Transgenetic investigations of prion diseases of humans and animals

Different isoforms of the non-integrin laminin receptor are present in mouse brain and bind PrP

The prion protein (PrP) plays a central role in prion diseases, and identifying its cellular receptor appears to be of crucial interest. We previously showed in the yeast two-hybrid system that PrP interacts with the 37 kDa precursor (LRP) of the high affinity 67 kDa laminin receptor (LR), which acts as the cellular receptor of PrP in cellular models. However, among the various isoforms of the receptor that have been identified so far, those which are present in the central nervous system and which bind PrP are still unknown. In this study, we have purified mouse brain fractions enriched in the laminin receptor and have performed overlay assays in order to identify those isoforms that interact with the prion protein. We demonstrate (i) the presence, in mouse brain, of several isoforms of the LRP/LR corresponding to different maturation states of the receptor (44, 60, 67 and 220 kDa) and (ii) the binding of all of these isoforms to PrP. Our data strongly support a physiological role of the laminin receptor/PrP interaction in the brain and highlight its relevance for transmissible spongiform encephalopathies.

Prions, BSE and food

Biochemical and biophysical properties of prions including possible inactivation methods are reviewed. Possible molecular markers of transmissible spongiform encephalopathy (TSE) and mechanisms behind infectivity and correlation with clinical symptoms are discussed. The risk of Bovine Spongiform Encephalopathy (BSE) for humans i.e. variant Creutzfeldt-Jakob Disease (cCJD) is addressed in detail. The consequences of the emergence of the new cCJD and the lack of information on the infectivity of cCJD at the clinical stage of the disease in relation to the need to reconsider the biological concepts currently used in microbiology.

Nervous and nonnervous cell transduction by recombinant adenoviruses that inducibly express the human PrP

The study of the prion protein (PrP) physiological functions or its specific role in transmissible spongiform encephalopathies (TSE) requires new tools, particularly those able to induce PrP overexpression in a large range of cells, in vivo as well as in vitro. Here we describe the construction of two recombinant adenoviruses encoding the human PrP either with a valine at position 129 (AdTRVal) or a methionine (AdTRMet). Both genes were put under the control of the tetracycline-responsive promoter, allowing tight regulation of PrP expression. AdTRVal and AdTRMet induced high expression of the human PrP in CHO-KI cells and in organotypic brain slices in culture. The proteins expressed from these viruses exhibited a glycosylphosphatidyl inositol (GPI) anchor, proper glycosylation and sensitivity to proteinase K digestion. AdTRVal and AdTRMet will allow future studies on the human PrP and on the role of the codon 129 polyphormism in human TSE.

Agents that cause transmissible subacute spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE) are characterised by a long incubation period which precedes clinical symptoms related to the degeneration of the central nervous system (CNS). The nature of their etiologic agents (TSA/prions) remains unknown, although there exists strong experimental data supporting the prion hypothesis. This hypothesis suggests a key role for the host derived protein (the prion protein, PrP) as the transmissible agent. In infected individuals, PrP accumulates proportionally to infectivity titre and resists proteinase K treatment (PrP-res). Iatrogenic Creutzfeldt-Jakob disease (CJD) cases have been described in humans after neurosurgery, treatment with pituitary derived hormones, and cornea and dura mater grafting. TSA-associated infectivity is dependent upon the nature of the organ in a given infected individual, though the CNS has the highest infectivity rate. In vitro, TSA/prions do not replicate easily: only cells of neuronal origin are susceptible, and the replication rate is very low. TSA/prions have unconventional properties; in particular, they resist to almost all the chemical and physical processes which inactivate conventional viruses. Only autoclaving at 134/136 degrees C for 1 h or treatment with either 1N NaOH or sodium hypochlorite (2% Cl) during 1 h at room temperature are considered to give inactivation that is compatible with public health criteria. In vivo, the distribution of infectivity is dependent upon strain and host, for a given inoculum injected by a given route. Although supported by numerous experimental data, the prion only hypothesis has not yet been convincingly demonstrated.

