Lead activates calmodulin sensitive processes

Calcium in Signaling: Its Specificity and Vulnerabilities toward Biogenic and Abiogenic Metal Ions

Divalent calcium ion (Ca²⁺) plays an indispensable role as a second messenger in a myriad of signal transduction processes. Of utmost importance for the faultless functioning of calcium-modulated signaling proteins is their binding selectivity of the native metal cation over rival biogenic/abiogenic metal ion contenders in the intra/extracellular fluids. In this Perspective, we summarize recent findings on the competition between the cognate Ca²⁺ and other biogenic or abiogenic divalent cations for binding to Ca²⁺-signaling proteins or organic cofactors. We describe the competition between the two most abundant intracellular biogenic metal ions (Mg²⁺ and Ca²⁺) for Ca²⁺-binding sites in signaling proteins, followed by the rivalry between native Ca²⁺ and "therapeutic" Li⁺ as well as "toxic" Pb²⁺. We delineate the key factors governing the rivalry between the native and non-native cations in proteins and highlight key implications for the biological performance of the respective proteins/organic cofactors.

Neurotoxicity of low-level lead exposure: History, mechanisms of action, and behavioral effects in humans and preclinical models

Lead is a neurotoxin that produces long-term, perhaps irreversible, effects on health and well-being. This article summarizes clinical and preclinical studies that have employed a variety of research techniques to examine the neurotoxic effects of low levels of lead exposure. A historical perspective is presented, followed by an overview of studies that examined behavioral and cognitive outcomes. In addition, a short summary of potential mechanisms of action is provided with a focus on calcium-dependent processes. The current level of concern, or reference level, set by the CDC is 5 μg/dL of lead in blood and a revision to 3.5 μg/dL has been suggested. However, levels of lead below 3 μg/dL have been shown to produce diminished cognitive function and maladaptive behavior in humans and animal models. Because much of the research has focused on higher concentrations of lead, work on low concentrations is needed to better understand the neurobehavioral effects and mechanisms of action of this neurotoxic metal.

Pb2+ Binds to Downstream Regulatory Element Antagonist Modulator (DREAM) and Modulates Its Interactions with Binding Partners: A Link between Neuronal Calcium Sensors and Pb2+ Neurotoxicity

Pb²⁺ exposure leads to diverse neurological disorders; however, the mechanism of Pb²⁺-induced neurotoxicity is not clearly understood. Here we demonstrate that Pb²⁺ binds to EF-hands in apo-DREAM (downstream regulatory element antagonist modulator) with a lower equilibrium dissociation constant ( K_d = 20 ± 2 nM) than Ca²⁺ ( K_d = 1 μM). Based on the Trp169 emission and CD spectra, we report that Pb²⁺ association triggers changes in the protein secondary and tertiary structures that are analogous to those previously observed for Ca²⁺-bound protein. The hydrophobic cavity in the C-terminal domain of DREAM is solvent exposed in the presence of Pb²⁺ as determined using a hydrophobic probe, 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS). Pb²⁺ association with DREAM also modulates interactions between DREAM and its intracellular partners as evident from the fact that Pb²⁺-bound DREAM associates with peptide-based model systems, presenilin-1 helix-9 "PS1HL9" K_V4.3(70-90) "site-2" and K_V4.3(2-22) "site 1". Namely, dissociation constants for Pb²⁺-bound DREAM interaction with PS1HL9 ( K_d = 2.4 ± 0.1 μM), site-2 ( K_d = 11.0 ± 0.5 μM) and site 1 ( K_d = 5.0 ± 0.6 μM) are nearly identical to those observed for Ca²⁺ bound DREAM. Isothermal titration calorimetry data reveal that Pb²⁺ binds to two high-affinity sites in Ca²⁺ bound DREAM with the overall apparent constant of 4.81 ± 0.06 μM and its binding to Ca²⁺ bound DREAM is entropy-driven. Taking into account the structural and sequence similarity between DREAM and other neuronal calcium sensor (NCS) proteins, these results strongly indicate that DREAM and possibly other NCS proteins bind Pb²⁺ with a higher affinity than that for Ca²⁺ and Pb²⁺ interactions with NCS proteins can contribute to Pb²⁺-induced neurotoxicity.

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.

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.

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.

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.

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.

The epidemiology of lead toxicity in adults: measuring dose and consideration of other methodologic issues

We review several issues of broad relevance to the interpretation of epidemiologic evidence concerning the toxicity of lead in adults, particularly regarding cognitive function and the cardiovascular system, which are the subjects of two systematic reviews that are also part of this mini-monograph. Chief among the recent developments in methodologic advances has been the refinement of concepts and methods for measuring individual lead dose in terms of appreciating distinctions between recent versus cumulative doses and the use of biological markers to measure these parameters in epidemiologic studies of chronic disease. Attention is focused particularly on bone lead levels measured by K-shell X-ray fluorescence as a relatively new biological marker of cumulative dose that has been used in many recent epidemiologic studies to generate insights into lead's impact on cognition and risk of hypertension, as well as the alternative method of estimating cumulative dose using available repeated measures of blood lead to calculate an individual's cumulative blood lead index. We review the relevance and interpretation of these lead biomarkers in the context of the toxico-kinetics of lead. In addition, we also discuss methodologic challenges that arise in studies of occupationally and environmentally exposed subjects and those concerning race/ethnicity and socioeconomic status and other important covariates.

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.

Cumulative lead exposure and prospective change in cognition among elderly men: the VA Normative Aging Study

Lead exposure has been found to affect cognitive function in several different populations. Whether chronic low-level environmental exposure to lead results in cognitive decline among adults has not been examined. The authors assessed the relation between biomarkers of lead exposure and change in Mini-Mental State Examination (MMSE) scores in the Normative Aging Study, a cohort of elderly US men. Bone lead was measured with K-shell x-ray fluorescence. A total of 466 men aged 67.4 (standard deviation, 6.6) years took the MMSE on two occasions that were an average of 3.5 (standard deviation, 1.1) years apart during the period 1993-2001 and had bone lead concentrations measured during the period 1991-2002. A one-interquartile range (20 microg/g of bone mineral) higher patella bone lead concentration was associated with a change in MMSE score of -0.24 (95% confidence interval: -0.44, -0.05) after adjustment for age, education, smoking, alcohol intake, and time between MMSE tests. This effect is approximately equivalent to that of aging 5 years in relation to the baseline MMSE score in study data. The association with tibia lead was weaker and that with blood lead was absent. The data suggest that higher patella bone lead levels, a marker of mobilizable accumulated lead burden, are associated with a steeper decline over time in performance on the MMSE test among nonoccupationally exposed elderly men.

