Microglial Contact Prevents Excess Depolarization and Rescues Neurons from Excitotoxicity

Abstract

Microglia survey and directly contact neurons in both healthy and damaged brain but the mechanisms and functional consequences of these contacts are not yet fully elucidated. Combining two-photon imaging and patch-clamping, we have developed an acute experimental model for studying the role of microglia in CNS excitotoxicity induced by neuronal hyperactivity. Our model allows us to simultaneously examine the effects of repetitive supramaximal stimulation on axonal morphology, neuronal membrane potential, and microglial migration, using cortical brain slices from Iba-1 eGFP mice. We demonstrate that microglia exert an acute and highly localized neuroprotective action under conditions of neuronal hyperactivity. Evoking repetitive action potentials (APs) in individual layer 2/3 pyramidal neurons elicited swelling of axons, but not dendrites, which was accompanied by a large, sustained depolarization of soma membrane potential. Microglial processes migrated to these swollen axons in a mechanism involving both ATP and glutamate release via volume-activated anion channels. This migration was followed by intensive microglial wrapping of affected axons, and in some cases removal of axonal debris, that induced a rapid soma membrane repolarization back to resting potentials. When the microglial migration was pharmacologically blocked, the activity-induced depolarization continued until cell death ensued, demonstrating that the microglia-axon contact served to prevent pathological depolarization of the soma and maintain neuronal viability. This is a novel aspect of microglia surveillance, detecting, wrapping and rescuing neuronal soma from damage due to excessive activity.

Significant Statement: Microglia, as immune cells in central nervous system, are highly motile cells, continuously expanding and retracting their processes as they monitor brain parenchyma. They can exert neuro-protective or neuro-toxic effects depending on their activation state. In this paper, we demonstrate that microglia are attracted to over-active axons, directly connecting to these axons to reduce membrane potential and exert neuro-protection. The attraction of microglia processes depends on ATP release through volume activated anion channels (VAACs). Blocking VAACs inhibited microglial attraction to axons and impaired the restoration of membrane potential and axonal survival.