Neurotoxicity testing: a discussion of in vitro alternatives

Recent advances and current challenges of new approach methodologies in developmental and adult neurotoxicity testing

Adult neurotoxicity (ANT) and developmental neurotoxicity (DNT) assessments aim to understand the adverse effects and underlying mechanisms of toxicants on the human nervous system. In recent years, there has been an increasing focus on the so-called new approach methodologies (NAMs). The Organization for Economic Co-operation and Development (OECD), together with European and American regulatory agencies, promote the use of validated alternative test systems, but to date, guidelines for regulatory DNT and ANT assessment rely primarily on classical animal testing. Alternative methods include both non-animal approaches and test systems on non-vertebrates (e.g., nematodes) or non-mammals (e.g., fish). Therefore, this review summarizes the recent advances of NAMs focusing on ANT and DNT and highlights the potential and current critical issues for the full implementation of these methods in the future. The status of the DNT in vitro battery (DNT IVB) is also reviewed as a first step of NAMs for the assessment of neurotoxicity in the regulatory context. Critical issues such as (i) the need for test batteries and method integration (from in silico and in vitro to in vivo alternatives, e.g., zebrafish, C. elegans) requiring interdisciplinarity to manage complexity, (ii) interlaboratory transferability, and (iii) the urgent need for method validation are discussed.

Neurotoxicity of Engineered Nanomaterials: Testing Considerations

As with toxicology in general, major challenges have emerged in its subfield neurotoxicology regarding the testing of engineered nanomaterials (ENM). This is on the one hand due to their complex physicochemical properties, like size, specific surface area, chemical composition as well as agglomeration and dissolution behavior in biological environments. On the other hand, toxicological risk assessment has faced an increasing demand for the development and implementation of non-animal alternative approaches. Regarding the investigation and interpretation of the potential adverse effects of ENM on the brain, toxicokinetic data are relatively scarce and thus hampers dose selection for in vitro neurotoxicity testing. Moreover, recent in vivo studies indicate that ENM can induce neurotoxic and behavioral effects in an indirect manner, depending on their physicochemical properties and route of exposure. Such indirect effects on the brain may proceed through the activation and spill-over of inflammatory mediators by ENM in the respiratory tract and other peripheral organs as well via ENM induced disturbance of the gut microbiome and intestinal mucus barrier. These ENM specific aspects should be incorporated into the ongoing developments of advanced in vitro neurotoxicity testing methods and strategies.

Neurotoxicants, the developing brain, and mental health

While life in urban environments may confer a number of benefits, it may also result in a variety of exposures, with toxic consequences for neurodevelopment and neuropsychological health. Neurotoxicants are any of a large number of chemicals or substances that interfere with normal function and/or compromise adaptation in the central and/or peripheral nervous system. Evidence suggests that neurotoxicant effects have a greater effect when occurring in utero and during early childhood. Recent findings exploring neural-level mechanisms provide a crucial opportunity to explore the ways in which environmental conditions may get “under the skin” to impact a number of psychological behaviors and cognitive processes, ultimately allowing for greater synergy between macro- and microlevel efforts to improve mental health in the presence of neurotoxicant exposures. In this review, we provide an overview of 3 types of neurotoxicants related to the built environment and relevant to brain development during childhood and adolescence: lead exposure, outdoor particulate matter pollution, and endocrine-disrupting chemicals. We also discuss mechanisms through which these neurotoxicants affect central nervous system function, including recent evidence from neuroimaging literature. Furthermore, we discuss neurotoxicants and mental health during development in the context of social determinants and how differences in the spatial distribution of neurotoxicant exposures result in health disparities that disproportionately affect low-income and minority populations. Multifaceted approaches incorporating social systems and their effect on neurotoxicant exposures and downstream mental health will be key to reduce societal costs and improve quality of life for children, adolescents, and adults.

Organotypic Brain Slice Cultures: An Efficient and Reliable Method for Neurotoxicological Screening and Mechanistic Studies

This paper reviews the current state of the use of organotypic brain slice cultures for neurotoxicological and neuropharmacological screening and mechanistic studies, as exemplified by excitotoxin application. At present, no in vitro systems have been approved by the regulatory authorities for neurotoxicity testing. For the evaluation of the slice culture method, organotypic hippocampal slice cultures were exposed to toxic doses of the excitotoxins, glutamate, N-methyl-D-aspartate (NMDA), kainic acid and 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and the glial toxin, DL-alpha-aminoadipic acid (DLAAA). Neuronal cell death was quantified by propidium iodide (PI) uptake, and visualised by Fluoro-Jade (FJ) staining. General cell death was monitored by lactate dehydrogenase (LDH) release into the culture medium. EC50 values for the different compounds, based on PI uptake after exposure for 48 hours in entire cultures, were: glutamate, 3.5 mM; DL-AAA, 2.3 mM; kainic acid, 13 microM; NMDA, 11 microM; and AMPA, 3.7 microM. In the slice cultures, the hippocampal subfields displayed the same differences in vulnerability as those observed in vivo. When subfield analysis was performed on the cultures, the CA1 subfield was most susceptible to glutamate, NMDA and AMPA, while CA3 was most susceptible to kainic acid. The amount of LDH release for DL-AAA was about four times that of L-glutamate, in accordance with the additional toxic effect on glial cells, which was also found by confocal microscopy to stain for FJ. In conclusion, it was found that organotypic brain slice culture, combined with standardised protocols and quantifiable markers, such as PI and FJ staining, is a relevant and feasible in vitro system for neurotoxicity testing. Considering the amount and quality of the available published data, it is recommended that the brain slice culture method could be subjected to pre-validation and formal validation for inclusion in a tiered in vitro neurotoxicity testing scheme to supplement and replace conventional animal tests.

