Paradoxical role of prion protein aggregates in redox-iron induced toxicity

Involvement of the Prion Protein in the Protection of the Human Bronchial Epithelial Barrier Against Oxidative Stress

Aim: Bronchial epithelium acts as a defensive barrier against inhaled pollutants and microorganisms. This barrier is often compromised in inflammatory airway diseases that are characterized by excessive oxidative stress responses, leading to bronchial epithelial shedding, barrier failure, and increased bronchial epithelium permeability. Among proteins expressed in the junctional barrier and participating to the regulation of the response to oxidative and to environmental stresses is the cellular prion protein (PrP^C). However, the role of PrP^C is still unknown in the bronchial epithelium. Herein, we investigated the cellular mechanisms by which PrP^C protein participates into the junctional complexes formation, regulation, and oxidative protection in human bronchial epithelium. Results: Both PrP^C messenger RNA and mature protein were expressed in human epithelial bronchial cells. PrP^C was localized in the apical domain and became lateral, at high degree of cell polarization, where it colocalized and interacted with adherens (E-cadherin/γ-catenin) and desmosomal (desmoglein/desmoplakin) junctional proteins. No interaction was detected with tight junction proteins. Disruption of such interactions induced the loss of the epithelial barrier. Moreover, we demonstrated that PrP^C protection against copper-associated oxidative stress was involved in multiple processes, including the stability of adherens and desmosomal junctional proteins. Innovation: PrP^C is a pivotal protein in the protection against oxidative stress that is associated with the degradation of adherens and desmosomal junctional proteins. Conclusion: Altogether, these results demonstrate that the loss of the integrity of the epithelial barrier by oxidative stress is attenuated by the activation of PrP^C expression, where deregulation might be associated with respiratory diseases.

Iron-Restricted Diet Affects Brain Ferritin Levels, Dopamine Metabolism and Cellular Prion Protein in a Region-Specific Manner

Iron is an essential micronutrient for several physiological functions, including the regulation of dopaminergic neurotransmission. On the other hand, both iron, and dopamine can affect the folding and aggregation of proteins related with neurodegenerative diseases, such as cellular prion protein (PrP^C) and α-synuclein, suggesting that deregulation of iron homeostasis and the consequential disturbance of dopamine metabolism can be a risk factor for conformational diseases. These proteins, in turn, are known to participate in the regulation of iron and dopamine metabolism. In this study, we evaluated the effects of dietary iron restriction on brain ferritin levels, dopamine metabolism, and the expression levels of PrP^C and α-synuclein. To achieve this goal, C57BL/6 mice were fed with iron restricted diet (IR) or with normal diet (CTL) for 1 month. IR reduced iron and ferritin levels in liver. Ferritin reduction was also observed in the hippocampus. However, in the striatum of IR group, ferritin level was increased, suggesting that under iron-deficient condition, each brain area might acquire distinct capacity to store iron. Increased lipid peroxidation was observed only in hippocampus of IR group, where ferritin level was reduced. IR also generated discrete results regarding dopamine metabolism of distinct brain regions: in striatum, the level of dopamine metabolites (DOPAC and HVA) was reduced; in prefrontal cortex, only HVA was increased along with the enhanced MAO-A activity; in hippocampus, no alterations were observed. PrP^C levels were increased only in the striatum of IR group, where ferritin level was also increased. PrP^C is known to play roles in iron uptake. Thus, the increase of PrP^C in striatum of IR group might be related to the increased ferritin level. α-synuclein was not altered in any regions. Abnormal accumulation of ferritin, increased MAO-A activity or lipid peroxidation are molecular features observed in several neurological disorders. Our findings show that nutritional iron deficiency produces these molecular alterations in a region-specific manner and provide new insight into the variety of molecular pathways that can lead to distinct neurological symptoms upon iron deficiency. Thus, adequate iron supplementation is essential for brain health and prevention of neurological diseases.

