Flow-enhanced adhesion regulated by a selectin interdomain hinge

Clustering endothelial E-selectin in clathrin-coated pits and lipid rafts enhances leukocyte adhesion under flow

During inflammation, E-selectin expressed on cytokine-activated endothelial cells mediates leukocyte rolling under flow. E-selectin undergoes endocytosis and may associate with lipid rafts. We asked whether distribution of E-selectin in membrane domains affects its functions. E-selectin was internalized in transfected CHO cells or cytokine-activated human umbilical vein endothelial cells (HUVECs). Confocal microscopy demonstrated colocalization of E-selectin with alpha-adaptin, a clathrin-associated protein. Deleting the cytoplasmic domain of E-selectin or disrupting clathrin-coated pits with hypertonic medium blocked internalization of E-selectin, reduced colocalization of E-selectin with alpha-adaptin, and inhibited E-selectin-mediated neutrophil rolling under flow. Unlike CHO cells, HUVECs expressed a small percentage of E-selectin in lipid rafts. Even fewer neutrophils rolled on E-selectin in HUVECs treated with hypertonic medium and with methyl-beta-cyclodextrin, which disrupts lipid rafts. These data demonstrate that E-selectin clusters in both clathrin-coated pits and lipid rafts of endothelial cells but is internalized in clathrin-coated pits. Distribution in both domains markedly enhances E-selectin's ability to mediate leukocyte rolling under flow.

Paradigms for glycan-binding receptors in cell adhesion

Diverse glycans found on the surfaces of mammalian cells provide a basis for selective adhesion between cells mediated by glycan-specific receptors. Well-understood examples of cell adhesion based on such interactions include selectin-mediated rolling of leukocytes on endothelia. Other receptors with similar selectivity for specific sugar epitopes on cell surfaces are being characterised. However, the simple paradigm of adhesion resulting from receptors on one cell binding to glycans on another cell applies in only a limited number of systems. Instead, glycans and receptor-glycan interactions often modulate adhesion in indirect ways, such as by changing the organisation of cell surface glycoproteins and by antagonising the effect of protein adhesion systems.

Inhibition of L-selectin binding by polyacrylamide-based conjugates under defined flow conditions

Selectins mediate tethering and rolling of leukocytes along the endothelium in a shear force-dependent manner. This key step in the cellular immune response is a target for experimental anti-inflammatory therapies. In the present paper we have examined the inhibitory activity of the minimal selectin ligand sialyl Lewis x (SiaLe(x)), its isomer sialyl Lewis a (SiaLe(a)) and sulfated tyrosine (sTyr) residues under dynamic flow reflecting the rheological conditions in the blood stream. The monomeric ligands were compared to multivalent polyacrylamide (PAA)-based conjugates under defined flow conditions on the molecular level, using surface plasmon resonance (SPR) technology, and on the cellular level, using a parallel-plate flow chamber. SPR measurements showed that a spatial arrangement of binding epitopes mimicking the selectin binding motif of the natural ligand PSGL-1 inhibits L-selectin binding successfully with IC(50) values in the nanomolar range. Using a flow chamber adhesion assay it could be shown that the multivalent inhibitors efficiently blocked rolling and tethering of NALM-6 pre-B cells transfected with human L-selectin to activated endothelium and that the inhibitory activity increased with rising shear stress. While PAA-conjugates were almost not inhibitory at low shear stress, NALM-6 cell rolling was nearly completely inhibited at high shear stress. The results indicate that multimeric conjugates of SiaLe(x), SiaLe(a) and sTyr are highly effective inhibitors of L-selectin-mediated cell adhesion particularly under flow conditions. Consequently, SiaLe(x), SiaLe(a) and/or sTyr on macromolecular carriers may be promising candidates for anti-inflammatory therapy.

Monitoring receptor-ligand interactions between surfaces by thermal fluctuations

We describe a new method for determining receptor-ligand association/dissociation events across the interface of two surfaces (two-dimensional binding) by monitoring abrupt decrease/resumption in thermal fluctuations of a biomembrane force probe. Our method has been validated by rigorous control experiments and kinetic experiments. We show that cellular on-rate of association can be measured by analysis of intervals from a dissociation event to the next association event (waiting times). Similarly, off-rate of molecular dissociation can be measured by analysis of intervals from an association event to the next dissociation event (bond lifetimes). Different types of molecular bonds could be distinguished by different levels of reduction in thermal fluctuations. This novel method provides a powerful tool to study cell adhesion and signaling mediated by single or multiple receptor-ligand species.

