Passage of murine scrapie prion protein across the mouse vascular blood–brain barrier

Cytoskeleton Elements Contribute to Prion Peptide-Induced Endothelial Barrier Breakdown in a Blood–Brain Barrier In Vitro System

The mechanisms involved in the interaction of PrP 106-126, a peptide corresponding to the prion protein amyloidogenic region, with the blood–brain barrier (BBB) were studied. PrP 106-126 treatment that was previously shown to impair BBB function, reduced cAMP levels in cultured brain endothelial cells, increased nitric oxide (NO) levels, and changed the activation mode of the small GTPases Rac1 (inactivation) and RhoA (activation). The latter are well established regulators of endothelial barrier properties that act via cytoskeletal elements. Indeed, liquid chromatography-mass spectrometry (LC-MS)-based proteomic profiling study revealed extensive changes in expression of cytoskeleton-related proteins. These results shed light on the nature of the interaction between the prion peptide PrP 106-126 and the BBB and emphasize the importance of the cytoskeleton in endothelium response to prion- induced stress.

Body-first Parkinson's disease and variant Creutzfeldt-Jakob disease - similar or different?

In several neurodegenerative disorders, proteins that typically exhibit an α-helical structure misfold into an amyloid conformation rich in β-sheet content. Through a self-templating mechanism, these amyloids are able to induce additional protein misfolding, facilitating their propagation throughout the central nervous system. This disease mechanism was originally identified for the prion protein (PrP), which misfolds into PrP^Sc in a number of disorders, including variant Creutzfeldt-Jakob disease (vCJD) and bovine spongiform encephalopathy (BSE). More recently, the prion mechanism of disease was expanded to include other proteins that rely on this self-templating mechanism to cause progressive degeneration, including α-synuclein misfolding in Parkinson's disease (PD). Several studies now suggest that PD patients can be subcategorized based on where in the body misfolded α-synuclein originates, either the brain or the gut, similar to patients developing sporadic CJD or vCJD. In this review, we discuss the human and animal model data indicating that α-synuclein and PrP^Sc misfolding originates in the gut in body-first PD and vCJD, and summarize the data identifying the role of the autonomic nervous system in the gut-brain axis of both diseases.

Prion pathogenesis is unaltered in a mouse strain with a permeable blood-brain barrier

Transmissible spongiform encephalopathies (TSEs) are caused by the prion, which consists essentially of PrPSc, an aggregated, conformationally modified form of the cellular prion protein (PrPC). Although TSEs can be experimentally transmitted by intracerebral inoculation, most instances of infection in the field occur through extracerebral routes. The epidemics of kuru and variant Creutzfeldt-Jakob disease were caused by dietary exposure to prions, and parenteral administration of prion-contaminated hormones has caused hundreds of iatrogenic TSEs. In all these instances, the development of postexposure prophylaxis relies on understanding of how prions propagate from the site of entry to the brain. While much evidence points to lymphoreticular invasion followed by retrograde transfer through peripheral nerves, prions are present in the blood and may conceivably cross the blood-brain barrier directly. Here we have addressed the role of the blood-brain barrier (BBB) in prion disease propagation using Pdgfbret/ret mice which possess a highly permeable BBB. We found that Pdgfbret/ret mice have a similar prion disease incubation time as their littermate controls regardless of the route of prion transmission. These surprising results indicate that BBB permeability is irrelevant to the initiation of prion disease, even when prions are administered parenterally.

Tau Proteins Cross the Blood-Brain Barrier

Tauopathies are a hallmark of many neurodegenerative diseases, including Alzheimer's disease and traumatic brain injuries. It has been demonstrated that amyloid-beta peptides, alpha-synuclein, and prion proteins cross the blood-brain barrier (BBB), contributing to their abilities to induce disease. Very little is known about whether tau proteins can cross the BBB. Here we systematically characterized several key forms of tau proteins to cross the BBB, including Tau-441 (2N4R), Tau-410 (2N3R), truncated tau 151-391 (0N4R), and truncated tau 121-227. All of these tau proteins crossed the BBB readily and bidirectonally; however, only Tau-410 had a saturable component to its influx. The tau proteins also entered the blood after their injection into the brain, with Tau 121-227 having the slowest exit from brain. The tau proteins varied in regards to their enzymatic stability in brain and blood and in their peripheral pharmacokinetics. These results show that blood-borne tau proteins could contribute to brain tauopathies. The result also suggest that the CNS can contribute to blood levels of tau, raising the possibility that, as suggested for other misfolded proteins, blood levels of tau proteins could be used as a biomarker of CNS disease.

