Electrophoretic properties of the scrapie agent in agarose gels

Detection and Quantification of CWD Prions in Fixed Paraffin Embedded Tissues by Real-Time Quaking-Induced Conversion

Traditional diagnostic detection of chronic wasting disease (CWD) relies on immunodetection of misfolded CWD prion protein (PrP^CWD) by western blotting, ELISA, or immunohistochemistry (IHC). These techniques require separate sample collections (frozen and fixed) which may result in discrepancies due to variation in prion tissue distribution and assay sensitivities that limit detection especially in early and subclinical infections. Here, we harness the power of real-time quaking induced conversion (RT-QuIC) to amplify, detect, and quantify prion amyloid seeding activity in fixed paraffin-embedded (FPE) tissue sections. We show that FPE RT-QuIC has greater detection sensitivity than IHC in tissues with low PrP^CWD burdens, including those that are IHC-negative. We also employ amyloid formation kinetics to yield a semi-quantitative estimate of prion concentration in a given FPE tissue. We report that FPE RT-QuIC has the ability to enhance diagnostic and investigative detection of disease-associated PrP^RES in prion, and potentially other, protein misfolding disease states.

CJD and Scrapie Require Agent-Associated Nucleic Acids for Infection

Unlike Alzheimer's and most other neurodegenerative diseases, Transmissible Spongiform Encephalopathies (TSEs) are all caused by actively replicating infectious particles of viral size and density. Different strain-specific TSE agents cause CJD, kuru, scrapie and BSE, and all behave as latent viruses that evade adaptive immune responses and can persist for years in lymphoreticular tissues. A foreign viral structure with a nucleic acid genome best explains these TSE strains and their endemic and epidemic spread in susceptible species. Nevertheless, it is widely believed that host prion protein (PrP), without any genetic material, encodes all these strains. We developed rapid infectivity assays that allowed us to reproducibly isolate infectious particles where >85% of the starting titer separated from the majority of host components, including PrP. Remarkably, digestion of all forms of PrP did not reduce brain particle titers. To ask if TSE agents, as other viruses, require nucleic acids, we exposed high titer FU-CJD and 22L scrapie particles to potent nucleases. Both agent-strains were propagated in GT1 neuronal cells to avoid interference by complex degenerative brain changes that can impede nuclease digestions. After exposure to nucleases that are active in sarkosyl, infectivity of both agents was reproducibly reduced by ≥99%. No gold-stained host proteins or any form of PrP were visibly altered by these nucleases. In contrast, co-purifying protected mitochondrial DNA and circular SPHINX DNAs were destroyed. These findings demonstrate that TSE agents require protected genetic material to infect their hosts, and should reopen investigation of essential agent nucleic acids. J. Cell. Biochem. 117: 1947-1958, 2016. © 2016 Wiley Periodicals, Inc.

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.

Proteinase K and the structure of PrPSc: The good, the bad and the ugly

Infectious proteins (prions) are, ironically, defined by their resistance to proteolytic digestion. A defining characteristic of the transmissible isoform of the prion protein (PrP(Sc)) is its partial resistance to proteinase K (PK) digestion. Diagnosis of prion disease typically relies upon immunodetection of PK-digested PrP(Sc) by Western blot, ELISA or immunohistochemical detection. PK digestion has also been used to detect differences in prion strains. Thus, PK has been a crucial tool to detect and, thereby, control the spread of prions. PK has also been used as a tool to probe the structure of PrP(Sc). Mass spectrometry and antibodies have been used to identify PK cleavage sites in PrP(Sc). These results have been used to identify the more accessible, flexible stretches connecting the β-strand components in PrP(Sc). These data, combined with physical constraints imposed by spectroscopic results, were used to propose a qualitative model for the structure of PrP(Sc). Assuming that PrP(Sc) is a four rung β-solenoid, we have threaded the PrP sequence to satisfy the PK proteolysis data and other experimental constraints.

