Molecular imaging assessment of periodontitis lesions in an experimental mouse model.

OBJECTIVE: We aimed to evaluate molecular imaging as a novel diagnostic tool for mice periodontitis model induced by ligature and Porphyromonas gingivalis (Pg) inoculation. MATERIALS AND METHODS: Twelve female mice were assigned to the following groups: no treatment as control group (n = 4); periodontitis group induced by ligature and Pg as Pg group (n = 4); and Pg group treated with glycyrrhizinic acid (GA) as Pg + GA group (n = 4). All mice were administered a myeloperoxidase (MPO) activity-specific luminescent probe and observed using a charge-coupled device camera on day 14. Image analysis on all mice was conducted using software to determine the signal intensity of inflammation. Additionally, histological and radiographic evaluation for periodontal inflammation and bone resorption at the site of periodontitis, and quantitative enzyme-linked immunosorbent assay (ELISA) were conducted on three mice for each group. Each experiment was performed three times. RESULTS: Levels of serum IgG antibody against P. gingivalis were significantly higher in the Pg than in the Pg + GA group. Histological analyses indicated that the number of osteoclasts and neutrophils were significantly lower in the Pg + GA than in the Pg group. Micro-CT image analysis indicated no difference in bone resorption between the Pg and Pg + GA groups. The signal intensity of MPO activity was detected on the complete craniofacial image; moreover, strong signal intensity was localized specifically at the periodontitis site in the ex vivo palate, with group-wise differences. CONCLUSIONS: Molecular imaging analysis based on MPO activity showed high sensitivity of detection of periodontal inflammation in mice. CLINICAL RELEVANCE: Molecular imaging analysis based on MPO activity has potential as a diagnostic tool for periodontitis.

Induction of migration of periodontal ligament cells by selective regulation of integrin subunits.

The recruitment of tissue-resident stem cells is important for wound regeneration. Periodontal ligament cells (PDL cells) are heterogeneous cell populations with stemness features that migrate into wound sites to regenerate periodontal fibres and neighbouring hard tissues. Cell migration is regulated by the local microenvironment, coordinated by growth factors and the extracellular matrix (ECM). Integrin-mediated cell adhesion to the ECM provides essential signals for migration. We hypothesized that PDL cell migration could be enhanced by selective expression of integrins. The migration of primary cultured PDL cells was induced by platelet-derived growth factor-BB (PDGF-BB). The effects of blocking specific integrins on migration and ECM adhesion were investigated based on the integrin expression profiles observed during migration. Up-regulation of integrins α3, α5, and fibronectin was identified at distinct localizations in migrating PDL cells. Treatment with anti-integrin α5 antibodies inhibited PDL cell migration. Treatment with anti-integrin α3, α3-blocking peptide, and α3 siRNA significantly enhanced cell migration, comparable to treatment with PDGF-BB. Furthermore, integrin α3 inhibition preferentially enhanced adhesion to fibronectin via integrin α5. These findings indicate that PDL cell migration is reciprocally regulated by integrin α3-mediated inhibition and α5-mediated promotion. Thus, targeting integrin expression is a possible therapeutic strategy for periodontal regeneration.

HMGB1-induced inflammatory response promotes bone healing in murine tooth extraction socket.