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.

The altered expression of perineuronal net elements during neural differentiation

Background

Perineuronal nets (PNNs), which are localized around neurons during development, are specialized forms of neural extracellular matrix with neuroprotective and plasticity-regulating roles. Hyaluronan and proteoglycan link protein 1 (HAPLN1), tenascin-R (TNR) and aggrecan (ACAN) are key elements of PNNs. In diseases characterized by neuritogenesis defects, the expression of these proteins is known to be downregulated, suggesting that PNNs may have a role in neural differentiation.

Methods

In this study, the mRNA and protein levels of HAPLN1, TNR and ACAN were determined and compared at specific time points of neural differentiation. We used PC12 cells as the in vitro model because they reflect this developmental process.

Results

On day 7, the HAPLN1 mRNA level showed a 2.9-fold increase compared to the non-differentiated state. However, the cellular HAPLN1 protein level showed a decrease, indicating that the protein may have roles in neural differentiation, and may be secreted during the early period of differentiation. By contrast, TNR mRNA and protein levels remained unchanged, and the amount of cellular ACAN protein showed a 3.7-fold increase at day 7. These results suggest that ACAN may be secreted after day 7, possibly due to its large amount of post-translational modifications.

Conclusions

Our results provide preliminary data on the expression of PNN elements during neural differentiation. Further investigations will be performed on the role of these elements in neurological disease models.

Diverse neurotoxicants target the differentiation of embryonic neural stem cells into neuronal and glial phenotypes

The large number of compounds that needs to be tested for developmental neurotoxicity drives the need to establish in vitro models to evaluate specific neurotoxic endpoints. We used neural stem cells derived from rat neuroepithelium on embryonic day 14 to evaluate the impact of diverse toxicants on their ability to differentiate into glia and neurons: a glucocorticoid (dexamethasone), organophosphate insecticides (chlorpyrifos, diazinon, parathion), insecticides targeting the GABA_A receptor (dieldrin, fipronil), heavy metals (Ni²⁺, Ag⁺), nicotine and tobacco smoke extract. We found three broad groupings of effects. One diverse set of compounds, dexamethasone, the organophosphate pesticides, Ni²⁺ and nicotine, suppressed expression of the glial phenotype while having little or no effect on the neuronal phenotype. The second pattern was restricted to the pesticides acting on GABA_A receptors. These compounds promoted the glial phenotype and suppressed the neuronal phenotype. Notably, the actions of compounds eliciting either of these differentiation patterns were clearly unrelated to deficits in cell numbers: dexamethasone, dieldrin and fipronil all reduced cell numbers, whereas organophosphates and Ni²⁺ had no effect. The third pattern, shared by Ag⁺ and tobacco smoke extract, clearly delineated cytotoxicity, characterized by major cell loss with suppression of differentiation into both glial and neuronal phenotypes; but here again, there was some selectivity in that glia were suppressed more than neurons. Our results, from this survey with diverse compounds, point to convergence of neurotoxicant effects on a specific "decision node" that controls the emergence of neurons and glia from neural stem cells.

Toxicological assessment of industrial solvents using human cell bioassays: assessment of short-term cytotoxicity and long-term genotoxicity potential

There is an increasing demand for simple toxicological screening methods to assess the human health risk associated with exposure to environmental toxicants. Such screening tools should allow for risk evaluation in terms of both short-term/acute toxicity and longer-term genetic damage, which may lead to mutagenicity and carcinogenicity. We employed a battery of human cell bioassays using the human hepatoma cell-line, HepG2, to assess the cytotoxic and genotoxic potential of environmental pollutants. Here, we demonstrate direct application of these human cell bioassays to the toxicological assessment of a number of industrial solvents that are in common use worldwide. HepG2 cells were exposed to various solvents at concentrations ranging from 25 to 500 ppm. The cells were then analysed using specific protocols for four different adverse effects: cell death/acute cytotoxicity using a neutral red uptake assay, altered enzyme function (often an indicator of cell stress) using the ethoxyresorufin O-deethylase (EROD) bioassay, DNA single strand breaks (SSB), and DNA repair induction, which evaluates mutagenic activity. Using the positive controls, linear dose-response curves were achieved for all four bioassays. The high sensitivity of the tests allowed for environmentally meaningful assessments, and precision studies showed excellent reproducibility for all four bioassays. Comparing the results of the four bioassays on each of 16 industrial solvents allowed for ranking of the anticipated relative human toxicity of these solvents, which were comparable with data from standard toxicity tests and human occupational data. Overall, the study clearly supports the application of the HepG2 cell bioassay system for rapid toxicological screening of many candidate toxicants for both short- and long-term toxicity potential.

