Selective neuronal targeting in prion disease

Inhibition of complex glycosylation increases the formation of PrPsc

N-linked glycans with complex structure have a major role in the biological activity of a wide variety of cell surface and secreted glycoproteins. Here, we show that geldanamycin, an inhibitor of Hsp90, interferes with the formation of complex glycosylated mammalian prion protein (PrPC). Similarly to inhibitors of alpha-mannosidases, geldanamycin stabilized a high mannose PrPC glycoform and prevented the subsequent processing into complex structures. Moreover, a PrP/Grp94 complex could be isolated from geldanamycin-treated cells, suggesting that Grp94 might play a role in the processing of PrPC in the endoplasmic reticulum. Inhibition of complex glycosylation did not interfere with the glycosylphosphatidylinositol (GPI) anchor attachment and cellular trafficking of high mannose PrPC to the outer leaflet of the plasma membrane. In scrapie-infected neuroblastoma cells, however, high mannose PrPC glycoforms were preferred substrates for the formation of PrP-scrapie (PrPSc). Our study reveals that complex glycosylation is dispensable for the cellular trafficking of PrPC, but modulates the formation of PrPSc.

Determinants of the in vivo folding of the prion protein. A bipartite function of helix 1 in folding and aggregation

Misfolding of the mammalian prion protein (PrP) is implicated in the pathogenesis of prion diseases. We analyzed wild type PrP in comparison with different PrP mutants and identified determinants of the in vivo folding pathway of PrP. The complete N terminus of PrP including the putative transmembrane domain and the first beta-strand could be deleted without interfering with PrP maturation. Helix 1, however, turned out to be a major determinant of PrP folding. Disruption of helix 1 prevented attachment of the glycosylphosphatidylinositol (GPI) anchor and the formation of complex N-linked glycans; instead, a high mannose PrP glycoform was secreted into the cell culture supernatant. In the absence of a C-terminal membrane anchor, however, helix 1 induced the formation of unglycosylated and partially protease-resistant PrP aggregates. Moreover, we could show that the C-terminal GPI anchor signal sequence, independent of its role in GPI anchor attachment, mediates core glycosylation of nascent PrP. Interestingly, conversion of high mannose glycans to complex type glycans only occurred when PrP was membrane-anchored. Our study indicates a bipartite function of helix 1 in the maturation and aggregation of PrP and emphasizes a critical role of a membrane anchor in the formation of complex glycosylated PrP.

Abbreviated incubation times for human prions in mice expressing a chimeric mouse-human prion protein transgene

Transgenic (Tg) mouse lines that express chimeric mouse-human prion protein (PrP), designated MHu2M, are susceptible to prions from patients with sporadic Creutzfeldt-Jakob disease (sCJD). With the aim of decreasing the incubation time to fewer than 200 days, we constructed transgenes in which one or more of the nine human residues in MHu2M were changed to mouse. The construct with murine residues at positions 165 and 167 was expressed in Tg(MHu2M,M165V,E167Q) mice and resulted in shortening the incubation time to approximately 110 days for prions from sCJD patients. The construct with a murine residue at position 96 resulted in lengthening the incubation time to more than 280 days for sCJD prions. When murine residues 96, 165, and 167 were expressed, the abbreviated incubation times for sCJD prions were abolished. Variant CJD prions showed prolonged incubation times between 300 and 700 days in Tg(MHu2M) mice on first passage and incubation times of approximately 350 days in Tg(MHu2M,M165V,E167Q) mice. On second and third passages of variant CJD prions in Tg(MHu2M) mice, multiple strains of prions were detected based on incubation times and the sizes of the protease-resistant, deglycosylated PrP(Sc) fragments. Our discovery of a previously undescribed chimeric transgene with abbreviated incubation times for sCJD prions should facilitate studies on the prion species barrier and human prion diversity.

Perspectives on prion biology, prion disease pathogenesis, and pharmacologic approaches to treatment

The main goals of this article have been to summarize our current understanding of the biology of PrP, the propagation of prions, and the etiology and pathogenesis of each form of prion disease (familial, sporadic, and infectious); and to review current rational pharmacologic strategies for treatment of prion diseases. Each of these subjects is presented primarily from the perspective of investigations performed by the prion disease research laboratories at the University of California in San Francisco and by its many collaborators in the United States and abroad. This review focuses on key results from the hundreds of transgenic mouse lines expressing different PrP constructs that have been used to determine the roles played by different PrPSc and PrPC domains in prion propagation and the prion disease phenotype.

Distance of sequons to the C-terminus influences the cellular N-glycosylation of the prion protein

Cell-specific differences in the utilization of the two N-glycosylation sequons (Asn180-Ile-Thr and Asn196-Phe-Thr) of the prion protein (PrP) have been proposed to influence the aetiology of the neurodegenerative prion diseases. As the N-glycosylation of PrP is ablated by deletion of the C-terminal glycosyl-phosphatidylinositol (GPI) anchor signal sequence, we have investigated the determinants for PrP sequon utilization in human neuronal cells using the novel approach of restoring N-glycosylation to secreted forms of PrP lacking a GPI anchor. N-glycosylation was restored to an efficiency comparable with that of GPI anchored PrP when the distance of the sequon to the C-terminus was increased so that it was sufficient to reach the active site of oligosaccharyltransferase before chain termination. Our findings indicate that sequon utilization in PrP is a co-translational process that precedes GPI anchor addition and, as such, will be greatly influenced by the dynamics of the translocon-oligosaccharyltransferase complex.

