TSE agent strains and PrP: reconciling structure and function

CJD and Scrapie Require Agent-Associated Nucleic Acids for Infection

Unlike Alzheimer's and most other neurodegenerative diseases, Transmissible Spongiform Encephalopathies (TSEs) are all caused by actively replicating infectious particles of viral size and density. Different strain-specific TSE agents cause CJD, kuru, scrapie and BSE, and all behave as latent viruses that evade adaptive immune responses and can persist for years in lymphoreticular tissues. A foreign viral structure with a nucleic acid genome best explains these TSE strains and their endemic and epidemic spread in susceptible species. Nevertheless, it is widely believed that host prion protein (PrP), without any genetic material, encodes all these strains. We developed rapid infectivity assays that allowed us to reproducibly isolate infectious particles where >85% of the starting titer separated from the majority of host components, including PrP. Remarkably, digestion of all forms of PrP did not reduce brain particle titers. To ask if TSE agents, as other viruses, require nucleic acids, we exposed high titer FU-CJD and 22L scrapie particles to potent nucleases. Both agent-strains were propagated in GT1 neuronal cells to avoid interference by complex degenerative brain changes that can impede nuclease digestions. After exposure to nucleases that are active in sarkosyl, infectivity of both agents was reproducibly reduced by ≥99%. No gold-stained host proteins or any form of PrP were visibly altered by these nucleases. In contrast, co-purifying protected mitochondrial DNA and circular SPHINX DNAs were destroyed. These findings demonstrate that TSE agents require protected genetic material to infect their hosts, and should reopen investigation of essential agent nucleic acids. J. Cell. Biochem. 117: 1947-1958, 2016. © 2016 Wiley Periodicals, Inc.

Insights into Mechanisms of Chronic Neurodegeneration

Chronic neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and prion diseases are characterised by the accumulation of abnormal conformers of a host encoded protein in the central nervous system. The process leading to neurodegeneration is still poorly defined and thus development of early intervention strategies is challenging. Unique amongst these diseases are Transmissible Spongiform Encephalopathies (TSEs) or prion diseases, which have the ability to transmit between individuals. The infectious nature of these diseases has permitted in vivo and in vitro modelling of the time course of the disease process in a highly reproducible manner, thus early events can be defined. Recent evidence has demonstrated that the cell-to-cell spread of protein aggregates by a “prion-like mechanism” is common among the protein misfolding diseases. Thus, the TSE models may provide insights into disease mechanisms and testable hypotheses for disease intervention, applicable to a number of these chronic neurodegenerative diseases.

Post-translational changes to PrP alter transmissible spongiform encephalopathy strain properties

The agents responsible for transmissible spongiform encephalopathies (TSEs), or prion diseases, contain as a major component PrP(Sc), an abnormal conformer of the host glycoprotein PrP(C). TSE agents are distinguished by differences in phenotypic properties in the host, which nevertheless can contain PrP(Sc) with the same amino-acid sequence. If PrP alone carries information defining strain properties, these must be encoded by post-translational events. Here we investigated whether the glycosylation status of host PrP affects TSE strain characteristics. We inoculated wild-type mice with three TSE strains passaged through transgenic mice with PrP devoid of glycans at the first, second or both N-glycosylation sites. We compared the infectious properties of the emerging isolates with TSE strains passaged in wild-type mice by in vivo strain typing and by the standard scrapie cell assay in vitro. Strain-specific characteristics of the 79A TSE strain changed when PrP(Sc) was devoid of one or both glycans. Thus infectious properties of a TSE strain can be altered by post-translational changes to PrP which we propose result in the selection of mutant TSE strains.

