial contributions to a better understanding of periodontitis pathogenesis and, reciprocally, the investigation of periodontitis has provided important insights into the field. This review discusses the molecular mechanisms underlying periodontal bone loss by focusing on the osteoimmune interactions and RANKL.

The blockade of kappa opioid receptors exacerbates alveolar bone resorption in rats

OBJECTIVES

Bone resorption associated to chronic diseases, such as arthritis and periodontitis, results from exacerbated immuno-inflammatory host response that leads to tissue breakdown. The significance of opioid pathways as endogenous modulators of inflammatory events has already been described. Thus, the aim of this work is to determine whether some of the main three opioid receptors are endogenously activated to prevent bone loss during experimentally-induced alveolar bone resorption.

DESIGN

This study used an experimental model of alveolar bone resorption induced by ligature in rats. A silk thread was placed around the 2nd maxillary molar of male Wistar rats. In the 3rd, 4th and 5th day after ligation the rats received a local injection of different concentrations of opioid antagonists Cyprodime, Naltrindole, or Nor-binaltorphimine, which specifically block mü, delta and kappa opioid receptors, respectively. In the 7th experimental day, rats were euthanized and their maxillae collected for evaluation of alveolar bone and fiber attachment loss, morphometric counting of osteoclasts and osteoblasts, as well as the levels of cytokines IL-1β, IFN-γ, and IL-6 by ELISA.

RESULTS

Selective antagonism of kappa opioid receptors, but not mü and delta, exacerbated alveolar bone resorption induced by ligature in rats. The increased bone loss associated with higher number of osteoclasts surrounding alveolar bone, although osteoblasts' counting remained unchanged. The concentrations of IL-1β and IL-6 in periodontal tissues were also significantly higher in the rats treated with the kappa antagonist.

CONCLUSION

Inhibiting kappa opioid receptors exacerbates alveolar bone resorption.

Osteoimmunology: Inflammatory osteolysis and regeneration of the alveolar bone

AIM

Osteoimmunology covers the cellular and molecular mechanisms responsible for inflammatory osteolysis that culminates in the degradation of alveolar bone. Osteoimmunology also focuses on the interplay of immune cells with bone cells during bone remodelling and regeneration. The aim of this review was to provide insights into how osteoimmunology affects alveolar bone health and disease.

METHOD

This review is based on a narrative approach to assemble mouse models that provide insights into the cellular and molecular mechanisms causing inflammatory osteolysis and on the impact of immune cells on alveolar bone regeneration.

RESULTS

Mouse models have revealed the molecular pathways by which microbial and other factors activate immune cells that initiate an inflammatory response. The inflammation-induced alveolar bone loss occurs with the concomitant suppression of bone formation. Mouse models also showed that immune cells contribute to the resolution of inflammation and bone regeneration, even though studies with a focus on alveolar socket healing are rare.

CONCLUSIONS

Considering that osteoimmunology is evolutionarily conserved, osteolysis removes the cause of inflammation by provoking tooth loss. The impact of immune cells on bone regeneration is presumably a way to reinitiate the developmental mechanisms of intramembranous and endochondral bone formation.

From Crosstalk between Immune and Bone Cells to Bone Erosion in Infection

Bone infection and inflammation leads to the infiltration of immune cells at the site of infection, where they modulate the differentiation and function of osteoclasts and osteoblasts by the secretion of various cytokines and signal mediators. In recent years, there has been a tremendous effort to understand the cells involved in these interactions and the complex pathways of signal transduction and their ultimate effect on bone metabolism. These crosstalk mechanisms between the bone and immune system finally emerged, forming a new field of research called osteoimmunology. Diseases falling into the category of osteoimmunology, such as osteoporosis, periodontitis, and bone infections are considered to have a significant implication in mortality and morbidity of patients, along with affecting their quality of life. There is a much-needed research focus in this new field, as the reported data on the immunomodulation of immune cells and their signaling pathways seems to have promising therapeutic benefits for patients.

Blockade of the angiotensin II type 1 receptor increases bone mineral density and left ventricular contractility in a mouse model of juvenile Paget disease

