The major polypeptide of scrapie-associated fibrils (SAF) has the same size, charge distribution and N-terminal protein sequence as predicted for the normal brain protein (PrP)

Prions and neurodegenerative diseases

The long-term, progressive decay of the central nervous system typifies prion diseases, a group of rare, transmissible maladies affecting humans, sheep, cattle and some other types of mammal. Little is known about the early molecular events in its pathogenesis but the diverse roles of PrP, the prion protein, in its destructive action have recently been re-emphasised.

Pitfalls in prion research

Prion research seems to get increasingly enigmatic since the protein only hypothesis has been established as almost the only working hypothesis. This may indicate that the hypothesis could be wrong, and should prompt the search for potential faults in past experiments. In fact some problematic experiments can be pinpointed, for example determination of the N-terminal cleavage site of the prion protein PrP, of the structure of PrP as determined by NMR, some conclusions concerning the function of PrP from gene knockout experiments including potential evidence against the protein only hypothesis, and some aspects of the prion purification procedure.

Screening Congo Red and its analogues for their ability to prevent the formation of PrP-res in scrapie-infected cells

Transmissible spongiform encephalopathies (TSEs) are incurable, fatal diseases. The dye Congo Red (CR) can cure cells infected with agents of the sheep TSE, scrapie, but is not used as a therapeutic or prophylactic agent in vivo, as its effects are small, possibly due to low blood-brain barrier permeability, and complicated by its intrinsic carcinogenicity. In this paper, the development is described of a structure-activity profile for CR by testing a series of analogues of this dye for their ability to inhibit the formation of the protease-resistant prion protein, PrP-res, a molecular marker for the infectious agent, in the scrapie-infected, SMB cell line. It was found that the central benzidine unit in CR, which gives the molecule potential carcinogenicity, can be replaced by other, less toxic moieties and that the sulphonate groups on the core molecule can be replaced by carboxylic acids, which should improve the brain permeability of these compounds. However, detailed dose-response curves were generated for several derivatives and they revealed that, while some compounds showed inhibition of PrP-res accumulation at high concentrations, at low concentrations they actually stimulated levels of PrP-res above control values.

Comparison of cattle and sheep colonic permeabilities to horseradish peroxidase and hamster scrapie prion protein in vitro

BACKGROUND

Paracellular permeability to solutes across the descending colon is much higher in cattle than sheep. This is a possible route for transmission of infective materials, such as scrapie prion.

AIMS

To compare the permeabilities of labelled scrapie prion protein and other macromolecules in bovine and ovine descending colons in vitro.

METHODS

Using fresh slaughterhouse material, transepithelial fluxes of macromolecules across colonic mucosae mounted in Ussing chambers were measured by monitoring transport of either enzyme activity or radioactivity.

RESULTS

The comparative bovine to ovine permeability ratio of the probes increased with molecular weight: from 3.1 (0.13) for PEG400 to 10.67 (0.20) (p<0.001) for PEG4000; and from 1.64 (0.17) for microperoxidase to 7.03 (0.20) (p<0.001) for horseradish peroxidase (HRP). The permeability of (125)I-labelled inactivated Syrian hamster scrapie prion protein (ShaPrP(sc)) was 7.02 (0.33)-fold higher in bovine than ovine colon (p<0.0025). In each species, the probe permeabilities decreased according to the formula: P = P(o). exp(-K.ra). The "ideal" permeabilities, P(o) are similar, however, K((ovine)) = 2.46 (0.20) cm/h/nm exceeds K((bovine)) = 0.85 (0.15) cm/h/nm (p<0.001) indicating that bovine colon has a higher proportion of wide pores than ovine. Image analysis confirmed that HRP permeated through the bovine mucosal layer via a pericryptal paracellular route much more rapidly than in sheep.

CONCLUSIONS

These data may imply that scrapie prion is transmitted in vivo more easily across the low resistance bovine colonic barrier than in other species.

Structural dependence of the cellular isoform of prion protein on solvent: spectroscopic characterization of an intermediate conformation

Using circular dichroism, fluorescence, and infrared spectroscopies, we studied the secondary structure of purified hamster PrP(C) in the presence of the mild, nonionic detergent octylglucoside. Under these native conditions, PrP(C) displayed an unexpectedly high beta-sheet component, intermediate between the values previously reported for PrP(Sc) and an isoform of PrP(C) isolated in a zwitterionic detergent. The structure of PrP(C) appeared to depend strongly on the detergent and/or phase. Switching from octylglucoside to zwitterion 3-14 drastically modified PrP secondary structure by increasing the alpha-helix while abolishing the beta-sheet component. In contrast, the conformation of PrP(C) in zwitterion was highly stable, since reverting to octylglucoside did not restore the original native structure. These and other results show that native PrP(C) in octylglucoside has some of the conformational characteristics that make the protein susceptible to conversion into PrP(Sc). Most importantly, this is the first study to demonstrate the intrinsic plasticity of the full-length native PrP(C) isolated from animal brains.

