Characterization of the multixenobiotic resistance (MXR) mechanism in embryos and larvae of the zebra mussel (Dreissena polymorpha) and studies on its role in tolerance to single and mixture combinations of toxicants.

The study of the cellular mechanisms of tolerance of organisms to pollution is a key issue in aquatic environmental risk assessment. Recent evidence indicates that multixenobiotic resistance (MXR) mechanisms represent a general biological defense of many marine and freshwater organisms against environmental toxicants. In this work, toxicologically relevant xenobiotic efflux transporters were studied in early life stages of zebra mussels (Dreissena polymorpha). Expression of a P-gp1 (ABCB1) transporter gene and its associated efflux activities during development were studied, using qRT-PCR and the fluorescent transporter substrates rhodamine B and calcein-AM combined with specific transporter inhibitors (chemosensitizers). Toxicity bioassays with the model P-gp1 chemotherapeutic drug vinblastine applied singly and in combination with different chemosensitizers were performed to elucidate the tolerance role of the P-gp1 efflux transporter. Results evidenced that the gene expression and associated efflux activities of ABC transporters were low or absent in eggs and increased significantly in 1-3d old trochophora and veliger larvae. Specific inhibitors of Pgp1 and/or MRP transport activities including cyclosporine A, MK571, verapamil and reversin 205 and the musk celestolide resulted in a concentration dependent inhibition of related transport activities in zebra mussel veliger larvae, with IC50 values in the lower micromolar range and similar to those reported for mammals, fish and mussels. Binary mixtures of the tested transporter inhibitors except celestolide with the anticancer drug and P-gp1 substrate vinblastine increased the toxicity of the former compound more than additively. These results indicate that MXR transporter activity is high in early life-stages of the zebra mussel and that may play an important role in the tolerance to environmental contaminants.