The effect of cyclic mechanical strain on the expression of adhesion-related genes by periodontal ligament cells in two-dimensional culture

Intermittent compressive force regulates matrix metalloproteinases and tissue inhibitors of metalloproteinases expression in human periodontal ligament cells

OBJECTIVE

This study aims to evaluate the effects of intermittent compressive force (ICF) on the expression of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) by human periodontal ligament cells (hPDLCs).

DESIGN

hPDLCs were subjected to ICF with a magnitude of 1.5 g/cm2 and loaded for 24 h. mRNA and protein expression of several MMPs and TIMPs were assessed using RT-PCR and ELISA analyses. An inhibitor of TGF-β (SB431542) was used to assess a possible role of TGF-β in the expression of MMPs and TIMPs under ICF.

RESULTS

mRNA and protein analyses showed that ICF significantly induced expression of TIMP1 and TIMP3, but decreased expression of MMP1. Incubation with the TGF-β inhibitor and applied to ICF showed a downregulation of TIMP3, but expression of MMP1 was not affected.

CONCLUSION

ICF is likely to affect ECM homeostasis by hPDLCs by regulating the expression of MMP1 and TIMPs. Moreover, TGF-β1 regulated expression of TIMP3. These findings suggest ICF may decrease the degradation of ECM and may thus be essential for maintaining PDL homeostasis.

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.

Under pressure-mechanisms and risk factors for orthodontically induced inflammatory root resorption: a systematic review

BACKGROUND

The application of orthodontic forces causes root resorption of variable severity with potentially severe clinical ramifications.

OBJECTIVE

To systematically review reports on the pathophysiological mechanisms of orthodontically induced inflammatory root resorption (OIIRR) and the associated risk factors based on in vitro, experimental, and in vivo studies.

SEARCH METHODS

We undertook an electronic search of four databases and a separate hand-search.

SELECTION CRITERIA

Studies reporting on the effect of orthodontic forces with/without the addition of potential risk factors on OIIRR, including (1) gene expression in in-vitro studies, the incidence root resorption in (2) animal studies, and (3) human studies.

DATA COLLECTION AND ANALYSIS

Potential hits underwent a two-step selection, data extraction, quality assessment, and systematic appraisal performed by duplicate examiners.

RESULTS

One hundred and eighteen articles met the eligibility criteria. Studies varied considerably in methodology, reporting of results, and variable risk of bias judgements.In summary, the variable evidence identified supports the notion that the application of orthodontic forces leads to (1) characteristic alterations of molecular expression profiles in vitro, (2) an increased rate of OIIRR in animal models, as well as (3) in human studies. Importantly, the additional presence of risk factors such as malocclusion, previous trauma, and medications like corticosteroids increased the severity of OIIRR, whilst other factors decreased its severity, including oral contraceptives, baicalin, and high caffeine.

CONCLUSIONS

Based on the systematically reviewed evidence, OIIRR seems to be an inevitable consequence of the application of orthodontic forces-with different risk factors modifying its severity. Our review has identified several molecular mechanisms that can help explain this link between orthodontic forces and OIIRR. Nevertheless, it must be noted that the available eligible literature was in part significantly confounded by bias and was characterized by substantial methodological heterogeneity, suggesting that the results of this systematic review should be interpreted with caution.

REGISTRATION

PROSPERO (CRD42021243431).

Optimizing a Diagnostic Model of Periodontitis by Using Targeted Proteomics

Periodontitis (PD), a widespread chronic infectious disease, compromises oral health and is associated with various systemic conditions and hematological alterations. Yet, to date, it is not clear whether serum protein profiling improves the assessment of PD. We collected general health data, performed dental examinations, and generated serum protein profiles using novel Proximity Extension Assay technology for 654 participants of the Bialystok PLUS study. To evaluate the incremental benefit of proteomics, we constructed two logistic regression models assessing the risk of having PD according to the CDC/AAP definition; the first one contained established PD predictors, and in addition, the second one was enhanced by extensive protein information. We then compared both models in terms of overall fit, discrimination, and calibration. For internal model validation, we performed bootstrap resampling (n = 2000). We identified 14 proteins, which improved the global fit and discrimination of a model of established PD risk factors, while maintaining reasonable calibration (area under the curve 0.82 vs 0.86; P < 0.001). Our results suggest that proteomic technologies offer an interesting advancement in the goal of finding easy-to-use and scalable diagnostic applications for PD that do not require direct examination of the periodontium.

