Affinity of heavy metal ions to intracellular Ca2+-binding proteins

Developmental Pb exposure increases AD risk via altered intracellular Ca2+ homeostasis in hiPSC-derived cortical neurons

Exposure to environmental chemicals such as lead (Pb) during vulnerable developmental periods can result in adverse health outcomes later in life. Human cohort studies have demonstrated associations between developmental Pb exposure and Alzheimer's disease (AD) onset in later life which were further corroborated by findings from animal studies. The molecular pathway linking developmental Pb exposure and increased AD risk, however, remains elusive. In this work, we used human iPSC-derived cortical neurons as a model system to study the effects of Pb exposure on AD-like pathogenesis in human cortical neurons. We exposed neural progenitor cells derived from human iPSC to 0, 15, and 50 ppb Pb for 48 h, removed Pb-containing medium, and further differentiated them into cortical neurons. Immunofluorescence, Western blotting, RNA-sequencing, ELISA, and FRET reporter cell lines were used to determine changes in AD-like pathogenesis in differentiated cortical neurons. Exposing neural progenitor cells to low-dose Pb, mimicking a developmental exposure, can result in altered neurite morphology. Differentiated neurons exhibit altered calcium homeostasis, synaptic plasticity, and epigenetic landscape along with elevated AD-like pathogenesis markers, including phosphorylated tau, tau aggregates, and Aβ42/40. Collectively, our findings provide an evidence base for Ca dysregulation caused by developmental Pb exposure as a plausible molecular mechanism accounting for increased AD risk in populations with developmental Pb exposure.

Direct Effects of Toxic Divalent Cations on Contractile Proteins with Implications for the Heart: Unraveling Mechanisms of Dysfunction

The binding of calcium and magnesium ions to proteins is crucial for regulating heart contraction. However, other divalent cations, including xenobiotics, can accumulate in the myocardium and enter cardiomyocytes, where they can bind to proteins. In this article, we summarized the impact of these cations on myosin ATPase activity and EF-hand proteins, with special attention given to toxic cations. Optimal binding to EF-hand proteins occurs at an ionic radius close to that of Mg²⁺ and Ca²⁺. In skeletal Troponin C, Cd²⁺, Sr²⁺, Pb²⁺, Mn²⁺, Co²⁺, Ni²⁺, Ba²⁺, Mg²⁺, Zn²⁺, and trivalent lanthanides can substitute for Ca²⁺. As myosin ATPase is not a specific MgATPase, Ca²⁺, Fe²⁺, Mn²⁺, Ni²⁺, and Sr²⁺ could support myosin ATPase activity. On the other hand, Zn²⁺ and Cu² significantly inhibit ATPase activity. The affinity to various divalent cations depends on certain proteins or their isoforms and can alter with amino acid substitution and post-translational modification. Cardiac EF-hand proteins and the myosin ATP-binding pocket are potential molecular targets for toxic cations, which could significantly alter the mechanical characteristics of the heart muscle at the molecular level.

Structural and biochemical insights into Zn2+-bound EF-hand proteins, EFhd1 and EFhd2

EF-hand proteins, which contain a Ca²⁺-binding EF-hand motif, are involved in regulating diverse cellular functions. Ca²⁺ binding induces conformational changes that modulate the activities of EF-hand proteins. Moreover, these proteins occasionally modify their activities by coordinating metals other than Ca²⁺, including Mg²⁺, Pb²⁺ and Zn²⁺, within their EF-hands. EFhd1 and EFhd2 are homologous EF-hand proteins with similar structures. Although separately localized within cells, both are actin-binding proteins that modulate F-actin rearrangement through Ca²⁺-independent actin-binding and Ca²⁺-dependent actin-bundling activity. Although Ca²⁺ is known to affect the activities of EFhd1 and EFhd2, it is not known whether their actin-related activities are affected by other metals. Here, the crystal structures of the EFhd1 and EFhd2 core domains coordinating Zn²⁺ ions within their EF-hands are reported. The presence of Zn²⁺ within EFhd1 and EFhd2 was confirmed by analyzing anomalous signals and the difference between anomalous signals using data collected at the peak positions as well as low-energy remote positions at the Zn K-edge. EFhd1 and EFhd2 were also found to exhibit Zn²⁺-independent actin-binding and Zn²⁺-dependent actin-bundling activity. This suggests the actin-related activities of EFhd1 and EFhd2 could be regulated by Zn²⁺ as well as Ca²⁺.

Structural Identification of Metalloproteomes in Marine Diatoms, an Efficient Algae Model in Toxic Metals Bioremediation

The biosorption of pollutants using microbial organisms has received growing interest in the last decades. Diatoms, the most dominant group of phytoplankton in oceans, are (i) pollution tolerant species, (ii) excellent biological indicators of water quality, and (iii) efficient models in assimilation and detoxification of toxic metal ions. Published research articles connecting proteomics with the capacity of diatoms for toxic metal removal are very limited. In this work, we employed a structural based systematic approach to predict and analyze the metalloproteome of six species of marine diatoms: Thalassiosira pseudonana, Phaeodactylum tricornutum, Fragilariopsis cylindrus, Thalassiosira oceanica, Fistulifera solaris, and Pseudo-nitzschia multistriata. The results indicate that the metalloproteome constitutes a significant proportion (~13%) of the total diatom proteome for all species investigated, and the proteins binding non-essential metals (Cd, Hg, Pb, Cr, As, and Ba) are significantly more than those identified for essential metals (Zn, Cu, Fe, Ca, Mg, Mn, Co, and Ni). These findings are most likely related to the well-known toxic metal tolerance of diatoms. In this study, metalloproteomes that may be involved in metabolic processes and in the mechanisms of bioaccumulation and detoxification of toxic metals of diatoms after exposure to toxic metals were identified and described.

Potential mechanism of lead poisoning to the growth and development of ovarian follicle

With the rapid development of economic globalization and industrialization, lead (Pb), one of the most important heavy metals, has been used widely since antiquity for several purposes. In fact, its impact on the health of animals and humans is a significant public health risk all the time. Pb could be accumulated in the body for a long time, causing irreversible damage to the health of animals and humans, including hostile reproductive health. Up to now, although there are some published studies on impeding the normal development of ovarian folliculogenesis of female resulted from Pb exposure, with the damage of structure in uterine tissue, the imbalance of female menstrual status, and the change of hormone levels. The potential mechanism of Pb exposure on female reproduction system, however, remains enigmatic. How to alleviate the damage of Pb toxicity to reproductive function of female has become an urgent problem. Therefore, the aim of the present review is to discuss the information on the growth and development of ovarian follicle of mammalians and the potential toxic mechanism when exposed to Pb. The literatures were collected via various websites and consulting books, reports, etc. In summary, Pb impair folliculogenesis of mammalians, which may be related to the interference to the hypothalamic-pituitary-gonadal (HPG) axis and the production of reactive oxygen species (ROS), in turn impairs various molecules including proteins, lipids and DNA, as well as the disruption of the antioxidant defense system, ionic equilibrium and endoplasmic reticulum homeostasis.

