Control of microglial dynamics by the Arp2/3 complex and the autism- and schizophrenia-associated protein CYFIP1

Microglia use a highly complex and dynamic network of branched processes to sense and respond to their surroundings. Despite emerging evidence that microglial motility plays important roles in brain development, neurodegeneration, and neuropsychiatric disease, little is known about the intracellular machinery orchestrating microglial process dynamics. Here, we identify roles for regulators of the actin cytoskeleton in controlling microglial behavior. We show that the actin branching Arp2/3 complex is critical for maintaining microglial morphology and is required for surveillance but not chemotactic motility. Neuropsychiatric disease-associated CYFIP1, a core component of the WAVE regulatory complex linking upstream signaling pathways to activation of the Arp2/3 complex, is highly expressed in microglia but has an unknown function. We report that conditional deletion of <i>Cyfip1</i> in mouse microglia reduces their morphological complexity and surveillance of the brain parenchyma, with no effect on chemotaxis. Deletion of <i>Cyfip1</i> also increased microglial CD68 positive lysosome volume and engulfment of presynapses. Thus, actin remodeling by CYFIP1 and the Arp2/3 complex controls microglial dynamics and shifts microglia away from a homeostatic state with potential implications for neuropsychiatric disease.