Methylmercury in northern pike (Esox lucius) liver and hepatic mitochondria is linked to lipid peroxidation

Methylmercury (MeHg) readily bioaccumulates and biomagnifies in aquatic food webs leading to elevated concentrations in fish and may thus induce toxicity. Oxidative stress is a suggested effect of MeHg bioaccumulation in fish. However, studies on how MeHg triggers oxidative stress in wild fish are scarce. The purpose of this study was to link the subcellular distribution of MeHg in the liver of northern pike from the St. Maurice River (Québec, Canada), affected by two run-of-river (RoR) dams, artificial wetlands, forest fires, and logging activity, to lipid peroxidation as an indicator of oxidative stress. We also evaluated the protective effects of the glutathione (GSH) system and selenium (Se), as they are known to alleviate MeHg toxicity. A customized subcellular partitioning protocol was used to separate the liver into metal-sensitive (mitochondria, microsome/lysosome and HDP - heat-denatured proteins) and metal-detoxified fractions (metal-rich granules and HSP - heat-stable proteins). We examined the relation among THg, MeHg, and Se concentration in livers and subcellular fractions, and the hepatic ratio of total GSH (GSHt) to oxidized glutathione (GSSG) on lipid peroxidation levels, using the concentrations of malondialdehyde (MDA), a product of lipid peroxidation. Results showed that hepatic MDA concentration was positively correlated with the combined MeHg and Se concentrations in northern pike liver (r2 = 0.88, p < 0.001) and that MDA concentrations were best predicted by MeHg associated with the mitochondria (r2 = 0.71, p < 0.001). This highlights the need for additional research on the MeHg influence on fish health and the interactions between Hg and Se in northern pike.

Selenization of Small Molecule Drugs: A New Player on the Board

There is an urgent need to develop safer and more effective modalities for the treatment of a wide range of pathologies due to the increasing rates of drug resistance, undesired side effects, poor clinical outcomes, etc. Throughout the years, selenium (Se) has attracted a great deal of attention due to its important role in human health. Besides, a growing body of work has unveiled that the inclusion of Se motifs into a great number of molecules is a promising strategy for obtaining novel therapeutic agents. In the current Perspective, we have gathered the most recent literature related to the incorporation of different Se moieties into the scaffolds of a wide range of known drugs and their feasible pharmaceutical applications. In addition, we highlight different representative examples as well as provide our perspective on Se drugs and the possible future directions, promises, opportunities, and challenges of this ground-breaking area of research.

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+.

Selenium Status: Its Interactions with Dietary Mercury Exposure and Implications in Human Health

Selenium is an essential trace element in humans and animals and its role in selenoprotein and enzyme antioxidant activity is well documented. Food is the principal source of selenium, and it is important that selenium status in the body is adequately maintained for physiological functions. There has been increasing attention on the role of selenium in mitigating the toxic effects of mercury exposure from dietary intake in humans. In contrast, mercury is a neurotoxin, and its continuous exposure can cause adverse health effects in humans. The interactions of selenium and mercury are multi-factorial and involve complex binding mechanisms between these elements at a molecular level. Further insights and understanding in this area may help to evaluate the health implications of dietary mercury exposure and selenium status. This review aims to summarise current information on the interplay of the interactions between selenium and mercury in the body and the protective effect of selenium on at-risk groups in a population who may experience long-term mercury exposure.

Cellular Conditions Responsible for Methylmercury-Mediated Neurotoxicity

Methylmercury (MeHg) is a widely known environmental pollutant that causes severe neurotoxicity. MeHg-induced neurotoxicity depends on various cellular conditions, including differences in the characteristics of tissues and cells, exposure age (fetal, childhood, or adulthood), and exposure levels. Research has highlighted the importance of oxidative stress in the pathogenesis of MeHg-induced toxicity and the site- and cell-specific nature of MeHg-induced neurotoxicity. The cerebellar granule cells and deeper layer cerebrocortical neurons are vulnerable to MeHg. In contrast, the hippocampal neurons are resistant to MeHg, even at high mercury accumulation levels. This review summarizes the mechanisms underlying MeHg-mediated intracellular events that lead to site-specific neurotoxicity. Specifically, we discuss the mechanisms associated with the redox ability, neural outgrowth and synapse formation, cellular signaling pathways, epigenetics, and the inflammatory conditions of microglia.

