In vitro assessment of developmental neurotoxicity: use of microelectrode arrays to measure functional changes in neuronal network ontogeny

International STakeholder NETwork (ISTNET): creating a developmental neurotoxicity (DNT) testing road map for regulatory purposes

A major problem in developmental neurotoxicity (DNT) risk assessment is the lack of toxicological hazard information for most compounds. Therefore, new approaches are being considered to provide adequate experimental data that allow regulatory decisions. This process requires a matching of regulatory needs on the one hand and the opportunities provided by new test systems and methods on the other hand. Alignment of academically and industrially driven assay development with regulatory needs in the field of DNT is a core mission of the International STakeholder NETwork (ISTNET) in DNT testing. The first meeting of ISTNET was held in Zurich on 23-24 January 2014 in order to explore the concept of adverse outcome pathway (AOP) to practical DNT testing. AOPs were considered promising tools to promote test systems development according to regulatory needs. Moreover, the AOP concept was identified as an important guiding principle to assemble predictive integrated testing strategies (ITSs) for DNT. The recommendations on a road map towards AOP-based DNT testing is considered a stepwise approach, operating initially with incomplete AOPs for compound grouping, and focussing on key events of neurodevelopment. Next steps to be considered in follow-up activities are the use of case studies to further apply the AOP concept in regulatory DNT testing, making use of AOP intersections (common key events) for economic development of screening assays, and addressing the transition from qualitative descriptions to quantitative network modelling.

Establishment of a Long-Term Chick Forebrain Neuronal Culture on a Microelectrode Array Platform

The biosensor system formed by culturing primary animal neurons on a microelectrode array (MEA) platform is drawing an increasing research interest for its power as a rapid, sensitive, functional neurotoxicity assessment, as well as for many other electrophysiological related research purposes. In this paper, we established a long-term chick forebrain neuron culture (C-FBN-C) on MEAs with a more than 5 month long lifespan and up to 5 month long stability in morphology and physiological function; characterized the C-FBN-C morphologically, functionally, and developmentally; partially compared its functional features with rodent counterpart; and discussed its pros and cons as a novel biosensor system in comparison to rodent counterpart and human induced pluripotent stem cells (hiPSCs). Our results show that C-FBN-C on MEA platform 1) can be used as a biosensor of its own type in a wide spectrum of basic biomedical research; 2) is of value in comparative physiology in cross-species studies; and 3) may have potential to be used as an alternative, cost-effective approach to rodent counterpart within shared common functional domains (such as specific types of ligand-gated ion channel receptors and subtypes expressed in the cortical tissues of both species) in large-scale environmental neurotoxicant screening that would otherwise require millions of animals.

Convergent regulation of neuronal differentiation and Erk and Akt kinases in human neural progenitor cells by lysophosphatidic acid, sphingosine 1-phosphate, and LIF: specific roles for the LPA1 receptor

The bioactive lysophospholipids lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) have diverse effects on the developing nervous system and neural progenitors, but the molecular basis for their pleiotropic effects is poorly understood. We previously defined LPA and S1P signaling in proliferating human neural progenitor (hNP) cells, and the current study investigates their role in neuronal differentiation of these cells. Differentiation in the presence of LPA or S1P significantly enhanced cell survival and decreased expression of neuronal markers. Further, the LPA receptor antagonist Ki16425 fully blocked the effects of LPA, and differentiation in the presence of Ki16425 dramatically enhanced neurite length. LPA and S1P robustly activated Erk, but surprisingly both strongly suppressed Akt activation. Ki16425 and pertussis toxin blocked LPA activation of Erk but not LPA inhibition of Akt, suggesting distinct receptor and G-protein subtypes mediate these effects. Finally, we explored cross talk between lysophospholipid signaling and the cytokine leukemia inhibitory factor (LIF). LPA/S1P effects on neuronal differentiation were amplified in the presence of LIF. Similarly, the ability of LPA/S1P to regulate Erk and Akt was impacted by the presence of LIF; LIF enhanced the inhibitory effect of LPA/S1P on Akt phosphorylation, while LIF blunted the activation of Erk by LPA/S1P. Taken together, our results suggest that LPA and S1P enhance survival and inhibit neuronal differentiation of hNP cells, and LPA1 is critical for the effect of LPA. The pleiotropic effects of LPA may reflect differences in receptor subtype expression or cross talk with LIF receptor signaling.

