RANKL deletion in periodontal ligament and bone lining cells blocks orthodontic tooth movement

Age-related mitophagy regulates orthodontic tooth movement by affecting PDLSCs mitochondrial function and RANKL/OPG

A thorough comprehension of age-related variances in orthodontic tooth movement (OTM) and bone remodeling response to mechanical force holds significant implications for enhancing orthodontic treatment. Mitophagy plays a crucial role in bone metabolism and various age-related diseases. However, the impact of mitophagy on the bone remodeling process during OTM remains elusive. Using adolescent (6 weeks old) and adult (12 months old) rats, we established OTM models and observed that orthodontic force increased the expression of the mitophagy proteins PTEN-induced putative kinase 1 (PINK1) and Parkin, as well as the number of tartrate-resistant acid phosphatase-positive osteoclasts and osteocalcin-positive osteoblasts. These biological changes were found to be age-related. In vitro, compression force loading promoted PINK1/Parkin-dependent mitophagy in periodontal ligament stem cells (PDLSCs) derived from adolescents (12-16 years old) and adults (25-35 years old). Furthermore, adult PDLSCs exhibited lower levels of mitophagy, impaired mitochondrial function, and a decreased ratio of RANKL/OPG compared to young PDLSCs after compression. Transfection of siRNA confirmed that inhibition of mitophagy in PDLSC resulted in decreased mitochondrial function and reduced RANKL/OPG ratio. Application of mitophagy inducer Urolithin A enhanced bone remodeling and accelerated OTM in rats, while the mitophagy inhibitor Mdivi-1 had the opposite effect. These findings indicate that force-stimulated PDLSC mitophagy contributes to alveolar bone remodeling during OTM, and age-related impairment of mitophagy negatively impacts the PDLSC response to mechanical stimulus. Our findings enhance the understanding of mitochondrial mechanotransduction and offer new targets to tackle current clinical challenges in orthodontic therapy.

Research progress on the regulatory mechanism of integrin-mediated mechanical stress in cells involved in bone metabolism

Mechanical stress is an internal force between various parts of an object that resists external factors and effects that cause an object to deform, and mechanical stress is essential for various tissues that are constantly subjected to mechanical loads to function normally. Integrins are a class of transmembrane heterodimeric glycoprotein receptors that are important target proteins for the action of mechanical stress stimuli on cells and can convert extracellular physical and mechanical signals into intracellular bioelectrical signals, thereby regulating osteogenesis and osteolysis. Integrins play a bidirectional regulatory role in bone metabolism. In this paper, relevant literature published in recent years is reviewed and summarized. The characteristics of integrins and mechanical stress are introduced, as well as the mechanisms underlying responses of integrin to mechanical stress stimulation. The paper focuses on integrin-mediated mechanical stress in different cells involved in bone metabolism and its associated signalling mechanisms. The purpose of this review is to provide a theoretical basis for the application of integrin-mediated mechanical stress to the field of bone tissue repair and regeneration.

Effect of age on orthodontic tooth movement in mice

Background/purpose

The number of middle-aged and elderly orthodontic patients is increasing due to changes in age composition. It is important to investigate the detailed mechanisms of bone remodeling in orthodontic tooth movement (OTM) in the elderly. However, there are few reports on the mechanism of tooth movement in the elderly. The purpose of the present study was to analyze OTM and osteoclastogenesis in aged mice and to elucidate the mechanism.

Materials and methods

It has been reported that tumor necrosis factor (TNF)-α plays an important role in osteoclast formation and OTM. First, 8-week-old and 78-week-old male C57BL/6J mice were subcutaneously injected with TNF-α into the calvaiae, and micro-CT, tartrate-resistant acid phosphatase (TRAP) staining, and real-time PCR were performed to evaluate osteoclast formation and bone resorption. Furthermore, osteoclastogenesis by TNF-α and receptor activator of nuclear factor-kappa B ligand (RANKL) using bone marrow cells was evaluated in vitro. Finally, a nickel-titanium closed-coil spring was attached, mesial movement of the maxillary left first molar was performed, and tooth movement distance and osteoclast formation were evaluated.

Results

Compared to 8-week-old mice, 78-week-old mice had decreased TNF-α-induced bone resorption, osteoclastogenesis, and TRAP and cathepsin K expression in the calvariae. In vitro osteoclast formation also decreased in 78-week-old mice. Furthermore, tooth movement distance and osteoclastogenesis were reduced.

Conclusion

OTM decreased in aged mice, which was shown to be caused by a decrease in osteoclastogenesis. Therefore, it was suggested that it is necessary to keep in mind that tooth movement may be suppressed when treating elderly patients.

Inhibition of RANKL improves the skeletal phenotype of adenine-induced chronic kidney disease in mice

Skeletal fragility and high fracture rates are common in CKD. A key component of bone loss in CKD with secondary hyperparathyroidism is high bone turnover and cortical bone deterioration through both cortical porosity and cortical thinning. We hypothesized that RANKL drives high bone resorption within cortical bone leading to the development of cortical porosity in CKD (study 1) and that systemic inhibition of RANKL would mitigate the skeletal phenotype of CKD (study 2). In study 1, we assessed the skeletal properties of male and female Dmp1-cre RANKLfl/fl (cKO) and control genotype (Ranklfl/fl; Con) mice after 10 wk of adenine-induced CKD (AD; 0.2% dietary adenine). All AD mice regardless of sex or genotype had elevated blood urea nitrogen and high PTH. Con AD mice in both sexes had cortical porosity and lower cortical thickness as well as high osteoclast-covered trabecular surfaces and higher bone formation rate. cKO mice had preserved cortical bone microarchitecture despite high circulating PTH as well as no CKD-induced increases in osteoclasts. In study 2, male mice with established AD CKD were either given a single injection of an anti-RANKL antibody (5 mg/kg) 8 wk post-induction of CKD or subjected to 3×/wk dosing with risedronate (1.2 μg/kg) for 4 wk. Anti-RANKL treatment significantly reduced bone formation rate as well as osteoclast surfaces at both trabecular and cortical pore surfaces; risedronate treatment had little effect on these bone parameters. In conclusion, these studies demonstrate that bone-specific RANKL is critical for the development of high bone formation/high osteoclasts and cortical bone loss in CKD with high PTH. Additionally, systemic anti-RANKL ligand therapy in established CKD may help prevent the propagation of cortical bone loss via suppression of bone turnover.

Role of vitamin D for orthodontic tooth movement, external apical root resorption, and bone biomarker expression and remodeling: A systematic review

Objective

: This systematic review aimed to evaluate the correlation between vitamin D levels and the rate of tooth movement, external apical root resorption, bone biomarker expression, and bone remodeling.

Methods

: Three databases (PubMed, Scopus, and Web of Science) were systematically searched from inception until 14th March 2023 to identify studies investigating the correlation between orthodontic tooth movement and vitamin D in animals and humans. The quality assessment was made in accordance with the Joanna Briggs Institute Critical Appraisal Checklist.

Results

: Overall, 519 records were identified, and 19 were selected for the qualitative synthesis. Eleven studies investigated the effect of local administration (injections in the periodontal ligament, to the gingiva distal to the teeth, or submucosae palatal area) and systemic administration (oral supplementation) of vitamin D on tooth movement, external apical root movement, pro-inflammatory cytokines, and bone remodeling factors. The remaining eight studies investigated the correlation between serum vitamin D levels and salivary vitamin D levels on bone turnover markers and tooth movement.

Conclusions

: The findings of this systematic review support that vitamin D3 local injections might increase the rate of tooth movement via the receptor activator of the nuclear factor-kB/osteoprotegerin axis. However, the non-uniform study designs and the different protocols and outcome methods make it challenging to draw reliable conclusions.

Sema3A secreted by sensory nerve induces bone formation under mechanical loads

Bone formation and deposition are initiated by sensory nerve infiltration in adaptive bone remodeling. Here, we focused on the role of Semaphorin 3A (Sema3A), expressed by sensory nerves, in mechanical loads-induced bone formation and nerve withdrawal using orthodontic tooth movement (OTM) model. Firstly, bone formation was activated after the 3rd day of OTM, coinciding with a decrease in sensory nerves and an increase in pain threshold. Sema3A, rather than nerve growth factor (NGF), highly expressed in both trigeminal ganglion and the axons of periodontal ligament following the 3rd day of OTM. Moreover, in vitro mechanical loads upregulated Sema3A in neurons instead of in human periodontal ligament cells (hPDLCs) within 24 hours. Furthermore, exogenous Sema3A restored the suppressed alveolar bone formation and the osteogenic differentiation of hPDLCs induced by mechanical overload. Mechanistically, Sema3A prevented overstretching of F-actin induced by mechanical overload through ROCK2 pathway, maintaining mitochondrial dynamics as mitochondrial fusion. Therefore, Sema3A exhibits dual therapeutic effects in mechanical loads-induced bone formation, both as a pain-sensitive analgesic and a positive regulator for bone formation.

