Copper binding to the prion protein: structural implications of four identical cooperative binding sites

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.

Studies of nitric oxide interaction with mono- and dinuclear copper(II) complexes of prion protein bis-octarepeat fragments

The interaction of nitric oxide with copper(ii) complexes of two octarepeat sequences belonging to the prion protein was studied, considering both mononuclear and dinuclear systems, i.e. Cu-Ac-(PHGGGWGQ)(2)-NH(2) and Cu(2)-Ac-(PHGGGWGQ)(2)-NH(2), respectively. The NO interaction with both systems was followed in aqueous solutions at physiological pH value, by using UV-Vis and EPR spectroscopic techniques as well as cyclic voltammetry. The mechanism of NO interaction with the mononuclear copper complex can be considered similar to that previously observed for the analogous copper systems with Ac-HGGG-NH(2) and Ac-PHGGGWGQ-NH(2). A more complicated behaviour was found with the copper dinuclear system, in which the involvement of two different intermediate complex species was evidenced. A positive cooperativity between the two copper ions, in the reduction process was inferred. When working with a large excess of the Ac-(PHGGGWGQ)(2)-NH(2) ligand, the frozen-solution EPR parameters pertain to the well characterized [Cu(N(im))(4)](2+) unit, which did not exhibit any interaction with NO. The presence of a free coordination site is the necessary requirement for the NO interaction to occur, as found only in the square-pyramidal geometry of [Cu(L)H(-2)] or [Cu(2)(L)H(-4)] complex species, which form when copper and ligand concentrations are similar.

Prion stability and infectivity in the environment

The biology of normal prion protein and the property of infectivity observed in abnormal folding conformations remain thinly characterized. However, enough is known to understand that prion proteins stretch traditional views of proteins in biological systems. Numerous investigators are resolving details of the novel mechanism of infectivity, which appears to feature a protein-only, homologous replication of misfolded isoforms. Many other features of prion biology are equally extraordinary. This review focuses on the status of infectious prions in various natural and man-made environments. The picture that emerges is that prion proteins are durable under extreme conditions of environmental exposure that are uncommon in biological phenomena, and this durability offers the potential for environmental reservoirs of persistent infectivity lasting for years. A recurrent theme in prion research is a propensity for these proteins to bind to mineral and metal surfaces, and several investigators have provided evidence that the normal cellular functions of prion protein may include metalloprotein interactions. This structural propensity for binding to mineral and metal ions offers the hypothesis that prion polypeptides are intrinsically predisposed to non-physiological folding conformations that would account for their environmental durability and persistent infectivity. Similarly, the avidity of binding and potency of prion infectivity from environmental sources also offers a recent hypothesis that prion polypeptides bound to soil minerals are actually more infectious than studies with purified polypeptides would predict. Since certain of the prion diseases have a history of epidemics in economically important animal species and have the potential to transmit to humans, urgency is attached to understanding the environmental transmission of prion diseases and the development of protocols for their containment and inactivation.

Copper binding sites in the C-terminal domain of mouse prion protein: A hybrid (QM/MM) molecular dynamics study

We present a hybrid QM/MM Car-Parrinello molecular dynamics study of the copper-loaded C-terminal domain of the mouse prion protein. By means of a statistical analysis of copper coordination in known protein structures, we localized the protein regions with the highest propensity for copper ion binding. The identified candidate structures were subsequently refined via QM/MM simulations. Their EPR characteristics were computed to make contact with the experimental data and to probe the sensitivity to structural and chemical changes. Overall best agreement with the experimental EPR data (Van Doorslaer et al., J Phys Chem B 2001; 105: 1631-1639) and the information currently available in the literature is observed for a binding site involving H187. Moreover, a reinterpretation of the experimental proton hyperfine couplings was possible in the light of the present computational findings.

The chemistry of copper binding to PrP: is there sufficient evidence to elucidate a role for copper in protein function?

There has been an enormous body of literature published in the last 10 years concerning copper and PrP (prion protein). Despite this, there is still no generally accepted role for copper in the function of PrP or any real consensus as to how and to what affinity copper associates with the protein. The present review attempts to look at all the evidence for the chemistry, co-ordination and affinity of copper binding to PrP, and then looks at what effect this has on the protein. We then connect this evidence with possible roles for PrP when bound to copper. No clear conclusions can be made from the available data, but it is clear from the present review what aspects of copper association with PrP need to be re-investigated.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

Reduced erythroid cell and erythropoietin production in response to acute anemia in prion protein-deficient (Prnp-/-) mice

Cellular prion protein (PrPc) participates in the pathogenesis of prion diseases but its normal function remains unclear. PrPc is expressed on hematopoietic cells, including erythroid precursors. We investigated the role of PrPc in erythropoiesis in vivo with phenylhydrazine-induced acute anemia. Induction of equivalent anemia in wild-type (WT) and Prnp-/- mice resulted in a higher number of circulating reticulocytes, hematocrits and spleen weights in WT mice than in Prnp-/- mice on Days 5 and 7. Examination of bone marrow erythroid precursor cells (Ter119+) on Day 5 revealed no significant differences in the number of these cells between the two types of animals. However, a higher percentage of Ter119+ cells were going through apoptosis in Prnp-/- mice than in WT mice. Plasma erythropoietin (Epo) levels and Epo mRNA in kidneys peaked on Day 3 in response to anemia for both types of animals but rose less in Prnp-/- (5500 pg/ml ) than in WT (18,000 pg/ml) animals. Administration of recombinant human Epo to mice produced an equivalent reticulocyte response in both types of animals suggesting that the potential for erythroid generation is intact in Prnp-/- animals. These observations indicate that PrPc may modulate tissue hypoxia-sensing mechanisms or effect hypoxia target gene expression.

Interaction of alpha-synuclein with divalent metal ions reveals key differences: a link between structure, binding specificity and fibrillation enhancement

The aggregation of alpha-synuclein (AS) is characteristic of Parkinson's disease and other neurodegenerative synucleinopathies. Interactions with metal ions affect dramatically the kinetics of fibrillation of AS in vitro and are proposed to play a potential role in vivo. We recently showed that Cu(II) binds at the N-terminus of AS with high affinity (K(d) approximately 0.1 microM) and accelerates its fibrillation. In this work we investigated the binding features of the divalent metal ions Fe(II), Mn(II), Co(II), and Ni(II), and their effects on AS aggregation. By exploiting the different paramagnetic properties of these metal ions, NMR spectroscopy provides detailed information about the protein-metal interactions at the atomic level. The divalent metal ions bind preferentially and with low affinity (millimolar) to the C-terminus of AS, the primary binding site being the (119)DPDNEA(124) motif, in which Asp121 acts as the main anchoring residue. Combined with backbone residual dipolar coupling measurements, these results suggest that metal binding is not driven exclusively by electrostatic interactions but is mostly determined by the residual structure of the C-terminus of AS. A comparative analysis with Cu(II) revealed a hierarchal effect of AS-metal(II) interactions on AS aggregation kinetics, dictated by structural factors corresponding to different protein domains. These findings reveal a strong link between the specificity of AS-metal(II) interactions and the enhancement of aggregation of AS in vitro. The elucidation of the structural basis of AS metal binding specificity is then required to elucidate the mechanism and clarify the role of metal-protein interactions in the etiology of Parkinson's disease.

