Overlapping profiles of Aβ peptides in the Alzheimer's disease and pathological aging brains

Introduction. A hallmark of Alzheimer's disease (AD) is the presence of senile plaques composed of aggregated amyloid (A) peptides. Pathological aging (PA) is a postmortem classification that has been used to describe brains with plaque pathology similar in extent to AD, minimal cortical tau pathology, and no accompanying history of cognitive decline in the brain donor prior to death. PA may represent either a prodromal phase of AD, a benign form of A accumulation, or inherent individual resistance to the toxic effects of A accumulation. To attempt to distinguish between these possibilities we have systematically characterized A peptides in a postmortem series of PA, AD and non-demented control (NDC) brains. Methods. A was sequentially extracted with tris buffered saline (TBS), radioimmunoprecipitation buffer (RIPA), 2% sodium dodecyl sulfate (SDS) and 70% formic acid (FA) from the pre-frontal cortex of 16 AD, eight PA, and six NDC patients. These extracts were analyzed by 1) a panel of A sandwich ELISAs, 2) immunoprecipitation followed by mass spectrometry (IP/MS) and 3) western blotting. These studies enabled us to asses A levels and solubility, peptide profiles and oligomeric assemblies. Results: In almost all extracts (TBS, RIPA, 2% SDS and 70% FA) the average levels of A1-40, A1-42, A total, and Ax-42 were greatest in AD. On average, levels were slightly lower in PA, and there was extensive overlap between A levels in individual PA and AD cases. The profiles of A peptides detected using IP/MS techniques also showed extensive similarity between the PA and AD brain extracts. In select AD brain extracts, we detected more amino-terminally truncated A peptides compared to PA patients, but these peptides represented a minor portion of the A observed. No consistent differences in the A assemblies were observed by western blotting in the PA and AD groups. Conclusions: We found extensive overlap with only subtle quantitative differences between A levels, peptide profiles, solubility, and SDS-stable oligomeric assemblies in the PA and AD brains. These cross-sectional data indicate that A accumulation in PA and AD is remarkably similar. Such data would be consistent with PA representing a prodromal stage of AD or a resistance to the toxic effects of A.