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

Non-amyloid and amyloid prion protein deposits in prion-infected mice differ in blockage of interstitial brain fluid

AIMS

Prion diseases are characterized by brain deposits of misfolded aggregated protease-resistant prion protein (PrP), termed PrPres. In humans and animals, PrPres is found as either disorganized non-amyloid aggregates or organized amyloid fibrils. Both PrPres forms are found in extracellular spaces of the brain. Thus, both might block drainage of brain interstitial fluid (ISF). The present experiments studied whether ISF blockage occurred during amyloid and/or non-amyloid prion diseases.

METHODS

Various-sized fluorescein-labelled ISF tracers were stereotactically inoculated into the striatum of adult mice. At times from 5 min to 77 h, uninfected and scrapie-infected mice were compared. C57BL/10 mice expressing wild-type anchored PrP, which develop non-amyloid PrPres similar to humans with sporadic Creutzfeldt-Jakob disease, were compared with Tg44+/+ mice (transgenic mice secreting anchorless PrP) expressing anchorless PrP, which develop amyloid PrPres similar to certain human familial prion diseases.

RESULTS

In C57BL/10 mice, extensive non-amyloid PrPres aggregate deposition was not associated with abnormal clearance kinetics of tracers. In contrast, scrapie-infected Tg44+/+ mice showed blockage of tracer clearance and colocalization of tracer with perivascular PrPres amyloid.

CONCLUSIONS

As tracer localization and clearance was normal in infected C57BL/10 mice, ISF blockage was not an important pathogenic mechanism in this model. Therefore, ISF blockage is unlikely to be a problem in non-amyloid human prion diseases such as sporadic Creutzfeldt-Jakob disease. In contrast, partial ISF blockage appeared to be a possible pathogenic mechanism in Tg44+/+ mice. Thus this mechanism might also influence human amyloid prion diseases where expression of anchorless or mutated PrP results in perivascular amyloid PrPres deposition and cerebral amyloid angiopathy.

Neuroanatomical distribution of abnormal prion protein in naturally occurring atypical scrapie cases in Great Britain

Scrapie belongs to a group of diseases known as the transmissible spongiform encephalopathies or prion diseases. Two different categories of naturally occurring scrapie have been identified: classical scrapie, which was first recorded around 1750, and atypical scrapie or 'Nor-98', which was first identified in Norway in 1998. The molecular characteristics of atypical scrapie have been well defined, but detailed descriptions of the neuropathological phenotype are rare since the majority of cases have been detected through active surveillance programmes where only brainstem and cerebellum are collected for statutory diagnosis. In order to characterise the neuropathology of naturally occurring atypical scrapie in sheep, we examined multiple brain levels from 15 whole brains from field cases of atypical scrapie, both clinical suspects and fallen stock, collected in Great Britain between 2004 and 2006. We found that the distribution of disease-associated prion protein (PrP(Sc)) and vacuolation in atypical scrapie cases are very different to both classical scrapie and experimental bovine spongiform encephalopathy in sheep. Immunolabelling for PrP(Sc) is mild and restricted at the obex and more intense and widespread rostrally, particularly in the cerebellum, substantia nigra, thalamus and basal nuclei. Intracellular immunolabelling types are not seen, but distinctive white matter immunolabelling is widespread. Vacuolation associated with PrP(Sc) deposits was not observed in the brainstem neuroanatomical areas commonly affected in classical scrapie and bovine spongiform encephalopathy, but was instead most prominent in the cerebellar cortex and neocortex. This is the largest comprehensive descriptive study of atypical scrapie pathology to date, and provides baseline data against which other natural or experimental cases can be compared. It also reinforces the current recommendation to collect cerebellum in addition to brainstem to enable confident confirmation of this distinct disease phenotype within surveillance programmes.

