Guanosine and synthetic organoselenium compounds modulate methylmercury-induced oxidative stress in rat brain cortical slices: involvement of oxidative stress and glutamatergic system

Intestinal microbiota protects against methylmercury-induced neurotoxicity

Methylmercury (MeHg) remains a global public health issue because of its frequent presence in human food sources obtained from the water. The excretion of MeHg in humans occurs slowly with a biological half-time of 32-47 days. Short-term MeHg exposure may cause long-lasting neurotoxicity. The excretion through feces is a major route in the demethylation of MeHg. Accumulating evidence suggests that the intestinal microbiota plays an important role in the demethylation of MeHg, thereby protecting the host from neurotoxic effects. Here, we discuss recent developments on the role of intestinal microbiota in MeHg metabolism, based on in vitro cell culture experiments, experimental animal studies and human investigations. Demethylation by intestinal bacteria is the rate-limiting step in MeHg metabolism and elimination. The identity of bacteria strains responsible for this biotransformation is currently unknown; however, the non-homogenous distribution of intestinal microbiota may lead to different demethylation rates in the intestinal tract. The maintenance of intestinal barrier function by intestinal microbiota may afford protection against MeHg-induced neurotoxicity, which warrant future investigations. We also discuss studies investigating the effects of MeHg exposure on the population structural stability of intestinal microbiota in several host species. Although this is an emerging area in metal toxicity, current research suggests that a change in certain phyla in the intestinal microbiota may indicate MeHg overexposure.

Mechanistic insights into Qiteng Xiaozhuo Granules' regulation of autophagy for chronic glomerulonephritis treatment: Serum pharmacochemistry, network pharmacology, and experimental validation

ETHNOPHARMACOLOGICAL RELEVANCE

Qiteng Xiaozhuo Granules (QTXZG), a traditional Chinese medicine prescription, is widely acknowledged for its therapeutic efficacy and lack of discernible toxicity in clinical practice, substantiating its potential in the treatment of chronic glomerulonephritis (CGN). Nevertheless, the specific effectiveness and underlying mechanisms of QTXZG remain insufficiently explored.

AIM OF THE STUDY

The purpose of this study was to explore the mechanism of the QTXZG in the treatment of CGN via targeting autophagy based on serum pharmacochemistry, network pharmacology, and experimental validation.

METHODS

Serum samples from SD rats orally administered QTXZG were analyzed using UPLC-QE/MS to identify contained compounds. Network and functional enrichment analyses elucidated QTXZG's targets and biological mechanisms. Reliability was ensured through molecular docking, in vivo and in vitro experiments.

RESULTS

After oral administration of QTXZG, 39 enriched compounds in serum samples collected 1 h later were identified as potential active agents, with 508 potential targets recognized as QTXZG-specific targets. Through integration of various databases, intersection analysis of QTXZG targets, CGN-related genes, and autophagy-related targets identified 10 core autophagy-related targets for QTXZG in CGN. GO and KEGG analyses emphasized their roles in autophagy, inflammation, and immune processes, particularly emphasizing the enrichment of the AMPK/mTOR signaling pathway. Molecular docking results demonstrated strong binding affinities between QTXZG's key compounds and the predicted core targets. In animal experiments, QTXZG was found to ameliorate renal tissue damage in CGN model mice, significantly reducing serum creatinine (Scr) and blood urea nitrogen (BUN) levels. Importantly, both animal and cell experiments revealed QTXZG's ability to decrease excessive ROS and inflammatory factor release in mesangial cells. Furthermore, in vitro and in vivo experiments confirmed QTXZG's capacity to upregulate Beclin1 and LC3II/I expression, decrease p62 expression, and induce CGN autophagy through modulation of the AMPK/mTOR pathway.

CONCLUSIONS

This study indicated that QTXZG can induce autophagy in CGN by affecting the AMPK/mTOR pathway, and induction of autophagy may be one of the possible mechanisms of QTXZG's anti-CGN.

Should ebselen be considered for the treatment of mercury intoxication? A minireview

Mercury is a ubiquitous environmental contaminant and can be found in inorganic (Hg0, Hg+ and Hg2+) and organic forms (chiefly CH3Hg+ or MeHg+). The main route of human, mammals and bird exposure occurs via predatory fish ingestion. Occupational exposure to Hg0 (and Hg2+) can also occur; furthermore, in gold mining areas the exposure to inorganic Hg can also be high. The toxicity of electrophilic forms of Hg (E+Hg) is mediated by disruption of thiol (-SH)- or selenol (-SeH)-containing proteins. The therapeutic approaches to treat methylmercury (MeHg+), Hg0 and Hg2+ are limited. Here we discuss the potential use of ebselen as a potential therapeutic agent to lower the body burden of Hg in man. Ebselen is a safe drug for humans and has been tested in clinical trials (for instance, brain ischemia, noise-induce hearing loss, diabetes complications, bipolar disorders) at doses varying from 400 to 3600 mg per day. Two clinical trials with ebselen in moderate and severe COVID are also approved. Ebselen can be metabolized to an intermediate with -SeH (selenol) functional group, which has a greater affinity to electrophilic Hg (E+Hg) forms than the available thiol-containing therapeutic agents. Accordingly, as observed in vitro and rodent models in vivo, Ebselen exhibited protective effects against MeHg+, indicating its potential as a therapeutic agent to treat MeHg+ overexposure. The combined use of ebselen with thiol-containing molecules (e.g. N-acetylcysteine and enaramide)) is also commented, because they can have synergistic protective effects against MeHg+.

Developmental Methylmercury Exposure Induced and Age-Dependent Glutamatergic Neurotoxicity in Caenorhabditis elegans

Developmental methylmercury (MeHg) exposures cause latent neurotoxic effects in adults; however, the mechanisms underlying the latent neurotoxicity are not fully understood. In the current study, we used C. elegans as an animal model to investigate the latent neurotoxic effects of developmental MeHg exposures on glutamatergic neurons. The young larvae stage 1 worms were exposed to MeHg (0.05 ~ 5 µM) for 48 h. The morphological and behavioral endpoints of glutamatergic neurons were compared when worms reached to adult stages including the young adult stage (day 1 adult) and the old adult stage (day 10 adult). Here, we showed that C. elegans glutamatergic neurons were morphologically intact following low or medium MeHg exposures (0.05 ~ 0.5 µM). The morphological damage of glutamatergic neurons appeared to be pronounced in day 10 adults developmentally exposed to 5 µM MeHg. Behavioral assays also showed an age-dependent latent effect of MeHg. In the nose touch response assay, only day 10 adult worms exhibited a functional decline following prior 5 µM MeHg exposure. Moreover, the disruption of NaCl memory appeared only in day 1 adults following MeHg exposures but not in day 10 adults. The expression of C. elegans homologs of mammalian vesicular glutamate transporter (eat-4) was repressed in day 1 adults, while the glutamate receptor homolog (glr-1) was upregulated in day 10 adults with 5 µM MeHg. In the comparison of age-dependent changes in the insulin-like pathway (daf-2/age-1/daf-16) following MeHg exposures, we showed that the daf-2/age-1/daf-16 pathway was mobilized in day 1 adults but repressed in day 10 adults. Collectively, our data supports a conclusion that MeHg-induced glutamatergic neurotoxicity exhibits an age-dependent pattern, possibly related to the prominent changes in age-dependent modulation in the glutamatergic neurotransmission and metabolic pathways.

