Mechanistic insight into neurotoxicity induced by developmental insults

Perfluorooctanoic acid exposure and its neurodegenerative consequences in C57BL6/J mice

Perfluorooctanoic acid (PFOA) is a member of Per- and polyfluoroalkyl substances (PFASs), an industrial pollutant that has been produced for decades and widely used in various industries. Accumulation of this compound in the environment and body of organisms led to increased concerns about this compound. The toxic effects of PFOA on the nervous system are unknown yet. We aimed to assess the myelination and neurogenesis in brain tissue. In this study, PFOA at doses of 1, 5, 10, and 20 mg/kg were injected intraperitoneally into C57BL/6 J mice for 14 days, and the myelin content, CD4 + and CD8 + cell infiltration to brain regions were evaluated. Also, bromodeoxyuridine (BrdU) labeling was performed to compare neurogenesis among the groups. Luxol Fast Blue (LFB) staining revealed a significant decrease in myelin content in both sex at high concentrations (p < 0.001). The BrdU incorporation changes were observed in both sexes especially females which was highly related to the dose of PFOA and region of the brain. The infiltration rates of CD4 + and CD8 + cells to the brain were shown to be decreased; meanwhile the lymphocyte count was not significantly changed among groups over time and vice versa for the monocyte and neutrophils. Our results showed that PFOA had a negative impact on neurogenesis and the myelination process through the specific region of the brain depending on the dose and sex. Also, PFOA could disturb the number of CD4 + and CD8 + cells infiltrating the brain, which plays a crucial role in neurogenesis, leading to toxicity and neurological abnormalities. It seems that more research is needed to determine the exact mechanisms of PFOA neurotoxicity and its long-term behavioral consequences.

The Threat Posed by Environmental Contaminants on Neurodevelopment: What Can We Learn from Neural Stem Cells?

Exposure to chemicals may pose a greater risk to vulnerable groups, including pregnant women, fetuses, and children, that may lead to diseases linked to the toxicants' target organs. Among chemical contaminants, methylmercury (MeHg), present in aquatic food, is one of the most harmful to the developing nervous system depending on time and level of exposure. Moreover, certain man-made PFAS, such as PFOS and PFOA, used in commercial and industrial products including liquid repellants for paper, packaging, textile, leather, and carpets, are developmental neurotoxicants. There is vast knowledge about the detrimental neurotoxic effects induced by high levels of exposure to these chemicals. Less is known about the consequences that low-level exposures may have on neurodevelopment, although an increasing number of studies link neurotoxic chemical exposures to neurodevelopmental disorders. Still, the mechanisms of toxicity are not identified. Here we review in vitro mechanistic studies using neural stem cells (NSCs) from rodents and humans to dissect the cellular and molecular processes changed by exposure to environmentally relevant levels of MeHg or PFOS/PFOA. All studies show that even low concentrations dysregulate critical neurodevelopmental steps supporting the idea that neurotoxic chemicals may play a role in the onset of neurodevelopmental disorders.

Phthalate toxicity mechanisms: An update

Phthalates are one of the most widely used plasticizers in polymer products, and they are increasingly being exposed to people all over the world, generating health concerns. Phthalates are often used as excipients in controlled-release capsules and enteric coatings, and patients taking these drugs may be at risk. In both animals and human, phthalates are mainly responsible for testicular dysfunction, ovarian toxicity, reduction in steroidogenesis. In this regard, for a better understanding of the health concerns corresponding to phthalates and their metabolites, still more research is required. Significantly, multifarious forms of phthalates and their biomedical effects are need to be beneficial to investigate in the various tissues or organs. Based on these investigations, researchers can decipher their toxicity concerns and related mechanisms in the body after phthalate's exposure. This review summarizes the chemical interactions, mechanisms, and their biomedical applications of phthalates in animals and human.

