Role of mechanical stress-induced glutamate signaling-associated molecules in cytodifferentiation of periodontal ligament cells

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.

Integrated bioinformatic analysis of protein landscape in gingival crevicular fluid unveils sequential bioprocess in orthodontic tooth movement

BACKGROUND

The biological mechanisms driving orthodontic tooth movement (OTM) remain incompletely understood. Gingival crevicular fluid (GCF) is an important indicator of the periodontal bioprocess, providing valuable cues for probing the molecular mechanisms of OTM.

METHODS

A rigorous review of the clinical studies over the past decade was conducted after registering the protocol with PROSPERO and adhering to inclusion criteria comprising human subjects, specified force magnitudes and force application modes. The thorough screening investigated differentially expressed proteins (DEPs) in GCF associated with OTM. Protein-protein interaction (PPI) analysis was carried out using the STRING database, followed by further refinement through Cytoscape to isolate top hub proteins.

RESULTS

A comprehensive summarization of the OTM-related GCF studies was conducted, followed by an in-depth exploration of biomarkers within the GCF. We identified 13 DEPs, including ALP, IL-1β, IL-6, Leptin, MMP-1, MMP-3, MMP-8, MMP-9, PGE2, TGF-β1, TNF-α, OPG, RANKL. Bioinformatic analysis spotlighted the top 10 hub proteins and their interactions involved in OTM. Based on these findings, we have proposed a hypothetic diagram for the time-course bioprocess in OTM, which involves three phases containing sequential cellular and molecular components and their interplay network.

CONCLUSIONS

This work has further improved our understanding to the bioprocess of OTM, suggesting biomarkers as potential modulating targets to enhance OTM, mitigate adverse effects and support real-time monitoring and personalized orthodontic cycles.

CD40-CD40 ligand interaction between periodontal ligament cells and cementoblasts enhances periodontal tissue remodeling in response to mechanical stress

OBJECTIVE

We analyzed the localization and expression of Cluster of differentiation 40 ligand (CD40L) in murine periodontal tissue applied with the orthodontic force to determine the CD40L-expressing cells under mechanical stress. Furthermore, we investigated whether CD40-CD40L interaction played an important role in transducing mechanical stress between periodontal ligament (PDL) cells and cementoblasts and remodeling the periodontal tissue for its homeostasis.

BACKGROUND

PDL is a complex tissue that contains heterogeneous cell populations and is constantly exposed to mechanical stress, such as occlusal force. CD40 is expressed on PDL cells and upregulated under mechanical stress. However, whether its ligand, CD40L, is upregulated in periodontal tissue in response to mechanical stress, and which functions the CD40-CD40L interaction induces by converting the force to biological functions between the cement-PDL complex, are not fully understood.

METHODS

The orthodontic treatment was applied to the first molars at the left side of the upper maxillae of mice using a nickel-titanium closed-coil spring. Immunohistochemistry was performed to analyze the localization of CD40L in the periodontal tissue under the orthodontic force. Human cementoblasts (HCEM) and human PDL cells were stretched in vitro and analyzed CD40L and CD40 protein expression using flow cytometry. A GFP-expressing CD40L plasmid vector was transfected into HCEM (CD40L-HCEM). CD40L-HCEM was co-cultured with human PDL cells with higher alkaline phosphatase (ALP) activity (hPDS) or lower ALP (hPDF). After co-culturing, cell viability and proliferation were analyzed by propidium iodide (PI) staining and bromodeoxyuridine (BrdU) assay. Furthermore, the mRNA expression of cytodifferentiation- and extracellular matrix (ECM)-related genes was analyzed by real-time PCR.

RESULTS

Immunohistochemistry demonstrated that CD40L was induced on the cells present at the cementum surface in periodontal tissue at the tension side under the orthodontic treatment in mice. The flow cytometry showed that the in vitro-stretching force upregulated CD40L protein expression on HCEM and CD40 protein expression on human PDL cells. Co-culturing CD40L-HCEM with hPDF enhanced cell viability and proliferation but did not alter the gene expression related to cytodifferentiation and ECM. In contrast, co-culturing CD40L-HCEM with hPDS upregulated cytodifferentiation- and ECM-related genes but did not affect cell viability and proliferation.

CONCLUSION

We revealed that in response to a stretching force, CD40L expression was induced on cementoblasts. CD40L on cementoblasts may interact with CD40 on heterogeneous PDL cells at the necessary time and location, inducing cell viability, proliferation, and cytodifferentiation, maintaining periodontal tissue remodeling and homeostasis.

Roux-en-Y Gastric Bypass Improves Insulin Sensitivity in Obese Rats with Type 2 Diabetes Mellitus by Regulating the Grin3a/AMPK Signal Axis in Hypothalamic Arcuate Nucleus

Objective

The objective of this study was to explore the effects and related mechanisms of Roux-en-Y gastric bypass (RYGB) on insulin sensitivity in obese rats with type 2 diabetes mellitus (T2DM).

Methods

The obese T2DM rat model was constructed by feeding a high-fat diet and injecting streptozotocin (STZ), and treated with RYGB. Grin3a shRNA was injected into the bilateral hypothalamic arcuate nucleus (ARC) to knockdown the Grin3a expression on T2DM rats. Eight weeks after operation, the body weight, fasting blood glucose (FBG), fasting serum insulin (FSI), homeostatic model assessment of insulin resistance (HOMA-IR), and plasma triglyceride (TG) levels were assessed. Hematoxylin & eosin (H&E) staining was adopted to observe the white adipose tissue (WAT) of rats. Western blot and qRT-PCR were used to detect the expression of Grin3a, adenosine 5' monophosphate-activated protein kinase (AMPK) and p-AMPK in ARC of rats. Later, the plasmid over-expressing or knocking down Grin3a was transfected into differentiated 3T3-L1 adipocytes, and the TG level and the formation of lipid droplets in adipocyte were assessed by TG kit and oil red O staining. The expression of lipogenic transcription factors in cells was detected by qRT-PCR.

