Prion protein binds copper within the physiological concentration range

Preferential Cu2+ Coordination by His96 and His111 Induces β-Sheet Formation in the Unstructured Amyloidogenic Region of the Prion Protein

The prion protein (PrP) is a Cu(2+) binding cell surface glycoprotein that can misfold into a beta-sheet-rich conformation to cause prion diseases. The majority of copper binding studies have concentrated on the octarepeat region of PrP. However, using a range of spectroscopic techniques, we show that copper binds preferentially to an unstructured region of PrP between residues 90 and 115, outside of the octarepeat domain. Comparison of recombinant PrP with PrP-(91-115) indicates that this prion fragment is a good model for Cu(2+) binding to the full-length protein. In contrast to previous reports we show that Cu(2+) binds to this region of PrP with a nanomolar dissociation constant. NMR and EPR spectroscopy indicate a square-planar or square-pyramidal Cu(2+) coordination utilizing histidine residues. Studies with PrP analogues show that the high affinity site requires both His(96) and His(111) as Cu(2+) ligands, rather than a complex centered on His(96) as has been previously suggested. Our circular dichroism studies indicate a loss of irregular structure on copper coordination with an increase in beta-sheet conformation. It has been shown that this unstructured region, between residues 90 and 120, is vital for prion propagation and different strains of prion disease have been linked with copper binding. The role of Cu(2+) in prion misfolding and disease must now be re-evaluated in the light of these findings.

Misfolding of the Prion Protein at the Plasma Membrane Induces Endocytosis, Intracellular Retention and Degradation

Suramin induces misfolding of the cellular prion protein (PrP(C)) and interferes with the propagation of infectious scrapie prions. A mechanistic analysis of this effect revealed that suramin-induced misfolding occurs at the plasma membrane and is dependent on the proximal region of the C-terminal domain (aa 90-158) of PrP(C). The conformational transition induces rapid internalization, mediated by the unstructured N-terminal domain, and subsequent intracellular degradation of PrP(C). As a consequence, PrP Delta N adopts a misfolded conformation at the plasma membrane; however, internalization is significantly delayed. We also found that misfolding and intracellular retention of PrP(C) can be induced by copper and that, moreover, copper interferes with the propagation of the pathogenic prion protein (PrP(Sc)) in scrapie-infected N2a cells. Our study revealed a quality control pathway for aberrant PrP conformers present at the plasma membrane and identified distinct PrP domains involved.

Anterograde and retrograde intracellular trafficking of fluorescent cellular prion protein

In order to investigate the microtubule-associated intracellular trafficking of the NH2-terminal cellular prion protein (PrPC) fragment [Biochem. Biophys. Res. Commun. 313 (2004) 818], we performed a real-time imaging of fluorescent PrPC (GFP-PrPC) in living cells. Such GFP-PrPC exhibited an anterograde movement towards the direction of plasma membranes at a speed of 140-180 nm/s, and a retrograde movement inwardly at a speed of 1.0-1.2 microm/s. The anterograde and retrograde movements of GFP-PrPC were blocked by a kinesin family inhibitor (AMP-PNP) and a dynein family inhibitor (vanadate), respectively. Furthermore, anti-kinesin antibody (alpha-kinesin) blocked its anterograde motility, whereas anti-dynein antibody (alpha-dynein) blocked its retrograde motility. These data suggested the kinesin family-driven anterograde and the dynein-driven retrograde movements of GFP-PrPC. Mapping of the interacting domains of PrPC identified amino acid residues indispensable for interactions with kinesin family: NH2-terminal mouse (Mo) residues 53-91 and dynein: NH2-terminal Mo residues 23-33, respectively. Our findings argue that the discrete N-terminal amino acid residues are indispensable for the anterograde and retrograde intracellular movements of PrPC.

Copper(II) complexes of peptide fragments of the prion protein. Conformation changes induced by copper(II) and the binding motif in C-terminal protein region

In this paper, we report the characterization of copper(II) complexes with two prion (PrP) protein peptide fragment analogues (VNITKQHTVTTTT), one with the N-terminus acetylated and the C-terminus amidated (PrP Ac180-193NH2) and the other with both the C- and N-termini free (PrP 180-193). Such peptide sequence almost entirely encompasses the PrPC's helix 2 in the C-terminal region. The stoichiometry, the binding modes and the conformational features of the copper(II) complexes with the above mentioned two peptides were investigated by electrospray ionization-mass spectrometry (ESI-MS), UV-visible (UV-Vis) spectrometry and electron paramagnetic resonance (EPR) spectrometry as well as by circular dichroism (CD) measurements. The binding site location of copper(II) in the structured region of the protein can be here suggested on the basis of our findings that show the involvement of His 187 residue. The similarity of the EPR parameters suggests that the anchoring imidazole residue drives the copper(II) coordination environment towards a common binding motif in different regions of the prion protein.