A Critical Review and Meta-Analysis of Impacts of Per- and Polyfluorinated Substances on the Brain and Behavior

Per- and polyfluoroalkyl substances (PFAS) are a class of structurally diverse synthetic organic chemicals that are chemically stable, resistant to degradation, and persistent in terrestrial and aquatic environments. Widespread use of PFAS in industrial processing and manufacturing over the last 70 years has led to global contamination of built and natural environments. The brain is a lipid rich and highly vascularized organ composed of long-lived neurons and glial cells that are especially vulnerable to the impacts of persistent and lipophilic toxicants. Generally, PFAS partition to protein-rich tissues of the body, primarily the liver and blood, but are also detected in the brains of humans, wildlife, and laboratory animals. Here we review factors impacting the absorption, distribution, and accumulation of PFAS in the brain, and currently available evidence for neurotoxic impacts defined by disruption of neurochemical, neurophysiological, and behavioral endpoints. Emphasis is placed on the neurotoxic potential of exposures during critical periods of development and in sensitive populations, and factors that may exacerbate neurotoxicity of PFAS. While limitations and inconsistencies across studies exist, the available body of evidence suggests that the neurobehavioral impacts of long-chain PFAS exposures during development are more pronounced than impacts resulting from exposure during adulthood. There is a paucity of experimental studies evaluating neurobehavioral and molecular mechanisms of short-chain PFAS, and even greater data gaps in the analysis of neurotoxicity for PFAS outside of the perfluoroalkyl acids. Whereas most experimental studies were focused on acute and subchronic impacts resulting from high dose exposures to a single PFAS congener, more realistic exposures for humans and wildlife are mixtures exposures that are relatively chronic and low dose in nature. Our evaluation of the available human epidemiological, experimental, and wildlife data also indicates heightened accumulation of perfluoroalkyl acids in the brain after environmental exposure, in comparison to the experimental studies. These findings highlight the need for additional experimental analysis of neurodevelopmental impacts of environmentally relevant concentrations and complex mixtures of PFAS.

Developmental exposure to methylmercury and ADHD, a literature review of epigenetic studies

Attention-deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder that affects the competence of academic performance and social wellness in children and adults. The causes of ADHD are unclear. Both genetic and environmental factors contribute to the development of ADHD. The behavioral impairments in ADHD are associated with epigenetic changes in genes that are important for neurodevelopment. Among environmental causes of ADHD, the neurotoxin methylmercury (MeHg) is associated with an increased risk for ADHD. Developing children are susceptible to neurotoxic effects of prenatal MeHg exposure. Human epidemiology studies have shown that prenatal MeHg exposure could invoke epigenetic changes in genes that are involved in ADHD. In addition, the pathogenesis of ADHD involves dopaminergic system, which is a target of developmental MeHg exposure. MeHg-induced alterations in the dopaminergic system have a profound impact on behavioral functions in adults. As a trace level of MeHg (around nM) can induce long-lasting behavioral alterations, potential mechanisms of MeHg-induced functional changes in the dopaminergic system may involve epigenetic mechanisms. Here, we review the relevant evidence on developmental MeHg exposures and the risk for ADHD. We also point out research gaps in understanding environmental causes of ADHD.

Persistent organic pollutants at the synapse: Shared phenotypes and converging mechanisms of developmental neurotoxicity

The developing nervous system is sensitive to environmental and physiological perturbations in part due to its protracted period of prenatal and postnatal development. Epidemiological and experimental studies link developmental exposures to persistent organic pollutants (POPs) including polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, polybrominated diphenyl ethers, and benzo(a)pyrene to increased risk for neurodevelopmental disorders in children. Mechanistic studies reveal that many of the complex cellular processes that occur during sensitive periods of rapid brain development are cellular targets for developmental neurotoxicants. One area of research interest has focused on synapse formation and plasticity, processes that involve the growth and retraction of dendrites and dendritic spines. For each chemical discussed in this review, we summarize the morphological and electrophysiological data that provide evidence that developmental POP exposure produces long-lasting effects on dendritic morphology, spine formation, glutamatergic and GABAergic signaling systems, and synaptic transmission. We also discuss shared intracellular mechanisms, with a focus on calcium and thyroid hormone homeostasis, by which these chemicals act to modify synapses. We conclude our review highlighting research gaps that merit consideration when characterizing synaptic pathology elicited by chemical exposure. These gaps include low-dose and nonmonotonic dose-response effects, the temporal relationship between dendritic growth, spine formation, and synaptic activity, excitation-inhibition balance, hormonal effects, and the need for more studies in females to identify sex differences. By identifying converging pathological mechanisms elicited by POP exposure at the synapse, we can define future research directions that will advance our understanding of these chemicals on synapse structure and function.

