Prion protein diversity and disease in the transmissible spongiform encephalopathies

Biophysical characterization of oligomerization and fibrillization of the G131V pathogenic mutant of human prion protein

The pathogenesis of fatal neurodegenerative prion diseases is closely associated with the conversion of α-helix-rich cellular prion protein into β-sheet-rich scrapie form. Pathogenic point mutations of prion proteins usually promote the conformational conversion and trigger inherited prion diseases. The G131V mutation of human prion protein (HuPrP) was identified to be involved in Gerstmann-Sträussler-Scheinker syndrome. Few studies have been carried out to address the pathogenesis of the G131V mutant. Here, we addressed the effects of the G131V mutation on oligomerization and fibrillization of the full-length HuPrP(23-231) and truncated HuPrP(91-231) proteins. The G131V mutation promotes the oligomerization but alleviates the fibrillization of HuPrP, implying that the oligomerization might play a crucial role in the pathogenic mechanisms of the G131V mutant. Moreover, the flexible N-terminal fragment in either the wild-type or the G131V mutant HuPrP increases the oligomerization tendencies but decreases the fibrillization tendencies. Furthermore, this mutation significantly alters the tertiary structure of human PrPC and might distinctly change the conformational conversion tendency. Interestingly, both guanidine hydrochloride denaturation and thermal denaturation experiments showed that the G131V mutation does not significantly change the thermodynamic stabilities of the HuPrP proteins. This work may be of benefit to a mechanistic understanding of the conformational conversion of prion proteins and also provide clues for the prevention and treatment of prion diseases.

Co-infection with the friend retrovirus and mouse scrapie does not alter prion disease pathogenesis in susceptible mice

Prion diseases are fatal, transmissible neurodegenerative diseases of the central nervous system. An abnormally protease-resistant and insoluble form (PrP(Sc)) of the normally soluble protease-sensitive host prion protein (PrP(C)) is the major component of the infectious prion. During the course of prion disease, PrP(Sc) accumulates primarily in the lymphoreticular and central nervous systems. Recent studies have shown that co-infection of prion-infected fibroblast cells with the Moloney murine leukemia virus (Mo-MuLV) strongly enhanced the release and spread of scrapie infectivity in cell culture, suggesting that retroviral coinfection might significantly influence prion spread and disease incubation times in vivo. We now show that another retrovirus, the murine leukemia virus Friend (F-MuLV), also enhanced the release and spread of scrapie infectivity in cell culture. However, peripheral co-infection of mice with both Friend virus and the mouse scrapie strain 22L did not alter scrapie disease incubation times, the levels of PrP(Sc) in the brain or spleen, or the distribution of pathological lesions in the brain. Thus, retroviral co-infection does not necessarily alter prion disease pathogenesis in vivo, most likely because of different cell-specific sites of replication for scrapie and F-MuLV.

Decomposition of extremely hard-to-degrade animal proteins by thermophilic bacteria

Hard-to-degrade animal proteins are ubiquitously present throughout animal bodies. Enormous numbers of these proteins generated in the meat industry are converted to industrial wastes, the disposal of which is tremendously difficult. Most hard-to-degrade animal proteins are currently disposed of by incineration; however, this method has ecological disadvantages in terms of an apparent energy loss and the production of a large amount of carbon dioxide. As a result, an innovative solution to these problems has been sought. In this review, we focus on the degradation of three hard-to-degrade animal proteins (extracellular matrix proteins, collagen in particular, keratin, and prion proteins) and discuss the decomposing capability of thermophilic bacteria. These proteins are strongly resistant to proteinases because of their structural features; therefore, new approaches employing bacterial proteases with strong activity and broad specificity are required for practical application.

