Evidence suggesting that PrP is not the infectious agent in Creutzfeldt-Jakob disease

Prion Protein PRNP: A New Player in Innate Immunity? The Aβ Connection

The prion protein PRNP has been centrally implicated in the transmissible spongiform encephalopathies (TSEs), but its normal physiological role remains obscure. We highlight emerging evidence that PRNP displays antimicrobial activity, inhibiting the replication of multiple viruses, and also interacts directly with Alzheimer's disease (AD) amyloid-β (Aβ) peptide whose own antimicrobial role is now increasingly secure. PRNP and Aβ share share membrane-penetrating, nucleic acid binding, and antiviral properties with classical antimicrobial peptides such as LL-37. We discuss findings that binding of abnormal nucleic acids to PRNP leads to oligomerization of the protein, and suggest that this may be an entrapment and sequestration process that contributes to its antimicrobial activity. Some antimicrobial peptides are known to be exploited by infectious agents, and we cover evidence that PRNP is usurped by herpes simplex virus (HSV-1) that has evolved a virus-encoded 'anti-PRNP'.unction. These findings suggest that PRNP, like LL-37 and Aβ, is likely to be a component of the innate immune system, with implications for the pathoetiology of both AD and TSE.

A brief history of prions

Proteins were described as distinct biological molecules and their significance in cellular processes was recognized as early as the 18th century. At the same time, Spanish shepherds observed a disease that compelled their Merino sheep to pathologically scrape against fences, a defining clinical sign that led to the disease being named scrapie. In the late 19th century, Robert Koch published his postulates for defining causative agents of disease. In the early 20th century, pathologists Creutzfeldt and Jakob described a neurodegenerative disease that would later be included with scrapie into a group of diseases known as transmissible spongiform encephalopathies (TSEs). Later that century, mounting evidence compelled a handful of scientists to betray the prevailing biological dogma governing pathogen replication that Watson and Crick so convincingly explained by cracking the genetic code just two decades earlier. Because TSEs seemed to defy these new rules, J.S. Griffith theorized mechanisms by which a pathogenic protein could encipher its own replication blueprint without a genetic code. Stanley Prusiner called this proteinaceous infectious pathogen a prion. Here we offer a concise account of the discovery of prions, the causative agent of TSEs, in the wider context of protein biochemistry and infectious disease. We highlight the discovery of prions in yeast and discuss the implication of prions as epigenomic carriers of biological and pathological information. We also consider expanding the prion hypothesis to include other proteins whose alternate isoforms confer new biological or pathological properties.

Investigating the conformational stability of prion strains through a kinetic replication model

Prion proteins are known to misfold into a range of different aggregated forms, showing different phenotypic and pathological states. Understanding strain specificities is an important problem in the field of prion disease. Little is known about which PrP^Sc structural properties and molecular mechanisms determine prion replication, disease progression and strain phenotype. The aim of this work is to investigate, through a mathematical model, how the structural stability of different aggregated forms can influence the kinetics of prion replication. The model-based results suggest that prion strains with different conformational stability undergoing in vivo replication are characterizable in primis by means of different rates of breakage. A further role seems to be played by the aggregation rate (i.e. the rate at which a prion fibril grows). The kinetic variability introduced in the model by these two parameters allows us to reproduce the different characteristic features of the various strains (e.g., fibrils' mean length) and is coherent with all experimental observations concerning strain-specific behavior.

Prion diseases are caused by the accumulation of a cellular prion protein with an altered conformation, which acts as a catalyst for the further recruitment and the modification of the normal form of the protein. Protein polymerization appears to have a central role in the progression of the disease, an aspect shared with several other neurodegenerative diseases. The aim of this work is to investigate at the kinetic level the “prion strain phenomenon”, i.e., the ability of prion proteins to misfold into a range of different aggregated forms exhibiting different replication and propagation properties. The dynamics of prion replication is investigated with the help of a mathematical model. We relate a measurement accessible in vitro (prion structural stability) to a mathematical description of the fibrils' kinetics in vivo. The analysis of the model suggests that the replication kinetics of the different prion strains is characterizable by means of two parameters, representing the rates of breakage and aggregation. This result is coherent with various experimental findings concerning strain-specific behavior, such as, for example, the observation of the fibril mean length of the various strains.

Quantitative recovery of scrapie agent with minimal protein from highly infectious cultures

There are few reports on the isolation, quantitative recovery, and relative purification of infectious particles that cause scrapie, Creutzfeldt-Jakob disease (CJD) and epidemic bovine spongiform encephalopathy (BSE). Because pure prion protein (PrP) has failed to show significant infectivity, it is critical to find other molecules that are integral agent components. Only complex diseased tissues such as degenerating brain have been fractionated, and agent recoveries have been quite low in concentrated abnormal prion protein (PrP-res) preparations. To simplify the purification of infectious particles, we evaluated a monotypic cell line that continuously produced high levels of the 22L scrapie agent (N2a-22L). A new rapid and accurate GT1 culture assay was used to titrate infectivity in six representative sucrose gradients. We developed a streamlined approximately 3-h procedure that yielded full recovery of starting infectivity in fractions with only a few selected protein bands (representing <1% of starting protein). Infectious particles reproducibly sedimented through >30% sucrose steps, whereas PrP and PrP-res sedimentation varied depending on the conditions used. Both normal and abnormal PrP could be largely separated from infectivity in a single short centrifugation. Because no foreign enzymes were added to achieve reasonably purified infectious particles, these preparations may be used to elicit diagnostic antibodies to foreign agent proteins.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

