Astrocytosis and amyloid deposition in scrapie-infected hamsters

Prion strains: shining new light on old concepts

Prion diseases are a group of inevitably fatal neurodegenerative disorders affecting numerous mammalian species, including humans. The existence of heritable phenotypes of disease in the natural host suggested that prions exist as distinct strains. Transmission of sheep scrapie to rodent models accelerated prion research, resulting in the isolation and characterization of numerous strains with distinct characteristics. These strains are grouped into categories based on the incubation period of disease in different strains of mice and also by how stable the strain properties were upon serial passage. These classical studies defined the host and agent parameters that affected strain properties, and, prior to the advent of the prion hypothesis, strain properties were hypothesized to be the result of mutations in a nucleic acid genome of a conventional pathogen. The development of the prion hypothesis challenged the paradigm of infectious agents, and, initially, the existence of strains was difficult to reconcile with a protein-only agent. In the decades since, much evidence has revealed how a protein-only infectious agent can perform complex biological functions. The prevailing hypothesis is that strain-specific conformations of PrPSc encode prion strain diversity. This hypothesis can provide a mechanism to explain the observed strain-specific differences in incubation period of disease, biochemical properties of PrPSc, tissue tropism, and subcellular patterns of pathology. This hypothesis also explains how prion strains mutate, evolve, and adapt to new species. These concepts are applicable to prion-like diseases such as Parkinson's and Alzheimer's disease, where evidence of strain diversity is beginning to emerge.

Astrocyte in prion disease: a double-edged sword

Prion diseases are infectious protein misfolding disorders of the central nervous system that result from misfolding of the cellular prion protein (PrPC) into the pathologic isoform PrPSc. Pathologic hallmarks of prion disease are depositions of pathological prion protein PrPSc, neuronal loss, spongiform degeneration and astrogliosis in the brain. Prion diseases affect human and animals, there is no effective therapy, and they invariably remain fatal. For a long time, neuronal loss was considered the sole reason for neurodegeneration in prion pathogenesis, and the contribution of non-neuronal cells like microglia and astrocytes was considered less important. Recent evidence suggests that neurodegeneration during prion pathogenesis is a consequence of a complex interplay between neuronal and non-neuronal cells in the brain, but the exact role of these non-neuronal cells during prion pathology is still elusive. Astrocytes are non-neuronal cells that regulate brain homeostasis under physiological conditions. However, astrocytes can deposit PrPSc aggregates and propagate prions in prion-infected brains. Additionally, sub-populations of reactive astrocytes that include neurotrophic and neurotoxic species have been identified, differentially expressed in the brain during prion infection. Revealing the exact role of astrocytes in prion disease is hampered by the lack of in vitro models of prion-infected astrocytes. Recently, we established a murine astrocyte cell line persistently infected with mouse-adapted prions, and showed how such astrocytes differentially process various prion strains. Considering the complexity of the role of astrocytes in prion pathogenesis, we need more in vitro and in vivo models for exploring the contribution of sub-populations of reactive astrocytes, their differential regulation of signaling cascades, and the interaction with neurons and microglia during prion pathogenesis. This will help to establish novel in vivo models and define new therapeutic targets against prion diseases. In this review, we will discuss the complex role of astrocytes in prion disease, the existing experimental resources, the challenges to analyze the contribution of astrocytes in prion disease pathogenesis, and future strategies to improve the understanding of their role in prion disease.

The role of astrocytes in prion-like mechanisms of neurodegeneration

Accumulating evidence suggests that neurodegenerative diseases are not merely neuronal in nature but comprise multicellular involvement, with astrocytes emerging as key players. The pathomechanisms of several neurodegenerative diseases involve the deposition of misfolded protein aggregates in neurons that have characteristic prion-like behaviours such as template-directed seeding, intercellular propagation, distinct conformational strains and protein-mediated toxicity. The role of astrocytes in dealing with these pathological prion-like protein aggregates and whether their responses either protect from or conspire with the disease process is currently unclear. Here we review the existing literature implicating astrocytes in multiple neurodegenerative proteinopathies with a focus on prion-like behaviour in this context.

Classical and Atypical Scrapie in Sheep and Goats. Review on the Etiology, Genetic Factors, Pathogenesis, Diagnosis, and Control Measures of Both Diseases

Prion diseases, such as scrapie, are neurodegenerative diseases with a fatal outcome, caused by a conformational change of the cellular prion protein (PrP^C), originating with the pathogenic form (PrP^Sc). Classical scrapie in small ruminants is the paradigm of prion diseases, as it was the first transmissible spongiform encephalopathy (TSE) described and is the most studied. It is necessary to understand the etiological properties, the relevance of the transmission pathways, the infectivity of the tissues, and how we can improve the detection of the prion protein to encourage detection of the disease. The aim of this review is to perform an overview of classical and atypical scrapie disease in sheep and goats, detailing those special issues of the disease, such as genetic factors, diagnostic procedures, and surveillance approaches carried out in the European Union with the objective of controlling the dissemination of scrapie disease.

Cellular Prion Protein (PrPc): Putative Interacting Partners and Consequences of the Interaction

Cellular prion protein (PrPc) is a small glycosylphosphatidylinositol (GPI) anchored protein most abundantly found in the outer leaflet of the plasma membrane (PM) in the central nervous system (CNS). PrPc misfolding causes neurodegenerative prion diseases in the CNS. PrPc interacts with a wide range of protein partners because of the intrinsically disordered nature of the protein’s N-terminus. Numerous studies have attempted to decipher the physiological role of the prion protein by searching for proteins which interact with PrPc. Biochemical characteristics and biological functions both appear to be affected by interacting protein partners. The key challenge in identifying a potential interacting partner is to demonstrate that binding to a specific ligand is necessary for cellular physiological function or malfunction. In this review, we have summarized the intracellular and extracellular interacting partners of PrPc and potential consequences of their binding. We also briefly describe prion disease-related mutations at the end of this review.

Gene Targeted Transgenic Mouse Models in Prion Research

The production of transgenic mice expressing different forms of the prion protein (PrP) or devoid of PrP has enabled researchers to study the role of PrP in the infectious process of a prion disease and its normal function in the healthy individual. A wide range of transgenic models have been produced ranging from PrP null mice, normal expression levels to overexpression models, models expressing different species of the Prnp gene and different mutations and polymorphisms within the gene. Using this range of transgenic models has allowed us to define the influence of PrP expression on disease susceptibility and transmission, assess zoonotic potential, define strains of human prion diseases, elucidate the function of PrP, and start to unravel the mechanisms involved in chronic neurodegeneration. This chapter focuses mainly on the use of the gene targeted transgenic models and summarizes the ways in which they have allowed us to study the role of PrP in prion disease and the insights they have provided into the mechanisms of neurodegenerative diseases.

