Oxidative damage is the earliest event in Alzheimer disease

Recently, we demonstrated a significant increase of an oxidized nucleoside derived from RNA, 8-hydroxyguanosine (8OHG), and an oxidized amino acid, nitrotyrosine in vulnerable neurons of patients with Alzheimer disease (AD). To determine whether oxidative damage is an early- or end-stage event in the process of neurodegeneration in AD, we investigated the relationship between neuronal 8OHG and nitrotyrosine and histological and clinical variables, i.e. amyloid-beta (A beta) plaques and neurofibrillary tangles (NFT), as well as duration of dementia and apolipoprotein E (ApoE) genotype. Our findings show that oxidative damage is quantitatively greatest early in the disease and reduces with disease progression. Surprisingly, we found that increases in A beta deposition are associated with decreased oxidative damage. These relationships are more significant in ApoE epsilon4 carriers. Moreover, neurons with NFT show a 40%-56% decrease in relative 8OHG levels compared with neurons free of NFT. Our observations indicate that increased oxidative damage is an early event in AD that decreases with disease progression and lesion formation. These findings suggest that AD is associated with compensatory changes that reduce damage from reactive oxygen.     

Lipid peroxidation is an early event in the brain in amnestic mild cognitive impairment

Multiple studies demonstrate that the brain in Alzheimer's disease (AD) contains extensive oxidative damage. Most of these studies used advanced-stage AD patients raising the question of whether oxidative damage is a late effect of neurodegeneration or precedes and contributes to the pathogenesis of AD. Here we describe F(2)-isoprostane (F(2)-IsoP) and F(4)-neuroprostane (F(4)-NP) levels in longitudinally followed, well documented autopsied normal control subjects and patients with amnestic mild cognitive impairment (MCI), and late-stage AD. Gas chromatography/negative ion chemical ionization/mass spectrometry was used to determine F(2)-IsoP and F(4)-NP levels. Significant increases in F(2)-IsoP levels were found in frontal, parietal and occipital lobes in MCI and late AD compared to controls but no significant differences were present between MCI and late AD. A significant increase in F(4)-NPs was present in parietal and occipital lobes in MCI compared to controls and a significant increase was present in these regions and hippocampus in late AD compared to controls. The only difference between MCI and late AD was significantly increased F(4)-NP in hippocampus in late AD. Our data indicate that lipid peroxidation is present in the brain of MCI patients and suggest that oxidative damage may play a role in the pathogenesis of AD.     

Remarkable increase in the concentration of 8-hydroxyguanosine in cerebrospinal fluid from patients with Alzheimer's disease

To investigate the possible role of oxidative RNA damage in the pathogenesis of Alzheimer's disease (AD), the concentrations of the oxidative stress marker 8-hydroxyguanosine (8-OHG) were measured in the cerebrospinal fluid (CSF) and the serum of patients with AD and control subjects. The concentration of 8-OHG in CSF in AD patients was approximately fivefold that in controls (P < 0.001). The concentration of 8-OHG in CSF decreased significantly with the duration of illness (r(s) = -0.48, P < 0.05) and the progression of cognitive dysfunctions (r(s) = 0.67, P < 0.01). However, the concentration of 8-OHG in CSF showed no correlation with that in serum in both the controls and AD patients. In addition, the concentration of 8-OHG in serum was not significantly altered in AD patients compared to that in controls, suggesting that the 8-OHG concentrations in the CSF do not reflect those in serum and may be probably reflect those in brain tissue. These in vivo findings suggest a possible role of 8-OHG and increased oxidative RNA damage in the early stage of the development of AD.     

Oxidative damage, protein synthesis, and protein degradation in Alzheimer's disease

A large number of studies has firmly established that increases in oxidative damage occurs in Alzheimer's disease (AD). Such studies have demonstrated that increased in oxidative damage selectively occurs within the brain regions involved in regulating cognitive performance. Studies from our laboratory and others have provided experimental evidence that increased levels of oxidative damage occur in subjects with Mild Cognitive Impairment (MCI), which is believed to be one of the earliest stages of AD, and is a condition which is devoid of dementia or the extensive neurofibrillary pathology and neuritic plaque deposition observed in AD. Together, these data support a role for the accrual of oxidative damage potentially serving as an early event that then initiates the development of cognitive disturbances and pathological features observed in AD. Recent studies from our laboratory have demonstrated that a decline in protein synthesis capabilities occurs in the same brain regions which exhibit increased levels of oxidative damage in MCI and AD subjects. The focus of this review is to describe the large number of studies which suggest protein synthesis may be one of the earliest cellular processes disrupted by oxidative damage in AD. Taken together, these findings have important implications for understanding the molecular and cellular basis of AD, understanding the basis for oxidative stress in AD, and may have important implications for studies involving proteomics and proteolysis in AD.     

