Evidence for a novel heme-binding protein, HasAh, in Alzheimer disease

Multiple lines of evidence indicate that oxidative stress is an integral component of the pathogenesis of Alzheimer disease (AD). The precipitating cause of such oxidative stress may be misregulated iron homeostasis because there are profound alterations in heme oxygenase-1 (HO-1), redox-active iron, and iron regulatory proteins. In this regard, HasA, a recently characterized bacterial protein involved in heme acquisition and iron metabolism, may also be important in the generation of reactive oxygen species (ROS) given its ability to bind heme and render iron available for free radical generation through the Fenton reaction. To study further the role of heme binding and iron metabolism in AD, we show an abnormal localization of anti-HasA to the neurofibrillary pathology of AD, but not in normal-appearing neurons in the brains of cases of AD or in age-matched controls. These results suggest the increased presence in AD of a HasA homologue or protein sharing a common epitope with HasA, which we term HasAh. We conclude that heme binding of HasAh is a potential source of free soluble iron and therefore toxic free radicals in AD and in aging. This furthers the evidence that redox-active iron and subsequent Fenton reaction generating reactive oxygen are critical factors in the pathogenesis of AD.

Protein disulfide isomerase in Alzheimer disease

There is a great deal of evidence that places oxidative stress as a proximal event in the natural history of Alzheimer disease (AD). In addition to increased damage, there are compensatory increases in the levels of free sulfhydryls, glucose-6-phosphate dehydrogenase, and NAD(P)H:quinone oxidoreductase 1. To investigate redox homeostasis further in AD, we analyzed protein disulfide isomerase (PDI), a multifunctional enzyme, which catalyzes the disruption and formation of disulfide bonds. PDI plays a pivotal role in both secreted and cell-surface-associated protein disulfide rearrangement. In this study, we show that PDI specifically localizes to neurons, where there is no substantial increase in AD compared to age-matched controls. These findings indicate that the neurons at risk of death in AD do not show a substantial change in PDI to compensate for the increased sulfhydryls and reductive state found during the disease. This suggests that, despite compensatory reductive changes in AD, the level of PDI is sufficiently high physiologically in neurons to accommodate a more reducing environment.

Metabolic, metallic, and mitotic sources of oxidative stress in Alzheimer disease

Cell bodies of neurons at risk of death in Alzheimer disease (AD) have increased lipid peroxidation, nitration, free carbonyls, and nucleic acid oxidation. These oxidative changes are uniform among neurons and are seen whether or not the neurons display neurofibrillary tangles and, in fact, are actually reduced in the latter case. In consideration of this localization of damage, in this review, we provide a summary of recent work demonstrating some key abnormalities that may initiate and promote neuronal oxidative damage. First, mitochondrial abnormalities might be the source of reactive oxygen species yielding perikaryal oxidative damage. The common 5-kb deletion mitochondrial (mt)DNA subtype was greatly increased in the AD cases, but only in neurons at risk. The importance of such mitochondrial abnormalities to oxidative stress was indicated by a high correlation coefficient between the extent of the mtDNA increase and RNA oxidative damage (r2 = 0.87). Nonetheless, because mitochondria in AD do not show striking oxidative damage, as one would expect if they were the direct producer of free radical species, we suspected that abnormal mitochondria supply a key reactant that, once in the cytoplasm, releases radicals. One such reactant, hydrogen peroxide, (H2O2), abundant in mitochondria, can react with iron via the Fenton reaction to produce.OH. To demonstrate this directly using a modified cytochemical technique that relies on the formation of mixed valence iron complexes, we found that redox-active iron is associated with vulnerable neurons. Interestingly, removal of iron was completely affected by using deferroxamine, after which iron could be rebound to re-establish lesion-dependent catalytic redox reactivity. Characterization of the iron-binding site suggests that binding is dependent on available histidine residues and on protein conformation. Taken together with our previous studies showing abnormalities in the iron homeostatic system including heme oxygenase, iron regulatory proteins 1 and 2, ceruloplasmin, and dimethylargininase, our results indicate that iron misregulation could play an important role in the pathogenesis of AD and therefore chelation therapy may be a useful therapeutic approach. Finally, we wanted to determine the proximal cause of mitochondrial abnormalities. One interesting mechanisms involves re-entry into the cell cycle, at which point organellokinesis and proliferation results in increased mitochondria. Supporting this, we have considerable in vivo and in vitro evidence for mitotic disturbances in AD and its relationship with the pathogenesis of AD.