Quantitative assessment of prion infectivity in tissues and body fluids by real-time quaking-induced conversion

Prions are amyloid-forming proteins that cause transmissible spongiform encephalopathies through a process involving the templated conversion of the normal cellular prion protein (PrP(C)) to a pathogenic misfolded conformation. Templated conversion has been modelled in several in vitro assays, including serial protein misfolding amplification, amyloid seeding and real-time quaking-induced conversion (RT-QuIC). As RT-QuIC measures formation of amyloid fibrils in real-time, it can be used to estimate the rate of seeded conversion. Here, we used samples from deer infected with chronic wasting disease (CWD) in RT-QuIC to show that serial dilution of prion seed was linearly related to the rate of amyloid formation over a range of 10(-3) to 10(-8) µg. We then used an amyloid formation rate standard curve derived from a bioassayed reference sample (CWD+ brain homogenate) to estimate the prion seed concentration and infectivity in tissues, body fluids and excreta. Using these methods, we estimated that urine and saliva from CWD-infected deer both contained 1-5 LD50 per 10 ml. Thus, over the 1-2 year course of an infection, a substantial environmental reservoir of CWD prion contamination accumulates.

Correlation of polydispersed prion protein and characteristic pathology in the thalamus in variant Creutzfeldt-Jakob disease: implication of small oligomeric species

The vacuolation, neuronal loss and gliosis that characterize human prion disease pathology are accompanied by the accumulation of an aggregated, insoluble and protease-resistant form (termed PrP(Sc)) of the host-encoded normal cellular prion protein (PrP(C)). In variant Creutzfeldt-Jakob disease the frontal cortex and cerebellum exhibit intense vacuolation and the accumulation of PrP(Sc) in the form of amyloid plaques and plaque-like structures. In contrast the posterior thalamus is characterized by intense gliosis and neuronal loss, but PrP(Sc) plaques are rare and vacuolation is patchy. We have used sucrose density gradient centrifugation coupled with conformation dependent immunoassay to examine the biochemical properties of the PrP(Sc) that accumulates in these different brain regions. The results show a greater degree of PrP(Sc) polydisperal in thalamus compared with frontal cortex or cerebellum, including a subpopulation PrP(Sc) molecules in the thalamus that have sedimentation properties resembling those of PrP(C). Much effort has focused on identifying aspects of PrP(Sc) biochemistry that distinguish between different forms of human prion disease and contribute to differential diagnosis. Here we show that PrP(Sc) sedimentation properties, which can depend on aggregation state, correlate with, and may underlie the distinct neurodegenerative processes occurring in different regions of the variant Creutzfeldt-Jakob disease brain.

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.

Discovering DNA encodes heredity and prions are infectious proteins

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

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.

Attempts to restore scrapie prion infectivity after exposure to protein denaturants

A wealth of experimental evidence argues that infectious prions are composed largely, if not entirely, of the scrapie isoform of the prion protein. We attempted to restore scrapie infectivity after exposure to protein denaturants including urea, chaotropic salts, and SDS. None of the procedures restored infectivity. Dialysis to remove slowly chaotropic ions and urea failed to restore scrapie infectivity. Attempts to create monomers of the scrapie isoform of the prion protein under nondenaturing conditions using a wide variety of detergents have been unsuccessful, to date, except for one report claiming that scrapie infectivity could be recovered from 12% polyacrylamide gels after SDS/PAGE [Brown, P., Liberski, P. P., Wolff, A. & Gajdusek, D. C. (1990) Proc. Natl. Acad. Sci. USA 87, 7240-7244]. We found that < 0.001% of the infectious prion titer could be recovered from the region of a polyacrylamide gel where the denatured proteinase K-resistant core of the scrapie isoform of the prion protein and other 30-kDa proteins migrate. We conclude that under the denaturing conditions used for SDS/PAGE, the scrapie isoform of the prion protein is denatured and little or no renaturation occurs upon injection of fractions eluted from gels into animals for bioassays.