Mechanism of PrP-amyloid formation in mice without transmissible spongiform encephalopathy

Gerstmann-Sträussler-Scheinker (GSS) P102L disease is a familial form of a transmissible spongiform encephalopathy (TSE) that can present with or without vacuolation of neuropil. Inefficient disease transmission into 101LL transgenic mice was previously observed from GSS P102L without vacuolation. However, several aged, healthy mice had large plaques composed of abnormal prion protein (PrP(d)). Here we perform the ultrastructural characterization of such plaques and compare them with PrP(d) aggregates found in TSE caused by an infectious mechanism. PrP(d) plaques in 101LL mice varied in maturity, with some being composed of deposits without visible amyloid fibrils. PrP(d) was present on cell membranes in the vicinity of all types of plaques. In contrast to the unicentric plaques seen in infectious murine scrapie, the plaques seen in the current model were multicentric and were initiated by protofibrillar forms of PrP(d) situated on oligodendroglia, astrocytes and neuritic cell membranes. We speculate that the initial conversion process leading to plaque formation begins with membrane-bound PrP(C) but that subsequent fibrillization does not require membrane attachment. We also observed that the membrane alterations consistently seen in murine scrapie and other infectious TSEs were not present in 101LL mice with plaques, suggesting differences in the pathogenesis of these conditions.

Characterization of the effect of heat on agent strains of the transmissible spongiform encephalopathies

The causal agents of the transmissible spongiform encephalopathy (TSE) diseases, sometimes called prion diseases, are characterized by high resistance to inactivation with heat. Results from thermal inactivation experiments on nine TSE strains, seven passaged in two PrP genotypes, showed differences in sensitivity to heat inactivation ranging over 17 °C. In addition, the rate of inactivation with increasing temperature varied between TSE models. In some cases passage in an alternative PrP genotype had little effect on the resulting inactivation properties, but for others the infectious agent was inactivated at lower temperatures. No strain with higher thermostability properties was selected. The effect of mixing two TSE strains, to see whether their properties were affected through interaction with each other, was also examined. The results showed that both strains behaved as expected from the behaviour of the unmixed controls, and that the strain responsible for inducing TSE disease could be identified. There was no evidence of a direct effect on intrinsic strain properties. Overall, the results illustrate the diversity in properties of TSE strains. They require intrinsic molecular properties of TSE agents to accommodate high resistance to inactivation and a mechanism, independent of the host, to directly encode these differences. These findings are more readily reconciled with models of TSE agents with two separate components, one of which is independent of the host and comprises a TSE-specific nucleic acid, than with models based solely on conformational changes to a host protein.

Distinct stability states of disease-associated human prion protein identified by conformation-dependent immunoassay

The phenotypic and strain-related properties of human prion diseases are, according to the prion hypothesis, proposed to reside in the physicochemical properties of the conformationally altered, disease-associated isoform of the prion protein (PrP(Sc)), which accumulates in the brains of patients suffering from Creutzfeldt-Jakob disease and related conditions, such as Gerstmann-Straussler-Scheinker disease. Molecular strain typing of human prion diseases has focused extensively on differences in the fragment size and glycosylation site occupancy of the protease-resistant prion protein (PrP(res)) in conjunction with the presence of mutations and polymorphisms in the prion protein gene (PRNP). Here we report the results of employing an alternative strategy that specifically addresses the conformational stability of PrP(Sc) and that has been used previously to characterize animal prion strains transmitted to rodents. The results show that there are at least two distinct conformation stability states in human prion diseases, neither of which appears to correlate fully with the PrP(res) type, as judged by fragment size or glycosylation, the PRNP codon 129 status, or the presence or absence of mutations in PRNP. These results suggest that conformational stability represents a further dimension to a complete description of potentially phenotype-related properties of PrP(Sc) in human prion diseases.

Spontaneous generation of prion infectivity in fatal familial insomnia knockin mice

A crucial tenet of the prion hypothesis is that misfolding of the prion protein (PrP) induced by mutations associated with familial prion disease is, in an otherwise normal mammalian brain, sufficient to generate the infectious agent. Yet this has never been demonstrated. We engineered knockin mice to express a PrP mutation associated with a distinct human prion disease, fatal familial insomnia (FFI). An additional substitution created a strong transmission barrier against pre-existing prions. The mice spontaneously developed a disease distinct from that of other mouse prion models and highly reminiscent of FFI. Unique pathology was transmitted from FFI mice to mice expressing wild-type PrP sharing the same transmission barrier. FFI mice were highly resistant to infection by pre-existing prions, confirming infectivity did not arise from contaminating agents. Thus, a single amino acid change in PrP is sufficient to induce a distinct neurodegenerative disease and the spontaneous generation of prion infectivity.

