Prions: health scare and biological challenge

A Pathogenic PrP Mutation and Doppel Interfere with Polarized Sorting of the Prion Protein

Several proteins linked to neurodegenerative diseases, such as the beta-amyloid precursor protein, amyloid beta-peptide, beta-secretase, and tau, undergo selective polarized sorting. We investigated polarized sorting of the mammalian prion protein (PrP(C)) and its homologue doppel (Dpl). In contrast to Dpl, which accumulates on the apical surface, PrP(C) is targeted selectively to the basolateral side in Madin-Darby canine kidney cells. An extensive deletion and domain swapping analysis revealed that the internal hydrophobic domain (HD) of PrP (amino acids 113-133) confers basolateral sorting in a dominant manner. PrP mutants lacking the HD are sorted apically, while Dpl chimeras containing the HD of PrP are directed to the basolateral membrane. Furthermore, a pathogenic PrP missense mutation within the HD leads to aberrant apical sorting of PrP as well.

Raman analysis of prion protein in blood cell membranes from naturally affected scrapie sheep

At present, there are no efficient antemortem diagnostic tests for transmissible spongiform encephalopathys (TSEs), particularly in blood. New strains of prion PrPSc, which causes TSEs, are currently appearing, and researchers remain concerned that if prion variants continue to emerge, some of them may escape detection by existing immunoassay tests. Because a common structural feature of PrPSc strains is their high content of beta sheets, Raman spectroscopy has proven to be a suitable technique to analyze a key membranous fraction of blood containing PrPSc. In this fraction, a significant increase in beta sheets has been correlated with the worsening of this TSE in naturally scrapie-infected animals in comparison with healthy controls. Since sensitivity and specificity were found to be 100% for each, this test may lead to a new and alternative diagnosis for prion diseases.

Metabotropic glutamate receptor/phospholipase C pathway: A vulnerable target to Creutzfeldt-Jakob disease in the cerebral cortex

Glutamate is the main excitatory neurotransmitter in the cerebral cortex. Altered glutamatergic transmission has been suggested as having a central role in many neurodegenerative diseases. Metabotropic glutamate receptors (mGluRs) are coupled to intracellular signal transduction via G proteins, and they mediate slower responses than ionotropic glutamate receptors. Group I mGluRs are positively coupled to phospholipase C beta1 (PLCbeta1). Creutzfeldt-Jakob disease (CJD) is a human transmissible spongiform encephalopathy associated with a dysfunction in the membrane glycoprotein PrP which is converted into an abnormal isoform, with a predominant beta-sheet structure, that is pathogenic and partially resistant to protease digestion. Proteins associated with the signal transduction of group I mGluRs were examined in the frontal cortex (area 8) of 12 cases with sCJD and four age-matched controls, by means of gel electrophoresis and Western blotting of total homogenates. Densitometric analysis of the bands demonstrated decreased expression levels of PLCbeta1 and PLCgamma, a non-related phospholipase which is a substrate of tyrosine kinase, in CJD cases when compared with controls. Novel protein kinase C delta (nPKCdelta) has also been found to be significantly decreased in CJD cases. However, no modifications in mGluR1 cPKCalpha expression levels are found in CJD when compared with controls. No modifications in PLCbeta1 solubility in PBS-, deoxycholate- and sodium dodecylsulphate-soluble fractions have been observed in CJD when compared with controls. Finally, no interactions between PLCbeta1 and PrP, as revealed by immunoprecipitation assays, have been found in CJD and controls. The present results show, for the first time, reduced expression levels of phospholipases, particularly PLCbeta1, which may interfere with group I mGluR signaling in the cerebral cortex in CJD. These abnormalities are not the result of abnormal PLC solubility or interactions with PrP. Selective involvement of group I mGluRs may have functional effects on glutamatergic transmission modulation and processing in CJD.

Intrinsic resistance of oligodendrocytes to prion infection

Within the CNS, the normal form of cellular prion protein (PrP(C)) is expressed on neurons, oligodendrocytes, and astrocytes. The contribution of these cell types to prion replication and pathogenesis is unclear. To assess the role of oligodendrocytes, we expressed PrP(C) under the control of the myelin basic protein (MBP) promoter in mice lacking endogenous PrP(C). PrP(C) was detected in oligodendrocytes and Schwann cells but not in neurons and astrocytes. MBP-PrP mice never developed scrapie after intracerebral, intraperitoneal, or intraocular challenge with scrapie prions. Transgenic brains did not contain protease-resistant prion protein and did not transmit scrapie when inoculated into PrP(C)-overexpressing indicator mice. To investigate whether prion spread within the CNS depends on oligodendrocytic PrP(C), we implanted PrP(C)-overexpressing neuroectodermal grafts into MBP-PrP brains. After intraocular prion inoculation, none of the grafts showed spongiform encephalopathy or prion infectivity. Hence oligodendrocytes do not support cell-autonomous prion replication, establishment of subclinical disease, and neural spread of prions. Prion resistance sets oligodendrocytes aside from both neurons and astrocytes.

