Treponema denticola outer membrane enhances the phagocytosis of collagen-coated beads by gingival fibroblasts

Mechanisms of IL-8 suppression by Treponema denticola in gingival epithelial cells

The purpose of this study was to investigate the mechanism(s) of interleukin (IL)-8 suppression by Treponema denticola, one of the major periodontal pathogens, in gingival epithelial cells. Immortalized human gingival epithelial HOK-16B cells were infected with wild-type (WT), dentilisin-deficient (K1) or flagellin-deficient (flgE) T. denticola in the presence or absence of 2% human serum for 24 h. The levels of IL-8 expression were measured with real-time reverse transcription PCR and ELISA. In the absence of human serum, the WT and flgE, but not K1, substantially reduced not only the levels of IL-8 protein but also of IL-8 mRNA. Such downregulation of IL-8 mRNA was independent of bacterial invasion. Degradation of cytokine mixture by the WT, K1 and flgE revealed dentilisin-dependent preferential degradation of tumor necrosis factor (TNF)-α, an IL-8-inducing cytokine. WT and flgE significantly decreased the levels of TNFα secreted by HOK-16B cells, suggesting modulation of IL-8 through dentilisin-mediated degradation of TNFα. The addition of human serum to the culture potentiated the suppressive effect of T. denticola, resulting in substantial reductions of IL-8 and TNFα levels, even by K1. The serum-dependent effects of T. denticola were attributed to its ability to suppress the accumulation of intracellular reactive-oxygen species (ROS), a group of ubiquitous signaling molecules. Pretreatment with an antioxidant suppressed TNFα-induced IL-8 expression, confirming the role of ROS in TNFα signaling. Collectively, T. denticola targeted a key inflammatory cytokine and its signaling molecule to modulate the host innate immune response, which provides a new insight into modulation of host immunity by a periodontal pathogen.

The major outer sheath protein of Treponema denticola inhibits the binding step of collagen phagocytosis in fibroblasts

Bacterial infections contribfute to the chronicity of connective tissue lesions in part by perturbing extracellular matrix remodelling processes. We examined a novel mechanism by which the major outer sheath protein (Msp) of the spirochaete Treponema denticola disrupts matrix remodelling mediated by intracellular digestion of collagen. The initial collagen-binding step of phagocytosis was examined in human gingival fibroblasts and Rat-2 fibroblasts. Cells were pretreated with Msp or vehicle, and binding of collagen-coated beads was measured by flow cytometry. Exposure to Msp induced a dose- and time-dependent decrease in cells that bound collagen beads; the inhibition of binding was reversed by absorption with anti-Msp antibodies. Msp-treated fibroblasts remained viable but underwent actin reorganization, including the assembly of a dense meshwork of subcortical actin filaments. Shear force assays showed that Msp abrogated collagen-binding interactions in the minimal affinity range required for stable adhesion. Fluorescence microscopy and immunoblotting showed equivalent amounts of beta1 integrin associated with collagen beads bound to Msp- and vehicle-treated cells. Photobleaching experiments found a similar percentage mobile fraction of beta1 integrins recovered in bleached areas of the plasma membrane. In contrast, Msp-induced inhibition of collagen binding was reversed by beta1 integrin affinity-activating antibodies and by latrunculin B, which prevented subcortical actin assembly. We conclude that native Msp of T. denticola inhibits the binding step of collagen phagocytosis in fibroblasts by inducing subcortical actin filament assembly and restricting affinity modulation of beta1 integrins. We suggest that, like Msp, bacterial toxins that target the cytoskeleton may also perturb the signalling networks required for cellular engagement of matrix ligands.

Regulation of smooth muscle actin expression and contraction in adult human mesenchymal stem cells

Prior studies have demonstrated the expression of a contractile actin isoform, alpha-smooth muscle actin, in bone marrow stromal cells. One objective of the current study was to correlate contractility with alpha-smooth muscle actin expression in human bone marrow stroma-derived mesenchymal stem cells. A second objective was to determine the effects of transforming growth factor-beta1, platelet derived growth factor-BB, and a microfilament-modifying agent on alpha-smooth muscle actin expression and alpha-smooth muscle actin-enabled contraction. Adult human bone marrow stromal cells were passaged in monolayer and their inducibility to chondrocytic, osteoblastic, and adipogenic phenotypes was demonstrated. Western blot analysis was employed along with densitometry to quantify the alpha-smooth muscle actin content of the cells and their contractility was evaluated by their contraction of a type I collagen-glycosaminoglycan sponge-like matrix into which they were seeded. Transforming growth factor-beta1 (1 ng/ml) significantly increased and platelet-derived growth factor-BB (10 ng/ml) decreased alpha-smooth muscle actin expression and the contractility of the cells. Cytochalasin D also blocked cell contraction. There was a notably high correlation of cell-mediated contraction normalized to the DNA content of the matrices with alpha-smooth muscle actin content of the cells by linear regression analysis (R(2) = 0.88). These findings lay the groundwork for considering the role of alpha-smooth muscle actin-enabled contraction in mesenchymal stem cells and in their connective tissue cell progeny.

