Developmental toxicity of tire wear particle (TWP) leachates produced in the presence or absence of simulated natural light
Tire wear particles (TWP) are small micro- or nano-particles resulting from the friction of tire tread against roads. These microplastics have been found in waterways, usually through rain and wind, posing a potential risk to exposed aquatic life. The goal of this project was to assess the toxicity of chemical leachates from TWP and characterize how they may impact the health and development of aquatic life, like Danio rerio, zebrafish. Furthermore, we also determined if solar-simulated or dark conditions intensified the effects of TWP, considering processes like photodegradation and photochemical oxidation occur in sunlight. Zebrafish were exposed from 0-4 days post fertilization (dpf) to a filtered TWP solution at varying concentrations, and each concentration was leached in either solar-simulated or dark conditions. Mortality and hatching rates were quantified throughout the exposures, and physical anomalies were recorded at 4 dpf using microscopy. Ethyoxyresorufin-O-deethylase (EROD) activity, an indicator of Phase I biotransformation, was performed using fluorescent microscopt. RNA sequencing was performed for 4 dpf larvae. TWP exposures, regardless of leaching light status, increased mortality, yolk sac edema, pericardial edema, spinal curvature, and craniofacial malformations, while decreasing hatching and swim bladder inflation. Regardless of lighting during leaching, EROD activity increased in the liver and brain and decreased in the gut CYP1A. RNA sequencing revealed some conserved processes in response to TWP, regardless of light status during leaching, including those involved in neurobiological function. However, divergent responses were also observed for processes such as amino acid metabolism, steroid biosynthesis, and fatty acid metabolism. Overall, these results suggest a disruption of embryonic development in zebrafish when exposed to TWP, and that sunlight during chemical leaching can alter the molecular responses to TWP. Overall design: All studies were performed using zebrafish. Zebrafish embryos were exposed to either control (egg water only) or TWP (10 g/L, leached for 1 or 6 days) from 0-4 dpf. For each exposure, leachates were proposed in the absence (dark) or presence (solar-simulated, to mimic southern California solar conditions) of light, to capture natural conditions. Samples were collected on 4 dpf. Four pooled samples (each containing 10 larvae at 4 dpf) were analyzed for each of the four exposure groups.