A catch to integrin activation

Integrins and the Metastasis-like Dissemination of Acute Lymphoblastic Leukemia to the Central Nervous System

Acute lymphoblastic leukemia (ALL) disseminates with high prevalence to the central nervous system (CNS) in a process resembling aspects of the CNS surveillance of normal immune cells as well as aspects of brain metastasis from solid cancers. Importantly, inside the CNS, the ALL blasts are typically confined within the cerebrospinal fluid (CSF)-filled cavities of the subarachnoid space, which they use as a sanctuary protected from both chemotherapy and immune cells. At present, high cumulative doses of intrathecal chemotherapy are administered to patients, but this is associated with neurotoxicity and CNS relapse still occurs. Thus, it is imperative to identify markers and novel therapy targets specific to CNS ALL. Integrins represent a family of adhesion molecules involved in cell-cell and cell-matrix interactions, implicated in the adhesion and migration of metastatic cancer cells, normal immune cells, and leukemic blasts. The ability of integrins to also facilitate cell-adhesion mediated drug resistance, combined with recent discoveries of integrin-dependent routes of leukemic cells into the CNS, have sparked a renewed interest in integrins as markers and therapeutic targets in CNS leukemia. Here, we review the roles of integrins in CNS surveillance by normal lymphocytes, dissemination to the CNS by ALL cells, and brain metastasis from solid cancers. Furthermore, we discuss whether ALL dissemination to the CNS abides by known hallmarks of metastasis, and the potential roles of integrins in this context.

Physical-Chemical Regulation of Membrane Receptors Dynamics in Viral Invasion and Immune Defense

Mechanical cues dynamically regulate membrane receptors functions to trigger various physiological and pathological processes from viral invasion to immune defense. These cues mainly include various types of dynamic mechanical forces and the spatial confinement of plasma membrane. However, the molecular mechanisms of how they couple with biochemical cues in regulating membrane receptors functions still remain mysterious. Here, we review recent advances in methodologies of single-molecule biomechanical techniques and in novel biomechanical regulatory mechanisms of critical ligand recognition of viral and immune receptors including SARS-CoV-2 spike protein, T cell receptor (TCR) and other co-stimulatory immune receptors. Furthermore, we provide our perspectives of the general principle of how force-dependent kinetics determine the dynamic functions of membrane receptors and of biomechanical-mechanism-driven SARS-CoV-2 neutralizing antibody design and TCR engineering for T-cell-based therapies.

The kinetics of E-selectin- and P-selectin-induced intermediate activation of integrin αLβ2 on neutrophils

Selectins and integrins are key players in the adhesion and signaling cascade that recruits leukocytes to inflamed tissues. Selectin binding induces β2 integrin binding to slow leukocyte rolling. Here, a micropipette was used to characterize neutrophil adhesion to E-selectin and intercellular adhesion molecule-1 (ICAM-1) at room temperature. The time-dependent adhesion frequency displayed two-stage kinetics, with an E-selectin-mediated fast increase to a low plateau followed by a slow increase to a high plateau mediated by intermediate-affinity binding of integrin αLβ2 to ICAM-1. The αLβ2 activation required more than 5 s contact to E-selectin and spleen tyrosine kinase (Syk) activity. A multi-zone channel was used to analyze αLβ2 activation by P-selectin in separate zones of receptors or antibodies, finding an inverse relationship between the rolling velocity on ICAM-1 and P-selectin dose, and a P-selectin dose-dependent change from bent to extended conformations with a closed headpiece that was faster at 37°C than at room temperature. Activation of αLβ2 exhibited different levels of cooperativity and persistent times depending on the strength and duration of selectin stimulation. These results define the precise timing and kinetics of intermediate activation of αLβ2 by E- and P-selectins.

