Early decrease in dna repair proteins, Ku70 and Ku86, and subsequent DNA fragmentation after transient focal cerebral ischemia in mice

Ku70 silencing aggravates oxygen-glucose deprivation/reperfusion-induced injury by activation of the p53 apoptotic pathway in rat cortical astrocytes

Oxidative stress-induced DNA damage is an important mechanism that leads to the death of neuronal cells after ischemic stroke. Our previous study found that Ku70 was highly expressed in ischemic brain tissue of rats after cerebral ischemia-reperfusion injury. However, the role of Ku70 in glucose-oxygen deprivation/reperfusion (OGD/R) in astrocytes has not been reported. Therefore, we investigated the effect and mechanism of Ku70 on OGD/R-induced astrocyte injury in rats. Rat astrocytes were cultured in vitro to establish the OGD/R-induced injury model and transfected with small interfering RNA (siRNA) to disturb Ku70 expression. Real-time quantitative polymerase chain reaction (RT-qPCR), western blotting, and immunofluorescence were performed to assay the expression of mRNA and proteins. Cell viability, apoptosis, and ROS accumulation were determined by CCK-8 assay, flow cytometry, and fluorescence microscopy, respectively. Our results showed Ku70 can be expressed in both the nucleus and cytoplasm of astrocytes, although mainly in the nucleus. Ku70 expression showed a trend of first increasing and then decreasing after OGD/R, reaching its highest change at 24 h of reoxygenation. OGD/R induced ROS production and DNA damage in rat astrocytes, and Ku70 silencing further increased ROS production and DNA lesions, which aggravated astrocyte injury and apoptosis. Furthermore, the expression of p53, Bax, and caspase 3 proteins significantly increased after OGD/R in astrocytes, and downregulation of Ku70 further enhanced expression of the above proteins. These results indicate that Ku70 silencing promotes OGD/R-induced astrocyte apoptosis, which may be associated with p53 apoptotic pathway activation. Our study suggests that Ku70 may be a novel target for cerebral ischemia-reperfusion injury therapy.

Cisplatin-induced cell death increases the degradation of the MRE11-RAD50-NBS1 complex through the autophagy/lysosomal pathway

Cisplatin and other platinum-based anticancer agents are among the most widely used chemotherapy drugs in the treatment of different types of cancer. However, it is common to find patients who respond well to treatment at first but later relapse due to the appearance of resistance to cisplatin. Among the mechanisms responsible for this phenomenon is the increase in DNA damage repair. Here, we elucidate the effect of cisplatin on the MRN (MRE11-RAD50-NBS1) DNA damage sensor complex. We found that the tumor suppressor FBXW7 is a key factor in controlling the turnover of the MRN complex by inducing its degradation through lysosomes. Inhibition of lysosomal enzymes allowed the detection of the association of FBXW7-dependent ubiquitylated MRN with LC3 and the autophagy adaptor p62/SQSTM1 and the localization of MRN in lysosomes. Furthermore, cisplatin-induced cell death increased MRN degradation, suggesting that this complex is one of the targets that favor cell death. These findings open the possibility of using the induction of the degradation of the MRN complex after genotoxic damage as a potential therapeutic strategy to eliminate tumor cells.

MiR-1224 downregulation inhibits OGD/R-induced hippocampal neuron apoptosis through targeting Ku protein

Ischemic cerebrovascular disease is the main cause of disability due to stroke. This study aimed to investigate the function of miR-1224 in OGD/R-induced hippocampal neuron apoptosis, as well as the regulatory mechanism of miR-1224 in ischemic cerebrovascular disease. The oxygen-glucose deprivation/reperfusion (OGD/R) model of primary mouse hippocampal neurons was established. RT-qPCR detected miR-1224, Ku70 and Ku86 levels. Western blotting was applied to measure the expression of Ku70/86 and apoptosis related proteins. Flow cytometry was used to assess apoptosis. JC-1 fluorescence was performed to test the mitochondrial membrane potential (MMP) in neurons. The double luciferase reporter assay was performed to investigate the relationship between miR-1224 and Ku70/86. OGD/R induced the apoptosis and mitochondrial injury in neuronal cells, while miR-1224 downregulation or Ku70/86 upregulation reversed this phenomenon. Meanwhile, miR-1224 negatively regulated the expression of Ku70/86 in neuronal cells through directly targeting Ku70/86. Furthermore, knockdown of Ku70/86 significantly reversed the inhibitory effect of miR-1224 silencing on apoptosis and mitochondrial injury in OGD/R-treated neuronal cells. Our findings indicated that miR-1224 downregulation suppressed OGD/R-induced hippocampal neuron apoptosis by targeting Ku protein, suggesting that miR-1224 could serve as a new target for ischemic cerebrovascular disease treatment.

Transcranial direct-current stimulation protects against cerebral ischemia-reperfusion injury through regulating Cezanne-dependent signaling

Transcranial direct-current stimulation (tDCS) is proved safe and shows therapeutic effect in cerebral ischemic stroke in clinical trials. But the underlying molecular mechanisms remain unclear. Here we show that tDCS treatment reduces the infarct volume after rat cerebral ischemia-reperfusion (I/R) injury and results in functional improvement of stroke animals. At the cellular and molecular level, tDCS suppresses I/R-induced upregulation of Cezanne in the ischemic neurons. Cezanne inhibition confers neuroprotection after rat I/R and oxygen glucose deprivation (OGD) in the cortical neuronal cultures. Inhibiting Cezanne increases the level of SIRT6 that is downregulated in the ischemic neurons. Suppressing SIRT6 blocks Cezanne inhibition-induced neuroprotective effect and overexpressing SIRT6 attenuates OGD-induced neuronal death. We further show that downregulating Cezanne reduces DNA double-strand break (DSB) through upregulation of SIRT6 in OGD-insulted neurons. Together, this study suggests that Cezanne-dependent SIRT6-DNA DSB signaling pathway may mediate the neuroprotective effect of tDCS in ischemic neurons.