Results

RYGB reduced FBG, FSI, HOMA-IR and plasma TG levels in T2DM rats while increasing Grin3a expression and p-AMPK/AMPK ratio in ARC. Knockdown of Grin3a not only reversed the decrease of FBG, FSI, HOMA-IR and plasma TG levels in T2DM rats induced by RYGB, but also reversed the up-regulation of p-AMPK/AMPK ratio in ARC affected by RYGB. Moreover, knocking down Grin3a significantly increased the TG level, promoted the formation of lipid droplets and up-regulated the expressions of lipogenic transcription factors in adipocytes.

Conclusion

RYGB improved the insulin sensitivity, reduced the plasma TG level and lessens the fat accumulation in obese T2DM rats by regulating the Grin3a/AMPK signal in ARC.

Personalized 3D-Printed Scaffolds with Multiple Bioactivities for Bioroot Regeneration

Recent advances in 3D printing offer a prospective avenue for producing transplantable human tissues with complex geometries; however, the appropriate 3D-printed scaffolds possessing the biological compatibility for tooth regeneration remain unidentified. This study proposes a personalized scaffold of multiple bioactivities, including induction of stem cell proliferation and differentiation, biomimetic mineralization, and angiogenesis. A brand-new bioink system comprising a biocompatible and biodegradable polymer is developed and reinforced with extracellular matrix generated from dentin tissue (treated dentin matrix, TDM). Adding TDM optimizes physical properties including microstructure, hydrophilicity, and mechanical strength of the scaffolds. Proteomics analysis reveals that the released proteins of the 3D-printed TDM scaffolds relate to multiple biological processes and interact closely with each other. Additionally, 3D-printed TDM scaffolds establish a favorable microenvironment for cell attachment, proliferation, and differentiation in vitro. The 3D-printed TDM scaffolds are proangiogenic and facilitate whole-thickness vascularization of the graft in a subcutaneous model. Notably, the personalized TDM scaffold combined with dental follicle cells mimics the anatomy and physiology of the native tooth root three months after in situ transplantation in beagles. The remarkable in vitro and in vivo outcomes suggest that the 3D-printed TDM scaffolds have multiple bioactivities and immense clinical potential for tooth-loss therapy.

Lipase-a single-nucleotide polymorphism rs143793106 is associated with increased risk of aggressive periodontitis by negative influence on the cytodifferentiation of human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Aggressive periodontitis (AgP) is characterized by general health and rapid destruction of periodontal tissue. The familial aggregation of this disease highlights the involvement of genetic factors in its pathogeny. We conducted a genome-wide association study (GWAS) to identify AgP-related genes in a Japanese population, and the lipid metabolism-related gene, lipase-a, lysosomal acid type (LIPA), was suggested as an AgP candidate gene. However, there is no report about the expression and function(s) of LIPA in periodontal tissue. Hence, we studied the involvement of how LIPA and its single-nucleotide polymorphism (SNP) rs143793106 in AgP by functional analyses of LIPA and its SNP in human periodontal ligament (HPDL) cells.

MATERIALS AND METHODS

GWAS was performed using the genome database of Japanese AgP patients, and the GWAS result was confirmed using Sanger sequencing. We examined the mRNA expression level of LIPA and the protein expression level of the encoded protein lysosomal acid lipase (LAL) in periodontium-composing cells using conventional and real-time polymerase chain reaction (PCR) and western blotting, respectively. Lentiviral vectors expressing LIPA wild-type (LIPA WT) and LIPA SNP rs143793106 (LIPA mut) were transfected into HPDL cells. Western blotting was performed to confirm the transfection. LAL activity of transfected HPDL cells was determined using the lysosomal acid lipase activity assay. Transfected HPDL cells were cultured in mineralization medium. During the cytodifferentiation of transfected HPDL cells, mRNA expression of calcification-related genes, alkaline phosphatase (ALPase) activity and calcified nodule formation were assessed using real-time PCR, ALPase assay, and alizarin red staining, respectively.

RESULTS

The GWAS study identified 11 AgP-related candidate genes, including LIPA SNP rs143793106. The minor allele frequency of LIPA SNP rs143793106 in AgP patients was higher than that in healthy subjects. LIPA mRNA and LAL protein were expressed in HPDL cells; furthermore, they upregulated the cytodifferentiation of HPDL cells. LAL activity was lower in LIPA SNP-transfected HPDL cells during cytodifferentiation than that in LIPA WT-transfected HPDL cells. In addition, ALPase activity, calcified nodule formation, and calcification-related gene expression levels were lower during cytodifferentiation in LIPA SNP-transfected HPDL cells than those in LIPA WT-transfected HPDL cells.

CONCLUSION

LIPA, identified as an AgP-related gene in a Japanese population, is expressed in HPDL cells and is involved in regulating cytodifferentiation of HPDL cells. LIPA SNP rs143793106 suppressed cytodifferentiation of HPDL cells by decreasing LAL activity, thereby contributing to the development of AgP.

