Prion protein binds copper within the physiological concentration range

Prion Protein Octarepeat Domain Forms Transient β-Sheet Structures upon Residue-Specific Binding to Cu(II) and Zn(II) Ions

Misfolding of the cellular prion protein (PrP^C) is associated with the development of fatal neurodegenerative diseases called transmissible spongiform encephalopathies (TSEs). Metal ions appear to play a crucial role in PrP^C misfolding. PrP^C is a combined Cu(II) and Zn(II) metal-binding protein, where the main metal-binding site is located in the octarepeat (OR) region. Thus, the biological function of PrP^C may involve the transport of divalent metal ions across membranes or buffering concentrations of divalent metal ions in the synaptic cleft. Recent studies have shown that an excess of Cu(II) ions can result in PrP^C instability, oligomerization, and/or neuroinflammation. Here, we have used biophysical methods to characterize Cu(II) and Zn(II) binding to the isolated OR region of PrP^C. Circular dichroism (CD) spectroscopy data suggest that the OR domain binds up to four Cu(II) ions or two Zn(II) ions. Binding of the first metal ion results in a structural transition from the polyproline II helix to the β-turn structure, while the binding of additional metal ions induces the formation of β-sheet structures. Fluorescence spectroscopy data indicate that the OR region can bind both Cu(II) and Zn(II) ions at neutral pH, but under acidic conditions, it binds only Cu(II) ions. Molecular dynamics simulations suggest that binding of either metal ion to the OR region results in the formation of β-hairpin structures. As the formation of β-sheet structures can be a first step toward amyloid formation, we propose that high concentrations of either Cu(II) or Zn(II) ions may have a pro-amyloid effect in TSE diseases.

Peptide aptamer targeting Aβ-PrP-Fyn axis reduces Alzheimer's disease pathologies in 5XFAD transgenic mouse model

Currently, no effective therapeutics exist for the treatment of incurable neurodegenerative diseases such as Alzheimer's disease (AD). The cellular prion protein (PrP^C) acts as a high-affinity receptor for amyloid beta oligomers (AβO), a main neurotoxic species mediating AD pathology. The interaction of AβO with PrP^C subsequently activates Fyn tyrosine kinase and neuroinflammation. Herein, we used our previously developed peptide aptamer 8 (PA8) binding to PrP^C as a therapeutic to target the AβO-PrP-Fyn axis and prevent its associated pathologies. Our in vitro results indicated that PA8 prevents the binding of AβO with PrP^C and reduces AβO-induced neurotoxicity in mouse neuroblastoma N2a cells and primary hippocampal neurons. Next, we performed in vivo experiments using the transgenic 5XFAD mouse model of AD. The 5XFAD mice were treated with PA8 and its scaffold protein thioredoxin A (Trx) at a 14.4 µg/day dosage for 12 weeks by intraventricular infusion through Alzet^® osmotic pumps. We observed that treatment with PA8 improves learning and memory functions of 5XFAD mice as compared to Trx-treated 5XFAD mice. We found that PA8 treatment significantly reduces AβO levels and Aβ plaques in the brain tissue of 5XFAD mice. Interestingly, PA8 significantly reduces AβO-PrP interaction and its downstream signaling such as phosphorylation of Fyn kinase, reactive gliosis as well as apoptotic neurodegeneration in the 5XFAD mice compared to Trx-treated 5XFAD mice. Collectively, our results demonstrate that treatment with PA8 targeting the AβO-PrP-Fyn axis is a promising and novel approach to prevent and treat AD.

Assessment of heavy metals and radionuclides in groundwater and associated human health risk appraisal in the vicinity of Rooppur nuclear power plant, Bangladesh

This study was carried out to assess the groundwater quality through estimating trace and heavy metal concentration and radionuclide levels in the vicinity of the Rooppur Nuclear Power Plant (RNPP) sites. Twenty-six (26) parameters, including major cations (K, Na, Mg, Ca) and anions (SO₄^-2, NO₃^-), trace and heavy metals (Mn, Fe, Zn, Ni, Co, Pb, Cd, As, Hg, Cu, Li, Be, B, V, Ga, Sr, Ag, Ba) and radionuclides (¹³⁷Cs, ²²⁶Ra, ²²⁸Ra, and ⁴⁰K) were estimated in water samples in the study area. This study revealed that the concentration values (μg/L) of Mn (667.091 ± 7.481), Fe (191.477 ± 3.756), Sr (105.218 ± 13.424), and Zn (23.493 ± 1.134) were the dominant metals in the study area. Different pollution evaluation indices (i.e., HPI, HEI, NI, C_d) data revealed that the study area was under a low to medium level of pollution due to the presence of metals in water. Subsequently, non-carcinogenic and carcinogenic health risks assessments for both adults and children were conducted, which indicated that health risk for the carcinogenic metals were below the threshold level except As through oral exposure for both adult and children. The activity concentrations of ²²⁶Ra, ²²⁸Ra, and ⁴⁰K were measured to demonstrate probable radioactivity pollution using Gamma-ray spectrometry (High-resolution HPGe detector). The highest activity concentration of ²²⁶Ra, ²²⁸Ra, and ⁴⁰K in groundwater samples were 4.9 ± 1.24 Bq/L (RNPP-15), 1.71 ± 0.43 Bq/L (RNPP-15), and 15.43 ± 3.08 Bq/L (RNPP-15). Among the three studied radionuclides, ⁴⁰K has the highest average activity concentration. The radiological indicators referred to the annual effective dose (AED) is 0.4273 mSv yr-1, which implies no significant cause of radiological risks and hazards (UNSCEAR guideline value). This study provides a baseline of trace and toxic metal contamination, radioactivity, and radiation levels in the groundwater of the nuclear power plant (being built) area.