Species-independent inhibition of abnormal prion protein (PrP) formation by a peptide containing a conserved PrP sequence

Conversion of the normal protease-sensitive prion protein (PrP) to its abnormal protease-resistant isoform (PrP-res) is a major feature of the pathogenesis associated with transmissible spongiform encephalopathy (TSE) diseases. In previous experiments, PrP conversion was inhibited by a peptide composed of hamster PrP residues 109 to 141, suggesting that this region of the PrP molecule plays a crucial role in the conversion process. In this study, we used PrP-res derived from animals infected with two different mouse scrapie strains and one hamster scrapie strain to investigate the species specificity of these conversion reactions. Conversion of PrP was found to be completely species specific; however, despite having three amino acid differences, peptides corresponding to the hamster and mouse PrP sequences from residues 109 to 141 inhibited both the mouse and hamster PrP conversion systems equally. Furthermore, a peptide corresponding to hamster PrP residues 119 to 136, which was identical in both mouse and hamster PrP, was able to inhibit PrP-res formation in both the mouse and hamster cell-free systems as well as in scrapie-infected mouse neuroblastoma cell cultures. Because the PrP region from 119 to 136 is very conserved in most species, this peptide may have inhibitory effects on PrP conversion in a wide variety of TSE diseases.

Cellular biology of prion diseases

Prion diseases are fatal neurodegenerative disorders of humans and animals that are important because of their impact on public health and because they exemplify a novel mechanism of infectivity and biological information transfer. These diseases are caused by conformational conversion of a normal host glycoprotein (PrPC) into an infectious isoform (PrPSc) that is devoid of nucleic acid. This review focuses on the current understanding of prion diseases at the cell biological level. The characteristics of the diseases are introduced, and a brief history and description of the prion hypothesis are given. Information is then presented about the structure, expression, biosynthesis, and possible function of PrPC, as well as its posttranslational processing, cellular localization, and trafficking. The latest findings concerning PrPSc are then discussed, including cell culture systems used to generate this pathogenic isoform, the subcellular distribution of the protein, its membrane attachment, proteolytic processing, and its kinetics and sites of synthesis. Information is also provided on molecular models of the PrPC-->PrPSc conversion reaction and the possible role of cellular chaperones. The review concludes with suggestions of several important avenues for future investigation.

Endogenous proteolytic cleavage of normal and disease-associated isoforms of the human prion protein in neural and non-neural tissues

We have investigated the proteolytic cleavage of the cellular (PrPC) and pathological (PrPSc) isoforms of the human prion protein (PrP) in normal and prion-affected brains and in tonsils and platelets from neurologically intact individuals. The various PrP species were resolved after deglycosylation according to their electrophoretic mobility, immunoreactivity, Sarkosyl solubility, and, as a novel approach, resistance to endogenous proteases. First, our data show that PrPC proteolysis in brain originates amino-truncated peptides of 21 to 22 and 18 (C1) kd that are similar in different regions and are not modified by the PrP codon 129 genotype, a polymorphism that affects the expression of prion disorders. Second, this proteolytic cleavage of PrPC in brain is blocked by inhibitors of metalloproteases. Third, differences in PrPC proteolysis, and probably in Asn glycosylation and glycosylphosphatidylinositol anchor composition, exist between neural and non-neural tissues. Fourth, protease-resistant PrPSc cores in sporadic Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker F198S disease brains all have an intact C1 cleavage site (Met111-His112), which precludes disruption of a domain associated with toxicity and fibrillogenesis. Fifth, the profile of endogenous proteolytic PrPSc peptides is characteristic of each disorder studied, thus permitting the molecular classification of these prion diseases without the use of proteinase K and even a recognition of PrPSc heterogeneity within type 2 CJD patients having different codon 129 genotype and neuropathological phenotype. This does not exclude the role of additional factors in phenotypic expression; in particular, differences in glycosylation that may be especially relevant in the new variant CJD. Proteolytic processing of PrP may play an important role in the neurotropism and phenotypic expression of prion diseases, but it does not appear to participate in disease susceptibility.

Specific inhibition of in vitro formation of protease-resistant prion protein by synthetic peptides

The transmissible spongiform encephalopathies are characterized by the conversion of the protease-sensitive prion protein (PrPsen) into a protease-resistant isoform (PrPres) associated with the neuropathogenic process in vivo. Recently, PrPres has been shown to be capable of directly inducing the conversion of PrPsen to PrPres in a cell-free in vitro system. In the present experiments, various PrP peptides were studied for their ability to enhance or inhibit this cell-free conversion reaction. None of the synthetic peptides was able to confer protease-resistance to the labeled PrPsen molecules on their own. On the contrary, peptides from the central part of the hamster PrP sequence from 106 to 141 could completely inhibit the conversion induced by preformed PrPres. The presence of residues 119 and 120 from the highly hydrophobic sequence AGAAAAGA (position 113 to 120) was crucial for an efficient inhibitory effect. Fourier transform infrared spectroscopy analysis indicated that inhibitory peptides formed high beta-sheet aggregates under the conditions of the conversion reaction, but this was also true of certain peptides that were not inhibitory. Thus, the potential to form beta-sheeted aggregates may be necessary, but not sufficient, for peptides to act as inhibitors of PrPres formation. Clearly, the amino acid sequence of the peptide is also important for inhibition. The sequence specificity of the inhibition is consistent with the idea that residues in the vicinity of positions 106-141 of PrPres and/or PrPsen are critically involved in the intermolecular interactions that lead to PrPres formation.