Role of mechano-sensitive non-coding RNAs in bone remodeling of orthodontic tooth movement: recent advances

Orthodontic tooth movement relies on bone remodeling and periodontal tissue regeneration in response to the complicated mechanical cues on the compressive and tensive side. In general, mechanical stimulus regulates the expression of mechano-sensitive coding and non-coding genes, which in turn affects how cells are involved in bone remodeling. Growing numbers of non-coding RNAs, particularly mechano-sensitive non-coding RNA, have been verified to be essential for the regulation of osteogenesis and osteoclastogenesis and have revealed how they interact with signaling molecules to do so. This review summarizes recent findings of non-coding RNAs, including microRNAs and long non-coding RNAs, as crucial regulators of gene expression responding to mechanical stimulation, and outlines their roles in bone deposition and resorption. We focused on multiple mechano-sensitive miRNAs such as miR-21, - 29, -34, -103, -494-3p, -1246, -138-5p, -503-5p, and -3198 that play a critical role in osteogenesis function and bone resorption. The emerging roles of force-dependent regulation of lncRNAs in bone remodeling are also discussed extensively. We summarized mechano-sensitive lncRNA XIST, H19, and MALAT1 along with other lncRNAs involved in osteogenesis and osteoclastogenesis. Ultimately, we look forward to the prospects of the novel application of non-coding RNAs as potential therapeutics for tooth movement and periodontal tissue regeneration.

Association of Early Childhood Caries with Bitter Taste Receptors: A Meta-Analysis of Genome-Wide Association Studies and Transcriptome-Wide Association Study

Although genetics affects early childhood caries (ECC) risk, few studies have focused on finding its specific genetic determinants. Here, we performed genome-wide association studies (GWAS) in five cohorts of children (aged up to 5 years, total N = 2974, cohorts: Center for Oral Health Research in Appalachia cohorts one and two [COHRA1, COHRA2], Iowa Fluoride Study, Iowa Head Start, Avon Longitudinal Study of Parents and Children [ALSPAC]) aiming to identify genes with potential roles in ECC biology. We meta-analyzed the GWASs testing ~3.9 million genetic variants and found suggestive evidence for association at genetic regions previously associated with caries in primary and permanent dentition, including the β-defensin anti-microbial proteins. We then integrated the meta-analysis results with gene expression data in a transcriptome-wide association study (TWAS). This approach identified four genes whose genetically predicted expression was associated with ECC (p-values < 3.09 × 10−6; CDH17, TAS2R43, SMIM10L1, TAS2R14). Some of the strongest associations were with genes encoding members of the bitter taste receptor family (TAS2R); other members of this family have previously been associated with caries. Of note, we identified the receptor encoded by TAS2R14, which stimulates innate immunity and anti-microbial defense in response to molecules released by the cariogenic bacteria, Streptococcus mutans and Staphylococcus aureus. These findings provide insight into ECC genetic architecture, underscore the importance of host-microbial interaction in caries risk, and identify novel risk genes.

Effect of Tension on Human Periodontal Ligament Cells: Systematic Review and Network Analysis

Orthodontic tooth movement is based on the remodeling of tooth-surrounding tissues in response to mechanical stimuli. During this process, human periodontal ligament cells (hPDLCs) play a central role in mechanosensing and mechanotransduction. Various in vitro models have been introduced to investigate the effect of tension on hPDLCs. They provide a valuable body of knowledge on how tension influences relevant genes, proteins, and metabolites. However, no systematic review summarizing these findings has been conducted so far. Aim of this systematic review was to identify all related in vitro studies reporting tension application on hPDLCs and summarize their findings regarding force parameters, including magnitude, frequency and duration. Expression data of genes, proteins, and metabolites was extracted and summarized. Studies' risk of bias was assessed using tailored risk of bias tools. Signaling pathways were identified by protein-protein interaction (PPI) networks using STRING and GeneAnalytics. According to our results, Flexcell Strain Unit^® and other silicone-plate or elastic membrane-based apparatuses were mainly adopted. Frequencies of 0.1 and 0.5 Hz were predominantly applied for dynamic equibiaxial and uniaxial tension, respectively. Magnitudes of 10 and 12% were mostly employed for dynamic tension and 2.5% for static tension. The 10 most commonly investigated genes, proteins and metabolites identified, were mainly involved in osteogenesis, osteoclastogenesis or inflammation. Gene-set enrichment analysis and PPI networks gave deeper insight into the involved signaling pathways. This review represents a brief summary of the massive body of knowledge in this field, and will also provide suggestions for future researches on this topic.

Scleraxis upregulated by transforming growth factor-β1 signaling inhibits tension-induced osteoblast differentiation of priodontal ligament cells via ephrin A2

During tooth movement in orthodontic treatment, bone formation and resorption occur on the tension and compression sides of the alveolar bone, respectively. Although the bone formation activity increases in the periodontal ligament (PDL) on the tension side, the PDL itself is not ossified and maintains its homeostasis, indicating that there are negative regulators of bone formation in the PDL. Our previous report suggested that scleraxis (Scx) has an inhibitory effect on ossification of the PDL on the tension side through the suppression of calcified extracellular matrix formation. However, the molecular biological mechanisms of Scx-modulated inhibition of ossification in the tensioned PDL are not fully understood. The aim of the present study is to clarify the inhibitory role of Scx in osteoblast differentiation of PDL cells and its underlying mechanism. Our in vivo experiment using a mouse experimental tooth movement model showed that Scx expression was increased during early response of the PDL to tensile force. Scx knockdown upregulated expression of alkaline phosphatase, an early osteoblast differentiation marker, in the tensile force-loaded PDL cells in vitro. Transforming growth factor (TGF)-β1-Smad3 signaling in the PDL was activated by tensile force and inhibitors of TGF-β receptor and Smad3 suppressed the tensile force-induced Scx expression in PDL cells. Tensile force induced ephrin A2 (Efna2) expression in the PDL and Efna2 knockdown upregulated alkaline phosphatase expression in PDL cells under tensile force loading. Scx knockdown eliminated the tensile force-induced Efna2 expression in PDL cells. These findings suggest that the TGF-β1-Scx-Efna2 axis is a novel molecular mechanism that negatively regulates the tensile force-induced osteoblast differentiation of PDL cells.