Determination of the frequency and distribution of vascular and parenchymal amyloid with polyclonal and N-terminal-specific PrP antibodies in scrapie-affected sheep and mice

Brains from 17 histopathologically confirmed cases of scrapie, five of which had congophilic vascular amyloid, were stained immunohistochemically for prion protein (PrP) using a polyclonal antibody. Two clinically suspect but pathologically unconfirmed cases of natural sheep scrapie and the brains of four mice infected with the 111A murine scrapie strain were also examined. Selected sections containing amyloid were stained with each of two peptide antibodies which recognise the N-terminal amino acid residues which are lost following protease digestion of the disease-specific isoform of PrP. The mice infected with the 111A murine scrapie strain had large numbers of hypermature plaques. All the amyloid plaques from both natural sheep scrapie brains and experimental murine brains were heavily immunostained by the polyclonal and both peptide antibodies. In addition, disease-specific accumulations of PrP were detected in endothelial cells or in the intima of blood vessels of the cerebral cortex of sheep scrapie brains. The affected blood vessels were located in areas which otherwise lacked typical scrapie pathology. Vascular accumulations of PrP were also found in leptomeningeal and choroid plexus blood vessels. Vascular amyloid was found mainly in the neocortex. Vascular amyloid and disease-specific parenchymal accumulations of PrP were found in two sheep which showed clinical signs of scrapie but lacked its typical vacuolar pathology. These results show that the mature amyloid of scrapie is composed of, or contains a substantial proportion of, whole length PrP protein. Thus truncation of PrP is not essential for the aggregation of PrP into amyloid. The vascular amyloid of natural sheep scrapie originates from the accumulation and release of PrP from endothelial cells presumably following systemic scrapie infection. The topography of vascular amyloid distribution in Great Britain differs from that reported in the Netherlands. As amyloid deposition in mice is largely controlled by the strain of the infecting agent it is possible that the strain of the agent may influence vascular amyloid deposition.

Astrocyte-specific expression of hamster prion protein (PrP) renders PrP knockout mice susceptible to hamster scrapie

Transmissible spongiform encephalopathies are characterized by spongiosis, astrocytosis and accumulation of PrPSc, an isoform of the normal host protein PrPC. The exact cell types responsible for agent propagation and pathogenesis are still uncertain. To determine the possible role of astrocytes, we generated mice devoid of murine PrP but expressing hamster PrP transgenes driven by the astrocyte-specific GFAP promoter. After inoculation with hamster scrapie, these mice accumulated infectivity and PrPSc to high levels, developed severe disease after 227 +/- 5 days and died 7 +/- 4 days later. Therefore, astrocytes could play an important role in scrapie pathogenesis, possibly by an indirect toxic effect on neurons. Interestingly, mice expressing the same transgenes but also endogenous murine PrP genes propagated infectivity without developing disease.

Prionics or the kinetic basis of prion diseases

A comparative kinetic analysis of mechanisms of prion diseases based on the "protein only" hypothesis is presented. The Prusiner mechanism of autocatalytic conversion of a host protein into a genetically identical, but conformationally different, prion state requires cooperativity in order to work, given realistic values of rate parameters. It then becomes phenomenologically indistinguishable from the Lansbury mechanism of plaque formation which is also a form of (passive) autocatalysis. Though the two kinds of mechanisms still may differ on the question which of the two monomeric protein conformations is the favoured equilibrium state they both require an aggregated state as the form that is eventually favored at equilibrium. While these considerations allow for a critical comparison of the mechanisms they do not yet tell us what the actual mechanism of infection is. Experiments rather indicate that the infectious unit in vivo may still differ from an in vitro form of aggregated prion proteins. Hence aggregation of the prionic form is most probably a necessary, but possibly not sufficient, prerequisite of infection. Be that as it may, the premise of a linkage between prion aggregation and infection offers a very sensitive method for diagnosing the disease at a very early stage, using fluorescence cross-correlation analysis. The possible analogies to Alzheimer's disease make such a prospect a "hot topic".