Changes in protein structure and distribution observed at pre-clinical stages of scrapie pathogenesis

Scrapie is a neurodegenerative disorder that involves the misfolding, aggregation and accumulation of the prion protein (PrP). The normal cellular PrP (PrP(C)) is rich in alpha-helical secondary structure, whereas the disease-associated pathogenic form of the protein (PrP(Sc)) has an anomalously high beta-sheet content. In this study, protein structural changes were examined in situ in the dorsal root ganglia from perorally 263K scrapie-infected and mock-infected hamsters using synchrotron Fourier Transform InfraRed Microspectroscopy (FTIRM) at four time points over the course of the disease (pre-clinical, 100 and 130 days post-infection (dpi); first clinical signs ( approximately 145 dpi); and terminal ( approximately 170 dpi)). Results showed clear changes in the total protein content, structure, and distribution as the disease progressed. At pre-clinical time points, the scrapie-infected animals exhibited a significant increase in protein expression, but the beta-sheet protein content was significantly lower than controls. Based on these findings, we suggest that the pre-clinical stages of scrapie are characterized by an overexpression of proteins low in beta-sheet content. As the disease progressed, the beta-sheet content increased significantly. Immunostaining with a PrP-specific antibody, 3F4, confirmed that this increase was partly - but not solely - due to the formation of PrP(Sc) in the tissue and indicated that other proteins high in beta-sheet were produced, either by overexpression or misfolding. Elevated beta-sheet was observed near the cell membrane at pre-clinical time points and also in the cytoplasm of infected neurons at later stages of infection. At the terminal stage of the disease, the protein expression declined significantly, likely due to degeneration and death of neurons. These dramatic changes in protein content and structure, especially at pre-clinical time points, emphasize the possibility for identifying other proteins involved in early pathogenesis, which are important for a further understanding of the disease.

Bovine Spongiform Encephalopathy in Sweden: An H-Type Variant

Bovine spongiform encephalopathy (BSE) had never been detected in Sweden until 2006, when the active surveillance identified a case in a 12-year-old cow. The case was an unusual form, because several molecular features of the protease-resistant prion protein (PrPres) were different from classical BSE. The differences included higher susceptibility for proteinase K, higher molecular weight of the PrPres bands, affinity to the N-terminus-specific antibodies 12B2 and P4, and peculiar banding pattern with antibody SAF84 showing an additional band at the 14 kDa position. The molecular characteristics were in accordance to previous descriptions of H-type BSE. This report shows that a range of Western blot techniques and antibodies can be applied to confirm H-type BSE and further describes that the ratio of the amounts of PrPres#1 and PrPres#2, after deglycosylation, depends on the antibody used during processing. Immunohistochemistry on sections of medulla at the level of the obex applying antibodies with epitopes covering a broad range of the PrP sequence showed accumulation of disease-specific PrP (PrP d ) in the gray matter. Fine punctate deposition in the neuropil was the most predominant type and was more severe in BSE target nuclei. The types of PrP d deposition are described in comparison with classical BSE. PrP-gene sequencing showed 6 copy octarepeat alleles and no abnormalities. It is postulated that the disease had a spontaneous origin, rather than having had been acquired in the BSE epidemic.

Scrapie-specific pathology of sheep lymphoid tissues

Transmissible spongiform encephalopathies (TSEs) or prion diseases often result in accumulation of disease-associated PrP (PrP^d) in the lymphoreticular system (LRS), specifically in association with follicular dendritic cells (FDCs) and tingible body macrophages (TBMs) of secondary follicles. We studied the effects of sheep scrapie on lymphoid tissue in tonsils and lymph nodes by light and electron microscopy. FDCs of sheep were grouped according to morphology as immature, mature or regressing. Scrapie was associated with FDC dendrite hypertrophy and electron dense deposit or vesicles. PrP^d was located using immunogold labelling at the plasmalemma of FDC dendrites and, infrequently, mature B cells. Abnormal electron dense deposits surrounding FDC dendrites were identified as immunoglobulins suggesting that excess immune complexes are retained and are indicative of an FDC dysfunction. Within scrapie-affected lymph nodes, macrophages outside the follicle and a proportion of germinal centre TBMs accumulated PrP^d within endosomes and lysosomes. In addition, TBMs showed PrP^d in association with the cell membrane, non-coated pits and vesicles, and also with discrete, large and random endoplasmic reticulum networks, which co-localised with ubiquitin. These observations suggest that PrP^d is internalised via the caveolin-mediated pathway, and causes an abnormal disease-related alteration in endoplasmic reticulum structure. In contrast to current dogma, this study shows that sheep scrapie is associated with cytopathology of germinal centres, which we attribute to abnormal antigen complex trapping by FDCs and abnormal endocytic events in TBMs. The nature of the sub-cellular changes in FDCs and TBMs differs from those of scrapie infected neurones and glial cells suggesting that different PrP^d/cell membrane interactions occur in different cell types.

