The Amyloid Phenomenon and Its Significance in Biology and Medicine

Abstract

The misfolding of proteins is now recognized to be the origin of a large number of medical disorders. One particularly important group of such disorders is associated with the aggregation of misfolded proteins into amyloid structures, and includes conditions ranging from Alzheimer's and Parkinson's diseases to type II diabetes. Such conditions already affect over 500 million people in the world, a number that is rising rapidly, and at present these disorders cannot be effectively treated or prevented. This review provides an overview of this field of science and discusses recent progress in understanding the nature and properties of the amyloid state, the kinetics and mechanism governing its formation, the origins of its links with disease, and the manner in which its formation may be inhibited or suppressed. This latter topic is of particular importance, both to enhance our knowledge of the maintenance of protein homeostasis in living organisms and also to address the development of therapeutic strategies through which to combat the loss of homeostasis and the associated onset and progression of disease.

Figure 1.

Model of one of the polymorphs of the amyloid fibrils formed from insulin as defined from cryogenic electron microscopy (cryo-EM) analysis. This particular fibril contains four protofilaments that twist around each other to form the mature fibril. Each of the protofilaments has a pair of nearly flat β-sheets, with the component strands oriented perpendicularly to the main fibril axis. (From Jiménez et al. 2002; reprinted, with permission, from the National Academy of Sciences © 2002.)

Figure 2.

The different states of a protein and the process of their interconversion. Schematic description of a selection of the possible states that can be formed by proteins under different circumstances. The populations and their rates of interconversion are determined by their different thermodynamic stabilities and free energy barriers associated with the various transitions as well as by the rates of synthesis and degradation, the propensity to interact with molecular chaperones, and to undergo posttranslational and other chemical modifications such as proteolytic cleavage. (From Knowles et al. 2014; reprinted, with permission, from Springer Nature © 2014.)

Figure 3.

The microscopic steps that can contribute to the macroscopic conversion of soluble proteins into amyloid fibrils. The mechanisms of these microscopic steps can be divided into nucleation (i.e., fibril-forming) and growth processes. Events producing new fibrils are further classified as primary or secondary processes on the basis of their dependence (secondary) or lack of dependence (primary) on the population of aggregates. Here, k_n, k_-, k₊, and k_off represent rate constants, and n_c and n₂ represent the reaction orders of primary and secondary (surface-catalyzed) nucleation. (From Michaels et al. 2018; reprinted, with permission, from Annual Reviews © 2018.)

Figure 4.

The interaction of different types of stabilized oligomers of α-synuclein with a lipid bilayer. Schematic representations of the binding of type A* (largely disordered) (left) and type B* (having both ordered [red] and disordered [gray] regions) (right) oligomers to a lipid bilayer. The amino-terminal regions of the type B* oligomers fold into amphipathic α-helices (blue) upon interaction with the bilayer, and the ordered regions, which are rich in β-sheet structure insert into the bilayer and disrupt its integrity. (From Fusco et al. 2017; reprinted, with permission, from the American Association for the Advancement of Science © 2017.)

Figure 5.

Kinetics of the 42 residue Aβ-peptide (Aβ42) aggregation in the presence of the molecular chaperone Brichos. (A–C) Reaction profiles from left (blue) to right (green) in the absence of Brichos and in the presence of increasing concentrations of Brichos up to a 1:1 stoichiometry. The blue dashed line is the integrated rate law for Aβ42 aggregation in the absence of Brichos using the rate constants determined previously. The green dashed lines show predictions of the resulting reaction profiles when each of (A) primary nucleation, (B) fibril elongation, and (C) secondary (surface-catalyzed) nucleation is inhibited by the chaperone. The thin dotted lines in (C) are theoretical predictions for the reaction profiles at the intermediate Brichos concentrations using the association and dissociation rate constants determined for its binding by means of surface plasmon resonance (SPR) measurements. (D–F) Time evolution of the nucleation rates calculated from the kinetic analysis. The blue line corresponds to the situation in the absence of Brichos and the green dashed lines show predictions for the cases when each of (D) primary nucleation, (E) fibril elongation, and (F) secondary nucleation is inhibited by the chaperone. The insets show the relative numbers of oligomers predicted to be generated during the aggregation reaction. (From Cohen et al. 2015; reprinted, with permission, from Nature Publishing Group © 2015 courtesy of the Open Access Licensing Policy.)

Figure 6.

Schematic diagram indicating some of the processes following protein synthesis that can potentially be perturbed for therapeutic purposes to combat amyloid disorders. The various possible strategies include (A) stabilizing the native state; (B) inhibiting enzymes that process proteins into peptides with a higher propensity to aggregate; (C) inhibiting protein synthesis; (D) stimulating clearance of misfolded proteins, for example, by boosting proteasomal degradation; (E) perturbing the assembly of fibrils; and (F) suppressing the formation of toxic oligomeric fibril precursors. (Image based on data in Ciryam et al. 2017.)

Figure 7.

Inhibition of Aβ42 aggregation by a small molecule. (A–C). Kinetic profiles of the aggregation of Aβ42 in the absence or presence of bexarotene (a small- molecule drug developed for cancer chemotherapy) in concentration ratios from 1:1 to 5:1 Aβ42: bexarotene. The solid lines show predictions for the resulting reaction profiles when secondary nucleation (A), fibril elongation (B), or primary nucleation (C) is inhibited by bexarotene. Only the prediction for inhibition of primary nucleation closely fits the experimental data. (D) Evolution of the apparent reaction rate constants with increasing concentrations of bexarotene. The definitions of the rate constants are as in Figure 3, and k represents in each case either k_nk₊ or k₂k₊. The data show the significant decrease in primary pathways when compared with secondary pathways as the concentration of bexarotene is increased. (From Habchi et al. 2016; reprinted, with permission, from The American Association for the Advancement of Science © 2016 under the terms of the Creative Commons Attribution-NonCommercial license.)

Figure 8.

The antibody scanning method produces antibodies that affect different microscopic steps in the aggregation of Aβ42. (A) Schematic of Aβ42 aggregation showing the primary (red arrow) and secondary (blue arrow) nucleation of the aggregation process and the elongation of fibrils (black). (B) The designed antibodies (DesAbs) were generated to target five different epitopes in the Aβ42 sequence (indicated on the left side) and the kinetics of aggregation were monitored at different concentrations of each DesAb at increasing (blue to green) concentrations relative to that of Aβ42. (C) Seeded aggregation at a low (blue) or high (green) concentration of added fibrils. (D) Bar plots showing that each DesAb inhibits the microscopic steps in the aggregation process in a different way; the fold change of each of the rate constants is shown at the top of the corresponding bar. (E) The relative number of oligomers generated during the aggregation reaction with 1:2 DesAb:Aβ42 ratios. (From Aprile et al. 2017; reprinted, with permission, from the American Association for the Advancement of Science © 2016 under the terms of the Creative Commons Attribution-NonCommercial license.)