Pathways of cadmium influx in mammalian neurons

Cadmium inhibits calcium activity in hippocampal CA1 neurons of freely moving mice

Cadmium (Cd) is a ubiquitous toxic heavy metal and a potential neurotoxicant due to its wide use in industrial manufacturing processes and commercial products, including fertilizers. The general population is exposed to Cd through food and smoking due to high transfer rates of Cd from contaminated soil. Cd has been shown to mimic calcium ions (Ca2+) and interfere with intracellular Ca2+ levels and Ca2+ signaling in in vitro studies. However, nothing is known about Cd's effects on Ca2+ activity in neurons in live animals. This study aimed to determine if Cd disrupts Ca2+ transients of neurons in CA1 region of the hippocampus during an associative learning paradigm. We utilized in vivo Ca2+ imaging in awake, freely moving C57BL/6 mice to measure Ca2+ activity in CA1 excitatory neurons expressing genetically encoded Ca2+ sensor GCaMP6 during an associative learning paradigm. We found that a smaller proportion of neurons are activated in Cd-treated groups compared with control during fear conditioning, suggesting that Cd may contribute to learning and memory deficit by reducing the activity of neurons. We observed these effects at Cd exposure levels that result in blood Cd levels comparable with the general U.S. population levels. This provides a possible molecular mechanism for Cd interference of learning and memory at exposure levels relevant to U.S. adults. To our knowledge, our study is the first to describe Cd effects on brain Ca2+ activity in vivo in freely behaving mice. This study provides evidence for impairment of neuronal calcium activity in hippocampal CA1 excitatory neurons in freely moving mice following cadmium exposure.

Metals on the Menu-Analyzing the Presence, Importance, and Consequences

Metals are integral components of the natural environment, and their presence in the food supply is inevitable and complex. While essential metals such as sodium, potassium, magnesium, calcium, iron, zinc, and copper are crucial for various physiological functions and must be consumed through the diet, others, like lead, mercury, and cadmium, are toxic even at low concentrations and pose serious health risks. This study comprehensively analyzes the presence, importance, and consequences of metals in the food chain. We explore the pathways through which metals enter the food supply, their distribution across different food types, and the associated health implications. By examining current regulatory standards for maximum allowable levels of various metals, we highlight the importance of ensuring food safety and protecting public health. Furthermore, this research underscores the need for continuous monitoring and management of metal content in food, especially as global agricultural and food production practices evolve. Our findings aim to inform dietary recommendations, food fortification strategies, and regulatory policies, ultimately contributing to safer and more nutritionally balanced diets.

Environmental exposures to cadmium and lead as potential causes of eye diseases

Humans are exposed to cadmium and lead in various regions of the world daily due to industrial development and climate change. Increasing numbers of preclinical and clinical studies indicate that heavy metals, such as cadmium and lead, play a role in the pathogenesis of eye diseases. Excessive exposure to heavy metals such as cadmium and lead can increase the risk of impaired vision. Therefore, it is essential to better characterize the role of these non-essential metals in disease etiology and progression. This article discusses the potential role of cadmium and lead in the development of age-related eye diseases, including age-related macular degeneration, cataracts, and glaucoma. Furthermore, we discuss how cadmium and lead affect ocular cells and provide an overview of putative pathological mechanisms associated with their propensity to damage the eye.

Mechanisms of Cadmium Neurotoxicity

Cadmium is a heavy metal that increasingly contaminates food and drink products. Once ingested, cadmium exerts toxic effects that pose a significant threat to human health. The nervous system is particularly vulnerable to prolonged, low-dose cadmium exposure. This review article provides an overview of cadmium's primary mechanisms of neurotoxicity. Cadmium gains entry into the nervous system via zinc and calcium transporters, altering the homeostasis for these metal ions. Once within the nervous system, cadmium disrupts mitochondrial respiration by decreasing ATP synthesis and increasing the production of reactive oxygen species. Cadmium also impairs normal neurotransmission by increasing neurotransmitter release asynchronicity and disrupting neurotransmitter signaling proteins. Cadmium furthermore impairs the blood-brain barrier and alters the regulation of glycogen metabolism. Together, these mechanisms represent multiple sites of biochemical perturbation that result in cumulative nervous system damage which can increase the risk for neurological and neurodegenerative disorders. Understanding the way by which cadmium exerts its effects is critical for developing effective treatment and prevention strategies against cadmium-induced neurotoxic insult.

Cadmium Transport in Maize Root Segments Using a Classical Physiological Approach: Evidence of Influx Largely Exceeding Efflux in Subapical Regions

The bidirectional fluxes of cadmium and calcium across the plasma membrane were assessed and compared in subapical maize root segments. This homogeneous material provides a simplified system for investigating ion fluxes in whole organs. The kinetic profile of cadmium influx was characterized by a combination of a saturable rectangular hyperbola (Km = 30.15) and a straight line (k = 0.0013 L h-1 g-1 fresh weight), indicating the presence of multiple transport systems. In contrast, the influx of calcium was described by a simple Michaelis-Menten function (Km = 26.57 µM). The addition of calcium to the medium reduced cadmium influx into the root segments, suggesting a competition between the two ions for the same transport system(s). The efflux of calcium from the root segments was found to be significantly higher than that of cadmium, which was extremely low under the experimental conditions used. This was further confirmed by comparing cadmium and calcium fluxes across the plasma membrane of inside-out vesicles purified from maize root cortical cells. The inability of the root cortical cells to extrude cadmium may have driven the evolution of metal chelators for detoxifying intracellular cadmium ions.

The mechanism of the neuroprotective effect of zinc against cadmium-induced behavioral impairments in male Wister rats: Focus on tryptophan degradation pathway, oxidative-inflammatory stress, and histologic evidence

The present study investigated the attenuating effects of Zn following Cd-exposure in the activities/expression of indoleamine 2, 3-dioxygenase (IDO), tryptophan 2, 3-dioxygenase (TDO), oxidative-inflammatory response, behavioral indices and histologic architecture in cerebral cortex and hippocampus of male rats. Adult male Wistar rats were exposed to 200μg/L and 100μg/L of Cd and/or Zn in drinking water for 42 days. Cd exposure significantly increased IDO and TDO activities, IDO 1 protein expression, inflammatory response, with attendant disruption in antioxidant systems and concomitant elevation in malondialdehyde (MDA) levels in the cerebral cortex and hippocampus. Following Zn co-treatment, Cd-mediated increase in IDO 1 protein expression, IDO, and TDO activities, and decrease in antioxidant enzymes, and an increase in markers of inflammatory response and MDA production were significantly (p < 0.05) reversed compared with control. Moreover, altered behavioral indices and histological architecture of brain sections following Cd exposure was evidently (p < 0.05) prevented by Zn co-treatment relative to control. Overall, Cd-induced alterations in IDO 1 expression, IDO and TDO activities, oxidative-inflammatory response, behavioral indices, and histological architecture in the cerebral cortex and hippocampus of rats within the time course of the investigation were prevented by Zn co-treatment.

ICP-MS Multi-Elemental Analysis of the Human Meninges Collected from Sudden Death Victims in South-Eastern Poland

Metals perform many important physiological functions in the human body. The distribution of elements in different tissues is not uniform. Moreover, some structures can be the site of an accumulation of essential or toxic metals, leading to multi-directional intracellular damage. In the nervous system, these disorders are especially dangerous. Metals dyshomeostasis has been linked to a variety of neurological disorders which end up leading to permanent injuries. The multi-elemental composition of the human brain is still the subject of numerous investigations and debates. In this study, for the first time, the meninges, i.e., the dura mater and the arachnoid, were examined for their elemental composition by means of inductively coupled plasma mass spectrometry (ICP-MS). Tissue samples were collected post mortem from those who died suddenly as a result of suicide (n = 20) or as a result of injuries after an accident (n = 20). The interactions between 51 elements in both groups showed mainly weak positive correlations, which dominated the arachnoid mater compared to the dura mater. The study showed differences in the distribution of some elements within the meninges in the studied groups. The significant differences concerned mainly metals from the lanthanide family (Ln), macroelements (Na, K, Ca, Mg), a few micronutrients (Co), and toxic cadmium (Cd). The performed evaluation of the elemental distribution in the human meninges sheds new light on the trace metals metabolism in the central nervous system, although we do not yet fully understand the role of the human meninges.

