Specific proteins associated with Creutzfeldt-Jakob disease and scrapie share antigenic and carbohydrate determinants

High-resolution structure and strain comparison of infectious mammalian prions

Within the extensive range of self-propagating pathologic protein aggregates of mammals, prions are the most clearly infectious (e.g., ∼10⁹ lethal doses per milligram). The structures of such lethal assemblies of PrP molecules have been poorly understood. Here we report a near-atomic core structure of a brain-derived, fully infectious prion (263K strain). Cryo-electron microscopy showed amyloid fibrils assembled with parallel in-register intermolecular β sheets. Each monomer provides one rung of the ordered fibril core, with N-linked glycans and glycolipid anchors projecting outward. Thus, single monomers form the templating surface for incoming monomers at fibril ends, where prion growth occurs. Comparison to another prion strain (aRML) revealed major differences in fibril morphology but, like 263K, an asymmetric fibril cross-section without paired protofilaments. These findings provide structural insights into prion propagation, strains, species barriers, and membrane pathogenesis. This structure also helps frame considerations of factors influencing the relative transmissibility of other pathologic amyloids.

Analysis of the Glycosylation Profile of Disease-Associated Water-Soluble Prion Protein Using Lectins

The disease-associated water-soluble form of hamster prion protein (ws-PrP^Sc) has recently been found to be less stable than classical PrP^Sc. Since the stability of PrP to degradation correlates with its glycosylation level, the aim of this study was to investigate whether there are differences between the glycosylation of ws-PrP^Sc and classical PrP^Sc of hamster which might account for the ws-PrP^Sc minor stability compared with that of the classical PrP^Sc. Thus, ws-PrP and classical PrP were captured from noninfected or scrapie-infected hamster brain homogenate [high-speed supernatant (S^HS) and high-speed pellet (P^HS)] and blood plasma by anti-PrP antibodies (3F4 and 6H4) and subjected to screening for glycans by lectins under denaturing or nondenaturing procedures in a sandwich lectin-ELISA. Glycans have been found in minor quantities and differently exposed on ws-PrP^Sc from S^HS and plasma compared with classical PrP^Sc from P^HS. These differences have been shown to be potentially responsible for the instability of ws-PrP^Sc. Treatment of infected blood with GdnHCl significantly (P<0.01) increased the detection of ws-PrP^Sc in ELISA, reflecting an increase in its stability, and showed efficacy in removing high-abundance proteins in silver-stained gels. This increase in ws-PrP^Sc stability is due to an interaction of GdnHCl not only with high-abundance proteins but also with the ws-PrP^Sc glycosylation with particular regard to the mannose sugar. Analysis of lectins immunoreactivity toward total proteins from plasma collected before and at different time points after infection revealed that mannose might exert a stabilizing effect toward all of hamster blood glycoproteins, regardless of scrapie infection. Since low levels of ws-PrP^Sc/soluble-infectivity have been estimated both in blood and brain of hamster, this glycosylation-related instability may have negatively influenced the propensity of ws-PrP^C to convert to ws-PrP^Sc both in blood and the brain. Therefore, PrP^C glycosylation characteristics may provide a tool for the determination risk of prion transmissibility.

A prokaryotic viral sequence is expressed and conserved in mammalian brain

A natural and permanent transfer of prokaryotic viral sequences to mammals has not been reported by others. Circular "SPHINX" DNAs <5 kb were previously isolated from nuclease-protected cytoplasmic particles in rodent neuronal cell lines and brain. Two of these DNAs were sequenced after Φ29 polymerase amplification, and they revealed significant but imperfect homology to segments of commensal Acinetobacter phage viruses. These findings were surprising because the brain is isolated from environmental microorganisms. The 1.76-kb DNA sequence (SPHINX 1.8), with an iteron before its ORF, was evaluated here for its expression in neural cells and brain. A rabbit affinity purified antibody generated against a peptide without homology to mammalian sequences labeled a nonglycosylated ∼41-kDa protein (spx1) on Western blots, and the signal was efficiently blocked by the competing peptide. Spx1 was resistant to limited proteinase K digestion, but was unrelated to the expression of host prion protein or its pathologic amyloid form. Remarkably, spx1 concentrated in selected brain synapses, such as those on anterior motor horn neurons that integrate many complex neural inputs. SPHINX 1.8 appears to be involved in tissue-specific differentiation, including essential functions that preserve its propagation during mammalian evolution, possibly via maternal inheritance. The data here indicate that mammals can share and exchange a larger world of prokaryotic viruses than previously envisioned.

What Makes a Prion: Infectious Proteins From Animals to Yeast

While philosophers in ancient times had many ideas for the cause of contagion, the modern study of infective agents began with Fracastoro's 1546 proposal that invisible "spores" spread infectious disease. However, firm categorization of the pathogens of the natural world would need to await a mature germ theory that would not arise for 300 years. In the 19th century, the earliest pathogens described were bacteria and other cellular microbes. By the close of that century, the work of Ivanovsky and Beijerinck introduced the concept of a virus, an infective particle smaller than any known cell. Extending into the early-mid-20th century there was an explosive growth in pathogenic microbiology, with a cellular or viral cause identified for nearly every transmissible disease. A few occult pathogens remained to be discovered, including the infectious proteins (prions) proposed by Prusiner in 1982. This review discusses the prions identified in mammals, yeasts, and other organisms, focusing on the amyloid-based prions. I discuss the essential biochemical properties of these agents and the application of this knowledge to diseases of protein misfolding and aggregation, as well as the utility of yeast as a model organism to study prion and amyloid proteins that affect human and animal health. Further, I summarize the ideas emerging out of these studies that the prion concept may go beyond proteinaceous infectious particles and that prions may be a subset of proteins having general nucleating or seeding functions involved in noninfectious as well as infectious pathogenic protein aggregation.