In situ oxidative catalysis by neurofibrillary tangles and senile plaques in Alzheimer's disease: a central role for bound transition metals

There is a great deal of evidence to support a pathogenic role of oxidative stress in Alzheimer's disease (AD), but the sources of reactive oxygen species have not been directly demonstrated. In this study, using a novel in situ detection system, we show that neurofibrillary tangles and senile plaques are major sites for catalytic redox reactivity. Pretreatment with deferoxamine or diethylenetriaminepentaacetic acid abolishes the ability of the lesions to catalyze the H2O2-dependent oxidation of 3,3'-diaminobenzidine (DAB), strongly suggesting the involvement of associated transition metal ions. Indeed, following chelated removal of metals, incubation with iron or copper salts reestablished lesion-dependent catalytic redox reactivity. Although DAB oxidation can also detect peroxidase activity, this was inactivated by H2O2 pretreatment before use of DAB, as shown by a specific peroxidase detection method. Model studies confirmed the ability of certain copper and iron coordination complexes to catalyze the H2O2-dependent oxidation of DAB. Also, the microtubule-associated protein tau, as an in vitro model for proteins relevant to AD pathology, was found capable of adventitious binding of copper and iron in a redox-competent manner. Our findings suggest that neurofibrillary tangles and senile plaques contain redox-active transition metals and may thereby exert prooxidant or possibly antioxidant activities, depending on the balance among cellular reductants and oxidants in the local microenvironment.

Prion protein glycosylation is sensitive to redox change

The conversion of soluble prion protein into an insoluble, pathogenic, protease-resistant isoform is a key event in the development of prion diseases. Although the mechanism by which the conversion engenders a pathogenic event is unclear, there is increasing evidence to suggest that this may depend on the function of the prion protein in preventing oxidative damage. Therefore, in this study, we assessed the interrelationship between redox-sensitive cysteine, glycosylation, and prion metabolism. Cells were treated with a thioreductant, dithiothreitol, to assess the effect of the cellular oxidation state on the synthesis of the prion protein. This change in redox balance affected the glycosylation of the prion protein, resulting in the sole production of glycosylated forms. The role of the single disulfide bridge in mediating this effect within the prion protein was confirmed by mutating the cysteine residues involved in its formation. These data suggest that conditions that increase the rate of formation of the disulfide bridge favor formation of the unglycosylated prion protein. Thus, since the presence of glycans on the prion protein is protective against its pathogenic conversion, a change in the redox status of the cell would increase the risk of developing a prion disease by favoring the production of the unglycosylated form.

Serum amyloid P is not present in amyloid beta deposits of a transgenic animal model

Serum amyloid P component (SAP) is associated with amyloid beta (A beta) deposition in Alzheimer disease (AD). Since SAP is exclusively synthesized by peripheral organs, its presence in the brain of AD suggests impairment of the blood-brain barrier (BBB). We studied the association of SAP with A beta deposits in a transgenic mouse model overexpressing beta-protein precursor (betaPP). Both SAP and another extracellular matrix binding protein, basic fibroblastic growth factor bind to the heparinase sensitive sites of A beta deposits in this model. However, no endogenous SAP immunoreactivity was found in the transgenic mouse brain. These results suggest that SAP is not required for A beta deposition, and that this mouse model does not develop the same BBB abnormalities as those seen in AD.

