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Chronic oral application of a periodontal pathogen results in brain inflammation, neurodegeneration and amyloid beta production in wild type mice

Ilievski V, Zuchowska PK, Green SJ, Toth PT, Ragozzino ME, Le K, Aljewari HW, O'Brien-Simpson NM, Reynolds EC, Watanabe K (2018) PLoS One.  Oct 3;13(10): e0204941. doi: 10.1371/journal.pone.0204941. eCollection 2018. 

Web URL: Read this and related abstracts on PubMed here

Abstract:

BACKGROUND:

The results from cross sectional and longitudinal studies show that periodontitis is closely associated with cognitive impairment (CI) and Alzhemer's Disease (AD). Further, studies using animal model of periodontitis and human post-mortem brain tissues from subjects with AD strongly suggest that a gram-negative periodontal pathogen, Porphyromonas gingivalis (Pg) and/or its product gingipain is/are translocated to the brain. However, neuropathology resulting from Pg oral application is not known. In this work, we tested the hypothesis that repeated exposure of wild type C57BL/6 mice to orally administered Pg results in neuroinflammation, neurodegeneration, microgliosis, astrogliosis and formation of intra- and extracellular amyloid plaque and neurofibrillary tangles (NFTs) which are pathognomonic signs of AD.

METHODS:

Experimental chronic periodontitis was induced in ten wild type 8-week old C57BL/6 WT mice by repeated oral application (MWF/week) of Pg/gingipain for 22 weeks (experimental group). Another 10 wild type 8-week old C57BL/6 mice received vehicle alone (control group) MWF per week for 22 weeks. Brain tissues were collected and the presence of Pg/gingipain was determined by immunofluorescence (IF) microscopy, confocal microscopy, and quantitative PCR (qPCR). The hippocampi were examined for the signs of neuropathology related to AD: TNFα, IL1β, and IL6 expression (neuroinflammation), NeuN and Fluoro Jade C staining (neurodegeneration) and amyloid beta1-42 (Aβ42) production and phosphorylation of tau protein at Ser396 were assessed by IF and confocal microscopy. Further, gene expression of amyloid precursor protein (APP), beta-site APP cleaving enzyme 1 (BACE1), a disintegrin and metalloproteinase domain-containing protein10 (ADAM10) for α-secretase and presenilin1 (PSEN1) for ɣ-secretase, and NeuN (rbFox3) were determined by RT-qPCR. Microgliosis and astrogliosis were also determined by IF microscopy.

RESULTS:

Pg/gingipain was detected in the hippocampi of mice in the experimental group by immunohistochemistry, confocal microscopy, and qPCR confirming the translocation of orally applied Pg to the brain. Pg/gingipain was localized intra-nuclearly and peri-nuclearly in microglia (Iba1+), astrocytes (GFAP+), neurons (NeuN+) and was evident extracellularly. Significantly greater levels of expression of IL6, TNFα and IL1β were evident in experimental as compared to control group (p<0.01, p<0.00001, p

CONCLUSIONS:

This study is the first to show neurodegeneration and the formation of extracellular Aβ42 in young adult WT mice after repeated oral application of Pg. The neuropathological features observed in this study strongly suggest that low grade chronic periodontal pathogen infection can result in the development of neuropathology that is consistent with that of AD.

FAB RESEARCH COMMENT:

Although numerous studies have shown associations between poor dental health and dementia, this is the first time that a clear causal effect has been demonstrated between the introduction of bacteria that cause periodontitis (gum disease), and not only their transmission into the brain, but the subsequent triggering of brain inflammation, neural 'plaques' and other neuropathological changes associated with Alzheimer's disease.

For obvious reasons this was an animal study, so as ever, generalisability to humans cannot be assumed - but the deliberate use of 'wild-type' mice (rather than mice specifically bred for high vulnerability to neurodegenerative disease) adds to the strength of these findings. 

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See also:
  • Hamilton et al 2017 - Atherosclerosis, Periodontal Disease, and Treatment with Resolvins