Morphine Withdrawal Modifies Prion Protein Expression in Rat Hippocampus

The hippocampus is a vulnerable brain structure susceptible to damage during aging and chronic stress. Repeated exposure to opioids may alter the brain so that it functions normally when the drugs are present, thus, a prolonged withdrawal might lead to homeostatic changes headed for the restoration of the physiological state. Abuse of morphine may lead to Reacting Oxygen Species-induced neurodegeneration and apoptosis. It has been proposed that during morphine withdrawal, stress responses might be responsible, at least in part, for long-term changes of hippocampal plasticity. Since prion protein is involved in both, Reacting Oxygen Species mediated stress responses and synaptic plasticity, in this work we investigate the effect of opiate withdrawal in rats after morphine treatment. We hypothesize that stressful stimuli induced by opiate withdrawal, and the subsequent long-term homeostatic changes in hippocampal plasticity, might modulate the Prion protein expression. Our results indicate that abstinence from the opiate induced a time-dependent and region-specific modification in Prion protein content, indeed during morphine withdrawal a selective unbalance of hippocampal Prion Protein is observable. Moreover, Prion protein overexpression in hippocampal tissue seems to generate a dimeric structure of Prion protein and α-cleavage at the hydrophobic domain. Stress factors or toxic insults can induce cytosolic dimerization of Prion Protein through the hydrophobic domain, which in turn, it stimulates the α-cleavage and the production of neuroprotective Prion protein fragments. We speculate that this might be the mechanism by which stressful stimuli induced by opiate withdrawal and the subsequent long-term homeostatic changes in hippocampal plasticity, modulate the expression and the dynamics of Prion protein.

Parkinson's Disease: The Mitochondria-Iron Link

Mitochondrial dysfunction, iron accumulation, and oxidative damage are conditions often found in damaged brain areas of Parkinson's disease. We propose that a causal link exists between these three events. Mitochondrial dysfunction results not only in increased reactive oxygen species production but also in decreased iron-sulfur cluster synthesis and unorthodox activation of Iron Regulatory Protein 1 (IRP1), a key regulator of cell iron homeostasis. In turn, IRP1 activation results in iron accumulation and hydroxyl radical-mediated damage. These three occurrences-mitochondrial dysfunction, iron accumulation, and oxidative damage-generate a positive feedback loop of increased iron accumulation and oxidative stress. Here, we review the evidence that points to a link between mitochondrial dysfunction and iron accumulation as early events in the development of sporadic and genetic cases of Parkinson's disease. Finally, an attempt is done to contextualize the possible relationship between mitochondria dysfunction and iron dyshomeostasis. Based on published evidence, we propose that iron chelation-by decreasing iron-associated oxidative damage and by inducing cell survival and cell-rescue pathways-is a viable therapy for retarding this cycle.

H ferritin silencing induces protein misfolding in K562 cells: A Raman analysis

The redox state of the cell is involved in the regulation of many physiological functions as well as in the pathogenesis of several diseases, and is strictly dependent on the amount of iron in its catalytically active state. Alterations of iron homeostasis determine increased steady-state concentrations of Reactive Oxygen Species (ROS) that cause lipid peroxidation, DNA damage and altered protein folding. Ferritin keeps the intracellular iron in a non-toxic and readily available form and consequently plays a central role in iron and redox homeostasis. The protein is composed by 24 subunits of the H- and L-type, coded by two different genes, with structural and functional differences. The aim of this study was to shed light on the role of the single H ferritin subunit (FHC) in keeping the native correct protein three-dimensional structure. To this, we performed Raman spectroscopy on protein extracts from K562 cells subjected to FHC silencing. The results show a significant increase in the percentage of disordered structures content at a level comparable to that induced by H2O2 treatment in control cells. ROS inhibitor and iron chelator were able to revert protein misfolding. This integrated approach, involving Raman spectroscopy and targeted-gene silencing, indicates that an imbalance of the heavy-to-light chain ratio in the ferritin composition is able to induce severe but still reversible modifications in protein folding and uncovers new potential pathogenetic mechanisms associated to intracellular iron perturbation.

