The Effects And Mechanisms Of Microglia On The Developing Cortical Circuits In Mice

INTRODUCTION:Many neurodevelopmental and neuropsychiatric disorders,including autism,schizophrenia,and intellectual disability,are characterized by abnormal development of cortical circuit.The normal function of cortical circuit is controlled by many factors,among which developmental synaptic pruning is a very important factor.Previous studies suggested that the astrocytes in the developing brain are involved in synaptic pruning,but microglia have also been found to be actively involved in synaptic pruning.Microglia are an important part of the developing brain and serve as an innate immune cell in the central nervous system.Studies have found that microglia play an important role in the development of the central nervous system,especially the synaptic pruning of microglia.However,little is known about the functional significance of microglia in shaping the developmental trajectory of cortical circuits.This study uses a variety of modern experimental methods to clarify this issue in depth.We hypothesized that the development of cortical neural circuits requires microglia to mature and establish normal circuit connectivity.METHODS:We applied pharmacological and genetic methods to eliminate most of the microglia in the brain.Whole-cell patch-clamp technique was used to study the effect of microglial depletion on the electrophysiological activity of excitatory pyramidal neurons in the visual cortex.The effect of microglia on the intracortical circuit connectivity in the visual cortex was investigated by laser scanning photostimulation(LSPS)combined with glutamate uncaging.In addition,the effect of microglial cells on dendritic spines of visual cortex neurons was tracked by in vivo two-photon dendritic spines imaging.Lastly,we use single-unit electrophysiological recording combined with a monocular deprivation paradigm to investigate how the critical period of cortical plasticity may be affected by microglia depletion.RESULTS:We administered PLX3397,a colony‐stimulating factor 1 receptor inhibitor,to mice starting at postnatal day 15 and through P28,which depletes >75% of microglia in the visual cortex(VC).This treatment largely covers the critical period(P19‐32)of VC maturation and plasticity.Patch clamp recording in VC layer 2/3(L2/3)and L5 neurons revealed increased m EPSC frequency and reduced amplitude,and decreased AMPA/NMDA current ratio,indicative of altered synapse maturation.Increased spine density was observed in these neurons,potentially reflecting impaired synapse pruning.In addition,VC intracortical circuit functional connectivity,assessed by LSPS combined with glutamate uncaging,was dramatically altered.Using two photon longitudinal dendritic spine imaging,we confirmed that spine elimination/pruning was diminished during VC critical period when microglia were depleted.Reduced spine pruning thus may account for increased spine density and disrupted connectivity of VC circuits.Lastly,using single‐unit recording combined with monocular deprivation,we found that ocular dominance plasticity in the VC was obliterated during the critical period as a result of microglia depletion.These data establish a critical role of microglia in developmental cortical synapse pruning,maturation,functional connectivity,and critical period plasticity.CONCLUSIONS:In this study,we have investigated how microglia depletion affects the development of the visual cortex.Our findings reveal that microglia depletion in critical period significantly affects the developmental trajectory of the primary visual cortex,leading to altered neuronal morphology,as well as affecting the maturation of glutamatergic synapses,intracortical functional connectivity,and dendritic spine pruning.In addition,ocular dominance plasticity caused by monocular visual deprivation is also impaired.Our findings reveal the novel role of microglia in shaping the trajectory of cortical circuit development.