Lead enters bovine adrenal medullary cells through calcium channels

Lead exposure induces neurodysfunction through caspase-1-mediated neuronal pyroptosis

Chronic lead (Pb) exposure causes neurodysfunction and contributes to the development of neurodegenerative disease. However, the mechanism of Pb-induced neurological dysfunction have yet to be fully elucidated. This study determined the role pyroptosis plays in Pb-induced neurodysfunction in neurons. We used both in vitro and in vivo approaches to explore whether Pb exposure induces caspase-1-mediated pyroptosis in neurons and its relationship to Pb-induced neurological disorders. Our findings showed that caspase-1-mediated pyroptosis in Pb-exposed neurons activated glycogen synthase kinase 3 protease activity by disrupting Ca2+/calmodulin-dependent protein kinase II/cAMP-response element binding protein pathway, leading to neurological disorders. Moreover, the caspase-1 inhibition VX-765 or the non-steroidal anti-inflammatory drug sodium para-aminosalicylic acid (PAS-Na) attenuated the Pb-induced neurological disorders by alleviating caspase-1 mediated neuronal pyroptosis. Our novel studies suggest that caspase-1-mediated pyroptosis in neurons represents a potential mechanism for Pb-induced neurodysfunction, identifying a putative target for attenuating the neurodegenerative effects induced by this metal.

Far-Infrared Ameliorates Pb-Induced Renal Toxicity via Voltage-Gated Calcium Channel-Mediated Calcium Influx

Far-infrared (FIR), characterized by its specific electromagnetic wavelengths, has emerged as an adjunctive therapeutic strategy for various diseases, particularly in ameliorating manifestations associated with renal disorders. Although FIR was confirmed to possess antioxidative and anti-inflammatory attributes, the intricate cellular mechanisms through which FIR mitigates lead (Pb)-induced nephrotoxicity remain enigmatic. In this study, we investigated the effects of FIR on Pb-induced renal damage using in vitro and in vivo approaches. NRK52E rat renal cells exposed to Pb were subsequently treated with ceramic-generated FIR within the 9~14 μm range. Inductively coupled plasma mass spectrometry (ICP-MS) enabled quantitative Pb concentration assessment, while proteomic profiling unraveled intricate cellular responses. In vivo investigations used Wistar rats chronically exposed to lead acetate (PbAc) at 6 g/L in their drinking water for 15 weeks, with or without a concurrent FIR intervention. Our findings showed that FIR upregulated the voltage-gated calcium channel, voltage-dependent L type, alpha 1D subunit (CaV1.3), and myristoylated alanine-rich C kinase substrate (MARCKS) (p < 0.05), resulting in increased calcium influx (p < 0.01), the promotion of mitochondrial activity, and heightened ATP production. Furthermore, the FIR intervention effectively suppressed ROS production, concurrently mitigating Pb-induced cellular death. Notably, rats subjected to FIR exhibited significantly reduced blood Pb levels (30 vs. 71 μg/mL; p < 0.01), attenuated Pb-induced glomerulosclerosis, and enhanced Pb excretion compared to the controls. Our findings suggest that FIR has the capacity to counteract Pb-induced nephrotoxicity by modulating calcium influx and optimizing mitochondrial function. Overall, our data support FIR as a novel therapeutic avenue for Pb toxicity in the kidneys.

Ionic displacement of Ca2+ by Pb2+ in calmodulin is affected by arrhythmia-associated mutations

Lead is a highly toxic metal that severely perturbs physiological processes even at sub-micromolar levels, often by disrupting the Ca²⁺ signaling pathways. Recently, Pb²⁺-associated cardiac toxicity has emerged, with potential involvement of both the ubiquitous Ca²⁺ sensor protein calmodulin (CaM) and ryanodine receptors. In this work, we explored the hypothesis that Pb²⁺ contributes to the pathological phenotype of CaM variants associated with congenital arrhythmias. We performed a thorough spectroscopic and computational characterization of CaM conformational switches in the co-presence of Pb²⁺ and four missense mutations associated with congenital arrhythmias, namely N53I, N97S, E104A and F141L, and analyzed their effects on the recognition of a target peptide of RyR2. When bound to any of the CaM variants, Pb²⁺ is difficult to displace even under equimolar Ca²⁺ concentrations, thus locking all CaM variants in a specific conformation, which exhibits characteristics of coiled-coil assemblies. All arrhythmia-associated variants appear to be more susceptible to Pb²⁺ than WT CaM, as the conformational transition towards the coiled-coil conformation occurs at lower Pb²⁺, regardless of the presence of Ca²⁺, with altered cooperativity. The presence of arrhythmia-associated mutations specifically alters the cation coordination of CaM variants, in some cases involving allosteric communication between the EF-hands in the two domains. Finally, while wild type CaM increases the affinity for the RyR2 target in the presence of Pb²⁺, no specific pattern could be detected for all other variants, ruling out a synergistic effect of Pb²⁺ and mutations in the recognition process.

Lead Inhibits Postecdysial Exoskeletal Calcification in the Blue Crab (Callinectes sapidus)

Postecdysial mineralization in crustaceans involves the deposition of carbonate salts, such as calcium carbonate, to the organic matrix. Because of the resemblance between Pb²⁺ and Ca²⁺ , the present study was carried out to investigate whether Pb is incorporated into the new shell during postecdysial mineralization using the blue crab (Callinectes sapidus) as the model crustacean. It was hypothesized that injected Pb would be deposited in the shell via calcium transporters in the epidermis during the mineralization process. Postecdysial blue crabs were injected with two doses of 5 µg Pb/g wet weight each in lead acetate, and then Pb, Ca, and Mg contents were analyzed in the exoskeleton, while only Pb bioaccumulation was quantified for the hepatopancreas, gills, muscles, and hemolymph. The results showed a statistically nonsignificant increase in exoskeletal Pb content in Pb-treated crabs compared to control, suggesting that exoskeletal Pb may not be a sensitive proxy for aquatic Pb pollution. There was a significant decrease in Ca content in Pb-treated crabs, suggesting that Pb hindered the deposition of Ca to crab exoskeleton, thereby obstructing calcification. A trend of a decrease in exoskeletal Mg was also observed in Pb-treated crabs. There was a significant increase in Pb content found in the gills, hepatopancreas, muscle, and hemolymph in Pb-treated crabs. The rank of the Pb level among three soft tissues in a decreasing order is hepatopancreas > gill > muscle. This is the first study to present evidence that Pb disrupts postecdysial exoskeletal calcification in a crustacean. Environ Toxicol Chem 2022;41:474-482. © 2021 SETAC.

Lead and zinc interactions - An influence of zinc over lead related toxic manifestations

BACKGROUND

Interaction between metals is known from earlier studies, in which one metal influences the absorption and functional role of other. Lead is known to cause debilitating effects in living organisms and also prevents several essential trace metals from functioning normally.

METHODS

The relevant literature using the key words lead toxicity, lead zinc interaction, zinc nutrition and the ability of zinc to act against lead has been reviewed.

RESULTS

Role of several nutrients in reducing the manifestations of toxic metals have been elucidated recently. Lead damages bio-membranes, causes cognitive disabilities and disturbs the normal process of DNA replication and transcription. Zinc on the other hand helps in proper maintenance of the cellular membranes and plays an important role as a metal cofactor in most of the proteins vital for membrane integrity. Zinc has essential role in cognitive functioning, zinc finger proteins and significantly neutralizes most toxic effects of lead.

