Host surface ectonucleotidase-CD73 and the opportunistic pathogen, Porphyromonas gingivalis, cross-modulation underlies a new homeostatic mechanism for chronic bacterial survival in human epithelial cells

Patients with periodontal disease demonstrates changes in purinergic and inflammatory markers in PBMCs, serum and saliva

Periodontal disease (PD) is characterized by the presence of a chronic inflammatory process, due to the accumulation of bacterial biofilm and the host's response to these pathogens, resulting in the destruction of the supporting tissues of dental structures. Studies have revealed that components of the purinergic system and inflammation are related to the development and progression of PD. The objective was to evaluate periodontal clinical parameters, modulation of the purinergic system and inflammation in patients with PD, compared to individuals without the disease. This is a cross-sectional study with 25 healthy individuals (CT group) and 57 individuals with PD, where blood and saliva collection and isolation of blood components were carried out. The results showed that there was a significant reduction in the hydrolysis of adenosine triphosphate (ATP; p < 0.0001), adenosine diphosphate (ADP; p < 0.05) and adenosine monophosphate (AMP; p < 0.01) in peripheral blood mononuclear cells (PBMCs) from individuals in the PD group compared to the CT group, indicating that individuals with PD showed reduced NTPDase 1 and Ecto-5'-nucleotidase activity. Adenosine deaminase activity in saliva (p < 0.0001) and serum p < 0.05) from individuals with PD were significantly higher compared to the CT group. Extracellular ATP and the serum concentration of C-Reactive Protein showed a statistically significant increase in the PD group ((p < 0.0001 and p < 0.001, respectively). Therefore, the enzymes of the purinergic system are present in the modulation of PD, leading individuals affected by the disease to a pro-inflammatory state, hindering the action of the immune system and increasing serum markers of inflammation.

Bacterial Protein Signatures Identified in Porphyromonas gingivalis Containing-Autophagic Vacuoles Reveal Co-Evolution Between Oral Red/Orange Complex Bacteria and Gut Bacteria

Modern oral bacterial species present as a concoction of commensal and opportunistic pathogens originating from their evolution in humans. Due to the intricate colonization mechanisms shared amongst oral and gut bacteria, these bacteria have likely evolved together to establish and adapt in the human oro-digestive tract, resulting in the transfer of genetic information. Our liquid chromatography-with-tandem-mass-spectrometry (LC-MS-MS) analyses have revealed protein signatures, Elongation Factor Tu, RagB/SusD nutrient uptake outer membrane protein and DnaK, specifically from Porphyromonas gingivalis -containing autophagic vacuoles isolated from the infected human primary gingival epithelial cells. Interestingly, our Mass-Spectrometry analysis reported similar proteins from closely related oral bacteria, Tannerella forsythia and Prevotella intermedia . In our phylogenetic study of these key protein signatures, we have established that pathogenic oral bacteria share extensive relatedness to each other and gut resident bacteria. We show that in the virulence factors identified from gut bacteria, Elongation Factor Tu and DnaK, there are several structural similarities and conservations with proteins from oral pathogenic bacteria. There are also major similarities in the RagB/SusD proteins of oral bacteria to prominent gut bacteria. These findings not only highlight the shared virulence mechanisms amongst oral bacterial pathogens/pathobionts but also gut bacteria and elucidate their co-evolutions in the human host.

Glutamate is a key nutrient for Porphyromonas gingivalis growth and survival during intracellular autophagic life under nutritionally limited conditions

