Dissecting the origin, regulation and function of microglial subsets in Alzheimer's disease

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PROJECT SUMMARY Recent advances in human genetics have unequivocally demonstrated mutations in microglia-specific genes, such as TREM2 R47H, to be some of the strongest risk factors for late-onset Alzheimer’s disease (AD). These breakthroughs point to microglia as a potential driver for AD pathology and thus a promising target for novel therapeutics. Remarkably, single-cell RNA sequencing studies have also uncovered a unique microglial subset, named disease-associated microglia (DAM), which are found to surround amyloid plaques in mouse models of AD. Compared to homeostatic microglia, DAM upregulate a cohort of signature genes, including AD risk genes TREM2 and APOE, which are also elevated in microglial subsets from human AD patients. Despite the apparent significance of DAM in AD, critical knowledge gaps exist regarding the cellular and molecular mechanisms that generate DAM as well as the functional contribution of DAM to different stages of AD pathology. The overall- objectives of this proposal are to determine the cellular origin, transcriptional regulation and subset-specific func- tion of DAM in AD. As microglial gene loci can be primed at the enhancers, microglial activation may lead to depositions of certain long-lasting epigenetic marks, which facilitate more rapid changes of gene expression upon a second hit later in life. Therefore, in theory, developmental activations, as we have observed for prolifer- ative region-associated microglia (PAM) in the normal developing white matter, may confer naïve homeostatic microglia such memory at the epigenetic level to allow their faster conversion to DAM in response to disease signals. Consistently, PAM and DAM share the same transcriptomic signature, which is presumably in turn reg- ulated by conserved transcription factors. In addition, analyses based on microglial depletion and global mutants of Trem2 and Apoe (partially controlling the DAM phenotype) have provided important hints for a possible neu- roprotective role of DAM by compacting amyloid plaques and limiting its spread. However, given the heteroge- neity of microglia and technical limitations (e.g. DAM resistant to drug depletion), the function of DAM during AD progression is still controversial. In this proposal, we will test the central hypothesis that conserved transcriptional and epigenetic mechanisms regulate DAM, which are ontogenically related to PAM and serve as the major neuroprotective microglial subset in AD amyloid pathology. Aim 1 will delineate the cellular origin of DAM in AD through genetic fate mapping and epigenetic profiling. Aim 2 will determine the transcriptional regulation of DAM in AD by transplantation of genetically modified primary microglial cells. Aim 3 will define the function of DAM in AD via microglia subset-specific manipulations. Upon completion of the proposed research, we expect to have elucidated the cellular origin, genetic and epigenetic mechanisms as well as in vivo function of the disease- associated microglia subset in AD. These results will provide mechanistic insights into selective subpopulations of microglia that may control AD progression, ultimately paving a new avenue for the development of precise microglia-based interventions to treat AD.