CONCLUSION

Increased lead exposure and limited resources for tackling lead poisoning may cause an increased possibility of future environmental emergencies. Interactions between essential nutrient metals and non-essential toxic metals may act as important factor which can be used to target the metal toxicities. An assumption is made that the lead toxicity can be reduced by maintaining the status of essential trace metals like zinc.

MOLECULAR MECHANISMS OF LEAD NEUROTOXICITY

Lead (Pb2+) is a non-essential metal with numerous industrial applications that have led to ts ubiquity in the environment. Thus, not only occupational-exposed individuals' health is compromised, but also that of the general population and in particular children. Notably, although the central nervous system is particularly susceptible to Pb2+, other systems are affected as well. The present study focuses on molecular mechanisms that underlie the effects that arise from the presence of Pb2+ in situ in the brain, and the possible toxic effects that follows. As the brain barriers represent the first target of systemic Pb2+, mechanisms of Pb2+ entry into the brain are discussed, followed by a detailed discussion on neurotoxic mechanisms, with special emphasis on theories of ion mimicry, mitochondrial dysfunction, redox imbalance, and neuroinflammation. Most importantly, the confluence and crosstalk between these events is combined into a cogent mechanism of toxicity, by intertwining recent and old evidences from humans, in vitro cell culture and experimental animals. Finally, pharmacological interventions, including chelators, antioxidants substances, anti-inflammatory drugs, or their combination are reviewed as integrated approaches to ameliorate Pb2+ harmful effects in both developing or adult organisms.

Cognitive Impairment Induced by Lead Exposure during Lifespan: Mechanisms of Lead Neurotoxicity

Lead (Pb) is considered a strong environmental toxin with human health repercussions. Due to its widespread use and the number of people potentially exposed to different sources of this heavy metal, Pb intoxication is recognized as a public health problem in many countries. Exposure to Pb can occur through ingestion, inhalation, dermal, and transplacental routes. The magnitude of its effects depends on several toxicity conditions: lead speciation, doses, time, and age of exposure, among others. It has been demonstrated that Pb exposure induces stronger effects during early life. The central nervous system is especially vulnerable to Pb toxicity; Pb exposure is linked to cognitive impairment, executive function alterations, abnormal social behavior, and fine motor control perturbations. This review aims to provide a general view of the cognitive consequences associated with Pb exposure during early life as well as during adulthood. Additionally, it describes the neurotoxic mechanisms associated with cognitive impairment induced by Pb, which include neurochemical, molecular, and morphological changes that jointly could have a synergic effect on the cognitive performance.

Mercury affects uterine myogenic activity even without producing any apparent toxicity in rats: Involvement of calcium-signaling cascades

BACKGROUND

Mercury is an established environmental toxicant reported to cause reproductive disorders in women, however, its direct action on myometrial activity is yet to be understood. Earlier we have reported the underlying mechanism of mercury-induced myometrial contractions following in vitro exposure; however, no such information on the effect of mercury on myometrial activity following in vivo exposure is available, therefore, the present study was undertaken.

OBJECTIVE

Present study was designed to evaluate the effect of mercury on myometrial activity following in vivo exposure of rats and unravel the possible underlying mechanism.

METHODS

Female Wistar rats were orally exposed to mercury (5, 50 and 500 μg/L in drinking water) for 28 days to investigate the toxicodynamics of mercuric chloride (HgCl₂)-induced alterations in myometrial activity. Response of the isolated myometrial strips to different spasmogens was recorded using polyphysiograph. Blood and uterine calcium, mercury, iron and zinc levels were estimated by atomic absorption spectrophotometry. Blood biochemicals and serum hormonal profiles (estradiol, progesterone) were also determined.

RESULTS

No systemic toxicity of mercury was observed in any of the treatment groups (5, 50 and 500 μg/L) in terms of alterations in body weight, organ weights, blood biochemical parameters including hormonal profile. Interestingly, mercury at 5 μg/L concentration significantly increased the receptor-dependent (PGF_2α-induced) and receptor-independent (CaCl₂-induced and high K⁺-depolarizing solution-induced) myometrial contractions and it was coupled with corresponding increase in the uterine calcium levels. However, mercury at higher dose levels (50 and 500 μg/L) did not significantly alter the myometrial response.

CONCLUSION

Our results evidently suggest that mercury at low level (5 μg/L) produced detrimental effect on myometrial activity by altering calcium entry into the smooth muscle and/or the release of calcium from intracellular stores without causing any apparent systemic toxicity in rats.

A pharmacokinetic model of lead absorption and calcium competitive dynamics

Lead is a naturally-occurring element. It has been known to man for a long time, and it is one of the longest established poisons. The current consensus is that no level of lead exposure should be deemed "safe". New evidence regarding the blood levels at which morbidities occur has prompted the CDC to reduce the screening guideline of 10 μg/dl to 2 μg/dl. Measurable cognitive decline (reduced IQ, academic deficits) have been found to occur at levels below 10 μg/dl, especially in children. Knowledge of lead pharmacology allows us to better understand its absorption and metabolization, mechanisms that produce its medical consequences. Based upon an original and very simplified compartmental model of Rabinowitz (1973) with only three major compartments (blood, bone and soft tissue), extensive biophysical models sprouted over the following two decades. However, none of these models have been specifically designed to use new knowledge of lead molecular dynamics to understand its deleterious effects on the brain. We build and analyze a compartmental model of lead pharmacokinetics, focused specifically on addressing neurotoxicity. We use traditional phase space methods, parameter sensitivity analysis and bifurcation theory to study the transitions in the system's behavior in response to various physiological parameters. We conclude that modeling the complex interaction of lead and calcium along their dynamic trajectory may successfully explain counter-intuitive effects on systemic function and neural behavior which could not be addressed by existing linear models. Our results encourage further efforts towards using nonlinear phenomenology in conjunction with empirically driven system parameters, to obtain a biophysical model able to provide clinical assessments and predictions.

Arabidopsis CNGC Family Members Contribute to Heavy Metal Ion Uptake in Plants

Heavy metal ions, including toxic concentrations of essential ions, negatively affect diverse metabolic and cellular processes. Heavy metal ions are known to enter cells in a non-selective manner; however, few studies have examined the regulation of heavy metal ion transport. Plant cyclic nucleotide-gated channels (CNGCs), a type of Ca2+-permeable-channel, have been suggested to be involved in the uptake of both essential and toxic cations. To determine the candidates responsible for heavy metal ion transport, a series of Arabidopsis CNGC mutants were examined for their response to Pb2+ and Cd2+ ions. The primary focus was on root growth and the analysis of the concentration of heavy metals in plants. Results, based on the analysis of primary root length, indicated that AtCNGC1, AtCNGC10, AtCNGC13 and AtCNGC19 play roles in Pb2+ toxicity, while AtCNGC11, AtCNGC13, AtCNGC16 and AtCNGC20 function in Cd2+ toxicity in Arabidopsis. Ion content analysis verified that the mutations of AtCNGC1 and AtCNGC13 resulted in reduced Pb2+ accumulation, while the mutations of AtCNGC11, AtCNGC15 and AtCNGC19 resulted in less Pb2+ and Cd2+ accumulation in plants. These findings provide functional evidence which support the roles of these AtCNGCs in the uptake and transport of Pb2+ or Cd2+ ion in plants.

