Biophysical and structural characterisation of a quasi-stable toxic dimer of amyloid Aβ42 peptide tethered at position 34

Abstract

Non-fibrillar oligomeric forms of amyloid β (Aβ) mediate toxicity in Alzheimer’s disease (AD) with Aβ accumulation preceding tau aggregation and synaptic damage. Aβ oligomerization is thought to initiate from the smallest stable unit, the dimer, yet structural details of this species remain unclear due to its transient nature. Here, we synthesized a quasi-stable E22P,L34DAZ-Aβ42 dimer incorporating a “toxic” turn at residues 22–23 and a flexible DAZ (l,l-2,8-diaminoazelaic acid) linker at L34, a residue critical in Aβ42 dimer and tetramer interfaces based on our crystallographic data. Thioflavin-T (ThT) assays, transmission electron microscopy (TEM), circular dichroism (CD) and electrospray ionization mass spectrometry (ESI-MS) showed that this dimer formed non-fibrillar aggregates predominantly tetramers (~16%) and dodecamers (~77%), as confirmed by small-angle X-ray scattering (SAXS). SAXS modelling based on crystallographic Aβ structures suggests compact, folded β-hairpin oligomers that retain the “toxic” turn at residues 22–23 and assemble into globular non-fibrillar aggregates. The dimer oligomers exhibited cytotoxicity toward THP-1 cells at low nanomolar levels.