Cellular prion protein (PrP(C)) and its role in stress responses

Investigation of the physiological function of cellular prion protein (PrP(C)) has been developed by the generation of transgenic mice, however, the pathological mechanisms related to PrP(C) in prion diseases such as transmissible spongiform encephalopathies (TSEs) are still abstruse. Regardless of some differences, most studies describe the neuroprotective role of PrP(C) in environmental stresses. In this review, we will update the current knowledge on the responses of PrP(C) to various stresses, especially those correlated with cell signaling and neural degeneration, including ischemia, oxidative stress, inflammation and autophagy.

Dopamine induces the accumulation of insoluble prion protein and affects autophagic flux

Accumulation of protein aggregates is a histopathological hallmark of several neurodegenerative diseases, but in most cases the aggregation occurs without defined mutations or clinical histories, suggesting that certain endogenous metabolites can promote aggregation of specific proteins. One example that supports this hypothesis is dopamine and its metabolites. Dopamine metabolism generates several oxidative metabolites that induce aggregation of α-synuclein, and represents the main etiology of Parkinson's diseases. Because dopamine and its metabolites are unstable and can be highly reactive, we investigated whether these molecules can also affect other proteins that are prone to aggregate, such as cellular prion protein (PrP^C). In this study, we showed that dopamine treatment of neuronal cells reduced the number of viable cells and increased the production of reactive oxygen species (ROS) as demonstrated in previous studies. Overall PrP^C expression level was not altered by dopamine treatment, but its unglycosylated form was consistently reduced at 100 μM of dopamine. At the same concentration, the level of phosphorylated mTOR and 4EBP1 was also reduced. Moreover, dopamine treatment decreased the solubility of PrP^C, and increased its accumulation in autophagosomal compartments with concomitant induction of LC3-II and p62/SQSTM1 levels. In vitro oxidation of dopamine promoted formation of high-order oligomers of recombinant prion protein. These results suggest that dopamine metabolites alter the conformation of PrP^C, which in turn is sorted to degradation pathway, causing autophagosome overload and attenuation of protein synthesis. Accumulation of PrP^C aggregates is an important feature of prion diseases. Thus, this study brings new insight into the dopamine metabolism as a source of endogenous metabolites capable of altering PrP^C solubility and its subcellular localization.

Prion protein promotes kidney iron uptake via its ferrireductase activity

Brain iron-dyshomeostasis is an important cause of neurotoxicity in prion disorders, a group of neurodegenerative conditions associated with the conversion of prion protein (PrP(C)) from its normal conformation to an aggregated, PrP-scrapie (PrP(Sc)) isoform. Alteration of iron homeostasis is believed to result from impaired function of PrP(C) in neuronal iron uptake via its ferrireductase activity. However, unequivocal evidence supporting the ferrireductase activity of PrP(C) is lacking. Kidney provides a relevant model for this evaluation because PrP(C) is expressed in the kidney, and ∼370 μg of iron are reabsorbed daily from the glomerular filtrate by kidney proximal tubule cells (PT), requiring ferrireductase activity. Here, we report that PrP(C) promotes the uptake of transferrin (Tf) and non-Tf-bound iron (NTBI) by the kidney in vivo and mainly NTBI by PT cells in vitro. Thus, uptake of (59)Fe administered by gastric gavage, intravenously, or intraperitoneally was significantly lower in PrP-knock-out (PrP(-/-)) mouse kidney relative to PrP(+/+) controls. Selective in vivo radiolabeling of plasma NTBI with (59)Fe revealed similar results. Expression of exogenous PrP(C) in immortalized PT cells showed localization on the plasma membrane and intracellular vesicles and increased transepithelial transport of (59)Fe-NTBI and to a smaller extent (59)Fe-Tf from the apical to the basolateral domain. Notably, the ferrireductase-deficient mutant of PrP (PrP(Δ51-89)) lacked this activity. Furthermore, excess NTBI and hemin caused aggregation of PrP(C) to a detergent-insoluble form, limiting iron uptake. Together, these observations suggest that PrP(C) promotes retrieval of iron from the glomerular filtrate via its ferrireductase activity and modulates kidney iron metabolism.

