Calpain-dependent endoproteolytic cleavage of PrPSc modulates scrapie prion propagation

Previous studies using post-mortem human brain extracts demonstrated that PrP in Creutzfeldt-Jakob disease (CJD) brains is cleaved by a cellular protease to generate a C-terminal fragment, referred to as C2, which has the same molecular weight as PrP-(27-30), the protease-resistant core of PrP(Sc) (1). The role of this endoproteolytic cleavage of PrP in prion pathogenesis and the identity of the cellular protease responsible for production of the C2 cleavage product has not been explored. To address these issues we have taken a combination of pharmacological and genetic approaches using persistently infected scrapie mouse brain (SMB) cells. We confirm that production of C2 is the predominant cleavage event of PrP(Sc) in the brains of scrapie-infected mice and that SMB cells faithfully recapitulate the diverse intracellular proteolytic processing events of PrP(Sc) and PrP(C) observed in vivo. While increases in intracellular calcium (Ca(2+)) levels in prion-infected cell cultures stimulate the production of the PrP(Sc) cleavage product, pharmacological inhibitors of calpains and overexpression of the endogenous calpain inhibitor, calpastatin, prevent the production of C2. In contrast, inhibitors of lysosomal proteases, caspases, and the proteasome have no effect on C2 production in SMB cells. Calpain inhibition also prevents the accumulation of PrP(Sc) in SMB and persistently infected ScN2A cells, whereas bioassay of inhibitor-treated cell cultures demonstrates that calpain inhibition results in reduced prion titers compared with control-treated cultures assessed in parallel. Our observations suggest that calpain-mediated endoproteolytic cleavage of PrP(Sc) may be an important event in prion propagation.

cDNA representational difference analysis of ileal Peyer’s patches in lambs after oral inoculation with scrapie

cDNA representational difference analysis (RDA) was used to study gene expression profiles in the ileal Peyer's patch of a lamb 1 week after oral inoculation with the scrapie agent. Twenty-five differentially expressed cDNA fragments were identified and cloned. Sequence analysis indicated seven novel gene sequences. Other clones shared sequence homology with genes encoding ribosomal and mitochondrial proteins, the translation initiation factor EIF4GII and the bovine pancreatic thread protein. Reverse Northern was used to confirm the differential expression in another four lambs inoculated with scrapie and the tissue distribution of the novel genes was examined using Northern blot analysis.

Comparison study on clinical and neuropathological characteristics of hamsters inoculated with scrapie strain 263K in different challenging pathways

OBJECTIVE

To understand the infectious characteristics of a hamster-adapted scrapie strain 263K with five different routes of infection including intracerebral (i.c.), intraperitoneal (i.p.), intragastrical (i.g.), intracardiac and intramuscular (i.m.) approaches.

METHODS

Hamsters were infected with crude- or fine-prepared brain extracts. The neuropathological changes, PrP(Sc) deposits, and patterns of PK-resistant PrP were analyzed by HE stain, immnunohistochemistry (IHC) assay and Western blot. Reactive gliosis and neuron loss were evaluated by glial fibrillary acidic protein (GFAP) and neuron specific enolase (NSE) specific IHC.

RESULTS

The animals inoculated in i.m. and i.p. ways with crude PrP(Sc) extracts showed clinical signs at the average incubation of 69.2 +/- 2.8 and 65.5 +/- 3.9 days. Inoculation in i.c. and intracardiac ways with fine PrP(Sc) extracts (0.00035 g) caused similar, but relative long incubation of around 90 days. Only one out of eight hamsters challenged in i.g. way with low dosage (0.01 g) became ill after a much longer incubation (185 d), while all animals (4/4) with high dosage (0.04 g) developed clinical signs 105 days postinfection. The most remarkable spongiform degeneration and PrP(Sc) deposits were found in brain stem among the five challenge groups generally. The number of GFAP-positive astrocytes increased distinctly in brain stems in all infection groups, while the number of NSE-positive cells decreased significantly in cerebrum, except i.c. group. The patterns of PK-resistant PrP in brains were basically identical among the five infection routes.