PrP knock-out and PrP transgenic mice in prion research

Spongiform encephalopathies such as scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle or Creutzfeldt-Jacob disease (CJD) and Gerstmann-Sträussler-Scheinker syndrome (GSS) in humans is caused by a transmissible agent designated prion. The 'protein only' hypothesis proposes that the prion consists partly or entirely of a conformational isoform of the normal host protein PrP(C), designated PrP(*)(1) and that the abnormal conformer, when introduced into the organism, causes the conversion of PrP(C) into a likeness of itself. PrP(*) may be congruent with PrP(Sc), a protease-resistant, aggregated conformer of PrP that accumulates mainly in brain of almost all prion-infected organisms. PrP(C) consists of a flexible N-terminal half, comprising Cu(2+)-binding octapeptide repeats, and a globular domain consisting of three alpha-helices, one short antiparallel beta-sheet and a single disulphide bond. It is anchored at the outer cell-surface by a glycosyl phosphatidylinositol (GPI) tail and is present in almost all tissues, however, mainly in brain. Compelling linkage between the prion and PrP was established by biochemical and genetic data and led to the prediction that animals devoid of PrP should be resistant to experimental scrapie and fail to propagate infectivity. This prediction was indeed borne out, adding substantial support to the 'protein only' hypothesis. In addition, the availability of PrP knock-out mice provided an approach to carry out reverse genetics on PrP, both in regard to prion disease and to its physiological role.

Transgenesis applied to transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative disorders present in various mammals. TSEs have been studies intensively, even more so following the BSE crisis and the subsequent threat of a human nvCJD epidemic. In the 'protein-only' hypothesis, the infectious agent, called prion, is assumed to be a misfolded host protein. Transgenesis has mainly been applied to study the role of this protein, its structure-function relationship with respect to its pathogenic properties and to assess the genetic origin of the well-recognised species barrier effect. This approach has somewhat supplemented the lack of in vitro models. This review will try to summarise the impressive work that has been done in this field. Although many questions remain unanswered, transgenic experiments have and will still improve our knowledge on this disease and might help us to develop critically needed therapeutic approaches.

Molecular basis of scrapie strain glycoform variation

Transmissible spongiform encephalopathies (TSE) are characterized by the conversion of a protease-sensitive host glycoprotein, prion protein or PrP-sen, to a protease-resistant form (PrP-res). PrP-res molecules that accumulate in the brain and lymphoreticular system of the host consist of three differentially glycosylated forms. Analysis of the relative amounts of the PrP-res glycoforms has been used to discriminate TSE strains and has become increasingly important in the differential diagnosis of human TSEs. However, the molecular basis of PrP-res glycoform variation between different TSE agents is unknown. Here we report that PrP-res itself can dictate strain-specific PrP-res glycoforms. The final PrP-res glycoform pattern, however, can be influenced by the cell and significantly altered by subtle changes in the glycosylation state of PrP-sen. Thus, strain-specific PrP-res glycosylation profiles are likely the consequence of a complex interaction between PrP-res, PrP-sen, and the cell and may indicate the cellular compartment in which the strain-specific formation of PrP-res occurs.

Changes in the glycosylation pattern of prion protein in murine scrapie. Implications for the mechanism of neurodegeneration in prion diseases

In prion diseases, the normal prion protein (PrP(c)) undergoes a conformational change that results in the abnormal form, named scrapie prion protein (PrP(sc)). The visual system of rodents provides a relatively simple neuronal model in which the cell bodies of neurons are confined to the retina and the axons constitute the optic nerve. We investigated by Western blot the profile of PrP(c) in the optic nerve and retina of normal hamsters and mice. We found that in the optic nerve the amount of PrP(c) is significantly higher than in the retina. A less abundant non-glycosylated band was observed in retinas compared with the optic nerve and brain. Similar results were found in the gray and white matter from normal mice and hamsters. After stereotaxic injection of ME7 or 139A in the superior colliculus, a visual target area, the proportion and glycopattern of PrP changed in the retina and optic nerve throughout the course of the disease. Similar results were found in the gray and white matter at terminal stage of scrapie after injection of ME7 and 139A in the dorsal hippocampus. This is the first time that changes in the distribution and glycopattern of PrP have been described in an in vivo model of prion diseases.

Prominent stress response of Purkinje cells in Creutzfeldt-Jakob disease

To examine the role of stress-related 70-kDa heat shock proteins (Hsp-s) in Creutzfeldt-Jakob disease (CJD), we performed immunocytochemistry to detect Hsp-72 and Hsp-73, together with the abnormal (PrP(Sc)) and the presumed cellular form (PrP(C)) of the prion protein, and TUNEL method to measure cellular vulnerability in different brain regions in CJD and control cases. While Hsp-73 showed uniform distribution in all the examined samples, an increase in the number of Purkinje cells with prominent accumulation of Hsp-72 in the CJD group was observed. These neurons also showed intense PrP(C) staining, but TUNEL-positive nuclei were only detected in the granular (Hsp-72-negative) cell layer. Fewer cells of the inferior olivary nucleus were immunoreactive for Hsp-72 in CJD than in control cases, and regions showing severe spongiform change and gliosis exhibited fewer Hsp-72-immunoreactive neurons. Our results indicate that accumulation of the inducible Hsp-72 in certain cell types may be part of a cytoprotective mechanism, which includes preservation of proteins like PrP(C).