Iron in neurodegenerative disorders of protein misfolding: a case of prion disorders and Parkinson's disease

SIGNIFICANCE

Intracellular and extracellular aggregation of a specific protein or protein fragments is the principal pathological event in several neurodegenerative conditions. We describe two such conditions: sporadic Creutzfeldt-Jakob disease (sCJD), a rare but potentially infectious and invariably fatal human prion disorder, and Parkinson's disease (PD), a common neurodegenerative condition second only to Alzheimer's disease in prevalence. In sCJD, a cell surface glycoprotein known as the prion protein (PrP(C)) undergoes a conformational change to PrP-scrapie, a pathogenic and infectious isoform that accumulates in the brain parenchyma as insoluble aggregates. In PD, α-synuclein, a cytosolic protein, forms insoluble aggregates that accumulate in neurons of the substantia nigra and cause neurotoxicity.

RECENT ADVANCES

Although distinct processes are involved in the pathogenesis of sCJD and PD, both share brain iron dyshomeostasis as a common associated feature that is reflected in the cerebrospinal fluid in a disease-specific manner.

CRITICAL ISSUES

Since PrP(C) and α-synuclein play a significant role in maintaining cellular iron homeostasis, it is important to understand whether the aggregation of these proteins and iron dyshomeostasis are causally related. Here, we discuss recent information on the normal function of PrP(C) and α-synuclein in cellular iron metabolism and the cellular and biochemical processes that contribute to iron imbalance in sCJD and PD.

FUTURE DIRECTIONS

Improved understanding of the relationship between brain iron imbalance and protein aggregation is likely to help in the development of therapeutic strategies that can restore brain iron homeostasis and mitigate neurotoxicity.

Iron-induced fibrin in cardiovascular disease

Accumulating evidence within the last two decades indicates the association between cardiovascular disease (CVD) and chronic inflammatory state. Under normal conditions fibrin clots are gradually degraded by the fibrinolytic enzyme system, so no permanent insoluble deposits remain in the circulation. However, fibrinolytic therapy in coronary and cerebral thrombosis is ineffective unless it is installed within 3-5 hours of the onset. We have shown that trivalent iron (FeIII) initiates a hydroxyl radical-catalyzed conversion of fibrinogen into a fibrin-like polymer (parafibrin) that is remarkably resistant to the proteolytic dissolution and thus promotes its intravascular deposition. Here we suggest that the persistent presence of proteolysis-resistant fibrin clots causes chronic inflammation. We study the effects of certain amphiphilic substances on the iron- and thrombin-induced fibrinogen polymerization visualized using scanning electron microscopy. We argue that the culprit is an excessive accumulation of free iron in blood, known to be associated with CVD. The only way to prevent iron overload is by supplementation with iron chelating agents. However, administration of free radical scavengers as effective protection against persistent presence of fibrin-like deposits should also be investigated to contribute to the prevention of cardiovascular and other degenerative diseases.

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

The secondary structures of amyloidogenic proteins are largely influenced by various intra and extra cellular microenvironments and metal ions that govern cytotoxicity. The secondary structure of a prion fragment, PrP(111-126), was determined using circular dichroism (CD) spectroscopy in various microenvironments. The conformational preferences of the prion peptide fragment were examined by changing solvent conditions and pH, and by introducing external stress (sonication). These physical and chemical environments simulate various cellular components at the water-membrane interface, namely differing aqueous environments and metal chelating ions. The results show that PrP(111-126) adopts different conformations in assembled and non-assembled forms. Aging studies on the PrP(111-126) peptide fragment in aqueous buffer demonstrated a structural transition from random coil to a stable β-sheet structure. A similar, but significantly accelerated structural transition was observed upon sonication in aqueous environment. With increasing TFE concentrations, the helical content of PrP(111-126) increased persistently during the structural transition process from random coil. In aqueous SDS solution, PrP(111-126) exhibited β-sheet conformation with greater α-helical content. No significant conformational changes were observed under various pH conditions. Addition of Cu²⁺ ions inhibited the structural transition and fibril formation of the peptide in a cell free in vitro system. The fact that Cu²⁺ supplementation attenuates the fibrillar assemblies and cytotoxicity of PrP(111-126) was witnessed through structural morphology studies using AFM as well as cytotoxicity using MTT measurements. We observed negligible effects during both physical and chemical stimulation on conformation of the prion fragment in the presence of Cu²⁺ ions. The toxicity of PrP(111-126) to cultured astrocytes was reduced following the addition of Cu²⁺ ions, owing to binding affinity of copper towards histidine moiety present in the peptide.

