Systematic delineation of signaling and epigenomic mechanisms underlying microglia inflammatory activity in acute and chronic brain pathologies

Microglia promptly mount an inflammatory response following detection of infectious agents or injuries in the central nervous system. Such function fundamentally depends upon dynamic modulation of gene expression. However, the signaling and epigenomic mechanisms that regulate the transcriptional process underlying microglial inflammatory activity are not well understood. To address this, we used RNA-seq, ChIP-seq and ATAC-seq to delineate gene signatures and activity across the repertoire of genomic regulatory elements of microglia engaged in acute and chronic neuroinflammatory activity. Systematic interrogations of the microglial population over time during a systemic inflammatory response revealed a coordinated, sequential activation of multiple gene programs associated with defense response, translation and cell cycling. Activation of these programs occurred in parallel with gain and loss of activity at 4,080 and 3,119 genomic cis-regulatory elements, respectively. Furthermore, computational analyses identified key transcriptional regulators, including Ets, AP-1, C/epb, Nf-κB, Irf, Runx, c-Myc and E2f family members, that display differential propensity for activity at gene promoters and promoter-distal cis-regulatory elements. Gene expression analyses also suggested that the transcriptional process likely contribute to the effective activity of numerous transcriptional regulators through the modulation of their mRNA levels. Finally, characterization of CD11c-positive microglia that emerge with chronic demyelinating brain lesions suggested that Egr2, Mef2 members and E-box-binding factors such as Tfeb and Mitf contribute to the enhanced phagosomal activity of this inflammatory subset. Loss-of-function experiments validated that Mef2a in microglia is necessary for the acquisition of the CD11c-positive phenotype. Collectively, these results demonstrate that the inflammatory activity of microglia arises through an intricate, ultimately context-dependent, interplay between signaling pathways, genomic regulatory elements and the transcriptional machinery.