Neuroinflammation changes with periodontal inflammation status during periodontitis in wild-type mice

Periodontitis-induced neuroinflammation impacts dendritic spine immaturity and cognitive impairment

OBJECTIVE

This study investigated the spinal changes in ligature-induced periodontitis and the role of periodontitis in cognitive impairment.

METHODS

Twenty mice were randomized into the control and chronic periodontitis (CP) groups, with the latter receiving ligature-induced periodontitis. Cognitive performance was assessed by fear conditioning test. Periodontal inflammation and alveolar bone resorption were evaluated by micro-computed tomography and histopathology. The hippocampal microglial activation was evaluated by immunohistochemistry (IHC). The expressions of hippocampal cytokines (TNF-α, iNOS, IL-1β, IL-4, IL-10, and TREM2) were measured by reverse transcription-polymerase chain reaction. The morphology and density of the dendritic spines were determined by Golgi-Cox staining.

RESULTS

The CP mice reported significant inflammatory cell infiltration and alveolar bone resorption, with marked increases in cytokine levels (TNF-α, iNOS, IL-1β, and TREM2) in the brain. Moreover, the CP mice showed significantly reduced freezing to the conditioned stimulus in the cued and contextual tests, indicating impaired memory. Further analyses revealed, in the hippocampus of the CP mice, enhanced microglial activation, decreased dendritic spine density, and increased proportion of thin dendritic spines.

CONCLUSIONS

Periodontitis-induced neuroinflammation may impair the cognitive function by activating hippocampal microglia and inducing dendritic spine immaturity.

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.

Glycolytic reprogramming controls periodontitis-associated macrophage pyroptosis via AMPK/SIRT1/NF-κB signaling pathway

Glycolysis has been demonstrated as a crucial metabolic process in bacteria infected diseases via modulating the activity of pyroptosis. Macrophages are the most abundant immune cells that infiltrated in the infected periodontal tissues, which significantly influence the outcome of periodontitis (PD). However, the effect of glycolysis in regulating macrophage pyroptosis during PD development remains unknown. This study aimed to explore the role of glycolysis in PD-associated macrophage pyroptosis and periodontal degeneration. Clinical specimens were used to determine the emergence of macrophage pyroptosis and glycolysis in periodontal tissues by immunohistochemical analysis and western blot. For an in-depth understanding of the regulatory effect of glycolysis in the progression of macrophage pyroptosis associated periodontitis, both in vivo PD model and in vitro PD model were treated with 2-DG (2-Deoxy-d-glucose), a glycolysis inhibitor. The data showed that the blockade of glycolysis could significantly suppress the lipopolysaccharide (LPS) induced macrophage pyroptosis, resulting in an attenuation of the inflammatory response and bone resorption in periodontal lesions. Furthermore, we revealed that the regulatory effect of glycolysis on macrophage pyroptosis can be mediated via AMPK/SIRT1/NF-κB signaling pathway. Our study unveiled that suppressed glycolysis restrains the activity of PD-associated macrophage pyroptosis, osteoclastogenesis, and subsequent periodontal tissue destruction. These findings extend our knowledge of glycolysis in regulating PD-associated macrophage pyroptosis and provide a potential novel target for PD therapy.

ProBDNF signaling is involved in periodontitis-induced depression-like behavior in mouse hippocampus

OBJECTIVE

Increasing evidence supports the association between periodontitis and depression. However, the specific mechanisms remain to be further elucidated. The present study aimed to mechanistically investigate the regional roles of proBDNF (the precursor of brain-derived neurotrophic factor) in periodontitis induced depression-like behavior in mice.

METHODS

Experimental periodontitis model was established by periodontal injection of Porphyromonas gingivalis lipopolysaccharide (Pg-LPS) in 8-week-old male Bdnf-HA/HA mice for 3 weeks. The depression-like behaviors, spontaneous exploratory activity and the level of anxiety were assessed by behavior tests. The activation of microglia and astrocytes, as well as the expression of Interleukin (IL)-1β and Tumor necrosis factor (TNF)-α in the hippocampus, prefrontal cortex, and cortex were further assessed by immunofluorescence and western blots. The levels of IL-1β in blood serum and expression of occludin as well as claudin5 in the hippocampus, prefrontal cortex, and cortex were further determined by enzyme-linked immunosorbent assay and western blot. Finally, the expression of proBDNF, its receptors, and mature BDNF (mBDNF), as well as neuronal activity were measured by western blots and immunofluorescence.