Abnormal properties of prion protein with insertional mutations in different cell types

Inherited forms of the human transmissible spongiform encephalopathy Creutzfeldt-Jakob disease (CJD) have been associated with mutations in the normal soluble, protease-sensitive form of the host prion protein (PrP-sen). Normal PrP protein contains five copies of a repeating eight-amino acid region, and PrP molecules with six or more copies of this region are associated with disease in familial CJD. It has been hypothesized that these mutations might facilitate spontaneous formation of the abnormal, aggregated protease-resistant PrP isoform, PrP-res, associated with clinical CJD and other transmissible spongiform encephalopathies (TSE). In the present experiments, hamster PrP molecules with 5 (wild-type), 7, 9, or 11 copies of this repeat region were generated and expressed in mouse fibroblast cells or mouse neuroblastoma cells. In mouse fibroblast cells, mutant hamster PrP molecules expressing 7, 9, and 11 copies of the octapeptide repeat sequence showed altered cell surface expression, but both mutant and wild-type hamster PrP-sen molecules demonstrated abnormal properties of aggregation and increased protease resistance. By contrast in mouse neuroblastoma cells, hamster PrP-sen with 5, 9, and 11 octapeptide repeats were expressed normally on the cell surface, but only PrP-sen molecules with 9 or 11 copies of the repeat motif had abnormal properties of aggregation and increased protease resistance. Overall, regardless of cell type, hamster PrP molecules with greater than 7 octapeptide repeats were more aggregated and more protease-resistant than molecules with 7 repeats or less. However, these abnormal molecules were at least 1000-fold less protease-resistant than bona fide PrP-res derived from TSE-infected brain tissue, and they showed no increased ability to form PrP-res in a cell-free system.

Mice with gene targetted prion protein alterations show that Prnp, Sinc and Prni are congruent

Classical genetic analysis has identified Sinc/Prni as the major gene controlling mouse scrapie incubation time. Sinc/Prni is linked to Prnp, the gene encoding the prion protein (PrP). Prnp alleles express distinct PrP protein variants, PrP A and PrP B, which arise from codon 108L/F and 189 T/V dimorphisms. Prnp genotype segregates with incubation time length which suggests, but does not prove, that incubation time is controlled by PrP dimorphisms, and that the Sinc/Prni and Prnp loci are congruent. We have used gene targetting to construct mice in which the endogenous Prnp allele has been modified to express PrP B instead of PrP A. Challenge with a mouse-adapted BSE strain results in dramatically shortened incubation times and demonstrates that PrP dimorphisms at codon 108 and/or 189 control incubation time, and that Sinc/Prni and Prnp are congruent.

Human prion proteins expressed in Escherichia coli and purified by high-affinity column refolding

An efficient method is presented for the production of intact mammalian prion proteins and partial sequences thereof. As an illustration we describe the production of polypeptides comprising residues 23-231, 81-231, 90-231 and 121-231 of the human prion protein (hPrP). Polypeptides were expressed as histidine tail fusion proteins into inclusion bodies in the cytoplasm of Escherichia coli and refolded and oxidized while N-terminally immobilized on a nickel-NTA agarose resin. This 'high-affinity column refolding' facilitates the preparation of prion proteins by preventing protein aggregation and intermolecular disulfide formation. After elution from the resin the histidine tail can be removed using thrombin without cleaving the prion protein polypeptide chain. The same protocol as used here for hPrP has been successfully applied with bovine and murine prion proteins. The protein preparations are stable for weeks at room temperature in concentrated solution and are thus suitable for detailed structural studies. Preliminary biophysical characterization of hPrP(23-231) suggests that the C-terminal half of the polypeptide chain forms a well-structured globular domain, and that the N-terminal half does not form extensive regular secondary structures.

RNA aptamers specifically interact with the prion protein PrP

We have isolated RNA aptamers which are directed against the recombinant Syrian golden hamster prion protein rPrP23-231 (rPrPc) fused to glutathione S-transferase (GST). The aptamers did not recognize the fusion partner GST or the fusion protein GST::rPrP90-231 (rPrP27-30), which lacks 67 amino acids from the PrP N terminus. The aptamer-interacting region of PrPc was mapped to the N-terminal amino acids 23 to 52. Sequence analyses suggest that the RNA aptamers may fold into G-quartet-containing structural elements. Replacement of the G residues in the G quartet scaffold with uridine residues destroyed binding to PrP completely, strongly suggesting that the G quartet motif is essential for PrP recognition. Individual RNA aptamers interact specifically with prion protein in brain homogenates from wild-type mice (C57BL/6), hamsters (Syrian golden), and cattle as shown by supershifts obtained in the presence of anti-PrP antibodies. No interaction was observed with brain homogenates from PrP knockout mice (prn-p(0/0)). Specificity of the aptamer-PrP interaction was further confirmed by binding assays with antisense aptamer RNA or a mutant aptamer in which the guanosine residues in the G tetrad scaffold were replaced by uridine residues. The aptamers did not recognize PrP27-30 in brain homogenates from scrapie-infected mice. RNA aptamers may provide a first milestone in the development of a diagnostic assay for the detection of transmissible spongiform encephalopathies.

