Force-Induced H2S by PDLSCs Modifies Osteoclastic Activity during Tooth Movement

Effects of mechanical loading on matrix homeostasis and differentiation potential of periodontal ligament cells: A scoping review

Various mechanical loadings, including mechanical stress, orthodontics forces, and masticatory force, affect the functions of periodontal ligament cells. Regulation of periodontal tissue destruction, formation, and differentiation functions are crucial processes for periodontal regeneration therapy. Numerous studies have reported that different types of mechanical loading play a role in maintaining periodontal tissue matrix homeostasis, and osteogenic differentiation of the periodontal ligament cells. This scoping review aims to evaluate the studies regarding the effects of various mechanical loadings on the secretion of extracellular matrix (ECM) components, regulation of the balance between formation and destruction of periodontal tissue matrix, osteogenic differentiation, and multiple differentiation functions of the periodontal ligament. An electronic search for this review has been conducted on two databases; MEDLINE via PubMed and SCOPUS. Study selection criteria included original research written in English that reported the effects of different mechanical loadings on matrix homeostasis and differentiation potential of periodontal ligament cells. The final 204 articles were mainly included in the present scoping review. Mechanical forces of the appropriate magnitude, duration, and pattern have a positive influence on the secretion of ECM components such as collagen, as well as regulate the secretion of matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases. Additionally, these forces regulate a balance between osteoblastic and osteoclast differentiation. Conversely, incorrect mechanical loadings can lead to abnormal formation and destruction of both soft and hard tissue. This review provides additional insight into how mechanical loadings impact ECM homeostasis and multiple differentiation functions of periodontal ligament cells (PDLCs), thus making it valuable for regenerative periodontal treatment. In combination with advancing technologies, the utilization of ECM components, application of different aspects of mechanical force, and differentiation potential of PDLCs could bring potential benefits to future periodontal regeneration therapy.

Force-induced Caspase-1-dependent pyroptosis regulates orthodontic tooth movement

Pyroptosis, an inflammatory caspase-dependent programmed cell death, plays a vital role in maintaining tissue homeostasis and activating inflammatory responses. Orthodontic tooth movement (OTM) is an aseptic force-induced inflammatory bone remodeling process mediated by the activation of periodontal ligament (PDL) progenitor cells. However, whether and how force induces PDL progenitor cell pyroptosis, thereby influencing OTM and alveolar bone remodeling remains unknown. In this study, we found that mechanical force induced the expression of pyroptosis-related markers in rat OTM and alveolar bone remodeling process. Blocking or enhancing pyroptosis level could suppress or promote OTM and alveolar bone remodeling respectively. Using Caspase-1-/- mice, we further demonstrated that the functional role of the force-induced pyroptosis in PDL progenitor cells depended on Caspase-1. Moreover, mechanical force could also induce pyroptosis in human ex-vivo force-treated PDL progenitor cells and in compressive force-loaded PDL progenitor cells in vitro, which influenced osteoclastogenesis. Mechanistically, transient receptor potential subfamily V member 4 signaling was involved in force-induced Caspase-1-dependent pyroptosis in PDL progenitor cells. Overall, this study suggested a novel mechanism contributing to the modulation of osteoclastogenesis and alveolar bone remodeling under mechanical stimuli, indicating a promising approach to accelerate OTM by targeting Caspase-1.

Fluid flow-induced modulation of viability and osteodifferentiation of periodontal ligament stem cell spheroids-on-chip

Developing physiologically relevant in vitro models for studying periodontitis is crucial for understanding its pathogenesis and developing effective therapeutic strategies. In this study, we aimed to integrate the spheroid culture of periodontal ligament stem cells (PDLSCs) within a spheroid-on-chip microfluidic perfusion platform and to investigate the influence of interstitial fluid flow on morphogenesis, cellular viability, and osteogenic differentiation of PDLSC spheroids. PDLSC spheroids were seeded onto the spheroid-on-chip microfluidic device and cultured under static and flow conditions. Computational analysis demonstrated the translation of fluid flow rates of 1.2 μl min-1 (low-flow) and 7.2 μl min-1 (high-flow) to maximum fluid shear stress of 59 μPa and 360 μPa for low and high-flow conditions, respectively. The spheroid-on-chip microfluidic perfusion platform allowed for modulation of flow conditions leading to larger PDLSC spheroids with improved cellular viability under flow compared to static conditions. Modulation of fluid flow enhanced the osteodifferentiation potential of PDLSC spheroids, demonstrated by significantly enhanced alizarin red staining and alkaline phosphatase expression. Additionally, flow conditions, especially high-flow conditions, exhibited extensive calcium staining across both peripheral and central regions of the spheroids, in contrast to the predominantly peripheral staining observed under static conditions. These findings highlight the importance of fluid flow in shaping the morphological and functional properties of PDLSC spheroids. This work paves the way for future investigations exploring the interactions between PDLSC spheroids, microbial pathogens, and biomaterials within a controlled fluidic environment, offering insights for the development of innovative periodontal therapies, tissue engineering strategies, and regenerative approaches.

Effective Orthodontic Tooth Movement via an Occlusion-Activated Electromechanical Synergistic Dental Aligner

Malocclusion is a prevalent dental health problem plaguing over 56% worldwide. Mechanical orthodontic aligners render directional teeth movement extensively used for malocclusion treatment in the clinic, while mechanical regulation inefficiency prolongs the treatment course and induces adverse complications. As a noninvasive physiotherapy, an appropriate electric field plays a vital role in tissue metabolism engineering. Here, we propose an occlusion-activated electromechanical synergistic dental aligner that converts occlusal energy into a piezo-excited alternating electric field for accelerating orthodontic tooth movement. Within an 18-day intervention, significantly facilitated orthodontic results were obtained from young and aged Sprague-Dawley rats, increasing by 34% and 164% in orthodontic efficiency, respectively. The different efficiencies were attributed to age-distributed periodontal tissue status. Mechanistically, the electromechanical synergistic intervention modulated the microenvironment, enhanced osteoblast and osteoclast activity, promoted alveolar bone metabolism, and ultimately accelerated tooth movement. This work holds excellent potential for personalized and effective treatment for malocclusions, which would vastly reduce the suffering of the long orthodontic course.

