Amyloid beta–heme peroxidase promoted protein nitrotyrosination: relevance to widespread protein nitration in Alzheimer’s disease

Cysteine in the R3 Tau Peptide Modulates Hemin Binding and Reactivity

Tau is a neuronal protein involved in axonal stabilization; however under pathological conditions, it triggers the deposition of insoluble neurofibrillary tangles, which are one of the biomarkers for Alzheimer's disease. The factors that might influence the fibrillation process are i) two cysteine residues in two pseudorepetitive regions, called R2 and R3, which can modulate protein-protein interaction via disulfide cross-linking; ii) an increase of reactive oxygen species affecting the post-translational modification of tau; and iii) cytotoxic levels of metals, especially ferric-heme (hemin), in hemolytic processes. Herein, we investigated how the cysteine-containing R3 peptide (R3C) and its Cys→Ala mutant (R3A) interact with hemin and how their binding affects the oxidative damage of the protein. The calculated binding constants are remarkably higher for the hemin-R3C complex (LogK1 = 5.90; LogK2 = 5.80) with respect to R3A (LogK1 = 4.44; LogK2 < 2), although NMR and CD investigations excluded the direct binding of cysteine as an iron axial ligand. Both peptides increase the peroxidase-like activity of hemin toward catecholamines and phenols, with a double catalytic efficiency detected for hemin-R3C systems. Moreover, the presence of cysteine significantly alters the susceptibility of R3 toward oxidative modifications, easily resulting in peptide dopamination and formation of cross-linked S-S derivatives.

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.

In vitro coordination of Fe-protoheme with amyloid β is non-specific and exhibits multiple equilibria

In addition to copper and zinc, heme is thought to play a role in Alzheimer's disease and its metabolism is strongly affected during the course of this disease. Amyloid β, the peptide associated with Alzheimer's disease, was shown to bind heme in vitro with potential catalytic activity linked to oxidative stress. To date, there is no direct determination of the structure of this complex. In this work, we studied the binding mode of heme to amyloid β in different conditions of pH and redox state by using isotopically labelled peptide in combination with advanced magnetic and vibrational spectroscopic methods. Our results show that the interaction between heme and amyloid β leads to a variety of species in equilibrium. The formation of these species seems to depend on many factors suggesting that the binding site is neither very strong nor highly specific. In addition, our data do not support the currently accepted model where a water molecule is bound to the ferric heme as sixth ligand. They also exclude structural models mimicking a peroxidatic site in the amyloid β-Fe-protoheme complexes.

Raman Imaging Reveals Accumulation of Hemoproteins in Plaques from Alzheimer's Diseased Tissues

Hemes have been suggested to play a central role in Alzheimer's disease since they show high peroxidase reactivity when bound to amyloid β peptides, leading to the production of reactive oxygen species. Here we used Fourier transform infrared and Raman imaging on Alzheimer's diseased mice and human brain tissue. Our finding suggests the accumulation of hemes in the senile plaques of both murine and human samples. We compared the Raman signature of the plaques to the ones of various hemeoproteins and to the hemin-Aβ-42 complex. The detected Raman signature of the plaques does not allow identifying the type of heme accumulating in the plaques.

Hesperetin Nanocrystals Improve Mitochondrial Function in a Cell Model of Early Alzheimer Disease

