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

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<0.00001 respectively). In addition, microgliosis and
astrogliosis were evident in the experimental but not in control group
(p <0.01, p<0.0001 respectively). Neurodegeneration was evident in
the experimental group based on a fewer number of intact neuronal cells
assessed by NeuN positivity and rbFOX3 gene expression, and there was a
greater number of degenerating neurons in the hippocampi of
experimental mice assessed by Fluoro Jade C positivity. APP and BACE1
gene expression were increased in experimental group compared with
control group (p<0.05, p<0.001 respectively). PSEN1 gene
expression was higher in experimental than control group but the
difference was not statistically significant (p = 0.07). ADAM10 gene
expression was significantly decreased in experimental group compared
with control group (p<0.01). Extracellular Aβ42 was detected in the
parenchyma in the experimental but not in the control group (p<
0.00001). Finally, phospho-Tau (Ser396) protein was detected and NFTs
were evident in experimental but not in the control group
(p<0.00001).

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.