Nutrient-sensing amyloid metastasis

Amyloids are organized suprastructural polypeptide arrangements. The prevalence of amyloid-related processes of pathophysiological relevance has been linked to aging-related degenerative diseases. Besides the role of genetic polymorphisms on the relative risk of amyloid diseases, the contributions of nongenetic ontogenic cluster of factors remain elusive. In recent decades, mounting evidences have been suggesting the role of essential micronutrients, in particular transition metals, in the regulation of amyloidogenic processes, both directly (such as binding to amyloid proteins) or indirectly (such as regulating regulatory partners, processing enzymes, and membrane transporters). The features of transition metals as regulatory cofactors of amyloid proteins and the consequences of metal dyshomeostasis in triggering amyloidogenic processes, as well as the evidences showing amelioration of symptoms by dietary supplementation, suggest an exaptative role of metals in regulating amyloid pathways. The self- and cross-talk replicative nature of these amyloid processes along with their systemic distribution support the concept of their metastatic nature. The role of amyloidosis as nutrient sensors would act as intra- and transgenerational epigenetic metabolic programming factors determining health span and life span, viability, which could participate as an evolutive selective pressure.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

The Crossroads between Host Copper Metabolism and Influenza Infection

Three main approaches are used to combat severe viral respiratory infections. The first is preemptive vaccination that blocks infection. Weakened or dead viral particles, as well as genetic constructs carrying viral proteins or information about them, are used as an antigen. However, the viral genome is very evolutionary labile and changes continuously. Second, chemical agents are used during infection and inhibit the function of a number of viral proteins. However, these drugs lose their effectiveness because the virus can rapidly acquire resistance to them. The third is the search for points in the host metabolism the effect on which would suppress the replication of the virus but would not have a significant effect on the metabolism of the host. Here, we consider the possibility of using the copper metabolic system as a target to reduce the severity of influenza infection. This is facilitated by the fact that, in mammals, copper status can be rapidly reduced by silver nanoparticles and restored after their cancellation.

Combination of Copper Ions and Nucleotide Generates Aggregates from Prion Protein Fragments in the N-Terminal Domain

BACKGROUND

It has been previously found that PrP_23-98, which contains four highly conserved octarepeats (residues 60-91) and one partial repeat (residues 92-96), polymerizes into amyloid-like and proteinase K-resistant spherical aggregates in the presence of NADPH plus copper ions.

OBJECTIVE

We aimed to determine the requirements for the formation of these aggregates.

METHODS

In this study, we performed an aggregation experiment using N-acetylated and Camidated PrP fragments of the N-terminal domain, Octa1, Octa2, Octa3, Octa4, PrP_84-114, and PrP_76-114, in the presence of NADPH with copper ions, and focused on the effect of the number of copper-binding sites on aggregation.

RESULTS

Among these PrP fragments, Octa4, containing four copper-binding sites, was particularly effective in forming aggregates. We also tested the effect of other pyridine nucleotides and adenine nucleotides on the aggregation of Octa4. ATP was equally effective, but NADH, NADP, ADP, and AMP had no effect.

CONCLUSION

The phosphate group on the adenine-linked ribose moiety of adenine nucleotides and pyridine nucleotides is presumed to be essential for the observed effect on aggregation. Efficient aggregation requires the presence of the four octarepeats. These insights may be helpful in the eventual development of therapeutic agents against prion-related disorders.

Dissecting molecular details and functional effects of the high-affinity copper binding site in plasminogen activator Inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the primary inhibitor for plasminogen activators, tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). As a unique member in the serine protease inhibitor (serpin) family, PAI-1 is metastable and converts to an inactive, latent structure with a half-life of 1-2 hr under physiological conditions. Unusual effects of metals on the rate of the latency conversion are incompletely understood. Previous work has identified two residues near the N-terminus, H2 and H3, which reside in a high-affinity copper-binding site in PAI-1 [Bucci JC, McClintock CS, Chu Y, Ware GL, McConnell KD, Emerson JP, Peterson CB (2017) J Biol Inorg Chem 22:1123-1,135]. In this study, neighboring residues, H10, E81, and H364, were tested as possible sites that participate in Cu(II) coordination at the high-affinity site. Kinetic methods, gel sensitivity assays, and isothermal titration calorimetry (ITC) revealed that E81 and H364 have different roles in coordinating metal and mediating the stability of PAI-1. H364 provides a third histidine in the metal-coordination sphere with H2 and H3. In contrast, E81 does not appear to be required for metal ligation along with histidines; contacts made by the side-chain carboxylate upon metal binding are perturbed and, in turn, influence dynamic fluctuations within the region encompassing helices D, E, and F and the W86 loop that are important in the pathway for the PAI-1 latency conversion. This investigation underscores a prominent role of protein dynamics, noncovalent bonding networks and ligand binding in controlling the stability of the active form of PAI-1.

Thermo-resistance of ESKAPE-panel pathogens, eradication and growth prevention of an infectious biofilm by photothermal, polydopamine-nanoparticles in vitro

Nanotechnology offers many novel infection-control strategies that may help prevent and treat antimicrobial-resistant bacterial infections. Here, we synthesized polydopamine, photothermal-nanoparticles (PDA-NPs) without further surface-functionalization to evaluate their potential with respect to biofilm-control. Most ESKAPE-panel pathogens in suspension with photothermal-nanoparticles showed three- to four-log-unit reductions upon Near-Infra-Red (NIR)-irradiation, but for enterococci only less than two-log unit reduction was observed. Exposure of existing Staphylococcus aureus biofilms to photothermal-nanoparticles followed by NIR-irradiation did not significantly kill biofilm-inhabitants. This indicates that the biofilm mode of growth poses a barrier to penetration of photothermal-nanoparticles, yielding dissipation of heat to the biofilm-surrounding rather than in its interior. Staphylococcal biofilm-growth in the presence of photothermal-nanoparticles could be significantly prevented after NIR-irradiation because PDA-NPs were incorporated in the biofilm and heat dissipated inside it. Thus, unmodified photothermal nanoparticles have potential for prophylactic infection-control, but data also constitute a warning for possible development of thermo-resistance in infectious pathogens.

