Spontaneous Formation of a Globally Connected Contractile Network in a Microtubule-Motor System

Abstract

Microtubule (MT) networks play key roles in cell division, intracellular transport, and cell motility. These functions of MT networks occur through interactions between MTs and various associated proteins, notably motor proteins that bundle and slide MTs. Our objective in this study was to address the question of how motors determine the nature of MT networks. We conducted in vitro assays using homotetrameric kinesin Eg5, a motor protein involved in the formation and maintenance of the mitotic spindle. The mixing of Eg5 and MTs produced a range of spatiotemporal dynamics depending on the motor/filament ratio. Low motor/filament ratios produced globally connected static MT networks with sparsely distributed contractile active nodes (motor-accumulating points with radially extending MTs). Increasing the motor/filament ratio facilitated the linking of contractile active nodes and led to a global contraction of the network. When the motor/filament ratio was further increased, densely distributed active nodes formed local clusters and segmented the network into pieces with their strong contractile forces. Altering the properties of the motor through the use of chimeric Eg5, which has kinesin-1 heads, resulted in the generation of many isolated asters. These results suggest that the spatial distribution of contractile active nodes determines the dynamics of MT-motor networks. We then developed a coarse-grained model of MT-motor networks and identified two essential features for reproducing the experimentally observed patterns: an accumulation of motors that form the active nodes necessary to generate contractile forces, and a nonlinear dependency of contractile force on motor densities. Our model also enabled us to characterize the mechanical properties of the contractile network. Our study provides insight into how local motor-MT interactions generate the spatiotemporal dynamics of macroscopic network structures.

Filament Rigidity and Connectivity Tune the Deformation Modes of Active Biopolymer Networks

“…While the role of cross-linking has been well described in terms of controlling force transmission (13,14,(21)(22)(23), our work suggests that it also plays an important role in controlling the direction of the deformation, namely changing it from extensile to contractile. This result unifies previous observations of both extensile and contractile behaviors in active microtubule systems (6,7,(17)(18)(19)(20), suggesting that network connectivity is a significant factor in determining which behavior predominates. In future work, it will be interesting to explore the transitions between other microscopic deformation modes in active motor-filament systems and see how these are controlled by local structure or composition (e.g., filament orientation or polarity organization).…”

“…Experimentally, in vitro networks constructed from actin filaments and myosin II motors are robustly contractile (13)(14)(15)(16). By contrast, systems of microtubules and molecular motors are either extensile (6,7,17,18) or contractile (19,20). One difference between these two active materials is that microtubules are significantly more rigid than actin.…”

Cytoskeleton polarity is essential in determining orientational order in basal bodies of multi-ciliated cells

“…Typically, we used −1 = 3.16 which corresponds to ~15 min in reality. This is comparable with the observed time scale of MT bundle dynamics in vitro [39]. In addition, we set = following the model for actin filament in [12].…”

Non-equilibrium assembly of microtubules: from molecules to autonomous chemical robots

“…Several recent high profile studies expanded upon this theme and demonstrated filament/motor systems exhibiting self-organization, ranging from networks formed by actin filaments and myosin motors, 119, 120 over vortex lattices emerging from microtubules moving on surface-adhered motors, 94, 97, 121 to microtubule bundle networks exhibiting motion and internal flows driven by kinesin motors. 74, 77118, 122 …”

“…(b) Different microtubule networks form depending on the concentration of motile cross-linkers (here the kinesin Eg5). Adapted from 118 . (c) Kinesin cross-linkers and depletion forces lead to the self-organization of microtubules into beating cilia.…”