RNA oxidation in Alzheimer disease and related neurodegenerative disorders

RNA oxidation and its biological effects are less well studied compared to DNA oxidation. However, RNA may be more susceptible to oxidative insults than DNA, for RNA is largely single-stranded and its bases are not protected by hydrogen bonding and less protected by specific proteins. Also, cellular RNA locates in the vicinity of mitochondria, the primary source of reactive oxygen species. Oxidative modification can occur not only in protein-coding RNAs, but also in non-coding RNAs that have been recently revealed to contribute towards the complexity of the mammalian brain. Damage to coding and non-coding RNAs will cause errors in proteins and disturbances in the regulation of gene expression. While less lethal than mutations in the genome and not inheritable, such sublethal damage to cells might be associated with underlying mechanisms of degeneration, especially age-associated neurodegeneration that is commonly found in the elderly population. Indeed, oxidative RNA damage has been described recently in most of the common neurodegenerative disorders including Alzheimer disease, Parkinson disease, dementia with Lewy bodies and amyotrophic lateral sclerosis. Of particular interest, the accumulating evidence obtained from studies on either human samples or experimental models coincidentally suggests that oxidative RNA damage is a feature in vulnerable neurons at early-stage of these neurodegenerative disorders, indicating that RNA oxidation actively contributes to the onset or the development of the disorders. Further investigations aimed at understanding of the processing mechanisms related to oxidative RNA damage and its consequences may provide significant insights into the pathogenesis of neurodegenerative disorders and lead to better therapeutic strategies.     

The earliest stage of cognitive impairment in transition from normal aging to Alzheimer disease is marked by prominent RNA oxidation in vulnerable neurons

Although neuronal RNA oxidation is a prominent and established feature in age-associated neurodegenerative disorders such as Alzheimer disease (AD), oxidative damage to neuronal RNA in aging and in the transitional stages from normal elderly to the onset of AD has not been fully examined. In this study, we used an in situ approachto identify an oxidized RNA nucleoside 8-hydroxyguanosine (8OHG) in the cerebral cortex of 65 individuals without dementia ranging in age from 0.3 to 86 years. We also examined brain samples from 20 elderly who were evaluated for their premortem clinicaldementia rating score and postmortem brain pathologic diagnoses to investigate preclinical AD and mild cognitive impairment. Relative density measurements of 8OHG-immunoreactivity revealed a statistically significant increase in neuronal RNA oxidation during aging in the hippocampus and the temporal neocortex. In subjects with mild cognitive impairment but not preclinical AD, neurons of the temporal cortex showed a higher burden of oxidized RNA compared to age-matched controls. These results indicate that, although neuronal RNA oxidation fundamentally occurs as an age-associated phenomenon, more prominent RNA damage than in normal aging correlates with the onset of cognitive impairment in the prodromal stage of AD.     

Aging in a dish: age-dependent changes of neuronal survival, protein oxidation, and creatine kinase BB expression in long-term hippocampal cell culture.

Results from different experimental systems demonstrate that increased oxidative damage plays a role in normal aging and age-associated pathology. In the current study, long-term cultures of hippocampal neurons were examined as a model system. It was established that neuronal survival in long-term culture decreases according to the Gompertz law and that neuronal "aging in the dish" is associated with increased oxidative damage of cell proteins. The increase of protein carbonyl formation in aged neurons was demonstrated both by Western blot analysis for oxidized proteins and by in situ immunocytochemical method, which was developed to analyze protein oxidation in fixed cells. In aging neuronal cultures, a gradual increase in creatine kinase (CK) content but decreased activity of enzyme per immunoreactive protein was found, suggesting the accumulation of inactive CK molecules. The increase in CK content was not a result of generalized protein elevation, since analysis of beta-actin content showed a time-dependent loss, probably reflecting decreased number of cellular processes with aging. These findings, showing "aging in a dish," consistent with the notion that aging is associated with increased protein oxidation, provide a system for study of age-related neurodegenerative disorders associated with oxidative stress.     

