Mutant PrPSc conformers induced by a synthetic peptide and several prion strains

Conformation-dependent high-affinity monoclonal antibodies to prion proteins

Prion diseases are fatal, neurodegenerative illnesses caused by the accumulation of PrP(Sc), an aberrantly folded isoform of the normal, cellular prion protein. Detection of PrP(Sc) commonly relies on immunochemical methods, a strategy hampered by the lack of Abs specific for this disease-causing isoform. In this article, we report the generation of eight mAbs against prion protein (PrP) following immunization of Prnp-null mice with rPrP. The eight mAbs exhibited distinct differential binding to cellular prion protein and PrP(Sc) from different species as well as PrP-derived synthetic peptides. Five of the eight mAbs exhibited binding to discontinuous PrP epitopes, all of which were disrupted by the addition of 2-ME or DTT, which reduced the single disulfide bond found in PrP. One mAb F20-29 reacted only with human PrP, whereas the F4-31 mAb bound bovine PrP; the K(D) values for mAbs F4-31 and F20-29 were ~500 pM. Binding of all five conformation-dependent mAbs to PrP was inhibited by 2-ME in ELISA, Western blots, and histoblots. One conformation-dependent mAb F4-31 increased the sensitivity of an ELISA-based test by nearly 500-fold when it was used as the capture Ab. These new conformation-dependent mAbs were found to be particularly useful in histoblotting studies, in which the low backgrounds after treatment with 2-ME created unusually high signal-to-noise ratios.

Protease-sensitive synthetic prions

Prions arise when the cellular prion protein (PrP^C) undergoes a self-propagating conformational change; the resulting infectious conformer is designated PrP^Sc. Frequently, PrP^Sc is protease-resistant but protease-sensitive (s) prions have been isolated in humans and other animals. We report here that protease-sensitive, synthetic prions were generated in vitro during polymerization of recombinant (rec) PrP into amyloid fibers. In 22 independent experiments, recPrP amyloid preparations, but not recPrP monomers or oligomers, transmitted disease to transgenic mice (n = 164), denoted Tg9949 mice, that overexpress N-terminally truncated PrP. Tg9949 control mice (n = 174) did not spontaneously generate prions although they were prone to late-onset spontaneous neurological dysfunction. When synthetic prion isolates from infected Tg9949 mice were serially transmitted in the same line of mice, they exhibited sPrP^Sc and caused neurodegeneration. Interestingly, these protease-sensitive prions did not shorten the life span of Tg9949 mice despite causing extensive neurodegeneration. We inoculated three synthetic prion isolates into Tg4053 mice that overexpress full-length PrP; Tg4053 mice are not prone to developing spontaneous neurological dysfunction. The synthetic prion isolates caused disease in 600–750 days in Tg4053 mice, which exhibited sPrP^Sc. These novel synthetic prions demonstrate that conformational changes in wild-type PrP can produce mouse prions composed exclusively of sPrP^Sc.

Prions are infectious proteins that cause heritable, sporadic, and transmissible diseases in humans and other mammals. These infectious proteins arise when the normal form of the prion protein (PrP) adopts a self-perpetuating conformation. This disease-causing PrP form is frequently distinguished from normal PrP by its resistance to digestion by proteases although considerable evidence shows that protease-sensitive prions occur naturally in humans and sheep. Here we describe the generation of novel protease-sensitive synthetic prions. After producing recombinant PrP of the wild-type mouse sequence in Escherichia coli, we polymerized the protein into an amyloid fiber conformation. Mice inoculated with these amyloid fibers developed extensive neurodegeneration characteristic of prion disease, but did not generate protease-resistant PrP. Prions from sick animals were transmitted to healthy animals, which likewise developed neurodegeneration but not protease-resistant prions. These novel synthetic prions demonstrate that truncated wild-type PrP can undergo a conformational change that becomes infectious yet the protein remains protease sensitive.

Chemical induction of misfolded prion protein conformers in cell culture

Prion-infected cells accumulate a heterogeneous population of aberrantly folded PrP conformers, including the disease-causing isoform (PrP(Sc)). We found that specific chemicals can modulate the levels of various PrP conformers in cultured cells. Positively charged polyamidoamines (dendrimers) eliminated protease-resistant (r) PrP(Sc) from prion-infected cells and induced the formation of insoluble, protease-sensitive PrP aggregates (designated PrP(A)). Larger, positively charged polyamidoamines more efficaciously induced the formation of PrP(A) and cleared rPrP(Sc), whereas negatively charged polyamidoamines neither induced PrP(A) nor cleared rPrP(Sc). Although the biochemical properties of PrP(A) were shown to be similar to protease-sensitive (s) PrP(Sc), bioassays of PrP(A) indicated that it is not infectious. Our studies argue that PrP(A) represents an aggregated PrP species that is off-pathway relative to the formation of rPrP(Sc). It remains to be established whether the formation of PrP(A) inhibits the formation of rPrP(Sc) by sequestering PrP(C) in the form of benign, insoluble aggregates.

Design and construction of diverse mammalian prion strains

Prions are infectious proteins that encipher biological information within their conformations; variations in these conformations dictate different prion strains. Toward elucidating the molecular language of prion protein (PrP) conformations, we produced an array of recombinant PrP amyloids with varying conformational stabilities. In mice, the most stable amyloids produced the most stable prion strains that exhibited the longest incubation times, whereas more labile amyloids generated less stable strains and shorter incubation times. The direct relationship between stability and incubation time of prion strains suggests that labile prions are more fit, in that they accumulate more rapidly and thus kill the host faster. Although incubation times can be changed by altering the PrP expression level, PrP sequence, prion dose, or route of inoculation, we report here the ability to modify the incubation time predictably in mice by modulating the prion conformation.

De novo mammalian prion synthesis

Prions are responsible for a heterogeneous group of fatal neurodegenerative diseases. They can be sporadic, genetic, or infectious disorders involving post-translational modifications of the cellular prion protein (PrP(C)). Prions (PrP(Sc)) are characterized by their infectious property and intrinsic ability to convert the physiological PrP(C) into the pathological form, acting as a template. The "protein-only" hypothesis, postulated by Stanley B. Prusiner, implies the possibility to generate de novo prions in vivo and in vitro. Here we describe major milestones towards proving this hypothesis, taking into account physiological environment/s, biochemical properties and interactors of the PrP(C).

Absence of spontaneous disease and comparative prion susceptibility of transgenic mice expressing mutant human prion proteins

Approximately 15 % of human prion disease is associated with autosomal-dominant pathogenic mutations in the prion protein (PrP) gene. Previous attempts to model these diseases in mice have expressed human PrP mutations in murine PrP, but this may have different structural consequences. Here, we describe transgenic mice expressing human PrP with P102L or E200K mutations and methionine (M) at the polymorphic residue 129. Although no spontaneous disease developed in aged animals, these mice were readily susceptible to prion infection from patients with the homotypic pathogenic mutation. However, while variant Creutzfeldt–Jakob disease (CJD) prions transmitted infection efficiently to both lines of mice, markedly different susceptibilities to classical (sporadic and iatrogenic) CJD prions were observed. Prions from E200K and classical CJD M129 homozygous patients, transmitted disease with equivalent efficiencies and short incubation periods in human PrP 200K, 129M transgenic mice. However, mismatch at residue 129 between inoculum and host dramatically increased the incubation period. In human PrP 102L, 129M transgenic mice, short disease incubation periods were only observed with transmissions of prions from P102L patients, whereas classical CJD prions showed prolonged and variable incubation periods irrespective of the codon 129 genotype. Analysis of disease-related PrP (PrP^Sc) showed marked alteration in the PrP^Sc glycoform ratio propagated after transmission of classical CJD prions, consistent with the PrP point mutations directly influencing PrP^Sc assembly. These data indicate that P102L or E200K mutations of human PrP have differing effects on prion propagation that depend upon prion strain type and can be significantly influenced by mismatch at the polymorphic residue 129.

