PPAD Activity Promotes Outer Membrane Vesicle Biogenesis and Surface Translocation by Porphyromonas gingivalis

Effects of bacterial extracellular vesicles derived from oral and gastrointestinal pathogens on systemic diseases

Oral microbiota and gastrointestinal microbiota, the two largest microbiomes in the human body, are closely correlated and frequently interact through the oral-gut axis. Recent research has focused on the roles of these microbiomes in human health and diseases. Under normal conditions, probiotics and commensal bacteria can positively impact health. However, altered physiological states may induce dysbiosis, increasing the risk of pathogen colonization. Studies suggest that oral and gastrointestinal pathogens contribute not only to localized diseases at their respective colonized sites but also to the progression of systemic diseases. However, the mechanisms by which bacteria at these local sites are involved in systemic diseases remain elusive. In response to this gap, the focus has shifted to bacterial extracellular vesicles (BEVs), which act as mediators of communication between the microbiota and the host. Numerous studies have reported the targeted delivery of bacterial pathogenic substances from the oral cavity and the gastrointestinal tract to distant organs via BEVs. These pathogenic components subsequently elicit specific cellular responses in target organs, thereby mediating the progression of systemic diseases. This review aims to elucidate the extensive microbial communication via the oral-gut axis, summarize the types and biogenesis mechanisms of BEVs, and highlight the translocation pathways of oral and gastrointestinal BEVs in vivo, as well as the impacts of pathogens-derived BEVs on systemic diseases.

An outer membrane vesicle specific lipoprotein promotes Porphyromonas gingivalis aggregation on red blood cells

Porphyromonas gingivalis uses a variety of mechanisms to actively interact with and promote the hydrolysis of red blood cells (RBCs) to obtain iron in the form of heme. In this study, we investigated the function of lipoprotein PG1881 which was previously shown to be up-regulated during subsurface growth and selectively enriched on outer membrane vesicles (OMVs). Our results show that wildtype strain W83 formed large aggregates encompassing RBCs whereas the PG1881 deletion mutant remained predominately as individual cells. Using a PG1881 antibody, immunofluorescence revealed that the wildtype strain's aggregation to RBCs involves an extracellular matrix enriched with PG1881. Our findings discover that RBCs elicit cell aggregation and matrix formation by P. gingivalis and that this process is promoted by an OMV-specific lipoprotein. We propose this strategy is advantageous for nutrient acquisition as well as dissemination from the oral cavity and survival of this periodontal pathogen.

Identification of a new genetic variant (G231N, E232T, N235D) of peptidylarginine deiminase from P. gingivalis in advanced periodontitis

Chronic periodontitis (CP), an inflammatory disease of periodontal tissues driven by a dysbiotic subgingival bacterial biofilm, is also associated with several systemic diseases, including rheumatoid arthritis (RA). Porphyromonas gingivalis, one of the bacterial species implicated in CP as a keystone pathogen produces peptidyl arginine deiminase (PPAD) that citrullinates C-terminal arginine residues in proteins and peptides. Autoimmunity to citrullinated epitopes is crucial in RA, hence PPAD activity is considered a possible mechanistic link between CP and RA. Here we determined the PPAD enzymatic activity produced by clinical isolates of P. gingivalis, sequenced the ppad gene, and correlated the results with clinical determinants of CP in patients from whom the bacteria were isolated. The analysis revealed variations in PPAD activity and genetic diversity of the ppad gene in clinical P. gingivalis isolates. Interestingly, the severity of CP was correlated with a higher level of PPAD activity that was associated with the presence of a triple mutation (G231N, E232T, N235D) in PPAD in comparison to W83 and ATCC 33277 type strains. The relation between mutations and enhanced activity was verified by directed mutagenesis which showed that all three amino acid residue substitutions must be introduced into PPAD expressed by the type strains to obtain the super-active enzyme. Cumulatively, these results may lead to the development of novel prognostic tools to assess the progress of CP in the context of associated RA by analyzing the ppad genotype in CP patients infected with P. gingivalis.

