New Molecular Insight into Mechanism of Evolution of Mammalian Synthetic Prions

Previous studies established that transmissible prion diseases could be induced by in vitro-produced recombinant prion protein (PrP) fibrils with structures that are fundamentally different from that of authentic PrP scrapie isoform (PrPSc). To explain evolution of synthetic prions, a new mechanism referred to as deformed templating was introduced. Here, we asked whether an increase in expression level of the cellular form of PrP (PrPC) speeds up the evolution of synthetic strains in vivo. We found that in transgenic mice that overexpress hamster PrPC, PrPC overexpression accelerated recombinant PrP fibril-induced conversion of PrPC to the abnormal proteinase K-resistant state, referred to as atypical PrPres, which was the first product of PrPC misfolding in vivo. However, overexpression of PrPC did not facilitate the second step of synthetic strain evolution-transition from atypical PrPres to PrPSc, which is attributed to the stochastic nature of rare deformed templating events. In addition, the potential of atypical PrPres to interfere with replication of a short-incubation time prion strain was investigated. Atypical PrPres was found to interfere strongly with replication of 263K in vitro; however, it did not delay prion disease in animals. The rate of deformed templating does not depend on the concentration of substrate and is hence more likely to be controlled by the intrinsic rate of conformational errors in templating alternative self-propagating states.