Brain-derived neurotrophic factor promotes orthodontic tooth movement by alleviating periodontal ligament stem cell senescence

Orthodontic treatment in older adults is more difficult than in younger adults, partially due to delayed osteogenesis caused by senescence of human periodontal ligament stem cells (hPDLSCs). The production of brain-derived neurotrophic factor (BDNF) which regulates the differentiation and survival of stem cells decreases with age. We aimed to investigate the relationship between BDNF and hPDLSC senescence and its effects on orthodontic tooth movement (OTM). We constructed mouse OTM models using orthodontic nickel‑titanium springs and compared the responses of wild-type (WT) and BDNF^+/- mice with or without addition of exogenous BDNF. In vitro, hPDLSCs subjected to the mechanical stretch were used to simulate the cell stretch environment during OTM. We extracted periodontal ligament cells from WT and BDNF^+/- mice to evaluate their senescence-related indicators. The application of orthodontic force increased BDNF expression in the periodontium of WT mice, while the mechanical stretch increased BDNF expression in hPDLSCs. Osteogenesis-related indicators, including RUNX2 and ALP decreased and cellular senescence-related indicators such as p16, p53 and β-galactosidase increased in BDNF^+/- mice periodontium. Furthermore, periodontal ligament cells extracted from BDNF^+/- mice exhibited more senescent compared with cells from WT mice. Application of exogenous BDNF decreased the expression of senescence-related indicators in hPDLSCs by inhibiting Notch3, thereby promoting osteogenic differentiation. Periodontal injection of BDNF decreased the expression of senescence-related indicators in periodontium of aged WT mice. In conclusion, our study showed that BDNF promotes osteogenesis during OTM by alleviating hPDLSCs senescence, paving a new path for future research and 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.

Stretchable Electrochemical Sensor Based on a Gold Nanowire and Carbon Nanotube Network for Real-Time Tracking Cell-Released H2S

Hydrogen sulfide (H₂S), as the third gas transporter in biological systems, plays a key role in the regulation of biological cells. Real-time detection of local H₂S concentration in vivo is an important and challenging task. Herein, we explored a novel and facile strategy to develop a flexible and transparent H₂S sensor based on gold nanowire (AuNW) and carbon nanotube (CNT) films embedded in poly(dimethylsiloxane) (PDMS) (AuNWs/CNTs/PDMS). Taking the advantage of the sandwich-like nanostructured network of AuNWs/CNTs, the prepared electrochemical sensing platform exhibited desirable electrocatalytic activity toward H₂S oxidation with a wide linear range (5 nM to 24.9 μM) and a low dete ction limit (3 nM). Furthermore, thanks to the good biocompatibility and flexibility of the sensor, HeLa cells can be cultured directly on the electrode, allowing real-time monitoring of H₂S released from cells under a stretched state. This work provides a versatile strategy for the construction of stretchable electrochemical sensors, which has potential applications in the study of H₂S-related signal mechanotransduction and pathological processes.

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).

Intermittent compressive force regulates human periodontal ligament cell behavior via yes-associated protein

Intermittent compressive force influences human periodontal ligament (PDL) cell behavior that facilitates periodontal tissue regeneration. In response to mechanical stimuli, Yes-associated protein (YAP) has been recognized as a mechanosensitive transcriptional activator that regulates cell proliferation and cell fate decisions. This study aimed to investigate whether compressive forces influence cell proliferation and cell fate decisions of human PDL cells via YAP signaling. YAP expression was silenced by shRNA. The effect of YAP on cell proliferation, adipogenesis and osteogenesis of PDL cells under ICF loading were determined. Adipogenic differentiation bias upon ICF loading was confirmed by fourier-transform infrared spectroscopy (FTIR). The results revealed that ICF-induced YAP promotes osteogenesis, but it inhibits adipogenesis in PDL cells. Depletion of YAP results in PDL cells that are irresponsive to ICF and, therefore, the failure of the PDL cells to undergo osteogenic differentiation. This was shown by a significant reduction in calcium deposited in the CF-derived osteoblasts of the YAP-knockdown (YAP-KD) PDL cells. As to control treatment, reduction of YAP promoted adipogenesis, whereas ICF-induced YAP inhibited this mechanism. However, the adipocyte differentiation in YAP-KD cells was not affected upon ICF treatment as the YAP-KD cells still exhibited a better adipogenic differentiation that was unrelated to the ICF. This study demonstrated that, in response to ICF treatment, YAP could be a crucial mechanosensitive transcriptional activator for the regulation of PDL cell behavior through a mechanobiological process. Our results may provide the possibility of facilitating PDL tissue regeneration by manipulation of the Hippo-YAP signaling pathway.

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YAP plays role as a mechanosensitive transcriptional activator of human PDL cells in response to ICF.

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ICF activates YAP and its target genes to promote cell proliferation and osteogenic differentiation of human PDL cells.

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Loss of YAP enhances adipogenic differentiation of human periodontal ligament cells.

Simvastatin encapsulated in exosomes can enhance its inhibition of relapse after orthodontic tooth movement

INTRODUCTION

Relapse after orthodontic treatment is a major clinical issue in the dental field. Studies indicate that simvastatin may, to some extent, decrease the rate and magnitude of relapse status. Recent evidence demonstrated that exosome-based drug delivery has a broad prospect of clinical application. Hence, this study investigates whether simvastatin encapsulated in exosomes can inhibit relapse after orthodontic tooth movement (OTM).

METHODS

Periodontal ligament stem cells (PDLSCs) and their exosomes (PDLSCs-Exo) were isolated and identified. Exosomal simvastatin was obtained by co-incubation of simvastatin and PDLSCs-Exo. An OTM rat model was established. During the relapse period, rats' local alveolar bone was injected with simvastatin, PDLSCs-Exo, and exosomal simvastatin to examine the effect on relapse. Finally, we analyzed the influence of exosomal simvastatin on osteogenesis at the molecular and histologic levels.

RESULTS

PDLSCs and PDLSCs-Exo were successfully extracted and characterized by multiple means. Simvastatin encapsulated in exosomes can increase the solubility of the drug. Exosomal simvastatin can enhance its inhibition of relapse after OTM in the rat model. The expression level of osteogenic-related genes and proteins in the exosomal simvastatin group is higher than in other groups. Histologic analysis showed a reduction of bone-resorptive lacunae in the exosomal simvastatin group.

CONCLUSIONS

Encapsulating simvastatin into the exosomes derived from PDLSCs can improve simvastatin solubility and enhance the inhibition effect of relapse in the rat model of OTM. Notably, local injection of PDLSCs-Exo alone can also block the relapse after OTM.

Periodontal ligament cells derived small extracellular vesicles are involved in orthodontic tooth movement

OBJECTIVES

Small extracellular vesicles (EVs) from human periodontal ligament cells (hPDLCs) are closely associated with periodontal homeostasis. Far less is known about EVs association with orthodontic tooth movement (OTM). This study aimed to explore the role of small EVs originated from hPDLCs during OTM.

MATERIALS AND METHODS

Adult C57BL/6 mice were used. Springs were bonded to the upper first molars of mice for 7 days to induce OTM in vivo. To block small EVs release, GW4869 was intraperitoneally injected and the efficacy of small EVs inhibition in periodontal ligament was verified by transmission electron microscope (TEM). Tooth movement distance and osteoclastic activity were studied. In vitro, hPDLCs were isolated and administered compressive force in the EV-free culture media. The cell morphologies and CD63 expression of hPDLCs were studied. Small EVs were purified and characterized using a scanning electron microscope, TEM, western blot, and nanoparticle tracking analysis. The expression of proteins in the small EVs was further processed and validated using a human immuno-regulated cytokines array and an enzyme-linked immunosorbent assay (ELISA).

