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Departments of Genomics and Pathobiology, and Pathology, and * Biomedical Engineering, University of Alabama at Birmingham, Birmingham, Alabama 35294; Department of Pathology, Louisiana State University Health Sciences Center, Shreveport, Louisiana 71130; and ¶ Research Service, Birmingham VA Medical Center, Birmingham, Alabama 35233
Correspondence: Address reprint requests to Dennis F. Kucik, MD, PhD, University of Alabama at Birmingham, Center for Biophysical Sciences and Engineering (CBSE), Rm. 258, 1025 18th St., South Birmingham, AL 35294. Tel.: 205-934-0062; Fax: 205-934-0480; E-mail: kucik{at}uab.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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,b; Shimaoka et al., 2002, 2003; Takagi et al., 2001; Hantgan et al., 1999, 2001; Vinogradova et al., 2000, 2002; Salas et al., 2002). The role of integrin rearrangement, however, remains less well understood.
Rearrangement of integrins on leukocytes upon cell activation occurs not by directed movement, but rather by diffusion, which occurs after release of cytoskeletal constraints on integrin motion (Kucik et al., 1996; Yauch et al., 1997; Sigal et al., 2000). Integrin rearrangement increases avidity both by allowing receptors to move to areas of high ligand density (Kucik et al., 1996), and also by receptor clustering (Lub et al., 1997). Theory predicts that clustering alone can lead to increased avidity, due to cooperative effects on resistance to bond breakage (Ward et al., 1994). Direct evidence that the cooperative binding associated with clustering increases adhesion has been provided by atomic force microscopy (Chen and Moy, 2000).
So far, the only adhesion molecules shown to adjust their avidity on a rapid timescale by rearrangement are the ß2-integrins, which are specialized for leukocyte adhesion. Both 
Lß2 (Lub et al., 1997) and Mß2 (Zhou and Li, 2000; Zhou et al., 2001; Jones et al., 1998) increase avidity by diffusion and rearrangement. The principal ligand for both of these leukocyte integrins is intercellular adhesion molecule 1 (ICAM-1) (Diamond et al., 1990; Lub et al., 1996).
Physiologically, activation of integrin-mediated adhesion occurs in the context of activation of the whole leukocyte. Leukocyte activation affects many functions, including adhesion, production of cytokines, and proliferation. To understand the effects of integrin rearrangement, therefore, control of integrin diffusion must be separated experimentally from other effects of cell activation. The phorbol ester PMA (phorbol 12-myristate 13-acetate) is often used to activate integrin adhesion experimentally, but PMA has myriad effects, including changes in surface morphology and induction of cell spreading. To isolate the effects of integrin rearrangement from other effects of cell activation, cytochalasin D, at a concentration that has minimal effects on cytoskeletal architecture, has proven to be a useful tool. We showed previously that a low dose of cytochalasin D (0.3–1 µg/ml), too small to grossly disrupt the structure of the actin cytoskeleton, is able to release integrins from cytoskeletal constraints on their motion, as measured directly by single particle tracking (SPT) (Kucik et al., 1996). This treatment caused integrin rearrangement and activated lymphocyte adhesion. Several studies from a number of laboratories have demonstrated that low-dose cytochalasin D increases diffusion of ß2-integrins on a variety of leukocyte types (van Kooyk and Figdor, 1997; Yauch et al., 1997; Stewart et al., 1998; van Kooyk et al., 1999), including the WEHI 274.1 cell line (Zhou and Li, 2000), with little effect on other cell functions. The utility of low-dose cytochalasin D in increasing integrin avidity has since been confirmed in a number of systems (Lub et al., 1997; Stewart et al., 1998; Yauch et al., 1997; Zhou and Li, 2000; van Kooyk et al., 1999). Although cytochalasin D effects on integrin diffusion and clustering are well documented, there is no evidence that cytochalasin D has any effect on integrin conformation (Kucik, 2002).
The biphasic effect of cytochalasin D on adhesion (activation at low concentrations, inhibition at higher concentrations) has been explained as follows (Kucik et al., 1996). At low doses, cytochalasin D activates ß2-integrin–mediated adhesion through its effects on integrin diffusion. These low doses sever the link between ß2-integrins and actin filaments, without major disruption of the cytoskeleton, while high doses prevent reinforcement of nascent contacts. (Kucik et al., 1996). Because no specific integrin-mobilizing drugs have yet been identified, cytochalasin D is still the best tool available to increase integrin diffusion.
