Laboratoire Matière et Systèmes Complexes

Actin networks are of paramount importance for cellular mechanics as they ensure cell integrity, deformability and migration. The relationship between the properties of the actin filaments, their associated proteins and the mechanics of the different organizations of the cytoskeleton remains elusive. Mechanics of actin networks has been studied so far with classical macro/micro rheometry at low actin concentration, or with demanding techniques such as atomic force microscopy at more physiological concentrations. We will present our new technique based on the dipolar attraction between magnetic colloids, which allows thousands of actin gels to be probed, orders of magnitude more than provided by previous experiments. We have first focused on dense branched networks assembled by the Arp2/3 complex with a minimum mix of purified proteins. By changing the proportion of nucleators or regulatory proteins, we have measured a strong dependence of the elasticity on the architecture of the networks. Our overall results are in agreement with an elasticity that arises from rigid fibers’ bending and stretching and not from entropic stretching of semi-flexible filaments as it is the case for less dense actin gels. We will also briefly present a new development using magnetic particles of controlled shapes (cubes, disks...), which will allow us to carry out dynamic measurements on growing actin gels and to study directly stress-strain curves to describe the non-linear mechanical behavior.

In parallel, with Alphée Michelot (CEA Grenoble) we study actin branched gels analogous to endocytic actin patches reconstituted from yeast extract. Compared to the purified system, these gels, which contain more than 80 regulatory proteins, are notably stiffer and present plastic deformations. Using mutant extracts, we observed that the absence of two cross-linkers softens the gels and modifies their long-term evolution whereas the absence of a protein involved in network disassembly, changes the plastic reorganization properties of the actin networks. This approach is a promising route to decipher the role of different regulatory proteins in the mechanics of actin cytoskeleton.