Using NMR spectroscopy to investigate the role played by copper in prion diseases

Prion diseases are a group of rare neurodegenerative disorders that develop as a result of the conformational conversion of normal prion protein (PrPC) to the disease-associated isoform (PrPSc). The mechanism that actually causes disease remains unclear. However, the mechanism underlying the conformational transformation of prion protein is partially understood-in particular, there is strong evidence that copper ions play a significant functional role in prion proteins and in their conformational conversion. Various models of the interaction of copper ions with prion proteins have been proposed for the Cu (II)-binding, cell-surface glycoprotein known as prion protein (PrP). Changes in the concentration of copper ions in the brain have been associated with prion diseases and there is strong evidence that copper plays a significant functional role in the conformational conversion of PrP. Nevertheless, because copper ions have been shown to have both a positive and negative effect on prion disease onset, the role played by Cu (II) ions in these diseases remains a topic of debate. Because of the unique properties of paramagnetic Cu (II) ions in the magnetic field, their interactions with PrP can be tracked even at single atom resolution using nuclear magnetic resonance (NMR) spectroscopy. Various NMR approaches have been utilized to study the kinetic, thermodynamic, and structural properties of Cu (II)-PrP interactions. Here, we highlight the different models of copper interactions with PrP with particular focus on studies that use NMR spectroscopy to investigate the role played by copper ions in prion diseases.

PrP (58-93) peptide from unstructured N-terminal domain of human prion protein forms amyloid-like fibrillar structures in the presence of Zn2+ ions

Many transition metal ions modulate the aggregation of different amyloid peptides. Substoichiometric zinc concentrations can inhibit aggregation, while an excess of zinc can accelerate the formation of cytotoxic fibrils. In this study, we report the fibrillization of the octarepeat domain to amyloid-like structures. Interestingly, this self-assembling process occurred only in the presence of Zn(ii) ions. The formed peptide aggregates are able to bind amyloid specific dyes thioflavin T and Congo red. Atomic force microscopy and transmission electron microscopy revealed the formation of long, fibrillar structures. X-ray diffraction and Fourier transform infrared spectroscopy studies of the formed assemblies confirmed the presence of cross-β structure. Two-component analysis of synchrotron radiation SAXS data provided the evidence for a direct decrease in monomeric peptide species content and an increase in the fraction of aggregates as a function of Zn(ii) concentration. These results could shed light on Zn(ii) as a toxic agent and on the metal ion induced protein misfolding in prion diseases.

Cellular prion protein-mediated protection against influenza A virus infection

The cellular prion protein, termed PrPC, is a glycoprotein abundantly expressed in brains and to a lesser extent in non-neuronal tissues including lungs. It was reported that PrPC is expressed by lung epithelial cells in mice, and that it may play a protective role against lethal infection with influenza A viruses (IAVs). This may occur by regulating Cu content and  superoxide dismutase (SOD) activity, eventually reducing oxidative stress in infected lungs. Antioxidative therapeutics have been demonstrated to protect mice from lethal infection with IAVs. Therefore, PrPC might be a new target molecule for development of IAV therapeutics. Here, we introduce the antiviral mechanism of PrPC against IAV infection and discuss perspectives of PrPC-targeting therapeutics against IAV infection.

Prion protein protects mice from lethal infection with influenza A viruses

The cellular prion protein, designated PrP^C, is a membrane glycoprotein expressed abundantly in brains and to a lesser extent in other tissues. Conformational conversion of PrP^C into the amyloidogenic isoform is a key pathogenic event in prion diseases. However, the physiological functions of PrP^C remain largely unknown, particularly in non-neuronal tissues. Here, we show that PrP^C is expressed in lung epithelial cells, including alveolar type 1 and 2 cells and bronchiolar Clara cells. Compared with wild-type (WT) mice, PrP^C-null mice (Prnp^0/0) were highly susceptible to influenza A viruses (IAVs), with higher mortality. Infected Prnp^0/0 lungs were severely injured, with higher inflammation and higher apoptosis of epithelial cells, and contained higher reactive oxygen species (ROS) than control WT lungs. Treatment with a ROS scavenger or an inhibitor of xanthine oxidase (XO), a major ROS-generating enzyme in IAV-infected lungs, rescued Prnp^0/0 mice from the lethal infection with IAV. Moreover, Prnp^0/0 mice transgenic for PrP with a deletion of the Cu-binding octapeptide repeat (OR) region, Tg(PrPΔOR)/Prnp^0/0 mice, were also highly susceptible to IAV infection. These results indicate that PrP^C has a protective role against lethal infection with IAVs through the Cu-binding OR region by reducing ROS in infected lungs. Cu content and the activity of anti-oxidant enzyme Cu/Zn-dependent superoxide dismutase, SOD1, were lower in Prnp^0/0 and Tg(PrPΔOR)/Prnp^0/0 lungs than in WT lungs. It is thus conceivable that PrP^C functions to maintain Cu content and regulate SOD1 through the OR region in lungs, thereby reducing ROS in IAV-infected lungs and eventually protecting them from lethal infection with IAVs. Our current results highlight the role of PrP^C in protection against IAV infection, and suggest that PrP^C might be a novel target molecule for anti-influenza therapeutics.

Influenza A virus (IAV) is an enveloped, negative sense, single-stranded RNA virus, causing seasonal epidemic outbreaks of influenza. Anti-influenza agents targeting viral molecules, such as neuraminidase inhibitors, are currently available. However, these agents have accelerated emergence of mutant IAVs that are resistant to these agents among human populations. Development of new types of anti-influenza agents is awaited. We show that the cellular prion protein PrP^C has a protective role against lethal infection with IAVs through the octapeptide repeat (OR) region by abrogating lung epithelial cell apoptosis induced by reactive oxygen species (ROS) in infected lungs. We also show that PrP^C could reduce ROS in IAV-infected lungs through the OR region by maintaining Cu ion homeostasis and thereby activating Cu/Zn-dependent superoxide dismutase, SOD1. These results highlight the protective role of PrP^C in IAV infection. Elucidation of the exact mechanism underlying the PrP^C-mediated protection against IAV infection would be important for further understanding the pathogenesis of IAV infection and could be useful for development of new types of anti-influenza therapeutics.