Juvenile Paget disease (JPD1), an autosomal-recessive disorder, is characterized by extremely rapid bone turnover due to osteoprotegerin deficiency. Its extra-skeletal manifestations, such as hypertension and heart failure, suggest a pathogenesis with shared skeletal and cardiovascular system components. In spite of this, the effects of anti-hypertensive drugs on bone morphometry remain unknown. We administered an angiotensin II type 1 receptor blocker, olmesartan (5 mg/kg/day) to 8-week-old male mice lacking the osteoprotegerin gene, with and without 1 μg/kg/min of angiotensin II infusion for 14 days. Olmesartan treatment decreased systolic blood pressure, and echocardiography showed increased left ventricular systolic contractility. Three-dimensional micro-computed tomography scans demonstrated that olmesartan treatment increased trabecular bone volume (sham, +176%; angiotensin II infusion, +335%), mineral density (sham, +150%; angiotensin II infusion, +313%), and trabecular number (sham, +407%; angiotensin II infusion, +622%) in the tibia. Olmesartan increased cortical mineral density (sham, +19%; angiotensin II infusion, +24%), decreased the cortical bone section area (sham, -16%; angiotensin II infusion, -18%), decreased thickness (sham, -18%; angiotensin II infusion, -31%), and decreased the lacunar area (sham, -41%; angiotensin II infusion, -27%) in the tibia. Similar trend was observed in the femur. Moreover, olmesartan decreased angiotensin II-induced increases in tartrate-resistant acid phosphatase concentrations in plasma, but it affected neither type I procollagen N-terminal propeptides, nor the receptor activator of nuclear factor kappa-B ligand. Our data suggest that blockade of the angiotensin II type 1 receptor improves bone vulnerability, and helps to maintain the heart's structural integrity in osteoprotegerin-deficient mice.

Genetically-achieved disturbances to the expression levels of TNFSF11 receptors modulate the effects of zoledronic acid on growing mouse skeletons

Zoledronic acid (ZOL), a nitrogen bisphosphonate (N-BP), is currently used to treat and control pediatric osteolytic diseases. Variations in the intensity of the effects and side effects of N-BPs have been reported with no clear explanations regarding their origins. We wonder if such variations could be associated with different levels of RANKL signaling activity in growing bone during and after the treatment with N-BPs. To answer this question, ZOL was injected into neonate C57BL/6J mice with different genetically-determined RANKL signaling activity levels (Opg^+/+\Rank^Tg-, Opg^+/+\Rank^Tg+, Opg^+/-\Rank^Tg-, Opg^+/-\Rank^Tg+, Opg^-/-\Rank^Tg- and Opg^-/-\Rank^Tg+ mice) following a protocol (4 injections from post-natal day 1 to 7 at the dose of 50 μg/kg) that mimics those used in onco-pediatric patients. At the end of pediatric growth (1 and half months) and at an adult age (10 months), the bone morphometric and mineral parameters were measured using μCT in the tibia and skull for the different mice. A histologic analysis of the dental and periodontal tissues was also performed. At the end of pediatric growth, a delay in long bone and skull bone growth, a blockage of tooth eruption, some molar root alterations and a neoplasia-like structure associated with incisor development were found. Interestingly, the magnitude of these side effects was reduced by Opg deficiency (Opg^-/-) but increased by Rank overexpression (Rank^Tg). Analysis of the skeletal phenotype at ten months confirmed respectively the beneficial and harmful effects of Opg deficiency and Rank overexpression. These results validated the hypothesis that the RANKL signaling activity level in the bone microenvironment is implicated in the modulation of the response to ZOL. Further studies will be necessary to understand the underlying molecular mechanisms, which will help decipher the variability in the effects of N-BPs reported in the human population. SIGNIFICANT STATEMENTS: The present study establishes that in mice the RANKL signaling activity level is a major modulator of the effects and side-effects of bisphosphonates on the individual skeleton during growth. However, the modulatory actions are dependent on the ways in which this level of activity is increased. A decrease in OPG expression is beneficial to the skeletal phenotype observed at the end of growth, while RANK overexpression deteriorates it. Far removed from pediatric treatment, in adults, the skeletal phenotypes initially observed at the end of growth for the different levels of RANKL signaling activity were maintained, although significant improvement was associated only with reductions in OPG expression.

Osteoimmunology: evolving concepts in bone-immune interactions in health and disease

In terrestrial vertebrates, bone tissue constitutes the 'osteoimmune' system, which functions as a locomotor organ and a mineral reservoir as well as a primary lymphoid organ where haematopoietic stem cells are maintained. Bone and mineral metabolism is maintained by the balanced action of bone cells such as osteoclasts, osteoblasts and osteocytes, yet subverted by aberrant and/or prolonged immune responses under pathological conditions. However, osteoimmune interactions are not restricted to the unidirectional effect of the immune system on bone metabolism. In recent years, we have witnessed the discovery of effects of bone cells on immune regulation, including the function of osteoprogenitor cells in haematopoietic stem cell regulation and osteoblast-mediated suppression of haematopoietic malignancies. Moreover, the dynamic reciprocal interactions between bone and malignancies in remote organs have attracted attention, extending the horizon of osteoimmunology. Here, we discuss emerging concepts in the osteoimmune dialogue in health and disease.