Mechanisms of Osteoclastogenesis in Orthodontic Tooth Movement and Orthodontically Induced Tooth Root Resorption

Orthodontic tooth movement (OTM) is achieved by the simultaneous activation of bone resorption by osteoclasts and bone formation by osteoblasts. When orthodontic forces are applied, osteoclast-mediated bone resorption occurs in the alveolar bone on the compression side, creating space for tooth movement. Therefore, controlling osteoclastogenesis is the fundamental tenet of orthodontic treatment. Orthodontic forces are sensed by osteoblast lineage cells such as periodontal ligament (PDL) cells and osteocytes. Of several cytokines produced by these cells, the most important cytokine promoting osteoclastogenesis is the receptor activator of nuclear factor-κB ligand (RANKL), which is mainly supplied by osteoblasts. Additionally, osteocytes embedded within the bone matrix, T lymphocytes in inflammatory conditions, and PDL cells produce RANKL. Besides RANKL, inflammatory cytokines, such as interleukin-1, tumor necrosis factor-α, and prostaglandin E2 promote osteoclastogenesis under OTM. On the downside, excessive osteoclastogenesis activation triggers orthodontically-induced external root resorption (ERR) through pro-osteoclastic inflammatory cytokines. Therefore, understanding the mechanisms of osteoclastogenesis during OTM is essential in reducing the adverse effects of orthodontic treatment. Here, we review the current concepts of the mechanisms underlying osteoclastogenesis in OTM and orthodontically induced ERR.

A review of tamoxifen administration regimen optimization for Cre/loxp system in mouse bone study

Gene knockout is a technique routinely used in basic experimental research, particularly in mouse skeletal and developmental studies. Tamoxifen-induced Cre/loxp system is known for its temporal and spatial precision and commonly utilized by researchers. However, tamoxifen has been shown its side effects on affecting the phenotype of mouse bone directly. This review aimed to optimize tamoxifen administration regimens including its dosage and duration, to identify an optimal induction strategy that minimizes potential side effects while maintaining recombination efficacy. This study will help researchers in designing gene knockout experiments in bone when using tamoxifen.

Tissue-Engineered Nanomaterials Play Diverse Roles in Bone Injury Repair

Nanomaterials with bone-mimicking characteristics and easily internalized by the cell could create suitable microenvironments in which to regulate the therapeutic effects of bone regeneration. This review provides an overview of the current state-of-the-art research in developing and using nanomaterials for better bone injury repair. First, an overview of the hierarchical architecture from the macroscale to the nanoscale of natural bone is presented, as these bone tissue microstructures and compositions are the basis for constructing bone substitutes. Next, urgent clinical issues associated with bone injury that require resolution and the potential of nanomaterials to overcome them are discussed. Finally, nanomaterials are classified as inorganic or organic based on their chemical properties. Their basic characteristics and the results of related bone engineering studies are described. This review describes theoretical and technical bases for the development of innovative methods for repairing damaged bone and should inspire therapeutic strategies with potential for clinical applications.

Osteoimmunology in Periodontitis and Orthodontic Tooth Movement

PURPOSE OF REVIEW

To review the role of the immune cells and their interaction with cells found in gingiva, periodontal ligament, and bone that leads to net bone loss in periodontitis or bone remodeling in orthodontic tooth movement.

RECENT FINDINGS

Periodontal disease is one of the most common oral diseases causing inflammation in the soft and hard tissues of the periodontium and is initiated by bacteria that induce a host response. Although the innate and adaptive immune response function cooperatively to prevent bacterial dissemination, they also play a major role in gingival inflammation and destruction of the connective tissue, periodontal ligament, and alveolar bone characteristic of periodontitis. The inflammatory response is triggered by bacteria or their products that bind to pattern recognition receptors that induce transcription factor activity to stimulate cytokine and chemokine expression. Epithelial, fibroblast/stromal, and resident leukocytes play a key role in initiating the host response and contribute to periodontal disease. Single-cell RNA-seq (scRNA-seq) experiments have added new insight into the roles of various cell types in the response to bacterial challenge. This response is modified by systemic conditions such as diabetes and smoking. In contrast to periodontitis, orthodontic tooth movement (OTM) is a sterile inflammatory response induced by mechanical force. Orthodontic force application stimulates acute inflammatory responses in the periodontal ligament and alveolar bone stimulated by cytokines and chemokines that produce bone resorption on the compression side. On the tension side, orthodontic forces induce the production of osteogenic factors, stimulating new bone formation. A number of different cell types, cytokines, and signaling/pathways are involved in this complex process. Inflammatory and mechanical force-induced bone remodeling involves bone resorption and bone formation. The interaction of leukocytes with host stromal cells and osteoblastic cells plays a key role in both initiating the inflammatory events as well as inducing a cellular cascade that results in remodeling in orthodontic tooth movement or in tissue destruction in periodontitis.

How does alendronate affect orthodontic tooth movement in osteogenesis imperfecta: an in vivo study on a mice model

OBJECTIVES

The purpose of this study was to evaluate the effects of alendronate on orthodontic tooth movement (OTM) and bone modelling/remodelling in an osteogenesis imperfecta (OI) mice model.

MATERIALS AND METHODS

Ten-week-old male and female OI mice (Col1a2oim, n = 32) were divided into four groups: 1. Alendronate male (AM, n = 8), 2. Alendronate female (AF, n = 8), 3. saline male (SM, n = 8), and 4. saline female (SF, n = 8). The mice in all four groups received either Alendronate (0.05 mg/kg) or vehicle (saline 0.05 mg/kg) subcutaneously for 2 weeks prior to the placement of orthodontic spring. A nickel-titanium spring applying 3-5 cN of force was used to perform the OTM for 1 week. After 7 days of OTM, the OI mice were euthanized with CO2 inhalation and microfocus computed tomography and histological analyses were performed.

RESULTS

AM and AF mice showed a significant decrease (P < 0.05) in the rate of OTM compared with SM and SF mice, respectively. In addition, AM and AF mice showed a significant increase (P < 0.05) in the bone volume fraction (BVF) and tissue density (TD) compared with SM and SF mice. Histological analysis of haematoxylin-eosin staining revealed a hyalinization zone in AM and AF mice compared with SM and SF mice. Furthermore, tartrate-resistant acid phosphatase staining indicated decreased number of osteoclasts in AM and AF mice compared with SM and SF mice. Picrosirius red staining showed, Alendronate treatment led to thick uniform and smooth morphology of collagen fibres as compared with saline group. Similarly, second harmony generation images also revealed thicker collagen fibres at the periodontal ligament (PDL)-cementum entheses and PDL-alveolar bone entheses in AM and AF mice compared with SM and SF mice.

CONCLUSIONS

Alendronate led to a decrease in the rate of OTM, increase in BVF and TD, decrease in the number of osteoclasts, and smooth and thick collagen fibres compared with saline in both male and female OI mice.

Role of noncoding RNAs in orthodontic tooth movement: new insights into periodontium remodeling

Orthodontic tooth movement (OTM) is biologically based on the spatiotemporal remodeling process in periodontium, the mechanisms of which remain obscure. Noncoding RNAs (ncRNAs), especially microRNAs and long noncoding RNAs, play a pivotal role in maintaining periodontal homeostasis at the transcriptional, post-transcriptional, and epigenetic levels. Under force stimuli, mechanosensitive ncRNAs with altered expression levels transduce mechanical load to modulate intracellular genes. These ncRNAs regulate the biomechanical responses of periodontium in the catabolic, anabolic, and coupling phases throughout OTM. To achieve this, down or upregulated ncRNAs actively participate in cell proliferation, differentiation, autophagy, inflammatory, immune, and neurovascular responses. This review highlights the regulatory mechanism of fine-tuning ncRNAs in periodontium remodeling during OTM, laying the foundation for safe, precise, and personalized orthodontic treatment.

Current perspectives of residual ridge resorption: Pathological activation of oral barrier osteoclasts

PURPOSE

Tooth extraction is a last resort treatment for resolving pathological complications of dentition induced by infection and injury. Although the extraction wound generally heals uneventfully, resulting in the formation of an edentulous residual ridge, some patients experience long-term and severe residual ridge reduction. The objective of this review was to provide a contemporary understanding of the molecular and cellular mechanisms that may potentially cause edentulous jawbone resorption.

STUDY SELECTION

Clinical, in vivo, and in vitro studies related to the characterization of and cellular and molecular mechanisms leading to residual ridge resorption.

