Understanding Mechanistic Details of Neuroinflammatory Pathways Stimulated by the Alzheimer\u27s Disease Amyloid-Beta Protein

Alzheimer’s disease (AD) is characterized by neuroinflammation. Senile plaques composed of aggregated amyloid-β protein (Aβ) are found in AD patients’ brains. The Aβ is formed by the proteolytic cleavage of the amyloid precursor protein (APP) resulting in Aβ fragments that are 39-42 amino acids in length. The two most common peptides are Aβ(1-40) and Aβ(1-42), which differ by two amino acids, isoleucine and alanine. Within the brain of AD patients, Aβ monomer self-assembles to form several aggregate morphologies, including oligomers, protofibrils, and fibrils. Activated microglial cells and associated secreted proinflammatory cytokines surround these plaques producing a localized inflammatory environment in the brain. Several innate-immune pathways, including Toll-like receptors (TLRs) and the NLRP3 inflammasome, have been implicated in AD inflammation. Aβ plays a primary role in activating these pathways likely contributing to the progressive neurodegeneration in AD. In order to better understand the complexities of this interaction, I investigated the inflammatory response of microglia to Aβ(1-42) and Aβ(1-42)/Aβ(1-40) protofibrils, along with additional biophysical properties. Increased understanding of these pathological events will expand the current model of Aβ neuroinflammatory pathways and help identify new therapeutic targets for AD. My research has demonstrated that the Aβ(1-42) protofibrils triggered a time- and TLR/MyD88-dependent process that produced inflammatory cytokines tumor necrosis factor alpha (TNFα) and interleukin-1β (IL-1β) mRNA and intracellular pro and mature forms of IL-1β protein. Despite previous reports suggesting that NLRP3 activation requires two signals from two distinct molecules, my research indicated that Aβ(1-42) protofibrils alone could efficiently prime (TLR-dependent pro-IL-1β production) and activate (cleavage of pro-IL-1β to mature-IL-1β) the NLRP3 inflammasome. However, the increased intracellular mature-IL-1β did not translate into greater IL-1β secretion. Instead, we found that Aβ was able to elicit a very rapid, unsustainable, yet re-inducible quantized burst of secreted IL-1β, which occurred prior to Aβ priming of the microglia. These findings suggested a basal level of either pro- or mature-IL-1β in the cultured primary microglia yet revealed multiple sites of IL-1β regulation by Aβ(1-42) protofibrils. These sites, which are potential therapeutic targets, include TLR/MyD88-mediated priming, NLRP3 inflammasome activation, and modulation of the IL-1β secretory process