Differential induction of heme oxygenase and other stress proteins in cultured hippocampal astrocytes and neurons by inorganic lead

We examined the effects of exposure to inorganic lead (Pb2+) on the induction of stress proteins in cultured hippocampal neurons and astrocytes, with particular emphasis on the induction of heme oxygenase-1 (HO-1). In radiolabeled neuronal cultures, Pb2+ exposure had no significant effect on the synthesis of any protein at any concentration (up to 250 microM) or duration of exposure (up to 4 days). In radiolabeled astrocyte cultures, however, Pb2+ exposure (100 nM to 100 microM; 1-4 days) increased synthesis of proteins with approximate molecular weights of 23, 32, 45, 57, 72, and 90 kDa. Immunoblot experiments showed that Pb2+ exposure (100 nM to 10 microM, 1-14 days) induces HO-1 synthesis in astrocytes, but not in neurons; this is probably the 32-kDa protein. The other heme oxygenase isoform, HO-2, is present in both neurons and astrocytes, but is not inducible by Pb2+ at concentrations up to 100 microM. HO-1 can be induced by a variety of stimuli. We found that HO-1 induction in astrocytes is increased by combined exposure to Pb2+ and many other stresses, including heat, nitric oxide, H2O2, and superoxide. One of the stimuli that may induce HO-1 is oxidative stress. Lead exposure causes oxidative stress in many cell types, including astrocytes. Induction of HO-1 by Pb2+ is reduced by the hydroxyl radical scavengers dimethylthiourea (DMTU) and mannitol, but not by inhibitors of calmodulin, calmodulin-dependent protein kinases, protein kinase C, or extracellular signal-regulated kinases (ERK). Therefore, we conclude that oxidative stress is an important mechanism by which Pb2+ induces HO-1 synthesis in astrocytes.

Lead exposure biomarkers and mini-mental status exam scores in older men

BACKGROUND

Lead is neurotoxic; yet, whether cognitive decline in older persons is associated with lead exposure is unknown. We studied whether lead exposure biomarkers are associated with cognitive test scores, as well as the modifying effects of age on the lead-cognition relationship.

METHODS

Lead exposure biomarkers and Mini-Mental Status Exam (MMSE) scores were measured among subjects in the Normative Aging Study. Multiple linear and logistic regression analyses were performed to examine the cross-sectional association of these 2 variables.

RESULTS

We found an odds ratio (OR) of 2.1 for MMSE <24 with an increase from the lowest to the highest quartile of patella lead levels (95% confidence interval [CI] = 1.1 to 4.1). From the lowest to the highest quartile of blood lead the OR for low MMSE was 3.4 (CI = 1.6 to 6.2). There was an interaction between lead biomarkers and age. Among subjects in the lowest quartile of patella lead levels, MMSE score decreased by 0.03 points per year (CI = -0.07 to 0.005), whereas in the highest quartile, MMSE score decreased by 0.13 points per year (CI = -0.19 to -0.07). Similar interactions were found between blood lead levels and age.

CONCLUSIONS

Increased levels of lead in bone and blood are inversely associated with cognitive performance among older men. Lead exposure might accelerate age-associated cognitive decline.

Vascular endothelial growth factor mediates vasogenic edema in acute lead encephalopathy

Brain injury from inorganic Pb(2+) is considered the most important environmental childhood health hazard worldwide. The microvasculature of the developing brain is uniquely susceptible to high level Pb(2+) toxicity (ie, Pb(2+) encephalopathy) characterized by cerebellar hemorrhage, increased blood-brain barrier permeability, and vasogenic edema. However, the specific molecular mediators of Pb(2+) encephalopathy have been elusive. We found that Pb(2+) induces vascular endothelial growth factor/vascular permeability factor (VEGF) in cultured astrocytes (J Biol Chem, 2000;275:27874-27882). The study presented here asks if VEGF dysregulation contributes mechanistically to Pb(2+) encephalopathy. Neonatal rats exposed to 4% Pb-carbonate develop the histopathological features of Pb(2+) encephalopathy seen in children. Cerebellar VEGF expression increased approximately twofold (p < 0.01) concurrent with the development of cerebellar microvascular hemorrhage, enhanced vascular permeability to serum albumin, and vasogenic cerebellar edema (p < 0.01). No change in VEGF expression occurred in cerebral cortex that does not develop these histopathological complications of acute Pb(2+) intoxication. Pb(2+) exposure increased phosphorylation of cerebellar Flk-1 VEGF receptors and the Flk-1 inhibitor CEP-3967 completely blocked cerebellar edema formation without affecting microhemorrhage formation or blood-brain barrier permeability. This establishes that Pb(2+)-induced vasogenic edema formation develops via a Flk-1-dependent mechanism and suggests that the vascular permeability caused by Pb(2+) is Flk-1 independent.

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.

An epidemiological re-appraisal of the association between blood pressure and blood lead: a meta-analysis

Studies on the possible association between blood pressure and blood lead have reached divergent conclusions. In a previous meta-analysis, a doubling of the blood lead concentration was associated with a 1.0/0.6 mm Hg increase in systolic and diastolic blood pressure (BP). This meta-analysis updates the analysis originally performed in 1994. Articles on the association between BP and blood lead were identified from computer searches from January 1980 to February 2001 using the Medical Literature Analysis and Retrieval System. Of the studies reviewed, 31 provided sufficient details to be considered. The meta-analysis included 58518 subjects recruited from the general population in 19 surveys and from occupationally exposed groups in 12 studies. In all but four studies, the results were adjusted for age, and most studies took into account additional confounding factors such as body mass index and the use of alcohol and medication. Weighted joint P-values were calculated using Stouffer's procedure. The association between BP and blood lead was similar in both men and women. In the combined studies, a two-fold increase in blood lead concentration was associated with a 1.0 mm Hg rise in the systolic pressure (95% CI +0.5 to +1.4 mm Hg; P < 0.001) and with a 0.6 mm Hg increase in the diastolic pressure (95% CI +0.4 to +0.8 mm Hg; P < 0.001). On balance, this meta-analysis suggests that there can only be a weak association between BP and blood lead.