Integrin-Mediated Adhesion and Chemoresistance of Acute Lymphoblastic Leukemia Cells Residing in the Bone Marrow or the Central Nervous System

Acute Lymphoblastic Leukemia (ALL) is the most common cancer in childhood. Despite a significantly improved prognosis over the last decade with a 5-years survival rate of ~90%, treatment-related morbidity remains substantial and relapse occurs in 10–15% of patients (1). The most common site of relapse is the bone marrow, but early colonization and subsequent reoccurrence of the disease in the central nervous system (CNS) also occurs. Integrins are a family of cell surface molecules with a longstanding history in cancer cell adherence, migration and metastasis. In chronic lymphoblastic leukemia (CLL), the VLA-4 integrin has been acknowledged as a prognostic marker and mounting evidence indicates that this and other integrins may also play a role in acute leukemia, including ALL. Importantly, integrins engage in anti-apoptotic signaling when binding extracellular molecules that are enriched in the bone marrow and CNS microenvironments. Here, we review the current evidence for a role of integrins in the adherence of ALL cells within the bone marrow and their colonization of the CNS, with particular emphasis on mechanisms adding to cancer cell survival and chemoresistance.

Atomic Force Microscopy-Based Nanoscopy of Chondrogenically Differentiating Human Adipose-Derived Stem Cells: Nanostructure and Integrin β1 Expression

Integrin β1 is known to be involved in differentiation, migration, proliferation, wound repair, tissue development, and organogenesis. In order to analyze the binding probability between integrin β1 ligand and cluster of differentiation 29 (CD29) receptors, atomic force microscopy (AFM) was used to detect native integrin β1-coupled receptors on the surface of human adipose-derived stem cells (hADSc). The binding probability of integrin β1 ligand-receptor interaction was probed by integrin β1-functionalized tips on hADSc during early chondrogenic differentiation at the two-dimensional cell culture level. Cell morphology and ultrastructure of hADSc were measured by AFM, which demonstrated that long spindled cells became polygonal cells with decreased length/width ratios and increased roughness during chondrogenic induction. The binding of integrin β1 ligand and CD29 receptors was detected by β1-functionalized tips for living hADSc. A total of 1200 curves were recorded at 0, 6, and 12 days of chondrogenic induction. Average rupture forces were, respectively, 61.8 ± 22.2 pN, 60 ± 20.2 pN, and 67.2 ± 22.0 pN. Rupture events were 19.58 ± 1.74%, 28.03 ± 2.05%, and 33.4 ± 1.89%, respectively, which demonstrated that binding probability was increased between integrin β1 ligand and receptors on the surface of hADSc during chondrogenic induction. Integrin β1 and the β-catenin/SOX signaling pathway were correlated during chondrogenic differentiation. The results of this investigation imply that AFM offers kinetic and visual insight into the changes in integrin β1 ligand-CD29 receptor binding on hADSc during chondrogenesis. Changes in cellular morphology, membrane ultrastructure, and the probability of ligand-transmembrane receptor binding were demonstrated to be useful markers for evaluation of the chondrogenic differentiation process.

Effects of Ketamine on Icam-1 Expression during Lipopolysaccharide Induced Acute Lung Injury in Rats

Introduction

Intercellular adhesion molecule-1 (ICAM-1) serves very important roles in lung injury, and increases in ICAM-1 are associated with severity of lung injury. We intended to examine whether ketamine could have favourable effects on ICAM-1 expression in lipopolysaccharide (LPS)-induced acute lung injury in the rat.

Methods

The acute lung injury was induced by instilling LPS into the tracheas of 72 anaesthetised rats. The animals were divided into four groups including a sham group with intratracheal saline, an LPS group with intratracheal LPS and intraperitoneal saline, and two ketamine treatment groups with intratracheal LPS and treated with 1 mg/kg or 10 mg/kg doses of intraperitoneal ketamine hydrochloride. Half of the animals were sacrificed at three hours and the remaining animals were sacrificed at six hours. Real time PCR was performed on the lung tissues obtained. Concentration of the soluble ICAM-1 was measured in the bronchoalveolar lavage fluid. Expression of ICAM-1 was measured.

Results

The transcriptional activity of ICAM-1 mRNA increased 9.92 fold in the LPS group compared to the sham group but decreased by 84.1% and 83.3%, respectively, in the 1 and 10 mg/kg ketamine treatment groups. The concentration of soluble ICAM-1 in bronchoalveolar lavage fluid increased 2.23 fold in the LPS group compared to the sham group while decreasing by 75.0% and 74.5% respectively in the 1 mg/kg and 10 mg/kg ketamine treatment groups. The intensity of immunohistochemical staining for ICAM-1 was also decreased in both ketamine treatment groups.

