Mechanism of misfolding of the human prion protein revealed by a pathological mutation.

The misfolding and aggregation of the human prion protein (PrP) is associated with transmissible spongiform encephalopathies (TSEs). Intermediate conformations forming during the conversion of the cellular form of PrP into its pathological scrapie conformation are key drivers of the misfolding process. Here, we analyzed the properties of the C-terminal domain of the human PrP (huPrP) and its T183A variant, which is associated with familial forms of TSEs. We show that the mutation significantly enhances the aggregation propensity of huPrP, such as to uniquely induce amyloid formation under physiological conditions by the sole C-terminal domain of the protein. Using NMR spectroscopy, biophysics, and metadynamics simulations, we identified the structural characteristics of the misfolded intermediate promoting the aggregation of T183A huPrP and the nature of the interactions that prevent this species to be populated in the wild-type protein. In support of these conclusions, POM antibodies targeting the regions that promote PrP misfolding were shown to potently suppress the aggregation of this amyloidogenic mutant.

The role of the immune system in prion infection.

Prion diseases are a unique group of chronic neurodegenerative diseases that affect humans and certain domestic and free-ranging animal species. Many natural prion diseases are acquired peripherally, such as by ingestion of contaminated food or pasture. Although the pathology during prion disease appears to be restricted to the central nervous system, where it causes extensive neurodegeneration, some prion diseases accumulate to high levels within the secondary lymphoid tissues of the host's immune system as they make their journey from the site of infection to the brain. The replication of prions within these tissues is essential for the efficient spread of disease to the brain. Moreover, the immune system has a profound influence on the development of disease within the central nervous system. This chapter describes the interactions between prions and the host's immune system. Particular emphasis is given to studies which have helped to identify the key tissues, cells, and molecules which the prions exploit to facilitate their propagation from peripheral sites of exposure (such as the intestine) to the brain. This chapter also describes how prion disease pathogenesis and susceptibility may be influenced by inflammation, co-infection with other pathogens, and aging. A thorough understanding of the factors which influence prion disease susceptibility is important as it may help to identify important targets for therapeutic intervention and to help determine the risk of susceptibility to novel peripherally acquired prion diseases.

Protein Misfolding Cyclic Amplification Cross-Species Products of Mouse-Adapted Scrapie Strain 139A and Hamster-Adapted Scrapie Strain 263K with Brain and Muscle Tissues of Opposite Animals Generate Infectious Prions.

Transmission of prions between mammalian species is limited by a "species barrier," a biological effect involving an increase in incubation period to decrease the percentage of animals succumbing to disease. In this study, we used protein misfolding cyclic amplification (PMCA) technique, which accelerates the conversion of prion proteins in vitro. Direct interspecies PMCA involving 144 cycles confirmed that both mouse-adapted scrapie strain 139A and hamster-adapted 263K could use brain homogenates of opposite species to form proteinase K (PK)-resistant PrP proteins (PrPres). Newly formed interspecies prions could stably propagate themselves in subsequent serial PMCA passages. The two types of PMCA-generated cross-species PrPres changed their glycosylation profiles, which was similar to that observed during interspecies infection by the mouse agent 139A in vivo. These profiles were distinct from individual seeded PrPSc and possessed properties of new hosts. Comparative analysis with respect to PK resistance showed no significant diversity between PMCA-PrPres and native PrPSc or between brain and muscle PrPres. However, PrPres from the relatively early cycles of serial PMCA showed lower PK resistance than those from later cycles. Inoculation of these PMCA products amplified with homogeneous or heterogeneous brain tissues (cross-species products) induced experimental transmissible spongiform encephalopathies. These results suggested that PMCA can help prion strains to overcome species barrier and to propagate efficiently both in vitro and in vivo.

Insights into Mechanisms of Transmission and Pathogenesis from Transgenic Mouse Models of Prion Diseases.

Prions represent a new paradigm of protein-mediated information transfer. In the case of mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, sometimes referred to as transmissible spongiform encephalopathies (TSEs), which frequently occur as epidemics. An increasing body of evidence indicates that the canonical mechanism of conformational corruption of cellular prion protein (PrPC) by the pathogenic isoform (PrPSc) that is the basis of prion formation in TSEs is common to a spectrum of proteins associated with various additional human neurodegenerative disorders, including the more common Alzheimer's and Parkinson's diseases. The peerless infectious properties of TSE prions, and the unparalleled tools for their study, therefore enable elucidation of mechanisms of template-mediated conformational propagation that are generally applicable to these related disease states. Many unresolved issues remain including the exact molecular nature of the prion, the detailed cellular and molecular mechanisms of prion propagation, and the means by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological role of the normal form of the prion protein remains unclear and it is uncertain whether or not loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, advances in our understanding of prions have occurred because of their transmissibility to experimental animals, and the development of transgenic (Tg) mouse models has done much to further our understanding about various aspects of prion biology. In this review, we will focus on advances in our understanding of prion biology that occurred in the past 8 years since our last review of this topic.

A closer look at prion strains: characterization and important implications.

Prions are infectious proteins that are responsible for transmissible spongiform encephalopathies (TSEs) and consist primarily of scrapie prion protein (PrP (Sc) ), a pathogenic isoform of the host-encoded cellular prion protein (PrP (C) ). The absence of nucleic acids as essential components of the infectious prions is the most striking feature associated to these diseases. Additionally, different prion strains have been isolated from animal diseases despite the lack of DNA or RNA molecules. Mounting evidence suggests that prion-strain-specific features segregate with different PrP (Sc) conformational and aggregation states. Strains are of practical relevance in prion diseases as they can drastically differ in many aspects, such as incubation period, PrP (Sc) biochemical profile (e.g., electrophoretic mobility and glycoform ratio) and distribution of brain lesions. Importantly, such different features are maintained after inoculation of a prion strain into genetically identical hosts and are relatively stable across serial passages. This review focuses on the characterization of prion strains and on the wide range of important implications that the study of prion strains involves.