PROJECT SUMMARY In Alzheimer's disease (AD) research, much focus has been on oligomeric assemblies of the amyloid-β (Aβ) peptide (a small protein). Oligomers are small nanoparticles produced by the aggregation of 50 or fewer peptide molecules. Peptide aggregation most readily produces amyloid fibrils, and it is a mystery what prevents some oligomers from aggregating further into fibrils. Even within oligomeric and fibrillar aggregate classes, peptide aggregates with a range of structures have been detected. Nevertheless, structural biology methods require stable and structurally homogeneous samples to achieve high-resolution information. To elucidate what oligomer structures are possible and which of these structures should be targeted by AD therapies, experiments are designed to understand Aβ assembly in general terms. Preliminary data show that it is possible to produce stable homogeneous samples of a 150 Aβ oligomer (32 Aβ peptide molecules) and study its structure using solid-state nuclear magnetic resonance (NMR) and electron microscopy (EM). Notably, the 150 kDa oligomer structure is unlike any previously understood Aβ assembly, and this observation presents a unique opportunity to probe how oligomer and fibril structures may differ. The project will pursue high-resolution atomic-level characterization of the 150 kDa oligomer by integrating solid-state NMR, cryo-EM, and computational modeling. The work further seeks to generalize understanding of Aβ assembly mechanisms by testing hypothesized assembly pathways inspired by the 150 kDa oligomer. The proposed work will: achieve a 3-dimensional image of the 150 kDa oligomer using cryo-EM (Aim 1); measure complementary atomic-level structural constraints with solid-state NMR (Aim 2); and test mechanistic hypotheses to produce new oligomers compatible with EM and NMR workflows (Aim 3). Knowledge of what oligomer structures are possible for Aβ (and how to isolate specific structures) is critical for developing new Alzheimer's therapeutics, implementing strategies for early detection, and designing mechanistic studies in disease models.