Myelin glycoproteins targeted by lead in the rodent model of prolonged exposure

Lead exposure is associated with functional and microstructural changes in the healthy human brain

Lead is a toxin known to harm many organs in the body, particularly the central nervous system, across an individual's lifespan. To date, no study has yet investigated the associations between body lead level and the microstructural properties of gray matter areas, and brain activity during attention-demanding tasks. Here, utilizing data of diffusion tensor imaging, functional magnetic resonance imaging and cognitive measures among 920 typically developing young adults, we show greater hair lead levels are weakly but significantly associated with (a) increased working memory-related activity in the right premotor and pre-supplemental motor areas, (b) lower fractional anisotropy (FA) in white matter areas near the internal capsule, (c) lower mean diffusivity (MD) in the dopaminergic system in the left hemisphere and other widespread contingent areas, and (d) greater MD in the white matter area adjacent to the right fusiform gyrus. Higher lead levels were also weakly but significantly associated with lower performance in tests of high-order cognitive functions, such as the psychometric intelligence test, greater impulsivity measures, and higher novelty seeking and extraversion. These findings reflect the weak effect of daily lead level on the excitability and microstructural properties of the brain, particularly in the dopaminergic system.

Cypermethrin Impairs Hippocampal Neurogenesis and Cognitive Functions by Altering Neural Fate Decisions in the Rat Brain

Neurogenesis is a developmental process that involves fine-tuned coordination between self-renewal, proliferation, and differentiation of neural stem cells (NSCs) into neurons. However, early-life assault with environmental toxicants interferes with the regular function of genes, proteins, and other molecules that build brain architecture resulting in attenuated neurogenesis. Cypermethrin is a class II synthetic pyrethroid pesticide extensively used in agriculture, veterinary, and residential applications due to its low mammalian toxicity, high bio-efficacy, and enhanced stability. Despite reports on cypermethrin-mediated behavioral and biochemical alterations, till now, no study implicates whether cypermethrin exposure has any effect on neurogenesis. Therefore, the present study was undertaken to comprehend the effects of cypermethrin treatment on embryonic and adult neurogenesis. We found that cypermethrin exposure led to a considerable decrease in the BrdU/Sox-2+, BrdU/Dcx+, and BrdU/NeuN+ co-labeled cells indicating that cypermethrin treatment decreases NSC proliferation and generation of mature and functional neurons. On the contrary, the generation of BrdU/S100β+ glial cells was increased resulting in neurogliogenesis imbalance in the hippocampus. Further, cypermethrin treatment also led to an increased number of BrdU/cleaved caspase-3+ and Fluoro-Jade B+ cells suggesting an induction of apoptosis in NSCs and increased degeneration of neurons in the hippocampus. Overall, these results explicate that cypermethrin exposure not only reduces the NSC pool but also disturbs the neuron-astrocyte ratio and potentiates neurodegeneration in the hippocampus, leading to cognitive dysfunctions in rats.

Effects of endocrine disrupting chemicals on myelin development and diseases

In the central and peripheral nervous systems, myelin is essential for efficient conduction of action potentials. During development, oligodendrocytes and Schwann cells differentiate and ensure axon myelination, and disruption of these processes can contribute to neurodevelopmental disorders. In adults, demyelination can lead to important disabilities, and recovery capacities by remyelination often decrease with disease progression. Among environmental chemical pollutants, endocrine disrupting chemicals (EDCs) are of major concern for human health and are notably suspected to participate in neurodevelopmental and neurodegenerative diseases. In this review, we have combined the current knowledge on EDCs impacts on myelin including several persistent organic pollutants, bisphenol A, triclosan, heavy metals, pesticides, and nicotine. Besides, we presented several other endocrine modulators, including pharmaceuticals and the phytoestrogen genistein, some of which are candidates for treating demyelinating conditions but could also be deleterious as contaminants. The direct impacts of EDCs on myelinating cells were considered as well as their indirect consequences on myelin, particularly on immune mechanisms associated with demyelinating conditions. More studies are needed to describe the effects of these compounds and to further understand the underlying mechanisms in relation to the potential for endocrine disruption.

Long-term exposure to lead reduces antioxidant capacity and triggers motor neurons degeneration and demyelination in spinal cord of adult rats

Lead is a toxic metal found in environment with great neurotoxic potential. The main effect is associated with impairments in hippocampus and cerebellum, driving to cognitive and motor dysfunctions, however, there is a lack of evidences about the effects over the spinal cord. In this way, we aimed to investigate in vivo the effects of long-term exposure to lead acetate in oxidative biochemistry and morphology of rats' spinal cord. For this, 36 male Wistar rats (Rattus norvegicus) were divided into the group exposed to 50 mg/kg of lead acetate and control group, which received only distilled water, both groups through intragastric gavage, for 55 days. After the exposure period, the animals were euthanized and the spinal cords were collected to perform the analyses of lead levels quantification, oxidative biochemistry evaluation by levels of malondialdehyde (MDA), nitrites and the antioxidant capacity against peroxyl radicals (ACAP). Besides, morphological evaluation with quantitative analysis of mature and motor neurons and reactivity to myelin basic protein (MBP). Our results showed high levels of lead in spinal cord after long-term exposure; there was a reduction on ACAP level; however, there was no difference observed in MDA and nitrite levels. Moreover, there was a reduction of mature and motor neurons in all three regions, and a reduction of immunolabeling of MBP in the thoracic and lumbar segments. Therefore, we conclude that long-term exposure to lead is able of increasing the levels of the metal in spinal cord, affecting the antioxidant capacity and inducing morphological impairments in spinal cord parenchyma. Our results also suggest that the tissue impairments triggered by lead may be resultant from others molecular mechanisms besides the oxidative stress.

