Selective loss of glial glutamate transporter GLT‐1 in amyotrophic lateral sclerosis

Abstract

The pathogenesis of sporadic amyotrophic lateral sclerosis (ALS) is unknown, but defects in synaptosomal high-affinity glutamate transport have been observed. In experimental models, chronic loss of glutamate transport can produce a loss of motor neurons and, therefore, could contribute to the disease. With the recent cloning of three glutamate transporters, i.e., EAAC1, GLT-1, and GLAST, it has become possible to determine if the loss of glutamate transport in ALS is subtype specific. We developed C-terminal, antioligopeptide antibodies that were specific for each glutamate transporter. EAAC1 is selective for neurons, while GLT-1 and GLAST are selective for astroglia. Tissue from various brain regions of ALS patients and controls were examined by immunoblot or immunocytochemical methods for each transporter subtype. All tissue was matched for age and postmortem delay. GLT-1 immunoreactive protein was severely decreased in ALS, both in motor cortex (71% decrease compared with control) and in spinal cord. In approximately a quarter of the ALS motor cortex specimens, the loss of GLT-1 protein (90% decrease from control) was dramatic. By contrast, there was only a modest loss (20% decrease from control) of immunoreactive protein EAAC1 in ALS motor cortex, and there was no appreciable change in GLAST. The minor loss of EAAC1 could be secondary to loss of cortical motor neurons. As a comparison, glial fibrillary acidic protein, which is selectively localized to astroglia, was not changed in ALS motor cortex. Because there is no loss of astroglia in ALS, the dramatic abnormalities in GLT-1 could reflect a primary defect in GLT-1 protein, a secondary loss due to down regulation, or other toxic processes.

Glutamate transporters in platelets: EAAT1 decrease in aging and in Alzheimer’s disease

“…Clearance of the excessive neurotoxic concentrations of glutamate is ensured by a high affinity glutamate uptake system, present in neurons and glial cells [32,39]. Defects in glutamate uptake were described in autoptic brain samples in neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS) [5,45,47] and Alzheimer's disease (AD) [37].…”

“…The first three transporters are involved in high affinity, sodium-dependent glutamate uptake and knock-out models indicate a specific role of each transporter in preventing excitotoxicity and neuronal degeneration [46,54]. Moreover, reduced expression of GLT1 (EAAT2) in global transient ischemia animal model [56] and in ALS patients [30,47] seems responsible for the elevation of extracellular glutamate concentration and neuronal death. Molecular and/or functional characterizations of glutamate transporters were performed in different cell culture models from CNS and peripheral tissues, in order to investigate the regulation of their expression, translocation and activity by various endogenous and exogenous factors [11][12][13]21,35,38,49].…”

Amino acid composition of brain cysts: levels of excitatory amino acids in cyst fluid fail to predict seizures

“…It is conceivable that the high levels of aspartate and/or glutamate in the cysts are due to insufficient capacity of glutamate transporters either in the cyst epithelium and/or the solid parts of the tumor, thus leading to extracellular accumulation of aspartate and/or glutamate. In fact, in the sporadic form of amyotrophic lateral sclerosis (ALS), a marked reduction of EAAT2 was found in motor cortex and spinal cord (Rothstein et al, 1992(Rothstein et al, , 1995 and an increase of aspartate/glutamate levels in the CSF (Rothstein et al, 1990). So far, no genetic epilepsy has been identified in man with a primary defect involving glutamate transporters (Meldrum et al, 1999).…”

Les cellules gliales

“…Les astrocytes ont le pouvoir de nettoyer l'environnement synaptique du surplus de glutamate auquel les motoneurones sont particulièrement sensibles [11]. Le fait qu'une perte des transporteurs astrocytaires au glutamate est observée dans les modèles de rats et de souris ainsi que chez les patients atteints de SLA [12,13] renforce cette hypothèse, également confortée par le fait que le seul traitement actuellement disponible (Riluzol, aux effets modestes) pour les patients atteints de SLA agit sur les voies glutamatergiques. Un autre dénominateur commun qui fait l'objet d'une attention particulière est la présence d'une réaction inflammatoire représentée dans la moelle épinière NOUVELLE…”

“…Les astrocytes ont le pouvoir de nettoyer l'environnement synaptique du surplus de glutamate auquel les motoneurones sont particulièrement sensibles [11]. Le fait qu'une perte des transporteurs astrocytaires au glutamate est observée dans les modèles de rats et de souris ainsi que chez les patients atteints de SLA [12,13] [14][15][16][17], de même que le remplacement des cellules microgliales/macrophages exprimant la SOD1 mutée par des cellules contrôles [18,19], entraînent une augmentation notable de la survie des souris SLA.…”