Lead poisoning: acute exposure of the heart to lead ions promotes changes in cardiac function and Cav1.2 ion channels

Lead ions (Pb²⁺) possess characteristics similar to Ca²⁺. Because of this and its redox capabilities, lead causes different toxic effects. The neurotoxic effects have been well documented; however, the toxic effects on cardiac tissues remain allusive. We utilized isolated guinea pig hearts and measured the effects of Pb²⁺ on their contractility and excitability. Acute exposure to extracellular Pb²⁺ had a negative inotropic effect and increased diastolic tension. The speed of contraction and relaxation were affected, though the effects were more dramatic on the speed of contraction. Excitability was also altered. Heart beat frequency increased and later diminished after lead ion exposure. Pro-arrhytmic events, such as early after-depolarization and a reduction of the action potential plateau, were also observed. In isolated cardiomyocytes and tsA 201 cells, extracellular lead blocked currents through Cav1.2 channels, diminished their activation, and enhanced their fast inactivation, negatively affecting their gating currents. Thus, Pb²⁺ was cardiotoxic and reduced cardiac contractility, making the heart prone to arrhythmias. This was due, in part, to Pb²⁺ effects on the Cav1.2 channels; however, other channels, transporters or pathways may also be involved. Acute cardiotoxic effects were observed at Pb²⁺ concentrations achievable during acute lead poisoning. The results suggest how Cav1.2 gating can be affected by divalent cations, such as Pb², and also suggest a more thorough evaluation of heart function in individuals affected by lead poisoning.

Chronic Kidney Disease and Exposure to Nephrotoxic Metals

Chronic kidney disease (CKD) is a common progressive disease that is typically characterized by the permanent loss of functional nephrons. As injured nephrons become sclerotic and die, the remaining healthy nephrons undergo numerous structural, molecular, and functional changes in an attempt to compensate for the loss of diseased nephrons. These compensatory changes enable the kidney to maintain fluid and solute homeostasis until approximately 75% of nephrons are lost. As CKD continues to progress, glomerular filtration rate decreases, and remaining nephrons are unable to effectively eliminate metabolic wastes and environmental toxicants from the body. This inability may enhance mortality and/or morbidity of an individual. Environmental toxicants of particular concern are arsenic, cadmium, lead, and mercury. Since these metals are present throughout the environment and exposure to one or more of these metals is unavoidable, it is important that the way in which these metals are handled by target organs in normal and disease states is understood completely.

Lead-Induced ERK Activation Is Mediated by GluR2 Non-containing AMPA Receptor in Cortical Neurons

Lead is a persistent environmental pollutant and exposure to high environmental levels causes various deleterious toxicities, especially to the central nervous system (CNS). The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor that is devoid of the glutamate receptor 2 (GluR2) subunit is Ca²⁺-permeable, which increases the neuronal vulnerability to excitotoxicity. We have previously reported that long-term exposure of rat cortical neurons to lead acetate induces decrease of GluR2 expression. However, it is not clarified whether lead-induced GluR2 decrease is involved in neurotoxicity. Therefore, we investigated the contribution of GluR2 non-containing AMPA receptor to lead-induced neurotoxic events. Although the expression of four AMPA receptor subunits (GluR1, GluR2, GluR3, and GluR4) was decreased by lead exposure, the decrease in GluR2 expression was remarkable among four subunits. Lead-induced neuronal cell death was rescued by three glutamate receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, a non-selective AMPA receptor blocker), MK-801 (N-methyl-D-aspartate (NMDA) receptor blocker), and 1-naphthyl acetyl spermine (NAS, a specific Ca²⁺-permeable AMPA receptor blocker). Lead exposure activated extracellular signal-regulated protein kinase (ERK) 1/2, which was significantly ameliorated by CNQX. In addition, lead exposure activated p38 mitogen-activated protein kinase (MAPK p38), and protein kinase C (PKC), which was partially ameliorated by CNQX. Our findings indicate that Ca²⁺-permeable AMPA receptors resulting from GluR2 decrease may be involved in lead-induced neurotoxicity.

Metal toxicity and opportunistic binding of Pb(2+) in proteins

Lead toxicity is associated with various human diseases. While Ca(2+) binding proteins such as calmodulin (CaM) are often reported to be molecular targets for Pb(2+)-binding and lead toxicity, the effect of Pb(2+) on the Ca(2+)/CaM regulated biological activities cannot be described by the primary mechanism of ionic displacement (e.g., ionic mimicry). The focus of this study was to investigate the mechanism of lead toxicity through binding differences between Ca(2+) and Pb(2+) for CaM, an essential intracellular trigger protein with two EF-Hand Ca(2+)-binding sites in each of its two domains that regulates many molecular targets via Ca(2+)-induced conformational change. Fluorescence changes in phenylalanine indicated that Pb(2+) binds with 8-fold higher affinity than Ca(2+) in the N-terminal domain. Additionally, NMR chemical shift changes and an unusual biphasic response observed in tyrosine fluorescence associated with C-terminal domain sites EF-III and EF-IV suggest a single higher affinity Pb(2+)-binding site with a 3-fold higher affinity than Ca(2+), coupled with a second site exhibiting affinity nearly equivalent to that of the N-terminal domain sites. Our results further indicate that Pb(2+) displaces Ca(2+) only in the N-terminal domain, with minimal perturbation of the C-terminal domain, however significant structural/dynamic changes are observed in the trans-domain linker region which appear to be due to Pb(2+)-binding outside of the known calcium-binding sites. These data suggest that opportunistic Pb(2+)-binding in Ca(2+)/CaM has a profound impact on the conformation and dynamics of the essential molecular recognition sites of the central helix, and provides insight into the molecular toxicity of non-essential metal ions.

Nephroprotective effect of calcium channel blockers against toxicity of lead exposure in mice

Exposure to lead (Pb) can induce kidney damage, which is related to induction of oxidative damage and disturbance of intracellular calcium homeostasis. Pb can readily permeate through dihydropyridine-sensitive L-type calcium channels and accumulate within cells. The objective of this study was to investigate protective effects of calcium channel blockers (CCBs) verapamil and nimodipine on nephrotoxicity induced by Pb acetate in mice. One hundred and twenty male mice were randomly divided into 6 groups: control, Pb, low-dose verapamil, high-dose verapamil, low-dose nimodipine and high-dose nimodipine (n=20 per group). Pb acetate was injected intraperitoneally (i.p.) at 40 mg/kg body weight/day for 10 days to establish the Pb toxicity model. While control mice received saline, mice of the treated groups simultaneously received i.p. injections of verapamil or nimodipine daily for 10 days. Both verapamil and nimodipine showed protection against Pb-induced kidney injury, including alleviation of renal pathological damage and decreasing the level of Pb in kidney homogenate and extent of apoptosis in nephrocytes. Moreover, verapamil and nimodipine significantly down-regulated levels of blood urea nitrogen and creatinine in the serum. In addition, verapamil and nimodipine administration decreased malondialdehyde content and increased activities of super oxide dismutase activity and glutathione peroxidase in the kidney homogenate. The findings in the present study implicate the therapeutic potential of CCBs for Pb-induced nephrotoxicity, which were at least partly due to the decrease of Pb uptake and inhibition of lipid peroxidation.

