A Quantitative Analysis of Contractility in Active Cytoskeletal Protein Networks

Abstract

Cells actively produce contractile forces for a variety of processes including cytokinesis and motility. Contractility is known to rely on myosin II motors which convert chemical energy from ATP hydrolysis into forces on actin filaments. However, the basic physical principles of cell contractility remain poorly understood. We reconstitute contractility in a simplified model system of purified F-actin, muscle myosin II motors, and alpha-actinin cross-linkers. We show that contractility occurs above a threshold motor concentration and within a window of cross-linker concentrations. We also quantify the pore size of the bundled networks and find contractility to occur at a critical distance between the bundles. We propose a simple mechanism of contraction based on myosin filaments pulling neighboring bundles together into an aggregated structure. Observations of this reconstituted system in both bulk and low-dimensional geometries show that the contracting gels pull on and deform their surface with a contractile force of approximately 1 microN, or approximately 100 pN per F-actin bundle. Cytoplasmic extracts contracting in identical environments show a similar behavior and dependence on myosin as the reconstituted system. Our results suggest that cellular contractility can be sensitively regulated by tuning the (local) activity of molecular motors and the cross-linker density and binding affinity.

Cell-sized liposomes reveal how actomyosin cortical tension drives shape change

“…S3). This observation indicates that contraction requires sufficiently high network connectivity, consistent with prior findings for bulk actomyosin networks (22). Note that streptavidin can mediate both filament-membrane and filamentfilament attachments (i.e., that the strong attachment and weak attachment cases differ in both membrane attachment strength and network connectivity).…”

“…2 i and ii and Fig. 4, right bar), consistent with previous observations in bulk networks (22)(23)(24)(25)33). The striking finding is that actin-membrane anchoring governs the outcome of cortex contraction.…”

“…These approaches have been successfully used to show that contractility of bulk actomyosin networks depends on the kinetic parameters of the motors (21)(22)(23), as well as on the presence of actin cross-linkers that allow build-up of stress (24,25). A recent study of 2D actomyosin networks attached to flat model biomembranes showed that actin-membrane anchoring also influences cortex contractility (26).…”

Active contractility in actomyosin networks

“…We further construct a phase diagram for active contractility as a function of motor concentration and network connectivity at a given motor susceptibility. This diagram identifies the threshold motor concentrations and contains a window of network connectivity for active macroscopic contraction, consistent with observation (16). We also find that, at high connectivity, contraction can still occur (at intermediate motor concentrations) but only if the excluded volume effect is negligibly small.…”

“…Recently Bendix et al have reconstituted contractility in a simplified system of F-actin, muscle myosin II motors, and α-actinin cross-links (16). The well-controlled nature of this in vitro system allows a systematic study of the dependence of contractility on microscopic parameters, such as the number and activity of myosin motors, cross-link density, and actin network connectivity.…”