RESULTS

The alveolar processes of the maxillary and mandibular bones uniquely juxtapose the gingival tissue. The gingival oral mucosa is an active barrier tissue that maintains homeostasis of the internal organs through its unique barrier immunity. Tooth extraction not only generates a bony socket but also injures oral barrier tissue. In response to wounding, the alveolar bone socket initiates regeneration and remodeling through coupled bone formation and osteoclastic resorption. Osteoclasts are also found on the external surface of the alveolar bone, interfacing the oral barrier tissue. Osteoclasts in the oral barrier region are not coupled with osteoblastic bone formation and often remain active long after the completion of wound healing, leading to a net decrease in the alveolar bone structure.

CONCLUSIONS

The novel concept of oral barrier osteoclasts may provide important clues for future clinical strategies to maintain residual ridges for successful prosthodontic and restorative therapies.

Osteoblastic STAT3 Is Crucial for Orthodontic Force Driving Alveolar Bone Remodeling and Tooth Movement

Mechanical force is essential to shape the internal architecture and external form of the skeleton by regulating the bone remodeling process. However, the underlying mechanism of how the bone responds to mechanical force remains elusive. Here, we generated both orthodontic tooth movement (OTM) model in vivo and a cyclic stretch-loading model in vitro to investigate biomechanical regulation of the alveolar bone. In this study, signal transducer and activator of transcription 3 (STAT3) was screened as one of the mechanosensitive proteins by protein array analysis of cyclic stretch-loaded bone mesenchymal stem cells (BMSCs) and was also proven to be activated in osteoblasts in response to the mechanical force during OTM. With an inducible osteoblast linage-specific Stat3 knockout model, we found that Stat3 deletion decelerated the OTM rate and reduced orthodontic force-induced bone remodeling, as indicated by both decreased bone resorption and formation. Both genetic deletion and pharmacological inhibition of STAT3 in BMSCs directly inhibited mechanical force-induced osteoblast differentiation and impaired osteoclast formation via osteoblast-osteoclast cross-talk under mechanical force loading. According to RNA-seq analysis of Stat3-deleted BMSCs under mechanical force, matrix metalloproteinase 3 (Mmp3) was screened and predicted to be a downstream target of STAT3. The luciferase and ChIP assays identified that Stat3 could bind to the Mmp3 promotor and upregulate its transcription activity. Furthermore, STAT3-inhibitor decelerated tooth movement through inhibition of the bone resorption activity, as well as MMP3 expression. In summary, our study identified the mechanosensitive characteristics of STAT3 in osteoblasts and highlighted its critical role in force-induced bone remodeling during orthodontic tooth movement via osteoblast-osteoclast cross-talk. © 2022 American Society for Bone and Mineral Research (ASBMR).

Ribosomal Stress Couples with the Hypoxia Response in Dec1-Dependent Orthodontic Tooth Movement

This study characterized the effects of a deficiency of the hypoxia-responsive gene, differentiated embryonic chondrocyte gene 1 (Dec1), in attenuating the biological function of orthodontic tooth movement (OTM) and examined the roles of ribosomal proteins in the hypoxic environment during OTM. HIF-1α transgenic mice and control mice were used for hypoxic regulation of periodontal ligament (PDL) fibroblasts. Dec1 knockout (Dec1KO) and wild-type (WT) littermate C57BL/6 mice were used as in vivo models of OTM. The unstimulated contralateral side served as a control. In vitro, human PDL fibroblasts were exposed to compression forces for 2, 4, 6, 24, and 48 h. HIF-1α transgenic mice had high expression levels of Dec1, HSP105, and ribosomal proteins compared to control mice. The WT OTM mice displayed increased Dec1 expression in the PDL fibroblasts. Micro-CT analysis showed slower OTM in Dec1KO mice compared to WT mice. Increased immunostaining of ribosomal proteins was observed in WT OTM mice compared to Dec1KO OTM mice. Under hypoxia, Dec1 knockdown caused a significant suppression of ribosomal protein expression in PDL fibroblasts. These results reveal that the hypoxic environment in OTM could have implications for the functions of Dec1 and ribosomal proteins to rejuvenate periodontal tissue homeostasis.

Influence of Single-Nucleotide Polymorphisms on Vitamin D Receptor Expression in Periodontal Ligament Fibroblasts as a Response to Orthodontic Compression

This study aimed to evaluate if single-nucleotide polymorphisms (SNPs) in the vitamin D receptor (VDR) gene are associated with gene expression in human periodontal ligament (hPDL) fibroblasts under simulated orthodontic compressive force. hPDL samples from 57 patients were used. A physiological compressive strain was performed to simulate orthodontic tooth movement in pressure areas under cell culture conditions. The RNA from hPDL fibroblasts was isolated to determine the relative gene expression (mRNA) of the VDR. The DNA was also isolated for the genotyping analysis of five SNPs in the VDR gene: BglI (rs739837, G/T), BsmI (rs1544410, T/C), ApaI (rs7975232, A/C), FokI (rs2228570, A/G), and TaqI (rs731236, A/G). Real-time polymerase chain reaction was used for both analyses. Kruskal−Wallis tests were used to compare VDR expression among genotypes of each SNP. A linear regression analysis was performed to evaluate SNP−SNP interaction. An established alpha of 5% was used. The relative mRNA VDR expression according to the genotypes in the SNPs BglI, BsmI, ApaI, FokI, and TaqI was not statistically significantly different (p > 0.05). The SNP−SNP interaction evaluated by regression analysis did not demonstrate any statistically significant association. No association was observed (p > 0.05). In conclusion, the SNPs BglI (rs739837), BsmI (rs1544410), ApaI (rs7975232), FokI (rs2228570), and TaqI (rs731236) did not show an impact on VDR gene expression in hPDL fibroblasts under simulated orthodontic compressive force.

The Effect of IFT80 Deficiency in Osteocytes on Orthodontic Loading-Induced and Physiologic Bone Remodeling: In Vivo Study

Osteocytes are the main mechanosensory cells during orthodontic and physiologic bone remodeling. However, the question of how osteocytes transmit mechanical stimuli to biological responses remains largely unanswered. Intraflagellar transport (IFT) proteins are important for the formation and function of cilia, which are proposed to be mechanical sensors in osteocytes. In particular, IFT80 is highly expressed in mouse skulls and essential for ciliogenesis. This study aims to investigate the short- and long-term effects of IFT80 deletion in osteocytes on orthodontic bone remodeling and physiological bone remodeling in response to masticatory force. We examined 10-week-old experimental DMP1 CRE+.IFT80f/f and littermate control DMP1 CRE-.IFT80f/f mice. After 5 and 12 days of orthodontic force loading, the orthodontic tooth movement distance and bone parameters were evaluated using microCT. Osteoclast formation was assessed using TRAP-stained paraffin sections. The expression of sclerostin and RANKL was examined using immunofluorescence stain. We found that the deletion of IFT80 in osteocytes did not significantly impact either orthodontic or physiologic bone remodeling, as demonstrated by similar OTM distances, osteoclast numbers, bone volume fractions (bone volume/total volume), bone mineral densities, and the expressions of sclerostin and RANKL. Our findings suggest that there are other possible mechanosensory systems in osteocytes and anatomic limitations to cilia deflection in osteocytes in vivo.

Impact of phosphorylation of heat shock protein 27 on the expression profile of periodontal ligament fibroblasts during mechanical strain

PURPOSE

Orthodontic tooth movement is a complex process involving the remodeling of extracellular matrix and bone as well as inflammatory processes. During orthodontic treatment, sterile inflammation and mechanical loading favor the production of receptor activator of NF-κB ligand (RANKL). Simultaneously, expression of osteoprotegerin (OPG) is inhibited. This stimulates bone resorption on the pressure side. Recently, heat shock protein 27 (HSP27) was shown to be expressed in the periodontal ligament after force application and to interfere with inflammatory processes.

METHODS

We investigated the effects of phosphorylated HSP27 on collagen synthesis (COL1A2 mRNA), inflammation (IL1B mRNA, IL6 mRNA, PTGS2 protein) and bone remodeling (RANKL protein, OPG protein) in human periodontal ligament fibroblasts (PDLF) without and with transfection of a plasmid mimicking permanent phosphorylation of HSP27 using real-time quantitative polymerase chain reaction (RT-qPCR), western blot and enzyme-linked immunosorbent assays (ELISAs). Furthermore, we investigated PDLF-induced osteoclastogenesis after compressive strain in a co-culture model with human macrophages.

RESULTS

In particular, phosphorylated HSP27 increased gene expression of COL1A2 and protein expression of PTGS2, while IL6 mRNA levels were reduced. Furthermore, we observed an increasing effect on the RANKL/OPG ratio and osteoclastogenesis mediated by PDLF.

CONCLUSION

Phosphorylation of HSP27 may therefore be involved in the regulation of orthodontic tooth movement by impairment of the sterile inflammation response and osteoclastogenesis.

