PrPc on the road: trafficking of the cellular prion protein

What is the role of lipids in prion conversion and disease?

The molecular cause of transmissible spongiform encephalopathies (TSEs) involves the conversion of the cellular prion protein (PrP^C) into its pathogenic form, called prion scrapie (PrP^Sc), which is prone to the formation of amorphous and amyloid aggregates found in TSE patients. Although the mechanisms of conversion of PrP^C into PrP^Sc are not entirely understood, two key points are currently accepted: (i) PrP^Sc acts as a seed for the recruitment of native PrP^C, inducing the latter's conversion to PrP^Sc; and (ii) other biomolecules, such as DNA, RNA, or lipids, can act as cofactors, mediating the conversion from PrP^C to PrP^Sc. Interestingly, PrP^C is anchored by a glycosylphosphatidylinositol molecule in the outer cell membrane. Therefore, interactions with lipid membranes or alterations in the membranes themselves have been widely investigated as possible factors for conversion. Alone or in combination with RNA molecules, lipids can induce the formation of PrP in vitro-produced aggregates capable of infecting animal models. Here, we discuss the role of lipids in prion conversion and infectivity, highlighting the structural and cytotoxic aspects of lipid-prion interactions. Strikingly, disorders like Alzheimer's and Parkinson's disease also seem to be caused by changes in protein structure and share pathogenic mechanisms with TSEs. Thus, we posit that comprehending the process of PrP conversion is relevant to understanding critical events involved in a variety of neurodegenerative disorders and will contribute to developing future therapeutic strategies for these devastating conditions.

Prion Protein Biology Through the Lens of Liquid-Liquid Phase Separation

Conformational conversion of the α-helix-rich cellular prion protein into the misfolded, β-rich, aggregated, scrapie form underlies the molecular basis of prion diseases that represent a class of invariably fatal, untreatable, and transmissible neurodegenerative diseases. However, despite the extensive and rigorous research, there is a significant gap in the understanding of molecular mechanisms that contribute to prion pathogenesis. In this review, we describe the historical perspective of the development of the prion concept and the current state of knowledge of prion biology including structural, molecular, and cellular aspects of the prion protein. We then summarize the putative functional role of the N-terminal intrinsically disordered segment of the prion protein. We next describe the ongoing efforts in elucidating the prion phase behavior and the emerging role of liquid-liquid phase separation that can have potential functional relevance and can offer an alternate non-canonical pathway involving conformational conversion into a disease-associated form. We also attempt to shed light on the evolutionary perspective of the prion protein highlighting the potential role of intrinsic disorder in prion protein biology and summarize a few important questions associated with the phase transitions of the prion protein. Delving deeper into these key aspects can pave the way for a detailed understanding of the critical molecular determinants of the prion phase transition and its relevance to physiology and neurodegenerative diseases.

Plasma PrPC and ADAM-10 as novel biomarkers for traumatic brain injury and concussion: a pilot study

BACKGROUND

Cellular prion protein (PrPC) is a lipid raft protein abundant within CNS. It is regulated by a disintegrin and metalloproteinase domain containing protein 10 (ADAM10). PrPC has previously been implicated as a biomarker for TBI. ADAM10 has not been investigated as a TBI biomarker.

OBJECTIVE

We evaluated PrPC and ADAM10 as candidate biomarkers for TBI.

METHODS

We performed ELISA for ADAM10 and PrPC on plasma samples of patients with TBI admitted to Brigham and Women's Hospital. Plasma samples from 20 patients admitted for isolated TBI were acquired from a biobank with clinical information. Control plasma (37 samples) was acquired from a commercial source. GraphPad was used to conduct statistical analysis.

RESULTS

37 controls and 20 TBI samples were collected. Of the patients with TBI, eight were mild, three were moderate, and nine were severe. Both PrPC and ADAM10 were elevated in patients with TBI compared with control (p < .001). ADAM10 exhibited greater expression in patients with worse clinical grade. There was no significant association of either PrPC or ADAM10 with time after injury.

CONCLUSIONS

Our results indicate that PrPC and ADAM10 appear to be useful potential tools for screening of TBI. ADAM10 is closely associated with clinical grade.

A New Take on Prion Protein Dynamics in Cellular Trafficking

The mobility of cellular prion protein (PrP^C) in specific cell membrane domains and among distinct cell compartments dictates its molecular interactions and directs its cell function. PrP^C works in concert with several partners to organize signaling platforms implicated in various cellular processes. The scaffold property of PrP^C is able to gather a molecular repertoire to create heterogeneous membrane domains that favor endocytic events. Dynamic trafficking of PrP^C through multiple pathways, in a well-orchestrated mechanism of intra and extracellular vesicular transport, defines its functional plasticity, and also assists the conversion and spreading of its infectious isoform associated with neurodegenerative diseases. In this review, we highlight how PrP^C traffics across intra- and extracellular compartments and the consequences of this dynamic transport in governing cell functions and contributing to prion disease pathogenesis.

The Quest for Cellular Prion Protein Functions in the Aged and Neurodegenerating Brain

Cellular (also termed ‘natural’) prion protein has been extensively studied for many years for its pathogenic role in prionopathies after misfolding. However, neuroprotective properties of the protein have been demonstrated under various scenarios. In this line, the involvement of the cellular prion protein in neurodegenerative diseases other than prionopathies continues to be widely debated by the scientific community. In fact, studies on knock-out mice show a vast range of physiological functions for the protein that can be supported by its ability as a cell surface scaffold protein. In this review, we first summarize the most commonly described roles of cellular prion protein in neuroprotection, including antioxidant and antiapoptotic activities and modulation of glutamate receptors. Second, in light of recently described interaction between cellular prion protein and some amyloid misfolded proteins, we will also discuss the molecular mechanisms potentially involved in protection against neurodegeneration in pathologies such as Alzheimer’s, Parkinson’s, and Huntington’s diseases.

The cellular prion protein and its derived fragments in human prion diseases and their role as potential biomarkers

Introduction: Human prion diseases are a heterogeneous group of incurable and debilitating conditions characterized by a progressive degeneration of the central nervous system. The conformational changes of the cellular prion protein and its formation into an abnormal isoform, spongiform degeneration, neuronal loss, and neuroinflammation are central to prion disease pathogenesis. It has been postulated that truncated variants of aggregation-prone proteins are implicated in neurodegenerative mechanisms. An increasing body of evidence indicates that proteolytic fragments and truncated variants of the prion protein are formed and accumulated in the brain of prion disease patients. These prion protein variants provide a high degree of relevance to disease pathology and diagnosis. Areas covered: In the present review, we summarize the current knowledge on the occurrence of truncated prion protein species and their potential roles in pathophysiological states during prion diseases progression. In addition, we discuss their usability as a diagnostic biomarker in prion diseases. Expert opinion: Either as a primary factor in the formation of prion diseases or as a consequence from neuropathological affection, abnormal prion protein variants and fragments may provide independent information about mechanisms of prion conversion, pathological states, or disease progression.