Microarray analysis of differential gene expression in lead-exposed astrocytes

The toxic metal lead is a widespread environmental health hazard that can adversely affect human health. In an effort to better understand the cellular and molecular consequences of lead exposure, we have employed cDNA microarrays to analyze the effects of acute lead exposure on large-scale gene expression patterns in immortalized rat astrocytes. Our studies identified many genes previously reported to be differentially regulated by lead exposure. Additionally, we have identified novel putative targets of lead-mediated toxicity, including members of the family of calcium/phospholipid binding annexins, the angiogenesis-inducing thrombospondins, collagens, and tRNA synthetases. We demonstrate the ability to distinguish lead-exposed samples from control or sodium samples solely on the basis of large-scale gene expression patterns using two complementary clustering methods. We have confirmed the altered expression of candidate genes and their encoded proteins by RT-PCR and Western blotting, respectively. Finally, we show that the calcium-dependent phospholipid binding protein annexin A5, initially identified as a differentially regulated gene by our microarray analysis, is directly bound and activated by nanomolar concentrations of lead. We conclude that microarray technology is an effective tool for the identification of lead-induced patterns of gene expression and molecular targets of lead.

Synaptotagmin I is a molecular target for lead

Lead poisoning can cause a wide range of symptoms with particularly severe clinical effects on the CNS. Lead can increase spontaneous neurotransmitter release but decrease evoked neurotransmitter release. These effects may be caused by an interaction of lead with specific molecular targets involved in neurotransmitter release. We demonstrate here that the normally calcium-dependent binding characteristics of the synaptic vesicle protein synaptotagmin I are altered by lead. Nanomolar concentrations of lead induce the interaction of synaptotagmin I with phospholipid liposomes. The C2A domain of synaptotagmin I is required for lead-mediated phospholipid binding. Lead protects both recombinant and endogenous rat brain synaptotagmin I from proteolytic cleavage in a manner similar to calcium. However, lead is unable to promote the interaction of either recombinant or endogenous synaptotagmin I and syntaxin. Finally, nanomolar concentrations of lead are able to directly compete with and inhibit the ability of micromolar concentrations of calcium to induce the interaction of synaptotagmin I and syntaxin. Based on these findings, we conclude that synaptotagmin I may be an important, physiologically relevant target of lead.

Dendritic alterations of cerebellar Purkinje neurons in postnatally lead-exposed kittens

Many investigations have sought to determine the effect of lead exposure on the development of the cerebellum. This study addresses the effects of postnatal lead exposure in kittens on dendritic development of Purkinje cells. Golgi-Cox filled cells were used to measure dendritic branching patterns, spine density, height, width and distance from the cerebellar surface. The results revealed a significant increase in spine density and altered patterns of dendritic branching. Complex dendritic branching was evident with a progressive shift in peak branching peripherally. Lead-exposed Purkinje cells showed early sprouting with subsequent pruning. At 5 weeks of age dendritic branches on experimental cells were increased along the entire dendritic extent. Control Purkinje cells showed initial sprouting with subsequent pruning. Normal developmental growth spurts and lead-induced effects were evident on dendritic height, width and distance from the surface. Cerebella stained with hematoxylin and eosin and cresyl violet acetate showed no evidence of vascular damage or other pathologies. These findings corroborate the evidence of hyperspiny dendritic formation representing an important mechanism of neuronal plasticity. In regard to morphological effects of lead on rodents, the hyperspiny Purkinje cell dendrites and patterns of dendritic growth in lead-treated kittens offer an alternative interpretation of neurobehavioral findings of lead-burdened children. The results are discussed with reference to other aspects of lead exposure and neural development.

Induction of vascular endothelial growth factor in human astrocytes by lead. Involvement of a protein kinase C/activator protein-1 complex-dependent and hypoxia-inducible factor 1-independent signaling pathway

The mechanism(s) underlying lead neurotoxicity are not fully elucidated. cDNA expression microarray analysis identified lead-sensitive genes in immortalized human fetal astrocytes (SV-FHA). Of the represented genes expressed, vascular endothelial growth factor (VEGF) was one of the most sensitive. Lead induced VEGF mRNA 3-fold and VEGF protein approximately 2-fold with maximum mRNA induction following incubation with 10 micrometer lead acetate for 24 h. Phorbol 12-myristate 13-acetate (PMA), a potent protein kinase C (PKC) activator, increased VEGF mRNA 2-fold and PKC inhibition by GF-109203 completely blocked VEGF induction by lead. Expression of dominant-negative PKC-epsilon, but not PKC-alpha, completely inhibited VEGF mRNA induction by lead. Lead activated the transcription factor AP-1 and increased AP-1-dependent luciferase expression >2-fold. Transfection of cells with a c-jun dominant-negative effectively inhibited both AP-1 activation and VEGF mRNA induction by lead. Hypoxia-inducible factor 1 (HIF-1) activity in SV-FHAs was moderately increased by lead (86%) and PMA (96%). Pretreatment with GF-109203 completely inhibited these effects of lead and PMA. However, lead did not alter HIF-1-dependent luciferase expression and a HIF-1alpha dominant-negative had no effects on the induction of VEGF mRNA by lead. These findings indicate that lead induces VEGF expression in SV-FHAs via a PKC/AP-1-dependent and HIF-1-independent signaling pathway.

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.

Increased AP-1 DNA binding activity in PC12 cells treated with lead

The possibility that the mechanism of lead neurotoxicity may be at the level of transcription was investigated in PC12 cells. In electrophoretic mobility gel shift assays Pb2+ was found to increase activator protein-1 complex (AP-1) DNA binding activity in PC12 cells; the increase was time- and concentration-dependent. Exposure to Pb2+ also resulted in an increase in AP-1-driven transcription in cerebellar granule cells transfected with a luciferase gene reporter construct. The increase in AP-1 DNA binding activity by Pb2+ required protein synthesis. The increase was mediated by protein kinase C because depletion of protein kinase C and an inhibitor of protein kinase C prevented the increase in AP-1 DNA binding activity by Pb2+. Fra-2 and JunD were found in supershift assays to be the major components of the AP-1 that was increased by Pb2+. In summary, our studies indicate that Pb2+ increases AP-1 DNA binding activity in PC12 cells by a pathway that requires protein kinase C and new protein synthesis.