Study on the role of pyroptosis in bone resorption induced by occlusal trauma with or without periodontitis

BACKGROUND AND OBJECTIVE

Occlusal trauma is considered to be a contributing factor to bone loss associated with inflammatory periodontal disease. We hypothesized that pyroptosis, a recently discovered inflammation-induced programmed cell death pathway, plays a role in occlusal trauma.

MATERIALS AND METHODS

The occlusal trauma model was established using a cemented 1-mm elevated computer-aided design and manufacturing (CAD/CAM) metal crown. The periodontitis model was established by periodontal wire ligation with lipopolysaccharide (LPS) injection. The rats were sacrificed at 1, 2, 3, and 4 weeks. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to analyze the expression of pyroptosis-, inflammation-, and osteoclast-related markers. Micro-computed tomography (micro-CT) was used to determine bone morphology parameters. Tissue morphology was evaluated using hematoxylin and eosin staining (H&E). Osteoclasts were identified using tartrate-resistant acid phosphatase (TRAP) staining. The expression and distribution of factors related to pyroptosis and inflammation were evaluated by immunohistochemistry (IHC). The colocalization of dead cells and cysteinyl aspartate-specific proteinase-1 (caspase-1)-positive cells was analyzed by immunofluorescence.

RESULTS

Quantitative real-time polymerase chain reaction and IHC results showed that occlusal trauma induced the expression of pyroptotic factors during the early stages, while occlusal trauma with periodontitis upregulated the expression of pyroptotic factors at the later stages. The results of qRT-PCR, TRAP staining, and micro-CT showed that occlusal trauma with periodontitis increased the production of proinflammatory cytokines, leading to severe bone loss. Glyburide, an NOD-like receptor pyrin domain containing protein 3 (NLRP3)inhibitor, reduced the expression of pyroptosis markers induced by occlusal trauma with periodontitis and reversed bone resorption.

CONCLUSIONS

Pyroptosis was involved in bone loss induced by occlusal trauma with or without periodontitis, while glyburide reversed inflammation and bone resorption.

Enhancement of orthodontic tooth movement and root resorption in ovariectomized mice

Background/purpose

As the number of patients with osteoporosis requiring orthodontic treatment is increasing with the aging of society, it is necessary to evaluate the relations between bone metabolism in old age and orthodontic tooth movement (OTM). However, the effects of changes in bone metabolism due to osteoporosis on OTM and root resorption are still unclear. Therefore, we investigated the effects of OTM and root resorption in a mouse ovariectomy (OVX)-induced osteoporosis model.

Materials and methods

Eight-week-old female wild-type mice underwent OVX or sham surgery (Sham) as controls. One month after treatment, a nickel titanium coil spring was used to apply a mesial force to the maxillary left first molars of OVX or Sham mice for 12 days. The distance between the maxillary first molar and the second molar changed due to OTM and osteoclast formation was evaluated. The odontoclast formation and root resorption along the root surface of the distobuccal root of the first molar was also evaluated by histological analysis and scanning electron microscopy.

Results

Distance of tooth movement and osteoclast formation were significantly increased in OVX mice compared to Sham controls. Furthermore, root resorption in the mesial surface of the distal molars induced by orthodontic force was significantly increased in OVX mice.

Conclusion

The amount of OTM was significantly increased, and the accompanying root resorption was also increased in OVX mice. Therefore, attention should be paid to the risk of root resorption associated with orthodontic treatment in patients with osteoporosis.

Acceleration of orthodontic tooth movement and root resorption with near and distant surgical insults: An in-vivo study on a rat model

OBJECTIVE

The purpose of this study was to determine the effects of alveolar decortications (AD) closer and farther from the tooth on Orthodontic tooth movement (OTM) and root resorption.

MATERIALS AND METHODS

Twenty-four Wistar rats (age 17 weeks) were used in the study. Three groups were formed by randomly dividing the rats into: 1) Control Group (N=8): OTM (14 days) without any surgical insult; 2) Near Group (N=8) OTM (14 days)+Alveolar Decortications (AD) adjacent to roots of the maxillary first molar; 3) Far Group (N=8) OTM (14 days)+AD 5mm away from the roots of the maxillary first molar. OTM was performed using a closed coil Nickel-Titanium (NiTi) spring to apply a mesial force on maxillary first molars of 8-10 grams. AD was performed with high-speed quarter round bur on the palatal aspect of alveolar bone. The rats were euthanized after 2 weeks of OTM and microfocus computed tomography and histological analysis were performed.

RESULTS

Near-AD and Far-AD groups exhibited significantly increased OTM and reduction of Bone volume fraction compared to control. The root volume was significantly decreased (increased root resorption) in the Near-AD group. Histological analysis showed no statistically significant difference between the groups.

CONCLUSIONS

Alveolar decortications lead to a significant increase in the rate of OTM. Increased root resorption was observed when the alveolar decortications were performed close to the maxillary molar.

Osteoblast lineage cells and periodontal ligament fibroblasts regulate orthodontic tooth movement that is dependent on Nuclear Factor-kappa B (NF-kB) activation

OBJECTIVES

To investigate the role of NF-κB in osteoblast lineage cells and periodontal ligament (PDL) fibroblasts during orthodontic tooth movement (OTM).

MATERIALS AND METHODS

Transgenic mice that expressed a dominant negative mutant of the inhibitor of kB kinase (IKK-DN) with lineage specific expression in osteoblastic cells and PDL fibroblasts driven by a response element in the collagen1α1 promoter and matched wild-type (WT) mice were examined. A 10-12 g force was applied by a NiTi coil and maintained for 5 or 12 days. OTM distance, PDL width, and bone volume fraction were measured using micro computed tomography. Osteoclast numbers were counted in tartrate-resistant acid phosphatase-stained sections. Activation of nuclear factor kappa B (NF-kB) was assessed by nuclear localization of p65, and the receptor activator of nuclear factor-κB ligand (RANKL) was measured by immunofluorescence and compared to control specimens with no orthodontic force.

RESULTS

OTM-induced NF-kB activation (p65 nuclear localization) in WT mice was largely blocked in transgenic (TG) mice. OTM was significantly reduced in the TG mice compared to WT mice along with reduced osteoclastogenesis, narrower PDL width, higher bone volume fraction, and reduced RANKL expression.

CONCLUSIONS

Osteoblast lineage cells and PDL fibroblasts are key contributors to alveolar bone remodeling in OTM through IKKβ dependent NF-κB activation.

The role of 25-hydroxyvitamin-D3 and vitamin D receptor gene in human periodontal ligament fibroblasts as response to orthodontic compressive strain: an in vitro study

Background

This study aimed to investigate, if different physiological concentrations of vitamin D (25(OH)D₃) and single nucleotide polymorphisms in vitamin D receptor (VDR) gene have an impact on gene expression in human periodontal ligament (hPDL) fibroblasts induced by simulated orthodontic compressive strain.

Methods

A pool of hPDL fibroblasts was treated in absence or presence of 25(OH)D₃ in 3 different concentrations (10, 40 and 60 ng/ml). In order to evaluate the role of single nucleotide polymorphisms in the VDR gene, hPDL fibroblasts from 9 patients were used and treated in absence or presence of 40 ng/ml 25(OH)D₃. Each experiment was performed with and without simulated orthodontic compressive strain. Real-time PCR was used for gene expression and allelic discrimination analysis. Relative expression of dehydrocholesterol reductase (DHCR7), Sec23 homolog A, amidohydrolase domain containing 1 (AMDHD1), vitamin D 25-hydroxylase (CYP2R1), Hydroxyvitamin D-1-α hydroxylase, receptor activator of nuclear factor-κB ligand (RANKL), osteoprotegerin (OPG), cyclooxygenase-2 (COX-2) and interleukin-6 (IL6) was assessed. Three single nucleotide polymorphisms in VDR were genotyped. Parametric or non-parametric tests were used with an alpha of 5%.

Results

RANKL, RANKL:OPG ratio, COX-2, IL-6, DHCR7, CYP2R1 and AMDHD1 were differentially expressed during simulated orthodontic compressive strain (p < 0.05). The RANKL:OPG ratio was downregulated by all concentrations (10 ng/ml, 40 ng/ml and 60 ng/ml) of 25(OH)D₃ (mean = 0.96 ± 0.68, mean = 1.61 ± 0.66 and mean = 1.86 ± 0.78, respectively) in comparison to the control (mean 2.58 ± 1.16) (p < 0.05). CYP2R1 gene expression was statistically modulated by the different 25(OH)D₃ concentrations applied (p = 0.008). Samples from individuals carrying the GG genotype in rs739837 presented lower VDR mRNA expression and samples from individuals carrying the CC genotype in rs7975232 presented higher VDR mRNA expression (p < 0.05).

Conclusions

Simulated orthodontic compressive strain and physiological concentrations of 25(OH)D₃ seem to regulate the expression of orthodontic tooth movement and vitamin-D-related genes in periodontal ligament fibroblasts in the context of orthodontic compressive strain. Our study also suggests that single nucleotide polymorphisms in the VDR gene regulate VDR expression in periodontal ligament fibroblasts in the context of orthodontic compressive strain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-021-01740-8.

