Hsp40 function in yeast prion propagation: Amyloid diversity necessitates chaperone functional complexity

It has been estimated that cerebrospinal fluid (CSF) contains approximately 80 proteins that significantly increase or decrease in response to various clinical conditions. Here we have evaluated the CSF protein PrPC (cellular prion protein) for possible increases or decreases following spinal cord injury. The physiological function of PrPC is not yet completely understood; however, recent findings suggest that PrPC may have neuroprotective properties. Our results show that CSF PrPC is decreased in spinal cord injured patients 12 hours following injury and is absent at 7 days. Given that normal PrPC has been proposed to be neuroprotective we speculate that the decrease in CSF PrPC levels may influence neuronal cell survival following spinal cord injury.     

PrPC has nucleic acid chaperoning properties similar to the nucleocapsid protein of HIV-1

The function of the cellular prion protein (PrPC) remains obscure. Studies suggest that PrPC functions in several processes including signal transduction and Cu2+ metabolism. PrPC has also been established to bind nucleic acids. Therefore we investigated the properties of PrPC as a putative nucleic acid chaperone. Surprisingly, PrPC possesses all the nucleic acid chaperoning properties previously specific to retroviral nucleocapsid proteins. PrPC appears to be a molecular mimic of NCP7, the nucleocapsid protein of HIV-1. Thus PrPC, like NCP7, chaperones the annealing of tRNA(Lys) to the HIV-1 primer binding site, the initial step of retrovirus replication. PrPC also chaperones the two DNA strand transfers required for production of a complete proviral DNA with LTRs. Concerning the functions of NCP7 during budding, PrPC also mimices NCP7 by dimerizing the HIV-1 genomic RNA. These data are unprecedented because, although many cellular proteins have been identified as nucleic acid chaperones, none have the properties of retroviral nucleocapsid proteins.     

Dividing roles of prion protein in staurosporine-mediated apoptosis

Prion protein (PrPC) is a normal cellular glycoprotein that is expressed in almost all tissues including the central nervous system. Much attention has been focused on this protein because conversion of the normal PrPC to the diseased form (PrPSc) plays an essential role in transmissible spongiform encephalopathies such as mad cow disease and Creutzfeldt-Jakob disease. In spite of the extensive effort, the normal physiological function of PrPC remains elusive. Emerging evidence suggests that PrPC plays a protective role against cellular stresses including apoptosis induced by various pro-apoptotic agents such as Bax and staurosporine (STS), however, other reports showed overexpression of PrPC enhances STS-mediated apoptosis. In this study, we took a different approach by depleting endogenous PrPC using specific interfering RNA technique and compared the depleting and overproducing effects of PrPC on STS-induced apoptosis in neuro-2a (N2a) cells. We demonstrate here that down-regulation of PrPC sensitizes N2a cells to STS-induced cytotoxicity and apoptosis. The enhanced apoptosis induced by STS was shown by increased DNA fragmentation, immunoreactivity of Bax, and caspase-3 cleavage. We also showed that overproduction of PrPC had little or no effect on STS-mediated DNA fragmentation in N2a cells but it augments STS-mediated apoptosis in HEK293 cells, suggesting a cell line-specific effect. In addition, the inhibitory effect of PrPC on STS-mediated cellular stress appears to be modulated in part through induction of cell cycle G2 accumulation. Together, our data suggest that physiological level of endogenous PrPC plays a protective role against STS-mediated cellular stress. Loss of this protection could render cells more prone to cellular insults such as STS.     

Cellular isoform of the scrapie agent protein participates in lymphocyte activation

The scrapie agent protein (Sp33-37 or PrPSc) is the disease-associated isoform of a normal cellular membrane protein (Cp33-37 or PrPC) of unknown function. We report that normal human lymphocytes and lymphoid cell lines, but not erythrocytes or granulocytes, express PrPC mRNA and protein. PrPC is detectable on the surface of lymphocytes; the surface immunoreactivity is sensitive to phosphatidylinositol-specific phospholipase C, indicating glycosyl-phosphatidylinositol membrane anchorage. Lymphocyte PrPC surface abundance is increased by cell activation, and polyclonal antibodies to PrPC suppress mitogen-induced activation. We conclude that PrPC is a lymphocyte surface molecule that may participate in cell activation.     

