Further studies of the infectivity and stability of extracts and homogenates derived from scrapie affected mouse brains

Cellular toxicity of scrapie prions in prion diseases; a biochemical and molecular overview

Transmissible spongiform encephalopathies (TSEs) or prion diseases consist of a broad range of fatal neurological disorders affecting humans and animals. Contrary to Watson and Crick's 'central dogma', prion diseases are caused by a protein, devoid of DNA involvement. Herein, we briefly review various cellular and biological aspects of prions and prion pathogenesis focusing mainly on historical milestones, biosynthesis, degradation, structure-function of cellular and scrapie forms of prions .

Stability of BSE infectivity towards heat treatment even after proteolytic removal of prion protein

The unconventional infectious agents of transmissible spongiform encephalopathies (TSEs) are prions. Their infectivity co-appears with PrPSc, aberrant depositions of the host's cellular prion protein (PrPC). Successive heat treatment in the presence of detergent and proteolysis by a keratinase from Bacillus licheniformis PWD-1 was shown before to destroy PrPSc from bovine TSE (BSE) and sheep scrapie diseased brain, however data regarding expected reduction of infectivity were still lacking. Therefore, transgenic Tgbov XV mice which are highly BSE susceptible were used to quantify infectivity before and after the bovine brain treatment procedure. Also four immunochemical analyses were applied to compare the levels of PrPSc. After heating at 115 °C with or without subsequent proteolysis, the original BSE infectivity of 106.2-6.4 ID50 g-1 was reduced to a remaining infectivity of 104.6-5.7 ID50 g-1 while strain characteristics were unaltered, even after precipitation with methanol. Surprisingly, PrPSc depletion was 5-800 times higher than the loss of infectivity. Similar treatment was applied on other prion strains, which were CWD1 in bank voles, 263 K scrapie in hamsters and sheep PG127 scrapie in tg338 ovinized mice. In these strains however, infectivity was already destroyed by heat only. These findings show the unusual heat resistance of BSE and support a role for an additional factor in prion formation as suggested elsewhere when producing prions from PrPC. Leftover material in the remaining PrPSc depleted BSE preparation offers a unique substrate for searching additional elements for prion infectivity and improving our concept about the nature of prions.

What Makes a Prion: Infectious Proteins From Animals to Yeast

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

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.

Biology and genetics of prions causing neurodegeneration

Prions are proteins that acquire alternative conformations that become self-propagating. Transformation of proteins into prions is generally accompanied by an increase in β-sheet structure and a propensity to aggregate into oligomers. Some prions are beneficial and perform cellular functions, whereas others cause neurodegeneration. In mammals, more than a dozen proteins that become prions have been identified, and a similar number has been found in fungi. In both mammals and fungi, variations in the prion conformation encipher the biological properties of distinct prion strains. Increasing evidence argues that prions cause many neurodegenerative diseases (NDs), including Alzheimer's, Parkinson's, Creutzfeldt-Jakob, and Lou Gehrig's diseases, as well as the tauopathies. The majority of NDs are sporadic, and 10% to 20% are inherited. The late onset of heritable NDs, like their sporadic counterparts, may reflect the stochastic nature of prion formation; the pathogenesis of such illnesses seems to require prion accumulation to exceed some critical threshold before neurological dysfunction manifests.

Prion propagation: the role of protein dynamics

The transfer of phenotypes from one individual to another is a fundamental aspect of biology. In addition to traditional nucleic acid-based genetic determinants, unique proteins known as prions can also act as elements of inheritance, infectivity, and disease. Nucleic acids and proteins encode genetic information in distinct ways, either in the sequence of bases in DNA or RNA or in the three dimensional structure of the polypeptide chain. Given these differences in the nature of the genetic repository, the mechanisms underlying the transmission of nucleic acid-based and protein-based phenotypes are necessarily distinct. While the appearance, persistence and transfer of nucleic acid determinants require the synthesis of new polymers, recent studies indicate that prions are propagated through dynamic transitions in the structure of existing protein.