Cu2+ binding triggers alphaBoPrP assembly into insoluble laminar polymers

Cu(2+) binding is so far the best characterized property of the prion protein. This interaction has been mapped to the N-terminal domain of the prion protein where multiple His residues occur largely embedded within the repetitive PHGGGWGQ sequence known as octarepeats. When Cu(2+) interaction is studied using a solution of full-length bovine prion protein containing six octarepeats at protein concentrations above 25 microM, a drastic increase in solution turbidity is observed due to the formation of insoluble cation-protein complexes that appear as bidimensional polymer meshes. These bidimensional meshes consist of a single layer of protein molecules crosslinked by Cu(2+) cations. Polymer formation is a cooperative process that proceeds by nucleation of protein molecules with a Cu(2+) site occupancy of above 2. These results support the hypothesis that the N-terminal domain of prion protein is a ligand binding module that promotes crosslinked assembly, and suggest the existence of inter-repeat Cu(2+) sites.

Prion protein suppresses perturbation of cellular copper homeostasis under oxidative conditions

Prion protein (PrP) binds copper and exhibits superoxide dismutase-like activity, while the roles of PrP in copper homeostasis remain controversial. Using Zeeman graphite furnace atomic absorption spectroscopy, we quantified copper levels in immortalized PrP gene (Prnp)-deficient neuronal cells transfected with Prnp and/or Prnd, which encodes PrP-like protein (PrPLP/Dpl), in the presence or absence of oxidative stress induced by serum deprivation. In the presence of serum, copper levels were not significantly affected by the expression of PrP and/or PrPLP/Dpl, whereas serum deprivation induced a decrease in copper levels that was inhibited by PrP but not by PrPLP/Dpl. The inhibitory effect of PrP on the decrease of copper levels was prevented by overexpression of PrPLP/Dpl. These findings indicate that PrP specifically stabilizes copper homeostasis, which is perturbed under oxidative conditions, while PrPLP/Dpl overexpression prevents PrP function in copper homeostasis, suggesting an interaction of PrP and PrPLP/Dpl and distinct functions between PrP and PrPLP/Dpl on metal homeostasis. Taken together, these results strongly suggest that PrP, in addition to its antioxidant properties, plays a role in stabilizing cellular copper homeostasis under oxidative conditions.

Inter- and Intra-octarepeat Cu(II) Site Geometries in the Prion Protein

Cu(II) binding to the alpha prion protein (alphaPrP) can be both intramolecular and intermolecular. X-ray absorption spectroscopy at the copper K-edge has been used to explore the site geometry under each binding mode using both insoluble polymeric Cu(II).alphaBoPrP-(24-242) (bovine PrP) complexes and soluble Cu(II) complexes of peptides containing one, two, and four copies of the octarepeat. Analysis of the extended region of the spectra using a multiple scattering approach revealed two types of sites differing in the number of His residues in the first coordination shell of Cu(II). Peptides containing one and two-octarepeat copies in sub-stoichiometric Cu(II) complexes showed the direct binding of a single His in accord with crystallographic intra-repeat geometry. Alternatively, the polymeric Cu(II).alphaBoPrP-(24-242) complex and Cu(II) in its soluble complex with a four-octarepeat peptide at half-site-occupancy showed Cu(II) directly bound to two His residues, consistent with an inter-repeat binding mode. Increasing the Cu(II) site occupancy from 0.5 to 0.75 in the peptide containing four octarepeats resulted in spectral features that are intermediate to those of the inter- and intra-repeat modes. The transition from His-Cu-His (inter-repeat) to Cu-His (intra-repeat) on increasing Cu(II) saturation offers a structural basis for the positive cooperativity of the cation binding process and explains the capacity of alphaPrP to participate in Cu(II)-mediated intermolecular interactions.

Microtubules-associated intracellular localization of the NH2-terminal cellular prion protein fragment

By utilizing double-labeled fluorescent cellular prion protein (PrPC), we revealed that the NH2-terminal and COOH-terminal PrPC fragments exhibit distinct distribution patterns in mouse neuroblastoma neuro2a (N2a) cells and HpL3-4, a hippocampal cell line established from prnp gene-ablated mice [Nature 400 (1999) 225]. Of note, the NH2-terminal PrPC fragment, which predominantly localized in the intracellular compartments, congregated in the cytosol after the treatment with a microtubule depolymerizer (nocodazole). Truncated PrPC with the amino acid residues 1-121, 1-111, and 1-91 in mouse (Mo) PrP exhibited a proper distribution profile, whereas those with amino acid residues 1-52 and 1-33 did not. These data indicate the microtubules-associated intracellular localization of the NH2-terminal PrPC fragment containing at least the 1-91 amino acid residues.