Although it is well established that rearrangement of integrins activates adhesion to purified ICAM-1 under static conditions, its role in leukocyte adhesion to endothelial cells in the presence of shear is less clear. Shear changes the biophysics of adhesion, requiring rapid formation of adhesive bonds to resist the forces that would pull a leukocyte free of the endothelium. Static assays of adhesion, therefore, poorly simulate the adhesion that occurs in blood or lymph vessels. The relative importance of affinity changes and integrin rearrangement in the presence of shear stress has not been worked out.
Similarly, although much is known about the role of integrin rearrangement in activating adhesion to purified ICAM-1, less is known regarding diffusion-mediated receptor rearrangement in the context of simultaneous binding of multiple sets of adhesion receptors. When leukocytes bind to vascular endothelium, a number of adhesion receptors and their ligands are available for binding. The importance of cooperativity among endothelial selectins, ß2-integrins, and other adhesion receptors is increasingly becoming appreciated (Steeber et al., 1998, 1999; Henderson et al., 2001).
To address the role of integrin rearrangement in adhesion under flow and the contribution of cooperative binding among different classes of adhesion molecules, we used a laminar flow adhesion assay system. We measured the effect of cytochalasin D mobilization of integrins on adhesion of a leukocyte cell line to both cultured endothelium and purified ligand substrates. We chose the WEHI 274.1 cell line, a well-established monocyte-like cell line used to study monocyte adhesion (Warner et al., 1979; Yue et al., 2000; Zhou and Li, 2000). In previous work, we demonstrated that the adhesive properties of this cell line on TNF- stimulated endothelium were similar to those of whole blood monocytes (Kevil et al., 2001). In addition, it has been established that low doses of cytochalasin D increase ß2-integrin diffusion on the WEHI 274.1 cell line (Zhou and Li, 2000) and that this increased diffusion activates adhesion in static assays (Zhou and Li, 2000; Zhou et al., 2001). In this study, we found that receptor rearrangement has consequences for both monocyte rolling and firm adhesion, and that these effects are enhanced by cooperative interactions among classes of adhesion molecules. This confirms that receptor rearrangement by diffusion can play an important role in adhesion of leukocytes to vascular endothelium under flow. In addition, these findings suggest that cooperativity between the ICAM-1–integrin receptor-ligand pair and other sets of adhesion molecules may be more important in the adhesion process than previously supposed.
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MATERIALS AND METHODS |
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) and cultured in dishes precoated with 1% gelatin (Sigma, St. Louis, MO). The cells were maintained for seven days in MCDB 131 media (Gibco, Grand Island, NY) supplemented with 10% FBS (HyClone, Logan, UT), 10 mg/L bovine brain extract (Biomedical Technologies, Stoughton, MA), 1000 units/L heparin (American Pharmaceutical Partners, Los Angeles, CA), 1 mg/L hydrocortisone (Sigma, St. Louis, MO), and 10 ml penicillin (Gibco). The cells were seeded in 8-cm2 tissue culture plastic dishes (Corning, Corning, NY) coated with 1% gelatin (Sigma). The cells formed confluent monolayers 6–7 days after seeding and were used for experiments within 2–3 days. WEHI 274.1 cells were obtained from ATCC and were cultured in DMEM (Gibco) containing 10% FBS and 4 µl 2-mercaptoethanol (Sigma).
In vitro rolling and adhesion assay
Cells of the WEHI 274.1 monocyte-like cell line were loaded with fluorescent dye by 20 min incubation at room temperature with 1 µM BCECF (2', 7' -bis-carboxyethyl-5-carboxyfluorescein; Molecular Probes, Eugene, OR) and suspended in Hanks' balanced salt solution (HBSS) (Sigma) at a concentration of 0.5 x 106/ml. The cells were then treated with cytochalasin D (Sigma) at 0, 0.3, 1.0, or 3.0 µg/ml for 30 min at 37°C, washed once in HBSS, resuspended in HBSS at 106 cells/ml, and loaded into a 25-ml syringe.