Amelioration strategies fail to prevent tobacco smoke effects on neurodifferentiation: Nicotinic receptor blockade, antioxidants, methyl donors

Tobacco smoke exposure is associated with neurodevelopmental disorders. We used neuronotypic PC12 cells to evaluate the mechanisms by which tobacco smoke extract (TSE) affects neurodifferentiation. In undifferentiated cells, TSE impaired DNA synthesis and cell numbers to a much greater extent than nicotine alone; TSE also impaired cell viability to a small extent. In differentiating cells, TSE enhanced cell growth at the expense of cell numbers and promoted emergence of the dopaminergic phenotype. Nicotinic receptor blockade with mecamylamine was ineffective in preventing the adverse effects of TSE and actually enhanced the effect of TSE on the dopamine phenotype. A mixture of antioxidants (vitamin C, vitamin E, N-acetyl-l-cysteine) provided partial protection against cell loss but also promoted loss of the cholinergic phenotype in response to TSE. Notably, the antioxidants themselves altered neurodifferentiation, reducing cell numbers and promoting the cholinergic phenotype at the expense of the dopaminergic phenotype, an effect that was most prominent for N-acetyl-l-cysteine. Treatment with methyl donors (vitamin B12, folic acid, choline) had no protectant effect and actually enhanced the cell loss evoked by TSE; they did have a minor, synergistic interaction with antioxidants protecting against TSE effects on growth. Thus, components of tobacco smoke perturb neurodifferentiation through mechanisms that cannot be attributed to the individual effects of nicotine, oxidative stress or interference with one-carbon metabolism. Consequently, attempted amelioration strategies may be partially effective at best, or, as seen here, can actually aggravate injury by interfering with normal developmental signals and/or by sensitizing cells to TSE effects on neurodifferentiation.

The potential of induced pluripotent stem cells in models of neurological disorders: implications on future therapy

There is an urgent need for new and advanced approaches to modeling the pathological mechanisms of complex human neurological disorders. This is underscored by the decline in pharmaceutical research and development efficiency resulting in a relative decrease in new drug launches in the last several decades. Induced pluripotent stem cells represent a new tool to overcome many of the shortcomings of conventional methods, enabling live human neural cell modeling of complex conditions relating to aberrant neurodevelopment, such as schizophrenia, epilepsy and autism as well as age-associated neurodegeneration. This review considers the current status of induced pluripotent stem cell-based modeling of neurological disorders, canvassing proven and putative advantages, current constraints, and future prospects of next-generation culture systems for biomedical research and translation.

In vitro models for neurotoxicology research

The nervous system has a highly complex organization, including many cell types with multiple functions, with an intricate anatomy and unique structural and functional characteristics; the study of its (dys)functionality following exposure to xenobiotics, neurotoxicology, constitutes an important issue in neurosciences.

Effects of tobacco smoke on PC12 cell neurodifferentiation are distinct from those of nicotine or benzo[a]pyrene

Although nicotine accounts for a great deal of the neurodevelopmental damage associated with maternal smoking or second-hand exposure, tobacco smoke contains thousands of potentially neurotoxic compounds. We used PC12 cells, a standard in vitro model of neurodifferentiation, to compare tobacco smoke extract (TSE) to nicotine, matching TSE exposure (with its inherent nicotine content) to parallel concentrations of nicotine, or to benzo[a]pyrene, a tobacco combustion product. TSE promoted the transition from cell replication to differentiation, resulting in fewer, but larger cells with greater neurite extension. TSE also biased differentiation into the dopaminergic versus the cholinergic phenotype, evidenced by an increase in tyrosine hydroxylase activity but not choline acetyltransferase. Nicotine likewise promoted differentiation at the expense of cell numbers, but its effect on growth and neurite extension was smaller than that of TSE; furthermore, nicotine did not promote the dopaminergic phenotype. Benzo[a]pyrene had effects opposite to those of TSE, retarding neurodifferentiation, which resulted in higher cell numbers, smaller cells, reduced neurite information, and impaired emergence of both dopaminergic and cholinergic phenotypes. Our studies show that the complex mixture of compounds in tobacco smoke exerts direct effects on neural cell replication and differentiation that resemble those of nicotine in some ways but not others, and most importantly, that are greater in magnitude than can be accounted for from just the nicotine content of TSE. Thus, fetal tobacco smoke exposure, including lower levels associated with second-hand smoke, could be more injurious than would be anticipated from measured levels of nicotine or its metabolites.

Morphological and functional differentiation in BE(2)-M17 human neuroblastoma cells by treatment with Trans-retinoic acid

Background

Immortalized neuronal cell lines can be induced to differentiate into more mature neurons by adding specific compounds or growth factors to the culture medium. This property makes neuronal cell lines attractive as in vitro cell models to study neuronal functions and neurotoxicity. The clonal human neuroblastoma BE(2)-M17 cell line is known to differentiate into a more prominent neuronal cell type by treatment with trans-retinoic acid. However, there is a lack of information on the morphological and functional aspects of these differentiated cells.