Cerebellar compartmentation of prion pathogenesis

In prion diseases, the brain lesion profile is influenced by the prion "strain" properties, the invasion route to the brain, and still unknown host cell-specific parameters. To gain insight into those endogenous factors, we analyzed the histopathological alterations induced by distinct prion strains in the mouse cerebellum. We show that 22L and ME7 scrapie prion proteins (PrP^22L , PrP^ME7 ), but not bovine spongiform encephalopathy PrP^6PB1 , accumulate in a reproducible parasagittal banding pattern in the cerebellar cortex of infected mice. Such banding pattern of PrP^22L aggregation did not depend on the neuroinvasion route, but coincided with the parasagittal compartmentation of the cerebellum mostly defined by the expression of zebrins, such as aldolase C and the excitatory amino acid transporter 4, in Purkinje cells. We provide evidence that Purkinje cells display a differential, subtype-specific vulnerability to 22L prions with zebrin-expressing Purkinje cells being more resistant to prion toxicity, while in stripes where PrP^22L accumulated most zebrin-deficient Purkinje cells are lost and spongiosis accentuated. In addition, in PrP^22L stripes, enhanced reactive astrocyte processes associated with microglia activation support interdependent events between the topographic pattern of Purkinje cell death, reactive gliosis and PrP^22L accumulation. Finally, we find that in preclinically-ill mice prion infection promotes at the membrane of astrocytes enveloping Purkinje cell excitatory synapses, upregulation of tumor necrosis factor-α receptor type 1 (TNFR1), a key mediator of the neuroinflammation process. These overall data show that Purkinje cell sensitivity to prion insult is locally restricted by the parasagittal compartmentation of the cerebellum, and that perisynaptic astrocytes may contribute to prion pathogenesis through prion-induced TNFR1 upregulation.

Defining the Microglia Response during the Time Course of Chronic Neurodegeneration

Inflammation has been proposed as a major component of neurodegenerative diseases, although the precise role it plays has yet to be defined. We examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a prion disease model of chronic neurodegeneration. Initially, we performed an extensive reanalysis of a large study of prion disease, where the transcriptome of mouse brains had been monitored throughout the time course of disease. Our analysis has provided a detailed classification of the disease-associated genes based on cell type of origin and gene function. This revealed that the genes upregulated during disease, regardless of the strain of mouse or prion protein, are expressed predominantly by activated microglia. In order to study the microglia contribution more specifically, we established a mouse model of prion disease in which the 79A murine prion strain was introduced by an intraperitoneal route into BALB/cJ(Fms-EGFP/-) mice, which express enhanced green fluorescent protein under the control of the c-fms operon. Samples were taken at time points during disease progression, and histological analysis of the brain and transcriptional analysis of isolated microglia was carried out. The analysis of isolated microglia revealed a disease-specific, highly proinflammatory signature in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the importance of microglia within the prion disease process and identifies the nature of the response through gene expression analysis of isolated microglia.

IMPORTANCE

Inflammation has been proposed as a major component of neurodegenerative diseases. We have examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a murine prion disease model of chronic neurodegeneration. Our study demonstrates that genes upregulated throughout the disease process are expressed predominantly by microglia. A disease-specific, highly proinflammatory signature was observed in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the important contribution of microglia to a chronic neurodegenerative disease process.

Efficient uptake and dissemination of scrapie prion protein by astrocytes and fibroblasts from adult hamster brain

Prion infections target neurons and lead to neuronal loss. However, the role of non-neuronal cells in the initiation and spread of infection throughout the brain remains unclear despite the fact these cells can also propagate prion infectivity. To evaluate how different brain cells process scrapie prion protein (PrPres) during acute infection, we exposed neuron-enriched and non-neuronal cell cultures from adult hamster brain to fluorescently-labeled purified PrPres and followed the cultures by live cell confocal imaging over time. Non-neuronal cells present in both types of cultures, specifically astrocytes and fibroblasts, internalized PrPres more efficiently than neurons. PrPres was trafficked to late endosomal/lysosomal compartments and rapidly transported throughout the cell bodies and processes of all cell types, including contacts between astrocytes and neurons. These observations suggest that astrocytes and meningeal fibroblasts play an as yet unappreciated role in prion infections via efficient uptake and dissemination of PrPres.

A comparative study of modified confirmatory techniques and additional immuno-based methods for non-conclusive autolytic bovine spongiform encephalopathy cases

Background

In the framework of the Bovine Spongiform Encephalopathy (BSE) surveillance programme, samples with non-conclusive results using the OIE confirmatory techniques have been repeatedly found. It is therefore necessary to question the adequacy of the previously established consequences of this non-conclusive result: the danger of failing to detect potentially infected cattle or erroneous information that may affect the decision of culling or not of an entire bovine cohort. Moreover, there is a very real risk that the underreporting of cases may possibly lead to distortion of the BSE epidemiological information for a given country.

In this study, samples from bovine nervous tissue presenting non-conclusive results by conventional OIE techniques (Western blot and immunohistochemistry) were analyzed. Their common characteristic was a very advanced degree of autolysis. All techniques recommended by the OIE for BSE diagnosis were applied on all these samples in order to provide a comparative study. Specifically, immunohistochemistry, Western blotting, SAF detection by electron microscopy and mouse bioassay were compared. Besides, other non confirmatory techniques, confocal scanning microscopy and colloidal gold labelling of fibrils, were applied on these samples for confirming and improving the results.

Results

Immunocytochemistry showed immunostaining in agreement with the positive results finally provided by the other confirmatory techniques. These results corroborated the suitability of this technique which was previously developed to examine autolysed (liquified) brain samples. Transmission after inoculation of a transgenic murine model TgbovXV was successful in all inocula but not in all mice, perhaps due to the very scarce PrPsc concentration present in samples.

Electron microscopy, currently fallen into disuse, was demonstrated to be, not only capable to provide a final diagnosis despite the autolytic state of samples, but also to be a sensitive diagnostic alternative for resolving cases with low concentrations of PrPsc.

Conclusions

Demonstration of transmission of the disease even with low concentrations of PrPsc should reinforce that vigilance is required in interpreting results so that subtle changes do not go unnoticed. To maintain a continued supervision of the techniques which are applied in the routine diagnosis would prove essential for the ultimate eradication of the disease.

Prion replication elicits cytopathic changes in differentiated neurosphere cultures

The molecular mechanisms of prion-induced cytotoxicity remain largely obscure. Currently, only a few cell culture models have exhibited the cytopathic changes associated with prion infection. In this study, we introduced a cell culture model based on differentiated neurosphere cultures isolated from the brains of neonatal prion protein (PrP)-null mice and transgenic mice expressing murine PrP (dNP0 and dNP20 cultures). Upon exposure to mouse Chandler prions, dNP20 cultures supported the de novo formation of abnormal PrP and the resulting infectivity, as assessed by bioassays. Furthermore, this culture was susceptible to various prion strains, including mouse-adapted scrapie, bovine spongiform encephalopathy, and Gerstmann-Sträussler-Scheinker syndrome prions. Importantly, a subset of the cells in the infected culture that was mainly composed of astrocyte lineage cells consistently displayed late-occurring, progressive signs of cytotoxicity as evidenced by morphological alterations, decreased cell viability, and increased lactate dehydrogenase release. These signs of cytotoxicity were not observed in infected dNP0 cultures, suggesting the requirement of endogenous PrP expression for prion-induced cytotoxicity. Degenerated cells positive for glial fibrillary acidic protein accumulated abnormal PrP and exhibited features of apoptotic death as assessed by active caspase-3 and terminal deoxynucleotidyltransferase nick-end staining. Furthermore, caspase inhibition provided partial protection from prion-mediated cell death. These results suggest that differentiated neurosphere cultures can provide an in vitro bioassay for mouse prions and permit the study of the molecular basis for prion-induced cytotoxicity at the cellular level.