Purification and properties of the cellular prion protein from Syrian hamster brain.

The cellular prion protein (PrPC) is encoded by a chromosomal gene, and its scrapie isoform (PrPSc) features in all aspects of the prion diseases. Prior to the studies reported here, purification of PrPC has only been accomplished using immunoaffinity chromatography yielding small amounts of protein. Brain homogenates contain two PrPC forms designated PrPC-I and -II. These proteins were purified from a microsomal fraction by detergent extraction and separated by immobilized Cu2+ ion affinity chromatography. PrPC-II appears to be generated from PrPC-I by limited proteolysis of the N-terminus. Fractions enriched for PrPC-I were purified further by cation-exchange chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Greater than 90% of the final product migrated as a broad band of M(r) 33-35 kDa as judged by silver staining after SDS-PAGE. Digestion of PrPC-I with peptide-N-glycosidase (PNGase) compressed the band and shifted its mobility giving an M(r) of 27 kDa. The protocol described should be amenable to large-scale preparation of PrPC, enabling physical comparisons of PrPC and PrPSc.     

Hsp70 binds to PrPC in the process of PrPC release via exosomes from THP-1 monocytes

PrPC (cellular prion protein) is a GPI (glycophosphatidylinositol)-anchored protein present on the surface of a number of peripheral blood cells. PrPC must be present for the generation and propagation of pathogenic conformer [PrPSc (scrapie prion protein)], which is a conformational conversion form of PrPC and has a central role in transmissible spongiform encephalopathies. It is important to determine the transportation mechanism of normal PrPC between cells. Exosomes are membrane vesicles released into the extracellular space upon fusion of multivesicular endosomes with the plasma membrane. We have identified that THP-1 monocytes can secrete exosomes to culture medium, and the secreted exosomes can bear PrPC. We also found that Hsp70 interacts with PrPC not only in intracellular environment, but in the secreted exosomes. However, the specific markers of exosomes, Tsg101 and flotillin-1, were found with no interaction with PrPC. Our results demonstrated that PrPC can be released from THP-1 monocytes via secreted exosomes, and in this process, Hsp70 binds to PrPC, which suggests that Hsp70 may play a potential functional role in the release of PrPC.     

Prion and TNFα: TAC(E)it agreement between the prion protein and cell signaling

Prion diseases are rare and fatal neurodegenerative disorders that occur when the cellular prion protein (PrPC) is converted into a conformationally modified isoform that originates the novel infectious agent, called prion. Although much information is now available on the different routes of prion infection, both the mechanisms underlying prion neurotoxicity and the physiologic role of PrPC remain unclear. By use of a novel paradigm, we have shown in a recent paper that - following a myotoxin-induced degenerative challenge - PrPC is implicated in the morphogenesis of the skeletal muscle of adult mice. PrPC accomplished this task by modulating signaling pathways central to the myogenic process, in particular the p38 kinase pathway. The possibility that PrPC acts in cell signaling has already been suggested after in vitro studies. Using our in vivo approach, we have instead provided proof of the physiologic relevance of PrPC commitment in signaling events, and that PrPC likely performed the task by controlling the activity of the enzyme (TACE) secreting the signaling TNF-α molecule. After a brief summary of our data, here we will discuss the suggestion, arising from our and other recent findings, implying that regulation of TACE, and of other members of the protease family TACE belongs to, may be exploited by PrPC in different cell contexts. Notably, this advancement of knowledge on PrPC physiology could also shed light on the defense mechanisms against the onset of a more common neurodegenerative disorder than prion disease, such as Alzheimer disease.     