Eight hours before the flow chamber experiment, a confluent monolayer of endothelial cells in tissue culture dishes were treated with 10 ng/ml of murine TNF- (Sigma) to activate the endothelium. For flow experiments, a GlycoTech (Rockville, MD) flow chamber insert and gasket were inserted into the dish to form a laminar flow chamber that can be viewed on a microscope. Cells were injected into the flow chamber in HBSS at a controlled physiological shear stress of 2.5 dynes/cm2 using a programmable syringe pump (KD Scientific, New Hope, PA). Cells were viewed on an Axiovert 100 microscope (Zeiss, Thornwood, NY) equipped with a CCD camera (Model 300T-RC, Dage-MTI, Michigan City, IN) and viewed both as brightfield and fluorescent images. Video was recorded onto sVHS videotape, and selected sequences were digitized to TIF files using the Perception video editing package (Perception PVR-2500, Digital Processing Systems, Markham, Ontario, Canada). Video images were analyzed to yield position measurements every 1/30 s using Metamorph software (Universal Imaging Corporation, West Chester, PA). Position measurements were processed by programs written for this purpose by Dennis Kucik to determine velocities, accelerations, and arrest durations. Counts of rolling and firmly adherent cells were determined by visual review of video sequences and confirmed by computer analysis.
Analysis of leukocyte adhesion
Definition of adhesion duration
Even firmly adherent cells move slightly under shear stress. Therefore, an objective definition of adhesion must be used, especially when cells are observed at high time resolution. A cell was considered firmly adhered if its velocity fell below 5 µ/s for five video frames (0.16 s). This is equivalent to movement of one cell diameter in 3 s. (Note that average adhesion durations were much longer, averaging 5–25 s, as seen in Fig. 5 a). Duration of adhesion was defined as the time until the cell resumed motion at a velocity greater than 5 µ/s for more than five frames. In general, the transition from arrest to rolling was not subtle, but rather was an abrupt resumption of rolling motion.
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). Rolling was then defined as cell motion that does not meet the criteria for adhesion (above) but is below Vcrit. Average rolling velocity is the mean of the instantaneous velocities (measured at 1/30-s intervals) during the period that the cell is rolling. Rolling duration is the total time that the cell is rolling (number of frames x 0.033 s/frame). If a cell rolled intermittently (i.e., rolling was interrupted by either free flow or firm adhesion), only the frames during which the cell met the criteria for rolling were averaged.
Cell deformability index measurements
Images of rolling cells were observed and recorded onto videotape, and selected sequences were digitized at 30 frames/s into TIF images at a resolution of 720 x 486 pixels using DPS Reality hardware and software (Leitch Technology Corporation, Florence, KY). The TIF sequences were played as movies and reviewed to identify rolling and adherent cells, and individual TIF images were selected for analysis. Using these original, sharp, still pictures, length and width of cells interacting with the endothelium were measured using Metamorph software (Universal Imaging Corporation) to an accuracy of ±0.44 µ, based on the magnification of the system and assuming single-pixel resolution. Ratios were then calculated to yield cell deformability indices (Finger et al., 1996). Between 10 and 50 cells were measured for each cytochalasin D concentration.
Scanning electron microscopy
Thermanox coverslips (Electron Microscopy Sciences, Fort Washington, PA) were incubated with 500 µl poly-L-lysine (PLL, Sigma) for 30 min at 37°C, the PLL was removed, and the coverslips were washed 5 times with SEM buffer (0.1 M cacodylate buffer in water, pH 7.2, Electron Microscopy Sciences). For each coverslip, 0.2 x 106 WEHIs were washed once with HBSS and incubated for 45 min at 37°C. Cells were then fixed with 2.5% electron microscopy grade glutaraldehyde (Electron Microscopy Sciences) in SEM buffer for 20 min at room temperature. They were then washed with SEM buffer 3 times, immersed in 1% osmium tetroxide in SEM buffer for 1 h at room temperature, and then washed with SEM buffer three times. Coverslips were rinsed with distilled H2O and dehydrated at room temperature with 50%, 75%, and 95% EtOH for 4 min each, and then 3 times with 100% EtOH for 4 min each time. Cells were then dehydrated at room temperature with 1:1 100% EtOH and hexamethyldisilizane (HMDS, Electron Microscopy Sciences) for 4 min, then twice with 100% HMDS for 4 min, and allowed to air dry overnight with a shallow covering of HMDS. Coverslips were then coated with gold and attached to an SEM stub with tape or rubber cement before SEM scanning.