A versatile prion replication assay in organotypic brain slices

Methods enabling prion replication ex vivo are important for advancing prion studies. However, few such technologies exist, and many prion strains are not amenable to them. Here we describe a prion organotypic slice culture assay (POSCA) that allows prion amplification and titration ex vivo under conditions that closely resemble intracerebral infection. Thirty-five days after contact with prions, mouse cerebellar slices had amplified the abnormal isoform of prion protein, PrP(Sc), >10(5)-fold. This is quantitatively similar to amplification in vivo, but fivefold faster. PrP(Sc) accumulated predominantly in the molecular layer, as in infected mice. The POSCA detected replication of prion strains from disparate sources, including bovines and ovines, with variable detection efficiency. Pharmacogenetic ablation of microglia from POSCA slices led to a 15-fold increase in prion titers and PrP(Sc) concentrations over those in microglia-containing slices, as well as an increase in susceptibility to infection. This suggests that the extensive microglial activation accompanying prion diseases represents an efficacious defensive reaction.

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.

Potential involvement of retroviral elements in human dementias

Creutzfeldt-Jakob disease (CJD) is a dementia of humans caused by a class of infectious agents with several biological properties similar to those of conventional viruses. The molecular nature of this group of agents is enigmatic, for neither an agent-specific nucleic acid nor a non-host protein has yet been identified. Recent transmissions of familial CJD dementias to rodents suggest that this class of agent can be integrated into the germline. Furthermore, tissue culture studies indicate that CJD causes transformation of cells in a manner reminiscent of slowly oncogenic retroviruses. Currently characterized retroviral-like elements include many forms that do not have 'typical' retroviral ultrastructural morphology; several forms are also known to be resistant to various types of standard physicochemical inactivation. We suggest that CJD agents are either constituted by retroviral-like nucleic acids or interact with endogenous retroviral sequences to elicit a slowly progressive disease of the central nervous system. Several overlapping properties between infectious CJD and 'non-infectious' dementias, such as Alzheimer's disease, implicate potential common pathogenic mechanisms.

Bistability and the species barrier in prion diseases: stepping across the threshold or not

The infectious agent of transmissible spongiform encephalopathies is thought to be a cellular protein, the prion protein, which undergoes, under some circumstances, a dramatic conformational change leading to pathogenesis. The conversion between the normal and pathogenic isoforms corresponds to a autocatalytic mechanism and the metabolism of the prion protein exhibits switches between a normal, stable steady state and a pathogenic one. When the disease can be transmitted between two species, a primary infection from a heterologous donor has to be followed by two passages in the same host species so that the incubation period is stabilized. Sometimes, no pathogenic isoform of the prion protein is detected after the first passage, although corresponding brain extracts remain infectious. The observation that three and only three passages are needed in order to stabilize the strain strongly suggests that, during the course of the primary infection by the heterologous donor, an intermediary conformational species is formed. Within this assumption, a common mechanism involving only conformational changes of the prion protein can give a unifying interpretation of the problem of species barrier, lag characteristics and apparent lack of detection of the pathogenic isoform after the first passage in experiments dealing with interspecies transmission of prion diseases.

A 25 nm virion is the likely cause of transmissible spongiform encephalopathies

The transmissible spongiform encephalopathies (TSEs) such as endemic sheep scrapie, sporadic human Creutzfeldt-Jakob disease (CJD), and epidemic bovine spongiform encephalopathy (BSE) may all be caused by a unique class of "slow" viruses. This concept remains the most parsimonious explanation of the evidence to date, and correctly predicted the spread of the BSE agent to vastly divergent species. With the popularization of the prion (infectious protein) hypothesis, substantial data pointing to a TSE virus have been largely ignored. Yet no form of prion protein (PrP) fulfills Koch's postulates for infection. Pathologic PrP is not proportional to, or necessary for infection, and recombinant and "amplified" prions have failed to produce significant infectivity. Moreover, the "wealth of data" claimed to support the existence of infectious PrP are increasingly contradicted by experimental observations, and cumbersome speculative notions, such as spontaneous PrP mutations and invisible strain-specific forms of "infectious PrP" are proposed to explain the incompatible data. The ability of many "slow" viruses to survive harsh environmental conditions and enzymatic assaults, their stealth invasion through protective host-immune defenses, and their ability to hide in the host and persist for many years, all fit nicely with the characteristics of TSE agents. Highly infectious preparations with negligible PrP contain nucleic acids of 1-5 kb, even after exhaustive nuclease digestion. Sedimentation as well as electron microscopic data also reveal spherical infectious particles of 25-35 nm in diameter. This particle size can accommodate a viral genome of 1-4 kb, sufficient to encode a protective nucleocapsid and/or an enzyme required for its replication. Host PrP acts as a cellular facilitator for infectious particles, and ultimately accrues pathological amyloid features. A most significant advance has been the development of tissue culture models that support the replication of many different strains of agent and can produce high levels of infectivity. These models provide new ways to rapidly identify intrinsic viral and strain-specific molecules so important for diagnosis, prevention, and fundamental understanding.

