Shear thickening of F-actin networks crosslinked with non-muscle myosin IIB.

The material properties of cytoskeletal F-actin networks facilitate a broad range of cellular behaviors, whereby in some situations cell shape is preserved in the presence of force and, at other times, force results in irreversible shape change. These behaviors strongly suggest that F-actin networks can variably deform elastically or viscously. While a significant amount is known about the regulation of the elastic stiffness of F-actin networks, our understanding of the regulation of viscous behaviors of F-actin networks is largely lacking. Here, we study the rheological behavior of F-actin networks formed with heavy meromyosin non-muscle IIB (NMMIIB). We show that NMMIIB quenched with ADP crosslinks F-actin into networks that, for sufficient densities, display stress stiffening behavior. By performing a series of creep tests, we show that densely crosslinked actin/NMMIIB-ADP networks undergo viscous deformation over a wide range of stresses, ranging from 0.001 to 10 Pa. At high stresses, networks that stress stiffen are also observed to shear thicken, whereby the effective viscosity increases as a function of stress. Shear thickening results in a reduction in the extent of irreversible, viscous deformation in actin/NMMIIB-ADP networks at high stresses compared to that expected for a linear viscoelastic material. Thus, viscous deformation contributes less to the overall mechanical response at high levels of applied force. Our results indicate mechanisms by which the fluid-like nature of the actomyosin cytoskeleton can be reduced under high load.