Effects of melanin and manganese on DNA damage and repair in PC12-derived neurons

Activated or Impaired: An Overview of DNA Repair in Neurodegenerative Diseases

As the population ages, age-related neurodegenerative diseases have become a major challenge in health science. Currently, the pathology of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease, is still not fully understood. Remarkably, emerging evidence indicates a role of genomic DNA damage and repair in various neurodegenerative disorders. Here, we summarized the current understanding of the function of DNA damage repair, especially base excision repair and double strand break repair pathways, in a variety of neurodegenerative diseases. We concluded that exacerbation of DNA lesions is found in almost all types of neurodegenerative diseases, whereas the activities of different DNA repair pathways demonstrate distinct trends, depending on disease type and even brain region. Specifically, key enzymes involved in base excision repair are likely impaired in Alzheimer's disease and amyotrophic lateral sclerosis but activated in Parkinson's disease, while nonhomologous end joining is likely downregulated in most types of neurodegenerative diseases. Hence, impairment of nonhomologous end joining is likely a common etiology for most neurodegenerative diseases, while defects in base excision repair are likely involved in the pathology of Alzheimer's disease and amyotrophic lateral sclerosis but are Parkinson's disease, based on current findings. Although there are still discrepancies and further studies are required to completely elucidate the exact roles of DNA repair in neurodegeneration, the current studies summarized here provide crucial insights into the pathology of neurodegenerative diseases and may reveal novel drug targets for corresponding neurodegenerative diseases.

Effects of Manganese on Genomic Integrity in the Multicellular Model Organism Caenorhabditis elegans

Although manganese (Mn) is an essential trace element, overexposure is associated with Mn-induced toxicity and neurological dysfunction. Even though Mn-induced oxidative stress is discussed extensively, neither the underlying mechanisms of the potential consequences of Mn-induced oxidative stress on DNA damage and DNA repair, nor the possibly resulting toxicity are characterized yet. In this study, we use the model organism Caenorhabditis elegans to investigate the mode of action of Mn toxicity, focusing on genomic integrity by means of DNA damage and DNA damage response. Experiments were conducted to analyze Mn bioavailability, lethality, and induction of DNA damage. Different deletion mutant strains were then used to investigate the role of base excision repair (BER) and dePARylation (DNA damage response) proteins in Mn-induced toxicity. The results indicate a dose- and time-dependent uptake of Mn, resulting in increased lethality. Excessive exposure to Mn decreases genomic integrity and activates BER. Altogether, this study characterizes the consequences of Mn exposure on genomic integrity and therefore broadens the molecular understanding of pathways underlying Mn-induced toxicity. Additionally, studying the basal poly(ADP-ribosylation) (PARylation) of worms lacking poly(ADP-ribose) glycohydrolase (PARG) parg-1 or parg-2 (two orthologue of PARG), indicates that parg-1 accounts for most of the glycohydrolase activity in worms.

A matter of concern - Trace element dyshomeostasis and genomic stability in neurons

Neurons are post-mitotic cells in the brain and their integrity is of central importance to avoid neurodegeneration. Yet, the inability of self-replenishment of post-mitotic cells results in the need to withstand challenges from numerous stressors during life. Neurons are exposed to oxidative stress due to high oxygen consumption during metabolic activity in the brain. Accordingly, DNA damage can occur and accumulate, resulting in genome instability. In this context, imbalances in brain trace element homeostasis are a matter of concern, especially regarding iron, copper, manganese, zinc, and selenium. Although trace elements are essential for brain physiology, excess and deficient conditions are considered to impair neuronal maintenance. Besides increasing oxidative stress, DNA damage response and repair of oxidative DNA damage are affected by trace elements. Hence, a balanced trace element homeostasis is of particular importance to safeguard neuronal genome integrity and prevent neuronal loss. This review summarises the current state of knowledge on the impact of deficient, as well as excessive iron, copper, manganese, zinc, and selenium levels on neuronal genome stability.

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Post-mitotic neurons show an increased vulnerability to oxidative stress.

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Trace element dyshomeostasis impairs neuronal genome maintenance, affecting DNA damage response as well as DNA repair.