Force-velocity characteristics of isolated myocardium preparations from rats exposed to subchronic intoxication with lead and cadmium acting separately or in combination

This investigation continues our study of the effects of Pb-Cd poisoning on the heart, extending the enquiry from isometric to auxotonic contractions, thereby examining the effect on the ability of myocardial tissues to perform mechanical work. Different shifts were revealed in myocardial force-velocity relations following subchronic exposure of rats to lead acetate and cadmium chloride acting separately, in combination, or in combination with a bioprotective complex (BPC). The experiments were conducted on isolated preparations of trabecules and papillary muscles of the right ventricle in physiological loading conditions and on isolated heart muscle contractile proteins examined by the in vitro motility assay. The results of the latter correlate with the shifts in the ratio of cardiac myosin isoforms. The amount of work performed by the myocardium was calculated on the basis of the tension-shortening loop area and was found to be similar in the preparations from all experimental groups. This fact presumably reflects adaptive capacity of the myocardial function even when contractility is damaged due to the metallic intoxication of a moderate severity. Some characteristics of rat myocardium altered by the impact of lead-cadmium intoxication became fully or partly normalized if intoxication developed against background administration of a bioprotective complex (BPC). Together with previously reported results obtained in the isometric mode of contractility, all these results strengthen the scientific foundations of risk assessment and risk management projects in the occupational and environmental conditions characterized by human exposure to lead and/or cadmium.

Further verification of some postulates of the combined toxicity theory: New animal experimental data on separate and joint adverse effects of lead and cadmium

Outbred male rats were repeatedly injected intraperitoneally two-level sub-lethal doses of lead acetate and/or cadmium chloride solutions 3 times a week during 6 weeks. The animals developed explicit, even if moderate, subchronic intoxication characterized by a large number of indices, both common to both metals (including increased DNA fragmentation coefficient) and lead-specific. Special attention was paid to hemodynamic and electrocardiographic effects. The combined action of lead and cadmium was modeled with the help of the Response Surface Methodology to obtain additional support for the previously substantiated postulates of combined toxicity's typological ambiguity. This is dependent on which particular effect comes under consideration, on its level, and on the acting dose ratio. For one and the same toxic combination, the type of combined toxic action can vary from synergistic to contra-directional. In particular, the actions of lead and cadmium on blood pressure were found to be opposite in direction. Furthermore, it is shown once again that the systemic toxic effects of a metal combination, its in vivo genotoxicity included, can be more or less attenuated by background administration of a theoretically justified composition of biologically active agents.

Systems Biology-Based Analysis Indicates Global Transcriptional Impairment in Lead-Treated Human Neural Progenitor Cells

Lead poisoning effects are wide and include nervous system impairment, peculiarly during development, leading to neural damage. Lead interaction with calcium and zinc-containing metalloproteins broadly affects cellular metabolism since these proteins are related to intracellular ion balance, activation of signaling transduction cascades, and gene expression regulation. In spite of lead being recognized as a neurotoxin, there are gaps in knowledge about the global effect of lead in modulating the transcription of entire cellular systems in neural cells. In order to investigate the effects of lead poisoning in a systemic perspective, we applied the transcriptogram methodology in an RNA-seq dataset of human embryonic-derived neural progenitor cells (ES-NP cells) treated with 30 µM lead acetate for 26 days. We observed early downregulation of several cellular systems involved with cell differentiation, such as cytoskeleton organization, RNA, and protein biosynthesis. The downregulated cellular systems presented big and tightly connected networks. For long treatment times (12 to 26 days), it was possible to observe a massive impairment in cell transcription profile. Taking the enriched terms together, we observed interference in all layers of gene expression regulation, from chromatin remodeling to vesicle transport. Considering that ES-NP cells are progenitor cells that can originate other neural cell types, our results suggest that lead-induced gene expression disturbance might impair cells’ ability to differentiate, therefore influencing ES-NP cells’ fate.

Further analysis of rat myocardium contractility changes associated with a subchronic lead intoxication

A moderate subchronic lead intoxication was observed in male rats after repeated intraperitoneal injections of lead acetate. Right ventricular trabeculae and papillary muscles were isolated for in vitro studying of the contraction-relaxation cycle under isotonic and physiological loading. The contractile function of the myocardium was also assessed by measuring the velocity of thin filament movement over myosin. Lead intoxication led in papillary muscles to a decrease in the maximal rate of isotonic shortening for all afterloads and a decrease in the thin filament sliding velocity. Papillary muscles from lead-exposed rats displayed marked changes in most of the main characteristics of afterload contraction-relaxation cycles, but in trabeculae these changes were less pronounced. The reported changes were attenuated to some extent in rats treated with a Ca-containing bioprotector. The amount of work produced by both types of heart muscle preparations was not changed by lead. Only in papillary muscles the load-dependent relaxation index was significantly increased in the lead-treated groups. Thus subchronic lead intoxication affects the peak rate of force development and relaxation properties of cardiac muscle contracting in isotonic/physiological regimes rather than the total amount of mechanical work, which may reflect adaptive changes in the myocardial function under decreased contractility.

Diabetes and Exposure to Environmental Lead (Pb)

Although the increased incidence of type 2 diabetes since the 1950s is thought to be primarily due to coincident alterations in lifestyle factors, another potential contributing factor in industrialized countries is exposure of the population to environmental pollutants and industrial chemicals. Exposure levels of many environmental toxicants have risen in the same time-frame as the disease incidence. Of particular interest in this regard is the metal lead. Although overall lead exposure levels have diminished in recent decades, there is an under-recognized but persistent occurrence of lead exposure in poor underserved urban populations. Although the neural developmental pathologies induced by lead exposures have been well documented, very little is known about the effect of lead exposure on the incidence of chronic metabolic diseases such as type 2 diabetes. Although our understanding of the metabolic health effects of lead exposure is incomplete, there are studies in model systems and a small amount of epidemiological data that together suggest a deleterious effect of environmental lead exposure on metabolic health. This article reviews the human, animal and in vitro studies that have examined the effects of lead exposure on the development of diabetes and related metabolic conditions.

Effects of subchronic lead intoxication of rats on the myocardium contractility

Outbred male rats were repeatedly injected IP with sub-lethal doses of lead acetate 3 times a week during 5 weeks. They developed an explicit, even if moderate, lead intoxication characterized by typical hematological and some other features. The next day after the last injection the heart of each animal was excised, and the trabecules and papillary muscles from the right ventricle were used for modeling in vitro isometric (with varying starting length of the preparation) regimes of the contraction-relaxation cycle with different preloads. Several well-established parameters of this model were found changed compared with the preparations taken from the hearts of healthy control rats. Background in vivo calcium treatment attenuated both systemic and cardiotoxic effects of lead to an extent. We show for the first time that subchronic intoxication with lead caused myocardial preparations in a wide range of lengths to respond by a decrease in the time and speed parameters of the isometric contraction while maintaining its amplitude and by a decrease in the passive stiffness of trabecules. The responses of the various heart structures are outlined, and the isomyosin ratio is shown to have shifted towards the slow isoform. Mechanistic and toxicological inferences from the results are discussed.

