[Biology and pathology of prion diseases]

During the last few decades, intensive research of the nature of causes of transmissible spongiform encephalophatics (TSE) have been undertaken. The interest in this field of research has risen especially after the outbreak of mad cow disease epidemy. According to proposed Prion hypothesis, PSE agents lack nucleic acid, with protein as their primary component. Various research has detected the presence of Prion genes which code normal cellular Prion proteins. Normal and pathogen protein are isomers but of different conformations. It is hypothesed that a pathogen protein when in contact with normal protein causes its conversion into a pathogen form which leads to disease.

The role of the GATA factors Gln3p, Nil1p, Dal80p and the Ure2p on ASP3 regulation in Saccharomyces cerevisiae

The role of Gln3p, Nil1p, Dal80p and Ure2p in the nitrogen regulation of ASP3, which codes for the periplasmic Saccharomyces cerevisiae asparaginase II, was investigated. Analysis of enzyme levels and mRNA(ASP3) in two wild-type strains and gln3, nil1, gln3nil1, gln3ure2, nil1ure2, nil1dal80, ure2, dal80 and ure2dal80 mutant cells allowed the study of the qualitative and quantitative regulatory role of the GATA factors and Ure2p on ASP3 expression. The simultaneous presence of Gln3p and Nil1p is a required condition for full gene transcription. Enzyme activity doubled upon nitrogen starvation of either ammonium-grown (possibly due to Nil2p/Deh1p derepression) or proline-grown (due to Dal80p derepression) cells. The ure2 mutation increased enzyme levels five-fold in fresh ammonium-grown cells and ten-fold in fresh proline-grown cells. The combined effects of the ure2 mutation and nitrogen starvation on ammonium- or proline-grown cells resulted in an overall 10-20-fold enzyme activity increase, respectively, in comparison with the wild-type cells.

Dental practice implications of prion diseases

This review article discusses dental practice implications of prion diseases, including Creutzfeldt-Jakob disease. The current universal precautions used for infection control in dentistry do not inactivate infectious prions. There is a theoretical, yet real risk of prion disease transmission through dental treatment, although the magnitude of that risk has not yet been determined. Medical, family, and travel histories can demonstrate the need for implementing improved levels of infection control. Best practices include the following: For certain cases, single-use disposable dental instruments should be used. Those instruments that are not disposable, should have a thorough physical cleaning, be soaked in hot 1N sodium hydroxide solution for 1 hour, and then autoclaved in a vacuum or porous-load autoclave at 134 degrees C to 138 degrees C for 18 to 20 minutes. Dental and other health care professionals need to understand prion diseases, and for best practice, consider implementing recommended changes to infection-control measures, since current practices do not destroy infectious prions.

TSE strain variation

Studies in mice have revealed considerable strain variation in the agents causing transmissible spongiform encephalopathies (TSEs). TSE strains interact with genetic factors in the host (in particular PrP genotype) to influence characteristics of the disease such as incubation period and neuropathology. TSE strains can retain their identity after propagation in different host species or PrP genotypes, showing that these agents carry their own strain-specific information. It is not known whether this information resides in specific self-perpetuating modifications of PrP, or whether a separate informational molecule is required. Strain typing in mice can be used to explore links between TSEs occurring naturally in different species. Such studies have demonstrated that the strain causing BSE in cattle has also infected domestic cats and exotic ungulates. Most importantly, the BSE strain has also been isolated from patients with variant CJD. In contrast, different TSE strains are associated with sporadic CJD and sheep scrapie.

Mutant prion protein-mediated aggregation of normal prion protein in the endoplasmic reticulum: implications for prion propagation and neurotoxicity

