Haemin binding as a factor in the virulence of Porphyromonas gingivalis

A Porphyromonas gingivalis mutant defective in a putative glycosyltransferase exhibits defective biosynthesis of the polysaccharide portions of lipopolysaccharide, decreased gingipain activities, strong autoaggregation, and increased biofilm formation

The Gram-negative anaerobic bacterium Porphyromonas gingivalis is a major pathogen in periodontal disease, one of the biofilm-caused infectious diseases. The bacterium possesses potential virulence factors, including fimbriae, proteinases, hemagglutinin, lipopolysaccharide (LPS), and outer membrane vesicles, and some of these factors are associated with biofilm formation; however, the precise mechanism of biofilm formation is still unknown. Colonial pigmentation of the bacterium on blood agar plates is related to its virulence. In this study, we isolated a nonpigmented mutant that had an insertion mutation within the new gene PGN_1251 (gtfB) by screening a transposon insertion library. The gene shares homology with genes encoding glycosyltransferase 1 of several bacteria. The gtfB mutant was defective in biosynthesis of both LPSs containing O side chain polysaccharide (O-LPS) and anionic polysaccharide (A-LPS). The defect in the gene resulted in a complete loss of surface-associated gingipain proteinases, strong autoaggregation, and a marked increase in biofilm formation, suggesting that polysaccharide portions of LPSs influence attachment of gingipain proteinases to the cell surface, autoaggregation, and biofilm formation of P. gingivalis.

Hemin-dependent modulation of the lipid A structure of Porphyromonas gingivalis lipopolysaccharide

Porphyromonas gingivalis is a periopathogen strongly associated with the development of adult-type periodontitis. Both the virulence characteristics of periopathogens and host-related factors are believed to contribute to periodontitis. P. gingivalis lipopolysaccharide (LPS) displays a significant amount of lipid A structural heterogeneity, containing both penta- and tetra-acylated lipid A structures. However, little is known concerning how the lipid A structural content of P. gingivalis is regulated. Alterations in the lipid A content may facilitate the ability of P. gingivalis to modulate the innate host response to this bacterium. In this report, it is shown that the concentration of hemin in the growth medium significantly modulates the lipopolysaccharide lipid A structural content of P. gingivalis. Hemin is a key microenvironmental component of gingival cervicular fluid which is believed to vary depending upon the state of vascular ulceration. At low hemin concentrations, one major penta-acylated lipid A structure was found, whereas at high concentrations of hemin, multiple tetra- and penta-acylated lipid A structures were observed. Hemin concentrations, not iron acquisition, were responsible for the alterations in the lipid A structural content. The modifications of the lipid A structural content were independent of the LPS extraction procedure and occurred in a variety of laboratory strains as well as a freshly obtained clinical isolate. The known hemin binding proteins Kgp and HmuR contributed to the lipid A modulation sensing mechanism. To the best of our knowledge, this is the first report that hemin, a clinically relevant microenvironmental component for P. gingivalis, can modulate the lipid A structure found in a bacterium. Since tetra- and penta-acylated P. gingivalis lipid A structures have opposing effects on Toll-like receptor 4 activation, the alteration of the lipid A structural content may have significant effects on the host response to this bacterium.

LuxS involvement in the regulation of genes coding for hemin and iron acquisition systems in Porphyromonas gingivalis

