Cloning of rat "prion-related protein" cDNA

Rat prion-related protein (PrP) cDNA has been cloned and sequenced. Comparison of this cDNA with those from human, hamster, and mouse indicates extremely high conservation (about 95%). The deduced partial rat PrP possesses: (a) a highly conserved region composed of repetitive sequences in what is presumably an extracellular domain, (b) a hydrophobic transmembrane domain, (c) a highly charged region which should stop membrane transfer, (d) a substantial cytoplasmic domain (which contains all of the nonconservative substitutions and a high proportion of conservative substitutions), and (e) a hydrophobic C-terminus. Dot and Northern blot analyses suggest a limited expression of PrP in rat tissues and indicate that PrP expression is decreased in the brain during the acute phase response systemically. Our results lend support to the notion that PrP is a highly conserved, normal cellular membrane protein of essential (although unknown) biologic function, which may be deposited in fibrillar amyloid form as a result of abnormal processing.

Changes in the localization of brain prion proteins during scrapie infection

Prion proteins (PrP) were localized in the brains of normal and scrapie-infected hamsters by immunohistochemistry and Western blotting. PrP monoclonal antibodies and monospecific anti-PrP peptide sera, which react with both the cellular (PrPC) and scrapie (PrPSc) isoforms of the prion protein, were used to locate PrP in tissue sections. In normal hamsters, PrPC was located primarily in nerve cell bodies throughout the CNS; whereas, in the terminal stages of scrapie, PrP immunoreactivity was shifted to the neuropil and was absent from most nerve cell bodies. Prion proteins were not uniformly dispersed throughout the gray matter of scrapie-infected hamster brains; rather, they were concentrated in those regions that exhibited spongiform degeneration and reactive astrogliosis. Since earlier studies showed that the level of PrPC remains constant during scrapie infection as measured in whole brain homogenates and no antibodies are presently available that can distinguish PrPC from PrPSc, we analyzed individual brain regions by Western blotting. Analysis of proteinase K-digested homogenates of dissected brain regions showed that most of the regional changes in PrP immunoreactivity that are seen during scrapie infection are due to the accumulation of PrPSc. These observations indicate that the tissue pathology of scrapie can be directly correlated with the accumulation of PrPSc in the neuropil, and they suggest that the synthesis and distribution of the prion protein has a central role in the pathogenesis of this disorder.

The comparative immunoreactivities of brain amyloids in Alzheimer's disease and scrapie

An antibody was raised to a synthetic peptide corresponding to a published sequence for the first 24 residues of a cerebrovascular amyloid peptide (CVAP). Immunohistochemical staining of tissue sections revealed that the antibody bound extensively to cerebrovascular amyloid in Alzheimer disease (AD/SDAT) and Down's syndrome cases. The antibody bound less extensively to neuritic plaques (primitive and mature) and indetectably to neurofibrillary tangles. The antibody did not label scrapie plaques, scrapie-associated fibrils, or Gerstmann-Sträussler syndrome plaques. Immunoblotting experiments showed that the cerebrovascular amyloid peptide epitopes contaminating the neurofibrillary tangle preparations could be extracted with urea, leaving the neurofibrillary tangles intact. These data confirm that the cerebrovascular amyloid peptide is a component of cerebrovascular amyloid, and suggest that its epitopes are also components of neuritic plaque amyloid. The reduced level of immunostaining on amyloid cores in tissue sections suggests that either the cerebrovascular amyloid peptide epitopes are a minor component of amyloid cores, or that their mode of packing or state of processing in amyloid cores renders them relatively inaccessible to the antibody. We also conclude that the cerebrovascular amyloid peptide is not a component of neurofibrillary tangles. The synthetic cerebrovascular amyloid peptide possesses amyloid-like properties: at neutral pH it forms insoluble aggregates consisting of 5-7-nm fibrils, which form red-green birefringent adducts with Congo red and fluoresce with thioflavine S.