Impacts of Porphyromonas gingivalis periodontitis on rheumatoid arthritis autoimmunity

In RA patients' synovial sites, citrullinated RA-related antigens such as type II collagens, fibrin (ogen), vimentin, and α-enolase could be targeted by ACCPAs. Since ACCPA production can be initiated a long time before RA sign appearance, primary auto-immunization against these citrullinated proteins can be originated from extra-articular sites. It has been shown that there is a significant association between P. gingivalis periodontitis, anti- P. gingivalis antibodies, and RA. P. gingivalis gingipains (Rgp, Kgp) can degrade proteins such as fibrin and α-enolase into some peptides in the form of Arg in the C-terminal which is converted to citrulline by PPAD. Also, PPAD can citrullinate type II collagen and vimentins (SA antigen). P. gingivalis induces inflammation and chemoattraction of immune cells such as neutrophils and macrophages through the increase of C5a (gingipain C5 convertase-like activity) and SCFA secretion. Besides, this microorganism stimulates anoikis, a special type of apoptosis, and NETosis, an antimicrobial form of neutrophil death, leading to the release of PAD1-4, α-enolase, and vimentin from apoptotic cells into the periodontal site. In addition, gingipains can degrade macrophages CD14 and decrease their ability in apoptotic cell removal. Gingipains also can cleave IgGs in the Fc region and transform them into rheumatoid factor (RF) antigens. In the present study, the effects of P. gingivalis on rheumatoid arthritis autoimmune response have been reviewed, which could attract practical insight both in bench and clinic.

Response of genes related to iron and porphyrin transport in Porphyromonas gingivalis to blue light

BACKGROUND

Antimicrobial blue light (aBL) kills a variety of bacteria, including Porphyromonas gingivalis. However, little is known about the transcriptomic response of P. gingivalis to aBL therapy. This study was designed to evaluate the selective cytotoxicity of aBL against P. gingivalis over human cells and to further investigate the genetic response of P. gingivalis to aBL at the transcriptome level.

METHODS

Colony forming unit (CFU) testing, confocal laser scanning microscopy (CLSM), and scanning electron microscopy (SEM) were used to investigate the antimicrobial effectiveness of blue light against P. gingivalis. The temperatures of the irradiated targets were measured to prevent overheating. Multiple fluorescent probes were used to quantify reactive oxygen species (ROS) generation after blue-light irradiation. RNA sequencing (RNA-seq) was used to investigate the changes in global gene expression. Following the screening of target genes, real-time quantitative polymerase chain reaction (RT-qPCR) was performed to confirm the regulation of gene expression.

RESULTS

A 405 nm aBL at 100 mW/cm² significantly killed P. gingivalis within 5 min while sparing human gingival fibroblasts (HGFs). No obvious temperature changes were detected in the irradiated surface under our experimental conditions. RNA-seq showed that the transcription of multiple genes was regulated, and RT-qPCR revealed that the expression levels of the genes RgpA and RgpB, which may promote heme uptake, as well as the genes Ftn and FetB, which are related to iron homeostasis, were significantly upregulated. The expression levels of the FeoB-2 and HmuR genes, which are related to hydroxyl radical scavenging, were significantly downregulated.

CONCLUSIONS

aBL strengthens the heme uptake and iron export gene pathways while reducing the ROS scavenging pathways in P. gingivalis, thus improving the accumulation of endogenous photosensitizers and enhancing oxidative damage to P. gingivalis.

Growth of Porphyromonas gingivalis on human serum albumin triggers programmed cell death

Aims

Gingival crevicular fluid (GCF) constitutes the primary growth substrate for Porphyromonas gingivalis in vivo. The goal of this work was to evaluate the growth of different strains of P. gingivalis on human serum albumin (HSA), a major constituent of GCF.

Methods

Growth of five different strains of P. gingivalis in the HSA medium was examined and, surprisingly, three of the strains underwent autolysis within 24 h. Comparative transcriptomic analysis was used to identify genes involved in autolysis.

