Assessment of environmental contaminants' neurotoxicity : in vitro and in vivo experimental studies

Abstract

Accumulating evidence over several decades shows that environmental contaminants may lead to neurodevelopmental alterations. An essential prerequisite to prevention is recognition of a chemical‟s ability to harm the developing nervous system. Knowledge that environmental contaminants are neurotoxic can prompt efforts to restrict its use and to control exposure. Many studies on environmental toxicants have been performed so far, but the knowledge available on the effects of exposures at low environmentally relevant doses and on cell-specific modes of action is still very limited. The work included in this thesis, is based on an experimental strategy including in vitro studies on cellular models and behavioral analyses in mice aimed at investigating the potential neurotoxic effects of selected food contaminants, such as methylmercury (MeHg), polychlorinated biphenyls (PCBs) and perfluorinated compounds (PFCs). Using in vitro models, we found that MeHg and PCBs cause cell death in the HT22 hippocampal cell line via a parallel activation of calpains and lysosomal proteases, with no involvement of caspases. Oxidative stress does not play a major role in PCB toxicity opposite to MeHg. Co-exposure to MeHg and PCBs shows mostly antagonistic interactions. We have also investigated the effects of PCBs and MeHg on primary cultures of rat neural stem cells (NSCs), focusing on the non-dioxin like (NDL)-PCBs 153 and 180. Both PCBs affect spontaneous neuronal differentiation with a significant increase in the number of neurite-bearing Tuj1 positive cells and a concomitant decrease in proliferating cells. In agreement, measurements of spontaneous Ca2+ oscillations showed a decreased number of cells with Ca2+ activity after PCB exposure, further confirming the increase in neuronal cells. Conversely, exposure to methylmercury (MeHg), which we evaluated in parallel, led to an increased number of cells with Ca2+ activity, in agreement with the previously observed inhibition of neuronal differentiation. Analyses by q-PCR of the Notch pathway revealed that PCBs have a repressive action on Notch signaling, whereas MeHg activates it. All together, the data indicate that nM concentrations of the selected NDL-PCBs and MeHg interfere in opposite directions with neuronal spontaneous differentiation of NSCs through Notch signalling. Combined exposures to PCBs and MeHg results in an induction of apoptosis and an antagonistic interaction on spontaneous neuronal differentiation. In the in vivo study performed in the frame of this project, mice offspring in utero exposed to perfluorooctanesulfonate (PFOS) or perfluorooctanoic acid (PFOA) were assessed for changes in motor function, circadian activity and emotion-related behaviour. Exposure to PFOS results in decreased locomotion in a novel environment and reduced muscle strength only in male offspring. Prenatal exposure to PFOA is associated with changes in exploratory behavior in male and female offspring, and increased home cage global activity only in males. In conclusion, our studies show that the developing nervous system is very susceptible to the neurotoxic effects of the selected food contaminants and that a combined approach with both in vitro and in vivo experimental models is most valuable for developmental neurotoxicity testing.