Involvement of astrocytes in transmissible spongiform encephalopathies: a confocal microscopy study

Astroglial proliferation associated with pathological prion protein (PrPsc) deposition is widely described in Transmissible Spongiform Encephalopathies (TSEs). However, little is known of the actual role played by glia in their pathogenesis. The aim of the study has been to determine whether PrPsc is located exclusively in neurons or in both neurons and glial cells present in the central nervous system in a natural Scrapie model. Samples of cerebellum from 25 Scrapie sheep from various flocks were sectioned. Following epitope retrieval with formic acid, proteinase K and heat treatment, primary antibody L42 and primary antibodies against glial fibrillary acidic protein were applied as prion- and astrocytic-specific markers, respectively. For visualization, a suitable mixture of fluorochrome-conjugated secondary antibodies was used. Relevant controls were processed in the same manner. As determined by confocal microscopy, PrPsc deposits co-localized with glial cells in all samples. Our results suggest that these cells can sustain active prion propagation, in agreement with similar findings from other studies of primary cell cultures and inoculated mice. Furthermore, despite ongoing debate regarding whether varied TSE sources show differences in their tropism for different cell lineages in the brains of affected animals, no differences in co-localization results were seen.

Styryl-based and tricyclic compounds as potential anti-prion agents

Prion diseases currently have no effective therapy. These illnesses affect both animal and human populations, and are characterized by the conformational change of a normal self protein PrP^C (C for cellular) to a pathological and infectious conformer, PrP^Sc (Sc for scrapie). We used a well characterized tissue culture model of prion infection, where mouse neuroblastoma cells (N2a) were infected with 22L PrP^Sc, to screen compounds for anti-prion activity. In a prior study we designed a library of styryl based, potential imaging compounds which were selected for high affinity binding to Alzheimer's disease β-amyloid plaques and good blood-brain barrier permeability. In the current study we screened this library for activity in the N2a/22L tissue culture system. We also tested the anti-prion activity of two clinically used drugs, trimipramine and fluphenazine, in the N2a/22L system. These were selected based on their structural similarity to quinacrine, which was previously reported to have anti-prion activity. All the compounds were also screened for toxicity in tissue culture and their ability to disaggregate amyloid fibrils composed of PrP and β-amyloid synthetic peptides in vitro. Two of the imaging agents, 23I and 59, were found to be both effective at inhibiting prion infection in N2a/22L tissue culture and to be non-toxic. These two compounds, as well as trimipramine and fluphenazine were evaluated in vivo using wild-type CD-1 mice infected peripherally with 139A PrP^Sc. All four agents significantly prolonged the asymptomatic incubation period of prion infection (p<0.0001 log-rank test), as well as significantly reducing the degree of spongiform change, astrocytosis and PrP^Sc levels in the brains of treated mice. These four compounds can be considered, with further development, as candidates for prion therapy.

T2-Weighted MRI Signal Alterations in the Early-Clinical Phase of Transmissible Spongiform Encephalopathy in a Scrapie Rodent Model

Background and Purpose

Transmissible spongiform encephalopathy (TSE) diseases are fatal, progressive neurodegenerative disorders affecting both humans and animals. Clinical signs typically appear after years and even decades of silent disease progression. This study was aimed at investigating whether altered brain MRI patterns may precede clinical signs in a TSE rodent model.

Methods

In vivo T2-weighted (T2W) MRI examinations (4.7 T) were performed on Golden Syrian hamsters (GSH) intracerebrally, orally, or intraperitoneally (i.p.) infected with the 263K scrapie strain. Histopathological analyses were performed on i.p. infected GSH at the end of one-day or longitudinal MRI sessions.

Results

T2W-MRI hyperintensity was detected in the thalamic nuclei of GSH with clinical signs, irrespective of the infection route. Hyperintensity in the thalamus was also observed in pre-clinical animals, between 106 and 121 days post-infection (dpi), while normal T2W intensity was detected in four animals examined between 72 and 96 dpi. Pathological prion protein deposition (but no astrogliosis and only occasionally, weak spongiosis) was detected between 106 and 121 dpi.

Conclusions

The altered T2W-MRI pattern detected in the thalamus of asymptomatic i.p. infected GSH provides a useful basis for evaluating the effectiveness of possible therapeutic approaches at early stages of TSE disease.

Accelerated prion replication in, but prolonged survival times of, prion-infected CXCR3-/- mice

Prion diseases have a significant inflammatory component. Glia activation, which is associated with increased production of cytokines and chemokines, may play an important role in disease development. Among the chemokines upregulated highly and early upregulated during scrapie infections are ligands of CXCR3. To gain more insight into the role of CXCR3 in a prion model, CXCR3-deficient (CXCR3(-/-)) mice were infected intracerebrally with scrapie strain 139A and characterized in comparison to similarly infected wild-type controls. CXCR3(-/-) mice showed significantly prolonged survival times of up to 30 days on average. Surprisingly, however, they displayed accelerated accumulation of misfolded proteinase K-resistant prion protein PrP(Sc) and 20 times higher infectious prion titers than wild-type mice at the asymptomatic stage of the disease, indicating that these PrP isoforms may not be critical determinants of survival times. As demonstrated by immunohistochemistry, Western blotting, and gene expression analysis, CXCR3-deficient animals develop an excessive astrocytosis. However, microglia activation is reduced. Quantitative analysis of gliosis-associated gene expression alterations demonstrated reduced mRNA levels for a number of proinflammatory factors in CXCR3(-/-) compared to wild-type mice, indicating a weaker inflammatory response in the knockout mice. Taken together, this murine prion model identifies CXCR3 as disease-modifying host factor and indicates that inflammatory glial responses may act in concert with PrP(Sc) in disease development. Moreover, the results indicate that targeting CXCR3 for treatment of prion infections could prolong survival times, but the results also raise the concern that impairment of microglial migration by ablation or inhibition of CXCR3 could result in increased accumulation of misfolded PrP(Sc).

