Exposure to lead-acetate modulates the developmental expression of myelin genes in the rat frontal lobe

Lead Neurotoxicity on Human Neuroblastoma Cell Line SH-SY5Y is Mediated via Transcription Factor EGR1/Zif268 Induced Disrupted in Scherophernia-1 Activation

Lead (Pb²⁺) is a well-known type of neurotoxin and chronic exposure to Pb²⁺ induces cognition dysfunction. In this work, the potential role of early growth response gene 1 (EGR1) in the linkage of Pb²⁺ exposure and disrupted in scherophernia-1 (DISC1) activity was investigated. Human neuroblastoma cell line SH-SY5Y was subjected to different concentrations of lead acetate (PbAc) to determine the effect of Pb²⁺ exposure on the cell viability, apoptosis, and activity of EGR1 and DISC1. Then the expression of EGR1 in SH-SY5Y cells was knocked down with specific siRNA to assess the function of EGR1 in Pb²⁺ induced activation of DISC1. The interaction between EGR1 and DISC1 was further validated with dual luciferase assay, Supershift electrophoretic mobility shift assay (EMSA), and chromatin immunoprecipitation (ChIP)-PCR. Administration of PbAc decreased cell viability and induced apoptosis in SH-SY5Y cells in a dose-dependent manner. Additionally, exposure to PbAc also up-regulated expression of EGR1 and DISC1 at all concentrations. Knockdown of EGR1 blocked the effect of PbAc on SH-SY5Y cells, indicating the central role of EGR1 in the function of Pb²⁺ on activity of DISC1. Based on the results of dual luciferase assay, Supershift EMSA, and ChIP-PCR, EGR1 mediated the effect of Pb²⁺ on DISC1 by directly bound to the promoter region of DISC1 gene. The current study elaborated the mechanism involved in the effect of Pb²⁺ exposure on expression of DISC1 for the first time: EGR1 activated by Pb²⁺ substitution of zinc triggered the transcription of DISC1 gene by directly binding to its promoter.

Bisphenol-A impairs myelination potential during development in the hippocampus of the rat brain

Myelin is the functional implication of oligodendrocytes (OLs), which is involved in insulation of axons and promoting rapid propagation of action potential in the brain. OLs are derived from oligodendrocyte progenitor cells (OPCs), which proliferate, differentiate, and migrate throughout the central nervous system. Defects in myelination process lead to the onset of several neurological and neurodegenerative disorders. Exposure to synthetic xenoestrogen bisphenol-A (BPA) causes cognitive dysfunction, impairs hippocampal neurogenesis, and causes onset of neurodevelopmental disorders. However, the effects of BPA on OPC proliferation, differentiation and myelination, and associated cellular and molecular mechanism(s) in the hippocampus of the rat brain are still largely unknown. We found that BPA significantly decreased bromodeoxyuridine (BrdU)-positive cell proliferation and number and size of oligospheres. We observed reduced co-localization of BrdU with myelination markers CNPase and platelet-derived growth factor receptor-α (PDGFR-α), suggesting impaired proliferation and differentiation of OPCs by BPA in culture. We studied the effects of BPA exposure during prenatal and postnatal periods on cellular and molecular alteration(s) in the myelination process in the hippocampus region of the rat brain at postnatal day 21 and 90. BPA exposure both in vitro and in vivo altered proliferation and differentiation potential of OPCs and decreased the expression of genes and levels of proteins that are involved in myelination. Ultrastructural electron microscopy analysis revealed that BPA exposure caused decompaction of myelinated axons and altered g-ratio at both the developmental periods as compared to control. These results suggest that BPA exposure both during prenatal and postnatal periods alters myelination in the hippocampus of the rat brain leading to cognitive deficits.

Proton magnetic resonance spectroscopy in adults with childhood lead exposure

BACKGROUND

Childhood lead exposure adversely affects neurodevelopment. However, few studies have examined changes in human brain metabolism that may underlie known adverse cognitive and behavioral outcomes.

OBJECTIVE

We examined the association between mean childhood blood lead levels and in vivo brain metabolite concentrations as adults, determined by proton magnetic resonance spectroscopy (MRS) in a birth cohort with documented low-to-moderate lead exposure.

