Nano-TiO2 aggravates bioaccumulation and developmental neurotoxicity of difenoconazole in zebrafish larvae via oxidative stress and apoptosis: Protective role of vitamin C

Titanium dioxide nanoparticles (n-TiO2) could enhance the bioavailability and toxicity of coexisting organic contaminants in the aquatic environment. This study attempted to investigate the combined effects of n-TiO2 and difenoconazole (DIF) on the neurodevelopment of zebrafish and the underlying mechanisms. In this study, zebrafish embryos were exposed to n-TiO2 (100 μg/L), DIF (0, 0.1 and 0.5 mg/L) and their mixtures from 4 to 96 h post fertilization (hpf) and neurotoxicity was evaluated. Our results indicated that n-TiO2 adsorbed DIF into the brain of zebrafish and significantly enhanced the bioaccumulation of DIF and n-TiO2 in the 0.5 mg/L co-exposure group. 100 μg/L n-TiO2 was not developmentally toxic to the zebrafish larvae, but it exacerbated DIF-induced neurobehavioral alterations in the zebrafish larvae. n-TiO2 also aggravated DIF-induced suppression of central nervous system (CNS) neurogenesis in Tg (HuC:egfp) zebrafish, motor neuron axon length in Tg (hb9:egfp) zebrafish, and downregulation of neurodevelopmental genes (elavl3, ngn1, gap43, gfap and mbp). In addition, DIF elevated oxidative stress by accumulation of reactive oxygen species (ROS) and inhibition of antioxidant enzymes, and triggered apoptosis by upregulation of p53, bax, bcl-2 and caspase-3, which were markedly intensified in the presence of n-TiO2. Moreover, vitamin C (VC) ameliorated n-TiO2/DIF-induced abnormal locomotor behaviors and neurotoxicity by inhibiting oxidative stress and apoptosis, indicating that oxidative stress and apoptosis are involved in n-TiO2/DIF-induced neurotoxicity. Taken together, our data indicated that n-TiO2 enhanced the accumulation of DIF and heightened oxidative stress and apoptosis, thereby inducing neurotoxicity. This study exemplifies the importance of the toxicity assessment of chemical mixtures and novel insights to mitigate their combined toxicity.