RESULTS

The small EV depletion significantly decreased the distance and osteoclastic activity of OTM in the mice. The hPDLCs displayed different morphologies under force compression and CD63 expression level decreased verified by western blot and immunofluorescence staining. Small EVs purified from supernatants of the hPDLCs showed features with <200 nm diameter, the positive EVs marker CD63, and the negative Golgi body marker GM130. The number of small EVs particles increased in hPDLCs suffering force stimuli. According to the proteome array, the level of soluble intercellular adhesion molecule-1 (sICAM-1) displayed the most significant fold change in small EVs under compressive force and this was further confirmed using an ELISA.

LIMITATIONS

Further mechanism studies are warranted to validate the hPDLC-originated small EVs function in OTM through proteins delivery.

CONCLUSIONS

The notable decrease in the OTM distance after small EV blocking and the significant alteration of the sICAM-1 level in the hPDLC-originated small EVs under compression provide a new vista into small EV-related OTM biology.

Inhibition of osteoclastogenesis by interleukin-33 administration in the periodontal ligament under mechanical loading

BACKGROUND AND OBJECTIVES

The molecular mechanisms mediating external root resorption are poorly understood. Interleukin-33 (IL-33) expression increased remarkably in the periodontal ligament (PDL) under orthodontic loading. The IL-33-driven responses are delicately cell type- and tissue context-dependent. It is unknown how IL-33 act on osteoclastogenesis in the context of root surface. This study aimed to investigate the effect of IL-33 on osteoclastogenesis in the PDL under mechanical loading.

MATERIALS AND METHODS

C57BL/6J mice were treated with injections of phosphate buffer saline (PBS) or recombinant mouse IL-33 (rmIL-33, 6 μl, 30 μg/ml), and subjected to models of orthodontic tooth movement. Tartrated resistant acid phosphates (TRAP)-positive cells and IL-33 expressions were examined in the PDL. IL-33 release from human PDL cells (hPDLCs) was detected by ELISA. Cementoblast-like (OCCM-30) cells were cultured in the presence of rmIL-33 to examine the release of osteoclast-regulatory proteins. The effects of rmIL-33 on osteoclastogenesis were examined in vitro in cultures of bone marrow macrophages (BMMs) and in BMMs-OCCM-30 cocultures. Expressions of osteoclast-specific or -related genes and proteins were investigated in BMMs-OCCM-30 cocultures treated with or without rmIL-33, in the presence or absence of granulocyte-macrophage colony-stimulating factor (GM-CSF) neutralizing antibody.

RESULTS

Interleukin-33 expressions were upregulated in the PDL under orthodontic loading. Static compressive force enhanced expression and release of IL-33 from hPDLCs. Administration of rmIL-33 resulted in reduced number of TRAP-positive cells in the PDL, and inhibited osteoclast differentiation from BMMs in vitro. OCCM-30 cells had varied osteoprotegerin (OPG) / receptor activator for nuclear factor-κB ligand (RANKL) secretion and increased release of GM-CSF under rmIL-33 stimulation. Treatment with rmIL-33 in BMMs-OCCM-30 cocultures resulted in inhibited differentiation and decreased activity of osteoclasts, and these effects were partially reversed by GM-CSF neutralizing antibody.

CONCLUSIONS

Interleukin-33 inhibits osteoclastogenesis in the PDL under orthodontic loading. The anti-osteoclastogenic effects were mediated partly by directly affecting osteoclast precursors and partly by cementoblast-mediated release of GM-CSF.

Mechanical force-promoted osteoclastic differentiation via periodontal ligament stem cell exosomal protein ANXA3

Exosomes play a critical role in intracellular communication. The biogenesis and function of exosomes are regulated by multiple biochemical factors. In the present study, we find that mechanical force promotes the biogenesis of exosomes derived from periodontal ligament stem cells (PDLSCs) and alters the exosomal proteome profile to induce osteoclastic differentiation. Mechanistically, mechanical force increases the level of exosomal proteins, especially annexin A3 (ANXA3), which facilitates exosome internalization to activate extracellular signal-regulated kinase (ERK), thus inducing osteoclast differentiation. Moreover, the infusion of exosomes derived from PDLSCs into mice promotes mechanical force-induced tooth movement and increases osteoclasts in the periodontal ligament. Collectively, this study demonstrates that mechanical force treatment promotes the biogenesis of exosomes from PDLSCs and increases exosomal protein ANXA3 to facilitate exosome internalization, which activates ERK phosphorylation, thus inducing osteoclast differentiation. Our findings shed light on new mechanisms for how mechanical force regulates the biology of exosomes and bone metabolism.

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Mechanical force promotes the biogenesis of exosomes derived from PDLSCs by RAB27B

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Mechanical force increases exosomal protein ANXA3 to facilitate exosome internalization

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ANXA3 activates ERK phosphorylation to induce osteoclast differentiation

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PDLSC exosomes enhance mechanical force-induced tooth movement

SM22α-lineage niche cells regulate intramembranous bone regeneration via PDGFRβ-triggered hydrogen sulfide production

Bone stromal cells are critical for bone homeostasis and regeneration. Growing evidence suggests that non-stem bone niche cells support bone homeostasis and regeneration via paracrine mechanisms, which remain to be elucidated. Here, we show that physiologically quiescent SM22α-lineage stromal cells expand after bone injury to regulate diverse processes of intramembranous bone regeneration. The majority of SM22α-lineage cells neither act as stem cells in vivo nor show their expression patterns. Dysfunction of SM22α-lineage niche cells induced by loss of platelet-derived growth factor receptor β (PDGFRβ) impairs bone repair. We further show that PDGFRβ-triggered hydrogen sulfide (H₂S) generation in SM22α-lineage niche cells facilitates osteogenesis and angiogenesis and suppresses overactive osteoclastogenesis. Collectively, these data demonstrate that non-stem SM22α-lineage niche cells support the niche for bone regeneration with a PDGFRβ/H₂S-dependent regulatory mechanism. Our findings provide further insight into non-stem bone stromal niche cell populations and niche-regulation strategy for bone repair.

Sclerostin in Periodontal Ligament: Homeostatic Regulator in Biophysical Force-Induced Tooth Movement

AIM

This study elucidates the role of sclerostin in periodontal ligament (PDL) as a homeostatic regulator in biophysical force-induced tooth movement (BFTM).

MATERIALS AND METHODS

BFTM was performed in rats, followed by microarray, immunofluorescence, in situ hybridization, and real-time PCR for detection and identification of the molecules. The periodontal space was analyzed via micro-computed tomography. Effects on osteoclastogenesis and bone resorption were evaluated in mouse bone marrow-derived cells. In vitro human PDL cells were subjected to biophysical forces.

RESULTS

In the absence of BFTM, sclerostin was hardly detected in the periodontium except the PDL and alveolar bone in the furcation region and apex of the molar roots. However, sclerostin was upregulated in the PDL in vivo by adaptable force, which induced typical transfiguration without changes in periodontal space as well as in vitro PDL cells under compression and tension. In contrast, the sclerostin level was unaffected by heavy force, which caused severe degeneration of the PDL and narrowed periodontal space. Sclerostin inhibited osteoclastogenesis and bone resorption, which corroborates the accelerated tooth movement by the heavy force.

