Uptake of TiO₂ Nanoparticles into C. elegans Neurons Negatively Affects Axonal Growth and Worm Locomotion Behavior

We employ model organism Caenorhabditis elegans to effectively study the toxicology of anatase and rutile phase titanium dioxide (TiO₂) nanoparticles (NPs). The experimental results show that nematode C. elegans can take up fluorescein isothiocyanate-labeled TiO₂ NPs and that both anatase and rutile TiO₂ NPs can be detected in the cytoplasm of cultured primary neurons imaged by transmission electron microscopy. After TiO₂ NP exposure, these neurons also grow shorter axons, which may be related to the detected impeded worm locomotion behavior. Furthermore, anatase TiO₂ NPs did not affect the worm's body length; however, we determined that a concentration of 500 μg/mL of anatase TiO₂ NPs reduced the worm population by 50% within 72 h. Notably, rutile TiO₂ NPs negatively affect both the body size and worm population. Worms unable to enter the L4 larval stage explain a severe reduction in the worm population at TiO₂ NPs LC₅₀/3d. To obtain a better understanding of the cellular mechanisms involved in TiO₂ NP intoxication, DNA microarray assays were employed to determine changes in gene expression in the presence or absence of TiO₂ NP exposure. Our data reveal that three genes (with significant changes in expression levels) were related to metal binding or metal detoxification (mtl-2, C45B2.2, and nhr-247), six genes were involved in fertility and reproduction (mtl-2, F26F2.3, ZK970.7, clec-70, K08C9.7, and C38C3.7), four genes were involved in worm growth and body morphogenesis (mtl-2, F26F2.3, C38C3.7, and nhr-247), and five genes were involved in neuronal function (C41G6.13, C45B2.2, srr-6, K08C9.7, and C38C3.7).