Structural studies of the scrapie prion protein using mass spectrometry and amino acid sequencing

The only component of the infectious scrapie prion identified to date is a protein designated PrPSc. A posttranslational process converts the cellular PrP isoform (PrPC) into PrPSc. Denatured PrPSc was digested with endoproteases, and the resulting fragments were isolated by HPLC. By both mass spectrometry and Edman sequencing, the primary structure of PrPSc was found to be the same as that deduced from the PrP gene sequence, arguing that neither RNA editing nor protein splicing feature in the synthesis of PrPSc. Mass spectrometry also was used to search for posttranslational chemical modifications other than the glycosylinositol phospholipid anchor attached to the C-terminus and two Asn-linked oligosaccharides already known to occur on both PrPSc and PrPC. These results contend that PrPSc molecules do not differ from PrPC at the level of an amino acid substitution or a posttranslational chemical modification; however, we cannot eliminate the possibility that a small fraction of PrPSc is modified by an as yet unidentified posttranslational process or that PrPC carries a modification that is removed in the formation of PrPSc. It seems likely that PrPSc differs from PrPC in its secondary and tertiary structure, but the possibility of a tightly bound, disease-specific molecule which purifies with PrPSc must also be considered.

[Prions]

Os autores se propõem a revisar alguns aspectos básicos sobre os prions, alertando sobre a possível participação destes na etiologia de algumas enfermidades degenerativas do sistema nervoso.

Molecular biology and transgenetics of prion diseases

Considerable progress has been made deciphering the role of an abnormal isoform of the prion protein (PrP) in scrapie of animals and Gerstmann-Sträussler syndrome (GSS) of humans. Some transgenic (Tg) mouse (Mo) lines that carry and express a Syrian hamster (Ha) PrP gene developed scrapie 75 d after inoculation with Ha prions; non-Tg mice failed to show symptoms after greater than 500 d. Brains of these infected Tg(HaPrP) mice featured protease-resistant HaPrPSc, amyloid plaques characteristic for Ha scrapie, and 10(9) ID50 units of Ha-specific prions upon bioassay. Studies on Syrian, Armenian, and Chinese hamsters suggest that the domain of the PrP molecule between codons 100 and 120 controls both the length of the incubation time and the deposition of PrP in amyloid plaques. Ataxic GSS in families shows genetic linkage to a mutation in the PrP gene, leading to the substitution of Leu for Pro at codon 102. Discovery of a point mutation in the Prp gene from humans with GSS established that GSS is unique among human diseases--it is both genetic and infectious. These results have revised thinking about sporadic Creutzfeldt-Jakob disease, suggesting it may arise from a somatic mutation. These findings combined with those from many other studies assert that PrPSc is a component of the transmissible particle, and the PrP amino acid sequence controls the neuropathology and species specificity of prion infectivity. The precise mechanism of PrPSc formation remains to be established. Attempts to demonstrate a scrapie-specific nucleic acid within highly purified preparations of prions have been unrewarding to date. Whether transmissible prions are composed only of PrPSc molecules or do they also contain a second component such as small polynucleotide remains uncertain.

Prion liposomes

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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.

The nature of the scrapie agent

There now seems little doubt that the infective agent of scrapie cannot be accommodated within current concepts of virology/molecular biology. It is proposed: that the basic infective entity is a nucleic acid fragment (oligonucleotide) of some 40 bp coupled with specific (but host encoded) protein totalling approximately 10(5) daltons, a significant proportion of which is in the form of proteolipid; that the nucleic acid fragment reprograms the host cell on the chemically switched microprocessor network analogy already proposed; that the nucleic acid fragment has no initiation sequence for replication: it is therefore non-infective; that the nucleic acid fragment can replicate when associated with the specific protein component because the resulting complex is able to displace mobile genetic element flanking sequences (similar to the yeast delta sequence). The function of the protein is to provide a scaffolding which allows the nucleic acid fragment to be assimilated into the replication cycle of the mobile genetic element as a whole.

