Modulation of Na(+),K(+) pumping and neurotransmitter uptake by beta-amyloid.

Micromolar concentrations of beta-amyloid (Abeta), a 40/42-amino-acid-long proteolytic fragment (Abeta(1-40/42)) of the amyloid precursor protein, was shown previously to play a crucial role in pathogenesis of Alzheimer's disease. We used the Xenopus oocyte expression system to investigate specific effects of micromolar concentrations of Abeta(1-42) on the neurotransmitter transporters for gamma-aminobutyric acid (GABA), GAT1, and for the excitatory amino acid glutamate, EAAC1, which are driven by the transmembrane Na(+) gradient that is regulated by the Na(+),K(+)-ATPase. Brief treatment with Abeta(1-42), up to 80 min, leads to a significant inhibition of ion translocation by the Na(+),K(+)-ATPase (30-40%); also glutamate uptake is inhibited (20%) while GABA uptake is not affected. Since reduced glutamate uptake will result in elevated, neurotoxic concentrations of extracellular glutamate, we investigated the effects of Abeta(1-42) and the smaller fragments, Abeta(12-28) and Abeta(25-35), on EAAC1 in more detail. Prolonged incubation in 1 microM Abeta(1-42) leads to further, strong inhibition of glutamate uptake and EAAC1-mediated current (after 4 h inhibition amounts to more than 80%). Abeta(12-28) is less effective with 50% inhibition after 4 h of incubation at 20 microM. Abeta(1-42) and Abeta(12-28) affect EAAC1-mediated current to a similar extent as the rate of glutamate uptake. The effects on EAAC1-mediated current are irreversible if Abeta were applied for longer time periods. Peptides directly microinjected into the oocyte are ineffective suggesting that the observed effect were mediated by extracellular proteins. Abeta(25-35) hardly affects EAAC1-mediated current or glutamate uptake. The results demonstrate that Abeta specifically inhibits the Na(+),K(+) pump and EAAC1. The domain between amino acids 12 and 28 of Abeta seems to play a crucial role for inhibition of EAAC1. The inhibition of EAAC1 by neurotoxic, elevated extracellular glutamate levels may contribute to Alzheimer's pathogenesis.