Differential display detects host nucleic acid motifs altered in scrapie-infected brain

The transmissible spongiform encephalopathies (TSEs) including scrapie have been attributed to an infectious protein or prion. Infectivity is allied to conversion of the endogenous nucleic-acid-binding protein PrP to an infectious modified form known as PrP(sc). The protein-only theory does not easily explain the enigmatic properties of the agent including strain variation. It was previously suggested that a short nucleic acid, perhaps host-encoded, might contribute to the pathoetiology of the TSEs. No candidate host molecules that might explain transmission of strain differences have yet been put forward. Differential display is a robust technique for detecting nucleic acid differences between two populations. We applied this technique to total nucleic acid preparations from scrapie-infected and control brain. Independent RNA preparations from eight normal and eight scrapie-infected (strain 263K) hamster brains were randomly amplified and visualized in parallel. Though the nucleic acid patterns were generally identical in scrapie-infected versus control brain, some rare bands were differentially displayed. Molecular species consistently overrepresented (or underrepresented) in all eight infected brain samples versus all eight controls were excised from the display, sequenced, and assembled into contigs. Only seven ros contigs (RNAs over- or underrepresented in scrapie) emerged, representing <4 kb from the transcriptome. All contained highly stable regions of secondary structure. The most abundant scrapie-only ros sequence was homologous to a repetitive transposable element (LINE; long interspersed nuclear element). Other ros sequences identified cellular RNA 7SL, clathrin heavy chain, visinin-like protein-1, and three highly specific subregions of ribosomal RNA (ros1-3). The ribosomal ros sequences accurately corresponded to LINE; retrotransposon insertion sites in ribosomal DNA (p<0.01). These differential motifs implicate specific host RNAs in the pathoetiology of the TSEs.

Cell-free propagation of prion strains

Prions are the infectious agents responsible for prion diseases, which appear to be composed exclusively by the misfolded prion protein (PrP(Sc)). Disease is transmitted by the autocatalytic propagation of PrP(Sc) misfolding at the expense of the normal prion protein. The biggest challenge of the prion hypothesis has been to explain the molecular mechanism by which prions can exist as different strains, producing diseases with distinguishable characteristics. Here, we show that PrP(Sc) generated in vitro by protein misfolding cyclic amplification from five different mouse prion strains maintains the strain-specific properties. Inoculation of wild-type mice with in vitro-generated PrP(Sc) caused a disease with indistinguishable incubation times as well as neuropathological and biochemical characteristics as the parental strains. Biochemical features were also maintained upon replication of four human prion strains. These results provide additional support for the prion hypothesis and indicate that strain characteristics can be faithfully propagated in the absence of living cells, suggesting that strain variation is dependent on PrP(Sc) properties.

Glycosylation of prion strains in transmissible spongiform encephalopathies

This is a review of prion replication in the context of the cell biology of membrane proteins especially folding quality control in the endoplasmic reticulum (ER). Transmissible spongiform encephalopathies, such as scrapie and BSE, are infectious lethal diseases of mammalian neurons characterised by conversion of the normal membrane protein PrPC to the disease-associated conformational isomer called PrPSc. PrPSc, apparently responsible for infectivity, forms a number of different conformations and specific N-glycosylation site occupancies that correlate with TSE strain differences. Dimerisation and specific binding of PrPc and PrPSc seems critical in PrPSc biosynthesis and is influenced by N-glycosylation and disulfide bond formation. PrPsc can be amplified in vitro but new glycosylation cannot occur in cell free environments without the special conditions of microsome mediated in vitro translation, thus strain specific glycosylation of PrPSc formed in vitro in the absence of these conditions must take place by imprintation of PrPc from existing glycosylation site-occupancies. PrPSc formed in cell free homogenates is not infectious pointing to events necessary for infectivity that only occur in intact cells. Such events may include glycosylation site occupancy and ER folding chaperone activity. In the biosynthetic pathway of PrPSc, early acquisition of sensitivity of the GPI anchor to phospholipase C can be distinguished from the later acquisition of protease resistance and detergent insolubility. By analogy to the co-translational formation of the MHC I loading complex, it is postulated that PrPSc or its specific peptides could imprint nascent PrPc chains thereby ensuring its own folds and the observed glycosylation site occupancy ratios of strains.

