Targeting of neurotrophic factors, their receptors, and signaling pathways in the developmental neurotoxicity of organophosphates in vivo and in vitro

Assessment of neurotransmitter release in human iPSC-derived neuronal/glial cells: a missing in vitro assay for regulatory developmental neurotoxicity testing

Human induced pluripotent stem cell (hiPSC)-derived neural stem cells (NSCs) and their differentiated neuronal/glial derivatives have been recently considered suitable to assess in vitro developmental neurotoxicity (DNT) triggered by exposure to environmental chemicals. The use of human-relevant test systems combined with in vitro assays specific for different neurodevelopmental events, enables a mechanistic understanding of the possible impact of environmental chemicals on the developing brain, avoiding extrapolation uncertainties associated with in vivo studies. Currently proposed in vitro battery for regulatory DNT testing accounts for several assays suitable to study key neurodevelopmental processes, including NSC proliferation and apoptosis, differentiation into neurons and glia, neuronal migration, synaptogenesis, and neuronal network formation. However, assays suitable to measure interference of compounds with neurotransmitter release or clearance are at present not included, which represents a clear gap of the biological applicability domain of such a testing battery. Here we applied a HPLC-based methodology to measure the release of neurotransmitters in a previously characterized hiPSC-derived NSC model undergoing differentiation towards neurons and glia. Glutamate release was assessed in control cultures and upon depolarization, as well as in cultures repeatedly exposed to some known neurotoxicants (BDE47 and lead) and chemical mixtures. Obtained data indicate that these cells have the ability to release glutamate in a vesicular manner, and that both glutamate clearance and vesicular release concur in the maintenance of extracellular glutamate levels. In conclusion, analysis of neurotransmitter release is a sensitive readout that should be included in the envisioned battery of in vitro assays for DNT testing.

Prediction of in vivo prenatal chlorpyrifos exposure leading to developmental neurotoxicity in humans based on in vitro toxicity data by quantitative in vitro-in vivo extrapolation

Introduction: Epidemiological studies in children suggested that in utero exposure to chlorpyrifos (CPF), an organophosphate insecticide, may cause developmental neurotoxicity (DNT). We applied quantitative in vitro-in vivo extrapolation (QIVIVE) based on in vitro concentration and non-choline esterase-dependent effects data combined with Benchmark dose (BMD) modelling to predict oral maternal CPF exposure during pregnancy leading to fetal brain effect concentration. By comparing the results with data from epidemiological studies, we evaluated the contribution of the in vitro endpoints to the mode of action (MoA) for CPF-induced DNT. Methods: A maternal-fetal PBK model built in PK-Sim^® was used to perform QIVIVE predicting CPF concentrations in a pregnant women population at 15 weeks of gestation from cell lysate concentrations obtained in human induced pluripotent stem cell-derived neural stem cells undergoing differentiation towards neurons and glia exposed to CPF for 14 days. The in vitro concentration and effect data were used to perform BMD modelling. Results: The upper BMD was converted into maternal doses which ranged from 3.21 to 271 mg/kg bw/day. Maternal CPF blood levels from epidemiological studies reporting DNT findings in their children were used to estimate oral CPF exposure during pregnancy using the PBK model. It ranged from 0.11 to 140 μg/kg bw/day. Discussion: The effective daily intake doses predicted from the in vitro model were several orders of magnitude higher than exposures estimated from epidemiological studies to induce developmental non-cholinergic neurotoxic responses, which were captured by the analyzed in vitro test battery. These were also higher than the in vivo LOEC for cholinergic effects. Therefore, the quantitative predictive value of the investigated non-choline esterase-dependent effects, although possibly relevant for other chemicals, may not adequately represent potential key events in the MoA for CPF-associated DNT.

Modulation of CREB and its associated upstream signaling pathways in pesticide-induced neurotoxicity

Human beings are exposed to various environmental xenobiotics throughout their life consisting of a broad range of physical and chemical agents that impart bodily harm. Among these, pesticide exposure that destroys insects mainly by damaging their central nervous system also exerts neurotoxic effects on humans and is implicated in the etiology of several degenerative disorders. The connectivity between CREB (cAMP Response Element Binding Protein) signaling activation and neuronal activity is of broad interest and has been thoroughly studied in various diseased states. Several genes, as well as protein kinases, are involved in the phosphorylation of CREB, including BDNF (Brain-derived neurotrophic factor), Pi3K (phosphoinositide 3-kinase), AKT (Protein kinase B), RAS (Rat Sarcoma), MEK (Mitogen-activated protein kinase), PLC (Phospholipase C), and PKC (Protein kinase C) that play an essential role in neuronal plasticity, long-term potentiation, neuronal survival, learning, and memory formation, cognitive function, synaptic transmission, and suppressing apoptosis. These elements, either singularly or in a cascade, can result in the modulation of CREB, making it a vulnerable target for various neurotoxic agents, including pesticides. This review provides insight into how these various intracellular signaling pathways converge to bring about CREB activation and how the activated or deactivated CREB levels can affect the gene expression of the upstream molecules. We also discuss the various target genes within the cascade vulnerable to different types of pesticides. Thus, this review will facilitate future investigations associated with pesticide neurotoxicity and identify valuable therapeutic targets.

Upscaling biological complexity to boost neuronal and oligodendroglia maturation and improve in vitro developmental neurotoxicity (DNT) evaluation

Human induced pluripotent stem cell (iPSC)-derived neuronal and glial cell models are suitable to assess the effects of environmental chemicals on the developing brain. Such test systems can recapitulate several key neurodevelopmental features, such as neural stem cell formation and differentiation towards different neuronal subtypes and astrocytes, neurite outgrowth, synapse formation and neuronal network formation and function, which are crucial for brain development. While monolayer, two-dimensional (2D) cultures of human iPSC-neuronal or glial derivatives are generally suited for high-throughput testing, they also show some limitations. In particular, differentiation towards myelinating oligodendrocytes can only be achieved after extended periods in differentiation. In recent years, the implementation of three-dimensional (3D) neuronal and glial models obtained from human iPSCs has been shown to compensate for such limitations, enabling robust differentiation towards both neuronal and glial cell populations, myelination and formation of more mature neuronal network activity. Here we compared the differentiation capacity of human iPSC-derived neural stem cells cultured either as 2D monolayer or as 3D neurospheres, and assessed chlorpyrifos (CPF) effects. Data indicate that 3D neurospheres differentiate towards neurons and oligodendroglia more rapidly than 2D cultures; however, the 2D model is more suitable to assess neuronal functionality by analysis of spontaneous electrical activity using multielectrode array. Moreover, 2D and 3D test systems are diversely susceptible to CPF treatment. In conclusion, the selection of the most suitable in vitro test system (either 2D or 3D) should take into account the context of use and intended research goals ('fit for purpose' principle).

Combining in vitro assays and mathematical modelling to study developmental neurotoxicity induced by chemical mixtures

Prenatal and postnatal co-exposure to multiple chemicals at the same time may have deleterious effects on the developing nervous system. We previously showed that chemicals acting through similar mode of action (MoA) and grouped based on perturbation of brain derived neurotrophic factor (BDNF), induced greater neurotoxic effects on human induced pluripotent stem cell (hiPSC)-derived neurons and astrocytes compared to chemicals with dissimilar MoA. Here we assessed the effects of repeated dose (14 days) treatments with mixtures containing the six chemicals tested in our previous study (Bisphenol A, Chlorpyrifos, Lead(II) chloride, Methylmercury chloride, PCB138 and Valproic acid) along with 2,2'4,4'-tetrabromodiphenyl ether (BDE47), Ethanol, Vinclozolin and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)), on hiPSC-derived neural stem cells undergoing differentiation toward mixed neurons/astrocytes up to 21 days. Similar MoA chemicals in mixtures caused an increase of BDNF levels and neurite outgrowth, and a decrease of synapse formation, which led to inhibition of electrical activity. Perturbations of these endpoints are described as common key events in adverse outcome pathways (AOPs) specific for DNT. When compared with mixtures tested in our previous study, adding similarly acting chemicals (BDE47 and EtOH) to the mixture resulted in a stronger downregulation of synapses. A synergistic effect on some synaptogenesis-related features (PSD95 in particular) was hypothesized upon treatment with tested mixtures, as indicated by mathematical modelling. Our findings confirm that the use of human iPSC-derived mixed neuronal/glial models applied to a battery of in vitro assays anchored to key events in DNT AOP networks, combined with mathematical modelling, is a suitable testing strategy to assess in vitro DNT induced by chemical mixtures.