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

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

Molecular aspects of disease pathogenesis in the transmissible spongiform encephalopathies

The transmissible spongiform encephalopathies (TSE), or prion diseases, are a group of rare, fatal, and transmissible neurodegenerative diseases of mammals for which there are no known viral or bacterial etiological agents. The bovine form of these diseases, bovine spongiform encephalopathy (BSE), has crossed over into humans to cause variant Creutzfeldt-Jakob disease. As a result, BSE and the TSE diseases are now considered a significant threat to human health. Understanding the basic mechanisms of TSE pathogenesis is essential for the development of effective TSE diagnostic tests and anti-TSE therapeutic regimens. This review provides an overview of the molecular mechanisms that underlie this enigmatic group of diseases.

Visualization of prion infection in transgenic mice expressing green fluorescent protein-tagged prion protein

Tg(PrP-EGFP) mice express an enhanced green fluorescent protein (EGFP)-tagged version of the prion protein (PrP) that behaves like endogenous PrP in terms of its posttranslational processing, anatomical localization, and functional activity. In this study, we describe experiments in which Tg(PrP-EGFP) mice were inoculated intracerebrally with scrapie prions. Although PrP-EGFP was incapable of sustaining prion infection in Tg(PrP-EGFP)/Prn-p(0/0) mice, it acted as a dominant-negative inhibitor that bound to, and fluorescently marked, deposits of PrPSc generated from endogenous PrP in Tg(PrP-EGFP)/Prn-p(+/+) mice. Scrapie infection of these latter animals caused a progressive accumulation of fluorescent PrP-EGFP aggregates in neuropil, axons, and prominently in the Golgi apparatus of neurons. Our results provide an entirely new picture of PrPSc localization during the course of prion infection, and they identify for the first time intracellular sites of PrPSc formation that are not well visualized with conventional immunohistochemical techniques.

Assembly of natural and recombinant prion protein into fibrils

The conversion of the alpha-helical, cellular isoform of the prion protein (PrP C ) to the insoluble, beta-sheet-rich, infectious, disease-causing isoform (PrP Sc ) is the fundamental event in the prion diseases. The C-terminal fragment of PrP Sc (PrP 27-30) is formed by limited proteolysis and retains infectivity. Unlike full-length PrP Sc , PrP 27-30 polymerizes into rod-shaped structures with the ultra-structural and tinctorial properties of amyloid. To study the folding of PrP, both with respect to the formation of PrP Sc from PrP C and the assembly of rods from PrP 27-30, we solubilized Syrian hamster (sol SHa) PrP 27-30 in low concentrations (0.2%) of sodium dodecyl sulfate (SDS) under conditions previously used to study the structural transitions of this protein. Sol SHaPrP 27-30 adopted a beta-sheet-rich structure at SDS concentrations between 0.02% and 0.04% and remained soluble. Here we report that NaCl stabilizes SHaPrP 27-30 in a soluble, beta-sheet-rich state that allows fibril assembly to proceed over several weeks. Under these conditions, fibril formation occurred not only with sol PrP 27-30, but also with native SHaPrP C . Addition of sphingolipids seems to increase fibril growth. When recombinant (rec) SHaPrP(90-231) was exposed to low concentrations of SDS, similar to those used to polymerize sol SHaPrP 27-30 in the presence of 250 mM NaCl, fibril formation occurred regularly. When fibrils formed from PrP 27-30 or PrP C were bioassayed in transgenic mice overexpressing full-length SHaPrP, no infectivity was obtained, whereas amyloid fibrils formed of rec mouse PrP(89-230) were infectious. At present, it cannot be determined whether the lack of infectivity is caused by a difference in the structure of the fibrils or in the bioassay conditions.