Ultrastructural observations of spinal cord lesions and blood-brain barrier changes in scrapie-infected mice

Spinal cord samples from IM or VM mice injected intracerebrally with the 87V scrapie agent were examined ultrastructurally at the clinical stage of disease for changes in blood vessel permeability and for pathological alterations. In several animals, (3 of 16), massive changes were noted in the cervical spinal cords in the subependymal area of the cortical gray matter immediately surrounding the central canal including ependymal cell changes, the presence of amyloid plaque in close association with microglial cells, extensive neuropil vacuolation, the appearance of reactive astrocytes, degenerating neurites and vacuolated neurons. In those regions showing structural damage, localized increased permeability to horseradish peroxidase across the blood-brain barrier was noticed along with the appearance of numerous vesiculo-canalicular profiles in micro-blood vessel endothelial cells with extravasation of the tracer to the neuropil. Some damaged neurons appeared flooded with this tracer. These changes were not observed in either the thoracic or lumbar spinal cord regions. The occurrence of pathological changes in the spinal cords of a small percentage of intracerebrally injected mice was probably due to a high concentration of the scrapie agent which localized in the cervical spinal cord, presumably after entering the spinal fluid via the lateral ventricle at the time of injection.

Chromatin structure in scrapie and Alzheimer's disease

Scrapie affected brains exhibit a number of pathological features in common with the human neurodegenerative condition, Alzheimer's disease. The present report describes studies on chromatin structure seen in these two disease processes. Chromatin associated proteins influence transcriptional activity of DNA through an effect upon chromatin structure. We examined chromatin structure by: measuring the capacity of the enzyme micrococcal nuclease to release mono- and dinucleosomes from isolated nuclei and measuring DNA-histone interactions by examining the effect of ambient tonicity upon the release of chromatin proteins. In two strains of mice infected with two strains of scrapie agent there was reduced accessibility to micrococcal nuclease and an increased content on dinucleosomes of the histone H1 and H1(0) types. These changes precede clinical signs of scrapie and resemble those found in the human conditions of Alzheimer's and Pick's disease. Scrapie mouse brain differs from Alzheimer brain in that scrapie does not alter histone-DNA interactions as monitored by ionically induced histone release from chromatin. Despite similarities, the scrapie agent appears to operate upon different molecular mechanisms than those found in Alzheimer's disease.

Immunological comparison of scrapie-associated fibrils isolated from animals infected with four different scrapie strains

Scrapie-associated fibrils (SAFs) are abnormal filamentous structures that are uniquely associated with unconventional slow virus diseases. The antigenic relationships of SAFs from animals infected with four biologically distinct scrapie strains were investigated by using antisera raised to purified SAF proteins. Rabbit antisera were raised to SAFs isolated from mice infected with the ME7 scrapie strain and to SAFs isolated from hamsters infected with the 263K scrapie strain. A strong antigenic relationship was shown among SAF proteins (PrPs) isolated from all scrapie-infected animals (ME7, 139A, and 87V in mice and 263K in hamsters), and this relationship was demonstrable regardless of which antiserum was used. SAF proteins were antigenically distinct from those of paired helical filaments or amyloid isolated from patients with Alzheimer disease. Distinct Western blot profiles were demonstrated for SAFs isolated from animals infected with each scrapie strain. Differences seen among SAFs were independent, at least in part, of host species or genotype, implying that certain specific structural and molecular properties of SAFs are mediated by the strain of scrapie agent.

Brain histamine metabolism in transmissible spongiform encephalopathy (scrapie)

The histamine (Hi) content and the activity of L-histidine decarboxylase (HD) in brains of scrapie infected hamsters were measured. No significant changes in Hi levels in particular brain areas were found when compared to controls. Decreased activity of HD was found in hypothalamus (p less than 0.02). Increased activity of the enzyme was observed in "rest of brain", which consisted mainly of thalamus and striatum (p less than 0.05).