Inactivation of transmissible degenerative encephalopathy agents: A review

The unconventional agents that cause transmissible degenerative encephalopathies, such as bovine spongiform encephalopathy, scrapie, and Creutzfeldt-Jakob disease (CJD), are relatively resistant to inactivation by standard decontamination procedures. The only methods that appear to be completely effective under worst-case conditions are strong sodium hypochlorite solutions or hot solutions of sodium hydroxide. Other procedures that result in significant degrees of inactivation are described. The infectivity levels in histologically-fixed tissue can be reduced substantially by treatment with concentrated formic acid without adversely affecting the microscopic quality of the tissue.

Prion rods contain an inert polysaccharide scaffold

A polysaccharide consisting of mainly 1,4-linked glucose units was found associated with prion rods, which are composed mainly of insoluble aggregates of the N-terminally truncated prion protein (PrP 27-30) exhibiting the ultrastructural and tinctorial properties of amyloid. The polysaccharide differs in composition from the Asn-linked oligosaccharides and the GPI-anchor of the prion protein. Prion rods were prepared from scrapie-infected hamster brains using two different purification protocols. Prolonged digestion of rods with proteinase K reduced PrP by a factor of at least 500, leaving about 10% (w/w) of the sample as an insoluble remnant. Only glucose was obtained by acid hydrolysis of the remnant and methylation analysis showed 80% 1,4-, 15% 1,6- and 5% 1,4,6-linked glucose units. The physical and chemical properties as well as the absence of terminal glucose units indicate a very high molecular mass of the polysaccharide. No evidence was found for covalent bonds between PrP and the polysaccharide. The polysaccharide certainly contributes to the unusual chemical and physical stability of prion rods, acting like a scaffold. A potential structural and/or functional relevance of the polysaccharide scaffold is discussed.

Comparative analysis of scrapie agent inactivation methods

A scrapie-infected hamster brain homogenate was subjected to several different potential inactivation methods. Methods included autoclaving for various lengths of time, either alone or in combination with different concentrations of sodium hydroxide or LpH, an aqueous acid phenolic derivative (Calgon Vestal Laboratories in St. Louis, MO). Inactivation treatments utilizing either NaOH or LpH alone were also evaluated. It was determined that several of the treatments inactivated all of the detectable infectivity.

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

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

Molecular biology of prion diseases

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

[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.

Conservation of infectivity in purified fibrillary extracts of scrapie-infected hamster brain after sequential enzymatic digestion or polyacrylamide gel electrophoresis

Infectious extracts of scrapie-infected hamster brain enriched for scrapie-associated fibrils and scrapie amyloid protein (PrP) were partially denatured and subjected to either polyacrylamide gel electrophoresis with subsequent isolation of the PrP band or sequential enzymatic digestion with deglycosidase, phospholipase, proteinase, and several different nucleases. Infectivity measurements of these various specimens revealed a convincing association between infectivity and scrapie amyloid protein, with or without its sugar chains and disulfide bonds, and did not support the hypothesis that nucleic acid is involved in replication.

Can potential hazard of Creutzfeldt-Jakob disease infectivity be reduced in the production of human growth hormone? Inactivation experiments with the 263K strain of scrapie. Rapid communication

Scrapie infectivity is reduced 5-6 logs following filtration through 100,000 MW cut-off filter plus overnight treatment with 6 M urea. These steps, applied to purified human Growth Hormone (hGH), increase the margin of safety of hGH.

Molecular hybridization studies with scrapie brain nucleic acids. I. Search for specific DNA sequences

Chromatography and hybridization techniques employing scrapie enriched fractions of hamster brains were employed to detect a scrapie-specific DNA molecule. 125I-labeled DNA from eight different scrapie-enriched hamster brain fractions was hybridized to total DNA and RNA from normal and scrapie hamster and mouse and to DNA from normal human brain and brain tissue from patients dying with Creutzfeldt-Jakob disease. Enrichment for infectivity was obtained by cellular partition, gel filtration and gel electrophoresis. Reassociation of the probes with the scrapie DNA did not have a higher value than with the normal. The level of detection in these studies indicated that if scrapie were a DNA virus replicating through DNA its specific infectivity would be lower than 687 molecules per infectious unit. These findings weaken the possibility that scrapie is a DNA virus.

