The mRNA encoding the scrapie agent protein is present in a variety of non-neuronal cells

The Zoonotic Potential of Chronic Wasting Disease-A Review

Prion diseases are transmissible neurodegenerative disorders that affect humans and ruminant species consumed by humans. Ruminant prion diseases include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats and chronic wasting disease (CWD) in cervids. In 1996, prions causing BSE were identified as the cause of a new prion disease in humans; variant Creutzfeldt-Jakob disease (vCJD). This sparked a food safety crisis and unprecedented protective measures to reduce human exposure to livestock prions. CWD continues to spread in North America, and now affects free-ranging and/or farmed cervids in 30 US states and four Canadian provinces. The recent discovery in Europe of previously unrecognized CWD strains has further heightened concerns about CWD as a food pathogen. The escalating CWD prevalence in enzootic areas and its appearance in a new species (reindeer) and new geographical locations, increase human exposure and the risk of CWD strain adaptation to humans. No cases of human prion disease caused by CWD have been recorded, and most experimental data suggest that the zoonotic risk of CWD is very low. However, the understanding of these diseases is still incomplete (e.g., origin, transmission properties and ecology), suggesting that precautionary measures should be implemented to minimize human exposure.

Prion protein protects mice from lethal infection with influenza A viruses

The cellular prion protein, designated PrP^C, is a membrane glycoprotein expressed abundantly in brains and to a lesser extent in other tissues. Conformational conversion of PrP^C into the amyloidogenic isoform is a key pathogenic event in prion diseases. However, the physiological functions of PrP^C remain largely unknown, particularly in non-neuronal tissues. Here, we show that PrP^C is expressed in lung epithelial cells, including alveolar type 1 and 2 cells and bronchiolar Clara cells. Compared with wild-type (WT) mice, PrP^C-null mice (Prnp^0/0) were highly susceptible to influenza A viruses (IAVs), with higher mortality. Infected Prnp^0/0 lungs were severely injured, with higher inflammation and higher apoptosis of epithelial cells, and contained higher reactive oxygen species (ROS) than control WT lungs. Treatment with a ROS scavenger or an inhibitor of xanthine oxidase (XO), a major ROS-generating enzyme in IAV-infected lungs, rescued Prnp^0/0 mice from the lethal infection with IAV. Moreover, Prnp^0/0 mice transgenic for PrP with a deletion of the Cu-binding octapeptide repeat (OR) region, Tg(PrPΔOR)/Prnp^0/0 mice, were also highly susceptible to IAV infection. These results indicate that PrP^C has a protective role against lethal infection with IAVs through the Cu-binding OR region by reducing ROS in infected lungs. Cu content and the activity of anti-oxidant enzyme Cu/Zn-dependent superoxide dismutase, SOD1, were lower in Prnp^0/0 and Tg(PrPΔOR)/Prnp^0/0 lungs than in WT lungs. It is thus conceivable that PrP^C functions to maintain Cu content and regulate SOD1 through the OR region in lungs, thereby reducing ROS in IAV-infected lungs and eventually protecting them from lethal infection with IAVs. Our current results highlight the role of PrP^C in protection against IAV infection, and suggest that PrP^C might be a novel target molecule for anti-influenza therapeutics.

Influenza A virus (IAV) is an enveloped, negative sense, single-stranded RNA virus, causing seasonal epidemic outbreaks of influenza. Anti-influenza agents targeting viral molecules, such as neuraminidase inhibitors, are currently available. However, these agents have accelerated emergence of mutant IAVs that are resistant to these agents among human populations. Development of new types of anti-influenza agents is awaited. We show that the cellular prion protein PrP^C has a protective role against lethal infection with IAVs through the octapeptide repeat (OR) region by abrogating lung epithelial cell apoptosis induced by reactive oxygen species (ROS) in infected lungs. We also show that PrP^C could reduce ROS in IAV-infected lungs through the OR region by maintaining Cu ion homeostasis and thereby activating Cu/Zn-dependent superoxide dismutase, SOD1. These results highlight the protective role of PrP^C in IAV infection. Elucidation of the exact mechanism underlying the PrP^C-mediated protection against IAV infection would be important for further understanding the pathogenesis of IAV infection and could be useful for development of new types of anti-influenza therapeutics.

Transmission and Replication of Prions

Transmissible spongiform encephalopathies (TSEs) are a group of progressive, invariably fatal diseases that affect the nervous system of many mammals including humans. The key molecular event in the pathogenesis of TSEs is the conversion of the cellular prion protein PrP^C into a disease-associated isoform PrP^Sc. The "protein-only hypothesis" argues that PrP^Sc itself is the infectious agent. In effect, PrP^Sc can adopt several structures that represent different prion strains. The interspecies transmission of TSEs is difficult because of differences between the host and donor primary PrP sequence. However, transmission is not impossible as this occurred when bovine spongiform encephalopathy spread to humans causing variant Creutzfeldt-Jakob disease (vCJD). This event determined a need for a thorough understanding of prion replication and transmission so that we could be one step ahead of further threats for human health. This chapter focuses on these concepts and on new insights gained into prion propagation mechanisms.

Cell Biology of Prion Protein

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. The precise function of PrP^C remains elusive but may depend upon its cellular localization. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. Nonetheless some forms of prion disease develop in the apparent absence of infectious PrP^Sc, suggesting that molecular species of PrP distinct from PrP^Sc may represent the primary neurotoxic culprits. Indeed, in some inherited cases of human prion disease, the predominant form of PrP detectable in the brain is not PrP^Sc but rather ^CtmPrP, a transmembrane form of the protein. The relationship between the neurodegeneration occurring in prion diseases involving PrP^Sc and that associated with ^CtmPrP remains unclear. However, the different membrane topology of the PrP mutants, as well as the presence of the GPI anchor, could influence both the function and the intracellular localization and trafficking of the protein, all being potentially very important in the pathophysiological mechanism that ultimately causes the disease. Here, we review the latest findings on the fundamental aspects of prions biology, from the PrP^C biosynthesis, function, and structure up to its intracellular traffic and analyze the possible roles of the different topological isoforms of the protein, as well as the GPI anchor, in the pathogenesis of the disease.