The effects of cocaine exposure in adolescence: Behavioural effects and neuroplastic mechanisms in experimental models

Drug addiction is a devastating disorder with a huge economic and social burden for modern society. Although an individual may slip into drug abuse throughout his/her life, adolescents are at higher risk, but, so far, only a few studies have attempted to elucidate the underlying cellular and molecular bases of such vulnerability. Indeed, preclinical evidence indicates that psychostimulants and adolescence interact and contribute to promoting a dysfunctional brain. In this review, we have focused our attention primarily on changes in neuroplasticity brought about by cocaine, taking into account that there is much less evidence from exposure to cocaine in adolescence, compared with that from adults. This review clearly shows that exposure to cocaine during adolescence, acute or chronic, as well as contingent or non‐contingent, confers a vulnerable endophenotype, primarily, by causing changes in neuroplasticity. Given the close relationship between drug abuse and psychiatric disorders, we also discuss the translational implications providing an interpretative framework for clinical studies involving addictive as well as affective or psychotic behaviours.

Chlorpyrifos induces cell proliferation in MCF-7 and MDA-MB-231 cells, through cholinergic and Wnt/β-catenin signaling disruption, AChE-R upregulation and oxidative stress generation after single and repeated treatment

Chlorpyrifos (CPF) biocide, is associated with breast cancer. The processes underlying this association have not been elucidated to date. CPF increases MCF-7 and MDA-MB-231 cell proliferation after acute and long-term treatment, partially through KIAA1363 overexpression and aryl-hydrocarbon receptor activation but also through estrogen receptor-alpha activation after 24 h exposure in MCF-7 cells, suggesting other mechanisms may be involved. CPF induces reactive oxygen species (ROS) generation, acetylcholine accumulation, and overexpression of acetylcholinesterase-R/S (AChE-R/S) variants, while it also alters the Wnt/β-catenin pathway, both in vitro and in vivo, in processes different from cancer. These latter mechanisms are also linked to cell proliferation and could mediate this effect induced by CPF. Our results show that CPF (0.01-100 μM), following one-day and fourteen-days treatment, respectively, induced ROS generation and lipid peroxidation, and acetylcholine accumulation due to AChE inhibition, Wnt/β-catenin up- or downregulation depending on the CPF treatment concentration, and AChE-R and AChE-S overexpression, with the latter being mediated through GSK-3β activity alteration. Finally, CPF promoted cell division through ACh and ROS accumulation, AChE-R overexpression, and Wnt/β-catenin signaling disruption. Our results provide novel information on the effect of CPF on human breast cancer cell lines that may help to explain its involvement in breast cancer.

Integrating biokinetics and in vitro studies to evaluate developmental neurotoxicity induced by chlorpyrifos in human iPSC-derived neural stem cells undergoing differentiation towards neuronal and glial cells

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Human iPSC-derived NSCs undergoing differentiation possess some metabolic competence.

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CPF entered the cells and was biotrasformed into its two main metabolites (CPFO and TCP).

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After repeated exposure, very limited bioaccumulation of CPF was observed.

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Treatment with CPF decreased neurite outgrowth, synapse number and electrical activity.

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Treatment with CPF increased BDNF levels and the percentage of astrocytes.

For some complex toxicological endpoints, chemical safety assessment has conventionally relied on animal testing. Apart from the ethical issues, also scientific considerations have been raised concerning the traditional approach, highlighting the importance for considering real life exposure scenario. Implementation of flexible testing strategies, integrating multiple sources of information, including in vitro reliable test methods and in vitro biokinetics, would enhance the relevance of the obtained results. Such an approach could be pivotal in the evaluation of developmental neurotoxicity (DNT), especially when applied to human cell-based models, mimicking key neurodevelopmental processes, relevant to human brain development.

Here, we integrated the kinetic behaviour with the toxicodynamic alterations of chlorpyrifos (CPF), such as in vitro endpoints specific for DNT evaluation, after repeated exposure during differentiation of human neural stem cells into a mixed culture of neurons and astrocytes. The upregulation of some cytochrome P450 and glutathione S-transferase genes during neuronal differentiation and the formation of the two major CPF metabolites (due to bioactivation and detoxification) supported the metabolic competence of the used in vitro model. The alterations in the number of synapses, neurite outgrowth, brain derived neurotrophic factor, the proportion of neurons and astrocytes, as well as spontaneous electrical activity correlated well with the CPF ability to enter the cells and be bioactivated to CPF-oxon. Overall, our results confirm that combining in vitro biokinetics and assays to evaluate effects on neurodevelopmental endpoints in human cells should be regarded as a key strategy for a quantitative characterization of DNT effects.

Red Beetroot Extract Abrogates Chlorpyrifos-Induced Cortical Damage in Rats

Organophosphorus insecticides including chlorpyrifos (CPF) are mainly used for agriculture, household, and military purposes; their application is associated with various adverse reactions in animals and humans. This study was conducted to evaluate the potential neuroprotective effect of red beetroot methanolic extract (RBR) against CPF-induced cortical damage. Twenty-eight adult male Wistar albino rats were divided into 4 groups (n = 7 in each group): the control group was administered physiological saline (0.9% NaCl), the CPF group was administered CPF (10 mg/kg), the RBR group was administered RBR (300 mg/kg), and the RBR+CPF group was treated with RBR (300 mg/kg) 1 hr before CPF (10 mg/kg) supplementation. All groups were treated for 28 days. Rats exposed to CPF exhibited a significant decrease in cortical acetylcholinesterase activity and brain-derived neurotrophic factor and a decrease in glial fibrillary acidic protein. CPF intoxication increased lipid peroxidation, inducible nitric oxide synthase expression, and nitric oxide production. This was accompanied by a decrease in glutathione content and in the activities of glutathione peroxidase, glutathione reductase, superoxide dismutase, and catalase in the cortical tissue. Additionally, CPF enhanced inflammatory response, indicated by increased levels and expression of interleukin-1β and tumor necrosis factor-α. CPF triggered neuronal apoptosis by upregulating Bax and caspase-3 and downregulating Bcl-2. However, RBR reversed the induced neuronal alterations following CPF intoxication. Our findings suggest that RBR can minimize and prevent CPF neurotoxicity through its antioxidant, anti-inflammatory, and antiapoptotic activities.

Assessment of developmental neurotoxicity induced by chemical mixtures using an adverse outcome pathway concept

BACKGROUND

In light of the vulnerability of the developing brain, mixture risk assessment (MRA) for the evaluation of developmental neurotoxicity (DNT) should be implemented, since infants and children are co-exposed to more than one chemical at a time. One possible approach to tackle MRA could be to cluster DNT chemicals in a mixture on the basis of their mode of action (MoA) into 'similar' and 'dissimilar', but still contributing to the same adverse outcome, and anchor DNT assays to common key events (CKEs) identified in DNT-specific adverse outcome pathways (AOPs). Moreover, the use of human in vitro models, such as induced pluripotent stem cell (hiPSC)-derived neuronal and glial cultures would enable mechanistic understanding of chemically-induced adverse effects, avoiding species extrapolation.

