also obtained in isolated heart mitochondria, supporting the idea that selective interaction between EAAC1 and NCX1 may 17328890 be a rather general mechanism in tissues where both of these transporters are expressed. Results and Discussion Glutamate ability to stimulate ATP synthesis in purified rat brain mitochondria To establish whether glutamate enhances oxidative metabolism by a direct mitochondrial effect, we exposed purified mitochondria from rat hippocampus and cortex, two regions thought to be among the most sensitive to the neurotransmitter, to 1 mM exogenous glutamate. We found that ATP synthesis increased significantly in mitochondria from both regions and that it depended on the activation of oxidative metabolism, as demonstrated by its abrogation by the F1FO-ATP synthase inhibitor oligomycin . To exclude a dependence of glutamate-induced ATP synthesis in mitochondria on possible cytoplasm contamination of our preparations, we performed experiments with glucose, which requires cytosolic glycolytic enzymes and was, as expected, unable to induce ATP synthesis. Moreover we found that in isolated mitochondria lactate dehydrogenase activity was virtually undetectable, strengthening the absence of cytosol contamination. namely GLutamate ONO4059 site aspartate Transporter, Glutamate Transporter 1 and EAAC1. To assess the role of mitochondrial EAATs in sustaining energy metabolism under physiological conditions, we further tested the ability of glutamate to stimulate ATP production even in the presence of other metabolic intermediates such as malate and pyruvate. As shown in EAATs involvement in glutamate-stimulated ATP synthesis Since glutamate transamination to a-ketoglutarate takes place in the mitochondrial matrix, the question arises of how 22694778 it accesses this compartment. The mechanisms responsible for its transport have been well characterized in the neuronal and glial plasma membrane, leading to identification of a family of highly specialized proteins, the Excitatory Amino Acid Transporters, which are Na-dependent glutamate transporters. Therefore, we explored the possibility that EAATs could also be involved in glutamate transport in brain mitochondria, a process that is held to be mediated by the aspartate/glutamate carriers Aralar/AGC1 and Citrin/AGC2. Interestingly, we found that the glutamate-stimulated ATP synthesis in rat hippocampal and cortical mitochondria was inhibited by the selective non-transportable EAATs blocker DL-TBOA in a dose-dependent manner. In addition, three different EAATs were detected in protein extracts of hippocampal and cortical mitochondria, Glutamate entry into mitochondria is sustained by EAAC1 activity DL-TBOA does not discriminate between GLAST, GLT1 and EAAC1, and, therefore, does not provide any information as to which subtype was involved in mediating the effect of glutamate on mitochondrial metabolism. However, EAAC1 protein was detected in SH-SY5Y and C6 
cell mitochondria where, as in brain, DL-TBOA inhibited glutamate-stimulated ATP synthesis, whereas GLAST mRNA and protein were barely detectable and GLT1 mRNA was virtually absent. To establish whether EAAC1 was the transporter subtype mediating stimulation of glutamate-induced metabolism, we investigated the effect of selective EAAC1 knockdown with antisense oligonucleotides on ATP responsiveness to glutamate in SH-SY5Y and C6 cells. Treatment with EAAC1 AsODN completely abolished glutamate-induced ATP synthesis in both systems. Since selective knock-down