Disturbance of Key Cellular Subproteomes upon Propofol Treatment Is Associated with Increased Permeability of the Blood-Brain Barrier

Background: Propofol is a short-acting anesthetic, which is often used for induction and maintenance of general anesthesia, sedation for mechanically ventilated adults and procedural sedation. Several side effects of propofol are known and a substantial number of patients suffer from post-operative delirium after propofol application. In this study, we analyzed the effect of propofol on the function and protein expression profile on a proteome-wide scale. Methods: We cultured human brain microvascular endothelial cells in absence and presence of propofol and analyzed the permeability of the blood-brain barrier (BBB) by fluorescein passage and protein abundance on a proteome-wide scale by mass spectrometry. Results: Propofol interfered with the function of the blood-brain barrier. This was not due to decreased adhesion of propofol-treated human brain microvascular endothelial cells. The proteomic analysis revealed that some key pathways in these cells were disturbed, such as oxygen metabolism, DNA damage recognition and response to stress. Conclusions: Propofol has strong effects on protein expression which could explain several side effects of propofol.

Chronic Toxic Effects of Waterborne Mercury on Silver Carp (Hypophthalmichthys molitrix) Larvae

Mercury (Hg) is a kind of heavy metal pollutant widely existing in the aquatic environment, and it is also recognized to have a highly toxic effect on fish. In this study, silver carp (Hypophthalmichthys molitrix) larvae were exposed to 0 (control), 1, 5, and 10 μg/L Hg2+ for 2 weeks. Antioxidant ability, neurotoxicity, and thyroid hormones (THs) content were evaluated. In comparison with the control, the superoxide dismutase (SOD) activity and the glutathione (GSH) activity were lower in silver carp exposed to 10 μg/L Hg2+. The lowest catalase (CAT) activity was found in the 10 μg/L Hg2+, while malondialdehyde (MDA) content was not significantly different among all groups. Compared with the control, monoamine oxidase (MAO) activity and nitric oxide (NO) content were significantly higher in the 10 μg/L Hg2+, while acetylcholinesterase (AChE) activity significantly decreased. Compared with the control, triiodothyronine (T3) content was significantly higher in the 1 μg/L Hg2+ and significantly lower in the 10 μg/L Hg2+; the 1 μg/L and 5 μg/L Hg2+ groups had significantly higher thyroxine (T4) content than the other groups. In the 1 μg/L Hg2+, the integrated biomarker response (IBR) index value was the highest. In summary, exposure to Hg could decrease the antioxidant ability, cause changes in neurotoxic parameters, and induce disorders of the thyroid hormone system in silver carp larvae. The results of this study may contribute to the understanding of the adverse effects of chronic mercury poisoning on fish.

Purines and Pyrimidines: Metabolism, Function and Potential as Therapeutic Options in Neurodegenerative Diseases

Various neurodegenerative disorders have various molecular origins but some common molecular mechanisms. In the current scenario, there are very few treatment regimens present for advanced neurodegenerative diseases. In this context, there is an urgent need for alternate options in the form of natural compounds with an ameliorating effect on patients. There have been individual scattered experiments trying to identify potential values of various intracellular metabolites. Purines and Pyrimidines, which are vital molecules governing various aspects of cellular biochemical reactions, have been long sought as crucial candidates for the same, but there are still many questions that go unanswered. Some critical functions of these molecules associated with neuromodulation activities have been identified. They are also known to play a role in foetal neurodevelopment, but there is a lacuna in understanding their mechanisms. In this review, we have tried to assemble and identify the importance of purines and pyrimidines, connecting them with the prevalence of neurodegenerative diseases. The leading cause of this class of diseases is protein misfolding and the formation of amyloids. A direct correlation between loss of balance in cellular homeostasis and amyloidosis is yet an unexplored area. This review aims at bringing the current literature available under one umbrella serving as a foundation for further extensive research in this field of drug development in neurodegenerative diseases.

Hydrogen-saturated saline mediated neuroprotection through autophagy via PI3K/AKT/mTOR pathway in early and medium stages of rotenone-induced Parkinson's disease rats

Some cardiovascular symptoms in the early stage of Parkinson's disease (PD) were related to degeneration of the rostral ventrolateral medulla (RVLM) catecholaminergic neurons. To date, little is known about the effects of hydrogen water on early stage of PD. Here, protective actions of hydrogen-saturated saline (HS) on rotenone-induced PD rats, as well as its underlying mechanisms were investigated. HS was used to treat PD rats at three general stages; early, medium and late, which were represented by rotenone induced rats for 0, 7 and 14 days. HS treatment significantly alleviated the cardiovascular and motor symptoms in rotenone-induced PD rats, improved the survival number of RVLM catecholaminergic neurons and nigral dopamine neurons only in early and medium stages of PD rats. Decreased levels of reactive oxygen species (ROS) and alpha-synuclein (α-Syn), transformation of microtubule associated protein 1 light chain 3 (LC3)-I/II and degradation of sequestosome 1 (p62) were detected, as well as increased expression level of autophagy related protein 5 (ATG5) and B-cell lymphoma-2 interacting protein 1 (Beclin-1) in the RVLM and substantia nigra (SN) after HS treatment in early and medium stages of PD rats. In addition, phosphorylation levels of phosphatidylinositol-3-kinase (PI3K), protein kinase B (Akt) and mammalian rapamycin target protein (mTOR) decreased after HS treatment in early and medium stages of PD rats. The results suggested that HS treatment exerted beneficial effects in early and medium stages before motor impairments emerged but not in the late stage of rotenone-induced PD rats. It exerted neuroprotection with RVLM catecholaminergic neurons and nigral dopamine neurons, mediated in part by decreasing levels of ROS and α-Syn through increasing autophagy machinery which were partly via inhibiting PI3K-Akt-mTOR pathway.

(PhSe)2 and (pCl-PhSe)2 organochalcogen compounds inhibit Candida albicans adhesion to human endocervical (HeLa) cells and show anti-biofilm activities

Adhesion capacity on biological surfaces and biofilm formation is considered an important step in the infection process by Candida albicans. The ability of (PhSe)₂ and (pCl-PhSe)₂, two synthetic organic selenium (organochalcogen) compounds, to act on C. albicans virulence factors related to adhesion to human endocervical (HeLa) cell surfaces and their anti-biofilm activities was analyzed. Both organochalcogen compounds inhibited C. albicans adhesion to HeLa cells, dependent on compound concentrations. (PhSe)₂ (at 20 µM; p = 0.0012) was significantly more effective than (pCl-PhSe)₂ (at 20 µM; p = 0.0183) compared with the control. (PhSe)₂ inhibited biofilm formation and decreased biofilm viability in both early and mature biofilms more efficiently than (pCl-PhSe)₂. Overall, the organochalcogen compounds, especially (PhSe)₂, were demonstrated to be effective antifungal drugs against C. albicans virulence factors related to epithelial cell surface adhesion and the formation and viability of biofilms.

Water temperature modulates mercury accumulation and oxidative stress status of common goby (Pomatoschistus microps)

Mercury (Hg) is a widespread pollutant across estuarine and coastal areas, raising concern on its potential impact on aquatic organisms. Hg may origin from natural and anthropogenic sources, being persistent and potentially toxic to biota, ultimately representing a serious risk to human health. Hg accumulation and toxicity may also induce reactive oxygen species (ROS) production in marine organisms, responsible for cell and tissue damage. Additionally, the temperature is undoubtedly an important environmental factor to consider regarding accumulation, due to its marked influence on the physiology and ecology of aquatic organisms. This study aimed to investigate the effect of different temperature scenarios (15, 20 and 25 °C) on the Hg accumulation in Pomatoschistus microps (Krøyer, 1838) liver and muscle, as well as on oxidative stress responses and energy metabolism, after short-term exposure to a naturally contaminated sediment with an environmentally relevant [Hg] (1.2 μg g^-1). The results showed that Hg accumulation tends to increase along the temperature gradient with higher values of Hg accumulated in liver than in muscle tissue. The action of antioxidant enzymes and stress proteins seems to be effective in combating oxidative stress in the liver. Despite the action of antioxidant defences in the muscle, oxidative damage was observed at the protein level concomitantly with a decrease in aerobic energy production after exposure to Hg at higher temperatures. These findings are ecologically relevant and highlight the importance of further investigation of combined effects of Hg and other stressors, especially in a scenario of a changing climate where events leading to rapid alterations on water parameters are more frequent.

