Integrin-dependent pathologies

Cell adhesion and migration are essential for embryonic development, tissue regeneration, and immune defence. The physical link between the extracellular substrate and the actin cytoskeleton is mediated by receptors of the integrin family and a large set of adaptor proteins. During cell migration this physical link is dynamically modified, allowing the cell to sense and adapt to the microenvironment. This includes the formation of integrin clusters at the cell front, their stabilization in the cell body and subsequent disassembly of these clusters at the rear of the cell. The modulation of the adhesion strength of the cell to the substrate is regulated by the affinity switch of integrin molecules and increased avidity through clustering of integrins. Here we explain how integrins mediate cell migration and how genetic defects of integrins and their adaptors lead to cellular dysfunction and generate pathological situations.     

Integrin α chain cytoplasmic tails regulate “antibody-redirected” cell adhesion,independently of ligand binding

Here we describe a novel “antibody-redirected cell adhesion” (ARCA) assay. This assay measures heterotypic cell-cell adhesion, resulting from antibody bridging between Fcγ receptors type II (CD32) on leukocytes, and clustered intergrins on adherent cell monolayers. This ARCA activity, facilitated by integrins αβ₁ or α⁴β₁, required an intact cytoskeleton, but did not involve typical integrin ligand binding sites or divalent cations. Furthermore, deletion of the α⁴ cytoplasmic tail almost completely abrogated integrin ARCA activity, suggesting an alteration of integrin recruitment into adhesive sites. If two or more tail residues were present after the conserved GFFKR motif, then ARCA activity was largely restored. Although α⁴ tail deletion caused loss of ARCA activity, it had no effect on the binding of VCAM-1 to intact α⁴-transfected K562 cells. In conclusion, the integrin α chain tail can positively regulate integrin-dependent cell adhesion by a receptor recruitment/clustering mechanism independent of conventional integrin ligand-binding considerations.     

A role for Syk-kinase in the control of the binding cycle of the beta2 integrins (CD11/CD18) in human polymorphonuclear neutrophils

A fine control of beta(2) integrin (CD11/CD18)-mediated firm adhesion of human neutrophils to the endothelial cell monolayer is required to allow ordered emigration. To elucidate the molecular mechanisms that control this process, intracellular protein tyrosine signaling subsequent to beta(2) integrin-mediated ligand binding was studied by immunoprecipitation and Western blotting techniques. The 72-kDa Syk-kinase, which was tyrosine-phosphorylated upon adhesion, was found to coprecipitate with CD18, the beta-subunit of the beta(2) integrins. Moreover, inhibition of Syk-kinase by piceatannol enhanced adhesion and spreading but diminished N-formyl-Met-Leu-Phe-induced chemotactic migration. The enhancement of adhesiveness was associated with integrin clustering, which results in increased integrin avidity. In contrast, piceatannol had no effect on the surface expression or on the affinity of beta(2) integrins. Altogether, this suggests that Syk-kinase controls alternation of beta(2) integrin-mediated ligand binding with integrin detachment.     

Shape and shift changes related to the function of leukocyte integrins LFA-1 and Mac-1

Integrin activity on leukocytes is controlled tightly, ensuring that ligand binding occurs only when leukocytes are in contact with their targets. For an integrinlike LFA-1, this ligand-binding activity comes about as a result of increased integrin clustering. Affinity regulation of integrins also plays a role, but the conformational changes giving rise to increased affinity appear to be secondary to clustering. Conformationally altered LFA-1 can be created artificially by deletion of the I domain, which is the key domain involved in ligand binding for many but not all integrins. Although I domain-deleted LFA-1 (DeltaI-LFA-1) cannot bind ligand, it is able to signal constitutively into the cell. One measure of this signaling activity is the ability of DeltaI-LFA-1 to activate beta1 integrins on the same T lymphocyte. Leukocytes use LFA-1 to migrate across the endothelium. Active beta1 integrins may be required subsequently to bind the matrix proteins encountered by leukocytes as they continue their voyage into the tissue interior.     