Combined effects of cyclic stretch and TNF-α on the osteogenic differentiation in MC3T3-E1 cells

OBJECTIVE

The study aimed to investigate the combined effects of cyclic stretch and tumor necrosis factor-alpha (TNF-α) on the osteogenic differentiation of MC3T3-E1 cells and the role of the nuclear factor-kappaB (NF-κB) signaling pathway in this process.

DESIGN

MC3T3-E1 cells were treated with TNF-α (0.5 and 10 ng/mL) and cyclically stretched using the Flexcell tension system 4000 with 12 % elongation for 12 h. Furthermore, to explore which cytokines might be regulated by the NF-κB signaling pathway in osteogenic differentiation, the cells were pre-treated with NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC), and then cyclically stretched for 12 h in the presence of 10 ng/mL of TNF-α. RT-PCR and western blot were utilized to detect the expression of type Ⅰ collagen (COL1), osteocalcin (OCN), runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL), NF-κB, and phosphorylated NF-κB (p-NF-κB) at gene and protein levels.

RESULTS

Cyclic stretch alone increased the expression of COL1, OCN, Runx2, ALP, and OPG, decreasing the expression of RANKL and the RANKL/OPG ratio. The upregulation or downregulation induced by cyclic stretch were restrained in the presence of TNF-α. The p-NF-κB/NF-κB ratio increased at any stimulation. Inhibition of NF-κB signaling pathway restrained the expression variations of COL1, OCN, ALP, OPG, and RANKL induced by TNF-α combined with cyclic stretch.

CONCLUSION

The results indicated that TNF-α inhibited the osteogenic differentiation of MC3T3-E1 cells induced by cyclic stretch and NF-κB signaling pathway might play a role in this process.

Biomechanical and biological responses of periodontium in orthodontic tooth movement: up-date in a new decade

Nowadays, orthodontic treatment has become increasingly popular. However, the biological mechanisms of orthodontic tooth movement (OTM) have not been fully elucidated. We were aiming to summarize the evidences regarding the mechanisms of OTM. Firstly, we introduced the research models as a basis for further discussion of mechanisms. Secondly, we proposed a new hypothesis regarding the primary roles of periodontal ligament cells (PDLCs) and osteocytes involved in OTM mechanisms and summarized the biomechanical and biological responses of the periodontium in OTM through four steps, basically in OTM temporal sequences, as follows: (1) Extracellular mechanobiology of periodontium: biological, mechanical, and material changes of acellular components in periodontium under orthodontic forces were introduced. (2) Cell strain: the sensing, transduction, and regulation of mechanical stimuli in PDLCs and osteocytes. (3) Cell activation and differentiation: the activation and differentiation mechanisms of osteoblast and osteoclast, the force-induced sterile inflammation, and the communication networks consisting of sensors and effectors. (4) Tissue remodeling: the remodeling of bone and periodontal ligament (PDL) in the compression side and tension side responding to mechanical stimuli and root resorption. Lastly, we talked about the clinical implications of the updated OTM mechanisms, regarding optimal orthodontic force (OOF), acceleration of OTM, and prevention of root resorption.

Effects of Incretin-Related Diabetes Drugs on Bone Formation and Bone Resorption

Patients with type 2 diabetes have an increased risk of fracture compared to the general population. Glucose absorption is accelerated by incretin hormones, which induce insulin secretion from the pancreas. The level of the incretin hormone, glucagon-like peptide-1 (GLP-1), shows an immediate postprandial increase, and the circulating level of intact GLP-1 is reduced rapidly by dipeptidyl peptidase-4 (DPP-4)-mediated inactivation. Therefore, GLP-1 receptor agonists and DPP-4 inhibitors are effective in the treatment of type 2 diabetes. However, these incretin-related diabetic agents have been reported to affect bone metabolism, including bone formation and resorption. These agents enhance the expression of bone markers, and have been applied to improve bone quality and bone density. In addition, they have been reported to suppress chronic inflammation and reduce the levels of inflammatory cytokine expression. Previously, we reported that these incretin-related agents inhibited both the expression of inflammatory cytokines and inflammation-induced bone resorption. This review presents an overview of current knowledge regarding the effects of incretin-related diabetes drugs on osteoblast differentiation and bone formation as well as osteoclast differentiation and bone resorption. The mechanisms by which incretin-related diabetes drugs regulate bone formation and bone resorption are also discussed.

Immune Changes Induced by Orthodontic Forces: A Critical Review

Orthodontic tooth movement (OTM) is generated by a mechanical force that induces an aseptic inflammatory response in the periodontal tissues and a subsequent coordinated process of bone resorption and apposition. In this review, we critically summarize the current knowledge on the immune processes involved in OTM inflammation and provide a novel insight into the relationship between classical inflammation and clinical OTM phases. We found that most studies focused on the acute inflammatory process, which ignites the initial alveolar bone resorption. However, the exact mechanisms and the immune reactions involved in the following OTM phases remain obscure. Recent studies highlight the existence of a typical innate response of resident and extravasated immune cells, including granulocytes and natural killer (NK), dendritic, and γδT cells. Based on few available studies, we shed light on an active, albeit incomplete, process of resolution in the lag phase, supported by continuously elevated ratios of M1/M2 macrophage and receptor activator of nuclear factor κB ligand/osteoprotegerin ratio. This partial resolution enables tissue formation and creates the appropriate environment for a transition between the innate and adaptive arms of the immune system, essential for the tissue's return to homeostasis. Nevertheless, as the mechanical trigger persists, the resolution turns into a low-grade chronic inflammation, which underlies the next, acceleration/linear OTM phase. In this stage, the acute inflammation dampens, and a simultaneous process of bone resorption and formation occurs, driven by B and T cells of the adaptive immune arm. Excessive orthodontic forces or tooth movement in periodontally affected inflamed tissues may hamper resolution, leading to "maladaptive homeostasis" and tissue loss due to uncoupled bone resorption and formation. The review ends with a brief description of the translational studies on OTM immunomodulation. Future studies are necessary for further uncovering cellular and molecular immune targets and developing novel strategies for controlling OTM by local and sustained tuning of the inflammatory process.

Cigarette Smoke Exposure Inhibits Osteoclast Apoptosis via the mtROS Pathway

It is widely known that smoking is a risk factor for bone loss and plays a key role in osteopenia. Despite this well-known association, the mechanisms by which smoking affects bone have not been definitively established. Since smoking increases bone loss and potentially affects bone resorption in response to mechanical force, we investigated the impact of cigarette smoke on osteoclast numbers and underlying mechanisms in a mouse model of orthodontic tooth movement (OTM). The experimental group was exposed to once-daily cigarette smoke while the control group was not, and tooth movement distance and osteoclast numbers were assessed. In addition, the effect of cigarette smoke extract (CSE) on osteoclast precursor proliferation and osteoclast apoptosis was assessed in vitro. We found that cigarette smoke exposure enhanced bone remodeling stimulated by mechanical force and increased osteoclast numbers in vivo. Also, CSE increased the number of osteoclasts by inhibiting osteoclast apoptosis via the mitochondrial reactive oxygen species/cytochrome C/caspase 3 pathway in vitro. Moreover, exposure of mice to cigarette smoke affected bone marrow cells, leading to increased formation of osteoclasts in vitro. This study identifies a previously unknown mechanism of how smoking has a detrimental impact on bone.

Mechanistic Insight into Orthodontic Tooth Movement Based on Animal Studies: A Critical Review

Alveolar bone remodeling in orthodontic tooth movement (OTM) is a highly regulated process that coordinates bone resorption by osteoclasts and new bone formation by osteoblasts. Mechanisms involved in OTM include mechano-sensing, sterile inflammation-mediated osteoclastogenesis on the compression side and tensile force-induced osteogenesis on the tension side. Several intracellular signaling pathways and mechanosensors including the cilia and ion channels transduce mechanical force into biochemical signals that stimulate formation of osteoclasts or osteoblasts. To date, many studies were performed in vitro or using human gingival crevicular fluid samples. Thus, the use of transgenic animals is very helpful in examining a cause and effect relationship. Key cell types that participate in mediating the response to OTM include periodontal ligament fibroblasts, mesenchymal stem cells, osteoblasts, osteocytes, and osteoclasts. Intercellular signals that stimulate cellular processes needed for orthodontic tooth movement include receptor activator of nuclear factor-κB ligand (RANKL), tumor necrosis factor-α (TNF-α), dickkopf Wnt signaling pathway inhibitor 1 (DKK1), sclerostin, transforming growth factor beta (TGF-β), and bone morphogenetic proteins (BMPs). In this review, we critically summarize the current OTM studies using transgenic animal models in order to provide mechanistic insight into the cellular events and the molecular regulation of OTM.