A novel hamster prion protein mRNA contains an extra exon: increased expression in scrapie

Prion protein (PrP) is the only known constituent of the agents (called prions) that cause fatal neurodegenerative diseases in animals and humans. PrP derives from a host protein encoded by a single copy gene having three known exons in mice, cattle and sheep but only two exons in hamsters and humans. We have identified and sequenced the missing exon from the hamster PrP gene. The new hamster PrP exon is 83% identical to mouse exon 2 and 76% identical to exon 2 from cattle and sheep. PrP mRNAs containing the new exon 2 (mRNA[1+2+3]) were expressed in the colliculi, frontal cortex and hippocampus of normal hamsters at approximately 30% to approximately 50% of the levels of the mRNA without exon 2 (mRNA[1+3]). Expression of PrP mRNA[1+2+3] was increased in the colliculi beginning 49 days after inoculation with scrapie prions and reached a level 2.5 times normal by day 77. Increased expression of PrP mRNA[1+2+3] in the colliculi correlated with expression of glial fibrillary acidic protein (GFAP) mRNA. Expression of GFAP and PrP mRNAs was not significantly increased in the hippocampus or the frontal cortex during the disease. Our study shows that exon 2 plays a role in regulating the cellular expression of hamster PrP and suggests that mRNA[1+2+3] may be preferentially expressed in hamster astrocytes.

A neurotoxic and gliotrophic fragment of the prion protein increases plasma membrane microviscosity

Prion-related encephalopathies are characterized by astrogliosis and nerve cell degeneration and loss. These lesions might be the consequence of an interaction between the abnormal isoform of the cellular prion protein that accumulates in nervous tissue and the plasma membranes. Previously we found that a synthetic peptide, homologous to residues 106-126 of the human prion protein, is fibrillogenic and toxic to neurons and trophic to astrocytes in vitro. This study dealt with the ability of the peptide to interact with membranes. Accordingly, we compared PrP 106-126 with different synthetic PrP peptides (PrP 89-106, PrP 127-147, a peptide with a scrambled sequences of 106-126, and PrP 106-126 amidated at the C-terminus) as to the ability to increase the microviscosity of artificial and natural membranes. The first three had no effect on nerve and glial cells in vitro, whereas the amidated peptide caused neuronal death. Using a fluorescent probe that becomes incorporated into the hydrocarbon core of the lipid bilayer and records the lipid fluidity, we found PrP 106-126 able to increase significantly the membrane microviscosity of liposomes and of all cell lines investigated. This phenomenon was associated with the distribution of the peptide over the cell surface, but not with changes in the membrane lipid or protein content, or with membrane lipid phase transitions. Accordingly, we deduced that increased membrane microviscosity was unrelated to changes in the membrane native components and was the result of increased lipid density following PrP 106-126 embedding into the lipid bilayer. No control peptides had comparable effects on the membrane microviscosity, except PrP 106-126 amidated at the C-terminus. Since the latter was as neurotoxic, but not as fibrillogenic, as PrP 106-126, we argued that the ability of PrP 106-126 to increase membrane microviscosity was unrelated to the propensity of the peptide to raise fibrils. Rather, it could be connected with the primary structure of PrP 106-126, characterized by two opposing regions, one hydrophilic and the other hydrophobic, that enabled the peptide to interact with the lipid bilayer. Based on these findings, we speculated that the glial and nerve cell involvement occurring in prion-related encephalopathies might be caused by the interaction with the plasma membrane of a PrP 106-126-like fragment or of the sequence spanning residues 106-126 of the abnormal isoform of the prion protein.

Metabolic changes associated with vacuolation in murine models of scrapie using in vitro 1H-NMR spectroscopy

In this study, metabolic changes in the 79A/C3H, ME7/VM, ME7/C3H, 87V/VM and 22A/SV scrapie mouse models were investigated during the clinical phase of the disease, using in vitro proton nuclear magnetic resonance spectroscopy. N-acetyl-aspartate was found to be significantly reduced in infected mice of the ME7/ C3H (40% reduction), ME7/VM (26%) and 79A/C3H (17%) models when compared to control mice, but not in the 87V/VM and 22A/SV models. The concentration of choline containing compounds and creatine remain unchanged in all models when compared with control murine brains. The level of N-acetyl-aspartate reduction correlated with the extent of grey-matter brain vacuolation. The levels of myo-inositol were found to be significantly increased in the ME7/VM (143%) and 79A/C3H (132%) models only and no significant changes were observed in the ME7/C3H, 87V/VM and 22A/SV models. These changes did not correspond to the severity of gliosis.

Partial unfolding and refolding of scrapie-associated prion protein: evidence for a critical 16-kDa C-terminal domain

The conversion of the normal form of prion protein (PrPC) to a disease-specific form (PrPSc) is a central event in scrapie and other transmissible spongiform encephalopathies. PrPSc is distinguished from PrPC by its insolubility and its resistance to proteolysis. PrPSc is also capable of converting 35S-PrPC, in vitro, into a form which is indistinguishable from PrPSc with respect to its protease-sensitivity. Both the "converting activity" and the protease-resistance of isolated hamster PrPSc can be at least partially eliminated by denaturation and recovered by renaturation, provided that the concentration of denaturant does not exceed a threshhold. This study was undertaken in order to localize the regions of native PrPSc structure that must remain intact to allow refolding. Proteinase K was used to digest exposed, denatured PrPSc sequences, and the residual fragments were characterized using anti-PrP antibodies directed toward four PrP epitopes. A 16-kDa fragment marked by an epitope within residues 143-156 remained protease-resistant under conditions which at least partially unfolded epitopes within residues 90-115 and 217-232. However, dilution of denaturant restored protease-resistance to these epitopes. This reversible unfolding was observed with both purified PrPSc and PrPSc in crude brain homogenates. Size fractionation of partially GdnHCl-solubilized PrPSc revealed that only the insoluble aggregates retained the ability to refold, consistent with the hypothesis that native PrPSc is an ordered aggregate. When the threshold denaturant concentration was exceeded, both protease-resistance of the 16-kDa C-terminal domain and converting activity were irreversibly destroyed. These results suggest that the in vitro converting activity requires ordered, protease-resistant PrPSc aggregates and that a critical aspect of the PrPSc structure is the folding of a particularly stable approximately 16-kDa C-terminal domain.