Cells infected with scrapie and Creutzfeldt-Jakob disease agents produce intracellular 25-nm virus-like particles

We had repeatedly found approximately 25-nm-diameter virus-like particles in highly infectious brain fractions with little prion protein (PrP), and therefore we searched for similar virus-like particles in situ in infected cell lines with high titers. Neuroblastoma cells infected with the 22L strain of scrapie as well as hypothalamic GT cells infected with the FU Creutzfeldt-Jakob disease agent, but not parallel mock controls, displayed dense 25-nm virus-like particles in orthogonal arrays. These particles had no relation to abnormal PrP amyloid in situ, nor were they labeled by PrP antibodies that faithfully recognized rough endoplasmic reticulum membranes and amyloid fibrils, the predicted sites of normal and pathological intracellular PrP. Additionally, phorbol ester stimulated the production of abnormal PrP gel bands by >5-fold in infected N2a + 22L cells, yet this did not increase either the number of virus-like arrays or the infectious titer of these cells. Thus, the 25-nm infection-associated particles could not be prions. Synaptic differentiation and neurodegeneration, as well as retroviruses that populate the rough endoplasmic reticulum of neuroblastoma cells, were not required for particle production. The 25-nm particle arrays in cultured cells strongly resembled those first described in 1968 in synaptic regions of scrapie-infected brain and subsequently identified in many natural and experimental TSEs. The high infectivity of comparable, isolated virus-like particles that show no intrinsic PrP by antibody labeling, combined with their loss of infectivity when nucleic acid-protein complexes are disrupted, make it likely that these 25-nm particles are the causal TSE virions that induce late-stage PrP brain pathology.

Acid inactivation of prions: efficient at elevated temperature or high acid concentration

Scrapie prion rods isolated from hamster and non-infectious aggregates of the corresponding recombinant protein rPrP(90–231) were incubated with hydrochloric acid. The amount of PrP and of infectivity that survived incubation in HCl at varying times, acid concentrations and temperatures was quantified by Western blot densitometry and bioassays, respectively. Prion rods and rPrP aggregates showed similar HCl hydrolysis kinetics of PrP, indicating structural homology. For 1 M HCl and 25 °C, the rate of PrP hydrolysis follows first-order kinetics at 0·014 h−1; the rate of infectivity inactivation is 0·54 h−1. Hydrolysis for 1 h at 25 °C was only slightly proportional to HCl concentration up to 5 M, but complete loss of infectivity and PrP reduction to <2 % was observed at 8 M HCl. The temperature dependence of unhydrolysed PrP, as well as infectivity at 1 M HCl for 1 h, showed a slight decrease up to 45 °C, but a sigmoidal decrease by several orders of magnitude at higher temperatures. The slow hydrolysis of PrP and inactivation of infectivity by acid treatment at room temperature are attributed to solvent inaccessibility of prion rods and rPrP aggregates, respectively. The more effective hydrolysis and inactivation at temperatures above 45 °C are interpreted as thermally induced disaggregation with an activation energy of 50–60 kJ mol−1. Most importantly, infectivity was always inactivated faster or to a higher extent than PrP was hydrolysed at several incubation times, HCl concentrations and temperatures.

Preparation of soluble infectious samples from scrapie-infected brain: a new tool to study the clearance of transmissible spongiform encephalopathy agents during plasma fractionation

BACKGROUND

Concern about the safety of blood, blood components, and plasma-derived products with respect to prions has increased since the report of two blood-related infections of variant Creutzfeldt-Jakob disease in the United Kingdom. Efforts were directed toward the development of procedures able to remove or inactivate prions from blood components or plasma-derived products with brain fractions of transmissible spongiform encephalopathy (TSE)-infected rodents as spiking materials. These spiking materials, however, are loaded with pathological prion protein (PrP(TSE)) aggregates that are likely not associated to blood infectivity. The presence of these aggregates may invalidate these studies.

STUDY DESIGN AND METHODS

Brains from 263K scrapie-infected hamsters were suspended in 10 percent phosphate-buffered saline. After low-speed centrifugation, the supernatant was collected and ultracentrifuged at 220,000 x g at 25 degrees C for 30 minutes. The high-speed supernatants (S(HS)) and pellets were collected; the proteinase-resistant PrP(TSE) was measured by Western blot and infectivity by intracerebral inoculation into weanling hamsters.

RESULTS

A substantial amount of prion infectivity (more than 10(5) LD(50) per mL of a 10% suspension of brain tissues) is present in the S(HS) fraction of 263K scrapie-infected hamster brains. Concomitantly, this fraction contains none or only traces of PrP(TSE) in its aggregate form.

CONCLUSION

This study describes a simple and fast protocol to prepare infectious material from 263K scrapie-infected brains that is not contaminated with PrP(TSE) aggregates. This S(HS) fraction is likely to be the most relevant material for endogenous spiking of human blood in validation experiments aimed at demonstrating procedures to remove or inactivate TSE infectious agents.

The state of the prion

There is little doubt that the main component of the transmissible agent of spongiform encephalopathies - the prion - is a conformational variant of the ubiquitous host protein PrP(C), and that the differing properties of various prion strains are associated with different abnormal conformations of this protein. The precise structure of the prion is not yet known, nor are the mechanisms of infection, conformational conversion and pathogenesis understood.