Scrapie PrP 27-30 is a sialoglycoprotein

The major scrapie prion protein, designated PrP 27-30, exhibited both charge and size heterogeneity after purification from infected hamster brains. Eight or more discrete charge isomers of PrP 27-30 with isoelectric points ranging from approximately pH 4.6 to 7.9 were found by using non-equilibrium pH gradient electrophoresis in the first dimension followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the second dimension. The charge isomers were detected by silver staining as well as by radioiodination. The procedures used to disaggregate PrP 27-30 before electrophoresis in the first dimension do not appear to be responsible for the charge heterogeneity. However, heating PrP 27-30 to 100 degrees C for 15 min in 0.1 N NaOH or 0.1 N HCl resulted in modification of the protein and alteration of its electrophoretic pattern. A PrP 27-30 fragment (molecular weight, 17,100 to 21,900) obtained by cyanogen bromide cleavage also exhibited charge and size heterogeneity. Periodic acid-Schiff staining of PrP 27-30 electrophoresed into sodium dodecyl sulfate-polyacrylamide gels demonstrated that carbohydrate residues are attached to the protein. Digestion of PrP 27-30 with neuraminidase and endo-beta-N-acetylglucosaminidase H resulted in significant changes in the isoelectric pH of PrP 27-30 isomers, whereas digestion with alkaline phosphatase had no effect. Our results demonstrate that PrP 27-30 is a sialoglycoprotein; this is consistent with several properties of this protein and of the scrapie prion.

Disinfection studies with two strains of mouse-passaged scrapie agent. Guidelines for Creutzfeldt-Jakob and related agents

A variety of disinfection procedures were tested on two strains of scrapie agent, treated either as brain macerates (autoclaving) or as 10% homogenates (chemical treatments). It is suggested that a given treatment should produce a titre loss, of both strains of scrapie, of at least 10(4) units before it be regarded as useful for the disinfection of the agents of scrapie and Creutzfeldt-Jakob disease (CJD). By this criterion, treatment at room temperature with about 4% Hycolin (0.6% chlorinated phenols), 0.2% permanganate, 5% Tego (dodecyl-di(aminoethyl)-glycine) or 5% sodium dodecyl sulphate (SDS) are unsuitable. However, data indicate that SDS might be used to reduce the heat stability of scrapie agent. Hypochlorite (Sterilex) was the only satisfactory chemical reagent tested. At least 10(4)-10(5) units of infectivity were lost by treatment with hypochlorite containing 1,000 ppm available chlorine after a 4-16 h exposure, or containing 10,000 ppm available chlorine after a half-hour exposure. The latter result points to the use of concentrated hypochlorite (about 2% available chlorine; approximately 20% Sterilex) to decontaminate surfaces. We suggest that the cleaning action of SDS, or other strong detergents, might also help to decontaminate surfaces, but studies on this are needed. Autoclaving at 126 degrees C for 1-2 h reduced titres by 10(3)-10(7) units, depending on the strain of agent. However, total disinfection of brain containing high titres of infectivity was approached only at 136 degrees C when titre losses of about 10(6) units were obtained by autoclaving for 4-32 min. Further studies are needed before we can make simple, general recommendations for the disinfection of CJD agents in hospital practice.

Novel proteinaceous infectious particles cause scrapie

After infection and a prolonged incubation period, the scrapie agent causes a degenerative disease of the central nervous system in sheep and goats. Six lines of evidence including sensitivity to proteases demonstrate that this agent contains a protein that is required for infectivity. Although the scrapie agent is irreversibly inactivated by alkali, five procedures with more specificity for modifying nucleic acids failed to cause inactivation. The agent shows heterogeneity with respect to size, apparently a result of its hydrophobicity; the smallest form may have a molecular weight of 50,000 or less. Because the novel properties of the scrapie agent distinguish it from viruses, plasmids, and viroids, a new term "prion" is proposed to denote a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids. Knowledge of the scrapie agent structure may have significance for understanding the causes of several degenerative diseases.

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.

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.