Lead acetate toxicity in vitro: Dependence on the cell composition of the cultures

It is well known that exposure to low doses of lead causes long-lasting neurobehavioural deficits, but the cellular changes underlying these behavioural changes remain to be elucidated. A protective role of glial cells on neurons through lead sequestration by astrocytes has been proposed. The possible modulation of lead neurotoxicity by neuron-glia interactions was examined in three-dimensional cultures of foetal rat telencephalon. Mixed-brain cell cultures or cultures enriched in either neurons or glial cells were treated for 10 days with lead acetate (10(-6) m), a concentration below the limit of cytotoxicity. Intracellular lead content and cell type-specific enzyme activities were determined. It was found that in enriched cultures neurons stored more lead than glial cells, and each cell type alone stored more lead than in co-culture. Moreover, glial cells but not neurons were more affected by lead in enriched culture than in co-culture. These results show that neuron-glia interactions attenuate the cellular lead uptake and the glial susceptibility to lead, but they do not support the idea of a protective role of astrocytes.

Predicting Ca2+ -binding sites using refined carbon clusters

Identifying Ca(2+) -binding sites in proteins is the first step toward understanding the molecular basis of diseases related to Ca(2+) -binding proteins. Currently, these sites are identified in structures either through X-ray crystallography or NMR analysis. However, Ca(2+) -binding sites are not always visible in X-ray structures due to flexibility in the binding region or low occupancy in a Ca(2+) -binding site. Similarly, both Ca(2+) and its ligand oxygens are not directly observed in NMR structures. To improve our ability to predict Ca(2+) -binding sites in both X-ray and NMR structures, we report a new graph theory algorithm (MUG(C) ) to predict Ca(2+) -binding sites. Using carbon atoms covalently bonded to the chelating oxygen atoms, and without explicit reference to side-chain oxygen ligand co-ordinates, MUG(C) is able to achieve 94% sensitivity with 76% selectivity on a dataset of X-ray structures composed of 43 Ca(2+) -binding proteins. Additionally, prediction of Ca(2+) -binding sites in NMR structures was obtained by MUG(C) using a different set of parameters, which were determined by the analysis of both Ca(2+) -constrained and unconstrained Ca(2+) -loaded structures derived from NMR data. MUG(C) identified 20 of 21 Ca(2+) -binding sites in NMR structures inferred without the use of Ca(2+) constraints. MUG(C) predictions are also highly selective for Ca(2+) -binding sites as analyses of binding sites for Mg(2+) , Zn(2+) , and Pb(2+) were not identified as Ca(2+) -binding sites. These results indicate that the geometric arrangement of the second-shell carbon cluster is sufficient not only for accurate identification of Ca(2+) -binding sites in NMR and X-ray structures but also for selective differentiation between Ca(2+) and other relevant divalent cations.

Novel approach for generation of low calcium reagents for investigations of heavy metal effects on calcium signaling

INTRODUCTION

Lead exposure can cause learning disabilities, memory loss and severe damage to the nervous system. However, the exact mechanism by which lead causes learning disabilities is not fully understood. The effects of lead on calcium-regulated signaling pathways are difficult to study biochemically; with the traditional method of controlling the free calcium concentration with EGTA, the exact concentrations of free lead and calcium ions in solution are interdependent and prone to error because EGTA also buffers lead.

METHODS AND RESULTS

In our approach, we first reduced the free calcium concentration in the solution using calcium-binding resins before adding lead to buffers. The solution was sequentially treated with Chelex-100 ion exchange resin, followed by immobilized BAPTA resin. The final concentration of free calcium in the solution was measured with Fluo-3 indicator. Our protocol successfully produced buffers with free calcium levels below 15 nM, which is substantially below threshold for activation of calcium-dependent enzymes in signaling pathways (which is typically a few hundred nanomolar calcium, when determined in vitro).

CONCLUSION

This method provides an improved approach to study the effect of heavy metals on calcium-stimulated signaling pathways.

The relationships between blood lead levels and serum follicle stimulating hormone and luteinizing hormone in the National Health and Nutrition Examination Survey 1999-2002

The relationships between blood lead levels and serum follicle stimulating hormone and luteinizing hormone were assessed in a nationally representative sample of women, 35-60 years old, from the National Health and Nutrition Examination Survey 1999-2002. The blood lead levels of the women ranged from 0.2 to 17.0 μg/dL. The estimated geometric mean was 1.4 μg/dL, and the estimated arithmetic mean was 1.6 μg/dL. As the blood lead level increased, the concentration of serum follicle stimulating hormone increased in post-menopausal women, women who had both ovaries removed, and pre-menopausal women. The concentration of luteinizing hormone increased as blood lead level increased in post-menopausal women and women who had both ovaries removed. The lowest concentrations of blood lead at which a relationship was detected were 0.9 μg/dL for follicle stimulating hormone and 3.2 μg/dL for luteinizing hormone. Lead may act directly or indirectly at ovarian and non-ovarian sites to increase the concentrations of follicle stimulating hormone and luteinizing hormone.

The interactions of iron with other divalent metals in the intestinal tract of a freshwater teleost, rainbow trout (Oncorhynchusmykiss)

This study examined the concentration-dependent interactive effects of four essential (Cu(2+), Zn(2+), Ni(2+), Co(2+)) and two non-essential (Pb(2+) and Cd(2+)) divalent metals on intestinal iron (Fe(2+)) absorption in freshwater rainbow trout (Oncorhynchusmykiss) using an invitro gut sac technique. All of the divalent metals except cobalt inhibited the intestinal Fe(2+) absorption in fish, and the magnitude of inhibition followed the order of: Ni(2+)~Pb(2+)>Cd(2+)~Cu(2+)>Zn(2+). The mucosal epithelium of the intestine was found to be the most sensitive to inhibition relative to the mucus or blood compartment, suggesting that these interactions likely occur via the divalent metal transporter-1 (DMT1). In addition, the reciprocal effects of Fe(2+) on intestinal accumulation of lead and cadmium were investigated. Elevated Fe(2+) did not affect lead accumulation in the intestine, indicating a greater affinity of Pb(2+) to the Fe(2+) transport pathway and/or the existence of additional pathways for lead absorption. In contrast, the accumulation of cadmium in the intestine decreased considerably in the presence of excess Fe(2+), indicating the importance of the Fe(2+) absorption pathway in dietary cadmium accumulation in fish. Overall, our study provides important insights into the mechanisms of dietary uptake of several divalent metals in freshwater fish.

