Reactive oxygen species are derivatives of aerobic metabolism. Their production is finely regulated to protect cells against severe and permanent damage to cellular components that could ultimately lead to programmed cell death. However at low levels, ROS act as second messagers in several signalling pathways leading to growth, differentiation, senescence or cell death. This dual function of ROS appears to be due to differences in their concentrations, their sub-cellular localization and the pulse-duration. Using a newly developed fluorescent redox biosensor that allows dynamic live imaging of the intracellular glutathione redox state, I will study the role of endogenous oxidants in cell fate decision at the G1 checkpoint. I will follow the intracellular redox state of 3 primary cell types as they grow in culture, respond to mitogenic stimuli or differentiation factors and undergo senescence. Oxygen sensitivity of these processes will be also evaluated by modifying the oxygen culture conditions. Murine adult neural stem cells will serve as a model to study the balance between self-renewal and differentiation while mouse embryonic fibroblasts and human adult skin fibroblasts will be studied in terms of proliferative capacity versus senescence induction. Furthermore, we plan to generate transgenic mice constitutively expressing the redox sensor. This will allow us to compare endogenous redox levels in different tissues, over time and under pathological conditions.