DNA damage and chronic neuronal degenerations

Modulation of Synaptic Plasticity Genes Associated to DNA Damage in a Model of Huntington's Disease

Huntington's disease (HD) is a disease characterized by the progressive degeneration of nerve cells in the brain. DNA damage has been implicated in many neurological disorders; however, the association between this damage and the impaired signaling related to neurodegeneration is still unclear. The transcription factor c-AMP-responsive element binding protein (CREB) has a relevant role in the neuronal plasticity process regulating the expression of several genes, including brain-derived neurotrophic factor (BDNF). Here we analyzed the direct link between DNA damage and the expression of genes involved in neuronal plasticity. The study was performed in model cell lines STHdhQ7 (wild type) and STHdhQ111 (HD model). Treatment with Etoposide (Eto) was used to induce double-strand breaks (DSBs) to evaluate the DNA damage response (DDR) and the expression of synaptic plasticity genes. Eto treatment induced phosphorylation of ATM (p-ATM) and H2AX (γH2AX), markers of DDR, in both cell lines. Interestingly, upon DNA damage, STHdhQ7 cells showed increased expression of activity-regulated cytoskeleton associated protein (Arc) and BDNF when compared to the HD cell line model. Additionally, Eto induced CREB activation with a differential localization of its co-activators in the cell types analyzed. These results suggest that DSBs impact differentially the gene expression patterns of plasticity genes in the normal cell line versus the HD model. This effect is mediated by the impaired localization of CREB-binding protein (CBP) and histone acetylation in the HD model. Our results highlight the role of epigenetics and DNA repair on HD and therefore we suggest that future studies should explore in depth the epigenetic landscape on neuronal pathologies with the goal to further understand molecular mechanisms and pinpoint therapeutic targets.

The Prevailing Role of Topoisomerase 2 Beta and its Associated Genes in Neurons

Topoisomerase 2 beta (TOP2β) is an enzyme that alters the topological states of DNA by making a transient double-strand break during the transcription process. The direct interaction of TOP2β with DNA strand results in transcriptional regulation of certain genes and some studies have suggested that a particular set of genes are regulated by TOP2β, which have a prominent role in various stages of neuron from development to degeneration. In this review, we discuss the role of TOP2β in various phases of the neuron's life. Based on the existing reports, we have compiled the list of genes, which are directly regulated by the enzyme, from different studies and performed their functional classification. We discuss the role of these genes in neurogenesis, neuron migration, fate determination, differentiation and maturation, generation of neural circuits, and senescence.

DNA Repair in Huntington's Disease and Spinocerebellar Ataxias: Somatic Instability and Alternative Hypotheses

The use of genome wide association studies (GWAS) in Huntington's disease (HD) research, driven by unbiased human data analysis, has transformed the focus of new targets that could affect age at onset. While there is a significant depth of information on DNA damage repair, with many drugs and drug targets, most of this development has taken place in the context of cancer therapy. DNA damage repair in neurons does not rely on DNA replication correction mechanisms. However, there is a strong connection between DNA repair and neuronal metabolism, mediated by nucleotide salvaging and the poly ADP-ribose (PAR) response, and this connection has been implicated in other age-onset neurodegenerative diseases. Validation of leads including the mismatch repair protein MSH3, and interstrand cross-link repair protein FAN1, suggest the mechanism is driven by somatic CAG instability, which is supported by the protective effect of CAA substitutions in the CAG tract. We currently do not understand: how somatic instability is triggered; the state of DNA damage within expanding alleles in the brain; whether this damage induces mismatch repair and interstrand cross-link pathways; whether instability mediates toxicity, and how this relates to human ageing. We discuss DNA damage pathways uncovered by HD GWAS, known roles of other polyglutamine disease proteins in DNA damage repair, and a panel of hypotheses for pathogenic mechanisms.

Age-Related Oxidative Changes in Primary Porcine Fibroblasts Expressing Mutated Huntingtin

BACKGROUND

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by CAG triplet expansions in the huntingtin gene. Oxidative stress is linked to HD pathology, although it is not clear whether this is an effect or a mediator of disease. The transgenic (TgHD) minipig expresses the N-terminal part of human-mutated huntingtin and represents a unique model to investigate therapeutic strategies towards HD. A more detailed characterization of this model is needed to fully utilize its potential.

METHODS

In this study, we focused on the molecular and cellular features of fibroblasts isolated from TgHD minipigs and the wild-type (WT) siblings at different ages, pre-symptomatic at the age of 24-36 months and with the onset of behavioural symptoms at the age of 48 months. We measured oxidative stress, the expression of oxidative stress-related genes, proliferation capacity along with the expression of cyclin B1 and D1 proteins, cellular permeability, and the integrity of the nuclear DNA (nDNA) and mitochondrial DNA in these cells.

RESULTS

TgHD fibroblasts isolated from 48-month-old animals showed increased oxidative stress, which correlated with the overexpression of SOD2 encoding mitochondrial superoxide dismutase 2, and the NEIL3 gene encoding DNA glycosylase involved in replication-associated repair of oxidized DNA. TgHD cells displayed an abnormal proliferation capacity and permeability. We further demonstrated increased nDNA damage in pre-symptomatic TgHD fibroblasts (isolated from animals aged 24-36 months).

CONCLUSIONS

Our results unravel phenotypic alterations in primary fibroblasts isolated from the TgHD minipig model at the age of 48 months. Importantly, nDNA damage appears to precede these phenotypic alterations. Our results highlight the impact of fibroblasts from TgHD minipigs in studying the molecular mechanisms of HD pathophysiology that gradually occur with age.

