Normand Mousseau
Professor of Physics and Chair
Université de Montréal

Onset of aggregation of amyloid proteins

I started working on amyloid peptides and proteins about 10 years ago, in collaboration with Derreumaux. We were among the first computational groups to look at these systems. We were the first to identify the beta-barrel self-assembly for amyloid peptides in equilibrium with the standard cross beta-sheet motif (Wei, NM, Derreumaux, Biophys. J 2004).

Focusing on the simulation of unbiased assembly and structural organisation, we could identify assembly mechanisms for small peptides but also for full size proteins such as beta-amyloid and amylin (for example, Laghaei et al, 2010; Côté et al, 2011). To produce these results, but also assess their validity, we have used a variety of methods and forcefields, either developed in-house (ART, OPEP) or standard (MD, REMD, CHARMM, GROMOS, etc.).

This regular comparison between various techniques and descriptions has clearly strengthened some of our predictions and conclusions. Nevertheless, since our primary consideration is always the quality of sampling, we have focused our work on reliable but simple forcefields, allowing us to perform longer simulations and reach equilibrium even with relatively large systems (60 to 120 residues) (see, for example, Laghaei et al, 2011; Nasica-Labouze et al., 2012).

For example, OPEP simulations of dimers of polyglutamine have led us to predict the existence of an double-helix anti-parallel beta-sheet nanotube, starting from random configurations, a structure that has yet to be confirmed experimentally (Laghaei and NM, 2010).

Selected work on the topic

  • V. Binette, S. Côté, N. Mousseau, Free energy Landscape of the Amino-terminal Fragment of Huntingtin in Aqueous Solution, Biophysical Journal (2016).
    Tags: amyloide.

  • S. Côté, V. Binette, E.  S. Salnikov, B. Bechinger, N. Mousseau, Probing the Huntingtin 1-17 Membrane Anchor on a Phospholipid Bilayer by Using All-Atom Simulations, Biophysical Journal 108, 1187-1198 (2015).
    Abstract: Mislocalization and aggregation of the huntingtin protein are related to Huntington?s disease. Its first exon?more specifically the first 17 amino acids (Htt17)?is crucial for the physiological and pathological functions of huntingtin. It regulates huntingtin?s activity through posttranslational modifications and serves as an anchor to membrane-containing organelles of the cell. Recently, structure and orientation of the Htt17 membrane anchor were determined using a combined solution and solid-state NMR approach. This prompted us to refine this model by investigating the dynamics and thermodynamics of this membrane anchor on a POPC bilayer using all-atom, explicit solvent molecular dynamics and Hamiltonian replica exchange. Our simulations are combined with various experimental measurements to generate a high-resolution atomistic model for the huntingtin Htt17 membrane anchor on a POPC bilayer. More precisely, we observe that the single α-helix structure is more stable in the phospholipid membrane than the NMR model obtained in the presence of dodecylphosphocholine detergent micelles. The resulting Htt17 monomer has its hydrophobic plane oriented parallel to the bilayer surface. Our results further unveil the key residues interacting with the membrane in terms of hydrogen bonds, salt-bridges, and nonpolar contributions. We also observe that Htt17 equilibrates at a well-defined insertion depth and that it perturbs the physical properties?order parameter, thickness, and area per lipid?of the bilayer in a manner that could favor its dimerization. Overall, our observations reinforce and refine the NMR measurements on the Htt17 membrane anchor segment of huntingtin that is of fundamental importance to its biological functions.
    Tags: amyloide.

