The Electrophysiology of Stroke: The Role of Glutamate Antiporter, System xc-, in the Ischemic Cascade
thesisposted on 01.08.2020, 00:00 by Bradley Stavros Heit
Ischemic stroke remains the third leading cause of death and leading cause of adult disability worldwide. Efforts to reduce stroke severity, however, have been plagued by translational failure due to gaps in our understanding of cellular mechanisms leading to brain damage after metabolic insult. Loss of blood supply to brain tissue (ischemia) depletes neurons of energy (ATP), which induces an excitatory effect in the synaptic network marked by the excessive release of glutamate. Within minutes, this excitation results in irreversible tissue damage triggered by anoxic depolarization (AD) - an electrophysiological event with predictive value for stroke outcome. Importantly, much of the “ischemic cascade” can be reproduced by transient deprivation of O2 to in vitro hippocampal slices. Using a battery of electrophysiological assays, we reveal several modifiable steps of the ischemic cascade. First, adenosine release during oxygen deprivation does not extend AD in CA1 of hippocampus, while antagonism of glutamate receptors does. Moreover, intact feedforward connections from CA3 contribute to the timing of AD in CA1 insomuch that glutamate expulsion in CA3 propagates AD from CA3 to CA1. System xc-, the cystine/glutamate antiporter, mediates ambient extracellular glutamate levels in the hippocampus, however, its influence during acute ischemia remains unexplored and enigmatic. We therefore compared hippocampal slices from wild type (WT) and xCT-KO (xCT-/-) mice to investigate the role of system xc- during oxygen deprivation. Our results show that both genetic deletion and pharmacological antagonism of system xc- increase latency to AD and attenuate depolarizing waves. This suggests that anoxia-induced excitation in the synaptic network is enhanced due to the antiporter’s putative effect on extracellular glutamate levels. Experiments where extracellular glutamate concentrations were manipulated, as well as experiments performed in conditions of glutamate receptor antagonism, further confirm this hypothesis. These data reveal that the antiporter is a salient driver of the ischemic cascade. Furthermore, after graded hypoxic episodes, xCT-/- mice display accelerated rate of recovery and post-hypoxic potentiation, which are likely functions of enhanced intracellular calcium. Taken together, our findings implicate system xc- as a regulator of ischemia tolerance and an attractive target for the amelioration of stroke pathology.