Prion diseases of humans and animals: their causes and molecular basis

Prion diseases are transmissible neurodegenerative conditions that include Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE) and scrapie in animals. Prions appear to be composed principally or entirely of abnormal isoforms of a host-encoded glycoprotein, prion protein. Prion propagation involves recruitment of host cellular prion protein, composed primarily of alpha-helical structure, into a disease specific isoform rich in beta-sheet structure. The existence of multiple prion strains has been difficult to explain in terms of a protein-only infections agent, but recent studies suggest that strain specific phenotypes can be encoded by different prion protein conformations and glycosylation patterns. The ability of a protein to encode phenotypic information has important biological implications. The appearance of a novel human prion disease, variant CJD, and the clear experimental evidence that it is caused by exposure to BSE has highlighted the need to understand the molecular basis of prion propagation, pathogenesis, and the barriers limiting intermammalian transmission. It is unclear if a large epidemic of variant CJD will occur in the years ahead.

Intracellular re-routing of prion protein prevents propagation of PrP(Sc) and delays onset of prion disease

Prion diseases are fatal and transmissible neurodegenerative disorders linked to an aberrant conformation of the cellular prion protein (PrP(c)). We show that the chemical compound Suramin induced aggregation of PrP in a post-ER/Golgi compartment and prevented further trafficking of PrP(c) to the outer leaflet of the plasma membrane. Instead, misfolded PrP was efficiently re-routed to acidic compartments for intracellular degradation. In contrast to PrP(Sc) in prion-infected cells, PrP aggregates formed in the presence of Suramin did not accumulate, were entirely sensitive to proteolytic digestion, had distinct biophysical properties, and were not infectious. The prophylactic potential of Suramin-induced intracellular re-routing was tested in mice. After intraperitoneal infection with scrapie prions, peripheral application of Suramin around the time of inoculation significantly delayed onset of prion disease. Our data reveal a novel quality control mechanism for misfolded PrP isoforms and introduce a new molecular mechanism for anti-prion compounds.

Scrapie strains maintain biological phenotypes on propagation in a cell line in culture

Bovine spongiform encephalopathy (BSE) and its human equivalent, variant Creutzfeldt-Jakob disease (vCJD), are caused by the same strain of infectious agent, which is similar to, but distinct from, >20 strains of their sheep scrapie homologue. A better understanding of the molecular strain determinants could be obtained from cells in monoculture than from whole animal studies where different cell targeting is commonly a strain-related feature. Although a few cell types can be infected with different strains, the phenotypes of the emergent strains have not been studied. We have cured the scrapie-infected, clonal SMB cell line with pentosan sulfate, stably re-infected it with a different strain of scrapie and shown that biological properties and prion protein profiles characteristic of each original strain are propagated faithfully in this single non-neuronal cell type. These findings attest to the fact that scrapie strain determinants are stable and host-independent in isolated cells.

Early-onset amyloid deposition and cognitive deficits in transgenic mice expressing a double mutant form of amyloid precursor protein 695

We have created early-onset transgenic (Tg) models by exploiting the synergistic effects of familial Alzheimer's disease mutations on amyloid beta-peptide (Abeta) biogenesis. TgCRND8 mice encode a double mutant form of amyloid precursor protein 695 (KM670/671NL+V717F) under the control of the PrP gene promoter. Thioflavine S-positive Abeta amyloid deposits are present at 3 months, with dense-cored plaques and neuritic pathology evident from 5 months of age. TgCRND8 mice exhibit 3,200-4,600 pmol of Abeta42 per g brain at age 6 months, with an excess of Abeta42 over Abeta40. High level production of the pathogenic Abeta42 form of Abeta peptide was associated with an early impairment in TgCRND8 mice in acquisition and learning reversal in the reference memory version of the Morris water maze, present by 3 months of age. Notably, learning impairment in young mice was offset by immunization against Abeta42 (Janus, C., Pearson, J., McLaurin, J., Mathews, P. M., Jiang, Y., Schmidt, S. D., Chishti, M. A., Horne, P., Heslin, D., French, J., Mount, H. T. J., Nixon, R. A., Mercken, M., Bergeron, C., Fraser, P. E., St. George-Hyslop, P., and Westaway, D. (2000) Nature 408, 979-982). Amyloid deposition in TgCRND8 mice was enhanced by the expression of presenilin 1 transgenes including familial Alzheimer's disease mutations; for mice also expressing a M146L+L286V presenilin 1 transgene, amyloid deposits were apparent by 1 month of age. The Tg mice described here suggest a potential to investigate aspects of Alzheimer's disease pathogenesis, prophylaxis, and therapy within short time frames.

Quantitative traits of prion strains are enciphered in the conformation of the prion protein

Variations in prions, which cause different disease phenotypes, are often referred to as strains. Strains replicate with a high degree of fidelity, which demands a mechanism that can account for this phenomenon. Prion strains differ by qualitative characteristics such as clinical symptoms, brain pathology, topology of accumulated PrP(Sc), and Western blot patterns of glycosylated or deglycosylated PrP(Sc). Since none of these qualitative features can directly explain quantitative strain traits such as incubation time or dose response, we analyzed conformational parameters of PrP(Sc) and the rate of accumulation in different prion strains. Using the conformation-dependent immunoassay (CDI), we were able to discriminate among PrP(Sc) molecules from eight different prion strains propagated in Syrian hamsters. CDI quantifies PrP isoforms by simultaneously following antibody binding to both the denatured and native forms of a protein. In a plot of the ratio of antibody binding to denatured/native PrP graphed as a function of the concentration of PrP(Sc), each strain occupied a unique position, indicating that each strain accumulated different concentrations of particular PrP(Sc) conformers. This conclusion was supported by a unique pattern of equilibrium unfolding of PrP(Sc) found within each strain. By comparing the PrP(Sc) levels before and after limited proteinase K digestion, we found that each strain produces a substantial fraction of protease-sensitive PrP(Sc). We asked whether this fraction of PrP(Sc) might reflect those PrP(Sc) molecules that are most readily cleared by cellular proteases. When the protease-sensitive PrP(Sc) fraction was plotted as a function of the incubation time, a linear relationship was found with an excellent correlation coefficient (r = 0.94). Combined with the data on time courses of prion infection in Tg(MHu2M) and Tg(SHaPrP) mice, the results argue that different incubation times of various prion strains may arise predominantly from distinct rates of PrP(Sc) clearance rather than from different rates of PrP(Sc) formation.