Increased neuronal glucose-6-phosphate dehydrogenase and sulfhydryl levels indicate reductive compensation to oxidative stress in Alzheimer disease

We analyzed glucose-6-phosphate dehydrogenase, the rate-controlling enzyme of the pentose phosphate pathway and free sulfhydryls, to study redox balance in Alzheimer disease. Glucose-6-phosphate dehydrogenase plays a pivotal role in homeostatic redox control by providing reducing equivalents to glutathione, the major nonenzymatic cellular antioxidant. There is a multitude of evidence that marks oxidative stress proximally in the natural history of Alzheimer disease. Consistent with a role for glutathione in defense against increased reactive oxygen, we found an upregulation of glucose-6-phosphate dehydrogenase together with increased sulfhydryls in Alzheimer disease. These data indicate that reductive compensation may play an important role in combating oxidative stress in Alzheimer disease.

Morphological features of regeneration of rabbit aortic endothelium after cryoinduced vascular damage

Regeneration of endothelium after damage is an important factor, which limits the development of atherogenesis. This study examines the topographical characteristics of regenerating endothelial cells (EC) in rabbit aorta after de-endothelialization by cryodestruction. The effects of cloricromene on these processes were also studied. Vessels were harvested from 6-month-old NZW rabbits, 1 and 3 days after cryodestruction. The vessels were evaluated using scanning electron microscopy (SEM). One day after cryodestruction, there were defects in the endothelial monolayer in the zone of injury in saline treated animals. Large numbers of platelets and monocytes were observed in association with endothelium in the damaged zone. Three days after de-endothelialization the size of the area of the damage had decreased. On the surface of the new endothelial layer and below this defect, the number of adhering monocytes was increased, and many microdefects between endothelium could be seen. Administration of cloricromene for 1 or 3 days after damage reduced the number of endothelium-adherent platelets and monocytes, and microdefects in endothelium. The feature of endothelial repair in rabbits is a relatively large involvement of monocytes and platelets, which are visible below regenerated endothelium. Administration of cloricromene essentially restored re-endothelialization and significantly decreased the number of adherent monocytes and microdefects in the new endothelium.

Alexander disease: Alzheimer disease of the developing brain?

Alexander disease is a leukodystrophy-like neurodegenerative disease that typically presents in infancy or childhood. The disease is essentially a sporadic condition, and there is no known genetic predisposition or metabolic abnormality. The hallmark of the disease is the diffuse accumulation of Rosenthal fibers (RF) throughout the central nervous system. Although an etiological relationship of the RF to disease pathogenesis has been suspected since the initial description of Alexander disease, such a relationship has not been confirmed. We previously identified a number of oxidative post-translational modifications, including advanced glycation end products and lipid peroxidation adducts, in intimate association with the RF of Alexander disease. Such oxidative protein damage provides a mechanism, through protein crosslinking, for insolubility and accumulation of RF. Notably, these findings show a striking parallel with the biochemical features of age-related neurodegenerative diseases such as Alzheimer disease. Therefore, Alexander disease and Alzheimer disease likely share a common pathogenesis, namely oxidative injury as a potential primary process in the etiology and pathogenesis.

Neuronal apoptosis--ISN satellite symposium. 5-7 August 1999, Tübingen, Germany

This satellite meeting of the International Society of Neurochemistry was attended by over 100 investigators and was organized by Jörg Schulz and Pierluigi Nicotera (Neurodegeneration Laboratory, Department of Neurology, Universitat Tubingen). The symposium brought together leaders from various aspects of the field of cell death with the goal of understanding how neurons die in degenerative conditions. Rapid growth in this area has led to greater biological, pathological and therapeutic understanding of diseases considered to be intractable.

Role of oxidative stress in frontotemporal dementia

Recent reports have established oxidative stress and damage as playing a role in the pathogenesis of a number of neurodegenerative diseases including Alzheimer disease, Parkinson disease, corticobasal degeneration, Pick's disease and Alexander's disease. Here we present evidence that oxidative damage is also one of the earliest cytopathological markers of neuronal dysfunction in frontotemporal dementia.

Activation of neuronal extracellular receptor kinase (ERK) in Alzheimer disease links oxidative stress to abnormal phosphorylation

Responses to increased oxidative stress may be the common mechanism responsible for the varied cytopathology of Alzheimer disease (AD). A possible link in support of this hypothesis is that one of the most striking features of AD, the abnormal accumulation of highly phosphorylated tau and neurofilament proteins, may be brought about by extracellular receptor kinase (ERK) whose activation is a common response to oxidative stress. In this study, we demonstrate that activated ERK is specifically increased in the same vulnerable neurons in AD that are the site of oxidative damage and abnormal phosphorylation. These findings suggest that ERK dysregulation, likley resulting from oxidative stress, could play an important role in the increased phosphorylation of cytoskeletal proteins observed in AD.