A Novel Diselenide-Probucol-Analogue Protects Against Methylmercury-Induced Toxicity in HT22 Cells by Upregulating Peroxide Detoxification Systems: a Comparison with Diphenyl Diselenide

Methylmercury (MeHg) is a ubiquitous environmental neurotoxicant whose mechanisms of action involve oxidation of endogenous nucleophilic groups (mainly thiols and selenols), depletion of antioxidant defenses, and disruption of neurotransmitter homeostasis. Diphenyl diselenide-(PhSe)₂-a model diaryl diselenide, has been reported to display significant protective effects against MeHg-induced neurotoxicity under both in vitro and in vivo experimental conditions. In this study, we compared the protective effects of (PhSe)₂ with those of RC513 (4,4'-diselanediylbis(2,6-di-tert-butylphenol), a novel diselenide-probucol-analog) against MeHg-induced toxicity in the neuronal (hippocampal) cell line HT22. Although both (PhSe)₂ and RC513 significantly mitigated MeHg- and tert-butylhydroperoxide (t-BuOOH)-cytotoxicity, the probucol analog exhibited superior protective effects, which were observed earlier and at lower concentrations compared to (PhSe)₂. RC513 treatment (at either 0.5 µM or 2 µM) significantly increased glutathione peroxidase (GPx) activity, which has been reported to counteract MeHg-toxicity. (PhSe)₂ was also able to increase GPx activity, but only at 2 µM. Although both compounds increased the Gpx1 transcripts at 6 h after treatments, only RC513 was able to increase mRNA levels of Prx2, Prx3, Prx5, and Txn2, which are also involved in peroxide detoxification. RC513 (at 2 µM) significantly increased GPx-1 protein expression in HT22 cells, although (PhSe)₂ displayed a minor (nonsignificant) effect in this parameter. In agreement, RC513 induced a faster and superior capability to cope with exogenously-added peroxide (t-BuOOH). In summary, when compared to the prototypical organic diaryl diselenide [(PhSe)₂], RC513 displayed superior protective properties against MeHg-toxicity in vitro; this was paralleled by a more pronounced upregulation of defenses related to detoxification of peroxides, which are well-known MeHg-derived intermediate oxidant species.

3,3'-Diselenodipropionic acid (DSePA) forms 1:1 complex with Hg (II) and prevents oxidative stress in cultured cells and mice model

Mercury is one of the most toxic heavy metal for mammals particularly in inorganic form. In present study, 3,3'-diselenodipropionic acid (DSePA), a well-known pharmacological diselenide was evaluated for its interaction with HgCl₂ and ability to prevent HgCl₂-induced toxicity in experimental cellular and mice models. UV-visible, stopped flow, Fourier-transform infrared spectroscopy and ¹H nuclear magnetic resonance spectroscopy studies confirmed that DSePA sequestered Hg (II) ions with stoichiometry of 1:1 and binding constant of ~10⁴ M^-1. X-ray photoelectron spectroscopy and X-ray powder diffraction analysis suggested that diselenide group of DSePA was involved in the complexation with Hg (II) ions. Further, Hg-DSePA complex degraded within 10 days to form excretable HgSe. The binding constant of DSePA and Hg (II) was comparable with that of dihydrolipoic acid, a standard disulfide compound used in heavy metal detoxification. Corroborating these observations, pre-treatment of DSePA (10 μM) significantly prevented the HgCl₂ (50 μM)-induced glutathione oxidation (GSH/GSSG), decrease of thioredoxin reductase (TrxR) and glutathione peroxidase (GPx) activities and cell death in Chinese Hamster Ovary (CHO) cells. Similarly, intraperitoneal administration of DSePA at a dosage of 2 mg/kg for 5 consecutive days prior to exposure of HgCl₂ (1 mg/kg) significantly suppressed oxidative stress in renal and hepatic tissues of C57BL/6 mice. In conclusion, the protective effect of DSePA against Hg induced oxidative stress is attributed to its ability to rescue the activities of GPx, TrxR and GSH by sequestering Hg (II) ions. DSePA being a relatively safer selenium-compound for in vivo administration can be explored for mercury detoxification.