Pathological adhesion of primary human schwannoma cells is dependent on altered expression of integrins

Mutations in the tumor suppressor gene coding for merlin cause Neurofibromatosis type 2 (NF2), all spontaneous schwannomas, and a majority of meningiomas. Merlin links transmembrane proteins to the cytoskeleton. Accordingly, primary human schwannoma cells lacking merlin show an increased number of lamellipodia and filopodia as well as increased cell spreading. We show enhanced adhesion in primary human schwannoma cells and present evidence that this is dependent on the integrin chains alpha6beta1 and alpha6beta4. We further demonstrate that the integrin chains beta1 and beta4 are upregulated in schwannomas using different complementary methods, and report higher expression of these integrins per schwannoma cell by fluorescence assisted cell sorting (FACS). Finally we report clustering of the integrin chains alpha6, beta1, and beta4 on schwannoma cells. Our findings fit well into recent data on the role of merlin in signaling cascades connected to integrins and help explain pathological ensheathment of extracellular matrix or pseudomesaxon formation which is a hallmark of schwannoma histopathology.     

Modulation of integrin-mediated intercellular adhesion during the interaction of thymocytes with stromal cells expressing VLA-4 and LFA-1 ligands

Mature peripheral T cells closely regulate their intercellular interactions by modulating integrin adhesion functions. The ability of members of the integrin family to mediate intercellular adhesion is dependent on signals from within the cells (inside-out signaling) that increase the avidity of integrins for their ligands. These changes in avidity are independent of the quantitative changes on the number of receptors, and there is evidence to suggest that phosphorylation events play a predominant role in the regulation of the avidity state of the integrins. Whether such regulatory mechanisms are operative during T cell development had hitherto been an opened question. In the present work, we have used an in vitro adhesion assay between thymocytes and target cells expressing VLA-4 and LFA-1 counter ligands to determine how thymocytes can discriminate between integrin-specific signals during T cell development. Our findings are that VLA-4, but not LFA-1, is constitutively expressed in its high-avidity state during the early stages of T cell development, and that the high-avidity state of thymocytes for VCAM-1-expressing cells is closely regulated by signaling through protein kinase C and protein tyrosine kinase pathways. At later stages of development, mature thymocytes prior to leaving the thymus turn off both VLA-4 and LFA-1 adhesion functions. Our results show that the low-affinity state of integrins on peripheral mature T cells is established before mature thymocytes leave the thymus. Only when mature T cells recognize antigenic peptides in the context of major histocompatibility complex in the periphery will they turn on the adhesion function of VLA-4 and/or LFA-1 integrins.     

Integrin activation and structural rearrangement

Summary: Among adhesion receptor families, integrins are particularly important in biological processes that require rapid modulation of adhesion and de-adhesion. Activation on a timescale of < 1 s of β2 integrins on leukocytes and β3 integrins on platelets enables deposition of these cells at sites of inflammation or vessel wall injury. Recent crystal, nuclear magnetic resonance (NMR), and electron microscope (EM) structures of integrins and their domains lead to a unifying mechanism of activation for both integrins that contain and those that lack an inserted (I) domain. The I domain adopts two alternative conformations, termed open and closed. In striking similarity to signaling G-proteins, rearrangement of a Mg²⁺-binding site is linked to large conformational movements in distant backbone regions. Mutations that stabilize a particular conformation show that the open conformation has high affinity for ligand, whereas the closed conformation has low affinity. Movement of the C-terminal α-helix 10 Å down the side of the domain in the open conformation is sufficient to increase affinity at the distal ligand-binding site 9000-fold. This C-terminal "bell-rope" provides a mechanism for linkage to conformational movements in other domains. Recent structures and functional studies reveal interactions between β-propeller, I, and I-like domains in the integrin headpiece, and a critical role for integrin epidermal growth factor (EGF) domains in the stalk region. The headpiece of the integrin faces down towards the membrane in the inactive conformation, and extends upward in a "switchblade"-like opening upon activation. These long-range structural rearrangements of the entire integrin molecule involving interdomain contacts appear closely linked to conformational changes within the I and I-like domains, which result in increased affinity and competence for ligand binding.     

The significance of integrin ligand nanopatterning on lipid raft clustering in hematopoietic stem cells

Hematopoietic stem cells (HSCs) are the vital, life-long source of all blood cell types. They are found in stem cell niches, specific anatomic locations that offer all the factors and signals necessary for the maintenance of the stem cell potential of HSCs. Much attention has been paid to the biochemical composition of the niches, but only little is known about the influence of physical parameters, such as ligand nanopatterns, on HSCs. To investigate the impact of nanometer-scale spacing between cell ligands on HSC adhesion, integrin distribution and signal transduction, we employed geometrically defined, nanostructured, bio-functionalized surfaces. HSCs proved to be sensitive to the lateral distance between the presented ligands with regard to adhesion and lipid raft clustering, the latter being a prerequisite for the formation of signaling complexes. Furthermore, an extensive redistribution of stem cell markers, integrins and phosphorylated proteins in HSCs was observed. In conclusion, integrin-mediated adhesion and signaling of HSCs proved to depend on the nanostructured presentation of ligands in their environment. In this work, we show that the nanostructure of the matrix is an important parameter influencing HSC behavior that should be integrated into biomaterial-based approaches aiming at HSC multiplication or differentiation.     