Developing novel computational prediction models for assessing chemical-induced neurotoxicity using naïve Bayes classifier technique

Development of reliable and efficient alternative in vivo methods for evaluation of the chemicals with potential neurotoxicity is an urgent need in the early stages of drug design. In this investigation, the computational prediction models for drug-induced neurotoxicity were developed by using the classical naïve Bayes classifier. Eight molecular properties closely relevant to neurotoxicity were selected. Then, 110 classification models were developed with using the eight important molecular descriptors and 10 types of fingerprints with 11 different maximum diameters. Among these 110 prediction models, the prediction model (NB-03) based on eight molecular descriptors combined with ECFP_10 fingerprints showed the best prediction performance, which gave 90.5% overall prediction accuracy for the training set and 82.1% concordance for the external test set. In addition, compared to naïve Bayes classifier, the recursive partitioning classifier displayed worse predictive performance for neurotoxicity. Therefore, the established NB-03 prediction model can be used as a reliable virtual screening tool to predict neurotoxicity in the early stages of drug design. Moreover, some structure alerts for characterizing neurotoxicity were identified in this research, which could give an important guidance for the chemists in structural modification and optimization to reduce the chemicals with potential neurotoxicity.

Dexmedetomidine Alleviates Neurogenesis Damage Following Neonatal Midazolam Exposure in Rats through JNK and P38 MAPK Pathways

Midazolam, a widely used anesthetic, inhibits proliferation of neural stem cells (NSCs) and induces neuroapoptosis in neonates. Dexmedetomidine, an effective auxiliary medicine in clinical anesthesia, protects the developing brain against volatile anesthetic-induced neuroapoptosis. Whether dexmedetomidine protects against neurogenesis damage induced by midazolam remains unknown. This study aims to clarify the protective effect of dexmedetomidine on midazolam-induced neurogenesis damage and explore its potential mechanism. Postnatal 7-day-old Sprague-Dawley (SD) rats and cultured NSCs were treated with either normal saline, midazolam, or dexmedetomidine combined with midazolam. The rats were sacrificed at 1, 3, and 7 days after treatment. Cell proliferation was assessed by 5-bromodeoxyurdine (BrdU) incorporation. Cell viability was determined using MTT assay. Cell differentiation and apoptosis were detected by immunofluorescent staining and terminal dUTP nick-end labeling (TUNEL), respectively. The protein levels of p-JNK, p-P38, and cleaved caspase-3 were quantified using Western blotting. Midazolam decreased cell proliferation and increased cell apoptosis in the subventricular zone (SVZ), the subgranular zone (SGZ) of the hippocampus, and cultured NSCs. Moreover, midazolam decreased cell viability and increased the expression of p-JNK and p-P38 in cultured NSCs. Co-treatment with dexmedetomidine attenuated midazolam-induced changes in cell proliferation, viability, apoptosis, and protein expression of p-JNK and p-P38 in cultured NSCs. Midazolam and dexmedetomidine did not affect the differentiation of the cultured NSCs. These results indicate that dexmedetomidine alleviated midazolam-induced neurogenesis damage via JNK and P38 MAPK pathways.

