Brain copper content and cuproenzyme activity do not vary with prion protein expression level

Prion and doppel proteins bind to granule cells of the cerebellum

We reported that expression of the cellular prion protein (PrPC) rescues doppel (Dpl)-induced cerebellar degeneration in mice. To search for protein(s) that mediate this process, we fused the C-termini of mouse (Mo) PrP and Dpl to the Fc portion of an IgG. Although both MoPrP-Fc and MoDpl-Fc bound to many regions of the brain, we observed restricted binding to granule cells in the cerebellum, suggesting a scenario in which granule cells express a protein that mediates Dpl-induced neurodegeneration. Because granule cells do not express PrPC, it seems unlikely that MoPrP-Fc binding reflects a ligand that is involved in the conversion of PrPC into PrPSc, the disease-causing isoform. In contrast, the dominant-negative MoPrP(Q218K)-Fc not only binds to granule cells but also binds to neurons of the molecular layer where PrPC is expressed. These findings raise the possibility that the cells of the molecular layer express an auxiliary protein, provisionally designated protein X, which is involved in prion formation and is likely to be distinct from the protein that mediates Dpl-induced degeneration. Although the binding of the dominant-negative MoPrP(Q218K)-Fc to cells in the molecular layer expressing PrPC is consistent with a scenario for the binding of MoPrP(Q218K)-Fc to protein X, the absence of PrPSc deposition in the molecular layer requires that PrP(Sc), once formed there, be readily transported to the cerebellar white matter where PrPSc is found. Identifying the ligands to which PrP-Fc, Dpl-Fc, and dominant-negative PrP bind may provide new insights into the functions of PrPC and Dpl as well as the mechanism of PrPSc formation.

Transgenesis applied to transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative disorders present in various mammals. TSEs have been studies intensively, even more so following the BSE crisis and the subsequent threat of a human nvCJD epidemic. In the 'protein-only' hypothesis, the infectious agent, called prion, is assumed to be a misfolded host protein. Transgenesis has mainly been applied to study the role of this protein, its structure-function relationship with respect to its pathogenic properties and to assess the genetic origin of the well-recognised species barrier effect. This approach has somewhat supplemented the lack of in vitro models. This review will try to summarise the impressive work that has been done in this field. Although many questions remain unanswered, transgenic experiments have and will still improve our knowledge on this disease and might help us to develop critically needed therapeutic approaches.

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.

Metal ions and prion diseases

Recent data demonstrate that transition metal ions such as copper not only bind the prion protein with high affinities, but also modify its biochemical properties. This has important consequences for the potential function of the protein in metal-ion transport or as an anti-oxidant molecule. In addition, this relationship between the prion protein and metal ions is likely to play a critical role in the physiopathology of prion diseases.

Molecular features of the copper binding sites in the octarepeat domain of the prion protein

Recent evidence suggests that the prion protein (PrP) is a copper binding protein. The N-terminal region of human PrP contains four sequential copies of the highly conserved octarepeat sequence PHGGGWGQ spanning residues 60-91. This region selectively binds Cu2+ in vivo. In a previous study using peptide design, EPR, and CD spectroscopy, we showed that the HGGGW segment within each octarepeat comprises the fundamental Cu2+ binding unit [Aronoff-Spencer et al. (2000) Biochemistry 40, 13760-13771]. Here we present the first atomic resolution view of the copper binding site within an octarepeat. The crystal structure of HGGGW in a complex with Cu2+ reveals equatorial coordination by the histidine imidazole, two deprotonated glycine amides, and a glycine carbonyl, along with an axial water bridging to the Trp indole. Companion S-band EPR, X-band ESEEM, and HYSCORE experiments performed on a library of 15N-labeled peptides indicate that the structure of the copper binding site in HGGGW and PHGGGWGQ in solution is consistent with that of the crystal structure. Moreover, EPR performed on PrP(23-28, 57-91) and an 15N-labeled analogue demonstrates that the identified structure is maintained in the full PrP octarepeat domain. It has been shown that copper stimulates PrP endocytosis. The identified Gly-Cu linkage is unstable below pH approximately 6.5 and thus suggests a pH-dependent molecular mechanism by which PrP detects Cu2+ in the extracellular matrix or releases PrP-bound Cu2+ within the endosome. The structure also reveals an unusual complementary interaction between copper-structured HGGGW units that may facilitate molecular recognition between prion proteins, thereby suggesting a mechanism for transmembrane signaling and perhaps conversion to the pathogenic form.

