Periodontitis Deteriorates Cognitive Function and Impairs Neurons and Glia in a Mouse Model of Alzheimer's Disease

Porphyromonas gingivalis virulence factors induce toxic effects in SH-SY5Y neuroblastoma cells: GRK5 modulation as a protective strategy

Periodontitis (PDS) is a chronic inflammatory disease initiated by a dysbiosis of oral pathogenic bacterial species, such as Porphyromonas gingivalis (Pg). These bacteria can penetrate the bloodstream, releasing various endo and exotoxins that fuel the infection, and stimulate toxic inflammation in different compartments, including the brain. However, the specific mechanisms by which PDS/Pg contribute to brain disorders, such as Alzheimer's disease (AD), remain unclear. This study assessed the effects of Pg's virulence factors - lipopolysaccharide (LPS-Pg) and gingipains (gps) K (Kgp) and Rgp - on SH-SY5Y cells. Our results demonstrated that LPS-Pg activated signaling through the Toll-like receptor (TLR)-2/4 induced a significant downregulation of G protein-coupled receptor kinase 5 (GRK5). Additionally, LPS-Pg stimulation resulted in a robust increase in Tau phosphorylation (pTau) and p53 levels, while causing a marked reduction in Bcl2 and increased cell death compared to unstimulated cells (Ns). LPS-Pg also elevated inducible nitric oxide synthase (iNOS) expression, leading to oxidative damage. In cells overexpressing GRK5 via Adenovirus, LPS-Pg failed to increase iNOS and pTau levels compared to GFP control cells. High GRK5 levels also prevented the nuclear accumulation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB). Furthermore, the overexpression of a GRK5 mutant form lacking the nuclear localization signal (ΔNLS) nearly abolished LPS-Pg induced p53 and iNOS upregulation. Finally, we tested whether Kgp and Rgp mediated similar effects and our data showed that both gps caused a marked downregulation of GRK5 leading to increased p53 and pTau levels. In conclusion, this study provides further insight into the toxic effects elicited by Pg in cells and suggests that preventing GRK5 deficiency may be a valid strategy to mitigate Pg-induced toxic effects (i.e. cell death, oxidative damage, and Tau hyperphosphorylation) in SH-SY5Y cells, which are typical molecular hallmarks of neurodegenerative disorders.

DO ORAL CARE AND REHABILITATION IMPROVE COGNITIVE FUNCTION? A SYSTEMATIC REVIEW OF CLINICAL STUDIES

OBJECTIVES

An increasing number of studies have identified an association between oral health status and cognitive function. However, the effect of oral interventions, including oral health care, dental treatment and oral motor exercises, on cognitive function remains unclear. This systematic review examined whether oral interventions contribute to the long-term improvement of cognitive status.

METHODS

Four databases were searched (MEDLINE, Web of Science, Cochrane Library, and ICHUSHI Web) to identify randomized and nonrandomized controlled trial studies and prospective cohort studies from inception until 1 September 2023, published in English or Japanese. The Cochrane risk of bias tool for randomized controlled trials and the risk of bias assessment tool for nonrandomized studies were used to assess bias risk.

RESULTS

A total of 20 articles were included in the qualitative analysis; 13 articles were published in English, and 7 were published in Japanese. The implemented interventions were oral care in 8 studies, dental treatment in 8 studies, and oral motor exercise in 4 studies. One study found a significant effect on attention following oral care intervention. Some dental treatments influenced cognitive function, although a clear positive effect was not determined. In 1 study, attention and working memory improved in the chewing exercise group.

CONCLUSIONS

Several studies verified the improvement effects of oral interventions, such as oral care, dental treatment, and oral motor exercise, on cognitive function or impairment. However, there was still a lack of conclusive evidence that such an intervention clearly improved cognitive function. To clarify the effects of oral interventions on cognitive function, it is necessary to examine participants, interventions, and outcome measures in detail.

The oral-brain axis: can periodontal pathogens trigger the onset and progression of Alzheimer's disease?