Defining potential roles of Pb(2+) in neurotoxicity from a calciomics approach

Metal ions play crucial roles in numerous biological processes, facilitating biochemical reactions by binding to various proteins. An increasing body of evidence suggests that neurotoxicity associated with exposure to nonessential metals (e.g., Pb(2+)) involves disruption of synaptic activity, and these observed effects are associated with the ability of Pb(2+) to interfere with Zn(2+) and Ca(2+)-dependent functions. However, the molecular mechanism behind Pb(2+) toxicity remains a topic of debate. In this review, we first discuss potential neuronal Ca(2+) binding protein (CaBP) targets for Pb(2+) such as calmodulin (CaM), synaptotagmin, neuronal calcium sensor-1 (NCS-1), N-methyl-d-aspartate receptor (NMDAR) and family C of G-protein coupled receptors (cGPCRs), and their involvement in Ca(2+)-signalling pathways. We then compare metal binding properties between Ca(2+) and Pb(2+) to understand the structural implications of Pb(2+) binding to CaBPs. Statistical and biophysical studies (e.g., NMR and fluorescence spectroscopy) of Pb(2+) binding are discussed to investigate the molecular mechanism behind Pb(2+) toxicity. These studies identify an opportunistic, allosteric binding of Pb(2+) to CaM, which is distinct from ionic displacement. Together, these data suggest three potential modes of Pb(2+) activity related to molecular and/or neural toxicity: (i) Pb(2+) can occupy Ca(2+)-binding sites, inhibiting the activity of the protein by structural modulation, (ii) Pb(2+) can mimic Ca(2+) in the binding sites, falsely activating the protein and perturbing downstream activities, or (iii) Pb(2+) can bind outside of the Ca(2+)-binding sites, resulting in the allosteric modulation of the protein activity. Moreover, the data further suggest that even low concentrations of Pb(2+) can interfere at multiple points within the neuronal Ca(2+) signalling pathways to cause neurotoxicity.

How plants cope with heavy metals

Heavy metals are naturally occurring in the earth's crust but anthropogenic and industrial activities have led to drastic environmental pollutions in distinct areas. Plants are able to colonize such sites due to several mechanisms of heavy metal tolerance. Understanding of these pathways enables different fruitful approaches like phytoremediation and biofortification.Therefore, this review addresses mechanisms of heavy metal tolerance and toxicity in plants possessing a sophisticated network for maintenance of metal homeostasis. Key elements of this are chelation and sequestration which result either in removal of toxic metal from sensitive sites or conduct essential metal to their specific cellular destination. This implies shared pathways which can result in toxic symptoms especially in an excess of metal. These overlaps go on with signal transduction pathways induced by heavy metals which include common elements of other signal cascades. Nevertheless, there are specific reactions some of them will be discussed with special focus on the cellular level.

Activity-dependent regulation of NMDA receptors in substantia nigra dopaminergic neurones

N-Methyl-d-aspartate receptors (NMDARs) are Ca(2+)-permeable glutamate receptors that play a critical role in synaptic plasticity and promoting cell survival. However, overactive NMDARs can trigger cell death signalling pathways and have been implicated in substantia nigra pars compacta (SNc) pathology in Parkinson's disease. Calcium ion influx through NMDARs recruits Ca(2+)-dependent proteins that can regulate NMDAR activity. The surface density of NMDARs can also be regulated dynamically in response to receptor activity via Ca(2+)-independent mechanisms. We have investigated the activity-dependent regulation of NMDARs in SNc dopaminergic neurones. Repeated whole-cell agonist applications resulted in a decline in the amplitude of NMDAR currents (current run-down) that was use dependent and not readily reversible. Run-down was reduced by increasing intracellular Ca(2+) buffering or by reducing Ca(2+) influx but did not appear to be mediated by the same regulatory proteins that cause Ca(2+)-dependent run-down in hippocampal neurones. The NMDAR current run-down may be mediated in part by a Ca(2+)-independent mechanism, because intracellular dialysis with a dynamin-inhibitory peptide reduced run-down, suggesting a role for clathrin-mediated endocytosis in the regulation of the surface density of receptors. Synaptic NMDARs were also subject to current run-down during repeated low-frequency synaptic stimulation in a Ca(2+)-dependent but dynamin-independent manner. Thus, we report, for the first time, regulation of NMDARs in SNc dopaminergic neurones by changes in intracellular Ca(2+) at both synaptic and extrasynaptic sites and provide evidence for activity-dependent changes in receptor trafficking. These mechanisms may contribute to intracellular Ca(2+) homeostasis in dopaminergic neurones by limiting Ca(2+) influx through the NMDAR.

Mechanisms of lead and manganese neurotoxicity

Human exposure to neurotoxic metals is a global public health problem. Metals which cause neurological toxicity, such as lead (Pb) and manganese (Mn), are of particular concern due to the long-lasting and possibly irreversible nature of their effects. Pb exposure in childhood can result in cognitive and behavioural deficits in children. These effects are long-lasting and persist into adulthood even after Pb exposure has been reduced or eliminated. While Mn is an essential element of the human diet and serves many cellular functions in the human body, elevated Mn levels can result in a Parkinson's disease (PD)-like syndrome and developmental Mn exposure can adversely affect childhood neurological development. Due to the ubiquitous presence of both metals, reducing human exposure to toxic levels of Mn and Pb remains a world-wide public health challenge. In this review we summarize the toxicokinetics of Pb and Mn, describe their neurotoxic mechanisms, and discuss common themes in their neurotoxicity.

Relation between heavy metals and left ventricular diastolic function in patients with coronary artery disease

One of the earliest detectable abnormalities in patients with coronary artery disease is left ventricular diastolic dysfunction. Its main cause is myocardial ischaemia, but still many of other mechanisms are possible. The aim of this study was to investigate the relation of cadmium, lead, copper, zinc, and selenium with left ventricular diastolic function indices in patients with coronary artery disease. The study included 33 patients (14 with single-vessel disease and 19 with multivessel disease on angiography) who were not exposed occupationally to heavy metals. Control group consisted of 18 patients with normal coronary arteries. The blood cadmium and lead concentrations and serum zinc, selenium and copper concentrations were determined using atomic absorption spectrophotometry. Left ventricular diastolic function parameters were estimated by echo-Doppler and colour M-mode. All heavy metal concentrations were within the standard ranges in all studied groups. Patients with single-vessel disease, multivessel disease and control group did not differ in concentrations of studied metals. On univariate and multivariate analysis only cadmium significantly correlated with echocardiographic parameters of left ventricular diastolic function: peak early to peak late mitral inflow velocity ratio (r = -0.36, p < 0.05) and deceleration time of early mitral inflow (r = 0.43, p < 0.01) in patients with CAD. No significant correlations between concentrations of heavy metals and indices of left ventricular diastolic function were found in control group. The present data suggest that cadmium in concentrations even below the threshold limit value for unexposed population may contribute to development of left ventricular diastolic dysfunction in patients with CAD, potentializing the effect of myocardial ischaemia.

Comparative study of Pb-phytoextraction potential in Sesuvium portulacastrum and Brassica juncea: tolerance and accumulation

Lead phytoextraction from salty soils is a difficult task because this process needs the use of plants which are able to tolerate salt and accumulate Pb(2+) within in their shoots. It has recently been suggested that salt-tolerant plants are more suitable for heavy metals extraction than salt-sensitive ones commonly used in this approach. The aim of this study was to investigate Pb-phytoextraction potential of the halophyte Sesuvium portulacastrum in comparison with Brassica juncea commonly used in Pb-phytoextraction. Seedlings of both species were exposed in nutrient solution to 0, 200, 400, 800 and 1000 μM Pb(2+) for 21 days. Lead strongly inhibited growth in B. juncea but had no impact on S. portulacastrum. Exogenous Pb(2+) reduced nutrients uptake mainly in B. juncea as compared to S. portulacastrum. Lead was preferentially accumulated in roots in both species. S. portulacastrum accumulated more Pb(2+) in the shoot than B. juncea. Hence, the amounts of Pb(2+) translocated at 1000 μM Pb(2+) were 3400 μg g(-1) DW and 2200 μg g(-1) DW in S. portulacastrum and B. juncea, respectively. These results suggest that S. portulacastrum is more efficient to extract Pb(2+) than B. juncea.

