Functional and molecular responses of the blue mussel Mytilus edulis' hemocytes exposed to cadmium - An in vitro model and transcriptomic approach.

The bivalve mollusk, Mytilus edulis, is used as a sentinel species in several monitoring programs due to its ability to bio-accumulate contaminants. Its immune system consists of hemocytes and humoral components, which constitute the main part of the hemolymph. The present study is aimed at understanding the effects of Cd on the differentially expressed genes involved in the phagocytosis of M. edulis' hemocytes. Our approach focuses on an in vitro model by exposing hemocytes to different concentrations of Cd ranging from 10-9 M to 10-3 M. Phagocytosis and cell viability as functional markers were measured using flow cytometry. The molecular mechanisms regulated by Cd were investigated using RNA-seq and DGE analysis. Results showed that viability and phagocytosis of hemocytes exposed to 10-3 M of Cd were significantly decreased after 21 h of exposure. RNA sequencing data showed that 1112 transcripts (out of 352,976 contigs) were differentially regulated by the highest concentration of Cd. Among these identified transcripts, 1028 and 84 were up and down-regulated respectively. The induction of super oxide dismutase (SOD), glutathion-s-transferase (GST), cytochrome P450 2C8 (CYP2C8), multidrug resistance protein (MRP1) and heat shock protein 70 (HSP70) suggests that Cd can regulate key molecular mechanisms. In addition, several toll-like receptors (TLR) as well as genes involved in phagocytosis (actin and CDC42) and apoptosis (caspase 8 and XIAP/IAP) were induced by Cd. Thus, our model highlights the effect of Cd on the phagocytic function of M. edulis' hemocytes along with the regulation of gene expression involved in innate immunity, detoxification and apoptosis. Further investigations need to be pursued to unravel the effects of Cd on the molecular mechanisms identified in this study.