Identification of a mutant amyloid peptide that predominantly forms neurotoxic protofibrillar aggregates

Abstract

The amyloid peptide (Aβ), derived from the proteolytic cleavage of the amyloid precursor protein (APP) by β- and γ-secretases, undergoes multistage assemblies to fibrillar depositions in the Alzheimer's brains. Aβ protofibrils were previously identified as an intermediate preceding insoluble fibrils. While characterizing a synthetic Aβ variant named EV40 that has mutations in the first two amino acids (DIE/A2V), we discerned unusual aggregation profiles of this variant. In comparison of the fibrillogenesis and cellular toxicity of EV40 to the wild-type Aβ peptide (Aβ40), we found that Aβ40 formed long fibrillar aggregates while EV40 formed only protofibrillar aggregates under the same in vitro incubation conditions. Cellular toxicity assays indicated that EV40 was slightly more toxic than Aβ40 to human neuroblastoma SHEP cells, rat primary cortical, and hippocampal neurons. Like Aβ40, the neurotoxicity of the protofibrillar EV40 could be partially attributed to apoptosis since multiple caspases such as caspase-9 were activated after SHEP cells were challenged with toxic concentrations of EV40. This suggested that apoptosis-induced neuronal loss might occur before extensive depositions of long amyloid fibrils in AD brains. This study has been the first to show that a mutated Aβ peptide formed only protofibrillar species and mutations of the amyloid peptide at the N-terminal side affect the dynamic amyloid fibrillogenesis. Thus, the identification of EV40 may lead to further understanding of the structural perturbation of Aβ to its fibrillation.