PrP-dependent cell adhesion in N2a neuroblastoma cells

The cellular isoform of prion protein (PrP(C)) is a ubiquitous glycoprotein expressed by most tissues and with a biological function yet to be determined. Here, we have used a neuroblastoma cell model to investigate the involvement of PrP in cell adhesion. Incubation of single cell suspension induced cell-cell adhesion and formation of cell aggregates. Interestingly, cells overexpressing PrP exhibit increased cation-independent aggregation. Aggregation was reduced after phosphatidylinositol-specific phospholipase C release of the protein and by pre-incubation of cells with an antibody raised against the N-terminal part of PrP(C). Our paradigm allows the study of the function of PrP as an intercellular adhesion molecule and a cell surface ligand or receptor.

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

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

Regulation of the cellular prion protein gene expression depends on chromatin conformation

Conversion of the normal cellular prion protein (PrPc), whose physiological function is still under investigation, to an infectious form called prion is the cause of some neurodegenerative diseases. Therefore, the elucidation of PrPc gene regulation is important both to define a strategy to control the infection and to better understand PrPc function. We cloned the rat PrPc gene promoter region into a luciferase reporter vector, transfected C6 and PC-12 cells, and isolated clones with stable enzyme expression. The dependence of chromatin conformation on PrPc promoter activity was evaluated using the histone deacetylase inhibitor, trichostatin A, which was able to highly increase not only promoter activity but also PrPc mRNA and protein levels. The phorbol ester (12-O-tetradecanoylphorbol-13-acetate) and cAMP poorly induced promoter activity; retinoic acid decreased it by 50%, whereas nerve growth factor and dexamethasone had no effect. When 12-O-tetradecanoylphorbol-13-acetate or cAMP but not retinoic acid was associated with trichostatin A, a potentiation of the primary effects was observed. These new data indicate that PrPc gene regulation is highly dependent on disruption of chromatin fiber assembly, which allows some ubiquitous transcription factors accession to specific DNA elements.

Lack of prion protein expression results in a neuronal phenotype sensitive to stress

The prion protein is a highly conserved glycoprotein expressed most highly in the synapse. Evidence has recently been put forward to suggest that the prion protein is an antioxidant. However, the functional importance of the prion protein has been disputed; it is claimed that mice genetically ablated to lack prion protein expression are normal and have no specific phenotype. We have reexamined the phenotype of prion protein knockout mice and found that there are multiple biochemical changes in the mice, including increased levels of nuclear factor NF-kappaB and Mn superoxide dismutase, COX-IV decreased levels of Cu/Zn superoxide dismutase activity, decreased p53, and altered melatonin levels. Additionally, cultured cells from these mice are more sensitive to a range of insults, all linked to increased neuronal sensitivity to oxidative stress. These results imply that prion protein knockout mice are more sensitive to oxidative stress and have an altered phenotype that must be taken into account when considering the additional effects of increased levels of proteins such as Doppel. The implication of these results is that the consequence of genetic ablation of genes must include biochemical analysis as well as analyses of possible developmental and behavioral changes.

Nonneuronal cellular prion protein

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

Altered insulin receptor processing and function in scrapie-infected neuroblastoma cell lines

The underlying neurochemical changes contributing to prion-induced neurodegeneration remain largely unknown. This study shows that scrapie infection induced a 2-fold increase of insulin receptor (IR) protein and aberrantly processed IR beta-chain in scrapie-infected N2a neuroblastoma cells (ScN2a) as measured by Western blot of immunoprecipitated IR, in the absence of increased IR mRNA. Elevated IR protein level was further confirmed in an independently scrapie-infected neuroblastoma cell line N1E-115 (ScN1E-115). Proliferation studies showed that the increased IR level in ScN2a did not result in an increased insulin-mediated cell growth compared to normal N2a cells. Binding studies indicated that this apparent paradox was due to a 65% decrease in specific [(125)I]insulin binding sites in ScN2a when compared to the amount of immunoreactive IR, although the IR binding affinity was unchanged. Analysis of insulin stimulated IR tyrosine phosphorylation showed a slight but not significant reduction in ScN2a, when related to the increased level of immunoreactive IR. However, comparing the IR tyrosine phosphorylation to the loss of binding sites in ScN2a, we demonstrated an increased IR tyrosine phosphorylation of the remaining functional IR. In addition to these differences in IR properties, the basal extracellular signal regulated kinase-2 (ERK2) phosphorylation detected by Western blot, was significantly elevated and the insulin stimulated ERK2 phosphorylation was subsequently decreased in ScN2a. Together, these data show that scrapie infection affects the level and processing of the IR and signal transduction mediated by the IR in neuroblastoma cells, as well as induces an elevated basal ERK2 phosphorylation. Aberrant regulation of neuroprotective receptors may contribute to neurodegeneration in prion diseases.