Molecular Mechanisms of Environmental Metal Neurotoxicity: A Focus on the Interactions of Metals with Synapse Structure and Function

Environmental exposure to neurotoxic metals and metalloids such as arsenic, cadmium, lead, mercury, or manganese is a global health concern affecting millions of people worldwide. Depending on the period of exposure over a lifetime, environmental metals can alter neurodevelopment, neurobehavior, and cognition and cause neurodegeneration. There is increasing evidence linking environmental exposure to metal contaminants to the etiology of neurological diseases in early life (e.g., autism spectrum disorder) or late life (e.g., Alzheimer’s disease). The known main molecular mechanisms of metal-induced toxicity in cells are the generation of reactive oxygen species, the interaction with sulfhydryl chemical groups in proteins (e.g., cysteine), and the competition of toxic metals with binding sites of essential metals (e.g., Fe, Cu, Zn). In neurons, these molecular interactions can alter the functions of neurotransmitter receptors, the cytoskeleton and scaffolding synaptic proteins, thereby disrupting synaptic structure and function. Loss of synaptic connectivity may precede more drastic alterations such as neurodegeneration. In this article, we will review the molecular mechanisms of metal-induced synaptic neurotoxicity.

Superoxide dismutase 1 (SOD1) and cadmium: A three models approach to the comprehension of its neurotoxic effects

Cadmium (Cd) is a widespread toxic environmental contaminant, released by anthropogenic activities. It interferes with essential metal ions homeostasis and affects protein structures and functions by substituting zinc, copper and iron. In this study, the effect of cadmium on SOD1, a CuZn metalloenzyme catalyzing superoxide conversion into hydrogen peroxide, has been investigated in three different biological models. We first evaluated the effects of cadmium combined with copper and/or zinc on the recombinant GST-SOD1, expressed in E. coli BL21. The enzyme activity and expression were investigated in the presence of fixed copper and/or zinc doses with different cadmium concentrations, in the cellular medium. Cadmium caused a dose-dependent reduction in SOD1 activity, while the expression remains constant. Similar results were obtained in the cellular model represented by the human SH-SY5Y neuronal cell line. After cadmium treatment for 24 and 48 h, SOD1 enzymatic activity decreased in a dose- and time-dependent way, while the protein expression remained constant. Finally, a 16 h cadmium treatment caused a 25 % reduction of CuZn-SOD activity without affecting the protein expression in the Caenorhabditis elegans model. Taken together our results show an inhibitory effect of cadmium on SOD1 enzymatic activity, without affecting the protein expression, in all the biological models used, suggesting that cadmium can displace zinc from the enzyme catalytic site.

Using FluoZin-3 and fura-2 to monitor acute accumulation of free intracellular Cd2+ in a pancreatic beta cell line

The understanding of cellular Cd²⁺ accumulation and toxicity is hampered by a lack of fluorescent indicators selective for intracellular free Cd²⁺ ([Cd²⁺]_i). In this study, we used depolarized MIN6 mouse pancreatic beta cells as a model for evaluating [Cd²⁺]_i detection with commercially available fluorescent probes, most of which have been traditionally used to visualize [Ca²⁺]_i and [Zn²⁺]_i. We trialed a panel of 12 probes including fura-2, FluoZin-3, Leadmium Green, Rhod-5N, indo-1, Fluo-5N, and others. We found that the [Zn²⁺]_i probe FluoZin-3 and the traditional [Ca²⁺]_i probe fura-2 responded most consistently and robustly to [Cd²⁺]_i accumulation mediated by voltage-gated calcium channels. While selective detection of [Cd²⁺]_i by fura-2 required the omission of Ca²⁺ from extracellular buffers, FluoZin-3 responded to [Cd²⁺]_i similarly in the presence or absence of extracellular Ca²⁺. Furthermore, we showed that FluoZin-3 and fura-2 can be used together for simultaneous monitoring of [Ca²⁺]_i and [Cd²⁺]_i in the same cells. None of the other fluorophores tested were effective [Cd²⁺]_i detectors in this model.

Molecular Neuroprotection Induced by Zinc-Dependent Expression of Hepatitis C-Derived Protein NS5A Targeting Kv2.1 Potassium Channels

We present the design of an innovative molecular neuroprotective strategy and provide proof-of-concept for its implementation, relying on the injury-mediated activation of an ectopic gene construct. As oxidative injury leads to the intracellular liberation of zinc, we hypothesize that tapping onto the zinc-activated metal regulatory element (MRE) transcription factor 1 system to drive expression of the Kv2.1-targeted hepatitis C protein NS5A (hepatitis C nonstructural protein 5A) will provide neuroprotection by preventing cell death-enabling cellular potassium loss in rat cortical neurons in vitro. Indeed, using biochemical and morphologic assays, we demonstrate rapid expression of MRE-driven products in neurons. Further, we report that MRE-driven NS5A expression, induced by a slowly evolving excitotoxic stimulus, functionally blocks injurious, enhanced Kv2.1 potassium whole-cell currents and improves neuronal viability. We suggest this form of "on-demand" neuroprotection could provide the basis for a tenable therapeutic strategy to prevent neuronal cell death in neurodegeneration.

Selenium and zinc: Two key players against cadmium-induced neuronal toxicity

Cadmium (Cd), a worldwide occupational pollutant, is an extremely toxic heavy metal, capable of damaging several organs, including the brain. Its toxicity has been related to neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. The neurotoxic potential of Cd has been attributed to the changes induced in the brain enzyme network involved in counteracting oxidative stress. On the other hand, it is also known that trace elements, such as zinc (Zn) and selenium (Se), required for optimal brain functions, appears to have beneficial effects on the prevention of Cd intoxication. Based on this protective effect of Zn and Se, we aimed to investigate whether these elements could protect neuronal cells from Cd-induced excitotoxicity. The experiments, firstly carried out on SH-SY5Y catecholaminergic neuroblastoma cell line, demonstrated that the treatment with 10 μM cadmium chloride (CdCl₂) for 24 h caused significant modifications both in terms of oxidative stress and neuronal sprouting, triggered by endoplasmic reticulum (ER) stress. The evaluation of the effectiveness of 50 μM of zinc chloride (ZnCl₂) and 100 nM sodium selenite (Na₂SeO₃) treatments showed that both elements were able to attenuate the Cd-dependent neurotoxicity. However, considering that following induction with retinoic acid (RA), the neuroblastoma cell line undergoes differentiation into a cholinergic neurons, our second aim was to verify the zinc and selenium efficacy also in this neuronal phenotype. Our data clearly demonstrated that, while zinc played a crucial role on neuroprotection against Cd-induced neurotoxicity independently from the cellular phenotype, selenium is ineffective in differentiated cholinergic cells, supporting the notion that the molecular events occurring in differentiated SH-SY5Y cells are critical for the response to specific stimuli.