Bis(2-ethylhexyl) phthalate induces DNA strand breaks and gene expression alterations in larval zebrafish Danio rerio

Contamination of the aquatic environment by plastic industrial products and their by-products is remarkable. Because of their physical, chemical, and biological degradation resistance, plasticizers can enter the food chain of living organisms, accumulate in the body and generate toxic effects. Here we determined the potential toxic effects of bis(2-ethylhexyl) phthalate (DEHP) plasticizer to larval (72 h post fertilization) zebrafish ( Danio rerio) by analyzing changes in expression levels of stress-related genes ( p53, rad51, and xrcc5) by the quantitative real-time polymerase chain reaction. Also, possible DNA damage by DEHP in larvae was determined. The concentration of DEHP (0–160 mg/l) that killed 50% of the larval zebrafish within 96 h was 54.02 mg/l. There was a concentration-related increase in DNA damage in cells from larvae exposed (96 h) to DEHP. DNA damage of 31.13% (mean ± standard error of the mean) was observed in larvae at the highest sublethal DEHP concentration (10 mg/l). Some significant differences in the induction of stress-related genes were also observed in larvae exposed to DEHP relative to control ( p < 0.05). The conclusion drawn from this ecotoxicological risk assessment is that, under present use and exposure patterns, DEHP presents a small hazard to zebrafish larvae.

The Role of Sirt1 in Ischemic Stroke: Pathogenesis and Therapeutic Strategies

Silent mating type information regulation 2 homolog 1 (Sirt1), a nicotine adenine dinucleotide (NAD⁺)-dependent enzyme, is well-known in playing a part in longevity. Ischemic stroke is a major neurological disorder and is a leading cause of death and adult disability worldwide. Recently, many studies have focused on the role of Sirt1 in ischemic stroke. Numerous studies consider Sirt1 as a protective factor and investigate the signaling pathways involved in the process under ischemic stress. However, the answer to whether upregulation of Sirt1 improves the outcome of stroke is still a controversy. In this review, we discuss the role and mechanisms of Sirt1 in the setting of ischemic stroke.

DNA binding activity of Ku during chemotherapeutic agent-induced early apoptosis

Ku protein is a heterodimer composed of two subunits, and is capable of both sequence-independent and sequence-specific DNA binding. The former mode of DNA binding plays a crucial role in DNA repair. The biological role of Ku protein during apoptosis remains unclear. Here, we show characterization of Ku protein during apoptosis. In order to study the DNA binding properties of Ku, we used two methods for the electrophoresis mobility shift assay (EMSA). One method, RI-EMSA, which is commonly used, employed radiolabeled DNA probes. The other method, WB-EMSA, employed unlabeled DNA followed by western blot and detection with anti-Ku antiserum. In this study, Ku-DNA probe binding activity was found to dramatically decrease upon etoposide treatment, when examined by the RI-EMSA method. In addition, pre-treatment with apoptotic cell extracts inhibited Ku-DNA probe binding activity in the non-treated cell extract. The inhibitory effect of the apoptotic cell extract was reduced by DNase I treatment. WB-EMSA showed that the Ku in the apoptotic cell extract bound to fragmented endogenous DNA. Interestingly, Ku in the apoptotic cell extract purified by the Resource Q column bound 15-bp DNA in both RI-EMSA and WB-EMSA, whereas Ku in unpurified apoptotic cell extracts did not bind additional DNA. These results suggest that Ku binds cleaved chromosomal DNA and/or nucleosomes in apoptotic cells. In conclusion, Ku is intact and retains DNA binding activity in early apoptotic cells.

Genotoxic effects and gene expression changes in larval zebrafish after exposure to ZnCl2 and ZnO nanoparticles

Engineered nanoparticles (NPs) can potentially generate adverse effects at the tissue, organ, cellular, subcellular, DNA, and protein levels due to their unique physico-chemical properties. Dissoluble NPs (e.g. nZnO) can be toxic in aquatic organisms. We compared effects of nZnO and corresponding concentrations of released Zn(II) by water-soluble ZnCl(2) on larval zebrafish Danio rerio (72 h post fertilization) by analyzing changes in expression levels of stress-related genes (p53, rad51, mt2) by qRT-PCR. Additionally, genotoxicity of nZnO and Zn(II) was assessed. The lethal concentrations for 50% mortality (LC(50)) in larval zebrafish exposed for 96 h to 0 to 70 mg l(-1) nZnO and Zn(II) were 21.37 ± 1.81 mg l(-1) (95% CI) and 4.66 ± 0.11 mg l(-1), respectively. A concentration-dependent increase in DNA strand breaks was detected in cells from larvae exposed (96 h) to nZnO and Zn(II). DNA damage was higher in Zn(II)- than nZnO-exposed larvae. Induction of stress-related genes in larvae was complex and was not directly related to nZnO and Zn(II) concentrations, although there was significant induction in the mt2 gene of larvae exposed to Zn(II) and nZnO relative to controls. mt2 induction of 20.5 ± 1.9-fold and 2.5 ± 0.8-fold change (mean ± SEM) was observed in larvae at the highest Zn(II) and nZnO concentrations (3 and 6 mg l(-1)), respectively. The results suggest that toxicity associated with nZnO is primarily due to the release of Zn(II).