Bovine spastic paresis: etiological hypotheses

Bovine spastic paresis (BSP) is a rare and little-known disease of cattle. The etiology remains unknown. A pathogenesis close to the sub-acute transmissible spongiform encephalopathies (TSEs) has repeatedly been advanced. In order to confirm an infectious origin, intra- and inter-species transmission studies are proposed. A positive result would bring a major advance in knowledge.

Prion diseases: what is the neurotoxic molecule?

A great deal of effort has been devoted during the past 20 years to defining the chemical nature of prions, the infectious agents responsible for transmissible spongiform encephalopathies. In contrast, much less attention has been paid to elucidating how prions actually damage the central nervous system. Although it is commonly assumed that PrP(Sc), the protein constituent of infectious prions, is the primary culprit, increasing evidence indicates that this may not be the case. Several alternative molecular forms of PrP are reasonable candidates for the neurotoxic species in prion diseases, although it is still too early to tell whether these or other ones will turn out to be the true instigating factors. The cellular pathways activated by neurotoxic forms of PrP that ultimately result in neuronal death are also being investigated, and several possible mechanisms have been uncovered, including the operation of quality control processes in the endoplasmic reticulum. Elucidating the distinction between the infectious and neurotoxic forms of PrP has important implications for designing therapy of prion diseases, as well as for understanding pathogenic mechanisms operative in other neurodegenerative disorders and the role of prion-like states in biology.

Prion diseases: contribution of high-resolution immunomorphology

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

Up-regulation of functionally impaired insulin-like growth factor-1 receptor in scrapie-infected neuroblastoma cells

A growing body of evidence suggests that an altered level or function of the neurotrophic insulin-like growth factor-1 receptor (IGF-1R), which supports neuronal survival, may underlie neurodegeneration. This study has focused on the expression and function of the IGF-1R in scrapie-infected neuroblastoma cell lines. Our results show that scrapie infection induces a 4-fold increase in the level of IGF-1R in two independently scrapie-infected neuroblastomas, ScN2a and ScN1E-115 cells, and that the increased IGF-1R level was accompanied by increased IGF-1R mRNA levels. In contrast to the elevated IGF-1R expression in ScN2a, receptor binding studies revealed an 80% decrease in specific (125)I-IGF-1-binding sites compared with N2a cells. This decrease in IGF-1R-binding sites was shown to be caused by a 7-fold decrease in IGF-1R affinity. Furthermore, ScN2a showed no significant difference in IGF-1 induced proliferative response, despite the noticeable elevated IGF-1R expression, putatively explained by the reduced IGF-1R binding affinity. Additionally, IGF-1 stimulated IGF-1Rbeta tyrosine phosphorylation showed no major change in the dose-response between the cell types, possibly due to altered tyrosine kinase signaling in scrapie-infected neuroblastoma cells. Altogether these data indicate that scrapie infection affects the expression, binding affinity, and signal transduction mediated by the IGF-1R in neuroblastoma cells. Altered IGF-1R expression and function may weaken the trophic support in scrapie-infected neurons and thereby contribute to neurodegeneration in prion diseases.

Prion diseases of humans and animals: their causes and molecular basis

Prion diseases are transmissible neurodegenerative conditions that include Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE) and scrapie in animals. Prions appear to be composed principally or entirely of abnormal isoforms of a host-encoded glycoprotein, prion protein. Prion propagation involves recruitment of host cellular prion protein, composed primarily of alpha-helical structure, into a disease specific isoform rich in beta-sheet structure. The existence of multiple prion strains has been difficult to explain in terms of a protein-only infections agent, but recent studies suggest that strain specific phenotypes can be encoded by different prion protein conformations and glycosylation patterns. The ability of a protein to encode phenotypic information has important biological implications. The appearance of a novel human prion disease, variant CJD, and the clear experimental evidence that it is caused by exposure to BSE has highlighted the need to understand the molecular basis of prion propagation, pathogenesis, and the barriers limiting intermammalian transmission. It is unclear if a large epidemic of variant CJD will occur in the years ahead.

