Effects of microtubule length and crowding on active microtubule network organization

SUMMARY Active filament networks can organize into various dynamic architectures driven by crosslinking motors. Densities and kinetic properties of motors and microtubules have been shown previously to determine active microtubule network self-organization, but the effects of other control parameters are less understood. Using computer simulations, we study here how microtubule lengths and crowding effects determine active network architecture and dynamics. We find that attractive interaction mimicking crowding effects or long microtubules both promote the formation of nematic networks of extensile bundles instead of contractile networks. When microtubules are very long and the network is highly percolated, a new isotropically motile network state resembling a ‘crawling mesh’ is predicted. Using in vitro reconstitutions, we confirm the existence of this crawling mesh experimentally. These results provide a better understanding of how active microtubule network organization can be controlled, with implications for cell biology and active materials in general.