Resolving distinct molecular origins for copper effects on PAI-1

Components of the fibrinolytic system are subjected to stringent control to maintain proper hemostasis. Central to this regulation is the serpin plasminogen activator inhibitor-1 (PAI-1), which is responsible for specific and rapid inhibition of fibrinolytic proteases. Active PAI-1 is inherently unstable and readily converts to a latent, inactive form. The binding of vitronectin and other ligands influences stability of active PAI-1. Our laboratory recently observed reciprocal effects on the stability of active PAI-1 in the presence of transition metals, such as copper, depending on the whether vitronectin was also present (Thompson et al. Protein Sci 20:353–365, 2011). To better understand the molecular basis for these copper effects on PAI-1, we have developed a gel-based copper sensitivity assay that can be used to assess the copper concentrations that accelerate the conversion of active PAI-1 to a latent form. The copper sensitivity of wild-type PAI-1 was compared with variants lacking N-terminal histidine residues hypothesized to be involved in copper binding. In these PAI-1 variants, we observed significant differences in copper sensitivity, and these data were corroborated by latency conversion kinetics and thermodynamics of copper binding by isothermal titration calorimetry. These studies identified a copper-binding site involving histidines at positions 2 and 3 that confers a remarkable stabilization of PAI-1 beyond what is observed with vitronectin alone. A second site, independent from the two histidines, binds metal and increases the rate of the latency conversion.

Stable Europium(III) Complexes with Short Linkers for Site-Specific Labeling of Biomolecules

In this study, two new terpyridine-based Eu^III complexes were synthesized, the structures of which were optimized for luminescence resonance energy-transfer (LRET) experiments. The complexes showed high quantum yields (32 %); a single long lifetime (1.25 ms), which was not influenced by coupling to protein; very high stability in the presence of chelators such as ethylenediamine-N,N,N',N'-tetraacetate and ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid; and no interaction with cofactors such as adenosine triphosphate and guanosine triphosphate. A special feature is the short length of the linker between the Eu^III ion and the maleimide or hydrazide function, which allows for site-specific coupling of cysteine mutants or unnatural keto amino acids. As a consequence, the new complexes appear particularly suited for accurate distance measurements in biomolecules by LRET.

Copper- and Zinc-Promoted Interdomain Structure in the Prion Protein: A Mechanism for Autoinhibition of the Neurotoxic N-Terminus

The function of the cellular prion protein (PrP^C), while still poorly understood, is increasingly linked to its ability to bind physiological metal ions at the cell surface. PrP^C binds divalent forms of both copper and zinc through its unstructured N-terminal domain, modulating interactions between PrP^C and various receptors at the cell surface and ultimately tuning downstream cellular processes. In this chapter, we briefly discuss the molecular features of copper and zinc uptake by PrP^C and summarize evidence implicating these metal ions in PrP-mediated physiology. We then focus our review on recent biophysical evidence revealing a physical interaction between the flexible N-terminal and globular C-terminal domains of PrP^C. This interdomain cis interaction is electrostatic in nature and is promoted by the binding of Cu²⁺ and Zn²⁺ to the N-terminal octarepeat domain. These findings, along with recent cellular studies, suggest a mechanism whereby NC interactions serve to regulate the activity and/or toxicity of the PrP^C N-terminus. We discuss this potential mechanism in relation to familial prion disease mutations, lethal deletions of the PrP^C central region, and neurotoxicity induced by certain globular domain ligands, including bona fide prions and toxic amyloid-β oligomers.

Mechanism of Unfolding of Human Prion Protein

Misfolding and aggregation of prion proteins are associated with several neurodegenerative diseases. Therefore, understanding the mechanism of the misfolding process is of enormous interest in the scientific community. It has been speculated and widely discussed that the native cellular prion protein (PrP^C) form needs to undergo substantial unfolding to a more stable PrP^C* state, which may further oligomerize into the toxic scrapie (PrP^Sc) form. Here, we have studied the mechanism of the unfolding of the human prion protein (huPrP) using a set of extensive well-tempered metadynamics simulations. Through multiple microsecond-long metadynamics simulations, we find several possible unfolding pathways. We show that each pathway leads to an unfolded state of lower free energy than the native state. Thus, our study may point to the signature of a PrP^C* form that corresponds to a global minimum on the conformational free-energy landscape. Moreover, we find that these global minima states do not involve an increased β-sheet content, as was assumed to be a signature of PrP^Sc formation in previous simulation studies. We have further analyzed the origin of metastability of the PrP^C form through free-energy surfaces of the chopped helical segments to show that the helices, particularly H2 and H3 of the prion protein, have the tendency to form either a random coil or a β-structure. Therefore, the secondary structural elements of the prion protein are only weakly stabilized by tertiary contacts and solvation forces so that relatively weak perturbations induced by temperature, pressure, pH, and so forth can lead to substantial unfolding with characteristics of intrinsically disordered proteins.

Dual role of cellular prion protein in normal host and Alzheimer's disease

Using PrPC-knockout cell lines, it has been shown that the inhibition of apoptosis through STI1 is mediated by PrPC-dependent SOD activation. Antioxidant PrPC may contribute to suppression of inflammasome activation. PrPC is functionally involved in copper metabolism, signal transduction, neuroprotection, and cell maturation. Recently several reports have shown that PrPC participates in trans-membrane signaling processes associated with hematopoietic stem cell replication and neuronal differentiation. In another role, PrPC also tends to function as a neurotoxic protein. Aβ oligomer, which is associated with neurodegeneration in Alzheimer's disease (AD), has also been reported to act as a ligand of PrPC. However, the physiological role of PrPC as an Aβ42-binding protein is not clear. Actually, PrPC is critical in Aβ42-mediated autophagy in neurons. PrPC shows a beneficial role in lipid rafts to promote autophagy. Further search for PrPC-interaction molecules using Prnp-/- mice and various types of Prnp-/- cell lines under various conditions may elucidate other important PrPC important functions.