Alzheimer's disease (AD) is the most prevalent form of dementia, characterized by a progressive cognitive decline. Sporadic AD, accounting for more than 95% of cases, may arise due to the influence of environmental factors. It was reported that periodontitis, a common oral ailment, shares several risk factors with AD, including advanced age, smoking, diabetes, and hypertension, among others. Periodontitis is an inflammatory disease triggered by dysbiosis of oral microorganisms, whereas Alzheimer's disease is characterized by neuroinflammation. Many studies have indicated that chronic inflammation can instigate brain AD-related pathologies, including amyloid-β plaques, Tau protein hyperphosphorylation, neuroinflammation, and neurodegeneration. The potential involvement of periodontal pathogens and/or their virulence factors in the onset and progression of AD by the oral-brain axis has garnered significant attention among researchers with ongoing investigations. This review has updated the periodontal pathogens potentially associated with AD, elucidating their impact on the central nervous system, immune response, and related pathological processes in the brain to provide valuable insights for future research on the oral-brain axis.

Periodontitis salivary microbiota exacerbates colitis-induced anxiety-like behavior via gut microbiota

The gut-brain axis is a bidirectional communication system between the gut and central nervous system. Many host-related factors can affect gut microbiota, including oral bacteria, making the brain a vulnerable target via the gut-brain axis. Saliva contains a large number of oral bacteria, and periodontitis, a common oral disease, can change the composition of salivary microbiota. However, the role and mechanism of periodontitis salivary microbiota (PSM) on the gut-brain axis remain unclear. Herein, we investigated the nature and mechanisms of this relationship using the mice with dextran sulfate sodium salt (DSS)-induced anxiety-like behavior. Compared with healthy salivary microbiota, PSM worsened anxiety-like behavior; it significantly reduced the number of normal neurons and activated microglia in DSS mice. Antibiotic treatment eliminated the effect of PSM on anxiety-like behavior, and transplantation of fecal microbiota from PSM-gavaged mice exacerbated anxiety-like behavior. These observations indicated that the anxiety-exacerbating effect of PSM was dependent on the gut microbiota. Moreover, the PSM effect on anxiety-like behavior was not present in non-DSS mice, indicating that DSS treatment was a prerequisite for PSM to exacerbate anxiety. Mechanistically, PSM altered the histidine metabolism in both gut and brain metabolomics. Supplementation of histidine-related metabolites had a similar anxiety-exacerbating effect as that of PSM, suggesting that histidine metabolism may be a critical pathway in this process. Our results demonstrate that PSM can exacerbate colitis-induced anxiety-like behavior by directly affecting the host gut microbiota, emphasizing the importance of oral diseases in the gut-brain axis.

Current status and future prospects of stomatology research

Research in stomatology (dental medicine) continues to expand globally and is oriented towards solving clinical issues, focusing on clarifying the clinical relevance and potential mechanisms of oral‍-‍systemic connections via clinical epidemiology, oral microecological characterization, and the establishment of animal models. Interdisciplinary integration of materials science and tissue engineering with stomatology is expected to lead to the creation of innovative materials and technologies to better resolve the most prevalent and challenging clinical issues such as peri-implantitis, soft and hard tissue defects, and dentin hypersensitivity. With the rapid development of artificial intelligence (AI), 5th generation mobile communication technology (5G), and big data applications, "intelligent stomatology" is emerging to build models for better clinical diagnosis and management, accelerate the reform of education, and support the growth and advancement of scientific research. Here, we summarized the current research status, and listed the future prospects and limitations of these three aspects, aiming to provide a basis for more accurate etiological exploration, novel treatment methods, and abundant big data analysis in stomatology to promote the translation of research achievements into practical applications for both clinicians and the public.