A toxicological comparison between two uranium compounds in Artemia salina: Artificial seawater containing CaCO3

Uranium (U) mining is an aquatic environmental concern because most of these harmful compounds are discharged into freshwater, reaching the saline environment as the final destination of this contaminated water. Carbonates are present in ocean waters and are essential for benthic organisms, however they may influence the U-induced toxicity. Thus, the aim of this study was to compare the toxicity of uranium nitrate (UN) and uranium acetate (UA) in Artemia salina (AS), which is one of the leading representatives of the marine biota. The cultures of AS (instar II) maintained in artificial seawater containing CaCO₃ were exposed for 24 h to different concentrations of U compounds. The results showed that AS were more sensitive to UN (LC₅₀ ≈ 15 μM) when compared with UA (LC₅₀ ≈ 245 μM) indicating higher toxicity of this U compound. Calculated U speciation indicated that Ca₂UO₂(CO₃)₃ and (UO₂)2CO₃(OH)₃^- complexes predominated under our experimental conditions. The immobilization/lethality was observed after 9 h of exposure for both U compounds. However, only UN caused a significant decrease (≈40%) in the acetylcholinesterase (AChE) activity when compared with control. In order to observe preliminary toxicity effects, we evaluated oxidative stress parameters, such as catalase (CAT) activity, TBARS formation, radical species (RS) generation and cell membrane injury and/or apoptosis (CMI). In this study, we demonstrate that U compounds caused a significant decrease in CAT activity. Similarly, we also observed that UN increased TBARS levels in AS at concentrations 5 times lower than AU (10 μM and 50 μM, respectively). Furthermore, RS generation and CMI were enhanced only on AS treated with UN. Overall, the effects observed here were remarkably significant in AS exposed to UN when compared with AU. In this study, we showed different profiles of toxicity for both U compounds, contributing significantly to the current and scarce understanding of the aquatic ecotoxicity of this heavy metal.

Novel Targets for Fast Antidepressant Responses: Possible Role of Endogenous Neuromodulators

The available medications for the treatment of major depressive disorder have limitations, particularly their limited efficacy, delayed therapeutic effects, and the side effects associated with treatment. These issues highlight the need for better therapeutic agents that provide more efficacious and faster effects for the management of this disorder. Ketamine, an N-methyl-D-aspartate receptor antagonist, is the prototype for novel glutamate-based antidepressants that has been shown to cause a rapid and sustained antidepressant effect even in severe refractory depressive patients. Considering the importance of these findings, several studies have been conducted to elucidate the molecular targets for ketamine's effect. In addition, efforts are under way to characterize ketamine-like drugs. This review focuses particularly on evidence that endogenous glutamatergic neuromodulators may be able to modulate mood and to elicit fast antidepressant responses. Among these molecules, agmatine and creatine stand out as those with more published evidence of similarities with ketamine, but guanosine and ascorbic acid have also provided promising results. The possibility that these neuromodulators and ketamine have common neurobiological mechanisms, mainly the ability to activate mechanistic target of rapamycin and brain-derived neurotrophic factor signaling, and synthesis of synaptic proteins in the prefrontal cortex and/or hippocampus is presented and discussed.

Oxidative Stress in Methylmercury-Induced Cell Toxicity

Methylmercury (MeHg) is a hazardous environmental pollutant, which elicits significant toxicity in humans. The accumulation of MeHg through the daily consumption of large predatory fish poses potential health risks, and the central nervous system (CNS) is the primary target of toxicity. Despite well-described neurobehavioral effects (i.e., motor impairment), the mechanisms of MeHg-induced toxicity are not completely understood. However, several lines of evidence point out the oxidative stress as an important molecular mechanism in MeHg-induced intoxication. Indeed, MeHg is a soft electrophile that preferentially interacts with nucleophilic groups (mainly thiols and selenols) from proteins and low-molecular-weight molecules. Such interaction contributes to the occurrence of oxidative stress, which can produce damage by several interacting mechanisms, impairing the function of various molecules (i.e., proteins, lipids, and nucleic acids), potentially resulting in modulation of different cellular signal transduction pathways. This review summarizes the general aspects regarding the interaction between MeHg with regulators of the antioxidant response system that are rich in thiol and selenol groups such as glutathione (GSH), and the selenoenzymes thioredoxin reductase (TrxR) and glutathione peroxidase (Gpx). A particular attention is directed towards the role of the PI3K/Akt signaling pathway and the nuclear transcription factor NF-E2-related factor 2 (Nrf2) in MeHg-induced redox imbalance.

Neurotransmitter amines and antioxidant agents in neuronal protection against methylmercury-induced cytotoxicity in primary cultures of mice cortical neurons

Methylmercury (MeHg) is an environmental toxicant with detrimental effects on the developing brain and adult nervous system. The main mechanisms identified include oxidative stress, changes in intracellular calcium, mitochondrial changes, inhibition of glutamate uptake, of protein synthesis and disruption of microtubules. However, little is known about mechanisms of protection against MeHg neurotoxicity. We found that resveratrol (10 μM) and ascorbic acid (200 μM) protected MeHg-induced cell death in primary cultures of cortical neurons. In this work, we aimed at finding additional targets that may be related to MeHg mode of action in cell toxicity with special emphasis in cell protection. We wonder whether neurotransmitters may affect the MeHg effects on neuronal death. Our findings show that neurons exposed to low MeHg concentrations exhibit less mortality if co-exposed to 10 μM dopamine (DA). However, DA metabolites, HVA (homovanillic acid) and DOPAC (3,4-dihydroxyphenylacetic acid) are not responsible for such protection. Furthermore, both DA D1 and D2 receptors agonists showed a protective effect against MeHg toxicity. It is striking though that DA receptor antagonists SKF83566 (10 μM) and haloperidol (10 μM) did not inhibit DA protection against MeHg. In addition, the protective effect of 10 μM DA against MeHg-induced toxicity was not affected by additional organochlorine pollutants exposure. Our results also demonstrate that cells exposed to MeHg in presence of 100 μM acetylcholine (ACh), show an increase in cell mortality at the "threshold value" of 100 nM MeHg. Finally, norepinephrine (10 μM) and serotonin (20 μM) also had an effect on cell protection. Altogether, we propose to further investigate the additional mechanisms that may be playing an important role in MeHg-induced cytotoxicity.