Are Charge-State Distributions a Reliable Tool Describing Molecular Ensembles of Intrinsically Disordered Proteins by Native MS?

Native mass spectrometry (MS) has become a central tool of structural proteomics, but its applicability to the peculiar class of intrinsically disordered proteins (IDPs) is still object of debate. IDPs lack an ordered tridimensional structure and are characterized by high conformational plasticity. Since they represent valuable targets for cancer and neurodegeneration research, there is an urgent need of methodological advances for description of the conformational ensembles populated by these proteins in solution. However, structural rearrangements during electrospray-ionization (ESI) or after the transfer to the gas phase could affect data obtained by native ESI-MS. In particular, charge-state distributions (CSDs) are affected by protein conformation inside ESI droplets, while ion mobility (IM) reflects protein conformation in the gas phase. This review focuses on the available evidence relating IDP solution ensembles with CSDs, trying to summarize cases of apparent consistency or discrepancy. The protein-specificity of ionization patterns and their responses to ligands and buffer conditions suggests that CSDs are imprinted to protein structural features also in the case of IDPs. Nevertheless, it seems that these proteins are more easily affected by electrospray conditions, leading in some cases to rearrangements of the conformational ensembles. Graphical Abstract ᅟ.

Copper-induced structural conversion templates prion protein oligomerization and neurotoxicity

Prion protein (PrP) misfolding and oligomerization are key pathogenic events in prion disease. Copper exposure has been linked to prion pathogenesis; however, its mechanistic basis is unknown. We resolve, with single-molecule precision, the molecular mechanism of Cu(2+)-induced misfolding of PrP under physiological conditions. We also demonstrate that misfolded PrPs serve as seeds for templated formation of aggregates, which mediate inflammation and degeneration of neuronal tissue. Using a single-molecule fluorescence assay, we demonstrate that Cu(2+) induces PrP monomers to misfold before oligomer assembly; the disordered amino-terminal region mediates this structural change. Single-molecule force spectroscopy measurements show that the misfolded monomers have a 900-fold higher binding affinity compared to the native isoform, which promotes their oligomerization. Real-time quaking-induced conversion demonstrates that misfolded PrPs serve as seeds that template amyloid formation. Finally, organotypic slice cultures show that misfolded PrPs mediate inflammation and degeneration of neuronal tissue. Our study establishes a direct link, at the molecular level, between copper exposure and PrP neurotoxicity.

Specific binding modes of Cu(I) and Ag(I) with neurotoxic domain of the human prion protein

Prion diseases are neurodegenerative disorders associated with a conformational change of the normal cellular isoform of the prion protein (PrP(C)) to an abnormal scrapie isoform (PrP(Sc)). human prion protein (hPrP(C)) is able to bind up to six Cu(II) ions. Four of them are distributed in the octarepeat domain, containing four tandem-repetitions of the sequence PHGGGWGQ. Immediately outside the octarepeat domain, in so called PrP amyloidogenic region, two additional and independent Cu(II) binding sites, encompassing His96 and His111 residues, respectively, are present. Considering the potential involvement of PrP in cellular redox homeostasis, investigations on Cu(I)-PrP interaction might be also biologically relevant. Interestingly, the amyloidogenic fragment of PrP contains a -M(X)nM- motif, known to act as Cu(I) binding site in different proteins. In order to shed more light on this issue, copper(I) and silver(I) interactions with model peptides derived from that region were analyzed. The results of our studies reveal that both metal ions are anchored to two thioether sulfurs of Met109 and Met112, respectively. Subsequent metal interaction and coordination to His96 and His111 imidazoles are primarily found for Cu(I) at physiological pH. Metal binding was also investigated in the presence of negatively charged micelles formed by the anionic surfactant, sodium dodecyl sulfate (SDS). Our results strongly support that metal binding mode strongly depends on the protein backbone structure. In particular we show that α-helix structuring of the amyloid PrP domain influences both the metal coordination sphere and the binding affinity.

Behind the Link between Copper and Angiogenesis: Established Mechanisms and an Overview on the Role of Vascular Copper Transport Systems

Angiogenesis critically sustains the progression of both physiological and pathological processes. Copper behaves as an obligatory co-factor throughout the angiogenic signalling cascades, so much so that a deficiency causes neovascularization to abate. Moreover, the progress of several angiogenic pathologies (e.g. diabetes, cardiac hypertrophy and ischaemia) can be tracked by measuring serum copper levels, which are being increasingly investigated as a useful prognostic marker. Accordingly, the therapeutic modulation of body copper has been proven effective in rescuing the pathological angiogenic dysfunctions underlying several disease states. Vascular copper transport systems profoundly influence the activation and execution of angiogenesis, acting as multi-functional regulators of apparently discrete pro-angiogenic pathways. This review concerns the complex relationship among copper-dependent angiogenic factors, copper transporters and common pathological conditions, with an unusual accent on the multi-faceted involvement of the proteins handling vascular copper. Functions regulated by the major copper transport proteins (CTR1 importer, ATP7A efflux pump and metallo-chaperones) include the modulation of endothelial migration and vascular superoxide, known to activate angiogenesis within a narrow concentration range. The potential contribution of prion protein, a controversial regulator of copper homeostasis, is discussed, even though its angiogenic involvement seems to be mainly associated with the modulation of endothelial motility and permeability.

Health risk assessment of heavy metals and metalloid in drinking water from communities near gold mines in Tarkwa, Ghana

Concentrations of heavy metals and metalloid in borehole drinking water from 18 communities in Tarkwa, Ghana, were measured to assess the health risk associated with its consumption. Mean concentrations of heavy metals (μg/L) exceeded recommended values in some communities. If we take into consideration the additive effect of heavy metals and metalloid, then oral hazard index (HI) results raise concerns about the noncarcinogenic adverse health effects of drinking groundwater in Huniso. According to the US Environmental Protection Agency's (USEPA) guidelines, HI values indicating noncarcinogenic health risk for adults and children in Huniso were 0.781 (low risk) and 1.08 (medium risk), respectively. The cancer risk due to cadmium (Cd) exposure in adults and children in the sampled communities was very low. However, the average risk values of arsenic (As) for adults and children through drinking borehole water in the communities indicated medium cancer risk, but high cancer risk in some communities such as Samahu and Mile 7. Based on the USEPA assessment, the average cancer risk values of As for adults (3.65E-05) and children (5.08E-05) indicated three (adults) and five (children) cases of neoplasm in a hundred thousand inhabitants. The results of this study showed that residents in Tarkwa who use and drink water from boreholes could be at serious risk from exposure to these heavy metals and metalloid.