The Ninth Datta Lecture. Molecular biology of transmissible spongiform encephalopathies

The prion, the transmissible agent that causes spongiform encephalopathies such as scrapie, bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease, is believed to be devoid of nucleic acid and identical with PrPSc, a modified form of the normal host protein PrPC which is encoded by the single copy gene Prnp. The 'protein only' hypothesis proposes that PrPSc, when introduced into a normal host, causes the conversion of PrPC into PrPSc; it therefore predicts that an animal devoid of PrPC should be resistant to prion diseases. We generated homozygous Prnp(olo) ('PrP knockout') mice and showed that, after inoculation with prions, they remained free of scrapie for at least 2 years while wild-type controls all died within 6 months. There was no propagation of prions in the Prnp(olo) animals. Surprisingly, heterozygous Prnp(ol+) mice, which express PrPC at about half the normal level, also showed enhanced resistance to scrapie disease despite high levels of infectious agent and PrPSc in the brain early on. After introduction of murine PrP transgenes Prnp(olo) mice became highly susceptible to mouse but not to hamster prions, while the insertion of Syrian hamster PrP transgenes rendered them susceptible to hamster but to a much lesser extent to mouse prions. These complementation experiments paved the way to the application of reverse genetics. We have prepared animals transgenic for genes encoding PrP with amino terminal deletions of various lengths and have found that PrP lacking 48 amino proximal amino acids, which comprise four of the five octa repeats of PrP, is still biologically active.

Effect of the D178N mutation and the codon 129 polymorphism on the metabolism of the prion protein

Prion diseases are thought to be caused by the conversion of the normal, or cellular, prion protein (PrPC)(PrPres). There are three familial forms of human prion disease, Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker syndrome, and fatal familial insomnia (FFI) which are all expressed at advanced age despite the congenital presence of the mutant prion protein (PrPM). The cellular mechanisms that result in the age-dependent conversion of PrPM into PrPres and the unique phenotypes associated with each PrPM are unknown. FFI and a familial type of Creutzfeldt-Jakob disease (CJD178), share the D178N mutation in the PrP gene but have distinct phenotypes linked to codon 129, the site of a methionine/valine polymorphism (129M/V). We analyzed PrP processing in cells transfected with constructs reproducing the FFI and CJD178 genotypes. The D178N mutation results in instability of the mutant PrP which is partially corrected by N-glycosylation. Hence, only the glycosylated forms of PrPM reach the cell surface whereas the unglycosylated PrPM is also under-represented in the brain of FFI patients validating the cell model. These results offer new insight into the effect of the D178N mutation on the metabolism of the prion protein.

High-level expression and characterization of a purified 142-residue polypeptide of the prion protein

The major, and possible only, component of the infectious prion is the scrapie prion protein (PrPSc); the protease resistant core of PrPSc is PrP 27-30, a protein of approximately 142 amino acids. PrPSc is derived from the cellular PrP isoform (PrPC) by a post-transliatonal process in which a profound conformational change occurs. Syrian hamster (SHa) PrP genes of varying length ranging from the N- and C- terminally truncated 90-228 up to the full-length mature protein 23-231 were inserted into various secretion and intracellular expression vectors that were transformed into Escherichia coli deficient for proteases. Maximum expression was obtained for a truncated SHaPrP containing residues 90-231, which correspond to the sequence of PrP 27-30; disruption of the bacteria using a microfluidizer produced the highest yields of this protein designated rPrP. After solubilization of rPrP in 8 M GdnHC1, it was purified by size exclusion chromatography and reversed phase chromatography. During purification the recovery was approximately 50%, and from each liter of E. coli culture, approximately 50 mg of purified rPrP was obtained. Expression of the longer species containing the basic N-terminal region was less successful and was not pursued further. The primary structure of rPrP was verified by Edman sequencing and mass spectrometry, and secondary structure determined by circular dichroism and Fourier transform infrared spectroscopy. When rPrP was purified under reducing conditions, it had a high beta-sheet content and relatively low solubility similar to PrPSc, particularly at pH values > 7. Refolding of rPrP by oxidation to form a disulfide bond between the two Cys residues of this polypeptide produced a soluble protein with a high alpha-helical content similar to PrPC. These multiple conformations of rPrP are reminiscent of the structural plurality that characterizes the naturally occurring PrP isoforms. The high levels of purified rPrP which can now be obtained should facilitate determination of the multiple tertiary structures that Prp can adopt.

Prion protein amyloidosis

The prion protein (PrP) plays an essential role in the pathogenesis of a group of sporadic, genetically determined and infectious fatal degenerative diseases, referred to as "prion diseases", affecting the central nervous system of humans and other mammals. The cellular PrP is encoded by a single copy gene, highly conserved across mammalian species. In prion diseases, PrP undergoes conformational changes involving a shift from alpha-helix to beta-sheet structure. This conversion is important for PrP amyloidogenesis, which occurs to the highest degree in the genetically determined Gerstmann-Sträussler-Scheinker disease (GSS) and prion protein cerebral amyloid angiopathy (PrP-CAA), while it is less frequently seen in other prion diseases. GSS and PrP-CAA are associated with point mutations of the prion protein gene (PRNP); these conditions show a broad spectrum of clinical presentation, the main signs being ataxia, spastic paraparesis, extrapyramidal signs and dementia. In GSS, parenchymal amyloid may be associated with spongiform changes or neurofibrillary lesions; in PrP-CAA, vascular amyloid is associated with neurofibrillary lesions. A major component of the amyloid fibrils in the two diseases is a 7 kDa peptide, spanning residues 81-150 of PrP.