Local administration of high-dose diabetes medicine exendin-4 inhibits orthodontic tooth movement in mice

OBJECTIVES

To investigate the effects of exendin-4 on orthodontic tooth movement distance, root resorption, and expression levels of osteoclast-related cytokines in a mouse model.

MATERIALS AND METHODS

A 10-g NiTi coil spring was placed between the anterior alveolar bone and upper left first molar of 8-week-old male C57BL/6 mice. Twenty microliters of exendin-4 solution (containing 0.2 μg, 4 μg, or 20 μg exendin-4) or phosphate-buffered saline (PBS) were injected on the buccal side of the upper left first molar at 2-day intervals (4 mice per group). Mice were sacrificed on day 12; silicone impressions were taken to record tooth movement distance. The left maxillae of the PBS and 20 μg exendin-4 groups were also excised for histological analysis and quantitative reverse transcription polymerase chain reaction analysis.

RESULTS

Orthodontic tooth movement distance was smaller in the 20 μg exendin-4 group than in the PBS group (P < .01). Compared with the PBS group, the 20 μg exendin-4 group showed lower osteoclast number (P < .05), odontoclast number (P < .05), and root resorption surface percentage (P < .05). Relative to maxillae with PBS injections, maxillae with 20 μg exendin-4 injections had lower receptor activator of nuclear factor kappa-B ligand (RANKL) mRNA expression (P < .05), TNF-α mRNA expression (P < .05), and RANKL/osteoprotegerin (OPG) ratio (P < .01). There were no differences in the expression of OPG mRNA.

CONCLUSIONS

Exendin-4 inhibits orthodontic tooth movement. Therefore, additional attention is needed for orthodontic patients who receive exendin-4 for diabetes treatment. GLP-1 receptor may be a treatment target for patients with severe root resorption.

γδT Cells Are Essential for Orthodontic Tooth Movement

Sustained mechanical forces applied to tissue are known to shape local immunity. In the oral mucosa, mechanical stress, either naturally induced by masticatory forces or externally via mechanical loading during orthodontic tooth movement (OTM), is translated, in part, by T cells to alveolar bone resorption. Nevertheless, despite being considered critical for OTM, depletion of CD4⁺ and CD8⁺ T cells is reported to have no impact on tooth movement, thus questioning the function of αβT cells in OTM-associated bone resorption. To further address the role of T cells in OTM, we first characterized the leukocytes residing in the periodontal ligament (PDL), the tissue of interest during OTM, and compared it to the neighboring gingiva. Unlike the gingiva, monocytes and neutrophils represent the major leukocytes of the PDL. These myeloid cells were also the main leukocytes in the PDL of germ-free mice, although at lower levels than SPF mice. T lymphocytes were more enriched in the gingiva than the PDL, yet in both tissues, the relative fraction of the γδT cells was higher than the αβ T cells. We thus sought to examine the role of γδT cells in OTM. γδT cells residing in the PDL were mainly Vγ6⁺ and produced interleukin (IL)-17A but not interferon-γ. Using Tcrd-GDL mice allowing conditional ablation of γδT cells in vivo, we demonstrate that OTM was greatly diminished in the absence of γδT cells. Further analysis revealed that ablation of γδT cells decreased early IL-17A expression, monocyte and neutrophil recruitment, and the expression of the osteoclastogenic molecule receptor activator of nuclear factor-κβ ligand. This, eventually, resulted in reduced numbers of osteoclasts in the pressure site during OTM. Collectively, our data suggest that γδT cells are essential in OTM for translating orthodontic mechanical forces to bone resorption, required for relocating the tooth in the alveolar bone.

Chemokine C-C motif ligand 8 in periodontal ligament during orthodontic tooth movement

OBJECTIVES

To investigate the roles of chemokine (C-C motif) ligand 8 (CCL8) in periodontal ligament during orthodontic tooth movement (OTM).

METHODS

Bioinformatics analyzed 100 genes in human periodontal ligament cells that were most upregulated after 48 hours of mechanical stress, and these genes were classified through GO and KEGG databases. Nickel-titanium closed-coil springs were placed between right first molar and incisors to produce 20 cN of orthodontic force in eight-week-old male SD rats for 1 and 2 days, followed by immunohistochemical staining of CCL8. Human periodontal ligament fibroblasts (hPDLFs) were stimulated by 14% cyclic tension force (Flexcell FX-5000 T Tension System) or hypoxia conditions to mimic OTM for 1 and 2 days, then the resulting CCL8 were examined through ELISA. Scratching assay was performed by treating hPDLFs with different concentrations of CCL8 (1 ng/ml, 10 ng/ml, 100 ng/ml). The migration, proliferation, and adhesion abilities of 100 ng/ml CCL8-treated hPDLFs were also examined. qRT-PCR and western blot detected matrix metalloproteinase 3, periostin, and osteoprotegrin expressions of hPDLFs under 100 ng/ml CCL8.

RESULTS

Bioinformatic analysis demonstrated that CCL8 was upregulated after applying mechanical stress for 48 hours. CCL8 secretion showed upregulation after 24 hours of OTM applicationsin vivo and in vitro. CCL8-treated hPDLFs showed significant positive effects on cell proliferation and matrix metalloproteinase 3. It also inhibited periostin and osteoprotegrin expressions.

CONCLUSIONS

CCL8 was upregulated in periodontal ligament during initial stage of OTM. Although CCL8 in human periodontal ligaments showed no significant effects on cell migration ability, it did enhance cell proliferation and osteoclastogenesis.

The Role of the Immune Microenvironment in Bone Regeneration

Bone is an active tissue, being constantly renewed in healthy individuals with participation of the immune system to a large extent. Any imbalance between the processes of bone formation and bone resorption is linked to various inflammatory bone diseases. The immune system plays an important role in tissue formation and bone resorption. Recently, many studies have demonstrated complex interactions between the immune and skeletal systems. Both of immune cells and cytokines contribute to the regulation of bone homeostasis, and bone cells, including osteoblasts, osteoclasts, osteocytes, also influence the cellular functions of immune cells. These crosstalk mechanisms between the bone and immune system finally emerged, forming a new field of research called osteoimmunology. Therefore, the immune microenvironment is crucial in determining the speed and outcome of bone healing, repair, and regeneration. In this review, we summarise the role of the immune microenvironment in bone regeneration from the aspects of immune cells and immune cytokines. The elucidation of immune mechanisms involved in the process of bone regeneration would provide new therapeutic targets for improving the curative effects of bone injury treatment.

The roles of osteocytes in alveolar bone destruction in periodontitis

Periodontitis, a bacterium-induced inflammatory disease that is characterized by alveolar bone loss, is highly prevalent worldwide. Elucidating the underlying mechanisms of alveolar bone loss in periodontitis is crucial for understanding its pathogenesis. Classically, bone cells, such as osteoclasts, osteoblasts and bone marrow stromal cells, are thought to dominate the development of bone destruction in periodontitis. Recently, osteocytes, the cells embedded in the mineral matrix, have gained attention. This review demonstrates the key contributing role of osteocytes in periodontitis, especially in alveolar bone loss. Osteocytes not only initiate physiological bone remodeling but also assist in inflammation-related changes in bone remodeling. The latest evidence suggests that osteocytes are involved in regulating bone anabolism and catabolism in the progression of periodontitis. The altered secretion of receptor activator of NF-κB ligand (RANKL), sclerostin and Dickkopf-related protein 1 (DKK1) by osteocytes affects the balance of bone resorption and formation and promotes bone loss. In addition, the accumulation of prematurely senescent and apoptotic osteocytes observed in alveolar bone may exacerbate local destruction. Based on their communication with the bloodstream, it is noteworthy that osteocytes may participate in the interaction between local periodontitis lesions and systemic diseases. Overall, further investigations of osteocytes may provide vital insights that improve our understanding of the pathophysiology of periodontitis.

Osteocytes promote osteoclastogenesis via autophagy-mediated RANKL secretion under mechanical compressive force

Osteocytes sense extracellular mechanical stimuli and transduce them into biochemical signals to regulate bone remodeling. The function is also evidenced in orthodontic tooth movement. But the underlying mechanisms haven't been clarified. Autophagy is an evolutionarily conserved cellular catabolic process which affects cellular secretory capabilities. We hypothesized that mechanical force activated osteocyte autophagy through TFE3-related signaling and further promoted osteocyte-mediated osteoclastogenesis. In the present study, we demonstrated that osteocyte autophagy was activated under mechanical compressive force using murine orthodontic tooth movement model since the number of LC3B-positive osteocytes increased by 3-fold in the compression side. In addition, both in vitro mechanical compression and chemical autophagy agonist increased the secretion of RANKL in osteocytes by 3-fold and 4-fold respectively, which is a crucial cytokine for osteoclastogenesis. Lastly, conditioned medium collected from compressed osteocytes promoted the development of osteoclasts. These results suggest that osteocytes could promote osteoclastogenesis via autophagy-mediated RANKL secretion under mechanical compressive force. Our research might provide evidence for exploring methods to accelerate tooth movement in clinic.