Phenotype of disease-associated PrP accumulation in the brain of bovine spongiform encephalopathy experimentally infected sheep

In view of the established link between bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease and of the susceptibility of sheep to experimental BSE, the detection of potential cases of naturally occurring BSE in sheep has become of great importance. In this study, the immunohistochemical (IHC) phenotype of disease-associated prion protein (PrP(d)) accumulation has been determined in the brain of 64 sheep, of various breeds and PrP genotypes, that had developed neurological disease after experimental BSE challenge with different inocula by a range of routes. Sheep BSE was characterized by neuron-associated intra- and extracellular PrP(d) aggregates and by conspicuous and consistent deposits in the cytoplasm of microglia-like cells. The stellate PrP(d) type was also prominent in most brain areas and marked linear deposits in the striatum and midbrain were distinctive. Sheep of the ARR/ARR and ARQ/AHQ genotypes displayed lower levels of PrP(d) than other sheep, and intracerebral BSE challenge resulted in higher levels of PrP(d) accumulating in the brain compared with other routes. The PrP genotype and the route of challenge also appeared to affect the incubation period of the disease, giving rise to complex combinations of magnitude of PrP(d) accumulation and incubation period. Despite these differences, the phenotype of PrP(d) accumulation was found to be very consistent across the different factors tested (notably after subpassage of BSE in sheep), thus highlighting the importance of detailed IHC examination of the brain of clinically affected sheep for the identification of potential naturally occurring ovine BSE.

Subcellular localization of disease-associated prion protein in the human brain

Disease-associated prion protein (PrP(TSE)) deposits in distinct immunostaining patterns in the brain in Creutzfeldt-Jakob disease, including synaptic, extracellular, and cell-associated localizations. After having developed an appropriate pretreatment protocol to enhance immunostaining for PrP(TSE) without damaging epitopes of other antigens, we systematically evaluated co-localization patterns of distinct PrP(TSE) immunodeposits by confocal laser microscopy, including optical serial sectioning. As shown by quantification, the most prominent co-localization of PrP(TSE) is with synaptophysin, but PrP(TSE) may also co-deposit with connexin-32, a gap junction-related protein. Furthermore, neuronal cell bodies, dendrites, axons, astrocytes, and microglia harbor granular PrP(TSE) deposits. Highly aggregated deposits are focally ubiquitinated. We conclude that PrP(TSE) is not exclusively associated with chemical but also with electric synapses, axonal transport may be a relevant route of PrP(TSE) spread in the brain, and activated microglia and astrocytes may play a role in PrP(TSE) processing, degradation, or removal.

Scrapie-specific neuronal lesions are independent of neuronal PrP expression

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

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

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

Effects of agent strain and host genotype on PrP accumulation in the brain of sheep naturally and experimentally affected with scrapie

Different cellular and neuroanatomical types of disease-specific prion protein (PrP(d)) accumulation in the brain were identified in sheep of different breeds and PrP genotypes exposed to experimental or natural scrapie infection. Immunohistochemical examination of the brains of 43 sheep with clinical signs compatible with scrapie revealed 12 different PrP(d)types, which were subjectively quantified in eight different brain regions. The PrP(d)types were grouped into four PrP(d)patterns, the relative magnitude of which provided the PrP(d)profile of each sheep examined. The analysis of the differences in magnitude and relative proportion of each of these PrP(d)types and patterns indicated (1) an effect of the scrapie strain on the PrP(d)profile, and (2) a possible effect of the host genotype on the magnitude of PrP(d)accumulation in the brain, apparently related to the incubation period. Furthermore, intraneuronal deposition of PrP(d)was the type most closely associated with the development of clinical disease. We conclude that different scrapie strains can be distinguished by PrP immunohistochemical examination of brains of affected animals.

Prion diseases: what is the neurotoxic molecule?

A great deal of effort has been devoted during the past 20 years to defining the chemical nature of prions, the infectious agents responsible for transmissible spongiform encephalopathies. In contrast, much less attention has been paid to elucidating how prions actually damage the central nervous system. Although it is commonly assumed that PrP(Sc), the protein constituent of infectious prions, is the primary culprit, increasing evidence indicates that this may not be the case. Several alternative molecular forms of PrP are reasonable candidates for the neurotoxic species in prion diseases, although it is still too early to tell whether these or other ones will turn out to be the true instigating factors. The cellular pathways activated by neurotoxic forms of PrP that ultimately result in neuronal death are also being investigated, and several possible mechanisms have been uncovered, including the operation of quality control processes in the endoplasmic reticulum. Elucidating the distinction between the infectious and neurotoxic forms of PrP has important implications for designing therapy of prion diseases, as well as for understanding pathogenic mechanisms operative in other neurodegenerative disorders and the role of prion-like states in biology.