All the Same? The Secret Life of Prion Strains within Their Target Cells

Prions are infectious β-sheet-rich protein aggregates composed of misfolded prion protein (PrP^Sc) that do not possess coding nucleic acid. Prions replicate by recruiting and converting normal cellular PrP^C into infectious isoforms. In the same host species, prion strains target distinct brain regions and cause different disease phenotypes. Prion strains are associated with biophysically distinct PrP^Sc conformers, suggesting that strain properties are enciphered within alternative PrP^Sc quaternary structures. So far it is unknown how prion strains target specific cells and initiate productive infections. Deeper mechanistic insight into the prion life cycle came from cell lines permissive to a range of different prion strains. Still, it is unknown why certain cell lines are refractory to infection by one strain but permissive to another. While pharmacologic and genetic manipulations revealed subcellular compartments involved in prion replication, little is known about strain-specific requirements for endocytic trafficking pathways. This review summarizes our knowledge on how prions replicate within their target cells and on strain-specific differences in prion cell biology.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Modifiers of prion protein biogenesis and recycling identified by a highly parallel endocytosis kinetics assay

The cellular prion protein, PrP^C, is attached by a glycosylphosphatidylinositol anchor to the outer leaflet of the plasma membrane. Its misfolded isoform PrP^Sc is the causative agent of prion diseases. Conversion of PrP^C into PrP^Sc is thought to take place at the cell surface or in endolysosomal organelles. Understanding the intracellular trafficking of PrP^C may, therefore, help elucidate the conversion process. Here we describe a time-resolved fluorescence energy transfer (FRET) assay reporting membrane expression and real-time internalization rates of PrP^C The assay is suitable for high-throughput genetic and pharmaceutical screens for modulators of PrP^C trafficking. Simultaneous administration of FRET donor and acceptor anti-PrP^C antibodies to living cells yielded a measure of PrP^C surface density, whereas sequential addition of each antibody visualized the internalization rate of PrP^C (Z' factor >0.5). RNA interference assays showed that suppression of AP2M1 (AP-2 adaptor protein), RAB5A, VPS35 (vacuolar protein sorting 35 homolog), and M6PR (mannose 6-phosphate receptor) blocked PrP^C internalization, whereas down-regulation of GIT2 and VPS28 increased PrP^C internalization. PrP^C cell-surface expression was reduced by down-regulation of RAB5A, VPS28, and VPS35 and enhanced by silencing EHD1. These data identify a network of proteins implicated in PrP^C trafficking and demonstrate the power of this assay for identifying modulators of PrP^C trafficking.

The Biological Function of the Prion Protein: A Cell Surface Scaffold of Signaling Modules

The prion glycoprotein (PrP^C) is mostly located at the cell surface, tethered to the plasma membrane through a glycosyl-phosphatydil inositol (GPI) anchor. Misfolding of PrP^C is associated with the transmissible spongiform encephalopathies (TSEs), whereas its normal conformer serves as a receptor for oligomers of the β-amyloid peptide, which play a major role in the pathogenesis of Alzheimer’s Disease (AD). PrP^C is highly expressed in both the nervous and immune systems, as well as in other organs, but its functions are controversial. Extensive experimental work disclosed multiple physiological roles of PrP^C at the molecular, cellular and systemic levels, affecting the homeostasis of copper, neuroprotection, stem cell renewal and memory mechanisms, among others. Often each such process has been heralded as the bona fide function of PrP^C, despite restricted attention paid to a selected phenotypic trait, associated with either modulation of gene expression or to the engagement of PrP^C with a single ligand. In contrast, the GPI-anchored prion protein was shown to bind several extracellular and transmembrane ligands, which are required to endow that protein with the ability to play various roles in transmembrane signal transduction. In addition, differing sets of those ligands are available in cell type- and context-dependent scenarios. To account for such properties, we proposed that PrP^C serves as a dynamic platform for the assembly of signaling modules at the cell surface, with widespread consequences for both physiology and behavior. The current review advances the hypothesis that the biological function of the prion protein is that of a cell surface scaffold protein, based on the striking similarities of its functional properties with those of scaffold proteins involved in the organization of intracellular signal transduction pathways. Those properties are: the ability to recruit spatially restricted sets of binding molecules involved in specific signaling; mediation of the crosstalk of signaling pathways; reciprocal allosteric regulation with binding partners; compartmentalized responses; dependence of signaling properties upon posttranslational modification; and stoichiometric requirements and/or oligomerization-dependent impact on signaling. The scaffold concept may contribute to novel approaches to the development of effective treatments to hitherto incurable neurodegenerative diseases, through informed modulation of prion protein-ligand interactions.

Flotillin-1 mediates PrPc endocytosis in the cultured cells during Cu²⁺ stimulation through molecular interaction

Flotillins are membrane association proteins consisting of two homologous members, flotillin-1 (Flot-1) and flotillin-2 (Flot-2). They define a clathrin-independent endocytic pathway in mammal cells, which are also distinct from some other endocytosis mechanisms. The implicated cargoes of the flotillin-dependent pathway are mainly some GPI-anchored proteins, such as CD59 and Thy-1, which positionally colocalize with flotillins at the plasma membrane microdomains. To see whether flotillins are involved in the endocytosis of PrP(C), the potential molecular interaction between PrP(C) and flotillins in a neuroblastoma cell line SK-N-SH was analyzed. Co-immunoprecipitation assays did not reveal a detectable complex in the cell lysates of a normal feeding situation. After stimulation of Cu(2+), PrP(C) formed a clear complex with Flot-1, but not with Flot-2. Immunofluorescent assays illustrated that PrP(C) colocalized well with Flot-1, and the complexes of PrP(C)-Flot-1 shifted from the cell membrane to the cytoplasm along with the treatment of Cu(2+). Down-regulating the expression of Flot-1 in SK-N-SH cells by Flot-1-specific RNAi obviously abolished the Cu(2+)-stimulated endocytosis process of PrP(C). Moreover, we also found that in the cell line human embryonic kidney 293 (HEK293) without detectable PrP(C) expression, the distribution of cellular Flot-1 maintained almost unchanged during Cu(2+) treatment. Cu(2+)-induced PrP(C)-Flot-1 molecular interaction and endocytosis in HEK293 cells were obtained when expressing wild-type human PrP (PrP(PG5)), but not in the preparation expressing octarepeat-deleted PrP (PrP(PG0)). Our data here provide direct evidences for the molecular interaction and endocytosis of PrP(C) with Flot-1 in the presence of copper ions, and the octarepeat region of PrP(C) is critical for this process, which strongly indicates that the Flot-1-dependent endocytic pathway seems to mediate the endocytosis process of PrP(C) in the special situation.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

