Characterization of Porphyromonas gingivalis-induced degradation of epithelial cell junctional complexes

Role for fimbriae and lysine-specific cysteine proteinase gingipain K in expression of interleukin-8 and monocyte chemoattractant protein in Porphyromonas gingivalis-infected endothelial cells

Recent cross-sectional and prospective epidemiological studies have demonstrated an association between periodontal disease and atherosclerosis and human coronary heart disease. Previously, we have established that the periodontal pathogen Porphyromonas gingivalis is capable of invading aortic, heart, and human umbilical vein endothelial cells (HUVEC). Since atherosclerosis is a chronic inflammatory response initiated at the vascular wall, interactions of P. gingivalis with endothelial cells and the subsequent host cell response to infection may be important in the pathogenesis of atherosclerosis. In this study we examined the consequences of P. gingivalis infection of HUVEC on the expression of the chemokines interleukin-8 (IL-8) and monocyte chemotactic protein 1 (MCP-1). HUVEC were found to constitutively produce low levels of IL-8 and MCP-1. The addition of P. gingivalis fimbrillin-specific peptides, lipopolysaccharides (LPS), or heat-killed whole cell preparations to HUVEC stimulated modest IL-8 and MCP-1 responses. In contrast, coculture of HUVEC with live P. gingivalis strain A7436, 33277, or 381 abolished the IL-8 and MCP-1 responses. Inhibition of IL-8 and MCP-1 production was not dependent on bacterial adherence since similar results were obtained with the nonadherent P. gingivalis fimA mutant DPG3 or when P. gingivalis was preincubated with fimbrillin peptide antisera prior to the addition to HUVEC. Furthermore, treatment of P. gingivalis-infected HUVEC with cytochalsin D, which prevented P. gingivalis invasion, also abolished the constitutive IL-8 and MCP-1 responses. Treatment of HUVEC with E. coli LPS stimulated robust IL-8 and MCP-1 responses that were abolished when stimulated cells were cocultured with live P. gingivalis. Analysis of P. gingivalis-infected HUVEC cultures by an RNase protection assay revealed an increase in the IL-8 transcript relative to uninfected HUVEC. Pretreatment of P. gingivalis with protease inhibitors prior to the addition to HUVEC prevented the inhibition of IL-8 and MCP-1 production in P. gingivalis-infected HUVEC, indicating that the inhibition was proteolytically mediated. Coculture of HUVEC with a P. gingivalis mutant deficient in lysine-specific cysteine proteinase (gingipain K [Kgp]) resulted in an increase in both IL-8 transcription and protein expression relative to that observed in HUVEC cocultured with the P. gingivalis wild-type strain. These results indicate that P. gingivalis can temporally modulate the chemokine response in endothelial cells through both fimbriae and gingipain-mediated mechanisms.

Microbial/host interactions: mechanisms involved in host responses to microbial antigens

The indigenous oral microflora and the host are normally in a state of equilibrium; however, the introduction of a pathogen can result in innate and adaptive immune responses that either contribute to the development of the disease or lead to host immunity. The interactions between the microorganisms and the host are very dynamic, thus allowing the complex interplay between host molecules and bacterial antigens. In this article, we focus on the mechanisms involved in the pathogenesis of and host responses to two oral pathogens: the Gram-negative bacterium Porphyromonas gingivalis, implicated in the etiology of periodontal disease, and the Gram-positive Streptococcus mutans, a primary agent involved in dental caries formation. Furthermore, we address mechanisms involved in the ability of select adjuvants and delivery systems to potentiate mucosal and systemic immune responses to microbial vaccine antigens.

Hepatocyte growth factor alters renal epithelial cell susceptibility to uropathogenic Escherichia coli

The urinary tract is frequently the source of Escherichia coli bacteremia. Bacteria from the urinary tract must cross an epithelial layer to enter the bloodstream. Hepatocyte growth factor (HGF) alters the polarity of Madin-Darby canine kidney (MDCK) epithelial cells. The role of cell polarity in determining renal epithelial resistance to Escherichia coli invasion is not well known. A model of polarized and HGF-treated MDCK epithelial cells grown on filters was used to study the role of epithelial cell polarity during the interaction of nonvirulent (XL1-Blue) and uropathogenic (J96) strains of Escherichia coli with renal epithelium. Basolateral exposure of MDCK cells to J96, but not XL1-Blue, resulted in loss of transepithelial resistance (TER), which was due to epithelial cytotoxicity and not degradation of epithelial junctional proteins by bacterial proteases. Apical exposure to both J96 and XL1-Blue did not alter TER. Pretreatment of polarized MDCK cell monolayers with HGF renders the cells sensitive to loss of TER and cytotoxicity by apical exposure to J96. Analysis by confocal microscopy demonstrated that HGF treatment of MDCK cell monolayers also greatly enhances adherence of J96 to the apical surface of the cell monolayer. These data demonstrate that the basolateral surface of polarized epithelia is more susceptible to J96 cytotoxicity. The data also support the hypothesis that processes that alter epithelial cell polarity increase sensitivity of epithelia to bacterial injury and adherence from the apical compartment.