Results

We studied the effects of trans-retinoic acid treatment on (a) some differentiation marker proteins, (b) types of voltage-gated calcium (Ca²⁺) channels and (c) Ca²⁺-dependent neurotransmitter ([³H] glycine) release in cultured BE(2)-M17 cells. Cells treated with 10 μM trans-retinoic acid (RA) for 72 hrs exhibited marked changes in morphology to include neurite extensions; presence of P/Q, N and T-type voltage-gated Ca²⁺ channels; and expression of neuron specific enolase (NSE), synaptosomal-associated protein 25 (SNAP-25), nicotinic acetylcholine receptor α7 (nAChR-α7) and other neuronal markers. Moreover, retinoic acid treated cells had a significant increase in evoked Ca²⁺-dependent neurotransmitter release capacity. In toxicity studies of the toxic gas, phosgene (CG), that differentiation of M17 cells with RA was required to see the changes in intracellular free Ca²⁺ concentrations following exposure to CG.

Conclusion

Taken together, retinoic acid treated cells had improved morphological features as well as neuronal characteristics and functions; thus, these retinoic acid differentiated BE(2)-M17 cells may serve as a better neuronal model to study neurobiology and/or neurotoxicity.

Chlorpyrifos developmental neurotoxicity: interaction with glucocorticoids in PC12 cells

Prenatal coexposures to glucocorticoids and organophosphate pesticides are widespread. Glucocorticoids are elevated by maternal stress and are commonly given in preterm labor; organophosphate exposures are virtually ubiquitous. We used PC12 cells undergoing neurodifferentiation in order to assess whether dexamethasone enhances the developmental neurotoxicity of chlorpyrifos, focusing on models relevant to human exposures. By themselves, each agent reduced the number of cells and the combined exposure elicited a correspondingly greater effect than with either agent alone. There was no general cytotoxicity, as cell growth was actually enhanced, and again, the combined treatment evoked greater cellular hypertrophy than with the individual compounds. The effects on neurodifferentiation were more complex. Chlorpyrifos alone had a promotional effect on neuritogenesis whereas dexamethasone impaired it; combined treatment showed an overall impairment greater than that seen with dexamethasone alone. The effect of chlorpyrifos on differentiation into specific neurotransmitter phenotypes was shifted by dexamethasone. Either agent alone promoted differentiation into the dopaminergic phenotype at the expense of the cholinergic phenotype. However, in dexamethasone-primed cells, chlorpyrifos actually enhanced cholinergic neurodifferentiation instead of suppressing this phenotype. Our results indicate that developmental exposure to glucocorticoids, either in the context of stress or the therapy of preterm labor, could enhance the developmental neurotoxicity of organophosphates and potentially of other neurotoxicants, as well as producing neurobehavioral outcomes distinct from those seen with either individual agent.

Developmental neurotoxicity: some old and new issues

The developing central nervous system is often more vulnerable to injury than the adult one. Of the almost 200 chemicals known to be neurotoxic, many are developmental neurotoxicants. Exposure to these compounds in utero or during childhood can contribute to a variety of neurodevelopmental and neurological disorders. Two established developmental neurotoxicants, methylmercury and lead, and two classes of chemicals, the polybrominated diphenyl ether flame retardants and the organophosphorus insecticides, which are emerging as potential developmental neurotoxicants, are discussed in this paper. Developmental neurotoxicants may also cause silent damage, which would manifest itself only as the individual ages, and may contribute to neurodegenerative diseases such as Parkinson's or Alzheimer's diseases. Guidelines for developmental neurotoxicity testing have been implemented, but there is still room for their improvement and for searching and validating alternative testing approaches.

Does mechanism matter? Unrelated neurotoxicants converge on cell cycle and apoptosis during neurodifferentiation

Mechanistically unrelated developmental neurotoxicants often produce neural cell loss culminating in similar functional and behavioral outcomes. We compared an organophosphate pesticide (diazinon), an organochlorine pesticide (dieldrin) and a metal (Ni(2+)) for effects on the genes regulating cell cycle and apoptosis in differentiating PC12 cells, an in vitro model of neuronal development. Each agent was introduced at 30μM for 24 or 72h, treatments devoid of cytotoxicity. Using microarrays, we examined the mRNAs encoding nearly 400 genes involved in each of the biological processes. All three agents targeted both the cell cycle and apoptosis pathways, evidenced by significant transcriptional changes in 40-45% of the cell cycle-related genes and 30-40% of the apoptosis-related genes. There was also a high degree of overlap as to which specific genes were affected by the diverse agents, with 80 cell cycle genes and 56 apoptosis genes common to all three. Concordance analysis, which assesses stringent matching of the direction, magnitude and timing of the transcriptional changes, showed highly significant correlations for pairwise comparisons of all the agents, for both cell cycle and apoptosis. Our results show that otherwise disparate developmental neurotoxicants converge on common cellular pathways governing the acquisition and programmed death of neural cells, providing a specific link to cell deficits. Our studies suggest that identifying the initial mechanism of action of a developmental neurotoxicant may be strategically less important than focusing on the pathways that converge on common final outcomes such as cell loss.