Canadian Association of Neurosciences Review: prion protein and prion diseases: the good and the bad

In the 1700's a strange new disease affecting sheep was recognized in Europe. The disease later became known as "Scrapie" and was the first of a family of similar diseases affecting a number of species that are now known as the Transmissible Spongiform Encephalopathies (TSEs). The appearance of a new disease in humans linked to the consumption of meat products from infected cattle has stimulated widespread public concern and scientific interest in the prion protein and related diseases. Nearly 300 years after the first report, these diseases still merit the descriptor "strange". This family of diseases is characterized by a unique profile of histological changes, can be transmitted as inherited or acquired diseases, as well as apparent sporadic spontaneous generation of the disease. These diseases are believed by many, to be caused by a unique protein only infectious agent. The "prion protein" (PrPC), a term first coined by Stanley Prusiner in 1982 is crucial to the development of these diseases, apparently by acting as a substrate for an abnormal disease associated form. However, aside from being critical to the pathogenesis of the disease, the function of PrPC, which is expressed in all mammals, has defied definitive description. Several roles have been proposed on the basis of in vitro studies, however, thus far, in vivo confirmation has not been forthcoming. The biological features of PrPC also seem to be unusual. Numerous mouse models have been generated in an attempt to understand the pathogenesis of these diseases. This review summarizes the current state of histological features, the etiologic agent, the normal metabolism and the function of the prion protein, as well as the limitations of the mouse models.

Investigation by bioassay of the efficacy of sodium hydroxide treatment on the inactivation of mouse-adapted scrapie

Sodium hydroxide (NaOH) has been shown to reduce the infectivity of transmissible spongiform encephalopathy (TSE) agents. This study investigated the efficacy of sodium hydroxide at 0.1M, 0.25M and 0.5M concentrations for the inactivation of mouse-adapted scrapie strain ME7. Times and temperatures modelled conditions used in an industrial plasma fractionation plant for sanitisation of ultrafilters, and the sodium hydroxide component of Clean In Place sanitisation. The concentration of scrapie ME7 brain homogenate in NaOH test solutions was 1% (w/v). At the end of incubation periods, the samples were adjusted to neutral pH prior to intracerebral inoculation into mice for bioassay. The conditions of 0.1M NaOH at 60 degrees C for 2min and 0.25M NaOH at 30 degrees C for 60min were found to inactivate 3.96 and 3.93logs of scrapie, respectively. Use of 0.5M NaOH at 30 degrees C for 60 or 75min was found to inactivate >or=4.23 and 4.15logs of scrapie. This indicates that the use of these conditions in an industrial process would substantially reduce prion infectivity.

Quantification of Peyer's patches in Cheviot sheep for future scrapie pathogenesis studies

Peyer's patches (PPs) are the most probable sites of intestinal uptake of the transmissible spongiform encephalopathy (TSE) agent. The amount of PP tissue varies considerably between different age groups of individuals, and whether this variation is related to susceptibility to TSE infection raises an intriguing possibility. The purpose of this study was to determine the surface area of PP tissue and the number of associated lymphoid follicles in different age groups of Neuropathogenesis Unit (NPU) Cheviot sheep. Terminal ilea were obtained from 33 sheep of different ages. Samples of ileal tissue were collected for immunocytochemistry and immunolabelled for prion protein (PrP). Specimens were then fixed in acetic acid, stained with methylene blue and transilluminated. Image analysis software was used to calculate the area of intestinal and PP tissue. The number of associated lymphoid follicles was determined using a dissecting microscope. Results showed a marked fall in surface area of PP tissue and lymphoid follicle density around puberty (about 8-9 months of age in NPU Cheviot sheep) and both measures remained low throughout adulthood. Using the Spearman's rank correlation coefficient, r(s), these two measures were found to be closely correlated (r(s)=0.899, n=33, P<0.0001). There was also a significant (negative) correlation between age and the two respective measures (surface area of PP tissue versus age, r(s)=-0.879 (n=33, P<0.0001); lymphoid follicle density versus age r(s)=-0.943 (n=33, P<0.0001). Immunolabelling for PrP was observed primarily in the light zone of lymphoid follicles. Results obtained from this study are useful for future oral pathogenesis studies of the NPU Cheviot flock. They may also offer a possible biological explanation for the apparent age-susceptibility relationship observed in natural cases of TSEs and might help to explain the young age-distribution of cases.

