Expression of prion protein increases cellular copper binding and antioxidant enzyme activities but not copper delivery

Prion protein self-peptides modulate prion interactions and conversion

Background

Molecular mechanisms underlying prion agent replication, converting host-encoded cellular prion protein (PrP^C) into the scrapie associated isoform (PrP^Sc), are poorly understood. Selective self-interaction between PrP molecules forms a basis underlying the observed differences of the PrP^C into PrP^Sc conversion process (agent replication). The importance of previously peptide-scanning mapped ovine PrP self-interaction domains on this conversion was investigated by studying the ability of six of these ovine PrP based peptides to modulate two processes; PrP self-interaction and conversion.

Results

Three peptides (octarepeat, binding domain 2 -and C-terminal) were capable of inhibiting self-interaction of PrP in a solid-phase PrP peptide array. Three peptides (N-terminal, binding domain 2, and amyloidogenic motif) modulated prion conversion when added before or after initiation of the prion protein misfolding cyclic amplification (PMCA) reaction using brain homogenates. The C-terminal peptides (core region and C-terminal) only affected conversion (increased PrP^res formation) when added before mixing PrP^C and PrP^Sc, whereas the octarepeat peptide only affected conversion when added after this mixing.

Conclusion

This study identified the putative PrP core binding domain that facilitates the PrP^C-PrP^Sc interaction (not conversion), corroborating evidence that the region of PrP containing this domain is important in the species-barrier and/or scrapie susceptibility. The octarepeats can be involved in PrP^C-PrP^Sc stabilization, whereas the N-terminal glycosaminoglycan binding motif and the amyloidogenic motif indirectly affected conversion. Binding domain 2 and the C-terminal domain are directly implicated in PrP^C self-interaction during the conversion process and may prove to be prime targets in new therapeutic strategy development, potentially retaining PrP^C function. These results emphasize the importance of probable PrP^C-PrP^C and required PrP^C-PrP^Sc interactions during PrP conversion. All interactions are probably part of the complex process in which polymorphisms and species barriers affect TSE transmission and susceptibility.

Metal ion physiopathology in neurodegenerative disorders

Metal dyshomeostasis in the brain (BMD) has often been proposed as a possible cause for several neurodegenerative disorders (NDs). Nevertheless, the precise nature of the biochemical mechanisms of metal involvement in NDs is still largely unknown. Mounting evidence suggests that normal aging itself is characterized by, among other features, a significant degree of metal ion dysmetabolism in the brain. This is probably the result of a progressive deterioration of the metal regulatory systems and, at least in some cases, of life-long metal exposure and brain accumulation. Although alterations of metal metabolism do occur to some extent in normal aging, they appear to be highly enhanced under various neuropathological conditions, causing increased oxidative stress and favoring abnormal metal-protein interactions. Intriguingly, despite the fact that most common NDs have a distinct etiological basis, they share striking similarities as they are all characterized by a documented brain metal impairment. This review will primarily focus on the alterations of metal homeostasis that are observed in normal aging and in Alzheimer's disease. We also present a brief survey on BMD in other NDs (Amyotrophic Lateral Sclerosis, Parkinson's, and Prion Protein disease) in order to highlight what represents the most reliable evidence supporting a crucial involvement of metals in neurodegeneration. The opportunities for metal-targeted pharmacological strategies in the major NDs are briefly outlined as well.

Functional implications of multistage copper binding to the prion protein

The prion protein (PrP) is responsible for a group of neurodegenerative diseases called the transmissible spongiform encephalopathies. The normal function of PrP has not yet been discovered, but indirect evidence suggests a linkage to its ability to bind copper. In this article, low-copper-concentration bindings of Cu(2+) to PrP are investigated by using a recently developed hybrid density functional theory (DFT)/DFT method. It is found that at the lowest copper concentrations, the binding site consists of 4 histidine residues coordinating the copper through epsilon imidazole nitrogens. At higher concentrations, 2 histidines are involved in the binding, one of them in the axial position. These results are in good agreement with existing experimental data. Comparison of free energies for all modes of coordination shows that when enough copper is available, the binding sites will spontaneously rearrange to accommodate more copper ions, despite the fact that binding energy per copper ion decreases with concentration. These findings support the hypothesis that PrP acts as a copper buffer in vivo, protecting other proteins from the attachment of copper ions. Using large-scale classical molecular dynamics, we also probe the structure of full-length copper-bound PrP, including its unfolded N-terminal domain. The results show that copper attachment leads to rearrangement of the structure of the Cu-bonded octarepeat region and to development of turns in areas separating copper-bound residues. These turns make the flexible N-terminal domain more rigid and thus more resistant to misfolding. The last result suggests that copper binding plays a beneficial role in the initial stages of prion diseases.