Neurodegeneration, demyelination, and astrogliosis in rat spinal cord by chronic lead treatment

Early exposure to lead (Pb) has been associated with an elevated risk of developing neurodegenerative diseases. There is evidence that neuronal damage in chronic Pb exposure can be caused by the convergence of glial damage. Apoptosis may be a possible mechanism of Pb-induced cell death in the central nervous system. We tested cellular damage and apoptosis in the spinal cord of Wistar rats treated with Pb. Twelve rats were divided into two groups (n = 6): the control group was treated with only drinking water and the other group received 500 ppm of Pb acetate. After 3 months of Pb treatment, all animals were euthanized and spinal cords were extracted. Morphology was evaluated by Nissl and Kluver-Barrera stainings. Apoptosis was detected by terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Specific antibodies were used to evaluate Pb damage in oligodendrocytes, astrocytes, and microglia. A large number of apoptotic bodies was observed in the white matter of the Pb-treated group. The Pb-treated group also showed a reduced number of neurons and oligodendrocytes but had an increased number of astrocytes compared with the nontreated group. Our results demonstrate that chronic Pb treatment induces neurodegeneration, demyelination, and astrogliosis in the rat spinal cord.

Heavy metal pollutants in selected organs of African giant rats from three agro-ecological zones of Nigeria: evidence for their role as an environmental specimen bank

An assessment of the concentration of heavy metals in the liver, brain, kidney, bone, and lungs of African giant rats (AGRs) from three agro-ecological zones of Nigeria having different industrial activities was carried out using atomic absorption spectrophotometer. Twenty adult AGRs from cities in mangrove/freshwater swamp, rainforest, and woodland/tall grass savanna agro-ecological zones of Nigeria were used for this study. AGRs were euthanized, carefully dissected, and the brains, liver, lungs, bone, and kidneys were harvested, digested, and analyzed for concentrations of vanadium (V), lead (Pb), cadmium (Cd), zinc (Zn), selenium (Se), copper (Cu), and iron (Fe). All data generated were evaluated for statistical significance using one-way ANOVA with Tukey's multiple post-test comparison. Results showed the major environmental heavy metal pollutants of the mangrove/freshwater swamp to be vanadium and selenium while those of woodland/tall grass savanna agro-ecological zones were lead, selenium, and zinc. The vanadium concentration was more than twofold higher in the observed tissues of AGR from the mangrove/freshwater swamp, and this may be related to increased exploitation of minerals and the activities of militants in pipeline vandalization in this zone. Interestingly, the highest concentration of this metal was seen in the lungs suggestive of a respiratory route of exposure. Among the potential adverse effects derived from exposure to metals, developmental toxicity is a serious risk. This type of investigation can assist in knowing the level of animal and human exposure to environmental pollutants both in highly industrialized and non-industrialized areas and is more ideal in environmental monitoring. This study therefore suggests AGR as model for ecotoxicological research and environmental specimen banks (ESBs) in this part of Africa.

Atypical prefrontal connectivity in attention-deficit/hyperactivity disorder: pathway to disease or pathological end point?

Functional neuroimaging studies have identified multiple nodes of dysfunction in frontostriatal and mesocorticolimbic networks in attention-deficit/hyperactivity disorder (ADHD). Yet relatively few studies have examined how structural and functional connectivity between nodes in these networks might relate to the behavioral symptoms of ADHD. Moreover, it is unknown whether abnormalities in connectivity are a primary cause of symptoms or arise secondary to common etiologic mechanisms. We review the most recent diffusion tensor imaging and functional magnetic resonance imaging studies of connectivity in ADHD to characterize associations between frontostriatal connectivity abnormalities and the behavioral symptoms of inattention and impulsivity in ADHD. Furthermore, we examine how structural and functional connectivity measures relate to environmental and genetic pathways to ADHD. Diffusion tensor imaging studies indicate that ADHD is associated with significant irregularities in white matter microstructure, especially in frontostriatal and select corticocortical tracts. Resting state functional magnetic resonance imaging studies implicate altered connectivity within a default mode network of structures active during introspective, task-free processes and disrupted interactions between this network and frontostriatal attentional systems. Deficits in functional connectivity within frontostriatal and mesocorticolimbic networks might give rise, in part, to ADHD symptoms. Conversely, structural connectivity deficits and ADHD symptoms might arise incidentally from a common etiologic mechanism, involving altered modulation of synaptic potentiation and pruning by dopamine and other factors during development. Collectively, these studies suggest that the core symptoms of ADHD might derive from dysregulated modulation of cortical plasticity in the developing brain, resulting in altered patterns of corticocortical connectivity that might persist into adulthood.

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.

Iron, Lead, and Children's Behavior and Cognition

Iron deficiency (ID) is the most common micronutrient deficiency in the world, with consequences of ID and ID anemia (IDA) in young children including behavioral and cognitive deficits. In turn, lead exposure is one of the most common environmental toxicants affecting children. Elevated blood lead levels (BLLs) in young children are also associated with behavioral and cognitive deficits. The metabolic and physiological connections between iron and lead, including a common route of entry into the body and similar neural targets, suggest a considerable overlap in their effects on functional outcomes. Very few studies have examined the existence of increased susceptibility to lead neurotoxicity in children with ID, but there is evidence that ID and BLL are independently associated with cognition and behavior. Children's susceptibility to both ID and elevated BLLs will likely depend on the timing and severity of both exposures, something that should be investigated systematically.

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.