Chronic lead exposure alters presynaptic calcium regulation and synaptic facilitation in Drosophila larvae

Prolonged exposure to inorganic lead (Pb(2+)) during development has been shown to influence activity-dependent synaptic plasticity in the mammalian brain, possibly by altering the regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)). To explore this possibility, we studied the effect of Pb(2+) exposure on [Ca(2+)](i) regulation and synaptic facilitation at the neuromuscular junction of larval Drosophila. Wild-type Drosophila (CS) were raised from egg stages through the third larval instar in media containing either 0 microM, 100 microM or 250 microM Pb(2+) and identified motor terminals were examined in late third-instar larvae. To compare resting [Ca(2+)](i) and the changes in [Ca(2+)](i) produced by impulse activity, the motor terminals were loaded with a Ca(2+) indicator, either Oregon Green 488 BAPTA-1 (OGB-1) or fura-2 conjugated to a dextran. We found that rearing in Pb(2+) did not significantly change the resting [Ca(2+)](i) nor the Ca(2+) transient produced in synaptic boutons by single action potentials (APs); however, the Ca(2+) transients produced by 10 Hz and 20 Hz AP trains were larger in Pb(2+)-exposed boutons and decayed more slowly. For larvae raised in 250 microM Pb(2+), the increase in [Ca(2+)](i) during an AP train (20 Hz) was 29% greater than in control larvae and the [Ca(2+)](i) decay tau was 69% greater. These differences appear to result from reduced activity of the plasma membrane Ca(2+) ATPase (PMCA), which extrudes Ca(2+) from these synaptic terminals. These findings are consistent with studies in mammals showing a Pb(2+)-dependent reduction in PMCA activity. We also observed a Pb(2+)-dependent enhancement of synaptic facilitation at these larval neuromuscular synapses. Facilitation of EPSP amplitude during AP trains (20 Hz) was 55% greater in Pb(2+)-reared larvae than in controls. These results showed that Pb(2+) exposure produced changes in the regulation of [Ca(2+)](i) during impulse activity, which could affect various aspects of nervous system development. At the mature synapse, this altered [Ca(2+)](i) regulation produced changes in synaptic facilitation that are likely to influence the function of neural networks.

A glutamatergic component of lead toxicity in adult brain: the role of astrocytic glutamate transporters

Astroglial cells have a variety of roles in the central nervous system (CNS), providing a homeostasis for the proper functioning of neuronal cells. The classical view concerning the supportive role of astroglia towards associated neurons has to be extended. A great number of new evidences suggest that astrocytes interact closely with neurons being involved in the active control of neuronal activity and metabolism, forming with pre- and postsynaptic nerve terminals a tripartite synapse. Astrocytes control many aspects of brain function. Regulation of extracellular glutamate concentration, potentially neurotoxic neurotransmitter, is fundamental. Glial glutamate transporters system is of importance in protection against glutamate excitotoxicity and antioxidant defence system which is glutathione. When astrocytes fail to function properly, they influence the degree of neuronal damage. Thus, astrocytes are involved to a very great extent into numerous brain pathologies, including toxicity of heavy metals, like lead (Pb). Under pathological conditions they appear to express two opposite features: they are neuroprotective (until they can) or deleterious for neurons and may participate in neuronal damage. The very well known affinity of Pb to astroglia and the changes in glutamatergic transmission upon Pb toxicity, led us to discuss the role of astroglia and astrocytic glutamate transporters in the neurotoxicity of this metal. Our observations are viewed against a background of other results.

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.

Molecular and ionic mimicry and the transport of toxic metals

Despite many scientific advances, human exposure to, and intoxication by, toxic metal species continues to occur. Surprisingly, little is understood about the mechanisms by which certain metals and metal-containing species gain entry into target cells. Since there do not appear to be transporters designed specifically for the entry of most toxic metal species into mammalian cells, it has been postulated that some of these metals gain entry into target cells, through the mechanisms of ionic and/or molecular mimicry, at the site of transporters of essential elements and/or molecules. The primary purpose of this review is to discuss the transport of selective toxic metals in target organs and provide evidence supporting a role of ionic and/or molecular mimicry. In the context of this review, molecular mimicry refers to the ability of a metal ion to bond to an endogenous organic molecule to form an organic metal species that acts as a functional or structural mimic of essential molecules at the sites of transporters of those molecules. Ionic mimicry refers to the ability of a cationic form of a toxic metal to mimic an essential element or cationic species of an element at the site of a transporter of that element. Molecular and ionic mimics can also be sub-classified as structural or functional mimics. This review will present the established and putative roles of molecular and ionic mimicry in the transport of mercury, cadmium, lead, arsenic, selenium, and selected oxyanions in target organs and tissues.

Study of calcium-dependent lead-tolerance on plants differing in their level of Ca-deficiency tolerance

The main aim of the study was to determine the role of calcium in the amelioration of lead toxic effects in plants with accordingly high/low level of Pb-tolerance and high/low Ca-deficiency tolerance. The study was performed on maize, rye, tomato and mustard. Plants were cultivated in modified Knop's solution. They were subjected to Ca-deficiency, and to lead nitrate administered in the presence of four calcium nitrate concentrations 3.0, 2.4, 1.2, 0.3mM. Lead-tolerance and tolerance to Ca-deficiency were determined, as were concentration of the studied elements in plant tissues, and the Pb deposition pattern at the ultrastructural level (electron microscopy study, X-ray microanalysis). In all studied plants, lead toxicity increased as medium calcium content decreased, however, only in the Ca-deficiency sensitive mustard with low Pb-tolerance was it accompanied by a rise in tissue lead concentration. In contrast, lead root and shoot levels did not increase in the highly Ca-deficiency tolerant tomato, mustard and rye with high Pb-tolerance irrespective of the Ca(2+) regimens applied. Thus, in these plants, lead's unfavourable effects resulted only from the higher toxicity of the same amount of lead in tissues at low calcium in the medium. Of particular relevance is the finding by electron microscopy and X-ray microanalysis, that under low calcium in both highly Ca-deficiency tolerant and Ca-deficiency sensitive plants, less efficient Pb(2+) detoxification was accompanied by the restriction of the formation of large lead deposits in cell walls. Obtained results are novel in demonstrating calcium involvement in the lead deposition in the cell wall, thus in the regulation of the internal lead detoxification.

Effects of calcium on the uptake and elimination of cadmium and zinc in Asiatic clams

We examined the effects of Ca, one of the major ions contributing to water hardness, on the uptake and elimination of Cd and Zn in the Asiatic clam Corbicula fluminea, a common bivalve species in the rivers and streams of Southern China. Over the wide range of dissolved Cd (4.5 to 446 nM) and Zn (15.3 to 1529 nM) concentrations, uptake of both metals increased with increasing dissolved metal concentration, showing Michaelis-Menten-type saturation kinetics. At each tested metal concentration, increased Ca concentration led to a significantly lower metal uptake. When the Ca, Cd, Zn concentrations were expressed as free-ion activities, Cd and Zn uptake data fitted the Michaelis-Menten inhibition model well. We also quantified the effects of the two Ca blockers on metal uptake by the clams. Verapamil significantly inhibited the uptake Cd and Zn, but the inhibitive effect of lanthanum on metal uptake was more evident for Cd than for Zn. Ca did not significantly affect the assimilation efficiency of either metal from ingested phytoplankton, nor did it affect the elimination of the two metals during a 1-month depuration period. Our study has shown that Ca inhibited the uptake of metals from water; such an effect could be predicted based on the free-metal ion activities. Ca did not seem to have a direct effect on metal assimilation from food or metal elimination from the bivalve.