Cellular and subcellular morphological localization of normal prion protein in rodent cerebellum

Normal cellular prion protein, a necessary protagonist in fatal neurodegenerative prion diseases, was mapped in rodent cerebellum to establish its cellular and ultrastuctural localization. Existing morphological data about native prion protein distribution in brain tissues remain, indeed, contradictory and do not fit with biochemical and cell biological results. Using ultrastructural preembedding immunocytochemistry and a monoclonal anti-mouse prion protein antibody, this report shows that cellular prion protein is present in all cortico-cerebellar and deep nuclei neuronal cell types, as well as in all glial cell types. The heaviest expression appears on parallel fibres and astrocytic processes. The protein is exclusively located on the outer cell membrane and in Golgi and endosomal intracytoplasmic organelles, with no cytoplasmic or synaptic vesicle labelling. Most important, and in contrast with previous ultrastructural data, cellular prion protein is shown to be distributed on all portions of neurons, without any preferential synaptic targeting. The present morphological report shows, for the first time in vivo, that the cellular prion protein is present on the entire cell surface membrane of all neuronal and glial cell types of the rat cerebellum. This ubiquitous presence supports the notion that prion protein has a generalized cellular function in brain tissue rather than a specialized role restricted to synaptic transmission.

Cellular and sub-cellular localisation of PrP in the lymphoreticular system of mice and sheep

Using immunocytochemistry or immunogold electron microscopy, abnormal PrP accumulation was found in lymphoreticular tissues of Suffolk sheep naturally exposed to scrapie and in the spleens of ME7 infected C57 BL mice at 70 days after infection and at the terminal stage of disease at 170 days. Clinically diseased scrapie affected sheep show widespread PrP accumulation within tingible body macrophages (TBMs) and follicular dendritic cells (FDCs) of secondary lymphoid follicles. Serial tonsillar biopsies taken from 171 ARQ/ARQ sheep at 4 months of age did not contain abnormal PrP accumulations but 80% of biopsies were positive by 14 months. In contrast, whole body necropsies of sheep not previously biopsied failed to detect PrP in the tonsil of sheep at 4, 8, 12 or 16 months of age. These findings suggest that the biopsy procedure of susceptible sheep but not resistant sheep may induce tonsillar infection. In spleen of mice both at 70 and 170 dpi, accumulations of PrP were found within lysosomes of TBMs and also at the plasma-lemma of FDCs. In the light zone of follicles of terminally diseased mice, all FDC dendrites were arranged in the form of highly reactive or hyperplastic labrynthine glomerular complexes. PrP was consistently seen between FDC dendrites in association with abundant electron dense antigen-antibody complexes. At 70 days after challenge, labrynthine complexes were rare and invariably labelled for PrP. However, sparse PrP labelling was also seen on simple FDC dendrites at this stage. These observations suggests that scrapie infected FDCs continually release PrP from the cell surface where it accumulates in excess in association with trapped immune complexes and dendritic extension. It is likely that TBMs acquire lysosomal PrP following phagocytosis of effete FDC processes or from the extracellular space. We suggest that the normal function of PrP may involve cell process extension or immune complex trapping.

Partial dissociation of PrP(Sc) deposition and vacuolation in the brains of scrapie and BSE experimentally affected goats

The diagnosis of transmissible spongiform encephalopathies (TSEs) depends on the detection of vacuolation in brain sections taken from affected individuals and/or the identification of the disease-associated isoform of the PrP (prion) protein (PrP(Sc)). During the course of an investigation, goats clinically affected following experimental infection with three different sources of TSE (SSBP/1, CH1641 and BSE) developed widespread vacuolar degeneration in the brain. With BSE, PrP(Sc) was clearly recognized in affected goat brain by immunocytochemistry (icc) and Western blotting, but in contrast the experimental scrapie sources SSBP/1 and CH1641 showed almost no or very little PrP(Sc) by icc. Western blot analysis of PrP(Sc) from BSE-affected and SSBP/1-affected goat brain showed that the protein was present in brain affected by both TSE sources, but could not be used to determine how much protein was present. It became clear that PrP(Sc) and vacuolation could be partially dissociated following challenge with two of the three TSE sources. Subtle differences in glycosylation patterns between BSE- and SSBP/1-associated PrP protein isoforms could also be recognized, although these experimentally generated results should not be regarded as a BSE/scrapie differential test. However, our study warns that the reliance on PrP(Sc) determination by icc alone as a means by which to diagnose TSE infection may generate false negative results.