Porphyromonas gingivalis survives in special autophagic vacuoles that serve as major replicative habitats in human primary gingival epithelial cells (GECs). As an asaccharolytic strict anaerobe, P. gingivalis is dependent on amino acids and peptides for nutrient sources. However, it is largely unknown as to P. gingivalis' metabolic processing under the nutritionally limited intracellular environments such the vacuoles, especially the preferred amino acids and associated-metabolic machineries. Here we elucidate that a Glutamate (Glu) catabolic enzyme, glutamate dehydrogenase (GdhA) is highly enriched in the isolated P. gingivalis -containing vacuoles. Interestingly, we found that P. gingivalis induces conversion of intracellular glutamine pool to Glu determined by analyses of the P. gingivalis- containing vacuoles and the whole infected-GECs. Critically, exogenous Glu-Glu dipeptide, a simple precursor of Glu, significantly increases the size of isolated intact P. gingivalis containing-vacuoles and live wild-type P. gingivalis numbers in GECs. In contrast, the isogenic GdhA-deficient-strain, Δ gdhA displayed a significant growth defect with collapsed-vacuoles in GECs. Next, we confirmed that P. gingivalis uptakes 14 C-Glu and it preferentially utilizes Glu-Glu-dipeptide using a nutritionally reduced Tryptic-Soy-Broth (TSB) media supplemented with Glu-Glu. Contrary, Δ gdhA -strain showed no detectable growth especially in nutritionally reduced TSB media with Glu-Glu. Using Atomic-Force-Microscopy, we observed that, wild-type P. gingivalis but not Δ gdhA strain notably increased the cell volume upon Glu-Glu supplementation, an indicator of higher metabolism and growth. Utilization of a human gingiva-mimicking organoid-system further validated the importance of Glu as an essential nutrient for the intramucosal colonization of P. gingivalis via the protected replicative vacuoles in GECs.

Importance

This study reveals that P. gingivalis heavily depends on preferential utilization of Glutamate (Glu) for autophagic vacuolar growth and survival in human GECs. Several novel observations are made to support this: (i) GdhA of P. gingivalis is highly enriched in these vacuoles, (ii) P. gingivalis induces a large conversion of intracellular glutamine to Glu, (iii) size of vacuoles are significantly increased in the presence of Glu-Glu in P. gingivalis wild-type strain infection which is opposite in a Δ gdhA strain, (iv) P. gingivalis uptakes 14 C-Glu and preferentially utilizes Glu-Glu dipeptide, (v) similarly, wild-type strain shows growth increase in a nutritionally reduced bacterial culture media, and (vi) finally, Glu-Glu supplementation increases bacterial cell-volume of P. gingivalis wild-type but not Δ gdhA strain, an indicator of higher metabolism and growth. Taken together, this study highlights the pathophysiological importance of Glu for P. gingivalis growth-rate, biomass induction and survival in nutritionally limited host subcellular environments.

Porphyromonas gingivalis activates Heat-Shock-Protein 27 to drive a LC3C-specific probacterial form of select autophagy that is redox sensitive for intracellular bacterial survival in human gingival mucosa

Porphyromonas gingivalis , a major oral pathobiont, evades canonical host pathogen clearance in human primary gingival epithelial cells (GECs) by initiating a non-canonical variant of autophagy consisting of Microtubule-associated protein 1A/1B-light chain 3 (LC3)-rich autophagosomes, which then act as replicative niches. Simultaneously, P. gingivalis inhibits apoptosis and oxidative-stress, including extracellular-ATP (eATP)-mediated reactive-oxygen-species (ROS) production via phosphorylating Heat Shock Protein 27 (HSp27) with the bacterial nucleoside-diphosphate-kinase (Ndk). Here, we have mechanistically identified that P. gingivalis -mediated induction of HSp27 is crucial for the recruitment of the LC3 isoform, LC3C, to drive the formation of live P. gingivalis -containing Beclin1-ATG14-rich autophagosomes that are redox sensitive and non-degrading. HSp27 depletions of both infected GECs and gingiva-mimicking organotypic-culture systems resulted in the collapse of P. gingivalis -mediated autophagosomes, and abolished P. gingivalis -induced LC3C-specific autophagic-flux in a HSp27-dependent manner. Concurrently, HSp27 depletion accompanied by eATP treatment abrogated protracted Beclin 1-ATG14 partnering and decreased live intracellular P. gingivalis levels. These events were only partially restored via treatments with the antioxidant N-acetyl cysteine (NAC), which rescued the cellular redox environment independent of HSp27. Moreover, the temporal phosphorylation of HSp27 by the bacterial Ndk results in HSp27 tightly partnering with LC3C, hindering LC3C canonical cleavage, extending Beclin 1-ATG14 association, and halting canonical maturation. These findings pinpoint how HSp27 pleiotropically serves as a major platform-molecule, redox regulator, and stepwise modulator of LC3C during P. gingivalis -mediated non-canonical autophagy. Thus, our findings can determine specific molecular strategies for interfering with the host-adapted P. gingivalis ' successful mucosal colonization and oral dysbiosis.