Prion protein oligomers in Creutzfeldt-Jakob disease detected by gel-filtration centrifuge columns

Prion diseases are diagnosed by the detection of accumulation of abnormal prion protein (PrP) using immunohistochemistry or the detection of protease-resistant abnormal PrP (PrP(res)). Although the abnormal PrP is neurotoxic by forming aggregates, recent studies suggest that the most infectious units are smaller than the amyloid fibrils. In the present study, we developed a simplified method by applying size-exclusion gel-filtration chromatography to examine PrP oligomers without proteinase K digestion in Creutzfeldt-Jakob disease (CJD) samples, and evaluated the correlation between disease severity and the polymerization degree of PrP. Brain homogenates of human CJD and non-CJD cases were applied to the gel-filtration spin columns, and fractionated PrP molecules in each fraction were detected by western blot. We observed that PrP oligomers could be detected by the simple gel-filtration method and distinctly separated from monomeric cellular PrP (PrP(c)). PrP oligomers were increased according to the disease severity, accompanied by the depletion of PrP(c). The separated PrP oligomers were already protease-resistant in the case with short disease duration. In the cases with quite severe pathology the oligomeric PrP reached a plateau, which may indicate that PrP molecules could mostly develop into amyloid fibrils in the advanced stages. The increase of PrP oligomers correlated with the degree of histopathological changes such as spongiosis and gliosis. The decrease of monomeric PrP(c) was unexpectedly obvious in the diseased cases. Dynamic changes of both oligomerization of the human PrP and depletion of normal PrP(c) require further elucidation to develop a greater understanding of the pathogenesis of human prion diseases.

Surround optical fiber immunoassay (SOFIA): an ultra-sensitive assay for prion protein detection

We describe the development of a new technology (SOFIA) and demonstrate its utility by establishing a sensitive and specific assay for PrP(Sc). SOFIA is a surround optical fiber immunoassay which is comprised of a set of specific monoclonal antibodies and comprehensive capture of high energy fluorescence emission. In its current format, this system is capable of detecting less than 10 attogram (ag) of hamster, sheep and deer recombinant PrP. Approximately 10 ag of PrP(Sc) from 263 K-infected hamster brains can be detected with similar lower limits of PrP(Sc) detection from the brains of scrapie-infected sheep and deer infected with chronic wasting disease. These detection limits allow protease treated and untreated material to be diluted beyond the point where PrP(C), non-specific proteins or other extraneous material may interfere with PrP(Sc) signal detection and/or specificity. This not only eliminates the issue of specificity of PrP(Sc) detection but also increases sensitivity since the possibility of partial PrP(Sc) proteolysis is no longer a concern. SOFIA will likely lead to early antemortem detection of transmissible encephalopathies and is also amenable for use with additional target amplification protocols. SOFIA represents a sensitive means for detecting specific proteins involved in disease pathogenesis and/or diagnosis that extends beyond the scope of the transmissible spongiform encephalopathies.

Aggregated, wild-type prion protein causes neurological dysfunction and synaptic abnormalities

The neurotoxic forms of the prion protein (PrP) that cause neurodegeneration in prion diseases remain to be conclusively identified. Considerable evidence points to the importance of noninfectious oligomers of PrP in the pathogenic process. In this study, we describe lines of Tg(WT) transgenic mice that over-express wild-type PrP by either approximately 5-fold or approximately 10-fold (depending on whether the transgene array is, respectively, hemizygous or homozygous). Homozygous but not hemizygous Tg(WT) mice develop a spontaneous neurodegenerative illness characterized clinically by tremor and paresis. Both kinds of mice accumulate large numbers of punctate PrP deposits in the molecular layer of the cerebellum as well as in several other brain regions, and they display abnormally enlarged synaptic terminals accompanied by a dramatic proliferation of membranous structures. The over-expressed PrP in Tg(WT) mice assembles into an insoluble form that is mildly protease-resistant and is recognizable by aggregation-specific antibodies, but that is not infectious in transmission experiments. Together, our results demonstrate that noninfectious aggregates of wild-type PrP are neurotoxic, particularly to synapses, and they suggest common pathogenic mechanisms shared by prion diseases and nontransmissible neurodegenerative disorders associated with protein misfolding.

Specific biarsenical labeling of cell surface proteins allows fluorescent- and biotin-tagging of amyloid precursor protein and prion proteins

Fluorescent tagging is a powerful tool for imaging proteins in living cells. However, the steric effects imposed by fluorescent tags impair the behavior of many proteins. Here, we report a novel technique, Instant with DTT, EDT, And Low temperature (IDEAL)-labeling, for rapid and specific FlAsH-labeling of tetracysteine-tagged cell surface proteins by using prion protein (PrP) and amyloid precursor protein (APP) as models. In prion-infected cells, FlAsH-labeled tetracysteine-tagged PrP converted from the normal isoform (PrPsen) to the disease-associated isoform (PrPres), suggesting minimal steric effects of the tag. Pulse-chase analysis of PrP and APP by fluorescent gel imaging demonstrated the utility of IDEAL labeling in investigating protein metabolism by identifying an as-yet-unrecognized C-terminal fragment (C3) of PrPsen and by characterizing the kinetics of PrPres and APP metabolism. C3 generation and N-terminal truncation of PrPres were inhibited by the anti-prion compound E64, a cysteine protease inhibitor. Surprisingly, E64 did not inhibit the synthesis of new PrPres, providing insight into the mechanism by which E64 reduces steady-state PrPres levels in prion-infected cells. To expand the versatility of tetracysteine tagging, we created new Alexa Fluor- and biotin-conjugated tetracysteine-binding molecules that were applied to imaging PrP endocytosis and ultrastructural localization. IDEAL-labeling extends the use of biarsenical derivatives to extracellular proteins and beyond microscopic imaging.