Ultrastructural evidence that ependymal cells are infected in experimental scrapie

During the last stage of infection in the experimental scrapie-infected hamster model, light microscopy reveals typical immunostaining of PrPsc in the subependymal region and at the apical ependymal cell borders. Whereas the subependymal immuno-staining is known to originate from extracellular amyloid filaments and residual membranes of astrocytes as constituents of plaque-like structures, the ultrastructural correlate of the supraependymal PrPsc staining remains uncertain. To decipher this apical PrPsc immunopositivity and subsequently the ependymocyte-scrapie agent interaction, we employed highly sensitive immuno-electron microscopy for detecting PrPsc in 263K scrapie-infected hamster brains. The results revealed the supraependymal PrPsc signal to be correlated not only with extracellular accumulation of amyloid filaments, but also with three distinct ependymal cell structures: (1) morphologically intact or altered microvilli associated with filaments, (2) the ependymal cell cytoplasm in proximity of apical cell membrane, and (3) intracytoplasmic organelles such as endosomes and lysosomal-like structures. These findings suggest a strong ependymotrope feature of the scrapie agent and recapitulate several aspects of the cell-prion interaction leading to the formation and production of PrPsc amyloid filaments. Our data demonstrate that in addition to neurons and astrocytes, ependymocytes constitute a new cellular target for the scrapie agent. In contrast, the absence of PrPsc labeling in choroid plexus and brain vascular endothelial cells indicates that these cells are not susceptible to the infection and may inhibit passage of the infectious agent across the blood-brain barrier.

Pharmacokinetics and distribution of clioquinol in golden hamsters

Clioquinol (5-chloro-7-iodo-8-quinolinol) is a zinc and copper chelator that can dissolve amyloid deposits and may be beneficial in Alzheimer's disease. Prion diseases are also degenerative CNS disorders characterised by amyloid deposits. The pharmacokinetics and tissue distribution of drugs active against prions may clarify their targets of action. We describe the pharmacokinetics of clioquinol in hamster plasma, spleen and brain after single and repeated oral or intraperitoneal administration (50 mg kg(-1)), as well as after administration with the diet. A single intraperitoneal administration led to peak plasma clioquinol concentrations after 15 min (Tmax), followed by a decay with an apparent half-life of 2.20 +/- 1.1 h. After oral administration, Tmax was reached after 30 min and was followed by a similar process of decay; the AUC(0-last) was 16% that recorded after intraperitoneal administration. The Cmax and AUC values in spleen after a single administration were about 65% (i.p.) and 25% (p.o.) those observed in blood; those in liver were 35% (p.o.) those observed in blood and those in brain were 20% (i.p.) and 10% (p.o.) those observed in plasma. After repeated oral doses, the plasma, brain and spleen concentrations were similar to those observed at the same times after a single dose. One hour after intraperitoneal dosing, clioquinol was also found in the ventricular CSF. Clioquinol was also given with the diet; its morning and afternoon concentrations were similar, and matched those after oral administration. No toxicity was found after chronic administration. Our results indicate that clioquinol, after oral administration with the diet, reaches concentrations in brain and peripheral tissues (particularly spleen) that can be considered effective in preventing prion accumulation, but are at least ten times lower than those likely to cause toxicity.

Differential expression of interferon responsive genes in rodent models of transmissible spongiform encephalopathy disease

Background

The pathological hallmarks of transmissible spongiform encephalopathy (TSE) diseases are the deposition of a misfolded form of a host-encoded protein (PrP^res), marked astrocytosis, microglial activation and spongiosis. The development of powerful gene based technologies has permitted increased levels of pro-inflammatory cytokines to be demonstrated. However, due to the use of assays of differing sensitivities and typically the analysis of a single model system it remained unclear whether this was a general feature of these diseases or to what extent different model systems and routes of infection influenced the relative levels of expression. Similarly, it was not clear whether the elevated levels of cytokines observed in the brain were accompanied by similar increases in other tissues that accumulate PrP^res, such as the spleen.

Results

The level of expression of the three interferon responsive genes, Eif2ak2, 2'5'-OAS, and Mx2, was measured in the brains of Syrian hamsters infected with scrapie 263K, VM mice infected with bovine spongiform encephalopathy and C57BL/6 mice infected with the scrapie strain ME7. Glial fibrillary acidic expression confirmed the occurrence of astrocytosis in all models. When infected intracranially all three models showed a similar pattern of increased expression of the interferon responsive genes at the onset of clinical symptoms. At the terminal stage of the disease the level and pattern of expression of the three genes was mostly unchanged in the mouse models. In contrast, in hamsters infected by either the intracranial or intraperitoneal routes, both the level of expression and the expression of the three genes relative to one another was altered. Increased interferon responsive gene expression was not observed in a transgenic mouse model of Alzheimer's disease or the spleens of C57BL/6 mice infected with ME7. Concurrent increases in TNFα, TNFR1, Fas/ApoI receptor, and caspase 8 expression in ME7 infected C57BL/6 mice were observed.

Conclusion

The identification of increased interferon responsive gene expression in the brains of three rodent models of TSE disease at two different stages of disease progression suggest that this may be a general feature of the disease in rodents. In addition, it was determined that the increased interferon responsive gene expression was confined to the CNS and that the TSE model system and the route of infection influenced the pattern and extent of the increased expression. The concurrent increase in initiators of Eif2ak2 mediated apoptotic pathways in C57BL/6 mice infected with ME7 suggested one mechanism by which increased interferon responsive gene expression may enhance disease progression.

Determination of 5-chloro-7-iodo-8-quinolinol (clioquinol) in plasma and tissues of hamsters by high-performance liquid chromatography and electrochemical detection

This paper describes a method of determining clioquinol levels in hamster plasma and tissue by means of HPLC and electrochemical detection. Clioquinol was separated on a Nucleosil C18 300 mm x 3.9 mm i.d. 7 microm column at 1 ml/min using a phosphate/citrate buffer 0.1M (400 ml) with 600 ml of a methanol:acetonitrile (1:1, v/v) mobile phase. The retention times of clioquinol and the IS were, respectively, 11.6 and 8.1 min; the quantitation limit (CV>8%) was 5 ng/ml in plasma and 10 ng/ml in tissues. The intra- and inter-assay accuracies of the method were more than 95%, with coefficients of variation between 3.0 and 7.7%, and plasma and tissue recovery rates of 72-77%. There was a linear response to clioquinol 5-2000 ng/ml in plasma, and 10-1000 ng/g in tissues. The method is highly sensitive and selective, makes it possible to study the pharmacokinetics of plasma clioquinol after oral administration and the distribution of clioquinol in tissues, and could be used to monitor plasma clioquinol levels in humans.

Differential gene expression in central nervous system tissues of sheep with natural scrapie

The expression of nine genes was analyzed by real-time RT-PCR in the central nervous system in order to investigate the molecular pathogenesis of natural scrapie. An up-regulation of genes related to glial activation (GFAP) and apoptosis (CASP3) was detected in obex and cerebrum, indicating a reactive glia. Another glial activation-related gene (CTSS) was slightly up-regulated in obex, whereas constitutive expression was detected for SOD1, YWHAZ, PRNP, and the apoptosis-related genes BCL2, MCL1, and BAX. This differential gene expression might reflect a spatial-temporal and tissue-specific molecular pathogenesis of scrapie.

