Scrapie-associated precursor proteins: antigenic relationship between species and immunocytochemical localization in normal, scrapie, and Creutzfeldt-Jakob disease brains

Reflections on Cerebellar Neuropathology in Classical Scrapie

In this review, the most important neuropathological changes found in the cerebella of sheep affected by classical natural scrapie are discussed. This disease is the oldest known of a group of unconventional “infections” caused by toxic prions of different origins. Scrapie is currently considered a “transmissible spongiform encephalopathy” (due to its neuropathological characteristics and its transmission), which is the paradigm of prion pathologies as well as many encephalopathies (prion-like) that present aberrant deposits of insoluble protein with neurotoxic effects due to errors in their catabolization (“misfolding protein diseases”). The study of this disease is, therefore, of great relevance. Our work data from the authors’ previous publications as well as other research in the field. The four most important types of neuropathological changes are neuron abnormalities and loss, neurogliosis, tissue vacuolization (spongiosis) and pathological or abnormal prion protein (PrP) deposits/deposition. These findings were analyzed and compared to other neuropathologies. Various aspects related to the presentation and progression of the disease, the involution of different neuronal types, the neuroglial responses and the appearance of abnormal PrP deposits are discussed. The most important points of controversy in scrapie neuropathology are presented.

Prion protein: structural features and related toxicity

Transmissible spongiform encephalopathies, or prion diseases, is a group of infectious neurodegenerative disorders. The conformational conversion from cellular form (PrP(C)) to disease-causing isoform (PrP(Sc)) is considered to be the most important and remarkable event in these diseases, while accumulation of PrP(Sc) is thought to be the main reason for cell death, inflammation and spongiform degeneration observed in infected individuals. Although these rare but unique neurodegenerative disorders have attracted much attention, there are still many questions that remain to be answered. Knowledge of the scrapie agent structures and the toxic species may have significance for understanding the causes of the diseases, and could be helpful for rational design of novel therapeutic and diagnostic methods. In this review, we summarized the available experimental evidence concerning the relationship among the structural features, aggregation status of misfolded PrP and related neurotoxicity in the course of prion diseases development. In particular, most data supports the idea that the smaller oligomeric PrP(Sc) aggregates, rather than the mature amyloid fibers, exhibit the highest toxicity to the host.

Involvement of astrocytes in transmissible spongiform encephalopathies: a confocal microscopy study

Astroglial proliferation associated with pathological prion protein (PrPsc) deposition is widely described in Transmissible Spongiform Encephalopathies (TSEs). However, little is known of the actual role played by glia in their pathogenesis. The aim of the study has been to determine whether PrPsc is located exclusively in neurons or in both neurons and glial cells present in the central nervous system in a natural Scrapie model. Samples of cerebellum from 25 Scrapie sheep from various flocks were sectioned. Following epitope retrieval with formic acid, proteinase K and heat treatment, primary antibody L42 and primary antibodies against glial fibrillary acidic protein were applied as prion- and astrocytic-specific markers, respectively. For visualization, a suitable mixture of fluorochrome-conjugated secondary antibodies was used. Relevant controls were processed in the same manner. As determined by confocal microscopy, PrPsc deposits co-localized with glial cells in all samples. Our results suggest that these cells can sustain active prion propagation, in agreement with similar findings from other studies of primary cell cultures and inoculated mice. Furthermore, despite ongoing debate regarding whether varied TSE sources show differences in their tropism for different cell lineages in the brains of affected animals, no differences in co-localization results were seen.

Neuropathologic characteristics of spinal cord lesions in sporadic Creutzfeldt-Jakob disease

We investigated the neuropathologic features of spinal cord lesions in 23 patients with sporadic Creutzfeldt-Jakob disease (sCJD), paying particular attention to neuronal loss and gliosis, pyramidal tract degeneration and prion protein (PrP) deposition. The study included 9 cases of subacute spongiform encephalopathy, 13 cases of panencephalopathic-type sCJD and 1 case of sporadic fatal insomnia (sFI). In the spinal gray matter, although gliosis was present in some patients with disease of relatively long duration, the number of neurons, including large motor neurons, was well preserved regardless of disease duration. Pyramidal tract degeneration was observed in some patients with disease lasting more than 14 months but not in the patient with sFI. PrP deposition was present in the spinal cord of all sCJD patients, and was identified predominantly in the posterior horn, particularly in the substantia gelatinosa, regardless of disease duration or disease classification based on cerebral pathology. Relatively prominent PrP deposition was also observed in Clarke's column. The density of PrP deposition in the sCJD spinal cord was not associated with disease duration or neuronal degeneration. Our results indicate that PrP deposition in the spinal cord is an early pathologic event in sCJD and may remain to the end stage. Although no VV1, VV2 or MV2 cases were included in our study, we suggest that stereotypic accumulation of PrP is a consistent pathologic feature of sCJD and that the spinal cord remains relatively resistant to the pathologic process of sCJD, at least in patients with MM1 sCJD.