Periodontal Pathogens and Their Links to Neuroinflammation and Neurodegeneration

Pathogens that play a role in the development and progression of periodontitis have gained significant attention due to their implications in the onset of various systemic diseases. Periodontitis is characterized as an inflammatory disease of the gingival tissue that is mainly caused by bacterial pathogens. Among them, Porphyromonas gingivalis, Treponema denticola, Fusobacterium nucleatum, Aggregatibacter actinomycetemcomitans, and Tannerella forsythia are regarded as the main periodontal pathogens. These pathogens elicit the release of cytokines, which in combination with their virulence factors induce chronic systemic inflammation and subsequently impact neural function while also altering the permeability of the blood-brain barrier. The primary objective of this review is to summarize the existing information regarding periodontal pathogens, their virulence factors, and their potential association with neuroinflammation and neurodegenerative diseases. We systematically reviewed longitudinal studies that investigated the association between periodontal disease and the onset of neurodegenerative disorders. Out of the 24 studies examined, 20 showed some degree of positive correlation between periodontal disease and neurodegenerative disorders, with studies focusing on cognitive function demonstrating the most robust effects. Therefore, periodontal pathogens might represent an exciting new approach to develop novel preventive treatments for neurodegenerative diseases.

Association between Periodontal Disease and Cognitive Impairment in Adults

INTRODUCTION

Periodontitis is a severe oral infection that can contribute to systemic inflammation. A large body of evidence suggests a role for systemic inflammation in the initiation of neurodegenerative disease. This systematic review synthesized data from observational studies to investigate the association between periodontitis and neuroinflammation in adults.

METHODS AND MATERIALS

A systematic literature search of PubMed, Web of Science, and Cumulative Index to Nursing and Allied Health Literature (CINAHL) was performed for studies published from the date of inception up to September 2021. Search terms for the exposure "oral disease" and outcome "dementia", "neuroinflammation" and "cognitive decline" were used. Study selection and data extraction were independently undertaken by two reviewers. The final eligible articles were included only if the exposure is periodontitis and the outcome is cognitive impairment or dementia or a topic related to this condition, and if the study was conducted in an adult population. The quality and risk of bias were assessed by Newcastle Ottawa Scale (NOS). Qualitative synthesis was used to narratively synthesize the results. Six cohort studies, three cross-sectional studies, and two case-control studies met the inclusion criteria. These eleven studies were only narratively synthesized. Meta-analysis was not performed due to the methodological heterogeneity of the studies.

RESULTS

The results of included studies show that chronic periodontitis patients with at least eight years of exposure are at higher risk of developing cognitive decline and dementia. Oral health measures such as gingival inflammation, attachment loss, probing depth, bleeding on probing, and alveolar bone loss are associated with cognitive impairment. The reduction of epidermal growth factor (EGF), interleukin 8 (IL-8), interferon γ-induced protein 10 (IP-10), and monocyte chemoattractant protein-1 (MCP-1) in addition to over expression of interleukin 1-β (IL-1β) are significant in patients suffering from cognitive decline with pre-existing severe periodontitis.

CONCLUSIONS

All the included studies show evidence of an association between periodontitis and cognitive impairment or dementia and Alzheimer's disease pathology. Nonetheless, the mechanisms responsible for the association between periodontitis and dementia are still unclear and warrant further investigation.

Porphyromonas gingivalis bacteremia increases the permeability of the blood-brain barrier via the Mfsd2a/Caveolin-1 mediated transcytosis pathway

Bacteremia induced by periodontal infection is an important factor for periodontitis to threaten general health. P. gingivalis DNA/virulence factors have been found in the brain tissues from patients with Alzheimer's disease (AD). The blood-brain barrier (BBB) is essential for keeping toxic substances from entering brain tissues. However, the effect of P. gingivalis bacteremia on BBB permeability and its underlying mechanism remains unclear. In the present study, rats were injected by tail vein with P. gingivalis three times a week for eight weeks to induce bacteremia. An in vitro BBB model infected with P. gingivalis was also established. We found that the infiltration of Evans blue dye and Albumin protein deposition in the rat brain tissues were increased in the rat brain tissues with P. gingivalis bacteremia and P. gingivalis could pass through the in vitro BBB model. Caveolae were detected after P. gingivalis infection in BMECs both in vivo and in vitro. Caveolin-1 (Cav-1) expression was enhanced after P. gingivalis infection. Downregulation of Cav-1 rescued P. gingivalis-enhanced BMECs permeability. We further found P. gingivalis-gingipain could be colocalized with Cav-1 and the strong hydrogen bonding between Cav-1 and arg-specific-gingipain (RgpA) were detected. Moreover, P. gingivalis significantly inhibited the major facilitator superfamily domain containing 2a (Mfsd2a) expression. Mfsd2a overexpression reversed P. gingivalis-increased BMECs permeability and Cav-1 expression. These results revealed that Mfsd2a/Cav-1 mediated transcytosis is a key pathway governing BBB BMECs permeability induced by P. gingivalis, which may contribute to P. gingivalis/virulence factors entrance and the subsequent neurological impairments.