Helix 3 is necessary and sufficient for prion protein's anti-Bax function

To identify the structural elements of the prion protein (PrP) necessary for its protective function against Bcl-2 associated protein X (Bax), we performed structure-function analyses of the anti-Bax function of cytosolic PrP (CyPrP) in MCF-7 cells. Deletions of 1, 2, or 3 N-terminal Bcl-2 homology domain 2-like octapeptide repeats (BORs), but not deletion of all four BORs, abolish CyPrPs anti-Bax function. Deletion of alpha-helix 3 (PrP23-199) or further C-terminal deletions of alpha-helix 1 and 2, and beta-strand 1 and 2 (PrP23-172, PrP23-160, PrP23-143, and PrP23-127) eliminates CyPrPs protection against Bax-mediated cell death. The substitution of helix 3 amino acid residues K204, V210, and E219 by proline inhibits the anti-Bax function of CyPrP. The substitution of K204, but not V210 and E219, by alanine residues also prevents CyPrPs anti-Bax function. Expression of PrPs helix 3 displays anti-Bax activity in MCF-7 cells and in human neurons. Together, these results indicate that although the BOR domain has an influence on PrPs anti-Bax function, the helix 3 is necessary and sufficient for the anti-Bax function of CyPrP. Identification of helix 3 as the structural element for the anti-Bax function thus provides a molecular target to modulate PrPs anti-Bax function in cancer and neurodegeneration.

Prion expression is activated by Adenovirus 5 infection and affects the adenoviral cycle in human cells

The prion protein is a cell surface glycoprotein whose physiological role remains elusive, while its implication in transmissible spongiform encephalopathies (TSEs) has been demonstrated. Multiple interactions between the prion protein and viruses have been described: viruses can act as co-factors in TSEs and life cycles of different viruses have been found to be controlled by prion modulation. We present data showing that human Adenovirus 5 induces prion expression. Inactivated Adenovirus did not alter prion transcription, while variants encoding for early products did, suggesting that the prion is stimulated by an early adenoviral function. Down-regulation of the prion through RNA interference showed that the prion controls adenovirus replication and expression. These data suggest that the prion protein could play a role in the defense strategy mounted by the host during viral infection, in a cell autonomous manner. These results have implications for the study of the prion protein and of associated TSEs.

Prion protein protects against zinc-mediated cytotoxicity by modifying intracellular exchangeable zinc and inducing metallothionein expression

PrPC contains several octapeptide repeats sequences toward the N-terminus which have binding affinity for divalent metals such as copper, zinc, nickel and manganese. However, the link between PrPC expression and zinc metabolism remains elusive. Here we studied the relationship between PrPC and zinc ions intracellular homeostasis using a cell line expressing a doxycycline-inducible PrPC gene. No significant difference in 65Zn2+ uptake was observed in cells expressing PrPC when compared with control cells. However, PrPC-expressing cells were more resistant to zinc-induced toxicity, suggesting an adaptative mechanism induced by PrPC. Using zinquin-ethyl-ester, a specific fluorophore for vesicular free zinc, we observed a significant re-localization of intracellular exchangeable zinc in vesicles after PrPC expression. Finally, we demonstrated that PrPC expression induces metallothionein (MT) expression, a zinc-upregulated zinc-binding protein. Taken together, these results suggest that PrPC modifies the intracellular localization of zinc rather than the cellular content and induces MT upregulation. These findings are of major importance since zinc deregulation is implicated in several neurodegenerative disorders. It is postulated that in prion diseases the conversion of PrPC to PrPSc may deregulate zinc homeostasis mediated by metallothionein.

The octarepeat region of prion protein, but not the TM1 domain, is important for the antioxidant effect of prion protein

The cellular prion protein (PrP(c)) plays a crucial role in the pathogenesis of prion diseases, but its physiological function is far from understood. Several candidate functions have been proposed including binding and internalization of metal ions, a superoxide dismutase-like activity, regulation of cellular antioxidant activities, and signal transduction. The transmembrane (TM1) region of PrP(c) (residues 110-135) is particularly interesting because of its very high evolutionary conservation. We investigated a possible role of TM1 in the antioxidant defense, by assessing the impact of overexpressing wt-PrP or deletion mutants in N(2)A mouse neuroblastoma cells on intracellular reactive oxygen species (ROS) levels. Under conditions of oxidative stress, intracellular ROS levels were significantly lowered in cells overexpressing either wild-type PrP(c) (wt-PrP) or a deletion mutant affecting TM1 (Delta8TM1-PrP), but, as expected, not in cultures overexpressing a deletion mutant lacking the octapeptide region (Deltaocta-PrP). Overexpression of wt-PrP, Delta8TM1-PrP, or Deltaocta-PrP did not affect basal ROS levels. Interestingly, the mitochondrial membrane potential was significantly lowered in Deltaocta-PrP-transfected cultures in the absence of oxidative stress. We conclude that the protective effect of PrP(c) against oxidative stress involves the octarepeat region but not the TM1 domain nor the high-affinity copper binding site described for human residues His96/His111.

