A quantitative analysis of contractility in active cytoskeletal protein networks
UNDER CONSTRUCTION -- Original Entry by Holly McIlwee, AP225 Fall 09
A quantitative analysis of contractility in active cytoskeletal protein networks P. Bendix, G. Koenderink, D. Cuvelier, Z. Dogic, B. Koeleman, W. Brieher, C. Field, L. Mahadevan and D. Weitz, Biophysical Journal, 94, 3126, 2008.
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 a-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;1mN, or
- 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.
myosin, actin, cellular contraction
Contractile forces are important for cell function. Forces are transmitted by the cytoskeleton, a dynamic scaffold of protein filaments throughout the cytoplasm connected to the plasma membrane. Actin and Myosin II have been identified as the main components in this contraction. F-actin provides the structure upon which myosin performs its job, powered by ATP hydrolysis. Contraction of cross-linked actin-myosin networks is mediated by internal stresses that are actively generated by the myosin motors. Myosin assembles and generates gliding of actin filaments past one another. Though there have been many successful attempts to model the contractile actin cortex, there is still a limited understanding of the dependence of contractility and pattern formation in actin-myosin gels on microscopic parameters such as the number, activity, and processivity of the myosin motors or the local cross-linker density and actin network connectivity.
Experiments have shown that contraction is accelerated by proteins that crosslink the actin filaments, but systematic and quantitative investigation of contraction is difficult because of the complexity of the system. Contraction has been studied in simplified systems and even then the importance of F-actin crosslinker was apparent. It was also shown that myosin cannot adequately do its job without the crosslinks.
Here Weitz et al. focus on <math>alpha</math>-actinin, a protein expressed in contractile cytoskeletal assemblies, for example muscle myofibrils. A simplified model of actin, myosin, and <math>alpha</math>-actinin was used. Microstructure and macroscopic behavior is imaged using confocal microscopy. The number of crosslinks and myosin motors per actin filament is varied. It is sown that contractility is achieved at a critical concentration of <math>alpha</math>-actinin. It is concluded that contractility is caused by myosin filaments pulling on actin bundles without changing the bundle dimensions. The results reveal the molecular mechanisms underlying macroscopic force generation by a collection of myosin motors embedded in a random network of actin filaments.