An Elastic Actomyosin Network in Motile Fish Keratocytes

Abstract

Crawling motility of eukaryotic cells is driven by net assembly of an actin network at the leading edge and net disassembly at the cell rear, orchestrated by a combination of biochemical and mechanical processes. To understand the distribution and function of mechanical forces in motile cells, it is necessary to determine the mechanical properties and connectivities of the structural components of the cell.We have investigated the mechanical properties of the actin network using detergent-extracted cytoskeletons from fish epithelial keratocytes. When stretched by a microneedle, the extracted lamellipodial network stretched significantly, up to several-fold its original dimensions. At moderate strains, the network was found to be highly elastic, at least over the timescales of 0.2 s to 30 s. These observations suggest that there is strong mechanical coupling within the cytoskeleton over distance scales comparable to the whole cell.Previously we have shown that activation of myosin II in detergent-extracted cytoskeletons by addition of ATP results in contraction and disassembly of the rear network. We have found that ATP analogs (AMP-PNP and ADP-fluoroberyllate) also cause partial disintegration and inward sliding of the rear network, toward the cell body, but without observable contraction within the network. The accompanied deformation of the cell body suggests that the cell body is stretched laterally in the live cell, anchored by the flanking actomyosin networks, and that release of actomyosin bonds (triggered by binding of ATP analogs to myosin) is sufficient for the network to rupture, allowing the cell body to relax. This raises the possibility that myosin may be playing a structural role, maintaining the integrity of the rear network under tension, while simultaneously driving its contraction. The cell body may serve as a reservoir for elastic energy, stress being distributed across the rear actomyosin network.