Copper and the structural biology of the prion protein

PrP (prion-related protein) is a cell-surface Cu2+-binding glycoprotein which, when misfolded, is responsible for a number of transmissible spongiform encephalopathies. The co-ordination geometry, stoichiometry and affinity of Cu2+ for PrP are the subject of much debate. In the present paper, we review the recent progress we have made in these areas. As many as six Cu2+ ions bind to PrP with submicromolar affinity. Initially, two Cu2+ ions bind to full-length PrP in the amyloidogenic region, between the octarepeats and the structured domain, at His95 and His110. Only subsequent Cu2+ ions bind to single histidine residues within the octarepeat region. Competitive chelators have been used to determine the affinity of the first molar equivalent of Cu2+ bound to full-length PrP; this approach places the affinity in the nanomolar range. The affinity and number of Cu2+-binding sites support the suggestion that PrP could act as an antioxidant by binding potentially harmful Cu2+ ions and sacrificially quenching of free radicals generated as a result of copper redox cycling. Finally, the effect of Cu2+ on the prion structure and misassembly into oligomers and fibres is discussed.

Increased PrPC expression correlates with endoglin (CD105) positive microvessels in advanced carotid lesions

Normal cellular prion protein (PrP(C)) has multiple functions but its role in the development of atherosclerosis has not been studied. Our pilot microarray data showed increased expression of PrP(C) in tissue samples of complicated carotid lesions. Therefore in this study, we aimed to investigate its localisation within atherosclerotic arteries and its concentration in patient plasma. PrP(C) expression was examined using an enzyme immunometric assay (EIA) in plasma from patients undergoing endarterectomy. Carotid specimens and control vascular transplants were studied for PrP(C) and CD105 (endoglin, a marker of active vessels) expression by immunohistochemistry and real-time PCR. Patients with carotid disease had higher levels of plasma PrP(C) than the control group [4.35 ng/ml (n = 22; 3.1-5.3) vs. 1.95 ng/ml (n = 21; 1.1-2.5), P < 0.001]. Furthermore, CD105-positive plaques had higher PrP(C) expression which colocalized with CD105 in neovessels. There was a significant correlation between mRNA expression of PrP(C) and CD105 in tested plaques (P < 0.001; r = 0.7) supporting our immunohistochemical findings. We conclude that PrP(C) is expressed in carotid specimens and may be associated with neovessel growth or survival in these plaques. Our results suggest a role for PrP(C) in modulating neovessel formation in complicated plaques.

Increased oxidation, glycoxidation, and lipoxidation of brain proteins in prion disease

The basic molecular underpinnings of the pathological changes that unfold in prion disease remain elusive. A key role of increased oxidative stress has been hypothesized. Given the transient nature of most intermediate molecules implicated, increased oxidative stress is better assessed by quantitating the damage it causes to macromolecules. We used mass spectrometry-based methods to measure specific products of protein oxidation, glycoxidation, and lipoxidation in brains from patients suffering from Creutzfeldt-Jakob disease and Syrian hamsters affected by scrapie. In both cases, increased amounts of glutamic and aminoadipic semialdehydes, products of metal-catalyzed oxidation, malondialdehydelysine (a product of lipoxidation), N-epsilon-carboxyethyllysine (a product of glycoxidation), and N-epsilon-carboxymethyllysine (generated by lipoxidation and glycoxidation) were measured. PrP(Sc), the infectious isoform of the prion protein that accumulates in prion disease, was itself shown to be a target of increased oxidative modification. These changes were accompanied by alterations in fatty acid composition and increased phosphorylation of ERK(1/2) and p38, protein kinases known to respond to increased flows of ROS. These data support an important role of oxidative damage in the pathology of prion disease.

Absence of the cellular prion protein exacerbates and prolongs neuroinflammation in experimental autoimmune encephalomyelitis

Although the physiological roles of the cellular prion protein (PrP C) remain to be fully elucidated, PrP C has been proposed to represent a potential regulator of cellular immunity. To test this hypothesis, we evaluated the consequences of PrP C deficiency on the course of experimental autoimmune encephalomyelitis induced by immunization with myelin oligodendrocyte glycoprotein peptide. Consistent with augmented proliferative responses and increased cytokine gene expression by myelin oligodendrocyte glycoprotein-primed Prnp-/- T cells, PrP C-deficient mice demonstrated more aggressive disease onset and a lack of clinical improvement during the chronic phase of experimental autoimmune encephalomyelitis. Acutely, Prnp-/- spinal cord, cerebellum, and forebrain exhibited higher levels of leukocytic infiltrates and pro-inflammatory cytokine gene expression, as well as increased spinal cord myelin basic protein and axonal loss. During the chronic phase, a remarkable persistence of leukocytic infiltrates was present in the forebrain and cerebellum, accompanied by an increase in interferon-gamma and interleukin-17 transcripts. Attenuation of T cell-dependent neuroinflammation thus represents a potential novel function of PrP C.