Role of Treponema denticola in periodontal diseases

Among periodontal anaerobic pathogens, the oral spirochetes, and especially Treponema denticola, have been associated with periodontal diseases such as early-onset periodontitis, necrotizing ulcerative gingivitis, and acute pericoronitis. Basic research as well as clinical evidence suggest that the prevalence of T denticola, together with other proteolytic gram-negative bacteria in high numbers in periodontal pockets, may play an important role in the progression of periodontal disease. The accumulation of these bacteria and their products in the pocket may render the surface lining periodontal cells highly susceptible to lysis and damage. T. denticola has been shown to adhere to fibroblasts and epithelial cells, as well as to extracellular matrix components present in periodontal tissues, and to produce several deleterious factors that may contribute to the virulence of the bacteria. These bacterial components include outer-sheath-associated peptidases, chymotrypsin-like and trypsin-like proteinases, hemolytic and hemagglutinating activities, adhesins that bind to matrix proteins and cells, and an outer-sheath protein with pore-forming properties. The effects of T. denticola whole cells and their products on a variety of host mucosal and immunological cells has been studied extensively (Fig. 1). The clinical data regarding the presence of T. denticola in periodontal health and disease, together with the basic research results involving the role of T. denticola factors and products in relation to periodontal diseases, are reviewed and discussed in this article.

A spirochete surface protein uncouples store-operated calcium channels in fibroblasts: a novel cytotoxic mechanism

The cytotoxicity of infectious agents can be mediated by disruption of calcium signaling in target cells. Outer membrane proteins of the spirochete Treponema denticola, a periodontal pathogen, inhibit agonist-induced Ca(2+) release from internal stores in gingival fibroblasts, but the mechanism is not defined. We determined here that the major surface protein (Msp) of T. denticola perturbs calcium signaling in human fibroblasts by uncoupling store-operated channels. Msp localized in complexes on the cell surface. Ratio fluorimetry showed that in cells loaded with fura-2 or fura-C18, Msp induced cytoplasmic and near-plasma membrane Ca(2+) transients, respectively. Increased conductance was confirmed by fluorescence quenching of fura-2-loaded cells with Mn(2+) after Msp treatment. Calcium entry was blocked with anti-Msp antibodies and inhibited by chelating external Ca(2+) with EGTA. Msp pretreatment reduced the amplitude of [Ca(2+)](i) transients upon challenge with ATP or thapsigargin. In experiments using cells loaded with mag-fura-2 to report endoplasmic reticulum Ca(2+), Msp reduced Ca(2+) efflux from endoplasmic reticulum stores when ATP was used as an agonist. Msp alone did not induce Ca(2+) release from these stores. Msp inhibited store-operated influx of extracellular calcium following intracellular Ca(2+) depletion by thapsigargin and also promoted the assembly of subcortical actin filaments. This actin assembly was blocked by chelating intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester. The reduced amplitude of agonist-induced transients and inhibition of store-operated Ca(2+) entry due to Msp were reversed by latrunculin B, an inhibitor of actin filament assembly. Thus, Msp retards Ca(2+) release from endoplasmic reticulum stores, and it inhibits subsequent Ca(2+) influx by uncoupling store-operated channels. Actin filament rearrangement coincident with conformational uncoupling of store-operated calcium fluxes is a novel mechanism by which surface proteins and toxins of pathogenic microorganisms may damage host cells.