Role of copper in folding and stability of cupredoxin-like copper-carrier protein CopC

CopC is a periplasmic copper carrier that, in contrast to cytoplasmic copper chaperones, has a beta-barrel fold and two metal-binding sites distinct for Cu(II) and Cu(I). The copper sites are located in each end of the molecule: the Cu(I) site involves His and Met coordination whereas the Cu(II) site consists of charged residues. To reveal biophysical properties of this protein, we have explored the effects of the cofactors on CopC unfolding in vitro. We demonstrate that Cu(II) coordination affects both protein stability and unfolding pathway, whereas Cu(I) has only a small effect on stability. Apo-CopC unfolds in a two-state reaction between pH 4 and 7.5 with maximal stability at pH 6. In contrast, Cu(II)-CopC unfolds in a three-state reaction at pH6 that involves a partly folded intermediate that retains Cu(II). This intermediate exhibits high thermal and chemical stability. Unique energetic and structural properties of different metalated CopC forms may help facilitate metal transport to many partners in vivo.

Copper(II) Interaction with Prion Peptide Fragments Encompassing Histidine Residues Within and Outside the Octarepeat Domain: Speciation, Stability Constants and Binding Details

A 31-mer polypeptide, which encompasses residues 84-114 of human prion protein HuPrP(84-114) and contains three histidyl residues, namely one from the octarepeat (His85) and two histidyl residues from outside the octarepeat region (His96 and His111), and its mutants with two histidyl residues HuPrP(84-114)His85Ala, HuPrP(84-114) His96Ala, HuPrP(84-114)His111Ala and HuPrP(91-115) have been synthesised and their Cu2+ complexes studied by potentiometric and spectroscopic (UV/Vis, CD, EPR, ESI-MS) techniques. The results revealed a high Cu2+-binding affinity of all peptides, and the spectroscopic studies made it possible to clarify the coordination mode of the peptides in the different complex species. The imidazole nitrogen donor atoms of histidyl residues are the exclusive metal-binding sites below pH 5.5, and they have a preference for macrochelate structure formation. The deprotonation and metal-ion coordination of amide functions take place by increasing the pH; all of the histidines can be considered to be independent metal-binding sites in these species. As a consequence, di- and trinuclear complexes can be present even in equimolar samples of the metal ion and peptides, but the ratios of polynuclear species do not exceed the statistically expected ones; this excludes the possibility of cooperative Cu2+ binding. The species with a (N(im),N,N)-binding mode are favoured around pH 7, and their stability is enhanced by the macrochelation from another histidyl residue in the mononuclear complexes. The independence of the histidyl sites results in the existence of coordination isomers and the preference for metal binding follows the order of: His111>His96>His85. Deprotonation and metal-ion coordination of the third amide functions were detected in slightly alkaline solutions at each of the metal-binding sites; all had a (N(im),N,N,N)-coordination mode. Spectroscopic measurements also made it clear that the four lysyl amino groups of the peptides are not metal-binding sites in any cases.

Molecular dynamics simulations of two tandem octarepeats from the mammalian prion protein: fully Cu2+-bound and metal-free forms

Molecular dynamics simulations have been conducted on a model fragment (Ac-PHGGGWGQPHGGGW-NH(2)) of the prion protein octarepeat domain, both in the Cu(2+)-bound and metal-free forms. The copper-bound models are based on the consensus structure of the core Cu(2+)-binding site of an individual octarepeat, relevant to the fully Cu(2+)-occupied prion protein octarepeat region. The model peptides contain Cu(2+) bound through a His imidazole ring and two deprotonated amide N-atoms in the peptide backbone supplied by the following two Gly residues. Both the copper-bound and metal-free models have been simulated with the OPLS all-atom force field with the GROMACS molecular dynamics package. These simulations, with two tandem copper-binding sites, represent the minimum model necessary to observe potential structuring between the copper-binding sites in the octarepeat region. The GWGQ residues constitute a flexible linker region that predominantly adopts a turn, serving to bring adjacent His residues into close proximity. The consequent formation of stable structures demonstrates that the copper-bound octarepeat region allows the copper-coordinating sites to come into van der Waals contact, packing into particular orientations to further stabilize the bend in the GWGQ linker region.

Copper and the prion protein: methods, structures, function, and disease

The transmissible spongiform encephalopathies (TSEs) arise from conversion of the membrane-bound prion protein from PrP(C) to PrP(Sc). Examples of the TSEs include mad cow disease, chronic wasting disease in deer and elk, scrapie in goats and sheep, and kuru and Creutzfeldt-Jakob disease in humans. Although the precise function of PrP(C) in healthy tissues is not known, recent research demonstrates that it binds Cu(II) in an unusual and highly conserved region of the protein termed the octarepeat domain. This review describes recent connections between copper and PrP(C), with an emphasis on the electron paramagnetic resonance elucidation of the specific copper-binding sites, insights into PrP(C) function, and emerging connections between copper and prion disease.

Environmental effects on a prion's helix II domain: copper(II) and membrane interactions with PrP180-193 and its analogues

An abnormal interaction between copper and the prion protein is believed to play a pivotal role in the pathogenesis of prion diseases. Copper binding has been mainly attributed to the N-terminal domain of the prion protein, but this hypothesis has recently been challenged in some papers which suggest that the C-terminal domain might also compete for metal anchoring. In particular, the segment corresponding to the helix II region of the prion protein, namely PrP180-193, has been shown both to bind copper and to exhibit a copper-enhanced cytotoxicity, as well as to interact with artificial membranes. The present work is aimed at extending these results by choosing the most representative model of this domain and by determining its copper affinity. With this aim, the different role played by the electrostatic properties of the C- and N-termini of PrP180-193 (VNITIKQHTVTTTT) in determining its conformational behaviour, copper coordination and ability to perturb model membranes was investigated. Owing to the low solubility of PrP180-193, its copper affinity was evaluated by using the shorter PrPAc184-188NH2 (IKQHT) analogue as a model. ESI-MS, ESR, UV/Vis, and CD measurements were carried out on the copper(II)/PrPAc184-188NH2 and copper(II)/PrP180-193NH2 systems, and showed that PrPAc184-188NH2 is a reliable model for the metal interaction with the helix II domain. The affinity of copper(II) for the helix II fragment is higher than that for the octarepeat and PrP106-126 peptides. Finally, the different ability of PrP180-193 analogues to perturb the DPPC model membrane was assessed by DSC measurements. The possible biological consequences of these findings are also discussed briefly.