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.

Global gene expression profile of periodontal ligament cells submitted to mechanical loading: A systematic review

OBJECTIVE

To evaluate the evidence reporting gene expression array data of human in vitro cultured periodontal ligament cells (PDLCs) submitted to static mechanical loading compared to a control group.

DESIGN

Systematic searches were performed in MEDLINE/PubMed, Scopus, Web of Science, Virtual Health Library, The Cochrane Library and the System for Information on Grey Literature in Europe up to June 2019. A narrative synthesis was performed to summarize differentially expressed genes (DEGs). These were grouped according to the culture method (2D or 3D), force type (compression or tension) and observation time. Additionally, gene ontology (GO) analysis was performed using the Database for Annotation Visualization and Integrated Discovery. The risk of bias (RoB) and certainty of evidence (CoE) were assessed using a modified CONSORT checklist and the GRADE tool, respectively.

RESULTS

Of eight studies included (all rated as having moderate RoB), only two provided the complete list of DEGs and four studies performed GO, gene network or pathways analysis. "Cell proliferation", "cell-cell signaling", "response to hypoxia and to mechanical stimulus" were among the significantly enriched biological processes in 3D-cultured compressed PDLCs (moderate CoE); while "collagen catabolic process", "extracellular matrix organization" and "cell proliferation" were associated with DEGs of 3D-cultured PDLCs submitted to tension (very low CoE). Biological processes significantly enriched in 2D-cultured PDLCs under compression were "extracellular matrix organization", "canonical glycolysis" and "glycolytic process" (very low CoE).

CONCLUSION

Genes such as NR4A2, NR4A3, NAMPT, PGK1, and REDD1 are suggested as novel biomarkers for orthodontic tooth movement. Limited amount of evidence on the complete gene expression profile and the high heterogeneity in methodologies make it impossible to obtain definite conclusions. New studies following standardized and well-designed in vitro model and reporting complete gene expression datasets are needed.

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.

Periodontal cell mechanotransduction

The periodontium is a structurally and functionally complex tissue that facilitates the anchorage of teeth in jaws. The periodontium consists of various cell types including stem cells, fibroblasts and epithelial cells. Cells of the periodontium are constantly exposed to mechanical stresses generated by biological processes such as the chewing motions of teeth, by flows generated by tongue motions and by forces generated by implants. Mechanical stresses modulate the function of cells in the periodontium, and may play a significant role in the development of periodontal disease. Here, we review the literature on the effect of mechanical forces on periodontal cells in health and disease with an emphasis on molecular and cellular mechanisms.

Effects of mechanical forces on osteogenesis and osteoclastogenesis in human periodontal ligament fibroblasts: A systematic review of in vitro studies

OBJECTIVES

Many in vitro studies have investigated the mechanism by which mechanical signals are transduced into biological signals that regulate bone homeostasis via periodontal ligament fibroblasts during orthodontic treatment, but the results have not been systematically reviewed. This review aims to do this, considering the parameters of various in vitro mechanical loading approaches and their effects on osteogenic and osteoclastogenic properties of periodontal ligament fibroblasts.

METHODS

Specific keywords were used to search electronic databases (EMBASE, PubMed, and Web of Science) for English-language literature published between 1995 and 2017.

RESULTS

A total of 26 studies from the 555 articles obtained via the database search were ultimately included, and four main types of biomechanical approach were identified. Compressive force is characterized by static and continuous application, whereas tensile force is mainly cyclic. Only nine studies investigated the mechanisms by which periodontal ligament fibroblasts transduce mechanical stimulus. The studies provided evidence from in vitro mechanical loading regimens that periodontal ligament fibroblasts play a unique and dominant role in the regulation of bone remodelling during orthodontic tooth movement.

CONCLUSION

Evidence from the reviewed studies described the characteristics of periodontal ligament fibroblasts exposed to mechanical force. This is expected to benefit subsequent research into periodontal ligament fibroblasts and to provide indirectly evidence-based insights regarding orthodontic treatment. Further studies should be performed to explore the effects of static tension on cytomechanical properties, better techniques for static compressive force loading, and deeper analysis of underlying regulatory systems.Cite this article: M. Li, C. Zhang, Y. Yang. Effects of mechanical forces on osteogenesis and osteoclastogenesis in human periodontal ligament fibroblasts: A systematic review of in vitro studies. Bone Joint Res 2019;8:19-31. DOI: 10.1302/2046-3758.81.BJR-2018-0060.R1.

