Unmodified and pyroglutamylated amyloid β peptides form hypertoxic hetero-oligomers of unique secondary structure.

Amyloid beta (A beta) peptide plays a major role in Alzheimer's disease (AD) and occurs in multiple forms, including pyroglutamylated A beta (A beta pE). Identification and characterization of the most cytotoxic A beta species is necessary for advancement in AD diagnostics and therapeutics. While in brain tissue multiple A beta species act in combination, structure/toxicity studies and immunotherapy trials have been focused on individual forms of A beta. As a result, the molecular composition and the structural features of "toxic A beta oligomers" have remained unresolved. Here, we have used a novel approach, hydration from gas phase coupled with isotope-edited Fourier transform infrared (FTIR) spectroscopy, to identify the prefibrillar assemblies formed by A beta and A beta pE and to resolve the structures of both peptides in combination. The peptides form unusual beta-sheet oligomers stabilized by intramolecular H-bonding as opposed to intermolecular H-bonding in the fibrils. Time-dependent morphological changes in peptide assemblies have been visualized by atomic force microscopy. A beta/A beta pE hetero-oligomers exert unsurpassed cytotoxic effect on PC12 cells as compared to oligomers of individual peptides or fibrils. These findings lead to a novel concept that A beta/A beta pE hetero-oligomers, not just A beta or A beta pE oligomers, constitute the main neurotoxic conformation. The hetero-oligomers thus present a new biomarker that may be targeted for development of more efficient diagnostic and immunotherapeutic strategies to combat AD.