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The review summarises the effects of excessive and deficient trace element levels neuronal genome stability maintenance.

Critical Role of Oxidatively Damaged DNA in Selective Noradrenergic Vulnerability

An important pathology in Parkinson's disease (PD) is the earlier and more severe degeneration of noradrenergic neurons in the locus coeruleus (LC) than dopaminergic neurons in the substantia nigra. However, the basis of such selective vulnerability to insults remains obscure. Using noradrenergic and dopaminergic cell lines, as well as primary neuronal cultures from rat LC and ventral mesencephalon (VM), the present study compared oxidative DNA damage response markers after exposure of these cells to hydrogen peroxide (H₂O₂). The results showed that H₂O₂ treatment resulted in more severe cell death in noradrenergic cell lines SK-N-BE(2)-M17 and PC12 than dopaminergic MN9D cells. Furthermore, there were higher levels of oxidative DNA damage response markers in noradrenergic cells and primary neuronal cultures from the LC than dopaminergic cells and primary cultures from the VM. It included increased tail moments and tail lengths in Comet assay, and increased protein levels of phosphor-p53 and γ-H2AX after treatments with H₂O₂. Consistent with these measurements, exposure of SK-N-BE(2)-M17 cells to H₂O₂ resulted in higher levels of reactive oxygen species (ROS). Further experiments showed that exposure of SK-N-BE(2)-M17 cells to H₂O₂ caused an increased level of noradrenergic transporter, reduced protein levels of copper transporter (Ctr1) and 8-oxoGua DNA glycosylase, as well as amplified levels of Ca_v1.2 and Ca_v1.3 expression. Taken together, these experiments indicated that noradrenergic neuronal cells seem to be more vulnerable to oxidative damage than dopaminergic neurons, which may be related to the intrinsic characteristics of noradrenergic neuronal cells.

Regulated Necrosis Orchestrates Microglial Cell Death in Manganese-Induced Toxicity

Microglia, the brain resident immune cells, play prominent roles in immune surveillance, tissue repair and neural regeneration. Despite these pro-survival actions, the relevance of these cells in the progression of several neuropathologies has been established. In the context of manganese (Mn) overexposure, it has been proposed that microglial activation contributes to enhance the neurotoxicity. However, the occurrence of a direct cytotoxic effect of Mn on microglial cells remains controversial. In the present work, we investigated the potential vulnerability of immortalized mouse microglial cells (BV-2) toward Mn²⁺, focusing on the signaling pathways involved in cell death. Evidence obtained showed that Mn²⁺ induces a decrease in cell viability which is associated with reactive oxygen species (ROS) generation. In this report we demonstrated, for the first time, that Mn²⁺ triggers regulated necrosis (RN) in BV-2 cells involving two central mechanisms: parthanatos and lysosomal disruption. The occurrence of parthanatos is supported by several cellular and molecular events: (i) DNA damage; (ii) AIF translocation from mitochondria to the nucleus; (iii) mitochondrial membrane permeabilization; and (iv) PARP1-dependent cell death. On the other hand, Mn²⁺ induces lysosomal membrane permeabilization (LMP) and cathepsin D (CatD) release into the cytosol supporting the lysosomal disruption. Pre-incubation with CatB and D inhibitors partially prevented the Mn²⁺-induced cell viability decrease. Altogether these events point to lysosomes as players in the execution of RN. In summary, our results suggest that microglial cells could be direct targets of Mn²⁺ damage. In this scenario, Mn²⁺ triggers cell death involving RN pathways.

Transplantable Melanomas in Hamsters and Gerbils as Models for Human Melanoma. Sensitization in Melanoma Radiotherapy-From Animal Models to Clinical Trials

The focus of the present review is to investigate the role of melanin in the radioprotection of melanoma and attempts to sensitize tumors to radiation by inhibiting melanogenesis. Early studies showed radical scavenging, oxygen consumption and adsorption as mechanisms of melanin radioprotection. Experimental models of melanoma in hamsters and in gerbils are described as well as their use in biochemical and radiobiological studies, including a spontaneously metastasizing ocular model. Some results from in vitro studies on the inhibition of melanogenesis are presented as well as radio-chelation therapy in experimental and clinical settings. In contrast to cutaneous melanoma, uveal melanoma is very successfully treated with radiation, both using photon and proton beams. We point out that the presence or lack of melanin pigmentation should be considered, when choosing therapeutic options, and that both the experimental and clinical data suggest that melanin could be a target for radiosensitizing melanoma cells to increase efficacy of radiotherapy against melanoma.