Distribution of cellular prion protein in normal human cerebral cortex--does it have relevance to Creutzfeldt-Jakob disease?

Creutzfeldt-Jakob disease and bovine spongiform encephalopathy are the best known forms of prion diseases. A basis for their pathogenesis is the transformation of normal prion protein to abnormal prion protein. This would mean that either loss of normal function or a gain of a toxic function of the prion protein would play a major role. Since the prime target for Creutzfeldt-Jakob disease in humans is the neocortex, and the intracortical distribution of the destructive process in prion diseases appears not to be haphazard, it may be that a clear cortical study of normal prion protein production in the premorbid human neocortex might contribute to insight in the pathogenesis of prion diseases. As no such study is available, we performed a detailed study in normal human cortex using immunohistochemistry for prion protein, in both frozen and vibratomised tissue, and in situ hybridisation for prion protein mRNA. We have found normal prion protein production mainly in the upper cortical neurons in neocortex and Purkinje cells in the cerebellum. This finding implicates that normal prion protein is more important as an anti-apoptotic signal in disease than abnormal prion protein is as a toxic substance.

Differential expression of cellular prion protein in mouse brain as detected with multiple anti-PrP monoclonal antibodies

The normal cellular prion protein (PrP(C)) plays an essential role in the development of prion diseases. Indirect evidence has suggested that different PrP(C) glycoforms may be expressed in different brain regions and perform distinct functions. However, due to a lack of monoclonal antibodies (Mabs) that are specific for mouse PrP(C), the expression of PrP(C) in the mouse brain has not been studied in great detail. We used Mabs specific for either the N-terminus or the C-terminus of the mouse PrP(C) to study its expression in the mouse brain by immunoblotting and immunohistochemistry. Immunoblotting studies demonstrated that the expression of PrP(C) differed quantitatively as well as qualitatively in different regions of the brain. The anti-C-terminus Mabs reacted with all three molecular weight bands of PrP(C); the anti-N-terminus Mabs only reacted with the 39-42 kDa PrP(C). The results from immunohistochemical staining revealed the spatial distribution of PrP(C) in the mouse brain, which were consistent with that from immunoblotting. Although expression of PrP(C) has been reported to be required for long-term survival of Purkinje cells, we were unable to detect PrP(C) in the Purkinje cell layer in the cerebellum with multiple anti-PrP Mabs. Our findings suggest that PrP(C) variants, i.e. various glycoforms and truncated forms, might be specifically expressed in different regions of mouse brain and might have different functions.

Prions: pathogenesis and reverse genetics

Spongiform encephalopathies are a group of infectious neurodegenerative diseases. The infectious agent that causes transmissible spongiform encephalopathies was termed prion by Stanley Prusiner. The prion hypothesis states that the partially protease-resistant and detergent-insoluble prion protein (PrPsc) is identical with the infectious agent, and lacks any detectable nucleic acids. Since the latter discovery, transgenic mice have contributed many important insights into the field of prion biology. The prion protein (PrPc) is encoded by the Prnp gene, and disruption of Prnp leads to resistance to infection by prions. Introduction of mutant PrPc genes into PrPc-deficient mice was used to investigate structure-activity relationships of the PrPc gene with regard to scrapie susceptibility. Ectopic expression of PrPc in PrPc knockout mice proved a useful tool for the identification of host cells competent for prion replication. Finally, the availability of PrPc knockout and transgenic mice overexpressing PrPc allowed selective reconstitution experiments aimed at expressing PrPc in neurografts or in specific populations of hemato- and lymphopoietic cells. The latter studies helped in elucidating some of the mechanisms of prion spread and disease pathogenesis.

The molecular biology of prion propagation

Prion diseases such as Creutzfeldt-Jakob disease (CJD) in humans and scrapie and bovine spongiform encephalopathy (BSE) in animals are associated with the accumulation in affected brains of a conformational isomer (PrP(Sc)) of host-derived prion protein (PrP(C)). According to the protein-only hypothesis, PrP(Sc) is the principal or sole component of transmissible prions. The conformational change known to be central to prion propagation, from a predominantly alpha-helical fold to one predominantly comprising beta structure, can now be reproduced in vitro, and the ability of beta-PrP to form fibrillar aggregates provides a plausible molecular mechanism for prion propagation. The existence of multiple prion strains has been difficult to explain in terms of a protein-only infectious agent but recent studies of human prion diseases suggest that strain-specific phenotypes can be encoded by different PrP conformations and glycosylation patterns. The experimental confirmation that a novel form of human prion disease, variant CJD, is caused by the same prion strain as cattle BSE, has highlighted the pressing need to understand the molecular basis of prion propagation and the transmission barriers that limit their passage between mammalian species. These and other advances in the fundamental biology of prion propagation are leading to strategies for the development of rational therapeutics.

