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Adhesive dynamics (AD) is a method for simulating the dynamic response of biological systems in response to force. Biological bonds are mechanical entities that exert force under strain, and applying forces to biological bonds modulates their rate of dissociation. Since small numbers of events usually control biological interactions, we developed a simple method for sampling probability distributions for the formation or failure of individual bonds. This method allows a simple coupling between force and strain and kinetics, while capturing the stochastic response of biological systems. Biological bonds are dynamically reconfigured in response to applied mechanical stresses, and a detailed spatio-temporal map of molecules and the forces they exert emerges from AD. The shape or motion of materials bearing the molecules is easily calculated from a mechanical energy balance provided the rheology of the material is known. AD was originally used to simulate the dynamics of adhesion of leukocytes under flow, but new advances have allowed the method to be extended to many other applications, including but not limited to the binding of viruses to surface, the clustering of adhesion molecules driven by stiff substrates, and the effect of cell-cell interaction on cell capture and rolling dynamics. The technique has also been applied to applications outside of biology. A particular exciting recent development is the combination of signaling with AD (so-called integrated signaling adhesive dynamics, or IS AD), which allows facile integration of signaling networks with mechanical models of cell adhesion and motility. Potential opportunities in applying AD are summarized. [ABSTRACT FROM AUTHOR]

The selectin family of leukocyte adhesion receptors is principally recognized for mediating transient rolling interactions during the inflammatory response. Recent studies using ultrasensitive force probes to characterize the force-lifetime relationship between P- and L-selectin and their endogenous ligands have underscored the ability of increasing levels of force to initially extend the lifetime of these complexes before disrupting bond integrity. This so-called "catch-slip" transition has provided an appealing explanation for shear threshold phenomena in which increasing levels of shear stress stabilize leukocyte rolling under flow. We recently incorporated catch-slip kinetics into a mechanical model for cell adhesion and corroborated this hypothesis for neutrophils adhering via L-selectin. Here, using adhesive dynamics simulations, we demonstrate that biomembrane force probe measurements of various P- and L-selectin catch bonds faithfully predict differences in cell adhesion patterns that have been described extensively in vitro. Using phenomenological parameters to characterize the dominant features of molecular force spectra, we construct a generalized phase map that reveals that robust shear-threshold behavior is possible only when an applied force very efficiently stabilizes the bound receptor complex. This criteria explains why only a subset of selectin catch bonds exhibit a shear threshold and leads to a quantitative relationship that may be used to predict the magnitude of the shear threshold for families of catch-slip bonds directly from their force spectra. Collectively, our results extend the conceptual framework of adhesive dynamics as a means to translate complex single-molecule biophysics to macroscopic cell behavior. [ABSTRACT FROM AUTHOR]

Abstract: Activated lymphocyte function-associated antigen-1 (LFA-1, αLβ2 integrin) found on leukocytes facilitates firm adhesion to endothelial cell layers by binding to intercellular adhesion molecule-1 (ICAM-1), which is up-regulated on endothelial cells at sites of inflammation. Recent work has shown that LFA-1 in a pre-activation, low-affinity state may also be involved in the initial tethering and rolling phase of the adhesion cascade. The inserted (I) domain of LFA-1 contains the ligand-binding epitope of the molecule, and a conformational change in this region during activation increases ligand affinity. We have displayed wild-type I domain on the surface of yeast and validated expression using I domain specific antibodies and flow cytometry. Surface display of I domain supports yeast rolling on ICAM-1-coated surfaces under shear flow. Expression of a locked open, high-affinity I domain mutant supports firm adhesion of yeast, while yeast displaying intermediate-affinity I domain mutants exhibit a range of rolling phenotypes. We find that rolling behavior for these mutants fails to correlate with ligand binding affinity. These results indicate that unstressed binding affinity is not the only molecular property that determines adhesive behavior under shear flow. [Copyright &y& Elsevier]

