This joint research project is promoted by three partner groups in Switzerland (Prof. Dr. Karl Fent, University of Applied Sciences Northwestern Switzerland, School of Life Sciences, Muttenz and ETH Zurich, Department of Environmental Sciences), Croatia (Dr. Tvrtko Smital, Laboratory for Molecular Ecotoxicology, Rudjer Boskovic Institute, Zagreb) and Serbia (Prof. Dr. Radmila Kovacevic, Laboratory for Ecotoxicology, University of Novi Sad). The project team consists of environmental toxicologists, who are unified by a common research question: to understand the cellular biology of key defence mechanisms that protect aquatic organisms against hazardous chemicals found in the environment. The main project goal is capacity-building and promoting ecotoxicological research groups in Serbia and Croatia, and this overall goal will be fulfilled by performing two core project activities. The first part of the project will be directed to the transfer of knowledge and expertise that will enable the Serbian and Croatian partners to improve their overall research capacity, performing part of the research using zebrafish (Danio rerio) and related cell lines as highly established and promising models in biomedical and ecotoxicological research. Stable zebrafish cell lines (PAC1 and ZFL) and related cell culture protocols will be transferred to partner laboratories in Serbia and Croatia. Graduate students from both countries will be trained in Switzerland, in order to get knowledge and skills needed for work with these cell culture systems. A small scale zebrafish facility will be established at the Serbian partner laboratory, and equipment needed for maintenance of fish cell cultures will be installed. Both Serbian and Croatian partners will acquire knowledge needed for performing classical zebrafish acute toxicity assays, as well as the recently established zebrafish embryotoxicity DarT test. The second part of the project, which is focused on research, will be directed to better understanding of critical cellular, evolutionary conserved xenobiotic defence systems in aquatic organisms, using zebrafish as a model. Aquatic organisms deploy a wide range of protective systems against the adverse effects of toxic chemicals originating from both exogenous (environment) and endogenous sources. Taking into account the complexity of defence mechanisms contributing to the overall adsorption, disposition, metabolism, excretion, and finally toxicity of xenobiotics, in this project we will specifically focus on the uptake of environmental chemicals and metabolites by specific transmembrane proteins, detoxification by phase I and II biotransformation enzyme systems; and finally, active elimination of xenobiotics and metabolites through specific transmembrane proteins (ABC transporters). Specifically, our proposal for a SCOPES joint research project comprises the following specific research topics:

• Identification and phylogenetic analyses of zebrafish genes encoding for members of the organic anion transporting polypeptides (OATPs), phase I enzymes belonging to cytochrome P450 (CYP) 1A/3A family, phase II enzymes (glutathione-S-transferases (GSTs) and uridine diphosphate glucuronyltransferases (UDPGTs), and ABC efflux transporters;
• Determination of tissue and age specific expression pattern of corresponding genes in zebrafish in vivo, and in PAC1 and ZFL zebrafish cell lines in vitro;
• Development, standardization and subsequent use of research tools needed for cloning amolecular characterization of selected, highly expressed members of the OATP, CYP1A/3A, GST, UDPGT and ABC family; In vitro analyses of gene expression regulation and interactions between uptake transporters, CYP1A/3A, GSTs, UDPGTs and ABC transporters in zebrafish;
• Evaluation of usability of acquired and/or developedin vitroandin vivotools in determination of interactions of environmental contaminants with critical cellular defence systems.

Apart from its research context, an important novel feature of this collaborative proposal is that it links environmental toxicology in three countries. Thus, by using a well established model species (zebrafish) and a range of cellular model systems, we provide a unique multidisciplinary training opportunity across multiple areas of ecotoxicology. The research fields covered by this proposal include xenobiotic uptake, disposition and elimination, adverse drug and chemical effects, and finally, identification of the presence of potentially hazardous chemicals in complex environmental matrices. Therefore, by realizing this project through the described research activities, exchange of students and capacity building, we will enable a direct transfer of knowledge and expertise among involved laboratories, fostering a high quality and competitive ecotoxicological research in all partner countries.