Expression of unglycosylated mutated prion protein facilitates PrP(Sc) formation in neuroblastoma cells infected with different prion strains

Prion replication involves conversion of the normal, host-encoded prion protein PrP(C), which is a sialoglycoprotein bound to the plasma membrane by a glycophosphatidylinositol anchor, into a pathogenic isoform, PrP(Sc). In earlier studies, tunicamycin prevented glycosylation of PrP(C) in scrapie-infected mouse neuroblastoma (ScN2a) cells but it was still expressed on the cell surface and converted into PrP(Sc); mutation of PrP(C) at glycosylation consensus sites (T182A, T198A) produced low steady-state levels of PrP that were insufficient to propagate prions in transgenic mice. By mutating asparagines to glutamines at the consensus sites, we obtained expression of unglycosylated, epitope-tagged MHM2PrP(N180Q,N196Q), which was converted into PrP(Sc) in ScN2a cells. Cultures of uninfected neuroblastoma (N2a) cells transiently expressing mutated PrP were exposed to brain homogenates prepared from mice infected with the RML, Me7 or 301V prion strains. In each case, mutated PrP was converted into PrP(Sc) as judged by Western blotting. These findings raise the possibility that the N2a cell line can support replication of different strains of prions.

Pathogenesis of prion diseases: a progress report

Almost 20 years have passed since Stanley Prusiner proposed that the agent causing transmissible spongiform encephalopathies consists exclusively of a protein and termed it prion. A mixed balance can be drawn from the enormous research efforts that have gone into prion research during this time. On the negative side, the protein-only hypothesis has not been conclusively proven yet. On the positive side, our understanding of spongiform encephalopathies has experienced tremendous advances, mostly through human genetics, mouse transgenetics, and biophysical methods. Perhaps the most astonishing development is the realization that many human neurodegenerative diseases for which transmissibility has been more or less stringently excluded, may follow pathogenetic principles similar to those of prion diseases. Also, the hypothesis that prion-like phenomena may underlie certain non-genetic traits observed in yeast has resulted in the surprising recognition that the instructional self-propagating changes in protein conformation may be much more prevalent in nature than previously thought. The latter developments have been astonishingly successful, and one could now argue that the prion principle is much more solidly established in yeast than in mammals.

Mapping the parameters of prion-induced neuropathology

We present a theoretical framework that enables us to dissect out the parametric dependencies of the pathogenesis of prion diseases. We are able to determine the influence of both host-dependent factors (connectivity, cell density, protein synthesis rate, and cell death) and strain-dependent factors (cell tropism, virulence, and replication rate). We use a model based on a linked system of differential equations on a lattice to explore how the regional distribution of central nervous system pathology in Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia relates to each of these factors. The model then is used to make qualitative predictions about the pathology for two possible hypothetical triggers of neuronal loss in prion diseases. Pathological progression in overexpressing mouse models has been shown to depend on the site of initial infection. The model allows us to compare the pathologies resulting from different inoculation routes.

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.

Prion protein genes and prion diseases: studies in transgenic mice

In the past decade, manipulation of PrP genes by transgenesis in mice has provided important insights into mechanisms of prion propagation and the molecular basis of prion strains and species barriers. Despite these advances, our understanding of these unique pathogens is far from complete. This review focuses on PrP gene knockout and gene replacement studies, PrP structure and function, and transgenic models of human and animal prion diseases. Transgenic approaches will doubtless remain the cornerstone of investigations into the prion diseases in the coming years, which will include mechanistic studies of prion pathogenesis and prion transmission barriers. Transgenic models will also be important tools for the evaluation of potential therapeutic agents for prion diseases.

Functional implications of the noradrenergic-cholinergic switch induced by retinoic acid in NB69 neuroblastoma cells

Some neuroblastoma cell lines change their neurotransmitter phenotype from noradrenergic to cholinergic under retinoic acid treatment. Such "neurotransmitter switch" seems to be a consequence of changes in the expression and activity of the biosynthetic machinery for both neurotransmitters. In this study, we have characterized this "neurotransmitter switch" induced by retinoic acid in a human neuroblastoma cell line (NB69) showing catecholaminergic characteristics. Retinoic acid treatment reduced tyrosine hydroxylase activity and noradrenaline levels in NB69 cells but did not modify the expression of this enzyme. Moreover, the calcium-dependent release of [(3)H]noradrenaline in control cells was highly reduced by retinoic acid treatment. On the other hand, NB69 cells treated with retinoic acid enhanced the expression of choline acetyltransferase and acquired the capability to release [(3)H]acetylcholine in a calcium-dependent way. In addition, we found that the expression of the vesicular monoamine transporter 2 (VMAT2) and the vesicular acetylcholine transporter (VAChT) was increased in those cells treated with retinoic acid. Immunostaining revealed that retinoic acid treatment changed the cellular distribution of both vesicular monoamine transporter 2 and vesicular acetylcholine transporter. In conclusion, retinoic acid induces a noradrenergic to cholinergic switch in NB69 cells by acting at several levels of the neurotransmitter phenotypic expression.

Glycosylation of prions and its effects on protein conformation relevant to amino acid mutations

The three-dimensional coordinates from a nuclear magnetic resonance (NMR)-averaged structure containing residues 121-226 of mouse prion were used as the starting geometry for MD of prion either with or without glycan in both mutant and wild-type forms. The following mutants were studied: Asp-178 to Asn, Thr-183 to Ala, Phe-198 to Ser, Glu-200 to Lys, and Gln-217 to Arg. NMR data vs structural models were compared to observe any major differences. Simulations of the change in protein structure with and without glycan were performed, as they cannot be tested by NMR analysis. Several mutants were expressed and analyzed for altered glycosylation and the results interpreted in terms of molecular modeling. N-linked glycosylation is likely to play an important role in prion biology as shown by visualization of glycoprotein conformation.