Efficient prion disease transmission through common environmental materials

Prion diseases are a group of fatal neurodegenerative diseases associated with a protein-based infectious agent, termed prion. Compelling evidence suggests that natural transmission of prion diseases is mediated by environmental contamination with infectious prions. We hypothesized that several natural and man-made materials, commonly found in the environments of wild and captive animals, can bind prions and may act as vectors for disease transmission. To test our hypothesis, we exposed surfaces composed of various common environmental materials (i.e. wood, rocks, plastic, glass, cement, stainless steel, aluminum, and brass) to hamster-adapted 263K scrapie prions and studied their attachment and retention of infectivity in vitro and in vivo Our results indicated that these surfaces, with the sole exception of brass, efficiently bind, retain, and release prions. Prion replication was studied in vitro using the protein misfolding cyclic amplification technology, and infectivity of surface-bound prions was analyzed by intracerebrally challenging hamsters with contaminated implants. Our results revealed that virtually all prion-contaminated materials transmitted the disease at high rates. To investigate a more natural form of exposure to environmental contamination, we simply housed animals with large contaminated spheres made of the different materials under study. Strikingly, most of the hamsters developed classical clinical signs of prion disease and typical disease-associated brain changes. Our findings suggest that prion contamination of surfaces commonly present in the environment can be a source of disease transmission, thus expanding our understanding of the mechanisms for prion spreading in nature.

Prions efficiently cross the intestinal barrier after oral administration: Study of the bioavailability, and cellular and tissue distribution in vivo

Natural forms of prion diseases frequently originate by oral (p.o.) infection. However, quantitative information on the gastro-intestinal (GI) absorption of prions (i.e. the bioavailability and subsequent biodistribution) is mostly unknown. The main goal of this study was to evaluate the fate of prions after oral administration, using highly purified radiolabeled PrP(Sc). The results showed a bi-phasic reduction of PrP(Sc) with time in the GI, except for the ileum and colon which showed sustained increases peaking at 3-6 hr, respectively. Plasma and whole blood (125)I-PrP(Sc) reached maximal levels by 30 min and 3 hr, respectively, and blood levels were constantly higher than plasma. Upon crossing the GI-tract (125)I-PrP(Sc) became associated to blood cells, suggesting that binding to cells decreased the biological clearance of the agent. Size-exclusion chromatography revealed that oligomeric (125)I-PrP(Sc) were transported from the intestinal tract, and protein misfolding cyclic amplification showed that PrP(Sc) in organs and blood retained the typical prion self-replicating ability. Pharmacokinetic analysis found the oral bioavailability of (125)I-PrP(Sc) to be 33.6%. Interestingly, (125)I-PrP(Sc) reached the brain in a quantity equivalent to the minimum amount needed to initiate prion disease. Our findings provide a comprehensive and quantitative study of the fate of prions upon oral infection.

Magnetic microparticle-based multimer detection system for the detection of prion oligomers in sheep