METHODS

HiPSC-derived neural progenitors differentiated into mixed cultures of neurons and astrocytes were used to assess the effects of acute (3 days) and repeated dose (14 days) treatments with single chemicals and in mixtures belonging to different classes (i.e., lead(II) chloride and methylmercury chloride (heavy metals), chlorpyrifos (pesticide), bisphenol A (organic compound and endocrine disrupter), valproic acid (drug), and PCB138 (persistent organic pollutant and endocrine disrupter), which are associated with cognitive deficits, including learning and memory impairment in children. Selected chemicals were grouped based on their mode of action (MoA) into 'similar' and 'dissimilar' MoA compounds and their effects on synaptogenesis, neurite outgrowth, and brain derived neurotrophic factor (BDNF) protein levels, identified as CKEs in currently available AOPs relevant to DNT, were evaluated by immunocytochemistry and high content imaging analysis.

RESULTS

Chemicals working through similar MoA (i.e., alterations of BDNF levels), at non-cytotoxic (IC20/100), very low toxic (IC5), or moderately toxic (IC20) concentrations, induce DNT effects in mixtures, as shown by increased number of neurons, impairment of neurite outgrowth and synaptogenesis (the most sensitive endpoint as confirmed by mathematical modelling) and increase of BDNF levels, to a certain extent reproducing autism-like cellular changes observed in the brain of autistic children.

CONCLUSIONS

Our findings suggest that the use of human iPSC-derived mixed neuronal/glial cultures applied to a battery of assays anchored to key events of an AOP network represents a valuable approach to identify mixtures of chemicals with potential to cause learning and memory impairment in children.

Developmental exposure to lead at environmentally relevant concentrations impaired neurobehavior and NMDAR-dependent BDNF signaling in zebrafish larvae

Lead (Pb) is one of the predominant heavy metals in e-waste recycling arears and recognized as a notorious environmental neurotoxic substance. However, whether Pb at environmentally relevant concentrations could cause neurobehavioral alteration and even what kind of signaling pathway Pb exposure would disrupt in zebrafish were not fully uncovered. In the present study, 6 h postfertilization (hpf) zebrafish embryos were exposed to Pb at the concentrations of 0, 5, 10, and 20 μg/L until 144 hpf. Then the neurobehavioral indicators including locomotor, turnings and social behaviors, and the expressions of selected genes concerning brain-derived neurotrophic factor (BDNF) signaling were investigated. The results showed that significant changes were obtained under 20 μg/L Pb exposure. The hypoactivity of zebrafish larvae in locomotor and turning behaviors was induced during the dark period, while hyperactivity was observed in a two-fish social assay during the light period. The significantly downregulation of genes encoding BDNF, its receptor TrkB, and N-methyl-D-aspartate glutamate receptor (NMDAR) suggested the involvement of NMDAR-dependent BDNF signaling pathway. Overall, our study demonstrated that developmental exposure to Pb at environmentally relevant concentrations caused obvious neurobehavioral impairment of zebrafish larvae by disrupting the NMDAR-dependent BDNF signaling, which could exert profound ecological consequences in the real environment.

Knockdown of Butyrylcholinesterase but Not Inhibition by Chlorpyrifos Alters Early Differentiation Mechanisms in Human Neural Stem Cells

Butyrylcholinesterase (BChE) is the evolutionary counterpart to acetylcholinesterase (AChE). Both are expressed early in nervous system development prior to cholinergic synapse formation. The organophosphate pesticide chlorpyrifos (CPF) primarily exerts toxicity through the inhibition of AChE, which results in excess cholinergic stimulation at the synapse. We hypothesized that the inhibition of AChE and BChE by CPF may impair early neurogenesis in neural stem cells (NSCs). To model neurodevelopment in vitro, we used human NSCs derived from induced pluripotent stem cells (iPSCs) with a focus on the initial differentiation mechanisms. Over the six days of NSC differentiation, the BChE activity and mRNA expression significantly increased, while the AChE activity and expression remained unchanged. The CPF treatment (10 μM) caused 82% and 92% inhibition of AChE and BChE, respectively. The CPF exposure had no effect on the cell viability or the expression of the differentiation markers HES5, DCX, or MAP2. However, the shRNA-knockdown of the BChE expression resulted in the decreased or delayed expression of the transcription factors HES5 and HES3. BChE may have a role in the differentiation of NSCs independent of, or in addition to, its enzymatic activity.

Effects of Chlorpyrifos or Methyl Parathion on Regional Cholinesterase Activity and Muscarinic Receptor Subtype Binding in Juvenile Rat Brain

The effects of developmental exposure to two organophosphorus (OP) insecticides, chlorpyrifos (CPF) and methyl parathion (MPS), on cholinesterase (ChE) activity and muscarinic acetylcholine receptor (mAChR) binding were investigated in preweanling rat brain. Animals were orally gavaged daily with low, medium, and high dosages of the insecticides using an incremental dosing regimen from postnatal day 1 (PND1) to PND20. On PND12, PND17 and PND20, the cerebral cortex, corpus striatum, hippocampus, and medulla-pons were collected for determination of ChE activity, total mAChR density, and the density of the individual mAChR subtypes. ChE activity was inhibited by the medium and high dosages of CPF and MPS at equal levels in all four brain regions at all three ages examined. Exposure to both compounds decreased the levels of the M1, M2/M4, and M3 subtypes and the total mAChR level in all brain regions, but the effects varied by dosage group and brain region. On PND12, only the high dosages induced receptor changes while on PND17 and PND20, greater effects became evident. In general, the effects on the M1 subtype and total receptor levels appeared to be greater in the cerebral cortex and hippocampus than in the corpus striatum and medulla-pons. This did not appear to be the case for the M2/M4 and M3 subtypes effects. The differences between CPF and MPS were minimal even though in some cases, CPF exerted statistically greater effects than MPS did. In general, repeated exposure to organophosphorus insecticides can alter the levels of the various mAChR subtypes in various brain regions which could induce perturbation in cholinergic neurochemistry during the maturation of the brain regions.

Developmental neurotoxicity of succeeding generations of insecticides

Insecticides are by design toxic. They must be toxic to effectively kill target species of insects. Unfortunately, they also have off-target toxic effects that can harm other species, including humans. Developmental neurotoxicity is one of the most prominent off-target toxic risks of insecticides. Over the past seven decades several classes of insecticides have been developed, each with their own mechanisms of effect and toxic side effects. This review covers the developmental neurotoxicity of the succeeding generations of insecticides including organochlorines, organophosphates, pyrethroids, carbamates and neonicotinoids. The goal of new insecticide development is to more effectively kill target species with fewer toxic side effects on non-target species. From the experience with the developmental neurotoxicity caused by the generations of insecticides developed in the past advice is offered how to proceed with future insecticide development to decrease neurotoxic risk.

Targeting neurotrophic factors and their receptors, but not cholinesterase or neurotransmitter, in the neurotoxicity of TDCPP in Chinese rare minnow adults (Gobiocypris rarus)

Organophosphate flame retardants (OPFRs) have been detected at high concentrations in various environmental and biotic samples, but little is known about their toxicity. In this study, the potential neurotoxicity of three OPFRs (TCEP, TDCPP, and TPP) and Chlorpyrifos (CPF, an organophosphate pesticide) were compared in Chinese rare minnow using an acute toxicity test and a 21-day fish assay. The acute test demonstrated significant inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) by CPF. Although significant AChE inhibition at high concentration of TPP was also observed, none of the OPFRs had effects similar to CPF on these enzymes, indicating that their acute toxicities to Chinese rare minnow may be unrelated to cholinesterase inhibition. In addition, the 21-day fish assay with TDCPP demonstrated no significant effects on cholinesterase activities or neurotransmitter levels. Nonetheless, this OPFR exhibited widespread effects on the neurotrophic factors and their receptors (e.g., ntf3, ntrk1, ntrk2, ngfr, and fgf2, fgf11, fgf22, fgfr4), indicating that TDCPP or other OPFRs may elicit neurological effects by targeting neurotrophic factors and their receptors in Chinese rare minnow.