Familial prion disorders are believed to result from spontaneous conversion of mutant prion protein (PrPM) to the pathogenic isoform (PrPSc). While most familial cases are heterozygous and thus express the normal (PrPC) and mutant alleles of PrP, the role of PrPC in the pathogenic process is unclear. Plaques from affected cases reveal a heterogeneous picture; in some cases only PrPM is detected, whereas in others both PrPC and PrPM are transformed to PrPSc. To understand if the coaggregation of PrPC is governed by PrP mutations or is a consequence of the cellular compartment of PrPM aggregation, we coexpressed PrPM and PrPC in neuroblastoma cells, the latter tagged with green fluorescent protein (PrPC-GFP) for differentiation. Two PrPM forms (PrP231T, PrP217R/231T) that aggregate spontaneously in the endoplasmic reticulum (ER) were generated for this analysis. We report that PrPC-GFP aggregates when coexpressed with PrP231T or PrP217R/231T, regardless of sequence homology between the interacting forms. Furthermore, intracellular aggregates of PrP231T induce the accumulation of a C-terminal fragment of PrP, most likely derived from a potentially neurotoxic transmembrane form of PrP (CtmPrP) in the ER. These findings have implications for prion pathogenesis in familial prion disorders, especially in cases where transport of PrPM from the ER is blocked by the cellular quality control.

Fundamentals of prion biology and diseases

One of the most remarkable changes in medicine during the last 20 years of the 20th century was the shift from the clinical-neuropathological classification of Creutzfeldt-Jakob disease (CJD) and related disorders as 'transmissible spongiform encephalopathies' to a molecular-etiologic classification as 'prion diseases'. We now know that these diseases are caused by abnormalities of the prion protein (PrP). Specifically, CJD is caused by the conversion of the normal, protease-sensitive PrP isoform, designated PrP(C), to a protease resistant isoform, designated PrP(Sc). PrP(Sc) forms into an infectious particle, named a 'prion', that can transmit the disease. Accumulation of PrP(Sc) in the brain causes neurodegeneration. The main goals of this review are to summarize our understanding of the attributes of the PrP molecule that give it the properties of an infectious agent and to describe how different alterations of the PrP molecule cause the multiple known prion disease variants. Finally, the emergence of a new variant of CJD in Great Britain and to a lesser extent in Europe and its relationship to the emergence of a particularly virulent form of bovine spongiform encephalopathy will be discussed.

A possible role of keratinocytes of skin and mucous membranes in prion propagation and transmission

Prion diseases or transmissible spongiform encephalopathies are lethal neurodegenerative diseases caused by proteinaceous agents that consist of an abnormal form of a host protein designated PrP and are devoid of nucleic acids. In laboratory settings these diseases are usually transmitted by intracerebral or peripheral inoculation. In the field they have been shown to be transmitted by uptake of contaminated food but in most instances the route of transmission remains obscure. Both nervous and lymphatic tissues in peripheral organs have been implicated in the spread and propagation of prions. The exact sites of uptake and initial propagation of the infectious agents have not yet been determined, however. As the expression of PrPc is required for the propagation of the infectious agent the search for peripheral cells positive for PrPc may reveal potential routes of entry and transmission. Recently epidermal and mucosal keratinocytes have been found to express PrPc. These data together with the recent finding that epithelial cells are able to support prion replication in vitro suggest that keratinocytes might play a role in the pathogenesis and/or transmission of prion diseases.

[Prions as the driving force in transmissible spongiform encephalopathies]

Transmissible spongiform encephalopathies are degenerative disorders affecting the central nervous system (CNS) occurring in a variety of species. The causative agent is thought to be composed of an abnormal form of the host encoded prion protein (PrPC), termed PrPSc. The conformational change of PrPC into PrPSc can occur spontaneously, however, it can also be induced by PrPSc. Prion diseases such as bovine spongiform encephalopathy (BSE), scrapie and variant Creutzfeldt-Jakob-Disease (vCJD) are most likely caused by peripheral uptake of prions. The process by which prions proceed to the CNS following peripheral uptake is referred to as neuroinvasion. Infection with prions is thought to occur in two phases: After ingestion prions first replicate in lymphatic tissue and then gain access to the CNS via peripheral nerves. Studies looking at the biochemical and clinical characteristics of BSE and vCJD demonstrated that BSE is most likely responsible for vCJD in humans.

Transmission of prions

The "protein only" hypothesis holds that the infectious agent causing transmissible spongiform encephalopathies is a conformational isomer of PrP, a host protein that is predominantly expressed in the brain. This hypothesis is strongly supported by many lines of evidence. To date, prion diseases are unique among conformational diseases in that they are transmissible-experimentally and by natural routes (mainly by ingestion). The pathway of prions to the brain has been elucidated in outline. A striking feature of prions is their extraordinary resistance to conventional sterilization procedures and their capacity to bind to surfaces of metal and plastic without losing infectivity. This property, first observed in a clinical setting, is now being investigated in experimental settings, both in animals and in cell culture.