The mitochondrial impairment, oxidative stress and neurodegeneration connection: reality or just an attractive hypothesis?

Aging is the most important risk factor for common neurodegenerative disorders such as Parkinson's and Alzheimer's diseases. Aging in the central nervous system has been associated with elevated mutation load in mitochondrial DNA, defects in mitochondrial respiration and increased oxidative damage. These observations support a 'vicious cycle' theory which states that there is a feedback mechanism connecting these events in aging and age-associated neurodegeneration. Despite being an extremely attractive hypothesis, the bulk of the evidence supporting the mitochondrial vicious cycle model comes from pharmacological experiments in which the modes of mitochondrial enzyme inhibition are far from those observed in real life. Furthermore, recent in vivo evidence does not support this model. In this review, we focus on the relationship among the components of the putative vicious cycle, with particular emphasis on the role of mitochondrial defects on oxidative stress.     

Proteomics: a new approach to investigate oxidative stress in Alzheimer''s disease brain

In Alzheimer's disease (AD) brain oxidative stress is observed indexed by several markers, among which are protein carbonyls and 3-nitrotyrosine, markers for protein oxidation. We hypothesized that identity of these oxidatively modified proteins would lead to greater understanding of some of the potential molecular mechanisms involved in neurodegeneration in this dementing disorder. Proteomics is an emerging method for identification of proteins, and its application to neurodegenerative disorders, especially AD, is just beginning. Posttranslational modification of brain proteins, particularly that due of oxidation of proteins, provides an effective means of screening a subset of proteins within the brain proteome that likely reflects the extensive oxidative stress under which the AD brain exists, and this new methodology provides insights into mechanisms of neurodegeneration in and new therapeutic targets for AD. In this review, the use of proteomics to identify specifically oxidized proteins in AD brain is presented, from which new insights into mechanisms of neurodegeneration and synapse loss in this dementing disorder that is associated with oxidative stress have emerged.     

DNA damage and repair: relevance to mechanisms of neurodegeneration

DNA damage is a form of cell stress and injury that has been implicated in the pathogenesis of many neurologic disorders, including amyotrophic lateral sclerosis, Alzheimer disease, Down syndrome, Parkinson disease, cerebral ischemia, and head trauma. However, most data reveal only associations, and the role for DNA damage in direct mechanisms of neurodegeneration is vague with respect to being a definitive upstream cause of neuron cell death, rather than a consequence of the degeneration. Although neurons seem inclined to develop DNA damage during oxidative stress, most of the existing work on DNA damage and repair mechanisms has been done in the context of cancer biology using cycling nonneuronal cells but not nondividing (i.e. postmitotic) neurons. Nevertheless, the identification of mutations in genes that encode proteins that function in DNA repair and DNA damage response in human hereditary DNA repair deficiency syndromes and ataxic disorders is establishing a mechanistic precedent that clearly links DNA damage and DNA repair abnormalities with progressive neurodegeneration. This review summarizes DNA damage and repair mechanisms and their potential relevance to the evolution of degeneration in postmitotic neurons.     

Peripheral markers of oxidative stress and excitotoxicity in neurodegenerative disorders: tools for diagnosis and therapy?

Oxidative stress has been implicated as a common pathogenetic mechanism in neurodegenerative disorders. Central nervous system is particularly exposed to free radical injury, given its high metal content, which can catalyze the formation of oxygen free radicals, and the relatively low content of antioxidant defenses. Indeed, several studies show markers of oxidative damage - lipid peroxidation, protein oxidation, DNA oxidation and glycoxidation markers - in brain areas affected by neurodegenerative disorders. Oxidative stress damage is intimately linked to glutamate neurotoxicity - known as "excitotoxicity". An excessive concentration of extracellular glutamate over-activates ionotropic glutamate receptors, resulting in intracellular calcium overload and a cascade of events leading to neural cell death. In this study we reviewed pathogenetic mechanisms that link oxidative stress and excitotoxicity in three neurodegenerative disorders (Alzheimer's disease, amyotrophic lateral sclerosis and Parkinson's disease) and described peripheral markers of these mechanisms, that may be analyzed in patients as possible diagnostic and therapeutic tools.     