Conclusion

Ketamine attenuates ICAM-1 expression during acute lung injury in rats. (Hong Kong j.emerg.med. 2011;18:397-405)

Molecular Mechanoneurobiology: An Emerging Angle to Explore Neural Synaptic Functions

Neural synapses are intercellular asymmetrical junctions that transmit biochemical and biophysical information between a neuron and a target cell. They are very tight, dynamic, and well organized by many synaptic adhesion molecules, signaling receptors, ion channels, and their associated cytoskeleton that bear forces. Mechanical forces have been an emerging factor in regulating axon guidance and growth, synapse formation and plasticity in physiological and pathological brain activity. Therefore, mechanical forces are undoubtedly exerted on those synaptic molecules and modulate their functions. Here we review current progress on how mechanical forces regulate receptor-ligand interactions, protein conformations, ion channels activation, and cytoskeleton dynamics and discuss how these regulations potentially affect synapse formation, stabilization, and plasticity.

Biomechanics of cell adhesion: how force regulates the lifetime of adhesive bonds at the single molecule level

Cell adhesion proteins play critical roles in positioning cells during development, segregating cells into distinct tissue compartments and in maintaining tissue integrity. The principle function of these proteins is to bind cells together and resist mechanical force. Adhesive proteins also enable migrating cells to adhere and roll on surfaces even in the presence of shear forces exerted by fluid flow. Recently, several experimental and theoretical studies have provided quantitative insights into the physical mechanisms by which adhesion proteins modulate their unbinding kinetics in response to tensile force. This perspective reviews these biophysical investigations. We focus on single molecule studies of cadherins, selectins, integrins, the von Willebrand factor and FimH adhesion proteins; the effect of mechanical force on the lifetime of these interactions has been extensively characterized. We review both theoretical models and experimental investigations and discuss future directions in this exciting area of research.

Interleukin (IL)-4 induces production of cytokine-induced neutrophil chemoattractants (CINCs) and intercellular adhesion molecule (ICAM)-1 in lungs of asthmatic rats

The present study aimed to examine the effect of interleukin (IL)-4 on neutrophil chemotaxis in airway inflammation in asthmatic rats and the possible mechanism. Male Wistar rats were intranasally instilled with recombinant rat (rr) IL-4 (rrIL-4) at different doses [2, 4 or 8 μg/animal, dissolved in 200 μL normal saline (NS)] or rrIL-4 at 4 μg/animal (dissolved in 200 μL NS). NS (200 μL) and LPS (6 mg/kg/animal, dissolved in 200 μL NS) were intranasally given respectively in the negative and positive control groups. Moreover, the asthmatic lung inflammation was induced in rats which were then intranasally treated with rrIL-4 (4 μg/animal) or LPS (6 mg/kg/animal). The normal rats treated with different doses of rrIL-4 and those asthmatic rats were sacrificed 6 h later. And animals instilled with rrIL-4 at 4 μg were sacrificed 6, 12 or 24 h later. The bronchoalveolar lavage fluid (BALF) and lungs were harvested for detection of leukocyte counts by Wright-Giemsa staining and lung histopathology by haematoxylin-eosin (HE) staining. The levels of cytokine-induced neutrophil chemoattractant (CINC)-1 and intercellular adhesion molecule (ICAM)-1 in BALF were determined by ELISA. Real-time PCR was used to measure the mRNA expression of CINCs (CINC-1, CINC-2α, CINC-2β, CINC-3) and ICAM-1 in lung tissues. The results showed that the intranasal instillation of IL-4 did not induce a recruitment of neutrophils in BALF in rats. However, IL-4 could increase the CINC-1 level in BALF in a dose-dependent manner at 6 h. But the mRNA expression levels of CINC-1, CINC-2α, CINC-2β, CINC-3 were not significantly increased in lungs of IL-4-treated rats relative to NS negative control group. Moreover, IL-4 was found to augment the mRNA expression of ICAM-1 in lungs and the ICAM-1 level in BALF at 6 h. However, the increase in CINC-1 and ICAM-1 levels in BALF of IL-4-treated asthmatic rats was not significantly different from that in untreated asthmatic rats. These findings indicate that IL-4 does not directly recruit neutrophils in the rat lungs, but it may contribute to airway neutrophilia through up-regulation of CINC-1 and ICAM-1.

