Project Summary Our proposal focuses on the epigenetic mechanisms of microglia-mediated regulation of brain homeostasis and neuro-degeneration. Our recent studies revealed brain region-specific microglia specification and suggested that this specification matches distinct neuron phenotypes in functionally distinct brain areas. We showed that regional microglia specification depends on the Polycomb Repressive Complex 2 (PRC2), which silences microglia genes in a brain-region specific fashion. Ablation of PRC2 “relaxes” specification of microglia followed by neurodegenerative-like changes in neuronal function and behavior. These findings suggest a model where interaction between “matching” neurons and microglia renders microglia from the aberrant production of factors responsible for microglial-mediated neuronal damage. To test our hypothesis, we propose to identify PRC2 targets in microglia in different brain regions and to determine the impact of PRC2 inactivation on regional microglia specification. PRC2 has been shown to operate downstream of different signaling pathways including the RAF/Erk signaling pathway, which has been implicated in microglia-mediated neurotoxicity. We hypothesize that activation of these signaling pathways may trigger the aberrant expression of genes controlling microglia proliferation, phagocytosis and/or proinflammatory activity by directly affecting PRC2 function. We will address the link between signal-induced microglia-driven neurotoxicity and PRC2-mediated gene silencing. Much of the microglia-mediated toxicity during neurodegeneration involves the activation of inflammatory responses. Our proposal aims at identification of the gene regulatory mechanisms supporting the proinflammatory activity of microglia. We found that BET proteins, which link histone acetylation and activation of RNA Pol II, play a key role in the signal-induced transcription of proinflammatory genes in microglia. We show that the pharmacological inhibition of BET leads to the selective suppression of microglia inflammatory gene expression in vitro and in vivo. The BET family includes the structurally different BRD2, BRD3 and BRD4 proteins, all of which are expressed in microglia. We previously observed differential binding of individual BET proteins to distinct gene targets in macrophages and neurons. Using mice with conditional microglia-specific inactivation, we will determine the contribution of individual BET proteins to brain region-specific microglia phenotypes in the healthy brain and during neurodegeneration. In summary, the proposed research will identify novel epigenetic mechanisms of region-specific microglia specification and the contribution of these mechanisms to neurodegeneration. Identification of proteins controlling distinct states of microglia activity will facilitate the development of novel therapeutic approaches for the prevention and/or attenuation of neurodegeneration.