Aurora Semerano, MD
@semerano_aurora
Microglia are the main resident immune cell population of the central nervous system and play a key role in brain development, homeostasis, and repair. During ischemic stroke, microglia are rapidly activated and are characterized by morphological, proliferative, and functional alterations. The role of microglia activation in ischemic stroke remains highly controversial in the preclinical setting and depends on multiple factors, including the experimental conditions and the phase of the disease. More recently, an additional role for microglial cells has been proposed, since they have been found to be implied in the occurrence, the sensing, and the response to cortical spreading depolarization (CSD).1 CSD is defined as a slowly propagating (2–5 mm/min) wave of rapid, near-complete depolarization of neurons and astrocytes followed by a period of electrical suppression of a distinct population of cortical neurons. CSD is considered as the biological substrate of migraine aura, but it has been shown to occur in other neurological conditions, such as ischemic stroke, subarachnoid hemorrhage, and traumatic brain injury.2 In other words, CSD consists in a deep perturbation of the ionic environment in the brain, which has been associated with excitotoxicity damage and vaso-occlusive phenomenons after brain injury.
In this paper, Liu et al.3 aim at exploring if microglia are involved in ionic perturbation after ischemic stroke, specifically addressing intracellular calcium influx. The authors employed two experimental mouse lines expressing genetically encoded indicators of calcium, which enable to visualize and follow over time intracellular calcium influxes in cortical microglial cells by means of in vivo imaging techniques. Imaging sessions were performed in an earlier (0-6 h) and a later (6-24 h) phase of stroke, after permanent middle cerebral artery occlusion (MCAO). Interestingly, waves of microglial intracellular calcium activity in a directionally progressive fashion could be detected in stroke mice compared to sham mice during both phases. In order to evaluate whether these patterns of calcium activity were related to the CSD phenomenon, naïve mice were treated with microinjections of KCl in the cerebral cortex, which have been shown to reliably induce CSD in previous studies. Similar calcium waves were observed in cortical microglia after KCl administration, even though at a faster speed compared to stroke-induced wavefronts. In addition, stimulating non-specific systemic inflammation with the administration of lipopolysaccharide (LPS) prior to KCl microinjection resulted to enhance microglial calcium activity. Finally, the authors tested if the wave-like calcium influx in microglia could be mediated by specific channels, known as calcium-release activated calcium (CRAC), which are expressed (though not exclusively) by immune cells. A selective inhibitor of CRAC was employed in naïve mice treated with KCl microinjections, and a partial decrease in intensity of calcium transients was observed, suggesting that the CRAC channels are at least in part responsible for calcium activity in microglia.
In this paper, the authors describe a new behaviour of microglial cells after stroke. Recording microglial calcium activity in vivo is a powerful tool which helps to move a further step into the comprehension of microglial involvement in the dysregulation of brain network activity in cerebral ischemia. Future studies are needed to further elucidate the specific triggers, the temporal dynamics within the CSD phenomenon, the role of sterile inflammation, and — importantly — the exact functional significance of this microglial calcium activity, in order to ultimately define the proper targets to address within the described mechanism. A previous study4 employed in vivo calcium imaging and a microglia ablation model to analyze the influence of microglia on neuronal activity and survival after stroke: Absence of microglia was shown to lead to dysregulated neuronal calcium responses, calcium overload, and increased neuronal death. A very recent study5 suggests that microglia could protect neurons from calcium overload after experimental stroke, through specialized neuron-microglia somatic junctions. In the complex microglial pathophysiology, the pathway for translating research to new effective treatments is long but exciting. Both (1) the selective impairment of the multiple microglial functions and (2) the emphasis on the phase of the disease and timing of intervention will probably play a fundamental role.
References:
1 Shibata M, Suzuki N. Exploring the role of microglia in cortical spreading depression in neurological disease. J Cereb Blood Flow Metab. 2017;37:1182-1191.
2 Dreier JP. The role of spreading depression, spreading depolarization and spreading ischemia in neurological disease. Nat Med. 2011;17:439-47.
3 Liu L, Kearns KN, Eli I, Sharifi KA, Soldozy S, Carlson EW, et al. Microglial Calcium Waves During the Hyperacute Phase of Ischemic Stroke. Stroke. 2020.
4 Szalay G, Martinecz B, Lénárt N, Környei Z, Orsolits B, Judák L, et al. Microglia protect against brain injury and their selective elimination dysregulates neuronal network activity after stroke. Nat Commun. 2016;7:11499.
5 Cserép C, Pósfai B, Lénárt N, Fekete R, László ZI, Lele Z, et al. Microglia monitor and protect neuronal function through specialized somatic purinergic junctions. Science. 2020;367:528-537.
