Structuring the puzzle of prion propagation

Perspectives on CRISPR Genome Editing to Prevent Prion Diseases in High-Risk Individuals

Prion diseases are neurodegenerative disorders caused by misfolded prion proteins. Although rare, the said diseases are always fatal; they commonly cause death within months of developing clinical symptoms, and their diagnosis is exceptionally difficult pre-mortem. There are no known cures or treatments other than symptomatic care. Given the aggressiveness of prion diseases on onset, therapies after disease onset could be challenging. Prevention to reduce the incidence or to delay the disease onset has been suggested to be a more feasible approach. In this perspective article, we summarize our current understandings of the origin, risk factors, and clinical manifestations of prion diseases. We propose a PCR testing of the blood to identify PRNP gene polymorphisms at codons 129 and 127 in individuals with familial PRNP mutations to assess the risk. We further present the CRISPR/Cas9 gene editing strategy as a perspective preventative approach for these high-risk individuals to induce a polymorphic change at codon 127 of the PRNP gene, granting immunity to prion diseases in selected high-risk individuals, in particular, in individuals with familial PRNP mutations.

Heterogeneity and Architecture of Pathological Prion Protein Assemblies: Time to Revisit the Molecular Basis of the Prion Replication Process?

Prions are proteinaceous infectious agents responsible for a range of neurodegenerative diseases in animals and humans. Prion particles are assemblies formed from a misfolded, β-sheet rich, aggregation-prone isoform (PrP^Sc) of the host-encoded cellular prion protein (PrP^C). Prions replicate by recruiting and converting PrP^C into PrP^Sc, by an autocatalytic process. PrP^Sc is a pleiomorphic protein as different conformations can dictate different disease phenotypes in the same host species. This is the basis of the strain phenomenon in prion diseases. Recent experimental evidence suggests further structural heterogeneity in PrP^Sc assemblies within specific prion populations and strains. Still, this diversity is rather seen as a size continuum of assemblies with the same core structure, while analysis of the available experimental data points to the existence of structurally distinct arrangements. The atomic structure of PrP^Sc has not been elucidated so far, making the prion replication process difficult to understand. All currently available models suggest that PrP^Sc assemblies exhibit a PrP^Sc subunit as core constituent, which was recently identified. This review summarizes our current knowledge on prion assembly heterogeneity down to the subunit level and will discuss its importance with regard to the current molecular principles of the prion replication process.

Mysterious oligomerization of the amyloidogenic proteins

Misfolding and subsequent self-assembly of protein molecules into various aggregates is a common molecular mechanism for a number of important human diseases. Curing protein misfolding pathologies and designing successful drugs for the inhibition or reversal of protein aggregation depends on understanding the peculiarities of the misfolding process. Protein aggregation is a very complex process characterized by a remarkable polymorphism, where soluble amyloid oligomers, amyloid fibrils and amorphous aggregates are found as final products. This polymorphism is associated with the existence of multiple independent and competing assembly pathways leading to aggregation. Regardless of the aggregation mechanism, soluble oligomers are inevitably formed during the self-association process. Some of these oligomers are now considered to be major initiators of the neurodegenerative cascades of corresponding diseases. However, not all oligomers are equally harmful, and several amyloidogenic proteins have been shown to form nontoxic oligomers, some of which were efficient fibrillation inhibitors. Unfortunately, the information on the structural properties of soluble oligomers and the mechanisms of their formation, interconversion and toxicity is sparse. This review provides an overview of some topics related to soluble oligomers and represents several illustrative examples of toxic, nontoxic, productive and off-pathway amyloid oligomers.

Curcumin binds to the alpha-helical intermediate and to the amyloid form of prion protein - a new mechanism for the inhibition of PrP(Sc) accumulation

Conversion of the native, predominantly alpha-helical conformation of prion protein (PrP) into the beta-stranded conformation is characteristic for the transmissible spongiform encephalopathies such as Creutzfeld-Jakob disease. Curcumin, an extended planar molecule and a dietary polyphenol, inhibits in vitro conversion of PrP and formation of protease resistant PrP in neuroblastoma cell lines. Curcumin recognizes the converted beta-form of the PrP both as oligomers and fibrils but not the native form. Curcumin binds to the prion fibrils in the left-handed chiral arrangement as determined by circular dichroism. We show that curcumin labels the plaques of the brain sections of variant Creutzfeld-Jakob disease cases and stains the same structures as antibodies against the PrP. In contrast to thioflavin T, curcumin also binds to the alpha-helical intermediate of PrP present at acidic pH at stoichiometry of 1 : 1. Congo red competes with curcumin for binding to the alpha-intermediate as well as to the beta-form of PrP but is toxic and binds also to the native form of PrP. We therefore show that the partially unfolded structural intermediate of the PrP can be targeted by non-toxic compound of natural origin.

Water molecules as structural determinants among prions of low sequence identity

The nature of the factors leading to the conversion of the cellular prion protein (PrP(C)) into its amyloidogenic isoform (PrP(Sc)) is still matter of debate in the field of structural biology. The NMR structures of non-mammalian PrP(C) (non-mPrP) from frog, chicken and turtle [Calzolai, L., Lysek, D.A., Perez, D.R., Guntert, P. and Wuthrich, K. (2005) Prion protein NMR structures of chickens, turtles, and frogs. Proc. Natl. Acad. Sci. USA 102, 651-655] have provided some new and valuable information on the scaffolding elements that preserve the PrP(C) folding, despite their low sequence identity with the mammalian prions (mPrP). The present molecular dynamics study of non-mPrP(C) focuses on the hydration properties of these proteins in comparison with the mammalian ones. The data reveal new insights in the PrP hydration and focus on the implications for PrP(C) folding stability and its propensity for interactions. In addition, for the first time, a role in disfavoring the PrP(C) aggregation is suggested for a conserved beta-bulge which is stabilized by the local hydration.