Transmissible spongiform encephalopathies (TSEs) are zoonotic fatal neurodegenerative diseases in animals and humans. TSEs are commonly known as bovine spongiform encephalopathy in cattle, scrapie in sheep and goats, chronic wasting disease in cervids, and Creutzfeldt–Jakob disease in humans. The putative transmissible agents are infectious prion proteins (PrP^Sc), which are formed by the conversion of the normal prion protein on the glycoprotein cell surface in the presence of other PrP^Sc. Reports of the transmission of TSEs through blood raised considerable concern about the safety of blood and blood products. To address this issue, many laboratories attempted to develop a sensitive and accurate blood diagnostic test to detect PrP^Sc. Previously, we reported that, compared to normal controls, the multimer detection system (MDS) was more efficient in detecting PrP^Sc in infected hamster brain homogenate, mouse plasma spiked with purified PrP^Sc from scrapie mouse brain, and scrapie-infected hamster plasmas. MDS differentiates prion multimers from the cellular monomer through the multimeric expression of epitopes on prion multimers, in contrast to the monomeric form. In this study, MDS detected PrP^Sc in plasma samples from scrapie-infected sheep expressing clinical symptoms, demonstrating 100% sensitivity and specificity in these samples. Plasma samples from asymptomatic lambs at the preclinical stage (8-month-old naturally infected offspring of scrapie-infected parents expressing a highly susceptible genotype) tested positive with 50% sensitivity and 100% specificity. In the first of two coded analyses using clinical scrapie-infected sheep and normal healthy samples, MDS successfully identified all but one of the clinical samples with 92% sensitivity and 100% specificity. Similar results were obtained in the second coded analysis using preclinical samples. MDS again successfully identified all but one of the samples with 87% sensitivity and 100% specificity. The false-negative sample was subjected to a protease pretreatment. In conclusion, MDS could accurately detect scrapie in plasma samples at both preclinical and clinical stages. From these studies, we conclude that MDS could be a promising tool for the early diagnosis of TSEs from blood samples.

Peripherally administrated prions reach the brain at sub-infectious quantities in experimental hamsters

The mechanisms implicated in prion infection and tissue distribution are not completely understood. In this study we investigated the levels of 263K prions in brain and spleen of Syrian hamsters few days after intra-peritoneal challenge. For this purpose we utilized the PMCA technology which permits to detect as little as few PrP(Sc) molecules. Our results show that peripherally administered prions directly reach the brain, although at levels below the minimum necessary to produce disease. PrP(Sc) remains in the brain several days after administration suggesting inefficient clearance or early replication. Understanding the fate of the infectious agent after administration and its uptake in different organs and fluids may provide useful information to develop strategies to minimize further spreading of prion diseases.

Classical scrapie prions in ovine blood are associated with B lymphocytes and platelet-rich plasma

Background

Classical scrapie is a naturally occurring transmissible spongiform encephalopathy of sheep and goats characterized by cellular accumulation of abnormal isoforms of prion protein (PrP^Sc) in the central nervous system and the follicles of peripheral lymphoid tissues. Previous studies have shown that the whole blood and buffy coat blood fraction of scrapie infected sheep harbor prion infectivity. Although PrP^Sc has been detected in peripheral blood mononuclear cells (PBMCs), plasma, and more recently within a subpopulation of B lymphocytes, the infectivity status of these cells and plasma in sheep remains unknown. Therefore, the objective of this study was to determine whether circulating PBMCs, B lymphocytes and platelets from classical scrapie infected sheep harbor prion infectivity using a sheep bioassay.

Results

Serial rectal mucosal biopsy and immunohistochemistry were used to detect preclinical infection in lambs transfused with whole blood or blood cell fractions from preclinical or clinical scrapie infected sheep. PrP^Sc immunolabeling was detected in antemortem rectal and postmortem lymphoid tissues from recipient lambs receiving PBMCs (15/15), CD72⁺ B lymphocytes (3/3), CD21⁺ B lymphocytes (3/3) or platelet-rich plasma (2/3) fractions. As expected, whole blood (11/13) and buffy coat (5/5) recipients showed positive PrP^Sc labeling in lymphoid follicles. However, at 549 days post-transfusion, PrP^Sc was not detected in rectal or other lymphoid tissues in three sheep receiving platelet-poor plasma fraction.

Conclusions

Prion infectivity was detected in circulating PBMCs, CD72⁺ pan B lymphocytes, the CD21⁺ subpopulation of B lymphocytes and platelet-rich plasma of classical scrapie infected sheep using a sheep bioassay. Combining platelets with B lymphocytes might enhance PrP^Sc detection levels in blood samples.