How to Limit the Spread of Creutzfeldt-Jakob Disease

Transmissible spongiform encephalopathies are rare lethal diseases induced in humans and animals by unconventional agents called transmissible spongiform encephalopathy agents (TSEAs), virions, or prions. Several cases of iatrogenic Creutzfeldt-Jakob disease (CJD) have been reported in the literature after neuro-surgery, treatment with pituitary-derived hormones, corneal grafting, and use of dura mater lyophilisates. In a given infected individual, TSEA-associated infectiousness depends on the nature of the organ: the central nervous system has the highest infectiousness, spleen and lymph nodes a medium infectiousness, and organs such as bone, skin, or skeletal muscles do not harbor any detectable infectiousness in experimental models. Transmissible spongiform encephalopathy/prions have unconventional properties; in particular, they resist almost all the chemical and physical processes that inactivate conventional viruses. Therefore, prevention of CJD agent transmission must be taken into account in daily hospital practice. Efficient sterilization procedures should be determined. In tissue and blood donation, donors with a neurologic history must be excluded, and patients treated with pituitary-derived hormones should be considered potentially infected with TSEA and excluded.

The Ninth Datta Lecture. Molecular biology of transmissible spongiform encephalopathies

The prion, the transmissible agent that causes spongiform encephalopathies such as scrapie, bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease, is believed to be devoid of nucleic acid and identical with PrPSc, a modified form of the normal host protein PrPC which is encoded by the single copy gene Prnp. The 'protein only' hypothesis proposes that PrPSc, when introduced into a normal host, causes the conversion of PrPC into PrPSc; it therefore predicts that an animal devoid of PrPC should be resistant to prion diseases. We generated homozygous Prnp(olo) ('PrP knockout') mice and showed that, after inoculation with prions, they remained free of scrapie for at least 2 years while wild-type controls all died within 6 months. There was no propagation of prions in the Prnp(olo) animals. Surprisingly, heterozygous Prnp(ol+) mice, which express PrPC at about half the normal level, also showed enhanced resistance to scrapie disease despite high levels of infectious agent and PrPSc in the brain early on. After introduction of murine PrP transgenes Prnp(olo) mice became highly susceptible to mouse but not to hamster prions, while the insertion of Syrian hamster PrP transgenes rendered them susceptible to hamster but to a much lesser extent to mouse prions. These complementation experiments paved the way to the application of reverse genetics. We have prepared animals transgenic for genes encoding PrP with amino terminal deletions of various lengths and have found that PrP lacking 48 amino proximal amino acids, which comprise four of the five octa repeats of PrP, is still biologically active.

Mutant and infectious prion proteins display common biochemical properties in cultured cells

Prion diseases are unusual neurodegenerative disorders that can be both infectious and inherited. Both forms are hypothesized to result from a posttranslational structural alteration in the cell surface glycoprotein PrPc (cellular isoform of the prion protein) that converts it into the protease-resistant isoform PrPSc (scrapie isoform of the prion protein). However, a direct comparison of molecular events underlying these two manifestations of prion diseases has not been possible, because there has been no cell culture model for the familial forms. We report here that when mutant prion proteins associated with three different inherited prion disorders of humans are expressed as their murine homologues in cultured Chinese hamster ovary cells, the proteins are protease-resistant and detergent-insoluble, two biochemical properties characteristic of infectious PrPSc. In addition, each mutant protein remains tightly associated with the plasma membrane after enzymatic cleavage of its glycosylphosphatidylinositol anchor, a property that we now show is also typical of infectious PrPSc. The cell culture system described here is the first in vitro model for familial prion diseases and provides compelling evidence that infectious and genetic cases share common molecular features.

