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

Dissecting the copper bioinorganic chemistry of the functional and pathological roles of the prion protein: Relevance in Alzheimer's disease and cancer

The cellular prion protein (PrP^C) is a metal-binding biomolecule that can interact with different protein partners involved in pivotal physiological processes, such as neurogenesis and neuronal plasticity. Recent studies profile copper and PrP^C as important players in the pathological mechanisms of Alzheimer's disease and cancer. Although the copper-PrP^C interaction has been characterized extensively, the role of the metal ion in the physiological and pathological roles of PrP^C has been barely explored. In this article, we discuss how copper binding and proteolytic processing may impact the ability of PrP^C to recruit protein partners for its functional roles. The importance to dissect the role of copper-PrP^C interactions in health and disease is also underscored.

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.

Prions Strongly Reduce NMDA Receptor S-Nitrosylation Levels at Pre-symptomatic and Terminal Stages of Prion Diseases

Prion diseases are fatal neurodegenerative disorders characterized by the cellular prion protein (PrP^C) conversion into a misfolded and infectious isoform termed prion or PrP^Sc. The neuropathological mechanism underlying prion toxicity is still unclear, and the debate on prion protein gain- or loss-of-function is still open. PrP^C participates to a plethora of physiological mechanisms. For instance, PrP^C and copper cooperatively modulate N-methyl-D-aspartate receptor (NMDAR) activity by mediating S-nitrosylation, an inhibitory post-translational modification, hence protecting neurons from excitotoxicity. Here, NMDAR S-nitrosylation levels were biochemically investigated at pre- and post-symptomatic stages of mice intracerebrally inoculated with RML, 139A, and ME7 prion strains. Neuropathological aspects of prion disease were studied by histological analysis and proteinase K digestion. We report that hippocampal NMDAR S-nitrosylation is greatly reduced in all three prion strain infections in both pre-symptomatic and terminal stages of mouse disease. Indeed, we show that NMDAR S-nitrosylation dysregulation affecting prion-inoculated animals precedes the appearance of clinical signs of disease and visible neuropathological changes, such as PrP^Sc accumulation and deposition. The pre-symptomatic reduction of NMDAR S-nitrosylation in prion-infected mice may be a possible cause of neuronal death in prion pathology, and it might contribute to the pathology progression opening new therapeutic strategies against prion disorders.

Prion protein and copper cooperatively protect neurons by modulating NMDA receptor through S-nitrosylation

AIMS

Several neurodegenerative disorders show alterations in glutamatergic synapses and increased susceptibility to excitotoxicity. Mounting evidence suggests a central role for the cellular prion protein (PrP(C)) in neuroprotection. Therefore, the loss of PrP(C) function occurring in prion disorders may contribute to the disease progression and neurodegeneration. Indeed, PrP(C) modulates N-methyl-d-aspartate receptors (NMDAR), thus preventing cell death. In this study, we show that PrP(C) and copper cooperatively inhibit NMDAR through S-nitrosylation, a post-translational modification resulting from the chemical reaction of nitric oxide (NO) with cysteines.

RESULTS

Comparing wild-type Prnp (Prnp(+/+)) and PrP(C) knockout (Prnp(0/0)) mouse hippocampi, we found that GluN1 and GluN2A S-nitrosylation decrease in Prnp(0/0). Using organotypic hippocampal cultures, we found that copper chelation decreases NMDAR S-nitrosylation in Prnp(+/+) but not in Prnp(0/0). This suggests that PrP(C) requires copper to support the chemical reaction between NO and thiols. We explored PrP(C)-Cu neuroprotective role by evaluating neuron susceptibility to excitotoxicity in Prnp(+/+) and Prnp(0/0) cultures. We found that (i) PrP(C)-Cu modulates GluN2A-containing NMDAR, those inhibited by S-nitrosylation; (ii) PrP(C) and copper are interdependent to protect neurons from insults; (iii) neuronal NO synthase inhibition affects susceptibility in wild-type but not in Prnp(0/0), while (iv) the addition of a NO donor enhances Prnp(0/0) neurons survival.

INNOVATION AND CONCLUSIONS

Our results show that PrP(C) and copper support NMDAR S-nitrosylation and cooperatively exert neuroprotection. In addition to NMDAR, PrP(C) may also favor the S-nitrosylation of other proteins. Therefore, this mechanism may be investigated in the context of the different cellular processes in which PrP(C) is involved.

The Rich Electrochemistry and Redox Reactions of the Copper Sites in the Cellular Prion Protein

This paper reviews recent electrochemical studies of the copper complexes of prion protein (PrP) and its related peptides, and correlates their redox behavior to chemical and biologically relevant reactions. Particular emphasis is placed on the difference in redox properties between copper in the octarepeat (OR) and the non-OR domains of PrP, as well as differences between the high and low copper occupancy states in the OR domain. Several discrepancies in literature concerning these differences are discussed and reconciled. The PrP copper complexes, in comparison to copper complexes of other amyloidogenic proteins/peptides, display a more diverse and richer redox chemistry. The specific protocols and caveats that need to be considered in studying the electrochemistry and redox reactions of copper complexes of PrP, PrP-derived peptides, and other related amyloidogenic proteins are summarized.

Copper(II) complexation to 1-octarepeat peptide from a prion protein: insights from theoretical and experimental UV-visible studies

The octarepeat domain in cellular prion protein (PrP(C)) has attracted much attention over the last 10 years because of its importance in the complexation of copper with PrP(C). The aim of this research was to study the UV-vis spectra of a peptide similar to the 1-repeat of the octarepeat region in PrP(C) using experimental and theoretical approaches and to gain insight into the complexation of the PrP(C) octarepeat domain with copper(II) ions in solution. We found that the copper atom was responsible for the peptide conformation, which allows for charge transfers between its two terminal residues.

Nitric oxide reactivity of copper(II) complexes of bidentate amine ligands: effect of substitution on ligand nitrosation

Three copper(ii) complexes with bidentate ligands L(1), L(2) and L(3) [L(1), N,N(/)-dimethylethylenediamine; L(2), N,N(/)-diethylethylenediamine and L(3), N,N(/)-diisobutylethylenediamine], respectively, were synthesized as their perchlorate salts. The single crystal structures for all the complexes were determined. The nitric oxide reactivity of the complexes was studied in acetonitrile solvent. The formation of thermally unstable [Cu(II)-NO] intermediate on reaction of the complexes with nitric oxide in acetonitrile solution was observed prior to the reduction of copper(II) centres to copper(I). The reduction was found to result with a simultaneous mono- and di-nitrosation at the secondary amine sites of the ligand. All the nitrosation products were isolated and characterized. The ratio of the yield of mono- and di-nitrosation product was found to be dependent on the N-substitution present in the ligand framework.