Purified substrate coating
Tissue culture dishes (35 mm) were marked with a diamond pen to outline a small area in the center. Then 10µl of 20µg/ml protein A (Sigma) in PBS was placed in the marked area and spread with the pipette tip, incubated at 37°C for 1 h, and then washed three times with PBS. Nonspecific binding was blocked with 2% human serum albumin (HSA, Sigma) in PBS for 2 h at 4°C followed by 3 washes with PBS. The marked areas were then coated with 50 µl purified substrate (25 µg/ml recombinant mouse ICAM-1/Fc chimera, 5 µg/ml recombinant mouse VCAM-1/Fc chimera, 0.5 µg/ml recombinant mouse E-selectin/Fc chimera, 0.5 µg/ml recombinant mouse P-selectin/Fc chimera, 16.8 µg/ml ICAM-1/10 µg/ml E-selectin double-coated, or 25 µg/ml ICAM-1/0.5 µg/ml E-selectin double-coated) overnight in 4°C. All purified coatings were obtained from R&D Systems, Minneapolis, MN.
One-minute adhesion assay
WEHI 274.1 cells were suspended in HBSS at a concentration of 1.25 x 106/ml. The cells were then treated with cytochalasin D at 0, 0.3, 1, and 3 µg/ml or 100 ng/ml phorbol 12-myristate 13-acetate (PMA) (Sigma) for 30 min at 37°C and washed once with HBSS. The cells were allowed to settle on the substrate-coated dishes for 1 min without flow, and then subjected to increasing shear stresses, incrementing every 7 seconds. Flow rates and durations were controlled by a PC using software written in the laboratory to operate a programmable syringe pump (KD Scientific). At the end of each interval the number of cells that remained bound was determined relative to the number of cells that had originally settled under static conditions. For each shear rate, adhesion was quantified in several fields of view.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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(10 ng/ml x 8 h). WEHI 274.1 cells were then perfused at a physiologic shear stress of 2.5 dynes/cm2 and the number of adherent cells per minute was determined (Fig. 1 a). A low concentration of cytochalasin D activated adhesion in a dose-dependent manner, whereas at higher concentrations adhesion was low, similar to untreated controls. Peak adhesion occurred at 0.3 µg/ml, where adhesion was increased greater than fivefold. This dose dependence is consistent with earlier studies measuring adhesion of lymphocytes to purified ICAM-1 under static conditions (Kucik et al., 1996).
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Cytochalasin D activates rolling adhesion in an ICAM-1-dependent manner
Binding of ß2-integrins to ICAM-1 has been shown to cooperate with selectins in stabilization of leukocyte rolling (Steeber et al., 1998, 1999; Kevil et al., 2003; Henderson et al., 2001). Therefore, we tested whether cytochalasin D could activate WEHI 274.1 rolling on cultured endothelium from both wild-type and ICAM-1 -/- mice. Treatment of the WEHI cells with cytochalasin D dramatically increased the number of rolling cells in a dose-dependent manner, but only on endothelium that expressed ICAM-1 (Fig. 2).
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; Finger et al., 1996; Ting-Beall et al., 1995), which might affect cell deformability. This effect can be quantified by calculating a deformation index, which is the ratio of cell length to cell width (Finger et al., 1996). To quantify deformability of WEHI 274.1 cells under our flow conditions, we calculated deformability indices for rolling cells as a function of cytochalasin D concentration. Lengths and widths of cells were measured with an accuracy of ± 0.44 µ. We found no significant increase in deformability at the cytochalasin D concentrations and shear stress used in this study (Fig. 4).
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Cytochalasin D does not affect rolling velocity
Rolling velocities for WEHI cells on wild-type endothelium were calculated by computer 30 times per second, and an average velocity for each cell was determined. Velocities were then plotted as histograms (Fig. 6) to compare the distribution of rolling velocities with and without cytochalasin D. Between 48 and 220 cell velocities were measured to produce each velocity histogram; more velocity data were available at 0.3 and 1.0 µg/ml cytochalasin D, due to increased rolling efficiency at those doses (cf. Fig. 2). We found that, even though cytochalasin D treatment affected the number of cells rolling on wild-type endothelium, average velocities were not significantly different. Similarly, cytochalasin D had no effect on rolling velocities in ICAM-1 -/- endothelium (data not shown).