CONCLUSION

Typical TSE could be induced in hamsters by inoculating strain 263K in the five infection ways. The incubation periods in bioassays depend on infective dosage, administrating pathway and preparation of PrP(Sc). The neuropathological changes and PrP(Sc) deposits seem to be related with regions and inoculating pathways.

Dynamic analyses of PrP and PrP(Sc) in brain tissues of golden hamsters infected with scrapie strain 263K revealed various PrP forms

OBJECTIVE

To expatiate dynamic changes in hamsters infected with scrapie strain 263K, to observe the presence and aggravation of various forms of PrP and PrP(Sc) during incubation period, and to probe primarily the relationship between the onset of clinic manifestations and the presence of different PrP(Sc) forms.

METHODS

Hamster-adapted scrapie strain 263K was intracerebrally inoculated into hamsters. Different forms of PrP and PrP(Sc) were monitored dynamically by Western blot and immuno-histochemical assays. The presence of scrapie-associated fibril (SAF) was assayed by electron microscopy analysis (EM) and immuno-golden EM.

RESULTS

PrP(Sc) was initially detected in the brain tissues of the animals in 20 days post-inoculation by immunohistochemistry and 40 days with Western blot. Quantitative evaluations revealed that the amounts of PrP and PrP(Sc) in brain tissues increased along with the incubation. Several high and low molecular masses of PrP were seen in the brains of the long-life span infected animals. Deglycosylation assays identified that the truncated PrP in the infected brains showed similar glycosylation patterns as the full-length PrP. The presence of short fragments was seemed to relate with the onset of clinical conditions.

CONCLUSION

These results indicate that infectious agents exist and accumulate in central nerve system prior to the onset of the illness. Various molecular patterns of PrP(Sc) may indwell in brain tissues during the infection.

Scrapie-infected GT1-1 cells show impaired function of voltage-gated N-type calcium channels (Cav 2.2) which is ameliorated by quinacrine treatment

Prions are transmissible pathogens that cause neurodegenerative diseases, although the mechanisms behind the nervous system dysfunctions are unclear. To study the effects of a prion infection on voltage-gated calcium channels, scrapie-infected gonadotropin-releasing hormone neuronal cells (ScGT1-1) in culture were depolarized by KCl and calcium responses recorded. Lower calcium responses were observed in infected compared to uninfected cells. This effect was still observed when L-type calcium channels were blocked by nimodipine. After inhibition of N-type calcium channels with omega-conotoxin GVIA, there was no difference in calcium responses. The calcium responses after nimodipine treatment became progressively lower during infection, but there was no major loss of the cellular prion protein (PrP(C)) or marked increase in accumulation of the abnormal prion protein (PrP(Sc)) in the cultures. These results indicate that scrapie infection causes a dysfunction of voltage-gated N-type calcium channels, which is exacerbated slowly over time. Quinacrine treatment cleared PrP(Sc) and restored calcium responses in the ScGT1-1 cultures.

Reduced proteinase K resistance and infectivity of prions after pressure treatment at 60 °C

High hydrostatic pressure is a mild technology compared with high temperatures and is commonly used for food pasteurization. Crude brain homogenates of terminally diseased hamsters infected with scrapie 263K strain were heated at 60 degrees C and/or pressurized up to 1000 MPa for 2 h. Prion proteins were analysed for their proteinase K sensitivity using a Western blot technique. PrP(Sc) pressurized with 500 MPa or above proved to be proteinase K sensitive. To test the remaining infectivity of the pressurized material, hamsters were infected intracerebrally. Results showed a greatly delayed onset of disease (from 80 up to 153 days) when samples had been pressurized at 500 MPa and above. An increase in the survival rate was also observed: 47 % survival over 180 days was seen following infection with homogenates pressurized at 700-1000 MPa.

