The monomer to oligomer transition initiates the aggregation and pathogenic transformation of Alzheimer amyloid-␤ (A␤) peptide. However, the monomeric state of this aggregationprone peptide has remained beyond the reach of most experimental techniques, and a quantitative understanding of this transition is yet to emerge. Here, we employ single-molecule level fluorescence tools to characterize the monomeric state and the monomer-oligomer transition at physiological concentrations in buffers mimicking the cerebrospinal fluid (CSF). Our measurements show that the monomer has a hydrodynamic radius of 0.9 ؎ 0.1 nm, which confirms the prediction made by some of the in silico studies. Surprisingly, at equilibrium, both A␤ 40 and A␤ 42 remain predominantly monomeric up to 3 M, above which it forms large aggregates. This concentration is much higher than the estimated concentrations in the CSF of either normal or diseased brains. If A␤ oligomers are present in the CSF and are the key agents in Alzheimer pathology, as is generally believed, then these must be released in the CSF as preformed entities. Although the oligomers are thermodynamically unstable, we find that a large kinetic barrier, which is mostly entropic in origin, strongly impedes their dissociation. Thermodynamic principles therefore allow the development of a pharmacological agent that can catalytically convert metastable oligomers into nontoxic monomers.Alzheimer disease (AD) 2 is a degenerative brain disorder that is associated with the presence of extracellular aggregates of amyloid-␤ (A␤) (1), which is an ϳ4.5-kDa peptide containing 39 -42 residues. Recent studies indicate that small soluble oligomers are key to A␤ toxicity (2-4). In the AD brain, both A␤ monomers and dimers have been isolated, and the dimers have been shown to impair synaptic plasticity in mouse hippocampal slices (5). In contrast, A␤ monomers have been shown to be devoid of neurotoxicity (5) and have in fact been suggested to be neuroprotective (6, 7). The monomer to oligomer transition is therefore not only the obligatory first event of aggregation, it is also the key event determining the transformation of a benign protein to a neurotoxic one.We address this transition from a thermodynamic viewpoint: an aggregation-capable molecule should have a defined equilibrium between monomers and dimers (or oligomers), such that it is primarily monomeric below a certain concentration. Any oligomer-enriched solution prepared below such a concentration must be thermodynamically unstable and must dissociate to monomers at a given rate. To understand AD in terms of A␤ aggregation, we need to understand how this concentration compares with the in vivo concentrations of A␤ (which is estimated to be Ͻ Ͻ1 M) (8 -11) and what the kinetics of A␤ oligomer dissociation is.However, experiments probing the monomer to oligomer transition have been difficult to perform due to the low concentration at which this transition most likely occurs, and they have yielded rather confusing results. Some studies have ...

Small oligomers of the amyloid β (Aβ) peptide, rather than the monomers or the fibrils, are suspected to initiate Alzheimer's disease (AD). However, their low concentration and transient nature under physiological conditions have made structural investigations difficult. A method for addressing such problems has been developed by combining rapid fluorescence techniques with slower two-dimensional solid-state NMR methods. The smallest Aβ40 oligomers that demonstrate a potential sign of toxicity, namely, an enhanced affinity for cell membranes, were thus probed. The two hydrophobic regions (residues 10-21 and 30-40) have already attained the conformation that is observed in the fibrils. However, the turn region (residues 22-29) and the N-terminal tail (residues 1-9) are strikingly different. Notably, ten of eleven known Aβ mutants that are linked to familial AD map to these two regions. Our results provide potential structural cues for AD therapeutics and also suggest a general method for determining transient protein structures.

Observations like high Zn(2+) concentrations in senile plaques found in the brains of Alzheimer's patients and evidences emphasizing the role of Zn(2+) in amyloid-β (Aβ)-induced toxicity have triggered wide interest in understanding the nature of Zn(2+)-Aβ interaction. In vivo and in vitro studies have shown that aggregation kinetics, toxicity, and morphology of Aβ aggregates are perturbed in the presence of Zn(2+). Structural studies have revealed that Zn(2+) has a binding site in the N-terminal region of monomeric Aβ, but not much is precisely known about the nature of binding of Zn(2+) with aggregated forms of Aβ or its effect on the molecular structure of these aggregates. Here, we explore this aspect of the Zn(2+)-Aβ interaction using one- and two-dimensional (13)C and (15)N solid-state NMR. We find that Zn(2+) causes major structural changes in the N-terminal and the loop region connecting the two β-sheets. It breaks the salt bridge between the side chains of Asp(23) and Lys(28) by driving these residues into nonsalt-bridge-forming conformations. However, the cross-β structure of Aβ(42) aggregates remains unperturbed though the fibrillar morphology changes distinctly. We conclude that the salt bridge is not important for defining the characteristic molecular architecture of Aβ(42) but is significant for determining its fibrillar morphology and toxicity.

Background: Curcumin reduces the risk of Alzheimer disease via an unknown mechanism. Results: Curcumin-incubated A␤ 42 aggregates retain the hairpin architecture but have disruptions in the turn region (surprising similarity with Zn 2ϩ incubation). Conclusion: Salt bridge-containing turn region is a major determinant of morphology and toxicity. Significance: Identification of crucial structural changes provides a checkpoint for developing effective AD therapeutics.

Small amyloid-β (Aβ) oligomers have much higher membrane affinity compared to the monomers, but the structural origin of this functional change is not understood. We show that as monomers assemble into small n-mers (n < 10), Aβ acquires a tertiary fold that is consistent with the mature fibrils. This is an early and defining transition for the aggregating peptide, and possibly underpins its altered bioactivity.