Intracellular adhesion molecule-1 is regulated by porphyromonas gingivalis through nucleotide binding oligomerization domain-containing proteins 1 and 2 molecules in periodontal fibroblasts

BACKGROUND

The mechanism by which Porphyromonas gingivalis regulates intracellular adhesion molecule 1 (ICAM-1) expression in human periodontal ligament cells (hPDLCs) and human gingival fibroblasts (hGFs) is unknown. The aim of this study is to investigate whether nucleotide binding oligomerization domain-containing protein (NOD) 1 and NOD2 are involved in this process and the clinical significance of ICAM-1 in periodontitis.

METHODS

hPDLCs and hGFs were treated with P. gingivalis, l-Ala-γ-d-glutamyl-mesodiaminopimelic acid (an agonist for NOD1), and muramyl dipeptide (an agonist for NOD2). Alternatively, cells transfected with small interfering RNA targeting NOD1and NOD2 were treated with P. gingivalis. ICAM-1, NOD1, and NOD2 were detected at mRNA and protein levels. In addition, clinical examinations were performed in 30 healthy controls and 40 patients with chronic periodontitis (CP) before and after treatment, and serum-soluble ICAM-1 (sICAM-1) levels in these individuals were detected by enzyme-linked immunosorbent assay.

RESULTS

This study shows that P. gingivalis caused an increase in ICAM-1, NOD1, and NOD2 expression in periodontal fibroblasts. There was a linear correlation between ICAM-1 and NOD1 and NOD2 levels. Activation of NOD1 and NOD2 by the specific agonist led to the upregulation of ICAM-1, whereas knocking down NOD1 and NOD2 caused a reduction in P. gingivalis-induced ICAM-1 production. Furthermore, sICAM-1 levels were higher in patients with CP than in healthy controls and were positively related to the clinical periodontal parameters. After periodontal treatment, sICAM-1 levels decreased significantly.

CONCLUSIONS

The present results indicate that sICAM-1 levels are correlated to the severity of periodontitis. NOD1 and NOD2 mediate P. gingivalis-induced ICAM-1 production in periodontal fibroblasts. NOD1 and NOD2 could be considered potential targets for periodontal therapy.

A role of kindlin-3 in integrin αMβ2 outside-in signaling and the Syk-Vav1-Rac1/Cdc42 signaling axis

Integrins mediate cell-cell and cell-extracellular matrix attachments. Integrins are signaling receptors because their cytoplasmic tails are docking sites for cytoskeletal and signaling proteins. Kindlins are a family of band 4.1-ezrin-radixin-moesin-containing intracellular proteins. Apart from regulating integrin ligand-binding affinity, recent evidence suggests that kindlins are involved in integrin outside-in signaling. Kindlin-3 is expressed in platelets, hematopoietic cells and endothelial cells. In humans, loss of kindlin-3 expression accounts for the rare autosomal disease leukocyte adhesion deficiency (LAD) type III that is characterized by bleeding disorders and defective recruitment of leukocytes into sites of infection. Studies have shown that the loss of kindlin-3 expression leads to poor ligand-binding properties of β1, β2 and β3 integrin subfamilies. The leukocyte-restricted β2 integrin subfamily comprises four members, namely αLβ2, αMβ2, αXβ2 and αDβ2. Integrin αMβ2 mediates leukocyte adhesion, phagocytosis, degranulation and it is involved in the maintenance of immune tolerance. Here we provide further evidence that kindlin-3 is required for integrin αMβ2-mediated cell adhesion and spreading using transfected K562 cells that expressed endogenous kindlin-3 but not β2 integrins. K562 stable cell line expressing si-RNA targeting kindlin-3, but not control-si-RNA, and transfected with constitutively activated integrin αMβ2N329S adhered and spread poorly on iC3b. We also show that kindlin-3 is required for the integrin αMβ2-Syk-Vav1 signaling axis that regulates Rac1 and Cdc42 activities. These findings reinforce a role for kindlin-3 in integrin outside-in signaling.

