Immunohistochemical detection and distribution of prion protein in a goat with natural scrapie

Differential Accumulation of Misfolded Prion Strains in Natural Hosts of Prion Diseases

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs), are a group of neurodegenerative protein misfolding diseases that invariably cause death. TSEs occur when the endogenous cellular prion protein (PrP^C) misfolds to form the pathological prion protein (PrP^Sc), which templates further conversion of PrP^C to PrP^Sc, accumulates, and initiates a cascade of pathologic processes in cells and tissues. Different strains of prion disease within a species are thought to arise from the differential misfolding of the prion protein and have different clinical phenotypes. Different strains of prion disease may also result in differential accumulation of PrP^Sc in brain regions and tissues of natural hosts. Here, we review differential accumulation that occurs in the retinal ganglion cells, cerebellar cortex and white matter, and plexuses of the enteric nervous system in cattle with bovine spongiform encephalopathy, sheep and goats with scrapie, cervids with chronic wasting disease, and humans with prion diseases. By characterizing TSEs in their natural host, we can better understand the pathogenesis of different prion strains. This information is valuable in the pursuit of evaluating and discovering potential biomarkers and therapeutics for prion diseases.

Detection of Pathognomonic Biomarker PrPSc and the Contribution of Cell Free-Amplification Techniques to the Diagnosis of Prion Diseases

Transmissible spongiform encephalopathies or prion diseases are rapidly progressive neurodegenerative diseases, the clinical manifestation of which can resemble other promptly evolving neurological maladies. Therefore, the unequivocal ante-mortem diagnosis is highly challenging and was only possible by histopathological and immunohistochemical analysis of the brain at necropsy. Although surrogate biomarkers of neurological damage have become invaluable to complement clinical data and provide more accurate diagnostics at early stages, other neurodegenerative diseases show similar alterations hindering the differential diagnosis. To solve that, the detection of the pathognomonic biomarker of disease, PrP^Sc, the aberrantly folded isoform of the prion protein, could be used. However, the amounts in easily accessible tissues or body fluids at pre-clinical or early clinical stages are extremely low for the standard detection methods. The solution comes from the recent development of in vitro prion propagation techniques, such as Protein Misfolding Cyclic Amplification (PMCA) and Real Time-Quaking Induced Conversion (RT-QuIC), which have been already applied to detect minute amounts of PrP^Sc in different matrixes and make early diagnosis of prion diseases feasible in a near future. Herein, the most relevant tissues and body fluids in which PrP^Sc has been detected in animals and humans are being reviewed, especially those in which cell-free prion propagation systems have been used with diagnostic purposes.

Abnormal Prion Accumulation Associated with Retinal Pathology in Experimentally Inoculated Scrapie-Affected Sheep

The purpose of this study was to characterize the patterns of PrPSc immunoreactivity in the retinae of scrapie-affected sheep and to determine the extent of retinal pathology as indicated by glial fibrillary acidic protein immunoreactivity (GFAP-IR) of Müller glia. Sections from the retina of 13 experimentally inoculated scrapie-affected and 2 negative control sheep were examined with immunohistochemical staining for PrPSc, GFAP, and PrPSc/GFAP double staining. GFAP-IR of Müller glia is suggestive of retinal pathology in the absence of morphologic abnormality detected by light microscopy. Sheep with the least amount of PrPSc in the retina have multifocal punctate aggregates of prion staining in the outer half of the inner plexiform layer and rarely in the outer plexiform layer. In these retinae, GFAP-IR is not localized with prion accumulation, but rather is present in moderate numbers of Müller glia throughout the sections of retina examined. The majority of sheep with retinal accumulation of PrPSc have intense, diffuse PrPSc staining in both plexiform layers, with immunoreactivity in the cytoplasm of multiple ganglion cells and lesser amounts in the optic fiber layer and between nuclei in nuclear layers. This intense PrPSc immunoreactivity is associated with diffuse, intense GFAP-IR that extends from the inner limiting membrane to the outer limiting membrane. This is the first report of a prion disease in a natural host that describes the accumulation of PrPSc in retina associated with retinal pathology in the absence of overt morphologic changes indicative of retinal degeneration.