Pathogenic mutations located in the hydrophobic core of the prion protein interfere with folding and attachment of the glycosylphosphatidylinositol anchor

Abnormal folding of the cellular prion protein (PrPC) is a key feature in prion diseases. Here we show that two pathogenic mutations linked to inherited prion diseases in humans severely affect folding and maturation of PrPC in the secretory pathway of neuronal cells. PrP-T183A and PrP-F198S adopt a misfolded and partially protease-resistant conformation, lack the glycosylphosphatidylinositol anchor, and are not complex glycosylated. These misfolded PrP mutants are not retained in the endoplasmic reticulum and are not subjected to the endoplasmic reticulum-associated degradation pathway. They rather are secreted, moreover, these mutants can be internalized by heterologous cells. Structural studies indicated that the side chains of Thr183 and Phe198 contribute to interactions between secondary structure elements in the C-terminal globular domain of PrPC. Consequently, we reasoned that a destabilized tertiary structure of these mutants could account for the defect in maturation. Indeed, mutations predicted to interfere selectively with the packing of the hydrophobic core of PrPC prevented the addition of the glycosylphosphatidylinositol anchor. Our study reveals that formation of the C-terminal globular domain of PrPC has an impact on membrane anchoring and indicates that misfolded secreted forms of the prion protein are linked to inherited prion diseases in humans.

Antiprion immunotherapy: to suppress or to stimulate?

Although human prion diseases are rare, they are invariably fatal, and treatments remain elusive. Hundreds of iatrogenic prion transmissions have occurred in the past two decades, and the bovine spongiform encephalopathy epidemic has raised concerns about prion transmission from cattle to humans. Research into therapeutics for prion disease is being pursued in several centres and prominently includes immunological strategies. Currently, the options that are being explored aim either to mobilize the innate and adaptive immune systems towards prion destruction or to suppress or dedifferentiate the lymphoreticular compartments that replicate prions. This article reviews the pathophysiology of prion diseases in mouse models and discusses their relevance to immunotherapeutic and immunoprophylactic antiprion strategies.

Progress and problems in the biology, diagnostics, and therapeutics of prion diseases

The term "prion" was introduced by Stanley Prusiner in 1982 to describe the atypical infectious agent that causes transmissible spongiform encephalopathies, a group of infectious neurodegenerative diseases that include scrapie in sheep, Creutzfeldt-Jakob disease in humans, chronic wasting disease in cervids, and bovine spongiform encephalopathy in cattle. Over the past twenty years, the word "prion" has been taken to signify various subtly different concepts. In this article, we refer to the prion as the transmissible principle underlying prion diseases, without necessarily implying any specific biochemical or structural identity. When Prusiner started his seminal work, the study of transmissible spongiform encephalopathies was undertaken by only a handful of scientists. Since that time, the "mad cow" crisis has put prion diseases on the agenda of both politicians and the media. Significant progress has been made in prion disease research, and many aspects of prion pathogenesis are now understood. And yet the diagnostic procedures available for prion diseases are not nearly as sensitive as they ought to be, and no therapeutic intervention has been shown to reliably affect the course of the diseases. This article reviews recent progress in the areas of pathogenesis of, diagnostics of, and therapy for prion diseases and highlights some conspicuous problems that remain to be addressed in each of these fields.

Pharmacokinetics and distribution of sodium 3,4-diaminonaphthalene-1-sulfonate, a Congo Red derivative active in inhibiting PrP(res) replication

Sodium 3,4-diaminonaphthalene-1-sulfonate (CRA) is a compound, synthesised by our group from Congo Red (CR), that is active in preventing the pathological conversion of normal prion protein (PrP). As the precise mechanisms controlling the ways in which prions are distributed and infect the brain and other organs are not fully understood, studying the pharmacokinetics of drugs that are active against prions may clarify their targets and their means of inhibiting prion infection. This paper describes the pharmacokinetics of CRA in plasma, spleen and brain after single or repeated intraperitoneal or subcutaneous administration, as determined by means of specific and sensitive fluorimetric HPLC. A single intraperitoneal administration led to peak plasma CRA concentrations after 15 min, followed by biphasic decay with an apparent half-life of 4.3 h. After subcutaneous administration, T(max) was reached after 30 min, and was followed by a similar process of decay: Cmax and the AUC0-last were 25% those recorded after intraperitoneal administration. The mean peak concentrations and AUCs of CRA after a single intraperitoneal or subcutaneous administration in peripheral tissue (spleen) were similar to those observed in blood, whereas brain concentrations were about 2% those in plasma. After repeated intraperitoneal or subcutaneous doses, the Cmax values in plasma, brain and spleen were similar to those observed at the same times after a single dose. After repeated intraperitoneal doses, CRA was also found in the ventricular cerebrospinal fluid at concentrations of 1.8 +/- 0.2 microg(-1) mL, which is similar to, or slightly higher than, those found in brain. Brain concentrations may be sufficient to explain the activity of CRA on PrP reproduction in the CNS. However, peripheral involvement cannot be excluded because the effects of CRA are more pronounced after intraperitoneal than after intracerebral infection.