Ebselen, a multi-target compound: its effects on biological processes and diseases

Ebselen is a well-known synthetic compound mimicking glutathione peroxidase (GPx), which catalyses some vital reactions that protect against oxidative damage. Based on a large number of in vivo and in vitro studies, various mechanisms have been proposed to explain its actions on multiple targets. It targets thiol-related compounds, including cysteine, glutathione, and thiol proteins (e.g., thioredoxin and thioredoxin reductase). Owing to this, ebselen is a unique multifunctional agent with important effects on inflammation, apoptosis, oxidative stress, cell differentiation, immune regulation and neurodegenerative disease, with anti-microbial, detoxifying and anti-tumour activity. This review summarises the current understanding of the multiple biological processes and molecules targeted by ebselen, and its pharmacological applications.

Twists and turns of an oyster's life: effects of different depuration periods on physiological biochemical functions of oysters

Aquaculture activities are often established in the vicinity of highly populated, potentially contaminated areas. Animals cultured at such locations, namely bivalves, are frequently used as test organisms in ecotoxicological testing. In this case, a period of depuration is required to allow the normalization of physiological processes, which are likely to be altered after exposure to a multiplicity of waterborne contaminants occurring in the wild. One of the most important species in modern marine aquaculture is the oyster species Crassostrea gigas. The aim of this study was to assess if the current depuration time frame of 24 h (adopted by most aquaculture facilities), is long enough to permit oysters to revert potential toxic effects exerted by environmental contaminants, allowing their use in laboratory-based ecotoxicological studies. The selected approach involved the monitoring of biochemical (antioxidant defence, oxidative damage, phase II metabolism, and neurological homeostasis) and physiological (condition index) parameters, along a period of 42 days. The obtained results showed that a period of 24 h does not revert any of the potential toxic effects caused by environmental contaminants to which animals may have been previously subjected; even a period of 42 days was not long enough for the oysters to completely normalize the levels of their antioxidant defences, namely total GPx activity, which increased over time. Lipid peroxidation was also increased during the depuration period, and the activity of the metabolic isoenzymes GSTs was significantly decreased. Furthermore, AChE activity measured in the adductor muscle of oysters was increased over time. These assumptions suggest that a period of depuration longer than 24 h is mandatory to obtain adequate test organisms of this oyster species, to be used for ecotoxicological testing purposes.

(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.

Selenium modulates inorganic mercury induced cytotoxicity and intrinsic apoptosis in PC12 cells

Mercury (Hg) in its all forms, including inorganic Hg (iHg) is an environmental contaminant due to toxicity and diseases in human. However, a little is known about the underlying mechanisms responsible for iHg toxicity. Selenium (Se) is an essential trace element, recognized as an antioxidant and protective agent against metal toxicities. The purpose of this research was to investigate ameliorations of Se counter to iHg-mediated toxicity in PC12 cells. Cytotoxic assays have been shown that iHg (5 μM) caused oxidative stress and intrinsic apoptosis via ROS generation, oxidizing glutathione, damaging DNA, degrading cell membrane integrity, down-regulating mTOR, p-mTOR, akt and ERK1, and up-regulating cleaved caspase 3 and cytochrome c release in PC12 cells 48 h after incubation. Co-treatment of Se (5 μM) inhibited intrinsic apoptosis and oxidative stress induced by iHg (5 μM) via inhibiting ROS formation, boosting GPx contents, increasing reduced glutathione, limiting DNA degradation, improving cell membrane integrity, up-regulating mTOR, p-mTOR, akt, ERK1 and caspase 3, and down-regulating cleaved caspase 3 and cytochrome c leakage in PC12 cells. In conclusion, these results recommended that excessive ROS generation acts a critical role in iHg-influenced oxidative stress and co-treatment of Se attenuates iHg-cytotoxicity through its antioxidant properties.

Mechanisms of ebselen as a therapeutic and its pharmacology applications

Ebselen is a synthetic organoselenium radical scavenger compound that possesses glutathione peroxidase-like activity and its own unique bioactivity by reacting with thiols, hydroperoxides and peroxynitrites. Owing to its high affinity toward several essential reactions, ebselen protects cellular components from oxidative and free radical damage, and it has been employed as a useful tool for studying redox-related mechanisms. Based on numerous in vitro and in vivo research, mechanisms are proposed to understand the biomedical and molecular actions of ebselen in health and disease, and it is currently under clinical trials for the prevention and treatment of various human disorders. Based on these outstanding discoveries, this review summarizes the current understanding of the biochemical and molecular characteristics, pharmacological applications and future directions of ebselen.

