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

Metal-catalyzed proteolysis, conformational antigenicity, photosensitized oxidation, and electrical dysfunction explain the pathogenicity of protein aggregates

It is widely accepted that protein aggregates tend to be pathologic, although little is known about why they are pathologic. Here, we summarize published findings about protein aggregates which have implications for pathology, but which have not yet been covered in any review or hypothesis on the subject, to the best of our knowledge. These findings suggest that protein aggregates can: (i) act as proteases, using exposed surface serines, (ii) function as immunogens, using novel conformational epitopes, (iii) behave as photosensitization-aids, using a novel peptide-based fluorescence, and (iv) act as electrical conductors, using electrons tunneling through hydrogen-bonded networks of peptide bonds. The potential pathological consequences of each finding are speculated upon.

Tryptophan scanning analysis of the membrane domain of CTR-copper transporters

Membrane proteins of the CTR family mediate cellular copper uptake in all eukaryotic cells and have been shown to participate in uptake of platinum-based anticancer drugs. Despite their importance for life and the clinical treatment of malignancies, directed biochemical studies of CTR proteins have been difficult because high-resolution structural information is missing. Building on our recent 7A structure of the human copper transporter hCTR1, we present the results of an extensive tryptophan-scanning analysis of hCTR1 and its distant relative, yeast CTR3. The comparative analysis supports our previous assignment of the transmembrane helices and shows that most functionally and structurally important residues are clustered around the threefold axis of CTR trimers or engage in helix packing interactions. The scan also identified residues that may play roles in interactions between CTR trimers and suggested that the first transmembrane helix serves as an adaptor that allows evolutionarily diverse CTRs to adopt the same overall structure. Together with previous biochemical and biophysical data, the results of the tryptophan scan are consistent with a mechanistic model in which copper transport occurs along the center of the trimer.

The octarepeat region of the prion protein is conformationally altered in PrP(Sc)

Background

Prion diseases are fatal neurodegenerative disorders characterized by misfolding and aggregation of the normal prion protein PrP^C. Little is known about the details of the structural rearrangement of physiological PrP^C into a still-elusive disease-associated conformation termed PrP^Sc. Increasing evidence suggests that the amino-terminal octapeptide sequences of PrP (huPrP, residues 59–89), though not essential, play a role in modulating prion replication and disease presentation.

Methodology/Principal Findings

Here, we report that trypsin digestion of PrP^Sc from variant and sporadic human CJD results in a disease-specific trypsin-resistant PrP^Sc fragment including amino acids ∼49–231, thus preserving important epitopes such as the octapeptide domain for biochemical examination. Our immunodetection analyses reveal that several epitopes buried in this region of PrP^Sc are exposed in PrP^C.

Conclusions/Significance

We conclude that the octapeptide region undergoes a previously unrecognized conformational transition in the formation of PrP^Sc. This phenomenon may be relevant to the mechanism by which the amino terminus of PrP^C participates in PrP^Sc conversion, and may also be exploited for diagnostic purposes.

Identification of the minimal copper(II)-binding alpha-synuclein sequence

Parkinson's disease has been long linked to environmental factors, such as transition metals and recently to alpha-synuclein, a presynaptic protein. Using tryptophan-containing peptides, we identified the minimal Cu(II)-binding sequence to be within the first four residues, MDV(F/W), anchored by the alpha-amino terminus. In addition, mutant peptide 1-10 (Lys --> Arg) verified that neither Lys6 nor Lys10 are necessary for Cu(II) binding. Interestingly, Trp4 excited-state decay kinetics measured for peptides and proteins reveal two quenching modes, possibly arising from two distinct Cu(II)-polypeptide structures.

