Micro- and Macrorheological Properties of Isotropically Cross-Linked Actin Networks

Abstract

Cells make use of semiflexible biopolymers such as actin or intermediate filaments to control their local viscoelastic response by dynamically adjusting the concentration and type of cross-linking molecules. The microstructure of the resulting networks mainly determines their mechanical properties. It remains an important challenge to relate structural transitions to both the molecular properties of the cross-linking molecules and the mechanical response of the network. This can be achieved best by well defined in vitro model systems in combination with microscopic techniques. Here, we show that with increasing concentrations of the cross-linker heavy meromyosin, a transition in the mechanical network response occurs. At low cross-linker densities the network elasticity is dominated by the entanglement length l(e) of the polymer, whereas at high heavy meromyosin densities the cross-linker distance l(c) determines the elastic behavior. Using microrheology the formation of heterogeneous networks is observed at low cross-linker concentrations. Micro- and macrorheology both report the same transition to a homogeneous cross-linked phase. This transition is set by a constant average cross-linker distance l(c) approximately 15 microm. Thus, the micro- and macromechanical properties of isotropically cross-linked in vitro actin networks are determined by only one intrinsic network parameter.

The role of structure in the nonlinear mechanics of cross-linked semiflexible polymer networks

“…28,38,41 The contributions from entanglement effects and crosslinks to the mechanics at small strains have been studied theoretically and experimentally previously in the context of actin networks. 42,43 It was found that, below a critical value of R, the concentration-dependent G 0 agrees with the theoretical prediction for entangled actin solution entanglements effects, and this has been described in terms of the Odijk length 44 and the resulting suppression of thermal excitation. 45 At the other end, above the critical value of R, G 0 is strongly dependent on R, indicating the role of cross-links in the network mechanics.…”

“…45 At the other end, above the critical value of R, G 0 is strongly dependent on R, indicating the role of cross-links in the network mechanics. 43 A comparison between bulk rheology and microrheological measurements reveals significant heterogeneity within biopolymer networks and an increasing degree of homogeneity with cross-linker concentration. 36,43 Moreover, when spatially averaged, microrheology leads to results consistent with those from bulk rheology, 43 as we also find.…”

“…43 A comparison between bulk rheology and microrheological measurements reveals significant heterogeneity within biopolymer networks and an increasing degree of homogeneity with cross-linker concentration. 36,43 Moreover, when spatially averaged, microrheology leads to results consistent with those from bulk rheology, 43 as we also find. Taken together, these studies therefore provide experimental support for our hypothesis that the relative roles of fiber entanglement and cross-links need to be considered in the context of network heterogeneity and measurement length scale.…”

Computational Analysis of a Cross-linked Actin-like Network

“…This means that large prestrain transforms the network into a highly elastic one that exhibits a phase delay close to 0 at all frequencies and results in G' comparable to in vivo values [9][10][11], wherein it has been postulated that prestress or prestrain plays a significant role [14]. G" also exhibits interesting behavior; at high prestrain, it increases slightly at low frequency, similar to observations in vitro using heavy meromyosin (HMM) [18,41], and a similar increase in G" was also observed in vivo [19,38]. This suggests that at low frequencies, viscous effects play an important role.…”

Actomyosin purse strings: Renewable resources that make morphogenesis robust and resilient

“…Because the secondary purse string assembles during a recoil phase, matures during a plateau phase, and then begins to shorten, this leaves little opportunity for prestretching the bulk of the newly formed purse string. Nevertheless, we cannot rule out that some compliance due to passive elastic properties is in part contributed by actin or myosin filaments themselves (reviewed in Smith et al, 2005) or by entropic properties of the actomyosin purse string (Luan et al, 2008). The prestretching that is required for such passive elasticity might occur as a consequence of active contractility or might be attributable to the mechanical properties of elements of actomyosin-rich purse strings that were assembled before the turning point and full maturation of a functional, secondary purse string.…”