Mitochondrial failures in Alzheimer's disease

Mitochondrial dysfunction and free radical-induced oxidative damage have been implicated in the pathogenesis of several different neurodegenerative diseases such as Parkinson disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and Alzheimer's disease (AD). The defective adenosine triphosphate (ATP) production and increased oxygen radicals may induce mitochondria-dependent cell death because damaged mitochondria are unable to maintain the energy demands of the cell. The role of vascular hypoperfusion-induced mitochondria failure in the pathogenesis of AD now has been widely accepted. However, the exact cellular mechanisms behind vascular lesions and their relation to oxidative stress markers identified by RNA oxidation, lipid peroxidation, or mitochondrial DNA (mtDNA) deletion remain unknown. Future studies comparing the spectrum of mitochondrial damage and the relationship to oxidative stress-induced damage during the aging process or, more importantly, during the maturation of AD pathology are warranted.     

Expression of 8-oxoguanine DNA glycosylase 1 (OGG1) and the level of p53 and TNF-αlpha proteins in peripheral lymphocytes of patients with Alzheimer's disease

The aim of the study was to determine the extent of oxidative DNA damage (levels of 8-oxo2dG) and expression of OGG1 and p53 and TNF-α proteins in lymphocytes of Alzheimer's disease (AD) patients and a control group. The studies were conducted on 41 patients with AD, including 25 women and 16 men aged 34-84 years. The control group included 51 individuals, 20 women and 31 men aged 22-83 years. The level of 8-oxo2dG was determined using HPLC/EC/UV, and the level of OGG1 and p53 and TNF-α proteins was determined with the Western blot method. The results showed that both proteins participating in DNA repair (OGG1, p53) and the inflammatory protein TNF-α are involved in pathogenesis of neurodegenerative diseases. It also seems that a specific system for DNA repair (OGG1) may contribute to downregulation of the inflammatory factor (TNF-α) level, especially in the early stages of dementia. Moreover, the results showed that p53 protein can fulfil its function in DNA damage repair only in early stages of dementia. It is possible that OGG1 and p53 and TNF-α proteins together or separately may be involved in pathogenesis of AD by repair of oxidative DNA damage and/or apoptosis.     

Oxidatively modified proteins in Alzheimer’s disease (AD), mild cognitive impairment and animal models of AD: role of Abeta in pathogenesis

Oxidative stress has been implicated in the pathogenesis of a number of diseases including Alzheimer’s disease (AD). The oxidative stress hypothesis of AD pathogenesis, in part, is based on β-amyloid peptide (Aβ)-induced oxidative stress in both in vitro and in vivo studies. Oxidative modification of the protein may induce structural changes in a protein that might lead to its functional impairment. A number of oxidatively modified brain proteins were identified using redox proteomics in AD, mild cognitive impairment (MCI) and Aβ models of AD, which support a role of Aβ in the alteration of a number of biochemical and cellular processes such as energy metabolism, protein degradation, synaptic function, neuritic growth, neurotransmission, cellular defense system, long term potentiation involved in formation of memory, etc. All the redox proteomics-identified brain proteins fit well with the appearance of the three histopathological hallmarks of AD, i.e., synapse loss, amyloid plaque formation and neurofibrillary tangle formation and suggest a direct or indirect association of the identified proteins with the pathological and/or biochemical alterations in AD. Further, Aβ models of AD strongly support the notion that oxidative stress induced by Aβ may be a driving force in AD pathogenesis. Studies conducted on arguably the earliest stage of AD, MCI, may elucidate the mechanism(s) leading to AD pathogenesis by identifying early markers of the disease, and to develop therapeutic strategies to slow or prevent the progression of AD. In this review, we summarized our findings of redox proteomics identified oxidatively modified proteins in AD, MCI and AD models.     

Are mitochondria critical in the pathogenesis of Alzheimer's disease?