The central role of DNA damage and repair in CAG repeat diseases

Diseases such as Huntington's disease and certain spinocerebellar ataxias are caused by the expansion of genomic cytosine-adenine-guanine (CAG) trinucleotide repeats beyond a specific threshold. These diseases are all characterised by neurological symptoms and central neurodegeneration, but our understanding of how expanded repeats drive neuronal loss is incomplete. Recent human genetic evidence implicates DNA repair pathways, especially mismatch repair, in modifying the onset and progression of CAG repeat diseases. Repair pathways might operate directly on repeat sequences by licensing or inhibiting repeat expansion in neurons. Alternatively, or in addition, because many of the genes containing pathogenic CAG repeats encode proteins that themselves have roles in the DNA damage response, it is possible that repeat expansions impair specific DNA repair pathways. DNA damage could then accrue in neurons, leading to further expansion at repeat loci, thus setting up a vicious cycle of pathology. In this review, we consider DNA damage and repair pathways in postmitotic neurons in the context of disease-causing CAG repeats. Investigating and understanding these pathways, which are clearly relevant in promoting and ameliorating disease in humans, is a research priority, as they are known to modify disease and therefore constitute prevalidated drug targets.

Methylation analysis of DNA repair genes in Alzheimer's disease

There is substantial evidence of impaired DNA repair activities in Alzheimer's disease (AD) neurons and peripheral tissues, inducing some investigators to speculate that this could partially result from promoter hypermethylation of DNA repair genes, resulting in gene silencing in those tissues. In the present study a screening cohort composed by late-onset AD (LOAD) patients and healthy matched controls was evaluated with a commercially available DNA methylation array for the assessment of the methylation levels of a panel of 22 genes involved in major DNA repair pathways in blood DNA. We then applied a cost-effective PCR based methylation-sensitive high-resolution melting (MS-HRM) technique, in order to evaluate the promoter methylation levels of the following DNA repair genes: OGG1, PARP1, MRE11A, BRCA1, MLH1, and MGMT. The analysis was performed in blood DNA from 56 LOAD patients and 55 matched controls, including the samples previously assessed with the DNA methylation array as validating samples. Both approaches revealed that all the investigated genes were largely hypomethylated in LOAD and control blood DNA, and no difference between groups was observed. Collectively, present data do not support an increased promoter methylation of some of the major DNA repair genes in blood DNA of AD patients.

SIRT1 in neurodevelopment and brain senescence

Sirtuins are nicotinamide adenine dinucleotide (NAD+)-dependent deacylases that have traditionally been linked with calorie restriction and aging in mammals. These proteins also play an important role in maintaining neuronal health during aging. During neuronal development, the SIR2 ortholog SIRT1 is structurally important, promoting axonal elongation, neurite outgrowth, and dendritic branching. This sirtuin also plays a role in memory formation by modulating synaptic plasticity. Hypothalamic functions that affect feeding behavior, endocrine function, and circadian rhythmicity are all regulated by SIRT1. Finally, SIRT1 plays protective roles in several neurodegenerative diseases including Alzheimer's, Parkinson's, and motor neuron diseases, which may relate to its functions in metabolism, stress resistance, and genomic stability. Drugs that activate SIRT1 may offer a promising approach to treat these disorders.

Effects of DSP4 on the noradrenergic phenotypes and its potential molecular mechanisms in SH-SY5Y cells

Dopamine β-hydroxylase (DBH) and norepinephrine (NE) transporter (NET) are the noradrenergic phenotypes for their functional importance to noradrenergic neurons. It is known that in vivo N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment induces degeneration of noradrenergic terminals by interacting with NET and depleting intracellular NE. However, DSP4's precise mechanism of action remains unclear. In this study various biochemical approaches were employed to test the hypothesis that DSP4 down-regulates the expression of DBH and NET, and to determine molecular mechanisms that may be involved. The results showed that treatment of SH-SY5Y neuroblastoma cells with DSP4 significantly decreased mRNA and protein levels of DBH and NET. DSP4-induced reduction of DBH mRNA and proteins, as well as NET proteins showed a time- and concentration-dependent manner. Flow cytometric analysis demonstrated that DSP4-treated cells were arrested predominantly in the S-phase, which was reversible. The arrest was confirmed by several DNA damage response markers (phosphorylation of H2AX and p53), suggesting that DSP4 causes replication stress which triggers cell cycle arrest via the S-phase checkpoints. Moreover, the comet assay verified that DSP4 induced single-strand DNA breaks. In summary, the present study demonstrated that DSP4 down-regulates the noradrenergic phenotypes, which may be mediated by its actions on DNA replication, leading to replication stress and cell cycle arrest. These action mechanisms of DSP4 may account for its degenerative consequence after systematic administration for animal models.

Unraveling 50-Year-Old Clues Linking Neurodegeneration and Cancer to Cycad Toxins: Are microRNAs Common Mediators?

