In line with the strong association between periodontitis and Alzheimer’s disease (AD) clinically, preclinical studies have shown that systemic exposure to Porphyromonas gingivalis (Pg) initiates AD pathologies. However, the involvement of periodontitis in promoting AD pathologies is unclear. In the present study, we provided evidence that chronic systemic exposure to lipopolysaccharide derived from Pg (PgLPS, 1 mg/kg, daily, intraperitoneally) prompted neuroinflammation and tau hyperphosphorylation in 10-month-old of amyloid precursor protein (APP) knock-in mice, a model of AD, carrying the Swedish and Beyreuther/Iberian mutation (APP^NL-F/NL-F). The learning and memory function were assessed using the passive avoidance test. The production of APP, Amyloid (A)β_1-42, cytokines, synaptic proteins and the activation of glycogen synthase kinase (GSK)-3β as well as phosphorylation of tau were analyzed by immunohistochemistry, Western blotting or an enzyme-linked immunosorbent assay (ELISA) in the cortex of APP^NL-F/NL-F mice. We found that systemic exposure of PgLPS for three consecutive weeks induced learning and memory deficits with significantly reduced postsynaptic density protein (PSD95). Increased hyperphosphorylation of tau in multiple residues, including Ser202, Thr231 and Ser396, but not the accumulation of Aβ_1-42 was detected in the neurons of APP^NL-F/NL-F mice. Furthermore, PgLPS increased the GSK3β activity by reducing its phosphorylation of the serine residue at position 9 (Ser9) and promoted neuroinflammation by increasing the expression of interleukin-1β (IL-1β) and tumor necrosis factor (TNF-α) while decreasing that of interleukin-10 (IL-10) and transforming growth factor (TGFβ) in the cortex of APP^NL-F/NL-F mice. Moreover, the PgLPS-increased GSK3β activity was detected in both microglia and neurons, while the PgLPS-increased TNF-α expression was mainly detected in the microglia in the cortex of APP^NL-F/NL-F mice. In in vitro studies, PgLPS (1 µg/ml) stimulation increased the mRNA and protein level of TNF-α in MG6 microglia, which were significantly inhibited by the GSK3β-specific inhibitor TWS119. In contrast, the tau hyperphosphorylation and activation of GSK3β in N2a neurons were enhanced after treatment with conditioned medium from PgLPS-stimulated microglia, which was attenuated after pre-treatment with TNF-α inhibitor. Taken together, these findings indicate that GSK3β is involved in prompting microglia (TNF-α)-dependent tau hyperphosphorylation in neurons, resulting in learning and memory deficits in APP^NL-F/NL-F mice without changes in the Aβ expression during chronic systemic exposure to PgLPS. We propose that dampening GSK3β activation may help delay the periodontitis-promoted pathological progression of AD.

根据临床上牙周炎和阿尔茨海默病 (AD) 之间的密切关联，临床前研究表明，全身暴露于牙龈卟啉单胞菌( Pg ) 会引发 AD 病变。然而，牙周炎在促进 AD 病变中的作用尚不清楚。在本研究中，我们提供的证据表明，慢性全身暴露于Pg衍生的脂多糖（Pg LPS，1 mg/kg，每天，腹膜内注射）促使 10 个月大的淀粉样前体蛋白 (APP) 敲除后出现神经炎症和 tau 过度磷酸化。在小鼠中，AD 模型携带瑞典和 Beyreuther/Iberian 突变（APP ^NL-F/NL-F）。使用被动回避测试评估学习和记忆功能。通过免疫组织化学、蛋白质印迹或酶联免疫吸附测定分析APP、淀粉样蛋白 (A)β _1-42 、细胞因子、突触蛋白的产生和糖原合酶激酶 (GSK)-3β 的激活以及 tau 的磷酸化(ELISA) 在 APP ^NL-F/NL-F小鼠的皮质中。我们发现连续三周全身暴露Pg LPS 会导致学习和记忆障碍，并显着降低突触后密度蛋白 (PSD95)。^{在 APP NL-F/NL-F}的神经元中检测到多个残基中 tau 的过度磷酸化增加，包括 Ser202、Thr231 和 Ser396，但未检测到 Aβ _{1-42的积累}老鼠。此外，Pg LPS 通过降低其第 9 位丝氨酸残基 (Ser9) 的磷酸化来增加 GSK3β 的活性，并通过增加白介素 1β (IL-1β) 和肿瘤坏死因子 (TNF-α) 的表达来促进神经炎症，同时降低APP ^NL-F/NL-F小鼠皮质中白细胞介素 10 (IL-10) 和转化生长因子 (TGFβ) 的研究。此外，在小胶质细胞和神经元中均检测到Pg LPS 增加的 GSK3β 活性，而Pg LPS 增加的 TNF-α 表达主要在 APP ^NL-F/NL-F小鼠皮层的小胶质细胞中检测到。在体外研究中，PgLPS (1 µg/ml) 刺激增加了 MG6 小胶质细胞中 TNF-α 的 mRNA 和蛋白质水平，这被 GSK3β 特异性抑制剂 TWS119 显着抑制。相比之下，用来自Pg LPS 刺激的小胶质细胞的条件培养基处理后，N2a 神经元中的 tau 过度磷酸化和 GSK3β 的活化得到增强，在用 TNF-α 抑制剂预处理后减弱。总之，这些发现表明 GSK3β 参与促使神经元中的小胶质细胞 (TNF-α) 依赖性 tau 过度磷酸化，导致 APP ^NL-F/NL-F小鼠的学习和记忆缺陷，而慢性全身性系统期间 Aβ 表达没有变化。暴露于Pg脂多糖。我们提出抑制 GSK3β 活化可能有助于延缓牙周炎促进的 AD 病理进展。