Molecular neurobiology of lead (Pb(2+)): effects on synaptic function

Lead (Pb(2+)) is a ubiquitous environmental neurotoxicant that continues to threaten public health on a global scale. Epidemiological studies have demonstrated detrimental effects of Pb(2+) on childhood IQ at very low levels of exposure. Recently, a mechanistic understanding of how Pb(2+) affects brain development has begun to emerge. The cognitive effects of Pb(2+) exposure are believed to be mediated through its selective inhibition of the N-methyl-D: -aspartate receptor (NMDAR). Studies in animal models of developmental Pb(2+) exposure exhibit altered NMDAR subunit ontogeny and disruption of NMDAR-dependent intracellular signaling. Additional studies have reported that Pb(2+) exposure inhibits presynaptic calcium (Ca(2+)) channels and affects presynaptic neurotransmission, but a mechanistic link between presynaptic and postsynaptic effects has been missing. Recent work has suggested that the presynaptic and postsynaptic effects of Pb(2+) exposure are both due to inhibition of the NMDAR by Pb(2+), and that the presynaptic effects of Pb(2+) may be mediated by disruption of NMDAR activity-dependent signaling of brain-derived neurotrophic factor (BDNF). These findings provide the basis for the first working model to describe the effects of Pb(2+) exposure on synaptic function. Here, we review the neurotoxic effects of Pb(2+) exposure and discuss the known effects of Pb(2+) exposure in light of these recent findings.

Transport pathways for cadmium in the intestine and kidney proximal tubule: focus on the interaction with essential metals

Cadmium (Cd) is a toxic metal with a propensity to accumulate in the proximal tubules cells (PTC) of the kidney where it can lead to tubular dysfunction and eventually renal failure. Although Cd(2+)-induced nephrotoxicity has been well described there is still uncertainty about how this metal gains entry into these cells to induce toxicity. As a non-essential metal, specific transport proteins for Cd are unlikely to exist. Rather transport proteins/channels used by essential metals (iron, zinc, calcium) are thought to be responsible. When these dietary essential metals are in short supply and deficiencies develop, Cd absorption and toxicity are enhanced. This is primarily due to increased expression of essential metal transport proteins such as divalent metal transporter 1 (DMT1) which can transport Cd in the intestine and enhance toxicity in the kidney. The zinc/bicarbonate sympoters ZIP8 and 14 are expressed at the apical membrane of enterocytes and PTC, and can transport Cd into cells. TRPV5 and 6 are major transporters for calcium in intestine and kidney and may be involved in Cd transport in these locations. Cd in the circulation is bound to proteins such as metallothioneins (MT) which are readily filtered. Two multiligand receptors, megalin and cubulin, reabsorb filtered proteins including albumin and MT by the process of receptor-mediated endocytosis. This review summarises the transport pathways for Cd in the intestine and kidney proximal tubule focusing in particular at how Cd uses essential metal transport processes to gain entry to the circulation and the kidney.

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.

Mechanisms of lead-induced hypertension and cardiovascular disease

Lead is a ubiquitous environmental toxin that is capable of causing numerous acute and chronic illnesses. Population studies have demonstrated a link between lead exposure and subsequent development of hypertension (HTN) and cardiovascular disease. In vivo and in vitro studies have shown that chronic lead exposure causes HTN and cardiovascular disease by promoting oxidative stress, limiting nitric oxide availability, impairing nitric oxide signaling, augmenting adrenergic activity, increasing endothelin production, altering the renin-angiotensin system, raising vasoconstrictor prostaglandins, lowering vasodilator prostaglandins, promoting inflammation, disturbing vascular smooth muscle Ca(2+) signaling, diminishing endothelium-dependent vasorelaxation, and modifying the vascular response to vasoactive agonists. Moreover, lead has been shown to cause endothelial injury, impede endothelial repair, inhibit angiogenesis, reduce endothelial cell growth, suppress proteoglycan production, stimulate vascular smooth muscle cell proliferation and phenotypic transformation, reduce tissue plasminogen activator, and raise plasminogen activator inhibitor-1 production. Via these and other actions, lead exposure causes HTN and promotes arteriosclerosis, atherosclerosis, thrombosis, and cardiovascular disease. In conclusion, studies performed in experimental animals, isolated tissues, and cultured cells have provided compelling evidence that chronic exposure to low levels of lead can cause HTN, endothelial injury/dysfunction, arteriosclerosis, and cardiovascular disease. More importantly, these studies have elucidated the cellular and molecular mechanisms of lead's action on cardiovascular/renal systems, a task that is impossible to accomplish using clinical and epidemiological investigations alone.

Potential marker enzymes and metal-metal interactions in Helisoma duryi and Lymnaea natalensis exposed to cadmium

In this study, we investigate the effects of exposure to cadmium and copper on Lymnaea natalensis and Helisoma duryi. The snails were dosed with Cd2+ or Cu2+ for a period of 96h. Snails dosed with Cd accumulated the metal significantly (P<0.05) in tissues but not in shells. Mortality was observed at approximately 1mg Cd/l of culture water. In tissues and shells of snails dosed with Cd or Cu, synergistic and antagonistic metal-metal interactions involving Cd, Cu, Zn, and Pb were observed and these may affect metal toxicity. Glutamate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase were assayed in whole snail tissue sub-cellular fractions of Cd-dosed snails. Generally, enzyme activity significantly increased at lower concentrations of Cd but decreased at high concentrations of the metal. However, mitochondrial alanine aminotransferase activity progressively declined with increasing Cd concentration. The changes in some of the enzymes' activities suggest biomarker potential.

A structural insight into lead neurotoxicity and calmodulin activation by heavy metals

Calmodulin is a calcium sensor that is also capable of binding and being activated by other metal ions. Of specific interest in this respect is lead, which is known to be neurotoxic and to have a very high affinity towards calmodulin. Crystal structures of human calmodulin complexed with lead and barium ions have been solved. The results will help in understanding the activation mechanisms of calmodulin by different heavy metals and will provide a detailed view of a putative target for lead neurotoxicity in humans.

Ca(2+) -dependent plant response to Pb(2+) is regulated by LCT1

Tobacco plants transformed with TaLCT1 were cultured on Knop's medium with modified calcium concentrations (0.01-3 mM) in the presence of Pb(2+), and in soil contaminated by lead. A 4-5 microM Pb(2+) administered in the presence of 1 mM Ca(2+) inhibited the root growth of transgenic plants to much lesser degree than of control plants, whereas in the presence of 3mM Ca(2+) no differences were found between the studied lines. The reduction of Pb(2+) toxicity in the presence of 1 mM Ca(2+) was not accompanied by a change in the lead tissue concentration. However, when Ca(2+) level in the medium was lowered to 0.01 mM, several fold higher root/shoot Pb ratio in transgenic plants was observed, twofold increase in the total amount of metal accumulated, and lower concentration of Pb in the xylem sap. Results suggest the involvement of TaLCT1 in the regulation of Ca-dependent Pb-detoxification, and under conditions of low calcium in lead uptake and distribution.