This review summarizes recent findings that suggest a causal connection between mitochondrial abnormalities and sporadic Alzheimer's disease (AD). Genetic causes of AD are known only for a small proportion of familial AD patients, but for a majority of sporadic AD patients, genetic causal factors are still unknown. Currently, there are no early detectable biomarkers for sporadic AD, and there is a lack of understanding of the pathophysiology of the disease. Findings from recent genetic studies of AD pathogenesis suggest that mitochondrial defects may play an important role in sporadic AD progression, and that mitochondrial abnormalities and oxidative damage may play a significant role in the progression of familial AD. Findings from biochemical studies, in vitro studies, gene expression studies, and animal model studies of AD are reviewed, and the possible contribution of mitochondrial mutations to late-onset sporadic AD is discussed.     

Oxidative stress in mild cognitive impairment and Alzheimer disease: a continuum

Although several studies show the importance of oxidative stress in the pathogenesis of Alzheimer's disease (AD), there are few evidences on the role of free radicals in Mild Cognitive Impairment (MCI). Our results showing a marked decrease of the main components of the antioxidant defense system of the organism support the hypothesis that in MCI there is a condition of oxidative stress. This work also gives an overview on the existing evidence of the early occurrence of oxidative processes in the development of dementing disorders of the Alzheimer type. Since MCI represents a condition of increased risk for AD, use of antioxidants in MCI could be of importance for prevention.     

Expression of 8-oxoguanine DNA glycosylase is reduced and associated with neurofibrillary tangles in Alzheimer's disease brain

Recent studies have confirmed the role of reactive oxygen species in the pathogenesis of Alzheimer's disease (AD). 8-Oxo-2'-deoxyguanosine accumulation in AD brain has been discussed, but few studies of DNA repair enzymes in AD brain have been done. Further, a relationship between mitochondrial function and oxidative stress has been noticed. In this study, to evaluate the repair mechanism for oxidative DNA damage in AD brain, we investigated brain tissues from autopsy cases of AD and control cases using an antibody against the mitochondrial form of 8-oxoguanine DNA glycosylase (hOGG1-2a), an enzyme that repairs 8-oxo-2'-deoxyguanosine. hOGGI-2a is expressed mainly in the neuronal cytoplasm in both AD and control cases in regionally different manners. Expression of hOGG1-2a is decreased in the orbitofrontal gyrus and entorhinal cortex in AD compared to that in control cases. Immunoreactivity to hOGG1-2a is associated with neurofibrillary tangles, dystrophic neurites and reactive astrocytes in AD. Our results indicate that the repair enzyme for oxidative damage in mitochondrial DNA may not function appropriately in AD, and thus oxidative DNA damage in mitochondria may be involved in the pathomechanism of AD.     

Alzheimer's disease: the two-hit hypothesis

There are many lines of evidence showing that oxidative stress and aberrant mitogenic changes have important roles in the pathogenesis of Alzheimer's disease (AD). However, although both oxidative stress and cell cycle-related abnormalities are early events, occurring before any cytopathology, the relation between these two events, and their role in pathophysiology was, until recently, unclear. However, on the basis of studies of mitogenic and oxidative stress signalling pathways in AD, we proposed a "two-hit hypothesis" which states that although either oxidative stress or abnormalities in mitotic signalling can independently serve as initiators, both processes are necessary to propagate disease pathogenesis. In this paper, we summarise evidence for oxidative stress and abnormal mitotic alterations in AD and explain the two-hit hypothesis by describing how both mechanisms are necessary and invariant features of disease.     

Lipid peroxidation and oxidative imbalance: early functional events in Alzheimer's disease

Alzheimer's disease (AD) is a growing public health problem worldwide. Clinically, AD is a progressive neurodegenerative disorder characterized by a global cognitive decline. Accumulating evidence indicates that reactive oxygen species-mediated reactions, particularly of neuronal lipids, are extensive in those AD brain areas directly involved in the disease processes. Traditional views claim that oxidative-mediated tissue injury in the AD brain is the result of neurodegeneration. In recent years, numerous investigations have pointed to the functional importance of oxidative imbalance as a crucial event in mediating AD pathogenesis. The availability of specific and sensitive markers to monitor in vivo oxidative stress, in combination with studies performed in living patients with clinical diagnosis of AD are helping us to elucidate these issues. The evidence we have accumulated so far clearly indicates that oxidative imbalance and subsequent oxidative stress are early events during the evolution of the disease, and secondary to specific mechanism(s) present in AD but not in other neurodegenerative diseases. These new concepts implicate that this phenomenon may play a more important role in AD pathogenesis than previously anticipated, and that any therapeutic intervention targeting oxidative stress should be initiated at the earliest possible stage of the disease.     