Membranal cholesterol is not required for L-selectin adhesiveness in primary lymphocytes but controls a chemokine-induced destabilization of L-selectin rolling adhesions

Cholesterol-enriched lipid microdomains regulate L-selectin signaling, but the role of membrane cholesterol in L-selectin adhesion is unclear. Arrest chemokines are a subset of endothelial chemokines that rapidly activate leukocyte integrin adhesiveness under shear flow. In the absence of integrin ligands, these chemokines destabilize L-selectin-mediated leukocyte rolling. In the present study, we investigated how cholesterol extraction from the plasma membrane of peripheral blood T or B cells affects L-selectin adhesions and their destabilization by arrest chemokines. Unlike the Jurkat T cell line, whose L-selectin-mediated adhesion is cholesterol dependent, in primary human PBLs and in murine B cells and B cell lines, cholesterol depletion did not impair any intrinsic adhesiveness of L-selectin, consistent with low selectin partitioning into lipid rafts in these cells. However, cholesterol raft disruption impaired the ability of two arrest chemokines, CXCL12 and CXCL13, but not of a third arrest chemokine, CCL21, to destabilize L-selectin-mediated rolling of T lymphocytes. Actin capping by brief incubation with cytochalasin D impaired the ability of all three chemokines to destabilize L-selectin rolling. Blocking of the actin regulatory phosphatidylinositol lipid, phosphatidylinositol 4,5-bisphosphate, did not affect chemokine-mediated destabilization of L-selectin adhesions. Collectively, our results suggest that L-selectin adhesions are inhibited by actin-associated, cholesterol-stabilized assemblies of CXCL12- and CXCL13-binding receptors on both T and B lymphocytes. Thus, the regulation of L-selectin by cholesterol-enriched microdomains varies with the cell type as well as with the identity of the destabilizing chemokine.

Shear-induced capping of L-selectin on the neutrophil surface during centrifugation

L-selectin on leukocytes is critical in leukocyte tethering and adhesion to inflamed endothelium and lymphocyte homing to lymphoid organs. The spatial distribution of L-selectin on leukocytes controls cellular adhesive function in hydrodynamic shear. How L-selectin changes its position on the cell membrane remains an open question, but a possible candidate is shear stress encountered on the cell surface. Here we demonstrate shear-induced L-selectin polarization on the membrane during the process of centrifugation of resting neutrophils via immunofluorescent microscopy. It was found that randomly distributed L-selectin on neutrophils moves to a polar cap at one end of the cell after centrifugation (300 x g for 2 min) without inflammatory stimuli. This L-selectin redistribution under shear was predicted by Monte Carlo simulations that show how convection dominates over diffusion, leading to L-selectin cap formation during centrifugation at 280 x g or during leukocyte adhesion to the endothelial wall at 1 dyn/cm(2). Those results point to a role for shear stress in the modulation of L-selectin distribution, and suggest a possible alternate mechanism and reinterpretation of previous in vitro studies of L-selectin mediated adhesion of neutrophils isolated via centrifugation.

Interdomain Interaction in the FimH Adhesin of Escherichia coli Regulates the Affinity to Mannose

FimH is a mannose-specific adhesin located on the tip of type 1 fimbriae of Escherichia coli that is capable of mediating shear-enhanced bacterial adhesion. FimH consists of a fimbria-associated pilin domain and a mannose-binding lectin domain, with the binding pocket positioned opposite the interdomain interface. By using the yeast two-hybrid system, purified lectin and pilin domains, and docking simulations, we show here that the FimH domains interact with one another. The affinity for mannose is greatly enhanced (up to 300-fold) in FimH variants in which the interdomain interaction is disrupted by structural mutations in either the pilin or lectin domains. Also, affinity to mannose is dramatically enhanced in isolated lectin domains or in FimH complexed with the chaperone molecule that is wedged between the domains. Furthermore, FimH with native structure mediates weak binding at low shear stress but shifts to strong binding at high shear, whereas FimH with disrupted interdomain contacts (or the isolated lectin domain) mediates strong binding to mannose-coated surfaces even under low shear. We propose that interactions between lectin and pilin domains decrease the affinity of the mannose-binding pocket via an allosteric mechanism. We further suggest that mechanical force at high shear stress separates the two domains, allowing the lectin domain to switch from a low affinity to a high affinity state. This shift provides a mechanism for FimH-mediated shear-enhanced adhesion by enabling the adhesin to form catch bond-like interactions that are longer lived at high tensile force.