Separation and properties of cellular and scrapie prion proteins

Purified preparations of scrapie prions contain a sialoglycoprotein of Mr 27,000-30,000, designated PrP 27-30, which is derived from the scrapie prion protein [Mr, 33,000-35,000 (PrP 33-35Sc)] by limited proteolysis. Under these same conditions of proteolysis, a cellular protein of the same size (PrP 33-35C) is completely degraded. Subcellular fractionation of hamster brain showed that both PrP 33-35Sc and PrP 33-35C were found only in membrane fractions. NaCl, EDTA, and osmotic shock failed to release the prion proteins from microsomal membranes. Electron microscopy of these microsomal fractions showed membrane vesicles but not prion amyloid rods. Detergent treatment of scrapie-infected membranes solubilized PrP 33-35C, while PrP 33-35Sc aggregated into amyloid rods; the concentration of PrP 33-35C was similar to that recovered from analogous fractions prepared from uninfected control brains. The apparent amphipathic character of the PrP 33-35Sc may explain the association of scrapie infectivity with both membranes and amyloid filaments.

Scrapie infectious agent is virus-like in size and susceptibility to inactivation

The virions of all known viruses are composed of small amounts of genomic nucleic acid enveloped by proteins and other macromolecules. The aetiological agents of scrapie disease and the other subacute spongiform virus encephalopathies (SSVE), a group of slow, fatal degenerative diseases of the central nervous system, are, based on their resistance to sterilization and on indirect measurements suggesting subviral size, thought to have non-viral structures (see refs 1-3 for reviews). The kinetic studies reported here demonstrate that scrapie's resistance to many inactivants is limited to small subpopulations of the total infectivity, the majority population being highly sensitive to inactivation. Moreover, control inactivations of conventional viruses provide examples of both scrapie-like resistant subpopulations and complete insensitivity to virucidal agents, especially when those viruses, like scrapie, are suspended in hamster brain homogenate. Virus controls further establish that the ability of the scrapie agent to penetrate dilute agarose-acrylamide electrophoretic gels is shared by conventional viruses. Direct comparison of scrapie's resistance to ionizing radiation with the resistances of other viruses places scrapie with the smaller viruses, as opposed to requiring a subviral size as claimed.

Towards purification of the scrapie agent

A method for the partial purification of scrapie infectivity from hamster brain is described. About a 100-1000-fold, 20-fold, and 200-fold enrichment in scrapie infectivity with respect to protein, RNA, and DNA content has been achieved using differential centrifugation, enzyme and detergent treatment. The inbred CLAC strain of hamsters used in our experiments contained about 10 times less infectivity in brain than has been found in randomly bred animals or other inbred strains.

Conditions for the chemical and physical inactivation of the K. Fu. strain of the agent of Creutzfeldt-Jakob disease

The unusual resistance of the "unconventional viruses" to inactivation by the commonly used disinfectants has led to a high degree of apprehension regarding patients with any form of dementia. The rapid adaptation of a newly acquired isolate of the agent of Creutzfeldt-Jakob Disease (CJD) to mice made possible this large scale study of its heat and chemical stability. The agent showed a decrease in titer of approximately two logs following incubation at 80 degrees C for 30 minutes with no additional loss at 80 degrees C for up to 500 minutes. There was greater than a three log decrease in titer at 100 degrees C for 30 minutes and temperatures of 115 degrees and 130 degrees C completely inactivated the agent. Treatment with sodium hypochlorite at three concentrations (0.33 per cent, 0.66 per cent and 1.31 per cent) showed inactivation of greater than 99 per cent at each. Crude agent preparations were not inactivated by sodium dodecylsulfate at detergent to protein ratios up to 4:1. These results suggest that those hospital supplies which resist autoclaving may be adequately disinfected by autoclaving for at least 30 minutes. Treatment of surfaces with solutions of sodium hypochlorite (household bleach) at concentrations of 15 to 25 per cent is also effective. Detergent treatment of contaminated surfaces or materials is inadequate for proper decontamination.