ELISA for IgG-class antibody to hepatitis E virus based on a highly conserved, conformational epitope expressed in Escherichia coli

In assays based on most recombinant hepatitis E virus (HEV) antigens, the IgG antibody responses to HEV are observed commonly to wane or disappear after the acute phase of infection. Such IgG assays have therefore been used for the diagnosis of acute HEV infection, but they have limited usefulness in seroepidemiological studies. Using western immunoblotting, it was shown previously that the open reading frame (ORF) 2.1 antigen, representing the carboxy-terminal 267 amino acids (aa) of the capsid protein, exposes a conformational epitope which allows optimal detection of convalescent antibody compared to other proteins expressed in Escherichia coli. This conformational epitope is shown to be highly conserved between divergent human HEV isolates, and the development of a sensitive and highly specific enzyme immunoassay (ELISA) based on this recombinant antigen is described. The ORF2.1 ELISA allows the detection and quantitation of both acute- and convalescent phase HEV-specific IgG, and will help to define better the antibody responses to the virus and the prevalence of HEV infection worldwide.

Is increased redox-active iron in Alzheimer disease a failure of the copper-binding protein ceruloplasmin?

One of the most striking features of Alzheimer disease (AD) is an accumulation of iron in neurofibrillary tangles and senile plaques. Intriguingly, this iron is found as both iron (II) and iron (III) and is redox-active. To address the issue of whether such iron participates in redox cycling, it was essential to investigate how iron (II) accumulates, since oxidation of iron (II) can lead to the generation of reactive oxygen species. To begin to address this issue, here we investigated ceruloplasmin, a key protein involved in the regulation of the redox state of iron by converting iron (II) to iron (III). Cases of AD and age-matched controls, obtained at autopsy with similar postmortem intervals, display similar levels of ceruloplasmin immunoreactivity that is mainly confined to neurons. However, in marked contrast, cases of AD show a significant increase in ceruloplasmin within the neuropil determined by immunoblot analysis of tissue homogenates as well as a generalized increased neuropil staining. Together, these findings suggest that neuronal induction of ceruloplasmin is feeble in AD, even while there is an increase in tissue ceruloplasmin. Therefore, a failure of neuronal ceruloplasmin to respond to iron may be an important factor that then leads to an accumulation of redox-active iron in neurons in AD.

Parkinson's disease is associated with oxidative damage to cytoplasmic DNA and RNA in substantia nigra neurons

Oxidative damage, including modification of nucleic acids, may contribute to dopaminergic neurodegeneration in the substantia nigra (SN) of patients with Parkinson's disease (PD). To investigate the extent and distribution of nucleic acid oxidative damage in these vulnerable dopaminergic neurons, we immunohistochemically characterized a common product of nucleic acid oxidation, 8-hydroxyguanosine (8OHG). In PD patients, cytoplasmic 8OHG immunoreactivity was intense in neurons of the SN, and present to a lesser extent in neurons of the nucleus raphe dorsalis and oculomotor nucleus, and occasionally in glia. The proportion of 8OHG immunoreactive SN neurons was significantly greater in PD patients compared to age-matched controls. Midbrain sections from patients with multiple system atrophy-Parkinsonian type (MSA-P) and dementia with Lewy bodies (DLB) also were examined. These showed increased cytoplasmic 8OHG immunoreactivity in SN neurons in both MSA-P and DLB compared to controls; however, the proportion of positive neurons was significantly less than in PD patients. The regional distribution of 8OHG immunoreactive neurons within the SN corresponded to the distribution of neurodegeneration for these three diseases. Nuclear 8OHG immunoreactivity was not observed in any individual. The type of cytoplasmic nucleic acid responsible for 8OHG immunoreactivity was analyzed by preincubating midbrain sections from PD patients with RNase, DNase, or both enzymes. 8OHG immunoreactivity was substantially diminished by either RNase or DNase, and completely ablated by both enzymes. These results suggest that oxidative damage to cytoplasmic nucleic acid is selectively increased in midbrain, especially the SN, of PD patients and much less so in MSA-P and DLB patients. Moreover, oxidative damage to nucleic acid is largely restricted to cytoplasm with both RNA and mitochondrial DNA as targets.