Transmission of sheep-bovine spongiform encephalopathy to pigs

Experimental transmission of the bovine spongiform encephalopathy (BSE) agent has been successfully reported in pigs inoculated via three simultaneous distinct routes (intracerebral, intraperitoneal and intravenous). Sheep derived BSE (Sh-BSE) is transmitted more efficiently than the original cattle-BSE isolate in a transgenic mouse model expressing porcine prion protein. However, the neuropathology and distribution of Sh-BSE in pigs as natural hosts, and susceptibility to this agent, is unknown. In the present study, seven pigs were intracerebrally inoculated with Sh-BSE prions. One pig was euthanized for analysis in the preclinical disease stage. The remaining six pigs developed neurological signs and histopathology revealed severe spongiform changes accompanied by astrogliosis and microgliosis throughout the central nervous system. Intracellular and neuropil-associated pathological prion protein (PrP^Sc) deposition was consistently observed in different brain sections and corroborated by Western blot. PrP^Sc was detected by immunohistochemistry and enzyme immunoassay in the following tissues in at least one animal: lymphoid tissues, peripheral nerves, gastrointestinal tract, skeletal muscle, adrenal gland and pancreas. PrP^Sc deposition was revealed by immunohistochemistry alone in the retina, optic nerve and kidney. These results demonstrate the efficient transmission of Sh-BSE in pigs and show for the first time that in this species propagation of bovine PrP^Sc in a wide range of peripheral tissues is possible. These results provide important insight into the distribution and detection of prions in non-ruminant animals.

Sensitive and specific detection of classical scrapie prions in the brains of goats by real-time quaking-induced conversion

Real-time quaking-induced conversion (RT-QuIC) is a rapid, specific and highly sensitive prion seeding activity detection assay that uses recombinant prion protein (rPrPSen) to detect subinfectious levels of the abnormal isoforms of the prion protein (PrPSc). Although RT-QuIC has been successfully used to detect PrPSc in various tissues from humans and animals, including sheep, tissues from goats infected with classical scrapie have not yet been tested. Therefore, the aims of the present study were to (1) evaluate whether prion seeding activity could be detected in the brain tissues of goats with scrapie using RT-QuIC, (2) optimize reaction conditions to improve scrapie detection in goats, and (3) compare the performance of RT-QuIC for the detection of PrPSc with the more commonly used ELISA and Western blot assays. We further optimized RT-QuIC conditions for sensitive and specific detection of goat scrapie seeding activity in brain tissue from clinical animals. When used with 200  mM sodium chloride, both full-length sheep rPrPSen substrates (PrP genotypes A136R154Q171 and V136R154Q171) provided good discrimination between scrapie-infected and normal goat brain samples at 10(- )3 dilution within 15  h. Our findings indicate that RT-QuIC was at least 10,000-fold more sensitive than ELISA and Western blot assays for the detection of scrapie seeding activity in goat brain samples. In addition to PRNP WT samples, positive RT-QuIC reactions were also observed with three PRNP polymorphic goat brain samples (G/S127, I/M142 and H/R143) tested. Taken together, these findings demonstrate that RT-QuIC sensitively detects prion seeding activity in classical scrapie-infected goat brain samples.

Evaluation of two commercial, rapid, ELISA kits testing for scrapie in retro-pharyngeal lymph nodes in sheep

AIMS

To estimate the number of cases of scrapie that would occur in sheep of different prion protein (PrP) genotypes if scrapie was to become established in New Zealand, and to compare the performance of two commercially available, rapid ELISA kits using ovine retro-pharyngeal lymph nodes (RLN) from non-infected and infected sheep of different PrP genotypes.

METHODS

Using published data on the distribution of PrP genotypes within the New Zealand sheep flock and the prevalence of cases of scrapie in these genotypes in the United Kingdom, the annual expected number of cases of scrapie per genotype was estimated, should scrapie become established in New Zealand, assuming a total population of 28 million sheep. A non-infected panel of RLN was collected from 737 sheep from New Zealand that had been culled, found in extremis or died. Brain stem samples were also collected from 131 of these sheep. A second panel of infected samples comprised 218 and 117 RLN from confirmed scrapie cases that had originated in Europe and the United States of America, respectively. All samples were screened using two commercial, rapid, transmissible spongiform encephalopathy ELISA kits: Bio-Rad TeSeE ELISA (ELISA-BR), and IDEXX HerdChek BSE-Scrapie AG Test (ELISA-ID).

RESULTS

If scrapie became established in New Zealand, an estimated 596 cases would occur per year; of these 234 (39%) and 271 (46%) would be in sheep carrying ARQ/ARQ and ARQ/VRQ PrP genotypes, respectively. For the non-infected samples from New Zealand the diagnostic specificity of both ELISA kits was 100%. When considering all infected samples, the diagnostic sensitivity was 70.4 (95% CI=65.3-75.3)% for ELISA-BR and 91.6 (95% CI=88.2-94.4)% for ELISA-ID. For the ARQ/ARQ genotype (n=195), sensitivity was 66.2% for ELISA-BR and 90.8% for ELISA-ID, and for the ARQ/VRQ genotype (n=107), sensitivity was 81.3% for ELISA-BR and 98.1% for ELISA-ID.

CONCLUSIONS

In this study, the ELISA-ID kit demonstrated a higher diagnostic sensitivity for detecting scrapie in samples of RLN from sheep carrying scrapie-susceptible PrP genotypes than the ELISA-BR kit at comparable diagnostic specificity.