The intricate mechanisms of neurodegeneration in prion diseases

Prion diseases are a group of infectious neurodegenerative diseases with an entirely novel mechanism of transmission, involving a protein-only infectious agent that propagates the disease by transmitting protein conformational changes. The disease results from extensive and progressive brain degeneration. The molecular mechanisms involved in neurodegeneration are not entirely known but involve multiple processes operating simultaneously and synergistically in the brain, including spongiform degeneration, synaptic alterations, brain inflammation, neuronal death and the accumulation of protein aggregates. Here, we review the pathways implicated in prion-induced brain damage and put the pieces together into a possible model of neurodegeneration in prion disorders. A more comprehensive understanding of the molecular basis of brain degeneration is essential to develop a much needed therapy for these devastating diseases.

Octarepeat region flexibility impacts prion function, endoproteolysis and disease manifestation

The cellular prion protein (PrP(C)) comprises a natively unstructured N-terminal domain, including a metal-binding octarepeat region (OR) and a linker, followed by a C-terminal domain that misfolds to form PrP(S) (c) in Creutzfeldt-Jakob disease. PrP(C) β-endoproteolysis to the C2 fragment allows PrP(S) (c) formation, while α-endoproteolysis blocks production. To examine the OR, we used structure-directed design to make novel alleles, 'S1' and 'S3', locking this region in extended or compact conformations, respectively. S1 and S3 PrP resembled WT PrP in supporting peripheral nerve myelination. Prion-infected S1 and S3 transgenic mice both accumulated similar low levels of PrP(S) (c) and infectious prion particles, but differed in their clinical presentation. Unexpectedly, S3 PrP overproduced C2 fragment in the brain by a mechanism distinct from metal-catalysed hydrolysis reported previously. OR flexibility is concluded to impact diverse biological endpoints; it is a salient variable in infectious disease paradigms and modulates how the levels of PrP(S) (c) and infectivity can either uncouple or engage to drive the onset of clinical disease.

Prion protein (PrP) gene-knockout cell lines: insight into functions of the PrP

Elucidation of prion protein (PrP) functions is crucial to fully understand prion diseases. A major approach to studying PrP functions is the use of PrP gene-knockout (Prnp (-/-)) mice. So far, six types of Prnp (-/-) mice have been generated, demonstrating the promiscuous functions of PrP. Recently, other PrP family members, such as Doppel and Shadoo, have been found. However, information obtained from comparative studies of structural and functional analyses of these PrP family proteins do not fully reveal PrP functions. Recently, varieties of Prnp (-/-) cell lines established from Prnp (-/-) mice have contributed to the analysis of PrP functions. In this mini-review, we focus on Prnp (-/-) cell lines and summarize currently available Prnp (-/-) cell lines and their characterizations. In addition, we introduce the recent advances in the methodology of cell line generation with knockout or knockdown of the PrP gene. We also discuss how these cell lines have provided valuable insights into PrP functions and show future perspectives.

Calorimetric investigation of copper binding in the N-terminal region of the prion protein at low copper loading: evidence for an entropically favorable first binding event

Although the Cu²⁺-binding sites of the prion protein have been well studied when the protein is fully saturated by Cu²⁺, the Cu²⁺-loading mechanism is just beginning to come into view. Because the Cu²⁺-binding modes at low and intermediate Cu²⁺ occupancy necessarily represent the highest-affinity binding modes, these are very likely populated under physiological conditions, and it is thus essential to characterize them in order to understand better the biological function of copper–prion interactions. Besides binding-affinity data, almost no other thermodynamic parameters (e.g., ΔH and ΔS) have been measured, thus leaving undetermined the enthalpic and entropic factors that govern the free energy of Cu²⁺ binding to the prion protein. In this study, isothermal titration calorimetry (ITC) was used to quantify the thermodynamic parameters (K, ΔG, ΔH, and TΔS) of Cu²⁺ binding to a peptide, PrP(23–28, 57–98), that encompasses the majority of the residues implicated in Cu²⁺ binding by full-length PrP. Use of the buffer N-(2-acetomido)-aminoethanesulfonic acid (ACES), which is also a well-characterized Cu²⁺ chelator, allowed for the isolation of the two highest affinity binding events. Circular dichroism spectroscopy was used to characterize the different binding modes as a function of added Cu²⁺. The K_d values determined by ITC, 7 and 380 nM, are well in line with those reported by others. The first binding event benefits significantly from a positive entropy, whereas the second binding event is enthalpically driven. The thermodynamic values associated with Cu²⁺ binding by the Aβ peptide, which is implicated in Alzheimer’s disease, bear striking parallels to those found here for the prion protein.

The thermodynamics (K, ΔG, ΔH, and TΔS) of the two highest affinity Cu²⁺-binding events of the prion protein were investigated using isothermal titration calorimetry. Peptide PrP(23−28, 57−98) was used as a model system for the metal-binding region. The first binding event had a K_d of 7 nM and was entropically driven (+ΔS), whereas the second binding event had a K_d of 380 nm and was enthalpically driven (−ΔH).