PrP octarepeats region determined the interaction with caveolin-1 and phosphorylation of caveolin-1 and Fyn

Caveolin-1 is one of the major constituents of caveolae. Both Cav-1 and PrP are plasma membrane proteins, which show active capacities for molecular interactions with many other proteins or agents, including themselves. Using yeast two-hybrid system and immunoprecipitation, we reconfirmed the molecular interaction between human Cav-1 and PrP. With co-immunoprecipitation tests, PrP(C)-Cav-1 and PrP(Sc)-Cav-1 complexes were identified in the brain homogenates of normal and scrapie agent 263K-infected hamsters, respectively. Transient expression of wild-type PrP (PrP-PG5) in HEK293 cells did not change the situation of Cav-1 and subsequent signal transduction pathways, while cross-linking of the expressed PrP with specific antibody induced remarkable colocalization of PrP and Cav-1 on the plasma membrane and significant increases of phosphorylated Cav-1 and phosphorylated Fyn. With deleted and inserted PrP mutants within octarepeat region, we observed obvious octarepeat-associated phenomena, including lower binding capacity with Cav-1 in vitro, unable to co-localize with Cav-1 in the cells and to induce up-regulation of p-Cav-1 and p-Fyn when removal of octarepeats in the context of full-length PrP. Moreover, we found that treatment on HEK293 cells with fibrous form of recombinant PrP protein led to up-regulating the levels of p-Cav-1 and p-Fyn. Our data here provide strong evidence that octarepeats of PrP are critical for the interaction between PrP and Cav-1. Significant alterations in the cultured cells, either the distributions of PrP and Cav-1 morphologically or the up-regulations of p-Cav-1 and p-Fyn, induced by antibody-mediated cross-linking or fibrous forms of PrP may suggest a possible internalization process of PrP(Sc).

Cellular prion protein: from physiology to pathology

The human cellular prion protein (PrP(C)) is a glycosylphosphatidylinositol (GPI) anchored membrane glycoprotein with two N-glycosylation sites at residues 181 and 197. This protein migrates in several bands by Western blot analysis (WB). Interestingly, PNGase F treatment of human brain homogenates prior to the WB, which is known to remove the N-glycosylations, unexpectedly gives rise to two dominant bands, which are now known as C-terminal (C1) and N-terminal (N1) fragments. This resembles the β-amyloid precursor protein (APP) in Alzheimer disease (AD), which can be physiologically processed by α-, β-, and γ-secretases. The processing of APP has been extensively studied, while the identity of the cellular proteases involved in the proteolysis of PrP(C) and their possible role in prion biology has remained limited and controversial. Nevertheless, there is a strong correlation between the neurotoxicity caused by prion proteins and the blockade of their normal proteolysis. For example, expression of non-cleavable PrP(C) mutants in transgenic mice generates neurotoxicity, even in the absence of infectious prions, suggesting that PrP(C) proteolysis is physiologically and pathologically important. As many mouse models of prion diseases have recently been developed and the knowledge about the proteases responsible for the PrP(C) proteolysis is accumulating, we examine the historical experimental evidence and highlight recent studies that shed new light on this issue.

Allosteric function and dysfunction of the prion protein

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases associated with progressive oligo- and multimerization of the prion protein (PrP(C)), its conformational conversion, aggregation and precipitation. We recently proposed that PrP(C) serves as a cell surface scaffold protein for a variety of signaling modules, the effects of which translate into wide-range functional consequences. Here we review evidence for allosteric functions of PrP(C), which constitute a common property of scaffold proteins. The available data suggest that allosteric effects among PrP(C) and its partners are involved in the assembly of multi-component signaling modules at the cell surface, impose upon both physiological and pathological conformational responses of PrP(C), and that allosteric dysfunction of PrP(C) has the potential to entail progressive signal corruption. These properties may be germane both to physiological roles of PrP(C), as well as to the pathogenesis of the TSEs and other degenerative/non-communicable diseases.

Characterization of intracellular localization of PrP(Sc) in prion-infected cells using a mAb that recognizes the region consisting of aa 119-127 of mouse PrP

Generation of an abnormal isoform of the prion protein (PrP(Sc)) is a key aspect of the propagation of prions. Elucidation of the intracellular localization of PrP(Sc) in prion-infected cells facilitates the understanding of the cellular mechanism of prion propagation. However, technical improvement in PrP(Sc)-specific detection is required for precise analysis. Here, we show that the mAb 132, which recognizes the region adjacent to the most amyloidogenic region of PrP, is useful for PrP(Sc)-specific detection by immunofluorescence assay in cells pre-treated with guanidine thiocyanate. Extensive analysis of the intracellular localization of PrP(Sc) in prion-infected cells using mAb 132 revealed the presence of PrP(Sc) throughout endocytic compartments. In particular, some of the granular PrP(Sc) signals that were clustered at peri-nuclear regions appeared to be localized in an endocytic recycling compartment through which exogenously loaded transferrin, shiga and cholera toxin B subunits were transported. The granular PrP(Sc) signals at peri-nuclear regions were dispersed to the peripheral regions including the plasma membrane during incubation at 20 °C, at which temperature transport from the plasma membrane to peri-nuclear regions was impaired. Conversely, dispersed PrP(Sc) signals appeared to return to peri-nuclear regions within 30 min during subsequent incubation at 37 °C, following which PrP(Sc) at peri-nuclear regions appeared to redisperse again to peripheral regions over the next 30 min incubation. These results suggest that PrP(Sc) is dynamically transported along with the membrane trafficking machinery of cells and that at least some PrP(Sc) circulates between peri-nuclear and peripheral regions including the plasma membrane via an endocytic recycling pathway.