Oxidative damage and progression to Alzheimer's disease in patients with mild cognitive impairment

Recent studies show that most of the oxidative changes found in Alzheimer's disease (AD) are already present in mild cognitive impairment (MCI) patients. The question arises as to whether oxidative stress has a role in the progression of MCI to AD. We conducted a longitudinal study on 70 MCI patients, and the peripheral blood levels of a broad spectrum of non-enzymatic and enzymatic antioxidant defenses, as well as lipid and protein oxidation markers and nitrogen oxidative species were determined. At baseline, there were no differences in any of the indexes of oxidative damage between stable MCI patients (MCI-MCI) and patients that progressed to AD (MCI-AD). Cellular levels of lipid peroxidation markers increased in both groups and this was accompained in MCI-AD, but not in MCI-MCI patients, by a significant decrease in cellular antioxidant defenses (oxidyzed/reduced glutathione ratio and vitamin E). Among MCI-AD patients, the longitudinal decrease in cellular vitamin E was associated with the deterioration in cognitive performance. These results suggest that accumulation of oxidative damage may start in pre-symptomatic phases of AD pathology and that progression to AD might be related to depletion of antioxidant defenses.     

Damage to lipids, proteins, DNA, and RNA in mild cognitive impairment

Free radical-mediated oxidative damage is thought to play a role in the pathogenesis of Alzheimer disease. Previous studies have shown oxidative damage to lipids, proteins, DNA, and RNA in multiple brain regions in late-stage Alzheimer disease. Recent studies on patients with amnestic mild cognitive impairment who have undergone autopsy have shown increased lipid peroxidation as well as protein, DNA, and RNA oxidation in multiple brain regions. These studies establish oxidative damage as an early event in the pathogenesis of Alzheimer disease that can serve as a therapeutic target to slow the progression or perhaps the onset of the disease.     

Oxidative damage in the olfactory system in Alzheimer's disease

Increased oxidative damage is a prominent and early feature of vulnerable neurons in Alzheimer's disease (AD). However, while damage to proteins, sugars, lipids, nucleic acids and organelles such as lysosomes, mitochondria, and endoplasmic reticulum are evident, the source of increased reactive oxygen species has not been determined. Furthermore, a major limitation in further determining the source, as well as finding a means to arrest damage, is the paucity of cellular models directly homologous to AD since the vulnerable neurons of the brain in AD cannot be studied in vitro. Here, we examined the olfactory epithelium in situ to see if neurons there exhibit a similar pathological oxidative balance to vulnerable neurons in AD. In biopsy specimens, (eight AD and three controls) we found that neurons, and also the surrounding epithelial cells, show an increase in oxidative damage for a subset of the markers increased in the brain of cases of AD. Lipid peroxidation and heme oxygenase-1, a stress response protein, were increased, while nucleic acid or protein oxidation, demonstrated in vulnerable neurons in AD, were not increased. These findings highlight the systemic nature of oxidative abnormalities in AD, but that different cell types may express this abnormality by a different array of oxidative stress markers, supporting the potential for using olfactory neurons or other cells derived from AD patients in culture to understand the mechanistic basis for increased oxidative damage in AD and as a model to screen compounds for therapeutic intervention.     

Levels of reduced and oxidized coenzyme Q-10 and 8-hydroxy-2′-deoxyguanosine in the CSF of patients with Alzheimer’s disease demonstrate that mitochondrial oxidative damage and/or oxidative DNA damage contributes to the neurodegenerative process

To investigate the possibility that mitochondrial oxidative damage, oxidative DNA damage or both contribute to the neurodegenerative process of Alzheimer’s disease (AD), we employed high-performance liquid chromatography using an electrochemical detector to measure the concentrations of the reduced and oxidized forms of coenzyme Q-10 (CoQ-10) and 8-hydroxy-2′-deoxyguanosine (8-OHdG) in the cerebrospinal fluid (CSF) of 30 patients with AD and in 30 age-matched controls with no neurological disease. The percentage of oxidized/total CoQ-10 (%CoQ-10) in the CSF of the AD group (78.2 ± 18.8%) was significantly higher than in the control group (41.3 ± 10.4%) (P < 0.0001). The concentration of 8-OHdG in the CSF of AD patients was greater than in the CSF of controls (P < 0.0001) and was positively correlated with the duration of illness (r _s = 0.95, P < 0.0001). The %CoQ-10 was correlated with concentrations of 8-OHdG in the CSF of AD patients (r _s = 0.66, P < 0.001). The present study suggests that both mitochondrial oxidative damage and oxidative DNA damage play important roles in the pathogenesis of early AD development.     