Identification of the copper(II) coordinating residues in the prion protein by metal-catalyzed oxidation mass spectrometry: evidence for multiple isomers at low copper(II) loadings

While the Cu(II) binding sites of the prion protein have been well studied under Cu-saturation conditions, the identity of the residues involved in coordinating Cu(II) at low stoichiometries and the order in which the binding sites load with Cu(II) remain unresolved. In this study, we have used two mass spectrometry based methods to gather insight into Cu(II)-prion binding under different stoichiometric loadings of Cu(II). The first method uses metal-catalyzed oxidation reactions to site specifically modify the residues bound to Cu(II) in solution, and the second method determines Cu binding sites based on the protection of His from modification by diethyl pyrocarbonate when this residue binds Cu(II) in solution. For both methods, the residues that are labeled by these reactions can then be unambiguously identified using tandem mass spectrometry. Upon applying these two complementary methods to a construct of the prion protein that contains residues 23-28 and 57-98, several noteworthy observations are made. Coordination of Cu(II) by multiple His imidazoles is found at 1:1 and 1:2 PrP:Cu(II) ratios. Notably, there appear to be four to seven isomers of this multiple histidine coordination mode in the 1:1 complex. Furthermore, our data clearly show that His96 is the dominant Cu(II) binding ligand, as in every isomer His96 is bound to Cu(II). The individual octarepeat binding sites begin to fill at ratios of 1:3 PrP:Cu(II) with no clear preference for the order in which they load with Cu(II), although the His77 octarepeat appears to saturate last. The existence of several "degenerate" Cu binding modes at low PrP:Cu(II) ratios may allow it to more readily accept additional Cu(II) ions, thus allowing PrP to transition from a singly Cu(II) bound state to a multiply Cu(II) bound state as a function of cellular Cu(II) concentration.

Protective effect of prion protein via the N-terminal region in mediating a protective effect on paraquat-induced oxidative injury in neuronal cells

Transmissible spongiform encephalopathies are a group of neurodegenerative disorders caused by a posttranslational, conformational change in the cellular isoform of the prion protein (PrP(C)) into an infectious, disease-associated form (PrP(Sc)). Increasing evidence supports a role for PrP(C) in the cellular response to oxidative stress. We investigated the effect of oxidative stress mediated by paraquat exposure on SH-SY5Y neuroblastoma cells. A loss of mitochondrial membrane potential and subsequent reduction in ATP production were demonstrated in untransfected SH-SY5Y cells, an effect that was ameliorated by the expression of PrP(C). Cells expressing either PrP-DeltaOct, which lacks the octapeptide repeats, or PrP-DA, in which the N-terminus is tethered to the membrane, showed increased sensitivity to paraquat compared with cells expressing wild-type PrP(C) as shown by reduced viability, loss of their membrane integrity, and reduced mitochondrial bioenergetic measurements. Exposure of prion-infected mouse SMB15S cells to paraquat resulted in a reduction in viability to levels similar to those seen in the untransfected SH-SY5Y cells. However, "curing" the cells with pentosan sulfate restored the viability to the level observed in the SH-SY5Y cells expressing PrP(C). These data would indicate that the molecular mechanism promoting cellular resistance to oxidative stress had been compromised in the infected SMB15S cells, which could be reinstated upon curing. Our study supports the hypothesis that PrP(C) expression protects cells against paraquat-induced oxidative injury, demonstrates the significance of the N-terminal region of the protein in mediating this protective effect, and also shows that the biochemical consequences of prion infection may be reversed with therapeutic intervention.

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.

PRNP M129V homozygosity in multiple system atrophy vs. Parkinson's disease

Multiple system atrophy (MSA) is a neurodegenerative disorder of unknown etiology characterized by extrapyramidal, pyramidal, cerebellar, and autonomic dysfunction in any combination. We report a patient with a 4-year history of MSA who developed dementia associated with sporadic Creutzfeldt-Jakob disease (CJD). Our proband was MM homozygous for the M129V polymorphism within the prion protein gene (PRNP), a known risk factor for CJD. We conducted a case-control study to test the hypothesis that homozygosity for the M129V polymorphism of PRNP occurs more frequently in MSA in comparison to Parkinson's disease and healthy volunteers. A total of 63 patients with MSA, 54 age-, race- and gendermatched controls with Parkinson's disease, and 126 matched healthy volunteers were studied. The genotype analysis revealed no significant difference in the codon 129 genotype distribution in MSA as compared to controls. Nonetheless, the frequencies of the MM and VV genotypes were higher in MSA than in Parkinson's disease. Thus, homozygosity, particularly VV homozygosity, at codon 129 of PRNP is associated with MSA compared to a clinically related but pathophysiologically distinct alpha-synucleinopathy. Considering the possibility that the prion protein contributes to the pathogenesis of MSA would require confirmation of these findings in an independent patient population.