Transgenesis applied to transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative disorders present in various mammals. TSEs have been studies intensively, even more so following the BSE crisis and the subsequent threat of a human nvCJD epidemic. In the 'protein-only' hypothesis, the infectious agent, called prion, is assumed to be a misfolded host protein. Transgenesis has mainly been applied to study the role of this protein, its structure-function relationship with respect to its pathogenic properties and to assess the genetic origin of the well-recognised species barrier effect. This approach has somewhat supplemented the lack of in vitro models. This review will try to summarise the impressive work that has been done in this field. Although many questions remain unanswered, transgenic experiments have and will still improve our knowledge on this disease and might help us to develop critically needed therapeutic approaches.

Variant Creutzfeldt-Jakob disease: an unfolding epidemic of misfolded proteins

Variant Creutzfeldt-Jakob disease (vCJD) is an emerging infectious disease believed to be the human manifestation of bovine spongiform encephalopathy (BSE). Variant CJD belongs to a family of human and animal diseases called transmissible spongiform encephalopathies (TSE). The pathogenesis of TSE is not fully understood, but a modified form of a normal cellular protein plays a central role. Current measures to control vCJD aim to prevent transmission of the infectious agent from animals to humans through food or pharmaceutical products and to prevent transmission from person to person via medical interventions. The anticipated development of preclinical diagnostic tests and treatments for vCJD will create new control options and difficult choices.

The physiological functions of prion protein

Prion proteins are mentioned predominantly as unprecedented infectious pathogens in the context of transmissible spongiform encephalopathies. Since prions are devoid of nucleic acids, disease transmission must be mediated by an entirely novel mechanism. The general accepted theory proposes the conversion of cellular prion protein (PrP(C)) into the pathological isoform solely through conformational changes. This process favors the development of insoluble protein aggregates in the central nervous system typical for prion diseases. However, progress to elucidate the physiological functions of PrP(C) is still slow besides recent indications of a multifaceted network, in which PrP(C) seems to play a fundamental role. Possible contributions of interrupted or disturbed physiological signaling events due to the pathological prion protein isoform are presented in terms of recent findings.

The biology of the cellular prion protein

Prions are the etiological agents for infectious degenerative encephalopaties acting by inducing conformational changes in the cellular prion protein (PrPc), which is a cell membrane GPI anchored glycoprotein. Besides its conservation among species and expression in most tissues, and in particular, in high levels in the nervous system, the role for cellular prion protein remained obscure for some time. Initial skepticism about such a role was mainly due to the absence of a gross phenotype alteration in cellular prion protein null mice. In the last few years, some possible biological functions for cellular prion protein have been described. Copper binds to the molecule and the resulting complex may be responsible for cell protection against oxidative stress. Cellular prion protein is also a high-affinity ligand for laminin, and induces neuronal cell adhesion, neurite extension and maintenance. The binding site resides in a carboxy-terminal peptide of the gamma-1 chain, which is very conserved among all laminin types, indicating that this interaction may be relevant in other tissues besides the brain. Moreover, cellular prion protein association with a peptide that mimics a putative ligand at the cell surface, p66, triggers neuroprotective signals through a cAMP/PKA-dependent pathway. Since PrPc recycles from membrane to an intracellular compartment, which is induced by copper binding, it is also possible that the internalization mechanism allows switching off elicited signals.

Doppel and PrP(C) do not share the same membrane microenvironment

Doppel is a paralog of the normal prion protein, PrP(C). It has been suggested that Doppel can compensate for the absence of PrP(C) in PrP(0/0) mice. In this work, we tested whether Doppel and PrP(C) share the same cell location, thereby sharing the same neighboring cell components, probably required to share the same cell function. Our results show that, at detergent conditions in which membrane rafts were intact, neither PrP(C) and Doppel co-immunoprecipitate with the appropriate antibodies, nor was Doppel retained by a Cu(2+)IMAC resin, as PrP(C) does. This indicates that, although Doppel is a raft-associated protein as is PrP(C), both proteins are not present in the same membrane microenvironment, and they probably do not perform the same function.