A single exposure to cocaine during development elicits regionally-selective changes in basal basic Fibroblast Growth Factor (FGF-2) gene expression and alters the trophic response to a second injection

RATIONALE

During adolescence, the brain is maturing and more sensitive to drugs of abuse that can influence its developmental trajectory. Recently, attention has been focused on basic fibroblast growth factor (FGF-2) given that its administration early in life enhances the acquisition of cocaine self-administration and sensitization at adulthood (Turner et al. (Pharmacol Biochem Behav 92:100-4, 2009), Clinton et al. (Pharmacol Biochem Behav103:6-17, 2012)). Additionally, we found that abstinence from adolescent cocaine exposure long lastingly dysregulates FGF-2 transcription (Giannotti et al. (Psychopharmacology (Berl) 225:553-60, 2013 ).

OBJECTIVES

The objectives of the study are to evaluate if (1) a single injection of cocaine (20 mg/kg) at postnatal day 35 alters FGF-2 messenger RNA (mRNA) levels and (2) the first injection influences the trophic response to a second injection (10 mg/kg) provided 24 h or 7 days later.

RESULTS

We found regional differences in the FGF-2 expression pattern as either the first or the second injection of cocaine by themselves upregulated FGF-2 mRNA in the medial prefrontal cortex and nucleus accumbens while downregulating it in the hippocampus. The first injection influences the trophic response of the second. Of note, 24 h after the first injection, accumbal and hippocampal FGF-2 changes produced by cocaine in saline-pretreated rats were prevented in cocaine-pretreated rats. Conversely, in the medial prefrontal cortex and hippocampus 7 days after the first injection, the cocaine-induced FGF-2 changes were modified by the subsequent exposure to the psychostimulant.

CONCLUSIONS

These findings show that a single cocaine injection is sufficient to produce enduring changes in the adolescent brain and indicate that early cocaine priming alters the mechanisms regulating the trophic response in a brain region-specific fashion.

Developmental exposure to organophosphate flame retardants elicits overt toxicity and alters behavior in early life stage zebrafish (Danio rerio)

Organophosphate flame retardants (OPFRs) are common replacements for the phased-out polybrominated diphenyl ethers (PBDEs) and have been detected at high concentrations in environmental samples. OPFRs are structurally similar to organophosphate pesticides and may adversely affect the developing nervous system. This study evaluated the overt toxicity, uptake, and neurobehavioral effects of tris (1,3-dichloro-2-propyl) phosphate (TDCPP), tris (2-chloroethyl) phosphate (TCEP), tris (1-chloro-2-propyl) phosphate (TCPP), and tris (2,3-dibromopropyl) phosphate (TDBPP) in early life stage zebrafish. Chlorpyrifos was used as a positive control. For overt toxicity and neurobehavioral assessments, zebrafish were exposed from 0 to 5 days postfertilization (dpf). Hatching, death, or malformations were evaluated daily. Teratogenic effects were scored by visual examination on 6 dpf. To evaluate uptake and metabolism, zebrafish were exposed to 1 µM of each organophosphate (OP) flame retardant and collected on 1 and 5 dpf to monitor accumulation. Larval swimming activity was measured in 6 dpf larvae to evaluate neurobehavioral effects of exposures below the acute toxicity threshold. TDBPP elicited the greatest toxicity at >1 µM. TDCPP and chlorpyrifos were overtly toxic at concentrations ≥10 µM, TCEP, and TCPP were not overtly toxic at the doses tested. Tissue concentrations increased with increasing hydrophobicity of the parent chemical after 24 h exposures. TDCPP and TDBPP and their respective metabolites were detected in embryos on 5 dpf. For all chemicals tested, developmental exposures that were not overtly toxic significantly altered larval swimming activity. These data indicate that OPFRs adversely affect development of early life stage zebrafish.

Repeated gestational exposure of mice to chlorpyrifos oxon is associated with paraoxonase 1 (PON1) modulated effects in maternal and fetal tissues

Chlorpyrifos oxon (CPO), the toxic metabolite of the organophosphorus (OP) insecticide chlorpyrifos, causes developmental neurotoxicity in humans and rodents. CPO is hydrolyzed by paraoxonase-1 (PON1), with protection determined by PON1 levels and the human Q192R polymorphism. To examine how the Q192R polymorphism influences fetal toxicity associated with gestational CPO exposure, we measured enzyme inhibition and fetal-brain gene expression in wild-type (PON1(+/+)), PON1-knockout (PON1(-/-)), and tgHuPON1R192 and tgHuPON1Q192 transgenic mice. Pregnant mice exposed dermally to 0, 0.50, 0.75, or 0.85 mg/kg/d CPO from gestational day (GD) 6 through 17 were sacrificed on GD18. Biomarkers of CPO exposure inhibited in maternal tissues included brain acetylcholinesterase (AChE), red blood cell acylpeptide hydrolase (APH), and plasma butyrylcholinesterase (BChE) and carboxylesterase (CES). Fetal plasma BChE was inhibited in PON1(-/-) and tgHuPON1Q192, but not PON1(+/+) or tgHuPON1R192 mice. Fetal brain AChE and plasma CES were inhibited in PON1(-/-) mice, but not in other genotypes. Weighted gene co-expression network analysis identified five gene modules based on clustering of the correlations among their fetal-brain expression values, allowing for correlation of module membership with the phenotypic data on enzyme inhibition. One module that correlated highly with maternal brain AChE activity had a large representation of homeobox genes. Gene set enrichment analysis revealed multiple gene sets affected by gestational CPO exposure in tgHuPON1Q192 but not tgHuPON1R192 mice, including gene sets involved in protein export, lipid metabolism, and neurotransmission. These data indicate that maternal PON1 status modulates the effects of repeated gestational CPO exposure on fetal-brain gene expression and on inhibition of both maternal and fetal biomarker enzymes.

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.

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.

Analysis of the fibroblast growth factor system reveals alterations in a mouse model of spinal muscular atrophy

The monogenetic disease Spinal Muscular Atrophy (SMA) is characterized by a progressive loss of motoneurons leading to muscle weakness and atrophy due to severe reduction of the Survival of Motoneuron (SMN) protein. Several models of SMA show deficits in neurite outgrowth and maintenance of neuromuscular junction (NMJ) structure. Survival of motoneurons, axonal outgrowth and formation of NMJ is controlled by neurotrophic factors such as the Fibroblast Growth Factor (FGF) system. Besides their classical role as extracellular ligands, some FGFs exert also intracellular functions controlling neuronal differentiation. We have previously shown that intracellular FGF-2 binds to SMN and regulates the number of a subtype of nuclear bodies which are reduced in SMA patients. In the light of these findings, we systematically analyzed the FGF-system comprising five canonical receptors and 22 ligands in a severe mouse model of SMA. In this study, we demonstrate widespread alterations of the FGF-system in both muscle and spinal cord. Importantly, FGF-receptor 1 is upregulated in spinal cord at a pre-symptomatic stage as well as in a mouse motoneuron-like cell-line NSC34 based model of SMA. Consistent with that, phosphorylations of FGFR-downstream targets Akt and ERK are increased. Moreover, ERK hyper-phosphorylation is functionally linked to FGFR-1 as revealed by receptor inhibition experiments. Our study shows that the FGF system is dysregulated at an early stage in SMA and may contribute to the SMA pathogenesis.

