The image shows a microscopic view of prions. [Case Western Reserve University]

Scientists at Case Western Reserve University and NIH have used cryo-electron microscopy to generate the highest-ever resolution imaging of an infectious prion. Their study has provided the first atomic-level data of how these abnormal proteins are assembled to cause fatal neurodegenerative diseases in people and animals, which could inform on how prion diseases could potentially be targeted by new therapies.

“These detailed prion structures provide a new premise for understanding and targeting these currently untreatable diseases,” said Allison Kraus, PhD, 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, build-up, and toxicity of prions.

Kraus and colleagues report on their study in Molecular Cell, in a paper titled, “High-resolution structure and strain comparison of infectious mammalian prions.”

Prions are proteins in brain tissue that transmit their irregular “misfolded” shapes onto the regular version of the same protein. They 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. “These and other prion diseases are untreatable and fatal,” the team noted. Though instances are rare, prion diseases can be transmitted between people; others are readily transmissible between animals, such as chronic wasting disease.

Similar prion-like mechanisms occur in proteins that are suspected to be involved in the development of other neurodegenerative conditions, including Parkinson’s disease, amyotrophic lateral sclerosis (ALS), chronic traumatic encephalopathy (CTE), and Alzheimer’s disease. As the authors wrote, “The widely reported ‘prion-like’ spreading characteristics of many pathological protein aggregates associated with neurodegenerative diseases such as Alzheimer’s and Parkinson’s have raised questions about their potential infectivity.”

Scientists have to date not had detailed structures of abnormal prion proteins. “Although it has long been apparent that prions have high ß sheet content and propagate via templated conformational conversion of the host’s normal prion protein (PrP) isoform, the detailed 3D structures that drive this pathological process have remained elusive,” the investigators wrote.

For their newly reported study, the team imaged rodent-adapted scrapie prions derived from the brains of clinically ill hamsters. 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, PhD, (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.

By suspending the prions in ice, the researchers were able to use the cryo-electron technology to take thousands of images of the protein assemblies, and build 3D atomic-resolution models using proprietary software. “Here we report a near-atomic core structure of a brain-derived, fully infectious prion (263K strain),” the authors noted. “The results highlighted amyloid fibrils assembled with parallel in-register intermolecular ß sheets.”

Source: Case Western Reserve University

Kraus commented, “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.” The study also obtained lower resolution images of another distinct prion strain that revealed structural differences between the two strains. “Comparison to another prion strain (aRML) revealed major differences in fibril morphology but, like 263K, an asymmetric fibril cross-section without paired protofilaments,” the team continued. “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.”

“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.”

The authors concluded, “With respect to proteopathic aggregates generally, comparisons of our infectious prion amyloid structure to those of non-infectious synthetic PrP amyloids illustrate the dramatic extent to which differences in core structure can determine relative infectivity.” They say that although both fully infectious and non-infectious PrP amyloids can be propagated indefinitely through seeded polymerization in vitro, they can differ greatly in infectivity per unit protein when inoculated in vivo. “Such distinctions will be critical in assessing biohazards that may be posed by the many disease-associated self-seeded protein aggregates that are not composed of PrP.”

Co-authors of the research were Forrest Hoyt, Cindi L. Schwartz, and 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, MT.

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