Dendritic and synaptic alterations of hippocampal pyramidal neurones in scrapie-infected mice

Neurone damage and eventual loss may underlie the clinical signs of disease in the transmissible spongiform encephalopathies (TSEs). Although neurone death appears to be through apoptosis, the trigger for this form of cell death in the TSEs is not known. Using two different murine scrapie models, hippocampal pyramidal cells were studied through microinjection of fluorescent dye, and synaptic integrity, using p38-immunoreactivity (p38-IR), both visualized using confocal laser scanning microscopy. Intradendritic distensions and dendritic spine loss were found to co-localize to areas of vacuolar and prion protein pathology in the hippocampus of mice infected with ME7 or 87 V scrapie. A significant reduction in p38-IR was found concomitantly in the hippocampus in ME7 scrapie mice. These results indicate that both pre- and post-synaptic sites are altered by scrapie infection; this would disrupt neuronal circuitry and may initiate apoptotic cell death, giving rise to the neurological disturbances manifested in clinical TSE cases.

Successful transmission of three mouse-adapted scrapie strains to murine neuroblastoma cell lines overexpressing wild-type mouse prion protein

Propagation of the agents responsible for transmissible spongiform encephalopathies (TSEs) in cultured cells has been achieved for only a few cell lines. To establish efficient and versatile models for transmission, we developed neuroblastoma cell lines overexpressing type A mouse prion protein, MoPrP(C)-A, and then tested the susceptibility of the cells to several different mouse-adapted scrapie strains. The transfected cell clones expressed up to sixfold-higher levels of PrP(C) than the untransfected cells. Even after 30 passages, we were able to detect an abnormal proteinase K-resistant form of prion protein, PrP(Sc), in the agent-inoculated PrP-overexpressing cells, while no PrP(Sc) was detectable in the untransfected cells after 3 passages. Production of PrP(Sc) in these cells was also higher and more stable than that seen in scrapie-infected neuroblastoma cells (ScN2a). The transfected cells were susceptible to PrP(Sc)-A strains Chandler, 139A, and 22L but not to PrP(Sc)-B strains 87V and 22A. We further demonstrate the successful transmission of PrP(Sc) from infected cells to other uninfected cells. Our results corroborate the hypothesis that the successful transmission of agents ex vivo depends on both expression levels of host PrP(C) and the sequence of PrP(Sc). This new ex vivo transmission model will facilitate research into the mechanism of host-agent interactions, such as the species barrier and strain diversity, and provides a basis for the development of highly susceptible cell lines that could be used in diagnostic and therapeutic approaches to the TSEs.

Prion protein glycosylation is sensitive to redox change

The conversion of soluble prion protein into an insoluble, pathogenic, protease-resistant isoform is a key event in the development of prion diseases. Although the mechanism by which the conversion engenders a pathogenic event is unclear, there is increasing evidence to suggest that this may depend on the function of the prion protein in preventing oxidative damage. Therefore, in this study, we assessed the interrelationship between redox-sensitive cysteine, glycosylation, and prion metabolism. Cells were treated with a thioreductant, dithiothreitol, to assess the effect of the cellular oxidation state on the synthesis of the prion protein. This change in redox balance affected the glycosylation of the prion protein, resulting in the sole production of glycosylated forms. The role of the single disulfide bridge in mediating this effect within the prion protein was confirmed by mutating the cysteine residues involved in its formation. These data suggest that conditions that increase the rate of formation of the disulfide bridge favor formation of the unglycosylated prion protein. Thus, since the presence of glycans on the prion protein is protective against its pathogenic conversion, a change in the redox status of the cell would increase the risk of developing a prion disease by favoring the production of the unglycosylated form.

Transgenic Models for Prion Disease: Have They Outlived Their Useful Purpose?

Prions are a recently identified class of proteinaceous pathological agents. Prion diseases are fatal neurological disorders, the importance of which is exemplified by the recent emergence of a novel variant of Creutzfeldt-Jacob disease (CJD) in humans. During pathogenesis, prion proteins undergo a conformational change, which converts the normal isoform to a pathogenic isoform. Several approaches are available for studying prion disease. The predominant approach has involved in vivo studies, especially involving transgenic mice. In vitro alternatives available for studying prion disease include a cell-free conversion assay, cell culture systems, and an immunoassay for the pathogenic form of the prion protein. Prion-like proteins have been identified in yeast, and therefore this constitutes another non-animal approach. Four main areas of prion research are discussed in this paper, to illustrate the potential applications and limitations of the in vivo and alternative systems. From this study, we conclude that, while current in vitro approaches can be used initially, in vivo studies are still needed to confirm data obtained in vitro. Priority should be given to the non-animal alternatives, as well as to developing new methods, and these should be given primary consideration at the outset of a project.

Subcellular trafficking abnormalities of a prion protein with a disrupted disulfide loop

The single disulfide loop (Cys178-Cys213) of the prion protein (PrP) may stabilize the conformation of this protein by bridging the C-terminal alpha-helices. The substitution mutant Cys178Ala fails to form the prion isoform PrPSc when expressed in scrapie-infected neuroblastoma ScN2a cells (Muramoto et al., Proc. Natl. Acad. Sci. USA 93, 15457-15462). To investigate the reasons for this failure, we introduced the C178A substitution in the full length mouse PrP gene as well as in its N-terminally truncated delta23-88 version. The resulting mutants (C178A and deltaC178A, respectively) were transiently expressed in N2a and CHO cells. Wild-type PrP, wild-type delta23-88 and the point mutant E199K served as controls in these experiments. Compared to the wild-type controls, the C178A mutants were markedly resistant to proteolysis and they were also vastly insoluble in sarcosyl. Studying the metabolic fate of the C178A mutants, we found that in contrast to control PrP molecules, these mutants (i) remained sensitive to the diagnostic endoglycosidase EndoH, (ii) failed to reach the cell surface and (iii) congregated in large juxtanuclear spots. We surmise that these severe trafficking abnormalities may contribute both to the spontaneous aggregation of the C178A mutants and to their reported inability to form PrP(Sc).

