Low level lead neurotoxicity in a pregnant guinea pigs model: neuroglial enzyme activities and brain trace metal concentrations

Effects of Sub-chronic Lead Exposure on Essential Element Levels in Mice

Lead (Pb), a corrosion-resistant heavy non-ferrous metal, is one of the most common environmental neurotoxic metals. The effects of Pb on other essential metal elements are contradictory. Therefore, this in vivo study addressed the effects of sub-chronic Pb exposure on the distribution of other divalent metals, exploring the relationships between Pb levels in blood, teeth, bones, hair, and brain tissues. Thirty-two healthy male C57BL/6 mice received intragastric administration (i.g.) with 0, 12.5, 25, and 50 mg/kg Pb acetate, once a day for 8 weeks. Levels of Pb and other metal elements [including iron(Fe), zinc (Zn), magnesium (Mg), copper (Cu), and calcium(Ca)] in the whole blood, teeth, the right thighbone, hair, and brain tissues (including cortex, hippocampus, striatum, and hypothalamus) were detected with inductively coupled plasma-mass spectrometry (ICP-MS). Pb levels in all detected organs were increased after Pb-exposed for 8 weeks. The results of relationship analysis between Pb levels in the tissues and lifetime cumulative Pb exposure (LCPE) showed that Pb levels in the blood, bone, and hair could indirectly reflect the Pb accumulation in the murine brain. These measures might serve as valuable biomarkers for chronic Pb exposure reflective of the accumulation of Pb in the central nervous system (CNS). Sub-chronic Pb exposure for 8 weeks altered Ca, Cu, Fe, and Zn levels, but no effects were noted on Mg levels in any of the analyzed tissues. Pb decreased Ca in teeth, Cu in thighbone and teeth, Zn in whole blood and hair, and Fe in hair. In contrast, Pb increased Ca levels in corpus striatum and hypothalamus, Cu levels in striatum, Zn levels in teeth, and Fe levels in hippocampus, thighbone, and teeth. The Pb-induced changes in metal ratios in various tissues may serve as valuable biomarkers for chronic Pb exposure as they are closely related to the accumulations of Pb in the murine CNS. The results suggest that altered distribution of several essential metal elements may be involved in Pb-induced neurotoxicity. Additional studies should address the interaction between Pb and essential metal elements in the CNS and other organs.

Analysis of lead distribution in avian organs by LA-ICP-MS: Study of experimentally lead-exposed ducks and kites

Lead poisoning of wild birds by ingestion of lead ammunition occurs worldwide. Histopathological changes in organs of lead-intoxicated birds are widely known, and lead concentration of each organ is measurable using mass spectrometry. However, detailed lead localization at the suborgan level has remained elusive in lead-exposed birds. Here we investigated the detailed lead localization in organs of experimentally lead-exposed ducks and kites by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In both the ducks and kites, lead accumulated diffusely in the liver, renal cortex, and brain. Lead accumulation was restricted to the red pulp in the spleen. With regard to species differences in lead distribution patterns, it is noteworthy that intensive lead accumulation was observed in the arterial walls only in the kites. In addition, the distribution of copper in the brain was altered in the lead-exposed ducks. Thus, the present study shows suborgan lead distribution in lead-exposed birds and its differences between avian species for the first time. These findings will provide fundamental information to understand the cellular processes of lead poisoning and the mechanisms of species differences in susceptibility to lead exposure.

Interaction of four low dose toxic metals with essential metals in brain, liver and kidneys of mice on sub-chronic exposure

This study reports on interactions between low dose toxic and essential metals. Low dose Pb (0.01mg/L), Hg (0.001mg/L), Cd (0.005mg/L) and As (0.01mg/L) were administered singly to four groups of 3-week old mice for 120 days. Pb exposure increased brain Mg and Cu by 55.5% and 266%, respectively. Increased brain Mg resulted from metabolic activity of brain to combat insults, whiles Cu overload was due to alteration and dysfunction of CTR1 and ATP7A molecules. Reduction of liver Ca by 56.0% and 31.6% (on exposure to As and Cd, respectively) resulted from inhibition of Ca-dependent ATPase in nuclei and endoplasmic reticulum through binding with thiol groups. Decreased kidney Mg, Ca and Fe was due to uptake of complexes of As and Cd with thiol groups from proximal tubular lumen. At considerably low doses, the study establishes that, toxic metals disturb the homeostasis of essential metals.

