An active biopolymer network controlled by molecular motors

Abstract

We describe an active polymer network in which processive molecular motors control network elasticity. This system consists of actin filaments cross-linked by filamin A (FLNa) and contracted by bipolar filaments of muscle myosin II. The myosin motors stiffen the network by more than two orders of magnitude by pulling on actin filaments anchored in the network by FLNa cross-links, thereby generating internal stress. The stiffening response closely mimics the effects of external stress applied by mechanical shear. Both internal and external stresses can drive the network into a highly nonlinear, stiffened regime. The active stress reaches values that are equivalent to an external stress of 14 Pa, consistent with a 1-pN force per myosin head. This active network mimics many mechanical properties of cells and suggests that adherent cells exert mechanical control by operating in a nonlinear regime where cell stiffness is sensitive to changes in motor activity. This design principle may be applicable to engineering novel biologically inspired, active materials that adjust their own stiffness by internal catalytic control.active soft matter ͉ cytoskeleton ͉ filamin A ͉ rheology ͉ myosin II R econstituted biopolymer networks are a class of soft material that exhibits pronounced solidlike behavior while comprising only a few percent by volume of protein. They exhibit a rich mechanical behavior and pronounced nonlinearity (1). Moreover, they model important features of living cells, because the mechanics of cells are largely controlled by a network of filamentous proteins known as the cytoskeleton. Reconstituted networks comprising one of the principal components of the cytoskeleton, filamentous actin (F-actin), are interesting models for the rheology of semiflexible polymers. Like most filamentous protein networks, they are highly responsive to external stress. Cross-linked F-actin networks exhibit strong nonlinear stiffening with strain (1, 2). This phenomenon is particularly pronounced for the case of F-actin cross-linked by filamin A (FLNa), a widely represented protein that is essential for fetal development and cell locomotion (3). FLNa is a large, highly flexible dimer that promotes orthogonal F-actin cross-linking (4). FLNa-F-actin networks are very soft in their linear regime, with shear moduli as low as 1 Pa, yet they stiffen by up to three orders of magnitude in response to external shear stress (5-8). In this respect, FLNa-F-actin resembles the behavior of living cells, which also stiffen if subjected to (tensile) stress (9, 10). Cells, however, are not passive materials, but rather use molecular motors to convert chemical energy into mechanical work within the cytoskeleton. There is evidence that cells employ internal stress generated by myosin motors to modulate their mechanics (11)(12)(13)(14)(15)(16)(17)(18)(19). This suggests a strategy of creating a new class of active materials, whose elastic properties are controlled by enzymatic activity, through addition of myosin motors to a passive F-actin network....

Intracellular transport driven by cytoskeletal motors: General mechanisms and defects

“…The geometry for the simulations was an ellipse of half-axes a = 19.2 µm, b = 6.6 µm with the MTOC being 1.2 µm off the center of the ellipse in x-and in y-direction which are representative parameters for the cell geometry discussed in [206]. Figures (A-B) (simulations) are taken from [110] and figures (C-D) (experiments) from [206] have been studied in in vitro experiments [52,203,11,104]. We shall not address this large field of research in this review.…”

Viscoelastic and elastomeric active matter: Linear instability and nonlinear dynamics

“…In Ref. [48], the modulus of a (contractile) actin gel was found to increase by a factor of 10 compared to the passive case in the presence of myosin motor activity, suggesting an upper bound to the dimensionless activity parameter of ζ /G Q ≈ 10.…”

Mimicking the mechanical properties of the cell cortex by the self-assembly of an actin cortex in vesicles

“…4,[10][11][12] The latter relies on the reconstitution of the actin cytoskeleton with purified or synthesized components in in vitro systems. [13][14][15][16][17][18][19][20][21] Previous measurements conducted with in vivo and in vitro systems suggest that the mechanical properties of the actin cortex depend on the force-dependent affinities of all of these proteins to F-actin as well as their concentrations. Despite the numerous experiments conducted in live cells, only a limited number of genes and proteins can be deleted or inhibited simultaneously, and when expressing genes of interest in cells, it can be difficult to control expression precisely.…”

“…One type of in vitro assay is to assemble the actin meshwork into a 2D flat sheet and measure its viscoelastic properties by particle tracking methods and shear micro-rheology. [14][15][16]18,[22][23][24] These assays provided many deep insights into the mechanical properties of cytoskeletal proteins. But the curvature effect of cytoskeletonmembrane composite in cells is missing in these measurements.…”