Detection and characterization of proteinase K-sensitive disease-related prion protein with thermolysin

Disease-related PrP^Sc [pathogenic PrP (prion protein)] is classically distinguished from its normal cellular precursor, PrP^C(cellular PrP) by its detergent insolubility and partial resistance to proteolysis. Although molecular diagnosis of prion disease has historically relied upon detection of protease-resistant fragments of PrP^Sc using PK (proteinase K), it is now apparent that a substantial fraction of disease-related PrP is destroyed by this protease. Recently, thermolysin has been identified as a complementary tool to PK, permitting isolation of PrP^Sc in its full-length form. In the present study, we show that thermolysin can degrade PrP^C while preserving both PK-sensitive and PK-resistant isoforms of disease-related PrP in both rodent and human prion strains. For mouse RML (Rocky Mountain Laboratory) prions, the majority of PK-sensitive disease-related PrP isoforms do not appear to contribute significantly to infectivity. In vCJD (variant Creutzfeldt–Jakob disease), the human counterpart of BSE (bovine spongiform encephalopathy), up to 90% of total PrP present in the brain resists degradation with thermolysin, whereas only ∼15% of this material resists digestion by PK. Detection of PK-sensitive isoforms of disease-related PrP using thermolysin should be useful for improving diagnostic sensitivity in human prion diseases.

Resistance of bovine spongiform encephalopathy (BSE) prions to inactivation

Distinct prion strains often exhibit different incubation periods and patterns of neuropathological lesions. Strain characteristics are generally retained upon intraspecies transmission, but may change on transmission to another species. We investigated the inactivation of two related prions strains: BSE prions from cattle and mouse-passaged BSE prions, termed 301V. Inactivation was manipulated by exposure to sodium dodecyl sulfate (SDS), variations in pH, and different temperatures. Infectivity was measured using transgenic mouse lines that are highly susceptible to either BSE or 301V prions. Bioassays demonstrated that BSE prions are up to 1,000-fold more resistant to inactivation than 301V prions while Western immunoblotting showed that short acidic SDS treatments reduced protease-resistant PrP(Sc) from BSE prions and 301V prions at similar rates. Our findings argue that despite being derived from BSE prions, mouse 301V prions are not necessarily a reliable model for cattle BSE prions. Extending these comparisons to human sporadic Creutzfeldt-Jakob disease and hamster Sc237 prions, we found that BSE prions were 10- and 10(6)-fold more resistant to inactivation, respectively. Our studies contend that any prion inactivation procedures must be validated by bioassay against the prion strain for which they are intended to be used.

Deep membrane insertion of prion protein upon reduction of disulfide bond

The membrane may play a role in the pathogenesis of the prion protein (PrP). Cytoplasmic expression of PrP causes the conversion of PrP to a self-perpetuating PrP(Sc)-like conformation and the interaction of polypeptide chain with the hydrophobic core of the membrane is believed to be closely correlated with neurodegeneration. However, it is still elusive what factors govern the membrane interaction of PrP. Here, we show that PrP penetrates deeply into the membrane when the single disulfide bond is reduced, which results in membrane disruption and leakage. The proteinase K treatment and the fluorescence quenching assays showed that a predicted transmembrane domain of PrP penetrates into the membrane when the disulfide bond was reduced. Therefore, the oxidation state of PrP might be an important factor that influences its neurotoxicity or pathogenesis.

Chemical and biophysical insights into the propagation of prion strains

Transmissible spongiform encephalopathies (TSEs) are lethal infectious neurodegenerative diseases. TSEs are caused by prions, infectious agents lacking informational nucleic acids, and possibly identical with higher-order aggregates of the cellular glycolipoprotein PrP(C). Prion strains are derived from TSE isolates that, even after inoculation into genetically identical hosts, cause disease with distinct patterns of protein aggregate deposition, incubation times, morphology of the characteristic brain damage, and cellular tropism. Most of these traits are relatively stable across serial passages. Here we review current techniques for studying prion strain differences in vivo and in cells, and discuss the strain phenomena in the general context of the knowledge gained from modeling prion fibril growth in vitro and in simple organisms.

The identification of disease-induced biomarkers in the urine of BSE infected cattle

Background

The bovine spongiform encephalopathy (BSE) epidemic and the emergence of a new human variant of Creutzfeldt-Jakob Disease (vCJD) have led to profound changes in the production and trade of agricultural goods. The rapid tests currently approved for BSE monitoring in slaughtered cattle are all based on the detection of the disease related isoform of the prion protein, PrP^d, in brain tissue and consequently are only suitable for post-mortem diagnosis. Objectives: In instances such as assessing the health of breeding stock for export purposes where post-mortem testing is not an option, there is a demand for an ante-mortem test based on a matrix or body fluid that would permit easy access and repeated sampling. Urine and urine based analyses would meet these requirements.

Results

Two dimensional differential gel eletrophoresis (2D-DIGE) and mass spectrometry analyses were used to identify proteins exhibiting differential abundance in the urine of BSE infected cattle and age matched controls over the course of the disease. Multivariate analyses of protein expression data identified a single protein able to discriminate, with 100% accuracy, control from infected samples. In addition, a subset of proteins were able to predict with 85% ± 13.2 accuracy the time post infection that the samples were collected.

Conclusion

These results suggest that in principle it is possible to identify biomarkers in urine useful in the diagnosis, prognosis and monitoring of disease progression of transmissible spongiform encephalopathy diseases (TSEs).

A novel human disease with abnormal prion protein sensitive to protease

OBJECTIVE

To report a novel prion disease characterized by distinct histopathological and immunostaining features, and associated with an abnormal isoform of the prion protein (PrP) that, contrary to the common prion diseases, is predominantly sensitive to protease digestion.

METHODS

Eleven subjects were investigated at the National Prion Disease Pathology Surveillance Center for clinical, histopathological, immunohistochemical, genotypical, and PrP characteristics.

RESULTS

Patients presented with behavioral and psychiatric manifestations on average at 62 years, whereas mean disease duration was 20 months. The type of spongiform degeneration, the PrP immunostaining pattern, and the presence of microplaques distinguished these cases from those with known prion diseases. Typical protease-resistant PrP was undetectable in the cerebral neocortex with standard diagnostic procedures. After enrichment, abnormal PrP was detected at concentrations 16 times lower than common prion diseases; it included nearly 4 times less protease-resistant PrP, which formed a distinct electrophoretic profile. The subjects examined comprised about 3% of sporadic cases evaluated by the National Prion Disease Pathology Surveillance Center. Although several subjects had family histories of dementia, no mutations were found in the PrP gene open reading frame.

INTERPRETATION

The distinct histopathological, PrP immunohistochemical, and physicochemical features, together with the homogeneous genotype, indicate that this is a previously unidentified type of disease involving the PrP, which we designated "protease-sensitive prionopathy" (or PSPr). Protease-sensitive prionopathy is not rare among prion diseases, and it may be even more prevalent than our data indicate because protease-sensitive prionopathy cases are likely also to be classified within the group of non-Alzheimer's dementias.

Transmission and detection of prions in feces

In chronic wasting disease (CWD) in cervids and in scrapie in sheep, prions appear to be transmitted horizontally. Oral exposure to prion-tainted blood, urine, saliva, and feces has been suggested as the mode of transmission of CWD and scrapie among herbivores susceptible to these prion diseases. To explore the transmission of prions through feces, uninoculated Syrian hamsters (SHas) were cohabitated with or exposed to the bedding of SHas orally infected with Sc237 prions. Incubation times of 140 days and a rate of prion infection of 80%-100% among exposed animals suggested transmission by feces, probably via coprophagy. We measured the disease-causing isoform of the prion protein (PrP(Sc)) in feces by use of the conformation-dependent immunoassay, and we titrated the irradiated feces intracerebrally in transgenic mice that overexpressed SHa prion protein (SHaPrP). Fecal samples collected from infected SHas in the first 7 days after oral challenge harbored 60 ng/g PrP(Sc) and prion titers of approximately 10(6.6) ID(50)/g. Excretion of infectious prions continued at lower levels throughout the asymptomatic phase of the incubation period, most likely by the shedding of prions from infected Peyer patches. Our findings suggest that horizontal transmission of disease among herbivores may occur through the consumption of feces or foodstuff tainted with prions from feces of CWD-infected cervids and scrapie-infected sheep.

