Leukocytes roll on selectins at nearly constant velocities over a wide range of wall shear stresses. Ligand-coupled microspheres roll faster on selectins and detach quickly as wall shear stress is increased. To examine whether the superior performance of leukocytes reflects molecular features of native ligands or cellular properties that favor selectin-mediated rolling, we coupled structurally defined selectin ligands to microspheres or K562 cells and compared their rolling on P-selectin. Microspheres bearing soluble P-selectin glycoprotein ligand (sPSGL)-1 or 2-glycosulfopeptide (GSP)-6, a GSP modeled after the NH2-terminal P-selectin–binding region of PSGL-1, rolled equivalently but unstably on P-selectin. K562 cells displaying randomly coupled 2-GSP-6 also rolled unstably. In contrast, K562 cells bearing randomly coupled sPSGL-1 or 2-GSP-6 targeted to a membrane-distal region of the presumed glycocalyx rolled more like leukocytes: rolling steps were more uniform and shear resistant, and rolling velocities tended to plateau as wall shear stress was increased. K562 cells treated with paraformaldehyde or methyl-β-cyclodextrin before ligand coupling were less deformable and rolled unstably like microspheres. Cells treated with cytochalasin D were more deformable, further resisted detachment, and rolled slowly despite increases in wall shear stress. Thus, stable, shear-resistant rolling requires cellular properties that optimize selectin–ligand interactions.

Selectins are adhesion molecules that initiate tethering and rolling of leukocytes on the vessel wall. Rolling requires rapid formation and breakage of selectin-ligand bonds that must have mechanical strength to resist premature dissociation by the forces applied in shear flow. P-and L-selectin bind to the N-terminal region of P-selectin glycoprotein ligand-1 (PSGL-1), a mucin on leukocytes. To define determinants on PSGL-1 that contribute to the kinetic and mechanical properties of bonds with selectins, we compared rolling of transfected preB cells expressing P-or L-selectin on transfected cell monolayers expressing wild-type PSGL-1 or PSGL-1 constructs with substitutions in targeted N-terminal residues. Rolling through P-or L-selectin required a Thr or Ser at a specific position on PSGL-1, the attachment site for an essential O-glycan, but required only one of three nearby Tyr residues, which are sites for Tyr-SO3 formation. The adhesive strengths and numbers of cells rolling through P-or L-selectin were similar on wild-type PSGL-1 and on each of the three PSGL-1 constructs containing only a single Tyr. However, the cells rolled more irregularly on the single-Tyr forms of PSGL-1. Analysis of the lifetimes of transient tethers on limiting densities of PSGL-1 revealed that L-selectin dissociated faster from single-Tyr than wild-type PSGL-1 at all shears examined. In sharp contrast, P-selectin dissociated faster from single-Tyr than wild-type PSGL-1 at higher shear but not at lower shear. Thus, tyrosine replacements in PSGL-1 affect distinct kinetic and mechanical properties of bonds with P-and L-selectin. T he initial step in leukocyte accumulation during inflammation is a rolling interaction on the blood vessel wall. This adhesive event is primarily mediated by binding of the selectins to cell-surface glycoconjugates that must be modified with sialic acid, fucose, and in some cases, sulfate (1, 2). L-selectin, expressed on most leukocytes, binds to ligands on endothelial cells and other leukocytes. P-and E-selectin, expressed on activated platelets and͞or endothelial cells, bind to ligands on leukocytes and some endothelial cells.Cell rolling is a dynamic process that requires the rapid formation and breakage of adhesive bonds that are subjected to hydrodynamic force in shear flow (3). Both the kinetic and mechanical properties of selectin-ligand interactions affect rolling adhesion (4). Biochemical and biophysical studies indicate that the dissociation rate, or k off , for L-selectin-ligand bonds is 7-to 10-fold higher than for P-selectin or E-selectin bonds (5, 6). However, the mechanical strength of L-selectin bonds, i.e., their ability to resist accelerated dissociation by force, is greater than that of P-or E-selectin bonds. The structural features that dictate the kinetic and mechanical properties of selectin-ligand interactions are not understood. Mild periodate treatment cleaves the exocyclic C8 and C9 carbons from sialic acids on the mucin CD34 and markedly increases the mechanical strength of its inte...

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