Organoselenium compounds as mimics of selenoproteins and thiol modifier agents

Selenium is an essential trace element for animals and its role in the chemistry of life relies on a unique functional group: the selenol (-SeH) group. The selenol group participates in critical redox reactions. The antioxidant enzymes glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) exemplify important selenoproteins. The selenol group shares several chemical properties with the thiol group (-SH), but it is much more reactive than the sulfur analogue. The substitution of S by Se has been exploited in organic synthesis for a long time, but in the last 4 decades the re-discovery of ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) and the demonstration that it has antioxidant and therapeutic properties has renovated interest in the field. The ability of ebselen to mimic the reaction catalyzed by GPx has been viewed as the most important molecular mechanism of action of this class of compound. The term GPx-like or thiol peroxidase-like reaction was previously coined in the field and it is now accepted as the most important chemical attribute of organoselenium compounds. Here, we will critically review the literature on the capacity of organoselenium compounds to mimic selenoproteins (particularly GPx) and discuss some of the bottlenecks in the field. Although the GPx-like activity of organoselenium compounds contributes to their pharmacological effects, the superestimation of the GPx-like activity has to be questioned. The ability of these compounds to oxidize the thiol groups of proteins (the thiol modifier effects of organoselenium compounds) and to spare selenoproteins from inactivation by soft-electrophiles (MeHg⁺, Hg²⁺, Cd²⁺, etc.) might be more relevant for the explanation of their pharmacological effects than their GPx-like activity. In our view, the exploitation of the thiol modifier properties of organoselenium compounds can be harnessed more rationally than the use of low mass molecular structures to mimic the activity of high mass macromolecules that have been shaped by millions to billions of years of evolution.

Characterization of mercury-binding proteins in human neuroblastoma SK-N-SH cells with immobilized metal affinity chromatography

Metal-binding proteins play important roles in biological functions of metals. However, only very limited mercury-binding proteins with high abundance were characterized in cells or organisms. Characterization of mercury-binding proteins in proteome-wide is important for elucidating mechanisms of mercury toxicity comprehensively. In this study, a method based on immobilized mercury ion affinity chromatography was developed for identification of putative mercury-binding proteins. The method was then successfully applied to profile mercury-binding proteins in human neuroblastoma SK-N-SH cells. In total, 38 proteins were identified as mercury-binding proteins, in which most of them were uncharacterized to associate with mercury in cells. The identified mercury-binding proteins did not show obvious relevance to protein abundance and were mainly involved in protein processing in endoplasmic reticulum, protein folding, and cytoskeleton organization. The newly built metalloproteomic approach provided valuable information on the possible molecular mechanisms and protein candidates for mercury transport and toxicity.

Response of oxidative stress transcripts in the brain of wild yellow perch (Perca flavescens) exposed to an environmental gradient of methylmercury

Methylmercury (MeHg) exposure and adverse health effects in fishes have been documented, but the molecular mechanisms involved in toxicity have not been fully characterized. The objectives of the current study were to (1) determine whether total Hg (THg) in the muscle was predictive of MeHg concentrations in the brain of wild female yellow perch (Perca flavescens) collected from four lakes in Kejimkujik National Park, a known biological mercury (Hg) hotspot in Nova Scotia, Canada and (2) to determine whether transcripts involved in the oxidative stress response were altered in abundance in fish collected across five lakes representing a MeHg gradient. In female yellow perch, MeHg in whole brain (0.38 to 2.00μg/g wet weight) was positively associated with THg in muscle (0.18 to 2.13μg/g wet weight) (R²=0.61, p<0.01), suggesting that muscle THg may be useful for predicting MeHg concentrations in the brain. Catalase (cat) mRNA levels were significantly lower in brains of perch collected from lakes with high Hg when compared to those individuals from lakes with relatively lower Hg (p=0.02). Other transcripts (cytochrome c oxidase, glutathione peroxidase, glutathione-s-transferase, heat shock protein 70, protein disulfide isomerase, and superoxide dismutase) did not show differential expression in the brain over the gradient. These findings suggest that MeHg may be inversely associated with catalase mRNA abundance in the central nervous system of wild fishes.

Chemical Speciation of Selenium and Mercury as Determinant of Their Neurotoxicity

The antagonism of mercury toxicity by selenium has been well documented. Mercury is a toxic metal, widespread in the environment. The main target organs (kidneys, lungs, or brain) of mercury vary depending on its chemical forms (inorganic or organic). Selenium is a semimetal essential to mammalian life as part of the amino acid selenocysteine, which is required to the synthesis of the selenoproteins. This chapter has the aim of disclosing the role of selenide or hydrogen selenide (Se^-2 or HSe^-) as central metabolite of selenium and as an important antidote of the electrophilic mercury forms (particularly, Hg²⁺ and MeHg). Emphasis will be centered on the neurotoxicity of electrophile forms of mercury and selenium. The controversial participation of electrophile mercury and selenium forms in the development of some neurodegenerative disease will be briefly presented. The potential pharmacological use of organoseleno compounds (Ebselen and diphenyl diselenide) in the treatment of mercury poisoning will be considered. The central role of thiol (-SH) and selenol (-SeH) groups as the generic targets of electrophile mercury forms and the need of new in silico tools to guide the future biological researches will be commented.

Guanosine and its role in neuropathologies

Guanosine is a purine nucleoside thought to have neuroprotective properties. It is released in the brain under physiological conditions and even more during pathological events, reducing neuroinflammation, oxidative stress, and excitotoxicity, as well as exerting trophic effects in neuronal and glial cells. In agreement, guanosine was shown to be protective in several in vitro and/or in vivo experimental models of central nervous system (CNS) diseases including ischemic stroke, Alzheimer's disease, Parkinson's disease, spinal cord injury, nociception, and depression. The mechanisms underlying the neurobiological properties of guanosine seem to involve the activation of several intracellular signaling pathways and a close interaction with the adenosinergic system, with a consequent stimulation of neuroprotective and regenerative processes in the CNS. Within this context, the present review will provide an overview of the current literature on the effects of guanosine in the CNS. The elucidation of the complex signaling events underlying the biochemical and cellular effects of this nucleoside may further establish guanosine as a potential therapeutic target for the treatment of several neuropathologies.

The Guanine-Based Purinergic System: The Tale of An Orphan Neuromodulation

Guanine-based purines (GBPs) have been recently proposed to be not only metabolic agents but also extracellular signaling molecules that regulate important functions in the central nervous system. In such way, GBPs-mediated neuroprotection, behavioral responses and neuronal plasticity have been broadly described in the literature. However, while a number of these functions (i.e., GBPs neurothophic effects) have been well-established, the molecular mechanisms behind these GBPs-dependent effects are still unknown. Furthermore, no plasma membrane receptors for GBPs have been described so far, thus GBPs are still considered orphan neuromodulators. Interestingly, an intricate and controversial functional interplay between GBPs effects and adenosine receptors activity has been recently described, thus triggering the hypothesis that GBPs mechanism of action might somehow involve adenosine receptors. Here, we review recent data describing the GBPs role in the brain. We focus on the involvement of GBPs regulating neuronal plasticity, and on the new hypothesis based on putative GBPs receptors. Overall, we expect to shed some light on the GBPs world since although these molecules might represent excellent candidates for certain neurological diseases management, the lack of putative GBPs receptors precludes any high throughput screening intent for the search of effective GBPs-based drugs.