Presynaptic disruption of transmitter release by lead

Low concentrations of inorganic lead ions (Pb2+) disrupt transmitter release by causing aberrant augmentation of spontaneous and suppression of evoked release. These effects result from high affinity interactions of Pb2+ with the voltage-gated calcium channels (VGCC) as well as Ca2+ binding proteins which regulate the synaptic vesicle mobilization, docking, and exocytosis processes. Augmentation of spontaneous release may involve stimulation of vesicle mobilization consequent to Pb2+ activation of CaMKII-dependent phosphorylation of synapsin I and/or stimulation of asynchronous exocytosis via direct Pb2+ activation of the putative exocytotic Ca2+-sensor protein synaptotagmin I. In addition, synergistic stimulation of PLC and DAG/Pb2+-dependent activation of PKC may enhance the secretagogue effects of Pb2+ by increasing metal sensitivity of exocytosis and/or modulating calcium channel activity. In contrast to intracellularly-mediated actions of Pb2+ resulting in augmentation of spontaneous release, the inhibition of evoked transmitter release by Pb2+ is largely attributable to extracellular block of the voltage-gated calcium channels.

Effects of toxic environmental contaminants on voltage-gated calcium channel function: from past to present

Voltage-gated Ca2+ channels are targets of the number of naturally occurring toxins, therapeutic agents as well as environmental toxicants. Because of similarities of their chemical structure to Ca2+ in terms of hydrated ionic radius, electron orbital configuration, or other chemical properties, polyvalent cations from aluminum to zinc variously interact with multiple types of voltage-gated Ca2+ channels. These nonphysiological metals have been used to study the structure and function of the Ca2+ channel, especially its permeability characteristics. Two nonphysiological cations, Pb2+ and Hg2+, as well as their organic derivatives, are environmental neurotoxicants which are highly potent Ca2+ channel blockers. These metals also apparently gain intracellular access in part by permeating through Ca2+ channels. In this review the history of Ca2+ channel block produced by Pb2+ and Hg2+ as well as other nonphysiological cations is traced. In particular the characteristics of Ca2+ channel block induced by these environmental neurotoxic metals and the consequences of this action for neuronal function are discussed.

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.

The generation of free radicals by nitric oxide synthase

Nitric oxide synthase (NOS) is an example of a family of heme-containing monooxygenases that, under the restricted control of a specific substrate, can generate free radicals. While the generation of nitric oxide (NO*) depends solely on the binding of L-arginine, NOS produces superoxide (O(2)*(-)) and hydrogen peroxide (H(2)O(2)) when the concentration of the substrate is low. Not surprisingly, effort has been put forth to understand the pathway by which NOS generates NO*, O(2)*(-) and H(2)O(2), including the role of substrate binding in determining the pathways by which free radicals are generated. By binding within the distal heme pocket near the sixth coordination position of the NOS heme iron, L-arginine alters the coordination properties of the heme iron that promotes formation of the perferryl complex NOS-[Fe(5+)=O](3+). This reactive iron intermediate has been shown to abstract a hydrogen atom from a carbon alpha to a heteroatom and generate carbon-centered free radicals. The ability of NOS to produce free radicals during enzymic cycling demonstrates that NOS-[Fe(5+)=O](3+) behaves like an analogous iron-oxo complex of cytochrome P-450 during aliphatic hydroxylation. The present review discusses the mechanism(s) by which NOS generates secondary free radicals that may initiate pathological events, along with the cell signaling properties of NO*, O(2)*(-) and H(2)O(2).

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.

Lead-induced endoplasmic reticulum (ER) stress responses in the nervous system

Lead (Pb) poisoning continues to be a significant health risk because of its pervasiveness in the environment, its known neurotoxic effects in children, and potential endogenous exposure from Pb deposited in bone. New information about mechanisms by which Pb enters cells and its organelle targets within cells are briefly reviewed. Toxic effects of Pb on the endoplasmic reticulum (ER) are considered in detail, based on recent evidence that Pb induces the expression of the gene for 78-kD glucose-regulated protein (GRP78) and other ER stress genes. GRP78 is a molecular chaperone that binds transiently to proteins traversing through the ER and facilitates their folding, assembly, and transport. Models are presented for the induction of ER stress by Pb in astrocytes, the major cell type of the central nervous system, in which Pb accumulates. A key feature of the models is disruption of GRP78 function by direct Pb binding. Possible pathways by which Pb-bound GRP78 stimulates the unfolded protein response (UPR) in the ER are discussed, specifically transduction by IRE1/ATF6 and/or IRE1/JNK. The effect of Pb binding to GRP78 in the ER is expected to be a key component for understanding mechanisms of Pb-induced ER stress gene expression.

Characteristics of block by Pb2+ of function of human neuronal L-, N-, and R-type Ca2+ channels transiently expressed in human embryonic kidney 293 cells

Lead (Pb(2+)) is a well-known inhibitor of voltage-dependent Ca(2+) channels in their native environments in several types of cells. However, its effects on discrete Ca(2+) channel phenotypes in isolation have not been well studied. We compared how specific subtypes of human neuronal high-voltage-activated Ca(2+) channels were affected by acute exposure to Pb(2+). Expression cDNA clones of human alpha(1C), alpha(1B), or alpha(1E) subunit genes encoding neuronal L-, N-, and R-subtypes of Ca(2+) channels, respectively, along with a constant alpha(2)delta and beta(3) subunits were transfected into human embryonic kidney 293 cells. Currents through the respective transiently expressed channels were measured using whole-cell recording techniques with Ba(2+) (20 mM) as charge carrier. Extracellular bath applications of Pb(2+) significantly reduced current amplitude through all three types of Ca(2+) channels in a concentration-dependent manner. The order of potency was: alpha(1E) (IC(50) = 0.10 microM), followed by alpha(1C) (IC(50) = 0.38 microM) and alpha(1B) (IC(50) = 1.31 microM). Pb(2+)-induced perturbation of function of alpha(1C) and alpha(1B) containing Ca(2+) channels was more easily reversed than for alpha(1E)-containing Ca(2+) channels after washing with Pb(2+) free solution. The current-voltage relationships were not altered after 3-min exposure to Pb(2+) for any of the three types. However, the steady-state inactivation relationships were shifted to more negative potentials for channels containing alpha(1B) and alpha(1E) subunits, but not for those containing alpha(1C) subunits. Pb(2+) accelerated the inactivation time of current in all three subtypes of Ca(2+) channels in a concentration- and voltage-dependent manner. Therefore, different subtypes of Ca(2+) channels exhibit differential susceptibility to Pb(2+) even when expressed in the same cell type. Current expressed by alpha(1E)-containing channels is more sensitive to Pb(2+) than that expressed by alpha(1C)- or alpha(1B)-containing channels. Several Ca(2+) channel phenotypes are quite sensitive to the inhibitory action of Pb(2+). Furthermore, it seems that Pb(2+) is more likely to combine with Ca(2+) channels in the closed state.