Murine osteoclasts secrete serine protease HtrA1 capable of degrading osteoprotegerin in the bone microenvironment

Osteoclasts are multinucleated cells responsible for bone resorption. The differentiation of osteoclasts from bone marrow macrophages (BMMs) is induced by receptor activator of NF-κB ligand (RANKL). Osteoprotegerin (OPG), a decoy receptor of RANKL, inhibits osteoclastogenesis by blocking RANKL signaling. Here we investigated the degradation of OPG in vitro. Osteoclasts, but not BMMs, secreted OPG-degrading enzymes. Using mass spectrometry and RNA-sequencing analysis, we identified high-temperature requirement A serine peptidase 1 (HtrA1) as an OPG-degrading enzyme. HtrA1 did not degrade OPG pre-reduced by dithiothreitol, suggesting that HtrA1 recognizes the three-dimensional structure of OPG. HtrA1 initially cleaved the amide bond between leucine 90 and glutamine 91 of OPG, then degraded OPG into small fragments. Inhibitory activity of OPG on RANKL-induced osteoclastogenesis was suppressed by adding HtrA1 in RAW 264.7 cell cultures. These results suggest that osteoclasts potentially prepare a microenvironment suitable for osteoclastogenesis. HtrA1 may be a novel drug target for osteoporosis.

Ferulic acid inhibits interleukin 17-dependent expression of nodal pathogenic mediators in fibroblast-like synoviocytes of rheumatoid arthritis

Interleukin 17 (IL-17), a proinflammatory cytokine produced by T helper (Th) 17 cells, potentially controls fibroblast-like synoviocytes (FLS)-mediated disease activity of rheumatoid arthritis (RA) via IL-17/ IL-17 receptor type A (IL-17RA)/signal transducer and activator of transcription 3 (STAT-3) signaling cascade. This has suggested that targeting IL-17 signaling could serve as an important strategy to treat FLS-mediated RA progression. Ferulic acid (FA), a key polyphenol, attenuates the development of gouty arthritis and cancer through its anti-inflammatory effects, but its therapeutic efficiency on IL-17 signaling in FLS-mediated RA pathogenesis remains unknown. In the current study, FA markedly inhibited the IL-17-mediated expression of its specific transmembrane receptor IL-17RA in FLS isolated from adjuvant-induced arthritis (AA) rats. Importantly, FA dramatically suppressed the IL-17-mediated expression of toll-like receptor 3 (TLR-3), cysteine-rich angiogenic inducer 61 (Cyr61), IL-23, granulocyte-macrophage colony stimulating factor (GM-CSF) in AA-FLS via the inhibition of IL-17/IL-17RA/STAT-3 signaling cascade. In addition, FA significantly decreased the formation of osteoclast cells and bone resorption potential in a coculture system consisting of IL-17 treated AA-FLS and rat bone marrow derived monocytes/macrophages. Furthermore, FA remarkably inhibited the IL-17-mediated expression of receptor activator of nuclear factor κ-Β ligand (RANKL) and increased the expression of osteoprotegerin (OPG) in AA-FLS via the regulation of IL-17/IL-17RA/STAT-3 signaling cascade. The therapeutic efficiency of FA on IL-17 signaling was further confirmed by knockdown of IL-17RA using small interfering RNA or blocking of STAT-3 activation with S3I-201. The molecular docking analysis revealed that FA manifests significant ligand efficiency toward IL-17RA, STAT-3, IL-23, and RANKL proteins. This study provides new evidence that FA can be used as a potential therapeutic agent for inhibiting IL-17-mediated disease severity and bone erosion in RA.

BMAL1 deficiency promotes skeletal mandibular hypoplasia via OPG downregulation

OBJECTIVES

Skeletal mandibular hypoplasia (SMH), a common type of developmental deformities, results in impaired aesthetics of facial profile, occlusal dysfunction and poor life quality. In this study, BMAL1 deficiency leads to SMH formation, and we aim to investigate the mechanism by which BMAL1 deficiency induces SMH.

MATERIALS AND METHODS

Circadian rhythm-disordered mouse models were constructed by placing animals in a jet lag schedule of 6-h light advance every 7 days for 4 or 8 weeks. The OPG expression was evaluated by histomorphometry, immunohistochemistry and western blot analysis. The mechanism by which BMAL1 affects OPG expression was investigated by chromatin immunoprecipitation and luciferase reporter assays. The phenotypes caused by BMAL1 knockout can be rescued by exogenous supplementation with OPG.

