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

Amyloid ß (Aß) peptide plays a major role in Alzheimer's disease (AD) and occurs in multiple forms, including pyroglutamylated Aß (AßpE). Identification and characterization of the most cytotoxic Aß species is necessary for advancement in AD diagnostics and therapeutics. While in brain tissue multiple Aß species act in combination, structure/toxicity studies and immunotherapy trials have been focused on individual forms of Aß. As a result, the molecular composition and the structural features of "toxic Aß 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 Aß and AßpE and to resolve the structures of both peptides in combination. The peptides form unusual ß-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. Aß/AßpE 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 Aß/AßpE hetero-oligomers, not just Aß or AßpE 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.