Generation of a Triple-Transgenic Zebrafish Line for Assessment of Developmental Neurotoxicity during Neuronal Differentiation

: The developing brain is extremely sensitive to many chemicals. Exposure to neurotoxicants during development has been implicated in various neuropsychiatric and neurological disorders, including autism spectrum disorders and schizophrenia. Various screening methods have been used to assess the developmental neurotoxicity (DNT) of chemicals, with most assays focusing on cell viability, apoptosis, proliferation, migration, neuronal differentiation, and neuronal network formation. However, assessment of toxicity during progenitor cell differentiation into neurons, astrocytes, and oligodendrocytes often requires immunohistochemistry, which is a reliable but labor-intensive and time-consuming assay. Here, we report the development of a triple-transgenic zebrafish line that expresses distinct fluorescent proteins in neurons (Cerulean), astrocytes (mCherry), and oligodendrocytes (mCitrine), which can be used to detect DNT during neuronal differentiation. Using in vivo fluorescence microscopy, we could detect DNT by 6 of the 10 neurotoxicants tested after exposure to zebrafish from 12 h to 5 days' post-fertilization. Moreover, the chemicals could be clustered into three main DNT groups based on the fluorescence pattern: (i) inhibition of neuron and oligodendrocyte differentiation and stimulation of astrocyte differentiation; (ii) inhibition of neuron and oligodendrocyte differentiation; and (iii) inhibition of neuron and astrocyte differentiation, which suggests that reporter expression reflects the toxicodynamics of the chemicals. Thus, the triple-transgenic zebrafish line developed here may be a useful tool to assess DNT during neuronal differentiation.

Biophysical and biomechanical properties of neural progenitor cells as indicators of developmental neurotoxicity

Conventional in vitro toxicity studies have focused on identifying IC₅₀ and the underlying mechanisms, but how toxicants influence biophysical and biomechanical changes in human cells, especially during developmental stages, remain understudied. Here, using an atomic force microscope, we characterized changes in biophysical (cell area, actin organization) and biomechanical (Young's modulus, force of adhesion, tether force, membrane tension, tether radius) aspects of human fetal brain-derived neural progenitor cells (NPCs) induced by four classes of widely used toxic compounds, including rotenone, digoxin, N-arachidonoylethanolamide (AEA), and chlorpyrifos, under exposure up to 36 h. The sub-cellular mechanisms (apoptosis, mitochondria membrane potential, DNA damage, glutathione levels) by which these toxicants induced biochemical changes in NPCs were assessed. Results suggest a significant compromise in cell viability with increasing toxicant concentration (p < 0.01), and biophysical and biomechanical characteristics with increasing exposure time (p < 0.01) as well as toxicant concentration (p < 0.01). Impairment of mitochondrial membrane potential appears to be the most sensitive mechanism of neurotoxicity for rotenone, AEA and chlorpyrifos exposure, but compromise in plasma membrane integrity for digoxin exposure. The surviving NPCs remarkably retained stemness (SOX2 expression) even at high toxicant concentrations. A negative linear correlation (R² = 0.92) exists between the elastic modulus of surviving cells and the number of living cells in that environment. We propose that even subtle compromise in cell mechanics could serve as a crucial marker of developmental neurotoxicity (mechanotoxicology) and therefore should be included as part of toxicology assessment repertoire to characterize as well as predict developmental outcomes.

Mitotically heritable effects of BMAA on striatal neural stem cell proliferation and differentiation

The widespread environmental contaminant β-methylamino-L-alanine (BMAA) is a developmental neurotoxicant that can induce long-term learning and memory deficits. Studies have shown high transplacental transfer of 3H-BMAA and a significant uptake in fetal brain. Therefore, more information on how BMAA may influence growth and differentiation of neural stem cells is required for assessment of the risk to the developing brain. The aim of this study was to investigate direct and mitotically inherited effects of BMAA exposure using primary striatal neurons and embryonic neural stem cells. The neural stem cells were shown to be clearly more susceptible to BMAA exposure than primary neurons. Exposure to 250 µM BMAA reduced neural stem cell proliferation through apoptosis and G2/M arrest. At lower concentrations (50–100 µM), not affecting cell proliferation, BMAA reduced the differentiation of neural stem cells into astrocytes, oligodendrocytes, and neurons through glutamatergic mechanisms. Neurons that were derived from the BMAA-treated neuronal stem cells demonstrated morphological alterations including reduced neurite length, and decreased number of processes and branches per cell. Interestingly, the BMAA-induced changes were mitotically heritable to daughter cells. The results suggest that early-life exposure to BMAA impairs neuronal stem cell programming, which is vital for development of the nervous system and may result in long-term consequences predisposing for both neurodevelopmental disorders and neurodegenerative disease later in life. More attention should be given to the potential adverse effects of BMAA exposure on brain development.