Sites of prion protein accumulation in scrapie-infected mouse spleen revealed by immuno-electron microscopy

Prion protein (PrP) from the brains of animals with transmissible spongiform encephalopathies is partially protease resistant (PrP(res)) compared with fully sensitive PrP (PrP(sen)) from uninfected brains. In most experimental models, PrP(res) is a reliable indicator of infectivity. Light microscopic studies have suggested that both PrP(sen) and disease-specific accumulations of PrP are associated with follicular dendritic cells (FDCs). Using immunogold electron microscopy, this study has demonstrated disease-specific accumulation of PrP in the spleens of C57 BL mice, 70 days after intracerebral infection with the ME7 strain of scrapie and at the terminal stage of disease at 170 days. At both stages, tingible body macrophages contained PrP within lysosomes and PrP was also detected at the plasmalemma of FDCs. In the light zone of follicles of terminally diseased mice, all FDC dendrites were arranged in the form of highly reactive or hyperplastic labyrinthine glomerular complexes, within which PrP was consistently seen between FDC processes in association with abundant electron dense material, interpreted as antigen-antibody complexes. Within some glomeruli, fibrillar forms of PrP consistent with amyloid were seen. At 70 days after challenge, large or hyperplastic labyrinthine complexes were rare and invariably labelled for PrP. However, sparse PrP labelling was also seen on simple FDC processes at this stage. The ubiquitous accumulation of extracellular PrP in complex glomerular dendrites of FDCs in spleens from terminally affected mice, contrasted with simple FDC profiles, sparse PrP and limited electron dense deposits in all but a few FDCs of 70-day post-infected mice. This suggests that FDCs continually release PrP from the cell surface, where it is associated with trapped antigen-antibody complexes and dendritic extension. It is likely that tingible body macrophages acquire PrP following phagocytosis of PrP within iccosomes or from the extracellular space around FDC dendrites. These studies would not support an intracellular phase of PrP accumulation in FDCs but show that PrP is produced in excess by scrapie-infected cells from where it is released into the extracellular space. We suggest that PrP(sen) is involved in dendritic extension or in the process of antibody-antigen trapping, perhaps as part of the binding mechanism for antigen-antibody complexes. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Ultrastructural localization of prion proteins: physiological and pathological implications

The transmissible spongiform encephalopathies (TSE) or prion diseases are fatal neurodegenerative disorders in which the central event is the conversion of a normal host-encoded protein (PrP(c)) into an abnormal isoform (PrP(sc)) which accumulates as amyloid in TSE brain. The two PrP(c) and PrP(sc) prion protein isoforms are membrane sialoglycoproteins synthesized in the central nervous system and various peripheral organ tissues. In this review, we describe the ultrastructural localization of prion proteins in human and animal cerebral and non-cerebral tissues whether or not infected by TSE agents. In addition to the plasma membrane of several cells, PrP(c) was found in association with cytoplasmic organelles of central and nerve-muscle synapses, and secretory granules of epithelial cells. Fibrils of amyloid plaques, synaptic structures, and lysosome-like organelles constitute the subcellular sites harboring PrP(sc). These findings have led to discussions on the physiological role of PrP(c) and the pathological mechanisms underlying prion spongiform encephalopathies.

Prion protein immunoreactivity in brain samples from an unselected autopsy population: findings in 200 consecutive cases

Immunostaining for prion protein (PrP) using the KG9 monoclonal antibody was undertaken on brain sections from an unselected group of 200 post-mortem cases. One case of clinically diagnosed vCJD was confirmed and showed widespread abundant PrP immunostaining with KG9 and somewhat less abundant PrP with another monoclonal antibody, 3F4. PrP immunostaining seen with KG9 was insensitive to proteinase K pretreatment in sections from this case of vCJD. Among the remaining 199 cases, sections from 84 (42%) showed small amounts of PrP immunoreactivity with the KG9 antibody, mainly localized to neurones, neural processes and argyrophilic plaques of the type seen in ageing and Alzheimer's disease. Purkinje cells, swollen (ischaemic) axons, macrophages and microglials cells were also occasionally labelled with this antibody in non-CJD cases. Pre-treatment of adjacent sections from non-CJD cases with positive KG9 staining abolished this staining, indicating that it represented the cellular form of PrP. There were differences in age, sex and cause of death in non-CJD cases with some PrP immunostaining patterns compared with cases lacking any staining. Specifically, a younger mean age, more females and fewer cardiac deaths were found among those with neuronal PrP staining patterns. Staining of some features was also significantly associated. These findings need to be taken into account when PrP immunostaining is used to diagnose prion diseases. They may indicate that cellular PrP is increased in the human brain under some circumstances and provide insight into the handling of this protein by human brain cells.