Depletion of multidrug-resistant uropathogenic Escherichia coli BC1 by ebselen and silver ion

Ebselen, an organo‐selenium compound with well‐characterized toxicology and pharmacology, recently exhibited potent antibacterial activity against glutathione (GSH)‐negative bacteria by disrupting redox homeostasis. In this paper, we show that ebselen and silver ion in combination exert strong bactericidal activity against multidrug‐resistant (MDR) uropathogenic Escherichia coli (UPEC) BC1, a model MDR GSH‐positive bacterium. The mechanisms were found to involve consumption of total intracellular GSH and inhibition of thioredoxin reductase activity, which was highly related to reactive oxygen species up‐regulation. Furthermore, the therapeutic efficacy of ebselen and silver ion against UPEC‐induced cystitis was assessed in a mouse model. Treatment with ebselen and silver ion significantly reduced bacterial loads, down‐regulated the expression levels of tumour necrosis factor‐α (TNF‐α) and interferon‐γ (IFN‐γ) on‐site and decreased white/red blood cell counts in mild cystitis model mice, which demonstrated the anti‐inflammatory property of these agents. In addition, ebselen and silver ion also exhibited significantly high protective ability (100%) against acute cystitis infections. These results together may lay the foundation for further analysis and development of ebselen and silver ion as antibacterial agents for treatment of MDR UPEC infections.

Sulfhydryl groups as targets of mercury toxicity

The present study addresses existing data on the affinity and conjugation of sulfhydryl (thiol; -SH) groups of low- and high-molecular-weight biological ligands with mercury (Hg). The consequences of these interactions with special emphasis on pathways of Hg toxicity are highlighted. Cysteine (Cys) is considered the primary target of Hg, and link its sensitivity with thiol groups and cellular damage. In vivo, Hg complexes play a key role in Hg metabolism. Due to the increased affinity of Hg to SH groups in Cys residues, glutathione (GSH) is reactive. The geometry of Hg(II) glutathionates is less understood than that with Cys. Both Cys and GSH Hg-conjugates are important in Hg transport. The binding of Hg to Cys mediates multiple toxic effects of Hg, especially inhibitory effects on enzymes and other proteins that contain free Cys residues. In blood plasma, albumin is the main Hg-binding (Hg2+, CH3Hg+, C2H5Hg+, C6H5Hg+) protein. At the Cys34 residue, Hg2+ binds to albumin, whereas other metals likely are bound at the N-terminal site and multi-metal binding sites. In addition to albumin, Hg binds to multiple Cys-containing enzymes (including manganese-superoxide dismutase (Mn-SOD), arginase I, sorbitol dehydrogenase, and δ-aminolevulinate dehydratase, etc.) involved in multiple processes. The affinity of Hg for thiol groups may also underlie the pathways of Hg toxicity. In particular, Hg-SH may contribute to apoptosis modulation by interfering with Akt/CREB, Keap1/Nrf2, NF-κB, and mitochondrial pathways. Mercury-induced oxidative stress may ensue from Cys-Hg binding and inhibition of Mn-SOD (Cys196), thioredoxin reductase (TrxR) (Cys497) activity, as well as limiting GSH (GS-HgCH3) and Trx (Cys32, 35, 62, 65, 73) availability. Moreover, Hg-thiol interaction also is crucial in the neurotoxicity of Hg by modulating the cytoskeleton and neuronal receptors, to name a few. However, existing data on the role of Hg-SH binding in the Hg toxicity remains poorly defined. Therefore, more research is needed to understand better the role of Hg-thiol binding in the molecular pathways of Hg toxicology and the critical role of thiols to counteract negative effects of Hg overload.

Vascular Dysfunction Induced by Mercury Exposure

Methylmercury (MeHg) causes severe damage to the central nervous system, and there is increasing evidence of the association between MeHg exposure and vascular dysfunction, hemorrhage, and edema in the brain, but not in other organs of patients with acute MeHg intoxication. These observations suggest that MeHg possibly causes blood-brain barrier (BBB) damage. MeHg penetrates the BBB into the brain parenchyma via active transport systems, mainly the l-type amino acid transporter 1, on endothelial cell membranes. Recently, exposure to mercury has significantly increased. Numerous reports suggest that long-term low-level MeHg exposure can impair endothelial function and increase the risks of cardiovascular disease. The most widely reported mechanism of MeHg toxicity is oxidative stress and related pathways, such as neuroinflammation. BBB dysfunction has been suggested by both in vitro and in vivo models of MeHg intoxication. Therapy targeted at both maintaining the BBB and suppressing oxidative stress may represent a promising therapeutic strategy for MeHg intoxication. This paper reviews studies on the relationship between MeHg exposure and vascular dysfunction, with a special emphasis on the BBB.

