Isotopic fractionation of natural uranium in a human neuronal cell model a proof of concept

International audienceThe study of isotopic fractionations of metals has recently been identified as a possible new biomarker for the investigation of metabolic processes, and as a very promising tool for the early diagnosis of diseases such as cancer or neurodegenerative disorders. In vitro isotopic fractionation studies in cell cultures display an outstanding potential to help in understanding biological processes governing isotopic fractionation, and eventually to help identify cellular pathways for metals. However, the use of cellular models for these types of investigations has been under-exploited so far.With the aim of investigating uranium cellular pathways and potential toxicity mechanisms during chronic exposure to non-toxic amounts, human cells differentiated into neurons were exposed to non-toxic natural uranium concentrations for seven days. The isotopic fractionation of uranium during its uptake by cells was studied. Given the small size of these samples, the low amounts of U incorporated by cells, and the complexity of the matrices, efforts were initially focused on the development and validation of a fit-for-purpose analytical procedure for U isotope ratio measurements. A highly efficient micro-flow sample introduction system working at 10 et61549;L/min was used, allowing the determination of et61540;238 values for the ratio n(238U)/n(235U), with an uncertainty of ~0.1 for uranium amounts as little as 20 ng. An isotopic fractionation was observed for intracellular uranium, the lightest isotope 235U being preferentially incorporated by cells. These results seem compatible with processes of uranium uptake such as passive diffusion according to a mass dependent fractionation, or with the incorporation via a transport protein and a mass-independent nuclear volume effect. The homeostatic modulation of endogen elements (P, Mg, Ca, Fe, Cu, Zn, Mn, Rb) induced by uranium accumulation will be discussed as well, since it can provide clues for the identification of uranium incorporation routes and potential toxicity mechanisms. Results of this work demonstrate the great potential of in vitro studies in cellular models as a tool to help identify cellular pathways of metals and for the understanding of processes leading to isotopic fractionations in living organisms. This information would be of prime importance in the framework of toxicological studies, as well as for the interpretation of clinical results in case of contamination or diseases