Fate of Prions in Soil: Adsorption Kinetics of Recombinant Unglycosylated Ovine Prion Protein onto Mica in Laminar Flow Conditions and Subsequent Desorption

Prions can be disseminated in soils. Their interaction with soil minerals is a key factor for the assessment of risks associated with the transport of their infectivity. We did not examine here the infectivity itself but the adsorption kinetics of an ovine recombinant prion protein (ovine PrPrec), as a noninfectious model protein, on muscovite mica, a phyllosilicate with surface properties analogous to soil clays, in conditions of laminar flow through a channel. The protein was labeled with (125)I, and its adsorption examined between pH 4.0 and 9.0. At wall shear rate 100 s(-1), we found the process to be controlled mainly by transport at the beginning of the adsorption process. Additional experiments at 1000 s(-1) (pH 5 and 6) determined that the diffusion coefficient was in accordance with the hydrodynamic radius measured by size exclusion chromatography. The pseudo-plateau of the interfacial concentration with time was compatible with more than a monolayer and suggests the presence of aggregates. Desorption was not observed in the presence of buffer between pH 4 and 9 and sheep plasma, while the addition of alkaline detergent or 10(-1) M NaOH allowed an almost complete removal from the interface. The ensemble of results suggests that the largely irreversible adsorption of the ovine PrPrec onto mica is mainly due to electrostatic attraction between the protein and the highly negatively charged mica surface. Possible consequences for the mode of dissemination of prion proteins in soils are indicated.

Local environmental effects on the structure of the prion protein

Prion diseases are neurodegenerative diseases causally linked to the partial unfolding and subsequent misfolding and aggregation of the prion protein (PrP). While most proteins fold into a single low energy state, PrP can fold into two distinct isoforms. In its innocuous state, denoted as PrPC, the protein has predominantly alpha-helical secondary structure, however, PrPC can misfold into an isoform rich in extended structure capable of forming toxic and infectious aggregates. While prion disease is believed to be a protein-only disease, one not requiring any non-protein elements for propagation, the different environments the protein finds itself in vivo likely influence its ability to misfold and aggregate. In this review we will examine various molecules, covalent modifications and environments PrP faces in vivo and the effect they have on PrP's local environment and, potentially, conformation. Included in this discussion are: (1) pH, (2) carbohydrates, (3) lipid membranes, (4) metal ions, and (5) small molecules.

Monoclonal antibodies specific for the native, disease-associated isoform of the prion protein

Reviewing the circumstances that have led to the first monoclonal antibody against the disease-associated form of PrP, we consider the availability of PrP knockout mice and recombinant PrP, as well as a reliable conformational screening protocol as being important prerequisites for a successful immunization approach. When considering presenting an antigen to a mouse with the goal of obtaining specific monoclonal antibodies against a misfolded or aggregated form of a host protein, it is desirable to increase the definition of a subtle conformational difference. This can be achieved by immunizing an antigen knockout mouse that has not developed self-tolerance against the respective antigen. Furthermore, if conformational isoforms and/or oligomeric forms of a protein sequence are understood to exist in an equilibrium, high and pure amounts of recombinant protein may increase the likelihood that a particular population of protein conformation passes an antigenic threshold necessary to start an immunogenic response. Pulling out the monoclonal antibodies by correct screening is essential. Screening against the pure misfolded or aggregated protein is often complicated by its poor solubility and hence the ability to immobilize. In the present case, immobilization of disease-associated PrP on nitrocellulose had been established as a conformation-sensitive screening method, allowing to "freeze" PrP in its distinguishable, disease-associated conformation. We are cautious to generalize conclusions of how to assess the generation of monoclonal antibodies against these particular protein isoforms to other diseases of protein misfolding and/or aggregation, but ultimately the present approach may inspire respective experiments.