Cadmium-induced neurotoxicity: still much ado

Cadmium (Cd) is a highly toxic heavy metal that accumulates in living system and as such is currently one of the most important occupational and environmental pollutants. Cd reaches into the environment by anthropogenic mobilization and it is absorbed from tobacco consumption or ingestion of contaminated substances. Its extremely long biological half-life (approximately 20–30 years in humans) and low rate of excretion from the body cause cadmium storage predominantly in soft tissues (primarily, liver and kidneys) with a diversity of toxic effects such as nephrotoxicity, hepatotoxicity, endocrine and reproductive toxicities. Moreover, a Cd-dependent neurotoxicity has been also related to neurodegenerative diseases such as Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, and multiple sclerosis. At the cellular level, Cd affects cell proliferation, differentiation, apoptosis and other cellular activities. Among all these mechanisms, the Cd-dependent interference in DNA repair mechanisms as well as the generation of reactive oxygen species, seem to be the most important causes of its cellular toxicity. Nevertheless, there is still much to find out about its mechanisms of action and ways to reduce health risks. This article gives a brief review of the relevant mechanisms that it would be worth investigating in order to deep inside cadmium toxicity.

Novel 1, 4-dihydropyridines for L-type calcium channel as antagonists for cadmium toxicity

The present study, we design and synthesize the novel dihydropyridine derivatives, i.e., 3 (a-e) and 5 (a-e) and evaluated, anticonvulsant activity. Initially due to the lacuna of LCC, we modeled the protein through modeller 9.15v and evaluated through servers. Docking studies were performed with the synthesized compounds and resulted two best compounds, i.e., 5a, 5e showed the best binding energies. The activity of intracellular Ca²⁺ measurements was performed on two cell lines: A7r5 (rat aortic smooth muscle cells) and SH-SY5Y (human neuroblastoma cells). The 5a and 5e compounds was showing the more specific activity on L-type calcium channels, i.e. A7r5 (IC₅₀ = 0.18 ± 0.02 and 0.25 ± 0.63 μg/ml, respectively) (containing only L-type channels) than SH-SY5Y (i.e. both L-type and T-type channels) (IC₅₀ = 8 ± 0.23 and 10 ± 0.18 μg/ml, respectively) with intracellular calcium mobility similar to amlodipine. Finally, both in silico and in vitro results exploring two derivatives 5a and 5e succeeded to treat cadmium toxicity.

Dendropanax morbifera Léveille extract ameliorates cadmium-induced impairment in memory and hippocampal neurogenesis in rats

Background

Cadmium leads to learning and memory impairment. Dendropanax morbifera Léveille stem extract (DMS) reduces cadmium-induced oxidative stress in the hippocampus. We investigated the effects of DMS on cadmium-induced impairments in memory in rats.

Methods

Cadmium (2 mg/kg), with or without DMS (100 mg/kg), was orally administered to 7-week-old Sprague-Dawley rats for 28 days. Galantamine (5 mg/kg), an acetylcholinesterase inhibitor, was intraperitoneally administered as a positive control. A novel-object recognition test was conducted 2 h after the final administration. Cell proliferation and neuroblast differentiation were assessed by immunohistochemistry for Ki67 and doublecortin, respectively. Acetylcholinesterase activity in the synaptosomes of the hippocampus was also measured based on the formation of 5,5′-dithio-bis-acid nitrobenzoic acid.

Results

An increase in the preferential exploration time of new objects was observed in both vehicle-treated and cadmium-treated rats. In addition, DMS administration increased cell proliferation and neuroblast differentiation in the dentate gyrus of vehicle-treated and cadmium-treated rats. Acetylcholinesterase activity in the hippocampal synaptosomes was also significantly higher in the DMS-treated group than in the vehicle-treated group. The effect of DMS on cadmium-induced memory impairment and cell proliferation in the hippocampus was comparable to that of galantamine.

Conclusions

These results suggest that DMS ameliorates cadmium-induced memory impairment via increase in cell proliferation, neuroblast differentiation, and acetylcholinesterase activity in the hippocampus. The consumption of DMS may reduce cadmium-induced neurotoxicity in animals or humans.

From the Cover: Cadmium Exposure Differentially Alters Odorant-Driven Behaviors and Expression of Olfactory Receptors in Juvenile Coho Salmon (Oncorhynchus kisutch)

Salmon exposed to waterborne metals can experience olfactory impairment leading to disrupted chemosensation. In the current study, we investigated the effects of cadmium (Cd) on salmon olfactory function by modeling an exposure scenario where juvenile salmon transiently migrate through a polluted waterway. Coho were exposed to environmentally relevant concentrations of waterborne Cd (2 and 30 µg/L) for 48 h and (0.3 and 2 μg/L) for 16 days, followed by a 16-day depuration associated with outmigration. Cadmium exposures inhibited behavioral responses towards L-cysteine and conspecific odorants, with effects persisting following the depuration. Behavioral alterations following the 30 µg/L exposure were associated with increased olfactory epithelial gene expression of metallothionein (mt1a) and heme oxygenase (hmox1); reduced expression of olfactory signal transduction (OST) molecules; and reduced expression of mRNAs encoding major coho odorant receptors (ORs). Salmon OR array analysis indicated that Cd preferentially impacted expression of OST and OR markers for ciliated olfactory sensory neurons (OSNs) relative to microvillus OSNs, suggesting a differential sensitivity of these two major OSN populations. Behavioral alterations on exposure to 0.3 and 2 µg/L Cd were associated with increased mt1a, but not with major histological or OR molecular changes, likely indicating disrupted OST as a major mechanism underlying the behavioral dysfunction at the low-level Cd exposures. Laser-ablation mass spectrometry analysis revealed that the OSN injury and behavioral dysfunction was associated with significant Cd bioaccumulation within the olfactory sensory epithelium. In summary, low-level Cd exposures associated with polluted waterways can induce differential and persistent olfactory dysfunction in juvenile coho salmon.

Neuroglial alterations in the zebrafish brain exposed to cadmium chloride

Cadmium is an extremely toxic heavy metal that widely occurs in industrial workplaces with various hazardous effects on brain functions. The cytotoxic effects of cadmium chloride (CdCl₂ ) on the neuroglial components of the zebrafish brain were analysed by detecting the glial fibrillary acidic protein (GFAP) expression and the mRNA levels of myelin genes mbp, mpz and plp1 in adult specimens exposed to cadmium for 2, 7 and 16 days. A significant decrease in the GFAP protein by Western blotting experiments was observed after 2 days of treatment, reaching 55% after 16 days. No change was observed in the mRNA levels. Using immunohistochemistry, a reduction in GFAP-positive structures was revealed with a progressive trend in all the brains at 2, 7 and 16 days of treatment. In particular, a considerable reduction in GFAP-positive fibres, with a different course, was observed in the ventricle areas and at the pial surface and in blood vessels after 16 days. Our experiments also showed a structural and chemical alteration of myelin and upregulation of mpz mRNA levels, the oligodendrocyte gene that is upregulated in experiments of neuronal injury, but not of plp1 and mbp mRNA levels, other myelin structural genes. These data confirm the toxic action of cadmium on the zebrafish brain. This action is time-dependent and involves the glial cells, key components of the protection and function of nerve cells, hence the basis for many neurological diseases. Copyright © 2016 John Wiley & Sons, Ltd.