Involvement of CTR1 and ATP7A in lead (Pb)-induced copper (Cu) accumulation in choroidal epithelial cells

The blood-cerebrospinal fluid barrier (BCB) plays a key role in maintaining copper (Cu) homeostasis in the brain. Cumulative evidences indicate that lead (Pb) exposure alters cerebral Cu homeostasis, which may underlie the development of neurodegenerative diseases. This study investigated the roles of Cu transporter 1 (CTR1) and ATP7A, two Cu transporters, in Pb-induced Cu accumulation in the choroidal epithelial cells. Pb exposure resulted in increased intracellular (64)Cu retention, accompanying with up-regulated CTR1 level. Knockdown of CTR1 using siRNA before Pb exposure diminished the Pb-induced increase of (64)Cu uptake. The expression level of ATP7A was down-regulated following the Pb exposure. ATP7A siRNA knockdown, or PCMB treatment, inhibited the (64)Cu efflux from the cells, while the following additional incubation with Pb failed to further increase the intracellular (64)Cu retention. Cu exposure, or intracellular Cu accumulation following the tetracycline (Tet)-induced overexpression of CTR1, did not result in significant change in ATP7A expression. Taken together, these data indicate that CTR1 and ATP7A play important roles in Cu transport in choroidal epithelial cells, and the Pb-induced intracellular Cu accumulation appears to be mediated, at least in part, via the alteration of CTR1 and ATP7A expression levels following Pb exposure.

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.

Level of oxidative stress and damage in erythrocytes in apprentices indirectly exposed to lead

BACKGROUND

Oxidative stress (OS) may result in damage to critical macromolecules, and an association between lead (Pb) toxicity and OS is a matter of research. Therefore, the aim of the present study was to investigate the effect of Pb on the oxidative system in indirectly Pb-exposed male apprentices.

METHODS

Established parameters of Pb toxicity (aminolevulinic acid dehydratase index [ALAD index], zinc-protoporphyrin [ZPP]) as well as Pb-level in blood were determined in Pb-exposed apprentices (n > 25) and controls (n > 24). Enzymatic (glutathione peroxides [GPx], superoxide dismutase [SOD], catalase [CAT]) and non-enzymatic (alpha-tocopherol, beta-carotene) indices of OS, and malondialdehyde (MDA) level were also determined.

RESULTS

There was a statistically significant increase in Pb level, ALAD index, ZPP concentration, GPx activity and MDA concentration in Pb-exposed apprentices when compared to controls. Although a statistically significant decrease in alpha-tocopherol and beta-carotene levels was seen, SOD and CAT activities were unaltered in Pb-exposed apprentices. Pb level and duration of Pb exposure were correlated with each other, as well as various indices of OS and MDA concentration.

CONCLUSIONS

Chronic indirect Pb exposure results in lipid peroxidation in erythrocytes of apprentices via OS, and duration of Pb exposure is a reliable marker of Pb toxicity.

In vitro models for assessing neurotoxicity of mixtures

Rapid and inexpensive methods are needed to investigate the interactions of complex mixtures. This commentary addresses the use of cell cultures to detect neurotoxicity of simple binary mixtures, which is a first step in the development of such methods. A small number of recent studies from our laboratory are examined. Though such studies are few, they offer guidance for optimizing the value of cell cultures as systems for chemical toxicity screening and mechanistic research. The same issues that apply to in vitro neurotoxicity studies of single agents also apply to the study of mixtures, such as relevance of endpoints tested, biological usefulness and limitations of cell culture models, and relevance of exposures tested. In this commentary we will focus on several aspects of these issues.

The involvement of copper transporter in lead-induced oxidative stress in astroglia

Lead (Pb), depositing primarily in astroglia in the brain, is a well-known neurotoxicant and a risk factor for neurologic disorders. Pb has been reported to induce oxidative stress by probably the disturbance of copper (Cu) homeostasis in astroglia. Thus, we hypothesized that Pb-induced oxidative stress is initiated by interfering with Cu transporter in astroglia. In this study, we observed Pb-induced oxidative stress as indicated by reactive oxygen species (ROS) augmentation and GRP78 and GRP94 protein induction, and it was parallel to Cu accumulation intracellularly by Pb. To further address Cu transporter as a potential Pb target, a heavy metal-binding (HMB) domain of Cu-transporting ATPase (Atp7a) was overexpressed and purified. Evidence showed that one molecule of HMB chelated 11 Pb ions or seven Cu ions and that Pb competed with Cu for binding to HMB. These findings suggest that Pb-induced oxidative stress results from the impairment of Cu metabolism by Pb targeting of Atp7a.