Expression of E-selectin ligands on circulating tumor cells: cross-regulation with cancer stem cell regulatory pathways?

Although significant progress has been made in the fight against cancer, successful treatment strategies have yet to be developed to combat those tumors that have metastasized to distant organs. Poor characterization of the molecular mechanisms of cancer spread is a major impediment to designing predictive diagnostics and effective clinical interventions against late stage disease. In hematogenous metastasis, it is widely suspected that circulating tumor cells (CTCs) express specific adhesion molecules that actively initiate contact with the vascular endothelium lining the vessel walls of the target organ. This "tethering" is mediated by ligands expressed by CTCs that bind to E-selectin expressed by endothelial cells. However, it is currently unknown whether expression of functional E-selectin ligands on CTCs is related to cancer stem cell regulatory or maintenance pathways, particularly epithelial-to-mesenchymal transition and the reverse, mesenchymal-to-epithelial transition. In this hypothesis and theory article, we explore the potential roles of these mechanisms on the dynamic regulation of selectin ligands mediating CTC trafficking during metastasis.

Signal-dependent slow leukocyte rolling does not require cytoskeletal anchorage of P-selectin glycoprotein ligand-1 (PSGL-1) or integrin αLβ2

In inflamed venules, neutrophils roll on P- or E-selectin, engage P-selectin glycoprotein ligand-1 (PSGL-1), and signal extension of integrin α(L)β(2) in a low affinity state to slow rolling on intercellular adhesion molecule-1 (ICAM-1). Cytoskeleton-dependent receptor clustering often triggers signaling, and it has been hypothesized that the cytoplasmic domain links PSGL-1 to the cytoskeleton. Chemokines cause rolling neutrophils to fully activate α(L)β(2), leading to arrest on ICAM-1. Cytoskeletal anchorage of α(L)β(2) has been linked to chemokine-triggered extension and force-regulated conversion to the high affinity state. We asked whether PSGL-1 must interact with the cytoskeleton to initiate signaling and whether α(L)β(2) must interact with the cytoskeleton to extend. Fluorescence recovery after photobleaching of transfected cells documented cytoskeletal restraint of PSGL-1. The lateral mobility of PSGL-1 similarly increased by depolymerizing actin filaments with latrunculin B or by mutating the cytoplasmic tail to impair binding to the cytoskeleton. Converting dimeric PSGL-1 to a monomer by replacing its transmembrane domain did not alter its mobility. By transducing retroviruses expressing WT or mutant PSGL-1 into bone marrow-derived macrophages from PSGL-1-deficient mice, we show that PSGL-1 required neither dimerization nor cytoskeletal anchorage to signal β(2) integrin-dependent slow rolling on P-selectin and ICAM-1. Depolymerizing actin filaments or decreasing actomyosin tension in neutrophils did not impair PSGL-1- or chemokine-mediated integrin extension. Unlike chemokines, PSGL-1 did not signal cytoskeleton-dependent swing out of the β(2)-hybrid domain associated with the high affinity state. The cytoskeletal independence of PSGL-1-initiated, α(L)β(2)-mediated slow rolling differs markedly from the cytoskeletal dependence of chemokine-initiated, α(L)β(2)-mediated arrest.

Physiology and pathophysiology of selectins, integrins, and IgSF cell adhesion molecules focusing on inflammation. A paradigm model on infectious endocarditis

The development of adhesion bonds, either among cells or among cells and components of the extracellular matrix, is a crucial process. These interactions are mediated by some molecules collectively known as adhesion molecules (CAMs). CAMs are ubiquitously expressed proteins playing a central role in controlling cell migration, proliferation, survival, and apoptosis. Besides their key function in physiological maintenance of tissue integrity, CAMs play an eminent role in various pathological processes such as cardiovascular disorders, atherogenesis, atherosclerotic plaque progression and regulation of the inflammatory response. CAMs such as selectins, integrins, and immunoglobulin superfamily take part in interactions between leukocyte and vascular endothelium (leukocyte rolling, arrest, firm adhesion, migration). Experimental data and pathologic observations support the assumption that pathogenic microorganisms attach to vascular endothelial cells or sites of vascular injury initiating intravascular infections. In this review a paradigm focusing on cell adhesion molecules pathophysiology and infective endocarditis development is given.

