Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are lethal and infectious neurodegenerative diseases of humans and animals. The misfolding of the normal, or cellular isoform of the prion protein (PrPC) into the abnormal disease-associated isoform of PrP (PrPSc) could change the properties of PrP, consequently, PrPSc has lethal infectivity to transmit diseases. The proteinaceous infectious particle consisting mainly of PrPSc is called prion. Transmissibility of prions is strongly influenced by multiple factors including PrP polymorphisms, species barriers (PrP sequence specificity) and prion strains (conformational specificity) by unknown mechanisms. Even though the ability of prions to cross a species barrier has been recognized, the precise mechanisms of interspecies prion transmission remain unclear. This dissertation research was conducted in order to learn more about the molecular mechanisms of conversion, propagation and transmission of PrPSc; about determinants of genetic susceptibility to infection in prion diseases; and about understanding those mechanisms, which might govern the zoonotic potential of prion diseases. First, we investigated the transmissibility risk of multiple strains of Chronic Wasting Disease, which is a cervid TSE, with humanized transgenic mice and showed that the transmission barriers between cervid and the humanized mice are high. Next, the structural factors underlying the species barrier of prion diseases were studied using cell culture systems by systematically introducing amino acid substitutions in the regions of PrP, where the most divergences of different PrP species are recognized. Thirdly, we investigated the effects of the genetic susceptibility to prions as well as conversion kinetics and properties of PrPSc using Tg mice expressing ovine PrP polymorphism (OvPrP) at codon 136 either alanine (A) or valine (V). The templating characteristics of OvPrPSc-V136 were dominant over OvPrPSc-A136 under co-expressions of OvPrPC-A136 and OvPrPC-V136. Finally, the function of PrP was studied in relation to the pathogenesis of Alzheimer’s disease. These studies demonstrated that the conformational compatibility between PrPC and PrPSc contributed to the conversion kinetics and species barrier. We concluded that the conformational compatibility of PrPC to PrPSc is controlled not only by the PrP sequence specificity but also by the tertiary structure of PrPC.

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Prion diseases are fatal neurodegenerative diseases that affect mammals, including humans, which are characterized by accumulation of pathologi- cal prion protein isoform (PrPTSE ) in the brain. The animals were commonly used for the prion disease research in the past but in recent years, the tissue cultures are being used as well. Tissue cultures have many advantages com- pared with animals. E.g. the possibility of a detailed study of the biochemical processes associated with prion diseases, and rapid and sensitive PrPTSE de- tecting method. However no reliable in vitro model was developed for human prion diseases so far. We focused on monitoring of transmission and propagation efficiency of different prion strains and on the influence of cultivation conditions on the transfer of the neuronal cell line CAD5, which is highly sensitive to prion infection. We confirmed the sensitivity of CAD5 cells to mouse-adapted scra- pie prion strains and we presented new facts about their ability to propagate mouse adapted prions of human strains and bovine spongiform encepha- lopathy. We have used CAD5 cell sensitivity to be infected with different prion strains in other parts of this work. In the second part, we focused on the cell sensitivity to prion infection and propagation of prion strains under different culture...