Microrheology of Highly Crosslinked Microtubule Networks is Dominated by Force-Induced Crosslinker Unbinding

Abstract

We determine time- and force-dependent viscoelastic responses of reconstituted networks of microtubules that have been strongly crosslinked by biotin-streptavidin bonds. To measure the microscale viscoelasticity of such networks, we use a magnetic tweezers device to apply localized forces. At short time scales, the crosslinked networks respond nonlinearly to applied force, with stiffening at small forces and softening at high forces, which we attribute to the force-induced unbinding of crosslinks. At long time scales, force-induced bond unbinding leads to local network rearrangement and significant bead creep. Interestingly, the network retains its elastic modulus even under conditions of significant plastic flow, suggesting that crosslinker breakage is balanced by the formation of new bonds. To better understand this effect, we developed a finite element model of such a stiff filament network with labile crosslinkers obeying force dependent Bell model unbinding dynamics. We confirm that for rigid MT filaments having many crosslinks, the coexistence of dissipation and elastic recovery of the network is possible as a result of bond unbinding and rebinding events. Elastic recovery can occur as long as a sufficient number of the original crosslinkers are preserved under the loading period. Plastic flow increases with the decreasing fraction of original crosslinkers preserved. It is interesting to note that the dissipative and plastic responses of such networks to applied loads are more similar to crack propagation in solids than to standard polymer rheology, where standard mechanisms for energy dissipation are hydrodynamics, filament contour fluctuations, etc. Our results will have important implications for understanding mechanical properties of cytoskeletons, where networks of MTs and Factin bundles crosslinked by different flexible and transient crosslinks are locally deformed by transport of intracellular cargos and by the large-scale structural changes in cell division, motility and morphogenesis.