High mobility group box 1 (HMGB1) is a non-histone DNA-binding protein that is secreted into the extracellular milieu in response to inflammatory stimuli. The secreted HMGB1 has been suggested to mediate various inflammatory diseases. However, it is still unknown whether HMGB1 is involved in a healing process in the tooth extraction socket, the tissue containing gingival epithelium, and alveolar bone that is exposed to oral bacteria. In this study, we constructed a murine tooth extraction model with anti-HMGB1 neutralization antibody administration and observed the inflammatory response and bone healing process in tooth extraction sockets by molecular imaging of myeloperoxidase (MPO) activity, histological analysis, and quantitative RT-PCR. The translocation of HMGB1 from the nucleus to the cytoplasm in gingival epithelial cells and inflammatory cells was inhibited by anti-HMGB1 antibody administration. The MPO activity around the tooth extraction socket was significantly reduced, and the numbers of CD31- and CD68-positive cells were significantly lower in the anti-HMGB1 antibody treatment samples than in the control samples. The TRAP-positive cells, osteocalcin positive cells, and the neoplastic bone area were significantly lower in anti-HMGB1 antibody treatment samples than in control samples. The expression levels of IL-1ß and VEGF-A were also decreased in anti-HMGB1 antibody treatment samples compared to that in control samples. Secreted HMGB1 induced initial acute inflammation and inflammatory cells recruitment after tooth extraction. HMGB1 was associated with angiogenesis and bone remodeling by osteoclast and osteoblast activation and promoted bone healing in the tooth extraction socket.

Modulation of microenvironment for controlling the fate of periodontal ligament cells: the role of Rho/ROCK signaling and cytoskeletal dynamics.

Cells behave in a variety of ways when they perceive changes in their microenvironment; the behavior of cells is guided by their coordinated interactions with growth factors, niche cells, and extracellular matrix (ECM). Modulation of the microenvironment affects the cell morphology and multiple gene expressions. Rho/Rho-associated coiled-coil-containing protein kinase (ROCK) signaling is one of the key regulators of cytoskeletal dynamics and actively and/or passively determines the cell fate, such as proliferation, migration, differentiation, and apoptosis, by reciprocal communication with the microenvironment. During periodontal wound healing, it is important to recruit the residential stem cells into the defect site for regeneration and homeostasis of the periodontal tissue. Periodontal ligament (PDL) cells contain a heterogeneous fibroblast population, including mesenchymal stem cells, and contribute to the reconstruction of tooth-supporting tissues. Therefore, bio-regeneration of PDL cells has been the ultimate goal of periodontal therapy for decades. Recent stem cell researches have shed light on intrinsic ECM properties, providing paradigm shifts in cell fate determination. This review focuses on the role of ROCK activity and the effects of Y-27632, a specific inhibitor of ROCK, in the modulation of ECM-microenvironment. Further, it presents the current understanding of how Rho/ROCK signaling affects the fate determination of stem cells, especially PDL cells. In addition, we have also discussed in detail the underlying mechanisms behind the reciprocal response to the microenvironment.

Aggregatibacter actinomycetemcomitans regulates the expression of integrins and reduces cell adhesion via integrin α5 in human gingival epithelial cells.

Gingival epithelial cells form a physiological barrier against bacterial invasion. Excessive bacterial invasion destroys the attachment between the tooth surface and the epithelium, resulting in periodontitis. Integrins play a significant role in cell attachment; therefore, we hypothesized that bacterial infection might decrease the expressions of these integrins in gingival epithelial cells, resulting in reduced cell adhesion. Immortalized human gingival epithelial cells were co-cultured with Aggregatibacter actinomycetemcomitans Y4 (Aa Y4), and the gene expression levels of IL-8, proliferating cell nuclear antigen (PCNA), and integrins (α2, α3, α5, ß4, and ß6) were measured using quantitative reverse transcription polymerase chain reaction. Expression of PCNA and integrins, except integrin α5, was significantly downregulated, while expression of IL-8 and integrin α5 was significantly upregulated in the cells co-cultured with Aa Y4. The number of adherent cells significantly decreased when co-cultured with Aa Y4, as determined using cell adhesion assays. In the cells co-cultured with Aa Y4 and an integrin α5 neutralizing antibody, there was no effect on the expression of IL-8 and PCNA, while the expressions of integrins α2, α3, ß4, and ß6, and the number of adherent cells did not decrease. The number of invading bacteria in the cells was reduced in the presence of the antibody and increased in the presence of TLR2/4 inhibitor. Therefore, integrin α5 might be involved in Aa Y4 invasion into gingival epithelial cells, and the resulting signal transduction cascade reduces cell adhesion by decreasing the expression of integrins, while the TLR2/4 signaling cascade regulates IL-8 expression.

