The Kinetics of L-selectin Tethers and the Mechanics of Selectin-mediated Rolling

Abstract

Two mechanisms have been proposed for regulating rolling velocities on selectins. These are (a) the intrinsic kinetics of bond dissociation, and (b) the reactive compliance, i.e., the susceptibility of the bond dissociation reaction to applied force. To determine which of these mechanisms explains the 7.5–11.5-fold faster rolling of leukocytes on L-selectin than on E- and P-selectins, we have compared the three selectins by examining the dissociation of transient tethers. We find that the intrinsic kinetics for tether bond dissociation are 7–10-fold more rapid for L-selectin than for E- and P-selectins, and are proportional to the rolling velocities through these selectins. The durations of pauses during rolling correspond to the duration of transient tethers on low density substrates. Moreover, applied force increases dissociation kinetics less for L-selectin than for E- and P-selectins, demonstrating that reactive compliance is not responsible for the faster rolling through L-selectin. Further measurements provide a biochemical and biophysical framework for understanding the molecular basis of rolling. Displacements of tethered cells during flow reversal, and measurements of the distance between successive pauses during rolling provide estimates of the length of a tether and the length of the adhesive contact zone, and suggest that rolling occurs with as few as two tethers per contact zone. Tether bond lifetime is an exponential function of the force on the bond, and the upper limit for the tether bond spring constant is of the same order of magnitude as the estimated elastic spring constant of the lectin–EGF unit. Shear uniquely enhances the rate of L-selectin transient tether formation, and conversion of tethers to rolling adhesions, providing further understanding of the shear threshold requirement for rolling through L-selectin.

Direct observation of catch bonds involving cell-adhesion molecules

“…With the use of flow chambers, several groups have shown that lifetimes of tethers of PSGL-1-expressing cells to P-selectin-coated surfaces decreased with increasing wall shear stress, which is consistent with slip bond behaviour 7,9,[11][12][13] . These experiments were conducted at wall shear stresses ranging from 0.17 to 3.0 dyn cm 22 , corresponding to tether forces of 20-400 pN.…”

“…It has been suggested that force could either shorten bond lifetimes, because work done by the force could lower the energy barrier between the bound and free states 4 ('slip'), or prolong bond lifetimes by deforming the molecules such that they lock more tightly 5,6 ('catch'). Whereas slip bonds have been widely observed [7][8][9][10][11][12][13][14] , catch bonds have not been demonstrated experimentally. Here, using atomic force microscopy and flow-chamber experiments, we show that increasing force first prolonged and then shortened the lifetimes of P-selectin complexes with P-selectin glycoprotein ligand-1, revealing both catch and slip bond behaviour.…”

“…This pattern remained unchanged as more data were accumulated (about 400 lifetime measurements for each form of PSGL-1), which allowed us to examine the distributions of lifetimes. As with published flow-chamber data 7,9,[11][12][13] , lifetimes at a given force seemed to follow an exponential distribution, which was made linear by plotting ln(number of events with a lifetime of t or more) against t; examples are given in Fig. 3d, e. The negative reciprocal of the slope of the linear fit was used as a lifetime measure and was plotted against force with the 95% confidence interval as error bar.…”

“…Here, using atomic force microscopy and flow-chamber experiments, we show that increasing force first prolonged and then shortened the lifetimes of P-selectin complexes with P-selectin glycoprotein ligand-1, revealing both catch and slip bond behaviour. Transitions between catch and slip bonds might explain why leukocyte rolling on selectins first increases and then decreases as wall shear stress increases 9,15,16 . This dual response to force provides a mechanism for regulating cell adhesion under conditions of variable mechanical stress.…”

Influence of Anatomical Location on Arterial Thrombosis

“…[25][26][27][28][29] Both selectin-and glycoprotein Ib␣-binding have been shown to have a shear threshold dependence whereby resistance to detachment increases proportional to increasing shear. These shear requirements are important not only for adhesion promotion but also maintenance.…”