Selenium and zinc protections against metal-(loids)-induced toxicity and disease manifestations: A review

Metals are ubiquitous in the environment due to huge industrial applications in the form of different chemicals and from extensive mining activities. The frequent exposures to metals and metalloids are crucial for the human health. Trace metals are beneficial for health whereas non-essential metals are dangerous for the health and some are proven etiological factors for diseases including cancers and neurological disorders. The interactions of essential trace metals such as selenium (Se) and zinc (Zn) with non-essential metals viz. lead (Pb), cadmium (Cd), arsenic (As), and mercury (Hg) in biological system are very critical and complex. A huge number of studies report the protective role of Se and Zn against metal toxicity, both in animal and cellular levels, and also explain the numerous mechanisms involved. However, it has been considered that a tiny dyshomeostasis in the metals/trace metals status in biological system could induce severe deleterious effects that can manifest to numerous diseases. Thus, in this particular review, we have demonstrated the critical protection mechanism/s of Se and Zn against Cd, Pb, As and Hg toxicity in a one by one manner to clarify the up-to-date findings and perspectives. Furthermore, biomolecular consequences are comprehensively presented in light of particular cellular/biomolecular events which are somehow linked to a subsequent disease. The analyzed reports support significant protection potential of Se and Zn, either alone or in combination with other agents, against each of the abovementioned non-essential metals. However, Se and Zn are still not being used as detoxifying agents due to some unexplained reasons. We hypothesized that Se could be a potential candidate for detoxifying As and Hg regardless of their chemical speciations, but requires intensive clinical trials. However, particularly Zn-Hg interaction warrants more investigations both in animal and cellular level.

Methylmercury's chemistry: From the environment to the mammalian brain

Methylmercury is a neurotoxicant that is found in fish and rice. MeHg's toxicity is mediated by blockage of -SH and -SeH groups of proteins. However, the identification of MeHg's targets is elusive. Here we focus on the chemistry of MeHg in the abiotic and biotic environment. The toxicological chemistry of MeHg is complex in metazoans, but at the atomic level it can be explained by exchange reactions of MeHg bound to -S(e)H with another free -S(e)H group (R¹S(e)-HgMe + R²-S(e)H ↔ R¹S(e)H + R²-S(e)-HgMe). This reaction was first studied by professor Rabenstein and here it is referred as the "Rabenstein's Reaction". The absorption, distribution, and excretion of MeHg in the environment and in the body of animals will be dictated by Rabenstein's reactions. The affinity of MeHg by thiol and selenol groups and the exchange of MeHg by Rabenstein's Reaction (which is a diffusion controlled reaction) dictates MeHg's neurotoxicity. However, it is important to emphasize that the MeHg exchange reaction velocity with different types of thiol- and selenol-containing proteins will also depend on protein-specific structural and thermodynamical factors. New experimental approaches and detailed studies about the Rabenstein's reaction between MeHg with low molecular mass thiol (LMM-SH) molecules (cysteine, GSH, acetyl-CoA, lipoate, homocysteine) with abundant high molecular mass thiol (HMM-SH) molecules (albumin, hemoglobin) and HMM-SeH (GPxs, Selenoprotein P, TrxR1-3) are needed. The study of MeHg migration from -S(e)-Hg- bonds to free -S(e)H groups (Rabenstein's Reaction) in pure chemical systems and neural cells (with special emphasis to the LMM-SH and HMM-S(e)H molecules cited above) will be critical to developing realistic constants to be used in silico models that will predict the distribution of MeHg in humans.

Glutathione antioxidant system and methylmercury-induced neurotoxicity: An intriguing interplay