Higher levels of myelin phospholipids in brains of neuronal α-Synuclein transgenic mice precede myelin loss

α-Synuclein is a protein involved in the pathogenesis of synucleinopathies, including Parkinson’s disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). We investigated the role of neuronal α-Syn in myelin composition and abnormalities. The phospholipid content of purified myelin was determined by ³¹P NMR in two mouse lines modeling PD, PrP-A53T α-Syn and Thy-1 wt-α-Syn. Significantly higher levels of phospholipids were detected in myelin purified from brains of these α-Syn transgenic mouse models than in control mice. Nevertheless, myelin ultrastructure appeared intact. To further investigate the effect of α-Syn on myelin abnormalities, we systematically analyzed the striatum, a brain region associated with neurodegeneration in PD. An age and disease-dependent loss of myelin basic protein (MBP) signal was detected by immunohistochemistry in striatal striosomes (patches). The age-dependent loss of MBP signal was associated with lower P25α levels in oligodendrocytes. In addition, we found that α-Syn inhibited oligodendrocyte maturation and the formation of membranous sheets in vitro. Based on these results we concluded that neuronal α-Syn is involved in the regulation and/or maintenance of myelin phospholipid. However, axonal hypomyelination in the PD models is evident only in progressive stages of the disease and associated with α-Syn toxicity.

Mechanisms of prion-induced neurodegeneration

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders characterised by long incubation period, short clinical duration, and transmissibility to susceptible species. Neuronal loss, spongiform changes, gliosis and the accumulation in the brain of the misfolded version of a membrane-bound cellular prion protein (PrP(C)), termed PrP(TSE), are diagnostic markers of these diseases. Compelling evidence links protein misfolding and its accumulation with neurodegenerative changes. Accordingly, several mechanisms of prion-mediated neurotoxicity have been proposed. In this paper, we provide an overview of the recent knowledge on the mechanisms of neuropathogenesis, the neurotoxic PrP species and the possible therapeutic approaches to treat these devastating disorders.

Review of studies that have used knockout mice to assess normal function of prion protein under immunological or pathophysiological stress

Deletion of cellular isoform of prion protein (PrP(C)) increases neuronal predisposition to damage by modulating apoptosis and the negative consequences of oxidative stress. In vivo studies have demonstrated that PrP(C)-deficient mice are more prone to seizure, depression, and induction of epilepsy and experience extensive cerebral damage following ischemic challenge or viral infection. In addition, adenovirus-mediated overexpression of PrP(C) reduces brain damage in rat models of cerebral ischemia. In experimental autoimmune encephalomyelitis, PrP(C)-deficient mice reportedly have a more aggressive disease onset and less clinical improvement during the chronic phase than wild-type mice mice. In mice given oral dextran sulfate, PrP(C) has a potential protective role against inflammatory bowel disease. PrP(C)-deficient mice demonstrate significantly greater increases in blood glucose concentrations after intraperitoneal injection of glucose than wild-type mice. Further in vivo challenges to PrP gene-deficient models and conditional knockout models with siRNA and in vivo administration of PrP-ligating agents may assist in refining knowledge of the lymphoid function of PrP(C) and predicting the effects of anti-PrP treatment on the immune system. Together, these findings indicate that PrP(C) may have multiple neuroprotective and anti-inflammatory roles, which explains why this protein is so widely expressed.

Motor neuron cell-nonautonomous rescue of spinal muscular atrophy phenotypes in mild and severe transgenic mouse models

Survival of motor neuron (SMN) deficiency causes spinal muscular atrophy (SMA), but restoring SMN in motor neurons only partially rescues SMA in mouse models. Hua et al. address the relative importance of SMN restoration in the CNS versus peripheral tissues in mouse models by using a therapeutic splice-switching antisense oligonucleotide to restore SMN and a complementary decoy oligonucleotide to neutralize its effects in the CNS. Increasing SMN exclusively in peripheral tissues completely rescued necrosis in mild SMA mice and robustly extended survival in severe SMA mice, with significant improvements in vulnerable tissues and motor function.

Survival of motor neuron (SMN) deficiency causes spinal muscular atrophy (SMA), but the pathogenesis mechanisms remain elusive. Restoring SMN in motor neurons only partially rescues SMA in mouse models, although it is thought to be therapeutically essential. Here, we address the relative importance of SMN restoration in the central nervous system (CNS) versus peripheral tissues in mouse models using a therapeutic splice-switching antisense oligonucleotide to restore SMN and a complementary decoy oligonucleotide to neutralize its effects in the CNS. Increasing SMN exclusively in peripheral tissues completely rescued necrosis in mild SMA mice and robustly extended survival in severe SMA mice, with significant improvements in vulnerable tissues and motor function. Our data demonstrate a critical role of peripheral pathology in the mortality of SMA mice and indicate that peripheral SMN restoration compensates for its deficiency in the CNS and preserves motor neurons. Thus, SMA is not a cell-autonomous defect of motor neurons in SMA mice.

Life cycle of cytosolic prions

Prions are self-templating protein aggregates that were originally identified as the causative agent of prion diseases in mammals, but have since been discovered in other kingdoms. Mammalian prions represent a unique class of infectious agents that are composed of misfolded prion protein. Prion proteins usually exist as soluble proteins but can refold and assemble into highly ordered, self-propagating prion polymers. The prion concept is also applicable to a growing number of non-Mendelian elements of inheritance in lower eukaryotes. While prions identified in mammals are clearly pathogens, prions in lower eukaryotes can be either detrimental or beneficial to the host. Prion phenotypes in fungi are transmitted vertically from mother to daughter cells during cell division and horizontally during mating or abortive mating, but extracellular phases have not been reported. Recent findings now demonstrate that in a mammalian cell environment, protein aggregates derived from yeast prion domains exhibit a prion life cycle similar to mammalian prions propagated ex vivo. This life cycle includes a soluble state of the protein, an induction phase by exogenous prion fibrils, stable replication of prion entities, vertical transmission to progeny and natural horizontal transmission to neighboring cells. Our data reveal that mammalian cells contain all co-factors required for cytosolic prion propagation and dissemination. This has important implications for understanding prion-like properties of disease-related protein aggregates. In light of the growing number of identified functional amyloids, cell-to-cell propagation of cytosolic protein conformers might not only be relevant for the spreading of disease-associated proteins, but might also be of more general relevance under non-disease conditions.