Senescence suppressors: their practical importance in replicative lifespan extension in stem cells

Recent animal and clinical studies report promising results for the therapeutic utilization of stem cells in regenerative medicine. Mesenchymal stem cells (MSCs), with their pluripotent nature, have advantages over embryonic stem cells in terms of their availability and feasibility. However, their proliferative activity is destined to slow by replicative senescence, and the limited proliferative potential of MSCs not only hinders the preparation of sufficient cells for in vivo application, but also draws a limitation on their potential for differentiation. This calls for the development of safe and efficient means to increase the proliferative as well as differentiation potential of MSCs. Recent advances have led to a better understanding of the underlying mechanisms and significance of cellular senescence, facilitating ways to manipulate the replicative lifespan of a variety of primary cells, including MSCs. This paper introduces a class of proteins that function as senescence suppressors. Like tumor suppressors, these proteins are lost in senescence, while their forced expression delays the onset of senescence. Moreover, treatments that increase the expression or the activity of senescence suppressors, therefore, cause expansion of the replicative and differentiation potential of MSCs. The nature of the activities and putative underlying mechanisms of the senescence suppressors will be discussed to facilitate their evaluation.

The prion protein family: a view from the placenta

Based on its developmental pattern of expression, early studies suggested the implication of the mammalian Prion protein PrP, a glycosylphosphatidylinositol-anchored ubiquitously expressed and evolutionary conserved glycoprotein encoded by the Prnp gene, in early embryogenesis. However, gene invalidation in several species did not result in obvious developmental abnormalities and it was only recently that it was associated in mice with intra-uterine growth retardation and placental dysfunction. A proposed explanation for this lack of easily detectable developmental-related phenotype is the existence in the genome of one or more gene (s) able to compensate for the absence of PrP. Indeed, two other members of the Prnp gene family have been recently described, Doppel and Shadoo, and the consequences of their invalidation alongside that of PrP tested in mice. No embryonic defect was observed in mice depleted for Doppel and PrP. Interestingly, the co-invalidation of PrP and Shadoo in two independent studies led to apparently conflicting observations, with no apparent consequences in one report and the observation of a developmental defect of the ectoplacental cone that leads to early embryonic lethality in the other. This short review aims at summarizing these recent, apparently conflicting data highlighting the related biological questions and associated implications in terms of animal and human health.

The many "faces" of copper in medicine and treatment

Copper (Cu) is an essential microelement found in all living organisms with the unique ability to adopt two different redox states-in the oxidized (Cu(2+)) and reduced (Cu(+)). It is required for survival and serves as an important catalytic cofactor in redox chemistry for proteins that carry out fundamental biological functions, important in growth and development. The deficit of copper can result in impaired energy production, abnormal glucose and cholesterol metabolism, increased oxidative damage, increased tissue iron (Fe) accrual, altered structure and function of circulating blood and immune cells, abnormal neuropeptides synthesis and processing, aberrant cardiac electrophysiology, impaired myocardial contractility, and persistent effects on the neurobehavioral and the immune system. Increased copper level has been found in several disorders like e.g.: Wilson's disease or Menke's disease. New findings with the great potential for impact in medicine include the use of copper-lowering therapy for antiangiogenesis, antifibrotic and anti-inflammatory purposes. The role of copper in formation of amyloid plaques in Alzheimer's disease, and successful treatment of this disorder in rodent model by copper chelating are also of interest. In this work we will try to describe essential aspects of copper in chosen diseases. We will represent the evidence available on adverse effect derived from copper deficiency and copper excess. We will try to review also the copper biomarkers (chosen enzymes) that help reflect the level of copper in the body.

Evidence for synergistic effects of PRNP and ATP7B mutations in severe neuropsychiatric deterioration

Background

Wilson’s disease (WD), a rare cause of neuropsychiatric deterioration, is associated with mutations in the ATP7B gene. Prion diseases are also rare causes of neuropsychiatric deterioration that can occur sporadically without an identifiable cause, or can be attributed to mutations in the PRNP gene.

Case presentation

Here we describe a biological “experiment of nature” in which a patient presented with severe neuropsychiatric decline and strong biochemical evidence of WD. Genetic analysis revealed that he was a compound heterozygote for two ATP7B sequence variants (c.2165dupT, p.Arg723Glufs*32; and c.4039G > A, p.Gly1347Ser), the first having been reported once previously, and the second being novel. In addition, the patient was heterozygous for a PRNP variant, c.160G > A, p.Gly54Ser, that has been reported in a neuropsychiatric patient only once previously in association with a similarly severe clinical course of neuropsychiatric disease and early age of onset, but no accompanying information on ATP7B genotype. Of particular interest was the observation that the patient’s older sister, who carried the same ATP7B genotype and laboratory evidence for biochemical WD but was clinically asymptomatic, lacked the PRNP variant allele.

Conclusions

We propose that synergism may occur between at least some allelic variants of ATP7B and PRNP, possibly exerted through effects on cellular copper metabolism.

Bovine Spongiform Encephalopathy in Japan: History and Recent Studies on Oxidative Stress in Prion Diseases