Cyclic tensile strain affects the response of human periodontal ligament stromal cells to tumor necrosis factor-α

Objectives

Orthodontic treatment in adult patients predisposed to mild or severe periodontal disease is challenging for orthodontists. Orthodontic malpractice or hyper-occlusal forces may aggravate periodontitis-induced destruction of periodontal tissues, but the specific mechanism remains unknown. In the present study, the combined effect of mechanical stress and tumor necrosis factor (TNF)-α on the inflammatory response in human periodontal ligament stromal cells (hPDLSCs) was investigated.

Materials and methods

hPDLSCs from 5 healthy donors were treated with TNF-α and/or subjected to cyclic tensile strain (CTS) of 6% or 12% elongation with 0.1 Hz for 6- and 24 h. The gene expression of interleukin (IL)-6, IL-8 and cell adhesion molecules VCAM and ICAM was analyzed by qPCR. The protein levels of IL-6 and IL-8 in conditioned media was measured by ELISA. The surface expression of VCAM-1 and ICAM-1 was quantified by immunostaining followed by flow cytometry analysis.

Results

TNF-α-induced IL-6 gene and protein expression was inhibited by CTS, whereas TNF-α-induced IL-8 expression was decreased at mRNA expression level but enhanced at the protein level in a magnitude-dependent manner. CTS downregulated the gene expression of VCAM-1 and ICAM-1 under TNF-α stimulation, but the downregulation of the surface expression analyzed by flow cytometry was observed chiefly for VCAM-1.

Conclusions

Our findings show that mechanical force differentially regulates TNF-α-induced expression of inflammatory mediators and adhesion molecules at the early stage of force application. The effect of cyclic tensile strain is complex and could be either anti-inflammatory or pro-inflammatory depending on the type of pro-inflammatory mediators and force magnitude.

Clinical relevance

Orthodontic forces regulate the inflammatory mediators of periodontitis. The underlying mechanism may have significant implications for future strategies of combined periodontal and orthodontic treatment.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00784-021-04039-8.

MMPs and TIMPs Expression Levels in the Periodontal Ligament during Orthodontic Tooth Movement: A Systematic Review of In Vitro and In Vivo Studies

Background: During orthodontic tooth movement (OTM), applied orthodontic forces cause an extensive remodeling of the extracellular matrix (ECM) in the periodontal ligament (PDL). This is mainly orchestrated by different types of matrix metalloproteinases (MMPs) and their tissue inhibitors of matrix metalloproteinases (TIMPs), which are both secreted by periodontal ligament (PDL) fibroblasts. Multiple in vitro and in vivo studies already investigated the influence of applied orthodontic forces on the expression of MMPs and TIMPs. The aim of this systematic review was to explore the expression levels of MMPs and TIMPs during OTM and the influence of specific orthodontic force-related parameters. Methods: Electronic article search was performed on PubMed and Web of Science until 31 January 2021. Screenings of titles, abstracts and full texts were performed according to PRISMA, whereas eligibility criteria were defined for in vitro and in vivo studies, respectively, according to the PICO schema. Risk of bias assessment for in vitro studies was verified by specific methodological and reporting criteria. For in vivo studies, risk of bias assessment was adapted from the Joanna Briggs Institute Critical Appraisal Checklist for analytical cross-sectional study. Results: Electronic article search identified 3266 records, from which 28 in vitro and 12 in vivo studies were included. The studies showed that orthodontic forces mainly caused increased MMPs and TIMPs expression levels, whereas the exact effect may depend on various intervention and sample parameters and subject characteristics. Conclusion: This systematic review revealed that orthodontic forces induce a significant effect on MMPs and TIMPs in the PDL. This connection may contribute to the controlled depletion and formation of the PDLs' ECM at the compression and tension site, respectively, and finally to the highly regulated OTM.