Role of c-Fos in orthodontic tooth movement: an in vivo study using transgenic mice

Objectives

The transcription factor c-Fos controls the differentiation of osteoclasts and is expressed in periodontal ligament cells after mechanical stimulation in vitro. However, it is unclear how c-Fos regulates orthodontic tooth movement (OTM) in vivo. The aim of this study was therefore to analyse OTM in transgenic mice with overexpression of c-Fos.

Materials and methods

We employed c-Fos transgenic mice (c-Fos tg) and wild-type littermates (WT) in a model of OTM induced by Nitinol tension springs that were bonded between the left first maxillary molars and the upper incisors. The unstimulated contralateral side served as an internal control. Mice were analysed by contact radiography, micro-computed tomography, decalcified histology and histochemistry.

Results

Our analysis of the unstimulated side revealed that alveolar bone and root morphology were similar between c-Fos tg and control mice. However, we observed more osteoclasts in the alveolar bone of c-Fos tg mice as tartrate-resistant acid phosphatase (TRAP)-positive cells were increased by 40%. After 12 days of OTM, c-Fos tg mice exhibited 62% increased tooth movement as compared with WT mice. Despite the faster tooth movement, c-Fos tg and WT mice displayed the same amount of root resorption. Importantly, we did not observe orthodontically induced tissue necrosis (i.e. hyalinization) in c-Fos tg mice, while this was a common finding in WT mice.

Conclusion

Overexpression of c-Fos accelerates tooth movement without causing more root resorption.

Clinical relevance

Accelerated tooth movement must not result in more root resorption as higher tissue turnover may decrease the amount of mechanically induced tissue necrosis.

Electronic supplementary material

The online version of this article (10.1007/s00784-020-03503-1) contains supplementary material, which is available to authorized users.

Estrogen effects on orthodontic tooth movement and orthodontically-induced root resorption

Estrogen is an essential regulator of the bone tissue. The remodeling of the alveolar bone and periodontal ligament is the basis of orthodontic tooth movement (OTM). There is a negative coregulation between physiological estrogen levels and the rate of OTM. As a possible inhibitory factor of OTM, estrogen suppresses bone resorption by inhibiting osteoclastic differentiation and restraining osteoclast lifespan though multiple pathways and cytokines, leading to the suppression of the initiation step of bone remodeling. On the other hand, estrogen stimulates osteoblastic differentiation and function. Estrogen receptor-α (ERα) involves in the osteogenic responses to mechanical stimulation, and the ERα expression is regulated positively by the levels of circulatory estrogen. Orthodontically induced root resorption (OIRR) is a common side-effect of orthodontic treatment. Estrogen may have some inhibitory effects on OIRR, but more studies are needed to get an effective conclusion.

LGRs in Skeletal Tissues: An Emerging Role for Wnt-Associated Adult Stem Cell Markers in Bone

Leucine-rich repeat-containing G protein-coupled receptors (LGRs) are adult stem cell markers that have been described across various stem cell niches, and expression of LGRs and their corresponding ligands (R-spondins) has now been reported in multiple bone-specific cell types. The skeleton harbors elusive somatic stem cell populations that are exceedingly compartment-specific and under tight regulation from various signaling pathways. Skeletal progenitors give rise to multiple tissues during development and during regenerative processes of bone, requiring postnatal endochondral and intramembranous ossification. The relevance of LGRs and the LGR/R-spondin ligand interaction in bone and tooth biology is becoming increasingly appreciated. LGRs may define specific stem cell and progenitor populations and their behavior during both development and regeneration, and their role as Wnt-associated receptors with specific ligands poses these proteins as unique therapeutic targets via potential R-spondin agonism. This review seeks to outline the current literature on LGRs in the context of bone and its associated tissues, and points to key future directions for studying the functional role of LGRs and ligands in skeletal biology. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Vitamin D3 and Dental Mesenchymal Stromal Cells

Vitamin D3 is a hormone involved in the regulation of bone metabolism, mineral homeostasis, and immune response. Almost all dental tissues contain resident mesenchymal stromal cells (MSCs), which are largely similar to bone marrow-derived MSCs. In this narrative review, we summarized the current findings concerning the physiological effects of vitamin D3 on dental MSCs. The existing literature suggests that dental MSCs possess the ability to convert vitamin D3 into 25(OH)D3 and subsequently to the biologically active 1,25(OH)2D3. The vitamin D3 metabolites 25(OH)D3 and 1,25(OH)2D3 stimulate osteogenic differentiation and diminish the inflammatory response of dental MSCs. In addition, 1,25(OH)2D3 influences the immunomodulatory properties of MSCs in different dental tissues. Thus, dental MSCs are both producers and targets of 1,25(OH)2D3 and might regulate the local vitamin D3-dependent processes in an autocrine/paracrine manner. The local vitamin D3 metabolism is assumed to play an essential role in the local physiological processes, but the mechanisms of its regulation in dental MSCs are mostly unknown. The alteration of the local vitamin D3 metabolism may unravel novel therapeutic modalities for the treatment of periodontitis as well as new strategies for dental tissue regeneration.

The expression of extracellular matrix metalloproteinase inducer (EMMPRIN) in the compression area during orthodontic relapse

OBJECTIVE

This study investigated the expression of extracellular matrix metalloproteinase inducer (EMMPRIN) in the compression area during orthodontic relapse in rat molars.

MATERIALS AND METHODS

Thirty Wistar rats (6 weeks old) underwent orthodontic tooth movement (OTM) of the left first maxillary molar for 21 days, followed by removal of the force device. The contralateral maxillary molar served as a control with no mechanical force stimuli. Animals were sacrificed at 0, 1, 3, 7, and 14 days of relapse after force withdrawal. Tooth relapse and alveolar bone parameters were measured using microcomputed tomography (micro-CT). Maxilla sections were obtained for haematoxylin and eosin (HE), immunohistochemical staining [EMMPRIN, nuclear factor kappa B ligand (RANKL) and vascular endothelial growth factor (VEGF)] and tartrate-resistant acid phosphatase (TRAP). Correlation analyses were then performed.

RESULTS

After force removal, nearly 79.88% of the total relapse occurred within the initial 3 days. The number of osteoclasts clearly increased while the alveolar bone density decreased on the pressure side on Day 3 of relapse. Moreover, the EMMPRIN expression level significantly increased on Day 1, peaked up on Day 3 and decreased on Days 7 and 14. Statistically, a strong positive correlation was found between EMMRPIN expression and the osteoclast number and RANKL and VEGF expression.

CONCLUSION

EMMPRIN was highly expressed on the pressure side during the orthodontic tooth relapse, which could be involved in osteoclastogenesis and alveolar bone resorption in association with RANKL and VEGF expression.

Osteoclast formation at the bone marrow/bone surface interface: Importance of structural elements, matrix, and intercellular communication

Osteoclasts, the multinucleated cells responsible for bone resorption, have an enormous destructive power which demands to be kept under tight control. Accordingly, the identification of molecular signals directing osteoclastogenesis and switching on their resorptive activity have received much attention. Mandatory factors were identified, but a very essential aspect of the control mechanism of osteoclastic resorption, i.e. its spatial control, remains poorly understood. Under physiological conditions, multinucleated osteoclasts are only detected on the bone surface, while their mono-nucleated precursors are only in the bone marrow. How are pre-osteoclasts targeted to the bone surface? How is their progressive differentiation coordinated with their approach to the bone surface sites to be resorbed, which is where they finally fuse? Here we review the information on the bone marrow distribution of differentiating pre-osteoclasts relative to the position of the mandatory factors for their differentiation as well as relative to physical entities that may affect their access to the remodelling sites. This info allows recognizing an "osteoclastogenesis route" through the bone marrow and leading to the coincident fusion/resorption site - but also points to what still remains to be clarified regarding this route and regarding the restriction of fusion at the resorption site. Finally, we discuss the mechanism responsible for the start of resorption and its spatial extension. This review underscores that fully understanding the control of bone resorption requires to consider it in both space and time - which demands taking into account the context of bone tissue.

