Review
Amyloid-β fibrillogenesis: Structural insight and therapeutic intervention

https://doi.org/10.1016/j.expneurol.2009.08.032Get rights and content

Abstract

Structural insight into the conformational changes associated with aggregation and assembly of fibrils has provided a number of targets for therapeutic intervention. Solid-state NMR, hydrogen/deuterium exchange and mutagenesis strategies have been used to probe the secondary and tertiary structure of amyloid fibrils and key intermediates. Rational design of peptide inhibitors directed against key residues important for aggregation and stabilization of fibrils has demonstrated effectiveness at inhibiting fibrillogenesis. Studies on the interaction between Aβ and cell membranes led to the discovery that inositol, the head group of phosphatidylinositol, inhibits fibrillogenesis. As a result, scyllo-inositol is currently in clinical trials for the treatment of AD. Additional small-molecule inhibitors, including polyphenolic compounds such as curcumin, (−)-epigallocatechin gallate (EGCG), and grape seed extract have been shown to attenuate Aβ aggregation through distinct mechanisms, and have shown effectiveness at reducing amyloid levels when administered to transgenic mouse models of AD. Although the results of ongoing clinical trials remain to be seen, these compounds represent the first generation of amyloid-based therapeutics, with the potential to alter the progression of AD and, when used prophylactically, alleviate the deposition of Aβ.

Introduction

The amyloidoses are protein-misfolding diseases characterized by the conversion of peptides or proteins from a soluble state into insoluble, β-sheet rich, fibrillar aggregates termed amyloid fibrils (Stefani and Dobson, 2003). The deposition of proteins, either extracellularly as plaques or intracellularly as inclusions, is a characteristic of approximately 20 amyloid disorders, including those which affect the brain or peripheral tissue. Although the proteins that comprise these deposits do not share sequence homology, amyloid fibrils formed by different proteins share biophysical properties and, possibly, a common pathogenic mechanism (Kayed et al., 2003). The major constituent of amyloid in Alzheimer's disease is Aβ, which is produced after proteolytic cleavage of the amyloid precursor protein to form peptides of predominantly 40–42 amino acid in length.

Section snippets

Structural analysis of amyloid fibrils

The biophysical definition of amyloid includes filaments of any polypeptide with a diameter of approximately 10 nm, which have a cross-β-sheet structure, and are commonly identified by birefringence under polarized light upon binding Congo red (Eanes and Glenner, 1968, Goedert and Spillantini, 2006). Electron microscopy (EM) studies have revealed that amyloid fibrils are typically long, straight, and unbranched, ranging from 0.1 to 10 μm in length (Serpell et al., 2000). Early studies using

Amyloid aggregation

The aggregation of Aβ and its assembly into fibrils does not occur in a linear fashion; rather, distinct aggregation intermediates or oligomers are formed, which either give rise to fibrils (termed on-pathway) or do not (termed off-pathway; Fig. 2; Wetzel, 2006). Regardless of which aggregation pathway is utilized, all Aβ species equilibrate into an array characterized by size, not the number of Aβ peptides within each assembly. Elucidation of the mechanisms involved in aggregation, including

Surface-facilitated assembly of Aβ peptides on synthetic or inorganic templates

A number of studies have characterized the assembly of Aβ fibrils in solution using thioflavin T fluorescence, and studied their morphology by EM, atomic force microscopy (AFM), or fluorescence microscopy of fibril assembly on solid surfaces (Stine et al., 1996, Antzutkin, 2004, Ban et al., 2006, Karsai et al., 2006). Peptide concentration, changes in Aβ primary sequence, pH, and interactions with other elements such as other proteins and lipid species can influence the formation of fibers

Therapeutic strategies aimed at inhibiting fibrillogenesis

The characteristics of nucleation-dependent growth can be used to direct pharmacologic approaches to inhibit fibrillogenesis. To illustrate this point we will utilize a few inhibitor molecules that have been reported in the literature to have distinct mechanisms of action and direct effects on Aβ aggregation, rather than detail a list of all reported inhibitors.

Novel screening tools for fibrillogenesis inhibitors

Common screening tools for fibrillogenesis inhibitors include analyses of fibril formation through thioflavin or Congo red binding, and turbidity; analysis of morphology by EM and AFM; spectroscopic techniques such as circular dichroism and NMR; and biochemical readouts including Western and dot blots, and cell culture viability assays (Kirschner et al., 2008). However, novel techniques have emerged which permit detailed analyses of the mechanism of action and affinity of inhibitors, as well as

Conclusion

Structural insights into the conformational changes associated with aggregation and assembly of monomers into oligomers, protofibrils, and fibrils have provided a number of targets for therapeutic intervention. Knowledge of key residues important for β-sheet formation and stabilization of fibrils permits rational design of fibrillogenesis inhibitors. Many of these compounds, including peptide-based inhibitors and small molecules, have demonstrated effectiveness in vitro and in vivo at

Acknowledgments

The authors acknowledge support from the Ontario Alzheimer's Society (J.M.), Canadian Institutes of Health Research (K.A.D. and J.M.), Alzheimer Society of Canada (J.E.S.), and Natural Science and Engineering Research Council of Canada (J.M.).

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