CONCLUSIONS

Sclerostin in PDL may be a key homeostatic molecule in the periodontium and a biological target for the therapeutic modulation of BFTM. This article is protected by copyright. All rights reserved.

CXCL12/CXCR4 Mediates Orthodontic Root Resorption via Regulating the M1/M2 Ratio

Mechanical force-induced external root resorption is a major clinical side effect of orthodontic treatment. Recent work has revealed that M1 macrophages play a vital role in promoting orthodontic root resorption (ORR), but the mechanism of how mechanical force stimulation increases the M1/M2 macrophage ratio in periodontal tissue is poorly understood. In the current study, we showed that C-X-C motif chemokine 12 (CXCL12)⁺ periodontal ligament cells (PDLCs) and C-X-C chemokine receptor type 4 (CXCR4)⁺ monocytes in the periodontal ligament (PDL) were significantly increased after force application with ongoing root resorption, and these effects were partially rescued after force removal in mice. The expression of CXCL12 in PDLCs was increased by force stimulation in a time- and intensity-dependent manner in vitro. Blockage of the CXCL12/CXCR4 axis using CXCR4 antagonist AMD3100 was sufficient to alleviate ORR and reverse the force-enhanced M1/M2 macrophage ratio. Further mechanism exploration showed that Ly6C^hi inflammatory monocytes homed in a CXCL12/CXCR4 axis-dependent manner. The number and proportion of CD11b⁺ Ly6C^hi inflammatory monocytes in cervical lymph nodes were significantly increased by force loading, accompanied by decreased CD11b⁺ Ly6C^hi monocytes in the blood. These changes were blunted by intraperitoneal injection of AMD3100. In addition, blockage of the CXCL12/CXCR4 axis effectively reversed M2 suppression and promoted M1 polarization. Collectively, results indicate that force-induced CXCL12/CXCR4 axis mediates ORR by increasing the M1/M2 ratio in periodontal tissues through attracting Ly6C^hi inflammatory monocytes and modulating macrophage polarization. The results also imply that AMD3100 is potentially inhibitory to root resorption.

Identification osteogenic signaling pathways following mechanical stimulation: A systematic review

INTRODUCTION

It has been shown that mechanical forces can induce or promote osteogenic differentiation as well as remodeling of the new created bone tissues. To apply this characteristic in bone tissue engineering, it is important to know which mechanical stimuli through which signaling pathway has a more significant impact on osteogenesis.

METHODS

In this systematic study, an electronic search was conducted using PubMed and Google Scholar databases. This study has been prepared and organized according to the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. Included studies were first categorized according to the in vivo and in vitro studies.

RESULTS

Six types of mechanical stresses were used in these articles and the most commonly used mechanical force and cell source were tension and bone marrow-derived mesenchymal stem cells (BMMSCs), respectively. These forces were able to trigger twelve signaling pathways in which Wnt pathway was so prominent.

CONCLUSION

1) Although specific signaling pathways are induced through specific mechanical forces, Wnt signaling pathways are predominantly activated by almost all types of force/stimulation, 2) All signaling pathways regulate expression of RUNX2, which is known as a master regulator of osteogenesis, 3) In Tension force, the mode of force administration, i.e, continuous or non-continuous tension is more important than the percentage of elongation.

Periodontal ligament stem cells in the periodontitis niche: inseparable interactions and mechanisms

Periodontitis is characterized by the periodontium's pathologic destruction due to the host's overwhelmed inflammation to the dental plaque. The bacterial infections and subsequent host immune responses have shaped a distinct microenvironment, which generally affects resident periodontal ligament stem cells (PDLSCs). Interestingly, recent studies have revealed that impaired PDLSCs may also contribute to the disturbance of periodontal homeostasis. The putative vicious circle underlying the interesting "positive feedback" of PDLSCs in the periodontitis niche remains a hot research topic, whereas the inseparable interactions between resident PDLSCs and the periodontitis niche are still not fully understood. This review provides a microscopic view on the periodontitis progression, especially the quick but delicate immune responses to oral dysbacterial infections. We also summarize the interesting crosstalk of the resident PDLSCs with their surrounding periodontitis niche and potential mechanisms. Particularly, the microenvironment reduces the osteogenic properties of resident PDLSCs, which are closely related to their reparative activity. Reciprocally, these impaired PDLSCs may disrupt the microenvironment by aggravating the host immune responses, promoting aberrant angiogenesis, and facilitating the osteoclastic activity. We further recommend that more in-depth studies are required to elucidate the interactions of PDLSCs with the periodontal microenvironment and provide novel interventions for periodontitis.

Force-Induced Autophagy in Periodontal Ligament Stem Cells Modulates M1 Macrophage Polarization via AKT Signaling

Autophagy, a lysosomal degradation pathway, serves as a protective cellular mechanism in maintaining cell and tissue homeostasis under mechanical stimulation. As the mechanosensitive cells, periodontal ligament stem cells (PDLSCs) play an important role in the force-induced inflammatory bone remodeling and tooth movement process. However, whether and how autophagy in PDLSCs influences the inflammatory bone remodeling process under mechanical force stimuli is still unknown. In this study, we found that mechanical force stimuli increased the expression of the autophagy protein LC3, the number of M1 macrophages and osteoclasts, as well as the ratio of M1/M2 macrophages in the compression side of the periodontal ligament in vivo. These biological changes induced by mechanical force were repressed by the application of an autophagy inhibitor 3-methyladenine. Moreover, autophagy was activated in the force-loaded PDLSCs, and force-stimulated PDLSC autophagy further induced M1 macrophage polarization in vitro. The macrophage polarization could be partially blocked by the administration of autophagy inhibitor 3-methyladenine or enhanced by the administration of autophagy activator rapamycin in PDLSCs. Mechanistically, force-induced PDLSC autophagy promoted M1 macrophage polarization via the inhibition of the AKT signaling pathway. These data suggest a novel mechanism that force-stimulated PDLSC autophagy steers macrophages into the M1 phenotype via the AKT signaling pathway, which contributes to the inflammatory bone remodeling and tooth movement process.

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.

Osteoclastic effects of mBMMSCs under compressive pressure during orthodontic tooth movement

Background

During orthodontic tooth movement (OTM), alveolar bone remodelling is closely related to mechanical force. It is unclear whether stem cells can affect osteoclastogenesis to promote OTM. This study aimed to investigate the role of mouse bone marrow mesenchymal stem cells (mBMMSCs) under compression load in OTM.

Methods

A mouse OTM model was established, and GFP-labelled mBMMSCs and normal saline were injected into different groups of mice by tail vein injection. OTM distance was measured using tissue specimens and micro-computed tomography (micro-CT). The locations of mBMMSCs were traced using GFP immunohistochemistry. Haematoxylin-eosin staining, tartrate-resistant acid phosphate (TRAP) staining and immunohistochemistry of Runx2 and lipoprotein lipase were used to assess changes in the periodontal ligament during OTM. mBMMSCs under compression were co-cultured with mouse bone marrow-derived macrophages (mBMMs), and the gene expression levels of Rankl, Mmp-9, TRAP, Ctsk, Alp, Runx2, Ocn and Osterix were determined by RT-PCR.