Methylmercury (MeHg) is a toxic chemical compound naturally produced mainly in the aquatic environment through the methylation of inorganic mercury catalyzed by aquatic microorganisms. MeHg is biomagnified in the aquatic food chain and, consequently, piscivorous fish at the top of the food chain possess huge amounts of MeHg (at the ppm level). Some populations that have fish as main protein's source can be exposed to exceedingly high levels of MeHg and develop signs of toxicity. MeHg is toxic to several organs, but the central nervous system (CNS) represents a preferential target, especially during development (prenatal and early postnatal periods). Though the biochemical events involved in MeHg-(neuro)toxicity are not yet entirely comprehended, a vast literature indicates that its pro-oxidative properties explain, at least partially, several of its neurotoxic effects. As result of its electrophilicity, MeHg interacts with (and oxidize) nucleophilic groups, such as thiols and selenols, present in proteins or low-molecular weight molecules. It is noteworthy that such interactions modify the redox state of these groups and, therefore, lead to oxidative stress and impaired function of several molecules, culminating in neurotoxicity. Among these molecules, glutathione (GSH; a major thiol antioxidant) and thiol- or selenol-containing enzymes belonging to the GSH antioxidant system represent key molecular targets involved in MeHg-neurotoxicity. In this review, we firstly present a general overview concerning the neurotoxicity of MeHg. Then, we present fundamental aspects of the GSH-antioxidant system, as well as the effects of MeHg on this system.

Diphenyl diselenide protects neuronal cells against oxidative stress and mitochondrial dysfunction: Involvement of the glutathione-dependent antioxidant system

Oxidative stress and mitochondrial dysfunction are critical events in neurodegenerative diseases; therefore, molecules that increase cellular antioxidant defenses represent a future pharmacologic strategy to counteract such conditions. The aim of this study was to investigate the potential protective effect of (PhSe)2 on mouse hippocampal cell line (HT22) exposed to tert-BuOOH (in vitro model of oxidative stress), as well as to elucidate potential mechanisms underlying this protection. Our results showed that tert-BuOOH caused time- and concentration-dependent cytotoxicity, which was preceded by increased oxidants production and mitochondrial dysfunction. (PhSe)2 pre-incubation significantly prevented these cytotoxic events and the observed protective effects were paralleled by the upregulation of the cellular glutathione-dependent antioxidant system: (PhSe)2 increased GSH levels (> 60%), GPx activity (6.9-fold) and the mRNA expression of antioxidant enzymes Gpx1 (3.9-fold) and Gclc (2.3-fold). Of note, the cytoprotective effect of (PhSe)2 was significantly decreased when cells were treated with mercaptosuccinic acid, an inhibitor of GPx, indicating the involvement of GPx modulation in the observed protective effect. In summary, the present findings bring out a new action mechanism concerning the antioxidant properties of (PhSe)2. The observed upregulation of the glutathione-dependent antioxidant system represents a future pharmacologic possibility that goes beyond the well-known thiol-peroxidase activity of this compound.

Post-translational modifications in MeHg-induced neurotoxicity

Mercury (Hg) exposure remains a major public health concern due to its widespread distribution in the environment. Organic mercurials, such as MeHg, have been extensively investigated especially because of their congenital effects. In this context, studies on the molecular mechanism of MeHg-induced neurotoxicity are pivotal to the understanding of its toxic effects and the development of preventive measures. Post-translational modifications (PTMs) of proteins, such as phosphorylation, ubiquitination, and acetylation are essential for the proper function of proteins and play important roles in the regulation of cellular homeostasis. The rapid and transient nature of many PTMs allows efficient signal transduction in response to stress. This review summarizes the current knowledge of PTMs in MeHg-induced neurotoxicity, including the most commonly PTMs, as well as PTMs induced by oxidative stress and PTMs of antioxidant proteins. Though PTMs represent an important molecular mechanism for maintaining cellular homeostasis and are involved in the neurotoxic effects of MeHg, we are far from understanding the complete picture on their role, and further research is warranted to increase our knowledge of PTMs in MeHg-induced neurotoxicity.

Diphenyl diselenide abrogates brain oxidative injury and neurobehavioural deficits associated with pesticide chlorpyrifos exposure in rats