Molecular mechanisms of lead neurotoxicity

Epidemiological studies have shown a strong relationship between the level of lead in blood and bone as assessed by performance on IQ tests and other psychometric tests. Approximately 1 out of 10 children in the United States have blood lead levels above 10 microg/dl, which has been established as the level of concern. Studies on experimental animals exposed to lead after birth have shown learning deficits at similar blood lead levels. Since learning requires the remodeling of synapses in the brain, lead may specifically affect synaptic transmission. Although the molecular targets for lead are unknown, a vast amount of evidence accumulated over many years has shown that lead disrupts processes that are regulated by calcium. Our laboratory has been studying the effect of lead on protein kinase C, a family of isozymes some of which require calcium for activity. We and others have shown that picomolar concentrations of lead can replace micromolar concentrations of calcium in a protein kinase C enzyme assay. Furthermore, lead activates protein kinase C in intact cells and induces the expression of new genes by a mechanism dependent on protein kinase C. We propose that the learning deficits caused by lead are due to events regulated by protein kinase C that most likely occur at the synapse.

Low levels of inorganic lead noncompetitively inhibit mu-calpain

Calpain is a ubiquitous calcium-dependent cysteine protease, whose cytoskeletal protein substrates suggest that it may be important in neuronal differentiation. Lead (Pb2+) is known to substitute for Ca2+ in a variety of intracellular processes, and interferes with the development of hippocampal neurons in vitro. We found that free Pb2+ at 1 nM does not activate calpain in the absence of Ca2+. Pb2+ inhibited the activity of calpain; the degree of calpain inhibition was dependent on an interaction between concentrations of both Ca2+ and Pb2+. In the presence of 1 microM free Ca2+, 10 pM free Pb2+ reduced calpain activity, but in the presence of 100 microM free Ca2+, 1 nM free Pb2+ failed to inhibit calpain. This provides evidence that Pb2+ competes for the Ca2+ binding sites on calpain. In the presence of 40 microM free Ca2+, 1 nM free Pb2+ significantly reduces Vmax without altering Km, suggesting that Pb2+ acts as a noncompetitive inhibitor of calpain. Inhibition of calpain is one mechanism by which Pb2+ may interfere with neuronal development.

Repair of wounded monolayers of cultured vascular endothelial cells after simultaneous exposure to lead and zinc

We investigated the interaction between lead and zinc on the repair of wounded monolayers of cultured bovine aortic endothelial cells. A half area of the monolayers was wounded and then incubated in the presence of lead (5.0 and 10 microM) and/or zinc (10 microM). It was morphologically observed that the appearance of the cells in the wounded area was strongly decreased by lead alone but considerably increased by zinc alone. The repair of wounded area after simultaneous exposure to lead and zinc showed that lead inhibits not only spontaneous but also zinc-promoted repair of endothelial cell layers without a change of the leakage of lactate dehydrogenase. Interaction between lead and zinc on the DNA synthesis of growing endothelial cells was similar to that on the repair, suggesting that the repair reflected the proliferation. In growing endothelial cells, the intracellular accumulation of lead was significantly increased by zinc; that of zinc was unaffected by lead; and that of metallothionein was slightly increased by zinc and lead but the effect of zinc was suppressed in the presence of lead. Although zinc significantly decreased the intracellular accumulation of radioactive calcium, lead increased it in the presence or absence of zinc. It was therefore concluded that lead inhibits not only spontaneous but also zinc-promoted repair of the damaged endothelial cell layers through an inhibition of the proliferation mediated by the calcium-mediated signalling pathways and/or a disturbance of intracellular calcium homeostasis.

A study of the cellular mechanism by which lead affects catecholamine secretion

Our purpose was to determine the role of protein kinases in the mediation of the stimulatory effects of lead on catecholamine secretion. Pheochromocytoma cells were incubated for 90 minutes with W-7 (calmodulin antagonist), calphostin C (protein kinase C inhibitor), Sp-cAMPS (cAMP agonist), Rp-cAMPS (cAMP antagonist), forskolin (activator of adenylyl cyclase), or lead nitrate. Catecholamines were measured by liquid chromatography. Lead had a stimulatory effect on catecholamine secretion, whereas W-7 was inhibitory. In the presence of both lead and W-7, the response was markedly decreased compared to that seen with lead alone. Calphostin C suppressed the secretion of catecholamines; however, in the presence of lead and calphostin C, the secretion was similar to that seen with lead alone. Compared to control, Sp-cAMPS was stimulatory. Co-incubation of Sp-cAMPS and lead had a slight synergistic effect. Rp-cAMPS decreased catecholamine secretion, but co-incubation of Rp-cAMPS and lead resulted in a slight reduction compared to lead alone. Forskolin markedly increased the secretion of catecholamines, and co-incubation of lead and forskolin resulted in a synergistic increase. In the absence of calcium, lead had no effect. We conclude that lead stimulates catecholamine secretion by acting through the calcium/calmodulin-dependent protein kinase II system and not through the protein kinase C or protein kinase A system, and requires the presence of calcium for its action.

Effects of lead-arsenic combined exposure on central monoaminergic systems

Lead acetate (116 mg/kg/day), arsenic (11 or 13.8 mg/kg/day as sodium arsenite), a lead-arsenic mixture or vehicle were administered to adult mice through gastric intubation during 14 days. Then, the regional content of norepinephrine (NE), dopamine (DA), serotonin (5-HT), 3,4 dihydroxyphenyl-acetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HIAA), arsenic, and lead were quantified. Compared with the accumulation after single element exposures, the mixture elicited a higher accumulation of lead and a lower arsenic accumulation in the brain. Compared to controls, lead induced only an augmentation of DOPAC (200%) in the hypothalamus. By contrast, the mixture provoked increases of DOPAC in the hypothalamus (250%), DA and 5-HIAA in the striatum (67 and 187%, respectively) and NE decreased in the hypothalamus (45%). Although these alterations were similar to those produced by arsenic alone, the mixture provoked a 38% decrease of NE in the hippocampus and increases of 5-HT in midbrain and frontal cortex (100 and 90%, respectively) over control values, alterations that were not elicited by either metal alone. These results demonstrate an interaction arsenic/lead on the central monoaminergic systems of the adult mouse.

