Amyotrophic Lateral Sclerosis (ALS) is an adult neurodegenerative disease characterized by a progressive and fatal loss of motor neurons in the spinal cord and brainstem and neurodegeneration in the motor cortex leading to paralysis and muscular atrophy. There is convincing evidence that excitotoxicity could participate in this neurodegeneration as impairment in the selective clearance of glutamate assumed by the astroglial glutamate transporter 1 (GLT-1) was demonstrated both in patients with sporadic ALS and in transgenic animal models during the development and the progression of the disease. In opposition, no modification was detected for the glutamate-aspartate transporter (GLAST). However, the mechanisms behind the loss of GLT-1 activity remain to be elucidated. In the present work, the GLT-1 activity and the expression of the two splice variant isoforms GLT-1a and GLT-1b were investigated in cultured cortical astrocytes from transgenic rats developing ALS. These animals express a mutated form of the human superoxide dismutase 1 gene (hSOD1^G93A) and exhibit features of the human disease. Activity of the transporters was evaluated measuring the D-[³H]-aspartate uptake velocity in presence of the selective GLT-1 inhibitor DHK or the GLAST inhibitor L-SOS. Interestingly, only the DHK-sensitive uptake was drastically reduced indicating a selective loss of GLT-1 activity in cultured astrocytes from mutated hSOD1^G93A rats as compared to wild-type animals. Besides, the transcription of the two splice variant isoforms GLT-1a and GLT-1b was inversely regulated as expression of the former was 2-fold decreased whereas the GLT-1b mRNAs were conversely 2-fold increased in cortical astrocytes from transgenic rats in comparison to control. Finally, effects of the Peptide Histidine Isoleucine (PHI) that could prevent the development of acute excitotoxic lesions and regulate the glutamate transporter activity were also reported. Short-term treatment with 0.1 mM PHI for 24 hours selectively conducted to a 4.5-fold increase of the GLT-1b transcription in cultured astrocytes from hSOD1^G93A rats and nearly did not modify the GLT-1a mRNA expression. Besides, the neuropeptide also enhanced the DHK-sensitive uptake in these cortical astroglial cells inducing the recover and the up-regulation of the GLT1 activity. For the first time in this report, we have highlighted that the GLT-1b transporter could show a potential anti-excitotoxic activity and that its up-regulation could represent a benefit for the glutamate clearance in murine transgenic models of familial form of ALS.

This work was supported by the National Fund for Scientific Research (F.N.R.S., Belgium, Convention FRSM 3.4560.07 and crédit au chercheur 1.5.109.09.F) and by the Belgian Queen Elisabeth Medical Foundation (Belgium).