Aberrant synaptic connectivity and abnormal sensory perception are hallmark features of autism spectrum disorders (ASDs). Interestingly, these defects are often accompanied by abnormally reactive microglia, resident brain immune cells. However, it is unknown whether microglia and circuit abnormalities are mechanistically linked and whether they manifest in behavioral changes. The goal of my research is to determine how neuron-microglia interactions regulate neural circuit development. Long term, we will apply this basic science to identify new therapeutic targets in ASDs. This line of research builds off my unexpected and exciting finding that microglia sculpt synaptic connectivity (Schafer et al. Neuron 2012). During development, synaptic connections first form in excess. Sensory experience (touch, vision, etc.) then regulates the removal of less active synapses and maintenance of more active synapses. We found that microglia engulf and remove less active synapses in the developing visual system. This finding compels us to consider microglia as regulators of brain wiring and inspires exciting new questions: Is microglia-mediated synapse removal a universal mechanism regulating circuit refinement across sensory modalities and different types of synapses? Do changes in sensory experience directly regulate microglial gene expression and synaptic engulfment? To address these questions, we will manipulate somatosensory and visual experience in the developing mouse (whisker manipulation and dark rearing). We will then fluorescently label microglia and synapses and assess microglia-mediated synapse removal using our newly developed super-resolution imaging and 2-photon in vivo live imaging approaches. We will also identify new microglia-specific genes that are regulated by sensory experience using next generation RNA sequencing followed by rapid validation with a new CRISPR/Cas9 in vitro screen. Answers will revolutionize our understanding of how sensory experience regulates neural circuit refinement, will identify novel molecular mechanisms underlying microglia function, and will provide new insight into ASDs with underlying defects in microglia, sensory perception, and synapses.