Hemin interactions and alterations of the subcellular localization of prion protein

Hemin (iron protoporphyrin IX) is a crucial component of many physiological processes acting either as a prosthetic group or as an intracellular messenger. Some unnatural, synthetic porphyrins have potent anti-scrapie activity and can interact with normal prion protein (PrPC). These observations raised the possibility that hemin, as a natural porphyrin, is a physiological ligand for PrPC. Accordingly, we evaluated PrPC interactions with hemin. When hemin (3-10 microM) was added to the medium of cultured cells, clusters of PrPC formed on the cell surface, and the detergent solubility of PrPC decreased. The addition of hemin also induced PrPC internalization and turnover. The ability of hemin to bind directly to PrPC was demonstrated by hemin-agarose affinity chromatography and UV-visible spectroscopy. Multiple hemin molecules bound primarily to the N-terminal third of PrPC, with reduced binding to PrPC lacking residues 34-94. These hemin-PrPC interactions suggest that PrPC may participate in hemin homeostasis, sensing, and/or uptake and that hemin might affect PrPC functions.     

The functional links between prion protein and copper

Prion diseases are fatal neurodegenerative disorders associated with the conversion of the cellular prion protein (PrPC) into a pathologic isoform. Although the physiological function of PrPC remains unknown, evidence relates PrPC to copper metabolism and oxidative stress as suggested by its copper-binding properties in the N-terminal octapeptide repeat region. This region also reduces copper ions in vitro, and this reduction ability is associated with the neuroprotection exerted by the octarepeat region against copper in vivo. In addition, the promoter region of the PrPC gene contains putative metal response elements suggesting it may be regulated by heavy metals. Here we address some of the evidence that support a physiological link between PrPC and copper. Also, in vivo experiments suggesting the physiological relevance of PrPC interaction with heparan sulfate proteoglycans are discussed.     

Silencing of cellular prion protein (PrPC) expression by DNA-antisense oligonucleotides induces autophagy-dependent cell death in glioma cells

Malignant gliomas are the most common and lethal primary central nervous system neoplasms. Several intriguing lines of evidence have recently emerged indicating that the cellular prion protein (PrPC) may exert neuro- and cyto-protective functions: PrPC overexpression protects cultured neurons and also tumor cell lines exposed to various pro-apoptotic stimuli while, on the contrary, PrPC silencing sensitizes Adriamycin-resistant human breast carcinoma cells to TRAIL-mediated cell death. In order to determine if PrPC is involved in the resistance of glial tumors to cell death, the effects of cellular prion protein downregulation by antisense approach were investigated in different human malignant glioma cell lines. PrPC downregulation induced profound morphological changes and significant cell death. In addition, a significant tumor volume reduction was noted after PrPC silencing in a EGFP-GL261 glioma murine model. Investigations of the molecular effects induced by PrPC silencing were carried out on T98G human glioma cells by analysing autophagic as well as typical apoptotic markers (nuclear morphology, caspase-3/7, p53 and PARP-1). The results indicated that apoptosis was not induced after PrPC downregulation while, on the contrary, electron microscopy analysis, and an accumulation of GFP-LC3-II in autophagosomal membranes of GFP-LC3 transfected cells, indicated a predominant activation of autophagy. PrPC silencing also led to induction of LC3-II, increase in Beclin-1 and a concomitant decrease in p62, Bcl-2 and in the phosphorylation of 4E-BP1, a target of mTOR autophagy signaling. In conclusion, our results show for the first time that interfering with the cellular prion protein expression could modulate autophagy-dependent cell death pathways in glial tumor cells.     

Silencing of cellular prion protein (PrPC) expression by DNA-antisense oligonucleotides induces autophagy-dependent cell death in glioma cells

Malignant gliomas are the most common and lethal primary central nervous system neoplasms. Several intriguing lines of evidence have recently emerged indicating that the cellular prion protein (PrPC) may exert neuro- and cyto-protective functions: PrPC overexpression protects cultured neurons and also tumor cell lines exposed to various pro-apoptotic stimuli while, on the contrary, PrPC silencing sensitizes Adriamycin-resistant human breast carcinoma cells to TRAIL-mediated cell death. In order to determine if PrPC is involved in the resistance of glial tumors to cell death, the effects of cellular prion protein downregulation by antisense approach were investigated in different human malignant glioma cell lines. PrPC downregulation induced profound morphological changes and significant cell death. In addition, a significant tumor volume reduction was noted after PrPC silencing in a EGFP-GL261 glioma murine model. Investigations of the molecular effects induced by PrPC silencing were carried out on T98G human glioma cells by analysing autophagic as well as typical apoptotic markers (nuclear morphology, caspase-3/7, p53 and PARP-1). The results indicated that apoptosis was not induced after PrPC downregulation while, on the contrary, electron microscopy analysis, and an accumulation of GFP-LC3-II in autophagosomal membranes of GFP-LC3 transfected cells, indicated a predominant activation of autophagy. PrPC silencing also led to induction of LC3-II, increase in Beclin-1 and a concomitant decrease in p62, Bcl-2 and in the phosphorylation of 4E-BP1, a target of mTOR autophagy signaling. In conclusion, our results show for the first time that interfering with the cellular prion protein expression could modulate autophagy-dependent cell death pathways in glial tumor cells.     