Voltage-Induced Ca²⁺ Release in Postganglionic Sympathetic Neurons in Adult Mice

Recent studies have provided evidence that depolarization in the absence of extracellular Ca²⁺ can trigger Ca²⁺ release from internal stores in a variety of neuron subtypes. Here we examine whether postganglionic sympathetic neurons are able to mobilize Ca²⁺ from intracellular stores in response to depolarization, independent of Ca²⁺ influx. We measured changes in cytosolic ΔF/F₀ in individual fluo-4 –loaded sympathetic ganglion neurons in response to maintained K⁺ depolarization in the presence (2 mM) and absence of extracellular Ca²⁺ ([Ca²⁺]_e). Progressive elevations in extracellular [K⁺]_e caused increasing membrane depolarizations that were of similar magnitude in 0 and 2 mM [Ca²⁺]_e. Peak amplitude of ΔF/F₀ transients in 2 mM [Ca²⁺]_e increased in a linear fashion as the membrane become more depolarized. Peak elevations of ΔF/F₀ in 0 mM [Ca²⁺]_e were ~5–10% of those evoked at the same membrane potential in 2 mM [Ca²⁺]_e and exhibited an inverse U-shaped dependence on voltage. Both the rise and decay of ΔF/F₀ transients in 0 mM [Ca²⁺]_e were slower than those of ΔF/F₀ transients evoked in 2 mM [Ca²⁺]_e. Rises in ΔF/F₀ evoked by high [K⁺]_e in the absence of extracellular Ca²⁺ were blocked by thapsigargin, an inhibitor of endoplasmic reticulum Ca²⁺ ATPase, or the inositol 1,4,5-triphosphate (IP₃) receptor antagonists 2-aminoethoxydiphenyl borate and xestospongin C, but not by extracellular Cd²⁺, the dihydropyridine antagonist nifedipine, or by ryanodine at concentrations that caused depletion of ryanodine-sensitive Ca²⁺ stores. These results support the notion that postganglionic sympathetic neurons possess the ability to release Ca²⁺ from IP₃-sensitive internal stores in response to membrane depolarization, independent of Ca²⁺ influx.

Citrate coated silver nanoparticles change heavy metal toxicities and bioaccumulation of Daphnia magna

Citrate-coated AgNPs (c-AgNPs) have negatively charged surfaces and their surface interactions with heavy metals can affect metal toxicity in aquatic environments. This study used Daphnia magna to compare the acute toxicities and bioaccumulation of As(V), Cd, and Cu when they interact with c-AgNPs. The 24-h acute toxicities of As(V) and Cu were not affected by the addition of c-AgNPs, while bioaccumulation significantly decreased in the presence of c-AgNPs. In contrast, both the 24-h acute toxicity and bioaccumulation of Cd increased in the presence of c-AgNPs. These toxicity and bioaccumulation trends can be attributed to the interactions between the AgNP surface and the heavy metals. As(V) and c-AgNPs compete by negative charge, decreasing As(V) toxicity. Copper adheres readily to c-AgNP citrate, decreasing Cu bioavailability, and thus reducing Cu toxicity and bioaccumulation. Citrate complexes with divalent cations such as Ca and Mg reduce the competition between divalent cations and Cd on biotic ligand, increasing toxicity and bioaccumulation of Cd. This study shows that surface properties determine the effect of c-AgNPs on heavy metal toxicities and bioaccumulations; hence, further studies on the effect of nanoparticle by it surface properties are warranted.

Dendropanax morbifera Léveille extract facilitates cadmium excretion and prevents oxidative damage in the hippocampus by increasing antioxidant levels in cadmium-exposed rats

BACKGROUND

Dendropanax morbifera Léveille is used in herbal medicine as a cancer treatment. In this study, we investigated the effects of Dendropanax morbifera stem extract (DMS) on cadmium (Cd) excretion from the blood and kidney and brain tissues of rats exposed to cadmium, as well as the effects of DMS on oxidative stress and antioxidant levels in the hippocampus after Cd exposure.

METHODS

Seven-week-old Sprague-Dawley rats were exposed to 2 mg/kg of cadmium by intragastric gavage and were orally administered 100 mg/kg of DMS for 4 weeks. Animals were sacrificed and Cd determination was performed using inductively coupled plasma mass spectrometry. In addition, the effects of Cd and/or DMS on oxidative stress were assayed by measuring reactive oxygen species production, protein carbonyl modification, lipid peroxidation levels, and antioxidant levels in hippocampal homogenates.

RESULTS

Exposure to Cd significantly increased Cd content in the blood, kidneys, and hippocampi. DMS treatment significantly reduced Cd content in the blood and kidneys, but not in the hippocampi. Exposure to Cd significantly increased reactive oxygen species production, protein carbonyl modification, lipid peroxidation, total sulfhydryl content, reduced glutathione content, and glutathione reductase activity. In contrast, Cu, Zn-superoxide dismutase (SOD1), catalase (CAT), glutathione peroxidase (GPx), and glutathione-S-transferase (GST) activity in the hippocampus were significantly decreased after exposure to Cd, and administration of DMS significantly inhibited these Cd-induced changes.

CONCLUSION

These results indicate that DMS facilitates cadmium excretion from the kidneys, reduces cadmium-induced oxidative stress in the hippocampus, and modulates SOD1, CAT, GPx, and glutathione-S-transferase activities.

Interaction of metal ions with neurotransmitter receptors and potential role in neurodiseases

There is increasing evidence that toxic metals play a role in diseases of unknown etiology. Their action is often mediated by membrane proteins, and in particular neurotransmitter receptors. This brief review will describe recent findings on the direct interaction of metal ions with ionotropic γ-aminobutyric acid (GABAA) and glutamate receptors, the main inhibitory and excitatory neurotransmitter receptors in the mammalian central nervous system, respectively. Both hyper and hypo function of these receptors are involved in neurological and psychotic syndromes and modulation by metal ions is an important pharmacological issue. The focus will be on three xenobiotic metals, lead (Pb), cadmium (Cd) and nickel (Ni) that have no biological function and whose presence in living organisms is only detrimental, and two trace metals, zinc (Zn) and copper (Cu), which are essential for several enzymatic functions, but can mediate toxic actions if deregulated. Despite limited access to the brain and tight control by metalloproteins, exogenous metals interfere with receptor performances by mimicking physiological ions and occupying one or more modulatory sites on the protein. These interactions will be discussed as a potential cause of neuronal dysfunction.

Cadmium

Cadmium is not an essential element for life. It is geologically marginal but anthropogenic activities have contributed significantly to its dispersion in the environment and to cadmium exposure of living species. The natural speciation of the divalent cation Cd2+ is dominated by its high propensity to bind to sulfur ligands, but Cd2+ may also occupy sites providing imidazole and carboxylate ligands. It binds to cell walls by passive adsorption (bio-sorption) and it may interact with surface receptors. Cellular uptake can occur by ion mimicry through a variety of transporters of essential divalent cations, but not always. Once inside cells, Cd2+ preferentially binds to thiol-rich molecules. It can accumulate in intracellular vesicles. It may also be transported over long distances within multicellular organisms and be trapped in locations devoid of efficient excretion systems. These locations include the renal cortex of animals and the leaves of hyper-accumulating plants. No specific regulatory mechanism monitors Cd2+ cellular concentrations. Thiol recruitment by cadmium is a major interference mechanism with many signalling pathways that rely on thiolate-disulfide equilibria and other redox-related processes. Cadmium thus compromises the antioxidant intracellular response that relies heavily on molecules with reactive thiolates. These biochemical features dominate cadmium toxicity, which is complex because of the diversity of the biological targets and the consequent pleiotropic effects. This chapter compares the cadmium-handling systems known throughout phylogeny and highlights the basic principles underlying the impact of cadmium in biology.