The effect of cyclic tensile force on the actin cytoskeleton organization and morphology of human periodontal ligament cells

To explore Girdin/Akt pathway protein expression and morphology change by cyclic tension in the periodontal ligament cells. Human periodontal ligament cells were exposed to cyclic tension force at 4000 μstrain and 0.5 Hz for 6 h though a four-point bending system. Cyclic tension force upregulated F-actin, Girdin and Akt expression in hPDL. In transmission electron microscope assay showed that there are more and bigger mitochondria, more and longer cynapses, more cellular organisms after tension force stimulation than control. The actin filament was changed to be regular lines and pointed to poles of cells. However, we found that the Girdin-depleted cells are small and there are more micro-organisms including more lysosomes and matrix vesicles than control. These finding suggest that the STAT3/Girdin/Akt pathway in PDL to response to mechanical stimulation as well, and Girdin may play a significant role in triggering cell proliferation and migration during orthodontic treatment. It provided an insight into the molecular basis for development of a vitro cell model in studying orthodontic treatment.

R-spondin 2 promotes osteoblastic differentiation of immature human periodontal ligament cells through the Wnt/β-catenin signaling pathway

OBJECTIVE

In this study, we measured the expression of R-spondin 2 (RSPO2) in periodontal ligament (PDL) tissue and cells. Further, we examined the effects of RSPO2 on osteoblastic differentiation of immature human PDL cells (HPDLCs).

BACKGROUND

R-spondin (RSPO) family proteins are secreted glycoproteins that play important roles in embryonic development and tissue homeostasis through activation of the Wnt/β-catenin signaling pathway. RSPO2, a member of the RSPO family, has been reported to enhance osteogenesis in mice. However, little is known regarding the roles of RSPO2 in PDL tissues.

METHODS

Expression of RSPO2 in rat PDL tissue and primary HPDLCs was examined by immunohistochemical and immunofluorescence staining, as well as by semiquantitative RT-PCR. The effects of stretch loading on the expression of RSPO2 and Dickkopf-related protein 1 (DKK1) were assessed by quantitative RT-PCR. Expression of receptors for RSPOs, such as Leucine-rich repeat-containing G-protein-coupled receptors (LGRs) 4, 5, and 6 in immature human PDL cells (cell line 2-14, or 2-14 cells), was investigated by semiquantitative RT-PCR. Mineralized nodule formation in 2-14 cells treated with RSPO2 under osteoblastic inductive condition was examined by Alizarin Red S and von Kossa stainings. Nuclear translocation of β-catenin and expression of active β-catenin in 2-14 cells treated with RSPO2 were assessed by immunofluorescence staining and Western blotting analysis, respectively. In addition, the effect of Dickkopf-related protein 1 (DKK1), an inhibitor of Wnt/β-catenin signaling, was also examined.

RESULTS

Rat PDL tissue and HPDLCs expressed RSPO2, and HPDLCs also expressed RSPO2, while little was found in 2-14 cells. Expression of RSPO2 as well as DKK1 in HPDLCs was significantly upregulated by exposure to stretch loading. LGR4 was predominantly expressed in 2-14 cells, which expressed low levels of LGR5 and LGR6. RSPO2 enhanced the Alizarin Red S and von Kossa-positive reactions in 2-14 cells. In addition, DKK1 suppressed nuclear translocation of β-catenin, activation of β-catenin, and increases of Alizarin Red S and von Kossa-positive reactions in 2-14 cells, all of which were induced by RSPO2 treatment.

CONCLUSION

RSPO2, which is expressed in PDL tissue and cells, might play an important role in regulating the osteoblastic differentiation of immature human PDL cells through the Wnt/β-catenin signaling pathway.

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.

[The role of extracellular signal regulated kinase 1/2 in mediating osteodifferentiation of human periodontal ligament cells induced by cyclic stretch]

OBJECTIVE

This study aimed to investigate the mechanism of cyclic stretch that promotesthe osteogenic differentiation of human periodontal ligament cells (hPDLCs) through the mediation of extracellular-signal-regulated kinase 1/2 (ERK1/2).

METHODS

hPDLCs were isolated through the explant method and cultured in vitro. hPDLCs were mechanically stimulated by a multi-channel cell-stress-loading system for 1, 3, 6, 12, and 24 h. The magnitude of stretch was 10% deformation, and the frequency was 0.5 Hz. Nonloaded cells were used as control group. ERK1/2 activation was blocked by U0126, a specific ERK1/2 pathway inhibitor. Additionally, hPDLCs were transfected with adenoviral vector encoding dominant negative ERK1/2 (DN-ERK1/2) to continuouslyinhibit ERK1/2 activation. The mRNA and protein levels of target geneswere detected through real-time polymerase chain reaction and Western blot.

RESULTS

Cyclic stretching promoted the expression of ERK1/2, osteocalcin (OCN) mRNA, and bone sialoprotein (BSP) mRNA. The expression of runt-related transcription factor (Runx) 2 protein and mRNA also increased at 3 and 6 h of cyclic stretching. The inhibition of ERK1/2 by U0126 and DN-ERK1/2 suppressed the expressionof Runx2 mRNA, OCN mRNA, BSP mRNA, Runx 2 protein, and p-ERK1/2 protein relative to that in stretched cells without the ERK1/2 inhibitor.

CONCLUSIONS

ERK1/2 is a critical molecule in the mediation ofthe osteogenic differentiation of hPDLCs under mechanical stimulation. ERK1/2 activation induced the elevation of Runx2 protein levels, which may be involved in the stretch-induced expressions of OCN and BSP.

Gene Expression Analysis of CCN Protein in Bone Under Mechanical Stress

To investigate mechanical-dependent bone remodeling, we had previously applied various types of mechanical loading onto the teeth of rats and mice. In vitro cultured bone cells were then used to elucidate the mechanisms underlying the specific phenomenon revealed by in vivo experiments. This review describes the techniques used to upregulate CCN2 expression in bone cells produced by different types of mechanical stress, such as fluid shear stress and substrate strain in vitro, and compression or tension force in vivo.

