Summary: Cryogenic-electron microscopy allowed researchers to determine the basic building blocks of prion proteins, including the placements of their amino acids.

Source: Case Western Reserve

The highest-ever resolution imaging of an infectious prion provides the first atomic-level data of how these abnormal proteins are assembled to cause fatal neurodegenerative diseases in people and animals—and how they can be potentially targeted by new therapies.

Conducted by Case Western Reserve University and the National Institutes of Health (NIH), the research is available at Molecular Cell.

“These detailed prion structures provide a new premise for understanding and targeting these currently untreatable diseases,” said Allison Kraus, lead and co-corresponding author of the research and an assistant professor in the Department of Pathology at the Case Western Reserve School of Medicine. “It will now be much easier to develop and test hypotheses about how prions are assembled as highly infectious and deadly protein structures.

Seeing the basic building blocks of these lethal proteins, she said, provides a foundation for therapeutic strategies to block the spread, buildup and toxicity of prions. 

Prions are proteins in brain tissue that transmit their irregular “misfolded” shapes onto the regular version of the same protein—and are the source of mammalian diseases, including human conditions like Creutzfeldt–Jakob disease (CJD) and its variant, known as vCJD, as well as Gerstmann–Sträussler–Scheinker syndrome, and others.

Similar prion-like mechanisms occur in the characteristic proteins suspected in the development of other neurodegenerative conditions, including Parkinson’s disease, Lou Gehrig’s disease (also known as ALS, or amyotrophic lateral sclerosis), chronic traumatic encephalopathy (CTE) and Alzheimer’s disease.

Though instances are rare, prion diseases can be transmitted between people; others are readily transmissible between animals, such as chronic wasting disease.

For this study, researchers imaged rodent-adapted scrapie prions derived from the brains of clinically ill hamsters.

New level of resolution

Using cryogenic-electron microscopy (cryo-EM)—at both NIH and the Cleveland Center for Structural and Membrane Biology Cryo-Electron Microscopy Core facilities at Case Western Reserve—and a collaborative pipeline between the Kraus (CWRU), Byron Caughey (NIH), and Research Technologies Branch (NIH) groups, researchers were able to determine aspects of the basic building blocks of these proteins, including the placements of their amino acids.

The normal form of the prion protein (PrPC) is shown tethered to a cell membrane beside the corrupted infectious form that forms a prion fibril. Credit: Case Western Reserve University/National Institute of Allergy and Infectious Diseases

By suspending the prions in ice, cryo-electron technology allowed researchers take thousands of images of the protein assemblies to build 3D atomic-resolution models using proprietary software.

This successful first-ever imaging to reach atomic-level detail of a brain-derived prion opens the door for similar “solving of other prion structures,” said Kraus. The study also obtained lower resolution images of another distinct prion strain that revealed structural differences between the two strains.

“It’s thought that there are many variations in prion structures as they relate to different diseases,” said Kraus. “Higher-resolution images provide clarity to many aspects of the cause and progression of these infectious diseases that are uniquely caused in nature by proteins—not viruses or bacteria.”

Co-authors of the research are: Forrest Hoyt, Cindi L. Schwartz, Bryan Hansen of NIH’s Rocky Mountain Laboratories (RML) Research Technologies Branch, and Efrosini Artikis, Andrew G. Hughson, Gregory J. Raymond, Brent Race, Gerald S. Baron and Caughey of RML’s Laboratory of Persistent Viral Diseases—both at the NIH’s National Institute of Allergy and Infectious Diseases in Hamilton, Montana.

About this neuroscience research news

Author: Bill Lubinger
Source: Case Western Reserve
Contact: Bill Lubinger – Case Western Reserve
Image: The image is credited to Case Western Reserve University/National Institute of Allergy and Infectious Diseases

Original Research: Closed access.
“High-resolution structure and strain comparison of infectious mammalian prions” by Allison Kraus et al. Molecular Cell


Abstract

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High-resolution structure and strain comparison of infectious mammalian prions

Highlights

  • Cryo-EM reveals parallel in-register structure for an infectious brain-derived prion
  • N-linked glycans and GPI anchor project outward from the fibril core
  • Comparison to another prion strain reveals distinct conformational templates
  • In silico modeling suggests a structural basis for a prion transmission barrier

Summary

Within the extensive range of self-propagating pathologic protein aggregates of mammals, prions are the most clearly infectious (e.g., ∼109 lethal doses per milligram). The structures of such lethal assemblies of PrP molecules have been poorly understood. Here we report a near-atomic core structure of a brain-derived, fully infectious prion (263K strain).

Cryo-electron microscopy showed amyloid fibrils assembled with parallel in-register intermolecular β sheets. Each monomer provides one rung of the ordered fibril core, with N-linked glycans and glycolipid anchors projecting outward. Thus, single monomers form the templating surface for incoming monomers at fibril ends, where prion growth occurs.

Comparison to another prion strain (aRML) revealed major differences in fibril morphology but, like 263K, an asymmetric fibril cross-section without paired protofilaments. These findings provide structural insights into prion propagation, strains, species barriers, and membrane pathogenesis.

This structure also helps frame considerations of factors influencing the relative transmissibility of other pathologic amyloids.



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