Expression profile of the excitatory amino acid transporter 2 (EAAT2) in normal and diseased brain

The excitatory amino acid transporter 2 (EAAT2)/glutamate transporter 1 (GLT1) is responsible for the majority of glutamate uptake in the brain. GLT1 is alternatively spliced in an extremely complex manner, creating transcripts with alternate 5' and 3' ends, as well as transcripts with skipped exons or retained introns. This study involved a detailed quantitative analysis of all of the major isoforms using quantitative RT-PCR. The relative abundance of the GLT1a, GLT1b, and GLT1c transcripts was determined in normal rat and human brain in the prefrontal cortex, hippocampus, nucleus accumbens, and amygdala. Part of the 3' UTR may exist as an independent noncoding molecule, based on evidence from northern blotting. A region of the GLT1b transcript previously believed to be intronic was shown to be part of the coding sequence. A monoclonal antibody was raised against the GLT1b isoform and characterized using rat brain tissue. Transcripts with retained introns and skipped exons were found to be present at lower levels than previously thought in normal human brain. The splice variants were also measured in tissue from human brains at different developmental stages. There was a substantial increase in most isoforms between 1-2 months of age and 1-2 years of age. The low expression of EAAT2 during the neonatal period coincides with a period when seizure susceptibility is highest. There was no difference in EAAT2 expression between subjects with schizophrenia and normal controls or between rats treated with haloperidol and saline. Although EAAT2 protein is substantially decreased in the motor cortex and spinal cord of subjects with amyotrophic lateral sclerosis, the present study demonstrated that expression of certain splice forms is increased at the RNA level in ALS tissue. One transcript with increased expression encodes an upstream open reading frame, which could potentially contribute to decreased protein levels by leading to decreased expression of the main protein product.