Fragment length influences affinity for Cu2+ and Ni2+ binding to His96 or His111 of the prion protein and spectroscopic evidence for a multiple histidine binding only at low pH

The prion protein (PrP) is a Cu2+-binding cell-surface glycoprotein. Using various PrP fragments and spectroscopic techniques, we show that two Cu2+ ions bind to a region between residues 90 and 126. This region incorporates the neurotoxic portion of PrP, vital for prion propagation in transmissible spongiform encephalopathies. Pentapeptides PrP-(92-96) and PrP-(107-111) represent the minimum motif for Cu2+ binding to the PrP-(90-126) fragment. Consequently, we were surprised that the appearance of the visible CD spectra for two fragments of PrP, residues 90-126 and 91-115, are very different. We have shown that these differences do not arise from a change in the co-ordination geometry within the two fragments; rather, there is a change in the relative preference for the two binding sites centred at His111 and His96. These preferences are metal-, pH- and chain-length dependent. CD indicates that Cu2+ initially fills the site at His111 within the PrP-(90-126) fragment. The pH-dependence of the Cu2+ co-ordination is studied using EPR, visible CD and absorption spectroscopy. We present evidence that, at low pH (5.5) and sub-stoichiometric amounts of Cu2+, a multiple histidine complex forms, but, at neutral pH, Cu2+ binds to individual histidine residues. We have shown that changes in pH and levels of extracellular Cu2+ will affect the co-ordination mode, which has implications for the affinity, folding and redox properties of Cu-PrP.

Mass spectrometric determination of the coordination geometry of potential copper(II) surrogates for the mammalian prion protein octarepeat region

The N-terminal domain of mammalian prion proteins contains several tandem repeats of the octapeptide PHGGGWGQ, each one capable of selectively binding up to 1 equiv of Cu2+. Under saturating conditions Cu2+ is known to coordinate the HGG portion of the repeat sequence via the histidine imidazole side chain, two deprotonated amide N-atoms, and a backbone carbonyl O-atom. Using appropriate selection criteria, we have generated a short list of candidate metal ions (Co3+, Ni2+, Pd2+, Pt2+) that can serve as potential surrogates for Cu2+. The selected metal ions were screened for binding interactions with the OR-derived peptide fragment AcHGGGWNH2 (Ac = acetyl, amino acid residues in italics) using electrospray ionization mass spectrometry. The coordination geometries of these metal ions with the synthetic OR peptide were subsequently determined from fragment analysis using collision-induced dissociation tandem mass spectrometry. Our results indicate that, although Co3+, Pd2+, and Pt2+ all bind to the OR fragment via the peptide backbone to varying extents, each of these metal ions appears to associate with the peptide in a unique manner, which is distinct from the way in which Cu2+ is coordinated. This work illustrates the extremely strong selectivity for Cu2+ of this highly conserved region of the mammalian prion protein.

The role of the octarepeat region in neuroprotective function of the cellular prion protein

Structural alterations of the cellular prion protein (PrP^C) seem to be the core of the pathogenesis of prion diseases. However, the physiological function of PrP^C remains an enigma. Cell culture experiments have indicated that PrP^C and in particular its N‐terminal octarepeat region together with the phosphatidylinositol 3‐kinase (PI3K)/Akt signaling pathways have a fundamental involvement in neuroprotection and oxidative stress reactions. We used wild‐type mice, PrP knockout (Prnp^−/−) animals and transgenic mice that lack the octarepeat region (C4/−) and subjected them to controlled ischemia. We identified an increased cleavage and synthesis of PrP^C in ischemic brain areas of wild‐type mice compared with sham controls. The infarct size in Prnp^−/− animals was increased threefold when compared with wild‐type mice. The infarct size in C4/− animals was identical to Prnp^−/− mice, that is, around three times larger than in wild‐type mice. We showed that the PrP in C4/− mice does not functionally rescue the Prnp^−/− phenotype; furthermore it is unable to undergo β cleavage, although an increased amount of C1 fragments was found in ischemic brain areas compared with sham controls. We demonstrated that the N‐terminal octarepeat region has a lead function in PrP^C physiology and neuroprotection against oxidative stress in vivo.

Role of copper gluconate/triethanolamine in irinotecan encapsulation inside the liposomes

A novel method for encapsulating irinotecan into liposomes containing copper gluconate buffered to pH 7.0 with triethanolamine (TEA) has recently been developed. In the present study, the mechanism dictating drug encapsulation and retention inside those liposomes was investigated. Spectroscopic analyses revealed that irinotecan interacted with copper gluconate/TEA in solution. Fourier transformed infrared (FT-IR) spectroscopy indicated a strengthening of the hydrogen bonds involving the hydroxyl groups when solutions of irinotecan and copper gluconate/TEA are mixed at a 1:1 molar ratio. The intensity of the circular dichroism (CD) signal of copper gluconate/TEA increased in the presence of equimolar amounts of irinotecan. The addition of irinotecan to liposomes containing copper gluconate/TEA at 50 degrees C induced a shift of the absorption bands from 370 nm to 378 nm as well as a 60% quenching of the drug fluorescence at 440 nm suggesting the occurrence of irinotecan self association. Irinotecan encapsulation was found to be kinetically and stoichiometrically correlated with the release of TEA from the liposomes. The results suggested that the encapsulation of irinotecan was mediated by TEA in association with copper gluconate, leading to a final drug complex that is retained inside the liposomes. A neutral antiport exchange loading mechanism between irinotecan and TEA is proposed.

