Activity and TREM2‐dependent phagocytic cup formation by microglia in the mouse cortex

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AbstractBackgroundMicroglia are the main innate immune cells capable of engulfing dead cells and cellular debris in the brain. Trem2 receptor is expressed in microglia and plays an important role in phagocytosis. This receptor has been linked to Alzheimer’s disease and involved in the uptake of amyloid plaques by microglia. However, how microglia engulf dead cells and the role of Trem2 in this process remain unclear.MethodWe used two‐photon microscopy to image microglia engulfing of dead cells in the primary motor cortex of Cx3cr1 gfp/+ mice, Cx3cr1gfp/+:Trem2‐/‐ mice, Cx3cr1gfp/+:Trem2R47H/+ mice, and ThyGCaMP6 mice. These mice were placed on a custom made, free‐floating treadmill and subjected to five motor‐running sessions over one hour. Electron microscopy was used to determine the ultrastructure of microglial phagocytic cups engulfing dead cells. We manipulated neuronal activity by using AAV viral infection to express ChR2 in thalamic neurons or by administrating MK‐801 or muscimol in the motor cortex.ResultOur time‐lapse imaging data show that microglial processes form phagocytic cups to engulf dead cells within tens of minutes in the motor cortex during running. Ultrastructural examination of phagocytic cups confirmed the engulfment of dead cell body by microglial processes as revealed by live imaging. Pharmacological inhibition of neuronal activity was sufficient to prevent running‐dependent phagocytic cup formation. On the other hand, optogenetic stimulation of thalamocortical projections induced the formation of microglial phagocytic cups. In Trem2‐deficient mice and Trem2 R47H mice, running‐dependent phagocytic cup formation was reduced as compared to wild type controls. There was an increase of propidium iodide (PI)‐positive cell bodies in Trem2 deficient/haplodeficient mice comparted to the control mice.ConclusionThese findings reveal that the formation of phagocytic cups by microglia is dependent on neuronal activity and the Trem2 receptor in the mouse cortex. They also suggest that the lack of dead‐cell clearance mediated by microglia might play a role in the pathogenesis of Alzheimer’s disease.