PROJECT SUMMARY Alzheimer's disease (AD) is the most common neurodegenerative disease that is characterized by pathological protein aggregation and inflammation in the brain (neuroinflammation). Microglia are the key immune cells of the brain that mediate neuroinflammation. Novel therapeutic strategies and AD biomarkers can be identified if we can define the molecular changes occurring in microglia in AD at the protein level, and not just at the mRNA (transcriptomic) level. Our knowledge about microglial disease mechanisms in AD are mostly shaped by transcriptomic studies although proteins are the enactors of biological processes and the correlation between mRNA and protein is poor. The major barrier to microglial proteomic studies is the dependence on isolation strategies to purify microglia before analyses. Isolation of microglia from brain induces artefacts, yields very little protein and provides highly biased sampling. To overcome this barrier, we will use a novel strategy (ciBONCAT) for microglia-specific proteomic labeling in-vivo. CiBONCAT allows us to label newly synthesized proteins in microglia with an azide tag (azidonorleucine). These azide-tagged proteins can be easily isolated from the brain without need for cell isolation. We have demonstrated the feasibility of using ciBONCAT to label neuronal and astrocytic proteomes in adult mice, and have optimized the in-vivo and mass spectrometry pipelines for proteomics using brain, cerebrospinal fluid (CSF) and other biofluids. This novel strategy, coupled with our extensive expertise in mass spectrometry (proteomics) methods, will allow us to test our central hypothesis that amyloid beta (Aβ) accumulation, APOEε4, and aging independently and synergistically impact proteomic phenotypes of microglia, and that these microglia-mediated mechanisms are reflected in the CSF via immune biomarkers of AD pathology. In Aim 1, we will use ciBONCAT to define microglia-specific protein alterations occurring in AD using two models of progressive amyloid beta pathology (APP-PS1 and 5xFAD). In Aim 2, we will use knock-in mouse models to determine the effect of human APOEε4 and APOEε3 expression on microglial proteins changes and how APOEε4 impacts with Aβ pathology. In Aim 3, we will identify microglia-derived proteins in the cerebrospinal fluid in mouse models of AD pathology to identify novel biomarkers of neuroinflammation that reflect activation or depletion of microglia in the brain in AD mouse models. Through the successful completion of this R01 proposal, we will obtain novel molecular insights into microglia-mediated AD mechanisms and will establish ciBONCAT as a powerful approach to investigate cell type-specific mechanisms of neurodegeneration. Our multidisciplinary expertise in microglial biology and transgenic models of AD pathology (Dr Rangaraju), and quantitative proteomics, systems biology and CSF biomarker discovery (Dr Seyfried), uniquely position us to execute this innovative R01 proposa...