Solvent microenvironments and copper binding alters the conformation and toxicity of a prion fragment

Neurometals in the Pathogenesis of Prion Diseases

Prion diseases are progressive and transmissive neurodegenerative diseases. The conformational conversion of normal cellular prion protein (PrP^C) into abnormal pathogenic prion protein (PrP^Sc) is critical for its infection and pathogenesis. PrP^C possesses the ability to bind to various neurometals, including copper, zinc, iron, and manganese. Moreover, increasing evidence suggests that PrP^C plays essential roles in the maintenance of homeostasis of these neurometals in the synapse. In addition, trace metals are critical determinants of the conformational change and toxicity of PrP^C. Here, we review our studies and other new findings that inform the current understanding of the links between trace elements and physiological functions of PrP^C and the neurotoxicity of PrP^Sc.

X-ray Absorption Spectroscopy Investigations of Copper(II) Coordination in the Human Amyloid β Peptide

Alzheimer's disease (AD) is the main cause of age-related dementia and currently affects approximately 5.7 million Americans. Major brain changes associated with AD pathology include accumulation of amyloid beta (Aβ) protein fragments and formation of extracellular amyloid plaques. Redox-active metals mediate oligomerization of Aβ, and the resultant metal-bound oligomers have been implicated in the putative formation of harmful, reactive species that could contribute to observed oxidative damage. In isolated plaque cores, Cu(II) is bound to Aβ via histidine residues. Despite numerous structural studies of Cu(II) binding to synthetic Aβ in vitro, there is still uncertainty surrounding Cu(II) coordination in Aβ. In this study, we used X-ray absorption spectroscopy (XAS) and high energy resolution fluorescence detected (HERFD) XAS to investigate Cu(II) coordination in Aβ(1-42) under various solution conditions. We found that the average coordination environment in Cu(II)Aβ(1-42) is sensitive to X-ray photoreduction, changes in buffer composition, peptide concentration, and solution pH. Fitting of the extended X-ray absorption fine structure (EXAFS) suggests Cu(II) is bound in a mixture of coordination environments in monomeric Aβ(1-42) under all conditions studied. However, it was evident that on average only a single histidine residue coordinates Cu(II) in monomeric Aβ(1-42) at pH 6.1, in addition to 3 other oxygen or nitrogen ligands. Cu(II) coordination in Aβ(1-42) at pH 7.4 is similarly 4-coordinate with oxygen and nitrogen ligands, although an average of 2 histidine residues appear to coordinate at this pH. At pH 9.0, the average Cu(II) coordination environment in Aβ(1-42) appears to be 5-coordinate with oxygen and nitrogen ligands, including two histidine residues.

The effect of a membrane-mimicking environment on the interactions of Cu2+ with an amyloidogenic fragment of chicken prion protein

Prion proteins (PrP) from different species have the ability to tightly bind Cu²⁺ ions. Copper coordination sites are located in the disordered and flexible N-terminal region which contains several His anchoring sites. Among them, two His residues are found in the so called amyloidogenic PrP region which is believed to play a key role in the process leading to oligomer and fibril formation. Both chicken and human amyloidogenic regions have a hydrophobic C-terminal region rich in Ala and Val amino acids. Recent findings revealed that this domain undergoes random coil to α-helix structuring upon interaction with membrane models. This interaction might strongly impact metal binding abilities either in terms of donor sets or affinity. In this study we investigated Cu²⁺ interaction with an amyloidogenic fragment, chPrP105-140, derived from chicken prion protein (chPrP), in different solution environments. The behavior of the peptide and its metal complexes was analyzed in water and in the presence of negative and positive charged membrane mimicking environments formed by sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium chloride (DTAC) micelles. The metal coordination sphere, the metal binding affinity and stoichiometry were evaluated by combining spectroscopic and potentiometric methods. Finally we compare copper(ii) interactions with human and chicken amyloidogenic fragments. Our results indicate that the chicken amyloidogenic fragment is a stronger copper ligand than the human amyloidogenic fragment.

Conformational dynamics of a hydrophobic prion fragment (113-127) in different pH and osmolyte solutions

Prion diseases are characterized by a conformational change in prion protein from its native state into beta-sheet rich aggregates that are neurotoxic. The central domain that contain a highly conserved hydrophobic region of the protein play an important role in the toxicity. The conformation of the proteins is largely influenced by various solvent environments. Here we report results of study of hydrophobic prion fragment peptide PrP(113-127) under different pH and osmolytes solution conditions. The secondary structure and the folding of PrP(113-127) was determined using circular dichroism and fluorescence spectroscopic methods. The results indicate that PrP(113-127) adopts a random coil conformation in aqueous buffer at neutral pH and that converted into beta sheet on aging. Even though the initial random coil conformation was similar in different pH conditions, the acidic as well as basic pH conditions delays the conformational transition to beta sheet. FRET results indicate that the distance between N and C-terminal regions increased on aging due to unfolding by self-assembly of the peptide into an organized beta sheet structure. Presence of osmolytes, prevented or decelerated the aggregation process of PrP(113-127) peptide.