RESULTS

We demonstrate that the expressions of BMAL1 and OPG decreased in SMH patients. Circadian rhythm-disordered mice and Bmal1-/- mice exhibited decreased expression of OPG, reduced bone mass and bone size of mandibles. Our results revealed that BMAL1 bound directly to the Opg promoter and upregulated its expression, thus inhibiting osteoclast differentiation. BMAL1 deficiency increased osteoclast differentiation by downregulating OPG expression. In vitro, the enhancement effect of osteoclast differentiation caused by BMAL1 knockdown was significantly reversed by exogenous supplementation with OPG. Importantly, bone loss caused by BMAL1 knockout can be partially reversed by injecting OPG Intraperitoneally.

CONCLUSIONS

These results indicate that the circadian clock plays a critical role in the growth and development of mandible by regulating OPG expression, and present a potential therapeutic strategy to prevent SMH.

Age-related changes of CD4+ T cell migration and cytokine expression in germ-free and SPF mice periodontium

OBJECTIVE

Increasing age is a potential risk factor for periodontal tissue breakdown, which may be affected by commensal flora. The aim of this study evaluated age-related changes in CD4⁺ T cells, C-C chemokine ligand 5 (CCL5), interleukin (IL)-17A, and receptor activator of nuclear factor-kappa B ligand (RANKL) expression using germ-free (GF) and conventionally reared (SPF) mice.

DESIGN

GF and SPF mice at 8 (n = 6/group) and 22 weeks old (n = 6/group) were used. Immunohistochemical analyses were performed to determine the effects of aging on protein expression in periodontal tissues. Age-related changes in alveolar bone were quantified using micro-CT analysis.

RESULTS

SPF mice, but not GF mice, showed an age-related increase in alveolar bone loss (P < 0.01). SPF mice at 22 weeks of age increased expression of CD4⁺ T cells, CCL5, IL-17A, and RANKL compared to those at 8 weeks of age in connective tissue and alveolar bone surface (P < 0.01). Furthermore, there was increased CD4⁺ T cells, which were co-expressed with IL-17A and RANKL in SPF mice at 22 weeks of age. On the other hand, the GF mice did not show any significant differences in CD4⁺ T cells, CCL5, IL-17A and RANKL expression between the two age groups.

CONCLUSIONS

SPF mice induced an age-related increase in CD4⁺ T cells co- expressed with IL-17A and RANKL, with occurring alveolar bone loss. In contrast, GF mice did not show age-related changes in CD4⁺ T cell migration and cytokine expression.

Genes Critical for Developing Periodontitis: Lessons from Mouse Models

Since the etiology of periodontitis in humans is not fully understood, genetic mouse models may pinpoint indispensable genes for optimal immunological protection of the periodontium against tissue destruction. This review describes the current knowledge of genes that are involved for a proper maintenance of a healthy periodontium in mice. Null mutations of genes required for leukocyte cell–cell recognition and extravasation (e.g., Icam-1, P-selectin, Beta2-integrin/Cd18), for pathogen recognition and killing (e.g., Tlr2, Tlr4, Lamp-2), immune modulatory molecules (e.g., Cxcr2, Ccr4, IL-10, Opg, IL1RA, Tnf-α receptor, IL-17 receptor, Socs3, Foxo1), and proteolytic enzymes (e.g., Mmp8, Plasmin) cause periodontitis, most likely due to an inefficient clearance of bacteria and bacterial products. Several mechanisms resulting in periodontitis can be recognized: (1) inefficient bacterial control by the polymorphonuclear neutrophils (defective migration, killing), (2) inadequate antigen presentation by dendritic cells, or (3) exaggerated production of pro-inflammatory cytokines. In all these cases, the local immune reaction is skewed toward a Th1/Th17 (and insufficient activation of the Th2/Treg) with subsequent osteoclast activation. Finally, genotypes are described that protect the mice from periodontitis: the SCID mouse, and mice lacking Tlr2/Tlr4, the Ccr1/Ccr5, the Tnf-α receptor p55, and Cathepsin K by attenuating the inflammatory reaction and the osteoclastogenic response.

Treatment of OPG-deficient mice with WP9QY, a RANKL-binding peptide, recovers alveolar bone loss by suppressing osteoclastogenesis and enhancing osteoblastogenesis