Copper binding extrinsic to the octarepeat region in the prion protein

Current research suggests that the function of the prion protein (PrP) is linked to its ability to bind copper. PrP is implicated in copper regulation, copper buffering and copper-dependent signaling. Moreover, in the development of prion disease, copper may modulate the rate of protein misfolding. PrP possesses a number of copper sites, each with distinct chemical characteristics. Most studies thus far have concentrated on elucidating chemical features of the octarepeat region (residues 60-91, hamster sequence), which can take up to four equivalents of copper, depending on the ratio of Cu2+ to protein. However, other sites have been proposed, including those at histidines 96 and 111, which are adjacent to the octarepeats, and also at histidines within PrP's folded C-terminal domain. Here, we review the literature of these copper sites extrinsic to the octarepeat region and add new findings and insights from recent experiments.

Insights into prion protein function from atomistic simulations

Computer simulations are a powerful tool for studies of biological systems. They have often been used to study prion protein (PrP), a protein responsible for neurodegenerative diseases, which include "mad cow disease" in cattle and Creutzfeldt-Jacob disease in humans. An important aspect of the prion protein is its interaction with copper ion, which is thought to be relevant for PrP's yet undetermined function and also potentially play a role in prion diseases. for studies of copper attachment to the prion protein, computer simulations have often been used to complement experimental data and to obtain binding structures of Cu-PrP complexes. This paper summarizes the results of recent ab initio calculations of copper-prion protein interactions focusing on the recently discovered concentration-dependent binding modes in the octarepeat region of this protein. In addition to determining the binding structures, computer simulations were also used to make predictions about PrP's function and the role of copper in prion diseases. The results demonstrate the predictive power and applicability of ab initio simulations for studies of metal-biomolecular complexes.

NMR studies of zinc binding in a multi-histidinic peptide fragment

A multi-histidinic peptide and its minimal models have been investigated for Zn(ii) binding. We have used NMR spectroscopy to probe the binding of zinc to the three repeats (T(1)R(2)S(3)R(4)S(5)H(6)T(7)S(8)E(9)G(10))(3) and to its mono-histidinic minimal models, the 9- and 10-aminoacid fragment. (1)H-(1)H TOCSY, (1)H-(13)C HSQC, (1)H-(1)H NOESY and (1)H-(1)H ROESY multidimensional NMR techniques were performed to understand the details of metal binding sites and the conformational behaviour of the peptides at different pH values and at different ligand to metal molar ratios. Zinc coordination involves imidazole N(delta) of His6 and carboxyl gamma-O of Glu9 residues; interaction with peptide oxygens of the His6-Thr7 or Thr7-Ser8 bonds in a tetrahedral arrangement with the minimal model peptides, cannot be excluded. Zinc coordination involves, at physiologic pH, all the three imidazole N(delta) donors of His6, His16 and His26 as well as carboxyl gamma-O of Glu residues in a tetra, penta or octahedral arrangement with the three repeats, the 30-aminoacid fragment. Zinc complexation induces important structural changes with the C-terminal portion of the ligand, constraining it to leave its disordered conformation. Our results give rise to a model of the induced structure of the peptides when bound to zinc. At high pH, amide deprotonation does not take place and hydroxo or high molecular weight polymeric species may be formed.

De novo mammalian prion synthesis

Prions are responsible for a heterogeneous group of fatal neurodegenerative diseases. They can be sporadic, genetic, or infectious disorders involving post-translational modifications of the cellular prion protein (PrP(C)). Prions (PrP(Sc)) are characterized by their infectious property and intrinsic ability to convert the physiological PrP(C) into the pathological form, acting as a template. The "protein-only" hypothesis, postulated by Stanley B. Prusiner, implies the possibility to generate de novo prions in vivo and in vitro. Here we describe major milestones towards proving this hypothesis, taking into account physiological environment/s, biochemical properties and interactors of the PrP(C).