Phagocytic cell responses to silica-coated dithiocarbamate-functionalized iron oxide nanoparticles and mercury co-exposures in Anguilla anguilla L

Immune system responses in fish are considered as suitable and sensitive biomarkers for monitoring aquatic pollution. However, a clear knowledge gap persists in the literture on the immunotoxic potential of engineered nanoparticles toward aquatic organisms such as fish. Employing major enzymatic- (glutathione reductase, GR; glutathione peroxidase, GPX; glutathione sulfo-transferase, GST; catalase, CAT) and thiol- (non-protein thiols, NP-SH; total glutathione, TGSH)-based defense biomarkers, this study assessed the response of phagocytes isolated from peritoneum (P-phagocytes), gill (G-phagocytes), head kidney (HK-phagocytes), and spleen (S-phagocytes) of European eel (Anguilla anguilla L.) to silica-coated magnetite particles (Fe3O4@SiO2/SiDTC, hereafter called IONP; size range: 82 ± 21 to 100 ± 30 nm; 2.5 mg L(-1)) alone and IONP and mercury (Hg; 50 μg L(-1)) concomitant exposures. Responses of previous biomarkers were studied in P-phagocytes, G-phagocytes, HK-phagocytes, and S-phagocytes collected during 0, 2, 4, 8, 16, 24, 48, and 72 h of exposures. Contingent to hour of exposure to IONP, Hg, and IONP + Hg GST, GPX, CAT, NP-SH, and TGSH exhibited their differential responses in all the phagocytic cells considered. In particular, under IONP exposure, the potential occurrence of the GSH-independent antioxidant defense was indicated by the observed herein inhibition in the enzymatic- and thiol-based defense in A. anguilla phagocytes. In contrast, the response of P-, G-, HK-, and S-phagocytes to the increasing Hg exposure period reflected an increased detoxification activity. Notably, the occurrence of an antagonism between IONP and Hg was depicted during late hours (72 h) under IONP + Hg concomitant exposure, where elevations in the defense biomarkers were depicted. Overall, the P-, G-, HK-, and S-phagocytic cells exhibited a differential induction in the studied enzymes and thiols to counteract impacts of IONP, Hg, and IONP + Hg concomitant exposures. Future studies on the fish immunotoxicity responses to IONP exposure in multi-pollution conditions can be benefited with the major outcomes of the present study.

Insights into neurosensory toxicity of mercury in fish eyes stemming from tissue burdens, oxidative stress and synaptic transmission profiles

This study aims to contribute to fill a knowledge gap related with Hg effects in fish eyes. As a pioneering strategy, Hg bioaccumulation in eye wall of the wild grey mullet (Liza aurata) was assessed, together with oxidative stress and synaptic transmission profiles. This approach was complemented by the characterisation of environmental contamination (both in water and sediment). Sampling was conducted in winter and summer in two sites of a Portuguese coastal lagoon (Aveiro lagoon): Largo do Laranjo (LAR) - located in an Hg contaminated/confined area; São Jacinto (SJ) - closer to the lagoon inlet and selected as reference site. Levels of total Hg (tHg), inorganic Hg (iHg) and methylmercury (MeHg) in eye wall were higher at LAR than SJ, both in winter and summer, reflecting the environmental contamination patterns. Moreover, fish caught at LAR in winter showed a significant decrease of catalase and superoxide dismutase activities, in line with the occurrence of peroxidative damage. A different spatial pattern was recorded in summer, being characterised by the increment of glutathione peroxidase and glutathione reductase activities at LAR, as well as total glutathione content, preventing the occurrence of lipid peroxidation. Also in summer, a significant decrease of acetylcholinesterase activity was recorded in fish eyes at LAR, pointed out Hg as an anticholinergic agent. Besides Hg, water salinity had probably an indirect effect on spatial and winter-summer variation patterns of AChE. Current data pointed out that Hg (in iHg and MeHg forms) could exert ocular toxicity both by the promotion of oxidative stress and by the interference with neurotransmission processes.

Sulforaphane Prevents Methylmercury-Induced Oxidative Damage and Excitotoxicity Through Activation of the Nrf2-ARE Pathway

Methylmercury (MeHg) is a prominent environmental neurotoxicant, which induces oxidative damage and an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. However, the interaction between oxidative damage and excitotoxicity in MeHg-exposed rats has not been fully recognized. Here, we explored the interaction between oxidative damage and excitotoxicity and evaluated the preventive effects of sulforaphane (SFN) on MeHg-induced neurotoxicity in rat cerebral cortex. Seventy-two rats were randomly assigned to four groups: control group, MeHg-treated groups (4 and 12 μmol/kg), and SFN pretreatment group. After treatment (28 days), the rats were killed and the cerebral cortex was analyzed. Then, Hg, glutathione (GSH), malondialdehyde (MDA), protein sulfhydryl, protein carbonyl, 8-hydroxy-2-deoxyguanosine (8-OHdG), and the levels of reactive oxygen species (ROS) and apoptosis were examined. Glu and glutamine (Gln) levels, glutamine synthetase (GS), phosphate-activated glutaminase (PAG), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), Na⁺-K⁺-ATPase and Ca²⁺-ATPase activities, intracellular Ca²⁺ levels, and the mRNA and protein expressions of Nrf2, Nrf2-regulated gene products, and N-methyl-D-aspartate receptors (NMDARs) were investigated in rat cerebral cortex. In our study, MeHg exposure not only induced Hg accumulation, apoptosis, ROS formation, GSH depletion, inhibition of antioxidant enzyme activities, and activation of Nrf2-ARE pathway signaling but also caused lipid, protein, and DNA peroxidative damage in a dose-dependent manner in rat cerebral cortex. Moreover, MeHg treatment significantly altered Gln/Glu cycling and NMDAR expression and resulted in calcium overloading. Furthermore, the present study also indicated that SFN pretreatment significantly reinforced the activation of the Nrf2-ARE pathway, which could prevent the toxic effects of MeHg exposure. Collectively, MeHg initiates multiple additive or synergistic disruptive mechanisms that lead to oxidative damage and excitotoxicity in rat cerebral cortex; pretreatment with SFN might prevent the MeHg-induced neurotoxicity by reinforcing the activation of the Nrf2-ARE pathway and then downregulating the interaction between oxidative damage and excitotoxicity pathways.

The neuroprotective effect of berberine in mercury-induced neurotoxicity in rats

The central nervous system is one of the most vulnerable organs affected by mercury toxicity. Both acute and chronic exposure to mercury is also known to cause a variety of neurological or psychiatric disorders. Here, the neuroprotective effect of berberine (BN; 100 mg/kg bwt) on mercuric chloride (HgCl2; 0.4 mg/kg bwt) induced neurotoxicity and oxidative stress was examined in rats. Adult male albino Wistar rats were injected with HgCl2 for 7 days. HgCl2 treatment induced oxidative stress by increasing lipid peroxidation (LPO) and nitrite/nitrate (nitric oxide; NO) production along with a concomitant decrease in glutathione (GSH) and various antioxidant enzymes, namely superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. Pre-treatment of rats with BN inhibited LPO and NO production, whereas it increased GSH content. Activities of antioxidant enzymes were also restored concomitantly when compared to the control rats after BN administration. Berberine also caused decrease in TNF-α level and caspase-3 activity which was higher with HgCl2. Furthermore, treatment with BN inhibited apoptosis, as indicated by the reduction of Bax/Bcl-2 ratio in brain tissue. These data indicated that BN augments antioxidant defense with anti-inflammatory and anti-apoptotic activities against HgCl2-induced neurotoxicity and provides evidence that it has a therapeutic potential as neuroprotective agent.

Diphenyl diselenide (PhSe)2 inhibits biofilm formation by Candida albicans, increasing both ROS production and membrane permeability

PROJECT

The opportunistic fungal Candida albicans can produce superficial and systemic infections in immunocompromised patients. An essential stage to both colonization and virulence by C. albicans is the transition from budding yeast form to filamentous form, producing biofilms.

PROCEDURE

In this work, we studied the effect of the organochalcogenide compound (PhSe)2 on both cell growth and biofilm formation by C. albicans.

RESULTS

(PhSe)2 inhibited both growth and biofilm formation by C. albicans. The inhibitory effects of (PhSe)2 depended on the cell density and (PhSe)2 concentration. We have also observed that (PhSe)2 stimulated ROS production (67%) and increased cell membrane permeability (2.94-fold) in C. albicans. In addition, (PhSe)2 caused a marked decrease in proteinase activity (6.8-fold) in relation to non-treated group.

CONCLUSIONS

(PhSe)2 decreased both cell growth and biofilm development, decreasing the release of extracellular proteinases, which is an important facet of C. albicans pathogenicity. The toxicity of (PhSe)2 towards C. albicans can be associated with an increase in ROS production, which can increase cell permeability. The permanent damage to the cell membranes can culminate in cell death.