Increase in the conformational flexibility of beta 2-microglobulin upon copper binding: a possible role for copper in dialysis-related amyloidosis

A key pathological event in dialysis-related amyloidosis is the fibril formation of beta(2)-microglobulin (beta 2-m). Because beta 2-m does not form fibrils in vitro, except under acidic conditions, predisposing factors that may drive fibril formation at physiological pH have been the focus of much attention. One factor that may be implicated is Cu(2+) binding, which destabilizes the native state of beta 2-m and thus stabilizes the amyloid precursor. To address the Cu(2+)-induced destabilization of beta 2-m at the atomic level, we studied changes in the conformational dynamics of beta 2-m upon Cu(2+) binding. Titration of beta 2-m with Cu(2+) monitored by heteronuclear NMR showed that three out of four histidines (His13, His31, and His51) are involved in the binding at pH 7.0. (1)H-(15)N heteronuclear NOE suggested increased backbone dynamics for the residues Val49 to Ser55, implying that the Cu(2+) binding at His51 increased the local dynamics of beta-strand D. Hydrogen/deuterium exchange of amide protons showed increased flexibility of the core residues upon Cu(2+) binding. Taken together, it is likely that Cu(2+) binding increases the pico- to nanosecond fluctuation of the beta-strand D on which His51 exists, which is propagated to the core of the molecule, thus promoting the global and slow fluctuations. This may contribute to the overall destabilization of the molecule, increasing the equilibrium population of the amyloidogenic intermediate.

Modulation of L-type voltage-gated calcium channels by recombinant prion protein

The prion protein (PrPC) has a primary role in the pathogenesis of transmissible spongiform encephalopathies. Here we analysed in detail the effect of recombinant PrPC and N- and C-terminal fragments of PrPC on the whole-cell current amplitude through voltage-gated calcium channels (VGCCs) of cultured wild-type cerebellar granule cells. With the application of full-length recombinant PrPC (50-500 nm), a highly significant reduction of the whole-cell current amplitude was observed in a dose-dependent manner. Amplitude reduction was abolished when cells were pre-incubated with nifedipine, a specific blocker of voltage-gated L-type calcium channels. N-terminal PrP fragments also led to a dose-dependent reduction of the maximal current amplitude, whereas a C-terminal fragment did not affect the current amplitude. These data demonstrate that nanomolar concentrations of PrPC modulate L-type VGCCs in mouse cerebellar granule cells, an effect that is dependent upon the copper-binding amino-terminal domain of PrPC.

Copper binding in the prion protein

A conformational change of the prion protein is responsible for a class of neurodegenerative diseases called the transmissible spongiform encephalopathies that include mad cow disease and the human afflictions kuru and Creutzfeldt-Jakob disease. Despite the attention given to these diseases, the normal function of the prion protein in healthy tissue is unknown. Research over the past few years, however, demonstrates that the prion protein is a copper binding protein with high selectivity for Cu(2+). The structural features of the Cu(2+) binding sites have now been characterized and are providing important clues about the normal function of the prion protein and perhaps how metals or loss of protein function play a role in disease. The link between prion protein and copper may provide insight into the general, and recently appreciated, role of metals in neurodegenerative disease.

Effects of different experimental conditions on the PrPSc core generated by protease digestion: implications for strain typing and molecular classification of CJD

The discovery of molecular subtypes of the pathological prion protein PrPSc has provided the basis for a novel classification of human transmissible spongiform encephalopathies (TSEs) and a potentially powerful method for strain typing. However, there is still a significant disparity regarding the understanding and nomenclature of PrPSc types. In addition, it is still unknown whether a specific PrPSc type is associated with each TSE phenotypic variant. In sporadic Creutzfeldt-Jakob disease (sCJD), five disease phenotypes are known, but only two major types of PrPSc, types 1 and 2, have been consistently reproduced. We further analyzed PrPSc properties in sCJD and variant CJD using a high resolution gel electrophoresis system and varying experimental conditions. We found that pH varies among CJD brain homogenates in standard buffers, thereby influencing the characteristics of protease-treated PrPSc. We also show that PrPSc type 1 and type 2 are heterogeneous species which can be further distinguished into five molecular subtypes that fit the current histopathological classification of sCJD variants. Our results shed light on previous disparities in PrPSc typing, provide a refined classification of human PrPSc types, and support the notion that the pathological TSE phenotype is related to PrPSc structure.

Micellar environments induce structuring of the N-terminal tail of the prion protein

In the physiological form, the prion protein is a glycoprotein tethered to the cell surface via a C-terminal glycosylphosphatidylinositol anchor, consisting of a largely alpha-helical globular C-terminal domain and an unstructured N-terminal portion. This unstructured part of the protein contains four successive octapeptide repeats, which were shown to bind up to four Cu(2+) ions in a cooperative manner. To mimic the location of the protein on the cell membrane and to analyze possible structuring effects of the lipid/water interface, the conformational preferences of a single octapeptide repeat and its tetrameric form, as well of the fragment 92-113, proposed as an additional copper binding site, were comparatively analyzed in aqueous and dodecylphosphocholine micellar solution as a membrane mimetic. While for the downstream fragment 92-113 no conformational effects were detectable in the presence of DPC micelles by CD and NMR, both the single octapeptide repeat and, in an even more pronounced manner, its tetrameric form are restricted into well-defined conformations. Because of the repetitive character of the rigid structural subdomain in the tetrarepeat molecule, the spatial arrangement of these identical motifs could not be resolved by NMR analysis. However, the polyvalent nature of the repetitive subunits leads to a remarkably enhanced interaction with the micelles, which is not detectably affected by copper complexation. These results strongly suggest interactions of the cellular form of PrP (PrP(c)) N-terminal tail with the cell membrane surface at least in the octapeptide repeat region with preorganization of these sequence portions for copper complexation. There are sufficient experimental facts known that support a physiological role of copper complexation by the octapeptide repeat region of PrP(c) such as a copper-buffering role of the PrP(c) protein on the extracellular surface.

