Low copper and high manganese levels in prion protein plaques

Zinc ions trigger the prion protein liquid-liquid phase separation

Prion diseases are characterized by the misfolding and conversion of the monomeric prion protein (PrP) to a multimeric aggregated pathogenic form, known as PrPSc. We and others have recently shown that biomolecular condensates formed via liquid-liquid phase separation of PrP can undergo maturation to solid-like species that resemble pathological aggregates, and this process is modulated by DNA, RNA, and oxidative conditions. Conversely, the most well-studied ligand of PrP, copper ions, induce liquid-like condensates of PrP that accumulate Cu2+in vitro, and live PrPC-expressing cells show condensation at the cell surface as triggered by physiologically relevant conditions of Cu2+ and protein concentrations. Since PrP can also bind to Zn2+ through its intrinsically disordered N-terminal domain, though with different affinities and binding modes than Cu2+, we hypothesized that Zn2+ could modulate PrP phase separation differently from copper ions. Using an appropriate buffer with negligible metal ion binding, as well as relevant pH, ionic strength, molecular crowding, and Zn2+ concentrations, we show that recombinant PrP undergoes phase separation with Zn2+. Furthermore, we show that metal ion-induced condensation of PrP is dependent on the N-terminal domain (residues 23-90). In vitro Fluorescence Recovery After Photobleaching (FRAP) experiments and thioflavin T aggregation kinetics support key differences in the molecular properties of PrP:Zn2+versus PrP:Cu2+ phase separated states. FRAP analysis indicated that both Cu2+ and Zn2+ promote liquid-like PrP condensates; however, PrP:Zn2+condensates exhibit a faster recovery. Cu2+ pronouncedly inhibits seed-induced PrP misfolding, whereas Zn2+ provides a milder delay in PrP aggregation. Our findings provide insights on Zn2+-induced phase separation of PrP, supporting a variety of previously proposed functions of PrP in metal sequestering and uptake, processes that could be effectively regulated through biomolecular condensation.

Copper drives prion protein phase separation and modulates aggregation

Prion diseases are characterized by prion protein (PrP) transmissible aggregation and neurodegeneration, which has been linked to oxidative stress. The physiological function of PrP seems related to sequestering of redox-active Cu2+, and Cu2+ dyshomeostasis is observed in prion disease brain. It is unclear whether Cu2+ contributes to PrP aggregation, recently shown to be mediated by PrP condensation. This study indicates that Cu2+ promotes PrP condensation in live cells at the cell surface and in vitro through copartitioning. Molecularly, Cu2+ inhibited PrP β-structure and hydrophobic residues exposure. Oxidation, induced by H2O2, triggered liquid-to-solid transition of PrP:Cu2+ condensates and promoted amyloid-like PrP aggregation. In cells, overexpression of PrPC initially protected against Cu2+ cytotoxicity but led to PrPC aggregation upon extended copper exposure. Our data suggest that PrP condensates function as a buffer for copper that prevents copper toxicity but can transition into PrP aggregation at prolonged oxidative stress.

Movement of Chronic Wasting Disease Prions in Prairie, Boreal and Alpine Soils

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy negatively impacting cervids on three continents. Soil can serve as a reservoir for horizontal transmission of CWD by interaction with the infectious prion protein (PrPCWD) shed by diseased individuals and from infected carcasses. We investigated the pathways for PrPCWD migration in soil profiles using lab-scale soil columns, comparing PrPCWD migration through pure soil minerals (quartz, illite and montmorillonite), and diverse soils from boreal (Luvisol, Brunisol) and prairie (Chernozem) regions. We analyzed the leachate of the soil columns by immunoblot and protein misfolding cyclic amplification (PMCA) and detected PrP in the leachates of columns composed of quartz, illite, Luvisol and Brunisol. Animal bioassay confirmed the presence of CWD infectivity in the leachates from quartz, illite and Luvisol columns. Leachates from columns with montmorillonite and prairie Chernozems did not contain PrP detectable by immunoblotting or PMCA; bioassay confirmed that the Chernozemic leachate was not infectious. Analysis of the solid phase of the columns confirmed the migration of PrP to lower layers in the illite column, while the strongest signal in the montmorillonite column remained close to the surface. Montmorillonite, the prevalent clay mineral in prairie soils, has the strongest prion binding ability; by contrast, illite, the main clay mineral in northern boreal and tundra soils, does not bind prions significantly. This suggests that in soils of North American CWD-endemic regions (Chernozems), PrPCWD would remain on the soil surface due to avid binding to montmorillonite. In boreal Luvisols and mountain Brunisols, prions that pass through the leaf litter will continue to move through the soil mineral horizon, becoming less bioavailable. In light-textured soils where quartz is a dominant mineral, the majority of the infectious prions will move through the soil profile. Local soil properties may consequently determine the efficiency of environmental transmission of CWD.