Illuminating the oral microbiome: cellular microbiology

Epithelial cells line mucosal surfaces such as in the gingival crevice and provide a barrier to the ingress of colonizing microorganisms. However, epithelial cells are more than a passive barrier to microbial intrusion, and rather constitute an interactive interface with colonizing organisms which senses the composition of the microbiome and communicates this information to the underlying cells of the innate immune system. Microorganisms, for their part, have devised means to manipulate host cell signal transduction pathways to favor their colonization and survival. Study of this field, which has become known as cellular microbiology, has revealed much about epithelial cell physiology, bacterial colonization and pathogenic strategies, and innate host responses.

Porphyromonas gingivalis infection upregulates the endothelin (ET) system in brain microvascular endothelial cells

Endothelin-1 (ET-1), the most potent vasoconstrictor identified to date, contributes to cerebrovascular dysfunction and brain ET-1 levels were shown to be related to Alzheimer's disease and related dementias (ADRD) progression. ET-1 also contributes to neuroinflammation, especially in infections of the central nervous system. Recent studies causally linked chronic periodontal infection with an opportunistic anaerobic bacterium Porphyromonas gingivalis (Coykendall et al.) Shah & Collins to AD development. Thus, the goal of the study was to determine the impact of P. gingivalis infection on the ET system and cell senescence in brain microvascular endothelial cells. Cells were infected with a multiplicity of infection 50 P. gingivalis with and without extracellular ATP-induced oxidative stress for 24 h. Cell lysates were collected for analysis of endothelin A receptor (ETA)/endothelin B receptor (ETB) receptor as well as senescence markers. ET-1 levels in cell culture media were measured with enzyme-linked immunosorbent assay. P. gingivalis infection increased ET-1 (pg/mL) secretion, as well as the ETA receptor expression, whereas decreased lamin A/C expression compared to control. Tight junction protein claudin-5 was also decreased under these conditions. ETA or ETB receptor blockade during infection did not affect ET-1 secretion or the expression of cell senescence markers. Current findings suggest that P. gingivalis infection may compromise endothelial integrity and activate the ET system.

Leaky Gum: The Revisited Origin of Systemic Diseases

The oral cavity is the gateway for microorganisms into your body where they disseminate not only to the directly connected respiratory and digestive tracts but also to the many remote organs. Oral microbiota, travelling to the end of the intestine and circulating in our bodies through blood vessels, not only affect a gut microbiome profile but also lead to many systemic diseases. By gathering information accumulated from the era of focal infection theory to the age of revolution in microbiome research, we propose a pivotal role of "leaky gum", as an analogy of "leaky gut", to underscore the importance of the oral cavity in systemic health. The oral cavity has unique structures, the gingival sulcus (GS) and the junctional epithelium (JE) below the GS, which are rarely found anywhere else in our body. The JE is attached to the tooth enamel and cementum by hemidesmosome (HD), which is structurally weaker than desmosome and is, thus, vulnerable to microbial infiltration. In the GS, microbial biofilms can build up for life, unlike the biofilms on the skin and intestinal mucosa that fall off by the natural process. Thus, we emphasize that the GS and the JE are the weakest leaky point for microbes to invade the human body, making the leaky gum just as important as, or even more important than, the leaky gut.

