Sub-lethal levels of amyloid ß-peptide oligomers decrease non-transferrin-bound iron uptake and do not potentiate iron toxicity in primary hippocampal neurons.

Two major lesions are pathological hallmarks in Alzheimer's disease (AD): the presence of neurofibrillary tangles formed by intracellular aggregates of the hyperphosphorylated form of the cytoskeletal tau protein, and of senile plaques composed of extracellular aggregates of amyloid beta (Aß) peptide. Current hypotheses regard soluble amyloid beta oligomers (AßOs) as pathological causative agents in AD. These aggregates cause significant calcium deregulation and mediate neurotoxicity by disrupting synaptic activity. Additionally, the presence of high concentrations of metal ions such as copper, zinc, aluminum and iron in neurofibrillary tangles and senile plaques, plus the fact that they accelerate the rate of formation of Aß fibrils and AßOs in vitro, suggests that accumulation of these metals in the brain is relevant to AD pathology. A common cellular response to AßOs and transition metals such as copper and iron is the generation of oxidative stress, with the ensuing damage to cellular components. Using hippocampal neurons in primary culture, we report here the effects of treatment with AßOs on the (+)IRE and (-)IRE mRNA levels of the divalent metal transporter DMT1. We found that non-lethal AßOs concentrations decreased DMT1 (-)IRE without affecting DMT1 (+)IRE mRNA levels, and inhibited non-transferrin bound iron uptake. In addition, since both iron and AßOs induce oxidative damage, we studied whether their neurotoxic effects are synergistic. In the range of concentrations and times used in this study, AßOs did not potentiate iron-induced cell death while iron chelation did not decrease AßOs-induced cell death. The lack of synergism between iron and AßOs suggests that these two neurotoxic agents converge in a common target, which initiates signaling processes that promote neurodegeneration.

The antioxidant N-acetylcysteine prevents the mitochondrial fragmentation induced by soluble amyloid-ß peptide oligomers.

BACKGROUND: Soluble amyloid-ß peptide oligomers (AßOs), which are centrally involved in the pathogenesis of Alzheimer's disease, trigger Ca(2+) influx through N-methyl-D-aspartate receptors and stimulate reactive oxygen species generation in primary hippocampal neurons. We have previously reported that AßOs promote Ca(2+) release mediated by ryanodine receptors (RyR), which in turn triggers mitochondrial fragmentation. We have also reported that the antioxidant N-acetylcysteine (NAC) prevents AßOs-induced Ca(2+) signal generation. OBJECTIVES: To determine if RyR-mediated Ca(2+) release activated by the specific agonist 4-chloro-m-cresol (4-CMC) induces fragmentation of the mitochondrial network, and to ascertain if NAC prevents the mitochondrial fragmentation induced by AßOs and/or 4-CMC. METHODS: Mature primary rat hippocampal neurons were incubated for 24 h with sublethal concentrations of AßOs (500 nM) or for 1-3 h with 4-CMC (0.5-1 mM), ± 10 mM NAC. Mitochondrial morphology was assessed by confocal microscopy of fixed neurons stained with anti-mHsp70. Intracellular Ca(2+) levels were determined by time series microscopy of neurons preloaded with Fluo-4 AM. RESULTS: Preincubation of neurons for 30 min with NAC prevented the mitochondrial fragmentation induced by AßOs or 4-CMC. In addition, we confirmed that preincubation with NAC abolished the stimulation of RyR-mediated Ca(2+) release induced by AßOs or 4-CMC. CONCLUSION: The present results strongly suggest that the general antioxidant NAC prevents AßO-induced mitochondrial fragmentation by preventing RyR-mediated Ca(2+)-induced Ca(2+) release.