METHODS

Adult participants from the Cincinnati Lead Study [n = 159; mean age (± SD), 20.8 ± 0.9 years] completed a quantitative, short-echo proton MRS protocol evaluating seven regions to determine brain concentrations of N-acetyl aspartate (NAA), creatine and phosphocreatine (Cr), cholines (Cho), myo-inositol, and a composite of glutamate and glutamine (GLX). Correlation and multiple linear regression analyses were conducted.

RESULTS

Mean childhood blood lead levels were associated with regionally specific brain metabolite concentrations adjusted for age at imaging and Full-Scale intelligence quotient. Adjusted analyses estimated for a unit (micrograms per deciliter) increase in mean childhood blood lead concentrations, a decrease of NAA and Cr concentration levels in the basal ganglia, a decrease of NAA and a decrease of Cho concentration levels in the cerebellar hemisphere, a decrease of GLX concentration levels in vermis, a decrease of Cho and a decrease of GLX concentration levels in parietal white matter, and a decrease of Cho concentration levels in frontal white matter.

CONCLUSIONS

Gray-matter NAA reductions associated with increasing childhood blood lead levels suggest that sustained childhood lead exposure produces an irreversible pattern of neuronal dysfunction, whereas associated white-matter choline declines indicate a permanent alteration to myelin architecture.

Effects of early low-level lead exposure on human brain structure, organization and functions

Advanced neuroimaging techniques offer unique insights into how childhood lead exposure impacts the brain. Volumetric magnetic resonance imaging affords anatomical information about the size of global, regional and subcomponent structures within the brain. Diffusion tensor imaging provides information about white matter architecture by quantitatively describing how water molecules diffuse within it. Proton magnetic resonance spectroscopy generates quantitative measures of neuronal, axonal and glial elements via concentration levels of select metabolites. Functional magnetic resonance imaging infers neuronal activity associated with a given task performed. Employing these techniques in the study of the Cincinnati Lead Study, a relatively homogeneous birth cohort longitudinally monitored for over 30 years, one can non-invasively and quantitatively explore how childhood lead exposure is associated with adult brain structure, organization and function. These studies yield important findings how environmental lead exposure impacts human health.

Altered myelination and axonal integrity in adults with childhood lead exposure: a diffusion tensor imaging study

Childhood lead exposure is associated with adverse cognitive, neurobehavioral and motor outcomes, suggesting altered brain structure and function. The purpose of this work was to assess the long-term impact of childhood lead exposure on white matter integrity in young adults. We hypothesized that childhood lead exposure would alter adult white matter architecture via deficits in axonal integrity and myelin organization. Adults (22.9+/-1.5 years, range 20.0-26.1 years) from the Cincinnati Lead Study were recruited to undergo a study employing diffusion tensor imaging (DTI). The anatomic regions of association between water diffusion characteristics in white matter and mean childhood blood lead level were determined for 91 participants (52 female). Fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) were measured on an exploratory voxel-wise basis. In adjusted analyses, mean childhood blood lead levels were associated with decreased FA throughout white matter. Regions of the corona radiata demonstrated highly significant lead-associated decreases in FA and AD and increases in MD and RD. The genu, body, and splenium of the corpus callosum demonstrated highly significant lead-associated decreases in RD, smaller and less significant decreases in MD, and small areas with increases in AD. The results of this analysis suggest multiple insults appear as distinct patterns of white matter diffusion abnormalities in the adult brain. Neurotoxic insults from the significant lead burden the participants experienced throughout childhood affect neural elements differently and may be related to the developmental stage of myelination at periods of exposure. This study indicates that childhood lead exposure is associated with a significant and persistent impact on white matter microstructure as quantified with diffusivity changes suggestive of altered myelination and axonal integrity.

Environmental risk factors and the developmental basis for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder whose clinical manifestations appear in old age. The hallmark pathological features of AD (amyloid plaques and associated proteins) are present in normal aging indivduals, suggesting that AD may result from the acceleration of normal age-related processes in the brain. The sporadic nature of most AD cases strongly argues for an environmental link that may drive AD pathogenesis; however, it is unclear when this environmental stress may occur. Therefore it is important to identify an environmental trigger(s) and to pinpoint the period during which such factors pose the greatest risk. Recently, we reported that developmental exposure of rats to the xenobiotic metal lead (Pb) resulted in a delayed overexpression (20 months later) of the amyloid precursor protein (APP) and its amyloidogenic Abeta product. Similarly, aged monkeys exposed to Pb as infants also responded in the same way. These data suggest that environmental influences occurring during brain development predetermine the expression and regulation of APP later in life, potentially influencing the course of amyloidogenesis, and argue for both an environmental trigger and a developmental origin of AD. In this review, we present evidence for the developmental basis of neurodegeneration and discuss mechanisms that may explain how perturbations during development can have long-term or delayed consequences in the aging brain.