In vivo microdialysis study of the relationship between lead-induced impairment of learning and neurotransmitter changes in the hippocampus

Chronic exposure to lead during development is associated with cognitive dysfunction in children and animals and impairment of release of neurotransmitters in the brain. Some amino acid neurotransmitters in the CNS are critical for the induction of LTP, which is considered a potential mechanism of learning and memory. In this study, the extracellular levels of amino acids in the dentate gyrus (DG) of the hippocampus of early postnatal rats exposed to lead were measured by in vivo microdialysis, before and after 50 days of training. Samples of cerebrospinal fluid were analyzed by high-pressure liquid chromatography (HPLC) and fluorescence detection. Compared to pre-training, the concentration of glutamate in the post-training samples increased by 164.2 and 222.6% in the control and lead-exposure rats, respectively. After training, the extracellular concentration of GABA and glycine decreased by 49.4 and 44.3% in lead-exposed rats, respectively, whereas in the after-training samples of control rats, the concentration of GABA was unchanged and glycine decreased by 21.8%. The results of this study may suggest that concentrations of the neurotransmitters were changed during the learning process and lead impaired the neurotransmitter systems, especially glutamate and GABA systems.

Lead-stimulated p38MAPK-dependent Hsp27 phosphorylation

Lead (Pb2+) is a cytotoxic metal ion whose mechanism of action is not established. However, Pb2+ is known to interact with a wide variety of molecules involved in signal transduction. In this study the effect of Pb2+ on protein phosphorylation in bovine adrenal chromaffin cells and human SH SY5Y cells was examined. Cells were incubated with 32P(i) for 1 h in the presence of Pb2+ (1-10 microM) and the proteins were separated by two-dimensional PAGE. An increase in the phosphorylation of a number of proteins was observed in response to Pb2+, including three spots, MW 25 kDa, and pI's in the range 4.0-4.5. These proteins were immunoidentified as three isoforms of the heat-shock protein 27 kDa (Hsp27), and the identity of the most basic spot was confirmed by amino acid sequencing. Phosphorylation of p38MAPK was increased by Pb2+ and the effect of Pb2+ on Hsp27 phosphorylation was blocked by the p38MAPK inhibitor SB203580 (1 microM). The results were similar for bovine chromaffin cells and human SH SY5Y cells. This is the first report showing that Pb2+ can modulate the phosphorylation state of Hsp27 via activation of the p38MAPK pathway.

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.

Mercuric chloride, but not methylmercury, inhibits glutamine synthetase activity in primary cultures of cortical astrocytes

Methylmercury (MeHg) is highly neurotoxic with an apparent dose-related latency period between time of exposure and the appearance of symptoms. Astrocytes are known targets for MeHg toxicity and a site of mercury localization within the central nervous system (CNS). Glutamine synthetase (GS) is an enzyme localized predominately within astrocytes. GS converts two potentially toxic molecules, glutamate and ammonia, to the relatively non-toxic amino acid, glutamine. During prolonged exposure to MeHg, inorganic mercury (I-Hg) accumulates within the brain, suggesting in situ demethylation of MeHg to I-Hg. To determine if speciation of mercurials would differentially alter GS activity and expression, neonatal rat primary astrocyte cultures were exposed to MeHg or mercuric chloride (HgCl2) for 1 or 6 h. MeHg produced no changes in GS activity, protein, or mRNA at any time or dose tested. In contrast, HgCl2 produced a dose dependent decrease in astrocytic GS activity at both 1 and 6 h. There were no changes in GS protein or mRNA levels following HgCl2 exposure. Additional studies were carried out to determine GS activity in cell lysates incubated with HgCl2 or MeHg. In cell lysates, HgCl2 was three-times more potent than MeHg in inhibiting GS activity. The inhibition of GS activity in cell lysates by HgCl2 was reversed by the addition of dithiothreitol (DTT), while DTT did not restore GS activity following MeHg. These data suggest that astrocytic GS activity is not inhibited by physiologically relevant concentrations of MeHg, but is inhibited by I-Hg, which is present in CNS following chronic MeHg exposure.

