Sensitivity-Enhanced Solid-State NMR Detection of Structural Differences and Unique Polymorphs in Pico- to Nanomolar...

Amyloid-β (Aβ) fibrils in neuritic plaques are a hallmark of Alzheimer's disease (AD). Since the 42-residue Aβ (Aβ42) fibril is the most pathogenic among different Aβ species, its structural characterization is crucial to our understanding of AD. While several polymorphs have been reported for Aβ40, previous studies of Aβ42 fibrils prepared at neutral pH detected essentially only one structure, with an S-shaped β-sheet arrangement (e.g., Xiao et al. Nat. Struct. Mol. Biol. 2015, 22, 499). Herein, we demonstrate the feasibility of characterizing the structure of trace amounts of brain-derived and synthetic amyloid fibrils by sensitivity-enhanced ¹H-detected solid-state NMR (SSNMR) under ultrafast magic angle spinning. By taking advantage of the high sensitivity of this technique, we first demonstrate its applicability for the high-throughput screening of trace amounts of selectively ¹³C- and ¹⁵N-labeled Aβ42 fibril prepared with ∼0.01% patient-derived amyloid (ca. 4 pmol) as a seed. The comparison of 2D ¹³C/¹H SSNMR data revealed marked structural differences between AD-derived Aβ42 (∼40 nmol or ∼200 μg) and synthetic fibrils in less than 10 min, confirming the feasibility of assessing the fibril structure from ∼1 pmol of brain amyloid seed in ∼2.5 h. We also present the first structural characterization of synthetic fully protonated Aβ42 fibril by ¹H-detected 3D and 4D SSNMR. With procedures assisted by automated assignments, main-chain resonance assignments were completed for trace amounts (∼42 nmol) of a fully protonated amyloid fibril in the ¹H-detection approach. The results suggest that this Aβ42 fibril exhibits a novel fold or polymorph structure.