CLINICAL RELEVANCE

The diagnostic performance of the ELISA-ID kit using ovine RLN merits the consideration of including this assay in the national scrapie surveillance programme in New Zealand.

Methods for olfactory fMRI studies: Implication of respiration

Human olfactory system is under-studied using fMRI compared with other sensory systems. Because the perception (intensity, threshold, and valence) and detection of odors are tightly involved with respiration, the subject's respiration pattern modulates and interacts with the experimental paradigm, which presents difficulties for olfactory fMRI data acquisition, post-processing, and interpretation. Based on our investigation on the interactions of odor presentation and subject's respiration, we propose a respiration-triggered event-related olfactory fMRI technique and a data post-processing method that effectively captures precise onsets of olfactory blood-oxygen-level-dependent (BOLD) signal in the primary olfactory cortex. We compared the olfactory BOLD signals from seventeen normal healthy adults with diverse respiratory patterns and showed that the subjects' respiratory patterns modulated the olfactory stimulation paradigm, which significantly confounded the BOLD signal. The presented experimental technique provides a simple and effective means for generating reliable olfactory fMRI results.

Lysine at position 222 of the goat prion protein inhibits the binding of monoclonal antibody F99/97.6.1

Prion protein (PrP) is encoded by the PRNP gene, which is highly polymorphic in goats, with polymorphisms encoding amino acid substitutions at the protein level. In the current study, the reactivity of monoclonal antibody (mAb) F99/97.6.1 in binding PrP from goats polymorphic at PRNP codon 222 was investigated. Nervous tissue from 30 scrapie-negative goats with 3 different genotypes (222Q/Q, 222Q/K, and 222K/K) was analyzed by Western blot using mAbs P4 and F99/97.6.1. Although PrP was detected in all 30 samples by mAb P4, detection of PrP by mAb F99/97.6.1 was limited to 222Q/Q (12/12). No PrP was detected by mAb F99/97.6.1 in the 222K/K samples (n = 6), and the signal intensity of mAb F99/97.6.1 for PrP was lower for the 222Q/K samples (12/12 samples). To further investigate these results, additional Western blot analyses were performed, and the PrP signals detected by mAbs F99/97.6.1 and SAF84 were then quantified. The mean F99/SAF84 ratio (± standard deviation) calculated for the 222Q/Q group was 0.73 ± 1.26, and the mean for the 222Q/K group was 0.27 ± 1.31. Statistical analysis of these values evidenced statistically significant differences between the 222Q/Q and 222Q/K samples. The results of the study thus revealed an inhibition by lysine at position 222 on the binding of mAb F99/97.6.1 to goat PrP. This has implications for the use of mAb F99/97.6.1 for diagnostic purposes. Because the 222K allele could be a target for genetic selection in goats, the differential reactivity of mAb F99/97.6.1 could be exploited with a genotyping test setup.

Disease-associated prion protein in neural and lymphoid tissues of mink (Mustela vison) inoculated with transmissible mink encephalopathy

Transmissible spongiform encephalopathies (TSEs) are diagnosed by immunodetection of disease-associated prion protein (PrP(d)). The distribution of PrP(d) within the body varies with the time-course of infection and between species, during interspecies transmission, as well as with prion strain. Mink are susceptible to a form of TSE known as transmissible mink encephalopathy (TME), presumed to arise due to consumption of feed contaminated with a single prion strain of ruminant origin. After extended passage of TME isolates in hamsters, two strains emerge, HY and DY, each of which is associated with unique structural isoforms of PrP(TME) and of which only the HY strain is associated with accumulation of PrP(TME) in lymphoid tissues. Information on the structural nature and lymphoid accumulation of PrP(TME) in mink is limited. In this study, 13 mink were challenged by intracerebral inoculation using late passage TME inoculum, after which brain and lymphoid tissues were collected at preclinical and clinical time points. The distribution and molecular nature of PrP(TME) was investigated by techniques including blotting of paraffin wax-embedded tissue and epitope mapping by western blotting. PrP(TME) was detected readily in the brain and retropharyngeal lymph node during preclinical infection, with delayed progression of accumulation within other lymphoid tissues. For comparison, three mink were inoculated by the oral route and examined during clinical disease. Accumulation of PrP(TME) in these mink was greater and more widespread, including follicles of rectoanal mucosa-associated lymphoid tissue. Western blot analyses revealed that PrP(TME) accumulating in the brain of mink is structurally most similar to that accumulating in the brain of hamsters infected with the DY strain. Collectively, the results of extended passage in mink are consistent with the presence of only a single strain of TME, the DY strain, capable of inducing accumulation of PrP(TME) in the lymphoid tissues of mink but not in hamsters. Thus, mink are a relevant animal model for further study of this unique strain, which ultimately may have been introduced through consumption of a TSE of ruminant origin.

