Biophysical description of multiple events contributing blood leukocyte arrest on endothelium

Abstract

Blood leukocytes have a remarkable capacity to bind to and stop on specific blood vessel areas. Many studies have disclosed a key role of integrin structural changes following the interaction of rolling leukocytes with surface-bound chemoattractants. However, the functional significance of structural data and mechanisms of cell arrest are incompletely understood. Recent experiments revealed the unexpected complexity of several key steps of cell-surface interaction: (i) ligand-receptor binding requires a minimum amount of time to proceed and this is influenced by forces. (ii) Also, molecular interactions at interfaces are not fully accounted for by the interaction properties of soluble molecules. (iii) Cell arrest depends on nanoscale topography and mechanical properties of the cell membrane, and these properties are highly dynamic. Here, we summarize these results and we discuss their relevance to recent functional studies of integrin-receptor association in cells from a patient with type III leukocyte adhesion deficiency. It is concluded that an accurate understanding of all physical events listed in this review is needed to unravel the precise role of the multiple molecules and biochemical pathway involved in arrest triggering.

Keywords: LAD-III; adhesion; avidity; bond strength; clustering; dynamics; integrin; ligand-receptor interaction.

Figure 1

Hydrodynamic forces on cells bound to blood vessel walls. (A) In a laminar viscous shear flow near a plane, the blood velocity at any point near the wall is parallel to the plane and equal to the distance z to the wall times the wall shear rate G (in s-1). The shear stress is the shear rate times the fluid viscosity μ (μ ∼ 0.001 Pa.s in aqueous medium). It represents the viscous force applied by the fluid on an unit area on the wall. (B) The fluid exerts on a sphere of radius bound to the wall a total force F ∼32 μa²G and a torque G ∼ 11.9 μa³G (Goldman et al., 1967). (C) if the sphere is maintained at rest by a single bond of length L and the contact between the surface and the wall is assumed to be frictionless, the tension T on the bond is ∼ 31 μa²G (a/L)^1/2 (Pierres et al., 1995).

Figure 2

Effect of forces on the kinetics of bond rupture. The simplest approximation consists of representing the free energy of a ligand-receptor complex as a simple function of the distance between ligand and receptor surfaces (red curve). Rupture requires the crossing of an energy barrier of height W. The rupture rate may be viewed as the product of the frequency of attempts at crossing times the success probability that is proportional to Boltzmann’s factor exp(−W/k_BT). Applying a force will decrease the barrier height by the product F.d, i.e., the force times the distance between the barrier and the equilibrium distance, thus multiplying the escape frequency by exp(Fd/k_BT).

Figure 3

Stabilization of leukocyte attachment to the blood vessels. (A) When a cell studded with protrusions (mv) of several hundreds of nm length encounters a plane surface, contact between membrane receptors such as selectins or integrins (red disks) and their ligands (red half circles) of total length lower than 50–100 nm can only occur on the tip of protrusions, allowing formation of a low number of bonds. As indicated in text, very high association rates are needed to tether freely flowing leukocytes within a molecular contact shorter than a few milliseconds and initiate rolling. (B) Within the following tens of seconds, rolling cells undergo (i) micrometer-scale flattening similarly to liquid droplets encountering a wettable surface, (ii) submicrometer-scale smoothing of microvilli, first due to compressive forces, and possibly later to intracellular signaling triggered by chemokines. (iii) lateral diffusion of membrane receptors that are trapped into the contact area. At some moment, these phenomena induce cell arrest. (C) Further attachment strengthening may involve a more extensive increase of contact area as a consequence of spreading, increase of membrane stiffness due to local cytoskeleton reinforcement, and possibly increase of the strength of membrane receptors attachment to underlying cytoskeleton, thus preventing further lateral displacement.