Functional relevance during lymphocyte migration and cellular localization of activated β1 integrins

The state of integrin activation can be assessed by monoclonal antibodies (mAb) that selectively recognize integrins in their active form. We demonstrate herein that the expression of the epitope recognized by mAb HUTS-21 is induced on T lymphoblasts upon binding of soluble vascular cell adhesion molecule (VCAM)-1 and an 80-kDa tryptic fragment of fibronectin (FN80) to the β1 integrins very late activation antigen (VLA)-4 and VLA-5, and that this effect is dependent on ligand concentration and is specific for β1 integrins. On T lymphoblasts adhering to immobilized fibronectin, the HUTS-21 epitope localized exclusively to sites of integrin binding to fibronectin. These results indicate that mAb HUTS-21 recognizes a ligand-induced binding site (LIBS) on the common β1 subunit of VLA proteins. Engagement of β1 integrins through this LIBS epitope inhibited T lym-phoblast movement on fibronectin, as determined by quantitative time-lapse video microscopy studies. Furthermore, the HUTS-21 mAb also prevented T lymphoblast-directed migration through gradients of substratum-immobilized β1 integrin ligands such as fibronectin or VCAM-1, whereas it did not affect migration on intercellular adhesion molecule (ICAM)-1. This anti-LIBS mAb stimulated cell adhesion through postreceptor events, without affecting receptor affinity for ligand, and appears to interfere with cell migration by a mechanism distinct from that of other anti-β1 activating antibodies.     

Vav1 controls integrin clustering and MHC/peptide-specific cell adhesion to antigen-presenting cells

Integrin-mediated adhesion is essential for the formation of stable contacts between T cells and antigen-presenting cells (APCs). We show that Vav1 controls integrin-mediated adhesion of thymocytes and T cells to ECM proteins and ICAM1 following TCR stimulation. In a peptide-specific system, Vav1 is required for T cell adhesion to peptide-loaded APCs. Intriguingly, TCR-induced cell adhesion and aggregation of integrins occurs independent of WASP. Whereas LFA-1 and actin caps colocalize in wasp(-/-) T cells in response to TCR stimulation, loss of WASP uncouples TCR caps from actin patches. Our data reveal a novel role for Vav1 and WASP in the regulation of TCR-induced integrin clustering and cell adhesion and show that integrin and TCR clustering are controlled by distinct pathways.     

Integrins regulate NMDA receptor-mediated synaptic currents

Synapses contain high concentrations of integrins, adhesion receptors known to influence the operation of neighboring transmembrane proteins. Evidence that integrins are important for consolidation of long-term potentiation suggests that these adhesion proteins may modulate activities of synaptic glutamate receptors. The present study provides a first test of the possibility that integrins modulate synaptic N-methyl-d-aspartate (NMDA)-type glutamate receptor activities. Excitatory postsynaptic currents (EPSCs) were recorded with whole cell clamp from hippocampal slices in which AMPA-type glutamate receptors and GABA(A) receptors were pharmacologically blocked. Microperfusion of the peptide integrin ligand gly-arg-gly-asp-ser-pro (GRGDSP) caused an approximately twofold increase in the amplitude and duration of NMDA receptor-gated synaptic currents. Control peptides had no effect. Paired-pulse facilitation was unchanged, indicating that the ligand did not modify neurotransmitter release probabilities. Infusion of the Src kinase antagonist PP2 but not the control drug 4-amino-7-phenylpyrazolo[3,4-d]pyrimidine eliminated the enhancing effect of GRGDSP. Integrins regulate Src kinases that are known to phosphorylate NMDA receptors. It is concluded that integrins act through this route to exert potent modulatory effects on the operation of NMDA receptors.     