Recognition of overlapping molecular signaling activated by a chemical trigger of cancer and neurodegeneration is new, but the path to this discovery has been long and potholed. Six conferences (1962–1972) examined the puzzling neurotoxic and carcinogenic properties of a then-novel toxin [cycasin: methylazoxymethanol (MAM)-β-d-glucoside] in cycad plants used traditionally for food and medicine on Guam where a complex neurodegenerative disease plagued the indigenous population. Affected families showed combinations of amyotrophic lateral sclerosis (ALS), parkinsonism (P), and/or a dementia (D) akin to Alzheimer’s disease (AD). Modernization saw declining disease rates on Guam and remarkable changes in clinical phenotype (ALS was replaced by P-D and then by D) and in two genetically distinct ALS-PDC-affected populations (Kii-Japan, West Papua-Indonesia) that used cycad seed medicinally. MAM forms DNA lesions – repaired by O⁶-methylguanine methyltransferase (MGMT) – that perturb mouse brain development and induce malignant tumors in peripheral organs. The brains of young adult MGMT-deficient mice given a single dose of MAM show DNA lesion-linked changes in cell-signaling pathways associated with miRNA-1, which is implicated in colon, liver, and prostate cancers, and in neurological disease, notably AD. MAM is metabolized to formaldehyde, a human carcinogen. Formaldehyde-responsive miRNAs predicted to modulate MAM-associated genes in the brains of MGMT-deficient mice include miR-17-5p and miR-18d, which regulate genes involved in tumor suppression, DNA repair, amyloid deposition, and neurotransmission. These findings marry cycad-associated ALS-PDC with colon, liver, and prostate cancer; they also add to evidence linking changes in microRNA status both to ALS, AD, and parkinsonism, and to cancer initiation and progression.

The cycad genotoxin MAM modulates brain cellular pathways involved in neurodegenerative disease and cancer in a DNA damage-linked manner

Methylazoxymethanol (MAM), the genotoxic metabolite of the cycad azoxyglucoside cycasin, induces genetic alterations in bacteria, yeast, plants, insects and mammalian cells, but adult nerve cells are thought to be unaffected. We show that the brains of adult C57BL6 wild-type mice treated with a single systemic dose of MAM acetate display DNA damage (O⁶-methyldeoxyguanosine lesions, O⁶-mG) that remains constant up to 7 days post-treatment. By contrast, MAM-treated mice lacking a functional gene encoding the DNA repair enzyme O⁶-mG DNA methyltransferase (MGMT) showed elevated O⁶-mG DNA damage starting at 48 hours post-treatment. The DNA damage was linked to changes in the expression of genes in cell-signaling pathways associated with cancer, human neurodegenerative disease, and neurodevelopmental disorders. These data are consistent with the established developmental neurotoxic and carcinogenic properties of MAM in rodents. They also support the hypothesis that early-life exposure to MAM-glucoside (cycasin) has an etiological association with a declining, prototypical neurodegenerative disease seen in Guam, Japan, and New Guinea populations that formerly used the neurotoxic cycad plant for food or medicine, or both. These findings suggest environmental genotoxins, specifically MAM, target common pathways involved in neurodegeneration and cancer, the outcome depending on whether the cell can divide (cancer) or not (neurodegeneration). Exposure to MAM-related environmental genotoxins may have relevance to the etiology of related tauopathies, notably, Alzheimer's disease.

Reduced DNA oxidation in aged prostaglandin H synthase-1 knockout mice

Prostaglandin H synthase (PHS)-2 (COX-2) is implicated in the neurodegeneration of Alzheimer and Parkinson diseases. Multiple mechanisms may be involved, including PHS-catalyzed bioactivation of neurotransmitters, precursors, and metabolites to neurotoxic free radical intermediates. Herein, in vitro studies with the purified PHS-1 (COX-1) isoform and in vivo studies of aging PHS-1 knockout mice were used to evaluate the potential neurodegenerative role of PHS-1-catalyzed bioactivation of endogenous neurotransmitters to free radical intermediates that enhance reactive oxygen species formation and oxidative DNA damage. The brains of 2-year-old wild-type (+/+) PHS-1 normal and heterozygous (+/-) and homozygous (-/-) PHS-1 knockout mice were analyzed for 8-oxo-2'-deoxyguanosine formation, characterized by high-performance liquid chromatography with electrochemical detection and by immunohistochemistry. Compared to aging PHS-1(+/+) normal mice, aging PHS-1(-/-) knockout mice had less oxidative DNA damage in the cortex, hippocampus, cerebellum, and brain stem. This PHS-1-dependent oxidative damage was not observed in young mice. In vitro incubation of purified PHS-1 and 2'-deoxyguanosine with dopamine, L-DOPA, and epinephrine, but not glutamate or norepinephrine, enhanced oxidative DNA damage. These results suggest that PHS-1-dependent accumulation of oxidatively damaged macromolecules including DNA may contribute to the mechanisms and risk factors of aging-related neurodegeneration.

Pim-1 kinase as activator of the cell cycle pathway in neuronal death induced by DNA damage

DNA damage is a critical component of neuronal death underlying neurodegenerative diseases and injury. Neuronal death evoked by DNA damage is characterized by inappropriate activation of multiple cell cycle components. However, the mechanism regulating this activation is not fully understood. We demonstrated previously that the cell division cycle (Cdc) 25A phosphatase mediates the activation of cyclin-dependent kinases and neuronal death evoked by the DNA damaging agent camptothecin. We also showed that Cdc25A activation is blocked by constitutive checkpoint kinase 1 activity under basal conditions in neurons. Presently, we report that an additional factor is central to regulation of Cdc25A phosphatase in neuronal death. In a gene array screen, we first identified Pim-1 as a potential factor up-regulated following DNA damage. We confirmed the up-regulation of Pim-1 transcript, protein and kinase activity following DNA damage. This induction of Pim-1 is regulated by the nuclear factor kappa beta (NF-kappaB) pathway as Pim-1 expression and activity are significantly blocked by siRNA-mediated knockdown of NF-kappaB or NF-kappaB pharmacological inhibitors. Importantly, Pim-1 activity is critical for neuronal death in this paradigm and its deficiency blocks camptothecin-mediated neuronal death. It does so by activating Cdc25A with consequent activation of cyclin D1-associated kinases. Taken together, our results demonstrate that Pim-1 kinase plays a central role in DNA damage-evoked neuronal death by regulating aberrant neuronal cell cycle activation.