Metal transporters in intestine and brain: their involvement in metal-associated neurotoxicities

The transport of essential metals and other nutrients across tight membrane barriers such as the gastrointestinal tract and blood-brain barrier is mediated by specific transport mechanisms. Specific transporters take up metals at the apical surface and export them at the basolateral surface, and are involved in their intracellular distribution. Transporters for each of the major essential metals, calcium, iron and zinc, have been identified. These transporters also mediate the transport of non-essential metals across tight membrane barriers. For example, the intestinal iron transporter divalent metal transporter 1 mediates the uptake of lead and cadmium. The levels of essential metals are strictly regulated by transporters. When dietary levels of essential metals are low, levels of the corresponding transporters increase in the intestine, after which there is a greater potential for increased transport of toxic metals. In the brain, the strict regulation of metals prevents injury that potentially would result from oxidative damage induced by the essential metals iron, copper and zinc. Indeed, the oxidative damage found in neurodegenerative diseases is likely to be due to higher levels of these metals. Involvement of intracellular transporters for copper and zinc has been shown in animal models of Alzheimer's disease, raising the possibility that higher levels of iron, zinc and copper might be due to a disruption in the activity of transporters. Accordingly, exposure to toxicants that affect the activity of transporters potentially could contribute to the aetiology/progression of neurodegenerative diseases.

Reciprocal inhibition of Cd and Pb sulfocomplexes for uptake in Caco-2 cells

Cadmium-lead interactions for uptake were studied in the TC7 clone of human enterocytic-like Caco-2 cells as a function of inorganic metal speciation. We have previously shown that Cd uptake in these cells involves both the free cation Cd2+ and chlorocomplex (CdCln(2-n)) species. Here we show 1.9 times higher uptake levels for 109CdCln(2-n) compared to 210PbCln(2-n). Reciprocal inhibitions of chlorocomplexes were observed with a much higher inhibitory effect of Cd compared to Pb. Replacing Cl- by NO3- increased both the level of aquo ion 109Cd2+ and 109Cd accumulation. In contrast, higher levels of 210Pb2+ did not favor 210Pb uptake. For both metals, higher uptake data were recorded in the presence of SO4(2-), leading to sulfocomplex formation, compared with Cl-. Reciprocal inhibitions were minimal at high-cation levels but were significant and comparable in the presence of sulfo-complexes. We conclude that, in addition to Cd2+ (but not Pb2+), sulfocomplexes of both metals would preferentially be taken up compared to chlorocomplexes. NRAMP2 is not involved in Pb2+ uptake, and the NRAMP2-mediated Cd2+ uptake is insensitive to Pb. Uptake of Pb chlorocomplexes could involve specific mechanisms but of very low affinity, whereas uptake of Pb sulfocomplexes occurs with high affinity.

Lead neurotoxicity: From exposure to molecular effects

The effects of lead (Pb(2+)) on human health have been recognized since antiquity. However, it was not until the 1970s that seminal epidemiological studies provided evidence on the effects of Pb(2+) intoxication on cognitive function in children. During the last two decades, advances in behavioral, cellular and molecular neuroscience have provided the necessary experimental tools to begin deciphering the many and complex effects of Pb(2+) on neuronal processes and cell types that are essential for synaptic plasticity and learning and memory in the mammalian brain. In this review, we concentrate our efforts on the effects of Pb(2+) on glutamatergic synapses and specifically on the accumulating evidence that the N-methyl-D-aspartate type of excitatory amino acid receptor (NMDAR) is a direct target for Pb(2+) effects in the brain. Our working hypothesis is that disruption of the ontogenetically defined pattern of NMDAR subunit expression and NMDAR-mediated calcium signaling in glutamatergic synapses is a principal mechanism for Pb(2+)-induced deficits in synaptic plasticity and in learning and memory documented in animal models of Pb(2+) neurotoxicity. We provide an introductory overview of the magnitude of the problem of Pb(2+) exposure to bring forth the reality that childhood Pb(2+) intoxication remains a major public health problem not only in the United States but worldwide. Finally, the latest research offers some hope that the devastating effects of childhood Pb(2+) intoxication in a child's ability to learn may be reversible if the appropriate stimulatory environment is provided.

Overexpression of LCT1 in tobacco enhances the protective action of calcium against cadmium toxicity

Wheat cDNA LCT1, a nonspecific transporter for Ca2+, Cd2+, Na+ and K+, was overexpressed in tobacco. Transformants were tested for their sensitivity to a range of Ca2+-concentrations [0.01-10 mM Ca(NO3)2] with or without the presence of 0.05 mM Cd(NO3)2. Calcium and cadmium accumulation was also determined. LCT1-transformed plants expressed a phenotype distinct from controls only under conditions of low calcium (0.01-1 mM Ca2+). They grew significantly better and had slightly higher shoot calcium concentration. Transformants subjected to 0.05 mM Cd(NO3)2 in the presence of 1 mM Ca2+ displayed a substantially higher level of tolerance to cadmium and accumulated less Cd in roots. These results are the first to demonstrate the involvement of LCT1 in calcium acquisition and in the regulation of amelioration of Cd-toxicity by calcium.

Lead-dependent effects on arachidonic acid accumulation and the proliferation of vascular smooth muscle

Lead (Pb(2+)) has been implicated in the development of hypertension and atherosclerosis. The proliferation of vascular smooth muscle cells (VSMC) is a central feature of both conditions and there is evidence that Pb(2+) potentiates serum-dependent cell growth. The aim of this work was to examine the role of phospholipase A(2) in mitogen-dependent VSMC proliferation and determine if Pb(2+) interacts with this system in order to potentiate mitotic events. It was observed that cell proliferation induced by angiotensin II, or fetal bovine serum, required the activation of a Ca(2+)-dependent cytosolic phospholipase A(2) and the subsequent release of unesterified arachidonic acid. This path was affected by Pb(2+) as the metal increased the amount of arachidonic acid accumulation induced by either mitogen. In addition, Pb(2+) potentiated mitogen-induced DNA synthesis when present at lower doses (0.02 or 0.2 mg%), but had no effect on DNA synthesis, or cell numbers, in unstimulated cells. However, a high dose (2 mg%) of Pb(2+) attenuated the DNA synthesis stimulated by angiotensin II, or serum, but induced the accumulation of unesterified arachidonic acid in unstimulated cells. A biphasic effect of Pb(2+) on cell numbers and viability was also observed as 0.02 or 0.2 mg% Pb(2+) did not affect cell numbers or trypan blue exclusion in unstimulated cells, while 2 mg% Pb(2+) reduced cell numbers and viability. It appeared, therefore, that the lower concentrations of Pb(2+) increased arachidonic acid release and DNA synthesis only in stimulated VSMC, perhaps due to further activation of a Ca(2+)-dependent processes. In contrast, the high dose of Pb(2+) reduced DNA synthesis in stimulated cells and reduced cell numbers and viability in unstimulated cells, which may relate to the noted increase in unesterified arachidonic acid.

Lead enters Rcho-1 trophoblastic cells by calcium transport mechanisms and complexes with cytosolic calcium-binding proteins

Within the placenta, a specialized Ca(2+) transport pathway develops in trophoblasts to promote growth of the fetus and hypothetically to enhance fetal uptake of Pb(2+). This hypothesis could not be tested until a method to monitor Pb(2+) influx by indo-1 fluorescence quench became available. We have applied this new method to cultured undifferentiated and differentiated Rcho-1 trophoblastic cells. Pb(2+) concentrations of 1 and 10 microM are equivalent to blood levels of 20 and 200 microg/dl in pregnant women. Over this range, Pb(2+) uptake increased with time and concentration in medium containing 1 mM Ca(2+) but was greater in Ca(2+)-omitted solutions. Activation of capacitative Ca(2+) entry (CCE) with thapsigargin, an endoplasmic reticulum (ER) Ca(2+) pump inhibitor, increased Pb(2+) uptake, while inhibition of CCE by La(3+) decreased influx. Parathyroid hormone-related peptide (PTHrP) stimulates the synthesis of Ca(2+)-binding proteins (CaBPs), as well as Ca(2+) transporters, during trophoblastic differentiation. Pretreatment for 72 h with PTHrP increased Pb(2+) uptake by undifferentiated Rcho-1 cells but had little effect on the quench in differentiated cells, probably due to their greater content of CaBPs which competed for Pb(2+)-binding with indo-1. This competition was most evident in differentiated cells when 1 microM Pb(2+) caused an initial quench, followed by a rise in fluorescence. This rise was not inhibited by thapsigargin, thereby ruling out sequestration into the ER and leaving complexation of Pb(2+) by CaBPs as the most plausible interpretation. We conclude that trophoblasts have the ability to clear Pb(2+) from the maternal circulation and deliver it to the fetus.