Periodontitis-related salivary microbiota aggravates Alzheimer's disease via gut-brain axis crosstalk

The oral cavity is the initial chamber of digestive tract; the saliva swallowed daily contains an estimated 1.5 × 10¹² oral bacteria. Increasing evidence indicates that periodontal pathogens and subsequent inflammatory responses to them contribute to the pathogenesis of Alzheimer's disease (AD). The intestine and central nervous system jointly engage in crosstalk; microbiota-mediated immunity significantly impacts AD via the gut-brain axis. However, the exact mechanism linking periodontitis to AD remains unclear. In this study, we explored the influence of periodontitis-related salivary microbiota on AD based on the gut-brain crosstalk in APP^swe/PS1^ΔE9 (PAP) transgenic mice. Saliva samples were collected from patients with periodontitis and healthy individuals. The salivary microbiota was gavaged into PAP mice for two months. Continuous gavage of periodontitis-related salivary microbiota in PAP mice impaired cognitive function and increased β-amyloid accumulation and neuroinflammation. Moreover, these AD-related pathologies were consistent with gut microbial dysbiosis, intestinal pro-inflammatory responses, intestinal barrier impairment, and subsequent exacerbation of systemic inflammation, suggesting that the periodontitis-related salivary microbiota may aggravate AD pathogenesis through crosstalk of the gut-brain axis. In this study, we demonstrated that periodontitis might participate in the pathogenesis of AD by swallowing salivary microbiota, verifying the role of periodontitis in AD progression and providing a novel perspective on the etiology and intervention strategies of AD.

COR388 (atuzaginstat): an investigational gingipain inhibitor for the treatment of Alzheimer disease

INTRODUCTION

Evidence from in vitro and in vivo studies demonstrates that amyloid beta (Aβ) oligomers have potent, broad-spectrum antimicrobial properties created by fibrils that entrap pathogens and disrupt their membranes. Data suggest that Aβ may play a protective role in the innate immune response to microbial infections and that Aβ in the brain plays a damaging role when the inflammatory response is not well controlled.

AREAS COVERED

This paper describes the relationship between periodontal disease and Alzheimer disease (AD), the role of Porphyromonas gingivalis and its secreted gingipains in AD, and the potential of the gingipain inhibitor atuzaginstat (COR388) to modulate AD neuropathologies.

EXPERT OPINION

P. gingivalis is opsonized by Aβ42, is capable of entering the brain, and is an accelerant of neuropathologies in rodent models of AD. Thus, in our opinion, this bacteria is highly likely to be a pathogen capable of initiating or precipitating the progression of AD, which agrees with the pathogen hypothesis of clinical AD development.

Outer membrane vesicles of Porphyromonas gingivalis trigger NLRP3 inflammasome and induce neuroinflammation, tau phosphorylation, and memory dysfunction in mice

Background

Porphyromonas gingivalis (Pg), the keystone pathogen in chronic periodontitis, is reported to initiate Alzheimer’s disease pathologies in preclinical studies. However, the specific mechanisms and signaling pathways acting on the brain still need to be further explored. Outer membrane vesicles are derived from Gram-negative bacteria and contain many virulence factors of bacteria. We hypothesized that outer membrane vesicles are an important weapon of Porphyromonas gingivalis to initiate Alzheimer’s disease pathologies.