Results

Ten days after mBMMSCs were injected into the tail vein of mice, the OTM distance increased from 176 (normal saline) to 298.4 μm, as determined by tissue specimen observation, and 174.2 to 302.6 μm, as determined by micro-CT metrological analysis. GFP-labelled mBMMSCs were mostly located on the compressed side of the periodontal ligament. Compared to the saline group, the number of osteoclasts in the alveolar bone increased significantly (P < 0.01) on the compressed side in the mBMMSC group. Three days after mBMMSC injection, the number of Runx2-GFP double-positive cells on the tension side was significantly higher than that on the compression side. After applying compressive force on the mBMMSCs in vitro for 2 days, RANKL expression was significantly higher than in the non-compression cells, but expression of Alp, Runx2, Ocn and Osterix was significantly decreased (P < 0.05). The numbers of osteoclasts differentiated in response to mBMMs co-cultured with mBMMSCs under pressure load and expression of osteoclast differentiation marker genes (Mmp-9, TRAP and Ctsk) were significantly higher than those in mBMMs stimulated by M-CSF alone (P < 0.05).

Conclusions

mBMMSCs are not only recruited to the compressed side of the periodontal ligament but can also promote osteoclastogenesis by expressing Rankl, improving the efficiency of OTM.

Mechanical loading and the control of stem cell behavior

OBJECTIVE

Mechanical stimulation regulates many cell responses. The present study describes the effects of different in vitro mechanical stimulation approaches on stem cell behavior.

DESIGN

The narrative review approach was performed. The articles published in English language that addressed the effects of mechanical force on stem cells were searched on Pubmed and Scopus database. The effects of extrinsic mechanical force on stem cell response was reviewed and discussed.

RESULTS

Cells sense mechanical stimuli by the function of mechanoreceptors and further transduce force stimulation into intracellular signaling. Cell responses to mechanical stimuli depend on several factors including type, magnitude, and duration. Further, similar mechanical stimuli exhibit distinct cell responses based on numerous factors including cell type and differentiation stage. Various mechanical applications modulate stemness maintenance and cell differentiation toward specific lineages.

CONCLUSIONS

Mechanical force application modulates stemness maintenance and differentiation. Modification of force regimens could be utilized to precisely control appropriate stem cell behavior toward specific applications.

Mechanical force modulates periodontal ligament stem cell characteristics during bone remodelling via TRPV4

OBJECTIVES

Mechanical force plays an important role in modulating stem cell fate and behaviours. However, how periodontal ligament stem cells (PDLSCs) perceive mechanical stimulus and transfer it into biological signals, and thereby promote alveolar bone remodelling, is unclear.

MATERIALS AND METHODS

An animal model of force-induced tooth movement and a compressive force in vitro was used. After force application, tooth movement distance, mesenchymal stem cell and osteoclast number, and proinflammatory cytokine expression were detected in periodontal tissues. Then, rat primary PDLSCs with or without force loading were isolated, and their stem cell characteristics including clonogenicity, proliferation, multipotent differentiation and immunoregulatory properties were evaluated. Under compressive force in vitro, the effects of the ERK signalling pathway on PDLSC characteristics were evaluated by Western blotting.

RESULTS

Mechanical force in vivo induced PDLSC proliferation, which was accompanied with inflammatory cytokine accumulation, osteoclast differentiation and TRPV4 activation; the force-stimulated PDLSCs showed greater clonogenicity and proliferation, reduced differentiation ability, improved induction of macrophage migration, osteoclast differentiation and proinflammatory factor expression. The biological changes induced by mechanical force could be partially suppressed by TRPV4 inhibition. Mechanistically, force-induced activation of TRPV4 in PDLSCs regulated osteoclast differentiation by affecting the RANKL/OPG system via ERK signalling.

CONCLUSIONS

Taken together, we show here that TRPV4 activation in PDLSCs under mechanical force contributes to changing their stem cell characteristics and modulates bone remodelling during tooth movement.

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.

EZH2 reduction is an essential mechanoresponse for the maintenance of super-enhancer polarization against compressive stress in human periodontal ligament stem cells

Despite the ubiquitous mechanical cues at both spatial and temporal dimensions, cell identities and functions are largely immune to the everchanging mechanical stimuli. To understand the molecular basis of this epigenetic stability, we interrogated compressive force-elicited transcriptomic changes in mesenchymal stem cells purified from human periodontal ligament (PDLSCs), and identified H3K27me3 and E2F signatures populated within upregulated and weakly downregulated genes, respectively. Consistently, expressions of several E2F family transcription factors and EZH2, as core methyltransferase for H3K27me3, decreased in response to mechanical stress, which were attributed to force-induced redistribution of RB from nucleoplasm to lamina. Importantly, although epigenomic analysis on H3K27me3 landscape only demonstrated correlating changes at one group of mechanoresponsive genes, we observed a genome-wide destabilization of super-enhancers along with aberrant EZH2 retention. These super-enhancers were tightly bounded by H3K27me3 domain on one side and exhibited attenuating H3K27ac deposition and flattening H3K27ac peaks along with compensated EZH2 expression after force exposure, analogous to increased H3K27ac entropy or decreased H3K27ac polarization. Interference of force-induced EZH2 reduction could drive actin filaments dependent spatial overlap between EZH2 and super-enhancers and functionally compromise the multipotency of PDLSC following mechanical stress. These findings together unveil a specific contribution of EZH2 reduction for the maintenance of super-enhancer stability and cell identity in mechanoresponse.

Overview of noncoding RNAs involved in the osteogenic differentiation of periodontal ligament stem cells

Periodontal diseases are infectious diseases that are characterized by progressive damage to dental support tissue. The major goal of periodontal therapy is to regenerate the periodontium destroyed by periodontal diseases. Human periodontal ligament (PDL) tissue possesses periodontal regenerative properties, and periodontal ligament stem cells (PDLSCs) with the capacity for osteogenic differentiation show strong potential in clinical application for periodontium repair and regeneration. Noncoding RNAs (ncRNAs), which include a substantial portion of poly-A tail mature RNAs, are considered “transcriptional noise.” Recent studies show that ncRNAs play a major role in PDLSC differentiation; therefore, exploring how ncRNAs participate in the osteogenic differentiation of PDLSCs may help to elucidate the underlying mechanism of the osteogenic differentiation of PDLSCs and further shed light on the potential of stem cell transplantation for periodontium regeneration. In this review paper, we discuss the history of PDLSC research and highlight the regulatory mechanism of ncRNAs in the osteogenic differentiation of PDLSCs.

Mechanosensing by Gli1+ cells contributes to the orthodontic force-induced bone remodelling

Objectives

Gli1⁺ cells have received extensive attention in tissue homeostasis and injury mobilization. The aim of this study was to investigate whether Gli1⁺ cells respond to force and contribute to bone remodelling.