Highly infectious CJD particles lack prion protein but contain many viral-linked peptides by LC-MS/MS

It is widely believed that host prion protein (PrP), without nucleic acid, converts itself into an infectious form (PrP‐res) that causes transmissible encephalopathies (TSEs), such as human sporadic CJD (sCJD), endemic sheep scrapie, and epidemic BSE. There are many detailed investigations of PrP, but proteomic studies of other proteins in verified infectious TSE particles have not been pursued, even though brain homogenates without PrP retain their complete infectious titer. To define proteins that may be integral to, process, or protect an agent genome, we developed a streamlined, high‐yield purification of infectious FU‐CJD mouse brain particles with minimal PrP. Proteinase K (PK) abolished all residual particle PrP, but did not reduce infectivity, and viral‐size particles lacking PrP were ∼70S (vs. 90–120S without PK). Furthermore, over 1,500 non‐PrP proteins were still present and positively identified in high titer FU‐CJD particles without detectable PrP by mass spectrometry (LC‐MS/MS); 114 of these peptides were linked to viral motifs in the environmental–viral database, and not evident in parallel uninfected controls. Host components were also identified in both PK and non‐PK treated particles from FU‐CJD mouse brain and human sCJD brain. This abundant cellular data had several surprises, including finding Huntingtin in the sCJD but not normal human brain samples. Similarly, the neural Wiskott–Aldrich sequence and multivesicular and endosome components associated with retromer APP (Alzheimer amyloid) processing were only in sCJD. These cellular findings suggest that new therapies directed at retromer–vesicular trafficking in other neurodegenerative diseases may also counteract late‐onset sCJD PrP amyloid pathology. J. Cell. Biochem. 115: 2012–2021, 2014. © 2014 Wiley Periodicals, Inc.

A rapid accurate culture assay for infectivity in Transmissible Encephalopathies

The molecular and structural features of infectious agents that cause CJD, scrapie and BSE remain controversial. A major impediment for agent resolution is the very long and expensive animal assays of infectivity. It is crucial to develop a rapid and broadly applicable cell culture assay to titer and compare different TSE agent strains. Because we found GT1 hypothalamic cells, unlike neuroblastoma N2a clones, were highly susceptible to a variety of TSE agents, and could stably produce high agent titers for >1 year, we studied the progressive display of abnormal prion protein (PrP-res) in GT1 cells following exposure to serially diluted 22L scrapie brain homogenates; PrP-res was used as a surrogate, but non-quantitative marker of GT1 infection. Even as early as the first cell split after 22L exposure, GT1 cells produced their own PrP-res bands that were clearly different than brain bands. Plots from passages 3-7 showed a good discrimination of 3 fold differences in titer over a range of >2 logs, with the same endpoint sensitivity (2 x 10(8) LD(50)/gm) as animal assays. Interestingly, the rapid production of de novo PrP-res suggested that GT1 PrP-res might be induced by interaction with an early-intermediate form of a particle that was not fully infectious. The GT1 assay here was also invaluable for rapidly identifying cell cultures with variant titers, even after detergent lysis. Additionally, in-situ PrP amyloid staining provided an independent measure of the minimum infectious dose per cell. Standardized GT1 assays can be used for direct comparison of different agent strains, and will facilitate the rapid isolation of essential agent components.

A modified host protein model of scrapie

The scrapie agent is still not completely characterized biochemically and ultrastructurally, but its requirement for a functional protein has been established. Purification of the scrapie agent by methods using digestion with proteinase K yields a glycoprotein with an apparent mass of 27-30 kDa (PrP 27-30). In contrast, a 33-37 kDa glycoprotein, called Sp33-37, is the major protein component isolated from scrapie-affected brain when protease digestion is not used. Sp33-37 is the product of a normal host gene and is a larger form of PrP 27-30. We propose a model in which Sp33-37, a modified host protein, is the critical component of the scrapie agent; a non-host nucleic acid is not part of the agent. We postulate that Sp33-37, perhaps in concert with other unidentified host components, is capable of inducing the disease and directing the production of more of itself by acting on the normal protein directly or by affecting one of the steps in protein processing. Agent replication requires that: 1) a constant supply of the substrate protein Cp33-37 is available, 2) aggregates of Sp33-37 are resistant to degradation and accumulate in cells or cell membranes, and 3) membrane damage and cell death facilitate spread to adjacent cells. The model predicts that disease can be transmitted by the scrapie agent or initiated by a spontaneous metabolic error resulting in accumulation of the abnormal protein.

Novel mechanisms of degeneration of the central nervous system--prion structure and biology

Prion is a term for the novel infectious agents which cause scrapie and Creutzfeldt-Jakob disease; these infectious pathogens are composed largely, if not entirely, of prion protein (PrP) molecules. No prion-specific polynucleotide has been identified. Considerable evidence indicates that PrP 27-30 is required for and inseparable from scrapie infectivity. PrP 27-30 is derived from a larger protein, denoted PrPSc. A cellular isoform, designated PrPC, and PrPSc are both encoded by a single copy chromosomal gene and both proteins appear to be translated from the same 2.1 kb mRNA. Monoclonal antibodies to PrP 27-30 as well as antisera to PrP synthetic peptides, react with both PrPC and PrPSc, establishing the relatedness of these proteins. PrPC is completely digested by proteinase K; PrPSc is converted to PrP 27-30 under the same conditions. Detergent extraction of microsomal membranes isolated from scrapie-infected hamster brains solubilizes PrPC but induces PrPSc to polymerize into amyloid rods. This procedure allows separation of the two prion protein isoforms and the demonstration that PrPSc accumulates during scrapie infection while the level of PrPC does not change. The prion amyloid rods generated by detergent extraction are identical morphologically, except for length, to extracellular collections of prion amyloid filaments which form plaques in scrapie- and CJD-infected brains. The prion amyloid plaques stain with antibodies to PrP 27-30 and PrP peptides. Prion rods composed of PrP 27-30 dissociate into phospholipid vesicles with full retention of scrapie infectivity. The murine PrP gene (Prn-p) is linked to the Prn-i gene, which controls the length of the scrapie incubation period. Prolonged incubation times are a cardinal feature of scrapie and CJD. While the central role of PrPSc in scrapie pathogenesis is well established, the chemical and conformational differences between PrPC and PrPSc are unknown but presumably arise from post-translational events.

Scrapie-associated fibrils, PrP protein and the Sinc gene

Scrapie-associated fibrils (SAF) are disease-specific structures found in extracts of the brains of animals affected with scrapie. These structures are pathological aggregates of a normal host protein called PrP. In collaboration with Konrad Beyreuther (Heidelberg), we have characterized the multiple forms of PrP found in SAF fractions from mouse brain affected by the ME7 strain of scrapie. There is no in vivo N-terminal cleavage of the most abundant forms of PrP. However, N-terminal cleavage of some minor forms of PrP does occur in vivo within a domain of repetitive sequences at sites similar to but distinct from those cut by proteinase K in vitro. We suggest that such covalently modified forms of PrP may be the result of enzymic degradation occurring as a consequence rather than as a cause of disease. We also found a novel, as yet unidentified, amino acid derivative of the arginine residue at position 3 in both hamster and mouse PrP 33-35, which may predispose PrP to form SAF. Carlson and colleagues have discovered a linkage between the PrP gene and the murine gene provisionally called Prn-i which, from the work of Carp and coworkers, appears identical to the Sinc gene. The Sinc gene is the major gene determining the incubation period of all strains of scrapie in mice. We have evidence for a linkage of the PrP gene and Sinc using inbred mice of known Sinc genotype, including VM(Sincp7) and VM(Sincs7) congenic mice. PrP may even be the protein product of the Sinc gene.

