Amyloid-beta peptide decreases glutamate uptake in cultured astrocytes: Involvement of oxidative stress and mitogen-activated protein kinase cascades

Alzheimer's disease (AD) is a progressive neurodegenerative disorder primarily characterized by excessive deposition of amyloid-β (Aβ) peptides in the brain. One of the earliest neuropathological changes in AD is the presence of a high number of reactive astrocytes at sites of Aβ deposition. Disturbance of glutamatergic neurotransmission and consequent excitotoxicity is also believed as implicated in the progression of this dementia. Therefore, the study of astrocyte responses to Aβ, the main cellular type involved in the maintenance of synaptic glutamate concentrations, is crucial for understanding the pathogenesis of AD. This study aims to investigate the effect of Aβ on the astrocytic glutamate transporters, glutamate transporter-1 (GLT-1) and glutamate–aspartate transporter (GLAST), and their relative participation to glutamate clearance. In addition we have also investigated the involvement of mitogen-activated protein (MAP) kinases in the modulation of GLT-1 and GLAST levels and activity and the putative contribution of oxidative stress induced by Aβ to the astrocytic glutamate transport function. Therefore, we used primary cultures of rat brain astrocytes exposed to Aβ synthetic peptides. The data obtained show that Aβ1-40 peptide decreased astroglial glutamate uptake capacity in a non-competitive mode of inhibition, assessed in terms of tritium radiolabeled d-aspartate (d-[3H]aspartate) transport. The activity of GLT-1 seemed to be more affected than that of GLAST, and the levels of both transporters were decreased in Aβ1-40-treated astrocytes. We demonstrated that MAP kinases, extracellular signal-regulated kinase (ERK), p38 and c-Jun N-terminal kinase, were activated in an early phase of Aβ1-40 treatment and the whole pathways differentially modulated the glutamate transporters activity/levels. Moreover it was shown that oxidative stress induced by Aβ1-40 may lead to the glutamate uptake impairment observed. Taken together, our results suggest that Aβ peptide downregulates the astrocytic glutamate uptake capacity and this effect may be in part mediated by oxidative stress and the differential activity and complex balance between the MAP kinase signaling pathways.