ROS dependent copper toxicity in Hydra-biochemical and molecular study

Copper, an essential microelement, is known to be toxic to aquatic life at concentrations higher than that could be tolerated. Copper-induced oxidative stress has been documented in vitro, yet the in vivo effects of metal-induced oxidative stress have not been extensively studied in the lower invertebrates. The objective of the present study has been to find the effect of ROS-mediated toxicity of environmentally relevant concentrations of copper at organismal and cellular levels in Hydra magnipapillata. Exposure to copper at sublethal concentrations (0.06 and 0.1 mg/L) for 24 or 48 h resulted in generation of significant levels of intracellular reactive oxygen species (ROS). We infer that the free radicals here originate predominantly at the lysosomes but partly at the mitochondria also as visualized by H2-DHCFDA staining. Quantitative real-time PCR of RNA extracted from copper-exposed polyps revealed dose-dependent up-regulation of all antioxidant response genes (CAT, SOD, GPx, GST, GR, G6PD). Concurrent increase of Hsp70 and FoxO genes suggests the ability of polyps to respond to stress, which at 48 h was not the same as at 24 h. Interestingly, the transcript levels of all genes were down-regulated at 48 h as compared to 24 h incubation period. Comet assay indicated copper as a powerful genotoxicant, and the DNA damage was dose- as well as duration-dependent. Western blotting of proteins (Bax, Bcl-2 and caspase-3) confirmed ROS-mediated mitochondrial cell death in copper-exposed animals. These changes correlated well with changes in morphology, regeneration and aspects of reproduction. Taken together, the results indicate increased production of intracellular ROS in Hydra on copper exposure.