Deletion of <em>Cyfip1</em> in mouse microglia affects microglial actin dynamics, motility and functions, unveiling a dendritic spine maturation defect

Abstract

Microglia are the innate immune cells in the CNS, serving as the first line of defense in the brain parenchyma. During mouse embryogenesis, microglial progenitor cells originate from the yolk sac and migrate into the CNS via the circulatory system. As the developmental stage progresses, microglia develop ramified and motile processes which constantly survey the brain parenchyma. The high motility allows microglia to respond to insults by migrating to the affected sites, and clear up the pathogens and damage. Moreover, microglia also interact with neuronal synapses with their processes. Microglial motility is driven by the intracellular force generated by actin cytoskeleton rearrangement. Essential to this process is the cytoplasmic fragile-X mental messenger ribonucleoprotein (FMRP) interacting protein 1 (CYFIP1), an essential component of the WAVE regulatory complex (WRC), which activates the ARP2/3 complex to regulate branched actin networks. <br /> In this study, we show that CYFIP1 is the sole CYFIP isoform in microglia, and is highly expressed compared to the other cell types in the CNS. We determine that microglia possess a unique WRC, which is composed of CYFIP1, HEM1, WAVE2 and ABI3, and that depleting CYFIP1 leads to the loss of all other WRC components. We then show that in the absence of CYFIP1, microglial branching is reduced and microglial homeostasis is impaired in the mouse brain. We also, for the first time, identify the molecular role of the CYFIP1-WRC in microglial actin dynamics, as a regulator of the dynamic filamentous actin formation, which is involved in microglial phagocytosis, migration and membrane ruffling. Indeed, the absence of the CYFIP1-WRC disrupted lamellipodium formation and impaired microglial directional migration. Interestingly, in the brain of <em>Cyfip1</em> conditional knockout mice, there was an unexpected alteration in dendritic spine maturation in layer V pyramidal neurons of the mouse cortex. <br /> In summary, in this study we demonstrate the pivotal role of the CYFIP1-WRC in microglial homeostasis, motility and phagocytic function. Exploring the role of CYFIP1-dependent actin dynamics on the interaction of microglia with synapses and its significance for the establishment of neuronal circuits are intriguing topics for future research.