Scrapie-specific neuronal lesions are independent of neuronal PrP expression

In the transmissible spongiform encephalopathies (TSE), accumulation of the abnormal disease-specific prion protein is associated with neurodegeneration. Previous data suggested that abnormal prion protein (PrP) could induce neuronal pathology only when neurons expressed the normal form of PrP, but conflicting evidence also has been reported. Understanding whether neuronal PrP expression is required for TSE neuropathological damage in vivo is essential for determining the mechanism of TSE pathogenesis. Therefore, these experiments were designed to study scrapie pathogenesis in vivo in the absence of neuronal PrP expression. Hamster scrapie (strain 263K) was used to infect transgenic mice expressing hamster PrP in the brain only in astrocytes. These mice previously were shown to develop clinical scrapie, but it was unclear whether the brain pathology was caused by damage to astrocytes, neurons, or other cell types. In this electron microscopic study, neurons demonstrated TSE-specific pathology despite lacking PrP expression. Abnormal PrP was identified around astrocytes, primarily in the extracellular spaces of the neuropil, but astrocytes showed only reactive changes and no damage. Therefore, in this model the pathogenesis of the disease appeared to involve neuronal damage associated with extracellular astrocytic accumulation of abnormal PrP acting upon nearby PrP-negative neurons or triggering the release of non-PrP neurotoxic factors from astrocytes.

On scrapie interference and artificial prions

The mechanisms responsible for neuronal death in transmissible spongiform encephalopathies (TSEs) are still not completely understood, and at least two major hypotheses have been formulated, based on the peculiar aspects of prion protein biology. In fact, the neuronal spreading of the prion conformational change may lead either to gain toxic properties, or to loose the normal function of this protein. In order to investigate the relative contribution of these two opposite mechanisms, two theoretical approaches may be proposed: RNA interference (RNAi) and artificial prion engineering. In fact, RNAi techniques offer now an extremely exciting new tool for investigating the effects of gene silencing both in prion, and other neurological disorders. On the other hand, the gain-of-toxic-function hypothesis might be definitely evaluated by creating an artificial prion choosing a protein target whose loss of function could be bypassed in the experimental set. In this paper the two aforementioned strategies are outlined, briefly discussing the consequent implications for TSE therapy.

Depleting neuronal PrP in prion infection prevents disease and reverses spongiosis

The mechanisms involved in prion neurotoxicity are unclear, and therapies preventing accumulation of PrPSc, the disease-associated form of prion protein (PrP), do not significantly prolong survival in mice with central nervous system prion infection. We found that depleting endogenous neuronal PrPc in mice with established neuroinvasive prion infection reversed early spongiform change and prevented neuronal loss and progression to clinical disease. This occurred despite the accumulation of extraneuronal PrPSc to levels seen in terminally ill wild-type animals. Thus, the propagation of nonneuronal PrPSc is not pathogenic, but arresting the continued conversion of PrPc to PrPSc within neurons during scrapie infection prevents prion neurotoxicity.

RNA molecules stimulate prion protein conversion

Much evidence supports the hypothesis that the infectious agents of prion diseases are devoid of nucleic acid, and instead are composed of a specific infectious protein. This protein, PrP(Sc), seems to be generated by template-induced conformational change of a normally expressed glycoprotein, PrP(C) (ref. 2). Although numerous studies have established the conversion of PrP(C) to PrP(Sc) as the central pathogenic event of prion disease, it is unknown whether cellular factors other than PrP(C) might be required to stimulate efficient PrP(Sc) production. We investigated the biochemical amplification of protease-resistant PrP(Sc)-like protein (PrPres) using a modified version of the protein-misfolding cyclic amplification method. Here we report that stoichiometric transformation of PrP(C) to PrPres in vitro requires specific RNA molecules. Notably, whereas mammalian RNA preparations stimulate in vitro amplification of PrPres, RNA preparations from invertebrate species do not. Our findings suggest that host-encoded stimulatory RNA molecules may have a role in the pathogenesis of prion disease. They also provide a practical approach to improve the sensitivity of diagnostic techniques based on PrPres amplification.