Astrocytosis and amyloid deposition in scrapie-infected hamsters

In scrapie infection, prion protein (PrPSc) is localized in areas where there is neurodegeneration and astrocytosis. It is thought that PrPSc is toxic to neurons and trophic for astrocytes. In our study, paraffin sections from scrapie infected (263K and 139H) and control hamsters were examined with histological and immunocytochemical staining. We found that PrPSc was present in the ependymal cells of both 263K- and 139H-infected hamsters. In 139H-infected hamsters, PrPSc was found in the cytoplasm of neurons in cerebral cortex and in hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. In contrast, neuronal cytoplasm and nuclei, were positive for PrPSc in most areas such as cortex, hippocampus, and thalamus in 263K-infected hamsters. Many aggregations of PrPSc could be seen in the cortex, hippocampus, substantia nigra and around the Pia mater, corpus callosum, fimbria, ventricles, and blood vessels in sections from 139H- and/or 263K-positive animals. Furthermore, PrPSc was also co-localized with glial fibrillary acidic protein (GFAP) in many reactive astrocytes (approximately 90%) in certain areas such as the hippocampus in 263K-infected hamsters, but not 139H-infected hamsters. The patterns of astrocytosis and PrPSc formation were different between 139H- and 263K-infected hamsters, which may be used for a diagnosis purpose. Our results suggest a hypothesis that multiple cell-types are capable of PrPSc production. Our results also confirm that reactive astrocytes can produce and/or accumulate PrPSc during some scrapie strain infections. The findings suggest a 'snowball effect', that is: astrocytosis might play an important role in amyloidosis, while amyloidosis may induce further astrocytosis at least in 263K-infected hamsters.

Sensitive enzyme-linked immunosorbent assay for detection of PrP(Sc) in crude tissue extracts from scrapie-affected mice

An enzyme-linked immunosorbent assay (ELISA) was developed that detects PrP(Sc) in crude extracts from brain and spleen tissue of scrapie-affected mice with high sensitivity and specificity. Brain tissue was homogenized in 8% Zwittergent 3-12 and 0.5% Sarkosyl. The homogenate was treated with collagenase and DNase I and then subjected to proteinase K digestion. Precipitates containing PrP(Sc) were obtained by ultracentrifugation. Spleen tissue was homogenized in 4% Triton X-100 and 0.5% Sarkosyl, and the homogenate was treated firstly with collagenase and DNase I, and secondly with proteinase K. PrP(Sc) was then extracted with 6.25% Sarkosyl and precipitated through salting-out with NaCl and by ultracentrifugation. When PrP(Sc) was dissolved in 3-4 M guanidine thiocyanate and adsorbed to microtiter plates, strong and specific reactions to the formation of antigen-antibody complexes could be detected by ELISA. The sensitivity of PrP(Sc)-detection for this ELISA, as measured by serial dilution of scrapie material in tissue homogenates from uninfected animals, was equal or higher than that attained by Western blot. This ELISA is more rapid than Western blot and seems to be more suitable for screening large numbers of animals. It also has potential application for the diagnosis of the transmissible spongiform encephalopathies.

Prion protein accumulation in the spinal cords of patients with sporadic and growth hormone associated Creutzfeldt-Jakob disease

An immunohistological study of the spinal cord in 20 cases of sporadic and 4 iatrogenic (growth hormone) cases of Creutzfeldt-Jakob (CJD) disease patients was performed to detect the presence of disease specific prion protein using a number of different antisera. Prion protein was present in all the growth hormone recipients and in 11 of the 20 sporadic CJD cases. Plaque-like deposits of prion protein were found in all the growth hormone cases and three of the sporadic cases. This is the first demonstration of the topographic immunolocalisation of prion protein in the spinal cord of CJD patients, a feature which could help elucidate some important aspects of the pathogenesis of CJD.