Prion protein: From physiology to cancer biology

Prion protein (PrPc) was originally viewed solely as being involved in prion disease, but now several intriguing lines of evidence have emerged indicating that it plays a fundamental role not only in the nervous system, but also throughout the human body. PrPc is expressed most abundantly in the brain, but has also been detected in other non-neuronal tissues as diverse as lymphoid cells, lung, heart, kidney, gastrointestinal tract, muscle, and mammary glands. Recent data indicate that PrPc may be implicated in biology of glioblastoma, breast cancer, prostate and gastric cancer. Over expression of PrPc is correlated to the acquisition by tumor cells of a phenotype for resistance to cell death induced by TNF alpha and TRAIL or antitumor drugs such as paclitaxel and anthracyclines. PrPc may promote tumorigenesis, proliferation and G1/S transition in gastric cancer cells. This review revisits the physiological functions of PrPc, and its possible implications for cancer biology.

Copper-responsive magnetic resonance imaging contrast agents

The design, synthesis, and evaluation of the Copper-Gad (CG) family, a new class of copper-activated magnetic resonance imaging (MRI) contrast agents, are presented. These indicators comprise a Gd(3+)-DO3A core coupled to various thioether-rich receptors for copper-induced relaxivity switching. In the absence of copper ions, inner-sphere water binding to the Gd(3+) chelate is restricted, resulting in low longitudinal relaxivity values (r(1) = 1.2-2.2 mM(-1) s(-1) measured at 60 MHz). Addition of Cu(+) to CG2, CG3, CG4, and CG5 and either Cu(+) or Cu(2+) to CG6 triggers marked enhancements in relaxivity (r(1) = 2.3-6.9 mM(-1) s(-1)). CG2 and CG3 exhibit the greatest turn-on responses, going from r(1) = 1.5 mM(-1) s(-1) in the absence of Cu(+) to r(1) = 6.9 mM(-1) s(-1) upon Cu(+) binding (a 360% increase). The CG sensors are highly selective for Cu(+) and/or Cu(2+) over competing metal ions at cellular concentrations, including Zn(2+) at 10-fold higher concentrations. (17)O NMR dysprosium-induced shift and nuclear magnetic relaxation dispersion measurements support a mechanism in which copper-induced changes in the coordination environment of the Gd(3+) core result in increases in q and r(1). T(1)-weighted phantom images establish that the CG sensors are capable of visualizing changes in copper levels by MRI at clinical field strengths.

The consequences of pathogenic mutations to the human prion protein

Prion diseases, in which the conformational transition of the native prion protein (PrP) to a misfolded form causes aggregation and subsequent neurodegeneration, have fascinated the scientific community as this transmissible disease appears to be purely protein-based. Disease can arise due to genetic factors only. At least 30 single point mutations have been indicated to cause disease in humans. Somehow, these mutations must influence the stability, processing and/or cellular interactions of PrP, such that aggregation can occur and disease develops. In this review, the current evidence for such effects of single point mutations is discussed, indicating that PrP can be affected in many different ways, although questions remain about the mechanism by which mutations cause disease.

The biochemistry of disease: desperately seeking syzygy

The elucidation of the precise molecular structure and dynamics of biological processes is the great work of biochemistry. From this, insights into the changes leading to process dysfunction or disease are derived, as well as the possible approaches to restore healthy function. Translating this information into effective and safe treatments for disease requires a coordinated interdisciplinary effort, a fusion of creativity and practicality, and a healthy dose of luck. Using several reviews in this volume as springboards, I discuss the broader issues of drug development, highlighting some recent successes and future directions. Such occurrences inspire awe but remain too rare for comfort.

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.

Fishing for prion protein function

The prion protein is infamous for its role in devastating neurological diseases, but its normal, physiological function has remained mysterious. A new study uses the experimentally tractable zebrafish model to obtain fresh clues to this puzzle.

Structural features of the Zn(2+) complex with the single repeat region of "prion related protein" (PrP-rel-2) of zebrafish zPrP63-70 fragment

The interaction between Zn(2+) and the single repeat of PrP-rel-2 of zebrafish at physiological pH was investigated by NMR spectroscopy; the chemical shift mapping and the proton-proton distances were used to obtain the structural model of the Zn(2+) complex.