Multi-well microelectrode array recordings detect neuroactivity of ToxCast compounds

Spontaneous activity in neuronal cultures on microelectrode arrays (MEAs) is sensitive to effects of drugs, chemicals, and particles. Multi-well MEA (mwMEA) systems have increased throughput of MEAs, enabling their use for chemical screening. The present experiments examined a subset of EPA's ToxCast compounds for effects on spontaneous neuronal activity in primary cortical cultures using 48-well MEA plates. A first cohort of 68 compounds was selected from the ToxCast Phase I and II libraries; 37 were positive in one or more of 20 individual ToxCast Novascreen assays related to ion channels (NVS_IC), with the remainder selected based on known neuroactivity. A second cohort of 25 compounds was then tested with 20 originating from the ToxCast Phase I and II libraries (not hits in NVS_IC assays) and 5 known negatives from commercial vendors. Baseline activity (1h) was recorded prior to exposing the networks to compounds for 1h, and the weighted mean firing rate (wMFR) was determined in the absence and presence of each compound. Compounds that altered activity by greater than the weighted change of DMSO-treated wells plus 2SD were considered "hits". Of the first set of 68 compounds, 54 altered wMFR by more than the threshold, while in the second set, 13/25 compounds were hits. MEAs detected 30 of 37 (81.1%) compounds that were hits in NVS_IC assays, as well as detected known neurotoxicants that were negative in NVS_IC assays, primarily pyrethroids and GABAA receptor antagonists. Conversely, wMFR of cortical neuronal networks on MEAs was insensitive to nicotinic compounds, as only one neonicotinoid was detected by MEAs; this accounts for the bulk of non-concordant compounds between MEA and NVS_IC assays. These data demonstrate that mwMEAs can be used to screen chemicals efficiently for potential neurotoxicity, and that the results are concordant with predictions from ToxCast NVS_IC assays for interactions with ion channels.

PCB 136 atropselectively alters morphometric and functional parameters of neuronal connectivity in cultured rat hippocampal neurons via ryanodine receptor-dependent mechanisms

We recently demonstrated that polychlorinated biphenyl (PCB) congeners with multiple ortho chlorine substitutions sensitize ryanodine receptors (RyRs), and this activity promotes Ca²⁺-dependent dendritic growth in cultured neurons. Many ortho-substituted congeners display axial chirality, and we previously reported that the chiral congener PCB 136 (2,2',3,3',6,6'-hexachlorobiphenyl) atropselectively sensitizes RyRs. Here, we test the hypothesis that PCB 136 atropisomers differentially alter dendritic growth and other parameters of neuronal connectivity influenced by RyR activity. (-)-PCB 136, which potently sensitizes RyRs, enhances dendritic growth in primary cultures of rat hippocampal neurons, whereas (+)-PCB 136, which lacks RyR activity, has no effect on dendritic growth. The dendrite-promoting activity of (-)-PCB 136 is observed at concentrations ranging from 0.1 to 100 nM and is blocked by pharmacologic RyR antagonism. Neither atropisomer alters axonal growth or cell viability. Quantification of PCB 136 atropisomers in hippocampal cultures indicates that atropselective effects on dendritic growth are not due to differential partitioning of atropisomers into cultured cells. Imaging of hippocampal neurons loaded with Ca²⁺-sensitive dye demonstrates that (-)-PCB 136 but not (+)-PCB 136 increases the frequency of spontaneous Ca²⁺ oscillations. Similarly, (-)-PCB 136 but not (+)-PCB 136 increases the activity of hippocampal neurons plated on microelectrode arrays. These data support the hypothesis that atropselective effects on RyR activity translate into atropselective effects of PCB 136 atropisomers on neuronal connectivity, and suggest that the variable atropisomeric enrichment of chiral PCBs observed in the human population may be a significant determinant of individual susceptibility for adverse neurodevelopmental outcomes following PCB exposure.

Stem cells and stem cell-derived tissues and their use in safety assessment

Toxicology has long relied on animal models in a tedious approach to understanding risk of exposure to an uncharacterized molecule. Stem cell-derived tissues can be made in high purity, quality, and quantity to enable a new approach to this problem. Currently, stem cell-derived tissues are primarily "generic" genetic backgrounds; the future will see the integration of various genetic backgrounds and complex three-dimensional models to create truly unique in vitro organoids. This minireview focuses on the state of the art of a number of stem cell-derived tissues and details their application in toxicology.