Characterisation of antisera raised against species-specific peptide sequences from scrapie-associated fibril protein and their application for post-mortem immunodiagnosis of spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE), such as scrapie or Creutzfeldt-Jakob disease (CJD), are fatal neurodegenerative diseases of the central nervous system caused by a yet unidentified virus. They are accompanied by a brain specific amyloidosis, during which a host coded protein irreversibly aggregates to form the scrapie-associated fibrils. The diagnosis of TSE relies on histopathological detection of spongiform lesions, on electron microscopical detection of fibrils, or on the immunological detection of SAF protein, which is the most specific diagnostic marker. In order to improve the diagnosis of TSE, we developed a protocol for rapid tissue fractionation and enrichment of SAF protein which subsequently allows the specific detection of SAF protein by western blotting and immunodetection. Using some new antisera raised against synthetic peptides with sequences specific for the hamster, sheep, cattle and human SAF protein, several samples can be diagnosed for TSE within 24 hours, starting with only 10-100 mg of brain tissue from different species.

Immunohistochemical detection of prion protein in sheep with scrapie

Prion protein (PrP), which is involved in the pathogenesis of scrapie, occurs in 2 forms. The form extracted from scrapie brain is protease resistant (PrP-res), whereas PrP from normal brain is protease sensitive (PrP-sen). This study examined whether PrP-res could be detected in brains of sheep with scrapie by immunohistochemistry (IHC). A suitable IHC procedure was developed using brain tissue from hamsters that had been inoculated with the transmissible mink encephalopathy agent. Tissue samples were fixed in PLP (periodate, lysine, paraformaldehyde) that contained paraformaldehyde at a concentration of 0.125%. Before application of the IHC technique, tissue sections were deparaffinized and treated with formic acid to simultaneously enhance PrP-res immunoreactivity and degrade PrP-sen. Primary antibody was obtained from a rabbit immunized to PrP-res extracted from brains of mice with experimentally induced scrapie. Brain from 21 sheep with histopathologically confirmed scrapie were examined by IHC. In all 21 brains, PrP-res was widely distributed throughout the brain stem. Staining was particularly intense in neuronal cell bodies and around blood vessels. The IHC technique successfully detected PrP-res in brain samples that had been frozen or that were severely autolyzed before fixation in PLP. Brains from 11 scrapie-suspect sheep that were not considered histologically positive were also examined by IHC. PrP-res was found in 4 of these brains. Sections of brains from 14 clinically normal sheep did not have detectable PrP-res. Results of this study indicate that IHC detection of PrP-res is equivalent, and perhaps superior, to histopathology for the diagnosis of scrapie in sheep. Furthermore, IHC is applicable to tissues that have autolytic changes or processing artifacts that prevent satisfactory histopathologic evaluation for lesions of scrapie.

Quantitative assessment of the astrocytic response in natural scrapie of sheep

Scrapie of sheep and goats belongs to the spongiform encephalopathies, a term derived from the characteristic vacuolar degeneration in the central nervous system. Astrocytosis has been described, but a systematic quantitative study has not been made. Such a study is important in resolving the still controversial issues as to whether the astrocytic response is caused directly by the infectious agent or represents a secondary reaction to tissue damage. In this study the numbers of astrocytes in 12 sheep with scrapie and 12 healthy sheep of a similar age were assessed and compared. Sections from eight planes of the brain were immunostained with anti-glial fibrillary acidic protein (GFAP) for astrocytes and their numbers counted in 40 areas of 1 mm2. The main findings were as follows. (1) A significant increase of astrocytes was detected in sheep with scrapie. (2) Astrocytosis was not usually related to the severity of the characteristic vacuolar lesions, indicating that it was at least partly due to a direct effect of the infectious agent but was not a secondary response to the neuronal damage. (3) The astrocytic response varied considerably between individual affected sheep; this may have been due to differences in agent strains, host response, or both.

Purification of non-infectious ganglioside preparations from scrapie-infected brain tissue

The extraction and purification of gangliosides from brains of animals infected with the scrapie agent was evaluated by scaling-down a large-scale procedure currently used with bovine brains. Inactivation experiments employed hamster brains infected with the 263 K strain of scrapie. Residual infectivity was determined at different points of the procedure and in the final preparation by an in vivo animal bioassay. The efficacy of single steps, which included chemicals or physical techniques known to affect the viability of the scrapie agent, was verified by spiking experiments where known amounts of infectivity were added just before each single step. Infectivity was significantly diminished at early stages of the purification, and no infectivity was detected in the final preparation containing purified gangliosides. The significant reduction of infectivity at intermediate steps and the total absence of detectable infectivity in the final product confirmed that a combination of heat and exposure to sodium hydroxide, even in the presence of organic solvents, completely inactivated the scrapie agent.

