Neuroprotective signaling and transcriptional pathways in microglia associated with Alzheimer's disease

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Functional genomics studies have identified an enrichment for Alzheimer’s disease (AD) risk variants in active enhancers of human myeloid cells. These data, along with myeloid-specific expression of several AD risk genes, strongly implicate these cells in the etiology of AD. Partial loss-of-function mutations in Triggering Receptor Expressed on Myeloid cells 2 (TREM2), linked to increased risk for AD, point to a protective role for TREM2 against neurodegeneration. Microglia are known to play numerous beneficial functions in the brain, including clearance of dying cells as well as cellular debris and orchestrate complex responses to tissue stress or damage. However, chronic activation of microglia can lead to toxic neuroinflammation and impairment of homeostatic functions. A better understanding of the mechanisms underlying the neuroprotective functions of microglia is critical to identifying therapeutic targets to prevent or delay AD onset. Using fine mapping approaches, we have identified likely causal genes within many AD GWAS loci. In one locus, the gene encoding embryonic ectoderm development (EED) is a strong candidate causal gene and a putative target of a myeloid cell enhancer containing an AD-associated functional variant. EED is an essential, non-catalytic component of the polycomb repressive complex 2 (PRC2), which functions to maintain the repressive histone mark H3K27me3. Modulation of H3K27me3 is critical for epigenetic remodeling following inflammatory stimuli and has been implicated in microglial clearance of apoptotic cells. Cytoplasmic PRC2 (cPRC2), also exhibits direct signaling activity, controlling receptor-driven activation of Erk as well as actin polymerization in immune cells, cellular processes implicated by our AD genetics studies. Furthermore, our preliminary data show that microglial Eed-/- on the 5XFAD background phenocopies TREM2-/-. Based on these genetic and biochemical observations we hypothesize that EED and PRC2 operate as a master regulator of the TREM2 signaling pathway and that factors that control PRC2 expression levels and activity define the TREM2 driven neuroprotective microglial state. To test this hypothesis, we propose 4 aims to examine the impact of Eed/PRC2 on both human and mouse microglial function and Trem2 signaling in vitro and in vivo. We will determine the role of EED/PRC2 loss of function on microglial function and TREM2 signaling using both mouse and human microglia in vitro (aim 1) and in vivo in 5XFAD mice (aim 2). In aim 3 we will examine the impact of cytoplasmic PRC2 on microglial function and Trem2 signaling in vitro and in vivo. In aim 4 we will use human genetic data to test for enrichment of AD risk alleles within the Trem2 signaling pathway genes and in the Eed regulatory network and determine the impact of select alleles on microglial function and Trem2 signaling. Finally, we will test for epistasis by examining the impact of Eed+/- on Trem2 haploinsufficiency in 5XFAD mice in vivo and in hiPSC-derived microglia in vitro.