Abstract: Selectins (E- and P-selectin) and other endothelial expressed leukocyte adhesion molecules (ELAMs) are potential targets for site-specific delivery of therapeutics to the vascular endothelium due to their specific and highly regulated expression in vascular disease. It was recently shown that degradable microspheres coated with antibodies against E-selectin or other ELAMs can target inflammation in vivo. However, targeting ELAMs alone cannot differentiate between normal and diseased state, as a basal level of these LAMs are expressed on endothelium in healthy tissues. Furthermore, leukocytes usually employ two separate adhesion molecules in parallel to home to diseased tissues, and we recently quantified the advantages of a two-receptor display for the targeting of leukocyte mimetics (Eniola AO, Willcox PJ, Hammer DA. Interplay between rolling and firm adhesion elucidated with a cell-free system engineered with two distinct receptor–ligand pairs. Biophys J 2003;85:2720–31). Here, we describe a leukocyte mimetic for targeting therapeutics to the vasculature in inflammatory diseases via two receptors, selectin and intercellular cell adhesion molecule-1 (ICAM-1), where biodegradable, polymer microspheres were co-functionalized with the selectin ligand, sialyl LewisX (sLeX), and an antibody against ICAM-1, anti-ICAM-1 (aICAM-1). These two-receptor targeted particles, at given ratios of sLeX/aICAM-1, firmly adhere to substrate surface in flow only when both targeting ligands can interact with their respective receptors, mimicking the multi-step in vivo leukocyte adhesion in inflammation. Thus, we have faithfully recreated the specificity and extent of leukocyte adhesion in a platform that can allow for local delivery of therapeutics. [Copyright &y& Elsevier]

The site-specific expression of selectins (P- and E-selectin) on endothelial cells of blood vessels during inflammation provides an opportunity for the targeted delivery of anti-inflammatory drugs to sites of chronic inflammation. It is well documented that the selectins mediate the initial interaction (rolling) of leukocytes in an inflamed vessel by binding to carbohydrate-presenting counter-receptors displayed on leukocytes. Previous work in our laboratory has shown that artificial capsules with the adhesive properties of leukocytes can be made by attaching leukocyte adhesive ligands to polymer microspheres (Biomaterials 23(10) (2002) 2167). Specifically, we showed that drug-loaded poly (lactic-co-glycolic-acid) (PLGA) microspheres coated with biotinylated-Sialyl LewisX (sLeX), a carbohydrate that serves as a ligand to selectins, mimic the adhesive behavior of leukocytes on selectins in flow chambers, displaying slow rolling under flow, suggesting that these drug-loaded particles can potentially target inflammatory sites in vivo. Since the effectiveness of this delivery system might depend on the degradation of polymer microspheres as well as the degradation of sLeX molecules, we measured the effect of polymer and ligand degradation on the adhesiveness of microspheres over time. We show that degrading sLeX microspheres maintain the ability to recognize selectin surfaces under flow for at least 2 weeks and that the ability to sustain recognition depends upon the extent at which microspheres are loaded. We also show that microsphere rolling velocity increases as microsphere degrade and that this increase is due to a combination of increase in average microsphere size and loss of sLeX molecules on microsphere surface—a result of microsphere degradation confirmed by flow cytometry. Control experiments show that microsphere, not sLeX, degradation limits the lifetime of our targeted delivery system; therefore, factors affecting degradation such as type of polymer, type of drug, extent of drug loading and microsphere size, provide an opportunity for engineering the time-scale of activity for the delivery system. [Copyright &y& Elsevier]

