Sensory Deprivation Induces Microglial Synapse Engulfment

Abstract

Synaptic connectivity is highly plastic in early development and undergoes extensive remodeling in response to changes in neuronal activity and sensory experience. Microglia, the resident central nervous system macrophages, participate in shaping mature neuronal circuits by dynamically surveying the brain parenchyma and pruning away less active synaptic connections. However, it is unknown how changes in neuronal activity regulates microglial pruning within circuits and whether this activity-dependent pruning is necessary to achieve plasticity. Using the rodent somatosensory circuit, I identified that microglia engulf and eliminate synapses in the cortex following early postnatal (P4) unilateral removal of mouse whiskers. I found this early life microglial synaptic remodeling requires specific chemokine signaling between neurons and microglia. Mice that lack expression of either the neuronal chemokine CX3CL1 (fractalkine), or its microglial receptor CX3CR1, have significantly reduced microglial synapse engulfment and fail to eliminate synapses following whisker removal. To gain more insight into how this signaling is regulated, I performed both single-cell RNA sequencing of the primary somatosensory cortex as well as microglia-specific Translating Ribosome Affinity Purification (TRAP) sequencing. I identified that the majority of central nervous system (CNS) cell populations in the somatosensory cortex, including microglia, undergo transcriptional changes following whisker removal. Further, the transcriptional changes in microglia after whisker cauterization require expression of the receptor CX3CR1. Importantly, I also found that Adam10, a gene encoding the metalloprotease known to post-translationally cleave CX3CL1 into a soluble chemokine, is upregulated in the deprived cortex after whisker ablation. Pharmacological inhibition of ADAM10 inhibits microglia-mediated removal of synapses in the deprived cortex. These data support a mechanism by which cleavage of membrane-bound CX3CL1 by ADAM10 is necessary for neuronal signaling to microglia via CX3CR1 to induce transcriptional changes within microglia upstream of synaptic engulfment and elimination following sensory deprivation.