Inflammasome Activation in Gingival Epithelial Cells

Inflammasomes are multiprotein complexes that assemble in host cells upon recognition of infection or danger via pattern recognition receptors and/or danger recognition receptors. The assembly of inflammasomes results in the activation of caspase-1 and is followed by the enzymatic maturation and secretion of inflammatory cytokines like interleukin 1β (IL-1β) and IL-18.In the oral cavity, gingival epithelial cells (GECs) line the mucosa and have a barrier role for invading pathogens. In these cells, the NLRP3 inflammasome is the best studied and has been shown to play a role in the inflammatory immune response against a variety of oral pathogens. As such, in order to study gingivitis and other oral pathologic inflammatory conditions, studying the activation of inflammasomes is important. The simplest way to detect inflammasome activation is to detect the activated caspase-1, as well as the secretion of mature IL-1β and IL-18, via ELISA, Western blot, and immunofluorescence techniques.Here we describe the detection of NLRP3 inflammasome activation by the oral pathogen Porphyromonas gingivalis in human GECs via these three methods and mention other methods and techniques that we have successfully used together with these in our quest to understand the host-pathogen interaction.

Immune Tolerance in the Oral Mucosa

The oral mucosa is a site of intense immune activity, where a large variety of immune cells meet to provide a first line of defense against pathogenic organisms. Interestingly, the oral mucosa is exposed to a plethora of antigens from food and commensal bacteria that must be tolerated. The mechanisms that enable this tolerance are not yet fully defined. Many works have focused on active immune mechanisms involving dendritic and regulatory T cells. However, epithelial cells also make a major contribution to tolerance by influencing both innate and adaptive immunity. Therefore, the tolerogenic mechanisms concurring in the oral mucosa are intertwined. Here, we review them systematically, paying special attention to the role of oral epithelial cells.

Purinergic signaling in the modulation of redox biology

Purinergic signaling is a cell communication pathway mediated by extracellular nucleotides and nucleosides. Tri- and diphosphonucleotides are released in physiological and pathological circumstances activating purinergic type 2 receptors (P2 receptors): P2X ion channels and P2Y G protein-coupled receptors. The activation of these receptors triggers the production of reactive oxygen and nitrogen species and alters antioxidant defenses, modulating the redox biology of cells. The activation of P2 receptors is controlled by ecto-enzymes named ectonucleotidases, E-NTPDase1/CD39 and ecto-5'-nucleotidase/CD73) being the most relevant. The first enzyme hydrolyzes adenosine triphosphate (ATP) and adenosine diphosphate (ADP) into adenosine monophosphate (AMP), and the second catalyzes the hydrolysis of AMP to adenosine. The activity of these enzymes is diminished by oxidative stress. Adenosine actives P1 G-coupled receptors that, in general, promote the maintenance of redox hemostasis by decreasing reactive oxygen species (ROS) production and increase antioxidant enzymes. Intracellular purine metabolism can also contribute to ROS generation via xanthine oxidase activity, which converts hypoxanthine into xanthine, and finally, uric acid. In this review, we describe the mechanisms of redox biology modulated by purinergic signaling and how this signaling may be affected by disturbances in the redox homeostasis of cells.

Key Elements of Gingival Epithelial Homeostasis upon Bacterial Interaction

Epithelia are structurally integral elements in the fabric of oral mucosa with significant functional roles. Similarly, the gingival epithelium performs uniquely critical tasks in responding to a variety of external stimuli and dangers through the regulation of specific built-in molecular mechanisms in a context-dependent fashion at cellular levels. Gingival epithelial cells form an anatomic architecture that confers defense, robustness, and adaptation toward external aggressions, most critically to colonizing microorganisms, among other functions. Accordingly, recent studies unraveled previously uncharacterized response mechanisms in gingival epithelial cells that are constructed to rapidly exert biocidal effects against invader pathobiotic bacteria, such as Porphyromonas gingivalis, through small danger molecule signaling. The host-adapted bacteria, however, have developed adroit strategies to 1) exploit the epithelia as privileged growth niches and 2) chronically target cellular bactericidal and homeostatic metabolic pathways for successful bacterial persistence. As the overgrowth of colonizing microorganisms in the gingival mucosa can shift from homeostasis to dysbiosis or a diseased state, it is crucial to understand how the innate modulatory molecules are intricately involved in antibacterial pathways and how they shape susceptibility versus resistance in the epithelium toward pathogens. Thus, in this review, we highlight recent discoveries in gingival epithelial cell research in the context of bacterial colonizers. The current knowledge outlined here demonstrates the ability of epithelial cells to possess highly organized defense machineries, which can jointly regulate host-derived danger molecule signaling and integrate specific global responses against opportunistic bacteria to combat microbial incursion and maintain host homeostatic balance. These novel examples collectively suggest that the oral epithelia are equipped with a dynamically robust and interconnected defense system encompassing sensors and various effector molecules that arrange and achieve a fine-tuned and advanced response to diverse bacteria.