Selectins are optimal biological molecules for targeted delivery of therapeutic agents because of their localized and carefully regulated expression in several human diseases, and their highly specific interactions with their counter receptors. In this study, we describe a targeted delivery system that can potentially deliver anti-inflammatory drug to sites of chronic inflammation using Poly(lactic-co-glycolic acid) (PLGA) and selectin–ligand chemistry. Biotinylated-sialyl Lewisx (sLex), a carbohydrate that serves as a ligand to selectins, was attached to the surface of avidin-linked PLGA microspheres. These carbohydrate-coated microspheres mimic the adhesive behavior of leukocytes on selectins in flow chambers, displaying slow rolling under flow. The rolling velocity of these artificial leukocytes is similar to that displayed by leukocytes rolling on P- or E-selectin coated surfaces. We can tune rolling velocity, and hence residence time of capsules on surfaces, by changing the density of sialyl Lewisx on the microsphere surfaces. Therefore, we have made a targeted drug delivery vehicle that mimics the adhesive properties of leukocytes and is biodegradable. [Copyright &y& Elsevier]

Investigates the ability of hydrodynamic collisions between cells to induce capture of free-stream leukocytes to a selection-bearing surface under shear flow. Use of cell-free assay in the study; Attachment of carbohydrate-coated spherical beads to the adhesive wall; Occurrence of hydrodynamic recruitment in the absence of buoyancy effects.

A recent study shows that the leukocyte adhesion molecules known as selectins form ‘catch’ bonds, the dissociation rate of which decreases with increasing applied force. The ability of selectins to switch between catch and slip bonds, where dissociation increases with force, can explain the shear threshold effect, in which leukocyte adhesion goes through a maximum with increasing shear rate. [Copyright &y& Elsevier]

The inserted (I) domain of αLβ2 integrin (LFA-1) contains the entire binding site of the molecule. It mediates both rolling and firm adhesion of leukocytes at sites of inflammation depending on the activation state of the integrin. The affinity change of the entire integrin can be mimicked by the I domain alone through mutations that affect the conformation of the molecule. High-affinity mutants of the I domain have been discovered previously using both rational design and directed evolution. We have found that binding affinity fails to dictate the behavior of I domain adhesion under shear flow. In order to better understand I domain adhesion, we have developed a novel panning method to separate yeast expressing a library of I domain variants on the surface by adhesion under flow. Using conditions analogous to those experienced by cells interacting with the post-capillary vascular endothelium, we have identified mutations supporting firm adhesion that are not found using typical directed evolution techniques that select for tight binding to soluble ligands. Mutants isolated using this method do not cluster with those found by sorting with soluble ligand. Furthermore, these mutants mediate shear-driven cell rolling dynamics decorrelated from binding affinity, as previously observed for I domains bearing engineered disulfide bridges to stabilize activated conformational states. Characterization of these mutants supports a greater understanding of the structure–function relationship of the αL I domain, and of the relationship between applied force and bioadhesion in a broader context. [ABSTRACT FROM AUTHOR]

OBJECTIVE:: The aim of this study was to characterize the hydrodynamic interactions between rolling and free-stream leukocytes in an in vivo model of selectin-mediated rolling, and to identify those physical mechanisms that influence the dynamics of transient adhesion with the walls of postcapillary venules. METHODS:: Postcapillary venules of diameter 22-37 µm in the cheek pouch of anesthetized hamsters were visualized using intravital microscopy, with selectin-mediated rolling occurring in response to surgical preparation. RESULTS:: Rolling velocity was found to be a strong function of the center-to-center separation with the nearest cell, and also was found to correlate strongly with the number of nearby cells. These effects are shown to be beyond that attributable to variations along the length of the vessel. Adherent leukocytes were observed to provide a nucleation site, precipitating further adhesion events of free-stream cells. CONCLUSIONS:: The dynamics of the transient adhesion of leukocytes to the vessel wall in postcapillary venules is strongly dependent on the local concentration of adherent leukocytes, due to the complex hydrodynamics induced by their presence. The results are shown to agree well with theoretical considerations of the flow field induced by multiple nearby cells, suggesting a need for the future exploration of multicellular effects in the microcirculation.Microcirculation (2003) 10, 401-409. doi:10.1038/sj.mn.7800203 [ABSTRACT FROM AUTHOR]