Linking cell cycle reentry and DNA damage in neurodegeneration

Aberrant cell cycle activity and DNA damage have been observed in neurons in association with various neurodegenerative conditions. While there is strong evidence for a causative role for these events in neurotoxicity, it is unclear how they are triggered and why they are toxic. Here, we introduce a brief background of the current view on cell cycle activity and DNA damage in neurons and speculate on their relevance to neuronal survival. Furthermore, we suggest that the two events may be triggered in common by deregulation of fundamental processes, such as chromatin modulation, which are required for maintaining both DNA integrity and proper regulation of cell cycle gene expression.

Deregulation of HDAC1 by p25/Cdk5 in neurotoxicity

Aberrant cell-cycle activity and DNA damage are emerging as important pathological components in various neurodegenerative conditions. However, their underlying mechanisms are poorly understood. Here, we show that deregulation of histone deacetylase 1 (HDAC1) activity by p25/Cdk5 induces aberrant cell-cycle activity and double-strand DNA breaks leading to neurotoxicity. In a transgenic model for neurodegeneration, p25/Cdk5 activity elicited cell-cycle activity and double-strand DNA breaks that preceded neuronal death. Inhibition of HDAC1 activity by p25/Cdk5 was identified as an underlying mechanism for these events, and HDAC1 gain of function provided potent protection against DNA damage and neurotoxicity in cultured neurons and an in vivo model for ischemia. Our findings outline a pathological signaling pathway illustrating the importance of maintaining HDAC1 activity in the adult neuron. This pathway constitutes a molecular link between aberrant cell-cycle activity and DNA damage and is a potential target for therapeutics against diseases and conditions involving neuronal death.

Accumulation of nuclear DNA damage or neuron loss: molecular basis for a new approach to understanding selective neuronal vulnerability in neurodegenerative diseases

According to a long-standing hypothesis, aging is mainly caused by accumulation of nuclear (n) DNA damage in differentiated cells such as neurons due to insufficient nDNA repair during lifetime. In line with this hypothesis it was until recently widely accepted that neuron loss is a general consequence of normal aging, explaining some degree of decline in brain function during aging. However, with the advent of more accurate procedures for counting neurons, it is currently widely accepted that there is widespread preservation of neuron numbers in the aging brain, and the changes that do occur are relatively specific to certain brain regions and types of neurons. Whether accumulation of nDNA damage and decline in nDNA repair is a general phenomenon in the aging brain or also shows cell-type specificity is, however, not known. It has not been possible to address this issue with the biochemical and molecular-biological methods available to study nDNA damage and nDNA repair. Rather, it was the introduction of autoradiographic methods to study quantitatively the relative amounts of nDNA damage (measured as nDNA single-strand breaks) and nDNA repair (measured as unscheduled DNA synthesis) on tissue sections that made it possible to address this question in a cell-type-specific manner under physiological conditions. The results of these studies revealed a formerly unknown inverse relationship between age-related accumulation of nDNA damage and age-related impairment in nDNA repair on the one hand, and the age-related, selective, loss of neurons on the other hand. This inverse relation may not only reflect a fundamental process of aging in the central nervous system but also provide the molecular basis for a new approach to understand the selective neuronal vulnerability in neurodegenerative diseases, particularly Alzheimer's disease.

The aging brain: Accumulation of DNA damage or neuron loss?

Age-related molecular and cellular alterations in the central nervous system are known to show selectivity for certain cell types and brain regions. Among them age-related accumulation of nuclear (n) DNA damage can lead to irreversible loss of genetic information content. In the present study on the aging mouse brain, we observed a substantial increase in the amount of nDNA single-strand breaks in hippocampal pyramidal and granule cells as well as in cerebellar granule cells but not in cerebellar Purkinje cells. The reverse pattern was found for age-related reductions in total numbers of neurons. Only the total number of cerebellar Purkinje cells was significantly reduced during aging whereas the total numbers of hippocampal pyramidal and granule cells as well as of cerebellar granule cells were not. This formerly unknown inverse relation between age-related accumulation of nDNA damage and age-related loss of neurons may reflect a fundamental process of aging in the central nervous system.

The Chk1/Cdc25A pathway as activators of the cell cycle in neuronal death induced by camptothecin

Cell cycle regulators appear to play a paradoxical role in neuronal death. We have shown previously that cyclin-dependent kinases (CDKs), along with their downstream effectors, Rb (retinoblastoma) and E2F/DP1 (E2 promoter binding factor/deleted in polyposis 1), regulate neuronal death evoked by the DNA damaging agent camptothecin. However, the mechanism by which CDKs are activated in this model is unclear. The cell division cycle 25A (Cdc25A) phosphatase is a critical regulator of cell cycle CDKs in proliferating cells. In cortical neurons, we presently show that expression of Cdc25A promotes death even in the absence of DNA damage. Importantly, Cdc25A activity is rapidly increased during DNA damage treatment. Inhibition of Cdc25A blocks death and reduces cyclin D1-associated kinase activity and Rb phosphorylation. This indicates that endogenous Cdc25A activity is important for regulation of cell cycle-mediated neuronal death. We also examined how Cdc25A activity is regulated after DNA damage. Cultured embryonic cortical neurons have a significant basal activity of checkpoint kinase 1 (Chk1), a kinase that regulates cell cycle arrest. During camptothecin treatment of neurons, this activity is rapidly downregulated with a concomitant increase in Cdc25A activity. Importantly, expression of wild-type Chk1, but not kinase-dead Chk1, inhibits the camptothecin-induced increase in Cdc25A activity. In addition, Chk1 expression also promotes survival in the presence of the DNA-damaging agent. Together, our data suggest that a Chk1/Cdc25A activity participates in activation of a cell cycle pathway-mediated death signal in neurons. These data also define how a proliferative signal may be abnormally activated in a postmitotic environment.