Correlative studies support lipid peroxidation is linked to PrPres propagation as an early primary pathogenic event in prion disease

To assess whether heightened oxidative stress plays an early and primary pathogenic role in transmissible spongiform encephalopathies (TSE), we undertook detailed correlative studies using a mouse-adapted model of human disease. The spatio-temporal evolution of the abnormal, protease-resistant isoform of the prion protein (PrP(res)) and neuropathological changes were correlated with the occurrence and type of oxidative stress. Heightened oxidative stress was demonstrated, but restricted to elevated levels of free aldehydic breakdown products of lipid peroxidation, affecting all brain regions to varying extents. The increase in lipid peroxidation was highest over the mid-incubation period, with the onset showing close temporal and general topographical concordance with the first detection of PrP(res) with both pre-empting the typical neuropathological changes of spongiform change, gliosis and neuronal loss. Further, prion propagation over the disease course was assessed using murine bioassay. This revealed that the initial rapid increase in infectivity titres was contemporaneous with the abrupt onset and maximisation of lipid peroxidation. The present results are an important extension to previous studies, showing that heightened oxidative stress in the form of lipid peroxidation is likely to constitute an early primary pathogenic event in TSE, associated temporally with the integral disease processes of prion propagation and PrP(res) formation, and consistent with causal links between these events and subsequent typical neuropathological changes.

Neuronal cell death in transmissible spongiform encephalopathies (prion diseases) revisited: from apoptosis to autophagy

Neuronal autophagy, like apoptosis, is one of the mechanisms of the programmed cell death (PCD). In this review, we summarize the presence of autophagic vacuoles in experimentally induced scrapie, Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Initially, a part of the neuronal cytoplasm was sequestrated by concentric arrays of double membranes; the enclosed cytoplasm appeared relatively normal except that its density was often increased. Next, electron density of the central area dramatically increased. The membranes then proliferated within the cytoplasm in a labyrinth-like manner and the area sequestrated by these membranes enlarged into a more complex structure consisting of vacuoles, electron-dense areas and areas of normally-looking cytoplasm connected by convoluted membranes. Of note, autophagic vacuoles form not only in neuronal perikarya but also in neurites and synapses. Finally, a large area of the cytoplasm was transformed into a collection of autophagic vacuoles of different sizes. On a basis of ultrastructural studies, we suggest that autophagy plays a major role in transmissible spongiform encephalopathies (TSEs) and may even participate in a formation of spongiform change.

Role of cytokines and chemokines in prion infections of the central nervous system

Prion infections of the central nervous system (CNS) are characterised by a reactive gliosis and the subsequent degeneration of neuronal tissue. The activation of glial cells, which precedes neuronal death, is likely to be initially caused by the deposition of misfolded, proteinase K-resistant, isoforms (termed PrP(res)) of the prion protein (PrP) in the brain. Cytokines and chemokines released by PrP(res)-activated glia cells may contribute directly or indirectly to the disease development by enhancement and generalisation of the gliosis and via cytotoxicity for neurons. However, the actual role of prion-induced glia activation and subsequent cytokine/chemokine secretion in disease development is still far from clear. In the present work, we review our present knowledge concerning the functional biology of cytokines and chemokines in prion infections of the CNS.

Cellular prion protein/laminin receptor: distribution in adult central nervous system and characterization of an isoform associated with a subtype of cortical neurons

The 67-kDa LR protein was originally discovered as a non-integrin laminin receptor. Several more recent in vitro studies demonstrated the function of 67-kDa LR and its related 'precursor' form 37-kDa LRP as receptors of cellular prion protein and their implication in abnormal prion protein propagation in vitro. In addition, expression of both proteins was shown to increase considerably in the brain of scrapie-infected mice and hamsters. While LRP/LR are thus likely to play important roles in neuronal cell adhesion, survival and homeostasis and during pathological disorders, little is known so far about their fine cellular distribution in adult central nervous system. Using immunocytochemistry and western blotting, we show here that the 67-kDa LR is the major receptor form in adult rat brain and spinal cord, expressed within the cytoplasm and at the plasma membrane of most neurons and in a subset of glial cells. The overall distribution of LR correlates well with that reported for laminin-1 but also with brain regions classically associated with prion-related neurodegeneration. In contrast to LR, the 37-kDa LRP form is much less abundant in adult than in postnatal central nervous system. Characterization of a novel antibody allowed us to study the distribution across tissues of cell membrane-associated LRP. Interestingly, this form is almost exclusively found on a subclass of parvalbumin-immunoreactive cortical interneurons known to degenerate during the early stages of Creutzfeldt-Jakob disease. Our demonstration of local differences in the expression of particular LRP/LR isoforms may be a first step towards unraveling their specific molecular interactions.

Role of interleukin-1 in prion disease-associated astrocyte activation

Prion-induced chronic neurodegeneration has a substantial inflammatory component, and the activation of glia cells may play an important role in disease development and progression. However, the functional contribution of cytokines to the development of the gliosis in vivo was never systematically studied. We report here that the expression of interleukin-1beta (IL-1beta), IL-1beta-converting enzyme, and IL-1 receptor type 1 (IL-1RI) is up-regulated in a murine scrapie model. The scrapie-induced gliosis in IL-1RI(-/-) mice was characterized by an attenuated activation of astrocytes in the asymptomatic stage of the disease and a reduced expression of CXCR3 ligands. Furthermore, the accumulation of the misfolded isoform of the prion protein PrP(Sc) was significantly delayed in the IL-1RI(-/-) mice. These observations indicate that IL-1 is a driver of the scrapie-associated astrocytosis and possibly the accompanying amyloid deposition. In addition, scrapie-infected IL-1RI-deficient (IL-1RI(-/-)) mice showed a delayed disease onset and significantly prolonged survival times suggesting that an anti-inflammatory therapeutical approach to suppress astrocyte activation and/or glial IL-1 expression may help to delay disease onset in established prion infections of the central nervous system.

Prions can infect primary cultured neurons and astrocytes and promote neuronal cell death

Transmissible spongiform encephalopathies arise as a consequence of infection of the central nervous system by prions, where neurons and glial cells are regarded as primary targets. Neuronal loss and gliosis, associated with the accumulation of misfolded prion protein (PrP), are hallmarks of prion diseases; yet the mechanisms underlying such disorders remain unclear. Here we introduced a cell system based on primary cerebellar cultures established from transgenic mice expressing ovine PrP and then exposed to sheep scrapie agent. Upon exposure to low doses of infectious agent, such cultures, unlike cultures originating from PrP null mice, were found to accumulate de novo abnormal PrP and infectivity, as assessed by mouse bioassay. Importantly, using astrocyte and neuron/astrocyte cocultures, both cell types were found capable of sustaining efficient prion propagation independently, leading to the production of proteinase K-resistant PrP of the same electrophoretic profile as in diseased brain. Moreover, contrasting with data obtained in chronically infected cell lines, late-occurring apoptosis was consistently demonstrated in the infected neuronal cultures. Our results provide evidence that primary cultured neural cells, including postmitotic neurons, are permissive to prion replication, thus establishing an approach to study the mechanisms involved in prion-triggered neurodegeneration at a cellular level.

