Comparison of ZnO nanoparticles and ZnCl₂ induced germ cell apoptosis in Caenorhabditis elegans

Abstract

There is inadequate research data available on manufactured ZnO nanoparticles. Manufactured nanoparticles are widely used in various products and production of manufactured nanoparticles is becoming increasingly abundant. Humans are exposed to ZnO nanoparticles in products such as sunscreen, toothpaste, and cosmetic products. Due to the frequent human contact with nanoparticles, such as ZnO, research on the resulting biological effects is highly significant. In this study, we utilized the model organism Caenorhabditis elegans (C. elegans) to investigate the effects ZnO nanoparticle exposure can have on germ cell apoptosis and key genes involved in the apoptosis pathway. ZnCl₂ (Zn²⁺) serves as a comparison of toxicity with ZnO nanoparticles in this study. It is known that ZnO nanoparticle exposure can have a severe effect on the reproduction process in C. elegans, however, it has yet to be tested whether or not ZnO nanoparticles affect germ cell apoptotic machinery as a possible mechanism of reproductive toxicity. Worms were exposed on an agar medium containing concentrations of ZnO nanoparticles and ZnCl₂ (range, 6.14[times]10-1, 61.4, and 614 µM Zn). ZnO nanoparticles and ZnCl₂ both significantly increased the number of apoptotic cells as compared to the control in the 61.4 and 614 µM treatment groups (p<0.05). However, ZnO nanoparticles significantly induced nearly 2[times] more average apoptotic cells in the 61.4 µM treatment than the ZnCl₂ at the same concentration. This relationship was observed in both the bristol N2 wild type (N2) and MD701 (bcIs39 [(lim-7)ced-1p::GFP + lin-15(+)]) strains. Genes involved in the apoptosis pathway (ced-13, ced-3, ced-4, ced-9, cep-1, dpl-1, efl-1, efl-2, egl-1, egl-38, lin-35, pax-2, and sir-2.1 were changed in response to ZnO nanoparticle exposure. Cep-1/p53 was significantly up-regulated in the 614 µM treatment (p<0.05). In the CEP-1 loss of function mutant, no significant increases were observed in germ cell apoptosis as compared to the control in any treatment group (p>0.05). Therefore, the increased apoptosis resulting from ZnO nanoparticle exposure is likely CEP-1 dependent. Following exposure, possible ZnO nanoparticles were observed in vivo in the worms using transmission electron microscopy, suggesting that the worms can possibly absorb the nanoparticles via various routes.