Chitosan-Mangafodipir nanoparticles designed for intranasal delivery of siRNA and DNA to brain

The overall objective of the present research was to develop a nanocarrier system for non-invasive delivery to brain of molecules useful for gene therapy. Manganese-containing nanoparticles (mNPs) carrying anti-eGFP siRNA were tested in cell cultures of eGFP-expressing cell line of mouse fibroblasts (NIH3T3). The optimal mNPs were then tested in vivo in mice. Following intranasal instillation, mNPs were visualized by 7T MRI throughout brain at 24 and 48 hrs. mNPs were effective in significantly reducing GFP mRNA expression in Tg GFP+ mice in olfactory bulb, striatum, hippocampus and cortex. Intranasal instillation of mNPS loaded with dsDNA encoding RFP also resulted in expression of the RFP in multiple brain regions. In conclusion, mNPs carrying siRNA, or dsDNA were capable of delivering the payload from nose to brain. This approach for delivery of gene therapies to humans, if successful, will have a significant impact on disease-modifying therapeutics of neurodegenerative diseases.

The response to oxidative stress and metallomics analysis in a twin study: The role of the environment

Inefficient response to oxidative stress has been associated with ageing and health risk. Metals are known to inhibit DNA repair and may modify the antioxidant response. How genetic variability and lifestyle factors modulate the response to oxidative stress is poorly explored. Our study aims to disentangle the contribution of genetics and environmental exposures to oxidative stress response using data from twin pairs. The non-enzymatic antioxidant capacity (NEAC), the repair capacity of 8-oxo-7,8-dihydroguanine (OGG activity) and the levels of 12 metals were measured in blood of 64 monozygotic and 31 dizygotic twin pairs. The contributions of genetic and environmental effects were assessed using standard univariate twin modelling. NEAC and OGG activity significantly decreased with age. Gender-, age- and body mass index-associated differences were identified for some metals. Principal Component Analysis identified two groups of metals whose levels in blood were highly correlated: As, Hg, Pb, Se, Zn and Al, Co, Cr, Mn, Ni. The environmental influence was predominant on OGG activity and NEAC variance whereas for most metals the best-fitting model incorporated additive genetic and unique environmental sources of variance. NEAC and OGG activity were both inversely correlated with blood levels of various metals. The inhibition of OGG activity by Cd was largely explained by smoking. Our data show a substantial role of environmental factors in NEAC and OGG activity variance that is not explained by twins' age. Exogenous environmental factors such as metals contribute to oxidative stress by decreasing NEAC and inhibiting repair of oxidatively-induced DNA damage.

Metal-mediated DNA damage and cell death: mechanisms, detection methods, and cellular consequences

Metal ions cause various types of DNA damage by multiple mechanisms, and this damage is a primary cause of cell death and disease.

Repair system of 7, 8-dihydro-8-oxoguanine as a defense line against carcinogenesis

Reactive oxygen species (ROS) are essentially harmful for living organisms, including human beings. It is well known that ROS-induced damage of cellular components may lead to human diseases, such as inflammatory diseases, degenerative diseases, or cancer. In particular, oxidative DNA damage is premutagenic, and thus, the generation of DNA damage and the failure of its removal are critical events for tumorigenesis or carcinogenesis. To prevent this disadvantage, living organisms have defense mechanisms against ROS-induced gene instability. Studies of 8-oxo-Gua and its main repair enzyme, 8-oxoguanine DNA glycosylase 1 (OGG1), are informative and useful, because 8-oxo-Gua is commonly observed in DNA, and OGG1 enzymes exist in a wide variety of living organisms. The importance of OGG1 was confirmed by polymorphism analyses and studies using knockout mice. Moreover, analyses of the influences of environmental factors on DNA damage and repair systems have confirmed the effects of heavy metals on 8-oxo-Gua formation and OGG1 expression. These studies revealed that the 8-oxo-Gua repair system is crucial for the prevention of mutation-related diseases, such as cancer. In this review, the advances in this field during the last two decades are described.