Dissociation of bimolecular αIIbβ3-fibrinogen complex under a constant tensile force

The regulated ability of integrin αIIbβ3 to bind fibrinogen plays a crucial role in platelet aggregation, adhesion, and hemostasis. Employing an optical-trap-based electronic force clamp, we studied the thermodynamics and kinetics of αIIbβ3-fibrinogen bond formation and dissociation under constant unbinding forces, mimicking the forces of physiologic blood shear on a thrombus. The distribution of bond lifetimes was bimodal, indicating that the αIIbβ3-fibrinogen complex exists in two bound states with different mechanical stability. The αIIbβ3 antagonist, abciximab, inhibited binding without affecting the unbinding kinetics, whereas Mn²(+) biased the αIIbβ3-fibrinogen complex to the strong bound state with reduced off-rate. The average bond lifetimes decreased exponentially with increasing pulling force from ∼5 pN to 50 pN, suggesting that in this force range the αIIbβ3-fibrinogen interactions are classical slip bonds. We found no evidence for catch bonds, which is consistent with the known lack of shear-enhanced platelet adhesion on fibrinogen-coated surfaces. Taken together, these data provide important quantitative and qualitative characteristics of αIIbβ3-fibrinogen binding and unbinding that underlie the dynamics of platelet adhesion and aggregation in blood flow.

Specificities of β1 integrin signaling in the control of cell adhesion and adhesive strength

Cells exert actomyosin contractility and cytoskeleton-dependent force in response to matrix stiffness cues. Cells dynamically adapt to force by modifying their behavior and remodeling their microenvironment. This adaptation is favored by integrin activation switch and their ability to modulate their clustering and the assembly of an intracellular hub in response to force. Indeed integrins are mechanoreceptors and mediate mechanotransduction by transferring forces to specific adhesion proteins into focal adhesions which are sensitive to tension and activate intracellular signals. α(5)β(1) integrin is considered of major importance for the formation of an elaborate meshwork of fibronectin fibrils and for the extracellular matrix deposition and remodeling. Here we summarize recent progress in the study of mechanisms regulating the activation cycle of β(1) integrin and the specificity of α(5)β(1) integrin in mechanotransduction.

Regulation of catch binding by allosteric transitions

An allosteric model is used to describe changes in lifetimes of biological receptor-ligand bonds subjected to an external force. Force-induced transitions between the two states of the allosteric site lead to changes in the receptor conformation. The ligand bound to the receptor fluctuates between two different potentials formed by the two receptor conformations. The effect of the force on the receptor-ligand interaction potential is described by the Bell mechanism. The probability of detecting the ligand in the bound state is found to depend on the relaxation times of both ligand and allosteric sites. An analytic expression for the bond lifetime is derived as a function of force. The formal theoretical results are used to explain the anomalous force and time dependences of the integrin-fibronectin bond lifetimes measured by atomic force microscopy (Kong, F.; et al J. Cell Biol. 2009, 185, 1275-1284). The analytic expression and model parameters describe very well all anomalous dependences identified in the experiments.

Demonstration of catch bonds between an integrin and its ligand

Binding of integrins to ligands provides anchorage and signals for the cell, making them prime candidates for mechanosensing molecules. How force regulates integrin-ligand dissociation is unclear. We used atomic force microscopy to measure the force-dependent lifetimes of single bonds between a fibronectin fragment and an integrin alpha(5)beta(1)-Fc fusion protein or membrane alpha(5)beta(1). Force prolonged bond lifetimes in the 10-30-pN range, a counterintuitive behavior called catch bonds. Changing cations from Ca(2+)/Mg(2+) to Mg(2+)/EGTA and to Mn(2+) caused longer lifetime in the same 10-30-pN catch bond region. A truncated alpha(5)beta(1) construct containing the headpiece but not the legs formed longer-lived catch bonds that were not affected by cation changes at forces <30 pN. Binding of monoclonal antibodies that induce the active conformation of the integrin headpiece shifted catch bonds to a lower force range. Thus, catch bond formation appears to involve force-assisted activation of the headpiece but not integrin extension.