A proautophagic antiviral role for the cellular prion protein identified by infection with a herpes simplex virus 1 ICP34.5 mutant

The cellular prion protein (PrP) often plays a cytoprotective role by regulating autophagy in response to cell stress. The stress of infection with intracellular pathogens can stimulate autophagy, and autophagic degradation of pathogens can reduce their replication and thus help protect the infected cells. PrP also restricts replication of several viruses, but whether this activity is related to an effect on autophagy is not known. Herpes simplex virus 1 (HSV-1) effectively counteracts autophagy through binding of its ICP34.5 protein to the cellular proautophagy protein beclin-1. Autophagy can reduce replication of an HSV-1 mutant, Δ68H, which is incapable of binding beclin-1. We found that deletion of PrP in mice complements the attenuation of Δ68H, restoring its capacity to replicate in the central nervous system (CNS) to wild-type virus levels after intracranial or corneal infection. Cultured primary astrocytes but not neurons derived from PrP(-/-) mice also complemented the attenuation of Δ68H, enabling Δ68H to replicate at levels equivalent to wild-type virus. Ultrastructural analysis showed that normal astrocytes exhibited an increase in the number of autophagosomes after infection with Δ68H compared with wild-type virus, but PrP(-/-) astrocytes failed to induce autophagy in response to Δ68H infection. Redistribution of EGFP-LC3 into punctae occurred more frequently in normal astrocytes infected with Δ68H than with wild-type virus, but not in PrP(-/-) astrocytes, corroborating the ultrastructural analysis results. Our results demonstrate that PrP is critical for inducing autophagy in astrocytes in response to HSV-1 infection and suggest that PrP positively regulates autophagy in the mouse CNS.

Ablation of the cellular prion protein, PrPC, specifically on follicular dendritic cells has no effect on their maturation or function

Follicular dendritic cells (FDC) are situated in the primary follicles of lymphoid tissues where they maintain the structural integrity of the B-lymphocyte follicle, and help to drive immunoglobulin class-switch recombination, somatic hypermutation and affinity maturation during the germinal centre response. FDC can also provide a reservoir for pathogens that infect germinal centres including HIV and prions. FDC express high levels of the normal cellular form of the prion protein (PrP(C) ), which makes them susceptible to prion infection. The function of PrP(C) is uncertain and it is not known why FDC require such high levels of expression of a protein that is found mainly on cells of the central nervous system. In this study, the function of FDC was assessed in mice that had PrP(C) ablated specifically in their FDC. In mice with FDC-specific PrP(C) ablation, our analysis revealed no observable deficits in lymphoid follicle microarchitecture and FDC status. No effects on FDC ability to trap immune complexes or drive antigen-specific antibody responses and affinity maturation in B lymphocytes were observed. These data clearly demonstrate that PrP(C) expression is dispensable for the functional maturation of FDC and their ability to maintain antigen-specific antibody responses and affinity maturation.

Detection and control of prion diseases in food animals

Transmissible spongiform encephalopathies (TSEs), or prion diseases, represent a unique form of infectious disease based on misfolding of a self-protein (PrP^C) into a pathological, infectious conformation (PrP^Sc). Prion diseases of food animals gained notoriety during the bovine spongiform encephalopathy (BSE) outbreak of the 1980s. In particular, disease transmission to humans, to the generation of a fatal, untreatable disease, elevated the perspective on livestock prion diseases from food production to food safety. While the immediate threat posed by BSE has been successfully addressed through surveillance and improved management practices, another prion disease is rapidly spreading. Chronic wasting disease (CWD), a prion disease of cervids, has been confirmed in wild and captive populations with devastating impact on the farmed cervid industries. Furthermore, the unabated spread of this disease through wild populations threatens a natural resource that is a source of considerable economic benefit and national pride. In a worst-case scenario, CWD may represent a zoonotic threat either through direct transmission via consumption of infected cervids or through a secondary food animal, such as cattle. This has energized efforts to understand prion diseases as well as to develop tools for disease detection, prevention, and management. Progress in each of these areas is discussed.

Exacerbation of experimental autoimmune encephalomyelitis in prion protein (PrPc)-null mice: evidence for a critical role of the central nervous system

BACKGROUND

The cellular prion protein (PrPc) is a host-encoded glycoprotein whose transconformation into PrP scrapie (PrPSc) initiates prion diseases. The role of PrPc in health is still obscure, but many candidate functions have been attributed to the protein, both in the immune and the nervous systems. Recent data show that experimental autoimmune encephalomyelitis (EAE) is worsened in mice lacking PrPc. Disease exacerbation has been attributed to T cells that would differentiate into more aggressive effectors when deprived of PrPc. However, alternative interpretations such as reduced resistance of neurons to autoimmune insult and exacerbated gliosis leading to neuronal deficits were not considered.

METHOD

To better discriminate the contribution of immune cells versus neural cells, reciprocal bone marrow chimeras with differential expression of PrPc in the lymphoid or in the central nervous system (CNS) were generated. Mice were subsequently challenged with MOG35-55 peptide and clinical disease as well as histopathology were compared in both groups. Furthermore, to test directly the T cell hypothesis, we compared the encephalitogenicity of adoptively transferred PrPc-deficient versus PrPc-sufficient, anti-MOG T cells.

RESULTS

First, EAE exacerbation in PrPc-deficient mice was confirmed. Irradiation exacerbated EAE in all the chimeras and controls, but disease was more severe in mice with a PrPc-deleted CNS and a normal immune system than in the reciprocal construction. Moreover, there was no indication that anti-MOG responses were different in PrPc-sufficient and PrPc-deficient mice. Paradoxically, PrPc-deficient anti-MOG 2D2 T cells were less pathogenic than PrPc-expressing 2D2 T cells.

CONCLUSIONS

In view of the present data, it can be concluded that the origin of EAE exacerbation in PrPc-ablated mice resides in the absence of the prion protein in the CNS. Furthermore, the absence of PrPc on both neural and immune cells does not synergize for disease worsening. These conclusions highlight the critical role of PrPc in maintaining the integrity of the CNS in situations of stress, especially during a neuroinflammatory insult.

Prion protein at the crossroads of physiology and disease

The presence of the cellular prion protein (PrP(C)) on the cell surface is critical for the neurotoxicity of prions. Although several biological activities have been attributed to PrP(C), a definitive demonstration of its physiological function remains elusive. In this review, we discuss some of the proposed functions of PrP(C), focusing on recently suggested roles in cell adhesion, regulation of ionic currents at the cell membrane and neuroprotection. We also discuss recent evidence supporting the idea that PrP(C) may function as a receptor for soluble oligomers of the amyloid β peptide and possibly other toxic protein aggregates. These data suggest surprising new connections between the physiological function of PrP(C) and its role in neurodegenerative diseases beyond those caused by prions.