Synapse loss associated with abnormal PrP precedes neuronal degeneration in the scrapie-infected murine hippocampus

Numbers of neurones, synapses and axon terminals were quantified in a murine scrapie model with severe hippocampal pyramidal cell loss, in which definite clinical scrapie is evident from 226 days post-infection (dpi) and death occurs around 250 dpi. Disease-specific PrP accumulations were first seen at 70 dpi (28% of the incubation period (IP)) in thalamus and as sparse foci within the stratum pyramidale of CA1. By 98 dpi (39% IP), PrP was seen in the stratum radiatum and was found at later stages throughout all levels of the hippocampus. At the ultrastructural level in the stratum radiatum of CA1, a decrease in the numbers of simple synapses from 84 dpi (34% IP) and in perforated synapses from 98 dpi (42% IP) was found using an unbiased stereological method, the disector analysis. Degeneration of axon terminals was found from 98 dpi (39% IP) onwards. Neuronal loss was detected in CA1 from 180 dpi (72% IP). The results suggest that the fundamental lesion in the hippocampus of ME7-infected mice is associated with PrP release from CA1 pyramidal neurones, which perturbs synaptic function and leads to degeneration of preterminal axons, and that subsequent pathological changes including neurone loss are sequelae to this initial insult.

Methods for studying prion protein (PrP) metabolism and the formation of protease-resistant PrP in cell culture and cell-free systems. An update

Transmissible spongiform encephalopathies (TSE) or prion diseases result in aberrant metabolism of prion protein (PrP) and the accumulation of a protease-resistant, insoluble, and possibly infectious form of PrP, PrP-res. Studies of PrP biosynthesis, intracellular trafficking, and degradation has been studied in a variety of tissue culture cells. Pulse-chase metabolic labeling studies in scrapie-infected cells indicated that PrP-res is made posttranslationally from an apparently normal protease-sensitive precursor, PrP-sen, after the latter reaches the cell surface. Cell-free reactions have provided evidence that PrP-res itself can induce the conversion of PrP-sen to PrP-res in a highly species- and strain-specific manner. These studies have shed light on the mechanism of PrP-res formation and suggest molecular bases for TSE species barrier effects and agent strain propagation.

In situ formation of protease-resistant prion protein in transmissible spongiform encephalopathy-infected brain slices

The transmissible spongiform encephalopathies (TSEs) comprise a group of fatal neurodegenerative diseases that are characterized by the conversion of the normal host cellular prion protein (PrPC), to the abnormal protease-resistant prion protein isoform (PrP-res). It has been proposed, though not proven, that the infectious TSE agent consists solely of PrP-res and that PrP-res-induced conformational conversion of PrPC to additional PrP-res represents agent replication. In this study we demonstrate in situ conversion of protease-sensitive PrPC to PrP-res in TSE-infected brain slices. One step in this process is the binding of soluble PrPC to endogenous PrP-res deposits. The newly formed PrP-res associated with the slices in a pattern that correlated with the pre-existing brain distribution of PrP-res. Punctate in situ PrP conversion was observed in brain regions containing PrP-res amyloid plaques, and a more dispersed conversion product was detected in areas containing diffuse PrP-res deposits. These studies provide direct evidence that PrP-res formation involves the incorporation of soluble PrPC into both nonfibrillar and fibrillar PrP-res deposits in TSE-infected brain. Our findings suggest that the in situ PrP conversion reaction leads to additional polymerization of endogenous PrP-res aggregates and is analogous to the process of PrP-res fibril and subfibril growth in vivo.

