Structural Insights into Aβ42 Oligomers Using Site-directed Spin Labeling

Oligomerization of the 42-residue peptide Aβ42 plays a key role in the pathogenesis of Alzheimer disease. Despite great academic and medical interest, the structures of these oligomers have not been well characterized. Site-directed spin labeling combined with electron paramagnetic resonance spectroscopy is a powerful approach for studying structurally ill-defined systems, but its application in amyloid oligomer structure study has not been systematically explored. Here we report a comprehensive structural study on a toxic Aβ42 oligomer, called globulomer, using site-directed spin labeling complemented by other techniques. Transmission electron microscopy shows that these oligomers are globular structures with diameters of ∼7–8 nm. Circular dichroism shows primarily β-structures. X-ray powder diffraction suggests a highly ordered intrasheet hydrogen-bonding network and a heterogeneous intersheet packing. Residue-level mobility analysis on spin labels introduced at 14 different positions shows a structured state and a disordered state at all labeling sites. Side chain mobility analysis suggests that structural order increases from N- to C-terminal regions. Intermolecular distance measurements at 14 residue positions suggest that C-terminal residues Gly-29–Val-40 form a tightly packed core with intermolecular distances in a narrow range of 11.5–12.5 Å. These intermolecular distances rule out the existence of fibril-like parallel in-register β-structures and strongly suggest an antiparallel β-sheet arrangement in Aβ42 globulomers.