The impacts of pressure and trimethylamine N-oxide on pollutant metabolism in European eels

The aim of this master thesis was to assess the role of trimethylamine N-oxide (TMAO) in the inhibitory effect of pressure on pollutant metabolism in European eels. By measuring TMAO levels in liver slices exposed to pressures going up to 30MPa, we first wanted to prove correlation between increasing pressure and TMAO levels in eel liver cells. Further, we wanted to compare transcript levels of associated genes implicated in pollutant metabolism under high pressure (30MPa) with those remaining at atmospheric pressure (0.1MPa). This would have allowed us to evaluate whether or not pressure inhibits the metabolism of pollutants in the liver of the European eel. As safety measures related to the COVID-19 crisis prevented us from carrying out the planned experiments, this thesis explains our intention and the importance of this work. First, the complete life cycle as well as the different life stages of the eel have made it possible to understand the importance of its spawning migration towards the Sargasso Sea, announcing the end of its life but simultaneously the beginning of that of its offspring. Over years, wild eel stocks have been gradually declining to a point where they are now classified as “Critically endangered species”. Among the different lethal obstacles involved in this decline, POPs are largely responsible. Indeed, scientific reports have shown that pollutants in high concentrations in the bloodstream are able to cause disturbances in the reproductive, immune, endocrine and nervous systems. They are also able to induce lipolysis thus depleting energy reserves needed for migration and so, endangering European eel’s life. After questioning which factor was preventing eels from getting rid of those persistent pollutants, high hydrostatic pressure appeared to be overwhelmingly implicated. The second part of this work has therefore focused on the effects of pressure on global fish, and more precisely, pollutant metabolism. As pressure experiences had not yet been done on eels, we relied on results from other fish species. These data demonstrated the inhibitory effect of pressure on the expression of genes involved in metabolic reactions of pollutants. Thus, CYP1A, HSP70 and HSP90 expression levels decreased drastically after being exposed to high pressure (15MPa representative 1500 m depth) and pollutant (3-MC 25 μM) for 6 and 15 hours in both pressure and non-pressure adapted species. Consequently, third part of the work focused on the potential implication of the osmolyte TMAO in the inhibitory effect of pressure. It was demonstrated that accumulated TMAO levels were significantly higher in deep-sea fish, making it possible to establish a linear relationship between TMAO concentrations and depth (and thus pressure). These experiments conducted on other species than eels have led to the postulate that HHP induces the production of FMO3 in fish liver cells and that the increased production of TMAO may interfere with phase 1 detoxification process thus affecting the migration of European eels. In conclusion, only by carrying out the planned eel experiments will we be able to validate the hypothesis of the involvement of TMAO in the pressure inhibition of detoxification process in European eels. These experiments would thus help to explain the current decline in European eels.