Pathogenesis of scrapie: study of the temporal development of clinical symptoms, of infectivity titres and scrapie-associated fibrils in brains of hamsters infected intraperitoneally

After an intraperitoneal infection of hamsters with scrapie agent, early low and constant titres of about 100 LD50/brain between days 10 to 50 were followed by a dramatic increase to maximum levels of 3 X 10(9) LD50/brain within about 15 days. The plateau of maximum infectivity remained unchanged from day 70 to the time of the first and final signs of disease at 95 and 123 days post-infection, respectively. Scrapie-associated fibrils (SAF) as measured by immunoblotting of SAF protein could not be detected before 79 days post-infection even when a total brain was used for analysis. Subsequently, the concentration of SAF increased gradually by about 100,000-fold until the time of clinical disease. The kinetics suggest a virus-induced amyloidosis of the brain as the cause of disease.

The major protein of SAF is absent from spleen and thus not an essential part of the scrapie agent

Scrapie-associated fibrils (SAF) play a controversial role in the discussion about the nature of the scrapie agent. In purification experiments SAF can be detected in brains of infected animals but not in spleen samples with similar titers of infectivity. Thus, SAF are not a constituent of the scrapie virus.

Immunoreactivity of a synthetic pentadecapeptide corresponding to the N-terminal region of the scrapie prion protein

A pentadecapeptide with an amino acid sequence corresponding to the amino-terminal region of the scrapie prion protein was synthesized. Immunization of a rabbit with the peptide conjugated with ovalbumin induced specific antibodies. The antibodies reacted with all three of the major polypeptides in a proteinase K-treated fraction obtained from brains of mice infected with the Obihiro strain of scrapie agent. Some peptides in the proteinase-untreated fraction also shared antigenicity with the three major polypeptides. Specific polypeptides were also detected by the antiserum in a fraction prepared from spleens, but only two of the three major polypeptides were found and the amounts of the polypeptides were less than in brain.

Scrapie and cellular prion proteins share polypeptide epitopes

Purified preparations of scrapie prions contain one major protein, PrP 27-30, and aggregates of rod-shaped structures. On the basis of the NH2-terminal amino acid sequence of PrP 27-30, a synthetic peptide (PrP-P1) was constructed. Monospecific rabbit antisera to PrP-P1 were found by immunoblotting to react with PrP 27-30 and its precursor (PrP 33-35Sc), as well as with a related protease-sensitive cellular homologue (PrP 33-35C). An enzyme-linked immunosorbent assay showed that rabbit antiserum to PrP 27-30 was more reactive with PrP 27-30 than with PrP-P1; conversely, antiserum to PrP-P1 was more reactive with the peptide than with the prion proteins. In addition, antibodies to PrP-P1 decorate purified prion rods and stain amyloid plaques in scrapie-infected hamster brain. The peptide epitopes shared by PrP 27-30, PrP 33-35Sc, and PrP 33-35C clearly establish a relationship among these three proteins.

Detection of scrapie-associated fibril (SAF) proteins using anti-SAF antibody in non-purified tissue preparations

Antisera raised to scrapie-associated fibril (SAF) proteins were used to detect scrapie-specific polypeptides in three different non-purified brain preparations: a synaptosomal-mitochondrial fraction, 20% brain homogenate and 20% brain homogenate extracted with Sarkosyl. The concentration of SAF proteins in the preparations was greater than the quantity of SAF as detected by negative stain electron microscopy. This suggests that not all of the protein exists in the form of SAF. An immunologically reactive 33K to 35K protein was detected in both normal and scrapie brain preparations. This protein was susceptible to complete proteinase K (PK) digestion in normal brain preparations and it is suggested that scrapie infection is responsible for post-translational modifications which confer PK resistance in scrapie preparations. These modifications may also play a role in the antigenic differences seen in a variety of scrapie agents. SAF-specific proteins were also detected in the spinal cords and spleens from scrapie-affected animals. Detergent extraction of material followed by PK treatment and Western blot analysis is a highly specific and sensitive method for the detection of SAF proteins. This procedure could be applied to human neurological diseases of unknown aetiology.