Normal cellular prion protein protects against manganese-induced oxidative stress and apoptotic cell death

The normal prion protein is abundantly expressed in the central nervous system, but its biological function remains unclear. The prion protein has octapeptide repeat regions that bind to several divalent metals, suggesting that the prion proteins may alter the toxic effect of environmental neurotoxic metals. In the present study, we systematically examined whether prion protein modifies the neurotoxicity of manganese (Mn) by comparing the effect of Mn on mouse neural cells expressing prion protein (PrP(C)-cells) and prion-knockout (PrP(KO)-cells). Exposure to Mn (10microM-10mM) for 24 h produced a dose-dependent cytotoxic response in both PrP(C)-cells and PrP(KO)-cells. Interestingly, PrP(C)-cells (EC(50) 117.6microM) were more resistant to Mn-induced cytotoxicity, as compared to PrP(KO)-cells (EC(50) 59.9microM), suggesting a protective role for PrP(C) against Mn neurotoxicity. Analysis of intracellular Mn levels showed less Mn accumulation in PrP(C)-cells as compared to PrP(KO)-cells, but no significant changes in the expression of the metal transporter proteins transferrin and DMT-1. Furthermore, Mn-induced mitochondrial depolarization and reactive oxygen species (ROS) generation were significantly attenuated in PrP(C)-cells as compared to PrP(KO)-cells. Measurement of antioxidant status revealed similar basal levels of glutathione (GSH) in PrP(C)-cells and PrP(KO)-cells; however, Mn treatment caused greater depletion of GSH in PrP(KO)-cells. Mn-induced mitochondrial depolarization and ROS production were followed by time- and dose-dependent activation of the apoptotic cell death cascade involving caspase-9 and -3. Notably, DNA fragmentation induced by both Mn treatment and the oxidative stress inducer hydrogen peroxide (100microM) was significantly suppressed in PrP(C)-cells as compared to PrP(KO)-cells. Together, these results demonstrate that prion protein interferes with divalent metal Mn uptake and protects against Mn-induced oxidative stress and apoptotic cell death.

Comparative analysis of the prion protein (PrP) gene in cetacean species

The partial PrP gene sequence and the deduced protein of eight cetacean species, seven of which have never been reported so far, have been determined in order to extend knowledge of sequence variability of the PrP genes in different species and to aid in speculation on cetacean susceptibility to prions. Both the nucleotide and the deduced amino acid sequences have been analysed in comparison with some of the known mammalian PrPs. Cetacean PrPs present typical features of eutherian PrPs. The PrP gene from the species of the family Delphinidae gave identical nucleic acid sequences, while differences in the PrP gene were found in Balaenopteridae and Ziphidae. The phylogenetic tree resulting from analysis of the cetacean PrP gene sequences, together with reported sequences of some ungulates, carnivores and primates, showed that the PrP gene phylogenesis mirrors the species phylogenesis. The PrP gene of cetaceans is very close to species where natural forms of TSEs are known. From an analysis of the sequences and the phylogenesis of the PrP gene, susceptibility to or occurrence of prion diseases in cetaceans can not be excluded.

Mutagenesis studies in transgenic Xenopus intermediate pituitary cells reveal structural elements necessary for correct prion protein biosynthesis

The cellular prion protein (PrP(C)) is generally accepted to be involved in the development of prion diseases, but its physiological role is still under debate. To obtain more insight into PrP(C) functioning, we here used stable Xenopus transgenesis in combination with the proopiomelanocortin (POMC) gene promoter to express mutated forms of Xenopus PrP(C) fused to the C-terminus of the green fluorescent protein (GFP) specifically in the neuroendocrine Xenopus intermediate pituitary melanotrope cells. Similar to GFP-PrP(C), the newly synthesized GFP-PrP(C)K81A mutant protein was stepwise mono- and di-N-glycosylated to 48- and 51-kDa forms, respectively, and eventually complex glycosylated to yield a 55-kDa mature form. Unlike GFP-PrP(C), the mature GFP-PrP(C)K81A mutant protein was not cleaved, demonstrating the endoproteolytic processing of Xenopus PrP(C) at lysine residue 81. Surprisingly, removal of the glycosylphosphatidylinositol (GPI) anchor signal sequence or insertion of an octarepeat still allowed N-linked glycosylation, but the GFP-PrP(C)DeltaGPI and GFP-PrP(C)octa mutant proteins were not complex glycosylated and not cleaved, indicating that the GPI/octa mutants did not reach the mid-Golgi compartment of the secretory pathway. The transgene expression of the mutant proteins did not affect the ultrastructure of the melanotrope cells nor POMC biosynthesis and processing, or POMC-derived peptide secretion. Together, our findings reveal the evolutionary conservation of the site of metabolic cleavage and the importance of the presence of the GPI anchor and the absence of the octarepeat in Xenopus PrP(C) for its correct biosynthesis.

Phased Fiber Growth in a Peptide Conjugate: Aggregation and Disaggregation Studies

A glycine-rich, short pentapeptide conjugate 6, derived from the highly conserved copper-binding octarepeat region of the prion protein, exhibits a tendency to self-aggregate in a time-dependent fashion. Aging of 6 afforded an insight into the phased growth of spherical prefibrillar structures to fibers of long persistence length, as observed by a combination of microscopic techniques. Interestingly, growth of these fibers was inhibited by colchicine, a known inhibitor of microtubule polymerization in a concentration dependent fashion. This study offers an intriguing insight into the occurrence of prefibrillar intermediates on the path to the formation of full length peptide fibers. It is also envisaged that constructs such as 6 may also serve as simple models to study chemical intervention of protein aggregation.

Empirical rules for rationalising visible circular dichroism of Cu2+and Ni2+histidine complexes: Applications to the prion protein

A natively unfolded region of the prion protein, PrP(90-126) binds Cu(2+) ions and is vital for prion propagation. Pentapeptides, acyl-GGGTH(92-96) and acyl-TNMKH(107-111), represent the minimum motif for this Cu(2+) binding region. EPR and (1)H NMR suggests that the coordination geometry for the two binding sites is very similar. However, the visible CD spectra of the two sites are very different, producing almost mirror image spectra. We have used a series of analogues of the pentapeptides containing His(96) and His(111) to rationalise these differences in the visible CD spectra. Using simple histidine-containing tri-peptides we have formulated a set of empirical rules that can predict the appearance of Cu(2+) visible CD spectra involving histidine and amide main-chain coordination.

Formation of a Stable Oligomer of β-2 Microglobulin Requires only Transient Encounter with Cu(II)

Beta-2 Microglobulin (beta2m) is a small, globular protein, with high solubility under conditions comparable to human serum. A complication of hemodialysis in renal failure patients is the deposition of unmodified beta2m as amyloid fibers. In vitro, exposure of beta2m to equimolar Cu(2+) under near-physiological conditions can result in self-association leading to amyloid fiber formation. Previously, we have shown that the early steps in this process involve a catalyzed structural rearrangement followed by formation of discrete oligomers. These oligomers, however, have a continued requirement for Cu(2+) while mature fibers are resistant to addition of metal chelate. Here, we report that the transition from Cu(2+) dependent to chelate resistant states occurs in the context of small oligomers, dimeric to hexameric in size. These species require Cu(2+) to form, but once generated, do not need metal cation for stability. Importantly, this transition occurs gradually over several days and the resulting oligomers are isolatable and kinetically stable on timescales exceeding weeks. In addition, formation is enhanced by levels of urea similar to those found in hemodialysis patients. Our results are consistent with our hypothesis that transient encounter of full-length wild-type beta2m with transition metal cation at the dialysis membrane interface is causal to dialysis related amyloidosis.