Materials and methods

We established orthodontic tooth movement (OTM) model to assess the bone response for mechanical force. The transgenic mice were utilized to label and inhibit Gli1⁺ cells, respectively. Additionally, mice that conditional ablate Yes‐associated protein (Yap) in Gli1⁺ cells were applied in the present study. The tooth movement and bone remodelling were analysed.

Results

We first found Gli1⁺ cells expressed in periodontal ligament (PDL). They were proliferated and differentiated into osteoblastic cells under tensile force. Next, both pharmacological and genetic Gli1 inhibition models were utilized to confirm that inhibition of Gli1⁺ cells led to arrest of bone remodelling. Furthermore, immunofluorescence staining identified classical mechanotransduction factor Yap expressed in Gli1⁺ cells and decreased after suppression of Gli1⁺ cells. Additionally, conditional ablation of Yap gene in Gli1⁺ cells inhibited the bone remodelling as well, suggesting Gli1⁺ cells are force‐responsive cells.

Conclusions

Our findings highlighted that Gli1⁺ cells in PDL directly respond to orthodontic force and further mediate bone remodelling, thus providing novel functional evidence in the mechanism of bone remodelling and first uncovering the mechanical responsive property of Gli1⁺ cells.

Cystathionine gamma-lyase aggravates periodontal damage in traumatic occlusion mouse models

BACKGROUND AND OBJECTIVE

Though impacts of traumatic occlusion (TO) on periodontal tissues and roles of cystathionine γ-lyase (Cth) gene in the regulation of bone homeostasis have been studied by many, no consensus has been reached so far on whether TO deteriorates the periodontium and precise roles of Cth in occlusal trauma. Therefore, this study aims to investigate the impacts of TO on periodontal tissues and the involvement of Cth gene.

METHODS

Eighty C57BL/6 wild-type (WT) mice and Cth knockout (Cth^-/- ) mice, 8 weeks old, were used in this study. The TO model was established using composite resin bonding on the left maxillary molar for one, two, and three weeks, respectively. Morphological and histological changes in the periodontium were assessed by micro-computed tomography (micro-CT), hematoxylin and eosin (H&E) staining, and tartrate-resistant acid phosphatase (TRAP) staining. Osteoclast-related genes were analyzed by real-time polymerase chain reaction (qPCR).

RESULTS

It was found that decreased alveolar bone height, expanded bone resorption area, and increased width of periodontal ligament (PDL) occurred in TO models, accompanied by an increased number of osteoclasts in a time-dependent manner by micro-CT and histological staining. Osteoclast-related genes including Ctsk, Mmp9, Rank, Trap, and Rankl/Opg were also up-regulated after one week of modeling. The up-regulated expressions of Cth gene and its protein CTH were observed in TO mouse models. After 1, 2, or 3 weeks of modeling, WT mice showed more severe alveolar bone resorption, wider PDL, higher osteoclast count, and higher levels of osteoclast-related genes Ctsk, Rank, and Rankl/Opg than Cth^-/- mice.

CONCLUSION

TO causes a reduction in alveolar bone height and PDL morphological disorder with their severity increases in a time-dependent manner. Cth aggravates periodontal damage caused by TO.

Role of autophagy in the periodontal ligament reconstruction during orthodontic tooth movement in rats

Background/purpose

Autophagy, a lysosome-based degradation pathway that is reportedly activated by mechanical stress and nutrient deprivation, plays an important role in various physiological and pathological events. The present study investigated the level of autophagy and tumor necrosis factor-α(TNF-α) expression in the periodontal ligaments (PDLs) of Sprague-Dawley (SD) rats to analyze the involvement of autophagy and inflammatory cytokines in orthodontic tooth movement (OTM) and maintaining periodontal tissue homeostasis.

Materials and methods

SD rats (n = 100) were randomly divided into a control group (n = 10) and an experimental group (n = 90). An orthodontic appliance was placed in each rat in the experimental group, and 10 rats were randomly euthanized 15 min, 30 min, 1 h, 2 h, 4 h, 12 h, 1 d, 3 d and 7 d after mechanical loading. The OTM distance was then measured. Hematoxylin and eosin (HE) staining was used to analyze the morphology of the PDL. Immunohistochemical (IHC) staining and tartrate-resistant acid phosphatase (TRAP) staining were also performed.

Results

After the application of orthodontic force and under the dual effects of mechanical force and starvation caused by compressed vessels, the level of autophagy and TNF-α expression in the PDL fluctuated and exhibited a similar trend.

Conclusion

Our data suggest a significant correlation between the initiation of autophagy and TNF-α expression, which both exerted positive effects on PDL remodeling during OTM in rats.

Bovine Bone Promotes Osseous Protection via Osteoclast Activation

The current study aimed at investigating the long-term biological mechanisms governing bone regeneration in osseous defects filled with bovine bone (BB). Tooth extraction sockets were filled with BB or left unfilled for natural healing in a C57BL/6 mouse alveolar regeneration bone model (n = 12). Seven weeks later, the alveolar bone samples were analyzed histologically with hematoxylin/eosin and tartrate-resistant acid phosphatase staining. A separate group (n = 10) was used for RNA sequencing. Osteoclast inhibition was induced by zoledronic acid (ZA) administration at 2 wk postextraction in a third group (n = 28) for examination of osseous changes and cellular functions with micro-computed tomography and quantitative reverse transcription polymerase chain reaction, respectively. Histological and radiological osseous healing was observed in both BB-filled and normal-healing sockets. However, BB regenerated bone showed significant robust expression of genes associated with bone homeostasis and osteoclasts' function. Osteoclasts' inhibition in BB-filled sockets led to decreased bone resorption markers and reduced bone formation to a greater extent than that observed in osteoclasts' inhibition with natural healing. BB displays long-term biologically active properties, despite a naive osseous histological appearance. These include activation of osteoclasts, which in turn promotes osseous remodeling and maturation of ossified bone.

Mechanical load-induced H2S production by periodontal ligament stem cells activates M1 macrophages to promote bone remodeling and tooth movement via STAT1

Background

Tooth movement is a unique bone remodeling process induced by mechanical stimulation. Macrophages are important in mediating inflammatory processes during mechanical load-induced tooth movement. However, how macrophages are regulated under mechanical stimulation remains unclear. Mesenchymal stem cells (MSCs) can modulate macrophage polarization during bone remodeling. Hydrogen sulfide (H₂S) can be produced by MSCs and have been linked to bone homeostasis. Therefore, this study aimed to investigate whether H₂S contributed to periodontal ligament stem cell (PDLSC)-regulated macrophage polarization and bone remodeling under mechanical stimulation.

Methods

An experimental mechanical load-induced tooth movement animal model was established. Changes in cystathionine-β-synthase (CBS), markers of M1/M2 macrophages, tooth movement distance, and the number of osteoclasts were examined. The conditioned medium of PDLSCs with or without mechanical loading was utilized to treat THP-1 derived macrophages for 24 h to further investigate the effect of PDLSCs on macrophage polarization. Different treatments with H₂S donor, CBS inhibitor, or the inhibitor of STAT1 were used to investigate the related mechanism. Markers of M1/M2 polarization and STAT1 pathway expression were evaluated in macrophages.