Structural differences between TSEs strains investigated by FT-IR spectroscopy

Strain diversity in transmissible spongiform encephalopathies (TSEs) has been suggested to be "enciphered" in the structure of the misfolded prion protein isoform PrP(Sc). We have recently demonstrated the strain typing potential of the FT-IR spectroscopy technique, analyzing four different TSE agents adapted to Syrian hamsters [A. Thomzig, S. Spassov, M. Friedrich, D. Naumann and M. Beekes, Discriminating scrapie and BSE isolates by infrared spectroscopy of pathological prion protein J. Biol. Chem. 279 (2004) 33847-33854.] [1]. In the present paper, we have extended the FT-IR study, exploring the secondary structure, temperature stability, and hydrogen-deuterium exchange characteristics of PrP27-30, from the TSE agents 263K, ME7-H, 22A-H, and BSE-H. The strain differentiation capacity of the FT-IR approach was objectively proven for the first time by multivariate cluster analysis. The second derivative FT-IR spectra obtained from dried protein films or samples hydrated in H(2)O or D(2)O consistently exhibited strain-specific infrared characteristics in the secondary structure sensitive amide I region, complemented by strain dependent spectral traits in the amide II and amide A absorption regions, and the different H/D-exchange behaviour of the various PrP27-30 samples. FT-IR spectra of PrP27-30 samples from 263K, ME7-H and 22A-H exposed to increasing temperature (up to 90 degrees C) showed that a strain-specific response to heat treatment is associated with strain specific thermostability of distinct secondary structure elements, providing additional means for TSEs strain discrimination.

Role of N-terminal Familial Mutations in Prion Protein Fibrillization and Prion Amyloid Propagation in Vitro*

A self-perpetuating conformational conversion of the prion protein (PrP) is believed to underlie pathology and transmission of prion diseases. Here we explore the effects of N-terminal pathogenic mutations (P102L, P105L, A117V) and the residue 129 polymorphism on amyloid fibril formation by the human PrP fragment 23-144, an in vitro conversion model that can reproduce certain characteristics of prion replication such as strains and species barriers. We find that these amino acid substitutions neither affect PrP23-144 amyloidogenicity nor introduce barriers to cross-seeding of soluble protein. However, the polymorphism strongly influences the conformation of the amyloid fibrils, as determined by infrared spectroscopy. Intriguingly, unlike conformational features governed by the critical amyloidogenic region of PrP23-144 (residues 138-139), the structural features distinguishing Met-129 and Val-129 PrP23-144 amyloid fibrils are not transmissible by cross-seeding. While based only on in vitro data, these findings provide fundamental insight into the mechanism of prion-based conformational transmission, indicating that only conformational features controlling seeding specificity (e.g. those in critical intermolecular contact sites of amyloid fibrils) are necessarily transmissible by cross-seeding; conformational traits in other parts of the PrP molecule may not be "heritable" from the amyloid template.

The Effect of Disease-associated Mutations on the Folding Pathway of Human Prion Protein

Propagation of transmissible spongiform encephalopathies is believed to involve the conversion of cellular prion protein, PrP(C), into a misfolded oligomeric form, PrP(Sc). An important step toward understanding the mechanism of this conversion is to elucidate the folding pathway(s) of the prion protein. We reported recently (Apetri, A. C., and Surewicz, W. K. (2002) J. Biol. Chem. 277, 44589-44592) that the folding of wild-type prion protein can best be described by a three-state sequential model involving a partially folded intermediate. Here we have performed kinetic stopped-flow studies for a number of recombinant prion protein variants carrying mutations associated with familial forms of prion disease. Analysis of kinetic data clearly demonstrates the presence of partially structured intermediates on the refolding pathway of each PrP variant studied. In each case, the partially folded state is at least one order of magnitude more populated than the fully unfolded state. The present study also reveals that, for the majority of PrP variants tested, mutations linked to familial prion diseases result in a pronounced increase in the thermodynamic stability, and thus the population, of the folding intermediate. These data strongly suggest that partially structured intermediates of PrP may play a crucial role in prion protein conversion, serving as direct precursors of the pathogenic PrP(Sc) isoform.

Prion protein and the molecular features of transmissible spongiform encephalopathy agents

Transmissible spongiform encephalopathy (TSE) diseases, or prion diseases, are neurodegenerative diseases found in a number of mammals, including man. Although they are generally rare, TSEs are always fatal, and as of yet there are no practical therapeutic avenues to slow the course of disease. The epidemic of bovine spongiform encephalopathy (BSE) in the UK greatly increased the awareness of TSE diseases. Although it appears that BSE has not spread to North America, chronic wasting disease (CWD), a TSE found in cervids, is causing significant concern. Despite decades of investigation, the exact nature of the infectious agent of the TSEs is still controversial. Although many questions remain, substantial efforts have been made to understand the molecular features of TSE agents, with the hope of enhancing diagnosis and treatment of disease, as well as understanding the fundamental nature of the infectious agent itself. This review summarizes the current understanding of these molecular features, focusing on the role of the prion protein (PrP(c)) and its relationship to the disease-associated isoform (PrP(Sc)).