Sirt1 in cerebral ischemia

Cerebral ischemia is among the leading causes of death worldwide. It is characterized by a lack of blood flow to the brain that results in cell death and damage, ultimately causing motor, sensory, and cognitive impairments. Today, clinical treatment of cerebral ischemia, mostly stroke and cardiac arrest, is limited and new neuroprotective therapies are desperately needed. The Sirtuin family of oxidized nicotinamide adenine dinucleotide (NAD⁺)-dependent deacylases has been shown to govern several processes within the central nervous system as well as to possess neuroprotective properties in a variety of pathological conditions such as Alzheimer's Disease, Parkinson's Disease, and Huntington's Disease, among others. Recently, Sirt1 in particular has been identified as a mediator of cerebral ischemia, with potential as a possible therapeutic target. To gather studies relevant to this topic, we used PubMed and previous reviews to locate, select, and resynthesize the lines of evidence presented here. In this review, we will first describe some functions of Sirt1 in the brain, mainly neurodevelopment, learning and memory, and metabolic regulation. Second, we will discuss the experimental evidence that has implicated Sirt1 as a key protein in the regulation of cerebral ischemia as well as a potential target for the induction of ischemic tolerance.

Neuronal apoptosis in cerebral ischemia/reperfusion area following electrical stimulation of fastigial nucleus

Previous studies have indicated that electrical stimulation of the cerebellar fastigial nucleus in rats may reduce brain infarct size, increase the expression of Ku70 in cerebral ischemia/reperfusion area, and decrease the number of apoptotic neurons. However, the anti-apoptotic mechanism of Ku70 remains unclear. In this study, fastigial nucleus stimulation was given to rats 24, 48, and 72 hours before cerebral ischemia/reperfusion injury. Results from the electrical stimulation group revealed that rats exhibited a reduction in brain infarct size, a significant increase in the expression of Ku70 in cerebral ischemia/reperfusion regions, and a decreased number of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells. Double immunofluorescence staining revealed no co-localization of Ku70 with TUNEL-positive cells. However, Ku70 partly co-localized with Bax protein in the cytoplasm of rats with cerebral ischemia/reperfusion injury. These findings suggest an involvement of Ku70 with Bax in the cytoplasm of rats exposed to electrical stimulation of the cerebellar fastigial nucleus, and may thus provide an understanding into the anti-apoptotic activity of Ku70 in cerebral ischemia/reperfusion injury.

MicroRNA-124 (miR-124) regulates Ku70 expression and is correlated with neuronal death induced by ischemia/reperfusion

MicroRNAs are small, non-coding RNA molecules that regulate gene expression, and miR-124 is the most abundant miRNA in the brain. Studies have shown that miR-124 is clearly reduced in the ischemic brain after stroke; however, the role of miR-124 after stroke is less well studied. Using TargetScan, MicroCosm Targets version 5, and microRNA.org databases, we identified miR-124 as a possible regulator of the DNA repair protein Ku70. We validated that Ku70 is a target for miR-124 with a luciferase reporter activity assay. Moreover, adult rats subjected to focal cerebral ischemia exhibited a substantial reduction of miR-124 expression, which was inversely upregulated by Ku70 expression. In vivo treatment with miR-124 antagomir effectively enhanced Ku70 mRNA and protein levels in the ischemic region. Furthermore, knockdown of cerebral miR-124 reduced cell death and infarct size and improved neurological outcomes. Our data demonstrate that miR-124 is an endogenous regulator of Ku70 that improves ischemia/reperfusion (I/R)-induced brain injury and dysfunction.

Acute responses of DNA repair proteins and StarD6 in rat hippocampus after domoic acid-induced excitotoxicity

StarD6, which might be considered to be neuroprotective, and DNA repair proteins can play a role against oxidative damages by excitotoxin in the nervous system. In order to investigate the relationship between StarD6 and DNA repair proteins, excitotoxicity was induced by domoic acid in male Sprague-Dawley rats. Western blot analysis revealed transitorily elevated levels in StarD6, apurinic/apyrimidinic endonuclease (APE) and 8-oxoguanine DNA-glycosylase (Ogg1) in accord with the DNA damage marker phosphorylated H2AX. Immunohistochemistry revealed that increased intensity was transiently seen not only in the Stratum (Str.) radiatum and Str. lacunosum-moleculare with StarD6 and APE, but also in the Str. pyramidale with Ogg1. Intensities decreased 24h after domoic acid injection in CA3 with APE and Ogg1 as well as in the Str. radiatum and Str. lacunosum-moleculare with StarD6 and APE. These results suggested that StarD6 may not be closely related with DNA repair proteins in the hippocampus after domoic acid-induced excitotoxicity, although the activities of these proteins might be positively affected by excitotoxic stimuli.