Osteoblasts express two key molecules for osteoclast differentiation, receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG), a soluble decoy receptor for RANKL. RANKL induces osteoclastogenesis, while OPG inhibits it by blocking the binding of RANKL to RANK, a cellular receptor of RANKL. OPG-deficient (OPG^–/–) mice exhibit severe alveolar bone loss with enhanced bone resorption. WP9QY (W9) peptide binds to RANKL and blocks RANKL-induced osteoclastogenesis. W9 is also reported to stimulate bone formation in vivo. Here, we show that treatment with W9 restores alveolar bone loss in OPG^–/–mice by suppressing osteoclastogenesis and enhancing osteoblastogenesis. Administration of W9 or risedronate, a bisphosphonate, to OPG^–/–mice significantly decreased the osteoclast number in the alveolar bone. Interestingly, treatment with W9, but not risedronate, enhanced Wnt/β-catenin signaling and induced alveolar bone formation in OPG^–/–mice. Expression of sclerostin, an inhibitor of Wnt/β-catenin signaling, was significantly lower in tibiae of OPG^–/–mice than in wild-type mice. Treatment with risedronate recovered sclerostin expression in OPG^–/–mice, while W9 treatment further suppressed sclerostin expression. Histomorphometric analysis confirmed that bone formation-related parameters in OPG^–/–mice, such as osteoblast number, osteoblast surface and osteoid surface, were increased by W9 administration but not by risedronate administration. These results suggest that treatment of OPG^–/–mice with W9 suppressed osteoclastogenesis by inhibiting RANKL signaling and enhanced osteoblastogenesis by attenuating sclerostin expression in the alveolar bone. Taken together, W9 may be a useful drug to prevent alveolar bone loss in periodontitis.

Osteocyte regulation of orthodontic force-mediated tooth movement via RANKL expression

Orthodontic tooth movement is achieved by the remodeling of the alveolar bone surrounding roots of teeth. Upon the application of orthodontic force, osteoclastic bone resorption occurs on the compression side of alveolar bone, towards which the teeth are driven. However, the molecular basis for the regulatory mechanisms underlying alveolar bone remodeling has not been sufficiently elucidated. Osteoclastogenesis is regulated by receptor activator of nuclear factor-κB ligand (RANKL), which is postulated to be expressed by the cells surrounding the tooth roots. Here, we show that osteocytes are the critical source of RANKL in alveolar bone remodeling during orthodontic tooth movement. Using a newly established method for the isolation of periodontal tissue component cells from alveolar bone, we found that osteocytes expressed a much higher amount of RANKL than other cells did in periodontal tissue. The critical role of osteocyte-derived RANKL was confirmed by the reduction of orthodontic tooth movement in mice specifically lacking RANKL in osteocytes. Thus, we provide in vivo evidence for the key role of osteocyte-derived RANKL in alveolar bone remodeling, establishing a molecular basis for orthodontic force-mediated bone resorption.

RANK/RANKL/OPG Signalization Implication in Periodontitis: New Evidence from a RANK Transgenic Mouse Model

Periodontitis is based on a complex inflammatory over-response combined with possible genetic predisposition factors. The RANKL/RANK/OPG signaling pathway is implicated in bone resorption through its key function in osteoclast differentiation and activation, as well as in the inflammatory response. This central element of osteo-immunology has been suggested to be perturbed in several diseases, including periodontitis, as it is a predisposing factor for this disease. The aim of the present study was to validate this hypothesis using a transgenic mouse line, which over-expresses RANK (R^Tg) and develops a periodontitis-like phenotype at 5 months of age. R^Tg mice exhibited severe alveolar bone loss, an increased number of TRAP positive cells, and disorganization of periodontal ligaments. This phenotype was more pronounced in females. We also observed dental root resorption lacunas. Hyperplasia of the gingival epithelium, including Malassez epithelial rests, was visible as early as 25 days, preceding any other symptoms. These results demonstrate that perturbations of the RANKL/RANK/OPG system constitute a core element of periodontitis, and more globally, osteo-immune diseases.

Preventive Effects of Drinking Hydrogen-Rich Water on Gingival Oxidative Stress and Alveolar Bone Resorption in Rats Fed a High-Fat Diet

Obesity induces gingival oxidative stress, which is involved in the progression of alveolar bone resorption. The antioxidant effect of hydrogen-rich water may attenuate gingival oxidative stress and prevent alveolar bone resorption in cases of obesity. We examined whether hydrogen-rich water could suppress gingival oxidative stress and alveolar bone resorption in obese rats fed a high-fat diet. Male Fischer 344 rats (n = 18) were divided into three groups of six rats each: a control group (fed a regular diet and drinking distilled water) and two experimental groups (fed a high-fat diet and drinking distilled water or hydrogen-rich water). The level of 8-hydroxydeoxyguanosine was determined to evaluate oxidative stress. The bone mineral density of the alveolar bone was analyzed by micro-computerized tomography. Obese rats, induced by a high-fat diet, showed a higher gingival level of 8-hydroxydeoxyguanosine and a lower level of alveolar bone density compared to the control group. Drinking hydrogen-rich water suppressed body weight gain, lowered gingival level of 8-hydroxydeoxyguanosine, and reduced alveolar bone resorption in rats on a high-fat diet. The results indicate that hydrogen-rich water could suppress gingival oxidative stress and alveolar bone resorption by limiting obesity.

