Decreased epileptogenesis in mice lacking the System x_c⁻ transporter occurs in association with a reduction in AMPA receptor subunit GluA1

Objective: Although the cystine/glutamate antiporter System x_c⁻ (Sx_c⁻) plays a permissive role in glioma-associated seizures, its contribution to other acquired epilepsies has not been determined. As such, the present study investigates whether and how Sx_c⁻ contributes to the pentylenetetrazole (PTZ) chemical kindling model of epileptogenesis. Methods: Male Sx_c⁻ null (sut/sut) mice and their wild-type littermates were administered PTZ (i.p.) daily for up to 21 days (kindling paradigm). Seizure severity was scored on a 5-point behavioral scale. Mossy fiber sprouting, cellular degeneration, and Sx_c⁻ light chain (xCT) messenger RNA (mRNA) were explored using Timm staining, thionin staining, and real-time quantitative polymerase chain reaction (qPCR), respectively. Levels of reduced and oxidized glutathione and cysteine were determined via high-performance liquid chromatography (HPLC). Plasma membrane protein levels of glutamate and γ-aminobutyric acid (GABA) receptor subunits as well as the K⁺/Cl⁻ co-transporter KCC2 were quantified via western blot analysis. Results: Repeated administration of PTZ produced chemical kindling in only 50% of Sx_c⁻ null mice as compared to 82% of wild-type littermate control mice. Kindling did not result in any changes in xCT mRNA levels assessed in wild-type mice. No cellular degeneration or mossy fiber sprouting was discernible in either genotype. Except for a small, but significant, decrease in oxidized cysteine in the hippocampus, no other change in measured redox couples was determined in Sx_c⁻ null mice. Cortical levels of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA1 were decreased in Sx_c⁻ null mice as compared to wild-type littermates, whereas all other proteins tested showed no difference between genotypes. Significance: This study provides the first evidence that Sx_c⁻ signaling contributes to epileptogenesis in the PTZ kindling model of acquired epilepsy. Further data indicate that a reduction in AMPA receptor signaling could underlie the resistance to PTZ kindling uncovered in Sx_c⁻ null mice.