Cerebroprotective effects of red ginseng extract pretreatment against ischemia-induced oxidative stress and apoptosis

Panax ginseng C.A. Meyer has been traditionally used as a medicinal plant and has beneficial effects due to pharmacological properties. Although ginseng is thought to be protective under abnormal conditions, the effects of pretreatment with red ginseng (RG) extract on ischemic stroke have not been fully elucidated. We investigated the protective effects of RG extract after focal cerebral ischemia in mice. Crude RG extract (360 mg/kg) was administered intraperitoneally for 2 weeks. Mice were then subjected to occlusion of the middle cerebral artery for 1 hour, followed by reperfusion for 4 and 24 hours. Pretreatment with RG extract followed by ischemia/reperfusion (I/R) resulted in significant reduction of oxidized hydroethidine signals in ischemic areas. At 4 and 24 hours after I/R, the number of 8-hydroxyguanosine and apoptosis signal-regulating kinase 1 (ASK1)-positive cells decreased in the ischemic penumbra as seen using immunofluorescent staining. Western blotting showed that RG efficiently attenuated the protein levels of activated ASK1 in the ischemic penumbra. Consequently, DNA fragmentation and the infarct volume were reduced by RG extract pretreatment 24 hours after I/R. Also, RG extract resulted in better performance in rotarod test after I/R. Thus, RG pretreatment demonstrates a protective effect at suppressing ischemia-induced oxidative stress and apoptosis in ischemic lesions. Pretreatment with crude RG extract may be an effective strategy for preventing brain injury after an ischemic stroke.

Protective effects and mechanisms of sirtuins in the nervous system

Silent information regulator two proteins (sirtuins or SIRTs) are a group of histone deacetylases whose activities are dependent on and regulated by nicotinamide adenine dinucleotide (NAD(+)). They suppress genome-wide transcription, yet upregulate a select set of proteins related to energy metabolism and pro-survival mechanisms, and therefore play a key role in the longevity effects elicited by calorie restriction. Recently, a neuroprotective effect of sirtuins has been reported for both acute and chronic neurological diseases. The focus of this review is to summarize the latest progress regarding the protective effects of sirtuins, with a focus on SIRT1. We first introduce the distribution of sirtuins in the brain and how their expression and activity are regulated. We then highlight their protective effects against common neurological disorders, such as cerebral ischemia, axonal injury, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. Finally, we analyze the mechanisms underlying sirtuin-mediated neuroprotection, centering on their non-histone substrates such as DNA repair enzymes, protein kinases, transcription factors, and coactivators. Collectively, the information compiled here will serve as a comprehensive reference for the actions of sirtuins in the nervous system to date, and will hopefully help to design further experimental research and expand sirtuins as therapeutic targets in the future.

Apoptosis signal-regulating kinase 1 (Ask1) targeted small interfering RNA on ischemic neuronal cell death

Apoptosis signal-regulating kinase 1 (Ask1) is one of mitogen-activated protein kinase kinase kinase (MAPKKK) for cell differentiation and apoptosis. The aim of the present study is to evaluate whether RNA interference against Ask1 (Ask1-siRNA) down-regulates the expression of Ask1 and prevents apoptotic neuronal cell death after ischemia/reperfusion (I/R) in mice. Mice were subjected to intraluminal suture occlusion of the middle cerebral artery for 1h, followed by reperfusion. The Ask1-siRNA or a control-siRNA was introduced using osmotic pump intracerebroventricularly at 3days before I/R. The expression and mRNA of Ask1 were evaluated by Western blot and RT-PCR after I/R with time. Immunohistochemistry and TUNEL assay were also investigated to evaluate the effect of Ask1 on cerebral infarction by Ask1-siRNA treatment. The expression of Ask1 was increased significantly at 8h after I/R. The level of mRNA and protein of Ask1 down-regulated after treatment of Ask1-siRNA and subsequently cerebral infarction volume was reduced. Our results suggest the increased Ask1 expression induce apoptotic cell death after I/R. And we also demonstrated that Ask1-siRNA attenuates upregulation of Ask1, which was followed by the reduction of infarction in ischemic brain after I/R. Ask1-siRNA could represent a molecular target for prevention of ischemic stroke.

Mechanistic insight into DNA damage and repair in ischemic stroke: exploiting the base excision repair pathway as a model of neuroprotection

Stroke is a common cause of death and serious long-term adult disability. Oxidative DNA damage is a severe consequence of oxidative stress associated with ischemic stroke. The accumulation of DNA lesions, including oxidative base modifications and strand breaks, triggers cell death in neurons and other vulnerable cell populations in the ischemic brain. DNA repair systems, particularly base excision repair, are endogenous defense mechanisms that combat oxidative DNA damage. The capacity for DNA repair may affect the susceptibility of neurons to ischemic stress and influence the pathological outcome of stroke. This article reviews the accumulated understanding of molecular pathways by which oxidative DNA damage is triggered and repaired in ischemic cells, and the potential impact of these pathways on ischemic neuronal cell death/survival. Genetic or pharmacological strategies that target the signaling molecules in DNA repair responses are promising for potential clinically effective treatment. Further understanding of mechanisms for oxidative DNA damage and its repair processes may lead to new avenues for stroke management.

Hydroethidine- and MitoSOX-derived red fluorescence is not a reliable indicator of intracellular superoxide formation: another inconvenient truth

Hydroethidine (HE; or dihydroethidium) is the most popular fluorogenic probe used for detecting intracellular superoxide radical anion. The reaction between superoxide and HE generates a highly specific red fluorescent product, 2-hydroxyethidium (2-OH-E(+)). In biological systems, another red fluorescent product, ethidium, is also formed, usually at a much higher concentration than 2-OH-E(+). In this article, we review the methods to selectively detect the superoxide-specific product (2-OH-E(+)) and the factors affecting its levels in cellular and biological systems. The most important conclusion of this review is that it is nearly impossible to assess the intracellular levels of the superoxide-specific product, 2-OH-E(+), using confocal microscopy or other fluorescence-based microscopic assays and that it is essential to measure by HPLC the intracellular HE and other oxidation products of HE, in addition to 2-OH-E(+), to fully understand the origin of red fluorescence. The chemical reactivity of mitochondria-targeted hydroethidine (Mito-HE, MitoSOX red) with superoxide is similar to the reactivity of HE with superoxide, and therefore, all of the limitations attributed to the HE assay are applicable to Mito-HE (or MitoSOX) as well.