Effects of prenatal exposure to chrysotile asbestos on hippocampal neurogenesis and long-term behavioral changes in adult male rat offspring

Prenatal development is a critical period of life that many environmental pollutants have been suggested to influence fetal growth. Nevertheless, there are still a few investigations into the prenatal exposure to chrysotile asbestos and its neurodevelopmental and behavioral outcome in offspring. In this study, twenty-eight pregnant Wistar rats were divided into four groups and received three-times repeated intraperitoneal injections of normal saline, chrysotile, ascorbic acid and the combination of chrysotile and ascorbic acid on gestational days 11, 14 and 17. The maternal serum levels of malondialdehyde (MDA) and prooxidant-antioxidant balance (PAB) and hippocampal MDA content in adult male offspring were measured. At postnatal day (PND) 60, elevated plus maze was performed to determine anxiety-like behavior, also depression-like behavior was examined using a forced swim test at PND 61- 62. Thereafter, the quantitative analysis of Ki-67, NeuN and GFAP positive cells in the hippocampal dentate gyrus were studied by immunostaining. Our data showed that prenatal exposure to chrysotile increased the maternal serum level of MDA and PAB as well as hippocampal MDA content in adult male offspring, also increased the depression- and anxiety-like behaviors of adult male offspring and decreased the hippocampal Ki-67⁺, NeuN⁺ and GFAP⁺ cells in dantate gyrus of adult male offspring. However, co-administration of ascorbic acid and chrysotile decreased hippocampal lipid peroxidation and increased the Ki-67⁺, NeuN⁺ and GFAP⁺ cells in adult male offspring. In summary, these results indicated that oxidative stress induced by prenatal exposure to chrysotile, lead to the long-lasting decrease of the hippocampal cell proliferation and neuronal differentiation as well as astrogliosis of adult male offspring that exhibit more depression- and anxiety-like behaviors in adulthood and co-treatment of ascorbic acid with chrysotile asbestos attenuated the changes.

Environmental Mechanisms of Neurodevelopmental Toxicity

PURPOSE OF REVIEW

With the incidence of neurodevelopmental disorders on the rise, it is imperative to identify and understand the mechanisms by which environmental contaminants can impact the developing brain and heighten risk. Here, we report on recent findings regarding novel mechanisms of developmental neurotoxicity and highlight chemicals of concern, beyond traditionally defined neurotoxicants.

RECENT FINDINGS

The perinatal window represents a critical and extremely vulnerable period of time during which chemical insult can alter the morphological and functional trajectory of the developing brain. Numerous chemical classes have been associated with alterations in neurodevelopment including metals, solvents, pesticides, and, more recently, endocrine-disrupting compounds. Although mechanisms of neurotoxicity have traditionally been identified as pathways leading to neuronal cell death, neuropathology, or severe neural injury, recent research highlights alternative mechanisms that result in more subtle but consequential changes in the brain and behavior. These emerging areas of interest include neuroendocrine and immune disruption, as well as indirect toxicity via actions on other organs such as the gut and placenta. Understanding of the myriad ways in which the developing brain is vulnerable to chemical exposures has grown tremendously over the past decade. Further progress and implementation in risk assessment is critical to reducing risk of neurodevelopmental disorders.