In vitro evaluation of osteoprotegerin in chitosan for potential bone defect applications

Background

The receptor activator of nuclear factor kappa-B (RANK)/RANK ligand/osteoprotegerin (OPG) system plays a critical role in bone remodelling by regulating osteoclast formation and activity. OPG has been used systemically in the treatment of bone diseases. In searching for more effective and safer treatment for bone diseases, we investigated newly formulated OPG-chitosan complexes, which is prepared as a local application for its osteogenic potential to remediate bone defects.

Methods

We examined high, medium and low molecular weights of chitosan combined with OPG. The cytotoxicity of OPG in chitosan and its proliferation in vitro was evaluated using normal, human periodontal ligament (NHPL) fibroblasts in 2D and 3D cell culture. The cytotoxicity of these combinations was compared by measuring cell survival with a tetrazolium salt reduction (MTT) assay and AlamarBlue assay. The cellular morphological changes were observed under an inverted microscope. A propidium iodide and acridine orange double-staining assay was used to evaluate the morphology and quantify the viable and nonviable cells. The expression level of osteopontin and osteocalcin protein in treated normal human osteoblast cells was evaluated by using Western blot.

Results

The results demonstrated that OPG in combination with chitosan was non-toxic, and OPG combined with low molecular weight chitosan has the most significant effect on NHPL fibroblasts and stimulates proliferation of cells over the period of treatment.

Smad2 overexpression induces alveolar bone loss and up regulates TNF-α, and RANKL

OBJECTIVE

The aim of the current study was to investigate whether Smad2 overexpression in JE cells induced alveolar bone loss, and to understand the mechanisms regulating the bone loss.

METHODS

A mouse line was created that used a cytokeratin 14 (K14) promoter to overexpress Smad2 in the epithelium of the transgenic mice (K14-Smad2). Micro CT radiographs (μCT) were used to assess bone loss, bone volume, and bone density. The expression of Tnfα, Il1-β, Ifγ, Rankl, and Opg were assessed by RT-PCR. Western blots were used to detect the protein levels of TNF-α and IL1-β. Tartrate-resistant acid phosphatase (TRAP) was used as a marker for osteoclasts. Wild type (WT) mice were used as controls in all steps of the current study.

RESULTS

K14-Smad2 mice had 52.5% (±4.2) root exposed compared to 32.4%(±3.2) in the WT mice. There was a significant difference in alveolar bone volume in the K14-Smad2 mice when compared to WT mice 2.65mm³ (±0.3) and 4.3mm³ (±0.35) respectively. K14-Smad2 mice also had reduced bone density 696.8mg/cc (±70) at 12 months when compared to WT mice 845.9mg/cc(±10). The mRNA levels of Tnfα and Rankl increased by 3.26- and 2.5-fold respectively in the K14-Smad2 mice when compared to controls. The protein level of TNF-α was also significantly increased to 2.8-fold in K14-Smad2 mice when compared to WT mice. Smad2 overexpression increased the total numbers of osteoclasts in K14-Smad2 mice (3.4±0.2)-fold when compared to WT mice.

CONCLUSION

Smad2 overexpression induces alveolar bone loss and increases the numbers of osteoclasts. Also, Smad2 overexpression up-regulates TNF-α and RANKL.

Effects of IL-10 and glucose on expression of OPG and RANKL in human periodontal ligament fibroblasts

The effects of interleukin-10 (IL-10) and glucose on mRNA and protein expression of osteoprotegerin (OPG), and its ligand, receptor activator of nuclear factor-κB ligand (RANKL), were investigated in human periodontal ligament fibroblasts (HPDLFs). Primary HPDLFs were treated with different concentrations of IL-10 (0, 1, 10, 25, 50, and 100 ng/mL) or glucose (0, 5.5, 10, 20, 30, and 40 mmol/L). Changes in mRNA and protein expression were examined using the reverse-transcription polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. After IL-10 treatment, mRNA and protein levels of OPG were increased, while mRNA and protein levels of RANKL were decreased (P<0.05), both in a concentration-dependent manner. Glucose stimulation had the opposite concentration-dependent effect to that of IL-10 on OPG and RANKL expression. IL-10 upregulated OPG expression and downregulated RANKL expression, whereas high glucose upregulated RANKL and downregulated OPG in HDPLFs. Abnormal levels of IL-10 and glucose may contribute to the pathogenesis of periodontal disease.