Absence of Spiroplasma or other bacterial 16s rRNA genes in brain tissue of hamsters with scrapie

Spiroplasma spp. have been proposed to be the etiological agents of the transmissible spongiform encephalopathies (TSEs). In a blind study, a panel of 20 DNA samples was prepared from the brains of uninfected hamsters or hamsters infected with the 263K strain of scrapie. The brains of the infected hamsters contained > or =10(10) infectious doses/g. The coded panel was searched for bacterial 16S rRNA gene sequences, using primers selective for spiroplasma sequences, primers selective for mollicutes in general, and universal bacterial primers. After 35 PCR cycles, no samples were positive for spiroplasma or any other bacterial DNA, while control Spiroplasma mirum genomic DNA, spiked at 1% of the concentration required to account for the scrapie infectivity present, was readily detected. After 70 PCR cycles, nearly all samples yielded amplified products which were homologous to various bacterial 16S rRNA gene sequences, including those of frequent environmental contaminants. These sequences were seen in uninfected as well as infected samples. Because the concentration of scrapie infectivity was at a known high level, it is very unlikely that a bacterial infection at the same concentration could have escaped detection. We conclude that the infectious agent responsible for TSE disease cannot be a spiroplasma or any other eubacterial species.

Fate of prions in soil: trapped conformation of full-length ovine prion protein induced by adsorption on clays

Studying the mechanism of retention of ovine prion protein in soils will tackle the environmental aspect of potential dissemination of scrapie infectious agent. We consider the surface-induced conformational changes that the recombinant ovine prion protein (ovPrP) may undergo under different pH conditions when interacting with soil minerals of highly adsorptive capacities such as montmorillonite. The conformational states of the full-length ovine prion protein adsorbed on the electronegative clay surface are compared to its solvated state in deuterated buffer in the pD range 3.5-9, using FTIR spectroscopy. The in vitro pH-induced conversion of the alpha-helical monomer of ovPrP into oligomers of beta-like structure prone to self-aggregation does not occur when the protein is adsorbed on the clay surface. The conformation of the trapped ovPrP molecules on montmorillonite is pH-independent and looks like that of the ovPrP solvated state at pD higher than 7, suggesting the major role of Arg and Lys residues in the electrostatic origin of adsorption. The uneven distribution of positively and negatively charged residues of the ovPrP protein would promote a favored orientation of the protein towards the clay, so that not only the basic residues embedded in the N-terminal flexible part but also external basic residues in the globular part of the protein might participate to the attractive interaction. From these results, it appears unlikely that the interaction of normal prions (PrP(C)) with soil clay surfaces could induce a change of conformation leading to the pathogenic form of prions (PrP(Sc)).

Frequencies ofPrPgene haplotypes in British sheep flocks and the implications for breeding programmes

AIMS

To analyse the frequencies of prion (PrP) gene haplotypes in UK sheep flocks and evaluate their relevance to transmissible spongiform encephalopathies (TSEs) and TSE resistance breeding programmes in sheep.

METHODS AND RESULTS

Genomic DNA isolated from sheep blood was PCR amplified for the coding region of the PrP gene and then sequenced. This study has analysed the sequence of PrP between codons 110 and 245 in 6287 ARQ haplotypes revealing a total of eight variant sequences, which represent a higher than expected 41% of all ARQ haplotypes. The additional PrP gene dimorphisms were M112T, L141F, M137T, H143R, H151C, P168L, Q175E and P241S.

CONCLUSION

The results do not suggest a correlation between the occurrence of a specific ARQ haplotype and the scrapie disease status of a flock. The ARQ haplotype variability appears to be different in the UK sheep flocks compared with sheep flocks from outside the UK.

SIGNIFICANCE AND IMPACT OF THE STUDY

Additional PrP dimorphisms may impact on the methodologies used for standard PrP genotyping in sheep breeding programmes. Some of these polymorphisms were found with significant frequencies in the UK sheep flocks and should therefore be considered in breeding programmes.