Scrapie: how much do we really understand?

Biological studies have produced convincing evidence for different scrapie strains, some of which undergo mutation. This argues strongly in favour of the infectious scrapie agent having a genome. The length of incubation period is influenced by the strain of agent but is also under strict host control. In mice, this control is exerted by a gene called Sinc which affects the overall rate of agent replication in the CNS. After peripheral infection, invasion of the CNS from lymphoreticular sites of agent replication is a key step in pathogenesis. Evidence from one scrapie model indicates spread of infection along autonomic nerves to the thoracic spinal cord and then to other parts of the CNS. Other studies have shown that infection can spread in neurons. There are close relationships between the presence of replicating agent and the development of vacuolation, and also of cerebral amyloid when it occurs. We can, therefore, begin to understand the patterns of lesion development in the brain in terms of the targeting of infection and its replication at certain sites. Structures known as SAF (Scrapie Associated Fibrils) have been discovered in extracts of scrapie brain (but not uninfected brain) and a glycoprotein (PrP 27-30: SAF protein) is a major constituent of purified SAF. The glycoprotein is coded by a single gene which is present in several species and expressed in uninfected brain. The normal protein seems to be modified in scrapie infected brain so that it accumulates as SAF. The modified protein may also be deposited as extracellular amyloid because there appear to be common epitopes between SAF and scrapie amyloid. The biochemical nature of the scrapie agent remains in doubt and the association between infectivity and purified SAF may arise fortuitously from the fact that scrapie agent is 'sticky'.

Distributions of amyloid plaques in four regions of the brains of mice infected with scrapie by intracerebral and intraperitoneal routes of injection

VM mice were inoculated by intracerebral and intraperitoneal routes with brain homogenates containing the 87V strain of scrapie. The distribution and numbers of plaques were found for the parietal cortex, cingulate cortex, corpus callosum and hippocampus/dentate in coronal sections cut at the level of the thalamus and stained with Masson's trichrome. For the intracerebrally injected animals, the greatest numbers were seen on the side of the brain that had been injected. In the four regions, they were most numerous in the corpus callosum and least numerous in the parietal cortex and hippocampus/dentate. Following an intraperitoneal injection, plaques were absent from the corpus callosum and, in the remaining three regions, they were less numerous than in intracerebrally injected animals, although their relative numbers were similar. The distribution of the plaques was suggestive of an initial passive spread of inoculum following intracerebral injection and a tendency for plaques to be associated with myelinated axon tracts.

Lectin histochemistry of scrapie amyloid plaques

Peroxidase-labeled lectins were used for detection of specific monosaccharide residues in amyloid plaques in brains of scrapie-infected mice. The lectins tested recognize the following residues: beta-D-galactosyl (Ricinus communis agglutinin 120, RCA-1), alpha-D-galactosyl and alpha-D-galactopyranoside (Bandeirea simplicifolia aggl., BSA), alpha-D-mannosyl and alpha-D-glucosyl (Concanavalin A, Con A), N-acetylglucosaminyl and sialyl (Wheat germ aggl., WGA), sialoglycoconjugates (Limulus polyphemus aggl., LPA), alpha-L-fucosyl (Ulex europeus aggl., UEA-1 and Tetragonolobus aggl., TPA), N-acetyl-D-galactosaminyl (Helix pomatia aggl., HPA). The most intense staining reaction in amyloid plaques was observed with BSA and WGA; it was less intense with RCA-1, Con A, and HPA. This indicates that the plaque material contains glycoproteins with abundance of accessible residues of alpha- and beta-galactose, N-acetyl-D-glucosamine and N-acetyl-D-galactosamine, and some types of sialo-glycoconjugates recognized by WGA. Such residues, like alpha-L-flucosyl recognized by UEA-1 and TPA, were almost undetectable in the examined plaques. There were also some differences in the staining intensity between small and large plaques (WGA and HPA) and between central and peripheral areas of the plaques. In the wall of micro-blood vessels relatively strong staining reaction was observed with RCA and BSA and less intense with WGA and Con A.