Disruption of Doppel prevents neurodegeneration in mice with extensive Prnp deletions

The Prnp gene encodes the cellular prion protein PrP(C). Removal of its ORF does not result in pathological phenotypes, but deletions extending into the upstream intron result in cerebellar degeneration, possibly because of ectopic cis-activation of the Prnd locus that encodes the PrP(C) homologue Doppel (Dpl). To test this hypothesis, we removed Prnd from Prnp(o/o) mice by transallelic meiotic recombination. Balanced loxP-mediated ablation yielded mice lacking both PrP(C) and Dpl (Prn(o/o)), which developed normally and showed unimpaired immune functions but suffered from male infertility. However, removal of the Prnd locus abolished cerebellar degeneration, proving that this phenotype is caused by Dpl upregulation. The absence of compound pathological phenotypes in Prn(o/o) mice suggests the existence of alternative compensatory mechanisms. Alternatively, Dpl and PrP(C) may exert distinct functions despite having partly overlapping expression profiles.

Variant Creutzfeldt–Jakob disease: between lymphoid organs and brain

Prion diseases are often caused by peripheral uptake of the infectious agent. To reach their ultimate target, the central nervous system (CNS), prions enter their host, replicate in lymphoid organs and spread via peripheral nerves. Once the agent has reached the CNS disease progression is rapid, resulting in neurodegeneration and death. many of these mechanisms have been uncovered using genetically modified mice. A recently published study demonstrated the presence of pathological prion protein in sympathetic ganglia of patients suffering from variant Creutzfeldt-Jakob disease, suggesting that these mechanisms might apply to humans.

Lymphoid follicle destruction and immunosuppression after repeated CpG oligodeoxynucleotide administration

DNA containing unmethylated cytidyl guanosyl (CpG) sequences, which are underrepresented in mammalian genomes but prevalent in prokaryotes, is endocytosed by cells of the innate immune system, including macrophages, monocytes and dendritic cells, and activates a pathway involving Toll-like receptor-9 (TLR9). CpG-containing oligodeoxynucleotides (CpG-ODN) are potent stimulators of innate immunity, and are currently being tested as adjuvants of antimicrobial, antiallergic, anticancer and antiprion immunotherapy. Little is known, however, about the consequences of repeated CpG-ODN administration, which is advocated for some of these applications. Here we report that daily injection of 60 microg CpG-ODN dramatically alters the morphology and functionality of mouse lymphoid organs. By day 7, lymphoid follicles were poorly defined; follicular dendritic cells (FDC) and germinal center B lymphocytes were suppressed. Accordingly, CpG-ODN treatment for > or =7 d strongly reduced primary humoral immune responses and immunoglobulin class switching. By day 20, mice developed multifocal liver necrosis and hemorrhagic ascites. All untoward effects were strictly dependent on CpG and TLR9, as neither the CpG-ODN treatment of Tlr9(-/-) mice nor the repetitive challenge of wild-type mice with nonstimulatory ODN (AT-ODN) or with the TLR3 agonist polyinosinic:cytidylic acid (polyI:C) were immunotoxic or hepatotoxic.

Primary Cultured Neurons Devoid of Cellular Prion Display Lower Responsiveness to Staurosporine through the Control of p53 at Both Transcriptional and Post-transcriptional Levels

We assessed the contribution of the cellular prion protein (PrPc) in the control of neuronal apoptosis by examining cell death in both human cells and murine primary cultured neurons. We first confirmed our previous finding that staurosporine-induced caspase activation is increased by PrPc overexpression in HEK293 cells. We show here that this phenotype is fully dependent on p53 and that the control of p53 activity by PrPc occurs at both transcriptional and post-transcriptional levels in human cells. Of most interest, we demonstrate that neuronal endogenous PrPc also controls a p53-dependent pro-apoptotic phenotype. Thus, DNA fragmentation and TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling)-positive cells were lower in primary cultured neurons derived from Zrch-1 mice embryos in which PrPc has been abrogated than in wild-type neurons. PrPc knock-out neurons also displayed drastically diminished caspase-3-like activity and immunoreactivity together with reduced p53 expression and transcriptional activity, a phenotype complemented in part by PrPc transfection. Interestingly, p53 expression was also reduced in the brain of adult Prnp-/- mice. Neuronal PrPc likely controls p53 at a post-transcriptional level because the deletion of cellular prion protein is accompanied by a higher Mdm2-like immunoreactivity and reduced phosphorylated p38 MAPK expression. We therefore propose that the physiological function of endogenous cellular prion could be to regulate p53-dependent caspase-3-mediated neuronal cell death. This phenotype likely occurs through up-regulation of p53 promoter transactivation as well as downstream by controlling p53 stability via Mdm2 expression.

Human prion diseases: epidemiology and integrated risk assessment

Human prion diseases are devastating and incurable, but are very rare. Fears that the bovine spongiform encephalopathy epizootic would lead to a large epidemic of its presumed human counterpart, variant Creutzfeldt-Jakob disease (vCJD), have not been realised. Yet a feeling of uncertainty prevails in the general public and in the biomedical world. The lack of data on the prevalence of asymptomatic carriers of vCJD compounds this uncertainty. In addition to this problem, Switzerland is currently faced with another issue of major public concern: a recent rise in the incidence of CJD. Here we examine the plausibility of several scenarios that may account for the increase in CJD incidence, including ascertainment bias due to improved reporting of CJD, iatrogenic transmission, and transmission of a prion zoonosis. In addition, we present the design and current status of a Swiss population-wide study of subclinical vCJD prevalence.