Transmissible spongiform encephalopathy strain, PrP genotype and brain region all affect the degree of glycosylation of PrPSc

Transmissible spongiform encephalopathies (TSEs), sometimes known as prion diseases, are caused by an infectious agent whose molecular properties have not been determined. Traditionally, different strains of TSE diseases are characterized by a series of phenotypic properties after passage in experimental animals. More recently it has been recognized that diversity in the degree to which an abnormal form of the host protein PrP, denoted PrPSc, is glycosylated and the migration of aglycosyl forms of PrPSc on immunoblots may have some differential diagnostic potential. It has been recognized that these factors are affected by the strain of TSE agent but also by other factors, e.g. location within the brain. This study shows in some cases, but not others, that host PrP genotype has a major influence on the degree of PrPSc glycosylation and migration on gels and provides further evidence of the effect of brain location. Accordingly both the degree of glycosylation and the apparent molecular mass of PrPSc may be of some value for differential diagnosis between TSE strains, but only when host effects are taken into account. Furthermore, the data inform the debate about how these differences arise, and favour hypotheses proposing that TSE agents affect glycosylation of PrP during its biosynthesis.

Nucleic Acid and Prion Protein Interaction Produces Spherical Amyloids which can Function in vivo as Coats of Spongiform Encephalopathy Agent

The infectious agent of transmissible spongiform encephalopathies (TSE) has been considered to be PrP(SC), a structural isoform of cellular prion protein PrP(C). PrP(SC) can exist as oligomers and/or as amyloid polymers. Nucleic acids induce structural conversion of recombinant prion protein PrP and PrP(C) to PrP(SC) form in solution and in vitro. Here, we report that nucleic acids, by interacting with PrP in solution, produce amyloid fibril and fibres of different morphologies, similar to those identified in the diseased brains. In addition, the same interaction produces polymer lattices and spherical amyloids of different dimensions (15-150 nm in diameters). The polymer lattices show apparent morphological similarity to the two-dimensional amyloid crystals obtained from linear amyloids isolated in vivo. The spherical amyloids structurally resemble "spherical particles" observed in natural spongiform encephalopathy (SE) and in scrapie-infected brains (TSE). We suggest that spherical amyloids, PrP(SC)-amylospheroids, are probable constituents of the coat of the spherical particles found in vivo and the latter can act as protective coats of the SE and TSE agents in vivo.

Partitioning of TSE infectivity during ethanol fractionation of human plasma

The practice of validating processes for their capacity to inactivate a range of non-enveloped and enveloped viruses also provides confidence that plasma products will be safe from emerging viral pathogens with known aetiology. Of greater concern are diseases of unknown or poorly defined aetiology such as the group of neurological diseases collectively called the transmissible spongiform encephalopathies (TSEs), or prion diseases, for which the best known human disease is Creutzfeldt-Jakob Disease (CJD) and its variant form (vCJD). The goal of the current study was to investigate the potential for manufacturing steps used in the production of albumin and immunoglobulin products by Kistler-Nitschmann fractionation, and the utility of nanofiltration of immunoglobulin to remove TSE agents. Two different scrapie model systems were used. In the first system infectious material used for spiking was scrapie sheep brain homogenate with infectivity titres being measured in hamsters. In the second system purified scrapie agent was used (PrP fibrils) with Western blot analysis measuring reduction in the proteinase K resistant form being used as a measure of removal. The data demonstrated substantial removal of the infectious agent by the manufacturing process in both model systems although some differences were observed in partitioning of the two different infectious materials. The hamster infectivity studies were shown to be approximately 1000 fold more sensitive than the Western Blot assay. The data from both studies provide added confidence that these plasma products are safe with respect to their potential to transmit TSE.