A human pluripotent carcinoma stem cell-based model for in vitro developmental neurotoxicity testing: effects of methylmercury, lead and aluminum evaluated by gene expression studies

The major advantage of the neuronal cell culture models derived from human stem cells is their ability to replicate the crucial stages of neurodevelopment such as the commitment of human stem cells to the neuronal lineage and their subsequent stages of differentiation into neuronal and glial-like cell. In these studies we used mixed neuronal/glial culture derived from the NTERA-2 (NT-2) cell line, which has been established from human pluripotent testicular embryonal carcinoma cells. After characterization of the different stages of cell differentiation into neuronal- and glial-like phenotype toxicity studies were performed to evaluate whether this model would be suitable for developmental neurotoxicity studies. The cells were exposed during the differentiation process to non-cytotoxic concentrations of methylmercury chloride, lead chloride and aluminum nitrate for two weeks. The toxicity was then evaluated by measuring the mRNA levels of cell specific markers (neuronal and glial). The results obtained suggest that lead chloride and aluminum nitrate at low concentrations were toxic primarily to astrocytes and at the higher concentrations it also induced neurotoxicity. In contrast, MetHgCl was toxic for both cell types, neuronal and glial, as mRNA specific for astrocytes and neuronal markers were affected. The results obtained suggest that a neuronal mixed culture derived from human NT2 precursor cells is a suitable model for developmental neurotoxicity studies and gene expression could be used as a sensitive endpoint for initial screening of potential neurotoxic compounds.

Neuronal connectivity as a convergent target of gene × environment interactions that confer risk for Autism Spectrum Disorders

Evidence implicates environmental factors in the pathogenesis of Autism Spectrum Disorders (ASD). However, the identity of specific environmental chemicals that influence ASD risk, severity or treatment outcome remains elusive. The impact of any given environmental exposure likely varies across a population according to individual genetic substrates, and this increases the difficulty of identifying clear associations between exposure and ASD diagnoses. Heritable genetic vulnerabilities may amplify adverse effects triggered by environmental exposures if genetic and environmental factors converge to dysregulate the same signaling systems at critical times of development. Thus, one strategy for identifying environmental risk factors for ASD is to screen for environmental factors that modulate the same signaling pathways as ASD susceptibility genes. Recent advances in defining the molecular and cellular pathology of ASD point to altered patterns of neuronal connectivity in the developing brain as the neurobiological basis of these disorders. Studies of syndromic ASD and rare highly penetrant mutations or CNVs in ASD suggest that ASD risk genes converge on several major signaling pathways linked to altered neuronal connectivity in the developing brain. This review briefly summarizes the evidence implicating dysfunctional signaling via Ca(2+)-dependent mechanisms, extracellular signal-regulated kinases (ERK)/phosphatidylinositol-3-kinases (PI3K) and neuroligin-neurexin-SHANK as convergent molecular mechanisms in ASD, and then discusses examples of environmental chemicals for which there is emerging evidence of their potential to interfere with normal neuronal connectivity via perturbation of these signaling pathways.

Evaluation of microelectrode array data using Bayesian modeling as an approach to screening and prioritization for neurotoxicity testing

The need to assess large numbers of chemicals for their potential toxicities has resulted in increased emphasis on medium- and high-throughput in vitro screening approaches. For such approaches to be useful, efficient and reliable data analysis and hit detection methods are also required. Assessment of chemical effects on neuronal network activity using microelectrode arrays (MEAs) has been proposed as a screening tool for neurotoxicity. The current study examined a Bayesian data analysis approach for assessing effects of a 30 chemical training set on activity of primary cortical neurons grown in multi-well MEA plates. Each well of the MEA plate contained 64 microelectrodes and the data set contains the number of electrical spikes registered by each electrode over the course of each experiment. A Bayesian data analysis approach was developed and then applied to several different parsings of the data set to produce probability determinations for hit selection and ranking. This methodology results in an approach that is approximately 74% sensitive in detecting chemicals in the training set known to alter neuronal function (23 expected positives) while being 100% specific in detecting chemicals expected to have no effect (7 expected negatives). Additionally, this manuscript demonstrates that the Bayesian approach may be combined with a previously published weighted mean firing rate approach in order to produce a more robust hit detection method. In particular, when combined with the weighted mean firing rate approach, the joint analysis produces a sensitivity of approximately 96% and a specificity of 100%. These results demonstrate the utility of a novel approach to analysis of MEA data and support the use of neuronal networks grown on MEAs as a for neurotoxicity screening approach.