Pharmacokinetics and distribution of sodium 3,4-diaminonaphthalene-1-sulfonate, a Congo Red derivative active in inhibiting PrP(res) replication

Sodium 3,4-diaminonaphthalene-1-sulfonate (CRA) is a compound, synthesised by our group from Congo Red (CR), that is active in preventing the pathological conversion of normal prion protein (PrP). As the precise mechanisms controlling the ways in which prions are distributed and infect the brain and other organs are not fully understood, studying the pharmacokinetics of drugs that are active against prions may clarify their targets and their means of inhibiting prion infection. This paper describes the pharmacokinetics of CRA in plasma, spleen and brain after single or repeated intraperitoneal or subcutaneous administration, as determined by means of specific and sensitive fluorimetric HPLC. A single intraperitoneal administration led to peak plasma CRA concentrations after 15 min, followed by biphasic decay with an apparent half-life of 4.3 h. After subcutaneous administration, T(max) was reached after 30 min, and was followed by a similar process of decay: Cmax and the AUC0-last were 25% those recorded after intraperitoneal administration. The mean peak concentrations and AUCs of CRA after a single intraperitoneal or subcutaneous administration in peripheral tissue (spleen) were similar to those observed in blood, whereas brain concentrations were about 2% those in plasma. After repeated intraperitoneal or subcutaneous doses, the Cmax values in plasma, brain and spleen were similar to those observed at the same times after a single dose. After repeated intraperitoneal doses, CRA was also found in the ventricular cerebrospinal fluid at concentrations of 1.8 +/- 0.2 microg(-1) mL, which is similar to, or slightly higher than, those found in brain. Brain concentrations may be sufficient to explain the activity of CRA on PrP reproduction in the CNS. However, peripheral involvement cannot be excluded because the effects of CRA are more pronounced after intraperitoneal than after intracerebral infection.

The occurrence of vacuolation, and periodic acid-Schiff (PAS)-positive granules and plaques in the brains of C57BL/6J, AKR, senescence-prone (SAMP8) and senescence-resistant (SAMR1) mice infected with various scrapie strains

Scrapie is a fatal, but slow, infectious disease. C57BL/6J, SAMP8 (a strain that develops early senescence), SAMR1 (a strain that is resistant to senescence) and AKR/J (a progenitor of the SAM strains) mice were infected with 22A, 139A, 22L and ME7 scrapie strains. Histopathological stains included haematoxylin and eosin (HE), and periodic acid-Schiff (PAS). Vacuolation was found in the brains of all scrapie-infected mice. The 22A strain caused more extensive vacuolation in the brains of SAMP8 and SAMR1 mice than in C57BL mice. PAS-positive plaques (PP) were found in 22A-infected mice in cortex, corpus callosum, hippocampus, subependymal zone area and thalamus. PP were significantly increased in 22A-infected SAMR1 mice compared to mice from other scrapie-infected strains. Clusters of small, round, homogeneous PAS-positive granular structures (PGS) were found in all mouse strains, especially in aging control and 22A-infected C57BL mice, predominantly in the stratum radiatum of the CA1, CA2 and CA3 areas of the hippocampus. Some of these structures were also observed in stratum oriens and piriform cortex, and in cerebellar Purkinje cell areas. Some of the PGS were associated with astrocytes and blood vessels. Each granule was 1-5 microm in diameter and there were clusters consisting of several to 40 PGS; the sizes of the clusters ranged from 10 to 80 microm in diameter. There were more PGS clusters in uninfected C57BL and AKR mice than in uninfected SAMP8 and SAMR1 mice. PGS were not increased in scrapie-infected mice. These findings suggest that PGS accumulation was more dependent on the genetic information of the mouse strain, whereas PP and vacuolation patterns depended on the scrapie strain-mouse strain combination.

Neuronal and astrocytic responses involving the serotonergic system in human spongiform encephalopathies

The relationships between the degree of cortical prion protein (PrP) deposition, tissue vacuolation and astrocytosis were studied in the frontal cortex of 27 cases of human spongiform encephalopathy, encompassing 13 cases of sporadic Creutzfeldt-Jakob disease (sCJD), four cases of familial CJD (fCJD) (one owing to E200K mutation, one owing to 144 bp insertion, one owing to P102L mutation and one owing to A117V mutation), five cases of iatrogenic CJD (iCJD) owing to growth hormone therapy and five cases of variant CJD (vCJD). The size and number of tryptophan hydroxylase (TPH) positive cells in the dorsal raphe were determined as an index of the function of the brain's serotonergic system. The amount of PrP deposited in frontal cortex in vCJD was significantly greater than that in both sCJD and iCJD, which did not differ significantly from each other. The extent of grey matter deposition of PrP correlated with that of white matter deposition. Deposition of PrP as plaques was greater in cases of sCJD bearing valine at codon 129 of PrP gene, especially when homozygous. However, all cases of vCJD displayed florid plaque formation yet these were homozygous for methionine at codon 129. Prion protein deposition as plaques was greater in cases of sCJD with 2A PrP isotype than those with 1 PrP isotype, similar to that seen in cases of vCJD all of which are 2B PrP isotype. There were no significant differences in the extent of astrocytosis between the different aetiological groups, in either grey or white matter, as visualized with glial fibrillary acidic protein (GFAP) or 5HT-2A receptor (5HT-2AR) immunostaining, although there was a strong correlation between the severity of 5HT-2AR and GFAP reactions within both grey and white matter. The extent of PrP deposition within the grey, but not white, matter correlated with the degree of astrocytosis for both GFAP and 5HT-2AR and the extent of tissue vacuolation in grey and white matter, although the latter did not correlate with degree of astrocytosis for either GFAP or 5HT-2AR. Astrocytes may be responding directly to the presence of PrP within the tissue, rather than the vacuolar damage to neurones. Although S100beta immunoreactivity was present in astrocytes in control cases, no S100beta staining was seen in astrocytes in either grey or white matter in most CJD cases. There were no differences in the number of TPH-positive cells between CJD and control cases, although the mean TPH-positive cell size was significantly greater, and cells were more intensely stained, in CJD compared to controls, suggesting a pathological overactivity of the brain's serotonergic system in CJD. This may result in excessive release of 5HT within the brain triggering increased 5HT-2AR expression within activated astrocytes leading to release and depletion of S100beta protein from such cells. The clinical symptoms of fluctuating attention and arousal could be mediated, at least in part, by such alterations in function of the serotonergic system.