The effects of dietary calcium during lactation on lead in bone mobilization: implications for toxicology

Under a normal 1.0% calcium (Ca) concentration in the diet during and after chronic lead (Pb) intoxication, there was a significant increment in the concentration of Pb in the blood, kidney, liver and brain during lactation, parallel to a decrement of Pb in the bone. The increment of Pb in the brain was accompanied by an enhanced lipid oxidation (increase in conjugated dienes). During lactation, on the first 14 days, when dietary Ca was reduced to 0.05%, bone Ca concentration was decreased by 15%, bone resorption measured as acid phosphatase activity in plasma increased three times and Pb bone concentration dropped by 30%. Under a 0.05% Ca in the diet in the nonlactating rats, Ca in the bone decreases also, but there were neither increments in bone resorption nor Pb efflux from the bone. These results suggested that Pb efflux in the bone was related to bone resorption during lactation. Interestingly, when dietary Ca was enhanced to 2.5% in lactating rats, Ca concentration in the bone increased by 21%, but resorption did not decrease and Pb bone concentration decreased by 28%, enhancing toxicity. In the control Pb-exposed nonlactating rats under a 2.5% Ca in the diet, Ca concentration in the bone was increased (18%), and Pb concentration in the bone was unaltered. Since Ca metabolism changes drastically in humans during pregnancy and lactation, and it is likely that bone Pb is mobilized and transferred to the more available compartments of the maternal circulation, the increment in daily intake of Ca during lactation could enhance Pb efflux from the bone.

Low level Pb(2+) exposure affects hippocampal protein kinase C gamma gene and protein expression in rats

We examined the effect of chronic exposure to lead (Pb(2+)) on protein kinase C (PKC) in 50-day-old rat hippocampus. Cytosolic and membrane fractions of hippocampus from Pb(2+)-exposed rats showed reduced expression of PKC gamma protein. In contrast, a significant elevation of PKC gamma mRNA was observed in pyramidal and dentate granule cell layers. Protein expression of alpha, beta I, beta II and epsilon isoenzymes were unchanged in Pb(2+)-exposed rats, as was [(3)H]phorbol 12,13 dibutyrate (PDBu) binding in tissue slices. Differences were not observed in Ca(2+)-dependent or -independent PKC activity, or in PKC-specific back-phosphorylation of hippocampal homogenates from Pb(2+)-exposed rats. Reduced subcellular levels of PKC gamma in Pb(2+)-exposed rats suggest that signal transduction in the hippocampus may be selectively altered and may be important in manifesting Pb(2+)-induced impairments of synaptic plasticity, learning and memory.

Nicotinic receptors on hippocampal cultures can increase synaptic glutamate currents while decreasing the NMDA-receptor component

Activation of presynaptic nicotinic acetylcholine receptors (nAChRs) can enhance the release of glutamate from synapses in hippocampal slices and cultures. In hippocampal cultures making autaptic connections, rapid application of a high concentration of nicotine activated presynaptic, postsynaptic, and somatic nAChRs, which consequently enhanced the amplitude of evoked excitatory postsynaptic currents (eEPSCs) mediated by glutamate receptors. The increased eEPSC amplitudes arose from enhanced glutamate release caused by presynaptic nAChRs (Radcliffe and Dani, 1998, Journal of Neuroscience 18, 7075). The same whole-cell nicotine applications that enhanced non-NMDA eEPSCs often decreased the NMDA-receptor component of the eEPSCs. Furthermore, whole-cell activation of nAChRs by nicotine selectively reduced the amplitude of the whole-cell NMDA-receptor currents without affecting the non-NMDA receptor currents. The inhibition by nicotine was prevented by the alpha7-specific antagonist, methyllycaconitine, and required the presence of extracellular Ca(2+). The calmodulin antagonist fluphenazine prevented inhibition of the NMDA-receptor current by nAChR activity, suggesting that a Ca(2+)-calmodulin-dependent process mediated the effect of nicotine. Our results indicate that activation of nAChRs can modulate glutamatergic synapses in several ways. Presynaptic nAChR activity enhances synaptic transmission by increasing transmitter release. Additionally, somatic or postsynaptic nAChRs can initiate a Ca(2+) signal that can act via calmodulin to reduce the responsiveness of NMDA receptors.

Activation of recombinant human SK4 channels by metal cations

The effects of metal cations on the activation of recombinant human SK4 (also known as hIK1 or hKCa4) channels, expressed in HEK 293 cells, were tested using patch clamp recording. Of the nine metals tested, cobalt, iron, magnesium, and zinc did not activate the SK4 channels when applied, at concentrations up to 100 microM, to the inside of SK4 channel-expressing membrane patches. Barium, cadmium, calcium, lead, and strontium activated SK4 channels in a concentration-dependent manner. The rank order of potency was at Ca2+ > Pb2+ > Cd2+ > Sr2+ > Ba2+.

Protein kinase C modulation of recombinant NMDA receptor currents: roles for the C-terminal C1 exon and calcium ions

Protein kinase C (PKC) positively modulates NMDA receptor (NMDAR) currents. In contrast to previous reports, this study determines the importance of individual exons in the mechanism underlying the potentiation process by examining the complete set of eight naturally occurring splice variants expressed in Xenopus oocytes both as homomers and as heteromeric NR1/NR2A or NR1/NR2B complexes. After PKC stimulation, homomeric currents demonstrated a high level of potentiation ( approximately 500% of untreated baseline currents) that reduced to a lower level ( approximately 300% of baseline) in variants containing the first C-terminal exon (C1). An ANOVA showed that only C1 and no other exon or interaction of exons determined the degree of NMDAR current modulation by PKC. When recordings were performed in solutions in which barium replaces calcium, only the lower form of potentiation was observed, regardless of the splice variant exon composition. This suggested an important role for calcium in the PKC modulation of homomeric NMDA splice variant currents in which the C1 exon also participates. The effectiveness of the C1 exon to reduce the higher form of potentiation is modulated by heteromeric assemblies with NR2A heteromers yielding smaller levels of potentiation and a larger C1 exon effect compared with NR2B heteromers. The heteromers demonstrated the higher form of potentiation even in the absence of calcium. Furthermore, calcium had different effects in the potentiation of the heteromers depending on the NR2 subunit. This study refines the region of the NR1 subunit involved in a modulation crucial to the function of NMDA receptors and provides evidence that the NR2A and NR2B subunits realize this modulation differentially.