  • S. Côté, G. Wei, N. Mousseau, Atomistic mechanisms of huntingtin N-terminal fragment insertion on a phospholipid bilayer revealed by molecular dynamics simulations, Proteins: Structure, Function, and Bioinformatics 82, 1409-1427 (2014).
    Abstract: The huntingtin protein is characterized by a segment of consecutive glutamines (QN) that is responsible for its fibrillation. As with other amyloid proteins, misfolding of huntingtin is related to Huntington's disease through pathways that can involve interactions with phospholipid membranes. Experimental results suggest that the N-terminal 17-amino-acid sequence (httNT) positioned just before the QN region is important for the binding of huntingtin to membranes. Through all-atom explicit solvent molecular dynamics simulations, we unveil the structure and dynamics of the httNTQN fragment on a phospholipid membrane at the atomic level. We observe that the insertion dynamics of this peptide can be described by four main steps—approach, reorganization, anchoring, and insertion—that are very diverse at the atomic level. On the membrane, the httNT peptide forms a stable α-helix essentially parallel to the membrane with its nonpolar side-chains—mainly Leu-4, Leu-7, Phe-11 and Leu-14—positioned in the hydrophobic core of the membrane. Salt-bridges involving Glu-5, Glu-12, Lys-6, and Lys-15, as well as hydrogen bonds involving Thr-3 and Ser-13 with the phospholipids also stabilize the structure and orientation of the httNT peptide. These observations do not significantly change upon adding the QN region whose role is rather to provide, through its hydrogen bonds with the phospholipids' head group, a stable scaffold facilitating the partitioning of the httNT region in the membrane. Moreover, by staying accessible to the solvent, the amyloidogenic QN region could also play a key role for the oligomerization of httNTQN on phospholipid membranes. Proteins 2014; 82:1409–1427. © 2014 Wiley Periodicals, Inc.
    Tags: amyloide.

  • C. Eugène, R. Laghaei, N. Mousseau, Early oligomerization stages for the non-amyloid component of alpha-synuclein amyloid, J. Chem. Phys. 141, 135103 (2014).
    Tags: amyloide.

  • S. Côté, G. Wei, N. Mousseau, All-Atom Stability and Oligomerization Simulations of Polyglutamine Nanotubes with and without the 17-Amino-Acid N-Terminal Fragment of the Huntingtin Protein, The Journal of Physical Chemistry B 116, 12168-12179 (2012).
    Abstract: Several neurodegenerative diseases are associated with the polyglutamine (polyQ) repeat disorder in which a segment of consecutive glutamines in the native protein is produced with too many glutamines. Huntington?s disease, for example, is related to the misfolding of the Huntingtin protein which occurs when the polyQ segment has more than approximately 36 glutamines. Experimentally, it is known that the polyQ segment alone aggregates into ?-rich conformations such as amyloid fibrils. Its aggregation is modulated by the number of glutamine residues as well as by the surrounding amino acid sequences such as the 17-amino-acid N-terminal fragment of Huntingtin which increases the aggregation rate. Little structural information is available, however, regarding the first steps of aggregation and the atomistic mechanisms of oligomerization are yet to be described. Following previous coarse-grained replica-exchange molecular dynamics simulations that show the spontaneous formation of a nanotube consisting of two intertwined antiparallel strands (Laghaei, R.; Mousseau, N. J. Chem. Phys.2010, 132, 165102), we study this configuration and some extensions of it using all-atom explicit solvent MD simulations. We compare two different lengths for the polyQ segment, 40 and 30 glutamines, and we investigate the impact of the Huntingtin N-terminal residues (httNT). Our results show that the dimeric nanotubes can provide a building block for the formation of longer nanotubes (hexamers and octamers). These longer nanotubes are characterized by large ?-sheet propensities and a small solvent exposure of the main-chain atoms. Moreover, the oligomerization between two nanotubes occurs through the formation of protein/protein H-bonds and can result in an elongation of the water-filled core. Our results also show that the httNT enhances the structural stability of the ?-rich seeds, suggesting a new mechanism by which it can increase the aggregation rate of the amyloidogenic polyQ sequence.
    Tags: amyloide.