Infectious prion protein alters manganese transport and neurotoxicity in a cell culture model of prion disease

Protein misfolding and aggregation are considered key features of many neurodegenerative diseases, but biochemical mechanisms underlying protein misfolding and the propagation of protein aggregates are not well understood. Prion disease is a classical neurodegenerative disorder resulting from the misfolding of endogenously expressed normal cellular prion protein (PrP(C)). Although the exact function of PrP(C) has not been fully elucidated, studies have suggested that it can function as a metal binding protein. Interestingly, increased brain manganese (Mn) levels have been reported in various prion diseases indicating divalent metals also may play a role in the disease process. Recently, we reported that PrP(C) protects against Mn-induced cytotoxicity in a neural cell culture model. To further understand the role of Mn in prion diseases, we examined Mn neurotoxicity in an infectious cell culture model of prion disease. Our results show CAD5 scrapie-infected cells were more resistant to Mn neurotoxicity as compared to uninfected cells (EC(50)=428.8 μM for CAD5 infected cells vs. 211.6 μM for uninfected cells). Additionally, treatment with 300 μM Mn in persistently infected CAD5 cells showed a reduction in mitochondrial impairment, caspase-3 activation, and DNA fragmentation when compared to uninfected cells. Scrapie-infected cells also showed significantly reduced Mn uptake as measured by inductively coupled plasma-mass spectrometry (ICP-MS), and altered expression of metal transporting proteins DMT1 and transferrin. Together, our data indicate that conversion of PrP to the pathogenic isoform enhances its ability to regulate Mn homeostasis, and suggest that understanding the interaction of metals with disease-specific proteins may provide further insight to protein aggregation in neurodegenerative diseases.

Amyloid-beta oligomers increase the localization of prion protein at the cell surface

In Alzheimer's disease, the amyloid-β peptide (Aβ) interacts with distinct proteins at the cell surface to interfere with synaptic communication. Recent data have implicated the prion protein (PrP(C)) as a putative receptor for Aβ. We show here that Aβ oligomers signal in cells in a PrP(C)-dependent manner, as might be expected if Aβ oligomers use PrP(C) as a receptor. Immunofluorescence, flow cytometry and cell surface protein biotinylation experiments indicated that treatment with Aβ oligomers, but not monomers, increased the localization of PrP(C) at the cell surface in cell lines. These results were reproduced in hippocampal neuronal cultures by labeling cell surface PrP(C). In order to understand possible mechanisms involved with this effect of Aβ oligomers, we used live cell confocal and total internal reflection microscopy in cell lines. Aβ oligomers inhibited the constitutive endocytosis of PrP(C), but we also found that after Aβ oligomer-treatment PrP(C) formed more clusters at the cell surface, suggesting the possibility of multiple effects of Aβ oligomers. Our experiments show for the first time that Aβ oligomers signal in a PrP(C)-dependent way and that they can affect PrP(C) trafficking, increasing its localization at the cell surface.

Aptamer-based silver nanoparticles used for intracellular protein imaging and single nanoparticle spectral analysis

Aptamer-adapted silver nanoparticles (Apt-AgNPs) were developed as a novel optical probe for simultaneous intracellular protein imaging and single nanoparticle spectral analysis, wherein AgNPs act as an illuminophore and the aptamer as a biomolecule specific recognition unit, respectively. It was found that streptavidin-conjugated and aptamer-functionalized AgNPs show satisfactory biocompatibility and stability in cell culture medium, and thus not only can act as a high contrast imaging agent for both dark-field light scattering microscope and TEM imaging but also can inspire supersensitive single nanoparticle spectra for potential intercellular microenvironment analysis. Further investigations showed that caveolae-related endocytosis is likely a necessary pathway for Apt-AgNPs labeled PrP(c) internalization in human bone marrow neuroblastoma cells (SK-N-SH cells). The integrated capability of Apt-AgNPs to be used as light scattering and TEM imaging agents, along with their potential use for single nanoparticle spectral analysis, makes them a great promise for future biomedical imaging and disease diagnosis.

ACAT-1, Cav-1 and PrP expression in scrapie susceptible and resistant sheep

Scrapie is a prion disease for which no means of ante-mortem diagnosis is available. We recently found a relationship between cell susceptibility to scrapie and altered cholesterol homeostasis. In brains and in skin fibroblasts and peripheral blood mononuclear cells from healthy and scrapie-affected sheep carrying a scrapie-susceptible genotype, the levels of cholesterol esters were consistently higher than in tissues and cultures derived from animals with a scrapie-resistant genotype. Here we show that intracellular accumulation of cholesterol esters (CE) in fibroblasts derived from scrapie-susceptible sheep was accompanied by parallel alterations in the expression level of acyl-coenzymeA: cholesterol-acyltransferase (ACAT1) and caveolin-1 (Cav-1) that are involved in the pathways leading to intracellular cholesterol esterification and trafficking. Comparative analysis of cellular prion protein (PrPc) mRNA, showed an higher expression level in cells from animals carrying a susceptible genotype, with or without Scrapie. These data suggest that CE accumulation in peripheral cells, together with the altered expression of some proteins implicated in intracellular cholesterol homeostasis, might serve to identify a distinctive lipid metabolic profile associated with increased susceptibility to develop prion disease following infection.

Glimepiride reduces the expression of PrPc, prevents PrPSc formation and protects against prion mediated neurotoxicity in cell lines

BACKGROUND

A hallmark of the prion diseases is the conversion of the host-encoded cellular prion protein (PrP(C)) into a disease related, alternatively folded isoform (PrP(Sc)). The accumulation of PrP(Sc) within the brain is associated with synapse loss and ultimately neuronal death. Novel therapeutics are desperately required to treat neurodegenerative diseases including the prion diseases.

PRINCIPAL FINDINGS

Treatment with glimepiride, a sulphonylurea approved for the treatment of diabetes mellitus, induced the release of PrP(C) from the surface of prion-infected neuronal cells. The cell surface is a site where PrP(C) molecules may be converted to PrP(Sc) and glimepiride treatment reduced PrP(Sc) formation in three prion infected neuronal cell lines (ScN2a, SMB and ScGT1 cells). Glimepiride also protected cortical and hippocampal neurones against the toxic effects of the prion-derived peptide PrP82-146. Glimepiride treatment significantly reduce both the amount of PrP82-146 that bound to neurones and PrP82-146 induced activation of cytoplasmic phospholipase A(2) (cPLA(2)) and the production of prostaglandin E(2) that is associated with neuronal injury in prion diseases. Our results are consistent with reports that glimepiride activates an endogenous glycosylphosphatidylinositol (GPI)-phospholipase C which reduced PrP(C) expression at the surface of neuronal cells. The effects of glimepiride were reproduced by treatment of cells with phosphatidylinositol-phospholipase C (PI-PLC) and were reversed by co-incubation with p-chloromercuriphenylsulphonate, an inhibitor of endogenous GPI-PLC.

CONCLUSIONS

Collectively, these results indicate that glimepiride may be a novel treatment to reduce PrP(Sc) formation and neuronal damage in prion diseases.