DNA damage in tissues and organs of mice treated with diphenyl diselenide

Diphenyl diselenide (DPDS) is an organoselenium compound with interesting pharmacological activities and various toxic effects. In previous reports, we demonstrated the pro-oxidant action and the mutagenic properties of this molecule in bacteria, yeast and cultured mammalian cells. This study investigated the genotoxic effects of DPDS in multiple organs (brain, kidney, liver, spleen, testes and urinary bladder) and tissues (bone marrow, lymphocytes) of mice using in vivo comet assay, in order to determine the threshold of dose at which it has beneficial or toxic effects. We assessed the mechanism underlying the genotoxicity through the measurement of GSH content and thiobarbituric acid reactive species, two oxidative stress biomarkers. Male CF-1 mice were given 0.2-200 micromol/kg BW DPDS intraperitonially. DPDS induced DNA damage in brain, liver, kidney and testes in a dose response manner, in a broad dose range at 75-200 micromol/kg with the brain showing the highest level of damage. Overall, our analysis demonstrated a high correlation among decreased levels of GSH content and an increase in lipid peroxidation and DNA damage. This finding establishes an interrelationship between pro-oxidant and genotoxic effects. In addition, DPDS was not genotoxic and did not increase lipid peroxidation levels in any organs at doses < 50 micromol/kg. Finally, pre-treatment with N-acetyl-cysteine completely prevented DPDS-induced oxidative damage by the maintenance of cellular GSH levels, reinforcing the positive relationship of DPDS-induced GSH depletion and DNA damage. In summary, DPDS induces systemic genotoxicity in mammals as it causes DNA damage in vital organs like brain, liver, kidney and testes.

Analysis of plasmid DNA damage induced by melanin with capillary electrophoresis

Dilute linear poly(N-isopropylacrylamide) (PNIPAM) in Tris-Mes-EDTA (TME) buffer was used as sieving matrix for capillary electrophoresis (CE) of plasmid DNA and plasmid topological isomers induced by melanin in uncoated capillary. At the optimized condition of 0.1% (w/v) PNIPAM in TME buffer, base line separation of the plasmid DNA ladder (2-12 kbp) was achieved within 15 min. Three positive clones with inserts of 468, 1147 and 1566 bp can be distinguished from the plasmid pUC 18 vector within 13 min. The migration order of the plasmid topological isomers in the dynamic coating matrix was confirmed by the enzymatically prepared and UV-induced plasmids. The covalently closed circular form appeared firstly, followed by the linear plasmid form and then the open circular form. The effect of bacterial melanin obtained from Pseudomonas maltophilia AT18 on plasmid pUC 18 was investigated by CE in uncoated capillary in vitro. Plasmid pUC 18 incubated with either melanin or copper ions alone sustained little DNA damage. The combination of melanin with Cu(II) can cause the plasmid pUC 18 conformational changes from covalently closed circular form to open form. Understanding the damage effect of melanin with copper ions on DNA would be important for the melanin-related application, such as photoprotective antioxidant in protecting the skin from cancer, pathophysiology research in clinic. The investigation of melanin induced plasmid conformational changes by CE in uncoated capillary also revealed that the application of the dynamic coating matrix could be extended to the study of plasmid conformational changes in other plasmid-based biological technologies.