Verification of γ-Amino-Butyric Acid (GABA) Signaling System Components in Periodontal Ligament Cells In Vivo and In Vitro

CNS key neurotransmitter γ-amino-butyric acid (GABA) and its signaling components are likewise detectable in non-neuronal tissues displaying inter alia immunomodulatory functions. This study aimed at identifying potential glutamate decarboxylase (GAD)65 and GABA receptor expression in periodontal ligament (PDL) cells in vivo and in vitro, with particular regard to inflammation and mechanical loading. Gene expression was analyzed in human PDL cells at rest or in response to IL-1ß (5 ng/ml) or TNFα (5 ng/ml) challenge via qRT-PCR. Western blot determined constitutive receptor expression, and confocal laser scanning fluorescence microscopy visualized expression changes induced by inflammation. ELISA quantified GAD65 release. Immunocytochemistry was performed for GABA component detection in vitro on mechanically loaded PDL cells, and in vivo on rat upper jaw biopsies with mechanically induced root resorptions. Statistical significance was set at p < 0.05. GABAB1, GABAB2, GABAA1, and GABAA3 were ubiquitously expressed both on gene and protein level. GABAA2 and GAD65 were undetectable in resting cells, but induced by inflammation. GABAB1 exhibited the highest basal gene expression (6.97 % ± 0.16). IL-1ß markedly increased GABAB2 on a transcriptional (57.28-fold ± 12.40) and protein level seen via fluorescence microscopy. TNFα-stimulated PDL cells released GAD65 (3.68 pg/ml ± 0.17 after 24 h, 5.77 pg/ml ± 0.65 after 48 h). Immunocytochemistry revealed GAD65 expression in mechanically loaded PDL cells. In vivo, GABA components were varyingly expressed in an inflammatory periodontal environment. PDL cells differentially express GABA signaling components and secrete GAD65. Inflammation and mechanical loading regulate these neurotransmitter molecules, which are also detectable in vivo and are potentially involved in periodontal pathophysiology.

Metabotropic glutamate receptor 1 promotes cementoblast proliferation via MAP kinase signaling pathways

PURPOSE/AIM

Glutamate is one of the signaling molecules responsible for transmission in the central nervous system. Periodontal ligament (PDL) cells were recently reported to express metabotropic glutamate receptors (mGluRs). However, the functions of mGluR signaling in PDL cells or PDL-related cells remain largely unknown. The aim of this study was to investigate the expression and function of mGluRs in PDL-related cells.

MATERIALS AND METHODS

OCCM-30 cells, immortalized murine cementoblasts, were stimulated with l-glutamate or mGluRs antagonists. The cells' proliferative response was evaluated using a colorimetric assay and gene expression was assessed using real-time polymerase chain reaction. The nuclear translocation of cyclin D1 was evaluated by immunohistochemistry.

RESULTS

l-Glutamate promoted the proliferation of OCCM-30 cells, which expressed mGluR1, but not mGluR5. Dihydroxyphenylglycine (DHPG), an agonist of group I mGluRs (mGluR1 and mGluR5), also promoted cell proliferation, and this was inhibited by LY456236, an mGluR1 antagonist. DHPG increased the expression of cyclin D1, a key regulator of cell proliferation, and its nuclear translocation. DHPG also increased the expression of Bcl2A1, an antiapoptotic oncogene and simultaneously reduced the expression of Bax, a pro-apoptotic marker. Furthermore, the DHPG-induced proliferation of OCCM-30 cells was reduced by pretreatment with SB203580, SP600125, and PD98059, inhibitors of p38, JNK, and ERK1/2, respectively.

CONCLUSIONS

These findings indicate that activation of mGluR1 expressed by OCCM-30 cells induces cell proliferation in a manner that is dependent on mitogen-activated protein kinase pathways and that cyclin D1 and Bcl2A1/Bax may be involved. Our results provide useful information for elucidating the mechanisms underlying cementum homeostasis and regeneration.

Transcriptome Reveals Cathepsin K in Periodontal Ligament Differentiation

Periodontal ligaments (PDLs) play an important role in remodeling the alveolar bond and cementum. Characterization of the periodontal tissue transcriptome remains incomplete, and an improved understanding of PDL features could aid in developing new regenerative therapies. Here, we aimed to generate and analyze a large human PDL transcriptome. We obtained PDLs from orthodontic treatment patients, isolated the RNA, and used a vector-capping method to make a complementary DNA library from >20,000 clones. Our results revealed that 58% of the sequences were full length. Furthermore, our analysis showed that genes expressed at the highest frequencies included those for collagen type I, collagen type III, and proteases. We also found 5 genes whose expressions have not been previously reported in human PDL. To access which of the highly expressed genes might be important for PDL cell differentiation, we used real-time polymerase chain reaction to measure their expression in differentiating cells. Among the genes tested, the cysteine protease cathepsin K had the highest upregulation, so we measured its relative expression in several tissues, as well as in osteoclasts, which are known to express high levels of cathepsin K. Our results revealed that PDL cells express cathepsin K at similar levels as osteoclasts, which are both expressed at higher levels than those of the other tissues tested. We also measured cathepsin K protein expression and enzyme activity during cell differentiation and found that both increased during this process. Immunocytochemistry experiments revealed that cathepsin K localizes to the interior of lysosomes. Last, we examined the effect of inhibiting cathepsin K during cell differentiation and found that cathepsin K inhibition stimulated calcified nodule formation and increased the levels of collagen type I and osteocalcin gene expression. Based on these results, cathepsin K seems to regulate collagen fiber accumulation during human PDL cell differentiation into hard tissue-forming cells.

