Cytoplasm fluidization by bulk actin rearrangements for organelle positioning – CytoActinPos

The regulation and function of bulk cytoplasm mechanics remain poorly understood. The cytoplasm is thought to be highly viscous (typically 1000X more than water) and also elastic, like a soft gel. These properties set challenges for cellular organization and organelle transport. Indeed, a stiff cytoplasm is needed to stabilize organelle positions, but the cytoplasm also needs to be fluid enough to allow for the transport of organelles of different sizes (e.g. vesicle cargos, or large nuclei and spindles). As cytoplasm mechanics is largely dependent on entangled bulk actin meshworks, based on our preliminary results, we here hypothesize that organelle motion and transport may locally break or rearrange actin networks, to create local soft paths and regions that allow them to be displaced by molecular motors. In a first work package, we will use in vivo magnetic tweezers to directly displace endogenous and foreign objects in the cytoplasm of live large zygote cells, and develop in vitro and in silico reconstitutions of bulk actin networks to understand how they reorganize around objects moved under force using optical tweezers. These experiments and models will allow us to explore the molecular requirements of actin networks that promote cytoplasm elasticity as well as force-dependent fluidization. In a second work package, we will use in vivo and in vitro experiments to test how transport along microtubules may locally reorganize actin networks. This shall allow us to unveil important local mechano-regulation of the cytoplasm by both cytoskeletal networks and their functions in regulating centrosome and spindle positioning as well as chromosome segregation. Overall, these studies shall bring prime understanding on the regulation of bulk actin networks and cytoplasm mechanics for cellular organization.