298. MICROGLIA INTEGRATES MULTIPLE STRESS SIGNALS VIA DISTINCT MOLECULAR PATHWAYS FOR BEHAVIORAL DYSFUNCTIONS

Abstract Background Clinical studies have reported the association between depression and inflammation. Rodent studies have demonstrated that chronic stress activates microglia, leading to neuronal dysfunctions and depression-related behaviors via proinflammatory molecules. However, since previous studies have relied on a limited number of microglial activation markers, the entire view of stress-induced microglial response remains elusive. Thus, essential questions, such as how differently microglia respond to acute and chronic stress, how differently microglia respond to stress among individuals, and what determines these properties of microglial responses, are also unanswered. Aims & Objectives Using a rodent model of social defeat stress, combined with multiomics analyses and genetic manipulations, we investigated the specific characteristics of stress that drive microglial responses, their molecular mechanisms, and their impact on behavior. Method We subjected male C57BL/6N mice to acute or chronic social defeat stress. We categorized the mice after chronic social defeat stress into susceptible and resilient mice according to the level of social avoidance, a typical depression-related behavior. Then, we isolated microglia from multiple brain areas of these mice, including the medial prefrontal cortex (mPFC), primary sensory and motor cortices, hippocampus, nucleus accumbens (NAc), and hypothalamus, and subjected these cells to single-cell RNA-seq. In addition, we more deeply analyzed the transcriptome of microglia from the mPFC and NAc, two representative brain areas with distinct responses to social defeat stress, with bulk RNA seq. We then predicted transcription factors responsible for stress-induced changes in the microglial transcriptome and examined their functional significance using genetic and surgical manipulations. Results Single-cell RNA-seq revealed that acute stress altered microglial transcriptome in all the examined brain regions. The microglial transcriptome was further changed with chronic stress in susceptible mice but returned to the original state in resilient mice. Microglial transcriptome also varied among brain regions and even more after stress. Thus, microglial transcriptome manifested chronic stress selectivity, stress susceptibility selectivity, and brain region selectivity. Clustering analysis revealed that gene expressions selective to chronic stress and those selective to stress susceptibility were mostly segregated and that brain region selectivity emerged within the former gene expressions. Nonetheless, these selectivities coincided in the same microglial clusters. Bulk RNA-seq analysis of mPFC and NAc microglia reproduced the segregation of chronic stress and stress susceptibility selectivities. Epigenomic analyses with H3K27ac ChIP-seq and ATAC-seq predicted distinct transcription factors for the respective gene expressions. Surgical and genetic manipulations suggested that gene expressions selective to chronic stress and stress susceptibility are mediated by different molecular pathways and involved in distinct behavioral domains. Discussion & Conclusions These findings demonstrate that microglia integrate chronic stress, stress susceptibility, and brain region selectivity in their gene expression through distinct molecular pathways, causing multiple neural dysfunctions and aggravating stress-related mental illness.