A hypothesis on prion disorders: Are infectious, inherited, and sporadic causes so distinct?

Prion diseases include a group of either sporadic, inherited or infectious disorders characterized by spongiform neurodegeneration and reactive glyosis in several brain regions. Whatever the origin, the neuropathological hallmark of prion diseases is the presence of brain aggregates containing an altered isoform of a cellular protein, named prion protein. Recent findings show the potential toxicity of the normal cellular prion protein, which occurs when its physiological metabolism is altered. In particular, several studies demonstrate that accumulation of the prion protein in the cytosol can be a consequence of an increased amount of misfolded prion proteins, a derangement of the correct protein trafficking or a reduced activity of the ubiquitin-proteasome system. The same effects can be a consequence of a mutation in the gene coding for the prion protein. In all these conditions, one assists to accumulation and self-replication of insoluble prion proteins which leads to a severe disease resembling what observed following typical "prion infections". This article provides an opinion aimed at reconciling the classic Prusiner's theory concerning the "prion concepts" with the present knowledge arising from experimental studies on neurodegenerative disorders, suggesting a few overlapping steps in the pathogenesis of these diseases.

Novel antibody-lectin enzyme-linked immunosorbent assay that distinguishes prion proteins in sporadic and variant cases of Creutzfeldt-Jakob disease

We used different anti-prion protein (anti-PrP) monoclonal antibodies to capture either full-length or truncated PrP species and then used biotinylated lectin to compare the nature of the glycans on bound PrP species present in control, sporadic Creutzfeldt-Jakob disease (sCJD), or variant CJD (vCJD) brains. When full-length PrP species in these three groups were compared, no significant difference in the binding of concanavalin A or Aleuria aurantia lectin was detected. However, the binding of Ricinus communis agglutinin I (RCA) to sCJD and vCJD samples was significantly increased. In contrast, when only truncated PrP species were compared, only vCJD samples had more RCA binding activity. Therefore, while most of the RCA binding activity in sCJD is restricted to the full-length PrP species, the RCA binding activity in vCJD is associated with truncated and full-length PrP species. Furthermore, the RCA binding activity in sCJD and vCJD samples is mostly associated with proteinase K-resistant PrP species, a known signature of infectious prion. Therefore, PrP species in sCJD and vCJD have dissimilar lectin immunoreactivity, which reflects differences in their N-linked glycans. These differences may account for the distinct phenotypes of sCJD and vCJD.

Virus-like interference in the latency and prevention of Creutzfeldt-Jakob disease

We previously showed that intracerebral (ic) inoculation of the attenuated SY strain of Creutzfeld-Jakob disease in mice could delay clinical signs and widespread neuropathology evoked by subsequent ic challenge with the more virulent FU strain. Using lower doses of SY and FU ic, we here demonstrate that mice can be protected well into old age without demonstrable neuropathology or pathologic prion protein (PrP-res). In contrast, parallel FU only controls became terminally diseased 1 year earlier. To determine whether factors elaborated in response to SY might be part of this effect, we evaluated brain and serum samples from additional parallel mice at 90 days after SY infection and just before FU challenge. The infectivity of FU preparations was significantly reduced by mixing with these fresh SY brain homogenates but not by mixing with SY serum samples, suggesting that brain cells were elaborating labile inhibitory factors that were part of the protective response. SY infectivity was too low to be detected in these brain homogenates. Although suppression could be overcome by higher FU doses ic, strong protection against maximal doses of FU was observed by using i.v. inoculations. Because myeloid microglia are infectious and also elaborate many factors in response to the foreign Creutzfeld-Jakob disease agent, it is likely that innate immunity underlies the profound protection shown here. In principle, it should be possible to artificially stimulate relevant myeloid pathways to better prevent and/or delay the clinical and pathological sequelae of these infections.

N-terminal truncation of prion protein affects both formation and conformation of abnormal protease-resistant prion protein generated in vitro

Transmissible spongiform encephalopathy diseases are characterized by conversion of the normal protease-sensitive host prion protein, PrP-sen, to an abnormal protease-resistant form, PrP-res. In the current study, deletions were introduced into the flexible tail of PrP-sen (23) to determine if this region was required for formation of PrP-res in a cell-free assay. PrP-res formation was significantly reduced by deletion of residues 34-94 relative to full-length hamster PrP. Deletion of another nineteen amino acids to residue 113 further reduced the amount of PrP-res formed. Furthermore, the presence of additional proteinase K cleavage sites indicated that deletion to residue 113 generated a protease-resistant product with an altered conformation. Conversion of PrP deletion mutants was also affected by post-translational modifications to PrP-sen. Conversion of unglycosylated PrP-sen appeared to alter both the amount and the conformation of protease-resistant PrP-res produced from N-terminally truncated PrP-sen. The N-terminal region also affected the ability of hamster PrP to block mouse PrP-res formation in scrapie-infected mouse neuroblastoma cells. Thus, regions within the flexible N-terminal tail of PrP influenced interactions required for both generating and disrupting PrP-res formation.

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

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

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.

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.

Viral particles are required for infection in neurodegenerative Creutzfeldt-Jakob disease

Several models have been proposed for the infectious agents that cause human Creutzfeldt-Jakob disease (CJD) and sheep scrapie. Purified proteins and extracted nucleic acids are not infectious. To further identify the critical molecular components of the CJD agent, 120S infectious material with reduced prion protein (PrP) was treated with guanidine hydrochloride or SDS. Particulate and soluble components were then separated by centrifugation and molecularly characterized. Conditions that optimally solubilized residual PrP and/or nucleic acid-protein complexes were used to produce subfractions that were assayed for infectivity. All controls retained > 90% of the 120S titer (approximately 15% of that in total brain) but lost > 99.5% of their infectivity after heat-SDS treatment (unlike scrapie fractions enriched for PrP). Exposure to 1% SDS at 22 degrees C produced particulate nucleic acid-protein complexes that were almost devoid of host PrP. These sedimenting complexes were as infectious as the controls. In contrast, when such complexes were solubilized with 2.5 M guanidine hydrochloride, the infectious titer was reduced by > 99.5%. Sedimenting PrP aggregates with little nucleic acid and no detectable nucleic acid-binding proteins had negligible infectivity, as did soluble but multimeric forms of PrP. These data strongly implicate a classical viral structure, possibly with no intrinsic PrP, as the CJD infectious agent. CJD-specific protective nucleic acid-binding protein(s) have already been identified in 120S preparations, and preliminary subtraction studies have revealed several CJD-specific nucleic acids. Such viral candidates deserve more attention, as they may be of use in preventing iatrogenic CJD and in solving a fundamental mystery.

