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 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, the 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.

Neuron–Microglia Interactions in Mental Health Disorders: “For Better, and For Worse”

“…In separate studies microglia were shown to play a neuroprotective role during excitotoxic injury, with microglia depletion leading to enhanced susceptibility to neuronal death (114). Consistent with these studies, recent work showed that microglia processes contacted swollen axonal segments during excitotoxic conditions, which normalized the excitability of affected neurons and preserved their viability (115). In these instances, neuron–microglia interactions are critical to maintain homeostasis in the brain but likely lead to undesirable inflammatory consequences.…”

Regulation of Physical Microglia–Neuron Interactions by Fractalkine Signaling after Status Epilepticus

“…The existence of a novel form of microglia–neuron physical interaction termed microglial process convergence (MPCs) was reported that increased upon extracellular calcium reduction (Eyo et al, 2015), a paradigm known to lead to epileptiform burst activity (Bikson et al, 1999) and prolonged neuronal depolarization (Kato et al, 2016). Typically, MPCs consist of several microglial processes that spontaneously converge at distinct sites and make transient focal aggregations (see movies).…”

“…Microglia are the predominant source of inflammation in the brain and are now recognized to play significant roles in brain homoeostasis (Hanisch and Kettenmann, 2007; Ransohoff and Perry, 2009). Particularly, microglia make transient physical interactions with neuronal elements and, in doing so, are suggested to monitor and alter synaptic activity (Wake et al, 2009; Tremblay et al, 2010; Kato et al, 2016). Moreover, neuronal hyperactivity, including after status epilepticus, triggered increasing microglial process extension to interact with neurons (Dissing-Olesen et al, 2014; Eyo et al, 2014).…”

“…Moreover, neuronal hyperactivity, including after status epilepticus, triggered increasing microglial process extension to interact with neurons (Dissing-Olesen et al, 2014; Eyo et al, 2014). Previous studies reported a novel microglia–neuron physical interaction named microglial process convergence (MPC) toward neuronal dendrites under conditions of reduced extracellular calcium (Eyo et al, 2015) as well as after repetitive neuronal stimulation (Kato et al, 2016). However, molecular regulators guiding MPC and whether they are functionally relevant in epilepsy remain to be elucidated.…”

Transient Developmental Purkinje Cell Axonal Torpedoes in Healthy and Ataxic Mouse Cerebellum

“…For example, could the swelling of a torpedo mark where a recent axonal collateral has been pruned? To address this, we examined the localization of activated microglia in the granule cell layer, which have been associated with axonal refinement and pruning during development (Pont-Lezica et al, 2011), and can be recruited to axonal swellings (di Penta et al, 2013; Kato et al, 2016). We looked at two ages: at P11, the peak of developmental torpedo density, and at P30 ( Figure 3A ), when torpedoes are larger but less numerous.…”