Global gene expression and Ingenuity biological functions analysis on PCBs 153 and 138 induced human PBMC in vitro reveals differential mode(s) of action in developing toxicities

Several reports have indicated that low level of polychlorinated biphenyl (PCB) exposure can adversely affect a multitude of physiological disorders and diseases in in vitro, in vivo, and as reported in epidemiological studies. This investigation is focused on the possible contribution of two most prevalent PCB congeners in vitro in developing toxicities. We used PCBs 138 and 153 at the human equivalence level as model agents to test their specificity in developing toxicities. We chose a global approach using oligonucleotide microarray technology to investigate modulated gene expression for biological effects, upon exposure of PCBs, followed by Ingenuity Pathway Analysis (IPA), to understand the underlying consequence in developing disease and disorders. We performed in vitro studies with human peripheral blood mononuclear cells (PBMC), where PBMC cells were exposed to respective PCBs for 48 h. Overall, our observation on gene expression indicated that PCB produces a unique signature affecting different pathways, specific for each congener. While analyzing these data through IPA, the prominent and interesting disease and disorders were neurological disease, cancer, cardiovascular disease, respiratory disease, as well as endocrine system disorders, genetic disorders, and reproductive system disease. They showed strong resemblances with in vitro, in vivo, and in the epidemiological studies. A distinct difference was observed in renal and urological diseases, organisimal injury and abnormalities, dental disease, ophthalmic disease, and psychological disorders, which are only revealed by PCB 138 exposure, but not in PCB 153. The present study emphasizes the challenges of global gene expression in vitro and was correlated with the results of exposed human population. The microarray results give a molecular mechanistic insight and functional effects, following PCB exposure. The extent of changes in genes related to several possible mode(s) of action highlights the changes in cellular functions and signaling pathways that play major roles. In addition to understanding the pathways related to mode of action for chemicals, these data could lead to the identification of genomic signatures that could be used for screening of chemicals for their potential to cause disease and developmental disorders.

The neurotoxic effects of monocrotophos on the formation of the serotonergic nervous system and swimming activity in the larvae of the sea urchin Hemicentrotus pulcherrimus

The neurotoxicity of monocrotophos (MCP) in the development of the serotonergic nervous system and swimming activity of larvae of the sea urchin, Hemicentrotus pulcherrimus, was examined. Lethal dose 50% of MCP was 43μg/ml. Overall morphology was not affected in larvae that received up to 30μg/ml of MCP soon after fertilization until the 53h post-fertilization pluteus stage. However, while 70±0.6% of larvae in 5μg/ml MCP swam actively, the proportion decreased to 30±1.7% in 30μg/ml MCP. Accordingly, immunoblotting indicated that MCP decreased the relative intensity of immunoreaction of serotonin receptor protein. Whole-mount immunohistochemistry indicated that MCP inhibited serotonergic axon growth, reduced the number of serotonergic cells at the apical ganglion, and perturbed formation of the serotonin receptor cell network. The present study demonstrated that sea urchin larva is a useful model for evaluating the working mechanism of environmental toxicants in neurogenesis and behavior.

Mass spectrometric analyses of organophosphate insecticide oxon protein adducts

OBJECTIVE

Organophosphate (OP) insecticides continue to be used to control insect pests. Acute and chronic exposures to OP insecticides have been documented to cause adverse health effects, but few OP-adducted proteins have been correlated with these illnesses at the molecular level. Our aim was to review the literature covering the current state of the art in mass spectrometry (MS) used to identify OP protein biomarkers.

DATA SOURCES AND EXTRACTION

We identified general and specific research reports related to OP insecticides, OP toxicity, OP structure, and protein MS by searching PubMed and Chemical Abstracts for articles published before December 2008.

DATA SYNTHESIS

A number of OP-based insecticides share common structural elements that result in predictable OP-protein adducts. The resultant OP-protein adducts show an increase in molecular mass that can be identified by MS and correlated with the OP agent. Customized OP-containing probes have also been used to tag and identify protein targets that can be identified by MS.

CONCLUSIONS

MS is a useful and emerging tool for the identification of proteins that are modified by activated organophosphate insecticides. MS can characterize the structure of the OP adduct and also the specific amino acid residue that forms the key bond with the OP. Each protein that is modified in a unique way by an OP represents a unique molecular biomarker that with further research can lead to new correlations with exposure.

Evaluation of the importance of astrocytes when screening for acute toxicity in neuronal cell systems

Reliable, high throughput, in vitro preliminary screening batteries have the potential to greatly accelerate the rate at which regulatory neurotoxicity data is generated. This study evaluated the importance of astrocytes when predicting acute toxic potential using a neuronal screening battery of pure neuronal (NT2.N) and astrocytic (NT2.A) and integrated neuronal/astrocytic (NT2.N/A) cell systems derived from the human NT2.D1 cell line, using biochemical endpoints (mitochondrial membrane potential (MMP) depolarisation and ATP and GSH depletion). Following exposure for 72 h, the known acute human neurotoxicants trimethyltin-chloride, chloroquine and 6-hydroxydopamine were frequently capable of disrupting biochemical processes in all of the cell systems at non-cytotoxic concentrations. Astrocytes provide key metabolic and protective support to neurons during toxic challenge in vivo and generally the astrocyte containing cell systems showed increased tolerance to toxicant insult compared with the NT2.N mono-culture in vitro. Whilst there was no consistent relationship between MMP, ATP and GSH log IC(50) values for the NT2.N/A and NT2.A cell systems, these data did provide preliminary evidence of modulation of the acute neuronal toxic response by astrocytes. In conclusion, the suitability of NT2 neurons and astrocytes as cell systems for acute toxicity screening deserves further investigation.