The periodontal pathogen Porphyromonas gingivalis employs a variety of mechanisms for the uptake of hemin and inorganic iron. Previous work demonstrated that hemin uptake in P. gingivalis may be controlled by LuxS-mediated signaling. In the present study, the expression of genes involved in hemin and iron uptake was determined in parent and luxS mutant strains by quantitative real-time reverse transcription-PCR. Compared to the parental strain, the luxS mutant showed reduced levels of transcription of genes coding for the TonB-linked hemin binding protein Tlr and the lysine-specific protease Kgp, which can degrade host heme-containing proteins. In contrast, there was up-regulation of the genes for another TonB-linked hemin binding protein, HmuR; a hemin binding lipoprotein, FetB; a Fe(2+) ion transport protein, FeoB1; and the iron storage protein ferritin. Differential expression of these genes in the luxS mutant was maximal in early-exponential phase, which corresponded with peak expression of luxS and AI-2 signal activity. Complementation of the luxS mutation with wild-type luxS in trans rescued expression of hmuR. Mutation of the GppX two-component signal transduction pathway caused an increase in expression of luxS along with tlr and lower levels of message for hmuR. Moreover, expression of hmuR was repressed, and expression of tlr stimulated, when the luxS mutant was incubated with AI-2 partially purified from the culture supernatant of wild-type cells. A phenotypic outcome of the altered expression of genes involved in hemin uptake was impairment of growth of the luxS mutant in hemin-depleted medium. The results demonstrate a role of LuxS/AI-2 in the regulation of hemin and iron acquisition pathways in P. gingivalis and reveal a novel control pathway for luxS expression.

Effects of various growth conditions in a chemostat on expression of virulence factors in Porphyromonas gingivalis

Porphyromonas gingivalis, one of the gram-negative organisms associated with periodontal disease, possesses potential virulence factors, including fimbriae, proteases, and major outer membrane proteins (OMPs). In this study, P. gingivalis ATCC 33277 was cultured in a chemostat under hemin excess and presumably peptide-limiting conditions to better understand the mechanisms of expression of the virulence factors upon environmental changes. At higher growth rates, the amounts of FimA and the 75-kDa protein, forming long and short fimbriae, respectively, increased significantly, whereas gingipains decreased in amount and activity. In a nutrient-limited medium, lesser amounts of the above two fimbrial proteins were observed, whereas clear differences were not found in the amounts of gingipains. In addition, two-dimensional electrophoresis revealed that proteins in cells were generally fewer in number during nutrient-limited growth. Under aeration, a considerable reduction in gingipain activity was found, whereas several proteins associated with intact cells significantly increased. However, the expression of major OMPs, such as RagA, RagB, and the OmpA-like proteins, was almost constant under all conditions tested. These results suggest that P. gingivalis may actively control expression of several virulence factors to survive in the widely fluctuating oral environment.

Iron and heme utilization in Porphyromonas gingivalis

Porphyromonas gingivalis is a Gram-negative anaerobic bacterium associated with the initiation and progression of adult periodontal disease. Iron is utilized by this pathogen in the form of heme and has been shown to play an essential role in its growth and virulence. Recently, considerable attention has been given to the characterization of various secreted and surface-associated proteins of P. gingivalis and their contribution to virulence. In particular, the properties of proteins involved in the uptake of iron and heme have been extensively studied. Unlike other Gram-negative bacteria, P. gingivalis does not produce siderophores. Instead it employs specific outer membrane receptors, proteases (particularly gingipains), and lipoproteins to acquire iron/heme. In this review, we will focus on the diverse mechanisms of iron and heme acquisition in P. gingivalis. Specific proteins involved in iron and heme capture will be described. In addition, we will discuss new genes for iron/heme utilization identified by nucleotide sequencing of the P. gingivalis W83 genome. Putative iron- and heme-responsive gene regulation in P. gingivalis will be discussed. We will also examine the significance of heme/hemoglobin acquisition for the virulence of this pathogen.

In vitro environmental regulation of Porphyromonas gingivalis growth and virulence

Porphyromonas gingivalis appears to be a major contributor to periodontal disease, especially soft tissue destruction, which is reflected by the ability to cause invasive, spreading lesions, and tissue inflammation in a murine abscess model. This study investigated the role of hemin on the regulation of growth and virulence of P. gingivalis strains. P. gingivalis strains W50, A7A1-28, 3079, 381, W50/BEI, and NG4B19 were grown in broth and on blood agar plates. P. gingivalis cells grown under iron-depleted conditions for multiple passages showed significantly decreased lesion size in mice, in contrast to cells grown under iron-normal (5 microg/ml) and iron-elevated conditions. Statistically significant (P < 0.01) decreases in gingipain enzyme activity were found among the strains grown under iron-depleted conditions. P. gingivalis grown in the presence of blood induced significantly different lesion type, lesion size, lesion onset, and mortality. Elevated hemin resulted in increased cell-associated iron in P. gingivalis, which increased the capacity of the microorganism to survive at times of iron deprivation. These results indicate that hemin or iron availability regulates multiple aspects related to P. gingivalis virulence, including growth, survival, gingipain levels, and iron accumulation.