Comparison of the neuropathological characteristics of bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) in mice

Bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) belong to a group of diseases called the transmissible spongiform encephalopathies (TSEs). Transmission studies in inbred mice (strain typing) provided overwhelming evidence that vCJD arose from BSE. In this study, we compare the patterns of neuropathology in a panel of three inbred mouse strains (RIII, C57BL and VM) and one cross (C57BL x VM) infected with either vCJD or BSE. For each mouse strain, patterns of abnormal prion protein (PrPres) deposition, astrocytosis and vacuolation were similar in the vCJD- and BSE-challenged mice. Prion protein (PrP)-positive plaques were prominent in the VM and C57BL x VM mice in addition to diffuse PrPres accumulation, whereas only diffuse PrPres labelling was observed in the RIII and C57BL mice. The hippocampus was targeted in all mouse strains, as was the cochlear nucleus in the medulla, both showing consistent severe vacuolation and heavy PrPres deposition. Although the targeting of PrPres was similar in the BSE- and vCJD-infected brains, the amount and intensity of PrPres observed in the brains treated with formic acid during fixation was reduced considerably. The distribution of astrocytosis was similar to the targeting of PrPres deposition in the brain, although some differences were observed in the hippocampi of mice challenged with vCJD. We conclude that there are no significant differences in the targeting of neuropathological changes observed in the BSE- and vCJD-infected mice, consistent with the previous evidence of a link between BSE and vCJD.

Loss of lipopolysaccharide-induced nitric oxide production and inducible nitric oxide synthase expression in scrapie-infected N2a cells

In scrapie-infected cells, the conversion of the cellular prion protein to the pathogenic prion has been shown to occur in lipid rafts, which are suggested to function as signal transduction platforms. Neuronal cells may respond to bacterial lipopolysaccharide (LPS) treatment with a sustained and elevated nitric oxide (NO) release. Because prions and the major LPS receptor CD14 are colocalized in lipid rafts, the LPS-induced NO production in scrapie-infected neuroblastoma cells was studied. This study shows that LPS induces a dose- and time-dependent increase in NO release in the murine neuroblastoma cell line N2a, with a 50-fold increase in NO production at 1 microg/ml LPS after 96 hr, as measured by nitrite in the medium. This massive NO release was not caused by activation of the neuronal NO synthase (nNOS), but by increased expression of the inducible NOS (iNOS) mRNA and protein. However, in scrapie-infected N2a cells (ScN2a), the LPS-induced NO production was completely abolished. The absence of LPS-induced NO production in ScN2a was due not to abolished enzymatic activity of iNOS but to a complete inhibition of the LPS-induced iNOS gene expression as measured by Western blot and RT-PCR. These results indicate that scrapie infection inhibits the LPS-mediated signal transduction upstream of the transcriptional step in the signaling cascade and may reflect the important molecular and cellular changes induced by scrapie infection.

Differences in the activation of the GFAP gene promoter by prion and viral infections

The expression of glial fibrillary acidic protein (GFAP), a component of astroglial intermediate filaments, is regulated under developmental and pathological conditions. After surgical injury or viral infections, an increase in this protein reflects reactive gliosis in the brain. We analyzed the activation of the GFAP gene in transgenic mice using a prion and two different viruses (rabies and Theiler viruses). Inoculation of the transgenic mice with the C506M3 mouse prion strain resulted in activation of the GFAP-lacZ transgene. Expression of the GFAP transgene increased concomitantly with the expression of GFAP in astrocytes from the infected mice. In contrast, infection with rabies or Theiler's virus had no effect on the expression of the GFAP transgene, showing that the glial reactions to these infectious agents involved different mechanisms. These findings indicate that the activation of the endogenous GFAP gene as a consequence of viral infection could involve different regulatory pathways than activation as a result of prion infection. The first 2 kb upstream from the start codon of the GFAP gene seems to provide enough activation domains to produce efficient activation of the reporter gene in prion-infected mice.

Increased c-Fos protein in the brains of scrapie-infected SAMP8, SAMR1, AKR and C57BL mice

Scrapie is a neurodegenerative disease that occurs naturally in sheep and goats. The histopathological changes include vacuolation, neuronal apoptosis and astrocytosis. The mechanisms involved in neuronal apoptosis are still unknown. Recently, we observed that activated p38 immunohistostaining was increased in scrapie-infected mice. In many neurodegenerative diseases, activation of the p38 pathway and of the immediate-early gene termed c-Fos appears to be required for the initiation of apoptosis. There are similarities in histopathological changes seen in scrapie-infected mice and in an uninfected senescence-accelerated mouse strain (SAMP8). This led us to investigate c-Fos protein levels in the brains of both uninfected and scrapie-infected SAMP8, SAMR1, AKR and C57BL mice using immunohistochemical methods. The SAMR1 strain served as a control in that it is a mouse strain that does not show accelerated ageing, but has a background that is similar to the SAMP8 strain. AKR was used because it is one of the progenitor strains of both SAM strains and, finally, C57BL is a completely unrelated strain. The results showed a low basal c-Fos expression in controls and a marked increase in c-Fos staining in scrapie-infected mice. In scrapie-positive mice, c-Fos immunoreactivity was observed in neurones in the cortex, hippocampus, thalamus, hypothalamus, medulla, midbrain, brainstem, paraterminal body, internal capsule and cerebellar Purkinje cells. Immunoreactivity of c-Fos was also observed in astrocytes in many brain areas of scrapie-infected mice, particularly in the hippocampus and cortex. Our results show that normal mouse brain (NMB)-injected AKR and SAMP8 mice had more c-Fos production than NMB-injected SAMR1 or C57BL mice; scrapie-infection induces significant increases in c-Fos immunoreactivity in all four mouse strains. Our study suggests that the increase in c-Fos levels may play a role in the neuronal apoptosis observed in scrapie-infected mice.

Nonneuronal cellular prion protein

The normal cellular prion protein (PrP(c)) is a membrane sialoglycoprotein of unknown function having the unique property of adopting an abnormal tertiary conformation. The pathological conformer PrP(sc) would be the agent of transmissible spongiform encephalopathies or prion diseases. They include scrapie and bovine spongiform encephalopathy in animals and Creutzfeldt-Jakob disease in humans. The conversion of PrP(c) into PrP(sc) in the brain governs the clinical phenotype of the disease. However, the three-dimensional structure change of PrP(c) can also take place outside the central nervous system, in nonneuronal cells particularly of lymphoid tissue where the agent replicates. In natural infection, PrP(c) in nonneuronal cells of peripheral extracerebral organs may play a key role as the receptor required to enable the entry of the infectious agent into the host. In the present review we have undertaken a first evaluation of compelling data concerning the PrP(c)-expressing cells of nonneuronal origin present in cerebral and extracerebral tissues. The analysis of tissue, cellular, and subcellular localization of PrP(c) may help us better understand the biological function of PrP(c) and provide some information on physiopathological processes underlying prion diseases.

The role of cytokines, astrocytes, microglia and apoptosis in Creutzfeldt-Jakob disease

In order to investigate inflammation and apoptosis in Creutzfeldt-Jakob disease (CJD) patients, we analyzed astrocytes, microglia and apoptotic neurons in brain and IL-1beta in cerebrospinal fluid (CSF). Our results showed increased numbers of astrocytes in CJD and increased numbers of microglia and apoptotic neurons both in CJD and Alzheimer's disease (AD) as compared to controls. All these markers correlated (P < 0.001) with the severity of the neuropathological lesions. An increased IL-1beta concentration was found in AD and CJD CSF that correlated with the number of microglia and which did not change in the disease course of CJD.In conclusion, apoptotic neurons in CJD correlates to the neuropathological lesions and are probably related to the presence of inflammatory cells and cytokines which are present during the whole CJD disease process.