Identification of Metal-binding Proteins in Human Hepatoma Lines by Immobilized Metal Affinity Chromatography and Mass Spectrometry

The metalloproteome is defined as the set of proteins that have metal-binding capacity by being metalloproteins or having metal-binding sites. A different metalloproteome may exist for each metal. Mass spectrometric characterization of metalloproteomes provides valuable information relating to cellular disposition of metals physiologically and in metal-associated diseases. We examined the Cu and Zn metalloproteomes in three human hepatoma lines: Hep G2 and Mz-Hep-1, which retain many functional characteristics of normal human hepatocytes, and SK-Hep-1, which is poorly differentiated. Additionally we studied a single specimen of normal human liver and Hep G2 cells depleted in vitro of cellular copper. We used matrix-assisted laser desorption ionization and electrospray ionization quadrupole time-of-flight mass spectrometry to analyze peptide sequences of tryptic digests obtained by either in-gel digestion of metal-binding proteins or peptides on an immobilized metal affinity chromatography column loaded with either Cu or Zn. Mainly high abundance proteins were identified. Cu-binding proteins identified included enolase, albumin, transferrin, and alcohol dehydrogenase as well as certain intracellular chaperone proteins. The Cu metalloproteome was not identical to the Zn metalloproteome. Peptide binding experiments demonstrated that Cu coordination prefers the order of residues histidine > methionine > cysteine. Although the Cu metalloproteome was similar from line to line, subtle differences were apparent. Gel profiling showed more extensive variation in expression of annexin II in SK-Hep-1 and Mz-Hep-1 than in Hep G2 and normal liver tissue. Glycerylphosphorylethanolamine was identified as a post-translational modification at residue Glu-301 of elongation factor 1-alpha in Hep G2. Intracellular copper depletion was associated with loss of the glycerylphosphoryl side group. These findings suggest that post-translational modification could be affected by intracellular actions of copper. Comparison of the Cu and Zn metalloproteomes in Hep G2 with a published general proteome of Hep G2 disclosed little overlap (Seow, T. K., et al. (2001) Proteomics 1, 1249-1263). Proteins in the metalloproteomes of human hepatocytes can be identified by these methods. Variations in these metalloproteomes may have important physiological relevance.

Copper Chelation Delays the Onset of Prion Disease

The prion protein (PrP) binds copper and under some conditions copper can facilitate its folding into a more protease resistant form. Hence, copper levels may influence the infectivity of the scrapie form of prion protein (PrPSc). To determine the feasibility of copper-targeted therapy for prion disease, we treated mice with a copper chelator, D-(-)-penicillamine (D-PEN), starting immediately following intraperitoneal scrapie inoculation. D-PEN delayed the onset of prion disease in the mice by about 11 days (p = 0.002), and reduced copper levels in brain by 29% (p < 0.01) and in blood by 22% (p = 0.03) compared with control animals. Levels of other metals were not significantly altered in the blood or brain. Modest correlation was observed between incubation period and levels of copper in brain (p = 0.08) or blood (p = 0.04), indicating that copper levels are only one of many factors that influence the rate of progression of prion disease. In vitro, copper dose-dependently enhanced the proteinase K resistance of the prion protein, and this effect was counteracted in a dose-dependent manner by co-incubation with D-PEN. Overall, these findings indicate that copper levels can influence the conformational state of PrP, thereby enhancing its infectivity, and this effect can be attenuated by chelator-based therapy.

Amino terminal interaction in the prion protein identified using fusion to green fluorescent protein

In contrast to the well-characterized carboxyl domain, the amino terminal half of the mature cellular prion protein has no defined structure. Here, following fusion of mouse prion protein fragments to green fluorescence protein as a reporter of protein stability, we report extreme variability in fluorescence level that is dependent on the prion fragment expressed. In particular, exposure of the extreme amino terminus in the context of a truncated prion protein molecule led to rapid degradation, whereas the loss of only six amino terminal residues rescued high level fluorescence. Study of the precise endpoints and residue identity associated with high fluorescence suggested a domain within the amino terminal half of the molecule defined by a long-range intramolecular interaction between 23KKRPKP28 and 143DWED146 and dependent upon the anti-parallel beta-sheet ending at residue 169 and normally associated with the structurally defined carboxyl terminal domain. This previously unreported interaction may be significant for understanding prion bioactivity and for structural studies aimed at the complete prion structure.