Developmental neurotoxicity of organophosphates targets cell cycle and apoptosis, revealed by transcriptional profiles in vivo and in vitro

Developmental organophosphate exposure reduces the numbers of neural cells, contributing to neurobehavioral deficits. We administered chlorpyrifos or diazinon to newborn rats on postnatal days 1-4, in doses straddling the threshold for barely-detectable cholinesterase inhibition, and evaluated gene expression in the cell cycle and apoptosis pathways on postnatal day 5. Both organophosphates evoked transcriptional changes in 20-25% of the genes in each category; chlorpyrifos and diazinon targeted the same genes, with similar magnitudes of change, as evidenced by high concordance. Furthermore, the same effects were obtained with doses above or below the threshold for cholinesterase inhibition, indicating a mechanism unrelated to anticholinesterase actions. We then evaluated the effects of chlorpyrifos in undifferentiated and differentiating PC12 cells and found even greater targeting of cell cycle and apoptosis genes, affecting up to 40% of all genes in the pathways. Notably, the genes affected in undifferentiated cells were not concordant with those in differentiating cells, pointing to dissimilar outcomes dependent on developmental stage. The in vitro model successfully identified 60-70% of the genes affected by chlorpyrifos in vivo, indicating that the effects are exerted directly on developing neural cells. Our results show that organophosphates target the genes regulating the cell cycle and apoptosis in the developing brain and in neuronotypic cells in culture, with the pattern of vulnerability dependent on the specific stage of development. Equally important, these effects do not reflect actions on cholinesterase and operate at exposures below the threshold for any detectable inhibition of this enzyme.

Developmental exposure to organophosphates triggers transcriptional changes in genes associated with Parkinson's disease in vitro and in vivo

Epidemiologic studies support a connection between organophosphate pesticide exposures and subsequent risk of Parkinson's disease (PD). We used differentiating, neuronotypic PC12 cells to compare organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)) for their effects on the transcription of PD-related genes. Both of the organophosphates elicited significant changes in gene expression but with differing patterns: chlorpyrifos evoked both up- and downregulation whereas diazinon elicited overall reductions in expression. Dieldrin was without effect but Ni(2+) produced a pattern resembling that of diazinon. We then exposed neonatal rats to chlorpyrifos or diazinon for the first 4 days after birth and examined the expression of PD-related genes in the brainstem and forebrain. Chlorpyrifos had no significant effect whereas diazinon produced significant increases and decreases in expression of the same PD genes that were targeted in vitro. Our results provide some of the first evidence for a mechanistic relationship between developmental organophosphate exposure and the genes known to confer PD risk in humans; but they also point to disparities between different organophosphates that reinforce the concept that their neurotoxic actions do not rest solely on their shared property as cholinesterase inhibitors. The parallel effects of diazinon and Ni(2+) also show how otherwise unrelated developmental neurotoxicants can nevertheless produce similar outcomes by converging on common molecular pathways, further suggesting a need to examine metals such as Ni(2+) as potential contributors to PD risk.

Novel avenues of drug discovery and biomarkers for diabetes mellitus

Globally, developed nations spend a significant amount of their resources on health care initiatives that poorly translate into increased population life expectancy. As an example, the United States devotes 16% of its gross domestic product to health care, the highest level in the world, but falls behind other nations that enjoy greater individual life expectancy. These observations point to the need for pioneering avenues of drug discovery to increase life span with controlled costs. In particular, innovative drug development for metabolic disorders such as diabetes mellitus becomes increasingly critical given that the number of diabetic people will increase exponentially over the next 20 years. This article discusses the elucidation and targeting of novel cellular pathways that are intimately tied to oxidative stress in diabetes mellitus for new treatment strategies. Pathways that involve wingless, β-nicotinamide adenine dinucleotide (NAD(+)) precursors, and cytokines govern complex biological pathways that determine both cell survival and longevity during diabetes mellitus and its complications. Furthermore, the role of these entities as biomarkers for disease can further enhance their utility irrespective of their treatment potential. Greater understanding of the intricacies of these unique cellular mechanisms will shape future drug discovery for diabetes mellitus to provide focused clinical care with limited or absent long-term complications.

Is the PentaBDE replacement, tris (1,3-dichloro-2-propyl) phosphate (TDCPP), a developmental neurotoxicant? Studies in PC12 cells

Organophosphate flame retardants (OPFRs) are used as replacements for the commercial PentaBDE mixture that was phased out in 2004. OPFRs are ubiquitous in the environment and detected at high concentrations in residential dust, suggesting widespread human exposure. OPFRs are structurally similar to neurotoxic organophosphate pesticides, raising concerns about exposure and toxicity to humans. This study evaluated the neurotoxicity of tris (1,3-dichloro-2-propyl) phosphate (TDCPP) compared to the organophosphate pesticide, chlorpyrifos (CPF), a known developmental neurotoxicant. We also tested the neurotoxicity of three structurally similar OPFRs, tris (2-chloroethyl) phosphate (TCEP), tris (1-chloropropyl) phosphate (TCPP), and tris (2,3-dibromopropyl) phosphate (TDBPP), and 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), a major component of PentaBDE. Using undifferentiated and differentiating PC12 cells, changes in DNA synthesis, oxidative stress, differentiation into dopaminergic or cholinergic neurophenotypes, cell number, cell growth and neurite growth were assessed. TDCPP displayed concentration-dependent neurotoxicity, often with effects equivalent to or greater than equimolar concentrations of CPF. TDCPP inhibited DNA synthesis, and all OPFRs decreased cell number and altered neurodifferentiation. Although TDCPP elevated oxidative stress, there was no adverse effect on cell viability or growth. TDCPP and TDBPP promoted differentiation into both neuronal phenotypes, while TCEP and TCPP promoted only the cholinergic phenotype. BDE-47 had no effect on cell number, cell growth or neurite growth. Our results demonstrate that different OPFRs show divergent effects on neurodifferentiation, suggesting the participation of multiple mechanisms of toxicity. Additionally, these data suggest that OPFRs may affect neurodevelopment with similar or greater potency compared to known and suspected neurotoxicants.

Pesticide use modifies the association between genetic variants on chromosome 8q24 and prostate cancer

Genome-wide association studies have identified 8q24 region variants as risk factors for prostate cancer. In the Agricultural Health Study, a prospective study of licensed pesticide applicators, we observed increased prostate cancer risk with specific pesticide use among those with a family history of prostate cancer. Thus, we evaluated the interaction among pesticide use, 8q24 variants, and prostate cancer risk. The authors estimated odds ratios (OR) and 95% confidence intervals (95% CI) for interactions among 211 8q24 variants, 49 pesticides, and prostate cancer risk in 776 cases and 1,444 controls. The ORs for a previously identified variant, rs4242382, and prostate cancer increased significantly (P<0.05) with exposure to the organophosphate insecticide fonofos, after correction for multiple testing, with per allele ORnonexposed of 1.17 (95% CI, 0.93-1.48), per allele OR(low) of 1.30 (95% CI, 0.75-2.27), and per allele ORhigh of 4.46 (95% CI, 2.17-9.17; P-interaction=0.002, adjusted P-interaction=0.02). A similar effect modification was observed for three other organophosphate insecticides (coumaphos, terbufos, and phorate) and one pyrethroid insecticide (permethrin). Among ever users of fonofos, subjects with three or four risk alleles at rs7837328 and rs4242382 had approximately three times the risk of prostate cancer (OR, 3.14; 95% CI, 1.41-7.00) compared with subjects who had zero risk alleles and never used fonofos. We observed a significant interaction among variants on chromosome 8q24, pesticide use, and risk of prostate cancer. Insecticides, particularly organophosphates, were the strongest modifiers of risk, although the biological mechanism is unclear. This is the first report of effect modification between 8q24 and an environmental exposure on prostate cancer risk.

