Evidence from three taxonomically distinct species for a non-AhR mechanism of developmental neurotoxicity of an environmentally derived mixture of polycyclic aromatic hydrocarbons

In the natural world, many pollutants do not arrive as single, isolated chemicals but as complex soups of substances mixed together. Among these, a large family of chemicals called polycyclic aromatic hydrocarbons, or PAHs, is common in the environment, often found in soot, oil, and industrial waste. Scientists have long known that certain PAHs can harm living things, and for decades, a primary focus of research has been on how these chemicals interact with a specific protein inside cells known as the Aryl hydrocarbon Receptor, or AhR. This receptor acts like a switch; when it is flipped on by a chemical, it triggers a chain of events that can lead to the production of enzymes to break down the toxin. While this pathway is well understood, most previous studies have looked at how single chemicals or very simple mixtures affect this switch, leaving a gap in our knowledge about how the messy, real-world blends of pollutants found in nature actually work. Understanding whether these complex mixtures harm developing brains through this known switch or through some other, hidden pathway is crucial for assessing the true risks of environmental contamination.

To address this gap, researchers turned to a specific, heavily polluted site in the Elizabeth River in Norfolk, Virginia, known as the Republic Creosoting site. From the sediment at the bottom of this river, they created a complex mixture of PAHs, which they named the Republic Sediment Extract. Instead of testing this mixture on just one type of animal, the team chose three very different species to see if the effects were consistent across the animal kingdom: Atlantic killifish, zebrafish, and a tiny roundworm called Caenorhabditis elegans. These species were selected because they represent distinct branches of the evolutionary tree, allowing the scientists to see if the results were unique to one type of animal or if they pointed to a fundamental biological truth. The researchers exposed the developing embryos of these animals to the sediment extract, carefully watching for signs of harm while also checking if the known AhR switch was being activated.

The results showed a clear split between what the scientists expected and what actually happened. In both the killifish and the zebrafish, the sediment extract successfully flipped the AhR switch, causing the fish to produce a specific enzyme called CYP1A, a standard sign that the receptor was active. However, the extract also caused significant damage to the developing nervous systems of the fish, altering their behavior, even at levels that were not high enough to cause visible physical deformities in the body. To figure out if this brain damage was caused by the AhR switch, the team used a special population of killifish that had adapted to the polluted river over many generations. These adapted fish are famous for their ability to ignore the AhR switch; when exposed to the same sediment, they did not produce the CYP1A enzyme. Yet, despite their resistance to the switch, these fish still suffered from the same harmful changes in behavior. This finding strongly suggested that the brain damage was happening through a route that did not involve the AhR receptor.

The evidence for this alternative pathway became even clearer when the researchers looked at the roundworms. Unlike fish and humans, roundworms do not have an AhR receptor that can be turned on by PAHs in the same way. When the worms were exposed to the sediment extract, they still developed damage to their dopamine-producing neurons, which are critical for movement and behavior. Since the receptor mechanism was impossible in these worms, the fact that they still got sick proved that the mixture was using a different method to cause harm. By digging deeper into the biology of the worms, the researchers found that the sediment extract was disrupting the cells' energy balance and changing their internal chemical state, specifically affecting how the cells managed their energy and their protection against oxidative stress.

This study demonstrates that the danger posed by complex environmental mixtures of PAHs is more intricate than previously thought. While these chemicals can certainly activate the well-known AhR pathway, the research shows they can also cause serious developmental harm to the nervous system through other mechanisms that bypass this switch entirely. The fact that this non-AhR damage appeared in fish, worms, and even in fish that had evolved to ignore the switch suggests that this alternative pathway is a conserved and robust way that these pollutants can injure developing brains. The work highlights that relying solely on the AhR pathway to predict the safety of environmental mixtures may miss significant risks, urging a broader look at how complex chemical soups affect life.