Interleukin-1β Induced Matrix Metalloproteinase Expression in Human Periodontal Ligament-Derived Mesenchymal Stromal Cells under In Vitro Simulated Static Orthodontic Forces

The periodontal ligament (PDL) responds to applied orthodontic forces by extracellular matrix (ECM) remodeling, in which human periodontal ligament-derived mesenchymal stromal cells (hPDL-MSCs) are largely involved by producing matrix metalloproteinases (MMPs) and their local inhibitors (TIMPs). Apart from orthodontic forces, the synthesis of MMPs and TIMPs is influenced by the aseptic inflammation occurring during orthodontic treatment. Interleukin (IL)-1β is one of the most abundant inflammatory mediators in this process and crucially affects the expression of MMPs and TIMPs in the presence of cyclic low-magnitude orthodontic tensile forces. In this study we aimed to investigate, for the first time, how IL-1β induced expression of MMPs, TIMPs and how IL-1β in hPDL-MSCs was changed after applying in vitro low-magnitude orthodontic tensile strains in a static application mode. Hence, primary hPDL-MSCs were stimulated with IL-1β in combination with static tensile strains (STS) with 6% elongation. After 6- and 24 h, MMP-1, MMP-2, TIMP-1 and IL-1β expression levels were measured. STS alone had no influence on the basal expression of investigated target genes, whereas IL-1β caused increased expression of these genes. In combination, they increased the gene and protein expression of MMP-1 and the gene expression of MMP-2 after 24 h. After 6 h, STS reduced IL-1β-induced MMP-1 synthesis and MMP-2 gene expression. IL-1β-induced TIMP-1 gene expression was decreased by STS after 6- and 24-h. At both time points, the IL-1β-induced gene expression of IL-1β was increased. Additionally, this study showed that fetal bovine serum (FBS) caused an overall suppression of IL-1β-induced expression of MMP-1, MMP-2 and TIMP-1. Further, it caused lower or opposite effects of STS on IL-1β-induced expression. These observations suggest that low-magnitude orthodontic tensile strains may favor a more inflammatory and destructive response of hPDL-MSCs when using a static application form and that this response is highly influenced by the presence of FBS in vitro.

An in-vitro mechanical strain three-dimensional culture model: periodontal ligament cell viability, apoptosis, and endoplasmic reticulum stress response

To develop a model to investigate a potential relationship between mechanical strain, cell responses, and endoplasmic reticulum stress in periodontal ligament (PDL) cells, primary PDL cell cultures were obtained from extracted premolars. Cells were cultured in hydrogel and subjected to 24 h of static mechanical strain, resulting in 18% dimensional substrate elongation. Cell viability, caspase-3/7 activity, and mRNA levels for 28 genes, including unfolded protein response (UPR)-related and mechanically responsive genes, serving as positive controls for stress induction, were examined. Compared with unstrained cultures, no difference in caspase activity was observed; however, viability responses differed between cell lines. Multiple UPR-related genes were differentially upregulated, with marginal statistical significance, including cAMP responsive element binding protein 3 like 3 (CREB3L3) (mean fold-regulation = 1.91), an adenosine monophosphate-dependent transcription factor with roles in UPR activation and the acute inflammatory response; and the pro-apoptotic UPR gene, endoplasmic reticulum to nucleus signaling 2 (ERN2) (mean fold-regulation = 4.01). The observed effect on cell viability following strain with no change in caspase activity suggests that reduction in viability may be mediated via caspase-3/7-independent mechanisms. Three-dimensional mechanical strain PDL cell culture models offer a method to study the role of endoplasmic reticulum stress and UPR, and provide a framework and potential UPR targets for future investigations.

Identification of Aberrantly Expressed lncRNAs Involved in Orthodontic Force Using a Subpathway Strategy

Background

The aim of the study was to identify key long noncoding RNAs (lncRNA) and related subpathways in the periodontal ligament tissue following orthodontic force.

Methods

We adopt a novelty subpathway strategy to identify lncRNAs competitively regulated functions and the key competitive lncRNAs in periodontal ligament disorders after undergoing orthodontic force. To begin with, patients with orthodontics in our hospital were enrolled in our research. The relationship of lncRNA-mRNA was established through shared predicted miRNA by using the hypergeometric test, Jaccard coefficient standardization, and the Pearson coefficient to determine the valid interaction relationship. After embedding screened lncRNA interactions to pathways, the significant subpathways were recognized by lenient distance and Wallenius approximation methods to calculate the false discovery rate value of each subpathway.