Risk of scrapie in British sheep of different prion protein genotype

There is a well-established association between sheep prion protein (PrP) genotype and the risk of death from scrapie. Certain genotypes are clearly associated with susceptibility to the disease and others to resistance. However, there have been no attempts to quantify the disease risk for all 15 PrP genotypes. Here, datasets of the PrP genotypes of nearly 14 000 British sheep and of more than 1500 confirmed scrapie cases were combined to yield an estimate of scrapie risk (reported cases per annum per million sheep of the genotype, or RCAM) for British sheep. The greatest scrapie risk by far, ranging from 225 to 545 RCAM, was for the VRQ-encoding genotypes ARQ/VRQ, ARH/VRQ and VRQ/VRQ. The next greatest risk (37 RCAM) was for the ARQ/ARQ genotype. The ARR/ARR genotype was the only numerically significant genotype for which no scrapie cases have been reported. The AHQ allele conferred resistance and the risk of scrapie in AHQ/VRQ sheep was very low (0·7 RCAM), although there was a higher and moderate risk for the AHQ homozygote (5 RCAM). The ARH allele appeared to confer susceptibility when encoded with VRQ, but possible resistance when encoded with other alleles. Scrapie risk varied with age: for VRQ/VRQ and ARH/VRQ the risk peaked at 2 years of age; that for ARQ/VRQ peaked at 3 years. There was some evidence that, following the lower risk at 4 and 5 years, a second rise occurred from about 6 years. Comparison with other published data indicated that the scrapie risk of certain PrP genotypes may differ between Great Britain and other countries.

Investigation of trace elements in soil as risk factors in the epidemiology of scrapie

Scrapie is a fatal transmissible spongiform encephalopathy (TSE) of sheep and goats which is thought to be caused by a conformational change of the normal prion protein to its pathological isoform. It has been speculated that this change may be mediated by an interaction between the prion protein and various trace elements, in particular manganese and copper, and that the levels of trace elements in soils may therefore be risk factors for TSEs. This hypothesis was tested by comparing the level of trace elements in the soils on farms with and without scrapie and on those with a higher and lower incidence of the disease. The levels of trace elements were obtained from the UK's National Soil Inventory and deficiencies reported by farmers. The results provide no evidence that trace elements are risk factors for scrapie on farms, and the variations in the levels of trace elements in soils at regional scales do not account for the regional differences in the prevalence of scrapie.

Prospects for the development of pre-mortem laboratory diagnostic tests for Creutzfeldt-Jakob disease

At present the diagnosis of Creutzfeldt-Jakob disease (CJD) and related transmissible spongiform encephalopathies in humans is based on clinical criteria and (at post-mortem) the histopathological and immunological examination of brain tissue. The misfolded prion protein, PrPSc, is the single most significant marker, but its recognition by standard serological methods is complicated by its antigenic similarity to the normal prion protein, PrPC. Although there are commercial diagnostic assays available for bovine spongiform encephalopathy using brain specimens taken at slaughter, there are no suitable pre-mortem assays for cattle and none either for pre-mortem human disease. Especially in view of the recent report of variant CJD transmission by blood transfusion, it is important that tests for pre-symptomatic infections are developed. This will safeguard the blood supply and, for example, prevent the transmission of CJD in neurosurgery. This paper reviews the current and prospective approaches to the pre-mortem diagnosis of CJD, in particular its variant form.

Sodium hydroxide renders the prion protein PrPSc sensitive to proteinase K

Sodium hydroxide (NaOH) solutions are widely used for the purification of contaminated equipment, as they are known to inactivate a variety of pathogens. However, information about their effect on agents causing transmissible spongiform encephalopathy (TSE) is sparse and contradictory. Scrapie hamster brain homogenate, containing the disease-associated form of the prion protein (PrP(Sc)), was exposed to NaOH. Kinetics studies showed that treatment of brain homogenate with millimolar concentrations of NaOH rapidly abolished the proteinase K-resistant form of the prion protein (PrP(res)). NaOH treatment converted PrP(Sc) into a protease-sensitive form, either in solution or when adsorbed to a metallic surface. If infectivity of TSEs is linked with PrP(res), the results imply that inactivation of TSE occurs more efficiently than currently assumed.

The prion protein and neuronal zinc homeostasis

Although the prion protein (PrP) is known to be the causative agent of the neurodegenerative transmissible spongiform encephalopathies, its normal cellular function remains elusive. Octapeptide repeats in the N terminus of PrP bind metal ions and are required for the endocytosis of PrP upon exposure of cells to copper or zinc. As the concentration of zinc in the extracellular spaces of the brain is higher than that for copper, we put forward the hypothesis that PrP is involved in neuronal zinc homeostasis; PrP might be involved in transport of zinc into the cell or might act as a zinc sensor. In prion disease, when the protein undergoes a conformational change to the infectious form, this function of PrP in zinc homeostasis might be compromised.