Early onset prion disease from octarepeat expansion correlates with copper binding properties

Insertional mutations leading to expansion of the octarepeat domain of the prion protein (PrP) are directly linked to prion disease. While normal PrP has four PHGGGWGQ octapeptide segments in its flexible N-terminal domain, expanded forms may have up to nine additional octapeptide inserts. The type of prion disease segregates with the degree of expansion. With up to four extra octarepeats, the average onset age is above 60 years, whereas five to nine extra octarepeats results in an average onset age between 30 and 40 years, a difference of almost three decades. In wild-type PrP, the octarepeat domain takes up copper (Cu²⁺) and is considered essential for in vivo function. Work from our lab demonstrates that the copper coordination mode depends on the precise ratio of Cu²⁺ to protein. At low Cu²⁺ levels, coordination involves histidine side chains from adjacent octarepeats, whereas at high levels each repeat takes up a single copper ion through interactions with the histidine side chain and neighboring backbone amides. Here we use both octarepeat constructs and recombinant PrP to examine how copper coordination modes are influenced by octarepeat expansion. We find that there is little change in affinity or coordination mode populations for octarepeat domains with up to seven segments (three inserts). However, domains with eight or nine total repeats (four or five inserts) become energetically arrested in the multi-histidine coordination mode, as dictated by higher copper uptake capacity and also by increased binding affinity. We next pooled all published cases of human prion disease resulting from octarepeat expansion and find remarkable agreement between the sudden length-dependent change in copper coordination and onset age. Together, these findings suggest that either loss of PrP copper-dependent function or loss of copper-mediated protection against PrP polymerization makes a significant contribution to early onset prion disease.

Prion diseases are neurodegenerative disorders involving the prion protein, a normal component of the central nervous system. An unusual class of inherited mutations giving rise to prion disease involves elongation of the so-called octarepeat domain, near the protein's N-terminus. Research from our lab and others shows that this domain binds the micronutrient copper, an essential element for proper neurological function. We investigated how octarepeat elongation influences copper binding by examining both the molecular features and the binding equilibrium. We find that elongation beyond a specific threshold, which confers profound early onset disease, gives rise to concomitant changes in copper uptake. The remarkable agreement between onset age and altered copper binding points to loss of copper protein function as significant in prion neurodegeneration.

Copper(II) coordination outside the tandem repeat region of an unstructured domain of chicken prion protein

Combined potentiometric, calorimetric and spectroscopic methods were used to investigate the Cu(2+) binding ability and coordination behaviour of some peptide fragments related to the neurotoxic region of chicken Prion Protein. The systems studied were the following protein fragments: chPrP(106-114), chPrP(119-126), chPrP(108-127), chPrP(105-127) and chPrP(105-133).The complex formation always starts around pH 4 with the coordination of an imidazole nitrogen, followed by the deprotonation and binding of amide nitrogens from the peptidic backbone. At neutral pH, the {N(im), 3N(-)} binding mode is the preferred one. The amide nitrogens participating in the binding to the Cu(2+) ion derive from residues from the N-terminus side, with the formation of a six-membered chelate ring with the imidazolic side chain.Comparison of thermodynamic data for the two histydyl binding domains (around His-110 and His-124), clearly indicates that the closest to the hexarepeat domain (His-110) has the highest ability to bind Cu(2+) ions, although both of them have the same coordination mode. Conversely, in the case of the human neurotoxic peptide region, between the two binding sites, located at His-96 and His-111, the farthest from the tandem repeat region is the strongest one. Finally, thermodynamic data show that chicken peptide is a distinctly better ligand for coordination of copper ions with respect to the human fragment.