Differential roles of nuclear and cytoplasmic cyclin-dependent kinase 5 in apoptotic and excitotoxic neuronal death

Cyclin-dependent kinase 5 (cdk5) is a member of the cyclin-dependent kinase family whose activity is localized mainly to postmitotic neurons attributable to the selective expression of its activating partners p35 and p39. Deregulation of cdk5, as a result of calpain cleavage of p35 to a smaller p25 form, has been suggested to be a central component of neuronal death underlying numerous neurodegenerative diseases. However, the relevance of cdk5 in apoptotic death that relies on the mitochondrial pathway is unknown. Furthermore, evidence that cdk5 can also promote neuronal survival has necessitated a more complex understanding of cdk5 in the control of neuronal fate. Here we explore each of these issues using apoptotic and excitotoxic death models. We find that apoptotic death induced by the DNA-damaging agent camptothecin is associated with early transcription-mediated loss of p35 and with late production of p25 that is dependent on Bax, Apaf1, and caspases. In contrast, during excitotoxic death induced by glutamate, neurons rapidly produce p25 independent of the mitochondrial pathway. Analysis of the localization of p35 and p25 revealed that p35 is mainly cytoplasmic, whereas p25 accumulates selectively in the nucleus. By targeting a dominant-negative cdk5 to either the cytoplasm or nucleus, we show that cdk5 has a death-promoting activity within the nucleus and that this activity is required in excitotoxic death but not apoptotic death. Moreover, we also find that cdk5 contributes to pro-survival signaling selectively within the cytoplasm, and manipulation of this signal can modify death induced by both excitotoxicity and DNA damage.

Nuclear factor-(kappa)B modulates the p53 response in neurons exposed to DNA damage

Previous studies have shown that DNA damage-evoked death of primary cortical neurons occurs in a p53 and cyclin-dependent kinase-dependent (CDK) manner. The manner by which these signals modulate death is unclear. Nuclear factor-kappaB (NF-kappaB) is a group of transcription factors that potentially interact with these pathways. Presently, we show that NF-kappaB is activated shortly after induction of DNA damage in a manner independent of the classic IkappaB kinase (IKK) activation pathway, CDKs, ATM, and p53. Acute inhibition of NF-kappaB via expression of a stable IkappaB mutant, downregulation of the p65 NF-kappaB subunit by RNA interference (RNAi), or pharmacological NF-kappaB inhibitors significantly protected against DNA damage-induced neuronal death. NF-kappaB inhibition also reduced p53 transcripts and p53 activity as measured by the p53-inducible messages, Puma and Noxa, implicating the p53 tumor suppressor in the mechanism of NF-kappaB-mediated neuronal death. Importantly, p53 expression still induces death in the presence of NF-kappaB inhibition, indicating that p53 acts downstream of NF-kappaB. Interestingly, neurons cultured from p65 or p50 NF-kappaB-deficient mice were not resistant to death and did not show diminished p53 activity, suggesting compensatory processes attributable to germline deficiencies, which allow p53 activation still to occur. In contrast to acute NF-kappaB inhibition, prolonged NF-kappaB inhibition caused neuronal death in the absence of DNA damage. These results uniquely define a signaling paradigm by which NF-kappaB serves both an acute p53-dependent pro-apoptotic function in the presence of DNA damage and an anti-apoptotic function in untreated normal neurons.

Presenilin-1 mutations sensitize neurons to DNA damage-induced death by a mechanism involving perturbed calcium homeostasis and activation of calpains and caspase-12

Mutations in presenilin-1 (PS1) can cause early onset familial Alzheimer's disease (AD). Studies of cultured cells and mice expressing mutant PS1 suggest that PS1 mutations may promote neuronal dysfunction and degeneration by altering cellular calcium homeostasis. On the other hand, it has been suggested that age-related damage to DNA in neurons may be an important early event in the pathogenesis of AD. We now report that PC12 cells and primary hippocampal neurons expressing mutant PS1 exhibit increased sensitivity to death induced by DNA damage. The hypersensitivity to DNA damage is correlated with increased intracellular Ca(2+) levels, induction of p53, upregulation of the Ca(2+)-dependent protease m-calpain, and mitochondrial membrane depolarization. Moreover, activation of caspase-12, an endoplasmic reticulum (ER)-associated caspase, is greatly increased in cells expressing mutant PS1. DNA damage-induced death of cells expressing mutant PS1 was attenuated by inhibitors of calpains I and II, by an intracellular Ca(2+) chelator, by the protein synthesis inhibitor cycloheximide, and by a broad-spectrum caspase inhibitor, but not by an inhibitor of caspase-1. Agents that release Ca(2+) from the ER increased the vulnerability of cells expressing mutant PS1 to DNA damage. By promoting ER-mediated apoptotic proteolytic cascades, PS1 mutations may sensitize neurons to DNA damage.