Prion protein: From physiology to cancer biology

Prion protein (PrPc) was originally viewed solely as being involved in prion disease, but now several intriguing lines of evidence have emerged indicating that it plays a fundamental role not only in the nervous system, but also throughout the human body. PrPc is expressed most abundantly in the brain, but has also been detected in other non-neuronal tissues as diverse as lymphoid cells, lung, heart, kidney, gastrointestinal tract, muscle, and mammary glands. Recent data indicate that PrPc may be implicated in biology of glioblastoma, breast cancer, prostate and gastric cancer. Over expression of PrPc is correlated to the acquisition by tumor cells of a phenotype for resistance to cell death induced by TNF alpha and TRAIL or antitumor drugs such as paclitaxel and anthracyclines. PrPc may promote tumorigenesis, proliferation and G1/S transition in gastric cancer cells. This review revisits the physiological functions of PrPc, and its possible implications for cancer biology.

Lipid rafts and clathrin cooperate in the internalization of PrP in epithelial FRT cells

BACKGROUND

The cellular prion protein (PrP(C)) plays a key role in the pathogenesis of Transmissible Spongiform Encephalopathies in which the protein undergoes post-translational conversion to the infectious form (PrP(Sc)). Although endocytosis appears to be required for this conversion, the mechanism of PrP(C) internalization is still debated, as caveolae/raft- and clathrin-dependent processes have all been reported to be involved.

METHODOLOGY/PRINCIPAL FINDINGS

We have investigated the mechanism of PrP(C) endocytosis in Fischer Rat Thyroid (FRT) cells, which lack caveolin-1 (cav-1) and caveolae, and in FRT/cav-1 cells which form functional caveolae. We show that PrP(C) internalization requires activated Cdc-42 and is sensitive to cholesterol depletion but not to cav-1 expression suggesting a role for rafts but not for caveolae in PrP(C) endocytosis. PrP(C) internalization is also affected by knock down of clathrin and by the expression of dominant negative Eps15 and Dynamin 2 mutants, indicating the involvement of a clathrin-dependent pathway. Notably, PrP(C) co-immunoprecipitates with clathrin and remains associated with detergent-insoluble microdomains during internalization thus indicating that PrP(C) can enter the cell via multiple pathways and that rafts and clathrin cooperate in its internalization.

CONCLUSIONS/SIGNIFICANCE

These findings are of particular interest if we consider that the internalization route/s undertaken by PrP(C) can be crucial for the ability of different prion strains to infect and to replicate in different cell lines.

Immunolocalisation of PrPSc in scrapie-infected N2a mouse neuroblastoma cells by light and electron microscopy

The causative agent of transmissible spongiform encephalopathies (TSE) is PrPSc, an infectious, misfolded isoform of the cellular prion protein (PrPC). The localisation and trafficking of PrPSc and sites of conversion from PrPC to PrPSc are under debate, particularly since most published work did not discriminate between PrPC and PrPSc. Here we describe the localisation of PrPC and PrPSc in a scrapie-infected neuroblastoma cell line, ScN2a, by light and electron microscopic immunolocalisation. After eliminating PrPC with proteinase K, PrPSc was detected at the plasma membrane, endocytosed via clathrin-coated pits and delivered to early endosomes. Finally, PrPSc was detected in late endosomes/lysosomes. As we detected PrPSc at the cell surface, in early endosomes and in late endosomes/lysosomes, i.e. locations where PrPC is also present, our data imply that the conversion process could take place at the plasma membrane and/or along the endocytic pathway. Finally, we observed the release of PrPC/PrPSc via exocytotic pathways, i.e. via exosomes and as an opaque electron-dense mass which may represent a mechanism of intercellular spreading of infectious prions.

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.

Trafficking of the microdomain scaffolding protein reggie-1/flotillin-2

The reggie/flotillin proteins oligomerize and associate into clusters which form scaffolds for membrane microdomains. Besides their localization at the plasma membrane, the reggies/flotillins reside at various intracellular compartments; however, the trafficking pathways used by reggie-1/flotillin-2 remain unclear. Here, we show that trafficking of reggie-1/flotillin-2 is BFA sensitive and that deletion mutants of reggie-1/flotillin-2 accumulate in the Golgi complex in HeLa, Jurkat and PC12 cells, suggesting Golgi-dependent trafficking of reggie-1/flotillin-2. Using total internal reflection fluorescence microscopy, we observed fast cycling of reggie-1/flotillin-2-positive vesicles at the plasma membrane, which engaged in transient interactions with the plasma membrane only. Reggie-1/flotillin-2 cycling was independent of clathrin, but was inhibited by cholesterol depletion and microtubule disruption. Cycling of reggie-1/flotillin-2 was negatively correlated with cell-cell contact formation but was stimulated by serum, epidermal growth factor and by cholesterol loading mediated by low density lipoproteins. However, reggie-1/flotillin-2 was neither involved in endocytosis of the epidermal growth factor itself nor in endocytosis of GPI-GFPs or the GPI-anchored cellular prion protein (PrP(c)). Reggie-2/flotillin-1 and stomatin-1 also exhibited cycling at the plasma membrane similar to reggie-1/flotillin-2, but these vesicles and microdomains only partially co-localized with reggie-2/flotillin-1. Thus, regulated vesicular cycling might be a general feature of SPFH protein-dependent trafficking.

Cell models of prion infection

Due to recent renewal of interest and concerns in prion diseases, a number of cell systems permissive to prion multiplication have been generated in the last years. These include established cell lines, neuronal stem cells and primary neuronal cultures. While most of these models are permissive to experimental, mouse-adapted strains of prions, the propagation of natural field isolates from sheep scrapie and chronic wasting disease has been recently achieved. These models have improved our knowledge on the molecular and cellular events controlling the conversion of the PrP(C) protein into abnormal isoforms and on the cell-to-cell spreading of prions. Infected cultured cells will also facilitate investigations on the molecular basis of strain identity and on the mechanisms that lead to neurodegeneration. The ongoing development of new cell models with improved characteristics will certainly be useful for a number of unanswered critical issues in the prion field.