Temporal association between human upper respiratory and gut bacterial microbiomes during the course of COVID-19 in adults

SARS-CoV-2 is the cause of COVID-19. It infects multiple organs including the respiratory tract and gut. Dynamic changes of regional microbiomes in infected adults are largely unknown. Here, we performed longitudinal analyses of throat and anal swabs from 35 COVID-19 and 19 healthy adult controls, as well as 10 non-COVID-19 patients with other diseases, by 16 S rRNA gene sequencing. The results showed a partitioning of the patients into 3-4 categories based on microbial community types (I-IV) in both sites. The bacterial diversity was lower in COVID-19 patients than healthy controls and decreased gradually from community type I to III/IV. Although the dynamic change of microbiome was complex during COVID-19, a synchronous restoration of both the upper respiratory and gut microbiomes from early dysbiosis towards late more diverse status was observed in 6/8 mild COVID-19 adult patients. These findings reveal previously unknown interactions between upper respiratory and gut microbiomes during COVID-19.

Rong Xu, Renfei Lu, Tao Zhang, Qunfu Wu, and colleagues perform a longitudinal analysis of throat and gut microbiomes on adult COVID-19 patients. They find initially lower bacterial diversity in COVID-19 patients, but demonstrate a restoration of microbiome diversity throughout disease progression for patients with mild forms of the disease.

In Situ Intraepithelial Localizations of Opportunistic Pathogens, Porphyromonas gingivalis and Filifactor alocis, in Human Gingiva

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The gingival epithelium serves as a growth reservoir for opportunistic bacteria.

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Intraepithelial P. gingivalis and F. alocis colonies are detected together in dysbiotic mucosa.

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Increased metabolically active dual species can lead to higher microvasculature.

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Invasion of intraepithelial microvessels leads to systemic pathogen dissemination.

Porphyromonas gingivalis and Filifactor alocis are fastidious oral pathogens and etiological agents associated with chronic periodontitis. Although previous studies showed increased levels of the two obligate anaerobic species in periodontitis patients, methodologies for this knowledge were primarily limited to sampling subgingival plaque, saliva, or gingival crevicular fluid. To evaluate the extent to which P. gingivalis and F. alocis may invade the periodontal tissues, an in situ cross-sectional study was comparatively conducted on the gingival biopsy specimens of patients diagnosed with periodontal health or chronic periodontitis. Immunostained tissue sections for each organism were imaged by Super-Resolution Confocal Scanning Microscopy to determine the relative presence and localization of target bacterial species. Fluorescence-in-situ-hybridization (FISH) coupled with species specific 16S rRNA method was utilized to confirm whether detected bacteria were live within the tissue. In periodontitis, P. gingivalis and F. alocis revealed similarly concentrated localization near the basement membrane or external basal lamina of the gingival epithelium. The presence of both bacteria was significantly increased in periodontitis vs. healthy tissue. However, P. gingivalis was still detected to an extent in health tissue, while only minimal levels of F. alocis were spotted in health. Additionally, the micrographic analyses displayed heightened formation of epithelial microvasculature containing significantly co-localized and metabolically active dual species within periodontitis tissue. Thus, this study demonstrates, for the first-time, spatial arrangements of P. gingivalis and F. alocis in both single and co-localized forms within the complex fabric of human gingiva during health and disease. It also exhibits critical visualizations of co-invaded microvascularized epithelial layer of the tissue by metabolically active P. gingivalis and F. alocis from patients with severe periodontitis. These findings collectively uncover novel visual evidence of a potential starting point for systemic spread of opportunistic bacteria during their chronic colonization in gingival epithelium.