Two Creutzfeldt-Jakob disease agents reproduce prion protein-independent identities in cell cultures

Human Creutzfeldt-Jakob disease (CJD) and similar neurodegenerative diseases such as sheep scrapie are caused by a variety of related infectious agents. They are associated with abnormal host prion protein (PrP), which is assessed by limited proteolysis to yield resistant PrP bands (PrP-res). Although PrP-res has been posited as the infectious agent, purified PrP-res itself is not infectious. To establish the independence of CJD agent characteristics from those of PrP-res, two different mouse-passaged CJD strains were propagated in neuronal cell lines whose PrP-res patterns differ markedly from each other and from those found in infected brain. In mouse brain, the fast CJD strain, FU, elicits many PrP-res deposits, whereas the slow SY strain elicits few. Both strains evoked PrP-res in cultured murine cells, although SY induced PrP-res only transiently. PrP-res patterns in FU- and SY-infected GT1 cells were identical, and were significantly different from those in brain and in N2a cells. Nevertheless, all FU-infected cell lines reproduced their original fast disease in mice, even after extensive subculture, whereas SY-infected cells produced only slow disease. These data indicate PrP-res neither encodes nor alters agent-specific characteristics. PrP-res was also a poor predictor of infectivity because SY cells that had lost PrP-res were approximately 10-fold more infectious than PrP-res-positive cultures. Furthermore, FU titers increased 650-fold, whereas PrP-res remained constant. Passaged FU-infected cells had titers comparable to brain, and >30% of cells displayed abundant cytoplasmic PrP-res aggregates that may trap agent. The continuous substantial replication of CJD in monotypic cells will further the discrimination of agent-specific molecules from pathological host responses to infection.

Transmissible encephalopathies: speculations and realities

Virtually all transmissible encephalopathies (TSEs), such as scrapie, CJD, and BSE, are caused by a type of infectious particle that remains enigmatic. The language of prion theory supersedes the reality of what is, and what is not known. This review questions the predictive value, consistency and accuracy of this now dominant assumption. Many people believe the normal cellular prion protein (PrP) self-converts into an infectious amyloid protein or prion. Although the amyloidogenic capacity of proteins is well established, the concept of an infectious protein without nucleic acid was "revolutionary." Diverse experiments have repeatedly shown, however, that this protein alone, in any form, is incapable of reproducing transmissible infection. In contrast, the infectious agent copurifies with many other molecules, including nucleic acids, while it separates from the majority of PrP. The infectious particle has a homogeneous viral size of ~25 nm, and infectivity is markedly reduced by conditions that disrupt viral core components but do not disrupt multimers of PrP amyloid. Additionally, the infectious agent replicates to high levels before any PrP abnormalities can be detected. Hence, we initially proposed that PrP changes are part of the host's pathologic response to high levels of infectious agent, but not the agent itself. Newer data clarifying a role for myeloid cells in the spread of infection, the unique character of two different agent strains propagated in a single animal, and the demonstration of long nucleic acids in a variety of simplified high titer preparations continue to raise serious questions for the prion hypothesis. Moreover, the epidemic spread of TSEs, and the activation of host innate immune mechanisms by infection, further indicate these agents are recognizably foreign, and probably viral.

Thermostability of mouse-passaged BSE and scrapie is independent of host PrP genotype: implications for the nature of the causal agents

Five experimentally maintained strains of scrapie and BSE agents have been passaged in two PrP genotypes of mice. Brain macerates were autoclaved at 126 degrees C and the levels of surviving infectivity were measured by titration. There was a large difference in the survival properties of transmissible spongiform encephalopathy (TSE) infectivity between TSE strains. PrP genotype had little effect. Phenotypic properties of the TSE strains were not affected with the exception that with one strain (ME7), incubation periods of the heated sample were longer than the controls given equivalent doses. It is concluded that PrP is probably not responsible for differences in thermostability between strains. More likely, a host-independent molecule which differs in covalent structure between strains accounts for these properties.

TSE agent strains and PrP: reconciling structure and function

The molecular structures of the infectious agents that cause transmissible spongiform encephalopathy (TSE) diseases are still not known despite the current wide acceptance of the prion hypothesis as the basis for their resolution. Here, data supporting and challenging the prion hypothesis in relation to both the biochemical and biological properties of TSE infectious agents are discussed. The need for the independent transmission of TSE agent-specific genetic information is described and the requirements for the molecule to carry this information are proposed. Such a molecule is likely to be a small nucleic acid encoding information to determine the diversity of the pathogenesis of TSE agents.

Microglia from Creutzfeldt-Jakob disease-infected brains are infectious and show specific mRNA activation profiles

Neurons are often assumed to be the principal sites for replication of the infectious agents causing Creutzfeldt-Jakob disease (CJD), scrapie, and bovine spongiform encephalopathy because they express high levels of normal and pathological prion protein (PrP). However, isolated brain cell types have not been evaluated for either infection or gene expression. Microglia purified from CJD-infected mice showed infectivity comparable to that of starting brain homogenate but expressed approximately 50-fold less PrP. CJD-infected microglia also displayed morphological changes indicative of cellular activation. To determine the molecular pathways of activation, we evaluated pertinent transcripts, including those linked to inflammation. Semiquantitative reverse transcription-PCR showed a >4-fold increase in cathepsin S, an enzyme important in antigen presentation, the cytokine interleukin-1beta, and the chemokine B-lymphocyte chemoattractant. The profile of microglial changes induced by the CJD agent differed substantially from activation induced by bacterial lipopolysaccharide or by beta-amyloid, a structure comparable to pathological PrP. These microglial studies emphasize migratory hematopoietic cells in the dispersion, and possibly replication, of the CJD agent. The low PrP levels in these highly infectious and activated cells further support the concept that pathological PrP is the result of infection rather than the infectious agent itself. Because microglia develop a specific pattern of responses to the CJD agent, microglial markers may be exploited in the diagnosis of these spongiform encephalopathies.