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4ß1 mediates both rolling and firm adhesion via binding to VCAM-1. In addition, like the ß2-integrins, adhesion via 4ß1 is activatable. To determine whether 4ß1-mediated adhesion was affected by cytochalasin D, we measured binding of WEHIs to purified VCAM-1 in the flow chamber. As with selectin substrates, cytochalasin D did not activate adhesion to purified VCAM-1 under flow (Fig. 7 a). When the cells were allowed to settle for 1 min and then exposed to flow (Fig. 7 b), cytochalasin D was still unable to increase adhesion significantly, though adhesion could be activated by PMA.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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It was shown previously that cytochalasin D, at low concentrations, increases ß2-integrin diffusion in the WEHI 274.1 cell line with little effect on other cell functions (Zhou and Li, 2000). Increased diffusion then results in increased integrin clustering (van Kooyk and Figdor, 1997; van Kooyk et al., 1999). The pro-adhesive effect of low doses of cytochalasin D on ß2-integrin–mediated adhesion is well established for static adhesion (van Kooyk and Figdor, 1997; Yauch et al., 1997; Stewart et al., 1998; van Kooyk et al., 1999; Zhou and Li, 2000), but had not been shown to operate under flow.
For flow studies, it was important to confirm that low-dose cytochalasin D does not alter cell morphology. This is because alterations in cell surface features can change the profile presented to the flowing fluid, and, thus, the forces exerted on the leukocyte. We demonstrated by scanning electron microscopy that the cytochalasin D concentration that activates adhesion is lower than that needed to alter surface morphology. Fig. 3 demonstrates that at higher concentrations of cytochalasin D (3 µg/ml), microvilli collapse, making the cell surface smoother. This might increase contact area and/or decrease the forces exerted on the cell by the fluid flow. Since the pro-adhesive effect of cytochalasin D was at lower concentrations, though, where effects on surface morphology are not obvious (Fig. 3, b and c), it cannot be attributed to these cell shape changes. Note, however, that this does not rule out effects on the mechanical properties of microvilli with low-dose cytochalasin D.
It was also important, however, to examine changes in morphology under flow. If low concentrations of cytochalasin D were to significantly decrease cell stiffness, deformation of cells under shear stress might be increased. In neutrophils, cytochalasin D can have significant effects on cortical tension and cytoplasmic viscosity, even at relatively low doses (Ting-Beall et al., 1995; Wakatsuki et al., 2001; Finger et al., 1996). To determine whether decreased cortical tension affected WEHI 274.1 deformability and, thus, cell shape in the presence of shear, we used digital image processing to calculate deformation indices (Finger et al., 1996) for rolling and adherent cells as a function of cytochalasin D concentration. Using an imaging system developed for high temporal and spatial resolution (Kevil et al., 2003), we were able to image our cells under flow at high enough resolution to reliably determine cell shape (with an accuracy of ±0.44 µ). Under the relatively low shear stress conditions used in this study (2.5 dynes/cm2), there was no significant effect of cytochalasin D on cell deformability (Fig. 4). This does not rule out changes in the mechanical properties of the cells that might be detected by a more sensitive assay. It does clearly show, though, that low-dose cytochalasin D did not change the mechanical properties of the cells enough to cause them to be significantly more deformable under the conditions used for our adhesion assays.
It should be noted that effects of cytochalasin D on the physical properties of microvillous tethers between leukocytes and endothelial cells cannot be ruled out by this study. It is possible that changes in microvillus compliance might also influence adhesive interactions. At low-dose cytochalasin D, no obvious effect on the shape or number of microvilli on WEHI 274.1 cells was detected by SEM (Fig. 3). This does not, however, rule out changes in mechanical properties that might be apparent only in the presence of shear stress. These would not be detected in our system, since microvilli tethers are too small to be reliably visualized using our flow chamber. If the compliance of microvilli were increased by cytochalasin D, this could affect rolling adhesion. This would not be expected to have a large effect on ß2-integrin–mediated adhesion, since ß2-integrins are localized between microvilli, not at their tips (Erlandsen et al., 1993; Abitorabi et al., 1997). There could be an effect, however, on selectin-mediated rolling. Both L-selectin and PSGL-1 (the major ligand for P-selectin) are localized to microvillus tips (Picker et al., 1991; Moore et al., 1995). Any positive effect on PSGL-1–mediated adhesion would have to involve cooperation with other receptors, though, since cytochalasin D did not increase adhesion to purified P-selectin substrates (Fig. 8).
