Actin polymers in the cellular cortex: a population dynamics approach – ACTIPOP

The mechanical properties of embryonic cells drive a wide diversity of cell behaviors, ranging from cell division to polarization and cell shape changes. These mechanical properties play a critical role during embryonic development and morphogenesis, as they contribute to the position, shape and fate of cell and organs, but also in pathological processes such as metastatic conversion in cancer. Cell mechanics are largely defined by the dynamics of a biological composite polymer, the actomyosin cytoskeleton. This skeleton is shaped in particular by the interactions between the actin polymers present in the cellular cortex, which are modulated by other proteins such as formins, Arp-2/3 and capping proteins. Thinking of this dynamics as that of a population of individual polymers interacting in a (potentially structured and/or fluctuating) environment is the key point of view at the core of this project. In previous work, the consortium developped a first stochastic model of such populations, without spatial structure or temporal fluctuations, but incorporating the stochasticity of molecular encounters. A C++ code was developped for efficient simulation of these complex population dynamics. Another model of randomly evolving trees was also created to understand the branching and (de)polymerisation dynamics of a single branched polymer. In this project, we shall confront the existing models to experimental data to test their empirical validity, and we shall further develop the mathematical modelling and analysis of these stochastic (measure-valued or tree-valued) Markov processes to incorporate spatially constrained interactions, the impact of an environment fluctuating in space and/or time, and finer interactions rules or more complex phenomena such as polymer fragmentation by cofilins.