Lead may affect glucocorticoid signal transduction in cultured hepatoma cells through inhibition of protein kinase C

Specific cellular sites of lead action have not been completely defined. To elucidate the effects of lead exposure on glucocorticoid-mediated signal transduction in hepatic hormonal target tissues, the induction of tyrosine aminotransferase (TAT) specific activity in the H4-IIE-C3 hepatoma cell culture model system was employed. It had been found that lead acetate (3-10 microM) exposure of HTC cells significantly reduced TAT specific activity in a concentration- and time-dependent manner. Two possible molecular targets of the lead-induced effect were investigated: interference with calcium-mediated cellular processes and calcium- and phospholipid-dependent protein kinase C (PKC) activity and isoform-type interactions. Lead acetate treatment (5 microM) reduced TAT specific activity below sodium acetate treated controls by 31%. One-half of the TAT specific activity was recovered by co-treatment with 5 microM lead acetate and 10 mM calcium chloride. As the concentration of lead acetate was increased to 10 microM, interference in calcium-mediated events also increased. Potentiation of glucocorticoid induction by phorbol myristate acetate (PMA) (300 nM) in control cells was 34%, but was abolished by exposure of cells to 10 microM lead acetate (48 h). Treatment with the kinase inhibitor genistein decreased TAT specific activity by 55% and 45% in control and lead acetate exposed cells, respectively. Following treatment with dexamethasone (100 nM), significant increases in both cytosolic and particulate PKC were noted in control cells but not lead acetate exposed cells. Western blot results indicated that lead exposure may increase PKC beta and decrease PKC alpha translocation from cytosolic to particulate fractions, respectively. Taken together, these results suggest that glucocorticoid signal transduction pathways in HTC cells involve calcium-mediated cellular events and PKC isoforms. Exposure of cells to lead results in interference with calcium-mediated events and aberrant modulation of PKC activities. Within hormonal target cells, these may be toxic molecular sites of action of the heavy metal lead and may contribute to the overall toxicity of lead exposure.

Calcium-cadmium interaction on sugar absorption across the rabbit jejunum

The element Cd is considered to have no biological function and is highly toxic to humans and animals. Toxic effects of this metal upon cell membrane structure and function have been shown. On the other hand, Ca is an essential element in a wide variety of cellular activities. The present study was initiated to research whether the interaction between Ca and Cd could affect D-galactose absorption across the rabbit jejunum in vitro. In media with Ca2+, when CdCl2 was present at 0.5 or 1 mM, Cd was found to significantly reduce the sugar absorption. In Ca2+ -free media, where CaCl2, was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by Cd, but when CaCl2 was replaced isotonically with MgCl2, the inhibition is observed. Verapamil at 10(-6)M (blocking mainly Ca2+ transport) did not modify the inhibitory effect of cadmium on D-galactose transport. When 10(-6)M of A 23187 (Ca2+ specific ionophore) was added in media with/without Ca2+; CdCl2 produced no change in D-galactose transport. These results suggest that Ca and Cd could have affinity for the same chemical groups of enterocyte membrane, which would be related with the intestinal absorption of D-galactose.

The effect of barium on [3H]noradrenalin release from the human neuroblastoma SH-SY5Y

Replacement of Ca2+ with Ba2+ in HEPES-buffered saline stimulated [3H]noradrenalin release in the human neuroblastoma clone SH-SY5Y by up to 20% of the cell content in the absence of other secretory stimuli. The Ba(2+)-evoked release was inhibited by 85% by 3 microM tetrodotoxin and 95% by 5 microM nifedipine. Ba2+ also increased the potency of K(+)-evoked release of [3H]noradrenalin, as maximal release was observed with 60 mM K+ compared with the 100 mM K+ necessary to achieve maximal release in the presence of Ca2+. In contrast, replacing Ca2+ with Ba2+ had little effect on carbachol- and bradykinin-evoked release of [3H]noradrenalin. No evidence was obtained from studies on changes in [Ca2+]i (in response to 100 microM carbachol) using fura-2 that Ba2+ could enter intracellular stores in SH-SY5Y cells. Whole-cell patch-clamp studies showed that Ba2+ depolarizes SH-SY5Y cells as well as enhancing inward Ca2+ channel currents and shifting their voltage dependence to more negative values. These results are discussed in terms of the hypothesis that Ba2+ blocks K+ channels, leading to depolarization followed by opening of voltage-sensitive Na+ channels. This in turn opens voltage-sensitive L-type Ca2+ channels, which are coupled to the release of [3H]noradrenalin in SH-SY5Y cells.

Differential effects of heavy metal ions on Ca(2+)-dependent K+ channels

1. The ability of various divalent metal ions to substitute for Ca2+ in activating distinct types of Ca(2+)-dependent K+ [K+(Ca2+)] channels has been investigated in excised, inside-out membrane patches of human erthrocytes and of clonal N1E-115 mouse neuroblastoma cells using the patch clamp technique. The effects of the various metal ions have been compared and related to the effects of Ca2+. 2. At concentrations between 1 and 100 microM Pb2+, Cd2+ and Co2+ activate intermediate conductance K+(Ca2+) channels in erythrocytes and large conductance K+(Ca2+) channels in neuroblastoma cells. Pb2+ and Co2+, but not Cd2+, activate small conductance K+(Ca2+) channels in neuroblastoma cells. Mg2+ and Fe2+ do not activate any of the K+(Ca2+) channels. 3. Rank orders of the potencies for K+(Ca2+) activation are Pb2+, Cd2+ > Ca2+, Co2+ >> Mg2+, Fe2+ for the intermediate erythrocyte K+(Ca2+) channel, and Pb2+, Cd2+ > Ca2+ > Co2+ >> Mg2+, Fe2+ for the small, and Pb2+ > Ca2+ > Co2+ >> Cd2+, Mg2+, Fe2+ for the large K+(Ca2+) channel in neuroblastoma cells. 4. At high concentrations Pb2+, Cd2+, and Co2+ block K+(Ca2+) channels in erythrocytes by reducing the opening frequency of the channels and by reducing the single channel amplitude. The potency orders of the two blocking effects are Pb2+ > Cd2+, Co2+ >> Ca2+, and Cd2+ > Pb2+, Co2+ >> Ca2+, respectively, and are distinct from the potency orders for activation. 5. It is concluded that the different subtypes of K+(Ca2+) channels contain distinct regulatory sites involved in metal ion binding and channel opening. The K+(Ca2+) channel in erythrocytes appears to contain additional metal ion interaction sites involved in channel block.

Purification and properties of rat cysteine-rich intestinal protein

Cysteine-rich intestinal protein (CRIP) is a zinc-binding protein where the binding domain is in the so-called LIM double zinc finger motif. Methods are described for the preparation of CRIP from rat small intestine. Gel-filtration and ion-exchange chromatography and preparative PAGE gave homogeneous CRIP, based upon analytical PAGE, mass spectrometry and microsequencing. Initial localization of CRIP during chromatography was based on binding of 65Zn radioisotope introduced into the intestine. The stoichiometry of binding by CRIP is less than 2 atoms of zinc per molecule. The metal-binding affinity in vitro is zinc > cadmium > copper > iron, at low metal concentrations. Zinc is the predominant metal bound when these metals are taken up from the intestinal lumen. Zinc binding was not influenced by pH between values of 4.5 to 7.5. Metallothionein has a much greater zinc-binding affinity than CRIP. The tissue concentration of CRIP is of the order of 15-20 micrograms/g of mucosal tissue, suggesting that the protein is more abundant than zinc-finger-containing transcription factors. The metal-binding properties of CRIP are consistent with proposed zinc-related functions for this cytoplasmic protein, which is expressed in the small intestine during the postnatal period.

