Nascent focal adhesions are responsible for the generation of strong propulsive forces in migrating fibroblasts

Fibroblast migration involves complex mechanical interactions with the unde rlying substrate. Although tight substrate contact at focal adhesions has b een studied for decades, the role of focal adhesions in force transduction remains unclear. To address this question, we have mapped traction stress g enerated by fibroblasts expressing green fluorescent protein (GFP)-zyxin. S urprisingly, the overall distribution of focal adhesions only partially res embles the distribution of traction stress. In addition, detailed analysis reveals that the faint, small adhesions near the leading edge transmit stro ng propulsive tractions, whereas large, bright, mature focal adhesions exer t weaker forces. This inverse relationship is unique to the leading edge of motile cells, and is not observed in the trailing edge or in stationary ce lls. Furthermore, time-lapse analysis indicates that traction forces decrea se soon after the appearance of focal adhesions, whereas the size and zyxin concentration increase. As focal adhesions mature, changes in structure, p rotein content, or phosphorylation may cause the focal adhesion to change i ts function from the transmission of strong propulsive forces, to a passive anchorage device for maintaining a spread cell morphology.