Neuroprotective effect of quercetin against oxidative damage and neuronal apoptosis caused by cadmium in hippocampus

The purpose of the present investigation was to evaluate cadmium (Cd)-induced neurotoxicity in hippocampal tissues and beneficial effect of quercetin (QE) against neuronal damage. A total of 30 male rats were divided into 3 groups: control, Cd-treated, and Cd + QE-treated groups. After the treatment, the animals were killed and hippocampal tissues were removed for biochemical and histopathological investigation. Cd significantly increased tissue malondialdehyde (MDA) and protein carbonyl (PC) levels and also decreased superoxide dismutase (SOD) and catalase (CAT) enzyme activities in hippocampal tissue compared with the control. Administration of QE with Cd significantly decreased the levels of MDA and PC and significantly elevated the levels of antioxidant enzymes in hippocampal tissue. In the Cd-treated group, the neurons of both tissues became extensively dark and degenerated with pyknotic nuclei. The morphology of neurons in Cd + QE group was well protected, but not as neurons of the control group. The caspase-3 immunopositivity was increased in degenerating neurons of the Cd-treated group. Treatment of QE markedly reduced the immunoreactivity of degenerating neurons. The results of the present study show that QE therapy causes morphologic improvement in neurodegeneration of hippocampus after Cd exposure in rats.

Role of calcium channels in heavy metal toxicity

The role of voltage-dependent Ca channels (VDCC) in the membrane permeation of two toxic metals, lead (Pb) and cadmium (Cd), was studied in mammalian cells. Both metals interact with Ca-binding sites, but, while Cd influx appears to occur mainly through the same pathways as Ca, Pb is also rapidly taken up by different passive transport systems. Furthermore, I compared the effect of Cd in two Chinese hamster ovary (CHO) cell lines, a wild-type and a modified cell line, which were permanently transfected with an L-type VDCC. When cultures were subjected to a brief (30–60 min) exposure to 50–100 μM Cd, apoptotic features, metal accumulation, and death were comparable in both cell lines although, in transfected cells, the effect of Cd treatment was partially prevented by nimodipine (VDCC antagonist) and enhanced by BayK8644 (VDCC agonist). Thus, expression of L-type Ca channels is not sufficient to modify Cd accumulation and sensitivity to a toxicological significant extent and while both Cd and Pb can take advantage of VDCC to permeate the membrane, these transport proteins are not the only, and frequently not the most important, pathways of permeation.

Effect of divalent metals on the neuronal proteasomal system, prion protein ubiquitination and aggregation

The role of normal cellular prion protein (PrP) remains to be fully elucidated; however, the protein is crucial for the infection and progression of prion diseases. Recent evidence indicates that PrP is a metalloprotein since the octapeptide repeat sequences in the protein have high affinity for various divalent cations and the binding sites appear to play a role in the pathogenesis of prion diseases. In our present study, we tested several divalent metals including manganese and cadmium and determined their effects on protein degradation and protein aggregation in mouse neuronal cells expressing PrP. Cadmium was more neurotoxic than manganese following 24h exposure. Manganese did not show any significant effect on the inhibition of proteasomal activity or formation of high molecular weight ubiquitinated PrPs. Interestingly, treatment with cadmium profoundly inhibited proteasomal activity, which resulted in greatly increased formation of high molecular weight ubiquitinated PrPs. Immunohistochemical analysis also revealed a dramatic increase in formation of oligomers after cadmium treatment. Cadmium also increased the formation of ubiquitinated PrP, but it did not lead to the formation of proteinase-K resistant PrP. Collectively, our results show that a divalent metal, cadmium affects proteasomal function and PrP aggregation, which promote neurotoxicity.

Gap junction-mediated spontaneous Ca(2+) waves in differentiated cholinergic SN56 cells

Neuronal gap junctions are receiving increasing attention as a physiological means of intercellular communication, yet our understanding of them is poorly developed when compared to synaptic communication. Using microfluorimetry, we demonstrate that differentiation of SN56 cells (hybridoma cells derived from murine septal neurones) leads to the spontaneous generation of Ca(2+) waves. These waves were unaffected by tetrodotoxin (1microM), but blocked by removal of extracellular Ca(2+), or addition of non-specific Ca(2+) channel inhibitors (Cd(2+) (0.1mM) or Ni(2+) (1mM)). Combined application of antagonists of NMDA receptors (AP5; 100microM), AMPA/kainate receptors (NBQX; 20microM), nicotinic AChR receptors (hexamethonium; 100microM) or inotropic purinoceptors (brilliant blue; 100nM) was also without effect. However, Ca(2+) waves were fully prevented by carbenoxolone (200microM), halothane (3mM) or niflumic acid (100microM), three structurally diverse inhibitors of gap junctions, and mRNA for connexin 36 was detected by PCR. Whole-cell patch-clamp recordings revealed spontaneous inward currents in voltage-clamped cells which we inhibited by Cd(2+), Ni(2+) or niflumic acid. Our data suggest that differentiated SN56 cells generated spontaneous Ca(2+) waves which are propagated by intercellular gap junctions. We propose that this system can be exploited conveniently for the development of neuronal gap junction modulators.

Endoplasmic reticulum is a major target of cadmium toxicity in yeast

Cadmium (Cd(2+)) is a very toxic metal that causes DNA damage, oxidative stress and apoptosis. Despite many studies, the cellular and molecular mechanisms underlying its high toxicity are not clearly understood. We show here that very low doses of Cd(2+) cause ER stress in Saccharomyces cerevisiae as evidenced by the induction of the unfolded protein response (UPR) and the splicing of HAC1 mRNA. Furthermore, mutant strains (Delta ire1 and Delta hac1) unable to induce the UPR are hypersensitive to Cd(2+), but not to arsenite and mercury. The full functionality of the pathways involved in ER stress response is required for Cd(2+) tolerance. The data also suggest that Cd(2+)-induced ER stress and Cd(2+) toxicity are a direct consequence of Cd(2+) accumulation in the ER. Cd(2+) does not inhibit disulfide bond formation but perturbs calcium metabolism. In particular, Cd(2+) activates the calcium channel Cch1/Mid1, which also contributes to Cd(2+) entry into the cell. The results reinforce the interest of using yeast as a cellular model to study toxicity mechanisms in eukaryotic cells.

Effects of maternal cadmium administration on development of monoaminergic, GABAergic and glutamatergic systems

The effects of maternal exposure to 10mgCd/l (as cadmium acetate) in drinking water during gestation and lactation on the development of monoaminergic and aminoacidergic systems were studied in discrete brain areas of the pups: striatum, cerebral cortex, dorsal hippocampus and basal-medial hypothalamus. Hippocampal levels of serotonin and 5-hydroxyindolacetic acid were significantly reduced in rats exposed to Cd whereas the dopamine content was not significantly affected by Cd. Glutamate concentration decreased in hypothalamus and increased in hippocampus, while gamma-aminobutiric acid content decreased only in cerebral cortex. The present results demonstrate that maternal exposure to 10mg/l of Cd leads to neurochemical disturbances on serotoninergic and aminoacidergic systems during development.

Endoplasmic reticulum stress and alteration in calcium homeostasis are involved in cadmium-induced apoptosis

Cadmium, a toxic environmental contaminant, exerts adverse effects on different cellular pathways such as cell proliferation, DNA damage and apoptosis. In particular, the modulation of Ca(2+) homeostasis seems to have an important role during Cd(2+) injury, but the precise assessment of Ca(2+) signalling still remains poorly understood. We used aequorin-based probes specifically directed to intracellular organelles to study Ca(2+) changes during cadmium injury. We observed that cadmium decreased agonist-evoked endoplasmic reticulum (ER) Ca(2+) signals and caused a 40% inhibition of sarcoplasmic-ER calcium ATPases activity. Moreover, time course experiments correlate morphological alterations, processing of xbp-1 mRNA and caspase-12 activation during cadmium administration. Finally, the time response of ER to cadmium injury was compared with that of mitochondria. In conclusion, we highlighted a novel pathway of cadmium-induced cell death triggered by ER stress and involving caspase-12. Mitochondria and ER pathways seemed to share common time courses and a parallel activation of caspase-12 and caspase-9 seemed likely to be involved in acute cadmium toxicity.