The adenoviral vector-mediated increase in apurinic/apyrimidinic endonuclease inhibits the induction of neuronal cell death after transient ischemic stroke in mice

Despite the correlation between changes in the levels of apurinic/apyrimidinic endonuclease and ischemic neuronal damage, no studies have addressed the question of whether increased APE/Ref-1 can prevent ischemic neuronal cell death in vivo. Using an adenoviral vector, we investigated whether increased APE/Ref-1 can inhibit the loss of APE/Ref-1 and thereby prevent oxidative DNA damage after transient focal cerebral ischemia. Mice were subjected to intraluminal suture occlusion of the middle cerebral artery for 1 h, followed by reperfusion. Pre-ischemic treatment of the adenoviral vector was introduced intracerebrally. An adenoviral vector harboring the entire APE/Ref-1 gene sequence or a control virus without the APE/Ref-1 sequence was introduced 3 days before ischemia/reperfusion (I/R). The reduction of APE/Ref-1 occurred before DNA fragmentation, which was shown by temporal and spatial analysis. Increased APE/Ref-1 significantly decreased DNA damage and infarct volume after I/R. In conclusion, increased APE/Ref-1 enhanced DNA repair and inhibited the induction of ischemic oxidative DNA damage and cerebral infarction after I/R.

Cerebral expression of DNA repair protein, Ku70, and its association with cell proliferation following cerebral hypoxia-ischemia in neonatal rats

We hypothesized that increased Ku70 expression could be involved in recovery following cerebral hypoxia-ischemia. We investigated the progression of cerebral alterations in Ku70 expression at different time points (24 h, 72 h, 1 week, 4 weeks and 8 weeks) after hypoxia-ischemia (right carotid artery occlusion plus 1.5h of hypoxia) in neonatal rats. To determine whether in addition to its known role of DNA repair, Ku70 was associated with cell death or cell proliferation we performed double staining for Ku70 and DNA fragmentation or bromodeoxyuridine, respectively. The results show that Ku70 expression was increased in the infarct core and peri-infarct regions at 24h following hypoxia-ischemia. The increased Ku70 expression was transient in the infarct core with a loss of Ku70 positive cells over days. In contrast, in the peri-infarct region the expression of Ku70 remained increased at chronic times 8 weeks following the insult. Cells positive for DNA fragmentation were not co-localized with cells positive for Ku70 after an insult. However, most of the cells positive for bromodeoxyuridine indicative of cell proliferation were positive for Ku70 in the peri-infarct region at 8 weeks after the insult. Considering the roles of Ku70 in DNA repair or inhibiting apoptosis and its co-localization within cells that had undergone proliferation, Ku70 may be considered a potential novel target to enhance recovery following hypoxia-ischemia.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

DNA repair in neurons: so if they don't divide what's to repair?

Neuronal DNA repair remains one of the most exciting areas for investigation, particularly as a means to compare the DNA repair response in mitotic (cancer) vs. post-mitotic (neuronal) cells. In addition, the role of DNA repair in neuronal cell survival and response to aging and environmental insults is of particular interest. DNA damage caused by reactive oxygen species (ROS) such as generated by mitochondrial respiration includes altered bases, abasic sites, and single- and double-strand breaks which can be prevented by the DNA base excision repair (BER) pathway. Oxidative stress accumulates in the DNA of the human brain over time especially in the mitochondrial DNA (mtDNA) and is proposed to play a critical role in aging and in the pathogenesis of several neurological disorders including Parkinson's disease, ALS, and Alzheimer's diseases. Because DNA damage accumulates in the mtDNA more than nuclear DNA, there is increased interest in DNA repair pathways and the consequence of DNA damage in the mitochondria of neurons. The type of damage that is most likely to occur in neuronal cells is oxidative DNA damage which is primarily removed by the BER pathway. Following the notion that the bulk of neuronal DNA damage is acquired by oxidative DNA damage and ROS, the BER pathway is a likely area of focus for neuronal studies of DNA repair. BER variations in brain aging and pathology in various brain regions and tissues are presented. Therefore, the BER pathway is discussed in greater detail in this review than other repair pathways. Other repair pathways including direct reversal, nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination and non-homologous end joining are also discussed. Finally, there is a growing interest in the role that DNA repair pathways play in the clinical arena as they relate to the neurotoxicity and neuropathy associated with cancer treatments. Among the numerous side effects of cancer treatments, major clinical effects include neurocognitive dysfunction and peripheral neuropathy. These symptoms occur frequently and have not been effectively studied at the cellular or molecular level. Studies of DNA repair may help our understanding of how those cells that are not dividing could succumb to neurotoxicity with the clinical manifestations discussed in the following article.