A key to our ability to detect the role of ß2-integrin diffusion in activation of adhesion under flow was the use of endothelium from ICAM-1–deficient mice. Endothelial monolayers have distinct advantages over purified substrates for understanding adhesion under flow. Although studies using purified substrates have been valuable in advancing understanding of the properties of individual receptors, they cannot truly mimic physiologic cell-cell adhesion, because, in vivo, cell adhesion involves the simultaneous interaction of multiple sets of adhesion molecules. Use of ICAM-1–deficient, gene-targeted mouse endothelial cells enabled us to detect previously unsuspected effects of ß2-integrin diffusion, such as enhancement of capture and rolling. This also allowed us to avoid the use of blocking antibodies, which can themselves have effects on cell activation, confounding interpretation of results.
High temporal and spatial resolution imaging (Kevil et al., 2003) made possible a detailed analysis of rolling motion to accurately quantify rolling adhesion and durations. Of course, at high enough temporal resolution, even smooth rolling motion consists of a series of discrete steps (Smith et al., 1999; Chen and Springer, 1999). However, by using strict criteria for rolling motion (see Materials and Methods) and quantifying motion 30 times per second, it was possible to measure velocities only during rolling motion, excluding periods when the leukocytes either arrested or released from the endothelium. Quantifying motion in this way gives a truer measure of rolling velocity than lower sampling frequencies, in which nonrolling behavior is often included in the average. Thus, it could be determined, for example, that cytochalasin D increased duration of rolling with no significant effect on rolling velocity.
The ICAM-1–dependent effect of cytochalasin D on rolling (as opposed to firm adhesion) is particularly interesting. The ß2-integrins are classically associated with firm adhesion and cell motility, and, in some systems, ß2-integrins do not mediate tethering or rolling in shear flow (Lawrence and Springer, 1991; Von Andrian et al., 1991). It is becoming increasingly clear, however, that the ß2-integrin–ICAM-1 interaction can contribute to rolling adhesion. An isolated L I-domain linked to a glycophosphatidylinositol GPI anchor and expressed in baby hamster kidney cells mediated rolling better than firm adhesion, demonstrating that, under the right conditions, the integrin binding site has rolling-receptor properties (Knorr and Dustin, 1997). Similarly, when LFA-1 was transfected into K562 cells, it mediated rolling better than firm adhesion on purified ICAM-1 (Sigal et al., 2000). In the same study, however, the Jurkat T-cell line, in which regulation of ß2-integrins is more like that of primary cells, mediated firm adhesion rather than rolling under the same conditions.
Structural studies of the LFA-1 molecule demonstrating multiple affinity states provide clues as to how the LFA-1–ICAM-1 interaction mediates both rolling and firm adhesion, and how the form of adhesion can depend on the activation state of the cell. On a leukocyte surface, ß2-integrins are thought to be present in multiple affinity states, the ratio of which can change with activation (Lollo et al., 1993). Salas et al., using isolated I-domains mutationally locked in either the high-affinity ("open") or low-affinity ("closed") conformations, demonstrated that the open conformation mediates firm adhesion, whereas the closed conformation mediates rolling (Salas et al., 2002). They also showed that, when expressed in K562 cells, the wild-type Lß2 was mostly in the closed conformation, and these cells rolled. This makes sense in terms of the binding kinetics of these constructs. Rapid kinetics for both association and dissociation are an essential feature of rolling adhesion (Alon et al., 1995). The engineered disulfide bond that stabilizes the open conformation increases the on-rate 50-fold and decreases the off-rate 200-fold. The faster dissociation rate of the low-affinity form would be predicted to favor rolling adhesion, as was observed.
Recently, an intermediate affinity state for Lß2 has been defined (Shimaoka et al., 2003). The intermediate conformation has a relatively fast on-rate as well as a fast off-rate, and it proves to be more effective as a rolling receptor than the low-affinity form. It is likely, then, that rolling interactions of ß2-integrins with ICAM-1 are mediated by low and/or intermediate conformations of the integrins.
This raises the question of whether release of integrins from cytoskeletal constraints affects affinity. The available evidence argues against this. A change in ß2-integrin affinity in response to cytochalasin D has not been demonstrated. Indeed, comparison of LFA-1 affinity states among K562 systems that rolled and those that adhered firmly indicated that the ability to roll depended on integrin arrangement, with no change in affinity state (Sigal et al., 2000). However, affinity measurements in that study and many others generally depend on binding of soluble ligand. This might not distinguish the intermediate from the low-affinity state, since both bind soluble ligand poorly. Therefore, it is possible that, in addition to increasing integrin diffusion, release from cytoskeletal constraints also converts integrins from low to intermediate affinity, which would also promote rolling. Given the well-known effects of cytochalasin D on ß2-integrin diffusion, though, and the lack of evidence that release from cytoskeletal constraints affects affinity, we feel that integrin rearrangement is the simplest interpretation of the effect of cytochalasin D on WEHI 274.1 cell adhesion.