Prion diseases: pathogenesis and public health concerns

Transmissible spongiform encephalopathy (TSE) agents or prions induce neurodegenerative fatal diseases in humans and in some mammalian species. Human TSEs include Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, kuru and fatal familial insomnia. In animals, scrapie in sheep and goats, feline spongiform encephalopathy, transmissible mink encephalopathy, chronic wasting disease in wild ruminants, and bovine spongiform encephalopathy (BSE), which appeared in the UK in the mid-1980s [Wells, G.A.H. et al. (1987) Vet. Rec. 121, 419-420], belong to the TSE group. Prions have biological and physicochemical characteristics that differ significantly from those of other microorganisms; for example, they are resistant to inactivation processes that are effective against conventional viruses, including those that alter nucleic acid structure or function. Alternatively, infectivity is highly susceptible to procedures that modify protein conformation. Today, the exact nature of prions remains unknown even though it is likely that they consist of protein only. At the biochemical level, TSEs are characterised by the accumulation, within the central nervous system of the infected individual, of an abnormal isoform of a particular protein from the host, the prion protein [Prusiner, S.B. (1982) Science 216, 136-144]. TSEs are transmissible among their species of origin, but they can also cross the species barrier and induce chronic infection and/or disease in other species. Transmissibility has been proven in natural situations such as the outbreak of CJD among patients treated with pituitary-derived hormones and the appearance of BSE that affected UK cattle in the mid-1980s.

Copper-dependent functions for the prion protein

Prion diseases such as bovine spongiform encephalopathy and Creutzfeldt-Jakob disease are fatal neurodegenerative diseases. These diseases are characterized by the conversion of a normal cellular protein, the prion protein, to an abnormal isoform that is thought to be responsible for both pathogenesis in the disease and the infectious nature of the disease agent. Understanding the biology and metabolism of the normal prion protein is therefore important for understanding the nature of these diseases. This review presents evidence for the normal function of the cellular prion protein, which appears to depend on its ability to bind copper (Cu). There is now considerable evidence that the prion protein is an antioxidant. Once the prion protein binds Cu, it may have an activity like that of a superoxide dismutase. Conversion of the prion protein to an abnormal isoform might lead to a loss of antioxidant protection that could be responsible for neurodegeneration in the disease.

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.

Unhampered prion neuroinvasion despite impaired fast axonal transport in transgenic mice overexpressing four-repeat tau

Transmissible spongiform encephalopathies often are caused by peripheral uptake of infectious prions, and the peripheral nervous system is involved in prion spread to the brain. Although the cellular prion protein is subjected to fast axonal transport, the mechanism of intranerval transport of infectious prions is unclear. Here we administered prions intranervally to transgenic mice overexpressing the four-repeat human tau protein, which exhibit defective fast axonal transport. These mice showed unaltered neuroinvasion, suggesting that transport mechanisms distinct from fast axonal transport effect prion neuroinvasion along peripheral nerves. Surprisingly, scrapie-sick tau transgenic mice accumulated intraneuronal deposits of hyperphosphorylated tau protein. The coincidence of tau and prion pathology resembled Gerstmann-Sträussler-Scheinker syndrome. These findings identify tau pathology as a possible end stretch of prion-induced neurodegeneration.

Amino acid residue 184 of yeast Hsp104 chaperone is critical for prion-curing by guanidine, prion propagation, and thermotolerance

Inactivation of Hsp104 by guanidine is contended to be the mechanism by which guanidine cures yeast prions. We now find an Hsp104 mutation (D184N) that confers resistance to guanidine-curing of the yeast [PSI(+)] prion. In an independent screen we isolated an HSP104 allele altered in the same residue (D184Y) that dramatically impairs [PSI(+)] propagation in a temperature-dependent manner. Directed mutagenesis of HSP104 produced additional alleles that conferred varying degrees of resistance to guanidine-curing or impaired [PSI(+)] propagation. The mutations similarly affected propagation of the [URE3] prion. Basal and induced abundance of all mutant proteins was normal. Thermotolerance of cells expressing mutant proteins was variably resistant to guanidine, and the degree of thermotolerance did not correlate with [PSI(+)] stability. We thus show that guanidine cures yeast prions by inactivating Hsp104 and identify a highly conserved Hsp104 residue that is critical for yeast prion propagation. Our data suggest that Hsp104 activity can be reduced substantially without affecting [PSI(+)] stability, and that Hsp104 interacts differently with prion aggregates than with aggregates of thermally denatured protein.