All clinically-relevant blood components transmit prion disease following a single blood transfusion: a sheep model of vCJD

Variant CJD (vCJD) is an incurable, infectious human disease, likely arising from the consumption of BSE-contaminated meat products. Whilst the epidemic appears to be waning, there is much concern that vCJD infection may be perpetuated in humans by the transfusion of contaminated blood products. Since 2004, several cases of transfusion-associated vCJD transmission have been reported and linked to blood collected from pre-clinically affected donors. Using an animal model in which the disease manifested resembles that of humans affected with vCJD, we examined which blood components used in human medicine are likely to pose the greatest risk of transmitting vCJD via transfusion. We collected two full units of blood from BSE-infected donor animals during the pre-clinical phase of infection. Using methods employed by transfusion services we prepared red cell concentrates, plasma and platelets units (including leucoreduced equivalents). Following transfusion, we showed that all components contain sufficient levels of infectivity to cause disease following only a single transfusion and also that leucoreduction did not prevent disease transmission. These data suggest that all blood components are vectors for prion disease transmission, and highlight the importance of multiple control measures to minimise the risk of human to human transmission of vCJD by blood transfusion.

Initial fate of prions upon peripheral infection: half-life, distribution, clearance, and tissue uptake

Prion diseases are infectious neurodegenerative disorders associated with the misfolded prion protein (PrP(Sc)), which appears to be the sole component of the infectious agent (termed prion). To produce disease, prions have to be absorbed into the body and reach sufficient quantities in the brain. Very little is known about the biological mechanisms controlling the initial fate of prions. Here, we studied the systemic pharmacokinetics and biodistribution of PrP(Sc) in vivo. After an intravenous injection of highly purified radiolabeled or native unlabeled PrP(Sc), the protein was eliminated rapidly from the serum (half-life of 3.24 h), mostly through tissue uptake. The quantity of intact PrP(Sc) reaching the brain was ∼ 0.2% of the injected dose per gram of brain tissue (ID/g). The highest levels were found in liver (∼ 20% ID/g), spleen (∼ 13% ID/g), and kidney (∼ 7.4% ID/g). Cell surface PrP(C) does not appear to play a role in PrP(Sc) pharmacokinetics, since the infectious protein distributed similarly in wild-type and PrP-null mice. To measure tissue uptake kinetics and biodistribution accurately, vascular space in tissues was measured with radioactively labeled albumin coinjected with radioactively labeled PrP(Sc). Our results provide a fundamental pharmacokinetic characterization of PrP(Sc) in vivo, which may be relevant to estimate tissue risks and mechanisms of prion neuroinvasion and to identify novel therapeutic strategies.

Interactions of the prion peptide (PrP 106-126) with brain capillary endothelial cells: coordinated cell killing and remodeling of intercellular junctions

We studied here the interactions of PrP 106-126, a peptide corresponding to the prion protein (PrP) amyloidogenic region, with a blood-brain barrier in vitro model consisting of confluent porcine brain endothelial cells (PBEC). PrP 106-126 interacted selectively with PBEC via their luminal side, and caused cumulative cell death, as shown by lactate dehydrogenase release, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, Caspase 3 induction and direct cell counting. In addition, PrP 106-126, but not its corresponding scrambled peptide, produced a 50% reduction of the trans-endothelial electrical resistance, while the PBEC maintained confluency. This process was accompanied by a 23% increase of PBEC average size and the selective disappearance from the cell borders of the junction proteins occludin, claudin-5 and VE-cadherin (but not ZO-1), as evaluated by immunostaining. These results fit with a mechanism by which PrP 106-126 initiates a coordinated cell killing process ultimately causing the remaining cells to undergo a coordinated remodeling of the intercellular junctions and an expansion of their cell territory.