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.

Electrochemical biosensors for biometal-protein interactions in neurodegenerative diseases

Electrochemical biosensors have been adopted into a wide range of applications in the study of biometal-protein interactions in neurodegenerative diseases. Transition metals such as zinc, copper, and iron that are significant to biological functions have been shown to have strong implications in the progressive neural degeneration in Alzheimer's disease (AD), Parkinson's disease (PD), and prion protein diseases. This review presents a summative examination of the progress made in the design, fabrication, and applications of electrochemical biosensors in recent literature at understanding the metal-protein interactions in neurodegenerative diseases. The focus will be drawn on disease-causing biomarkers such as amyloid-β (Aβ) and tau proteins for AD, α-synuclein (α-syn) for PD, and prion proteins (PrP). Topics such as the use of electrochemical biosensing in monitoring biometal-induced conformational changes, elucidation of complexation motifs, production of reactive oxygen species (ROS) as well as the influence on downstream biomolecular interactions will be discussed. Major results and important concepts presented in these studies will be summarized in the hope to spark inspiration for the next generation of electrochemical sensors.

The Effects of Divalent Cation-Chelated Prion Fibrils on the Immune Response of EOC 13.31 Microglia Cells

Transmissible spongiform encephalopathies (TSEs) are epidemic neurodegenerative diseases caused by prion proteins; in particular, they are induced by misfolded prion proteins (PrPSc). PrPSc tend to aggregate into insoluble amyloid prion fibrils (fPrPWT), resulting in apoptosis of neuron cells and sequential neurodegeneration. Previous studies indicate that microglia cells play an important role in the innate immune system, and that these cells have good neuroprotection and delay the onset of TSEs. However, microglia can be a double-sided blade. For example, both Cu2+ and Mn2+ can induce microglia activation and secrete many inflammatory cytokines that are fatal to neuron cells. Unfortunately, PrP have cation binding sites at the N-terminus. When PrPSc accumulate during microglial phagocytosis, microglia may change the phenotype to secrete pro-inflammation cytokines, which increases the severity of the disease. Some studies have revealed an increase in the concentration of Mn2+ in the brains of patients. In this study, we treated microglia with fPrPWT and cations and determined IκBα and IL-1β expression by Western blotting and quantitative polymerase chain reaction. The results showed that Mn-fPrPWT decreased IκBα levels and dramatically increased IL-1β mRNA expression. In addition, competing binding between Cu2+ and Mn2+ can decrease the effect of Mn-fPrPWT on IκBα and IL-1β. The effects of divalent cations and fPrPWT in microglia inflammation are also discussed.

Copper brain protein protection against free radical-induced neuronal death: Survival ratio in SH-SY5Y neuroblastoma cell cultures