Serum Calmodulin Activity in Male Lead-exposed Workers

Serum calmodulin (CaM) activity was studied in 75 lead-exposed and 21 non-exposed male workers. The lead-exposed workers were divided into groups with low blood lead (BPb < 50 µg/dL) and high blood lead (BPb>/=50 µg/dL). The concentrations of lead, calcium, magnesium, copper, zinc, and free erythrocytic protoporphyrin (FEP) in blood were determined. Serum samples were heated in a water bath (100 degrees C) for 3 minutes and centrifuged for 15 minutes at 4 degrees C (18,000 x g). The supernatants obtained were used to measure CaM activity. The results showed that: 1) Average blood lead concentrations in workers with both low and high levels of exposure were significantly higher than those in controls (p < 0.05). 2) Serum CaM activity in the high-exposure group (31.09 +/- 7.84 µg/dL) was significantly lower than that in controls (78.11 +/- 15.13 µg/dL, p < 0.05). The biological threshold of BPb inhibition of CaM activity was less than 50 µg/dL. 3) Multiple correlation analysis showed a negative dose-response relationship between BPb and CaM activity. The stepwise regression procedure indicated that lead had negative, and calcium and magnesium positive, effects on serum CaM activity. The regression equation was Y = 66.1383 - 1.0857 X&inf1; + 2.9676 X&inf2; + 5.2222 X&inf3; (Y:CaM; X&inf1;:Pb; X&inf2;: Ca; X&inf3;:Mg). These results of the first such study carried out in male lead-exposed workers suggest that lead can inhibit CaM activity in humans.

Lead reduces depolarization-induced calcium entry in cultured DRG neurons without crossing the cell membrane: fura-2 measurements

1. Cultured dorsal root ganglion of rat pups were depolarized by exposure to 50 mM K+ and the rise of [Ca2+]i was measured using fura-2 as an indicator. 2. Lead in the extracellular solution reduced the rise of [Ca2+]i in a concentration-dependent manner, with a threshold concentration of 0.25 microM. More than 80% of the calcium entry was prevented by approximately 5 microM lead. The IC50 and the Hill coefficient were 3.1 microM and 1, respectively. 3. This effect was considered to be due to a reduction of VACCCs, since applications of NMDA did not result in any rise of [Ca2+]i. 4. Since Pb2+ itself changes the fura-2 signal in a typical and characteristic manner, fura-2 is also an indicator for Pb2+. No changes in fura-2 signals were detected when lead (5 microM) was applied for several minutes in the absence of calcium, indicating that Pb2+ did not enter the cells. 5. Thus it is concluded that lead prevents calcium entry by reducing VACCCs but does not cross the cell membrane itself.

Lead perturbs the regulation of spontaneous release of tissue plasminogen activator and plasminogen activator inhibitor-1 from vascular smooth muscle cells and fibroblasts in culture

To evaluate the toxicity of lead on the blood fibrinolytic system during hemostasis, human aortic smooth muscle cells and human fetal lung fibroblasts were cultured in the presence of lead chloride. Tissue plasminogen activator antigen (t-PA:Ag) and plasminogen activator inhibitor-1 antigen (PAI-1:Ag) released were determined by enzyme immunoassay. It was found that lead decreased the release of both t-PA:Ag and PAI-1:Ag from vascular smooth muscle cells. On the other hand, in fibroblasts, the release of t-PA:Ag was markedly decreased whereas that of PAI-1:Ag was markedly increased by the metal. Fibrin zymography showed that lead reduced the plasminogen activator activity in the conditioned medium of both cell types. However, lead did not cause a nonspecific cell damage and an alteration of protein synthesis when evaluated by lactate dehydrogenase leakage and [14C]leucine incorporation, respectively. Lead accumulated within either vascular smooth muscle cells or fibroblasts in a dose-dependent manner; intracellular accumulation of calcium could be increased by lead. However, the effects of lead on the release of t-PA:Ag and PAI-1:Ag were different from those of calcium ionophore A23187. It was therefore suggested that regulation of spontaneous release of fibrinolytic proteins from subendothelial cells is disturbed by lead through intracellular calcium-independent pathway.

Effects of triethyl lead administration on the expression of glutathione S-transferase isoenzymes and quinone reductase in rat kidney and liver

The effects of triethyl lead chloride (TEL) on the expression of two detoxication enzyme families, glutathione S-transferases (GSTs) and NAD(P)H:quinone oxidoreductase (QR) were determined in rat liver and kidney. Fischer 344 rats were given one intraperitoneal (i.p.) injection of TEL. GST activity, GST isoenzyme levels, mRNA levels of alpha class GST isoenzymes Ya1, Ya2, and Yc1 and activity of QR were determined. Treatment of rats with TEL caused a significant increase in GST activity in kidney. In kidney, the levels of all GST subunits were significantly elevated; the largest increase was a 3.2-fold increase in GST Yb1. The levels of GST Ya1, Ya2, and Yc1 mRNA also increased after injection of TEL. In liver, TEL injection resulted in decreased GST activity and lower levels of hepatic GSTs Yb2, Yb3, Ya1, and Ya2. The largest decrease was a 40% reduction of GST Ya1. In contrast, the level of liver GST Yc1 increased from day 4 through day 14 after injection of 10 mg/kg TEL and Yp was increased 1.4-fold 4 days after injection of 12 mg/kg TEL. The levels of liver mRNAs coding for alpha class GSTs Ya1, Ya2, and Yc1 were reduced 12 h after injection of TEL. The mRNA levels of GST Ya1 and Ya2 returned to basal level while Yc1 message increased to a level higher than controls 24 h after TEL injection. The increase in Yc1 protein between days 4 and 14 is consistent with the increase in the corresponding mRNA. The activity of QR was elevated 1.5-fold in kidney and 2.7-fold in liver 14 days after the injection of TEL. This report demonstrates that administration of organic lead significantly affects GST expression and QR activity in a tissue-specific and isoenzyme-specific manner. These results indicate that GST expression and QR activity are not co-regulated.

