Nature of the Amyloid-β Monomer and the Monomer-Oligomer Equilibrium

Abstract

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 ...

Comparing atomistic molecular mechanics force fields for a difficult target: a case study on the Alzheimer’s amyloid β-peptide

“…The other four force fields produced more compact structures, with statistically indistinguishable values of R g . The R g values produced by AMBER03, OPLS-AA, and both GROMOS96 parameter sets were in good agreement with available experimental evidence that suggests the R g of monomeric Aβ 40 in solution is .9 ± .1 nm (Nag et al, 2011).…”

Intra-membrane Oligomerization and Extra-membrane Oligomerization of Amyloid-β Peptide Are Competing Processes as a Result of Distinct Patterns of Motif Interplay

“…evokes strong pressure increase in monolayer assays, A␤1-36 fails to stably associate with membrane, despite that the amino acid 30 -36 sequence is the most hydrophobic stretch in the sequence of A␤ and aa [25][26][27][28][29][30][31][32][33][34][35] have been reported to insert and form ion channels within the membrane (46,47). These results thus suggest an antagonistic interaction between aa 29 -36 and aa 1-28, the extracellular domain of A␤.…”

“…Because A␤ monomers and soluble A␤ oligomers, such as ADDL, are more or less in continuous interconversion (29,30), we reasoned that the preferential incorporation of A␤ monomers into membrane might gradually shift the equilibrium of monomers 7 ADDL transition, thereby leading to the accumulation and oligomerization of A␤ in liposomes. Indeed, although ADDL showed no detectable membrane association after a 2-h incubation (Figs.…”

Amyloid Beta monomers regulate cyclic adenosine monophosphate response element binding protein functions by activating type‐1 insulin‐like growth factor receptors in neuronal cells

“…Although synthetic Aβ monomers might oligomerize in the culture medium, spectroscopy techniques operating at single‐molecule sensitivity levels have predicted that Aβ stays monomeric up to at least 3 μ m (Nag et al ., 2011) (i.e., a concentration three times higher than the highest concentration of monomeric Aβ used in this study). Moreover, the slow kinetic of Aβ self‐association in vitro (Kusumoto et al ., 1998) is not consistent with the timing of CREB phosphorylation.…”