Games played by rogue proteins in prion disorders and Alzheimer's disease

The incidence of Alzheimer's disease (AD) and that of prion disorders (PrD) could not be more different. One-third of octogenarians succumb to AD, whereas Creutzfeldt-Jakob disease typically affects one individual in a million each year. However, these diseases have many common features impinging on the metabolism of neuronal membrane proteins: the amyloid precursor protein APP in the case of AD, and the cellular prion protein PrPC in PrD. APP begets the Abeta peptide, whereas PrPC begets the malignant prion protein PrPSc. Both Abeta and PrPSc are associated with disease, but we do not know what triggers their accumulation and neurotoxicity. A great deal has been learned, however, about protein folding, misfolding, and aggregation; an entirely new class of intramembrane proteases has been identified; and unsuspected roles for the immune system have been uncovered. There is reason to expect that prion research will profit from advances in the understanding of AD, and vice versa.

Extraneural pathologic prion protein in sporadic Creutzfeldt-Jakob disease

BACKGROUND

In patients with sporadic Creutzfeldt-Jakob disease, pathologic disease-associated prion protein (PrPSc) has been identified only in the central nervous system and olfactory-nerve tissue. Understanding the distribution of PrPSc in Creutzfeldt-Jakob disease is important for classification and diagnosis and perhaps even for prevention.

METHODS

We used a highly sensitive method of detection--involving the concentration of PrPSc by differential precipitation with sodium phosphotungstic acid, which increased the sensitivity of Western blot analysis by up to three orders of magnitude--to search for PrPSc in extraneural organs of 36 patients with sporadic Creutzfeldt-Jakob disease who died between 1996 and 2002.

RESULTS

PrPSc was present in the brain tissue of all patients. In addition, we found PrPSc in 10 of 28 spleen specimens and in 8 of 32 skeletal-muscle samples. Three patients had PrPSc in both spleen and muscle specimens. Patients with extraneural PrPSc had a significantly longer duration of disease and were more likely to have uncommon molecular variants of sporadic Creutzfeldt-Jakob disease than were patients without extraneural PrPSc.

CONCLUSIONS

Using sensitive techniques, we identified extraneural deposition of PrPSc in spleen and muscle samples from approximately one third of patients who died with sporadic Creutzfeldt-Jakob disease. Extraneural PrPSc appears to correlate with a long duration of disease.

Positioning of follicular dendritic cells within the spleen controls prion neuroinvasion

Peripheral infection is the natural route of transmission in most prion diseases. Peripheral prion infection is followed by rapid prion replication in lymphoid organs, neuroinvasion and progressive neurological disease. Both immune cells and nerves are involved in pathogenesis, but the mechanisms of prion transfer from the immune to the nervous system are unknown. Here we show that ablation of the chemokine receptor CXCR5 juxtaposes follicular dendritic cells (FDCs) to major splenic nerves, and accelerates the transfer of intraperitoneally administered prions into the spinal cord. Neuroinvasion velocity correlated exclusively with the relative locations of FDCs and nerves: transfer of CXCR5-/- bone marrow to wild-type mice induced perineural FDCs and enhanced neuroinvasion, whereas reciprocal transfer to CXCR5-/- mice abolished them and restored normal efficiency of neuroinvasion. Suppression of lymphotoxin signalling depleted FDCs, abolished splenic infectivity, and suppressed acceleration of pathogenesis in CXCR5-/- mice. This suggests that prion neuroimmune transition occurs between FDCs and sympathetic nerves, and relative positioning of FDCs and nerves controls the efficiency of peripheral prion infection.

Post-translational import of the prion protein into the endoplasmic reticulum interferes with cell viability: a critical role for the putative transmembrane domain

Aberrant folding of the mammalian prion protein (PrP) is linked to prion diseases in humans and animals. We show that during post-translational targeting of PrP to the endoplasmic reticulum (ER) the putative transmembrane domain induces misfolding of PrP in the cytosol and interferes with its import into the ER. Unglycosylated and misfolded PrP with an uncleaved N-terminal signal sequence associates with ER membranes, and, moreover, decreases cell viability. PrP expressed in the cytosol, lacking the N-terminal ER targeting sequence, also adopts a misfolded conformation; however, this has no adverse effect on cell growth. PrP processing, productive ER import, and cellular viability can be restored either by deleting the putative transmembrane domain or by using a N-terminal signal sequence specific for co-translational ER import. Our study reveals that the putative transmembrane domain features in the formation of misfolded PrP conformers and indicates that post-translational targeting of PrP to the ER can decrease cell viability.

The sympathetic nervous system is involved in variant Creutzfeldt-Jakob disease

Prion epizoonoses spread from animals consumed by humans raise the question of which pathways lead to prion neuroinvasion after oral exposure of humans. Here we show that neurons of sympathetic ganglia of patients with variant Creutzfeldt-Jakob disease (vCJD) accumulate the abnormal isoform of the protein prion. This observation shows the involvement of the sympathetic nervous system in the pathogenesis of vCJD and suggests a role for GUT-associated sympathetic neurons in prion propagation in humans after oral contamination.