Resistance to classical scrapie in experimentally challenged goats carrying mutation K222 of the prion protein gene

Susceptibility of sheep to scrapie, a transmissible spongiform encephalopathy of small ruminants, is strongly influenced by polymorphisms of the prion protein gene (PRNP). Breeding programs have been implemented to increase scrapie resistance in sheep populations; though desirable, a similar approach has not yet been applied in goats. European studies have now suggested that several polymorphisms can modulate scrapie susceptibility in goats: in particular, PRNP variant K222 has been associated with resistance in case-control studies in Italy, France and Greece. In this study we investigated the resistance conferred by this variant using a natural Italian goat scrapie isolate to intracerebrally challenge five goats carrying genotype Q/Q 222 (wild type) and five goats carrying genotype Q/K 222. By the end of the study, all five Q/Q 222 goats had died of scrapie after a mean incubation period of 19 months; one of the five Q/K 222 goats died after 24 months, while the other four were alive and apparently healthy up to the end of the study at 4.5 years post-challenge. All five of these animals were found to be scrapie negative. Statistical analysis showed that the probability of survival of the Q/K 222 goats versus the Q/Q 222 goats was significantly higher (p = 0.002). Our study shows that PRNP gene mutation K222 is strongly associated with resistance to classical scrapie also in experimental conditions, making it a potentially positive target for selection in the frame of breeding programs for resistance to classical scrapie in goats.

Cell-surface expression of PrPC and the presence of scrapie prions in the blood of goats

Although host-encoded prion protein (PrP(C)) expression in ovine PBMCs and prion infectivity in scrapie-infected sheep blood have been demonstrated, such studies have not been reported in goats. Therefore, this study characterized cell-surface expression of PrP(C) on PBMC subsets derived from normal goats and sheep, by flow cytometry, and determined prion infectivity in blood from a scrapie-infected goat using a transfusion bioassay in goat kids. Cell-surface PrP(C) expression was detected on all subsets of goat PBMCs. The highest PrP(C) cell-surface expression was found in CD2(+) T lymphocytes in goats. Transmission of infection was detected in all three recipients who received whole blood from a goat with classical scrapie. It was concluded that caprine PBMCs express PrP(C) similarly to sheep but with relative differences among PBMCs subsets, and that blood-borne infectious prions can be detected in scrapie-infected goats. Thus, similar to sheep, goat blood may be a suitable diagnostic target for the detection of scrapie infection.

Detection of disease-associated prion protein in the optic nerve and the adrenal gland of cattle with bovine spongiform encephalopathy by using highly sensitive immunolabeling procedures

A sensitive immunohistochemical procedure, the tyramide signal amplification (TSA) system, was applied to detect the localization of immunolabeled disease-associated prion protein (PrP(Sc)) in cattle affected with bovine spongiform encephalopathy (BSE). In this procedure, immunolabeling could be visualized in the optic nerve and the adrenal medulla. In the optic nerve, the dual immunofluorescent technique showed that the granular PrP(Sc) was occasionally detected in the astrocytes, microglia, and myelin sheath adjacent to the axon. Clustered PrP(Sc) was also scattered in association with microglial cells and astrocytes of the optic nerve. In the adrenal gland, PrP(Sc) immunolabeling was confined within the sympathetic nerve fibers and endings. The results suggest that (1) PrP(Sc) might centrifugally spread within and between glial cells and/or the non-axonal (also known as ad-axonal) region of nerve fibers, rather than the axonal and/or extracellular space pathway in the optic nerve, and (2) the sympathetic innervations might be important for the trafficking of BSE agent in the adrenal glands of cattle. This study also suggests that tyramide-based immunochemical analysis should be performed to detect immunolabeled PrP(Sc) in the extracerebral tissues of BSE-affected cattle.

State-of-the-art review of goat TSE in the European Union, with special emphasis on PRNP genetics and epidemiology

Scrapie is a fatal, neurodegenerative disease of sheep and goats. It is also the earliest known member in the family of diseases classified as transmissible spongiform encephalopathies (TSE) or prion diseases, which includes Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy (BSE), and chronic wasting disease in cervids. The recent revelation of naturally occurring BSE in a goat has brought the issue of TSE in goats to the attention of the public. In contrast to scrapie, BSE presents a proven risk to humans. The risk of goat BSE, however, is difficult to evaluate, as our knowledge of TSE in goats is limited. Natural caprine scrapie has been discovered throughout Europe, with reported cases generally being greatest in countries with the highest goat populations. As with sheep scrapie, susceptibility and incubation period duration of goat scrapie are most likely controlled by the prion protein (PrP) gene (PRNP). Like the PRNP of sheep, the caprine PRNP shows significantly greater variability than that of cattle and humans. Although PRNP variability in goats differs from that observed in sheep, the two species share several identical alleles. Moreover, while the ARR allele associated with enhancing resistance in sheep is not present in the goat PRNP, there is evidence for the existence of other PrP variants related to resistance. This review presents the current knowledge of the epidemiology of caprine scrapie within the major European goat populations, and compiles the current data on genetic variability of PRNP.