Dose-related gene expression changes in forebrain following acute, low-level chlorpyrifos exposure in neonatal rats

Chlorpyrifos (CPF) is a widely used organophosphorus insecticide (OP) and putative developmental neurotoxicant in humans. The acute toxicity of CPF is elicited by acetylcholinesterase (AChE) inhibition. We characterized dose-related (0.1, 0.5, 1 and 2mg/kg) gene expression profiles and changes in cell signaling pathways 24h following acute CPF exposure in 7-day-old rats. Microarray experiments indicated that approximately 9% of the 44,000 genes were differentially expressed following either one of the four CPF dosages studied (546, 505, 522, and 3,066 genes with 0.1, 0.5, 1.0 and 2.0mg/kg CPF). Genes were grouped according to dose-related expression patterns using K-means clustering while gene networks and canonical pathways were evaluated using Ingenuity Pathway Analysis®. Twenty clusters were identified and differential expression of selected genes was verified by RT-PCR. The four largest clusters (each containing from 276 to 905 genes) constituted over 50% of all differentially expressed genes and exhibited up-regulation following exposure to the highest dosage (2mg/kg CPF). The total number of gene networks affected by CPF also rose sharply with the highest dosage of CPF (18, 16, 18 and 50 with 0.1, 0.5, 1 and 2mg/kg CPF). Forebrain cholinesterase (ChE) activity was significantly reduced (26%) only in the highest dosage group. Based on magnitude of dose-related changes in differentially expressed genes, relative numbers of gene clusters and signaling networks affected, and forebrain ChE inhibition only at 2mg/kg CPF, we focused subsequent analyses on this treatment group. Six canonical pathways were identified that were significantly affected by 2mg/kg CPF (MAPK, oxidative stress, NFΚB, mitochondrial dysfunction, arylhydrocarbon receptor and adrenergic receptor signaling). Evaluation of different cellular functions of the differentially expressed genes suggested changes related to olfactory receptors, cell adhesion/migration, synapse/synaptic transmission and transcription/translation. Nine genes were differentially affected in all four CPF dosing groups. We conclude that the most robust, consistent changes in differential gene expression in neonatal forebrain across a range of acute CPF dosages occurred at an exposure level associated with the classical marker of OP toxicity, AChE inhibition. Disruption of multiple cellular pathways, in particular cell adhesion, may contribute to the developmental neurotoxicity potential of this pesticide.

Genome-wide gene expression analysis in response to organophosphorus pesticide chlorpyrifos and diazinon in C. elegans

Organophosphorus pesticides (OPs) were originally designed to affect the nervous system by inhibiting the enzyme acetylcholinesterase, an important regulator of the neurotransmitter acetylcholine. Over the past years evidence is mounting that these compounds affect many other processes. Little is known, however, about gene expression responses against OPs in the nematode Caenorhabditis elegans. This is surprising because C. elegans is extensively used as a model species in toxicity studies. To address this question we performed a microarray study in C. elegans which was exposed for 72 hrs to two widely used Ops, chlorpyrifos and diazinon, and a low dose mixture of these two compounds. Our analysis revealed transcriptional responses related to detoxification, stress, innate immunity, and transport and metabolism of lipids in all treatments. We found that for both compounds as well as in the mixture, these processes were regulated by different gene transcripts. Our results illustrate intense, and unexpected crosstalk between gene pathways in response to chlorpyrifos and diazinon in C. elegans.

Diverse neurotoxicants converge on gene expression for neuropeptides and their receptors in an in vitro model of neurodifferentiation: effects of chlorpyrifos, diazinon, dieldrin and divalent nickel in PC12 cells

Unrelated developmental neurotoxicants can produce similar neurobehavioral outcomes. We examined whether disparate agents affect neuromodulators that control numerous neurotransmitters and circuits, employing PC12 cells to explore the targeting of neuroactive peptides by organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)); we utilized microarrays to profile gene expression for the peptides and their receptors. Chlorpyrifos evoked robust upregulation of cholecystokinin, corticotropin releasing hormone, galanin, neuropeptide Y, neurotensin, preproenkephalin and tachykinin 1; this involved a critical period at the commencement of neurodifferentiation, since the effects were much less notable in undifferentiated PC12 cells. Diazinon targeted a similar but smaller repertoire of neuropeptide genes and the magnitude of the effects was also generally less. Surprisingly, dieldrin shared many of the same neuropeptide targets as the organophosphates and concordance analysis showed significant overlap among all three pesticides. However, dieldrin had more notable effects on neuropeptide receptors, and overlap between diazinon and dieldrin for the receptors led to a stronger resemblance of these two agents than of chlorpyrifos and dieldrin. Ni(2+) was unique, evoking upregulation of only one of the peptides affected by the other agents, while causing downregulation of several others. Nevertheless, there was still significant concordance between Ni(2+) and either diazinon or dieldrin, reflecting similarities toward the receptors. Our results show that neuropeptides are likely to be a prominent target for the developmental neurotoxicity of organophosphates and other neurotoxicants, and further, that the convergence of disparate agents on the same genes and pathways may contribute to similar neurobehavioral outcomes.

Transcriptional profiles for glutamate transporters reveal differences between organophosphates but similarities with unrelated neurotoxicants

The developmental neurotoxicity of organophosphates involves mechanisms other than their shared property as cholinesterase inhibitors, among which are excitotoxicity and oxidative stress. We used PC12 cells as a neurodevelopmental model to compare the effects of chlorpyrifos and diazinon on the expression of genes encoding glutamate transporters. Chlorpyrifos had a greater effect in cells undergoing nerve growth factor-induced neurodifferentiation as compared to undifferentiated PC12 cells, with peak sensitivity at the initiation of differentiation, reflecting a global upregulation of all the glutamate transporter genes expressed in this cell line. In differentiating cells, chlorpyrifos had a significantly greater effect than did diazinon and concordance analysis indicated no resemblance in their expression patterns. At the same time, the smaller effects of diazinon were highly concordant with those of an organochlorine pesticide (dieldrin) and a metal (divalent nickel). We also performed similar evaluations for the cystine/glutamate exchanger, which provides protection against oxidative stress by moving cystine into the cell; again, chlorpyrifos had the greatest effect, in this case reducing expression in undifferentiated and differentiating cells. Our results point to excitotoxicity and oxidative stress as major contributors to the noncholinesterase mechanisms that distinguish the neurodevelopmental outcomes between different organophosphates while providing a means whereby apparently unrelated neurotoxicants may produce similar outcomes.

Diabetes mellitus: channeling care through cellular discovery

Diabetes mellitus (DM) impacts a significant portion of the world's population and care for this disorder places an economic burden on the gross domestic product for any particular country. Furthermore, both Type 1 and Type 2 DM are becoming increasingly prevalent and there is increased incidence of impaired glucose tolerance in the young. The complications of DM are protean and can involve multiple systems throughout the body that are susceptible to the detrimental effects of oxidative stress and apoptotic cell injury. For these reasons, innovative strategies are necessary for the implementation of new treatments for DM that are generated through the further understanding of cellular pathways that govern the pathological consequences of DM. In particular, both the precursor for the coenzyme beta-nicotinamide adenine dinucleotide (NAD(+)), nicotinamide, and the growth factor erythropoietin offer novel platforms for drug discovery that involve cellular metabolic homeostasis and inflammatory cell control. Interestingly, these agents and their tightly associated pathways that consist of cell cycle regulation, protein kinase B, forkhead transcription factors, and Wnt signaling also function in a broader sense as biomarkers for disease onset and progression.

