Capturing Intermediate Structures of Alzheimer's β-Amyloid, Aβ(1−40), by Solid-State NMR Spectroscopy

Molecular structures of diffusible amyloid intermediates, commonly observed in misfolding of amyloid proteins into fibrils, have attracted broad interest because the intermediates may be potent neurotoxins responsible for amyloid diseases such as Alzheimer's disease (AD) and because the intermediate structures provide an experimental basis for defining the misfolding pathway. However, owing to the intrinsically unstable and noncrystalline nature of the systems, traditional approaches such as X-ray crystallography and solution NMR have been ineffective for elucidating molecular-level structures of the amyloid intermediates. We present a novel approach using solid-state NMR (SSNMR) that permitted the first site-resolved structural measurement of an intermediate species in fibril formation for a 40-residue Alzheimer's β-amyloid peptide, Aβ(1−40). In this approach, we combined detection of conformation and morphology changes by fluorescence spectroscopy and electron microscopy and quantitative structural examination for freeze-trapped intermediates by SSNMR. The results provide the initial evidence that a spherical amyloid intermediate of 15−30 nm in diameter exists prior to fibril formation of Aβ(1−40) and that the intermediate involves well-ordered β-sheets in the C-terminal and hydrophobic core regions. The SSNMR-based approach presented here could be applied to intermediate species of diverse amyloid proteins.