Mitochondrial dysfunction represents a hallmark of both brain aging and age-related neurodegenerative disorders including Alzheimer disease (AD). AD-related mitochondrial dysfunction is characterized by an impaired electron transport chain (ETC), subsequent decreased adenosine triphoshpate (ATP) levels, and elevated generation of reactive oxygen species (ROS). The bioactive citrus flavanone hesperetin (Hst) is known to modulate inflammatory response, to function as an antioxidant, and to provide neuroprotective properties. The efficacy in improving mitochondrial dysfunction of Hst nanocrystals (HstN) with increased bioavailability has not yet been investigated. Human SH-SY5Y cells harboring neuronal amyloid precursor protein (APP₆₉₅) acted as a model for the initial phase of AD. MOCK-transfected cells served as controls. The energetic metabolite ATP was determined using a luciferase-catalyzed bioluminescence assay. The activity of mitochondrial respiration chain complexes was assessed by high-resolution respirometry using a Clarke electrode. Expression levels of mitochondrial respiratory chain complex genes were determined using quantitative real-time polymerase chain reaction (qRT-PCR). The levels of amyloid β-protein (Aβ_1-40) were measured using homogeneous time-resolved fluorescence (HTRF). ROS levels, peroxidase activity, and cytochrome c activity were determined using a fluorescence assay. Compared to pure Hst dissolved in ethanol (HstP), SH-SY5Y-APP₆₉₅ cells incubated with HstN resulted in significantly reduced mitochondrial dysfunction: ATP levels and respiratory chain complex activity significantly increased. Gene expression levels of RCC I, IV, and V were significantly upregulated. In comparison, the effects of HstN on SY5Y-MOCK control cells were relatively small. Pure Hst dissolved in ethanol (HstP) had almost no effect on both cell lines. Neither HstN nor HstP led to significant changes in Aβ_1-40 levels. HstN and HstP were both shown to lower peroxidase activity significantly. Furthermore, HstN significantly reduced cytochrome c activity, whereas HstP had a significant effect on reducing ROS in SH-SY5Y-APP₆₉₅ cells. Thus, it seems that the mechanisms involved may not be linked to altered Aβ production. Nanoflavonoids such as HstN have the potential to prevent mitochondria against dysfunction. Compared to its pure form, HstN showed a greater effect in combatting mitochondrial dysfunction. Further studies should evaluate whether HstN protects against age-related mitochondrial dysfunction and thus may contribute to late-onset AD.

Condition-Dependent Coordination and Peroxidase Activity of Hemin-Aβ Complexes

The peroxidase activity of hemin-peptide complexes remains a potential factor in oxidative damage relevant to neurodegeneration. Here, we present the effect of temperature, ionic strength, and pH relevant to pathophysiological conditions on the dynamic equilibrium between high-spin and low-spin hemin-Aβ40 constructs. This influence on peroxidase activity was also demonstrated using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and dopamine (DA) oxidation rate analyses with increasing ratios of Aβ16 and Aβ40 (up to 100 equivalents). Interaction and reactivity studies of aggregated Aβ40-hemin revealed enhanced peroxidase activity versus hemin alone. Comparison of the results obtained using Aβ16 and Aβ40 amyloid beta peptides revealed marked differences and provide insight into the potential effects of hemin-Aβ on neurological disease progression.

Molecular crowding effects on the biochemical properties of amyloid β-heme, Aβ-Cu and Aβ-heme-Cu complexes

Heme as a cofactor has been proposed to bind with β-amyloid peptide (Aβ) and the formed Aβ-heme complex exhibits enhanced peroxidase-like activity. So far, in vitro studies on the interactions between heme, Cu and Aβ have been exclusively performed in dilute solution. However, the intracellular environment is highly crowded with biomolecules. Therefore, exploring how Aβ-heme-Cu complexes behave under molecular crowding conditions is critical for understanding the mechanism of Aβ neurotoxicity in vivo. Herein, we selected PEG-200 as a crowding agent to mimic the crowded cytoplasmic environment for addressing the contributions of crowded physiological environments to the biochemical properties of Aβ-heme, Aβ-Cu and Aβ-heme-Cu complexes. Surprisingly, experimental studies and theoretical calculations revealed that molecular crowding weakened the stabilization of the Aβ-heme complex and decreased its peroxidase activity. Our data attributed this consequence to the decreased binding affinity of heme to Aβ as a result of the alterations in water activity and Aβ conformation. Our findings highlight the significance of hydration effects on the interaction of Aβ-heme and Aβ-Cu and their peroxidase activities. Molecular crowding inside cells may potentially impose a positive effect on Aβ-Cu but a negative effect on the interaction of Aβ with heme. This indicates that Aβ40-Cu but not Aβ40-heme may play more important roles in the oxidative damage in the etiology of AD. Therefore, this work provides a new clue for understanding the oxidative damage occurring in AD.

Metallotexaphyrins as MRI-Active Catalytic Antioxidants for Neurodegenerative Disease: A Study on Alzheimer's Disease

The complex etiology of neurodegeneration continues to stifle efforts to develop effective therapeutics. New agents elucidating key pathways causing neurodegeneration might serve to increase our understanding and potentially lead to improved treatments. Here, we demonstrate that a water-soluble manganese(II) texaphyrin (MMn) is a suitable magnetic resonance imaging (MRI) contrast agent for detecting larger amyloid beta constructs. The imaging potential of MMn was inferred on the basis of in vitro studies and in vivo detection in Alzheimer's disease C. elegans models via MRI and ICP-MS. In vitro antioxidant- and cellular-based assays provide support for the notion that this porphyrin analog shows promise as a therapeutic agent able to mitigate the oxidative and nitrative toxic effects considered causal in neurodegeneration. The present report marks the first elaboration of an MRI-active metalloantioxidant that confers diagnostic and therapeutic benefit in Alzheimer's disease models without conjugation of a radioisotope, targeting moiety, or therapeutic payload.