Genetic evidence for oxidative stress in Alzheimer's disease

The cause and proximal consequences of Alzheimer's disease (AD), a progressive debilitating dementia remain largely unknown. Nonetheless an increasing number of genetic risk factors, including most recently bleomycin hydrolase, have been shown to be associated with the disease, offering the hope of revealing the mechanism of disease pathogenesis. Here we show that bleomycin hydrolase, known to be induced in an oxidative environment, is specifically increased in neurons marked for degeneration in AD. These findings support a key proximal role for bleomycin hydrolase, and oxidative stress in AD.

Redox metals and neurodegenerative disease

Multiple lines of evidence implicate redox-active transition metals as mediators of oxidative stress in neurodegenerative diseases. Among the recent research discoveries is the finding that transition metals bind to proteins associated with neurodegeneration, including the prion protein. Whereas binding in the latter case may serve an antioxidant function, adventitious binding of metals to other proteins appears to preserve their catalytic redox activity in a manner that disturbs free radical homeostasis. Alterations in the levels of copper- and iron-containing metalloenzymes, involved in processing partially reduced oxygen species, are also likely to contribute to altered redox balance in neurodegenerative diseases. Nonetheless, even in familial forms of amyotrophic lateral sclerosis linked to mutations in superoxide dismutase, it is unclear whether an altered enzyme activity or, indirectly, a disturbance in transition-metal homeostasis is involved in the disease pathogenesis.

RNA oxidation is a prominent feature of vulnerable neurons in Alzheimer's disease

In this study we used an in situ approach to identify the oxidized nucleosides 8-hydroxydeoxyguanosine (8OHdG) and 8-hydroxyguanosine (8OHG), markers of oxidative damage to DNA and RNA, respectively, in cases of Alzheimer's disease (AD). The goal was to determine whether nuclear and mitochondrial DNA as well as RNA is damaged in AD. Immunoreactivity with monoclonal antibodies 1F7 or 15A3 recognizing both 8OHdG and 8OHG was prominent in the cytoplasm and to a lesser extent in the nucleolus and nuclear envelope in neurons within the hippocampus, subiculum, and entorhinal cortex as well as frontal, temporal, and occipital neocortex in cases of AD, whereas similar structures were immunolabeled only faintly in controls. Relative density measurement showed that there was a significant increase (p < 0.0001) in 8OHdG and 8OHG immunoreactivity with 1F7 in cases of AD (n = 22) as compared with senile (n = 13), presenile (n = 10), or young controls (n = 4). Surprisingly, the oxidized nucleoside was associated predominantly with RNA because immunoreaction was diminished greatly by preincubation in RNase but only slightly by DNase. This is the first evidence of increased RNA oxidation restricted to vulnerable neurons in AD. The subcellular localization of damaged RNA showing cytoplasmic predominance is consistent with the hypothesis that mitochondria may be a major source of reactive oxygen species that cause oxidative damage in AD.

The mosaic of brain glial hyperactivity during normal ageing and its attenuation by food restriction