The transmissible spongiform encephalopathies: pathogenic mechanisms and strategies for therapeutic intervention

Primary neurodegenerative diseases tend to be intractable and largely affect the elderly. There is rarely the opportunity to identify individuals at risk and the appearance of clinical symptoms usually signifies the occurrence of irreversible neurological damage. This situation describes sporadic Creutzfeldt-Jakob disease which occurs world-wide, affecting one person per million per annum. The epidemic of bovine spongiform encephalopathy in the UK in the 1980s and the subsequent causal appearance of variant Creutzfeldt-Jakob disease in young UK residents in the 1990s has refocused attention on this whole group of diseases, known as the transmissible spongiform encephalopathies or prion diseases. The potentially lengthy incubation period of variant Creutzfeldt-Jakob disease, including perhaps an obligate peripheral phase, prior to neuroinvasion, marks variant Creutzfeldt-Jakob disease out as different from sporadic Creutzfeldt-Jakob disease. The formal possibility of detecting individuals infected with the bovine spongiform encephalopathy agent during this asymptomatic peripheral phase provides a strong incentive for the development of therapies for transmissible spongiform encephalopathies. This review focuses on recent advances in the understanding of the pathogenesis of these diseases, with particular reference to in vitro and animal model systems. Such systems have proved invaluable in the identification of potential therapeutic strategies that either specifically target the prion protein or more generally target peripheral pathogenesis. Furthermore, recent experiments in animal models suggest that even after neuroinvasion there may be pharmacological avenues to explore that might retard or even halt the degenerative process.

Fluoro-Jade and silver methods: application to the neuropathology of scrapie, a transmissible spongiform encephalopathy

Traditional methods for evaluating neurodegeneration include variations of Nauta's selective silver-staining techniques. The Fluoro-Jade (FJ) method applies a novel fluorescent, anionic stain for localizing degenerating neurons. FJ has produced comparable results to the silver methods, when both have been applied to detect neurodegeneration in animals treated acutely with a variety of neurotoxins, including kainic acid (KA), ibogaine (IBO), 3-nitropropionic acid (3-NPA), domoic acid and others. The potential value of methods selective for neurodegeneration in elucidating the pathophysiology of transmissible spongiform encephalopathies (TSEs), such as the prion disease 'scrapie', has not yet been investigated. Using frozen or paraffin sections stained with FJ or silver, we evaluated the brains of hamsters inoculated with either the 263K or the 139H strains of scrapie, originally passaged from sheep into mice and then into hamsters. As a positive control, we also examined sections from IBO-treated rats, which experience degeneration restricted to small clusters of Purkinje neurons located in the paravermal region of the cerebellum. As expected, both FJ and silver methods delineated this identical pattern of neurodegeneration, characteristic of IBO exposure. Surprisingly, only a small number of FJ or silver-labeled cortical neurons were observed in scrapie-infected hamsters evaluated near the end of their incubation period but before obvious spongiform pathology. Instead, there was intense fluorescent staining of astrocytes in scrapie-infected hamsters, especially in the cortex, corpus callosum, and hypothalamus. Detailed protocols describing the application of the degeneration-selective methods we utilized are presented and compared.

Immunohistochemical detection of scrapie prion proteins in clinically normal sheep in Pennsylvania

Following diagnosis of scrapie in a clinically suspect Suffolk sheep, 7 clinically normal flockmates were purchased by the Pennsylvania Department of Agriculture to determine their scrapie status using an immunohistochemical procedure. Two of the 7 euthanized healthy sheep had positive immunohistochemical staining of the prion protein of scrapie (PrP-Sc) in their brains, nictitating membranes, and tonsils. The PrP-Sc was localized in the areas of the brain where, histopathologically, there was neurodegeneration and astrocytosis. The PrP-Sc occurred within germinal centers of the affected nictitating membranes and tonsils and was located in the cytoplasm of the dendrite-like cells, lymphoid cells, and macrophages. These results confirm that immunohistochemical examination of the nictitating membrane can be used as a screen for the presence of scrapie infection in clinically normal sheep at a capable veterinary diagnostic laboratory. In sheep with a PrP-Sc-positive nictitating membrane, the diagnosis of scrapie should be confirmed by histopathology and immunohistochemical examination of the brain following necropsy. Following full validation, immunohistochemistry assays for detection of PrP-Sc in nictitating membrane lymphoid tissues can improve the effectiveness of the scrapie control and eradication program by allowing diagnosis of the disease in sheep before the appearance of clinical signs.

Synaptic prion protein immuno-reactivity in the rodent cerebellum

The cellular prion protein PrP(c) is a neurolemmal glycoprotein essential for the development of the transmissible spongiform encephalopathies. In these neurodegenerative diseases, host PrP(c) is converted to infectious protease-resistant isoforms PrP(res) or prions. Prions provoque predictable and distinctive patterns of PrP(res) accumulation and neurodegeneration depending on the prion strain and on regional cell-specific properties modulating PrP(c) affinity for infectious PrP(res) in the host brain. Synaptolysis and synaptic accumulation of PrP(res) during PrP-related diseases suggests that the synapses could be primary sites able to propagate PrP(res) and neurodegeneration in the central nervous system. In the rodent cerebellum, the present light and electron microscopic immuno-cytochemical analysis shows that distinct types of synapses display differential expression of PrP(c), suggesting that synapse-specific parameters could influence neuroinvasion and neurodegeneration following cerebral infection by prions. Although the physiological functions of PrP(c) remain unknown, the concentration of PrP(c) almost exclusively at the Purkinje cell synapses in the cerebellum suggests its critical involvement in the synaptic relationships between cerebellar neurons in agreement with their known vulnerability to PrP deficiencies.

Astrocytosis and proliferating cell nuclear antigen expression in brains of scrapie-infected hamsters

Scrapie is a neurodegenerative disease in sheep and goats. Neuropathological examination shows astrocytosis. One issue is whether the astrocytosis seen in scrapie is a function of an increase in reactivity of individual cells, or whether there is actual replication of astrocytes. We used double-label immunohistochemistry for proliferating cell nuclear antigen (PCNA) and for glial fibrillary acidic protein (GFAP) to determine the mitotic state of cells and to confirm their identity as astrocytes. Brain sections from hamsters (strain LVG/LAK) infected with 139H or 263K scrapie isolates were examined. GFAP immunostaining was increased in astrocytes in most regions of the brains of scrapie-infected hamsters. These qualitative observations were confirmed by computerized image analysis quantification. A proportion of the hypertrophic astrocytes (0.5-10.8%, depending on specific location) were PCNA immunoreactive. The PCNA-immunopositive astrocytes were most frequently found in cerebral cortex, corpus callosum, subependymal areas, fimbria, caudate, thalamus, hypothalamus, hippocampus, and dentate gyrus. Our results suggest that the astrocytosis seen in scrapie-infected animals is, at least in part, owing to actual replication of astrocytes in these animals. We hypothesize that the astrocytes may be an important locus for the disease process.