Hydrogen Peroxide Is Generated during the Very Early Stages of Aggregation of the Amyloid Peptides Implicated in Alzheimer Disease and Familial British Dementia

Abstract

Alzheimer disease and familial British dementia are neurodegenerative diseases that are characterized by the presence of numerous amyloid plaques in the brain. These lesions contain fibrillar deposits of the ␤-amyloid peptide (A␤) and the British dementia peptide (ABri), respectively. Both peptides are toxic to cells in culture, and there is increasing evidence that early "soluble oligomers" are the toxic entity rather than mature amyloid fibrils. The molecular mechanisms responsible for this toxicity are not clear, but in the case of A␤, one prominent hypothesis is that the peptide can induce oxidative damage via the formation of hydrogen peroxide. We have developed a reliable method, employing electron spin resonance spectroscopy in conjunction with the spin-trapping technique, to detect any hydrogen peroxide generated during the incubation of A␤ and other amyloidogenic peptides. Here, we monitored levels of hydrogen peroxide accumulation during different stages of aggregation of A␤-(1-40) and ABri and found that in both cases it was generated as a short "burst" early on in the aggregation process. Ultrastructural studies with both peptides revealed that structures resembling "soluble oligomers" or "protofibrils" were present during this early phase of hydrogen peroxide formation. Mature amyloid fibrils derived from A␤-(1-40) did not generate hydrogen peroxide. We conclude that hydrogen peroxide formation during the early stages of protein aggregation may be a common mechanism of cell death in these (and possibly other) neurodegenerative diseases.There is mounting evidence for the importance of oxidative damage to the brain in a wide range of neurodegenerative diseases based on detection of markers such as elevated levels of redox-active transition metal ions, lipid peroxidation, DNA and protein oxidation, and the introduction of carbonyl groups into proteins (reviewed, for example, in Refs. 1-6). These are hallmarks of attack by reactive oxygen species (ROS), 3 including superoxide, hydrogen peroxide, and the hydroxyl radical. The ␤-amyloid peptide (A␤), which is responsible for senile plaque formation in Alzheimer disease (AD), has been reported to generate hydrogen peroxide from molecular oxygen through electron transfer interactions involving bound redox-active metal ions (7-10). Hydrogen peroxide is readily converted into the aggressive hydroxyl radical by Fenton chemistry and these two ROS could be responsible for some of the oxidative damage seen in the brain in AD. Familial British dementia (FBD) is an inherited neurodegenerative disorder that is strikingly similar in neuropathology to AD, including the presence of extracellular amyloid plaques and intracellular neurofibrillary tangles. FBD is due to a stop codon mutation in the BRI gene, the protein product of which undergoes proteolytic cleavage to release an abnormally long peptide fragment (ABri) that rapidly aggregates in vitro into toxic oligomers (11,12). Only ABri with an intact intramolecular disulfide bond can do this, whereas the correspondi...

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