Toxicogenomic profiling in maternal and fetal rodent brains following gestational exposure to chlorpyrifos

Considering the wide variety of effects that have been reported to occur in the developmental neurotoxicity of chlorpyrifos (CP) and the lack of consensus on their dependence of brain acetylcholinesterase (AChE) activity inhibition, we applied microarray technology to explore dose-dependent alterations in transcriptional response in the fetal and maternal C57BL/6 mouse brain after daily gestational exposure (days 6 to 17) to CP (2, 4, 10, 12 or 15 mg/kg, sc). We identified significantly altered genes across doses and assessed for overrepresentation of Gene Ontology (GO) biological processes and KEGG pathways. We further clustered genes based on their expression profiles across doses and repeated the GO/pathways analysis for each cluster. The dose-effect relationship of CP on gene expression, both at the gene and pathway levels was non-monotonic and not necessarily related to brain AChE inhibition. The largest impact was observed in the 10mg/kg dose group which was also the LOAEL for brain AChE inhibition. In the maternal brain, lower doses (4 mg/kg) influenced GO categories and pathways such as cell adhesion, behavior, lipid metabolism, long-term potentiation, nervous system development, neurogenesis, synaptic transmission. In the fetal brain, lower doses (2 and/or 4 mg/kg) significantly altered cell division, translation, transmission of nerve impulse, chromatin modification, long-term potentiation. In addition, some genes involved in nervous system development and signaling were shown to be specifically influenced by these lower CP doses. Our approach was sensitive and reflected the diversity of responses known to be disrupted by CP and highlighted possible additional consequences of CP neurotoxicity, such as disturbance of the ubiquitin proteasome system.

Oxidative stress: Biomarkers and novel therapeutic pathways

Oxidative stress significantly impacts multiple cellular pathways that can lead to the initiation and progression of varied disorders throughout the body. It therefore becomes imperative to elucidate the components and function of novel therapeutic strategies against oxidative stress to further clinical diagnosis and care. In particular, both the growth factor and cytokine erythropoietin (EPO) and members of the mammalian forkhead transcription factors of the O class (FoxOs) may offer the greatest promise for new treatment regimens since these agents and the cellular pathways they oversee cover a range of critical functions that directly influence progenitor cell development, cell survival and degeneration, metabolism, immune function, and cancer cell invasion. Furthermore, both EPO and FoxOs function not only as therapeutic targets, but also as biomarkers of disease onset and progression, since their cellular pathways are closely linked and overlap with several unique signal transduction pathways. However, biological outcome with EPO and FoxOs may sometimes be both unexpected and undesirable that can raise caution for these agents and warrant further investigations. Here we present the exciting as well as complicated role EPO and FoxOs possess to uncover the benefits as well as the risks of these agents for cell biology and clinical care in processes that range from stem cell development to uncontrolled cellular proliferation.

Childhood brain tumors, residential insecticide exposure, and pesticide metabolism genes

BACKGROUND

Insecticides that target the nervous system may play a role in the development of childhood brain tumors (CBTs). Constitutive genetic variation affects metabolism of these chemicals.

METHODS

We analyzed population-based case-control data to examine whether CBT is associated with the functional genetic polymorphisms PON1C-108T, PON1Q192R, PON1L55M, BCHEA539T, FMO1C-9536A, FMO3E158K, ALDH3A1S134A, and GSTT1 (null). DNA was obtained from newborn screening archives for 201 cases and 285 controls, <or= 10 years of age, and born in California or Washington State between 1978 and 1990. Conception-to-diagnosis home insecticide treatment history was ascertained by interview.

RESULTS

We observed no biologically plausible main effects for any of the metabolic polymorphisms with CBT risk. However, we observed strong interactions between genotype and insecticide exposure during childhood. Among exposed children, CBT risk increased per PON1-108T allele [odds ratio (OR) = 1.8; 95% confidence interval (CI), 1.1-3.0] and FMO1-9536A (*6) allele (OR = 2.7; 95% CI, 1.2-5.9), whereas among children never exposed, CBT risk was not increased (PON1: OR = 0.7; 95% CI, 0.5-1.0, interaction p = 0.005; FMO1: OR = 1.0; 95% CI, 0.6-1.6, interaction p = 0.009). We observed a similar but statistically nonsignificant interaction between childhood exposure and BCHEA539T (interaction p = 0.08). These interactions were present among both Hispanic and non-Hispanic white children.

CONCLUSION

Based on known effects of these variants, these results suggest that exposure in childhood to organophosphorus and perhaps to carbamate insecticides in combination with a reduced ability to detoxify them may be associated with CBT. Confirmation in other studies is required.

Disparate developmental neurotoxicants converge on the cyclic AMP signaling cascade, revealed by transcriptional profiles in vitro and in vivo

Cell-signaling cascades are convergent targets for developmental neurotoxicity of otherwise unrelated agents. We compared organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)) for their effects on neuronotypic PC12 cells, assessing gene transcription involved in the cyclic AMP pathway. Each agent was introduced during neurodifferentiation at a concentration of 30 microM for 24 or 72 h and we assessed 69 genes encoding adenylyl cyclase isoforms and regulators, G-protein alpha-and beta,gamma-subunits, protein kinase A subtypes and the phosphodiesterase family. We found strong concordance among the four agents across all the gene families, with the strongest relationships for the G-proteins, followed by adenylyl cyclase, and lesser concordance for protein kinase A and phosphodiesterase. Superimposed on this pattern, chlorpyrifos and diazinon were surprisingly the least alike, whereas there was strong concordance of dieldrin and Ni(2+) with each other and with each individual organophosphate. Further, the effects of chlorpyrifos differed substantially depending on whether cells were undifferentiated or differentiating. To resolve the disparities between chlorpyrifos and diazinon, we performed analyses in rat brain regions after in vivo neonatal exposures; unlike the in vitro results, there was strong concordance. Our results show that unrelated developmental neurotoxicants can nevertheless produce similar outcomes by targeting cell signaling pathways involved in neurodifferentiation during a critical developmental period of vulnerability. Nevertheless, a full evaluation of concordance between different toxicants requires evaluations of in vitro systems that detect direct effects, as well as in vivo systems that allow for more complex interactions that converge on the same pathway.

Oxidative stress from diverse developmental neurotoxicants: antioxidants protect against lipid peroxidation without preventing cell loss

Oxidative stress has been hypothesized to provide a mechanism by which apparently unrelated chemicals can nevertheless produce similar developmental neurotoxic outcomes. We used differentiating PC12 cells to compare the effects of agents from four different classes and then to evaluate antioxidant amelioration: fipronil, perfluorooctanesulfonamide (PFOSA), dieldrin and chlorpyrifos. The rank order for lipid peroxidation corresponded to the ability to evoke cell loss: fipronil>PFOSA>dieldrin>chlorpyrifos. The same sequence was found for an index of cell enlargement (protein/DNA ratio) but the effects on neurite outgrowth (membrane/total protein) diverged, with fipronil producing a decrease and PFOSA an increase. Cotreatment with antioxidants reduced (ascorbate) or eliminated (Vitamin E) lipid peroxidation caused by each of the agents but failed to protect against cell loss, with the sole exception of chlorpyrifos, for which we earlier showed partial protection by Vitamin E; addition of higher NGF concentrations protected neither against oxidative stress nor cell loss. Despite the failure to prevent cell loss, ascorbate protected the cells from the effects of PFOSA on neuritic outgrowth; NGF, and to a lesser extent, ascorbate, offset the effects of fipronil on both cell enlargement and neuritogenesis. At the same time, the ameliorant treatments also worsened some of the other toxicant effects. Our results point out the problems in concluding that, just because a neurotoxicant produces oxidative stress, antioxidant therapy will be effective in preventing damage. Instead, additional mechanisms for each agent may provide alternative routes to neurotoxicity, or may be additive or synergistic with oxidative stress.

New strategies for Alzheimer's disease and cognitive impairment

Approximately five million people suffer with Alzheimer's disease (AD) and more than twenty-four million people are diagnosed with AD, pre-senile dementia, and other disorders of cognitive loss worldwide. Furthermore, the annual cost per patient with AD can approach $200,000 with an annual population aggregate cost of $100 billion. Yet, complete therapeutic prevention or reversal of neurovascular injury during AD and cognitive loss is not achievable despite the current understanding of the cellular pathways that modulate nervous system injury during these disorders. As a result, identification of novel therapeutic targets for the treatment of neurovascular injury would be extremely beneficial to reduce or eliminate disability from diseases that lead to cognitive loss or impairment. Here we describe the capacity of intrinsic cellular mechanisms for the novel pathways of erythropoietin and forkhead transcription factors that may offer not only new strategies for disorders such as AD and cognitive loss, but also function as biomarkers for disease onset and progression.