Ab initio simulations of Cu binding sites on the N-terminal region of prion protein

The human prion protein binds Cu2+ ions in the octarepeat domain of the N-terminal tail up to full occupancy at pH 7.4. Recent experiments have shown that the HGGG octarepeat subdomain is responsible for holding the metal bound in a square-planar configuration. By using first principle ab initio molecular dynamics simulations of the Car-Parrinello type, the coordination of copper to the binding sites of the prion protein octarepeat region is investigated. Simulations are carried out for a number of structured binding sites. Results for the complexes Cu(HGGGW)(wat), Cu(HGGG), and [Cu(HGGG)]2 are presented. While the presence of a Trp residue and a water molecule does not seem to affect the nature of the copper coordination, high stability of the bond between copper and the amide nitrogen of deprotonated Gly residues is confirmed in all cases. For the more interesting [Cu(HGGG)]2 complex, a dynamically entangled arrangement of the two domains with exchange of amide nitrogen bonds between the two copper centers emerges, which is consistent with the short Cu-Cu distance observed in experiments at full copper occupancy.

Structural and Dynamic Characterization of Copper(II) Binding of the Human Prion Protein Outside the Octarepeat Region

Human prion protein (hPrP) fragments encompassing the 91-120 region, namely hPrP92-100 (SP1), hPrP106-113 (SP2), hPrP91-120 (LP1), and hPrP91-114 (LP2), were considered for delineation of the Cu(II)-binding site(s). NMR and EPR spectroscopy results obtained from LP1 or LP2 were compared with those obtained from SP1 and SP2. The coexistence of two binding sites, one centered at His96 and the other at His111, was evidenced and ratified by ESI mass spectrometry at low and high metal:peptide ratios. While room-temperature NMR spectroscopy data were consistent with the binding site centered on His111 being approximately fourfold stronger than that centered on His96, low-temperature EPR spectroscopy results yielded evidence for the opposite trend. This disagreement, which has also occurred in the literature, was clarified by temperature-dependent molecular dynamics runs that demonstrated Met112 approaching the metal at room temperature, a process that is expected to stabilize the His111-centered binding site through hydrophobic shielding of the metal coordination sphere.

Ni K-edge XAS suggests that coordination of NiII to the unstructured amyloidogenic region of the human prion protein produces a Ni2 bis-μ-hydroxo dimer

Prion diseases are thought to be caused by the misfolding of the ubiquitous neuronal membrane prion protein (PrP) through an unknown mechanism that may involve Cu(II) coordination to the PrP. Previous work has utilized Ni(II) as a diamagnetic probe for Cu(II) coordination [C.E. Jones, M. Klewpatinond, S.R. Abdelraheim, D.R. Brown, J.H. Viles, J. Mol. Biol. 346 (2005) 1393-1407]. Herein we investigate Ni(II) coordination to the PrP fragment PrP(93-114) (AcN-GGTHSQWNKPSKPKTNMKHMAG) at pH=10.0 by Ni K-edge X-ray absorption spectroscopy (XAS). We find that two equivalents of Ni(II) will coordinate to PrP(93-114) by UV/Vis titrations and mass spectrometry. Ni K-edge XAS data is consistent with Ni(II) ligated by five N/O based ligands (three N/O ligands at 2.01(2) Angstrom and two at 1.855(2) Angstrom). We were also able to locate a Ni-Ni vector at 3.1(1) Angstrom, which suggests the two Ni(II) centers are contained in a bis-mu-hydroxo dimer. We therefore suggest that Ni(II) may not be a suitable diamagnetic mimic for Cu(II) coordination within the PrP since differential coordination modes for the two metals exist.

Copper ions disrupt dopamine metabolism via inhibition of V-H+-ATPase: a possible contributing factor to neurotoxicity

The involvement of copper in the pathophysiology of neurodegeneration has been well documented but is not fully understood. Commonly, the effects are attributed to increased reactive oxygen species (ROS) production due to inherent redox properties of copper ions. Here we show copper can have physiological effects distinct from direct ROS production. First, we show that extragranular free copper inhibits the vesicular H(+)-ATPase of resealed chromaffin granule ghosts. Extragranular ascorbate potentiates this inhibition. The inhibition is mixed type with K(is) = 6.8 +/- 2.8 micromol/L and K(ii) = 3.8 +/- 0.6 micromol/L, with respect to ATP. Second, extracellular copper causes an inhibition of the generation of a pH-gradient and rapid dissipation of pre-generated pH and catecholamine gradients. Copper chelators and the ss-amyloid peptide 1-42 were found to effectively prevent the inhibition. The inhibition is reversible and time-independent suggesting the effects of extracellular copper on H(+)-ATPase is direct, and not due to ROS. The physiological significance of these observations was shown by the demonstration that extracellular copper causes a dramatic perturbation of dopamine metabolism in SH-SY5Y cells. Thus, we propose that the direct inhibition of the vesicular H(+)-ATPase may also contribute to the neurotoxic effects of copper.

NMR characterization of the pH 4 beta-intermediate of the prion protein: the N-terminal half of the protein remains unstructured and retains a high degree of flexibility

Prion diseases are associated with the misfolding of the PrP (prion protein) from a largely alpha-helical isoform to a beta-sheet-rich oligomer. CD has shown that lowering the pH to 4 under mildly denaturing conditions causes recombinant PrP to convert from an alpha-helical protein into one that contains a high proportion of beta-sheet-like conformation. In the present study, we characterize this soluble pH 4 folding intermediate using NMR. (15)N-HSQC (heteronuclear single-quantum correlation) studies with mPrP (mouse PrP)-(23-231) show that a total of 150 dispersed amide signals are resolved in the native form, whereas only 65 amide signals with little chemical shift dispersion are observable in the pH 4 form. Three-dimensional (15)N-HSQC-TOCSY and NOESY spectra indicate that the observable residues are all assigned to amino acids in the N-terminus: residues 23-118. (15)N transverse relaxation measurements indicate that these N-terminal residues are highly flexible with additional fast motions. These observations are confirmed via the use of truncated mPrP-(112-231), which shows only 16 (15)N-HSQC amide peaks at pH 4. The loss of signals from the C-terminus can be attributed to line broadening due to an increase in the molecular size of the oligomer or exchange broadening in a molten-globule state.