Barium evokes glutamate release from rat brain synaptosomes by membrane depolarization: involvement of K+, Na+, and Ca2+ channels

During K(+)-induced depolarization of isolated rat brain nerve terminals (synaptosomes), 1 mM Ba2+ could substitute for 1 mM Ca2+ in evoking the release of endogenous glutamate. In addition, Ba2+ was found to evoke glutamate release in the absence of K(+)-induced depolarization. Ba2+ (1-10 mM) depolarized synaptosomes, as measured by voltage-sensitive dye fluorescence and [3H]-tetraphenylphosphonium cation distribution. Ba2+ partially inhibited the increase in synaptosomal K+ efflux produced by depolarization, as reflected by the redistribution of radiolabeled 86Rb+. The release evoked by Ba2+ was inhibited by tetrodotoxin (TTX). Using the divalent cation indicator fura-2, cytosolic [Ca2+] increased during stimulation by approximately 200 nM, but cytosolic [Ba2+] increased by more than 1 microM. Taken together, our results indicate that Ba2+ initially depolarizes synaptosomes most likely by blocking a K+ channel, which then activates TTX-sensitive Na+ channels, causing further depolarization, and finally enters synaptosomes through voltage-sensitive Ca2+ channels to evoke neurotransmitter release directly. Though Ba(2+)-evoked glutamate release was comparable in level to that obtained with K(+)-induced depolarization in the presence of Ca2+, the apparent intrasynaptosomal level of Ba2+ required for a given amount of glutamate release was found to be several-fold higher than that required of Ca2+.

Divalent cations activate small- (SK) and large-conductance (BK) channels in mouse neuroblastoma cells: selective activation of SK channels by cadmium

Effects of Cd2+, Co2+, Fe2+ and Mg2+ (1 microM and 100 microM) and Pb2+ (1 microM and 90 microM) on single-channel properties of the small-conductance (SK) and large-conductance (BK) Ca(2+)-activated K+ channels were investigated in inside-out patches of N1E-115 mouse neuroblastoma cells. Cd2+, Co2+ and Pb2+, but not Fe2+ and Mg2+, cause SK channel opening. The potency of the metals in enhancing the SK channel-open probability follows the sequence Cd2+ approximately Pb2+ > Ca2+ > Co2+ >> Mg2+, Fe2+. The four metals that cause SK channel opening are equipotent in enhancing the opening frequency of SK channels. The BK channel is activated by Pb2+ and Co2+, whereas Cd2+, Fe2+ and Mg2+ are ineffective. The potency of the metals in enhancing BK channel-open probability, open time and opening frequency follows the sequence Pb2+ > Ca2+ > Co2+ >> Cd2+, Mg2+, Fe2+. The results show that SK channels are much more sensitive to Cd2+ than BK channels and indicate that Cd2+ is a selective agonist of SK channels. It is concluded that the various metal ions bind to the same regulatory site(s) at which Ca2+ activates the SK and BK channels under physiological conditions. The different potency sequences of metal ions with respect to BK and SK channel activation indicate that the regulatory sites of these Ca(2+)-activated K+ channels have distinct chemical and physical properties.

Distinct metal ion binding sites on Ca(2+)-activated K+ channels in inside-out patches of human erythrocytes

Effects of Cd2+, Co2+, Pb2+, Fe2+ and Mg2+ (1-100 microM) on single-channel properties of the intermediate conductance Ca(2+)-activated K+ (CaK) channels were investigated in inside-out patches of human erythrocytes in a physiological K+ gradient. Cd2+, Co2+ and Pb2+, but not Fe2+ and Mg2+, were able to induce CaK channel openings. The potency of the metals to open CaK channels in human erythrocytes follows the sequence Pb2+, Cd2+ > Ca2+ > or = Co2+ >> Mg2+, Fe2+. At higher concentrations Pb2+, Cd2+ and Co2+ block the CaK channel by reducing the opening frequency and the single-channel current amplitude. The potency of the metals to reduce CaK channel opening frequency follows the sequence Pb2+ > Cd2+, Co2+ >> Ca2+, which differs from the potency sequence Cd2+ > Pb2+, Co2+ >> Ca2+ to reduce the unitary single-channel current amplitude. Fe2+ reduced the channel opening frequency and enhanced the two open times of CaK channels activated by Ca2+, whereas up to 100 microM Mg2+ had no effect on any of the measured single-channel parameters. It is concluded that the activation of CaK channels of human erythrocytes by various metal ions occurs through an interaction with the same regulatory site at which Ca2+ activates these channels. The different potency orders for the activating and blocking effects suggest the presence of at least one activation and two blocking sites. A modulatory binding site for Fe2+ exists as well. In addition, the CaK channels in human erythrocytes are distinct from other subtypes of Ca(2+)-activated K+ channels in their sensitivity to the metal ions.

Cadmium exposure inhibits the prolactin secretory response to thyrotrophin releasing hormone (TRH) in vitro

In vitro dose response (0.1-100 microM) and time course studies were performed on anterior pituitary cells isolated from female Sprague-Dawley rats in the diestrous stage of the estrous cycle. All doses of TRH produced a significant increase in prolactin release by 5 min which was sustained for 30 min. Pretreatment with 25 or 50 microM Cd for 2 or 4 h produced a significant decrease in prolactin secretion during Cd exposure and in the prolactin secretory response to 1 microM TRH. These results suggest that Cd pretreatment alters the subsequent ability of the lactotrophs to respond to the physiological secretagogue TRH. This alteration does not appear to be due to a rebound phenomenon since basal prolactin release was the same in Cd-pretreated and control cells once the metal was removed. These results suggest that Cd pretreatment produces a significant decrease in prolactin release from the anterior pituitary lactotrophs in response to subsequent TRH stimulation in vitro.

Cadmium inhibits stimulus-response coupling in skate (Raja erinacea) electric organ

1. The effects of cadmium on stimulus-response coupling in the skate (Raja erinacea) electric organ were examined. 2. Cadmium decreased the evoked electrical discharge and the evoked release of 3H-ACh in a concentration-related fashion. 3. Cadmium (100 microM) also blocked voltage-dependent 45Ca uptake. 4. Both d-tubocurarine and nifedipine blocked Ca uptake and evoked potential, but not 3H-ACh release, thus most of the 45Ca uptake measured was post-synaptic through L Ca channels. 5. Nickel, cadmium, and verapamil inhibited 3H-ACh release and evoked potential, indicating a block of pre-synaptic T Ca channels.

Mechanisms of lead neurotoxicity

During the past several years, there has been a renewed interest in the mechanisms by which lead poisoning disrupts brain function. In part, this is related to clinical observations that imply an absence of threshold for toxicity in the immature brain. Many of the neurotoxic effects of lead appear related to the ability of lead to mimic or in some cases inhibit the action of calcium as a regulator of cell function. At a neuronal level, exposure to lead alters the release of neurotransmitter from presynaptic nerve endings. Spontaneous release is enhanced and evoked release is inhibited. The former may be due to activation of protein kinases in the nerve endings and the latter to blockade of voltage-dependent calcium channels. This disruption of neuronal activity may, in turn, alter the developmental processes of synapse formation and result in a less efficient brain with cognitive deficits. Brain homeostatic mechanisms are disrupted by exposure to higher levels of lead. The final pathway appears to be a breakdown in the blood-brain barrier. Again, the ability of lead to mimic or mobilize calcium and activate protein kinases may alter the behavior of endothelial cells in immature brain and disrupt the barrier. In addition to a direct toxic effect upon the endothelial cells, lead may alter indirectly the microvasculature by damaging the astrocytes that provide signals for the maintenance of blood-brain barrier integrity.