The role of prion heterogeneity in transmission and transport

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The protein-only hypothesis states that infectious prions consist of a misfolded isoform (PrPSc) of the cellular prion protein (PrPC) without encoding nucleic acid which represents an exceptional biological principle. Prions replicate by inducing conformational conversion of the mainly a-helical PrPC into PrPSc with b-sheet structure, which makes it prone to aggregation and resilient to proteolytic digestion. The conformation of PrPSc can vary despite identical primary structure, and these conformational variants are termed prion strains. Prion strains can evolve or be selected upon passage through different hosts with distinct PrPC primary structures, or in the same host species expressing species-specific allelic variants of PrPC arising from polymorphic codons and subsequent single amino acid substitutions. Prion strains encode and inherit their biological and biochemical properties in their conformations, and consist of a mixture of PrPSc aggregates of different molecular weights, representing fractions of monomeric, oligomeric or fibrillary PrPSc. Oligomeric PrPSc aggregates contain the highest levels of infectivity when injected intracerebrally into rodent models of prion disease. However, the role of PrPSc aggregate size in natural infection at peripheral body sites, and the impact of host factors on the aggregate profile such as the PrP genotype have not been investigated but are critically important to gain a full understanding of how transmissible information is encoded by prions. Interestingly, our preliminary data indicate a role of PrPSc aggregate size in targeting specific brain regions for replication. Our proposed research program addresses the basic questions which information is stored and transmitted by the subsets of PrPSc aggregates that compose prion strains, and how this information is modulated upon interaction with the host organism. Chronic wasting disease (CWD) prions of cervids are a highly relevant model as the disease is spreading among wild cervids in North America and two European countries, and threatens endangered caribou populations in Canada. We will use our newly generated, unique knock-in mouse models expressing wild-type and selected allelic variants of cervid PrP under control of the endogenous PrP promoter for infection with size-fractionated PrPSc aggregates. These will be complemented by innovative cell culture, primary neuron and in vitro prion amplification models to address three objectives: (1) to determine the impact of specific polymorphisms in cervid PrPC on cellular prerequisites for prion conversion, (2) to analyse PrPSc aggregate size profiles of CWD isolates and the role in prion transport and infectivity and (3) to investigate PrPSc aggregate size inheritance and its role in host cell selection in the brain. Results from this research will have a significant impact on our knowledge about prion-host interaction and the range of information that can be encoded by protein structure.