The Rapid Exchange of Zinc(2+) Enables Trace Levels to Profoundly Influence Amyloid-ß Misfolding and Dominates Assembly Outcomes in Cu(2+)/Zn(2+) Mixtures.

The misfolding and self-assembly of amyloid-ß (Aß) into oligomers and fibres is fundamental to Alzheimer's disease pathology. Alzheimer's disease is a multifaceted disease. One factor that is thought to have a significant role in disease aetiology is Zn(2+) homeostasis, which is disrupted in the brains of Alzheimer's disease sufferers and has been shown to modulate Alzheimer's symptoms in animal models. Here, we investigate how the kinetics of Aß fibre growth are affected at a range of Zn(2+) concentrations and we use transmission electron microscopy to characterise the aggregate assemblies formed. We demonstrate that for Aß(1-40), and Aß(1-42), as little as 0.01mol equivalent of Zn(2+) (100nM) is sufficient to greatly perturb the formation of amyloid fibres irreversibly. Instead, Aß(1-40) assembles into short, rod-like structures that pack tightly together into ordered stacks, whereas Aß(1-42) forms short, crooked assemblies that knit together to form a mesh of disordered tangles. Our data suggest that a small number of Zn(2+) ions are able to influence a great many Aß molecules through the rapid exchange of Zn(2+) between Aß peptides. Surprisingly, although Cu(2+) binds to Aß 10,000 times tighter than Zn(2+), the effect of Zn(2+) on Aß assembly dominates in Cu(2+)/Zn(2+) mixtures, suggesting that trace levels of Zn(2+) must have a profound effect on extracellular Aß accumulation. Trace Zn(2+) levels profoundly influence Aß assembly even at concentrations weaker than its affinity for Aß. These observations indicate that inhibitors of fibre assembly do not necessarily have to be at high concentration and affinity to have a profound impact.

Cu²âº accentuates distinct misfolding of Aß1₋40 and Aß1₋42 peptides, and potentiates membrane disruption.

Central to Alzheimer's disease is the misfolding of amyloid-beta (Aß) peptide, which generates an assorted population of amorphous aggregates, oligomers and fibres. Metal ion homoeostasis is disrupted in the brains of sufferers of Alzheimer's disease and causes heightened Alzheimer's disease phenotype in animal models. In the present study, we demonstrate that substochiometric Cu²âº affects the misfolding pathway of Aß1₋40, and the more toxic Aß1₋42, in markedly different ways. Cu²âº accelerates Aß1₋40 fibre formation. In contrast, for Aß1₋42, substoichiometric levels of Cu²âº almost exclusively promote the formation of oligomeric and protofibrillar assemblies. Indeed, mature Aß1₋42 fibres are disassembled into oligomers when Cu²âº is added. These Cu²âº stabilized oligomers of Aß1₋42 interact with the lipid bilayer, disrupting the membrane and increasing permeability. Our investigation of Aß1₋40/Aß1₋42 mixtures with Cu²âº revealed that Aß1₋40 neither contributed to nor perturbed formation of Aß1₋42 oligomers, although Cu²âº-Aß1₋42 does frustrate Cu²âº-Aß1₋40 fibre growth. Small amounts of Cu²âº accentuate differences in the propensity of Aß1₋40 and Aß1₋42 to form synaptotoxic oligomers, providing an explanation for the connection between disrupted Cu²âº homoeostasis and elevated Aß1₋42 neurotoxicity in Alzheimer's disease.