Biology of the prion gene complex.

The prion protein gene Prnp encodes PrPSc, the major structural component of prions, infectious pathogens causing a number of disorders including scrapie and bovine spongiform encephalopathy (BSE). Missense mutations in the human Prnp gene, PRNP, cause inherited prion diseases such as familial Creutzfeldt-Jakob Disease. In uninfected animals, Prnp encodes a GPI-anchored protein denoted PrPC, and in prion infections, PrPC is converted to PrPSc by templated refolding. Although Prnp is conserved in mammalian species, attempts to verify interactions of putative PrP-binding proteins by genetic means have proven frustrating in that two independent lines of Prnp gene ablated mice (Prnp0/0 mice: ZrchI and Npu) lacking PrPC remain healthy throughout development. This indicates that PrPC serves a function that is not apparent in a laboratory setting or that other molecules have overlapping functions. Shuttling or sequestration of synaptic Cu(II) via binding to N-terminal octapeptide residues and (or) signal transduction involving the fyn kinase are possibilities currently under consideration. A new point of entry into the issue of prion protein function has emerged from identification of a paralog, Prnd, with 25% coding sequence identity to Prnp. Prnd lies downstream of Prnp and encodes the Dpl protein. Like PrPC, Dpl is presented on the cell surface via a GPI anchor and has three alpha-helices: however, it lacks the conformationally plastic and octapeptide repeat domains present in its well-known relative. Interestingly, Dpl is overexpressed in two other lines of Prnp0/0 mice (Ngsk and Rcm0) via intergenic splicing events. These lines of Prnp0/0 mice exhibit ataxia and apoptosis of cerebellar cells, indicating that ectopic synthesis of Dpl protein is toxic to CNS neurons: this inference has now been confirmed by the construction of transgenic mice expressing Dpl under the direct control of the PrP promoter. Remarkably, Dpl-programmed ataxia is rescued by wt Prnp transgenes. The interaction between the Prnp and Prnd genes in mouse cerebellar neurons may have a physical correlate in competition between Dpl and PrPC within a common biochemical pathway that, when misregulated, leads to apoptosis.     

Cholesterol depletion and modification of COOH-terminal targeting sequence of the prion protein inhibit formation of the scrapie isoform [published erratum appears in J Cell Biol 1995 Jul;130(2):501]

After the cellular prion protein (PrPC) transits to the cell surface where it is bound by a glycophosphatidyl inositol (GPI) anchor, PrPC is either metabolized or converted into the scrapie isoform (PrPSc). Because most GPI-anchored proteins are associated with cholesterol-rich membranous microdomains, we asked whether such structures participate in the metabolism of PrPC or the formation of PrPSc. The initial degradation of PrPC involves removal of the NH2 terminus of PrPC to produce a 17-kD polypeptide which was found in a Triton X-100 insoluble fraction. Both the formation of PrPSc and the initial degradation of PrPC were diminished by lovastatin-mediated depletion of cellular cholesterol but were insensitive to NH4Cl. Further degradation of the 17-kD polypeptide did occur within an NH4Cl-sensitive, acidic compartment. Replacing the GPI addition signal with the transmembrane and cytoplasmic domains of mouse CD4 rendered chimeric CD4PrPC soluble in cold Triton X-100. Both CD4PrPC and truncated PrPC without the GPI addition signal (Rogers, M., F. Yehieley, M. Scott, and S. B. Prusiner. 1993. Proc. Natl. Acad. Sci. USA. 90:3182-3186) were poor substrates for PrPSc formation. Thus, it seems likely that both the initial degradation of PrPC to the 17-kD polypeptide and the formation of PrPSc occur within a non-acidic compartment bound by cholesterol-rich membranes, possibly glycolipid-rich microdomains, where the metabolic fate of PrPC is determined. The pathway remains to be identified by which the 17-kD polypeptide and PrPSc are transported to an acidic compartment, presumably endosomes, where the 17-kD polypeptide is hydrolyzed and limited proteolysis of PrPSc produces PrP 27-30.     