A gamma-secretase inhibitor and quinacrine reduce prions and prevent dendritic degeneration in murine brains

In prion-infected mice, both the Notch-1 intracellular domain transcription factor (NICD) and the disease-causing prion protein (PrP(Sc)) increase in the brain preceding dendritic atrophy and loss. Because the drug LY411575 inhibits the gamma-secretase-catalyzed cleavage of Notch-1 that produces NICD, we asked whether this gamma-secretase inhibitor (GSI) might prevent dendritic degeneration in mice with scrapie. At 50 d postinoculation with Rocky Mountain Laboratory (RML) prions, mice were given GSI orally for 43-60 d. Because we did not expect GSI to produce a reduction of PrP(Sc) levels in brain, we added quinacrine (Qa) to the treatment regimen. Qa inhibits PrP(Sc) formation in cultured cells. The combination of GSI and Qa reduced PrP(Sc) by approximately 95% in the neocortex and hippocampus but only approximately 50% in the thalamus at the site of prion inoculation. The GSI plus Qa combination prevented dendritic atrophy and loss, but GSI alone did not. Even though GSI reduced NICD levels to a greater extent than GSI plus Qa, it was unable to prevent dendritic degeneration. Whether a balance between NICD and dendrite growth-stimulating factors was achieved with GSI plus Qa but not GSI alone remains to be determined. Although the combination of GSI and Qa diminished PrP(Sc) in the brains of RML-infected mice, GSI toxicity prevented us from being able to assess the effect the GSI plus Qa combination on incubation times. Whether less toxic GSIs can be used in place of LY411575 to prolong survival remains to be determined.

Prions and the proteasome

Prion diseases are fatal neurodegenerative disorders that include Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in animals. They are unique in terms of their biology because they are caused by the conformational re-arrangement of a normal host-encoded prion protein, PrPC, to an abnormal infectious isoform, PrPSc. Currently the precise mechanism behind prion-mediated neurodegeneration remains unclear. It is hypothesised than an unknown toxic gain of function of PrPSc, or an intermediate oligomeric form, underlies neuronal death. Increasing evidence suggests a role for the ubiquitin proteasome system (UPS) in prion disease. Both wild-type PrPC and disease-associated PrP isoforms accumulate in cells after proteasome inhibition leading to increased cell death, and abnormal beta-sheet-rich PrP isoforms have been shown to inhibit the catalytic activity of the proteasome. Here we review potential interactions between prions and the proteasome outlining how the UPS may be implicated in prion-mediated neurodegeneration.

CRBL cells: establishment, characterization and susceptibility to prion infection

The cerebellum is involved in complex physiological functions including motor control, sensory perception, cognition, language, and emotion. Humans and animals with prion diseases are characterized clinically by ataxia, postural abnormalities and cognitive decline. Pathology in the cerebellum affected by prions includes spongiform degeneration, neuronal loss, and gliosis. To develop an in vitro model system for studying prion biology in cerebellar cells, we established and characterized an immortal cell line (CRBL) isolated from the cerebellum of mice lacking expression of a protein involved in cell cycle arrest. The characteristics of the cells include morphological heterogeneity, rapid proliferation, serum responsiveness during growth, and a change in the number of chromosomes. CRBL cells expressed both neuronal and glial cell markers as well as a considerable level of cellular prion protein, PrP(C). Upon in vitro infection, CRBL cells exhibited selective susceptibility to prions isolated from different sources. These cells chronically propagated prions from SMB cells. Strain-specific prion infection in CRBL cells was not due to instability of the cell line, allelic variance, or mutations in the PrP gene. Molecular properties of prions derived from SMB cells were maintained in the infected CRBL cells. Our results suggest that the specific interaction between a prion strain and hosts determined the selective susceptibility of CRBL cells, which reflects the conditions in vivo. In addition to the future studies revealing cellular and molecular mechanism involved in prion pathogenesis, CRBL cells will contribute to the studies dealing with prion strain properties and host susceptibilities.

Transgenic mouse models of prion diseases

Prions represent a new biological paradigm of protein-mediated information transfer. In mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, often referred to as transmissible spongiform encephalopathies. Many unresolved issues remain, including the exact molecular nature of the prion, the detailed mechanism of prion propagation, and the mechanism by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological function of the normal form of the prion protein remains unclear, and it is uncertain whether loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, dramatic advances in our understanding of prions have occurred because of their transmissibility to experimental animals and the development of transgenic mouse models has done much to further our understanding about various aspects of prion biology. In this chapter, I review recent advances in our understanding of prion biology that derive from this powerful and informative approach.

Prion detection by an amyloid seeding assay

Polymerization of recombinant prion protein (recPrP), which was produced in bacteria, into amyloid fibers was accompanied by the acquisition of prion infectivity. We report here that partially purified preparations of prions seed the polymerization of recPrP into amyloid as detected by a fluorescence shift in the dye Thioflavin T. Our amyloid seeding assay (ASA) detected PrP(Sc), the sole component of the prion, in brain samples from humans with sporadic Creutzfeldt-Jakob disease, as well as in rodents with experimental prion disease. The ASA detected a variety of prion strains passaged in both mice and hamsters. The sensitivity of the ASA varied with strain type; for hamster Sc237 prions, the limit of detection was approximately 1 fg. Some prion strains consist largely of protease-sensitive PrP(Sc) (sPrP(Sc)), and these strains were readily detected by ASA. Our studies show that the ASA provides an alternative methodology for detecting both sPrP(Sc) and protease-resistant PrP(Sc) that does not rely on protease digestion or immunodetection.

Prion strain discrimination in cell culture: the cell panel assay

Prions are thought to consist mainly or entirely of misfolded PrP, a constitutively expressed host protein. Prions associated with the same PrP sequence may occur in the form of different strains; the strain phenotype is believed to be encoded by the conformation of the PrP. Some cell lines can be persistently infected by prions and, interestingly, show preference for certain strains. We report that a cloned murine neuroblastoma cell population, N2a-PK1, is highly heterogeneous in regard to its susceptibility to RML and 22L prions. Remarkably, sibling subclones may show very different relative susceptibilities to the two strains, indicating that the responses can vary independently. We have assembled four cell lines, N2a-PK1, N2a-R33, LD9 and CAD5, which show widely different responses to prion strains RML, 22L, 301C, and Me7, into a panel that allows their discrimination in vitro within 2 weeks, using the standard scrapie cell assay (SSCA).