Up-regulation of cathepsin B and cathepsin L activities in scrapie-infected mouse Neuro2a cells

Prion diseases are characterized by the accumulation of an abnormal, proteinase K-resistant isoform of the prion protein, PrP(Sc), which is generated by a post-translational conversion of the protease-sensitive normal cell-surface glycoprotein PrP(c) involving major conformational changes. The conversion is thought to occur at the plasma membrane or along the endocytic pathway towards the lysosome. PrP(Sc) aggregates have been found to accumulate in secondary lysosomes. In our study, the activities of two major lysosomal cysteine proteases, cathepsins B and L, were found to be significantly increased in scrapie-infected Neuro2a cells compared with uninfected cells using biochemical and cytochemical methods. We hypothesize that lysosomal proteases may be involved in a 'second autocatalytic loop' of PrP(Sc) formation, acting in concert with the well-known autocatalytic enhancement of PrP conversion in the presence of PrP(Sc).

Overview of the needs and realities for developing new and improved vaccines in the 21st century

The science of present day vaccinology is based on the pioneering discoveries of the late 18th and late 19th centuries and the technologic breakthroughs of the past 60 years. The driving force for the development of new vaccines resides in technologic feasibility, public need and economic incentive for translating the basic knowledge into a product. Past efforts by government to define which particular vaccines to develop were mostly irrelevant to the realistic choices which were made. There is a vast array of viral, bacterial, parasitic and fungal disease agents against which preventative vaccines may be developed, and to this may be added cancer and certain amyloidoses such as Alzheimer's and 'mad cow' diseases. The proven past for vaccines has relied on live, killed, protein and polysaccharide antigens plus the single example of recombinant-expressed hepatitis B vaccine. The validity of redirection of vaccinology to exploration of simplified vaccines such as recombinant vectored and DNA preparations and reductionist vaccines based on peptides of contrived epitope composition remains to be proved. Reductionism imposes vastly increased complexity and difficulty on vaccine development and might not be capable of achievement. The challenge in the 21st century will involve new and uncertain pathways toward worthwhile accomplishments.

Soluble dimeric prion protein binds PrP(Sc) in vivo and antagonizes prion disease

Conversion of cellular prion protein (PrP(C)) into a pathological conformer (PrP(Sc)) is thought to be promoted by PrP(Sc) in a poorly understood process. Here, we report that in wild-type mice, the expression of PrP(C) rendered soluble and dimeric by fusion to immunoglobulin Fcgamma (PrP-Fc(2)) delays PrP(Sc) accumulation, agent replication, and onset of disease following inoculation with infective prions. In infected PrP-expressing brains, PrP-Fc(2) relocates to lipid rafts and associates with PrP(Sc) without acquiring protease resistance, indicating that PrP-Fc(2) resists conversion. Accordingly, mice expressing PrP-Fc(2) but lacking endogenous PrP(C) are resistant to scrapie, do not accumulate PrP-Fc(2)(Sc), and do not transmit disease to others. These results indicate that various PrP isoforms engage in a complex in vivo, whose distortion by PrP-Fc(2) affects prion propagation and scrapie pathogenesis. The unique properties of PrP-Fc(2) suggest that soluble PrP derivatives may represent a new class of prion replication antagonists.

Oral prion infection requires normal numbers of Peyer's patches but not of enteric lymphocytes

Prion pathogenesis following oral exposure is thought to involve gut-associated lymphatic tissue, which includes Peyer's patches (PPs) and M cells. Recruitment of activated B lymphocytes to PPs requires alpha(4)beta(7) integrin; PPs of beta 7(-/-) mice are normal in number but are atrophic and almost entirely devoid of B cells. Here we report that minimal infectious dose and disease incubation after oral exposure to logarithmic dilutions of prion inoculum were similar in beta 7(-/-) and wild-type mice, and PPs of both beta 7(-/-) and wild-type mice contained 3-4 log LD(50)/g prion infectivity > or =125 days after challenge. Despite marked reduction of B cells, M cells were present in beta 7(-/-) mice. In contrast, mice deficient in both tumor necrosis factor and lymphotoxin-alpha (TNF alpha(-/-) x LT alpha(-/-)) or in lymphocytes (RAG-1(-/-), mu MT), in which numbers of PPs are reduced in number, were highly resistant to oral challenge, and their intestines were virtually devoid of prion infectivity at all times after challenge. Therefore, lymphoreticular requirements for enteric and for intraperitoneal uptake of prions differ from each other. Although susceptibility to prion infection following oral challenge correlates with the number of PPs, it is remarkably independent of the number of PP-associated lymphocytes.

