Effects of Zinc Oxide Nanoparticles on the Pharyngeal Pumping and Neurological Behavior of C. elegans

Abstract

Cogent data regarding the effects of manufactured zinc oxide nanoparticles (ZnO-NPs) on human health and other organisms is limited. With the prominence of manufactured ZnO - NPs increasing rapidly in recent years, the identification of prolonged consequences of exposure to the NPs is crucial. Thus, further study regarding their biological effects is highly significant. The purpose of this research is to discern the potential toxicological effects of manufactured zinc oxide nanoparticles (0.614, 61.4, and 614 [micro]M Zn corresponding to 0.05, 5, and 50 [micro]g/L ZnO) on the pharyngeal pumping and neurological behaviors of the nematode Caenorhabditis elegans (C. elegans). While several studies have examined endpoints regarding behavior, reproduction efficacy, lethality and transgene expression after exposure to ZnO - NPs, insufficient attention has been allocated to the effects of the ZnO - NPs on the "heart" of the nematode, the pharynx. This experiment is designed to test hypotheses that exposure to ZnO - NPs results in aberrant neurological behavior and alternations in the pharyngeal pumping of C. elegans. Behavioral assays have been performed on C. elegans exposed to three different concentrations of manufactured ZnO - NPs (0.614, 61.4, and 614 [micro]M Zn) for different exposure times. Results suggest that C. elegans at different developmental stages respond to ZnO NPs exposure differently, and different exposure durations also result in different behavioral responses. Exposure of L4 C. elegans to ZnO-NPs at 61.4 and 614 [micro]M for 1 hour significantly reduced pharyngeal pumping and body bends. Adult 3-day-old C. elegans exposed to ZnO-NPs at 0.614, 61.4 and 614 [micro]M for 24 hours displayed significantly increased pharyngeal pumping as compared to the control. The enhanced pumping rate on 3-day-old adult worms followed a temporal pattern and lasted until at least 72 hours of ZnO NP treatment for the 61.4 [micro]M group. Whereas for the 4-day-old C.elegans, treatment with ZnO 0.614, 61.4 and 614 [micro]M for relatively short periods of time (1 hr and 4 hrs) do not have significant effects on pharyngeal pumping as compared to control, while prolonged treatment to 24 hours significantly decreased pharyngeal pumping, head thrashes, and body bends. A total of 23 genes that are related to pharyngeal pumping and stress response were selected to test their expressions on 3-day-old C.elegans treated with ZnO NPs at 61.4 [micro]M for 72 hours: ceh-22, ceh-24, daf-12, drp-1, dyn-1, eat-2, eat-5, eat-18, F02A9.4, glc-2, inx-10, lev-11, myo-1, myo-2, myo-5, nas-3, nas-15, nfi-1, pha-1, pha-2, pha-4, sod-1 and unc-2. Overall, a majority of the said genes, 17 out of 23, were significantly upregulated. Genes that exhibited significant > 2 fold upregulation include ceh-22, daf-12, drp-1, dyn-1, eat-5, eat-18, F02A9.4, glc-2, myo-1, myo-5, nas-3, nas-15, nfi-1, pha-1, pha-2, sod-1 and unc-2. Only three genes were downregulated: ceh-24, myo-2 and pha-4. Since the number of conserved genes from C. elegans to humans is relatively high, the data collected in these studies can be indicative of human physiological responses stemming from the exposure to ZnO - NPs.