Isotope-edited FTIR reveals distinct aggregation and structural behaviors of unmodified and pyroglutamylated amyloid β peptides

Abstract

Amyloid β peptide (Aβ) is causatively associated with Alzheimer’s disease (AD), and N-terminally truncated and pyroglutamylated Aβ peptides (AβpE) exert hypertoxic effect by an unknown mechanism. Recent evidence has identified the prefibrillar oligomers of Aβ, not the fibrils, as the prevalent cytotoxic species. Structural characterization of Aβ and AβpE oligomers is therefore important for better understanding of their toxic effect. Here we have used isotope-edited Fourier transform infrared (FTIR) spectroscopy to identify the conformational changes in Aβ1-42 and AβpE3-42 upon aggregation, individually and in 1:1 molar combination. During the first two hours of exposure to aqueous buffer, the peptides undergo transition from mostly α-helical to mostly β-sheet structure. Data on peptides 13C,15N-labeled at K16L17V18 or V36G37G38V39 allowed construction of structural models for the monomer and early oligomers. The peptide monomer comprises a β-hairpin that involves residues upstream of the K16L17V18 sequence and an N-terminal α-helix. The oligomers form by non-H-bonding interactions between the β-strands of neighboring β-hairpins, in lateral or staggered manner, with the strands running parallel or antiparallel. Relative α-helical and β-sheet propensities of Aβ1-42 and AβpE3-42 depend on the ionic strength of the buffer, emphasizing the importance of ionic interactions in Aβ peptide structure and aggregation. It is inferred that N-terminal modification of AβpE3-42 affects the helix stability and thereby modulates β-sheet oligomer formation. The data thus provide new insight into the molecular mechanism of Aβ oligomerization by emphasizing the role of the N-terminal transient α-helical structure and by identifying structural constraints for molecular organization of the oligomers.

An Account of Amyloid Oligomers: Facts and Figures Obtained from Experiments and Simulations

“…It has been suggested that the lateral or partially stacked non‐H‐bonding interactions are between the N‐terminal strand of one peptide and the C‐terminal strand of the other in a parallel or antiparallel manner. Moreover, the presence of a residual α‐helical segment at the N terminus of the peptide is another characteristic of this oligomer structure . Further evidence for the contribution of α‐helices to oligomer secondary structure was provided by Kirkidatze et al .…”

“…Moreover,t he presence of ar esidual a-helicals egment at the Nterminus of the peptidei sa nother characteristic of this oligomer structure. [118] Further evidencef or the contributiono fa-helices to oligomer secondary structure was providedb yK irkidatze et al [119] They analyzedd ifferent Ab alloforms by CD and found between 20 and 30 % a-helicity for an intermediate structure based on a minimum of 23 Ab monomers.…”

“…With the aid of isotope‐edited FTIR spectroscopy, the oligomerization of Aβ42 has been compared with that of the N‐terminally truncated Aβ pE3–42. Assemblies formed within the first two hours of dissolution are characterized by intramolecular hydrogen bonding and intermolecular non‐hydrogen contacts . It has been suggested that the lateral or partially stacked non‐H‐bonding interactions are between the N‐terminal strand of one peptide and the C‐terminal strand of the other in a parallel or antiparallel manner.…”

“…Assemblies formed within the first two hours of dissolution are characterized by intramolecular hydrogen bonding and intermolecularn on-hydrogen contacts. [118] It has been suggested that the lateral or partially stacked non-H-bonding interactions are betweent he N-terminal strand of one peptidea nd the C-terminal strand of the other in ap arallel or antiparallel manner. Moreover,t he presence of ar esidual a-helicals egment at the Nterminus of the peptidei sa nother characteristic of this oligomer structure.…”

Pyroglutamate-Modified Amyloid-β(3–42) Shows α-Helical Intermediates before Amyloid Formation

“…The N-terminal region might support the aggregation-prone sequence stabilizing b-sheet formation as described by Abedini and Raleigh (50,51). Similarly, it was shown, using Fourier transform infrared (FTIR) spectroscopy, that Ab forms a transient a-helical structure at the N-terminal region and that the helix stability is modulated by the ionic strength and the N-terminal modification (62).…”