How microglia sense and regulate neuronal activity

Abstract

Microglia are innate immune cells of the central nervous system that sense extracellular cues. Brain injuries, inflammation, and pathology evoke dynamic structural responses in microglia, altering their morphology and motility. The dynamic motility of microglia is hypothesized to be a critical first step in sensing local alterations and engaging in pattern-specific responses. Alongside their pathological responses, microglia also sense and regulate neuronal activity. In this review, we consider the extracellular molecules, receptors, and mechanisms that allow microglia to sense neuronal activity changes under both hypoactivity and hyperactivity. We also highlight emerging in vivo evidence that microglia regulate neuronal activity, ranging from physiological to pathophysiological conditions. In addition, we discuss the emerging role of calcium signaling in microglial responses to the extracellular environment. The dynamic function of microglia in monitoring and influencing neuronal activity may be critical for brain homeostasis and circuit modification in health and disease.

Keywords: calcium imaging; dynamics; microglia; neuronal activity; two-photon microscopy.

Figure 1:. Microglial responses and regulation during neuronal hyperactivity.

Neurons exhibit multiple mechanisms for ATP release, including voltage-activated anion channels (VACCs), vesicular release, and NMDAR-driven release through a probenecid-sensitive mechanism. Once ATP is released, it is hydrolyzed by CD39 into ADP—a high-affinity ligand of the P2Y12 receptor, which promotes microglial process recruitment towards the neuronal purine source. The actions of P2Y12 occur both through canonical Gi signaling and secondary activation of potassium leak currents through THIK-1. Once recruited, close apposition of the microglial membrane to the synapse allows for effective hydrolysis of ADP into AMP by CD39, followed by AMP hydrolysis to Adenosine by CD73. Adenosine then signals to the presynaptic A1 receptor to decrease neurotransmitter release, closing this negative feedback loop. Additional studies indicate that CX3CL1-CX3CR1 signaling may additionally enhance microglial recognition and regulation of hyperactivity.

Figure 2:. Mechanisms of microglial process surveillance during awake and hypoactive conditions.

(A) In the awake state, resting norepinephrine (NE) tone set by locus coeruleus (LC) neurons signals to the microglial Beta-2 receptor (β2R) to increase cAMP signaling (see inset). Under conditions of high intracellular cAMP, large process motility is prevented, favoring smaller, filopodial outgrowth. These conditions result in a smaller surveillance territory for microglia in the awake state (dashed lines). (B) During periods of hypoactivity (TTX, anesthesia) or selective blockade of NE→β2R signaling, microglial cells become ‘disinhibited.’ Lack of Gs signaling through the β2R promotes lower cAMP levels inside microglia, favoring conditions for larger process outgrowth and expanded microglial surveillance territory.

Figure 3:. A simplified model of purine-driven microglial calcium activity.

Purinergic signaling is the major identified driver of microglial calcium signaling in vivo under conditions of inflammation, injury, and certain pathologies. Transcript and protein studies identify four major purine receptors expressed by microglia. The ionotropic P2X4 and P2X7 receptors, with preferential affinity for ATP, allow for extracellular calcium entry. The metabotropic P2Y1 and P2Y6 receptors are respectively ADP- and UDP-sensitive and couple with Gq→Phospholipase C (PLC)→Inositol triphosphate (IP₃)→IP₃ receptor signaling to release high levels of calcium from intracellular stores, including the endoplasmic reticulum (ER).