Role of NF-κB and DNA Repair Protein Ku on Apoptosis in Pancreatic Acinar Cells

Reactive oxygen species have been known to cause DNA damage and induce apoptosis. During DNA damage, DNA repair proteins Ku70 and Ku80 prevent cell death, but severe DNA damage beyond the repair capacity of the DNA repair proteins triggers necrosis or apoptosis. Recent reports have shown that NF-kappaB plays a critical role in protecting the cells from apoptosis. We investigated whether glucose oxidase acting on beta-D-glucose (G/GO), which continuously produces H(2)O(2), induces apoptosis, and whether NF-kappaB and Ku are involved in G/GO-induced apoptosis in pancreatic acinar AR42J cells. Electron microscopic observation showed that apoptotic cells with characteristic nuclear condensation and shrinkage as well as large vacuoles were detected after G/GO treatment. G/GO treatment induced apoptotic cell death, as determined by viable cell count and DNA fragmentation. G/GO-induced apoptosis was increased in the cells transfected with the Ku-dominant negative mutant (Ku D/N) and mutated IkappaBalpha gene (IkappaB mt) as compared to the wild-type cells (Wild) and the cells transfected with the control pcDNA3 vector (pcN-3). G/GO treatment caused nuclear loss of both Ku70 and Ku80 in Wild cells and pcN-3 cells. Even without G/GO treatment, nuclear loss of Ku proteins was observed in IkappaB mt cells. These results suggest that oxidative stress-induced reduction of nuclear Ku proteins may cause loss of defense against DNA damage and thus induce apoptosis in pancreatic acinar cells. The novel finding is that nuclear translocation of Ku proteins may be mediated by NF-kappaB.

Iron-induced oxidative brain injury after experimental intracerebral hemorrhage

We investigated the occurrence of DNA damage in brain after intracerebral hemorrhage (ICH) and the role of iron in such injury. Male Sprague-Dawley rats received an infusion of 100 microL autologous whole blood or 30 microL FeCl2 into the right basal ganglia and were sacrificed 1, 3, or 7 days later. 8-hydroxyl-2'-deoxyguanosine (8-OHdG) was analyzed by immunohistochemistry, while the number of apurinic/apyrimidinic abasic sites (AP sites) was also quantified. 8-OHdG and AP sites are two hallmarks of DNA oxidation. DNA damage was also examined using PANT and TUNEL labeling. Dinitrophenyl (DNP) was measured by Western blot to compare the time course of protein oxidative damage to that of DNA. DNA repair APE/Ref-1 and Ku-proteins were also measured by Western blot. Bipyridine, a ferrous iron chelator, was used to examine the role of iron in ICH-induced oxidative brain injury. An increase in 8-OHdG, AP sites, and DNP levels, and a decrease in APE/Ref-1 and Ku levels were observed. Abundant PANT-positive cells were also observed in the perihematomal area 3 days after ICH. Bipyridine attenuated ICH-induced changes in PANT and DNP. These results suggest that iron-induced oxidation causes DNA damage in brain after ICH and that iron is a therapeutic target for ICH.

Early nuclear translocation of endonuclease G and subsequent DNA fragmentation after transient focal cerebral ischemia in mice

We investigated whether the endonuclease G (endoG) translocated from mitochondria to nucleus after transient focal cerebral ischemia (tFCI), thereby contributed to subsequent DNA fragmentation. Adult male mice were subjected to 60min of focal cerebral ischemia by intraluminal suture blockade of the middle cerebral artery. Western blot analysis for endoG was performed at various time points of tFCI. Nuclear endoG was detected as early as 4h after tFCI in the ischemic brain, and correspondingly mitochondrial endoG showed a significant reduction at 4h after reperfusion (p<0.01). Immunohistochemistry of endoG confirmed that the nuclear translocation of endoG was detected as early as 4h after tFCI in the middle cerebral artery (MCA) territory of the ischemic brain. Double immunofluorescent staining with endoG and AIF showed that endoG was predominantly colocalized with AIF at 24h after tFCI. Double staining with endoG immunohistochemistry and TdT-mediated dUTP-biotin nick end labeling showed a spatial relationship between endoG expression and DNA fragmentation at 24h after tFCI. These data suggest that the early nuclear translocation of endoG occurs and could induce DNA fragmentation in the ischemic brain after tFCI.

Oxidative DNA injury after experimental intracerebral hemorrhage

Our previous studies have demonstrated that DNA injury occurs in the brain after intracerebral hemorrhage (ICH). DNA damage can result from at least two pathways, either endonuclease-mediated DNA fragmentation or oxidative injury. The present study investigated the occurrence of the latter after ICH and the role of iron in such injury. Male Sprague-Dawley rats received an infusion of autologous whole blood or ferrous iron into the right basal ganglia. Control rats just had a needle insertion (sham). The rats were sacrificed 1, 3, or 7 days later. 8-Hydroxyl-2'-deoxyguanosine (8-OHdG) was analyzed by immunohistochemistry while the number of apurnic/apyrimidinic abasic sites (AP sites) was also quantified. 8-OHdG and AP sites are two hallmarks of DNA oxidation. Dinitrophenyl (DNP) was measured by Western blotting to compare the time course of protein oxidative damage to that of DNA. DNA repair Ku proteins were measured by Western blot analysis. DNA damage was also examined using DNA polymerase I-mediated biotin-dATP nick translation (PANT) labeling. An increase of 8-OHdG, AP sites and DNP levels and a decrease of Ku levels were observed. Abundant PANT-positive cells were also observed in the perihematomal area 3 days after ICH. In addition, intracerebral infusion of iron increased brain DNP levels and resulted in DNA injury. These results suggest that oxidative stress contributes to DNA damage and brain injury after ICH. Reducing DNA oxidative damage (for example, through iron chelation) may be a therapeutic target for ICH.