Hyperphosphorylation of RNA polymerase II and reduced neuronal RNA levels precede neurofibrillary tangles in Alzheimer disease

Affected neurons of Alzheimer disease (AD) brain are distinguished by the presence of the cell cycle cdc2 kinase and mitotic phosphoepitopes. A significant body of previous data has documented a decrease in neuronal RNA levels and nucleolar volume in AD brain. Here we present evidence that integrates these seemingly distinct findings and offers an explanation for the degenerative outcome of the disease. During mitosis cdc2 phosphorylates and inhibits the major transcriptional regulator RNA polymerase II (RNAP II). We therefore investigated cdc2 phosphorylation of RNAP II in AD brain. Using the H5 and H14 monoclonal antibodies specific for the cdc2-phosphorylated sites in RNAP II, we found that the polymerase is highly phosphorylated in AD. Moreover, RNAP II in AD translocates from its normally nuclear compartment to the cytoplasm of affected neurons, where it colocalizes with cdc2. These M phase-like changes in RNAP II correlate with decreased levels of poly-A RNA in affected neurons. Significantly, they precede tau phosphorylation and neurofibrillary tangle formation. Our data support the hypothesis that inappropriate activation of the cell cycle cdc2 kinase in differentiated neurons contributes to neuronal dysfunction and degeneration in part by inhibiting RNAP II and cellular processes dependent on transcription.

Comparison of base-excision repair capacity in proliferating and differentiated PC 12 cells following acute challenge with dieldrin

Dieldrin, an organochlorine pesticide and known neurotoxicant, is ubiquitously distributed in the environment. Dieldrin depletes brain monoamines in some animal species and is toxic for dopaminergic neurons in vitro. Dieldrin interferes with mitochondrial electron transport and increases generation of superoxide anion. Reactive oxygen species have been shown to produce oxidative lesions to DNA bases, i.e., 8-hydroxy-2'-deoxyguanosine (8-oxodGuo). Accumulation of 8-oxodGuo has been shown to be promutagenic in proliferating cells, and can lead to degeneration in fully differentiated cells. The objective of this study was to determine the effects of dieldrin exposure on the activity of the enzyme responsible for removing 8-oxodGuo, OGG1, from undifferentiated (untreated with NGF) and differentiated (NGF-treated) PC 12 cells. Proliferating PC 12 cells exhibited a mild upregulation of glycosylase activity, reaching a maximum by 1 h and returning to baseline by 6 h. Differentiated (+) NGF cells showed a time-dependent decline in activity reaching a nadir at 3 h with a return towards baseline by 6 h. Levels of the damaged base, 8-oxodGuo, in the differentiated PC12 cells appeared to be regulated by the activity of OGG1. In contrast, levels of the damaged base in actively proliferating cells were independent of the OGG1 activity. This difference between actively dividing and differentiated cells in the regulation of base-excision repair and DNA damage accumulation explains, in part, the vulnerability of postmitotic neurons to oxidative stresses and neurotoxins.

Caspase-mediated suppression of glutamate (AMPA) receptor channel activity in hippocampal neurons in response to DNA damage promotes apoptosis and prevents necrosis: implications for neurological side effects of cancer therapy and neurodegenerative disorders

DNA damage in neurons is implicated in the pathogenesis of several neurodegenerative disorders and may also contribute to the often severe neurological complications in cancer patients treated with chemotherapeutic agents. DNA damage can trigger apoptosis, a form of controlled cell death that involves activation of cysteine proteases called caspases. The excitatory neurotransmitter glutamate plays central roles in the activation of neurons and in processes such as learning and memory, but overactivation of ionotropic glutamate receptors can induce either apoptosis or necrosis. Glutamate receptors of the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) type mediate such physiological and pathological processes in most neurons. We now report that DNA damage can alter glutamate receptor channel activity by a mechanism involving activation of caspases. Whole-cell patch clamp analyses revealed a marked decrease in AMPA-induced currents after exposure of neurons to camptothecin, a topoisomerase inhibitor that induces DNA damage; N-methyl-d-aspartate (NMDA)-induced currents were unaffected by camptothecin. The decrease in AMPA-induced current was accompanied by a decreased calcium response to AMPA. Pharmacological inhibition of caspases abolished the effects of camptothecin on AMPA-induced current and calcium responses, and promoted excitotoxic necrosis. Combined treatment with glutamate receptor antagonists and a caspase inhibitor prevented camptothecin-induced neuronal death. Caspase-mediated suppression of AMPA currents may allow neurons with damaged DNA to withdraw their participation in excitatory circuits and undergo apoptosis, thereby avoiding widespread necrosis. These findings have important implications for treatment of patients with cancer and neurodegenerative disorders.

8OHdG levels in brain do not indicate oxidative DNA damage in Alzheimer's disease

Accumulation of oxidative DNA damage has been proposed to underlie aging and neurodegenerative diseases such as Alzheimer's Disease (AD). The DNA adduct 8-hydroxy-2'-deoxyguanosine (8OHdG) is considered a good indicator of oxidative DNA damage. To investigate whether this type of DNA damage is involved in AD etiology, 8OHdG levels were determined in postmortem human brain tissue of controls and AD patients (in frontal, occipital, and temporal cortex and in hippocampal tissue). Parametric studies in rat revealed no influences of postmortem delay, repeated freezing/thawing or storage time. In human brain, approximately two 8OHdG molecules were present per 10(5) 2'-deoxyguanosines. In AD patients and controls, 8OHdG-levels were not related to age, sex, or brain region. Also, no differences were found between controls and AD patients. It was concluded that 8OHdG in nuclear DNA, although present throughout the brain in fairly high amounts, does not accumulate with age, nor does it appear to be involved in AD. More detailed studies are required to extend this conclusion to other types of oxidative damage.