Cadmium neurotoxicity

The Cd has been recognized as one of the most toxic environmental and industrial pollutants due to its ability to induce disturbances in several organs and tissues following either acute or chronic exposure. This review accounts for the recent evidence on its mechanisms to induce neurotoxicity, the role of the blood-brain barrier, oxidative stress, interference with calcium, and zinc-dependent processes and apoptosis induction as well as the modulatory effect of metallothionein. Discussion about cadmium neurotoxicity is centered on mechanisms of induction of cellular disfunctions. Future investigations must address those neuronal mechanisms in detail in order to understand cadmium-induced neurotoxicity.

Cadmium toxicity in animal cells by interference with essential metals

Cadmium is found in the environment as part of several, mainly zinc-rich, ores. It has been used in many technological applications, but biological systems generally failed to safely deal with this element. In mammalian biology, cadmium exposure jeopardizes health and mechanisms of cadmium toxicity are multifarious. Mainly because bioavailable cadmium mimics other metals that are essential to diverse biological functions, cadmium follows a Trojan horse strategy to get assimilated. Metals susceptible to cadmium deceit include calcium, zinc, and iron. The wealth of data addressing cadmium toxicity in animal cells is briefly reviewed with special emphasis on disturbance of the homeostasis of calcium, zinc, and iron. A limited number of tissues and cell types are considered as main targets for cadmium toxicity. Still, the diversity of pathways affected by cadmium exposure points to a more general threat to basic cellular functions. The poor efficiency of cellular export systems for cadmium explains the long residence time of the element in mammals. Therefore, proper disposal and educated uses of this technologically appealing, but biologically malicious, element should be favored in the future. The comprehensive knowledge of cadmium biological effects is indeed a necessary step to protect human and animal populations from environmental and anthropological exposures.

Silencing of ZnT-1 expression enhances heavy metal influx and toxicity

ZnT-1 reduces intracellular zinc accumulation and confers resistance against cadmium toxicity by a mechanism which is still unresolved. A functional link between the L-type calcium channels (LTCC) and ZnT-1 has been suggested, indicating that ZnT-1 may regulate ion permeation through this pathway. In the present study, immunohistochemical analysis revealed a striking overlap of the expression pattern of LTCC and ZnT-1 in cardiac tissue and brain. Using siRNA to silence ZnT-1 expression, we then assessed the role of ZnT-1 in regulating cation permeation through the L-type Ca(2+) channels in cells that are vulnerable to heavy metal permeation. Transfection of cortical neurons with ZnT-1 siRNA resulted in about 70% reduction of ZnT-1 expression and increased Ca(2+) influx via LTCC by approximately fourfold. Moreover, ZnT-1 siRNA transfected neurons showed approximately 30% increase in synaptic release, monitored using the FM1-43 dye. An increased cation influx rate, through the LTCC, was also recorded for Zn(2+) and Cd(2+) in cells treated with the ZnT-1 siRNA. Furthermore, Cd(2+)-induced neuronal death increased by approximately twofold after transfection with ZnT-1 siRNA. In addition, ZnT-1 siRNA transfection of the ovarian granulosa cell line, POGRS1, resulted in a twofold increase in Cd(2+) influx rate via the LTCC. Finally, a robust nimodipine-sensitive Cd(2+) influx was observed using a low extracellular Cd(2+) concentration (5 muM) in neurons and testicular slice cultures, attesting to the relevance of the LTCC pathway to heavy metal toxicity. Taken together, our results indicate that endogenously-expressed ZnT-1, by modulating LTCC, has a dual role: regulating calcium influx, and attenuating Cd(2+) and Zn(2+) permeation and toxicity in neurons and other cell types.

Cadmium-induced astroglial death proceeds via glutathione depletion

Cadmium is a heavy metal that accumulates in the body, and its accumulation in the brain damages both neurons and glial cells. In the current study, we explored the mechanism underlying cadmium toxicity in primary cortical astroglia cultures. Chronic treatment with 10 microM cadmium was sufficient to cause 90% cell death in 18 hr. However, unlike that observed in neurons, cadmium-induced astroglial toxicity was not attenuated by the antioxidants trolox (100 microM), caffeic acid (1 mM), and vitamin C (1 mM). In contrast, extracellular 100 microM glutathione (GSH; gamma-Glu-Cys-Gly) or 100 microM cysteine almost completely blocked cadmium-induced astroglial death, whereas 300 microM oxidized GSH (GSSG) or 300 microM cystine, which do not have the free thiol group, were ineffective. In addition, cadmium toxicity was noticeably inhibited or enhanced when intracellular GSH was, respectively, increased by using the cell-permeable glutathione ethyl ester (GSH-EE) or depleted by using buthionine sulfoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase. In agreement with these data, intracellular GSH levels were found to be depressed in cadmium-treated astrocytes. These results suggest that the toxic effect of cadmium on primary astroglial cells involves GSH depletion and, furthermore, that GSH administration can potentially be used to counteract cadmium-induced astroglial cell death therapeutically.

Persistent calcium current in rat suprachiasmatic nucleus neurons

The suprachiasmatic nuclei contain the primary circadian clock, and suprachiasmatic nuclei neurons exhibit a diurnal modulation of spontaneous firing rate. The present study examined the voltage-gated persistent Ca(2+) current, in acutely isolated rat suprachiasmatic nuclei neurons using a ramp-type voltage-clamp protocol. Slow triangular voltage-clamp commands from a holding potential of -85 mV to +5 mV elicited inward current (100-400 pA) that was completely blocked by Cd(2+). This current showed little or no hysteresis, and was identified as persistent Ca(2+) current. The threshold for persistent Ca(2+) current ranged between -60 and -45 mV, and it was maximal at about -8 mV. Nifedipine at 10-20 microM blocked 80-100%. To assess the role of persistent Ca(2+) current in the generation of spontaneous action potentials in both acutely isolated and intact suprachiasmatic nuclei neurons, the effect of Cd(2+) and nifedipine on firing rate was studied using on-cell recording. Application of Cd(2+) exerted a weak excitatory effect and nifedipine had no significant effect on the spontaneous firing rate of isolated suprachiasmatic nuclei neurons. In all intact suprachiasmatic nuclei neurons in slice preparations (n=15), Cd(2+) slowly inhibited spontaneous firing; in high-frequency firing cells (four of 15), a transient increase of firing rate preceded inhibition. No significant effect of nifedipine on firing rate of intact suprachiasmatic nuclei neurons was found. Therefore, persistent Ca(2+) current itself (as carrier of charge) does not appear to contribute significantly to spontaneous firing of suprachiasmatic nuclei neurons. A slowly developing inhibitory effect of Cd(2+) on spontaneous firing of intact suprachiasmatic nuclei neurons in slice preparations may be due to penetration of Cd(2+) through Ca(2+) channels, and its subsequent effect on intracellular mechanisms, while the transient increase of firing rate in high-frequency firing neurons is probably due to inhibition of Ca(2+)-activated K(+) current.