In Creutzfeldt Jakob, Alzheimer and Parkinson diseases, copper metalloproteins such as prion, amyloid protein precursor and α-synuclein are able to protect against free radicals by reduction from cupric Cu⁺² to cupreous Cu⁺. In these pathologies, a regional copper (Cu) brain decrease correlated with an iron, zinc or manganese (Mn) increase has previously been observed, leading to local neuronal death and abnormal deposition of these metalloproteins in β-sheet structures. In this study we demonstrate the protective effect of Cu metalloproteins against deleterious free-radical effects. With neuroblastoma SH-SY5Y cell cultures, we show that bovine brain prion protein in Cu but not Mn form prevents free radical-induced neuronal death. The survival ratio of SH-SY5Y cells has been measured after UV irradiation (free radical production), when the incubating medium is supplemented with bovine brain homogenate in native, Cu or Mn forms. This ratio, about 28% without any addition or with bovine brain protein added in Mn form, increases by as much as 54.73% with addition to the culture medium of native bovine brain protein and by as much as 95.95% if the addition is carried out in cupric form. This protective effect of brain copper protein against free radical-induced neuronal death has been confirmed with Inductively Coupled Plasma Mass Spectrometry Mn and Cu measurement in bovine brain homogenates: respectively lower than detection limit and 9.01μg/g dry weight for native form; lower than detection limit and 825.85μg/g dry weight for Cu-supplemented form and 1.75 and 68.1μg/g dry weight in Mn-supplemented brain homogenate.

Proteomics applications in prion biology and structure

Prion diseases are a heterogeneous class of fatal neurodegenerative disorders associated with misfolding of host cellular prion protein (PrP(C)) into a pathological isoform, termed PrP(Sc). Prion diseases affect various mammals, including humans, and effective treatments are not available. Prion diseases are distinguished from other protein misfolding disorders - such as Alzheimer's or Parkinson's disease - in that they are infectious. Prion diseases occur sporadically without any known exposure to infected material, and hereditary cases resulting from rare mutations in the prion protein have also been documented. The mechanistic underpinnings of prion and other neurodegenerative disorders remain poorly understood. Various proteomics techniques have been instrumental in early PrP(Sc) detection, biomarker discovery, elucidation of PrP(Sc) structure and mapping of biochemical pathways affected by pathogenesis. Moving forward, proteomics approaches will likely become more integrated into the clinical and research settings for the rapid diagnosis and characterization of prion pathogenesis.

Distinct effects of Cu2+-binding on oligomerization of human and rabbit prion proteins

The cellular prion protein (PrP(C)) is a kind of cell-surface Cu(2+)-binding glycoprotein. The oligomerization of PrP(C) is highly related to transmissible spongiform encephalopathies (TSEs). Cu(2+) plays a vital role in the oligomerization of PrP(C), and participates in the pathogenic process of TSE diseases. It is expected that Cu(2+)-binding has different effects on the oligomerization of TSE-sensitive human PrP(C) (HuPrP(C)) and TSE-resistant rabbit PrP(C) (RaPrP(C)). However, the details of the distinct effects remain unclear. In the present study, we measured the interactions of Cu(2+) with HuPrP(C) (91-230) and RaPrP(C) (91-228) by isothermal titration calorimetry, and compared the effects of Cu(2+)-binding on the oligomerization of both PrPs. The measured dissociation constants (Kd) of Cu(2+) were 11.1 ± 2.1 μM for HuPrP(C) and 21.1 ± 3.1 μM for RaPrP(C). Cu(2+)-binding promoted the oligomerization of HuPrP(C) more significantly than that of RaPrP(C). The far-ultraviolet circular dichroism spectroscopy experiments showed that Cu(2+)-binding induced more significant secondary structure change and increased more β-sheet content for HuPrP(C) compared with RaPrP(C). Moreover, the urea-induced unfolding transition experiments indicated that Cu(2+)-binding decreased the conformational stability of HuPrP(C) more distinctly than that of RaPrP(C). These results suggest that RaPrP(C) possesses a low susceptibility to Cu(2+), potentially weakening the risk of Cu(2+)-induced TSE diseases. Our work sheds light on the Cu(2+)-promoted oligomerization of PrP(C), and may be helpful for further understanding the TSE-resistance of rabbits.

Prion infection in cells is abolished by a mutated manganese transporter but shows no relation to zinc

The cellular prion protein has been identified as a metalloprotein that binds copper. There have been some suggestions that prion protein also influences zinc and manganese homeostasis. In this study we used a series of cell lines to study the levels of zinc and manganese under different conditions. We overexpressed either the prion protein or known transporters for zinc and manganese to determine relations between the prion protein and both manganese and zinc homeostasis. Our observations supported neither a link between the prion protein and zinc metabolism nor any effect of altered zinc levels on prion protein expression or cellular infection with prions. In contrast we found that a gain of function mutant of a manganese transporter caused reduction of manganese levels in prion infected cells, loss of observable PrP(Sc) in cells and resistance to prion infection. These studies strengthen the link between manganese and prion disease.