Electrophoretic properties of the scrapie agent in agarose gels

The molecular properties of the scrapie agent were investigated by subjecting partially purified preparations to electrophoresis on agarose gels. When electrophoresis was performed at room temperature in the presence of sodium dodecyl sulfate (NaDodSO4), most of the recoverable agent was found at the top of the gel, consistent with previous studies indicating aggregation of the agent upon exposure to elevated temperatures. In addition, less than 5% of the agent applied to the gel was found after electrophoresis, even though the study was performed with a low concentration of NaDodSO4 (0.1%). Further studies on the inactivation of the agent by NaDodSO4 suggest that this may be, in part, a function of the NaDodSO4: protein ratio in the sample. In contrast, sodium N-lauroyl sarcosinate (Sarkosyl) did not inactivate the agent in concentrations as high as 5% (wt/vol). Virtually all of the infectivity could be recovered after electrophoresis of the agent into 0.6% agarose gels at 4 degrees C in the presence of 0.2% Sarkosyl. Digestion of the preparations with micrococcal nuclease and proteinase K prior to Sarkosyl electrophoresis caused a substantial portion of the agent to migrate ahead of DNA fragments of 1 x 10(6) daltons. The behavior of the scrapie agent in electrophoretic gels is consistent with earlier studies showing that the monomeric form of the agent has a sedimentation coefficient of less than or equal to 40 S. Thus, the smallest or monomeric form of the agent is smaller than any known animal virus.

Precautions in medical care of, and in handling materials from, patients with transmissible virus dementia (Creutzfeldt-Jakob disease)

We have formulated a series of precautions to be observed in caring for patients with Creutzfeldt-Jakob disease and in handling their tissues. The virus resists inactivation by simple boiling in water. Also ineffective are 10 per cent formalin, 70 per cent alcohol and ionizing and ultraviolet radiation. Autoclaving for one hour at 121 degrees C and 20 psi inactivates the agent completely. Five per cent hypochlorite, 0.03 per cent permanganate, phenolics and iodine solutions are adequate disinfectants inactivating large infective doses of the virus. Special isolation wards for afflicted patients seem unwarranted. Workers exposed to infected saliva, nasopharyngeal secretions, urine or feces need to and should wash thoroughly with ordinary soap. Needles and needle electrodes should be autoclaved or incinerated and discarded. Demented persons should not be used for donations of blood or other tissues. Although precautions are necessary, the epidemiologic evidence does not suggest an unusual risk of Creutzfeldt-Jakob disease for medical workers.

Nature of the scrapie agent: evidence against a viroid

The postulated viroid nature of scrapie agent was tested. Since the agent could not be recovered after a carefully controlled phenol extraction of infected mouse brains, it is suggested that this agent is not a viroid.

Scrapie and transmissible mink encephalopathy: search for infectious nucleic acid

Brain preparations from animals with scrapie or transmissible mink encephalopathy were phenol extracted and examined for the presence of pathogenic nucleic acid. Animals inoculated with various extracts remained healthy, and analysis on 2.6 to 5% polyacrylamide gels failed to detect a difference in extractable RNA species between infected and normal mink brain.

Multiplicity of Scrapie virus in infected mouse spleen cells in vivo

Subpopulations of spleen cells from scrapie virus-infected mice were used to determine the average virus content of infected cells in vivo at a time when virus was rapidly increasing in titer in lymphoreticular tissues. Comparison of the mean lethal doses of lysed to intact cells indicated averages of 2 to 6 infectious units per infected cell. In another experiment, preparations of cytoplasmic nucleic acids extracted from spleen cells of infected mice had no detectable infectivity, which suggests that the transmissible form of the virus is not a free nucleic acid.

Scrapie-induced changes in the percentage of polymorphonuclear neutrophils in mouse peripheral blood

A decrease in the percentage of polymorphonuclear neutrophils (PMN) in the peripheral blood of mice appeared 3 days after intracerebral (IC) inoculation with scrapie mouse brain homogenate. Mice inoculated IC with normal mouse brain had PMN percentages similar to those found for uninoculated mice. This difference between normal and scrapie-inoculated mice continued throughout the preclinical phase of the disease. In the clinical phase of the disease, the percentage of PMN was either higher or lower than that found in normals. The factor causing the decrease in PMN percentages was found in the filtrates from 220-, 100-, and 50-nm filters, but not in the filtrates from a 25-nm filter. Sodium periodate treatment of the scrapie brain samples eliminated their ability to cause the decrease in PMN percentages, whereas sodium iodate had no effect. In addition to two genetically different scrapie mouse brain isolates, homogenates of mouse spleen, sheep brain, and sheep spleen from scrapie-affected animals caused a decrease in percent PMN, whereas the corresponding normal tissue homogenates did not.