Caractérisation de la microphase séparation hypoxique de Dictyostelium

Recently, a new form of aggregation was observed in Dictyostelium Discoideum under a millimetric film of nutrient medium and after several days of cell division. This aggregation is characterized by the appearance, under a bright-field microscope, of black spots, which are aggregates of cells, surrounded by a gas of cells. Remarkably, these aggregates rapidly stabilize in size while remaining highly mobile. The aim of this thesis is to propose an initial characterization of this aggregation. To this end, we have developed a method for detecting aggregates. At the same time, we carried out experiments under different media height and atmospheric conditions. This first series of manipulations and analyses demonstrated that oxygen, and more specifically hypoxia, is the driving force behind our aggregation. We then developed a microphase separation model and simulation, which successfully reproduced the experimental results. However, this initial study failed to explain the observed dynamics. We then used tracking algorithms to calculate the mean square displacement (MSD) associated with these trajectories. We observed that the MSD of these aggregates follows a three-regime law. The reasons for the first diffusive regime of the aggregates were not determined. The reasons for the pesistent and diffusive long-term regimes are due to the natural movements of the cells, but their values (much higher than those of the cells) seem to indicate the precense of other mechanisms, and more particularly the presence of cell-cell cooperation. In addition, there is a correlation between these values and the size of the aggregates. Larger aggregates are more diffusive and persistent over long time scales, while smaller aggregates are more diffusive over short time scales. This type of observation has already been reported in a theoretical study. We therefore decided to develop a simulation that would combine the ideas of microphase separation (competition between aerotaxis and adhesion) and cell-scale cooperation (polarization vector alignment). We used the Cellular Potts model (a Monte-Carlo algorithm particularly well-suited to representing cell membrane fluctuations). We then obtained a simulation with aggregates having a characteristic size and dynamics similar to those observed experimentally. Finally, we also investigated the role of a number of genes and biological mechanisms in this new aggregation, using drug, mutant screening and QPCR assays. These initial tests enabled us to rule out a number of genes that might be involved. We have also determined that several adhesion genes are required to stabilize our aggregations. In conclusion, Dictyostelium is a model organism used to understand and study a wide range of biological phenomena. The adaptation of this organism to hypoxia, and in particular to aerotaxis, remains little studied. We have studied a new aggregation phenomenon in Dictyostelium that combines hypoxia, adhesion and aerotaxis. When proliferating cells are too numerous, they consume all the oxygen and aggregate to a preferred size. This size results from a balance between adhesion and aetotactic repulsion. The aggregates show surprising mobility, suggesting mechanisms of internal cooperation. In biological terms, we studied the expression of a number of genes classically linked to oxygen and adhesion, to test their involvement in this new aggregation phase. The model, the knowledge of key parameters and mathematical modelling tools could contribute to a better understanding of hypoxia.