Introduction

Cell migration plays an essential role in the development of most organisms (1,2) and in adult life (3). A migrating cell exerts the traction forces to the substratum to realize its migration. Traction forces are generated by intracellular molecular machinery. At the anterior of the cell, actin polymerization is considered the source of the driving forces for extension of the leading edge (3,4,5,6). On the other hand, the detachment of the rear of the cell from the substratum and its retraction is thought to occur through contraction via a myosin II-dependent process (7,8,9,10,11). At the front of the cell, actin polymerization-mediated counterforces should therefore be exerted onto the substratum in the opposite direction of extension. On the other hand, prior to detachment of the rear of the cell from the substratum, myosin II accumulation and a subsequent increase in traction forces should be exerted onto the substratum via a myosin II-dependent process. To detect the traction forces exerted by migrating cells, Dembo and colleagues (12,13) developed force microscopy, in which the measurement of deformation of the elastic substratum under a migrating cell was converted into stress in the substratum using finite element methods. They showed a detailed map under migrating fibroblasts (14,15,16,17,18,19) and revealed dynamic traction stresses at the leading edge of migrating fibroblasts (15,16). On the other hand, Butler et al. (20) proposed another kind of calculation method, namely Fourier-transform traction cytometry, and showed a detailed traction map under smooth muscle cells.

Fluorescence of green fluorescent protein (GFP)-zyxin in a migrating fibroblast and traction forces were simultaneously observed using fluorescence microscopy (17), and in some cases, traction forces appeared at nascent focal adhesions. Simultaneous observation of more detailed molecular dynamics, such as polymerization of actin at the leading edge or accumulation of filamentous myosin II at the rear end of migrating cells, as well as exertion of traction forces onto the substratum, could provide important information about the relationships between molecular dynamics and their production of traction forces.

Dictyostelium is one of the most appropriate materials to investigate the relationships between cell migration and molecular dynamics because Dictyostelium cells are known as fast moving irregularly shaped cells and molecular genetic approaches can be easily applied to them, such as expressing GFP fusion proteins. Although it had been difficult to measure the traction force of migrating Dictyostelium cells because of their small size, several authors recently showed the distribution of traction forces under a migrating Dictyostelium cell (11,20,21,22,23).

We therefore developed a new method for simultaneous recording of molecular dynamics and traction forces under migrating cells, in which total internal refractive fluorescence (TIRF) and force microscopy were combined. Here we describe the method and its application to simultaneous recording of enhanced GFP-myosin II and traction forces under a migrating Dictyostelium cell, as well as the recording of traction forces produced by individual filopodia.

Materials and Methods

Cell Culture

Dictyostelium discoideum cells were cultured in HL5 medium (1.3% w/v bacteriological peptone, 0.75% w/v yeast extract, 85.5 mM D-glucose, 3.5 mM Na₂HPO₄12H₂O, 3.5 mM KH₂PO₄, pH 6.4) and developed until the cells became aggregation-competent in a balanced salt solution (BSS; 10 mM NaCl, 10 mM KCl, 3 mM CaCl₂). Two cell lines, AX2 (referred to as wild-type in this paper and an axenic derivative of the wild-type strain NC4) expressing GFP-ABD120k (24) and myosin heavy chain null cells (HS1) expressing GFP-myosin II (25), were used. ABD120k is the actinbinding domain of an actin cross-linking protein, ABP120k. ABD120k can bind to filamentous but not globular actin. Therefore, fluorescence imaging of GFP-ABD120k reflects the distribution of filamentous actin. The GFP-ABD120k gene was kindly provided from David A. Knecht (University of Connecticut, Storrs, CT). This fused gene was inserted into the pBIG expression vector by Taro Q. Uyeda (AIST, Tsukuba, Japan).

Preparation of Elastic Substrata

To simultaneously observe the dynamics of molecular machinery and traction forces in a migrating cell, we combined TIRF and force microscopy. To create an evanescent field above the surface of elastic substrata, the refractive index of the substrata must be higher than 1.33, the index of the medium surrounding the cells. An evanescent field could not be created on the surface of elastic substrata made with previously described materials such as polyacrylamide (15) and gelatin (26) because of their low refractive indexes and/or low degree of transparency. Therefore, we selected a pair of liquid silicones (CY52-276A and B; Dow Corning Toray, Tokyo, Japan) as materials for the elastic substrata. These are platinum-catalyzed silicones that use a platinum complex to participate in hydrosilylation of a vinyl functional siloxane polymer by a hydride functional siloxane polymer. The gel made from the pair of silicones is clear and colorless, and its refractive index is 1.40. Moreover, the elasticity of the gel can be modulated by changing the mixing ratio of the two liquid silicones.