Assessment of internalization and viability of Porphyromonas gingivalis in KB epithelial cells by confocal microscopy

Porphyromonas gingivalis (P. gingivalis) is considered to be one of the main periodontal pathogens. The goal of this work was to confirm the ability of P. gingivalis to invade host cells. We detected P. gingivalis inside KB cells by confocal microscopy and analyzed the various aspects of the adherence and internalization process. Lysates of P. gingivalis-infected KB cells were also examined using anaerobic growth techniques. The results showed the viability and ability to replicate, inside the host cells, of the internalized pathogen. The production of vesicles was also tracked for the first time. Confocal microscopy revealed P. gingivalis in a perinuclear position.

Molecular and cellular permeability control at the blood-brain barrier

The blood-brain barrier (BBB) is formed by brain capillary endothelial cells. These cells have at least three properties which distinguish them from their peripheral counterparts: (1) tight junctions (TJs) of extremely low permeability; (2) low rates of fluid-phase endocytosis; (3) specific transport and carrier molecules. In combination, these features restrict the nonspecific flux of ions, proteins, and other substances into the central nervous system (CNS) environment. The restriction protects neurons from harmful compositional fluctuations occurring in the blood and allows uptake of essential molecules. Breakdown of the BBB is associated with a variety of CNS disorders and results in aggravation of the condition. Restoration of the BBB is thus one strategy during therapy of CNS diseases. Its success depends on a precise knowledge of the structural and functional principles underlying BBB functionality. In this review we have tried to summarise the current knowledge of TJs, including information gained from non-neuronal systems, and describe selected mechanisms involved in permeability regulation.

Calprotectin expression in vitro by oral epithelial cells confers resistance to infection by Porphyromonas gingivalis

Calprotectin, an S100 calcium-binding protein with broad-spectrum antimicrobial activity in vitro, is expressed in neutrophils, monocytes, and gingival keratinocytes. In periodontitis, calprotectin appears upregulated and is detected at higher levels in gingival crevicular fluid and tissue specimens. How calprotectin contributes to the pathogenesis of periodontal diseases is unknown. To isolate the effects of calprotectin, a calprotectin-negative oral epithelial cell line was transfected with calprotectin genes to enable expression. Porphyromonas gingivalis was permitted to bind and invade transfected cells expressing calprotectin and sham transfectants. Rates of invasion into both cell lines were compared using the antibiotic protection assay. Transfected cells expressing calprotectin showed 40 to 50% fewer internalized P. gingivalis than sham transfectants. Similarly, binding to calprotectin expressing cells was reduced approximately twofold at all time points (15, 30, 45, and 60 min) as estimated by immunofluorescence analysis. Independent of invasion, however, prolonged exposure to P. gingivalis induced epithelial cell rounding and detachment from the substratum. These morphological changes were delayed, however, in cells expressing calprotectin. Using P. gingivalis protease-deficient mutants, we found that Arg-gingipain and Lys-gingipain contributed to epithelial cell rounding and detachment. In conclusion, expression of calprotectin appears to protect epithelial cells in culture against binding and invasion by P. gingivalis. In addition, cells expressing calprotectin are more resistant to detachment mediated by Arg-gingipain and Lys-gingipain. In periodontal disease, calprotectin may augment both the barrier protection and innate immune functions of the gingival epithelium to promote resistance to P. gingivalis infection.

Inhibition of epithelial cell apoptosis by Porphyromonas gingivalis

Porphyromonas gingivalis is periodontal pathogen that is capable of invading gingival epithelial cells (GECs). Apoptotic responses of primary cultures of GECs to P. gingivalis were investigated with a DNA fragmentation ELISA assay. P. gingivalis induced a transient increase in GEC DNA fragmentation; however, after prolonged incubation GECs did not undergo apoptosis. Furthermore, P. gingivalis blocked apoptosis in GECs following stimulation with camptothecin. Immunoblotting of GECs with Bcl-2 or Bax antibodies showed that P. gingivalis up-regulated Bcl-2 levels in GECs, whereas Bax levels were transiently elevated and declined after 24 h stimulation. Streptococcus gordonii did not affect levels of either molecule. RT-PCR demonstrated that induction of Bcl-2 occurs at the transcriptional level. The results suggest that P. gingivalis can inhibit apoptosis in GECs by up-regulation of the anti-apoptotic molecule Bcl-2. The prevention of host cell apoptosis may represent a strategy for P. gingivalis survival within invaded GECs.

