Summary: Researchers have uncovered the 3D structure of infectious prions.
Infectious prions or PrPSc–misfolded versions of the normal cellular prion protein PrPC–convert their normal counterparts into copies of themselves and thereby cause fatal disease. How this conversion works at the molecular level has remained largely a mystery. A study published on September 8th in PLOS Pathogens reports the three-dimensional structure of a large part of PrPSc. The structure argues against existing theories of conversion and suggests how the process might actually work.
The discovery of the structure of DNA in 1953 made it immediately obvious how DNA could be copied, or replicated. The three-dimensional structure of PrPSc has remained elusive, but the hope is that its discovery would likewise promote the understanding of prion replication, as well as lead to the development of structure-based therapeutic interventions. Convinced that the structure of what they call ‘infectious conformers’–PrPSc from the brain of diseased animals–will be most informative, a team led by Holger Wille and Howard Young from the University of Alberta in Edmonton, Canada, and Jesús Requena from the University of Santiago de Compostela, Spain, is applying electron cryomicroscopy (cryo-EM) to the problem.
In this study, they used cryo-EM to record and analyze the structure of PrPSc isolated from the brain of infected mice. Prion-infected mouse and human brains contain a mix of different versions of PrPSc because different types of molecules such as lipids and sugars have been attached to the core protein. The heterogeneity of these modified brain-derived PrPSc makes it difficult to analyze their structure. To avoid this difficulty, the researchers started with PrPSc molecules that were truncated to delete the attachment of one type of modification, the so-called GPI lipid anchor. By using as a source the brains of transgenic mice expressing a GPI-anchorless form of the prion protein, they were able to analyze a more homogeneous version of PrPSc that nonetheless retained its ability to cause disease and convert normal cellular prion proteins.
In the diseased brain, PrPSc molecules are often arranged in fibrils. The cryo-EM images of the mouse GPI-anchorless PrPSc fibrils, and their subsequent analysis, showed that they consist of two intertwined protofilaments of defined volume. As cryo-EM preserves the native structure of specimens, this information sets a structural restraint for the conformation of GPI-anchorless PrPSc, with the implication that PrPSc molecules can form protofilaments with the observed dimensions only if they are folded up onto themselves.
Based on their own analyses (and consistent with data from related studies), the researchers conclude that the cryo-EM data reveal a four-rung ß-solenoid architecture as the basic element for the structure of the mammalian prion GPI-anchorless PrPSc. ß-solenoids are protein structures that consist of an array of repetitive elements with secondary structures that are predominantly beta sheets. These PrPSc beta-sheet rungs, the researchers propose, serve as templates for new unfolded PrPSc molecules.
What they have learned about the structure of GPI-anchorless PrPSc and its four-rung ß solenoid architecture, the researchers say, allows them to rule out all previously proposed templating mechanisms for the replication of infectious prions in vivo. Discussing their ideas for the conversion of PrPC to PrPSc, the researchers note that the molecular forces responsible for the templating are fundamentally similar to those operating during the replication of DNA. “Because the exquisite specificity of the A:T and G:C pairings is lacking”, they conclude that “a much more complex array of forces controls the pairing of the pre-existing and nascent ß-rungs”.
“Templating based on a four-rung ß-solenoid architecture”, they say, “must involve the upper- and lowermost ß-solenoid rungs [which] are inherently aggregation-prone”. “Once an additional ß-rung has formed”, they propose, “it creates a fresh “sticky” edge ready to continue templating until the incoming unfolded PrP molecule has been converted into another copy of the infectious conformer”.
The researchers acknowledge that higher resolution structures and resolution of structures of other PrPSc molecules will be needed. Nonetheless, they conclude, “we present data based on cryo-EM analysis that strongly support the notion that GPI-anchorless PrPSc fibrils consist of stacks of four-rung ß-solenoids. Two of such protofilaments intertwine to form double fibrils. The four-rung ß-solenoid architecture of GPI-anchorless PrPSc provides unique and novel insights into the molecular mechanism by which mammalian prions replicate”.