MicroRNAs in Parkinson's disease and emerging therapeutic targets

Parkinson's disease(PD) is the second most common age-related neurodegenerative disorder, with the clinical main symptoms caused by a loss of dopaminergic neurons in the substantia nigra, corpus striatum and brain cortex. Over 90% of patients with PD have sporadic PD and occur in people with no known family history of the disorder. Currently there is no cure for PD. Treatment with medications to increase dopamine relieves the symptoms but does not slow down or reverse the damage to neurons in the brain. Increasing evidence points to inflammation as a chief mediator of PD with inflammatory response mechanisms, involving microglia and leukocytes, activated following loss of dopaminergic neurons. Oxidative stress is also recognized as one of the main causes of PD, and excessive reactive oxygen species(ROS) and reactive nitrogen species can lead to dopaminergic neuron vulnerability and eventual death. Micro RNAs control a range of physiological and pathological functions, and may serve as potential targets for intervention against PD to mitigate damage to the brain. Several studies have demonstrated that micro RNAs can regulate oxidative stress and prevent ROS-mediated damage to dopaminergic neurons, suggesting that specific micro RNAs may be putative targets for novel therapeutic strategies in PD. Recent human and animal studies have identified a large number of dysregulated micro RNAs in PD brain tissue samples, many of which were downregulated. The dysregulated micro RNAs affect downstream targets such as SNCA, PARK2, LRRK2, TNFSF13 B, LTA, SLC5 A3, PSMB2, GSR, GBA, LAMP-2 A, HSC. Apart from one study, none of the studies reviewed had used agomirs or antagomirs to reverse the levels of downregulated or upregulated micro RNAs, respectively, in mouse models of PD or with isolated human or mouse dopaminergic cells. Further large-scale studies of brain tissue samples collected with short postmortem interval from human PD patients are warranted to provide more information on the micro RNA profiles in different brain regions and to test for gender differences.     

Mouse models of age-related mitochondrial neurosensory hearing loss

Hearing loss is the most common sensory disorder in the elderly population. Overall, 10% of the population has a hearing loss in the US, and this age-related hearing disorder is projected to afflict more than 28 million Americans by 2030. Age-related hearing loss is associated with loss of sensory hair cells (sensory hearing loss) and/or spiral ganglion neurons (neuronal hearing loss) in the cochlea of the inner ear. Many lines of evidence indicate that oxidative stress and associated mitochondrial dysfunction play a central role in age-related neurodegenerative diseases and are a cause of age-related neurosensory hearing loss. Yet, the molecular mechanisms of how oxidative stress and/or mitochondrial dysfunction lead to hearing loss during aging remain unclear, and currently there is no treatment for this age-dependent disorder. Several mouse models of aging and age-related diseases have been linked to age-related mitochondrial neurosensory hearing loss. Evaluation of these animal models has offered basic knowledge of the mechanism underlying hearing loss associated with oxidative stress, mitochondrial dysfunction, and aging. Here we review the evidence that specific mutations in the mitochondrial DNA or nuclear DNA that affect mitochondrial function result in increased oxidative damage and associated loss of sensory hair cells and/or spiral ganglion neurons in the cochlea during aging, thereby causing hearing loss in these mouse models. Future studies comparing these models will provide further insight into fundamental knowledge about the disordered process of hearing and treatments to improve the lives of individuals with communication disorders. This article is part of a Special Issue entitled ‘Mitochondrial function and dysfunction in neurodegeneration’.     

