Differential abundance of glutamate transporter subtypes in amyotrophic lateral sclerosis (ALS)-vulnerable versus ALS-resistant brain stem motor cell groups.

Previous studies have suggested that defective high-affinity glutamate uptake, due mainly to a major loss of the astroglial-specific GLT-1 glutamate transporter, underlies the selective motoneuron degeneration observed in sporadic ALS (24, 28). If a defect in glutamate transport underlies the pathogenesis of sporadic ALS, the glutamate transporter subtype found to be lost in sporadic ALS should be present in abundance in the affected motor nuclei under normal conditions. To investigate this, we used immunohistochemical methods to analyze the localization of two subtypes of high-affinity glutamate transporters in the cranial motor nuclei of normal monkey brain stem: GLT-1, localized to astroglia; and EAAC1, localized to neurons. Our results indicated that all motor cell groups of monkey brain stem are rich in the GLT-1 glutamate transporter, which is localized to astroglial cells and processes that surround and envelop motoneuron cell bodies and dendrites. Image analysis indicated that the abundance of GLT-1 immunoreactive astroglial elements in ALS-vulnerable motor cell groups (i.e., the trigeminal, facial, and hypoglossal motor cell groups) is higher than in ALS-resistant motor cell groups (i.e., the oculomotor, trochlear, and abducens motor cell groups), and statistical analysis showed that this difference is significant. Our results also indicated that both ALS-vulnerable and ALS-resistant motor cell groups of monkey brain stem are relatively poor in EAAC1 immunoreactivity. Therefore, in the case of a loss in the GLT-1 glutamate transporter in sporadic ALS, glutamate may increase in the vicinity of motoneurons in all brain-stem motor cell groups, but especially in the ALS-vulnerable motor cell groups, which are normally richer in GLT-1. Increased extracellular glutamate could lead to excess entry of Ca2+ into motoneurons via glutamate-gated or voltage-activated Ca2+ channels and produce degeneration of those motoneurons unable to resist the insult. Since motoneurons in the ALS-resistant motor cell groups of the brain stem are enriched in the Ca2+ buffering protein parvalbumin, they should be better able to resist the damage than the majority of motoneurons in the ALS-vulnerable motor cell groups, which lack parvalbumin (20).