Characterization of antisera against scrapie-associated fibrils (SAF) from affected hamster and cross-reactivity with SAF from scrapie-affected mice and from patients with Creutzfeldt-Jakob disease

Antisera raised in rabbits and also for the first time in mice against scrapie-associated fibril (SAF) protein from hamster brain have been quantified by a modified ELISA technique (NC-ELISA) and used for a detailed analysis of SAF proteins obtained from hamster, mouse, and from patients who died of Creutzfeldt-Jakob disease. The antisera predominantly detected five bands in a Western blot analysis with apparent molecular weights of about 26000 (26K), 24K, 20K, 18K and 16K. By gel electrophoresis these antigens seem to be identical in mouse, hamster and man. The amount of material in the various bands, however, varies according to host or agent. In control materials from healthy brain SAF protein was found to be absent even when this material was used in a 50-fold excess compared to diseased brain.

Genetic control of amyloid plaque production and incubation period in scrapie-infected mice

The extent of amyloid plaque production was investigated in three inbred mouse strains carrying the p7 allele of the scrapie incubation (Sinc) gene (VM, IM and MB). With either ME7 or 87V scrapie, many more plaques were seen in the MB strain than in VM or IM mice. A backcrossing experiment using 87V suggested the involvement of more than one gene. Within this backcrossing experiment there was a positive correlation between mean plaque count and mean incubation period for the various strains and crosses. Also male mice tended to have higher plaque counts and longer incubation periods than female mice of the same genotype. These results suggest that some of the genes controlling minor variation in the incubation period also influence plaque production. This is consistent with previous evidence that the number of amyloid plaques depends, to some extent, on the duration of agent replication within the brain. This study has also identified a high plaque model (MB mice infected with 87V) for future investigation of the nature of the amyloid protein.

Identification of prion amyloid filaments in scrapie-infected brain

Extracellular collections of abnormal filaments composed of prion proteins have been identified in the brains of scrapie-infected hamsters using immunoelectron microscopy. Some of the filaments were 1500 nm in length; generally, they exhibited a uniform diameter of 16 nm. Rarely, the filaments had a twisted appearance, raising the possibility that they are flattened cylinders or are composed of helically wound protofilaments. The prion filaments possess the same diameter and limited twisting as the shorter rod-shaped particles observed in purified preparations of prions. Both the filaments and rods are composed of PrP 27-30 molecules, as determined by immunoelectron microscopy using affinity-purified antibodies. The ultrastructural features of the prion filaments are similar to those reported for amyloid in many tissues including brain. These results provide the first evidence that prion proteins assemble into filaments within the brain and that these filaments accumulate in extracellular spaces to form amyloid plaques.

Characterization of lipids in membrane vesicles from scrapie-infected hamster brain

The lipid compositions of membrane vesicles from scrapie-infected and uninfected hamster brains were examined before and after detergent extraction. No differences were observed in polar lipids, glycolipids, gangliosides or neutral lipids examined by thin-layer chromatography. Analysis of detergent-extracted CsCl gradient fractions with high scrapie infectivity failed to reveal any glycerolphosphatides, although neutral lipids were demonstrated. The major neutral lipid associated with detergent-extracted membrane vesicles from both infected and uninfected brain was an unidentified lipid which was found to absorb u.v. radiation strongly from 250 to 300 nm wavelengths. Membrane neutral lipids that strongly absorb u.v. radiation at wavelengths normally used to inactivate viruses may protect a small nucleic acid essential for scrapie infectivity.

Comparison of RNA from healthy and scrapie-infected hamster brain

Density gradient fractions prepared from healthy or scrapie-infected hamster brain tissue enriched in plasma membrane vesicles were treated with nucleases prior to phenol extraction and ethanol precipitation. The recovered nucleic acids were 3' end-labelled and run on one-dimensional polyacrylamide gels. Autoradiography revealed the presence of low molecular weight RNAs (4S) in both healthy and scrapie samples. Two-dimensional fingerprint analysis indicated that the RNAs isolated from scrapie-infected hamsters contained oligonucleotides that were not present in RNAs isolated from healthy hamsters.