Distinct PrP properties suggest the molecular basis of strain variation in transmissible mink encephalopathy

The molecular basis of strain variation in scrapie diseases is unknown. The only identified component of the agent is the posttranslationally modified host prion protein (PrPSc). The biochemical and physical properties of PrP from two strains of transmissible mink encephalopathy (TME), called hyper (HY) and drowsy (DY), were compared to investigate if PrP heterogeneity could account for strain diversity. The degradation rate of PrPTME digested with proteinase K was found to be strain specific and correlated with inactivation of the TME titer. Edman protein sequencing revealed that the major N-terminal end of HY PrPTME commenced at least 10 amino acid residues prior to that of DY PrPTME after digestion with proteinase K. Analysis of the brain distribution of PrPTME exhibited a strain-specific pattern and localization of PrPTME to the perikarya of specific neuron populations. Our findings are consistent with HY and DY PrPTME having distinct protein conformations and/or strain-specific ligand interactions that influence PrPTME properties. We propose that PrPTME conformation could play a role in targeting TME strains to different neuron populations in which strain-specific formation occurs. These data are consistent with the idea that PrPTME protein structure determines the molecular basis of strain variation.

Dementias, neurodegeneration, and viral mechanisms of disease from the perspective of human transmissible encephalopathies

Our transmission experiments with human CJD emphasize the centrality of an exogenous infectious pathogen that can exist in symbiosis with its host for extended periods. Many latent or persistent viruses can cause neurodegenerative disease and may have a role in late onset dementias. There are reasons to believe that CJD infections may share properties with some of these latent viruses in causing dementia, and several retroviral mechanisms may be operative in CJD. In order to clarify viral-like attributes of the CJD agent we have closely followed infectivity and find the following: 1) the CJD agent has a virus-like size and density, and is biochemically separable from most host-encoded prion protein (PrP); 2) Endogenous retroviral IAP RNA sequences of 5,000 bases, as well as several gag-like nucleic acid binding proteins, co-purify with infectivity in preparations treated with high concentrations of anionic detergents and exhaustive nuclease digestion. They signify the purification of true viral cores rather than aggregation artifacts, and diminish claims that there are no protected nucleic acids of > 50 bases in highly purified infectious preparations; 3) In established hamster CJD, temporal studies show the agent has an effective doubling time of approximately 7.5 days in brain, consistent with complex host-viral interactions common to slow viral infections; 4) PrP-res does not correspond to titered levels of infectivity either in a biochemical or an in vivo setting but may function as a viral receptor that can modulate disease expression. Interestingly, functional changes in glial cells occur earlier than PrP-res changes, and indicate an important role for glial cells in evolving infections; 5) Human-rodent transmission studies suggest that CJD, or a CJD-like variant can be a common but latent infection of humans, with relatively infrequent expression of neurological disease. Susceptibility to disease can rest on host attributes and possibly age-related co-factors. Nonetheless, fundamental viral principles are also operative. Agent strain variants, viral burden, and the routes of infection are critical parameters for latency and disease expression. The properties described above have led me to return to the inclusion of CJD (and scrapie) in the panorama of conventional slow viral infections of the brain, as originally proposed by Sigurdsson. Identification of virus-specific molecules are essential for elucidating the role of these agents in the spectrum of human dementias.

Molecular biology and genetics of prion diseases

Scrapie was thought for many years to be caused by a virus. Enriching fractions from Syrian hamster (SHa) brain for scrapie infectivity led to the discovery of the prion protein (PrP). To date, no scrapie-specific nucleic acid has been found. As well as scrapie, prion diseases include bovine spongiform encephalopathy (BSE) of cattle, as well as Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker syndrome (GSS) of humans. Transgenic (Tg) mice expressing both SHa and mouse (Mo) PrP genes were used to probe the molecular basis of the species barrier and the mechanism of scrapie prion replication. The prion inoculum was found to dictate which prions are synthesized de novo, even though the cells express both PrP genes. Discovery of mutations in the PrP genes of humans with GSS and familial CJD established that prion diseases are both genetic and infectious. Tg mice expressing MoPrP with the GSS point mutation spontaneously develop neurologic dysfunction, spongiform degeneration and astrocytic gliosis. Inoculation of brain extracts prepared from these Tg(MoPrP-P101L) mice produced neurodegeneration in many of the recipient animals after prolonged incubation times. These and other results suggest that prions are devoid of foreign nucleic acid and are thus different from viruses and viroids. Studies on the structure of PrPSc and PrPC suggest that the difference is conformational. Whether one or more putative alpha-helices in PrPC are converted into beta-sheets during synthesis of PrPSc is unknown. Distinct prion isolates or 'strains' exhibit different patterns of PrPSc accumulation which are independent of incubation times. Whether variations in PrPSc conformation are responsible for prion diversity remains to be established. Prion studies have given new insights into the etiologies of infectious, sporadic and inherited degenerative diseases.