Results

The lncRNA-mRNA intersections including 263 lncRNAs, 1,599 mRNAs, and 3,762 interacting pairs were obtained. The enriched mRNAs were further enriched into various candidate pathways such as the PI3K-Akt signaling pathway. Several subpathways were screened, including the PI3K-Akt signaling pathway, 04510_1 focal adhesion, and p53 signaling pathway, respectively. The network of pathway-lncRNA-mRNA was constructed. Several key lncRNAs including DNAJC3-AS1, WDFY3-AS2, LINC00482, and DLEU2 were screened.

Conclusions

DNAJC3-AS1, WDFY3-AS2, LINC00482, and DLEU2 as aberrantly expressed lncRNAs involved in orthodontic force might play crucial roles in periodontal ligament disease pathogenesis.

Closer to Nature Through Dynamic Culture Systems

Mechanics in the human body are required for normal cell function at a molecular level. It is now clear that mechanical stimulations play significant roles in cell growth, differentiation, and migration in normal and diseased cells. Recent studies have led to the discovery that normal and cancer cells have different mechanosensing properties. Here, we discuss the application and the physiological and pathological meaning of mechanical stimulations. To reveal the optimal conditions for mimicking an in vivo microenvironment, we must, therefore, discern the mechanotransduction occurring in cells.

Cyclic tensile force-upregulated IL6 increases MMP3 expression by human periodontal ligament cells

OBJECTIVE

Cyclic tensile force (CTF) modulates physiological responses of periodontal ligament (PDL) cells. PDL cells are mechanosensitive and are able to maintain tissue homeostasis; a process mediated by the expression of particular cytokines including interleukin 6 (IL6). It is unknown whether CTF-induced IL6 regulates the expression of MMPs, enzymes needed for tissue remodeling.

DESIGN

Human PDL cells were subjected to 10% elongation strain of CTF at a frequency of 60 rpm continuously for 6 h. RNA and proteins were extracted and analyzed for IL6 and MMP expression by quantitative real-time PCR and ELISA, respectively. Using a neutralizing anti-IL6 antibody and addition of recombinant human IL6 at concentrations of 0.1, 1, 10 ng.mL^-1 were performed to clarify whether CTF-upregulated IL6 increased MMP expression. Inhibitors of intracellular signaling molecules were employed to reveal possible pathway(s) of IL6-induced MMP expression.

RESULTS

CTF-induced IL6 expression coincided with an increased MMP3 expression. A neutralizing anti-IL6 antibody attenuated the CTF-increased MMP3 expression, whereas stimulating the cells with recombinant human IL6 increased MMP3 expression. Both PI3K and MAPK pathways were essential in the IL6 induced expression of MMP3.

CONCLUSION

Our findings suggest a role of CTF in the modulation of expression of IL6 and MMP3 and thus in the regulation of homeostasis and remodeling of the periodontal ligament.

Gene expression in hip soft tissues in incipient canine hip dysplasia and osteoarthritis

Canine hip dysplasia and developmental dysplasia of the human hip share demographic, phenotypic, and clinical features including the predisposition to develop osteoarthritis in affected joints. To support the results of genetic mapping studies for CHD and its concomitant osteoarthritis with functional information, we performed RNA-seq on hip capsule and teres ligament of affected and unaffected dogs. RNA seq showed that expressed genes segregated according age, capsule or ligament, and hip phenotype. Expression of HHIP, DACT2, and WIF1 was significantly higher in capsule from control hips than dysplastic hips indicating a disruption of the hedgehog signaling pathway. Expression of SPON 1, a key component of the WNT pathway, was increased significantly in both dysplastic capsule and ligament while FBN2 and EMILIN3 were significantly increased in dysplastic capsule. Of genes associated with human hip osteoarthritis, expression of ACAN, IGF1, CILP2, COL11A1, COL8A1, and HAPLN was increased significantly in dysplastic capsule. The significant increase in expression of PLA2F, TNFRSF, TMEM, and IGFBP in dysplastic capsule indicated an injury response. Gene set enrichment analysis revealed that genes involved in extracellular matrix structure, epithelial to mesenchymal transition, myogenesis, growth factor signaling, cancer and immune pathways were enriched in dysplastic capsule. For teres ligament from dysplastic joints, genes in retinoic signaling pathways and those encoding extracellular matrix molecules, but not proteoglycans, were enriched. Hip tissues respond to abnormal mechanics early in dysplastic hip development and these pathways present targets for intervention in the early synovitis and capsulitis secondary to canine and human hip dysplasia. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:313-324, 2019.