RESULTS

Pg-LPS successfully induced periodontitis in mice and caused obvious depression-like behavior. Furthermore, we observed an increased activation of astrocytes and microglia, as well as a significant increase in expression of IL-1β and TNF-α in the hippocampus of mice treated with Pg-LPS, with elevated level of IL-1β in serum and decreased expression of occludin and claudin5 in the hippocampus. Importantly, we found that the levels of proBDNF and its receptors, SorCS2 and p75NTR, were increased significantly; however, the level of mBDNF was decreased, therefor leading to greater ratio of proBDNF/mBDNF. In addition, we also detected decreased neuronal activity in the hippocampus of mice treated with Pg-LPS.

CONCLUSIONS

Our results indicate that Pg-LPS-induced periodontitis could cause depression-like behaviors in mice, and the proBDNF signaling is involved in the process.

How Periodontitis or Periodontal Bacteria Can Influence Alzheimer's Disease Features? A Systematic Review of Pre-Clinical Studies

BACKGROUND

The negative effects of periodontitis on systemic diseases, including diabetes, cardiovascular diseases, and Alzheimer's disease (AD), have been widely described.

OBJECTIVE

This systematic review aimed to gather the current understanding of the pathophysiological mechanisms linking periodontitis to AD.

METHODS

An electronic systematic search of the PubMed/MEDLINE, Scopus, and Embase databases was performed using the following PECO question: How can periodontitis or periodontal bacteria influence Alzheimer's disease features?". Only preclinical studies exploring the biological links between periodontitis and AD pathology were included. This study was registered at the International Prospective Register of Systematic Reviews (PROSPERO), and the Syrcle and Camarades protocols were used to assess the risk of bias.

RESULTS

After a systematic screening of titles and abstracts (n = 3,307), thirty-six titles were selected for abstract reading, of which 13 were excluded (k = 1), resulting in the inclusion of 23 articles. Oral or systemic exposure to periodontopathogens or their byproducts is responsible for both in situ brain manifestations and systemic effects. Significant elevated rates of cytokines and amyloid peptides (Aβ) and derivate products were found in both serum and brain. Additionally, in infected animals, hyperphosphorylation of tau protein, hippocampal microgliosis, and neuronal death were observed. Exposure to periodontal infection negatively impairs cognitive behavior, leading to memory decline.

CONCLUSIONS

Systemic inflammation and brain metastatic infections induced by periodontal pathogens contribute to neuroinflammation, amyloidosis, and tau phosphorylation, leading to brain damage and subsequent cognitive impairment.

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.

Humanized Mouse Models for the Study of Periodontitis: An Opportunity to Elucidate Unresolved Aspects of Its Immunopathogenesis and Analyze New Immunotherapeutic Strategies

Periodontitis is an oral inflammatory disease in which the polymicrobial synergy and dysbiosis of the subgingival microbiota trigger a deregulated host immune response, that leads to the breakdown of tooth-supporting tissues and finally tooth loss. Periodontitis is characterized by the increased pathogenic activity of T helper type 17 (Th17) lymphocytes and defective immunoregulation mediated by phenotypically unstable T regulatory (Treg), lymphocytes, incapable of resolving the bone-resorbing inflammatory milieu. In this context, the complexity of the immune response orchestrated against the microbial challenge during periodontitis has made the study of its pathogenesis and therapy difficult and limited. Indeed, the ethical limitations that accompany human studies can lead to an insufficient etiopathogenic understanding of the disease and consequently, biased treatment decision-making. Alternatively, animal models allow us to manage these difficulties and give us the opportunity to partially emulate the etiopathogenesis of periodontitis by inoculating periodontopathogenic bacteria or by placing bacteria-accumulating ligatures around the teeth; however, these models still have limited translational application in humans. Accordingly, humanized animal models are able to emulate human-like complex networks of immune responses by engrafting human cells or tissues into specific strains of immunodeficient mice. Their characteristics enable a viable time window for the study of the establishment of a specific human immune response pattern in an in vivo setting and could be exploited for a wider study of the etiopathogenesis and/or treatment of periodontitis. For instance, the antigen-specific response of human dendritic cells against the periodontopathogen Porphyromonas gingivalis favoring the Th17/Treg response has already been tested in humanized mice models. Hypothetically, the proper emulation of periodontal dysbiosis in a humanized animal could give insights into the subtle molecular characteristics of a human-like local and systemic immune response during periodontitis and support the design of novel immunotherapeutic strategies. Therefore, the aims of this review are: To elucidate how the microbiota-elicited immunopathogenesis of periodontitis can be potentially emulated in humanized mouse models, to highlight their advantages and limitations in comparison with the already available experimental periodontitis non-humanized animal models, and to discuss the potential translational application of using these models for periodontitis immunotherapeutics.