Porphyromonas gingivalis, a periodontitis causing bacterium, induces memory impairment and age-dependent neuroinflammation in mice

Abstract

Background: A possible relationship between periodontitis and Alzheimer's disease (AD) has been reported. However, there is limited information on the association between the Porphyromonas gingivalis (P. gingivalis) periodontal infection and the pathological features of AD. The hypothesis that P. gingivalis periodontal infection may cause cognitive impairment via age-dependent neuroinflammation was tested.

Results: Thirty 4-week-old (young) female C57BL/6 J mice were randomly divided into two groups, the control group and the experimental group. Thirty 12-month-old (middle-aged) were grouped as above. The mouth of the mice in the experimental group was infected with P. gingivalis. Morris water maze(MWM) was performed to assess the learning and memory ability of mice after 6 weeks. Moreover, the expression levels of the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β in the mice brain tissues were determined by Quantitative real-time polymerase chain reaction (qRT-PCR), Enzyme Linked Immunosorbent Assay(ELISA) and immunohistochemistry. Our results showed that the learning and memory abilities of the middle-aged P. gingivalis infected mice were impaired. Moreover, the expression levels of the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β in the brain tissues of the middle-aged P. gingivalis infected mice were increased.

Conclusions: These results suggest that P. gingivalis periodontal infection may cause cognitive impairment via the release of the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β in the brain tissues of middle-aged mice.

Keywords: Alzheimer’s disease(AD); Cognition; Neuroinflammation; Periodontitis; Porphyromonas gingivalis (P. gingivalis).

Fig. 1

Spatial learning and memory assessed by the MWM test in young and middle-aged mice. (a) Escape latency; (b) the number of platform crossings. Data presented as mean ± SEM. “*” indicates statistically significant difference (P < 0.05) in the escape latency time of mice in the same group, between day 1 and the corresponding day. “#” indicates statistically significant difference (P < 0.05) in the escape latency time between P. gingivalis infected mice and the corresponding control mice, on the same day. “a” indicates statistically significant difference (P < 0.05) in the number of crossing times between P. gingivalis infected mice and the corresponding mice control group

Fig. 2

Analysis of the TNF-α, IL-6, IL-1β mRNA expression levels in mice brain tissues by qRT-PCR. (a) the mRNA expression of TNF-α in middle-aged P. gingivalis infected mice was increased; (b) P. gingivalis infection increased the mRNA levels of IL-6 in middle-aged mice; (c) the mRNA expression levels of IL-1β were increased in middle-aged P. gingivalis infected mice when compared to the control group. “**” indicates statistical significant difference (P < 0.01) between P. gingivalis infected mice and the corresponding control group

Fig. 3

Age-dependent expression of TNF-α, IL-6, IL-1β in mice brain tissues, assessed by ELISA and immunohistochemistry. The protein expression levels of (a) TNF-α, (b) IL-6, (c) IL-1β in brain samples from young and middle-aged mice, measured by ELISA. Immunostaining of mice cerebral cortex tissue samples with antibodies against (d-h) TNF-α, (i-m) IL-6, and (n-r) IL-1β. (d, i, n): young control mice. (e, j, o): young P. gingivalis infected mice. (f, k, p): middle-aged control mice. (g, l, q): middle-aged P. gingivalis infected mice. Scale bar: 50 μm. “**” indicates statistically significant differences (P < 0.01) between the P. gingivalis infected mice and the mice in the corresponding control group