Mechanics of the F-actin cytoskeleton

Abstract

Dynamic regulation of the filamentous actin (F-actin) cytoskeleton is critical to numerous physical cellular processes, including cell adhesion, migration and division. Each of these processes require precise regulation of cell shape and mechanical force generation which, to a large degree, is regulated by the dynamic mechanical behaviors of a diverse assortment of F-actin networks and bundles. In this review, we review the current understanding of the mechanics of F-actin networks and identify areas of further research needed to establish physical models. We first review our understanding of the mechanical behaviors of F-actin networks reconstituted in vitro, with a focus on the nonlinear mechanical response and behavior of "active" F-actin networks. We then explore the types of mechanical response measured of cytoskeletal F-actin networks and bundles formed in living cells and identify how these measurements correspond to those performed on reconstituted F-actin networks formed in vitro. Together, these approaches identify the challenges and opportunities in the study of living cytoskeletal matter.

Figure 1

Fluorescent Images (left) and schematics (right) of F-actin networks observed in living cells (top) and in vitro (bottom). F-actin (green), a focal adhesion marker (blue, paxillin) and myosin II (red) are indicated in the image of the cell; in vitro network is stained for F-actin using fluorescent phalloidin. Cell schematic indicates the lamellipodium (green cross-hatches), lamella (thin gridded lines), stress fibers (thick lines) and cortex (surrounding boundary). By contrast, F-actin networks in vitro are homogeneous networks. Scale bar is 10 microns in both images.

Figure 2

Schematic diagrams of four distinct F-actin cytoskeletal networks discussed here showing the geometry of force application (black arrow) for the mechanical measurement of elastic modulus (G′) or viscous modulus (G″). For the lamellar network, force arrow indicates cell-generated force measured in the underlying extracellular matrix.