High titers of transmissible spongiform encephalopathy infectivity associated with extremely low levels of PrPSc in vivo

Diagnosis of transmissible spongiform encephalopathy (TSE) disease in humans and ruminants relies on the detection in post-mortem brain tissue of the protease-resistant form of the host glycoprotein PrP. The presence of this abnormal isoform (PrP(Sc)) in tissues is taken as indicative of the presence of TSE infectivity. Here we demonstrate conclusively that high titers of TSE infectivity can be present in brain tissue of animals that show clinical and vacuolar signs of TSE disease but contain low or undetectable levels of PrP(Sc). This work questions the correlation between PrP(Sc) level and the titer of infectivity and shows that tissues containing little or no proteinase K-resistant PrP can be infectious and harbor high titers of TSE infectivity. Reliance on protease-resistant PrP(Sc) as a sole measure of infectivity may therefore in some instances significantly underestimate biological properties of diagnostic samples, thereby undermining efforts to contain and eradicate TSEs.

Classical sheep transmissible spongiform encephalopathies: pathogenesis, pathological phenotypes and clinical disease

Scrapie is a prion disease or transmissible spongiform encephalopathy (TSE) of sheep, goats and moufflon. As with its human counterparts, pathology consists of vacuolation, gliosis and accumulations of abnormal forms of a host prion protein (PrPd) in the brain of affected individuals. Immunohistochemical methods can be used to identify both the intracellular truncation sites of PrPd in different cell types (PrPd epitope mapping) and the different morphological patterns of accumulation (PrPd profiling). Differences in the inferred truncation sites of PrPd are found for different strains of sheep TSEs and for different infected cell types within individual strains. Immunochemical methods of characterizing strains broadly correspond to PrPd mapping discriminatory results, but distinct PrPd profiles, which provide strain- and source-specific information on both the cell types which sustain infection (cellular tropisms) and the cellular processing of PrPd, have no immunoblotting counterparts. The cause of neurological dysfunction in human is commonly considered to be neuronal loss secondary to a direct or indirect effect of the accumulation of PrPd. However, in sheep scrapie there is no significant neuronal loss, and relationships between different magnitudes, topographical and cytological forms of PrPd accumulation and clinical signs are not evident. PrPd accumulation also occurs in lymphoid tissues, for which there is indirect evidence of a pathological effect, in the peripheral nervous system and in other tissues. It is generally assumed that neuroinvasion results from infection of the enteric nervous system neurones subsequent to amplification of infectivity in lymphoid tissues and later spread via sympathetic and parasympathetic pathways. The evidence for this is, however, circumstantial. Accumulation of PrPd and presence of infectivity in tissues other than the nervous and lymphoreticular systems gives insights on the ways of transmission of infection and on food safety.

Discovering DNA encodes heredity and prions are infectious proteins

The resemblance between the discoveries that DNA is the basis of heredity and that prions are infectious proteins is remarkable. Though four decades separated these two discoveries, the biochemical methodologies and scientific philosophies that were employed are surprisingly similar. In both cases, bioassays available at the time that the projects were initiated proved to be inadequate to support purification studies. Improved bioassays allowed the transforming principle (TP) to be purified from pneumococci and prions from scrapie-infected hamster brains. Publications describing TP as composed of DNA prompted some scientists to contend that undetected proteins must contaminate TP enriched fractions. The simplicity of DNA was thought to prevent it from encoding genetic information. By the time prions were discovered, the genomes of all infectious pathogens including viruses, bacteria, fungi and parasites had been shown to be comprised of nucleic acids and so an antithetical refrain became widely echoed: DNA or RNA molecules must be hiding among the proteins of prions. Finding the unexpected and being asked to demonstrate unequivocally the absence of a possible contaminant represent uncanny parallels between the discoveries that DNA encodes the genotype and that prions are infectious proteins.

Accumulation of prion protein in the brain that is not associated with transmissible disease

Prion diseases or transmissible spongiform encephalopathies are characterized histopathologically by the accumulation of prion protein (PrP) ranging from diffuse deposits to amyloid plaques. Moreover, pathologic PrP isoforms (PrP(Sc)) are detected by immunoblot analysis and used both as diagnostic markers of disease and as indicators of the presence of infectivity in tissues. It is not known which forms of PrP are associated with infectivity. To address this question, we performed bioassays using human brain extracts from two cases with phenotypically distinct forms of familial prion disease (Gerstmann-Sträussler-Scheinker P102L). Both cases had PrP accumulations in the brain, but each had different PrP(Sc) isoforms. Only one of the brains had spongiform degeneration. Tissue from this case transmitted disease efficiently to transgenic mice (Tg PrP101LL), resulting in spongiform encephalopathy. In contrast, inoculation of tissue from the case with no spongiform degeneration resulted in almost complete absence of disease transmission but elicited striking PrP-amyloid deposition in several recipient mouse brains. Brains of these mice failed to transmit any neurological disease on passage, but PrP-amyloid deposition was again observed in the brains of recipient mice. These data suggest the possible isolation of an infectious agent that promotes PrP amyloidogenesis in the absence of a spongiform encephalopathy. Alternatively, the infectious agent may be rendered nonpathogenic by sequestration in amyloid plaques, or PrP amyloid can seed amyloid accumulation in the brain, causing a proteinopathy that is unrelated to prion disease. Formation of PrP amyloid may therefore not necessarily be a reliable marker of transmissible spongiform encephalopathy infectivity.

Profiling the culprit in Alzheimer's disease (AD): bacterial toxic proteins - Will they be significant for the aetio-pathogenesis of AD and the transmissible spongiform encephalopathies?

The aetiology of Alzheimer's disease (AD) and the transmissible spongiform encephalopathies (tSEs) is still elusive. The concept that prion protein (PrP(Sc)) is the aetiological agent (infectious protein) in the tSEs has recently been questioned. In AD, the cause of the aberrant cleavage of the beta-amyloid precursor protein (APP), resulting in the production of amyloidogenic Abeta fragments, has yet remained obscure. Moreover, the amyloid hypothesis of AD has been seriously challenged. In both AD and the tSEs, pathogens of various nature, including bacteria, have been discussed as possible causal factors. However, aetiological considerations have completely neglected microbial products such as the bacterial toxic proteins (BTPs). The present paper is aimed at drawing a "culprit profile" of these toxic molecules that can exert, at low-dosage, neuro-degeneration through various effects. Clearly, BTPs may affect cell-surface receptors including modulatory amine transmitter receptor expression, block neuro-transmitter release, increase intra-cellular Ca(2+) levels, affect intra-cellular signal transduction, change cyto-skeletal processing, alter synaptic transmission, influence APP proteolysis, interact with cell surface proteins like PrP(C) or their GPI anchors, act as chaperones inducing conformational change in proteins (e.g., PrP(C) to PrP(Sc)), alter lipid membrane integrity by affecting phospholipases or forming pores and channels, induce vacuolar (spongiform) change and elicit inflammatory reactions with cytokine production including cytokines that were demonstrated in the AD brain. Like PrP(Sc), BTPs can be heat-stable and acid-resistant. BTPs can meet the key-proteins of AD and tSEs in the lipid-rich domains of the plasma membrane called rafts. Basically, this might enable them to initiate a large variety of unfavourable molecular events, eventually resulting in pathogenetic cascades as in AD and the tSEs. All in all, their profile lends support to the hypothesis that BTPs might represent relevant culprits capable to cue and/or promote neuro-degeneration in both AD and the tSEs.