Reversible precipitation of bovine serum albumin by metal ions and synthesis, structure and reactivity of new tetrathiometallate chelating agents

Independent research is an important component of any undergraduate chemistry program. This article reports the findings of two of many undergraduate research projects directed by Ed Stiefel in the hopes that the results will be inspiring and useful to the scientific community. The neurological disorders associated with insufficient copper in Menkes disease and an excess of copper in Wilson's disease are well established; however, recent evidence suggests that copper may also be involved in other disorders, such as Alzheimer's, angiogenesis, and prion diseases. The exact role of copper, however, is uncertain. This study examines the role of copper and zinc in the formation of protein deposits and the chelation and removal of the metal ions to reverse the process. The bovine serum albumin (BSA) protein forms a precipitate after the addition of approximately 6 copper(II) atoms or 8 zinc(II) atoms. Other metal ions, such as Ca(II), Al(III), Ni(II), and Co(II), did not precipitate the BSA even when the metal ion to BSA ratios were in excess of 1000. The copper and zinc protein precipitates returned to solution after addition of the chelating agents, ethylenediaminetetraacetic acid (EDTA) or tetrathiometallates [(MS(4)(2-)), where M=Mo, W]. Two new choline and acetylcholine tetrathiomolybdate and tetrathiotungstate chelating agents have been synthesized and characterized. The infrared (IR) and X-ray crystal structures of the complexes revealed that the (MS(4)(2-)) cores had approximate T(d) symmetry in the choline (Ch) salts and C(2v) symmetry in the acetylcholine (AcCh) salts. The AcCh salts hydrolyzed more slowly than the ammonium or Ch salts and the tetrathiotungstate salts hydrolyzed approximately two orders of magnitude more slowly than the tetrathiomolybdate salts. The slower hydrolysis of tetrathiotungstate may make it more useful as an inorganic reagent and therapeutic agent.

Biological characteristics of Chinese hamster ovary cells transfected with bovine Prnp

A normal prion protein (PrP^c) is converted to a protease-resistant isoform by an apparent self-propagating activity in transmissible spongiform encephalopathy, a neurodegenerative disease. The cDNA encoding open reading frame (ORF) of the bovine prion protein gene (Prnp) was cloned from Korean cattle by PCR, and was transfected into Chinese hamster ovary (CHO-K1) cells using lipofectamine. The gene expression of the cloned cDNA was confirmed by RT-PCR and Western blotting with the monoclonal antibody, 6H4. Cellular changes in the transfected CHO-K1 cells were investigated using parameters such as MTT, lactate dehydrogenase (LDH), and superoxide dismutase (SOD) activities, as well as nitric oxide (NO) production, and an apoptosis assay. In the MTT and LDH assays, the bovine PrnP-transfectant showed a lower proliferation rate than the wild-type (p < 0.05). Production of NO, after LPS or ConA stimulation, was not detected in either transfectants or CHO-K1 cells. In SOD assay under ConA stimulation, the SOD activity of transfectants was 10 times higher than that of CHO-K1 cells at 6 h after treatment (p < 0.05). The genomic DNA of both the transfectants and control cells began to be fragmented at 6 h after treatment with cyclohexamide. Caspase-3 activity was reduced by transfection with the bovine Prnp (p < 0.05). Conclusively, the viability of transfectants expressing exogenous bovine Prnp was decreased while the capacities for cellular protection against antioxidative stress and apoptosis were increased.

Canadian Association of Neurosciences Review: prion protein and prion diseases: the good and the bad

In the 1700's a strange new disease affecting sheep was recognized in Europe. The disease later became known as "Scrapie" and was the first of a family of similar diseases affecting a number of species that are now known as the Transmissible Spongiform Encephalopathies (TSEs). The appearance of a new disease in humans linked to the consumption of meat products from infected cattle has stimulated widespread public concern and scientific interest in the prion protein and related diseases. Nearly 300 years after the first report, these diseases still merit the descriptor "strange". This family of diseases is characterized by a unique profile of histological changes, can be transmitted as inherited or acquired diseases, as well as apparent sporadic spontaneous generation of the disease. These diseases are believed by many, to be caused by a unique protein only infectious agent. The "prion protein" (PrPC), a term first coined by Stanley Prusiner in 1982 is crucial to the development of these diseases, apparently by acting as a substrate for an abnormal disease associated form. However, aside from being critical to the pathogenesis of the disease, the function of PrPC, which is expressed in all mammals, has defied definitive description. Several roles have been proposed on the basis of in vitro studies, however, thus far, in vivo confirmation has not been forthcoming. The biological features of PrPC also seem to be unusual. Numerous mouse models have been generated in an attempt to understand the pathogenesis of these diseases. This review summarizes the current state of histological features, the etiologic agent, the normal metabolism and the function of the prion protein, as well as the limitations of the mouse models.

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.