Interaction between caspase-3 and caspase-5 in the stretch-induced programmed cell death in the human periodontal ligament cells

In our previous studies, programmed cell death (PCD) was induced in human periodontal ligament (PDL) cells, through activation of caspase-3 and upregulation of CASP5 gene (encoding caspase-5 protein), in response to mechanical stretch loading. The aim of this study is to explore the relationship between the inflammatory caspase, caspase-5, and the apoptotic executioner protein, caspase-3, in human PDL cells. Here, we found that cyclic stretching upregulated the activity and the protein expression level of caspase-3 and -5 and the addition of the caspase-3 inhibitor or caspase-5 inhibitor significantly inhibited the stretch-induced PCD. Meanwhile, the inhibition of caspase-5 inhibited the activation of caspase-3 and vice versa. The result of coimmunoprecipitation also demonstrated that the expression of caspase-3 was immunoprecipitated with caspase-5. Thus, our study revealed that the in vitro application of cyclic stretching induced PCD by activation of caspase-3 and -5 in human PDL cells, and these two caspases could interact with each other after mechanical stretch loading. The study may facilitate further studies on the mechanism of stretch-induced PCD and help us understand the force-related periodontal homeostasis and remodeling better.

Cyclic tensile force stimulates BMP9 synthesis and in vitro mineralization by human periodontal ligament cells

Periodontal ligament (PDL) cells are mechanosensitive and have the potential to differentiate into osteoblast-like cells under the influence of cyclic tensile force (CTF). CTF modulates the expression of regulatory proteins including bone morphogenetic proteins (BMPs), which are essential for the homeostasis of the periodontium. Among the BMPs, BMP9 is one of the most potent osteogenic BMPs. It is yet unknown whether CTF affects the expression of BMP9 and mineralization. Here, we demonstrated that continuously applied CTF for only the first 6 hr stimulated the synthesis of BMP9 and induced mineral deposition within 14 days by human PDL cells. Stimulation of BMP9 expression depended on ATP and P2Y ₁ receptors. Apyrase, an ecto-ATPase, inhibited CTF-mediated ATP-induced BMP9 expression. The addition of ATP increased the expression of BMP9. Loss of function experiments using suramin (a broad-spectrum P2Y antagonist), MRS2179 (a specific P2Y ₁ receptor antagonist), MRS 2365 (a specific P2Y ₁ agonist), U-73122 (a phospholipase C [PLC] inhibitor), and thapsigargin (enhancer of intracytosolic calcium) revealed the participation of P2Y ₁ in regulating the expression of BMP9. This was mediated by an increased level of intracellular Ca ²⁺ through the PLC pathway. A neutralizing anti-BMP9 antibody decreased mineral deposition, which was stimulated by CTF for almost 45% indicating a role of BMP9 in an in vitro mineralization. Collectively, our findings suggest an essential modulatory role of CTF in the homeostasis and regeneration of the periodontium.

Nicotine inhibits osteogenic differentiation of human periodontal ligament cells under cyclic tensile stress through canonical Wnt pathway and α7 nicotinic acetylcholine receptor

BACKGROUND AND OBJECTIVES

Nicotine, the main psychoactive component of tobacco, affects cell metabolism, proliferation, adhesion and, importantly, the osteogenic differentiation of fibroblasts. Approximately 15% of all orthodontic patients are adults among who one-fifth are smokers. Hence, it is necessary to have insight into the effects of nicotine on the osteogenic differentiation of hPDLCs during orthodontic tooth movement. This study aimed to investigate the effects and mechanisms of nicotine on the osteogenic differentiation of human periodontal ligament cells (hPDLCs) under the application of cyclic tensile stress.

