Global characterization of dose-dependent effects of cadmium in clam Ruditapes philippinarum

Cadmium (Cd) is being frequently detected in marine organisms. However, dose-dependent effects of Cd challenged unraveling the toxicological mechanisms of Cd to marine organisms and developing biomarkers. Here, the dose-dependent effects of Cd on clams Ruditapes philippinarum following exposure to 5 doses of Cd (3, 9, 27, 81, 243 μg/L) were investigated using benchmark dose (BMD) method. By model fitting, calculation of BMD values was performed on transcriptomic profiles, metals concentrations, and antioxidant indices. Cd exposure induced not only significant Cd accumulation in clams, but also marked alterations of essential metals such as Ca, Cu, Zn, Mn, and Fe. Gene regulation posed little influence on essential metal homeostasis, indicated by poor enrichment of differentially expressed genes (DEGs) associated with metal binding and metal transport in lower concentrations of Cd-treated groups. BMD analysis on biological processes and pathways showed that peptide cross-linking was the most sensitive biological process to Cd exposure, followed by focal adhesion, ubiquitin mediated proteolysis, and apoptosis. Occurrence of apoptosis was also confirmed by TUENL-positive staining in gills and hepatopancreas of clams treated with Cd. Furthermore, many DEGs, such as transglutaminases (TGs), metallothionein (MT), STEAP2-like and laccase, which presented linear or monotonic curves and relatively low BMD values, were potentially preferable biomarkers in clams to Cd. Overall, BMD analysis on transcriptomic profiles, metals concentrations and biochemical endpoints unraveled the sensitiveness of key events in response to Cd treatments, which provided new insights in exploring the toxicological mechanisms of Cd in clams as well as biomarker selection.