Methods

The outer membrane vesicles of Porphyromonas gingivalis (Pg OMVs, 4 mg/kg) or saline were delivered to 14-month-old mice by oral gavage every other day for eight weeks. Behavioral alterations were assessed by the open field test, Morris water maze, and Y-maze test. Blood–brain barrier permeability, neuroinflammation, tau phosphorylation, and NLRP3 inflammasome-related protein were analyzed.

Results

Pg OMVs impaired memory and learning ability of mice and decreased tight junction–related gene expression ZO-1, occludin, claudin-5, and occludin protein expression in the hippocampus. Pg OMVs could be detected in the hippocampus and cortex three days after oral gavage. Furthermore, Pg OMVs activated both astrocytes and microglia and elevated IL-1β, tau phosphorylation on the Thr231 site, and NLRP3 inflammasome–related protein expression in the hippocampus. In in vitro studies, Pg OMV (5 µg/ml) stimulation increased the mRNA and immunofluorescence of NLRP3 in BV2 microglia, which were significantly inhibited by the NLRP3 inhibitor MCC950. In contrast, the tau phosphorylation in N2a neurons was enhanced after treatment with conditioned media from Pg OMV-stimulated microglia, which was attenuated after pretreatment with MCC950.

Conclusions

These results indicate that Pg OMVs prompt memory dysfunction, neuroinflammation, and tau phosphorylation and trigger NLRP3 inflammasome in the brain of middle-aged mice. We propose that Pg OMVs play an important role in activating neuroinflammation in the AD-like pathology triggered by Porphyromonas gingivalis, and NLRP3 inflammasome activation is a possible mechanism.

Experimental Periodontitis Deteriorates Cognitive Function and Impairs Insulin Signaling in a Streptozotocin-Induced Alzheimer’s Disease Rat Model

Background: With advancements in periodontal medicine, the relationship between periodontitis and systemic diseases has garnered increasing attention. Recently, emerging evidence has indicated that periodontitis may be involved in the pathogenesis of Alzheimer’s disease (AD). Objective: To assess the impact of experimental periodontitis on cognitive function deficits in a rat model of streptozotocin-induced AD and determine the mechanisms underlying these effects. Methods: Rats were randomly assigned to the control (C), experimental periodontitis (P), Alzheimer’s disease (AD), and experimental periodontitis with streptozotocin-induced AD (AD-P) groups. Experimental periodontitis was induced using ligation and coating with Porphyromonas gingivalis. In the AD-P group, AD was induced by intracerebroventricular injection of streptozotocin after 6 weeks of experimental periodontitis induction. Results: Compared with the group C rats, those in group P exhibited alveolar bone resorption, learning and memory function impairment, and decreased insulin sensitivity and insulin signaling-related protein expression. Glial cell activation and cognitive impairment in streptozotocin-induced groups with significantly increased phosphorylated tau levels were more pronounced relative to the C group. The number of neurons and insulin sensitivity and insulin signaling-related protein expression in group AD-P rats were lower than those in the AD alone group, while the expressions of glial fibrillary acidic protein, tau phosphorylation, interleukin-6, and cyclooxygenase-2 were significantly increased. Conclusion: Periodontitis may be a risk factor exacerbating cognitive deficits in an AD-like neurodegenerative context, possibly by impairing the insulin signaling pathway and stimulating gliosis and neuroinflammation.

Periodontal microorganisms and Alzheimer disease - A causative relationship?

In the initiation or exacerbation of Alzheimer disease, the dissemination of oral microorganisms into the brain tissue or the low‐level systemic inflammation have been speculated to play a role. However, the impact of oral microorganisms, such as Porphyromonas gingivalis, on the pathogenesis of Alzheimer disease and the potential causative relationship is still unclear. The present review has critically reviewed the literature by examining the following aspects: (a) the oral microbiome and the immune response in the elderly population, (b) human studies on the association between periodontal and gut microorganisms and Alzheimer disease, (c) animal and in vitro studies on microorganisms and Alzheimer disease, and (d) preventive and therapeutic approaches. Factors contributing to microbial dysbiosis seem to be aging, local inflammation, systemic diseases, wearing of dentures, living in nursing homes and no access to adequate oral hygiene measures. Porphyromonas gingivalis was detectable in post‐mortem brain samples. Microbiome analyses of saliva samples or oral biofilms showed a decreased microbial diversity and a different composition in Alzheimer disease compared to cognitively healthy subjects. Many in‐vitro and animal studies underline the potential of P gingivalis to induce Alzheimer disease‐related alterations. In animal models, recurring applications of P gingivalis or its components increased pro‐inflammatory mediators and β‐amyloid in the brain and deteriorated the animals' cognitive performance. Since periodontitis is the result of a disturbed microbial homoeostasis, an effect of periodontal therapy on the oral microbiome and host response related to cognitive parameters may be suggested and should be elucidated in further clinical trials.