Erythropoietin, forkhead proteins, and oxidative injury: biomarkers and biology

Oxidative stress significantly impacts multiple cellular pathways that can lead to the initiation and progression of varied disorders throughout the body. It therefore becomes imperative to elucidate the components and function of novel therapeutic strategies against oxidative stress to further clinical diagnosis and care. In particular, both the growth factor and cytokine erythropoietin (EPO), and members of the mammalian forkhead transcription factors of the O class (FoxOs), may offer the greatest promise for new treatment regimens, since these agents and the cellular pathways they oversee cover a range of critical functions that directly influence progenitor cell development, cell survival and degeneration, metabolism, immune function, and cancer cell invasion. Furthermore, both EPO and FoxOs function not only as therapeutic targets, but also as biomarkers of disease onset and progression, since their cellular pathways are closely linked and overlap with several unique signal transduction pathways. Yet, EPO and FoxOs may sometimes have unexpected and undesirable effects that can raise caution for these agents and warrant further investigations. Here we present the exciting as well as the complex role that EPO and FoxOs possess to uncover the benefits as well as the risks of these agents for cell biology and clinical care in processes that range from stem cell development to uncontrolled cellular proliferation.

The vitamin nicotinamide: translating nutrition into clinical care

Nicotinamide, the amide form of vitamin B(3) (niacin), is changed to its mononucleotide compound with the enzyme nicotinic acide/nicotinamide adenylyltransferase, and participates in the cellular energy metabolism that directly impacts normal physiology. However, nicotinamide also influences oxidative stress and modulates multiple pathways tied to both cellular survival and death. During disorders that include immune system dysfunction, diabetes, and aging-related diseases, nicotinamide is a robust cytoprotectant that blocks cellular inflammatory cell activation, early apoptotic phosphatidylserine exposure, and late nuclear DNA degradation. Nicotinamide relies upon unique cellular pathways that involve forkhead transcription factors, sirtuins, protein kinase B (Akt), Bad, caspases, and poly (ADP-ribose) polymerase that may offer a fine line with determining cellular longevity, cell survival, and unwanted cancer progression. If one is cognizant of the these considerations, it becomes evident that nicotinamide holds great potential for multiple disease entities, but the development of new therapeutic strategies rests heavily upon the elucidation of the novel cellular pathways that nicotinamide closely governs.

Oxidative and excitatory mechanisms of developmental neurotoxicity: transcriptional profiles for chlorpyrifos, diazinon, dieldrin, and divalent nickel in PC12 cells

BACKGROUND

Oxidative stress and excitotoxicity underlie the developmental neurotoxicity of numerous chemicals.

OBJECTIVES

We compared the effects of organophosphates (chlorpyrifos and diazinon), an organo-chlorine (dieldrin), and a metal [divalent nickel (Ni2+)] to determine how these mechanisms contribute to similar or dissimilar neurotoxic outcomes.

METHODS

We used PC12 cells as a model of developing neurons and evaluated transcriptional profiles for genes for oxidative stress responses and glutamate receptors.

RESULTS

Chlorpyrifos had a greater effect on oxidative-stress-related genes in differentiating cells compared with the undifferentiated state. Chlorpyrifos and diazinon showed significant concordance in their effects on glutathione-related genes, but they were negatively correlated for effects on catalase and superoxide dismutase isoforms and had no concordance for effects on ionotropic glutamate receptors. Surprisingly, the correlations were stronger between diazinon and dieldrin than between the two organophosphates. The effects of Ni2+ were the least similar for genes related to oxidative stress but had significant concordance with dieldrin for effects on glutamate receptors.

CONCLUSIONS

Our results point to underlying mechanisms by which different organophosphates produce disparate neurotoxic outcomes despite their shared property as cholinesterase inhibitors. Further, apparently unrelated neurotoxicants may produce similar outcomes because of convergence on oxidative stress and excitotoxicity. The combined use of cell cultures and microarrays points to specific end points that can distinguish similarities and disparities in the effects of diverse developmental neurotoxicants.

Protein kinase C is a target for diverse developmental neurotoxicants: transcriptional responses to chlorpyrifos, diazinon, dieldrin and divalent nickel in PC12 cells

Unrelated developmental neurotoxicants can elicit similar functional outcomes, whereas agents in the same class may differ. We compared two organophosphate insecticides (chlorpyrifos, diazinon) with an organochlorine (dieldrin) and a metal (Ni(2+)) for similarities and differences in their effects on gene expression encoding subtypes of protein kinase C and their modulators, a cell signaling cascade that integrates the actions of neurotrophic factors involved in brain development. We conducted evaluations in PC12 cells, a model for neuronal development, with each agent introduced at 30 microM for 24 or 72 h, treatments devoid of cytotoxicity. Chlorpyrifos evoked by far the largest effect, with widespread upregulation of multiple genes; the effects were greater during neurodifferentiation than when cells were exposed prior to differentiation. Diazinon had smaller and less widespread effects, consistent with its lesser long-term impact on synaptic function and behavior noted for in vivo exposures in developing rats. Surprisingly, the effects of diazinon, dieldrin and Ni(2+) showed basic similarities despite the fact that all three come from different classes of toxicants. Our findings provide some of the first evidence for a specific mechanistic cascade contributing to the cholinesterase-independent developmental neurotoxicant actions of chlorpyrifos and its differences from diazinon, while at the same time identifying mechanistic convergence between otherwise unrelated toxicants that provides predictions about common neurodevelopmental outcomes. These results further show how combined use of cell cultures and microarray technology can guide future in vivo work on diverse developmental neurotoxicants.

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.

Exposure of neonatal rats to parathion elicits sex-selective impairment of acetylcholine systems in brain regions during adolescence and adulthood

BACKGROUND

Organophosphates elicit developmental neurotoxicity through multiple mechanisms other than their shared property as cholinesterase inhibitors. Accordingly, these agents may differ in their effects on specific brain circuits.

OBJECTIVES

We gave parathion to neonatal rats [postnatal days (PNDs) 1-4], at daily doses of 0.1 or 0.2 mg/kg, spanning the threshold for barely detectable cholinesterase inhibition and systemic effects.

METHODS

We assessed neurochemical indices related to the function of acetylcholine (ACh) synapses (choline acetyltransferase, presynaptic high-affinity choline transporter, nicotinic cholinergic receptors) in brain regions comprising all the major ACh projections, with determinations carried out from adolescence to adulthood (PNDs 30, 60, and 100).

RESULTS

Parathion exposure elicited lasting alterations in ACh markers in the frontal/parietal cortex, temporal/occipital cortex, midbrain, hippocampus, and striatum. In cerebrocortical areas, midbrain, and hippocampus, effects in males were generally greater than in females, whereas in the striatum, females were targeted preferentially. Superimposed on this general pattern, the cerebrocortical effects showed a nonmonotonic dose-response relationship, with regression of the defects at the higher parathion dose; this relationship has been seen also after comparable treatments with chlorpyrifos and diazinon and likely represents the involvement of cholinesterase-related actions that mask or offset the effects of lower doses.

CONCLUSIONS

Neonatal exposure to parathion, at doses straddling the threshold for cholinesterase inhibition, compromises indices of ACh synaptic function in adolescence and adulthood. Differences between the effects of parathion compared with chlorpyrifos or diazinon and the non-monotonic dose-effect relationships reinforce the conclusion that various organophosphates diverge in their effects on neurodevelopment, unrelated to their anticholinesterase actions.