Project 3 – Mitochondrial and Cellular Mechanisms of Neurotoxicity of Superfund Chemical Co-Exposures

Project Leaders

Goals and Importance of Research

Some developmental exposures impact the proper functioning of mitochondria, resulting in altered behavior and increased risk of neurodegeneration and related diseases. This project will study the underlying mechanisms of developmental neurotoxicity from co-exposures to polycyclic aromatic hydrocarbons (PAHs) and the metals lead and cadmium in the nematode worm C. elegans. One plausible mechanism is that exposures change how neurons develop, leading to altered cell fate, structure, and connectivity (“hardwiring”). An alternative mechanism is that epigenetic changes result in altered function of neurons that otherwise look normal (epigenetic “programming”). This work will study these potential mechanisms, thus informing adverse outcome pathways and methods to intervene and reverse health impacts.

Summary

In ongoing work by this Project, researchers have found that developmental exposures to mitochondrial toxicants cause neurotoxic outcomes in Caenorhabditis elegans, including morphological alterations in neurons, altered behavior, and, in the long term, increased susceptibility to neurodegeneration. In line with the U.S. Environmental Protection Agency, these are described as “developmental neurotoxicity (DNT),” because they result from exposures that occur during development.

Two important and overarching mechanisms of DNT are:

  • Changes to neurogenesis resulting in altered cell fate, morphology, and connectivity (“hardwiring”).
  • Persistent changes to the function of neurons that appear to be morphologically normal (epigenetic “programming”).

Distinguishing these is challenging; the project uses a novel and powerful way to assess each possibility. The research team begins with an in vivo yet relatively high-throughput and economic model, C. elegans. C. elegans offers an additional, key benefit: developmental neurogenesis is normally invariant, permitting clear identification of variation in hardwiring as well as behavioral and stress-responsive changes without morphological alteration (programming). Work in C. elegans is followed by testing in human neuronal stem cells (hNSCs) that permit human-relevant DNT testing, plus the opportunity to identify sex-specific differences and epigenetic modifications.

Relatively few chemicals have been rigorously evaluated for DNT. The paucity of information is even more pronounced for chemical co-exposures, despite the fact that combined exposures are the reality. This lack of testing of mixtures results partly from regulatory policy, and partly from technical challenges in laboratory testing of co-exposures. The team’s combined in vivo-in vitro approach permits them to rigorously test for DNT resulting from both complex environmental mixtures, and from defined combinations of individual Superfund chemicals that they evaluate for non-additive effects. Researchers will test the effects of the prototypical developmental neurotoxicants lead, cadmium, and polycyclic aromatic hydrocarbons, singly and in combinations dictated by known environmental concentrations.

The team compares their outcomes in C. elegans and hNSCs, to those obtained by other projects in fish, rats, and people. Demonstration that C. elegans can be reliably used to investigate mixture DNT will add a powerful new model for testing and regulation of environmental mixtures. Finally, the scientists test the degree to which mitochondrial dysfunction, key to neurodevelopment, drives DNT by these prototypical chemicals. These chemicals have multiple molecular targets, including but not limited to different mitochondrial macromolecules. The fact that these chemicals individually all affect mitochondria and neurons, but by different mechanisms, is why synergistic interactions are predicted. However, while mitochondria are known targets of these chemicals, the extent to which mitochondrial toxicity drives their DNT is not known. This work will establish the contribution of mitochondrial dysfunction in single and combined chemical DNT, informing development of adverse outcome pathways and intervention efforts.

Project Aims

  1. Test if co-exposures to PAHs and the metals lead and cadmium cause developmental and later-life neurotoxicity by altering hardwiring.
  2. Test if co-exposures cause developmental and later-life neurotoxicity via altered programming.