Mammalian prions are notoriously difficult as structural biology subjects, given their insolubility and tendency to aggregate. Researchers have now overcome these challenges to figure out the preliminary structure of a shortened form of infectious prion (PrPSc), which they report today (September 8) in PLOS Pathogens.
“For the first time, we have a structure of an infectious mammalian prion,” said Giuseppe Legname of Scuola Internazionale Superiore di Studi Avanzati in Trieste, Italy, who was not involved in this study. “It’s a very important paper,” he added.
“What we have done is to obtain a very simple, very preliminary idea of what the structure of these mammalian prions are,” said study coauthor Jesús Requena of the University of Santiago de Compostela in Spain.
Requena and colleagues generated a shortened form of PrPSc by injecting a laboratory strain of prions into transgenic mice that express a truncated form of normal cellular prion protein (PrPC), which lacks the attachment of a membrane anchor present in full-length PrPSc. In nature, PrPC transforms into full-length PrPSc, which causes Creutzfeldt-Jakob disease in humans, scrapie in sheep, and mad cow disease. The absence of the membrane anchor in shortened PrPSc from the transgenic mice allowed the researchers to isolate a fairly homogeneous population of PrPSc. They confirmed that this population was infectious by inoculating wild-type mice, which then developed symptoms of prion disease.
The authors performed electron cryomicroscopy (cryo-EM) on the PrPSc molecules, which aggregate into amyloid fibrils in the brains of infected animals. They examined thousands of images, containing hundreds of fibrils each, and generated 3-D reconstructions of the fibrils. Their analysis indicated that the basic element of PrPSc structure is a four-rung β-solenoid, not unlike a mattress spring with four turns.
Coauthor Holger Wille of the University of Alberta, Canada, had previously used X-ray diffraction techniques to probe PrPSc structure, and those results also suggested the prion consists of a four-rung β-solenoid. “When you get the same result with different techniques, it makes you confident that you’re on the right track,” he said.
The work “gives a higher resolution picture of the structure of the infectious prion particles than has been possible before,” said David Harris of Boston University School of Medicine who did not participate in the work. “It’s a very important question and one that’s very difficult to answer,” he said.
One open question is how PrPC transforms into PrPSc to become infectious, Legname said. Requena’s team has shown the structure of the infectious prion, which is the endpoint, he added. “What you don’t know is what goes in between, how the prion protein then folds into that structure. That is the most challenging and most interesting thing that we still need to work on.”
The authors acknowledge that there is plenty left to explore. They are already at work on the structure of full-length PrPSc isolated from human brain tissue and from bovines infected with mad cow disease. They are also interested in obtaining a higher-resolution structure, which would provide much more detail and perhaps yield insights into other brain diseases.
“The structure of the prion protein is important by itself, but we’re also seeing a converging of paradigms in the neurodegeneration field,” said coauthor Howard Young of the University of Alberta. He pointed to similarities between prion-associated diseases and Alzheimer’s disease and Parkinson’s disease. “Getting at those fundamental mechanisms, you never know where the overlap’s going to come and insights are going to be gained in other areas,” he said. “That’s what really makes [this work] important and exciting.”
E. Vazquez-Fernandez et al., “The structural architecture of an infectious mammalian prion using electron cryomicroscopy,” PLOS Pathogens, doi:10.1371/journal.ppat.1005835, 2016.