MATERIAL AND METHODS

hPDLCs were obtained from donor third molars. The hPDLCs were treated with nicotine and/or cyclic tensile stress that was applied with a cell stress plus unit. The effect of nicotine on cell viability was analyzed using the MTT assay. The osteogenic differentiation of hPDLCs was detected by alkaline phosphatase staining, Alizarin Red S staining, quantitative real-time polymerase chain reaction and western blotting.

RESULTS

In combination with cyclic tensile stress, nicotine prevented the tensile stress-induced increase in alkaline phosphatase activity, formation of mineralization nodules and the upregulation of mRNA and protein expression of Runt-related transcription factor 2, transcription factor Sp7 and collagen type I; however, canonical Wnt pathway was activated. Furthermore, the addition of Dickkopf-related protein 1 and α-bungarotoxin counteracted the negative effect of nicotine and rescued the osteogenic differentiation of hPDLCs, respectively.

CONCLUSION

These results indicate that nicotine prevents the increased osteogenic potential of hPDLCs induced by cyclic tensile stress by binding to an α7 nicotinic acetylcholine receptor and activating the canonical Wnt pathway.

Caspase-8 and Caspase-9 Functioned Differently at Different Stages of the Cyclic Stretch-Induced Apoptosis in Human Periodontal Ligament Cells

Background

Human periodontal ligament (PDL) cells underwent apoptosis after mechanical stretch loading. However, the exact signalling pathway remains unknown. This study aimed to elucidate how the apoptotic caspases functioned in the cyclic stretch-induced apoptosis in human PDL cells.

Materials and Methods

In the present study, 20% cyclic stretch was selected to load the cells for 6 or 24 h. The following parameters were analyzed: apoptotic rates, the protein levels of caspase-3, -7, -8 and -9 and the activities of caspase-8 and -9. Subsequently, the influences of caspase-8 and caspase-9 inhibitors on the apoptotic rate and the protein level of the activated caspase-3 were assessed as well.

Results

The apoptotic rates increased in response to cyclic stretch, but the cells entered different apoptotic stages after 6 and 24 h stretches. Caspase-3, -7, -8 and -9 were all activated after stretch loading. The stretch-induced apoptosis and the protein level of the activated caspase-3 were inhibited after inhibiting both caspase-8 and caspase-9 in both 6 and 24 h stretched cells and after inhibiting caspase-9 in 24 h stretched cells.

Conclusion

Caspase-8 and -9 functioned differently at different apoptotic stages in human PDL cells after cyclic stretch.

Dynamic Mechanical and Nanofibrous Topological Combinatory Cues Designed for Periodontal Ligament Engineering

Complete reconstruction of damaged periodontal pockets, particularly regeneration of periodontal ligament (PDL) has been a significant challenge in dentistry. Tissue engineering approach utilizing PDL stem cells and scaffolding matrices offers great opportunity to this, and applying physical and mechanical cues mimicking native tissue conditions are of special importance. Here we approach to regenerate periodontal tissues by engineering PDL cells supported on a nanofibrous scaffold under a mechanical-stressed condition. PDL stem cells isolated from rats were seeded on an electrospun polycaprolactone/gelatin directionally-oriented nanofiber membrane and dynamic mechanical stress was applied to the cell/nanofiber construct, providing nanotopological and mechanical combined cues. Cells recognized the nanofiber orientation, aligning in parallel, and the mechanical stress increased the cell alignment. Importantly, the cells cultured on the oriented nanofiber combined with the mechanical stress produced significantly stimulated PDL specific markers, including periostin and tenascin with simultaneous down-regulation of osteogenesis, demonstrating the roles of topological and mechanical cues in altering phenotypic change in PDL cells. Tissue compatibility of the tissue-engineered constructs was confirmed in rat subcutaneous sites. Furthermore, in vivo regeneration of PDL and alveolar bone tissues was examined under the rat premaxillary periodontal defect models. The cell/nanofiber constructs engineered under mechanical stress showed sound integration into tissue defects and the regenerated bone volume and area were significantly improved. This study provides an effective tissue engineering approach for periodontal regeneration—culturing PDL stem cells with combinatory cues of oriented nanotopology and dynamic mechanical stretch.