Smad2 overexpression enhances adhesion of gingival epithelial cells.

OBJECTIVE: Gingival epithelial cells play an important role in preventing the initiation of periodontitis, by their hemidesmosomal adhesion to the tooth root surface. Adhesion requires integrin-extracellular matrix (ECM) interactions that are intricately regulated by transforming growth factor-ß (TGF-ß) signaling. However, the mechanisms underlying the interplay between adhesion molecules and TGF-ß, especially the respective roles of Smad2 and Smad3, remain elusive. In this study, we examined the effects of Smad overexpression on gingival epithelial cell adhesion and expression profiles of integrin and ECM-related genes. METHODS: Human gingival epithelial cells immortalized by the SV40 T-antigen were transfected with Smad2- and Smad3-overexpression vectors. A cell adhesion assay involving fluorescence detection of attached cells was performed using the ArrayScan imaging system. Real-time PCR was performed to examine the kinetics of integrin and ECM gene expression. In vitro and in vivo localization of adhesion molecules was examined by immunofluorescence analysis. RESULTS: By using SB431542, a specific inhibitor of the TGF-ß type I receptor, Smad2/3 signaling was confirmed to be dominant in TGF-ß1-induced cell adhesion. The Smad2-transfectant demonstrated higher potency for cell adhesion and integrin expression (α2, α5, ß4, and ß6) than the Smad3-transfectant, whereas little or no change in ECM expression was observed in either transfectant. Moreover, the gingival epithelium of transgenic mice that overexpressed Smad2 driven by the keratin 14 promoter showed increased integrin α2 expression. CONCLUSION: These findings indicate the crucial role of Smad2 in increased adhesion of gingival epithelial cells via upregulation of integrin α2.

Osteogenic differentiation regulated by Rho-kinase in periodontal ligament cells.

The periodontal ligament is a multifunctional soft connective tissue, which functions not only as a cushion supporting the teeth against occlusal force, but is also a source of osteogenic cells that can regenerate neighboring hard tissues. Periodontal ligament cells (PDL cells) contain heterogeneous cell populations, including osteogenic cell progenitors. However, the precise mechanism underlying the differentiation process remains elusive. Cell differentiation is regulated by the local biochemical and mechanical microenvironment that can modulate gene expression and cell morphology by altering actin cytoskeletal organization mediated by Rho-associated, coiled-coil containing protein kinase (ROCK). To determine its role in PDL cell differentiation, we examined the effects of ROCK on cytoskeletal changes and kinetics of gene expression during osteogenic differentiation. PDL cells were isolated from human periodontal ligament on extracted teeth and cultured in osteogenic medium for 14 days. Y-27632 was used for ROCK inhibition assay. Osteogenic phenotype was determined by monitoring alkaline phosphatase (ALP) activity and calcium deposition by Alizarin Red staining. ROCK-induced cytoskeletal changes were examined by immunofluorescence analysis of F-actin and myosin light chain 2 (MLC2) expression. Real-time PCR was performed to examine the kinetics of osteogenic gene expression. F-actin and phospho-MLC2 were markedly induced during osteogenic differentiation, which coincided with upregulation of ALP activity and mineralization. Subsequent inhibition assay indicated that Y-27632 significantly inhibited F-actin and phospho-MLC2 expression in a dose-dependent manner with concomitant partial reversal of the PDL cell osteogenic phenotype. PCR array analysis of osteogenic gene expression indicated that extracellular matrix genes, such as fibronectin 1, collagen type I and III, and biglycan, were significantly downregulated by Y27632. These findings indicated crucial effects of ROCK in cytoskeletal reorganization and differentiation of PDL cells toward osteogenic cells. ROCK contributes to induction of osteogenic differentiation by synergistic increases in extracellular matrix gene expression in PDL cells.