Human prion diseases

Transmissible spongiform encephalopathy (TSE) is a group of rare, sub-acute, fatal neurodegenerative diseases in humans and animals. TSE includes Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease, fatal familial insomnia, and Kuru (a disease confined to the Fore linguistic group, a tribe in Papua-New Guinea). From the onset, it was recognized that some of these spongiform diseases occur in clusters, in an inherited, familial manner. This article describes these diseases, current treatment modalities, and suggests directions for future research.

Characterization of thermodynamic diversity between transmissible spongiform encephalopathy agent strains and its theoretical implications

Some transmissible spongiform encephalopathy (TSE) (or "prion") strains, notably those derived from bovine spongiform encephalopathy, are highly resistant to total inactivation by heat. When three TSE strains derived from sheep with scrapie were heated, little inactivation took place at low temperatures, but at higher temperatures, considerable inactivation occurred. The temperature at which substantial inactivation first occurred varied according to TSE strain, and it was calculated to be 70 degrees C for the 22C strain, 84 degrees C for ME7, and 97 degrees C for 22A by fitting the data to a model based on competition between a destructive and a protective reaction. However, PrP(Sc) from mice infected with a range of TSE strains retained similar resistance to proteinase K digestion after heating to below or above these temperatures, showing that the properties of PrP(Sc) responsible for proteinase resistance do not correlate with those conferring thermostability on the TSE agent. The simplest explanation of these data is that the causal agent contains a macromolecular component that is structurally independent of the host, that it varies covalently between TSE strains, and that it is protected by other macromolecular components. The model is in accord with the virino hypothesis, which proposes a host-independent informational molecule protected by the host protein PrP.

Neuroinvasion by a Creutzfeldt-Jakob disease agent in the absence of B cells and follicular dendritic cells

With the potential spread of bovine spongiform encephalopathy to people as a variant Creutzfeldt-Jakob disease (CJD), it becomes critical to identify cells in the periphery that carry infection. Initial work with scrapie agents suggested that B cells were central vectors for neuroinvasion. Subsequent studies indicated that B cells played an indirect role by promoting the development of follicular dendritic cells (FDCs) that accumulate abnormal prion protein (PrP). The mechanism for the role of FDCs, however, has not been clear. To further dissect potential B cell functions that contribute to neuroinvasion, we inoculated a CJD agent into mutant mice that (i) lacked B cells, (ii) had B cells unable to secrete Ig, or (iii) could secrete only IgM. Remarkably, all these mice developed disease with practically indistinguishable incubation times. The demonstration that neither immune complexes nor B cells were required for neuroinvasion from the periphery mandates a reanalysis of the accepted view of the essential role of B cells and FDC in these infections. Moreover, immune complexes were not required for the accumulation of pathologic PrP on the surface of FDCs, suggesting that PrP can bind to FDCs autonomously or by means of another factor. Wild-type mice had incubation times approximately 50 days less than all mutant mice at the same peripheral doses, indicating that an intact immune system may increase agent uptake and delivery, but this condition is not essential. Specifically, the evidence to date suggests that IgG may enhance pivotal agent interactions with migratory myeloid cells.

The measured level of prion infectivity varies in a predictable way according to the aggregation state of the infectious agent

Transmissible spongiform encephalopathies are believed to be caused by an infectious form of the prion protein, designated PrP(Sc). The concentration of PrP(Sc) is often poorly correlated to the level of infectivity. Infectivity can be measured in two ways, namely endpoint titration and the incubation time assay, but patterns of infectivity vary depending on which method is used. These discrepancies can be explained by variation in the aggregation state of PrP(Sc). Both methods of measuring infectivity are modelled mathematically, and the theoretical results are in agreement with published data. It was found to be theoretically impossible to characterise prion infectivity by a multiple of a single quantity representing 'one prion', no matter how it is measured. Infectivity is instead characterised by both the number and sizes of the PrP(Sc) aggregates. Apparent discrepancies arise when these complexities are reduced to a single number.

Protease-resistant and detergent-insoluble prion protein is not necessarily associated with prion infectivity

PrPSc, an abnormal isoform of PrPC, is the only known component of the prion, an agent causing fatal neurodegenerative disorders such as bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease (CJD). It has been postulated that prion diseases propagate by the conversion of detergent-soluble and protease-sensitive PrPC molecules into protease-resistant and insoluble PrPSc molecules by a mechanism in which PrPSc serves as a template. We show here that the chemical chaperone dimethyl sulfoxide (Me2SO) can partially inhibit the aggregation of either PrPSc or that of its protease-resistant core PrP27-30. Following Me2SO removal by methanol precipitation, solubilized PrP27-30 molecules aggregated into small and amorphous structures that did not resemble the rod configuration observed when scrapie brain membranes were extracted with Sarkosyl and digested with proteinase K. Interestingly, aggregates derived from Me2SO-solubilized PrP27-30 presented less than 1% of the prion infectivity obtained when the same amount of PrP27-30 in rods was inoculated into hamsters. These results suggest that the conversion of PrPC into protease-resistant and detergent-insoluble PrP molecules is not the only crucial step in prion replication. Whether an additional requirement is the aggregation of newly formed proteinase K-resistant PrP molecules into uniquely structured aggregates remains to be established.