Update on human prion disease

The recognition that variant Creutzfeldt-Jakob disease (vCJD) is caused by the same prion strain as bovine spongiform encephalopathy in cattle has dramatically highlighted the need for a precise understanding of the molecular biology of human prion diseases. Detailed clinical, pathological and molecular data from a large number of human prion disease patients indicate that phenotypic diversity in human prion disease relates in part to the propagation of disease-related PrP isoforms with distinct physicochemical properties. Incubation periods of prion infection in humans can exceed 50 years and therefore it will be some years before the extent of any human vCJD epidemic can be predicted with confidence.

An overview of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders of humans and animals associated with an accumulation of abnormal isoforms of prion protein (PrP) in nerve cells. The pathogenesis of TSEs involves conformational conversions of normal cellular PrP (PrPc) to abnormal isoforms of PrP (PrPSc). While the protein-only hypothesis has been widely accepted as a causal mechanism of prion diseases, evidence from more recent research suggests a possible involvement of other cellular component(s) or as yet undefined infectious agent(s) in PrP pathogenesis. Although the underlying mechanisms of PrP strain variation and the determinants of interspecies transmissibility have not been fully elucidated, biochemical and molecular findings indicate that bovine spongiform encephalopathy in cattle and new-variant Creutzfeldt–Jakob disease in humans are caused by indistinguishable etiological agent(s). Cumulative evidence suggests that there may be risks of humans acquiring TSEs via a variety of exposures to infected material. The development of highly precise ligands is warranted to detect and differentiate strains, allelic variants and infectious isoforms of these PrPs. This article describes the general features of TSEs and PrP, the current understanding of their pathogenesis, recent advances in prion disease diagnostics, and PrP inactivation.

Proteinase K-sensitive disease-associated ovine prion protein revealed by conformation-dependent immunoassay

PrPSc [abnormal disease-specific conformation of PrP (prion-related protein)] accumulates in prion-affected individuals in the form of amorphous aggregates. Limited proteolysis of PrPSc results in a protease-resistant core of PrPSc of molecular mass of 27-30 kDa (PrP27-30). Aggregated forms of PrP co-purify with prion infectivity, although infectivity does not always correlate with the presence of PrP27-30. This suggests that discrimination between PrPC (normal cellular PrP) and PrPSc by proteolysis may underestimate the repertoire and quantity of PrPSc subtypes. We have developed a CDI (conformation-dependent immunoassay) utilizing time-resolved fluorescence to study the conformers of disease-associated PrP in natural cases of sheep scrapie, without using PK (proteinase K) treatment to discriminate between PrPC and PrPSc. The capture-detector CDI utilizes N-terminal- and C-terminal-specific anti-PrP monoclonal antibodies that recognize regions of the prion protein differentially buried or exposed depending on the extent of denaturation of the molecule. PrPSc was precipitated from scrapie-infected brain stem and cerebellum tissue following sarkosyl extraction, with or without the use of sodium phosphotungstic acid, and native and denatured PrPSc detected by CDI. PrPSc was detectable in brain tissue from homozygous VRQ (V136 R154 Q171) and ARQ (A136 R154 Q171) scrapie-infected sheep brains. The highest levels of PrPSc were found in homozygous VRQ scrapie-infected brains. The quantity of PrPSc was significantly reduced, up to 90% in some cases, when samples were treated with PK prior to the CDI. Collectively, our results show that the level of PrPSc in brain samples from cases of natural scrapie display genotypic differences and that a significant amount of this material is PK-sensitive.

The reconstitution of mammalian prion infectivity de novo

The discovery of prion disease transmission in mammals, as well as a non-Mendelian type of inheritance in yeast, has led to the establishment of a new concept in biology, the prion hypothesis. The prion hypothesis postulates that an abnormal protein conformation propagates itself in an autocatalytic manner using the normal isoform of the same protein as a substrate and thereby acts either as a transmissible agent of disease (in mammals), or as a heritable determinant of phenotype (in yeast and fungus). While the prion biology of yeast and fungus supports this idea strongly, the direct proof of the prion hypothesis in mammals, specifically the reconstitution of the disease-associated isoform of the prion protein (PrP(Sc)) in vitro de novo from noninfectious prion protein, has been difficult to achieve despite many years of effort. The present review summarizes our current knowledge about the biochemical nature of the prion infectious agent and structure of PrP(Sc), describes potential strategies for generating prion infectivity de novo and provides some insight on why the reconstitution of infectivity has been difficult to achieve in vitro. Several hypotheses are proposed to explain the apparently low infectivity of the first generation of recently reported synthetic mammalian prions.

Accelerated accumulation of misfolded prion protein and spongiform degeneration in a Drosophila model of Gerstmann-Sträussler-Scheinker syndrome

Prion diseases are CNS disorders that can occur in sporadic, infectious, and inherited forms. Although all forms of prion disease are associated with the accumulation of pathogenic conformers of the prion protein, collectively termed PrP(Sc), the mechanisms by which PrP(Sc) molecules form and cause neuronal degeneration are unknown. Using the bipartite galactosidase-4-upstream activating sequence expression system, we generated transgenic Drosophila melanogaster heterologously expressing either wild-type (WT) or mutant, disease-associated (P101L) mouse PrP molecules in cholinergic neurons. Transgenic flies expressing neuronal P101L PrP molecules exhibited severe locomotor dysfunction and premature death as larvae and adults. These striking clinical abnormalities were accompanied by age-dependent accumulation of misfolded PrP molecules, intracellular PrP aggregates, and neuronal vacuoles. In contrast, transgenic flies expressing comparable levels of WT PrP displayed no clinical, pathological, or biochemical abnormalities. These results indicate that transgenic Drosophila expressing neuronal P101L PrP specifically exhibit several hallmark features of human Gerstmann-Sträussler-Scheinker (GSS) syndrome. Because the rates of abnormal PrP accumulation and clinical progression are highly accelerated in Drosophila compared with the rates of these processes in rodents or humans, the P101L mutant may be used for future genetic and pharmacologic studies as a novel invertebrate model of GSS.

Transmission of elk and deer prions to transgenic mice

Chronic wasting disease (CWD) is a fatal prion disease in deer and elk. Unique among the prion diseases, it is transmitted among captive and free-ranging animals. To facilitate studies of the biology of CWD prions, we generated five lines of transgenic (Tg) mice expressing prion protein (PrP) from Rocky Mountain elk (Cervus elaphus nelsoni), denoted Tg(ElkPrP), and two lines of Tg mice expressing PrP common to white-tailed deer (Odocoileus virginianus) and mule deer (Odocoileus hemionus), denoted Tg(DePrP). None of the Tg(ElkPrP) or Tg(DePrP) mice exhibited spontaneous neurologic dysfunction at more than 600 days of age. Brain samples from CWD-positive elk, white-tailed deer, and mule deer produced disease in Tg(ElkPrP) mice between 180 and 200 days after inoculation and in Tg(DePrP) mice between 300 and 400 days. One of eight cervid brain inocula transmitted disease to Tg(MoPrP)4053 mice overexpressing wild-type mouse PrP-A in approximately 540 days. Neuropathologic analysis revealed abundant PrP amyloid plaques in the brains of ill mice. Brain homogenates from symptomatic Tg(ElkPrP) mice produced disease in 120 to 190 days in Tg(ElkPrP) mice. In contrast to the Tg(ElkPrP) and Tg(DePrP) mice, Tg mice overexpressing human, bovine, or ovine PrP did not develop prion disease after inoculation with CWD prions from among nine different isolates after >500 days. These findings suggest that CWD prions from elk, mule deer, and white-tailed deer can be readily transmitted among these three cervid species.

