Intrinsic toxicity of the cellular prion protein is regulated by its conserved central region

Evidence for preexisting prion substrain diversity in a biologically cloned prion strain

Prion diseases are a group of inevitably fatal neurodegenerative disorders affecting numerous mammalian species, including Sapiens. Prions are composed of PrPSc, the disease specific conformation of the host encoded prion protein. Prion strains are operationally defined as a heritable phenotype of disease under controlled transmission conditions. Treatment of rodents with anti-prion drugs results in the emergence of drug-resistant prion strains and suggest that prion strains are comprised of a dominant strain and substrains. While much experimental evidence is consistent with this hypothesis, direct observation of substrains has not been observed. Here we show that replication of the dominant strain is required for suppression of a substrain. Based on this observation we reasoned that selective reduction of the dominant strain may allow for emergence of substrains. Using a combination of biochemical methods to selectively reduce drowsy (DY) PrPSc from biologically-cloned DY transmissible mink encephalopathy (TME)-infected brain resulted in the emergence of strains with different properties than DY TME. The selection methods did not occur during prion formation, suggesting the substrains identified preexisted in the DY TME-infected brain. We show that DY TME is biologically stable, even under conditions of serial passage at high titer that can lead to strain breakdown. Substrains therefore can exist under conditions where the dominant strain does not allow for substrain emergence suggesting that substrains are a common feature of prions. This observation has mechanistic implications for prion strain evolution, drug resistance and interspecies transmission.

Multisite interactions of prions with membranes and native nanodiscs

Although prions are known as protein-only infectious particles, they exhibit lipid specificities, cofactor dependencies and membrane-dependent activities. Such membrane interactions play key roles in how prions are processed, presented and regulated, and hence have significant functional consequences. The expansive literature related to prion protein interactions with lipids and native nanodiscs is discussed, and provides a unique opportunity to re-evaluate the molecular composition and mechanisms of its infectious and cellular states. A family of crystal and solution structures of prions are analyzed here for the first time using the membrane optimal docking area (MODA) program, revealling the presence of structured binding elements that could mediate specific lipid recognition. A set of motifs centerred around W99, L125, Y169 and Y226 are consistently predicted as being membrane interactive and form an exposed surface which includes α helical, β strand and loop elements involving the prion protein (PrP) structural domain, while the scrapie form is radically different and doubles the size of the membrane interactive site into an extensible surface. These motifs are highly conserved throughout mammalian evolution, suggesting that prions have long been intrinsically attached to membranes at central and N- and C-terminal points, providing several opportunities for stable and specific bilayer interactions as well as multiple complexed orientations. Resistance or susceptibility to prion disease correlates with increased or decreased membrane binding propensity by mutant forms, respectively, indicating a protective role by lipids. The various prion states found in vivo are increasingly resolvable using native nanodiscs formed by styrene maleic acid (SMA) and stilbene maleic acid (STMA) copolymers rather than classical detergents, allowing the endogenous states to be tackled. These copolymers spontaneously fragment intact membranes into water-soluble discs holding a section of native bilayer, and can accommodate prion multimers and mini-fibrils. Such nanodiscs have also proven useful for understanding how β amyloid and α synuclein proteins contribute to Alzheimer's and Parkinson's diseases, providing further biomedical applications. Structural and functional insights of such proteins in styrene maleic acid lipid particles (SMALPs) can be resolved at high resolution by methods including cryo-electron microscopy (cEM), motivating continued progress in polymer design to resolve biological and pathological mechanisms.