The association between PrP and infectivity in scrapie and BSE infected mouse brain

The structure of the scrapie agent remains unknown. However, scrapie infectivity tends to co-sediment with an infection specific fraction of the glycoprotein PrP (PrPSc) under conditions which solubilise the normal form of this protein (PrPc); accordingly, PrP has been proposed as a candidate component of the agent. To investigate this further we have been examining a new scrapie-related murine model in conjunction with established scrapie models. A bovine spongiform encephalopathy (BSE) derived murine model has short incubation periods, high infectivity titre and low amounts of PrP deposited in the brain. A membrane fraction from scrapie/BSE infected brain is solubilised with Sarkosyl at pH > or = 9.0. Most PrP is also solubilised. In models of the disease with little deposition of the PrP in the brain, this solubilisation step is particularly effective in reducing the amounts of PrP sedimented from brain extracts. Gradient centrifugation of the sedimented fraction shows further separation of infectivity and the residual PrP. It is concluded that at least some PrPSc in the brain need not be associated directly with infectious agents but is deposited in brain solely as a pathological product of infection. However, a residual sedimentable fraction contains PrP which may be a component of the agent.

Aggregates of scrapie-associated prion protein induce the cell-free conversion of protease-sensitive prion protein to the protease-resistant state

INTRODUCTION

Scrapie infection instigates the in vivo conversion of normal, protease-sensitive prion protein (PrPC) into a protease-resistant form (PrPSc) by an unknown mechanism. In vitro studies have indicated that PrPSc can induce this conversion, consistent with proposals that PrPSc itself might be the infectious scrapie agent. Using this cell-free model of the PrPC to PrPSc conversion, we have studied the dependence of conversion on reactant concentration, and the properties of the PrPSc-derived species that has converting activity.

RESULTS

The cell-free conversion of 35S PrPC to the proteinase K-resistant form was dependent on the reaction time and initial concentrations of PrPSc (above an apparent minimum threshold concentration) and 35S PrPC. Analysis of the physical size of the converting activity indicated that detectable converting activity was associated only with aggregates. Under mildly chaotropic conditions, which partially disaggregated PrPSc and enhanced the converting activity, the active species were heterogeneous in size, but larger than either effectively solubilized PrP or molecular weight standards of approximately 2000 kDa.

CONCLUSIONS

The entity responsible for the converting activity was many times larger than a soluble PrP monomer and required a threshold concentration of PrPSc. These results are consistent with a nucleated polymerization mechanism of PrPSc formation and inconsistent with a heterodimer mechanism.

Truncated forms of the human prion protein in normal brain and in prion diseases

The cellular form of the prion protein (PrPc) is a glycoprotein anchored to the cell membrane by a glycosylphosphatidylinositol moiety. An aberrant form of PrPc that is partially resistant to proteases, PrPres, is a hallmark of prion diseases, which in humans include Cruetzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia. We have characterized the major forms of PrP in normal and pathological human brains. A COOH-terminal fragment of PrPc, designated C1, is abundant in normal and CJD brains as well as in human neuroblastoma cells. Sequence analysis revealed that C1 contains alternative NH2 termini starting at His-111 or Met-112. Like PrPc, C1 is glycosylated, anchored to the cell membrane, and is heat-stable. Consistent with the lack of the NH2-terminal region of PrPc, C1 is more acidic than PrPc and does not bind heparin. An additional fragment longer than C1, designated C2, is present in substantial amounts in CJD brains. Like PrPres, C2 is resistant to proteases and is detergent-insoluble. Our data indicate that C1 is a major product of normal PrPc metabolism, generated by a cleavage that disrupts the neurotoxic and amyloidogenic region of PrP comprising residues 106-126. This region remains intact in C2, suggesting a role for C2 in prion diseases.

Overexpression of active Syrian golden hamster prion protein PrPc as a glutathione S-transferase fusion in heterologous systems

This article describes a procedure which permits for the first time the isolation of the prion protein PrPc from the Syrian golden hamster in heterologous systems. Using a glutathione S-transferase (GST) fusion approach, milligram amounts of stable, soluble, and homogeneous GST::PrPc protein were obtained in Escherichia coli and with baculovirus-infected insect cells. Authentic PrPc was released from the immobilized fusion protein by direct cleavage with thrombin. GST::PrPc expressed in these two expression systems and also authentic PrPc released by thrombin cleavage were recognized by a polyclonal antibody directed against amino acid 95 to 110 of the golden hamster PrPc protein. GST::PrPc was not detected by a monoclonal antibody recognizing the region encompassing amino acids 138 to 152 of the human prion protein. The fusion protein was sensitive to proteinase K digestion, demonstrating that the cellular rather than the proteinase K-resistant scrapie isoform was produced.

