Atomic Resolution Insights into pH Shift Induced Deprotonation Events in LS-Shaped Aβ(1-42) Amyloid Fibrils

Alzheimer's disease is a neurodegenerative disorder associated with the deposition of misfolded aggregates of the amyloid-fi protein (Afi). Afi(1-42) is one of the most aggregationprone components in senile plaques of AD patients. We demonstrated that relatively homogeneous Afi(1-42) fibrils with one predominant fold visible in solid-state NMR spectra can be obtained at acidic pH. The structure of these fibrils differs remarkably from some other polymorphs obtained at neutral pH. In particular, the entire N-terminal region is part of the rigid fibril core. Here, we investigate the effects of a pH shift on the stability and the fold of these fibrils at higher pH values. Fibril bundling at neutral pH values renders cryo-EM studies impractical, but solidstate NMR spectroscopy, molecular dynamics simulations, and biophysical methods provide residue-specific structural information under these conditions. The LS-fold of the Afi(1-42) fibrils does not change over the complete pH range from pH 2 to pH 7; in particular, the N-terminus remains part of the fibril core. We observe changes in the protonation state of charged residues starting from pH 5 on a residue-specific level. The deprotonation of the C-terminal carboxyl group of A42 in the intermolecular salt bridge with D1 and K28 is slow on the NMR time scale, with a local pKa of 5.4, and local conformations of the involved residues are affected by deprotonation of A42. Thus, we demonstrate that this fibril form is stable at physiological pH values.

Automated summary

Alzheimer's disease is characterized by the accumulation of misfolded amyloid-fi protein (Afi). We found that Afi(1-42) fibrils with a predominant fold can be obtained at acidic pH, and their structure differs from polymorphs obtained at neutral pH.