Synthetic prions

The prion theory postulates that prions are novel infectious agents that are composed largely, if not entirely, of abnormally folded host-encoded prion proteins. However, the existence of different prion strains is enigma, if these novel infectious agents lack a genetic element, such as a nucleic acid. The best proof for this 'protein-only' concept would be the in vitro generation of prions from synthetic sources. Indeed, a substantial body of evidence has meanwhile been accumulated in favour of this postulate. This mini review recapitulates all relevant studies and experimental data on the generation of synthetic prions.

Converting the prion protein: what makes the protein infectious

The discovery of prion disease transmission in mammals, as well as a non-Mendelian type of inheritance in yeast, has led to the establishment of a new concept in biology, the prion hypothesis. The prion hypothesis postulates that an abnormal protein conformation propagates itself in an autocatalytic manner via recruitment of the normal isoform of the same protein as a substrate, and thereby acts either as a transmissible agent of disease (in mammals) or as a heritable determinant of phenotype (in yeast and fungus). Although reconstitution of fully infectious PrP(Sc)in vitro from synthetic components has not yet been achieved, numerous lines of evidence indicate that the prion protein is the major and essential component, if not the only one, of the prion infectious agent. This article summarizes our current knowledge about the chemical nature of the prion infectious agent, describes potential strategies and challenges related to the generation of prion infectivity de novo, proposes new hypotheses to explain the apparently low infectivity observed in the first synthetic mammalian prions, and describes plausible effects of chemical modifications on prion conversion.

Gerstmann-Sträussler-Scheinker

Gerstmann-Sträussler-Scheinker (GSS) is a hereditary prion disease typically associated with prion protein (PrP)-containing plaques. The protease-resistant, scrapie PrP (PrPSc) is represented by internal fragments, whereas the C-terminal fragments associated with the other prion diseases are generally underrepresented. Different histopathologic and PrPSc features associated with at least 13 PrP gene (PRNP) mutations have been described in GSS. We report the histopathology and PrP characteristics in a father and son carrying a mutation at PRNP codon 187 that substitutes histidine (H) with arginine (R) and is coupled with valine (V) at position 129 (H187R-129V). The PrP plaques were present in both cases but with different structure and topography and minimal spongiform degeneration. A distinctive, "curly" PrP immunostaining was prominent in one case. The protease-resistant PrPSc differed in amount in the 2 cases, possibly depending on whether plaques or the curly immunostain was present. Two protease-resistant PrP fragments of 14 kDa and 7 kDa with, in at least one case, N-terminus between residues 90-99 and 82-90, respectively, codistributed with the plaques, whereas only very small amounts of the PK-resistant PrP were present in the curly staining regions. PK-resistant PrP recovered from the plaque and curly staining regions appeared to be full length.

Two-rung model of a left-handed beta-helix for prions explains species barrier and strain variation in transmissible spongiform encephalopathies

In this study, a new beta-helical model is proposed that explains the species barrier and strain variation in transmissible spongiform encephalopathies. The left-handed beta-helix serves as a structural model that can explain the seeded growth characteristics of beta-sheet structure in PrP(Sc) fibrils. Molecular dynamics simulations demonstrate that the left-handed beta-helix is structurally more stable than the right-handed beta-helix, with a higher beta-sheet content during the simulation and a better distributed network of inter-strand backbone-backbone hydrogen bonds between parallel beta-strands of different rungs. Multiple sequence alignments and homology modelling of prion sequences with different rungs of left-handed beta-helices illustrate that the PrP region with the highest beta-helical propensity (residues 105-143) can fold in just two rungs of a left-handed beta-helix. Even if no other flanking sequence participates in the beta-helix, the two rungs of a beta-helix can give the growing fibril enough elevation to accommodate the rest of the PrP protein in a tight packing at the periphery of a trimeric beta-helix. The folding of beta-helices is driven by backbone-backbone hydrogen bonding and stacking of side-chains in adjacent rungs. The sequence and structure of the last rung at the fibril end with unprotected beta-sheet edges selects the sequence of a complementary rung and dictates the folding of the new rung with optimal backbone hydrogen bonding and side-chain stacking. An important side-chain stack that facilitates the beta-helical folding is between methionine residues 109 and 129, which explains their importance in the species barrier of prions. Because the PrP sequence is not evolutionarily optimised to fold in a beta-helix, and because the beta-helical fold shows very little sequence preference, alternative alignments are possible that result in a different rung able to select for an alternative complementary rung. A different top rung results in a new strain with different growth characteristics. Hence, in the present model, sequence variation and alternative alignments clarify the basis of the species barrier and strain specificity in PrP-based diseases.

Probing the conformation of the prion protein within a single amyloid fibril using a novel immunoconformational assay

The coexistence of multiple strains or subtypes of the disease-related isoform of prion protein (PrP) in natural isolates, together with the observed conformational heterogeneity of PrP amyloid fibrils generated in vitro, indicates the importance of probing the conformation of single particles within heterogeneous samples. Using an array of PrP-specific antibodies, we report the development of a novel immunoconformational assay. Uniquely, application of this new technology allows the conformation of multimeric PrP within a single fibril or particle to be probed without pretreatment of the sample with proteinase K. Using amyloid fibrils prepared from full-length recombinant PrP, we demonstrated the utility of this assay to define (i) PrP regions that are surface-exposed or buried, (ii) the susceptibility of defined PrP regions to GdnHCl-induced denaturation, and (iii) the conformational heterogeneity of PrP fibrils as measured for either the entire fibrillar population or for individual fibrils. Specifically, PrP regions 159-174 and 224-230 were shown to be buried and were the most resistant to denaturation. The 132-156 segment of PrP was found to be cryptic under native conditions and solvent-exposed under partially denaturing conditions, whereas the region 95-105 was solvent-accessible regardless of the solvent conditions. Remarkably, a subfraction of fibrils showed immunoreactivity to PrPSc-specific antibodies designated as IgGs 89-112 and 136-158. The immunoreactivity of the conformational epitopes was reduced upon exposure to partially denaturing conditions. Unexpectedly, PrPSc -specific antibodies revealed conformational polymorphisms even within individual fibrils. Our studies provide valuable new insight into fibrillar substructure and offer a new tool for probing the conformation of single PrP fibrils.