A 60-kDa prion protein (PrP) with properties of both the normal and scrapie-associated forms of PrP

Scrapie is a transmissible spongiform encephalopathy of sheep and other mammals in which disease appears to be caused by the accumulation of an abnormal form of a host protein, prion protein (PrP), in the brain and other tissues. The process by which the normal protease-sensitive form of PrP is converted into the abnormal protease-resistant form is unknown. Several hypotheses predict that oligomeric forms of either the normal or abnormal PrP may act as intermediates in the conversion process. We have now identified a 60-kDa PrP derived from hamster PrP expressed in murine neuroblastoma cells. Peptide mapping studies provided evidence that the 60-kDa PrP was composed solely of PrP and, based on its molecular mass, appeared to be a PrP dimer. The 60-kDa PrP was not dissociated under several harsh denaturing conditions, which indicated that it was covalently linked. It was similar to the disease-associated form of PrP in that it formed large aggregates. However, it resembled the normal form of PrP in that it was sensitive to proteinase K and had a short metabolic half-life. The 60-kDa PrP, therefore, had characteristics of both the normal and disease-associated forms of PrP. Formation and aggregation of the 60-kDa hamster PrP occurs in uninfected mouse neuroblastoma cells, which suggests that hamster PrP has a predisposition to aggregate even in the absence of scrapie infectivity. Similar 60-kDa PrP bands were identified in scrapie-infected hamster brain but not in uninfected brain. Therefore, a 60-kDa molecule might participate in the scrapie-associated conversion of protease-sensitive PrP to protease-resistant PrP.

There is nothing permanent except change. The emergence of new virus diseases

The sudden appearance of apparently new viruses with pathogenic potential is of fundamental importance in medical microbiology and a constant threat to humans and animals. The emergence of a "new" pathogen is not an isolated event, as for instance the frequent appearance of new influenza virus strains demonstrates. Often the new virus strains co-circulate with the older strains in a susceptible population, but a replacement of the older strains has been also observed. In rare instances the new viruses can cause dramatic epidemics or pandemics, such as those observed with the human immunodeficiency virus, canine parvovirus, or most recently, with the agent of bovine spongiform encephalopathy in the United Kingdom. The mechanisms of the emergence are not always clearly understood, but an altered host range appears to be a common event. Whether a true change in host range occurs, or whether the virus adapted to the host and replicated more efficiently, is often unknown. This review tries to summarize the facts that are known about a wide variety of "new" viruses of mammals, such as the simian, human and feline lentiviruses, the feline coronaviruses, the feline parvoviruses, the carnivore morbilliviruses, the influenza A viruses, and the transmissible spongiform encephalopathies. A particular emphasis will be put on the genetic mechanisms that might have taken place and that might have been responsible for their sudden appearance.

Capillary electrophoresis of the scrapie prion protein from sheep brain

Scrapie in sheep and goats causes a progressive, degenerative disease of the central nervous system and is the prototype of other transmissible spongiform encephalopathies (TSE) found in humans and in animals. In samples of TSE-affected brains, unique rod-shaped structures are found and are infectious. These rods are composed of a protease-resistant, post-translationally modified cellular protein (PrPsc) that has a molecular mass of ca. 27,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Laboratory tests used for the diagnosis of scrapie detect PrPsc. The overall concentration of PrPsc in tissues is low. The present methods to diagnose scrapie are lengthy, require relatively large quantities of starting material to detect PrPsc and lack sensitivity. We explored the use of free zone capillary electrophoresis and immunocomplex formation to detect PrPsc in the brain tissue of infected sheep. Brain tissue from both infected (as confirmed by histological and biological tests) and from normal animals was used to prepare the PrPsc. After treatment with proteinase K and non-ionic detergents, PrPsc was solubilized and reacted with a rabbit antiserum specific for a peptide of the prion protein. Immunocomplex formation was observed for the samples from scrapie-infected brain but not for samples from normal brain. When a fluorescein-labeled goat anti-rabbit immunoglobulin was used as a second antibody, the detection of immunocomplex formation was enhanced both by the immunological technique and by using laser-induced fluorescence for detection. This same rabbit antiserum was used on immunoblot analysis. Three bands were observed for material from an infected sheep but none in preparations from brain material from normal sheep.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell-free formation of protease-resistant prion protein

The infectious agent (or 'prion') of the transmissible spongiform encephalopathies (TSEs) such as scrapie resembles a virus in that it replicates in vivo and has distinct strains, but it was postulated long ago to contain only protein. More recently, PrPSc, a pathogenic, scrapie-associated form of the host-encoded prion protein (PrP), was identified as a possible primary TSE agent protein. PrPSc is defined biochemically by its insolubility and resistance to proteases and is derived post-translationally from normal, protease-sensitive PrP (PrPc). The conversion seems to involve conformational change rather than covalent modification. However, the conversion mechanism and the relationship of PrPSc formation to TSE agent replication remain unclear. Here we report the conversion of PrPc to protease-resistant forms similar to PrPSc in a cell-free system composed of substantially purified constituents. This conversion was selective and required the presence of preexisting PrPSc, providing direct evidence that PrPSc derives from specific PrPc-PrPSc interactions.

Heterologous PrP molecules interfere with accumulation of protease-resistant PrP in scrapie-infected murine neuroblastoma cells

Mutations within a host cellular protein, PrP, have been associated with disease in the transmissible spongiform encephalopathies. Murine neuroblastoma cells persistently infected with mouse scrapie accumulate protease-resistant PrP (PrP-res), the abnormal form of PrP associated with disease in the transmissible spongiform encephalopathies. These cells provide a controlled system in which to study the molecular interactions which are important in the formation of PrP-res. We have expressed recombinant PrP molecules in mouse scrapie-infected murine neuroblastoma cells and assayed the effect of these heterologous PrP genes on the formation and accumulation of PrP-res. The results demonstrate that expression of heterologous PrP molecules which differ from the endogenous PrP by as little as one amino acid can profoundly interfere with the overall accumulation of PrP-res. The data suggest that precise interactions between homologous PrP molecules are important in PrP-res accumulation and that heterologous PrP molecules can block these interactions.

