Microglial Activity During the Hyperacute Phase of Ischemic Stroke – The Cardiology Advisor

Calcium activity was detected in microglia calcium during the hyperacute phase of ischemic stroke, according to study results presented at the American Heart Association (AHA) Scientific Sessions 2020, held virtually from November 13 to 17, 2020.

Mice from a line which expressed RNA Polymerase II Subunit A (Polr2a)-based GCaMP5 and Cre-dependent tdTomato were used for this study. Cerebral ischemia was induced between 8 and 10 weeks by occlusion of the middle cerebral artery with silicon-coated filaments. The brains of the mice were imaged 2 hours after the artery occlusion or sham procedure to record early (ie, 0-6 hours) and late (ie, 6-24 hours) hyperacute phenotypes.

Initial recordings indicated periodical directional calcium events in the parenchymal microglia. Calcium activity in meningeal macrophages were not initially involved.

At 20 hours after occlusion, 18 wave-like calcium activities were recorded in 3 mice who were imaged for 5 hours each. These microglial calcium waves decreased in frequency from 8 during the first 6 hours, to 6 between 6- and 12-hours post occlusion, to 4 waves in the 12- to 20-hour period after induced stroke. Despite the decrease of events, the amplitude of calcium increased over time.

High-speed recordings revealed robust calcium transients with an average (among 2 mice) of 15 seconds, which were more pronounced during the early hyperacute phase.

Microinjections of potassium chloride elicited similar waves of calcium, 60 seconds after injection, which traveled along the same trajectory from the injection site. These waves had a velocity of 6.71.99 mm/min which was more rapid than waves detected after stroke (4.91.71 mm/min).

Peripheral inflammation was induced by a lipopolysaccharide injection. During this experiment, wave velocity was not significantly different than in control animals (6.91.10 mm/min; P =.06). Microglial cell soma size increased not significantly from 269.630.45 mm3 to 343.731.46 mm3 after lipopolysaccharide treatment (P =.1).

Compared with controls, calcium transients in 15 lipopolysaccharide-affected microglia had greater: maximum peak intensity (increase, 47%; P =.025), peak duration (increase, 16%; P =.04), and area under the curve (increase, 59%; P =.007).

Mice were given the calcium release-activated channel inhibitor CM-EX-137 orally 4 hours before stroke induction. The cells which had the greatest response had a 20% decrease in microglial calcium peak intensity compared with controls (0.610.05 to 0.490.03, respectively; P =.04) and a 26.7% decrease in the area under the curve (10.641.01 to 7.800.57, respectively; P =.014); indicating a partial role of store-operated calcium channels during the calcium wave response.

This study was limited by the fact that a single channel type mediating the calcium response was investigated.

These observations indicate that microglial calcium activity followed a cerebral ischemia event and likely involved store-operated calcium channels. Further studies are needed to better understand the molecular mechanisms underlying the calcium response after stroke.

Disclosure: An author declared affiliations with industry. Please refer to the original article for a full list of disclosures.

Reference

Liu L, Kearns K N, Eli I, et al. Microglial calcium waves during the hyperacute phase of ischemic stroke. Presented at: AHA Scientific Sessions 2020; November 13-17, 2020. Presentation 370. doi:10.1161/STROKEAHA.120.032766

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Microglial Activity During the Hyperacute Phase of Ischemic Stroke - The Cardiology Advisor