Regulation of cell-matrix adhesion strength by cytoskeleton re-organization

Upon spreading on an extracellular matrix (ECM), fibroblast cells may create discrete focal adhesion complexes, which intimately connect cytoskeleton actin to the ECM via specialized adhesion molecules and the cell receptor integrin. However, a quantitative relation between this cytoskeleton re-organization and the overall cell-matrix adhesion strength has not been established. Using analytical immuno-fluorescence confocal microscopy and atomic force spectroscopy, we systematically measured the time-dependent density, spatial distribution, molecular composition, and local strength of focal adhesions in Swiss3T3 fibroblasts for various cell-matrix contact times. These parameters were directly compared to the time-dependent extent of spreading and overall adhesion strength of cells to substrates coated with a wide concentration range of the ECM protein fibronectin (FN). We found that at short cell-matrix contact times, for which few vinculin-containing focal adhesions were present, the overall cell-matrix was regulated by FN concentration. For long contact times, however, cell-matrix adhesion strength was high but independent of FN concentration, which instead dramatically increased the number of focal adhesions. Morphometric measurements showed that the extent of cell spreading increased with FN concentration at short contact time and was instead high and constant at long contact time. Together with complementary data on cells that do not express fibronectin, these results question the conventional wisdom that the presence of focal adhesions correlates with firm cell adhesion.