Inhibition of mitochondrial cytochrome c oxidase potentiates Aβ-induced ER stress and cell death in cortical neurons

Previously we reported that amyloid-beta (A beta) leads to endoplasmic reticulum (ER) stress in cultured cortical neurons and that ER-mitochondria Ca2+ transfer is involved in A beta-induced apoptotic neuronal cell death. In cybrid cells which recreate the defect in mitochondrial cytochrome c oxidase (COX) activity observed in platelets from Alzheimer's disease (AD) patients, we have shown that mitochondrial dysfunction affects the ER stress response triggered by A beta. Here, we further investigated the impact of COX inhibition on A beta-induced ER dysfunction using a neuronal model. Primary cultures of cortical neurons were challenged with toxic concentrations of A beta upon chemical inhibition of COX with potassium cyanide (KCN). ER Ca2+ homeostasis was evaluated under these conditions, together with the levels of ER stress markers, namely the chaperone GRP78 and XBP-1, a mediator of the ER unfolded protein response (UPR). We demonstrated that COX inhibition potentiates the A beta-induced depletion of ER Ca2+ content. KCN pre-treatment was also shown to enhance the rise of cytosolic Ca2+ levels triggered by A beta and thapsigargin, a widely used ER stressor. This effect was reverted in the presence of dantrolene, an inhibitor of ER Ca2+ release through ryanodine receptors. Similarly, the increase in GRP78 and XBP-1 protein levels was shown to be higher in neurons treated with A beta or thapsigargin in the presence of KCN in comparison with levels determined in neurons treated with the neurotoxins alone. Although the decrease in cell survival, the activation of caspase-9- and -3-mediated apoptotic cell death observed in A beta- and thapsigargin-treated neurons were also potentiated by KCN, this effect is less pronounced than that observed in Ca2+ signalling and UPR. Furthermore, in neurons treated with A beta, the potentiating effect of the COX inhibitor in cell survival and death was not prevented by dantrolene. These results show that inhibition of mitochondrial COX activity potentiates A beta-induced ER dysfunction and, to a less extent, neuronal cell death. Furthermore, data supports that the effect of impaired COX on A beta-induced cell death occurs independently of Ca2+ release through ER ryanodine receptors. Together, our data demonstrate that mitochondria dysfunction in AD enhances the neuronal susceptibility to toxic insults, namely to A beta-induced ER stress, and strongly suggest that the close communication between ER and mitochondria can be a valuable future therapeutic target in AD. (c) 2012 Elsevier Inc. All rights reserved.