The configuration of the Cu2+ binding region in full-length human prion protein

The cellular prion protein (PrP(C)) is a Cu(2+) binding protein connected to the outer cell membrane. The molecular features of the Cu(2+) binding sites have been investigated and characterized by spectroscopic experiments on PrP(C)-derived peptides and the recombinant human full-length PrP(C )(hPrP-[23-231]). The hPrP-[23-231] was loaded with (63)Cu under slightly acidic (pH 6.0) or neutral conditions. The PrP(C)/Cu(2+)-complexes were investigated by extended X-ray absorption fine structure (EXAFS), electron paramagnetic resonance (EPR), and electron nuclear double resonance (ENDOR). For comparison, peptides from the copper-binding octarepeat domain were investigated in different environments. Molecular mechanics computations were used to select sterically possible peptide/Cu(2+) structures. The simulated EPR, ENDOR, and EXAFS spectra of these structures were compared with our experimental data. For a stoichiometry of two octarepeats per copper the resulting model has a square planar four nitrogen Cu(2+) coordination. Two nitrogens belong to imidazole rings of histidine residues. Further ligands are two deprotonated backbone amide nitrogens of the adjacent glycine residues and an axial oxygen of a water molecule. Our complex model differs significantly from those previously obtained for shorter peptides. Sequence context, buffer conditions and stoichiometry of copper show marked influence on the configuration of copper binding to PrP(C).

The Copper(II) Adduct of the Unstructured Region of the Amyloidogenic Fragment Derived from the Human Prion Protein is Redox-Active at Physiological pH

Prion diseases are caused by the misfolding and aggregation of the prion protein (PrP). Herein we provide evidence that the CuII adduct of the unstructured amyloidogenic fragment of the human PrP (PrP(91-126)) is redox active under physiological conditions. We have identified that the relevant high-affinity CuII binding region of PrP(91-126) is contained between residues 106 and 114. Both [CuII(PrP(91-126))] and [CuII(PrP(106-114))] have CuII Kd values of approximately 90 microM. Furthermore, the smaller PrP fragment PrP(106-114) coordinates CuII producing an electronic absorption spectrum nearly identical with [CuII(PrP(91-126))] (lambda max approximately 610 nm (epsilon approximately 125 M-1 cm-1)) suggesting a similar coordination environment for CuII. Cu K-edge X-ray absorption spectroscopy (XAS) reveals a nearly identical CuN(N/O)2S coordination environment for these two metallopeptides (2N/O at approximately 1.97 A; 1S at approximately 2.30 A; 1 imidazole N at approximately 1.95 A). Both display quasireversible CuII/CuI redox couples at approximately -350 mV vs Ag/AgCl. ESI-MS indicates that both peptides will coordinate CuI. However, XAS indicates differential coordination environments between [CuI(PrP(91-126))] and [CuI(PrP(106-114))]. These data indicate that [CuI(PrP(91-126))] contains Cu in a four coordinate (N/O)2S2 environment with similar (N/O)-Cu bond distances (Cu-(N/O) r = 2.048(4) A), while [CuI(PrP(106-114))] contains Cu in a four coordinate (N/O)2S2 environment with differential (N/O)-Cu bond distances (Cu-(N/O) r1 = 2.057(6) A; r2 = 2.159(3) A). Despite the differential coordination environments both Cu-metallopeptides will catalytically reduce O2 to O2*- at comparable rates.

On the Copper(II) Ion Coordination by Prion Protein HGGGW Pentapeptide Model

The interaction of the octapeptide domain of the prion protein with the transition-metal-ion Cu2+ was studied at the DFT level by using the HGGGW pentapeptide as a model to mimic the PHGGGWGQ octarepeat sequence. Ten complexes, in which the metal ion exhibits different coordinations, were considered. Our results indicate that the lowest-energy structure is characterized by a tetracoordinated metal center and that this tendency of the ion to assume the square planar geometry is strong enough to prevent the addition of a further water molecule in its coordination sphere. The role of tryptophan was found to cause a lowering of the system energy due to the stabilizing effect of the electrostatic interaction between the Trp aromatic indole and histidine imidazole rings.

Nitrogen oxide interaction with copper complexes formed by small peptides belonging to the prion protein octa-repeat region

The interaction between NO and copper(II) complexes formed by peptides coming from the N-terminal prion protein octa-repeat region was studied. Aqueous solutions of the Cu-Ac-HGGG-NH(2) and the Cu-Ac-PHGGGWGQ-NH(2) systems around pH 7.5 were tested after the addition of NONOates as a source of NO. UV-Vis, room temperature and frozen solution EPR spectra showed the occurrence of copper(ii) reduction in all these complexes. The reduction of these complexes is probably mediated by the formation of a labile NO adduct, which, after re-oxidation, leads to a relatively stable NO(2)(-) adduct through the apical coordination along the void site of their square pyramidal structure. In fact, the most significant shifts in EPR magnetic parameters (g(||) and A(||) or g(iso) and A(iso)) as well as in the optical parameters (lambda(max) and epsilon(max)) gave a reason for geometrical changes of the copper coordination polyhedron from a distorted square pyramid to a pseudo-octahedron. The presence of oxygen in the aqueous solution hindered the reduction ability of NO towards copper, but it made it easier to return to the original species. In order to elucidate the possible mechanism of this interaction, the reduction of copper complexed by these ligands was followed by means of zinc powder addition. The further addition of nitrite to the solution containing reduced copper led to the conclusion that nitrite could easily form an adduct, which after re-oxidation presented the same spectral features of the species obtained when the NO interaction was followed. The complexity of this interaction could involve both an inner or an outer-sphere electron transfer mechanism.

Copper-induced oligomerization of peptides: a model study

In this work, copper-binding of the tetraglycine peptide (Gly-Gly-Gly-Gly) was studied by electrospray ionization mass spectrometry. Experiments were performed under alkaline conditions, in the presence of ethanolamine (pH 10.95). We observed that the presence of copper(II) ions induces the aggregation of the peptide and the formation of copper-bound complexes with higher molecular mass is favored, such as the oligomer complexes [3M+2Cu-3H](+) and [4M+3Cu-5H](+). At 1:1 peptide-copper(II) ion ratio, the singly charged [3M+2Cu-3H](+) oligomer complex is the base peak in the mass spectrum. Metal ion-induced oligomer-ization of neurotoxic peptides is well known in the literature; however, there are very few examples in which such oligomerization was directly observed by mass spectrometry. Our results show that application of short peptides can be useful to study the -mechanism of metal ion binding and metal ion-induced oligomerization of peptides.

Immunoreactivity enhancement with chelators for increasing the detection sensitivity of human PrPSc by Western blotting

Prion diseases are neurodegenerative disorders affecting humans as Creutzfeldt-Jakob disease. The host-encoded prion protein (PrP(C)) will be converted into a structurally altered isoform (PrP(Sc)). PrP(Sc) differ in sizes and glycoform patterns and can be identified using molecular typing with Western blotting. The electrophoretic mobility of PrP(Sc) changes on treatment with metal ions or chelators prior to digestion with proteases. The effects of chelators applied to PrP(Sc) after protease digestion had not been examined in detail, we investigated these effects in this study. Application of EDTA, NTA and DTPA, and to a lesser extent EGTA, significantly enhanced PrP(Sc) signals in immunoblots. PrP(Sc) intensities increased two- to three-fold compared with untreated PrP(Sc). Since the immunoblot method is highly specific, sensitivity is the limiting factor. Enhancing sensitivity might be important in the determination of PrP(Sc) at levels close to or just below the limits of detection. It is to be expected that application of chelators to digested protein samples will increase the sensitivity of PrP(Sc) detection using the Western blot technique.