Scrapie-associated PrP accumulation and its inhibition: revisiting the amyloid-glycosaminoglycan connection

An abnormal protease-resistant isoform of the protein PrP accumulates in the brain of hosts with transmissible spongiform encephalopathies (TSEs) and appears to be centrally involved in TSE pathogenesis. Studies with scrapie-infected tissue culture cells have indicated that this abnormal PrP is formed from an apparently normal precursor on the plasma membrane or along an endocytic pathway to the lysosomes. Inhibitors of protease-resistant PrP accumulation might serve as tools for studying the basic mechanism of protease-resistant PrP formation and as potential drugs for TSE therapy. Using scrapie-infected neuroblastoma cells to screen for such compounds in vitro, we found that the amyloid binding dye Congo red and certain sulfated glycans potently inhibited the accumulation of protease-resistant PrP in scrapie-infected cells without apparent effects on the metabolism of the normal isoform. The relative potencies of the sulfated glycans corresponded with their previously determined anti-scrapie activities in vivo, suggesting that the prophylactic effects of sulfated polyanions may be due to inhibition of protease-resistant PrP accumulation. Since protease-resistant PrP amyloid is known to contain sulfated glycosaminoglycans, as do other naturally derived amyloids, we hypothesize that these sulfated inhibitors competitively block binding between PrP and endogenous glycosaminoglycans that is important for its accumulation in a protease-resistant, potentially amyloidogenic state. Drugs which interfere with this (pre)amyloid-glycosaminoglycan interaction may be useful for treating a variety of amyloidoses.

Murine scrapie-infected neurons in vivo release excess prion protein into the extracellular space

An originally heretical proposition that the transmissible spongiform encephalopathies are caused by a host-coded protein (the prion hypothesis) is now current dogma. Indeed these disorders are commonly called prion diseases but the prion hypothesis provides no readily acceptable explanation for the source of the informational component of the agent necessary to code for the diversity of strains of scrapie. Ultrastructural immunolocalisation of prion protein (PrP) in murine scrapie shows that PrP accumulates in association with the plasmalemma of neurones, diffusing from the neuronal cell surface into the extracellular space around small neurites prior to aggregation and fibril assembly. These events occur without the involvement of other cell types. The area of neuropil infiltrated with extracellular PrP around infected neurons and neurites indicates that the form of PrP initially produced is not immediately amyloidogenic.

Properties of the scrapie prion protein: quantitative analysis of protease resistance

The disease-specific isoform of the prion protein (PrPSc) is an essential part of the infectious particle which causes spongiform degeneration in various mammalian species. PrPSc differs from PrP of normal animals (PrPc) by its relative protease resistance. The physical nature of this difference is still unknown. We analyzed the protease resistance of PrPSc quantitatively using an enzyme-linked immunofiltration assay. PrPSc was rendered completely protease-sensitive at alkaline pH or in > 1.5 M guanidinium thiocyanate (GdnSCN). Denaturation in 4 M GdnSCN completely abolished the protease resistance of PrPSc within 15 min, while denaturation in 7.2 M urea showed a slower time course. In the presence of ethanol, PrPSc was protected from denaturation by GdnSCN or alkaline pH. Denaturation curves were used to calculate the free energy (delta GD) as a function of different denaturant concentrations. Linear regression of delta GD values was used to extrapolate the free energy in the absence of denaturants (delta GH2O), yielding similar values (delta GH2O,GdnSCN = -2.3 kcal/mol; delta GH2O,urea = -3.1 kcal/mol). The linear relationship between delta GD and the denaturant concentration is suggestive of a two-state model involving the conformational change of a single protein domain. This is also reflected in the small number of side chains (11.6) additionally exposed to the solvent upon conversion of PrPSc to its protease-sensitive isoform. Our results suggest that only minor rearrangements of the structure of PrP are needed to abolish the protease resistance of PrPSc.

Correlation between the results of a histopathological examination and the detection of abnormal brain fibrils in the diagnosis of bovine spongiform encephalopathy

A statistical comparison was made between the results of the statutory neurohistopathological method for the post mortem diagnosis of bovine spongiform encephalopathy (BSE) and the detection of abnormal brain fibrils (SAF). A total of 773 suspect cases was examined by both methods; it comprised 531 animals born before the feeding of ruminant-derived protein to ruminant species was prohibited and 242 born after the ban. The relative sensitivities and specificities of the methods were calculated for the diagnosis of clinically suspected BSE. The agreement between the results of the methods was excellent for 331 of the cases born before the ban and for all the cases born after it. In these cases the samples were not autolysed. For the 200 cases in which autolysis was recorded there was poor agreement between the diagnostic methods and this was attributed to an apparently reduced specificity of the histopathological diagnosis in the autolysed material. Despite the potentially greater specificity of fibril detection in the diagnosis of scrapie-like disease, this study indicates that a reliance on fibril detection alone may result in some false negative diagnoses, probably owing to the inadequate sampling of the tissues.