From rolling to arrest on blood vessels: leukocyte tap dancing on endothelial integrin ligands and chemokines at sub-second contacts

In order to extravasate the bloodstream at specific sites of inflammation or antigen presentation, circulating leukocytes must rapidly translate specific adhesive and stimulatory signals into firm adhesion. Leukocyte arrest is nearly exclusively mediated by integrin receptors. Recent in vitro and in vivo evidence suggests that specialized integrins support reversible tethers that slow down selectin-initiated rolling of leukocytes prior to their arrest. In situ activation of integrin avidity by ligand and chemokine signaling can take place within fractions of seconds, resulting either in augmented reversible adhesions or immediate arrest on the vascular endothelium. The ability of leukocyte integrins to rapidly respond to these in situ avidity modulators appears to depend on preformed affinity and clustering states, which are internally regulated by cytoskeletal constraints on integrin conformation and mobility. We discuss potential regulatory mechanisms by which a given set of chemokine receptors and integrins may interact to rapidly generate high avidity, shear-resistant integrin-mediated leukocyte arrest on vascular endothelium.     

Manipulation of the adhesive behaviour of skeletal muscle cells on soft and stiff polyelectrolyte multilayers

Polyelectrolyte multilayer coatings have emerged as substrates to control a variety of cell behaviour, including adhesion, proliferation and differentiation. In particular, it is possible to modulate film stiffness by physical or chemical cross-linking. In this study, we evaluate the adhesive behaviour of skeletal muscle cells (C2C12 myoblasts) during the initial steps of spreading on layer-by-layer films of controlled stiffness made of poly(l-lysine) and hyaluronan as model biomaterial surfaces for muscle tissue engineering. We show that integrin clustering, integrin actin cytoskeleton connection and focal adhesion formation for cell spreading can be decoupled by controlling film stiffness. This made it possible to switch the cells morphologically between round and spreading shapes depending on the stiffness of the microenvironment. Although hyaluronan is one of the main components of cross-linked multilayer films, the HA receptor CD44 did not appear to mediate early adhesion as suggested by the use of blocking antibodies. In contrast, integrins were found to play a pivotal role in early adhesion: their activation significantly enhanced C2C12 myoblast spreading on soft films, where they were otherwise round. Integrin clustering was also induced by the softer films and enhanced on the stiffest films. Conversely, the use of soluble inhibitors or blocking antibodies directed against integrins induced a round phenotype on stiff films, where cells were well spread out in control conditions. We show that specific integrins were involved in the adhesion process as blocking β₃, but not β₁, integrins inhibited cell adhesion. These soft, stiff films can thus be used to tune the adhesion of C2C12 myoblasts, an early key event in myogenesis, via integrin clustering and subsequent signalling. They may be further used to decorticate the signalling pathways associated with β₃ integrins.     

Adhesion to fibronectin promotes the activation of the p125(FAK)/Zap-70complex in human T cells

The beta1 integrins are a family of heterodimeric adhesion receptors involved in cell-to-cell contacts and cell-to-extracellular matrix interactions. Through their adhesive role, integrins participate in transduction of outside/inside signals and contribute to trigger a multitude of cellular events such as differentiation, cell activation, and motility. The fibronectin integrin receptors, alpha4beta1 and alpha5beta1, can function as costimulatory molecules in T-cell receptor (TCR)-dependent T-cell activation. In the current study the Jurkat T-cell line was used as a model system to investigate the TCR-independent role of cell adhesion to fibronectin in the activation of Zap-70, a central molecule in the signalling events in T cells. Upon adhesion to plastic immobilized fibronectin but not to bovine serum albumin (BSA) the phosphorylation of p125FAK, a protein kinase that localizes to focal adhesion sites, was induced. Moreover, clustering of fibronectin receptors led to the detection of a p125FAK/Zap-70 complex. Finally, while the complex between fak-B, another protein kinase localized to focal adhesion sites, and Zap-70 was detected in cells plated either on BSA or on fibronectin, the formation of the p125FAK/Zap-70 complex appeared specifically induced following fibronectin-mediated integrin clustering. These data suggest the existence of a high degree of specificity when the members of the beta1 integrin family mediate signalling pathways in T cells.     