Magnetic resonance imaging of the visual system in vivo: Transsynaptic illumination of V1 and V2 visual cortex

Brain nuclei directly receiving retinal projections are readily labeled in magnetic resonance images following intraocular injection of manganese (Mn). To assess whether Mn in retinal ganglion cell axons can be transsynaptically delivered to visual cortex, mice that had previously received intraocular Mn injection were anesthetized with isoflurane, and T1-weighted data sets were acquired of the eyes and brain using a 7-T magnetic resonance imaging machine. Image intensity within contralateral brain structures was evaluated by assessing 1) signal-to-noise ratios, 2) mean image intensity, and 3) mean image intensity normalized to facial muscle intensity. Image intensity was increased throughout the visual pathway including within contralateral visual cortex areas V1 and V2L. Mean normalized image intensity was greater by 53% in the ipsilateral optic nerve and by 31% and 28% in the contralateral lateral geniculate nucleus and superior colliculus, respectively (N=5, P<0.02, paired t test). In contralateral visual cortex areas V1 and V2L, image intensity was increased by 7.5% and 6.8%, respectively (P<0.02 for both, paired t test). Power analysis of the different evaluation methods yielded evidence of superior sensitivity using the normalization method. Reconstruction of the visual system based upon threshold analysis allowed simultaneous visualization of all portions of the major retinal projections to the brain. These results support use of high magnetic field MRI imaging and data normalization for in vivo quantitative analysis of the mouse brain visual system including visual cortex.

Mechanism for manganese enhancement of dopamine-induced oxidative DNA damage and neuronal cell death

Although the cause of dopaminergic cell death in Parkinson's disease is still poorly understood, there is accumulating evidence suggesting that metal ions can be involved in the processes. We investigated the effect of manganese on cell death and DNA damage in PC12 cells treated with dopamine. Mn(II) enhanced cell death induced by dopamine. Mn(II) also increased the 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) contents of DNA in PC12 cells treated with dopamine. To clarify the mechanism of cellular DNA damage, we investigated DNA damage induced by dopamine and Mn(II) using (32)P-labeled DNA fragments. Mn(II) enhanced Cu(II)-dependent DNA damage by dopamine. The Mn(II)-enhanced DNA damage was greatly increased by NADH. Piperidine and formamidopyrimidine-DNA glycosylase treatment induced cleavage sites mainly at T and G of the 5'-TG-3' sequence, respectively. Bathocuproine, a Cu(I) chelator, and catalase inhibited the DNA damage. Oxygen consumption and UV-visible spectroscopic measurements showed that Mn(II) enhanced autoxidation of dopamine with H(2)O(2) formation. These results suggest that reactive species derived from the reaction of H(2)O(2) with Cu(I) participates in Mn(II)-enhanced DNA damage by dopamine plus Cu(II). Therefore, it is concluded that oxidative DNA damage induced by dopamine in the presence of Mn(II), NADH, and Cu(II) is possibly linked to the degeneration of dopaminergic neurons.

Oxidized guanine lesions and hOgg1 activity in lung cancer

In humans, the oxidatively induced DNA lesion 8-hydroxyguanine (8-oxoG) is removed from DNA by hOgg1, a DNA glycosylase/AP lyase that specifically incises 8-oxoG opposite cytosine. We analysed the expression of hOGG1 mRNA in 18 lung cancer and three normal cell lines. Although hOGG1 was overexpressed in most cell lines, 2/18 (11.1%) showed a lower hOGG1 mRNA and protein expression (approximately 80% decrease) relative to normal cell lines. Liquid chromatography/mass spectrometry analysis showed increased levels of 8-oxoG in the two cell lines with the lowest hOGG1 mRNA expression. We examined the ability of nuclear and mitochondrial extracts to incise 8-oxoG lesion in cell lines H1650 and H226 expressing lower hOGG1 mRNA and H1915 and H1975 with higher than normal hOGG1 mRNA expression. Both nuclear and mitochondrial extracts from H1915 and H1975 cells were proficient in 8-oxoG removal. However, both cell lines with the lowest hOGG1 mRNA expression exhibited a severe reduction in 8-oxoG incision in both nuclear and mitochondrial extracts. Under-expression of hOGG1 mRNA and hOgg1 protein was associated with a decrease in mitochondrial DNA repair in response to oxidative damaging agents. These results provide evidence for defective incision of 8-oxoG in both nuclear and mitochondria of H1650 and H226 lung cancer cell lines. These results may implicate 8-oxoG repair defects in certain lung cancers.