A human prion protein peptide (PrP(59-91)) protects against copper neurotoxicity

Human cellular prion protein (PrP(C)) is involved in several neurodegenerative disorders; however, its normal function is unknown. We report here that a synthetic peptide corresponding to the four-octarepeat sequence of the PrP(C) (PrP(59-91)) protects hippocampal neurons against copper neurotoxic effects in vivo. Using a rat bilateral intrahippocampal injection model, we found that PrP(59-91) protects against copper-induced neurotoxicity, including a recovery in spatial learning performance and a reduced neuronal cell loss and astrogliosis. Previous studies from our laboratory indicated that a tryptophan (Trp) residue plays a key role in the reduction of copper(II) to copper(I); therefore several PrP(59-91) fragments lacking histidine (His) and Trp residues were tested for their capacity to protect from copper toxicity. A PrP(59-91) peptide lacking His residue shows as much neuroprotection as the native peptide; however, PrP(59-91) without Trp residues only partially protected against copper toxicity. The neuroprotective effect not only occurs with PrP(59-91), in fact a full neuroprotection was also observed using just one octamer of the N-terminal region of prion protein. We conclude that the N-terminal tandem octarepeat of the human PrP(C) protects neurons against copper toxicity by a differential contribution of the binding (His) and reducing (Trp) copper activities of PrP(59-91). Our results are consistent with the idea that PrP(C) function is related to copper homeostasis.

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

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

Cellular prion protein function in copper homeostasis and redox signalling at the synapse

The fundamental physiological function of native cellular prion (PrPC) remains unknown. Herein, the most salient observations as regards prion physiology are critically evaluated. These include: (i) the role of PrPC in copper homeostasis, particularly at the pre-synaptic membrane; (ii) involvement of PrPC in neuronal calcium disturbances; and (iii) the neuroprotective properties of PrPC in response to copper and oxidative stress. Ultimately, a tentative hypothesis of basic prion function is derived, namely that PrPC acts as a sensor for copper and/or free radical stimuli, thereby triggering intracellular calcium signals that finally translate into modulation of synaptic transmission and maintenance of neuronal integrity.

Is loss of function of the prion protein the cause of prion disorders?

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases that involve misfolding of the prion protein. Recent studies have provided evidence that normal prion protein might have a physiological function in neuroprotective signaling, suggesting that loss of prion protein activity might contribute to the pathogenesis of prion disease. However, studies using knockout animals do not support the loss-of-function hypothesis and argue that prion neurodegeneration might be associated with a gain of a toxic activity by the misfolded prion protein. Thus, the mechanism of neurodegeneration in spongiform encephalopathies remains enigmatic.

The human prion octarepeat fragment prevents and reverses the inhibitory action of copper in the P2X4 receptor without modifying the zinc action

Human prion protein fragments (PrP60-67 or PrP59-91) prevented and reversed the inhibition elicited by 5 micro m copper on the P2X4 receptor expressed in Xenopus laevis oocytes. A 60-s pre-application of 5 micro m copper caused a 69.2 +/- 2.6% inhibition of the 10 micro m adenosine triphosphate (ATP)-evoked currents, an effect that was prevented by mixing 5 micro m copper with 0.01-10 micro m of the PrP fragments 1-min prior to application. This interaction was selective, as PrP59-91 did not alter the facilitatory action of zinc. The EC50 of PrP60-67 and PrP59-91 for the reduction of the copper inhibition were 4.6 +/- 1 and 1.3 +/- 0.4 micro m, respectively. A synthetic PrP59-91 variant in which all four His were replaced by Ala was inactive. However, the replacement of Trp in each of the four putative copper-binding domains by Ala slightly decreased its potency. Furthermore, the application of 10 micro m PrP59-91 reversed the copper-evoked inhibition, restoring the ATP concentration curve to the same level as the non-inhibited state. Fragment 139-157 of betaA4 amyloid precursor protein also prevented the action of copper; its EC50 was 1.6 +/- 0.1 micro m; the metal chelator penicillamine was equipotent with PrP60-67, but carnosine was significantly less potent. Our findings highlight the role of PrP in copper homeostasis and hint at its possible role as a modulator of synapses regulated by this trace metal.

Aggregate formation in Cu,Zn superoxide dismutase-related proteins

Aggregation of Cu,Zn superoxide dismutase (SOD1) protein is a pathologic hallmark of familial amyotrophic lateral sclerosis linked to mutations in the SOD1 gene, although the structural motifs within mutant SOD1 that are responsible for its aggregation are unknown. Copper chaperone for SOD1 (CCS) and extracellular Cu,Zn superoxide dismutase (SOD3) have some sequence identity with SOD1, particularly in the regions of metal binding, but play no significant role in mutant SOD1-induced disease. We hypothesized that it would be possible to form CCS- or SOD3-positive aggregates by making these molecules resemble mutant SOD1 via the introduction of point mutations in codons homologous to a disease causing G85R SOD1 mutation. Using an in vitro assay system, we found that expression of wild type human CCS or a modified intracellular wild type SOD3 does not result in significant aggregate formation. In contrast, expression of G168R CCS or G146R SOD3 produced aggregates as evidenced by the presence of high molecular weight protein complexes on Western gels or inclusion bodies on immunofluorescence. CCS- and SOD3-positive inclusions appear to be ubiquitinated and localized to aggresomes. These results suggest that proteins sharing structural similarities to mutant SOD1 are also at risk for aggregate formation.

