Point Mutations in Aβ Result in the Formation of Distinct Polymorphic Aggregates in the Presence of Lipid Bilayers

Abstract

A hallmark of Alzheimer's disease (AD) is the rearrangement of the β-amyloid (Aβ) peptide to a non-native conformation that promotes the formation of toxic, nanoscale aggregates. Recent studies have pointed to the role of sample preparation in creating polymorphic fibrillar species. One of many potential pathways for Aβ toxicity may be modulation of lipid membrane function on cellular surfaces. There are several mutations clustered around the central hydrophobic core of Aβ near the α-secretase cleavage site (E22G Arctic mutation, E22K Italian mutation, D23N Iowa mutation, and A21G Flemish mutation). These point mutations are associated with hereditary diseases ranging from almost pure cerebral amyloid angiopathy (CAA) to typical Alzheimer's disease pathology with plaques and tangles. We investigated how these point mutations alter Aβ aggregation in the presence of supported lipid membranes comprised of total brain lipid extract. Brain lipid extract bilayers were used as a physiologically relevant model of a neuronal cell surface. Intact lipid bilayers were exposed to predominantly monomeric preparations of Wild Type or different mutant forms of Aβ, and atomic force microscopy was used to monitor aggregate formation and morphology as well as bilayer integrity over a 12 hour period. The goal of this study was to determine how point mutations in Aβ, which alter peptide charge and hydrophobic character, influence interactions between Aβ and the lipid surface. While fibril morphology did not appear to be significantly altered when mutants were prepped similarly and incubated under free solution conditions, aggregation in the lipid membranes resulted in a variety of polymorphic aggregates in a mutation dependent manner. The mutant peptides also had a variable ability to disrupt bilayer integrity.

Stability of Transmembrane Amyloid β-Peptide and Membrane Integrity Tested by Molecular Modeling of Site-Specific Aβ42 Mutations

“…The same kind of experiments with Iowa mutant (D23N) A β 40 revealed the formation of stable oligomeric aggregates on the TBLE surface within 2–3 hours [36]. However, after longer exposure (10–12 hours), the bilayer structural integrity was highly disrupted in small areas arising from D23N A β 40 oligomers inside the bilayer [36]. Our molecular simulations revealed that E22G A β has a higher propensity to stay inside a membrane compared to D23G, while D23G has a greater tendency to bind to the surface.…”

“…These areas were seen to be populated with stable oligomers composed of 10–15 peptides per oligomer, rather than with fibrillar aggregates as observed for WT A β 40 . The same kind of experiments with Iowa mutant (D23N) A β 40 revealed the formation of stable oligomeric aggregates on the TBLE surface within 2–3 hours [36]. However, after longer exposure (10–12 hours), the bilayer structural integrity was highly disrupted in small areas arising from D23N A β 40 oligomers inside the bilayer [36].…”

Familial Alzheimer’s Disease Osaka Mutant (ΔE22) β-Barrels Suggest an Explanation for the Different Aβ1–40/42 Preferred Conformational States Observed by Experiment

“…13 These mutants formed fibrils in solution morphologically similar to those formed by WT Aβ peptides and presented polymorphic aggregates on a lipid membrane. 12 Taken together, these results emphasized the importance of the point substitution at Aβ position 22 in FAD and CAA.…”

“…11 The Arctic mutant increased bilayer disruption due to high hydrophobicity. The Italian mutant was observed to increase the rate of aggregation 12 as did the Dutch mutant which aggregated faster than WT Aβ. 13 These mutants formed fibrils in solution morphologically similar to those formed by WT Aβ peptides and presented polymorphic aggregates on a lipid membrane.…”

Single nucleotide variations: Biological impact and theoretical interpretation

“…Roughly three quarters of amino acid substitutions that result in Mendelian diseases do affect protein stability, proving the value of this assumption 56,57. Impacting protein stability typically implies local or total unfolding of the protein, but occasionally deleterious aggregates like amyloid fibrils58,59 may form. Rarely, single mutations have been known to cause a switch between stable folds 60.…”