Endogenous viral complexes with long RNA cosediment with the agent of Creutzfeldt-Jakob disease

A class of viruslike agents that induces Creutzfeldt-Jakob Disease (CJD) and scrapie remains undefined at the molecular level. Several investigators believe this infectious agent is constituted by a single host protein or 'prion', and have emphasized data that would seem to exclude the presence of any viral nucleic acids. However, more rigorous evaluations in scrapie have shown reasonably abundant nucleic acids. Additionally, in highly purified 120S CJD preparations that have been treated with nucleases, RNAs as long as 6,000 bases have been detected. Few nucleic acids have been characterized in either scrapie or CJD, but previous cloning experiments delineated relatively short LTR regions of the endogenous IAP retrovirus in 120S CJD preparations. We therefore used specific primers encompassing the entire IAP genome to test for the presence of long viral RNAs, and here show approximately 5,000 contiguous bases of the IAP RNA genome can be recovered from reasonable amounts of starting brain. The 3' env region of IAP is comparably truncated in CJD and normal preparations, and we find no evidence for IAP transduction of CJD-specific sequences. Because IAP cores can coencapsidate unrelated sequences, and are unusually resistant to physical and chemical treatments, it was relevant to find if cosedimenting cognate proteins of the IAP core, such as gag, could be detected. The predicted approximately 65 kd acidic gag protein, showing appropriate antigenic and nucleic acid binding features, was apparent in both one and 2-D Western blots. This data strongly indicates specific viral complexes cofractionate with the CJD agent. Interestingly, these nuclease resistant IAPs do not appear to be in morphologically recognizable 'R' particles. This cosedimenting viral assembly therefore provides a paradigm for non-particulate CJD complexes in infectious preparations. In developing strategies to identify a CJD specific sequence, cosedimenting IAPs can be used to assess the quality, length and recovery of RNAs extracted from highly resistant viral complexes.

A transmissible Creutzfeldt-Jakob disease-like agent is prevalent in the human population

The etiology of most human dementias is unknown. Creutzfeldt-Jakob disease (CJD), a relatively uncommon human dementia, is caused by a transmissible virus-like agent. Molecular markers that are specific for the agent have not yet been defined. However, the infectious disease can be transmitted to rodents from both brain and infected buffy coat (blood) samples. To determine whether human CJD infections are more widespread than is apparent from the low incidence of neurological disease, we attempted to transmit CJD from buffy coat samples of 30 healthy volunteers who had no family history of dementing illness. Primary transmissions from 26 of 30 individuals produced CJD-like spongiform changes in the brains of recipient hamsters at 200-500 days postinoculation. This positive evidence of viremia was found for individuals in all age groups (20-30, 40-50, and 61-71 years old), whereas 12 negatively scored brain samples failed to produce similar changes in hamsters observed for > 900 days in the same setting. We suggest that a CJD agent endemically infects humans but only infrequently produces an infectious dementia. Disease expression is likely to be influenced by several host factors in combination with viral variants that have altered neurovirulence.

Nucleic acid binding proteins in highly purified Creutzfeldt-Jakob disease preparations

The nature of the infectious agent causing human Creutzfeldt-Jakob disease (CJD), a slowly progressive dementia, is controversial. As in scrapie, no agent-specific proteins or nucleic acids have been identified. However, biological features of exponential replication and agent strain variation, as well as physical size and density data, are most consistent with a viral structure--i.e., a nucleic acid-protein complex. It is often assumed that nuclease treatment, which does not reduce infectious titer, leaves no nucleic acids of > 50 bp. However, nucleic acids of 500-6000 bp can be extracted from highly purified infectious complexes with a mass of approximately 1.5 x 10(7) daltons. It was therefore germane to search for nucleic acid binding proteins that might protect an agent genome. We here use Northwestern blotting to show that there are low levels of nonhistone nucleic acid binding proteins in highly purified infectious 120S gradient fractions. Several nucleic acid binding proteins were clearly host encoded, whereas others were apparent only in CJD, but not in parallel preparations from uninfected brain. Small amounts of residual host Gp34 (prion protein) did not bind any 32P-labeled nucleic acid probes. Most of the minor "CJD-specific" proteins had an acidic pI, a characteristic of many viral core proteins. Such proteins deserve further study, as they probably contribute to unique properties of resistance described for these agents. It remains to be seen if any of these proteins are agent encoded.

Attempts to convert the cellular prion protein into the scrapie isoform in cell-free systems

The scrapie prion protein (PrPSc) is derived from a cellular isoform (PrPC) that acquires protease resistance posttranslationally. We have used several different experimental approaches in attempts to reconstitute in vitro the processes leading to protease-resistant PrPSc molecules. In the first study, we performed mixing experiments by adding mouse PrP 27-30 (MoPrP27-30), the protease-resistant core of PrPSc, to PrPC and then incubating the mixture to investigate the possibility of heterodimer formation as a first step in prion replication. We used epitopically tagged PrP molecules, synthesized in murine neuroblastoma (N2a) cells transfected with the chimeric mouse/Syrian hamster MHM2 PrP construct, which are recognized by the Syrian hamster-specific monoclonal antibody 3F4. After as long as 24 h of incubation, the reaction mixture was assayed for heterodimeric intermediates of MHM2 PrPC and MoPrPSc and for protease-resistant 3F4-reactive PrP. We were unable to identify any aggregates of MHM2 PrPC and MoPrPSc on immunoblots; furthermore, we did not observe de novo formation of protease-resistant MHM2 PrP. In a second study, MoPrPC was metabolically radiolabeled in scrapie prion-infected N2a cultured cells, and then the cell extract was homogenized and incubated under various conditions to allow for the formation of protease-resistant MoPrPSc. We observed no radiolabeled MoPrPSc by immunoprecipitation after as long as 24 h of in vitro incubation. In a third approach, Syrian hamster PrP (SHaPrP) was synthesized in a cell-free translation system supplemented with microsomal membranes derived from either normal or scrapie prion-infected cultured cells. We found that all SHaPrP species translocated across microsomal membranes from scrapie prion-infected cells were protease sensitive in the presence of detergents and displayed the same topology as those generated by microsomes from normal cells or from dog pancreas. We also studied PrP molecules that encode the codon 102 mutation that causes the rare human prion disease Gerstmann-Sträussler-Scheinker (GSS) syndrome. On the basis of our data, GSSPrP appears to yield topological forms similar to those of the wild-type PrP when processed by either normal or scrapie prion-derived microsomes.

N-terminal truncation of the scrapie-associated form of PrP by lysosomal protease(s): implications regarding the site of conversion of PrP to the protease-resistant state

Scrapie and related transmissible spongiform encephalopathies result in the accumulation of a protease-resistant form of an endogenous brain protein called PrP. As an approach to understanding the scrapie-associated modification of PrP, we have studied the processing and sedimentation properties of protease-resistant PrP (PrP-res) in scrapie-infected mouse neuroblastoma cells. Like brain-derived PrP-res, the neuroblastoma cell PrP-res aggregated in detergent lysates, providing evidence that the tendency to aggregate is an intrinsic property of PrP-res and not merely a secondary consequence of degenerative brain pathology. The PrP-res species had lower apparent molecular masses than the normal, protease-sensitive PrP species and were not affected by moderate treatments with proteinase K. This suggested that the PrP-res species were partially proteolyzed by the neuroblastoma cells. Immunoblot analysis of PrP-res with a panel of monospecific anti-PrP peptide sera confirmed that the PrP-res species were quantitatively truncated at the N terminus. The metabolic labeling of PrP-res in serum-free medium did not prevent the proteolysis of PrP-res, showing that the protease(s) involved was cellular rather than serum-derived. The PrP-res truncation was inhibited in intact cells by leupeptin and NH4Cl. This provided evidence that a lysosomal protease(s) was involved, and therefore, that PrP-res was translocated to lysosomes. When considered with other studies, these results imply that the conversion of PrP to the protease-resistant state occurs in the plasma membrane or along an endocytic pathway before PrP-res is exposed to endosomal and lysosomal proteases.