Architecture of Ure2p prion filaments: the N-terminal domains form a central core fiber

The [URE3] prion is an inactive, self-propagating, filamentous form of the Ure2 protein, a regulator of nitrogen catabolism in yeast. The N-terminal "prion" domain of Ure2p determines its in vivo prion properties and in vitro amyloid-forming ability. Here we determined the overall structures of Ure2p filaments and related polymers of the prion domain fused to other globular proteins. Protease digestion of 25-nm diameter Ure2p filaments trimmed them to 4-nm filaments, which mass spectrometry showed to be composed of prion domain fragments, primarily residues approximately 1-70. Fusion protein filaments with diameters of 14-25 nm were also reduced to 4-nm filaments by proteolysis. The prion domain transforms from the most to the least protease-sensitive part upon filament formation in each case, implying that it undergoes a conformational change. Intact filaments imaged by cryo-electron microscopy or after vanadate staining by scanning transmission electron microscopy (STEM) revealed a central 4-nm core with attached globular appendages. STEM mass per unit length measurements of unstained filaments yielded 1 monomer per 0.45 nm in each case. These observations strongly support a unifying model whereby subunits in Ure2p filaments, as well as in fusion protein filaments, are connected by interactions between their prion domains, which form a 4-nm amyloid filament backbone, surrounded by the corresponding C-terminal moieties.

The fate of the prion protein in the prion/plasminogen complex

The cellular prion protein (PrP(c)) forms complexes with plasminogen. Here, we show that the PrP(c) in this complex is cleaved to yield fragments of PrP(c). The cleavage is accelerated by plasmin but does not appear to be dependent on it.

Prion: toxic or infectious agent?

Prions are proteins that cause a number of invariably fatal neuro-degenerative diseases, which can be classified into two groups: genetic or sporadic diseases (GSD) and transmissible spongiform encephalopathies (TSE). Both types of disease require the development of both normal prion (PrP) and abnormal prion (PrP(sc)) which differs from PrP in having a tertiary structure rich in beta-sheets. In fact, PrP(sc) is a totally dehydrated protein with an anhydrous environment, probably a thin carbon dioxide gas gap, that is why it appears highly resistant to proteases, to chemical disinfectants in water phase except in certain conditions to sodium hydroxide and sodium hypochlorite, to heat and to radiation. GSD and TSE diseases differ in incubation time, primary symptoms, and nature of CNS lesions. This paper argues that diseases of the GSD type as inherited or hereditary metabolic disorders and diseases of the TSE type could be regarded as chemical poisonings. TSE is caused by a deficiency in the chemo-defense system (CDS), which is unable to destroy or eliminate PrP(sc). As a result, the immune defense system (IDS) accommodates PrP(sc) as an inert particle if not a virus lure and routes it through to the nervous central system and the brain via the body's lymphoreticular system. In TSE PrP(sc) acts inside the cells as a toxic disruptor of post-translational phase of PrP biosynthesis. Unfortunately, CDS and IDS appear unable to neutralize PrP(sc).

Prion protein conversions: insight into mechanisms, TSE transmission barriers and strains

Conversion of PrP(C) to aberrant forms such as PrP(Sc) appears to be critical in the transmission and pathogenesis of transmissible spongiform encephalopathies (TSEs) or prion diseases. In vitro studies have shown that TSE-associated, protease-resistant forms of PrP can cause PrP(C) to convert to forms that are similarly protease-resistant under a wide variety of conditions. These observations have provided evidence that pathological forms of PrP have at least limited capacity to propagate themselves, which is necessary for them to be infectious. PrP conversion reactions have proven to be highly specific and appear to account, at least in part, for TSE species barriers and the propagation of strains. Such in vitro conversion systems have yielded insights into the molecular mechanisms of TSE disease and are being exploited as screens for anti-TSE drugs and as bases for diagnostic tests.