Effects of methylmercury on human neuronal L-type calcium channels transiently expressed in human embryonic kidney cells (HEK-293)

Methylmercury (MeHg) disrupts the function of native, high voltage-activated neuronal Ca(2+) channels in several types of cells. However, the effects of MeHg on isolated Ca(2+) channel phenotypes have not been examined. The aim of the present study was to examine the action of MeHg on recombinant, neuronal L-type voltage-sensitive Ca(2+) channels. Human embryonic kidney cells (HEK-293) were transfected with human neuronal cDNA clones of the alpha(1C-1) subunit in combination with alpha(2b) and beta(3a) Ca(2+) channel subunits and the reporter jellyfish green fluorescent protein for transient expression. Current from expressed channels (I(Ba)) and their response to MeHg applied acutely were measured using whole-cell voltage-clamp recording techniques and Ba(2+) (5 mM) as charge carrier. Amplitude of I(Ba) in these cells was reduced by the dihydropyridine (DHP), nimodipine, and enhanced by Bay K8644 [S-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3 pyridine carboxylic acid methyl ester]. MeHg (0.125-5.0 microM) caused a time- and concentration-dependent reduction in amplitude of the peak and sustained current through these channels. However, even at the highest concentration of MeHg tested, reduction of current amplitude by MeHg was incomplete. Washing with MeHg-free solution could not reverse its effects. The steady-state inactivation curve was unaltered by MeHg. Increasing the stimulation frequency or the extracellular Ba(2+) concentration each attenuated slightly the reduction in amplitude of I(Ba) by MeHg. In the presence of MeHg (5.0 microM), Bay K8644 still increased the remaining current, and nimodipine (10 microM) reduced residual current that was resistant to MeHg. Thus, although MeHg reduces the amplitude of recombinant, heterologously expressed L-type channel current, a portion of current is resistant to reduction by MeHg. Furthermore, DHP agonists and antagonists retain their ability to affect L-type Ca(2+) channel current even in the presence of MeHg.

The activation of neuronal nitric-oxide synthase by various divalent cations

Nitric-oxide synthase (NOS; EC 1.14.13.39) catalyzes the oxidation of L-arginine to nitric oxide (NO(.)) and L-citrulline via the intermediate N(omega)-hydroxy-L-arginine. Of the three distinct isoforms of NOS that have been characterized, the constitutive neuronal NOS (NOS I) generates NO(.) associated with long-term potentiation (LTP) and early brain development. All of the NOS isoforms contain an N-terminal oxidase and a C-terminal reductase domain connected by a Ca(2+)/calmodulin binding region. To activate NOS I, Ca(2+) has to bind to calmodulin, allowing electron transport through both domains. Calcium ions are tightly regulated in cells. However, a number of other metal ions that bind and activate calmodulin may also activate NOS I. One such metal ion may be Pb(2+), which is associated with neurobehavioral and psychological alterations, including the inhibition of LTP. The effect of various divalent cations on NOS I activity was tested, and the results presented herein demonstrate that Pb(2+) and Sr(2+) can activate NOS I to a level similar to that found for Ca(2+). Finally, there is a synergy between Pb(2+) and Ca(2+) resulting in maximal activation of NOS I using minimal concentrations of both metal ions.

Uptake of lead and iron by divalent metal transporter 1 in yeast and mammalian cells

Although the divalent metal transporter (DMT1) was suggested to transport a wide range of metals in Xenopus oocytes, recent studies in other models have provided contrasting results. Here, we provide direct evidence demonstrating that DMT1 expressed in yeast mutants defective for high affinity iron transport facilitates the transport of iron with an 'apparent K(m)' of approximately 1.2 microM, and transport of lead with an 'apparent K(m)' of approximately 1.8 microM. DMT1-dependent lead transport was H(+)-dependent and was inhibited by iron. Human embryonic kidney fibroblasts (HEK293 cells) overexpressing DMT1 also showed a higher uptake of lead than HEK293 cells without overexpressing DMT1. These results show that DMT1 transports lead and iron with similar affinity in a yeast model suggesting that DMT1 is a transporter for lead.

Mobilization of intracellular calcium in kidney epithelial cells is inhibited by lead

The effect of lead (Pb) on intracellular calcium (Cai) after stimulation with agonists was studied in Madin-Darby canine kidney (MDCK) cells. In response to the agonist ADP, the levels of Cai increased by approximately threefold in MDCK cells bathed in a buffer with calcium (Ca) or in a buffer with nominal Ca. Pb inhibited the response to ADP in MDCK cells bathed in either buffer. The inhibition by Pb was observed after a 5 and 20-min exposure to Pb, but not after 2-min. Very high concentrations of ADP did not reverse the effects of Pb. Concentrations of Pb of 1 microM or more inhibited the response to ADP. Similarly, the response to bradykinin was also inhibited by Pb. Protein kinase C did not play a role since the protein kinase C inhibitor GF 109203X did not reverse the effects of Pb. Interestingly, MDCK cells treated with Pb at concentrations above 1 microM, for periods of 5-20 min, displayed elevated levels of inositol 1,4,5-trisphosphate. In conclusion, Pb inhibits mobilization of Cai after agonist stimulation by a mechanism that is unrelated to the type of agonist used. Evidence is presented suggesting that the inhibition is due to increases in levels of inositol 1,4,5-trisphosphate, which possibly decreases the amount of Cai available for mobilization.

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.

Astroglia as metal depots: molecular mechanisms for metal accumulation, storage and release

The brain is an organ that concentrates metals, and these metals are often localized to astroglia. An examination of metal physiology of brain cells, particularly astroglia, offers insights into the developmental neurotoxicity of certain metals, including lead (Pb), mercury (Hg), manganese (Mn), and copper (Cu). Xenobiotic metals probably accumulate in cells by exploiting the normal functions of proteins that transport and handle essential metals. In addition, essential metals may become toxic by accumulating at levels that exceed the normal metal buffering capacity of the cell. This review considers the uptake, accumulation, storage, and release of two xenobiotic metals, Pb and Hg, as well as two essential nutrient metals that are neurotoxic in high amounts, Mn and Cu. Evidence that each metal accumulates in astroglia is evaluated, together with the mechanisms the host cell may invoke to protect itself from cytoxicity.

Calmodulin as a versatile calcium signal transducer in plants

The complexity of Ca²⁺ patterns observed in eukaryotic cells, including plants, has led to the hypothesis that specific patterns of Ca²⁺ propagation, termed Ca²⁺ signatures, encode information and relay it to downstream elements (effectors) for translation into appropriate cellular responses. Ca²⁺ -binding proteins (sensors) play a key role in decoding Ca²⁺ signatures and transducing signals by activating specific targets and pathways. Calmodulin is a Ca²⁺ sensor known to modulate the activity of many mammalian proteins, whose targets in plants are now being actively characterized. Plants possess an interesting and rapidly growing list of calmodulin targets with a variety of cellular roles. Nevertheless, many targets appear to be unique to plants and remain uncharacterized, calling for a concerted effort to elucidate their functions. Moreover, the extended family of calmodulin-related proteins in plants consists of evolutionarily divergent members, mostly of unknown function, although some have recently been implicated in stress responses. It is hoped that advances in functional genomics, and the research tools it generates, will help to explain themultiplicity of calmodulin genes in plants, and to identify their downstream effectors. This review summarizes current knowledge of the Ca²⁺ -calmodulin messenger system in plants and presents suggestions for future areas of research. Contents I. Introduction 36 II. CaM isoforms and CaM-like proteins 37 III. CaM-target proteins 42 IV. CaM and nuclear functions 46 V. Regulation of ion transport 49 VI. CaM and plant responses to environmental stimuli 52 VII. Conclusions and future studies 58 Acknowledgements 59 References 59.