Unrelated developmental neurotoxicants elicit similar transcriptional profiles for effects on neurotrophic factors and their receptors in an in vitro model

Diverse developmental neurotoxicants can often produce similar functional and behavioral outcomes. We examined an organophosphate pesticide (diazinon), an organochlorine pesticide (dieldrin) and a metal (Ni(2+)) for effects on the expression of neurotrophic factors and their receptors and modulators in differentiating PC12 cells, an in vitro model of neuronal development. Each agent was introduced at 30 microM for 24 or 72 h, treatments devoid of cytotoxicity. Using microarrays, we examined the mRNAs encoding members of the fibroblast growth factor (fgf) family, the neurotrophins (ntfs), brain-derived neurotrophic factor (bdnf), nerve growth factor (ngf), the wnt and fzd gene families, and the receptors and modulators for each class. All three agents evoked highly concordant patterns of effects on genes encoding the fgf family, whereas the correlations were poor for the group comprising bdnf, ngf and their respective receptors. For wnt, fzd and their receptors/modulators, the relationships between diazinon and dieldrin were highly concordant, whereas the effect of Ni(2+) was less similar, albeit still significantly correlated with the others. Our results show that otherwise disparate developmental neurotoxicants converge on common sets of neurotrophic pathways known to control neuronal differentiation, likely contributing to similarities in functional outcomes. Further, cell culture models can provide a useful initial screen to identify members of a given class of compounds that may be greater or lesser risks for developmental neurotoxicity, or to provide an indication of agents in different classes that might produce similar effects.

Is fipronil safer than chlorpyrifos? Comparative developmental neurotoxicity modeled in PC12 cells

Fipronil, a GABA(A) receptor antagonist, is replacing many insecticide uses formerly fulfilled by organophosphates like chlorpyrifos. Few studies have addressed the potential for fipronil to produce developmental neurotoxicity. We compared the neurotoxicity of fipronil and chlorpyrifos in undifferentiated and differentiating neuronotypic PC12 cells, evaluating indices of cell replication, cell number, differentiation, and viability for short- and long-term exposures. Fipronil inhibited DNA and protein synthesis in undifferentiated PC12 cells and evoked oxidative stress to a greater extent than did chlorpyrifos, resulting in reduced cell numbers even though cell viability was maintained. In differentiating cells, fipronil displayed an even lower threshold for disruption of development, reducing cell numbers without impairing cell growth, and promoting emergence of neurotransmitter phenotypes; superimposed on this effect, the phenotypic balance was shifted in favor of dopamine as opposed to acetylcholine. Differentiation also enhanced the susceptibility to fipronil-induced oxidative stress, although antioxidant administration failed to provide protection from cell loss. At low concentrations maintained for prolonged periods, fipronil had a biphasic effect on cell numbers, increasing them slightly at low concentrations, implying interference with apoptosis, while nevertheless reducing cell numbers at higher concentrations. Our results suggest that fipronil is inherently a more potent disruptor of neuronal cell development than is chlorpyrifos. The neurodevelopmental effects are not predicated on GABA(A) antagonist properties, since PC12 cells lack the GABA(A) receptor. If fipronil is intended to provide greater safety than chlorpyrifos, then this will have to entail advantages from factors that are yet unexamined: exposure, persistence, pharmacokinetics.

Transcriptional profiles reveal similarities and differences in the effects of developmental neurotoxicants on differentiation into neurotransmitter phenotypes in PC12 cells

Unrelated developmental neurotoxicants nevertheless converge on common functional and behavioral outcomes. We used PC12 cells, a model of neuronal development, to explore similarities and differences for organophosphate pesticides (chlorpyrifos, diazinon), an organochlorine pesticide (dieldrin) and a metal (Ni(2+)), focusing on transcriptional profiles related to differentiation into acetylcholine, dopamine and norepinephrine phenotypes. Agents were introduced at 30 microM for 24 or 72 h, treatments devoid of cytotoxicity. Using microarrays, we examined the mRNAs encoding the proteins involved in neurotransmitter biosynthesis, storage, and degradation, along with the complete panoply of receptors for each transmitter. All three pesticides evoked concordant patterns of effects on genes involved in neural growth and neurite extension, with a distinctly different pattern for Ni(2+). All four toxicants promoted differentiation into the dopamine phenotype at the expense of the acetylcholine phenotype, involving separable effects of each agent on the various gene families; however, there were major differences in the ability of each to promote or repress the norepinephrine phenotype. Chlorpyrifos and diazinon, although displaying many similarities in their transcriptional profiles, also showed major disparities in keeping with their known differences in synaptic and behavioral outcomes after neonatal exposures to these agents in vivo. Surprisingly, there were closer similarities among diazinon, dieldrin and Ni(2+) than for each agent to chlorpyrifos. Our results illustrate how cell culture systems, combined with microarray technology, can screen for developmental neurotoxicants, serving as a model for alternative approaches to the detection and characterization of the impact of exogenous chemicals on brain development.

Developmental neurotoxicity of perfluorinated chemicals modeled in vitro

BACKGROUND

The widespread detection of perfluoroalkyl acids and their derivatives in wildlife and humans, and their entry into the immature brain, raise increasing concern about whether these agents might be developmental neurotoxicants.

