The mechanobiochemistry of motile actin networks
Dr. Peter Bieling, UC Berkeley
Branched actin networks generate forces required for cell morphogenesis, motility and organization of sub-cellular structures. Despite the importance of polymerization-mediated force generation in many biological processes, surprisingly little is known about how dynamic actin networks with physiological architecture respond to forces at the molecular level. We combined micropatterning with atomic force microscopy and multi-color TIRF imaging to visualize the force-generating region of growing, lamellipodial networks in vitro. We demonstrate that branched actin networks are highly load-sensitive: Network velocity decreases exponentially with counteracting forces. However, the actin density increases strongly with elevated loads. Measurement of the force-dependent rate of nucleation demonstrates that enhanced branching by the Arp2/3 complex is not responsible for increased actin densities. Nevertheless, the concentration of free polymerizing ends rises with the the load force, an effect that can be explained by the force-dependent decrease in capping. Our AFM-TIRF measurements provide a detailed characterization of the force-dependent changes in actin network assembly. Furthermore, we have investigated how mechanical properties of branched networks respond to external forces. Our results show that the elasticity and mechanical resilience of dendritic networks are strongly dependent on the mechanical constraints they experience during assembly: Networks grown under high load are stiffer and more resilient to subsequent mechanical stresses, while networks grown under low force are easily plastically deformed. This shows that networks permanently adapt to the mechanical environment through changes in their architecture.