Cytoskeletal response to external forces: Contractile to extensile transition

Cellular cytoskeleton is a highly complex material composed of various filaments, crosslinkers and molecular motors which drive the system out-of-equilibrium (i.e., active) via imparting forces at microscopic scale. Due to it's architechtural complexity and activity, the cytoskeletal structures may exhibit non-trivial response to external purturbations. Here we employ a simplified and coarse-grained agent-based description of the actin cytoskeleton using polar filaments and motors to understand it's response to the external forces. Interestingly, we find the actomyosin self-assembly to undergo a morphological transition from an aster to a bundle in response to the external forces. Based on the microscopic dynamics, we build a mesoscopic theorectical description to show that underlying the mentioned morphological transition, there is a transition of the active stress from being contractile in aster to become extensile in the bundles. We show this transition in local active stress arises due to the broken detailed balance at the microscopic scale. The emerging response to external force seen in our work may help in understanding the underlying physical principle of the physiologically important phenomenon of cellular mechanosensing.