Cytochalasin D affected not only frequency of rolling and firm adhesion, but also their duration. Adhesion of leukocytes to endothelium is not irreversible. Many cells that arrest on endothelium do so briefly, resuming rolling after a few seconds. Similarly, rolling cells often release from the endothelium after a short time to be carried away by the flow. Although cytochalasin D increased both duration of rolling and duration of firm adhesion, the dose-dependence differed. The increase in firm adhesion duration peaked at 0.3 µg/ml, but increased rolling duration was sustained even at a concentration of cytochalasin D (3 µg/ml) high enough to disrupt cytoskeletal architecture (as seen in Fig. 3). The classic interpretation for the biphasic effect of cytochalasin D is that, whereas low concentrations are pro-adhesive due to their effects on integrin rearrangement, doses high enough to substantially disrupt the cytoskeleton interfere with the ability of actin to reinforce and stabilize nascent contacts (Kucik et al., 1996). Our data suggest that cytoskeletal reinforcement is less important for maintenance of rolling than for arrest.
Clearly, ICAM-1 plays an important role in adhesion between monocytes and endothelium, because far fewer cells per minute rolled and arrested on endothelium lacking ICAM-1. Not all effects of cytochalasin D could be explained by the ß2-integrin–ICAM-1 interaction, however. The low level of adhesion on ICAM-1 -/- endothelium could still be slightly increased by pretreatment of the leukocytes with cytochalasin D (as seen in Fig. 1 b). One possible explanation for this is that, in the absence of ICAM-1, the ß2-integrins bind to other ligands. Mathematical modeling (Bell, 1978) predicts that increased diffusion of ß2-integrins will increase avidity for any ligand, not just ICAM-1. Although ICAM-1 is the major ß2-integrin ligand found on endothelial cells, ICAM-2, another Lß2 ligand, is also present (Issekutz et al., 1999). In addition, Mß2 is known to have a number of poorly characterized ligands that may be present on activated endothelial cells (Davis, 1992). Interactions between ß2-integrins and any of these other ligands could account for the small amount of ICAM-1–independent adhesive effects.
Alternatively, effects of cytochalasin D on other adhesion molecules might also result in ICAM-1–independent increased adhesion. Although, so far, increased adhesiveness in response to cytochalasin D has been described only for ß2-integrins, we tested for effects of cytochalasin D on other adhesion molecules that might be involved in activation of leukocyte adhesion. We first tested 4ß1 because, as with the ß2-integrins, leukocyte adhesion to VCAM-1 via 4ß1 is activatable. In addition, 4ß1 associates with the cytoskeleton in a regulated manner, clustering and binding to the cytoskeleton at sites of cell-cell contact (Sanchez-Mateos et al., 1993). To determine whether cytochalasin D affected 4ß1 avidity, we perfused cells over a purified VCAM-1 substrate, specific for 4ß1-mediated adhesion. We found no increase in 4ß1-mediated rolling or arrest under flow in response to cytochalasin D treatment (Fig. 7 a). Since there was very little firm adhesion under these conditions, we also measured adhesion in a brief static assay in the same flow chamber to confirm that the substrate could mediate adhesion and that 4ß1 was indeed activatable on our cells. Cells were allowed to settle for 1 min, and then nonadherent cells were washed away by exposure to high flow rates. Although PMA activated adhesion fourfold under these conditions, cytochalasin D had no significant effect (Fig. 7 b). This lack of effect of cytochalasin D on 4ß1 function is evidence that 4ß1 is not responsible for the ICAM-1–independent increase in firm adhesion observed on ICAM-1 -/- endothelium. This is consistent with earlier studies in which 4ß1 was transfected into Chinese hamster ovary (CHO) cells (Yauch et al., 1997). In those cells, 4ß1-mediated adhesion could be activated by PMA, but not by low-dose cytochalasin D. Direct measurement of 4ß1 diffusion in that system demonstrated that cytochalasin D did not affect 4ß1 lateral mobility.