PrP deposition, microglial activation, and neuronal apoptosis in murine scrapie

The present study investigated the relationship among PrP deposition, microglial activation, vacuolation, and neuronal death in the hippocampus of the 301V/VM murine scrapie model (mean incubation period 117 +/- 1 days). PrP deposition was first detected after 30 days and microglial activation after 60 days. Vacuolation in the CA1 and CA2 pyramidal layer was present from 90 days onward. Only occasional in situ end labeling (ISEL)-positive neurons were present in the hippocampus of scrapie-infected mice from 75 days postinoculation (d.p.i.), except at 105 d.p.i. when relatively large numbers of apoptotic, ISEL-positive neurons in the CA1 hippocampal region were observed. Terminally ill animals showed almost complete loss of CA1 pyramidal neurons. Electron microscopy of the CA1 region at 105 days confirmed that these neurons were dying by apoptosis. These data suggest that microglial activation in scrapie is a response to abnormal PrP deposition rather than a response to neuronal cell loss.

The Ninth Datta Lecture. Molecular biology of transmissible spongiform encephalopathies

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

Normal host prion protein necessary for scrapie-induced neurotoxicity

Accumulation of the prion protein PrPSc, a pathological and protease-resistant isoform of the normal host protein PrPC, is a feature of prion disease such as scrapie. It is still unknown whether scrapie pathology comes about by neurotoxicity of PrPSc, acute depletion of PrPC, or some other mechanism. Here we investigate this question by grafting neural tissue overexpressing PrPC into the brain of PrP-deficient mice which are scrapie-resistant and do not propagate infectivity. After intracerebral inoculation with scrapie prions, the grafts accumulated high levels of PrPSc and infectivity and developed the severe histopathological changes characteristic of scrapie. Moreover, substantial amounts of graft-derived PrPSc migrated into the host brain. Even 16 months after inoculation no pathological changes were seen in PrP-deficient tissue, not even in the immediate vicinity of the grafts. Therefore, in addition to being resistant to scrapie infection, brain tissue devoid of PrPC is not damaged by exogenous PrPSc.

Pathology of the transmissible spongiform encephalopathies with special emphasis on ultrastructure

The transmissible spongiform encephalopathies are a group of genetic and infectious disorders which are exemplified by scrapie in animals and Creutzfeldt-Jakob disease in humans. The spongiform encephalopathies are characterized by symmetrical vacuolation of neurons and neuropil. Amyloid plaque formation similar to that found in Alzheimer's disease is conspicuous in many, but not all, of these diseases. The sub-cellular pathology features of the spongiform encephalopathies have been studied by conventional transmission electron microscopy, scanning electron microscopy, freeze fracture, negative staining and most recently by application of immunogold labelling methods. Although these studies have revealed many unusual structures, convincing virus-like particles have not been demonstrated. Considerable data, including important transgenic mouse studies, now suggest that a single cellular protein, designated prion protein, is necessary for infection. Ultrastructural immunogold studies have shown that prion protein is released from the surface of neurons and neurites, diffuses through the extracellular space around infected cells where it accumulates and finally becomes aggregated as amyloid fibrils. It is likely that the accumulation of prion protein within the extracellular space is instrumental in causing nerve cell dysfunction and, ultimately, neurological disease.

Morphogenesis of amyloid plaques in 87V murine scrapie

Amyloid plaques of scrapie-infected mouse brains are composed of fibrillar forms of a host coded, cell surface sialoglycoprotein called PrP (prion protein). Serial ultrastructural immunogold staining was performed on plaques identified by light microscopic immunocytochemistry of brains of VM mice infected with the 87V strain of scrapie. Classical plaques, of a kuru-type morphology, were composed of a central core of bundles of amyloid fibrils. Amyloid fibrils of classical plaques were immunoreactive for PrP. In addition, PrP was also found at the plaque periphery, in the absence of fibrils, at the plasmalemma of cell processes and in the associated extracellular spaces. Frequent microglial cells and occasional astrocytes contained PrP within lysosomes. Other plaques with few or no recognizable amyloid fibrils were frequent and were termed primitive plaques. PrP could be demonstrated in a non-fibrillar form at the plasmalemma and in the extracellular spaces between neurites of such plaques. Many primitive plaques showed little or no sub-cellular pathology associated with the PrP accumulation. PrP was closely associated with the plasmalemma of occasional dendrites passing towards the centre of primitive plaques. These results suggest that plaques are formed around one or more PrP releasing dendrites. PrP accumulates in the extracellular spaces adjacent to such processes prior to its spontaneous aggregation into fibrils. Lysosomal accumulation of PrP in microglia and astrocytes located at the periphery of plaques suggest that these cells are involved in the phagocytosis of excess or abnormal PrP.