Detection of trace processes in the networks of neurons cultured on microelectrode arrays

We compared the effects of electrical stimulation of primary dissociated neuronal cultures cultured on microelectrode arrays in terms on the expression of c-fos transcriptional factor that is involved in plastic reorganization in neurons. Dissociated hippocampal neurons cultured on multielectrode arrays were exposed to two stimulation protocols: high-frequency and low-frequency stimulations. Expression of c-fos was evaluated using immunofluorescence. Both high-frequency and low-frequency stimulations significantly increased c-fos expression in comparison with non-stimulated control. These findings indicate that c-fos expression can be induced in neuronal cell culture by different types of electrical stimulations and can be used for studying plasticity processes in microphysiological in vitro systems.

Developmental neurotoxicity testing: a path forward

Great progress has been made over the past 40 years in understanding the hazards of exposure to a small number of developmental neurotoxicants. Lead, polychlorinated biphenyls, and methylmercury are all good examples of science-based approaches to characterizing the hazard to the developing nervous systems from environmental contaminants. However, very little effort has been spent to address the challenge of assessing the potential developmental neurotoxic hazard of the thousands of other chemicals in common commercial use. The extensive time, financial and animal resource requirements for current regulatory testing guideline methods make this an untenable solution to this challenge. A new testing paradigm is needed that uses time and cost-efficient methods to screen large numbers of chemicals for developmental neurotoxicity (DNT). In silico models are needed to provide rapid chemical structure-based screening. In vitro techniques are being developed to provide rapid and efficient testing in cell-free and cell-based systems. In addition, the use of alternative species, such as zebrafish, will provide efficient models for testing the effects of chemicals in organisms with intact developing nervous systems. Finally, these methods and models need to be used in an integrated fashion to provide the data needs for hazard assessment in a manner that is problem-driven and cost-efficient. This paper summarizes discussions on these issues from the symposium 'Developmental neurotoxicity testing: Scientific approaches towards the next generation to protecting the developing nervous system of children' held at the 2011 annual meeting of the Japanese Teratology Society.

Neurite outgrowth and differentiation of rat cortex progenitor cells are sensitive to lithium chloride at non-cytotoxic exposures

Neuron-specific in vitro screening strategies have the potential to accelerate the evaluation of chemicals for neurotoxicity. We examined neurite outgrowth as a measure of neuronal response with a commercially available rat cortex progenitor cell model, where cells were exposed to a chemical during a period of cell differentiation. In control cultures, the fraction of beta-III-tubulin positive neurons and their neurite length increased significantly with time, indicating differentiation of the progenitor cells. Expression of glial fibrillary acidic protein, an astrocyte marker, also increased significantly with time. By seeding progenitor cells at varying densities, we demonstrated that neurite length was influenced by cell-cell spacing. After ten days, cultures seeded at densities of 1000 cells/mm(2) or lower had significantly shorter neurites than cultures seeded at densities of 1250 cells/mm(2) or higher. Progenitor cells were exposed to lithium, a neuroactive chemical with diverse modes of action. Cultures exposed to 30 mmol/L or 10 mmol/L lithium chloride (LiCl) had significantly lower metabolic activity than control cultures, as reported by adenosine triphosphate content, and no neurons were observed after ten days of exposure. Cultures exposed to 3 mmol/L, 1 mmol/L, or 0.3 mmol/L LiCl, which encompass lithium's therapeutic range, had metabolic activity similar to control cultures. These cultures exhibited concentration-dependent decreases in neurite outgrowth after ten days of LiCl exposure. Neurite outgrowth results were relatively robust, regardless of the evaluation methodology. This work demonstrates that measurement of neurite outgrowth in differentiating progenitor cell cultures can be a sensitive endpoint for neuronal response under non-cytotoxic exposure conditions.

Evaluation of multi-well microelectrode arrays for neurotoxicity screening using a chemical training set

Microelectrode array (MEA) approaches have been proposed as a tool for detecting functional changes in electrically excitable cells, including neurons, exposed to drugs, chemicals or particles. However, conventional single well-MEA systems lack the throughput necessary for screening large numbers of uncharacterized compounds. Recently, multi-well MEA (mwMEA) formats have become available to address the need for increased throughput. The current experiments examined the effects of a training set of 30 chemicals on spontaneous activity in networks of cortical neurons grown on mwMEA plates. Each plate contained 12 wells with 64 microelectrodes/well, for a total of 768 channels. Of the 30 chemicals evaluated, 23 were known to alter neuronal function in vivo ("positives"), including 6 GABAergic and 3 glutamatergic antagonists/agonists, 4 pyrethroids, 3 metals, 2 cholinesterase inhibitors, 2 nicotinic acetylcholine receptor agonists, valproic acid, verapamil, and fluoxetine. Seven compounds expected to have no effect on neuronal function were tested as "negatives" (glyphosate, acetaminophen, salicylic acid, paraquat, saccharin, d-sorbitol and amoxicillin). Following collection of 33 min of baseline activity, chemical effects (50 μM or highest soluble concentration) were recorded for 33 min. Twenty of the positives altered the mean network spike rate by more than the 14% threshold (two standard deviations from the mean for DMSO control). The three positives without effect were bifenthrin, nicotine and imidacloprid. None of the negative compounds caused a change in activity beyond the threshold. Based on these results, the mwMEA assay has both high sensitivity (87% identification of positive compounds) and specificity (100% identification of negative compounds). These experiments demonstrate the capacity of mwMEAs to screen compounds for neurotoxic effects mediated by a broad variety of mechanisms.