Early destruction of the extracellular matrix around parvalbumin-immunoreactive interneurons in Creutzfeldt-Jakob disease

GABA-interneurons immunoreactive (IR) for the calcium-binding protein parvalbumin are lost during the early stages of Creutzfeldt-Jakob disease (CJD) and diminution in their number may partially account for the neurological disturbances manifested in patients suffering from this condition. The disease is characterized by a transformation of the prion protein, PrP(c)-a host-coded sialoglycoprotein-to its protease-resistant and putatively pathological form, PrP(CJD). And since this conversion is likely to take place at the cell surface, we were curious to know whether the "perineuronal net"-a characteristic accumulation of extracellular matrix in intimate contact with the surface of parvalbumin-IR neurons-is implicated in the early disappearance of the mantled cells. Using various lectins and antibodies as markers for the perineuronal net in brains of 21 CJD victims, we observed that this meshwork of extracellular matrix molecules is lost before the embraced parvalbumin-IR neurons themselves disappear. Hence, an interaction of PrP(c) and/or PrP(CJD) with components of the extracellular matrix around this subpopulation of nerve cells precipitates a sequence of as yet unknown events which culminates in the replacement of perineuronal nets by deposits of insoluble PrP(CJD). This change in the environment of the GABA-interneurons IR for parvalbumin may ultimately provoke their death.

Cellular biology of prion diseases

Prion diseases are fatal neurodegenerative disorders of humans and animals that are important because of their impact on public health and because they exemplify a novel mechanism of infectivity and biological information transfer. These diseases are caused by conformational conversion of a normal host glycoprotein (PrPC) into an infectious isoform (PrPSc) that is devoid of nucleic acid. This review focuses on the current understanding of prion diseases at the cell biological level. The characteristics of the diseases are introduced, and a brief history and description of the prion hypothesis are given. Information is then presented about the structure, expression, biosynthesis, and possible function of PrPC, as well as its posttranslational processing, cellular localization, and trafficking. The latest findings concerning PrPSc are then discussed, including cell culture systems used to generate this pathogenic isoform, the subcellular distribution of the protein, its membrane attachment, proteolytic processing, and its kinetics and sites of synthesis. Information is also provided on molecular models of the PrPC-->PrPSc conversion reaction and the possible role of cellular chaperones. The review concludes with suggestions of several important avenues for future investigation.

Molecular biology of prion propagation

The occurrence of new variant Creutzfeldt-Jakob disease and the experimental confirmation that it is caused by the same prion strain as BSE has dramatically highlighted the need for a precise understanding of the molecular basis of prion propagation. The molecular basis of prion-strain diversity, previously a major challenge to the protein-only model, is now becoming clearer. The conformational change thought to be central to prion propagation, from a predominantly alpha-helical fold to one predominantly comprising beta-structure, can now be reproduced in vitro, and the ability of beta-PrP to form fibrillar aggregates provides a plausible molecular mechanism for prion propagation. These and other advances in the fundamental biology of prion propagation are leading to prion diseases becoming arguably the best understood of the neurodegenerative conditions and strategies for the development of rational therapeutics are becoming clearer.

The prion diseases: Creutzfeldt-Jakob, Gerstmann-Sträussler-Scheinker, and related disorders

The prion diseases are an interesting group of neurodegenerative disorders for a variety of reasons. The most obvious is their property of transmissibility, but beyond that they constitute a fascinating example of the diversity of disease expression possible from a common etiologic factor. Thought of as "strains" in animals and phenotypes in humans, these varied expressions of prion disease are most likely due to subtle conformational changes in the pathogenic form of the prion protein. These strain-like characteristics are best exemplified in the genetic varieties of human prion disease in which specific mutations are associated with specific phenotypic profiles. This review attempts to highlight the clinical and pathologic features of the prion diseases with a particular focus on the genetic determinants that define the various familial forms and that modify sporadic and iatrogenic forms of the disease.

Prions

Prions are unprecedented infectious pathogens that cause a group of invariably fatal neurodegenerative diseases by an entirely novel mechanism. Prion diseases may present as genetic, infectious, or sporadic disorders, all of which involve modification of the prion protein (PrP). Bovine spongiform encephalopathy (BSE), scrapie of sheep, and Creutzfeldt-Jakob disease (CJD) of humans are among the most notable prion diseases. Prions are transmissible particles that are devoid of nucleic acid and seem to be composed exclusively of a modified protein (PrPSc). The normal, cellular PrP (PrPC) is converted into PrPSc through a posttranslational process during which it acquires a high beta-sheet content. The species of a particular prion is encoded by the sequence of the chromosomal PrP gene of the mammals in which it last replicated. In contrast to pathogens carrying a nucleic acid genome, prions appear to encipher strain-specific properties in the tertiary structure of PrPSc. Transgenetic studies argue that PrPSc acts as a template upon which PrPC is refolded into a nascent PrPSc molecule through a process facilitated by another protein. Miniprions generated in transgenic mice expressing PrP, in which nearly half of the residues were deleted, exhibit unique biological properties and should facilitate structural studies of PrPSc. While knowledge about prions has profound implications for studies of the structural plasticity of proteins, investigations of prion diseases suggest that new strategies for the prevention and treatment of these disorders may also find application in the more common degenerative diseases.