The ultrastructural diversity of scrapie-associated fibrils isolated from experimental scrapie and Creutzfeldt-Jakob disease

Several different samples of scrapie-associated fibrils (SAF) were extracted in identical fashion from the brains of golden Syrian hamsters infected with the 263K strain of scrapie agent and NIH Swiss mice infected with the Fujisaki strain of Creutzfeldt-Jakob disease (CJD) agent. Based on a total of over 500 measurements in individual fibrils in different extracts, hamster fibrils were more abundant, thicker and had better defined substructure than mouse fibrils. Hamster protofibrils were usually either twisted helically or in parallel arrays, whereas mouse protofibrils were often twisted, occasionally parallel, or could not be morphologically defined. Thus, SAF preparations from scrapie-affected hamsters can be ultrastructurally distinguished from those of CJD-affected mice, an observation that presumably reflects differences in their respective host-encoded amyloid protein subunits.

Molecular biology and pathology of scrapie and the prion diseases of humans

Scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans are transmissible and genetic neurodegenerative diseases caused by prions. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein which is encoded by a chromosomal gene. An as yet unidentified post-translational process converts the cellular prion protein into an abnormal isoform. Scrapie neuropathology, incubation times, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neurodegeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Studies of prion diseases may advance investigations of other neurodegenerative disorders and of how neurons differentiate, function for decades and grow senescent.

Molecular biology of prion diseases

Prions cause transmissible and genetic neurodegenerative diseases, including scrapie and bovine spongiform encephalopathy of animals and Creutzfeldt-Jakob and Gerstmann-Sträussler-Scheinker diseases of humans. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is encoded by a chromosomal gene. A posttranslational process, as yet unidentified, converts the cellular prion protein into an abnormal isoform. Scrapie incubation times, neuropathology, and prion synthesis in transgenic mice are controlled by the prion protein gene. Point mutations in the prion protein genes of animals and humans are genetically linked to development of neuro-degeneration. Transgenic mice expressing mutant prion proteins spontaneously develop neurologic dysfunction and spongiform neuropathology. Understanding prion diseases may advance investigations of other neurodegenerative disorders and of the processes by which neurons differentiate, function for decades, and then grow senescent.

The scrapie fibril protein and its cellular isoform

Proteins need help to fold and attain their functional conformation (Ellis and Hemmingsen 1989), and mechanisms have evolved to prevent the accumulation of misfolded protein aggregates within cells (Pelham 1988). These mechanisms fail to prevent the formation of protease-resistant, misfolded forms of PrP (ScPrP) during the development of scrapie and other transmissible spongiform encephalopathies, and ScPrP is a biochemical marker of these diseases. Much is now known about the structure and expression of the PrP gene, but the physiological function of the PrP protein and the mechanism by which the TDE pathogen replicates and specifically interferes with PrP metabolism remain a mystery--a mystery which will entertain prion-ophiliacs for some time yet.

In vitro expression and biosynthesis of prion protein

In addition to whatever function PrP may have normally, its involvement in scrapie-like neurodegenerative diseases has become clearer in recent years. In vitro studies have made important contributions to the understanding of normal PrP biosynthesis and turnover and how they can be influenced by scrapie infection. Cell-free transcription and translation experiments have indicated that PrP gene translation products are capable of assuming two different topologies, one spanning microsomal membranes and the other completely translocated into the microsomal lumen (Hay et al. 1987a, b). A novel stop transfer signal in the polypeptide is critical to the formation of the transmembrane topology (Yost et al. 1990). Expression of recombinant PrP genes has been accomplished in mouse (Caughey et al. 1988b), monkey (Scott et al. 1988), frog (Hay et al. 1987a), and insect (Scott et al. 1988) tissue culture cells. PrP products encoded by PrP cDNAs cloned from scrapie-infected brain tissues are not infectious and do not have the protease-resistance characteristic of the scrapie-associated form of PrP isolated from diseased tissue (Caughey et al. 1988b; Scott et al. 1988). Studies of PrP encoded by the endogenous gene of mouse neuroblastoma cells have identified the precursors (Caughey et al. 1989) and products (Race et al. 1988; Caughey et al. 1989) of normal PrP biosynthesis and shown that most of the PrP of normal cells is linked to the cell surface by phosphatidylinositol (Stahl et al. 1987; Caughey et al. 1989, 1990; Borchelt et al. 1990). In scrapie-infected clones, and additional pool of PrP is present which, unlike the normal PrP, aggregates (B. Caughey, unpublished observations) and is partially protease resistant (Butler et al. 1988; Caughey et al. 1990; Borchelt et al. 1990; Stahl et al. 1990). This scrapie-associated pool of PrP differs from the normal PrP in that it is primarily intracellular (Caughey et al. 1990; Borchelt et al. 1990; Taraboulos et al. 1990) and resistant to removal from cells by phospholipase or protease (Caughey et al. 1990; Borchelt et al. 1990; Stahl et al. 1990) treatments. Kinetic studies have shown that while PrP-sen is synthesized and degraded relatively rapidly (Caughey et al. Borchelt et al. 1990), PrP-res is synthesized slowly and has a very long half-life (Borchelt et al. 1990). Further studies with the scrapie-infected mouse neuroblastoma cells should lead toward the elucidation of the molecular details of the scrapie-associated modification of PrP and whether the modification is directly related to scrapie agent replication.(ABSTRACT TRUNCATED AT 400 WORDS)

Nuclease treatment results in high specific purification of Creutzfeldt-Jakob disease infectivity with a density characteristic of nucleic acid-protein complexes

Representative preparations of partially purified Creutzfeldt-Jakob disease (CJD), including disaggregated density gradient fractions, were treated with a variety of nucleases. RNases as well as exhaustive digestions with micrococcal nuclease did not significantly diminish infectivity, but resulted in an approximately 7,000-fold specific purification of infectivity with respect to nucleic acid. Protected nucleic acids included species of up to 2,000 bases in length. After nuclease treatment, infectivity co-migrated with nucleic acid-protein complexes at a density of 1.27 g/cm3 in sucrose. Substantial specific protein purification were also achieved in the gradient step (approximately 11,000-fold), where 70% the host Gp34 ("prion protein") as well as other free proteins separated from infectivity. These CJD purifications are better than those previously attained in scrapie, and may be useful for further studies of non-host protein and nucleic acid species. The data are consistent with the hypothesis that CJD-like agents are composed of nucleic acid-protein complexes.