Retinal Function and Morphology Are Altered in Cattle Infected with the Prion Disease Transmissible Mink Encephalopathy

Transmissible spongiform encephalopathies (TSEs) are a group of diseases that result in progressive and invariably fatal neurologic disease in both animals and humans. TSEs are characterized by the accumulation of an abnormal protease-resistant form of the prion protein in the central nervous system. Transmission of infectious TSEs is believed to occur via ingestion of prion protein–contaminated material. This material is also involved in the transmission of bovine spongiform encephalopathy (“mad cow disease”) to humans, which resulted in the variant form of Creutzfeldt-Jakob disease. Abnormal prion protein has been reported in the retina of TSE-affected cattle, but despite these observations, the specific effect of abnormal prion protein on retinal morphology and function has not been assessed. The objective of this study was to identify and characterize potential functional and morphologic abnormalities in the retinas of cattle infected with a bovine-adapted isolate of transmissible mink encephalopathy. We used electroretinography and immunohistochemistry to examine retinas from 10 noninoculated and 5 transmissible mink encephalopathy–inoculated adult Holstein steers. Here we show altered retinal function, as evidenced by prolonged implicit time of the electroretinogram b-wave, in transmissible mink encephalopathy–infected cattle before the onset of clinical illness. We also demonstrate disruption of rod bipolar cell synaptic terminals, indicated by decreased immunoreactivity for the alpha isoform of protein kinase C and vesicular glutamate transporter 1, and activation of Müller glia, as evidenced by increased glial fibrillary acidic protein and glutamine synthetase expression, in the retinas of these cattle at the time of euthanasia due to clinical deterioration. This is the first study to identify both functional and morphologic alterations in the retinas of TSE-infected cattle. Our results support future efforts to focus on the retina for the development of new strategies for the diagnosis of TSEs.

Role of the lymphoreticular system in prion neuroinvasion from the oral and nasal mucosa

Prion neuroinvasion from peripheral tissues involves agent replication in the lymphoreticular system (LRS) prior to entry into the nervous system. This study investigated the role of the LRS in prion neuroinvasion from the oral and nasal mucosa in wild-type and immunodeficient mice and in hamsters infected with the HY and DY strains of the transmissible mink encephalopathy (TME) agent. Following inoculation at neural sites, all hosts were susceptible to prion disease and had evidence of prion infection in the brain, but infection of the LRS was found only in scrapie-infected wild-type mice and HY TME-infected hamsters. In the LRS replication-deficient models, prion neuroinvasion was not observed following intraperitoneal or oral inoculation. However, immunodeficient mice, which have impaired follicular dendritic cells, were susceptible to scrapie following intratongue and intranasal inoculation despite the absence of PrP(Sc) in the tongue or the nasal cavity. For DY TME, hamsters were susceptible following intratongue but not intranasal inoculation and PrP(Sc) was limited to nerve fibers of the tongue. These findings indicate that neuroinvasion from the tongue and nasal cavity can be independent of LRS infection but neuroinvasion was partially dependent on the strain of the prion agent and/or the host species. The paucity of PrP(Sc) deposition in the oral and nasal mucosa from LRS replication-deficient hosts following neuroinvasion from these tissues suggests an infection of nerve fibers that is below the threshold of PrP(Sc) detection and/or the transport of the prion agent along cranial nerves without agent replication.

Atypical PrPsc distribution in goats naturally affected with scrapie

The brain and spinal cord of 48 goats from two Greek herds in which scrapie had been reported were examined. All animals were symptomless at the time of euthanasia. Notably, no lesions were observed either at the level of the obex or at other regions of the brain and spinal cord. Immunohistochemical examination revealed PrPsc labelling of the linear and fine punctuate types, mainly in the cerebral cortices, of 36 goats. Twenty-seven of them were negative by ELISA (designed to detect proteinase-resistant PrP) at the level of the obex but positive in a pooled brain sample, and the majority carried PrP genotypes associated with scrapie susceptibility. Surprisingly, in 16 of the 27 animals, PrPsc deposits were detected only in the rostral parts of the brain. In addition, nine animals which were ELISA-positive at the level of the obex exhibited positive immunoreactivity, but not in the dorsal vagal nucleus. The findings indicate that this unusual scrapie type may have been underdiagnosed previously and may be of importance in scrapie surveillance programmes.