Results

Mechanical load promoted tooth movement and increased the number of M1-like macrophages, M1-associated pro-inflammatory cytokines, and the expression of CBS on the compression side of the periodontal ligament. The injection of CBS inhibitor or H₂S donor could further repress or increase the number of M1-like macrophages, tartrate-resistant acid phosphatase-positive osteoclasts and the distance of tooth movement. Mechanistically, load-induced PDLSCs enhanced H₂S production, which increased the expression of M1-associated cytokines in macrophages. These effects could be blocked by the administration of CBS inhibitor. Moreover, load-induced H₂S steered M1 macrophage polarization via the STAT1 signaling pathway.

Conclusions

These data suggest a novel mechanism indicating that mechanical load-stimulated PDLSCs produce H₂S to polarize macrophages toward the M1 phenotype via the STAT1 signaling pathway, which contributes to bone remodeling and tooth movement process. These results provide new insights into the role of PDLSCs in regulating macrophage polarization and mediating bone remodeling under mechanical stimulation, and indicate that appropriate H₂S supplementation may accelerate tooth movement.

Electronic supplementary material

Supplementary information accompanies this paper at 10.1186/s13287-020-01607-9.

Hydrogen Sulfide in Bone Tissue Regeneration and Repair: State of the Art and New Perspectives

The importance of hydrogen sulfide (H₂S) in the regulation of multiple physiological functions has been clearly recognized in the over 20 years since it was first identified as a novel gasotransmitter. In bone tissue H₂S exerts a cytoprotective effect and promotes bone formation. Just recently, the scientific community has begun to appreciate its role as a therapeutic agent in bone pathologies. Pharmacological administration of H₂S achieved encouraging results in preclinical studies in the treatment of systemic bone diseases, such as osteoporosis; however, a local delivery of H₂S at sites of bone damage may provide additional opportunities of treatment. Here, we highlight how H₂S stimulates multiple signaling pathways involved in various stages of the processes of bone repair. Moreover, we discuss how material science and chemistry have recently developed biomaterials and H₂S-donors with improved features, laying the ground for the development of H₂S-releasing devices for bone regenerative medicine. This review is intended to give a state-of-the-art description of the pro-regenerative properties of H₂S, with a focus on bone tissue, and to discuss the potential of H₂S-releasing scaffolds as a support for bone repair.

Compressive force-induced autophagy in periodontal ligament cells downregulates osteoclastogenesis during tooth movement

BACKGROUND

Autophagy has recently emerged as a protective mechanism in response to compressive force and an important process in maintenance of bone homeostasis. It appears to be involved in the degradation of osteoclasts, osteoblasts, and osteocytes. The aim of this study was to investigate the role of compressive force-induced autophagy in periodontal ligament (PDL) cells in regulating osteoclastogenesis of orthodontic tooth movement (OTM).

METHODS

An OTM model and compressive force on PDL cells were employed to investigate the expression of autophagy markers in vivo and in vitro, respectively. Autophagosomes and autolysosomes were observed in PDL cells by transmission electron microscope (TEM) and autophagy LC3 double labelling. 3-Methyladenine (3-MA) and rapamycin were respectively used to inhibit and promote autophagy, and the effect of autophagy on osteoclastogenesis was explored via microcomputed tomography, hematoxylin and eosin (H&E) staining, histochemistry of titrate-resistant acid phosphatase, and real-time polymerase chain reaction (RT-PCR) in vivo. Receptor activator of nuclear factor-kappa B ligand/osteoprotegerin (RANKL/OPG) was investigated by RT-PCR and ELISA in vitro.

RESULTS

Orthodontic force-induced autophagy was prominent on the pressured side of PDL tissues. Administration of 3-MA downregulated bone density and upregulated osteoclasts, while rapamycin had reverse results in OTM. The autophagy activity increased initially then decreased in PDL cells during compressive force application and responded to light force. In PDL cells, administration of 3-MA upregulated while rapamycin downregulated the RANKL/OPG ratio.

CONCLUSION

Autophagy is activated by compressive force in PDL cells. Besides, it could modulate OTM by negatively regulating osteoclastogenesis and keep bone homeostasis via RANKL/OPG signaling.

Mechanical Stress Modulates the RANKL/OPG System of Periodontal Ligament Stem Cells via α7 nAChR in Human Deciduous Teeth: An In Vitro Study

The aim of this study was to investigate the mechanism by which periodontal ligament stem cells (PDLSCs) modulate root resorption of human deciduous teeth under mechanical stress. In this investigation, the PDLSCs were derived from deciduous and permanent teeth at different stages of root resorption. A cyclic hydraulic pressure was applied on the PDLSCs to mimic chewing forces in the oral environment. The cultured cells were characterized using osteogenic and adipogenic differentiation assays, quantitative real-time polymerase chain reaction (qRT-PCR), and Western blotting analysis. The PDLSCs exhibited the ability to induce osteoclast differentiation under certain mechanical stresses. As the expressions of RUNX2, alkaline phosphatase (ALP), and osteoprotegerin (OPG) were significantly reduced, the receptor activator of the nuclear factor kappa-B ligand (RANKL) was upregulated increasing the RANKL/OPG ratio. Under hydrodynamic pressure at 0-135 kPa, the expressions of alpha 7 nicotinic acetylcholine receptors (α7 nAChR), p-GSK-3β, and active-β-catenin were markedly upregulated in PDLSCs from unresorbed deciduous teeth. Treatment with the α7 nAChR inhibitor alpha-bungarotoxin (α-BTX) and the Wnt pathway inhibitor DKK1 may reverse the mechanical stress inducing upregulation of RANKL and reduction of RUNX2, ALP, and OPG. Alizarin red staining confirmed these results. The mechanical stress applied on the deciduous tooth PDLSCs can induce osteoclastic effects through upregulation of α7 nAChR and activation of the canonical Wnt pathway. It can be suggested that chewing forces may play a major role at the beginning of the physiological root resorption of deciduous teeth.

Identification and characterization of circular RNAs involved in mechanical force-induced periodontal ligament stem cells

Circular RNAs (circRNAs) play critical roles in signal transduction during cell proliferation, differentiation, and apoptosis in a posttranscriptional manner. Recently, circRNAs have been proved to be a large class of animal RNAs with regulatory potency. However, whether circRNAs can respond to mechanical force (MF) and impact on human periodontal ligament stem cells (PDLSCs) and the orthodontic tooth movement (OTM) process remain unknown. Here, we investigated the circRNAs expression patterns in PDLSCs induced by MF and found that circRNAs were responsive to the MF in PDLSCs. Through the valid reads' distribution analysis, we found that the majority of reads in both the control PDLSCs and the MF-induced PDLSCs were distributed in exons. Then we analyzed Gene Ontology terms of genes that overlap with or are neighbors of the stress-responsive circRNAs and found unique enrichment patterns in biological processes, molecular function, and cellular component of PDLSCs. Next, we predicted the possible functions of circRNAs through circRNAs-miRNAs networks. We found that one circRNA may regulate one or several miRNA/miRNAs and one miRNA may interact with one or multiple circRNA/circRNAs. Importantly, a number of circRNAs were predicted to directly or indirectly regulate miRNAs-mediated osteogenic differentiation in mesenchymal stem cells. For instance, circRNA3140 was highly and widely associated with microRNA-21, which plays a critical role in MF-induced osteogenic differentiation of PDLSCs. Taken together, these findings reveal a previously unrecognized mechanism that MF can induce the expression changes of circRNAs in PDLSCs, which may modulate the OTM process and the alveolar bone remodeling.

