Alzheimer's disease involves inflammatory sequalae, as supposed by the overall hypothesis guiding this Program, and microglia make a substantial contribution to neuroinflammatory molecules and events. Although much has been learned about the initial stages of microglial activation by amyloid (3-peptide and other Alzheimer-related stimuli, little is known about the subsequent stages that are likely to make a larger contribution to brain biology in a slow, chronic disease like Alzheimer's. The relationship between feedback signals, a refractory state (""""""""innate tolerance""""""""), and alternative activation are not clear. Our ongoing studies conducted under this Program have pointed to a role for genetics in the resolution of microglial activation. The human interleukin-1a (IL-1a) gene contains single-nucleotide polymorphism (SNP) at position +4845, creating an amino acid coding change one residue from the site of proteolytic maturation of the cytokine; possession of the rarer allele 2 at this SNP is correlated with increased risk for Alzheimer's disease. We have found that microglia expressing pro-Illa of the allele 1 sequence become desensitized to activation by other stimuli (a phenomenon known as """"""""innate cross-tolerance"""""""")whereas those expressing from allele 2 do not. We have also found roles for calcium regulation and cellular oxidation in critical aspects of microglial activation, including their release of excitotoxins. We intend to examine these events in three main areas of investigation. The role of IL-1a (or pro-IL1a) in tolerance will be studied in a mechanistic and genotype- specific manner. Genotypic differences in hydrolysis by the calcium-dependent protease calpain will be examined along with other impacts of calcium regulation on microglial activation. The interplay between calcium and cellular oxidation will be elucidated, taking particular advantage of a novel, targeted mutation in mice that accumulates lipid peroxidation products. Finally, the impact of AD pathology and oxidative stress on the memory-related behavior of these mice will be assessed. Together, these studies will enhance our understanding of novel aspects of microglial behavior, specifically, the meaningful interactions between proinflammatory cytokines, oxidation, calcium, and excitotoxicity. Elucidation of these issues will provide insight into events that might be amenable to therapeutic intervention, as well as explaining phenotypic differences resulting from genetic variability in the IL1A gene.
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