Surface characteristics of nanoparticles determine their intracellular fate in and processing by human blood-brain barrier endothelial cells in vitro

A polarized layer of endothelial cells that comprises the blood-brain barrier (BBB) precludes access of systemically administered medicines to brain tissue. Consequently, there is a need for drug delivery vehicles that mediate transendothelial transport of such medicines. Endothelial cells use a variety of endocytotic pathways for the internalization of exogenous materials, including clathrin-mediated endocytosis, caveolar endocytosis, and macropinocytosis. The different modes of endocytosis result in the delivery of endocytosed material to distinctive intracellular compartments and therewith correlated differential processing. To obtain insight into the properties of drug delivery vehicles that direct their intracellular processing in brain endothelial cells, we investigated the intracellular processing of fixed-size nanoparticles in an in vitro BBB model as a function of distinct nanoparticle surface modifications. Caveolar endocytosis, adsorptive-mediated endocytosis, and receptor-mediated endocytosis were promoted by the use of uncoated 500-nm particles, attachment of the cationic polymer polyethyleneimine (PEI), and attachment of prion proteins, respectively. We demonstrate that surface modifications of nanoparticles, including charge and protein ligands, affect their mode of internalization by brain endothelial cells and thereby their subcellular fate and transcytotic potential.

Pathogenesis of chronic wasting disease in cervidized transgenic mice

Chronic wasting disease (CWD) is a fatal, endemic prion disease of wild and captive cervids, including deer, elk, and moose. Typical of prion diseases, CWD is characterized by the conversion of the native, protease-sensitive protein PrP(C) to a protease-resistant isoform, denoted as PrP(RES). Here we have studied the expression of cervid PrP(C) and the pathogenesis of CWD infection in transgenic mice expressing the normal cervid prion protein (Tg[CerPrP] mice). Using tissue-based in situ immunohistochemistry protocols, we first identified cervid PrP(C) expression in the lymphoid, nervous, hemopoietic, endocrine, and certain epithelial tissues of Tg[CerPrP] mice. Tg[CerPrP] mice were then inoculated with CWD via one of four routes (intracerebral, intravenous, intraperitoneal, or oral); all groups developed spongiform encephalopathy, although the oral route required a larger infecting dose. Incubation periods were 184 +/- 13, 218 +/- 15, 200 +/- 7, and 350 +/- 27 days after inoculation, respectively. In longitudinal studies, we tracked the appearance of PrP(RES) in the brain, spleen, Peyer's patches, lymph nodes, pancreatic islets of Langerhans, bone marrow, and salivary glands of preclinical and terminal mice. In addition, we documented horizontal transmission of CWD from inoculated mice and to un-inoculated cohabitant cage-mates. This work documents the multiroute susceptibility, pathogenesis, and lateral transmission of CWD infection in Tg[CerPrP] mice, affirming this model as a robust system to study this cervid transmissible spongiform encephalopathy.

Choroid plexus: biology and pathology

The choroid plexus is an epithelial-endothelial vascular convolute within the ventricular system of the vertebrate brain. It consists of epithelial cells, fenestrated blood vessels, and the stroma, dependent on various physiological or pathological conditions, which may contain fibroblasts, mast cells, macrophages, granulocytes or other infiltrates, and a rich extracellular matrix. The choroid plexus is mainly involved in the production of cerebrospinal fluid (CSF) by using the free access to the blood compartment of the leaky vessels. In order to separate blood and CSF compartments, choroid plexus epithelial cells and tanycytes of circumventricular organs constitute the blood-CSF-brain barrier. As non-neuronal cells in the brain and derived from neuroectoderm, choroid plexus epithelia are defined as a subtype of macroglia. The choroid plexus is involved in a variety of neurological disorders, including neurodegenerative, inflammatory, infectious, traumatic, neoplastic, and systemic diseases. Abeta and Biondi ring tangles accumulate in the Alzheimer's disease choroid plexus. In multiple sclerosis, the choroid plexus could represent a site for lymphocyte entry in the CSF and brain, and for presentation of antigens. Recent studies have provided new diagnostic markers and potential molecular targets for choroid plexus papilloma and carcinoma, which represent the most common brain tumors in the first year of life. We here revive some of the classical studies and review recent insight into the biology and pathology of the choroid plexus.