Mapping of a copper-binding site on the small CP12 chloroplastic protein of Chlamydomonas reinhardtii using top-down mass spectrometry and site-directed mutagenesis

CP12 is a small chloroplastic protein involved in the Calvin cycle that was shown to bind copper, a metal ion that is involved in the transition of CP12 from a reduced to an oxidized state. In order to describe CP12's copper-binding properties, copper-IMAC experiments and site-directed mutagenesis based on computational modelling, were coupled with top-down MS [electrospray-ionization MS and MS/MS (tandem MS)]. Immobilized-copper-ion-affinity-chromatographic experiments allowed the primary characterization of the effects of mutation on copper binding. Top-down MS/MS experiments carried out under non-denaturing conditions on wild-type and mutant CP12–Cu2+ complexes then allowed fragment ions specifically binding the copper ion to be determined. Comparison of MS/MS datasets defined three regions involved in metal ion binding: residues Asp16–Asp23, Asp38–Lys50 and Asp70–Glu76, with the two first regions containing selected residues for mutation. These data confirmed that copper ligands involved glutamic acid and aspartic residues, a situation that contrasts with that obtaining for typical protein copper chelators. We propose that copper might play a role in the regulation of the biological activity of CP12.

Copper(II) complexes with peptide fragments encompassing the sequence 122-130 of human doppel protein

Copper(II) complexes of the peptide fragment (Dpl122-130) encompassing the sequence 122-130 of human doppel protein were characterized by potentiometric, UV-Visible, CD and EPR spectroscopic methods. An analogous peptide, in which the aspartate residue was substituted by an asparagine amino acid, was synthesized in order to provide evidence on the possible role of carboxylate group in copper(II) coordination. It was found that the carboxylic group is directly involved in copper(II) coordination at acidic pH, forming the CuLH(2) species with Dpl122-130. This copper(II) complex displayed EPR parameters very similar to those of the analogous complex with the whole doppel protein. At pH higher than 7, the complexes showed magnetic parameters similar to those of the major species of protein formed in the pH range 7-8, with the metal coordination environment consisting of one imidazole and three amide nitrogen atoms. The comparison of Cu-Dpl122-130 binding constant values with those of the prion peptide fragments (PrP106-114), showed that doppel peptide had a higher metal binding affinity at acidic pH whereas the prion peptide fragment binds the metal tightly at physiological pH.

Metal binding sheds light on mechanisms of amyloid assembly

Beta-2 microglobulin (beta2m) is the protein responsible for amyloid deposition in Dialysis-Related Amyloidosis (DRA). Aggregation can be induced by various solution conditions including exposure to divalent metal, incubation at acidic pH, and limited proteolysis. Using Cu(2+) as a trigger, we have trapped, isolated, and crystallized a stable oligomer of beta2m that is populated under amyloidogenic solution conditions (Calabrese et al. Nat Struct Mol Biol 2008; 15:965-71). This structure reveals that Cu(2+)-binding is associated with dramatic conformational rearrangements. This has allowed us to postulate a set of structural changes common to all beta2m aggregation pathways. Cu(2+) serves as a potential trigger in other aggregation systems such as Abeta, alpha-synuclein, and mammalian Prion (PrP). A comparison of Cu(2+) binding to beta2m and PrP reveals common features. Therefore, in addition to providing insight into DRA, induction of structure by Cu(2+) binding appears to be a recurring structural motif for pathological changes in conformation.

Can copper binding to the prion protein generate a misfolded form of the protein?

The native prion protein (PrP) has a two domain structure, with a globular folded alpha-helical C-terminal domain and a flexible extended N-terminal region. The latter can selectively bind Cu(2+) via four His residues in the octarepeat (OR) region, as well as two sites (His96 and His111) outside this region. In the disease state, the folded C-terminal domain of PrP undergoes a conformational change, forming amorphous aggregates high in beta-sheet content. Cu(2+) bound to the ORs can be redox active and has been shown to induce cleavage within the OR region, a process requiring conserved Trp residues. Using computational modeling, we have observed that electron transfer from Trp residues to copper can be favorable. These models also reveal that an indole-based radical cation or Cu(+) can initiate reactions leading to protein backbone cleavage. We have also demonstrated, by molecular dynamics simulations, that Cu(2+) binding to the His96 and His111 residues in the remaining PrP N-terminal fragment can induce localized beta-sheet structure, allowing us to suggest a potential mechanism for the initiation of beta-sheet misfolding in the C-terminal domain by Cu(2+).