The PrP-like protein Doppel binds copper

Doppel (Dpl) is a glycosylphosphatidylinositol-anchored protein expressed in the testis. It exhibits 26% sequence identity with the prion protein (PrP) but lacks the octarepeat region implicated as the major copper-binding domain. Contrary to expectations, Cu(II) induced a 26% reduction in the intrinsic fluorescence of Dpl(27-154) and a calculated K(d) for a single-site model of 0.16 +/- 0.08 microm. Other metals had minimal effects on fluorescence quenching. Matrix-assisted laser desorption ionization mass spectrometry of a Dpl peptide revealed binding of copper (but not other metals) to the helical alphaB/B'-loop-alphaC subregion of Dpl. Fluorescence quenching and equilibrium dialysis analyses of this Dpl(101-145) peptide were compatible with a binding site of K(d) = 0.4 microm. Diethylpyrocarbonate footprinting (Qin, K., Yang, Y., Mastrangelo, P., and Westaway, D. (2002) J. Biol. Chem. 277, 1981-1990) of Dpl(27-154) defined one residue/molecule was protected by copper from diethylpyrocarbonate adduct formation, and reiteration of this analysis with Dpl(101-145) suggested that His(131) may contribute to Cu(II) binding. Taken together, our data indicate that the alpha-helical region of mouse Dpl possesses a selective copper-binding site with a submicromolar K(d) and perhaps one or more lower affinity sites. Although metallated forms of Dpl might exist in vivo, analyses of Tg(Dpl)10329 mice were inconsistent with reports that Dpl expression is associated with increased carbonylation and nitrosylation of brain proteins. Thus, rather than comprising an important source of free radical damage, copper binding may serve to modulate the activity, stability, or localization of the Dpl protein.

Stability and Cu(II) binding of prion protein variants related to inherited human prion diseases

All inherited forms of human prion diseases are linked with mutations in the prion protein (PrP) gene. Here we have investigated the stability and Cu(II) binding properties of three recombinant variants of murine full-length PrP(23-231)-containing destabilizing point mutations that are associated with human Gerstmann-Sträussler-Scheinker disease (F198S), Creutzfeld-Jakob disease (E200K), and fatal familial insomnia (D178N) by electron paramagnetic resonance and circular dichroism spectroscopy. Furthermore, we analyzed the variants H140S, H177S, and H187S of the isolated C-terminal domain of murine PrP, mPrP(121-231), to test a role of the histidine residues in Cu(II) binding. The F198S and E200K variants of PrP(23-231) differed in Cu(II) binding from the wild-type mPrP(23-231). However, circular dichroism spectroscopy indicated that the variants and the wild type did not undergo conformational changes in the presence of Cu(II). The D178N variant showed a high tendency to aggregate at pH 7.4 both with and without Cu(II). At lower pH values, it showed the same Cu(II) binding behavior as the wild type. The analysis allowed for a better location of the Cu(II) binding sites in the C-terminal part of the protein. Our present data indicate that hereditary forms of prion diseases cannot be rationalized on the basis of altered Cu(II) binding or mutation-induced protein destabilization alone.

Copper binding to the octarepeats of the prion protein. Affinity, specificity, folding, and cooperativity: insights from circular dichroism

The prion protein (PrP) is a Cu(2+) binding cell surface glycoprotein. There is increasing evidence that PrP functions as a copper transporter. In addition, strains of prion disease have been linked with copper binding. We present here CD spectroscopic studies of Cu(2+) binding to various fragments of the octarepeat region of the prion protein. We show that glycine and l-histidine will successfully compete for all Cu(2+) ions bound to the PrP octapeptide region, suggesting Cu(2+) coordinates with a lower affinity for PrP than the fm dissociation constant reported previously. We show that each of the octarepeats do not form an isolated Cu(2+) binding motif but fold up cooperatively within multiple repeats. In addition to the coordinating histidine side chain residues, we show that the glycine residues and the proline within each octarepeat are also necessary to maintain the coordination geometry. The highly conserved octarepeat region in mammals is a hexarepeat in birds that also binds copper but with different coordination geometry. Finally, in contrast to other reports, we show that Mn(2+) does not bind to the octarepeat region of PrP.

