Glial Glutamate Transporter-Mediated Plasticity: System xc-/xCT/SLC7A11 and EAAT1/2 in Brain Diseases

Glial cells play an essential role in the complex function of the nervous system. In particular, astrocytes provide nutritive support for neuronal cells and are involved in regulating synaptic transmission. Oligodendrocytes ensheath axons and support information transfer over long distances. Microglial cells constitute part of the innate immune system in the brain. Glial cells are equipped with the glutamate-cystine-exchanger xCT (SLC7A11), the catalytic subunit of system xc-, and the excitatory amino acid transporter 1 (EAAT1, GLAST) and EAAT2 (GLT-1). Thereby, glial cells maintain balanced extracellular glutamate levels that enable synaptic transmission and prevent excitotoxic states. Expression levels of these transporters, however, are not fixed. Instead, expression of glial glutamate transporters are highly regulated in reaction to the external situations. Interestingly, such regulation and homeostasis is lost in diseases such as glioma, (tumor-associated) epilepsy, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis or multiple sclerosis. Upregulation of system xc- (xCT or SLC7A11) increases glutamate export from the cell, while a downregulation of EAATs decreases intracellular glutamate import. Occurring simultaneously, these reactions entail excitotoxicity and thus harm neuronal function. The release of glutamate via the antiporter system xc- is accompanied by the import of cystine-an amino acid essential in the antioxidant glutathione. This homeostasis between excitotoxicity and intracellular antioxidant response is plastic and off-balance in central nervous system (CNS) diseases. System xc- is highly expressed on glioma cells and sensitizes them to ferroptotic cell death. Hence, system xc- is a potential target for chemotherapeutic add-on therapy. Recent research reveals a pivotal role of system xc- and EAAT1/2 in tumor-associated and other types of epilepsy. Numerous studies show that in Alzheimer's disease, amyotrophic lateral sclerosis and Parkinson's disease, these glutamate transporters are dysregulated-and disease mechanisms could be interposed by targeting system xc- and EAAT1/2. Interestingly, in neuroinflammatory diseases such as multiple sclerosis, there is growing evidence for glutamate transporter involvement. Here, we propose that the current knowledge strongly suggest a benefit from rebalancing glial transporters during treatment.

Automated summary

Glial cells, such as astrocytes, oligodendrocytes, and microglial cells, play important roles in the function of the nervous system, including providing nutritive support, regulating synaptic transmission, and participating in the immune response. Glial cells are equipped with various glutamate transporters to maintain balanced extracellular glutamate levels and prevent excitotoxicity. Dysregulation of these transporters is observed in various CNS diseases, including glioma, epilepsy, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis, leading to harmful effects on neuronal function. Targeting these transporters, such as system xc- and EAAT1/2, may have therapeutic benefits in treating these diseases.