Receptor-cytoskeletal unbinding in detachment of P-selectin from PSGL-1 on leukocytes

Using a biomembrane force probe decorated with P-selectin, discrete bonds were formed to PSGL-1 receptors on PMN surfaces and detached at speeds from ∼1 - 100 μm/sec. High resolution tracking of the distance between probe tip and PMN revealed an initial elastic deformation that was either terminated by abrupt detachment or interrupted by yield and fluid-like extrusion of a macroscale tether plus subsequent detachment. Selecting tests that exhibited first yield then a single detachment step, we were able to quantify cohesive strengths between single PSGL-1 receptors and the PMN cytoskeleton. Prior to yield, the constant force rate was set by elastic stiffness (∼0.25 pN/nm) of the cytostructure and the pulling speed. Collected at rates over a span from 265 pN/sec to 38000 pN/sec, distributions of yield forces were found to agree precisely with probability densities for rupture of a single bond defined by a spontaneous dissociation rate of ∼0.5/sec and an energy barrier projected at ∼0.25 nm along the direction of force. By comparison, single P-selectin bonds to PSGL-1 covalently attached to microspheres were slightly stronger at all loading rates as characterized by a spontaneous dissociation rate of ∼0.15/sec and an energy barrier projected at ∼0.22 nm. Weaker anchoring to the cytoskeleton implies frequent tether formation that can reduce the hydrodynamic load applied to selectin bonds and prolog PMN attachments to vessel walls under conditions of flow.