Living soft matter
Gijsje Koenderink (FOM Institute AMOLF)
One of the defining qualities of soft matter is that it is readily driven far from thermodynamic equilibrium by external stress. Driving forces such as those due to an electric field or shear can drive colloidal suspensions and polymer networks into fascinating non-equilibrium patterns, such as banded or ordered steady states. By contrast, living cells naturally exhibit a unique form of internal driving in the form of chemomechanical activity. A prominent example is the cytoskeleton, a meshwork of protein polymers and force-generating motor proteins that constitutes the scaffold of cells. The cytoskeleton is responsible for driving vital cellular functions such as growth, division, and movement. In this talk, I will present two examples of our research on active cytoskeletal polymer gels. The first example concerns active contractility of the actin cortex, which lies underneath the cell membrane and drives shape changes by means of myosin motors. By reconstituting a simple model system composed of purified proteins, we could show how myosin motors and actin filaments collectively self-organize into force-generating arrays. We discovered that motors contract actin networks only above a sharp threshold in crosslink density, corresponding to a connectivity percolation transition. Surprisingly, the motors tend to drive initially well-connected networks robustly to this critical point. The second example I will discuss concerns cell shape polarization directed by interactions of actin filaments with microtubules. I will show that active force generation by growing and shrinking microtubules leads to feedback between the organization of the actin filaments and microtubules, explaining earlier observations made in living cells.