Substrate-induced modulation of glutamate uptake in human platelets.
In the central nervous system (CNS), glutamate rapidly upregulates the activities of different excitatory amino-acid transporter subtypes (EAATs) in order to help protect neurons from excitotoxicity. Since human platelets display a specific sodium-dependent glutamate uptake activity, and express the three major glutamate transporters, which may be affected in neurological disorders, we investigated whether platelets are subject to substrate-induced modulation as described for CNS. A time- and dose-dependent upregulation of [3H]-glutamate uptake (up to two-fold) was observed in platelets preincubated with glutamate. There was an increase in maximal velocity rate without affinity changes. Glutamate receptor agonists and antagonists did not modulate this upregulation and preincubation with glutamate analogues failed to mimic the glutamate effect. Only aspartate preincubation increased the uptake, albeit approximately 35% less with respect to glutamate. The effect of glutamate preincubation on the expression of the three major transporters was studied by Western blotting, showing an increase of approximately 70% in EAAT1 immunoreactivity that was completely blocked by cycloheximide (CEM). However, L-serine-O-sulphate, at a concentration (200 microM) known to block EAAT1/3 selectively, did not completely inhibit the effect of glutamate stimulation, indicating the possible involvement of EAAT2. In fact, glutamate stimulation was completely abolished only when, following CEM pre-incubation, the experiment was run in the presence of the selective EAAT2 inhibitor dihydrokainic acid. Since surface biotinylation experiments failed to show evidence of EAAT2 translocation, our results suggest the existence of a different way of regulating EAAT2 activity. These findings indicate that human platelets display a substrate-dependent modulation of glutamate uptake mediated by different molecular mechanisms and confirm that ex vivo platelets are a reliable model to investigate the dysfunction of glutamate uptake regulation in patients affected by neurological disorders.
Begni
Barbara
B
Department of Neuroscience and Biomedical Technologies, University of Milano-Bicocca, Via Cadore, Monza (MI) 48-20052, Italy.
Tremolizzo
Lucio
L
D'Orlando
Cristina
C
Bono
Maria Serena
MS
Garofolo
Rosanna
R
Longoni
Marco
M
Ferrarese
Carlo
C
D027322:Amino Acid Transport System X-AG / Q000378:metabolism; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D003513:Cycloheximide / Q000494:pharmacology; D026603:D-Aspartic Acid / Q000494:pharmacology; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D027341:Excitatory Amino Acid Transporter 1 / Q000096:biosynthesis*; D027342:Excitatory Amino Acid Transporter 2 / Q000378:metabolism; D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000493:pharmacokinetics* / Q000494:pharmacology; D005998:Glycine / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D007608:Kainic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D027981:Symporters / Q000378:metabolism
D027322:Amino Acid Transport System X-AG; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D027341:Excitatory Amino Acid Transporter 1; D027342:Excitatory Amino Acid Transporter 2; D050491:Glutamate Plasma Membrane Transport Proteins; C493397:SLC1A2 protein, human; C493394:SLC1A3 protein, human; D027981:Symporters; C474696:methyl-(4-carboxyphenyl)glycine; D018698:Glutamic Acid; D026603:D-Aspartic Acid; C020889:dihydrokainic acid; D003513:Cycloheximide; D007608:Kainic Acid; D005998:Glycine