Event-Tracking Model of Adhesion Identifies Load-Bearing Bonds in Rolling Leukocytes

Abstract

Objectives-P-selectin binding to P-selectin glycoprotein ligand (PSGL)-1 mediates leukocyte rolling under conditions of inflammation and injury. The objectives were to develop an efficient, high temporal resolution model for direct simulation of leukocyte rolling, and then to conduct a study of load-bearing bonds using the model. Methods-A stochastic π-calculus-driven Event TrackingModel of Adhesion was developed and compared with experimental data. Multiple simulations for each case were conducted to obtain high confidence numerical characteristics of leukocyte rolling.Results-Leukocyte rolling and the underlying P-selectin-PSGL-1 bonds were studied under low wall shear rate (25-50 s -1 ) conditions from measured parameters of leukocyte rolling and bond properties. For the first time, the location, number, lifetime, history, and kinetics of load-bearing bonds and their influence on cell rolling are identified. Instantaneous cell displacements, translational and rotational velocities, and cell-endothelium distances are derived. The model explains the commonly observed "stop-start" type rolling behavior and reveals that a few load-bearing bonds are sufficient to support rolling while a large number of bonds dissociate before becoming load-bearing.Conclusions-The presented model provides a method for precise and direct simulation of leukocyte rolling, and sets a foundation upon which further refinements can be introduced.

Live Cell Imaging of Paxillin in Rolling Neutrophils by Dual-Color Quantitative Dynamic Footprinting

“…qDF microscopy is an adaptation of TIRFM, allowing realtime measurements of ''height'' of the hills and valleys in the footprint of rolling neutrophils. qDF revealed that a rolling neutrophil has a fourfold larger footprint than that predicted by theoretical models [39,40] as well as in vivo measurements [12,18]. In addition, qDF revealed three to four long tethers at the rear of each rolling neutrophil.…”

“…Over the last two decades, P-selectinmediated neutrophil rolling has been studied in vivo using intravital microscopy [8,12,56] and in in vitro flow chambers using trans-illumination [1,28,59], epifluorescence [50], and DIC microscopy [42,47]. These studies have provided cellular and molecular measurements that have led to the creation of in silico theoretical models [7,25,27,39,40], which provide invaluable insight into the molecular and cellular mechanisms that govern neutrophil rolling.…”

Studies of Bending Effects of Microvilli of Leukocyte on Rolling Adhesion

“…The time interval ∆t of 10µs is used, as suggested by Ref. (20), to simulate leukocyte rolling for a period of 3s. All parameters used in the AD are presented in Table 1 and Table 2.…”

“…This idea was first proposed by Shao et al (18). Pawar et al (19) (20,21). However, these studies were limited to microvillus extensional deformation.…”

“…Munn et al (23) suggested that when microvilli are flattened along the cell surface, the receptors at the bases of microvilli (instead of at the tips of microvilli) may obtain additional adhesion with ligands on the endothelial cells or on the ligand-coated substrate. While Pospieszalska et al first simulated the pivoting of microvilli around their bases, they did not address the flexural stiffness of microvilli (20,21). Based on the importance of flexibility in microvilli, Yao and Shao (24,25) experimentally measured the flexural stiffness of the microvilli on lymphocytes and neutrophils under small deformation of the microvilli.…”