The role of iron-induced fibrin in the pathogenesis of Alzheimer's disease and the protective role of magnesium

Amyloid hypothesis of Alzheimer's disease (AD) has recently been challenged by the increasing evidence for the role of vascular and hemostatic components that impair oxygen delivery to the brain. One such component is fibrin clots, which, when they become resistant to thrombolysis, can cause chronic inflammation. It is not known, however, why some cerebral thrombi are resistant to the fibrinolytic degradation, whereas fibrin clots formed at the site of vessel wall injuries are completely, although gradually, removed to ensure proper wound healing. This phenomenon can now be explained in terms of the iron-induced free radicals that generate fibrin-like polymers remarkably resistant to the proteolytic degradation. It should be noted that similar insoluble deposits are present in AD brains in the form of aggregates with Abeta peptides that are resistant to fibrinolytic degradation. In addition, iron-induced fibrin fibers can irreversibly trap red blood cells (RBCs) and in this way obstruct oxygen delivery to the brain and induce chronic hypoxia that may contribute to AD. The RBC-fibrin aggregates can be disaggregated by magnesium ions and can also be prevented by certain polyphenols that are known to have beneficial effects in AD. In conclusion, we argue that AD can be prevented by: (1) limiting the dietary supply of trivalent iron contained in red and processed meat; (2) increasing the intake of chlorophyll-derived magnesium; and (3) consumption of foods rich in polyphenolic substances and certain aliphatic and aromatic unsaturated compounds. These dietary components are present in the Mediterranean diet known to be associated with the lower incidence of AD and other degenerative diseases.

Prion protein conversion induced by trivalent iron in vesicular trafficking

Iron dyshomeostasis has been observed in prion diseases; however, little is known regarding the contribution of the oxidation state of iron to prion protein (PrP) conversion. In this study, PrP(C)-deficient HpL3-4 cells were exposed to divalent [Fe(II)] or trivalent [Fe(III)] iron, followed by exogenous recombinant PrP (rPrP) treatment. We then analyzed the accumulation of internalized rPrP and its biochemical properties, including its resistance to both proteinase K (PK) digestion and detergent solubility. Fe(III), but not Fe(II), induced the accumulation of internalized rPrP, which was partially converted to detergent-insoluble and PK-resistant PrP (PrP(res)). The Fe(III)-induced PrP(res) generation required an intact cell structure, and it was hindered by U18666A, an inhibitor of vesicular trafficking, but not by NH4Cl, an inhibitor of endolysosomal acidification. These observations implicated that the Fe(III)-mediated PrP(res) conversion likely occurs during endosomal vesicular trafficking rather than in the acidic environment of lysosomes.