Retinal cell types are differentially affected in sheep with scrapie

Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases characterized microscopically by spongiform lesions (vacuolation) in the neuropil, neuronal loss, and gliosis. Accumulation of the abnormal form of the prion protein (PrP(Sc)) has been demonstrated in the retina of natural and non-natural TSE-affected hosts, with or without evidence of microscopically detectable retinal pathology. This study was conducted to investigate the effect of PrP(Sc) accumulation on retinal neurons in a natural host lacking overt microscopical evidence of retinal degeneration by comparing the distribution of retinal cell type-specific markers in control and scrapie-affected sheep. In retinas with PrP(Sc)-immunoreactivity, there was disruption of the normal immunoreactivity patterns of the alpha isoform of protein kinase C (PKCalpha) and vesicular glutamate transporter 1 (VGLUT1), markers of retinal bipolar cells. Altered immunoreactivity was also observed for microtubule-associated protein 2 (MAP2), a marker of a subset of retinal ganglion cells, and glutamine synthetase (GS), a marker of Müller glia. These results demonstrate alterations of immunoreactivity patterns for proteins associated with specific cell types in retinas with PrP(Sc) accumulation, despite an absence of microscopical evidence of retinal degeneration.

Atypical scrapie in a Swiss goat and implications for transmissible spongiform encephalopathy surveillance

Different types of transmissible spongiform encephalopathies (TSEs) affect sheep and goats. In addition to the classical form of scrapie, both species are susceptible to experimental infections with the bovine spongiform encephalopathy (BSE) agent, and in recent years atypical scrapie cases have been reported in sheep from different European countries. Atypical scrapie in sheep is characterized by distinct histopathologic lesions and molecular characteristics of the abnormal scrapie prion protein (PrP(sc)). Characteristics of atypical scrapie have not yet been described in detail in goats. A goat presenting features of atypical scrapie was identified in Switzerland. Although there was no difference between the molecular characteristics of PrP(sc) in this animal and those of atypical scrapie in sheep, differences in the distribution of histopathologic lesions and PrP(sc) deposition were observed. In particular the cerebellar cortex, a major site of PrP(sc) deposition in atypical scrapie in sheep, was found to be virtually unaffected in this goat. In contrast, severe lesions and PrP(sc) deposition were detected in more rostral brain structures, such as thalamus and midbrain. Two TSE screening tests and PrP(sc) immunohistochemistry were either negative or barely positive when applied to cerebellum and obex tissues, the target samples for TSE surveillance in sheep and goats. These findings suggest that such cases may have been missed in the past and could be overlooked in the future if sampling and testing procedures are not adapted. The epidemiological and veterinary public health implications of these atypical cases, however, are not yet known.

Interaction between dendritic cells and nerve fibres in lymphoid organs after oral scrapie exposure

In transmissible spongiform encephalopathies (TSEs), the infectious agent, called PrPsc, an abnormal isoform of the cellular prion protein, accumulates and replicates in lymphoid organs before affecting the nervous system. To clarify the cellular requirements for the neuroinvasion of the scrapie agent from the lymphoid organs to the central nervous system, we have studied, by confocal microscopy, the innervations within Peyer's patches, mesenteric lymph nodes and the spleen of mice in physiological conditions and after oral exposure to prion. Contacts between nerve fibres and PrPsc-associated cells, dendritic cells (DCs) and follicular dendritic cells (FDCs), were evaluated in preclinical prion-infected mice. Using a double immunolabelling strategy, we demonstrated the lack of innervation of PrPsc-accumulating cells (FDCs). Contacts between nerve fibers and PrPsc-propagating cells (DCs) were detected in T-cell zones and cell-trafficking areas. This supports, for the first time, the possible implication of dendritic cells in the prion neuroinvasion process.

Histopathological and immunohistochemical features of natural goat scrapie

Histopathological and immunohistochemical examinations were performed on the brain and spinal cord of 37 goats from two Greek herds in which scrapie had been reported. Of the 37 animals, 18 were from a herd consisting only of goats and 19 were from a herd of goats mixed with sheep. The goats studied were grouped on the basis of the presence or absence of clinical signs. Distinctive lesions and PrP(sc) (PrP, prion protein) deposition were found in the central nervous system (CNS) of eight clinically affected animals and six symptomless animals. The lesion profile and PrP(sc) distribution varied both between and within groups, variation being particularly pronounced in the symptomless goats. The results concerning the latter group suggested a poor correlation between the intensity of lesions, the amount of PrP(sc) in the CNS, and the manifestation of clinical signs. Immunohistochemical examination revealed 10 different PrP(sc) types, four of which are reported for the first time in goats. All scrapie-affected animals carried the VV(21)II(142)HH(143)RR(154) genotype, with the exception of two goats that carried the HR(143) dimorphism and had detectable PrP(sc) deposits. The results suggest that the histopathological and immunohistochemical profile of the natural disease in goats is influenced by the PrP genotype and age of the animals but may not be directly associated with the presence or otherwise of clinical signs.