The transmissible amyloidoses: genetical control of spontaneous generation of infectious amyloid proteins by nucleation of configurational change in host precursors: kuru-CJD-GSS-scrapie-BSE

Kuru, Creutzfeld-Jakob disease, Gerstmann-Sträussler syndrome, scrapie, and bovine spongiform encephalopathy are caused by so-called unconventional viruses which are really replicating proteins which induce by auto nucleation and autopatterning a configurational change in the precursor protein to produce an infectious amyloid form. Crystallography and NMR may eventually determine how amyloid precursor protein is converted to this infectious form by configurational changes in all tertiary and quaternary structure of the normal precursor. Most sporadic cases of CJD arise by de novo spontaneous conversion of the normal precursor to the infectious form, a rare event occurring at the frequency of one per million persons per year (the annual incidence of CJD throughout the world). In the familial forms of CJD and GSS, where the occurrence is an autosomal dominant trait, each family has one of five different mutations causing a single amino acid change or one of five insertions of 5, 6, 7, 8 or 9 octapeptide repeats. Each mutation causes a million-fold increased probability of the spontaneous configurational change to an infectious polypeptide, and appears as an autosomal dominant trait. Thus, the behavior of the transmissible brain amyloidosis parallels completely that of the transthyretin amyloidoses causing familial amyloidotic polyneuropathy, in which there are 19 different point mutations, each one of which increases enormously the likelihood of configurational change of transthyretin prealbumin to amyloid.

Production and characterization of antibodies to mouse scrapie-amyloid protein elicited by non-carrier linked synthetic peptide immunogens

Two polyclonal antibodies were raised by immunizing rabbits with two non carrier-linked synthetic peptides whose amino acid sequences corresponded to codons 89-107 (peptide P1) and 219-233 (peptide P2) of the translated cDNA sequence of murine PrP protein. These free peptides, whose structural characteristics in solution were studied by circular dichroism, elicited a reasonable immunologic response in animals. Both antibodies still recognized the corresponding immunogens after affinity chromatography purification. However, only antibodies raised to the former sequence reacted by immunoblot with a purified preparation of murine scrapie amyloid protein. These findings are discussed together with their correlation to peptide structure and the effectiveness of this simplified immunization procedure.

Scrapie-associated prion protein accumulates in astrocytes during scrapie infection

In the course of scrapie, a transmissible spongiform encephalopathy caused by an unconventional agent, a normal cellular protein is converted to an abnormal form that copurifies with infectivity and aggregates to form deposits of amyloid. We have used immunocytochemistry and methods that enhance detection of amyloidogenic proteins to investigate the types of cells in the central nervous system which are involved in the formation of the abnormal scrapie-associated protein. We show that this protein accumulates in astrocytes prior to the cardinal neuropathological changes in scrapie--astrogliosis, vacuolation, neuron loss, and amyloid deposition. These findings implicate the astrocyte in the formation of the scrapie isoform of the prion protein and amyloid in scrapie and suggest that this cell type might also be involved in the replication of the scrapie agent.

Molecular mass, biochemical composition, and physicochemical behavior of the infectious form of the scrapie precursor protein monomer

A highly purified fraction obtained from scrapie (263-K strain)-infected hamsters' brains by an alternative procedure without proteinase K treatment contained a protease-resistant form of the scrapie precursor protein (PrPSc) and infectivity of 9.9 +/- 0.7 log LD50/ml. Polyclonal antibodies produced against hamster scrapie amyloid protein (PrP27-30) and used in a neutralization test diminished infectivity of the PrPSc preparations by 1.6 log after intracerebral inoculation and by 1 log after intraperitoneal inoculation. PrPSc was subjected to size-exclusion HPLC; greater than or equal to 60% of the eluted infectious units were recovered from the peak with an apparent mass of 30.4 +/- 0.6 kDa. Characterization by UV absorption spectra, SDS/PAGE, immunoblots, N-terminal amino acid sequence, and neutral sugar and amino sugar analyses demonstrated homogeneity of the infectious units. The neutral sugar and amino sugar compositional analyses revealed high mannose, glucosamine, fucose, and sialic acid content. This demonstrated an extensive posttranslational modification by the complex type of N-linked glycosylation and glycane core of C-terminal glycolipid of PrPSc. The results correspond to the predicted size, composition, and sequence of PrPSc and indicate that this protein may be the only component of scrapie infectious unit or the infectious form of scrapie precursor.