Reveromycin A Administration Prevents Alveolar Bone Loss in Osteoprotegerin Knockout Mice with Periodontal Disease

Chronic periodontal disease is characterized by alveolar bone loss and inflammatory changes. Reveromycin A (RMA) was recently developed and is a unique agent for inhibiting osteoclast activity. This study analysed the effects of RMA in an experimental mouse model of periodontitis involving osteoprotegerin (OPG)-knockout mice, specifically, whether it could control osteoclasts and reduce inflammation in periodontal tissue. We examined wild-type (WT) and OPG knockout mice (OPG KO) ligated with wire around contact points on the left first and second molars. RMA was administered twice a day to half of the mice. Using micro-computed tomography, we measured the volume of alveolar bone loss between the first and second molars, and also performed histological analysis. The OPG KO RMA+ group had significantly decreased osteoclast counts, alveolar bone loss, attachment loss, and inflammatory cytokine expression 8 weeks after ligation. Thus, RMA may reduce alveolar bone loss and inflamed periodontal tissues in patients with periodontitis.

Bone resorption: an actor of dental and periodontal development?

Dental and periodontal tissue development is a complex process involving various cell-types. A finely orchestrated network of communications between these cells is implicated. During early development, communications between cells from the oral epithelium and the underlying mesenchyme govern the dental morphogenesis with successive bud, cap and bell stages. Later, interactions between epithelial and mesenchymal cells occur during dental root elongation. Root elongation and tooth eruption require resorption of surrounding alveolar bone to occur. For years, it was postulated that signaling molecules secreted by dental and periodontal cells control bone resorbing osteoclast precursor recruitment and differentiation. Reverse signaling originating from bone cells (osteoclasts and osteoblasts) toward dental cells was not suspected. Dental defects reported in osteopetrosis were associated with mechanical stress secondary to defective bone resorption. In the last decade, consequences of bone resorption over-activation on dental and periodontal tissue formation have been analyzed with transgenic animals (RANK^Tg and Opg^−∕− mice). Results suggest the existence of signals originating from osteoclasts toward dental and periodontal cells. Meanwhile, experiments consisting in transitory inhibition of bone resorption during root elongation, achieved with bone resorption inhibitors having different mechanisms of action (bisphosphonates and RANKL blocking antibodies), have evidenced dental and periodontal defects that support the presence of signals originating bone cells toward dental cells. The aim of the present manuscript is to present the data we have collected in the last years that support the hypothesis of a role of bone resorption in dental and periodontal development.

Wnt16 regulates osteoclast differentiation in conjunction with Wnt5a

The canonical Wnt/β-catenin signaling pathway in osteoblast-lineage cells inhibits osteoclastogenesis through the expression of osteoprotegerin (Opg), a decoy receptor of receptor activator of Nf-κb (Rank) ligands. Wnt5a, a typical non-canonical Wnt ligand, enhances the expression of Rank in osteoclast precursors, which, in turn, promotes the Rank ligand (Rankl)-induced formation of osteoclasts. In contrast, Wnt16 and Wnt4 have been shown to inhibit the Rankl-induced formation of osteoclasts through non-canonical Wnt signals. However, the relationships among these Wnt ligands in osteoclastogenesis remained to be elucidated. We herein showed that Wnt16, but not Wnt4, inhibited the Rankl-induced osteoclastogenesis in bone marrow-derived macrophage (BMM) cultures. Wnt3a and Wnt4 inhibited the 1α,25-dihydroxy vitamin D3 (1,25D3)-induced osteoclastogenesis in co-cultures prepared from wild-type mice, but not in those from Opg(-/-) nice. Wnt16 inhibited the 1,25D3-induced formation of osteoclasts in both wild-type and Opg(-/-) co-cultures. Wnt16, Wnt4, and Wnt3a failed to inhibit the pit-forming activity of osteoclasts. Wnt16 failed to inhibit the Wnt5a-induced expression of Rank in osteoclast precursors. In contrast, Wnt5a abrogated the inhibitory effects of Wnt16 on Rankl-induced osteoclastogenesis. These results suggested that Wnt16 inhibited osteoclastogenesis, but not the function of osteoclasts and that Wnt16, an inhibitory Wnt ligand for osteoclastogenesis, regulates bone resorption in conjunction with Wnt5a.

RANKL/OPG; Critical role in bone physiology

After it was proposed that the osteoblast lineage controlled the formation of osteoclasts, cell culture methods were developed that established this to be the case. Evidence was obtained that cytokines and hormones that promote osteoclast formation act first on osteoblast lineage cells to promote the production of a membrane-bound regulator of osteoclastogenesis. This proved to be receptor activator of NF-kB ligand (RANKL) a member of the tumor necrosis factor ligand family that acts upon its receptor RANK in the hematopoietic lineage, with interaction restricted by a decoy soluble receptor osteoprotegerin (OPG), also a product of the osteoblast lineage. The physiological roles of these factors were established through genetic and pharmacological studies, have led to a new physiology of bone, with complete revision of older ideas over the last 15 years, ultimately leading to the development of new pharmaceutical agents for bone disease.