Preventive effects of dextromethorphan on methylmercury-induced glutamate dyshomeostasis and oxidative damage in rat cerebral cortex

Methylmercury (MeHg) is a well-known environmental pollutant leading to neurotoxicant associated with aberrant central nervous system (CNS) functions, but its toxic mechanisms have not yet been fully recognized. In the present study, we tested the hypothesis that MeHg induces neuronal injury via glutamate (Glu) dyshomeostasis and oxidative damage mechanisms and that these effects are attenuated by dextromethorphan (DM), a low-affinity and noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist. Seventy-two rats were randomly divided into four groups of 18 animals in each group: control group, MeHg-treated group (4 and 12 μmol/kg), and DM-pretreated group. After the 4-week treatment, we observed that the administration of MeHg at a dose of 12 μmol/kg significantly increased in total mercury (Hg) levels, disrupted Glu metabolism, overexcited NMDARs, and led to intracellular calcium overload in the cerebral cortex. We also found that MeHg reduced nonenzymatic and enzymatic antioxidants, enhanced neurocyte apoptosis, induced reactive oxygen species (ROS), and caused lipid, protein, and DNA peroxidative damage in the cerebral cortex. Moreover, glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) appeared to be inhibited by MeHg exposure. These alterations were significantly prevented by the pretreatment with DM at a dose of 13.5 μmol/kg. In conclusion, these findings strongly implicate that DM has potential to protect the brain from Glu dyshomeostasis and oxidative damage resulting from MeHg-induced neurotoxicity in rat.

The potential therapeutic effect of guanosine after cortical focal ischemia in rats

BACKGROUND AND PURPOSE

Stroke is a devastating disease. Both excitotoxicity and oxidative stress play important roles in ischemic brain injury, along with harmful impacts on ischemic cerebral tissue. As guanosine plays an important neuroprotective role in the central nervous system, the purpose of this study was to evaluate the neuroprotective effects of guanosine and putative cerebral events following the onset of permanent focal cerebral ischemia.

METHODS

Permanent focal cerebral ischemia was induced in rats by thermocoagulation. Guanosine was administered immediately, 1 h, 3 h and 6 h after surgery. Behavioral performance was evaluated by cylinder testing for a period of 15 days after surgery. Brain oxidative stress parameters, including levels of ROS/RNS, lipid peroxidation, antioxidant non-enzymatic levels (GSH, vitamin C) and enzymatic parameters (SOD expression and activity and CAT activity), as well as glutamatergic parameters (EAAC1, GLAST and GLT1, glutamine synthetase) were analyzed.

RESULTS

After 24 h, ischemic injury resulted in impaired function of the forelimb, caused brain infarct and increased lipid peroxidation. Treatment with guanosine restored these parameters. Oxidative stress markers were affected by ischemic insult, demonstrated by increased ROS/RNS levels, increased SOD expression with reduced SOD activity and decreased non-enzymatic (GSH and vitamin C) antioxidant defenses. Guanosine prevented increased ROS/RNS levels, decreased SOD activity, further increased SOD expression, increased CAT activity and restored vitamin C levels. Ischemia also affected glutamatergic parameters, illustrated by increased EAAC1 levels and decreased GLT1 levels; guanosine reversed the decreased GLT1 levels and did not affect the EAAC1 levels.

CONCLUSION

The effects of brain ischemia were strongly attenuated by guanosine administration. The cellular mechanisms involved in redox and glutamatergic homeostasis, which were both affected by the ischemic insult, were also modulated by guanosine. These observations reveal that guanosine may represent a potential therapeutic agent in cerebral ischemia by preventing oxidative stress and excitotoxicity.

Effects of diphenyl diselenide on methylmercury toxicity in rats

This study investigates the efficacy of diphenyl diselenide [(PhSe)2] in attenuating methylmercury- (MeHg-)induced toxicity in rats. Adult rats were treated with MeHg [5 mg/kg/day, intragastrically (i.g.)] and/ or (PhSe)2 [1 mg/kg/day, intraperitoneally (i.p.)] for 21 days. Body weight gain and motor deficits were evaluated prior to treatment, on treatment days 11 and 21. In addition, hepatic and cerebral mitochondrial function (reactive oxygen species (ROS) formation, total and nonprotein thiol levels, membrane potential (ΔΨm), metabolic function, and swelling), hepatic, cerebral, and muscular mercury levels, and hepatic, cerebral, and renal thioredoxin reductase (TrxR) activity were evaluated. MeHg caused hepatic and cerebral mitochondrial dysfunction and inhibited TrxR activity in liver (38,9%), brain (64,3%), and kidney (73,8%). Cotreatment with (PhSe)2 protected hepatic and cerebral mitochondrial thiols from depletion by MeHg but failed to completely reverse MeHg's effect on hepatic and cerebral mitochondrial dysfunction or hepatic, cerebral, and renal inhibition of TrxR activity. Additionally, the cotreatment with (PhSe)2 increased Hg accumulation in the liver (50,5%) and brain (49,4%) and increased the MeHg-induced motor deficits and body-weight loss. In conclusion, these results indicate that (PhSe)2 can increase Hg body burden as well as the neurotoxic effects induced by MeHg exposure in rats.

Gliopreventive effects of guanosine against glucose deprivation in vitro

Guanosine, a guanine-based purine, is recognized as an extracellular signaling molecule that is released from astrocytes and confers neuroprotective effects in several in vivo and in vitro studies. Astrocytes regulate glucose metabolism, glutamate transport, and defense mechanism against oxidative stress. C6 astroglial cells are widely used as an astrocyte-like cell line to study the astrocytic function and signaling pathways. Our previous studies showed that guanosine modulates the glutamate uptake activity, thus avoiding glutamatergic excitotoxicity and protecting neural cells. The goal of this study was to determine the gliopreventive effects of guanosine against glucose deprivation in vitro in cultured C6 cells. Glucose deprivation induced cytotoxicity, an increase in reactive oxygen and nitrogen species (ROS/RNS) levels and lipid peroxidation as well as affected the metabolism of glutamate, which may impair important astrocytic functions. Guanosine prevented glucose deprivation-induced toxicity in C6 cells by modulating oxidative and nitrosative stress and glial responses, such as the glutamate uptake, the glutamine synthetase activity, and the glutathione levels. Glucose deprivation decreased the level of EAAC1, the main glutamate transporter present in C6 cells. Guanosine also prevented this effect, most likely through PKC, PI3K, p38 MAPK, and ERK signaling pathways. Taken together, these results show that guanosine may represent an important mechanism for protection of glial cells against glucose deprivation. Additionally, this study contributes to a more thorough understanding of the glial- and redox-related protective properties of guanosine in astroglial cells.

Mercury toxicity on sodium pump and organoseleniums intervention: a paradox

Mercury is an environmental poison, and the damage to living system is generally severe. The severity of mercury poisoning is consequent from the fact that it targets the thiol-containing enzymes, irreversibly oxidizing their critical thiol groups, consequently leading to an inactivation of the enzyme. The Na⁺/K⁺-ATPase is a sulfhydryl protein that is sensitive to Hg²⁺ assault. On the other hand, organoseleniums are a class of pharmacologically promising compounds with potent antioxidant effects. While Hg²⁺ oxidizes sulfhydryl groups of Na⁺/K⁺-ATPase under in vitro and in vivo conditions, the organoselenium compounds inhibit Na⁺/K⁺-ATPase in vitro but enhance its activities under in vivo conditions with concomitant increase in the level of endogenous thiols. Paradoxically, it appears that these two thiol oxidants can be used to counteract one another under in vivo conditions, and this hypothesis serves as the basis for this paper.

