Amyotrophic lateral sclerosis-linked glutamate transporter mutant has impaired glutamate clearance capacity

We have investigated the functional impact of a naturally occurring mutatio n of the human glutamate transporter GLT1 (EAAT2), which had been detected in a patient with sporadic amyotrophic lateral sclerosis, The mutation invo lves a substitution of the putative N-linked glycosylation site asparagine 206 by a serine residue (N206S) and results in reduced glycosylation of the transporter and decreased uptake activity. Electrophysiological analysis o f N206S revealed a pronounced reduction in transport rate compared with wil d-type, but there was no alteration in the apparent affinities for glutamat e and sodium. In addition, no change in the sensitivity for the specific tr ansport inhibitor dihydrokainate was observed, However, the decreased rate of transport was associated with a reduction of the N206S transporter in th e plasma membrane. Under ionic conditions, which favor the reverse operatio n mode of the transporter, N206S exhibited an increased reverse transport c apacity. Furthermore, if coexpressed in the same cell, N206S manifested a d ominant negative effect on the wild-type GLT1 activity, whereas it did not affect wild-type EAAC1, These findings provide evidence for a role of the N -linked glycosylation in both cellular trafficking and transport function. The resulting alteration in glutamate clearance capacity likely contributes to excitotoxicity that participates in motor neuron degeneration in amyotr ophic lateral sclerosis.