Cofilin 2 Acts as an Inflammatory Linker Between Chronic Periodontitis and Alzheimer's Disease in Amyloid Precursor Protein/Presenilin 1 Mice

Increasing evidence has shown a correlation between chronic periodontitis (CP) and Alzheimer’s disease (AD). Nevertheless, there is still a lack of direct evidence, and especially key molecules to connect the two diseases. This study aims to investigate potential protein links between CP and AD within the inflammatory aspect. The hippocampus of CP model mice and controls were collected, and changes in protein expression were evaluated using two-dimensional differential in-gel electrophoresis (2D-DIGE) analysis combined with liquid chromatography tandem mass spectrometry. A total of 15 differentially expressed proteins were identified in CP model mice, as compared with the controls. Among them, S100-A9, transthyretin, Cofilin 2, peroxiredoxin 2, and lipocalin-2 were validated by Western blot according to their dual function both in inflammation and AD. Based on 2D-DIGE analysis, CP animal model had higher levels of S100-A9, Cofilin 2, peroxiredoxin 2, and lipocalin-2 compared to controls. The level of Cofilin 2, one of the well-established proteins in the pathology of AD, was strongly correlated with the time course of CP pathology, indicating a specific molecular correlation between CP and AD. Moreover, the in vivo results showed the level of Cofilin 2 increased significantly along with a prominent increase of the phosphorylation of protein phosphatase 2 (PP2A) and tau protein in the cell lysates of Porphyromonas gingivalis (P.g-LPS)-treated SK-N-SH APPwt cells. Cofilin 2 inhibition resulted in a sharp decrease in PP2A dependent of tau phosphorylation. Furthermore, tumor growth factor (TGF)-β1 was one of the most important inflammatory cytokines for the Pg-LPS-induced Cofilin 2 upregulation in SK-N-SH APPwt cells. These results showed inflammation served as the bond between CP and AD, whereas inflammatory related proteins could be the key linkers between the two diseases. Determining the association between CP and AD at the molecular mechanism will not only hold the direct evidence of the association between the two diseases but also provide a new way of preventing and treating AD: the effective prevention and treatment of CP could serve as a useful method to alleviate the development of AD.

Application of Ligature-Induced Periodontitis in Mice to Explore the Molecular Mechanism of Periodontal Disease

Periodontitis is an inflammatory disease characterized by the destruction of the periodontium. In the last decade, a new murine model of periodontitis has been widely used to simulate alveolar bone resorption and periodontal soft tissue destruction by ligation. Typically, 3-0 to 9-0 silks are selected for ligation around the molars in mice, and significant bone loss and inflammatory infiltration are observed within a week. The ligature-maintained period can vary according to specific aims. We reviewed the findings on the interaction of systemic diseases with periodontitis, periodontal tissue destruction, the immunological and bacteriological responses, and new treatments. In these studies, the activation of osteoclasts, upregulation of pro-inflammatory factors, and excessive immune response have been considered as major factors in periodontal disruption. Multiple genes identified in periodontal tissues partly reflect the complexity of the pathogenesis of periodontitis. The effects of novel treatment methods on periodontitis have also been evaluated in a ligature-induced periodontitis model in mice. This model cannot completely represent all aspects of periodontitis in humans but is considered an effective method for the exploration of its mechanisms. Through this review, we aimed to provide evidence and enlightenment for future studies planning to use this model.