Plasticity of perisynaptic astroglia during ischemia-induced spreading depolarization

Abstract

High astroglial capacity for glutamate and potassium clearance aids in recovering spreading depolarization (SD)-evoked disturbance of ion homeostasis during stroke. Since perisynaptic astroglia cannot be imaged with diffraction-limited light microscopy, nothing is known about the impact of SD on the ultrastructure of a tripartite synapse. We used serial section electron microscopy to assess astroglial synaptic coverage in the sensorimotor cortex of urethane-anesthetized male and female mice during and after SD evoked by transient bilateral common carotid artery occlusion. At the subcellular level, astroglial mitochondria were remarkably resilient to SD compared to dendritic mitochondria that were fragmented by SD. Overall, 482 synapses in `Sham' during `SD' and `Recovery' groups were randomly selected and analyzed in 3D. Perisynaptic astroglia was present at the axon-spine interface (ASI) during SD and after recovery. Astrocytic processes were more likely found at large synapses on mushroom spines after recovery, while the length of the ASI perimeter surrounded by astroglia has also significantly increased at large synapses. These findings suggest that as larger synapses have a bigger capacity for neurotransmitter release during SD, they attract astroglial processes to their perimeter during recovery, limiting extrasynaptic glutamate escape and further enhancing the astrocytic ability to protect synapses in stroke.

Spreading depolarization causes reversible neuronal mitochondria fragmentation and swelling in healthy, normally perfused neocortex

“…To reduce bias, all 292 dendritic mitochondria from 150 dendritic segments contained in two serial sections from two mice of the SD group were reconstructed in 3D and compared to 142 3D reconstructed mitochondria from 73 dendritic segments in three serial sections from Sham mice. As previously reported for dendritic mitochondria 22, 36, 37, 47 , Sham mice had mostly long filamentous mitochondria ( Fig. 3G ).…”

“…EM datasets showing the absence of swollen dendrites with intact mitochondria might indirectly support this idea. Conversely, despite astrocytes swell during SD 20,69 , astroglial mitochondria maintaine their structure during ischemia-induced SD, which is in striking difference to the neighboring dendritic mitochondria 47 . Whether SD-induced water influx into intact neurons is sufficient and necessary for neuronal mitochondria fragmentation requires further investigation.…”

“…The length and maximum width of mitochondria were measured in 3D Next, we classified mitochondria into globules and tubules, selecting the aspect ratio of 3 (length/maximum diameter) as an arbitrary threshold between these categories 22,47 . As previously reported for dendrites of pyramidal neurons 22,36,37 , most reconstructed mitochondria in Sham mice were elongated tubular organelles (91.5% of the total), and globular mitochondria were infrequent (6.3% of the total) (Fig.…”