Dynamics in active actin networks

The actin cytoskeleton consists of a highly dynamic protein polymer network wich is regulated by a myriad of binding proteins. These dynamics originate not only from polymerization and degradation of actin filaments but also motor proteins can introduce an active component. This results in a highly flexible and adaptable scaffold that undergoes constant remodeling. Despite the fundamental importance of such active systems, the microscopic mechanisms and their consequences are poorly understood.

Movie 1: Dynamics in active actin networks.

We investigate the underlying physical principles of such active networks to examine their microscopic dynamics. To classify the thereby resulting dynamic structures we use a reconstituted active actin network consisting of three major components, actin filaments, fascin crosslinking proteins and myosin-II motor filaments. The presence of only motor filaments and ATP does not suffice to induce any reorganization or structure formation in an actin solution. Only the combination of motors and crosslinking molecules can result in structure formation (Movie 1).

Movie 2: Cluster formation.

In this minimal model system the key parameters are tightly controlled which allows unravelling the basic physical principles underlying the complex cytoskeletal dynamics. At sufficiently high motor concentrations and strengths, a coarsening process is observed resulting in a dynamic steady state, which is characterized by a broad cluster size distribution. Three distinct cluster sizes with their respective characteristic stability and dynamics can be identified (Movie 2). Only clusters of actin filaments above a critical size of 10–20 µm in diameter are able to grow and coarsen. Their growth up to several hundred µm in diameter is limited by the concomitantly decreasing connectivity within the actin network and the subsequently decreased possibility of active transport. This dependence of the overall mobility on the growth and coarsening rate proves robust for all varied parameters, suggesting this to be a generic feature of such active networks.

 

Movie 3: Correlated motion.

Increasing the maximal force the motor proteins can exert on the network results in the occurence of collective modes (Movie 3). They are dominated by the complex competition of crosslinking molecules and motor filaments in the network: Collective dynamics are only observed if the relative strength of the binding of myosin-II filaments to the actin network allows exerting high enough forces to unbind the crosslinking proteins. This prevents cluster formation and ensures a high connectivity in the network which is a prerequisite for the occurence of collective modes.

 

The resulting dynamics can be described by an anomalous diffusion of the network’s constituents reminiscent of the intracellular dynamics in living cells. The observed superdiffusive behavior can be traced back to a complex alternation of of runs and stalls of the individual network structures: Stalls are attributed to the binding of network structures by crosslinking molecules, while runs are originated in the forced unbinding of these crosslinks and the subsequent transport of the actin structures by motor filaments. Thus, the degree of superdiffusivity is set by the balance of motor activity and the degree of connectivity in the active network.

 

It is the excellent accessibility of the self organization principles and dynamics on all levels of description – from the molecular mechanisms to large scale macroscopic pattern formation – that makes this minimal active system based on fascin, myosin-II and actin to a versatile benchmark for the exploration of the broad material class of active gels.