Monitoring Alzheimer Amyloid Peptide Aggregation by EPR

Plaques containing the aggregated beta-Amyloid (A beta) peptide in the brain are the main indicators of Alzheimer's disease. Fibrils, the building blocks of plaques, can also be produced in vitro and consist of a regular arrangement of the peptide. The initial steps of fibril formation are not well understood and could involve smaller aggregates (oligomers) of A beta. Such oligomers have even been implicated as the toxic agents. Here, a method to study oligomers on the time scale of aggregation is suggested. We have labeled the 40 residue A beta peptide variant containing an N-terminal cysteine (cys-A beta) with the MTSL [1-oxyl-2,2,5,5-tetramethyl-Delta-pyrroline-3-methyl] methanethiosulfonate spin label (SL-A beta). Fibril formation in solutions of pure SL-A beta and of SL-A beta mixed with A beta was shown by Congo-red binding and electron microscopy. Continuous-wave 9 GHz electron paramagnetic resonance reveals three fractions of different spin-label mobility: one attributed to monomeric A beta, one to a multimer (8-15 monomers), and the last one to larger aggregates or fibrils. The approach, in principle, allows detection of oligomers on the time scale of aggregation.