Detection of bovine spongiform encephalopathy, ovine scrapie prion-related protein (PrPSc) and normal PrPc by monoclonal antibodies raised to copper-refolded prion protein

Prion-related protein (PrP) is a glycosylphosphatidylinositol-linked cell-surface protein expressed by a wide variety of cells, including those of the nervous system and the immune system. Several functions of normal cellular PrP (PrPc) have been proposed that may be associated with the capacity of this protein to bind copper. In the present study, we describe the generation of a panel of monoclonal antibodies raised to copper-refolded PrP, which may be used to analyse the normal and disease-associated forms of this protein. The anti-PrP monoclonal antibodies were reactive by Western blot and ELISA with recombinant murine PrPc refolded in the presence or absence of either copper or manganese, and with the disease-susceptible allelic form V136R154Q171 ('VRQ'; where single-letter amino-acid notation has been used) and disease-resistant allelic form A136R154R171 ('ARR') of recombinant ovine PrPc. FACS analysis of lymphoid cells using these monoclonal antibodies showed that wild-type non-activated mouse lymphocytes expressed little, if any, PrPc. These monoclonal antibodies were shown to react with the unglycosylated and monoglycosylated forms of PrPSc (abnormal disease-specific conformation of PrP) in prion-infected tissue samples from all of the different species tested by Western blot. In addition, this analysis allowed one to make a distinction between bovine spongiform encephalopathy ('BSE') and scrapie PrPSc) isolates from experimentally infected sheep on the basis of their different electrophoretic mobilities.

Molecular evolution of the mammalian prion protein

Prion protein (PrP) sequences are until now available for only six of the 18 orders of placental mammals. A broader comparison of mammalian prions might help to understand the enigmatic functional and pathogenic properties of this protein. We therefore determined PrP coding sequences in 26 mammalian species to include all placental orders and major subordinal groups. Glycosylation sites, cysteines forming a disulfide bridge, and a hydrophobic transmembrane region are perfectly conserved. Also, the sequences responsible for secondary structure elements, for N- and C-terminal processing of the precursor protein, and for attachment of the glycosyl-phosphatidylinositol membrane anchor are well conserved. The N-terminal region of PrP generally contains five or six repeats of the sequence P(Q/H)GGG(G/-)WGQ, but alleles with two, four, and seven repeats were observed in some species. This suggests, together with the pattern of amino acid replacements in these repeats, the regular occurrence of repeat expansion and contraction. Histidines implicated in copper ion binding and a proline involved in 4-hydroxylation are lacking in some species, which questions their importance for normal functioning of cellular PrP. The finding in certain species of two or seven repeats, and of amino acid substitutions that have been related to human prion diseases, challenges the relevance of such mutations for prion pathology. The gene tree deduced from the PrP sequences largely agrees with the species tree, indicating that no major deviations occurred in the evolution of the prion gene in different placental lineages. In one species, the anteater, a prion pseudogene was present in addition to the active gene.

Analysis of the metal-binding selectivity of the metallochaperone CopZ from Enterococcus hirae by electrospray ionization mass spectrometry

Metallochaperones are soluble proteins involved in metal transport and regulation in vivo. Copper metallochaperones belong to a structural family of metal binding domains displaying a ferredoxin-like fold (betaalphabetabetaalphabeta) and a consensus metal-binding motif MXCXXC. The metal-binding selectivities for this class of proteins are poorly documented so far. The present study focuses on the measurement of the selectivity of the copper metallochaperone CopZ from Enterococcus hirae for different metal ions using an experimental approach based on electrospray ionization mass spectrometry (ESI-MS). All the metal cations tested, i.e. Cu(I), Cu(II), Hg(II), Cd(II) and Co(II), form specific metal complexes with CopZ. The study of a chemically modified CopZ as well as variants of CopZ in the active site demonstrated that the complexes observed by ESI-MS, i.e. in the gas phase, corresponded to the complexes previously observed by other analytical methods in solution. Competition experiments allowed the classification of the metal ions by increasing affinities for CopZ as follows: Co << Cd < Hg < Cu. A dissociation constant in the range of 20 microM was determined for cobalt. The affinity of CopZ for the other metals tested was found to be higher, with dissociation constants smaller than micromolar.

Trafficking, turnover and membrane topology of PrP

Cell biological studies of PrP have contributed enormously to our understanding of prion diseases. Like other membrane proteins, PrP(C) is post-translationally processed in the endoplasmic reticulum and Golgi on its way to the cell surface after synthesis. Cell surface PrP(C) constitutively cycles between the plasma membrane and early endosomes via a clathrin-dependent mechanism, a pathway consistent with a suggested role for PrP(C) in cellular trafficking of copper ions. PrP molecules carrying mutations linked to inherited prion diseases display several abnormalities in their biochemical properties, maturation, and localisation that may explain their pathogenicity. Recent results have clarified the role of the proteasome in degradation of PrP, and the properties of a transmembrane form of PrP which may play a neurotoxic role in prion diseases.

Intercellular transfer of the cellular prion protein

The cellular prion protein (PrP(C)) is a glycosylphosphatidylinositol (GPI)-anchored protein. We investigated whether PrP(C) can move from one cell to another cell in a cell model. Little PrP(C) transfer was detected when a PrP(C) expressing human neuroblastoma cell line was cultured with the human erythroleukemia cells IA lacking PrP(C). Efficient transfer of PrP(C) was detected with the presence of phorbol 12-myristate 13-acetate, an activator of protein kinase C. Maximum PrP(C) transfer was observed when both donor and recipient cells were activated. Furthermore, PrP(C) transfer required the GPI anchor and direct cell to cell contact. However, intercellular protein transfer is not limited to PrP(C), another GPI-anchored protein, CD90, also transfers from the donor cells to acceptor cells after cellular activation. Therefore, this transfer process is GPI-anchor and cellular activation dependent. These findings suggest that the intercellular transfer of GPI-anchored proteins is a regulated process, and may have implications for the pathogenesis of prion disease.