Role of anion exchange and thiol groups in the regulation of potassium efflux by lead in human erythrocytes

Pb2+ is thought to enter erythrocytes through anion exchange (AE) and to remain in the cell by binding to thiol groups. To define the role of AE mechanisms and thiol groups in Pb2+ toxicity, we studied the effects of drugs and conditions that modify AE and that modify thiol groups on the ability of Pb2+ to stimulate potassium efflux as measured with 86Rb. The most potent stimulation of 86Rb efflux by Pb2+ occurred when conditions were optimal for the AE mechanism--that is, when bicarbonate was included in the buffer or a buffer made with Nal or NaCl rather than NaClO4 or NaNO3 was used. Furthermore, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfuonic acid, potent inhibitors of the AE mechanism, completely inhibited stimulation of the 86Rb efflux by Pb2+. These conditions or inhibitors did not affect stimulation of the 86Rb efflux by ionomycin plus Ca2+. A role for Ca2+ channels was dismissed because the inorganic Ca2+ channel blockers, Cd2+ or Mn2+, did not prevent stimulation of 86Rb efflux by Pb2+ but did inhibit stimulation by ionomycin plus Ca2+. 86Rb efflux was more sensitive to Pb2+ if erythrocytes were treated for 15 min with thiol-modifying reagents that enter cells, such as iodoacetamide, N-ethylmaleimide, or dithiothreitol, than to reduced glutathione, a thiol-modifying reagent that is not permeable to the cell. Thus, in erythrocytes the AE mechanism and internal thiol groups are critical factors that affect the stimulation of a Ca(2+)-dependent process by Pb2+.

Lead alters structure and function of mouse flexor muscle

To evaluate the effect of long-term exposure to heavy metals on skeletal muscle, chronic subcutaneous injections for 7 days of two level treatments (low dose, 0.1 mg/kg and high dose, 1 mg/kg) of lead acetate were investigated. Comparative analyses of in situ dorsiflexor muscle isometric contractile characteristics were studied in urethane-anesthetized (2 mg/g, i.p.) control and lead-exposed male mice. Control muscle-twitch tension reached an average of 1.81 +/- 0.06 g. Chronic lead (Pb2+) treatments did not affect muscle contractile speed, but reduced significantly the twitch tension in both high and low doses when compared to control animals. This effect was in a dose-dependent manner; 1.21 +/- 0.07 g for low dose and 0.90 +/- 0.05 g for high dose. These chronic Pb2+ treatments accelerated muscle fatigue after 250 stimuli (25 Hz for 10 sec) in both the low and high doses equally. However, marked elevation in tetanic (25 Hz) specific tension were observed in the high-dose, chronically treated animals, indicating some changes in contractile apparatus function. The high dose of chronic Pb2+ treatment induced ultrastructural changes, including reduced number of synaptic vesicles, disruption of mitochondria and increased number of smooth endoplasmic reticulum and myelin-like figures in the intramuscular axons and neuromuscular junctions. Chronic Pb2+ treatment caused extensive disruption of the sarcoplasmic mitochondria and increased the number of myelin-like figures in the muscle. These results suggest that exposure to Pb2+ at a low concentration can compromise the in situ skeletal muscle isometric contraction.

Inhibitory effect of lead on the proliferation of cultured vascular endothelial cells

We investigated the effect of lead nitrate (0.5, 1.0, 2.0 or 5.0 microM) on the proliferation of cultured bovine aortic endothelial cells. After exposure to lead, the number of cells and the incorporation of [3H]thymidine into the acid-insoluble fraction of the cells were reduced in parallel in a concentration-dependent manner. Histologically, lead treatment resulted in a decrease in the cell number accompanied by a change in the cell shape from polygonal to spindle; however, no degenerative change was observed except in 5.0 microM lead-treated cells. Furthermore, stimulation of [3H]thymidine incorporation by either basic or acidic fibroblast growth factor was significantly reduced by lead. However, the leakage of lactate dehydrogenase into the medium from the cells, a marker of nonspecific cell damage, was not changed by lead. From these results, it was revealed that lead inhibits the proliferation of cultured vascular endothelial cells without nonspecific cell damage. Although lead does not destroy the monolayer of endothelial cells, the metal may exhibit its noxious effect in the repair process of the vascular endothelium.

Calmodulin and cAMP dependent synaptic vesicle protein phosphorylation in rat cortex following lead exposure

The effect of in vivo and in vitro lead exposure on calmodulin and cAMP dependent synaptic vesicle protein phosphorylation has been investigated. Lead could enhance calmodulin activity following in vitro and in vivo lead exposure. The calmodulin dependent synaptic vesicle protein phosphorylation was enhanced following in vivo and in vitro lead exposure resulting in the depletion of the neurotransmitters norepinephrine and acetylcholine. The cAMP dependent synaptic vesicle protein phosphorylation was inhibited by both in vitro and in vivo lead treatment. The results suggest that lead adversely affects synaptic vesicle protein phosphorylation and this may ultimately effect synaptic functions.

Changes in the contractile responses to carbachol and in the inhibitory effects of verapamil and nitrendipine on isolated smooth muscle preparations from rats subchronically exposed to Pb2+ and Zn2+

Male Wistar rats were exposed to Pb2+ or Zn2+ and to Pb2+ + Zn2+, receiving Pb(CH3COO)2 or/and ZnSO4 with drinking water for 30 days. Cumulative concentration-effect curves for carbachol were obtained in ileum and trachea isolated from control and heavy metal-treated rats. The effect of the Ca2+ channel blockers on the carbachol-induced contractions was studied by addition of increasing concentrations of verapamil or nitrendipine to the bath solution 20 min. prior to carbachol. The results showed that exposure of rats to heavy metals in doses which did not change the body weight and behaviour, altered the contractile responses to carbachol. The sensitivity to carbachol was higher in preparations from the ileum of Zn(2+)-exposed rats as compared to controls, while a tendency towards decreasing this sensitivity was observed in ileal preparations from Pb(2+)-treated animals. The concentration-effect curves for carbachol in ileal preparations from Pb2+ + Zn(2+)-treated rats did not differ from those in the preparations from untreated rats. The inhibitory effect of the Ca2+ channel blockers on the contractility of ileal and tracheal preparations from treated rats was weaker as compared to that in controls.

