Novel model systems for in vitro neurotoxicity testing: Chemical stressors and neurotoxic hazard of extremely low frequency electromagnetic fields

Both structurally and functionally, the nervous system is one of the most complex organ systems. Its main function is to send and receive signals; so-called neurotransmission, which largely depends on the viability and structure of neurons as well as on proper regulation of the cellular and molecular mechanisms underlying neurotransmission, in particular intracellular calcium signaling. Moreover, the function of neuronal networks depends on the heterogeneity of the network, i.e. the presence of other neural cell types, including oligodendrocytes, microglia, and astrocytes.

Any adverse effect on its chemistry, structure and function, induced by chemical or physical influences is called neurotoxicity. Neurotoxicity is often investigated in vivo, but can also be investigated using in vitro model systems. The latter have the potential to improve understanding of basic physiological processes of neuronal cells and can be used to study the cellular and molecular mechanisms underlying neurotoxic effects. At the same time, they are more time- and cost effective than conventional animal experiments.

In this thesis, novel model systems for in vitro neurotoxicity testing were developed and characterized, with a special focus on developmental neurotoxicity. Subsequently, these models were used to investigate the neurotoxicity of exposure to 50 Hz ELF-EMF and elucidate the underlying mechanisms. To evaluate the neurotoxicity of environmental exposures, it is important to use appropriate models and to study both morphological and functional endpoints. The choice of the in vitro model should depend on the research question at hand, as each approach has its advantages and disadvantages. Moreover, thorough characterization of any new model system is critical prior to using it for (developmental) neurotoxicity testing. In the first part of this thesis both in vitro cell lines (naïve and chemically stressed PC12 cells) and ex vivo primary cell cultures (mouse neural progenitor cells and primary rat cortical cultures) were characterized as model systems for respectively the stressed or aging and developmental nervous system.

Effects of chemical exposures can successfully be investigated in vitro. However, more difficulties arise when investigating physical exposures, especially when they have limited energy and are therefore unlikely to cause direct effects on cell viability or function. A much investigated, and controversial, example of such an exposure involves electromagnetic fields (EMF), in particular extremely low frequency (ELF)-EMF.

ELF-EMF are defined as EMF with frequencies between 3 and 300 Hz. Exposure to these fields (mainly 50 Hz) has dramatically increased during the last decennia because of the growing electrical demand and advancing technologies. Great public and scientific concern was raised when early epidemiological studies indicated a correlation between ELF-EMF exposure and the development of childhood leukemia. Consequently, researchers have investigated possible health effects of ELF-EMF. Of the many suspected target organs of ELF-EMF, the nervous system could be particularly vulnerable since neuronal function and signaling is highly voltage-dependent.

Using the different in vitro models characterized in the first part of this thesis, we thoroughly investigated the potential neurotoxic effects of a variety of EMF exposure scenarios and conclude that the neurotoxic potential of ELF-EMF exposure in humans is limited.