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. 2012 Apr 18;102(8):1757-66.
doi: 10.1016/j.bpj.2012.03.018.

Leukocyte rolling on P-selectin: a three-dimensional numerical study of the effect of cytoplasmic viscosity

Damir B Khismatullin  1 , George A Truskey
Affiliations

Leukocyte rolling on P-selectin: a three-dimensional numerical study of the effect of cytoplasmic viscosity

Damir B Khismatullin et al. Biophys J. .

Abstract

Rolling leukocytes deform and show a large area of contact with endothelium under physiological flow conditions. We studied the effect of cytoplasmic viscosity on leukocyte rolling using our three-dimensional numerical algorithm that treats leukocyte as a compound droplet in which the core phase (nucleus) and the shell phase (cytoplasm) are viscoelastic fluids. The algorithm includes the mechanical properties of the cell cortex by cortical tension and considers leukocyte microvilli that deform viscoelastically and form viscous tethers at supercritical force. Stochastic binding kinetics describes binding of adhesion molecules. The leukocyte cytoplasmic viscosity plays a critical role in leukocyte rolling on an adhesive substrate. High-viscosity cells are characterized by high mean rolling velocities, increased temporal fluctuations in the instantaneous velocity, and a high probability for detachment from the substrate. A decrease in the rolling velocity, drag, and torque with the formation of a large, flat contact area in low-viscosity cells leads to a dramatic decrease in the bond force and stable rolling. Using values of viscosity consistent with step aspiration studies of human neutrophils (5-30 Pa·s), our computational model predicts the velocities and shape changes of rolling leukocytes as observed in vitro and in vivo.

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Figures

Figure 1
Figure 1
Sketch of the geometry used in the numerical simulation of leukocyte rolling. (A) The leukocyte is modeled as a compound viscoelastic drop of initially spherical shape consisting of the cell cytoplasm and nucleus. The plasma membrane is treated as a layer of infinitesimal thickness that possesses cortical tension. The cell has cylindrical microvilli with adhesion molecules (PSGL-1) on their tips. (B) The flow domain is a rectangular channel of length l, height h, and width w. The imposed boundary conditions reconstitute fully developed flow in a parallel-plate flow chamber or a rectangular microchannel. (C) The leukocyte is located sufficiently close to a P-selectin-coated substrate to form bonds between P-selectin and PSGL-1. The initial density of receptor-ligand bonds is zero. In the model, x axis coincides with the flow direction, z axis is perpendicular to the flow (from bottom to top); and y axis is perpendicular to the flow and parallel to the substrate. The origin is the left-front-bottom corner of the flow domain.
Figure 2
Figure 2
Instantaneous velocity of the leukocyte versus time for different values of the cytoplasmic viscosity. Significant temporal variations in the velocity leading to cell detachment are observed for high-viscosity cells.
Figure 3
Figure 3
(A) Mean rolling velocity of the leukocyte as a function of the cytoplasmic viscosity. (∗) In all runs, a 1000 P-viscosity cell detaches for the substrate within a simulation time of 1 s. (B) Effect of the nuclear viscosity on the mean rolling velocity. (C) Effect of the cortical tension on the mean rolling velocity. The cell continues to roll for a simulation time of 3 s even with a 10-fold increase in the cortical tension (from 30 to 300 μN m−1). (D) Nonlinear regression fit of the rolling velocity versus time data to the two-phase decay model. In panels B–D, the cytoplasmic viscosity is 100 P.
Figure 4
Figure 4
(A) Mean cell-substrate contact area as a function of the cytoplasmic viscosity. (Insets in A) The natural logarithm of the normalized mean contact area versus the cytoplasmic viscosity that illustrates a clean exponential relationship between the contact area and the viscosity. (B) Effect of the nuclear viscosity on the mean contact area. (C) Effect of the cortical tension on the mean contact area. (D) Mean contact area versus the deformation index of the cell.
Figure 5
Figure 5
(A) Total number of receptor-ligand bonds and (B) number of load-bearing (stressed) bonds as a function of the cytoplasmic viscosity of a rolling cell. (Insets in A) The natural logarithm of the total number of bonds versus the cytoplasmic viscosity that shows a clean exponential relationship between the total number of bonds and the viscosity. The viscosity has a small effect on the number of load-bearing bonds, but it dramatically decreases the number of unstressed bonds.
Figure 6
Figure 6
Bond and hydrodynamic forces on a rolling cell as a function of time for different values of the cytoplasmic viscosity. (Left panel) Drag force (upper curve, solid circles) and the translational (x-) component of the bond force (lower curve, open squares). (Right panel) Lift force (upper curve, solid circles) and the normal (z-) component of the bond force (lower curve, open squares). The bond force drops to zero (open circle) and the drag and lift forces decrease to very small values when the cell detaches.
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