Protective effects of guanosine against sepsis-induced damage in rat brain and cognitive impairment

The development of cognitive impairment in sepsis is associated with neurotoxic effects caused by oxidative stress. We have assessed the effects of acute and extended administration of guanosine (GUA) on brain oxidative stress parameters and cognitive impairment in rats submitted to sepsis by cecal ligation and perforation (CLP). To achieve this goal, male Wistar rats underwent either sham operation or CLP with GUA. Rats subjected to CLP were treated with intraperitoneal injection of GUA (8 mg/kg after CLP) or vehicle. Twelve and 24 h after CLP, the rats were sacrificed, and samples from brain (hippocampus, striatum, cerebellum, prefrontal cortex and cortex) were obtained and assayed for thiobarbituric acid reactive species (TBARS) formation and protein carbonyls. On the 10th day, another group of rats was submitted to the behavioral tasks. GUA administration reduced TBARS and carbonyl levels in some brain regions between 12 and 24 h after CLP, and ameliorated cognitive impairment evaluated 10 days after CLP. Our data provide the first experimental demonstration that GUA was able to reduce the consequences of CLP-induced sepsis in rats, by decreasing oxidative stress parameters in the brain and recovering the memory impairment.

Role of calcium and mitochondria in MeHg-mediated cytotoxicity

Methylmercury (MeHg) mediated cytotoxicity is associated with loss of intracellular calcium (Ca²⁺) homeostasis. The imbalance in Ca²⁺ physiology is believed to be associated with dysregulation of Ca²⁺ intracellular stores and/or increased permeability of the biomembranes to this ion. In this paper we summarize the contribution of glutamate dyshomeostasis in intracellular Ca²⁺ overload and highlight the mitochondrial dysfunctions induced by MeHg via Ca²⁺ overload. Mitochondrial disturbances elicited by Ca²⁺ may involve several molecular events (i.e., alterations in the activity of the mitochondrial electron transport chain complexes, mitochondrial proton gradient dissipation, mitochondrial permeability transition pore (MPTP) opening, thiol depletion, failure of energy metabolism, reactive oxygen species overproduction) that could culminate in cell death. Here we will focus on the role of oxidative stress in these phenomena. Additionally, possible antioxidant therapies that could be effective in the treatment of MeHg intoxication are briefly discussed.

Cooperation of non-effective concentration of glutamatergic system modulators and antioxidant against oxidative stress induced by quinolinic acid

Excessive formation of reactive oxygen species (ROS) and disruption of glutamate uptake have been hypothesized as key mechanisms contributing to quinolinic acid (QA)-induced toxicity. Thus, here we investigate if the use of diphenyl diselenide (PhSe)(2), guanosine (GUO) and MK-801, alone or in combination, could protect rat brain slices from QA-induced toxicity. QA (1 mM) increased ROS formation, thiobarbituric acid reactive substances (TBARS) and decreased cell viability after 2 h of exposure. (PhSe)(2) (1 μM) protected against this ROS formation in the cortex and the striatum and also prevented decreases in cell viability induced by QA. (PhSe)(2) (5 μM) prevented ROS formation in the hippocampus. GUO (10 and 100 μM) blocked the increase in ROS formation caused by QA and MK-801 (20 and 100 μM) abolished the pro-oxidant effect of QA. When the noneffective concentrations were used in combination produced a decrease in ROS formation, mainly (PhSe)(2) + GUO and (PhSe)(2) + GUO + MK-801. These results demonstrate that this combination could be effective to avoid toxic effects caused by high concentrations of QA. Furthermore, the data obtained in the ROS formation and cellular viability assays suggest different pathways in amelioration of QA toxicity present in the neurodegenerative process.

Antioxidant activity of β-selenoamines and their capacity to mimic different enzymes

The antioxidant properties of organoselenium compounds have been extensively investigated because oxidative stress is a hallmark of a variety of chronic human diseases. Here, we reported the influence of substituent groups in the antioxidant activity of β-selenoamines. We have investigated whether they exhibited glutathione peroxidase-like (GPx-like) activity and whether they could be substrate of thioredoxin reductase (TrxR). In the DPPH assay, the β-selenium amines did not exhibit antioxidant activity. However, the β-selenium amines with p-methoxy and tosyl groups prevented the lipid peroxidation. The β-selenium amine compound with p-methoxy substituent group exhibited thiol-peroxidase-like activity (GPx-like activity) and was reduced by the hepatic TrxR. These results contribute to understand the influence of structural alteration of non-conventional selenium compounds as synthetic mimetic of antioxidant enzymes of mammalian organisms.

Oxidative stress in MeHg-induced neurotoxicity

Methylmercury (MeHg) is an environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. Although the molecular mechanisms mediating MeHg-induced neurotoxicity are not completely understood, several lines of evidence indicate that oxidative stress represents a critical event related to the neurotoxic effects elicited by this toxicant. The objective of this review is to summarize and discuss data from experimental and epidemiological studies that have been important in clarifying the molecular events which mediate MeHg-induced oxidative damage and, consequently, toxicity. Although unanswered questions remain, the electrophilic properties of MeHg and its ability to oxidize thiols have been reported to play decisive roles to the oxidative consequences observed after MeHg exposure. However, a close examination of the relationship between low levels of MeHg necessary to induce oxidative stress and the high amounts of sulfhydryl-containing antioxidants in mammalian cells (e.g., glutathione) have led to the hypothesis that nucleophilic groups with extremely high affinities for MeHg (e.g., selenols) might represent primary targets in MeHg-induced oxidative stress. Indeed, the inhibition of antioxidant selenoproteins during MeHg poisoning in experimental animals has corroborated this hypothesis. The levels of different reactive species (superoxide anion, hydrogen peroxide and nitric oxide) have been reported to be increased in MeHg-exposed systems, and the mechanisms concerning these increments seem to involve a complex sequence of cascading molecular events, such as mitochondrial dysfunction, excitotoxicity, intracellular calcium dyshomeostasis and decreased antioxidant capacity. This review also discusses potential therapeutic strategies to counteract MeHg-induced toxicity and oxidative stress, emphasizing the use of organic selenocompounds, which generally present higher affinity for MeHg when compared to the classically studied agents.

Evidence that hyperprolinemia alters glutamatergic homeostasis in rat brain: neuroprotector effect of guanosine

This study investigated the effects of acute and chronic hyperprolinemia on glutamate uptake, as well as some mechanisms underlying the proline effects on glutamatergic system in rat cerebral cortex. The protective role of guanosine on effects mediated by proline was also evaluated. Results showed that acute and chronic hyperprolinemia reduced glutamate uptake, Na(+), K(+)-ATPase activity, ATP levels and increased lipoperoxidation. GLAST and GLT-1 immunocontent were increased in acute, but not in chronic hyperprolinemic rats. Our data suggest that the effects of proline on glutamate uptake may be mediated by lipid peroxidation and disruption of Na(+), K(+)-ATPase activity, but not by decreasing in glutamate transporters. This probably induces excitotoxicity and subsequent energy deficit. Guanosine was effective to prevent most of the effects promoted by proline, reinforcing its modulator role in counteracting the glutamate toxicity. However, further studies are needed to assess the modulatory effects of guanosine on experimental hyperprolinemia.