PrP 106-126 altered PrP mRNA gene expression in mouse microglia BV-2 cells

Prion diseases are infectious and fatal neurodegenerative diseases. The pathogenic agent is an abnormal prion protein aggregate. Microglial activation in the centre nervous system is a characteristic feature of prion disease. In this study, we examined the effect of PrP 106-126 on PrP mRNA gene expression in Mouse microglia cells BV-2 by real-time quantitative PCR. PrP mRNA expression level was found to be significantly increased after 18 h exposure of BV-2 cells to PrP 106-126, with 3-fold increase after 18 h and 4.5-fold increase after 24 h and BV-2 cells proliferating occurred correspondingly. Our results provide the first in vitro evidence of the increase of PrP mRNA levels in microglial cells exposed to PrP 106-126, and indicate that microglial cells might play a critical role in prion pathogenesis.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

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.

Prion proteins: Physiological functions and role in neurological disorders

Stanley Prusiner was the first to promote the concept of misfolded proteins as a cause for neurological disease. It has since been shown by him and other investigators that the scrapie isoform of prion protein (PrP(Sc)) functions as an infectious agent in numerous human and non-human disorders of the central nervous system (CNS). Interestingly, other organ systems appear to be less affected, and do not appear to lead to major co-morbidities. The physiological function of the endogenous cellular form of the prion protein (PrP(C)) is much less clear. It is intriguing that PrP(c) is expressed on most tissues in mammals, suggesting not only biological functions outside the CNS, but also a role other than the propagation of its misfolded isotype. In this review, we summarize accumulating in vitro and in vivo evidence regarding the physiological functions of PrP(C) in the nervous system, as well as in lymphoid organs.

Prion proteins: a biological role beyond prion diseases

The biological role of the scrapie isoform of prion protein (PrP(Sc)) as an infectious agent in numerous human and non-human disorders of the central nervous system is well established. In contrast, and despite decades of intensive research, the physiological function of the endogenous cellular form of the prion protein (PrP(C)) remains elusive. In mammals, the ubiquitous expression of PrP(C) suggests biological functions other than its pathological role in propagating the accumulation of its misfolded isotype. Other functions that have been attributed to PrP(C) include signal transduction, synaptic transmission and protection against cell death through the apoptotic pathway. More recently, immunoregulatory properties of PrP(C) have been reported. We review accumulating in vitro and in vivo evidence regarding physiological functions of PrP(C).

Prion protein facilitates hormone-induced differentiation of mammary gland epithelial cells

Expression of prion protein has been reported for a variety of cell types including neuronal cells, haematopoietic stem cells, lymphocytes, fibroblasts, and epithelial cells. However, the characterization of the physiological roles exhibited by this protein is still in progress and multiple biological functions have been described to date. In this study we have characterized the contribution of prion protein during hormone-induced differentiation of mouse mammary gland epithelial cells. We present evidence that prion expression enhances the differentiation-capabilities of these cells indicating novel physiological roles during mammary gland development. In addition we were able to demonstrate the presence of prion molecules resistant to mild proteinase digestion in differentiated mammary gland epithelial cells. This represents the first report of proteinase-resistant prion proteins in a physiological, non-pathogenic context.

Molecular discrimination of atypical bovine spongiform encephalopathy strains from a geographical region spanning a wide area in Europe

Transmissible spongiform encephalopathy strains can be differentiated by their behavior in bioassays and by molecular analyses of the disease-associated prion protein (PrP) in a posttranslationally transformed conformation (PrPSc). Until recently, isolates from cases of bovine spongiform encephalopathy (BSE) appeared to be very homogeneous. However, a limited number of atypical BSE isolates have recently been identified upon analyses of the disease-associated proteinase K (PK) resistance-associated moiety of PrPSc (PrPres), suggesting the existence of at least two additional BSE PrPres variants. These are defined here as the H type and the L type, according to the higher and lower positions of the nonglycosylated PrPres band in Western blots, respectively, compared to the position of the band in classical BSE (C-type) isolates. These molecular PrPres variants, which originated from six different European countries, were investigated together. In addition to the migration properties and glycosylation profiles (glycoprofiles), the H- and L-type isolates exhibited enhanced PK sensitivities at pH 8 compared to those of the C-type isolates. Moreover, H-type BSE isolates exhibited differences in the binding of antibodies specific for N- and more C-terminal PrP regions and principally contained two aglycosylated PrPres moieties which can both be glycosylated and which is thus indicative of the existence of two PrPres populations or intermediate cleavage sites. These properties appear to be consistent within each BSE type and independent of the geographical origin, suggesting the existence of different BSE strains in cattle. The choice of three antibodies and the application of two pHs during the digestion of brain homogenates provide practical and diverse tools for the discriminative detection of these three molecular BSE types and might assist with the recognition of other variants.

Immunohistochemical characterization of cell types expressing the cellular prion protein in the small intestine of cattle and mice

The gastrointestinal tract is thought to be the main site of entry for the pathological isoform of the prion protein (PrP(Sc)). Prion diseases are believed to result from a conformational change of the cellular prion protein (PrP(c)) to PrP(Sc). Therefore, PrP(c) expression is a prerequisite for the infection and spread of the disease to the central nervous system. However, the distribution of PrP(c) in the gut is still a matter of controversy. We therefore investigated the localization of PrP(c) in the bovine and murine small intestine. In cattle, most PrP(c) positive epithelial cells were detected in the duodenum, while a few positive cells were found in the jejunum. PrP(c) was expressed in serotonin producing cells. In bovine Peyer's patches, PrP(c) was distributed in extrafollicular areas, but not in the germinal centre of the jejunum and ileum. PrP(c) was expressed in myeloid lineage cells such as myeloid dendritic cells and macrophages. In mice, PrP(c) was expressed in some epithelial cells throughout the small intestine as well as in cells such as follicular dendritic cell in the germinal centre of Peyer's patches. In this study, we demonstrate that there are a number of differences in the localization of PrP(c) between the murine and bovine small intestines.

Novel Aspects of Prions, Their Receptor Molecules, and Innovative Approaches for TSE Therapy

1. Prion diseases are a group of rare, fatal neurodegenerative diseases, also known as transmissible spongiform encephalopathies (TSEs), that affect both animals and humans and include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep, chronic wasting disease (CWD) in deer and elk, and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are usually rapidly progressive and clinical symptoms comprise dementia and loss of movement coordination due to the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(c)). 2. This article reviews the current knowledge on PrP(c) and PrP(Sc), prion replication mechanisms, interaction partners of prions, and their cell surface receptors. Several strategies, summarized in this article, have been investigated for an effective antiprion treatment including development of a vaccination therapy and screening for potent chemical compounds. Currently, no effective treatment for prion diseases is available. 3. The identification of the 37 kDa/67 kDa laminin receptor (LRP/LR) and heparan sulfate as cell surface receptors for prions, however, opens new avenues for the development of alternative TSE therapies.