Casein kinase II interacts with prion protein in vitro and forms complex with native prion protein in vivo

The most essential and crucial step during the pathogenesis of transmissible spongiform encephalopathy is the conformational change of cellular prion protein to pathologic isoform. Casein kinase II (CK2) is a ubiquitously expressed and evolutionarily conserved pleiotropic protein kinase that is essential for viability. To explore the possible molecular interaction between CK2 and prion protein (PrP), the full-length sequences of human CK2alpha and CK2beta complementary DNA were amplified with reverse transcription-polymerase chain reaction using the total messenger RNA from cell line SH-SY5Y as the template; then, the fusion proteins histidine-CK2alpha and glutathione S-transferase-histidine-CK2beta were expressed in Escherichia coli. The interaction between CK2 and PrP was evaluated with co-immunoprecipitation and pull-down assays. The results demonstrated that recombinant PrP bound specifically with CK2alpha, but not with CK2beta. The native CK2 and PrP in hamster brains interacted with each other, forming protein complexes. Three different glycosylated forms of PrP (diglycosylated, monoglycosylated and unglycosylated PrP) from normal brains interacted with the CK2alpha subunit, though the unglycosylated PrP seemed to have a stronger binding ability with CK2alpha subunit. The domain responsible for interacting with CK2alpha was located at the C-terminal segment of PrP (residues 91-231). This study proposed reliable experimental data for the molecular interaction between PrP and CK2alpha (both in recombinant and native categories), scientific clues for further assessing the potential biological significance of the PrP-CK2 interaction, and the possible role of CK2 in the pathogenesis of prion diseases.

Metal-dependent generation of reactive oxygen species from amyloid proteins implicated in neurodegenerative disease

Using a method based on ESR spectroscopy and spin-trapping, we have shown that Aβ (amyloid β-peptide) (implicated in Alzheimer's disease), α-synuclein (implicated in Parkinson's disease), ABri (British dementia peptide) (responsible for familial British dementia), certain toxic fragments of the prion protein (implicated in the transmissible spongiform encephalopathies) and the amylin peptide (found in the pancreas in Type 2 diabetes mellitus) all have the common ability to generate H2O2in vitro. Numerous controls (reverse, scrambled and non-toxic peptides) lacked this property. We have also noted a positive correlation between the ability of the various proteins tested to generate H2O2 and their toxic effects on cultured cells. In the case of Aβ and ABri, we have shown that H2O2 is generated as a short burst during the early stages of aggregation and is associated with the presence of protofibrils or oligomers, rather than mature fibrils. H2O2 is readily converted into the aggressive hydroxyl radical by Fenton chemistry, and this extremely reactive radical could be responsible for much of the oxidative damage seen in all of the above disorders. We suggest that the formation of a redox-active complex involving the relevant amyloidogenic protein and certain transition-metal ions could play an important role in the pathogenesis of several different protein misfolding disorders.

Hydration effects on the HET-s prion and amyloid-beta fibrillous aggregates, studied with three-dimensional molecular theory of solvation

We study the thermodynamic properties of the experimental fragments of the amyloid fibril made of the HET-s prion proteins (the infectious element of the filamentous fungus Podospora anserina) and of amyloid-beta proteins (the major component of Alzheimer's disease-associated plaques) by using the three-dimensional molecular theory of solvation. The full quantitative picture of hydration effects, including the hydration thermodynamics and hydration structure around the fragments, is presented. For both the complexes, the hydration entropic effects dominate, which results in the entropic part offsetting the unfavorable energetic part of the free energy change upon the association. This is in accord with the fact that the hydrophobic cooperativity plays an essential role in the formation of amyloid fibrils. By calculating the partial molar volume of the proteins, we found that the volume change upon the association in both the systems is large and positive, with the implication that high pressure causes destabilization of the fibril. This observation is in good agreement with the recent experimental results. We also found that both the HET-s and amyloid-beta pentamers have loose intermolecular packing with voids. The three-dimensional molecular theory of solvation predicts that water molecules can be locked in the interior cavities along the fibril axis for both the HET-s and amyloid-beta proteins. We provide a detailed molecular picture of the structural water localized in the interior of the fibrils. Our results suggest that the interior hydration plays an important role in the structural stability of fibrils.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Expansion of the octarepeat domain alters the misfolding pathway but not the folding pathway of the prion protein