Leukocytes have a number of ligands for endothelial selectins. If cytochalasin D were to increase diffusion of any or all of these ligands, it might activate adhesion. We reasoned that P-selectin–mediated adhesion might be affected by cytochalasin D, because the major ligand for P selectin, PSGL-1 (Sako et al., 1993; Moore et al., 1995; Snapp et al., 1998), is linked to the actin cytoskeleton in leukocytes via moesin (Snapp et al., 2002), and, thus, might undergo regulation of diffusion similar to the ß2-integrins. However, low-dose cytochalasin D had no significant effect on adhesion to purified P-selectin in our system (Fig. 8 a).
A number of cell-surface molecules serve as ligands for E-selectin, including PSGL-1 (Sako et al., 1993), and ESL-1 (Steegmaier et al., 1995). (Although L-selectin binds directly to E-selectin in humans, it is not a ligand in mice (Zollner et al., 1997)). Other E-selectin ligands may exist, since no combination of antibodies has yet been demonstrated to completely block E-selectin binding (Kansas, 2001). Therefore, it is difficult to account for all of the possible E-selectin ligands on a given leukocyte. Even without identifying the specific ligands present, however, the global effect of cytochalasin D on all E-selectin ligands can be tested functionally by measuring the dose dependence of cytochalasin D on WEHI cell rolling on a purified E-selectin substrate. We found that the low doses that activate integrin-mediated adhesion had no significant effect on firm adhesion to E-selectin (Fig. 9 b), and actually decreased rolling interactions (Fig. 8 b).
One of the most interesting findings was the requirement for cooperativity among adhesion receptors for avidity effects under flow (Fig. 9). On purified ICAM-1 alone, there was very little firm adhesion at a shear stress of 2.5 dynes/cm2 (perhaps due to the inefficiency of tethering), and adhesion was not increased by cytochalasin D. Tethering was more efficient on E-selectin alone, and this resulted in some firm adhesion, but cytochalasin D had no effect. On a substrate of combined purified ICAM-1 and E-selectin, however, the pro-adhesive effect of cytochalasin D could be reconstituted. This confirms the role of ICAM-1 (and its receptors, the ß2-integrins) in the adhesive effects of cytochalasin D, but shows that cooperativity with another receptor is required. A simple interpretation would be that, for cytochalasin D to increase ß2-integrin adhesion to ICAM-1, cells must first be in contact with the endothelium, and this requires an additional receptor to mediate capture and rolling. This is consistent with previous work that suggested that E-selectin and ICAM-1 work together for optimal leukocyte adhesion. In that study, L-cells transfected with both E-selectin and ICAM-1 could support far greater adhesion, both rolling and firm, than those expressing either receptor alone (Gopalan et al., 1997).
The requirement for cooperativity between integrins and selectins may result either from physical cooperation between receptors or from receptor-mediated signaling. It has been demonstrated that efficiency of firm adhesion through ß2-integrins decreases as shear increases, suggesting a requirement for a collisional contact duration of at least 5 ms (Lynam et al., 1998; Neelamegham et al., 1998). Thus, interactions between E-selectin and its ligands may slow the cell and provide for increased contact duration.
E-selectin does more, however, than simply tethering the leukocyte to increase the contact time. E-selectin binding by neutrophils can activate ß2-integrin–mediated adhesion through mitogen-activated protein kinases (Simon et al., 2000). In this study, on a combined substrate of ICAM-1 and E-selectin, adhesion could be increased by cytochalasin D. This suggests that signaling through E-selectin binding is not sufficient to fully activate integrin-mediated adhesion. A possible explanation is that integrin rearrangement can work synergistically with effects on affinity, with the two regulated independently. For example, signaling through E-selectin might increase ß2-integrin affinity, yet independent control of rearrangement might allow for further avidity increases. Future studies will be aimed at further elucidating the biochemical basis of these effects.
In summary, using low-dose cytochalasin D to mimic the increased integrin diffusion that occurs upon leukocyte activation, we have demonstrated that avidity modulation can have major effects on leukocyte adhesion to endothelium under flow. Effects involve not only firm adhesion, a well-established ß2-integrin function, but also capture and rolling, in which ß2-integrins and ICAM-1 must cooperate with selectins to exert their effects. These functional assays provide a proof of principle for regulation of leukocyte adhesion under flow by avidity regulation, and lay the groundwork for investigation of the molecular mechanisms by which integrin diffusion is controlled during adhesion in the presence of shear. Our findings not only have important implications for the mechanism of activation of adhesion, but reveal that cooperativity among sets of adhesion molecules plays a larger role than expected.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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This work was supported by a Veterans Administration Merit Award to D.F.K.
Submitted on March 28, 2003; accepted for publication August 25, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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