Redox proteomic identification of 4-hydroxy-2-nonenal-modified brain proteins in amnestic mild cognitive impairment: insight into the role of lipid peroxidation in the progression and pathogenesis of Alzheimer's disease

Numerous investigations point to the importance of oxidative imbalance in mediating AD pathogenesis. Accumulated evidence indicates that lipid peroxidation is an early event during the evolution of the disease and occurs in patients with mild cognitive impairment (MCI). Because MCI represents a condition of increased risk for Alzheimer's disease (AD), early detection of disease markers is under investigation. Previously we showed that HNE-modified proteins, markers of lipid peroxidation, are elevated in MCI hippocampus and inferior parietal lobule compared to controls. Using a redox proteomic approach, we now report the identity of 11 HNE-modified proteins that had significantly elevated HNE levels in MCI patients compared with controls that span both brain regions: Neuropolypeptide h3, carbonyl reductase (NADPH), alpha-enolase, lactate dehydrogenase B, phosphoglycerate kinase, heat shock protein 70, ATP synthase alpha chain, pyruvate kinase, actin, elongation factor Tu, and translation initiation factor alpha. The enzyme activities of lactate dehydrogenase, ATP synthase, and pyruvate kinase were decreased in MCI subjects compared with controls, suggesting a direct correlation between oxidative damage and impaired enzyme activity. We suggest that impairment of target proteins through the production of HNE adducts leads to protein dysfunction and eventually neuronal death, thus contributing to the biological events that may lead MCI patients to progress to AD.     

Impact of gender on upregulation of antioxidant defence mechanisms in Alzheimer’s disease brain

Summary. Since oxidative stress plays an important role in the pathogenesis of Alzheimers disease (AD) and since the age-adjusted incidence of AD is higher in females than males, we examined a possible influence of gender on antioxidant metabolism in brains from male and female AD patients and age-matched controls. Activities of copper/zinc-dependent superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione-disulfide reductase (GR) were elevated in AD samples compared to controls. Upon in vitro stimulation, levels of malondialdehyde formation were significantly lower in AD samples, probably due to the increased antioxidant capacity. Overall, our results indicate that antioxidant metabolism is functionally still intact but increased in AD implying that oxidative damage is caused rather by overproduction than by insufficient detoxification of ROS. Among AD patients, a gender-specific partial upregulation of antioxidant defence was present: activities of SOD and GPx were even further increased in female patients, and levels of 4-hydroxynonenal, a marker of oxidative damage, were higher than in male patients. Importantly, our results are in line with epidemiological studies indicating a higher risk for AD in females. Thus, gender differences in oxidative stress parameters might be related to the higher prevalence of AD in females.Present address: Neurobiology Research Lab, Psychiatric University Clinic, Basel, Switzerland     

Decreased base excision repair and increased helicase activity in Alzheimer's disease brain

Recent studies show an increase in DNA oxidation in brain and cerebrospinal fluid (CSF), and decreased levels of the free repair product in CSF in Alzheimer's disease (AD). This is a study of the activity of the base excision repair enzyme, 8-oxoguanine glycosylase (responsible for the excision of 8-oxoguanine), and DNA helicase activity in nuclear protein samples from four brain regions of 10 AD and eight age-matched control subjects. Statistically significant (p<0.05) decreases in 8-oxoguanine glycosylase activity were observed in the nuclear fraction of AD hippocampal and parahippocampal gyri (HPG), superior and middle temporal gyri (SMTG), and inferior parietal lobule (IPL). DNA helicase activity was elevated in all nuclear samples except the IPL with statistically significant elevations in the HPG and CER. Statistically significant depletion of helicase activity was observed in the nuclear fraction in AD IPL. Our results demonstrate that the repair capabilities for 8-oxoguanine are decreased in AD. The modest increase in DNA helicase activity in some brain regions in AD may interfere with base excision repair mechanisms. Overall, the decreased repair of DNA damage could be involved in the pathogenesis of neurodegeneration in AD.