A systematic investigation of production of synthetic prions from recombinant prion protein.

According to the protein-only hypothesis, infectious mammalian prions, which exist as distinct strains with discrete biological properties, consist of multichain assemblies of misfolded cellular prion protein (PrP). A critical test would be to produce prion strains synthetically from defined components. Crucially, high-titre 'synthetic' prions could then be used to determine the structural basis of infectivity and strain diversity at the atomic level. While there have been multiple reports of production of prions from bacterially expressed recombinant PrP using various methods, systematic production of high-titre material in a form suitable for structural analysis remains a key goal. Here, we report a novel high-throughput strategy for exploring a matrix of conditions, additives and potential cofactors that might generate high-titre prions from recombinant mouse PrP, with screening for infectivity using a sensitive automated cell-based bioassay. Overall, approximately 20,000 unique conditions were examined. While some resulted in apparently infected cell cultures, this was transient and not reproducible. We also adapted published methods that reported production of synthetic prions from recombinant hamster PrP, but again did not find evidence of significant infectious titre when using recombinant mouse PrP as substrate. Collectively, our findings are consistent with the formation of prion infectivity from recombinant mouse PrP being a rare stochastic event and we conclude that systematic generation of prions from recombinant PrP may only become possible once the detailed structure of authentic ex vivo prions is solved.

Overexpression of the Hspa13 (Stch) gene reduces prion disease incubation time in mice.

Prion diseases are fatal neurodegenerative disorders that include bovine spongiform encephalopathy (BSE) and scrapie in animals and Creutzfeldt-Jakob disease (CJD) in humans. They are characterized by long incubation periods, variation in which is determined by many factors including genetic background. In some cases it is possible that incubation time may be directly correlated to the level of gene expression. To test this hypothesis, we combined incubation time data from five different inbred lines of mice with quantitative gene expression profiling in normal brains and identified five genes with expression levels that correlate with incubation time. One of these genes, Hspa13 (Stch), is a member of the Hsp70 family of ATPase heat shock proteins, which have been previously implicated in prion propagation. To test whether Hspa13 plays a causal role in determining the incubation period, we tested two overexpressing mouse models. The Tc1 human chromosome 21 (Hsa21) transchromosomic mouse model of Down syndrome is trisomic for many Hsa21 genes including Hspa13 and following Chandler/Rocky Mountain Laboratory (RML) prion inoculation, shows a 4% reduction in incubation time. Furthermore, a transgenic model with eightfold overexpression of mouse Hspa13 exhibited highly significant reductions in incubation time of 16, 15, and 7% following infection with Chandler/RML, ME7, and MRC2 prion strains, respectively. These data further implicate Hsp70-like molecular chaperones in protein misfolding disorders such as prion disease.