Investigating the Role of Periodontitis in Inducing Alzheimer’s Disease-like Pathology

Abstract

Several theories have been proposed to describe the pathology of Alzheimer’s disease (AD) such as the inflammatory hypothesis, whereby neuroinflammation is thought to be the process that drives neurodegeneration. Periodontitis (PD), which was described as the most common chronic disease in humans, has been linked to AD. Neuroinflammation driven by the virulence factors of the PD-associated bacteria such Porphyromonas gingivalis (Pg) have been proposed to give rise to AD-like pathology since Pg virulence factors, gingipains, were found in the brains of patients with AD. The aim of this thesis was to investigate the effects of virulence factors of the bacteria associated with PD in giving rise to AD-like pathology.
Firstly, neuroinflammation was recognized in the post-mortem brain tissues of patients with AD (n=2), which showed high reads of bacterial DNA content from the Porphyromonas genera within the Bacteroidetes phylum, compared to non-AD controls (n=1).
The effect of single dose and multiple doses of Pg LPS, was investigated on monocultures of immortalized human microglial cell lines, HMC3 and IM, an immortalized mouse cell line, BV2, and primary human astrocytes, HA. Pre-conditioning microglia with sub-threshold levels of Pg LPS was also tested. Pg LPS was found to induce reactive oxygen species (ROS) production in HMC3 cells only. Stimulating HMC3 cells with one dose of LPS at has resulted in increased expression of markers of activation as well as the production of IL-6. Priming HMC3 with multiple doses of LPS induced an even greater increase in some activation markers and IL-6 production. IM cells stimulated with LPS showed increased production of IL-6, and this production was greater when primed with multiple doses. An increased expression of markers of activation was recorded following priming with two doses of LPS. HA cells only exhibited a change in the markers of activation profile following LPS priming. BV2 displayed an increase in the markers of activation expression and IL-6 production following LPS exposure. IL-6 production increased even more when primed with multiple doses of LPS. Only BV2 cells showed a sign of pre-conditioning, as evidenced by an increased cell viability.
Cultures of microglia and astrocytes on 3D platforms resulted in a change in the markers of activation in HMC3, HA and BV2. Three-dimensional cultures of HMC3, IM and BV2 produced IL-6 following LPS exposure in similar manners to the monolayer cultures.
Co-culturing microglia and astrocytes provoked a change in the expression of some markers in HA. Also, IL-6 production was recorded at baseline and following LPS stimulation in this model.
Pg OMVs were isolated from Pg bacteria and used to stimulate monolayers of microglia and astrocytes. Stimulating HMC3 with OMVs led to the production of IL-6. Exposing IM to OMVs resulted in ROS production, changes in the markers of activation as well as increased IL-6 production. Increased ROS production, changes in the markers of activation, and increased IL-6 production was recorded after exposing HA to OMVs. In BV2 cells, OMVs resulted in increased ROS production, and resulted in IL-6 production.
These findings indicate that Pg virulence factors could induce neuroinflammation by activating microglia and astrocytes. The chronic activation of microglia and astrocytes by these virulence factors lead to aggravated responses by these cells, which in turn amplifies the effects of neuroinflammation in the brain. Understanding the role of Pg OMVs could further understanding of how PD in gives rise to neuroinflammation as OMVs pack various virulence factors which have showed different effects on microglia and astrocytes compared to Pg LPS.