With the respect to BSE and vCJD, compliance with the following three rules should strictly be observed: (i) Identification and destruction of all clinically affected cattle; (ii) destruction of all mammalian proteins used in feeding ruminant livestock; and (iii) destruction of all high-risk tissues for use in human consumption. Scrapie in sheep has been documented in the 18th century in the United Kingdom. Through studies of brain-to-brain transmission in the same species in 1935, Cuille et al. successfully isolated the culprit protein from the sheep brain. To transmit said protein from an animal to another, intracerebral inoculation was much more efficient than intraperitoneal or oral route in certain species; i.e. the hamster and mouse. Since discovery of the more efficacious infection route, studies and development of prion research have undergone 4 developmental phases. Phase I depicted discoveries of the pathological features of Creutzfeldt-Jakob disease (CJD) and scrapie with typical lesions of spongiform encephalopathy, while Phase II revealed individual-to-individual (or cross-species) transmissions of CJD, kuru and scrapie in animals. Phases I and II suggested the possible participation of a slow virus in the infection process. In Phase III, Prusiner et al. proposed the 'prion' theory in 1982, followed by the milestone development of the transgenic or gene-targeted mouse in prion research in Phase IV. By strain-typing of prions, CJD has been classified as type 2 or 4 by Parchi et al. and Wadsworth as type-2 or -4 and type-1 or -2, respectively. Wadsworth type 1 is detected in the cerebellum, while Wadsworth type 2 was detected in the prefrontal cortex of 10% of sporadic CJD patients. In 1999, Puoti et al. have reported the co-existence of two types of PrP(res) in a same patient. These reports indicated that PrP(res)-typing is a quantitative rather than a qualitative process, and the relationship between the molecular type and the prion strain is rather complex. In fact, previous findings of Truchot have correlated type-1 distribution with synaptic deposits, and type-2 with arrangement of diffuse deposits in neurons. Although the normal function of PrP(C) has not been fully understood, recent studies have shown that PrP(C) plays a role in copper metabolism, signal transduction, neuroprotection and cell maturation. Further search of PrP(C)-interacting molecules and detailed studies using Prnp(-/-) mice and various type of Prnp(-/-) cell lines under various conditions are the prerequisites in elucidating PrP functions. In the pathogenesis of prion diseases, present results support the hypothesis that 'loss-of-function' of PrP(C) decreases resistance to oxidative stress, and 'gain-of-function' of PrP(Sc) increases oxidative stress. The mechanisms of (i) the 'loss-of-function' of PrP(C) in enhanced susceptibility to oxidative stress and (ii) the 'gain-of-function' of PrP(Sc) in generation of oxidative stress remain to be elucidated, although their mechanisms of action, at least in part, involve the decrease and increase in SOD activity, respectively.

Prion disease: a tale of folds and strains

Research on prions, the infectious agents of devastating neurological diseases in humans and animals, has been in the forefront of developing the concept of protein aggregation diseases. Prion diseases are distinguished from other neurodegenerative diseases by three peculiarities. First, prion diseases, in addition to being sporadic or genetic like all other neurodegenerative diseases, are infectious diseases. Animal models were developed early on (a long time before the advent of transgenic technology), and this has made possible the discovery of the prion protein as the infectious agent. Second, human prion diseases have true equivalents in animals, such as scrapie, which has been the subject of experimental research for many years. Variant Creutzfeldt-Jakob disease (vCJD) is a zoonosis caused by bovine spongiform encephalopathy (BSE) prions. Third, they show a wide variety of phenotypes in humans and animals, much wider than the variants of any other sporadic or genetic neurodegenerative disease. It has now become firmly established that particular PrP(Sc) isoforms are closely related to specific human prion strains. The variety of human prion diseases, still an enigma in its own right, is a focus of this article. Recently, a series of experiments has shown that the concept of aberrant protein folding and templating, first developed for prions, may apply to a variety of neurodegenerative diseases. In the wake of these discoveries, the term prion has come to be used for Aβ, α-synuclein, tau and possibly others. The self-propagation of alternative conformations seems to be the common denominator of these "prions," which in future, in order to avoid confusion, may have to be specified either as "neurodegenerative prions" or "infectious prions."

Unique structural properties associated with mouse prion Δ105-125 protein

Murine prion protein deleted for residues 105-125 is intrinsically neurotoxic and mediates a TSE-like phenotype in transgenic mice. Equivalent and overlapping deletions were expressed in E.coli, purified and analyzed. Among mutants spanning the region 95-135, a construct lacking solely residues 105-125 had distinct properties when compared with the full-length prion protein 23-231 or other deletions. This distinction was also apparent followed expression in eukaryotic cells. Unlike the full-length protein, all deletion mutants failed to bind to synthetic membranes in vitro. These data suggest a novel structure for the 105-125 deleted variant that may relate to its biological properties.

Utilizing NMR and EPR spectroscopy to probe the role of copper in prion diseases

Copper is an essential nutrient for the normal development of the brain and nervous system, although the hallmark of several neurological diseases is a change in copper concentrations in the brain and central nervous system. Prion protein (PrP) is a copper-binding, cell-surface glycoprotein that exists in two alternatively folded conformations: a normal isoform (PrP(C)) and a disease-associated isoform (PrP(Sc)). Prion diseases are a group of lethal neurodegenerative disorders that develop as a result of conformational conversion of PrP(C) into PrP(Sc). The pathogenic mechanism that triggers this conformational transformation with the subsequent development of prion diseases remains unclear. It has, however, been shown repeatedly that copper plays a significant functional role in the conformational conversion of prion proteins. In this review, we focus on current research that seeks to clarify the conformational changes associated with prion diseases and the role of copper in this mechanism, with emphasis on the latest applications of NMR and EPR spectroscopy to probe the interactions of copper with prion proteins.

Effects of the pathological Q212P mutation on human prion protein non-octarepeat copper-binding site

Prion diseases are a class of fatal neurodegenerative disorders characterized by brain spongiosis, synaptic degeneration, microglia and astrocytes activation, neuronal loss and altered redox control. These maladies can be sporadic, iatrogenic and genetic. The etiological agent is the prion, a misfolded form of the cellular prion protein, PrP(C). PrP(C) interacts with metal ions, in particular copper and zinc, through the octarepeat and non-octarepeat binding sites. The physiological implication of this interaction is still unclear, as is the role of metals in the conversion. Since prion diseases present metal dyshomeostasis and increased oxidative stress, we described the copper-binding site located in the human C-terminal domain of PrP-HuPrP(90-231), both in the wild-type protein and in the protein carrying the pathological mutation Q212P. We used the synchrotron-based X-ray absorption fine structure technique to study the Cu(II) and Cu(I) coordination geometries in the mutant, and we compared them with those obtained using the wild-type protein. By analyzing the extended X-ray absorption fine structure and the X-ray absorption near-edge structure, we highlighted changes in copper coordination induced by the point mutation Q212P in both oxidation states. While in the wild-type protein the copper-binding site has the same structure for both Cu(II) and Cu(I), in the mutant the coordination site changes drastically from the oxidized to the reduced form of the copper ion. Copper-binding sites in the mutant resemble those obtained using peptides, confirming the loss of short- and long-range interactions. These changes probably cause alterations in copper homeostasis and, consequently, in redox control.