The periodontal pathogen Porphyromonas gingivalis harnesses the chemistry of the mu-oxo bishaem of iron protoporphyrin IX to protect against hydrogen peroxide

The major haem component in the black pigment of Porphyromonas gingivalis is the mu-oxo bishaem of iron protoporphyrin IX and formation and cell-surface binding of this haem species is proposed as an extracellular buffer against reactive oxidants [Smalley, J.W. et al. (1998) Biochem. J. 331, 681-685]. P. gingivalis cells grown in the presence of the mu-oxo bishaem were protected against H(2)O(2) compared to control cells grown without it. When added to the growth medium, soluble mu-oxo bishaem inactivated H(2)O(2) and supported cell growth. Cells carrying a surface layer of mu-oxo bishaem were less susceptible to peroxidation by H(2)O(2). Cell-surface haems were slowly destroyed during reaction with H(2)O(2). Binding of mu-oxo bishaem by P. gingivalis may aid survival during neutrophil attack through inactivation of hydrogen peroxide.

Porphyrin-mediated binding to hemoglobin by the HA2 domain of cysteine proteinases (gingipains) and hemagglutinins from the periodontal pathogen Porphyromonas gingivalis

Heme binding and uptake are considered fundamental to the growth and virulence of the gram-negative periodontal pathogen Porphyromonas gingivalis. We therefore examined the potential role of the dominant P. gingivalis cysteine proteinases (gingipains) in the acquisition of heme from the environment. A recombinant hemoglobin-binding domain that is conserved between two predominant gingipains (domain HA2) demonstrated tight binding to hemin (Kd = 16 nM), and binding was inhibited by iron-free protoporphyrin IX (Ki = 2.5 microM). Hemoglobin binding to the gingipains and the recombinant HA2 (rHA2) domain (Kd = 2.1 nM) was also inhibited by protoporphyrin IX (Ki = 10 microM), demonstrating an essential interaction between the HA2 domain and the heme moiety in hemoglobin binding. Binding of rHA2 with either hemin, protoporphyrin IX, or hematoporphyrin was abolished by establishing covalent linkage of the protoporphyrin propionic acid side chains to fixed amines, demonstrating specific and directed binding of rHA2 to these protoporphyrins. A monoclonal antibody which recognizes a peptide epitope within the HA2 domain was employed to demonstrate that HA2-associated hemoglobin-binding activity was expressed and released by P. gingivalis cells in a batch culture, in parallel with proteinase activity. Cysteine proteinases from P. gingivalis appear to be multidomain proteins with functions for hemagglutination, erythrocyte lysis, proteolysis, and heme binding, as demonstrated here. Detailed understanding of the biochemical pathways for heme acquisition in P. gingivalis may allow precise targeting of this critical metabolic aspect for periodontal disease prevention.

