E-selectin/sLea Form Catch-Slip Bonds Without Force-History Dependence

Abstract

It has been established that P- and L-selectins form catch-slip bonds with their ligands, with initially increasing and subsequent decreasing bond lifetimes with increasing load. Little, however, is known about the mechanics of E-selectin bonds with its ligands. We tested the load-dependence of bond rupture for E-selectin, and its dependence on the history of load application, by using the distribution of load between multiple receptor-ligand bonds to create a complex loading history. Briefly, an E-selectin-coated bead was held in a laser trap and touched to the vertical surface of a bead coated with sialyl lewis A (sLea), allowing one or more bonds to form with the ligand-coated surface. The laser trap was deflected away from the trapped bead, applying a nearly instantaneous load. When multiple bonds were present, we were able to discern the rupture of each bond as a step displacement of the trapped bead away from the stationary bead. In this way we were able to monitor both the number of bonds and the lifetime of each bond. We assumed that the load was evenly distributed between bonds and were thus able to monitor bond lifetimes across complex loading histories as the bonds ruptured asynchronously. Our data suggests that E-selectin/sLea bonds behave as catch-slip bonds with critical forces of approximately 35 pN. Further, the lifetime of single bonds is similar to the lifetime of bonds that have previously shared load with others. This implies that E-selectin/sLea bonds do not display strong force history-dependence. Rather, the bond lifetime is determined solely by the instantaneous load on the bond.