Characterization of proteins in membrane vesicles from scrapie-infected hamster brain

Previous studies have shown that the scrapie agent is highly membrane-associated. We examined the protein composition of gradient fractions enriched for large membrane vesicles prepared from scrapie-infected and uninfected hamster brain using various methods to extract membrane proteins. We also examined proteins in detergent-extracted membrane vesicles fractionated on CsCl gradients. No qualitative differences in protein composition were seen comparing scrapie-infected and uninfected samples by one-dimensional gel electrophoresis. Extraction of proteins from membrane vesicles by phenol, pyridine, perchloric acid or lithium diiodosalicylate also failed to reveal any unique proteins in scrapie-infected hamster brain. Attempts to solubilize hydrophobic proteins (proteolipids) from CsCl gradient fractions into organic solvents were unsuccessful. These findings indicate that any hydrophobic protein associated with the scrapie agent is not a proteolipid, and that the ability of solvents to reduce scrapie infectivity is not a result of extraction of a proteolipid.

Ultrastructural links between scrapie and Alzheimer's disease

The discovery of abnormal fibrillar structures, scrapie-associated fibrils (SAF), in fractions with high infectivity from scrapie-infected brains has led to the proposal that SAF are a form of the infectious agent. On the basis of this proposal and on the congophilia shared by SAF and amyloid, it has been speculated elsewhere that the amyloid in Alzheimer's disease is infectious. This speculation is not supported by available evidence and therefore a conventional origin for the amyloid in Alzheimer's disease is favoured--that it originates by partial degradation of a host protein, as occurs in all other forms of amyloidosis characterised so far.

Isonicotinic hydrazide causes seizures in scrapie-infected hamsters with shorter latency than in control animals: a possible GABAergic defect

Isonicotinic hydrazide, a drug that decreases the level of GABA, when injected subcutaneously in control and scrapie-infected hamsters induced tonic-clonic seizures in scrapie hamsters significantly earlier (P less than 0.0001) than in control animals. This suggests depression of the GABAergic system in scrapie-infected hamsters. To determine whether this lesion is pre or postsynaptic we measured the level of GABA, glutamate, cGMP and cAMP and the GABA-benzodiazepine receptor complex.

Molecular characteristics of the major scrapie prion protein

A major protein was identified that purifies with the scrapie agent extracted from infected hamster brains. The protein, designated PrP 27-30, was differentiated from other proteins in purified fractions containing the scrapie agent by its microheterogeneity (Mr 27000-30000) and its unusual resistance to protease digestion. PrP 27-30 was found in all fractions enriched for scrapie prions by discontinuous sucrose gradient sedimentation or sodium dodecyl sarcosinate-agarose gel electrophoresis. It is unlikely that PrP 27-30 is a pathologic product because it was found in fractions isolated from the brains of hamsters sacrificed prior to the appearance of histopathology. If PrP 27-30 is present in normal brain, its concentration must be 100-fold lower than that found in equivalent fractions from scrapie-infected hamsters. Three protease-resistant proteins similar to PrP 27-30 were found in fractions obtained by discontinuous sucrose gradient sedimentation of scrapie-infected mouse brain. These proteins were not evident in corresponding fractions prepared from normal mouse brain. One-dimensional peptide maps comparing PrP 27-30 and normal hamster brain proteins of similar molecular weight demonstrated that PrP 27-30 has a primary structure which is distinct from these normal proteins. Heating substantially purified scrapie fractions to 100 degrees C in sodium dodecyl sulfate inactivated the prion and rendered PrP 27-30 susceptible to protease digestion. Though the scrapie agent appears to be hydrophobic, PrP 27-30 remained in the aqueous phase after extraction with organic solvents, indicating that it is probably not a proteolipid. PrP 27-30 is the first structural component of the scrapie prion to be identified.