Viroids and prions

Viroids are small "naked" infectious RNA molecules that are pathogens of higher plants. The potato spindle tuber viroid (PSTV) is composed of a covalently closed circular RNA molecule containing 359 ribonucleotides. The properties of PSTV were compared with those of the scrapie agent, which causes a degenerative neurological disease in animals. PSTV was inactivated by ribonuclease digestion, psoralen photoadduct formation, Zn2+ -catalyzed hydrolysis, and chemical modification with NH2OH. The scrapie agent resisted inactivation by these procedures, which modify nucleic acids. The scrapie agent was inactivated by proteinase K and trypsin digestion, chemical modification with diethylpyrocarbonate, and by exposure to phenol, NaDodSO4, KSCN, or urea. PSTV resisted inactivation by these procedures, which modify proteins. Earlier evidence suggested that the scrapie agent is smaller than PSTV. Its small size seems to preclude the presence of a genome coding for the protein(s) of a putative capsid. The properties of the scrapie agent distinguish it from both viroids and viruses and have prompted the introduction of the term "prion" to denote a small proteinaceous infectious particle that resists inactivation by procedures that modify nucleic acids.

Measurement of the scrapie agent using an incubation time interval assay

The titer of the scrapie agent was determined by measurements of time intervals from inoculation to onset of illness and from inoculation to death. Both intervals were found to be inversely proportional to the size of the dose injected intracerebrally into random-bred weanling Syrian hamsters. The logarithms of the time intervals minus a time factor were linear functions of the logarithm of the inoculum size. The time factors were determined by regression analysis in order to maximize these linear relationships. An equation relating the titer of the inoculum to the dilution of the sample and the length of the time intervals was developed. This equation facilitates the use of a computerized data base. Validation of these relationships was provided by comparing samples for which the agent was measured both by end-point titration and by time interval assay. Agreement between the two methods was generally within +/-0.5 log10 median infective dose units. No differences between the molecular properties of the agents from hamster and murine sources were observed using primarily the incubation time interval method with the former and end-point titration with the latter. The advantages of this new approach based on time interval measurements are considerable with respect to time and resources.

Scrapie agent contains a hydrophobic protein

The scrapie agent causes a degenerative nervous system disorder of sheep and goats. Considerable evidence indicates that the scrapie agent contains a protein that is necessary for infectivity [Prusiner, S. B., Groth, D. F., Cochran, S. P., Masiarz, F. R., McKinley, M. P. & Martinez, H. M. (1980) Biochemistry 19, 4883-4891], but direct demonstration of a protein moiety has been hampered by lack of sufficiently purified preparations. Employing preparations of the scrapie agent enriched 100- to 1000-fold with respect to protein, we found that digestion by proteinase K destroyed more than 99.9% of the infectivity. Diethylpyrocarbonate, which chemically modifies amino acid residues in proteins with high efficiency, also inactivated the scrapie agent in these purified preparations. Reductions of infectivity by proteinase K and diethylpyrocarbonate were not observed with less purified preparations. The agent bound to phenyl-Sepharose could not be eluted with 8.5 M ethylene glycol; however, a combination of ethylene glycol and detergents did release the agent. These observations provide good evidence for a protein and for hydrophobic domains within the scrapie agent. Whether the protein required for infectivity is the same protein responsible for the hydrophobic properties of the scrapie agent remains to be established.