High affinity interactions of Coxsackievirus A9 with integrin alphavbeta3 (CD51/61) require the CYDMKTTC sequence of beta3, but do not require the RGD sequence of the CAV-9 VP1 protein

Integrins are transmembrane molecules involved in numerous cell matrix, cell-cell adhesion phenomena and also utilised as viral receptors. These interactions with integrins are mediated by brief oligopeptide recognition sequences. The Arg-Gly-Asp sequence (RGD), is recognized by many integrins, including integrin alphavbeta3 (CD51/61). Coxsackievirus A9 (CAV-9), a human pathogen that has an Arg-Gly-Asp sequence in the VP1 capsid protein, has been known to be one of the many viruses that utilise integrin alphavbeta3 as a receptor. In order to determine important binding sites of CAV-9 on integrin alphavbeta3, we performed binding studies of CAV-9 on CHO-alphavbeta3, CHO-alphavbeta1 and CHO-alphavbeta1-3-1 mutant cell line, in the presence of function blocking mAb specific for integrin alphavbeta3 and natural ligand vitronectin. Our experiments show that the CYDMKTTC sequence (187-193 residue) of integrin beta3, which has been shown to be involved in ligand specificity, is an important binding site for CAV-9. We also report that an RGD-less Coxsackievirus A9 mutant can bind efficiently on the ligand binding site of integrin alphavbeta3. Thus documenting the capability of this RNA virus to interact with integrin alphavbeta3, without the presence of an Arg-Gly-Asp sequence.     

Beta1 integrin activation on human neutrophils promotes beta2 integrin-mediated adhesion to fibronectin

Although the importance of beta1 integrin-mediated binding to adhesion molecules and extracellular matrix (ECM) molecules is well established for most types of leukocytes, the expression patterns and functional importance of beta1 integrins on neutrophils have remained controversial. Using flow cytometry, we found that human neutrophils express the alpha4, alpha5, alpha9 and beta1 integrin subunits. To examine whether the integrins VLA-4 (alpha4/beta1) and VLA-5 (alpha5/beta1) have a functional role on neutrophils, we studied adhesion to their ligand fibronectin. Treatment of neutrophils with antibody 8A2, which specifically binds and activates beta1 integrins, resulted in increased binding to fibronectin. However, addition of blocking mAb revealed that 8A2-induced adhesion did not depend on beta1 integrins, but on the beta2 integrin CD11b/CD18. Similarly, activation of beta1 integrins by 8A2 resulted in CD11b-dependent binding of neutrophils to fibrinogen. 8A2 treatment increased expression of an activation epitope of CD11b/CD18, which depended on phosphoinositide 3-OH kinase activity and an adequate concentration of intracellular free Ca2+. These data suggest that engagement of beta1 integrins on neutrophils results in a cross-talk signal that leads to activation of the beta2 integrin CD11b/CD18, followed by CD11b-mediated adhesion. As transmigrated neutrophils are surrounded by both beta1 and beta2 ligands in the ECM, this integrin cross-talk could play a role in modifying migration and cellular activation in inflamed tissues.     

T-cell receptor signaling to integrins

Summary:  Integrin adhesion receptors are critical for antigen recognition by T cells and for regulated recirculation and trafficking into and through various tissues in the body. T-cell receptor (TCR) signaling induces rapid increases in integrin function that facilitate T-cell activation by promoting stable contact with antigen-presenting cells and extracellular proteins in the environment. In this review, we outline the molecular mechanisms by which the TCR signals to integrins and present a model that highlights four key events: (i) initiation of proximal TCR signals nucleated by the linker for activated T cells (LAT) adapter protein and involving Itk, phospholipase C-γ1, Vav1, and Src homology 2 domain-containing leukocyte-specific phosphoprotein of 76 kDa; (ii) transmission of integrin activation signals from the LAT signalosome to integrins by protein kinase (PK) C and the adapter protein, adhesion and degranulation-promoting adapter protein; (iii) assembly of integrin-associated signaling complexes that include PKD, the guanosine triphosphatase Rap1 and its effectors, and talin; and (iv) reorganization of the actin cytoskeleton by WAVE2 and other actin-remodeling proteins. These events coordinate changes in integrin conformation and clustering that result in enhanced integrin functional activity following TCR stimulation.     

Structural specializations of immunoglobulin superfamily members for adhesion to integrins and viruses