Life below the gum line: pathogenic mechanisms of Porphyromonas gingivalis

Porphyromonas gingivalis, a gram-negative anaerobe, is a major etiological agent in the initiation and progression of severe forms of periodontal disease. An opportunistic pathogen, P. gingivalis can also exist in commensal harmony with the host, with disease episodes ensuing from a shift in the ecological balance within the complex periodontal microenvironment. Colonization of the subgingival region is facilitated by the ability to adhere to available substrates such as adsorbed salivary molecules, matrix proteins, epithelial cells, and bacteria that are already established as a biofilm on tooth and epithelial surfaces. Binding to all of these substrates may be mediated by various regions of P. gingivalis fimbrillin, the structural subunit of the major fimbriae. P. gingivalis is an asaccharolytic organism, with a requirement for hemin (as a source of iron) and peptides for growth. At least three hemagglutinins and five proteinases are produced to satisfy these requirements. The hemagglutinin and proteinase genes contain extensive regions of highly conserved sequences, with posttranslational processing of proteinase gene products contributing to the formation of multimeric surface protein-adhesin complexes. Many of the virulence properties of P. gingivalis appear to be consequent to its adaptations to obtain hemin and peptides. Thus, hemagglutinins participate in adherence interactions with host cells, while proteinases contribute to inactivation of the effector molecules of the immune response and to tissue destruction. In addition to direct assault on the periodontal tissues, P. gingivalis can modulate eucaryotic cell signal transduction pathways, directing its uptake by gingival epithelial cells. Within this privileged site, P. gingivalis can replicate and impinge upon components of the innate host defense. Although a variety of surface molecules stimulate production of cytokines and other participants in the immune response, P. gingivalis may also undertake a stealth role whereby pivotal immune mediators are selectively inactivated. In keeping with its strict metabolic requirements, regulation of gene expression in P. gingivalis can be controlled at the transcriptional level. Finally, although periodontal disease is localized to the tissues surrounding the tooth, evidence is accumulating that infection with P. gingivalis may predispose to more serious systemic conditions such as cardiovascular disease and to delivery of preterm infants.

The periodontopathogen Porphyromonas gingivalis binds iron protoporphyrin IX in the mu-oxo dimeric form: an oxidative buffer and possible pathogenic mechanism

Mössbauer spectroscopy was used to re-evaluate iron protoporphyrin IX, FePPIX, binding and the chemical nature of the black iron porphyrin pigment of Porphyromonas gingivalis. We demonstrate that FePPIX is bound to the cell in the mu-oxo dimeric form, [Fe(III)PPIX]2O, and that the iron porphyrin pigment is also composed of this material. P. gingivalis also assimilated monomeric Fe(II)- and Fe(III)PPIX into mu-oxo dimers in vitro. Scatchard analysis revealed a greater binding maximum of cells for mu-oxo dimers than for monomeric Fe(III)-or Fe(II)PPIX, although the relative affinity constant for the dimers was lower. Formation of [Fe(III)PPIX]2O via reactions of Fe(II)PPIX with oxygen, and its toxic derivatives, would serve as an oxidative buffer and permit P. gingivalis and other black-pigmenting anaerobes to engender and maintain a local anaerobic environment. Tying up of free oxygen species with iron protoporphyrin IX would also reduce and limit Fe(II)PPIX-mediated oxygen-radical cell damage. More importantly, formation of a cell-surface mu-oxo dimer layer may function as a protective barrier against assault by reactive oxidants generated by neutrophils. Selective interference with these mechanisms would offer the possibility of attenuating the pathogenicity of P. gingivalis and other iron protoporphyrin IX-binding pathogens whose virulence is regulated by this reactive molecule.

Albumin and hemalbumin degradation by Porphyromonas gingivalis

Degradation of bovine albumin and hemalbumin by Porphyromonas gingivalis W50 cells under non-reducing conditions at 37 degrees C was examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometry. Albumin and hemalbumins with heme:protein molar ratios of 1:1, 4:1 and 8:1 were degraded, yielding protease-resistant 55.6-kDa peptides. Cells of strains WPH 35, 11834 and Bg 381 also produced a similar digestion pattern. N-terminal sequencing of the 55.6-kDa albumin digestion fragment revealed two peptides with the sequences 82glu-thr-tyr-gly-asp-met-ala and 95gln-pro-glu-arg-asn-glu-cys, indicating cleavage in the N-terminal hinge region. Tosyllysylchloromethylketone and N-ethylmaleimide were the most effective in inhibiting breakdown of albumin and hemalbumin with a 1:1 heme:protein ratio. Initial degradation rates of albumin and all hemalbumins were similar, but the total amount of hemalbumins degraded over 7.5 h decreased with increased ratio of bound hemin. The specific proteolysis of hemalbumin may enable P. gingivalis to release hemin from a region of the molecule where heme binding is least avid.