Cellular prion protein sensitizes neurons to apoptotic stimuli through Mdm2-regulated and p53-dependent caspase 3-like activation

We examined the influence of cellular prion protein (PrP(c)) in the control of cell death in stably transfected TSM1 cells. PrP(c) expression enhanced staurosporine-stimulated neuronal toxicity and DNA fragmentation, caspase 3-like activity and immunoreactivity, and p53 immunoreactivity and transcriptional activities. Caspase activation was reduced by the chemical inhibitor of p53, pifithrin-alpha, as well as by PrP(c)- or p53-antisense approaches but remained insensitive to the Fyn kinase inhibitor PP2 (4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine). We establish that PrP(c) controls p53 at a post-transcriptional level and is reversed by Mdm2 transfection and p38 MAPK inhibitor. We propose that endogenous cellular prion protein sensitizes neurons to apoptotic stimuli through a p53-dependent caspase 3-mediated activation controlled by Mdm2 and p38 MAPK.

An evolutionary basis for scrapie disease: identification of a fish prion mRNA

Infectious prion proteins cause neurodegenerative disease in mammals owing to the acquisition of an aberrant conformation. We cloned a Fugu rubripes gene that encodes a structurally conserved prion protein, and found rapid rates of molecular divergence among prions from different vertebrate classes, along with molecular stasis within each class. We propose that a directional trend in the evolution of prion sequence motifs associated with pathogenesis and infectivity could account for the origin of scrapie in mammals.

Prion pathogenesis in the absence of Toll-like receptor signalling

To reach the brain from peripheral sites, prions must colonize various cell types within the lymphoreticular compartment. However, no prion entry receptors are yet known. Toll-like receptors (TLRs) are pattern-recognition receptors that bind a multitude of pathogens and are therefore candidates as effectors of prion entry. Moreover, injection of unmethylated CpG oligodinucleotides, which stimulate TLR9, has been reported to delay peripherally initiated scrapie. We therefore studied prion infection in MyD88(-/-) mice, which are defective in TLR signalling. Despite subtle defects in splenic microarchitecture, MyD88(-/-) mice challenged intraperitoneally or intracerebrally were fully susceptible to disease and died of scrapie after similar incubation times to those of wild-type mice. Splenic infectivity titres rose to similar levels with the same kinetics, and brains showed similar histopathological changes. TLR signalling therefore does not have any major role in prion pathogenesis, and the protective effect of TLR stimulation is unlikely to result from direct interactions with prions.

CD34+ cells from paroxysmal nocturnal hemoglobinuria (PNH) patients are deficient in surface expression of cellular prion protein (PrPc)

Cellular prion protein (PrP(c)) is a glycosylphosphatidylinositol (GPI)-anchored protein (GPI-AP) constitutively expressed by neurons but also in hematopoietic cells. In trasmissible spongiform encephalopathies, the protease-resistant form of prion (PrP (s c)) converts the host PrP(c) into the pathologic form. We have investigated PrP(c) expression in hematopoietic cells from paroxysmal nocturnal hemoglobinuria (PNH). In this disease, due to somatic mutations in PIG-A gene, biosynthesis of the (GPI)-anchor is impaired and affected cells lack membrane expression of all GPI-AP. Normal and PNH hematopoietic progenitors and paired wild-type (WT) and PIG-A mutant cell lines were used for analysis of intracellular and surface PrP(c) expression using flow cytometry and Western blot.By flow cytometry, PrP(c) was constitutively present on normal CD34(+) cells, including more immature CD38(dim) cells, as well as hematopoietic cell lines. Similar results were obtained in purified CD34(+). Phospholipase C treatment confirmed that PrP(c) was expressed on the membrane via the GPI-anchor. In PNH patients, GPI-AP-deficient CD34(+) cells lacked PrP(c) membrane expression. PIG-A-mutated cell lines (Jurkat, K562, C(EBV), A(EBV)), in contrast to their normal counterparts, did not express surface PrP(c). However, we detected intracellular PrP(c) at approximately equivalent levels in both normal and PIG-A-mutated cells using intracellular flow cytometry and Western blotting. Cells and cell lines with PNH phenotype together with their normal counterparts may be a suitable system to explore the function of membrane PrP(c) in the hematopoietic system. Conversely, PrP(c) is a good model to elucidate the fate of GPI-AP in PIG-A-deficient cells.

CNS pathogenesis of prion diseases

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases, clinically characterised by cognitive decline, paralleled by severe damage to the central nervous system. Prion diseases have attracted a broad interest because of their unique mechanisms of replication and propagation; however, the underlying pathogenic mechanisms are still highly speculative. In this review, current knowledge about the pathogenesis of prion diseases in the CNS will be highlighted and the most revealing animal models will be discussed, with future perspectives to address immediate questions about the pathogenesis.

Prions and the immune system: a journey through gut, spleen, and nerves

For more than two decades it has been contended that prion infection does not elicit immune responses: transmissible spongiform encephalopathies do not go along with conspicuous inflammatory infiltrates, and antibodies to the prion protein are typically undetectable. Why is it, then, that prions accumulate in lymphoid organs, and that various states of immune deficiency prevent peripheral prion infection? This review revisits the current evidence of the involvement of the immune system in prion diseases, while attempting to trace the elaborate mechanisms by which peripherally administered prions invade the brain and ultimately cause damage. The investigation of these questions leads to unexpected detours, including the neurophysiology of lymphoid organs, and even the function of a prion protein homolog in male fertility.

