Porphyromonas gingivalis –Induced NETs Mediate Neuroinflammation via TLR4 Activation

Periodontitis, caused by periodontal pathogens such as Porphyromonas gingivalis , is a risk factor for Alzheimer’s disease (AD) progression. Neutrophils, which are abundant in patients with periodontitis, release neutrophil extracellular traps (NETs) to resist microbial infection. We explored the mechanism and effects of P. gingivalis –induced NETs on the relationship between periodontitis and neuroinflammation. A murine periodontitis model was established via oral local application of P. gingivalis with or without tak242 (TLR4 inhibitor). Maxillary bones were evaluated via micro–computed tomography. The proportion of neutrophils was determined by flow cytometry. NET formation and morphology were analyzed via a cell-free DNA kit, a neutrophil elastase enzyme-linked immunosorbent assay (ELISA) and myeloperoxidase ELISA kit, reverse transcription polymerase chain reaction (RT-PCR), western blotting and immunofluorescence. Behavior tests were used to investigate cognitive ability. Neuroinflammation was assayed by immunohistochemistry (IHC) and RT-PCR. Amyloid precursor protein (APP) processing was measured by IHC. In vitro experiments explored the functional mechanism underlying the effects of P. gingivalis –induced NETs on the neuron–glia unit. We observed significant alveolar bone resorption with elevated neutrophil count and increased NET formation in mice with periodontitis. Cognitive abilities were impaired by periodontitis. Neuroinflammation manifested as glia activation and upregulated inflammatory cytokines, and APP processing was altered by the elevated expression of APP and PSEN1. These changes were specifically reversed by tak242. In vitro, P. gingivalis –induced NETs mediated M1 polarization in BV2 cells and changed APP processing in N2a cells, along with the activation of TLR4/Myd88/NF-κB and GSK3β/Akt, which is consistent with the in vivo findings. In conclusion, P. gingivalis –induced NETs play pivotal roles in the relationship between neuroinflammation and cognitive impairment. Furthermore, the effects of P. gingivalis –induced NETs on neuron–glia unit were related to TLR4 activation.