Unique Physicochemical Profile of β-Amyloid Peptide Variant Aβ1–40E22G Protofibrils: Conceivable Neuropathogen in Arctic Mutant Carriers

A new early-onset form of Alzheimerʹs disease (AD) was described recently where a point mutation was discovered in codon 693 of the β-amyloid (Aβ) precursor protein gene, the Arctic mutation. The mutation translates into a single amino acid substitution, glutamic acid→glycine, in position 22 of the Aβ peptide. The mutation carriers have lower plasma levels of Aβ than normal, while in vitro studies show that Aβ1–40E22G protofibril formation is significantly enhanced. We have explored the nature of the Aβ1–40E22G peptide in more detail, in particular the protofibrils. Using size-exclusion chromatography (SEC) and circular dichroism spectroscopy (CD) kinetic and secondary structural characteristics were compared with other Aβ1–40 peptides and the Aβ12–28 fragment, all having single amino acid substitutions in position 22. We have found that Aβ1–40E22G protofibrils are a group of comparatively stabile β-sheet-containing oligomers with a heterogeneous size distribution, ranging from >100 kDa to >3000 kDa. Small Aβ1–40E22G protofibrils are generated about 400 times faster than large ones. Salt promotes their formation, which significantly exceeds all the other peptides studied here, including the Dutch mutation Aβ1–40E22Q. Position 22 substitutions had significant effects on aggregation kinetics of Aβ1–40 and in Aβ12–28, although the qualitative aspects of the effects differed between the native peptide and the fragment, as no protofibrils were formed by the fragments. The rank order of protofibril formation of Aβ1–40 and its variants was the same as the rank order of the length of the nucleation/lag phase of the Aβ12–28 fragments, E22V>E22A≫E22G>E22Q≫E22, and correlated with the degree of hydrophobicity of the position 22 substituent. The molecular mass of peptide monomers and protofibrils were estimated better in SEC studies using linear rather than globular calibration standards. The characteristics of the Aβ1–40E22G suggest an important role for the peptide in the neuropathogenesis in the Arctic form of AD.