Antiprion activity of cholesterol esterification modulators: a comparative study using ex vivo sheep fibroblasts and lymphocytes and mouse neuroblastoma cell lines

Our studies on the role of cholesterol homeostasis in the pathogenesis of scrapie revealed abnormal accumulation of cholesterol esters in ex vivo peripheral blood mononuclear cells (PBMCs) and skin fibroblasts from healthy and scrapie-affected sheep carrying a scrapie-susceptible genotype compared to sheep with a resistant genotype. Similar alterations were observed in mouse neuroblastoma N2a cell lines persistently infected with mouse-adapted 22L and RML strains of scrapie that showed up to threefold-higher cholesterol ester levels than parental N2a cells. We now report that proteinase K-resistant prion protein (PrPres)-producing cell populations of subclones from scrapie-infected cell lines were characterized by higher cholesterol ester levels than clone populations not producing PrPres. Treatments with a number of drugs known to interfere with different steps of cholesterol metabolism strongly reduced the accumulation of cholesterol esters in ex vivo PBMCs and skin fibroblasts from scrapie-affected sheep but had significantly less or no effect in their respective scrapie-resistant or uninfected counterparts. In scrapie-infected N2a cells, inhibition of cholesterol esters was associated with selective antiprion activity. Effective antiprion concentrations of cholesterol modulators (50% effective concentration [EC(50)] range, 1.4 to 40 microM) were comparable to those of antiprion reference compounds (EC(50) range, 0.6 to 10 microM). These data confirm our hypothesis that abnormal accumulation of cholesterol esters may represent a biological marker of susceptibility to prion infection/replication and a novel molecular target of potential clinical importance.

PrPc does not mediate internalization of PrPSc but is required at an early stage for de novo prion infection of Rov cells

We have studied the interactions of exogenous prions with an epithelial cell line inducibly expressing PrPc protein and permissive to infection by a sheep scrapie agent. We demonstrate that abnormal PrP (PrPSc) and prion infectivity are efficiently internalized in Rov cells, whether or not PrPc is expressed. At odds with earlier studies implicating cellular heparan sulfates in PrPSc internalization, we failed to find any involvement of such molecules in Rov cells, indicating that prions can enter target cells by several routes. We further show that PrPSc taken up in the absence of PrPc was unable to promote efficient prion multiplication once PrPc expression was restored in the cells. This observation argues that interaction of PrPSc with PrPc has to occur early, in a specific subcellular compartment(s), and is consistent with the view that the first prion multiplication events may occur at the cell surface.

Oral scrapie infection modifies the homeostasis of Peyer's patches' dendritic cells

In transmitted prion diseases the immune system supports the replication and the propagation of the pathogenic agent (PrPSc). DCs, which are mobile cells present in large numbers within lymph organs, are suspected to carry prions through the lymphoid system and to transfer them towards the peripheral nervous system. In this study, C57Bl/6 mice were orally inoculated with PrPSc (scrapie strain 139A) and sacrificed at the preclinical stages of the disease. Immunolabelled cryosections of Peyer's patches were analysed by confocal microscopy. Membrane prion protein expression was studied by flow cytometry. In Peyer's patches (PP), dissected at day one and day 105 after oral exposure to scrapie, we observed an increased population of DCs localised in the follicular-associated epithelium. On day 105, PrPSc was found in the follicles inside the PP of prion-infected mice. A subset of Peyer's patches DCs, which did not express cellular prion protein on their surface in non-infected mice conditions, was prion-positive in scrapie conditions. Within Peyer's patches oral scrapie exposure thus induced modifications of the homeostasis of DCs at the preclinical stages of the disease. These results give new arguments in favour of the implication of DCs in prion diseases.

Signaling induced by hop/STI-1 depends on endocytosis

The co-chaperone hop/STI-1 is a ligand of the cell surface prion protein (PrP(C)), and their interaction leads to signaling and biological effects. Among these, hop/STI-1 induces proliferation of A172 glioblastoma cells, dependent on both PrP(C) and activation of the Erk pathway. We tested whether clathrin-mediated endocytosis affects signaling induced by hop/STI-1. Both hyperosmolarity induced by sucrose and monodansyl-cadaverine blocked Erk activity induced by hop/STI-1, without affecting the high basal Akt activity typical of A172. The endocytosis inhibitors also affected the sub-cellular distribution of phosphorylated Erk, consistent with blockade of the latter's activity. The data indicate that signaling induced by hop/STI-1 depends on endocytosis. These findings are consistent with a role of sub-cellular trafficking in signal transduction following engagement by PrP(C) by ligands such as hop/STI-1, and may help help unravel both the functions of the prion protein, as well as possible loss-of-function components of prion diseases.

The role of the cellular prion protein in the immune system

Prion protein (PrP) plays a key role in the pathogenesis of prion diseases. However, the normal function of the protein remains unclear. The cellular isoform (PrP(C)) is expressed widely in the immune system, in haematopoietic stem cells and mature lymphoid and myeloid compartments in addition to cells of the central nervous system. It is up-regulated in T cell activation and may be expressed at higher levels by specialized classes of lymphocyte. Furthermore, antibody cross-linking of surface PrP modulates T cell activation and leads to rearrangements of lipid raft constituents and increased phosphorylation of signalling proteins. These findings appear to indicate an important but, as yet, ill-defined role in T cell function. Although PrP(-/-) mice have been reported to have only minor alterations in immune function, recent work has suggested that PrP is required for self-renewal of haematopoietic stem cells. Here, we consider the evidence for a distinctive role for PrP(C) in the immune system and what the effects of anti-prion therapeutics may be on immune function.

Interaction of metals with prion protein: Possible role of divalent cations in the pathogenesis of prion diseases

Prion diseases are fatal neurodegenerative disorders that affect both humans and animals. The rapid clinical progression, change in protein conformation, cross-species transmission and massive neuronal degeneration are some key features of this devastating degenerative condition. Although the etiology is unknown, aberrant processing of cellular prion proteins is well established in the pathogenesis of prion diseases. Normal cellular prion protein (PrP(c)) is highly conserved in mammals and expressed predominantly in the brain. Nevertheless, the exact function of the normal prion protein in the CNS has not been fully elucidated. Prion proteins may function as a metal binding protein because divalent cations such as copper, zinc and manganese can bind to octapeptide repeat sequences in the N-terminus of PrP(c). Since the binding of these metals to the octapeptide has been proposed to influence both structural and functional properties of prion proteins, alterations in transition metal levels can alter the course of the disease. Furthermore, cellular antioxidant capacity is significantly compromised due to conversion of the normal prion protein (PrP(c)) to an abnormal scrapie prion (PrP(sc)) protein, suggesting that oxidative stress may play a role in the neurodegenerative process of prion diseases. The combination of imbalances in cellular transition metals and increased oxidative stress could further exacerbate the neurotoxic effect of PrP(sc). This review includes an overview of the structure and function of prion proteins, followed by the role of metals such as copper, manganese and iron in the physiological function of the PrP(c), and the possible role of transition metals in the pathogenesis of the prion disease.