Inhibitory effect of lead on the release of tissue plasminogen activator from human vascular endothelial cells in culture

To evaluate the toxicity of lead on the blood fibrinolytic system, vascular endothelial cells from human umbilical vein were cultured in the presence of lead and the content of tissue plasminogen activator antigen (t-PA:Ag) released into the medium was determined by enzyme immunoassay. It was found that lead significantly decreased the t-PA:Ag release from the cells. Although heavy metals including cadmium, mercury, cobalt, manganese, nickel, zinc and copper as well as lead each had an inhibitory effect, lead was the potent inhibitor. Lead significantly disturbed thrombin up-regulation of t-PA:Ag release and significantly amplified endothelin-1 down-regulation of it. Incorporation of [3H]thymidine into the acid-insoluble fraction of the cell layer was significantly increased by lead; however, that of [14C]leucine was unchanged by the metal. In lead-treated cells, a significant accumulation of lead was observed but calcium content was increased slightly. From these results, it was suggested that lead decreased the release of t-PA:Ag from cultured endothelial cells without nonspecific inhibition of protein synthesis; lead may stimulate the calcium-dependent down-regulation of endothelial cell t-PA:Ag release by calcium or by mimicking calcium.

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.

Interaction of metals with brain calmodulin purified from normal and cadmium exposed rats

Chronic exposure of cadmium (Cd) to rats (6 mg/kg body weight/day) led to a significant accumulation of Cd in brain and other organs. Calmodulin (CaM) isolated from brains of Cd exposed rats showed a decreased ability to stimulate CaM-dependent phosphodiesterase (PDE) as compared to that purified from unexposed animals. There was a dose dependent inhibition of CaM activity when CaM (from normal and Cd exposed rats) was incubated with different molar ratios of aluminium (Al3+), lead (Pb2+), manganese (Mn2+) and vanadium (V5+). Regression analysis of rat brain CaM activity versus varying metal ion concentration demonstrated negative slopes. However, CaM from the brains of Cd exposed rats was less sensitive to these metals in comparison to the normal rat brain CaM. These data suggest that CaM inhibition may be used as a biological marker of neurotoxicity and for elucidating the possible mechanism by which neurotoxic metals manifest toxic effects.

Pb2(+)-induced secretion from bovine chromaffin cells: fura-2 as a probe for Pb2+

The effect of Pb2+ on catecholamine release was studied in isolated intact and permeabilized bovine chromaffin cells. Fura-2 was used to monitor intracellular Pb2+. A characterization of Pb2(+)-fura-2 interactions in solutions simulating intracellular ionic composition showed that Pb2+ forms a 1:1 Pb2(+)-fura-2 complex (apparent dissociation constant = 4.2 x 10(-12) M, pH 7.05) whose fluorescence resembles that of the Ca2(+)-fura-2 complex. Spectra recorded from fura-2-loaded cells indicate entry of Pb2+ into the cells. Intracellular Pb2+ concentrations were proportional to extracellular Pb2+ concentrations and were found to be 10(-11)-10(-12) M in cells exposed to micromolar Pb2+ concentrations. Pb2+ elicited the release of tritiated norepinephrine from fura-2-loaded cells, indicating the extraordinary effectiveness of Pb2+ as a releasing agent. Permeabilization of cells with digitonin showed that Pb2+ is able, in the absence of Ca2+, to produce exocytotic release at concentrations of 3.2 x 10(-10) M or higher (3 orders of magnitude lower than Ca2+). These results support the notion that Pb2+ can act as a potent Ca2+ surrogate in triggering secretion.

Metal inhibition of calmodulin activity in monkey brain

Studies were conducted to investigate the effect of vanadium (V5+), cadmium (Cd2+), mercury (Hg2+), aluminium (Al3+), lead (Pb2+) and manganese (Mn2+) ions on calmodulin (CaM)-related Ca2+-ATPase activity in rhesus monkey (Macaca mulatta) brain (RMB). The cerebral cortex of RMB was dissected and homogenized in 10% sucrose buffer, and synaptic plasma membranes (SPM) were prepared. Ca2+-ATPase activity was determined by the inorganic phosphate method. Different concentrations of metal ions were incubated with RMB fractions containing endogenous CaM and RMB fractions without CaM. The results show that no metal ion inhibited the basal enzyme but that they showed a profound inhibition of total Ca2+-ATPase containing CaM. The order of potency of metals was Hg greater than Cd greater than Pb greater than Mn greater than Al greater than V. Exogenous addition of CaM restored the metal-inhibited enzyme activity to near the normal level. The basal enzyme, when reconstituted with CaM, showed sensitivity to metal ions. These data, presented for the first time in non-human primate brain, indicate that the metal ions inhibit CaM activity in RMB.

The effects of inorganic lead on the spontaneous and potassium-evoked release of 3H-5-HT from rat cortical synaptosome interaction with calcium

Interaction of lead with the serotonergic system has been studied in vitro in rat brain synaptosomal fraction prepared from cortical tissue. Synaptosomes were loaded with 3H-5-HT and spontaneous and K+-evoked release of the amine was examined in the presence and the absence of calcium. It was shown that lead itself induced the release of 3H-5-HT (EC50 = 27 microM). This effect decreased (40%) in the presence of calcium without modification of the EC50. Moreover, lead markedly inhibited the K+-evoked release of 3H-5-HT observed in the presence of calcium. This effect was obtained either in the presence of lead or using synaptosomes pretreated with lead and washed. These results indicate that lead interferes with neuronal 5-HT release by mechanism(s) involving calcium.

