Project Summary/Abstract. While activated microglia have been observed in the vicinity of neuritic amyloid plaques in Alzheimer's disease (AD), there have been no large-scale assessments of microglial activation in aging and neurodegenerative disease. Our long-term goal is to understand the genetic underpinning of microglial responses—particularly the proportion of microglia in a morphologically-defined state of activation—that increase susceptibility to AD, so we can develop more targeted forms of immune-based therapies to prevent cognitive decline and progression to dementia. Our objective is to refine the genetic architecture of microglial activation to identify novel loci that influence the proportion of activated microglia, and to understand the functional consequences of variants driving microglial activation in AD. Our central hypothesis is that identifiable gene variants influence microglial activation and susceptibility to AD. We will test this hypothesis by conducting genome-wide analysis and identifying associations between gene variants and microglial activation. Microglial activation will be measured in human autopsy tissue (ex vivo), living human brain using PET imaging (in vivo), and in monocyte-derived microglia-like cells (in situ and in vitro). Our rationale is that mapping the genetic architecture that drives the proportion of activated microglia will be important in developing a first generation of polygenic models for this trait and determine whether the proportion of activated microglia captures a causal element of the cascade leading to AD. Our specific aims are 1) Validation and discovery of loci influencing a postmortem measure of human microglial activation (proportion of activated microglia) in aging and Alzheimer's disease, 2) In vivo validation of GWAS and assessment of clinical relevance using TSPO PET imaging to measure microglial activation, and 3) In situ histological and in vitro functional characterization of validated variants in a human microglia-like system. For the first aim, we will perform GWAS and measure the proportion of activated microglia in autopsy brain tissue from 1,600 individuals, and then replicate our findings in an independent, diverse set of samples from three separate institutions. In the second aim, we will perform GWAS and TSPO PET imaging, using the state-of-the-art radioligand 11C-ER176, in a prospective cohort of 200 older human subjects (equal proportions of cognitively normal, mild cognitive impairment, and Alzheimer's disease) to identify gene variants associated with AD-related microglial activation. In the third aim, we will assess how gene variants identified in Aims 1 and 2 influence cellular function and protein expression in monocyte-derived microglia-like cells. Our innovative approach combines autopsy, PET, and human cell measures of microglial activation to create a multimodal investigation into the genetic architecture of microglial responses. The proposed research is significant, as mapping the variants that regulate microglial activation, and determining their functional consequences, will provide key insights in prioritizing potential therapeutic targets for drug development and in creating novel outcome measures with which to guide clinical trial design.

PROJECT NARRATIVE The proposed research is relevant to public health because determining how genetic regulation of microglia relates to neuroinflammatory changes seen in Alzheimer's disease is expected to inform future strategies to predict and prevent cognitive decline and progression to dementia. Thus, the proposed research is relevant to the NIA's mission that pertains to developing fundamental knowledge that will reduce the burden of Alzheimer's disease on society.