Direct Observation of Catch-Bonds in Focal Adhesions of Living Cells

Abstract

Single molecule force spectroscopy data have demonstrated that the chemical bonds between extracellular matrix proteins, integrins and several proteins of the focal adhesion complex show catch-bond behavior: the binding strength increases under mechanical load. It remains unknown, however, whether catch-bond mechanisms are of any relevance for stabilizing matrix adhesions in living cells. To measure adhesion strength, we bind RGD-coated magnetic beads to integrin adhesion receptors of living cells and apply forces of up to 80 nN with a magnetic tweezer. Under mechanical load, the beads detach stochastically from the cell surface, and the characteristic force at which 50% of the beads detach is a measure of the adhesion strength. In the case of a pulling force that increases linearly with time, the characteristic bead detachment force is expected to increase logarithmically with the loading rate for thermally activated Bell-type molecular bonds. We find that the detachment force tends to increase faster than logarithmically, demonstrating that the adhesion bonds strengthen under force. This may be indicative of catch bonds, but could also arise from a complex binding energy landscape that, as it is tilted under a mechanical load, presents different energy barriers against detachment. To distinguish between these two possibilities, we applied a staircase-like mechanical load with the same average loading rate but with forces that at all times exceeded those of the linear ramp protocol. We find significantly increased detachment forces under a staircase-like loading protocol compared to a linear force ramp, which rules out other mechanisms except catch-bond behavior.