Stimulatory and inhibitory effects of inorganic lead on calcineurin

Calcineurin is a phosphatase with activity dependent on both Ca(2+)/calmodulin binding to the catalytic A subunit and Ca(2+) binding to the regulatory B subunit. We have previously shown that Pb(2+) activates calmodulin with a threshold of about 100 pM free Pb(2+), and that Pb(2+) and Ca(2+) are roughly additive in calmodulin activation (Kern et al., NeuroToxicology 21, 353-364 (2000)). In the present study, we evaluated the effects of Pb(2+), with and without Ca(2+) and calmodulin, on calcineurin activity. In calmodulin-containing, Ca(2+)-free solutions, Pb(2+) activated calcineurin with a threshold of about 100 pM free Pb(2+). Maximum calcineurin activity (comparable to that induced by 10 microM Ca(2+)) was reached at about 200 pM free Pb(2+). Higher Pb(2+) concentrations reduced activity, although some activity remained even at 2000 pM free Pb(2+). Combined with subsaturating Ca(2+) concentrations, as little as 20 pM free Pb(2+) enhanced calcineurin activity, but free Pb(2+) concentrations greater than 200 pM still reduced activity below maximum. Extremely high Ca(2+) concentrations (10 microM) completely reversed the inhibition of activity by 2000 pM free Pb(2+). In the absence of calmodulin, Ca(2+) slightly stimulated calcineurin activity. Pb(2+) did not substitute for Ca(2+) in calmodulin-free activation; in fact, high concentrations of Pb(2+) inhibited Ca(2+)-mediated activation. We tentatively conclude that low concentrations of free Pb(2+) activate calcineurin by activating calmodulin. Higher concentrations reduce calcineurin activity, perhaps by binding to the B subunit.

Methylmercury affects multiple subtypes of calcium channels in rat cerebellar granule cells

We tested the ability of methylmercury (MeHg) to block calcium channel current in cultures of neonatal cerebellar granule cells using whole-cell patch clamp techniques and Ba(2+) as charge carrier. Low micromolar concentrations of MeHg (0.25-1 microM) reduced the amplitude of whole cell Ba(2+) current in a concentration- and time-dependent fashion; however, this effect was not voltage-dependent and the current-voltage relationship was not altered. Increasing the stimulation frequency hastened the onset and increased the magnitude of block at both 0.25 and 0.5 microM MeHg but not at 1 microM. In the absence of stimulation, all concentrations of MeHg were able to decrease current amplitude. The ability of several Ca(2+) channel antagonists (omega-conotoxin GVIA, omega-conotoxin MVIIC, omega-agatoxin IVA, calcicludine, and nimodipine) to alter the MeHg-induced effect was tested in an effort to determine if MeHg targets a specific subtype of Ca(2+) channel. Each of the antagonists tested was able to decrease a portion of whole cell Ba(2+) current under control conditions. However, none were able to attenuate the MeHg-induced block of whole cell Ba(2+) current, suggesting either that the mechanism of MeHg-induced block involves sites other than those influenced specifically by Ca(2+) channel antagonists or that MeHg was able to "outcompete" these toxins for their binding sites. These results show that acute exposure to submicromolar concentrations of MeHg can block Ba(2+) currents carried through multiple Ca(2+) channel subtypes in primary cultures of cerebellar granule cells. However, it is unlikely that the presence of a specific Ca(2+) channel subtype is able to render granule cells more susceptible to the neurotoxicologic actions of MeHg.

The role of anion exchange in the uptake of Pb by human erythrocytes and Madin-Darby canine kidney cells

Anion exchange (AE) plays a critical role in regulating intracellular pH in erythrocytes and epithelial cells and has been suggested to facilitate the transport of lead (Pb) across the erythrocyte cell membrane. In this study we examined the role of AE in the uptake of Pb by human erythrocytes and by Madin-Darby canine kidney (MDCK) cells, the kidney epithelial cell line. Functional AE in MDCK cells was evidenced by: increased uptake of SO(4)(2-) at pH 6.0 over pH 7.0, and inhibition of SO(4)(2-) uptake by the AE inhibitor 4, 4'-diisothiocyanostilbene-2, 2'- disulfonic acid (DIDS) as well as by non-halide anions. Accumulation of Pb into MDCK cells was time and temperature dependent. DIDS inhibited uptake of Pb into human erythrocytes but not MDCK cells. In conclusion, uptake of Pb into erythrocytes but not kidney epithelial cells occurs through AE.

Lead induced alterations in nitrite and nitrate levels in different regions of the rat brain

Nitric oxide (NO) is a free radical synthesized by nitric oxide synthase (NOS) during the conversion of L-arginine to citrulline. Lead (Pb) affects neuronal functioning in the rat brain. Nitric oxide, a neuronal messenger has a short half life and converts immediately into nitrite and nitrate. The present study is designed to determine lead-induced alterations in NO production by measuring nitrite and nitrate in the cerebellum, the hippocampus, the frontal cortex and the brain stem of the rat brain. Male Sprague-Dawley rats were treated with lead acetate (5 and 15 mg/kg body wt.) by intraperitoneal injection. The control and experimental rats were sacrificed at the end of 7 and 14 days after treatment and different regions of the brain were isolated. Nitrite and nitrate (NOx) levels were estimated by the chemiluminescent method using the NOA 280 (Sievers). The data suggested dose-dependent and region-specific responses to lead. Both treatments of lead reduced NOx levels in the cerebellum and the hippocampus. However, the frontal cortex and the brain stem responded differently to Pb exposure. NOx levels in the frontal cortex were significantly increased in rats treated with low and high doses of Pb for 7 days but not in rats treated for 14 days, whereas in the brain stem, NOx levels were increased in a dose- and time-dependent manner. Although, the response was time-dependent, the variation between 7- and 14-day treatment was not clearly delineated. These results provide additional evidence that Pb exposure alters NO-production in rat brain leading to neuronal dysfunction.

A tobacco plasma membrane calmodulin-binding transporter confers Ni2+ tolerance and Pb2+ hypersensitivity in transgenic plants

All organisms require a minimal amount of metal ions to sustain their metabolism, growth and development. At the same time, their intrinsic metal-uptake systems render them vulnerable to toxic levels of metals in the biosphere. Using radiolabeled recombinant calmodulin as a probe to screen a tobacco cDNA library, we have discovered a protein designated NtCBP4 (Nicotiana tabacum calmodulin-binding protein) that can modulate plant tolerance to heavy metals. Structurally, NtCBP4 is similar to vertebrate and invertebrate K+ and to non-selective cation channels, as well as to recently reported proteins from barley and Arabidopsis. Here we report on the subcellular localization of NtCBP4 and the phenotype of transgenic plants overexpressing this protein. The localization of NtCBP4 in the plasma membrane was manifested by fractionating tobacco membranes on sucrose gradients or by aqueous two-phase partitioning, and subsequently using immunodetection. Several independent transgenic lines expressing NtCBP4 had higher than normal levels of NtCBP4. These transgenic lines were indistinguishable from wild type under normal growth conditions. However, they exhibited improved tolerance to Ni2+ and hypersensitivity to Pb2+, which are associated with reduced Ni2+ accumulation and enhanced Pb2+ accumulation, respectively. To our knowledge this is the first report of a plant protein that modulates plant tolerance or accumulation of Pb2+. We propose that NtCBP4 is involved in metal uptake across the plant plasma membrane. This gene may prove useful for implementing selective ion tolerance in crops and improving phytoremediation strategies.