Change in the characteristics of ferritin induces iron imbalance in prion disease affected brains

Prion disease associated neurotoxicity is mainly attributed to PrP-scrapie (PrP(Sc)), the disease associated isoform of a normal protein, the prion protein (PrP(C)). Participation of other proteins and processes is suspected, but their identity and contribution to the pathogenic process is unclear. Emerging evidence implicates imbalance of brain iron homeostasis as a significant cause of prion disease-associated neurotoxicity. The underlying cause of this change, however, remains unclear. We demonstrate that iron is sequestered in heat and SDS-stable protein complexes in sporadic-Creutzfeldt-Jakob-disease (sCJD) brains, creating a phenotype of iron deficiency. The underlying cause is change in the characteristics of ferritin, an iron storage protein that becomes aggregated, detergent-insoluble, and partitions with denatured ferritin using conventional methods of ferritin purification. A similar phenotype of iron deficiency is noted in the lumbar spinal cord (SC) tissue of scrapie infected hamsters, a site unlikely to be affected by massive neuronal death and non-specific iron deposition. As a result, the iron uptake protein transferrin (Tf) is upregulated in scrapie infected SC tissue, and increases with disease progression. A direct correlation between Tf and PrP(Sc) suggests sequestration of iron in dysfunctional ferritin that either co-aggregates with PrP(Sc) or is rendered dysfunctional by PrP(Sc) through an indirect process. Surprisingly, amplification of PrP(Sc)in vitro by the protein-misfolding-cyclic-amplification (PMCA) reaction using normal brain homogenate as substrate does not increase the heat and SDS-stable pool of iron even though both PrP(Sc) and ferritin aggregate by this procedure. These observations highlight important differences between PrP(Sc)-protein complexes generated in vivo during disease progression and in vitro by the PMCA reaction, and the significance of these complexes in PrP(Sc)-associated neurotoxicity.

PrPC displays an essential protective role from oxidative stress in an astrocyte cell line derived from PrPC knockout mice

The PrP(C) protein, which is especially present in the cellular membrane of nervous system cells, has been extensively studied for its controversial antioxidant activity. In this study, we elucidated the free radical scavenger activity of purified murine PrP(C) in solution and its participation as a cell protector in astrocytes that were subjected to treatment with an oxidant. In vitro and using an EPR spin-trapping technique, we observed that PrP(C) decreased the oxidation of the DMPO trap in a Fenton reaction system (Cu(2+)/ascorbate/H(2)O(2)), which was demonstrated by approximately 70% less DMPO/OH(). In cultured PrP(C)-knockout astrocytes from mice, the absence of PrP(C) caused an increase in intracellular ROS (reactive oxygen species) generation during the first 3h of H(2)O(2) treatment. This rapid increase in ROS disrupted the cell cycle in the PrP(C)-knockout astrocytes, which increased the population of cells in the sub-G1 phase when compared with cultured wild-type astrocytes. We conclude that PrP(C) in solution acts as a radical scavenger, and in astrocytes, it is essential for protection from oxidative stress caused by an external chemical agent, which is a likely condition in human neurodegenerative CNS disorders and pathological conditions such as ischemia.

Acute exposure to prion infection induces transient oxidative stress progressing to be cumulatively deleterious with chronic propagation in vitro

Neuronal loss is a pathological feature of prion diseases for which increased reactive oxygen species (ROS) and consequent oxidative stress is one proposed mechanism. The processes underlying ROS production in prion disease and the precise relationship to misfolding of the prion protein remain obscure. Using cell culture models of prion infection we found that cells demonstrate a rapid, prion protein (PrP) dependent, increase in intracellular ROS following exposure to infectious inoculum. ROS production correlated with internalisation and increased intracellular protease resistant PrP (PrP(Res)). The ROS increase was predominantly lysosomal in origin but not sustained, with cells adapting within 48 hours. Overall ROS levels remained normal in the chronically prion infected cell population; however a subpopulation characterised by loss of membrane phosphatidylserine asymmetry exhibited highly peroxidised intracellular aggregates that localised with PrP and intense caspase activation. These apoptotic cells showed increased ROS closely correlating with increased PrP(Res). Our findings demonstrate that a PrP-dependent, transient, increase in intracellular ROS is characteristic of acute cellular prion infection, while chronic phases of prion infection in vitro are associated with a significant subpopulation manifesting apoptosis accompanying heightened oxidative stress and increased PrP(Res) burden. Such observations strengthen the direct links between heightened ROS and ongoing prion propagation with eventual cellular demise.