Pathological findings in retina and visual pathways associated to natural Scrapie in sheep

This work represents a comprehensive pathological description of the retina and visual pathways in naturally affected Scrapie sheep. Twenty naturally affected Scrapie sheep and 6 matched controls were used. Eyes, optic nerves and brain from each animal were fixed and histologically processed using hematoxylin-eosin, followed by immunohistochemical staining for prion protein (PrPsc) and glial fibrillar acidic protein (GFAP). Retinal histopathological changes were observed in only 7 clinically affected animals and mainly consisted of loss of outer limitant layer definition, outer plexiform layer atrophy, disorganization and loss of nuclei in both nuclear layers, and Müller glia hypertrophy. PrPsc was detected in the retina of 19 of the 20 sheep and characterized by a disseminated granular deposit across layers and intraneuronally in ganglion cells. The inner plexiform and the ganglion cell layers were the structures most severely affected by PrPsc deposits. PrPsc exhibited a tendency to spread from these two layers to the others. A marked increase in the number and intensity of GFAP-expressing Müller cells was observed in the clinical stage, especially at the terminal stage of the disease. Spongiosis and PrPsc were detected within the visual pathways at the preclinical stage, their values increasing during the course of the disease but varying between the areas examined. PrPsc was detected in only 3 optic nerves. The results suggest that the presence of PrPsc in the retina correlates with disease progression during the preclinical and clinical stages, perhaps using the inner plexiform layer as a first entry site and diffusing from the brain using a centrifugal model.

Rapid and discriminatory diagnosis of scrapie and BSE in retro-pharyngeal lymph nodes of sheep

BACKGROUND

Diagnosis based on prion detection in lymph nodes of sheep and goats can improve active surveillance for scrapie and, if it were circulating, for bovine spongiform encephalopathy (BSE). With sizes that allow repetitive testing and a location that is easily accessible at slaughter, retropharyngeal lymph nodes (RLN) are considered suitable organs for testing. Western blotting (WB) of brain homogenates is, in principle, a technique well suited to both detect and discriminate between scrapie and BSE. In this report, WB is developed for rapid diagnosis in RLN and to study biochemical characteristics of PrPres.

RESULTS

Optimal PrPres detection in RLN by WB was achieved by proper tissue processing, antibody choice and inclusion of a step for PrPresconcentration. The analyses were performed on three different sheep sources. Firstly, in a study with preclinical scrapie cases, WB of RLN from infected sheep of VRQ/VRQ genotype--VRQ represents, respectively, polymorphic PrP amino acids 136, 154, and 171--allowed a diagnosis 14 mo earlier compared to WB of brain stem. Secondly, samples collected from sheep with confirmed scrapie in the course of passive and active surveillance programmes in the period 2002-2003 yielded positive results depending on genotype: all sheep with genotypes ARH/VRQ, VRQ/VRQ, and ARQ/VRQ scored positive for PrPres, but ARQ/ARQ and ARR/VRQ were not all positive. Thirdly, in an experimental BSE study, detection of PrPres in all 11 ARQ/ARQ sheep, including 7 preclinical cases, was possible. In all instances, WB and IHC were almost as sensitive. Moreover, BSE infection could be discriminated from scrapie infection by faster electrophoretic migration of the PrPres bands. Using dual antibody staining with selected monoclonal antibodies like 12B2 and L42, these differences in migration could be employed for an unequivocal differentiation between BSE and scrapie. With respect to glycosylation of PrPres, BSE cases exhibited a greater diglycosylated fraction than scrapie cases. Furthermore, a slight time dependent increase of diglycosylated PrPres was noted between individual sheep, which was remarkable in that it occurred in both scrapie and BSE study.

CONCLUSION

The present data indicate that, used in conjunction with testing in brain, WB of RLN can be a sensitive tool for improving surveillance of scrapie and BSE, allowing early detection of BSE and scrapie and thereby ensuring safer sheep and goat products.

