Direct measurement of the lamellipodial protrusive force in a migrating cell

Abstract

There has been a great deal of interest in the mechanism of lamellipodial protrusion (Pollard, T., and G. Borisy. 2003. Cell. 112:453–465). However, one of this mechanism's endpoints, the force of protrusion, has never been directly measured. We place an atomic force microscopy cantilever in the path of a migrating keratocyte. The deflection of the cantilever, which occurs over a period of ∼10 s, provides a direct measure of the force exerted by the lamellipodial leading edge. Stall forces are consistent with ∼100 polymerizing actin filaments per micrometer of the leading edge, each working as an elastic Brownian ratchet and generating a force of several piconewtons. However, the force-velocity curves obtained from this measurement, in which velocity drops sharply under very small loads, is not sensitive to low loading forces, and finally stalls rapidly at large loads, are not consistent with current theoretical models for the actin polymerization force. Rather, the curves indicate that the protrusive force generation is a complex multiphase process involving actin and adhesion dynamics.

An historical perspective on cell mechanics

“…There is an extensive literature, dating back to the late 1800s, on wound healing and migration which are also highly mechanical in nature [153]. Recently, AFM has been used to measure the protrusive forces [44,154] at the edge of migrating cells in complement to traction force assays [155,156], micropipette and laser trap studies [157]. Migration is a key element in cancer metastasis, and in recent years, cells have been optically trapped and stretched in electromagnetic fields to measure mechanical properties in relation to metastatic potential [158][159][160][161] (complementary to early deformability assays [162]).…”

AFM as a tool to probe and manipulate cellular processes

“…Moreover, AFM allows for real-time, local monitoring of membrane fluctuations at the location on the cell surface that is undergoing extensive cytoskeleton movement and remodelling. The mechanical fluctuations recorded at the protruding edge of individual cells are larger in amplitude than those recorded in other regions of the cells, indicating that the cytoskeleton at the protruding edge is in a greater dynamical state [37,45,59].…”

“…To date, very few experiments have been designed to evaluate the mechanical contributions of individual cells to specific physiological responses. The importance of the mechanical responses and behaviour of single cells is especially relevant in dynamic physiological processes such as tissue remodelling, cell motility and signal transduction in various mechanically sensitive cells such as those involved in hearing, bone remodelling and pressure sensing in vascular systems [36,37,39,[41][42][43][44][45][46]. Several new concepts, describing mechanical responses in cells, have emerged since the development of AFM-thanks to its ability to apply or record minute forces on individual cells.…”

“…Several new concepts, describing mechanical responses in cells, have emerged since the development of AFM-thanks to its ability to apply or record minute forces on individual cells. The force-distance curves obtained by the direct indentation of the cell surface with an AFM tip have been used to quantify the mechanical properties of cells in contexts relevant to physiological processes such as cell migration, contraction and division [37,39,40,45]. The overall mechanical characteristics of a cell are largely determined by its plasma membrane, as well as by the different molecular components of the internal cytoskeleton and external glycocalix.…”

Biophysical Tools for Cellular and Subcellular Mechanical Actuation of Cell Signaling

“…For decades, glass microneedles have been used as force probes to investigate biological systems from single actin filament mechanics to muscle physiology (12,13). With the development of atomic force microscopy, more sophisticated cantilever force probes with sensitive force resolution have provided measurements of forces generated by single cells and in vitro cytoskeleton networks (14,15). Because of the requirement of a physical probe as a force transducer, force application is limited locally to the surface contact to a cell.…”