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.

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.

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.

Glycan chains modulate prion protein binding to immobilized metal ions

PrP(c) is the normal isoform of the prion protein which can be converted into PrP(Sc), the pathology-associated conformer in prion diseases. It contains two N-linked glycan chains attached to the C-proximal globular domain. While the biological functions of PrP(c) are still unknown, its ability to bind Cu(2+) is well documented. The main Cu(2+)-binding sites are located in the N-proximal, unstructured region of the molecule. Here we report that PrP(c) glycans influence the capacity of PrP(c) from sheep brain or cultured Rov cells to bind IMAC columns loaded with Cu(2+) or Co(2+). Using different anti-PrP antibodies and PrP(c) glycosylation mutants, we show that the full length non-glycosylated form of PrP(c) has a higher binding efficiency for column-bound Cu(2+) and Co(2+) than the corresponding glycosylated form. Our findings raise the possibility that the accessibility of the PrP(c) metal ion-binding sites might be controlled by the glycan chains.

Real-time kinetics of discontinuous and highly conformational metal-ion binding sites of prion protein

The prion protein (PrP) is a metalloprotein with an unstructured region covering residues 60-91 that bind two to six Cu(II) ions cooperatively. Cu can bind to PrP regions C-terminally to the octarepeat region involving residues His111 and/or His96. In addition to Cu(II), PrP binds Zn(II), Mn(II) and Ni(II) with binding constants several orders of magnitudes lower than those determined for Cu. We used for the first time surface plasmon resonance (SPR) analysis to dissect metal binding to specific sites of PrP domains and to determine binding kinetics in real time. A biosensor assay was established to measure the binding of PrP-derived synthetic peptides and recombinant PrP to nitrilotriacetic acid chelated divalent metal ions. We have identified two separate binding regions for binding of Cu to PrP by SPR, one in the octarepeat region and the second provided by His96 and His111, of which His96 is more essential for Cu coordination. The octarepeat region at the N-terminus of PrP increases the affinity for Cu of the full-length protein by a factor of 2, indicating a cooperative effect. Since none of the synthetic peptides covering the octarepeat region bound to Mn and recombinant PrP lacking this sequence were able to bind Mn, we propose a conformational binding site for Mn involving residues 91-230. A novel low-affinity binding site for Co(II) was discovered between PrP residues 104 and 114, with residue His111 being the key amino acid for coordinating Co(II). His111 is essential for Co(II) binding, whereas His96 is more important than His111 for binding of Cu(II).

Biological polyamines inhibit nucleic-acid-induced polymerisation of prion protein

Nucleic-acid-induced polymerisation of prion protein, when monitored by anilino naphthalene sulfonic acid dye, shows, successively, an immediate fluorescence increase of the dye upon mixing of the reactants, followed by a lag period in which the dye fluorescence remains unchanged, and then a phase in which dye fluorescence increases with time. The biological polyamines spermine and spermidine reduce the extent of the initial fluorescence increase, increase the lag period, and reduce both the rate and the extent of increase in fluorescence intensity of the dye in the final phase of the reaction. Spermidine is less effective than spermine in all of these processes. A nearly fivefold lower concentration of spermine can inhibit polymerisation of prion protein by tRNAs compared to the same process induced by double-stranded nucleic acid. The change in the secondary structure of the globular domain of the protein induced by nucleic acid is reversed by the addition of spermine, and it prevents structural destabilization of this domain induced by nucleic acids. It is suggested that physiological event(s) that would reduce the concentrations of intracellular biological amines may make nucleic acid available to induce oligomerization and polymerisation of cellular prion protein related to prion disease.

Substrates of the methionine sulfoxide reductase system and their physiological relevance

Posttranslational modifications can change a protein's structure, function, and solubility. One specific modification caused by reactive oxygen species is the oxidation of the sulfur atom in the methionine (Met) side chain. This modified amino acid is denoted as methionine sulfoxide (MetO). MetOs in proteins are of considerable interest as they are involved in early posttranslational modification events. Thus, various organisms produce specific enzymes that can reverse these modifications. MetO reductases, known collectively as the methionine sulfoxide reductase (Msr) system, are the only known enzymes that can reduce MetOs. The current research field of Met redox cycles is consumed with elucidating its role in regulation, redox homeostasis, prevention of irreversible modifications, pathogenesis, and the aging process. Substrates of the Msr system can be loosely classified by the overall effect of the MetO on the protein. Regulated substrates utilize Met as a molecular switch to modulate activation; scavenging substrates use Mets to detoxify oxidants and protect important regions of the protein; and modified substrates are altered by Met oxidation resulting in various changes in their properties, including function, activity, structure, and degradation resistance.

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.