Susceptibility of insulinoma cells to cadmium and modulation by L-type calcium channels

Cadmium (Cd), a toxic metal that induces apoptosis and necrosis in a variety of cells, accumulates in pancreas and may be a cause of diabetes in humans. In the insulinoma cells line HIT-T15 (HIT), we measured internal calcium (Ca) and Cd levels by the fluorescent dye Fura-2 and confirm that L-type voltage-dependent calcium channels (VDCC) play a major role in glucose response and represent a pathway of Cd influx in these cells. Therefore we examined the role of VDCC in acute Cd poisoning by comparing its accumulation and cytotoxic effect in HIT cells and in epithelial-like VDCC-free HeLa cells. Cultures were incubated with 10-300 microM Cd for 15 min-6 h. While negligible at the end of the treatment, HIT cell death was evident after 18-24 h, and it was time-, dose- and serum-dependent. Short (< or = 60 min) Cd treatments with lower doses (< or = 100 microM in serum-free medium) induced delayed apoptotic cell death, as demonstrated by DNA fragmentation on agarose gels and segmentation of DAPI-stained nuclei. Longer incubations and/or higher concentrations caused mainly necrosis. The same treatments were largely harmless in HeLa cells, in which neither death nor DNA fragmentation was observed. The Ca antagonist nimodipine was capable to prevent HIT cell death at lower doses of Cd and to restore the apoptotic condition at higher doses, indicating that reduction of Cd flux through VDCC modulates Cd toxicity. These data demonstrate a specific sensitivity to Cd of insulinoma cells that can be significant for pancreatic beta-cell pathology.

Reduction in functional potency of the neurotoxin domoic acid in the presence of cadmium and zinc ions

The tricarboxylic neurotoxin domoic acid (DA) binds trace metals such as iron and copper. In vitro brain slice recording (area CA1 of rat hippocampal slices) was used to assess changes in DA potency in the presence of cadmium and zinc. Cadmium or zinc alone had little or no effect on CA1 responses. DA alone produced hyperexcitability and, with prolonged administration, a robust suppression of CA1 responses. Coadministration of DA with either 2 or 4μM Cd(2+) produced significant reductions in the potency of DA; less striking effects were seen in the presence of 4μM Zn(2+). These findings suggest that interactions of Cd(2+) and Zn(2+) with DA result in the formation of trace metal-neurotoxin complexes which are either unavailable for binding to ionotropic glutamate receptors, or bind without producing full agonist activity.

Aequorin chimeras as valuable tool in the measurement of Ca2+ concentration during cadmium injury

The ability of cadmium to disrupt calcium homeostasis has been known since a long time, but the precise cellular targets of its toxic action are still debated. A great problem in the interpretation of data has been associated with the ability of cadmium to strongly bind traditional calcium probes. Aequorin, the well-characterized calcium-sensitive photoprotein, was used as intracellular calcium indicator during cadmium injury in NIH 3T3 murine fibroblasts. NIH 3T3 cells were transfected with a cDNA construct containing aequorin fused to a truncated glutamate receptor, which directs the probe to the outer surface of intracellular membranes. At first, we tested if different cadmium concentrations were able to modify the rate of light emission by aequorin showing that cadmium concentrations <15 microM were ineffective on aequorin luminescence. Hence, aequorin chimeras revealed as a useful tool in the analyses of Cd2+/Ca2+ interference. To directly investigate the role of Cd2+ in Ca2+ homeostasis, we have started to selectively measure the free Ca2+ concentration in different cell compartments. Here, we report that cadmium reduces the transient free calcium signal after stimulation of cells with bradykinin. Further studies are in progress to clarify the role of mitochondria and endoplasmic reticulum in cadmium-induced alterations of Ca2+ homeostasis in order to link signal transduction modifications with the onset of apoptosis induced by cadmium exposure.

A molecular technique for detecting the liberation of intracellular zinc in cultured neurons

We have previously reported that oxidative stimuli liberate Zn(2+) from metalloproteins, a phenomenon that can trigger neuronal cell death. Excessive intracellular Zn(2+) in many cell types triggers the expression of genes that encode metal binding proteins, such as metallothionein, via the activation and nuclear translocation of metal response element (MRE)-binding transcription factor-1 (MTF-1). Cd(2+) strongly induces nuclear translocation of MTF-1 in non-neuronal cells, but it does so by displacing Zn(2+) from its metal binding sites within the cell and increasing the intracellular concentration of this ion. Here, we describe the use of MRE-driven expression of a luciferase reporter gene as a sensitive molecular assay for detecting increases in intracellular zinc concentrations. MRE transactivation was induced in primary cortical neurons upon brief exposure to Zn(2+) or Cd(2+). Enhanced MRE transactivation was observed upon co-exposure of neurons to Cd(2+) together with NMDA, as this metal can permeate through the receptor channel. Luciferase expression was observed regardless of whether or not neurons had been co-transfected with an MTF-1-containing plasmid, suggesting the presence of an endogenous MTF-1-like protein. Indeed, RT-PCR revealed that MTF-1 mRNA is present in neurons. In contrast, MTF-1 deficient dko7 cells were only observed to have MRE transactivation when co-transfected with MTF-1. Our results indicate that Cd(2+) can effectively induce transactivation of MRE in neurons by liberating Zn(2+) from its intracellular binding sites.

Cadmium inhibits GABA-activated ion currents by increasing intracellular calcium level in snail neurons

Blocking of the GABA-activated chloride current by cadmium was investigated in identified nerve cells (RPeD1, RPaD1) of the pond snail (Lymnaea stagnalis L.), using a two-microelectrode voltage-clamp technique. Cd2+, at 50 microM extracellular concentration, inhibited GABA-activated chloride currents, both in normal and Ca2+-free solution. Intracellular injection of Ca2+ or the application of caffeine mimicked the inhibitory effect of Cd2+ on GABA-elicited currents. Cd2+-block was eliminated, or it was substantially decreased, when neurons were intracellularly loaded with EGTA, or when the Ca2+-release was blocked by ruthenium red. The blocking effect of Cd2+ was also eliminated by applying G-protein inhibitors: pertussis toxin, suramin or GTP-gamma-S. Finally, intracellularly injected Cd2+ was ineffective at eliciting an inward current on GABA-activated currents, suggesting that the Cd2+-binding site resides extracellularly. These results suggest that cadmium inhibited GABA-activated chloride currents by increasing the intracellular Ca2+ level, by releasing it from intracellular stores and by interacting with a putative G-protein-coupled cell-surface "metal-receptor".

Two-photon imaging of calcium accumulation in rat cerebellar granule cells

Topical accumulation of calcium ions in neurites and cell bodies of rat cerebellar granule cells was studied by two-photon microscopy in neurons loaded with the Ca-sensitive fluorescent indicator Oregon Green 488 Bapta. High potassium caused a rapid surge of internal calcium ([Ca2+]i) in the cell body, followed by a plateau. In neurites, [Ca2+]i reached a peak and then decreased back to the control level. In contrast, in neurons stimulated by NMDA, [Ca2+]i reached a steady level and remained constant as long as the agonist was present in the bath, either in the cell bodies or in neurites. In the latter, the response to NMDA treatment was smaller and heterogeneous, and [Ca2+]i increased in certain segments of the neurite, but not in others.

Apoptosis and necrosis: two distinct events induced by cadmium in cortical neurons in culture

(1) Cadmium is an extremely toxic metal commonly found in industrial workplaces, a food contaminant and a major component of cigarette smoke. Cadmium can severely damage several organs, including the brain. In this work, we have studied both the cadmium toxicity on rat cortical neurons in culture and the possible protective effect of serum. (2) Our results indicate that: (1) cadmium is taken up by the neurons in a dose and serum dependent way; (2) cadmium, at concentrations from 1 micro M or 10 micro M (depending on the absence or the presence of serum) up to 100 micro M, decreases the metabolic capacity, which was evaluated by the XTT (tetrazolium salt) test; (3) cadmium induces apoptosis and LDH (lactate dehydrogenase) release in a dose dependent way; (4) in a serum-free medium, the cadmium-induced apoptosis is accompanied by caspase-3 activation; (5) both the caspase-3 activation and the cadmium-induced apoptosis are reversed by N-acethyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO), a selective caspase-3 inhibitor, indicating that the caspase-3 pathway is involved in cadmium-induced apoptosis in cortical neurons; and (6) the cadmium concentrations which produce caspase-3 activation do not modify the intracellular ATP levels; however, higher cadmium concentrations lead to both intracellular ATP depletion and ATP release, but do not increase the caspase-3 activity, indicating that cadmium also produces cellular death by necrosis. (3) These results suggest that cadmium induces either apoptosis or necrosis in rat cortical neurons, depending on the cadmium concentration.