A misfolded conformation of the prion protein (PrP), PrP (Sc), is the essential component of prions, the infectious agents that cause transmissible neurodegenerative diseases. Insertional mutations that lead to an increase in the number of octarepeats (ORs) in PrP are linked to familial human prion disease. In this study, we investigated how expansion of the OR domain causes PrP to favor a prion-like conformation. Therefore, we compared the conformational and aggregation modulating properties of wild-type versus expanded OR domains, either as a fusion construct with the protein G B1 domain (GB1-OR) or as an integral part of full-length mouse PrP (MoPrP). Using circular dichroism spectroscopy, we first demonstrated that ORs are not unfolded but exist as an ensemble of three distinct conformers: polyproline helix-like, beta-turn, and "Trp-related". Domain expansion had little effect on the conformation of GB1-OR fusion proteins. When part of MoPrP however, OR domain expansion changed PrP's folding landscape, not by hampering the production of native alpha-helical monomers but by greatly reducing the propensity to form amyloid and by altering the assembly of misfolded, beta-rich aggregates. These features may relate to subtle pH-dependent conformational differences between wild-type and mutant monomers. In conclusion, we propose that PrP insertional mutations are pathogenic because they enhance specific misfolding pathways of PrP rather than by undermining native folding. This idea was supported by a trial bioassay in transgenic mice overexpressing wild-type MoPrP, where intracerebral injection of recombinant MoPrP with an expanded OR domain but not wild-type MoPrP caused prion disease.

Copper(II) binding to alpha-synuclein, the Parkinson's protein

Variations in tryptophan fluorescence intensities confirm that copper(II) interacts with α-synuclein, a protein implicated in Parkinson’s disease. Trp4 fluorescence decay kinetics measured for the F4W protein show that Cu(II) binds tightly (K_d ∼ 100 nM) near the N-terminus at pH 7. Work on a F4W/H50S mutant indicates that a histidine imidazole is not a ligand in this high-affinity site.

Parallels between pathogens and gluten peptides in celiac sprue

Pathogens are exogenous agents capable of causing disease in susceptible organisms. In celiac sprue, a disease triggered by partially hydrolyzed gluten peptides in the small intestine, the offending immunotoxins cannot replicate, but otherwise have many hallmarks of classical pathogens. First, dietary gluten and its peptide metabolites are ubiquitous components of the modern diet, yet only a small, genetically susceptible fraction of the human population contracts celiac sprue. Second, immunotoxic gluten peptides have certain unusual structural features that allow them to survive the harsh proteolytic conditions of the gastrointestinal tract and thereby interact extensively with the mucosal lining of the small intestine. Third, they invade across epithelial barriers intact to access the underlying gut-associated lymphoid tissue. Fourth, they possess recognition sequences for selective modification by an endogenous enzyme, transglutaminase 2, allowing for in situ activation to a more immunotoxic form via host subversion. Fifth, they precipitate a T cell–mediated immune reaction comprising both innate and adaptive responses that causes chronic inflammation of the small intestine. Sixth, complete elimination of immunotoxic gluten peptides from the celiac diet results in remission, whereas reintroduction of gluten in the diet causes relapse. Therefore, in analogy with antibiotics, orally administered proteases that reduce the host's exposure to the immunotoxin by accelerating gluten peptide destruction have considerable therapeutic potential. Last but not least, notwithstanding the power of in vitro methods to reconstitute the essence of the immune response to gluten in a celiac patient, animal models for the disease, while elusive, are likely to yield fundamentally new systems-level insights.