Prion protein 90-231 contains a streptavidin-binding motif

The biological function of prion protein (PrP) and the physiological relevance of its truncated subtypes and glycoforms is still enigmatic. In this paper, we adduce evidence that recombinant murine PrP fragment 90-231 (mPrP90-231) contains a biotin-mimicking sequence motif that causes binding of the bacterial protein streptavidin to mPrP90-231. As indicated by epitope mapping and proven by analysis of a deletion mutant (mPrP101-231), streptavidin binding is primarily mediated by the amino-terminus of mPrP90-231 with the core-binding sequence represented by residues 94-100. Competition with biotin significantly reduces the interaction pointing to an involvement of streptavidin's biotin-binding site (BBS). Since the BBS of streptavidin shares similarities with the active sites of proteins involved in biotin metabolism we speculate that biotin mimicry by truncated PrP-species may have an impact in vivo.

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.

The prion protein is neuroprotective against retinal degeneration in vivo

A common feature of neurodegenerative disorders is acute or progressive loss of neurons due to apoptosis. The pathological isoform of the prion protein is associated with retinal apoptosis and the cellular isoform (PrPc) has been shown to mediate protection from apoptosis in cell culture and in neonatal retinal explants. Using a model of light-induced photoreceptor apoptosis, we show in vivo that the levels of PrPc expression in the retina inversely correlate with the susceptibility of photoreceptors to light damage. Dissection of apoptotic signalling cascades suggests that PrPc acts neuroprotectively downstream of AP-1 induction. Our results reveal PrP as a neuroprotective/anti-apoptotic factor in vivo and suggest that PrPc may function as a guardian of neuronal integrity.

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

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

Green fluorescent protein as a reporter of prion protein folding

Background

The amino terminal half of the cellular prion protein PrP^c is implicated in both the binding of copper ions and the conformational changes that lead to disease but has no defined structure. However, as some structure is likely to exist we have investigated the use of an established protein refolding technology, fusion to green fluorescence protein (GFP), as a method to examine the refolding of the amino terminal domain of mouse prion protein.

Results

Fusion proteins of PrP^c and GFP were expressed at high level in E.coli and could be purified to near homogeneity as insoluble inclusion bodies. Following denaturation, proteins were diluted into a refolding buffer whereupon GFP fluorescence recovered with time. Using several truncations of PrP^c the rate of refolding was shown to depend on the prion sequence expressed. In a variation of the format, direct observation in E.coli, mutations introduced randomly in the PrP^c protein sequence that affected folding could be selected directly by recovery of GFP fluorescence.

Conclusion

Use of GFP as a measure of refolding of PrP^c fusion proteins in vitro and in vivo proved informative. Refolding in vitro suggested a local structure within the amino terminal domain while direct selection via fluorescence showed that as little as one amino acid change could significantly alter folding. These assay formats, not previously used to study PrP folding, may be generally useful for investigating PrP^c structure and PrP^c-ligand interaction.

Cu(II) induces small-size aggregates with amyloid characteristics in two alleles of recombinant ovine prion proteins

One of symptoms of transmissible spongiform encephalopathies is associated with the transformation of normal cellular prion protein, PrP, in its amyloid isoform resistant to proteolytic cleavage. The present study shows that interaction with copper ions converts both monomeric recombinant scrapie-susceptible PrP-VRQ and scrapie-resistant PrP-ARR variants into protease-resistant soluble oligomers with amyloid characteristics -- dominant beta-sheet secondary structure and interaction with thioflavine S. In contrast, binding of zinc ions resulting in the same resistance to proteolysis does not provoke transformation of alpha-helical monomeric structure of both PrP polymorphic variants. Cleavage of PrP N-terminus destabilises soluble form of such aggregates, and N-truncated PrPrec complexed with metal cations precipitate. N-truncated PrPrec complexed with Zn precipitated much faster than N-truncated PrPrec complexed with Cu. According to the hypothesis about the key role of small PrP oligomers in PrP(C)-PrP(Sc) transformation, formation of soluble oligomers of PrP complexed with Cu can constitute an additional element in TSE propagation. Identical metal-chelating behaviour of two studied polymorphic PrPrec variants conferring different susceptibilities of sheep to scrapie could indicate their different capabilities to form fibrils. This could imply also that other factors than physico-chemical differences between PrP-VRQ and PrP-ARR and the differences in PrP transformation are responsible for the onset of TSE.

Copper induces conformational changes in the N-terminal part of cell-surface PrPC

Prion diseases are caused by misfolding of the cellular prion protein, PrPC. In vitro studies have shown that PrP binds copper via the octarepeat region lying within the unstructured N-terminal segment of the protein, but the significance of copper in PrP metabolism remains unclear. Here, six specific antibodies recognizing different epitope regions of PrP were used to measure the effect of copper on the conformation of the molecule at the cell surface. Binding of an antibody, E149, to an epitope within the octarepeat domain of PrP is halved in the presence of copper, whereas binding of antibodies recognizing epitope motifs C-terminal to residue 90 of PrP remain relatively unaltered under equivalent conditions. These experiments strongly suggest that copper induces localized conformational change within the N-terminal portion of cell-surface PrPC.