Molecular targets of lead in brain neurotoxicity

The detrimental effects of lead poisoning have been well known since ancient times, but some of the most severe consequences of exposure to this metal have only been described recently. Lead [Pb(II)] affects the higher functions of the central nervous system and undermines brain growth, preventing the correct development of cognitive and behavioral functions. As an established neurotoxin, Pb(II) crosses the blood-brain barrier rapidly and concentrates in the brain. The mechanisms of lead neurotoxicity are complex and still not fully understood, but recent findings recognized that both Ca(II) dependent proteins and neurotransmitters receptors represent significant targets for Pb(II). In particular, acute and chronic exposure to lead would predominantly affect two specific protein complexes: protein kinase C and the N-methyl-D-aspartate subtype of glutamate receptor. These protein complexes are deeply involved in learning and cognitive functions and are also thought to interact significantly with each other to mediate these functions. This review outlines the most recent hypotheses and evidences that link lead poisoning to impairment of these protein functions, as well as the in vitro experimental approaches that are most likely to provide information on basic mechanicistic processes.

Heavy metal toxicity: cadmium permeates through calcium channels and disturbs the plant water status

Because plant wilting has been described as a consequence of cadmium (Cd2+) toxicity, we investigate Cd2+ effects on plant water losses, gas exchanges and stomatal behaviour in Arabidopsis thaliana L. Effects of 1-week Cd2+ application in hydroponic condition (CdCl2 10-100 micro m) were analyzed. A 10- micro m Cd2+ concentration had no significant effect on the plant-water relationship and carbon assimilation. At higher Cd2+ concentrations, a Cd2+ -dependent decrease in leaf conductance and CO2 uptake was observed despite the photosynthetic apparatus appeared not to be affected as probed by fluorescence measurements. In epidermal strip bioassays, nanomolar Cd2+ concentrations reduced stomatal opening under light in A. thaliana, Vicia faba and Commelina communis. Application of 5 micro m ABA limited the root-to-shoot translocation of cadmium. However, the Cd2+-induced stomatal closure was likely ABA-independent, since a 5-day treatment with 50 micro m Cd2+ did not affect the plant relative water content. Additionally, a similar Cd2+-induced stomatal closure was observed in the ABA insensitive mutant abi1-1. Interestingly, this mutant displayed a higher transpiration rate than the wild type but did not accumulate more Cd2+, arguing that Cd2+ uptake is not dependent only on the transpiration flow. Application of putative calcium channels inhibitors suppressed the inhibitory effect of Cd2+ in epidermal strip experiments, suggesting that Cd2+ could enter the guard cell through calcium channels. Patch-clamp studies with V. faba guard cell protoplasts showed that plasma membrane K+ channels were insensitive to external Cd2+ application whereas Ca2+ channels were found permeable to Cd2+. In conclusion, we propose that Cd2+ affects guard cell regulation in an ABA-independent manner by entering the cytosol via Ca2+ channels.

Multiple pathways of Pb(2+) permeation in rat cerebellar granule neurones

The pathways of lead (Pb(2+)) uptake were studied in fura-2-loaded cerebellar granule cells from 8-day-old rats. In a nominal Ca-free external bath, Pb(2+) (5-50 microM) determined an increase of the fluorescence emission ratio (R = E(340)/E(380)) even in the absence of any specific stimulus. This rise was dose-dependent, was not significantly affected by mM Mg(2+) or Ca(2+), but it was readily reversed by the membrane-permeant heavy metal chelator tetrakis(2-pyridylmethyl) ethylene-diamine (TPEN, 100 microM), indicating that it was due to Pb(2+) influx. The rate of rise, dR/dt, was increased up to a factor of 5 by depolarizing high-KCl solution, indicating a sizeable permeation through voltage-dependent channels. This effect was neither antagonized by nimodipine, nor enhanced by BayK8644, but it was slackened by omega-agatoxin IVA (200 nM), suggesting an involvement of non-L-type calcium channels. Pb(2+) influx was also stimulated by glutamic acid or NMDA in the presence of 10-30 microM glycine, but only in Mg-free solution, suggesting that glutamate channels of the NMDA type are an additional pathway of Pb(2+) uptake. Pb(2+) caused a time-, dose- and stimulus-dependent saturation of the dye, whose intracellular concentration is approximately 10 microM, indicating that intracellular Pb(2+) can readily reach a concentration in the micromolar range. These results indicate that the particular vulnerability of neurones to Pb(2+) poisoning is linked to the presence of specific transport systems, which mediate the rapid uptake of Pb(2+) into the neurone.

Involvement of DMT1 in uptake of Cd in MDCK cells: role of protein kinase C

The involvement of iron (Fe) transporters in the uptake of cadmium (Cd) was examined in Madin-Darby kidney cells (MDCK). The uptake of Cd displayed properties that are associated with the Fe transporter divalent metal transporter 1 (DMT1). For example, the uptake of Cd and Fe was reduced by altering the cell membrane potential. The uptake of Cd was blocked by Fe, and the uptake of Fe was blocked by Cd. Also, the uptake of Cd and Fe was higher in MDCK cells bathed in a buffer at low pH. Increased uptake of Fe and Cd was observed in the HEK-293 cell line overexpressing DMT1. Overnight treatment of MDCK cells with the protein kinase C activator phorbol 12,13-dibutyrate (PDBu) resulted in increased uptake of Cd and Fe and an increase in DMT1 mRNA. An increase in newly transcribed DMT1 mRNA was not observed, suggesting that PDBu does not increase DMT1 mRNA by activating transcription. Rather, the increase was most likely due to greater stability of DMT1 mRNA, because the rate of degradation of DMT1 mRNA was slower in MDCK cells treated with PDBu. Our results suggest that Fe and Cd are transported in MDCK cells by a transporter with biochemical properties similar to those of DMT1.

Alternate cadmium exposure differentially affects amino acid metabolism within the hypothalamus, median eminence, striatum and prefrontal cortex of male rats

This work was designed to analyze the possible changes in glutamate, aspartate and glutamine content induced by cadmium exposure in the hypothalamus, striatum and prefrontal cortex of rats, using an alternate schedule of metal administration. Pubertal-adult differences were also evaluated. In adult control rats, glutamate and aspartate contents in the anterior hypothalamus decreased as compared to pubertal controls. After cadmium administration from day 30 to 60 of life, the content of anterior hypothalamic glutamate and aspartate diminished. In adult control animals, the glutamine content increased in mediobasal hypothalamus as compared to pubertal controls. After cadmium exposure from day 30 to 60 of life, the mediobasal glutamine content increased, and after cadmium treatment from day 60 to 90 of life, the mediobasal aspartate content decreased. In adult control rats the content of glutamine, glutamate and aspartate of the posterior hypothalamus decreased significantly. After cadmium administration in pubertal animals, posterior hypothalamic contents of glutamine, glutamate and aspartate diminished. Cadmium treatment of adult animals caused a decrease in glutamine content, as compared to controls. In adult control rats, only glutamate and aspartate content increased in the prefrontal cortex as compared to the values found in pubertal controls. When cadmium was administered to adult animals, only the aspartate content decreased. In the striatum, cadmium decreased the glutamine and aspartate contents when administered from day 60 to 90 of life. These data suggest that cadmium differentially affects amino acid metabolism in the hypothalamus, striatum and prefrontal cortex. Age-dependent effects of cadmium on these brain areas appeared to have occurred.