The secret life of extracellular vesicles in metal homeostasis and neurodegeneration

Biologically active metals such as copper, zinc and iron are fundamental for sustaining life in different organisms with the regulation of cellular metal homeostasis tightly controlled through proteins that coordinate metal uptake, efflux and detoxification. Many of the proteins involved in either uptake or efflux of metals are localised and function on the plasma membrane, traffic between intracellular compartments depending upon the cellular metal environment and can undergo recycling via the endosomal pathway. The biogenesis of exosomes also occurs within the endosomal system, with several major neurodegenerative disease proteins shown to be released in association with these vesicles, including the amyloid-β (Aβ) peptide in Alzheimer's disease and the infectious prion protein involved in Prion diseases. Aβ peptide and the prion protein also bind biologically active metals and are postulated to play important roles in metal homeostasis. In this review, we will discuss the role of extracellular vesicles in Alzheimer's and Prion diseases and explore their potential contribution to metal homeostasis.

Potential role of soil properties in the spread of CWD in western Canada

Chronic wasting disease (CWD) is a horizontally transmissible prion disease of free ranging deer, elk and moose. Recent experimental transmission studies indicate caribou are also susceptible to the disease. CWD is present in southeast Alberta and southern Saskatchewan. This CWD-endemic region is expanding, threatening Manitoba and areas of northern Alberta and Saskatchewan, home to caribou. Soil can serve as a stable reservoir for infectious prion proteins; prions bound to soil particles remain infectious in the soils for many years. Soils of western Canada are very diverse and the ability of CWD prions to bind different soils and the impact of this interaction on infectivity is not known. In general, clay-rich soils may bind prions avidly and enhance their infectivity comparable to pure clay mineral montmorillonite. Organic components of soils are also diverse and not well characterized, yet can impact prion-soil interaction. Other important contributing factors include soil pH, composition of soil solution and amount of metals (metal oxides). In this review, properties of soils of the CWD-endemic region in western Canada with its surrounding terrestrial environment are described and used to predict bioavailability and, thus, potential spread of CWD. The major soils in the CWD-endemic region of Alberta and Saskatchewan are Chernozems, present in 60% of the total area; they are generally similar in texture, clay mineralogy and soil organic matter content, and can be characterized as clay loamy, montmorillonite (smectite) soils with 6-10% organic carbon. The greatest risk of CWD spread in western Canada relates to clay loamy, montmorillonite soils with humus horizon. Such soils are predominant in the southern region of Alberta, Saskatchewan and Manitoba, but are less common in northern regions of the provinces where quartz-illite sandy soils with low amount of humus prevail.

The role of copper ions in pathophysiology and fluorescent sensors for the detection thereof

Copper ions are crucial to life, and some fundamental roles of copper in pathophysiology have been elucidated using fluorescent sensors.

The relevance of metals in the pathophysiology of neurodegeneration, pathological considerations

Neurodegenerative disorders are featured by a variety of pathological conditions that share similar critical processes, such as oxidative stress, free radical activity, proteinaceous aggregations, mitochondrial dysfunctions, and energy failure. They are mediated or triggered by an imbalance of metal ions leading to changes of critical biological systems and initiating a cascade of events finally leading to neurodegeneration and cell death. Their causes are multifactorial, and although the source of the shift in oxidative homeostasis is still unclear, current evidence points to changes in the balance of redox transition metals, especially iron, copper, and other trace metals. They are present at elevated levels in Alzheimer disease, Parkinson disease, multisystem atrophy, etc., while in other neurodegenerative disorders, copper, zinc, aluminum, and manganese are involved. This chapter will review the recent advances of the role of metals in the pathogenesis and pathophysiology of major neurodegenerative diseases and discuss the use of chelating agents as potential therapies for metal-related disorders.