Parallels between pathogens and gluten peptides in celiac sprue

Pathogens are exogenous agents capable of causing disease in susceptible organisms. In celiac sprue, a disease triggered by partially hydrolyzed gluten peptides in the small intestine, the offending immunotoxins cannot replicate, but otherwise have many hallmarks of classical pathogens. First, dietary gluten and its peptide metabolites are ubiquitous components of the modern diet, yet only a small, genetically susceptible fraction of the human population contracts celiac sprue. Second, immunotoxic gluten peptides have certain unusual structural features that allow them to survive the harsh proteolytic conditions of the gastrointestinal tract and thereby interact extensively with the mucosal lining of the small intestine. Third, they invade across epithelial barriers intact to access the underlying gut-associated lymphoid tissue. Fourth, they possess recognition sequences for selective modification by an endogenous enzyme, transglutaminase 2, allowing for in situ activation to a more immunotoxic form via host subversion. Fifth, they precipitate a T cell-mediated immune reaction comprising both innate and adaptive responses that causes chronic inflammation of the small intestine. Sixth, complete elimination of immunotoxic gluten peptides from the celiac diet results in remission, whereas reintroduction of gluten in the diet causes relapse. Therefore, in analogy with antibiotics, orally administered proteases that reduce the host's exposure to the immunotoxin by accelerating gluten peptide destruction have considerable therapeutic potential. Last but not least, notwithstanding the power of in vitro methods to reconstitute the essence of the immune response to gluten in a celiac patient, animal models for the disease, while elusive, are likely to yield fundamentally new systems-level insights.

Prion sequence polymorphisms and chronic wasting disease resistance in Illinois white-tailed deer (Odocoileus virginianus)

Nucleic acid sequences of the prion gene (PRNP) were examined and genotypes compiled for 76 white-tailed deer from northern Illinois, which previously tested positive for chronic wasting disease (CWD), and 120 negative animals selected to control for geographic location and age. Nine nucleotide polymorphisms, seven silent and two coding, were found in the sampled population. All observed polymorphisms except two of very low frequency were observed in both negative and positive animals, although five polymorphic loci had significantly different distributions of alleles between infected and non-infected individuals. Nucleotide base changes 60C/T, 285A/C, 286G/A and 555C/T were observed with higher than expected frequencies in CWD negative animals suggesting disease resistance, while 153C/T was observed more than expected in positive animals, suggesting susceptibility. The two coding polymorphisms, 285A/C (Q95H) and 286G/A (G96S), have been described in white-tailed deer populations sampled in Colorado and Wisconsin. Frequency distributions of coding polymorphisms in Wisconsin and Illinois deer populations were different, an unexpected result considering the sampled areas are less than 150 km apart. The total number of polymorphisms per animal, silent or coding, was negatively correlated to disease status. The potential importance of silent polymorphisms (60C/T, 153C/T, 555C/T), either individually or cumulatively, in CWD disease status has not been previously reported.

The prion protein is a combined zinc and copper binding protein: Zn2+ alters the distribution of Cu2+ coordination modes

PrP binds copper in the highly conserved, unstructured N-terminal half of the protein. The octarepeat region consists of 4 tandem repeats of PHGGGWGQ and binds four equivalents of copper at full occupancy. Adjacent to the octarepeats are two additional histidines that may also bind copper. We recently showed that when the octarepeat region is titrated with Cu2+, the copper binding mode depends on the number of equivalents of copper bound. In addition to copper, other metals have been associated with PrP, however zinc is the only metal other than copper that induces PrP endocytosis, inhibits fibril formation and promotes inter-molecular interactions. In this work we show that even large excesses of zinc (> 1mM) are unable to displace copper from either the octarepeat region or the full-length protein. However, EPR reveals that physiologically relevant levels of zinc significantly alter the distribution of copper among the available binding modes. Diethyl pyrocarbonate (DEPC) modification and Mass Spectrometry is used to identify the octarepeat region as the zinc binding site and to confirm that the affinity of PrP for zinc is ~200 μM. PrP can simultaneously bind both copper and zinc by shifting to binding modes that minimize the ratio of histidines to copper.