Transcriptional involvement in neurotoxicity

Exposure to various chemicals and environmental hazards elicits changes in the expression of a variety of genes. The study of gene expression and transcriptional regulation is an important aspect of understanding the mechanisms associated with neurotoxicity. The availability of whole genome sequences and the development of new tools to identify and monitor transcriptional activity have accelerated the rate of discovery. This review surveys the historical steps taken to study gene expression in the brain and deals with recent advances in our understanding and classification of the roles of transcription factors. Disturbances in the regulation of gene expression associated with the neurotoxic response are also presented. Specific focus and detail is presented on the effects of heavy metals on the integrity and function of zinc finger proteins.

Differential ototoxicities induced by lead acetate and tetraethyl lead

Lead poisoning disrupts many biological structures and functions, including those of the auditory system. This study examined the ototoxic effects of lead acetate (LA) and tetraethyl lead (TEL) of equal lead content on cochlear function and the ability of alpha-phenyl-tert-butyl-nitrone (PBN) to attenuate such effects. Baseline 1.0 microV cochlear microphonic (CM) and compound action potential (CAP) responses were recorded and animals administered either PBN (100 mg/kg, i.p.) or an equal volume of 0.9% saline, followed by an i.p. injection of LA (50 mg/kg) in an ethanol vehicle, TEL (42.7 mg/kg) in a corn oil vehicle, corn oil or ethanol vehicle alone. Two hours after administration, post-exposure CM and CAP responses were recorded. CAP threshold shifts in the saline-LA group were elevated by 5-10 dB at mid to high frequencies relative to controls (20-24 kHz, P<0.05). Mean CAP threshold shifts in the saline-TEL were significantly greater than those of both control groups at all tested frequencies except 2 kHz (P<0.001). However, threshold shifts in the group receiving PBN prior to TEL were significantly smaller than shifts in the group receiving saline prior to TEL (P<0.01). These data suggest that TEL is more ototoxic than is LA and that free radicals partially mediate TEL-induced CAP disruption.

Molecular changes in glutamatergic synapses induced by Pb2+: association with deficits of LTP and spatial learning

What are the molecular bases for the neurotoxicity that occurs after developmental exposure to low levels of Pb2+, and are these effects persistent and detrimental in adults? Our inability to understand specific mechanisms behind Pb2+ neurotoxicity has long been one of many problem areas of this preventable childhood disease. The sensitivity of the developing brain to Pb2+-induced neurotoxicity is an outcome of the many unique characteristics that comprise the developing central nervous system. The developing brain can be exposed to significant concentrations of Pb2+ during vulnerable periods of development such as synapse formation, gene and protein expression, and other diverse molecular changes associated with these processes. Recently, changes in NMDA receptor subunits were identified in animals that showed cognitive deficits induced by exposure to Pb2+. This molecular association is important because it provides new evidence in the characterization of developmental Pb2+ neurotoxicity that supports physiological findings of impairments in synaptic plasticity and behavior. This review updates information from molecular studies that can be directly associated with impairments of behavior and synaptic plasticity, and outlines the functional consequences of molecular differences in Pb2+-exposed animals that illuminate potential mechanisms of Pb2+-induced neurotoxicity.