Results

Two highly related reference strains (W50 and W83) differed dramatically in their survival when grown on HSA. Strain W83 grew fast and lysed within 24 h, while W50 survived for an additional 20 h. Differential gene expression analysis led us to a gene cluster containing enzymes involved in arginine metabolism and a gene predicted to be lytic murein transglycosylase, which are known to play a role in autolysis. Deletion of this gene (PG0139) resulted in a mutant that did not lyse, and complementation restored the HSA lysis phenotype, indicating that this enzyme plays a central role in the autolysis of P. gingivalis.

Conclusions

P. gingivalis undergoes autolysis when provided with HSA as a substrate for growth.

Coating and Corruption of Human Neutrophils by Bacterial Outer Membrane Vesicles

Porphyromonas gingivalis is a keystone oral pathogen that successfully manipulates the human innate immune defenses, resulting in a chronic proinflammatory state of periodontal tissues and beyond. Here, we demonstrate that secreted outer membrane vesicles (OMVs) are deployed by P. gingivalis to selectively coat and activate human neutrophils, thereby provoking degranulation without neutrophil killing. Secreted granule components with antibacterial activity, especially LL-37 and myeloperoxidase (MPO), are subsequently degraded by potent OMV-bound proteases known as gingipains, thereby ensuring bacterial survival. In contrast to neutrophils, the P. gingivalis OMVs are efficiently internalized by macrophages and epithelial cells. Importantly, we show that neutrophil coating is a conserved feature displayed by OMVs of at least one other oral pathogen, namely, Aggregatibacter actinomycetemcomitans. We conclude that P. gingivalis deploys its OMVs for a neutrophil-deceptive strategy to create a favorable inflammatory niche and escape killing. IMPORTANCE Severe periodontitis is a dysbiotic inflammatory disease that affects about 15% of the adult population, making it one of the most prevalent diseases worldwide. Importantly, periodontitis has been associated with the development of nonoral diseases, such as rheumatoid arthritis, pancreatic cancer, and Alzheimer's disease. Periodontal pathogens implicated in periodontitis can survive in the oral cavity only by avoiding the insults of neutrophils while at the same time promoting an inflamed environment where they successfully thrive. Our present findings show that outer membrane vesicles secreted by the keystone pathogen Porphyromonas gingivalis provide an effective delivery tool of virulence factors that protect the bacterium from being killed while simultaneously activating human neutrophils.

Implications of Porphyromonas gingivalis peptidyl arginine deiminase and gingipain R in human health and diseases

Porphyromonas gingivalis is a major pathogenic bacterium involved in the pathogenesis of periodontitis. Citrullination has been reported as the underlying mechanism of the pathogenesis, which relies on the interplay between two virulence factors of the bacterium, namely gingipain R and the bacterial peptidyl arginine deiminase. Gingipain R cleaves host proteins to expose the C-terminal arginines for peptidyl arginine deiminase to citrullinate and generate citrullinated proteins. Apart from carrying out citrullination in the periodontium, the bacterium is found capable of citrullinating proteins present in the host synovial tissues, atherosclerotic plaques and neurons. Studies have suggested that both virulence factors are the key factors that trigger distal effects mediated by citrullination, leading to the development of some non-communicable diseases, such as rheumatoid arthritis, atherosclerosis, and Alzheimer’s disease. Thus, inhibition of these virulence factors not only can mitigate periodontitis, but also can provide new therapeutic solutions for systematic diseases involving bacterial citrullination. Herein, we described both these proteins in terms of their unique structural conformations and biological relevance to different human diseases. Moreover, investigations of inhibitory actions on the enzymes are also enumerated. New approaches for identifying inhibitors for peptidyl arginine deiminase through drug repurposing and virtual screening are also discussed.

