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Deposition of β-amyloid (Aβ) fibrils is an early event in the neurodegenerative processes associated with Alzheimer's disease. According to the ‘amyloid cascade’ hypothesis, Aβ aggregation, and its subsequent deposition as fibrils, is the underlying cause of disease. Aβ is a proteolytic product of amyloid precursor protein (APP); several mutations in APP have been identified that are associated with early onset of disease. Transgenic mice overexpressing APP with the Swedish mutation develop numerous plaques but, surprisingly, lack the neurofibrillary tangles and neuronal loss characteristic of Alzheimer's disease, in apparent contradiction of the amyloid cascade hypothesis. However, recent studies suggest that coproduction of sAPPα, an alternative proteolytic product of APP, increases synthesis of transthyretin that, in turn, interacts directly with Aβ to inhibit its toxicity. Here we report results from biophysical analysis of Aβ aggregation kinetics in the presence of transthryetin. At substoichiometric ratios, transthyretin drastically decreased the rate of aggregation without affecting the fraction of Aβ in the aggregate pool. Detailed analysis of the data using a mathematical model demonstrated that the decrease in aggregation rate was due to both a decrease in the rate of elongation relative to the rate of initiation of filaments and a decrease in lateral association of filaments to fibrils. Tryptophan quenching data indicated that transthyretin binds weakly to Aβ, with an estimated apparent Ks of 2300 M-1. Taken together, the data support a hypothesis wherein transthyretin preferentially binds to aggregated rather than monomeric Aβ and arrests further growth of the aggregates. [ABSTRACT FROM AUTHOR]

As the population ages, the need to develop methods to understand and intercept the processes responsible for protein aggregation diseases is becoming more urgent. The aggregation of the protein β-amyloid (Aβ) has been implicated in Alzheimer's Disease (AD); however, whether the toxic species is a large, insoluble aggregate or some lower order form is not yet known. Agents that can modulate the aggregation state of Aβ could resolve this controversy by facilitating our understanding of the consequences of aggregation and its underlying mechanism. To date, however, ligands that bind to specific forms of Aβ have not been identified. To address this deficiency, we tested whether phage display could yield such ligands by screening libraries against Aβ in two different states: monomeric or highly aggregated. Intriguingly, the peptides selected had different effects on Aβ aggregation. Peptides selected for binding to monomeric Aβ did not perturb aggregation, but those selected using highly aggregated Aβ increase the rate of aggregation drastically. The latter also alter the morphology of the resulting aggregate. The ability of a peptide to promote aggregation correlated with its affinity for the N-terminal 10 residues of Aβ. This result indicates that the mechanism by which the peptides influence aggregation is related to their affinity for the Aβ N-terminus. Thus, the identification of compounds that bind to this Aβ section can afford agents that affect aggregation. Moreover, the data suggest that endogenous ligands that interact with the N-terminal region can influence the propensity of Aβ to form aggregates and the morphology of those that form. Our data highlight the utility of phage display for identifying ligands that bind to target proteins in different states, and they indicate that such agents can be used to perturb protein aggregation. [ABSTRACT FROM AUTHOR]