Chapter Nine - Aggregation of disease-related peptides
Introduction
Numerous diseases affecting either the central nervous system or a variety of peripheral tissues result from the self-assembly of amyloid proteins. Disorders range from Alzheimer's disease (AD, implication of Aβ proteins of 39–43 amino acids and tau protein of 441 amino acids), Parkinson (α-synuclein protein of 140 amino acids) to amyotrophic lateral sclerosis or ALS (superoxide dismutase, SOD of 154 amino acids) and type II diabetes (IAPP or islet amyloid polypeptide of 37 amino acids). All these proteins differ in amino acid sequence and length, yet they all form amyloid fibrils with cross-β structure.1 This propensity to self-assembly into amyloid fibrils under given conditions is also observed for heptapeptides (e.g., Aβ16–22),2 tetrapeptides (e.g., KFFE),3 dipeptides4 and even by single amino acids.5
Experimentally, aggregation kinetics of amyloid proteins display a sigmoidal curve with a lag phase, during which monomers self-assembly into oligomers and undergo structural rearrangements, until the growth phase where fibril elongation and primary/secondary nucleation occur, followed by the saturation phase where the system is in equilibrium between fibrils and a small concentration of monomers. Note that our understanding of amyloid aggregation kinetics goes much beyond classical nucleation theory where primary nucleation event is sufficient to fit the experimental curves.1
Structural determination of all species along amyloid fibril formation pathways is challenging by experimental means because of their transient character despite the use of a large variety of biophysical techniques.6 Complexity comes from the metastable character of each species (a very large number of conformations for each aggregate), but also from the sensitivity of the kinetics to experimental conditions (solution pH, temperature, salt concentration, agitation, ions) and external conditions (presence of membrane, crowding, etc.).7
To add further complexity, amyloid aggregation kinetics is also modulated by the exact composition of the lipid bilayers. For example, although Aβ1–40, Aβ1–42 and tau oligomers are key players in AD,8 dietary PUFA (polyunsaturated fatty acids) supplementation change molecular phospholipids, and there is evidence that increased intake of omega-3 PUFA slows the progression of AD, while omega-6 PUFA is linked to higher risk of AD.9 Another example of the modulation of the aggregation upon membrane presence can be taken from experiments on α-synuclein. On the one hand, DPLS lipid bilayers significantly augment its aggregation rate, while DOPE lipid bilayers have no impact on its aggregation rate.10
Simulations on amyloids at different time and length scales can complement experiment, but require accurate potential energy models ranging from atomistic in explicit aqueous solution/lipid bilayer, coarse-grained with implicit solvent/membrane to mesoscopic or super-mesoscopic representations.11, 12, 13, 14 Molecular dynamics (MD), replica exchange molecular dynamics (REMD) simulations and other sampling methods are often used to generate the conformational ensemble of intrinsically disordered proteins (IDPs). These methods are described in Section 2 and we will see that their results vary with the protein force field used. Section 3 focuses on the application of computer simulations to a better understanding of small oligomers of short amyloid peptides. While an implicit solvent representation reduces the number of degrees of freedom, hydrodynamics, which deals with the motion of fluids and the forces acting on solid bodies immersed in fluids and in motion relative to them, plays a significant role in the early aggregation steps of amyloids using simplified models (Section 4). Section 5 reports on the simulations aimed at understanding the primary nucleation and the surface-catalyzed secondary nucleation. The final section reports on recent advances in the determination of oligomer structures of Aβ40/42 peptides in aqueous solution and lipid bilayers based on computer simulations. Our review will be mainly centered on Aβ peptides. For computer simulations of tau, hIAAP and synuclein, see Ref. [14].
Section snippets
Computer simulation models for amyloid protein aggregation
Numerous conformational sampling methods are used to study amyloid proteins. We will not describe them in detail but rather summarize their main features. Atomistic molecular dynamics (MD) simulations in explicit environment offer the most detailed dynamic and energetic pictures of protein folding and aggregation. The longest simulation on the fastest computer (Anton) does not exceed, however, 1 ms in explicit solvent, sufficient for sampling the monomeric state of amyloid proteins, but far too
Structures of small aggregates
Among all amyloid sequences, the Aβ16–22 peptide has been the most studied. Due to its simplicity, its amyloid fibril at neutral pH consists of antiparallel β-strands and parallel β-sheets,2 and the amino-acids 17–21 constitute the main driving force for the aggregation of the full-length Aβ peptides.8 Based on MD and REMD simulations with all force fields except AMBER99sb-disp, it is clear that the probability of Aβ16–22 fibril formation following the one-step nucleation is very small.
Using
Exploring the early aggregates of amyloid peptides at quasi-atomic level with hydrodynamics
Coarse-grained simplified molecular models with implicit solvent have been extensively used to explore the aggregation process of amyloid systems and to inspect the impact of the peptide β-propensity and amino acid sequence, peptide-peptide interactions, concentration, temperature,46, 47, 52, 53, 71, 72 and crowding.73 However, these simulations based on simplified models neglect the effect of solvent mediated interactions on the kinetic behavior of the system and the diffusion limited
Primary and secondary nucleation from simulations
The foundation of the nucleation for amyloid fibrils dates from the polymerization studies of actin by Oosawa83 and deoxyhemoglobin by Weaton.84 They theoretically explained the nucleation and subsequent polymerization processes by employing the homogeneous nucleation theory developed for vapor condensation. Their theories have served as a background for the development of kinetic and thermodynamic analyses for amyloid nucleation.85
Basically, in the first step, the fluctuations in the system
Recent advances in structures of Aβ40/42 oligomers from simulations
In this last section, we would like to review some recent and significant contributions in the field of Aβ40/42 peptide simulations. We already discussed the simulation results of Aβ42 dimer in aqueous solution (see Section 2).
In the case of Aβ40 dimer in aqueous solution, Nguyen et al. performed atomistic REMD simulations on the wild-type (WT) sequence, the A2V/A2V mutant and the mixed WT/A2V mutant.100, 101, 102 Experimentally, the A2V mutation protects from Alzheimer's disease in its
Conclusions
Understanding how amyloid aggregates actually become toxic is truly a real challenge in developing a treatment for neurodegenerative diseases, as only monomers are nontoxic. Thus far, all molecules (antibodies and drugs) targeting amyloid-β oligomers have failed to pass clinical trials. Many reasons have been put forward to explain this repetitive failure.112, 113
Atomistic and coarse-grained simulations with increased computer efficiency, improved force field accuracy,15, 114 coupling to
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
We acknowledge support by the “Initiative d'Excellence” program from the French State (Grant “DYNAMO,” ANR-11-LABX-0011-01, and “CACSICE,” ANR-11-EQPX-0008). PhD thanks Université de Paris, CNRS and PSL.
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