Quantifying the kinetic parameters of prion replication

The mechanism of protein-only prion replication is controversial. A detailed mathematical model of prion replication by nucleated polymerisation is developed, and its parameters are estimated from published data. PrP-res decay is around two orders of magnitude slower than PrP-sen decay, a plausible ratio of two parameters estimated from very different experiments. By varying the polymer breakage rate, we reveal that systems of short polymers grow the fastest. Drugs which break polymers could therefore accelerate disease progression. Growth in PrP-res seems slower than growth in infectious titre. This can be explained either by a novel hypothesis concerning inoculum clearance from a newly infected brain, or by the faster growth of compartments containing smaller polymers. The existence of compartments can also explain why prion growth sometimes reaches a plateau. Published kinetic data are all compatible with our mathematical model, so the nucleated polymerisation hypothesis cannot be ruled out on dynamic grounds.

Scrapie

Scrapie and other transmissible spongiform encephalopathies (TSEs) are characterized by similar pathology, biochemistry and genetics. The PrP protein and its conversion to the disease-related isoform, PrPSC, are crucial for the development of all TSEs. Although scrapie is more often studied in laboratory rodents, it is not a natural disease of these animals, and much can be learned from the normal hosts, sheep. Disease incidence is linked to polymorphisms and mutations of the PrP gene. The complex relationships between PrP genotype and the survival of sheep subjected to scrapie infection are now being investigated in terms of the different structure of the PrP protein molecules produced by each allele. It is these structures and their differing abilities to convert to PrPSC that hold the key to understanding why TSEs occur.

Vaccination with an attenuated Creutzfeldt-Jakob disease strain prevents expression of a virulent agent

Although slow and persistent viruses often escape host defenses infection may be prevented by live vaccines. To determine whether an attenuated "slow" strain of the Creutzfeldt-Jakob disease agent (SY) could block expression of a virulent "fast" strain (FU), outbred CD-1 mice were inoculated intracerebrally with low infectious doses of SY and challenged 80 days later with higher doses of FU. For comparison, the same SY and FU samples were inoculated in two parallel control groups. All 18 superinfected mice showed incubation times identical to those inoculated with only the SY strain, yielding clinical disease >110 days later than predicted for the FU strain. Neurological signs, such as scratching and an extended clinical phase, were also characteristic for SY but not FU infection. Moreover, the widespread cortical pathology of FU was not detectable in superinfected mice. Western blot analyses further showed no strain-specific differences in prion protein (PrP) band profiles for all experimental groups, although there was approximately 10-fold more protease-resistant PrP (PrP-res) in FU brains during terminal disease. In contrast, infectivity assays revealed an approximately 10,000-fold difference between SY and FU at terminal stages, indicating that PrP-res content does not correlate with infectivity. In summary, an attenuated strain of the Creutzfeldt-Jakob disease agent evokes substantial interference against a virulent agent. Because superinfected mice had little PrP-res just before the onset of clinical disease and retained abundant cellular PrP, cellular PrP was not the factor limiting FU replication. The mechanisms underlying SY interference are not understood but could be based on host recognition of foreign molecular features shared by this class of invasive agents involving antibody production, and possibly involve defective viral particles produced by attenuated variants.

Evolution of a strain of CJD that induces BSE-like plaques

Bovine spongiform encephalopathy (BSE) has become a public health issue because a recently evolved BSE agent has infected people, yielding an unusual form of Creutzfeld-Jakob disease (CJD). A new CJD agent that provokes similar amyloid plaques and cerebellar pathology was serially propagated. First-passage rats showed obvious clinical signs and activated microglia but had negligible PrP-res (the more protease-resistant form of host PrP) or cerebellar lesions. Microglia and astrocytes may participate in strain selection because the agent evolved, stabilized, and reproducibly provoked BSE-like disease in subsequent passages. Early vacuolar change involving activated microglia and astrocytes preceded significant PrP-res accumulation by more than 50 days. These studies reveal several inflammatory host reactions to an exogenous agent.

Prion protein (PrP) with amino-proximal deletions restoring susceptibility of PrP knockout mice to scrapie

The 'protein only' hypothesis postulates that the prion, the agent causing transmissible spongiform encephalopathies, is PrP(Sc), an isoform of the host protein PrP(C). Protease treatment of prion preparations cleaves off approximately 60 N-terminal residues of PrP(Sc) but does not abrogate infectivity. Disruption of the PrP gene in the mouse abolishes susceptibility to scrapie and prion replication. We have introduced into PrP knockout mice transgenes encoding wild-type PrP or PrP lacking 26 or 49 amino-proximal amino acids which are protease susceptible in PrP(Sc). Inoculation with prions led to fatal disease, prion propagation and accumulation of PrP(Sc) in mice expressing both wild-type and truncated PrPs. Within the framework of the 'protein only' hypothesis, this means that the amino-proximal segment of PrP(C) is not required either for its susceptibility to conversion into the pathogenic, infectious form of PrP or for the generation of PrP(Sc).