GFP-tagged PrP supports compromised prion replication in transgenic mice

The ability of green fluorescent protein (GFP)-prion protein (PrP) fusions to support prion propagation has not been demonstrated. Here, we show that while transgenic mice expressing PrP tagged at its amino terminus with enhanced GFP, referred to as EGFPrP-N, supported prion replication, disease onset was prolonged, the brains of diseased mice did not exhibit typical disease neuropathology and disease-associated EGFPrP-N lacked the full spectrum of biochemical properties normally associated with PrP(Sc). Co-expression of wild-type PrP and EGFPrP-N substantially reduced prion incubation times and resulted in accumulation of protease-resistant EGFPrP(Sc)-N in the brains of transgenic mice as well as chronically infected cultured cells, suggesting that wild-type PrP rescued a compromised amino terminal function in EGFPrP-N. While our results show that EGFPrP(C)-N adopts a conformation necessary for the production of infectious prions, the synergistic interaction of wild-type and EGFPrP-N underscores the importance of the amino terminus in modulating prion pathogenesis.

Changing Images of the Gene

During the twentieth century the gene emerged as the major driving force of biology. Initially, even the nature and behavior of gene vehicles, the chromosomes, were subjected to doubts. The basic or standard gene concept, as a unit of function, mutation, and recombination, had to be revised. Half a century was required for reaching a general consensus about the chemical nature of the genetic material, DNA and RNA. The relationship between single genes and individual proteins was a great milestone at the middle of the twentieth century, but within two decades it was realized that the relationship was more complex. Understanding of genetic coding, transcription, and translation during the 1960s laid a firm foundation to the "nucleic doctrine," harking back to the dicta of Lederberg (1959) and meaning that single nucleic acid genes alone were responsible for each separate function within the cell. However, important aspects of gene expression are recognized now as a function of the genome and many genes collaborate in circuits. It has come to light that genes may be mobile, exist in plasmids and cytoplasmic organelles, and can be imported by nonsexual means from other organisms or as synthetic products. Epigenetics has reborn as a new field of developmental genetics. The unorthodox prion proteins can even simulate some gene properties. Genetics was to an extent reincarnated as of the twenty-first century by assimilating the tools of cybernetics and of many formerly distant areas of science. This overview highlights some of the historical milestones that contributed to the development of our image of the gene, extending elements of issues laid down by Rédei (2003).

Prion clearance in bigenic mice

The clearance of prions from the brain was investigated in bigenic mice designated Tg(tTA : PrP(+/0))3, in which expression of the cellular prion protein (PrP(C)) was regulated by oral doxycycline administration. With suppression of PrP(C) expression, the incubation time for RML prions was prolonged almost threefold from approximately 150 to approximately 430 days. To determine the clearance rate of disease-causing PrP(Sc), bigenic mice were given oral doxycycline beginning 98 days after inoculation with RML prions and sacrificed at various time points over the subsequent 56 days. The half-life (t1/2) for PrP(Sc) was approximately 1.5 days in mouse brain, in reasonable agreement with the apparent t1/2 of 30 h that was determined in a separate study for scrapie-infected mouse neuroblastoma (ScN2a) cells in culture. Both protease-sensitive and -resistant conformers of PrP(Sc) were cleared at the same rate. The t1/2 value for PrP(C) clearance from brain was approximately 18 h, which was considerably longer than the t1/2 of 5 h found in ScN2a cells. The capability of the brain to clear prions raises the possibility that PrP(Sc) is normally made at low levels and continually cleared, and that PrP(Sc) may have a function in cellular metabolism. Moreover, these bigenic mice make it possible to determine both components of PrP(Sc) accumulation, i.e. the rates of formation and clearance, for various strains of prions exhibiting different incubation times.

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.

Search for a prion-specific nucleic acid

Diversity of prion strains was attributed to an elusive nucleic acid, yet a search spanning nearly two decades has failed to identify a prion-specific polynucleotide. In our search for a prion-specific nucleic acid, we analyzed nucleic acids in purified fractions from the brains of Syrian hamsters infected with Sc237 prions. Purification of Sc237 prions removed nucleic acids larger than 50 nucleotides as measured by return refocusing electrophoresis (RRGE). To determine the size of the largest polynucleotide present in purified fractions at an abundance of one molecule per infectious (ID50) unit, we measured prions present after inoculation. In order to account for the rapid clearance of prions after intracerebral inoculation, we determined the number of PrP(Sc) molecules and ID50 units of prions that were retained in brain. Factoring in clearance after inoculation, we estimate that the largest polynucleotide present in our purified fractions at one molecule per ID50 unit is approximately 25 nucleotides in length. In the same fractions, there were approximately 3,000 protease-resistant PrP(Sc) molecules per ID50 unit after accounting for clearance of PrP(Sc) following inoculation. We compared the resistance of Sc237 and 139H prions to inactivation by UV irradiation at 254 nm. Irradiation of homogenates and microsomes diminished prion infectivity by a factor of approximately 1,000 but did not alter the strain-specified properties of the Sc237 and 139H prions. The data reported here combined with the production of synthetic prions argue that the 25-mer polynucleotides found in purified prion preparations are likely to be host encoded and of variable sequence; additionally, these 25-mers are unlikely to be prion specific.

Use of a new immunoassay to measure PrP Sc levels in scrapie-infected sheep brains reveals PrP genotype-specific differences

The diagnosis of prion diseases, such as scrapie and BSE, has traditionally relied upon the identification of the disease-associated form of the prion protein, PrP(Sc), based on its resistance to digestion by proteinase K (PK). A more recent development is the conformation-dependent immunoassay (CDI), which distinguishes between PrP Sc and normal PrP (PrP C) based on their differing solubility in guanidine hydrochloride rather than resistance or sensitivity to PK. We have developed a CDI-formatted sandwich immunoassay for the measurement of PrP Sc in sheep brain, which discriminates between clinically affected scrapie cases (natural or experimental) and uninfected controls of the same PrP genotype. Using this method, we have shown for the first time that, in sheep, the PrP genotype has a significant influence on the amount of PrP Sc deposited in the brains of animals experimentally infected with scrapie.

Immunodetection of disease-associated mutant PrP, which accelerates disease in GSS transgenic mice

The absence of infectivity-associated, protease-resistant prion protein (PrP(Sc)) in the brains of spontaneously sick transgenic (Tg) mice overexpressing PrP linked to Gerstmann-Sträussler Scheinker syndrome, and the failure of gene-targeted mice expressing such PrP to develop disease spontaneously, challenged the concept that mutant PrP expression led to spontaneous prion production. Here, we demonstrate that disease in overexpressor Tg mice is associated with accumulation of protease-sensitive aggregates of mutant PrP that can be immunoprecipitated by the PrP(Sc)-specific monoclonal antibody designated 15B3. Whereas Tg mice expressing multiple transgenes exhibited accelerated disease when inoculated with disease-associated mutant PrP, Tg mice expressing mutant PrP at low levels failed to develop disease either spontaneously or following inoculation. These studies indicate that inoculated mutant PrP from diseased mice promotes the aggregation and accumulation of pre-existing pathological forms of mutant PrP produced as a result of transgene overexpression. Thus, while pathological mutant PrP possesses a subset of PrP(Sc) characteristics, we now show that the attribute of prion transmission suggested by previous studies is more accurately characterized as disease acceleration.