Activation of protease-activated receptors by gingipains from Porphyromonas gingivalis leads to platelet aggregation: a new trait in microbial pathogenicity

The bacterium Porphyromonas gingivalis is a major etiologic agent in the pathogenesis of adult periodontitis in humans. Cysteine proteinases produced by this pathogen, termed gingipains, are considered to be important virulence factors. Among many other potentially deleterious activities, arginine-specific gingipains-R (RgpB and HRgpA) efficiently activate coagulation factors. To further expand knowledge of the interaction between gingipains and the clotting cascade, this study examined their effects on cellular components of the coagulation system. The enzymes induced an increase in intracellular calcium in human platelets at nanomolar concentrations and caused platelet aggregation with efficiency comparable to thrombin. Both effects were dependent on the proteolytic activity of the enzymes. Based on desensitization studies carried out with thrombin and peptide receptor agonists, and immunoinhibition experiments, gingipains-R appeared to be activating the protease-activated receptors, (PAR)-1 and -4, expressed on the surface of platelets. This was confirmed by the finding that HRgpA and RgpB potently activated PAR-1 and PAR-4 in transfected cells stably expressing these receptors. Cumulatively, the results indicate the existence of a novel pathway of host cell activation by bacterial proteinases through PAR cleavage. This mechanism not only represents a new trait in bacterial pathogenicity, but may also explain an emerging link between periodontitis and cardiovascular disease. (Blood. 2001;97:3790-3797)

Protease-active extracellular protein preparations from Porphyromonas gingivalis W83 induce N-cadherin proteolysis, loss of cell adhesion, and apoptosis in human epithelial cells

BACKGROUND

The protease-induced cytotoxicity of P. gingivalis may partly result from alteration of the extracellular matrix and/or surface receptors that mediate interaction between the host cells and their matrix. While P. gingivalis-induced degradation of E-cadherin has been documented, there is no information on the effects of P. gingivalis proteases on other members of this family of cell adhesion proteins.

METHODS

Human epithelial KB cells were exposed to protease-active extracellular protein preparations from isogenic mutants of P. gingivalis. Quantification of apoptosis was performed by visualization of nuclei stained with 4,6'-diamidino-2-phenylindole. Alteration of cell adhesion proteins was examined by immunoblotting of cell lysates using monoclonal antibodies to those proteins.

RESULTS

Treated cells exhibited loss of cell adhesion properties with apoptotic cell death subsequently observed. These effects correlated with the different levels of cysteine-dependent proteolytic activities of the isogenic mutants tested. Cleavage of N-cadherin was observed in immunoblots of lysates from detached cells. There was a direct correlation between the kinetics of N-cadherin cleavage and loss of cell adhesion properties. Loss of cell adhesion, as well as N-cadherin cleavage, could be inhibited by preincubation of P. gingivalis protease active extracellular protein preparations with the cysteine protease inhibitor TLCK. In control experiments, the cleavage of N-cadherin was detected after treatment of KB cells with trypsin but not after cell dissociation by a non-enzymatic method.

CONCLUSIONS

These results suggest that extracellular proteases from P. gingivalis can induce degradation of N-cadherin, which could have implications for the pathogenicity of this bacterium.

Porphyromonas gingivalis gingipains and adhesion to epithelial cells

Porphyromonas gingivalis is one of the principal organisms associated with adult periodontitis. Bacterial surface proteins such as fimbriae and gingipain hemagglutinin domains have been implicated as adhesins that actuate colonization of epithelium lining the gingival sulcus. We investigated the genetics of P. gingivalis adhesion to monolayers of epithelial cells using wild-type and gingipain mutant strains. These experiments suggested that arginine-specific gingipain (Rgp) catalytic activity modulated adhesion. From the data obtained with rgp mutants, we constructed a working hypothesis predicting that attachment and detachment of P. gingivalis to epithelial cells were mediated by gingipain adhesin and Rgp catalytic domains, respectively. A membrane-based epithelial cell binding assay, used to locate adhesins in extracellular fractions of wild-type and mutant strains, recognized gingipain peptides as adhesins rather than fimbriae. We developed a capture assay that demonstrated the binding of gingipain adhesin peptides to oral epithelial cells. The adherence of fimbrillin to epithelial cells was detected after heat denaturation of cell fractions. The prediction that Rgp catalytic activities mediated detachment was substantiated when the high level of attachment of an rgp mutant was reduced in the presence of wild-type cell fractions that contained gingipain catalytic activities.

Molecular physiology and pathophysiology of tight junctions V. assault of the tight junction by enteric pathogens

Studies of the impact of enteric pathogens and their virulence factors on the proteins comprising the tight junction and zonula adherens offer a novel approach to dissection of tight junctional complex regulation. Most studies to date provide only tantalizing clues that select pathogens may indeed assault the tight junctional complex. Information on critical human pathogens such as Campylobacter jejuni and Shigella and Salmonella subspecies is lacking. Mechanistic studies are currently sparse, but available results on pathogenic Escherichia coli and specific virulence factors such as the Rho-modifying and protease bacterial toxins indicate four major mechanisms by which these pathogens may act: 1) direct cleavage of tight junctional structural proteins; 2) modification of the actin cytoskeleton; 3) activation of cellular signal transduction; and 4) triggering transmigration of polymorphonuclear cells across the epithelial cell barrier. New therapeutics may evolve from detailed studies of these pathogens and the cellular processes and proteins they disrupt.