Prion function and dysfunction: a structure-based scenario

Prion diseases are transmissible, neurodegenerative disorders associated with as yet incompletely defined isoforms of a cellular protein termed prion protein (PrP). We have now identified in PrP structural information compatible with nucleotide- and nucleic acid-binding. As such, PrP contains a putative nicotinamide adenine dinucleotide (NADH)-binding site. Moreover, the PrP octarepeats reveal homology to the nucleic acid-binding and strand-annealing octarepeats of mammalian heterogeneous ribonucleoprotein (RNP) A1. Therefore, PrP may have NADH-dependent oxidoreductase activity as well as A1-like functions such as nucleic acid annealing and splicing. Moreover, we propose that infectious prions are propagated through a dynamic molecular symbiosis between a ribozyme-like nucleic acid and a conformational isomer of the RNP-like prion protein. Thus, our model has important implications for the understanding and treatment of prion diseases.

The association between PrP and infectivity in scrapie and BSE infected mouse brain

The structure of the scrapie agent remains unknown. However, scrapie infectivity tends to co-sediment with an infection specific fraction of the glycoprotein PrP (PrPSc) under conditions which solubilise the normal form of this protein (PrPc); accordingly, PrP has been proposed as a candidate component of the agent. To investigate this further we have been examining a new scrapie-related murine model in conjunction with established scrapie models. A bovine spongiform encephalopathy (BSE) derived murine model has short incubation periods, high infectivity titre and low amounts of PrP deposited in the brain. A membrane fraction from scrapie/BSE infected brain is solubilised with Sarkosyl at pH > or = 9.0. Most PrP is also solubilised. In models of the disease with little deposition of the PrP in the brain, this solubilisation step is particularly effective in reducing the amounts of PrP sedimented from brain extracts. Gradient centrifugation of the sedimented fraction shows further separation of infectivity and the residual PrP. It is concluded that at least some PrPSc in the brain need not be associated directly with infectious agents but is deposited in brain solely as a pathological product of infection. However, a residual sedimentable fraction contains PrP which may be a component of the agent.

Concentration and distribution of infectivity and PrPSc following partial denaturation of a mouse-adapted and a hamster-adapted scrapie strain

PrPSc is a specific protein marker for slow infectious diseases known as the transmissible subacute spongiform encephalopathies. Although PrPSc is closely associated with infectivity, it is not known if it is the infectious agent itself, a component of the agent or merely adventitiously associated with infectivity. In the present study we demonstrate that the resistance of PrPSc to partial denaturation and of infectivity to inactivation differs markedly for two scrapie strains. Proteinase K treatment or electrophoretic analysis of partially denatured PrPSc preparations reveal a dissociation between infectivity and demonstrable PrPSc. Our findings support other evidence that not all PrPSc is required for infectivity. Our studies combined with previous biological analyses suggest that PrPSc cannot be the sole component associated with the infectious agent.

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.

Alkaloidal glycosidase inhibitors (AGIs) as the cause of sporadic scrapie, and the potential treatment of both transmissible spongiform encephalopathies (TSEs) and human immunodeficiency virus (HIV) infection

AGIs are produced by plants and microorgansims in the environment. They are absorbed from the gut, distributed throughout the body and are concentrated inside cells. AGIs alter the glycan chains of cellular glycoproteins (CGP) during their formation so that the same CGP produced by different clones of cells (and hence with different glycan chains) becomes structurally the same. Prion protein (PrP), a CGP, is rendered indestructable to cellular mechanisms (as PrPi) by the TSE infective process; it is suggested that AGIs could both cause and prevent this by altering the primary structure of PrP. HIV envelope protein, gp120, carries glycan chains that are decided by the clone of the cells by which it is produced. Each cellular clone would be expected to add a specific group of glycan chains, making the gp120 antigenically separate. As HIV infection progresses, infected clone numbers rise, the antigenic diversity of gp120 may rise as would antibody production, trying to keep pace. Antigenically stimulated CD4+ cells carrying HIV genes, increase HIV production with gp120 antigenically different from its stimulant. AGIs prevent the glycan diversity and may prevent the extension of HIV infection.

Molecular cloning of single-stranded DNA purified from scrapie-infected hamster brain

Homogenized normal and scrapie-infected hamster brains were subjected to subcellular fractionation. A single band of ssDNA corresponding to about 1.2 kb was purified by alkaline gel electrophoresis from the nucleic acid content of enriched preparations of mitochondria/tubulofilamentous particles. The ssDNA was synthesized into double-stranded DNA using Taq polymerase with four dNTP for extension. The cDNA synthesized was inserted in M13mp10, cloned and sequenced. An unusual palindromic six-base TACGTA repeat sequence was obtained and confirmed by an independent automated pathway and by cutting with a specific restriction enzyme. Comparison of the nucleotide sequence of the inserted DNA with the GenBank nucleotide database revealed no significant homology with those sequences. A probe prepared from the Nar 50 clone hybridized with the scrapie DNA band of about 1.2 kb noted above; however, no hybridization was observed with normal DNA, thus confirming the presence of ssDNA in scrapie. The presence of palindromic sequences in the scrapie genome could explain why many previous searches have revealed no evidence for a scrapie-specific nucleic acid.