Oxidative stress differentially induces tau dissociation from neuronal microtubules in neurites of neurons cultured from different regions of the embryonic Gallus domesticus brain

Abnormal phosphorylation of microtubule-associated proteins such as tau has been shown to play a role in neurodegenerative disorders. It is hypothesized that oxidative stress-induced aggregates of hyperphosphorylated tau could lead to the microtubule network degradation commonly associated with neurodegeneration. We investigated whether oxidative stress induced tau hyperphosphorylation and focused on neurite degradation using cultured neurons isolated from the embryonic chick brain as a model system. Cells were isolated from the cerebrum, cerebellum, and tectum of 14-day-old chicks, grown separately in culture, and treated with tert-Butyl hydroperoxide (to simulate oxidative stress) for 48 hr. Relative expression and localization of tau or phospho-tau and β-tubulin III in neurites were determined using quantitative immunocytochemistry and confocal microscopy. In untreated cells, tau was tightly colocalized with β-tubulin III. Increasing levels of oxidative stress induced an increase in overall tau expression in neurites of cerebral and tectal but not the cerebellar neurons, coupled with a decrease in phospho-tau expression in tectal but not the cerebral or cerebellar neurons. In addition, oxidative stress induced the degeneration of the distal ends of the neurites and redistribution of phospho-tau toward the neuronal soma in the cerebral but not the tectal and cerebellar neurons. These results suggest that oxidative stress induces changes in tau protein that precede cytoskeletal degradation and neurite retraction. Additionally, there is a differential susceptibility of neuronal subpopulations to oxidative stress, which may offer potential avenues for investigation of the cellular mechanisms underlying the differential manifestations of neurodegenerative disorders in different regions of the brain.     

Oxidative stress: apoptosis in neuronal injury

Apoptosis has been well documented to play a significant role in cell loss during neurodegenerative disorders, such as stroke, Parkinson disease, and Alzheimer's disease. In addition, reactive oxygen species (ROS) has been implicated in the cellular damage during these neurodegenerative disorders. These ROS can react with cellular macromolecular through oxidation and cause the cells undergo necrosis or apoptosis. The control of the redox environment of the cell provides addition regulation in the signal transduction pathways which are redox sensitive. Recently, many researches focus on the relationship between apoptosis and oxidative stress. However, till now, there is no clear and defined mechanisms that how oxidative stress could contribute to the apoptosis. This review hopes to make clear that generation of ROS during brain injury, particularly in ischemic stroke and Alzheimer's Disease, and the fact that oxidative state plays a key role in the regulation and control of the cell survival and cell death through its interaction with cellular macromolecules and signal transduction pathway, and ultimately helps in developing an unique therapy for the treatment of these neurodegenerative disorders.     

Altern in Teilen? Systemalterungen des Nervensystems

Neurons of the central nervous system in general do not multiply after birth. Therefore, no replacement or biological renewal of individual cells affected by aging or death is possible. Morphological changes occurring in the aging brain are found substantially more pronounced in neurodegenerative diseases. Systemic degenerations of selective brain areas in these disorders, e.g. in Alzheimer's, Parkinson's, Huntington's disease or in amyotrophic lateral sclerosis, may be considered as models of accelerated aging and may allow to study the genetic and environmental influences of selective aging and cell death in modules of the central nervous system. Although neurodegenerative diseases are disparate disorders on the basis of their symptomatology and the anatomic distribution of pathologic lesions, they actually share key attributes with respect to biochemical and cellular determinants of selective vulnerability. Most strikingly, many show a conversion of disease specific and only recently identified proteins into unsoluble aggregates which form intra- or extracellular deposits. These protein aggregates may, over time, affect neuronal function, eventually leading to neurodegeneration and neurodegenerative pathology. The pathological process is counterbalanced by protective mechanisms that may loose their efficacy during normal aging. This could explain the late onset of even the inherited neurodegenerative disorders. Since the expression of disease-specific proteins is often not restricted to the affected brain areas (as exemplified by the expression of polyglutamine containing proteins in trinucleotide repeat disorders in non-affected brain areas and even outside the brain), the anatomical specificity of the degenerative process may be determined by associated binding proteins. Therapeutic strategies include the reinforcement of physiological defense mechanisms and intervention at early phases of the pathological biochemistry of disease specific proteins.     