STAT-6 mediates TRAIL induced RANK ligand expression in stromal/preosteoblast cells

Receptor activator of nuclear factor kappa-B ligand (RANKL) is a critical osteoclastogenic factor expressed in bone marrow stromal/osteoblast lineage cells. Tumor necrosis factor (TNF) related apoptosis-inducing ligand (TRAIL) levels are elevated in pathologic conditions such as multiple myeloma and inflammatory arthritis, and have been positively correlated with osteolytic markers. Osteoprotegerin (OPG) which inhibits osteoclastogenesis is a decoy receptor for RANKL and also known to interact with TRAIL. Herein, we show that TRAIL increases DR5 and DcR1 receptors but no change in the levels of DR4 and DcR2 expression in human bone marrow derived stromal/preosteoblast (SAKA-T) cell line. We further demonstrated that TRAIL treatment significantly decreased OPG mRNA expression. Interestingly, TRAIL treatment induced RANKL mRNA expression in these cells. In addition, TRAIL significantly increased NF-kB and c-Jun N-terminal kinase (JNK) activity. Human transcription factor array screening by real-time RT-PCR identified TRAIL up-regulation of the signal transducers and activators of the transcription (STAT)-6 expression in SAKA-T cells. TRAIL stimulation induced p-STAT-6 expression in human bone marrow derived primary stromal/preosteoblast cells. Confocal microscopy analysis further revealed p-STAT-6 nuclear localization in SAKA-T cells. Chromatin immunoprecipitation (ChIP) assay confirmed p-STAT-6 binding to the hRANKL gene distal promoter region. In addition, siRNA suppression of STAT-6 expression inhibits TRAIL increased hRANKL gene promoter activity. Thus, our results suggest that TRAIL induces RANKL expression through a STAT-6 dependent transcriptional regulatory mechanism in bone marrow stromal/preosteoblast cells.

Role of periodontal ligament fibroblasts in osteoclastogenesis: a review

During the last decade it has become clear that periodontal ligament fibroblasts may contribute to the in vitro differentiation of osteoclasts. We surveyed the current findings regarding their osteoclastogenesis potential. Periodontal ligament fibroblasts have the capacity to select and attract osteoclast precursors and subsequently to retract and enable migration of osteoclast precursors to the bone surface. There, fusion of precursors takes place, giving rise to osteoclasts. The RANKL-RANK-osteoprotegerin (OPG) axis is considered crucial in this process. Periodontal ligament fibroblasts produce primarily OPG, an osteoclastogenesis-inhibitory molecule. However, they may be influenced in vivo by direct or indirect interactions with bacteria or by mechanical loading. Incubation of periodontal ligament fibroblasts with bacteria or bacterial components causes an increased expression of RANKL and other osteoclastogenesis-stimulating molecules, such as tumor necrosis factor-α and macrophage-colony stimulating factor. Similar results are observed after the application of mechanical loading to these fibroblasts. Periodontal ligament fibroblasts may be considered to play an important role in the remodelling of alveolar bone. In vitro experiments have demonstrated that periodontal ligament fibroblasts adapt to bacterial and mechanical stimuli by synthesizing higher levels of osteoclastogenesis-stimulating molecules. Therefore, they probably contribute to the enhanced osteoclast formation observed during periodontitis and to orthodontic tooth movement.

Icariin Augments Bone Formation and Reverses the Phenotypes of Osteoprotegerin-Deficient Mice through the Activation of Wnt/ β -Catenin-BMP Signaling

Icariin has been mostly reported to enhance bone fracture healing and treat postmenopausal osteoporosis in ovariectomized animal model. As another novel animal model of osteoporosis, there is few publication about the effect of Icariin on osteoprotegerin-deficient mice. Therefore, the goal of this study is to find the effect on bone formation and underlying mechanisms of Icariin in osteoprotegerin (OPG) knockout (KO) mice. We found that Icariin significantly stimulated new bone formation after local injection over the surface of calvaria at the dose of 5 mg/kg per day. With this dose, Icariin was also capable of significantly reversing OPG-deficient-induced bone loss and bone strength reduction. Real-time PCR analysis showed that Icariin significantly upregulated the expression of BMP2, BMP4, RUNX2, OC, Wnt1, and Wnt3a in OPG KO mice. Icariin also significantly increased the expression of AXIN2, DKK1, TCF1, and LEF1, which are the direct target genes of β -catenin signaling. The in vitro studies showed that Icariin induced osteoblast differentiation through the activation of Wnt/ β -catenin-BMP signaling by in vitro deletion of the β -catenin gene using β -catenin(fx/fx) mice. Together, our findings demonstrate that Icariin significantly reverses the phenotypes of OPG-deficient mice through the activation of Wnt/ β -catenin-BMP signaling.