Downregulation of genes involved in DNA repair and differential expression of transcription regulators and phosphatases precede IgM-induced apoptosis in the Burkitt's lymphoma cell line BL60-2

Apoptosis of lymphocytes recognizing self-antigens is an essential mechanism to protect the organism against autoimmune diseases. Programmed cell death of susceptible B cells occurs in response to surface IgM cross-linking mediated by self-antigens. This effect can be mimicked in the Burkitt's lymphoma line BL60-2 by addition of anti-IgM antibodies. In order to identify genes with differential expression in response to the apoptotic stimulus, total RNA prepared from BL60-2 cells before and at different points in time after IgM cross-linking was used for Atlas arrays, high-density oligonucleotide microarrays (GeneChip arrays, Affymetrix) and in RNase protection assays (RPA). One of our major observations was the downregulation of six genes involved in the ligation of DNA strand breaks, like DNA ligases and DNA-PK, indicating a shutdown of DNA repair mechanisms in apoptotic cells. In addition, we found changes on mRNA level for several transcription regulators, including early growth response genes 1 and 2, TAFII30 and topoisomerase I. Furthermore, we show accumulation of mRNA for the phosphatases CD45 and DUSP5 in anti-IgM stimulated BL60-2 cells. Our data provide a basis for further analysis of the differentially expressed genes and their roles in IgM-induced B cell death as well as in apoptosis in other cellular systems.

An Assessment of a Variant of the DNA Repair Gene XRCC3 as a Possible Nevus or Melanoma Susceptibility Genotype

Inheritance of the T allele in exon 7 (position 18067) of the DNA repair gene XRCC3 has been reported to be associated with susceptibility to melanoma in a study from Oxford. We report a study in which an attempt was made to confirm this association in a similar population. The most potent risk factor for melanoma in the general population is a phenotype characterized by the presence of multiple melanocytic nevi: the atypical mole syndrome. Our hypothesis is that the atypical mole syndrome may be a marker of genetic susceptibility to melanoma. We have therefore investigated whether the XRCC3 polymorphism influences the nevus phenotype. The XRCC3 genotype was investigated using PCR in a general-practice-based sample of 565 women and 475 patients from a cohort enriched for the atypical mole syndrome, of whom 140 had had melanoma. Allele frequencies were the same in the healthy women, the melanoma cases from this study, and the melanoma cases reported in the Oxford study, but were different from those in the Oxford control group. We found no evidence therefore that the T allele of this XRCC3 polymorphism is indicative of susceptibility to melanoma. There was a marginal relationship with nevus phenotype, but this was no longer statistically significant in multivariate analysis. The previous association between XRCC3 and melanoma may be a result of the choice of control group and we emphasize the need for appropriate choice of controls.

Is DNA repair compromised in Alzheimer's disease?

Mammalian cells utilize multiple mechanisms to repair DNA damage that occurs during normal cellular respiration and in response to genotoxic stress. This study sought to determine if chronic oxidative stress proposed to occur during Alzheimer's disease alters the expression or activity of DNA double-strand break repair or base excision repair proteins. Double-strand break repair requires DNA-dependent protein kinase, composed of a catalytic subunit, DNA-PKcs, and a regulatory component, Ku. Ku DNA binding activity was reduced in extracts of postmortem AD midfrontal cortex, but was not significantly different from the age-matched controls. Decreased Ku DNA binding correlated with reduced protein levels of Ku subunits, DNA-PKcs, and poly(ADP-ribose) polymerase-1. Expression of the base excision repair enzyme Ref-1, however, was significantly increased in AD extracts compared to controls. Ku DNA binding and DNA-PK protein levels in the AD cases correlated significantly with synaptophysin immunoreactivity, which is a measure of synaptic loss, a major correlate of cognitive deficits in AD. Immunohistochemical analysis suggested that DNA-PK protein levels reflected both number of neurons and regulation of cellular expression.

Molecular mechanisms of cerebral ischemia-induced neuronal death

The mode of neuronal death caused by cerebral ischemia and reperfusion appears on the continuum between the poles of catastrophic necrosis and apoptosis: ischemic neurons exhibit many biochemical hallmarks of apoptosis but remain cytologically necrotic. The position on this continuum may be modulated by the severity of the ischemic insult. The ischemia-induced neuronal death is an active process (energy dependent) and is the result of activation of cascades of detrimental biochemical events that include perturbion of calcium homeostasis leading to increased excitotoxicity, malfunction of endoplasmic reticulum and mitochondria, elevation of oxidative stress causing DNA damage, alteration in proapoptotic gene expression, and activation of the effector cysteine proteases (caspases) and endonucleases leading to the final degradation of the genome. In spite of strong evidence showing that brain infarction can be reduced by inhibiting any one of the above biochemical events, such as targeting excitotoxicity, up-regulation of an antiapoptotic gene, or inhibition of a down-stream effector caspase, it is becoming clear that targeting a single gene or factor is not sufficient for stroke therapeutics. An effective neuroprotective therapy is likely to be a cocktail aimed at all of the above detrimental events evoked by cerebral ischemia and the success of such therapeutic intervention relies upon the complete elucidation of pathways and mechanisms of the cerebral ischemia-induced active neuronal death.

Ku70 suppresses the apoptotic translocation of Bax to mitochondria

Bax induces mitochondrial-dependent cell death signals in mammalian cells. However, the mechanism of how Bax is kept inactive has remained unclear. Yeast-based functional screening of Bax inhibitors from mammalian cDNA libraries identified Ku70 as a new Bax suppressor. Bax-mediated apoptosis was suppressed by overexpression of Ku70 in mammalian cells, but enhanced by downregulation of Ku70. We found that Ku70 interacts with Bax, and that the carboxyl terminus of Ku70 and the amino terminus of Bax are required for this interaction. Bax is known to translocate from the cytosol to mitochondria when cells receive apoptotic stimuli. We found that Ku70 blocks the mitochondrial translocation of Bax. These results suggest that in addition to its previously recognized DNA repair activity in the nucleus, Ku70 has a cytoprotective function in the cytosol that controls the localization of Bax.