Lead activates protein kinase C in immature rat brain microvessels

We investigated the effects of inorganic lead upon calcium-, phospholipid-dependent protein kinase (protein kinase C) in brain microvessels isolated from 6-day-old rat pups. We found that (a) in broken cell preparations, lead at micromolar concentrations activates this enzyme to an extent equivalent to that of micromolar calcium (10.3 +/- 1.3 and 9.2 +/- 1.6 pmol/mg/min, respectively) and (b) preincubation of intact microvessels with lead results in a translocation of protein kinase C from the soluble to the particulate fraction. The cytosolic kinase activity stimulated by lead has the same requirements for diacylglycerol and phospholipid as the calcium-stimulated enzyme, suggesting that lead activates the kinase by mimicking calcium. The hypothesis that lead affects protein kinase C activity through a mechanism similar to that of calcium is supported by the similar time courses of substrate phosphorylation and dephosphorylation mediated by lead and calcium. When intact microvessels are preincubated with micromolar concentrations of lead, the translocation of protein kinase C occurs in a dose- and time-dependent manner. The relocalization is virtually complete at 0.1 microM lead and by 30 min of exposure. We propose that the sensitivity of protein kinase C to lead, described here in immature brain microvessels, makes this regulatory enzyme a potential mediator of lead toxicity.

Diminished regulation of mesolimbic dopaminergic activity in rat after chronic inorganic lead exposure

Previous studies in this laboratory have indicated that chronic lead (Pb) exposure during development induces a neurotoxicity in dopamine (DA) neurons that is primarily presynaptic in nature and at least partially related to altered regulation of DA synthesis. A primary form of DA synthesis regulation is the inhibition exerted on synaptic tyrosine hydroxylase activity via dopaminergic autoreceptors. This study assessed the functional status of this mechanism in Pb-exposed rats employing a pharmacological model. At parturition dams received 0.2% Pb acetate (1090 ppm) in the drinking water while control dams received distilled water. Offspring were weaned to and maintained on the same solution given their dams until termination at 125 days. Rats were given saline or 6,7-dihydroxy-2-dimethylaminotetralin (TL-99, 2.5-20 mg/kg ip) 40 min before termination followed 10 min later by 750 mg/kg ip of gamma-butyrolactone (GBL) or saline. The ability of TL-99 to prevent the GBL-induced increase in DA content was significantly diminished in nucleus accumbens (NAc) of exposed rats compared to controls, indicating that chronic Pb impairs receptor-mediated regulation of DA synthesis in mesolimbic neurons. No effect of Pb was observed in caudate-putamen. In animals receiving only saline injections concentrations of the DA metabolites, homovanillic acid and dihydroxyphenylacetic acid, were significantly decreased by Pb in the range of 17-31% and 12-24%, respectively. DA content was also significantly diminished by Pb in ventral tegmental area of these latter groups. These findings suggest that chronic Pb has multiple actions on central nervous system dopaminergic neurons consisting of an impaired regulation of DA synthesis that is apparently independent of a decrease in DA release.

Picomolar concentrations of lead stimulate brain protein kinase C

Recent growth studies in children suggest that there is no threshold for adverse effects from the universal exposure to inorganic lead. The biochemical mechanisms mediating low-level toxicity are unclear, but in several biological systems, lead alters calcium-mediated cellular processes and may mimic calcium in binding to regulatory proteins. Here we present evidence that lead stimulates diacylglycerol-activated calcium and phospholipid-dependent protein kinase, protein kinase C, partially purified from rat brain. Picomolar concentrations of lead are equivalent to micromolar calcium in kinase activation, so this regulatory enzyme is sensitive to the lead levels expected from current environmental exposure.

Action of di- and tri-valent cations on calcium-activated K+-efflux in rat erythrocytes

Isolated rat erythrocytes were labelled with [86Rb] as a tracer for intracellular K+. It was demonstrated that rat erythrocytes possess a Ca2+-mediated K+-efflux mechanism similar to that reported for human erythrocytes. This model was used to investigate the interactions of di- and tri-valent cations on potassium [86Rb] permeability in intact cells. Low concentrations of Ag2+ and Hg2+ haemolysed erythrocytes and Pb2+ produced a selective increase in [86Rb] efflux which became self-inhibitory at concentrations above 100 microM. The effects of Pb2+ were potentiated by A23187. Ni2+, Cu2+, Co2+, Zn2+, Fe2+, Mn2+, Y2+ and Ba2+ did not initiate [86Rb] efflux, even in the presence of 0.5 microM A23187 and at concentrations as high as 1 mM. All of these cations, except Ba2+, were potent inhibitors of [86Rb] efflux evoked by 50 microM Ca2+ + 0.5 microM A23187. The lanthanides Tb3+, Gd3+, Eu3+, Sm3+ and La3+ increased [86Rb] efflux at low concentrations in the presence of A23187, but were self inhibitory at higher concentrations. They also inhibited Ca2+-mediated [86Rb]-efflux. It is concluded that the effectiveness of a cation in activating [86Rb] efflux is, in part, related to its non-hydrated crystalline ionic radius, and that the site of activation may only accommodate ionic radii between 0.95 and 1.00 A.

Chronic low-level lead exposure. Its role in the pathogenesis of hypertension

Lead is a common element in the earth's crust, serving useful purposes in industry, but serving no purpose in the human body. Increase in blood pressure is an important public health problem with numerous factors contributing to many facets of the disease. The relationship of lead exposure and increased blood pressure has long been considered, but only recently critically investigated. Reports of subtle changes in calcium metabolism and renal function, as well as in vitro studies examining end-arteriolar smooth muscle contractility, link lead exposure and increased blood pressure. This paper critically examines the evidence associating chronic low-level lead exposure and increased blood pressure. The review focuses on epidemiological, clinical, and toxicological data. The epidemiological evidence is consistent with low-level exposure to lead causing an elevation in blood pressure. The strength of that association, and the dose-response characteristics, are less certain. Individual resistance and susceptibility could affect the degree of blood pressure elevation. The results of animal and in vitro studies are consistent with the epidemiological evidence, and suggest biologically plausible mechanisms for the association. The most probable mechanisms are intracellular perturbations in calcium metabolism mediated by direct lead effects at the end-arteriole, and indirect effects via renal dysfunction. Better indices of lead exposure and lead activity are needed to quantify these effects in humans. New and safer methods of chelating lead suggest interesting approaches for studying the relationship between lead and hypertension. This link could have significant implications in determining what constitutes a 'safe' level of environmental lead exposure, and whether a proportion of essential hypertension could be 'cured' by chelation therapy.