Protein molecular structure dictates the rigidity of amyloid nanofibrils

Researchers from the FOM Institute AMOLF have shown that the mechanical rigidity of amyloid nanofibrils depends on the molecular folding of the protein building block. Fibrils of the same chemical composition become 40 times stiffer when the proteins are folded into beta-sheets instead of alpha-helices.

This finding is important for understanding the toxicity of amyloid nanofibrils in human diseases where these fibrils form plaques or disrupt cell membranes. The results of this research will be published in the renowned Journal of the American Chemical Society (JACS).

Protein function is normally associated with folding of proteins into highly specific secondary structures. However, when proteins are denatured by heating or chemical treatment, they can start sticking together to form a universal type of structure known as amyloid fibrils. These fibrils are long and rigid and therefore useful as biomaterials for cell culture or drug release. However, similar nanofibrils also form during diseases such as Alzheimer’s and diabetes. Here, fiber rigidity controls whether the fibers can tangle to form plaques or cause damage to cellular membranes. The structure of amyloid fibers is known to depend on the molecular conformation of the protein building blocks, but it is unknown how this conformation dictates fiber rigidity.
The AMOLF research groups of Gijsje Koenderink and Mischa Bonn teamed up to directly relate fiber rigidity to protein molecular structure.

Combined imaging and vibration spectroscopy
The researchers combined atomic force microscopy (AFM) with vibrational sum frequency generation spectroscopy (vSFG) to correlate mesoscopic fibril mechanics to molecular conformation. With AFM the fibril persistence length, a measure of rigidity, was determined; the vSFG vibrational spectra reflect the protein molecular conformation. Amyloid nanofibrils were prepared from the model protein beta-lactoglobulin, for which the mechanical rigidity can be tuned by varying protein concentration. Strikingly, fibers made up of the same protein exhibit bending rigidities varying by a factor of 40 (see figure 1). Fibers prepared at low protein concentration where long and rigid, whereas fibers prepared at high concentration were short and wormlike. The infrared spectra of the same fibers revealed the origin of this huge difference in rigidity, namely changes in protein molecular structure. High fiber rigidity requires folding of the protein subunits into beta-sheets, which are held together by numerous hydrogen bonds. The beta-sheet structures assemble in an ordered fashion, by lining up. As soon as the beta-sheet content falls below 80%, proteins that do not posses beta-sheet structure will intercalate in the fibril and the fiber rigidity decreases dramatically (see figure 2).

Protein misfolding and new biomimetic materials
The finding that amyloid nanofibril rigidity is dictated by beta-sheet content is an important step forward in understanding the role of protein molecular structure in the toxicity of amyloid fibrils in diseases involving protein misfolding. A higher stiffness is thought to promote the disruption of cellular membranes by the fibrils, and also promotes entangling to form insoluble amyloid plaques. The relation between molecular structure and fiber rigidity is also useful in materials science, to guide the design of novel nanofibrils from synthetic peptides with applications as templates for nanowires and scaffolds for tissue repair or controlled drug release.

The research was financed by the Industrial Partnership Programme (IPP) Bio-(Related) Materials (BRM) of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Netherlands Organisation for Scientific Research (NWO). The IPP BRM is cofinanced by the Top Institute Food and the Dutch Polymer Institute.

Contact
Prof. Gijsje Koenderink, +31 20 754 7190, E-mail: g.koenderinkATamolf.nl
Prof. Mischa Bonn, +31 20 754 7100, E-mail: m.bonnATamolf.nl

Reference
Morphology and Persistence Length of Amyloid Fibrils are Correlated to Peptide Molecular Structure
Corianne C. van den Akker, Maarten F.M. Engel, Krassimir P. Velikov, Mischa Bonn, and Gijsje H. Koenderink
J. Am. Chem. Soc (2011). DOI: 10.1021/ja206513r

Figure 1. Amyloid fibrils with identical chemical composition (beta-lactoglobulin) but prepared from starter solutions of different protein concentration (3% in (a), 7.5% in (b)) differ dramatically in morphology and bending rigidity (AFM images, top-right). The vSFG spectra of the same fibrils show that this difference coincides with a change from nearly pure beta sheet structure to nearly pure alpha-helix and disordered structure (spectra, top-left). The persistence length increases strongly when the beta-sheet content is above 80% (right).