Involvement of superoxide in excitotoxicity and DNA fragmentation in striatal vulnerability in mice after treatment with the mitochondrial toxin, 3-nitropropionic acid

Oxidative stress and excitotoxicity have been implicated in selective striatal vulnerability caused by the mitochondrial toxin, 3-nitropropionic acid (3-NP), which may simulate Huntington's disease in animals and humans. The detailed mechanism of the role of superoxide in striatal vulnerability induced by 3-NP is still unknown. The authors investigated oxidative cellular injury and DNA fragmentation after systemic 3-NP injection in wild-type (Wt) mice and mutant mice with a deficiency in manganese superoxide dismutase (MnSOD; Sod2 -/+). Furthermore, they investigated the effects of decortication after 3-NP treatment in Sod2 -/+ mice, and copper/zinc SOD (CuZnSOD) treatment in recently developed Sod2 -/+ mice that overexpress CuZnSOD (SOD1 +/- / Sod2 -/+ mice). Oxidized hydroethidine, 8-hydroxyguanosine immunoreactivity, and nitrotyrosine immunoreactivity were increased in the Sod2 -/+ mice compared with the Wt mice after 3-NP treatment (P < 0.001). Decortication completely abolished oxidative striatal damage after 3-NP treatment in the Sod2 -/+ mice. Increased CuZnSOD attenuated DNA fragmentation and striatal lesion volume after 3-NP treatment in the Sod2 -/+ mice (P < 0.001). These data suggest that production of superoxide may be a critical step to excitotoxicity and subsequent DNA fragmentation in selective striatal vulnerability after 3-NP treatment.

Activation of mitochondrial ATP-dependent potassium channels protects neurons against ischemia-induced death by a mechanism involving suppression of Bax translocation and cytochrome c release

Neurons express a variety of plasma-membrane potassium channels that play important roles in regulating neuronal excitability and synaptic transmission, but also contain mitochondrial ATP-sensitive potassium channels, the functions of which are unknown. Studies of cardiac cells suggest that similar mitochondrial ATP-sensitive potassium channels are involved in the process of ischemic preconditioning, suggesting a role in regulating cell survival. The authors report that mice given diazoxide, an activator of mitochondrial ATP-sensitive potassium channels, exhibited a large (60% to 70%) decrease in cortical infarct size after permanent occlusion of the middle cerebral artery. Diazoxide decreases neuronal apoptosis and increases astrocyte survival and activation in the penumbral region of the ischemic cortex. The neuroprotective effect of diazoxide is abolished by 5-hydroxydecanoate, a selective antagonist of mitochondrial ATP-sensitive potassium channels. Studies of cultured hippocampal neurons reveal that diazoxide depolarizes mitochondria, prevents cytochrome c release, and protects cells against death induced by staurosporine and chemical hypoxia. Diazoxide increased the levels of Bcl2 and inhibited the association of Bax with mitochondria in neurons exposed to an apoptotic insult, suggesting that activation of mitochondrial ATP-sensitive potassium channels may stabilize mitochondrial function by differentially modulating proapoptotic and antiapoptotic proteins. Collectively, the data suggest that mitochondrial ATP-sensitive potassium channels play a key role in modulating neuronal survival under ischemic conditions, and identify agents that activate mitochondrial ATP-sensitive potassium channels as potential therapeutics for stroke and related neurodegenerative conditions.

Overexpression of copper/zinc superoxide dismutase in transgenic rats protects vulnerable neurons against ischemic damage by blocking the mitochondrial pathway of caspase activation

Mitochondria are known to be involved in the early stage of apoptosis by releasing cytochrome c, caspase-9, and the second mitochondria-derived activator of caspases (Smac). We have reported that overexpression of copper/zinc superoxide dismutase (SOD1) reduced superoxide production and ameliorated neuronal injury in the hippocampal CA1 subregion after global ischemia. However, the role of oxygen free radicals produced after ischemia/reperfusion in the mitochondrial signaling pathway has not been clarified. Five minutes of global ischemia was induced in male SOD1-transgenic (Tg) and wild-type (Wt) littermate rats. Cytosolic expression of cytochrome c and Smac and activation of caspases were evaluated by immunohistochemistry, Western blot, and caspase activity assay. Apoptotic cell death was characterized by DNA nick end and single-stranded DNA labeling. In the Wt animals, early superoxide production, mitochondrial release of cytochrome c, Smac, and cleaved caspase-9 were observed after ischemia. Active caspase-3 was subsequently increased, and 85% of the hippocampal CA1 neurons showed apoptotic DNA damage 3 d after ischemia. Tg animals showed less superoxide production and cytochrome c and Smac release. Subsequent active caspase-3 expression was not evident, and only 45% of the neurons showed apoptotic DNA damage. A caspase-3 inhibitor (N-benzyloxycarbonyl-val-ala-asp-fluoromethyl ketone) reduced cell death only in Wt animals. These results suggest that overexpression of SOD1 reduced oxidative stress, thereby attenuating the mitochondrial release of cytochrome c and Smac, resulting in less caspase activation and apoptotic cell death. Oxygen free radicals may play a pivotal role in the mitochondrial signaling pathway of apoptotic cell death in hippocampal CA1 neurons after global ischemia.