Tachykinins, neurotrophism and neurodegenerative diseases: a critical review on the possible role of tachykinins in the aetiology of CNS diseases

The tachykinins are a family of undecapeptides that are widely distributed throughout the body, including the central nervous system (CNS). They have several well defined roles in non-CNS sites as well as in the dorsal horn, where they are involved in the transmission of nociceptive information. However their function(s) in other CNS sites is unclear, but there is some evidence that they function as neuromodulators rather than neurotransmitters. This neuromodulation includes a possible role in maintaining the integrity of neuronal populations, analogous to the functions of neurotrophic factors. This review critically evaluates the role of tachykinins as neurotrophic factors, with particular reference to the common neurodegenerative diseases of the CNS.

Olfactory dysfunction in the Shy-Drager syndrome

The aetiology of the Shy-Drager syndrome (multiple system atrophy) is unknown. We reported previously a preliminary association between environmental-occupational risk factors and Shy-Drager syndrome. To further investigate this relationship, we evaluated olfactory function in eight patients in different stages of disease. When the eight patients' olfactory function was compared with 203 age- and sex-matched controls using a self-administered olfactory test, seven scored below the 39th percentile of this population. Five of the eight patients had total anosmia or microsmia. Additional studies will be required to elucidate the significance of this abnormal clinical observation.

DNA repair mechanisms in neurological diseases: facts and hypotheses

DNA repair mechanisms usually consist of a complex network of enzymatic reactions catalyzed by a large family of mutually interacting gene products. Thus deficiency, alteration or low levels of a single enzyme and/or of auxiliary proteins might impair a repair process. There are several indications suggesting that some enzymes involved both in DNA replication and repair are less abundant if not completely absent in stationary and non replicating cells. Postmitotic brain cell does not replicate its genome and has lower levels of several DNA repair enzymes. This could impair the DNA repair capacity and render the nervous system prone to the accumulation of DNA lesions. Some human diseases clearly characterized by a DNA repair deficiency, such as xeroderma pigmentosum, ataxia-telangiectasia and Cockayne syndrome, show neurodegeneration as one of the main clinical and pathological features. On the other hand there is evidence that some diseases characterized by primary neuronal degeneration (such as amyotrophic lateral sclerosis and Alzheimer disease) may have alterations in the DNA repair systems as well. DNA repair thus appears important to maintain the functional integrity of the nervous system and an accumulation of DNA damages in neurons as a result of impaired DNA repair mechanisms may lead to neuronal degenerations.

Substance P and neurodegenerative disorders. A speculative review

The causes of the neurodegenerative disorders of Parkinson's disease (PD), Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS) are unknown. It is proposed that all these disorders result primarily from a loss of trophic peptidergic neurotransmitter, possibly Substance P (SP). This loss in turn produces the classical neuronal degeneration seen in each of these diseases and occurs due to a combination of natural aging and chronic autoimmune destruction following a viral infection of the CNS, early in life. The loss is therefore slow and by the time of clinical presentation the inflammatory process is disappearing as the antigenic stimulus lessens with its removal. The implications of the theory in terms of future research and therapy are briefly discussed.

Dementia in cancer patients undergoing chemotherapy: implication of free radical injury and relevance to Alzheimer disease

A number of neurological disorders including Alzheimer and Parkinson disease have been suggested to be caused by processes leading to lipid peroxidation. Other theories implicate the accumulation of damaged DNA, resulting from a defect in DNA repair, in the pathogenesis of these disorders. I suggest that these theories might be related, since the hydroxy free radical is known to attack DNA and inactivate enzymes so that oxygen metabolism has the potential to interfere with the maintenance of genomic integrity. Since psychometric intelligence correlates highly with erythrocyte glutathione peroxidase activity, a free radical scavenger, perhaps this might explain the marked intellectual impairment caused by chemotherapeutic agents such as cytosine arabinoside, as well as in Alzheimer disease.

Environmental-occupational risk factors and familial associations in multiple system atrophy: a preliminary investigation

We studied 60 patients with multiple system atrophy and autonomic failure and 60 control subjects matched for age, sex and race. Their psychosocial history, pedigree and occupation were obtained by personal interview. An inventory of autonomic and neurologic symptoms was obtained from 148 first-degree relatives of the patients and 80 controls by a self-administered questionnaire. Patients with multiple system atrophy had significantly more potential exposures to metal dusts and fumes, plastic monomers and additives, organic solvents, and pesticides than the control population. The potential exposures were determined in most subjects by their reported usual occupation. Clinical symptoms of multiple system atrophy were reported by a significantly larger group of patients' relatives than controls. These findings are possibly consistent with the hypothesis that multiple system atrophy develops as a result of a genetically determined selective vulnerability in the nervous system. Specific neuronal systems may become targets for environmental insults or toxins, and the disease state may occur when ageing neuronal systems can no longer sustain functional capacity. This preliminary study supports the need to further explore possible environmental, occupational, and familial contributions to the aetiology of multiple system atrophy.

Decreased DNA repair in familial Alzheimer's disease

Alterations in the capacity of a cell to repair DNA lesions play an important role in a number of human diseases. We and others have demonstrated defective DNA repair of alkylation damage in cells from patients with Alzheimer's disease. It has been hypothesized that this defect is related to the cause of Alzheimer's disease and results in the accumulation of lesions in the central nervous system neurons. One prediction of this hypothesis is that in dominantly inherited Alzheimer's disease, the repair defect will be present in half of the offspring of affected patients long before they develop symptoms of the disease. In order to test the hypothesis that decreased DNA repair is responsible for familial Alzheimer's disease and their at-risk offspring we have studied DNA repair in these individuals after exposure of lymphoblasts to alkylating agents. Our results indicate that cell lines from affected patients repair significantly less damage in 3 h than cell lines from healthy controls. A small number of at-risk individuals were also studied and some of these had lower levels of repair, although more cell lines from individuals in this group must be studied. These findings provide further support for defective DNA repair playing a role in the pathogenesis of Alzheimer's disease.

