Role of inflammasomes in Alzheimer's Disease

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PROJECT SUMMARY/ABSTRACT Neuroinflammation is an important component of Alzheimer's disease (AD). However, the molecular mechanism by which inflammation modulates AD progression is not defined. We discovered that beta-amyloid 1-42 activates NLRP3 inflammasomes and that AD patients uniformly have evidence of activated inflammasomes in their brains. To test the role of NLRP3 inflammasomes in AD, we bred APP/PS1 mice (an amyloid-based murine model of AD) into NLRP3, ASC, or caspase-1 KOs (the three proteins comprising the NLRP3 inflammasome) and observed that these mice were completely protected from numerous AD features including learning/memory deficits and abnormalities in long-term potentiation. NLRP3 inflammasomes regulate the expression of IL-1beta and IL-18, two highly proinflammatory cytokines abundantly produced in microglial cells; in addition, astrocytes are strong expresses of pro-IL-18. IL-1beta is abundant in microglial cells on the periphery of amyloid plaques and can cause fever, a strong acute phase response, sepsis syndrome, and pyroptotic cell death. IL-18 is a member of the IL1 superfamily. Unlike IL- 1beta, IL-18 does not activate NF-kappaB or have pyrogenic activity. It is unknown if IL-1beta or IL-18 reduction was responsible for the protective phenotype in inflammasome-deficient APP/PS1 mice. We generated IL-18KO/APP/PS1 mice and, surprisingly, these mice developed a lethal seizure disorder, which was completely reversed by levetiracetam therapy. This is highly relevant as epidemiologic studies suggest that almost two-thirds of AD patients have seizures at some point during the course of their disease. In Aim 1, we will examine the role of inflammasome-dependent pyroptosis in microglial cells in the pathogenesis of AD using transgenic mice in which the inflammasome has been specifically deleted from microglial cells on an APP/PS1 background. We will also test APP/PS1 mice deficient for Gasdermin D (a caspase-1 substrate and the final effector molecule of pyroptosis). In Aim 2, we will examine if IL-18 counterbalances the proepileptic effects of IL-1beta in AD-related seizures. We will use a genetic approach (deleting IL-18 in other AD mouse models) as well as a pharmacological approach (an IL-1beta loss-of-function approach) in IL-18KO/APP/PS1 mice and assess animals for seizures. In Aim 3, we will determine the role of IL-18 in reducing neuronal network activity and modulating synaptic transmission. We will identify the types of synapses that are dysregulated in IL-18KO/APP/PS1 mice by performing morphological and electrophysiological studies as well as biochemical analysis using immunohistochemistry. We will specifically examine the role of microglial IL-18 using a floxed IL-18 transgenic mouse line. Successful completion of these Aims will elucidate the role of inflammasome-generated cytokines in AD, and could result in novel translational approaches designed to specifically halt the inflammation that drives AD, as well as mechanistically target seizures that affect the quality of life of AD patients.