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Positioning of follicular dendritic cells within the spleen controls prion neuroinvasion

Peripheral infection is the natural route of transmission in most prion diseases. Peripheral prion infection is followed by rapid prion replication in lymphoid organs, neuroinvasion and progressive neurological disease. Both immune cells and nerves are involved in pathogenesis, but the mechanisms of prion transfer from the immune to the nervous system are unknown. Here we show that ablation of the chemokine receptor CXCR5 juxtaposes follicular dendritic cells (FDCs) to major splenic nerves, and accelerates the transfer of intraperitoneally administered prions into the spinal cord. Neuroinvasion velocity correlated exclusively with the relative locations of FDCs and nerves: transfer of CXCR5-/- bone marrow to wild-type mice induced perineural FDCs and enhanced neuroinvasion, whereas reciprocal transfer to CXCR5-/- mice abolished them and restored normal efficiency of neuroinvasion. Suppression of lymphotoxin signalling depleted FDCs, abolished splenic infectivity, and suppressed acceleration of pathogenesis in CXCR5-/- mice. This suggests that prion neuroimmune transition occurs between FDCs and sympathetic nerves, and relative positioning of FDCs and nerves controls the efficiency of peripheral prion infection.

A quantitative, highly sensitive cell-based infectivity assay for mouse scrapie prions

Prions are usually quantified by bioassays based on intracerebral inoculation of mice that are slow, imprecise, and costly. We have isolated neuroblastoma N2a sublines highly susceptible to mouse prions, as evidenced by accumulation of infectivity and the scrapie form of prion protein (PrPSc), and developed quantitative in vitro assays for prion infectivity. In the scrapie cell (SC) assay, susceptible N2a cells are exposed to prion-containing samples for 3 days, grown to confluence, and split 1:10 three times, and the proportion of PrPSc-containing cells is determined with automated counting equipment. In a log/log plot, the dose-response is linear over two logs of prion concentrations. The SC assay is about as sensitive as the mouse bioassay, 10 times faster, >2 orders of magnitude less expensive, and suitable for robotization. SC assays performed in a more time-consuming end point titration format extend the sensitivity and show that infectivity titers measured in tissue culture and in the mouse are similar.

Neurochemical and behavioural modifications induced by scrapie infection in golden hamsters

Scrapie-infected hamsters were tested for spontaneous motor activity and passive avoidance at various times after infection. After testing, some animals were killed and their whole brains assayed for norepinephrine, dopamine, serotonin and their metabolites. The apparent rate of turnover was estimated in terms of metabolite/amine concentrations. After 70 days, there was a decrease in passive avoidance and dopamine and serotonin. Passive avoidance correlated with the apparent rate of turnover of dopamine, whereas motor activity correlated with that of serotonin and dopamine.

Dimethyl sulfoxide delays PrP sc accumulation and disease symptoms in prion-infected hamsters

PrP(Sc), an aberrantly folded protein, is the only identified component of the prion, an agent causing fatal neurodegenerative diseases such as scrapie and bovine spongiform encephalopathy. Dimethyl sulfoxide (DMSO) has been shown to reduce the accumulation of PrP(Sc) in scrapie-infected (ScN2a) cells, and to inhibit its aggregation in vitro. In humans, DMSO was used successfully in the treatment of various peripheral amyloidotic diseases. Here we show that administration of DMSO to scrapie-infected hamsters significantly prolonged disease incubation time, as well as delayed the accumulation of PrP(Sc) in Syrian hamster brains. Interestingly, administration of DMSO to scrapie sick hamsters resulted in increased clearance of protease-resistant PrP in their urine. We conclude that although DMSO by itself may not be sufficient to cure prion diseases, it may be considered as a component in a 'cocktail' drug approach for these disorders. Also, urine PrP testing should be considered for the assessment of treatment efficacy.