Copper-dependent degradation of recombinant ovine prion protein. Phosphatidylinositol stimulates aggregation and copper-driven disappearance of prion protein

Prion protein (PrP) plays an important role in cell protection from oxidative stress due to its action as copper-chelating protein. The present study demonstrates that PrP participates in reductions of Cu2+ to Cu+ ions, and that this process results in fragmentation of protein. The interaction with phosphatidylinositol, a natural phospholipid moiety bound to PrP, strongly enhances recombinant PrP aggregation and degradation. The copper-dependent PrP degradation could promote the formation of amyloid structures, destabilizing the PrP soluble form by the cleavage of the N-terminal part.

A spectroscopic and voltammetric study of the pH-dependent Cu(II) coordination to the peptide GGGTH: relevance to the fifth Cu(II) site in the prion protein

The GGGTH sequence has been proposed to be the minimal sequence involved in the binding of a fifth Cu(II) ion in addition to the octarepeat region of the prion protein (PrP) which binds four Cu(II) ions. Coordination of Cu(II) by the N- and C-protected Ac-GGGTH-NH(2) pentapeptide (P(5)) was investigated by using potentiometric titration, electrospray ionization mass spectrometry, UV-vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and cyclic voltammetry experiments. Four different Cu(II) complexes were identified and characterized as a function of pH. The Cu(II) binding mode switches from NO(3) to N(4) for pH values ranging from 6.0 to 10.0. Quasi-reversible reduction of the [Cu(II)(P(5))H(-2)] complex formed at pH 6.7 occurs at E (1/2)=0.04 V versus Ag/AgCl, whereas reversible oxidation of the [Cu(II)(P(5))H(-3)](-) complex formed at pH 10.0 occurs at E (1/2)=0.66 V versus Ag/AgCl. Comparison of our EPR data with those of the rSHaPrP(90-231) (Burns et al. in Biochemistry 42:6794-6803, 2003) strongly suggests an N(3)O binding mode at physiological pH for the fifth Cu(II) site in the protein.

Interactions of recombinant prions with compounds of therapeutical significance

The transformation of the cellular prion protein (PrP(C)) into the infectious form (PrP(Sc)) is implicated in the invariably fatal transmissible spongiform encephalopathies. To identify a mechanism to prevent the undesired PrP(C)-->PrP(Sc) transformation, we investigated the interactions of recombinant prion proteins with a number of potential therapeutic agents which inhibit the PrP(Sc) formation, infectivity, and the accumulation of the misfolded form. We show that the prion aggregates formed in the presence of six compounds have no beta-structure, which is typical of the infectious form, and possess considerably higher alpha-helical content than the normal PrP(C). The investigated compounds stimulate the formation of alpha-helices and the destruction of beta-structure. They prevent the transformation of alpha-helical structure into beta-sheets. Probably, this is the reason for the resistance to PrP(C)-->PrP(Sc) transformation in the presence of these compounds. The results may be useful for the future therapy of neurodegenerative diseases.

Deletion of Cellular Prion Protein Results in Reduced Akt Activation, Enhanced Postischemic Caspase-3 Activation, and Exacerbation of Ischemic Brain Injury

Background and Purpose— The physiological function of cellular prion protein (PrP c ) is not yet understood. Recent findings suggest that PrP c may have neuroprotective properties, and its absence increases susceptibility to neuronal injury. The purpose of this study was to elucidate the role of PrP c in ischemic brain injury in vivo.

Methods— PrP knockout (Prnp 0/0 ) and Prnp +/+ wild-type (WT) mice were subjected to 60-minute transient or permanent focal cerebral ischemia followed by infarct volume analysis 24 hours after lesion. To identify effects of PrP c deletion on mechanisms regulating ischemic cell death, expression analysis of several proapoptotic and antiapoptotic proteins was performed at 6 and 24 hours after transient ischemia and in nonischemic controls using Western blot or immunohistochemistry.

Results— Prnp 0/0 mice displayed significantly increased infarct volumes after both transient or permanent ischemia when compared with WT animals (70.2±23 versus 13.3±4 mm 3 ; 119.8±24 versus 86.4±25 mm 3 ). Expression of phospho-Akt (Ser473) was significantly reduced in Prnp 0/0 compared with WT animals both early after ischemia and in sham controls. Furthermore, postischemic caspase-3 activation was significantly enhanced in the basal ganglia and the parietal cortex of Prnp 0/0 mice. In contrast, expression of total Akt, Bax, and Bcl-2 did not differ between both groups.

Conclusions— These results demonstrate that PrP c deletion impairs the antiapoptotic phosphatidylinositol 3-kinase/Akt pathway by resulting in reduced postischemic phospho-Akt expression, followed by enhanced postischemic caspase-3 activation, and aggravated neuronal injury after transient and permanent cerebral ischemia.