Force as a Facilitator of Integrin Conformational Changes during Leukocyte Arrest on Blood Vessels and Antigen-Presenting Cells

Integrins comprise a large family of cell-cell and cell-matrix adhesion receptors that rapidly modulate their adhesiveness. The arrest of leukocyte integrins on target vascular beds involves instantaneous conformational switches generating shear-resistant adhesions. Structural data suggest that these integrins are maintained in low-affinity conformations and must rapidly undergo conformational switches transduced via cytoplasmic changes ("inside-out" signaling) and simultaneous ligand-induced rearrangements ("outside-in"). This bidirectional activation is accelerated by signals from endothelial chemoattractants (chemokines). Recent studies predict that shear forces in the piconewton (pN) range per integrin can facilitate these biochemical switches. After extravasation, antigen recognition involves smaller internal forces from cytoskeletal motors and actin polymers forming the immune synapse. In this review, we address how forces facilitate allosteric integrin activation by biochemical signals. Evidence suggests that preformed cytoskeletal anchorage rather than free integrin mobility is key for force-enhanced integrin activation by chemokines and TCR signals.

A structure-based sliding-rebinding mechanism for catch bonds

Catch bonds, whose lifetimes are prolonged by force, have been observed in selectin-ligand interactions and other systems. Several biophysical models have been proposed to explain this counterintuitive phenomenon, but none was based on the structure of the interacting molecules and the noncovalent interactions at the binding interface. Here we used molecular dynamics simulations to study changes in structure and atomic-level interactions during forced unbinding of P-selectin from P-selectin glycoprotein ligand-1. A mechanistic model for catch bonds was developed based on these observations. In the model, "catch" results from forced opening of an interdomain hinge that tilts the binding interface to allow two sides of the contact to slide against each other. Sliding promotes formation of new interactions and even rebinding to the original state, thereby slowing dissociation and prolonging bond lifetimes. Properties of this sliding-rebinding mechanism were explored using a pseudoatom representation and Monte Carlo simulations. The model has been supported by its ability to fit experimental data and can be related to previously proposed two-pathway models.

Transport governs flow-enhanced cell tethering through L-selectin at threshold shear

Flow-enhanced cell adhesion is a counterintuitive phenomenon that has been observed in several biological systems. Flow augments L-selectin-dependent adhesion by increasing the initial tethering of leukocytes to vascular surfaces and by strengthening their subsequent rolling interactions. Tethering or rolling might be influenced by physical factors that affect the formation or dissociation of selectin-ligand bonds. We recently demonstrated that flow enhanced rolling of L-selectin-bearing microspheres or neutrophils on P-selectin glycoprotein ligand-1 by force decreased bond dissociation. Here, we show that flow augmented tethering of these microspheres or cells to P-selectin glycoprotein ligand-1 by three transport mechanisms that increased bond formation: sliding of the sphere bottom on the surface, Brownian motion, and molecular diffusion. These results elucidate the mechanisms for flow-enhanced tethering through L-selectin.

For catch bonds, it all hinges on the interdomain region

Tensile mechanical force was long assumed to increase the detachment rates of biological adhesive bonds (Bell, 1978). However, in the last few years, several receptor–ligand pairs were shown to form “catch bonds,” whose lifetimes are enhanced by moderate amounts of force. These include the bacterial adhesive protein FimH binding to its ligand mannose (Thomas et al., 2002; Thomas et al., 2006), blood cell adhesion proteins P- and L-selectin binding to sialyl Lewis X (sLe^X)–containing ligands (Marshall et al., 2003; Evans et al., 2004; Sarangapani et al., 2004), and the myosin–actin motor protein interaction (Guo and Guilford, 2006). The structural mechanism behind this counterintuitive force–enhanced catch bond behavior is of great interest.