The stoichiometry of host PrPC glycoforms modulates the efficiency of PrPSc formation in vitro

A central event in the formation of infectious prions is the conformational change of a host-encoded glycoprotein, PrPC, into a pathogenic isoform, PrPSc. However, the molecular requirements for efficient PrP conversion remain unknown. In this study, we employed the recently developed protein misfolding cyclic amplification (PMCA) and scrapie cell assay (SCA) techniques to study the role of N-linked glycosylation on prion formation in vitro. The results show that unglycosylated PrPC molecules are required to propagate mouse RML prions, whereas diglycosylated PrPC molecules are required to propagate hamster Sc237 prions. Furthermore, the formation of Sc237 prions is inhibited by substoichiometric levels of hamster unglycosylated PrPC molecules. Thus, interactions between different PrPC glycoforms appear to control the efficiency of prion formation in a species-specific manner.     

Glycosylinositol phospholipid anchors of the scrapie and cellular prion proteins contain sialic acid.

The only identified component of the scrapie prion is PrPSc, a glycosylinositol phospholipid (GPI)-linked protein that is derived from the cellular isoform (PrPC) by an as yet unknown posttranslational event. Analysis of the PrPSc GPI has revealed six different glycoforms, three of which are unprecedented. Two of the glycoforms contain N-acetylneuraminic acid, which has not been previously reported as a component of any GPI. The largest form of the GPI is proposed to have a glycan core consisting of Man alpha-Man alpha-Man-(NeuAc-Gal-GalNAc-)Man-GlcN-Ino. Identical PrPSc GPI structures were found for two distinct isolates or "strains" of prions which specify different incubation times, neuropathology, and PrPSc distribution in brains of Syrian hamsters. Limited analysis of the PrPC GPI reveals that it also has sialylated glycoforms, arguing that the presence of this monosaccharide does not distinguish PrPC from PrPSc.     

Prion protein attenuates excitotoxicity by inhibiting NMDA receptors

It is well established that misfolded forms of cellular prion protein (PrP [PrPC]) are crucial in the genesis and progression of transmissible spongiform encephalitis, whereas the function of native PrPC remains incompletely understood. To determine the physiological role of PrPC, we examine the neurophysiological properties of hippocampal neurons isolated from PrP-null mice. We show that PrP-null mouse neurons exhibit enhanced and drastically prolonged N-methyl-D-aspartate (NMDA)-evoked currents as a result of a functional upregulation of NMDA receptors (NMDARs) containing NR2D subunits. These effects are phenocopied by RNA interference and are rescued upon the overexpression of exogenous PrPC. The enhanced NMDAR activity results in an increase in neuronal excitability as well as enhanced glutamate excitotoxicity both in vitro and in vivo. Thus, native PrPC mediates an important neuroprotective role by virtue of its ability to inhibit NR2D subunits.     

Copper and zinc cause delivery of the prion protein from the plasma membrane to a subset of early endosomes and the Golgi

The cellular isoform of prion protein (PrPC) is a plasma membrane glycoprotein whose conformational conversion into PrPSc is the central molecular event in the propagation of infectious prions. However, the physiological function of PrPC has remained uncertain. The finding that PrPC binds copper ions with low micromolar affinity, coupled with several other observations, has led to the proposal that the protein plays a role in copper homeostasis. Using biochemical techniques, we had shown previously that copper ions rapidly and reversibly stimulate endocytosis of PrPC from the cell surface. In this report, we employ immunofluorescence microscopy to further investigate the specificity and kinetics of metal effects on PrPC trafficking and to identify the intracellular compartments to which internalized PrPC is delivered in response to copper and zinc. We find that both of these metals stimulate redistribution of surface PrPC to a subset of transferrin-containing early endosomes as well as to Golgi compartments. These results are consistent with models in which PrPC plays a role in the cellular uptake or efflux of transition metals.     

