Cryo-electron microscopy: Alzheimer's fibrils

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  • Published: Oct 1, 2017
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Cryo-electron microscopy: Alzheimer's fibrils

Augmented reality

3D reconstruction of an amyloid fibril from two protofilaments (red/blue) calculated from cryo-electron microscopy images. Credit: Forschungszentrum Jülich/HHU Düsseldorf/Gunnar Schröder

A technique that can augment X-ray crystallographic and spectroscopic studies has been demonstrated to provide an atomic resolution in imaging of the amyloid fibrils present in Alzheimer's disease, thanks to European research.

Amyloid proteins are closely associated with several neurodegenerative diseases. Specifically, fibrous aggregates of amyloid-beta protein are found in high proportion in the senile plaques associated with Alzheimer's disease. As such, understanding their chemistry and biology are an important focus of research into this disease. A clearer understanding of amyloids will not only provide a firmer basis for explaining how the disease arises but might also lead to novel treatments or perhaps even preventative measures.

Now, a team of researchers from Germany and the Netherlands have determined the structure of an amyloid fibril at unprecedented resolution. The atomic-level three-dimensional structure obtained by teams from Forschungszentrum Jülich, Heinrich Heine University Düsseldorf, the Centre for Structural Systems Biology in Hamburg, and Maastricht University reveals structural details that have not been seen in previous studies. The new clues might open up studies into genetic and external risk factors. The team provides full details of the research in the journal Science.

Chilling structure

The cryo-electron microscopic studies obtained details that were not accessible with X-ray studies nor nuclear magnetic resonance (NMR) spectroscopic work. Indeed, the new structure reveals how the many single amyloid-beta protein molecules are staggered in layers on top of each other and are arranged into so-called protofilaments. Two of these protofilaments are twinned around each other to form a fibril. The team explains that if several such fibrils become entangled, then they form intractable deposits, or plaques, seen in post mortem examination of brain tissue from Alzheimer's patients.

"This is a milestone on the road to a fundamental understanding of amyloid structures and the related diseases,” explains Dieter Willbold. "The fibril structure answers many questions about the mechanism of fibril growth and identifies the role played by a whole series of familial mutations that lead to early onset of Alzheimer's disease," he adds.

Conformation confirmation

The technique allowed the researchers to achieve 4 angstrom resolution, which is within the typical magnitude of atomic radii and atomic bond lengths. Thus, they were able to show for the first time precise interactions of the protein fibres that make up the fibrils revealing a staggered arrangement of each relative to its neighbours in the bundle. Moreover, the structure reveals the positions and effect on conformation of all 42 amino acid residues within the protein chain, also for the first time.

A lack of stabilizing structural changes that occur through genetic effects might explain why wild type mice do not succumb to Alzheimer's disease despite otherwise similar biology to our own. The revelations also hint at what protects a certain population in Iceland that also seems to be resistant to the disease. The variants in amyloid-beta, differing by just three amino acid residues in mice and one in the Icelanders, respectively, seems sufficient to protect against the disease.

While the team used cryo-electron microscopy for the main structural study the results from those experiments were underpinned by measurements obtained using solid-state NMR spectroscopy and X-ray crystallography. The latter two techniques also helped validate the data from microscopy.

Gunnar Schröder explains how difficult it can be to extract information from proteins. "The individual images in cryo-electron microscopy are usually extremely noisy since proteins are very sensitive to electron radiation and the pictures can only be generated with very low radiation intensity. A computer-aided system allowed them to combined thousands of individual images and ultimately extract high-resolution structural data from them.

"This is a step that can be very complicated if the sample is heterogeneous, that is to say if it consists of differently formed fibrils. In the past, this was almost always the case with the amyloid fibrils and represented one of the major obstacles for the analysis. However, we now had a fairly unique specimen [obtained by Lothar Gremer] with very homogeneous fibrils – 90 % of them had the same shape and symmetry,” explains Schröder.

The solid-state NMR spectra provided useful data to allow the team to build a model and so validate the microscopy data. "NMR enabled us to obtain additional information such as which amino acid residues form salt bridges thus enhancing the stability of the fibrils,” explains Henrike Heise. X-ray data obtained by Jörg Labahn's team at the Centre for Structural Systems Biology in Hamburg, provided additional confirmation of the results.

Related Links

Science 2017, online: "Fibril structure of amyloid-beta by cryo-electron microscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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