Prion infection of oral and nasal mucosa

Centrifugal spread of the prion agent to peripheral tissues is postulated to occur by axonal transport along nerve fibers. This study investigated the distribution of the pathological isoform of the protein (PrP(Sc)) in the tongues and nasal cavities of hamsters following intracerebral inoculation of the HY strain of the transmissible mink encephalopathy (TME) agent. We report that PrP(Sc) deposition was found in the lamina propria, taste buds, and stratified squamous epithelium of fungiform papillae in the tongue, as well as in skeletal muscle cells. Using laser scanning confocal microscopy, PrP(Sc) was localized to nerve fibers in each of these structures in the tongue, neuroepithelial taste cells of the taste bud, and, possibly, epithelial cells. This PrP(Sc) distribution was consistent with a spread of HY TME agent along both somatosensory and gustatory cranial nerves to the tongue and suggests subsequent synaptic spread to taste cells and epithelial cells via peripheral synapses. In the nasal cavity, PrP(Sc) accumulation was found in the olfactory and vomeronasal epithelium, where its location was consistent with a distribution in cell bodies and apical dendrites of the sensory neurons. Prion spread to these sites is consistent with transport via the olfactory nerve fibers that descend from the olfactory bulb. Our data suggest that epithelial cells, neuroepithelial taste cells, or olfactory sensory neurons at chemosensory mucosal surfaces, which undergo normal turnover, infected with the prion agent could be shed and play a role in the horizontal transmission of animal prion diseases.

Suppression of prion protein in livestock by RNA interference

Given the difficulty of applying gene knockout technology to species other than mice, we decided to explore the utility of RNA interference (RNAi) in silencing the expression of genes in livestock. Short hairpin RNAs (shRNAs) were designed and screened for their ability to suppress the expression of caprine and bovine prion protein (PrP). Lentiviral vectors were used to deliver a transgene expressing GFP and an shRNA targeting PrP into goat fibroblasts. These cells were then used for nuclear transplantation to produce a cloned goat fetus, which was surgically recovered at 81 days of gestation and compared with an age-matched control derived by natural mating. All tissues examined in the cloned fetus expressed GFP, and PCR analysis confirmed the presence of the transgene encoding the PrP shRNA. Most relevant, Western blot analysis performed on brain tissues comparing the transgenic fetus with control demonstrated a significant (>90%) decrease in PrP expression levels. To confirm that similar methodologies could be applied to the bovine, recombinant virus was injected into the perivitelline space of bovine ova. After in vitro fertilization and culture, 76% of the blastocysts exhibited GFP expression, indicative that they expressed shRNAs targeting PrP. Our results provide strong evidence that the approach described here will be useful in producing transgenic livestock conferring potential disease resistance and provide an effective strategy for suppressing gene expression in a variety of large-animal models.

Comparison of immunohistochemistry and two rapid tests for detection of abnormal prion protein in different brain regions of sheep with typical scrapie

One of the "gold standard" techniques for postmortem confirmation of scrapie diagnosis in sheep and goats is immunohistochemical examination of brain tissue. Active surveillance for scrapie is mainly performed by rapid diagnostic tests on the basis of postmortem immunochemical detection of prion protein (PrP) in the obex tissue. The aim of this study was to determine the performance of 2 rapid tests, Prionics-Check LIA (a chemiluminescence sandwich enzyme-linked immunosorbent assay) and Prionics-Check Western blot for scrapie diagnosis when applied to brain areas other than the obex, in comparison with the recognized immunohistochemistry. Prion protein was detected in the obex, cervical spinal cord, and thalamus from all the scrapie-positive sheep by the 3 tests. Western blot and LIA were negative in other areas of the brain, although weak immunohistochemical staining was detected. The results show that the 2 rapid tests studied may detect PrP in brain areas other than the obex, although with a lower sensitivity than immunohistochemistry when there is minimal PrP deposition.

Polymorphisms in the prion precursor functional gene but not the pseudogene are associated with susceptibility to chronic wasting disease in white-tailed deer

Chronic wasting disease (CWD) status and PrP genotypes were determined for a group of 133 wild white-tailed deer in a 780 acre enclosure in western Nebraska, USA. Approximately half of the deer tested showed evidence of PrPd in the brainstem or lymphoid tissues. Four PRNP alleles encoding amino acid substitutions were identified, with substitutions at residues 95 (Q-->H), 96 (G-->S) or 116 (A-->G), each with serine (S) at residue 138. In addition, a processed pseudogene with two alleles encoding five or six copies of the octapeptide repeat was identified in 26 % of the deer. Both alleles encoded asparagine (N) at residue 138. The functional gene alleles sorted into five major diploid genotypes and four rare genotypes. Although all five major diploid genotypes were found in deer with CWD, unaffected deer were less likely to have the allele QGAS and more likely to have QSAS compared with CWD-affected deer. Late-stage disease (PrPd in brainstem) was noted in deer less than 1 year of age, although no single genotype was associated with this rapid neuroinvasion. Early-stage disease (PrPd distribution limited to the lymphoid system) was observed in deer estimated to be more than 5 years old, suggesting that they were infected as adults or that the incubation time might be extremely long in some individuals. The pseudogene was found in deer of all major PRNP genotypes and was not correlated with CWD status. The large number of susceptible genotypes and the possibility of adult-to-adult transmission suggest that much of the white-tailed deer population may be at risk for disease following exposure to CWD, despite the association of specific genotypes with CWD noted here.