Lymphoid follicles of the ileal Peyer's patch of lambs express low levels of PrP, as demonstrated by quantitative real-time RT-PCR on microdissected tissue compartments, in situ hybridization and immunohistochemistry

The expression level of normal cellular prion protein (PrPC) is thought to influence the transmission of transmissible spongiform encephalopathies (TSEs) from the peripheral entry site to the site of pathological changes in the central nervous system. In many TSEs, the clinical disease is preceded by a period in which the agent accumulates in lymphoid organs, particularly in association with follicular dendritic cells of lymphoid follicles. As the probable route of entry of the TSE agent is via the gut, the expression profile of PrP was examined in well-developed gut-associated lymphoid tissue of lambs, the ileal Peyer's patch, by laser microdissection and real-time RT-PCR. Lymphoid follicles were found to have very low levels of expression, whilst highest levels were detected in the outer submucosa and the muscular layer. These findings were supported by in situ hybridization and immunohistochemistry, which showed specific labelling in nerve cells in ganglia of the submucosal (Meissner's) and myenteric (Auerbach's) plexi of the enteric nervous system. Based on the assumption that potential sites for conversion to the scrapie-related prion protein (PrPSc) should display high levels of expression of PrPC, this study suggests that the accumulation of PrPSc in the lymphoid follicles of the Peyer's patch is not preceded by PrP conversion in the same tissue compartment.

Immunohistochemical expression of prion protein (PrPC) in the human forebrain during development

The cellular prion protein (PrPC) is a ubiquitous protein whose expression in the adult brain occurs mainly in synapses. We used monoclonal antibodies to study fetal and perinatal PrPC expression in the human forebrain. Double immunofluorescence and confocal microscopy with GFAP, Iba1, MAP2, doublecortin, synaptophysin, and GAP-43 were used to localize PrPC. PrPC immunoreactivity was observed in axonal tracts and fascicles from the 11th week to the end of gestation. Synapses expressed PrPC at increasing levels throughout synaptogenesis. At midgestation, a few PrPC-labeled neurons were detected in the cortical anlage and numerous ameboid and intermediate microglial cells were PrPC-positive. In contrast, at the end of gestation, microglial PrPC expression decreased to almost nothing, whereas neuronal PrPC expression increased, most notably in ischemic areas. In adults, PrPC immunoreactivity was restricted to the synaptic neuropil of the gray matter. At all ages, choroid plexus, ependymal, and endothelial cells were labeled, whereas astrocytes were only occasionally immunoreactive. In conclusion, the early expression of PrPC in the axonal field may suggest a specific role for this molecule in axonal growth during development. Moreover, PrPC may play a role in early microglial cell development.

Prion protein mRNA expression in Xenopus laevis: No induction during melanotrope cell activation

In mammals, the prion protein (PrP) is expressed in most tissues, but predominantly in neuronal tissues. Here, we investigated the temporal and spatial mRNA expression of PrP in the non-mammalian South African claw-toed frog Xenopus laevis. PrP transcripts were maternally expressed and detected throughout embryonic development, most strongly from neurulation onwards and including the tadpole stage. Microinjection of PrP mRNA into fertilized Xenopus eggs did not affect early embryonic development. In adult frogs, PrP mRNA expression was observed in all tissues examined, with high expression in brain, pituitary and testis. In Xenopus, the intermediate pituitary melanotrope cells are involved in background adaptation of the animal and produce high levels of the prohormone proopiomelanocortin (POMC) when the melanotrope cells are active (i.e. when the animal is black-adapted). Remarkably and in contrast to most secretory pathway components, PrP was not upregulated in the melanotropes of black-adapted animals, arguing against a direct role of this protein in POMC biosynthesis.

Expression of Prion Protein in the Gut of Mice Infected Orally with the 301V Murine Strain of the Bovine Spongiform Encephalopathy Agent

Transmissible spongiform encephalopathies (TSEs) are characterized by the accumulation of an abnormal, disease-associated prion protein (PrP(d)). Expression of its normal cellular counterpart (PrP(c)) by the host is a pre-requisite for the spread of infection to the central nervous system and the development of disease. Moreover, cells expressing PrP(c) at specific sites such as the gastrointestinal tract might be regarded as the initial point of PrP(c)-PrP(d) conversion after infection by the oral route. In this study, inbred mice of the I/M strain were infected orally with the 301V murine strain of the bovine spongiform encephalopathy agent. The expression of PrP(c) and the accumulation of PrP(d) in the intestine was then investigated immunohistochemically, together with the variations in immunoreactivity that resulted from different pretreatments of the tissue. After proteinase K (PK) pretreatment, abnormal PrP was still detectable only in the gut-associated lymphoid tissue (GALT) of clinically affected mice and, to a much more limited degree, in the enteric nervous system (ENS). Cellular PrP that disappeared after PK treatment was particularly conspicuous in the ENS and present to a lesser extent in the GALT of all mice examined after inoculation with 301V or with normal brain homogenates, as well as in uninoculated controls. These findings suggested that not all PrP found in infected mice was PrP(d) and that part of the PrP(d) was sensitive to PK treatment. Reactivity to PrP antibody 1A8 was consistently found in the absorptive epithelium of the intestinal villi, with or without PK pretreatment. However, epithelial immunolabelling was comparable in inoculated and uninoculated mice and was also consistently seen in PrP "knockout" mice used as controls. It is therefore concluded that immunohistochemically detectable accumulation of PrP(d) in the gut of mice is a relatively late event in the pathogenesis of experimental infection in this model and that the immunoreactivity observed in the intestinal epithelium does not correspond to PrP expression. While enterocytes may still play a role in the uptake of infection from the intestinal lumen, the results do not suggest that these cells are a site of initial accumulation of PrP(d).

Molecular characterization, phylogenetic relationships, and developmental expression patterns of prion genes in zebrafish (Danio rerio)

Prion diseases are characterized by the accumulation of a pathogenic misfolded form of a prion protein (PrP) encoded by the Prnp gene in humans. In the present study in zebrafish, two transcripts and the corresponding genes encoding prion proteins, PrP1 and PrP2, related to human PrP have been characterized with a relatively divergent deduced amino acid sequence, but a well preserved overall organization of structural prion protein motifs. Whole-mount in situ hybridization analysis performed during embryonic and larval development showed a high level of PrP1 mRNA spatially restricted to the anterior floor-plate of the central nervous system and in ganglia. Transcripts of prp2 were detected in embryonic cells from the mid-blastula transition to the end of the segmentation period. From 24 h postfertilization up to larval stages, prp2 transcripts were localized in distinct anatomical structures, including a major expression in the brain, eye, kidney, lateral line neuromasts, liver, heart, pectoral fins and posterior intestine. The observed differential developmental expression patterns of the two long PrP forms, prp1 and prp2, and the short PrP form prp3, a more divergent prion-related gene previously identified in zebrafish, should contribute to understanding of the phylogenetic and functional relationships of duplicated prion gene forms in the fish genome. Together, the complex history of prion-related genes, reflected in the deduced structural features, conserved amino acid sequence and repeat motifs of the corresponding proteins, and the presence of differential developmental expression patterns suggest possible acquisition or loss of prion protein functions during vertebrate evolution.