Immune system and peripheral nerves in propagation of prions to CNS

Prions are not only unique in the way they replicate. Also the sequence of events triggered by peripheral prion infection, generically termed 'peripheral pathogenesis', sets prions aside from all other known pathogens. Whereas most bacteria, parasites, and viruses trigger innate and adaptive immune responses, the mammalian immune system appears to be remarkably oblivious to prions. Transmissible spongiform encephalopathies (TSEs) do not go along with inflammatory infiltrates, and antibodies to the prion protein are not typically raised during the course of the disease. On the other hand, there is conspicuous involvement of lymphoid organs, which accumulate sizeable concentrations of the infectious agent early during disease. Moreover, various states of immune deficiency can abolish peripheral pathogenesis and prevent 'take' of infection when prions are administered to peripheral sites. Here, we critically re-visit the current evidence for an involvement of the immune system in prion diseases, and will attempt to trace the elaborate mechanisms by which prions, upon entry into the body from peripheral sites, reach the brain.

The thioredoxin system?From science to clinic

The thioredoxin system-formed by thioredoxin reductase and its characteristic substrate thioredoxin-is an important constituent of the intracellular redox milieu. Interactions with many different metabolic pathways such as DNA-synthesis, selenium metabolism, and the antioxidative network as well as significant species differences render this system an attractive target for chemotherapeutic approaches in many fields of medicine-ranging from infectious diseases to cancer therapy. In this review we will present and evaluate the preclinical and clinical results available today. Current trends in drug development are emphasized.

Alzheimer's and prion diseases: distinct pathologies, common proteolytic denominators

Alzheimer's and prion pathologies are often seen as distinct neurodegenerative diseases, particularly because the infectious character of some prion-associated pathology makes this stand apart from classical neurodegenerative, age-related syndromes. Are there specific common denominators that could link the two diseases? It appears that betaAPP (beta-amyloid precursor protein) and PrP(c) (cellular prion protein), the 'guilty' proteins involved in these pathologies, undergo protein-kinase-C-regulated proteolysis by identical proteases of the disintegrin family. This cleavage occurs in an analogous way, in the middle of the 'toxic' Abeta and PrP(c)106-126 domains of betaAPP and PrP(c), respectively. As these two sequences trigger similar caspase-dependent and -independent cascades, this proteolytic attack could be seen as an inactivating process aimed at clearing cells of these endogenous 'toxins' and, thus, preventing the associated proteinaceous accumulation usually detected in affected brains. It is our opinion that targeting these disintegrins with specific 'activators' could be a suitable strategy to slow down, or even arrest, betaAPP and PrP(c)-related aggregation and toxicity.

Overexpression of PrPc triggers caspase 3 activation: potentiation by proteasome inhibitors and blockade by anti-PrP antibodies

We examined the influence of cellular prion protein (PrPc) in the control of cell death in stably transfected HEK293 cell line and in the PrPc-inducible Rov9 cells. PrPc expression in stably transfected HEK293 human cells did not modify basal apoptotic tonus but drastically potentiated staurosporine-stimulated cellular toxicity and DNA fragmentation as well as caspase 3-like activity and immunoreactivity. An identical staurosporine-induced caspase 3 activation was observed after doxycycline in the PrPc-inducible Rov9 cell line. Interestingly, proteasome inhibitors increase PrPc-like immunoreactivity and unmasked a basal caspase 3 activation. Conversely, we show that anti-PrPc antibodies sequestrate PrPc at the cell surface and drastically lower PrPc-dependent caspase activation. We suggest that intracellular PrPc could sensitize human cells to pro-apoptotic phenotype and that blockade of PrPc internalization could be a track to prevent intracellular toxicity associated with PrPc overexpression.

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.

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.

Quantification of fetomaternal hemorrhage by fluorescence microscopy is equivalent to flow cytometry

BACKGROUND

The quantification of fetal cells in the maternal circulation remains an important goal to determine the amount of anti-D necessary to prevent active immunization of a D- mother giving birth to a D+ baby. Underestimation of fetomaternal hemorrhage (FMH) results in inefficient anti-D prophylaxis and maternal immunization; overestimation of FMH results in higher doses of passively transferred anti-D, higher costs, and the risk of disease transmission. Thus, a reliable method to quantitatively assess FMH is necessary.

STUDY DESIGN AND METHODS

Serial dilutions of artificial FMH were quantitatively measured by three different methods: flow cytometry, fluorescence microscopy (each after anti-D staining), and by the Kleihauer-Betke test. The accuracy and precision of the three methods were compared by statistical analysis.

RESULTS

Fluorescence microscopy and flow cytometry were comparably accurate and precise in quantifying FMH. In contrast, the accuracy of the Kleihauer-Betke test was poor, resulting in substantial overestimation of FMH in the samples with lower fetal cell concentrations.

CONCLUSION

Anti-D flow cytometry and fluorescence microscopy for detection of fetal cells offer equally reliable and precise methods in contrast to the Kleihauer-Betke test. Fluorescence microscopy may be established as standard to quantify FMH in clinical practice because it is comparable to flow cytometry; in addition, it is time saving and is less expensive.