Thiocyanate and hydroxyl ions inactivate the scrapie agent

To probe the macromolecular structure of the scrapie agent and explore conditions for monomerization, the stability of the agent in low concentrations of inorganic ions was determined. A reduction by a factor of 10(5) in scrapie titer was found on exposure of the agent to 1 M KSCN or 0.3 M NaOH. In addition to the inactivation by thiocyanate ions, other chaotropic ions such as guanidinium and trichloroacetate inactivate the scrapie agent. Removal of thiocyanate ions by dialysis or glass permeation chromatography prevented the reduction in scrapie agent infectivity. Addition of equimolar amounts of (NH4)2SO4, a nonchaotrope, to preparations containing 1 M KSCN also prevented the loss of scrapie infectivity. In contrast, neutralization of the alkali-treated fractions with HCl did not restore infectivity. Acidification of partially purified fractions did not cause inactivation of the agent but did result in precipitation of the infectious agent. Inactivation by relatively low concentrations of chaotropic ions is consistent with many observations, all of which suggest that the scrapie agent contains a protein component that is essential for the maintenance of infectivity. Thus, it is unlikely that the agent is composed only of a "naked" nucleic acid. Certainly, if the agent were a naked nucleic acid, its lability in alkali virtually eliminates the possibility that it is composed of a single-stranded molecule of DNA.

Determination of scrapie agent titer from incubation period measurements in hamsters

Progress in studies on the scrapie agent has been hampered by the slow and tedious endpoint titration assays in rodents. A new assay based on incubation period measurements has been developed. The incubation period is defined as the time interval from inoculation to the onset of clinically detectable neurological illness or as the interval from inoculation to death. Both of these intervals or incubation periods are inversely proportional to the size of the dose injected intracerebrally into random bred weanling syrian hamsters. The incubation period was found to be a linear function of the logarithm of the inoculum size over a wide range of dosages (10(2) - 10(8) ID50 units). From these studies an equation relating the titer of the inoculum to the dilution of the sample and the length of the incubation period has been developed facilitating the use of a computerized record system. Validation of the assay was provided by comparing samples where the agent was measured both by endpoint titration and incubation period methods. Agreement between the 2 methods was generally within +/- 0.5 log10 ID50 units. In addition, no differences between the molecular properties of the agents from hamster and murine sources have been detected using primarily the incubation period method with the former and endpoint titration with the latter. The advantages of the incubation period assay are considerable with respect to time and economy.

Experimental scrapie in the mouse: electrophoretic and sedimentation properties of the partially purified agent

Some biochemical and biophysical properties of the scrapie agent in a partially purified fraction P5 from murine spleen are described in this communication. The agent was stable in the nonionic detergents Triton X-100 and Nonidet P40 and stable in the nondenaturing, anionic detergents sodium cholate and sodium N-lauroyl sarcosinate. In contrast, sodium dodecyl sulfate (SDS) inactivated the agent at high concentrations (1% or >) when the detergent-to-protein ration approached 1.5 g SDS/g protein. The agent was resistant to inactivation by nucleases and proteases, even in the presence of 0.1% SDS. A broad peak of infectivity was exhibited in modified colloidal silica (Percoll) density gradients. Maximal titers were found at a Percoll density of 1.10 g/cm3 in the presence and absence of 0.05% SDS. Gel electrophoresis of the agent in the presence of 0.1% SDS resulted in inactivation of > 95% of the agent loaded onto the gel. Free-flow electrophoresis showed that > 99% of the agent in fraction P5 migrated toward the anode, but not as a discrete species. Sedimentation analysis of the agent in fraction P5 in the presence of 1% lysolecithin showed that the agent has a sedimentation coefficient of < 300S but > 30S. Heating P5 preparations caused the agent to associate with cellular elements and form aggregates with sedimentation coefficients > 10,000S. Removal by differential centrifugation of the large forms of the agent produced upon heating permitted characterization of a discrete subpopulation of scrapie agent particles. Rate-zonal sucrose gradient studies showed that > 95% of the infectivity in this subpopulation sedimented as uniform particles with a sedimentation coefficient of 240S.