PERK-eIF2α-ATF4 pathway mediated by endoplasmic reticulum stress response is involved in osteodifferentiation of human periodontal ligament cells under cyclic mechanical force

To prevent excess accumulation of unfolded proteins in endoplasmic reticulum (ER), eukaryotic cells have signaling pathways from the ER to the cytosol or nucleus. These processes are known as the endoplasmic reticulum stress (ERS) response. Protein kinase R like endoplasmic reticulum kinase (PERK) is a major transducer of the ERS response and it directly phosphorylate α-subunit of eukaryotic initiation factor 2 (eIF2α), resulting in translational attenuation. Phosphorylated eIF2α specifically promoted the translation of the activating transcription factor 4 (ATF4). ATF4 is a known important transcription factor which plays a pivotal role in osteoblast differentiation and bone formation. Furthermore, ATF4 is a downstream target of PERK. Studies have shown that PERK-eIF2α-ATF4 signal pathway mediated by ERS was involved in osteoblastic differentiation of osteoblasts. We have known that orthodontic tooth movement is a process of periodontal ligament cells (PDLCs) osteodifferentiation and alveolar bone remodeling under mechanical force. However, the involvement of PERK-eIF2α-ATF4 signal pathway mediated by ERS in osteogenic differentiation of PDLCs under mechanical force has not been unclear. In our study, we applied the cyclic mechanical force at 10% elongation with 0.5Hz to mimic occlusal force, and explored whether PERK-eIF2α-ATF4 signaling pathway mediated by ERS involved in osteogenic differentiation of PDLCs under mechanical force. Firstly, cyclic mechanical force will induce ERS and intensify several osteoblast marker genes (ATF4, OCN, and BSP). Next, we found that PERK overexpression increased eIF2α phosphorylation and expression of ATF4, furthermore induced BSP, OCN expression, thus it will promote osteodifferentiation of hPDLCs; mechanical force could promote this effect. However, PERK(-/-) cells showed the opposite changes, which will inhibit osteodifferentiation of hPDLCs. Taken together, our study proved that PERK-eIF2α-ATF4 signaling pathway mediated by ERS involved in osteoblast differentiation of PDLCs under cyclic mechanical force.

Phosphorylation of Runx2, induced by cyclic mechanical tension via ERK1/2 pathway, contributes to osteodifferentiation of human periodontal ligament fibroblasts

Occlusal force is an important stimulus for maintaining periodontal homeostasis. This is attributed to the quality of human periodontal ligament fibroblasts (hPDLFs) that could transfer occlusal force into biological signals modulating osteoblst differentiation. However, few studies investigated the mechanism of occlusal force-induced osteodifferentiation of hPDLFs. In our study, we used the cyclic mechanical tension (CMT) at 10% elongation with 0.5 Hz to mimic occlusal force, and explored its effects on osteogenesis of hPDLFs. Firstly, elevated expressions of several osteoblast marker genes (Runx2, ATF4, SP7, OCN, and BSP), as well as activated ERK1/2 pathway were detected during CMT loading for 1, 3, 6, 12, 18, and 24 h. To gain further insight into how CMT contributed to those effects, we focused on the classic ERK1/2-Runx2 pathway by inhibiting ERK1/2 and overexpressing Runx2. Our results reflected that Runx2 overexpression alone could induce osteodifferentiation of hPDLFs. Meanwhile, CMT loading could intensify while combined ERK1/2 blockage could weaken this process. Furthermore, we found that CMT promoted Runx2 transcription and phosphorylation via ERK1/2; protein level of phospho-Runx2 (p-Runx2), rather than Runx2, was in parallel with mRNA expressions of SP7, OCN, and BSP. Taken together, our study proved that p-Runx2, elevated by CMT via ERK1/2 pathway, is the predominate factor in promoting osteoblast differentiation of hPDLFs.

Mechanical force-induced specific MicroRNA expression in human periodontal ligament stem cells

It remains unclear how the expression of microRNAs (miRNAs) in human periodontal ligament stem cells (PDLSCs) might respond to mechanical stretch. To investigate specific miRNA expression in stretched PDLSCs, we used a Flexcell® FX-5000™ tension system to achieve external mechanical stimulation. Then, a custom-designed microarray assay was performed to investigate and describe the genome-wide differential expression of miRNAs in normal and stretched PDLSCs. Finally, we implemented integrative miRNA target prediction and network analysis approaches to construct an interaction network of the key miRNAs and their putative targets. We found that stretching induced morphological changes and increased alkaline phosphatase (ALP) activity, runt-related transcription factor 2 (RUNX2), osteocalcin (OCN), and bone sialoprotein (BSP) expression in PDLSCs. The microarray data showed that 53 miRNAs were differentially expressed with stretching. With an interaction network, we examined the connections between 10 selected key miRNAs and their putative target genes, which were related to mechanical force. The results from the interaction network provided a basis for postulating the functional roles of miRNAs in PDLSCs.

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.

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.

Adiponectin regulates functions of gingival fibroblasts and periodontal ligament cells

BACKGROUND AND OBJECTIVE

Adiponectin is a cytokine constitutively produced by adipocytes and exhibits multiple biological functions by targeting various cell types. However, the effects of adiponectin on primary gingival fibroblasts and periodontal ligament cells are still unexplored. Therefore, we investigated the effects of adiponectin on gingival fibroblasts and periodontal ligament cells.

