xCT is present in select blood/brain/CSF interface areas and in an astrocyte subpopulation, in sufficient quantities to support the notion that system xc- provides physiologically relevant transport activity.

The communication between the immune and central nervous system (CNS) is affected in many neurological disorders. Peripheral injections of the endotoxin lipopolysaccharide (LPS) are widely used to study this communication: an LPS challenge leads to a biphasic syndrome that starts with acute sickness and is followed by persistent brain inflammation and chronic behavioral alterations such as depressive-like symptoms. In vitro, the response to LPS treatment has been shown to involve enhanced expression of system x c - . This cystine-glutamate antiporter, with xCT as specific subunit, represents the main glial provider of extracellular glutamate in mouse hippocampus. Here we injected male xCT knockout and wildtype mice with a single intraperitoneal dose of 5 mg/kg LPS. LPS-injection increased hippocampal xCT expression but did not alter the mainly astroglial localization of the xCT protein. Peripheral and central inflammation (as defined by cytokine levels and morphological activation of microglia) as well as LPS-induced sickness and depressive-like behavior were significantly attenuated in xCT-deficient mice compared with wildtype mice. Our study is the first to demonstrate the involvement of system x c - in peripheral and central inflammation in vivo and the potential therapeutic relevance of its inhibition in brain disorders characterized by peripheral and central inflammation, such as depression.

Glutamate transport activities have been identified not only in the brain, but also in the liver, kidney, and intestine. Although glutamate transporter distributions in the central nervous system are fairly well known, there are still uncertainties with respect to the distribution of these transporters in peripheral organs. Quantitative information is mostly lacking, and few of the studies have included genetically modified animals as specificity controls. The present study provides validated qualitative and semi-quantitative data on the excitatory amino acid transporter (EAAT)1-3 subtypes in the mouse liver, kidney, and intestine. In agreement with the current view, we found high EAAT3 protein levels in the brush borders of both the distal small intestine and the renal proximal tubules. Neither EAAT1 nor EAAT2 was detected at significant levels in murine kidney or intestine. In contrast, the liver only expressed EAAT2 (but 2 C-terminal splice variants). EAAT2 was detected in the plasma membranes of perivenous hepatocytes. These cells also expressed glutamine synthetase. Conditional deletion of hepatic EAAT2 did neither lead to overt neurological disturbances nor development of fatty liver.

Objective:To test the hypothesis that glutamate and GABA are linked to the formation of epilepsy networks and the triggering of spontaneous seizures, we examined seizure initiation/propagation characteristics and neurotransmitter levels during epileptogenesis in a translationally relevant rodent model of mesial temporal lobe epilepsy.Methods:The glutamine synthetase (GS) inhibitor methionine sulfoximine was infused into one of the hippocampi in laboratory rats to create a seizure focus. Long-term video-intracranial EEG recordings and brain microdialysis combined with mass spectrometry were used to examine seizure initiation, seizure propagation, and extracellular brain levels of glutamate and GABA.Results:All seizures (n = 78 seizures, n = 3 rats) appeared first in the GS-inhibited hippocampus of all animals, followed by propagation to the contralateral hippocampus. Propagation time decreased significantly from 11.65 seconds early in epileptogenesis (weeks 1-2) to 6.82 seconds late in epileptogenesis (weeks 3 – 4, paired t-test, p = 0.025). Baseline extracellular glutamate levels were 11.6-fold higher in the hippocampus of seizure propagation (7.3 µM) vs. the hippocampus of seizure onset (0.63 µM, ANOVA/Fisher’s LSD, p = 0.01), even though the concentrations of the major glutamate transporter proteins EAAT1, EAAT2 and xCT were unchanged between the brain regions. Finally, extracellular GABA in the seizure focus decreased significantly from baseline several hours before a spontaneous seizure (paired t-test/FDR).Conclusion:The changes in glutamate and GABA suggest novel and potentially important roles of the amino acids in epilepsy network formation and in the initiation and propagation of spontaneous seizures.

The cystine/glutamate antiporter system xc− has been identified as the major source of extracellular glutamate in several brain regions as well as a modulator of neuroinflammation, and genetic deletion of its specific subunit xCT (xCT−/−) is protective in mouse models for age-related neurological disorders. However, the previously observed oxidative shift in the plasma cystine/cysteine ratio of adult xCT−/− mice led to the hypothesis that system xc− deletion would negatively affect life- and healthspan. Still, till now the role of system xc− in physiological aging remains unexplored. We therefore studied the effect of xCT deletion on the aging process of mice, with a particular focus on the immune system, hippocampal function, and cognitive aging. We observed that male xCT−/− mice have an extended lifespan, despite an even more increased plasma cystine/cysteine ratio in aged compared to adult mice. This oxidative shift does not negatively impact the general health status of the mice. On the contrary, the age-related priming of the innate immune system, that manifested as increased LPS-induced cytokine levels and hypothermia in xCT+/+ mice, was attenuated in xCT−/− mice. While this was associated with only a very moderate shift towards a more anti-inflammatory state of the aged hippocampus, we observed changes in the hippocampal metabolome that were associated with a preserved hippocampal function and the retention of hippocampus-dependent memory in male aged xCT−/− mice. Targeting system xc− is thus not only a promising strategy to prevent cognitive decline, but also to promote healthy aging.

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