Comparison of chemical-induced changes in proliferation and apoptosis in human and mouse neuroprogenitor cells

There is a need to develop rapid and efficient models to screen chemicals for their potential to cause developmental neurotoxicity. Use of in vitro neuronal models, including human cells, is one approach that allows for timely, cost-effective toxicity screening. The present study compares the sensitivity of human (ReN CX) and mouse (mCNS) neuroprogenitor cell lines to chemicals using a multiplex assay for proliferation and apoptosis, endpoints that are critical for neural development. Cells were exposed to 0.001-100 μM concentrations of 11 chemicals (cadmium, chlorpyrifos oxon, dexamethasone, dieldrin, ketamine, lead, maneb, methylmercury, nicotine, trans-retinoic acid, and trimethyltin) reported in the literature to affect proliferation and/or apoptosis, and 5 chemicals (dimethyl pthalate, glyphosate, omeprazole, saccharin, and d-sorbitol) with no reports of effects on either endpoint. High-content screening of markers for proliferation (BrdU incorporation) and apoptosis (activated caspase 3 and p53) was used to assess the effect of chemicals in both cell lines. Of the chemicals tested, methylmercury, cadmium, dieldrin, chlorpyrifos oxon, trans-retinoic acid, and trimethyltin decreased proliferation by at least 50% of control in either the ReN CX or mCNS cells. None of the chemicals tested activated caspase 3 or p53 in the ReN CX cells, while methylmercury, cadmium, dieldrin, chlorpyrifos oxon, trimethyltin, and glyphosate all induced at least a doubling in these apoptotic markers in the mCNS cells. Compared to control, cadmium, trans-retinoic acid, and trimethyltin decreased cell viability (ATP levels) by at least 50% in the ReN CX cells, while cadmium, dieldrin, and methylmercury decreased viability by at least 50% in the mCNS cells. Based on these results, BrdU is an appropriate marker for assessing chemical effects on proliferation, and human cells are more sensitive than mouse cells for this endpoint. By contrast, caspase 3 and p53 were altered by environmental chemicals in mouse, but not in human cells. Therefore, these markers are not appropriate to assess the ability of environmental chemicals to induce apoptosis in the ReN CX cells.

Effects of silver nanoparticles on primary mixed neural cell cultures: uptake, oxidative stress and acute calcium responses

In the body, nanoparticles can be systemically distributed and then may affect secondary target organs, such as the central nervous system (CNS). Putative adverse effects on the CNS are rarely investigated to date. Here, we used a mixed primary cell model consisting mainly of neurons and astrocytes and a minor proportion of oligodendrocytes to analyze the effects of well-characterized 20 and 40 nm silver nanoparticles (SNP). Similar gold nanoparticles served as control and proved inert for all endpoints tested. SNP induced a strong size-dependent cytotoxicity. Additionally, in the low concentration range (up to 10 μg/ml of SNP), the further differentiated cultures were more sensitive to SNP treatment. For detailed studies, we used low/medium dose concentrations (up to 20 μg/ml) and found strong oxidative stress responses. Reactive oxygen species (ROS) were detected along with the formation of protein carbonyls and the induction of heme oxygenase-1. We observed an acute calcium response, which clearly preceded oxidative stress responses. ROS formation was reduced by antioxidants, whereas the calcium response could not be alleviated by antioxidants. Finally, we looked into the responses of neurons and astrocytes separately. Astrocytes were much more vulnerable to SNP treatment compared with neurons. Consistently, SNP were mainly taken up by astrocytes and not by neurons. Immunofluorescence studies of mixed cell cultures indicated stronger effects on astrocyte morphology. Altogether, we can demonstrate strong effects of SNP associated with calcium dysregulation and ROS formation in primary neural cells, which were detectable already at moderate dosages.