Filament Nucleation Tunes Mechanical Memory in Active Polymer Networks

Abstract

Incorporating growth into contemporary material functionality presents a grand challenge in materials design. The F-actin cytoskeleton is an active polymer network that serves as the mechanical scaffolding for eukaryotic cells, growing and remodeling in order to determine changes in cell shape. Nucleated from the membrane, filaments polymerize and grow into a dense network whose dynamics of assembly and disassembly, or "turnover," coordinates both fluidity and rigidity. Here, the extent of F-actin nucleation is varied from a membrane surface in a biomimetic model of the cytoskeleton constructed from purified protein. It is found that nucleation of F-actin mediates the accumulation and dissipation of polymerization-induced F-actin bending energy. At high and low nucleation, bending energies are low and easily relaxed yielding an isotropic material. However, at an intermediate critical nucleation, stresses are not relaxed by turnover and the internal energy accumulates 100-fold. In this case, high filament curvatures template further assembly of F-actin, driving the formation and stabilization of vortexlike topological defects. Thus, nucleation coordinates mechanical and chemical timescales to encode shape memory into active materials.

VASP localization to lipid bilayers induces polymerization driven actin bundle formation

“…Since mDia1 only binds the barbed end of F-actin during polymerization, the filaments are free to diffuse away from the membrane. This is in contrast to when additional depleting agents force the filaments to the bilayer ( Yadav et al , 2019 ). Additionally, the CP in solution competes with mDia1 for the barbed end, thus causing filaments to detach from the bilayer.…”

Kinesin and Myosin Motors Compete to Drive Rich Multi-Phase Dynamics in Programmable Cytoskeletal Composites

“…Crosslinking proteins also connect and bundle filaments as needed for cellular processes [9][10][11][12] . This complex composite continuously restructures and reconfigures itself in response to the demands of the cell, to enable diverse processes from cytokinesis to mechano-sensing [3][4][5]7,8,[13][14][15][16][17][18][19][20][21] . In vitro systems of reconstituted cytoskeletal proteins, which display rich and tunable dynamics, are also intensely studied as model active matter platforms to shed light on the non-equilibrium physics underlying force-generating, reconfigurable systems 7,12,19,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] .…”

Active Cytoskeletal Composites Display Emergent Tunable Contractility and Restructuring

“…Other studies have shown that a critical motor density is required for contractility, but contraction rates do not change substantially above this density 17 . Further, organized contractile dynamics and restructuring has often required confining actomyosin networks to quasi-2D geometries crowded to a surface, and has typically been limited to ~5 min of observed activity 19,20 . Here, to increase the tunability, longevity, resilience, and viability of actomyosin active matter as a functional material, we couple microtubules to actomyosin networks and combine experiments with modeling to systematically explore a broad parameter space of composite formulations and spatiotemporal scales.…”