The prion gene complex encoding PrP(C) and Doppel: insights from mutational analysis

The prion protein gene, Prnp, encodes PrP(Sc), the major structural component of prions, infectious pathogens causing a number of disorders including scrapie and bovine spongiform encephalopathy (or BSE). Missense mutations in the human Prnp gene cause inherited prion diseases such as familial Creutzfeldt-Jakob disease. In uninfected animals Prnp encodes a glycophosphatidylinositol (GPI)-anchored protein denoted PrP(C) and in prion infections PrP(C) is converted to PrP(Sc) by templated refolding. Though Prnp is conserved in mammalian species, attempts to verify interactions of putative PrP binding proteins by genetic means have proven frustrating and the ZrchI and Npu lines of Prnp gene-ablated mice (Prnp(0/0) mice) lacking PrP(C) remain healthy throughout development. This indicates that PrP(C) serves a function that is not apparent in a laboratory setting or that other molecules have overlapping functions. Current possibilities involve shuttling or sequestration of synaptic Cu(II) via binding to N-terminal octapeptide residues and/or signal transduction involving the fyn kinase. A new point of entry into the issue of prion protein function has emerged from identification of a paralogue, Prnd, with 24% coding sequence identity to Prnp. Prnd lies downstream of Prnp and encodes the doppel (Dpl) protein. Like PrP(C), 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 the Ngsk and Rcm0 lines of Prnp(0/0) mice via intergenic splicing events. These lines of Prnp(0/0) mice exhibit ataxia and apoptosis of cerebellar cells, indicating that ectopic synthesis of Dpl protein is toxic to central nervous system 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 wild-type Prnp transgenes. The interaction between the Prnp and Prnd genes in mouse cerebellar neurons may have a physical correlate in competition between Dpl and PrP(C) within a common biochemical pathway that when mis-regulated leads to apoptosis.

Inhibitory effect of copper(II) on zinc(II)-induced aggregation of amyloid beta-peptide

Aggregation of amyloid beta-peptide (Abeta), a key pathological event in Alzheimer's disease, has been shown in vitro to be profoundly promoted by Zn(II). This fact suggests that some factors in the normal brain protect Abeta from the Zn(II)-induced aggregation. We demonstrate for the first time that Cu(II) effectively inhibits the Abeta aggregation by competing with Zn(II) for histidine residues. The Raman spectrum of a metal-Abeta complex in the presence of both Zn(II) and Cu(II) shows that the cross-linking of Abeta through binding of Zn(II) to the N(tau) atom of histidine is prevented by chelation of Cu(II) by the N(pi) atom of histidine and nearby amide nitrogens. The inhibitory effect is strongest at a Cu/Abeta molar ratio of around 4. Above this ratio, Cu(II) itself promotes the Abeta aggregation by binding to the phenolate oxygen of Tyr10. These results emphasize the importance of regulation of Cu(II) levels to inhibit Abeta aggregation, and are consistent with an altered metal homeostasis in Alzheimer's disease.

Location and properties of metal-binding sites on the human prion protein

Although a functional role in copper binding has been suggested for the prion protein, evidence for binding at affinities characteristic of authentic metal-binding proteins has been lacking. By presentation of copper(II) ions in the presence of the weak chelator glycine, we have now characterized two high-affinity binding sites for divalent transition metals within the human prion protein. One is in the N-terminal octapeptide-repeat segment and has a K(d) for copper(II) of 10(-14) M, with other metals (Ni(2+), Zn(2+), and Mn(2+)) binding three or more orders of magnitude more weakly. However, NMR and fluorescence data reveal a previously unreported second site around histidines 96 and 111, a region of the molecule known to be crucial for prion propagation. The K(d) for copper(II) at this site is 4 x 10(-14) M, whereas nickel(II), zinc(II), and manganese(II) bind 6, 7, and 10 orders of magnitude more weakly, respectively, regardless of whether the protein is in its oxidized alpha-helical (alpha-PrP) or reduced beta-sheet (beta-PrP) conformation. A role for prion protein (PrP) in copper metabolism or transport seems likely and disturbance of this function may be involved in prion-related neurotoxicity.