Prion protein (PrPc) immunocytochemistry and expression of the green fluorescent protein reporter gene under control of the bovine PrP gene promoter in the mouse brain

Expression of the cellular prion protein (PrP(c)) by host cells is required for prion replication and neuroinvasion in transmissible spongiform encephalopathies. As a consequence, identification of the cell types expressing PrP(c) is necessary to determine the target cells involved in the cerebral propagation of prion diseases. To identify the cells expressing PrP(c) in the mouse brain, the immunocytochemical localization of PrP(c) was investigated at the cellular and ultrastructural levels in several brain regions. In addition, we analyzed the expression pattern of a green fluorescent protein reporter gene under the control of regulatory sequences of the bovine prion protein gene in the brain of transgenic mice. By using a preembedding immunogold technique, neuronal PrP(c) was observed mainly bound to the cell surface and presynaptic sites. Dictyosomes and recycling organelles in most of the major neuron types also exhibited PrP(c) antigen. In the olfactory bulb, neocortex, putamen, hippocampus, thalamus, and cerebellum, the distribution pattern of both green fluorescent protein and PrP(c) immunoreactivity suggested that the transgenic regulatory sequences of the bovine PrP gene were sufficient to promote expression of the reporter gene in neurons that express immunodetectable endogenous PrP(c). Transgenic mice expressing PrP-GFP may thus provide attractive murine models for analyzing the transcriptional activity of the Prnp gene during prion infections as well as the anatomopathological kinetics of prion diseases.

Neuroprotective functions of prion protein

The normal function of prion protein (PrP) is usually disregarded at the expense of the more fascinating role of PrP in transmissible prion diseases. However, the normal PrP may play an important role in cellular function in the central nervous system, since PrP is highly expressed in neurons and motifs in the sequence of PrP are conserved in evolution. The finding that prion null mice do not have a significant overt phenotype suggests that the normal function of PrP is of minor importance. However, the absence of PrP in cells or in vivo contributes to an increased susceptibility to oxidative stress or apoptosis-inducing insults. An alternative explanation is that the PrP normal function is so important that it is redundant. Probing into the characteristics of PrP has revealed a number of features that could mediate important cellular functions. The neuroprotective actions so far identified with PrP are initiated through cell surface signaling, antioxidant activity, or anti-Bax function. Here, we review the characteristics of the PrP and the evidence that PrP protects against neurodegeneration and neuronal cell death.

Organ distribution of prion proteins in variant Creutzfeldt-Jakob disease

In this article we give an overview of the transmissible spongiform encephalopathies, with emphasis on the evidence for the distribution of abnormal prions in tissues. The normal prion protein is distributed ubiquitously throughout human body tissues. Endogenous expression of the normal prion protein, as well as auxiliary proteins, plays a part in accumulation of the abnormal prion protein. As exemplified by variant Creutzfeldt-Jakob disease (vCJD) the abnormal prion protein can accumulate in the host lymphoid system, in particular the follicular dendritic cells. The route for the disease-related prion neuroinvasion is likely to involve the peripheral nervous system. An alternative route may involve blood constituents. Both animal studies and studies on vCJD patients suggest a potential for abnormal prion distribution in several peripheral tissues other than the lymphoreticular system. In human beings the abnormal prion has been reported in the brain, tonsils, spleen, lymph node, retina, and proximal optic nerve. Infectivity, although present in peripheral tissues, is at lower levels than in the central nervous system (CNS). Animal models suggest that the growth of infectivity in the CNS is likely to be gradual with maximum values during the clinical phase of disease. That tissues may harbour the abnormal prion, at different levels of infectivity, during the incubation period of the disease raises concerns of iatrogenic transmission of the disease either after surgery, blood transfusion, or accidental organ transplantation from donors in the preclinical phase of the disease.

Tissue-specific expression pattern of bovine prion gene: quantification using real-time RT-PCR

In recent studies PrP mRNA was determined mostly by in situ hybridisation or Northern Blot analysis--methods not suitable for absolute quantification of mRNA copy numbers. Herein we report on bovine prion mRNA quantification using calibrated highly sensitive externally standardized real-time RT-PCR with LightCycler instrument. Total RNA was isolated from nine different regions of the CNS and seven peripheral organs. PrP(c) mRNA copy numbers could be determined in all tissues under study. In approval with prior studies high mRNA level was found in Neocortex and Cerebellum. Lymphatic organs showed at least as high expression levels of prion mRNA as overall brain. Lowest expression was detected in kidney. Results of our study provide insight into the involvement of different organs in pathogenesis with respect to prion mRNA expression. LightCycler technology is currently considered the most precise method for nucleic acid quantification and showed to be powerful tool for further studies on prion diseases pathogenesis.

Selective expression of prion protein in peripheral tissues of the adult mouse

The level of expression of normal cellular prion protein, PrP(c) (cellular prion protein), controls both the rate and the route of neuroinvasive infection, from peripheral entry portal to the CNS. Paradoxically, an overview of the distribution of PrP(c) within tissues outside the CNS is lacking. We have used novel antibodies that recognise cellular prion protein in glutaraldehyde-fixed tissue (in order to optimise immunohistochemical labelling of this conformationally labile protein), in combination with in situ hybridisation, to examine the expression of PrP(c) in peripheral tissues of the adult mouse. We found that although prion protein is expressed in many tissues, it is expressed at high levels only in discrete subpopulations of cells. Prominent amongst these are elements of the "hardwired neuroimmune network" that integrate the body's immune defence and neuroendocrine systems under CNS control. These prion protein-expressing elements include small diameter afferent nerves in the skin and the lamina propria of the aerodigestive tract, sympathetic ganglia and nerves, antigen presenting and processing cells (both follicular and non-follicular dendritic cells) and sub-populations of lymphocytes particularly in skin, gut- and bronchus-associated lymphoid tissues. Prion protein is also expressed in the parasympathetic and enteric nervous systems, in the dispersed neuroendocrine system, and in peripheral nervous system axons and their associated Schwann cells. This selective expression of cellular prion protein provides a variety of alternative routes for the propagation and transport of prion infection entering from peripheral sites, either naturally (via the aerodigestive tract or abraded skin) or experimentally (by intraperitoneal injection) to the brain. Key regulatory cells that express prion protein, and in particular enteroendocrine cells in the mucosal wall of the gut, and dendritic cells that convey pathogens from epithelial layers to secondary lymphoid organs, may be particularly important in the transmission of infection in the periphery.

