Alzheimer's Aβ Peptides with Disease-Associated N-Terminal Modifications: Influence of Isomerisation, Truncation and Mutation on Cu2+ Coordination

Abstract

BackgroundThe amyloid-β (Aβ) peptide is the primary component of the extracellular senile plaques characteristic of Alzheimer's disease (AD). The metals hypothesis implicates redox-active copper ions in the pathogenesis of AD and the Cu2+ coordination of various Aβ peptides has been widely studied. A number of disease-associated modifications involving the first 3 residues are known, including isomerisation, mutation, truncation and cyclisation, but are yet to be characterised in detail. In particular, Aβ in plaques contain a significant amount of truncated pyroglutamate species, which appear to correlate with disease progression.Methodology/Principal FindingsWe previously characterised three Cu2+/Aβ1–16 coordination modes in the physiological pH range that involve the first two residues. Based upon our finding that the carbonyl of Ala2 is a Cu2+ ligand, here we speculate on a hypothetical Cu2+-mediated intramolecular cleavage mechanism as a source of truncations beginning at residue 3. Using EPR spectroscopy and site-specific isotopic labelling, we have also examined four Aβ peptides with biologically relevant N-terminal modifications, Aβ1[isoAsp]–16, Aβ1–16(A2V), Aβ3–16 and Aβ3[pE]–16. The recessive A2V mutation preserved the first coordination sphere of Cu2+/Aβ, but altered the outer coordination sphere. Isomerisation of Asp1 produced a single dominant species involving a stable 5-membered Cu2+ chelate at the amino terminus. The Aβ3–16 and Aβ3[pE]–16 peptides both exhibited an equilibrium between two Cu2+ coordination modes between pH 6–9 with nominally the same first coordination sphere, but with a dramatically different pH dependence arising from differences in H-bonding interactions at the N-terminus.Conclusions/SignificanceN-terminal modifications significantly influence the Cu2+ coordination of Aβ, which may be critical for alterations in aggregation propensity, redox-activity, resistance to degradation and the generation of the Aβ3–× (× = 40/42) precursor of disease-associated Aβ3[pE]–x species.

Amyloid-β Peptide Aβ3pE-42 Induces Lipid Peroxidation, Membrane Permeabilization, and Calcium Influx in Neurons

“…pE-A␤ peptides also demonstrate increased ␤-sheet (aggregate structure) stability (16,17), differences in fibril ultrastructure (18,19), and altered interactions with copper ions (20,21) and synthetic lipid membranes (22,23). Notably, trace quantities of A␤3pE-42 have been observed to dramatically enhance the aggregation and neurotoxicity of A␤ (24), prompting descriptions of pE-A␤ as "prionlike."…”

“…Imbalances in ROS production and detoxification are strongly implicated in AD neurodegeneration, reflected by cerebral elevations in oxidized DNA, lipids, and proteins (35)(36)(37). Pyroglutamate formation alters A␤-Cu 2ϩ coordination modes (20,21), although it is not known whether this affects the capacity of pE-A␤ peptides to undertake redox cycling and produce cytotoxic ROS. We therefore aimed to determine whether full-length A␤ and pE-A␤ possess differences in their capacity to alter ROS flux and cause oxidative damage to neurons in vitro.…”

Pyroglutamate Amyloid-β (Aβ): A Hatchet Man in Alzheimer Disease

“…Other modifications include metal-induced oxidation (23) or phosphorylation (24,25). In general, N-terminal modifications significantly influence the Cu 2ϩ coordination of A␤, which may be critical for alterations in aggregation propensity, redox activity, and resistance to degradation (26). In addition, it has been shown that certain apolipoprotein E isoforms bind to A␤ and with less avidity to modified species, including isomerized and pyroglutamate-modified A␤ (A␤ pE3 ) (27).…”

CuII Binding to Various Forms of Amyloid‐β Peptides: Are They Friends or Foes?

“…More generally, N-terminally truncated peptides might also be considered for new therapeutic targets. [9] To conclude, and as previously pointed out in seminal works, [103,104,111,134,136] the interaction of Cu II with N-terminally truncated peptides might reshape our current view of the deleterious impact of Cu II binding to amyloid-peptides.…”

“…[9] The formation of the A 11-40/42 peptide is due to the activity of a secondary -secretase ( ′). 0.12 [134,136] [a] A s parameters refer to 63 Cu.…”