Inducible cytokine gene expression in the brain in the ME7/CV mouse model of scrapie is highly restricted, is at a strikingly low level relative to the degree of gliosis and occurs only late in disease

The temporal course of cerebral cytokine gene expression was investigated in the ME7/CV murine scrapie model to determine any association with neuropathological events. Analysis by RNase protection assay (RPA) demonstrated no transcripts for ILs 2, 3, 4, 5, 6, 7, 10, 12p40 and 13, granulocyte macrophage colony-stimulating factor, IFN-gamma or lymphotoxin-alpha at any time during the course of this disease. Transcripts for transforming growth factor-beta 1 were constitutively expressed in both control and scrapie-infected brain and were elevated at terminal disease. RPA and quantitative real-time RT-PCR detected low levels of transcripts for IL-1 alpha, IL-1 beta and TNF alpha in scrapie-infected brain but only IL-1 beta was elevated consistently in all mice studied. Although glial cell activation within the hippocampus was evident from 100 days post-infection (p.i.), elevated IL-1 beta transcripts (and immunoreactivity) were evident from 180 days p.i., around the time of hippocampal pyramidal neuron loss, and increased steadily thereafter to reach a 3.5-fold increase at terminal disease. Even at their maximum, levels of these transcripts were disproportionately low relative to the degree of glial cell activation. It is concluded that cytokine gene expression in the ME7 scrapie-infected mouse brain, relative to the degree of reactive gliosis, is highly restricted, temporally late and disproportionately low.

Up-regulation of cathepsin B and cathepsin L activities in scrapie-infected mouse Neuro2a cells

Prion diseases are characterized by the accumulation of an abnormal, proteinase K-resistant isoform of the prion protein, PrP(Sc), which is generated by a post-translational conversion of the protease-sensitive normal cell-surface glycoprotein PrP(c) involving major conformational changes. The conversion is thought to occur at the plasma membrane or along the endocytic pathway towards the lysosome. PrP(Sc) aggregates have been found to accumulate in secondary lysosomes. In our study, the activities of two major lysosomal cysteine proteases, cathepsins B and L, were found to be significantly increased in scrapie-infected Neuro2a cells compared with uninfected cells using biochemical and cytochemical methods. We hypothesize that lysosomal proteases may be involved in a 'second autocatalytic loop' of PrP(Sc) formation, acting in concert with the well-known autocatalytic enhancement of PrP conversion in the presence of PrP(Sc).

Immunohistochemical comparison of anti-prion protein (PrP) antibodies in the CNS of mice infected with scrapie

One of the pathological changes characteristic of the transmissible spongiform encephalopathies (TSEs) is the accumulation of disease-specific PrP (PrP(sc)). Immunolabeling of PrP(sc) was compared using a panel of monoclonal and polyclonal antibodies. To determine the effects of tissue fixation on immunostaining, we performed a supplementary investigation reviewing the fixatives formol saline and periodate-lysine-paraformaldehyde (PLP). The main target sites of the antibodies were similar. However the monoclonal antibodies (MAbs) 6H4, 7A12 and 8H4 revealed targeted PrP(sc) labeling with no background labeling. Although 7A12 and 8H4 did not detect early PrP deposition, we propose that during the later stages of disease 7A12 and 8H4 can be used with equal effectiveness in place of 6H4. Tissues taken during the early stages of disease that had been fixed in PLP displayed more PrP immunolabeling than tissues that had undergone formol fixation. PLP fixation on 6H4-immunostained tissue revealed interweaving granular linear PrP deposits in the hippocampus. This labeling was not observed in tissue that had undergone formol fixation, suggesting that PLP fixation might enhance the sensitivity of the immunohistochemical (IHC) detection of PrP. In the two scrapie mouse models studied here, PLP fixation and immunolabeling with the anti-PrP antibody 6H4 gave superior results.

