Tuning the Properties of Active Microtubule Networks by Depletion Forces

Abstract

Suspensions of microtubules and nonadsorbing particles form thick and long bundles due to depletion forces. Such interactions act at the nanometer scale and define the structural and dynamical properties of the resulting networks. In this study, we analyze the depletion forces exerted by two types of nonadsorbing particles, namely, the polymer, poly­(ethylene glycol) (PEG), and the block copolymer, Pluronic. We characterize their effects both in passive and active networks by adding motor proteins to the suspensions. By exploiting its bundling effect via entropic forces, we observed that PEG generates a network with thick structures showing a nematic order and larger mesh size. On the other hand, Pluronic builds up a much denser gel-like network without a recognizable mesh structure. This difference is also reflected in the network activity. PEG networks show moderate contraction in lateral directions while Pluronic networks exhibit faster and isotropic contraction. Interestingly, by mixing the two nonadsorbing polymers in different ratios, we observed that the system showed a behavior that exhibited properties of both agents, leading to a robust and fast responsive structure compared to the single-depletant networks. In conclusion, we show how passive osmotic compression modifies the distribution of biopolymers. Its combination with active motors results in a new active material with potential for nanotechnological applications.

Crosslinking and depletion determine spatial instabilities in cytoskeletal active matter

“…Besides bending and buckling instabilities, global and local contractions have been reported both in actin-myosin gels 8,10 and in MT-motors mixtures. 9,11,12,16 In the Dogic system, global contractions appear with taxol MTs, 6,[17][18][19] while local contractions arise with GMPCPP MTs. 18,19 Very recent experiments, however, have observed both types of contractions with GMPCPP MTs.…”

Self-emergent vortex flow of microtubule and kinesin in cell-sized droplets under water/water phase separation

“…Actin filaments have been reported to form such a cortex due to the depletion interaction induced by their large persistence length, typically a few 10 μm 17 , 42 . Since MTs have even larger persistence lengths (0.1–10 mm) 43 than actin filaments, we assume that depletion interaction also forms a cortex of MTs in the current system 44 . Upon the addition of kinesin without ATP, kinesin is distributed in the MT-rich region (see Supplementary Table S2 for detailed concentration).…”