A marked disparity between the expression of prion protein and its message by neurones of the CNS

Expression of the normal cellular form of prion protein is both necessary and rate-limiting in the spread of prion disease, yet its cellular expression in vivo is poorly understood. To optimise immunohistochemical labelling of this protein in mouse brain, we have developed novel antibodies that recognise cellular prion protein in glutaraldehyde-fixed tissue. Expression was found to be predominantly neuronal, and to differ between different classes of neurone. Thus, neurones immunoreactive for GABA expressed very high levels of normal prion protein; most projection neurones expressed much lower levels, particularly on their axons in the major fibre tracts, and some neurones (e.g. those positive for dopamine) displayed no detectable prion protein. In marked contrast, all neurones, even those that were immunonegative, expressed high levels of message for prion protein, shown by non-radioactive in situ hybridisation. Glia expressed very low levels of message, and undetectable levels of prion protein. We conclude that the steady-state level of prion protein, which differs so markedly between different neuronal types, is primarily controlled post-transcriptionally, possibly by differences in protein trafficking or degradation. These marked differences in the way different neurones produce and/or degrade their normal cellular prion protein may influence the selective spread and neurotoxic targeting of prion diseases within the CNS.

Nonneuronal cellular prion protein

The normal cellular prion protein (PrP(c)) is a membrane sialoglycoprotein of unknown function having the unique property of adopting an abnormal tertiary conformation. The pathological conformer PrP(sc) would be the agent of transmissible spongiform encephalopathies or prion diseases. They include scrapie and bovine spongiform encephalopathy in animals and Creutzfeldt-Jakob disease in humans. The conversion of PrP(c) into PrP(sc) in the brain governs the clinical phenotype of the disease. However, the three-dimensional structure change of PrP(c) can also take place outside the central nervous system, in nonneuronal cells particularly of lymphoid tissue where the agent replicates. In natural infection, PrP(c) in nonneuronal cells of peripheral extracerebral organs may play a key role as the receptor required to enable the entry of the infectious agent into the host. In the present review we have undertaken a first evaluation of compelling data concerning the PrP(c)-expressing cells of nonneuronal origin present in cerebral and extracerebral tissues. The analysis of tissue, cellular, and subcellular localization of PrP(c) may help us better understand the biological function of PrP(c) and provide some information on physiopathological processes underlying prion diseases.

Prion diseases: contribution of high-resolution immunomorphology

The transmisible spongiform encephalopathies or prion diseases are fatal neurological diseases that occur in animals and humans. They are characterized by the accumulation in the cerebral tissue of the abnormal form of prion protein (PrPsc) produced by a post-translational event involving conformational change of its normal cellular counterpart (PrPc). In this short review, we present some results on the biology of prion proteins which have benefited from morphological approaches combining the electron microscopy techniques and the immunodetection methods. We discuss data concerning in particular the physiological function of the normal cellular prion prion (PrPc) which have allowed to open up new vistas on prion diseases, the biogenesis of amyloid plaque and the cellular site involved in the prion protein conversion process.

Distribution of prion protein in the ileal Peyer's patch of scrapie-free lambs and lambs naturally and experimentally exposed to the scrapie agent

A sensitive immunohistochemical procedure was used to investigate the presence of prion protein (PrP) in the ileal Peyer's patch of PrP-genotyped lambs, including scrapie-free lambs and lambs naturally and experimentally exposed to the scrapie agent. The tyramide signal amplification system was used to enhance the sensitivity of conventional immunohistochemical procedures to show that PrP was widely distributed in the enteric nervous plexus supplying the gut wall. In scrapie-free lambs, PrP was also detected in scattered cells in the lamina propria and in the dome and interfollicular areas of the Peyer's patch. In the follicles, staining for PrP was mainly confined to the capsule and cells associated with vascular structures in the light central zone. In lambs naturally exposed to the scrapie agent, staining was prominent in the dome and neck region of the follicles and was also found to be associated with the follicle-associated epithelium. Similar observations were made in lambs that had received a single oral dose of scrapie-infected brain material from sheep with a homologous and heterologous PrP genotype 1 and 5 weeks previously. These studies show that the ileal Peyer's patch in young sheep may be an important site of uptake of the scrapie agent and that the biology of this major gut-associated lymphoid tissue may influence the susceptibility to oral infection in sheep. Furthermore, these studies suggest that homology or heterology between PrP genotypes or the presence of PrP genotypes seldom associated with disease does not impede uptake of PrP.

Ultrastructural localization of prion proteins: physiological and pathological implications

The transmissible spongiform encephalopathies (TSE) or prion diseases are fatal neurodegenerative disorders in which the central event is the conversion of a normal host-encoded protein (PrP(c)) into an abnormal isoform (PrP(sc)) which accumulates as amyloid in TSE brain. The two PrP(c) and PrP(sc) prion protein isoforms are membrane sialoglycoproteins synthesized in the central nervous system and various peripheral organ tissues. In this review, we describe the ultrastructural localization of prion proteins in human and animal cerebral and non-cerebral tissues whether or not infected by TSE agents. In addition to the plasma membrane of several cells, PrP(c) was found in association with cytoplasmic organelles of central and nerve-muscle synapses, and secretory granules of epithelial cells. Fibrils of amyloid plaques, synaptic structures, and lysosome-like organelles constitute the subcellular sites harboring PrP(sc). These findings have led to discussions on the physiological role of PrP(c) and the pathological mechanisms underlying prion spongiform encephalopathies.