Electron paramagnetic resonance evidence for binding of Cu(2+) to the C-terminal domain of the murine prion protein

Transmissible spongiform encephalopathies in mammals are believed to be caused by scrapie form of prion protein (PrP(Sc)), an abnormal, oligomeric isoform of the monomeric cellular prion protein (PrP(C)). One of the proposed functions of PrP(C) in vivo is a Cu(II) binding activity. Previous studies revealed that Cu(2+) binds to the unstructured N-terminal PrP(C) segment (residues 23-120) through conserved histidine residues. Here we analyzed the Cu(II) binding properties of full-length murine PrP(C) (mPrP), of its isolated C-terminal domain mPrP(121-231) and of the N-terminal fragment mPrP(58-91) in the range of pH 3-8 with electron paramagnetic resonance spectroscopy. We find that the C-terminal domain, both in its isolated form and in the context of the full-length protein, is capable of interacting with Cu(2+). Three Cu(II) coordination types are observed for the C-terminal domain. The N-terminal segment mPrP(58-91) binds Cu(2+) only at pH values above 5.0, whereas both mPrP(121-231) and mPrP(23-231) already show identical Cu(II) coordination in the pH range 3-5. As the Cu(2+)-binding N-terminal segment 58-91 is not required for prion propagation, our results open the possibility that Cu(2+) ions bound to the C-terminal domain are involved in the replication of prions, and provide the basis for further analytical studies on the specificity of Cu(II) binding by PrP.

Copper-catalyzed oxidation of the recombinant SHa(29-231) prion protein

Metal-catalyzed oxidation may result in structural damage to proteins and has been implicated in aging and disease, including neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis. The selective modification of specific amino acid residues with high metal ion affinity leads to subtle structural changes that are not easy to detect but may have dramatic consequences on physical and functional properties of the oxidized protein molecules. PrP contains a histidine-rich octarepeat domain that binds copper. Because copper-binding histidine residues are particularly prone to metal-catalyzed oxidation, we investigated the effect of this reaction on the recombinant prion protein SHaPrP(29-231). Using Cu2+/ascorbate, we oxidized SHaPrP(29-231) in vitro. Oxidation was demonstrated by liquid chromatography/mass spectrometry, which showed the appearance of protein species of higher mass, including increases in multiples of 16, characteristic of oxygen incorporation. Digestion studies using Lys C indicate that the 29-101 region, which includes the histidine-containing octarepeats, is particularly affected by oxidation. Oxidation was time- and copper concentration-dependent and was evident with copper concentrations as low as 1 microM. Concomitant with oxidation, SHaPrP(29-231) suffered aggregation and precipitation, which was nearly complete after 15 min, when the prion protein was incubated at 37 degrees C with a 6-fold molar excess of Cu2+. These findings indicate that PrP, a copper-binding protein, may be particularly susceptible to metal-catalyzed oxidation and that oxidation triggers an extensive structural transition leading to aggregation.

Imbalance of antioxidant defense in mice lacking cellular prion protein

Prion diseases are fatal neurodegenerative disorders resulting from conformational changes in the prion protein from its normal cellular isoform, PrPC, to the infectious scrapie isoform, PrP(Sc). In spite of many studies, the physiological function of PrPC remains unknown. Recent work shows that PrPC binds Cu2+, internalizing it into the cytoplasm. Since many antioxidant enzymes depend on Cu2+ (e.g., Cu/ZnSOD), their function could be affected in prion diseases. Here we investigate a possible relationship between PrP(C) and the cellular antioxidant systems in different structures isolated from PrPC knockout and wild-type mice by determining oxidative damage in protein and lipids and activity of antioxidant enzymes (CAT, SOD) and stress-adaptive enzymes (ODC). Our results show that, in the absence of PrPC, there is an increased oxidation of lipid and protein in all structures investigated. Decreased SOD activity and changes in CAT/ODC activities were also observed. Taking into account these results, we suggest that the physiological function of PrP(C) is related to cellular antioxidant defenses. Therefore, during development of prion diseases, the whole organism becomes more sensitive to ROS injury, leading to a progressive oxidative disruption of tissues and vital organs, especially the central nervous system.