Mechano-regulation of collagen biosynthesis in periodontal ligament

Periodontal ligament (PDL) plays critical roles in the development and maintenance of periodontium such as tooth eruption and dissipation of masticatory force. The mechanical properties of PDL are mainly derived from fibrillar type I collagen, the most abundant extracellular component. The biosynthesis of type I collagen is a long, complex process including a number of intra- and extracellular post-translational modifications. The final modification step is the formation of covalent intra- and intermolecular cross-links that provide collagen fibrils with stability and connectivity. It is now clear that collagen post-translational modifications are regulated by groups of specific enzymes and associated molecules in a tissue-specific manner; and these modifications appear to change in response to mechanical force. This review focuses on the effect of mechanical loading on collagen biosynthesis and fibrillogenesis in PDL with emphasis on the post-translational modifications of collagens, which is an important molecular aspect to understand in the field of prosthetic dentistry.

Hypoxia regulates the proliferation and osteogenic differentiation of human periodontal ligament cells under cyclic tensile stress via mitogen-activated protein kinase pathways

BACKGROUND

Previous studies have shown that periodontal ligament exists in a hypoxic microenvironment, especially under the condition of periodontitis or physical stress. The present study is designed to investigate the effects and mechanisms of hypoxia on regulating the proliferation and osteogenic differentiation of human periodontal ligament cells (hPDLCs) under cyclic tensile stress (CTS).

METHODS

hPDLCs were cultured in 2% O2 (hypoxia) or 20% O2 (normoxia) and then subjected to a cyclic in-plane tensile deformation of 10% at 0.5 Hz. The following parameters were measured: 1) cell proliferation by flow cytometry; 2) cell ultrastructure by transmission electron microscopy; 3) expression of hypoxia-inducible factor-1α (HIF-1α) and osteogenic relative factors (i.e., secreted phosphoprotein 1 [SPP1; also known as bone sialoprotein I/osteopontin], runt-related transcription factor 2 [RUNX2], and transcription factor Sp7 [SP7]) by real-time polymerase chain reaction and Western blot; and 4) involvement of mitogen-activated protein kinase (MAPK) signaling pathways by Western blot with specific inhibitor.

RESULTS

Proliferation index in the hypoxia with CTS group was significantly higher than in other groups. Significant increases in HIF-1α, SPP1, RUNX2, and SP7 occurred in the presence of hypoxia for 24 hours. In addition, MAPK inhibitor (PD 98,059) significantly attenuated hypoxia and CTS-induced phosphor-ERK1/2 (extracellular regulated kinase 1/2), phosphor-JNK (c-jun N-terminal kinase), and phosphor-P38 expression.

CONCLUSIONS

Hypoxia regulates CTS-responsive changes in proliferation and osteogenic differentiation of hPDLCs via MAPK pathways. Hypoxia-treated hPDLCs may serve as an in vitro model to explore the molecular mechanisms of periodontitis.

The role of SIRT6 in the differentiation of vascular smooth muscle cells in response to cyclic strain

Vascular smooth muscle cells (VSMCs) may switch their phenotype between a quiescent contractile phenotype and a synthetic phenotype in response to cyclic strain, and this switch may contribute to hypertension, atherosclerosis, and restenosis. SIRT 6 is a member of the sirtuin family, and plays an important role in different cell processes, including differentiation. We hypothesized that cyclic strain modulates the differentiation of VSMCs via a transforming growth factor-β1 (TGF-β1)-Smad-SIRT6 pathway. VSMCs were subjected to cyclic strain using a Flexercell strain unit. It was demonstrated that the strain stimulated the secretion of TGF-β1 into the supernatant of VSMCs. After exposed to the strain, the expressions of contractile phenotype markers, including smooth muscle protein 22 alpha, alpha-actin, and calponin, and phosphorylated Smad2, phosphorylated Smad5, SIRT6 and c-fos were up-regulated in VSMCs by western blot and immunofluorescence. And the expression of intercellular-adhesion molecule-1 (ICAM-1) was also increased detected by flow cytometry. The strained-induced up-regulation of SIRT6 was blocked by a TGF-β1 neutralizing antibody. Furthermore, the effects of strain on VSMCs were abrogated by SIRT6-specific siRNA transfection via the suppression c-fos and ICAM-1. These results suggest that SIRT6 may play a critical role in the regulation of VSMC differentiation in response to the cyclic strain.