Methylmercury-induced alterations in astrocyte functions are attenuated by ebselen

Methylmercury (MeHg) preferentially accumulates in glia of the central nervous system (CNS), but its toxic mechanisms have yet to be fully recognized. In the present study, we tested the hypothesis that MeHg induces neurotoxicity via oxidative stress mechanisms, and that these effects are attenuated by the antioxidant, ebselen. Rat neonatal primary cortical astrocytes were pretreated with or without 10 μM ebselen for 2h followed by MeHg (0, 1, 5, and 10 μM) treatments. MeHg-induced changes in astrocytic [(3)H]-glutamine uptake were assessed along with changes in mitochondrial membrane potential (ΔΨ(m)), using the potentiometric dye tetramethylrhodamine ethyl ester (TMRE). Western blot analysis was used to detect MeHg-induced ERK (extracellular-signal related kinase) phosphorylation and caspase-3 activation. MeHg treatment significantly decreased (p<0.05) astrocytic [(3)H]-glutamine uptake at all time points and concentrations. Ebselen fully reversed MeHg's (1 μM) effect on [(3)H]-glutamine uptake at 1 min. At higher MeHg concentrations, ebselen partially reversed the MeHg-induced astrocytic inhibition of [(3)H]-glutamine uptake [at 1 min (5 and 10 μM) (p<0.05); 5 min (1, 5 and 10 μM) (p<0.05)]. MeHg treatment (1h) significantly (p<0.05) dissipated the ΔΨ(m) in astrocytes as evidenced by a decrease in mitochondrial TMRE fluorescence. Ebselen fully reversed the effect of 1 μM MeHg treatment for 1h on astrocytic ΔΨ(m) and partially reversed the effect of 5 and 10 μM MeHg treatments for 1h on ΔΨ(m). In addition, ebselen inhibited MeHg-induced phosphorylation of ERK (p<0.05) and blocked MeHg-induced activation of caspase-3 (p<0.05-0.01). These results are consistent with the hypothesis that MeHg exerts its toxic effects via oxidative stress and that the phosphorylation of ERK and the dissipation of the astrocytic mitochondrial membrane potential are involved in MeHg toxicity. In addition, the protective effects elicited by ebselen reinforce the idea that organic selenocompounds represent promising strategies to counteract MeHg-induced neurotoxicity.

Brain as a critical target of mercury in environmentally exposed fish (Dicentrarchus labrax)--bioaccumulation and oxidative stress profiles

Although mercury is recognized as a potent neurotoxicant, information regarding its threat to fish brain and underlying mechanisms is still scarce. In accordance, the objective of this work was to assess vulnerability of fish to mercury neurotoxicity by evaluating brain pro-oxidant status in wild European sea bass (Dicentrarchus labrax) captured in an estuarine area affected by chlor-alkali industry discharges (Laranjo Basin, Ria de Aveiro, Portugal). To achieve this goal, brain antioxidant responses such as catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR), glutathione S-transferase (GST) activities and total glutathione (GSHt) content were measured. Additionally, damage was determined as lipid peroxidation. To ascertain the influence of seasonal variables on both mercury accumulation and oxidative stress profiles, surveys were conducted in contrasting conditions-warm and cold periods. In the warm period, brain of fish from mercury contaminated sites exhibited ambivalent antioxidant responses, viz. higher GR activity and lower CAT activity regarded, respectively, as possible signs of protective adaptation and increased susceptibility to oxidative stress challenge. Though the risk of an overwhelming ROS production cannot be excluded, brain appeared to possess compensatory mechanisms and was able to avoid lipid peroxidative damage. The warm period was the most critical for the appearance of oxidative damage as no inter-site alterations on oxidative stress endpoints were detected in the cold period. Since seasonal differences were found in oxidative stress responses and not in mercury bioaccumulation, environmental factors affected the former more than the latter. This work increases the knowledge on mercury neurotoxicity in feral fish, highlighting that the definition of critical tissue concentrations depends on environmental variables.

Modulation of methylmercury uptake by methionine: prevention of mitochondrial dysfunction in rat liver slices by a mimicry mechanism

Methylmercury (MeHg) is an ubiquitous environmental pollutant which is transported into the mammalian cells when present as the methylmercury-cysteine conjugate (MeHg-Cys). With special emphasis on hepatic cells, due to their particular propensity to accumulate an appreciable amount of Hg after exposure to MeHg, this study was performed to evaluate the effects of methionine (Met) on Hg uptake, reactive species (RS) formation, oxygen consumption and mitochondrial function/cellular viability in both liver slices and mitochondria isolated from these slices, after exposure to MeHg or the MeHg-Cys complex. The liver slices were pre-treated with Met (250 μM) 15 min before being exposed to MeHg (25 μM) or MeHg-Cys (25 μM each) for 30 min at 37 °C. The treatment with MeHg caused a significant increase in the Hg concentration in both liver slices and mitochondria isolated from liver slices. Moreover, the Hg uptake was higher in the group exposed to the MeHg-Cys complex. In the DCF (dichlorofluorescein) assay, the exposure to MeHg and MeHg-Cys produced a significant increase in DFC reactive species (DFC-RS) formation only in the mitochondria isolated from liver slices. As observed with Hg uptake, DFC-RS levels were significantly higher in the mitochondria treated with the MeHg-Cys complex compared to MeHg alone. MeHg exposure also caused a marked decrease in the oxygen consumption of liver slices when compared to the control group, and this effect was more pronounced in the liver slices treated with the MeHg-Cys complex. Similarly, the loss of mitochondrial activity/cell viability was greater in liver slices exposed to the MeHg-Cys complex when compared to slices treated only with MeHg. In all studied parameters, Met pre-treatment was effective in preventing the MeHg- and/or MeHg-Cys-induced toxicity in both liver slices and mitochondria. Part of the protection afforded by Met against MeHg may be related to a direct interaction with MeHg or to the competition of Met with the complex formed between MeHg and endogenous cysteine. In summary, our results show that Met pre-treatment produces pronounced protection against the toxic effects induced by MeHg and/or the MeHg-Cys complex on mitochondrial function and cell viability. Consequently, this amino acid offers considerable promise as a potential agent for treating acute MeHg exposure.

DietaryRRR-α-tocopherol succinate attenuates lipopolysaccharide-induced inflammatory cytokines secretion in broiler chicks

The anti-inflammatory effects of two esters of α-tocopherol (α-TOH),all-rac-α-TOH acetate (dl-α-TOA) andRRR-α-TOH succinate (d-α-TOS), on broilers repeatedly challenged with lipopolysaccharide (LPS) were investigated. Three hundred and twenty 1-d-old broiler chicks were allotted into four treatment groups and fed on a control diet (30 mg/kgdl-α-TOA) or diets containing 10, 30, 50 mg/kgd-α-TOS. Half of the birds from each treatment group were challenged with 0·9 % NaCl solution or LPS (250 μg/kg body weight) at 16, 18 and 20 d of age. The results indicated that the pretreatment of birds with 50 mg/kgd-α-TOS markedly reduced serum PGE2secretion and increased the concentrations of serum or hepatic α-TOH. When LPS-challenged birds were pretreated with 30 or 50 mg/kgd-α-TOS, the increases of plasma and splenic concentrations of interferon-γ, IL-1β, IL-2, IL-6, IL-4 and IL-10 were dramatically attenuated. Also, a significant decrease of hepatic reactive oxygen species (ROS) and hepatic or splenic phosphokinase C (PKC) activities was found in birds pretreated with 30 or 50 mg/kgd-α-TOS. Furthermore,d-α-TOS inhibited the activation of NF-κB by preventing the degradation of inhibitory-κBα. In conclusion, D-α-TOS is able to prevent LPS-induced inflammation responsein vivo.The beneficial effect may depend on suppressing the secretion of various plasma and splenic inflammatory mediators through inhibiting NF-κB activation and by blocking ROS signalling, in which PKC may play an assistant role.