Insights into the physiological function of cellular prion protein

Prions have been extensively studied since they represent a new class of infectious agents in which a protein, PrPsc (prion scrapie), appears to be the sole component of the infectious particle. They are responsible for transmissible spongiform encephalopathies, which affect both humans and animals. The mechanism of disease propagation is well understood and involves the interaction of PrPsc with its cellular isoform (PrPc) and subsequently abnormal structural conversion of the latter. PrPc is a glycoprotein anchored on the cell surface by a glycosylphosphatidylinositol moiety and expressed in most cell types but mainly in neurons. Prion diseases have been associated with the accumulation of the abnormally folded protein and its neurotoxic effects; however, it is not known if PrPc loss of function is an important component. New efforts are addressing this question and trying to characterize the physiological function of PrPc. At least four different mouse strains in which the PrP gene was ablated were generated and the results regarding their phenotype are controversial. Localization of PrPc on the cell membrane makes it a potential candidate for a ligand uptake, cell adhesion and recognition molecule or a membrane signaling molecule. Recent data have shown a potential role for PrPc in the metabolism of copper and moreover that this metal stimulates PrPc endocytosis. Our group has recently demonstrated that PrPc is a high affinity laminin ligand and that this interaction mediates neuronal cell adhesion and neurite extension and maintenance. Moreover, PrPc-caveolin-1 dependent coupling seems to trigger the tyrosine kinase Fyn activation. These data provide the first evidence for PrPc involvement in signal transduction.

Copper converts the cellular prion protein into a protease-resistant species that is distinct from the scrapie isoform

Several lines of evidence have suggested that copper ions play a role in the biology of both PrP(C) and PrP(Sc), the normal and pathologic forms of the prion protein. To further investigate this intriguing connection, we have analyzed how copper ions affect the biochemical properties of PrP(C) extracted from the brains of transgenic mice and from transfected cells. We report that the metal rapidly and reversibly induces PrP(C) to become protease-resistant and detergent-insoluble. Although these two properties are commonly associated with PrP(Sc), we demonstrate using a conformation-dependent immunoassay that copper-treated PrP is structurally distinct from PrP(Sc). The effect of copper requires the presence of at least one of the five octapeptide repeats normally present in the N-terminal half of the protein, consistent with the idea that the metal alters the biochemical properties of PrP by directly binding to this region. These results suggest potential roles for copper in prion diseases, as well as in the physiological function of PrP(C).

Immobilized prion protein undergoes spontaneous rearrangement to a conformation having features in common with the infectious form

It is hypothesized that infectious prions are generated as the cellular form of the prion protein (PrP(C)) undergoes pronounced conformational change under the direction of an infectious PrP(Sc) template. Conversion to the infectious conformer is particularly associated with major structural rearrangement in the central portion of the protein (residues 90-120), which has an extended flexible structure in the PrP(C) isoform. Using a panel of recombinant antibodies reactive with different parts of PrP, we show that equivalent major structural rearrangements occur spontaneously in this region of PrP immobilized on a surface. In contrast, regions more towards the termini of the protein remain relatively unaltered. The rearrangements occur even under conditions where individual PrP molecules should not contact one another. The propensity of specific unstructured regions of PrP to spontaneously undergo large and potentially deleterious conformational changes may have important implications for prion biology.

Pseudoknots in prion protein mRNAs confirmed by comparative sequence analysis and pattern searching

The human prion gene contains five copies of a 24 nt repeat that is highly conserved among species. An analysis of folding free energies of the human prion mRNA, in particular in the repeat region, suggested biased codon selection and the presence of RNA patterns. In particular, pseudoknots, similar to the one predicted by Wills in the human prion mRNA, were identified in the repeat region of all available prion mRNAs available in GenBank, but not those of birds and the red slider turtle. An alignment of these mRNAs, which share low sequence homology, shows several co-variations that maintain the pseudoknot pattern. The presence of pseudoknots in yeast Sup35p and Rnq1 suggests acquisition in the prokaryotic era. Computer generated three-dimensional structures of the human prion pseudoknot highlight protein and RNA interaction domains, which suggest a possible effect in prion protein translation. The role of pseudoknots in prion diseases is discussed as individuals with extra copies of the 24 nt repeat develop the familial form of Creutzfeldt-Jakob disease.