In vitro aberrant gene expression as the indicator of lead-induced neurotoxicity in U-373MG cells

Lead is an important neurobehavioral toxicant and may interfere with developmental processes in the brain resulting in impairment of its functions. U-373MG, a human glioma cell line, was cultured in Dulbecco's modified Eagles' medium supplemented with either 20 or 10% FBS (fetal bovine serum) to explore the possible indications for lead-induced toxicity. Although lead did not affect cell growth rate in concentrations ranging from 0.01 to 10 microM, it substantially altered gene expression analyzed by reverse-transcription polymerase chain reaction. With 10% FBS culture, lead affected the gene expression in a dose-dependent relationship. It enhanced the expression of tumor necrosis factor-alpha (TNF-alpha), but decreased those of interleukin-1beta (IL-1beta), interleukin-6 (IL-6), gamma-aminobutyric acid (GABA) transaminase, and glutamine synthetase. With 20% FBS culture, lead also profoundly increased TNF-alpha and IL-1beta; however, it did not extensively affect the other genes examined above. Thus, the highly sensitive changes of gene expression of these cytokines or metabolic enzymes after treatments with lead acetate evidenced their usefulness as indicators for in vitro measurement of lead-induced neurotoxicity.

Lead targets GRP78, a molecular chaperone, in C6 rat glioma cells

Exposure to potentially neurotoxic levels of lead (Pb) occurs in about 9% of American children under 6 years of age. Astroglia in the brain serve as a Pb depot, sequestering Pb and preventing its contact with the more sensitive neurons. Astroglia have the capacity to adapt to Pb exposure, and as such are able to tolerate relatively high intracellular Pb accumulation. This tolerance mechanism has yet to be defined in biochemical terms. In the present study, we present evidence that glucose-regulated protein (GRP78), a molecular chaperone in the ER, participates directly or indirectly in the tolerance mechanism. Exposure of cultured C6 rat glioma cells, an astroglia-like cell line, to 1 microM Pb acetate for 1 week raised the intracellular levels of two proteins, one of which was identified by sequence analysis as GRP78. GRP78 accumulation started within 1 day and progressed with time of exposure. Studies in vitro showed that GRP78 bound tightly to affinity columns with Pb(2+) as the affinity ligand and bound weakly when either Zn(2+) or Ni(2+) replaced the Pb(2+). The reduced form of GSH and BSA did not compete with GRP78 to chelate Pb(2+). However, the heavy metal binding domain (HMB) of Menkes protein competed with GRP78 for chelating Pb(2+). The data provide evidence that GRP78 may be a component of the Pb tolerance mechanism through its direct interaction with Pb(2+). Its increased synthesis could be part of the adaptive response to Pb exposure.

Effect of lead exposure and accumulation on copper homeostasis in cultured C6 rat glioma cells

C6 rat glioma cells resemble rat astroglia in culture in that both cell types accumulate lead (Pb) intracellularly from the medium. As such, C6 cells are a model for Pb accumulation by the brain. In this study, an increase in intracellular Pb accumulation induced by p-chloromercuribenzoate (PCMB) after exposure to 10 microM Pb acetate suggests a role for sulfhydryl groups in Pb retention. Stimulation of Pb accumulation by nifedipine suggests the entry of Pb into these cells by a novel path. Most of the intracellular Pb from exposure for 7 days to 1 microM Pb was associated with high-molecular weight components in cytosol. Pb exposure increased the abundance of three proteins with the following characteristics on two-dimensional gels: 81 kDa with pI of 5.6, 81 kDa with pI of 4. 9, and 71 kDa with pI of 5.6. The levels of five other proteins, ranging in size from 37-41 kDa with pIs of 6.0-6.8 declined. Exposed C6 cells accumulated copper (Cu) intracellularly, and Cu accumulation after Pb exposure was shown by kinetic analysis with 67Cu to result from an increased uptake and a decreased efflux for Cu. Pb-exposed cells also showed increased Cu binding to membranes, which is consistent with the increase of Cu uptake. These data indicate that intracellular Pb interacts with high molecular weight proteins in C6 cells, and exposure also alters membrane transport properties for copper.

