Polarité mécanique du cortex cellulaire

As they divide, differentiate and migrate, cells establish a polarity by asymmetrically distributing cellular components (proteins, organelles, cytoskeleton). They also undergo striking morphological changes during polarisation. These morphological changes, often triggered by certain polarity cues, are associated with the remodelling of the actin cortex: a dynamic, thin layer of actin filaments underlining the plasma membrane. While the molecular components of the actin cortex have been studied in detail, a deep investigation into its mechanical properties is still lacking in the literature. To study this, we use a tool developed in the team known as the Magnetic Pincher to make mechanical measurements at the actin cortex, in combination with optogenetics to induce cell polarisation. The Magnetic Pincher uses superparamagnetic beads that self-assemble to form linear chains in an external magnetic field. Cells are allowed to ingest a small quantity of the beads a priori, following which more beads are then added to their environment. The pair of beads, i.e. one inside the cell and one outside, attract each other to ‘pinch' the cortex. A 3D tracking of the beads is performed to obtain measurements on the local thickness and elasticity of the cortex. Ongoing work in the team has shown that the cortex is a non-linear material with a stiffness ranging from 1-10 kPa on applying a stress of 100-800 Pa. We use 3T3 fibroblasts with the Cry2-CIBN optogenetic system to locally recruit ARHGEF11 to the plasma membrane by exposing cells to blue light, resulting in an increased amount of active RhoA at the cortex. By illuminating roughly half the cell, we can create a ‘polarized rear' - a highly contracted region as a downstream effect of RhoA - and a ‘polarized front' corresponding to actin protrusions. Maintaining the cell in this polarized state, we use the Magnetic Pincher to characterize cortex mechanics at the cell rear and front. The project focuses on extracting the mechanical properties of the cortex such as elasticity, non-linearity and thickness locally, inaccessible with other existing tools. Understanding these properties in the light of cell polarization deepens our understanding of polarity-establishment mechanisms and emphasizes the link between intracellular cell signaling and cell surface mechanics.