Microrheometry of semiflexible actin networks through enforced single-filament reptation: Frictional coupling and heterogeneities in entangled networks

Abstract

Magnetic tweezers are applied to study the enforced motion of single actin filaments in entangled actin networks to gain insight into friction-mediated entanglement in semiflexible macromolecular networks. Magnetic beads are coupled to one chain end of test filaments, which are pulled by 5 to 20 pN force pulses through entangled solutions of nonlabeled actin, the test filaments thus acting as linear force probes of the network. The transient filament motion is analyzed by microfluorescence, and the deflectionversus-time curves of the beads are evaluated in terms of a mechanical equivalent circuit to determine viscoelastic parameters, which are then interpreted in terms of viscoelastic moduli of the network. We demonstrate that the frictional coefficient characterizing the hydrodynamic coupling of the filaments to the surrounding network is much higher than predicted by the tube model, suggesting that friction-mediated interfilament coupling plays an important role in the entanglement of non-cross-linked actin networks. Furthermore, the local tube width along the filament contour (measured in terms of the root-mean-square displacement characterizing the lateral Brownian motion of the test filament) reveals strong fluctuations that can lead to transient local pinching of filaments. N etworks of filamentous actin (F-actin) are of great interest from the point of view of both cell biology and polymer physics and have thus been the subject of intense experimental and theoretical studies. On the one hand, actin is a major structural component of the intracellular scaffold (the cytoskeleton) and plays a key role in various cellular processes, such as cell locomotion (1, 2), the transport processes within cells (3), or the control of cell adhesion on surfaces (4). To fulfill this multiplicity of tasks, nature uses a large number of helper proteins. These include severing proteins by which the length of actin filaments can be manipulated, monomer-sequestering proteins that allow the control of the polymer concentration, and finally a manifold of cross-linker proteins enabling the generation of randomly organized gels or arrangements of bundles acting as cell-stabilizing fibers or cables for intracellular transport (5).On the other hand, F-actin is a prototype of a semiflexible polymer. Highly versatile models of entangled and cross-linked networks of semiflexible macromolecules can be designed by controlling the structure through the manifold of manipulating proteins to study the distinct physical properties of this particular class of polymer networks. Such semiflexible polymer networks exhibit outstanding viscoelastic features, which are determined by a subtle interplay of entropic and enthalpic contributions to the elastic free energy of the individual filament (6-12).One distinct advantage of actin as model polymer is the large contour length (typically in the 10-m range) (13) and persistence length (L p Ӎ17 m) (10, 14, 15) enabling the design of networks with mesh sizes in the optical wavelength regime. Therefore, t...

Dynamic cross-correlations between entangled biofilaments as they diffuse

“…The experimental times probed ( Fig. 2A) are longer than required for dynamic equilibration of a filament in its tube but far below the reptation time (31,32). Fig.…”

“…Biopolymer filaments of F-actin in aqueous solution present an attractive test bed for theory and experiment to address this matter (9,23,31,32). They are globally isotropic yet heavily entangled at the concentrations studied here.…”

“…2B). Relative to the critical entanglement concentration, c e ≈ 0.4 mg/mL (31,32), these systems are highly entangled, with c/c e in the range 1 to 5. One fluorescent dye labels the chain backbone uniformly and hence its emission is diffraction-limited, whereas the chain is also labeled in sparse abundance with a second fluorescent dye to give subdiffraction resolution.…”

“…They are globally isotropic yet heavily entangled at the concentrations studied here. The intermediate time-scale regime of anisotropic diffusion, which is longer than the entanglement onset time (τ e ∼10 −1 s) (31) to equilibrate within a tube but far less than the terminal flow or reptation time (τ rep ∼10 3 s) (31,32), is experimentally accessible. The time and length scales we probe, and a cartoon of the system, are summarized in Fig.…”

Active microrheology of networks composed of semiflexible polymers: Theory and comparison with simulations

“…This approach was applied to analyze the micromechanical behavior of actin networks, the cytoplasm and cellular membranes 12,38,[41][42][43][44][45] In general, arising of the osmotic force discussed in this paper requires two conditions:…”