Developmental patterns of aluminum and five essential mineral elements in the central nervous system of the fetal and infant guinea pig

Al is found in the developing conceptus, but little information is available concerning its tissue distribution and its changes in concentration with age. Because Al has affinity for many of the same biological ligands as the essential mineral cations Ca, Mg, Zn, Fe, and Mn, we hypothesized that Al might show a pattern of developmental concentrations that was similar to one or more of these elements in the brain, a major target of Al toxicity. Concentrations of Al, Ca, Mg, Zn, Fe, and Mn were measured in spinal cord, brainstem, cerebellum, and forebrain of guinea pig fetuses on gestation day (GD) 30 and 45, at birth, and on postnatal day (PND) 3, 6, and 12. Dams were fed commercial guinea pig chow, which contained 47 micrograms Al/g. Tissue Al and Mn were measured with electrothermal atomic absorption spectrophotometry (ETAAS), and the other elements with inductively coupled axial plasma spectroscopy (ICAP-AES). Al concentrations in the brain regions were highest in spinal cord, brainstem, and cerebellum, and decreased during late gestation and lactation. Al did not show marked increases in regional brain concentrations during the final third of gestation as did Fe, Mg, and Zn. In contrast to Fe and Ca, Al did not accumulate in placenta. Al was the only element to show higher concentrations in spinal cord than in any other tissue at birth. In summary, the tissue distribution of Al did not follow that of essential cations as examined in this study.

Subsensitivity of lead-exposed rats to the accuracy-impairing and rate-altering effects of MK-801 on a multiple schedule of repeated learning and performance

Exposure to lead (Pb) has been reported to inhibit MK-801 binding and to alter other NMDA receptor complex-associated functions. These reported changes are provocative since both NMDA receptor antagonism and Pb exposure are know to impair learning processes. Whether the Pb-induced changes in NMDA function relate to the learning impairments associated with Pb exposure, however, has not been explored. The contention of this study was that if changes in NMDA function produced by Pb serve as the basis of Pb-associated learning impairments, then such changes should be of sufficient biological magnitude and clinical relevance to induce alterations in sensitivity at the level of the whole animal, i.e., changes in behavioral sensitivity to glutamatergic compounds. Thus, in this study, dose-effect curves of control and Pb-treated rats working on a multiple schedule of repeated learning (repeated acquisition, RA) and performance (P) were compared following acute administration of MK-801, the non-competitive NMDA antagonist. Based on the nature of the reported effects of Pb on NMDA systems, it was expected that the curves of Pb-exposed rats would be right-shifted relative to controls, if differential behavioral sensitivity was evident. Rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water from weaning and trained on the multiple RA and P schedule beginning at 55 days old. The RA component required the rat to learn a new 3-member sequence of responses during each experimental session (center right left, RLC, CLR, RCL, or LRC), while the correct sequence of responses for the P component was constant across sessions (LCR), requiring performance of an already learned response. Acute administration of MK-801 (0.05-0.3 mg/k, i.p.) resulted in decrements in accuracy in both the RA and P components of the schedule, indicative of non-specific effects on behavior rather than selective effects on learning. The declines in accuracy during the RA component of the schedule were primarily the result of increased perseverative responding, i.e., repetitive responding on a single lever. Both the decline in RA accuracy and the increases in perseverative responding produced by MK-801 were attenuated by Pb exposure. Moreover, dose-effect curves relating MK-801 dose to changes in rates of responding were significantly shifted to the right in Pb-exposed rats relative to controls. Taken together, these data demonstrate a subsensitivity of Pb-exposed rats to both the accuracy-impairing and response rate-altering properties of MK-801.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of low-level lead exposure on hypothalamic hormones and serum progesterone levels in pregnant guinea pigs

Pregnant guinea pigs were given a daily oral dose of 0, 5.5, or 11 mg lead (as lead acetate) per kg body weight during days 22-52 or 22-62 of gestation. Maternal serum progesterone levels were measured at the end of treatment, as well as hypothalamic levels of gonadotropin-releasing hormone (GnRH) and somatostatin (SRIF) in both the mothers and fetuses. Lead-treated dams had lower serum concentrations of progesterone at the end of treatment than did vehicle-treated animals. This effect was statistically significant for the higher Pb dose only. Hypothalamic levels of GnRH and SRIF were reduced in a dose-dependent manner by lead treatment in both dams and fetuses. The reduction of SRIF levels in 52-day-old fetuses was particularly severe (92%) in the 11 mg group. However, neither litter size nor body and organ weights, including placental weight, of the dams and fetuses was significantly affected. The relevance of these hormonal decreases is unknown, but could include decreased reproductive capacity in both the dams and fetuses that does not become apparent until later in the life-cycle.