Active stresses and self-organization in intra/extracellular networks

Much like the bones in our bodies, the cytoskeleton consisting of filamentous proteins

largely determines the mechanical response and stability of cells. Unlike passive materials,

however, living cells are kept far out of equilibrium by metabolic processes and energyconsuming

molecular motors that generate forces to drive the machinery behind various

cellular processes. We describe recent advances both in theoretical modeling of such

networks, as well as experiments on reconstituted in vitro acto-myosin networks and living

cells. We show how such internal force generation by motors can lead to dramatic

mechanical effects, including strong mechanical stiffening. Furthermore, stochastic motor

activity can give rise to diffusive-like motion in elastic networks. We also show how the

collective activity of myosin motors generically organizes actin filaments into contractile

structures, in a multistage non-equilibrium process. This can be understood in terms of the

highly asymmetric load response of actin filaments: they can support large tensions, but

they buckle easily under piconewton compressive loads.