Cyclic tensile stress during physiological occlusal force enhances osteogenic differentiation of human periodontal ligament cells via ERK1/2-Elk1 MAPK pathway

Physiological occlusal force constitutively exists in the oral environment and is important for periodontal homeostasis and remodeling. Cyclic tensile stress (CTS) triggers the biological response of periodontal ligament (PDL). However, a few reports have studied the correlation between CTS during physiological occlusal force and PDL cell activities such as osteogenic differentiation. In the present study, human PDL cells (hPDLCs) were subjected to 10% elongation CTS loading at 0.5 Hz for 24 h, which represents the physiological conditions of occlusal force. Gene expression microarray was used to investigate the mechano-induced differential gene profile and pathway analysis in vitro. The osteogenic relative factors, that is, SPP1, RUNX2, and SP7, were assessed by real-time PCR and Western blot. The involvement of mitogen-activated protein kinase (MAPK) signaling pathways was investigated by Western blot with a specific inhibitor. The expressions of SPP1, RUNX2, SP7, p-ERK1/2, and p-Elk1 were up-regulated after 10% CTS exposure. However, these up-regulated expressions were prevented by ERK1/2 inhibitor U0126 in the physiological occlusal force-applied hPDLCs. These results showed that 10% CTS could enhance osteogenic differentiation of hPDLCs via ERK1/2-Elk1 MAPK pathway, indicating that CTS during physiological occlusal force is a potent agent for PDL remodeling.

Engineering three-dimensional constructs of the periodontal ligament in hyaluronan-gelatin hydrogel films and a mechanically active environment

BACKGROUND AND OBJECTIVE

Periodontal ligament (PDL) cells in stationary two-dimensional culture systems are in a double default state. Our aim therefore was to engineer and characterize three-dimensional constructs, by seeding PDL cells into hyaluronan-gelatin hydrogel films (80-100 μm) in a format capable of being mechanically deformed.

MATERIAL AND METHODS

Human PDL constructs were cultured with and without connective tissue growth factor (CTGF) and fibroblast growth factor (FGF)-2 in (i) stationary cultures, and (ii) mechanically active cultures subjected to cyclic strains of 12% at 0.2 Hz each min, 6 h/d, in a Flexercell FX-4000 Strain Unit. The following parameters were measured: cell number and viability by laser scanning confocal microscopy; cell proliferation with the MTS assay; the expression of a panel of 18 genes using real-time RT-PCR; matrix metalloproteinases (MMPs) 1-3, TIMP-1, CTGF and FGF-2 protein levels in supernatants from mechanically activated cultures with Enzyme-linked immunosorbent assays. Constructs from stationary cultures were also examined by scanning electron microscopy and immunostained for actin and vinculin.

RESULTS

Although initially randomly distributed, the cells became organized into a bilayer by day 7; apoptotic cells remained constant at approximately 5% of the total. CTGF/FGF-2 stimulated cell proliferation in stationary cultures, but relative quantity values suggested modest effects on gene expression. Two transcription factors (RUNX2 and PPARG), two collagens (COL1A1, COL3A1), four MMPs (MMP-1-3, TIMP-1), TGFB1, RANKL, OPG and P4HB were detected by gel electrophoresis and Ct values < 35. In mechanically active cultures, with the exception of P4HB, TGFB1 and RANKL, each was upregulated at some point in the time scale, as was the synthesis of MMPs and TIMP-1. SOX9, MYOD, SP7, BMP2, BGLAP or COL2A1 were not detected in either stationary or mechanically active cultures.

CONCLUSION

Three-dimensional tissue constructs provide additional complexity to monolayer culture systems, and suggest some of the assumptions regarding cell growth, differentiation and matrix turnover based on two-dimensional cultures may not apply to cells in three-dimensional matrices. Primarily developed as a transitional in vitro model for studying cell-cell and cell-matrix interactions in tooth support, the system is also suitable for investigating the pathogenesis of periodontal diseases, and importantly from the clinical point of view, in a mechanically active environment.