Project Summary This proposal addresses a novel way in which microglia functional states are regulated by the transcription factor PU.1 in the context of amyloid pathology and its potential contribution to neuroprotection against Alzheimer’s disease (AD). The expression level of PU.1 is crucial for instructing commitment to myeloid versus lymphoid lineages and is enriched in microglia. PU.1 also determines the inflammatory activation state of macrophages by priming cis-regulatory regions to elicit appropriate gene expression upon stimulation. Recently, genome wide association studies (GWAS) have identified a variant in Spi1 (encoding PU.1) that modulates AD risk. Unlike other variants, the Spi1 variant is unique in that it leads to a reduction in PU.1 expression and has a protective effect against AD risk. In a 5xFAD amyloid mouse model of AD, I observed heterogeneous protein expression of PU.1 in microglia. Recent work supports the idea that specific microglia activation states have unique impacts on AD pathogenesis. The Disease Associated Microglia (DAMs) are thought to serve a protective role in AD by robustly responding to amyloid plaques whereas inflammatory populations potentially play a harmful role in disease. I hypothesize that low PU.1 expression in microglia induces neuroprotective functional states in microglia against AD. We generated genetic mouse models that increase (Spi1cOE) or decrease (Spi1cKD) PU.1 expression specifically in microglia and crossed them to the 5xFAD amyloid disease model. Ribosomal profiling of these microglia revealed significant bulk transcriptional changes. To understand how PU.1 expression level regulates microglia activation states during amyloid pathology, I performed single nuclei RNA-sequencing. These new data reveal that Spi1cKD in 5xFAD favors microglia states associated with neuroprotection (DAM) while attenuating the proportion of cells in toxic states (inflammatory). Knowing the location of these microglia in relation to amyloid plaques will enhance our understanding of where engagement of such states occur and where these microglia may be exerting protective or harmful effects. In my first aim, I will use multiplexed error-robust in situ hybridization (MERFISH) to spatially identify microglia in DAM and inflammatory activation states with Spi1cKD or Spi1cOE in 5xFAD. In addition to these transcriptional changes, we showed a reduction in the complement protein C1qa in Spi1cKD 5xFAD animals. We further found that Spi1cKD microglia prevented synapse loss that is characteristic in human AD and recapitulated in the 5xFAD model. It has been shown that pathological activation of complement occurs with inflammatory activation and contributes to synapse loss in AD. Therefore, I propose to assess the production of C1qa by microglia with differential PU.1 expression level in vitro and synaptic co-localization of C1qa in 5xFAD mice with Spi1cKD and Spi1cOE microglia.