MATERIAL AND METHODS

The expression of adiponectin receptors (AdipoR1 and AdipoR2) on human gingival fibroblasts (HGFs), mouse gingival fibroblasts (MGFs) and human periodontal ligament (HPDL) cells was examined using RT-PCR and western blotting. HGFs and MGFs were stimulated with interleukin (IL)-1β in the presence or absence of adiponectin, and the expression of IL-6 and IL-8 at both mRNA and protein levels was measured by real-time PCR and ELISA, respectively. Furthermore, small interfering RNAs (siRNAs) in MGFs were used to knock down the expression of mouse AdipoR1 and AdipoR2. The effects of adiponectin on the expression of alkaline phosphatase (ALP) and runt-related transcription factor 2 (Runx2) genes were evaluated by real-time PCR. Mineralized nodule formation of adiponectin-treated HPDL cells was revealed by Alizarin Red staining.

RESULTS

AdipoR1 and AdipoR2 were expressed constitutively in HGFs, MGFs and HPDL cells. Adiponectin decreased the expression of IL-6 and IL-8 in IL-1β-stimulated HGFs and MGFs. AdipoR1 siRNA in MGFs revealed that the effect of adiponectin on reduction of IL-6 expression was potentially mediated via AdipoR1. Adiponectin-treated HPDL cells promoted the expression of ALP and Runx2 mRNAs and up-regulated ALP activity. Furthermore, adiponectin enhanced mineralized nodule formation of HPDL cells.

CONCLUSION

Our observations demonstrate that adiponectin exerts anti-inflammatory effects on HGFs and MGFs, and promotes the activities of osteoblastogenesis of HPDL cells. We conclude that adiponectin has potent beneficial functions to maintain the homeostasis of periodontal health, improve periodontal lesions, and contribute to wound healing and tissue regeneration.

Implications of cultured periodontal ligament cells for the clinical and experimental setting: a review

The periodontal ligament (PDL) is a key contributor to the process of regeneration of the periodontium. The heterogeneous nature of the PDL tissue, its development during early adulthood, and the different conditions to which the PDL tissue is exposed to in vivo impart on the PDL unique characteristics that may be of consequence during its cultivation in vitro. Several factors affecting the in vivo setting influence the behaviour of PDL fibroblasts in culture. The purpose of this review is to address distinct factors that influence the behaviour of PDL fibroblasts in culture -in vivo-in vitro transitions, cell identification/isolation markers, primary PDL cultures and cell lines, tooth-specific factors, and donor-specific factors. Based on the reviewed studies, the authors recommendations include the use of several identification markers to confirm cell identity, use of primary cultures at early passage to maintain unique PDL heterogeneic characteristics, and noting donor conditions such as age, systemic health status, and tooth health status. Continued efforts will expand our understanding of the in vitro and in vivo behaviour of cells, with the goal of orchestrating optimal periodontal regeneration. This understanding will lead to improved evidence-based rationales for more individualized and predictable periodontal regenerative therapies.

Genomic predictors of the maximal O₂ uptake response to standardized exercise training programs

Low cardiorespiratory fitness is a powerful predictor of morbidity and cardiovascular mortality. In 473 sedentary adults, all whites, from 99 families of the Health, Risk Factors, Exercise Training, and Genetics (HERITAGE) Family Study, the heritability of gains in maximal O(2) uptake (VO(2max)) after exposure to a standardized 20-wk exercise program was estimated at 47%. A genome-wide association study based on 324,611 single-nucleotide polymorphisms (SNPs) was undertaken to identify SNPs associated with improvements in VO(2max) Based on single-SNP analysis, 39 SNPs were associated with the gains with P < 1.5 × 10(-4). Stepwise multiple regression analysis of the 39 SNPs identified a panel of 21 SNPs that accounted for 49% of the variance in VO(2max) trainability. Subjects who carried ≤9 favorable alleles at these 21 SNPs improved their VO(2max) by 221 ml/min, whereas those who carried ≥19 of these alleles gained, on average, 604 ml/min. The strongest association was with rs6552828, located in the acyl-CoA synthase long-chain member 1 (ACSL1) gene, which accounted by itself for ~6% of the training response of VO(2max). The genes nearest to the SNPs that were the strongest predictors were PR domain-containing 1 with ZNF domain (PRDM1); glutamate receptor, ionotropic, N-methyl-D-aspartate 3A (GRIN3A); K(+) channel, voltage gated, subfamily H, member 8 (KCNH8); and zinc finger protein of the cerebellum 4 (ZIC4). The association with the SNP nearest to ZIC4 was replicated in 40- to 65-yr-old, sedentary, overweight, and dyslipidemic subjects trained in Studies of a Targeted Risk Reduction Intervention Through Defined Exercise (STRRIDE; n = 183). Two SNPs were replicated in sedentary obese white women exercise trained in the Dose Response to Exercise (DREW) study (n = 112): rs1956197 near dishevelled associated activator of morphogenesis 1 (DAAM1) and rs17117533 in the vicinity of necdin (NDN). The association of SNPs rs884736 in the calmodulin-binding transcription activator 1 (CAMTA1) locus and rs17581162 ~68 kb upstream from regulator of G protein signaling 18 (RGS18) with the gains in VO(2max) in HERITAGE whites were replicated in HERITAGE blacks (n = 247). These genomic predictors of the response of Vo(2max) to regular exercise provide new targets for the study of the biology of fitness and its adaptation to regular exercise. Large-scale replication studies are warranted.