Mechanobiology of Periodontal Ligament Stem Cells in Orthodontic Tooth Movement

Periodontal ligament stem cells (PDLSCs) possess self-renewal, multilineage differentiation, and immunomodulatory properties. They play a crucial role in maintaining periodontal homeostasis and also participated in orthodontic tooth movement (OTM). Various studies have applied controlled mechanical stimulation to PDLSCs and investigated the effects of orthodontic force on PDLSCs. Physical stimuli can regulate the proliferation and differentiation of PDLSCs. During the past decade, a variety of studies has demonstrated that applied forces can activate different signaling pathways in PDLSCs, including MAPK, TGF-β/Smad, and Wnt/β-catenin pathways. Besides, recent advances have highlighted the critical role of orthodontic force in PDLSC fate through mediators, such as IL-11, CTHRC1, miR-21, and H₂S. This perspective review critically discusses the PDLSC fate to physical force in vitro and orthodontic force in vivo, as well as the underlying molecular mechanism involved in OTM.

In Vitro Weight-Loaded Cell Models for Understanding Mechanodependent Molecular Pathways Involved in Orthodontic Tooth Movement: A Systematic Review

Cells from the mesenchymal lineage in the dental area, including but not limited to PDL fibroblasts, osteoblasts, and dental stem cells, are exposed to mechanical stress in physiological (e.g., chewing) and nonphysiological/therapeutic (e.g., orthodontic tooth movement) situations. Close and complex interaction of these different cell types results in the physiological and nonphysiological adaptation of these tissues to mechanical stress. Currently, different in vitro loading models are used to investigate the effect of different types of mechanical loading on the stress adaptation of these cell types. We performed a systematic review according to the PRISMA guidelines to identify all studies in the field of dentistry with focus on mechanobiology using in vitro loading models applying uniaxial static compressive force. Only studies reporting on cells from the mesenchymal lineage were considered for inclusion. The results are summarized regarding gene expression in relation to force duration and magnitude, and the most significant signaling pathways they take part in are identified using protein-protein interaction networks.

Hydrogen sulfide maintains dental pulp stem cell function via TRPV1-mediated calcium influx

Hydrogen sulfide (H₂S), an endogenous gasotransmitter, mediated a variety of biological processes through multiple signaling pathways, and aberrant H₂S metabolism has been associated with mesenchymal stem cell (MSC) dysfunction. Here we employed the small interfering RNA treatment for cystathionine β-synthase (CBS), cystathionine γ-lyase, the main enzymes to synthesize H₂S, and CBS-knockout mice to analyze the effect of H₂S on dental pulp homeostasis. We showed that H₂S deficiency attenuated dental pulp stem cell (DPSC) osteogenic/dentinogenic differentiation in vitro and in vivo with enhanced cell proliferation. Mechanically, H₂S facilitated the transient receptor potential action channel subfamily V member 1-mediated calcium (Ca²⁺⁾ influx, which subsequently activated the β-catenin pathway. While H₂S deficiency decreased Ca²⁺, resulting in glycogen synthase kinase-3β-mediated β-catenin degradation, which controls proliferation and differentiation of DPSCs. Consistently, H₂S-deficient mice displayed disturbed pattern of dental pulp and less dentin formation. In this study, we identified a previously unknown mechanism by which H₂S regulates DPSC lineage determination and dental pulp homeostasis.

Hydrogen sulfide promotes immunomodulation of gingiva-derived mesenchymal stem cells via the Fas/FasL coupling pathway

BACKGROUND

Mesenchymal stem cells derived from gingiva (GMSCs) display profound immunomodulation properties in addition to self-renewal and multilineage differentiation capacities. Hydrogen sulfide (H₂S) is not only an environmental pollutant, but also is an important biological gas transmitter in health and disease.

METHODS

We used an in-vitro coculture system and a mouse colitis model to compare the immunomodulatory effects between control and H₂S-deficient GMSCs. The flow cytometry analysis was used for T-cell apoptosis and T-helper 17 (Th17) and regulatory T (Treg) cell differentiation.

RESULTS

We revealed that GMSCs exerted their immunomodulatory effect by inducing T-cell apoptosis, promoting Treg cell polarization, and inhibiting Th17 cell polarization in vitro. The levels of H₂S regulated the immunomodulatory effect of GMSCs. Mechanically, H₂S deficiency downregulated the expression of Fas in GMSCs, resulting in reduced secretion of monocyte chemotactic protein 1 (MCP-1), which in turn led to decreased T-cell migration to GMSCs mediated by MCP-1. Moreover, H₂S deficiency downregulated the expression of Fas ligand (FasL) in GMSCs. The Fas/FasL coupling-induced T-cell apoptosis by GMSCs was attenuated in H₂S-deficient GMSCs. Consistent with this, H₂S-deficient GMSCs showed attenuated therapeutic effects on colitis in vivo, which could be restored by treatment with the H₂S donor, NaHS.

CONCLUSIONS

These findings showed that H₂S was required to maintain immunomodulation of GMSCs, which was mediated by Fas/FasL coupling-induced T-cell apoptosis.

Tension force-induced bone formation in orthodontic tooth movement via modulation of the GSK-3β/β-catenin signaling pathway

Orthodontic force-induced osteogenic differentiation and bone formation at tension sites play a critical role in orthodontic tooth movement. However, the molecular mechanism underlying this phenomenon is poorly understood. In the current study, we investigated the involvement of the GSK-3β/β-catenin signaling pathway, which is critical for bone formation during tooth movement. We established a rat tooth movement model to test the hypothesis that orthodontic force may stimulate bone formation at the tension site of the moved tooth and promote the rate of tooth movement via regulation of the GSK-3β/β-catenin signaling pathway. Our results showed that continued mechanical loading increased the distance between the first and second molar in rats. In addition, the loading force increased bone formation at the tension site, and also increased phospho-Ser9-GSK-3β expression and β-catenin signaling pathway activity. Downregulation of GSK-3β activity further increased bone parameters, including bone mineral density, bone volume to tissue volume and trabecular thickness, as well as ALP- and osterix-positive cells at tension sites during tooth movement. However, ICG-001, the β-catenin selective inhibitor, reversed the positive effects of GSK-3β inhibition. In addition, pharmaceutical inhibition of GSK-3β or local treatment with β-catenin inhibitor did not influence the rate of tooth movement. Based on these results, we concluded that GSK-3β/β-catenin signaling contributes to the bone remodeling induced by orthodontic forces, and can be used as a potential therapeutic target in clinical dentistry.