OBJECTIVES

We evaluated perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorooctane sulfonamide (PFOSA), and perfluorobutane sulfonate (PFBS) in undifferentiated and differentiating PC12 cells, a neuronotypic line used to characterize neurotoxicity.

METHODS

We assessed inhibition of DNA synthesis, deficits in cell numbers and growth, oxidative stress, reduced cell viability, and shifts in differentiation toward or away from the dopamine (DA) and acetylcholine (ACh) neurotransmitter phenotypes.

RESULTS

In general, the rank order of adverse effects was PFOSA > PFOS > PFBS approximately PFOA. However, superimposed on this scheme, the various agents differed in their underlying mechanisms and specific outcomes. Notably, PFOS promoted differentiation into the ACh phenotype at the expense of the DA phenotype, PFBS suppressed differentiation of both phenotypes, PFOSA enhanced differentiation of both, and PFOA had little or no effect on phenotypic specification.

CONCLUSIONS

These findings indicate that all perfluorinated chemicals are not the same in their impact on neurodevelopment and that it is unlikely that there is one simple, shared mechanism by which they all produce their effects. Our results reinforce the potential for in vitro models to aid in the rapid and cost-effective screening for comparative effects among different chemicals in the same class and in relation to known developmental neurotoxicants.

Developmental neurotoxicity testing in vitro: models for assessing chemical effects on neurite outgrowth

In vitro models may be useful for the rapid toxicological screening of large numbers of chemicals for their potential to produce toxicity. Such screening could facilitate prioritization of resources needed for in vivo toxicity testing towards those chemicals most likely to result in adverse health effects. Cell cultures derived from nervous system tissue have proven to be powerful tools for elucidating cellular and molecular mechanisms of nervous system development and function, and have been used to understand the mechanism of action of neurotoxic chemicals. Recently, it has been suggested that in vitro models could be used to screen for chemical effects on critical cellular events of neurodevelopment, including differentiation and neurite growth. This review examines the use of neuronal cell cultures as an in vitro model of neurite outgrowth. Examples of the cell culture systems that are commonly used to examine the effects of chemicals on neurite outgrowth are provided, along with a description of the methods used to quantify this neurodevelopmental process in vitro. Issues relating to the relevance of the methods and models currently used to assess neurite outgrowth are discussed in the context of hazard identification and chemical screening. To demonstrate the utility of in vitro models of neurite outgrowth for the evaluation of large numbers of chemicals, efforts should be made to: (1) develop a set of reference chemicals that can be used as positive and negative controls for comparing neurite outgrowth between model systems, (2) focus on cell cultures of human origin, with emphasis on the emerging area of neural progenitor cells, and (3) use high-throughput methods to quantify endpoints of neurite outgrowth.

The sea urchin embryo, an invertebrate model for mammalian developmental neurotoxicity, reveals multiple neurotransmitter mechanisms for effects of chlorpyrifos: therapeutic interventions and a comparison with the monoamine depleter, reserpine

Lower organisms show promise for the screening of neurotoxicants that might target mammalian brain development. Sea urchins use neurotransmitters as embryonic growth regulatory signals, so that adverse effects on neural substrates for mammalian brain development can be studied in this simple organism. We compared the effects of the organophosphate insecticide, chlorpyrifos in sea urchin embryos with those of the monoamine depleter, reserpine, so as to investigate multiple neurotransmitter mechanisms involved in developmental toxicity and to evaluate different therapeutic interventions corresponding to each neurotransmitter system. Whereas reserpine interfered with all stages of embryonic development, the effects of chlorpyrifos did not emerge until the mid-blastula stage. After that point, the effects of the two agents were similar. Treatment with membrane permeable analogs of the monoamine neurotransmitters, serotonin and dopamine, prevented the adverse effects of either chlorpyrifos or reserpine, despite the fact that chlorpyrifos works simultaneously through actions on acetylcholine, monoamines and other neurotransmitter pathways. This suggests that different neurotransmitters, converging on the same downstream signaling events, could work together or in parallel to offset the developmental disruption caused by exposure to disparate agents. We tested this hypothesis by evaluating membrane permeable analogs of acetylcholine and cannabinoids, both of which proved effective against chlorpyrifos- or reserpine-induced teratogenesis. Invertebrate test systems can provide both a screening procedure for mammalian neuroteratogenesis and may uncover novel mechanisms underlying developmental vulnerability as well as possible therapeutic approaches to prevent teratogenesis.

Tiered toxicity testing: evaluation of toxicity-based decision triggers for human health hazard characterization

A set of biologically-based toxicity testing decision triggers was developed and analyzed within a tiered testing and decision-making framework for evaluating potential human health hazards and risks associated with chemical exposures. The proposed three-tiered toxicity testing approach starts from a base set of toxicity studies (acute toxicity, in vitro genetic toxicity, in vitro cytogenetics, repeat dose/subchronic toxicity, developmental toxicity, reproductive toxicity) and then uses the toxicity triggers to identify which specific additional tests are needed to adequately characterize a substance's hazard potential. The toxicity triggers were initially evaluated using published information for eight chemicals, representing diverse classes. A retrospective validation study was then conducted using seven chemicals which had completed the USEPA's Voluntary Children's Chemical Evaluation Program (VCCEP). The toxicity triggers were shown to identify appropriate higher tier tests and to be reasonably predictive of the results expected in higher tiered tests. Employing these toxicity triggers within a tiered testing framework could lead to a reduction in the number of laboratory animals without diminishing the degree of scientific certainty necessary for hazard evaluations. The toxicity triggers appear to be suitable for identifying which specific endpoints and tests warrant further evaluation, and which do not, and for documenting the scientific basis for such decisions.

