Characterization of monomeric and soluble aggregated Aβ in Down's syndrome and Alzheimer's disease brains

The major characteristics of Alzheimer's disease (AD) are amyloid plaques, consisting of aggregated beta amyloid (A beta) peptides, together with tau pathology (tangles, neuropil treads and dystrophic neurites surrounding the plaques), in the brain. Down's syndrome (DS) individuals are at increased risk to develop AD-type pathology; most DS individuals have developed substantial pathology already at the age of 40. DS individuals have an extra copy of chromosome 21, harbouring the amyloid precursor protein gene (APP). Our aim was to investigate the A beta peptide pattern in DS and AD brains to investigate differences in their amyloid deposition and aggregation, respectively. Cortical tissue from patients with DS (with amyloid pathology), sporadic AD and controls were homogenized and fractionated into TBS (water soluble) and formic acid (water insoluble) fractions. Immunoprecipitation (IP) was performed using a variety of antibodies targeting different A beta species including oligomeric A beta. Mass spectrometry was then used to evaluate the presence of A beta species in the different patient groups. A large number of A beta peptides were identified including A beta 1-X, 2-X, 3-X, 4-X, 5-X, 11-X, and A beta peptides extended N terminally of the BACE1 cleavage site and ending at amino 15 in the A beta sequence APP/A beta(-X to 15), as well as peptides post-translationally modified by pyroglutamate formation. Most A beta peptides had higher abundance in AD and DS compared to controls, except the APP/A beta(-X to 15) peptides which were most abundant in DS followed by controls and AD. Furthermore, the abundancies of A?X-40 and A beta X-34 were increased in DS compared with AD. A beta 1-40, A beta 1-42, and A beta 4-42 were identified as the main constitutes of protofibrils (IP'd using mAb158) and higher relative A beta 1-42 signals were obtained compared with samples IP'd with 6E10 + 4G8, indicating that the protofibrils/oligomers were enriched with peptides ending at amino acid 42. All A? peptides found in AD were also present in DS indicating similar pathways of A beta peptide production, degradation and accumulation, except for APP/A beta(-X to 15). Likewise, the A beta peptides forming protofibrils/oligomers in both AD and DS were similar, implying the possibility that treatment with clinical benefit in sporadic AD might also be beneficial for subjects with DS.

Automated summary

The patterns of A beta peptides in the brains of Alzheimer's disease (AD) and Down's syndrome (DS) patients differ, with DS patients showing higher amyloid pathology, and varying abundances of main A beta species in different patient groups.