Distinct expression and regulation of the glutamate transporter isoforms GLT-1a and GLT-1b in cultured astrocytes from a rat model of amyotrophic lateral sclerosis (hSOD1G93A)

Impaired glutamate uptake associated with accumulation of extracellular glutamate is a well-documented feature of amyotrophic lateral sclerosis (ALS) and related excitotoxicity is frequently proposed to participate in the progression of the disease. We herein characterised the expression and activity of the glutamate transporter glutamate transporter 1 (GLT-1) in cultured cortical astrocytes derived from a transgenic rat strain expressing an ALS-related mutated form of human superoxide dismutase 1 (hSOD1G93A). Measurements of d-[3H]-aspartate uptake velocity in the presence of selective glutamate transporter blockers demonstrated that astrocytes from the transgenic rats showed an impaired GLT-1 activity as compared to cells from wild-type animals. In addition, the density of GLT-1a mRNA in cells from hSOD1G93A animals appeared nearly 2-fold lower while the density of GLT-1b mRNA was nearly 2-fold higher. Besides, we observed that exposing the astrocytes from hSOD1G93A rats to the neuroprotective transmitter Peptide Histidine Isoleucine (PHI) for 24 h caused a 4.5-fold increase in the GLT-1b mRNA level without influencing the expression of the other key isoform GLT-1a. This selective upregulation of GLT-1b by the neuropeptide was correlated with a significant increase in d-[3H]-aspartate uptake activity. The possibility to specifically regulate a single isoform of the high-affinity transporter GLT-1 is an unprecedented observation which sheds light on new perspectives for the pharmacological manipulation of glutamate transmission in diseases such as ALS.