Prions: Protein Aggregation and Infectious Diseases

Transmissible spongiform encephalopathies (TSEs) are inevitably lethal neurodegenerative diseases that affect humans and a large variety of animals. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrPC) of the host. PrPCis necessary for prion replication and for prion-induced neurodegeneration, yet the proximal causes of neuronal injury and death are still poorly understood. Prion toxicity may arise from the interference with the normal function of PrPC, and therefore, understanding the physiological role of PrPCmay help to clarify the mechanism underlying prion diseases. Here we discuss the evolution of the prion concept and how prion-like mechanisms may apply to other protein aggregation diseases. We describe the clinical and the pathological features of the prion diseases in human and animals, the events occurring during neuroinvasion, and the possible scenarios underlying brain damage. Finally, we discuss potential antiprion therapies and current developments in the realm of prion diagnostics.

Transport of prion protein across the blood-brain barrier

The cellular form of the prion protein (PrP(c)) is necessary for the development of prion diseases and is a highly conserved protein that may play a role in neuroprotection. PrP(c) is found in both blood and cerebrospinal fluid and is likely produced by both peripheral tissues and the central nervous system (CNS). Exchange of PrP(c) between the brain and peripheral tissues could have important pathophysiologic and therapeutic implications, but it is unknown whether PrP(c) can cross the blood-brain barrier (BBB). Here, we found that radioactively labeled PrP(c) crossed the BBB in both the brain-to-blood and blood-to-brain directions. PrP(c) was enzymatically stable in blood and in brain, was cleared by liver and kidney, and was sequestered by spleen and the cervical lymph nodes. Circulating PrP(c) entered all regions of the CNS, but uptake by the lumbar and cervical spinal cord, hypothalamus, thalamus, and striatum was particularly high. These results show that PrP(c) has bidirectional, saturable transport across the BBB and selectively targets some CNS regions. Such transport may play a role in PrP(c) function and prion replication.

Olfactory system involvement in natural scrapie disease

The olfactory system (OS) is involved in many infectious and neurodegenerative diseases, both human and animal, and it has recently been investigated in regard to transmissible spongiform encephalopathies. Previous assessments of nasal mucosa infection by prions following intracerebral challenge suggested a potential centrifugal spread along the olfactory nerve fibers of the pathological prion protein (PrP(Sc)). Whether the nasal cavity may be a route for centripetal prion infection to the brain has also been experimentally studied. With the present study, we wanted to determine whether prion deposition in the OS occurs also under field conditions and what type of anatomical localization PrP(Sc) might display there. We report here on detection by different techniques of PrP(Sc) in the nasal mucosa and in the OS-related brain areas of sheep affected by natural scrapie. PrP(Sc) was detected in the perineurium of the olfactory nerve bundles in the medial nasal concha and in nasal-associated lymphoid tissue. Olfactory receptor neurons did not show PrP(Sc) immunostaining. PrP(Sc) deposition was found in the brain areas of olfactory fiber projection, chiefly in the olfactory bulb and the olfactory cortex. The prevalent PrP(Sc) deposition patterns were subependymal, perivascular, and submeningeal. This finding, together with the discovery of an intense PrP(Sc) immunostaining in the meningeal layer of the olfactory nerve perineurium, at the border with the subdural space extension surrounding the nerve rootlets, strongly suggests a probable role of cerebrospinal fluid in conveying prion infectivity to the nasal submucosa.