Activities of lysosomal enzymes in rabbit brain with experimental neurofibrillary changes

Rabbits were injected intracerebrally with aluminum salt leading to experimental neurofibrillary change formation as a model of Alzheimer neurofibrillary change. Eleven days after the injection, the brain tissues were excised from the cortex, hippocampus, and cervical region of spinal cord. Five lysosomal enzymes (cathepsin D, beta-glucuronidase, acid phosphatase, acid DNase, alkaline DNase) were assayed and compared with the control. Cathepsin D, acid DNase and beta-glucuronidase activities increased significantly in all 3 areas of aluminum-injected brain. On the other hand, acid phosphatase and alkaline DNase activities remained at the same level. The results showed the lysosomal enzymes did not change in parallel after aluminum administration, suggesting a role of the increased enzymes in the brain with neurofibrillary changes.

Astrocyte RNA in relation to neuronal RNA depletion in Alzheimer's disease

A new double-staining procedure, in which the techniques of immunocytochemistry of glial fibrillary acidic protein (GFAP) and quantitative microdensitometry of azure B-RNA were combined, was used to study nucleic acid alterations in fibrous astrocytes in Alzheimer's disease (AD). RNA contents of GFAP-positive cells of the hippocampal endplate (Rose's H3-H5 fields) and the dentate gyrus molecular layer were determined in ten autopsy-proven AD patients (ages 51-88) and ten age-matched, non-demented controls. In addition, RNA contents of pyramidal neurons of the endplate were examined. While there were no differences in RNA contents of astrocytes of either region between AD patients and controls, neuronal RNA was markedly depleted. These data suggest that astrocytes maintain protein synthetic capabilities in AD and that RNA loss is limited to the neuronal compartment.

Response to DNA-damaging agents in cultured cells from patients with X-linked duchenne muscular dystrophy phenotype: male DMD, female DMD, possible carriers

Using blood cultures the response to gamma (γ) radiation was examined in a male DMD and his mother, in a female DMD and her mother and in a normal control. In a series of experiments chromosome aberrations were determined after 3 separate γ-irradiation dose levels: 0, 150, 300 rads. The DMD patients showed a response to ionising radiations different from control, in fact the percentage of aberrations was lower than the control. In this preliminary study a slight difference between normal and possible carriers was also found.

Deficient DNA repair in amyotrophic lateral sclerosis cells

We studied survival and DNA repair capacity in cultured sporadic ALS and control skin fibroblasts after treatment with DNA damaging agents producing different types of lesions. Mean survival in ALS and control fibroblasts was similar after exposure to ultraviolet (UV) light, x-rays and mitomycin C (MMC). Both mean survival and mean unscheduled (repair) DNA synthesis (UDS) were significantly reduced in ALS fibroblasts following treatment with the alkylating agent methyl methane sulfonate (MMS). These data suggest that ALS cells are relatively deficient in the repair of alkylation damage, possibly of apurinic/apyrimidinic sites, and that they are not unduly sensitive to DNA damage produced by UV light, x-rays and MMC. Normal survival and UDS seen in some patients' cells after MMS treatment indicate a spectrum of repair efficiency, and suggest heterogeneity of the biochemical defect in ALS.

Alzheimer's disease cells exhibit defective repair of alkylating agent-induced DNA damage

The most common cause of senile and presenile dementia is Alzheimer's disease, a disorder with an undetermined cause. A number of studies have indicated that neurons from patients with Alzheimer's disease have decreased ribonucleic acid levels and reduced protein synthesis. Recent studies using lymphoblasts from patients with Alzheimer's disease have indicated that these cells are more sensitive to deoxyribonucleic acid (DNA)-alkylating agents. We have used cell survival, unscheduled DNA synthesis, and alkaline elution to assess the capacity for DNA repair in skin fibroblasts from normal control subjects, control subjects with central nervous system disease, and patients with Alzheimer's disease. Our results indicate that the Alzheimer's disease cells, unlike normal cells, fail to repair methylmethane sulfonate-induced DNA damage. Both normal and Alzheimer's disease cells are able to ameliorate the effects of ultraviolet light. These results indicate that a specific pathway for DNA repair is affected in Alzheimer's disease. The repair defect may be related to the cause of the disease or may be the cause of the disease.

Tissue culture studies of muscle disorders: Part 1. Techniques, cell growth, morphology, cell surface

Tissue culture has been used extensively in studies of human inherited disorders, and its application in the field of the neuromuscular disorders has increased rapidly in recent years. This review, covering the period 1977 to 1984, deals with tissue culture studies of both human and animal muscle disorders, although Duchenne muscular dystrophy (DMD) figures prominently because of the overwhelming interest in that disorder. The review is in two parts. In the first part, I discuss technical innovations in the field, the morphology and growth of cells, and a variety of studies related to the cell surface. Important findings in relation to DMD include reports of abnormal growth rates and reduced lifespan of DMD cells, hypersensitivity to DNA-damaging agents, abnormal cell-to-cell and cell-to-substratum adhesion, and a more "fluid" cell membrane. However, these findings are controversial or have so far been reported only by single laboratories.