Absence of the prion protein homologue Doppel causes male sterility

The agent that causes prion diseases is thought to be identical with PrP(Sc), a conformer of the normal prion protein PrP(C). PrP(C)-deficient mice do not exhibit major pathologies, perhaps because they express a protein termed Dpl, which shares significant biochemical and structural homology with PrP(C). To investigate the physiological function of Dpl, we generated mice harbouring a homozygous disruption of the Prnd gene that encodes Dpl. Dpl deficiency did not interfere with embryonic and postnatal development, but resulted in male sterility. Dpl protein was expressed at late stages of spermiogenesis, and spermatids of Dpl mutants were reduced in numbers, immobile, malformed and unable to fertilize oocytes in vitro. Mechanical dissection of the zona pellucida partially restored in vitro fertilization. We conclude that Dpl regulates male fertility by controlling several aspects of male gametogenesis and sperm-egg interaction.

Profound sex-specific effects on incubation times for transmission of bovine spongiform encephalopathy to mice

Four strains of mice were inoculated intracerebrally with a primary isolate of bovine spongiform encephalopathy (BSE) and the cloned mouse-adapted scrapie strain ME7. Clinical prion disease diagnosis was made at the appearance of three or more neurological symptoms and their persistence for 3 consecutive weeks and confirmed by neuropathological criteria. For BSE, incubation periods were profoundly different between the sexes in all four mouse strains, being longer in the females. In contrast, ME7 scrapie incubation times were similar between the sexes. Our results indicate that sex-specific processes are involved in the course of primary BSE transmission. Research into this phenomenon may provide clues to the prophylaxis of BSE and have possible implications for new variant Creutzfeldt-Jakob disease in humans.

Filipin prevents pathological prion protein accumulation by reducing endocytosis and inducing cellular PrP release

Conversion of the normal membrane-bound prion protein (PrP-sen) to its pathological isoform (PrP-res) is a key event in the pathogenesis of transmissible spongiform encephalopathies. Although the subcellular sites of conversion are poorly characterized, several lines of evidence have suggested the involvement of membrane lipid rafts in the conversion process. Here we report that copper stimulates the endocytosis of PrP-sen via a caveolin-dependent pathway in both microglia and neuroblastoma cells. We show that the polyene antibiotic filipin both limits endocytosis of PrP-sen and dramatically reduces the amount of membrane-bound PrP-sen. This reduction results from a rapid and massive release of full matured PrP-sen into the culture medium. Finally, we demonstrate that filipin is a potent inhibitor of PrP-res formation into chronically infected neuroblastoma cells. Our results reinforce the role of rafts in PrP trafficking and raise the possibility that the release of PrP-sen from the plasma membrane decreases the amount of available substrate PrP-sen at the conversion sites.

Kuru: the old epidemic in a new mirror

The kuru epidemic lasted almost a century; it started in 1901-1902, reached epidemic proportions in the mid-1950s, and disappeared in the 1990s. Kuru is the prototype member of a group of disorders known as transmissible spongiform encephalopathies (TSEs) or prion diseases. Recent data on the genetics and pathogenesis of TSEs contribute to a better understanding of the documented kuru phenomena, and vice versa, observations made during the kuru epidemic are immensely helpful in understanding the epidemic of variant Creutzfeldt-Jakob disease that is currently developing in Europe. The major goal of this review is to identify and illustrate these points.

Prions: a mystery unravelled?

Prions result in fatal degeneration of the central nervous system (CNS) in the form of diseases known as transmissible spongiform encephalopathies (TSEs). The discovery in 1996 of a new variant of Creutzfeldt-Jakob disease (a human TSE) and experimental confirmation that it is caused by the prion strain responsible for bovine spongiform encephalopathy (BSE) has greatly spurred research in this field. The mechanism underlying prion propagation is now reasonably clear. Prions multiply, in fact, by stimulating their hosts to produce proteins that are initially normal, but acquire an abnormal, prion-like conformation during the coiling stage. A fuller understanding of this mechanism could lead to the employment of molecules capable of making prion proteins revert to the normal conformation in the treatment of both TSEs and other serious CNS disorders.