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.

Traffic of prion protein between different compartments on the neuronal surface, and the propagation of prion disease

The key mechanism in prion disease is the conversion of cellular prion protein into an altered, pathogenic conformation, in which cellular mechanisms play a poorly understood role. Both forms of prion protein are lipid-anchored and reside in rafts that appear to protect the native conformation against conversion. Neurons rapidly traffic their cellular prion protein out of its lipid rafts to be endocytosed via coated pits before recycling back to the cell surface. It is argued in this review that understanding the mechanism of this trafficking holds the key to understanding the cellular role in the conformational conversion of prion protein.

Structural differences between allelic variants of the ovine prion protein revealed by molecular dynamics simulations

We have modeled ovine prion protein (residues 119-233) based on NMR structures of PrP from other mammalian species. Modeling of the C-terminal domain of ovine PrP predicts three helices: helix-1 (residues 147-155), flanked by two short beta-strands; helix-2 (residues 176-197), and helix-3 (residues 203-229). Molecular dynamics simulations on this model of ovine PrP have determined structural differences between allelic variants. At neutral pH, limited root mean-squared (RMS) fluctuations were seen in the region of helix-1; between beta-strand-2 and residue 171, and the loop connecting helix-2 and helix-3. At low pH, these RMS fluctuations increased and showed allelic variation. The extent of RMS fluctuation between beta-strand 2 and residue 171 was ARR > ARQ > VRQ. This order was reversed for the loop region connecting helix-2 and helix-3. Although all three variants have the potential to display an extended helix at the C-terminal region of helix-1, the major influence of the VRQ allele was to restrict the conformations of the Asn162 and Arg139 side-chains. Variations observed in the simulations in the vicinity of helix-1 correlated with reactivity of C-terminal specific anti-PrP monoclonal antibodies with peripheral blood cells from scrapie-susceptible and -resistant genotypes of sheep: cells from VRQ homozygous sheep showed uniform reactivity, while cells from ARQ and ARR homozygous sheep showed variable binding. Our data show that molecular dynamics simulations can be used to determine structural differences between allelic variants of ovine PrP. The binding of anti-PrP monoclonal antibodies to ovine blood cells may validate these structural predictions.

The "ins" and "outs" of the high-affinity choline transporter CHT1

Maintenance of acetylcholine (ACh) synthesis depends on the activity of the high-affinity choline transporter (CHT1), which is responsible for the reuptake of choline from the synaptic cleft into presynaptic neurons. In this review, we discuss the current understanding of mechanisms involved in the cellular trafficking of CHT1. CHT1 protein is mainly found in intracellular organelles, such as endosomal compartments and synaptic vesicles. The presence of CHT1 at the plasma membrane is limited by rapid endocytosis of the transporter in clathrin-coated pits in a mechanism dependent on a dileucine-like motif present in the carboxyl-terminal region of the transporter. The intracellular pool of CHT1 appears to constitute a reserve pool of transporters, important for maintenance of cholinergic neurotransmission. However, the physiological basis of the presence of CHT1 in intracellular organelles is not fully understood. Current knowledge about CHT1 indicates that stimulated and constitutive exocytosis, in addition to endocytosis, will have major consequences for regulating choline uptake. Future investigations of CHT1 trafficking should elucidate such regulatory mechanisms, which may aid in understanding the pathophysiology of diseases that affect cholinergic neurons, such as Alzheimer's disease.

Uptake and neuritic transport of scrapie prion protein coincident with infection of neuronal cells

Invasion of the nervous system and neuronal spread of infection are critical, but poorly understood, steps in the pathogenesis of transmissible spongiform encephalopathies or prion diseases. To characterize pathways for the uptake and intraneuronal trafficking of infectious, protease-resistant prion protein (PrP-res), fluorescent-labeled PrP-res was used to infect a neuronally derived murine cell line (SN56) and adult hamster cortical neurons in primary culture. Concurrent with the establishment of persistent scrapie infection, SN56 cells internalized PrP-res aggregates into vesicles positive for markers for late endosomes and/or lysosomes but not synaptic, early endocytic, or raft-derived vesicles. Internalized PrP-res was then transported along neurites to points of contact with other cells. Similar trafficking was observed with dextran, Alzheimer's Abeta1-42 fibrils and noninfectious recombinant PrP fibrils, suggesting that PrP-res is internalized by a relatively nonspecific pinocytosis or transcytosis mechanism. Hamster cortical neurons were also capable of internalizing and disseminating exogenous PrP-res. Similar trafficking of exogenous PrP-res by cortical neurons cultured from the brains of PrP knock-out mice showed that uptake and neuritic transport did not require the presence of endogenous cellular PrP. These experiments visualize and characterize the initial steps associated with prion infection and transport within neuronal cells.

Absence of evidence for the participation of the macrophage cellular prion protein in infection with Brucella suis

Brucella spp. are stealthy bacteria that enter host cells without major perturbation. The molecular mechanism involved is still poorly understood, although numerous studies have been published on this subject. Recently, it was reported that Brucella abortus utilizes cellular prion protein (PrP(C)) to enter the cells and to reach its replicative niche. The molecular mechanisms involved were not clearly defined, prompting us to analyze this process using blocking antibodies against PrP(C). However, the behavior of Brucella during cellular infection under these conditions was not modified. In a next step, the behavior of Brucella in macrophages lacking the prion gene and the infection of mice knocked out for the prion gene were studied. We observed no difference from results obtained with the wild-type control. Although some contacts between PrP(C) and Brucella were observed on the surface of the cells by using confocal microscopy, we could not show that Brucella specifically bound recombinant PrP(C). Therefore, we concluded from our results that prion protein (PrP(C)) was not involved in Brucella infection.

Prion proteins from susceptible and resistant sheep exhibit some distinct cell biological features

It is well established that natural polymorphisms in the coding sequence of the PrP protein can control the expression of prion disease. Studies with a cell model of sheep prion infection have shown that ovine PrP allele associated with resistance to sheep scrapie may confer resistance by impairing the multiplication of the infectious agent. To further explore the biochemical and cellular mechanisms underlying the genetic control of scrapie susceptibility, we established permissive cells expressing two different PrP variants. In this study, we show that PrP variants with opposite effects on prion multiplication exhibit distinct cell biological features. These findings indicate that cell biological properties of ovine PrP can vary with natural polymorphisms and raise the possibility that differential interactions of PrP variants with the cellular machinery may contribute to permissiveness or resistance to prion multiplication.