Multiple forms of copper (II) co-ordination occur throughout the disordered N-terminal region of the prion protein at pH 7.4

Although the physiological function of the prion protein remains unknown, in vitro experiments suggest that the protein may bind copper (II) ions and play a role in copper transport or homoeostasis in vivo. The unstructured N-terminal region of the prion protein has been shown to bind up to six copper (II) ions, with each of these ions co-ordinated by a single histidine imidazole and nearby backbone amide nitrogen atoms. Individually, these sites have micromolar affinities, which is weaker than would be expected of a true cuproprotein. In the present study, we show that with subsaturating levels of copper, different forms of co-ordination will occur, which have higher affinity. We have investigated the copper-binding properties of two peptides representing the known copper-binding regions of the prion protein: residues 57-91, which contains four tandem repeats of the octapeptide GGGWGQPH, and residues 91-115. Using equilibrium dialysis and spectroscopic methods, we unambiguously demonstrate that the mode of copper co-ordination in both of these peptides depends on the number of copper ions bound and that, at low copper occupancy, copper ions are co-ordinated with sub-micromolar affinity by multiple histidine imidazole groups. At pH 7.4, three different modes of copper co-ordination are accessible within the octapeptide repeats and two within the peptide comprising residues 91-115. The highest affinity copper (II)-binding modes cause self-association of both peptides, suggesting a role for copper (II) in controlling prion protein self-association in vivo.

The affinity of copper binding to the prion protein octarepeat domain: evidence for negative cooperativity

The prion protein (PrP) binds Cu(2+) in its N-terminal octarepeat domain, composed of four or more tandem PHGGGWGQ segments. Previous work from our laboratory demonstrates that copper interacts with the octarepeat domain through three distinct coordination modes at pH 7.4, depending upon the precise ratio of Cu(2+) to protein. Here, we apply both electron paramagnetic resonance (EPR) and fluorescence quenching to determine the copper affinity for each of these modes. At low copper occupancy, which favors multiple His coordination, the octarepeat domain binds Cu(2+) with a dissociation constant of 0.10 (+/-0.08) nM. In contrast, high copper occupancy, involving coordination through deprotonated amide nitrogens, exhibits a weaker affinity characterized by dissociation constants in the range of 7.0-12.0 microM. Decomposition of the EPR spectra reveals the proportions of all coordination species throughout the copper concentration range and identifies significant populations of intermediates, consistent with negative cooperativity. At most copper concentrations, the Hill coefficient is less than 1.0 and approximately 0.7 at half copper occupancy. These findings demonstrate that the octarepeat domain is responsive to a remarkably wide copper concentration range covering approximately 5 orders of magnitude. Consideration of these findings, along with the demonstrated ability of the protein to quench copper redox activity at high occupancy, suggests that PrP may function to protect cells by scavenging excess copper.

The expanding universe of prion diseases

Prions cause fatal and transmissible neurodegenerative disease. These etiological infectious agents are formed in greater part from a misfolded cell-surface protein called PrP(C). Several mammalian species are affected by the diseases, and in the case of "mad cow disease" (BSE) the agent has a tropism for humans, with negative consequences for agribusiness and public health. Unfortunately, the known universe of prion diseases is expanding. At least four novel prion diseases--including human diseases variant Creutzfeldt-Jakob disease (vCJD) and sporadic fatal insomnia (sFI), bovine amyloidotic spongiform encephalopathy (BASE), and Nor98 of sheep--have been identified in the last ten years, and chronic wasting disease (CWD) of North American deer (Odocoileus Specis) and Rocky Mountain elk (Cervus elaphus nelsoni) is undergoing a dramatic spread across North America. While amplification (BSE) and dissemination (CWD, commercial sourcing of cervids from the wild and movement of farmed elk) can be attributed to human activity, the origins of emergent prion diseases cannot always be laid at the door of humankind. Instead, the continued appearance of new outbreaks in the form of "sporadic" disease may be an inevitable outcome in a situation where the replicating pathogen is host-encoded.

A reassessment of copper(II) binding in the full-length prion protein

It has been shown previously that the unfolded N-terminal domain of the prion protein can bind up to six Cu2+ ions in vitro. This domain contains four tandem repeats of the octapeptide sequence PHGGGWGQ, which, alongside the two histidine residues at positions 96 and 111, contribute to its Cu2+ binding properties. At the maximum metal-ion occupancy each Cu2+ is co-ordinated by a single imidazole and deprotonated backbone amide groups. However two recent studies of peptides representing the octapeptide repeat region of the protein have shown, that at low Cu2+ availability, an alternative mode of co-ordination occurs where the metal ion is bound by multiple histidine imidazole groups. Both modes of binding are readily populated at pH 7.4, while mild acidification to pH 5.5 selects in favour of the low occupancy, multiple imidazole binding mode. We have used NMR to resolve how Cu2+ binds to the full-length prion protein under mildly acidic conditions where multiple histidine co-ordination is dominant. We show that at pH 5.5 the protein binds two Cu2+ ions, and that all six histidine residues of the unfolded N-terminal domain and the N-terminal amine act as ligands. These two sites are of sufficient affinity to be maintained in the presence of millimolar concentrations of competing exogenous histidine. A previously unknown interaction between the N-terminal domain and a site on the C-terminal domain becomes apparent when the protein is loaded with Cu2+. Furthermore, the data reveal that sub-stoichiometric quantities of Cu2+ will cause self-association of the prion protein in vitro, suggesting that Cu2+ may play a role in controlling oligomerization in vivo.

Investigating pH and Cu (II) effects on lipase activity and enantioselectivity via kinetic and spectroscopic methods

For Candida rugosa lipase (CRL) catalyzed hydrolysis of racemic 1-phenethyl acetate, both the weakly acidic pH (pH 6.0) and the addition of 1 mM copper (II) ion enhanced the enzyme activity and enantioselectivity (E value) about twofold, as compared with that under neutral pH and noadditive conditions. The decrease of activation free energy (DeltaG) and increase of k(cat)(R)/k(cat)(S) at weakly acidic pH and/or in the presence of copper (II) characterized the kinetic behavior of CRL. On the other hand, for providing reasonable insights into the catalytic mechanism and the structural basis for enantioselectivity alteration, spectroscopic techniques were employed to probe conformational changes of the enzyme in each medium assayed. The fluorescence emission spectra revealed that pH and copper (II) might exert different effects on the microenvironment of Trp residue and thereby on the protein conformation, which could be further verified by UV-visible and Raman spectra. The conformational modulation of CRL associated with either pH or copper (II) concentration in the reaction medium could be attributed to the flexible and sensitive conformation of the enzyme, which is responsible for the significant variation of apparent activity and enantioselectivity with the tuning of biocatalyst microenvironment.