Funding: This project has been supported by grants from the Alberta Prion Research Institute / Alberta Innovates Bio Solutions (201100010; 201100011; 201300012; 201300024), the Alberta Livestock & Meat Agency (2012A001R), the Canada Foundation for Innovation (NIF 21633 and IOF 21633 awards to D. Westaway), the European Commission grant FP7 222887 “Priority”, a Spanish Ministry of Education grant (BFU2006-04588/BMC), and Spanish Ministry of Economy and Competitiveness grants (BFU2013-48436-C2-1-P & TIN2012-37483-C03-02). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: MRV is an employee of FEI Company (Eindhoven, The Netherlands), this does not alter our adherence to all PLoS Pathogens policies on sharing data and materials.
Image Source: This NeuroscienceNews.com image is credited to Ester Vázquez-Fernández, Howard Young, Holger Wille, Jesús Requena.
Original Research: Full open access research for “The Structural Architecture of an Infectious Mammalian Prion Using Electron Cryomicroscopy” by Ester Vázquez-Fernández, Matthijn R. Vos, Pavel Afanasyev, Lino Cebey, Alejandro M. Sevillano, Enric Vidal, Isaac Rosa, Ludovic Renault, Adriana Ramos, Peter J. Peters, José Jesús Fernández, Marin van Heel, Howard S. Young, Jesús R. Requena, and Holger Wille in PLOS Pathogens. Published online September 8 2016 doi:10.1371/journal.ppat.1005835
[cbtabs][cbtab title=”MLA”]PLOS. “Newly Deciphered Structure Suggests How Infectious Prions Replicate.” NeuroscienceNews. NeuroscienceNews, 11 September 2016.
<https://neurosciencenews.com/prion-replication-neuroscience-5010/>.[/cbtab][cbtab title=”APA”]PLOS. (2016, September 11). Newly Deciphered Structure Suggests How Infectious Prions Replicate. NeuroscienceNews. Retrieved September 11, 2016 from https://neurosciencenews.com/prion-replication-neuroscience-5010/[/cbtab][cbtab title=”Chicago”]PLOS. “Newly Deciphered Structure Suggests How Infectious Prions Replicate.” https://neurosciencenews.com/prion-replication-neuroscience-5010/ (accessed September 11, 2016).[/cbtab][/cbtabs]
The Structural Architecture of an Infectious Mammalian Prion Using Electron Cryomicroscopy
The structure of the infectious prion protein (PrPSc), which is responsible for Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy, has escaped all attempts at elucidation due to its insolubility and propensity to aggregate. PrPSc replicates by converting the non-infectious, cellular prion protein (PrPC) into the misfolded, infectious conformer through an unknown mechanism. PrPSc and its N-terminally truncated variant, PrP 27–30, aggregate into amorphous aggregates, 2D crystals, and amyloid fibrils. The structure of these infectious conformers is essential to understanding prion replication and the development of structure-based therapeutic interventions. Here we used the repetitive organization inherent to GPI-anchorless PrP 27–30 amyloid fibrils to analyze their structure via electron cryomicroscopy. Fourier-transform analyses of averaged fibril segments indicate a repeating unit of 19.1 Å. 3D reconstructions of these fibrils revealed two distinct protofilaments, and, together with a molecular volume of 18,990 Å3, predicted the height of each PrP 27–30 molecule as ~17.7 Å. Together, the data indicate a four-rung β-solenoid structure as a key feature for the architecture of infectious mammalian prions. Furthermore, they allow to formulate a molecular mechanism for the replication of prions. Knowledge of the prion structure will provide important insights into the self-propagation mechanisms of protein misfolding.
“The Structural Architecture of an Infectious Mammalian Prion Using Electron Cryomicroscopy” by Ester Vázquez-Fernández, Matthijn R. Vos, Pavel Afanasyev, Lino Cebey, Alejandro M. Sevillano, Enric Vidal, Isaac Rosa, Ludovic Renault, Adriana Ramos, Peter J. Peters, José Jesús Fernández, Marin van Heel, Howard S. Young, Jesús R. Requena, and Holger Wille in PLOS Pathogens. Published online September 8 2016 doi:10.1371/journal.ppat.1005835