[Evaluation of transmission of unconventional agents by human albumin]

Transmissible spongiform encephalopathy agents (TSA) or prions induce neurodegenerative diseases in humans and animals. Their nature is still unknown, even if the main component of infectivity is identified as an abnormal isoform of a host-encoded protein, the prion protein (PrP). Today, no diagnostic test is available routinely for the detection of infected patients. TSA are resistant to most of the physical and chemical procedures that are efficient against other micro-organisms. Iatrogenic transmissions of TSA have been reported in the past: they always involved either brain derivatives or instruments that have been in contact with infected central nervous system. In an infected individual, infectivity is mostly detectable in brain. However, a persistent low-level viremia can be demonstrated in association with the white blood cells; infectivity is never found in plasma, serum or in red blood cells. Epidemiological data do not evidence any relationship between spongiform encephalopathies and blood transfusion. Therefore, in 1995, TSA transmission trough albumin is only a theoretical risk.

Pathology of the transmissible spongiform encephalopathies with special emphasis on ultrastructure

The transmissible spongiform encephalopathies are a group of genetic and infectious disorders which are exemplified by scrapie in animals and Creutzfeldt-Jakob disease in humans. The spongiform encephalopathies are characterized by symmetrical vacuolation of neurons and neuropil. Amyloid plaque formation similar to that found in Alzheimer's disease is conspicuous in many, but not all, of these diseases. The sub-cellular pathology features of the spongiform encephalopathies have been studied by conventional transmission electron microscopy, scanning electron microscopy, freeze fracture, negative staining and most recently by application of immunogold labelling methods. Although these studies have revealed many unusual structures, convincing virus-like particles have not been demonstrated. Considerable data, including important transgenic mouse studies, now suggest that a single cellular protein, designated prion protein, is necessary for infection. Ultrastructural immunogold studies have shown that prion protein is released from the surface of neurons and neurites, diffuses through the extracellular space around infected cells where it accumulates and finally becomes aggregated as amyloid fibrils. It is likely that the accumulation of prion protein within the extracellular space is instrumental in causing nerve cell dysfunction and, ultimately, neurological disease.

Infectious amyloid precursor gene sequences in primates used for experimental transmission of human spongiform encephalopathy

Based on the analysis of genomic DNA from single healthy animals of each of five primate species, nucleotide and predicted amino acid sequences of the infectious amyloid precursor gene of higher apes (Gorilla and Pan) and Old World (Macaca) and New World (Ateles, Saimiri) monkeys showed 95-99% homology to the human sequences, corresponding to their phylogenetic distance from humans. Two of 18 amino acids that differed from humans resulted from nucleotide changes at sites of mutations in humans with familial forms of spongiform encephalopathy (a deleted codon within the codon 51-91 region of 24 bp repeats and a substitution at codon 198). In each of the five animals, codon 129 specified methionine, the more common of the two polymorphic genotypes in humans. Because genotypic homology did not correlate with experimental transmission rates of human spongiform encephalopathy, primary structural similarity of the infectious amyloid precursor protein in humans and experimental primates may not be an important factor in disease transmissibility.

Detection of apoptosis induced DNA cleavage in scrapie-infected sheep brain

The pathogenesis and molecular basis of nerve cell death which accompanies scrapie infections in sheep are not well understood. Degeneration of neurons in culture caused by prion protein fragments has recently been reported to be consistent with mechanisms of cell death by apoptosis or programmed cell death. Apoptosis activation during prion-related encephalopathies has not yet been established in vivo. We report here the detection of DNA damage consistent with apoptosis in the brain cells of sheep infected with scrapie using laser scanning microscopic analysis of the single cell gel assay. We suggest that this DNA fragmentation is the result of the activation of the mechanisms characteristic of apoptotic cell death.

Molecular biology of prion diseases

What is the nature of the transmissible agent responsible for neurodegenerative diseases such as scrapie and mad-cow disease in animals and Creutzfeldt-Jakob disease in man? There is now weighty evidence that PrP(Sc), a modified version of the ubiquitously expressed host protein PrP(C), is responsible for pathogenesis of these diseases and that conversion of PrP(C) into PrP(Sc) under the influence of PrP(Sc) is the process leading to the propagation of PrP(Sc) and disease progression.