Screening for developmental neurotoxicity using PC12 cells: comparisons of organophosphates with a carbamate, an organochlorine, and divalent nickel

BACKGROUND

In light of the large number of chemicals that are potential developmental neurotoxicants, there is a need to develop rapid screening techniques.

OBJECTIVES

We exposed undifferentiated and differentiating neuronotypic PC12 cells to different organophosphates (chlorpyrifos, diazinon, parathion), a carbamate (physostigmine), an organochlorine (dieldrin), and a metal (divalent nickel; Ni2+) and examined indices of cell replication and differentiation for both short- and long-term exposures.

RESULTS

In undifferentiated cells, all the agents inhibited DNA synthesis, with the greatest effect for diazinon, but physostigmine eventually produced the largest deficits in the total number of cells after prolonged exposure. The onset of differentiation intensified the adverse effects on DNA synthesis and changed the rank order in keeping with a shift away from noncholinergic mechanisms and toward cholinergic mechanisms. Differentiation also worsened the effects of each agent on cell number after prolonged exposure, whereas cell growth was not suppressed, nor were there any effects on viability as assessed with trypan blue. Nevertheless, differentiating cells displayed signs of oxidative stress from all of the test compounds except Ni2+, as evidenced by measurements of lipid peroxidation. Finally, all of the toxicants shifted the transmitter fate of the cells away from the cholinergic phenotype and toward the catecholaminergic phenotype.

CONCLUSIONS

These studies point out the feasibility of developing cell-based screening methods that enable the detection of multiple end points that may relate to mechanisms associated with developmental neurotoxicity, revealing some common targets for disparate agents.

Evaluation of a proposed in vitro test strategy using neuronal and non-neuronal cell systems for detecting neurotoxicity

The European Commission White Paper, "Strategy for a future chemicals policy" (EC, 2001) is estimated to require the testing of approximately 30,000 "existing" chemicals by 2012. Recommended in vitro tests require validation. As the White Paper (EC, 2001) requires neurotoxic data, this study evaluated an in vitro testing strategy for predicting in vivo neurotoxicity. The sensitivities of differentiated PC12 cells and primary cerebellum granule cells (CGC) were compared to undifferentiated PC12 cells which can indicate basal cytotoxicity. Cytotoxicants and neurotoxicants selected for testing covered a range of mechanisms and potencies. Neurotoxicants were not distinguished from cytotoxicants despite significantly different cell system responses using all endpoints; cell viability/activity, ATP depletion, MMP depolarisation, ROS production and cytoskeleton modifications. For all chemicals tested, neuronal-like cell systems were generally less sensitive than undifferentiated PC12 cells. Acute oral rodent LD(50) values correlated with cytotoxicity IC(50) values for the respective chemicals tested in each cell system. This study concluded that although simple non-specific assays are required to distinguish basal cytotoxicity from specific neurotoxicity by using different cell systems with different states of neuronal differentiation, further work is required to determine suitable combinations of cell systems and endpoints capable of distinguishing neurotoxicants from cytotoxicants.

Arsenic induced changes in growth development and apoptosis in neonatal and adult brain cells in vivo and in tissue culture

Arsenic at a nonlethal level in drinking water consumed over a period of time has been reported to produce chronic toxicity and various types of health problems ranging from skin cancer to disturbance in memory. Neurotoxic effects have been reported in clinical cases with chronic exposure to arsenic. Physiological detoxication of arsenic occurs partially through methylation. Arsenic and its methylated derivatives are distributed in different organs and systems. The present study examined the possible interference in the neuronal development and differentiation due to the exposure to arsenic during gestation. The experiments were carried out to examine short and long term effects of arsenic on brain explants and cells grown and maintained in tissue culture system. The effects of arsenic exposure showed changes in brain cell membrane function indicated by generation and release of reactive oxygen-nitrogen intermediates. On the morphological aspect the explants' growth was reduced, ground matrix was lost and neural networking was inhibited. Cells showed signs of apoptotic changes. Arsenic toxicity may induce damage to brain cells prior to more visible clinical conditions. The deleterious effects also pass from the maternal to fetal tissue across the transplacental barrier.

Biochemical and molecular neurotoxicology: relevance to biomarker development, neurotoxicity testing and risk assessment

Biochemical and molecular approaches are most useful to define potential mechanisms of neurotoxicity. Information on the mechanisms of action of neurotoxicants can play a key role in neurotoxicology by allowing, among others, the development of potential biomarkers of effect, the refinement of in vitro testing procedures, and the improvement of the risk assessment process. An important class of insecticides, the organophosphates, are discussed as an example of how knowledge of molecular mechanisms is useful in various aspects of neurotoxicology. The utilization of such information in the area of biomarkers of exposure and effects, and of in vitro testing is presented. Additionally, mechanistic issues related to genetic polymorphisms and risk assessment are discussed.