Cortisol Promotes Surface Translocation of Porphyromonas gingivalis

Studies are showing that the stress hormone cortisol can reach high levels in the gingival sulcus and induce shifts in the metatranscriptome of the oral microbiome. Interestingly, it has also been shown that cortisol can influence expression levels of Type IX Secretion System (T9SS) genes involved in gliding motility in bacteria belonging to the phylum Bacteroidota. The objective of this study was to determine if cortisol impacts gene expression and surface translocation of Porphyromonas gingivalis strain W50. To conduct these experiments, P. gingivalis was stabbed to the bottom of soft agar plates containing varying cortisol concentrations (0 μM, 0.13 μM, 1.3 μM, and 13 μM), and surface translocation on the subsurface was observed after 48 h of incubation. The results show that when grown with certain nutrients, i.e., in rich medium with the addition of sheep blood, lactate, or pyruvate, cortisol promotes migration of P. gingivalis in a concentration-dependent manner. To begin to examine the underlying mechanisms, quantitative PCR was used to evaluate differential expression of genes when P. gingivalis was exposed to cortisol. In particular, we focused on differential expression of T9SS-associated genes, including mfa5, since it was previously shown that Mfa5 is required for cell movement and cell-to-cell interactions. The data show that mfa5 is significantly up-regulated in the presence of cortisol. Moreover, an mfa5 deletion mutant showed less surface translocation compared to the wild-type P. gingivalis in the presence of cortisol, and the defects of the mfa5 deletion mutant were restored by complementation. Overall, cortisol can stimulate P. gingivalis surface translocation and this coincides with higher expression levels of T9SS-associated genes, which are known to be essential to gliding motility. Our findings support a high possibility that the stress hormone cortisol from the host can promote surface translocation and potentially virulence of P. gingivalis.

An Investigation into the Effects of Outer Membrane Vesicles and Lipopolysaccharide of Porphyromonas gingivalis on Blood-Brain Barrier Integrity, Permeability, and Disruption of Scaffolding Proteins in a Human in vitro Model

BACKGROUND

The effects of the key pathogens and virulence factors associated with gum disease such as Porphyromonas gingivalis (P. gingivalis) on the central nervous system is of great interest with respect to development of neuropathologies and hence therapeutics and preventative strategies. Chronic infections and associated inflammation are known to weaken the first line of defense for the brain, the blood-brain barrier (BBB).

OBJECTIVE

The focus of this study is to utilize an established human in vitro BBB model to evaluate the effects of P. gingivalis virulence factors lipopolysaccharide (LPS) and outer membrane vesicles (OMVs) on a primary-derived human model representing the neurovascular unit of the BBB.

METHODS

Changes to the integrity of the BBB after application of P. gingivalis LPS and OMVs were investigated and correlated with transport of LPS. Additionally, the effect of P. gingivalis LPS and OMVs on human brain microvascular endothelial cells in monolayer was evaluated using immunofluorescence microscopy.

RESULTS

The integrity of the BBB model was weakened by application of P. gingivalis LPS and OMVs, as measured by a decrease in electrical resistance and a recovery deficit was seen in comparison to the controls. Application of P. gingivalis OMVs to a monoculture of human brain microvascular endothelial cells showed disruption of the tight junction zona occludens protein (ZO-1) compared to controls.

CONCLUSION

These findings show that the integrity of tight junctions of the human BBB could be weakened by association with P. gingivalis virulence factors LPS and OMVs containing proteolytic enzymes (gingipains).

Inhibition of Porphyromonas gingivalis peptidyl arginine deiminase, a virulence factor, by antioxidant-rich Cratoxylum cochinchinense: In vitro and in silico evaluation

Porphyromonas gingivalis, the cause of periodontitis, is also linked to many systemic disorders due to its citrullination capability from a unique peptidyl arginine deiminase (PPAD). Protein citrullination is able to trigger an autoimmune response, increasing the severity of rheumatoid arthritis. The main objective of this study is to evaluate the inhibitory activity of Cratoxylym cochinchinense leaves extract towards the PPAD in vitro and in silico. Methanolic extract of Cratoxylum cochinchinense (CCM) was tested for total phenolic and flavonoid contents along with antioxidative assays. Inhibition of PPAD activities was conducted thereafter using recombinant PPAD in cell lysate. Phytocompounds postulated present in the CCM such as mangiferin, vismiaquinone A, δ-tocotrienol and α-tocotrienol and canophyllol were used as ligands in a simulated docking study against PPAD. Results obtained indicated high antioxidant potential in CCM while recording abundant phenolic (129.0 ± 2.5495 mg GA/g crude extract) and flavonoid (159.0 ± 2.1529 mg QE/g crude extract) contents. A dose-dependent inhibition of PPAD was observed when CCM was evaluated at various concentrations. CCM at 1 mg/mL exhibited citrulline concentration of 24.37 ± 3.25 mM which was 5 times lower than the negative control (114.23 ± 3.31 mM). Molecular docking simulation revealed that mangiferin and vismiaquinone A engaged in H-bonding and pi-pi interactions with important active site residues (Asp130, Arg152, Arg154 and Trp127) of PPAD and could be the potential phytochemicals that accounted for the inhibitory activities observed in the methanolic leaves extract. As such, CCM could be further explored for its therapeutic properties not only for periodontitis, but also for other systemic diseases like rheumatoid arthritis.