In-situ analysis of cellular poly(ADP-ribose) production in scrapie-infected mouse neuroblastoma cells

Transmissible spongiform encephalopathies (TSEs), also called prion diseases, are characterized by formation of the disease-associated isoform of prion protein (PrP(Sc)), which arises from a normal isoform termed PrP(c) by a post-translational conversion process occurring in an autocatalytic fashion. Oxidative stress has been proposed as a pathogenetic mechanism in TSEs and increased lipid peroxidation has recently been described in prion-infected cell cultures, suggesting an intrinsic link between the presence of prions and oxidative stress. We investigated if poly(ADP-ribose) formation can be detected in cultured cells upon prion infection, as this NAD+-consuming and DNA strand break-activated nuclear enzymatic reaction has the potential to cause rapid and lethal NAD+ depletion in cells under severe oxidative stress. Poly(ADP-ribose) production was analysed by immunofluorescence in freshly scrapie-infected Neuro2a-D11 mouse neuroblastoma cells, which had been confirmed by immunocytochemistry to produce PrP(Sc), and in uninfected controls. No spontaneous poly(ADP-ribose) specific signals were observed in infected or in uninfected cells, while both cell types readily reacted to H2O2 treatment with poly(ADP-ribose) synthesis in a dose-dependent manner, with no obvious difference in staining intensity at any dose tested. In summary, our data reveal that replication of scrapie agent in neuroblastoma cells can proceed without detectable stimulation of the cellular poly(ADP-ribosyl)ation system.

Small is not beautiful: antagonizing functions for the prion protein PrP(C) and its homologue Dpl

A conformational variant of the normal prion protein PrP(C) is believed to be identical to PrP(Sc), the agent that causes prion diseases. Recently, a novel protein, named Doppel (Dpl), was identified that shares significant biochemical and structural homology with PrP(C). In specific strains of PrP(C)-deficient mouse lines, Dpl is overexpressed and causes a neurological disease. Dpl neurotoxicity is counteracted and prevented by PrP(C), but the mechanism of antagonistic PrP(C)-Dpl interaction remains elusive. In contrast to its homologue PrP(C), initial studies suggest that Dpl is dispensable for prion disease progression and for the generation of PrP(Sc). Although we are only beginning to understand its function, the discovery of Dpl has already provided some answers to long-standing questions and is transforming our understanding of prion biology.

Similar turnover and shedding of the cellular prion protein in primary lymphoid and neuronal cells

The cellular prion protein (PrP(C)) is essential for pathogenesis and transmission of prion diseases. Although prion replication in the brain is accompanied by neurodegeneration, prions multiply efficiently in the lymphoreticular system without any detectable pathology. We have used pulse-chase metabolic radiolabeling experiments to investigate the turnover and processing of PrP(C) in primary cell cultures derived from lymphoid and nervous tissues. Similar kinetics of PrP(C) degradation were observed in these tissues. This indicates that the differences between these two organs with respect to their capacity to replicate prions is not due to differences in the turnover of PrP(C). Substantial amounts of a soluble form of PrP that lacks the glycolipid anchor appeared in the medium of splenocytes and cerebellar granule cells. Soluble PrP was detected in murine and human serum, suggesting that it might be of physiological relevance.

Prevention of scrapie pathogenesis by transgenic expression of anti-prion protein antibodies

Variant Creutzfeldt-Jakob disease and bovine spongiform encephalopathy are initiated by extracerebral exposure to prions. Although prion transmission from extracerebral sites to the brain represents a potential target for prophylaxis, attempts at vaccination have been limited by the poor immunogenicity of prion proteins. To circumvent this, we expressed an anti-prion protein (anti-PrP) mu chain in Prnp(o/o) mice. Transgenic mice developed sustained anti-PrP titers, which were not suppressed by introduction of Prnp+ alleles. Transgene expression prevented pathogenesis of prions introduced by intraperitoneal injection in the spleen and brain. Expression of endogenous PrP (PrP(C)) in the spleen and brain was unaffected, suggesting that immunity was responsible for protection. This indicates the feasibility of immunological inhibition of prion disease in vivo.

The shifting biology of prions

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are rare fatal neurodegenerative diseases of humans and animals. Although some TSEs, like scrapie in sheep, have been known to exist for centuries, bovine spongiform encephalopathy (BSE) was recognized only 15 years ago. New variant Creutzfeldt-Jakob disease (nvCJD) of humans is probably caused by consumption of BSE-infected materials. The nature of the infectious agent is not fully elucidated, but substantial evidence suggests that it is devoid of nucleic acids and consists at least in part of an abnormal form of a host protein termed PrP(C). Despite their rarity, prion diseases have become an important topic in public health and basic research because of the connection between nvCJD and BSE and also because of the unusual biological attributes of the infectious agent.

Interventional strategies against prion diseases

Only a few years ago, the idea that transmissible spongiform encephalopathies could be treated pharmacologically would have met with considerable scepticism. Even now, there is no way to cure a patient or animal suffering from a manifest prion disease. But recent, exciting developments seem to indicate that immunological and pharmacological interventions could have some potential for the pre-exposure and post-exposure prophylaxis of prion diseases. Although it is unlikely that we will be able to cure the clinically overt stages of prion diseases in the foreseeable future, palliative and even life-prolonging interventions might no longer be confined to the realm of science fiction.