Impaired exercise capacity, but unaltered mitochondrial respiration in skeletal or cardiac muscle of mice lacking cellular prion protein

The studies of physiological roles for cellular prion protein (PrP(c)) have focused on possible functions of this protein in the CNS, where it is largely expressed. However, the observation that PrP(c) is expressed also in muscle tissue suggests that the physiological role of PrP(c) might not be limited to the central nervous system. In the present study, we investigated possible functions of PrP(c) in muscle using PrP(c) gene (Prnp) null mice (Prnp(0/0)). For this purpose, we submitted Prnp(0/0) animals to different protocols of exercise, and compared their performance to that of their respective wild-type controls. Prnp(0/0) mice showed an exercise-dependent impairment of locomotor activity. In searching for possible mechanisms associated with the impairment observed, we evaluated mitochondrial respiration (MR) in skeletal or cardiac muscle from these mice during resting or after different intensities of exercise. Baseline MR (states 3 and 4), respiratory control ratio (RCR) and mitochondrial membrane potential (DeltaPsi) were evaluated and were not different in skeletal or cardiac muscle tissue of Prnp(0/0) mice when compared with wild-type animals. We concluded that Prnp(0/0) mice show impairment of swimming capacity, perhaps reflecting impairment of muscular activity under more extreme exercise conditions. In spite of the mitochondrial abnormalities reported in Prnp(0/0) mice, our observation seems not to be related to MR. Our results indicate that further investigations should be conducted in order to improve our knowledge about the function of PrP(c) in muscle physiology and its possible role in several different neuromuscular pathologies.

Assigning functions to distinct regions of the N-terminus of the prion protein that are involved in its copper-stimulated, clathrin-dependent endocytosis

The cellular prion protein (PrPC) is essential for the pathogenesis and transmission of prion diseases. Although PrPC is known to be located in detergent-insoluble lipid rafts at the surface of neuronal cells, the mechanism of its internalisation is unclear, with both raft/caveolae-based and clathrin-mediated processes being proposed. We have investigated the mechanism of copper-induced internalisation of PrPC in neuronal cells by immunofluorescence microscopy, surface biotinylation assays and buoyant sucrose density gradient centrifugation in the presence of Triton X-100. Clathrin-mediated endocytosis was selectively blocked with tyrphostin A23, which disrupts the interaction between tyrosine motifs in the cytosolic domains of integral membrane proteins and the adaptor complex AP2, and a dominant-negative mutant of the adaptor protein AP180. Both these agents inhibited the copper-induced endocytosis of PrPC. Copper caused PrPC to move laterally out of detergent-insoluble lipid rafts into detergent-soluble regions of the plasma membrane. Using mutants of PrPC that lack either the octapeptide repeats or the N-terminal polybasic region, and a construct with a transmembrane anchor, we show that copper binding to the octapeptide repeats promotes dissociation of PrPC from lipid rafts, whereas the N-terminal polybasic region mediates its interaction with a transmembrane adaptor protein that engages the clathrin endocytic machinery. Our results provide an experimental basis for reconciling the apparently contradictory observations that the prion protein undergoes clathrin-dependent endocytosis despite being localised in lipid rafts. In addition, we have been able to assign distinct functions in the endocytic process to separate regions of the protein.

Anterograde axonal transport of the exogenous cellular isoform of prion protein in the chick visual system

The cellular isoform of endogenous, newly synthesized prion protein (PrPc) can be transported by axons in the anterograde direction. To determine whether a mechanism exists for secreted PrPc to be internalized and then axonally transported, we analyzed internalization and anterograde axonal transport of radiolabeled recombinant PrPc after its intraocular injection in chick embryos. Internalization and axonal transport of exogenous PrPc to the midbrain by retinal ganglion cells (RGCs) is efficient, saturable and likely receptor-mediated. Ultrastructural quantitative localization of radiolabeled PrPc within RGC soma showed significant labeling of vesicular/endosomal compartments and much less labeling present over the Golgi apparatus and lysosomes, which indicates slow degradation of exogenous PrPc in this system. These data show that a mechanism exists to internalize a secreted form of PrPc and then to axonally transport such PrPc in an anterograde direction. This may provide an additional, novel mechanism for prion protein to spread among neurons.

The scrapie prion protein is present in flotillin-1-positive vesicles in central- but not peripheral-derived neuronal cell lines

Abstract Transmissible prion diseases are fatal neurodegenerative diseases associated with the conversion of the normal host prion protein (PrP c) into an abnormal isoform (PrP Sc) that accumulates in brain. This pathology affects neurons of the central nervous system whereas no clear toxic effect has been reported for peripheral neurons. We examined the subcellular distribution of PrP c and PrP Sc in the scrapie-infected mouse neuronal cell lines GT1-7 and N2a, derived, respectively, from the central and peripheral nervous system. We observed that in both cell types, PrP c is present in the endocytic compartment, mainly in LAMP-1-positive late endosomes, but excluded from LYAAT-1-lysosomes. In contrast, PrP Sc was distributed differently in the two cell lines. In infected N2a, PrP Sc and PrP c had comparable distribution patterns. In infected GT1-7, PrP Sc is present in an additional vesicular compartment which is flotillin-1-positive. The level of expression of flotillin-1 is higher in GT1-7 than in N2a cells, but no difference is observed between infected and noninfected cells. In Alzheimer's disease patients, it has been reported that flotillin-1 is abundant in brain areas containing the beta-amyloid protein, which accumulates in endosomal vesicles in primary neurons. We propose that the flotillin compartment could store aggregated proteins and play a role in these neurodegenerative pathologies.

An unusual soluble beta-turn-rich conformation of prion is involved in fibril formation and toxic to neuronal cells

A key molecular event in prion diseases is the conversion of the prion protein (PrP) from its normal cellular form (PrPC) to the disease-specific form (PrPSc). The transition from PrPC to PrPSc involves a major conformational change, resulting in amorphous protein aggregates and fibrillar amyloid deposits with increased beta-sheet structure. Using recombinant PrP refolded into a beta-sheet-rich form (beta-PrP) we have studied the fibrillization of beta-PrP both in solution and in association with raft membranes. In low ionic strength thick dense fibrils form large networks, which coexist with amorphous aggregates. High ionic strength results in less compact fibrils, that assemble in large sheets packed with globular PrP particles, resembling diffuse aggregates found in ex vivo preparations of PrPSc. Here we report on the finding of a beta-turn-rich conformation involved in prion fibrillization that is toxic to neuronal cells in culture. This is the first account of an intermediate in prion fibril formation that is toxic to neuronal cells. We propose that this unusual beta-turn-rich form of PrP may be a precursor of PrPSc and a candidate for the neurotoxic molecule in prion pathogenesis.