Protected endogenous retroviral sequences copurify with infectivity in experimental Creutzfeldt-Jakob disease

Scrapie and Creutzfeldt-Jakob disease (CJD) are caused by infectious agents that are defined phenomenologically. No agent-specific molecules or particles have been identified. Biological properties, such as exponential agent replication and strain variation, as well as physical characteristics of infectivity indicate a protected viral structure. A host membrane glycoprotein of 34 kDa ("prion" protein) that aggregates at end stages of disease is clearly important in pathology and susceptibility to infection, but has no demonstrable infectivity in any purified or recombinant form. Thus a characterization of more viral-like molecules is important. In order to identify viral-like nucleic acids we previously developed methods to substantially purify the human CJD agent from experimentally infected hamster brains, and demonstrated selected retroviral-like LTR bands at pg levels that were insufficient for sequencing. To further define these and other viral-like sequences we cloned nucleic acids from highly infectious CJD fractions, and tested the efficacy of our methods using a selected retroviral probe. RNA extracted from an infectious 120 S Gaussian peak, which is reproducibly purified approximately 100,000 fold with respect to starting nucleic acids, and contains approximately 20% of the initial brain infectivity, was used to generate a cDNA library in a sequence independent amplification strategy for low levels of RNA (< 6 ng). Reconstituted strong stop experiments using several retroviral tRNA primers had indicated that Syrian hamster IAP (SHIAP) sequences should be present in both CJD and uninfected control fractions. Because SHIAP particles are extremely resistant to denaturation, their representation in a cDNA library would imply adequate extraction of other protected RNAs of viral origin. At least 900 bases of the Syrian hamster retroviral IAP genome were unambiguously identified in the cDNA library, and in independent PCR walks with selected primers, all of which were based on our cloned sequences. Sequencing confirmed the presence of protected LTR and adjacent retroviral motifs. Because these sequences were also present in control preparations they may represent normal endogenous viral contaminants that cosediment with infectivity in size and density gradients. On the other hand, LTRs can drive the expression of many diverse sequences, and it remains to be seen if CJD specific sequences are either transduced, or copackaged with, protected IAP complexes. The effective extraction and amplification of highly protected SHIAP nucleic acids of significant length sets the stage for identifying additional protected viral elements that may specify the CJD agent.

Amphotericin B treatment dissociates in vivo replication of the scrapie agent from PrP accumulation

Scrapie and related animal and human disorders are neurodegenerative diseases characterized by the formation of a modified, partly proteinase-resistant protein (PrP) of the host, which tends to aggregate as amyloid fibrils and accumulate in the brain of infected individuals. There is a general consensus that the pathological form of PrP (PrPSc) is essential for the clinical appearance of the disease, but whether it is part of the scrapie agent or a by-product of viral infection is still controversial. Here we report that treatment of scrapie-infected hamsters with amphotericin B delays the accumulation in the brain of the proteinase-resistant portion of PrPSc by about 30 days without affecting scrapie replication. The consequence is that hamsters treated with amphotericin B developed clinical signs of disease later than infected controls. We argue that the proteinase-resistant portion of PrPSc is necessary for the development of the disease but that it is unlikely to be essential for scrapie replication.

Transmissable spongiform encephalopathy (TSE) agents as crystalline forms of the prion protein (PrP) that multiply by allowing normal metabolic forms of PrP to join the crystal

The prion protein (PrP), is found only in the brain of animals infected with TSE. It is a modified form of a normal protein (PrPn) produced from the genome of the animal. The modification prevents breakdown by proteinases and hence its total chemical or physical structure is unknown. The finding of fibre-like structures in microglia and neurones that cross-react with antibodies produced against PrP and the rapid turnover of PrPn may mean that normal biochemical pathway PrPn forms can join a crystal seed of PrP to produce these fibres. This hypothesis, that the modification of PrP is physical rather than chemical, avoids the major problems with theories of PrP as the infective agent.

A 'unified theory' of prion propagation

There is now very persuasive evidence that the transmissible agent for spongiform encephalopathies such as scrapie, consists of a modified form of the normal host protein PrPc, devoid of any nucleic acid. On the other hand, because there are many different strains of scrapie agent with distinct phenotypes which can be propagated in animals homozygous for the PrPc gene, it has been suggested that a nucleic acid must be a component of the agent. Can the two views be reconciled?

"Life, Jim, but not as we know it"? Transmissible dementias and the prion protein

The spongiform encephalopathies are unusual in several respects. Firstly, they are transmissible, and in some cases inheritable. Secondly, variants of these disorders occur in many species and can be transmitted by consumption of infected material; this has led to concern as to the potential risk from eating contaminated animal products. Thirdly, increasing evidence suggests that a 'prion' protein is central to their aetiology and pathogenesis, and that no nucleic acid is involved in the infective process. The role of the prion gene and its protein is outlined and proposed as the basis for an improved classification of the transmissible dementias.

Protease sensitivity and nuclease resistance of the scrapie agent propagated in vitro in neuroblastoma cells

The scrapie agent has been propagated in vitro in mouse neuroblastoma cells. To further characterize the tissue culture-derived scrapie agent, we studied the effects of protease and nuclease digestion on the agent derived from these cells. The scrapie agent in these cells was found to be resistant to protease digestions for short times but was inactivated by prolonged digestion at high protease concentrations. In contrast, digestion with a variety of nucleases did not alter the agent titer. These results demonstrate that the agent requires an essential protein or proteins for infectivity. If the agent also contains a nucleic acid genome, it must be more nuclease resistant than the majority of cellular DNA and RNA. These properties of the tissue culture-derived scrapie agent were identical to those of brain-derived scrapie agent and thus cannot be attributed to secondary effects of tissue pathology, since the infected cell cultures show no cytopathic effects as a result of infection.

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.