Non linear mechanics of dense dendritic actin networks

Actin is a protein which self-assembles into dynamic filaments organized in cells in different kind of meshworks. The combination of their mechanics and their dynamics enables the cell to achieve essential processes like migration, deformation or integration of external mechanical cues. Rheology of actin suspensions has been extensively studied at low concentrations at which the elastic response of filament is entropic, in agreement with their semi-flexible nature. Here we focus on more concentrated actin networks whose density and microscopic architecture is closer to the one found in cells. We use a cellular biochemical machinery, called Arp2/3 machinery, that assembles dense and dendritic networks; the mesh size is small compared to the actin persistence length implicating that actin filaments behave as rigid rod. We probed the elastic responses of these particular actin networks by a new technique based on magnetic particles and have shown that the mechanics is strongly non linear with a non linear modulus that decreases linearly towards low values when applied stress decreases. We attribute this behavior at low stress to the low connectivity of the networks as Arp2/3 mediates connection between three actin strands and not four as a more conventional crosslinker would do.