Mammalian prion proteins: enigma, variation and vaccination

Although misfolding of the cellular prion protein PrP(C) into an alternative form, denoted PrP(Sc), is a key event in prion infections, the normal function of PrP(C) remains to be clearly defined. Many PrP(C)-binding proteins have been identified, but authentication of these interactions in functional assays is incomplete. Doppel (Dpl), a recently discovered PrP-like protein, might provide a new avenue by which to explore physiological and pathological functions of PrP. For example, overexpression of Dpl causes apoptotic cerebellar cell death that is abrogated by PrP(C), indicating that these two proteins can act in a common pathway. Despite our incomplete understanding of PrP(C), immunological targeting of this PrP(Sc) precursor has produced encouraging results, indicating a potential point of intervention against these fatal diseases.

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.

Cellular phenotyping of secretory and nuclear prion proteins associated with inherited prion diseases

The pathogenic mechanisms leading from mutations in the prion protein (PrP) gene to infectious disease are not understood. To investigate the possibility that cellular processing of mutant prion protein may contribute to the formation of infectious particles, a mouse PrP model system has been established using the green fluorescent protein. Three novel PrP mutants were examined employing this model system and compared with wild type as well as known mutant PrPs. Two Creutzfeldt-Jakob disease-associated PrP mutants, PrP T188K and PrP T188R, revealed a secretory pathway to the cell membrane and PrP(Sc)-like properties, i.e. enhanced proteinase K resistance and detergent insolubility similar to other mutant PrPs associated with familial prion diseases. Moreover, a recently described disease-related truncated PrP mutant, PrP Q160(Stop), showed an almost exclusive localization in the nucleus and a catabolism along the proteasomal pathway. Therefore, various distinct pathological mechanisms may cause prion diseases, and aberrant cellular processing may be included in the pathogenesis of prion diseases.

Copper-induced conformational change in a marsupial prion protein repeat peptide probed using FTIR spectroscopy

We report the first Fourier transform infrared analysis of prion protein (PrP) repeats and the first study of PrP repeats of marsupial origin. Large changes in the secondary structure and an increase in hydrogen bonding within the peptide groups were evident from a red shift of the amide I band by >7 cm(-1) and an approximately five-fold reduction in amide hydrogen-deuterium exchange for peptide interacting with Cu(2+) ions. Changes in the tertiary structure upon copper binding were also evident from the appearance of a new band at 1564 cm(-1), which arises from the ring vibration of histidine. The copper-induced conformational change is pH dependent, and occurs at pH >7.

Mapping Cu(II) binding sites in prion proteins by diethyl pyrocarbonate modification and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometric footprinting

Although Cu(II) ions bind to the prion protein (PrP), there have been conflicting findings concerning the number and location of binding sites. We have combined diethyl pyrocarbonate (DEPC)-mediated carbethoxylation, protease digestion, and mass spectrometric analysis of apo-PrP and copper-coordinated mouse PrP23-231 to "footprint" histidine-dependent Cu(II) coordination sites within this molecule. At pH 7.4 Cu(II) protected five histidine residues from DEPC modification. No protection was afforded by Ca(II), Mn(II), or Mg(II) ions, and only one or two residues were protected by Zn(II) or Ni(II) ions. Post-source decay mapping of DEPC-modified histidines pinpointed residues 60, 68, 76, and 84 within the four PHGGG/SWGQ octarepeat units and residue 95 within the related sequence GGGTHNQ. Besides defining a copper site within the protease-resistant core of PrP, our findings suggest application of DEPC footprinting methodologies to probe copper occupancy and pathogenesis-associated conformational changes in PrP purified from tissue samples.

The metal site as a template for the metalloprotein structure formation

Achieving a thorough explanation of the behavior of metal sites in the formation of native metalloprotein structures is an exciting challenge in the biochemistry of metallobiomacromolecules. This study presents a personal insight into the subject. It is proposed that a metal center and its exogenous ligand compose a template. A template may impose a clear stereochemical preference on the loose peptide chains, and organize them into natural stereospecificity via the metal-ligand interaction, a long-range and strong interaction. Therefore, the stable peptide conformation induced by the template effect surrounding a template polyhedron could be called a template-mediated structural motif (TMSM).

The hunchback and its neighbours: proline as an environmental modulator

The unique ability of Pro or Pro-rich repeats to affect the stability and function of proteins has recently been highlighted by biophysical studies on fragments from prions, signalling domains and muscle proteins. Pro-rich regions have been observed to either occupy disordered states or adopt various helical structures; some are also able to undergo an environmental-dependent transformation between these states. Such a transformation could explain some of the inherent functional properties of the parent proteins and, additionally, can be efficiently exploited to generate novel temperature- and pH-switches in more conventional globular proteins.

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.

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.