Gamma-glutamylcysteine ethyl ester protection of proteins from Abeta(1-42)-mediated oxidative stress in neuronal cell culture: a proteomics approach

Protein oxidation mediated by amyloid beta-peptide (1-42) (Abeta[1-42]) has been proposed to play a central role in the pathogenesis of Alzheimer's disease (AD), a neurodegenerative disorder associated with aging and the loss of cognitive function. The specific mechanism by which Abeta(1-42), the primary component of the senile plaque and a pathologic hallmark of AD, contributes to the oxidative damage evident in AD brain is unknown. Moreover, the specific proteins that are vulnerable to oxidative damage induced by Abeta(1-42) are unknown. Identification of such proteins could contribute to our understanding of not only the role of Abeta(1-42) in the pathogenesis of AD, but also provide insight into the mechanisms of neurodegeneration at the protein level in AD. We report the proteomic identification of two proteins found to be oxidized significantly in neuronal cultures treated with Abeta(1-42): 14-3-3zeta and glyceraldehyde-3-phosphate dehydrogenase. We also report that pretreatment of neuronal cultures with gamma-glutamylcysteine ethyl ester, a compound that supplies the limiting substrate for the synthesis of glutathione and results in the upregulation of glutathione in neuronal cultures, protects both proteins against Abeta(1-42)-mediated protein oxidation.     

Therapeutic applications of chelating drugs in iron metabolic disorders of the brain and retina

Iron is essential for normal cellular function, however, excessive accumulation of iron in neural tissue has been implicated in both cortical and retinal diseases. The exact role of iron in the pathogenesis of neurodegenerative disorders remains incompletely understood. However, iron-induced damage to the brain and retina is often attributed to the redox ability of iron to generate dangerous free radicals, which exacerbates local oxidative stress and neuronal damage. Iron chelators are compounds designed to scavenge labile iron, aiding to regulate iron bioavailability. Recently there has been growing interest in the application of chelating agents for treatment of diseases including neurodegenerative conditions, characterized by increased oxidative stress. This article reviews both clinical and preclinical evidence relating to the effectiveness of iron chelation therapy in conditions of iron dyshomeostasis linked to neurodegeneration in the brain and retina. The limitations as well as future opportunities iron chelation therapy are discussed.     

Current status and future directions of gene expression profiling in Parkinson's disease

Parkinson's disease (PD) is a common age-associated neurodegenerative disorder. Motor symptoms are the cardinal component of PD, but non-motor symptoms, such as dementia, depression, and autonomic dysfunction are being increasingly recognized. Motor symptoms are primarily caused by selective degeneration of substantia nigra dopamine (SNDA) neurons in the midbrain; non-motor symptoms may be referable to well-described pathology at multiple levels of the neuraxis. Development of symptomatic and disease-modifying therapies is dependent on an accurate and comprehensive understanding of the pathogenesis and pathophysiology of PD. Gene expression profiling has been recently employed to assess function on a broad level in the hopes of gaining greater knowledge concerning how individual mechanisms of disease fit together as a whole and to generate novel hypotheses concerning PD pathogenesis, diagnosis, and progression. So far, the majority of studies have been performed on postmortem brain samples from PD patients, but more recently, studies have targeted enriched populations of dopamine neurons and have begun to explore extra-nigral neurons and even peripheral tissues. This review will provide a brief synopsis of gene expression profiling in parkinsonism and its pitfalls to date and propose several potential future directions and uses for the technique. It will focus on the use of microarray experiments to stimulate hypotheses concerning mechanisms of neurodegeneration in PD, since the majority of studies thus far have addressed that complicated issue.     

