Published!: Infections caused by proteins: Fact or Fiction?

(by Natalie Nady) Each month we will review/feature an article published by one of our very own MBP students in the biology and/or physics stream.

This article features research that was carried out in the labs of Dr. Emil Pai and Dr. Avi Chakrabartty and co-first authors were grad students Braden Sweeting and Qasim Khan from Biochemistry [Link to Paper].

This article talks about prions, a proteinaceous infectious particles which affect the brain or other neural tissue leading to various neurodegenerative diseases such as bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt–Jakob disease (CJD) in humans. All prion associated diseases are currently untreatable and universally fatal. Fortunately mammals vary in their prion disease susceptibility: hamsters and mice show relatively high susceptibility, whereas rabbits, horses, and dogs show low susceptibility. The mechanism of pathogenesis and the causes for varying susceptibility in mammals are not known. What is known is that in the native non-disease state of the protein, PrP, is alpha-helical. After infection, PrP is converted to predominantly beta-sheet form that aggregates into oligomers and amyloid fibrils. The transition between these secondary structure elements and oligomerization are not well defined. The authors used a combination of various biochemical techniques and structural biology to get a better understanding of how populated the beta-state is, its pathogenicity, and what drives the conversion between normal alpha and pathogenic beta-state.

First, the group used a novel two-wavelength approach in circular dichroism (CD), a technique that looks at the secondary structure elements, to quantitate the concentration of beta-state. After testing the prion protein from different species, they found that PrP from species that are susceptible to prion disease formed a higher fraction of the beta-state compared to those that were more resistant.

Second, the beta-state of PrP forms oligomers and amyloid, but the existence of a monomeric form of the beta-state has not been shown clearly before. The authors were able to isolate monomeric beta-state PrP using a combination of sedimentation equilibrium and size exclusion chromatography. They also showed that the beta-state is toxic to neural cells in cell culture.

Lastly, in an effort to find structural basis for the observed differences in beta-state conversion, the authors determined the structure of PrP from rabbits, a species with very low susceptibility to prion disease, using X-ray crystallography. Careful examination of the structure revealed presence of a hydrophobic-staple helix-cap that increases the stability of the normal alpha-state of rabbit PrP. Other species, which are more prone to the disease, have a sequence difference eliminating this motif and increasing the propensity to form the pathogenic beta-state.

The authors, however, fall short of making structure-based mutations and trying to “convert” a protein from highly susceptible species to that with low infectivity by introducing the mutation in a hydrophobic patch capping the helix. Despite that, the paper provides important advancements in our understanding as to why some species are more susceptible to prion infection than others. It also provides structural information that can be used to design antibodies that stabilize the prion protein in its non-infectious alpha-state.