Origin and pathophysiology of protein carbonylation, nitration and chlorination in age-related brain diseases and aging

Non-enzymatic protein modifications occur inevitably in all living systems. Products of such modifications accumulate during aging of cells and organisms and may contribute to their age-related functional deterioration. This review presents the formation of irreversible protein modifications such as carbonylation, nitration and chlorination, modifications by 4-hydroxynonenal, removal of modified proteins and accumulation of these protein modifications during aging of humans and model organisms, and their enhanced accumulation in age-related brain diseases.

Free Heme and Amyloid-β: A Fatal Liaison in Alzheimer's Disease

While the etiology of Alzheimer's disease (AD) is still unknown, an increased formation of amyloid-β (Aβ) peptide and oxidative processes are major pathological mechanism of the disease. The interaction of Aβ with free heme leads to the formation of peroxidase-active Aβ-heme complexes. However, enzyme-kinetic data and systematic mutational studies are still missing. These aspects were addressed in this study to evaluate the role of Aβ-heme complexes in AD. The enzyme-kinetic measurements showed peroxidase-specific pH- and H2O2-dependencies. In addition, the enzymatic activity of Aβ-heme complexes constantly increased at higher peptide excess. Moreover, the role of the Aβ sequence for the named enzymatic activity was tested, depicting human-specific R5, Y10, and H13 as essential amino acids. Also by studying Y10 as an endogenous peroxidase substrate for Aβ-heme complexes, ratio-specific effects were observed, showing an optimal dityrosine formation at an about 40-fold peptide excess. As dityrosine formation promotes Aβ fibrillation while free heme disturbs protein aggregation, we also investigated the effect of Aβ-heme complex-derived peroxidase activity on the formation of Aβ fibrils. The fluorescence measurements showed a different fibrillation behavior at strong peroxidase activity, leading also to altered fibril morphologies. The latter was detected by electron microscopy. As illustrated by selected in vivo measurements on a mouse model of AD, the disease is also characterized by Aβ-derived microvessel destructions and hemolytic processes. Thus, thrombo-hemorrhagic events are discussed as a source for free heme in brain tissue. In summary, we suggest the formation and enzymatic activity of Aβ-heme complexes as pathological key features of AD.

Copper Binding Induces Nitration of NPY under Nitrative Stress: Complicating the Role of NPY in Alzheimer's Disease

Neuropeptide Y (NPY) is a 36 amino acid peptide that regulates a multitude of physiological functions in the central nervous system and has been shown to be involved in Alzheimer's disease (AD). A change in copper homeostasis is a remarkable feature of AD, and the dysregulation may contribute to toxicity in neural cells. Moreover, it has been shown that copper could interact with many neuropeptides and result in catalyzing the production of reactive oxygen species, which may lead to peptide oxidation. Besides, copper could also catalyze protein tyrosine nitration under oxidative stress, and there are two tyrosine residues playing an important role in NPY. Therefore, it is also likely that copper has an action on NPY and potentially influences its functions through tyrosine nitration. In this paper, the studies of the interaction of copper with NPY and the copper-catalyzed NPY nitration were performed. The electrochemical techniques, UV-vis spectroscopy, mass spectrometry, and fluorescence titration, have been applied to show that copper can interact with NPY to form a Cu-NPY complex with a conditional dissociation constant of 0.021 μmol/L, and the binding promotes the generation of •OH. Dot blotting results reveal that NPY can be nitrated upon binding with copper under nitrative stress. Furthermore, liquid chromatography-mass spectrometry (LC-MS) identify that the tyrosine residues in NPY are all nitrated during the nitration process, which will cause the inactivation of NPY shown by our previous study. This study supports the hypothesis that copper has a close correlation with NPY and implicates the peptide in AD. These data may provide a new insight into understanding the pathology and pathogenesis of AD.