Presence of mitochondrial D-loop DNA in scrapie-infected brain preparations enriched for the prion protein

The prion preparation has, in recent years, been the focal point of scrapie research. The inability to identify agent-specific nucleic acids in this sample has led to the formulation of the infectious protein or prion hypothesis. In this study, we analyzed three different prion protein-enriched preparations and found all to contain significant amounts of mitochondrial nucleic acid. Southern blot analyses indicated that they are enriched for a specific component of the mitochondrial genome, the single-stranded displacement loop fragment. Our results suggest that if mitochondrial nucleic acids are involved in scrapie infection, it is the displacement loop fragment that is specifically responsible.

Potential retroviral RNAs in Creutzfeldt-Jakob disease

The molecular nature of the related infectious agents that cause Creutzfeldt-Jakob disease (CJD) and scrapie is poorly understood, and an agent-specific nucleic acid genome has not yet been identified. Several biological manifestations of these agents resemble those seen in retrovirus-induced diseases. We therefore attempted to identify an agent-specific retrovirus-like RNA transcript in CJD infectious fractions. A series of synthetic oligonucleotides complementary to known mammalian retroviral primer binding sites were used in a primer extension assay. Substrate nucleic acids isolated from partially purified hamster brain CJD infectious fractions and from parallel normal brain fractions were compared with total starting brain RNA. This sensitive exogenous strong-stop reaction revealed that CJD infectious fractions contained a series of potential retroviral RNAs including apparent transcripts of endogenous hamster IAP genes. Most transcripts selectively recovered in the fractions were substantially protected from micrococcal nuclease digestion, and at least one substrate RNA, consistent with an intracisternal A particle, was packaged in a form that had the same buoyant density as CJD infectivity. Although a completely CJD-specific transcript was not identified, the copurification of potential retroviral transcripts with CJD infectivity suggests that models of disease involving retrovirus-like nucleic acid elements deserve further consideration.

Two alleles of a neural protein gene linked to scrapie in sheep

Sheep are the natural hosts of the pathogens that cause scrapie, an infectious degenerative disease of the central nervous system. Scrapie-associated fibrils [and their major protein, prion protein (PrP)] accumulate in the brains of all species affected by scrapie and related diseases. PrP is encoded by a single gene that is linked to (and may be) the major gene controlling the incubation period of the various strains of scrapie pathogens. To investigate the role of PrP in natural scrapie, we have determined its gene structure and expression in the natural host. We have isolated two sheep genomic DNA clones that encode proteins of 256 amino acids with high homology to the PrPs of other species. Sheep PrPs have an arginine/glutamine polymorphism at position 171 that may be related to the alleles of the scrapie incubation-control gene in this species.

Normal and scrapie-associated forms of prion protein differ in their sensitivities to phospholipase and proteases in intact neuroblastoma cells

Previous studies have indicated that scrapie infection results in the accumulation of a proteinase K-resistant form of an endogenous brain protein generally referred to as prion protein (PrP). The molecular nature of the scrapie-associated modification of PrP accounting for proteinase K resistance is not known. As an approach to understanding the cellular events associated with the PrP modification in brain tissue, we sought to identify proteinase K-resistant PrP (PrP-res) in scrapie-infected neuroblastoma cells in vitro and to compare properties of PrP-res with those of its normal proteinase K-sensitive homolog, PrP-sen. PrP-res was detected by immunoblot in scrapie-infected but not uninfected neuroblastoma clones. Densitometry of immunoblots indicated that there was two- to threefold more PrP-res than PrP-sen in one infected clone. Metabolic labeling and membrane immunofluorescence experiments indicated that PrP-sen was located on the cell surface and could be removed from intact cells by phosphatidylinositol-specific phospholipase C and proteases. In contrast, PrP-res was not removed after reaction with these enzymes. Thus, either the scrapie-associated PrP-res was not on the cell surface or it was there in a form that is resistant to these hydrolytic enzymes. Attempts to detect intracellular PrP-res by immunofluorescent staining of fixed and permeabilized cells revealed that PrP was present in discrete perinuclear Golgi-like structures. However, the staining pattern was similar in both scrapie-infected and uninfected clones, and thus the intracellular staining may have represented only PrP-sen. Analysis of scrapie infectivity in cells treated with extracellular phospholipase, proteinase K, and trypsin indicated that, like PrP-res, the scrapie agent was not removed from the infected cells by any of these enzymes.

Diversity of oligosaccharide structures linked to asparagines of the scrapie prion protein

Prion proteins from humans and rodents contain two consensus sites for asparagine-linked glycosylation near their C-termini. The asparagine-linked oligosaccharides of the scrapie isoform of the hamster prion protein (PrP 27-30) were released quantitatively from the purified molecule by hydrazinolysis followed by N-acetylation and NaB3H4 reduction. The radioactive oligosaccharides were fractionated into one neutral and three acidic oligosaccharide fractions by anion-exchange column chromatography. All oligosaccharides in the acidic fractions could be converted to neutral oligosaccharides by sialidase digestion. Structural studies on these oligosaccharides including sequential exoglycosidase digestion in combination with methylation analysis revealed that PrP 27-30 contains a mixture of bi-, tri-, and tetraantennary complex-type sugar chains with Man alpha 1----6(GlcNAc beta 1----4)(Man alpha 1----3)Man beta 1----4GlcNAc beta 1----4-(Fuc alpha 1----6)GlcNAc as their core. Variation is produced by the different combination of the oligosaccharides Gal beta 1----4GlcNAc beta 1----, Gal beta 1----4(Fuc alpha 1----3)GlcNAc beta 1----, GlcNAc beta 1----, Sia alpha 2----3Gal beta 1----4GlcNAc beta 1----, and Sia alpha 2----6Gal beta 1----4GlcNAc beta 1---- in their outer chain moieties. When both asparagine-linked consensus sites are glycosylated, the diversity of oligosaccharide structures yields over 400 different forms of the scrapie prion protein. Whether these diverse asparagine-linked oligosaccharides participate in scrapie prion infectivity or modify the function of the cellular prion protein remains to be established.