Endogenous proteolytic cleavage of normal and disease-associated isoforms of the human prion protein in neural and non-neural tissues

We have investigated the proteolytic cleavage of the cellular (PrPC) and pathological (PrPSc) isoforms of the human prion protein (PrP) in normal and prion-affected brains and in tonsils and platelets from neurologically intact individuals. The various PrP species were resolved after deglycosylation according to their electrophoretic mobility, immunoreactivity, Sarkosyl solubility, and, as a novel approach, resistance to endogenous proteases. First, our data show that PrPC proteolysis in brain originates amino-truncated peptides of 21 to 22 and 18 (C1) kd that are similar in different regions and are not modified by the PrP codon 129 genotype, a polymorphism that affects the expression of prion disorders. Second, this proteolytic cleavage of PrPC in brain is blocked by inhibitors of metalloproteases. Third, differences in PrPC proteolysis, and probably in Asn glycosylation and glycosylphosphatidylinositol anchor composition, exist between neural and non-neural tissues. Fourth, protease-resistant PrPSc cores in sporadic Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker F198S disease brains all have an intact C1 cleavage site (Met111-His112), which precludes disruption of a domain associated with toxicity and fibrillogenesis. Fifth, the profile of endogenous proteolytic PrPSc peptides is characteristic of each disorder studied, thus permitting the molecular classification of these prion diseases without the use of proteinase K and even a recognition of PrPSc heterogeneity within type 2 CJD patients having different codon 129 genotype and neuropathological phenotype. This does not exclude the role of additional factors in phenotypic expression; in particular, differences in glycosylation that may be especially relevant in the new variant CJD. Proteolytic processing of PrP may play an important role in the neurotropism and phenotypic expression of prion diseases, but it does not appear to participate in disease susceptibility.

Doxycycline control of prion protein transgene expression modulates prion disease in mice

Conversion of the cellular prion protein (PrPC) into the pathogenic isoform (PrPSc) is the fundamental event underlying transmission and pathogenesis of prion diseases. To control the expression of PrPC in transgenic (Tg) mice, we used a tetracycline controlled transactivator (tTA) driven by the PrP gene control elements and a tTA-responsive promoter linked to a PrP gene [Gossen, M. and Bujard, H. (1992) Proc. Natl. Acad. Sci. USA 89, 5547-5551]. Adult Tg mice showed no deleterious effects upon repression of PrPC expression (>90%) by oral doxycycline, but the mice developed progressive ataxia at approximately 50 days after inoculation with prions unless maintained on doxycycline. Although Tg mice on doxycycline accumulated low levels of PrPSc, they showed no neurologic dysfunction, indicating that low levels of PrPSc can be tolerated. Use of the tTA system to control PrP expression allowed production of Tg mice with high levels of PrP that otherwise cause many embryonic and neonatal deaths. Measurement of PrPSc clearance in Tg mice should be possible, facilitating the development of pharmacotherapeutics.

Eight prion strains have PrP(Sc) molecules with different conformations

Variations in prions, which cause different incubation times and deposition patterns of the prion protein isoform called PrP(Sc), are often referred to as 'strains'. We report here a highly sensitive, conformation-dependent immunoassay that discriminates PrP(Sc) molecules among eight different prion strains propagated in Syrian hamsters. This immunoassay quantifies PrP isoforms by simultaneously following antibody binding to the denatured and native forms of a protein. In a plot of the ratio of antibody binding to denatured/native PrP graphed as a function of the concentration of PrP(Sc), each strain occupies a unique position, indicative of a particular PrP(Sc) conformation. This conclusion is supported by a unique pattern of equilibrium unfolding of PrP(Sc) found with each strain. Our findings indicate that each of the eight prion strains has a PrP(Sc) molecule with a unique conformation and, in accordance with earlier results, indicate the biological properties of prion strains are 'enciphered' in the conformation of PrP(Sc) and that the variation in incubation times is related to the relative protease sensitivity of PrP(Sc) in each strain.

The prion diseases

The human prion diseases are fatal neurodegenerative maladies that may present as sporadic, genetic, or infectious illnesses. The sporadic form is called Creutzfeldt-Jakob disease (CJD) while the inherited disorders are called familial (f) CJD, Gerstmann-Straussler-Scheinker (GSS) disease and fatal familial insomnia (FFI). Prions are transmissible particles that are devoid of nucleic acid and seem to be composed exclusively of a modified protein (PrPSc). The normal, cellular PrP (PrPC) is converted into PrPSc through a posttranslational process during which it acquires a high beta-sheet content. In fCJD, GSS, and FFI, mutations in the PrP gene located on the short arm of chromosome 20 are the cause of disease. Considerable evidence argues that the prion diseases are disorders of protein conformation.

Structure-function aspects of prion proteins

Prions diseases are fatal neurodegenerative disorders resulting from conformational changes in the prion protein from the normal cellular form, PrPC, to the infectious scrapie isoform, PrPSc. High resolution structures for PrPC are now available, and biochemical investigations are shedding light on the nature and determinants of the conformational transition. Together, these studies are beginning to provide a framework to describe structure-function relationships of the prion protein.

Transmissible spongiform encephalopathies: transmission, mechanism of disease, and persistence

Prion protein is central to the control of development of all transmissible spongiform encephalopathies. Controversy exists as to whether the protein itself is responsible for disease manifestation, in one of perhaps several isoforms, or whether an additional informational molecule must be involved in conjunction with the protein. Recent studies have been trying to resolve these issues.