NPY binds with heme to form a NPY–heme complex: enhancing peroxidase activity in free heme and promoting NPY nitration and inactivation

NPY binding with heme enhances the peroxidase activity of free heme, resulting in the important tyrosine nitration, which will attenuate its bioactivity.

Is there correlation between Aβ-heme peroxidase activity and the peptide aggregation state? A literature review combined with hypothesis

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by aggregation of amyloid-β (Aβ) peptide, formation of neurofibrillary tangles, synaptic loss and neuronal cell death, and is manifested clinically by progressive cognitive dysfunction and memory loss. Disease pathogenesis is mainly linked to the formation of Aβ insoluble or soluble oligomeric assemblies. Binding of heme to Aβ has been suggested as the origin of the heme deficiency, peroxidase activity, as well as some oxidative stress-mediated AD pathologies, and then differential affinity of heme for human and rodent Aβ peptide has been proposed to account for the susceptibility of humans to AD. This review highlights whether there is any dependency of peroxidase activity of heme-bound Aβ on the Aβ aggregation state or not, with focusing on emerging role of heme in neurodegeneration. Here, several lines of evidence supporting existing contradictory conjectures are discussed.

Effect of protein structure and/or conformation on the dityrosine cross-linking induced by haem-hydrogen peroxide

BACKGROUND

Haem, an essential cofactor in aerobic organisms, can cause oxidative stress and impose toxic effects on tissues and organs. It can induce aggregation of proteins via dityrosine cross-linking and cause neurodegenerative diseases. Although dityrosine cross-linking in many proteins induced by haem has been reported, not all the proteins have the same effect or the efficiency of cross-linking varies, while the reason has not been clarified.

METHODS

The correlation of protein structure/conformation with its aggregation tendency via dityrosine induced by hematin (oxidized form of haem) in the presence of hydrogen peroxide (H2O2) was studied through reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), fluorescence and circular dichroism (CD) measurements, and the mechanism was investigated by performing UV-Vis absorbance, Raman spectroscopy and low-temperature electron spin resonance (ESR) experiments.

RESULTS

It was found that proteins in unstructured state are more readily to be cross-linked via dityrosine formation by hematin-H2O2. The unstructured protein without steric effect can coordinate with hematin to form six-coordinated protein-hematin complex, in which the produced tyrosyl radicals by H2O2 are with high tendency to dimerize to form dityrosine.

CONCLUSIONS

Our results demonstrate that protein structure/conformation can affect its coordination state with haem, and the tendency of reaction of two tyrosyl radicals, further influencing the yield and efficiency of dityrosine cross-linking in the presence of H2O2.

GENERAL SIGNIFICANCE

This research can help to deepen our understanding of the protein aggregation and inactivation mechanisms in varied sophisticated conditions, and especially give us the new insight into the toxic effects under haem stress.

Key roles of Tyr 10 in Cu bound Aβ complexes and its relevance to Alzheimer's disease

Recent studies show that the accumulation of redox-active Cu mediates the aggregation of amyloid β-peptide (Aβ) and conspicuous oxidative damage to the brain in Alzheimer's disease (AD). However, the key roles for Tyr 10 in Aβ-Cu(II) complex and its potential biological relevance to AD etiology under oxidative stress, were not stressed enough. Interestingly, our results indicated that Aβ40 (not Aβ16)-Cu(II) complex showed obviously enhanced peroxidase activity than free Cu(II). Although Tyr 10 was not the residue binding Cu(II), the mutation of Tyr 10 residue in Aβ40 decreased the peroxidase activity of Aβ40-Cu(II) complex, and the mutation of Tyr 10 could inhibit Aβ40 aggregation. Under oxidative and nitrative stress conditions, the Aβ-Cu(II) complex caused oxidation and nitration of the Aβ Tyr 10 residue through peroxidase-like reactions, where the formation of Cu(I) and hydroxyl radical (OH) was proposed as a chemical mechanism. We also showed that, when Aβ40 aggregates were bound to Cu(II), they retained peroxidase-like activity. Therefore, Tyr 10 residue is pivotal in Aβ-Cu(II) complex and shows important relevance to oxidative stress, implicating the novel significance of Tyr 10 residue as well as Aβ-Cu(II) complex in the pathology of AD.

