IL-1 Receptor Type 1 Regulates Microglial Synapse Displacement To Maintain Neuronal Excitatory Homeostasis

Microglial processes often contact and then phagocytize synapses to regulate synaptic plasticity in physiological and pathological conditions through C1a/C3/CR3 pathway.Microglia are also found to contact neuronal axons and induce synapse formation,which may be mainly related to neuronal calcium activity and skeleton.Besides,microglia can participate in the regulation of synaptic function by ensheathing neuronal cell body and displacement of the GABAergic synapses surrouding the neuronal soma.However,microglial synaptic displacement was reported in pathological setting,such as lipopolysaccharide induced neuroinflammation and febrile seizures.Whether it exists in physiological conditions and its mechanism remain unclear.In this study,we employed three-dimension reconstruction,in vivo two-photon calcium imaging,in vivo electrophysiology and Cre-lox P cell-specific gene knockout approaches to study the characteristics,mechanism and function of microglia-mediated synaptic displacement under physiological conditions,as well as the potential regulatory effects of related regulatory molecules on physiological and pathological processes.We discovered that microglia extensively associated with soma of glutamatergic neurons and displaced surrounding GABAergic synapses around the neurons in physiological condition.The degree of synapse displacement was heterogeneous in different brain regions.For example,there was more microglial association and synaptic displacement in anterior motor cortex than that in posterior motor cortex.Furthermore,in vivo two-photon calcium imaging showed that neurons extensively associated with microglia showed significantly more calcium activity and higher excitability than neurons not extensively associated with microglia cells.Through recording local field potential and analyzing the power spectral density,we found that brain regions with higher degree of synaptic displacement had higher overall neuronal excitability.Interlukin-1β(IL-1β)expression detected by RNAscope was negatively correlated to the degree of microglial association.Its expression was high in the region with low synaptic displacement,but low in the region with high synaptic displacement.Administration of IL-1β prevented microglial synaptic displacement in the anterior motor cortex,while antagonism of IL-1βor knockout of interlukin-1 receptor type 1(IL-1R1)resulted in more microglia approaching neurons and more synaptic displacement in the posterior motor cortex.The field potential recording and the energy analysis also showed that the excitability of the neural network in IL-1Ra treated mice with more synaptic displacement was higher than that in vehicle treated mice.We further investigated the effect of IL-1β mediated microglial synapse displacement on the physiological function by local injection of IL-1Ra in the posterior motor cortex.The results showed that IL-1Ra impaired the motor learning ability of mice,and administration of TRAM-34,a microglia inhibitor with inhibitory effect on synaptic displacement,could reverse the impairment of IL-1Ra on motor learning ability.By cell-specific knockdown of IL-1β in glutamatergic neuron of microglia,we found that IL-1β mainly derived from glutamatergic neuron mediated microglial synapse displacement.IL-1R1 on microglia,excitatory neurons or GABAergic neurons was knocked out by Cre-Lox P system,to study the cell-specific role of IL-1R1 in microglial synapse displacement.We found that IL-1R1 on microglia promoted synapse displacement and IL-1R1 on glutamatergic neurons inhibited synapse displacement,while IL-1R1 on GABAergic neurons may not impact microglial synapse displacement.We also examined the effect of IL-1R1 knockout on different types of cells on motor learning,and the results showed that the motor learning ability of mice with specific knockout IL-1R1 on glutamatergic neurons was significantly decreased,while the motor learning ability of mice with specific knockout IL-1R1 on microglia cells was significantly enhanced.We also found that in MPTP(1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced subacute Parkinson’s disease model,the inhibition of microglial association with neuron by IL-1β effectively improved motor learning ability.In conclusion,this study demonstrated the presence of brain regional heterogeneity in microglia-mediated synaptic displacement under physiological conditions.IL-1β is a critical negative regulator of microglia-mediated synaptic displacement,thereby regulating neuronal excitatory homeostasis and motor function.The effect of IL-1β on microglial synaptic displacement is cell-specific and brain-specific.IL-1β mainly derived from neurons inhibits microglial synapse displacement through neuronal own IL-1R1,while IL-1R1 on microglia had opposite effect.Moreover,IL-1β/IL-1R1 shows distinctive actions on microglial synaptic displacement between anterior and posterior motor cortex.This study provides a theoretical basis for explaining the mechanism and function of microglial synaptic displacement,a new paradigm of microglial regulating synaptic plasticity,and provides a new idea for improving PD and other motor dysfunction.