Cleavage of the amino terminus of the prion protein by reactive oxygen species

Relatively limited information is available on the processing and function of the normal cellular prion protein, PrP(C). Here it is reported for the first time that PrP(C) undergoes a site-specific cleavage of the octapeptide repeat region of the amino terminus on exposure to reactive oxygen species. This cleavage was both copper- and pH-dependent and was retarded by the presence of other divalent metal ions. The oxidative state of the cell also decreased detection of full-length PrP(C) and increased detection of amino-terminally fragmented PrP(C) within cells. Such a post-translational modification has vast implications for PrP(C), in its processing, because such cleavage could alter further proteolysis, and in the formation of the scrapie isoform of the prion protein, PrP(Sc), because abnormal cleavage of PrP(Sc) occurs into the octapeptide repeat region.

Prion infection impairs the cellular response to oxidative stress

The molecular mechanism of neurodegeneration in transmissible spongiform encephalopathies remains uncertain. In this study, it was demonstrated that prion-infected hypothalamic neuronal GT1 cells displayed a higher sensitivity to induced oxidative stress over noninfected cells. In addition, the infected cells presented an increased lipid peroxidation and signs of apoptosis associated with a dramatic reduction in the activities of the glutathione-dependent and superoxide dismutase antioxidant systems. This study indicates for the first time that prion infection results in an alteration of the molecular mechanisms promoting cellular resistance to reactive oxygen species. This finding is vital for future therapeutic approaches in transmissible spongiform encephalopathies and the understanding of the function of the prion protein.

Identification of the Cu2+ binding sites in the N-terminal domain of the prion protein by EPR and CD spectroscopy

Recent evidence indicates that the prion protein (PrP) plays a role in copper metabolism in the central nervous system. The N-terminal region of human PrP contains four sequential copies of the highly conserved octarepeat sequence PHGGGWGQ spanning residues 60-91. This region selectively binds divalent copper ions (Cu(2+)) in vivo. To elucidate the specific mode and site of binding, we have studied a series of Cu(2+)-peptide complexes composed of 1-, 2-, and 4-octarepeats and several sub-octarepeat peptides, by electron paramagnetic resonance (EPR, conventional X-band and low-frequency S-band) and circular dichroism (CD) spectroscopy. At pH 7.45, two EPR active binding modes are observed where the dominant mode appears to involve coordination of three nitrogens and one oxygen to the copper ion, while in the minor mode two nitrogens and two oxygens coordinate. ESEEM spectra demonstrate that the histidine imidazole contributes one of these nitrogens. The truncated sequence HGGGW gives EPR and CD that are indistinguishable from the dominant binding mode observed for the multi-octarepeat sequences and may therefore comprise the fundamental Cu(2+) binding unit. Both EPR and CD titration experiments demonstrate rigorously a 1:1 Cu(2+)/octarepeat binding stoichiometry regardless of the number of octarepeats in a given peptide sequence. Detailed spin integration of the EPR signals demonstrates that all of the bound Cu(2+) is detected thereby ruling out strong exchange coupling that is often found when there is imidazolate bridging between paramagnetic metal centers. A model consistent with these data is proposed in which Cu(2+) is bound to the nitrogen of the histidine imidazole side chain and to two nitrogens from sequential glycine backbone amides.

Differential contribution of superoxide dismutase activity by prion protein in vivo

Normal prion protein (PrP(C)) is a copper binding protein and may play a role in cellular resistance to oxidative stress. Recently, copper-bound recombinant PrP(C) has been shown to exhibit superoxide dismutase (SOD)-like activity. However, as PrP(C) affinity for copper is low in comparison to other cupro-proteins, the question remains as to whether PrP(C) could contribute SOD activity in vivo. To unravel this enigma, we compared the SOD activity in lysates extracted from different regions of the brain from wild-type mice before and after the depletion of PrP(C). We found that removal of PrP(C) from the brain lysates reduced the levels of total SOD activity. The level of contribution to the total SOD activity was correlated to the level of PrP expressed and to the predominant form of PrP present in the specific brain region. Collectively, these results provide strong evidence that PrP(C) differentially contributes to the total SOD activity in vivo.