TLR2 Activation by Porphyromonas gingivalis Requires Both PPAD Activity and Fimbriae

Porphyromonas gingivalis, a keystone oral pathogen implicated in development and progression of periodontitis, may also contribute to the pathogenicity of diseases such as arthritis, atherosclerosis, and Alzheimer's. P. gingivalis is a master manipulator of host immune responses due to production of a large variety of virulence factors. Among these, P. gingivalis peptidilarginine deiminase (PPAD), an enzyme unique to P. gingivalis, converts C-terminal Arg residues in bacterium- and host-derived proteins and peptides into citrulline. PPAD contributes to stimulation of proinflammatory responses in host cells and is essential for activation of the prostaglandin E2 (PGE2) synthesis pathway in gingival fibroblasts. Since P. gingivalis is recognized mainly by Toll-like receptor-2 (TLR2), we investigated the effects of PPAD activity on TLR2-dependent host cell responses to P. gingivalis, as well as to outer membrane vesicles (OMVs) and fimbriae produced by this organism. Using reporter cell lines, we found that PPAD activity was required for TLR2 activation by P. gingivalis cells and OMVs. We also found that fimbriae, an established TLR2 ligand, from wild-type ATCC 33277 (but not from its isogenic PPAD mutant) enhanced the proinflammatory responses of host cells. Furthermore, only fimbriae from wild-type ATCC 33277, but not from the PPAD-deficient strains, induced cytokine production and stimulated expression of genes within the PGE2 synthesis pathway in human gingival fibroblasts via activation of the NF-ĸB and MAP kinase-dependent signaling pathways. Analysis of ten clinical isolates revealed that type I FimA is preferable for TLR2 signaling enhancement. In conclusion, the data strongly suggest that both PPAD activity and fimbriae are important for TLR2-dependent cell responses to P. gingivalis infection.

Porphyromonas gingivalis survival skills: Immune evasion

Periodontitis is a chronic inflammatory condition that destroys the tooth-supporting tissues and eventually leads to tooth loss. As one of the most prevalent oral conditions, periodontitis endangers the oral health of 70% of people throughout the world. Periodontitis is also related to various systemic diseases, such as diabetes mellitus, atherosclerosis, and rheumatoid arthritis, which not only has a great impact on population health status and the quality of life but also increases the social burden. Porphyromonas gingivalis (P. gingivalis) is a gram-negative oral anaerobic bacterium that plays a key role in the pathogenesis of periodontitis. Porphyromonas gingivalis can express various of virulence factors to overturn innate and adaptive immunities, which makes P. gingivalis survive and propagate in the host, destroy periodontal tissues, and have connection to systemic diseases. Porphyromonas gingivalis can invade into and survive in host tissues by destructing the gingival epithelial barrier, internalizing into the epithelial cells, and enhancing autophagy in epithelial cells. Deregulation of complement system, degradation of antibacterial peptides, and destruction of phagocyte functions facilitate the evasion of P. gingivalis. Porphyromonas gingivalis can also suppress adaptive immunity, which allows P. gingivalis to exist in the host tissues and cause the inflammatory response persistently. Here, we review studies devoted to understanding the strategies utilized by P. gingivalis to escape host immunity. Methods for impairing P. gingivalis immune evasion are also mentioned.