Key roles of Arg(5), Tyr(10) and his residues in Aβ-heme peroxidase: relevance to Alzheimer's disease

Recent reports show that heme binds to amyloid β-peptide (Aβ) in the brain of Alzheimer's disease (AD) patients and forms Aβ-heme complexes, thus leading a pathological feature of AD. However, the important biological relevance to AD etiology, resulting from human Aβ-heme peroxidase formation, was not well characterized. In this study, we used wild-type and mutated human Aβ1-16 peptides and investigated their Aβ-heme peroxidase activities. Our results indicated that both histidine residues (His(13), His(14)) in Aβ1-16 and free histidine enhanced the peroxidase activity of heme, hence His residues were essential in peroxidase activity of Aβ-heme complexes. Moreover, Arg(5) was found to be the key residue in making the Aβ1-16-heme complex as a peroxidase. Under oxidative and nitrative stress conditions, the Aβ1-16-heme complexes caused oxidation and nitration of the Aβ Tyr(10) residue through promoting peroxidase-like reactions. Therefore, these residues (Arg(5), Tyr(10) and His) were pivotal in human Aβ-heme peroxidase activity. However, three of these residues (Arg(5), Tyr(10) and His(13)) identified in this study are all absent in rodents, where rodent Aβ-heme complex lacks peroxidase activity and it does not show AD, implicating the novel significance of these residues as well as human Aβ-heme peroxidase in the pathology of AD.

Nitrative modifications of α-enolase in hepatic proteins from diabetic rats: the involvement of myeloperoxidase

Many studies reported that oxidative and nitrative stress might be important in the pathogenesis of diabetes and the development of its complications. In this study, we showed that α-enolase (EC 4.2.1.11, 2-phospho-d-glycerate hydrolase) was identified as the important target for oxidative and nitrative modifications in diabetic hepatic proteins. After 6 weeks of streptozotocin-administration, α-enolase expression and nitration were clearly increased in diabetic rat liver, whereas the enolase activity and oxidation status were not significantly changed in diabetic group. By means of immunoprecipitation and liquid chromatography-tandem mass spectrometry analysis, it was found that Tyr 12 and Tyr 257 of α-enolase were the most susceptible to nitration in diabetic rat liver. Moreover, myeloperoxidase (MPO) as a likely alternative mechanism for nitrative modification of α-enolase in vivo was apparently facilitated by the presence of higher MPO level and activity in diabetic liver, and fact that Tyr 12 and Tyr 191 of enolase was nitrated by MPO/nitrite/H2O2 system in vitro. Further studies in vitro indicated that carbonyl formation, rather than tyrosine nitration, might make a major contribution to the inactivation of enolase. The present results provided the new evidence for α-enolase as a susceptive target for MPO-catalyzed nitrative modification in diabetes. They also suggested a potential contribution of nitrative and oxidative modifications of enolase to an impaired glycolytic activity in diabetic hepatic injury.

New insights into Alzheimer's disease amyloid inhibition: nanosized metallo-supramolecular complexes suppress aβ-induced biosynthesis of heme and iron uptake in PC12 cells

Nanosized metallo-supramolecular compounds, [Ni2 L3 ](4+) and [Fe2 L3 ](4+) , can not only strongly inhibit Aβ aggregation but also reduce the peroxidase activity of Aβ-heme. Further studies demonstrate that through blocking the heme-binding site, these two compounds can suppress Aβ-induced biosynthesis of heme and iron uptake in PC12 cells. This work provides new insights into molecular mechanisms of Aβ inhibitors on Aβ-mediated neurotoxicity.

Heme prevents amyloid beta peptide aggregation through hydrophobic interaction based on molecular dynamics simulation

Heme, which is abundant in hemoglobin and many other hemoproteins, is known to play an important role in electron transfer, oxygen transport, regulation of gene expression, and many other biological functions. With the belief that the aggregation of Aβ peptides forming higher order oligomers is one of the central pathological pathways in Alzheimer's disease, the formation of the Aβ-heme complex is essential as it inhibits Aβ aggregation and protects the neurons from degradation. In our studies, conventional molecular dynamics simulations were performed on the 1 Aβ + 1 heme and 2 Aβ + 4 hemes system, respectively, with the identification of several dominant binding motifs. We found that hydrophobic residues of the Aβ peptide have a high affinity to interact with heme instead of the histidine residue. We conclude that hydrophobic interaction plays a dominant role in the Aβ-heme complex formation which indirectly serves to physically prevent Aβ aggregation.