Exposure to pesticide chlorpyrifos (CPF) is associated with neurodevelopmental toxicity both in humans and animals. Diphenyl diselenide (DPDS) is a simple synthetic organoselenium well reported to possess antioxidant, anti-inflammatory and neuroprotective effects. However, there is paucity of information on the beneficial effects of DPDS on CPF-mediated brain injury and neurobehavioural deficits. The present study investigated the neuroprotective mechanism of DPDS in rats sub-chronically treated with CPF alone at 5 mg/kg body weight or orally co-treated with DPDS at 2.5 and 5 mg/kg body weight for 35 consecutive days. Endpoint analyses using video-tracking software in a novel environment revealed that co-treatment with DPDS significantly (p < 0.05) protected against CPF-mediated locomotor and motor deficits precisely the decrease in maximum speed, total distance travelled, body rotation, absolute turn angle, forelimb grip strength as well as the increase in negative geotaxis and incidence of fecal pellets. The enhancement in the neurobehavioral activities of rats co-treated with DPDS was verified by track plot analyses. Besides, DPDS assuaged CPF-induced decrease in acetylcholinesterase and antioxidant enzymes activities and the increase in myeloperoxidase activity and lipid peroxidation level in the mid-brain, cerebral cortex and cerebellum of the rats. Histologically, DPDS co-treatment abrogated CPF-mediated neuronal degeneration in the cerebral cortex, dentate gyrus and cornu ammonis3 in the treated rats. In conclusion, the neuroprotective mechanisms of DPDS is related to the prevention of oxidative stress, enhancement of redox status and acetylcholinesterase activity in brain regions of the rats. DPDS may be a promising chemotherapeutic agent against brain injury resulting from CPF exposure.

The cytoplasmic thioredoxin system in Caenorhabditis elegans affords protection from methylmercury in an age-specific manner

Methylmercury (MeHg) is an environmental pollutant linked to many neurological defects, especially in developing individuals. The thioredoxin (TRX) system is a key redox regulator affected by MeHg toxicity, however the mechanisms and consequences of MeHg-induced dysfunction are not completely understood. This study evaluated the role of the TRX system in C. elegans susceptibility to MeHg during development. Worms lacking or overexpressing proteins from the TRX family were exposed to MeHg for 1 h at different developmental stage: L1, L4 and adult. Worms without cytoplasmic thioredoxin system exhibited age-specific susceptibility to MeHg when compared to wild-type (wt). This susceptibility corresponded partially to decreased total glutathione (GSH) levels and enhanced degeneration of dopaminergic neurons. In contrast, the overexpression of the cytoplasmic system TRX-1/TRXR-1 did not provide substantial protection against MeHg. Moreover, transgenic worms exhibited decreased protein expression for cytoplasmic thioredoxin reductase (TRXR-1). Both mitochondrial thioredoxin system TRX-2/TRXR-2, as well as other thioredoxin-like proteins: TRX-3, TRX-4, TRX-5 did not show significant role in C. elegans resistance to MeHg. Based on the current findings, the cytoplasmic thioredoxin system TRX-1/TRXR-1 emerges as an important age-sensitive protectant against MeHg toxicity in C. elegans.

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.

Neurotoxicity Linked to Dysfunctional Metal Ion Homeostasis and Xenobiotic Metal Exposure: Redox Signaling and Oxidative Stress

SIGNIFICANCE

Essential metals such as copper, iron, manganese, and zinc play a role as cofactors in the activity of a wide range of processes involved in cellular homeostasis and survival, as well as during organ and tissue development. Throughout our life span, humans are also exposed to xenobiotic metals from natural and anthropogenic sources, including aluminum, arsenic, cadmium, lead, and mercury. It is well recognized that alterations in the homeostasis of essential metals and an increased environmental/occupational exposure to xenobiotic metals are linked to several neurological disorders, including neurodegeneration and neurodevelopmental alterations. Recent Advances: The redox activity of essential metals is key for neuronal homeostasis and brain function. Alterations in redox homeostasis and signaling are central to the pathological consequences of dysfunctional metal ion homeostasis and increased exposure to xenobiotic metals. Both redox-active and redox-inactive metals trigger oxidative stress and damage in the central nervous system, and the exact mechanisms involved are starting to become delineated.

CRITICAL ISSUES

In this review, we aim to appraise the role of essential metals in determining the redox balance in the brain and the mechanisms by which alterations in the homeostasis of essential metals and exposure to xenobiotic metals disturb the cellular redox balance and signaling. We focus on recent literature regarding their transport, metabolism, and mechanisms of toxicity in neural systems.

FUTURE DIRECTIONS

Delineating the specific mechanisms by which metals alter redox homeostasis is key to understand the pathological processes that convey chronic neuronal dysfunction in neurodegenerative and neurodevelopmental disorders. Antioxid. Redox Signal. 28, 1669-1703.

Modulation of thiol-dependent redox system by metal ionsviathioredoxin and glutaredoxin systems

Metal and metalloid ions regulate thioredoxin and glutaredoxin system-mediated biological functions by targeting mammalian thioredoxin reductase and mitochondrial glutaredoxin 2&5.

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.