Intracellular accumulation of the cellular prion protein after mutagenesis of its Asn-linked glycosylation sites

The cellular isoform of the prion protein (PrPC) is a sialoglycoprotein bound almost exclusively on the external surface of the plasma membrane by a glycosyl phosphatidylinositol anchor. The deduced amino acid sequence of Syrian hamster PrPC identifies two potential sites for the addition of Asn-linked carbohydrates at amino acids 181-183 (Asn-Ile-Thr) and 197-199 (Asn-Phe-Thr). We have altered these sites by replacing the threonine residues with alanine and expressed the mutant proteins transiently in CV1 cells utilizing a mutagenesis vector with the T7 promoter located upstream from the PrP gene. The T7 RNA polymerase was supplied by infection with a recombinant vaccinia virus. The 3 mutant proteins (PrPAla183, PrPAla199 and PrPAla183/199) have a reduced relative molecular weight compared to wild-type (wt) PrP. Deglycosylation as well as synthesis in the presence of tunicamycin reduced the relative molecular weight of all the PrP species to that of the double mutant PrPAla183/199. Our results indicate that both single-site mutant prion proteins are glycosylated at non-mutated sites and they suggest that both potential sites for Asn-linked glycosylation are utilized in wt PrPC. Immunofluorescence studies demonstrate that while wt PrPC localizes to the cell surface, all the mutant PrP molecules accumulate intracellularly. The site of accumulation of PrPAla183 is probably prior to the mid-Golgi stack since this protein does not acquire resistance to endoglycosidase H. Whether the intracellular locations of the mutant PrPC species are the same as those identified for the scrapie isoform of the prion protein (PrPSc) remains to be established.     

Functional implication of cellular prion protein in antigen-driven interactions between T cells and dendritic cells

The cellular prion protein (PrPC) is a host-encoded, GPI-anchored cell surface protein, expressed on a wide range of tissues including neuronal and lymphoreticular cells. PrPC may undergo posttranslational conversion, giving rise to scrapie PrP, the pathogenic conformer considered as responsible for prion diseases. Despite intensive studies, the normal function of PrPC is still enigmatic. Starting from microscope observations showing an accumulation of PrPC at the sites of contact between T cells and Ag-loaded dendritic cells (DC), we have studied the contribution of PrPC in alloantigen and peptide-MHC-driven T/DC interactions. Whereas the absence of PrPC on the DC results in a reduced allogeneic T cell response, its absence on the T cell partner has no apparent effect upon this response. Therefore, PrPC seems to fulfill different functions on the two cell partners forming the synapse. In contrast, PrPC mobilization by Ab reduces the stimulatory properties of DC and the proliferative potential of responding T cells. The contrasted consequences, regarding T cell function, between PrPC deletion and PrPC coating by Abs, suggests that the prion protein acts as a signaling molecule on T cells. Furthermore, our results show that the absence of PrPC has consequences in vivo also, upon the ability of APCs to stimulate proliferative T cell responses. Thus, independent of neurological considerations, some of the evolutionary constraints that may have contributed to the conservation of the Prnp gene in mammalians, could be of immunological origin.     

正常和反常PRION分子PrP~C和PrP~(Sc)的空间结构模建和电子结构计算

朊蛋白（ＰＲＩＯＮ） 是一种不同于细菌,真菌,病毒的新的病原体。１９９６ 年英国“疯牛病”引起了对ＰＲＩＯＮ 研究的高潮。ＰＲＩＯＮ 分子生物学研究的关键是解释ＰＲＩＯＮ 蛋白构象转换的生物学本质。本文从构象模建,量子化学整体从头算的水平上对两个构象的电子结构进行了计算比较,从中发现了ＰＲＩＯＮ 蛋白分子电子结构上区别于其他分子的特异性, 并对计算所得的活性部位进行了讨论。