Perinatal lead exposure alters the expression of neuronal nitric oxide synthase in rat brain

Environmental exposure to lead (Pb) is known to affect the developing nervous system causing cognitive deficits in children. The diffusible nitric oxide (NO) is a biological messenger known to be involved in brain development. We examined the developmental changes of neuronal nitric oxide synthase (nNOS) in cerebellum and hippocampus of developing rat brain by radiometric assay, Western blot analysis and immunohistochemistry. Pb-exposure (0.2% Pb acetate) was initiated on gestation day 6 through the drinking water of the dam and continued through birth and postnatal days (PNDs) 1 to 21. The pups were never exposed to Pb directly. Pb exposure was stopped on weaning of pups from mothers on PND 21. The changes in nNOS were measured in the offspring on PNDs 7, 14, 21, and 35. The nNOS activity was increased gradually from PNDs 7 to 35 in both cerebellum and hippocampus of control rats when the enzyme activity was determined in the presence of either 0.5 or 6 microM calcium (Ca2+) in the reaction mixture. However, Pb exposure decreased the nNOS activity significantly at PNDs 21 to 35 as compared to respective controls when the enzyme activity was determined in the presence of 6 microM Ca2+. The decrease of nNOS was even greater and evident at all PNDs tested when the enzyme activity was assayed in the presence of physiological concentration of Ca2+ (0.5 microM). These findings were further strengthened by the in vitro studies. The cerebellar nNOS activity was inhibited much more at low Ca2+ (0.5 microM) as compared to 6 microM Ca2+, with IC50 values of 35 and 50 nM Pb, respectively. The nNOS protein levels and immunoreactivity in the cerebellum and hippocampus of rats perinatally exposed to Pb were decreased as compared to controls at PNDs 21 and 35. These data suggest perinatal Pb exposure decreases the nNOS in the developing brain. The decrease of nNOS activity and protein may explain the Pb-mediated cognitive deficits because NO regulates long-term potentiation (LTP) and other neurophysiological events in the developing nervous system.

Lead exposure alters Egr-1 DNA-binding in the neonatal rat brain

Zinc finger proteins (ZFP) contain a structural motif (Cys-2 His-2) found in a large family of eukaryotic transcriptional regulatory proteins, such as Sp1. Previous studies have shown that Sp1 DNA-binding was disrupted by exposure to lead (Pb), due to action on its zinc finger domain. In this paper, we discuss the results of studies with another ZFP, Egr-1, an early growth response gene, which is functionally involved in cell proliferation and differentiation. Egr-1 DNA-binding was studied by gel shift mobility assays in several brain regions of developing rat pups. We observed a distinct developmental profile of Egr-1 DNA-binding with a gradual increase from the early to late postnatal days in all the brain regions examined. Lactational exposure to Pb resulted in a modulation of Egr-1 DNA binding manifested by premature peaks in DNA-binding reminiscent of the in vivo changes previously reported for Sp1. These data are consistent with earlier findings that exposure to Pb both in vivo and in vitro causes a modulation in the DNA-binding of ZFP such as Sp1, Egr-1 and TFIIIA. The commonality by which Pb exposure alters the DNA-binding patterns of ZFP suggests that divalent Pb may be interacting directly with the Zn moiety of these proteins.

Immunotoxic effects of inorganic lead on host resistance of mice with different circling behavior preferences

We have observed differential immune responses in mice with different circling preferences, which are posited to reflect interindividual immune response differences influenced by brain laterality effects on neuroimmune circuits. In this study, we have investigated the influence of inorganic lead (Pb) and/or Listeria monocytogenes (LM) infection on the cytokine and corticosterone (CORT) levels of mice grouped by lateralized behavior. Pb increased the LM susceptibility of mice with both left (LC)- and right-circling (RC) preferences; however, Pb did not inhibit the host resistance of mice with no circling preference (NP mice). The basal serum IFNgamma levels were lowered in all groups after Pb exposure, which coincided with a decrease in host resistance in LC and RC mice, but not NP mice. Pb also altered the basal serum CORT levels, and these changes appear to correlate better with changes in the host resistance of all groups. The basal CORT levels were significantly lowered by Pb in mice with a circling preference, and Pb significantly suppressed the host resistance of mice with a circling preference. However, Pb slightly increased the serum CORT level of NP mice, and their host resistance was slightly improved by Pb. After infection, the increase in CORT levels was associated with an increase in the serum IL-6 levels, which may reflect cytokine influences on the hypothalamic-pituitary-adrenal axis. At 3 days after infection, the serum IL-6 level seems to be a good indicator of the severity of the infection. We suggest that environmental stressors can reorder the observed differential susceptibility to LM in mice with different circling preferences, in that relatively resistant mice (RC mice) become more susceptible than NP mice after exposure to Pb. The results suggest that environmental stressors may have differential effects among individuals with endogenous differences in their neuroimmune circuits, since brain laterality is known to influence immune functions.