Involvement of actin filaments and integrins in the binding step in collagen phagocytosis by human fibroblasts

In physiological conditions, collagen degradation by fibroblasts occurs primarily via phagocytosis, an intracellular pathway that is thought to require collagen receptors and actin assembly for fibril internalization and degradation. Currently it is unclear which specific steps of collagen phagocytosis in fibroblasts involve actin filament assembly. As studies of phagocytosis in fibroblasts are complicated by the relatively slow rate of particle internalization compared to professional phagocytes, we have examined the role of collagen receptors and actin only in the initial collagen binding step. Prior to the binding of collagen-coated fluorescent beads by human gingival fibroblasts, a cell type that is avidly phagocytic in vitro, cells were treated with cytochalasin D (actin filament barbed-end capping) or swinholide A (actin dimer sequestering and severing) or latrunculin B (actin monomer sequestering). Bead binding and immunostaining of (alpha)(2)(beta)(1) and (alpha)(3)(beta)(1) integrin collagen receptors were measured by flow cytometry. After 1–3 hours of coincubation with beads, cytochalasin D or swinholide A eliminated actin filaments stained by rhodamine-phalloidin and inhibited collagen bead binding (reductions of 25% and 50%, respectively), possibly because of cell rounding and restricted interactions with beads. In contrast, latrunculin enhanced binding dose-dependently over controls (twofold at 1 microM) and induced the formation of brightly staining aggregates of actin and the retention of long cytoplasmic extensions. Latrunculin also reduced surface (beta)(1), (alpha)(2) and (alpha)(3) integrin staining up to 40% in bead-free and bead-loaded cells, indicating that latrunculin enhanced collagen receptor internalization. As determined by fluorescence recovery after photobleaching, latrunculin increased the mobility of surface-bound (beta)(1) integrin. The stimulatory effect of latrunculin on collagen bead binding was reduced to control levels by treatment with a (beta)(1) integrin inactivating antibody while a (beta)(1) integrin blocking antibody abrogated both bead binding and the latrunculin-induced stimulation. Immunoblotting of bead-associated proteins showed that latrunculin completely eliminated binding of (beta)-actin to collagen beads but did not affect (beta)(1) integrin binding. These data indicate that latrunculin-induced sequestration of actin monomers facilitates the disengagement of actin from (beta)(1) integrin receptors, increases collagen bead binding and enhances collagen receptor mobility. We suggest that these alterations increase the probability of adhesive bead-to-cell interactions.

Association of fibroblast orientation around titanium in vitro with expression of a muscle actin

The objective of this study was to investigate the association of cell orientation around a biomaterial with expression of a contractile actin isoform. Selected cytokines and a fungal metabolite known to alter the cytoskeleton were used to modulate the fibroblast orientation around titanium in vitro and the synthesis of a specific muscle actin in order to reveal an association between these processes. A novel culture system using a fibronectin-coated silicone surface was employed to evaluate the orientation of human gingival fibroblasts around titanium discs. Round glass cover slips, 25 mm in diameter, were coated with polydimethylsiloxane. During the heat-induced polymerization process, two commercially pure titanium discs, 5 mm in diameter, were placed on the silicone at a distance of approximately 0.5 mm apart. The rubbery consistency of the silicone stabilized the metal discs on the cover slip and eliminated the risk of developing a lip at the edge of the titanium sample. The cover slip was then heated to complete polymerization of the silicone and subsequently coated with fibronectin. One hundred thousand human gingival fibroblasts were plated onto each glass cover slip containing the titanium discs. The cells were treated with one of the following prior to seeding on the cover slips: transforming growth factor-beta1 (TGF-beta1), platelet-derived growth factor-BB (PDGF-BB), interferon-gamma (IFN-gamma) for cytochalasin-D. Untreated cells served as controls. The orientation of the cells at the surface of the titanium discs was evaluated microscopically and the cell content of alpha-smooth muscle actin (SMA) was determined by Western blot analysis and immunohistochemistry. A notable finding was the high correlation between the percentage of cells oriented perpendicular to the titanium surface and SMA synthesis. TGF-beta1, IFN-gamma and cytochalasin-D increased synthesis of SMA while PDGF-BB decreased it. The findings support the proposition that SMA-enabled cell contraction may play a role in the orientation of cells to a biomaterial surface.

Perturbation and exploitation of host cell cytoskeleton by periodontal pathogens

Some periodontal pathogens disrupt epithelial barriers and cellular adhesion to the extracellular matrix, which affects the cytoskeleton. Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans exploit the cytoskeleton during their uptake by epithelial cells. Treponema denticola perturbs actin and actin-regulating pathways in host cells. Cytoskeletal dysfunction due to pathogenic bacteria may impair physiologic remodeling and wound repair in the periodontium.