Food restriction of adult rodents increases lifespan, with commensurate attenuation of age-related pathological lesions in many organs, as well as attenuation of normal ageing changes that are distinct from gross lesions. Previous work showed that chronic food restriction attenuated age-associated astrocyte and microglial hyperactivity in the hippocampal hilus, as measured by expression of glial fibrillary acidic protein and major histocompatibility complex II antigen (OX6). Here, we examined other markers of astrocyte and microglial activation in gray and white matter regions of ad libitum-fed (Brown Norway x Fischer 344) F1 male rats aged three and 24 months and chronic food-restricted rats aged 24 months. In situ hybridization and immunohistochemical techniques evaluated glial expression of glial fibrillary acidic protein, apolipoprotein E, apolipoprotein J (clusterin), heme oxygenase-1, complement 3 receptor (OX42), OX6 and transforming growth factor-beta1. All markers were elevated in the corpus callosum during ageing and were attenuated by food restriction, but other regions showed marked dissociation of the extent and direction of changes. Astrocytic activation, as measured with glial fibrillary acidic protein expression (coding and intron-containing RNA, immunoreactivity), increased with age in the corpus callosum, basal ganglia and hippocampus. Generally, food restriction attenuated the age-related increase in glial fibrillary acidic protein messenger RNA and immunoreactivity. Food restriction also reduced the age-related increase in apolipoprotein J and E messenger RNA and heme oxygenase-1 immunoreactivity in the basal ganglia and corpus callosum. However, astrocytes in the hilus of the hippocampus showed an age-related decrease in apolipoprotein J and E messenger RNA, which was further intensified by food restriction. The age-associated microglial activation measured by OX6 and OX42 immunoreactivity was reduced by food restriction in most subregions. The localized subsets of glial age changes and effects of food restriction comprise a mosaic of ageing consistent with the regional heterogeneity of ageing changes reported by others. In particular, age has a differential effect on astrocytic and microglial hyperactivity in gray versus white matter areas. The evident mosaic of glial ageing and responses to food restriction suggests that multiple mechanisms are at work during ageing.

[Is the lesion produced by oxidation a central part in the pathogenesis of Alzheimer's disease?]

The two most striking features of Alzheimer's disease are: a) the multitude of abnormalities affecting essentially every system, and b) the strict age dependence. Recent work suggests that both features are linked to increased oxidative stress that damages lipids, proteins and nucleic acids and results in redox-active metal accumulations, mitochondrial damage and formation of advanced glycation endproducts. Interestingly, beta-protein precursor, amyloid-beta, presenilins and apolipoprotein E have all been linked to reactive oxygen species production or with apoptosis, a process intimately associated with oxidative stress. In therapeutics, the commonality between a number of efficacious agents appears to be oxidative stress reduction. Therefore, we contend that oxidative stress is the element that links the multitude of changes in Alzheimer's disease and that a reduction of oxidative stress will have a dramatic effect on reducing the incidence or progression of Alzheimer's disease.

Universal isolation of cross-linked peptides: application to neurofibrillary tangles

A universal procedure to isolate cross-linked peptides has been demonstrated. The procedure relies on initially converting the epsilon-amino groups of lysine to dimethyl lysine by reductive methylation with sodium cyanoborohydride and formaldehyde. The lysine-modified protein is proteolytically cleaved and the resulting alpha-amino groups derivatized with methoxypoly(ethylene glycol)5000 succinyl hydroxysuccinimide. Any unintentional derivatization of tyrosine side chains can be reversed by incubation under mildly alkaline conditions. The cross-linked polypeptides contain two poly(ethylene glycol)5000 chains while non-cross-linked peptides contain a single poly(ethylene glycol)5000 chain. The two populations of peptides can be separated by gel filtration chromatography. This procedure has been shown capable of isolating cross-linked peptides using glutathione, lysozyme, chemically cross-linked hemoglobin, and neurofibrillary tangles isolated from the brain of a case of Alzheimer's disease.

Striation is the characteristic neuritic abnormality in Alzheimer disease

In this study, we found that neuropil threads of Alzheimer disease, rather than being continuous filaments along cell processes, show multiple interruptions. They are segmental in nature and therefore appear as striations rather than continuous filaments along the length of the neurite. Neuritic striation is not an artifact of section thickness since the majority of abnormal filament accumulations are extremely short. The dominance of short striations demonstrates that argyrophilic grains, rather than being distinct structures, simply represent a short variant of striation and that longer striations are arbitrarily considered neuropil threads. Ultrastructural examination showed that the intervals between striations lack a cytoskeleton. We suggest that neuritic striations may interrupt the microtubule system functionally blocking fast neuritic transport as well as playing a role in loss of neuronal connectivity.