Ultrastructural evidence that ependymal cells are infected in experimental scrapie

During the last stage of infection in the experimental scrapie-infected hamster model, light microscopy reveals typical immunostaining of PrPsc in the subependymal region and at the apical ependymal cell borders. Whereas the subependymal immuno-staining is known to originate from extracellular amyloid filaments and residual membranes of astrocytes as constituents of plaque-like structures, the ultrastructural correlate of the supraependymal PrPsc staining remains uncertain. To decipher this apical PrPsc immunopositivity and subsequently the ependymocyte-scrapie agent interaction, we employed highly sensitive immuno-electron microscopy for detecting PrPsc in 263K scrapie-infected hamster brains. The results revealed the supraependymal PrPsc signal to be correlated not only with extracellular accumulation of amyloid filaments, but also with three distinct ependymal cell structures: (1) morphologically intact or altered microvilli associated with filaments, (2) the ependymal cell cytoplasm in proximity of apical cell membrane, and (3) intracytoplasmic organelles such as endosomes and lysosomal-like structures. These findings suggest a strong ependymotrope feature of the scrapie agent and recapitulate several aspects of the cell-prion interaction leading to the formation and production of PrPsc amyloid filaments. Our data demonstrate that in addition to neurons and astrocytes, ependymocytes constitute a new cellular target for the scrapie agent. In contrast, the absence of PrPsc labeling in choroid plexus and brain vascular endothelial cells indicates that these cells are not susceptible to the infection and may inhibit passage of the infectious agent across the blood-brain barrier.

Prions and peripheral nerves: A deadly rendezvous

The infectious particle causing transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease of humans and animals, has been termed prion. Its major component is an aggregated variant of the cellular prion protein, PrP(C). The main target of prion pathology is the central nervous system (CNS), yet most prion diseases are initiated or accompanied by prion replication at extracerebral locations, including secondary lymphoid organs, muscle and, in some instances, blood. How do prions travel from the periphery into the CNS? Is this an active or a passive process and does neuronal prion transport explain the long incubation times in prion diseases? Alternatively, if prion infectivity arises spontaneously in the CNS, as believed from sporadic Creutzfeldt-Jakob patients, how do prions manage to travel from the CNS into the periphery (e.g., spleen, muscle) of the infected host? The mechanisms of neuronal prion transport from the periphery into the CNS or vice versa are heavily investigated and debated but poorly understood. Although research in the past has accumulated knowledge on prion progression from the periphery to the brain, we are far from understanding the molecular mechanisms of neuronal prion transport.

Prion protein in the cerebrospinal fluid of healthy and naturally scrapie-affected sheep

The aim of this study was to characterize the cerebrospinal fluid (CSF) prion protein (PrP) of healthy and naturally scrapie-affected sheep. The soluble form of CSF PrPC immunoblotted with an anti-octarepeat and an anti-C terminus mAb showed two isoforms of approximately 33 and 26 kDa, corresponding to the biglycosylated and unglycosylated isoforms of brain PrPC. Neither the mean concentration nor the electrophoretic profile of CSF PrP differed between healthy and scrapie-affected sheep, whereas a slightly increased resistance of CSF PrP to mild proteolysis by proteinase K was evident in the CSF of scrapie-affected sheep. No difference in susceptibility to proteolysis was observed between the two ARR and VRQ genetic variants of the purified prokaryote recombinant PrP. It was concluded that the physicochemical properties of PrPC in the CSF could be altered during scrapie and that these changes might reflect the physiopathological process of prion disease.

Pathogenesis of prion diseases: current status and future outlook

The prion, a conformational variant of a host protein, is the infectious particle responsible for transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease of humans and animals. The principal target of prion pathology is the brain, yet most TSEs also display prion replication at extra-cerebral locations, including secondary lymphoid organs and sites of chronic inflammation. Despite significant progress in our understanding of this infectious agent, many fundamental questions relating to the nature of the prion, including the mechanism of replication and the molecular events underlying brain damage, remain unanswered. Here we focus on the unresolved issues pertaining to prion pathogenesis, particularly on the role played by the immune system.

Diagnosing prion diseases: needs, challenges and hopes

Prion diseases are among the most intriguing infectious diseases and are associated with unconventional proteinaceous infectious agents known as prions. Prions seem to lack nucleic acid and propagate by transmission of protein misfolding. The nature of prions and their unique mode of transmission present challenges for early diagnosis of prion diseases. In this article, state-of-the-art prion diagnostic techniques, together with the new strategies that are being used to develop sensitive, early and non-invasive diagnoses for these diseases are reviewed.