In vivo micro magnetic resonance imaging signal changes in scrapie infected mice

Signal abnormalities on magnetic resonance imaging (MRI) T2-weighted images (T2WI) have been described in patients with Creutzfeldt-Jakob disease; however, the pathology underlying these findings remains to be fully described. We investigated the time-course of signal alterations in a murine model of prion disease using in vivo 9.4 Tesla micro magnetic resonance imaging (muMRI). The topography of muMRI signal changes was correlated with the accumulation of proteinase resistant PrP(Sc) in corresponding brain sections. Increased signal intensity on T2WI was observed in the septum and in the hippocampus of presymptomatic mice 120 days post infection (dpi). Mildly symptomatic animals (150 dpi) and animals with apparent neurological deficit (180 dpi) had a greater increase of signal intensity on T2WI in the septum and the hippocampus; in addition, abnormalities in the cortex and in the thalamus were found. Neuropathological evaluation demonstrated accumulation of PrP(Sc) and astrogliosis but only minimal or no spongiform changes in structures where abnormal signal was detected. These observations suggest that early pathological changes related to the accumulation of PrP(Sc) may be detectable in presymptomatic subjects using MRI systems with higher magnetic field strength.

Specific inhibition of pathological prion protein accumulation by small interfering RNAs

Development of transmissible spongiform encephalopathies (TSEs) pathogenesis requires the presence of both the normal host prion protein (PrP-sen) and the abnormal pathological proteinase-K resistant isoform (PrP-res). PrP-res forms highly insoluble aggregates, with self-perpetuating properties, by binding and converting PrP-sen molecules into a likeness of themselves. In the present report, we show that small interfering RNA (siRNA) duplexes trigger specific Prnp gene silencing in scrapie-infected neuroblastoma cells. A non-passaged, scrapie-infected culture transfected with siRNA duplexes is depleted of PrP-sen and rapidly loses its PrP-res content. The use of different murine-adapted scrapie strains and host cells did not influence the siRNA-induced gene silencing efficiency. More than 80% of transfected cells were positive for the presence of fluorescein-labeled siRNA duplexes. No cytotoxicity associated with the use of siRNA was observed during the time course of these experiments. Despite a transient abrogation of PrP-res accumulation, our results suggest that the use of siRNA may provide a new and promising therapeutic approach against prion diseases.

Cooperative binding of dominant-negative prion protein to kringle domains

Conversion of the cellular prion protein (PrP(C)) to the pathogenic isoform (PrP(Sc)) is a major biochemical alteration in the progression of prion disease. This conversion process is thought to require interaction between PrP(C) and an as yet unidentified auxiliary factor, provisionally designated protein X. In searching for protein X, we screened a phage display cDNA expression library constructed from prion-infected neuroblastoma (ScN2a) cells and identified a kringle protein domain using full-length recombinant mouse PrP (recMoPrP(23-231), hereafter recMoPrP) expressing a dominant-negative mutation at codon 218 (recMoPrP(Q218K)). In vitro binding analysis using ELISA verified specific interaction of recMoPrP to kringle domains (K(1+2+3)) with higher binding by recMoPrP(Q218K) than by full-length recMoPrP without the mutation. This interaction was confirmed by competitive binding analysis, in which the addition of either a specific anti-kringle antibody or L-lysine abolished the interaction. Biochemical studies of the interactions between K(1+2+3) and various concentrations of both recMoPrP molecules demonstrated binding in a dose-dependent manner. A Hill plot analysis of the data indicates positive cooperative binding of both recMoPrP(Q218K) and recMoPrP to K(1+2+3) with stronger binding by recMoPrP(Q218K). Using full-length and an N-terminally truncated MoPrP(89-231), we demonstrate that N-terminal sequences enable PrP to bind strongly to K(1+2+3). Further characterization with truncated MoPrP(89-231) refolded in different conformations revealed that both alpha-helical and beta-sheet conformations bind to K(1+2+3). Our data demonstrate specific, high-affinity binding of a dominant-negative PrP as well as binding of other PrPs to K(1+2+3). The relevance of such interactions during prion pathogenesis remains to be established.

