Project Summary/Abstract The prenatal period is a sensitive and critical time for brain development characterized by waves of neurogenesis, neuronal migration, and formation of neural networks. In the first and second trimester, microglia are the dominant immune cells of the brain and participate in a variety of processes essential to brain development, including secreting neurotropic factors and engulfing apoptotic neural progenitor cells. Fetal microglia dysfunction can lead to aberrant cortical lamination, resulting in an increased risk of brain pathology. We have identified numerous ligand-receptor pairs involved in microglia-to-cortex and cortex-to-microglia signaling predicted to contribute to human fetal microglia function and fetal brain development. We observe concordant expression of these ligand-receptors pairs in cerebral organoids (COs) and induced pluripotent stem cell-derived microglia with our human fetal data. COs can model early human brain development, but current models lack the immune component of the brain. Our data suggest that induced pluripotent stem cell-derived microglia co- cultured with COs (oMGs) capture significant phenotypic characteristics of human fetal microglia. Thus, a systematic analysis of neural maturation following integration of microglia into COs is the first step in using this model system to interrogate the molecular mechanisms underlying how neuron-microglia interactions establish early brain circuitry. This proposal aims to use COs and oMGs to assess how brain environment signals and corresponding transcription factors contribute to fetal microglia behavior and microglial interaction with neurons in early fetal development. In Aim 1, completed in the K99 phase, I will test the hypothesis that integration of microglia into COs results in enhanced neural maturation. Additionally, I will test how perturbation of homeostatic brain environment signaling in microglia results in microglia dysfunction and altered neuronal subpopulations. In Aim 2, I will identify transcription factor networks underlying human and mouse microglia behavior throughout development, at homeostasis and after an inflammatory insult. The goal for Aim 2 is to uncover species- conserved mechanisms in microglia responses to inflammation for improved therapeutic targeting and murine modeling and to discover potential human-specific risk factors for disease. Additionally, I will test the hypothesis that microglial developmental transcriptional factors are re-wired following an inflammatory insult, leading to long- lasting changes in microglia behavior and disruption of brain circuitry. Studies in Aim 2 will be completed in the independent phase. My long-term goal is to elucidate the epigenetic mechanisms underlying neuronal-microglia communication in health and disease as an independent investigator. I have assembled a diverse group of highly skilled mentors who will ensure that I receive extensive training in neurodevelopment and assess...