Physical properties of the Creutzfeldt-Jakob disease agent

In this report, we present the first physical characterization of the Creutzfeld-Jakob disease agent. Preparations with high yields of infectivity (assayed infectious units) were obtained by a novel, gentle procedure in which initially sedimenting Gp34 ("prion" protein) was disaggregated by a variety of criteria with no subsequent loss of infectivity. Studies with this preparation indicate that most of the Creutzfeld-Jakob disease agent has both a viruslike size and density. In velocity sedimentation and isopycnic sucrose gradients, infectivity comigrated with nucleic acid-protein complexes of appreciable size.

Prion protein biosynthesis in scrapie-infected and uninfected neuroblastoma cells

Numerous studies have indicated that a modified proteinase K-resistant form of an endogenous brain protein, prion protein (PrP), is associated with scrapie infection in animals. This scrapie-associated PrP modification appears to occur posttranslationally in brain, but its molecular nature is not known. To learn about the normal PrP biosynthesis and whether it is altered by scrapie infection in vitro, we did metabolic labeling experiments with uninfected and scrapie-infected mouse neuroblastoma tissue culture cells. Pulse-chase labeling experiments indicated that, in both cell types, two major PrP precursors of 28 and 33 kilodaltons (kDa) were processed to mature 30- and 35- to 41-kDa forms. Endoglycosidase H, tunicamycin, and phospholipase treatments revealed that the 28- and 33-kDa precursors resulted from the addition of high-mannose glycans to a 25-kDa polypeptide containing a phosphatidylinositol moiety and that maturation of the precursors involved the conversion of the high-mannose glycans to hybrid or complex glycans. Treatments of the live cells with trypsin and phosphatidylinositol-specific phospholipase C indicated that the mature PrP species were expressed solely on the cell surface, where they were anchored by covalent linkage to phosphatidylinositol. Once on the cell surface, the major PrP forms had half-lives of 3 to 6 h. No differences in PrP biosynthesis were observed between the scrapie-infected versus uninfected neuroblastoma cells.

Fibrils from brains of cows with new cattle disease contain scrapie-associated protein

During the past two years, more than 1,000 cases of a neurological disorder of cattle, bovine spongiform encephalopathy (BSE), have been confirmed from farms throughout Great Britain. The neurological signs and brain pathology of BSE resemble those produced in other species by the pathogens of scrapie and related disorders. The discovery of fibrils similar to scrapie-associated fibrils in detergent extracts o BSE-affected brain supported the clinical and pathological diagnosis of the disease, but has been controversial. Scrapie-associated fibrils are found in brain extracts of all species affected by scrapie and diseases caused by related pathogens. They are pathological aggregates of a neuronal membrane protein termed PrP and a protease-resistant form of PrP is a molecular marker of scrapie-associated fibrils. In this report, we show the major protein of BSE fibrils is the bovine homologue of PrP as judged by its size, protease resistance, immunoreactivity, lectin binding and partial N-terminal protein sequence. This confirms that BSE is a scrapie-like disease.

Immunological analysis of host and agent effects on Creutzfeldt-Jakob disease and scrapie prion proteins

Creutzfeldt-Jakob disease (CJD) and scrapie are degenerative neurological diseases caused by unusual infectious pathogens. The term prion has been introduced to underscore the apparent distinctness of these agents from viruses and viroids. The only macromolecule shown to be associated with the infectious agent, the CJD or scrapie prion protein (PrPCJD or PrPSc, respectively), is encoded by the same gene as a normal cellular protein. In several studies biochemical differences have been reported in PrPScs derived from a common host species infected with different putative strains of the scrapie agent, suggesting agent-specific characteristics independent of the host. We analyzed various agent-host combinations by Western blotting of PrPs that were separated by size or charge. The profile of immunoreactive proteins for CJD prions isolated from mice, guinea pigs, and humans appeared distinct. Importantly, PrPCJDS purified from a human brain and from the corresponding first-passage mouse brains were clearly distinguishable. PrPCJDs isolated from CJD prions propagated in NAMRU or B10.Q mice, which are homozygous for a short-incubation-time gene; from the short-incubation-time backcross progeny of (B10.Q x I/LnJ)F1 x B10.Q; or from NAMRU mice inoculated with I/LnJ prions were identical to each other but distinguishable from those of I/LnJ mice, which are homozygous for the long-incubation-time gene. The PrPs from human CJD and ovine scrapie propagated in the same mouse strain appeared the same, but they were distinct from the same isolate of scrapie passaged in hamsters. Lastly, PrPScs purified from five different strains of scrapie propagated in C57BL mice were identical, including strains, ME7 and 139A, which were previously reported to be distinct. This evidence does not support, although it does not exclude, agent-mediated characteristics independent of host-mediated ones for scrapie and CJD.

Molecular pathology of scrapie-associated fibril protein (PrP) in mouse brain affected by the ME7 strain of scrapie

Scrapie-associated fibrils (SAF) are disease-specific structures found in extracts of the brains of animals affected with scrapie. These structures are pathological aggregates of a normal host protein (PrP). Abnormal post-translational modification of PrP has been suggested to explain its aberrant properties in scrapie-affected brains and although there is a form of PrP in SAF indistinguishable in size from the protein in uninfected brain, lower-molecular-mass variants of PrP are also found in SAF fractions. We report the characterisation of the multiple forms of PrP found in SAF fractions purified from mouse brain affected by the ME7 strain of scrapie. The quantitatively major forms of PrP in SAF prepared without the use of proteinase K have the amino-terminal sequence Lys-Lys-Arg-Pro-Lys-Pro-Gly-Gly-, identical to that predicted for the amino-terminus of normal mouse brain PrP. However N-terminal cleavage of some PrP does occur in vivo within a domain of repetitive sequences at sites similar to but distinct from those cut by proteinase K in vitro. This suggests the conformation of the protein in aggregates in vivo does not differ extensively from that in detergent-treated SAF in vitro. We conclude that the size diversity of PrP in SAF is only partly due to N-terminal proteolysis and is independent of the proteolysis that occurs if proteinase K is used in the purification of SAF. Apart from proteolytic changes in the structure of PrP, we found a novel, as yet unidentified, amino-acid derivative of the arginine residue at position 3 in mouse PrP, which may predispose PrP to form SAF.