Ca2+-ATPase isoforms are expressed in neuroprotection in rat, but not human, neurons

Glutamate excitotoxicity has been implicated in neuronal death and damage in many neurodegenerative disorders. The potential neuroprotective role of the plasma membrane calcium ATPase (PMCA) and the NMDA receptor were investigated in rat and human brain neurons after a glutamate insult. Investigation of potential mechanisms of neuronal survival revealed that surviving rat cerebellar granule cells expressed the mRNA of new PMCA isoforms 2b and 2c. There was no observable change in expression of PMCA isoforms or NMDA receptor NR2 subtypes in human cortical neurons. This study shows that subsets of rat and human neurons are resistant to glutamate-induced excitotoxicity and the mechanisms employed to enable survival differ between rat and human neurons.     

Gamma-glutamylcysteine ethyl ester-induced up-regulation of glutathione protects neurons against Abeta(1-42)-mediated oxidative stress and neurotoxicity: implications for Alzheimer's disease

Glutathione (GSH) is an important endogenous antioxidant found in millimolar concentrations in the brain. GSH levels have been shown to decrease with aging. Alzheimer's disease (AD) is a neurodegenerative disorder associated with aging and oxidative stress. Abeta(1-42) has been shown to induce oxidative stress and has been proposed to play a central role in the oxidative damage detected in AD brain. It has been shown that administration of gamma-glutamylcysteine ethyl ester (GCEE) increases cellular levels of GSH, circumventing the regulation of GSH biosynthesis by providing the limiting substrate. In this study, we evaluated the protective role of up-regulation of GSH by GCEE against the oxidative and neurotoxic effects of Abeta(1-42) in primary neuronal culture. Addition of GCEE to neurons led to an elevated mean cellular GSH level compared with untreated control. Inhibition of gamma-glutamylcysteine synthetase by buthionine sulfoximine (BSO) led to a 98% decrease in total cellular GSH compared with control, which was returned to control levels by addition of GCEE. Taken together, these results suggest that GCEE up-regulates cellular GSH levels which, in turn, protects neurons against protein oxidation, loss of mitochondrial function, and DNA fragmentation induced by Abeta(1-42). These results are consistent with the notion that up-regulation of GSH by GCEE may play a viable protective role in the oxidative and neurotoxicity induced by Abeta(1-42) in AD brain.     

Neurodegeneration and oxidative stress: prion disease results from loss of antioxidant defence

Prion diseases or transmissible spongiform encephalopathies (TSEs) are rare neurodegenerative disorders that can be acquired either by direct transmission, inherited through dominant mutations in the prion protein gene or via an unknown sporadic cause. This latter group constitutes the vast majority of cases. Like many neurodegenerative diseases the hallmarks of oxidative damage can be readily detected throughout the brain of the affected individual. However, unlike most other neurodegenerative diseases, prion diseases are connected with a dramatic loss of antioxidant defence. As abnormal protein accumulates in the diseased brain there is both an increase of oxidative substances and a loss of the defences that keep them in check. In particular the normal cellular prion protein has been shown to be an antioxidant. Conversion of this protein to the protease resistant isoform is accompanied by a loss of this antioxidant activity. This change creates a paradox as the loss of activity is not accompanied by a loss of protein expression. It is likely that this prevents other cellular defences from responding sufficiently to protect neurons from the heightened oxidative burden. Recent experiments with transgenic mice have shown that when prion protein expression is switched off during the course of prion disease, cell death is dramatically halted and the mouse recovers from the disease. This result clearly illustrates that the continued expression of non-function prion protein is essential for disease progression. This implies that the presence of this abnormal protein during prion disease causes a failure of cellular antioxidant defence. This failed defence is the fundamental cause of the massive neurodegeneration that results in the fatal nature of TSEs. The role of oxidative stress in TSEs and other neurodegenerative disorders are discussed in this review.     

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.