  • J. Nasica-Labouze, M. Meli, P. Derreumaux, G. Colombo, N. Mousseau, A Multiscale Approach to Characterize the Early Aggregation Steps of the Amyloid-Forming Peptide GNNQQNY from the Yeast Prion Sup-35, PLoS Comput Biol 7, e1002051 (2011).
    Abstract: Author Summary The formation of amyloid fibrils is associated with many neurodegenerative diseases such as Alzheimer's, Creutzfeld-Jakob, Parkinson's, the Prion disease and diabetes mellitus. In all cases, proteins misfold to form highly ordered insoluble aggregates called amyloid fibrils that deposit intra- and extracellularly and are resistant to proteases. All proteins are believed to have the instrinsic capability of forming amyloid fibrils that share common specific structural properties that have been observed by X-ray crystallography and by NMR. However, little is known about the aggregation dynamics of amyloid assemblies, and their toxicity mechanism is therefore poorly understood. It is believed that small amyloid oligomers, formed on the aggregation pathway of full amyloid fibrils, are the toxic species. A detailed atomic characterization of the oligomerization process is thus necessary to further our understanding of the amyloid oligomer's toxicity. Our approach here is to study the aggregation dynamics of a 7-residue amyloid peptide GNNQQNY through a combination of numerical techniques. Our results suggest that this amyloid sequence can form fibril-like structures and is polymorphic, which agrees with recent experimental observations. The ability to fully characterize and describe the aggregation pathway of amyloid sequences numerically is key to the development of future drugs to target amyloid oligomers.
    Tags: amyloide.

  • R. Laghaei, N. Mousseau, Spontaneous formation of polyglutamine nanotubes with molecular dynamics simulations, The Journal of Chemical Physics 132, 165102 (2010).
    Abstract: Expansion of polyglutamine (polyQ) beyond the pathogenic threshold (35–40 Gln) is associated with several neurodegenerative diseases including Huntington’s disease, several forms of spinocerebellar ataxias and spinobulbar muscular atrophy. To determine the structure of polyglutamine aggregates we perform replica-exchange molecular dynamics simulations coupled with the optimized potential for effective peptide forcefield. Using a range of temperatures from 250 to 700 K, we study the aggregation kinetics of the polyglutamine monomer and dimer with chain lengths from 30 to 50 residues. All monomers show a similar structural change at the same temperature from α -helical structure to random coil, without indication of any significant β -strand. For dimers, by contrast, starting from random structures, we observe spontaneous formation of antiparallel β -sheets and triangular and circular β -helical structures for polyglutamine with 40 residues in a 400 ns 50 temperature replica-exchange molecular dynamics simulation (total integrated time 20 μ s ). This ∼ 32 Å diameter structure reorganizes further into a tight antiparallel double-stranded ∼ 22 Å nanotube with 22 residues per turn close to Perutz’ model for amyloid fibers as water-filled nanotubes. This diversity of structures suggests the existence of polymorphism for polyglutamine with possibly different pathways leading to the formation of toxic oligomers and to fibrils.
    Tags: amyloide.
    Attachment Full Text PDF 851.2 kb (source)

  • A. Melquiond, G. Boucher, N. Mousseau, P. Derreumaux, Following the aggregation of amyloid-forming peptides by computer simulations, The Journal of Chemical Physics 122, 174904 (2005).
    Abstract: There is experimental evidence suggesting that the toxicity of neurodegenerative diseases such as Alzheimer’s disease may result from the soluble intermediate oligomers. It is therefore important to characterize extensively the early steps of oligomer formation at atomic level. As these structures are metastable and short lived, experimental data are difficult to obtain and they must be complemented with numerical simulations. In this work, we use the activation-relaxation technique coupled with a coarse-grained energy model to study in detail the mechanisms of aggregation of four lys–phe–phe–glu (KFFE) peptides. This is the shortest peptide known to form amyloidfibrilsin vitro. Our simulations indicate that four KFFE peptides adopt a variety of oligomeric states (tetramers, trimers, and dimers) with various orientations of the chains in rapid equilibrium. This conformational distribution is consistent with all-atom molecular-dynamics simulations in explicit solvent and is sequence dependent; as seen experimentally, the lys–pro–gly–glu (KPGE) peptides adopt disordered structures in solution. Our unbiased simulations also indicate that the assembly process is much more complex than previously thought and point to intermediate structures which likely are kinetic traps for longer chains.
    Tags: amyloide.
    Attachment Full Text PDF 734.8 kb (source)
Tuesday 1 July 2014

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