Disease-associated PrP in the enteric nervous system of scrapie-affected Suffolk sheep

Disease-associated prion protein (PrP(d)) in the enteric nervous system (ENS) of 20- to 24-month-old Suffolk sheep in the late subclinical and early clinical phase of scrapie was studied. Sites in the alimentary tract extending from the forestomachs and abomasum to the colon from scrapie-affected sheep (PrP(ARQ/ARQ)) and scrapie-resistant sheep (PrP(ARR/ARQ) and PrP(ARR/ARR)) were examined. PrP(d) was found only in scrapie-affected sheep and was most prominent in the ENS when abundant deposits of PrP(d) were also present in adjacent lymphoid nodules. Immunolabelling with the nerve fibre markers PgP 9.5 and neuron-specific enolase and the satellite cell marker glial fibrillary acidic protein revealed the extensive ganglionated networks of the myenteric and submucosal plexi. Fewer nerve fibres were present in the lamina propria, T-cell dominated interfollicular areas and dome regions of Peyer's patches. A substantial network of nerve fibres was detected in many lymphoid nodules of both the scrapie-affected and scrapie-resistant sheep. Nerve fibres were also detected within the capsule of lymphoid nodules. Electron microscopy revealed the presence of nerves in the lymphoid nodules, showing a close association with follicular dendritic cells, lymphocytes and tingible body macrophages. In demonstrating that lymphoid nodules in the Peyer's patches of scrapie-affected sheep possess a substantial network of nerve fibres, the present study shows that nodules provide close contact between nerve fibres and cell populations known to contain abundant PrP(d), including follicular dendritic cells and tingible body macrophages, and that gut-associated lymphoid nodules in sheep may represent an important site for neuroinvasion.

Widespread PrPSc accumulation in muscles of hamsters orally infected with scrapie

Scrapie, bovine spongiform encephalopathy and chronic wasting disease are orally communicable, transmissible spongiform encephalopathies (TSEs). As zoonotic transmissions of TSE agents may pose a risk to human health, the identification of reservoirs for infectivity in animal tissues and their exclusion from human consumption has become a matter of great importance for consumer protection. In this study, a variety of muscles from hamsters that were orally challenged with scrapie was screened for the presence of a molecular marker for TSE infection, PrPSc (the pathological isoform of the prion protein PrP). Sensitive western blotting revealed consistent PrPSc accumulation in skeletal muscles from forelimb and hindlimb, head, back and shoulder, and in tongue. Previously, our animal model has provided substantial baseline information about the peripheral routing of infection in naturally occurring and orally acquired ruminant TSEs. Therefore, the findings described here highlight further the necessity to investigate thoroughly whether muscles of TSE-infected sheep, cattle, elk and deer contain infectious agents.

Distinct profiles of PrP(d) immunoreactivity in the brain of scrapie- and BSE-infected sheep: implications for differential cell targeting and PrP processing

Previous studies have shown that the patterns of disease-specific prion protein (PrP(d)) accumulation in the brain (the 'PrP(d) profile') of scrapie-affected sheep are mainly influenced by the source of scrapie agent. We have now extended those studies to investigate the effect of different PrP antibodies on the PrP(d) profile of scrapie- and bovine spongiform encephalopathy (BSE)-affected sheep. Immunohistochemical examination of brains of 20 sheep was performed with four different PrP antibodies (P4, 521.7, 505.2 and R486), and the animals were allocated to four groups of five sheep each depending on the transmissible spongiform encephalopathy (TSE) agent source (two natural scrapie sources, SSBP/1 and BSE). Although the PrP(d) profiles depended on the antibody used, the four TSE sources could always be differentiated. Natural Suffolk scrapie showed the highest levels of glia-associated PrP(d), natural Welsh Mountain scrapie uniquely had consistent vascular PrP(d) plaques, SSBP/1 produced the highest intracellular accumulations of PrP(d) and BSE led to moderate accumulation of all PrP(d) patterns except for vascular plaques. The variations in PrP(d) profile between TSE sources appeared to be the result of variations in cell tropism and in PrP processing. These processing differences are possibly associated with changes in PrP(d) conformation, and are manifest as differences in intracellular truncation and in release to the extracellular space of the abnormal protein. Moreover, variations in PrP(d) conformation would appear to be also influenced by the cell type supporting infection, arguing that it is modulated by the interaction between the infectious agent and the host.