Summary: The circulation and migration of leukocytes are critical for immune surveillance and immune response to infection or injury. The key step of leukocyte recruitment involves the adhesion between immunoglobulin superfamily (IgSF) proteins on endothelium and integrin molecules on leukocyte surfaces. Some of the IgSF members are subverted as virus receptors. Four crystal structures of N-terminal two-domain fragments of these IgSF proteins have been determined: intercellular adhesion molecule-1 (IC.AM-l), ICAM-2, vascular adhesion molecule-1 (VCAM-1), and mucosal address in cell adhesion molecule-1 (MAdCAM-1), An acidic residue near the bottom of domain 1 plays a key role in integrin binding. For ICAM-1 and ICAM-2, this glutamic add residue is located on a flat surface, complementary to the flat surface of the 1 domain of the integrin to which they bind, lymphocyte function-associated antigen-1 (LFA-1). For VCAM-1 and MAdCAM-1, the acidic residue is aspartic acid, and it resides on a protruded CD loop which may be complementary to a more pocket-like structure in the a4 integrins to which they bind, which lack I domains. A number of unique structural features of this subclass of IgSF have been identified which are proposed to consolidate the domain structure to resist force during adhesion to integrins. Different mechanisms are proposed for the different CAMs to present the integrin-binding surface toward the opposing cell for adhesion, and prevent ds interaction with integrins on the same cell. Finally, CD4 and ICAM-1 are compared in the context of ligand binding and virus binding, which shows how human immunodeficiency virus and rhinovirus fit well with the distinct structural feature of their cognate receptors.     

Endothelial cell motility, integrin receptor clustering, and microfilament organization are inhibited by agents that increase intracellular cAMP

Adhesion of human umbilical vein endothelial cells (ECs) to various extracellular matrix proteins is mostly mediated by receptors of the integrin family. The interaction of ECs with extracellular matrix proteins is accompanied by cell spreading, cytoskeletal organization, and clustering of the specific integrin receptors in complex supramolecular structures known as adhesion plaques or focal contacts. Little is known on the functional role of focal contacts in EC adhesion and motility and on the possibility to modulate their organization. In this article we report that an increase in intracellular cAMP levels severely impaired focal contact formation. This process did not affect cell attachment, but increased cell adhesion and strongly inhibited cell motility. ECs were treated with the cAMP-increasing agents forskolin and 2-chloro-adenosine or with the cAMP analogue 8-bromo-cAMP. When treated cells were seeded on purified vitronectin, fibrinogen, or fibronectin little modification in the number of attached cell was observed. In contrast ECs showed impaired organization of microfilaments and poorly developed clusters of beta 3- and beta 1-integrin receptors. On a vitronectin substrate, vinculin followed the distribution of beta 3-receptors. It was typically enriched at the focal contacts in control cells but was fragmented in small dots at the cell periphery in treated cells, as were bundles of actin stress fibers. Similarly, when forskolin was added to ECs spread on vitronectin or on fibrinogen, there was a progressive but reversible disruption of actin microfilaments and diffusion of beta 3 receptors. This was accompanied by a tighter adhesion of the cells to substrata. Migration of ECs in response to different matrix proteins was severely inhibited by cAMP-increasing agents. These data indicate that EC adhesion can occur very efficiently in the absence of fully developed beta 3- or beta 1-integrin receptor-containing focal contacts but suggest that the capacity to normally assemble focal contacts and cytoskeletal proteins is required for full cell spreading and migration.     

A pseudosymmetric cell adhesion regulatory domain in the beta7 tail of the integrin alpha4beta7 that interacts with focal adhesion kinase and src

The beta7 integrins alpha4beta7 and alphaEbeta7 play key roles in forming the gut-associated lymphoid tissue, and contribute to chronic inflammation. The alpha4beta7 integrin-mediated adhesion of activated lymphocytes is largely due to a transient increase in avidity from ligand-induced clustering of alpha4beta7 at the cell-surface. Here, we report that L and D enantiomers of a cell-permeable peptide YDRREY encompassing residues 735-740 of the cytoplasmic tail of the beta7 subunit inhibit the adhesion of T cells to beta7 integrin ligands. The YDRREY peptide abrogated mucosal addressin cell adhesion molecule-1-induced clustering of alpha4beta7 on the surface of activated T cells. A mutated form of the YDRREY peptide carrying either single or double conservative mutations at Tyr(735)Phe and Tyr(740)Phe was unable to inhibit T cell adhesion, suggesting that both tandem tyrosines are critical for activity. The YDRREY peptide was bound and phosphorylated by focal adhesion kinase and src, which may serve to sequester cytoskeletal proteins to the cytoplasmic domain of alpha4beta7. The quasi-palindromic sequence YDRREY within the beta7 cytoplasmic tail constitutes a cell adhesion regulatory domain that modulates the interaction of beta7-expressing leukocytes with their endothelial and epithelial ligands. Cell-permeable peptidomimetics based on this motif have utility as anti-inflammatory reagents for the treatment of chronic inflammatory disease.