Transmigration of neutrophils across inflamed endothelium is signaled through LFA-1 and Src family kinase

Leukocyte capture on inflamed endothelium is facilitated by a shift in LFA-1 from low to high affinity that supports binding to ICAM-1. LFA-1 bonds help anchor polymorphonuclear leukocytes (PMN) to inflamed endothelium in shear flow, and their redistribution to the leading edge guides pseudopod formation, migration, and extravasation. These events can be disrupted at the plasma membrane by stabilizing LFA-1 in a low- or intermediate-affinity state with allosteric small molecules. We hypothesized that a minimum dimeric bond formation between high-affinity LFA-1 and ICAM-1 under shear stress is necessary to catalyze transmembrane signaling of directed cell migration. Microspheres and substrates were derivatized with monomeric or dimeric ICAM-1 to simulate the surface of inflamed endothelium under defined ligand valence. Binding to dimeric ICAM-1, and not monomeric ICAM-1, was sufficient to elicit assembly of F-actin and phosphorylation of Src family kinases that colocalized with LFA-1 on adherent PMN. Genetic deletion or small molecule inhibition of Src family kinases disrupted their association with LFA-1 that correlated with diminished polarization of arrested PMN and abrogation of transmigration on inflamed endothelium. We conclude that dimeric bond clusters of LFA-1/ICAM-1 provide a key outside-in signal for orienting cytoskeletal dynamics that direct PMN extravasation at sites of inflammation.

Extravasation of leukocytes in comparison to tumor cells

The multi-step process of the emigration of cells from the blood stream through the vascular endothelium into the tissue has been termed extravasation. The extravasation of leukocytes is fairly well characterized down to the molecular level, and has been reviewed in several aspects. Comparatively little is known about the extravasation of tumor cells, which is part of the hematogenic metastasis formation. Although the steps of the process are basically the same in leukocytes and tumor cells, i.e. rolling, adhesion, transmigration (diapedesis), the molecules that are involved are different. A further important difference is that leukocyte interaction with the endothelium changes the endothelial integrity only temporarily, whereas tumor cell interaction leads to an irreversible damage of the endothelial architecture. Moreover, tumor cells utilize leukocytes for their extravasation as linkers to the endothelium. Thus, metastasis formation is indirectly susceptible to localization signals that are literally specific for the immune system. We herein compare the extravasation of leukocytes and tumor cells with regard to the involved receptors and the localization signals that direct the cells to certain organs and sites of the body.

Separable requirements for cytoplasmic domain of PSGL-1 in leukocyte rolling and signaling under flow

In inflamed venules, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to roll on P-selectin and E-selectin and to activate integrin alphaLbeta2 (lymphocyte function-associated antigen-1, LFA-1) to slow rolling on intercellular adhesion molecule-1 (ICAM-1). Studies in cell lines have suggested that PSGL-1 requires its cytoplasmic domain to localize in membrane domains, to support rolling on P-selectin, and to signal through spleen tyrosine kinase (Syk). We generated "DeltaCD" mice that express PSGL-1 without the cytoplasmic domain. Unexpectedly, neutrophils from these mice localized PSGL-1 normally in microvilli, uropods, and lipid rafts. DeltaCD neutrophils expressed less PSGL-1 on their surfaces because of inefficient export from the endoplasmic reticulum. Limited digestion of wild-type neutrophils with O-sialoglycoprotein endopeptidase was used to reduce the PSGL-1 density to that on DeltaCD neutrophils. At matched PSGL-1 densities, both DeltaCD and wild-type neutrophils rolled similarly on P-selectin. However, DeltaCD neutrophils rolling on P-selectin did not trigger Syk-dependent activation of LFA-1 to slow rolling on ICAM-1. These data demonstrate that the PSGL-1 cytoplasmic domain is dispensable for leukocyte rolling on P-selectin but is essential to activate beta2 integrins to slow rolling on ICAM-1.