Immunorthodontics: in vivo gene expression of orthodontic tooth movement

Orthodontic tooth movement (OTM) is a “sterile” inflammatory process. The present study aimed to reveal the underlying biological mechanisms, by studying the force associated-gene expression changes, in a time-dependent manner. Ni-Ti springs were set to move the upper 1^st-molar in C57BL/6 mice. OTM was measured by μCT. Total-RNA was extracted from tissue blocks at 1,3,7 and 14-days post force application, and from two control groups: naïve and inactivated spring. Gene-expression profiles were generated by next-generation-RNA-sequencing. Gene Set Enrichment Analysis, K-means algorithm and Ingenuity pathway analysis were used for data interpretation. Genes of interest were validated with qRT-PCR. A total of 3075 differentially expressed genes (DEGs) were identified, with the greatest number at day 3. Two distinct clusters patterns were recognized: those in which DEGs peaked in the first days and declined thereafter (tissue degradation, phagocytosis, leukocyte extravasation, innate and adaptive immune system responses), and those in which DEGs were initially down-regulated and increased at day 14 (cell proliferation and migration, cytoskeletal rearrangement, tissue homeostasis, angiogenesis). The uncovering of novel innate and adaptive immune processes in OTM led us to propose a new term “Immunorthodontics”. This genomic data can serve as a platform for OTM modulation future approaches.

Core Matrisome Protein Signature During Periodontal Ligament Maturation From Pre-occlusal Eruption to Occlusal Function

The pre-occlusal eruption brings the molars into functional occlusion and initiates tensional strains during mastication. We hypothesized that upon establishment of occlusal contact, the periodontal ligament (PDL) undergoes cell and extracellular matrix maturation to adapt to this mechanical function. The PDL of 12 Wistar male rats were laser microdissected to observe the proteomic changes between stages of pre-occlusal eruption, initial occlusal contact and 1-week after occlusion. The proteome was screened by mass spectrometry and confirmed by immunofluorescence. The PDL underwent maturation upon establishment of occlusion. Downregulation of alpha-fetoprotein stem cell marker and protein synthesis markers indicate cell differentiation. Upregulated proteins were components of the extracellular matrix (ECM) and were characterized with the matrisome project database. In particular, periostin, a major protein of the PDL, was induced following occlusal contact and localized around collagen α-1 (III) bundles. This co-localization coincided with organization of collagen fibers in direction of the occlusal forces. Establishment of occlusion coincides with cellular differentiation and the maturation of the PDL. Co-localization of periostin and collagen with subsequent fiber organization may help counteract tensional forces and reinforce the ECM structure. This may be a key mechanism of the PDL to adapt to occlusal forces and maintain structural integrity.

Effects of orthodontic force magnitude on cell apoptosis and RANKL-induced osteoclastogenesis : Studies in a rat model

PURPOSE

The aim of this study was to evaluate the time course of orthodontic force-induced apoptosis and receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis in a rat model under light- and heavy-force conditions.

METHODS

Male Wistar rats were divided into light-force (10 cN) and heavy-force (60 cN) groups (N = 28/group). Each group was divided into four time-course subgroups to evaluate all phases of orthodontic tooth movement. Mesialization appliances were placed on three united maxillary molars unilaterally and activated. Tooth movements were calculated, and periodontal ligament (PDL) widths were measured. Expression of Bax, Bcl‑2, caspase 3, caspase 9, and RANK-RANKL were assessed by immunohistochemistry. Expression levels at the PDL-alveolar bone border were compared between experimental and control groups and force groups.

RESULTS

The rate of tooth movement did not differ between the force groups. PDL widths were higher on the tension side in the heavy-force group in the post-lag phase. Pro-apoptotic protein Bax expression was elevated in the heavy-force group, whereas anti-apoptotic protein Bcl‑2 expression was elevated in the light-force group. RANK expression on days 7 and 21 and RANKL expression on day 21 differed between the force groups.

CONCLUSIONS

Evidence of orthodontic force-induced apoptosis is more robust with stronger forces than with weaker forces. Exuberant RANKL-induced osteoclastogenesis that was seen when applying a low force results from increased RANK and RANKL expression in the post-lag phase.

Interleukin-1β is a potential therapeutic target for periodontitis: a narrative review

Interleukin(IL)-1β, a pro-inflammatory cytokine, was elevated and participates in periodontitis. Not only the link between IL-1β and periodontitis was proved by clinical evidence, but also the increased IL-1β triggers a series of inflammatory reactions and promotes bone resorption. Currently, IL-1β blockage has been therapeutic strategies for autoimmune and autoinflammatory diseases such as rheumatoid arthritis, cryopyrin-associated periodic syndromes, gout and type II diabetes mellitus. It is speculated that IL-1β be a potential therapeutic target for periodontitis. The review focuses on the production, mechanism, present treatments and future potential strategies for IL-1β in periodontitis.

Local orthodontic force initiates widespread remodelling of the maxillary alveolar bone

Objectives

To clarify the effects of a local orthodontic force on alveolar bone by analysing bone remodelling in different regions of the maxilla during orthodontic tooth movement (OTM).

Methods

An OTM model was established in rats. Histological changes in the maxilla were analysed using TRAP staining, IHC staining for CTSK and haematoxylin and eosin (H and E) staining. The root bifurcation region of the alveolar bone of the first (M1), second (M2) and third (M3) molars were selected as the regions of interest (ROIs), which were further divided into a cervical and an apical level. Sequential fluorochrome labelling was performed to analyse bone deposition rates.

Results

The maxillary left first molars were moved mesially. TRAP staining and IHC staining for CTSK showed orthodontic force increased osteoclast numbers in all six ROIs at both the cervical and apical levels. H and E staining indicated elevated osteoblast numbers in the OTM group in all induced regions. Sequential fluorochrome labelling exhibited increased bone deposition rates around M1, M2 and M3 in the OTM group.

Conclusions

An orthodontic force applied to the first molar could initiate widespread remodelling of the maxillary alveolar bone, which was not restricted to the tension and pressure sites. This may revise the orthodontic biomechanical theory and provide new insights for clinical work.

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.

Effects Of Triptolide On Tooth Movement And Root Resorption In Rats

Purpose

The aim of this study was to investigate the effects of triptolide on the tooth movement and root resorption in rats during orthodontic treatment.

Material and methods

A total of 48 male Wistar rats were divided into three groups of 16 each. The right maxillary first molars of rats were drawn mesially by closed coil nickel-titanium spring with a force of 50 g. The two experimental groups received intraperitoneal injections of triptolide for 14 days at a dose of 15 µg/kg/day and 30 µg/kg/day, respectively. The control group received vehicle injections. After 14 days, the rats were humanely killed. The amount of tooth movement was measured. Eight rats from each group were randomly chosen for analysis of the percentage of root resorption area by scanning electron microscopy. For the remaining eight rats in each group, the H&E staining, tartrate-resistant acid phosphatase (TRAP) staining and immunohistochemistry analysis were performed.

Results

The amount of tooth movement and the ratio of root resorption area were significantly decreased in the triptolide-treated rats. The number of TRAP-positive cells was significantly lower in triptolide-treated groups. Moreover, the expression of nuclear factor kappa B ligand (RANKL) was reduced. In contrast, the expression of osteoprotegerin was significantly up-regulated. In the tension side, the expressions of runt-related transcription factor 2 and osteocalcin were significantly enhanced by triptolide injection.

Conclusion

Triptolide injection could arrest orthodontic tooth movement and reduce root resorption in rats via inhibition of osteoclastogenesis. In addition, triptolide may exert a positive effect on osteoblastogenesis.

Genetic polymorphisms influence gene expression of human periodontal ligament fibroblasts in the early phases of orthodontic tooth movement

Genetic polymorphisms could be involved in the individual rate of OTM (orthodontic tooth movement) corresponding to the clinical phenomenon of "slow movers" and "fast movers". This study evaluated, if genetic polymorphisms in RANK, RANKL, OPG, COX2 and IL6 are associated with the expression of RANKL, OPG, COX2 and IL6 by human periodontal ligament (hPDL) fibroblasts during OTM. Primary hPDL fibroblasts from periodontal connective tissue of teeth extracted from 57 human subjects for medical reasons were collected, isolated, cultivated and characterized. To simulate orthodontic forces in PDL pressure areas, a physiological compressive force of 2 g/cm² was applied to the hPDL fibroblasts under cell culture conditions at 70% confluency for 48 h, using a glass disc. Thereafter we analysed relative expression of RANKL, OPG, COX2 and IL6 by RT-qPCR. We also performed genotyping analysis of seven genetic polymorphisms in RANK, RANKL, OPG, COX2 and IL6. Relative gene expression was compared among the genotypes. The genotype TT in polymorphism rs9594738 (RANKL) had a higher RANKL expression in the recessive model (p = 0.021; TT vs. CT + CC). For polymorphism rs9594738 (RANKL), in the recessive model, TT was associated with a higher RANKL/OPG expression ratio (p = 0.013; TT vs. CT + CC). In the dominant model, GG genotype in rs5275 (COX2) was associated with a lower gene expression of COX2 (p = 0.04; GG vs. AA + AG). Genetic polymorphisms in genes associated with OTM affect the relative force-induced upregulation of these genes in hPDL fibroblasts.