Early life exposure to air pollution particulate matter (PM) as risk factor for attention deficit/hyperactivity disorder (ADHD): Need for novel strategies for mechanisms and causalities

Epidemiological studies have demonstrated that air pollution particulate matter (PM) and adsorbed toxicants (organic compounds and trace metals) may affect child development already in utero. Recent studies have also indicated that PM may be a risk factor for neurodevelopmental disorders (NDDs). A pattern of increasing prevalence of attention deficit/hyperactivity disorder (ADHD) has been suggested to partly be linked to environmental pollutants exposure, including PM. Epidemiological studies suggest associations between pre- or postnatal exposure to air pollution components and ADHD symptoms. However, many studies are cross-sectional without possibility to reveal causality. Cohort studies are often small with poor exposure characterization, and confounded by traffic noise and socioeconomic factors, possibly overestimating the study associations. Furthermore, the mechanistic knowledge how exposure to PM during early brain development may contribute to increased risk of ADHD symptoms or cognitive deficits is limited. The closure of this knowledge gap requires the combined use of well-designed longitudinal cohort studies, supported by mechanistic in vitro studies. As ADHD has profound consequences for the children affected and their families, the identification of preventable risk factors such as air pollution exposure should be of high priority.

Recommendation on test readiness criteria for new approach methods in toxicology: Exemplified for developmental neurotoxicity

Multiple non-animal-based test methods have never been formally validated. In order to use such new approach methods (NAMs) in a regulatory context, criteria to define their readiness are necessary. The field of developmental neurotoxicity (DNT) testing is used to exemplify the application of readiness criteria. The costs and number of untested chemicals are overwhelming for in vivo DNT testing. Thus, there is a need for inexpensive, high-throughput NAMs, to obtain initial information on potential hazards, and to allow prioritization for further testing. A background on the regulatory and scientific status of DNT testing is provided showing different types of test readiness levels, depending on the intended use of data from NAMs. Readiness criteria, compiled during a stakeholder workshop, uniting scientists from academia, industry and regulatory authorities are presented. An important step beyond the listing of criteria, was the suggestion for a preliminary scoring scheme. On this basis a (semi)-quantitative analysis process was assembled on test readiness of 17 NAMs with respect to various uses (e.g. prioritization/screening, risk assessment). The scoring results suggest that several assays are currently at high readiness levels. Therefore, suggestions are made on how DNT NAMs may be assembled into an integrated approach to testing and assessment (IATA). In parallel, the testing state in these assays was compiled for more than 1000 compounds. Finally, a vision is presented on how further NAM development may be guided by knowledge of signaling pathways necessary for brain development, DNT pathophysiology, and relevant adverse outcome pathways (AOP).

Cerium oxide nanoparticles inhibit differentiation of neural stem cells

Cerium oxide nanoparticles (nanoceria) display antioxidant properties and have shown cytoprotective effects both in vitro and in vivo. Here, we explored the effects of nanoceria on neural progenitor cells using the C17.2 murine cell line as a model. First, we assessed the effects of nanoceria versus samarium (Sm) doped nanoceria on cell viability in the presence of the prooxidant, DMNQ. Both particles were taken up by cells and nanoceria, but not Sm-doped nanoceria, elicited a temporary cytoprotective effect upon exposure to DMNQ. Next, we employed RNA sequencing to explore the transcriptional responses induced by nanoceria or Sm-doped nanoceria during neuronal differentiation. Detailed computational analyses showed that nanoceria altered pathways and networks relevant for neuronal development, leading us to hypothesize that nanoceria inhibits neuronal differentiation, and that nanoceria and Sm-doped nanoceria both interfere with cytoskeletal organization. We confirmed that nanoceria reduced neuron specific β3-tubulin expression, a marker of neuronal differentiation, and GFAP, a neuroglial marker. Furthermore, using super-resolution microscopy approaches, we could show that both particles interfered with cytoskeletal organization and altered the structure of neural growth cones. Taken together, these results reveal that nanoceria may impact on neuronal differentiation, suggesting that nanoceria could pose a developmental neurotoxicity hazard.