Copper-dependent co-internalization of the prion protein and glypican-1

Heparan sulfate chains have been found to be associated with amyloid deposits in a number of diseases including transmissible spongiform encephalopathies. Diverse lines of evidence have linked proteoglycans and their glycosaminoglycan chains, and especially heparan sulfate, to the metabolism of the prion protein isoforms. Glypicans are a family of glycosylphosphatidylinositol-anchored, heparan sulfate-containing, cell-associated proteoglycans. Cysteines in glypican-1 can become nitrosylated by endogenously produced nitric oxide. When glypican-1 is exposed to a reducing agent, such as ascorbate, nitric oxide is released and autocatalyses deaminative cleavage of heparan sulfate chains. These processes take place while glypican-1 recycles via a non-classical, caveolin-associated pathway. We have previously demonstrated that prion protein provides the Cu2+ ions required to nitrosylate thiol groups in the core protein of glypican-1. By using confocal immunofluorescence microscopy and immunomagnetic techniques, we now show that copper induces co-internalization of prion protein and glypican-1 from the cell surface to perinuclear compartments. We find that prion protein is controlling both the internalization of glypican-1 and its nitric oxide-dependent autoprocessing. Silencing glypican-1 expression has no effect on copper-stimulated prion protein endocytosis, but in cells expressing a prion protein construct lacking the copper binding domain internalization of glypican-1 is much reduced and autoprocessing is abrogated. We also demonstrate that heparan sulfate chains of glypican-1 are poorly degraded in prion null fibroblasts. The addition of either Cu2+ ions, nitric oxide donors, ascorbate or ectopic expression of prion protein restores heparan sulfate degradation. These results indicate that the interaction between glypican-1 and Cu2+-loaded prion protein is required both for co-internalization and glypican-1 self-pruning.

Characterization of copper binding to the peptide amyloid-beta(1-16) associated with Alzheimer's disease

Amyloid-beta peptide (Abeta) is the principal constituent of plaques associated with Alzheimer's disease (AD) and is thought to be responsible for the neurotoxicity associated with the disease. Copper binding to Abeta has been hypothesized to play an important role in the neruotoxicity of Abeta and free radical damage, and Cu2+ chelators represent a possible therapy for AD. However, many properties of copper binding to Abeta have not been elucidated clearly, and the location of copper binding sites on Abeta is also in controversy. Here we have used a range of spectroscopic techniques to characterize the coordination of Cu2+ to Abeta(1-16) in solution. Electrospray ionization mass spectrometry shows that copper binds to Abeta(1-16) at pH 6.0 and 7.0. The mode of copper binding is highly pH dependent. Circular dichroism results indicate that copper chelation causes a structural transition of Abeta(1-16). UV-visible absorption spectra suggest that three nitrogen donor ligands and one oxygen donor ligand (3N1O) in Abeta(1-16) may form a type II square-planar coordination geometry with Cu2+. By means of fluorescence spectroscopy, competition studies with glycine and L-histidine show that copper binds to Abeta(1-16) with an affinity of Ka approximately 10(7) M(-1) at pH 7.8. Besides His6, His13, and His14, Tyr10 is also involved in the coordination of Abeta(1-16) with Cu2+, which is supported by 1H NMR and UV-visible absorption spectra. Evidence for the link between Cu2+ and AD is growing, and this work has made a significant contribution to understanding the mode of copper binding to Abeta(1-16) in solution.

Sequence variation of bovine prion protein gene in Japanese cattle (Holstein and Japanese Black)

To assess relationships between nucleotide polymorphisms of the prion protein (PRNP) gene and susceptibility to bovine spongiform encephalopathy (BSE), we investigated polymorphisms in the open reading frame (ORF) and 2 upper regions of the PRNP gene from 2 Japanese cattle breeds: 863 healthy Holstein cattle, 6 BSE-affected Holstein cattle, and 186 healthy Japanese Black (JB) cattle. In the ORF, we found single-nucleotide polymorphisms (SNPs) at nucleotide positions 234 and 576 and found 5 or 6 copies of the octapeptide repeat, but we did not find any amino acid substitutions. In the upper region, we examined 2 sites of insertion/deletion (indel) polymorphisms: a 23-bp indel in the upper region of exon 1, and a 12-bp indel in the putative promoter region of intron 1. A previous report suggests that the 23-bp indel polymorphism is associated with susceptibility to BSE, but we did not find a difference in allele frequency between healthy and BSE-affected Holstein cattle. There were differences in allele frequency between healthy Holstein and JB cattle at the 23- and 12-bp indels and at the SNPs at nucleotide positions 234 and 576, but there was no difference in allele frequency of the octapeptide repeat. We identified a unique PRNP gene lacking a 288-bp segment (96 amino acids) in DNA samples stocked in our laboratory, but this deletion was not found in any of the 1049 cattle examined in the present study. The present results provide data about variations and distribution of the bovine PRNP gene.

Metal complexes with superoxide dismutase-like activity as candidates for anti-prion drug

Various compounds were evaluated for ability to inhibit the formation of the abnormal protease-resistant form of prion protein (PrP-res) in two cell lines infected with different prion strains. Examination of the structure-activity relationships indicated that compounds with copper-selective chelating ability and whose copper complexes have high SOD-like activity are candidates for anti-prion drug.

Computational Comparison of Cation Coordination to Human Prion Peptide Models

The coordination of the cations Cu(II), Co(II), Rh(III), Ir(III), Ni(II), Pd(II), Pt(II), and Zn(II) to the copper-binding octapeptide region in the human prion protein has been compared through structural optimization. The initial coordination mode used in the calculations is a five-coordinated mode obtained from previously published crystallographic data for Cu(II). The computational results show that, among these cations, the coordinations of Co(II) and Rh(III) are the most similar to that of Cu(II). The cations Ni(II), Pd(II), and Pt(II) prefer a four-coordinate square-planar coordination by the peptide ligand. The paramagnetic Co(II) ion with its large quadrupole moment is not a good substitute for Cu(II) to be used in NMR spectroscopic studies of the coordinated peptide region. Rh(III) has more attractive NMR spectroscopic characteristics than Cu(II) and Co(II) and may represent a suitable substitute for Cu(II) in these types of studies. Some preliminary experimental studies using NMR spectroscopic methods indicate that Rh(III) coordinates the copper-binding octapeptide region of the human prion protein, although further studies are required to determine the mode of interaction in detail.