Epithelial and endothelial expression of the green fluorescent protein reporter gene under the control of bovine prion protein (PrP) gene regulatory sequences in transgenic mice

The expression of the cellular form of the prion protein (PrP(c)) gene is required for prion replication and neuroinvasion in transmissible spongiform encephalopathies. The identification of the cell types expressing PrP(c) is necessary to understanding how the agent replicates and spreads from peripheral sites to the central nervous system. To determine the nature of the cell types expressing PrP(c), a green fluorescent protein reporter gene was expressed in transgenic mice under the control of 6.9 kb of the bovine PrP gene regulatory sequences. It was shown that the bovine PrP gene is expressed as two populations of mRNA differing by alternative splicing of one 115-bp 5' untranslated exon in 17 different bovine tissues. The analysis of transgenic mice showed reporter gene expression in some cells that have been identified as expressing PrP, such as cerebellar Purkinje cells, lymphocytes, and keratinocytes. In addition, expression of green fluorescent protein was observed in the plexus of the enteric nervous system and in a restricted subset of cells not yet clearly identified as expressing PrP: the epithelial cells of the thymic medullary and the endothelial cells of both the mucosal capillaries of the intestine and the renal capillaries. These data provide valuable information on the distribution of PrP(c) at the cellular level and argue for roles of the epithelial and endothelial cells in the spread of infection from the periphery to the brain. Moreover, the transgenic mice described in this paper provide a model that will allow for the study of the transcriptional activity of the PrP gene promoter in response to scrapie infection.

Circadian regulation of prion protein messenger RNA in the rat forebrain: a widespread and synchronous rhythm

Although the expression of the normal prion protein in the host is critical to the development of transmissible spongiform encephalopathies, the physiological role of this protein and the processes regulating its expression remain obscure. We now report that the messenger RNA for the prion protein is regulated in the rat brain in a marked circadian manner not only in the suprachiasmatic nuclei, the principal site for the generation of mammalian circadian rhythms, but also in other forebrain regions. The data show a remarkable consistency in the concurrence of a single peak of prion protein messenger RNA at each of the sites early in the animal's phase of increased locomotor activity; behavioural arousal does not, however, appear to affect this expression. We believe this to be the first study demonstrating that the expression of prion protein messenger RNA can change over a relatively short period in vivo. The results are discussed with reference to the range of recently discovered "clock-related" transcripts which also have widespread tissue expression; these include the messenger RNAs for D-box binding protein and thyroid embryonic factor, transcription factors which bind to the prion protein promoter.

Ultrastructural localization of cellular prion protein (PrPc) at the neuromuscular junction

We examined the localization of the normal cellular isoform of prion protein (PrPc) in mammalian skeletal muscle. Using two anti-PrP antibodies, the neuromuscular junction (NMJ) was preferentially stained after immunohistofluorescence. The mouse, hamster, and human NMJ displayed a fluorescent signal specific for PrPc. Postembedding immunoelectron microscopy analysis performed in the mouse muscle showed that the PrPc-specific colloidal gold immunolabelling was concentrated over the sarcoplasmic cytoplasm. The membrane of the postsynaptic domain was devoid of gold particles, while a weak signal was occasionally observed close to the presynaptic vesicles of the terminal axons. These results indicate that the PrP gene is expressed in mammalian muscle at the NMJ. The subsynaptic sarcoplasm of the NMJ appears to be the privileged site where PrPc presumably associated with endosome membrane may play a role in either physiological activity or maintenance of the morphological integrity of the synapse.

Astrocytosis and amyloid deposition in scrapie-infected hamsters

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

Prion protein expression in muscle cells and toxicity of a prion protein fragment

The prion protein (PrP) is a cell surface glycoprotein normally associated with neurones. Expression of the prion protein in cultured mouse myoblasts and myotubes suggests that the prion protein may play a physiological role in skeletal muscle. When myotubes differentiate from myoblasts prion protein expression is upregulated. Accompanying this increase is an upregulation of Cu/Zn superoxide dismutase (SOD-1) in myotubes. Muscle cells derived from mice deficient in cellular PrP (PrPc) show little increase in SOD-1 after differentiation from myoblasts to myotubes. Myoblasts and myotubes are resistant to the toxicity of a neurotoxic prion protein peptide (PrP106-126). However, in the presence of murine microglia, PrP106-126 causes a reduction in cell number. This effect is greater on myotubes than myoblasts. Even in the presence of microglia PrP106-126 is not toxic to muscle cells derived from PrP-deficient mice. Our results suggest that PrPc expression is associated with regulation of cellular resistance to oxidative stress in skeletal muscle.

Sensitivity and specificity of newly proposed clinical criteria for possible vascular dementia

The objective of this study was to determine the sensitivity and specificity of clinical criteria for possible vascular dementia (VaD) recently developed independently by two groups: the State of California Alzheimer's Disease Diagnostic and Treatment Centers (ADDTC) and the National Institute for Neurological Disorders and Stroke with the Association Internationale pour la Recherche et l'Enseignement en Neurosciences (NINDS-AIREN). We also wished to compare the performance of the new criteria to that of the Hachinski Ischemic Score (HIS). The study was comprised of a retrospective chart review and clinicopathologic correlation, and took place in 304-bed acute-care geriatric hospital. The subjects were 113 autopsied elderly patients with dementia, who were assessed to determine sensitivity and specificity of the ADDTC and NINDS-AIREN criteria for possible VaD. Sensitivity and specificity were calculated using the neuropathologic diagnosis as a gold standard. Sensitivity was 0.63, and specificity was 0.64 for the ADDTC, 0.58 sensitivity and 0.80 specificity for the NINDS, and 0.43 sensitivity and 0.88 specificity for the HIS. Test combinations did not lead to substantial gains in sensitivity or specificity. The majority of patients with Alzheimer's disease were successfully excluded by the ADDTC (87%), the NINDS-AIREN (91%), and the HIS (97%). The proportion of mixed dementia cases clinically misclassified as VaD was 54% for the ADDTC, 29% for the NINDS-AIREN, and 18% for the HIS. Low sensitivity is the main weakness of the above clinical criteria for possible VaD. Mixed dementia is better excluded by the NINDS-AIREN than the ADDTC. Data from this validation study should provide valuable information to clinicians and researchers who wish to apply these criteria to the diagnosis of VaD.

A neurotoxic prion protein fragment enhances proliferation of microglia but not astrocytes in culture

The scrapie isoform of the prion protein (PrPSc) induces pathological changes in the central nervous system including neurodegeneration and gliosis. A synthetic prion protein (PrP) peptide corresponding to amino acid residues 106-126 has been shown to be toxic to neurons that express PrPC, the cellular isoform of PrP. Here we show that in mixed glial cultures PrP106-126 induces astroglial proliferation that is dependent on cellular PrPc expression. In purified cultures of glial subtypes only microglia proliferated in response to PrP106-126. This effect was independent of PrP expression. Destruction of microglia in mixed glial cultures by L-leucine methyl ester (LLME) treatment abolished enhanced proliferation caused by PrP106-126. This proliferative effect can be restored by co-culturing LLME-treated astrocytes with microglia. Microglia therefore seem to mediate the proliferative effect exerted by PrP106-126 on astrocytes.